³¹P Chemical Shift of Adsorbed Trialkylphosphine Oxides for Acidity Characterization of Solid Acids Catalysts

Abstract

A comprehensive study has been made to predict the adsorption structures and (31)P NMR chemical shifts of various trialkylphosphine oxides (R3PO) probe molecules, viz., trimethylphosphine oxide (TMPO), triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO), by density functional theory (DFT) calculations based on 8T zeolite cluster models with varied Si-H bond lengths. A linear correlation between the (31)P chemical shifts and proton affinity (PA) was observed for each of the homologous R3PO probe molecules examined. It is found that the differences in (31)P chemical shifts of the R3POH(+) adsorption complexes, when referring to the corresponding chemical shifts in their crystalline phase, may be used not only in identifying Brønsted acid sites with varied acid strengths but also in correlating the (31)P NMR data obtained from various R3PO probes. Such a chemical shift difference therefore can serve as a quantitative measure during acidity characterization of solid acid catalysts when utilizing (31)P NMR of various adsorbed R3PO, as proposed in our earlier report (Zhao; et al. J. Phys. Chem. B 2002, 106, 4462) and also illustrated herein by using a mesoporous H-MCM-41 aluminosilicate (Si/Al = 25) test adsorbent. It is indicative that, with the exception of (TMPO), variations in the alkyl chain length of the R3PO (R = C(n)H(2n+1); n > or = 2) probe molecules have only negligible effect on the (31)P chemical shifts (within experimental error of ca. 1-2 ppm) either in their crystalline bulk or in their corresponding R3POH(+) adsorption complexes. Consequently, an average offset of 8 +/- 2 ppm was observed for (31)P chemical shifts of adsorbed R3PO with n > or = 2 relative to TMPO (n = 1). Moreover, by taking the value of 86 ppm predicted for TMPO adsorbed on 8T cluster models as a threshold for superacidity (Zheng; et al. J. Phys. Chem. B 2008, 112, 4496), a similar threshold (31)P chemical shift of ca. 92-94 ppm was deduced for TEPO, TBPO, and TOPO.

Heteropolyacid-based ionic liquids as effective catalysts for the synthesis of benzaldehyde glycol acetal

“…82.2, 88.6, and 92.2 ppm should arise from TMPO adsorbed on Brønsted acid sites of the TPA with different acidic strengths. That the ␦ 31 P values observed for such TMPOH + adsorption complexes are in the proximity of the threshold value predicted for superacidity (86 ppm) [38,39,50,51], indicates that the parent H 3 PW 12 O 40 catalyst indeed possesses ultra-strong acidity. Moreover, additional resonance peaks were also observed with ␦ 31 P within the range of 55-80 ppm, which may be attributed to the presence of (TMPO) 2 H + adducts, i.e., the absorption of two TMPO molecules per protic site [52].…”

“…35-45 ppm [38][39][40][41][42]. Furthermore, a linear correlation exists between the 31 P NMR CS (␦ 31 P) of the adsorbed TMPO and proton affinity (PA) value (also known as deprotonation energy, i.e., energy required to remove a hydrogen ion (H + ) from a Brønsted acid site); as verified by density functional theory (DFT) theoretical calculations [50,51]. Thus, a higher observed ␦ 31 P value of the adsorbed TMPO would correspond to Brønsted acid sites with stronger acidic strength, and vice versa.…”

“…13 [38][39][40][41][42][43][44][45]. It has been demonstrated that a linear correlation between the observed 31 P NMR chemical shifts and the strengths of Brønsted acidity may be inferred [50,51] by means of such 31 P-TMPO NMR approach. Prior to the adsorption of TMPO, catalyst was first subjected to a thorough dehydration treatment at 150 • C for 24 h under vacuum (<10 −5 Torr).…”

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.…”

Novel Keggin-type H 4 PVMo 11 O 40 -based ionic liquid catalysts for n -caprylic acid esterification

“…Each sample was heated from room temperature to 873 K under the protection of nitrogen. As depicted in Fig In addition, the acidic properties of various samples were studied by solid-state 31 P MAS NMR of adsorbed trimethylphosphine oxide (TMPO); which was proven to be a powerful technique for acidity characterization of solid acid catalysts [26][27][28]38,39]. The base TMPO probe molecule tended to interact with the Brønsted acidic protons to form TMPOH + adducts, whose 31 P NMR chemical shift (δ 31 P) was directly proportional to the acidic strength of the catalyst [38,39] 40 4− ) of the heteropolyacid acid [40,41].…”