Accepted Manuscript One-pot catalytic conversion of cellulose into polyols with Pt/CNTs catalysts Li Yang, Xiaopei Yan, Qiwu Wang, Qiong Wang, Haian Xia PII: DOI: Reference:

S0008-6215(14)00437-6 http://dx.doi.org/10.1016/j.carres.2014.12.001 CAR 6903

To appear in:

Carbohydrate Research

Received Date: Revised Date: Accepted Date:

16 June 2014 28 November 2014 11 December 2014

Please cite this article as: Yang, L., Yan, X., Wang, Q., Wang, Q., Xia, H., One-pot catalytic conversion of cellulose into polyols with Pt/CNTs catalysts, Carbohydrate Research (2014), doi: http://dx.doi.org/10.1016/j.carres. 2014.12.001

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One-pot catalytic conversion of cellulose into polyolswith Pt/CNTs catalysts Li Yang, Xiaopei Yan, Qiwu Wang, Qiong Wang, Haian Xia*

Jiangsu Key Lab of Biomass-Based Green Fuels and Chemicals, College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China

* To whom correspondence should be addressed. Tel: +86-25-85427635; Fax: +86-25-85418873 E-mail: [email protected]

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Abstract A series of Pt nanoparticles supported on carbon nanotubes (CNTs) were synthesized using the incipient-wetness impregnation method. These catalysts were characterized by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Raman spectroscopy, and Transmission electron microscope (TEM) techniques. The characterization results indicate that the Pt nanoparticles were highly dispersed on the surface of the CNTs, and the mean size was less than 5 nm. These catalysts were utilized to convert cellulose to hexitol, ethylene glycerol (EG), and 1,2-propylene glycol (1,2-PG) under low H2 pressure. The total yields were as high as 71.4% for EG and 1,2-PG using 1Pt/CNTs as the catalyst in the hydrolytic hydrogenation of cellulose under mild reaction conditions.

Keywords: Pt/CNTs, ethylene glycerol, 1,2-propylene glycol, hydrogenation

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1. Introduction Cellulose, which is a chain of glucose molecules, is the primary fuel component in lignocellulosic biomass. However, unlike starch, it is not soluble in water. Highly efficient conversion of cellulose has received considerable attention due to a shortage of sources and environmental issues. The development of chemical techniques for transforming cellulose to chemicals and bio-fuels would have significant economic and environmental impacts. However, it is very difficult for cellulose to be selectively converted due to its insolubility and the existence of inter- and intramolecular H bonds.1, 2 During the past ten years, the hydrolytic hydrogenation method has been used to convert cellulose into valuable chemicals, such as sorbitol3-9 and ethylene glycol.10-20 Fukuoka et al. reported that the hydrolytic hydrogenation of cellulose over Pt/γ-Al2O3 catalyst produced high yields of sorbitol and mannitol.3 Zhang et al. described the use of a Ni promoted W2C catalyst, which resulted in high yields of EG.13 Liang et al. reported that CuCr catalysts exhibited a high yield for the production of EG and 1,2-PG even using highly concentrated cellulose.12 In addition, Ru/C,6, 21 Ru/CNTs,21 supported Ni catalysts,7, 20, 22and M(Pt, Ni, Sn, Cu)-SnOx/Al2 O317are active for the production of polyols. As described above, the metallic function enhances the hydrolysis of cellulose into oligomers and glucose by contributing H+ ions resulting from cleavage of H2 or by promotion of hydride steps with the exception of its hydrogenation activity.16 However, several disadvantages of heterogeneous catalysis must be overcome. 3

