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Shrimp Shell Catalyst for Biodiesel Production Linguo Yang,† Aiqing Zhang,‡ and Xinsheng Zheng*,† Department of Chemistry, College of Science, Hua Zhong Agriculture UniVersity, Wuhan, China 430070 and Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, HuBei ProVince South-Central UniVersity for Nationalities, Wuhan, China 430074 ReceiVed March 30, 2009. ReVised Manuscript ReceiVed June 10, 2009
A high-performance and environmentally friendly shrimp shell catalyst for biodiesel production was prepared by incomplete carbonization of shrimp shell, loading KF on the resultant, and activation at a desired temperature. The dependence of catalytic activity on preparation conditions such as carbonization temperature, loading amount of KF, and activation temperature was investigated. The shrimp shell catalyst was characterized by the Hammett indicator method, thermogravimetric analysis (TGA) and differential thermal analysis (DTA), scanning electron microscope (SEM), N2 adsorption-desorption, X-ray diffraction (XRD), energy dispersive spectrometer (EDS), element analyzer, and Fourier transform infrared spectrometer (FT-IR). The catalytic performance was evaluated by the transesterification of rapeseed oil with methanol. The results indicated that the optimum preparation conditions were carbonization at 450 °C, loading KF of 25 wt %, and activation at 250 °C. The conversion reached 89.1% using the shrimp shell catalyst when the reaction was carried out at 65 °C with a catalyst amount 2.5 wt %, a methanol/rapeseed oil molar ratio 9:1, and a reaction time of 3 h. The shrimp shell catalyst possesses a porous framework structure, and its catalytic activity for the transesterification came from the active sites formed by the reaction of incompletely carbonized shrimp shell with KF during the activation process. It was found that the shrimp shell catalyst shows high catalytic activity and ecologically friendly properties, having the potential opportunity to be used in biodiesel production process as heterogeneous base catalyst.
1. Introduction Depleting supplies of fossil fuel and increasing environmental concerns have stimulated the intense search for alternative renewable fuels that are capable of fulfilling an increasing energy demand.1,2 The energy supply of the world has relied heavily on nonrenewable crude oil for more than two centuries. Because the production demand gap of fossil fuel is fluctuating worldwide, the price of conventional fossil fuel continues to rise, and the economies of importing nations suffer significant disruption. From an environmental perspective, combustion of petroleum fuels such as carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), and sulfur-containing residues are the principal causes of global warming. These concerns have driven significant investment in identifying and channeling renewable (biomass) raw materials into the manufacture of liquid fuel alternatives because the development of such biomass-based power would ensure that new technologies are available to keep pace with the need of society for new renewable power alternatives in the future. Biodiesel (fatty acid methyl esters) derived from the transesterification of vegetable oils or animal fats with methanol is a potential substitute for petroleum-based diesel fuels, due to its nontoxic, sulfur- and aromatic-free, biodegradable, and renewable features.3,4 * Corresponding author. Phone: +86-27-87281187; fax: +86-2787282133; e-mail address:
[email protected]. † Hua Zhong Agriculture University. ‡ HuBei Province South-Central University for Nationalities. (1) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (2) Huber, G.; Iborra, S.; Corma, A. V. Chem. ReV. 2006, 106, 4044– 4098. (3) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Fuel. 2005, 84, 335–340.
