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
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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
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.%)
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
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
REFERENCES 1. H. Fukuda, A. Kondo, H. Noda, Biodiesel fuel production by transesterification of oils. J.
Biosci . Bioeng, 2001. 92: p. 405-416.
2. J. Jitputti, B. Kitiyanan, P. Rangsunvigit, and L.A.K. Bunyakiat, P. Jenvanitpanjakul,
Transesterification of crude palm kernel oil and crude coconut oil by different solid
catalysts. Chem. Eng. J. 2006. 116: p. 61-66.
3. M. Fangrui, M. Hanna, Biodiesel fuel production by transesterification of oils. Bioresour.
Technol. 1999. 70: p. 1-15.
4. S. Semwal, A.K. Arora., R.P. Badoni, D.K. Tuli, Biodiesel production using
heterogeneous catalysts. Bioresour. Technol. 2011. 102: p. 2151-2161.
5. S. Gryglewicz, Rapeseed oil methyl esters preparation using heterogeneous catalysts.
Bioresour. Technol. 1999. 70(3): p. 249-253.
6. M. Di Serio, R. Tesser, L. Pengmei and E. Santacesaria, Heterogeneous catalysts for
biodiesel production. Energy Fuels, 2008. 22: p. 207-217.
7. L. Bournay, D. Casanave, B. Delfort, G. Hillion, J.A. Chodorge, Synthesis of biodiesel
with heterogeneous NaOH/Alumina catalysts. Catal. Today, 2005. 106: p. 190-192.
8. D-W. Lee, Y.-m. Park, K-Y. Lee, Heterogeneous base catalysts for transesterification in
biodiesel. Catal.Surv. Asia, 2009. 13: p. 63-77.
9. A. Peugtong, k. Kapilakarn, A Comparison of Costs of Biodiesel Production from
Transesterification. Int. Energy. J. 2007. 8: p. 1-6.
10. M. Zabeti, W.M.A.Wan. Daud, M.K. Aroua, Activity of solid catalysts for biodiesel
production. Fuel Process. Technol. 2009. 90: p. 770-777.
11. M. Di Serio, R. Tesser, L. Pengmei and E. Santacesaria, Heterogeneous catalysts for
biodiesel production. Energy Fuels, 2008. 22: p. 207-217.
12. M. Di Serio, M. Ledda, M. Cozzolino, G. Minutillo, R. Tesser, and E. Santacesaria,
Transesterification of Soybean Oil to Biodiesel by Using Heterogeneous Basic Catalysts.
Ind. Eng. Chem. Res. 2006. 45: p. 3009-3014.
13. A.V. Ramaswamy, B. Viswanathan, Selection of solid heterogeneous catalysts for
transesterification reaction. 2007: Madras.
21
14. J.F. Gomes, J.F. Puna, J.C. Bordado, M.J.N. Correia, Development of heterogeneous
catalyst for transesterification triglycerides. Catal. Lett. 2008. 95(2): p. 273-279.
15. M. Hanna, M. Fangrui, Biodiesel production: a review. Bioresour. Technol. 1999. 70: p.
1-15.
16. A.N. Phan, T.M. Phan., Biodiesel production from waste cooking oils. Fuel, 2008. 87: p.
3490-3496.
17. L.L. Myint, M.M. El-Halwagi., Process analysis and optimization of biodiesel production
from soybean oil. Clean Techn. Environ. Policy, 2009. 11: p. 263-276.
18. A.V. Tomasevic, S.S. Siler-Marinkovic, Methanolysis of used frying oil. Fuel Process.
Technol. 2003. 81: p. 1-6.
19. P. Felizardo, M. Correia, I. Raposo, JF. Mendes, R. Berkemeier, JM. Bordado,
Production of biodiesel from waste frying oils. Waste Manage. 2006. 26: p. 487-494.
20. Y. Wang, S. Ou, P. Liu, Z. Zhang, Preparation of biodiesel from waste cooking oil via
two-step catalyzed process. Energy Convers. Manage. 2007. 48: p. 184-188.
21. A. Demirbas, Biodiesel fuels from vegetable oils via catalytic and non-catalytic
supercritical alcohol transesterifications and other methods: a survey. Energy Convers.
Manage. 2003. 44: p. 2093-2109.
