Clay materials modified with amino acids for purification

processes of biogas and natural gas

Joanna Juźków

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisor: Prof. Dr. Moisés Luzia Gonçalves Pinto

Examination Committee

Chairperson: Prof. Carlos Manuel Faria de Barros Henriques

Supervisor: Prof. Moisés Luzia Gonçalves Pinto

Memeber of the Committee: Prof. João Manuel Pires da Silva

July 2016

ACKNOWLEDGEMENTS

In the first place I would like to thank my supervisor, prof. Moisés Luzia Gonçalves Pinto for the

opportunity to conduct my work in Lisbon, valuable guidance during the research, sharing the

knowledge, patience and motivation that allowed me to successfully complete my thesis.

I would like to offer my special thanks to prof. João Pires da Silva for helpful suggestions,

encouragement and enjoyable cooperation.

I am also very grateful to Ana Cristina Fernandes for her assistance in the research and helping hand.

I am very thankful to all the workers and students of Grupo de Adsorção e Materiais Adsorventes of

Center of Chemistry and Biochemistry of the Faculty of Sciences of the University of Lisbon for

wonderful collaboration and friendly atmosphere.

I also want to thank my dearest parents and sister for their constant support, faith in me and love.

ABSTRACT

Biogas and natural gas purification is an issue of a great industrial importance. Due to decrease of

calorific value of the fuel, corrosion and dry ince formation as a result of carbon dioxide presence in

methane, the concentration of CO2 needs to be reduced to ppm level. Adsorption on a natural sorbent

has drawn the attention of researchers recently, as a low-cost, environmentaly friendly and effective

way of gas treatment. In the present research samples of montmorillonite were intercalated with amino

acids: glycine, arginine and L-histidine at pH 7 and pH 5 in order to enhance adsorption properties of

the clay materials. The obtained adsorbents were analyzed with FTIR, XRD, thermogravimetry with

DSC and nitrogen adsorption-desorption. Conducted tests confirmed retention of amino acid

molecules in the clay structure and increase of porosity of materials in the result of intercalation.

Investigations of methane and carbon dioxide adsorption on the prepared samples were conducted

and adsorption isotherms were plotted. Clays intercalated with arginine and L-histidine adsorbed more

CO2 than in case of glycine. Materials prepared at pH 5 showed better results than samples obtained

at pH 7. The adsorbent with the highest adsorption capacity was ARG-5 with 0.80 mmol/g of CO2

adsorbed. The most selective material for CH4/CO2 separation was L-HIST-5, which up to 0.7 molar

fraction of CH4 adsorbed only CO2 from the mixture at relatively low pressure 100 kPa. The obtained

results showed a promising possibility for further application of intercalated clay materials in industrial

gas treatment processes.

Keywords: adsorption, montmorillonite, amino acids, methane purification, carbon dioxide separation

OBJECTIVES

The main aim of present research was to characterize the adsorption properties of amino acid

intercalated monmorillonite materials for the methane and carbon dioxide separation. The samples

were investigated with analytical methods: FTIR, XRD, thermogravimetry and nitrogen adosrption-

desprtion to prove the amino acids renention in the caly structure and their influence on the surface

area and porosity of clay material. Adsorption measurements of CO2 and CH4 were conducted and

adsorption isotherms were plotted in order to evaluate the adsorption capacity of the intercalated clays

in the function of pressure. Selectivity of the samples was calculated with MathCAD software and

phase diagrams of adsorption of carbon dioxide and methane from binary mixture were prepared to

determine the separation abilities of prepared materials.

TABLE OF CONTENTS

I. INTRODUCTION...................................................................................................................1

II. THEORETICAL PART ..........................................................................................................2

1. Natural gas and biogas...................................................................................................2

1.1. Fossil fuels and CO2 issue .......................................................................................2

1.2. Biomass ..................................................................................................................2

1.3. Biogas energy .........................................................................................................3

1.4. Natural gas ..............................................................................................................6

1.5. Biogas production – anaerobic digestion ................................................................7

1.6. Biogas and natural gas upgrading ..........................................................................8

1.7. Gas purification methods ..................................................................................... 11

1.7.1. Physical adsorption .................................................................................. 11

1.7.2. Chemical adsorption ................................................................................ 11

1.7.3. Adsorption on a solid surface .................................................................. 11

1.7.4. Membrane separation .............................................................................. 12

1.7.5. Cryogenic separation ............................................................................... 12

1.7.6. Pressure swing adsorption....................................................................... 12

2. Clay materials as adsorbents ...................................................................................... 13

2.1. Low-cost adsorbents ............................................................................................ 13

2.2. Clays .................................................................................................................... 14

2.3. Clay structure ....................................................................................................... 14

2.4. Smectites ............................................................................................................. 16

2.5. Properties of clays ............................................................................................... 17

2.5.1. Ion exchange ........................................................................................... 17

2.5.2. Swelling .................................................................................................... 18

2.5.3. Acidity ....................................................................................................... 18

2.5.4. Intercalation ............................................................................................. 18

2.6. Montmorillonite ..................................................................................................... 19

2.7. Amino acid intercalation ....................................................................................... 20

2.8. Adsorption of carbon dioxide on amino acid intercalated montmorillonite .......... 22

III. EXPERIMENTAL PART ..................................................................................................... 24

1. Materials preparation ................................................................................................... 24

2. Characterization methods ........................................................................................... 25

2.1. Fourier-transform infrared spectroscopy .............................................................. 25

2.2. XRD ...................................................................................................................... 26

2.3. Thermogravimetry ................................................................................................ 27

2.4. Nitrogen adsorption-desorption ........................................................................... 28

3. High pressure adsorption line ...................................................................................... 28

4. Methodology of methane and carbon dioxide adsorption measurements .................. 29

5. Data evaluation ............................................................................................................ 30

IV. RESULTS AND DISCUSSION .......................................................................................... 31

1. Fourier-transform infrared spectroscopy ..................................................................... 31

2. XRD ............................................................................................................................. 35

3. Thermogravimetry ....................................................................................................... 37

4. Nitrogen adsorption-desorption ................................................................................... 39

5. Adsorption isotherms ................................................................................................... 41

5.1. Carbon dioxide and methane adsorption at pH 7 at 25 oC .................................. 41

5.2. Carbon dioxide and methane adsorption at pH 5 at 25 oC .................................. 42

5.3. Carbon dioxide adsorption at pH 5 at 25 oC and 45

oC ........................................ 43

6. Selectivity .................................................................................................................... 44

7. Phase diagrams ........................................................................................................... 45

V. CONCLUSIONS ................................................................................................................. 49

VI. REFERENCES................................................................................................................... 50

VII. ANNEX 1 ............................................................................................................................ 54

VIII. ANNEX 2 ............................................................................................................................ 56

LIST OF FIGURES

Figure 1. Comparison of different CO2 emissions connected with different options of electricity

production .................................................................................................................................................4

Figure 2. Number of plants and total installed capacity in Europe in years 2010-2014 ...........................5

Figure 3. Energy produced from biogas power plants in Poland in years 2003-2013 .............................6

Figure 4. Anaerobic digestion steps .........................................................................................................7

Figure 5. Dependence of Wobbe index of CO2 content and relative density of the biogas .................. 10

Figure 6. Origin of low-cost adsorbents ..................................................................................................13

Figure 7. Schema of 1:1 and 2:1 clay minerals structures .....................................................................15

Figure 8. Classification of clays ............................................................................................................. 16

Figure 9. The structure of monmorillonite .............................................................................................. 19

Figure 10. Structural formulas of glycine, arginine and L-histidine ........................................................20

Figure 11. Forms of glycine ions in aqueous solution depending on pH ...............................................21

Figure 12. Intercalation of amino acids on montmorillonite depending on pH value ..............................22

Figure 13. Mechanism of CO2 adsorption on amino acid intercalated montmorillonite ........................ 23

Figure 14. Scheme of amino-acid intercalated MMT materials preparation ......................................... 25

Figure 15. Simplified schema of the adsorption line ............................................................................. 29

Figure 16. FTIR spectra for pure glycine, arginine and l-histidine ......................................................... 31

Figure 17. The FTIR spectra for MMT, L-histidine intercalated MMT samples L-HIST-7 and L-HIST-5

and pure L-histidine ............................................................................................................................... 32

Figure 18. The FTIR spectra for MMT, arginine intercalated MMT samples GLY-7 and GLY-5 and pure

arginine .................................................................................................................................................. 33

Figure 19. The FTIR spectra for MMT, glycine intercalated MMT samples GLY-7 and GLY-5 and pure

glycine .................................................................................................................................................... 34

Figure 20. XRD plots for amino acid modified MMT materials obtained at pH 7 .................................. 35

Figure 21. XRD plots for amino acid modified MMT materials obtained at pH 5 .................................. 36

Figure 22. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials

prepared at pH 7 .................................................................................................................................... 37

Figure 23. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials

prepared at pH 5 .................................................................................................................................... 38

Figure 24. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated

MMT samples at pH 7 ........................................................................................................................... 39

Figure 25. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated

MMT samples at pH 5 ........................................................................................................................... 40

Figure 26. Adsorption isotherms of carbon dioxide and methane adsorption at 25oC on amino acid

intercalated MMT samples prepared at pH 7 ........................................................................................ 42

Figure 27. Adsorption isotherms of carbon dioxide and methane adsorption at 25oC on amino acid

intercalated MMT samples prepared at pH 5 ........................................................................................ 43

Figure 28. Adsorption isotherms of carbon dioxide adsorption at 25oC and 45

oC on amino acid

intercalated MMT samples prepared at pH 5 ........................................................................................ 44

Figure 29. Selectivity of aminoacid intercalated MMT adsorbents against pressure ............................ 45

Figure 30. Phase diagrams at 100 kPa describing separation of CH4 between gaseous and adsorbed

phase as a function of mole fraction of CH4 in adsorbed phase ........................................................... 46

Figure 31. Phase diagrams for L-HIST-7, ARG-7, L-HIST-5, ARG-5 and GLY-5 ................................. 47

LIST OF TABLES

Table 1. Top natural gas exporters in 2010 ..............................................................................................7

Table 2. Typical composition of natural gas and biogas ..........................................................................9

Table 3. Typical specifications of composition on feed to LNG plant and on pipeline gas ................... 10

Table 4. θ values for basal peak obtained in XRD plots for MMT and amino acid modified MMT

materials, corresponding interlayer spacing and expansion of MMT in the effect of amino acid

intercalation ........................................................................................................................................... 36

Table 5. Porosity and surface properties characteristics of pure MMT and amino acids modified MMT

materials ................................................................................................................................................ 41

LIST OF ABBREVIATIONS

ARG-5 montmorillonite intercalated with arginine prepared at pH 5

ARG-7 montmorillonite intercalated with arginine prepared at pH 7

ARG pure pure arginine

CEC concentration of exchangeable cations

CHP combined heat and power

C1 constant

C2 constant

C3 constants

d the interplanar distance [Ǻ]

FTIR Fourier-transfer infrared spectroscopy

G Gibbs free energy [J]

GHG greenhouse gas

GLY-5 montmorillonite intercalated with glycine prepared at pH 5

GLY-7 montomrillonite intercalated with glycine prepared at pH 7

GLY pure pure glycine

K equilibrium constant

L-HIST-5 montomrillonite intercalated with L-histidine prepared at pH 5

L-HIST-7 montomrillonite intercalated with L-histidine prepared at pH 7

L-HIST pure pure L-histidine

LNG liquiefied natural gas

MMT raw montmorillonite

n the positive integer

nads amount of moles of gas adsorbed on the unit of mass of adsorbent [mmol/g]

n0CO2 standard-state loading for pure CO2

n0CO2 standard-state loading for pure CO2

p pressure [kPa]

PSA pressure-swing adsorption

R gas constant

SCO2/CH4 selectivity of the clay material for CO2/CH4 mixture

T temperature [oC]

TG DSC thermogravimetry with differential scanning calorimetry

XRD X-ray diffraction

xaCO2 CO2 molar fraction in the adsorbed phase

xaCH4 CH4 molar fraction in the adsorbed phase

xgCO2 CO2 molar fraction in the gas phase

xgCH4 CH4 molar fraction in the gas phase

λ the wavelength of incident wave (λ=1.5406 Ǻ)

θ the scattering angle [o]

1

I. INTRODUCTION

Nowadays, due to proceeding exploitation of limited fossil fuel resources, alternative sources of energy

need to be developed. Moreover, because of high concentration of CO2 in the atmosphere steps have

to be undertaken in direction of cutting the emissions released to the environment. Implementation of

biomass as renewable energy source offers a considerable solution for both issues mentioned. Use of

this variant of green energy allows to avoid exploitation fossil fuels and rise of the level of CO2,

because of emission of so called neutral CO2. Among fuels derived from biomass, biogas gained a lot

of attention as a green alternative for natural gas. Biogas is a product of anaerobic digestion,

composed in major part of methane and carbon dioxide. In order to be applied as substituent for

natural gas, biogas needs to undergo treatment in order to remove contaminants and upgrading to

eliminate CO2, which has negative influence on the calorific value of biogas. Hence, an efficient way of

carbon dioxide and methane separation needs to be found, which could be possible to implement in a

large-scale industrial process.

