Maria Bevilacqua

FTIR study of species arising after NO adsorption and NO + O2 co-adsorption on CoY : comparison with Co-ZSM-5

Abstract CoY, with its low activity in selective catalytic reduction (SCR) of nitrogen oxides, differs from Co-exchanged pentasil zeolites (e.g. Co-ZSM-5). To obtain more information on the SCR mechanism, the NO x species formed after NO adsorption and NO+O 2 co-adsorption on CoY were studied by means of IR spectroscopy and the results were compared with those obtained for Co-ZSM-5. NO adsorption on CoY leads to the formation of Co 2+ (NO) 2 species ( ν s at 1900 and ν as at 1819 cm −1 ) which are characterised by a stability similar to the stability of the dinitrosyls formed on Co-ZSM-5 (1894 and 1812 cm −1 ). This suggests that the Co 2+ (NO) 2 species are not involved in the SCR. The stable species produced upon NO+O 2 co-adsorption on the two samples are very different. The principal compounds formed on Co-ZSM-5 are surface monodentate nitrates characterised by an IR band at ≈1540 cm −1 . These nitrates easily interact with hydrocarbons, which confirms that they are key species in SCR. No monodentate nitrates are formed on CoY. Stable symmetric nitrates (1488 and 1473 cm −1 ) and less stable species, probably bidentate nitrates (1620 and 1320 cm −1 ) appear instead. The symmetric nitrates are converted, during evacuation, into nitro-compounds (1563 and 1383 cm −1 ) that are not removed even by evacuation at 743 K. Interaction of methane with the nitrates on CoY only leads to their partial reduction to nitro-compounds. These results account for the low SCR activity of CoY.

A study of the external and internal sites of MFI-type zeolitic materials through the FT-IR investigation of the adsorption of nitriles

Abstract The adsorption of acetonitrile (AN) and pivalonitrile (2,2-dimethylpropionitrile, PN) has been investigated on pure silicalite S1, on a ZSM5 zeolite apparently free from extraframework material and another ZSM5 zeolite apparently rich in extraframework material, on a titanium silicalite TS1 and on a boralite sample. AN enters the cavities of MFI type structures and interacts with both internal and external acid sites while PN does not enter such cavities and only interacts with the external sites. Terminal silanols, whose acidity can vary from weak to medium, are present in all cases mostly at the external surface, while very acidic bridging Si–OH–Al sites of ZSM5 zeolite are apparently exclusively located at the internal channel surface. Lewis acidic Ti cations are evident both in the internal and on the external surface of TS1. Weakly acidic BOH sites of boralite are distributed among internal and external surface. Extraframework alumina-like species of ZSM5 zeolite are apparently located in the internal cavities. Terminal silanols and hydrolized defects are located mostly at the external surface of silicalite. The mechanism of incorporation and charge balance of boron atoms as well as the structure of the hydroxy groups of boralite are still not well established.

An FTIR study of the accessibility of the protonic sites of H-mordenites

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 flat 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 localization and accessibility of Brønsted and Lewis acid sites of H-mordenite through the FT-IR spectroscopy of adsorbed branched nitriles

Abstract Adsorption of propionitrile (PrN), isobutironitrile (IBN) and pivalonitrile (PN) with H-MOR (Si/Al=10) has been investigated. The accessibility of IBN and, more, of PN are limited, unlike PrN. Terminal silanols ( 3745 cm −1 ), weakly acidic, are located, as Lewis sites, at the external crystal surface. Bridging hydroxy groups absorbing at 3588 cm −1 are inside the side pockets, those absorbing at 3605 cm −1 are in the main channels and those absorbing at 3609 cm −1 are at the intersection between side pockets and main channels. OH’s due to extra framework materials ( 3650 cm −1 ) are in the side pockets.

An FT-IR study of the adsorption of aromatic hydrocarbons and of 2,6-lutidine on H-FER and H-ZSM-5 zeolites

Abstract The interaction of aromatic hydrocarbons benzene, toluene, o -, m - and p -xylene and of methyl pyridines (in particular 2,6-lutidine) with H-ZSM-5 and H-FER zeolites has been studied. Two different H-ZSM-5 samples with strongly different intensity ratios between the two main OH stretching bands have been used. Benzene, toluene and p -xylene enter easily the cavities and give rise to three different H-bonded complexes. Two of them are strongly bonded while the third is likely a very hindered and distorted one. o -Xylene enters slowly the cavities and m -xylene even more slowly. Faster diffusion occurs at higher temperatures. On the contrary, xylenes do not enter the FER cavities. In spite of its steric hindrance, supposed to be the same of m -xylene, 2,6-lutidine enters fast the ZSM channels and is protonated by the internal sites. On the contrary, it does not enter the FER cavities, but it is protonated too on the external silanols sites. Evidence is provided for some kind of heterogeneity of the internal sites of ZSM-5 zeolite. Additionally, it is concluded that other effects besides the molecular sieving effect may play a role in the access and diffusion of molecules into the zeolite channels.

FT-IR studies of internal, external and extraframework sites of FER, MFI, BEA and MOR type protonic zeolite materials

FT-IR studies of the adsorption of nitriles show that the external surface of H-FER, H-MFI, and H-MOR carries terminal silanols and strong Lewis sites. Terminal silanols present wide heterogeneity with respect their Bronsted acidity also in relation to the zeolite structure. Bridging Si-(OH)-Al exist at the internal channel surface only of H-FER and H-MFI and are stronger Bronsted acids than the external terminal silanols. The distinction of the bridging Si-(OH)-Al sites located in the different channels of H-BEA and H-MOR can be obtained. Extraframework material can be located both at the internal channel surface and at the external surtface of H-MFI and H-BEA.

13-P-25-FTIR studies of the interaction of aromatic and branched aliphatic compounds with internal, external and extraframework sites of MFI-type zeolite materials

Publisher Summary This chapter presents the Fourier transform infrared spectroscopy (FTIR) studies of the interaction of aromatic and branched aliphatic compounds with internal, external, and extraframework sites of MFI-type zeolite materials. The interaction of different nitriles of branched aliphatic compounds and of aromatics has been studied over four different ZSM5 zeolites, over silicalite-1 and titanium silicalite-1, and over boralite BOR-C. Internal, external, and extraframework sites have been characterized, and the access to the cavities are discussed.