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

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SiO₂模板剂的用量对其结构及催化性能的影响。通过N₂物理吸附、X射线衍射、NH₃-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, CH₃CHF₂)脱HF制备氯乙烯(VF,CH₂=CHF)为探针对其催化性能进行了研究。结果表明, SiO₂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SiO₂模板剂用量为14mol%时, 氟化镁比表面积可达304 m²/g, 是不添加SiO₂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, MgF₂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, MgF₂的催化活性迅速升高。因此, 以SiO₂为模板是制备高活性MgF₂催化剂的有效方法。

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

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-152a, 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 MgF₂. 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 MgF₂ 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 MgF₂ catalyst.