Among available adsorbents clay minerals drew attention, because of their extraordinary properties

allowing for their implementation in gas separation. Layered structure, porosity and cation exchange

properties are main characteristics, that make these materials highly interesting for investigation in the

field of adsorption. A lot of research over these materials has been already done, however, still new

applications for these materials are being discovered. The proper modification of clays tunes

properties allowing for adsorption of specific compounds with high selectivity and efficiency.

In the present work investigations on separation of carbon dioxide and methane were conducted using

montmorillonite – a clay material. Amino acid intercalation of montmorillonite was applied in order to

enhance its adsorptive properties. The obtained samples were characterized with different analytic

methods, what allowed for description of their structural, surface and porous properties. The

adsorption isotherms for retention of carbon dioxide and methane on modified clays were plotted and

analyzed. Finally, selectivity of the tested materials was evaluated and estimation of adsorption from

binary mixture was conducted using phase diagrams.

2

II. THEORETICAL PART

1. Natural gas and biogas

1.1. Fossil fuels and CO2 issue

Since ancient times mankind utilized bioenergy and biofuels. Burning of wood provided heat and light

needed for cooking, heating of shelters, illumination and pottery. Until 19th century wood remained the

main fuel for cooking and heating purposes and vegetable oil was used as a fuel for lightning [1].

Nowadays fossil fuels are by far the most dominant energy sources and provide about 80% of total

world energy consumption. There is a strong necessity to cut the utilization of fossil fuel energy

resources due to their significant environmental impact connected with the emission of greenhouse

gases and pollutants [2]. As effect of fossil fuels consumption, the atmospheric CO2 concentration has

been elevated from 280 ppm in pre-industrial period to nearly 400 ppm in present times, what

contributed to a significant climate change [1]. Due to this fact steps aiming in decreasing the GHG

emission have to be undertaken. The recent solution for this problem is the implementation of

renewable energy sources as an alternative of fossil fuels. This idea is supported by the European

Union, that on 9th December 2008 passed the “Renewable Energy Directive”, which set a target of

achieving the production of 20% of Europe’s energy from renewable sources by the year 2020 [3, 4].

1.2. Biomass

One of most promising renewable energy sources is biomass. Biomass can be defined as the

contemporaneous (non-fossil) biological material generated from the conversion of solar energy into

vegetable matter and it is considered as one of the most suitable ways of energy storage, being a real

alternative to fossil fuels, as it is abundant, clean and carbon neutral [5]. In 2009 biomass contributed

13,1% of global energy demand. The global primary energy supplied from biomass reached

approximately 55 EJ in 2012. Heating accounted for the majority of use of biomass (46 EJ), including

heat produced from modern use of biomass (biofuels) and also traditional uses in the form of wood

and peat. By the end of 2012, global bio-power approached 83 GW. In Europe, biomass currently

accounts for around 2/3 of renewable energy and will play a key role in reaching the target approved

by the renewable sources by 2020 [2].

Biomass resources can be defined as byproducts with no or low profit from agricultural crops or

industrial processes and as crops grown specially for the purpose of energy production. Other

biomass resources are part of agricultural or industrial waste streams representing negative profit. In

Europe and North America, agricultural byproducts used for energy production include wheat straw,

corn stalks and soybean residues, while large industrial waste streams, in e.g. the United States,

originate mainly from the paper-making industry. The major crops presently grown for energy include

3

sugar or starch crops such as sugar cane and corn. In addition, crops containing mainly

lignocelluloses e.g. willow and poplar are getting more attention with respect to their application for

energy production [6].

1.3. Biogas energy

Biogas is produced by anaerobic digestion of biomass; organic feedstock, the most common being:

animal waste and crop residues, dedicated energy crops, domestic food waste and municipal solid

waste (MSW) [7]. Industrial biogas is generated at sewage treatment plants, landfills, sites with

industrial processing industry and at digestion plants for agricultural organic waste [8].

Biogas is an alternative for energy production, that has advantages of being eco-friendly source of

energy, in that the calorific value of biogas is equal to that of half litre of diesel oil. (6 kWh/m3). This

biofuel is fully capable of replacing other rural energy sources like wood, hard coal, kerosene, plant

residues or propane [9]. Use of biogas is highly beneficial for the environment, as it contributes to

reduction of emission of greenhouse gas and air pollution in effect of reduction of the use of fossil

fuels. Moreover, the process of anaerobic digestion applied in biogas production reduces odours,

pathogens and other components, that could be harmful to plants (e.g. organic acids) [7].

Biogas is mainly used for Combined Heat and Power (CHP) and in electricity generation and feed-in to

the national grid. Like with any other renewable energy resources, the application of biogas

technology will contribute to reduction in greenhouse gas (GHG) emissions and air pollution, due to

the expected reduction of the use of fossil fuels. Fig. 1. compares GHG emissions associated with

different electricity production options from a life-cycle perspective for biogas, fossil fuels, and other

renewable energy sources. The calculated negative emissions for biogas-CHP are result of the

substitution of oil fuel with biogas [7].

4

Fig 1. Comparison of different CO2 emissions connected with different options of electricity production

[10].

To the countries with biggest biogas production are Germany, UK, France, Italy and Netherlands.

Biogas is utilized in combined heat and power (CHP) units to produce electricity and heat. In 2013,

primary production of biogas in Europe (including landfill and sewage gas) was estimated 13,4 million

tons of oil equivalents (Mtoe) [2, 11-13].

It is significant, that a clear increase in energy obtained from renewable sources in Europe is being

noted, for instance, in 2003 the proportion of renewable energy obtained was 11.1% of primary energy

in general, and in 2012 this value increased to 22.3%. In Fig. 2. the number of plants and total

installed capacity of biogas powered power plants in Europe during years 2010-2014 are presented.

Constant growth of the number of power plants can be observed, followed by the rise in amount of

energy produced.

5

Fig 2. Number of plants and total installed capacity in Europe in years 2010-2014 [14].

The rise in renewable energy utilization can be also noticed in Poland. In 2012 the share of the

obtained renewable energy was 11.7% of the primary energy in general, while in 2003 it was 5.2%.

Electricity production in power plants powered with biogas should also be considered, because the

progress in this field can be clearly seen. In the years 2003–2013 an almost twelve-fold increase in the

production of energy from biogas was recorded; it grew from 56 GWh in 2003 to 670 GWh in 2013

[15]. This tendency is presented in the Fig. 3. below, with specification of origin of biogas used for

energy production. It can be clearly seen, that renewable energy use has been intensively developed

in Poland during recent decade and is likely to increase even more in following years.

6

Fig. 3. Energy produced from biogas power plants in Poland in years 2003-2013 [15].

In Poland, due to a large area of agricultural lands (14.6 million hectares) and well developed cattle

and pig raising, the opportunities for development of the renewable energy market are seen in

agricultural biogas plants. Poland has a great biogas potential, which is comparable to that of

Germany, which is a leader country in generation energy from biogas in Europe [16].

1.4. Natural gas

Natural gas production is estimated to be over 3300 billion cubic meters per year worldwide [17]. In

2010 natural gas supplied 23,81% of world’s energy demand and the rise in consumption of natural

gas was noted by 7,4% comparing to 2009. The increase of the demand led to re-evaluation of gas

reserves, that earlier were considered as unviable economically due to contamination. Moreover,

many significant reserves of natural gas are located far from the gas markets in Western Europe,

Japan and South Korea. For this reason immerse volumes of gas have to be transported for long

distances from exporting countries by pipeline or in tankers as LNG – liquefied natural gas. The

production of LNG is essential to international trade and its importance is going to increase in following

decades [18]. In Table 1. top natural gas exporters were gathered with amounts of gas exported by

them in 2010. As newer resources of natural gas are being discovered, the natural gas market is

predicted to expand up to 65% in 2035 [19].

7

Table 1. Top natural gas exporters in 2010 [18].

Country Pipeline LNG Total

Russian Federation 186.5 13.4 199.9

Norway 95.9 4.7 100.6

Qatar 19.2 75.8 94.9

Canada 92.4 0.0 92.4

Algeria 36.5 19.3 55.3

Although natural gas cannot be referred to as the green energy, its use has many advantages over the

use of other conventional fuels. Burning NG produces less CO2 and more water vapour per energy

unit than burning gasoline or diesel. NG presents an alternative to oil-derived fuels. In the automotive

sector, NG provides a secure, clean and efficient combustion, almost free of sulphur and lead oxides,

benzene and solid particles. As it is less dense than air, it spreads in the case of leak, minimizing risk

of explosion. The emissions of carbon dioxide and carbon monoxide in NG-powered vehicles can be

reduced 23% and 85% in comparison to conventionally-powered vehicles. The main drawback of NG

is low volumetric heat of combustion than in case of liquid fuels [20].

1.5. Biogas production - anaerobic digestion

Anaerobic digestion is a natural biological process of biogas production, in which organic matter

(biomass) is broken down by bacteria in conditions of small or no access of oxygen. The final result of

a series of decomposition reactions are methane, carbon dioxide and water as main products. The

process of anaerobic digestion can be divided into three main steps, as shown in the Fig 4.

Fig. 4. Anaerobic digestion steps.

CH4 CO2 H2O carbosmolecules

Methanogenesis

Complex organics carbohydrates, fats, oils,

proteins

carbosmolecules

Simpler organics amino acids, fatty acids,

sugars

carbosmolecules

Organic acids formic, acetic,

propionic

carbosmolecules

Hydrolysis Liquefaction

carbosmolecules

Acetogenesis

8

In the first stage, called hydrolysis or liquefaction, the insoluble complex organic matter like e.g.

cellulose is converted by fermentative bacteria into soluble molecules like sugars, amino acids and

fatty acids.

