Biodiesel production over supported Zinc Oxide nano-particles

BY

MBALA MUKENGA

A thesis submitted to the Faculty of Engineering and the Built Environment,

University of Johannesburg, in partial fulfillment of the requirements for the

degree of Magister Technologiae

Supervisors: - Prof. Edison Muzenda

- Dr. Kalala Jalama

- Prof. Reinout Meijboom

2012

Major Subject: Chemical Engineering

i

Declaration

I understand what plagiarism is and am aware of the department’s policy in this regard.

I declare that this thesis is my own original work, any ideas or sentences from another source

have been carefully referenced.

………………………… ………………………

Signature Date

ii

Acknowledgment

I am grateful for the financial support given by the University of Johannesburg through the Next

Generation Scholarship (NGS) and the technical support from Meta–Catalysis group in the

chemistry department at the University of Johannesburg.

I really want to express my deepest gratitude to the following persons for their different

contributions throughout this project:

- My supervisors: Prof. Edison Muzenda, Dr Kalala Jalama and Prof. Reinout Meijboom for the

patience, guidance and also the time they spent on reading and correcting this work to make it as

it is today.

- My family: father and mother (Mukenga Tshamala and Mbuyi Mbala), my brothers and sisters,

uncles and aunts as well as cousins, nephews and nieces, for giving me the emotional support

and a peaceful environment what allowed me to work properly.

- My colleagues: all postgraduate students for the chemical engineering department at the

University of Johannesburg for their friendship and help when it was necessary.

- And finally, all those who have faith in me and encouraged me to take this beautiful journey of

masters studies.

iii

Research output

- Journal paper

Expanding the synthesis of Stöber spheres: towards the synthesis of nano-MgO and nano-ZnO.

(Submitted, October 2012)

Authors: Liberty L. Mguni, Mbala Mukenga, Edison Muzenda, Kalala Jalama*, Reinout

Meijboom*

- Conference paper

Biodiesel production from soybean oil over TiO2 supported nano-ZnO. WASET, Paris 25-26 April 2012

Authors: Mbala Mukenga, Edison Muzenda, Kalala Jalama*, Reinout Meijboom*

iv

Abstract

Stöber spheres of SiO2 with different diameters were synthesized via a modified Stöber synthesis

method in which the amount of ethanol, ammonia and water 0.46, 2.89, and 2.15 moles were

respectively kept constant, while the concentration of TEOS was varied from 0.250 M to 3.494

M. This method was expanded to the synthesis of nano-ZnO from zinc methoxide. The

synthesized spherical SiO2 and hexagonal nano-ZnO were characterized by X-ray diffraction

(XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM),

Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET). The average

crystallite sizes determined by XRD analysis were calculated using the Scherrer equation and

showed an ordinary tendency to increase with an increase of the precursor’s amount. Initial

catalytic investigations revealed good activity of nano-ZnO for the trans-esterification reaction

of soybean oil with methanol to biodiesel. The synthesized nano-ZnO particles were further

supported on titania by precipitation-deposition and tested as solid catalyst for the soybean oil

trans-esterification reaction.

The effect of reaction time (15, 30, 45, 60, 360 and 600 minutes), reaction temperature (150,

175, 200, 225oC), ZnO loading on TiO2 (5, 10, 20 wt.%) catalyst concentration in the reactor (0,

0.5, 1.5, 3 and 6wt.% catalyst to soybean oil), methanol to oil molar ratio (6:1, 12:1 and 18:1)

and catalyst reusability on soybean oil conversion during trans-esterification have been

investigated. The experiments were carried out in a stainless steel stirred batch reactor where

nitrogen was used to pressurize the reactor in order to maintain methanol in liquid phase at all

the reaction temperatures used in this study. A constant speed of the agitator (1100 rpm) was

used for all the runs and the reaction product was analyzed using proton nuclear magnetic

resonance (1H-NMR) from which the soybean oil conversion was calculated.

The soybean oil conversion increased with the increase in temperature and this was explained by

the fact that as the temperature increased, the viscosity of the reacting medium decreased. A

decrease in viscosity leads to a decrease in mass transfer restriction, hence increase rate of

reaction. The effect of ZnO loading on the oil conversion depends on the range of reaction

v

temperatures used. The lowest conversions were measured on catalysts with the highest ZnO

loading, i.e. 20% ZnO for the reactions performed at 150 and 175oC. However, the oil

conversion passes through a minimum on the catalyst with a 10% ZnO loading and increases on

the catalyst containing 20% ZnO for reactions performed at 200 and 225°C. These findings can

be explained by the ZnO dispersion on the support and the mass transfer limitations. The

soybean oil conversion increased also with reaction time up to 1 hour, no significant changes

were observed in the first 6 hours and a decrease in the conversion was noticed from 6 to 10

hours which can be explained by the glycerolysis process taking place in the reaction.

The highest methanol to oil ratio gave the best oil conversion which can be explained by the fact

that the trans-esterification reaction is a reversible reaction, an excess of methanol is necessary

for driving the reaction towards products. Both unsupported and supported catalyst particle sizes

did not significantly affect the oil conversion meaning varying the catalyst’s particle sizes did not

affect the biodiesel conversion and it was observed that supported catalyst gave high oil

conversion compared to the unsupported one. Therefore, the oil conversion is increased by

minimizing the mass transfer limitation of the catalyst through supporting.

The optimum reaction conditions for the supported catalyst were found to be 20% nano-ZnO

loaded on titania, 1.5 wt% of catalyst to oil, 18:1 alcohol to oil molar ratio, 225oC as reaction

temperature and 1 hour reaction time.

vi

Table of Contents

CHAPTER 1 ...............................................................................................................................1

INTRODUCTION ......................................................................................................................1

1.1 Background and motivation ...............................................................................................1

1.2 Catalysis ............................................................................................................................2

1.3 Objectives ..........................................................................................................................3

1.4 Thesis overview .................................................................................................................3

CHAPTER 2 ...............................................................................................................................4

LITERATURE REVIEW ............................................................................................................4

2.1 Biodiesel production. .........................................................................................................4

2.1.1 Background .................................................................................................................4

2.1.2 Production process ......................................................................................................8

2.1.3 Parameters affecting the yield of biodiesel ...................................................................9

2.1.4 Feedstock .................................................................................................................. 10

2.2 Catalysts .......................................................................................................................... 11

2.2.1 Introduction ............................................................................................................... 11

Homogeneous catalysts .................................................................................................. 11

Heterogeneous catalysts .................................................................................................. 12

Enzymatic catalysts ........................................................................................................ 13

2.2.2 Catalyst synthesis. ..................................................................................................... 14

2.2.3 Catalyst support ......................................................................................................... 16

REFERENCES ......................................................................................................................... 20

vii

CHAPTER 3 ............................................................................................................................. 24

RESEARCH DESIGN AND METHODOLOGY ...................................................................... 24

3.1 Introduction ..................................................................................................................... 24

3.2 Experimental procedure ................................................................................................... 25

3.2.1 Catalyst synthesis ...................................................................................................... 25

3.2.2 Catalyst testing .......................................................................................................... 33

REFERENCES ......................................................................................................................... 38

CHAPTER 4 ............................................................................................................................. 40

RESULTS AND DISCUSSION ................................................................................................ 40

4.1 Materials synthesis and characterization........................................................................... 40

4.1.1 Silica nanoparticles.................................................................................................... 40

4.1.2 Unsupported nano-ZnO ............................................................................................. 43

4.1.3 TiO2 Support ............................................................................................................. 49

4.1.4 Supported nano-ZnO ................................................................................................. 51

4.2 ZnO catalyst testing for soybean oil trans-esterification ................................................... 53

4.2.1 Unsupported nano-ZnO as catalyst ............................................................................ 53

4.2.2 TiO2 supported nano-ZnO catalyst ............................................................................. 55

REFERENCES: ........................................................................................................................ 63

CHAPTER 5 ............................................................................................................................. 65

CONCLUSION AND RECOMMENDATIONS ....................................................................... 65

5.1 Conclusion ....................................................................................................................... 65

5.2 Recommendations............................................................................................................ 67

Appendix .................................................................................................................................. 68

viii

List of figures

Figure 2.1: Schematic biodiesel production process .....................................................................8

Figure 3.1: SEM analysis equipment ......................................................................................... 27

Figure 3.2: XRD analysis equipment ......................................................................................... 30

Figure 3.3: XRF analysis equipment .......................................................................................... 31

Figure 3.4: BET analysis equipment .......................................................................................... 33

Figure 3.5: 300 cm3 Parr pressure reactor for biodiesel production ............................................ 34

Figure 3.6: NMR analysis equipment ........................................................................................ 35

Figure 3.7: Rotary evaporator for biodiesel, glycerol and methanol separation .......................... 36

Figure 3.8: ICP-OES analysis equipment................................................................................... 37

Figure 4.1: SEM images of silica produced at a reaction temperature of 0OC and fixed molar ratio

of NH3/H2O and CH3CH2OH: 0.46/2.89 and 2.15 respectively with following moles of TEOS (a)

0.005 mol (0.25 M) (b) 0.035 mol (1.75 M) (c) 0.06 mol (3 M) with their particles distribution

graphs. ...................................................................................................................................... 42

Figure 4.2: The dependence of silica particle size on concentration of TEOS. Reaction

conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h. ................... 43

Figure 4.3: IR results of the precursor and the zinc oxide obtained under these reaction

conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h. ................... 44

Figure 4.4: XRD results of the precursor and the zinc oxide ...................................................... 45

Figure 4.5: The dependence of nano-ZnO particle size on concentration of zinc methoxide.

Reaction conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h. .... 46

ix

Figure 4.6: TEM images of ZnO, with their particles size distribution, produced at a reaction

temperature of 0°C and fixed molar ratio of [NH3] : [H2O] : [CH3CH2OH] of 0.46 : 2.89 : 2.15

moles, respectively, with following moles of [Zn(OCH3)2] (a) 0.025 mol (b) 0.035 mol (c) 0.055

mol (d) 0.07 mol. ...................................................................................................................... 49

Figure 4.7: XRD of titania before and after calcination at 650oC ............................................... 50

Figure 4.8: XRD data for calcined (a) unsupported ZnO, (b) TiO2 supported ZnO and .............. 51

(c) blank TiO2 ........................................................................................................................... 51

Figure 4.9: Effect of precursor amount on supported nano-ZnO particle size. Reaction

conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h. ................... 52

Figure 4.10: Effect of unsupported nano-ZnO particle size on biodiesel conversion (alcohol to oil

molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction temperature of

225oC). ...................................................................................................................................... 54

Figure 4.11: Effect of reaction temperature and oxide loading on biodiesel conversion: alcohol

to oil molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time. ................................ 56

Figure 4.12: Effect of reaction time on biodiesel conversion: alcohol to oil molar ratio 18 to 1,

catalyst amount 1.5 wt.% and reaction temperature of 225oC..................................................... 57

Figure 4.13: Effect of catalyst amount on biodiesel conversion: alcohol to oil molar ratio 18 to 1,

20% ZnO/TiO2 and reaction temperature of 225oC .................................................................... 58

Figure 4.14: Effect of alcohol to oil molar ratio on biodiesel conversion: 1.5 wt.% of ZnO/TiO2

at 20% and reaction temperature of 225oC ................................................................................. 59

