organoamines-grafted on nano-sized silica for carbon dioxide capture

7/27/2019 Organoamines-Grafted on Nano-sized Silica for Carbon Dioxide Capture

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Organoamines-grafted on nano-sized silica for carbon dioxide capture

Miklos Czaun, Alain Goeppert, Robert B. May, Drew Peltier, Hang Zhang, G.K. Surya Prakash *,George A. Olah *

Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park Campus, Los Angeles, CA 90089-1661, USA

1. Introduction

The ever increasing consumption of fossil fuels by humankind

has resulted in a rapid increase of carbon dioxide concentration in

the atmosphere from 270 ppm at the dawn of the industrial

revolution to the present 395 ppm. It is generally accepted that this

higher atmospheric carbon dioxide concentration is one of the

major contributors to global warming. However, global warming is

not the only effect of the increasing anthropogenic emission of CO2.

The oceans of our planet are net sinks of CO2 but their absorption

capacity is also finite. Furthermore, the dissolution of CO2 in the

oceans lowers the pH of seawater resulting in a reduction in the

abundance and/or size of shellfish, corals and crustaceans [1]. In

order to avoid further increase in CO2 concentration, the

management of this greenhouse gas has to gain more attention

and we believe that the reduction of anthropogenic CO2 emission

should be among the highest priorities of this century. Various

sequestration techniques that have been suggested may provideonly a temporary answer to our CO2 management problem. For

example pumping CO2 to deep aquifers, coal bed, depleted oil or

natural gas fields are promising solutions [2,3] but these methods

need to be validated on a large scale to ensure the long term safe

storage of CO2. As an alternative to sequestration, the capture,

recycling and utilization of CO2 promises an ultimate solution. An

elegant way to recycle CO2, for example, is to use it in reforming

reactions such as dry reforming or bi-reforming to produce syngas,

a mixture of carbon monoxide and hydrogen [4]. Presently,

approximately 130 million tonnes per year [5] of CO2 are used in

the energy and chemical industries and the majority is converted

to urea. This is still a drop in the bucket considering that humanity

now emits more than 30 billion tonnes of CO2 per year.

Fortunately, there are an increasing number of scientific projects

[5–7] that consider CO2 as a valuable industrial feedstock rather

than just a greenhouse gas harmful for the Planet’s ecosystem. As a

result of the predicted improvements, the amount of CO2 utilized

in the industry may grow to 300 million tonnes per year [8] in the

short term and higher in the following decades.

The most widely used post-combustion capture technologies

are based on the chemisorption of CO2 (Scheme 1) in aqueous

alkanolamine solutions such as monoethanolamine (MEA) [9],

diethanolamine (DEA) and methyldiethanolamine (MDEA).

The continuous regeneration of alkanolamine solutions (recov-ery of CO2) is a very energy intensive process due to high heating

and pumping costs. While for example an aqueous solution of DEA/

MDEA has a heat capacity of approximately 4.50 J gÀ1 8CÀ1

( xH2O ¼ 0:6, xDEA / xMDEA = 0.24/0.16, t = 50 8C) [10], silica based

adsorbents show significantly lower heat capacities

(0.73 J gÀ1 8CÀ1)1 making this kind of adsorbents more energeti-

cally efficient candidates for large scale CO2 capture and recycling.

Journal of CO2 Utilization 1 (2013) 1–7

A R T I C L E I N F O

Article history:

Received 2 February 2013

Received in revised form 26 March 2013Accepted 26 March 2013

Available online 24 April 2013

Keywords:

Nanosilica

Chemically grafted organoamines

Carbon dioxide capture

A B S T R A C T

Organoamine–inorganic hybrid adsorbent materials were synthesized by covalent immobilization of

alkylaminotrimethoxysilanes and polyethyleneiminetrimethoxysilane onto fumed silica (nanosilica).

The obtained silica–organic hybrid materials were characterized by thermogravimetry and diffusereflectance infrared Fourier transform spectroscopy (DRIFT) confirming the successful grafting of the

amine derivatives to silica and their surface area measured using Brunauer–Emmett–Teller method

(BET). The influenceof reaction conditions on the graft density of organoamineswas investigatedand it

was found that the saturation of the silane coupling agents with carbon dioxide prior to surface

modification resulted in higher graft densities. Carbon dioxide uptake of the obtained hybridmaterials

were determined by thermogravimetric analysis at room temperature as well as higher temperatures

resulting in CO2 adsorption capacities from 32.4 to 69.7mggÀ1 adsorbent.

