Catalytic Conversion Of Biomass-derived Carbohydrates To Methyl Lactate

Methyl lactate is a kind of important hydroxyl ester. It is used in spices and herbicides mainly. In addition, it is a kind of important industrial solvents. And it is widely used in medicine, resin coating, adhesive, cleaning lotion, printing ink, cosmetics, cigarettes, drink, ice cream, and other food industries. Methyl lactate can be hydrolyzed into platform lactic acid. At present, lactic acid is produced by either bacterial fermentation. However, enzyme catalysis processes show many drawbacks, such as long reaction time, formation of large amount of calcium sulfate wastes, and very strict reaction condition. Currently, the growing demand for lactic acid based materials calls attention to the improvement of unpractical conventional processes with an efficient and sustainable chemical route used for lactic acid production.In this work, biomass-derived carbohydrates were used as substrates. Solid base, acidic zeolites, amphipathic transition metal oxide were used as catalysts. Catalysts were characterized by XRD, BET, TEM, CO2-TPD, NH3-TPD, and Py-IR. The influence factors of catalytic activity were researched. We prove the following conclusions:(1) Magnesium oxide can be used as solid base catalyst for the production of methyl lactate from biomass-derived carbohydrate in methanol for the first time. MgO with the highest concentration of weakly basic sites was found to show the highest catalytic activity for the conversion of sugars to methyl lactate. The yield of methyl lactate reached nearly30%under optimal reaction conditions. The catalyst can be reused without deactivation for many times. Water showed negative effect to the formation of methyl lactate.(2) Methanol also showed catalytic activity in this chemical system. Different monohydric alcohols were used as solvent for the blank run reactions. And methanol showed the best catalytic activity. When hexane, a kind of proton transfer inert solvent, was used as solvent, no methyl lactate was detected.(3) Sn-MCM-41and SnO2/SiO2-a with only partly MCM-41structure were prepared. Both catalysts showed high catalytic activity for the conversion of sugars to methyl lactate. The yield of methyl lactate reached nearly40%under optimal reaction conditions. The methyl lactate yield was closely related to reaction temperature, reaction times, and the type together with the amount of the acid sites of the catalyst. The catalytic activity was improved with the increase of the total acid content of catalysts. The acidic sites and mesoporous structures of reused catalysts did not change much. The catalysts were active and can be reused without significant decrease in the catalytic activity after being used for five recycles. The TEM images and Py-IR of reused catalysts indicated unchanged mesoporous structures and acidic sites.(4) NiO, La2O3, Cr2O3, and TiO2were prepared. Their catalytic activities were in following order:NiO> La2O3> Cr2O3> TiO2. The yield of methyl lactate reached nearly40%under optimal reaction conditions with NiO as catalyst. The catalytic activity was improved with the increase of the total acid content of catalysts. The base content of catalysts did not show significant influence in catalytic activity. The catalysts were active and can be reused without significant decrease in the catalytic activity after being used for three recycles. The XRD pattern and Py-IR of reused catalyst indicated unchanged crystal structures and acidic sites.