The complex polymeric matter is hydrolyzed to monomers, e.g. cellulose to sugars or alcohols and

proteins to peptides or amino acids, by hydrolytic enzymes, (lipases, proteases, cellulases, amylases,

etc.) secreted by microbes (1-4).

lipids → fatty acids (1)

polysaccharides → monosaccharides (2)

proteins → aminoacids (3)

nucleic acids → purines and pyrimidines (4)

In the second stage products of the first phase are converted by acetogenic bacteria to simple organic

acids, carbon dioxide and hydrogen. The acetogenesis reaction is presented below (5):

C6H12O6 → 2C2H5OH + 2CO2 (5)

In the final – third stage methane is produced by bacteria called metanogens in two possible ways –

by cleavage of acetic acid molecules to carbon dioxide and methane or by reduction of carbon dioxide

with hydrogen. The first reaction is dominant, as the reduction of CO2 is dependent of hydrogen

concentration in digesters. The methanogenesis reaction can have following forms (6-8) [21-23]:

CH3COOH → CH4 +CO2 (6)

2C2H5OH + CO2 → CH4 + 2CH3COOH (7)

CO2 + 4H2 → CH4 + 2H2O (8)

The obtained biofuel can be use as alternative energy source. When CO2 and other impurities are

removed during the upgrading process, the methane concentration increases and thus the resulting

biomethane can be utilized as an alternative to natural gas [24]. After treatment the biogas can be

used as a vehicle fuel or injected in existing natural gas grids [25].

1.6. Biogas and natural gas upgrading

The main aims of the biogas treatment are:

- The cleaning process, in which the trace components harmful to the natural gas grid,

appliances and end-users are removed

- The upgrading process, in which CO2 is removed in order to adjust the calorific value and

relative density and to meet the specifications of Wobbe index, which is dependent on both

calorific value and relative density [26].

9

Typical composition of natural gas and biogas are presented in the Table 2. below.

Table 2. Typical composition of natural gas and biogas [27].

For methane distribution through pipelines or liquefaction in order to transport it for long distances the

impurities contained in methane (in form of biogas or natural gas) need to be removed, because they

can cause corrosion in the pipelines, compressors, gas storage tanks and engines [12]. Methane for

domestic use is required to have 97% or higher purity. Presence of CO2, as it is a inert gas in terms of

combustion, lowers the calorific value of the fuel. In order to eliminate the formation of dry ice and

corrosion in the liquefaction step, impurities contained in the natural gas or biogas, such as CO2 or

H2O have to be reduced to ppm level [29].

Upgrading biogas to the quality of natural gas is a multiple step process. After removal of water

(vapour), H2S, siloxanes, carbon hydrates and NH3, the removal of CO2 is necessary to obtain quality

that meets the Wobbe index. As the CO2 in the upgraded gas is removed, the relative density

decreases and the calorific value increases, increasing the Wobbe index [26].

In the Fig. 5. dependence of the Wobbe index on CO2 content and relative density is shown.

Natural gas component %mol

methane 70-90

ethane

propane

butane

0-20

CO2 0-8

N2 0-5

H2S 0-5

O2 0-0.2

rare gases (He, Ar, Xe, Ne) trace

Biogas component %mol

methane 40-75

CO2 15-60

H2S 0.0005-2

H2O 0-10

volatile compounds

NH3

N2

O2

CO

trace

10

Fig. 5. Dependence of Wobbe index of CO2 content and relative density of the biogas [30].

After transformation, the final product is referred to as biomethane and typically contains 95-97% CH4

and 1-3% CO2 [26]. The calorific value of raw biogas is 22 000 to 25 000 kJ/m3. After CO2 is removed,

the methane gas increases calorific value up to 39 000 kJ/m3 [31].

Typical requirements, which have to be met in order to apply the gas on feed to NG plants and for

pipeline transport were gathered in the Table 3.

Table 3. Typical specifications of composition on feed to LNG plant and on pipeline gas [32].

Impurity Feed to LNG plant Pipeline gas

H2O <0.1 ppmv 150 ppmv

H2S <4 ppmv 5.7-22.9 mgSm-3

CO2 <50 ppmv 3-4 vol.%

total sulphur <20 ppmv 115-419 mgSm-3

N2 <1 vol.% 3 vol.%

Hg <0.01 mg/Nm3 -

C4 <2 vol.% -

C5+ < 0.1 vol.% -

aromatics <2 ppmv -

11

1.7. Gas purification methods

Removal of carbon dioxide from methane in order to meet specifications required for pipeline transport

(typically 2-3vol% CO2) or LNG production (typically less than 50 ppm CO2) can be achieved using

different technologies available [33]. Some of them are discussed below.

1.7.1. Physical adsorption

One of the easiest and cheapest method of the gas treatment is physical adsorption. It involves use

pressurized water as an adsorbent. Instead of water, organic solvents such as methanol and

dimethylethers of polyethyleneglycol (DMPEG) can be used to absorb CO2 [34]. The raw biogas is

pressed and introduced to the column from the top, while pressurized water is sprayed from the top.

The CO2 (as well as H2S) is dissolved in the water, which is collected in the bottom of the column.

Methane, which has very low solubility in water, is not retained. This method is effective also at low

flow rates, at which power plants are normally operating. It is also simple method, that does not

require much infrastructure and is cost-effective [35, 36].

1.7.2. Chemical adsorption

Chemical adsorption involves formation of a reversible chemical bond between solute and solvent.

Regeneration of the solvent requires breaking of the bonds, which needs more energy input. Solvents

used contain usually aqueous solutions of amines or aqueous solution of alkaline salts [36]. The

absorption with organic amine solution is highly efficient, but the regeneration of amine solution usually

involves high energy consumption and potential high corrosion, which is not environmentally friendly

[31].

1.7.3. Adsorption on a solid surface

Adsorption process involves transfer of solute in the gas stream to the surface of solid material, on

which they concentrate by the effect of physical or Van der Waals forces. Commercial adsorbents are

usually granular solids with a large surface area per unit volume. Depending on the choice of

adsorbent, simultaneous or selective removal of CO2, H2S, moisture and other impurities can be

obtained. Among commonly used adsorbents are silica, alumina, activated carbon and silicates.

This type of adsorption process is usually conducted at high temperature and pressure. The

advantages are significant moisture removal capacities, design simplicity and easiness of operation

[36].

12

1.7.4. Membrane separation

The principle of this method is the transport of some components of the raw through a thin membrane

(< 1mm), while others are retained. The transport of the molecules in driven by the difference in partial

pressure over the membrane and is dependent on the permeability of the membrane material. For the

high methane purity, the permeability of membrane needs to be high. For example, a solid membrane

constructed from acetate-cellulose polymer shows permeability for CO2 up to 20 times higher than for

CH4. The disadvantage of the process is that it requires application of high pressure of 25-40 bar [36,

37]. For the membrane separation, the membrane itself is expensive, often suffers thermal shock and

chemical corrosion, and is easily contaminated [31].

1.7.5. Cryogenic separation

The cryogenic method involves the separation of gas mixtures by fractional condensations and

distillations at low temperatures. During cryogenic the raw biogas is compressed to ca. 80 bar. Then

the gas is dried in order to avoid freezing in the cooling process. Next, the biogas is cooled down to -

45oC and condensed CO2 is removed in the separator. The CO2 is further processed to recover the

dissolved methane, which is recycled to the gas inlet. With this process methane of 97% purity can be

obtained. The advantage of the process is the recovery of the pure methane in the form of liquid,

which can be transported conveniently. Disadvantages of the method are high capital cost and

complicated equipment requirements [33].

1.7.6. Pressure swing adsorption (PSA)

Among the available adsorption technologies, pressure swing adsorption (PSA) has gained interest as

a method for separation and capture of CO2 due to the low energy requirements and low capital costs

in comparison to common separation methods. PSA is based on selective adsorption of the undesired

gas on a porous adsorbent at high pressure and recovery of the gas at low pressure. After the process

the porous adsorbent can be reused in a subsequent adsorption cycle [38].

13

2. Clay materials as adsorbents

2.1. Low-cost adsorbents

Nowadays there is a strong concern risen around the proceeding degradation of the natural

environment caused by human activity. It is essential to take the steps that would minimize the impact

on the nature. Hence, new criteria appeared in terms of design of new materials and processes. The

attention of the scientific research is focused to find not only cheap and effective materials that can be

applied in the big-scale industrial processes, but also environmental friendly. The recent trend in the

science is the development of materials that are biocompatible, non-toxic and neutral towards

ecosystem. Conventional adsorbents utilized in the industry usually involve high cost of production,

complicated treatment and regeneration, also generating pollutants and residues, which pose danger

to the natural environment. In the adsorption processes a new type of materials is gaining a growing

popularity in the last few decades. These are so-called low-cost adsorbents, a group of natural

materials, which are abundant in the nature, easily accessible and exploitable and relatively cheap in

comparison to conventional adsorbents. This group also encloses materials that are agricultural or

industrial residues, which after simple treatment can be successfully used as adsorbents. The origin of

these materials, diversity of properties and low costs enable their wide application in various

processes [39].

The division of low-cost adsorbents regarding their origin is presented in the Fig 6. below:

Fig. 6. Origin of low-cost adsorbents [40].

As presented above, mineral materials are one of the groups of low-cost adsorbents. They are

inorganic materials building the earth’s crust, abundant in the nature.

LOW-COST ADSORBENTS

Natural materials Agro-industrial residues Mineral and soil materials

orange peel

coconut shell

wheat straw

sawdust

fly ash

sawdust

clays

peat

iron oxides red mud

natural zeolites tea waste

14

Natural clay minerals are known to mankind since the beginning of the civilization. They are naturally

occurring nanomaterials, which are the youngest members of the family of minerals in the earth’s

crust. Regarding their abundance in most continents of the world, high sorption properties and

potential for ion exchange, clay materials found wide application in adsorption processes [41, 42].

2.2. Clays

Clays are natural, earthy, fine-grained materials that develop plasticity when mixed with limited amount

of water. They are cheap, easily available and green materials possessing a wide variety of structures

[43, 44]. Clays are the main components of the mineral fraction of soils and are one of most abundant

natural low-cost materials (~$50/ton) [45]. Clays are very versatile materials and hundreds of millions

of tons currently find application in ceramics and building materials, paper coating and fillings, drilling

muds, foundry moulds, pharmaceuticals etc. Clays have been extensively used as bricks and

insulation materials due to their abundance, high mechanical strength and good thermal stability. They

have attracted attention due to their unique properties such as selective adsorption and the capability

for the surface modification. They found application as adsorbents, catalysts or catalysts supports, ion

exchangers etc, depending on their specific properties [45-47]. Clays have also been incorporated as

fillers for fabricating nano-composite polymers either to enhance the mechanical/physical properties

[43].

Two broad classes of clays may be identified:

- Cationic clays (or clay minerals), widespread in nature. They have negatively charged

alumina-silicate layers, with small cations in the interlayer space to balance the charge

- Anionic clays (or layered double hydroxides LDHs), more rare in nature, but relatively simple

and inexpensive to synthesize in the laboratory and industrial scales. The anionic clays have

positively-charged brucite-type metal hydroxide layers with balancing ions and water

molecules located interstitially [46].

2.3. Clay structure

Clay minerals are built of layered silicates. They are crystalline materials of very fine particle size

varying from 150 to less than 1 micron. The basic building block for clay materials are tetrahedral and

octahedral layers [41].

Tetrahedral layers are composed of continuous sheets of silica tetrahedral linked via three corners to

form a hexagonal mesh and the fourth corner of each tetrahedron (normal to the plane of the sheet) is

shared with octahedral in adjacent layers. Octahedral layers in the clay minerals, are composed of flat

layers of edge-sharing octahedra, each formally containing cations in its centre (usually Mg2+

or Al3+

)

15

and OH- and O

2- at its apices. The Al

3+ is generally found in six fold or octahedron coordination , while

the Si4+

cation takes place in four fold or tetrahedral coordination with oxygen. Octahedral layers may

be trioctahedral or dioctaherdral, which depends on the degree of occupancy of the octahedral sites

[41, 43].

