Effects of Binders on the Performance of Shaped Hierarchical MFI Zeolites in Methanol-to-Hydrocarbons

Abstract

The scale-up of zeolite catalysts from powder to industrially relevant shapes is widely neglected in fundamental research because of the added preparative and analytical complexity. Binders incorporated to improve mechanical stability and related structuring steps can cause decisive performance alterations, which are currently difficult to predict. Here, by characterizing physical, extruded, and milled admixtures of MFI zeolites with common silica, alumina, or clay binders, we elucidate the cause and magnitude of property variations induced by shaping. Subsequent evaluation in the conversion of methanol to hydrocarbons confirms the corresponding catalytic impacts. Our findings show that binder effects can, without optimization, match or even exceed those of hierarchically structuring the porosity of the MFI crystals. The macroporosity and related mass transfer properties of the technical bodies are enhanced on extrusion with attapulgite and kaolin as a result of their larger particle size. Comparatively, the acid site density and speciation is binderdependent. Although a decreased Brønsted acidity, due to the partial dealumination or ion exchange of the zeolite framework, reduces the intrinsic activity of the catalysts, no direct correlation is observed with the selectivity or catalyst lifetime. The unique role of attapulgite in promoting the longevity and light-olefin selectivity of the zeolite is correlated with the reversible neutralization of the framework by mobile Mg species. Ball-milling proves a complementary tool to rapidly screen for potential reactions between component phases with small sample quantities. An improved understanding of the complex morphological and chemical interactions within zeolite−binder composites and of how they can be effectively tuned will ultimately accelerate the development of superior catalytic technologies.

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

“…For the data shown in Figure 7 and the characteristic results in section 2.1, extrusion with attapulgite provides a promising strategy to obtain a shaped catalyst with higher para-selectivity and prolonged catalyst lifetime. As has been shown by other authors before, Brønsted acid sites in zeolite extrudates could be partially neutralized due to the solid state ion-exchange with mobile alkaline species in binders, such as sodium in montmorillonite [32] or magnesium in attapulgite [29]. Revealed in SEM image, the attapulgite binder wrapped around the HZSM-5 zeolite particles, showing an intimate zeolite-binder interaction.…”

“…Compared with Z5-Bo, Z5-atp exhibited a lower total acidity and weaker acid strength. Since no evidence of pore blocking was detected in N2 physical adsorption, there may be a solid-state ion exchange of some of the mobile species in attapulgite (e.g., MgO) [29,32]. …”

“…The prolonged lifetime of Z5-atp could be ascribed to the less extraframework aluminum introduced in extrusion. 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].…”

“…Attapulgite is mainly used as sorbent in the removal of metal ions [24,25], catalyst support for various reactions [26], polymer additive for mechanical improvement [27] and synthesis additive for new materials [22,28]. The abundant hydroxyl groups on attapulgite's surface makes it a good binder as well [29]. These hydroxyl groups are dehydrated and cross-linked between adjacent binder particles after calcination, thereby increasing mechanical strength of the shaped catalyst [19].…”

Methane dehydroaromatisation and methanol activation over zeolite catalysts: an overview

“…Whilst to date there has only been limited attention to the effect of binders upon zeolite catalysed reactions in the academic literature, there is greater realisation of the often substantial effects that they can have [30][31][32]. Far from being inert, they can have very significant effects upon coking by, for example, trapping coke precursors.…”