Figure 4.15: Effect of the supported nano-ZnO particle size on biodiesel conversion (alcohol to

oil molar ratio 18 to 1, 20% ZnO/TiO2, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction

temperature of 225oC). .............................................................................................................. 60

x

Figure 4.16: Effect of ZnO/TiO2 reutilisation on soybean oil conversion (alcohol to oil molar

ratio 18 to 1, 20% ZnO/TiO2, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction

temperature of 225oC). R1 to R4 are different runs. ................................................................... 61

xi

List of tables

Table 2.1: Comparison of the standards for diesel and biodiesel based on American Society for

Testing and Materials (ASTM). ...................................................................................................5

Table 2.2: The average emissions of B100 and B20 (20% blend with diesel as compared to

normal diesel (in percentages). ....................................................................................................6

Table 2.3: Advantages of biodiesel over conventional petroleum based diesel .............................7

Table 2.4: Comparison of homogeneous and heterogeneous catalyzed trans-esterification ........ 13

Table 4.1: targeted ZnO loading on titania and XRF obtained results......................................... 52

Table 4.2: surface area and pore size diameter for blank support (TiO2) and supported ZnO

(ZnO/TiO2) ............................................................................................................................... 53

Table 4.3: ICP-OES results of biodiesel produced under the following conditions: alcohol to oil

molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time .......................................... 62

1

CHAPTER 1

INTRODUCTION

1.1 Background and motivation

Due to the increase in crude oil prices and environmental concerns, a search for alternative fuels

has gained significant attention recently. Among the different possible resources, diesel fuels

derived from triglycerides of vegetable oils and animal fats have shown potential as substitutes

for petroleum-based diesel fuels [1]. However, the direct use of vegetable oils in a diesel engine

can lead to a number of problems such as poor fuel atomization, poor cold engine start-up, oil

ring sticking, and the formation of gum and other deposits. Consequently, considerable efforts

have been made to develop alternative diesel fuels that have the same properties and

performance as the petroleum-based fuels, with the trans-esterification of triglycerides to fatty

acid alkyl esters showing the most promise [2]. Trans-esterification, often called alcoholysis [3],

is the reaction of a fat or oil with an alcohol to form esters and glycerol as shown by reaction 1.1

[4]. This reaction has been widely used to reduce the viscosity of vegetable oils (triglycerides).

The triglycerides in vegetable oil react with alcohol to form a mixture of glycerol and fatty acid

alkyl esters, called biodiesel.

H2C O R'

O

C

HC O R''

O

C

H2C O R"'

O

C

3ROH

R O C R'

O

R O C R''

O

R O C R'''

O

H2C O

HC O

H2C O

H

H

H

catalyst

Alcohol

Triglycerides biodiesel glycerol

R’, R”, R’’’ = hydrocarbon chain ranging from 15 to 21 carbon atoms

(1.1)

2

Biodiesel fuels produced from vegetable oils can be used as an alternative to diesel fuels because

their characteristics are similar to those of petroleum-based diesel fuels. For example, they have

a viscosity close to that of petroleum-based diesel fuel, their volumetric heating values are a little

lower, but they have high cetane and flash points [1]. Many types of alcohols such as methanol

and ethanol can be used in the trans-esterification reaction. If methanol is used, the resulting

biodiesel is fatty acid methyl ester (FAME), which has the desired viscosity, boiling point and a

high cetane number [5].

The motivation for this project comes from the fact that biodiesel is a green and renewable

energy source. The production and use of biodiesel will certainly contribute to the decrease on

global carbon footprint. However the production cost for biodiesel is still higher than that of

petroleum-derived fuel. More research and development are still needed to improve the biodiesel

production process. The catalytic performance for the biodiesel production process is one of the

significant parameters that affect production costs. This project will contribute to the

improvement of the process’s catalytic performance which will potentially make it more

attractive. With improved process economics, more investments in this field will be possible and

will lead to job creation and poverty alleviation.

1.2 Catalysis

The introduction of solid heterogeneous catalysts in biodiesel production could reduce its price,

becoming competitive with diesel. Therefore, great research efforts has been undertaken recently

to find suitable catalysts. As a matter of fact, heterogeneous catalysts can be separated more

easily from reaction products and the reaction conditions could be less drastic than the methanol

supercritical process [6]. The heterogeneous catalyst process requires neither catalyst recovery

nor aqueous treatment steps: the purification steps of products are then much more simplified

and very high yields of methyl esters, close to the theoretical value, are obtained. Glycerine is

directly produced with high purity (at least 98%) and is exempt from any salt contaminants. With

all these features, this process can be considered a green process [7].

3

Heterogeneous base catalysis is the most viable process for the trans-esterification of triglyceride

into biodiesel. The heterogeneous catalysis features are lower corrosiveness, environmental

friendliness, easy catalyst recovery and high process integrity.

1.3 Objectives

The main objective of this project is to determine the catalytic behaviour of zinc oxide (ZnO)

nanoparticle/support system to be used in the industry, as heterogeneous catalyst for the trans-

esterification reaction of soybean oil in order to produce biodiesel.

In particular, this project aims at evaluating the effect of the following parameters on the soybean

oil conversion during the trans-esterification to biodiesel: i) oil to methanol molar ratio; ii) ZnO

loading on TiO2 support; iii) catalyst to oil mass ratio; iv) reaction time; v) reaction temperature,

vi) ZnO particle size and vii) catalyst reutilization.

1.4 Thesis overview This thesis contains five chapters. In chapter one, a general introduction on biodiesel synthesis,

the motivation and also the aim of the thesis are introduced. In chapter two, a literature survey on

the biodiesel production from vegetable oil using both homogeneous and heterogeneous catalysts

are presented as well as the different preparation methods of the heterogeneous catalyst. In

chapter three, the specifications of the chemicals used in this study and also the single step sol-

gel method to synthesize the heterogeneous catalysts is explained in details. Also in this chapter,

the material characterization methods, such as X-Ray diffraction, the surface area measurement

using N2 adsorption, X-Ray fluorescence, Fourier transform infrared spectrum, scanning electron

microscopy, transmission electron microscopy, proton nuclear magnetic resonance are given. In

chapter four, the catalyst properties, such as crystallite size, as well as its catalytic activity on the

trans-esterification are presented and discussed to shed light on the relationship between the

catalyst activity and the textural and chemical properties of the catalysts studied in this thesis.

Finally, the conclusion and some recommendations are listed in chapter five.

4

CHAPTER 2

LITERATURE REVIEW

2.1 Biodiesel production.

2.1.1 Background Biodiesel synthesis is chemically described as the trans-esterification of triglycerides (oil

sources) into alkyl esters using alcohol, typically progressed using an acid, base, or enzyme

catalyst. The resulting alkyl esters, with various alkyl groups, are utilized as biodiesel in the

industry [9].

Nowadays, biodiesel is well accepted as a renewable energy source [10]. Recently, there has

been renewed interest in vegetable oils and animal fats to produce biodiesel since it offers many

advantages such as high flash point, high cetane number, low viscosity, high lubricity,

biodegradability, environmental friendliness due to lower carbon monoxide emissions, as well as

fewer emission profiles compared to conventional fossil fuels. Lower carbon monoxide

emissions are due in part to the oxygen in the ester bonds which allows more CO to be oxidized

to CO2. It can also be used in conventional compression-ignition engines without the need for

engine modifications. In addition, the fuel is used either pure or as a blend can reduce the

particulate emissions from engines [8]. The production of biodiesel is increasing hugely due to

its environmental benefits. However, production costs are still rather high, compared to

petroleum-based diesel fuels [11]. Biodiesel produced using homogeneous catalysts from the

trans-esterification reaction of vegetable oils, requires the vegetable oil to be refined before

using it. This means inserting a step in the process that leads to additional costs for the

production of biodiesel. Using a homogeneous catalyst also leads to the formation of soap, which

is not desired in the final product. This is due to the large amount of water found in both used

and raw oils. To remove soap, additional treatment is needed; therefore an increase in production

costs is unavoidable. The cost of biodiesel could certainly be reduced through the use of a

heterogeneous catalyst, instead of a homogeneous one, providing for higher-quality esters and

glycerol, which are more easily separated, and eliminating the need for further, expensive,

refining operations [12].

5

The research and development on heterogeneous base catalysis for biodiesel synthesis has

focused mainly on improving its slow reaction rate up to the level of its homogeneous

counterpart. The reaction performances were usually evaluated in the following respects: what

level of fatty acid methyl esters (FAME) yield can be achieved within a given time frame, how

low the temperature is and what methanol/oil molar ratio and catalyst amount can be used.

It is also to be noticed that biodiesel exhibits characteristics that are comparable to the traditional

diesel fuel as given in Table 2.1, where the ASTM standards for both these fuels are given [13].

Table 2.1: Comparison of the standards for diesel and biodiesel based on American Society for Testing and Materials (ASTM).

Property Diesel Biodiesel

Standard Number Composition Specific gravity (g/mL) Flash point (K) Cloud point (K) Pour point (K) Water (vol.%) Carbon (wt.%) Hydrogen (wt.%) Oxygen (wt.%) Sulphur (wt.%)

Cetane number

ASTM D975

Hydrocarbon (C10-C21) 0.85 333-353 258-278 243-258 0.05 87 13 0 0.05

40-55

ASTM D6751

Fatty acid methyl ester (C12-C22) 0.88 373-443 270-285 258-289 0.05 77 12 11 0.05

48-60

6

When a blend of biodiesel and “petro”-diesel is prepared, the concentration of biodiesel is

conventionally written as BXX where the ‘XX’ refers to the percentage volume of biodiesel.

For example, pure 100% biodiesel is referred to as B100 while B20 designates a mixture of 20%

biodiesel and 80% petro-diesel.

In Table 2.2, the emission characteristics of pure biodiesel (B100) and that of 20% blend with

80% diesel (B20) are given relative to that of diesel.

Table 2.2: The average emissions of B100 and B20 (20% blend with diesel as compared to normal diesel (in percentages).

Emission Diesel (%) B20 (%) B100 (%)

Carbon monoxide Total unburnt hydrocarbons Particulate matter Nitrogen oxides Sulphates Air toxics

100 100 100 100 100 100

- 12 - 20 - 12 + 2 - 20 - 12 to - 20

- 48 - 67 - 47 + 10 - 100 - 60 to - 90

From Table 2.2, biodiesel (B100) can be regarded as environmentally friendly with regards to the

gaseous emissions into the atmosphere [13]. Table 2.2 summarizes the typical emission profiles

of biodiesel and one of its blends, B20, which consists of 20% biodiesel and 80% diesel, using

petroleum-derived diesel emissions as the reference. The information in the table shows how

biodiesel significantly reduces emissions compared to diesel even when used as the minor

component of a fuel blend.

In addition to these factors, biodiesel has even more advantages over conventional fossil fuel

based diesel. These considerations are presented in Table 2.3.

7

Table 2.3: Advantages of biodiesel over conventional petroleum based diesel

Parameters Characteristics exhibited by biodiesel in comparison to petroleum

based diesel

Engine Modification

Source

Toxicity

Cetane number

Life of engine

Odor

Blends of 20% biodiesel with 80% petroleum diesel (B20) can be

used in unmodified diesel engines. The use of neat biodiesel (B100)

may require certain engine modifications to avoid performance and

maintenance problems.

