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

Abstract

31 P MAS NMR spectra of trimethylphosphane (TMP) adsorbed on monoclinic zirconia (ZrO 2 ) and on tetragonal sulfated zirconia (ZrO 2 -S) were measured. The 31 P NMR spectra show that protonic acidity, which does not exist on ZrO 2 , is induced on ZrO 2 -S. Lewis acid centers on ZrO 2 -S are stronger than those on ZrO 2 . The formation of trimethylphosphane oxide was also detected, indicating oxidizing properties of ZrO 2 -S that sulfate-free ZrO 2 does not possess. ESR spectra suggest the formation of SO 3 -radical anions during TMP oxidation. IntroductionSulfated zirconia (ZrO 2 -S) develops strong acidity of the Lewis and Brønsted type and has frequently been described as a solid superacid. 1,2 The acid properties of ZrO 2 -S have been studied by probe molecules such as CO and FTIR spectroscopy [3][4][5] and 1 H MAS (magic angle spinning) NMR gave proton chemical shifts that were more positive than those reported for H-ZSM-5. 5,6 However, O-H stretching frequency shifts induced by adsorbed CO were significantly lower than those observed for H-ZSM-5, 4 thus suggesting that the Brønsted acid strength is at least not higher than that of the zeolite. This is fully consistent with the conclusions of Adeeva et al. 7 who demonstrated, based on 1 H-chemical shift measurements after adsorption of CD 3 CN, that sulfated zirconia is not superacidic. In the present study we report on a qualitative analysis of the acid properties of ZrO 2 -S compared to sulfate-free zirconia ZrO 2 by using trimethylphosphane (TMP) as a probe molecule. The adsorption of TMP was studied by 31 P MAS NMR and by ESR (electron spin resonance).TMP was first introduced as a basic probe molecule for the detection of acid centers in zeolites when Schoonheydt et al. 8 studied such adsorption systems by IR spectroscopy. Rothwell et al., 9 taking advantage of the ideal nuclear properties of 31 P (I ) 1 / 2 , 100% natural abundance), first studied the adsorption of TMP in zeolites by NMR as early as 1984. Lunsford et al. 10,11 reported 31 P chemical shifts of -2 to -4 ppm for the protonated TMP (HTMP + ) in HY zeolites. These authors also observed a 1 J P-H coupling constant of 550 Hz. TMP bound to Lewis acid centers gave upfield shifts in the range -31 to -58 ppm, depending on the Lewis acid strength of the adsorption site. Zalewski et al. 12 demonstrated that TMP coordinated to Lewis sites and physisorbed TMP were instantaneously oxidized in HY zeolites in the presence of O 2 . 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.Besides several other reports 13-15 dealing with TMP adsorption for characterization of acid properties of solid materials, TMP has also been used earlier in conjunction with 31 P NMR as a p...

Characterization of WOx/CeO2 catalysts and their reactivity in the isomerization of hexane

“…Fȃrcaşiu et al [5] proposed an alternative mechanism consisting of a one-electron transfer from the substrate to the catalyst, followed by the cleavage of the radical cation. This supposition was supported by both spectroscopic and catalytic data [10,11], and quantum-chemical calculations [12].…”

31P NMR Chemical Shifts of Phosphorus Probes as Reliable and Practical Acidity Scales for Solid and Liquid Catalysts

“…Sulfated zirconia (SO 4 2– /ZrO 2 ; SZ) is an environmentally friendly catalyst with ultrahigh acidity and has been extensively utilized for various catalytic reactions such as isomerization, cracking, and alkylation . The 31 P-TMP NMR approach has also been used to identify acidity properties of zirconia (ZrO 2 ), sulfated zirconia (SZ), and metal-promoted SZ. ,− Although pristine ZrO 2 possesses only Lewis acidity, both BA and LA sites are found in SZ. The sulfation treatment normally leads to enhanced Lewis acidity compared to its pristine counterpart .…”

“…The peak at −16.6 ppm was most likely due to distorted PO 4 3– tetrahedra, whereas peaks at −26.1 and −3.9 ppm may be attributed to TMP adsorbed on surface LA and BA sites, respectively. It is noteworthy that the NMR chemical shift observed for TMP bound to coordinately unsaturated metal Nb centers of reduced Pd/NbOPO 4 (δ 31 P at −26.1 ppm) is comparable to those observed for sulfated metal oxides such as SO 4 2– /ZrO 2 ,− and SO 4 2– /TiO 2 , indicating the presence of ultrastrong Lewis acidity in the Pd/NbOPO 4 catalyst.…”

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

“…À20 B À55 ppm) when associated with various Lewis acid centers (see Scheme 1a) in different solid acids. 21,48,49,54,[56][57][58][59][60][61][62] On the other hand, when Brønsted acid sites are present in the substrate, TMP tends to form protonated adducts (i.e., TMPH + complexes; see Scheme 1c), giving rise to 31 P resonances in a much narrower range of ca. À2 to À5 ppm with a typical J P-H coupling constant of ca.…”