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Mechanistic study of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under microwave heating
Renewable Energy 147 (2020) 695e704
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Mechanistic study of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under microwave heating Linye Zhang a, Zongwu Xin a, Zihan Liu a, Guangtao Wei a, b, *, Zhongmin Li a, Yuning Ou a a b
School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China Guangxi Key Laboratory of Processing for Non-ferrous Metallic and Featured Materials, Guangxi Zhuang Autonomous Region, Nanning, 530004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 25 July 2019 Accepted 11 September 2019 Available online 13 September 2019
The use of microwave heating (MH) constitutes a promising way to upgrade biodiesel via catalytic transfer hydrogenation (CTH) catalyzed by Raney-Ni. However, mechanistic reports on this reaction are rare and the mechanisms involved in CTH process remains unclear. Herein, we presented a thorough mechanistic study of CTH of Jatropha oil biodiesel catalyzed by Raney-Ni under MH, using isopropyl alcohol as hydrogen donor and water as solvent. The intensification effect under microwave radiation has been studied from three aspects of catalyst, water, and hydrogen donor. It was found that microwave made an adverse effect on the activity of Raney-Ni. The solvent of water could provide hydrogen to some extent for the CTH of biodiesel, and the influence of Raney-Ni from microwave radiation had a positive role for the water to provide hydrogen during CTH reaction. Under microwave irradiation, the activity of isopropyl alcohol would be enhanced, and the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. Based on the characterization of catalyst by SEM, XRD, FTIR and TGA, it was found that the reason for the catalyst deactivation was mainly the carbonaceous deposits of fatty acid methanol esters (FAMEs) on the surface of the catalyst. A schematic representation of mechanism of microwave-assisted CTH has been presented. Furthermore, combining theory of heterogeneous catalytic process with mechanism of six-membered cyclic transition state, the reaction course for the CTH of Jatropha oil biodiesel under microwave heating has been explained successfully. Lastly, the performance improvement of the upgraded Jatropha oil biodiesel was briefly analyzed in view of composition change of biodiesel. Overall, our results provide a systematic understanding of CTH of biodiesel catalyzed by Raney-Ni under microwave heating, and our findings help to develop a green technology for the upgrading of biodiesel in industry. © 2019 Published by Elsevier Ltd.
Keywords: Microwave Catalytic transfer hydrogenation Jatropha oil Raney-Ni Biodiesel
1. Introduction Energy is one of the main pillars of modern civilization, and it is indispensable in social life. However, with the rapid development of human society, the non-renewable energy sources such as coal and oil are increasingly lacking, and the air pollution caused by the burning of fossil fuels is becoming increasingly prominent [1,2]. The world is up against the dual crisis of fossil fuel depletion and environmental deterioration. The search for sustainable and environment-friendly alternative fuels has become a very pressing issue in recent years [3]. Biodiesel, prepared from non-grain
* Corresponding author. School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China. E-mail address: [email protected] (G. Wei). https://doi.org/10.1016/j.renene.2019.09.035 0960-1481/© 2019 Published by Elsevier Ltd.
biomass, has become one of the effective ways for humans to solve the energy crisis and reduce air pollution owing to its renewable performance, higher cetane number and lower exhaust emissions as compared with petroleum-based diesel [4e6]. The biodiesel is composed of fatty acid methyl esters (FAMEs), produced via the transesterification of oils (vegetable oils, animal fats and waste oils) with methanol [7e10]. Although biodiesel shows many advantages, it has some drawbacks related to its properties such as poor cold flow property and low oxidative stability. These properties of biodiesel are truly related to the structural features of FAMEs. Biodiesel with high content of polyunsaturated FAMEs has a low oxidative stability. Whereas if a high content of saturated FAMEs, the biodiesel would show a poor cold flow property [11]. Monounsaturated FAME compositions are ideal for biodiesel [12,13]. Jatropha oil, as a non-edible vegetable oil, is an excellent raw material for biodiesel producing [14e16].
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However, Jatropha oil biodiesel generally has relatively high contents of polyunsaturated FAMEs, which are the needless compositions for gaining a biodiesel with ideal compositions of monounsaturated FAMEs. To upgrade biodiesel, namely increasing the cold flow properties and improving the oxidation property of biodiesel, selective hydrogenation has become a useful process to refine the FAMEs of biodiesel [17e19]. Selective hydrogenation for unsaturated organic compounds can proceed via conventional hydrogenation method or catalytic transfer hydrogenation (CTH) method. In comparison with conventional hydrogenation in which uses the explosive hydrogen gas, CTH utilizing molecular hydrogen as the hydrogen donor is a simple, safe and eco-friendly method of hydrogenation. It has been proven that CTH is an effective alternative to conventional hydrogenation to realize the selective hydrogenation [20e22]. Moreover, compared with non-noble catalysts, noble catalysts have been used in CTH process due to their higher catalytic activity and selectivity [23]. CTH reactions of vegetable oils or grease-like materials catalyzed by noble metal have been reported in the literature [24]. Martinelli et al. investigated the CTH of castor oil using limonene as the hydrogen donor and Pd/C as catalyst [25]. Tike and Mahajani investigated the CTH of soybean oil using ammonium formate as the donor over 5% Pd/C catalyst, and it was found that the CTH of soybean oil offered good selectivity in the complete reduction of linolenic acid [26]. Sancheti and Gogate investigated the CTH of soybean oil with Pd/C as catalyst under ultrasound, and reported that the CTH of soybean oil offered excellent selectivity with low trans-isomer formation [27]. Although the noble catalysts have excellent catalytic performance in CTH, the high cost limits their wide application in CTH of oils or grease-like materials. Thus, an inexpensive non-noble catalyst is needed for selective hydrogenations, which proceed via CTH method. Among the non-noble catalysts, nickel-based catalysts have been broadly used in hydrogenation reaction due to its high catalytic activity, low cost, and easy separation from the product. However, there are some drawbacks for CTH reaction catalyzed by nickel-based catalyst, which include its low catalytic selectivity and high reaction temperature [28]. Therefore, a process-intensifying method to increase the catalytic selectivity of nickel-based catalyst and reduce reaction temperature can make the process of CTH economical and efficient in operation. In view of the excellent properties of microwave as heating and process-intensifying method [29,30], the CTH of Jatropha oil biodiesel heated by microwave irradiation has been studied in our laboratory [31]. It was found that microwave was an excellent process-intensifying method for the CTH of biodiesel. Compared with the CTH of Jatropha oil biodiesel under conventional heating (CH), the process of CTH under microwave heating (MH) occurred very rapidly, and the upgraded Jatropha oil biodiesel obtained had a remarkable advantage in the high content of C18:1 composite. However, it should be pointed out that the mechanistic reports on the hydrogenation of biodiesel via CTH method are truly rare. As for the selective hydrogenation of Jatropha oil biodiesel catalyzed by Raney-Ni under MH, the mechanisms of CTH of biodiesel, including the mechanism of process intensification in the reaction system, are not clear. Hence, a thorough research on the mechanism of CTH of biodiesel catalyzed by Raney-Ni under MH would help to have a clear understanding of the reaction process, and would provide a theoretical basis for the further development of this upgrading technique for biodiesel. The present work is, therefore, focused on the mechanisms of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under MH. Firstly, the intensification effects on the CTH of biodiesel under microwave radiation have been studied from three aspects: catalyst, water and hydrogen donor. Moreover, based on the identifying of the catalytic activity and stability of Raney-Ni
after CTH reaction, the cause of catalyst deactivation in this reaction system has been explored in detail. Lastly, the reaction mechanism of CTH of biodiesel in this reaction system has also been proposed, and in view of the composition change, the performance improvement of the upgraded Jatropha oil biodiesel was also briefly analyzed.
