Chemisorption of pyridine

Very little is known about the behaviour of different catalysts only a few comparisons of alumina and silica—alumina have been made. On Al2-03—Si02, the disproportionation of diethylamine is more rapid by one order of magnitude than its deamination on A1203, the rates are comparable [149], The activity of alumina for aniline disproportionation is higher than of silica—alumina [150]. The steric demands of the alumina surface are higher than those of silica—alumina as the comparison of the chemisorption of pyridine and 2,6-dimethylpyridine has shown [158]. 

The thermal stability of NH4Y zeolite in which ammonium ions have been exchanged at various levels with La3+ ions was studied. The catalytic activity of these La zeolites in isooctane cracking was measured as a function of pretreatment temperature, and an IR study of the chemisorption of pyridine was used to determine the numbers of Bronsted and Lewis sites. The structural damage resulting from high temperature calcination was examined qualitatively. 

The main chemisorption of pyridine on alumina surfaces at temperatures roughly below 350°C is, thus, by coordination on to coordinatively unsaturated Al3+ ions ... 

In the present work we examine the microporosity of a TSLS complex formed from synthetic imogolite and natural montmorillonite. Nitrogen adsorption and desorption isotherms are reported and analyzed in terms of microporous volume and surface area. Also, the adsorption isotherm for an organic adsorbate, m-xylene, is reported. Preliminary FTIR results for the chemisorption of pyridine and catalytic studies of the dealkylation of cumene suggest that TSLS complexes are promising microporous acids for shape selective chemical conversions. 

Chemisorption of pyridine results in the disappearance of these groups and the formation of pyridinium ions. The concentration of pyridinium ions, and hence accessible Bronsted acid sites, follows a similar relationship to that of the hydroxyl group concentrations. Thus, the acid site concentration remains constant until about 16-18 calcium ions have been introduced. The acidity concentration then decreases rapidly as the calcium ions are exchanged for ammonium ions in accessible positions. At the same degree of exchange, the cation-pyridine band near 1444 cm" is observed first, confirming the appearance of calcium ions in accessible positions in the structure. 

Strong acid sites of the zeolite with and without silica binder were measured by the chemisorption of pyridine at 400 C. The acid sites were also measured in terms of the activity of the zeolite catalysts in acid catalyzed model reaction, disproportionation of toluene at 500 C. Acid sites on the external surface of zeolite crystals or intercrystalline acid sites of the zeolite catalysts were measured in terms of the iso-octane (which cannot enter in ZSM-5 zeolite channels even at 400°C [18, 19]) cracking activity at 400 C [11]. The results showing the influence of silica binder on both the intracrystalline and intercrystalline acidity of the zeolite catalyst are presented in Tables 1 and 2. 

The total acidity (i.e. both intra- and intercrystalline) of both the zeolite catalysts, obtained by the pyridine chemisorption, is consistent with that measured by the toluene disproportionation reaction. The acid sites measured by the chemisorption of pyridine (at 400°C) are essentially strong zeolitic acid sites. No chemisorption of pyridine (at 400 C) on... 

Time-on-stream aetivity/selectivity and product distribution in the aromatization of C2+ hydrocarbons from natural gas over Ga H-ZSM-5 and H-GaAlMFI zeolites (having same bulk composition) at 600 C have been investigated. The zeolites were characterized for their FW-Si/Al and Si/Ga ratios and non-FW Ga by Si MAS NMR and bulk chemical analysis, for crystal size and morphology by SEM and also for their acidity by the chemisorption of pyridine at 400 C. The H-GaAlMFI zeolite shows higher activity and aromatics selectivity and slower deactivation due to coking in the aromatization of C2+ hydrocarbons from natural gas. 

The selective hydrogenation of heterocyclic aromatic compounds is possible using strong acids. In a neutral medium pyridine is hydrogenated faster than benzene as its chemisorption is easier. In an acidic medium, the chemisorption of pyridine is greatly disfavored by protonation... 

