香京julia种子在线播放

Metal-based oxides are the most commonly exploited as a heterogeneous catalyst for transesterification. The surface structure of a metal oxide is presented in Figure 3. Zhang et al. (1988) demonstrated the basic properties of alkaline earth metal oxides using temperature-programmed desorption (TPD) of adsorbed carbon dioxide analysis. They showed that the amount of basic sites per unit weight approximately corresponds to that of the surface area and follows the sequence: CaO > MgO > SrO > BaO. The oxides of stronger basic sites promote the reaction more effectively.

Among the metal-based catalysts, CaO has been the most studied catalyst material for biodiesel production, as it presents many advantages, namely, long catalyst life, relatively high basic strength, high activity, and low solubility in methanol and requires only moderate reaction conditions (Roschat et al., 2016; Latchubugata et al., 2018). Arun et al. (2017) studied the biodiesel production from Terminalia belerica and Garcinia gummi-gutta using calcinated CaO as a heterogeneous catalyst. The calcination process was done in a muffle furnace at 605°C for 5 h, which was preceded by drying in an oven at 125°C for 12 h. The transesterification was carried out at 60°C, 9:1 M methanol/oil ratio, and 2% w/v catalyst for 3 h. Properties of biodiesel produced from these feedstocks were within the range suggested by the American Society for Testing and Materials (ASTM) standard establishing a successful production. Roschat et al. (2016) carried out the transesterification of palm olein oil (FFAs of 0.29 mg KOH/g) using hydrated lime-derived calcium oxide (CaO) as a catalyst. The catalyst was prepared by drying overnight in an oven at 100°C, crushed and sieved, and then calcined in a furnace at different temperatures (700, 800, and 900°C) in the air for 3 h. A biodiesel yield of 97.20% was achieved for 800°C calcined sample. This catalyst could be reused for at least five times without significant yield deterioration (more than 90%).

Du et al. (2019) synthesized three novel carbon-based MgO solid bases using sol–gel and calcination method for biodiesel production from castor oil. These catalysts were identified as MgO-PVA-800, MgO-UREA-800, and MgO-GLY-800, where 800 indicates the calcination temperature, and PVA, UREA, and GLY stands for polyvinyl acetate, urea, and ethylene glycol, respectively. The transesterification was carried out at 12:1 M ethanol/oil ratio, 6 wt.% catalyst concentration at 75°C. The highest conversion rate of 96.5% was obtained with MgO-UREA-800 after 50 min under these conditions. The biodiesel yield for MgO-PVA-800 and MgO-GLY-800 was 93.6 and 91.8%, respectively, for the same reaction time. Manríquez-Ramírez et al. (2013) studied the catalyst activity of impregnated MgO with KOH, NaOH, and cerium nitrate. The basic strength of the samples followed the order: MgO–KOH > MgO–NaOH > MgO–CeO₂ as determined by Fourier-transform infrared spectroscopy (FTIR). The reaction was carried out at 60°C with oil/methanol ratio of 4:1, and the biodiesel yields after 1 h of reaction were 44, 56, 78, and <99% for MgO and MgO–CeO₂, MgO–NaOH, and MgO–KOH, respectively. It has been reported that MgO has low activity in transesterification of vegetable oils to biodiesel (Liu et al., 2007).

Roschat et al. (2018) studied the biodiesel production process of palm oil using strontium oxide (SrO) via the ethanolysis reaction. The optimum reaction condition for this work was determined experimentally as 80°C reaction temperature, ethanol/oil ratio of 12:1 M, catalyst loading amount of 5 wt.%, and reaction time of 3 h. This resulted in a yield of 98.2% fatty acid ethyl ester. The catalyst can be reused for five times with more than 90% yield. Liu et al. (2007) studied the transesterification of soybean oil to biodiesel using SrO as a solid base catalyst. They proposed the mechanism of the catalytic reaction of SrO, which creates basic intermediate compounds, thereby making it a solid base catalyzed reaction. The results showed that biodiesel yield was <95% at temperatures below 70°C within 30 min. The reusability of SrO was high and could maintain sustained activity even after 10 cycles.

