Design And Modification Of Catalysts For One-step Synthesis Of Dimethyl Ether From Syngas

Dimethyl ether（DME）,as a kind of environmental friendly clean fuel,can be synthesized by the coal based syngas over bifunctional catalysts.In this study,CZ-x SA/ZSM-5 bifunctional catalyst prepared by physical mixing of hydrophobic Cu Zn catalyst modified by stearic acid（CZ-x SA）with ZSM-5,and hydrotalcite（LDH）derived（Cu Zn）_xAl_y bifunctional catalyst were applied to the one-step synthesis of DME respectively.The following conclusions were drawn:（1）The traditional methanol synthesis catalyst（Cu Zn）was hydrophobically modified with stearic acid（SA）.The contact angle increased with the increase of treatment concentration,indicated that hydrophobic surface was constructed successfully.（2）The prepared catalysts were characterized by XRD,FT-IR,TG,N₂ absorption and desorption,H₂-TPR,XPS etc.These results showed that the stearic acid could combine with zinc of Cu Zn catalyst and formed a hydrophobic surface.Moreover,the hydrophobic surface of catalyst exhibited good thermal stabilityand the decomposition temperature was even up to 350℃.（3）DME directly synthesis from syngas was tested over CZ and CZ-x SA/ZSM-5catalysts.The evaluation results showed that the hydrophobic catalyst exhibited a certain inhibitory effect for water-gas-shift side reaction.Moreover,an appropriate concentration of stearic acid treatment had no obvious effect on methanol formation.The CZ-4SA catalyst treated by 4 m M SA achieved higher DME selectivity and lower selectivity of CO₂ than those of untreated CZ catalyst.The CO/H₂O-TPSR-MS characterization showed that the modification of catalyst by stearic acid could effectively restrain the water-gas-shift side reaction.（4）A series of Cu Zn Al precursors with hydrotalcite structure with a wide range of M²⁺/M³⁺was designed and prepared by sedimentation reflux method.After calcination,a bifunctional catalyst with both methanol synthesis and methanol dehydration component was obtained,which was successfully applied to the one-step synthesis of dimethyl ether from syngas.（5）The prepared catalysts were characterized by XRD、SEM、TEM、BET、H₂-TPR、NH₃-TPD etc.The results showed that the（Cu Zn）/Al ratio of（Cu Zn）_xAl_yhydrotalcite precursor,or the content of Al,had a great influence on the structure of precursor,which further affected the structural properties of calcination catalysts.With the increasing of Al content,precursor chip became smaller,the specific surface area of calcination catalysts increased gradually,and the dispersion of active species Cu was markedly improved.At the same time,the strong acid sites in catalyst and total acid content were both increased.Therefore,for（Cu Zn）₁Al₃-LDH with highest Al content,more active sites exposed on catalysts surface and the synergy of active Cu species with Al₂O₃,was strengthened,which greatly contributed to high catalytic activity and selectivity of DME.