Catalytic Synthesis Of Organic Acid Esters From Bio-Based Polyols Under Mild Conditions

The continuous consumption of fossil resources has led to a series of increasingly serious environmental problems.Finding clean and renewable resources that can supplement or partially replace fossil resources is of great significance for achieving sustainable social development.Biomass resources are abundant in nature,with a short regeneration period,and can achieve zero carbon dioxide emissions during its recycling process,which is an ideal alternative resource.Due to the existence of a large number of oxygen-containing functional groups in the biomass structure,a large number of polyhydroxy compounds are often present during the conversion process.The conversion of bio-based polyhydroxy compounds to produce high value-added organic acids（esters）can make full use of the oxygen in the structure,which has attracted wide attention from researchers.In this paper,based on the three-carbon structure of biodiesel by-product glycerol（GLY）and its oxidation product 1,3-dihydroxyacetone（DHA）,through the design of an efficient catalytic system,the green synthesis of lactate and pyruvate was achieved.The synthesis of methyl lactate from GLY conversion mainly includes two steps:（1）glycerol is oxidized to 1,3-dihydroxyacetone;（2）1,3-dihydroxyacetone is converted into methyl lactate by dehydration-alcohol addition-isomerization.Therefore,the designed catalyst needs to contain both oxidation active sites and dehydration-addition-isomerization active sites.In this paper,Au/Cu O is used to provide oxidation active sites,and Sn-βmolecular sieve is used to provide heterogeneous active sites to prepare Au/Cu O-Sn-βcatalyst.Under low-temperature and alkali-free conditions,catalytic conversion of GLY to produce methyl lactate is achieved.First,the influence of the synthesis conditions such as Sn source and Sn content on the catalytic performance of Sn-βmolecular sieve was investigated.The results show that when tin tetrachloride（Sn Cl₄）is used as the Sn source and the Sn content is 1 wt%,the synthesized Sn-βmolecular sieve has excellent dehydration-addition-isomerism performance.Au/Cu O-Sn-βcatalysts were further synthesized using molecular sieves synthesized under this condition as precursors,and the effects of Cu O introduction and Au loading on the catalytic performance of Au/Cu O-Sn-βwere investigated.The excessively low introduction amount of Cu O is not conducive to the dispersion of Au,which leads to a larger size of Au and lower oxidation activity of Au/Cu O-Sn-β.When the introduction amount of Cu O is higher,the Cu O crystal is larger,and the promotion effect on the activity of Au is weaker.At the same time,when the loading of Au is too low or too high,it is not conducive to the oxidation activity of Au/Cu O-Sn-β.When the mass ratio of Au to Cu O is 1:4,the catalyst（1Au/4Cu O-Sn-β）has the best catalytic performance.Finally,the influence of reaction conditions was investigated.Using 1Au/4Cu O-Sn-βas a catalyst,the conversion of glycerol and the yield of methyl lactate can be reached 89%and 56%at90 ^oC for 4 h.Pyruvate is the oxidation product of lactate,and the synthesis of pyruvate from glycerol can further improve the process economy.However,there is a contradiction between the oxidative inertness of glycerol and the thermal instability of pyruvate.Therefore,we use 1,3-dihydroxyacetone,the oxidation product of glycerol,as the raw material to realize the direct conversion of 1,3-dihydroxyacetone to pyruvate under low temperature conditions.The direct conversion of 1,3-dihydroxyacetone to pyruvate is mainly divided into two steps:（1）1,3-dihydroxyacetone is converted to lactate by dehydration-alcohol addition-isomerization;（2）Oxidation of lactate to pyruvate.In this paper,Sn-βand TS-1 molecular sieve are used as mixed catalysts.Among them,Sn-βprovides dehydration-addition-isomerization activity to convert 1,3-dihydroxyacetone to lactate.TS-1 provides an oxidation center,using H₂O₂as an oxidant to oxidize lactate to pyruvate.Highly selective conversion of 1,3-dihydroxyacetone to pyruvate can be achieved through synergy between Sn-βand TS-1 molecular sieve.The effects of conditions such as the amount of Sn-βand TS-1added,the amount of methanol added,the amount of H₂O₂added,the reaction temperature and reaction time were systematically investigated and the reaction conditions were optimized.Under optimized conditions,the conversion rate of 1,3-dihydroxyacetone and the yield of methyl pyruvate can be reached 100%and 69%.