Mootabadi et al. (2010) carried out ultrasonic-assisted transesterification of palm oil using different alkaline earth metal oxides such as CaO, SrO, BaO, etc. as catalysts. They varied the reaction parameters, namely, reaction time of 10–60 min, alcohol/oil ratio of 3:1–15:1 M, catalyst loading of 0.5–3%, and ultrasonic amplitudes of 25–100%. Their results opposed the previously reported activity ranking of the catalysts as CaO < SrO < BaO. At optimum test conditions, 95% yield was reached in 1 h. When non-optimized conditions were used, the yield of 77.3, 95.2, and 95.2% was reported for CaO, SrO, and BaO catalysts, respectively, for 1 h reaction time. Dai et al. (2018) prepared lithium-based catalyst, lithium–iron oxide (LiFe₅O₈-LiFeO₂) by homogeneously mixing Fe₂O₃ in an aqueous solution of Li₂CO₃ and then heating this in a muffle furnace at different calcination temperatures and cooling to room temperature afterwards. The catalysts were then used to produce biodiesel from soybean oil where reactions were carried out at 65°C for 2 h utilizing methanol/oil ratio 36:1 M and 8 wt.% catalyst loading. The maximum conversion of 96.5% was achieved when the catalyst calcination temperature of 800°C was used. Moreover, the catalyst can be reused for at least five runs, maintaining the biodiesel yields close to 94%.

Mixed metal-based oxides are predominantly used as base catalyst depending on the catalyst mixture. Their basicity can be tuned by altering their chemical composition and synthesis procedure (Teo et al., 2017). The type of synthesis method, activation temperature, and structure have a strong influence in the final basicity of the mixed oxides (Mckenzie et al., 1992; Tichit et al., 1995). They were sometimes used as a mixed acid–base catalyst by some researchers. Salinas et al. (2018) studied the catalytic activity of 1–5 wt.% La₂O₃ in ZrO₂ mixed metal oxides produced using sol–gel method and calcined at 600°C in biodiesel production from canola oil through methanolysis reaction. The sol–gel method allows for improving catalytic activity by modifying acid–base properties. It was found that La₂O₃ doped in ZrO₂ forms a monoclinic-ZrO₂ structure and enhances its basicity, which plays a key role in catalysis, and 3 wt.% La₂O₃ was the optimum percentage for the transesterification of canola oil. Limmanee et al. (2013) studied the catalytic activity of nanocrystallite CaMgZn mixed oxides as heterogeneous catalyst prepared using coprecipitation method using Na₂CO₃ as a precipitant in the synthesis of palm kernel oil biodiesel via methanolysis. They reported a maximum yield of 97.5 wt.% over the CaMgZn mixed oxide, prepared with the Ca/Mg/Zn ratio of 3:1:1 under the Na₂CO₃ concentration of 0.75 mol/L and the CaMg(CO₃)₂ and CaZn(CO₃)₂ metal ions ratio of 1.0 when the reaction conditions were methanol/oil ratio of 20:1 M, 6 wt.% catalyst and 60°C. Santiago-Torres et al. (2014) studied Na₂ZrO₃ as a basic catalyst for the transesterification of soybean oil. They reported a maximum biodiesel conversion efficiency of 98.3% at 3% of catalyst in 3 h of reaction time at 65°C reaction temperature. Lee et al. (2015) synthesized bifunctional acid–base catalyst comprising of mixed metal oxides of Ca and La (CaO–La₂O₃) at various Ca/La atomic ratios via coprecipitation. This integration enhanced the catalytic activity due to the dispersion of CaO on the composite surface, thereby increasing the surface acidic and basic sites compared to individual oxides. They reported the highest biodiesel yield of 98.76% under conditions of 160°C, 3 h, 25:1 M methanol/oil ratio, and 3 wt.% catalyst.