The different arrangement of tetrahedral and octahedral layers leads to creation of two basic

structures, in which clay minerals can occur; namely 1:1 and 2:1 structure.

- 1:1 structure, also known as OT structure, has alternating tetrahedral and octahedral

sheets, occurs in e.g. kaolinite group

- 2:1 structure or TOT structure is composed of a sandwich of one octahedral sheet between

two tetrahedral sheets and can be found in e. g. smectite clay materials, like montmorillonite.

These layered crystals, which are approximately 1 nm thick with lateral dimensions from 30

nm to several microns, are piled parallel to each other and are bonded by local Van der Waals

and electrostatic forces [41, 43].

The schema of discussed clay structures are presented in Fig. 7. below:

1:1 structure (OT) 2:1 structure (TOT)

Fig. 7. Schema of 1:1 and 2:1 clay minerals structures [42].

Clays are a wide family of minerals, which can be divided into groups, considering their structure. The

general systematization of clay minerals is presented in the Fig 8. below:

16

Fig. 8. Classification of clays [42].

2.4. Smectites

Smectites belong to the family of 2:1 phylosilicates and are constructed from layers formed in the

result of the condensation of one central octahedral sheet and two tetrahedral sheets. [48].

Smectites are a group of clay minerals that have a dioctahedral or trioctahedral structure, with

isomorphous substitution that leads to a negative layer charge of less than 1.2 formula unit. Interlayer

spacings vary between ~10 and 15 Ǻ and are generally dependent of the nature of the exchangeable

cation and the relative humidity [41].

Smectites can be divided into four sub-classes, depending upon:

- The type of octahedral layer (dioctahedral or trioctahedral)

- The predominant location of the layer charge sites (octahedral or tetrahedral)

trioctahedral

the Serpentine Group

lizardite

antigorite

chrysotile

amesite

carlosturanite

greenalite

CLAYS

1:1 MINERALS

dioctahedral

the Kaolin Group

kaolinite

dickite

nacrite

halloysite

hinsingerite

beidelite

montmorillonite

hectorite

saponite

2:1 MINERALS

pyrophyllite

talc

micas

smectites

vermiculite

chlorite

illite

17

Montmorillonite and beidellite groups are dioctahedral smectites with composition presented in

formulas below:

Montmorillonite group: (Mx+)ex

[(Si8)tet

(M(III)4-x)M(II)x)oct

O20(OH)]x-4

Beidellite group: (Mx+)ex

[(Si8-xAlx)tet

(M(III)4oct

O20(OH)4]x-

Where M+ is an exchangeable cation present in the interlayer (e.g. Na

+) and M(III) and M(II) are non-

exchangable octahedrally coordinated trivalent and divalent cations (e.g. Al3+

and Mg2+

), respectively

and the layer charge is 0.5<x<1.2.

Minerals belonging to hectorite and saponite groups are trioctahedral smectites, characterized by

formulas:

Hectorite group: (Mx+)ex

[(Si8)tet

(M(II)6-xM(I)x)oct

O20(OH)4]x-

Saponite group: (Mx+)ex

[(Si8-xAlx)tet

(M(II)6)oct

O20(OH)4]x-

M(II) and M(I) are non-exchangeable octaherally coordinated divalent and univalent cations (e.g. Mg2+

and Li+) respectively and the layer charge is 0.5<x<1.2 [41].

2.5. Properties of clays

Clay minerals present a number of extraordinary properties, because of which these materials find

wide application as adsorbents, catalysts and supports. Most significant of them were shortly

described below.

2.5.1. Ion exchange

The clay materials posses ion exchange properties, which allow for isomorphous substitution of metal

cations in the lattice by lower-valent ions, e.g. the aluminium ions can be replaced by silicon ions, what

in effect leaves the residual negative charge in the lattice. The created negative charge can be

balanced by other cations. The cations can be exchanged when they are brought into contact with

other ions in aqueous solution [41].

The concentration of exchangeable cations of clay material is called CEC and is usually expressed in

miliequivalents per 100 g of the dried clay. Because smectites are clays with the highest concentration

of interlayer cations, they show highest cation exchange capacities (typically 70-120 mequiv./100 g).

Structural defects at layer edges additionally contribute CEC and also to a small amount of anion

exchange capacity.

18

The exchangeable ions influence microporosity and surface area affecting also adsorption capacity of

the clay material. Hence, the possibility to introduce desired cations to the interlayer position allows for

tuning the clay’s properties. It was reported, that saturation of clay material with exchangeable ions

Na+, K

+, Ca

2+, H

+, Al

3+ enhanced its adsorption capacity towards C2H2 and CO2. The gas adsorption

was influenced by the ionic radius of exchangeable cations, the interlayer separation and surface area

of montmorillonite [49].

2.5.2. Swelling

Clay minerals can absorb water between layers, moving them apart, what causes the swelling of clay.

In order to obtain efficient swelling the energy released by cations or layer salvation needs to be

sufficient to overcome the attractive forces (like hydrogen bonding) between the layers in the clay

structure. In 1:1 (OT) type clay minerals (kaolinite), water forms strong hydrogen bonds with hydroxyl

groups on hydrophilic octahedral layers, allowing swelling to occur. In 2:1 (TOT) clay minerals the

ability to swell depends on the solvation of interlayer cations and the layer charge. Clays with low layer

charge (e.g. talc and phyrophillite) have very low concentration of interlayer cations and their swelling

is more difficult. On the other hand, clay minerals that have very high charges (e.g. mica) present

strong electrostatic forces, which hold together anionic layers and the interlayer cations together, also

preventing from swelling. The expansion proceeds most easily in case of materials with univalent

cations in the interlayer. With the increase of valence number of the cation, swelling decreases

accordingly. The extent of swelling can be observed by measuring interlayer separations using powder

X-ray diffraction method [41].

2.5.3. Acidity

The cations located in the interlayer space contribute to the acidity of the clay minerals. The protons

H+ and polarizing cations (e.g. Al

3+) give rise to strong Brönsted acidity. The higher the

electronegativity of M+, the stronger are the acidic sites generated. Brönstedt acidity also derives from

the terminal hydroxyl groups and from the bridging oxygen atoms in the clay structure. Moreover,

presence of layer surface and edge defects in clay minerals results in weaker Brönsted or Lewis

acidity, specially at low concentrations [25].

2.5.4. Intercalation

Considering cation exchange properties and layered structure of clay materials, it is possible to

introduce molecules of other compounds, which can be used to change and tune their properties.

Different guest molecules can be introduced between layers of the clay, depending on the desired

19

result. Because of swelling properties of clays, exchanged molecules can be bigger than original ones,

what is compensated by expanse of interlayer spacing in the clay structure [42, 50, 51].

2.6. Montmorillonite

Montmorillonite is a smectite clay and many of its industrial uses are related to the manufacture of

wine, beer, oils, moulding sand, ore pellets, petroleum products, pesticides, catalysts, adsorbents,

cosmetics, ceramics, paintings, etc. Montmorillonite is a 2:1 (or T–O–T) layer phyllosilicate clay formed

by an octahedral sheet containing Al3+

or Mg2+

ions between two tetrahedral silica sheets The

isomorphic substitution in the octahedral and tetrahedral sheets results in a deficit of surface charges

that is balanced by exchangeable cations, e.g. Na+, K

+, Ca

2+, Mg

2+ situated in the interlayer position

[52].

The structure of montmorillonite is presented in Fig. 9.

Fig. 9. The structure of monmorillonite.

Montmorillonite presents a relatively high cationic exchange capacity and is easily expandable, which

allows the intercalation of a wide range of organic species [53].

Researchers have studied the properties of clays and found that clays contain weak base character,

due to presence of –OH groups in its structure and ion-exchange properties which can attract CO2 gas

easily [54]. The interactions performed by the formation of weak Van der Waals forces and the

immobilization of CO2 molecules. Thus, clays can potentially be applied as CO2 adsorbents [47].

Ca2+

Ca2+

Na+ Mg

2+ K

+ Mg

2+

Mg2+ K

+ Na

+ Na

+

20

2.7. Amino acid intercalation

Clay minerals can be substantially modified by replacing the natural inorganic interlayer cations with

selected organic cations. An example of organic compounds used for intercalation are amines [55] and

amino acids [51]. In the present research, in order to enhance the adsorption properties of

montmorillonite towards CO2 an intercalation of clay materials with amino acids was conducted.

Amino acids are cheap, under appropriate pH they are stable as cations and anions, so they can be

exchanged with interlayer ions of cationic clays. These compounds can be used for carbon dioxide

adsorption purposes in order to enhance the adsorption properties presented by clay materials. It was

proved that amino acids can be successfully intercalated to the montmorillonite, causing the extension

of intermolecular spacing. 20-50% increase of BET surface area was indicated [50].

Three types of amino acids were used for clay intercalation in the presented research: glicyne,

arginine and L-histidine. Their structural formulas are presented in the Fig. 10.

Fig. 10. Structural formulas of glycine, arginine and L-histidine [56].

Amino acids can be considered as green compounds, as being one of main building blocks of living

organisms are eco-friendly and do not cause contamination when released to the environment. Among

the amino acids three of them were chosen for the research: glycine, arginine and L-hisitidine. The

choice was made in order to compare the influence of functional groups of amino acid on adsorption

properties. Glycine, as the simplest amino acid, contains only one NH2 group in the structure. L-

histidine and arginine, having additional basic amino groups side chains are expected to show

enhanced adsorption properties [56].

21

Amino acids in water solutions dissociate and turn into ions. Depending on the pH applied, they can

be present in the form of cations, anions or zwitterions. They are adsorbed to MMT by exchange with

exchangeable ions present in interlayer space of clay. This amino acid can be located in the interlayer

space as both glycinium and zwitter ion [50]. There is an electrostatic interaction between the

carboxylic acid species and the edge surface sites. When smectite edge sites are saturated, glycine is

adsorbed by intercalation [57].

Forms, in which amino acids will be present in aqueous solution at given pH is dependent on their

isoelectric point. For the chosen amino acids the values of isoelectric points are as follows [58]:

pI = 6.0 for glycine

pI = 7.6 for l-histidine

pI = 11.2 for arginine

Predominant forms in which the amino acid is present in the aqueous solution depending on pH are

shown for the example of glycine on Fig. 11. below:

Fig. 11. Forms of glycine ions in aqueous solution depending on pH [59].

In case, when applied pH is lower than pI, the amino acids are protonated and NH3+ are dominant in

chains, the strong electrostatic interaction between NH3+ of amino acid molecules and negatively

charged surface of montmorillonite is the main driving force for intercalation, in case when pH > pI

there are more COO- groups in amino acid chains and the intercalation proceeds by coordination

between COO- groups and silicate layer of montmorillonite, the amino acids are inserted in the

interlayer and bind with the clay by carboxylate groups [60].

In general, positively charged basic amino acids are more strongly adsorbed than neutral or acidic

amino acids, due to ion-exchange reactions onto negatively charged clay surfaces [57].

Two possible ways of amino acid intercalation are presented in Fig. 12 below:

22

Fig. 12. Intercalation of amino acids on montmorillonite depending on pH value [60].

2.8. Adsorption of carbon dioxide on amino acid intercalated montmorillonite

Amino acid intercalated clay materials are capable of CO2 retention due to the presence of amine

groups in the amino acid structure. However, the exact mechanisms leading to the retention of carbon

dioxide inside the clay structure have not been investigated yet. According to one of hypothesis the

adsorption of CO2 can proceed through a reaction of a carbon dioxide molecule with amine group

creating carbamic acid group, which can transform to carbamate due to dissociation. This model is

based on better known system of CO2 adsorption on amine intercalated clays, already discussed in

another work [61].

