Production of ethylene glycol from direct catalytic conversion of cellulose over a binary catalyst of metal-loaded modified SBA-15 and phosphotungstic acid

This study presents the utilization of a binary catalyst composed of metal-loaded modified SBA-15 (M/SBA-15) and phosphotungstic acid (H3PW12O40) for ethylene glycol (EG) production from direct catalytic conversion of cellulose. M/SBA-15 (M = Ru, Au, Pd, Pt, Rh and Ni) catalysts were prepared using the impregnation method and characterized by means of XRD, N2 physisorption, TEM and H2-temperature-programmed reduction (H2-TPR) techniques. Their catalytic performance was then studied in detail on the basis of cellulose conversion and the selectivity of polyols and EG. The results showed that the mesoporous structure of the SBA-15 sample was well maintained after the metal-loaded modification, and almost all of the selected catalysts gave about 100% conversion of cellulose. However, the selectivity for EG was greatly different. Among the various binary catalysts, the combination of Rh/SBA-15 and H3PW12O40 gave the best selectivity to EG (55.5%), whereas the worst selectivity of EG (11%) was obtained over the Au/SBA-15 and H3PW12O40 system under identical conditions. In addition to phosphotungstic acid, other W compounds were also studied in combination with the Ru/SBA-15 catalyst. The results showed that the EG selectivity depended on the W compounds as follows: H4SiW12O40 < H2WO4 < H3PW12O40. Therefore, the binary catalyst of Rh/SBA-15 and H3PW12O40 showed the greatest potential for EG production from direct catalytic conversion of cellulose.


Introduction
The excessive consumption of fossil fuel and the ever-increasing emissions associated with their consumption have spurred a great deal of research for alternative energy sources. 1 Biomass as a clean and renewable carbon source on earth has received increasing attention. 2 With regards to the utilization of biomass, the conversion of cellulose holds great potential due to its nonedible nature and huge availability. 3 Nevertheless, cellulose, as a biopolymer bonded by b-1,4-glucosidic bonds, contains abundant hydrogen bonds; hence, it is difficult to convert it by solvents or catalysts under mild conditions. 4 Therefore, the robust structure of cellulose remains a great concern for its efficient transformation under mild conditions.
In order to achieve high energy efficiency and atom economy in the conversion of cellulose, different primary routes have been explored. [5][6][7][8] One of the most promising routes is direct catalytic conversion of cellulose to polyols, for which the polyol products can be directly used as chemicals or precursors to produce renewable chemicals. 9 This process couples the C-C breaking reaction of cellulose by an acid catalyst to form glucose and the hydrogenation of the glucose over the metal catalysts to generate the polyols. 7 Amongst the polyol products, EG has a large market and is widely used in the pharmaceutical, cosmetic, food, dye, plastic and automobile industries; thus, it is considered to be a high-value market product. 10 Currently, the manufacture of EG is mainly dependent on petroleum resources using the method of oxidation, followed by hydration of ethylene. 11 Therefore, obtaining EG from the direct transformation of cellulose is very signicant because of its possible environment benets and the current concern over the depletion of fossil fuel sources. For this catalytic approach, catalyst plays an important role in the selectivity of a specic polyol product. For example, using Ru/AC as the catalyst (AC ¼ activated carbon), Liu et al. obtained $40% yield of hexitols. 12 Zhang et al. obtained EG yield of up to $54% using H 2 WO 4 in combination with 1.2% Ru/AC catalyst. 13 It is well known that the dispersion and accessibility of active sites have a signicant impact on the activity and selectivity of the catalyst. To develop efficient catalysts, extensive studies have been conducted by using different technologies. For example, Zhang et al. used the traditional active carbon (AC) as support for dispersing Ni particles to improve the activity of the catalyst in the catalytic conversion of cellulose to EG. 14 Although the AC had large surface area, the active sites exhibited low dispersion and poor accessibility due to the microporous structure of AC. Additionally, the microporous carbon used as support may have a negative effect on the overall catalytic performance due to its steric hindrance and diffusion limitations. For this reason, mesoporous carbon (CMK-3) with WC x active sites was synthesized to catalyze cellulose. 15 The WC x /CMK-3 catalyst exhibited better catalytic performance than the WC x /AC catalyst due to its good accessibility and the dispersion of the active sites. However, the rigorous synthesis conditions and difficulty in replication greatly limited its application. In 2010, Zheng et al. developed a series of bimetallic catalysts for the conversion of cellulose, including Ru-W, Ni-W, Ir-W and Pt-W supported on different carriers. It was found that the maximum yield of EG (75.4%) was obtained over the Ni-W/SBA-15 catalyst. 16 In this context, the mesoporous material SBA-15 can be used as the catalyst for catalytic cracking of cellulose due to its ordered hexagonal mesostructure and large pore size, which can facilitate the conversion of bulky molecules. 17 However, the pure silica SBA-15 material had almost no catalytic activity because of the absence of active sites. 18 Therefore, the catalyst must be further modied and improved to solve this problem. Among all the modication methods, the impregnation method has remarkable advantages such as simple preparation and easier industrial application. 19 Up to now, metals, including Ni, 20 La, 21 Al, 22 Ca 23 and Mg, 24 modied with SBA-15 have shown good catalytic performance in the catalytic cracking of vegetable oils. It is worth mentioning that these catalysts were all prepared using the impregnation method. Nevertheless, there are few reports related to obtaining the desired products from catalytic conversion of cellulose over these catalysts.
In this study, we evaluated the catalytic performances of various binary catalysts of M/SBA-15 (M ¼ Ru, Au, Pd, Pt, Rh and Ni) mesoporous materials and different W compounds and chose the best binary catalyst to produce EG from the catalytic conversion of cellulose. To the best of our knowledge, there is no clear picture of the comparative performance of these catalysts. Therefore, it is expected that the results will provide the vital guidance for the rational selection of highly active binary catalysts for the catalytic conversion of cellulose to EG.

