The Synthesis And Application Of Solid Acid Catalysts For Methanol Dehydration

With the rapid growth of economy, the demand of energy becomes an exigent problem both in China and all over the world. Dimethyl ether (DME) could be utilized in vehicle engines instead of diesel-fuel, fuel additive and family cooking gas instead of liquefied petroleum gas (LPG). DME and fluoro-dimethyl ether are environmental friendly aerosol spray and green refrigerant for their zero ozone depletion potential (ODP) and lower globe warming potential (GWP) compared with the traditional chlorofluorocarbon (CFCs, Freon) and newer R-134a (HFC-134a). Production and utilization of DME have attracted many attentions in both environmental protection and increasing price of crude oil.DME was traditionally produced via methanol dehydration (eq. (1)) catalyzed by liquid sulfate acid (or phosphate acid) and/or solid acids:Several solid-acid catalysts such as HZSM-5, H-beta, alumina, modified-alumina and SAPOs were reported for methanol dehydration in a temperature range of 250-400℃. However, most of the solid-acid catalysts produce undesirable products such as hydrocarbons due to the presence of strong acid sites and the higher dehydrotion temperature. Extensive research has been made especially in order to find catalysts having higher selectivity for the ether formation and less tendency to coke formation. It has recently been found that DME formation is mainly related to sites with weak and medium acidity, as this catalyst may not be effective for coke deposition.In this research work, A series of combined Al₂O₃-HZSM-5 solid acids were prepared via impregnation of a colloidal solution of Al₂O₃ onto HZSM-5 powder, chemical precipitation of solvable Al(NO₃)₃ onto the HZSM-5 powder and mechanically mixing of Al₂O₃ powder and HZSM-5 powder. The physicochemical properties of these combined solid acids were characterized by X-ray diffractometer, N₂ adsorption and NH₃ temperature-programmed desorption. The activity of the combined solid acid for methanol dehydration to dimethyl ether was examined in a microreactor. It was found that both the catalytic activity and selectivity of combined Al₂O₃-HZSM-5 with lower Al₂O₃ (< 30wt%) were improved obviously. And the reaction kinetics of methanol dehydration over the combined Al₂O₃-HZSM-5 was deduced using a Rideal-Eley model.At the same time, a series of different sized zeolite Y (ranged from 500 ran to 4.0 were synthesized via optimization of the aging time of the initiating agents. The physicochemical properties of these samples were characterized by X-ray diffractometer, scanning electron microscopy (SEM), N₂ adsorption and NH₃ temperature-programmed desorption. Characterizations disclosed that the surface area, the accessibility of surface acid sites increased with the decreasing of the particle sizes. And small sized zeolite HY are more active for the direct synthesis of diphenylmethane (DPM) via condensation of benzene and formaldehyde.