The proposed mechanism of retention of CO2 by amino acid intercalated clay was presented in the

Fig. 13.

pH < pI pH > pI

23

Fig. 13. Mechanism of CO2 adsorption on amino acid intercalated montmorillonite.

The created carbamic acid group is unstable and can release CO2 upon heating, what allows for easy

regeneration of adsorbent, not requiring application of other chemical solutions, what is an important

advantage of this process [62].

The manner of bonding between clay material and amino acid molecules is also not known yet. It can

proceed through creation of a chemical bond or electrostatic forces. The bonding presented in Fig. 13

is a proposed model for the system.

24

III. EXPERIMENTAL PART

1. Materials preparation

Natural montmorillonite (MMT) clay from Wyoming, USA has been used as a raw material for

adsorbents preparation. For the clay’s surface functionalization three different amino acids: glycine,

arginine and l-histidine (Sigma Aldrich, purity >99%) were used.

Two samples, at pH 5 and pH 7 were prepared for each kind of amino acid and also a sample of non-

intercalated MMT. In order to prepare amino acid intercalated clay adsorbents the following procedure

was applied. 2 g of clay was added to 100 mL of distilled water and mixed for 3 h using magnetic

stirrer. 50 mL of 0.03 M solution of amino acid was prepared by dissolving adequate amount of amino

acid in distilled water. All solutions were obtained at room temperature. Then, the amino acid solution

was added to the clay and pH of the mixture was adjusted to the required value using 0.05 M HCl

solution. The acid solution was prepared by dilution of 37 % HCl (Carlo Erbo) with distilled water. The

clay mixture was mixed with amino acid solution overnight in the room temperature. The next day it

was removed from stirrer and centrifuged with centrifuge (NF400 – N400R Nüve). Next the

supernatant was discarded and the sample was washed with distilled water at room temperature and

dried in the oven overnight. The obtain material was milled with a mortar and stored in a plastic

container. For non-intercalated MMT sample preparation the same procedure as above was used,

without steps of amino acid solution addition and pH adjustment. Scheme of adsorbents preparation

steps are presented on the Fig. 14 below.

For the description simplification following abbreviations for prepared adsorbents were used:

MMT – for pure montmorillonite material

GLY-7 – for glycine intercalated montmorillonite prepared at pH 7

GLY-5 – for glycine intercalated montmorillonite prepared at pH 5

ARG-7 – for arginine intercalated montmorillonite prepared at pH 7

ARG-5 – for arginine intercalated montmorillonite prepared at pH 5

L-HIST-7 – for L-histidine intercalated montmorillonite prepared at pH 7

L-HIST-5 – for L-histidine intercalated montmorillonite prepared at pH 5

25

Fig. 14. Scheme of amino-acid intercalated MMT materials preparation.

2. Characterization methods

Several analytical methods were applied in order to characterize obtained adsorbent materials.

Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetry with differential scanning

calorimetry and nitrogen adsorption-desorption were used.

2.1. Fourier-transform infrared spectroscopy

Infrared spectroscopy is an analytical method to detect the vibrations of the atoms of a molecule. The

infrared spectrum is recorded by passing infrared radiation through a sample and determining which

part of incident radiation was absorbed at a particular energy. The energy at which peaks on the

obtained spectra appear indicate the frequency of vibration of a part of a given molecule. Fourier-

transform infrared spectroscopy uses the interference of radiation between two beams. The interaction

of the beams creates an interferogram as a signal resulting from a function of change of pathlength

between them. During the FTIR spectrophotometer experiment the radiation emitted from the source is

passed through an interferometer to the sample, which absorbs certain part of radiation. The

unabsorbed radiation reaches detector and is directed to an amplifier, which allows for elimination of

high-frequency disturbances. The data are converted to digital form by an analog-to-digital converter

and transferred to the computer for Fourier-transformation, which transforms the data of intensity of

radiation falling on the detector into wavelength value. By subtraction of background spectra from

100 mL distilled

water 2 g clay

50 mL 0.03 M

amino acid

solution

aa

0.05 M HCl for pH

adjustment

centrifugation drying mincing

AMINO ACID INTERCALATED

CLAY MATERIAL stirring

26

spectra of the sample, peaks resulting from interaction of radiation with molecules of investigated

compound are obtained [63].

In the present research infrared spectra of all the prepared adsorbent sample were collected. Also

spectra of pure amino acids used for montmorillonite intercalation – glycine, arginine and L-histidine

were recorded. The analysis was conducted using KBr pellet method. First KBr pellet was obtained by

milling KBr with a mortar and pressurizing it. The KBr pellet was used to obtain the background

spectra. The pellets of investigated samples were prepared by mixing adsorbent material with KBr in

proportion 2:3, milling with a mortar and pressurizing to produce the final pellet. The sample spectra

was collected by subtraction of background KBr spectrum from the spectrum of pellet containing KBr

and sample material. The investigation of FTIR was conducted with use of a Nicolet 6700 Fourier

transform IR spectrophotometer (256 scans, resolution 4 cm-1

) in the wavenumber range from 400 to

4000 cm-1

.

2.2. XRD

Diffraction effects can be observed when periodic structures are exposed to electromagnetic radiation

and their geometrical variations are of the length of scale of the wavelength of radiation. The

interatomic distances in crystals and molecules are in the range of 0.15 to 0.4 nm, which corresponds

to the electromagnetic spectrum with the wavelength of X-rays that have photon energies between 3

and 8 keV. The exposition of crystalline and molecular structures to X-rays results in observable

constructive and destructive interference. X-rays are generated when electrons with sufficiently high

kinetic energies (in the keV range or above) impinge on the matter. In the laboratory X-ray diffraction

(XRD) apparatus X-rays are emitted from the cathode filament and accelerated towards the anode

plate made usually from copper, chromium, molybdenum or another metal. When hitting upon anode

the electrons are deacelerated by interaction with metal atoms, what results with emissions of X-rays.

The obtained radiation is directed to the surface of the investigated sample. The diffraction pattern is

collected by varying the incidence angle of incoming X-ray beam by θ and the scattering angle by 2θ.

The scattered intensity is measured as the function of 2θ. In the type of apparatus used for described

research the X-ray source remains fixed, while the sample is rotated around θ and the detector is

moved by 2θ. The beam reaching detector is transformed to a digital signal and sent to the computer.

Analysis of X-ray diffraction spectra allows for investigation of the materials structure and interatomic

distances [64].

In the presented work pressed powder samples were prepared for pure MMT and each type of amino

acid-intercalated MMT. X-ray powder diffraction patterns in range from 3o to 10

o were obtained with a

Phillips PW 1730 diffractometer with automatic data acquisition (APD Phillips v3.6B software using a

Cu anode (λ=1.5406 Ǻ)).

27

The values of interatomic spacing were calculated on the basis of θ values from the Bragg law (9):

(9)

where:

d – the interplanar distance [Ǻ]

θ – the scattering angle [o]

n – the positive integer (n=1)

λ – the wavelength of incident wave (λ=1.5406 Ǻ).

The clay expansion in result of intercalation was calculated by subtracting the value of the interatomic

spacing of MMT from the value of interatomic spacing of intercalated materials.

2.3. Thermogravimetry

The thermogravimetric analysis with differential scanning calorimetry is used for material

characterisation, which enables following a fixed thermal cycle and obtain the mass loss of the sample

in the milligram range and identification of phase transistion changes based on a heat flow differences.

The technique has a wide application in different scientific fields. In the TG DSC apparatus the

investigated sample is placed into the platinum crucible, which is hanged along with an reference

crucible on the high-precision weighing scales inside a hermetic furnace. The reference crucible is

empty and it serves to compensate the influence of the recipient in the final measurement. For

measuring the temperature, three thermocouples are introduced inside the furnace – two for both

crucibles and one between the crucibles to measure the ambient temperature of the furnace. Nitrogen

is introduced to the furnace as the sweeping gas. An electric resistor is supplies the heat power

require to follow the set thermal cycle and is protected from external ambient by a layer of isolating

fibre. The collected data is passed to the computer, where analysis of data can be proceeded: mass

loss of the sample and heat flux exchanged calculated from the temperature difference between two

crucibles. Basing on these results it is possible to calculate the onset temperature and enthalpy, which

are main properties investigated in DSC analysis [65].

In the present research experiments of thermogravimetry with differential-scanning calorimetry were

conducted using an apparatus Setaram TG-DSC 111. All the samples were analyzed regarding mass

loss and heat flow during heating. The analysis was conducted in the temperature range from 25oC to

700oC and speed 5

oC/min.

28

2.4. Nitrogen adsorption-desorption

Gas adsorption is an important way of porous material characterization. Among gases used for this

purpose liquid nitrogen (at temperature -196oC) is most widely used as adsorptive gas. The

determination of adsorption isotherms is usually conducted using the approach of BET volumetric

method. During the experiment nitrogen is physically adsorbed on the surface of the investigated

material. The adsorption occurs on the outer surface and inside the pores. The formation of monolayer

of nitrogen on the surface, due to Langmuir theory, is used to determine the specific surface area of

investigated material. The capillary condensation of nitrogen inside pores allows for evaluation of

porous volume and pore size distribution in the sample [66].

In the presented research all the prepared samples were analyzed by nitrogen (Air Liquide, 99.999%)

adsorption-desorption with NOVA 2200e Quantachrome at -196oC. The samples were previously

degassed for 2.5 h at 150oC under vacuum conditions.

2.5. High pressure adsorption line

Pure gas adsorption isotherm experiments were performed on a high-pressure adsorption line. The

central element of the adsorption line was a stabilization cell of calibrated volume enclosed between

three valves and connected with pressure tranducer, which allowed for precise measurement of

pressure of the gas inside. The left valve was connected to the pressurized gas bottles (switched

between methane, carbon dioxide and helium), that supplied the gas used for adsorption properties

investigation. The right valve was connected to the diffusion pump and allowed for creating vacuum

conditions inside the line and degassing of the sample. The bottom valve was connected to the cell

with powder of investigated adsorbent material inside. The cell and the stabilization cell were

submerged in the water bath, in order to conduct the experiments in the conditions of controlled

temperature. For degassing of the sample vacuum pump and diffusion pump were applied in the line.

The liquid nitrogen was used for removal of contaminations from the line by their condensation in the

trap. The general scheme of high pressure adsorption line installation is presented below in the Fig 15.

29

Fig. 15. Simplified schema of the adsorption line.

2.6. Methodology of methane and carbon dioxide adsorption measurements

The sample powder ca. 1,5 g was placed inside the cell, which was connected to the adsorption line

and degassed under vacuum conditions with diffusion pump for 2,5 h at 150 oC using oven with

thermocouple for temperature control. Next, the cell was cooled down to the room temperature and the

cell factor measurements were conducted. In order to calculate the cell factor extrapure helium

(Praxair) was introduced into the cell at pressures increasing accordingly to the order: 200, 400, 600

and 1000 kPa. After opening the cell valve, the resulting pressure decrease was used to calculate the

cell factor. For adsorbents prepared at pH 7 the cell factor was calculated at 25oC, for adsorbents at

pH 5 at 25 and 45 oC. The cell with sample and calibrated cell were submerged in waterbath for

temperature control. After finishing the calibration helium was eliminated from the cell under vacuum.