One half of biodiesel can be manufactured from recycled oil or fat

and the other half can be harnessed from sources like soya bean, rape

seed oil, etc.

Biodiesel is not toxic and biodegradable. Nearly 80% less carbon

monoxide emission is produced, but emission of nitrogen oxides

(precursor of ozone) is higher (refer to Table 2).

Biodiesel has a cetane number of 100 as compared to the value of 40

for diesel. This parameter is related to ignition quality and hence

biodiesel will allow cold starts and less idle noise.

Biodiesel being a better lubricant can extend the life of the engine.

Biodiesel provides a smell of popcorn or French fries.

8

2.1.2 Production process

The production process of biodiesel can easily be represented by the following diagram:

Figure 2.1: Schematic biodiesel production process [14]

There are two main factors that affect the cost of biodiesel: the cost of raw materials and the cost

of processing. Processing costs could be reduced through simplified operations and eliminating

waste streams [6].

Biodiesel can be produced by either catalyzed or uncatalyzed trans-esterification. The former

process is currently used in commercial biodiesel production while the latter method typically

involves supercritical conditions for it to occur.

9

2.1.3 Parameters affecting the yield of biodiesel The yield of biodiesel is affected by different reaction variables such as methanol/oil ratio,

temperature, mixing rate, catalyst amount and reaction time. These reaction variables are

associated with the type of catalysts used [15].

Methanol ratio

In order to shift the equilibrium forward, a large stoichiometric ratio of methanol to oil is

required. Monoglycerides, diglycerides, and triglycerides are not water-soluble. Consequently,

when trans-esterification is incomplete, these un-reacted compounds are contained in the final

biodiesel product, since they are not washed away by water. Therefore, it is vital to employ the

reaction mechanism and conditions that provide a near-complete trans-esterification process.

Although the stoichiometric methanol to oil ratio required is 3:1, the trans-esterification is

commonly carried out with an extra amount of alcohol in order to shift the equilibrium to the

proposed product, methyl ester [16]. However, as the amount of excess methanol increases, not

only does the cost for raw materials increase, but so does the cost for methanol separation and

purification [17]. Thus a higher molar ratio, results in a higher conversion of oil, however using

too high an excess of methanol can obstruct glycerin separation.

Reaction temperature

Trans-esterification can occur at different temperatures, depending on the properties of the oils.

It could be at ambient temperature [18]; or at a temperature close to the boiling temperature of

methanol [19, 20]. However, high reaction temperatures speed up the reaction and shorten the

reaction time [3, 21]. Freedman [22] found that biodiesel yield depended on temperature in the

first 30 minutes of the reaction. The reaction temperature plays an important role on the quality

of the products. The temperature which is higher than the normal boiling point of methanol

(68oC) causes excessive vaporization of methanol (loss). On the other hand, a temperature which

is lower than 50oC results in higher viscosity of the biodiesel reaction mixture. Lower

temperatures slow down the rate of reactions, prolonging the reaction time required to achieve

the maximum alkyl ester production. On the other hand, higher reaction temperatures result in

lower maximal alkyl ester production due to reversible reaction steps.

10

Catalyst amount

The amount of catalyst is always calculated as a function of weight % of the oil. Increasing the

catalyst amount significantly accelerates the trans-esterification reaction with the same degree of

transformation. Further increases of the catalyst concentration (above 1.5 wt.%) decreases the

methyl ester yield [15].

Reaction time

The reaction time affects the concentration of methyl ester, such that the concentration increases

exponentially 5 minutes after the beginning of the reaction, then after 1 hour, the concentration

of triglyceride, diglycerides and monoglyceride slightly decreases and reaches steady state [15].

Mixing rate

The rate of the trans-esterification reaction of vegetable oil with alkaline methanol solution

strongly depends on the rate of mass transfer at the interface between glycerol–methanol and oil–

ester phases. Generally, low reaction rates are observed in transesterification as a result of a poor

dispersion of the methanol and oil phases, and an induction period can often been seen on the

kinetic curves (slow initial reaction before steady-state concentrations are reached). Therefore,

intense mixing is very important for the trans-esterification process [23].

2.1.4 Feedstock There are different types of oils and fats that can be used as feedstock for biodiesel production

(edible and non edible oil): babassu, canola, coconut, corn, cottonseed, linseed, olive, palm,

peanut, rapeseed, safflower, sesame, soybean, sunflower, butter, lard, yellow grease, tallow

(beef), etc. However, a careful choice of feedstock should be made. The type of feedstock has a

greater effect on the energy content of biodiesel than a particular processing method [17].

Vegetable oil is composed of over 100 substances, and different oils have different compositions

that can vary even for the same oil type. The source of feedstock for the production of biodiesel

should fulfill two requirements: price (low feedstock and production costs; more than 80% of the

production cost corresponds to the feedstock cost) and local availability (large and constant

production volume). It is also necessary to take into consideration the oil content of the seeds and

the yield of oil per hectare [23].

11

2.2 Catalysts

2.2.1 Introduction Catalysts can be defined as substances which are used to modify a chemical reaction by reducing

the activation energy, which is the energy needed to initiate the reaction, without being

consumed during the reaction. There are two different types of catalyst systems, heterogeneous

and homogeneous systems [24].

The basis of catalysis is the enhancement of rate of a reaction by a substance without overall

change in that substance, the catalyst. It is not an over simplification to state that the activity of

the catalyst arises because it assists the re-arranging or moving of atoms or electrons of the

bound reactants from one centre to another while forming products more quickly than would

happen in its absence. In other words, it reduces the free energy barriers to reaction, sometimes

by opening up a pathway different from that of the easiest un-catalyzed reaction [25].

Nanometer-sized catalysts have several advantages over conventional micrometer-sized

catalysts; these include higher surface area, better diffusion properties and longer catalytic

lifetimes.

Both heterogeneous and homogeneous catalysts are used for the trans-esterification reaction. An

overview on the behaviour of different catalysts is given below and also their advantages and

disadvantages on the trans-esterification reaction are included. The selection of an appropriate

catalyst is of fundamental importance for the design of a sustainable trans-esterification process.

Homogeneous catalysts Currently, homogeneous alkaline catalysts, such as sodium hydroxide and potassium hydroxide,

are most commonly used in industrial trans-esterification processes for biodiesel production,

mainly because they are able to efficiently promote the reaction at relatively low temperatures.

Homogeneous acid catalysts and heterogeneous (solid) catalysts are used to a lesser extent.

While the main advantages in the use of homogeneous acid and base catalysts are their cost-

effectiveness and good performance, both catalysts require the use of an excess of alcohol, can

provoke various problems, and are associated with technical difficulties. The use of

homogeneous catalysts is normally limited to batch-mode processing followed by a catalyst

12

separation step. The immiscible glycerol phase, which accumulates during the course of the

reaction, solubilizes the homogeneous catalyst and, therefore, withdraws it from the reaction

medium. Moreover, other difficulties of using homogeneous catalysts relate to their sensitivity to

free fatty acids (FFAs) and water and the resulting saponification phenomenon.

Usually, base catalysts are better suited to the trans-esterification of triglycerides, whereas acid

catalysts are better suited for the esterification of FFAs. Therefore, feedstocks containing FFAs

may require two different types of catalysts (acid and base catalysts) that are usually employed in

a two-stage process, which means that the acid catalyst from the first stage has to be removed

before the base catalyst is added in the second stage. A disadvantage of homogeneous acid

catalysts is also their increased corrosiveness [23].

Heterogeneous catalysts The heterogeneously catalysed process requires neither catalyst recovery nor aqueous treatment

steps: the purification steps of products are much simplified and very high yields of methyl

esters, close to the theoretical value, are obtained. Glycerine is directly produced with high purity

levels (at least 98%) and is exempt from any salt contaminants. With all these features, this

process can be considered as a green process [7].

Heterogeneous base catalysis is the most viable process for the trans-esterification of

triglycerides into biodiesel. The heterogeneous catalyst features are lower corrosiveness,

environmental friendliness, easy catalyst recovery and high process integrity.

The research and development on heterogeneous base catalysis for biodiesel synthesis has

focused mainly on improving its slow reaction rate up to the level of its homogeneous

counterpart. The reaction performances were usually evaluated in the following respects: what

level of fatty acid methyl esters (FAME) yield can be achieved within a given time frame, how

low the temperature is and what methanol/oil molar ratio and catalyst amounts can be used.

Generally, a higher reaction temperature (100-250oC) and/or high methanol amounts are required

for the performance of the heterogeneous process to equal that of its homogeneous counterparts

[9]. The use of heterogeneous catalysts does not lead to the formation of soaps through

neutralization of FFAs or saponification of triglycerides. Furthermore, solid acid catalysts can

13

indeed improve the sustainability of the biodiesel production process, eliminating the corrosion

problems associated with their use, and consequent environmental hazards posed by them.

However, the performance of heterogeneous catalysts is generally lower than that of the

commonly used homogeneous catalysts. Moreover, heterogeneously catalyzed trans-

esterification requires relatively high temperatures and pressures [23].

A summary of the homogeneous catalysis compared to the heterogeneous catalysis for the trans-

esterification reaction is presented in Table 2.4.

Table 2.4: Comparison of homogeneous and heterogeneous catalyzed trans-esterification [13].

Factors Homogeneous catalysis Heterogeneous catalysis

1. Reaction Rate

2. After treatment

3. Processing methodology

4. Presence of water/free fatty acids 5.Catalyst reuse

6. Cost

Fast and high conversion Catalyst cannot be recovered, must be neutralized leading to waste chemical production Limited use of continuous methodology Sensitive Not possible Comparatively costly

Moderate conversion Can be recovered Continuous fix bed operation possible Not sensitive Possible Potentially cheaper

Enzymatic catalysts Some problems associated with conventional homogeneous catalytic processes, such as removal

of glycerol and the catalyst, high energy requirements, and the need to pretreat feedstocks

containing FFAs or to post-treat large amounts of waste water, can be overcome by using

enzymes. Enzymatic catalysts such as lipases are able to effectively catalyze the trans-

esterification of triglycerides with high selectivity to FAMEs either in aqueous or in non-aqueous

14

systems. Several examples of the lipase-catalyzed production of biodiesel have been reported

using different feedstocks [23]. However, some disadvantages of enzyme catalysis include the

ease of deactivation of enzymes in these systems, generally low reaction rates, and low

conversions. For example, immobilized enzymes are easily deactivated in the absence of polar

compounds such as water and methanol. Moreover, immobilized enzymes are generally more

expensive than chemical catalysts [23]. This process can tolerate free fatty acid and water

without soap formation and thereby making separation of biodiesel and glycerol easier. Enzyme

cost and its deactivation due to feed impurities are major hindrance for commercial viability of

this process [26].

2.2.2 Catalyst synthesis. Different methods of catalyst synthesis have been intensively reported in the literature including

precipitation, spray pyrolysis, microemulsion, hydrothermal synthesis, electrochemical method,

laser ablation method, gas phase synthesis and sol–gel processes [27].