First, sub- and super-critical solvents, such as water, possess weak acidity that is detrimental to many solid catalysts especially supported metal catalysts, which leads to agglomeration of nanoparticles, leaching of active metal sites, loss of surface area or collapse of pore structure and solubility of the inorganic support.2, 11, 16 Carbon nanotubes (CNTs) are an excellent support due to their special properties including high hydrothermal stability, one-dimension pore structure, rich surface functional groups, and high H2 capability.23, 24 Therefore, carbon nanotubes (CNTs) are one of the most suitable choices for catalyst supports for the hydrolytic hydrogenation of cellulose under severe hydrothermal conditions(i.e., the sub- or super-critical conditions). In this study, Pt/CNTs catalysts with different Pt loadings were synthesized and used to convert microcrystalline cellulose to EG, 1,2-PG, hexitol, and other polyols. The effects of the Pt nanoparticle size and reaction conditions on the conversion of cellulose and product selectivity were investigated. The possible conversion pathways of cellulose to ethylene glycerol, 1,2-PG, hexitol, and unsaturated aldehydes or ketones are proposed. 2. Results and Discussion 2.1. Catalyst characterization The XRD patterns of CNTs and Pt/CNTs with varied Pt contents are shown in Fig. 1. In the pattern for the CNTs, a strong peak located at 2θ= 26.32°and a weak peak located at2θ= 42.9° were observed and correspond to the (0 0 2) and (1 0 0)

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diffraction patterns of typical graphite, 25 respectively. This result indicates that the CNTs are well graphitized, which is also supported by the Raman spectral result where the intensity ratio of the G/D band increased after HNO3 treatment (Supporting Information, Fig. S1). Therefore, HNO3 treatment results in high-purity MWCNTs by removing amorphous carbon and metallic impurities from the as-produced CNTs. In addition, HNO3 treatment increased the amount of oxygen-containing functional groups (e.g., carboxyl and carbonyl groups), which could favor high dispersion of Pt nanoparticles (Supporting Information, Table S2, Figs. S3 and S4). After Pt loading, three diffraction peaks were observed at a 2θof 39.5°, 46.3°, and 67.4°corresponding to the (1 1 1), (2 0 0), and (2 2 0) reflections of the face-centered cubic (f.c.c.) crystalline Pt nanoparticles,25 respectively, indicating that the formation of Pt nanoparticles on the surface of the CNTs. The average size of the Pt nanoparticles calculated by the Scherrer equation is shown in Table 1. The particle size of the two catalysts is very small, and the Pt nanoparticles are highly dispersed on the two catalysts. Fig. 2 shows representative TEM images of Pt/CNTs with different Pt loadings. Pt nanoparticles of the two Pt/CNTs catalysts with 0.5 and 1 wt.% Pt are narrowly sized (in the range of 1-6 nm), and their average sizes are 1.86 and 2.11 nm, respectively. However, the Pt nanoparticles of the 5Pt/CNTs catalyst have a larger average size of 4.60 nm. This result indicates that the Pt nanoparticles of three catalysts are well dispersed on the CNT surface. Most of the Pt nanoparticles appear to be on the external surface of the CNTs (Fig. 2). It is also possible that a few Pt 5

nanoparticles are inside the CNT channel because HNO3 pretreatment may open the end of the CNTs. 2.2. Catalytic conversion of cellulose The hydrolytic hydrogenation of microcrystalline cellulose was conducted in an aqueous medium. The liquid products are polyols, such as ethylene glycerol, 1,2-PG, hexitol, and some unknown products (Supporting Information, Figs. 3, S6 and S7). The unidentified compounds may be water-soluble oligosaccharides, which were not detected by HPLC and LC-MS due to their low content.2,

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Therefore, the

unidentified compounds lead to a low polyol yield. The total organic carbon analysis (Supporting Information, Table S1) indicated that approximately 70% of the cellulose was converted to liquid products, such as EG and 1,2-PG, suggesting that approximately 30% of the cellulose was transformed into non-condensable gas, such as CO and CH4, during the cellulose hydrogenation reaction.14 It is important to note that the fresh liquid product was colorless or light yellow, and its color became dark brown upon exposure to air for several hours or days (Supporting Information, Fig. S5). A similar phenomenon was also observed by Zheng et al., who determined that the liquid product contained no detectable level of aldehydes or reducing sugars.10 The catalytic activity for the hydrolytic hydrogenation of cellulose using Pt/CNTs is provided in Tables 2 and 3. Cellulose can be converted in hot water even in the absence of catalysts because H+ ions can be formed at high temperatures (above 200 °C) and act as an acid catalyst.2,

