Caracterização e avaliação catalítica de zeólitas HZSM-5 com estrutura hierárquica de poros obtidas por síntese direta e por dessilicação

Abstract

ZSM-5 is a microporous zeolite. Due to its unique pore structure and acidity it has been much studied as catalisty at methanol to olefins reactions (MTO) and another biomass platform molecules reactions as ethanol to hydrocarbon and glycols etherification reactions. In catalytic reactions with bulky molecules, the unique presence of micropores ( 5 - 6 Å) in zeolites can limit catalytic performance by restricting molecular transport within its crystals.Thus there has been increasing interest in mesoporous generation in this zeolites as a way to improve accessibility of active acid sites in the structure increasing its use as a catalyst in reactions involving bulk molecules. Given this panorama, the objective of this work was to investigate mesopore generation in ZSM-5 zeolite samples. It was evaluated two routes to synthesize ZSM-5 with a bimodal micro/mesoscopic pore system (i) HZSM-5 desilication by alkali treatment to generation of secondary mesoporosity (ii) directed Synthesis of Hierarchical ZSM-5 using an organosilane as mesopore structure-directing agent. Hhierarchical structure, and acidity of the obtained hierarchical ZSM-5 zeolites were systematically investigated through FRX, DRX, N2, adsorption, NH3-TPD, DRIFTS , 27Al MAS NMR and 29Si MAS NMR. The catalytic performance was evaluated by methanol to hydrocarbons, ethanol to hydrocarbons and glycerol etherification by tert -butanol reactions. Characterization of the materials confirmed highly crystallized MFI zeolites formed without any impurities and well-defined hierarchical structure. Compared with the as-received zeolite (HZNT) alkali-treatment led to a crystallinity reduction, mesoporous generation and a total acidity reduction of the samples gradually as the zeolites were desilicated Directed synthesis obtained samples, presented hierarchical micro/mesoporous structure, with a lower acidity density compared to the other samplesTo methanol to hydrocarbon reaction catalytic performance, desilicated samples presented higher stabillity and higher selectivity to propene and aromatics formationcompared to the precursor. Comparing the performance of the different zeolites it was noticed that the samples were all active for ethanol conversion, with ethylene, being the mayor product. HZNT sample showed more favorable acidic and structural characteristics to the formation of propylene and BTX aromatics. To glycerol etherefication reaction all samples presented a mono-tert-butylglicerol seletivity above 95%. It was not possible to establish activity-stability relationships to samples structural, textural and acirity properties to this reaction.