Solid-state n.m.r. and catalytic studies of mildly hydrothermally dealuminated HZSM-5

Observation of a compensation relation for monomolecular alkane cracking by zeolites: the dominant role of reactant sorption

“…With increasing severity of steaming, the apparent rate constant decreases in MFI, indicating that there is an optimum in the activity with steaming. These results are consistent with previous studies on mild steam dealumination of MFI and MOR zeolites [10][11][12][13][14][15][16][17]22,30].…”

“…In addition to the number of acid sites, it is thought that the strength of the Brønsted acid sites is affected by the acid site density, i.e., the Si/Al ratio [7], the T-O-T bond angle [8,9], the presence of nonframework aluminum [2,3,8,[10][11][12][13][14][15][16][17][18], or charge-balancing cations [18,19]. Theoretical and experimental evidence suggests that the acid strength per aluminum is a maximum when the framework aluminum ions have no second nearest neighbor aluminum [20,21].…”

Characterization of Zeolites — Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction

“…In the spectra of dealuminated zeolites HY (Fig. 4.43 d) [310], H-mordenite [311] and HZSM-5 [317,318], signals of hydroxyl protons at non-framework aluminum species appear at chemical shifts in the range between 2.6 and 3.6 ppm. The comparison of these resonance positions with those of AIOH groups on the outer surface of aluminum oxides [316] indicates a low-field shift of the resonance position by about 3 ppm.…”

“…Recent NMR studies on dehydrated zeolites HZSM-5 and Hbeta [313,317,[312][313][314][315][316][317][318][319][320][321][322][323][324] were determined by decomposition of the IH MAS NMR spectra depicted in Fig. 4.45.…”

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

“…This enhanced activity was attributed to superacidic Brønsted sites formed by a synergism of Lewis acid sites in the vicinity of bridging OH groups. According to 1 H and 27 Al MAS NMR spectroscopic studies of hydrothermally dealuminated zeolites H-ZSM-5 [171,172], the maximum of the n-hexane cracking rate occurs for zeolites H-ZSM-5 with an optimum number of bridging OH groups (Si(OH)Al) and immobilized extra-framework aluminum species as obtained after steaming at 810 K for 150 min with a water vapor pressure of 7 kPa [171,172]. An effect of paired aluminum atoms in the H-ZSM-5 framework on the enhanced n-hexane cracking rate could be excluded [171,172].…”