New insight into the alkylation-efficiency of methanol with toluene over ZSM-5: Microporous diffusibility significantly affects reacting-pathways

Here we show a new insight into the methanol alkylation-efficiency in toluene alkylation. Concretely, nanosheet (SH-), nano-particle (N-), alkali treated (AT-), conventional (C-) HZSM-5 zeolites with similar acidic properties, silica-modified C-HZSM-5 and HSAPO-34 were respectively tested in this catalytic reaction, at an enough high linear velocity of fluid in reactor with eliminating the influence of external diffusion. Combining with the characterizations such as XRD, BET, NH3-TPD, Py-IR, SEM, EDX, probe reactions etc., the effect of ZSM-5 microporous diffusibility on methanol reacting-pathways was studied from the two aspects of (i) the diffusion constraint at pore-openings and (ii) the effective length of diffusion-reaction path in intracrystal micropore. It was found that the excellent diffusibility (SH-, N- or AT-HZSM-5) can effectively improve the methanol alkylation with aromatics to form desired products. Significantly, up to 96.7% of alkylation-efficiency and 16.4% of toluene conversion were obtained over the similar to 10 nm thickness nanosheet HZSM-5 (normal pore-opening diffusion-constraint and shortest diffusion-reaction path) at the 6.0 molar ratio of toluene/methanol. On the contrary, the enhanced pore-opening diffusion-constraint (4Si-C-HZSM-5) or the overlong diffusion-reaction path (C-HZSM-5) resulted in the much lower alkylation-efficiency (38.2% and 67.4% respectively), owing to the enhanced methanol-to-olefins side-reaction with dual-cycle process which converted much more methanol into light aliphatic-hydrocarbons but not the alkyl of aromatics. Our work may provide a theoretical guidance for improving the shape-selective alkylation-efficiency for this reaction process.