Preparation And Performance Of CZA/SAPO-34Composite Catalyst For Light Hydrocarbon Synthesis From CO Hydrogenation

With the gradual depletion of petroleum reserves, it would be very desirable to effectively convert syngas (H2and CO) to light hydrocarbons for syngas can be obtained through the gasification of coal or natural gas. C2-C4hydrocarbons have environmentally benign characteristics and can been used as clean fuels and raw chemical feedstock. Therefore, the synthesis of C2-C4hydrocarbons from coal or biomass-derived syngas has drawn increasing attention for the resource constraints of petroleum. The present study focuses on the preparation of CZA/SAPO-34composite catalyst and explores its performance in direct production of hydrocarbons from syngas over the above catalyst. CZA (CuO/ZnO/Al2O3) catalyst is typical catalyst for methanol synthesis from syngas, and SAPO-34zeolite can catalyze methanol into light hydrocarbons. Therefore, the CZA/SAPO-34composite catalyst might catalyze the two reactions in a consecutive order and relieve the thermodynamic constraint. The application of CZA/SAPO-34composite catalyst in CO hydrogenation to produce hydrocarbons directly has theoretical and practical significance.Firstly, in this dissertation, the synthesis of C2-C4hydrocarbons from syngas was investigated over CZA/SAPO-34bifunctional catalyst at temperatures ranging from240to440℃. The results indicate higher temperatures make the Cu metal aggregated and lead to the deactivation of CZA. Reaction temperatures have significant effect on the selectivity of C2-C4hydrocarbons and coke species of SAPO-34. The ’active coke’ acts both as intermediates and as coke depositions. While the ’inert coke’ acts only as coke depositions. The predominant products are dimethyl ether (DME) and methanol at temperatures from280to360℃and a small amount of hydrocarbons are found due to the deactivation of SAPO-34caused by a type of coke named ’inert coke’. While96.5%selectivity of C2-C4hydrocarbons in products of all hydrocarbons can be obtained at400℃for less coke named ’active coke’ and the absent of ’inert coke’ Comprehensive consideration of the characteristics of bifunctional catalyst, the moderate temperature for C2-C4hydrocarbons synthesis from syngas might be approximately400℃. Secondly, the present study focuses on synthesis of SAPO-34zeolite membrane on the surface of CZA catalyst particles by intermediate layer to form CZA@SAPO-34core-shell structured catalyst. CZA particles have relatively brittle surface and it leads to big challenge to coat SAPO-34zeolite membrane on their surface. To overcome above shortcomings, the intermediate layer of alumina was introduced to the surface of CZA particles and acted as a barrier to the high-temperature hydrothermal and acidic condition on the one hand, on the other hand it also acted as a transition to enhance SAPO-34zeolite membrane adherence to the surface of CZA particles. With the help of alumina layer, continuous and dense zeolite membrane was obtained on the surface of CZA particles. The Al-0bonds on the surface of the intermediate layer prefer to combine with the tetrahedral units of T-O4of SAPO-34zeolite which acts as protector of CZA particles against wild synthetic conditions of zeolite membrane. Simultaneously, Al-0bonds of the intermediate layer involve in the formation of tetrahedral units of SAPO-34zeolite membrane which may promote the adherence of the zeolite membrane.The prepared CZA@SAPO-34core@shell structured catalyst has been used in CO hydrogenation to produce hydrocarbons. The selectivity of methanol on CZA@SAPO-34catalyst is lower than CZA+10%SAPO-34bifunctional catalyst which indicates the structural advantages of core@shell structured catalyst. However, CO conversion of CZA@SAPO-34catalyst is lower than CZA+10%SAPO-34bifunctional catalyst which is due to the catalytic activity of CZA is distroyed by the wild hydrothermal synthesis condition of SAPO-34zeolite membrane. The reason of the lower catalytic activity of CZA@SAPO-34is studied by exploring N-CZA, CuO, CA-CZ and AC-CZ as core catalyst. The change of ZnO in Cu/ZnO solid solution during the process of hydrothermal synthesis is considered as the main reason and it can reduce the number of the active sites of Cu/ZnO solid solution which is related to the degree of deactivation of the corresponding core@shell structured catalyst.Finally, a novel core@shell structured catalyst of CZA@Composite-SAPO-34is prepared and its advantage is the preparation method can solve the problem of the deactivation of the core catalyst by avoiding exploring the method of hydrothermal synthesis. The shell is composed of γ-Al2O3and SAPO-34zeolite which possesses excellent pore structures and CZA@Composite-SAPO-34catalyst can realize the advantages of the core@shell structure.