氧化钙改性分子筛对一步法合成二甲醚的影响(英文)

One-step synthesis of dimethyl ether over a bi-functional catalyst containing CaO-modified HZSM-5

采用浸渍法制备了一系列CaO改性的HZSM-5(Si/Al=38)分子筛,并以CaO/HZSM-5为脱水剂与JC207甲醇合成催化剂(靖江催化剂厂)按照一定的比例组成双功能催化剂,在固定床反应器上考察了其对一步法合成二甲醚的影响。XRD结果表明,CaO在HZSM-5上呈高度分散状态,没有发现新的物种生成。Pyridine-IR结果表明,CaO引入HZSM-5后,酸中心的类型和数量发生明显的变化,CaO的加人促使部分酸中心由B酸中心转变为L酸中心。NH3-TPD结果表明,随着CaO含量的增加,HZSM-5分子筛表面酸性较强的酸中心数目下降,总酸中心数目也下降,但酸性较强的酸中心下降较快。表明适量的CaO改性HZSM-5分子筛不是除去表面所有的酸中心,而是通过与表面强酸中心的作用,使其向弱酸和中强酸中心过渡,从而改变HZSM-5表面的酸强度分布,提高二甲醚的选择性。

Synthesis of dimethyl ether （DME） was carried out in a fixed bed reactor over a bifunctional catalyst CaO/HZSM-5 ＋ JC207, which was composed of a methanol-synthesis catalyst JC207 （Jingjing Catalyst Plant, China） and a dehydrator HZSM-5 （ Si/Al = 38 ） modified with CaO. The effects of zeolite modification with CaO on the performance of the bifunctional catalyst in one-step synthesis of DME were investigated. XRD analysis of the modified zeolite CaO/HZSM-5 indicates that CaO is highly dispersed on the zeolite HZSM-5 without any new species formed. Pyridine-IR analysis shows that the modification of HZSM-5 with CaO not only brings on significant changes in acid type and acid amount but also turns some of Bronstead acid sites into Lewis acid sites. NH3-temperature programming desorption analysis indicates that the number of acid sites on the zeolite surface, especially, that of the strong acid sites decreases with the increase of CaO loading. Modification of zeolite HZSM-5 with appropriate dose of CaO turns the strong acid sites into weak or less strong acid sites; such a redistribution of acid sites may be beneficial for the increase of DME selectivity.