Chapter Sixteen
Mesoporous Catalysts for Biodiesel Production
A New Approach
Abstract
Recently, mesostructured solid catalysts have been attracting attention for biodiesel synthesis owning to their remarkable characteristics, such as harmonized surface characteristic, large surface area, large and flexible pore size, specific crystallization, and high thermal stability. Principally, higher surface area can be obtained as a result of either porous creation or transforming bulk material to nanoparticles. The bigger pore diameters simplify the diffusion of the reactants into pore channels, which consequently enhances the accessibility of the reactants to the majority of the active sites. The considerable physicochemical, morphological, structural, and textural properties of mesoporous materials have found applicable as heterogeneous catalysts, support catalysts, and adsorbents in many fields of studies. The absorbent surface property of the mesoporous catalysts makes them to absorb long-chain organic molecules such as free fatty acids (FFAs). It highly improves the catalytic activity through reaction for transforming of high FFAs feedstocks to biodiesel.
Keywords
Biodiesel production; Mesoporous catalysts; Mesoporous materials; Surface modification; Transesterification reaction
16.1. Introduction
Energy is a principal necessity for human existence on the earth. The energy crisis has become the main concern for the human beings. This concern is strongly associated to the availability of natural sources of power (i.e., coal and natural gases) which are depleting at rapid rates. These natural sources have been hugely consumed for transportation, industrial, and domestic sectors. Apart from these facts, the increasing consumption of petroleum has been endangering planet earth and more particularly human health [1,2].
However, the world's most energy needs are met mainly by petrochemical sources; the depletion of these sources has warned human communities to discover alternative sources of energy. The increasing trend on the number of researches on biodiesel production might be a true evidence that biodiesel would be the best substitute for petroleum-based diesel. Biodiesel is a sulfur-free liquid fuel, which can enhance the world's energy security [3]. Biodiesel is generated from various feedstocks, which has comparable physical and chemical characteristics to petroleum [4]. In terms of energy generation, biodiesel is a promising source of energy which is nontoxic in nature, sustainable, and biodegradable with a low emission of greenhouse gases [5–7].
16.2. Biodiesel
The seed oil–based fuel is referred to as ester, well known as biodiesel, normally produced in presence of a catalyst with a short-chain alcohol. Biodiesel is considered as a sulfur-free, nontoxic, and biodegradable source of energy which burns cleaner than petroleum-based fuels.
16.2.1. Triglycerides As Diesel Fuel
The history of using triglyceride (TG) as a diesel fuel comes from 1885 where Rudolph Diesel designed a diesel engine and used peanut oil as raw material. Palm, sunflower, soybean, olive, coconut, and peanut oils have been utilized majorly for biodiesel production [8]. Climate condition and geographical location are two important factors that affect the cultivation of these feedstocks. Unfortunately, the number of drawbacks, such as high viscosity, flash point, cloud point, pour point, and carbon deposition, have limited the use of edible oils in industrial applications [9]. Besides, choosing food-based vegetable oils as raw material may affect both biodiesel and food industries. Rising demand for edible oils will consequentially increase their prices in both markets [10]. Moreover, there is a need to explore nonedible oils, such as waste cooking oils (WCOs) and animal fats, which are completely unfit for human food consumption. Thereby, the cost of the production process can be substantially reduced using nonedible oils as raw material.
16.2.2. Process of Biodiesel Production
Several synthesis methods have been utilized for biodiesel production, including: (1) direct utilization of vegetable oils, (2) micro emulsions, (3) thermal cracking, (4) transesterification, and (5) supercritical methanol (detailed in Table 16.1) [11,12].
