Catalytic activity of an economically sustainable fly-ash-metal-organic- framework composite towards biomass valorization

Low thermal stability and insufficient Brønsted acidity are the deficiencies of conventional metal-organic-frameworks (MOFs), which would greatly limit their applications particularly for high temperature and pressure reactions such as dehydration of carbohydrates. This work has successfully demonstrated the development of a composite of MOF and activated fly ash, and their catalytic application in the xylose-to-furfural dehydration process. The composite is capable of maintaining high stability under severe hydrothermal conditions and even in the acidic medium. In addition, the composite shows its excellent catalytic performance for ten consecutive reaction cycles, which is much better than the bare MOF catalyst, MIL-101 (Cr), obtaining the furfural yield and selectivity of 71% and 80%, respectively. It is proposed that such catalytic activity is mainly attributed to the mutual contribution from the Cr atoms of MIL-101 (Cr) and hydroxyl groups of activated fly ash, acting as Lewis acid centers and Brønsted acid sites, respectively. Besides, the effect of salt concentration on the efficiency of xylose conversion has been studied. The product yield can be enhanced to 78% in the presence of trace amount of sodium chloride solution (35 ppt). This provides a promising direction towards the applications when seawater is used as the reaction media. In summary, the work shows that the incorporation of fly ash as the composite material not only reduces the cost of synthesis, but also mitigates the fly ash disposal problems to some extent.