Influence of the calcination treatment on the catalytic properties of hierarchical zsm-5
The effect on the calcination conditions on the properties and catalytic performance of hierarchical and standard ZSM-5 zeolite samples have been investigated using different reactions as catalytic tests: acylation of aromatic substrates (anisole and 2- metoxynaphtalene) and rearrangement of linear and cyclic epoxides (1,2-epoxyoctane and isophorone oxide). The h-ZSM-5 material was prepared by crystallization of silanized protozeolitic units. The removal of the organics present in the as-synthesized forms of both ZSM-5 samples has been carried out using two different calcination procedures. The first one takes place under air atmosphere from room temperature to 550ºC for 5 hours, whereas the second one is performed under nitrogen at 400ºC for 4 hours, followed by treatment at 550ºC for 5 hours under air atmosphere. Clear differences are noticed related to the textural properties of the ZSM-5 samples. Thus, h- ZSM-5 presents not only higher BET surface than standard ZSM-5 zeolite, but 75% of its total area is shared by surface located outside the zeolite micropores. FTIR studies showed almost no differences between the acidic properties of the conventional zeolite calcined by both methods. However, noticeable changes were observed over the hierarchical zeolite. Thus, the concentration of Brønsted acid sites decreases by half in the case of one-step air calcination, but only a quarter when using two-step nitrogen/air calcination. These changes in the acidic properties denote that the aluminium present in hierarchical ZSM-5 is very sensitive to the calcination conditions. Furthermore, this has also significant effects on the catalytic properties of this material in both aromatic acylation and epoxide rearrangement reactions. For both types of reactions, the hierarchical sample calcined under a two-step nitrogen/air treatment presents higher activity and product yields than the same material when calcined under one-step air atmosphere. Therefore, controlling the calcination conditions is an effective method in preserving the acidic features of hierarchical ZSM-5, which in turn has a positive influence on its catalytic behaviour.
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