Dehydration of sugar mixtures to 5-hydroxymethylfurfural catalyzed by modified tin-mordenite

5-hydroxymethylfurfural (HMF) is a versatile compound that has great market potential in the future chemical industry. HMF production from fructose has a problem of higher cost, while HMF production from glucose has a problem of lower yield. Therefore, the use of relatively inexpensive biomass-derived syrup to produce HMF in order to achieve industrial production is currently a research hotspot. A series of Sn-MOR catalysts were prepared by using mordenite zeolite (H-MOR) as a carrier, which was modified with acid treatment and adding tin to remove Al and replenish Sn. The Sn-MOR catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance ultraviolet-visible spectra (UV-vis), ammonia temperature programmed desorption (NH₃-TPD), and X-ray fluorescence spectroscopy (XRF). The characterization results showed that the Sn-MOR still maintained the crystal structure of mordenite, with changes in strength and content of acid centers, and Sn was inserted into the zeolite skeleton. Glucose and fructose were used as substrates in the catalytic reaction of unmodified H-MOR and modified Sn-MOR, and the experimental results showed that H-MOR catalyzed the dehydration reaction of glucose poorly, with a HMF yield of only 7.08%, but its catalytic performance in dehydration of fructose was better, with a HMF yield of 76.78%. The modified Sn-MOR possessed isomerization activity, which improved the reactivity of glucose dehydration with a HMF yield of 38.65%, while the modified Sn-MOR still maintained the high catalytic activity of MOR for fructose dehydration to HMF. Using sugar mixtures (m_{hydrated glucose}∶m_fructose = 1∶1) as the substrate, the reaction performance of the catalysts with different tin metal additions to H-MOR was firstly investigated, and the results showed that 3.76-Sn-MOR with 3.76% tin addition catalyzed the dehydration of the sugar mixtures better. The reaction performance of the catalyst prepared by adding tin after H-MOR acid treatment was further investigated, and the results showed that the 3.76-Sn-MOR₁ prepared by acid treating H-MOR using 1 mol/L hydrochloric and adding tin could obtain better HMF yield (49.37%) and selectivity (58.09%) in dehydration of sugar mixtures. The reaction conditions were further optimized through orthogonal experiments using a 3.76-Sn-MOR₁ catalyst in terms of sugar concentration, reaction temperature, catalyst dosage, and reaction time. The results showed that neither too high nor too low sugar concentration was conducive to HMF formation, and increasing temperature and catalyst dosage were conducive to HMF formation, but increasing temperature reduced the selectivity of HMF. Prolongating reaction time had little effect on improving the yield of HMF, but decreased the selectivity of HMF. The optimal reaction conditions were as follows: 1.5 g of sugar mixtures, reaction temperature of 170 ℃, catalyst dosage of 0.3 g, and reaction time of 3 h. Under the above optimal reaction conditions, the superior catalyst 3.76-Sn-MOR₁ was finally applied to F55 fructose syrup, which has a dry matter ratio of glucose and fructose similar to that of sugar mixtures, and the HMF yield was 63.76%, the HMF selectivity was 69.43%, and the fructose syrup conversion was 91.82%. The catalyst was recycled five times and the HMF yield reduced to 49.50%, which still maintained a certain catalytic activity.