Table 16.1
Some Important Approaches to Produce Biodiesel
| Type of Process | Method | Pros | Cons |
| Direct mixing and dilution | Blending plant oil without any type of pretreatment with petrodiesel and using directly in engine | No engine modification required | Low ignition/low cold filter plugging point/carbon deposition on engine |
| Micro emulsion | Simple process | Low stability | |
| – | High viscosity | ||
| Thermal cracking | Converting of an organic substance to another one by heating | Simple process/nonpolluting | High temperature required/high cost of equipment/less purity/nonpolluting |
| Transesterification | Converting of TG into alkyl esters in presence of alcohol and catalyst | Similar fuel properties to diesel/high conversion/low cost/suitable to scale up to industrial scale | Undesired byproduct/separation costing |
| Supercritical methanol | Accelerate free catalytic transesterification using supercritical fluid at high pressure/temperature | Free catalyst/high conversion efficiency/very short reaction time/no product purification/simultaneously esterification and transesterification | High pressure and temperature required/high energy using/expensive equipment |
16.2.3. Transesterification
Alcoholysis known as transesterification to synthesize alcoholic ester as a result of interaction between an ester with an alcohol in the presence of a proper catalyst. The general equation of the transesterification reaction is illustrated as follows [3]:
(16.1)
where R′, R″, and R‴ are referred to as hydrocarbons (oils) or free fatty acid (FFA) chains.
The main reason to transesterify vegetable oils is to enhance their combustion by reducing their high viscosity [13]. Transesterification is a chemical reaction that includes a sequence of converting TG to diglyceride, and then diglyceride to monoglyceride, and finally monoglyceride to glycerol [14]. Typically, transesterification is performed to convert TG to methyl ester (ME). The molar ratio of alcohol to oil (MeOH:oil), catalyst amount, level of FFAs, operating temperature, and reaction time are some important reaction variables that affect the conversion rate [9]. Generally, the conversion percentage of TG to glycerol is generally obtained from:
(16.2)
where ∑Ai is the total maximum of methyl esters, A is the area of methyl ester, C is the strength in mg/ml of the methyl heptadecanoate, V is the volume in ml of the methyl heptadecanoate, and W is the weight in mg of the sample [15].
Biodiesel properties have to be in accordance with either ASTM D6751-07 or EN 14214:2003 standards, utilized as an ecofriendly fuel [16].
16.3. Catalysts
Typically, a proper catalyst should be active enough to catalyze transesterification reaction which consists of a large number of saturated fatty acids (SFAs) and long carbon chains [17–20]. Catalysts are used to maximize the biodiesel yield in a shorter reaction rate by increasing the number of active sites. Using a proper catalyst may provide better condition due to enhancing the miscibility of the reactants into alcohol.
16.3.1. Classification of Catalysts
Generally, transesterification is performed in the presence of acid or base catalysts considering the fatty acid content of feedstocks. Transesterification is a reversible reaction, therefore, catalyst dosage should be sufficient to catalyze both the forward and backward reactions [21,22]. Generally, catalysts are categorized into three major groups; homogeneous catalyst, heterogeneous catalyst, and biocatalyst, of which the subclassification is shown in Scheme 16.1 [23]. Commonly, biodiesel is produced using homogeneous base catalyst (such KOH and NaOH) or homogeneous acid catalysts (such as H2SO4), which are available in market but are corrosive in nature.
16.3.2. Problems With Homogeneous Catalysts
Conventionally, transesterification has been conducted using homogeneous acidic catalysts that are ironically nongreen and corrosive. Besides, loading higher MeOH:oil ratio is required to complete the reaction. Consequently, loading higher MeOH:oil ratio prolongs the transesterification reaction time. Homogeneous base catalysts (such as sodium methoxide and potassium hydroxide) are also used for catalyzing the transesterification reaction [24]. On the other hand, homogeneous base catalysts are quite sensitive to water and FFAs. Furthermore, huge amount of water is needed to separate catalyst from the final product which consequently increases the production cost of biodiesel production. Generally, the hygroscopic nature, water and soap formation, oil losses, and difficulty in separation are some undesired drawbacks of the homogenous catalysts during catalyzing the transesterification reaction. Thereby, research efforts have been switched onto heterogeneous catalysts [15,25].
16.3.3. Problems With Base and Acid Catalysts
Hana et al. [24] concluded that the base catalysts are not suitable in cases of high levels of FFAs (>1%) but according to 2012 reports, these types of catalysts can efficiently catalyze feedstocks with FFAs over 4% [26]. It is also reported that solid base catalysts have higher catalytic activity and stability than the solid acid catalysts [27]. On the other hand, acid content is an important feature of the feedstocks which increases by raising the reaction temperature [28]. As an example, the acid value of soybean oil will be increased from 0.04% to 1.51% as the temperature goes above 190°C [29]. In this case, using heterogeneous acid catalysts performs much better than heterogeneous base catalysts. Heterogeneous acid catalysts are mostly preferred than the base ones for the transesterification of feedstocks with high FFA [30]. Besides, high acid density is an important advantage of solid acid catalysts in organic reactions which significantly eases the adsorption of reactants to the catalyst' active sites. Thereby, presence of heterogeneous catalyst can increase the catalytic activity through transesterification reaction. Solid acid catalysts can be simply detached from the reactants after filtration process.
