Studies On Catalytic Transformation Of Products Derived From Staged Pyrolysis Of Biomass Into Value-Added Chemicals

This dissertation work focuses on the studies of catalytic transformation of two major model compunds derived from the pyrolysis of lignocellulosic biomass,i.e.acetol and levoglucosan,into value-added chemicals such as lactic acid and gluconic acid.A control on the activation of C-O bond is the key to achieving the selective conversion toward the desired products.For the conversion of acetol to lactic acid,dealuminated tin beta supported gold catalyst(Au/DeAl-Sn-Beta)was designed and prepared by post-synthesis method.The results of catalytic test showed that Au/DeAl-Sn-Beta catalyst exhibited outstanding performances,reaching 93.4%conversion and 73.2%selectivity toward lactic acid,and the volumetric productivity was higher than that of fermentation method.The catalyst also showed good stability.No significant changes for the activity and selectivity were observed during 3 recycling tests.FITIR,pyridine probed FTIR,UV-vis and titration characterization showed that 6.0 wt%Sn was incorporated into the framework of Beta zeolite,and this type of Sn species exhibited strong Lewis acidity.The results of catalytic tests suggest that Lewis acid Sn sites located in the framework of zeolite are vital to high selectivity toward lactic acid,and golcd nanoparticles are indispensable to achieving high yield of lactic acid.The combination of Lewis acid Sn sites and Au sites in close proximity favored the conversion of acetol to lactic acid.In addition,loading amount of gold and reaction conditions including reaction temperature,dioxygen partial pressure,acetol concentration also affected the selectivity to lactic acid.By optimizing these parameters,precise control on the activation of C-O bond could be realized during transformation of acetol.On the basis of experiment results of TOS tests using acetol,pyruvaldehyde and lactaldehyde as substrates respectively and simplified kinetic analysis,we speculate that the formation of lactic acid likely proceeds via the route with pyruvaldehyde as an intermediate,which first forms by oxidation of acetol on the active gold sites,followed by hydration and 1,2-hydride shift at the Lewis acid Sn sites.For the conversion of levoglucosan to gluconic acid,cesium salt of phosphotungstate supported gold(Au/Cs2.5H0.5PW12O40)catalyst was designed and prepared.Compared with three other typical solid acid supported gold catalysts,it exhibited suprior catalytic performance,achieving 93.6%levoglucosan conversion and 93.1%selectivity toward gluconic acid.This catalyst also showed good stability with gluconic acid yield nearly unchanged during 3 recycling tests.The experiment results suggest that hydrolysis of levoglucosan to glucose and oxidation of glucose to gluconic acid are invovled during the conversion of levoglucosan to gluconic acid.Acid sites with appropriate strength and amount over catalyst support are more beneficial to highly selective formation of gluconic acid.In addition,dioxygen pressure and reaction temperature were found to be two important parameters in selective conversion of levoglucosan.Kinetic analysis suggests that hydrolysis of levoglucosan is the rate-determining step in the overall conversion.Therefore,supported gold catalyst with good oxidative properties to enable the rapid transformation of intermediate into desired gluconic acid and ability to maintain a low glucose concentration during reaction is the key to precise control on the activation of C-O bond in the conversion of levoglucosan to gluconic acid.We also found that gluconic acid formed can act as a Br(?)nsted acid to further promote levoglucosan hydrolysis.For the conversion of levoglucosan to lactic acid,a series of binary catalysts composisting of metal nitrate and Mo-containing compounds were tested.The experiment results showed that the Al(NO3)3-MoO3 binary catalysts exhibited the best catalytic performances,achieving 61.3%lactic acid yield under the optimum reaction conditions.On the basis of the TOS testing results of levoglucosan,we assume that four sequential steps take place,namely hydrolysis of levoglucosan to glucose,isomerization of glucose to fructose via 1,2-hydride shift,retro-aldol reaction of fructose to glyceraldehyde and dihydroxyacetone and isomerization of pyruvaldehyde,coming from dehydration of glyceraldehyde,into lactic acid.By investigating the effects of Al(NO3)3-MoO3 catalyst amount,ratio of Al(NO3)3 to MoO3,and reaction temperature on levoglucosan conversion,we speculate that highly selective formation of gluconic acid results from the precise control on the activation of C-O bond,namely the synergies between isomerization and retro-aldol reaction.