Solid-state31P NMR mapping of active centers and relevant spatial correlations in solid acid catalysts

Solid acid catalysts are used in a wide variety of industrial catalytic processes for production of chemicals and petrochemicals. This protocol uses(31)P NMR of phosphorous probes to characterize acidity in detail for both Bronsted and Lewis acid sites. Solid acid catalysts are used extensively in various advanced chemical and petrochemical processes. Their catalytic performance (namely, activity, selectivity, and reaction pathway) mostly depends on their acid properties, such as type (Bronsted versus Lewis), location, concentration, and strength, as well as the spatial correlations of their acid sites. Among the diverse methods available for acidity characterization, solid-state nuclear magnetic resonance (SSNMR) techniques have been recognized as the most valuable and reliable tool, especially in conjunction with suitable probe molecules that possess observable nuclei with desirable properties. Taking(31)P probe molecules as an example, both trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO) adsorb preferentially to the acid sites on solid catalysts and thus are capable of providing qualitative and quantitative information for both Bronsted and Lewis acid sites. This protocol describes procedures for (i) the pretreatment of typical solid acid catalysts, (ii) adoption and adsorption of various(31)P probe molecules, (iii) considerations for one- and two-dimensional (1D and 2D, respectively) NMR acquisition, (iv) relevant data analysis and spectral assignment, and (v) methodology for NMR mapping with the assistance of theoretical calculations. Users familiar with SSNMR experiments can complete(31)P-H-1 heteronuclear correlation (HETCOR),P-31-P-31 proton-driven spin diffusion (PDSD), and double-quantum (DQ) homonuclear correlation with this protocol within 2-3 d, depending on the complexity and the accessible acid sites of the solid acid samples.