ß 2013 Elsevier Ltd. All rights reserved.

* Corresponding authors. Tel.: +1 213 740 5984; fax: +1 213 740 6679.

E-mail addresses: [email protected] (G.K. Surya Prakash), [email protected]

(G.A. Olah).

1 Average heat capacity of Sil-N2 determined by DSC in the temperature range

from 50 to 110 8C.

Contents lists available at SciVerse ScienceDirect

Journal of CO2 Utilization

jo urn al hom ep ag e: www.elsev ier .com/locat e/ jco u

2212-9820/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.jcou.2013.03.007














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Substituting aqueous adsorbents by solid analogues offers

therefore a potentially lower cost solution. Developing more

economical alternatives for CO2 adsorption is part of our ongoing

efforts (Methanol Economy1) [11] on CO2 capture and its

conversion to value added products such as methanol [11–13],

dimethyl ether, [12] formic acid [14,15], methyl formate and

eventually to most of the important petrochemical intermediates

such as ethylene and propylene, which are presently produced

from natural gas or petroleum oil.

Various approaches have been utilized to prepare inorganic–

organoamine hybrid materials. Based on the interactions that occurbetween the organic compounds and the inorganic supports, the

synthetic techniques can be divided into two main groups. Materials

in which there is physical interaction between the amines and the

supports fall into the first group (a),e.g. amine impregnated supports

[16,17]. In order to increase the stability of the adsorbents, amines

can be chemically attached to the support giving grafted organic–

inorganic hybrids constituting the second group (b). Functionaliza-

tion of surface accessible OH groups of inorganic materials using

silane coupling agents is a widely used technique to fabricate hybrid

materials for a variety of applications [18]. Chemically attached thin

films can be prepared via ‘‘grafting from’’ techniques (b1) where the

organic coating is prepared by surface-initiated oligomerization/

polymerization [19]. They can also be prepared by immobilization of

an organic compound (e.g. polymeric) bearing anchoring groupsreacting with the OH groups of the support. This latter technique is

often referred to as the ‘‘grafting to approach’’ (b2) [20–23]. While

inorganic–organoamine adsorbents in which the organic compound

is physically adsorbed often suffer from leaching of the amine

component [16,17], chemically grafted amines usually show a better

stability, that helps to maintain the adsorption capacity over many

adsorption/desorption cycles.

The application of silica–amine hybrid materials for CO2

capture has been reviewed [23–25] and it can be observed that

a majority of the reported adsorbent materials are based on porous

(high surface area) supports. Although, the high porosity and high

surface area are advantageous for gas adsorption, very often

impregnation or grafting of organic compounds to porous supports

results

in

a

significant

drop

in

the

surface

area

indicating

thenecessity to develop new adsorbent materials using more cost

effective non-porous supports such as fumed silica. It should be

added here that we use the term ‘‘adsorption’’ in this manuscript to

indicate the reversible capture of CO2 molecules as a surface

phenomenon rather than a bulk one. Since a chemical reaction

takes place between CO2 and organoamines, in fact here we deal

with the phenomenon of chemisorption.

2. Experimental

2.1. Materials

Fumed silica Aerosil1 380 (average primary particle size 7 nm)

was

obtained

from

Evonik

(formerly

Degussa).

Toluene

(Mallinckrodt, 99.5%), methanol (Mallinckrodt, 99.8%),

3-aminopropyl-trimethoxysilane (N1, Acros, 95%), 3-(2-aminoethy-

lamino)propyltrimethoxy-silane (N2, Acros, 97%), 3-[2-(2-ami-

noethylamino)ethylamino]-propyl-trimethoxysilane (N3, Acros,

technical grade), trimethoxysilylpropyl-polyethyleneimine (PEI,

Gelest, 50% in iPrOH) were used without further purification.

2.2. Preparation of adsorbents

Aerosil-380 (7.0 g) was mixed in 280 mL toluene and N2 wasbubbled through the vigorously stirred suspension for 30 min.

19.95 mmol of silane coupling agent was added dropwise and the

reaction mixture was stirred at room temperature for 10 min before

heating to 110 8C. The suspension was stirred at this temperature

under a nitrogen atmosphere for 12 h. The cold reaction mixture was

separated by centrifugation and the obtained solid was re-

suspended in cold toluene. This suspension was again separated

by centrifugation. The above described purification steps were

repeated three times using cold methanol as a solvent. Finally, the

solid was transferred to a round bottom flask and the solvent was

removed by heating at 50 8C under vacuum on a rotovapor followed

by vacuum treatment overnight (65 mTorr). A small sample of the

obtained white solid material was placed in a sealable glass tube and

kept under vacuum at 85 8C for 3 h and then used either forcharacterization or for measuring the adsorption capacity.

