Catalytic Properties of Lithium-Doped ZnO Catalysts Used for

Oct 6, 2007 - Biodiesel produced by the transesterification of vegetable oils or animal fats with short-chain alcohols (typically methanol) is a promi...
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Catalytic Properties of Lithium-Doped ZnO Catalysts Used for Biodiesel Preparations Wenlei Xie,* Zhenqiang Yang, and Hong Chun School of Chemistry and Chemical Engineering, Henan UniVersity of Technology, Zhengzhou 450052, People’s Republic of China

Biodiesel produced by the transesterification of vegetable oils or animal fats with short-chain alcohols (typically methanol) is a promising alternative fuel for diesel engines, because of the limited resources of fossil fuels and environmental concerns. In this work, Li/ZnO catalysts were prepared using an impregnation method followed by calcinations, and then they were tested for soybean oil transesterification. It was determined that Li/ZnO catalysts exhibited good catalytic activities, and the catalytic performance was greatly dependent on (i) the loading amount of lithium and (ii) the calcination temperature. This Li/ZnO catalyst, at an amount of 5 wt %, resulted in a soybean oil conversion of 96.3% in 3 h using a reflux of methanol and a 12:1 molar ratio of methanol to oil. Moreover, the catalyst was characterized using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, thermogravimetry-differental thermal analysis (TG-DTA), and the Hammett titration method. It was shown that the activity of the catalysts for the transesterification reaction is closely related to their basic properties. 1. Introduction Alternative fuels for diesel engines have become increasing important, because of diminishing petroleum reserves and the environmental consequences of exhaust gases from petroleumfueled engines. Biodiesel, which consists of fatty acid methyl esters and is produced via the transesterification reaction of vegetable oils or animal fats with short-chain alcohols (usually methanol), has attracted considerable attention during the past decade as a renewable, biodegradable, and nontoxic fuel.1-3 Currently, the transesterification of vegetable oils with methanol is commonly performed using acidic or basic catalysts. Acids used for the transesterification include sulfonic and hydrochloric acids. Although the transesterification by acid catalysis is much slower than that by alkali catalysis and a higher molar ratio of alcohol to oil is required,4 acid-catalyzed transesterifications are more suitable for vegetable oils that have relatively high free fatty acid contents and more water.5,6 Most biodiesel is prepared using homogeneous basic catalysts, such as potassium hydroxide or sodium hydroxide, as well as potassium and sodium alkoxides,7-9 because the process is faster and the reaction conditions are moderated. In this conventional homogeneous method, however, the removal of these base catalysts from the products after reaction entails further treatment of alkaline wastewater, thus leading to multiple process steps. Moreover, the utilization of the homogeneous base catalysts in vegetable oil transesterification can produce soaps by neutralizing the free fatty acid in the oil or triglyceride saponification. The soaps could promote the formation of stable emulsions that prevent the separation of biodiesel from the glycerin during processing, resulting in product loss and problems involving product separation and purification. More recently, alternative methods for the preparation of biodiesel include the enzymatic and supercritical transesterification. The enzymatic reactions, using lipase as a catalyst, do not produce side reactions,10,11 and the enzyme can be immobilized in the support materials.12 The drawbacks of enzy* To whom correspondence should be addressed. Tel.: +86 371 67789524. Fax: +86 371 67789524. E-mail address: [email protected].

matic methods are low space velocity and a relatively complex workup of the reaction mixture.13 In addition to this, the enzyme is very expensive and has an unstable activity. On the other hand, the method using supercritical alcohols has also attracted great interest and has been investigated extensively in recent years.14-17 However, this method requires high temperature and high-pressure conditions (such as 573 K and 20 MPa, respectively). Therefore, new catalytic materials are extremely desirable for the development of an environmentally benign process and the reduction of production cost in modern catalysis. During the past decade, increasing attention has been paid to heterogeneous catalysts, which could improve the synthesis methods by eliminating the additional process steps associated with homogeneous catalysts and minimizing the production of pollutants. For this reason, many different heterogeneous base catalysts such as Na/NaOH/γ-Al2O3,18 KI/Al2O3,19 and Li/CaO20,21 have been determined to be efficient for the transesterification reaction. In this work, a solid base catalyst of ZnO loaded with lithium was prepared via an impregnation method, followed by calcinations, and then adopted for the preparation of biodiesel. The catalytic efficiency of the catalysts in the transesterification of soybean oil with methanol was investigated in detail, and more attention was given to the effects of calcination temperature and lithium loading amounts. In addition, the transesterification conditions (such as the amount of catalyst, the molar ratio of methanol to oil, and the reaction time) were optimized with the Li/ZnO catalyst. Furthermore, the formation of the basic sites on the catalyst was investigated using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry-differential thermal analysis (TG-DTA), infrared (IR) spectroscopy, and the Hammett indicator method, in an attempt to correlate the catalytic activity with the basic properties of the catalyst. 2. Experimental Section 2.1. Catalyst Preparation. Zinc oxide, with particle sizes of ∼60 nm, was used as a support; it was purchased from