It is observed that the base catalysts are able to complete the reaction at a lower temperature at a shorter reaction time than the acid catalysts [31,32]. Activation of solid acid catalysts is completely low at low temperatures in the transesterification reaction. However, gaining higher conversion rate requires an increase in the reaction temperature (>170°C), but some solid acid catalysts are not stable enough against severe reaction temperatures. On the other hand, less surface area and less porosity may decline the performance of solid acid catalysts [33]. For instance, zirconia as an inorganic catalyst has fairly high stability at high temperatures, but its small surface area and small pore size reduce its activity. In order to enhance the activity of heterogeneous acid catalysts, they can be functionalized with other types of materials with a higher surface area [34].
16.4. Porous Materials
Generally, porous materials can be classified into three major groups by their size: micropores, mesopores, and macropores, which consist of pores with diameters less than 2 nm, between 2 and 50 nm, and more than 50 nm, respectively. The macropore channel leads to the penetration of bulky TGs into the active sites of the catalyst via mass transfer [35]. In contrast, the mesopore channel leads to the enhancement of the catalyst's active sites on the surface. Indeed, the function of each macropore and mesopore channel is to provide fast transportation channels and to enhance catalyst' active sites on the surface, respectively [36].
16.4.1. Mesoporous Material Characteristics
Utilizing heterogeneous catalysts causes a slow mass transfer in transesterification reaction. This may be attributed to their small surface area which causes low contact between the catalyst and reactants. This feature prolongs the reaction time and reduces the yield of the final product. In the last few decades, zeolites have drawn considerable interest owing to their high surface area, unique porosity, and high thermal stability. On the other hand, interrupting mass transfer (while reacting with large reactant particles, especially in liquid phase) has limited their effective utilization [37]. It is important to note that catalysts with a lower pore diameter are inappropriate for biodiesel preparation because of the low penetration of big-sized particles into the pore system [14]. Therefore, it is required to enhance the diffusion rate of reactants into the mesoporous channels to overcome these limitations.
Several research works have been carried out in the field of mesoporous materials to enhance porosity with thicker pore wall [38]. Gaining a higher pore diameter and thicker pore wall results in greater thermal stability [39,40]. The thermal stability is a considerable advantage of a mesoporous material specifically in the process of catalyst fabrication which requires high temperature stability [41]. Despite the dimensions of TG (5.8 nm), methyl oleate (2.5 nm), and glycerin (0.6 nm) [15,22] particles, the pore size of the synthesized catalysts has to be large enough to let the reactants diffuse into the pore structure of the catalyst [42]. Flexibility in the pore diameter is one of the remarkable advantage of mesoporous materials which make them favorable in catalytic reactions [43].
It is reported that the catalytic activity of mesoporous catalysts is not high at low temperatures. Therefore, a higher reaction temperature is needed to activate them [44,45]. In this regard, a microwave-assisted reactor is a promising technique that is capable of conducting transesterification at a higher temperature in a short time from hours to minutes. As a matter of fact, this attractive option vastly simplifies the process of biodiesel production in a shorter reaction time, because microwave irradiation can directly penetrate into the reactants [46–48].
16.5. Various Types of Mesoporous Catalysts
16.5.1. Mesoporous Silica Material
Mesoporous ordered silicate (mesoporous silica material, MSM) is a hexagonal ordered mesoporous structure material that has prominent properties, such as high porosity and thick pore wall. These inherited properties result in higher mass transfer through the active sites. High hydrothermal stability is another remarkable advantage which increases the demand for its application in many fields of studies [14]. So far, MSM has been functionalized by a number of metal oxides (such as Al2O3, MgO, CuO, and La2O3) to transform its function into a heterogeneous base catalyst [49]. It should be considered that interaction between the silicon element and strong alkaline destructs the mesoporous silicate structure. Therefore, weaker loading of the alkali has been recommended to avoid destroying of the mesopore channels [50].