Titanium oxide (TiO₂) and zinc oxide (ZnO) are among the transition metal oxides that are used as a heterogeneous base catalyst for biodiesel production (Yoo et al., 2010). Kaur et al. (2018) synthesized tungsten (W) supported TiO₂/SiO₂ catalyst using the sol–gel method to study transesterification of waste cottonseed oil to produce biodiesel. The complete transesterification reaction was achieved in 4 h with 1:30 M oil/methanol ratio, 5 wt.% catalyst at 65°C. They also reported good reusability with the catalyst being active in four catalytic runs without significant reduction in activity. Madhuvilakku and Piraman (2013) studied the catalytic activity of mixed oxides of Ti and Zn (TiO₂-ZnO) and ZnO by employing these in palm oil transesterification process. Biodiesel yield of 92% was attained with 200 mg catalyst loading of TiO₂-ZnO, 6:1 M methanol/oil ratio, and 60°C reaction temperature in 5 h. They also reported a better yield compared to ZnO catalyst (83%).

Boron group-based compounds, especially alumina (Al₂O₃), are widely utilized for supporting different metal oxides, halides, nitrates, and alloys (Chouhan and Sarma, 2011). Kesserwan et al. (2020) reported on novel hybrid CaO/Al₂O₃ aerogel to catalyze the methanolysis of waste cooking oil. The catalyst was prepared by the rapid epoxide-initiated sol–gel process followed by calcination at 700°C under supercritical carbon dioxide conditions. They reported that, at optimized reaction condition of 11:1 M methanol/oil ratio, 1 wt.% of 3:1 CaO/Al₂O₃ calcined aerogel, 65°C reaction temperature, and 4 h reaction time results in a maximum biodiesel yield of 89.9%. Benjapornkulaphong et al. (2009) studied various Al₂O₃-supported alkali and alkali earth metal nitrates, namely, LiNO₃/Al₂O₃, NaNO₃/Al₂O₃, Ca(NO₃)₂/Al₂O₃, KNO₃/Al₂O₃, and Mg(NO₃)₂/Al₂O₃ produced by impregnation method followed by calcination at 450–850°C. They employed these to catalyze the methanolysis of palm kernel oil and coconut oil. They found that calcination temperature affects the yield significantly with Ca(NO₃)₂/Al₂O₃ and LiNO₃/Al₂O₃ yielding <90% biodiesel when calcined at 450°C, and higher calcination temperature dropped the biodiesel yield. They recommended a catalyst amount of 10 wt.% and 15–20 wt.% of Ca(NO₃)₂/Al₂O₃ catalyst for transesterification of palm kernel oil and coconut oil, respectively, when methanol/oil ratio of 65:1 M, temperature of 60°C, and reaction time of 3 h were used.

“Hydrotalcites are a class of anionic and basic clays with a general formulas of M1-x2+M_x³⁺ (OH₂)^x+(A_x/n)ⁿ⁻. yH₂O where M²⁺ and M³⁺ are divalent and trivalent metals, respectively, Aⁿ⁻(CO3-, SO42-, Cl⁻, NO3-) is an n-valent anion, and x usually has a value between 0.25 and 0.33” (Helwani et al., 2009; Endalew et al., 2011). Hydrotalcites are predominantly used as heterogeneous base catalyst. The advantages of these catalysts are their tunable high basic strength obtained by changing the ratio of Mg/Al and the inherent better catalyst morphology compared to alkaline earth metal oxides. However, nonhomogeneity and sensitiveness to FFA and water are the major disadvantages for this type of catalysts. Navajas et al. (2018) studied the catalytic activity of Mg–Al hydrotalcite with Mg/Al ratios of 1.5–5 M, synthesized by coprecipitation, in transesterification of sunflower oil. They reported an optimum conversion of 96% under methanol/oil ratio of 48:1 M, 1 atm, 60°C, 2 wt.% of the catalyst after 24 h. They attributed the activity of catalyst to the presence of Brønsted-type basic sites at the edges of the crystal. Nowicki et al. (2016) studied the catalytic performance of Zr-doped Mg–Al hydrotalcite with varying Zr/Mg molar ratios in transesterification of refined rapeseed oil. They found that tetravalent cation of Zr (Zr⁴⁺) was effective in significantly increasing the catalytic activity of the catalyst. They reported the optimum oil conversion of 99.9% for hydrotalcite with the ratio of Zr/Mg/Al = 0.45:2.55:1 M and the reaction condition of 373–393 K temperature, 4.8−5.0 atm pressure, and 6 h reaction time. Trakarnpruk and Porntangjitlikit (2008) studied the catalytic activity of K-loaded calcined Mg–Al hydrotalcite (Mg/AL = 4) in transesterification of palm oil with methanol. They found that the catalyst acts as a basic catalyst. They also reported an 86.6% methyl ester yield at reaction parameters of 30:1 M methanol/oil ratio, 100°C, and 7 wt.% catalyst for 6 h. Zeng et al. (2009) reported that immobilization of Saccharomyces cerevisiae lipase (a biocatalyst) on Mg–Al hydrotalcite (Mg/Al molar ratio, 4.0) by physical adsorption markedly improved performance of the enzyme. The transesterification of refined rape oil was carried out with immobilized lipase (1.5 wt.%) and under atmospheric pressure at 45°C with 96% ester conversion in 4.5 h. The biocatalyst maintained high activity with 81.3% ester conversion after 10 reuse cycles.

