Preparation, Characterzation And Performance Of The Core-shell Catalysts For Dimethyl Ether Direct Synthesis From Syngas

With the development of industry, the tense situation about energy utilization is constantly serious in recent years. Being rich in coal and inadequate in petroleum and natural gas is the present energy situation in China, so it’s an effectively solutions for China to ease the tense energy situation by developing a new route which convert coal based energy sources to clean substitutive fuel. Dimethyl Ether (DME), which is called the "Clean Energy for21st Century", is not only an new essential chemicals, but also can substitute for diesel or LPG.Direct synthesis of dimethyl ether from syngas (Syngas to dimethyl ether, STD) has attracted worldwide attentions, because the STD process can use the advantage of the abundant coal resources in China and ease tense situation of oil resources, and STD process can achieve the clean use of coal. The preparation of the bifunctional catalysts with high catalytic activity and stability is key factor for the STD process can be widely applied. The bifunctional catalysts for the STD process usually contain methanol synthesis catalyst and methanol dehydration catalyst. This thesis has synthesized the core-shell structure bifunctional catalysts CZA@Al2O3by hydrothermal treatment, which contains methanol synthesis catalyst and methanol dehydration catalyst. In addition, by adding PVA as a binder bifunctional catalysts CZA@Al2O3-PVA was synthesized. Catalytic activity of bifunctional catalysts was tested in a fixed bed reactor, the reaction conditions are:260℃、5MPa and1500mL/(h·gcat). The main results are as follows:(1) Bifunctional catalysts CZA@Al2O3was prepared by a two-step method. Firstly, a hydrothermal carbon layer was coated on the surface of CZA by hydrothermal treating glucose, and this sample was denoted as CZA@C. Then, CZA@C was further treated in Al(NO3)3solution under hydrothermal condition. The synthesis mechanism of CZA@Al2O3has been studied by DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) and SEM (Scanning Electron Microscope):hydrothermal carbon sphere was formed and adsorbed on surface of CZA, then Al3+was adsorbed on hydrothermal carbon layer due to abundant functional groups on its surface. After calcination, bifunctional catalysts CZA@Al2O3was prepared with CZA as core and Al2O3as shell. The core-shell structure catalyst CZA@Al2O3shows higher DME selectivity than physically mixed catalyst CZA-M due to the core-shell structure Al2O3has special "Confinement effect", so that CZA@Al2O3shows higher methanol dehydration efficiency.(2) The core-shell structure bifunctional catalysts CZA@Al2O3with methanol synthesis catalyst CZA as core and methanol dehydration catalyst Al2O3as shell were synthesized by using glucose, sucrose and starch as templates and using aluminium nitrate as aluminium source in one-step synthesis procedure. In this procedure, no acid, alkali or amine template was used. The thickness of the shell can be altered by changing the synthesis parameters, such as synthesis temperature, synthesis time, concentration of aluminum nitrate and template. The core-shell structure catalysts CZA@Al2O3shows excellent performance in the STD process, the CO conversion achieves35.2%and DME selectivity achieves61.1%. Compared to the bifunctional catalysts preparation by two-step method, catalysts preparation by one-step method own higher CO conversion and DME selectivity, this is because shortening the contact time of glucose and CZA can reduce the damage to the CZA surface structure.(3) Bifunctional catalysts CZA@Al2O3-PVA was synthesized by adding PVA as a binder. Compared to the bifunctional catalysts preparation by one-step method, catalysts CZA@Al2O3-PVA own smoother surface and lower thickness. Chelating key is formed between Al3+and free hydroxyl groups in PVA solution, which could reduce the probability of Al3+combined with CZA. The thickness of the shell can be altered by changing the synthesis parameters, such as concentration of aluminum nitrate and PVA. The conversion of CO and the selectivity of DME on CZA@Al2O3-PVA achieved56.0%and50.9%, respectively.