Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

Abstract

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.

In situsolid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

“…The signal of Al(IV) c appears in the resonance region of the tetrahedral aluminium framework, and experiences a large quadrupolar interaction with the second order quadrupolar effect (SOQE) of 4.0 MHz. This large anisotropic quadrupolar broadening makes it difficult to identify in the usual 27 Al MAS NMR spectra, however, in the isotropic F 1 projection, it is clearly noticeable. At the same time one could note that the intensity of the Al(IV) a signal is decreased readily from the HBeta framework after impregnating Mo species.…”

“…0 ppm (Al(VI) a ) in the octahedral environment is also observed. When HBeta was impregnated with Mo species, the 27 Al MQ MAS spectra of Mo/HBeta reveals an additional species Al(IV) c with an isotropic chemical shift (d iso ) at 58.8 ppm besides the line at 14 ppm assigned to a hydrated species of aluminium molybdate (Al 2 (MoO 4 ) 3 ). The signal of Al(IV) c appears in the resonance region of the tetrahedral aluminium framework, and experiences a large quadrupolar interaction with the second order quadrupolar effect (SOQE) of 4.0 MHz.…”

“…Among the tested catalysts, the optimized 4.0 wt% Mo supported on a composite support of 70% HBeta zeolite and 30% g-alumina (HB-30Al) showed much higher performance for the metathesis of ethene and 2-butene to propene compared with the catalyst without alumina in the support. 128 Quantitative one-dimensional 27 Al MAS NMR measurements indicate that the framework Al content of HBeta zeolites decreases from 592 to 400 mmol g À1 after introduction of Mo species, which means the dealumination of the HBeta framework. However, the concentration of framework Al in HBeta zeolites increases to 486 mmol g À1 with addition of 30% alumina into the support.…”

“…97 Therefore, most Mo species were distributed in the internal zeolitic channels of HMCM-22. Two dimensional 27 Al multiple-quantum (MQ) MAS NMR experiments were also used to further investigate interaction between the Mo species and HMCM-22. 116 Combined DFT calculations and MQ MAS NMR measurements demonstrated that Mo species were directly bonded to the zeolite framework Al species with an isotropic chemical shift of 54.6 ppm and quadrupolar coupling constant of 6.2 MHz, which were responsible for the initial transformation of reactant in MDA.…”

“…This reveals that addition of alumina into the support may protect the HBeta framework from being destroyed. Two-dimensional 27 Al MQ MAS NMR spectra are employed to remove the anisotropic line broadening which allows species with similar isotropic chemical shifts but different quadrupolar coupling constants to be identified. 128 As demonstrated in Fig.…”