Biodiesel is a renewable fuel for use in diesel engines, and it is receiving increasing attention, mainly because of its environmentally friendly properties [1]. Biodiesel is produced commercially by the homogeneous transesterification of vegetable oils with methanol using KOH as a catalyst. Homogeneous base catalysts exhibit high catalytic activity, but a large amount of water is required to remove the residual homogeneous catalyst, and it is impossible to reuse homogeneous catalysts [2]. Heterogeneous catalysts are attractive because the catalytic system can be separated and reused [3, 4, 5]. However, the catalytic activity of heterogeneous catalysts for transesterification is lower than that of homogeneous catalysts, and leaching of active species into the liquid products is possible.
Supported alkali metal hydroxides or salts are among the most-studied solid heterogeneous catalytic systems for the transesterification of vegetable oils [6]. Supported K-based catalysts, particularly Al2O3-supported catalysts, are attractive because of their high basicity and relatively easy and cheap synthesis. Several authors have described solid K-based catalysts with high catalytic activity; for example, ester yields greater than 90% using K2CO3/Al2O3 [7, 8], KF/Al2O3 [9, 10], KOH/Al2O3 [11, 12], and KNO3/Al2O3 catalysts [13, 14] have been reported. However, the main problem in applications of these catalysts is their stability.
Most authors reported continuous drops in the activity of KNO3/Al2O3 [14], KF/Al2O3 [10, 15], K2CO3/Al2O3 [8, 16, 17], and KOH/Al2O3 catalysts [12, 18] after the first catalytic cycle. Catalyst deactivation was explained by the leaching of K into the liquid phases. The reason for this is still under discussion. It has been explained by attack of K by methanol [8, 16, 18] and glycerol [10, 19] or by covering of the catalyst surface by glycerol [10].
Isahak et al. [11] and Verziu et al. [20] reported stable activity of KOH/Al2O3 and KF/Al2O3, respectively, over four cycles in catalytic tests (95%-85% ester yields). However, significant decreases in the performance of KOH/Al2O3 and KF/Al2O3 catalysts were observed in cycles five (~70% ester yield) and six (~30% ester yield). The decrease in catalytic activity was associated with K leaching. Wen at al. [21] reported a biodiesel yield exceeding 91% even after 16 catalytic cycles. However, the concentration of K in the solid catalyst or in the liquid phases during the 16 catalytic cycles was not reported.
As discussed above, solid K-based catalysts have mainly been studied using batch reactors. Although there have only been a few studies focusing on the use of fixed-bed reactors with this type of catalyst in the transesterification of rapeseed oil, fixed-bed reactors have many advantages such as the absence of other catalyst separation and continual product formation. For example, Furuta et al. [22] reported ester yields of more than 90% using W-ZrO2/Al2O3 for up to 100 h of time-on-stream, with the reaction temperature kept at 250 °C.
Previously, we focused on the leaching of K species from solid K/Al2O3 catalysts into the liquid phases during transesterification [19]. It was found that K species were leached into the liquid phases as a result of attack by produced glycerol. The aim of this work is to study the catalytic behaviours of K/Al2O3 catalysts in batch and fixed-bed reactors. Attention is also focused on the stability of K2O and K-O-Al species in the reaction.
An alumina support with a particle diameter of 3-5 mm was supplied by Euro Support Manufacturing Czechia. The K/Al2O3 catalyst (16.1 wt% K) was prepared by the wet impregnation method. A mixture of the support in an aqueous solution of KOH (90% purity, Lachner, Czech Republic) was stirred at 95 °C for 4 h. Excess water was removed at the same temperature until dryness. The K/Al2O3 catalyst was then dried for 24 h at room temperature. Finally, the catalyst was calcined in an air flow at 475 °C for 4 h.
Fourier transform infrared (FT-IR) spectra of the samples were obtained on Nicolet 6700 FT-IR spectrometer using KBr pellets, with accumulation of 64 scans at a spectral resolution of 4 cm−1. The scanning range was from 400 to 4000 cm−1. All measurements were conducted at room temperature.
