Review of Recent Developments in Solid Acid, Base, and Enzyme

6162

Ind. Eng. Chem. Res. 2009, 48, 6162–6172

Review of Recent Developments in Solid Acid, Base, and Enzyme Catalysts (Heterogeneous) for Biodiesel Production via Transesterification Rajabathar Jothiramalingam*,† and Ming Kuang Wang*,‡ Graduate Institute of EnVironmental Engineering, National Taiwan UniVersity, Taipei, Taiwan, and Department of Agricultural Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106

Development of new and effective solid catalysts (solid acid base to immobilized enzyme) is necessary for renewable and green energy production. The various choices of heterogeneous solid catalysis were reported recently, and the specific research outcomes of trans-esterification on heterogeneous catalysis are discussed in the present article. The studies on biodiesel production are mainly focused on the following aspects: (i) examining the appropriate oil source, (ii) establishing the adequate alcohol to oil molar ratio, and (iii) evaluating the available solid acid catalysts, solid base catalysts, and lipase immobilized enzyme catalysts. Although homogeneous catalysis are superior in terms of reaction rate compared to heterogeneous catalysis, the focus of research on development of solid catalysts for heterogeneous catalytic transesterification is directed from the point of easy process and possible adoption for large scale production. The scope of this presentation is to examine these aspects in detail and to provide a comprehensive outlook on the aspects mentioned. 1. Introduction The world has been confronted with environmental and energetic crises due to depletion of resources and increased population as well as environmental pollution. Other than energy sources such as hydroelectricity and nuclear power, energy production from renewable energy source has been increased worldwide for transportation purposes. Hence, it is important to search for an alternate low-cost fuel for every day usage, which should be sustainable and also friendly to the environment. Many efforts have been underway to develop clean fuel in many countries like wind and solar energy. Among the many possible sources, biodiesel derived from vegetable oil (VO) attracts attention as a promising one for substitution or blending with conventional diesel-based fuels. If pure or blend biodiesel is used as fuel, the net production of carbon dioxide can be suppressed. Sharmer et al.1 have estimated that, in the case of 1.0 kg of pure biodiesel instead of fossil fuel, 3.2 kg of CO2 production could be reduced.1 Diesel fuel is largely utilized in the transport, agriculture, commercial, domestic, and industrial sectors. The increase in demand for crude oil and limited resources of fossil oil cause the need to focus on renewable fuel sources from vegetable oils and animal fats. The vegetable oil (VO) is renewable, nontoxic, and widely available from a variety of sources and has low sulfur contents close to zero. The vegetable oils like palm, soybean, sunflower, peanut, and olive oils are used as an alternative fuel for diesel engines. Depending upon the climate and environmental conditions, different countries are looking for different sources of vegetable oil as substitutes for diesel fuel. For example, soybean oil in the United States, rapeseed and sunflower oils in Europe, and palm oil and coconut oil in the Asian region are being considered. The production of oil seeds, the percentage of recovery, and their cost has been discussed in detail elsewhere.2 Fats and oils are primarily water-insoluble, hydrophobic substances in the plants and animals that are made up of one mole * Corresponding author. E-mail: [email protected]. Phone: (0118862) 2363-0231ext. 2491 or 3066 (O). Fax: (0118862) 2366-0751. † Graduate Institute of Environmental Engineering, National Taiwan University. ‡ Department of Agricultural Chemistry, National Taiwan University.

of glycerol and three moles of fatty acids and are commonly referred to as triglycerides (TG).3 Fatty acids vary in carbon chain length and in the number of unsaturated bonds. In beef tallow, the saturated fatty acid component accounts for almost 50% of the total fatty acids. The higher stearic and palmitic acid contents of beef tallow provide the unique properties of high melting point and high viscosity.4 Natural vegetable oils and animal fats are extracted to obtain crude oil, and they usually contain free fatty acids (FFAs), phospholipids, sterols, water, odorants and other impurities. The FFAs and water content have significant effects on the transesterification of glycerides with alcohols using solid acid or base catalysts, and they also interfere with the separation of fatty acid esters and glycerol. Considerable research has been carried out on vegetable oils as a possible alternate renewable oil source. The palm, soybean, sunflower, coconut, rapeseed, and tung oil and animal fats have also been considered as renewable oil sources for biodiesel production. Algae, bacteria, and fungi have also been studied as possible renewable oil sources for biodiesel production.5 Transesterification is the general term used to describe the important class of organic reactions where an ester is transformed into another through interchange of the alkoxy moiety. The transesterification is an equilibrium reaction, and the transformation occurs essentially by mixing the reactants. However, the presence of a catalyst (acid or base) could accelerate and control the equilibrium, to achieve the high yield of the ester; however, the alcohol has to be used in excess. The overall process is a sequence of three consecutive and reversible reactions, in which di- and monoglycerides are formed as intermediates. The stoichiometric reaction requires 1 mol of TG and 3 mol of the alcohol. However, an excess of the alcohol is used to increase the yield of the alkyl esters and to allow its physical separation from the glycerol formed. Many aspects determine the extent of the reaction such as type of catalyst (acid or base), alcohol-to-vegetable oil molar ratio, temperature, purity of the reactants, and free fatty acid content. Methyl esters of fatty acids (FAME) are produced by alcoholysis of triglycerides with methanol in the presence of a solid acid or base catalyst, as illustrated by the following chemical reaction (eqs 1, 2, and 3). Complete conversion of the TG via transesterification involves three consecutive reaction steps, and monoglyc-

10.1021/ie801872t CCC: $40.75  2009 American Chemical Society Published on Web 06/03/2009

Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009

eride and diglyceride act as intermediates. The transesterification reaction sequence is as follow: Triglycerides + CH3OH ) diglycerides + R1COOCH3 (1) diglycerides + CH3OH ) monoglycerides + R2COOCH3 (2) monoglycerides + CH3OH ) glycerine + R3COOCH3 (3) The combustion properties of biodiesel are similar to petroleumbased diesel, and thus, it can act as a substitute for diesel fuel or more commonly for blending with fuels. Pure biodiesel carries about 90% of the energy content of the normal diesel, and hence, it can be expected that the engine performance can be nearly the same. The flash point of biodiesel is higher compared to commercial diesel. In addition, biodiesel increases lubricity, which prolongs engine life and reduces the frequency of engine part replacement. Another significant advantage of biodiesel is that it has low emission profile and its oxygen content is 10-11%. A brief comparison of the ASTM standards for diesel and biodiesel is given in ref 6. Transesterification needs a catalyst in order to obtain reasonable conversion rates, and the nature of the catalyst is fundamental since it determines the compositional limits of the feedstock, the reaction conditions, and the postseparation steps. The preparation method of the catalyst also plays an important role in the catalytic transesterification process. The strong base (NaOH or KOH) catalyzed via a homogeneous transesterification process has certain limitations like product separation and results in higher production cost for biodiesel manufacture. The scope of the presentation is to review the differences in the catalytic activity of various solid acid and base catalysts and to evaluate the potential solid catalysts for biodiesel production via the heterogeneous transesterification process. Among the available solid acid/base catalysts, zeolites, sulfated metal oxides, hydrotalcites, and alkali metal loaded metal oxides are effective and exhibit interesting results for the conversion of TG to biodiesel via transesterification. Enzyme (lipase) catalysis is another growing area of research in the field of heterogeneous catalytic transesterification for biodiesel production. Hence, in the present article, we review the impact of solid acid, solid base, and enzyme catalysts for biodiesel production via transesterification of various oil sources. 2. Impact of Solid Catalysts on Different Oil Sources for Fuel Production 2.1. Canola Oil. Katikaneni et al.7 used a number of catalysts, including Al-PILC (aluminum pillared clay catalyst), to convert canola oil to fuel using a fixed-bed reactor. They examined the performance of each catalyst with respect to the yield of organic liquid product (OLP), the selectivity, and the extent of coke formation. They found that HZSM-5 (proton exchanged zeolite type acid catalyst) gave the highest yield of OLP (63 mass %). The OLP obtained with PILC catalyst contained more aliphatic hydrocarbons and the least amount of aromatic hydrocarbons as compared to OLP obtained with other catalysts. Their studies showed that the increase of the pore size of the catalyst increased the conversion of canola oil, the coke formation, and the selectivity for aliphatic compounds, while the yield of hydrocarbons and the selectivity for aromatics decreased. This led them to the conclusion that the medium pore size of the catalysts would enhance the initial cracking