C16:0 C18:2 c-C18:1 t-C18:1 C18:0 CRC182 k1 k2
methyl hexadecanoate methyl linoleate cis-methyl oleate trans-methyl oleate methyl stearate mass conversion ratio of C18:2 (wt%) reaction rate constant for C18:2 to C18:1 (min1) reaction rate constant for C18:1 to C18:0 (min1)
2. Materials and methods 2.1. Materials Jatropha oil was purchased from FMD Bio-Energy Ltd. (Guangxi, China). Jatropha oil biodiesel was synthesized by us in our laboratory, according to the synthesis method previously developed [31,32]. The composition of Jatropha oil biodiesel is shown in Table 1. Raney-Ni was obtained from Raney metal technology Ltd. (Jiangsu, China). All chemicals, including methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol and tert-butanol were analytical grade. 2.2. Catalytic transfer hydrogenation of Jatropha oil biodiesel catalyzed by Raney-Ni Microwave chemical reactor (MCR-3; Shijishuangke experimental instrument co., Led, Zhengzhou, China) with a temperature sensor and a motor-driven Teflon stirring rod was used in this work. With the automatic model, the microwave chemical reactor can automatically control the reaction temperature and the reaction time at predetermine values. 0.56 g of Raney-Ni (8 wt% of biodiesel) and 80 g of water were taken in a 250-ml three-necked flask, and then the flask was placed in the microwave chemical reactor. The microwave pretreatment of Raney-Ni was operated at different time (0e30 min) and different temperature (80e100 C). After pretreatment of Raney-Ni under microwave radiation, 24 g of isopropyl alcohol and 7 g of biodiesel were added into the above mixtures of Raney-Ni and water. Under CH, the CTH of biodiesel, in which the catalyst was pretreated Raney-Ni (p-Raney-Ni) by microwave radiation, was carried out under the conditions of temperature 85 C, reaction time 60 min and stirring speed 400 rpm. To understand the essence of catalyst effect under microwave radiation on the CTH reaction, different reaction systems with RaneyNi or p-Raney-Ni were carried out, including (1) water þ isopropyl alcohol þ p-Raney-Ni þ CH, (2) water þ isopropyl alcohol þ RaneyNi þ CH, and (3) water þ isopropyl alcohol þ Raney-Ni þ MH. On the base of understanding the essence of catalyst effect, two additional reaction systems of CTH without isopropyl alcohol under CH, i.e., (4) water þ p-Raney-Ni þ CH, and (5) water þ Raney-Ni þ CH, were also carried out. The p-Raney-Ni catalysts used in the reaction systems (1) and (4) were pretreated for 30 min under the temperature of 100 C by microwave radiation. Unless otherwise specified, the parameters of comparative experiments, if involved in the different reaction systems, were as follows: amount of water 80 g, amount of isopropyl alcohol 24 g, catalyst loading 8 wt%, amount of biodiesel 7 g, temperature 85 C, and stirring speed 400 rpm. In order to understand
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3. Results and discussion
hydrogenation process, less hydrogenation of C18:2 to t-C18:1 happened for Raney-Ni catalyst which had a longer pretreatment time. Furthermore, it was found that the content of C18:0 almost remained constant with the increase of microwave pretreatment time. The above-mentioned results indicated that the adverse effect of Raney-Ni from microwave was truly on the reaction of C18:2 to tC18:1 and not on the reaction of C18:1 to C18:0. The effects of Raney-Ni from microwave radiation with pretreatment time on the conversion ratio of C18:2 (CRC182) under different pretreatment temperature are shown in Fig. 2. As the increase of pretreatment temperature, the CRC182 decreased. The CRC182 catalyzed by p-Raney-Ni pretreated under high temperature was lower than that catalyzed by p-Raney-Ni pretreated under low temperature. The results mentioned above confirmed that the microwave truly made an adverse effect on the activity of Raney-Ni for the CTH of biodiesel. The results of different reaction systems with Raney-Ni or pRaney-Ni are shown in Fig. 3. The CRC182 under MH (reaction system of (3)) was higher than that under CH (reaction system of (2)), showing that microwaves had a promoting influence on the biodiesel hydrogenation [31]. However, at the same reaction time, the CRC182 obtained in the reaction system of (1) was significantly lower than the CRC182 obtained in the reaction system of (2), which further confirmed that the effect of Raney-Ni from microwave radiation had a negative role in the CTH of biodiesel. The p-Raney-Ni used in the reaction system of ‘p-RaneyNi þ CH’ and the Raney-Ni used in the reaction system of ‘RaneyNi þ CH’ were analyzed using TGA. As shown in Fig. 4, both used pRaney-Ni and used Raney-Ni had a significant weight loss peak around 300 C, probably due to disappear of carbonaceous deposits attached to the catalyst. Moreover, the amount of carbonaceous deposits on the used p-Raney-Ni was higher than that on the used Raney-Ni because of greater weight loss happened on the former catalyst. Hence, the adsorption capacity of Raney-Ni catalyst was enhanced under the action of microwave, which might be attributed to the surface activation of catalyst by microwave radiation. The carbonaceous deposits on catalyst could cover some active sites of catalyst, which would cause the catalytic activity of catalyst to decrease.
3.1. Effect of Raney-Ni from microwave radiation on CTH of Jatropha oil biodiesel
3.2. Effect of water with Raney-Ni under microwave radiation on CTH of Jatropha oil biodiesel
To explore the effect of Raney-Ni from microwave radiation on the CTH of biodiesel, Raney-Ni was pretreated by microwave radiation before using, and then its catalytic property in CTH reaction under CH was investigated. The changes of C18 FAMEs composition with pretreatment time under different pretreatment temperature are presented in Fig. 1 a-e. It was observed that with the increase of microwave pretreatment time from 0 to 30 min, the content of C18:2 increased, indicating that Raney-Ni truly might have an adverse effect from microwave radiation on the conversion of C18:2. Moreover, the content of t-C18:1 decreased with the microwave pretreatment time, and meanwhile, the content of c-C18:1 almost remained unchanged. The reason of this was that during the
To understand the effect of water on the CTH of biodiesel, two reaction systems of CTH without hydrogen donor (i.e., reaction systems (4) and (5)) were carried out. The results are presented in Fig. 3. The CRC182 increased with the reaction time from 10 to 50 min in the two reaction systems. For the reaction system of (5), the CRC182 increased to 26.03 wt% at reaction time 50 min, indicating that water could provide hydrogen to some extent for the CTH of biodiesel in the absence of hydrogen donor. It was further found that the CRC182 in the reaction system of (4) was higher than that in the reaction system of (1). For example, the CRC182 was 19.59 wt% at reaction time 50 min in the former reaction system, while the CRC182 reached 29.64 wt% at the same time in the latter
Table 1 Composition and content of Jatropha oil biodiesel. Composition
Content of biodiesel (wt%)
C16:0 C18:2 c-C18:1 t-C18:1 C18:0
12.10 30.53 51.40 0 6.06
more clearly, the difference of experimental conditions for the five reaction systems mentioned above are summarized in Table 2. Using different hydrogen donors (methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol or tert-butanol), CTH of biodiesel under CH or under MH was respectively investigated under the conditions as follows: quantity of water 80 g, amount of Jatropha oil biodiesel 7 g, catalyst loading 8 wt%, temperature 85 C, amount of hydrogen donor 24 g, reaction time 50 min and stirring speed 400 rpm. In order to ensure the accuracy of the results, each experiment was performed in duplicate or in triplicate. 2.3. Characterization of catalyst The structure of the Raney-Ni was studied by X-ray diffraction (XRD, SmartLab (3 KW)). XRD patterns were collected using Cu-Ka radiation with beam voltage and current of 40 kV and 30 mA, respectively. The samples were scanned in the range of 2q from 5 to 90 at the rate of 5 /min. The morphology of Raney-Ni was investigated by scanning electron microscope (SEM, Quanta 400). The composition of deposits on the surfaces of catalysts was analyzed by Fourier-transform infrared spectroscopy (FTIR, Nicolet 6700). FTIR analysis was collected using KBr pellet with 4 cm1 resolution. Thermogravimetric analysis (TGA) was used to investigate carbonaceous deposits on the catalyst. Under flowing air at a rate of 50 ml/min, the thermogravimetric curves of catalyst were obtained at a heating rate of 20 C/min between 40 C and 800 C by a Mettler TGA.
Table 2 Difference of the experimental conditions. Reaction systems (1) (2) (3) (4) (5)
water water water water water
þ þ þ þ þ
isopropyl alcohol þ p-Raney-Ni þ CH isopropyl alcohol þ Raney-Ni þ CH isopropyl alcohol þ Raney-Ni þ MH p-Raney-Ni þ CH Raney-Ni þ CH
Isopropyl alcohol
Catalyst used
Heating method
24 g 24 g 24 g / /
p-Raney-Ni Raney-Ni Raney-Ni p-Raney-Ni Raney-Ni
CH CH MH CH CH
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Fig. 1. Changes of composition of C18 FAMEs with pretreatment time under different pretreatment temperature for catalyst: (a) under 80 C, (b) under 85 C, (c) under 90 C, (d) under 95 C, (e) under 100 C.
reaction system. In the reaction system of (1), the acetone formed by dehydrogenation of isopropyl alcohol adhered to the surface of p-Raney-Ni, causing the decrease of the catalytic activity of catalyst. There was no acetone produced in the reaction system of (4), so no negative effect from acetone on the catalytic activity of p-Raney-Ni. Moreover, it was further found that compared with the reaction system of (5), the reaction system of (4) had a slight increase in the CRC182 at all reaction times. The difference showed that the effect of Raney-Ni from microwave radiation played a positive role for the water to provide hydrogen during the CTH of biodiesel.
3.3. Effect of hydrogen donor under microwave radiation on CTH of Jatropha oil biodiesel The effects of hydrogen donor of different alcohols, including methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol and tert-butanol, on the CTH of biodiesel under MH and under CH are presented in Fig. 5 and Fig. 6, respectively. Different types of alcohols had different activities in CTH of biodiesel, showing a considerably different efficiency for the degree of hydrogenation of biodiesel. Isopropyl alcohol showed the highest activity in CTH of
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Fig. 2. Changes of CRC182 with pretreatment time under different pretreatment temperature for catalyst.
Fig. 3. Changes of CRC182 with reaction time in different reaction systems.
Fig. 4. TGA curves of used p-Raney-Ni and used Raney-Ni.
biodiesel, followed by sec-butanol and sec-pentanol in decreasing order. The content of C18:2 in biodiesel hydrogenated for 50 min under MH with isopropyl alcohol, sec-butanol and sec-pentanol was 2.45, 15.01 and 16.95 wt%, respectively. Methanol did not exhibit hydrogen-donating activity in CTH of biodiesel. When
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Fig. 5. Changes of C18 FAMEs composition for different hydrogen donor under MH: (A) feed biodiesel, (B) methanol, (C) ethanol, (D) isopropyl alcohol, (E) sec-butanol, (F) secpentanol, (G) tert-butyl alcohol.