The reactions were carried out in a stainless-steel batch reactor (150 mL). Rapeseed oil (cold-pressed, filtered, free of erucic acid, acid number 1.2 mg KOH/g, water content 610 mg/kg, density 920 kg/m3 at 25 °C, kinematic viscosity 35.0 mm2/s at 40 °C; RPN Slatiňany) and methanol (p.a., 0.04% water content, determined using the Karl Fischer method) were used. Oil (50 g), methanol (methanol to oil molar ratio 24:1), and the catalyst (4 wt% of catalyst based on the mass of rapeseed oil) were put in the reactor and heated to the reaction temperature (117 °C). The stirring rate was 320 r/min. After reaction (8 h), the catalyst was filtered off and the methanol was removed from the reaction mixture by evaporation (2kPa and 65°C). The ester phase (upper phase) and glycerol phase (lower phase) were separated in a separating funnel and analysed. The reusability of the catalyst was checked over five consecutive cycles under the reaction conditions described above. No further purification of the catalyst was performed between each cycle. Additionally, the catalyst was placed in contact with pure glycerol after five consecutive catalytic cycles (1g of catalyst and 5.2 g of glycerol were used, the total K leaching was calculated based on the amount of K in the fresh catalyst).
The resistance of the K/Al2O3 catalyst to glycerol was checked over five consecutive cycles in the presence of glycerol. Pure glycerol was used in each cycle and the catalyst was not purified between cycles. The tests were performed by washing 3 and 6 g of K/Al2O3 catalyst with 15.6 and 31.2 g of pure glycerol, respectively. The amount of glycerol used was calculated based on the theoretical value for total oil conversion: 5.2 g of glycerol is produced if 50 g of oil and 1 g of catalyst are used. Reaction conditions: 65 °C, 320 r/min, and 7.5 h.
Catalytic test was performed after five consecutive washing cycles with glycerol under the same reaction conditions (batch reactor, rapeseed oil 50 g, 2 g of catalyst, 117 °C, 8 h, 320 r/min, methanol:oil = 24:1).
Catalytic tests were performed using a bench-scale catalytic apparatus equipped with an electrically heated fixed-bed reactor of inner diameter 17 mm. Food-grade rapeseed oil and methanol were used as liquid feeds. Prior to the catalytic tests, the catalyst (10 g) was diluted with silicon carbide (1.2 mm diameter; the catalyst:SiC volume ratio was set to keep the total volume of catalyst plus SiC constant; constant catalyst bed height). Once the reaction temperature of 115 °C was reached in the reactor (the temperature gradient was set at 40 °C/h), the catalyst was dried for 4 h under N2 (4 MPa, 10 Nl/h). Transesterification of the rapeseed oil was studied under the same N2 pressure and at 115 °C. The mass hourly space velocity (WHSV) was set at 1.43 g/(g·h) and the methanol:oil molar ratio was 12:1. After the reaction, the methanol was removed from the reaction mixture by evaporation (2.3 kPa and 65 °C) and the ester and glycerol phases were separated in a separating funnel and analysed.
The ester phase was analysed by GC, according to EN 14105, using a Shimadzu GC-2010 instrument, and linear calibration curves of pure methyl esters. Methyl esters were identified in the chromatogram; calibration standards were prepared by homogeneous transesterification of the same oil and purification by molecular distillation. The ester yield (YE) was calculated according to eq. (1).
where mE is the mass of methyl esters and mO is the amount of oil fed into the reaction.
The amount of glycerol in the glycerol phase, if formed, was determined by iodometric titration and the concentration of K was determined.
The concentration of K ions in each phase was determined using flame photometry (Flame photometer 410, Sherwood), using linear calibration curves. The calibration solutions for K determination in the ester phase were prepared by mixing a methanolic KOH solution (close to saturation) with a biodiesel matrix without any ions [23]. The biodiesel matrix was prepared by washing the biodiesel with an aqueous solution of citric acid. The concentration of K ions in the ester phase was directly determined. Aqueous solutions of KOH were used as calibration standards for the determination of K ions in the glycerol phase. A sample of the glycerol phase was diluted with water and the K ion concentration was determined. The influence of glycerol on the K determination can be neglected in this case, because highly dilute glycerol samples were used.
Table 1A shows the activity of the 16.1 wt% K/Al2O3 catalyst in five consecutive catalytic tests without any further purification of the catalyst. The fresh K/Al2O3 catalyst gave an ester yield of 80%. However, leaching of 7.3 and 116 mg of K species into the ester and glycerol phases, respectively, was observed. In total, 38.2% of the K were leached into the liquid phases (Table 1A, 1st cycle). The leaching of K indicated irreversible deactivation of the K/Al2O3 catalyst for further catalytic steps. The irreversible loss of 38.2% K resulted in a significant decrease in activity in the second catalytic run with 15% of ester yield. Similar ester yields (14%-15%) were observed in the second to fifth catalytic cycles. The small decreases in the ester yields were associated with minimum loss of K species into the liquid phases.
