氧化镁改性HZSM．5分子筛的酸性及在合成气直接制二甲醚反应中的应用--镁前驱体的影响

Acidity of Magnesium Oxide Modified HZSM-5 Zeolite and its Application in the Direct Synthesis of Dimethyl Ether from Syngas——The Effect of Magnesium Precursor

以乙酸镁为前驱体采用浸渍法制备了一系列不同氧化镁（MgO）含量（0％-20％）的镁改性HZSM-5分子筛，并采用X射线衍射（XRD）、N2吸附、X射线光电子能谱（XPS）、氨程序升温脱附（NH3-tPD）和吡啶吸附红外光谱（Py-IR）方法对其结构和酸性进行了表征。在连续流动加压固定床反应器中及温度260℃、压力4MPa和气体空速1500mL．（gcn·h）^-1条件下，考察了以上述HZSM-5分子筛为甲醇脱水活性组分与铜基甲醇合成活性组分（CuO-ZnO-Al2O3）所组成的双功能催化剂对合成气直接制二甲醚反应的催化性能。结果表明，HZSM-5分子筛经适量的MgO改性后，产物中二甲醚的选择性由49．1％提高到68％以上，而二氧化碳和烃类副产物的选择性则分别由37．1％和9．3％降低到27％和0．1％以下。烃类和二氧化碳副产物减少的原因是MgO改性降低了HZSM-5分子筛强酸中心的酸量，尤其是减少了强Broensted酸的酸量。另外，与以硝酸镁为前驱体制备的MgO／HZSM-5不同，MgO在较大的范围（2．5％-20％）内对其催化性能几乎无影响。

A series of HZSM-5 zeolites modified with various contents （0% -20%） of magnesium oxide （MgO） were prepared by impregnating with aqueous solutions of magnesium acetate, and their structural and acidic properties were characterized by X-ray diffraction （XRD）, N2-adsorption, X-ray photoelectron spectroscopy （XPS）, temperature-programmed desorption of ammonia （NH3-TPD） and pyridine adsorption infrared spectroscopy （Py-IR） methods. The direct synthesis of dimethyl ether （DME） from syngas was carried, with two admixed catalysts, Cu-ZnO-Al2O3 / HZSM-5 and Cu-ZnO-Al2O3 / HZSM-5 modified by MgO in a continuous flow pressurized fixed-bed under temperature of 260℃, pressure of 4 MPa and gas hourly space velocity of 1500 mL·（gcat·h）^-1. The results indicate that the modification of HZSM-5 with suitable amount of MgO can significantly enhance the selectivity for DME from 49% to higher than 68%, while the selectivity for the undesired byproducts CO2 and hydrocarbons is decreased from 37.1% and 9.3% to less than 27% and 0.1%, respectively. The decrease in the formation of hydrocarbons and carbon dioxide can be attributed to the significant decline in the amount of strong acid sites, especially the strong Broensted acid sites induced by the modification with magnesium oxide. Moreover, different from the modified HZSM-5 prepared by using magnesium nitrate as magnesium precursor, the amount of magnesium oxide in a wide range （2.5% -20%） exhibits almost no effect on the performance of the modified HZSM-5 prepared with magnesium acetate as magnesium precursor.