6163

and deoxygenation reactions needed for an optimum fuel yield.8 The effect of acidity, basicity, and shape selectivity of the catalyst on the conversion of canola oil was also examined by Idem et al.9 A comparative evaluation of the activity of the catalyst systems such as HZSM-5, silica-alumina, and γ-alumina has been made. Product distribution obtained for runs using silica-alumina and γ-alumina as catalysts is similar to what was obtained without the catalyst. The realized product selectivity and the increased OLP yield obtained using HZSM-5 has to be associated with the ZSM-5 shape selectivity and not with its acidity. The studies have been extended using calcium and magnesium oxide as catalysts to examine how the basicity of the catalyst affects the product yield. The results showed that the presence of basic centers inhibited the secondary cracking and produced large amounts of residual oil. Biodiesel production from canola oil sources in a homogeneous catalytic process instead of a heterogeneous solid catalytic process was recently reported.10 2.2. Palm Oil. Leng et al.11 used a fixed-bed reactor to crack palm oil over HZSM-5 catalyst. The maximum formation of gasoline-range hydrocarbons was achieved at 673 K with a low space velocity. The conversion of palm oil was low (40-70%) as compared to canola oil, where conversions up to 100% were achieved.10 This was attributed to the fact that palm oil contains more saturated fatty acids (palmitic acids) than canola oil and these saturated compounds have a greater stability than unsaturated fatty acids. Twaiq et al.12 have also studied the palm oil conversion using HZSM-5, Zeolite-β, and ultrastable Y zeolites. Conversions of up to 99 wt % with gasoline yields of 28% were achieved. They concluded that the HZSM-5 was the best catalyst from the points of view of conversion of the substrate, gasoline yield, selectivity for aromatics, and lower coke formation. Noiroj et al.13 recently reported the comparative study of KOH/Al2O3 and KOH/NaY catalysts for biodiesel production. The 25 wt % KOH/Al2O3 and 10 wt % KOH/NaY catalysts are suggested to be the best composition because of their biodiesel yield of 91.07% at temperatures below 70 °C within 2-3 h at a 1:15 molar ratio of palm oil to methanol and a catalyst amount of 3-6 wt %. The leaching of potassium species in both spent catalysts was observed. The amount of leached potassium species of the KOH/Al2O3 was somewhat higher compared to that of the KOH/NaY catalyst. Kansedo et al.14 reported the feasibility of biodiesel production from the world’s cheapest and most easily available vegetable oil like palm oil via heterogeneous transesterification in the presence of montmorillonite KSF as a heterogeneous catalyst with conversion up to 78.7% by adopting the following reaction conditions, such as reaction temperature of 155 °C, reaction period of 120 min, ratio of methanol/oil at 10:1 mol mol-1, and amount of catalyst at 4 wt %.14 The effects of catalysts on different oil sources and their biodiesel yield via transesterification is shown in Table. 1. 3. Solid Acid Catalysis (Heterogeneous) for Biodiesel Production Homogeneous catalysts are effective and feasible, but they lead to serious contamination problems that make essential the implementation of good separation and product purification protocols, resulting in increased production cost. To be economic and to compete commercially with petroleum-based diesel fuel, processes for the synthesis of biodiesel need to involve continuous processing in a flow system, have as few reaction steps as possible, limit the number of separation processes, and ideally use the potential solid catalyst for the heterogeneous

6164


Table 1. Impact of Solid Catalysts on Different Oil Sources for Heterogeneous Transesterification Process substrate source

catalyst

palm oil unrefined oil or waste oil

KOH/Al2O3 and KOH/NaY ZnO-La2O3

soybean frying oil

Mg-MCM-41, Mg-Al-hydrotalcite, K-Zirconia anhydrous sodium molybdate various alkali and alkali earth metal oxides supported alumina CaO-Ca(OCH3) heterogenous resin catalysts with trace amount of homogeneous CH3ONa Ca and Zn mixed oxides Mg-La catalyst

soybean oil palm kernel oil and coconut oil rapessed oil glycerides palm kernel oil edible and nonedible oil vegetable oil (sunflower and castor) fired oil palm kernel oil soybean oil cotton seed oil palm kernel oil/crude coconut oil soybean oil sunflower oil (in hexane solvent) tallow (in hexane)

(i) CaO supported on SBA-15, MCM-41, and fumed silica (ii) superacid sulfated TiO2 catalyst Y756 zeolite interchanged with KOH (i) modified dolomites-CaO catalyst (ii) Ca/Zn mixed oxide Ba-ZnO MgO, CaO, and MgO-Al2O3 various solid oxide catalysts: ZrO2, ZnO, sulfated SnO2, sulfated ZrO2, KNO3/KL zeolite (i) KF/CaO (ii) MgO/calcined hydrotalcite Rhizomucor miehei (lipase) Mucor miehei IM60 (lipase)

transesterification reaction. The appropriate solid catalysts can be easily incorporated into a packed-bed continuous flow reactor, simplifying product separation and purification and reducing waste generation. Research on the direct transesterification of lipid feedstocks into biodiesel by solid acid catalysts are not examined extensively. Among the catalysts tested, sulfuric acid prepared by impregnation method has shown the highest activity. The solid catalysts have shown varied activities depending on reaction conditions and preparation methods. Impregnation method prepared solid acid catalysts showed higher activities, such as activated montmorillonite showing a 100% conversion after 4 h of reaction at 493 K and at 52 bar. However, leaching of sulfate species restricted the reusability of the catalyst. It is also likely that some degree of homogeneous reaction was concurrently taking place due to sulfuric acid leaching. Attempts to prepare other solid acid catalysts with high activities for transesterification have been reported, as well. In particular, Kaita et al.15 designed aluminum phosphate catalysts with various metal-to-phosphoric acid molar ratios (1:3-1:0.01) and used these materials for the transesterification of kernel oil with methanol. The good reactivity and selectivity to methyl esters was observed.15 However, the use of these materials still needs high temperatures (473 K) and high methanol-to-oil molar ratio (60:1) for a feasible process. In a related study, Waghoo et al.16 reported on the transesterification of ethyl acetate with several alcohols over hydrous tin oxide. Linear and aromatic alcohols were tested in a temperature range of 443-483 K. All reactions were selective toward transesterification, and in particular, hydrous tin oxide catalyst showed appreciable activity for reactions involving n-butyl alcohol, n-octyl alcohol, and benzyl alcohol.16 Amberlyst-15 has also been studied for the transesterification reaction; however, mild reaction conditions are necessary to avoid catalyst degradation. At a relatively low temperature (333 K), the conversion of sunflower oil was

heterogeneous process

biodiesel yield (%)

cited literature

transesterification (TE) simultaneous esterification/ transesterification TE

91.1 96.0

Noiroj et al.13 Yan et al.69

97

Georgogianni et al.70

TE TE

95 93

Nakagaki et al.71 Benjapornkulaphong et al.72

TE TE

90 90

Kawashima et al.73 Kim et al.74

TE room temp. transesterification TE

>94 >90

TE TE

>40 98

95

Ngamcharussrivichai et al.75 Babu et al.76 Albuquerque et al.77 De Almeida78 Brito et al.79

TE

99.9

Ngamcharussrivichai et al.80,81

TE TE

96 90

Xie et al.82 Chen et al.83

TE

>90

Jitputti et al.84

TE TE TE TE

90 >90 94 94

Meng et al.85 Di Serio et al.86 Dossat et al.87 Nelson et al.88

reported to be only 0.7%, when the reaction was carried out in atmospheric pressure and a 6:1 methanol-to-oil molar ratio was employed.17 3.1. Zeolite Type Solid Acid Catalyst. Solid acid catalysts are generally more suitable for the biodiesel synthesis process instead of ion-exchange resins due to poor thermal stability and swelling behavior, except for a few commercially available, high-cost ion-exchange resins like Nafion and Amberlyst-15.18 Recently De Rezende et al.19 reported on the polymer based resin catalysts for the soybean and coconut oil transesterification reaction. Sulfonated poly(S-divineyl benzene) and poly(divineyl benzene) had shown effective transesterification of soybean oil and coconut oil, and TG conversion increased with increasing catalyst amount. Superiority of physical properties of modified resins may be a dominant factor for higher catalytic activity. The performance of synthesized resins was higher than the commercial resins Amberlyst-15 and Amberlyst-35. The highest values of FAME (>90%) were obtained with a sulfonated polyvinyl benzene (DVB) due to higher surface area (442 m2/ g). Among the different types of solid acid, the different characteristics of zeolites make them excellent catalysts for acid-base catalyzed reactions. For instance, zeolites can be synthesized with different crystal structures, pore sizes, framework Si/Al ratios, and proton-exchange levels. Too high acidity causes the deactivation of the catalyst by coking or undesirable byproducts that might be formed, requiring additional and expensive separation procedures.20 In addition, zeolites provide the possibility to choose among different pore structures and surface hydrophobicities, according to the substrate’s size and polarity. Multifunctionalization of mesoporous silica with both organosulfonic acid and hydrophobic organic groups was shown to be effective in esterifying the free fatty acid while excluding