2.3. Thermogravimetric analysis to determine the organic content of

the prepared adsorbent

Thermogravimetric measurements were carried out on a

Shimadzu TGA-50 thermogravimetric analyzer under an air flow

of 30 mL minÀ1 in a temperature range from 25 to 800 8C with a

heating rate of 10 8C minÀ1.

2.4. FTIR

Diffuse reflectance infrared Fourier transform spectroscopic

measurements

were conducted under vacuum

on a

Bruker

Vertex80v FTIRspectrometer using KBr as a reference. Samples were diluted

with KBr, placed into a PrayingMantisTM DRP-SAP diffuse reflection

accessory and scanned from 4000 to 500 cmÀ1 (number of

scans = 32).

2.5. Surface area and pore volume analysis

The supports were characterized by N2 adsorption/desorption

isotherm measurements on a Quantachrome NOVA 2200e instru-

ment. The surface area was determined by the multipoint BET

method. The total pore volume was evaluated at a P/P0 close to 0.995.

2.6. Heat capacity

Heat capacity of the solid adsorbent was measured on a PerkinElmer DSC 7 differential scanning calorimeter. In a typical

experiment, about 5 mg of the adsorbent (measured precisely

with a microbalance) was placed in an aluminium pan and inside

the sample cell of the DSC under a flow of nitrogen (20 mL minÀ1).

Temperature programme: heating from 0 to 120 8C with a heating

rate of 5 8C minÀ1; cooling from 120 to 0 8C with a rate of

20 8C minÀ1. This temperature programme was repeated two

times. The heat flow measured in the second cycle was considered

to determine the heat capacity.

2.7. Measurement of CO 2 adsorption capacity

6–13 mg of solid adsorbent was loaded in a platinum crucible

and placed into a Shimadzu TGA-50 thermogravimetric analyzer.

Scheme 1. Chemisorption of CO2 with amines.

M. Czaun et al. / Journal of CO 2 Utilization 1 (2013) 1–7 2



The sample was first heated to 110 8C under N2 atmosphere

(flow = 60 mL/min) and this temperature was maintained for

30 min for desorbing water and CO2 from the surface. The

adsorbent was exposed to pure CO2 (flow = 60 mL minÀ1) at

25 8C for 3 h, and then the sweeping gas was replaced with N2 at

85 8C for 90 min for desorption. The second adsorption cycle was

carried out under CO2 at 55 8C for 3 h followed by desorption under

N2 for 90 min at 85 8C.The third adsorption cycle was carried out at

85 8C for 3 h. Finally 10 adsorption/desorption cycles were carried

out isothermally at 85 8C. 15 min adsorption under pure CO2 wasfollowed by 25 min desorption under N2. For the 100 cycle

experiment, adsorption (10 min, 85 8C) was followed by desorp-

tion (15 min, 85 8C). Otherwise the experimental conditions were

similar to those described earlier.

3. Results

3.1. Preparation and characterization of adsorbent materials

Herein, we report on the facile synthesis of alkylamine-fumed

silica hybridmaterialsfromcommercially available and relatively

inexpensive starting materials. We have selected Aerosil-380

(fumedsilica, averageprimary particlesize7 nm [26], surfacearea

329 m2

gÀ1

) as a support forthe amine containing adsorbents. It isgenerally acceptedand widelypracticed that surface hydroxyls of

inorganic substrates can be functionalized with reactive groups

such as chlorosilanes [18,27], alkoxysilanes [18], and phosphonic

acid derivatives [28], establishing chemically attached organic

layers on the inorganic materials. Primary and secondary

trialkoxysilylalkylamines such as 3-aminopropyl-trimethoxysi-

lane (N1), 3-(2-aminoethylamino)propyltrimethoxysilane (N2),

3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane

(N3), and trimethoxysilylpropyl-polyethyleneimine (PEI) were

immobilized onto Aerosil-380 by the reaction of alkoxysilyl

anchoring groups andsurface silanol groups of the supportgiving

siloxane linkages (Scheme 2).

