Na Ji

Direct Catalytic Conversion of Cellulose into Ethylene Glycol Using Nickel-Promoted Tungsten Carbide Catalysts

Cellulose, the most abundant source of biomass, is currently regarded as a promising alternative for fossil fuels as it cannot be digested by human beings and thus its use, unlike corn and starch, will not impose a negative impact on food supplies. One of the most attractive routes for the reaction of cellulose utilization is its direct conversion into useful organic compounds. A recent example of the catalytic conversion of cellulose has been demonstrated by Fukuoka and Dhepe, who utilized Pt/Al2O3 as an effective catalyst to convert cellulose into sugar alcohols (Scheme 1, Route A). The product sugar alcohols can be used as chemicals in their own right or as new starting materials for the production of fuels, as demonstrated by Dumesic and co-workers. 6] Recently, Luo et al. have studied this process further. In their work, the reaction was conducted at elevated temperatures so that water could generate H ions to catalyze the hydrolysis reactions. The subsequent hydrogenation reaction was catalyzed by Ru/C. An increased sugar alcohol yield was obtained, which was attributed to the higher reaction temperatures and the wellknown high efficiency of Ru/C in the hydrogenation reaction. . A disadvantage of the above two studies is the use of precious-metal catalysts. The amount of precious metals needed for the degradation of cellulose was relatively high, 4– 10 mg per gram of cellulose. This is too expensive for the conversion of large quantities of cellulose, even though the solid catalyst could be reused. Therefore, it is highly desirable to develop a less expensive but efficient catalyst to replace precious-metal catalysts in this cellulose degradation process. The carbides of Groups 4–6 metals show catalytic performances similar to those of platinum-group metals in a variety of reactions involving hydrogen. In our previous work, tungsten and molybdenum carbides were found to exhibit excellent performances in the catalytic decomposition of hydrazine, which were comparable with those of expensive iridium catalysts. Tungsten carbides have been used as electrocatalysts because of their platinum-like catalytic behavior, stability in acidic solutions, and resistance to CO poisoning. 18] However, to the best of our knowledge, there have been no attempts so far to utilize metal carbides as catalysts for cellulose conversion. Herein we report the first observation that carbonsupported tungsten carbide (W2C/AC; AC = activated carbon) can effectively catalyze cellulose conversion into polyols (Scheme 1, Route B). More interestingly, when the catalyst is promoted with a small amount of nickel, the yield of polyols, especially ethylene glycol (EG) and sorbitol, can be significantly increased. These Ni-W2C/AC catalysts showed a remarkably higher selectivity for EG formation than Pt/Al2O3 [4] and Ru/C. After 30 minutes at 518 K and 6 MPa H2, the cellulose could be completely converted into polyols and the yield of EG was as high as 61 wt % with a 2% Ni-30% W2C/AC-973 catalyst. This value is the highest yield reported to date. Currently in the petrochemical industry, EG is mainly produced from ethylene via the intermediate ethylene oxide. The global production of EG in 2007 is estimated to be 17.8 million tonnes, an increase of Scheme 1. Catalytic conversion of cellulose into polyols.

Nickel‐Promoted Tungsten Carbide Catalysts for Cellulose Conversion: Effect of Preparation Methods

A series of Ni-promoted W(2) C catalysts was prepared by means of a post-impregnation method and evaluated for the catalytic conversion of cellulose into ethylene glycol (EG). Quite different from our previously reported Ni-W(2) C/AC catalysts, which were prepared by using the co-impregnation method, the introduction of Ni by the post-impregnation method did not cause catalyst sintering, but resulted in redispersion of the W component, which was identified and characterized by means of XRD, TEM, and CO chemisorption. The highly dispersed Ni-promoted W(2) C catalyst was very active and selective in cellulose conversion into EG, with a 100% conversion of cellulose and a 73.0% yield in EG. The underlying reason for the enhanced catalytic performance was most probably the significantly higher dispersion of active sites on the catalyst.