22. B. Freedman, E.H. Pryde, T.L. Mounts, Variables affecting the yields of fatty esters from
transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984. 61: p. 1638-1643.
23. A. Sivasamy, K.Y. Cheah, P. Fornasiero, F. Kemausuor, S. Zinoviev, S. Miertus,
Catalytic Applications in the Production of Biodiesel from Vegetable Oils. Chem. Sus.
Chem. 2009. 2: p. 278-300.
24. G. Vicente, M. Martinez, J. Aracil, Integrated biodiesel production: a comparison of
different homogeneous catalysts. Bioresour. Technol. 2004. 92(3): p. 297-305.
25. R.J.P. Williams, A comparison of types of catalyst: The quality of metallo-enzymes. J.
Inorg. Biochem. 2008. 102: p. 1-25.
26. N. Dizge, C. Aydiner, D.Y. Imer, M. Bayramoglu, A. Tanriseven, B. Keskinler, Biodiesel
production from sunflower, soybean and waste cooking oils by transesterification using
lipase immobilized onto a novel microporous polymer. Bioresour. Technol. 2009. 100: p.
1983-1991.
22
27. Y.L. Zhang, Y. Yang, J.H. Zhao, R.Q. Tan, P. Cui, W.J. Song, Preparation of ZnO
nanoparticles by a surfactant-assisted complex sol–gel method using zinc nitrate. J. Sol-
Gel Sci. Technol. 2009. 51: p. 198 - 203.
28. J-S. Lee, S. Saka, Biodiesel production by heterogeneous catalysts and supercritical
technologies. Bioresour. Technol. 2010. 101: p. 7191-7200.
29. Z. Mirjafary, H. Saeidian, A. Sadeghi, F.M. Moghaddam, ZnO nanoparticles: An
efficient nanocatalyst for the synthesis of β-acetamido ketones/esters via a muti
component reaction. Catal. Commun. 2008. 9: p. 299-306.
30. C.S. Chen, X.H. Chen., B. Yi, T.G. Liu, W.H. Li, L.S. Xu, Z. Yang, H. Zhang, Y.G.
Wang, Zinc oxide nanoparticle decorated multi-walled carbon nanotubes and their optical
properties. Acta Mater. 2006. 54: p. 5401-5407.
31. G. Ambrozic, S.D.Skapin, M. Zigon, Z.C. Orel, The synthesis of zinc oxide nanoparticles
from zinc acetylacetonate hydrate and 1-butanol or isobutanol. J. Colloid Interface Sci.
2010. 346: p. 317-323.
32. L.M. Bronstein., Nanoparticles made in mesoporous solids. Top. Curr. Chem. 2003. 226:
p. 55-89.
33. X. Xue, C. Liu, W. Xing, T. Lu, Physical and electrochemical characterizations of
PtRu/C catalysts by spray pyrolysis for electrocatalytic oxidation of methanol. J.
Electrochem. Soc. 2006. 153: p. E79-E84.
34. P.S. Patil., Versatility of chemical spray pyrolysis technique. Mater. Chem. Phys. 1999.
59: p. 185-198.
35. F. Maillard, M. Eikerling, O.V. Cherstiouk, S. Schreier, E. Savinova, U. Stimming. Size,
effects on reactivity of Pt nanoparticles in CO monolayer oxidation: the role of surface
mobility. Faraday Discuss. 2004. 125: p. 357-377.
36. H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, A review of anode
catalysis in the direct methanol fuel cell. J. Power Sources, 2006. 155: p. 95-110.
37. J. Stavek, M. Sipek, I. Hirasawa and K. Toyokura, Controlled Double- Jet Precipitation
of Sparingly Soluble Salts. A Method for the Preparation of High Added Value Materials.
Chem. Mater. 1992. 4: p. 545-555.
23
38. G.S. Sewell, C.T. O’Connor, E. van Steen, Effect of Activation Procedure and Support on
the Reductive Amination of Ethanol Using Supported Cobalt Catalysts. J. Catal. 1997.
167(2): p. 513-521.
39. B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Preparation of Catalysts II. 1979,
Amsterdam: Elsevier.
40. J.R. Anderson, K. Forger, T. Mole, R.A. Rajadhyaksha, and J.V. Sanders, Reactions on
ZSM-5-type zeolite catalysts. J. Catal. 1979. 58: p. 114-130.
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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.
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.
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