Fig. 6. Changes of C18 FAMEs composition for different hydrogen donor under CH: (A) feed biodiesel, (B) methanol, (C) ethanol, (D) isopropyl alcohol, (E) sec-butanol, (F) secpentanol, (G) tert-butyl alcohol.
ethanol was used as the hydrogen donor under MH, the C18:2content of Jatropha oil biodiesel declined from 30.53 wt% to 22.49 wt%, showing that ethanol had a certain hydrogen-donating activity in CTH of Jatropha oil biodiesel. However, ethanol did not exhibit activity under CH. The results showed that primary alcohols were inferior to secondary alcohols as hydrogen donors in CTH of Jatropha oil biodiesel. This was due to the fact that the aldehyde generated from primary alcohol during the reaction had high adhesion ability on Raney-Ni, which resulted in the deactivation of catalyst [33]. As expected, tert-butanol was found to have no hydrogen-donating activity, which was attributed to the fact that tert-butanol had not a-hydrogen atom to be used as hydrogen source for CTH reaction. To understand the effect of different secondary alcohols on the CTH of biodiesel under microwave radiation, the hydrogenation experiments under MH were carried out using sec-butanol and secpentanol as hydrogen donor, respectively. The results are presented in Fig. 7 and Fig. 8. It was found that the composition changes of two resulting upgraded biodiesels were in line with the composition change of upgraded biodiesel, which has been prepared using isopropyl alcohol as hydrogen donor in our published paper [31]. k1 k2 CTH of biodiesel was a series reaction C18 : 2/ C18 : 1/C18 : 0. 1 Reaction rate constants for C18:2 to C18:1 (k1, min ) and for C18:1
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Fig. 7. Changes of C18 FAMEs composition with reaction time using sec-butanol as hydrogen donor under MH (reaction conditions: catalyst dosage 8 wt%, alcohol amount 24 g, water amount 100 g, reaction temperature 85 C, and stirring rate 400 rpm).
three secondary alcohols decreases in the order of sec-pentanol > sec-butanol > isopropyl alcohol. Moreover, when the substituent group of a-carbon atom is an electron-donating substitute, the electron-releasing inductive effect of substituent group will decrease the activity of a-hydrogen atom as the hydrogen source for CTH reaction. Whether from the polarity secondary alcohol or the electron-releasing inductive effect of substituent group perspective, the order of reaction rate constant should be sec-pentanol > sec-butanol > isopropyl alcohol, which was not consistent with the results obtained in the present work. It should be noted that the hydrogen transfer using alcohol as hydrogen donor takes place via a concerted process involving a six-membered cyclic transition state [34]. In this concerted process, alcohol and longchain oil biodiesel would be adsorbed on a concerted site on the surface of catalyst at the same time. For different secondary alcohols, the steric hindrance from alkyl group of a-carbon atom played an essential role in reaction proceeding. In view of steric hindrance effect from different alkyl groups, the order of reaction rate for CTH of biodiesel under MH was isopropyl alcohol > sec-butanol > secpentanol. 3.4. Causes of Raney-Ni deactivation for CTH reaction
Fig. 8. Changes of C18 FAMEs composition with reaction time using sec-pentanol as hydrogen donor under MH (reaction conditions: catalyst dosage 8 wt%, alcohol amount 24 g, water amount 100 g, reaction temperature 85 C, and stirring rate 400 rpm).
to C18:0 (k2, min1) reflect the reaction process of the first reaction C18:2/C18:1 and the second reaction C18:1/C18:0, respectively. Both of reaction rate constants can be calculated through the data of composition change with reaction time according to the reported method [25,31]. For a better insight into the effect of secondary alcohols under microwave radiation on the CTH of biodiesel, k1 and k2 for different CTH reactions in which isopropyl alcohol, secbutanol and sec-pentanol were used as hydrogen donors, respectively, have been calculated. The values of reaction rate constants are listed in Table 3. It was found that for CTH reaction using different secondary alcohols as hydrogen donors, the reaction rate constant had the following order: isopropyl alcohol > secbutanol > sec-pentanol. Microwave is an electromagnetic wave sensitive to the polar molecules, and the polar molecules are more reactive under the action of microwave [17]. The polarity of the
Generally speaking, the heterogeneous catalyst used in hydrogenation of oils is difficult to be reused due to deactivation. To identify catalytic activity and stability of Raney-Ni after CTH reaction, the comparative tests catalyzed by fresh Raney-Ni and used Raney-Ni were carried out. It was observed from Fig. 9 that the content of C18:2 declined from 30.53 wt% to 8.87 wt% at reaction time 30 min with fresh Raney-Ni as catalyst. However, when used Raney-Ni was used as catalyst once again, the content of C18:2 only reduced to 25.82 wt% at reaction time 30 min. It was found that the catalyst of Raney-Ni was obviously deactivated after the first cycle of using. The main reasons for catalyst deactivation commonly include catalyst site poisoning, active phase sintering, change of the catalysts active phase structure, blockage of the catalyst active surface by adsorbed substances [35,36]. Catalyst site poisoning is usually caused by impurities such as arsenic, halogen, sulfur, lead and mercury occupying the active center of the catalyst, resulting in a decrease in catalyst activity. However, there was no introduction and production of toxic substances in the CTH of biodiesel. Therefore, except for the catalyst site poisoning, the decrease in the activity of used Raney-Ni catalyst might involve in the other three
Table 3 Kinetic parameters of CTH reaction with different hydrogen donors under MH. hydrogen donor
k1 (min1)
k2 (min1)
isopropyl alcohola sec-butanol sec-pentanol
0.0504 0.0142 0.0118
0.00080 0.00034 0.00021
a
obtained from Literature [31].
Fig. 9. Comparative experiments catalyzed by fresh Raney-Ni and used Raney-Ni (catalyst loading 8 wt%, amount of isopropyl alcohol 24 g, quantity of water 80 g, stirring speed 400 rpm, temperature 85 C, amount of Jatropha oil biodiesel 7 g, and reaction time 30 min).
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reasons. The insight into the catalyst deactivation for CTH process can be valuable for the development of regeneration method for used Raney-Ni. To investigate the reasons for the deactivation of Raney-Ni catalyst, both the fresh Raney-Ni and the used Raney-Ni were characterized. XRD characterization is a direct means to observe the effect of thermal deactivation of catalyst. The XRD patterns of fresh RaneyNi and used Raney-Ni are presented in Fig. 10. It was observed that there were no differences in the positions of XRD diffraction peaks between fresh Raney-Ni and used Raney-Ni, which showed that there was almost no change in the phase between fresh RaneyNi and used Raney-Ni. The diffraction peaks at 2q values of 44.5 , 51.8 and 76.3 represent Ni (1 1 1), Ni (2 0 0) and Ni (2 2 0) peaks, respectively (PDF:01e1260). The crystal grain sizes of catalysts calculated from Scherrer's equation for the strongest diffraction peak at 44.5 , corresponding to (1 1 1) plane of Ni, are 12.3 nm for fresh Raney-Ni and 11.2 nm for used Raney-Ni. Even though the used Raney-Ni showed a slight increase in the crystallinity compared with the fresh Raney-Ni, the results of crystal grain sizes indicated that no sintering of active phase happened in the used Raney-Ni. SEM images of fresh Raney-Ni and used Raney-Ni are presented in Fig. 11. As shown in Fig. 11, the surface of fresh Raney-Ni was clean and smooth, and there was no attachment on the catalyst. However, the surface of used Raney-Ni was rough and uneven. Moreover, obvious little attachments were found on the used Raney-Ni. This reason for this might be that the surface of the catalyst was covered by some carbon deposits after reaction. The carbon deposits could block the pores and cover the active site of the Raney-Ni catalyst, making the catalyst less active. FT-IR spectra of fresh Raney-Ni and used Raney-Ni are presented in Fig. 12. Compared with the spectrum of fresh Raney-Ni, the characteristic peak attributed to stretching vibration of C¼O was found at 1740 cm1 in the spectrum of used Raney-Ni [37]. Furthermore, two sharp absorption bands at 2800 cm1 and 3000 cm1 were also discovered in the spectrum of used Raney-Ni, which were due to ¼ CeH [38]. The FTIR results indicated that the deposits on used Raney-Ni were carbonaceous deposits. Acetone was generated from isopropyl alcohol during the hydrogenation of Jatropha oil biodiesel. Acetone, a typical ketone, has high water solubility. The generated acetone could not be the major ingredient of carbonaceous deposits because it was easily dissolved in the water solution during the hydrogenation of Jatropha oil biodiesel. It should be noted that the molecules of oil biodiesels (FAMEs) contain both C¼O and ¼ CeH groups, and oil biodiesels have high viscosity and high adherence capability. It was safely concluded
Fig. 10. XRD patterns of fresh Raney-Ni and used Raney-Ni.