In most cases, solid catalysts used in biodiesel production are prepared by impregnation of active

compounds onto the surface of porous support materials. Therefore, the catalytic activity of the

support and the density of the active compounds coated are the most important factors

determining the activity of solid catalysts [28]. To maximize the catalytic activity of solid

catalysts, it is necessary to initially screen highly active compounds that will be supported on

them. As the catalyst’s affinity for the active materials might depend on the type of support, a

good support material should be identified in order to enhance that affinity and, thereby, increase

activity. The morphology of the support material, including its porosity, pore volume and

internal structure, are additional important criteria for selection of a suitable support. The use of

solid catalysts has many advantages over homogeneous catalysts. Solid catalysts should have a

high mass transfer limitation, because most solid catalysts are prepared on porous supports, and

the reaction is mostly three-phase: solid catalyst, polar methanol and non-polar triglyceride

(solid–liquid–liquid). Design of catalysts of proper configuration and for proper operating

conditions is essential to minimizing the mass transfer limitation and achieving more highly

efficient biodiesel production [28].

15

Some catalyst synthesis techniques are given below:

Sol-gel method

The sol-gel synthesis begins with the formation of a liquid solution of suspended particles (a sol)

that is aged and dried to form a semi-solid suspension of particles in a liquid (a gel), which is

finally calcined, resulting in a mesoporous solid or powder. There are four distinct steps to the

sol-gel technique: (i) formation of the gel; (ii) aging to allow fine-tuning of the gel properties;

(iii) drying to remove the solvent from the gel; and (iv) calcination to permanently change the

physical and chemical properties of the solid. The aging and calcination steps allow for fine

control of the pore size distribution and volume by controlling experimental parameters like

time, temperature, heating rate, and pore liquid composition [27, 29-31].

Prefabricated nanoparticles can be incorporated into mesoporous solids by adding the particles

into the sol-gel mixture or, if the particles are formed by micro emulsions, the micro emulsion

can be incorporated into the preformed mesoporous structure. Alternatively, metal salts can be

added during gel formation, or after the mesoporous structure has formed [32].

Spray pyrolysis method

The experimental procedure of spray pyrolysis is simple. Firstly, an aqueous solution containing

the metal precursor is atomized into a carrier gas that is passed through a furnace. Secondly, the

atomized precursor solution deposits onto a substrate, where it reacts and forms the final product

[33]. The process has many advantages compared to other metal-forming techniques [34]: (i) it is

very easy to dope films or form alloys in any proportion by manipulating the spray solution; (ii)

neither high-purity targets and substrates nor vacuum set-ups are required; (iii) deposition rates

and therefore film thickness can easily be controlled by controlling the spray parameters; (iv)

moderate operation temperatures (100–500 °C) allow for deposition on temperature-sensitive

substrates and ensure that the overall process is less energy intensive; (v) the technique has

relatively limited environmental impact since aqueous precursor solutions can be used.

16

Impregnation method

Due to the ease of preparation, impregnation is one of the most commonly used techniques to

fabricate catalysts. Other methods of impregnating a substrate involve depositing an aliquot of

solution containing the catalyst precursor onto a substrate and allowing it to air dry [35].

Following the impregnation step, a reduction step is required to reduce the catalyst precursor to

its metallic state. As reduction occurs after the impregnation step, the nature of the support plays

a crucial role in controlling particle size [36].

Controlled double jet precipitation (CDJP)

In the concept of the CDJP technique the formation and growth of monodispersed micro crystals

and the nucleation of unstable nuclei or formation of primary particles occur simultaneously

during the whole run. However, monodispersed particles may be prepared if these unstable

nuclei will disappear from the system via their dissolution and diffusion of the matter to the

surface of growing monodispersed micro crystals (mechanism of controlled Ostwald ripening) or

via controlled agglomeration of primary particles to form uniform secondary particles

(mechanism of controlled agglomeration) [37].

2.2.3 Catalyst support It is well-known that the catalyst support facilitates the preparation of a well-dispersed, high

surface area catalytic phase, stabilizes the active phase against loss of surface area and

significantly influences the morphology, adsorption, and activity/selectivity properties of the

active phase [38]. Important factors on catalytic activity of solid catalysts are specific surface

area, pore size, pore volume and active site concentration on the surface of the catalyst.

Moreover, the type of precursor of active materials has a significant effect on the catalyst activity

of supported catalysts. However, active site concentration was found to be the most important

factor for solid catalyst performance. The use of catalyst supports such as alumina or silica could

improve the mass transfer limitation of the three phase reaction. Furthermore, by anchoring

metal oxides inside pores, catalyst supports could prevent active phases from sintering in the

reaction medium [8]. One of the ways to minimize the mass transfer limitation for heterogeneous

17

catalysts in liquid phase reactions is the use of catalyst supports. Supports can provide higher

surface area through the existence of pores where metal particles can be anchored [8].

The principal catalyst-preparation technique involves two stages. First, rendering a metal-salt

component into a finely divided form on a support and secondly; conversion of the supported

metal salt to a metallic or oxide state. The first stage is known as dispersion and is achieved by

impregnation, adsorption from solution, co-precipitation, or deposition, while the second stage is

variously called calcination or reduction [39].

The supporting process of a catalyst on a substrate is usually done by one of the following

methods:

- Impregnation;

- Deposition-precipitation;

- Adsorption from solution;

- Chemical vapor deposition

- Co-precipitation

Parameters to be looked at for a good catalyst/support are:

Activity - In general activity arises from maximizing both the dispersion and availability of the

active catalytic material. Ideally, from an activity viewpoint, the catalyst material should be

highly dispersed and concentrated on the external surface of the support. Already, however, there

is an inherent conflict as high concentrations of active material become progressively more

difficult to disperse [40].

Stability - By stability we refer to the loss in activity with time. This is due to one or several of

the four main causes; fouling of the active surface with non volatile reaction by-products,

sintering or crystal growth of the active material, poisoning of the active surface by feed

impurities, and blockage of the support pore structure [40].

Sintering during catalyst use is usually not a problem if catalysts are properly designed for their

end use, although it is perhaps an important problem during catalyst preparation, activation, and

18

reduction if the impregnated metal is not bound to the support surface. It also becomes an

important factor under the more severe conditions imposed during catalyst regeneration.

Fouling of the active surface by reaction by-products is a real problem, which typically can be

partially met by selective poisoning of the active ingredient. In a general sense, the use of

bimetallic supported catalysts would also commonly fall into this category, since selective

poisoning implies a close control over the ratio of poison to active material. In this case a severe

constraint is imposed upon catalyst design in that both active and moderating components should

ideally be in a constant ratio throughout the catalyst support, that is to say, the placement of both

should be the same.

Poisoning of the catalyst by impurities introduced with the reactants can often be minimized by

placing the active material deep within the catalyst support structure, and since most catalyst

supports are also good absorbents, poisons frequently can be selectively removed by such

absorption before reaching the active surface. An example would be the removal of traces of lead

and phosphorous from a car exhaust by the surface of the catalyst support. A catalyst design

modification of this same technique would be the deposition of a poison-resistant catalyst

component close to the surface and a poison-sensitive component deep within the support. This

technique can be taken even further; an inert material can be used as a poison trap close to the

support’s external surface. In this way, each catalyst support particle can be viewed as coming

complete with its own catalyst guard bed. Once again for poison resistance the location of the

active component becomes a critical factor in proper catalyst design.

Finally, blockage of the support-pore structure is critically dependent upon the pore-size

distribution of the support. Normally a correct balance of large and small pores is required; the

former to aid reactant transport and the latter to provide the large surface necessary for the

optimal dispersion of the active components.

Selectivity - Catalyst selectivity can change due to either physical or chemical reasons. For

sequential reactions diffusivity and mass transport through the pore structure can lead to apparent

loss in selectivity in the formation of intermediate products. Location of active ingredients and

pore-size distributions are therefore again of importance. Changes in selectivity can also arise

19

from changes in intrinsic chemical activity of the active component. Typically this can be

affected by use of multicomponent catalysts in which case, as we saw earlier for stability

improvement, the location of the different components should ideally be the same. A specific

example of this type of selectivity arises in the case of multifunctional catalysts in which a

hydrogenation function is combined with an acid function. Since the latter is typically provided

by the support and the former by the impregnated material, a uniform impregnation is required

[40].

Regenerability - Regenerability refers to the reactivation of a catalyst, which typically will

involve an air calcination followed in some cases by a redispersion of the active components.

From the catalyst design viewpoint this will generally imply enhanced thermal-hydrothermal

stability of the support itself, combined with stability of the active components under the high

temperature oxidizing environments required for the oxidation of the deactivating carbonaceous

deposits. It is now generally recognized that many metals sinter more readily under oxidizing

conditions and in extreme cases may even dissolve in the underlying support and become

effectively removed from the reaction system. A further complication arises with

multicomponent catalysts in which the combination ratio is all important, since such

combinations frequently are destroyed under oxidizing conditions [40].

20

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24

CHAPTER 3

RESEARCH DESIGN AND METHODOLOGY

3.1 Introduction

Zinc oxide (ZnO) is one of the most extensively studied materials because of its outstanding

optoelectronic properties, with potential applications in many different fields of technology [1].

As an important II–VI semiconductor with a wide and direct band gap (3.37 eV) and a large

exciton binding energy (60 meV), ZnO has attracted much interest, owing to its specific

electrical, catalytic, photochemical and optoelectronic properties [2].

Nanopowders, controlled to nanocrystalline size (less than 100 nm), can show atom-like

behaviors which result from higher surface energy due to their large surface area and wider band

gap between valence and conduction band when they are divided to near atomic size. Therefore,

these phenomena can effectively enhance properties of materials including optical, chemical,

electro-magnetic, etc. ZnO has been widely used in applications such as UV protection, photo

catalysis, field emission displays, varistors, functional devices, thermoelectric materials, etc. due

to its exceptional physical and chemical qualities [3].

However, to obtain this oxide with interesting properties and for the development of novel

devices, the structure and the micro-structure (surface quality, the shape and the size, etc.) of the

elaborated particles should be highly controlled. Several physical and chemical methods have

been developed to obtain these starting nanocrystals: the solid state reaction, chemical vapor

transport (CVT), mechanochemical processing, vapor phase oxidation of Zn powders,

hydrothermal process, sol–gel process and forced hydrolysis in polyol medium [4].

Stӧber’s method is a sol-gel process for the synthesis of silica particles. It is regarded as the

simplest and most effective route to prepare monodisperse silica spheres because the reactants

are normal and the reaction conditions are controllable and is easy to be carried out [5].

25

In this work, a version of Stӧber’s method was used to produce nano-ZnO where the TEOS was

replaced by zinc methoxide as catalyst precursor. These nano-ZnO were also supported on TiO2

and both the unsupported and supported ZnO were tested for catalytic activity for the trans-

esterification of soybean oil to biodiesel.

3.2 Experimental procedure

3.2.1 Catalyst synthesis Synthesis of silica particles

Silica particles were successfully produced via the Stöber synthesis version method.

In this version of Stöber synthesis, the following reagents were used [6]:

- Tetraethyl orthosilicate (TEOS) (Sigma Aldrich, 99%) as precursor alkoxide,

- Ethanol (Prolabo, 98%) as solvent,

- Ammonia (Acechem, 25%) as catalyst and,

- Distilled water as hydrolyzing agent.