16.5.2. Mesoporous Carbon Material
Mesoporous carbon material (MCM) could be a perfect candidate to enhance catalytic activity [51,52]. The unique surface area and uniform porosity are two excellent features of MCMs that lead to the absorption of long chains of FFAs and prevent the absorption of water [53]. High thermal stability is another important characteristic of MCMs, which promotes its application. In order to improve the catalytic activity of MCMs, they can be modified by grafting via electrochemistry [54,55], chemical reducing of aryl diazoniums, and by reduction of alkylation and arylation [56,57].
16.5.3. Mesoporous Metal Oxide Catalyst
A number of reports in the literature underscore the excellent performance of the metal oxides as filler in mesoporous materials. To date, a wide range of nanosized materials (such as Al2O3, ZnO, TiO2, and ZrO2) has been utilized as support elements in mesoporous catalyst preparations [58]. Coating nonmetal ions [59,60], doping of noble metal nanoparticles [61,62], and mixing different semiconductors [63,64] are some attractive approaches to modify the structural and textural properties of the mesoporous catalysts [65]. Nanotransition, high thermal stability, high firmness, and low-temperature deposition capacity are some significant aspects of mesoporous metal oxide catalysts. The combination of these properties makes them applicable in many fields, such as sensing of methanol, hydrogen production, sensors, solar cells, and biodiesel [66,67].
16.6. Application of Mesoporous Materials
Much attention has been focused on the application of mesoporous materials since 1990s. Some effective characteristics of mesoporous materials, such as large surface area, large and uniform pore structure, low density, and delivery capacity, have attracted the focus of the international scientific communities to utilize them for various bioapplications [68–70]. The combination of remarkable physicochemical, textural, and structural properties implies that the mesoporous solid catalysts are the most encouraging applicants in many branches of science, such as quantum dot, optic, electronic, optoelectronic, magnetic, mechanical, nanoreactor, as well as biofuel production [69,71]. Some important techniques to fabricate porous materials consist of solid-state reaction, sol–gel, coprecipitation, polymeric precursor, hydrothermal, and microwave hydrothermal.
A large number of research has been conducted on mesoporous materials to investigate the three principal considerations, including [69]:
1. synthesis of mesoporous materials for a required structure;
2. examination of textural, morphological, structural, and physicochemical characteristics;
3. functionalizing the surface of synthesized mesoporous materials for desirable applications.
16.7. Performance of the Mesoporous Catalyst
16.7.1. Catalytic Activity
Based on reports, there is modification proposed which may maximize the activity of mesoporous catalysts including [42,74,75] as suggested as follows:
1. formation of a wide number of active sites with large surface area;
2. increase accessibility of the reagent particles to active sites;
3. widen the pore diameter by shortening the length of the macro-perforated channels;
4. decrease the level of the by-products and deposition of carbon within the synthesis process.
16.7.2. Thermal Stability
The catalyst stability is an important feature that significantly reduces the cost of production in industrial scale. In order to commercialize the process of biodiesel production, the stability and particularly reusability of the catalyst should be improved. A stable catalyst never gets dissolved in the reactants, so the total quantity will remain the same at the end of the reaction [76]. After several runs, the adsorbed glycerol on the active sites prohibits interaction between the active sites and reactants, which causes deactivation of the catalyst. To some degree, it can handle with decontamination of the deposited particles by thermal degradation [42]. Furthermore, the low acid strength is another factor that causes lower thermal stability. This problem can be solved with postsulfonation treatment to attach acid functional groups to the surface of the active sites [71]. In order to increase the thermal stability of a product, we can functionalize the surface of the catalyst with phosphoric acid [77,78] or sulfuric acid (SO3/H2SO4) [79] functional groups. Postsulfonation is a promising approach that significantly enhances the thermal stability of the synthesized samples up to 500°C. Among the type of mesoporous catalyst, thermal stability of mesoporous carbon is quite high which is staying intact at temperatures as high as 1400°C.