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that the carbonaceous deposits on the surface of used Raney-Ni were mainly FAMEs, which were gradually accumulated on the catalyst during the entire CTH process. Fig. 13 shows the results of TGA of fresh Raney-Ni and used Raney-Ni. Different from the TGA curve of fresh Raney-Ni, a significant weight loss peak around 300 C was found in the TGA curve of used Raney-Ni, indicating a great mass loss happen in used Raney-Ni. The weight loss peak was mainly caused by the oxidative removal of carbonaceous deposits on the surface of Raney-Ni catalyst, namely the combustion of carbonaceous deposits. The carbon removal temperature was around 300 C, which indicated the carbonaceous deposits on Raney-Ni catalyst should belong to light carbon [39]. In one word, the carbonaceous deposit was the main cause of the deactivation of Raney-Ni catalyst. The blockage of catalyst active surface by carbonaceous deposit resulted in the decrease of catalytic activity of Raney-Ni. 3.5. Reaction mechanism of CTH and brief analysis of performance improvement of biodiesel As found in the present work, microwave made an adverse effect on the activity of Raney-Ni. The intensification on CTH mainly resulted from the effects of water with Raney-Ni and hydrogen donors under microwave radiation. However, considering the significant intensification effect on the CTH of Jatropha oil biodiesel under MH, it was safely concluded that the negative effect of the microwave on the catalyst is less than the positive effect of the microwave on the hydrogen donor, solvent and substrate. Microwave, as a transmitted medium and heated energy, can accelerate the reaction rate and even improve the selectivity of chemical reaction. Under microwave irradiation, the activity of isopropyl alcohol would enhance. Furthermore, the cleavage of carbonhydrogen bonds and hydrogen-oxygen bonds in isopropyl alcohol molecules also increased. Therefore, the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. As for FAMEs in Jatropha oil biodiesel, the carbon-carbon double bonds in C18:2 and C18:1 also would be weakened by microwaves during the CTH of biodiesel. Moreover, microwave can greatly enhance the molecular motion of polar molecules (reactants, solvents). Under the action of microwave, the effective collision between hydrogen donors and Jatropha oil biodiesel increased sharply. Moreover, it should be noted that the function of solvent water cannot be ignored during the CTH of biodiesel. In addition to the medium for the contact of hydrogen donor with Raney-Ni catalyst and with substrate in Jatropha oil biodiesel [31], the water also could provide hydrogen to Jatropha oil biodiesel in CTH reaction, even though the hydrogen supply by water was weaker than isopropyl alcohol in the reaction system of CTH of biodiesel. Based on the experiment result and above-mentioned analysis, the schematic representation of mechanism of microwave-assisted CTH is presented in Fig. 14. The CTH of biodiesel, which was carried out using Raney-Ni as catalyst, isopropyl alcohol as hydrogen donor and water as solvent, could be explained on the bases of theory of heterogeneous catalytic process and mechanism of six-membered cyclic transition state. The reaction course of CTH of biodiesel could be roughly divided into four stages. The stages were as follows (as depicted in Fig. 15): (i) Isopropyl alcohol and Jatropha oil biodiesel were adsorbed on Raney-Ni catalyst via the Van der Waals forces. At the same time, cleavage of carbon-hydrogen bond in the isopropyl alcohol happened. After that, the detached ahydrogen atom was transferred to the surface of catalyst and temporarily fixed by a nickel atom. In the above process, the d-orbital vacancies of nickel atom were activated by free
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Fig. 11. SEM images: (a) fresh Raney-Ni, (b) used Raney-Ni.
was transferred to the hydrogen acceptor of Jatropha oil biodiesel. (iii) As illustrated by the circular diagram of dotted line in Fig. 15, the transfer of the second hydrogen atom was achieved via the six-membered cyclic transition state with two contiguous Ni atom sites [34]. When molecules of Jatropha oil biodiesel were bonded to Ni atom through the double bond effect, the hydrogen atom in hydroxy group of alcohols which was adsorbed on the adjacent Ni atom could be transferred to the hydrogen acceptor of Jatropha oil biodiesel. (iv) When CTH reaction finished, isopropyl alcohol changed into acetone, and Jatropha oil biodiesel converted to a high saturated biodiesel. Finally, the products desorbed from the Raney-Ni catalyst.
Fig. 12. FTIR spectra of fresh Raney-Ni and used Raney-Ni.
Fig. 13. TGA curves of fresh Raney-Ni and used Raney-Ni.
electrons in the hydrogen atom, and an active intermediate of hydride was formed (marked as HeNieC(OH)(CH3)2) [40]. (ii) C¼C double bonds in Jatropha oil biodiesel were unsaturated and unstable. A coordination reaction between the NieH bond in HeNieC(OH)(CH3)2) and the C¼C double bond in Jatropha oil biodiesel could easily happen. After the C¼C double bond broke, the hydrogen atom in the NieH bond
The main compositions of upgraded biodiesel and their contents have been given in our published literature [41]. It has been found the content of C18:2 in upgraded Jatropha oil biodiesel finally decreased from 30.53 wt% of Jatropha oil biodiesel to 2.45 wt%, which is a very low level of C18:2 for upgraded biodiesel. Moreover, even a significant reduction in C18:2 content for the upgraded biodiesel, the content of C18:0 in the upgraded Jatropha oil biodiesel showed a slight increase to 8.56 wt% from 6.06 wt% of Jatropha oil biodiesel. It has been well known that the high content of polyunsaturated FAMEs in Jatropha oil biodiesel (C18:2) is the major cause of low oxidative stability of Jatropha oil biodiesel [30], which would make Jatropha oil biodiesel more susceptible to oxidative deterioration and reduce shelf life of biodiesel. Furthermore, a high content of C18:0 in upgraded biodiesel, for its high melting points (39 C), would directly affect the cold flow property of biodiesel. In view of the high content of C18:1 (76.7 wt%) and the very low levels of C18:2 (2.45 wt%) and C18:0 (8.56 wt%), it was safely concluded that the properties of Jatropha oil biodiesel, especially the oxidative stability and the combustibility, have been significantly improved. 4. Conclusions The work presented in this paper was a thorough mechanistic study of CTH of Jatropha oil biodiesel catalyzed by Raney-Ni under microwave heating, using isopropyl alcohol as hydrogen donor and water as solvent. It was found that microwave truly made an adverse effect on the activity of Raney-Ni for the CTH of biodiesel. The adsorption capacity of Raney-Ni catalyst was enhanced under the action of microwave, and the carbonaceous deposits adsorbed on the catalyst could result in the decrease of catalytic activity of
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Fig. 14. Schematic representation of mechanism of microwave-assisted CTH.
Fig. 15. Schematic representation of CTH mechanism for Jatropha oil biodiesel catalyzed by Raney-Ni (where RHC ¼ CHR0 represents C18:2).
catalyst. The solvent of water could provide hydrogen to some extent for the CTH of biodiesel, and the influence of Raney-Ni from microwave radiation had a positive role for the water to provide hydrogen during CTH reaction. Under microwave irradiation, the activity of isopropyl alcohol would be enhanced, and the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. Among different types of alcohols, secondary alcohol was found to be the most effective hydrogen donor for the CTH of biodiesel under MH. The order of reaction rate constant of CTH for three common secondary alcohols was isopropyl alcohol > secbutanol > sec-pentanol. Compared with the polarity of alcohol and the electron-releasing inductive effect of substituent group of alcohol, the steric hindrance effect from alkyl group of a-carbon atom of alcohol played an essential role to determine the order of reaction rate constant. It was disclosed that the carbonaceous deposit was the main cause of the deactivation of Raney-Ni catalyst when used in CTH of biodiesel. The carbonaceous deposits were mainly FAMEs and not acetone. Based on the research results, the mechanism of microwave-assisted CTH has been proposed. Combining theory of heterogeneous catalytic process with mechanism of six-membered cyclic transition state, the reaction course for the CTH of Jatropha oil biodiesel under microwave heating has been explained successfully. Future experimentation should focus on the regeneration of
deactivated catalyst, as well as the process analysis for the upgrading of biodiesel which hydrogenated by the CTH method under microwave heating. Acknowledgements Financial support from National Natural Science Foundation of China (21667004), Guangxi Science Foundation Funded Project (2018GXNSFAA281343, 2018GXNSFAA138039), Innovation Project of Guangxi Graduate Education (YCSW2019032), and Undergraduates' Innovation and Entrepreneurship Program in Guangxi (201810593194) is gratefully acknowledged. References [1] A. Demi_Rbas, Progress and recent trends in biodiesel fuels, Energy Convers. Manag. 50 (2009) 14e34. https://doi.org/10.1016/j.enconman.2008.09.001. [2] J. Chen, C.S. Zhou, S.J. Wang, S.J. Li, Impacts of energy consumption structure, energy intensity, economic growth, urbanization on PM2.5 concentrations in countries globally, Appl. Energy 230 (2018) 94e105. https://doi.org/10.1016/j. apenergy.2018.08.089. [3] A.K. Agarwal, Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines, Prog. Energ. Combust. 33 (2007) 233e271. https://doi. org/10.1016/j.pecs.2006.08.003. [4] Y. Louhasakul, B. Cheirsilp, S. Maneerat, P. Prasertsan, Potential use of flocculating oleaginous yeasts for bioconversion of industrial wastes into biodiesel feedstocks, Renew. Energy 136 (2019) 1311e1319. https://doi.org/10.