Materials
Microcrystalline cellulose (MCC, average molecular weight: 90 000) was purchased from Sigma-Aldrich and used without further purication.  20 ), and tetraethyl orthosilicate (TEOS), were all purchased from Aldrich and used aer drying without any further treatment.

Preparation and characterization of catalysts
SBA-15 was synthesized by the hydrothermal method according to the literature. 25 The molar composition of the mixture was 1TEOS : 0.02P 123 : 6HCl : Ru/SBA-15 catalyst was prepared by the wet impregnation method. First, 1.0 g SBA-15 powder was immersed into 20 mL RhCl 3 $3H 2 O solution (10 wt%) under stirring at 80 C. Aer evaporation at 80 C, the sample was dried at 100 C for 12 h and then calcined at 550 C for 6 h in air. The solid obtained was reduced with H 2 (20 cm 3 min À1 ) at 550 C for 2 h at a heating rate of 2 C min À1 , giving the Ru-SBA-15 catalyst. Other catalysts such as Ni (Pt, Pd, Au, Rh)/SBA-15 were also prepared using the same method.
XRD patterns were recorded on a XB-3A instrument operating at 40 kV and 100 mA with Cu Ka radiation (l ¼ 0.15418 nm) in the 2q range from 0.5 to 10 . The experiment conditions were as follows: a step width of 0.01 and scan speed of 2 min À1 . The BET specic surface area and pore size distribution of the catalysts were measured from N 2 adsorption/ desorption isotherms at À77 K using a Micromeritics ASAP 2000 system. TEM images were recorded to observe the dispersion degree of the metal phase. Transmission electron microscopy (TEM) was conducted on a JEOS-2020 F electron microscope operating at an accelerating voltage of 200 kV. The metal contents of the as-synthesized catalyst were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique on a Varian VISTA-PRO AX spectrophotometer.
H 2 -temperature-programmed reduction (H 2 -TPR) was performed in a xed reactor. Prior to the TPR, 100 mg samples were purged in Ar at 100 C for 1 h. Subsequently, the samples were heated in a ow of pure H 2 from 100 C to 800 C at a rate of 10 C min À1 and were analyzed by a Hiden QIC-20 mass spectrometer.

Catalytic cracking cellulose
The catalytic conversion of cellulose was carried out in a Teon-lined stainless autoclave (100 mL) at 245 C for 4 h under 5.0 MPa hydrogen pressure. Typically, the reaction mixture was composed of cellulose (W 1 ), binary catalyst (0.1 g M/SBA-15 and 0.02 g W compounds) and 25 mL of H 2 O, which were successively placed in the autoclave. Then, the reactor was purged with nitrogen at about 0.2 MPa to completely remove the air. The reactor was heated by external electrical resistance at a heating rate of 15 C min À1 and its temperature was measured using a calibrated thermocouple. Aer the reaction, the cellulose (W 2 ) remaining in the reactor was collected by centrifugation and used to calculate the cellulose conversion. The compositions of liquid-phase products were analyzed by HPLC equipped with RID (Refractive Index Detector). The separation was realized on a column of Bio-Rad HPX-87X column (8 Â 300 mm) with 0.05 M H 2 SO 4 aqueous solution as mobile phase, and each product (W i ) was quanti-ed using the external standard method. The conversion (wt%) of cellulose and selectivity of the desired product (wt%) were calculated as follows: 3. Results and discussion

