The oxidation of trimethylphosphine in zeolite Y: a solid-state NMR study

Abstract

The oxidation of trimethylphosphine (TMP) to trimethylphosphine oxide (TMPO) has been followed in Y-type zeolites by using 31P MAS-NMR spectroscopy. In a Na-Y zeolite, TMP coordinates weakly with Na+ ions to give a resonance at -59.5 ppm. This form of TMP is readily oxidized at 25 °C to TMPO, which may be coordinated to Na+ (48.6 ppm) or may be physisorbed (41.4 ppm). Similarly, TMP which is weakly bound to a Lewis acid site (-61.5 ppm) in a dealuminated zeolite may be easily oxidized to Lewis-bound TMPO (56 ppm). Liquidlike TMP (-67.1 to -67.9 ppm) is somewhat less easily oxidized, but it also forms physisorbed TMPO. By contrast, the protonated adduct of TMP (-1.0 to -2.8 ppm) is more difficult to oxidize and requires a temperature in excess of 100 °C. TMPO is a weaker Bronsted base than TMP, although the protonated adduct (65, 74 ppm) is formed when TMPO is adsorbed in a H-Y zeolite or when TMP is oxidized in a dealuminated zeolite, which is more strongly acidic.

31P Solid State NMR and ESR Investigation of Trimethylphosphane Adsorption on Zirconia and Sulfated Zirconia

“…The observation of a peak at +65.7 ppm on ZrO 2 −S is intriguing. This signal is clearly separated from spinning sidebands of HTMP + species at the MAS frequency of 8 kHz (see spectrum b of Figure ) and can be attributed to a protonated OTMP species . In contrast to Coster et al., Lunsford et al also observed a peak at approximately this position, which was, however, superimposed by spinning sidebands.…”

“…Zalewski et al were able to show that HOTMP + only formed on very strong Brønsted acid centers in HY zeolites. Lunsford et al, in their study of TMP adsorption on sulfated ZrO 2 , argued that the oxidation of TMP to OTMP occurred by traces of O 2 being present during the sample preparation and handling for NMR measurements.…”

“…In contrast, HTMP + was oxidized only very slowly. Chemical shifts around +41 to +42 ppm were attributed to physisorbed trimethylphosphane oxide (OTMP) by Zalewski et al, 12 while chemical shifts in the range +48 to +56 ppm and of +65 ppm were considered to be characteristic for OTMP coordinated to Lewis sites and bonded to Brønsted sites (protonated OTMP is denoted HOTMP + ), respectively.…”

Evaluation of Zeolite Acidity by 31P MAS NMR Spectroscopy of Adsorbed Phosphine Oxides: Quantitative or Not?

“…Another significant drawback of trialkylphosphines as probe molecules is their ability to oxidize during adsorption over zeolites. 17 , 21 …”

Double rotation NMR studies of zeolites and aluminophosphate molecular sieves

“…PMe3 is a sterically large, soft (class B) Lewis base, compared to H20, a small, hard (class A) Lewis base molecule[128]. Binding of PMe3 to Brcnsted and Lewis acid sites in zeolite Y has been probed in several studies using 31P MAS[69,144,145,146].Figure 6.7 features the 23Na DOR spectra of Na56Y zeolite, loaded with quantitative amounts of PMea guest molecules. The adsorbed PMe3 molecules clearly affect the environments of the sodium nuclei, causing profound changesin the positions and shapes of the _3Na signals, as shown in Figure 6.7.…”