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In-situ UV-visible spectroscopy was used to measure the extent of reduction of active centers in VO x /γ-Al 2 O 3 during oxidative dehydrogenation (ODH) of propane. Prevalent extents of reduction (0.062 to 0.30 e -/V) are much smaller than required for the formation of stoichiometric V 3+ or V 4+ suboxides. Surface oxygen atoms are the most abundant reactive intermediates during propane ODH, as previously suggested by kinetic and isotopic studies. These measurements involved the rigorous calibration of UV-visible intensities in the pre-edge region using quantitative reoxidation of a small number of centers reduced in H 2 . Transients observed during changes in C 3 H 8 and O 2 concentrations indicate that only a fraction of the prevalent reduced centers (∼30-40%) are active in catalytic turnovers, while the rest are reoxidized in time scales much longer than turnover times. The number of catalytically relevant reduced centers depends only on C 3 H 8 /O 2 ratios, and not on individual reactant concentrations, indicating that oxygen vacancies are the predominant reduced centers and that hydroxyls and alkoxides are present at much lower concentrations. The fraction of V-atoms that exist as catalytically reduced centers and the rate of propane ODH (per exposed V-atom) increase with increasing vanadia surface density and domain size up to surface densities typical of polyvanadate monolayers (∼7.5 V/nm 2 ) and then reach nearly constant values at higher surface densities. This relation between the extent of reduction during catalysis and the propene formation rates confirms the redox nature of catalytic cycles and the exclusive kinetic relevance of the reduction part of the cycle, in which C-H bonds are activated using lattice oxygen atoms. This method for measuring the extent of reduction during catalysis using preedge features in the UV-visible spectrum provides greater sensitivity and time resolution than X-ray absorption and UV-visible spectroscopic methods based on near-edge spectral features. The approach and initial results seem generally applicable to oxidation reactions using lattice oxygens as reactive intermediates.

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The interaction of different probes with two H-MOR samples has been studied by IR. In the case of the sample with Si/Al = 10 acetonitrile perturbs all the hydroxy groups while 2,2-dimethylpropionitrile (pivalonitrile) perturbs only very few. Pyridine also perturbs all the hydroxy groups but only some of them protonate the pyridine, the others only H-bond to it. n-Hexane and 2,2-dimethylbutane give the same result, perturbing only some of the bridging hydroxy groups. The results are interpreted by assuming that no OHs are located in the 8-ring channels, and that the hydrocarbons cannot interact (due to the steric hindrance of the methyl group) with the OHs located in the side pockets. On the contrary, the at molecule pyridine can enter slightly into the side pockets and H-bond with the OHs there. Pivalonitrile interacts only with the OHs which are well exposed in the main channels. It is concluded that the active sites for alkane isomerization are likely exclusively those that are well exposed in the main channels of H-MOR and that Al substitution in the T3 sites probably does not occur. The sample with Si/Al 45, taken as an example of a dealuminated sample, presents many less bridging OHs which are entirely available for interaction with even pivalonitrile

A study of the nature and accessibility of internal and external protonic and cationic sites has been carried out on protonic and partially Co-exchanged FER, MFI, and MOR through the use of UV−vis and FT-IR spectroscopy. The ion-exchange procedure was exactly the same for the three zeolites. A sample of Co-exchanged silica−alumina has also been investigated for comparison. UV−vis spectra recorded after outgassing at 773 K provide evidence of the presence of low-coordination Co2+ sites in all outgassed Co-containing samples. However, they show that the sites located in the zeolite cavities cannot be distinguished from those located at the open surfaces. FT-IR studies of the adsorption of benzonitrile and o-toluonitrile, used as differently hindered weak basic molecules, allowed us to distinguish sites located at the external surfaces or in different cavities. Benzonitrile enters only the main channels of H−MOR and Co−H−MOR, whereas it is not able to enter the mouths of the side pockets. Benzonitrile also enters the channels of H−MFI and Co−H−MFI. o-Toluonitrile enters the main channels of H−MOR but does not enter the main channels of Co−H−MOR. Similarly, it slowly enters the channels of H−MFI but does not enter those of Co−H−MFI. So, it provides evidence for the narrowing of the pores of MOR and MFI by Co exchange. Both nitriles do not enter at all the channels of H−FER and Co−H−FER but provide evidence for the location of Co ions at the outer surface. In all cases terminal silanols and Lewis sites are located at the external surfaces, whereas the strongly acidic bridging OHs are exclusively located in the interior of the cavities. In Co−H zeolites, Co2+ ions are distributed between internal and external surfaces. The extent of exchange of the internal OHs roughly depends directly on the cavity dimensions, increasing in this sense:  FER < MFI < MOR and MOR (side pockets) < MOR (main channels). The data also show that, at least for MFI, cation exchange essentially occurs at the mouth of the channels.

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