Studies On The Conversion Of Glucose To 5-hydroxymethylfurfural In Aqueous Phase Catalyzed By Niobium-based Catalysts

Biomass resources,as a green and renewable resource with wide sources and low prices,can be used as a potential alternative to fossil resources.The biomass-derived compound,5-hydroxymethylfurfural（HMF）,is considered as one of the most promising platform molecules.The one-step synthesis of HMF from glucose catalyzed by solid acid in pure aqueous phase is a green and environmentally friendly reaction route.This thesis focuses on the preparation of a series of porous niobium-based catalysts with high specific surface area,using niobium-based compound as the active center for solid acid-catalyzed reactions,and applied it to the reaction of glucose in pure aqueous phase for the synthesis of HMF,specifically:1.Three mesoporous niobium-based materials,niobium silicate,niobium phosphate and niobium oxide,were hydrothermally synthesized by surfactant sulfuric acid pre-carbonation method using stable and low-cost ammonium niobium oxalate as niobium precursors,and characterized by various techniques including small-angle XRD,N₂ adsorption-desorption and SEM.Only niobium silicate forms an uniform mesoporous structure with the highest BET specific surface area of 403 m² g^-1.2.A series of niobium phosphosilicate solid acid catalysts（Nb Si P）were synthesized hydrothermally using polyether F127 as a soft template,ammonium niobium oxalate as a niobium source,diammonium hydrogen phosphate as a phosphorus source,and tetraethyl orthosilicate as a silicon source.The comprehensive characterization of the mesoporous structure and surface acidity of the materials by XRD,N₂ adsorption-desorption,multinuclear and probe molecular solid-state NMR techniques showed that the decrease in silicon content led to the collapse of mesopores and decrease in specific surface area,and the concentration of Lewis and Br?nsted acidic sites increased simultaneously,but the L/B acid ratio increased first and then decreased.The synthesized catalysts were used for the conversion of glucose to HMF in pure aqueous phase,and it was found that the HMF yield was mainly determined by the concentration ratio of Lewis and Br?nsted acidic sites,which also showed an increasing and then decreasing trend with the decrease of silicon content in the catalyst.The best HMF yield（29.61%）was achieved when（Nb+P）:Si=8:2 in the catalyst.3.A mesoporous niobium phosphosilicate catalyst was synthesized using niobium tartrate as the niobium precursor and F127 as the soft template.The results showed that the mesoporous niobium phosphosilicate/carbon composite catalyst formed by in situ carbonization of the templating agent had the least number of accessible Lewis and Br?nsted acidic sites on the surface,but the carbon component provides a suitable hydrophobic microenvironment in the catalyst,which promotes the rapid departure of the generated HMF from the acidic surface,reduces the occurrence of side reactions and improves the HMF selectivity,resulting in the best HMF yield（49.18%）.4.A hierarchical pore niobium phosphosilicate catalyst with mesoporous-macropore graded pore structure was prepared by using polystyrene microspheres as a macroporous template agent.The introduction of macropores improved the flow diffusivity and effectively enhanced the catalytic efficiency,resulting in an increase in HMF selectivity from 57.96%to73.16%relative to mesoporous niobium phosphosilicate and a further increase in HMF yield from 49.18%to 56.03%.The catalyst has good stability and the catalytic activity did not decrease significantly after five cycles.In addition,the catalytic conversion of other sugar compounds to HMF using this catalyst also showed good activity.