The Hexose Dehydration By Co-operative Catalysis Of Adjacent Acid Sites

Nowadays, fossil resource which is main building blocks for chemical products is facing a danger of exhaustion. Also environmental pollution caused by the burning of fossil fuels is increasingly serious. So it is imminent to take advantage of reproducible and redegradative biomass resource to solve fossil resource depletion and reduce environmental pollution. In the past few years, efforts have been devoted to find ways to utilize biomass as feedstocks for the production of organic chemicals because of its abundance, renewability and worldwide distribution. 5-hydroxymethylfurfural (HMF) is a very important platform chemical and its derivatives can be applied as precursors for polymers, fuels, fine chemicals, polymer materials or solvents. Carbohydrate, especially hexose, is dehydrted to HMF over acid catalysts. It is very important to study the dehydration of carbohydrates to HMF in biomass conversion and utilization.The dehydration of hexose to HMF is a typical acid-catalyzed reaction and the reaction has been studied for several decades. The liqid catalyst is almost not considered now because of its dificult seperation and corrosion to equipment. The solid acid catalysts are used nowadays. The acidity and the density of acid sites in solid catalysts influence the catalytic process of the hexose dehydration to HMF, including conversion and selectivity. It is notable that polyol is full of hydroxyls in molecules and the interaction between hydroxyls and acid sites are expected. However, the influence of adjacent acid sites on the hexose conversion to HMF has not been clarifid to date. Here we study the dehydration of hexose (fructose and glucose) from the micro-meso perspective and discuss the synergistic effect of adjacent active sites for hexose dehydration.Firstly, phthalic acid and hydroxybenzoic acid (ortho-, meta-, para-) are used as model catalysts to study the effect of space proximities of active centers for acid-catalyzed reaction, and an acceleration of the dehydration of fructose to HMF by the adjacent active centers is found. Then HZSM-5 zeolites with varied Si/Al ratios are used as catalysts. The space proximities of these sites become prominent with the decrease of Si/Al ratio in HZSM-5 zeolites according to the 1H double-quantum magic angle spinning NMR. The solid state 13C MAS NMR spectra of the adsorbed n-butane in HZSM-5 zeolites demonstrate that with decrease in Si/Al ratios, the charge density on the reactant molecule increases compared with the molecule over the single site. The resulted higher charge density on the reactant would lead to new signals which refer to chemically adsorbed molecules more intensive and further shifted to lowfield. In situ 1H NMR spectra reveal that with decreasing in Si/Al ratio of the HZSM-5 zeolites, the muti-site interaction on one fructose molecule simultaneously leads to an increase of the measured rate constant compared with that over single active site. The measured activation energies of the reaction also decrease with the increased number of adjacent active sites.Lewis acids are efficient for D-glucose isomerizationthe into D-fructose which is an important step for the efficient formation of HMF. H-Beta and ion exchanged Beta zeolites (La-, Cu-, Ga-) are used as catalysts in glucose dehydration. We find that ion-exchanged metal atoms in the tetrahedral positions of zeolites are well-explored as Lewis acid. The synegistic effect is identified in glucose dehydration to HMF.Adjacent active sites (B and B or L acid) can interact with one reaction molecule simultaneously in spatial permission. This means that the synergistic effect of acid sites accelerates hexose dehydration to HMF. This finding and conceptual integrations offer strategies to design more effective catalysts within specific space for dehydration process.