Waste derived from industrial processes and surrounding environment can aid in the development of a low-cost solid base catalyst. These catalysts can promote a sustainable and environment-friendly approach toward biodiesel production (Majhi and Ray, 2016; Pandit and Fulekar, 2017). Calcium-enriched waste products, namely, shells of mussel, egg, cockle, snail, and oyster; fish scales; animal bones; and ash derived from plant species, etc. are easily available at low cost (Marwaha et al., 2018). Calcium obtained from these waste materials could be converted to CaO, which is the most versatile heterogeneous base catalyst, as discussed previously. Yaşar (2019) studied the catalytic activity of waste-eggshell-based CaO and compared it to the pure CaO. Transesterification of rapeseed oil was carried out at 9:1 M methanol/oil ratio, 4 wt.% catalyst, and 60°C reaction temperature for 1 h. The maximum yield for these conditions was 96.81 and 95.12% for CaO and waste-eggshell-based CaO, respectively. Sirisomboonchai et al. (2015) studied the catalytic activity of calcined scallop shell in the transesterification of waste cooking oil using methanol. A yield of 86% was observed in the presence of small amount of water with 5 wt.% catalyst loading, 6:1 M methanol/oil ratio, and 65°C reaction temperature for 2 h. The same catalyst was used for four cycles with 20% reduction of FAME yield owing to the formation of Ca-glyceroxide on its surface. Hu et al. (2011) studied the catalyst derived from waste freshwater mussel shell in transesterification of Chinese tallow oil. The catalyst was synthesized using the calcination–impregnation–activation method. The mussel shell was first calcined at 900°C followed by impregnation in deionized water and activated by calcination at 600°C for 3 h. Over 90% yields was obtainable with 12:1 M methanol/oil ratio, 5 wt.% catalyst, and reaction temperature of 70°C in 1.5 h. The catalyst also exhibited excellent reusability with only 10–15% decrease in yield after 12 runs.

Many researchers have used cation-exchange resin for biodiesel production at laboratory scale. Cation exchange resins are macroporous and contain numerous acidic sites to catalyze FFAs to biodiesel through heterogeneous esterification reactions and prevent saponification. Fu et al. (2015) studied the catalyst activity of sulfonated polystyrene-divinyl benzene (ST-DVB-SO₃H) macroporous resin in the esterification of high FFA (acid value, 64.9 mg KOH/g) oil. Maximum FFA conversion of 97.8% was achieved when the reaction was carried out under the following conditions: 10 wt.% catalyst loading, methanol/oil ratio of 15:1 M, and 100°C for 3 h. Feng et al. (2011) studied continuous esterification of waste fried oil with an acid value of 36.0 mg KOH/g in a fixed bed reactor using commercial cation-exchange resin (NKC-9). They reported a 98% conversion rate during 500 h run under 2.8:1 methanol/oleic acid mass ratio, 44.0 cm catalyst bed height, 0.62 ml/min feed flowrate, and 65°C reaction temperature.