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Mechanistic study of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under microwave heating Linye Zhang a, Zongwu Xin a, Zihan Liu a, Guangtao Wei a, b, *, Zhongmin Li a, Yuning Ou a a b
School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China Guangxi Key Laboratory of Processing for Non-ferrous Metallic and Featured Materials, Guangxi Zhuang Autonomous Region, Nanning, 530004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 25 July 2019 Accepted 11 September 2019 Available online 13 September 2019
The use of microwave heating (MH) constitutes a promising way to upgrade biodiesel via catalytic transfer hydrogenation (CTH) catalyzed by Raney-Ni. However, mechanistic reports on this reaction are rare and the mechanisms involved in CTH process remains unclear. Herein, we presented a thorough mechanistic study of CTH of Jatropha oil biodiesel catalyzed by Raney-Ni under MH, using isopropyl alcohol as hydrogen donor and water as solvent. The intensification effect under microwave radiation has been studied from three aspects of catalyst, water, and hydrogen donor. It was found that microwave made an adverse effect on the activity of Raney-Ni. The solvent of water could provide hydrogen to some extent for the CTH of biodiesel, and the influence of Raney-Ni from microwave radiation had a positive role for the water to provide hydrogen during CTH reaction. Under microwave irradiation, the activity of isopropyl alcohol would be enhanced, and the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. Based on the characterization of catalyst by SEM, XRD, FTIR and TGA, it was found that the reason for the catalyst deactivation was mainly the carbonaceous deposits of fatty acid methanol esters (FAMEs) on the surface of the catalyst. A schematic representation of mechanism of microwave-assisted CTH has been presented. Furthermore, combining theory of heterogeneous catalytic process with mechanism of six-membered cyclic transition state, the reaction course for the CTH of Jatropha oil biodiesel under microwave heating has been explained successfully. Lastly, the performance improvement of the upgraded Jatropha oil biodiesel was briefly analyzed in view of composition change of biodiesel. Overall, our results provide a systematic understanding of CTH of biodiesel catalyzed by Raney-Ni under microwave heating, and our findings help to develop a green technology for the upgrading of biodiesel in industry. © 2019 Published by Elsevier Ltd.
Keywords: Microwave Catalytic transfer hydrogenation Jatropha oil Raney-Ni Biodiesel
1. Introduction Energy is one of the main pillars of modern civilization, and it is indispensable in social life. However, with the rapid development of human society, the non-renewable energy sources such as coal and oil are increasingly lacking, and the air pollution caused by the burning of fossil fuels is becoming increasingly prominent [1,2]. The world is up against the dual crisis of fossil fuel depletion and environmental deterioration. The search for sustainable and environment-friendly alternative fuels has become a very pressing issue in recent years [3]. Biodiesel, prepared from non-grain
* Corresponding author. School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China. E-mail address: [email protected] (G. Wei). https://doi.org/10.1016/j.renene.2019.09.035 0960-1481/© 2019 Published by Elsevier Ltd.
biomass, has become one of the effective ways for humans to solve the energy crisis and reduce air pollution owing to its renewable performance, higher cetane number and lower exhaust emissions as compared with petroleum-based diesel [4e6]. The biodiesel is composed of fatty acid methyl esters (FAMEs), produced via the transesterification of oils (vegetable oils, animal fats and waste oils) with methanol [7e10]. Although biodiesel shows many advantages, it has some drawbacks related to its properties such as poor cold flow property and low oxidative stability. These properties of biodiesel are truly related to the structural features of FAMEs. Biodiesel with high content of polyunsaturated FAMEs has a low oxidative stability. Whereas if a high content of saturated FAMEs, the biodiesel would show a poor cold flow property [11]. Monounsaturated FAME compositions are ideal for biodiesel [12,13]. Jatropha oil, as a non-edible vegetable oil, is an excellent raw material for biodiesel producing [14e16].
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However, Jatropha oil biodiesel generally has relatively high contents of polyunsaturated FAMEs, which are the needless compositions for gaining a biodiesel with ideal compositions of monounsaturated FAMEs. To upgrade biodiesel, namely increasing the cold flow properties and improving the oxidation property of biodiesel, selective hydrogenation has become a useful process to refine the FAMEs of biodiesel [17e19]. Selective hydrogenation for unsaturated organic compounds can proceed via conventional hydrogenation method or catalytic transfer hydrogenation (CTH) method. In comparison with conventional hydrogenation in which uses the explosive hydrogen gas, CTH utilizing molecular hydrogen as the hydrogen donor is a simple, safe and eco-friendly method of hydrogenation. It has been proven that CTH is an effective alternative to conventional hydrogenation to realize the selective hydrogenation [20e22]. Moreover, compared with non-noble catalysts, noble catalysts have been used in CTH process due to their higher catalytic activity and selectivity [23]. CTH reactions of vegetable oils or grease-like materials catalyzed by noble metal have been reported in the literature [24]. Martinelli et al. investigated the CTH of castor oil using limonene as the hydrogen donor and Pd/C as catalyst [25]. Tike and Mahajani investigated the CTH of soybean oil using ammonium formate as the donor over 5% Pd/C catalyst, and it was found that the CTH of soybean oil offered good selectivity in the complete reduction of linolenic acid [26]. Sancheti and Gogate investigated the CTH of soybean oil with Pd/C as catalyst under ultrasound, and reported that the CTH of soybean oil offered excellent selectivity with low trans-isomer formation [27]. Although the noble catalysts have excellent catalytic performance in CTH, the high cost limits their wide application in CTH of oils or grease-like materials. Thus, an inexpensive non-noble catalyst is needed for selective hydrogenations, which proceed via CTH method. Among the non-noble catalysts, nickel-based catalysts have been broadly used in hydrogenation reaction due to its high catalytic activity, low cost, and easy separation from the product. However, there are some drawbacks for CTH reaction catalyzed by nickel-based catalyst, which include its low catalytic selectivity and high reaction temperature [28]. Therefore, a process-intensifying method to increase the catalytic selectivity of nickel-based catalyst and reduce reaction temperature can make the process of CTH economical and efficient in operation. In view of the excellent properties of microwave as heating and process-intensifying method [29,30], the CTH of Jatropha oil biodiesel heated by microwave irradiation has been studied in our laboratory [31]. It was found that microwave was an excellent process-intensifying method for the CTH of biodiesel. Compared with the CTH of Jatropha oil biodiesel under conventional heating (CH), the process of CTH under microwave heating (MH) occurred very rapidly, and the upgraded Jatropha oil biodiesel obtained had a remarkable advantage in the high content of C18:1 composite. However, it should be pointed out that the mechanistic reports on the hydrogenation of biodiesel via CTH method are truly rare. As for the selective hydrogenation of Jatropha oil biodiesel catalyzed by Raney-Ni under MH, the mechanisms of CTH of biodiesel, including the mechanism of process intensification in the reaction system, are not clear. Hence, a thorough research on the mechanism of CTH of biodiesel catalyzed by Raney-Ni under MH would help to have a clear understanding of the reaction process, and would provide a theoretical basis for the further development of this upgrading technique for biodiesel. The present work is, therefore, focused on the mechanisms of the catalytic transfer hydrogenation of biodiesel catalyzed by Raney-Ni under MH. Firstly, the intensification effects on the CTH of biodiesel under microwave radiation have been studied from three aspects: catalyst, water and hydrogen donor. Moreover, based on the identifying of the catalytic activity and stability of Raney-Ni
after CTH reaction, the cause of catalyst deactivation in this reaction system has been explored in detail. Lastly, the reaction mechanism of CTH of biodiesel in this reaction system has also been proposed, and in view of the composition change, the performance improvement of the upgraded Jatropha oil biodiesel was also briefly analyzed.
