The Exploitation And Investigation Of Efficient Catalysts For The Preparation Of Ethylene Glycol Via Ethylene Oxide Hydration

Monoethylene glycol (MEG) is an important chemical raw material, with excellent water solubility and high boiling point. It has been widely used as an intermediate to make various products, such as polyester fiber, antifreeze, lubricant and so on. The global total production capacity of MEG has reached 27.09 million tons per year. There are several synthesis routes including hydration of ethylene oxide (EO), two-step synthesis via carbonate, CO carbonyl coupling method and the emerging Path method of biomass resources. Now MEG is commercially produced by thermal hydration of EO with significant amounts of by-products diethylene glycol (DEG) and triethylene glycol (TEG). In pursuit of high selectivity to MEG, a large excess amount of water (about 30 mol water/mol EO) has to be used, which ultimately increases production cost due to high enormous energy consumption occurred during the distillation process to remove water. Therefore, considerable efforts have been made to explore efficient catalysts working at a low water/EO ratio. The catalysts reported in the literature include anion exchange resins, supported metal oxides and zeolites, quaternary phosphoniumhalides, polymeric organosilane ammonium salts and macrocyclic chelating compounds. However, none of the catalysts has been developed to commercial application. The resins swell and deactivate under reaction conditions. The preparation of other catalysts is too complicated or expensive. On the other hand, some low cost soluble salts were reported as good homogeneous catalysts for EO hydration. As a simple catalytic system with less product composition, this reaction can also be applied to test the catalytic capability and investigate the reaction mechanism of both solid and base catalyst.This paper introduces a systematic research on various homogeneous and heterogeneous catalysts for the catalytic hydration of EO, based on the summarizing of a large number of literatures and patents. The aim is to get a high MEG yield with the water amount as low as possible. Besides the water amount, the cost, lifetime, the complexity of catalyst preparation and the equipment requirements are considered. The effects of pH value and addition-esterification of anion were studied, with the alkaline transition phenomenon reported and a comprehensive mechanism proposed. Based on this results and principles, layered niobic acid was introduced as the effcient catalyst for EO hydration with self-exfoliatable nanosheets and adjustable acidity. These results are important and useful for the industrial production of ethylene grycol.The first chapter mainly reviewed the current industrial production techniques of ethylene glycol, which focused on the catalytic hydration of ethylene oxide. Various catalysts reported for EO hydration are summarized, including the performance and features.In chapter 2, selective hydration of ethylene oxide was investigated with several inorganic salt systems as homogeneous catalysts. By optimizing reaction conditions, the highest monoethylene glycol selectivity of 98% was obtained with> 99% EO conversion at a water/EO molar ratio=10. The effects of pH value and anion addition-esterification on MEG selectivity were systematically studied, and a comprehensive mechanism was proposed based on the results. The conclusion should be useful in developing high performance catalysts for the manufacture of MEG by EO hydration at a low water/EO ratio.In chapter 3, layered niobic acids (HxK1-xNb3O8, x=0-1) are reported as new solid acid catalysts for selective hydration of ethylene oxide. They are prepared by simply calcinating Nb2O5-K2CO3 mixture followed by an ion-exchange process in HNO3 solution of different concentrations. The highest selectivity for monoethylene glycol is achieved over 95% with EO conversion of>99% at x of 0.7 under H2O/EO ratio of 8. Combined with the results of first-principles density functional theory calculations and Hammett indicator method, it is revealed that the suitable acid strength is more crucial for MEG selectivity than acid amount. Furthermore, a self-exfoliation of layered HNbsOs is also found during EO hydration, which proves to be another important factor for its good catalytic performance by exposing the abundant acid sites among the Nb3O8- nanosheets. The thermal stability investigation of HNb3O8 also indicates a careful selection of characterization and application way for this layered niobic acid.Chapter 4 involves the EO hydration using zeolites as catalysts. The effects of the size, shape, Si/Al ratio and different elements modification (Nb modification) of the zeolites were simply introduced. Our target is to make a uniform dispersion of the efficient and hydrophilic niobic acid in the pores of mesoporous silica or zeolite materials, complete catalytic reaction. This part of the work is still not complete, which needs further discussion and investigation.Based on the research results as exhibited, chapter 5 gives the summary and outlook on the research subject of ethylene oxide catalytic hydration.