10.1021/ie070597s CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

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Luoyang Chemical Regent Factory (Luoyang, PRC). The support, in powder form and pretreated at 873 K for 3 h, was used as a starting material. An alkali-metal nitrate, such as LiNO3, was loaded onto the zinc oxide via an impregnation method from aqueous solutions, followed by drying at 393 K overnight. Prior to each reaction, the catalyst was calcined at desired temperatures (typically 873 K) in air for 5 h. After calcination, the lithium nitrate that was impregnated in the support decomposed to lithium oxide. The lithium loading amounts were calculated based on the amounts of the initial materials. Unless otherwise noted, the lithium was loaded at a dosage of 3 mmol/g support. 2.2. Catalyst Characterization. The powder XRD patterns were recorded on a Rigaku D/MAX-3B diffractometer, using Cu KR radiation (λ ) 0.1548 nm) over a 2θ range of 10°-80° with a scanning speed of 5°/min at an accelerating voltage of 40 kV and current of 50 mA. The XRD phases present in the samples were identified using the Joint Committee of the Powder Diffraction Standards (JCPDS) database files. The mean catalyst size was estimated from the half-height width of the characteristic diffraction peak for ZnO at 2θ ) 36.3° (using the Scherrer equation) and a Gaussian shape factor of 0.94. However, the results were used only for relative comparison of the mean catalyst size. The XPS measurements were performed with a PHI 1600 spectrometer under ultrahigh vacuum (UHV) conditions (base vacuum of ∼10-8 Pa) at room temperature. Mg KR radiation (hν ) 1253.6 eV) was used as the X-ray source. The elemental binding energies (BEs) were referenced to C 1s at 284.6 eV. The KBr pellet technique was applied to determine the IR spectra of the catalysts. Spectra were recorded on a Shimadzu IR-Prestige-21 spectrometer with 4 cm-1 resolution. The scanning range was from 400 cm-1 to 4000 cm-1. The thermal behavior of the catalysts was evaluated by a Shimadzu DT-40 differential thermal analyzer (DTA) and thermogravimetric analyzer (TG) operating under a flow of air at a heating rate of 10 K/min, up to 1073 K. The basic strengths of the catalyst (H-) were determined with various Hammett indicators, and the base amount (described as basicity) was assessed using Hammett indicator-benzene carboxylic acid (0.02 mol/L anhydrous methanol solution) titration.21,22 Using different Hammett indicators, the distribution of the basic sites was determined, according to their basic strength. The following Hammett indicators were used: bromothymol blue (H- ) 7.2), phenolphthalein (H- ) 9.8), 2,4dinitroaniline (H- ) 15.0), and nitroaniline (H- ) 18.4). 2.3. Transesterification Procedures. Commercial ediblegrade soybean oil was obtained. According to gas chromatography (GC) analysis (Shimadzu DC-9A), the fatty acid compositions of the used soybean oil were as follows: palmitic acid, 12.3%; stearic acid, 5.8%; oleic acid, 26.5%; linoleic acid, 49.4%; and linolenic acid, 5.9%. The acid value was H- > 15.0 18.4 > H- > 15.0 18.4 > H- > 15.0 18.4 > H- > 15.0 18.4 > H- > 15.0 18.4 > H- > 15.0

95.2 8.8 17.5 90.0 86.9 55.0

a Reaction conditions: molar ratio of methanol to oil, 12:1; catalyst amount, 5 wt %; reaction temperature, 338 K; and reaction time, 5 h. b The loading amount of alkali metal was 2.0 mmol/g support.

