Characterization of MFI and BEA embedded in mesoporous molecular sieve – Thermal stability

MFI and BEA zeolite seeds were embedded in MCM-41 gel mixture with or without additional aluminum isopropoxide. These novel meso–microporous molecular sieves, NK-MM-MFI and NK-MM-BEA, contain zeolite crystallites surrounded by a mesoporous ordered phase. A thorough characterization was made utilizing XRF, XRD, N2-physisorption and 27Al MAS NMR as well as acidity and acid site strength measurements by 1H MAS NMR and NH3-TPD. Al content was varied from 0.4 to 3.9 wt.% to tailor acidity and porosity. The embedded NK-MM-MFI and NK-MM-BEA materials were compared to pure zeolites and MCM-41. The mesoporous phase had a stronger effect on porosity and number of acid sites, while acid site strength was dominated by the microporous zeolite seeds. Total number of acid sites increased linearly with Al content, which also was true for Brønsted acidity in NK-MM-MFI, while no effect on Brønsted acidity was observed in NK-MM-BEA. The thermal stability of NK-MM-MFI and NK-MM-BEA, as well as of MFI, BEA and MCM-41, was studied by calcination in air at 773, 873 and 1173 K. The exposure to 1173 K caused a decrease in unit cell dimension of the MCM-41 phase. The unit cell shrinkage was the largest in pure MCM-41 and the smallest in NK-MM-MFI. The zeolite addition stabilized the mesoporous structure, effect of MFI was more pronounced than of BEA. Total acidity and Brønsted acidity decreased, while only minor loss of tetrahedral framework Al was observed. The loss in acidity was caused by thermal dehydroxylation and being thus reversible. It was shown that the structure, porosity and framework Al were thermally stable in NK-MM-MFI and NK-MM-BEA.

Zeolite embedding and Al addition to MCM-41 gel formed a grain boundary layer with secondary mesopores, pore diameter 3.5–4.5 nm was detected by N2 physisorption. The grain boundary layer surrounding the zeolite allows intimate interaction between OH-groups in the zeolite and Al in the grain boundary. Proton mobility from strong acid sites in MFI induces formation of Brønsted acidity, detected by 1H MAS NMR, while the weaker sites in BEA do not.Figure optionsDownload as PowerPoint slideHighlights
► Secondary mesoporous domains formed upon zeolite embedding in MCM-41.
► Pore diameter 3.5–4.5 nm, enriched with Al.
► Induce new Brønsted acid sites with MFI, not with BEA.
► Embedding, mutual reinforcement of zeolite and MCM-41.