We previously reported that the leaching of K species from K/Al2O3 catalysts was caused by attack of K species with glycerol formed during the reaction [19]. High leaching of K species in the first catalytic run can therefore be easily explained by glycerol phase formation, leading to the attack of K species in the K/Al2O3 catalyst with glycerol [19]. Transesterification involves three consecutive reversible reactions and glycerol is formed only in the final step (when monoglycerides are converted to glycerol and esters, so it is unlikely that the 14%-15% ester yields in the second to fifth catalytic cycles resulted from the absence of glycerol phase formation. The absence of attack of K species with glycerol led to a minimum loss of K species (our previous work shows that K species are resistant to methanol, water, esters, and oil [19]), and approximately the same ester yields as in the other catalytic cycles.
To support this fact, we tested the possible leaching of K from the used K/Al2O3 catalyst after the fifth consecutive catalytic test in pure glycerol. It was found that another 9% of K was leached from the catalyst to the glycerol. A total of 47.2% of K was therefore leached from the K/Al2O3 catalyst, i.e. 38.2% during the first catalytic cycle and 9% during washing in pure glycerol.
Figure 1 shows the amounts of K leached from the solid K/Al2O3 catalyst into pure glycerol. Glycerol was mixed with the catalyst under the following conditions: 65 °C, 320 r/min, 7.5 h, and the amount of glycerol that would be expected after complete transesterification on the amount of K/Al2O3 catalyst used. Two tests were performed to investigate the leaching of K species from 3 and 6 g of the catalyst into the appropriate amount of glycerol, i.e. 15.6 and 31.2 g of glycerol, respectively (Fig. 1). It can be clearly seen that the major part of the K was leached into the glycerol during the first and second washing cycles (46% and 18% of K in the first and second cycles, respectively). Very small amounts of K were leached into glycerol in the third to fifth cycles. These results indicate that some of the K species are fixed to the K/Al2O3 catalyst and are resistant to glycerol. It is also clearly seen that the percentage of K leached into glycerol was the same at different absolute amounts of K/Al2O3 catalyst. Comparable results obtained from several tests support this finding. It should also be mentioned that partial disintegration of the K/Al2O3 catalyst grains was observed after five washing cycles (Fig. 3(c)).
In our previous work, leaching of K species into methanol, oil, and esters was not observed under the same conditions as those used with glycerol [19].
Table 1B shows the activity of the K/Al2O3 catalyst after five washing cycles with glycerol, i.e. with the K/Al2O3 catalyst containing the residual K species resistant to attack of glycerol. We expected this K/Al2O3 catalyst resistant to glycerol to be active and stable in transesterification. Although the catalytic activity of the catalyst was high after five washing cycles with glycerol, mechanical destruction of the catalyst grains became a significant problem. Approximately 50% of the catalyst was lost in the liquid phases, and practically all the catalyst grains were destroyed (Fig. 3(d)). We were therefore unable to perform detailed product analyses of the ester and glycerol phases, and could not support our suggestion that the residual K species have high and stable catalytic activity.
To prevent such mechanical destruction, the activity and stability of the K/Al2O3 catalyst was studied in a fixed-bed reactor.
Figure 2 shows the transesterification of rapeseed oil over the K/Al2O3 catalyst at 115 °C in a fixed-bed reactor. In both tests, ester yields of approximately 83% were obtained, and the yield was almost constant for 100 h of time-on-stream. The amount of K leached from the catalyst into the liquid phase decreased with increasing reaction time, but the ester yield was constant. Leaching of K into the liquid phase was observed up to approximately 40 h of time-on-stream. After that, only small amounts of K were detected in the liquid phases, even if glycerol was formed. This supports our suggestion that although some of the K species are leached into the liquid phase, the remaining K species are resistant and have high transesterification activity.
Although mechanical destruction of the K/Al2O3 catalyst was observed in the batch reactor, it was not found in the fixed-bed reactor (Fig. 3(b)). This shows that the catalyst grains are stable under the reaction conditions, and the main problem is contact between the catalyst and the stirrer. This problem was not observed in the fixed-bed reactor, even after 100 h of time-on-stream.
The information on the amount of K leaching from the catalyst is very important. In the batch reactor, 66% and 65% of K were leached into glycerol from the catalyst over five consecutive washing cycles (Fig. 1). In the fixed-bed reactor, 60% of K was leached from the catalyst during reaction for 100 h of time-on-stream (Fig. 2).
Finally, 51% of K (Table 1A) was leached from the catalyst over five consecutive catalytic cycles in the batch reactor (42%) followed by washing with glycerol (9%).