C16:0 C18:2 c-C18:1 t-C18:1 C18:0 CRC182 k1 k2
methyl hexadecanoate methyl linoleate cis-methyl oleate trans-methyl oleate methyl stearate mass conversion ratio of C18:2 (wt%) reaction rate constant for C18:2 to C18:1 (min1) reaction rate constant for C18:1 to C18:0 (min1)
2. Materials and methods 2.1. Materials Jatropha oil was purchased from FMD Bio-Energy Ltd. (Guangxi, China). Jatropha oil biodiesel was synthesized by us in our laboratory, according to the synthesis method previously developed [31,32]. The composition of Jatropha oil biodiesel is shown in Table 1. Raney-Ni was obtained from Raney metal technology Ltd. (Jiangsu, China). All chemicals, including methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol and tert-butanol were analytical grade. 2.2. Catalytic transfer hydrogenation of Jatropha oil biodiesel catalyzed by Raney-Ni Microwave chemical reactor (MCR-3; Shijishuangke experimental instrument co., Led, Zhengzhou, China) with a temperature sensor and a motor-driven Teflon stirring rod was used in this work. With the automatic model, the microwave chemical reactor can automatically control the reaction temperature and the reaction time at predetermine values. 0.56 g of Raney-Ni (8 wt% of biodiesel) and 80 g of water were taken in a 250-ml three-necked flask, and then the flask was placed in the microwave chemical reactor. The microwave pretreatment of Raney-Ni was operated at different time (0e30 min) and different temperature (80e100 C). After pretreatment of Raney-Ni under microwave radiation, 24 g of isopropyl alcohol and 7 g of biodiesel were added into the above mixtures of Raney-Ni and water. Under CH, the CTH of biodiesel, in which the catalyst was pretreated Raney-Ni (p-Raney-Ni) by microwave radiation, was carried out under the conditions of temperature 85 C, reaction time 60 min and stirring speed 400 rpm. To understand the essence of catalyst effect under microwave radiation on the CTH reaction, different reaction systems with RaneyNi or p-Raney-Ni were carried out, including (1) water þ isopropyl alcohol þ p-Raney-Ni þ CH, (2) water þ isopropyl alcohol þ RaneyNi þ CH, and (3) water þ isopropyl alcohol þ Raney-Ni þ MH. On the base of understanding the essence of catalyst effect, two additional reaction systems of CTH without isopropyl alcohol under CH, i.e., (4) water þ p-Raney-Ni þ CH, and (5) water þ Raney-Ni þ CH, were also carried out. The p-Raney-Ni catalysts used in the reaction systems (1) and (4) were pretreated for 30 min under the temperature of 100 C by microwave radiation. Unless otherwise specified, the parameters of comparative experiments, if involved in the different reaction systems, were as follows: amount of water 80 g, amount of isopropyl alcohol 24 g, catalyst loading 8 wt%, amount of biodiesel 7 g, temperature 85 C, and stirring speed 400 rpm. In order to understand
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3. Results and discussion
hydrogenation process, less hydrogenation of C18:2 to t-C18:1 happened for Raney-Ni catalyst which had a longer pretreatment time. Furthermore, it was found that the content of C18:0 almost remained constant with the increase of microwave pretreatment time. The above-mentioned results indicated that the adverse effect of Raney-Ni from microwave was truly on the reaction of C18:2 to tC18:1 and not on the reaction of C18:1 to C18:0. The effects of Raney-Ni from microwave radiation with pretreatment time on the conversion ratio of C18:2 (CRC182) under different pretreatment temperature are shown in Fig. 2. As the increase of pretreatment temperature, the CRC182 decreased. The CRC182 catalyzed by p-Raney-Ni pretreated under high temperature was lower than that catalyzed by p-Raney-Ni pretreated under low temperature. The results mentioned above confirmed that the microwave truly made an adverse effect on the activity of Raney-Ni for the CTH of biodiesel. The results of different reaction systems with Raney-Ni or pRaney-Ni are shown in Fig. 3. The CRC182 under MH (reaction system of (3)) was higher than that under CH (reaction system of (2)), showing that microwaves had a promoting influence on the biodiesel hydrogenation [31]. However, at the same reaction time, the CRC182 obtained in the reaction system of (1) was significantly lower than the CRC182 obtained in the reaction system of (2), which further confirmed that the effect of Raney-Ni from microwave radiation had a negative role in the CTH of biodiesel. The p-Raney-Ni used in the reaction system of ‘p-RaneyNi þ CH’ and the Raney-Ni used in the reaction system of ‘RaneyNi þ CH’ were analyzed using TGA. As shown in Fig. 4, both used pRaney-Ni and used Raney-Ni had a significant weight loss peak around 300 C, probably due to disappear of carbonaceous deposits attached to the catalyst. Moreover, the amount of carbonaceous deposits on the used p-Raney-Ni was higher than that on the used Raney-Ni because of greater weight loss happened on the former catalyst. Hence, the adsorption capacity of Raney-Ni catalyst was enhanced under the action of microwave, which might be attributed to the surface activation of catalyst by microwave radiation. The carbonaceous deposits on catalyst could cover some active sites of catalyst, which would cause the catalytic activity of catalyst to decrease.
3.1. Effect of Raney-Ni from microwave radiation on CTH of Jatropha oil biodiesel
3.2. Effect of water with Raney-Ni under microwave radiation on CTH of Jatropha oil biodiesel
To explore the effect of Raney-Ni from microwave radiation on the CTH of biodiesel, Raney-Ni was pretreated by microwave radiation before using, and then its catalytic property in CTH reaction under CH was investigated. The changes of C18 FAMEs composition with pretreatment time under different pretreatment temperature are presented in Fig. 1 a-e. It was observed that with the increase of microwave pretreatment time from 0 to 30 min, the content of C18:2 increased, indicating that Raney-Ni truly might have an adverse effect from microwave radiation on the conversion of C18:2. Moreover, the content of t-C18:1 decreased with the microwave pretreatment time, and meanwhile, the content of c-C18:1 almost remained unchanged. The reason of this was that during the
To understand the effect of water on the CTH of biodiesel, two reaction systems of CTH without hydrogen donor (i.e., reaction systems (4) and (5)) were carried out. The results are presented in Fig. 3. The CRC182 increased with the reaction time from 10 to 50 min in the two reaction systems. For the reaction system of (5), the CRC182 increased to 26.03 wt% at reaction time 50 min, indicating that water could provide hydrogen to some extent for the CTH of biodiesel in the absence of hydrogen donor. It was further found that the CRC182 in the reaction system of (4) was higher than that in the reaction system of (1). For example, the CRC182 was 19.59 wt% at reaction time 50 min in the former reaction system, while the CRC182 reached 29.64 wt% at the same time in the latter
Table 1 Composition and content of Jatropha oil biodiesel. Composition
Content of biodiesel (wt%)
C16:0 C18:2 c-C18:1 t-C18:1 C18:0
12.10 30.53 51.40 0 6.06
more clearly, the difference of experimental conditions for the five reaction systems mentioned above are summarized in Table 2. Using different hydrogen donors (methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol or tert-butanol), CTH of biodiesel under CH or under MH was respectively investigated under the conditions as follows: quantity of water 80 g, amount of Jatropha oil biodiesel 7 g, catalyst loading 8 wt%, temperature 85 C, amount of hydrogen donor 24 g, reaction time 50 min and stirring speed 400 rpm. In order to ensure the accuracy of the results, each experiment was performed in duplicate or in triplicate. 2.3. Characterization of catalyst The structure of the Raney-Ni was studied by X-ray diffraction (XRD, SmartLab (3 KW)). XRD patterns were collected using Cu-Ka radiation with beam voltage and current of 40 kV and 30 mA, respectively. The samples were scanned in the range of 2q from 5 to 90 at the rate of 5 /min. The morphology of Raney-Ni was investigated by scanning electron microscope (SEM, Quanta 400). The composition of deposits on the surfaces of catalysts was analyzed by Fourier-transform infrared spectroscopy (FTIR, Nicolet 6700). FTIR analysis was collected using KBr pellet with 4 cm1 resolution. Thermogravimetric analysis (TGA) was used to investigate carbonaceous deposits on the catalyst. Under flowing air at a rate of 50 ml/min, the thermogravimetric curves of catalyst were obtained at a heating rate of 20 C/min between 40 C and 800 C by a Mettler TGA.
Table 2 Difference of the experimental conditions. Reaction systems (1) (2) (3) (4) (5)
water water water water water
þ þ þ þ þ
isopropyl alcohol þ p-Raney-Ni þ CH isopropyl alcohol þ Raney-Ni þ CH isopropyl alcohol þ Raney-Ni þ MH p-Raney-Ni þ CH Raney-Ni þ CH
Isopropyl alcohol
Catalyst used
Heating method
24 g 24 g 24 g / /
p-Raney-Ni Raney-Ni Raney-Ni p-Raney-Ni Raney-Ni
CH CH MH CH CH
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Fig. 1. Changes of composition of C18 FAMEs with pretreatment time under different pretreatment temperature for catalyst: (a) under 80 C, (b) under 85 C, (c) under 90 C, (d) under 95 C, (e) under 100 C.
reaction system. In the reaction system of (1), the acetone formed by dehydrogenation of isopropyl alcohol adhered to the surface of p-Raney-Ni, causing the decrease of the catalytic activity of catalyst. There was no acetone produced in the reaction system of (4), so no negative effect from acetone on the catalytic activity of p-Raney-Ni. Moreover, it was further found that compared with the reaction system of (5), the reaction system of (4) had a slight increase in the CRC182 at all reaction times. The difference showed that the effect of Raney-Ni from microwave radiation played a positive role for the water to provide hydrogen during the CTH of biodiesel.
3.3. Effect of hydrogen donor under microwave radiation on CTH of Jatropha oil biodiesel The effects of hydrogen donor of different alcohols, including methanol, ethanol, isopropyl alcohol, sec-butanol, sec-pentanol and tert-butanol, on the CTH of biodiesel under MH and under CH are presented in Fig. 5 and Fig. 6, respectively. Different types of alcohols had different activities in CTH of biodiesel, showing a considerably different efficiency for the degree of hydrogenation of biodiesel. Isopropyl alcohol showed the highest activity in CTH of
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Fig. 2. Changes of CRC182 with pretreatment time under different pretreatment temperature for catalyst.
Fig. 3. Changes of CRC182 with reaction time in different reaction systems.
Fig. 4. TGA curves of used p-Raney-Ni and used Raney-Ni.
biodiesel, followed by sec-butanol and sec-pentanol in decreasing order. The content of C18:2 in biodiesel hydrogenated for 50 min under MH with isopropyl alcohol, sec-butanol and sec-pentanol was 2.45, 15.01 and 16.95 wt%, respectively. Methanol did not exhibit hydrogen-donating activity in CTH of biodiesel. When
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Fig. 5. Changes of C18 FAMEs composition for different hydrogen donor under MH: (A) feed biodiesel, (B) methanol, (C) ethanol, (D) isopropyl alcohol, (E) sec-butanol, (F) secpentanol, (G) tert-butyl alcohol.