The probe molecule pyridine has often been used more recently in the study of clay surface acidity by IR methods. Figure 7 shows data of pyridine chemisorption on a synthetic mica-montmorillonite catalyst as an example (62,63) of the types of bands of interest. The spectra are interpreted as showing chemisorption of pyridine at both protic and aprotic sites. The clay was first heated for 15 h at 650°C under vacuum and then cooled and spectrum A taken. Note that there are no bands in the 1400-1700 cm region, the residual hydroxyl near 3450 cm S and edge silanol hydroxyl at 3747 cm . Pyridine vapor was then chemisorbed and spectrum B taken. The bands at 1456 cm and 1547 cm are assigned to Lewis and Bronsted sites, respectively. Since the 3747 cm edge silanol band decreases, it is assumed that these protons are involved in the mechanism. Lewis sites predominated under these particular conditions. 

The pH-independence of Tdhpz at Pt indicates that the driving force for coordination of the nitrogen heteroatom to the Pt surface is much larger than that for protonation even in molar acid. This behavior is in contrast to that of pyridine, where protonation of the nitrogen heteroatom in molar acid hinders N-coordination to the surface (H). Such a difference in chemisorption characteristics may be related to the fact that the basicity of the nitrogen heteroatom in pyridine (pKb =8.8) is much greater than that of the nitrogens in pyridazine (pKb = 11.8) (23.). ... 

Butanol, reaction over reduced nickel oxide catalysts, 35 357-359 effect of ammonia, 35 343 effect of hydrogen, 35 345 effect of pyridine, 35 344 effect of sodium, 35 342, 351 effect of temperature, 35 339 over nickel-Kieselguhr, 35 348 over supported nickel catalysts, 35 350 Butanone, hydrogenation of, 25 103 Butene, 33 22, 104-128, 131, 135 adsorption on zinc oxide, 22 42-45 by butyl alcohol dehydration, 41 348 chemisorption, 27 285 dehydrogenation, 27 191 isomerization, 27 124, 31 122-123, 32 305-308, 311-313, 41 187, 188 isomerization of, 22 45, 46 isomers... 

We begin by comparing fluxes of the permeant molecule pyridine in untreated and thiol-treated nanotubule membranes. An untreated membrane that contained tubules with diameters of approximately 2.6 nm showed a pyridine flux of 1.8 X 10 mol cm hr. After chemisorption of the R = -C2H4-OH thiol the flux increased to 4.2 X 10 mol cm hr. In contrast, after chemisorption of the R = -C16H33 thiol, the pyridine flux dropped to 2.7 X 10 mol cm hr. These data clearly show that thiol chemisorption has a dramatic effect on permeant flux in these nanotubule membranes. 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

Br0nsted acid sites depends on the structure of the amine. Chemisorption data for amorphous oxides (54) show that 2,6-dimethylpyridine (which contains methyl groups that hinder coordination of the nitrogen atom with Lewis acids) is a more selective reagent for the determination of Br0nsted acidity than an unhindered amine such as pyridine. Jacobs and Heylen (44) arrived at a similar conclusion on the basis of an infrared study of amines chemisorbed on Y zeolite. They also found that the poisoning effectiveness of 2,6-dimethylpyridine is much greater than that of pyridine for the catalytic titration of Y zeolite. 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

It is concluded from the foregoing considerations that pyridine may successfully be applied as a specific poison, provided the possible pitfalls are carefully kept in mind. The lower basicity of pyridine as compared to ammonia renders its chemisorption more selective. However, its basicity is in most cases still much higher than that of the commonly used reactants, so that one is usually able to determine an upper limit for the number of active sites by pyridine poisoning (239). On the other hand, the hardness of reactants or reaction products may be comparable with that of pyridine [e.g., dehydration of alcohols (47)] the poison will then be partially displaced. The molecular size of pyridine may bring about difficulties, since it restricts the accessibility of pyridine to narrow pores or even the approach to an adsorption site (214). In favorable cases, however, steric effects may be utilized to improve the specificity of poisoning (35, 36,241). 