In fact, biodiesel has already been commercially produced from renewable resources including rapeseed, sunflower, or soy bean oil and entered into the fuel market as a diesel substitute. Current biodiesel manufacturing processes primarily employ homogeneous strong bases5-8 or acids9-11 as catalysts, because they can make biodiesel production relatively fast and afford high conversion rates. However, some serious drawbacks limit the applications of such homogeneous catalysts. For instance, the catalyst cannot be recovered and must be neutralized and separated from the methyl ester phase at the end of the reaction, with the consequent generation of a large volume of wastewater.1,12 To address these issues, conventional homogeneous catalysts are expected to be replaced by environmentally friendly heterogeneous catalysts mainly in the near future because of environmental constraints and simplifications in the existing processes. Heterogeneous catalysts could be easily separated from the reaction mixture by filtration and then reused. Also, (4) Karmee, S. K.; Chadha, A. Bioresour. Technol. 2005, 96, 1425– 1429. (5) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J. Energ. Fuel. 2002, 16, 443–450. (6) Dmytryshyn, S. L.; Dalai, A. K.; Chaudhari, S. T. Bioresour. Technol. 2004, 92, 55–64. (7) Siler-Marinkovic, S.; Tomasevic, A. Fuel. 1998, 77, 1389–1391. (8) Dorado, M. P.; Ballesteros, E.; Lopez, J. F.; Mittelbach, M. Energ. Fuel. 2004, 18, 77–83. (9) Crabbe, E.; Hipolito, C. N.; Kobayashi, G.; Sonomoto, K.; Ishizaki, A. Process Biochem. 2001, 37, 65–71. (10) Serio, M. D.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi, M.; Santacesarua, E. J. Mol. Catal. A: Chem. 2005, 239, 111–115. (11) Aranda, D. A. G.; Santos, R. T. P.; Tapanes, N. C. O. Tapanes. Catal. Lett. 2008, 122, 20–25. (12) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresourc. Technol. 2003, 90, 229–240.
10.1021/ef900273y CCC: $40.75 2009 American Chemical Society Published on Web 07/13/2009
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they are less corrosive, leading to safer, cheaper, and more environment-friendly operations. Since the catalytic activity of basic catalysts is higher than that of acid solids, they have been preferably studied.13 At the laboratory scale, many different heterogeneous catalysts have been developed for the transesterification of vegetable oils with methanol. Leclercq et al.14 studied the alcoholysis of rapeseed oil in the presence of Cs-exchanged NaX faujasites and commercial hydrotalcite (KW2200) catalysts. Suppes et al.15 achieved conversions of 78% at 513 K and >95% at 533 K for the transesterification of vegetable oils using calcium carbonate rock as a catalyst. For both of the above systems, the reaction temperature must be higher than 473 K to achieve high conversions within the time scales of the experiments. Their high catalytic activities for the transesterification of triglyceride with low molecular alcohols were attributed to the strong base characters of alkali earth oxides, such as MgO and CaO.16 However, if the solid bases of alkali earth oxides were used as the catalysts for biodiesel synthesis, then catalytic activities are lost.17-19 The poor catalytic performance of such materials could result from their low surface area17,19 and limited concentrations of edge and corner defect sites.18,19 Thus, it is desirable to develop an environmentally friendly heterogeneous catalyst with high catalytic activity for biodiesel synthesis. Shrimp shell is an excellent raw material for the preparation of catalyst, due to its wide source, low price, favorable biodegradability, and environment-friendly property. Chitin as the main component of shrimp shell can be transformed into saccharides under certain conditions. Recently, a new class of sulfonated carbons derived through incomplete carbonization of saccharides20 has been reported to have relatively high catalytic performance for biodiesel production, exhibiting interesting acid catalytic properties. In this study, a novel tristep synthetic strategy was devised to prepare shrimp shell catalyst. Shrimp shell was first incompletely carbonized, loading KF on the resultant, and followed by activation. Shrimp shell catalyst for the transesterification of rapeseed oil with methanol exhibited interesting base catalytic properties. It also had relatively high catalytic activity, good thermal and chemical stability, and can make the production of biodiesel environmentally friendly. 2. Experimental Section 2.1. Materials. Shrimp shell was purchased from a local aquatic products market. KF (99.0 wt %) was supplied by Tianjin Suzhuang chemical Reagent factory. Anhydrous methanol (99.5 wt %) was acquired from Shanghai Zhengxing chemical factory. Food-grade refined rapeseed oil supplied by Xinyang Wanfu oil Co. Ltd. was used to carry out the dehydrated pretreatment. 