Pyridine, adsorption structure

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

Pyridine adsorption experiments have showed that the nickel containing smectites have Lewis acid sites and do not have Bronsted acid sites [9]. The Ni2+-substituted smectite catalysts have large surface areas even after 873 K treatment because many small fragments with the same smectite structure are intercalated in the interlayer region. The activities of the Ni2+ substituted catalysts are derived from Ni2+ Lewis acid sites located on the edge framework. 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

Although the basic principles behind this Intact ejection mechanism can be Illustrated with carbon monoxide, the extrapolation to large bloorganlc molecules Is not necessarily obvious. Calculations have been performed for a series of organic molecules adsorbed on a Ni(OOl) surface to understand the mechanisms of molecular ejection (8-12). The first molecules which have more than just a few atoms examined are benzene which it-bonds on a metal surface and pyridine which can either ir-bond or o-bond on a metal surface. Larger structures, whose sizes approach the diameter of bloorganlc molecules, are naphthalene, biphenyl and coronene whose adsorption structures are unknown. All the molecules except pyridine are assumed to ir-bond on the surface. 

Naturally, structures (d) and (f) do not exhaust all possible states of low-coordinated A1 atoms on the surface of the oxides considered. The calculations, however, seem quite sufficient to suggest that water molecule coordination by a LAS is energetically less favorable for aluminophosphate than for aluminosilicate surfaces. This conclusion is also in accordance with IR data, which indicate that LASs of the both oxides quite similarly interact with pyridine, whereas the LASs of aluminophosphates do not coordinate C02 molecules (136). Indeed, in the case of a sufficiently strong base (pyridine), adsorption interaction appears stronger than the structural coordination and therefore stabilizes the A1 atom in the adsorption state. On the contrary, for C02, which is certainly a very weak base, the interaction is strong enough in the case of aluminosilicates but is insufficient for the adsorption stabilization of aluminum in aluminophosphates. 

P. E. Eberly, Jr. 1 share your concern about structural changes with temperature. Consequently, we have consistently given consideration to what conditions would be most desirable for measuring acidity. We have effected a compromise. At reaction temperatures much above 260 °C, pyridine can undergo some decomposition, and this clouds the experimental results. We selected 260°C for pyridine adsorption. This is higher than that used by other investigators and hence should yield acidities which more nearly represent those at the cracking temperature of 274°C used in this study. 

Characterization of catalysts The zeolite structure was checked by X-ray diffraction patterns recorded on a CGR Theta 60 instrument using Cu Ka, filtered radiation. The chemical composition of the catalysts was determined by atomic absorption analysis after dissolution of the sample (SCA-CNRS, Solaize, France). Micropore volumes were measured by N2 adsorption at 77 K using a Micromeritics ASAP 2000 apparatus and by adsorption of cyclohexane (at P/Po=0.15) using a microbalance apparatus SET ARAM SF 85. Incorporation of tetrahedral cobalt (II) in the framework of Co-Al-BEA and Co-B-BEA was confirmed by electronic spectroscopy [18] using a Perkin Elmer Lambda 14 UV-visible diffuse reflectance spectrophotometer. Acidity measurements were performed by Fourier transform infrared spectroscopy (FT-IR, Nicolet FTIR 320) after pyridine adsorption. Self-supported wafer of pure zeolite (20 mg/cm ) was outgassed at 673 K for 6 hours at a pressure of lO Pa. After cooling at 423 K, the zeolite was saturated with pyridine vapour (30 kPa) for 5 min, evacuated at this temperature for 30 min and the IR spectrum was recorded. 

HT, HTA solids. It has been found that the l.f. band, in contrast to the h.f. one, is insensitive to pyridine in the case of non dealuminated HY zeolites (5,11) however, it tends to be sensitive after dealumination (5,11). In our case, the structural bands (h.f.,l.f.) disappear completely after pyridine adsorption whatever the physicochemical characteristics of the solids (figure 2). More surprising is the behaviour of the silanol band (3742 cm-1). While the low frequency part of it (3738 cm-1) appears to be insensitive to pyridine, the high frequency part (3743 cm-1) does interact with pyridine. This interaction persists even after evacuation at higher temperatures up to 723 K. Thus, two components are responsible for the silanol band. 

In the spectroelectrochemical experiment, the simulation of reflectivity responses can become quite complicated owing to the multiple interfaces and phases through which the optical beam must pass [104]. However, the models employed have gained in sophistication over the years as they have been needed to assist in the design of cells and optical systems and in the interpretation of spectra [23, 80, 104, 189-193]. An expanded discussion of the computational approach and its application to a four-phase model of the infrared spectroelectrochemical experiment was presented by Pettinger and coworkers recently [104]. An example of its use and the type of detailed structural information that can be gained is described for the case of pyridine adsorption on an Au electrode [103]. 

The available data concerning the adsorption of different bases indicate that adsorbed bases interact with zeolites more strongly than hydrocarbons of similar structure and molecular weight [21], A comparison of the heats of adsorption of various bases, such as ammonia, pyridine and -butylamine, with those of benzene on A, X, Y and mordenite zeolites, modified by ion exchange and aluminum extraction, has been carried out by Klyachko et al. [21]. Surprisingly, the heats of pyridine adsorption were found to be virtually the same on sodium and on hydrogen zeolites. Furthermore, the sorbed amounts of large molecules such as pyrichne and w-butylamine were veiy limited, due to the finite void... 

As a first example for illustrating the application of Raman spectroscopy in characteri2ing the orientation of surface species, we consider pyridine adsorption on an Ag surface [84], for several reasons. The first SERS experiment was carried out using pyridine as the adsorbed species. Secondly, pyridine has a large Raman cross section, relatively simple molecular structure, and a good assignment of bands appearing in its normal Raman spectrum and SER spectrum. Thirdly, pyridine is an excellent model molecule for surface coordination studies. Eourthly, interactions of the pyridine molecule with the metal surface involve both the it and lone-pair electrons. 

Fig. 19 Preferred perpendicular, tilted, and parallel adsorption structures of pyridine on an Au(lll) surface (calculated at DFT-D3 level of theory). Reproduced with permission from ref. 61. Copyright 2013 American Chemical Society.

Fig. 21 Perpendicular and parallel adsorption structures of pyridine derivatives (ordered along the increasing donor strength of the substituent) on a Au(lll) surface. The preferred orientation is marked in red. Reproduced from ref. 61.

The effect of preparation conditions on structural and surface properties of magnesium fluoride was studied in the aspect of its use as a catalyst support. Amorphous and spherical polyciystaUine Mgp2 supports were prepared and characterised by BET, XRD, TEM, and FTIR (pyridine adsorption) techniques. The influence of Mgp2 properties on the performance of Ru/MgF2 catalysts in selective reduction of ortho- and para-chloronitrobenzene to respective chloroanilines is reported as well. 

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