Solid Acid-catalyzed Conversion Of Carbohydrates Into 5-Hydroxymethylfurfural And Furfural

The efficient preparation of platform compounds was significant for conversion of biomass resources into fuels and bulk chemicals.Among them,5-hydroxymethylfurfural(HMF)and furfural have attracted extensive attention,because they can be converted into many important chemical derivatives,and can be prepared from various feedstocks like lignocellulose-derived glucan and xylan.Catalysts and solvents were the two key factors in the efficient conversion of carbohydrates to HMF and furfural.Among them,the solid acid catalyst was very important for the industrial production of platform compounds,because of the advantages of low equipment corrosion,less industrial waste water,controllable active sites and easy recycling.Thus,a series of new solid acid catalysts with different active sites were prepared from relatively inexpensive raw materials.This study focused on three aspects including stability of the solid acid catalysts,effect of acidic sites and the applicability of the catalytic system to different raw materials.Specifically,the stability of the solid acid catalyst was compared with that of carbon solid acid catalysts prepared from different carbon sources using different preparation methods.In order to investigate the effect of different active sites of solid acid on the yield of HMF and furfural,a series of new solid acid catalysts was prepared,such as the sucrose-derived carbon solid acid with Br(?)nsted acid,tobacco stem-derived multifunctional carbon solid acid with Br(?)nsted acids and a small amount of Lewis acids,and Na-doped porous Al2O3 with Lewis acids.Meanwhile,physicochemical properties of those solid acids were characterized in terms of morphology,structure,acid amount,acid type and surface functional group.Moreover,the synergistic effect between the solid catalyst and the solvent was analyzed to study the applicability of the catalytic system to different sugars.Firstly,a series of carbon solid acids were prepared,and the effect of sugar conversion and recycle stability of each catalyst was evaluated.The results showed that the sucrose-derived carbon solid acid prepared by template carbonization display better catalytic effect and excellent cycle performance.The HMF yield from fructose conversion was maintained at nearly 80.0%after the seventh cycle.The catalytic effect of glycerol-derived carbon solid acid,which was prepared by one-pot simultaneous carbonization and sulfonation,was modest,while the effect of wood-derived carbon solid acid prepared by microwave-assisted H3PO4 activation and cellulose-derived solid acid made by one-pot method were relatively poor.Secondly,the catalyst characteristics and the effects of sucrose-derived carbon solid Br(?)nsted acid were systematically evaluated.The results showed that the amount of strong acid(H+)reached 1.49 mmol/g.When fructose was used as raw material,the yield of HMF reached 78.1%.The catalytic performance of the catalyst in conversion of fructose into HMF hardly changed over 7 cycles.The main reason was that a large amount of benzenesulfonic acid groups filled the carbon pores,which significantly reduced the detachment of the sulfonic acid groups.When the glucose and cellulose were used as raw materials,the yields of HMF were 33.2%and 22.5%,respectively,and the total yields of HMF and furfural reached 42.1%and 33.7%,respectively.Thirdly,a series of porous tobacco stem-derived carbon solid acid catalysts were prepared.The catalysts were characterized by pyridine infrared,infrared,acid-base titration,elemental analysis and X-ray photoelectron spectroscopy,and so on.It was found that tobacco stem-derived carbon catalyst featured not only high-density of benzenesulfonic acid loading but also multifunctional active sites such as Lewis acid and oxygen-containing and nitrogen-containing functional groups.By studing the relationships between the characteristics of the catalysts and their conversion effects on different sugars,it was found that the catalyst prepared by the carrier at lower carbonization temperature showed higher acid loading and better catalytic effects.When fructose and glucose were used as raw materials,the yields of HMF reached 93.7%and 43.8%,respectively.In addition,yields of total furans plus HMF and furfural from fructose and glucose reached 96.8%and 54.1%,respectively.Among the various active sites of the catalyst,high-density of Br(?)nsted acid played a major role,and other active sites,such as weaker Lewis acid sites,oxygen-containing weak acid sites and protonated nitrogen-containing functional groups,play synergistic roles.The performance of the tobacco stem-derived carbon catalyst was better than the sucrose-derived Br(?)nsted carbon solid acid.Moreover,the catalyst had good stability.After repeated use for five times,the activity was almost unchanged,mainly because of a large amount of sulfonic acid group was grafted into the micropores and small/medium mesopores.Therefore,the detachment of sulfonic acid groups and the deposition of humin in the pores reduced,whereas the catalytic activity of free acid H+ protons was unchanged because of the small size of H+.Fourthly,to deeply study the applicability of different acid types and catalytic systems to different sugars,a Na-doped porous Al2O3 Lewis solid acid was developed.The DMSO and ionic liquid[Bmim][Cl]were used as solvents to investigate the synergistic effect between catalyst and solvents.When the glucose was used as the substrate,the yield of HMF was 81.8%.The solvent of DSMO and[Bmim][Cl]showed good synergy effect with the catalyst Na/Al2O3,while DMSO worked better.When cellulose was used as raw material and the[Bmim][Cl]/DMSO mass ratio of was fixed at 4:2,the HMF yield reached 66.4%.The results showed that the catalytic system Na/Al2O3-[Bmim][Cl]/DMSO displayed the comprehensive synergistic effect.When the catalyst was used for the conversion of xylose to furfural,the yield of furfural reached 83.6%.When corn stover was used as raw material,yields of furfural and HMF reached 99.5%and 48.7%,respectively.Moreover,the yield of total furans reached a maximum of 63.2%from corn stover.Therefore,the catalytic system was highly applicable to various carbohydrates.The conversion of glucan and xylan to HMF and furfural can be achieved synchronously and efficiently.