2.2. Catalyst Preparation. Shrimp shell catalyst was synthesized as follows: shrimp shell was first incompletely carbonized at the desired temperature for 2 h; then, KF was loaded on incom(13) Di Serio, M.; Tesser, R.; Pengmei, L.; Santacesaria, E. Energy Fuels. 2008, 22, 207–217. (14) Leclercq, E.; Finiels, A.; Moreau, C. J. Am. Oil Chem. Soc. 2001, 78, 1161–1165. (15) Suppes, G. J.; Bockwinkel, K.; Lucas, S.; Mason, J. B.; Heppert, J. A. J. Am. Oil Chem. Soc. 2001, 78, 139–145. (16) Corma, A.; Iborra, S.; Miquel, S.; Primo, J. J. Catal. 1998, 173, 315–321. (17) Lopez, D. E.; Goodwin, J. G.; Bruce, D. A.; Lotero, E. Appl. Catal., A 2005, 295, 97–105. (18) Watkins, R. S.; Lee, A. F.; Wilson, K. Green Chem. 2004, 6 (7), 335–340. (19) Reddy, C.; Reddy, V.; Oshel, R.; Verkade, J. G. Energy Fuels. 2006, 20, 1310–1314. (20) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Nature. 2005, 438, 178–178.
Yang et al. pletely carbonized shrimp shell by an impregnation method, followed by drying in air at 120 °C for 12 h; finally, the material was activated at the desired temperature for 2 h. To observe the influences of carbonization temperature, loading amount of KF, and activation temperature on the activity of the catalyst, the orthogonal design of three factors and three levels was used to elect the optimum preparation conditions. In the orthogonal design, carbonization temperature was 300, 400, and 500 °C, loading amount of KF was 15, 25, and 35 wt. % (on the mass of incomplete carbonized shrimp shell), and activation temperature was 200, 400, and 500 °C, respectively. The optimum condition of each factor can be preliminarily determined by measuring the conversion of rapeseed oil. The optimum conditions of carbonization temperature, loading amount of KF, and activation temperature were finally obtained using single factor method, respectively, by analyzing the results of further experiments on the base of the preliminary optimum condition. 2.3. Transesterification Reaction. A 250 mL two-necked glass flask with a water-cooled condenser and mechanical agitator was charged with 50 mL of rapeseed oil, 18.1 mL of anhydrous methanol (methanol/oil ratio 9:1), and shrimp shell catalyst at 2 wt % (on the mass of rapeseed oil). The mixture was vigorously stirred and refluxed at 65 °C for 3 h. After the transesterification reaction finished, the catalyst was recovered by filtration, and the residual methanol was removed from the liquid phase by decompressed distillation. Then, the resultant mixture was held in a separatory funnel for a certain time. Biodiesel product from the upper layer of the separatory funnel can be separated by decantation. The biodiesel product was quantitative analyzed in the presence of methyl salicylate as internal standard by gas chromatography using a HITACHI 163 instrument, equipped with a packed column injection system operating at 280 °C and sample size of 1 µL. The glass packed column of OV-17, 3 m in length, and 0.5 mm internal diameter, was employed, and the column temperature program was: initial temperature of 190 °C (2 min) and 10 °C/min to 260 °C (7 min). The detection system was equipped with a flame ionization detector operating at 280 °C. The carrier gas was high purity nitrogen. The amount of fatty acid methyl ester could be calculated by standard curve, and the conversion of rapeseed oil could be determined. 2.4. Catalyst Characterizations. The thermal stability of the catalyst was evaluated by NETZSH TG209 from room-temperature to 900 °C at a ramping rate of 10 °C min-1 under a flow of nitrogen. The structures of the samples were examined by means of XRD, CHNS/O analyzer, EDS, and FT-IR. XRD were performed on a Bruker D8 Power X-ray diffractometer, using Cu KR radiation, over a 2θ range from 5 to 90° with a step size of 0.04° at a scanning speed of 2° min-1. The contents of C, H, N, and S of the samples were measured by Perkin-Elmer Series II CHNS/O Analyzer 2400, and the content of O of the samples was measured by Vario Micro cube CHNS/O Analyzer. EDS was performed on an OXFORD INCA energy dispersive spectrometer. Measurement of the IR spectrum was performed on Thermo Nicolet avatar 330 FT-IR applying KBr pellet technique, with 2 cm-1 resolution over a scanning range of 400-4000 cm-1. Specific surface area of the samples was obtained using N2 adsorption-desorption at 77.3 K in Quantachrome Autosorb-1. Prior to N2 adsorption, the samples were thermally pretreated at 473 K under vacuum for 6 h. The surface structure was observed with a JSM-6390/LV scanning electron microscope (SEM) operating at 20 kV. With the basic property, strength of the basic site was determined by the indicator method21 and was expressed by an acidity function (21) Zhou, J.; Chun, Y.; Wang, Y.; Xu, Q. Catal.Today. 1999, 51, 103– 111.
