Heteropolyacid (H3PW12O40)-impregnated mesoporous KIT-6 catalyst for green synthesis of bio-diesel using transesterification of non-edible neem oil
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Mesoporous Ia3d cubic structured KIT-6 support was prepared by hydrothermal strategy and heteropoly acid content (10, 20 and 30%) was stacked on KIT-6 by wet-impregnation technique. The synthesized catalysts were characterized by XRD, N2-sorption, NH3-TPD, ATFT-IR, TEM and SEM–EDAX analyses. Acid properties of the supported catalysts were investigated by pyridine-adsorbed ATFT-IR and NH3-TPD, respectively. Poly-anion coordination present in the catalyst was confirmed by the DRS-UV spectrum. The dispersion of heteropoly acid on the catalyst surface was observed by TEM analysis. Also, the presence of the elements such as W and P on the catalyst and its morphology were represented by the HRSEM–EDAX technique. The catalytic activity of HPWA/KIT-6 was investigated by the transesterification of neem oil with methanol. Among the different wt% catalyst, 20% catalyst showed highest neem oil conversion and selectivity at an optimized reaction temperature of 60 °C. Further, the spent catalyst was recovered and recycled three times, and it showed activity losses of less than 4%.
KeywordsMesoporous KIT-6 Heteropolyacid Non edible neem oil Transesterification Biodiesel
The expansive usage of diesel fuels from petroleum derivative has made a serious ecological issue, which is environmental change and global warming . Besides, the consuming of diesel fuel could discharge the toxin gasses, which are more destructive to the human well-being . Further, energy security is considered as a fundamental factor in the decline of petroleum resources. Because of these environmental difficulties, sustainable and clean energy sources are pulled in enthusiasm for the worldwide . The current study demonstrates that the usage of biofuel from sustainable resources can essentially lessen the emission of ozone harming substances, like greenhouse gases (GHG) by utilizing as a mix part of transportation fuel [3, 4]. Moreover, the significance of using biodiesel is its feasibility to reduce dependence on diesel. The mono-alkyl esters of long-chain fatty acids are called biodiesel, which is obtained from edible oil, non-edible oil and animal fats by the trans-esterification process . Among the sustainable sources, non-edible oil sources are financially ideal in biodiesel generation, and it does not compete with the existing edible resources .
In this consideration, neem oil was an inexpensive feedstock for biodiesel production that effectively reduces production cost. Neem oil appears light to dark brown in color, has the strong pungent odor, and contains triglycerides with a huge quantity of triterpenoids, which was accountable for the bitter flavor. Also, it contains some steroids like campesterol, beta-sitosterol, and stigmasterol . The composition of neem oil fatty acids was Palmitic (12.01 wt%), Stearic (12.95 wt%), Oleic (34.09 wt%), Linoleic (38.26 wt%) and Lignoceric (0.3 wt%) and their acid value was 44 mg KOH/g of oil . Moreover, it was widely available in India and its subcontinent. The inedible neem oil was a promising alternative for edible oil. Further, the neem oil obtained from the neem tree was simply grown in any climate, and its seed contains 40% oil, which was more potential for biodiesel production . Besides, the high content of monosaturated fatty acid in neem oil has shown good biodiesel characteristics . However, the possible use of this oil in India is about 1 million tons per year .
Biodiesel synthesized by transesterification of vegetable oils and animal fats utilizing homogeneous acid and base catalyst has seen a fold augment in the most recent couple of years for their trade and use as a mixing part in transportation fuels. Indeed, the usage of homogeneous acid and base catalysts has created serious impediments in the reaction via corrosion and soap formation [11, 12]. Therefore, commercial biodiesel production utilizing ecologically well-disposed technique using heterogeneous catalysts has a significant interest in the industry. The production of biodiesel from non-edible oil via transesterification has been examined over numerous heterogeneous catalysts including metal oxides, sulphated zirconia, tungstated zirconia, etc. [13, 14]. However, these catalysts were suffered by mass transport because of their limited pore size and pore volume .
Concerned over this issue, it has focused attention on the mesoporous solid acid catalyst for biodiesel production because of their significant properties such as acidity and textural characteristics. Moreover, the mesoporous KIT-6 support has a three-dimensional cubic morphology with interpenetrating bicontinuous channels, which could be a superior catalytic support for solid acid catalyst synthesis [16, 17]. However, Heteropolytungstic acid (HPWA) is a strong Bronsted acid, which is significant in acid-catalyzed reactions and paid more attention to environmental protection and economic concern because of their non-toxic and inexpensive nature . Furthermore, compared with mineral acids, HPWA showed higher catalytic activity for a wide verity of catalytic reactions [19, 20, 21]. However, in homogeneous systems, it highly suffered from the disadvantages like difficulty in separation of the catalyst from the reaction mixture and inconvenience for continuous production. These issues were overcome using HPWA-supported KIT-6 heterogeneous catalyst, synthesized by the hydrothermal and impregnation method. The impregnation of HPWA on KIT-6 support could significantly alter the textural properties of the catalyst. Furthermore, in the literature, various acid catalysts have been studied for neem oil transesterification, in which considerable activities were observed [22, 23, 24, 25, 26, 27, 28]. However, the use of these acid catalysts has been of less interest because of their toxicity, non-reusability, and restricted stabilities. Therefore, this problem has been recovered by heteropoly acid-supported KIT-6 catalyst, which has superior catalytic properties, i.e., resistance against pore blockage, high surface area, thicker pore walls, and interconnected 3D mesopores, which enhance the diffusion of the bulky molecule .
