Impact of in situ MAS NMR techniques to the understanding of the mechanisms of zeolite catalyzed reactions

Abstract

The in situ MAS NMR studies of the mechanisms of zeolite catalyzed reactions are reviewed. The first part of the critical review contains brief information on the different experimental approaches used for the in situ MAS NMR studies under batch and flow conditions. In the second part, a cross reference index between the reactions studied, the catalysts used, the mechanistic information obtained and the corresponding literature sources is established. Finally, in the last part the most widely studied areas are discussed in more detail. In particular, the impact of in situ MAS NMR to unravel the mechanisms of olefin, alcohol and alkanes transformations over zeolite catalysts is analyzed (232 references).

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

“…This is the case of one PBS-Al which is not able to protonate lutidine, Si(Z 2 )AOHÁ Á ÁAl V (III 2 ). If the lifetime of the tert-butyl carbenium species is already questioned in the case of zeolites [51,67], it can be anticipated that the lifetime of such species on ASA will be even shorter. Whereas carbenium chemistry is often supposed to be at the core of hydrocarbon conversion for aluminosilicates, our present results suggest that it would hardly be the case on ASA.…”

“…Highly resonant species mainly could be observed in zeolites [45,47,48], even if indirect evidence was provided by 13 C NMR for the transient formation of short-live carbenium ions in the case of long-chain or branched olefins [49][50][51]. Alternative proposals to carbenium b-scission can thus be recorded.…”

“…Moreover, alkoxides (Fig. 3) were characterized in zeolites thanks to 13 C NMR [51,54,55] and FTIR [56,57] and were calculated as stable species by earlier quantum chemical calculations [58][59][60][61][62][63][64], which led some authors to refute the existence of carbenium ions as cracking intermediates in zeolites, even for tertiary carbenium. The intermediate or transition state nature of carbenium in zeolites as calculated by quantum chemistry, however, appears to be strongly dependant on the alkene considered [65], but also on the level of theory (DFT, with or without dispersion corrections, versus post-Hartree-Fock methods for example) [66,67], on the kind of model -small clusters versus periodic cells -and on the local geometry of the active site [65,[68][69][70][71][72][73][74][75].…”

Rotor design for high pressure magic angle spinning nuclear magnetic resonance

“…This technique enables collection of high resolution spectra of these types of samples because the line broadening that arises from various internal and external nuclear interactions is averaged away. The ability to make in situ MAS NMR measurements on a system under high pressure is important across fields such as biophysics/protein science [6], materials [7][8][9], catalysis [10,11], geochemistry [12], food preservation and processing [13], and medicine [14]. However, because of the unusual technical challenges intrinsic to the development of a reusable high-pressure cell (e.g., rotor cannot contain bulk metal), there are only a few publications where MAS NMR spectra are reported at pressures of up to 70 bars [15,16].…”

In situ magnetic resonance imaging study of the impregnation of γ-alumina pellets

“…NMR spectroscopy, especially under the conditions of magic angle spinning, has been extensively used to study catalysts and catalysis [25][26][27], offering solutions for in situ observation in batchlike [28] and flow [29] conditions. Magnetic resonance 1 H imaging ( 1 H MRI) is becoming an increasingly popular technique, allowing for 2-and 3-dimensional imaging of the liquid components within the catalyst in order to not only identify the molecular species but also to map out their positions in the solid support.…”