Two different solutions were prepared in separate containers: the first one, containing ammonia

and water with a fixed mole ratio and the second, containing TEOS and ethanol with variable

amount of the former and a fixed amount of the latter. The concentration of TEOS was varied

from 0.250 M to 3.494 M. The amount of ammonia, water and ethanol were fixed at: 0.46, 2.89,

and 2.15 mol respectively. Calculations were performed to determine the quantities to be mixed

for all the different reagents: 64.492 cm3 for ammonia, 10.62 cm3 for pure water and 125.1 cm3

for ethanol and variable amount of TEOS.

The first solution containing TEOS mixed with ethanol in desired ratio was equilibrated at 0oC in

an ice-water bath for 30 minutes. The second solution containing ammonia mixed with water

was also equilibrated at 0oC in an ice-water bath for 30 minutes. The two different solutions were

mixed and stirred for 3 hours in an ice-water bath to allow for the formation of silica particles.

The mixture formed was centrifuged for 5 minutes to separate the solids from the liquid. The

solids recovered were washed twice in distilled water, to remove the remaining ammonia. After

centrifugation, the solids obtained were dispersed in distilled water and silica slides were dipped

26

into the solution for the particles to be coated on the surface of the slides. The silica particles

were characterized by scanning electron microscopy (SEM) to determine the particles size.

SEM analysis.

SEM is the most versatile and widely used electron beam instrument in the world. It owes its

popularity to the easily interpreted nature of the micrographs that it generates, to the diversity of

types of information that it can produce, and to the fact that images and analytical information

can readily be combined. The use of the SEM for materials characterization is increasingly

motivated by the desire to obtain not just images but quantitative information in two, or even

three dimensions, about the microstructure, the chemistry, the crystallography and the electronic

properties of the material of interest [7]. The basic mode of use of the SEM has always been in

the imaging of surface topography. In SEM, a fine probe of electrons with energies typically up

to 40 keV is focused on a specimen, and scanned along a pattern of parallel lines. Various signals

are generated as a result of the impact of the incident electrons, which are collected to form an

image or to analyze the sample surface [8].

In this study SEM analysis was performed on a JEOL JSM-5600 equipment at a working

distance of 10 mm, spot size 20 and a voltage of 20 keV represented in figure 3.1. This technique

provides a high resolution image of the surface of a catalyst (topographical information). It

provides the information concerning catalytic particle morphology, active phase homogeneity

and composition near the surface regions of the material.

27

Figure 3.1: SEM analysis equipment

28

Synthesis of ZnO nanoparticles

The synthesis of ZnO nanoparticles started with the preparation of the precursor, which is in this

case, zinc methoxide with formula Zn(OCH3)2. Zinc methoxide was prepared in the laboratory

by reacting sodium metal, methanol (Acechem, 99%) and zinc chloride (Acechem, 98%) in a

fixed proportion. After obtaining the desired precursor, ZnO nanoparticles were synthesized

using a version of the Stöber synthesis method [6] using the same procedure as for silica

described above.

The zinc oxide nanoparticles produced were dried over night and then characterized using

Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX), X-ray

diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) for more details of the

product.

TEM and EDX analysis.

TEM is a powerful tool for microstructural analysis at high spatial resolution. In a TEM, a high

energy (100 - 300 KeV) electron beam is transmitted from the top through a thin section of a

sample and the image is formed below the sample. Unlike a SEM image, the TEM image

contains three-dimensional information from the thin section of the sample. The contrast

variation in the image is a result of complex beam-specimen interactions that are unique to a

TEM. The contrast is also sensitive to small variations in chemical, structural, and topographical

features of the sample. This property is frequently exploited to resolve subtle effects of crystal

defects and interfacial layers. In addition, the resolving power of the TEM is inherently better

than the SEM because of the smaller wavelength (~ 0.0025 nm at 200 keV) of the high-energy

electron beam [9].

TEM and EDX analyses were done on a JEOL JEM 2100 electron microscope at 200 KeV.

Synthesis of the catalyst/support system

ZnO nanoparticles were supported on titania by deposition-precipitation using a modified

version of Stöber synthesis method. The titania support (Degussa P25), used in this work, was

mixed with the other reagents zinc methoxide, distilled water, ammonia and ethanol. The mixture

was stirred for 3 hours in an ice-water bath solution as described by the Stöber synthesis method

29

and the ZnO nanoparticles were precipitating and depositing on the titania support to form the

desired system that was used for the trans-esterification reaction.

The catalyst/support system obtained was characterized by XRD, BET, XRF and was used as

catalyst in the trans-esterification reaction.

Preparation of the titania support.

Titania (Degussa P25) was used to prepare the catalyst support as follows:

Titania powder was mixed with distilled water in a ratio of 1:1 wt.% and the mixture was dried at

120oC overnight [10] and calcined at 650oC for 2 hours [11]. After calcination, the support was

screened to get particles of 50-100 µm diameters for the purpose of the experiment. The support

obtained was characterized by XRD and BET.

Preparation of the catalyst/support system.

The catalyst/support system was prepared by precipitating ZnO nanoparticles on titania using a

modified version of the Stöber synthesis.

- Firstly, 5% of ZnO loading on the support was targeted by reacting: 0.87 g of Zinc methoxide,

2.418 cm3 of distilled water, 14.684 cm3 of ammonia, 28.484 cm3 of ethanol and 10 g of titania.

- Secondly, 10% of ZnO loading on titania was also targeting by reacting the same amount of

reagents and varying the amount of titania to 5 g.

- Lastly, 2.5 g of titania was used to synthesize 20% of the catalyst loaded on the support and by

keeping constant the same amount of the other reagents used previously.

Characterization of the catalyst/support system.

The ZnO/TiO2 system produced was characterized by XRD, to make sure that the desired

compound was produced and was supported on the substrate and also by XRF to confirm the

targeted ZnO loading on the support. BET was also used to determine the pore size distribution

and the surface area of the catalytic system.

30

XRD analysis.

XRD is used to identify bulk phases, to monitor kinetics of bulk transformations, and also to

estimate particle sizes. The XRD pattern of a powdered sample is measured with stationary X-

ray source (usually Cu Kα) and a movable detector, which scans the intensity of the diffracted

radiation as a function of the angle 2θ between the incoming and the diffracted beams. In catalyst

characterization, diffraction patterns are mainly used to identify the crystallographic phases that

are present in the catalyst [12]. XRD analysis was performed on a PHILIPS PW 3040/60 XPERT

powder diffractometer (figure 3.2). A Cu-Kα radiation (40 mA, 40 kV) source was used. The

scan was taken from 2θ = 4° to 2θ = 80° with a step width of 2θ = 0.02969°. This technique

allows the identification of crystalline phases in bulk materials and the determination of

crystallite size and shape from diffraction peak characteristics.

Figure 3.2: XRD analysis equipment

31

XRF analysis.

XRF is a non-destructive method for the elemental analysis. The method has broad dynamic

range and can be used for the analysis of wide range of concentration of all elements beyond

beryllium. XRF is an instrumental method of qualitative and quantitative analysis for chemical

elements based on the measurement of wavelengths and intensities of their spectral lines emitted

[13]. The purpose of XRF analysis is to convert elemental peak intensities to elemental

concentrations and/or film thicknesses. This is achieved typically though a calibration step,

where the XRF response function (related to parameters that are independent of the sample

matrix) for each element is measured using a known standard of some kind. In this study XRF

analysis was performed using a Magix Pro XRF spectrometer to verify the amount of catalyst

loaded on the support (figure 3.3).

Figure 3.3: XRF analysis equipment

32

BET analysis.

BET is a characterization technique of pore size distribution and surface area which are

established by adsorption and desorption of N2 on a given catalyst. The samples were dried at

90oC over night and degassed the following day at 250oC for 4 hours, prior to introduction in the

equipment for analysis. The solid samples were evacuated under vacuum and high temperature

(250oC) in such a way that the catalytic surface was free from water and other impurities and

thus available for being occupied with nitrogen molecules. The physical properties such as

surface area and pore size distribution were measured using PORETECH Tristar 3000 equipment

(figure 3.4), following the multiple point BET method using the adsorption– desorption

isotherms of nitrogen. The degassed sample powder was kept inside a glass tube, nitrogen was

used as an adsorbate at 77 K to form a monolayer on the sample. At each step, certain volume

(V) of nitrogen was adsorbed by the wall of the pores and the corresponding change in partial

pressure (P/P0) was kept between 0.05 and 0.35 along with the pore volume filling at P/P0 ~ 1

[14] with P the pressure and P0 the saturation pressure.

33

Figure 3.4: BET analysis equipment

3.2.2 Catalyst testing The activity of the unsupported nano-ZnO and the supported nano-ZnO (ZnO/TiO2) obtained

was tested in the trans-esterification of soybean oil to biodiesel under specific conditions. The

moisture content of the raw material (soybean oil) determined by the KARL FISCHER titration,

was found to be 0.027%.

The prepared TiO2 supported ZnO catalyst was tested for the trans-esterification of soybean oil

in a 300 cm3 Parr batch reactor under a nitrogen pressure that was selected to keep all the

reactants in the liquid phase at different reaction temperatures. The curve for methanol vapor

pressure versus temperature (appendix A) allowed us to determine the pressure of the reaction at

the different reaction temperatures investigated in order to keep the methanol in a liquid phase

34

for better yield of the trans-esterification reaction.

Figure 3.5: 300 cm3 Parr pressure reactor for biodiesel production The reactor was fitted with a stirrer that was operated at a constant stirring speed of 1100 rpm for

all the runs. A K-type thermocouple in contact with the reaction medium was connected to a PID

controller which controlled the reaction temperature to the desired set-point by regulating the

current to the heating mantle around the reactor. After the reaction, methanol was removed by

evaporation using a rotary evaporator and the two remaining phases were separated by

centrifugation into glycerol as one phase and methyl esters and unreacted soybean oil as the

second phase.

The effect of the following parameters on the trans-esterification reaction were investigated:

molar ratio (alcohol to oil), metal oxide loading on the support, amount of the catalyst loading in

35

the reactor for the trans-esterification reaction to take place, reaction temperature, reaction time

as well as ZnO particle size. The reusability of the supported nano-ZnO was also tested. The

biodiesel produced was analyzed by 1H-NMR and ICP-OES.

1H-NMR.

The conversion of soybean oil to fatty acid methyl esters was determined using 1H-NMR on a

Bruker Avance 400 MHz instrument (figure 3.6). After complete removal of the methanol using

a rotary evaporator (figure 3.7), the remaining phases (glycerol and biodiesel) were separated by

centrifugation, then the biodiesel phase dissolved in CDCl3 was submitted to 1H-NMR analysis.

This technique gives the conversion of oil to biodiesel by measuring the ratio of areas of the 1H-

NMR signals at 3.68 ppm (methoxy groups of methyl esters) and 2.30 ppm (α-carbon CH2

groups of all fatty acid derivatives) [15].

Figure 3.6: NMR analysis equipment

36

Figure 3.7: Rotary evaporator for biodiesel, glycerol and methanol separation

ICP-OES.

ICP is one method for optical emission spectrometry. When plasma energy is given to an

analysis sample from outside, the component elements (atoms) are excited. When the excited

atoms return to low energy position, emission rays (spectrum rays) are released and the emission

rays that correspond to the photon wavelength are measured. The element type is determined

based on the position of the photon rays, and the content of each element is determined based on

the rays intensity [16]. The analyses were performed on a Spectro Arcos instrument (figure 3.8).