The present work reports for producing biodiesel via transesterification of neem oil using methanol as co-reactant. Further, a various weight percentage of H3PW12O40 (10, 20, 30%) was impregnated on KIT-6 and it was characterized by different physicochemical techniques. Moreover, its catalytic activity was examined by liquid-phase transesterification of neem oil under different experimental conditions. Besides, various reaction parameters such as temperature, time, catalysts amount, and methanol, to oil ratios have been optimized. Finally, the recyclability of the optimized catalyst has been studied and its activity has been evaluated under optimized reaction condition. Therefore, this study could afford significant information on properties of HPWA/KIT-6 catalyst for improving the biodiesel yield via neem oil transesterification.
Triblock copolymer (Pluronic P123, EO20 PO70 EO20; Aldrich: Mol. Wt. 5800), tetraethylorthosilicate was obtained from Aldrich and Merck (98% pure). Hydrochloric acid, methanol, and n-butanol were obtained from SRL Biochem (India) Ltd, which was an AR-grade chemical. Commercial heteropoly acid such as H3PW12O40·nH2O was obtained from SRL Biochem (India) Ltd. Neem oil was purchased from the local market. All the chemicals were used as such without treatment.
Preparation of the catalysts
Mesoporous KIT-6 support was synthesized hydrothermally with molar ratios of 1 TEOS: 0.017 P123:1.83 HCl (35%):1.3 n-BuOH: 195 H2O . In a polypropylene bottle, pluronic P123 4 g, 144-g distilled water, and 7.9 g of 35% HCl were mixed and stirred for 4 h at 35 °C to an obtained homogeneous solution. And then, 4-g n-butanol as a co-solvent was added and stirring continued for another 1 h. After that, 8.6-g TEOS was added to the mixture and it was stirred for 24 h at 35 °C. Then, the whole mixture was placed in an air oven at 100 °C for 24 h. The obtained product was filtered and dried overnight in an air oven at 100 °C. The dried material was powdered and calcined at 540 °C for 8 h under the air atmosphere to remove the template.
Different (10, 20, 30%) weight percentage of HPWA were loaded on support by wet-impregnation method using dodecatungstophosphoric acid as the precursor. Pre-dried KIT-6 1 g and 20-mL deionized water were taken in a round bottom flask and stirred for few minutes. A desired amount of HPWA (e.g., 0.1 g for 10 wt%) was weighed and dissolved in 20-mL deionized water and this solution was added drop by drop to support material and stirred for 6 h. The residue was filtered and washed with deionized water. Finally, the impregnated material was dried at 100 °C and calcined in air at 300 °C for 5 h . The obtained material is represented as X % HPWA/KIT-6 (X = 10, 20, 30%).
XRD pattern of calcined support and catalyst was analyzed by XRD (Philips, Holland) using nickel-filtered Cu Kα radiation (λ = 1.5406 Å). The surface area of the catalyst was measured by the ASAP-2010 volumetric adsorption analyzer manufactured by the micrometrics corporation (Norcross, GA). Before measurement, each sample was degassed at 350 °C of pressure 10−5 Torr for 12 h in an outgassing station of the sorption apparatus. The nitrogen-sorption isotherms were obtained by BET method over different relative pressures; the pore size measurement and wall thickness were obtained by BJH method using nitrogen-sorption isotherms. The total acid content of all the HPWA-supported catalyst was measured by temperature-programmed desorption of ammonia (NH3-TPD) in a flow reactor (micrometrics instrument corporation chemisoft TP × V1.02 unit 1-2750). The sample was pre-treated at 500 °C for 1 h in a flow of Helium, and then cooled to 100 °C with 10 vol.% NH3 adsorption for half an hour. Further, the physisorbed ammonia was removed by purging with Helium until the baseline was stable at 100 °C, and then desorption started from 100 to 600 °C. The Bronsted and Lewis acid sites on the impregnated catalysts were analyzed by ex situ pyridine-adsorbed AT-FTIR technique using Perkin Elmer Spectrum Two spectrophotometer. About 100 mg of the samples were dried at 100 °C for 1 h. After drying, the sample was wetted with 0.3 mL of pyridine for 5 h. To remove the physisorbed pyridine from the wetted sample, it was heated at 120 °C for 1 h. After that the pyridine chemisorbed sample was mixed with KBr pellet, and the pellet was placed into the IR cell for recording the spectrum. The metal concentration in the impregnated catalyst was calculated by inductively coupled plasma optical emission spectrometry (ICP-OES) Perkin Elmer Optima 5300Dv instrument. Thermal gravimetric analysis (TGA) of support and catalysts was done on the waters TA instrument SDTQ 600. Surface morphology and elemental composition present in the catalyst were analyzed by HR-SEM with EDX using FEI Quanta FEG 200 instrument. The TEM images of support and HPWA-supported catalyst were analyzed by TECHNAI 10-Philphs instrument. The samples for TEM analysis were prepared by ultrasonic dispersion of catalyst in acetone for 1 h and dispersed samples were placed on a copper grid and samples were dried till the solvent was evaporated and further analyzed by TEM instrument. A diffuse reflectance UV–visible spectrum of the sample was recorded using JASCO V650 spectrophotometer and BaSO4 was used as the reference.