was filtered and the excess methanol was recovered, with the help of a rotary evaporator under vacuum prior to the subsequent analyses. The conversion of soybean oil to fatty acid methyl esters was determined using a nuclear magnetic resonance (1H NMR) technique.23 Normally, the reaction mixture, after methanol was removed completely, was washed three times with a saturated aqueous NaCl solution, to remove the formed glycerin. The organic phase was separated by decantation, dried with anhydrous sodium sulfate, and then subjected to 1H NMR analysis (Bruker, model DPX-400) in CDCl3 using tetramethyl silane (TMS) as an internal standard. The conversion of the soybean oil to the methyl esters was determined by measuring the area of the 1H NMR signal relative to the methoxy (A1) and R-carbon CH2 groups (A2), respectively, according to the reference.23

conversion to methyl esters )

A1/3 A2/2

3. Results and Discussion 3.1. Screening of the Catalyst. Initial screening experiments were conducted to identify the most promising solid catalysts for the transesterification. The results are tabulated in Table 1. The reaction conditions were not optimized for the highest reaction yield for each catalyst; however, they provided a way to compare the activities of catalysts. The transesterification reaction was not performed in the presence of ANO3 (where A ) Li, Na, and K). Also, the non-loaded ZnO and ZrO2 that were tested as catalysts exhibited no activity. However, as shown in Table 1, when the alkali metal was loaded on the supports and activated at a high temperature, the supported catalyst showed activities toward the soybean oil transesterification. Among the catalysts screened, ZnO loaded with Li had the superior catalytic activity, compared to the other catalysts, giving the highest conversion of 95.2% over 5 h in a reflux of methanol (see entry 1 in Table 1). In addition, ZrO2 loaded with Li or Na was also determined to be highly active; respective conversions of 90.0% and 86.9% over them were obtained (see entries 4 and 5 in Table 1). Over Na/ZnO, K/ZnO, and K/ZrO2 catalysts, however, lower conversions of 8.8%, 17.5%, and 55.0%, respectively, were achieved (see entries 2, 3, and 6 in Table 1). Although different catalytic activities were observed among the tested catalysts, the same strength of basic sites with H- in the range of 15.0-18.4 was observed, as is evident in Table 1. As a result, it seems very likely that the activity of the catalysts is dependent not only on the strength of basic sites but also on their amounts. Based on the aforementioned results, the Li/ZnO catalyst showed a great potential for use as heterogeneous catalysts and, therefore, was chosen for the subsequent study in this work.

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Figure 1. X-ray diffractometry (XRD) patterns for ZnO samples with lithium: (a) 1 mmol/g Li/ZnO calcined at 873 K, (b) 3 mmol/g Li/ZnO calcined at 873 K, (c) 5 mmol/g Li/ZnO calcined at 873 K, (d) 3 mmol/g Li/ZnO calcined at 673 K and (e) 3 mmol/g Li/ZnO calcined at 1073 K. Open boxes (0) represent ZnO, and solid boxes (9) represent Li2ZnO2.

3.2. Catalyst Characterization. Figure 1 shows the powder XRD patterns of Li/ZnO catalysts. As can be seen, when the calcination temperature was 673 K, the catalysts exhibited a significant catalytic activity, suggesting that the high-temperature pretreatment is indispensable for the catalyst to obtain high catalytic activity for the reaction. The catalytic activity of Li/ZnO samples that were calcined at different temperatures and their basicities are shown in Table 2. As can been observed, the calcination temperature signifi-

Table 3. Catalytic Activities and Basicities of the 873 K-Calcined Li/ZnO Samples with Different Lithium Loading Amountsa Basicity (mmol/g) loading amount (mmol/g)

H- ) 7.2-9.8

H- ) 9.8-15.0

H- ) 15.0-18.4

total basicity (mmol/g)

conversion (%)

1 2 3 4 5

1.23 1.57 2.17 1.94 1.50

0.47 1.37 1.93 1.79 1.52

0.22 0.32 0.40 0.30 0.26

1.92 3.26 4.50 4.11 3.28

18.6 92.1 96.3 92.6 90.1

a Reaction conditions: molar ratio of methanol to oil, 12:1; catalyst amount, 5 wt %; reaction temperature, 338 K; and reaction time, 5 h.

