摘要
The VOx catalysts supported on dealuminated Beta zeolite(Si Beta) with varying V loadings(from 0.5 to 10 wt%) are prepared and tested for their catalytic activities in the reaction of direct dehydrogenation of propane to propylene(PDH). It is characterized that the VSi Beta catalysts possess different kinds of vanadium species on the Si Beta support, including monomeric or isolated VOx species at a low V loading, and polynuclear VOx species in different polymerization degrees at higher V loadings. The 3 VSi Beta catalyst(V loading is 3 wt%), containing isolated VOx species in monolayer, shows around 40% of propane conversion with 90% of propylene selectivity(reaction conditions: 600 o C, 4000 m L g–1 h–1) which are comparable to VSi Beta catalysts with higher V loadings. The catalytic activity exhibits a good linear relationship with the amount of generated acidic sites, which are derived from the interaction sites between VOx species and Si Beta support, and keeps stable after several regeneration cycles. Thus, as the VOx species directly contact with Si Beta support via V–O–Si bonds, a reactivity enhancement can be achieved. While, the initial valence state of V does not seem to influence the catalytic performance. Moreover, the aggregation degree of VOx species determines the propylene selectivity and deactivation rate, both of which increase as raising the V loading amount.
丙烯是一种非常重要的化工基础原料,主要用来生产高价值的化学品,如丙烯腈,异丙醇和甘油等.丙烯的传统来源主要是石油的催化裂化反应,以及石脑油和轻柴油的裂解过程.随着石油资源的减少,丙烷直接脱氢制丙烯技术逐渐成为一条重要的丙烯生产渠道.V基催化剂在丙烷氧化脱氢反应中被广泛研究,但被用于丙烷直接脱氢反应的报道还较少,且V基催化剂在丙烷直接脱氢制丙烯中的活性位目前还没有统一定论,可能是由于V的活性物种与载体性质密切相关,因此研究V基催化剂在不同载体上的活性位具有重要意义.Beta沸石分子筛具有规则微孔结构,高的比表面积以及可调节的酸性,是一种理想的金属催化剂载体.本文采用脱铝后的Beta分子筛作为V催化剂载体,通过调节V的负载量探究丙烷直接脱氢活性和VO_x结构的关系,以及催化剂的酸性对性能的影响.活性测试结果显示,V负载量分别为3wt%,7wt%和10wt%时(催化剂分别命名为3VSiBeta,7VSiBeta和10VSiBeta),三者的催化活性十分接近,此外,这些催化剂还具有较好的循环利用性,但碳沉积,丙烯选择性以及失活率都是随着V负载量的升高而增加.XRD和N2吸附结果揭示,VO_x在SiBeta上呈高度分散状态,并且当V负载量从3wt%升至10wt%时,表面V密度发生明显变化,VO_x物种在3VSiBeta中可基本实现孤立的单分散状态.同时,DRUV-vis,H2-TPR以及Raman测试结果表明,VO_x物种的聚合程度随V负载量的升高逐渐增加.NH^3-TPD结果表明,Beta载体在脱铝后本身的酸性位完全消除,但是负载V后引入了新的酸性位,并且3VSiBeta,7VSiBeta和10VSiBeta三者的酸量基本相当.尽管XPS结果显示,不同VSiBeta催化剂上的V价态分布有差异,但是相似的催化活性说明VSiBeta催化剂的活性位与形成的酸性位数目密切相关,而受V初始价态的影响不大.本文指出,酸性位可能是与V–O–Si键直接相关,V负载量从3wt%
基金
supported by the National Natural Science Foundation of China(21421001,21573115)
the 111 project(B12015)
the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering(2017-K13)~~