37

Figure 3.8: ICP-OES analysis equipment

38

REFERENCES:

1. G. Ambrozic, S.D. Skapin., M. Zigon, Z.C. Orel, The synthesis of zinc oxide

nanoparticles from zinc acetyacetonate hydrate and 1-butanol or isobutanol. J. Colloid

Interface Sci. 2010. 346: p. 317-323.

2. Y. Ni, X. Cao, G. Wu, G. Hu, Z. Yang, X. Wei, Preparation, characterization and

property study of zinc oxide nanoparticles via a simple solution- combustion method.

Nanotechnology, 2007. 18.

3. Y.J. Kwon, K.H. Kim, C.S. Lim, K.B. Shim, Characterization of ZnO nanopowders

synthesized by the polymerized complex method via an organochemical route. J. Ceram.

Process. Res. 2002. 3(3): p. 146-149.

4. A. Dakhlaoui, M. jendoubi, L.S. Smiri, A. Kanaev, N. Jouini, Synthesis, characterization

and optical properties of ZnO nanoparticles with controlled size and morphology. J.

Cryst. Growth, 2009. 311(16): p. 3989-3996.

5. X.-D. Wang, Z.-X. Shen, T. Sang, X.-B. Cheng, M.-F. Li, L.-Y. Chen, Z.-S. Wang,

Preparation of spherical silica particles by Stober process with high concentration of

tetraethyl-orthosilicate. J. Colloid Interface Sci. 2010. 341: p. 23-29.

6. D.A.S. Razo, L. Pallavidino, E. Garrone, F. Geobaldo, E. Descrovi, A. Chiodoni, F.

Giorgis, A version of stober synthesis enabling the facile prediction of silica nanospheres

size for the fabrication of opal photonic crystals. J. Nanopart. Res. 2008. 10: p. 1225-

1229.

7. D. C. Joy, Scanning electron microscopy for materials characterization. Current Opinion

in Solid State & Materials Science 1997. 2: p. 465-468.

8. A. Bogner, P.-H.Jouneau, G. Thollet, D. Basset, C. Gauthier, A history of scanning

electron microscopy developments: Towards ‘‘wet-STEM’’ imaging. Micron, 2007. 38:

p. 390-401.

9. S. R. Rai, S. Subramanian, Role of transmission electron microscopy in the

semiconductor industry for process development and failure analysis. Progress in Crystal

Growth and Characterization of Materials, 2009. 55: p. 63-97.

10. K. Jalama, N.J.Coville, D. Hildebrandt, L.L.Jewell, D. Glasser, Fischer-tropsch synthesis

over Co/TiO2: Effect of ethanol addition. Fuel, 2007. 86: p. 73-80.

39

11. R. Zennaro, M. Tagliabue, C.H. Bartholomew, Kinetics of Fischer-Tropsch synthesis on

titania-supported cobalt. Catal. Today, 2000. 58: p. 309-319.

12. J. W. Niemantsverdriet, Spectroscopy in catalysis. Third ed, ed. WILEY-VCH. 2007, weinheim.

13. N. L. Misra, K. D. S. Mudher, Total reflexion X-ray fluorescence: a technique for trace element analysis in materials. Progress in Crystal Growth and Characterization of materials (2002) 65-74, 2002: p. 65-74.

14. D. Dutta, S. Chatterjee., K. T. Pillai, P. K. Pujari, B. N. Ganguly, Pore structure of silica

gel: a comparative study through BET and PALS. Chemical Physics 2005. 312: p. 319-324.

15. W. Xie, Z. Yang, Ba-ZnO catalysts for soybean oil transesterification. Catal. Lett. 2007.

117(3-4): p. 159-165.

16. Inc, S.N. Description of ICP Optical Emission Spectrometry (ICP-OES). 2012.

40

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Materials synthesis and characterization

4.1.1 Silica nanoparticles Silica nanoparticles were successfully synthesized using a modified version of the Stöber

synthesis method and the results are shown in Figures 4.1 and 4.2. Figure 4.1 reports SEM data

for silica nanoparticles produced at a reaction temperature of 0oC and fixed molar ratio of

NH3/H2O and CH3CH2OH: 0.46/2.89 and 2.15 respectively using various amounts of TEOS. At

lower concentration of the precursor (picture a), mono layer particles with almost the same size,

as shown on the particle size distribution graph, were obtained. By increasing the precursor

amount (picture b), mono layer particles were still produced but with different particle sizes.

At higher concentrations of the precursor (picture c), the particles were no more separated but

rather aggregated.

Figure 4.2 shows the dependence between the amount of precursor and silica particle sizes

determined by SEM. These data show the dependence of particle diameter on the cubic root of

TEOS concentration. The dependence of diameter d and TEOS amount (molTEOS ) is illustrated

in equation (1) by a cubic-root relationship [1]:

In this work the proportionality constant k was found to have a standard deviation of 2.6%

(Figure 4.2). It depends on the molar ratios of NH3/H2O/C2H5OH [2] and is given by:

41

This dependence of particle diameter on the cubic root of TEOS concentration agrees with a

simplistic model in which the constant composition of ammonia, ethanol, and water plays a

major role in the nucleation process, as compared to the TEOS concentration at constant

composition of the other ingredients [1, 5-7].

42

300 400 500 600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

Fre

qu

en

cy

Particle Size (nm)

300 400 500 600 700 800 900 1000 1100

0

10

20

30

40

50

60

70

80

90

100F

req

ue

ncy

Particle size (nm)

Figure 4.1: SEM images of silica produced at a reaction temperature of 0OC and fixed molar ratio of NH3/H2O and CH3CH2OH: 0.46/2.89 and 2.15 respectively with following moles of TEOS (a) 0.005 mol (0.25 M) (b) 0.035 mol (1.75 M) (c) 0.06 mol (3 M) with their particles distribution graphs.

43

Figure 4.2: The dependence of silica particle size on concentration of TEOS. Reaction conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h.

4.1.2 Unsupported nano-ZnO The synthesized unsupported ZnO nanoparticles were characterized by FTIR, XRD, SEM and

TEM.

FTIR results

Figure 4.3 shows the FTIR spectrum for the precursor (zinc methoxide) and the synthesized

nano-ZnO particles. ZnO nanoparticles were synthesized using a synthetic methodology similar

to the synthesis of silica nanoparticles discussed in 4.1.1. It should be noted here that the sol-gel

synthesis directly produced ZnO, and no calcination was necessary to convert the product to the

oxide. The reaction was followed using IR spectroscopy. In the IR spectrum for the precursor,

the ν(C-H) was observed at 2900 cm-1, whereas the ν(C-O) was observed at 1060 cm-1. These

absorptions were only present in the precursor and were not observed in the nano-ZnO spectrum,

suggesting complete conversion of the precursor to the oxide. The spectrum of the oxide showed

a weak ν(O-H) absorption around 3200 cm-1, which was attributed to absorbed surface

hydroxyls.

44

Figure 4.3: IR results of the precursor and the zinc oxide obtained under these reaction conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h.

XRD results

Figure 4.4 shows XRD data for the precursor (zinc methoxide) and the synthesized ZnO. New

diffraction peaks at diffraction angles 34, 36, 47, 63, 68, 69, 72, 77 degrees which were not

present in the precursor spectrum were observed in the synthesized ZnO spectrum. That

confirmed the formation of a different compound from the precursor. The XRD pattern of ZnO

suggests a wurtzite crystal structure of the ZnO with a hexagonal shape. The lattice constants (a

= b = 0.32 nm and c = 0.52 nm) and diffraction peaks corresponding to the planes (100), (002)

and (101) obtained from X-ray diffraction data are consistent with reported literature data of

ZnO by Gupta et al. (2006) [8].

45

Figure 4.4: XRD results of the precursor and the zinc oxide

XRD data were also used to determine the particles size using the Scherrer equation as follows:

Dp = 0.9 /(1/2 cos) (3)

Where:

Dp : crystallite size (nm)

: wavelength of X-ray (Angstrom)

1/2 : full width at half maximum (radians)

: diffraction angle (radians)

Figure 4.5 shows the dependence of nano-ZnO particle size, determined from XRD data, on the concentration of zinc methoxide.

46

Figure 4.5: The dependence of nano-ZnO particle size on concentration of zinc methoxide. Reaction conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h.

As for SiO2, the relationship between the precursor quantity and the synthesized

particles sizes was established. Again, within experimental errors, a cubic root

relationship between precursor quantities and the synthesized particles sizes was

observed. The proportionality constant k for ZnO was found to be equal to:

47

SEM results

Unfortunately, the synthesized nano-ZnO crystallites could not be observed using SEM.

The crystallites size was so small and could not be visualized on the SEM equipment

JEOL JSM-5600. Therefore, a high resolution TEM was necessary.

TEM results TEM results are summarized in figure 4.6 below. The results show size increase of the

synthesized ZnO nanoparticles with an increase in precursor amounts, consistent with

XRD results. The data show that while increasing the concentration of zinc methoxide

(precursor) respectively from 1.25, 1.75, 2.75 to 3.5 Mol, ZnO particle sizes increase

also accordingly from 30, 40, 50 and 60 nm respectively. The feret diameter was used

to determine the particle size distribution. EDX spectra are added to the TEM data in

figure 4.6 to confirm the presence of Zn and O in the samples. The Cu peak was from

the TEM grid and no C was detected.

The operating conditions of the version of Stӧber’s synthesis were set up in fixing the

mole amount of ethanol, distilled water, ammonia and by varying the mole amount of

the precursor. Therefore, this method has successfully produced the particles size of

interest for this project, i.e. less than 100 nm. These sizes were controlled by varying

the precursor amount.

48

49

Figure 4.6: TEM images of ZnO, with their particles size distribution, produced at a reaction temperature of 0°C and fixed molar ratio of [NH3] : [H2O] : [CH3CH2OH] of 0.46 : 2.89 : 2.15 moles, respectively, with following moles of [Zn(OCH3)2] (a) 0.025 mol (b) 0.035 mol (c) 0.055 mol (d) 0.07 mol.

4.1.3 TiO2 Support The prepared TiO2 support was characterized by XRD and BET analyses.

50

XRD results

TiO2 was characterized by XRD before and after calcination. The data are presented in figure

4.7.

Figure 4.7: XRD of titania before and after calcination at 650oC

Major peaks for anatase and rutile were found at 26, 37, 48 and 28, 36, 42, 44, 70 degrees,

respectively. The anatase phase composition was higher than the rutile one before

calcination (70% and 30%, respectively). After calcining the sample at 650°C, a change in

phase composition of the sample was noticed (86% rutile and 14% anatase). Rutile peaks

became significantly larger, suggesting that the rutilation process occurred during the

calcination process. Also calcination increases the mechanical strength of the crystals. Thus,

increasing calcination temperature increases the transformation of anatase into rutile [9].

BET results

BET analysis of the calcined TiO2 support gave a surface area of 21.7 m2/ g and a pore size

of 30.2 nm.

