SBA-15负载β-Mg_2V_2O_7催化剂的制备及其催化丙烷选择氧化脱氢的性能

Preparation of SBA-15-Supported β-Mg_2V_2O_7 Catalysts and Their Properties in Oxidative Dehydrogenation of Propane

以MgO修饰的SBA-15为载体,采用浸渍法制备了负载β-Mg2V2O7催化剂,并运用X射线衍射、拉曼光谱、紫外-可见漫反射光谱和H2程序升温还原等技术对催化剂V中心的结构和还原性能进行了表征.结果表明,β-Mg2V2O7具有与α-Mg2V2O7相同的结构单元,但其催化丙烷氧化脱氢(ODH)反应的初始活性和初始选择性均低于后者.与体相β-Mg2V2O7相比,负载的β-Mg2V2O7上V中心分散度以及丙烷ODH反应活性和选择性更高,520oC时丙烷ODH反应的初始活性提高了约20倍,丙烯初始选择性也从体相的88.3%提高到94.1%,接近于α-Mg2V2O7(94.6%),并且在20%的丙烷转化率时也表现出相似的规律.这与表征催化剂选择性的两个本征动力学参数k1/k2(丙烷初级ODH和燃烧反应速率常数之比)和k3/k1(次级丙烯燃烧和初级丙烷ODH反应速率常数之比)反映出的规律一致.这些对体相和负载的Mg2V2O7催化剂催化丙烷ODH反应本征特性的认识将有助于设计合成更高效的Mg-V-O催化剂,如基于α-Mg2V2O7结构的高分散催化剂,以获得更高的丙烷ODH反应活性和选择性.

β-Mg2V2O7 catalysts supported on MgO-modified SBA-15 with V surface density of 4.1-8.5 nm-2 were synthesized by impregnation of the support with a pre-prepared aqueous solution of ammonium metavanadate,citric acid,and magnesium nitrate.The structure and reducibility of the catalysts were characterized by X-ray diffraction,Raman spectroscopy,diffuse reflectance UV-Vis spectroscopy,and temperature-programmed reduction in H2.For oxidative dehydrogenation（ODH） of propane,bulk β-Mg2V2O7 phase was much less active and selective than α-Mg2V2O7 although they have the same dimeric tetrahedral VO4 units connected by bridging V-O-V bonds.Compared to the bulk phase,the greater accessibility of V sites on the supported β-Mg2V2O7 catalysts led to an increase in not only the initial propane ODH rates by about 20 times but also the initial propene selectivity from 88.3 to 94.1%,as high as that for α-Mg2V2O7（94.6%） that has been considered as the most selective Mg-V-O phase for propane ODH reactions.The selectivity results are consistent with those reflected by the two kinetic parameters,k1/k2 and k3/k1,which represent the ratios of the rate constants for primary propane ODH（k1） to propane combustion（k2） and that for secondary propene combustion（k3） to k1.Such understanding on the intrinsic catalytic properties of the bulk and supported Mg2V2O7 catalyst provides useful information for design of new Mg-V-O-based catalysts,for example,dispersed α-Mg2V2O7 catalyst with high activity and selectivity for propane ODH reactions under mild conditions.