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Table 1. Chemical Compositions of the Shrimp Shell and the Incompletely Carbonized Shrimp Shell sample
carbon (%)
hydrogen (%)
nitrogen (%)
sulfur (%)
oxygen (%)
calciuma (%)
phosphorusa (%)
shrimp shell incompletely carbonized material
25.93 14.38
3.92 0.25
8.29 8.15
0.69 0.46
30.68 27.44
22.68 37.50
0.86 0.91
a
The content was calculated by the content of O measured by element analysis and the ratio of its content and the content of O measured by EDS.
(H-). The used indicators were as follows: bromthymol Blue (H) 7.2), phenolphthalein (H- ) 9.8), and 2,4-dinitroaniline (H- ) 15.0).
3. Results and Discussion 3.1. Synthesis Mechanism of the Shrimp Shell Catalyst. Shrimp shell mainly comprises chitin, crude protein, fat, ash, fiber, calcium salt, and phosphate. Chitin occupies 69% of the dry weight of shrimp shell.22 Chitin, which presents mutual staggered fibrous or random network structure in the shrimp shell, grows in parallel with surface layer of shrimp shell; protein, which regards chitin as skeleton, grows along shrimp shell layer in flake; inorganic salt, which presents honeycomb porous crystal structure, fills in the interspaces between the chitin layer and the protein layer.23 After the purification of shrimp shell, the preparation process of shrimp shell catalyst mainly included incompletely carbonization, loading KF, and activation. Shrimp shell was purified to remove the moisture of shrimp shell and impurities attached to shrimp shell. By incompletely carbonization, the evaporation of small molecules resulted in the formation of porous framework material, which increased its specific surface area. It is well-known that large specific surface area was one of the necessary characteristics of excellent heterogeneous catalysts. During the incomplete carbonization process, chitin was transformed into quasiaromatic compounds,20 which were available for the next chemical modification. Considering that F element possesses the strongest electronegativity, KF possesses strong reaction ability with other compounds. Generally speaking, the thermal stability of fluoride is good, which is one of characters of excellent catalyst. Hence, KF was employed as modifier. Activation can lead to the formation of active sites by the reaction of incompletely carbonized shrimp shell with KF on the surface of shrimp shell catalyst. The results of CHNS/O analysis and EDS of samples are shown in Table 1. It was found that the organic content of shrimp shell induced by 20 wt % during the incomplete carbonization process, which was probably attributed to the decomposition of protein and the evaporation of small molecules in shrimp shell. Meanwhile, the content of H in shrimp shell during the incomplete carbonization process decreased sharply. It suggested that chitin was probably transformed into quasiaromatic compounds by elimination of small molecules in the chitin. 3.2. Catalytic Mechanism of Shrimp Shell Catalyst. Specific surface area of shrimp shell catalyst prepared at the optimum preparation conditions was 13 m2/g. From the pore size distributions (PSD) of shrimp shell catalyst and incompletely carbonized shrimp shell (shown in Figure 1), they all possessed porous frame structure. The more porous frame structure that the shrimp shell catalyst possessed, the larger specific surface area the catalyst had. With the increase of specific surface area, catalytic activity of shrimp shell catalyst increased. The basicity of shrimp shell catalyst are shown in (22) Jiang, T. Chitin; China Environmental Science Press: Beijing, 1996; p 3. (23) Jiang, T. Chitin; China Environmental Science Press: Beijing, 1996; p 9-10.
