Rational Design of Engineered Multifunctional Heterogeneous Catalysts. The Role of Advanced EPR Techniques

Abstract

The importance of surface paramagnetic species owes much to that of surface phenomena which are involved in numerous areas of chemistry and material science such as heterogeneous catalysis, photochemistry and, in general terms, nano-sciences and technology. In the present contribution the opportunities offered by the use of EPR in the field of heterogeneous catalysis, with emphasis on the application of hyperfine techniques, will be illustrated taking as an example Ti-based catalytic materials. The reductive activation of framework titanium ions in the different materials and their sub-sequent reactivity towards NH 3 is followed, highlighting subtle differences in chemical reactivity related to the different matrixes.

Chromium Environment within Cr-Doped Silico-Aluminophosphate Molecular Sieves from Spin Density Studies

“…Above this value, additional peaks associated with multiple quantum transitions were observed in the simulation, which were not present in the experimental spectrum. Considering that the value of a 0 = 3367.76 MHz for the unit spin density in the 27 Al 3s orbital, the corresponding spin density in the Al 3s orbital is in the range ≈ 0.06–0.5%, in good agreement with other d 1 transition-metal ions involving M­(3 d 1 )–O–Al linkages. , It is interesting to note that a similar degree of spin density transfer has been observed for the isoelectronic Ti 3+ and V 4+ (3d 1 ) toward 31 P and 29 Si in microporous aluminophosphate and silicalites, and it appears to be a distinctive feature of isomorphous framework substitution . Moreover, a ridge with a maximum extension of about 2 MHz centered at the 31 P Larmor frequency is observed.…”

“…16,39 It is interesting to note that a similar degree of spin density transfer has been observed for the isoelectronic Ti 3+ and V 4+ (3d 1 ) toward 31 P and 29 Si in microporous aluminophosphate and silicalites, and it appears to be a distinctive feature of isomorphous framework substitution. 43 Moreover, a ridge with a maximum extension of about 2 MHz centered at the 31 P Larmor frequency is observed. This indicates that the electron spin, localized on Cr 5+ , interacts with distant (the fourth coordination shell) phosphorus nuclei, providing firm and unambiguous evidence that chromium ions were successfully incorporated at the phosphorus sites of SAPO-5.…”

Spectroscopic investigation into the design of solid–acid catalysts for the low temperature dehydration of ethanol

“…The ionic nature of the AlPO framework allows the Al 3+ and P 5+ ions to be isomorphously substituted for a heteroatom, generating a range of active sites. [8,[10][11][12] Purposeful combinations of a microporous framework with specific dopants, can then lead to the targeted formation of precisely engineered active sites. The most common dopant in AlPOs is silicon (to create a silicon-doped AlPO; SAPO), which leads to the generation of a Brønsted acid site within the framework ( Figure S1).…”

Structure and dynamics of catalytically competent but labile paramagnetic metal-hydrides: the Ti(iii)-H in homogeneous olefin polymerization

“…Both a iso and the dipolar matrix correlate with values observed in the case of VO 2+ -O-Al linkages in V doped aluminumphosphate 47 and Ti 3+ -O-Al linkages in Ti doped aluminosilicate materials. 48 Somewhat larger values have been reported for Ti 3+ ions coordinated to an anionic aluminum hydride (alanate) ligand 42 and for Ti 3+ ions on the surface of chlorinated aluminum oxide. 49 Finally, we note that no evidence of Cl interactions could be detected.…”