模板法制备高比表面积的氟化镁及其在HFC-152a脱HF反应中的应用

High-surface-area Magnesium Fluoride: Preparation by Template Method and Catalytic Activity for the Dehydrofluorination of HFC-152a

镁基固体酸催化剂在含氟化学品的合成中具有优异的性能。利用模板法制备了高表面积的氟化镁,并考察了SiO_2模板剂的用量对其结构及催化性能的影响。通过N_2物理吸附、X射线衍射、NH_3-程序升温脱附、透射电镜和X射线光电子能谱等表征手段进行了表征,以1,1-二氟乙烷(HFC-152a,CH_3CHF_2)脱HF制备氯乙烯(VF,CH_2=CHF)为探针对其催化性能进行了研究。结果表明, SiO_2模板剂用量对氟化镁的比表面积、晶粒度和酸性有较大影响。当SiO_2模板剂用量为14mol%时,氟化镁比表面积可达304m^2/g,是不添加SiO_2模板剂的2.5倍,而且Mg晶粒度更小,配位数更多。随着Mg配位数增多,MgF_2的酸性位急剧增多,在以Lewis酸为活性位的1,1-二氟乙烷脱HF反应中, MgF_2的催化活性迅速升高。因此,以SiO_2为模板是制备高活性MgF_2催化剂的有效方法。

Magnesium-based solid acid catalysts exhibit satisfactory catalytic performance for the preparation of fluorocarbons. In the present study, magnesium fluoride with high surface area was obtained by template method. The influences of SiO2 template doping amount on phase structure, porosity, and surface acidity of MgF2 were investigated. Physical and chemical properties of the as-prepared MgF2 were characterized by N2 physical adsorption-desorption iso-therms, X-ray powder diffraction（XRD）, NH3-temperature programmed desorption（NH3-TPD）, transmission electron microscope（TEM）, and X-ray photoelectron spectroscope（XPS）. The catalytic performance was evaluated for the dehydrofluorination of 1,1-difluoroethane（HFC-152 a, CH3CHF2） to vinyl fluoride（VF, CH2=CHF）. Results show that the doping amount of SiO2 template plays a major role in the specific surface area, crystal size and acidity of Mg F2. With the presence of 14 mol% SiO2 template in the Mg（CH3COO）2 and NH4F solution, the derived MgF2 possesses a specific surface area as high as 304 m2/g, which is almost 2.5 times higher than that of MgF2 prepared without SiO2 template. In addition, with suitable doping amounts of SiO2 template, smaller crystal sizes and under-coordinated sites（five-fold and four-fold coordination Mg species） are derived. Consequently, with enhanced amounts of under-coordinated sites, acidic sites（Lewis acid） of Mg F2 are improved significantly. As the dehydrofluorination of 1,1-difluoroethane catalyzed by Lewis acid, as a result, the conversion of 1,1-difluoroethane is increased dramatically. In summary, it is confirmed that template method with SiO2 as the template is an effective route for the fabrication of Mg F2 catalyst.