The obtained suspension was separated by centrifugation

and the

adsorbent particles

were

washed

repeatedly withtoluene and methanol and dried under reduced pressure

(65 mTorr) at 85 8C. Hybrid materials were characterized by

thermogravimetric analysis (TGA) and diffuse reflectance

infrared Fourier transform spectroscopy (DRIFT). The surface

area was determined by the Brunauer–Emmett–Teller method

(BET).

Graft density and nitrogen content of adsorbent-grafted silica

particles were calculated on the basis of weight losses indicated by

the thermograms. Representative thermograms of bare

Aerosil-380 (a), Sil-N3 (b), Sil-N3-CO2 (c), Sil-PEI (d) (heating

rate 10 8C minÀ1, 30 mL minÀ1 air) are shown in Fig. 1.

Weight losses (WL) were recorded in the temperature range from

25 to 800 8C and nitrogen contents (NC) are summarized in

Table

1.

Grafting of N1 onto Aerosil-380 resulted in Sil-N1 adsorbent,

which contained 1.89 mmol N atom gÀ1 silica while immobiliza-

tion of N2 containing two nitrogen atom per molecule resulted in

an almost double amine loading (3.42 mmol N atom gÀ1 silica).

Immobilization of N3 did not follow the same trend and resulted

only in slightly higher amine content (3.56 mmol N a-

tom gÀ1 silica) compared with Sil-N2.

Not surprisingly, covalent immobilization of alkylaminotri-

methoxysilanes onto Aerosil-380 resulted in significant decrease

in surface area compared to the bare support. While Aerosil-380had a surface area of 329 m2 gÀ1, Sil-N1, Sil-N2 and Sil-N3 were

found to have 159, 148 and 157 m2 gÀ1, respectively, indicating a

54–57% drop in surface area. It can also be observed that the

surface area of the grafted silica particles are very similar

regardless of the silane coupling agent applied. Diffuse reflectance

infrared Fourier transform spectroscopy is a very powerful method

to gain structural information about the molecules immobilized on

the surface of inorganic materials. Successful grafting of the silane

coupling agents N1–3 is indicated by the antisymmetric ( yasNH2)

and symmetric stretching vibrations ( ysNH2) of hydrogen bonded

NH2 groups (3366 and 3302 cmÀ1) [29,30], respectively, and the

corresponding vibrations of CH2 groups ( yasCH2 at 2928 and ysCH2

at 2856 cmÀ1) [31].

Deformation vibration of amino groups (dNH2) could also bedetected at 1595 cmÀ1 in Sil-N1–3 while CH2 deformations were

observed at 1475 and 1449 cmÀ1 [31] in Sil-N1. These peaks are

overlapped in Sil-N2–3 giving a relatively broad band at

1458 cmÀ1. The disappearance of the signal in the DRIFT spectrum

of Aerosil-380 at 3745 cmÀ1 attributed to surface OH groups [32]

Scheme 2. Immobilization of N1, N2, N3 and PEI onto Aerosil-380.

Fig. 1. TGA curves of bare Aerosil-380 (a), Sil-N3 (b), Sil-N3-CO2 (c), and Sil-PEI (d).




further confirmed the surface modification of surface OH groups

using methoxysilanes (Fig. 2).

3.2. Measurement of adsorption capacities

Adsorption capacities of the silica–alkylamine hybrids were

measured with a Shimadzu TGA-50 thermogravimetric analyzer at

room temperature, 55 8C and 85 8C using instrument grade CO2

under flow conditions. Adsorbents were heated at 110 8C for

30 min under N2 atmosphere prior to the measurements. The

weight increases were determined after an exposure to CO2 (3 h) at

each temperature. The adsorption and desorption cycles were

repeated ten times at 85 8C showing insignificant changes in the

adsorption capacities (Figs. 3 and 4).

For two selected adsorbents (Sil-N1 and Sil-PEI) the adsorp-

tion/desorption cycles were repeated 100 times at the same

temperature confirming the good stability of the hybrid adsorbents

(Fig. 5). It should be noted here that slightly lower adsorption

capacities were measured in the course of 100 cycles due to shorter

adsorption (10 min) and desorption (15 min) times.

The monoamino derivative (Sil-N1) showed moderate CO2

uptake at room temperature (32.4 mg CO2 gÀ1

adsorbent), but asexpected the adsorbents with higher nitrogen content (3.42 for Sil-

N2 and 3.56 mmol N gÀ1 for Sil-N3) exhibited a higher CO2

Table 1

Weight loss (WL), nitrogen content (NC), surface area ( ABET) and pure CO2 adsorption capacity of silica–organic hybrids.

