Pyridine, adsorption metal oxides

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Vasudevan, D. Dorley, P.J. Zhuang, X. (2001) Adsorption of hydroxy pyridines and quinolines at the metal oxide-water interface Role of tautomeric equilibrium. Environ. Sci. 

The technique has been fruitfully used to characterize acid and basic sites in many catalysts, in particular for zeoHtes and metal oxides [143]. It has also been applied for POMs [144]. It consists of measuring the differential heats of adsorption when adsorbing successive increments of a basic probe molecule such as ammonia or pyridine for acidity characterization or of an acid probe molecule such as GO2 or SO2 to characterize basicity. The technique produces a histogram of the acid-base strength as a function of coverage, in particular when heterogeneity in strength exists. The data should then be compared with ammonia or pyridine desorption data from IR and thermal desorption experiments (see above). 

Recently, there have been various studies of the effect of the adsorption temperature on the acidic properties of metal oxide catalysts. Tsutsumi and co-workers (84,85) studied calorimetrically the adsorption of ammonia and pyridine on H Y and NaY zeolites, silica-alumina, and silica between 313 and... 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Bulk Metal Oxides. Extensive Raman chemisorption studies on high surface area alumina and silica supports have been performed because of the industrial importance of these oxides and their weak background Raman vibrations. The most informative studies resulted from the adsorption of pyridine since this probe molecule is very sensitive to the type of acid sites (Brpnsted and Lewis) present on the silica and alumina surfaces. On the alumia support, Lewis pyridine was predominately observed and on the silica support both Lewis and Brpnsted pyridine were observed. " The surface concentrations of pyridine on the silica surface were very small in comparison to the pyridine coverages on the alumina surface. The signal intensities were dramatically enhanced by the... 

Maidanovskaya, L.G. and Skipko, T.V., Adsorption of pyridine and phenol from n-hexane by titanium-group metal oxides, Zh. Fiz. Khim., 46, 115, 1972. 

AIPO4 and AlPO -metal oxide catalysts exhibited high activity and selectivity in aniline alkylation to produce N-alkylated products in a consecutive first-order reaction process. N-methylaniline was easily formed at low temperatures/contact times and converted to N,N-dimethylaniline as temperature/contact time increased. N-alkylated products remained 100 mol% at 523-623 K. At 673 K, N,N-dimethyltoluidine (p->o-) also appeared although in very small amounts. The results obtained for pyridine adsorption at 373 (weak acidic) and 573 K (medium acidic) generally agreed with catalytic activity. 

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

Intercalation of ruthenium poly(pyridine) complexes into other layered materials has been reported (95-106). The intercalation is not as facile as in smectite systems, which is partly due to the higher charge densities of these host materials. Consequently, quantitative ion exchange of ruthenium complexes with the interlayer cations is difficult. Synthetic efforts have been made to introduce ruthenium polypyridine chelate complexes into magadiite, zirconium phosphate and phos-phonates, LDH, MnPSs, and a transition metal oxide and to control the adsorption states. 

The catalytic activity of transitional aluminas y-, r]-, S-, 0-AI2O3 are undoubtedly mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond. The alumina s Lewis sites have been well characterized by adsorption of probes such as pyridine, carbon monoxide, and several bases followed by IR [161], ammonia and amines followed by calorimetry [40,44,46], triphenylphosphine followed by NMR [163], to be the strongest among metal oxides, only... 

For further investigation of metal state in the rtraterials prepared XPS spectroscopy was applied and the results obtained are shown in Table 1. The oxidation state of the metal incorporated was estimated as +5 for both rtiobirrm arrd tantalum. The presence of metal at +5 oxidation state in the zeolite framework shotrld imply generation of Lewis acidity. Irrdeed, the pyridine adsorption followed by FTTR spectroscopy proved the presence of Lewis acid sites in all zeolites (spectra not shown here). As mentioned above, all alurttinum is tetrahedrally coordinated hence the origin of Lewis acidity should be associated with niobium and tantalum incorporated irrto the skeleton of zeolite. Nb and Ta in the zeolite framework forms positive charged tetrahedral species. 

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