Bronsted acidity site concentration measurement

Figure 4.13 Dependence of the concentration of Bronsted acid sites (as measured by the intensity of the band at 1540 cm r) on the catalyst preparation for supported boron trifluoride...

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

This has been attributed to the use of ammonia as a probe molecule, since this cannot distinguish between Lewis and Bronsted acidity. Cornaglia and coworkers [79] measured the acid sites using pyridine and acetonitrile. However, the pyridine results showed no correlahon between the activity and selectivity to maleic anhydride and either the Lewis-to-Bronsted acid sites ratio (L/B acidity ratio) or the Lewis acid site concentration. 

Figure 3. Yvp vs. concentration of Bronsted acid sites (measured at 1S0°C) of all the catalysts tested.

One can thus conclude that the greater is the concentration of surface Bronsted add sites, the higher should be the catalytic activity. However, when calculating the specific activity, i.e. the catalytic activity per Bronsted acid site (Fig.4a), some specific features of each catalyst can be evidenced. For instance, every site of F34 appears very active for HEP dehydration, in spite of the low overall activity, which is due only to the low concentration of surface acid sites. Other catalysts, such as HBETA 20, show a low specific activity, but a rather good overall performance, possessing a relatively high number of Bronsted acid sites. Of course, also the strength of the acid sites could play a role. In order to evaluate this parameter, for each catalyst we calculated the ratio between the concentration of Bronsted acid sites measured after... 

In the light of these remarks, it becomes quite clear that quantitative spectroscopic methods for determining the Bronsted acidity are a necessary prerequisite for the characterization and understanding of solid catalysts. Quantitative determination of Bronsted acidity, however, means the measurement of (i) the concentration, (ii) the strength of acidity, and (iii) the accessibility of the Bronsted acid sites. 

Contrary to this behavior, various structures with high Si/Al ratio (H-ZSM-5, H-ZSM-12, HY and H-mordenite) were found to exhibit almost the same adsorption energies measured at 480 K, suggesting that the isolated Bronsted acid sites in these materials are identical [76]. Authors observed that the strength of Bronsted acid sites associated with framework A1 atoms does not vary in a given zeolite, and they suggest that Bronsted acid sites in unsteamed zeolites are independent of the sample (what means, independent of Si/ A1 ratio) and equal in concentration to the framework aluminum content [77]. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

According to the XRD pattern all samples are well crystallized and show the typical feature of the MFI structure. Its largely pure formation is confirmed by the results of n-hexane adsorption. The values of the micropore volume (at p/ps = 0.5) are fairly close to the theoretical ones calculated for an ideal MFI-structure (0.19 cm /g, see Table 1). Table 1 gives the Si/Me ratios of the fnunework as further characteristic data. An equal concentration of Me in the lattice have been strived for. However, the results of the chemically determined Me concentration and the ammonium ion exchange capacity disagree especially for the In-Sil. It is less pronounced for Fe-Sil. Therefore the creation of extra-framework species in In-Sil and Fe-Sil has to be considered which do not contribute to the Bronsted acidity but to other kinds of acidic sites. This is in agreement with the results of the TPD measurements. 

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