Activation Conditions Play a Key Role in the Activity of Zeolite CaY:  NMR and Product Studies of Brønsted Acidity

Abstract

CaY, activated under different conditions, was characterized with 1H, 31P, and 1H/27Al double resonance MAS NMR. The 1H MAS NMR spectra of CaY, calcined in an oven at 500 °C, shows resonances from H2O (bound to Ca2+ and the zeolite framework), CaOH+, aluminum hydroxides, silanols, and Brønsted acid sites. No evidence for Lewis acidity is observed on adsorption of trimethylphosphine, and an estimate of ≈16 Brønsted acid sites per unit cell is obtained for this sample. CaY activated in an oven at higher temperatures contains less water, but all the other species are still present. In contrast, CaY activated by slow ramping of the temperature under vacuum to 500 or 600 °C shows a much lower concentration of Brønsted acid sites (<1/unit cell). Again, no evidence for Lewis acidity was observed. These NMR results have been utilized to understand the very different product distributions that are observed for reactions of 1,1- and 1,2-diarylethylenes in zeolite CaY activated in an oven (in air) and under vacuum. Samples with high concentrations of Brønsted acid sites react stoichiometrically with these sites, yielding diarylalkanes. At low concentrations, the Brønsted acid sites can act catalytically resulting in isomerization reactions.

Silver Phosphanes Partnered with Carborane Monoanions: Synthesis, Structures and Use as Highly Active Lewis Acid Catalysts in a Hetero-Diels–Alder Reaction

“…[56] It is also similar to the role metal-coordinated water is suggested to play in certain catalytic processes. [57] In support of this, preliminary DFT calculations on the model system [Ag(Me 3 P)(OH 2 )(MeNH CHMe)] indicate that a coordinated water molecule plays an important function in the silver promoted [24] addition of imine to diene. A putative intermediate in this process is shown in Scheme 4, in which the water molecule forms a hydrogen bond with the nitrogen atom.…”

The acidic nature of “NMR-invisible” tri-coordinated framework aluminum species in zeolites

“…Furthermore, the LAS is generally recognized as an extra-framework or framework aluminum species, formed upon calcination or steaming of zeolites. 8,9 The properties of BAS associated with the tetrahedral framework Al and LAS associated with various extra-framework Al (EFAL) species have been extensively studied, from their structural characteristics [10][11][12][13][14] to their functions in catalytic reactions. [15][16][17][18] In particular, the LAS in zeolites has been proved to play an important role in the catalytic transformations of hydrocarbons such as the hydrogen transfer process, 7,19,20 and also generates the Brønsted/Lewis acid synergy owing to the vicinity of the corresponding Al species in the local architecture of dealuminated zeolites.…”

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“…These results, however, implicate a polarised, silver-bound water molecule in the catalytic process, with the resulting Lewis-assisted Brønsted-acid similar to that previously reported in lanthanide catalysed aromatic electrophilic substitutions 6 and the catalytic role of coordinated water in certain zeolites. 7 Consonant with this idea, when the reaction was repeated in the presence of the hindered base 2,6-di-tertbutyl-4-methylpyridine no product was formed, while a control experiment taking I and II with just 50 mol% water also resulted in no product formation. On the bench, it is probably adventitious water that is made available to the reaction.…”