Figure 1. Pore size distribution curves for (a) incompletely carbonized shrimp shell carbonized at 450 °C. (b) Shrimp shell catalyst prepared at the optimum preparation conditions. Table 2. The Influence of Loading Amount of KF on the Basicity of the Catalyst loading amount of KF (wt %)
H- ) 7.2-9.8 (mmol/g)
H- ) 9.8-15.0 (mmol/g)
total basicity (mmol/g)
20 25 30
0.0138 0.051 0.0139
0.323 0.366 0.327
0.337 0.417 0.341
Table 3. The Influence of Activation Temperature on the Basicity of the Catalyst activation temperature (°C)
H- ) 7.2-9.8 (mmol/g)
H- ) 9.8-15.0 (mmol/g)
total basicity (mmol/g)
150 200 250 300
0.0061 0.0123 0.051 0.0093
0.269 0.315 0.366 0.286
0.275 0.327 0.417 0.295
Tables 2 and 3. Although the basicity of shrimp shell catalyst was much weaker than liquid base (seen below in Tables 2 and 3), it had a relatively high catalytic activity for the transesterification of rapeseed oil with methanol, most likely owing to the fact that its specific surface area was large and the basic sites dispersing on the surface could sufficiently contact with reactants to accelerate the transesterification reaction. On the other hand, as seen in Tables 2 and 3, the main basic sites with the H- in the range of 9.8-15.0 and the fewer basic sites with the H- in the range of 7.2-9.8 were observed. Consequently, it was likely that two types of basic sites are expected to be generated on the catalyst. The basic sites with the H- in the range of 9.8-15.0 probably came from -CH2OK, and the basic
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Scheme 1. The Transformation of Chitin Molecules during the Incompletely Carbonization Process
Figure 3. XRD patterns for (a) incompletely carbonized shrimp shell carbonized at 450 °C, (b) shrimp shell catalyst prepared at the optimum preparation conditions.
sites with the H- in the range of 7.2-9.8 probably came from solid inorganic base. Because of the evaporation of small molecules in shrimp shell and the elimination of water in or out of chitin molecules, a porous framework structure, which was probably a stereo polymer composed of six-member heterocyclic compounds, was generated from shrimp shell during the incompletely carbonization process (as shown in Scheme 1). As shown in Figure 2, the broader band at around 3450 cm-1 could be assigned to the stretching vibration O-K of R-O-K group (R: alkyl). The absorption band at around 1050 cm-1 could be attributed to the vibration of C-O of -CH2OK. Hence, KF may react with -CH2OH of the polymer to form -CH2OK during the activation process. As shown in Figure 3, compared with the XRD curve of the incompletely carbonized shrimp shell, several new peaks at 2θ ) 17.9, 28.6, 34.1, 41.4, and 81.8 appeared in the XRD curve of shrimp shell catalyst. Compared them with PDF-2 (powder diffraction file) of two common KF, we found that the new peaks were not diffraction angles of KF. It suggested that new crystal phase was generated during the activation process. Otherwise, the result of EDS indicated that the tiny white particles on the surface of the shrimp shell catalyst (shown in Figure 4b; see also Supporting Information) contained K, Ca, P, O, and F. Considering that shrimp shell contains Ca3(PO4)2 and KF was loaded onto incompletely carbonized shrimp shell, it was likely that Ca3(PO4)2 reacted with KF to form a solid inorganic base during the activation process. Figure 4. SEM imagine of (a) incompletely carbonized shrimp shell carbonized at 450 °C and (b) shrimp shell catalyst prepared at the optimum preparation conditions.
Figure 2. IR spectra of shrimp shell catalyst prepared at the optimum preparation.
