Improved Postsynthesis Of Sn-Beta Zeolites For Efficient Conversion Of Glucose To Alkyl Lactates

With the development of the global economy, petroleum-based fossil resources are being consumed rapidly. The exploration of clean and sustainable resources has become an urgent task in view of mitigating global warming and addressing the oil shortage. There is a growing interest in deriving platform molecules from biomass resources to produce fuels and chemicals. Carbohydrates are the main components of biomass, from which many platform compounds such as lactic acid, levulinic acid,5-hydroxymethyl furfural, etc. have been synthesized. Among these platform compounds, lactic acid and alkyl lactates are widely used as chemicals, food additives,green solvents and raw materials for synthesis of biodegradable polymer-polylactic acid.Currently, lactic acid and alkyl lactates are produced mainly by fermentation of glucose and sucrose, but novel chemocatalytic methods were also developed in recent years to overcome the low efficiency of fermentation route. Conversion of abundant and cheap C6 carbohydrates to alkyl lactates is a challenging and promising process of biomass conversion and is in line with the development of economical society. The hydrothermally synthesized Sn-Beta zeolite(Sn-Beta-F) is considered as the state-of-the-art catalyst for this process. However, the synthesis of Sn-Beta-F is difficult, which generally needs long time for crystallization and uses large amount of expensive organic template and poisonous HF as mineral agent. Conventional postsynthesis of Sn-Beta(Sn-Beta-p) can overcome these drawbacks; however,Sn-Beta-p is less selective for the conversion of C6 carbohydrates to lactic acid/alkyl lactate compared to Sn-Beta-F.Here, an improved postsynthesis method was developed to prepare Sn-Beta zeolite(Sn-Beta-p-x) without fluoride. Low concentration of tetraethylammonium hydroxide(TEAOH) was used to hydrothermally treat Sn-Beta-p. After the treatment, the yield of methyl lactate(MLA) from glucose was improved from 22% to 58%. Moreover,the yield of MLA over Sn-beta-p-x is higher than that of Sn-Beta-F(47%) at the samereaction conditions. The physicochemical properties of the prepared Sn-Beta-p-x were characterized by XRD, XRF, N2 physisorption, SEM, TEM, TG/DSC, UV-vis,ICP-AES, FT-IR of pyridine adsorption, and NH3-TPD. The results indicated that TEAOH played two roles during the hydrothermal treatment; one is as a structure directing agent to heal the defect sites and the other is as a base to desilicate. As a result, Sn-Beta-p-x with low concentration of defect sites and hierarchical pore structure was obtained.Subsequently, the effects of the physicochemical properties of Sn-Beta before and after the hydrothermal treatment on the three main steps of glucose to MLA were explored, which include(1) the isomerization of glucose to fructose,(2) cleavage of fructose to 1,3-dihydroxyactone(DHA) and glyceraldehydes(GLA), and(3)isomerization of DHA and GLA to MLA. It was found that the defect sites of hydroxyls were only favorable for the first and the third steps. However, the retro-aldol reaction of fructose to C3 sugars, the rate-limiting step of glucose conversion to alkyl lactate, was suppressed by the defect sites. The hydrothermal treatment is beneficial for the elimination of the large amount of hydroxyls present in Sn-Beta-p and thus increasing the yield of MLA from glucose. On the other hand, the hierarchical pore structure is in favor of mass transfer and thus advantageous for the formation of MLA to some extent.