water, which is an undesired reaction product from the proximity of the active sites.21 Both postsynthesis grafting and one-step co-condensation techniques were used to introduce the hydrophobic organic groups, preferentially to modify the external and internal surfaces, respectively. It was shown that the technique of incorporating the hydrophobic organic group into the mesoporous silica and knowledge of the reaction mixture are important for affecting the increased performance of the catalysts.22 Zeolites are active catalysts for the esterification of large carboxylic acids, but they catalyze the reaction rather slowly. Thus, only large-pore zeolites have been used with any success in fatty acid esterification. In general, the catalytic activity of zeolites for the esterification of free fatty acids increases with increasing Si/Al ratio, indicating that the reactivity is influenced by acid site strength as well as surface hydrophobicity. Pore size, dimensionality of the channel system (related to the diffusion of reagents and products), and aluminum content of the zeolite framework strongly affect the catalytic activity. 3.2. Heteropoly Acid Loaded MCM-41 Catalyst. Introducing aluminum, zirconium, titanium, or tin ions into the silica matrix of mesoporous solids can significantly improve their acid properties.23 However, metal-doped materials behave more like weak acids and can only be used for reactions that do not require a strong acid catalyst. For instance, the catalytic activity of AlMCM-41 in the esterification of oleic acid with glycerol was found to be significantly lower than that of zeolite beta with a similar Si/Al molar ratio and to enhance the catalytic activity; while keeping the benefits of large pore diameters, strong acid species have been introduced to the pore interior of mesoporous solids.23 In particular, MCM-41 supported heteropoly acids (HPAs) have been used as catalysts in the gas-phase esterification of acetic acid and 1-butanol, and these catalysts showed good activity at 383 K (95% conversion of 1-butanol).24 As expected, MCM-41 supported HPA was more active than pure HPA. The enhanced activity could be ascribed to high dispersion of the HPA on the MCM-41 internal surface, giving rise to the higher population of available acid sites than in pure HPA. However, the supported catalyst is considerably more hydrophilic than the original HPA. Water formation caused HPA migration from MCM-41 pores to the outer surface, thus facilitating the sintering of HPA species. Other composite catalysts such as silica mesoporous materials modified with sulfonic groups were used in the pre-esterification of mixtures of FFAs and soybean oil. Mbaraka et al.21 found that the activity of this hybrid mesoporous silica was highly dependent on their pore dimensions and probably on their hydrophobic character as well. Sakthivel et al.25 reported tungstophosphoric acid supported on cubic MCM-48 type mesoporous supported material for long-chain fatty acid esterification in supercritical carbon dioxide. The high performances of HPW in sc-CO2 medium are due to the effective mass transfer of reactants and products in the catalysis process under low temperature (373 K) and shorter reaction time (6 h). Chen et al.26 reported the SBA-15 as possible mesoporous support for sulfateted zirconia via direct incorporation of sulfated zirconia into SBA-15. Demirbas et al.27 revealed details regarding biodiesel production from vegetable oil in the presence and absence of catalyst in supercritical methanol (sc-CH3OH) medium for transesterification process. 3.3. Sulfated Zirconia and Tin Oxide Type Solid Acid Catalyst. Acid strength of the catalyst has an important role in the transesterification reaction. For instance, catalysts prepared from a stronger acid precursor containing benzene sulfonic acid

6165

groups are more active than those containing only propylsulfonic acid groups. Much improved catalytic performance under mild temperature conditions was observed for the esterification of lauric acid and palmitic acid in presence of as-prepared solid acid support catalyst compared to unsupported SZ. The good catalytic performance of the sulfated silica-zirconia materials is attributed to a higher dispersion of zirconia to give higher acid site density and also to better tolerance of water.26 Indeed, catalyst with a medium pore diameter of 50 Å with benzene sulfonic acid groups was shown to have effective activity comparable to that of sulfuric acid. Recently, sulfated zirconia (SO4/ZrO2) has been shown to be active for several acidcatalyzed reactions.28 For esterification, SO4/ZrO2 has shown promising catalytic activity due to its high acid strength; however, the sulfate leaching was enhanced by hydrolysis.29 Sulfate groups can leach out as H2SO4 and HSO4-, which in turn can give rise to homogeneous acid catalysis, interfering with measurements of the heterogeneous catalytic activity. To overcome the susceptibility of SO4/ZrO2 to water and to improve its general characteristics, new preparations of SO4/ZrO2 have recently been proposed. For instance, Yadav and Murkute30 reported different synthetic routes to prepare SO4/ZrO2 with improved sulfate loadings and resistance to leaching of sulfate by hydrolysis. This catalyst was prepared by using a chloro sulfonic acid precursor dissolved in an organic solvent, instead of the conventional impregnation of sulfuric acid. The prepared SO4/ZrO2 exhibited higher catalytic activity for esterification than the conventionally prepared SO4/ZrO2, and no leaching of sulfate was observed. Additionally, the catalyst demonstrated good retention of its activity for subsequent experiments. Sulfated tin oxide (SO42-/SnO2), prepared from meta-stannic acid, has shown activity superior to that of SO42-/ZrO2 for the esterification of n-octanoic acid with methanol at temperatures below 423 K due to its superior acid strength.31 However, a more widespread testing of SO42-/SnO2 has not been carried out because of inadequacies in the preparation method. New synthetic routes for preparing SO42-/SnO2 are reported, and additional studies are expected.32 Einloft et al.33 reported biodiesel synthesis from rice bran oil via transesterification by various tin complexes. Among the various tin complexes, dibutyl tin dilaurate (DBTDL) shows the best performance with a yield of 69% in 4 h using a molar ratio of 400:100:1 (methanol/oil/ catalyst). Transesterificaiton by tin complex (DBTDL) could be an alternative choice for biodiesel production with a suitable oil source.33 Furuta et al.34 carried out the biodiesel production over solid super acid catalysts in a fixed-bed reactor under atmospheric pressure. Solid superacid catalysts, such as sulfated tin and zirconium oxides prepared by coprecipitation method, and tungstated zirconia are prepared by mixing of an appropriate amount of metal salt followed by calcination at suitable temperature; they were also evaluated for the catalytic activity of the as-prepared catalysts for transesterification of soybean oil with methanol at 473-573 K and esterification of n-octanoic acid with methanol at 458-473 K. Tungsten doped zirconiaalumina is a promising solid acid catalyst for the production of biodiesel from soybean oil due to its effective performance (conversions were in the range of 90%). The transesterification of soybean oil with methanol to fatty acid ester was carried out over WZA (tungstated zirconia), SZA (sulfated zirconia), and STO (sulfated tin oxide) at 473-573 K. The conversions of soybean oil after 20 h of time on stream show that the WZA catalyst is quite effective for the reaction, with the conversions being more than 90% at temperatures over

6166


523 K. This high activity was maintained up to 100 h of time on stream, but a little amount of byproduct were detected by the gel permeation chromatographic (GPC) analysis. The esterification of n-octanoic acid with methanol to give methyl n-octanoate was carried out in the same manner as that of the transesterification of soybean oil; the yield of ester after 20 h and the catalysts such as WZA, STO, and SZA showed quite high activities close to 100% yield at temperatures above 448 K, and no byproducts were detected. Therefore, high performance of the WZA catalyst for transesterification is also based on its high activity for the esterification of the liberated freefatty acids. STO showed especially high activities for the esterification of n-octanoic acid due to the strong acidity of the catalyst. Very recently, the catalytic action of STO in comparison with that of SZA has been reported, along with temperatureprogrammed desorption (TPD) measurements using ammonia and pyridine as probes.35 The fact that the acid strength of STO is higher than those of SZA and WZA was confirmed by the adsorption heat of Ar, calculated from a temperature dependence of the amount of Ar adsorption around room temperature; hence, it has been reported that Ar is more suitable than ammonia as a probe molecule for acid sites.36,37 In summary, three types of solid super acid catalysts were prepared and evaluated for the transesterification of soybean oil with methanol and the esterification of n-octanoic acid with methanol. Tungstated zirconiaalumina is a promising catalyst for the production of biodiesel, because of its activity for the transesterification as well as the esterification. 3.4. Tungsten Trioxide Loaded Zirconia (WO3/ZrO2) Type Solid Acid Catalyst. Ramu et al.38 studied the esterification of palmitic acid with methanol over tungsten oxide supported on zirconia solid acid catalyst, prepared by both impregnation and coprecipitation methods; the extent of tungsten loading and calcination temperature were varied. They characterized the catalysts by X-ray diffraction (XRD) for structural elucidation and by temperature-programmed desorption (TPD) of ammonia for surface acidity. The impregnated catalysts exhibited both tetragonal and monoclinic phases of zirconia, at calcination temperatures beyond 773 K, and their coprecipitated analogues showed stability toward the tetragonal phase. The formation of monoclinic phases of zirconia and crystalline tungsten oxide phase decreased the esterification activity, as a consequence of decreased surface acidity. Coexistence of tetragonal zirconia with the amorphous tungsten oxide offered maximum esterification activity. A series of WO3/ZrO2 catalysts were prepared by the impregnation technique with tungsten concentration ranging from 2.5 to 25 wt %. These systems were used for the liquidphase esterification of palmitic acid with methanol. This reaction was carried out at two different reaction times: 3 and 6 h. It is seen that the esterification activity of the catalysts increased with WO3 loading up to 5%, and thereafter, the activity showed a decreasing trend. They tested two sets of catalysts with two different loadings of WO3, namely, 5 and 15 wt % prepared by coprecipitation method; two dried masses were subjected to calcination at the same temperature. The 15 wt % catalyst was selected based on the literature evidence that the coprecipitated catalysts containing 10-15% WO3 in ZrO2 stabilize the tetragonal phase.39 The 5 wt % WO3/ZrO2 exhibited the transformation of tetragonal to monoclinic phase of zirconia with increase in calcination temperature from 773 to 1173 K. However, the 15 wt % WO3/ZrO2 catalyst showed the presence of tetragonal phase predominantly, irrespective of the calcination temperature. These catalysts were evaluated for their esterifi-