Next, investigation of adsorption isotherms were conducted using pressurized gases: methane (Air

Liquide GSF) and carbon dioxide (Criolab). In order to obtain the isotherms portions of gas were

purged into the cell starting from 200 kPa and doubling the pressure each time up to the value of 1000

kPa. The value of pressure drop was noted after reaching equilibrium pressure and the amount of the

gas adsorbed was calculated. After finishing the experiment the powder was cleaned removing the

gas by heating for ca. 1 h at 150 oC in vacuum conditions.

vacuum gas inlet

cell

cell valve

vacuum valve

gas valve

calibrated cell

cell isolation

pressure sensor

30

2.7. Data evaluation

On the basis of obtained results adsorption isotherms of carbon dioxide and methane adsorption on

amino acid intercalated MMT were plotted. The isotherms show results obtained at 25 oC for clay

based adsorbents at pH 7 and at 25 and 45 oC for adsorbents produced at pH 5. In order to conduct

further calculations of selectivity and separation properties of intercalated towards binary mixture of

carbon dioxide and methane analytical expressions of the adsorption isotherms needed to be applied.

The obtained experimental data was fitted using the virial equation presented below (10) [67]:

(10)

where:

p – pressure [kPa]

nads – amount of moles of gas adsorbed on the unit of mass of adsorbent [mmol/g]

K – equilibrium constant

C1, C2, C3 – constants

Analytical integration of the virial equation allowed for obtaining the Gibbs free energy of desorption,

as following (11)

(11)

Phase diagrams for a given pressure, which determine the composition of the gas phase as a function

of composition of adsorbed phase were obtained by numerically solving to derivate the standard-state

loadings (n0) for pure carbon dioxide and methane (CO2 and CH4) at a given value of G. Next, the

system of two equations was solved simultaneously for the phase equilibrium to determine the

component molar fraction in the gas (xg) and adsorbed phases (x

a) (12, 13) [67]:

(12)

(13)

Next, the selectivity was calculated using following equation (14):

(14)

31

IV. RESULTS AND DISCUSSION

1. Fourier-transform infrared spectroscopy

The FTIR analysis was conducted in order to prove the amino acid retention in the structure of clay

material is the result of montmorillonite intercalation. Spectra of six samples were considered in

comparison to the data obtained for pure glycine, arginine and L-histidine and raw montmorillonite.

FTIR spectra of pure amino acids used for clay intercalation are presented in Fig. 16.

Fig. 16. FTIR spectra for pure glycine, arginine and l-histidine.

In the spectra of all the analyzed amino acids characteristic stretching N-H bands in the range of 3500-

3100 cm-1 can be observed together with stretching C-H bonds in 3000-2850 cm-1

. In the range

1150-1050 stretching C-O bonds from carboxylic groups are were noticed. Typical for O-H bond of

carboxylic group wide peak in the range of 3400-2500 was observed on the spectra of all the amino

acids. C=O bond stretching peak at 1690 was also noticed for all. Additionally, L-HIST showed N-H

stretching in the imizadole ring at 3300-3200 and C-H stretching in the ring at 3000.

All the data collected in the spectra corresponded to characteristic peaks for these compounds

described in the literature [68].

In the analysis of amino acid intercalated montmorillonite samples only spectra obtained in the range

of 1200 to 1800 cm-1

and 3000 to 3800 cm-1

were discussed, due to their importance for the present

work. Sections of other ranges of wavenumber were omitted because of minor significance for the

discussion. Full FTIR spectra obtained for the prepared adsorbents can be found in Annex 1.

The FTIR spectra recorded for L-HIST-7 and L-HIST-5 samples are presented in the Fig. 17.

0

2

4

6

8

10

12

400900140019002400290034003900

arb

itra

l in

it

wavenumber [cm-1]

GLY pure

ARG pure

L-HIST pure

32

Fig. 17. The FTIR spectra for MMT, L-histidine intercalated MMT samples L-HIST-7 and L-HIST-5 and

pure L-histidine.

As it can be noticed in presented graphs, both L-HIST-7 and L-HIST-5 spectra have plots strongly

corresponding to the spectrum of MMT. In L-HIST-7 graph peaks characteristic for L-histidine were not

recorded, what implies that only minor amount of amino acid was intercalated on the montmorillonite

surface at pH 7. The presence of L-histidine is covered by the much stronger signal coming from the

clay matrix. In case of L-HIST-5 small peaks resulting from the presence of L-histidine were recorded.

It shows, that at pH 5 intercalation of montmorillonite with the amino acid was more effective than at

pH 7.

The spectra of results obtained for arginine are shown in the Fig. 18.

30003200340036003800

wavenumber [cm-1]

MMT

L-HIST-7

L-HIST-5

L-HIST pure

1200140016001800

arb

itra

l un

it

wavenumber

33

Fig. 18. The FTIR spectra for MMT, arginine intercalated MMT samples ARG-7 and ARG-5 and pure

arginine.

For these plots similar tendency as in case of L-histidine can be observed. For the material

intercalated with arginine at pH 7 the spectrum is similar to the spectrum obtained for pure

montmorillonite. However, for ARG-7 some minor peaks corresponding to arginine spectrum can be

observed in the range of 1300-1500 cm-1

. For the material prepared at pH 5 these peaks were more

enhanced and visible, what was caused by greater amount of arginine intercalated in the interlayer

spacing on clay at this pH.

The spectra obtained for clay materials intercalated with glycine are presented in Fig. 19. below:

30003200340036003800

wavenumber [cm-1]

MMT

ARG-7

ARG-5

ARG pure

1200140016001800

arb

itra

l in

it

wavenumber

34

Fig. 19. The FTIR spectra for MMT, glycine intercalated MMT samples GLY-7 and GLY-5 and pure

glycine.

The results obtained for this group of intercalated clay materials follows the trend presented earlier by

montmorillonite intercalated with l-histidine and arginine. For the GLY-7 spectrum no significant

differences were observed with comparison to the raw material – MMT. For GLY-5 material there is a

series of slight peaks in the range 3300-3500 cm-1

, but it is not possible to find corresponding peaks

for them in spectrum obtained for glycine. The amino acid retained in the claysheet is very diluted, so it

is difficult to obtain visible peaks. Additionally, the impact of montmorillonite in the spectrum is very

strong and predominant.

Resuming the data obtained with FTIR method for amino acid intercalated samples, the FTIR analysis

could not provide reliable proof for the presence of amino acid molecules in the clay structure for all

the investigated materials. According to a big dilution of amino acids in clay material in was very

difficult to obtain noticeable peaks for respective peaks in the spectra. Also the peaks coming from the

raw material – montmorillonite influenced to the great extent the plot of spectra, covering the presence

of amino acids.

30003200340036003800wavenumber [cm-1]

MMT

GLY-7

GLY-5

GLY pure

1200140016001800

arb

itra

l in

it

wavenumber

35

2. XRD

The XRD analysis of prepared adsorbents allowed for evaluation of expansion of interlayer spacing

resulting from introduction and immobilization of amino acid molecules in the interlayers of

montmorillonite.

The data for modified MMT adsorbents at pH 7 and pH 5 were compared with the plot of unmodified

MMT material. The graphs are presented in Fig. 20 and Fig. 21 describe results obtained for materials

intercalated at pH 7 and pH 5, respectively.

Fig. 20. XRD plots for amino acid modified MMT materials obtained at pH 7.

0

20

40

60

80

100

3 4 5 6 7 8 9 10

inte

nsi

ty [a

. u

.]

2θ [o]

MMT

GLY-7

ARG-7

L-HIST-7

36

Fig. 21. XRD plots for amino acid modified MMT materials obtained at pH 5.

A characteristic, strong and sharp basal reflection for the MMT spectrum can be observed for θ equal

to 3.6o. For all the amino acid modified samples shift of the basal peak to smaller θ values was

noticed. It indicates increase of interlayer spacing in the structure of investigated materials, what can

be explained by expansion of clay structure caused by intercalation of amino acid molecules.

Using obtained θ values, the interlayer spacings for particular adsorbents were determined from the

Bragg law equation as described before. The results of calculated interlayer spacing along with values

of MMT structure expansion are presented in the Table 4.

Table 4. θ values for basal peak obtained in XRD plots for MMT and amino acid modified MMT

materials, corresponding interlayer spacing and expansion of MMT in the effect of amino acid

intercalation.

MMT pH 7 pH 5

GLY-7 ARG-7 L-HIST-7 GLY-5 ARG-5 L-HIST-5

θ [o] 3.600 3.4845 3.3230 3.4045 3.4300 3.4215 3.4170

d [Ǻ] 12.27 12.67 13.29 12.97 12.86 12.91 12.92

expansion [Ǻ] - 0.40 1.02 0.70 0.59 0.64 0.65

The basal spacing for the pure MMT was calculated as 12.27 Ǻ, which is characteristic value of basal

spacing for clay materials, usually enclosed between 12 to 14 Ǻ. It can be noticed that the greatest

expansion of spacing is present in case of ARG-7, equal to 1.02 Ǻ, which is followed by L-HIST-7 with

the value of 0.70 Ǻ. The least expanded material of all the investigated samples was GLY-7 with

0

20

40

60

80

100

3 4 5 6 7 8 9 10

inte

nsi

ty [a

. u

.]

2θ [o]

MMT

GLY-5

ARG-5

L-HIST-5

37

expansion corresponding to 0.40 Ǻ. Adsorbent samples obtained at pH 5 presented medium

expansion with values located between marginal results for materials at pH 7. Among them the most

expanded material was L-HIST-5 amounting to 0.65 Ǻ, followed by ARG-5 with minimally smaller

value of 0.64 Ǻ. The least expanded material was GLY-5 with expansion of 0.59 Ǻ.

Differences in expansion values between different types of amino acids can be explained by steric

effects. Glycine, as the smallest molecule among amino acids used in the research occupies the

smallest volume. Hence the expansion of clay intercalated with glycine is not significant. Arginine and

L-histidine have bigger molecules, what causes bigger structure expansion when they are intercalated

between clay layers. Arginine is the most branched compound of the three amino acids investigated,

that is the reason for arginine-intercalated clay materials to show the greatest increase in the

intermolecular distance.

3. Thermogravimetry

The thermogravimetric analysis was conducted to characterize mass lost and heat flow due to

decomposition of compounds adsorbed on the clay surface and indicate the presence of amino acids

in the intercalated montmorillonite. The loss of mass and heat flow were investigated and plotted

against the temperature, as presented on the Fig. 22. and 23. below:

Fig. 22. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials

prepared at pH 7.

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

70

75

80

85

90

95

100

15 115 215 315 415 515 615

he

at fl

ow

[m

V]

mas

s [%

]

temperature [oC]

pH7

MMT

GLY-7

ARG-7

L-HIST-7

38

For the raw montmorillonite material MMT the first loss of mass of ca. 7% was observed between 30

and 115 oC, which contributed to the release of water physically adsorbed on the clay. It corresponded

to the endothermic peak that could be observed on the heat flow in the same range of temperatures.

During further heating the mass of MMT remained constant until 600 oC. The sharper decrease of

mass beyond 600 oC resulted from destruction of clay structure under high temperature.

For intercalated clays prepared at pH 7 analogous mass loss due to water evaporation can be noticed,

but with smaller mass decrease indicated; approximately 4% for L-HIST-7 and 6% for GLY-7 and

ARG-7. Next, for all the amino acid intercalated samples another small decrease of mass (around 1%)

was observed, which also reflected in slight endothermic peak on the heat flow plot, that contributed to

the loss of water adsorbed in the pores. Further slow decrease of mass was observed for GLY-7 and

L-HIST-7 (ca. 5% and 6% respectively) in the region of 300 to 600 oC In case of ARG-7 no significant

mass decrease was observed in this region. However, corresponding strong endothermic decrease in

the heat flow can be noticed. It indicated decomposition of adsorbed amino acids from the interlayer

space of clays. Above 600 oC strong decrease of mass was observed for all the materials, indicating

clay structure destruction. It is reflected in a significant endothermic peaks on the heat flow plots in the

same region.

Fig. 23. TG DSC analysis of mass loss and heat flow against temperature for adsorbent materials

prepared at pH 5.

