Acid-base catalytic reaction, either in heterogeneous or homogeneous systems, is one of the most important chemical reactions that has provoked a wide variety of industrial catalytic processes for production of chemicals and petrochemicals over the past few decades. In view of the fact that the catalytic performances (e.g., activity, selectivity, and reaction mechanism) of acid-catalyzed reactions over acidic catalysts are mostly dictated by detailed acidic features, viz. type (Brønsted vs Lewis acidity), amount (concentration), strength, and local environments (location) of acid sites, information on and manipulation of their structure-activity correlation are crucial for optimization of catalytic performances as well as innovative design of novel effective catalysts. This review aims to summarize recent developments on acidity characterization of solid and liquid catalysts by means of experimental P nuclear magnetic resonance (NMR) spectroscopy using phosphorus probe molecules such as trialkylphosphine (TMP) and trialkylphosphine oxides (RPO). In particular, correlations between the observed P chemical shifts (δP) of phosphorus (P)-containing probes and acidic strengths have been established in conjuction with density functional theory (DFT) calculations, rendering practical and reliable acidity scales for Brønsted and Lewis acidities at the atomic level. As illustrated for a variety of different solid and liquid acid systems, such as microporous zeolites, mesoporous molecular sieves, and metal oxides, the P NMR probe approaches were shown to provide important acid features of various catalysts, surpassing most conventional methods such as titration, pH measurement, Hammett acidity function, and some other commonly used physicochemical techniques, such as calorimetry, temperature-programmed desorption of ammonia (NH-TPD), Fourier transformed infrared (FT-IR), and H NMR spectroscopies.

A brief review is presented on acidity characterization of solid acid catalysts by means of solid-state phosphor-31 magic-angle-spinning nuclear magnetic resonance ((31)P MAS NMR) spectroscopy using phosphor-containing molecules as probes. It is emphasized that such a simple approach using (31)P MAS NMR of adsorbed phosphorous probe molecules, namely trimethylphosphine (TMP) and trialkylphosphine oxides (R(3)PO), represents a unique technique in providing detailed qualitative and quantitative features, viz. type, strength, distribution, and concentration of acid sites in solid acid catalysts. In particular, it will be shown that when applied with a proper choice of probe molecules with varied sizes and results obtained from elemental analysis, the amounts and locations (intracrystalline vs. extracrystalline) of different types (Brønsted vs. Lewis) of acid sites may be determined. In addition, by incorporating the NMR results with that obtained from theoretical density functional theory (DFT) calculations, correlations between the (31)P chemical shifts (δ(31)P) and acidic strengths of Brønsted and Lewis acid sites may also be derived, facilitating a suitable acidity scale for solid acid catalysts.

A new methodology is reported for concurrent qualitative and quantitative characterization of internal and external acid sites in zeolitic catalysts. H-ZSM-5 zeolites with varied Si/Al ratios have been examined by solid-state 31 P MAS NMR using different adsorbed probe molecules, namely trimethylphosphine oxides (TMPO) and tributylphosphine oxide (TBPO), in conjunction with elemental analysis. Up to seven distinct 31 P resonance peaks at 86, 75, 67, 63, 53, 43, and 30 ppm were identified from the 31 P NMR spectra of adsorbed TMPO. The resonance peak at 30 ppm has never been observed previously and may be ascribed to mobile TMPO. The peak at 43 ppm is assigned to physisorbed TMPO. The rest of the peaks result from TMPOH + complexes at Brønsted sites with peaks at higher chemical shifts reflecting acid sites of higher strengths. 31 P NMR experiments, performed with TMPO and TBPO adsorbed on a mesoporous MCM-41 sample, respectively, provide further correlation of the internal and external acid sites. It is concluded that the peaks at 75 and 53 ppm arise exclusively from the internal Brønsted sites, whereas the peaks at 86, 67, and 63 ppm are associated with both internal and external acid sites. While the concentration and distribution of internal sites were found to increase with acid strengths, changing the Si/Al ratio of HZSM-5 has nearly no effect on the strength of the external acid sites.