Tiziana Armaroli

Selective saccharides dehydration to 5-hydroxymethyl-2-furaldehyde by heterogeneous niobium catalysts

Selective dehydration of different substrates, such as fructose, sucrose and inulin, to 5-hydroxymethyl-2-furaldehyde has been studied in aqueous medium by using heterogeneous niobium-based catalysts. Batch experiments have been performed in the presence of both commercial niobium phosphates and catalysts prepared by treatment of niobic acid with phosphoric acid. Flow experiments on catalyst packed beds have been also examined. Finally, batch catalytic experiments, characterized by combination of reaction and products extraction steps, have been also carried out.

Acid sites characterization of niobium phosphate catalysts and their activity in fructose dehydration to 5-hydroxymethyl-2-furaldehyde

Abstract The nature of different acid sites on the surface of various niobium-based catalysts has been spectroscopically investigated by means of FT-IR and UV–VIS techniques. Surface acidity has been further characterized by acetonitrile adsorption and subsequent FT-IR analysis. The catalytic activity of the different examined samples has been preliminarily tested in the fructose dehydration to 5-hydroxymethyl-2-furaldehyde, a reaction of relevant industrial interest.

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 spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity

Different titanium silicalite (TS-1) samples and TiO2/SiO2 aerogels samples have been prepared and characterized structurally by XRD, skeletal FT-IR and FT-Raman spectroscopies and diffuse reflectance UV spectroscopy. The surface acidity of all materials has been investigated by FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile. The TS-1 samples are found to be totally free from extra-framework Ti oxide species, while the characteristic features of framework tetrahedrally coordinated Ti cations are well evident. Framework Ti cations substituting for Si into the silicalite MFI structure act as quite strong Lewis acid sites towards acetonitrile. The presence of Ti neither changes significantly the Brønsted acidity of the silicalite silanols, nor results in the formation of new hydroxy groups. TS-1 samples present lower defectivity (smaller amount of clustered hydrolyzed Si–OH OH–Si bonds) than pure silicalite prepared in the same way. Extra-framework TiO2, instead, is well detectable into aerogel samples, although framework Ti cations are likely also present. The surface of Ti-containing aerogels is covered by large amounts of silanol groups, although these sites do not present enhanced Brønsted acidity. Additionally, Lewis sites are detected whose strength is similar to that observed on TS samples, and is superior to that of the predominant sites on TiO2. It is suggested that silica tends to cover the TiO2 core in the case of aerogels. The surface is consequently dominated by silica with Ti sites in substitutional position.

FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0–25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Bronsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si–(OH)–Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging “ zeolitic” Bronsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework.

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 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.

A FT-IR study of the adsorption of C5 olefinic compounds on NaX zeolite

Abstract The IR spectra of 1-pentene, 2-methyl-1-butene, 2-methyl-2-butene and isoprene (2-methyl-1,3-butadiene) liquid and adsorbed on NaX zeolite have been recorded. Empirical assignments for the observed spectrum (4000–1200 cm −1 ) are reported in all cases. Data on the adsorption of water, acetonitrile and C5 alkanes on NaX are also briefly discussed. The behaviour of NaX zeolite in the separation of alkenes from alkanes in selective adsorption processes is also briefly discussed.

Characterization of the Structural and Gas Adsorption Properties of ETS-10 Molecular Sieve

A sample of ETS-10 was prepared and characterized by spectroscopic (Raman, UV–vis, FT-IR) and TPD techniques. An interpretation of the vibrational spectra was proposed. Absorption in the UV–vis range was typical of charge-transfer transitions involving octahedral Ti. No surface –OH groups were detected on the dehydrated sample. FT-IR measurements showed that water was adsorbed in two different molecular modes while ammonia was adsorbed coordinatively and mostly desorbed below 600 K. These results were confirmed by TPD studies that showed hydrogen-bonded water and water coordinated to Na+ and K+ cations located in the main channels of ETS-10. Ammonia TPD measurements on the hydrated material provided evidence for two forms of adsorbed NH3, both bonded to water molecules. On the anhydrous material, NH3 was probably coordinated only to the more loosely bonded Na+ and K+ ions in the main channels of ETS-10. The amount of adsorbed NH3 was markedly reduced after heat treatment that led to a partial loss of crystallinity.

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.