Relationship between the scaling of the acid strength of Lewis sites by EPR and NMR probes

Journal Article · Thu Apr 01 04:00:00 EST 1993 · Journal of Catalysis; (United States)

DOI:https://doi.org/10.1006/jcat.1993.1101· OSTI ID:6465877

Coster, D J; Bendada, A; Chen, F R; Fripiat, J J ^[1]

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1. Univ. of Wisconsin, Milwaukee (United States)

While numerous techniques have been successful for scaling the acid strength of Broensted sites, the situation is not satisfactory at all, for the Lewis acid sites. This is most unfortunate, since Lewis sites are present in most acid catalysts. This contribution aims to show that the simultaneous use of EPR and NMR probes suggests solutions to the problem of scaling the acid strength of Lewis sites. As shown previously, the hyperfine splitting of the EPR spectrum of the aniline radical cation or of the O[sub 2][sup [minus]] superoxide ion is a measurement of the strength of the electron acceptor site. Other researchers have suggested the shielding of the [sup 31]P nucleus and the shift of its resonance lines at chemisorbed trimethylphosphine (TMP) as a measurement of the Lewis acid strength. The comparison of the scaling obtained on a set of superacids, namely, and in the decreasing order of acidity, the sulfated derivatives of ZrO[sub 2], HfO[sub 2], Al[sub 2]O[sub 3] and TiO[sub 2], by either the EPR or the NMR probes gives interesting information. While very strong Lewis acid centers in sulfated ZrO[sub 2] and HfO[sub 2] are revealed by [sup 31]P downfield shifted resonance lines, no such lines are observed in sulfated Al[sub 2]O[sub 3] or TiO[sub 2] or zeolites (such as dealuminated mordenite) which contains strong Lewis acid centers. In the latter samples, the most downfield shifted line corresponds to that generally assigned to TMPH[sup +]. Measurement of [sup 31]P relaxation rates suggests that TMP has a more restricted mobility on strong than on weak Lewis sites. They also show that the origin of the relaxation in TMPH[sup +] is ambiguous: either the mobility of TMPH[sup +] is very restricted and/or the extraproton is not as close to the phosphorus as anticipated. 20 refs., 5 figs., 4 tabs.

• OSTI ID: 6465877
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 140:2; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
AMINES
ANILINE
AROMATICS
CATALYSTS
CHALCOGENIDES
CHEMICAL PROPERTIES
ELECTRON SPIN RESONANCE
ELEMENTS
HAFNIUM COMPOUNDS
HAFNIUM OXIDES
HYDROGEN COMPOUNDS
INORGANIC ACIDS
INORGANIC ION EXCHANGERS
ION EXCHANGE MATERIALS
LEWIS ACIDS
MAGNETIC RESONANCE
MATERIALS
MINERALS
MOBILITY
MORDENITE
NONMETALS
NUCLEAR MAGNETIC RESONANCE
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOSPHINES
PHOSPHORUS
PHOSPHORUS COMPOUNDS
REFRACTORY METAL COMPOUNDS
RELAXATION
RESONANCE
SHIELDING
SILICATE MINERALS
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS
ZEOLITES
ZIRCONIUM COMPOUNDS
ZIRCONIUM OXIDES