Heteropoly acids (HPAs) and their salts as solid (heterogeneous) acid catalysts are also used frequently for the production of biodiesel (Hanif et al., 2017; Alcañiz-Monge et al., 2018). HPAs having a Keggin structure are preferably used because they have high thermal stability and can be synthesized easily compared to other types of HPAs. However, Keggin-type HPA has a low specific surface area, which can be overcome using appropriate supportive material. HPAs supported on the carriers are used in biodiesel production because of their structural mobility and superacidity. Kurhade and Dalai (2018) studied 12-tungstophosphoric acid (TPA) (H₃PW₁₂O₄₀.nH₂O) impregnated on the γ-Al₂O₃ catalyst for the biodiesel production. An optimized conversion of 94.9 ± 2.3% can be achieved with 10 wt.% of the catalyst loading, 17.5:1 M methanol/oil ratio, 200°C, and 4 MPa in 10 h. A conversion of 90.3 ± 4.3% was achieved after the second run. Siddiquee et al. (2011) studied the catalytic activity of mesoporous ordered silica, SBA-15 impregnated with the HPA in the production of biodiesel from the lipid of wastewater sludge. The experiment was carried out in a microreactor setup under varying operating conditions. A biodiesel yield of 30.14 wt.% was obtained with 15% HPA at a temperature of 135°C and a pressure of 135 psi in 3 h reaction time.

Sulfonic acid group catalysts are characterized by sulfonated cross-linked polystyrene and are generally less corrosive and environmentally benign (Mansir et al., 2017). These type of catalysts have enhanced activity due to the attraction of fatty acids and tails of alcohol by the polymer support. In addition, the strong sulfonic acid group attached to the polymer chains increases the acidity of the sites (Vaccari, 1999). The typical examples of these catalysts are nonporous Nafion resins and porous Amberlysts. Liu et al. (2008a) studied the synthesis of mesoporous solid acid catalysts based on sulfonic acid functionalized ordered mesoporous carbons (OMC-SO₃H) to use as a heterogeneous acid catalyst for esterification of oleic acid with ethanol. The esterification was performed under an N₂ atmosphere in a closed flask at 80°C. The results showed that this catalyst is highly efficient due to its high acid density and hydrophobic surface property. Andrijanto et al. (2012) studied sulfonic acid catalysts supported on hypercrosslinked polystyrene (D5082) for the esterification of oleic acid with methanol. The results showed that D5082 had high catalytic activity despite showing low concentrations of acid sites and acid site strengths, which they attributed to the high accessibility of acid sites throughout the catalyst particles.

Sulfated metal oxides generally work as an acid heterogeneous catalyst in esterification reaction (Chen et al., 2007; Shi et al., 2016). Kaur and Ali (2015) studied the efficacy and reusability of Ce/ZrO₂-TiO₂/SO42-, a catalyst in the esterification of oleic acid with methanol or ethanol. The catalyst activity of this catalyst is a function of its Brønsted acidic sites, which depends on the cerium concentration. A 2 wt.% cerium in the catalyst, when calcined at 600°C, showed the highest catalytic activity. With 5 wt.% catalyst, 6:1 M methanol/oil ratio at 65°C and 1 h reaction time achieved <98% conversion for the esterification of vegetable oil. They also reported no significant loss in the catalyst activity until the fifth cycle. Ropero-Vega et al. (2010) studied the catalytic activity of titania sulfated with ammonium sulfate [TiO₂/SO42--(NH₄)₂SO₄] as well as sulfuric acid (TiO₂/SO42--H₂SO₄). They found that sulfated samples showed strong acidity as determined using Hammet indicators. The FTIR analysis of TiO₂/SO42--(NH₄)₂SO₄ and TiO₂/SO42--H₂SO₄ showed the presence of both Lewis and Brønsted acid sites and only Lewis-type sites, respectively. Further analysis showed very high activity for the esterification of fatty acids with ethanol in a mixture of oleic acid (79%). Up to 82.2% conversion of oleic acid was achieved after 3 h of reaction at 80°C.

