Selective conversion of cellulose into 5-Hydroxymethylfurfural over modified zeolite catalysts in a biphasic solvent system

Lignocellulosic biomass mainly composed of cellulose which can be transformed through various thermochemical processes such as catalytic depolymerization, acid-catalyzed dehydration, pyrolysis, gasification, etc., into high-value chemicals for sustainable development and bio-based economy. 5-Hydroxymethylfurfural (HMF) is one of the potential platform bio-chemicals that can be obtained directly from lignocellulosic biomass via catalytic depolymerization and/or acid-catalyzed dehydration using a bifunctional catalyst for the manufacture of various renewable products. Protonic zeolites are bifunctional solid acid catalysts and are well known for their remarkable properties. Their large surface area could enable molecules to diffuse in and out of their pore systems. Moreso, their acidic properties can be easily controlled by various post-synthesis or modification procedures.  In this project, three protonic commercial zeolites namely, H-ZSM-5 (SiO2/Al2O3 ratio 24), H-USY (SiO2/Al2O3 ratio 6), and H-Beta (SiO2/Al2O3 ratio of 28) were used as a starting material for preparing a series of acid catalysts with bifunctionality for direct dehydration of glucose and one-pot conversion of cellulose to HMF in a biphasic water/tetrahydrofuran system. The pristine zeolites were mildly dealuminated by refluxing with dilute nitric acid solutions to adjust their acid properties and were further modified with tin metal using tin precursor, SnCl4.H2O. Although the acid treatment slightly altered the elemental composition, textural properties, and morphology of the zeolites, the total acidity and distribution of acid sites were significantly modified. Using 0.1 M acid-treated H-ZSM-5 as a reference, some non-framework aluminum (Al) oxide clusters were removed from the parent zeolite simultaneously with partial hydrolysis of the zeolitic framework. An increased fraction of coordinatively unsaturated framework Al species enhanced the number of Lewis acid sites. UV-vis spectroscopy revealed that tin was incorporated mostly at extraframework sites, thereby enhancing the weak acid sites and the overall total acidities of the zeolite catalysts. Using 0.1 M solution in the treatment process with H-ZSM-5 zeolite provided a suitable catalyst (0.1DeAl.H-ZSM-5) for glucose dehydration to HMF, giving the glucose conversion and HMF yield of >99% and 64.7%. Moreso, further modification with tin by using a weight of 3.84wt% on the 0.1DeAl.H-ZSM-5 zeolite provided a selective catalyst (3.84wt%Sn-0.1DeAl.H-ZSM-5) for  HMF synthesis from glucose, achieving a glucose conversion and HMF yield of ~84% and 62.3% at HMF selectivity of 74.3%, respectively, with good reusability under the optimized reaction conditions. Using a tin weight of 7 wt% on 0.1 M acid-treated  H-USY zeolite induced a very efficient and acidic catalyst (7wt%Sn-0.1DeAl.H-USY) for one-pot conversion of cellulose to HMF, obtaining a total cellulose conversion and HMF yield of 76.1%. These results explain the industrial potential of the proposed methods for the simple but efficient post-synthesis of protonic zeolite catalysts for producing HMF from cellulose and glucose