Hierarchical Micromesoporous Zeolites with Reduced External Brønsted Acidity for Long-Term Naphthalene Alkylation

Continuous alkylation of naphthalene with long-chain olefins over conventional microporous zeolites is often limited by diffusion constraints and rapid deactivation caused by the accumulation of heavy byproducts. Herein, a series of hierarchical zeolitic catalysts was synthesized by introducing hexamethylenimine (HMI) into the precursor gel of Y zeolite, thereby inducing a progressive evolution from FAU-rich to MWW-rich materials. Increasing the HMI content simultaneously increased mesopore volume and suppressed Brønsted acidity externally while preserving abundant Brønsted acid sites relevant to alkylation. In the alkylation of naphthalene with 1-tetradecene, the catalyst with the highest HMI content (MY-5) showed the best stability, retaining 66% naphthalene conversion after 9 h at WHSV = 12 h –1 and 85% conversion after 4000 h in continuous fixed-bed operation at WHSV = 0.2 h –1 . Py-IR, 2,6-DTBPy-IR, and textural characterization indicate that the improved stability is associated with the coupled effects of enhanced mesopore accessibility and reduced externally accessible Brønsted acidity. Thermogravimetric analysis combined with time-on-stream normalization showed that MY-5 exhibited a lower apparent coke accumulation rate than other samples. Periodic DFT calculations on representative FAU- and MWW-type models suggest that both naphthalene and 1-tetradecene are more strongly stabilized within the confined internal pore environment of the MWW-type framework, whereas the FAU-type external surface exhibits stronger adsorption of both reactants, which may favor nonconfined side reactions and coke-precursor formation. These results identify mesopore accessibility and suppression of externally accessible Brønsted acidity as key descriptors for improving long-cycle stability in fixed-bed naphthalene alkylation.