Zirconium dioxide (ZrO₂), also known as zirconia, is used as both acid and the base heterogeneous catalyst. The primary nature of this catalyst is acidic, as it has strong surface acidity (Lam et al., 2010). Other derivatives of zirconia include sulfated ZrO₂ (Shi et al., 2016), metal oxides with ZrO₂ (Guldhe et al., 2017), metal-supported zirconia (Wan Omar and Amin, 2011), zirconia supported metal oxides (Kim et al., 2012), etc. Ibrahim et al. (2019) studied the catalytic activity of ZrO₂ loaded into different supports, namely, Al₂O₃, Fe₂O₃, TiO₂, and SiO₂ using hybrid sol–gel autocombustion method. Highest conversion of biodiesel of 48.6% was achieved using alcohol/acid ratio of 120:1 M, 0.1 mass% catalyst, and 120°C reaction temperature for 3 h when ZrO₂/SiO₂ was used. In addition, the catalyst could be reused five times without significant loss of catalytic activity. Guldhe et al. (2017) studied tungstated zirconia (WO₃/ZrO₂) as a heterogeneous acid catalyst for the synthesis of biodiesel for S. obliquus lipids. FTIR characterization of catalyst showed the presence of both Brønsted and Lewis acid sites. Optimized biodiesel conversion of 94.58% was achieved at 100°C temperature, 12:1 M methanol/oil ratio, and 15 wt.% of catalyst amount in 3 h. Sun et al. (2010) studied ZrO₂ supported La₂O₃ as a catalyst for the transesterification reaction of sunflower oil with methanol to produce biodiesel. The catalyst was prepared by an incipient wetness impregnation method, followed by drying at 110°C overnight and calcination at 600°C for 4 h in air. They reported that biodiesel conversion was possible due to the basic nature of the catalyst, which was determined using TPD of adsorbed CO₂ and 21 wt.% La₂O₃ loaded on ZrO₂ showed the highest basicity. The optimized test conditions were 30:1 M methanol/oil ratio, 200°C reaction temperature, and 5 h of reaction time that resulted in 84.9% biodiesel yield for each case.

Zeolites occur naturally in the form of microporous crystalline aluminosilicates interlinked by oxygen atoms. The chemical composition, pore size structure, and ion exchange properties of zeolites are responsible for their versatile catalytic behavior (De Lima et al., 2016). Zeolite framework structure contains molecular pores and channels of equal sizes, which can absorb molecules that fit into these and exclude the larger ones. This property of zeolite helps to exchange ions that in turn produce negative ion within the structure of the catalyst, thereby emerging as a base catalyst (Hattori, 1995; Mansir et al., 2017). The base strength of the alkali ion-exchanged zeolite increases with increasing electropositivity of the exchange cation. Both synthetic and natural zeolites are being pursued as promising catalysts. Zeolites have unique properties as catalysts, namely, shape selectivity, ability to maintain electro-neutrality through cation–polar molecules reversible interactions, etc. The weak basic strength and small catalytic pore diameter are the main problems reported for basic zeolite catalysts (Endalew et al., 2011). These can also be used as an acid heterogeneous catalyst.

Li et al. (2019) studied alkaline Li/NaY zeolite catalysts with different molar ratios of Li₂CO₃ to NaY zeolite in the transesterification of castor oil with ethanol. The ideal catalyst was synthesized from fly ash using coprecipitation method with ratio of Li₂CO₃ to NaY zeolite of 1:1 M calcined at 750°C for 4 h. The fatty acid ethyl ester yield of 98.6% was obtained for the reaction conditions of ethanol/oil ratio of 18:1 M, 3 wt.% catalyst, and reaction temperature of 75°C for 2 h. Du et al. (2018) studied the catalytic activity of NaY zeolite-supported La₂O₃ catalysts in the production of castor oil biodiesel. They produced the optimized catalyst, S-La₂O₃/NaY-800, by physically mixing the zeolite NaY, sodium carboxymethyl cellulose (CMC), lanthanum oxide and kaolin (mass ratio of NaY/kaolin/La₂O₃/CMC = 70:20:10:2.5). A surfactant (4 wt.%) was added to this mixture and then calcined at 800°C. Under the optimized reaction conditions: ratio of ethanol to oil of 15:1 M, 10 wt.% catalyst, and reaction temperature of 70°C for 50 min; 84.6% fatty acid ethyl ester was obtained. Doyle et al. (2016) studied the acidic catalytic activity of zeolite Y with Si/Al ratio of 3.1, in the esterification of oleic acid. The optimum oleic acid conversion using the zeolite catalyst was 85% with 6:1 M ethanol/oleic acid ratio, 5 wt.% catalyst loading, and 70°C for 1 h reaction time.