Catalyst characterization
The  Table 1. It was found that the SBA-15 material had high specic surface area (580 m 2 g À1 ), but the surface areas of the M/SBA-15 samples were reduced within the range of 394-482 m 2 g À1 . Moreover, the M/SBA-15 samples exhibited smaller total pore volume and larger average pore size compared to those of the pure silica SBA-15 sample. This may be due to the accumulation of metal species on the surface of the mesoporous material or deep into the channel interior, thus hindering the interior channel of the mesoporous material SBA-15 and resulting in smaller pore volume. Moreover, the metal species loaded on SBA-15 will partially block most of the micropores in the M/SBA-15 samples, thereby increasing the average pore size. All these results indicated that the metal phases were dispersed on the surface of the SBA-15 support. According to Table 1, the specic surface areas and total pore volumes of the Pd (Ni, Pt)/SBA-15 samples were relatively low compared to those of the other samples, which may be due to the accumulation of excess metal species on the surface of the SBA-15 support.
The particle size distributions of the metal phase on the surface of the SBA-15 support were determined by TEM images. As shown in Fig. 3, the average particle size of different catalyst was different. The average particle sizes of Rh and Ru on the SBA-15 support were 5.78 nm and 4.25 nm, respectively (Fig. 3(c  and d)). However, the average particle sizes of Ni/SBA-15 and Pd/ SBA-15 were larger than 20 nm (Fig. 3(e and f)), which may be due to the aggregation of metal species on the surface of SBA-15. 27 These results showed that the metal particles Au, Ru and Rh have good dispersibility on the SBA-15 support, which is consistent with the structural parameters of the different samples obtained from the N 2 adsorption-desorption experiment. Fig. 4 displays the H 2 -TPR proles of the SBA-15 and the M/ SBA-15 samples. As can be seen from Fig. 4, different M/SBA-15 catalysts showed the hydrogen desorption peaks at different temperatures. This indicates that the metal species were successfully loaded on the SBA-15 sample and had catalytic hydrogenation capability. It is worth noting that the Ni (Pd)/ SBA-15 samples showed higher hydrogen desorption peaks compared to those observed for the other catalysts, which may be due to the accumulation of excess Ni and Pd metal species on the surface of the SBA-15 support, which can be conrmed by the TEM images.  Table 2 shows the distribution of the main products, namely, ethylene glycol (EG), sorbitol (SO), mannitol (MA), 1,2-propylene glycol (PG), and glycerol (GY), obtained using different tested binary catalysts. As can be seen from Table 2, cellulose was almost totally converted even in the absence of M/SBA-15 catalysts (Entry 1), indicating that TPA plays a key role in the cracking cellulose reaction.
The EG selectivity signicantly improved when the M/SBA-15 catalysts were introduced (Entries 3-8). According to the literature, 27 metal catalyst as a hydrogenation active centre has a synergistic effect with the W compound, that is, the cellulose is hydrolyzed by W compound, followed by the metal-catalyzed hydrogenation of the formed intermediate products to obtain     binary catalyst in the selective formation of EG from cellulose. From Table 2, it is observed that different combinations of the binary catalyst have a signicant impact on the selectivity of EG. The best selectivity to EG (55%) was observed when the binary catalyst of Ru/SBA-15 and TPA was used (Entry 5), whereas the worst selectivity to EG (11%) was observed when using Au/SBA-15 and TPA as the binary catalyst (Entry 8     and C-C breaking reactions of cellulose. Interestingly, regardless of the type of W compounds used, the polyols selectivity is always high in the presence of Ru/SBA-15 catalyst, indicating that Ru/SBA-15 exhibit excellent catalytic performance in the selective formation of polyols. This may be due to the fact that Ru particles are well dispersed on the SBA-15 support to form more hydrogenation active sites (seeing Fig. 3(d)). The effect of different binary catalysts on the EG selectivity was investigated, and the detailed results are shown in Fig. 5(b). It was found that the binary catalyst of Ru/SBA-15 and TPA gave the highest EG selectivity (55.5%), whereas the EG selectivity of 46.5% and 24.0% were obtained when the Ru/SBA-15 was combined with TA and TSA, respectively. This indicates that the combination of Ru/SBA-15 and TPA show better synergy in the selective formation of EG from cellulose than other binary catalysts. In order to further shed light on the potential of the Ru/SBA-15 catalyst in the catalytic conversion of cellulose, the effect of binary catalyst of Ru/SBA-15 and different W compounds on the distribution of polyol products was investigated. As shown in Fig. 5(c), the addition of W compounds can effectively improve the polyol products selectivity, particularly the EG selectivity, which increases from 13.0% to 55.5% aer the addition of TPA. At the same time, it can be seen that regardless of which W compound was used in combination with Ru/SBA-15, EG was always predominant product. These results indicate that W compounds play an important role in the selective formation of EG from cellulose. It is worth noting that the lower EG selectivity (24.0%) and higher SO selectivity (19.6%) were observed when using TSA and Ru/SBA-15 as the binary catalyst. The reason for higher SO selectivity and lower EG selectivity can be explained as follows: TSA catalyze the cleavage of C-C bonds of cellulose by retro-aldol reaction, but it is less active to C-C breaking reaction of cellulose so that it results in lower EG selectivity. 13 Based on the above analysis, we can conclude that the EG selectivity depended on the W compounds in the presence of the Ru/SBA-15 in the following order: TSA < TA < TPA. These results show the potential of the binary catalyst of Ru/SBA-15 and TPA in the selective formation of EG from cellulose. As Ru/SBA-15 and TPA system has good selectivity to EG, the binary catalyst was chosen for the further study.

