m-Xylene Isomerization over IWW Zeolite with a Three-Modal Pore Structure: The Effect of Crystal Morphology

Xylene isomerization is a key zeolite-catalyzed petrochemical process for the production of pxylene, a highly demanded intermediate in the polymer industry. While MFI-type zeolites are widely used in industry as shape-selective catalysts, xylene isomerization also serves as a benchmark reaction for evaluating the shape-selectivity of new zeolite catalysts in relation to theirporosity. In this study, IWW zeolite with a three-modal pore network of isolated 8- and 12-ring pores intersected by 10-ring channels was investigated for m-xylene isomerization, with a focus on how both the multidimensional pore system and crystal morphology affect catalytic performance. IWW zeolites were synthesized as germanosilicates with platelet-like and needle-like crystals, functionalized by post-synthetic Ge-to-Al substitution, and tested in gas-phase m-xylene isomerization in comparison with reference zeolite catalysts containing unimodal 8-, 10-, or 12-ring channels. Compared to MFI with a similar concentration of acid sites, Al-IWW catalysts exhibited higher m-xylene conversion across a wide range of WHSV values (4.4 – 40 h-1), regardless of crystal morphology. Needle-like Al-IWW crystals achieved para-selectivity comparable to that of MFI, while platelet-like Al-IWW outperformed MFI in p-xylene yield at short contact times (WHSV = 40 h-1). STEM analysis confirmed that the 10-ring channels are aligned along the length of the needle-like crystals, promoting shape-selective p-xylene formation. In contrast, the 12-ring channels running along the extended dimension of the platelet-like crystals facilitate the diffusion of reactant and product molecules to and from the active sites. All in all, the integration of multi-sized pores and tunable crystal morphology in IWW zeolites may offer a promising strategy for balancing selectivity and activity in p-xylene synthesis.