异丁烯作为一种常用的有机化工原料,具有重要的工业价值.与传统的化石路线制异丁烯相比,生物基丙酮制异丁烯引起了研究者的广泛关注.Brønsted酸催化剂常用作丙酮制异丁烯反应的催化剂,但反应过程产生的积碳会导致催化剂快速失活.与常用的Brønsted酸催化剂相比,Lewis酸催化剂具有更好的催化性能和更高的异丁烯选择性,近年来得到了研究者的广泛关注.然而,催化剂的失活仍不可避免,并且失活机理仍有待阐明.本文以具有Lewis酸的Y/Beta分子筛为丙酮转化制异丁烯反应的催化剂,发现该催化剂表现出较好的催化性能,然而随着反应时间的延长,其失活仍不可避免,并且反应温度对催化剂失活影响较大.采用程序升温表面反应(TPSR)、色质联用(GC-MS)、原位紫外-可见光(UV-vis)光谱和13C交叉极化魔角旋转核磁共振(CP MAS NMR)波谱等多种表征技术详细研究了Y/Beta催化剂上丙酮制异丁烯的催化反应过程.首先,通过TPSR监测了反应过程反应物、产物以及中间体的动态变化,并通过原位UV-vis光谱辅以验证.其次,利用13C CP MAS NMR以及GC-MS得到了反应过程重要有机中间物种的具体结构信息.基于以上谱学手段,实现了反应过程有机中间物种的动态监测,得到了失活物种生成和演变过程的谱学证据,并绘制了Lewis酸Y/Beta催化剂上丙酮转化制异丁烯的反应机制和失活路线.在低温反应条件下,丙酮在具有Lewis酸性的Y活性位点上的连续缩合和环化是主要的副反应,并且在反应过程会产生较多的佛尔酮等失活物种.高温则可导致环状不饱和醛/酮以及芳烃物种的快速形成和累积,比如2,4-二甲基苯甲醛等,该物种可较强地吸附在Y活性位点上,并最终导致催化剂失活.然而,该积碳物种经过简单的煅烧便可去除,催化剂的活性也可以很好地恢复.因此,通过优化反应温度以及催化剂活性位点的分布,有望提高丙酮制异丁烯反应的催化活性,同时延长催化剂的寿命.综上,本文通过系列谱学手段相结合的方式,实现了Lewis酸分子筛上丙酮制异丁烯反应过程有机中间物种动态变化的监测,阐明了丙酮制异丁烯的反应和失活机理,为Lewis酸催化的丙酮制异丁烯反应高效催化剂的优化和设计提供了新思路.
Deactivation mechanism of acetone to isobutene conversion over Y/Beta catalyst
The conversion of acetone derived from biomass to isobutene has attracted extensive attentions.In comparison with Brønsted acidic catalyst,Lewis acidic catalyst could exhibit a better catalytic per-formance with a higher isobutene selectivity.However,the catalyst stability remains a key problem for the long-running acetone conversion and the reasons for catalyst deactivation are poorly under-stood up to now.Herein,the deactivation mechanism of Lewis acidic Y/Beta catalyst during the acetone to isobutene conversion was investigated by various characterization techniques,including acetone-temperature-programmed surface reaction,gas chromatography-mass spectrometry,in situ ultraviolet-visible,and 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy.A successive aldol condensation and cyclization were observed as the main side-reactions during the acetone conversion at Lewis acidic Y sites.In comparison with the low reaction temperature,a rapid formation and accumulation of the larger cyclic unsaturated alde-hydes/ketones and aromatics could be observed,and which could strongly adsorb on the Lewis acidic sites,and thus cause the catalyst deactivation eventually.After a simple calcination,the coke deposits could be easily removed and the catalytic activity could be well restored.
Deactivation mechanismAcetone to isobuteneLewis acid sitesY/BetaSpectroscopy
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