Table 2. Effect of the Calcination Temperature on the Esterification Activity over the Catalysts Prepared by Coprecipitation conversion (%) sample number

calcination temperature (°C)

5 wt % WO3/ZrO2

15 wt % WO3/ZrO2

1 2 3 4 5

400 500 600 700 900

93 95 10 5 5

78 81 20 25 17

Table 3. Correlation between Preparation Method of the Catalysts and Structural Effect for Esterification of Palmitic Acid on 5 wt % WO3/ZrO2 Catalysts impregnation calcination temperature conversion (°C) (%) XRD phase 500 900

98 8

tetragonal ZrO2 monoclinic ZrO2/WO3

coprecipitation conversion (%) 95 17

XRD phase tetragonal ZrO2 monoclinic ZrO2/WO3

cation activity, and the results are shown in Table 2. With increase in calcination temperature, the activity of the catalysts decreased drastically. An interesting observation in this case is that, even though the catalyst calcined at 1173 K showed the presence of tetragonal phase of zirconia exclusively, its activity is found to be negligible. This implies that the presence of tetragonal phase alone was not sufficient for exhibiting the required activity. Lopez et al.40 studied the kinetics and mechanism of gas- and liquid-phase esterificaiton of acetic acid with methanol on tungstated zirconia cataysts. It can be concluded from these studies that the presence of zirconia in tetragonal phase is not the only criterion, but the coexistence of amorphous WO3 is also required for obtaining considerable activity. The presence of monoclinic phase of zirconia and crystalline WO3 had detrimental effects on the esterification of palmitic acid (Table 3). Literature data also support the presence of such species for obtaining high efficiency from the tungstated zirconia super acid catalysts. Furuta et al.41 and Larson and Petkovic42 concluded that the active sites of tungsten oxide supported on zirconia are generated by the interaction of amorphous tungsten trioxide and the crystalline zirconium oxide. Such an active species exists in the case of 5 wt % WO3/ZrO2 catalyst, prepared by impregnation, and it showed the maximum esterification activity. McNeff et al.43 studied the esterificaiton and transesterification in a continuous fixed-bed reactor process instead of conventional methods by porous zirconia, titania, and alumina type solid oxide catalysts under high pressure (2500 psi) and elevated temperature (573-800 K). 4. Solid Base Catalysis (Heterogeneous) for Biodiesel Production 4.1. Alkali Metal Salt-Loaded Alumina Solid Base Catalyst. Ebiura et al.44 studied the selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid base catalyst. The transesterification of triolein with methanol proceeded over a solid base catalyst at a lower reaction temperature than 473 K, at which the conversion of triolein was 3% without a catalyst; in the presence of alkali metal salt-loaded alumina catalyst, transesterificaiton of triolein gave a good yield for methyl oleate and glycerol. Nonloaded alumina did not yield methyl oleate or glycerol within 1 h, and after 12 h of reaction, 7% yield was observed for methyl oleate at 473 K without glycerol byproduct.


Thus, the catalytic activity of this material is generated by loading the alumina with alkali metal salts followed by evacuation of the samples at 673 or 823 K.44 Among the catalysts, alumina loaded with K2CO3, KF, LiNO3, and NaOH gave methyl oleate and glycerol in high yield over 1 h at 333 K. For example, K2CO3/Al2O3 shows 93% yield of methyl oleate and 87% yield of glycerol; however, KOH/Al2O3 exhibited low catalytic activity. To compare the catalytic activity of K2CO3/ Al2O3 with that of KOH as a solid base catalyst, for a transesterification at 273 K for 1 h, K2CO3/Al2O3 (0.05 g, K2CO3 loading of 0.13 mmol) exhibited nearly the same catalytic activity as 0.023 mmol of KOH. Since the transesterification is a reversible reaction, the effect of the molar ratio of methanol to triolein on the yield of esters such as methyl oleate and glycerol was examined by varying the amount of methanol for transesterification at 333 K. The initial amount of triolein was set at 1.0 mmol, and 5.0 mL of tetrahydrofuran (THF) was used as the solvent. The conversion of triolein and the yields of methyl oleate increased with increasing addition of methanol. The yield of methyl oleate reached (ca. 100%) when more than 25 mmol of methanol was present (methanol/triolein > 25). These results confirm that the transesterification of triolein with methanol in the presence of heterogeneous solid base catalyst proceeds effectively at 333 K in atmospheric pressure, and it may be the convenient route for biodiesel and glycerol production. Xie et al.45 recently reported the use of KF loaded ZnO instead of typical alkali loaded alumina type solid base. The KF loaded on ZnO is an active and promising heterogeneous solid base catalyst for the production of biodiesel from soybean oil. The improved active sites formation in solid base catalyst is due to the combination of KF mixed ZnO and their surface hydroxyls groups via strong basic sites formation in the catalyst. The catalyst with 15 wt % KF loaded on ZnO and after being calcined at 873 K for 5 h was found to be the optimum catalyst, which exhibited the highest basicity and the best catalytic activity for the transesterification reaction.45 Transesterification was carried out with methanol, with a molar ratio is 10:1 for methanol to soybean oil, and the reaction time is fixed at 9 h; the corresponding conversion of soybean oil was observed over 87% at the above reaction conditions. The catalytic activities showed a correlation with the corresponding basic properties toward soybean oil transesterification.45 Macedo et al.46 recently reported (Al2O3)4(SnO) and (Al2O3)4(ZnO) type heterogeneous mixed solid oxide catalyst for transesterification of vegetable oil using different chain length alcohol, and the highest yield of 84% was observed with methanol at 4 h reaction time. Differences were observed for the reaction yield of soybean methanolysis assisted by solid catalysts (Al2O3)4(SnO) and (Al2O3)4(ZnO); this is probably due to the fact that the solids possessed similar surface structures, independent of the presence of tin or zinc in the final composition of the solid catalyst. These heterogeneous solid catalysts show high activity with a shortchain linear alcohol (methanol) for transesterification of vegetable oil. Recently the group of Bournay et al.47 commercialized the new heterogeneous catalytic biodiesel production process to the diesel engines by utilizing mixed oxides of aluminum and zinc in heterogeneous catalytic process. The composition and preparation method of the as-synthesized catalysts are not discussed in detail because of commercialization. Kim et al.48 studied the transesterification of soybean oil, using solid base catalyst such as sodium and sodium hydroxide loaded γ-alumina, correlating the catalytic activity of the transesterification process with respect to the basic strength of the catalyst. They have also studied the role of cosolvent in the

6167

Figure 1. Methyl ester (wt %) at 120 °C as a function of intermediate electronegativity calculated from Sanderson intermediate rule at 120 °C. To calculate electronegativity, 60% ion exchange is assumed for cesium. To calculate electronegativity, 95% ion exchange is assumed for potassium.