51

4.1.4 Supported nano-ZnO The TiO2 supported ZnO nanoparticles were characterized by XRD, XRF and BET analyses.

XRD results

To facilitate comparison, XRD data for the blank calcined TiO2 support, the unsupported

ZnO and the TiO2 supported ZnO (20 wt.% ZnO/TiO2) are reported in Figure 4.8. After

calcination the proportion of rutile and anatase were respectively 86% and 14% which can

be explained by the fact that rutile is stable at higher temperatures compared to anatase. The

major peaks for the unsupported ZnO (Fig. 4.8 a) were detected at diffraction angles of ca.

32, 35, 37, 46, 48, 57, 63 and 68o consistent with data reported in literature [8, 10, 11]. The

crystallite size of the unsupported ZnO was ca. 50 nm. The XRD pattern for the TiO2

supported ZnO (Fig. 4.8 b) was a combination of the XRD patterns for the unsupported ZnO

(Fig. 4.8 a) and the blank TiO2 (Fig. 4.8 c). No new peaks indicating the formation of new

phases in addition to TiO2 and ZnO were detected. This could suggest that there were no

significant ZnO-TiO2 compounds formed and that the ZnO-TiO2 interactions were purely

physical. The crystallite size for the supported ZnO was ca. 17 nm, about a third of the

crystallite size for the unsupported ZnO. These findings indicate that the TiO2 support

stabilised ZnO in a dispersed form and limited the growth of ZnO crystallite size.

0 10 20 30 40 50 60 70 80 90

Coun

t

2θ [o]

(a)

(b)

(c)

Figure 4.8: XRD data for calcined (a) unsupported ZnO, (b) TiO2 supported ZnO and (c) blank TiO2

52

XRD data were also used to determine the nano-ZnO particle sizes as a function of precursor (zinc methoxide) amount. The results are summarized in figure 4.9.

Figure 4.9: Effect of precursor amount on supported nano-ZnO particle size. Reaction conditions: [NH3] : [H2O] : [C2H5OH] = 0.46 : 2.89 : 2.15 moles; T = 0°C; t = 3 h.

XRF results

The XRF analysis results for the synthesized TiO2 supported ZnO are summarized in table

4.1 below:

Table 4.1: targeted ZnO loading on titania and XRF obtained results. Calculated ZnO loading on titania (%) XRF results obtained (%)

5

10

20

3

8

15

53

BET results

The surface area and pore size diameter for the blank TiO2 support and the TiO2 supported

ZnO are summarized in table 4.2 below:

Table 4.2: surface area and pore size diameter for blank support (TiO2) and supported ZnO (ZnO/TiO2)

Blank titania (TiO2) Supported ZnO (ZnO/TiO2)

Surface area (m2/ g)

Pore size (nm)

21.7

30.2

15.9

28.5

BET ran on the supported 20% ZnO revealed a surface area of 15.9 m2/ g and a pore

diameter of 28.5 nm. It is possible that more ZnO was growing in the TiO2 pores and

therefore the resulting crystallite size was less than the pore size. Thus the TiO2 stabilized

ZnO particles in a more dispersed form.

4.2 ZnO catalyst testing for soybean oil trans-esterification

4.2.1 Unsupported nano-ZnO as catalyst

The catalytic activity of the synthesized nano-ZnO was evaluated for the trans-esterification

reaction of soybean oil to biodiesel. A mixture of soybean oil, methanol and an appropriate

amount of unsupported nano-ZnO catalyst were transferred to a stirred batch reactor (Parr

reactor). The reactor was sealed and pressurized to 42 bar using N2 and the reaction was

conducted for 1 hour at 225oC. After the reaction, the reactor content was discharged, the solids

filtered off and the biodiesel and un-reacted oil phase was separated from the glycerol and

methanol phase. Analysis by 1H-NMR gave the yield of the trans-esterification reaction.

The activities of various sizes of unsupported nano-ZnO catalyst on the trans-esterification

reaction of soybean oil are given in figure 4.10 in terms of soybean oil conversion.

54

Varying the nano-ZnO particle sizes showed no significant effect on the rate of the soybean

trans-esterification reaction. These results are in agreement with results reported previously, that

the rate of trans-esterification increases with crystallite size from 3 - 10 nm and then levels of

around 10 nm [15]. The particles in this study, however, were larger than 10 nm and the

calcination temperature of the catalysts was kept constant, thus no big difference in surface

defects in these particle sizes was expected hence no change in activity was expected.

The activity of ZnO in the trans-esterification reaction has been proven beyond doubt [12].

Karmee & Chadha studied the trans-esterification of a certain oil using three different catalysts

including ZnO and they observed on a higher activity of ZnO compared to the two others [13].

Stern et al. (1999) also reported ZnO as a heterogeneous catalyst for the production of alkyl

esters from vegetable oils with alcohol and they found that ZnO gives a high methyl ester yield

[14].

Figure 4.10: Effect of unsupported nano-ZnO particle size on biodiesel conversion (alcohol to oil molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction temperature of 225oC).

55

4.2.2 TiO2 supported nano-ZnO catalyst The synthesized ZnO/TiO2 systems were tested for catalytic activity in the trans-esterification

reaction of soybean oil to biodiesel. The effect of the following parameters on the soybean oil

conversion during the trans-esterification to biodiesel were investigated: i) oil to methanol

molar ratio; ii) ZnO loading on TiO2 support; iii) catalyst to oil mass ratio; iv) reaction time; v)

reaction temperature, vi) ZnO particle size and vii) catalyst reutilization.

Effect of reaction temperature and oxide loading on the support

The effect of temperature and ZnO loading on the TiO2 support as catalyst on the soybean trans-

esterification reaction with methanol are reported in Figure 4.11. The general trend of the data

shows that when catalysts with the same ZnO loading are used, the oil conversion increases with

reaction temperature. For example, soybean oil conversions of ca. 16, 55, 82 and 96%. were

respectively measured at 150, 175, 200 and 225oC when a catalyst containing 5% ZnO was used.

The effect of ZnO loading on the oil conversion depends on the range of reaction temperatures

used. The lowest conversions were measured on catalysts with the highest ZnO loading, i.e. 20%

ZnO for the reactions performed at 150 and 175oC. However, the oil conversion passes through a

minimum on the catalyst with a 10% ZnO loading and increases on the catalyst containing 20%

ZnO for reactions performed at 200 and 225°C. These findings can be explained by the ZnO

dispersion on the support and the mass transfer limitations. The crystallite size, determined by

XRD analysis, for the catalyst containing 5% ZnO was ca. 31 nm compared to ca. 17 nm for the

catalyst containing 20% ZnO. The ZnO crystallite size for the 5% ZnO catalyst was slightly

larger than the pore size of the TiO2 support suggesting that most of the ZnO particles were

stabilized on the outer surface of the support. The mass transfer resistance on these particles was

lower compared to the case of 20% ZnO catalyst, where most of the particles were probably

stabilized inside the pores of the TiO2 support. As the temperature increased to 200 and 225oC,

the viscosity of the reacting medium decreased and improved the mass transfer on particles

inside the pores of the 20% ZnO catalyst resulting in a significant increase in oil conversion. The

highest conversion was achieved with the reaction performed over a 20% ZnO catalyst at 225oC.

A temperature of 225oC was selected as the reaction temperature for the rest of the study.

56

0

20

40

60

80

100

120

0% 5% 10% 15% 20% 25%

Oil

conv

. [%

]

ZnO loading on TiO2

150oC 175oC

200oC 225oC

Figure 4.11: Effect of reaction temperature and oxide loading on biodiesel conversion: alcohol to oil molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time.

Effect of reaction time on biodiesel conversion

The oil conversion after 1, 6 and 10 hours of the soybean trans-esterification reaction over 5, 10

and 20% ZnO catalysts are reported in Figure 4.12. Oil conversions of ca. 96% were achieved

over 5 and 20% ZnO catalyst after 1 hour of reaction. No significant change in the oil conversion

was observed when the reaction time was extended to 6 and 10 hours on a 5% ZnO catalyst. An

almost total oil conversion was measured on a 20% ZnO catalyst when the reaction time was

extended to 6 hours but a decrease in conversion to ca. 90% was observed after 10 hours of

reaction. This could be due to the glycerolysis reaction reported to take place at extended trans-

esterification reaction times [19]. The lowest oil conversion (ca. 75%) was measured over 10%

ZnO catalyst after one hour of reaction followed by an increase to 85% after 6 hours of reaction.

No significant change in conversion was measured on 10% ZnO catalyst after 10 hours of

reaction. These findings revealed that a reaction time of ca. 1 hour over a 5 or 20% ZnO catalyst

was enough to achieve satisfactory levels of oil conversion.

57

Figure 4.12: Effect of reaction time on biodiesel conversion: alcohol to oil molar ratio 18 to 1, catalyst amount 1.5 wt.% and reaction temperature of 225oC

Effect of catalyst amount on biodiesel conversion

The effect of catalyst amount on the oil conversion has been evaluated by running the trans-

esterification reaction at 225oC without catalyst and subsequently with 0.5, 1.5, 3 and 6 wt.%

catalyst (20% ZnO/TiO2) with respect to the amount of oil loaded in the reactor. The results

summarized in Figure 4.13 show that after 15 minutes (Figure 4.13 a) of reaction the oil

conversion for the run without catalyst reached ca. 25% compared to 81, 87, 88, 89%

respectively for the runs with catalyst amounts equal to 0.5, 1.5, 3 and 6 % catalyst loaded in the

reactor. Oil conversions in excess of 90% were measured for the reactions with 0.5-6 wt.%

catalyst after 30 minutes of reaction (Figure 4.13 b). Within an experimental error, no difference

in oil conversions were noted after 45 minutes of reaction (Figure 4.13 c) with 1.5-6 wt.%

catalyst in the reactor. These conversions were at their maximum values of ca. 100% compared

to 54 and 95% respectively for 0 and 0.5 wt.% catalyst in the reactor. Further increase in reaction

time to 60 minutes (Figure 4.13 d) resulted in a decrease in conversion to ca. 99% for the run

with 1.5% catalyst and 96% for the reaction run with 3 and 6% catalyst respectively. This

58

decrease in oil conversions is suggestive of a glycerolysis reaction at extended reaction times as

discussed earlier in the effect of reaction time on the trans-esterification reaction. The activity of

the glycerolysis reaction, as indicated by a drop in measured oil conversion, was high for the

reaction with the highest amount of catalyst (6 wt.%). No indication of glycerolysis reaction

activity was observed for the reaction with 0 and 0,5 wt.% catalyst which showed monotone

increases in oil conversion with time up to 68 and 98% respectively after 60 minutes of reaction.