3.3. Influence of Catalyst Preparation Conditions on Catalytic Activity. The preliminarily optimized preparation conditions from the orthogonal designs experiment were as follows: carbonization at 400 °C, KF loading of 25 wt %, and activation at 200 °C. The influence of each factor on catalytic activity of shrimp shell catalyst was in the following order: activation temperature, carbonization temperature, and loading amount of KF. To further study their influences on the catalytic activity, each factor was further investigated by experiments, and the results were as follows. 3.3.1. Influence of Carbonization Temperature. In the orthogonal design experiment, shrimp shell catalyst prepared by carbonization at 400 °C was a little superior to that obtained at 500 °C. Therefore, in order to further investigate the influence
Shrimp Shell Catalyst for Biodiesel Production
Figure 5. Influence of carbonization temperature on the conversion. Preparation conditions: KF loading of 25 wt %, activated at 200 °C. Reaction conditions: methanol/oil molar ratio 9:1, catalyst amount 2 wt %, reaction time 3 h, methanol reflux temperature.
Figure 6. TGA and DTA analysis of shrimp shell.
of carbonization temperature on catalytic activity, a series of catalysts were prepared at different carbonization temperature ranging from 400 to 500 °C, and employed to catalyze the transesterification of rapeseed oil with methanol to evaluate their activities. As shown in Figure 5, with the increase of carbonization temperature from 400 to 450 °C, there was significant improvement in catalytic activity because of the increase of the specific surface area of shrimp shell catalyst and the amount of active sites. The incompletely carbonized shrimp shell had large specific surface area and relatively strong reacting capacity for KF, which would result in the formation of more active sites. When carbonization temperature increased from 450 to 500 °C, there was significant decrease in catalytic activity because of the decrease of the specific surface area of shrimp shell catalyst and the amount of active sites. The specific surface area of shrimp shell incompletely carbonized at 450 and 600 °C were determined by the BET method to be approximately 13 and 7 m2/g, respectively. When shrimp shell was carbonized at an exorbitant temperature such as 600 °C, the porous frame structure of shrimp shell was destroyed, which was attributed to the fracture of the ether bond of chitin molecule and the collapse of molecular skeleton. It had no reacting capacity for KF. As a result, shrimp shell incompletely carbonized at 600 °C had no catalytic activity. The influence of carbonization temperature on catalytic activity also can be illustrated by TGA and DTA analysis of shrimp shell. As shown in Figure 6, the second weight-loss peak in the range of 260-390 °C, which was accompanied by a mass loss of 18.5%, corresponded to the evaporation of small molecules in shrimp shell and incidental formation of porous framework structure. TGA curve dropped approximately in an invariableness speed in the range of 360-580 °C, which was
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Figure 7. Influence of loading amount of KF on the conversion. Preparation conditions: carbonized at 450 °C, activated at 250 °C. Reaction conditions: methanol/oil molar ratio 9:1, catalyst amount 2 wt %, reaction time 3 h, methanol reflux temperature.
