One-step Synthesis Of Dimethyl Ether Over Cu-Zn-Mn/zeolite-Y Hybrid Catalyst And The Promotion Effects Of Mn

Dimethyl ether(DME) is an important chemical product and potential clean energy,with wide application in many industrial fields.The One-step synthesis of dimethyl ether via syngas is of economic values and theoretical significance,because it is greatly favorable in thermodynamics.In this thesis,we have designed a series of hybrid catalyst by altering methodology of preparation and investigated the hydrogenation properties of those hybrid catalysts.In addition,the effects of Mn promoter and different precursors on the catalytic performance of hybrid Cu-based/zeolite-Y have been systematically studied.Cu-Mn/zeolite-Y catalysts prepared using a coprecipitation impregnation procedure have been investigated to develop active Cu-based catalysts for the dimethyl ether synthesis from syngas.A great enhancement of CO conversion and DME selectivity is observed on Mn-containing catalysts compared to Mn-free counterpart.However,excess addition of manganese suppresses the syngas-to-dimethyl ether(STD) activity.Manganese has been found to be interacting with copper to form copper manganese mixed oxides phase(Cu_1+xMn_2-xO₄) upon calcinations and the excess addition of copper or manganese will induce the segregation of copper or manganese as isolated CuO or Mn₂O₃ phase,respectively. The segregates CuO phases enhanced the reducibility of Cu_1+xMn_2-xO₄ phase and Mn₂O₃ phase by reducing spill-over species.Furthermore,it is found that addition of Mn to Cu/zeolite-Y catalyst decreases the average particle size of copper to same extent.Combining the TPR and XRD analysis,it suggests that catalyst with the excess CuO phases in conjunction with copper manganese mixed oxides are much more active than that with excess Mn₂O₃ phases together with copper manganese mixed oxides in the hydrogenation of CO to DME.The addition of a certain amount of manganese to the Cu-Zn/zeolite-Y catalyst remarkably improves the activity of the catalyst for the direct synthesis of dimethyl ether from syngas.The conversion of CO increases from 56.8%on Cu-Zn/zeolite-Y to 82.9%on Cu-Zn-Mn/zeolite-Y.The characterization of XRD reveals that a pronounced reduction in crystallite size of both CuO and ZnO is observed upon the addition of Mn.Combined with the HRTEM analysis results,it is deduced that the smaller crystallite size of both CuO and ZnO and higher inter-dispersion between the metal oxide components is responsible for the significantly high catalytic activity.A much higher resistance to sintering on Mn-modified than on Mn-free Cu-Zn/zeolite-Y catalyst is also found.The catalytic activity of Cu-Zn/zeolite-Y catalyst can be promoted by aging the precipitate during the precipitation and it can also be improved significantly by the addition of manganese during the precipitation without the aging process.The remarked increase in the catalytic activity of the MCZY catalyst is explained by that the incorporation of Mn during the co-precipitation process shorten the time of the development of a blue-green precursor,a structure in which the Cu and Zn are atomically dispersed in a hydroxycarbonate matrix.Finally,this leads to ideally mixed and uniformly distributed copper and zinc mixed oxides with smaller particle sizes than the CZY sample derived from independent Cu and Zn precursor phases. Undoubtedly,it appears to be a reasonable explanation for the promoting effect of manganese on the catalytic activity of CZY catalyst in DME synthesis to certain extentAn investigation is carried out to indentify the effects of manganese incorporation sequence on the catalytic structures and performance of Cu-Zn/zeolite-Y Catalyst for direct dimethyl ether synthesis from syngas.Three different catalysts are prepared:the Cu-Zn/zeolite-Y(CZY),and two Mn-promoted Cu-Zn/zeolite-Y Catalysts,MCZY-C and MCZY-S,which are prepared by simultaneous co-precipitation impregnation and sequential precipitation impregnation, respectively.The MCZY-C catalyst appears to be the most active for dimethyl ether synthesis,whereas the MCZY-S catalyst shows little increase in the catalytic activity with respect to the CZ-Y catalyst.The remarked increase in the catalytic activity of the MCZY-C catalyst is explained by that the incorporation of Mn during the simultaneous co-precipitation impregnation process favor the development of a blue-green precursor,a structure in which the Cu and Zn are atomically dispersed in a hydroxycarbonate matrix.Finally,this leads to ideally mixed and uniformly distributed copper and zinc mixed oxides with smaller particle sizes than the other two samples derived from independent Cu and Zn precursor phases.This study reveals significance of the methodology used for the incorporation of manganese. Meanwhile,it shows that the precipitation order has a remarkable influence on the properties of the precursors and calcined catalysts,and consequently,on the catalytic performance for the direct synthesis of dimethyl ether from syngas.Two different precipitating agents,Na₂CO₃ solution and NaHCO₃ solution,were used to explore the effect of method of preparations on the properties and catalytic perforamance of the catalysts for the dimethyl ether synthesis via syngas.The bi-functional catalysts obtained were investigated as synthesized or after calcination by various physico-chemical techniques.Cu-Zn/zeolite-Y catalyst obtained from the co-precipitation impregnation process using NaHCO₃ was found to be superior to that using Na₂CO₃ solution in terms of activity and stability.The conversion of CO and the selectivity to DME of the catalyst,prepared by using NaHCO₃ solution as precipiating agent,reaches 78.1%and 66.2%,respectively,under the reaction conditions of 2.0 MPa,250℃,1500 h^-1.Precipitation using sodium carbonate solution leads to the formation of an amorphous phase precursor whereas using NaHCO₃ results in a high-zincian malachite phase.Finally,smaller crystallite size of Cu and ZnO and higher microstrain of copper particle are found in calcined Cu-Zn/zeoltie-Y catalyst prepared by using NaHCO₃ solution.Therefore,the increase in the activity and stability of the catalyst prepared by using NaHCO₃ solution as precipiating agent do not only correlate with the specific Cu surface area but also coincides with an increase in the microstrain in the metallic copper presumably because of the improved contact area(interfacial area) between Cu and ZnO.Based on the characterization data,it has shown that the selection of a suitable precipitating agent for the Cu-Zn/zeoltie-Y catalyst preparation via co-precipitation impregnation process significantly influences the properties of the precursors and therefore the formation of the final structure and catalytic activity.