Pyridine, adsorption correlation

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]. 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

Since silica-alumina contains Br nsted as well as Lewis acid sites, a clear correlation between rates of a heterogeneously catalyzed reaction and surface acidity as measured by pyridine adsorption is only possible if a distinction between PyH+ and PyL is made. This is possible by infrared spectroscopy as shown in this section. Thus, Ward and Hansford (226) found a good linear correlation between the percent conversion of o-xylene and the Br nsted acidity of synthetic silica-alumina catalysts. This correlation is shown in Fig. 4, where the Br nsted acidity is expressed as peak height of the band at 1545 cm-1 per unity of catalyst weight. 

Results suggest that the OH groups absorbing near 3630 cm" are primarily responsible for cracking activity. Hence, the decrease in their concentration caused by more severe calcination appears to account for the decrease in catalytic activity. The acidity or protonation ability of these groups, as measured by pyridine adsorption, would be expected to be the prime variable. Correlations of catalytic activity with Bronsted acidity have been reported previously (20, 21, 24, 25). Results from the... 

A. E. Hirschler (Sun Oil Co., Marcus Hook, Pa. 19061) I should like to make a comment relating to your statement that activity is not a simple function of acidity. This statement appears to contain the implicit assumption that all acid sites able to protonate pyridine would have the same catalytic activity. Actually, one might expect an intensity factor (acid strength) to be operative in acid catalysis as well as an extensive factor, and the apparent lack of correlation you observed may result from an influence of acid strength on catalytic activity. Pyridine adsorption data alone do not completely define Bronsted acidity. One may need also some measure of acid strength. Acidity measurement with Hr indicators, such as we and others have reported, may supply useful information relative to acid strength as well as number of acid sites. 

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

Pyridine adsorption shows a dramatic increase of Bronsted acid centers paralleled by the decrease of Lewis acid centers (see upper spectrum of Fig. 2). This is in agreement with the observation of the growing amount of reduced Cu particles accompanied by a loss of Lewis acid sites. From our experiments a correlation between cluster size and the water and hydrogen partial pressure must be assumed. 

These methods suffer from the lack of complementarity, and thus the significance of results provided by any of them is limited. A standard practice to detect the Bronsted or Lewis character of surface sites is pyridine adsorption combined with FTIR measurements the number of Lewis or Bronsted sites is more difficult to count, however. Other titration methods use either color indicators and acid or base titrants in nonpolar solvents or the adsorption of gaseous acidic or basic probes. They do not, in general, give consistent quantitative information about the number of acid or base sites even when applied to the same sample. There are several reasons the applicability of titration methods is limited Either the state of the surface is different for different methods or adsorption equilibrium is not always achieved. Another more serious source of discrepancies between titration methods is that probe molecules of different basicities "see" different surface sites. The lack of a uniquely defined thermodynamic scale of acid strength of surface sites makes difficult any correlation between results obtained with different probe molecules. The use of standard catalytic tests for probing the so-called catalytic acidity is not always a better approach, because the mechanistic assumptions involved are neither straightforward nor subject to experimental proof. 

The results of a very recent study by Attia [3], summarized in Fig. 1, are a good illustration. Carbons of varying surface area were produced by carbonization and activation of date pits in steam, air, or carbon dioxide (at different temperatures and to different extents of burnoff). The correlations of water vapor and pyridine uptakes with the total surface area are seen to be very poor. The author concludes that water vapour adsorption is related to the chemistry of the surface rather than to the extent of the surface area but does not identify, let alone quantify, this chemistry. Regarding pyridine adsorption, the author notes that activation of carbons at low temperatures created acidic sites while treatment at high temperatures led to the generation of basic sites on the surface, but she does not identify these sites. 

A very intense band between 3496 and 3530 cm is assigned to the existence of OH groups associated with RE cations. A shift in this band after pyridine adsorption (or lutidine) is observed, which has been attributed to an increase in the acid strength of the OH groups associated to rare-earth cations. Indeed, a linear correlation between the ionic radius of the rare-earth cation and the wavenumber of the band can be observed. 

Consumption of acid OH groups upon introduction of Mn cations led to a decrease in Brpnsted and a concomitant increase in Lewis acidity, as indicated by IR using pyridine as a probe. The decrease in Bronsted acidity was indicated by a decrease in the intensity of the pyridinium ion band at 1540 cm" observed upon pyridine adsorption on Mn-ZSM-5 samples that had been prepared by SSIE. A corresponding increase in the Lewis acidity effected an enhancement of the absorbance around 1450-1454 cm" which is indicative of pyridine coordinated to electron pair acceptors or Lewis sites such as Mn +. This was compared with similar IR measurements on the parent H-ZSM-5. Figure 51 shows a linear correlation between the density of Bronsted and Lewis acid sites of the parent H-ZSM-5 as well as of Mn,H-ZSM-5 samples measured by this technique. 

Finally, two sets of physical properties have been correlated by the Hammett equation. Sharpe and Walker have shown that changes in dipole moment are approximately linearly correlated with ct-values, and Snyder has recently correlated the free energies of adsorption of a series of substituted pyridines with u-values. All the reaction constants for the series discussed are summarized in Table V. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

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.]...

The band centered at 3615 cm disappears by adsorption of pyridine and further outgassing at 150°C, thus confirming its assignment to stretching vibration of acidic OH groups. For samples with a different Si/Al ratio, a direct correlation is observed between the intensity of this band and that of protonated pyridine at 1540 cm (Table I). 

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.

In order to obtain quantitative measurements of hydrogenation activity and acidity, various schemes are employed. For example, metal surface area has been related to hydrogenation activity and the adsorption of bases such as pyridine and ammonia have been correlated with acidity ((3). Some authors have used certain key reactions involving pure compounds as an indication of catalytic properties (16). Each of these methods is useful however, because of the complex interdependence of the catalytic functions of the hydrocracking catalysts and changes in these functions with catalyst aging, results from each method must be interpreted with caution. 

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