cantly influenced the catalytic performance of Li/ZnO catalysts for the reaction. Furthermore, the conversion of soybean oil was markedly increased with the rise in the calcination temperature, from 573 K to 873 K. The optimal calcination temperature for the catalyst was observed at 873 K. At this calcination temperature, the highest conversion of 96.3% was achieved. However, when the calcination temperature was increased beyond 873 K, the catalytic activity toward the reaction dropped, and, therefore, a decreased conversion was obtained. Although the Li/ZnO samples calcined at temperatures of >673 K possessed the same basic strength (H-), in the range of 15.018.4, the basicity of the catalyst calcined at 873 K was the highest, resulting in the maximum conversion. Obviously, as illustrated in Table 2, the basicity changes with the calcination temperature parallel the changes in the catalytic activity for the transesterification reaction. As a consequence, the catalytic activity of the catalysts is significantly affected by their basicity. Because LiNO3 is inactive for this reaction, the activity variation of the catalysts calcined at different temperatures should be attributed to the different extents of LiNO3 decomposition and distributions of lithium on the ZnO support. The degree of LiNO3 decomposition is a function of temperature. However, the Li/ZnO sample that was calcined at 1073 K could generate a new species (Li2ZnO2) in the spinel structure, which may be one of causes of its lower catalytic activity, as mentioned previously. The catalytic activity and basicity of Li/ZnO samples with lithium loadings that range from 1 mmol/g to 5 mmol/g are presented in Table 3. This table shows that there was an optimum lithium loading for the catalytic activity, and, thus, the addition of small amounts of lithium could improve the activity of Li/ZnO catalysts for the reaction, but lithium loadings of >3 mmol/g caused the conversion to decrease. By drawing on the results, the optimum loading amount of lithium for the catalyst is 3 mmol/g. Note that, whenever the basicity of the catalysts increased, a corresponding increase in the conversion

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was observed in Table 3, again demonstrating that the activity of the catalysts is strongly dependent on their basicity. The number of basic sites on the catalyst surface cannot be related to the loading amount of LiNO3, but it is probably related to the amount of LiNO3 decomposition. Most probably, the lithium is loaded on the ZnO support in different ways, depending on its amount, thus affecting the amount of the active basic sites differently. When the amount of lithium loaded on ZnO is below the saturation uptake of lithium, it could be welldispersed. As a result, the number of basic sites, together with the catalytic activity of the catalysts, would increase as the lithium content increased. However, if too much lithium is loaded onto the ZnO, the lithium may not be well-dispersed and, mostly likely, because of this, not all but only part of the loaded LiNO3 could be decomposed. Moreover, the excess Li may lead to the agglomeration of active phases, which occurs during calcinations and, hence, reduces the surface area of active components and reduces the catalytic activity.34 In addition, it is worth noting that a modification of the distribution of basic sites with different strengths was observed as lithium was loaded onto ZnO. As listed in Tables 2 and 3, the main basic sites, with H- ranges of 7.2-9.8 and 9.8-15.0, and fewer basic sites, with H- in the range of 15.0-18.4, were found on the catalyst surface. Thus, it can be inferred that the basic sites are nonuniform and the different basic species are formed after the lithium loading onto ZnO, in agreement with the XPS results. For industrial applications, the sustained activity in catalysts has great significance. To study the ability to reuse the catalyst, the Li/ZnO catalyst was separated by filtration, washed with cyclohexane and methanol, dried at 383 K, and subsequently used again with fresh reactants in a second reaction cycle under the optimum reaction conditions. A decrease in the basic strength (H-), from 15.0 to -18.4 to 9.8-15.0, and in the basicity, from 4.50 mmol/g to 1.94 mmol/g, was observed, and mostly for this reason, the recovered catalyst provided a lower conversion of 42.7%, compared to the original catalyst. The reduced catalytic activity observed here could be, probably, due to the leaching of Li species from the supported catalyst. However, the spent Li/ZnO catalyst can be regenerated by impregnating it in an aqueous solution of LiNO3, as previously described. The obtained result shows that the regenerated catalyst could give a high conversion of 83.6% under reaction conditions identical to those in the case of fresh catalysts. 3.4. Transesterification of Soybean Oil. The molar ratio of methanol to soybean oil is one of the important variables that affect the conversion to methyl ester. Because the transesterification reaction is reversible, a large excess of methanol could shift the equilibrium favorably. Generally, the molar ratio is associated with the type of catalyst used. Previous studies indicate that an acid-catalyzed transesterification requires high molar ratios of methanol to oil (30-150:1), because of its relatively slow reaction rates,4 whereas the molar ratio of methanol to oil of 6-15:1 is usually used in the base-catalyzed processes.18-20 Using 5 wt % of catalyst, we tested the influence of the molar ratio of methanol to oil on the conversion. As reported in Figure 7, as the methanol loading amount increased, the conversion was increased considerably. When the molar ratio was very close to 12:1, the maximum conversion was obtained. However, beyond a molar ratio of 12:1, the excessively added methanol exhibited no significant effect on the conversion. Therefore, a 12:1 molar ratio of methanol to oil is suitable for obtaining high product yields, with the conversion being 96%.

