双介孔硅基材料负载Pd催化剂上甲苯氧化性能的研究

催化氧化技术是挥发性有机物(VOCs)减排与控制的主流技术之一,其关键之处在于高效催化材料的研究与开发,负载型贵金属催化材料由于其低温下优越的VOCs催化氧化性能,受到国内外研究者的广泛关注.对于负载型催化剂而言,载体的性质直接影响活性相的分散,反应物和生成物的扩散与吸脱附,是影响负载型催化剂性能的主要因素.近年来,多级孔结构硅基材料由于具有多级的孔道结构、高比表面积和大的孔体积,逐渐成为VOCs催化氧化材料的研究热点.本文采用溶胶凝胶法和浸渍法制备了系列双介孔结构硅基材料负载Pd催化剂(Pd/BMS-x),通过控制合成过程中氨水的用量以调节催化剂的介孔结构分布. X射线衍射(XRD)结果表明,所合成的Pd/BMS-x催化剂在~2.0°的衍射峰,类似于MCM-41的(100)晶面衍射峰,表明所有的样品均具有有序的介孔结构. N2吸脱附实验表明所有样品的比表面积均高于1000m2/g,孔径分布表明Pd/BMS-30样品为单一介孔结构,而Pd/BMS-5~Pd/BMS-20样品具有2.64 nm以及18–45 nm范围内的双介孔结构,且Pd/BMS-15样品介孔分布较为集中. Pd/BMS-x催化剂上甲苯催化氧化性能测试表明,双介孔结构的Pd/BMS-5~Pd/BMS-20催化剂上甲苯催化氧化活性远高于单一介孔结构的Pd/BMS-30催化剂,表明载体结构对催化剂性能有重要影响.其中, Pd/BMS-15催化剂性能最佳(T90为228°C)且具有较强的稳定性,250°C条件下,反应持续60h催化剂未见明显失活. SEM和TEM结果表明, Pd/BMS-15催化剂中Pd高度分散于载体上,平均粒径在~3 nm左右.而Pd/BMS-30催化剂中Pd颗粒间有明显的团聚,平均粒径在8~17 nm之间.分散度测试表明,单一介孔结构的Pd/BMS-30催化剂, Pd分散度仅为27%,而双介孔结构Pd/BMS-5–Pd/BMS-20催化剂介于39%到69%,其中Pd/BMS-15催化剂中Pd分散度高达69%.与常规单一介孔MCM-41和MCM-48负载Pd催化剂相比,在低空速(42000 h–1)条件下, Pd/BMS-15催化剂上甲苯催化氧化性能与Pd/MCM-41和Pd/MCM-48催化剂相当.高空速(70000h–1)条件下, Pd/BMS-15催化剂的活性远高于单一介孔的Pd/MCM-41和Pd/MCM-48催化剂. Pd/BMS-15催化剂独特的双介孔结构,有利于活性相Pd的分散、反应物的扩散和传输,特别是在高空速条件下,有利于反应物与活性相的接触,提高了材料的氧化反应性能.进一步考察了材料的水热稳定性,将11 vol%的水蒸气引入到反应体系中,测试结果表明水蒸气的加入导致Pd/MCM-41和Pd/MCM-48催化剂的甲苯催化氧化性能显著下降,反应500 min后甲苯转化率分别从100%下降到76%和81%,而对于Pd/BMS-15催化剂,水蒸气的引入并未导致其活性明显下降,从而表明Pd/BMS-15催化材料具有较高的水热稳定性.

High performance Pd catalysts supported on bimodal mesopore silica for the catalytic oxidation of toluene

A series of bimodal mesoporous silica (BMS-x)-supported Pd catalysts were successfully prepared by a facile sol-gel approach, followed by an impregnation method. The synthesized catalysts were characterized by several analytical techniques and the oxidation of toluene was used to evaluate their catalytic performance. Textural analysis showed that all samples had high surface areas (~1000 m2/g), large pore volumes (~1.2cm3/g) and uniform mesopore size (~2.6 nm). Defining the level of ammonia solution to within a certain range resulted in the catalysts possessing a typical bimodal mesoporous structure with an intraparticle framework mesopore and an interparticle textural mesopore (18–40 nm). Transmission electron microscopy observations and CO chemisorp-tion results revealed that this unique bimodal mesoporous structure helped to decrease the particle size of Pd nanoparticles and could further enhance their dispersion. Activity tests revealed the Pd/BMS-5–Pd/BMS-20 catalysts with a bimodal mesopore structure possessed superior catalytic performance for the oxidation of toluene compared to Pd/BMS-30 with a unimodal mesopore structure. More importantly, compared with the Pd/MCM-41 and Pd/MCM-48 catalysts, Pd/BMS-15 had improved hydrothermal stability and catalytic performance at a high gas hourly space velocity of 70000 h?1. These results indicate the potential application of the catalysts for the elimination of volatile organic compounds.