The effect of reaction conditions on the distribution of main products
In order to improve the EG selectivity, the reaction conditions for the catalytic conversion of cellulose using the binary catalyst of Ru/SBA-15 and TPA as catalyst were investigated. Fig. 6 shows the effect of catalyst dosage on the distribution of polyol products. Fig. 6(a) displays the effect of the Ru/SBA-15 dosage on the distribution of polyol products. As shown in Fig. 6(a), the selectivity of EG increased with the increase in the Ru/SBA-15 dosage from 0.0 g to 0.1 g. When the catalyst Ru/SBA-15 dosage was 0.1 g, EG selectivity reached the maximum of 55.5%. However, when the Ru/SBA-15 dosage was 0.14 g, the selectivity of EG decreased to 49.3%. This can be explained by the fact that the more the amount of Ru/SBA-15, the faster would be the reaction for EG hydrogenolysis. 11 Therefore, the optimal the amount of Ru/SBA-15 seems to be 0.1 g. Fig. 6(b) shows the effect of the TPA dosage on the distribution of polyol products. According to Fig. 6(b), when only Ru/SBA-15 was used, the polyol products such as EG, SO and MA were produced from the C-C breaking reaction of cellulose. This indicates that Ru-SBA-15 not only possesses hydrogenation properties, but also has hydrogenolysis ability. Compared with no TPA, the addition of TPA can greatly improve the EG selectivity. For example, when the amount of TPA was 0.02 g and on xing the amount of Ru/SBA-15 at 0.1 g, EG selectivity was enhanced from 13.0% to 55.5%. Nevertheless, when the TPA dosage was 0.03 g, the EG selectivity decreased to 51.2%. Similar to the inuence of the Ru/SBA-15 content, the selectivity for EG also showed a volcano curve with the change in the amount of TPA, signifying that a good balance between Ru/SBA-15 and TPA is required. Based on the above results, the binary catalyst combination of 0.1 g Ru/SBA-15 and 0.02 g TPA shows the best catalytic performance for EG selectivity from catalytic conversion cellulose.
The plot showing the effect of reaction temperature on the distribution of polyol products is displayed in Fig. 7(a). As the reaction temperature increased from 225 C to 245 C, the EG selectivity increased from 50.4% to 55.5%, respectively. This might be due to the fact that more H + was produced from hot compressed water at higher temperature to promote the conversion of cellulose, and/or the higher temperature might provide more energy for the cleavage of C-C bond of sugar intermediates. 30 When the temperature was increased to 255 C, the selectivity for EG decreased to 47.3%. This may be due to the deactivation of active sites or the degradation of EG product at higher temperature. Therefore, the optimum operating temperature was set as 245 C. Fig. 7(b) displays the effect of reaction time on the distribution of polyol products. When the reaction time was increased from 2.0 h to 4.0 h, the EG selectivity increased from 48.2% to 55.5%, respectively. With prolonged reaction time, the selectivity to EG decreased from 55.5% to 52.1%. This is ascribed to the long reaction time favoring side reactions such as hydrogenolysis of EG. 11 Therefore, the optimum reaction conditions for EG selectivity were as follows: Ru/SBA-15: 0.1 g; TPA: 0.02 g; 245 C, 4 h. Under the abovementioned conditions, the EG selectivity reached nearly 55.5%.

Conclusion
The direct catalytic conversion of cellulose into EG was investigated over the various binary catalysts composed of M/SBA-15 and W compounds. A remarkable advantage of these binary catalysts was that it not only led to a nearly 100% conversion of  This journal is © The Royal Society of Chemistry 2018 cellulose, but also gave the higher selectivity for EG. Among the binary catalysts, Ru/SBA-15 and TPA system showed the excellent performance for EG selectivity (about 55.5%). More importantly, compared with single catalyst, the use of the binary catalyst can signicantly improve the EG selectivity due to the synergistic effect between M/SBA-15 and W compounds. Moreover, the SBA-15 mesoporous material, which has a large surface area and ordered hexagonal mesopore, facilitates not only the high dispersion of metal element but also the transport of reactant and product molecules. Taking into account that the raw material used is cellulose and the importance of EG in the petrochemical industry, the new binary catalyst of Ru/SBA-15 and TPA is a promising candidate for the production of EG from cellulose in the future.

Conflicts of interest
There are no conicts to declare.