Mo/HZSM-5催化剂上甲烷脱氢芳构化反应—催化剂制备方法的影响

采用高温蒸汽处理的方法对机械混合法制备的Mo/HZSM-5催化剂进行了改良,利用XRD、XPS等表征催化剂的晶相组成、Mo物种的分散度。结果表明,Mo/HZSM-5催化剂的甲烷脱氢芳构化性能与其Mo物种的分散度密切相关。机械混合法制备的催化剂上,Mo物种主要以微晶的形式分布在分子筛的外表面,由于缺少分子筛孔道择形保护,使积炭副反应的容易发生。经高温蒸汽处理后,催化剂上Mo物种的分散度显著提高,有较多的Mo物种迁移至分子筛的孔道内,并与B酸发生强相互作用生成钼氧二聚体。因此在甲烷芳构化反应中,蒸汽处理的Mo/HZSM-5催化剂显示出较强的抗积炭能力,甲烷转化活性及芳烃的选择性较传统机械混合法制备的催化剂有明显提高。

Methane Dehydroaromatization over Mo/HZSM-5 Catalysts: Effect of Preparation Methods of Catalysts

A steam treatment was introduced to modify mechanically mixed Mo/HZSM-5 catalyst,and the crystal phase compositions and Mo dispersion of the catalysts were investigated by XRD and XPS techniques.It was found that the catalytic performance of Mo/HZSM-5 catalyst in methane dehydroaromatization(MDA) was strongly dependent on the dispersion of Mo species on the HZSM-5 zeolite.In the mechanically mixed Mo/HZSM-5 catalyst,the Mo species mainly existed as MoO3 crystallites on the external surface of the zeolite.Without protection by the shape-selective environment within zeolite channels,the Mo crystallites promoted the coke formation during the MDA reaction.After subjected to the steam treatment,the dispersion of the loaded Mo species considerably increased,and some Mo species with small particle size migrated into the zeolite channels and reacted with the Brnsted acid sites to form(MO2O5)2+ dimers.Thus,a remarkable improvement in catalytic durability and selectivity to aromatics was achieved for the steam-treated Mo/HZSM-5 catalyst compared to the mechanically mixed catalysts,mainly due to effective suppression of coke formation during the MDA reaction.