16.7.3. Basicity and Acidity
Another prominent factor that has huge impact on catalyst performance is the basicity and the acidity of the catalyst. Acid (or basic) modification can be done by introducing a proper acid (or base) functional group to the surface of the sample [80]. For instance, postsulfonation method is an effective method that transforms the nature of materials by enhancing the aromatic chains and reducing the aliphatic rings in the presence of oxidized bands [81]. To examine the basic sites of the basic solid catalyst, an acidic molecular particle is required to be adsorbed on the surface of the origin basic sites, which is usually preferred to be carbon dioxide. The level of the adsorption of the CO2 determines the quantity of the basic sites. On the other hand, to examine the acid sites of the acid solid catalyst, a basic molecular particle is required to be adsorbed on the surface of the original acid sites, which is usually preferred to be ammonia. The level of adsorption of the ammonia determines the quantity of the acid sites. It is noteworthy that the number of active sites plays a prominent role to enhance the yield of the product even more than the surface area [82]. In a later section, we discuss about preparation of base/acid mesoporous catalyst and their catalytic activity.
16.7.4. Pore Filling
Good preservation of mesostructure is highly challenging due to pore filling process which might occur during the saturation reaction. The pore filling is a reaction that might take place as a result of sulfate species. In such cases, postcalcination should be performed to prevent pore filling [83]. Calcination is an integral part of catalyst fabrication which enhances the leaching resistance of samples via the transformation of salt into a more stable form. It also helps in activating synthesized catalyst via catalysis reaction. For instance, mesoporous CaO catalyst can simply get poisoned by hydration and carbonation in air. In this regard, postcalcination process is required to activate poisoned sample [36,84]. Thereby, postcalcination treatment is an effective way to prevent pore filling and protect the channels through the saturation–diffusion process.
16.8. The Diffusion Process of the Reactants Into Mesopore Channels
The reaction between mesoporous catalyst and adsorbed reactants usually occurs in heterogeneous catalysis. In case of catalytic transesterification reaction, reactants diffuse through catalyst surface (mainly to the internal surface of pores) into porous channels. The rate of diffusion is highly associated with the accessibility of the reactants with majority of active sites through catalyst porous channels (Fig. 16.1) [85]. The chemisorption reaction occurs in order to convert the reactant (A) to product (B) (Scheme 16.2);
16.9. Surface Modifications
Several treatments have been established to fulfil surface modification of nanocompositions. In general, surface modification can be classified into physical and chemical treatments.
16.9.1. Physical Treatment
Physical treatment is applied to cover nanoparticles with an appropriate surfactant to generate secondary forces including electrostatic, hydrogen, and van der Waals forces [86].
16.9.1.1. Surfactant
A surfactant is a lengthy aliphatic chain with one or more polar bonds. The main role of surfactant treatment is to adsorb polar-band surfactant onto the surface of the fillers through electrostatic interactions. It results in the formation of ionic bonds between the filler's surface and surfactants which consequently decreases the surface energy of filler's surface by making a shield around it. Furthermore, surfactant treatment can increase the surface area of the treated particles [86].
16.9.1.2. Dispersant
Discernments are also used to modify the surface area by encapsulating inorganic particles with in situ–formed polymers. Typically, the polymeric dispersing agent contains two major components: (1) a functional group (such as –OH, –NH2, and –COOH) and (2) soluble polymeric chains (such as polyester, polyether, polyolefin, and so on). The function of these two components helps binding of the dispersing agent to the mesopore surface via hydroxide and electrostatic bonds. It also causes dispersion of the particles in different directions to facilitate creation of homogeneous coverage [86].
16.9.2 Chemical Treatment
Significantly, surface modification through chemical interaction occurs by applying modifier to create strong chemical bonding with the matrix. Surface modification by coupling agents (such as Zn and Al) is another treatment method that provides the bonding between inorganic fillers and matrix polymer. Another advantage is the elimination of the hydrophilic properties to some extent. Furthermore, unstable nanoparticle agglomerates become more strengthened due to the transformation of nanoparticles into a nanocomposite structure. Therefore, surface modification by coupling agents is a promising approach to create a unique surface coverage onto the mesopore channels [86].
Figure 16.1 Sequential process of diffusion with heterogeneous catalyst [85].
