2 1 I H I G H L I G H T S 2 0 2 1
Figure 8a shows that the shaped catalyst consists of doughnut-like spheres with diameters from 50 to 200 microns. The adsorption isotherm for the powdered zeolite (Figure 8b) is typical for a microporous material, whereas the isotherm for the isolated alumina binder has low micropore volume and fairly large mesopores. The shaped catalyst displays a combined behaviour. Figure 8c also shows that the catalyst lifetime is improved by the incorporation of Zn by either impregnation (ZB-1Zn has 1 wt% Zn by impregnation, etc.; ZB-3IE has 1 wt% Zn by three cycles of ion exchange). The C4 Hydrogen Transfer Index (C4-HTI) conveys mechanistic information. A high index suggests that the aromatics are formed by hydrogen transfer leading to paraffin formation, whereas a lower value implies that aromatic formation is associated with the formation of molecular hydrogen. Clearly, Zn shifts the reaction mechanism towards H2 evolution (Figure 8d).
Zn K-edge XAS experiments were performed at BM23 in transmission mode on self-supporting pellets. All the samples were characterised in their as-prepared state, at RT in air. Moreover, the two catalysts with Zn ~1% wt were further activated at 400°C in vacuum first and
later in oxygen inside a devoted glass cell equipped with Kapton windows. Once activated, the samples were cooled under vacuum, and measured at RT, inside the same glass cell. Figure 9 shows the linear combination fit (LCF) analysis of the XANES spectra collected for as-prepared and activated ZB-type catalysts based on three reference XANES spectra. These represent hydrated, mobile Zn(II) species (Hydr. Zn); dehydrated, framework-coordinated Zn ions (Z-3IE act); and Zn in the binder material (B-1Zn and B-10Zn).
The XANES LCF analysis shown in Figure 9 makes it possible to quantify the fraction of each Zn species in the different materials before and after activation. In the activated catalysts with 1 wt% Zn loading, LCF analysis reveals a quite similar Zn-speciation for the two samples: ca. 40% of total Zn is found to occur as binder-related ZnAl2O4, while zeolite-coordinated Zn(II) species account for the remaining ca. 60%. The activation procedure enhances the fractional contribution of Zn in the zeolite with respect to the as-prepared state, at the expense of binder-related Zn-phases. It thus appears plausible that migration of Zn species from the binder to the zeolite occurs upon activation.
Fig. 8: a) SEM image of the spray-dried catalyst (ZB-parent). b) N2 adsorption/desorption isotherms for the isolated zeolite, for the binder itself, and for the spray-dried catalyst. c) Catalyst deactivation profiles for the shaped Zn containing catalysts during MTH (WHSV = 5 h-1, Treaction = 400°C, methanol partial pressure 1 bar, and total pressure 20 bar). d) The C4 hydrogen transfer index, defined as the ratio of paraffinic C4 to total C4.