In case of materials obtained at pH 5 a mass loss along with characteristic endothermic peaks can be

observed in the range of 15 to 115 oC contributing to the water loss. Amount of water lost was equal to

4% of mass for ARG-5, 7% for GLY-5 and 11% for L-HIST-5. Then slow decrease of mass for all the

organoclays could be noticed until the end of experiment, connected with removal of water

physisorbed in the pores (up to 200-300oC) and amino acids intercalated on the surface of clay (above

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

70

75

80

85

90

95

100

15 115 215 315 415 515 615

he

at fl

ow

[m

V]

mas

s [%

]

temperature [oC]

pH5

MMT

GLY-5

ARG-5

L-HIST-5

39

300 oC). The mass loss contributing to amino acids removal amounted to 13% for ARG-5, 6% for GLY-

5 and 7% for L-HIST-5.

Concluding, intercalation of clays conducted at pH 5 resulted in more efficient intercalation of amino

acids on the clays than at pH 7. Arginine was the most retained amino acid at pH 5 (13%) on ARG-5.

4. Nitrogen adsorption

The nitrogen adsorption-desorption analysis allowed for investigation of influence of amino acid

intercalation on porosity and surface properties of montmorillonite. The data obtained for samples

were compared with raw clay material.

The adsorption isotherms referring to the amount of nitrogen adsorbed and desorbed during the

experiment are presented in the Fig. 24. and 25.

Fig. 24. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated MMT

samples at pH 7.

The analysis of the obtained graphs indicates that at pH 7 GLY-7 presented the biggest absorption

capacity of nitrogen, ARG-7 was a material with lower adsorption abilities and L-HIST-7 sample

0

0,5

1

1,5

2

2,5

0 0,2 0,4 0,6 0,8 1

nad

s[m

mo

l/g]

p/p0

pH7MMT

GLY-5

ARG-5

L-HIST-5

40

adsorbed the lowest amount of nitrogen of all intercalated clays. All the organoclays had higher

adsorption capacity than sample of pure MMT.

The increase of adsorption properties of the clay in result of amino acid intercalation can be explained

by creation of additional porosity and extension of adsorbent’s surface. At the same time molecules of

amino acids occupy volume inside pores of clay, what limits the amount of adsorptive that can be

adsorbed. For this reason GLY5, as glycine is amino acid with the lowest steric obstacle, has bigger

amount of available volume left for nitrogen adsorption.

Fig. 25. Isotherms of nitrogen adosprtion-desorption for pure MMT and amino acid intercalated MMT

samples at pH 5.

Comparing the materials prepared at pH 5, GLY-5 and ARG-5 presented almost the same adsorption

capacity, which was significantly higher than for materials prepared at pH 7. It is important to point out,

that while at pH 7 GLY-7 adsorbed higher amount of nitrogen than ARG-7, at pH 5 the amounts of

nitrogen adsorbed by these two amino acids were almost equal. Amount of nitrogen adsorbed by L-

HIST-5 was lower than in case of material prepared at pH 7 and also lower than amount adsorbed by

pure MMT.

The detailed adsorbent properties determined from the nitrogen adsorption-desorption analysis were

presented in Table 5.

0

0,5

1

1,5

2

2,5

0 0,2 0,4 0,6 0,8 1

nad

s[m

mo

l/g]

p/p0

pH5MMT

GLY-5

ARG-5

L-HIST-5

41

Table 5. Porosity and surface properties characteristics of pure MMT and amino acids modified MMT

materials.

MMT pH 7 pH 5

GLY-7 ARG-7 L-HIST-7 GLY-5 ARG-5 L-HIST-5

BET surface area [m2/g] 20 34 22 14 35 35 9

micropore volume [cm3/g] 0 0 0 0 0 0 0

micropore area [cm2/g] 1 0 0 0 0 0 2

external surface area [cm2/g] 18 34 22 14 35 35 7

total pore volume [cm3/g] 0.032 0.054 0.044 0.037 0.072 0.074 0.025

As it can be noticed in presented results, GLY-5 and ARG-5 show the best porosity and surface

properties among all the adsorbents prepared, with surface area amounting to 35 m2/g and total pore

volume 0.074 cm3/g for ARG-5 and 0.072 cm

3/g for GLY-5. Both adsorbents intercalated with L-

histidine; L-HIST-7 and L-HIST-5 exhibited relatively low development of surface area, 14 cm2/g and 7

cm2/g respectively, which was lower than for MMT with 18 cm

2/g. ARG-7 and GLY-7 presented

medium values of surface area with 22 cm2/g and 34 cm

2/g respectively and pore volume 0.044 cm

3/g

and 0.054 cm3/g respectively.

Concluding, the increase in the pore volume was noticed in result of amino acid intercalation process

for all the samples investigated with except of L-HIST-5. This effect was supported by aggregation of

amino acids in the clays interlayer space, what created additional porosity of the material.

5. Adsorption isotherms

In order to investigate adsorption properties of obtained amino acid intercalated MMT adsorbents

towards carbon dioxide and methane, adsorption isotherms were plotted. First adsorption of CO2 and

CH4 was compared at pH 7 and pH 5, next retention of CO2 at pH 5 at 25 and 45 oC was considered.

The data obtained in the adsorption experiments used for isotherm plotting can be found in Annex 2.

5.1. Carbon dioxide and methane adsorption at pH 7 at 25

oC

The adsorption data for retention of carbon dioxide and methane at 25 oC by materials obtained at pH

7 is presented on the graphs of Fig. 26. below.

42

Fig. 26. Adsorption isotherms of carbon dioxide and methane adsorption at 25oC on amino acid

intercalated MMT samples prepared at pH 7.

As presented on the plots, at pH 7 the highest adsorption capacity towards carbon dioxide was shown

by ARG-7 with the value of 0.46 mmol/g at high pressures and it was the only material among

intercalated clays, that showed better adsorption abilities than raw MMT with CO2 retention amounting

to 0.25 mmol/g at high pressures. Both GLY-7 and L-HIST-7 exhibited lower amount of CO2 adsorbed

with results of 0.08 and 0.023 mmol/g respectively. There is also noticeable difference in obtained

plots for different adsorbents. It can be seen that ARG-7 adsorbs significant amounts of carbon dioxide

adsorbed at lower pressures and its isotherm reaches plateau before L-HIST-7. In case of methane

adsorption, amounts of gas adsorbed were lower than for carbon dioxide for all the investigated

adsorbents. Amount of CH4 retained by GLY-7 was very low (0.003 mmol/g) and below the sensitivity

of the method. For this reason results of this adsorption isotherm are not presented on the graph.

Methane retained on ARG-7 and L-HIST-7 reached value of 0.11 mmol/g.

Due to low amounts of gases adsorbed at 25 oC, for the materials prepared at pH 7 adsorption at

higher temperature (45 oC) was not conducted.

5.2. Carbon dioxide and methane adsorption at pH 5 at 25 oC

Investigations of carbon dioxide and methane adsorption at pH 5 at 25 oC were also conducted. The

resulting adsorption isotherms were gathered in Fig. 27.

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nad

s[m

mo

l/g]

p [kPa]

CO2

ARG-7 L-HIST-7 GLY-7 MMT

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nad

s[m

mo

l/g]

p [kPa]

CH4

L-HIST-7 ARG-7

43

Fig. 27. Adsorption isotherms of carbon dioxide and methane adsorption at 25oC on amino acid

intercalated MMT samples prepared at pH 5.

As it can be seen in the graph, for pH 5 the amounts of carbon dioxide adsorbed increased for all the

intercalated clays in comparison to the values obtained at pH 7, with the best result for ARG-5, equal

to 0.80 mmol/g. It was the highest amount of carbon dioxide adsorbed among all experiments

conducted in presented research. The second value belonged to L-HIST-5 with 0.53 mmol/g of CO2

adsorbed. The material of lowest adsorptive properties was GLY-5, which retained 0.32 mmol/g. All

adsorbents prepared at pH 5 exhibited better adsorption of carbon dioxide than raw MMT material.

ARG-5 and L-HIST-5 are capable of adsorbing over 0.4 mmol/g of CO2 at pressures lower than 200

kPa. Their isotherms rise significantly at lower pressure, stabilizing the plot after reaching pressure

above 200 kPa. It indicates the possibility to conduct the adsorption process at lower pressure, what is

favourable from technical and economical point of view for further implementation to industrial-scale

processes. In all the isotherms the same tendency can be observed. After reaching relative adsorption

plateau at higher pressures, the amount of gas adsorbed increases slightly again at the last point, as

the result of gas purging at high pressures.

In the methane adsorption investigations ARG-5 showed the best adsorption abilities with 0.26 mmol/g

of gas adsorbed. The second material was GLY-5, which adsorbed 0.20 mmol/g. L-HIST-5 presented

very low adsorption of methane, after slight increase of amount of gas adsorbed to 0.06 mmol/g at 160

kPa, it dropped down to 0.01 mmol/g in effect of further purging of the gas at higher pressures.

5.3. Carbon dioxide adsorption at pH 5 at 25oC and 45

oC

Regarding satisfactory adsorption results for carbon dioxide adsorption on adsorbents prepared at pH

5, the investigation was repeated at higher temperature of 45oC. This time results were not fitted into

virial equation, so plots are represented only by points obtained from experimental data (Fig. 28.).

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nad

s[m

mo

l/g]

p [kPa]

CO2

ARG-5 L-HIST-5 GLY-5 MMT

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nab

s[m

mo

l/g]

p [kPa]

CH4

GLY-5 ARG-5 L-HIST-5

44

Fig. 28. Adsorption isotherms of carbon dioxide adsorption at 25oC and 45

oC on amino acid

intercalated MMT samples prepared at pH 5.

As it can be noticed, at 45oC ARG-5 showed the highest adsorption properties with 0.72 mmol/g of all

the samples investigated at pH 5. Although L-HIST-5 presented slightly lower results with final amount

of carbon dioxide adsorbed equal to 0.64 mmol/g, it obtained better results than ARG-5 at lower

pressures, reaching 0.5 mmol/g at 240 kPa. GLY-5 was the material with the lowest adsorptive

properties, reached 0.32 mmol/g. It should be pointed out, that even though it was expected, that at

higher temperature the adsorbents would exhibit lower properties of adsorption, GLY-5 and L-HIST-5

reached higher amounts of carbon dioxide adsorbed at 45 oC than in case of the experiment

conducted at 25oC.

6. Selectivity

Selectivity of investigated intercalated clays was calculated using MathCAD software, in order to

evaluate their properties of carbon dioxide and methane separation with relation to the pressure

change. GLY-7 was not regarded in the analysis, due to its low results for methane adsorption,

because of which selectivity plot could not be developed for this material. The results of calculations

were presented in the Fig. 29.

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nad

s[m

mo

l/g]

p [kPa]

25 oCL-HIST-5 ARG-5 GLY-5 MMT

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000

nad

s [m

mo

l/g]

p [kPa]

45 oCL-HIST-5 ARG-5 GLY-5

45

Fig. 29. Selectivity of amino acid intercalated MMT adsorbents against pressure.

Among materials used for experiences L-HIST-5 presented the best selectivity properties with

selectivity reaching 182 at 1000 kPa. The second adsorbent was ARG-7 with value 52 at 1000 kPa,

followed by ARG-5 with 22. The least selective materials were L-HIST-7 and GLY-5 with their

selectivities 6 and 3 respectively. It is remarkable, that the adsorbent with the highest amount of

carbon dioxide adsorbed – ARG-5 had not very good properties in terms of selectivity. L-HIST-5, with

its good adsorption performance towards carbon dioxide and very limited methane adsorption is the

most selective material of all the samples analyzed. It is remarkable, that for samples with low

selectivity, the selectivity is not much dependent on pressure, remaining relatively stable in all the

pressure range (GLY-5, L-HIST-7, ARG-5). For materials of high selectivity, selectivity is more

dependent on pressure and grows significantly with increase of pressure.