Soltani et al. [87] hydrothermally synthesized a polymeric mesoporous ZnAl2O4 catalyst, using polyethylene glycol as surfactant and D-glucose as a template. The as-prepared catalyst was further sulfonated in order to improve the hydrophobicity through catalysis reaction, and the proposed mechanism is presented in Fig. 16.2. The optimized mesoporous SO3H-ZnAl2O4 catalyst possessed unique characteristics, such as a surface area of 352.39 m2/g, the average pore diameter of 3.10 nm, a total pore volume of 0.13 cm3/g, and high acid density up to 1.95 mmol/g, simultaneously. The combination of the mesostrucure and functionalization methods (in situ doping Al source and sulfonation process) resulted in high catalytic performance of synthesized catalyst, giving FAME yield of 94.65%. It was claimed that the polymerization step provided a better occasion for the sulfonic groups to scatter on the surface of the formed mixed metal oxide particles.
16.10. The Effect of Mesoporous Catalyst on Transesterification Reaction
Different types of mesoporous catalysts were used for transesterifying TG to biodiesel. Kazemian et al. [84] functionalized an ordered mesoporous silicate catalyst (SBA-15) with cesium species (CsNO3) in order to catalyze transesterification of canola oil, using pressurized batch stirred tank reactor. A high surface area of 628.15 m2/g was obtained with a pore volume of 0.837 cm3/g. In order to gain high conversion, the effect of different variables were examined, including different ratio of methanol-oil (20:1 and 40:1), catalyst concentration (100–200 mg), different reaction times (3–24 h), and reaction temperatures (65–135°C). The high biodiesel yield (25.35%) was obtained under optimized condition: mesoporous 2wt%-CsNO3-SBA-15 catalyst concentration of 100 mg, MeOH:oil of 40:1, reaction time of 5 h, and operating temperature of 135°C.
Albuquerque et al. [41] prepared mesoporous solid basic catalyst through impregnation with different ratios of calcium acetate CaO as a support on SBA-15, using the incipient wetness method. The catalytic activity was examined through transesterification reaction. To obtain higher conversion rate, the effects of different quantities of catalyst (0.4 and 1.6 wt.%) were examined. The highest conversion (95%) was obtained by only loading 1 wt.% of the catalyst which proves high catalytic activity of the synthesized mesoporous catalyst.
In another study, [14] impregnated surface area of mesoporous SBA-15 with KOH solution. A high surface area of 539 m2/g, average pore diameter of 5.63 nm, and pore volume of 0.63 cm3/g were recorded, which illustrates the high potential of mesoporous K-SBA-15 catalyst. The response surface methodology was established to examine the impact of different variables on the biodiesel yield. The optimized condition found 3.91 wt.% for the catalyst concentration, 11:6 for MeOH:oil ratio, 70°C for the operating temperature, and 5 h for the reaction time. The optimum reaction condition results in 93% conversion of palm oil to biodiesel through transesterification reaction. The high catalyst activity was attributed to extremely high surface area and high porosity of the prepared catalyst, which simplified the diffusion of reagents into mesostructure.
Mesoporous MgO catalyst was prepared by Jeon et al. [36] in the presence of template-MgO, using the sol–gel method as shown in Fig. 16.3. The surface areas obtained were 32.9 and 79.6 m2/g for nontemplate-MgO and template-MgO, respectively. It shows that the surface area of the template-MgO sample is 2.4 times larger than that of nontemplate-MgO. The basicity was determined with CO2-TPD which shows that the basicity of template-MgO catalyst was 2.5 times higher than that of nontemplate-MgO catalyst. The transesterification of canola oil was performed with methanol/oil molar ratio of 20:3, catalyst loading of 3%, and operating temperature of 190°C for 2 h, using batch reactor. The rapid transportation structure of the synthesized catalyst resulted in 96.5% conversion of biodiesel through transesterification reaction.
Figure 16.2 Synthesis of the sulfonated mesoporous ZnAl2O4 catalyst and FAME production [87]. Copyright 2016 Elsevier Ltd.
In another study that was carried out by [49], mesoporous silica was used in MgO applying two methods: impregnated and in situ-coated method which resulted in different physical characteristics of SBA-15 catalyst. In comparison with impregnated method, less-blocked mesopores were reported via in situ-coating method which was mainly due to smooth formation of MgOx particles on SBA-15. Also, the X-ray photoelectron spectroscopy (XPS) data indicated low level of Mg particles on the surface area of the in situ-coated SBA-15 catalysts which resulted in a higher surface area and higher pore volume in comparison with the impregnated method. The catalytic activity was examined by transesterification of blended vegetable oils. The high conversion (96%) was obtained in the presence of mesoporous SBA-15-MgO catalyst through transesterification reaction.