probably attributed to the formation of conjugated molecules20 by the elimination of water molecule and other molecules in the chitin molecule. The third weight-loss peak in the range of 610-770 °C was ascribed to the damage of the porous frame structure of shrimp shell and the disintegration of the chitin molecule. The TGA and DTA analysis of shrimp shell showed that porous framework structure was formed in the range of 260-360 °C and accompanying specific surface area increased gradually. Conjugated molecules that supplied material base for the formation of active site were formed in the range of 360-580 °C. Hence, catalytic activity increased with the increase of carbonization temperature up to 450 °C. While carbonization temperature increased continually beyond 450 °C, catalytic activity decreased, which corresponded to the damage of porous frame structure. 3.3.2. Influence of Loading Amount of KF. A series of catalysts with KF loading ranging from 20 to 30 wt % were prepared and used to catalyze the transesterification reaction to evaluate their activities so as to further study the effect of loading amount of KF on catalytic activities on the base of the result of orthogonal design experiment. As illustrated in Figure 7, the highest conversion was achieved at KF loading of 25 wt %. With the rise of loading amount of KF from 20 to 25 wt %, the conversion increased gradually, mainly because the more loaded KF, the more active sites was formed by the reaction of incompletely carbonized shrimp shell with KF. When the amount of loaded KF was beyond 25 wt %, the conversion was decreased, most likely owing to that the excess KF would cover the basic sites on the surface of the catalysts and cause a lower catalytic activity. The influence of loading amount of KF on catalytic activity could result from the basicity of catalysts. The basicity of shrimp shell catalysts prepared at different loading amount of KF was shown in Table 2. The maximum basicity was obtained at loading KF of 25 wt %. With further increase in loading amount of KF, the basicity decreased. Comparing Figure 7 with Table 2, it can be readily observed that the activity of the catalysts for the transesterification was correlated closely with their basicity as determined by the Hammett method. 3.3.3. Influence of ActiVation Temperature. The dependence of the catalytic activity on activation temperature was further studied on the base of orthogonal design experiment. As shown in Figure 8, with the rise of activation temperature from 150 to 250 °C, the conversion increased steadily, probably owing to that the active sites increased with the increase of activation temperature. The reaction of incompletely carbonized shrimp with KF depended on reaction temperature. However, the
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Figure 8. Influence of activation temperature on the conversion. Preparation conditions: carbonized at 450 °C, loading KF of 25 wt %. Reaction conditions: methanol/oil molar ratio 9:1, catalyst amount 2 wt %, reaction time 3 h, methanol reflux temperature.
conversion decreased when activation temperature increased from 250 to 300 °C, which was probably ascribed to the damage of the active sites. As illustrated in Figure 4b, there were several white particles on the surface of shrimp shell catalyst, which was probably formed by the reaction of incompletely carbonized shrimp with KF during the activation process. Comparing panels a and b from Figure 4, it was found that several tiny white particles appeared on the surface of shrimp shell catalyst. The incompletely carbonized shrimp shell has no catalytic activity. Hence, the tiny white particles were likely to be the active component. The tiny white particles were determined by OXFORD INCA EDS. The result indicated that the contents of K, Ca, P, O, and F in the tiny white particles were 5.51, 20.56, 1.41, 51.86, and 20.66%, respectively. To further investigate the influence of activation temperature on catalytic activity, shrimp shell catalyst was characterized by the Hammett indicator method and TGA. The basicity of catalysts obtained at different activation temperatures was measured by the Hammett indicator method, and the results were presented in Table 3. When activation temperature increased from 150 to 250 °C, the catalyst basicity (total basicity) was markedly increased and came up to the maximum value at 250 °C. However, when the activation temperature was beyond 250 °C, the basicity of catalyst decreased. Comparing Figure 8 with Table 3, it was obvious that the change in the catalytic activity of shrimp shell catalyst was well correlated to the change of their basicity determined by the Hammett method, suggesting that the catalytic activity was dependent on the basicity of the catalyst if the base strengths remained constant. In addition, the thermal behaviors of incompletely carbonized shrimp shell and shrimp shell catalyst are shown in Figure 9. Compared with Figure 9a, there were three new weight-loss peaks in the ranges 35-100, 100-160, and 390-610 °C in Figure 9b. The first and second weight-loss peaks were attributed to the evaporation of moisture by physisorption and chemisorption during airexposure. The third weight-loss peak in the range of 390-610 °C was probably attributed to the decomposition of the particles formed during the activation process. As presented in Figure 9b, it was observed that there was no mass loss at transesterification reaction temperature of 65 °C and there was only mass loss of 8.2% at 600 °C. Moreover, the catalyst was insoluble in common chemical solvents such as water, methanol, ethanol, ether, chloroform, acetone, cyclohexane, and benzene. Therefore, the thermal and chemical stability of the catalyst were good. Otherwise, when shrimp shell catalyst prepared by carbonization at 450 °C, KF loading of 25 wt %, and activation at 250 °C was employed to catalyze the transesterification of rapeseed oil with methanol, and the reaction was carried out at 65 °C,
Figure 9. TGA and DTA analysis of (a) incompletely carbonized shrimp shell carbonized at 450 °C and (b) shrimp shell catalyst prepared at the optimum preparation. Table 4. Effect of Repeated Use of Shrimp Shell Catalyst on the Conversion of Transesterificationa repeated times
1
2
3
4
shrimp shell catalyst
89.1%
85.6%
73.5%
50.2%
a Catalyst amount, 2.5 wt. %; methanol/oil molar ratio, 9: 1; reaction temperature, 65 °C; and reaction time, 3 h.
