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Keywords:

  • acidity;
  • green chemistry;
  • heterogeneous catalysis;
  • metal–organic frameworks;
  • polyoxometalates

Abstract

Cellulose and cellobiose were selectively converted into sorbitol over water-tolerant phosphotungstic acid (PTA)/metal– organic-framework-hybrid-supported ruthenium catalysts, Ru-PTA/MIL-100(Cr), under aqueous hydrogenation conditions. The goal was to investigate the relationship between the acid/metal balance of bifunctional catalysts Ru-PTA/MIL-100(Cr) and their performance in the catalytic conversion of cellulose and cellobiose into sugar alcohols. The control of the amount and strength of acid sites in the supported PTA/MIL-100(Cr) was achieved through the effective control of encapsulated-PTA loading in MIL-100(Cr). This design and preparation method led to an appropriately balanced Ru-PTA/MIL-100(Cr) in terms of Ru dispersion and hydrogenation capacity on the one hand, and acid site density of PTA/MIL-100(Cr) (responsible for acid-catalyzed hydrolysis) on the other hand. The ratio of acid site density to the number of Ru surface atoms (nA/nRu) of Ru-PTA/MIL-100(Cr) was used to monitor the balance between hydrogenation and hydrolysis functions; the optimum balance between the two catalytic functions, that is, 8.84<nA/nRu<12.90, achieves maximum conversion of cellulose and cellobiose into hexitols. Under the applied reaction conditions, optimal results (63.2 % yield in hexitols with a selectivity for sorbitol of 57.9 % at complete conversion of cellulose, and 97.1 % yield in hexitols with a selectivity for sorbitol of 95.1 % at complete conversion of cellobiose) were obtained using a Ru-PTA/MIL-100(Cr) catalyst with loadings of 3.2 wt % for Ru and 16.7 wt % for PTA. This research thus opens new perspectives for the rational design of acid/metal bifunctional catalysts for biomass conversion.