Nitric oxide is rapidly desorbed on evacuation. The number of adsorption sites for this reaction was found to increase from 1.5/100 A2 after pretreatment at 25°C to only 2.3/100 A2 after pretreatment at 800°C for afi-Al203 (Degussa). These values are about an order of magnitude higher than those obtained by pyridine adsorption on the same type of alumina (121) and by NH3 adsorption on a y-Al203 (168). The number of a sites as determined by C02 adsorption by Peri (157) is still lower. The chemisorption of N02, therefore, seems to be less specific than that of pyridine, NH3, and C02. The weak absorptions at wave numbers above 2000 cm-1 were tentatively assigned by Parkyns to N02+ species. These may be comparable to the species found in zeolites by Naccache and Ben Taarit (242). 

The N03" species are fairly strongly held and can only be completely removed from aluminas on evacuation at 500°C. Thus, N02 is very specifically held by Lewis acid sites on surfaces that are evacuated at sufficiently high temperatures. Although the specificity of the N02 chemisorption seems to be less pronounced, for example, than that of pyridine, it might be a useful poison for certain catalytic studies. Unfortunately, no attempts of this kind have yet been undertaken. 

It is interesting to note that the cis-trans ratios obtained by Ghorbel and coworkers (312) on 1-butene isomerization were mostly unaffected by the chemisorption of the different poisons and corresponded to the thermodynamically determined ratio. The same result was obtained by Knozinger and Aounallah (377), when 1-butene was isomerized in a recirculating reactor at 120°C on tj-A1203 (activated at 500°C) that was partially poisoned by pyridine. In micro-catalytic pulse experiments, on the other hand, one observes kinetically determined primary product distributions with cis-trans ratios of about 2 (370). Different types of active sites appear, therefore, to be involved in the doublebond shift and in the cis-trans isomerization, the latter remaining unaffected by pyridine chemisorption. 

The problem with sulfide catalysts (hydrotreatment) is to determine the active centres, which represent only part of their total surface area. Chemisorption of O2, CO and NO is used, and some attempts concern NIL, pyridine and thiophene. Static volumetric methods or dynamic methods (pulse or frontal mode) may be used, but the techniques do not seem yet reliable, due to the possible modification (oxidation) of the surface or subsurface regions by O2 or NO probe molecules or the kinetics of adsorption. CO might be more promising. Infrared spectroscopy, especially FTIR seems necessary to characterise co-ordinativcly unsaturated sites, which are essential for catalytic activity. CO and NO can also be used to identify the chemical nature of sites (sulfided, partially reduced or reduced sites). For such... 

Oare earth forms of zeolites X and Y type faujasites possess superior catalytic properties for various reactions such as alkylation, isomerization, and cracking (9, 12, 18). Structural studies involving x-ray diffraction and CO chemisorption have been made to locate the positions of the rare earth (11, 14, 16). Hydroxyl groups and their relationship to surface acidity have been studied by infrared spectroscopy, utilizing the adsorption of pyridine and other basic molecules (2, 6, 21, 22, 23). Since much of the previous research has involved measurements on mixed rare earth faujasites, a need existed for a more systematic study of the individual rare earth zeolites, in regard to both structural and catalytic properties. The present investigation deals with the Y, La, Ce, Pr, Sm,... 

It should be noted that whenever equation (27) applies and dehydroxylation happens solely through the Uytterhoven step, assuming the stoechiometry of pyridine chemisorption is one molecule adsorbed per either Brbnsted or Lewis acid site, then the N/Al ratio must be equal to one. 

Conversely, it can be seen on Fig. 1, that chemisorption of thiophenol (C6H5SH) at room temperature does not involve ail the surface Ni atoms. This behavior has already been obsen/ed for another similar molecule, benzene the fraction of occupied Ni surface atoms at room temperature was ca. 0.65 over a 4.2 % Al Ni Raney pretreated in vacuo at 250°C (ref. 13). This kind of study has been extended to a series of poisons such as thiophene (ref. 14), ammonia (ref. 15), piperidine, pyridine and 3.5 dimethyl piperidine (ref. 16). which also lead to a non-corrosive chemisorption, involving only a fraction of surface atoms. It has also been shown (ref. 14) that after poison saturation, the free Ni atoms are accessible to H2 chemisorption but not to large molecules such as benzene. In contrast, COS and CS2 chemisorptions at room temperature are corrosive as for the case for H2S chemisorption (ref. 14). 

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... 

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