Fig. 6. Changes of C18 FAMEs composition for different hydrogen donor under CH: (A) feed biodiesel, (B) methanol, (C) ethanol, (D) isopropyl alcohol, (E) sec-butanol, (F) secpentanol, (G) tert-butyl alcohol.
ethanol was used as the hydrogen donor under MH, the C18:2content of Jatropha oil biodiesel declined from 30.53 wt% to 22.49 wt%, showing that ethanol had a certain hydrogen-donating activity in CTH of Jatropha oil biodiesel. However, ethanol did not exhibit activity under CH. The results showed that primary alcohols were inferior to secondary alcohols as hydrogen donors in CTH of Jatropha oil biodiesel. This was due to the fact that the aldehyde generated from primary alcohol during the reaction had high adhesion ability on Raney-Ni, which resulted in the deactivation of catalyst [33]. As expected, tert-butanol was found to have no hydrogen-donating activity, which was attributed to the fact that tert-butanol had not a-hydrogen atom to be used as hydrogen source for CTH reaction. To understand the effect of different secondary alcohols on the CTH of biodiesel under microwave radiation, the hydrogenation experiments under MH were carried out using sec-butanol and secpentanol as hydrogen donor, respectively. The results are presented in Fig. 7 and Fig. 8. It was found that the composition changes of two resulting upgraded biodiesels were in line with the composition change of upgraded biodiesel, which has been prepared using isopropyl alcohol as hydrogen donor in our published paper [31]. k1 k2 CTH of biodiesel was a series reaction C18 : 2/ C18 : 1/C18 : 0. 1 Reaction rate constants for C18:2 to C18:1 (k1, min ) and for C18:1
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Fig. 7. Changes of C18 FAMEs composition with reaction time using sec-butanol as hydrogen donor under MH (reaction conditions: catalyst dosage 8 wt%, alcohol amount 24 g, water amount 100 g, reaction temperature 85 C, and stirring rate 400 rpm).
three secondary alcohols decreases in the order of sec-pentanol > sec-butanol > isopropyl alcohol. Moreover, when the substituent group of a-carbon atom is an electron-donating substitute, the electron-releasing inductive effect of substituent group will decrease the activity of a-hydrogen atom as the hydrogen source for CTH reaction. Whether from the polarity secondary alcohol or the electron-releasing inductive effect of substituent group perspective, the order of reaction rate constant should be sec-pentanol > sec-butanol > isopropyl alcohol, which was not consistent with the results obtained in the present work. It should be noted that the hydrogen transfer using alcohol as hydrogen donor takes place via a concerted process involving a six-membered cyclic transition state [34]. In this concerted process, alcohol and longchain oil biodiesel would be adsorbed on a concerted site on the surface of catalyst at the same time. For different secondary alcohols, the steric hindrance from alkyl group of a-carbon atom played an essential role in reaction proceeding. In view of steric hindrance effect from different alkyl groups, the order of reaction rate for CTH of biodiesel under MH was isopropyl alcohol > sec-butanol > secpentanol. 3.4. Causes of Raney-Ni deactivation for CTH reaction
Fig. 8. Changes of C18 FAMEs composition with reaction time using sec-pentanol as hydrogen donor under MH (reaction conditions: catalyst dosage 8 wt%, alcohol amount 24 g, water amount 100 g, reaction temperature 85 C, and stirring rate 400 rpm).
to C18:0 (k2, min1) reflect the reaction process of the first reaction C18:2/C18:1 and the second reaction C18:1/C18:0, respectively. Both of reaction rate constants can be calculated through the data of composition change with reaction time according to the reported method [25,31]. For a better insight into the effect of secondary alcohols under microwave radiation on the CTH of biodiesel, k1 and k2 for different CTH reactions in which isopropyl alcohol, secbutanol and sec-pentanol were used as hydrogen donors, respectively, have been calculated. The values of reaction rate constants are listed in Table 3. It was found that for CTH reaction using different secondary alcohols as hydrogen donors, the reaction rate constant had the following order: isopropyl alcohol > secbutanol > sec-pentanol. Microwave is an electromagnetic wave sensitive to the polar molecules, and the polar molecules are more reactive under the action of microwave [17]. The polarity of the
Generally speaking, the heterogeneous catalyst used in hydrogenation of oils is difficult to be reused due to deactivation. To identify catalytic activity and stability of Raney-Ni after CTH reaction, the comparative tests catalyzed by fresh Raney-Ni and used Raney-Ni were carried out. It was observed from Fig. 9 that the content of C18:2 declined from 30.53 wt% to 8.87 wt% at reaction time 30 min with fresh Raney-Ni as catalyst. However, when used Raney-Ni was used as catalyst once again, the content of C18:2 only reduced to 25.82 wt% at reaction time 30 min. It was found that the catalyst of Raney-Ni was obviously deactivated after the first cycle of using. The main reasons for catalyst deactivation commonly include catalyst site poisoning, active phase sintering, change of the catalysts active phase structure, blockage of the catalyst active surface by adsorbed substances [35,36]. Catalyst site poisoning is usually caused by impurities such as arsenic, halogen, sulfur, lead and mercury occupying the active center of the catalyst, resulting in a decrease in catalyst activity. However, there was no introduction and production of toxic substances in the CTH of biodiesel. Therefore, except for the catalyst site poisoning, the decrease in the activity of used Raney-Ni catalyst might involve in the other three
Table 3 Kinetic parameters of CTH reaction with different hydrogen donors under MH. hydrogen donor
k1 (min1)
k2 (min1)
isopropyl alcohola sec-butanol sec-pentanol
0.0504 0.0142 0.0118
0.00080 0.00034 0.00021
a
obtained from Literature [31].
Fig. 9. Comparative experiments catalyzed by fresh Raney-Ni and used Raney-Ni (catalyst loading 8 wt%, amount of isopropyl alcohol 24 g, quantity of water 80 g, stirring speed 400 rpm, temperature 85 C, amount of Jatropha oil biodiesel 7 g, and reaction time 30 min).
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reasons. The insight into the catalyst deactivation for CTH process can be valuable for the development of regeneration method for used Raney-Ni. To investigate the reasons for the deactivation of Raney-Ni catalyst, both the fresh Raney-Ni and the used Raney-Ni were characterized. XRD characterization is a direct means to observe the effect of thermal deactivation of catalyst. The XRD patterns of fresh RaneyNi and used Raney-Ni are presented in Fig. 10. It was observed that there were no differences in the positions of XRD diffraction peaks between fresh Raney-Ni and used Raney-Ni, which showed that there was almost no change in the phase between fresh RaneyNi and used Raney-Ni. The diffraction peaks at 2q values of 44.5 , 51.8 and 76.3 represent Ni (1 1 1), Ni (2 0 0) and Ni (2 2 0) peaks, respectively (PDF:01e1260). The crystal grain sizes of catalysts calculated from Scherrer's equation for the strongest diffraction peak at 44.5 , corresponding to (1 1 1) plane of Ni, are 12.3 nm for fresh Raney-Ni and 11.2 nm for used Raney-Ni. Even though the used Raney-Ni showed a slight increase in the crystallinity compared with the fresh Raney-Ni, the results of crystal grain sizes indicated that no sintering of active phase happened in the used Raney-Ni. SEM images of fresh Raney-Ni and used Raney-Ni are presented in Fig. 11. As shown in Fig. 11, the surface of fresh Raney-Ni was clean and smooth, and there was no attachment on the catalyst. However, the surface of used Raney-Ni was rough and uneven. Moreover, obvious little attachments were found on the used Raney-Ni. This reason for this might be that the surface of the catalyst was covered by some carbon deposits after reaction. The carbon deposits could block the pores and cover the active site of the Raney-Ni catalyst, making the catalyst less active. FT-IR spectra of fresh Raney-Ni and used Raney-Ni are presented in Fig. 12. Compared with the spectrum of fresh Raney-Ni, the characteristic peak attributed to stretching vibration of C¼O was found at 1740 cm1 in the spectrum of used Raney-Ni [37]. Furthermore, two sharp absorption bands at 2800 cm1 and 3000 cm1 were also discovered in the spectrum of used Raney-Ni, which were due to ¼ CeH [38]. The FTIR results indicated that the deposits on used Raney-Ni were carbonaceous deposits. Acetone was generated from isopropyl alcohol during the hydrogenation of Jatropha oil biodiesel. Acetone, a typical ketone, has high water solubility. The generated acetone could not be the major ingredient of carbonaceous deposits because it was easily dissolved in the water solution during the hydrogenation of Jatropha oil biodiesel. It should be noted that the molecules of oil biodiesels (FAMEs) contain both C¼O and ¼ CeH groups, and oil biodiesels have high viscosity and high adherence capability. It was safely concluded
Fig. 10. XRD patterns of fresh Raney-Ni and used Raney-Ni.
