Surface Fingerprinting of Faceted Metal Oxides and Porous Zeolite Catalysts by Probe-Assisted Solid-State NMR Approaches

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Acid catalysis in heterogeneous systems such as metal oxides and porous zeolites has been widely involved in various catalytic processes for chemical and petrochemical industries. In acid-catalyzed reactions, the performance (e.g., activity and selectivity) is closely associated with the acidic features of the catalysts, viz., type (Lewis vs Brønsted acidity), distribution (external vs internal surface), strength (strong vs weak), concentration (amount), and spatial interactions of acidic sites. The characterization of local structure and acidic properties of these active sites has important implications for understanding the reaction mechanism and the practical catalytic applications of acidic catalysts. Among diverse acidity characterization approaches, the solid-state nuclear magnetic resonance (SSNMR) technique with suitable probe molecules has been recognized as a reliable and versatile tool. Such a probe-assisted SSNMR approach could provide qualitative (type, distribution, and spatial interactions) and quantitative (strength and concentration) information on each acidic site. This Account aims to integrate our recent important findings in determining the structures and acidic characteristics of some typical metal oxide and zeolite catalysts by using the probeassisted SSNMR technique, as well as clarifying the continuously evolving process of each discrete acidic site under hydrothermal or chemical treatments even at the molecular level with multiscale theoretical simulations. More specifically, we will describe herein the development and applications of the probe-assisted SSNMR methods, such as trimethylphosphine (TMP) and acetonitrile-d 3 (CD 3 CN) in conjunction with advanced two-dimensional (2D) homo-and heteronuclear correlation spectroscopy, for characterizing the structures and properties of acidic sites in varied solid catalysts. Moreover, relevant information regarding the surface fingerprinting of various facets on crystalline metal oxide nanoparticles and active centers inside porous zeolites, the mapping of relevant spatial interactions, and the verification of structure−activity correlation were investigated as well. Relevant discussions are mainly based on the recent NMR experiments of our collaborating research groups, including (i) determining the acidic characterization with probe-assisted SSNMR approaches, (ii) mapping various active centers (or crystalline facets), and (iii) revealing their influence on catalytic performance of solid acid catalyst systems. It is anticipated that this information may provide more in-depth insights toward our fundamental understanding of solid acid catalysis.

Mechanistic insights into positional and skeletal isomerization of cyclohexene in the H-BEA zeolite

“…For the second pathway, an adjacent carbon (C3, see Scheme 1a) loses one proton to the framework oxygen and forms a new double C2QC3 bond simultaneously. The combination of the last pathway with the protonation of the C1QC2 double bond is the positional isomerization of the double CQC bond, 12 which is confirmed by the H/D isotope tracer study in conjunction with 1 H NMR experiments 13,14 in the dehydration process of cyclohexanol in various zeolite pores. 15 Due to the lightest mass, hydrogen transfer is more feasible to achieve, which plays a significant role in the activation of reactants, transformation, and/or stabilization of intermediates toward products.…”

Quantifying the Influence of Water on the Mobility of Aluminum Species and Their Effects on Alkane Cracking in Zeolites

“…Since the loss of BAS represented by the peak at 4.2 ppm is compensated by the generation of AlOH species not classified as traditional framework BAS species, both steaming samples show increases in the peak corresponding to non-BAS AlOH species located at 2.8 and 12–15 ppm in comparison with the fresh catalysts. The signal at 2.8 ppm is commonly attributed to AlOH species, with the corresponding 12–15 ppm peak attributed to framework/extra-framework interactions in both HZSM5 and HFER catalysts . Recently, the appearance of these two signals in the 1 H NMR spectra of HZSM5 catalysts has also been attributed to Al–OH species that are best described as partially coordinated framework species. ,,, However, these partially coordinated species can also be precursors to the generation of EFAl species after exposure to more severe conditions, which may then interact synergistically with existing framework sites through a variety of previously proposed mechanisms. , For the purpose of this analysis, the presence of these features is solely meant to indicate a qualitative increased presence of Al species that are not traditional framework sites upon high-temperature water treatment, a subset of which correlates to enhancements in activity for hexane cracking at elevated temperatures.…”

Surface acidity of tin dioxide nanomaterials revealed with 31P solid-state NMR spectroscopy and DFT calculations

“…4b ). Since a stronger adsorption for TMP gives rise to a more positive 31 P chemical shift, 16 it is reasonable to attribute the resonances at around −6, −14, −23 and −29 ppm to TMP adsorbed on the tin cations on (001), (101), (110), and (100) surfaces, respectively. It has also been shown that 31 P chemical shift exhibits a linear correlation with the adsorption energies.…”