transesterification process in the presence of solid base catalyst. The vegetable oil (VO)-to-n-hexane molar ratio is found to be 5:1 for better results. When n-hexane was added, the immiscible two-phase system was changed to the homogeneous emulsion state. The maximum biodiesel production yield reached up to 94%, which was almost the same value as compared to the conventional homogeneous NaOH system;49 both the sodium aluminate formed by loading sodium hydroxide on γ-alumina and the ionization of sodium are responsible for the strong basic sites creation in the as-prepared catalyst. 4.2. Zeolite. The base strength of the alkali ion-exchanged zeolites increases with increasing electropositive nature of the exchanged cation. The occlusion of alkali-metal oxide clusters in zeolite cages via decomposition of impregnated alkali metal salts results in anincrease in the basicity of these materials. The zeolites faujasite NaX and titanosilicate structure-10 (ETS-10) were used for the alcoholeysis of soybean oil with methanol. These catalysts are predominantly alkali-cation-exchanged zeolite type materials. The basicities of zeolites NaX and ETS-10 were enhanced by ion exchange with higher electropositive metals like K and Cs using conventional techniques. Sodium oxide clusters in the faujasite zeolites were occluded in zeolite cages via decomposition of impregnated sodium acetate or sodium azide to further increase their basicity.50 ETS-10 is a new type of microporous inorganic titanium containing zeolite that has a three-dimensional 12-ring pore structure consisting of interlocking chains of octahedral titanium (TiO68-) and tetrahedral silicon (SiO44-) atoms.51 Its unique large pore structure, novel chemical composition, and strong basic character are the factors responsible for the performance advantages over other zeolites for liquid-phase reactions. Potassium- and cesiumexchanged ETS-10 type catalysts were prepared by partial ion exchanging of 15 g of ETS-10 zeolite using 0.5 M aqueous solutions of potassium hydroxide and cesium chloride at 333 K (30 mL g-1), respectively.52 Zeolite NaX was triply ionexchanged with K and Cs by contacting the zeolite in 1.0 M solutions of potassium acetate and cesium chloride, respectively, for 2 h at room temperature. Figure 1 graphically illustrates that the intermediate electronegativity of the solid is correlated with the yield of methyl ester. The intermediate electronegativity was calculated based on the principle of electronegativity equalization proposed by Sanderson,53 which states that, when two or more atoms are initially different in electronegativity, they adjust to have the same intermediate electronegativity

6168

Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009 Table 4. Surface Area of Hydrotalcite (HY) Samples As a Function of Mg Contents

Figure 2. Comparison of different catalyst systems to achieve 80% conversion to methyl esters. a and b are reference catalyst. The catalysts that have been underlined are believed to have a significant homogeneous component to their activity.

within the compound. This intermediate electronegativity is given by the geometric mean of the individual electronegativity of the component atoms. After 24 h of the reaction, the conversion of soybean oil to methyl ester is 15.4% for NaX, 22.5% for KX, and 18.7% for CsX. The decreased conversion for CsX compared to that of KX is unexpected and relates to the degree of ion exchange of these solids. The large size of cesium cations limits the exchange capacity compared to that for the smaller potassium, which affects the basicity associated with the framework oxygen. A similar phenomenon was observed (Figure 1) in the case of the ETS-10 and zeolites. The parent ETS-10 catalyst is reported to be approximately four times more basic than NaX.54 The more basic sites produced remarkably higher conversions to methyl esters in the case of ETS-10 compared to NaX zeolite on an identical mass basis. The ETS-10 catalyst generally had the greatest reactivity. Incorporation of occluded sodium oxide species results in an increase in the base strength and the number of basic sites over the parent zeolite. Further increase in the basicity of ETS-10 can be achieved by impregnating the porous solids with sodium acetate or sodium azide to form extra framework basic sites. ETS-10 catalyst has an activity to a level comparable to the occluded NaX zeolite, which had undergone a number of chemical treatments in order to reach that level of activity. Figure 2 compares the performance of ETS-10 zeolites and occluded faujasite NaX zeolites to other heterogeneous catalysts reported in the literature. A first-order linear regression model was used to obtain a least-squares rate constant. This rate constant was then used to calculate the time necessary to achieve 80% conversion to methyl esters. The ETS-10 zeolites and occluded faujasite NaX zeolites have activities greater than any other solid base catalyst that are reported in the literature.55 The few solid base catalysts reported to have higher activities are likely to follow a homogeneous mechanism for the transesterification reaction in the heterogeneous catalytic process.56 4.3. Hydrotalcite. Cantrell et al.57 studied the alkali free coprecipitate route to synthesized magnesium-aluminum hydrotalcites; the general composition of hydrotalcites is as follows, [Mg(1-x)Alx(OH)2]x+(CO3)x/n2- over the range of x ) 0.25-0.55. Hydrotalcites are another class of solid bases, and their acid-basic properties were controlled by changing their metal ion mole ratio. The structure of hydrotalcites is based on layered double hydroxides with brucite-like (Mg(OH)2) hydroxide layers containing octahedrally coordinated M2+ and M3+ cations An- as the counteranion. Variations in the Al content (x) are known to modify the basic properties of the material, and the pure hydrotalcites structures were reported to exhibit in the range of 0.25 < x < 0.44. The basic sites in the alkali

bulk Mg content (wt %)

surface area (m2/g)

glyceryl tributyrate conversion (%)

Mg13.4 (in HY) Mg17.9 (in HY) Mg20.5 (in HY) Mg24.0 (in HY) MgO

166 121 92 104 18

42.2 49.2 55.3 74.8 11.0

earth oxides can originate from O2- (strong basicity), O- species near hydroxyl groups (medium strength), and OH groups (weak). The addition of Al3+ alters the acid-base site distribution through the introduction of Al3+-O2- sites with moderate Lewis acidity and medium basicity. Mg content progressively reduces the surface area of the resulting hydrotalcites and their total area remaining in excess of 90 m2/g up to Mg/Al ) 3:1 (Table 4). This gradual loss of surface area has been previously observed and attributed to the effect of carbonate decomposition on the resulting formation of oxide morphology of the parent metal oxide, which can cause the porous texture for catalytic activity.58 Al-rich hydrotalcites tend to have higher carbonate contents, which favor porous open networks following thermal processing (calcination). Therefore, they exhibit larger pores than Al2O3 or MgO, which have a broad pore size distribution in the range of 20 nm. The reduced porosity of the calcined Al2O3 reference sample can be attributed to structural collapse at 473 K upon thermal decomposition. The catalytic transesterification of glyceryl tributyrate with methanol to form methyl butanoate was investigated using the series of Mg/Al hydrotalcites. Triglyceride conversion to the methyl ester occurred immediately without an induction period with concomitant formation of the diglyceride.57 The yield of these primary products increases linearly with triglyceride consumption over the first 15 min of reaction, after which secondary transesterification of the diglyceride to monoglyceride is initiated. This continued to boost the methyl butanoate yield, while the yield of diglyceride intermediately reached a plateau, before slowly declining concomitant with monoglyceride production. The surface area normalizes the initial rates for glyceryl tributyrate transesterification, and it increased continuously with Mg content across the hydrotalcite, while pure Al2O3 was completely inactive. The activities of the higher loaded 21 and 24 wt % Mg hydrotalcites are comparable to those reported for the best alternative Li-doped CaO solid base catalysts of 2.5 mmol min-1 g (cat)-1.59 Serio et al.60 recently reported the comparative catalytic activity of calcined hydrotalcite and MgO prepared by a different method, which showed efficient catalytic activity for transesterification of soybean oil versus the conventional-route-prepared bulk MgO catalyst. Pure MgO shows a lower activity and selectivity, which may be attributed to a reduced number of accessible basic sites associated with its low porosity compared to the hydrotalcite materials. The catalytic activities of these hydrotalcites show a striking correlation with their corresponding intralayer charge densities toward tributyrate transesterification. This increased intralayer electron density of the Mg rich hydrotalcites would be expected to correlate with an increase of the basicity of these materials. All hydrotalcite materials are effective catalysts for the transesterification of glyceryl tributyrate with methanol. The rate of tributyrate conversion and associated methyl butanoate and diglyceride formation was first order with respect to triglyceride concentration. The rate increases steadily with Mg content, and the most active Mg2.93Al catalyst was 10 times more active than MgO. They have also correlated the rate of reaction with the intralayer electron density, which can be associated with increased


6169

Figure 3. (a) Reaction profile for the transesterification of glyceryl tributyrate with methanol using a 2.4 wt % Li/CaO catalyst. (b) Conversion of glyceryl tributyrate observed for the series of Li/CaO catalysts: 9 ) CaO; O ) 0.25 wt % Li/CaO; b ) 1.23 wt % Li/CaO; ∆ ) 2.4 wt % Li/CaO; 2 ) 4.0 wt % Li/CaO.