3, 16

We determined that CNTs could also

effectively convert cellulose to glucose and other unknown products at 240 °C. The 6

addition of the Pt/CNT catalyst produced EG, 1,2-PG, hexitol, and some unknown products. The cellulose conversion over 5Pd/CNTs was approximately 60.8% at a reaction temperature of 220 °C, and the conversion of cellulose reached 100% as the reaction temperature increased to more than 240 °C (Table 2, entries 3-5).This results is due to the high temperatures enhancing the formation of H+ ions and promoting the cracking of the β-1,4-glycosidic bond in cellulose to form oligosaccharides and glucose.5, 13As the reaction temperature increased, the yield of EG decreased from 37.2% at 240 °C to 4.8% at 260 °C, and the yield of 1,2-PG decreased from 6.2% at 240 °C to 4.8% at 260 °C. Therefore, a temperature of 240 °C results in the best results (Table 2, entries 3 and 5) because a temperature greater than 240 °C favors further conversion of EG and 1,2-PG to produce non-condensable gases, such as CO and CH4.14This explanation is supported by the TOC result where the effective carbon yield decreased from 76.2% at 240 °C to 60.3% at 260 °C (Supporting Information, Table S1, entries 1-3). As the Pt content increased, the total yield of EG and 1,2-PG first increased and then decreased. 0.5Pt/CNTs, 1Pt/CNTs and 5Pt/CNTs afforded a polyol yield of 5.7, 73.9%, and 52.9%, respectively (Table 2, entries 1-3). It should be noted that the total yields of EG and 1,2-PG (i.e., as high as 71.4%) are higher than those of monometallic catalysts, such as Pd/C and Pt/C.10 However, the total yields of EG and 1,2-PG obtained from the Pt/CNT catalysts were lower than those obtained from bimetallic catalysts and combined catalysts(e.g., Ni-W2C catalyst,13 M(Pt, Ni, Sn, Cu)-SnOx/Al2O3,17 Ru/C combined with H2WO42). The effects of the cellulose concentration and reaction time on the conversion and 7

product selectivity were also investigated, and the results are shown in Table 3. An increase in the cellulose concentration results in a decrease in the EG and 1,2-PG yields but did not affect the cellulose conversion. In addition, a longer reaction time favored an increase in the EG yield simultaneously decreased the 1,2-PG yield over 1Pt/CNTs. 1Pt/CNTs exhibited the highest polyol yield of 43.9% at a reaction time of 4 h, which indicated that a long reaction time favored the formation of EG under high cellulose concentration. Zheng et al. reported that the cellulose conversion was 63.5% and the EG yield was 11.6% using a Pt/C catalyst under similar reaction conditions. 10

Therefore, the CNTs may provide a better support than activated carbon, and a

longer reaction time favored the production of EG. In addition, the color of the liquid product of the highly concentrated cellulose (500 mg cellulose / 50 mL H2O) changed faster than the lower concentration cellulose (200 mg cellulose / 50 mL H2O). This result suggests that the highly concentrated cellulose produced more unsaturated compounds due to incomplete hydrogenation. From the above result presented, it is found that the particle sizes have a small effect on the hydrogenation activity of glucose since the hexitol yield does not directly correlate with the Pt particle size (Table 2, entries 1-3). However, it seems that the particle sizes have a marked effect on the hydrogenolysis product, especially the 1,2-PG yield (Table 2, entries 1-3 and Table 3, entries 3-4). The different Pt particle sizes exhibit the different selectivities toward hexitol and 1,2-PG could be due to their different reaction mechanisms. The production pathway of hexitol is the hydrogenation of glucose derived from the hydrolysis reaction of cellulose, while that 8

of EG and 1,2-PG is that cellulose is firstly hydrolyzed into hexose (r1), which then undergoes retro-aldol condensation reaction to produce C2 and C3 intermediate (r2), glycolaldehyde and glyceraldehyde, which further are hydrogenated to EG and 1,2-PG (r3).2,14 Due to the instabilities of glycolaldehyde and glyceraldehyde as well as cellulose-derived sugars, the reaction rates should be r1