The activity of the catalyst was evaluated by the transesterification of neem oil using a liquid-phase reactor. In a typical transesterification reaction, 10 g of neem oil was added in 3 volumes of methanol with 0.35 g of the pre-treated catalyst. The reaction was carried out at 50–65 °C with reaction time 1–5 h; at the end of the reaction, the reaction mixture was cooled to room temperature and the catalyst was separated by filtration. The progress of the reaction was monitored by TLC. The crude reaction mixture was transferred to the water/n-hexane mixture of 1:3 ratios. The reaction mixture was separated into two layers using separating funnel. The lower layer consisted of water, glycerol, and methanol mixtures; whereas, the upper layer being the fatty acid alkyl esters, the un-reacted oil and intermediate products. After the removal of a solvent from the organic phase, the resulting composition was analyzed by the gas chromatograph (ShimazuGC-17A) equipped with an FID detector using (Rtx-5; 30 m × 0.25 mm id) column and the product was confirmed by 1H NMR spectroscopy.
Results and discussion
X-ray diffraction study
Textural properties, surface acidity, and metal content of support and supported catalysts
XRD despacinga (Å)
ICP–OES Wg (wt%)
Nitrogen-sorption isotherms and their pore size distribution for the KIT-6 and HPWA-supported KIT-6 materials are shown in Fig. 2a, b, respectively. The BET surface area, average pore size and pore volume of all the prepared materials are shown in Table 1. The isotherms were assigned as type IV with capillary condensation of pore steps that occurred at 0.6–0.8, which was the characteristic of a large pore mesostructured material . The surface area for KIT-6 support showed 704.6 m2 g−1. However, the surface area decreased in HPWA-supported catalysts on comparison with KIT-6 support, which may be due to the bulk HPWA species located on the external surface as well as on the pores of the catalyst . The pore size distribution of KIT-6 and HPWA-loaded KIT-6 was calculated by the BJH method. The H1 broad hysteresis loop revealed that materials have good structural ordering with narrow pore size distribution . However, the pore diameter was a maximum of 9.3 nm in KIT-6 support, whereas HPWA-supported catalyst shows 8.1 and 8.2 nm, respectively. The decreasing of pore diameter in HPWA-supported catalyst may be due to the partial pore block of support by bulk HPWA species.
Pyridine-adsorbed AT-FTIR spectroscopy study
Diffuse reflectance UV–visible spectroscopy (DRS-UV)
HR-SEM with EDAX analysis
Transmittance electron microscopy (TEM) study
Effect of heteropoly acid loading over KIT-6
Effect of reaction temperature
Effect of reaction time
Effect of MeOH/oil molar ratio
Recyclability of 20% HPWA/KIT-6 catalyst
Yield of FAME (%)
Three different weight percentages of heteropoly acid-supported mesoporous KIT-6 catalysts were synthesized by wet-impregnation method, characterized and applied to liquid-phase neem oil transesterification to fatty acid methyl ester. The order of trans-esterification activity was 20% HPWA/KIT-6 (90%) > 30% HPWA/KIT-6 (82%) > 10% HPWA/KIT-6 (70%), respectively. About 90% conversion and high FAME yield were observed in 20% HPWA catalyst. Furthermore, the catalyst possesses the nature of Bronsted acid sites, large surface area, pore size, and fine dispersion of small HPWA clusters on the surface was responsible for higher neem oil conversion, further these results well agreed with N2-Sorption and TEM analysis. Besides, the higher loading of 30% HPWA catalyst could not give higher conversion when compared with 20% HPWA catalyst, because their low surface area and bulk species of HPWA highly block the pore size of the catalyst. The leaching study of the 20% HPWA catalyst, combined with the analysis of leached tungsten, indicated that the tungsten species practically leached as maximum as 4% up to three cycles in the reaction mixture under hydrothermal reaction condition.
The author acknowledges UGC-BSR, New Delhi, India for providing financial support to carry out this research. Also, he thank the Department of Chemistry, IIT Madras, Chennai for providing instrumentation facility.
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