basicity.60 Barakos et al.61 reported the transesterification of triglycerides using high- and low-quality oil feeds over hydrotalcite type catalysts; the transesterification reaction of refined and acidic cottonseed oil for the production of methyl esters (biodiesel) has been studied. The basic Mg-Al-CO3 hydrotalcite catalyst had shown high activity for methanolysis and esterification reactions in a refined and an acidic cottonseed oil as well as in a representative high-water-content animal fat feed; 99% conversion of triglycerides was achieved within a time period of 3 h, and the fatty acids content decreased considerably up to 1 wt %. 4.4. Lithium-Calcium Oxide (Li-CaO). Watkins et al.59 studied the Li-CaO solid base catalyst for transesterification reaction, and they chose the glyceryl tributyrate as a model substrate for testing the catalytic activity in the presence of various amounts of lithium doped calcium oxide material. A series of LiNO3 was impregnated with CaO by the wet impregnation method at theoretical Li contents in the range of 1-20 wt %. Alkali earth oxides are potential as solid bases for use in triglyceride transesterification. The basic strength of the group 2 oxides and hydroxides increased in the order Mg < Ca < Sr < Ba; of these, Ca-derived bases are the most promising because they are inexpensive, exhibit low methanol solubility, and are the least toxic.62 The origin of basic sites in alkali earth oxides has been debated, and it is generally believed that they are generated by the presence of M2+- O2- ion pairs in different coordination environments.63 Figure 3 shows the conversion of glyceryl tributyrate as a function of Li loading. CaO exhibited poor activity in transesterification, with only 2.5% conversion being observed after 20 min of reaction. In contrast, catalysts with as little as 0.23 and 1.23 wt % Li loadings showed activity with 83 and 100% conversions, respectively, after the same reaction time. A further increase in Li content improved the activity up to 37% conversion. Negligible leaching of LiNO3 occurred during the reaction; catalysts were successfully filtered and recycled with minimal (10%) loss of activity. In conclusion, an optimum Li content of 1.23 wt % gave maximum activity for formation of methyl butanoate, and lithium incorporation in CaO increases the base strength of catalyst.59 4.5. Enzyme (Lipase) Catalysis for Biodiesel Production. Pure lipase or lipase supported catalyst system are a recently growing field of research for biodiesel production. Enzyme catalysis on transesterification of renewable oil is safe and clean without byproducts. Engineering of biodiesel synthesis in the presence of enzyme catalyst requires optimizing the molar ratio of substrates, the temperature of the reaction, the organic solvent, and the water activity. Al-Zhuair et al.64 reported that the production of biodiesel from simulated waste cooking oil

(SWCO) catalyzed by immobilized lipase from different sources in the presence and absence of n-hexane was investigated. Immobilized lipase on ceramic beads was effective in catalyzing the production of biodiesel from SWCO with high water contents. However, immobilized lipase from bacterial sources was more competent than that from a yeast source. It was also found that the presence of n-hexane has a negative effect on the production rate, due to dilution of reactants and addition of mass transfer resistances. A kinetic model was developed to describe the system, taking into consideration the mass transfer resistances of the reactants and the inhibition effects by both substrates. It was observed that the reaction was slightly more inhibited by methanol than by the ester bond on the triglyceride. This was reflected on the inhibition constant of alcohol, which was found to be slightly higher than that of the substrate. Dizge et al.65 reported the biodiesel production from canola oil using lipase immobilized on hydrophobic microporous styrene-divinylbenzene copolymer. Lipase from T. lanuginosus was successfully immobilized on styrene-divinylbenzenepolyglutaraldehyde (STY-DVB-PGA) copolymer prepared by emulsion technique. The STY-DVB-PGA copolymer was found to be a better support material for lipase than the STY-DVB copolymer in terms of protein loadings, activity yields, and operational stabilities. The highest amount of protein immobilized on STY-DVB-PGA copolymer was 11.4 mg of protein/g of polymer. The best result was obtained at pH 6, and the immobilized enzyme was used successfully for biodiesel production from canola oil with 97% yield. The microporous poly(STY-DVB-PGA) constitutes an excellent support for lipase immobilization because of the following properties: (a) the copolymer could be prepared economically in a short time in large amounts in any shape; (b) it could immobilize a large amount of protein (11.4 mg of protein/g of polymer); and (c) it could immobilize the enzymes by both adsorption and covalent binding due to the presence of polyglutaraldehyde functional groups.65 Tamalampudi et al.66 employed immobilized wholecell and commercial lipase as biocatalyst for biodiesel production from Jatropha oil. The lipase producing whole cells of Rhizopus oryzae (ROL) immobilized onto biomass support particles (BSPs) were used for the production of biodiesel from relatively low-cost nonedible oil from the seeds of Jatropha curcas. The activity of ROL was compared with that of the commercially available, most effective lipase (Novozym 435). The various alcohols were tested as a possible hydroxyl donor, and methanolysis of Jatropha oil progresses faster than other alcoholeysis regardless of the lipases used. The maximum methyl esters content in the reaction mixture reaches 80 wt % after 60 h using ROL, whereas it is 76% after 90 h using Novozym 435. Whole-

6170


cell R. oryzae immobilized onto BSP catalyzes the methanolysis of Jatropha oil more efficiently than Novozym 435. The presence of water in Jatropha oil has a significant effect on the rate of methanolysis, and ROL exhibits the highest activity in the presence of 5% (v/v) added water; in contrast to ROL, Novozym 435 activity is inhibited by the presence of added water. The results obtained here suggest that whole-cell ROL immobilized onto BSP can be used as a low-cost biocatalyst for the production of biodiesel from crude Jatropha oil compared to commercial catalyst. Du et al.67 studied the lipase catalyzed transesterification of soya bean oil via continuous batch operation. Lipase from Thermomyces lanuginosus immobilized on acrylic resin was used in continuous batch operation when shortchain alcohols were used as the acyl acceptor. In noncontinuous batch operation, the optimal oil/alcohol ratio and temperature were 1:4 and 303-313 K, and 92% yield of methyl ester was observed; however, during the continuous batch operation, the optimal oil/alcohol ratio and temperature were 1:1 and 30 °C, and 95% of enzymic activity remained after 10 batches when isopropanol was adopted to remove byproduct glycerol during repeated use of the lipase. Zheng et al.68 reported the lipase catalyzed transesterification process in a solvent-free system. Feruloylated diacylglycerol (FDAG) was synthesized using a selective lipase-catalyzed transesterification between ethyl ferulate and triolein. The highest reaction conversion and selectivity toward FDAG were 73.9% and 92.3%, respectively, at 338 K, reaction time ) 5.3 days, with enzyme loading of 30.4 mg/ mL; water activity is 0.08, and the substrate molar ratio is 3.7. The disadvantage of the above process is that it is timeconsuming compared to other lipase catalyzed transesterification. 5. Conclusions Transesterification is basically a sequential reaction. The monoglycerides are finally reduced to fatty acid methyl esters. The order of the reaction changes with the reaction conditions. The main factor affecting transesterification is the molar ratio of alcohol to glycerides. From the commercial point of view, solid base catalysts are more effective than acid catalysts and enzyme catalyst. The solid base catalyst system eliminates the corrosion problems and unwanted waste product formation. However, research studies dealing with the use of solid acid catalysts for biodiesel production have been limited due to the expectations on reaction rates and unfavorable side reactions. An ideal or a model of solid acid catalyst should possess interconnected large porous texture with moderate-to-high concentration of acid sites and a hydrophobic surface. A hydrophobic surface is essential to promote preferential adsorption of oily hydrophobic species on the catalyst surface and to avoid possible deactivation of catalytic sites by the strong adsorption of polar byproducts such as water and glycerol. Many metals and zeolites are efficient catalytic materials for a transesterification process. The most significant implication of the catalysis by iron, palladium, and nickel present in the reactor surfaces is that it might catalyze the alcoholeysis in the absence of catalysts for the noncatalytic transesterification process. Faujasite NaX and titanosilicate structure-10 (ETS-10) zeolites have shown higher conversion, nearly 92% for the transesterifcation process compared to other zeolite materials. Solid base catalyst like Na/NaOH/γ-Al2O3 shows almost the same conversion of vegetable oil under optimized conditions in the presence of cosolvent (n-hexane) compared to the conventional homogeneous NaOH catalyst. Lipase immobilized polymer support catalyst is another promising technique to produce the biodiesel from suitable substrate oil source, and