Hybrid WL (%) NCa (mmolN gÀ1) Adsorption capacityb (mg gÀ1) Ads. CO2 per N at 25 8C ABET (m2gÀ1) Total pore

volume (mL gÀ1)

25 8C 55 8C 85 8C

Sil-N1 9.9 1.89 32.4 26.2 19.7 0.43 159 1.66

Sil-N2 14.7 3.42 35.6 27.2 18.3 0.28 148 1.61

Sil-N3 14.6 3.56 41.3 33.2 24.6 0.31 157 1.63

Sil-N1-CO2 11.8 2.29 37.4 31.5 24.5 0.42 149 1.75

Sil-N2-CO2 18.6

4.51

47.3

39.3

30.1

0.29

132

1.47Sil-N3-CO2 21.0 5.52 49.8 42.9 33.7 0.26 127 1.26

Sil-PEI 47.0 20.6 24.0 50.2 68.0 0.05 43.0 0.76

Sil-PEI-CO2 46.3 20.0 30.1 58.8 69.7 0.06 19.6 0.31

a Millimoles of NH2 and NH groups per gram silica support.b Milligrams of adsorbed CO2 per gram adsorbent.

Fig. 2. DRIFT spectra of bare Aerosil-380 (a), Sil-N1 (b), Sil-N2 (c), Sil-N3 (d), and Sil-

PEI (e).

Fig. 3. A typical thermogram of adsorption/desorption measurements for Sil-N1.




adsorption capacities at 35.6 and 41.3 mg CO2 gÀ1 adsorbent,

respectively. CO2 adsorption was also measured at 55 and 85 8C

showing decreasing adsorption capacity, 26.2 and 19.7 mg gÀ1 for

Sil-N1, respectively, as the temperature increased. A trend could beobserved for the diamine and triamine functionalized silica giving

lower adsorption capacities, 27.2/18.3 mg gÀ1and 33.2/

24.6 mg gÀ1at 55 and 85 8C respectively.

In order to further increase the adsorption capacities of the

hybrid materials, we attempted to achieve higher graft densities byfollowing a different grafting procedure. It was shown that H-

Fig. 4. Adsorption capacities of functionalized silica particles at 25/55/85 8C followed by 10 cycles of adsorption/desorption at 85 8C.

Fig. 5. Adsorption capacity of Sil-N1 and Sil-PEI in repeated adsorption and desorption cycles at 85 8C. Inset: Adsorbent weight versus time diagram for Sil-PEI.




bonded aminopropylsilanes are less reactive and only a small

portion of alkoxysilane groups participates in Si–O–Si bond

formation [29].

It was also suggested that the saturation of alkylaminotri-

methoxysilane solutions with CO2 prior to introducing the solid

support facilitates covalent immobilization preventing the inter-

action of amine nitrogens of silane coupling agents with surface

silanols through hydrogen bonding [33]. Following these leads,

toluene solutions of N1–3were evacuated and then exposed to CO2

for 15 min before silica particles were added. The suspension was

then slowly heated to 60 8C and the temperature was maintained

for 30 min. After that, the reaction mixture was heated and kept at

110 8C for 12 h. In accordance with the results of Knowles et al.

[33], we obtained alkylamino-grafted silica with higher graft

densities (Sil-N1-CO2: 2.29; Sil-N2-CO2: 4.51; Sil-N3-CO2:

5.52 mmol N atom gÀ1 silica) which corresponds to a 21, 32 and

55% increase compared to the adsorbents prepared in the absence

of CO2. Interestingly the increase in graft density is higher when

the silane coupling agent contains more nitrogen which seems to

confirm that the reaction of amino groups with CO2 makes the

formation of hydrogen bond with surface silanol groups less

favourable. The higher graft densities resulted in higher CO2

adsorption capacities of 37.4, 47.3 and 49.8 mg CO2 gÀ1 adsorbent

at 25 8C, respectively, and similar trend could be observed at 55and 85 8C as well (Table 1).

Similarly to the adsorbents prepared from practically CO2 free

silane coupling agents and silica suspension these hybrid materials

also showed lower adsorption capacity at higher temperature

(Table 1). Immobilization of N1, N2 and N3 using solutions

saturated with CO2 resulted in slightly lower surface areas (149,

132 and 127 m2 gÀ1) than those we obtained for Sil-N1, Sil-N2 and

Sil-N3 but significantly lower than that of bare Aerosil-380.