Figure 4.13: Effect of catalyst amount on biodiesel conversion: alcohol to oil molar ratio 18 to 1, 20% ZnO/TiO2 and reaction temperature of 225oC

25

8187 88 90

0

20

40

60

80

100

1 2 3 4 5

Series1

0.0 % 6.0 %3.0 %1.5 %0.5 %

g Cat/g Oil

Oil

conv

. [%

]

a) After 15 minutes

40

91 91 93 95

0

20

40

60

80

100

1 2 3 4 5

Series1

0.0 % 6.0 %3.0 %1.5 %0.5 %g Cat/g Oil

Oil

conv

. [%

]

b) After 30 minutes

54

95 100 100 100

0

20

40

60

80

100

1 2 3 4 5

Series1

0.0 % 6.0 %3.0 %1.5 %0.5 %

g Cat/g Oil

Oil

conv

. [%

]

c) After 45 minutes

68

98 99 96 96

0

20

40

60

80

100

1 2 3 4 5

Series1

0.0 % 6.0 %3.0 %1.5 %0.5 %

g Cat/g Oil

Oil

conv

. [%

]

d) After 60 minutes

59

Effect of alcohol to oil molar ratio on biodiesel conversion

One of the most important variables affecting the conversion of oil is the molar ratio of alcohol

to oil. The stoichiometric ratio for trans-esterification requires 3 moles of methanol of each mole

of oil to yield 3 moles of fatty acid methyl ester and 1 mole of glycerol. Knowing that the trans-

esterification reaction is a reversible reaction, an excess of methanol is necessary for driving the

reaction towards products. Methanol to oil ratios of 6:1, 12:1 and 18:1 were used to evaluate the

effect of methanol to oil ratio on the soybean oil conversion during the trans-esterification

reaction. The results are presented in Figure 4.14 and show that up to 60 minutes of reaction, the

measured conversion was higher for the reaction that was performed with the highest methanol

to oil ratio (18:1 in this study) as also reported in other studies [20-22]. However, it can be

observed that the reaction with a methanol to oil ratio of 18:1 showed some decline in measured

oil conversion from ca. 30 minutes of reaction where the oil conversion was almost complete to

ca. 96% after 60 minutes. Although a high methanol to oil ratio increases the rate of oil trans-

esterification, it appears that the glycerolysis reaction is also favoured when the reaction time is

extended.

Figure 4.14: Effect of alcohol to oil molar ratio on biodiesel conversion: 1.5 wt.% of ZnO/TiO2 at 20% and reaction temperature of 225oC

60

Effect of particle size on biodiesel conversion

As discussed in section 4.1.4, varying the catalyst precursor (zinc methoxide) amount leads to

different ZnO particle sizes on the TiO2 support (Figure 4.9). TiO2 supported nano-ZnO with

particle sizes in the 40 – 60 nm range were synthesized and tested to evaluate the effect of

particle size on the soybean oil conversion during the trans-esterification reaction. The results

are summarized in figure 4.15. Varying supported nano-ZnO particle sizes showed no significant

effect on the conversion of the soybean oil during the trans-esterification reaction. These results

are in agreement with results reported previously, that the rate of trans-esterification increases

with crystallite size from 3 - 10 nm and then levels of around 10 nm [15]. The data in figure 4.15

for the TiO2 supported ZnO were compared to the data for the unsupported ZnO in figure 4.10

and show higher activities measured on TiO2 supported ZnO. Thus, supporting the nano-ZnO

increases the catalyst activity.

Figure 4.15: Effect of the supported nano-ZnO particle size on biodiesel conversion (alcohol to oil molar ratio 18 to 1, 20% ZnO/TiO2, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction temperature of 225oC).

61

Reusability of the catalyst/support system on biodiesel conversion

The reusability of the TiO2 supported ZnO catalyst was tested by determining the yield of

biodiesel obtained after a certain number of runs with the same catalyst sample. The results are

presented in figure 4.16 below.

Figure 4.16: Effect of ZnO/TiO2 reutilisation on soybean oil conversion (alcohol to oil molar ratio 18 to 1, 20% ZnO/TiO2, catalyst amount 1.5 wt.%, 1 hour reaction time, reaction temperature of 225oC). R1 to R4 are different runs.

The soybean oil conversion decreased with the number of reutilization runs. This could be due to

the deactivation of the basic sites of the catalyst [23] or to some possible loss of ZnO to the

reaction product. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used

to characterize the biodiesel produced in order to find out how much of the catalyst was lost

during the trans-esterification reaction. The results are presented in table 4.3.

62

Table 4.3: ICP-OES results of biodiesel produced under the following conditions: alcohol to oil molar ratio 18 to 1, catalyst amount 1.5 wt.%, 1 hour reaction time Metals Catalyst before reaction Metals in biodiesel

150oC 175oC 200oC 225oC

Zn 0.32 g 1.10-4 µg 2.10-4 µg 5.10-4 µg 13.10-4 µg

Ti 1 g 0.3.10-4 µg 0.3.10-4 µg 0.4.10-4 µg 1.10-4 µg

The data show the presence of trace amounts of Zn in biodiesel which suggests that some of the

ZnO was lost into liquids. Lee et al. [23] also found that the dissolution of active species into

liquids is a problem of heterogeneous process which makes the catalysis partly homogeneous

and then causes problems in biodiesel quality and limits the repeated utilization of catalyst.

63

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9. A. Ahmad, G.H.Awan, S. Aziz. Synthesis and applications of TiO2 nanoparticles.

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production. Energy Fuels, 2008. 22: p. 207-217.

64

13. S.K. Karmee, A. Chadha, Preparation of biodiesel from crude oil of Pongamia pinnata.

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14. R. Stern, G. Hillion, J.- J. Rouxel, S. Leporq, Process for the production of esters from

vegetable oil or animal oils alcohols. 1999: United State Patent.

15. J.M. Montero, P. Gai, K. Wilson, A.F. Lee, Structure-sensitive biodiesel synthesis over

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16. N.S. Babu, R. Sree, P.S.S. Prasad, N. Lingaiah, Room-Temperature Transesterification of

Edible and Nonedible Oils Using a Heterogeneous Strong Basic Mg/La Catalyst. Energy

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biodiesel synthesis. Catal. Surv. Asia, 2009. 13: p. 63-67.

18. W. Xie, Z. Yang, Ba-ZnO catalysts for soybean oil transesterification. Catal. Lett. 2007.

117(3-4): p. 159-165.

19. C.A. Ferretti, S. Fuente., N. Castellani, C.R. Apesteguia, J.I. Di Cosimo, Monoglyceride

synthesis by glycerolysis of methyl oleate on MgO: Catalytic and DFT study of the active

site. Appl. Catal., A, 2012. 413-414: p. 322-331.

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two-step catalyzed process. Energy Convers. Manage. 2007. 48: p. 184-188.

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65

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion The main objective of this project was to study the catalytic properties for TiO2 supported nano-

ZnO for the trans-esterification reaction of soybean oil in order to produce biodiesel. Nano-ZnO

particles were successfully produced from zinc methoxide, and supported on TiO2. The activities

of the unsupported nano-ZnO were also tested. The outcomes of these studies are summarized as

follows:

1) Nano-ZnO was successfully produced in the desired range of this project (~ 100 nm) via the

modified Stöber synthesis method;

2) Lowest conversions were measured on catalysts with the highest ZnO loading, i.e. 20% ZnO

for the reactions performed at 150 and 175oC. As the temperature increased to 200 and 225oC,

the viscosity of the reacting medium decreased and improved the mass transfer on particles

inside the pores of a 20% ZnO catalyst resulting in a significant increase in oil conversion. Thus,

225oC was found to be the better working temperature of the trans-esterification reaction

because at this temperature, the biodiesel conversion was the highest for all the different metal

oxide loadings on the support.

3) The oil conversion was investigated after 1, 6 and 10 hours of the soybean trans-esterification

reaction over 5, 10 and 20% ZnO catalysts. Oil conversions of ca. 96% were achieved over 5 and

20% ZnO catalyst after 1 hour of reaction. An almost total oil conversion was measured on a

20% ZnO catalyst when the reaction time was extended to 6 hours but a decrease in conversion

to ca. 90% was observed after 10 hours of reaction. This could be due to the glycerolysis reaction

reported to take place at extended trans-esterification reaction times. These findings revealed

that a reaction time of ca. 1 hour over a 5 or 20% ZnO catalyst was enough to achieve

satisfactory levels of oil conversion.

4) Catalysts containing 5%, 10% and 20% ZnO were used. The lowest conversions were

measured on catalysts with the highest ZnO loading, i.e. 20% ZnO for the reactions performed at

66

150 and 175oC. However, the oil conversion passes through a minimum on the catalyst with 10%

ZnO and increases on the catalyst containing 20% ZnO for reactions performed at 200 and

225°C. These findings can be explained by the ZnO dispersion on the support and the mass

transfer limitations. The crystallite size, determined by XRD analysis, for the catalyst containing

5% ZnO was ca. 31 nm compared to ca. 17 nm for the catalyst containing 20% ZnO. The ZnO

crystallite size for the 5% ZnO catalyst was slightly larger than the pore size of the TiO2 support

suggesting that most of the ZnO particles were stabilized on the outer surface of the support. The

mass transfer resistance on these particles was lower compared to the case of 20% ZnO catalyst,

where most of the particles were likely stabilized inside the pores of the TiO2 support. The

highest conversion was achieved with the reaction performed over a 20% ZnO catalyst

5) Different catalysts amount on the oil conversion have been evaluated by running the trans-

esterification reaction at 225oC without catalyst and subsequently with 0.5, 1.5, 3 and 6 wt.%

catalyst (20% ZnO/TiO2) with respect to the amount of oil loaded in the reactor. The results

show that after 15 minutes of reaction the oil conversion for the run without catalyst reached ca.

25% compared to 81, 87, 88, 89% respectively for the runs with catalyst amounts equal to 0.5,

1.5, 3 and 6 % catalyst loaded in the reactor. Oil conversions in excess of 90% were measured

for the reactions with 0.5-6 wt.% catalyst after 30 minutes of reaction Within an experimental

error, no difference in oil conversions were noted after 45 minutes of reaction with 1.5-6 wt.%

catalyst in the reactor. These conversions were at their maximum values of ca. 100% compared

to 54 and 95% respectively for 0 and 0.5 wt.% catalyst in the reactor. Further increase in reaction

time to 60 minutes resulted in a decrease in conversion to ca. 99% for the run with 1.5% catalyst

and 96% for the reaction run with 3 and 6% catalyst respectively. This decrease in oil

conversions is suggestive of a glycerolysis reaction at extended reaction times. After the

optimum reaction time of 1 hour, 1.5 Wt% of ZnO/TiO2 to oil was found to give the best oil

conversion.

6) Methanol to oil ratios of 6:1, 12:1 and 18:1 were used to evaluate the effect of methanol to oil

ratio on the soybean oil conversion during the trans-esterification reaction. The results show that

up to 60 minutes of reaction, the measured conversion was higher for the reaction that was

performed with the highest methanol to oil ratio (18:1 in this study). The reaction with a

67

methanol to oil ratio of 18:1 showed some decline in measured oil conversion from ca. 30

minutes of reaction where the oil conversion was almost complete to ca. 96% after 60 minutes.

Although a high methanol to oil ratio increases the rate of oil trans-esterification, it appears that

the glycerolysis reaction is also favoured when the reaction time is extended.

5.2 Recommendations. This research gave interesting results on the activity of the ZnO/TiO2 system as solid catalyst for

soybean oil trans-esterification. Further investigations can be undertaken in the future to expand

the understanding on the same topic. The following can be investigated:

- Trans-esterification of different types of oil using ZnO/TiO2;

- Selectivity of the catalyst;

- Catalyst regenerability;

- Economic studies for the entire process (from catalyst synthesis to biodiesel production);

- Deposition of nano-ZnO on different supports (alumina, zirconia, carbon nano-tubes,

silica, etc)

68

Appendix

69

Appendix A

Methanol vapor pressure vs temperature

Source: technical and safe handling for methanol. Version 3.0 (September 2006)