hybrid solid base and enzyme catalyst could yield more interesting results for the future biodiesel process in heterogeneous medium. Hence, adopting suitable reaction conditions such as optimized methanol-to-oil mole ratio and identification of potential solid acid or base catalysts with specific porous texture and hydrophobic property can facilitate the efficient transesterification process for biodiesel production. Acknowledgment The authors thank the National Science Council, Taiwan, Republic of China, for financial support (NSC 94-2811-Z-002006). Literature Cited (1) Sharmer, K. Umweltaspekte bei Herstellung und Verwendung Von RME; RME Hearing, Ministry for Agriculture: Vienna, Austria, 1993. (2) Barnwal, B. K.; Sharma, M. P. Prospects of biodiesel production from vegetable oils in India. Renewable Sustainable Energy ReV. 2005, 9, 363. (3) Sonntag, N. O. V. Composition and characteristics of individual fats and oils. In Bailey’s Industrial Oil and Fat Products, 4th ed.; John Wiley & Sons: New York, 1979; Vol. 1, p 343. (4) Ali, Y.; Hanna, M. A.; Cuppett, S. L. Fuel properties of tallow and soybean oil esters. J. Am. Oil Chem. Soc. 1997, 74, 1457. (5) Shay, E. G. Diesel fuel from vegetable oils: status and opportunities. Biomass Bioenergy 1993, 4, 227. (6) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Synthesis of Biodiesel via Acid Catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353. (7) Katikaneni, S. P. R.; Adjaye, J. D.; Bakhshi, N. N. Catalytic conversion of canola oil to fuels and chemicals over various cracking catalysts. Can. J. Chem. Eng. 1995, 73, 484. (8) Katikaneni, S. P. R.; Adjaye, J. D.; Bakhshi, N. N. Studies on the catalytic conversion of canola oil to hydrocarbons, influence of hybrid catalysts and steam. Energy Fuels 1995, 9, 599. (9) Idem, R. O.; Katikaneni, S. P. R.; Bakhshi, N. N. Catalytic conversion of canola oil to fuels and chemicals: Roles of catayst acidity, basicity and shape selectivity on product distribution. Fuel. Process. Technol. 1997, 51, 101. (10) Tremblay, A. Y.; Cao, P.; Dube, M. A. Biodiesel production using ultralow catalyst concentrations. Energy Fuels 2008, 22, 2748. (11) Leng, T. Y.; Mohamed, A. R.; Bhatia, S. Catalytic conversion of palm oil to fuels and chemicals. Can. J. Chem. Eng. 1999, 77, 156. (12) Twaiq, F. A.; Zabidi, N. A. M.; Bhatia, S. Catalytic conversion of palm oil to hydrocarbons: Performance of various zeolite catalysts. Ind. Eng. Chem. Res. 1999, 38, 3230. (13) Noiroj, K.; Intarapong, P.; Luengnaruemitchai, A.; Jai-In, S. A comparative study of KOH/Al2O3 and KOH/NaY catalysts for biodiesel production via transesterification from palm oil. Renewable Energy 2009, 34, 1145. (14) Kanesedo, J.; Lee, K. T.; Bhatia, S. Feasibility of palm oil as the feedstock for biodiesel production via heterogeneous transesterification. Chem. Eng. Technol. 2008, 31, 993. (15) Kaita, J.; Mimura, T.; Fukuoda, N.; Hattori, Y. Catalysts for transesterification. U.S. Patent 6407269, June 18, 2002. (16) Waghoo, G.; Jayaram, R. V.; Joshi, M. V. Heterogeneous, catalytic conversions with hydrous SnO2. Synth. Commun. 1999, 29, 513. (17) Vicente, G.; Coteron, A.; Martinez, M.; Aracil, J. Application of the factorial design of experiments and response surface methodology to optimize biodiesel production. Ind. Crops Prod. 1998, 8, 29. (18) Chavan, S. P.; Subbarao, Y. T.; Dantale, S. W.; Sivappa, R. Transesterification of ketoesters using Amberlyst-15. Synth. Commun. 2001, 31, 289. (19) De Rezende, S. M.; Castro Reis, M. D.; Reid, M. C.; Silva, P. L., Jr.; Coutinho, F.M. B.; Dasilva sangil, R. A.; Latcher, E. R. Transesterification of vegetable oils promoted by poly(styrene-divinylbenzene) and poly(divinylbenzene). Appl. Catal., A 2008, 349, 198. (20) Sasidharan, M.; Kumar, R. Transesterification over various zeolites under liquid-phase conditions. J. Mol. Catal., A 2004, 210, 93. (21) Mbaraka, I. K.; Radu, D. R.; Lin, V. S. Y.; Shanks, B. H. Organosulfonic acid-functionalized mesoporous silicas for the esterification of fatty acid. J. Catal. 2003, 219, 329.

Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009 (22) Perez-Pariente, J.; Diaz, I.; Mohino, F.; Sastre, E. Selective synthesis of fatty monoglycerides by using functionalised mesoporous catalysts. Appl. Catal., A 2003, 254, 173. (23) Corma, A.; Rodriguez, M.; Sanchez, N.; Aracil, J. Process for the selective production of monoesters of diols and triols using zeolitic catalysts. WO9413617, 1994. (24) Verhoef, M. J.; Kooyman, P. J.; Peters, J. A.; Van Bekkum, H. A study on the stability of MCM-41-supported heteropoly acids under liquidand gas-phase esterification conditions. Microporous Mesoporous Mater. 1999, 27, 365. (25) Sakthivel, A.; Komura, K.; Sugi, Y. MCM-48 Supported Tungstophosphoric Acid: An Efficient Catalyst for the Esterification of LongChain Fatty Acids and Alcohols in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 2008, 47, 2538. (26) Chen, X. R.; Ju, Y. H.; Mou, C. Y. Direct Synthesis of Mesoporous Sulfated Silica-Zirconia Catalysts with High Catalytic Activity for Biodiesel via Esterification. J. Phys. Chem. C 2007, 111, 18731. (27) Demirbas, A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog. Energy Combust. Sci. 2005, 31, 466. (28) Yadav, G. D.; Nair, J. J. Sulfated zirconia and its modified versions as promising catalysts for industrial processes. Microporous Mesoporous Mater. 1999, 33, 1. (29) Omota, F.; Dimian, A. C.; Bliek, A. Fatty acid esterification by reactive distillation. Part 2: Kinetics-based design for sulphated zirconia catalysts. Chem. Eng. Sci. 2003, 58, 3175. (30) Yadav, G. D.; Murkute, A. D. Preparation of a novel catalyst UDCaT-5: Enhancement in activity of acid-treated zirconia-effect of treatment with chlorosulfonic acid vis-a-vis sulfuric acid. J. Catal. 2004, 224, 218. (31) Furuta, S.; Matsuhashi, H.; Arata, K. Catalytic action of sulfated tin oxide for etherification and esterification in comparison with sulfated zirconia. Appl. Catal., A 2004, 269, 187. (32) Matsuhashi, H.; Miyazaki, H.; Kawamura, Y.; Nakamura, H.; Arata, K. Preparation of a solid superacid of sulfated tin oxide with acidity higher than that of sulfated zirconia and its applications to aldol condensation and benzoylation. Chem. Mater. 2001, 13, 3038. (33) Einloft, S.; Magalha˜es, T. O.; Donato, A.; Dullius, J.; Ligabue, R. Biodiesel from Rice Bran Oil: Transesterification by Tin Compounds. Energy Fuels 2008, 22, 671. (34) Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun. 2004, 5, 721. (35) Furuta, S.; Matsuhashi, H.; Arata, K. Catalytic action of sulfated tin oxide for etherification and esterification in comparison with sulfated zirconia. Appl. Catal., A 2004, 269, 187. (36) Matsuhashi, H.; Tanaka, T.; Arata, K. Measurement of Heat of Argon Adsorption for the Evaluation of Relative Acid Strength of Some Sulfated Metal Oxides and H-type Zeolites. J. Phys. Chem. B 2001, 105, 9669. (37) Matsuhashi, H.; Arata, K. Measurement of the relative acid strength and acid amount of solid acids by argon adsorption. Phys. Chem. Chem. Phys. 2004, 6, 2529. (38) Ramu, S.; Lingaiaha, N.; Prabhavathi, B. L. A.; Devi, R. B. N.; Prasadb, I.; Suryanarayanaa, P. S. Esterification of palmitic acid with methanol over tungsten oxide supported on zirconia solid acid catalysts: Effect of method of preparation of the catalyst on its structural stability and reactivity. Appl. Catal., A 2004, 276, 163. (39) Brei, V. V.; Prudius, S. V.; Melezhyk, O. S. Vapour-phase nitration of benzene over superacid WO3/ZrO2 catalysts. Appl. Catal., A 2003, 239, 11. (40) Lopez, D. E.; Suwannakarn, K.; Goodwin, G. J., Jr.; Bruce, D. A. Reaction Kinetics and Mechanism for the Gas- and Liquid-Phase Esterification of Acetic Acid with Methanol on Tungstated Zirconia. Ind. Eng. Chem. Res. 2008, 47, 2221. (41) Furuta, S.; Yano, K.; Hiromi, M.; Arata, M. Biodiesel fuel (BDF) production by the trans-esterification of soybean and castor oils and the esterification of fatty acid using fixed bed reactor with solid superacid and amorphous zirconia catalysts. Chem. Mater. 2001, 13, 3038. (42) Larson, G.; Petkovic, L. M. Effect of preparation method and selective poisoning on the performance of platinum supported on tungstated zirconia catalysts for alkane isomerization. Appl. Catal., A 1996, 148, 155. (43) McNeff, C. V.; McNeff, L. C.; Yan, B.; Nowlan, D. T.; Rasmussen, M.; Gyberg, A. E.; Krohn, B. J.; Fedie, R. L.; Hoye, T. R. A continuous catalytic system for biodiesel production. Appl. Catal., A 2008, 343, 39. (44) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst. Appl. Catal., A 2005, 283, 111.

