Alkaline Modification of MCM-22 to a 3D Interconnected Pore System and its Application in Toluene Disproportionation and Alkylation

Abstract

Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This 'desilication' process led for mild alkaline concentrations (*0.10-0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3 -acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Brønsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Brønsted acid site concentration. Decomposition and deconvolution of 29 Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q 3 SiOH groups were formed at the expense of the Q 4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite.

Interplay of Properties and Functions upon Introduction of Mesoporosity in ITQ‐4 Zeolite

“…[10] However, presently, no direct relationship between extraction of specific silicon sites and the resulting mesoporosity can be elucidated. Van Miltenburg et al [12] have also demonstrated the preferential extraction of silicon atoms (T6, T7) in Al-containing MCM-22 upon NaOH treatment. The 29 Si MAS NMR spectra in Figure 3a additionally show the relative increase of the defect groups (À102 ppm) and the aluminumcontaining groups (À104.5 ppm).…”

“…[4][5][6][7][8][9] Numerous works have proven the positive influence of the introduction of the secondary porosity on the catalytic performance, which is commonly attributed to the shortened diffusion pathlength resulting from the newly introduced mesoporosity. [6] However, aside from the introduced secondary (meso)porosity, it is possible that other changes in the properties of the hierarchical zeolite, e.g., chemical composition, [10] acid site distribution, [11,12] and surface effects, [13,14] are partially responsible for the altered catalytic behavior as well.…”

“…This postsynthesis treatment has proven to be capable of introducing a controllable amount of interconnected mesopores into various zeolite frameworks (MFI, MTW, MOR, BEA, AST, FER, MWW). [10,12,[22][23][24][25][26] Insights into the altered properties upon the generation of mesoporosity are obtained and discussed considering thorough characterization.…”

Full Compositional Flexibility in the Preparation of Mesoporous MFI Zeolites by Desilication

“…Previous studies varying alkalinity have primarily focused on individual zeolites of Si/Al ratios within or near the preferred range , adjusting according to either framework structure or morphology. 7,12,19,[33][34][35][36] Herein, we provide further insight into the mechanism of mesopore formation in basic media enabling full compositional flexibility in the preparation of hierarchical zeolites. Exploring the entire range of Si/Al ratios in MFI (10-1000), we probe the influence of base concentration on the properties of the treated zeolites.…”

The Influence of Si/Al Ratio on the Distribution of OH Groups in Zeolites with MWW Topology

“…Although both hexagonal and orthorhombic forms may be proposed for the MCM-22 zeolite [15,16], the NMR data determined on highly-siliceous zeolites favor the orthorhombic one with 13 non-equivalent T-sites, one of them being 'buried' within the structure, not accessible from either channel systems [17]. The sitting of aluminum, both in the parent samples [18], and modified by dealumination or desilication [19] has also been investigated. , in contrast to majority of other zeolites, contain hydroxyl groups located at the external surface of the zeolite crystals, namely inside the 12-ring pockets ('cups').…”