Cooperative action of heteropolyacids and carbon supported Ru catalysts for the conversion of cellulose

Acid hydrolysis and hydrogenation/hydrogenolysis reactions can be combined for catalytic conversion of cellulose into renewable biorefinery feedstocks by using heterogeneous bifunctional catalysts. In the present study a cooperative effect of heteropolyacids (HPA) and Ru nanoparticles supported on two carbon materials is demonstrated. The process can be suitable for the one-pot tandem reaction, yielding the conversion of cellulose into alkanediols (mainly propylene glycol and ethylene glycol). From a mechanistic point of view the differences in the distribution of polyol products, obtained from the cellulose reaction over monometallic Ru catalysts or over bifunctional Ru-HPAs materials, seem to be strongly determined by the competitive reactions of the sucrose (glucose + fructose) intermediate. HPA not only promote, as solid acids, the efficient hydrolysis of cellulose to glucose, but also catalyze the selective cleavage of the C-C bonds in glucose and fructose, leading to the formation of ethylene glycol and propylene glycol. These reactions are in competition with the sugar hydrogenation to the corresponding C-6 polyols (e.g. sorbitol), which takes place on the single Ru surface sites. The strong dependence of the product distribution on both catalytic functions is clarified by the kinetic analysis of the three competitive reactions of glucose, including its hydrogenation, isomerization and C-C bond cleavage. Finally, considering the applicability of this reaction, it should be raised that the ball-milling pretreatment of cellulose is compulsory. In fact, during this ball-milling the crystallinity and particle size of cellulose are reduced, which results in a much higher conversion of cellulose. Herein, mixed ball-milling of cellulose and solid catalysts together was presented, which remarkably accelerates the cellulose conversion into valuable products.