6171

(45) Xie, W.; Huang, X. Synthesis of biodiesel from soybean oil using heterogeneous KF/ZnO catalyst. Catal. Lett. 2006, 107, 53. (46) Macedo, C. C. S.; Abreu, F. R.; Tavares, A. P.; Alves, M. B.; Zara, L. F.; Rubima, J. C.; Suarez, P. A. Z. New Heterogeneous Metal-Oxides Based Catalyst for Vegetable Oil Trans-esterification. J. Braz. Chem. Soc. 2006, 17, 1291. (47) Bournay, L D.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. New heterogeneous process for biodiesel production: A way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Catal. Today 2005, 106, 192. (48) Kim, H. J.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 2004, 93-95, 315. (49) Boocock, D. G. B.; Konar, S. K.; Sidi, H. Phase diagrams for oil/ methanol/ether mixtures. J. Am. Oil Chem. Soc. 1996, 73, 1247. (50) Kang, M.; Lee, M. H. Synthesis and characterization of Al-, Bi-, and Fe-incorporated mesoporous titanosilicate (MPTS) materials and their hydrophilic properties. Appl. Catal., A 2005, 284, 215. (51) Das, T. K.; Chandwadkar, A. J.; Sivasanker, S. Studies on the synthesis, characterization and catalytic properties of the large pore titanosilicate, ETS-10. J Mol. Catal., A 1996, 107, 199. (52) Philippou, A.; Anderson, M. W. Aldol-Type Reactions over Basic Microporous Titanosilicate ETS-10 Type Catalysts. J. Catal. 2000, 189, 395. (53) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic Press: New York, 1976. (54) Philippou, A.; Rocha, J.; Anderson, M. W. The strong basicity of the microporous titanosilicate ETS-10. Catal. Lett. 1999, 57, 151. (55) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal., A 2004, 257, 213. (56) Lo´pez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A.; Lotero, E. Transesterification of triacetin with methanol on solid acid and base catalysts. Appl. Catal., A 2005, 295, 97. (57) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Structurereactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal., A 2005, 287, 183. (58) Di Cosimo, J. I.; Diez, V. K.; Xu, M.; Iglesia, E.; Apesteguia, C. R. Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides. J. Catal. 1998, 178, 499. (59) Watkins, R. S.; Lee, A. F.; Wilson, K. Li-CaO catalysed tri-glyceride transesterification for biodiesel applications. Green Chem. 2004, 6, 335. (60) Serio, M. D.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of soybean oil to biodiesel by using heterogeneous basic catalysts. Ind. Eng. Chem. Res. 2006, 45, 3009. (61) Barakos, N.; Pasias, S.; Papayannakos, N.; Barakos, N.; Pasias, S.; Papayannakos, N. Transesterification of triglycerides in high and low quality oil feeds over an HT2 hydrotalcite catalyst. Bioresour. Technol. 2008, 99, 5037. (62) Gryglewicz, S. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour. Technol. 1999, 70, 249. (63) Davydov, A. A.; Shepotko, M. L.; Budneva, A. A. Basic sites on the oxide surfaces: their effect on the catalytic methane coupling. Catal. Today 1995, 43, 225. (64) Al-Zuhair, S.; Dowaidar, A.; Kamal, H. Dynamic modeling of biodiesel production from simulated waste cooking oil using immobilized lipase. Biochem. Eng. J. 2009, 44, 256. (65) Dizge, N.; Keskinler, B.; Tanriseven, A. Biodiesel production from canola oil by using lipase immobilized onto hydrophobic microporous styrene-divinylbenzene copolymer. Biochem. Eng. J. 2009, 44, 220. (66) Tamalampudi, S.; Talukder, M. R.; Hamad, S.; Numata, T.; Kondo, A.; Fukuda, H. Enzymatic production of biodiesel from Jatropha oil: A comparative study of immobilized-whole cell and commercial lipases as a biocatalyst. Biochem. Eng. J. 2008, 39, 185. (67) Du, W.; Xu, Y.; Liu, D. Lipase-catalysed transesterification of soya bean oil for biodiesel production during continuous batch operation. Biotechnol. Appl. Biochem. 2003, 38, 103. (68) Zheng, Y.; Wua, X. M.; Christopher, B. W.; Jing, Q.; Zhu, L. M. Dual response surface-optimized process for feruloylated diacylglycerols by selective lipase-catalyzed transesterification in solvent free system. Bioresour. Technol. 2009, 100, 2896. (69) Yan, S.; Salley, S. O.; Simon Ng, K. L. Simultaneous transesterification and esterification of unrefined or waste oil over ZnO-La2O3 catalysts. Appl. Catal., A 2009, 353, 203. (70) Georgogianni, K. G.; Katsoulidis, A. P.; Pomonis, P. J.; Kontominas, M. G. Transesterification of soybean frying oil to biodiesel using heterogeneous catalysts. Fuel Process. Technol. 2009, (In Press).

6172


(71) Nakagaki, S.; Bail, A.; Santos, V. C.; Souza, V. H. R.; Vrubel, H.; Nunes, F. S.; Ramos, L. P. Use of anhydrous sodium molybdate as an efficient heterogeneous catalyst for soybean oil methanolysis. Appl. Catal., A 2008, 351, 267. (72) Benjapornkulaphong, S.; Ngamcharussrivichai, C.; Bunyakiat, K. Al2O3-supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil. Chem. Eng. J. 2009, 145, 468. (73) Kawashima, A.; Matsubara, K.; Honda, K. Acceleration of catalytic activity of calcium oxide for biodiesel production. Bioresour. Technol. 2009, 100, 696. (74) Kim, M.; Salley, S. O.; Ng, K. Y. S. Transesterification of glycerides using a heterogeneous resin catalyst combined with a homogeneous catalyst. Energy Fuels 2008, 22, 3594. (75) Ngamcharussrivichai, C.; Totarat, P.; Bunyakiat, K. Ca and Zn mixed oxide as a heterogeneous base catalyst for transesterification of palm kernel oil. Appl. Catal., A 2008, 341, 77. (76) Babu, N. S.; Sree, R.; Prasad, P. S. S.; Lingaiah, N. Roomtemperature transesterification of edible and nonedible oils using a heterogeneous strong basic Mg/La catalyst. Energy Fuels 2008, 22, 1965. (77) Albuquerque, M. C. G.; Jimeínez-Urbistondo, I.; SantamarıáGonzaílez, J.; Meírida-Robles, J. M.; Moreno-Tost, R.; Rodrı´guez-Castelloín, E.; Jimeínez-Lo´pez, A.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; MairelesTorres, P. CaO supported on mesoporous silicas as basic catalysts for transesterification reaction. Appl. Catal., A 2008, 334, 35. (78) De Almeida, R. M.; Noda, L. C.; Goncalves, N. S.; Meneghetti, S. M. P.; Meneghetti, M. R. Transesterification reaction of vegetable oils, using superacid sulfated TiO2-base catalysts. Appl. Catal., A 2008, 347, 100. (79) Brito, A.; Garcia, F.; Borges, M. E.; Diaztt, M. C.; Arvelo, R.; Otero, N. Reuse of fried oil to obtain biodiesel: Zeolites Y as a catalyst. Int. J. Chem. React. Eng. 2005, 5; Art. No. A104.

(80) Ngamcharussrivichai, C.; Wiwatnimit, W.; Wangnoi, S. Modified dolomites as catalysts for palm kernel oil transesterification. J. Mol. Catal., A 2007, 276, 24. (81) Ngamcharussrivichai, C.; Totarat, P.; Bunyakiat, K. Ca and Zn mixed oxide as heterogeneous base catalyst for transesterification of palm kernel oil. Appl. Catal., A 2008, 341, 77. (82) Xie, W.; Yang, Z. Ba-ZnO catalysts for soybean oil transesterification. Catal. Lett. 2007, 117, 159. (83) Chen, H.; Wang, J.-F. Biodiesel from transesterification of cotton seed oil by solid bases catalysis. J. Chem. Eng. Chin. UniV. 2006, 20, 593. (84) Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat, K.; Attanatho, L.; Jenvanitpanjakul, P. Transesterification of crude palm kernel oil and crude coconut oil by different solid catalysts. Chem. Eng. J. 2006, 116, 61. (85) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of soybean oil to biodiesel by using heterogeneous basic catalysts. Ind. Eng. Chem. Res. 2006, 45, 3009. (86) Meng, X.; Xin, Z. Preparation of biodiesel from soybean oil by transesterification on KF/CaO catalyst. Petrochem. Technol. 2005, 34, 282. (87) Dossat, V.; Combes, D.; Marty, A. Lipase-catalyzed transesterification of high oleic sunflower oil. Enzyme Microb. Technol. 2002, 30, 90. (88) Nelson, L. A.; Foglia, T. A.; Marmer, W. N. Lipase-catalyzed production of biodiesel. J. Am. Oil Chem Soc. 1996, 73, 1191.

ReceiVed for reView December 5, 2008 ReVised manuscript receiVed April 27, 2009 Accepted May 8, 2009 IE801872T

Review of Recent Developments in Solid Acid, Base, and Enzyme

Recommend Documents