Acid properties of solid acid catalysts characterized by solid-state 31P NMR of adsorbed phosphorous probe molecules

Abstract

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.

Acid–base characterization of heterogeneous catalysts: an up-to-date overview

“…A correlation between 31 P NMR chemical shifts of adsorbed probe phosphine molecules R 3 PO phosphine, with R = C n H 2n?1 , n = 1 TMPO, n = 2 TEPO, 3, BTPO, 4 T0PO, and Brønsted/Lewis acid strengths has been proposed by Zheng et al [87][88][89]. A linear correlation was found with chemical shifts d 31 P and the proton affinity analyses in Fig.…”

“…MAS-NMR technique has been used extensively to characterize acid-base properties of solid catalysts via 1 H [86], 2 D, 13 C, 15 N, 17 O, 27 Al, 29 Si, 31 P [87][88][89] nuclei. Many applications of NMR spectroscopy have been reported in literature for the determination of the types of hydroxyls via isotopic chemical shifts of 1 H (see Table 7) or H/D exchange reaction, of their concentration, accessibility, and mobility, and for the characterization of their acid strength and local structure.…”

Acid Properties of Protonated Titanate Nanotubes

“…3). Therefore, the Brønsted acidity of the titanate nanotubes (65 ppm) is comparable to those of niobic acid, H Y z e o l i t e , a n d H Ti N b O5 n a n o s h e e t c a t a l y s t (65 ppm) 31),35), 36) , but is weaker than those of H-ZSM-5 (86 ppm) and H resin (72 ppm) 31), 32) . The acid strength of the titanate nanotubes was also evaluated by density functional theory (DFT) calculations.…”

“…Adsorption of TMPO molecules on the acid sites causes decreased density of the electron cloud surrounding the 31 P nucleus neighboring the oxygen atom of the phosphine oxide proportional to the acid strength of the Brønsted acid sites, which causes the 31 P resonance peak position to shift downfield 31), 32) . Figure 6 shows 31 P NMR spectra for TMPO adsorbed on titanate materials.…”

Polystyrene sulphonic acid resins with enhanced acid strength via macromolecular self-assembly within confined nanospace

“…5b). The 31 P chemical shift of TEPO chemisorbed on the acid site is sensitive to acid strength, and stronger acid strength will usually lead to larger 31 P chemical shift of TEPO [42][43][44] . The 31 P NMR spectrum of Amberlyst-15 exhibits a signal at 89.4 p.p.m.…”