In another research, the sol–gel method was reported by Wu et al. [42] to impregnate K2SiO3 into mesoporous Al-SBA-15. By increasing the amount of K2SiO3, the surface area of parent material was reduced because the surface area of the support (K2SiO3) covered through the preparation process. Besides, the basicity of the synthesized mesoporous Al-SBA-15 was also increased by introducing higher concentration of potassium compounds. In order to examine the catalytic activity, transesterification was carried out, using Jatropha oil under batch condition. The highest conversion (95%) was obtained in the presence of mesoporous Al-SBA-15-K2SiO3 catalyst.
Xie and Zhao [88] hydrothermally impregnated Ca(NO3)2 and (NH4)6Mo7O24 into mesoporous SBA-15 to synthesize mesoporous CaO–MoO3–SBA-15. The effect of different postcalcination temperatures (623–823K) was examined to gain higher catalytic activity. It was found that catalytic activity was increased from 48.3% to 83.2% by increasing the postcalcination temperature from 623K to 823K.
Figure 16.3 Schematic illustration of the synthesis of the mesoporous MgO catalyst using a PDMS–PEO comb-like copolymer. Reproduced with permission from Jeon H, Kim DJ, Kim SJ, Kim JH. Synthesis of mesoporous MgO catalyst templated by a PDMS–PEO comb-like copolymer for biodiesel production. Fuel Processing Technology 2013;116:325–31. Copyright 2013 Elsevier Ltd.
Grafting method was established by Xie and Fan [89] in order to functionalize mesoporous SBA-15 with ammonium chloride organosilane. The high surface area of 810 m2/g and large pore diameter of 6.75 nm show the high potential of synthesized catalyst. The catalytic activity of mesoporous SBA-15-pr-NR3OH was examined via transesterification of soybean under various reaction conditions. A quite high yield (99.4%) was obtained under the suitable reflux condition, including: catalyst concentration 2.5 wt.%, MeOH:oil ratio12:1, and reaction time 30 min. The synthesized catalyst was recycled for several runs without even a slight drop of activity.
Xie et al. [90] synthesized SBA-15-pr-ILOH under hydrothermal condition through impregnation of 4-butyl-1,2,4-triazolium hydroxide onto SBA-15 silica. The surface area and pore size of the support were obtained at 590 m2/g and 6.64 nm, respectively. After surface functionalization, the surface area and pore diameter were significantly dropped to 341 m2/g and 5.58 nm, respectively. The catalytic activity of synthesized catalyst was examined through transesterification of soybean oil. It should be considered, however, that the textural properties were dropped in some extent after treatment, but the conversion (95.4 wt.%) confirmed high potential of synthesized mesoporous base catalyst.
16.11. Conclusion and Recommendation
Generally, small surface area is the main concern for heterogeneous catalysts, which causes a slow mass transfer through catalysis reaction. Consequently, lower contact between catalyst' active sites and reactants prolongs the reaction time. Furthermore, catalysts with a lower pore diameter are inappropriate for biodiesel preparation because of the lower penetration of big-sized particles (FFAs) into the pore system. The large surface area, uniform pore structure, and high thermal stability are the most important characteristics of mesoporous materials, which proposed them in many applications as catalysts or catalyst support. The excellent textural properties of mesoporous catalysts enhance the catalytic activity of them due to easy accessibility of the reactants into mesopores. Up to now, many scientists have utilized mesoporous catalysts in order to accelerate the process of biodiesel production. The interactions between variables such as reaction time, temperature, the amount of methanol, and catalyst loading contribute to the great potential of biodiesel production via transesterification reaction. In order to gain prominent success in the mesoporous solid materials technology, it is necessary to find out different compositions of the mesoporous materials with different physicochemical properties for different applications. It is also recommended to utilize novel metal oxide mesoporous materials rather than mesoporous silica as alternative materials for catalyzing chemical reaction.
Nomenclature/Abbreviation
FFA Free fatty acid
TG Triglyceride
WCO Waste cooking oil
ME Methyl ester
SFA Saturated fatty acid
MSM Mesoporous silica material
MCM Mesoporous carbon material
MeOH Methanol
Acknowledgments
The authors extend their appreciation to the International Scientific Partnership Program (ISPP) at King Saud University for funding this research work through ISPP# 0035.
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