The curious case of zeolite–clay/binder interactions and their consequences for catalyst preparation

Abstract

Zeolite-based catalyst bodies are commonly employed in a range of important industrial processes. Depending on the binder and shaping method chosen, vast differences in the reactivity, selectivity and stability are obtained. Here, three highly complementary micro-spectroscopic techniques were employed to study zeolite ZSM-5-binder interactions in SiO2-, Al2O3-, SiO2 : Al2O3- (2 : 1 mix) and kaolinite-bound catalyst pellets. We establish how their preparation influences the zeolite-clay/binder interactions. Using thiophene as an acid-catalyzed staining reaction, light absorbing oligomers produced in each sample were followed. To our surprise, kaolinite decreased the overall reactivity of the sample due to the phase change of the binder, creating a hard impenetrable outer layer. Aluminum migration to the zeolite was observed when Al2O3 was selected as a binder, creating additional Brønsted acid sites, which favored the formation of ring-opened thiophene oligomers compared to the larger oligomer species produced when SiO2 was used as a binder. In the latter case, the interaction of the Si-OH groups in the binder with thiophene was revealed to have a large impact in creating such large oligomer species. Furthermore, the combination of a SiO2 : Al2O3 mix as a binder enhanced the reactivity, possibly due to the creation of additional Brønsted acid sites between the two binder components during pellet preparation. It is evident that, independent of the shaping method, the intimate contact between the zeolite and binder heavily impacts the reactivity and product selectivity, with the type of binder playing a vital role.

Assessing Inter and Intra‐particle Heterogeneity in Alumina‐poor H‐ZSM‐5 Zeolites

“…During this process it is assumed that the zeolite crystals themselves are homogeneous in nature, that is, only small variations in properties between the zeolite crystals in a batch exist. However, if this assumption is wrong and large variations in, for example, acid site density are present, parts of the shaped catalyst could become more active than others, resulting in undesired coke formation and thus a loss in catalyst activity and lifetime …”

A Facile Strategy to Prepare Shaped ZSM-5 Catalyst with Enhanced para-Xylene Selectivity and Stability for Toluene Methylation

“…Calculated from the saturation adsorption amounts, the adsorption ratio for Z5-bo increased from 1.8 to 1.9 after regeneration, whereas the adsorption ratio for Z5-atp decreased from 1.9 to 1.7 after regeneration. It means that the initial para-selectivity increment for regenerated Z5-bo was not only due to the reduced acidity but partially due to the more restricted pore opening which might be caused by the aluminium species migration in alumina-bound ZSM-5 extrudates [21,31]. In the Z5-atp case, the extent of pore opening reduction decreased after regeneration, which indicated that the continuously decreased acidity of Z5-atp was the main reason for para-selectivity increment.…”

“…Prior studies have reported that extra-framework aluminum species catalyze the formation of coke (and methane) in methanol to olefins (MTO) conversion [17,29]. Moreover, aluminum in alumina-bound extrudates can migrate to the zeolite surface, forming additional Brønsted acid sites [21,31]. These extra Brønsted acid sites facilitate coking, resulting a higher coke formation rate.…”

“…Until recently, the need for a better understanding of shaping process is highlighted in the vibrant area of zeolite catalysis [16][17][18]. The zeolite-binder interactions result in multiple effects on catalyst activity, stability and product selectivity, with the type of binder playing a pivotal role [17,[19][20][21].…”

Concluding remarks: progress toward the design of solid catalysts

“…Imaging methods, some of them quite novel in catalysis research, 7,8 were also used widely in the work reported at the Faraday Discussion, and these included characterization of zeolitecontaining cracking catalysts investigated on the scale of microns. Important components of industrial catalysts for hydrocarbon cracking in the manufacture of petrol incorporate binders and amorphous components in addition to zeolite crystallites, and results presented at the Faraday Discussion demonstrate their high degrees heterogeneity-on the scale of micronsperhaps a surprising result in view of the enormous scale of application of cracking catalysts.…”