Study On Polymerization Mechanism Of Bio-oil Based Small Molecules In Catalytic Reaction

Bio-oil obtained from the thermal treatment of biomass,such as pyrolysis,contains furans,phenols,aldehydes,ketones,sugars and other small molecular compounds.Due to the complex composition and active chemical properties of such biomass-based molecules,the polymerization is very easy to occur in the process of storage and upgrading into high value-added chemicals.It could lead to the decreasing selectivity of the target product and the blockage of the pore structure in the catalyst.Therefore,scientists have been committed to developing catalysts with high catalytic performance and high selectivity to reduce the probability of polymerization.Br（?）nsted acid has strong acidity and high catalytic performance,which can improve the conversion of raw materials and shorten the reaction time as well.It is one of the common acid catalysts for the preparation of high value-added chemicals through acid catalytic conversion of biomass derivatives such as cellulose,polysaccharide and monosaccharide.However,in the process of catalytic conversion of bio-oil based small molecules,the polymerization will also occur,reducing the selectivity of the target product.The effect of inorganic salts on the production of 5-hydroxymethylfurfural（HMF）/levulinic acid（LA）from monosaccharide（glucose/fructose）catalyzed by Br（?）nsted acid was studied.The addition of metal salts will affect the product distribution of HMF,LA and the by-products（polymers）.Na₂SO₄ and K₂SO₄ can promote glucose isomerization to fructose while Mn SO₄,Co SO₄,Ni SO₄ and Zn SO₄ mainly promote the side polymerization.Cu SO₄ can promote the dehydration of fructose to HMF and further convert HMF to LA,but cannot promote the isomerization of glucose to fructose.Fe₂（SO₄）₃,La₂（SO₄）₃ and Ce（SO₄）₂can catalyze the conversion of glucose/fructose to LA.A cobalt sulfate（CoSO₄·7H₂O）/tetrahydrofuran（THF）catalytic system was constructed to catalyze the conversion of cellulose/inulin/glucose/fructose to produce HMF.The coordination of Co SO₄·7H₂O with THF is better than that with DMSO,alcohols,acetone or other organic solvents,which promotes the dehydration of fructose and inhibits the polymerization of HMF.The highest yield of HMF is 88.0%.The synergistic effect among cobalt ion,sulfate,crystal water and THF is very important to realize the high selectivity of fructose dehydration to HMF.Solid salt catalysts（FeCl_x-D008）with different metal salt contents were prepared by ion exchange method.Br（?）nsted acid site not only catalyzes the dehydration of xylan/xylose to furfural,but also catalyzes the polymerization of reaction intermediates and furfural.The content of iron in FeCl_x-D008 affects the selectivity of furfural and the polymerization activity of xylose.FeCl₃ can replace some Br（?）nsted acid sites of D008 itself,and the polymerization can be minimized by adjusting the iron content.The kinetic study shows that in the presence of FeCl_x-D008,the activation energy（AE）of xylose polymerization is higher than that in the presence of D008,which explains the high selectivity of furfural in the presence of FeCl_x-D008.In addition,the stability experiment showed that FeCl₃ species were stably fixed on the surface of FeCl_x/1-D008 catalyst.The basic research on the polymerization between furans and phenols in bio-oil is helpful to understand the polymerization behavior of complex products in the process of biomass-based upgrading and conversion.The polymerization reaction between furans and phenols with or without acid catalyst was conducted,and the effects of the molecular structure of the reaction substrate on the polymerization network and the morphology,structure and properties of the polymerization products were studied.It is confirmed that furans and phenols will undergo cross polymerization in addition to self-polymerization,so the coexistence of the two products will be easier to polymerize than storage alone.The reaction intermediates formed by furan compounds through ring opening can be cross-polymerized with guaiacol or vanillin to initiate polymerization.The type and content of functional groups in the final polymers can be changed by adding sulfuric acid.The presence/absence of H₂SO₄ and the molecular structure of the feedstock also significantly affect the morphology of the resulting polymer.2,5-Furan dimethyl alcohol（DHMF）were produced by hydrogenation of HMF with modified copper-based catalyst.The addition of alkali metals and alkaline earth metals significantly affected the physicochemical properties and catalytic behavior of copper-based catalysts.The addition of additives will reduce the specific surface area,but inhibit the sintering of Cu hydrogenation site and improve the hydrogenation activity.Na,K or Mg modified catalysts increase the weak base sites on the surface of support and improve the electron donor ability,which contribute to the selective adsorption of aldehyde groups in HMF,and promote the hydrogenation of HMF rather than hydrolysis or polymerization.The weak basic sites of additives play an important role in improving the low-temperature activity and determining the selectivity of HMF hydrogenation catalyst.