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that the carbonaceous deposits on the surface of used Raney-Ni were mainly FAMEs, which were gradually accumulated on the catalyst during the entire CTH process. Fig. 13 shows the results of TGA of fresh Raney-Ni and used Raney-Ni. Different from the TGA curve of fresh Raney-Ni, a significant weight loss peak around 300 C was found in the TGA curve of used Raney-Ni, indicating a great mass loss happen in used Raney-Ni. The weight loss peak was mainly caused by the oxidative removal of carbonaceous deposits on the surface of Raney-Ni catalyst, namely the combustion of carbonaceous deposits. The carbon removal temperature was around 300 C, which indicated the carbonaceous deposits on Raney-Ni catalyst should belong to light carbon [39]. In one word, the carbonaceous deposit was the main cause of the deactivation of Raney-Ni catalyst. The blockage of catalyst active surface by carbonaceous deposit resulted in the decrease of catalytic activity of Raney-Ni. 3.5. Reaction mechanism of CTH and brief analysis of performance improvement of biodiesel As found in the present work, microwave made an adverse effect on the activity of Raney-Ni. The intensification on CTH mainly resulted from the effects of water with Raney-Ni and hydrogen donors under microwave radiation. However, considering the significant intensification effect on the CTH of Jatropha oil biodiesel under MH, it was safely concluded that the negative effect of the microwave on the catalyst is less than the positive effect of the microwave on the hydrogen donor, solvent and substrate. Microwave, as a transmitted medium and heated energy, can accelerate the reaction rate and even improve the selectivity of chemical reaction. Under microwave irradiation, the activity of isopropyl alcohol would enhance. Furthermore, the cleavage of carbonhydrogen bonds and hydrogen-oxygen bonds in isopropyl alcohol molecules also increased. Therefore, the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. As for FAMEs in Jatropha oil biodiesel, the carbon-carbon double bonds in C18:2 and C18:1 also would be weakened by microwaves during the CTH of biodiesel. Moreover, microwave can greatly enhance the molecular motion of polar molecules (reactants, solvents). Under the action of microwave, the effective collision between hydrogen donors and Jatropha oil biodiesel increased sharply. Moreover, it should be noted that the function of solvent water cannot be ignored during the CTH of biodiesel. In addition to the medium for the contact of hydrogen donor with Raney-Ni catalyst and with substrate in Jatropha oil biodiesel [31], the water also could provide hydrogen to Jatropha oil biodiesel in CTH reaction, even though the hydrogen supply by water was weaker than isopropyl alcohol in the reaction system of CTH of biodiesel. Based on the experiment result and above-mentioned analysis, the schematic representation of mechanism of microwave-assisted CTH is presented in Fig. 14. The CTH of biodiesel, which was carried out using Raney-Ni as catalyst, isopropyl alcohol as hydrogen donor and water as solvent, could be explained on the bases of theory of heterogeneous catalytic process and mechanism of six-membered cyclic transition state. The reaction course of CTH of biodiesel could be roughly divided into four stages. The stages were as follows (as depicted in Fig. 15): (i) Isopropyl alcohol and Jatropha oil biodiesel were adsorbed on Raney-Ni catalyst via the Van der Waals forces. At the same time, cleavage of carbon-hydrogen bond in the isopropyl alcohol happened. After that, the detached ahydrogen atom was transferred to the surface of catalyst and temporarily fixed by a nickel atom. In the above process, the d-orbital vacancies of nickel atom were activated by free
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Fig. 11. SEM images: (a) fresh Raney-Ni, (b) used Raney-Ni.
was transferred to the hydrogen acceptor of Jatropha oil biodiesel. (iii) As illustrated by the circular diagram of dotted line in Fig. 15, the transfer of the second hydrogen atom was achieved via the six-membered cyclic transition state with two contiguous Ni atom sites [34]. When molecules of Jatropha oil biodiesel were bonded to Ni atom through the double bond effect, the hydrogen atom in hydroxy group of alcohols which was adsorbed on the adjacent Ni atom could be transferred to the hydrogen acceptor of Jatropha oil biodiesel. (iv) When CTH reaction finished, isopropyl alcohol changed into acetone, and Jatropha oil biodiesel converted to a high saturated biodiesel. Finally, the products desorbed from the Raney-Ni catalyst.
Fig. 12. FTIR spectra of fresh Raney-Ni and used Raney-Ni.
Fig. 13. TGA curves of fresh Raney-Ni and used Raney-Ni.
electrons in the hydrogen atom, and an active intermediate of hydride was formed (marked as HeNieC(OH)(CH3)2) [40]. (ii) C¼C double bonds in Jatropha oil biodiesel were unsaturated and unstable. A coordination reaction between the NieH bond in HeNieC(OH)(CH3)2) and the C¼C double bond in Jatropha oil biodiesel could easily happen. After the C¼C double bond broke, the hydrogen atom in the NieH bond
The main compositions of upgraded biodiesel and their contents have been given in our published literature [41]. It has been found the content of C18:2 in upgraded Jatropha oil biodiesel finally decreased from 30.53 wt% of Jatropha oil biodiesel to 2.45 wt%, which is a very low level of C18:2 for upgraded biodiesel. Moreover, even a significant reduction in C18:2 content for the upgraded biodiesel, the content of C18:0 in the upgraded Jatropha oil biodiesel showed a slight increase to 8.56 wt% from 6.06 wt% of Jatropha oil biodiesel. It has been well known that the high content of polyunsaturated FAMEs in Jatropha oil biodiesel (C18:2) is the major cause of low oxidative stability of Jatropha oil biodiesel [30], which would make Jatropha oil biodiesel more susceptible to oxidative deterioration and reduce shelf life of biodiesel. Furthermore, a high content of C18:0 in upgraded biodiesel, for its high melting points (39 C), would directly affect the cold flow property of biodiesel. In view of the high content of C18:1 (76.7 wt%) and the very low levels of C18:2 (2.45 wt%) and C18:0 (8.56 wt%), it was safely concluded that the properties of Jatropha oil biodiesel, especially the oxidative stability and the combustibility, have been significantly improved. 4. Conclusions The work presented in this paper was a thorough mechanistic study of CTH of Jatropha oil biodiesel catalyzed by Raney-Ni under microwave heating, using isopropyl alcohol as hydrogen donor and water as solvent. It was found that microwave truly made an adverse effect on the activity of Raney-Ni for the CTH of biodiesel. The adsorption capacity of Raney-Ni catalyst was enhanced under the action of microwave, and the carbonaceous deposits adsorbed on the catalyst could result in the decrease of catalytic activity of
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Fig. 14. Schematic representation of mechanism of microwave-assisted CTH.
Fig. 15. Schematic representation of CTH mechanism for Jatropha oil biodiesel catalyzed by Raney-Ni (where RHC ¼ CHR0 represents C18:2).
catalyst. The solvent of water could provide hydrogen to some extent for the CTH of biodiesel, and the influence of Raney-Ni from microwave radiation had a positive role for the water to provide hydrogen during CTH reaction. Under microwave irradiation, the activity of isopropyl alcohol would be enhanced, and the transfer of hydrogen atoms from hydrogen donors to hydrogen acceptors became easy. Among different types of alcohols, secondary alcohol was found to be the most effective hydrogen donor for the CTH of biodiesel under MH. The order of reaction rate constant of CTH for three common secondary alcohols was isopropyl alcohol > secbutanol > sec-pentanol. Compared with the polarity of alcohol and the electron-releasing inductive effect of substituent group of alcohol, the steric hindrance effect from alkyl group of a-carbon atom of alcohol played an essential role to determine the order of reaction rate constant. It was disclosed that the carbonaceous deposit was the main cause of the deactivation of Raney-Ni catalyst when used in CTH of biodiesel. The carbonaceous deposits were mainly FAMEs and not acetone. Based on the research results, the mechanism of microwave-assisted CTH has been proposed. Combining theory of heterogeneous catalytic process with mechanism of six-membered cyclic transition state, the reaction course for the CTH of Jatropha oil biodiesel under microwave heating has been explained successfully. Future experimentation should focus on the regeneration of
deactivated catalyst, as well as the process analysis for the upgrading of biodiesel which hydrogenated by the CTH method under microwave heating. Acknowledgements Financial support from National Natural Science Foundation of China (21667004), Guangxi Science Foundation Funded Project (2018GXNSFAA281343, 2018GXNSFAA138039), Innovation Project of Guangxi Graduate Education (YCSW2019032), and Undergraduates' Innovation and Entrepreneurship Program in Guangxi (201810593194) is gratefully acknowledged. References [1] A. Demi_Rbas, Progress and recent trends in biodiesel fuels, Energy Convers. Manag. 50 (2009) 14e34. https://doi.org/10.1016/j.enconman.2008.09.001. [2] J. Chen, C.S. Zhou, S.J. Wang, S.J. Li, Impacts of energy consumption structure, energy intensity, economic growth, urbanization on PM2.5 concentrations in countries globally, Appl. Energy 230 (2018) 94e105. https://doi.org/10.1016/j. apenergy.2018.08.089. [3] A.K. Agarwal, Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines, Prog. Energ. Combust. 33 (2007) 233e271. https://doi. org/10.1016/j.pecs.2006.08.003. [4] Y. Louhasakul, B. Cheirsilp, S. Maneerat, P. Prasertsan, Potential use of flocculating oleaginous yeasts for bioconversion of industrial wastes into biodiesel feedstocks, Renew. Energy 136 (2019) 1311e1319. https://doi.org/10.
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