Figure 7. Conversion of soybean oil, as a function of the molar ratio of methanol to oil. Reaction conditions: catalyst amount, 5 wt %; reaction time, 5 h; and reaction temperature, 338 K.

Figure 8. Conversion of soybean oil, as a function of catalyst amount. Reaction conditions: molar ratio of methanol to oil, 12:1, reaction time, 5 h; and reaction temperature, 338 K.

Note that the excess methanol used in the reaction can be recovered and reused via simple distillation in a subsequent cycle. The effect of the catalyst loading was investigated at a 12:1 molar ratio of methanol to soybean oil at 338 K for 5 h. The catalyst amount was varied in the range of 1-11 wt %, which was referenced to the starting oil weight, while the remaining variables were kept constant. As indicated in Figure 8, if the catalyst amount was increased from 1 wt % to 5 wt %, the corresponding conversion was increased gradually and attained the maximum value of 96.3%. However, with further increases in the amount of catalyst, a decreased conversion was observed. From the results, the catalyst amount of 5 wt % is adopted for the optimization of reaction time. Figure 9 shows the effect of reaction time on the conversion to methyl esters. It can be observed that the conversion increased continuously in the reaction time range from 1 h to 3 h, and thereafter remained almost constant, as a result of an almostequilibrium conversion. Based on the results, the maximum conversion is achieved after 3 h of reaction time. The composition of the obtained methyl esters for the transesterification reaction performed under optimized reaction

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reached 96.3%. The basicity of the catalysts had a significant impact on their catalytic activities, and the Li2O that was derived from the thermal decomposition of LiNO3 at high calcination temperatures represented the probable main active sites. Literature Cited

Figure 9. Conversion of soybean oil, as a function of reaction time. Reaction conditions: molar ratio of methanol to oil, 12:1; catalyst amount, 5 wt %; and reaction temperature, 338 K.

conditions in the presence of the catalyst was determined by GC analysis, and the following results were obtained: methyl palmitate, 11.8%; methyl stearate, 6.3%; methyl oleate, 25.8%; methyl linoleate, 50.2%; and methyl linolenate, 5.1%. The analysis also show a good conversion of the catalyst, because the fatty acid composition of the methyl esters was rather similar to that of the used soybean oil. Obviously, the Li/ZnO catalyst was identified as an effective catalyst for the transesterification reaction of soybean oil. The conversion to methyl esters in excess of 90% was achieved using the catalyst. However, the stablity of the catalyst was not satisfying to us. Several other heterogeneous catalysts are also reported in the literature. For example, studies on the transesterification of soybean oil with methanol in the presence of series NaX faujasites, zeolites, and ETS-10 zeolite showed that NaX that contained occluded sodium oxide and occluded sodium azide gave >90% conversion at 393 K in 24 h.35 In addition, Reddy et al. reported that nanocrystalline calcium oxides might allow the transesterification to occur in quantitative conversions at room temperature, which would afford operational simplicity and low-energy consumption.36 Schuchardt et al. studied the tranesterification of vegetable oil in the presence of alkyl guanidines supported on an organic polymer.37 Despite the fact that this catalyst showed a good conversion reaction yield (up to 95%) in a reflux of methanol, the amine leaching and its irreversible protonation caused rapid deactivation of the catalyst. Also, we tested the use of calcined hydrotalcites in the transesterification of soybean oil in a reflux of methanol, with low conversions of 64%, which were probably due to the low reaction temperature adopted in the experiments that were performed.38 3. Conclusions Zinc oxide (ZnO) loaded with lithium was demonstrated to be an effective catalyst for the transesterification of soybean oil with methanol. The catalyst with 3 mmol/g of lithium loaded on ZnO, and after being calcined at 873 K for 5 h, was determined to be the optimum catalyst, which could give the maximum basicity and the best catalytic activity. When the reaction was performed in a reflux of methanol (at 338 K) with a molar ratio of methanol to oil of 12:1, a reaction time of 3 h, and a catalyst amount of 5 wt %, the conversion of soybean oil

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ReceiVed for reView April 26, 2007 ReVised manuscript receiVed July 24, 2007 Accepted August 15, 2007 IE070597S