with a methanol/rapeseed oil molar ratio 9:1, a catalyst amount of 2.5 wt %, and a reaction time of 3 h, the highest conversion of 89.1% can be achieved. Shrimp shell catalyst exhibited relatively high catalytic activity. For instance, the catalyst organic quaternary ammonium functionalized silica gel (QN+OH-/ SiO2) was used to catalyze the transesterification of triacetin and methanol, and the reaction was carried out at 60 °C, a catalyst amount 0.88 wt %, a methanol/oil molar ratio 6:1, and a reaction time of 4 h, around 60% was achieved.24 3.4. The Deactivation and Reusability of Shrimp Shell Catalyst. The reusability of shrimp shell catalyst prepared at the optimum preparation conditions was investigated by carrying out subsequent reaction cycles. The catalyst after 3 h transesterification reaction was separated from the reaction mixture and used directly again in a second reaction cycle under the same reaction conditions as before. The results for consecutive reaction cycles were shown in Table 4. As seen in Table 4, the catalytic activity decreased through consecutive reaction cycles, which was probably attributed to some active sites of shrimp shell catalyst being covered by the resultant. To prove the speculation, shrimp shell catalyst after 3 h transesterification reaction was treated in Soxhlet’s apparatus with petroleum ether as solvent for 3 h, and used again with (24) Liu, Y.; Lotero, E.; Goodwin, J. G., Jr.; Lu, C. J. Catal. 2007, 246, 428–433.
Shrimp Shell Catalyst for Biodiesel Production
Figure 10. Effect of repeated use of shrimp shell catalyst on conversion. The used shrimp shell catalyst was treated in Soxhlet’s Apparatus before the next reaction cycle. Reaction conditions: methanol/oil molar ratio 9:1, catalyst amount 2.5 wt %, reaction time 3 h, and methanol reflux temperature.
fresh reactants in the next reaction cycle under the same reaction conditions as before. After this treatment, the catalyst mostly recovered its original activity. As shown in Figure 10, it largely maintained activity even after being used for 10 cycles, and the conversion only decreased 13%. 4. Conclusions Considering that it is biodegradable and environmentfriendly, shrimp shell is an excellent raw material for the
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preparation of catalyst. Shrimp shell catalyst was prepared by incomplete carbonization of shrimp shell, loading KF on the resultant, and subsequent activation. The preparation process of shrimp shell catalyst was simple and environmentally friendly. Shrimp shell catalyst exhibited relatively high catalytic activity, good thermal and chemical stability, and interesting base catalytic property. The transesterification process that involved shrimp shell catalyst lead to minimum pollution and wastes, and the catalyst itself was environmentally benign. The catalyst prepared by carbonization at 450 °C, KF loading of 25 wt %, and activation at 250 °C, was found to be the optimum, which exhibited the best catalytic activity for the transesterification. When the reaction was carried out at 65 °C with a catalyst amount of 2.5 wt %, a methanol/rapeseed oil molar ratio of 9:1, and a reaction time of 3 h, the highest conversion of 89.1% can be achieved. This finding provides opportunities for obtaining a novel environmentally friendly catalyst for biodiesel production. Acknowledgment. This research was supported by Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Wuhan, PR China. The author also thanks Dr. Hong Yuan for useful discussions. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. EF900273Y