As the increasing number of secondary amino groups in the

grafted layer resulted in better CO2 adsorption capacity per g

adsorbent, we attempted to immobilize polyethyleneimine

bearing trimethoxysilyl (PEI) anchoring groups. (The average

number of ethyleneimine groups in the PEI chain was 18 based on1

H NMR measurements).2

The high organic content of the hybridmaterials was reflected by the significant weight losses in TGA

experiments. Sil-PEI showed a weight loss of 47.0%

(20.6 mmolN gÀ1) between 200 and 800 8C.

DRIFT study of PEI-grafted silica revealed stretching vibration of

NH groups at 3255 cmÀ1 and a group of signals from 2947 to

2825 cmÀ1 due to antisymmetric and symmetric CH2 stretching

vibrations. Moreover the CH2 deformation bands overlapped

showing an intense and broad peak at 1450 cmÀ1.

Impregnation or covalent grafting of polymers onto supports

usually causes dramatic decrease of the surface area as it was

observed in the case of Sil-PEI. Surface area of Sil-PEI (43 m2 gÀ1)

showed a 87% decrease compared to that of Aerosil-380.

Regardless, the low surface area PEI-grafted silica exhibited

remarkable

adsorption

capacities

of

68.0

mg

CO2 gÀ1

at

85 8C

compared with mono-, di- and triamine functionalized silica.

We also attempted to obtain a higher graft density of amine

adsorbents on silica by saturating the toluene solution of silane

coupling agents with CO2 prior to the addition of Aerosil-380 using

PEI. Although the saturation of the silane coupling agent with CO2

resulted in higher graft density for the mono-, di- and triamine

derivatives it did not increase the graft density for PEI. (N content

of 20.0 mmolN gÀ1) compared to what we obtained under a

nitrogen atmosphere (20.6 mmolN gÀ1). Furthermore, while all the

mono-, di- and triamine-functionalized silica showed lower

adsorption capacity at higher temperature a reverse trend could

be observed in the adsorption capacity in the case of Sil-PEI and

Sil-PEI-CO2. Although the graft density of Sil-PEI-CO2 was

somewhat lower than for Sil-PEI, its CO2 adsorption was however

slightly higher. This hybrid material exhibited an adsorption

capacity of 24.0/50.2/68.0 mg gÀ1 and 30.1/58.8/69.7 mg gÀ1 at 25,

55 and 85 8C, respectively.

As reflected by a large number of publications, a variety of

supports such as mesoporous [34] or delaminated [35] materials,

and metal organic frameworks (MOFs) [36] were functionalized

with organoamine derivatives in order to develop high perfor-

mance CO2 adsorbent materials. If one intends to make a detailed

comparison of present results with those reported in the literature,

one should refer to a comprehensive review by Jones et al. [23]

Furthermore, the comparison of the adsorption capacity and

surface area of some hybrid materials with present aminopropyl-

functionalized fumed silica is also given in the supplementary

information (Table S1).

Supplementary material related to this article found, in the

online version, at doi:10.1016/j.jhazmat.2012.10.056.

4. Conclusions

Silica–organoamine

hybrid

materials

were

prepared

by

cova-lent grafting of alkylamines (with one, two or three amino groups)

and polyethyleneimine bearing reactive methoxysilane groups.

The characterization of the grafted silica particles was carried out

by thermogravimetry, DRIFT and BET methods. Both the nitrogen

contents and the adsorption capacities of hybrid materials

increased (although not linearly) as the molecular weight of the

organic component increased. However the CO2/N ratio decreased,

indicating that less amino functional groups were accessible or/

and active for adsorption. Higher graft densities and higher CO2

adsorption capacities could be obtained when silane coupling

agents containing one, two or three amino groups were saturated

with CO2 prior to the reaction with surface silanol groups. Mono-,

di- and triamine-functionalized silica sorbents showed decreasing

adsorption

capacities

as

a

function

of

temperature

while

thepolyamine-functionalized ones exhibited increasing adsorption

capacity at higher temperatures. Organoamine functionalized

fumed silica adsorbents are easy to prepare, high performance

adsorbents and more cost effective alternatives to other amine-

grafted porous adsorbents based on supports for carbon dioxide

capture. To the best of our knowledge it is also the first time that

trimethoxysilylpropyl-polyethyleneimine has been used to pre-

pare materials for CO2 adsorption.

Acknowledgement

Support of our work by the Loker Hydrocarbon Research

Institute and the United States Department of Energy is gratefully

acknowledged.

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