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借助ESCA技术,研究了Sb_2O_3对HZSM-5型沸石表面性质的影响。发现在焙烧过程中,在表面区间有一部分Al_2O_3形成了离析相。Si元素也呈现多种状态,Si峰的最大分裂达到4eV,接近于元素Si与SiO_2之间所曾见的分裂,与沸石吸附了强还原性气体吡啶(在150℃)的结果也相仿。鉴于主体检测技术(如X射线衍射方法等)从未观察到SiO_2还原为Si°,显然这种情况只可能发生于沸石表面层。很可能在焙烧过程中,Sb_2O_3作用于HZSM-5沸石,并改变了沸石的表面结构与表面化学性质(例如,表面酸性),从而强烈地变更了某种工作条件下焙烧产物(即样品)的催化性质,并使得在该催化剂上烃类能够高选择性地转变为对二甲苯。我们认为在HZSM-5型沸石微晶表面进行的烃类转化过程,其选择性主要由整个反应体系(即反应物、产物与催化剂)界面的化学特性所决定,而不只是由“分子筛效应”的几何因素所决定。我们可能通过某些改性剂适当地变更沸石的表面化学性质,以获得较为优异的选择性能。
With ESCA technology, the effect of Sb 2 O 3 on the surface properties of HZSM-5 zeolite was studied. It was found that during the calcination process, a part of Al 2 O 3 formed an isolated phase in the surface area. Si also showed a variety of state, the maximum peak Si split 4eV, close to the elemental Si and SiO2 between the splitting seen, and zeolite adsorption strong reductive gas pyridine (at 150 ℃) the results are similar. In view of the fact that no reduction of SiO_2 to Si ° has been observed by bulk detection techniques (such as X-ray diffraction methods), it is clear that this may only occur with zeolite surface layers. It is likely that Sb 2 O 3 acts on the HZSM-5 zeolite during firing and alters the surface structure and surface chemistry (eg, surface acidity) of the zeolite, strongly altering the roasting products (ie, samples) under certain operating conditions Catalytic properties, and enables hydrocarbons to be converted with high selectivity to para-xylene over this catalyst. In our opinion, the selectivity of hydrocarbon conversion process on the surface of HZSM-5 zeolite microcrystals is mainly determined by the chemical properties at the interface of the whole reaction system (ie reactant, product and catalyst), not just by the “molecular sieve effect” The geometry of the decision. We may change the surface chemical properties of the zeolite by some modifiers, so as to obtain more excellent selectivity.