Although K leaching into methanol [8, 16, 18] and glycerol [10, 19] has been observed, no K in the second to fifth catalytic cycles was lost, i.e. when no glycerol phase was formed but methanol was present in the reaction mixture. However, another 9% of K was leached from the used catalyst (after five consecutive catalytic cycles, shown in Table 1A) into glycerol. The leaching of K from the used catalyst after five consecutive catalytic tests into glycerol shows that the glycerol phase plays a critical role in K leaching. The reason for the lower amount of leached K after five consecutive catalytic tests in the batch reactor followed by washing of the used catalyst in glycerol (51%) is probably the absence of other washing cycles of the catalyst in glycerol (Table 1A); the loss was caused by mechanical damage to the catalyst (similar to Fig. 3(d)).
The decrease in the stability of the K/Al2O3 catalyst between the first and second catalytic cycles is in agreement with most reports on the stability of K-based catalysts in transesterification of rapeseed oil [12, 16, 17, 18, 24].
The results reported here support the proposal that K species are leached from the K/Al2O3 catalyst by the attack of glycerol [10, 19] and not methanol [8, 16, 18]. Moreover, the fact that K species are leached only if the glycerol phase is separated from the ester phase could be the reason for the discrepancies in the stability of K-based catalysts reported in the literature. Possible K leaching and its detailed analysis were not described by Wen et al. [21]; they reported biodiesel yields exceeding 91% even after 16 catalytic cycles. Decreases in the activity of K-based catalysts after the fifth and sixth consecutive cycles were reported by Isahak at el. [11] and Verziu et al. [20], respectively. Although K leaching was reported as the reason for deactivation, a detailed analysis of K in the glycerol and ester phases was not provided.
The stable activity of the K/Al2O3 catalyst in the fixed-bed reactor for 100 h shows that the residual K species present in the catalyst have high activity in transesterification. This is probably the reason for the very high activity of a K/CaO catalyst after 16 consecutive catalytic tests reported by Wen et al. [21]. However, this could not be proved because an analysis of the K present in the solid catalyst and/or liquid phases is not given in that paper. Our results for 100 h of time testing of the catalytic activity and stability are comparable to, or higher than, the average results reported in the literature. For example, Di Serio et al. [25] and Sankaranarayanan et al. [26] tested the activity and stability of Mg-Al mixed oxide and MoO3 supported on Al2O3 in transesterifications for 80 and 120 h, respectively. The MoO3/Al2O3 catalyst had long-term stability (conversion dropped only about 5% during the catalytic tests and 1% of MoO3 was leached) [26], but the Mg-Al mixed oxide catalyst was deactivated rapidly in the first hour on stream [25]. However, it was possible to regenerate the catalyst by simply washing with acetone [25]. Continuous testing for about 8 h was reported by Sherstyuk et al. [27]. It was found that the rapeseed oil conversion remained nearly constant during the tests with some binary catalytic systems. Melero et al. [28] tested the activity and resistance against deactivation of Zr-SBA-15/bentonite catalyst for more than 260 h of time-on-stream in transesterifications, and obtained promising results (the catalytic activity remained practically the same during the entire test and the physicochemical properties of the catalyst were similar before and after use). However, tests under flow conditions of a comparable catalytic system to that studied in this work have not yet been reported; therefore, it is hard to compare the reported results with those in this work.
The use of a fixed-bed reactor is favourable in heterogeneous catalysis. Other studies of the use of K catalysts in flow-type reactor have been reported. Buasri et al. [29] tested KOH supported on coconut-shell activated carbon in a packed-bed reactor for the transesterification of waste frying oil; there was no evidence of catalyst deactivation. However, the leaching of active species was not discussed. Furuta et al. [30] studied K/ZrO2 in the transesterification of soybean oil using a fixed-bed reactor. This catalyst gave almost 100% oil conversion at the beginning of the reaction, but it was deactivated rapidly due to K leaching from the catalyst.
In the case of the K/Al2O3 catalyst, the high and stable activity, even when some K leaching occurs at the beginning of the reaction, and the absence of mechanical destruction, show its high potential for use in transesterification. The main aim in the future will be to fix all the K on the support under the reaction conditions.
FT-IR spectroscopy was used to investigate the stability of K species in the K/Al2O3 catalysts. Figure 4 shows the spectra of the fresh and used catalysts after five consecutive cycles in the batch reactor and after 130 h of time-on-stream. All the samples showed a very intense broad band at around 3400 cm−1, which could be assigned to the O-H stretching vibration of the hydroxyl groups attached to Al2O3. This band could also be partly attributed to the stretching vibrations of Al-O-K groups [31, 32]. The minor absorption peak at about 1640 cm−1 may be assigned to the O-H bending vibration mode of H2O molecules adsorbed from air [13]. In addition, there were two bands at around 1540 and 1400 cm−1, which were attributed to the vibrations of CO32− ions; the adsorption of gaseous CO2 from the ambient atmosphere on the basic sites of K2O may account for the presence of carbonates [33, 34]. The band at 585 cm−1 (AlO6 groups) and the weak band at 792 cm−1 (AlO4 groups) were attributed to aluminate bands, and indicate the presence of Al-O-Al framework [31, 35].
