Ni–WO 3 /SBA-15 catalysts were prepared by an impregnation way and applied to the hydrogenolysis of cellulose in aqueous solution. The effect of nickel and WO 3 loading, catalysts reduction temperature and time on cellulose conversion were investigated. Up to 70.7% ethylene glycol yield was obtained over 3%Ni–15%WO 3 /SBA-15 catalysts. Several physicochemical methods such as XRD, Raman spectroscopy, NH 3 -TPD, H 2 -TPR and XPS were used to determine the character of catalysts. Based on these studies, it was found that there was a strong electronic interaction between NiO and WO 3 , which not only promoted the reduction of WO 3 , but restrained the reduction of NiO. Together with the experimental results, it can be concluded that the WO 3− X , which may be the active species for the selective cleavage of C C bond, was formed after catalysts pretreatment process. Additionally, a possible catalytic mechanism was proposed.
Hydrogenolysis of glucose and cellobiose, used as the model feedstocks, over Ni–WO3/SBA-15 catalysts has been investigated to probe the influencing factors for the conversion of cellulose to ethylene glycol (EG). The conversion of glucose and cellobiose to EG showed different dependencies on reaction temperature; a lower reaction temperature was needed for the former. Additionally, the surface atomic ratio of W to Ni on the Ni–WO3/SBA-15 catalysts was the key factor for the product distribution. Both glucose and cellobiose had their own optimum W–Ni ratio for the production of EG, and the ratio of W to Ni for glucose was slightly lower than that of cellobiose. On the other hand, the Ni–WO3/SBA-15 catalysts were thoroughly characterized by N2 adsorption–desorption, X-ray diffraction (XRD), hydrogen-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The results indicated that the reducibility of Ni–WO3/SBA-15 catalysts with high Ni loading was more similar to that of pure NiO. More importantly, the change of surface atomic content of Ni and W of Ni–WO3 catalysts with various Ni loadings resulted from the surface W species of the catalysts being gradually covered by Ni species with the increase of Ni loading.
Xinqiang Feng,Chun Shen,Kaiyue Ji,Jiabin Yin,Tianwei Tan
Mesoporous solid acid catalyst WO3/SBA-15 exhibited high performance for the production of bio-based p-xylene (PX). After immobilization on SBA-15, isolated [WO4]2− tetrahedral species which were coordinatively unsaturated, were generated and functioned as Lewis acid centers. The ratio of Lewis to Brønsted acid sites varied in the range of 7.4 to 13.4 by adjusting the calcination temperature. A high ratio of Lewis to Brønsted acid sites and the mesoporous structure contributed to the high activity and stability: a selectivity of 88.0% for PX with a conversion of 64.4% and a carbon balance of 90.1% was achieved over 0.20-WO3/SBA-15-700. Compared with microporous Sn-Beta, the as-prepared catalyst showed better resistance to carbon deposition with a 20% higher yield. The spent catalyst could be easily regenerated by calcination at high temperature. The conversion decreased by 3% and no loss in PX selectivity was observed after five regeneration cycles. The as-prepared WO3/SBA-15 catalyst is highly active, stable and recyclable, and has high potential towards bio-based PX production.
Supported tungsten oxide catalysts are promising candidates for olefin epoxidation with hydrogen peroxide (H2O2). Understanding the structure–activity relationship is of great importance for developing more efficient tungsten oxide-based catalysts. Herein, WO3–SBA-15 with an isolated tetrahedron WO4 is synthesized using a direct incorporation strategy, and its TOF is 191 h−1 in cis-cyclooctene epoxidation with H2O2, which is much higher than that of WO3/SBA-15 (51 h−1) prepared using an impregnation method and that of bulk-WO3 (0.4 h−1). Its lowest apparent activation energy of 34.6 kJ mol−1 also corresponded to the remarkable performance of WO3–SBA-15. Kinetic analysis indicated that WO3–SBA-15 bound H2O2 more strongly than other catalysts, resulting in the facilitated activation of H2O2. By correlating the specific activity and the structure characterization, we considered that the high reactivity of WO3–SBA-15 was attributed to (i) more isolated WO4 sites, which were of benefit to the coordination and activation of H2O2 and (ii) a stronger metal–support interaction derived from isolated WO4 species, which favored the dissociation of an O–O bond in the W–OOH intermediate and contributed to the insertion of an oxygen atom into an olefin double bond. Furthermore, DFT calculations indicated that the isolated tungsten oxide was beneficial for lowering the energy barrier of electrophilic oxygen atom transfer, thereby facilitating epoxidation.
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