Preparation And Characterization Of Modified Sn-? Catalyst And Its Application In Catalytic Conversion Of Biomass-derived Resources To Lactic Acid

Lignocellulose Biomass,as an important renewable energy source,has the potential for being utilized to produce chemicals and fuels sustainably.Compared with other approaches such as bio-chemical process or hydrothermal one usually involved with complex and costly pretreatment,fast/staged pyrolysis is considered as a more efficient way to generate liquid products directly from raw biomass.This thesis focuses on catalytic conversion of levoglucosan,a major pyrolysis product,to high value-added chemicals lactic acid under modified Sn-? catalysts.Due to the complex reactions involved such as hydrolysis,Meerwein-Ponndorf-Verley(MPV),retro-aldol-condensation etc.,the work in this paper was organized into two related parts.Firstly,we evaluated the structure-reactivity relationship of modified Sn-? catalysts toward MPV reaction,using methacrolein in ethanol solvent to produce methallyl alcohol and diethyl acetal as a probe reaction.Through careful characterization and experiments,we found that using untreated Sn-?,methallyl alcohol and diethyl acetal can be simultaneously synthesized from methacrolein in ethanol solvent,with the yield 71%and 95%(based on methacrolein respectively).However,via pre-heated by argon and post-exchanged with Na,the number of framework Lewis-Sn sites increased in the obtained catalysts(Sn-P-Ar-Na),while the weak Bronsted acid sites declines.Hence,upon Sn-?-Ar-Na the yield of methallyl alcohol and diethyl acetal was enhanced to 90%and 96%respectively,where the side reactions catalyzed by weak Br(?)nsted acid sites were restricted accordingly such as addition,acetalization,etherification.Based on the results from the probe reaction above,we applied the Sn-? catalyst through pre-heated by argon and post-exchanged with Na into the catalytic reaction of levoglucosan,where extra lactic acid as feed was also added as Br(?)nsted acid to catalyze the initial hydrolysis step.Furthermore,we found that the treated catalyst with bivalent ions(Zn2+m Ca2+,Mg2+)exchanging generally demonstrated a better selectivity of lactic acid than those with monovalent ions(Na+,K+,NH4+)exchanging.Among them,Sn-?-Ca exhibits outstanding selectivity to lactic acid,leading to the yield of lactic acid up to 60%.The mechanism analysis suggests that the Ca2+ exchanged Sn-? improved the selectivity of lactic acid mainly by promoting the rate of retro-aldol reaction,where the framework Sn and probably Ca-OH present in the close vicinity could play a key role through providing a synergetic combination of Lewis acid and Br(?)nsted base.The scope of the catalytic reaction system can be extended to a broad of reactants include polysaccharide.We can obtain 41%and 50%lactic acid directly from cellulose and inulin respectively,with the Sn-?-Ar-Ca catalyst above as well as extra lactic acid initially added.Therefore,the findings in this work not only shed light on the fundamental importance of the structure-reactivity relationship over modified Sn-?,but also provide a facile and promising method to the production of lactic acid from the abundant resources directly derived from lignocellulose biomass.