The intensity of the bands at 1540 and 1400 cm−1 attributed to K2O species for the K/Al2O3 catalyst after five consecutive cycles in the batch reactor and after 130 h of time-on-stream in the fixed-bed reactor decreased significantly compared with that of the fresh catalyst. In contrast, the intensity of the band at 3400 cm−1 partly reflected to the Al-O-K groups only decreased slightly compared with that of the fresh catalyst. It was therefore concluded that some K2O species were leached out of the catalyst during the reaction. This observation is in agreement with the previously reported suggestion that K2O species are leached from solid K-based catalysts into the liquid phase [33].
We previously reported that the leaching of K species was caused by the attack with the formed glycerol [10, 19], and not with methanol [8, 16, 18]. In order to support this proposal, we compared the spectra of the K/Al2O3 catalysts after washing with various substances, namely methanol, water, ester, oil, and glycerol (Fig. 4), with that of the fresh catalyst. Only minor decreases in the intensity of the bands corresponding to Al-O-K groups (the band at 3400 cm−1 partly reflects the presence of Al-O-K groups. We mentioned it in more detail at the beginning of this caption) and K2O species (1540 and 1400 cm−1) in the spectra of K/Al2O3 catalysts washed with methanol, water, oil, and ester were observed. However, decreases were observed in the intensity of the bands at 1540 and 1400 cm−1 in the catalyst after washing with glycerol. Washing with glycerol therefore mainly caused leaching of K2O species from the catalyst, in the same way as was previously observed during the catalytic reaction. This supports our previous suggestion that glycerol causes the leaching of some K species from the catalyst during transesterification. It also supports the suggestion that leaching of some K occurs during catalytic reactions carried out in batch and fixed-bed reactors (Table 1A). As the amount of leached K species approached 0 in the fixed-bed reactor (Fig. 2) and the catalytic activity of the residual K/Al2O3 catalyst was constant for more than 100 h (Fig. 2), it is suggested that Al-O-K groups and/or residual K2O species are responsible for the activity of the K/Al2O3 catalyst in transesterification.
Different opinions regarding the main active sites of K/Al2O3 catalysts have been reported. For example, Alonso et al. [24] and Ma et al. [18] reported that Al-O-K groups were responsible for the activity of KOH/Al2O3 catalyst, whereas Liu et al. [31] and Xie et al. [13] found that Al-O-K groups and K2O species on the catalyst surfaces were the main active sites. Recently, Li et al. [33] suggested that K2O species (more specifically K2O·CO2 species, i.e. K2O species after adsorption of CO2 from air) were possibly responsible for the activity of KOH/Al2O3 catalyst in transesterification. In our case, the fresh K/Al2O3 catalyst contains Al-O-K groups as well as K2O species, as can be seen from the band at around 3400 cm−1, and bands at 1540 and 1400 cm−1, respectively, as shown in Fig. 4. The K/Al2O3 catalysts tested in this work therefore contained both Al-O-K groups and K2O species, which can be responsible for the activity of K/Al2O3 catalysts.
Recently, many authors have optimized the reaction conditions [17, 36, 37] and reactor design [38] for the transesterification of rapeseed oil catalysed by K-based catalysts, to meet European standards. The fact that some K2O species are leached from the K/Al2O3 catalyst as a result of attack by produced glycerol, whereas K-O-Al species are resistant, is important information that could increase the use of K-based catalysts in repeated catalytic cycles or in fixed-bed reactors.
Mechanical destruction and potassium loss are the main problems that restrict the use of K/Al2O3 catalysts in batch reactors for transesterification of rapeseed oil, however, it showed high (ester yields of ~ca. 83%) and stable activity in a fixed-bed reactor for 100 h, despite some K leaching at the beginning of the reaction. The absence of mechanical destruction shows the high potential for use of K/Al2O3 in the transesterification of rapeseed oil carried out in fixed-bed reactor. The results indicated that most of the leached K species were K2O.
2014, Vol. 35 Issue (7): 1084-1090
PDF (422KB)
2014, Vol. 35 Issue (7): 1084-1090
PDF (422KB)