7. Phase diagrams

Phase diagrams were prepared in order to assess the separation capabilities in relation to phase

composition. The calculations were made for constant pressure 100 kPa. The results were presented

in Fig. 30.

1

21

41

61

81

101

121

141

161

181

201

0 200 400 600 800 1000

Sele

ctivity

p / kPa

L-HIST-5

ARG-5

GLY-5

L-HIST-7

ARG-7

46

Fig. 30. Phase diagrams at 100 kPa describing separation of CH4 between gaseous and adsorbed

phase as a function of mole fraction of CH4 in adsorbed phase.

The comparison of developed plots confirm, that the best separation properties among modified clays

belong to L-HIST-5, followed by ARG-7 and ARG-5. GLY-5 and L-HIST-7 present similar, low

separation properties.

The separation of binary mixture of carbon dioxide and methane was also analyzed as amount of

particular mixture component adsorbed in the function of the molar fraction of CH4 in gaseous phase.

The results were presented in the Fig. 31. The graphs were calculated for the constant pressure of

100 kPa.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,2 0,4 0,6 0,8 1

yC

H4

xCH4

L-HIST-5

ARG-5

GLY-5

L-HIST-7

ARG-7

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 0,5 1

ad

so

rbed

am

ount / m

mo

l g -1

yCH4

CO2

CH4

Total

L-HIST-7

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 0,5 1

ad

so

rbed

am

ount / m

mo

l g -

1

yCH4

CO2

CH4

Total

ARG-7

47

Fig. 31. Phase diagrams for L-HIST-7, ARG-7, L-HIST-5, ARG-5 and GLY-5.

The presented graphs show that L-HIST-5 has best adsorptive properties among all the investigated

samples, what was earlier noticed during analysis of selectivity and xy collective phase diagram. It is

significant, that up to 0.7 molar fraction of CH4 contained in the mixture, no adsorption of this gas is

exhibited and only CO2 is being retained with amount of 0.3 mmol/g of CO2 adsorbed. Values of pure

gases adsorption amount to 0.04 and 0.40 mmol/g for CH4 and CO2, respectively. It is important, that

up to high values of molar fraction of CH4, L-HIST-5 remains highly selective favouring CO2 retention.

The retention of CO2 remains predominant up to 0.96 molar fraction of CH4 in the binary mixture. This

property is particularly important for further application in industrial methane purification, which

requires production of high purity methane, what was earlier discussed in the present work.

The second most selective adsorbent following L-HIST-5 was ARG-7. This material shows exclusivity

of CO2 adsorption until 0.25 molar fraction of CH4 with retention of CO2 equal to 0.34 mmol/g at this

point. In case of this adsorbent amount of CO2 adsorbed decreases quicker with increase of CH4

fraction in the binary mixture in comparison to L-HIST-5. Values of pure gases adsorbed by ARG-7 are

0.04 and 0.36 mmol/g for CH4 and CO2 respectively.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 0,5 1

ad

so

rbed

am

ount / m

mo

l g -

1

yCH4

CO2

CH4

Total

L-HIST-5

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 0,5 1

ad

so

rbed

am

ount / m

mo

l g -

1

yCH4

CO2

CH4

Total

ARG-5

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 0,5 1

ad

so

rbed

am

ount / m

mo

l g -

1

yCH4

CO2

CH4

Total

GLY-5

48

Interesting properties were also shown by ARG-5 with the highest retention of both pure gases, with

values amounting to 0.44 mmol/g for CO2 and 0.1 mmol/g for CH4. This adsorbed retained CH4

exclusively up to 0.2 molar fraction of CH4 in the mixture, adsorbing 0.4 mmol/g CO2 at this mixture

composition. The amount of CO2 adsorbed drops significantly as the fraction of CH4 in the mixture

increases.

Very poor selectivity and adsorption properties were shown by L-HIST-7 and GLY-5. These materials

start to adsorb CH4 at low CH4 molar fraction, around 0.1. Both adsorbents retain low amount of pure

gases at 100 kPa, with values of 0.08 mmol/g for CO2 and 0.02 of CH4 for both materials. Retention of

CH4 became predominant at 0.8 molar fraction of CH4 for L-HIST-7 and 0.88 for GLY-5.

49

V. CONCLUSIONS

Investigated amino acid intercalated montmorillonite samples showed very interesting adsorption

properties. Intercalation process was successfull for all the materials prepared, what was proved on

FTIR spectra and thermogravimetry. Due to insertion of amino acid molecules into the interlayer space

expansion of montmorillonite structure was observed on XRD plots. Modification influenced the

porosity and surface properties of clay. For all the samples, except for L-HIST-5, increase of surface

area and total pore volume was noted. It can be explained by introduction of amino acid molecules in

the interlayer spacing, they formed clusters creating additional porosity.

Raw montmorillonite material showed minor adsorption capacity towards carbon dioxide. The CO2

adsorption was strongly enhanced by intercalation with amino acids for all samples analyzed except

for GLY-7. For all adsorbents better results were obtained, when adsorption was conducted at pH 5. It

can be explained by greater retention of amino acids by clay material at this pH due to protonation of

amine groups of amino acids (i.e. the amino acids are positively charged), what causes the increase of

their availability for carbon dioxide adsorption. The material with highest adsorption capacity towards

CO2 was ARG-5 with 0.80 mmol/g. Among materials prepared at pH 7 the best results were obtained

for L-HIST-7. Apparently, both arginine and L-histidine intercalated clays showed satisfactory results

and their adsorption capacity towards carbon dioxide was higher than in case of raw montmorillonite.

Adsorption on glycine intercalated samples did not give promising results. For GLY-7 the adsorption

capacity was lower than for MMT sample, for GLY-5 this value was only slightly higher than for MMT.

For the tests conducted at temperature 45 oC at pH 5, the amount of carbon dioxide adsorbed was

higher for ARG-5 and GLY-5 than at 25 oC. Retention of methane by intercalated clays gave lower

results than for carbon dioxide for all materials.

In terms of selectivity L-HIST-5 was the best adsorbent and provided noticeably higher separation of

CO2 and CH4 than other materials tested. It showed exclusive adsorption of CO2 up to 0.7 molar

fraction of CH4 in the binary mixture and predominant CO2 retention up to 0.96 molar fraction of CH4,

therefore providing satisfactory separation properties for further implementation in industrial gas

separation.

Concluding, amino acid modified montmorillonite adsorbents discussed in the present work showed

promising results for carbon dioxide and methane separation. Regarding their low-cost, environmental

friendly character and selectivity, they can be successfully applied in the industrial separation

processes.

50

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54

VII. ANNEX 1

Full FTIR spectra obtained in the range 400-4000 cm-1

for amino acid intercalated adsorbents, pure

amino acids and raw montmorillonite are presented below:

400900140019002400290034003900

arb

itra

l in

it

wavenumber [cm-1]

MMT

L-HIST-7

L-HIST-5

L-HIST pure

400900140019002400290034003900

arb

itra

l in

it

wavenumber [cm-1]

MMT

ARG pH7

ARG pH5

ARG pure

55

400900140019002400290034003900

arb

itra

l un

it

wavenumber [cm-1]

MMT

GLY-7

GLY-5

GLY pure

56

VIII. ANNEX 2

In the tables below experimental data of adsorption isotherms are presented.

MMT pure GLY-7 25 oC L-HIST-7 25

oC

CO2 CO2 CH4 CH4 CO2

p/bar nabs p/bar nabs p/bar nabs p/bar nabs p/bar nabs

0,0724 0,019811 0,0753 0,013509 0,0809 0,000899 0,0785 0,003504 0,0744 0,016809

0,1962 0,032863 0,2007 0,022453 0,2066 0,00137 0,205 0,007821 0,201 0,025957

0,431 0,045507 0,432 0,031477 0,4378 0,001691 0,4408 0,015373 0,3575 0,03607

0,7693 0,055016 0,7718 0,038174 0,7842 -0,00469 0,783 0,019684 0,7265 0,050318

1,6472 0,082778 1,6724 0,051669 1,6696 -0,02106 1,7299 0,027514 1,6426 0,099276

2,993 0,130387 2,9983 0,064081 2,2225 -0,0329 3,0452 0,048714 2,9712 0,152218

5,8199 0,189326 5,8034 0,068191 3,33156 -0,03379 5,5239 0,072575 6,0796 0,229875

7,5388 0,217048 7,7843 0,079198 5,774 0,002568 7,2013 0,105439

8,5868 0,2575

ARG-7 25 oC ARG-7 45

oC

CO2 CH4 CO2

p/bar nabs p/bar nabs p/bar nabs

0,0522 0,078729 0,0787 0,006297 0,064 0,050344

0,1704 0,157865 0,2035 0,013456 0,1752 0,104792

0,3859 0,247114 0,4405 0,023724 0,3957 0,166828

0,733 0,310747 0,7685 0,031616 0,7288 0,238484

1,688 0,400035 1,6555 0,051854 1,6213 0,340232

3,0616 0,45849 3,0127 0,096193 2,9787 0,435885

5,92 0,458757 4,822 0,126556 5,8001 0,512001

7,6646 0,462757 6,7158 0,118758 7,4564 0,563481

7,805 0,11294 8,5131 0,609941

GLY-5 25 oC GLY-5 45

oC

CH4 CO2 CO2

p/bar nabs p/bar nabs p/bar nabs

0,0789 0,001857 0,0753 0,016885 0,0756 0,014413

0,206 0,006594 0,2012 0,031126 0,1982 0,033955

0,4378 0,013829 0,433 0,047283 0,4253 0,058758

0,7849 0,020984 0,7823 0,063844 0,7567 0,085518

1,6862 0,043866 0,98501 0,0756 1,6418 0,139756

3,0201 0,072577 1,8063 0,111639 2,9535 0,210051

5,6806 0,112309 3,0808 0,178658 6,0926 0,308336

7,276 0,15541 6,2106 0,248336 7,8505 0,384909

8,2427 0,195115 8,0585 0,320282

57

ARG-5 25 oC ARG-5 45

oC

CH4 CO2 CO2

p/bar nabs p/bar nabs p/bar nabs

0,0769 0,009809 0,0437 0,087055 0,0677 0,031724

0,1983 0,022973 0,1427 0,184709 0,1864 0,064683

0,4257 0,045494 0,3585 0,288815 0,4131 0,101789

0,7582 0,072819 0,6902 0,390232 0,7507 0,135346

1,6235 0,130101 1,5641 0,520672 1,5996 0,235926

2,9345 0,192562 2,9001 0,622113 2,9124 0,376167

5,5898 0,230842 6,0169 0,72502 5,839 0,603692

7,2047 0,261745 7,7031 0,795395 7,571 0,724505

L-HIST-5 25 oC L-HIST-5 45

oC

CO2 CH4 CO2

p/bar nabs p/bar nabs p/bar nabs

0,048 0,163705 0,1094 0,008765 0,062 0,122981

0,2597 0,259119 0,3696 0,024271 0,2575 0,2364

0,54 0,306295 0,7405 0,035447 0,5234 0,308078

0,8935 0,353182 1,6125 0,060003 0,9015 0,368726

1,233 0,423544 3,2235 0,07295 1,3091 0,439405

1,6665 0,452347 6,0376 0,031636 2,4012 0,511103

3,2254 0,484005 7,7302 0,012817 3,8585 0,568039

4,8058 0,507003 8,7705 0,01155 5,2766 0,600314

5,882 0,513126 6,1565 0,637653

6,5426 0,525839