Pyridine, chemisorbed, adsorption

The strong acidity of the zeolite without and with silica binder were measured in terms of the pyridine chemisorbed at 400°C, GC adsorption/ desorption method [16. The zeolite without and with silica binder were also characterized for their acid function by carrying out over them two catalyzed model reactions - cracking of isooctane (at 400°C) for measuring external acidity and toluene disproportionation (at 500 C) for measuring the intracrystalline or overall acidity, using pulse micro-reactor (id 4.5 mm) made of quartz, connected to a gas chromatograph [11, 14, 17]. 

The bulk and framework compositions of the H-GaAlMFI and Ga/H-ZSM-5 zeolites are given in Table 1. The zeolites were prepared by the procedures described elsewhere (6,9). The MFI structure of the zeolites was confirmed by XRD. The incorporation of Ga in the framework of GaAlMFI zeolite was confirmed by the Ga MAS NMR peak at +156 ppm. The framework (FW) Si/Ga and Si/Al ratios of the zeolites were obtained from their 29gi mAS NMR peaks, Si(0Ga or OAl) at about -112 ppm and Si(lGa or lAl) at about -104 ppm by the method described earlier (10). The crystal size and morphology of the zeolites was studied by SEM. The zeolite bulk chemical composition was determined by the chemical analysis of Ga, Al, Si and Na. Strong acid sites on the zeolites were measured in terms of the pyridine chemisorbed at 400 C, using the GC adsorption/desorption method (11). 

The acid sites strength can be determined by measuring the heats of adsorption of basic probe molecules. The basic probes most commonly used are NH3 (pTTa = 9.24, proton affinity in gas-phase = 857.7 kJ/mol) and pyridine (pTTa = 5.19, proton affinity in gas-phase = 922.2 kJ/mol). The center of basicity of these probes is the electron lone pair on the nitrogen. When chemisorbed on a surface possessing acid properties, these probes can interact with acidic protons, electron acceptor sites, and hydrogen from neutral or weakly acidic hydroxyls. 

As can be seen in Table 13.2, the heats of NH3, pyridine, CO2 or SO2 adsorption clearly show that these molecules are chemisorbed on all aluminas (heats of adsorption higher than 100 kJ/mol) in spite of the different origins of AI2O3 and different pretreatment and adsorption temperatures used. 

The earliest NMR studies of oxide surfaces (362-364) involved wide-line proton NMR of adsorbed organic species. For example, Petrakis and Kiviat (363), who studied the adsorption of pyridine and thiophene on molybdena-modified alumina, found that chemisorbed and physisorbed species can be readily distinguished. When physically adsorbed, both compounds exhibited liquid-like NMR behavior with high molecular mobility even at low temperatures. Chemisorbed pyridine was much more rigidly held with essentially only a rotation about the C2 molecular axis persisting to - 130°C. Pyridine was sorbed both physically and chemically, and pretreatment of the surface was not particularly significant in this respect. By contrast, thiophene was physisorbed only on surfaces previously reduced with hydrogen, and underwent a reaction on calcined but unreduced surfaces. 

Tomida et al. (73) investigated the temperature-programmed desorption of n-butylamine from silica-alumina and alumina. The desorbed amine products were different in the two cases. n-Butylamine and n-butene were obtained from silica-alumina dibutylamine and n-butene were obtained from alumina. In a subsequent paper by Takahashi et al. (73a), the authors conclude that two types of adsorption sites on silica-alumina account for the desorption behavior of n-butylamine. One type chemisorbs the amine and the other catalyzes the decomposition of the amine to lower olefins at temperatures above 300°C. On the other hand, amine decomposition was not observed when pyridine was desorbed from silica-alumina. The effects of sodium poisoning on desorption behavior of n-butylamine and pyridine were also examined. 

Infrared spectroscopic studies regarding the adsorption of pyridine on both anatase and rutile have been reported (136, 176, 194, 216,217). Hydrogen-bonded pyridine is readily desorbed on pumping at room temperature, whereas pyridine held by coordinatively unsaturated Ti4+ ions is thermally stable up to approximately 400°C. As ammonia, pyridine forms two distinct coordinately held species (176, 217) indicating the existence of two types of Lewis acid sites, which should correspond to Ti4+ ions in different stereochemical environments. According to Primet et al. (176), the more stable species is chemisorbed on type... 

The formation of acidic and metallic sites has been investigated in detail at pressures of 2 to 5 Torr (0.3-0.6 kPa) after in-situ reduction of catalyst samples and by FT-IR investigation of pyridine adsorption as a function of temperature. The amounts of hydrogen chemisorbed proved to be pressure-independent under these conditions. 

The self-assembling complex [Os(bpy)2(dipy)Cl](PF6) (bpy is 2,2 -bipyridine, dipy is trimethylene-4,4 -bipyridine abbreviated as Osdipy Fig. 4(C)) chemisorbs onto platinum and gold surfaces through a dangling pyridine ring with a free energy of adsorption of the order of-50 kJ/mol to form well-ordered, redox-active monolayers... 

The adsorption of /J-picoline on stilbite(OlO) was characteristically slow, enabling us to observe its adsorption process. Upon contact of the surface with a / -picoline solution, a fuzzy, cloud-like surface is observed, which is attributed to multi-layer physisorbed /3-picoline molecules. Scraping off the physisorbed layer with the AFM tip revealed the presence of two chemisorbed monolayer phases one randomly adsorbed and the other well-ordered two-dimensional quasi-hexagonal phase. The development of the latter phase was very slow on this surface, allowing us to estimate its rate to be ca. 6 nm/min. The slow ordering process also allowed us to determine the interrelation between the adlayer and the substrate lattices. It is formd that one of the unit vector of the adlayer makes an angle of 90° with respect to the c axis of the substrate(OlO) plane. The ordered phase is incommensurate with the substrate surface as in the case of pyridine/zeolite(010). 

However, the behavior or function of spilt-over hydrogen on zeolite is not clear yet. Pyridine, which is a typical organic base, is chemisorbed on either Brpnsted (B) acid site or Lewis (L) acid site to form pyridinium ion or coordinately bonded pyridine complexes with B or L acid sites, respectively, and giving different IR-adsorption bands [7]. Therefore, the change in the IR band of adsorbed pyridine on B or L acid sites could be attributed to the change in the adsorbed pyridine on acid site. 

In this paper, pyridine adsorption was utilized to investigate the hydrogen spillover phenomenon from metal to acidic centers of zeolite by observing hydrogenation of chemisorbed pyridine on B or L acid sites of USY zeolite using hybrid catalyst composed of USY zeolite and Pt/SiOo. To give insight into spill-over process in the hybrid catalyst system, isomerization of n-pentane, which is one of the typical add catalyzed reaction, was also studied. 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

The present results clearly show that only 2-BIPY among the six pyridine derivatives used inhibits the electrode reaction of cytochrome c in the bulk of the solution. This is probably due to the formation of either stable Ag(2-BIPY)2 or a chemisorbed film of 2-BIPY on the silver electrode and these films inhibit both the adsorption of cytochrome c on the silver electrode and electron exchange between the electrode and cytochrome c in the bulk. 

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