Pyridine poisoning

Fig. 3. Correlation of the slopes p for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning (41), and deuterium kinetic isotope effects (68). [Reprinted with permission from Berdnek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]...

NICOTINE 1-Methyl-2-(3-pyrldyl) pyrrolidine, J-(l-Methyl-2-pyrrolldyl) pyridine Poisonous, Class B, II 4 1 0... 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

Fig. 5. Correlation of the Taft reaction parameter for the dehydration of secondary alcohols (see Table 4) on four different oxide catalysts with the heat of adsorption, A//ads> °f water and diethylether, with the sensitivity of the rate to pyridine poisoning 7> [55] and with the value of the deuterium kinetic isotope effect [123] for the same catalysts.

The plot of at versus poison compounds loading Is shown In Figure 3. It shows that ot is linearly dependent on the pyridine loading and independent of the cracking temperature. The parameter, at, can be expressed for pyridine poisoning as follows ... 

On the basis of the poisoning studies, the number of active sites of the catalyst were 1.63 x 10 per gram obtained from pyridine poisoning and cumene cracking reaction at 300°C. This number is close to the number reported by Jacobs and Heylen (13) in the study of poisoning with 2,6-methylpyridine of cumene cracking activity of the HY zeolites. 

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). 

FIG. 6. Adsorption of isobutanol on pyridine-poisoned 6-AI2O3. Integrated band areas (I.b.a., arbitrary units) of bands at 2870 and 1567 cm-1 as a function of pyridine coverage. [Reproduced with permission from Knozinger and Stolz (47).]... 

The coke was removed predominantly from pores in the range of 4-12 nm, resulting in a bimodal pore size distribution and an increase in the pore volume and surface area. The amount of coke removed depended on the extraction temperature, pressure and duration. In the most severe extraction conditions, the silica foulant of the catalyst could also be removed as fine particles. Pyridine poisoned the catalyst during extraction, however its removal by acetone wash could restore the catalyst activity. 

Pd-C, H2-balloon, Pyr (0.5 eq.), 24 h. The use of pyridine poisoned catalyst allows for the hydrogenation of benzyl ether in the presence of a phenolic PMB ether. Good selectivity is also obtained for the dimethyl and trimethyl-... 

From pyridine poisoning experiments and FT/IR measurements it can be concluded that this increased activity stems from the increased acidity of the resulting NiSMM (Ni-synthetic mica montmorillonite) clay, due to reduction of Ni. 

With the aid of selective pyridine-poisoning experiments, we will show that isomerization of alkanes over NiSMM is a bifunction-ally catalyzed reaction. 

The relative distribution of para + meta aromatics in methanol conversion was increased over Mo exchanged ZSM-5 but not on the pyridine poisoned sample. The same increase trend was also observed in the disproportionation reaction over Mo exchanged zeolites (Fig. 3), thus a reasonable explanation is the presence of internal Mo. Both methanol and toluene conversions performed on zeolites are molecular shape selective processes. Internal Mo will create diffusional hindrances which will favour the formation of para aromatics (product selectivity). 

Pyridine poisoning at 500°C has also been used to characterize... 

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