Adsorption microcalorimetry acid sites strength

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

Experiments of NH3 adsorption microcalorimetry, together with FTIR results from pyridine thermodesorption, have shown that the isomorphous substitution of A1 by Ga in various zeolite frameworks (offretite, faujasite, beta) leads to reduced acid site strength, density, and distribution [236-239]. To a lesser extent, a similar behavior has also been observed in the case of a MFI framework [240,241]. 

H-ZSM-5 zeolite modified by phosphorus was studied by means of adsorption microcalorimetry of ammonia for acidity characterization [255]. It was foimd that phosphorus neutralizes acidic sites primarily at the entrance of the channels of the zeolite particles. However, the strongest acid sites remained immodified, which suggested that the aluminum distribution and consequently the distribution of acid site strengths along the zeolite channels was heterogeneous. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The determination of acidity in FCCs from adsorption microcalorimetry of probe molecules was the object of a review article by Shen and Auroux [105], Adsorption microcalorimetry results obtained using anunonia as a probe molecule revealed that, as long as Lewis acid sites with strength greater than 100 kJ/mol are present and as long as these sites are available to gas oil, FCCs can retain their useful cracking activity and selectivity properties [221],... 

It was proven that microcalorimetry technique is quite well developed and very useful in providing information on the strength and distribution of acidic and basic sites of catalysts. When interpreting calorimetric data, caution needs to be exercised. In general, one must be careful to determine if the experiments are conducted under such conditions that equilibration between the probe molecules and the adsorption sites can be attained. By itself, calorimetry only provides heats of interaction. It does not provide any information about the molecular nature of the species involved. Therefore, other complementary techniques should be used to help interpreting the calorimetric data. For example, IR spectroscopy needs to be used to determine whether a basic probe molecule adsorbs on a Brpnsted or Lewis acid site. 

Increasing the pre-treatment temperature modifies the surface acidity of the solids. For y-alumina, there are numerous surface models, and various acid sites having different strengths are formed on the surface during dehydration. The influence of the pre-treatment temperature, between 573 and 1073 K, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry. The number and strength of the strong sites, which should be... 

Adsorption microcalorimetry is probably the most direct method for describing in detail both the quantitative and the energetic features of surface sites. The ability of the microcalorimetric technique to readily reveal subtle differences among samples is worth stressing. Even though one has to be cautious about the nature of the acidic and basic sites, calorimetric information on the concentration and strength of the sites can be used with confidence for interpreting the catalytic behavior. 

For most effective utilization in heterogeneous catalysis research, adsorption microcalorimetry must be used in combination with other techniques which probe the nature of the surface-adsorbed species. In the case of acidity studies, for example, IR spectroscopy is needed to identify which regions of the acid strength distribution correspond to Lewis verus Brpnsted acid sites. As the application of adsorption microcalorimetry in heterogeneous catalysis evolves from studies involving primarily probe molecules to studies involving more reactive molecules, it will become even more important to combine these calorimetric studies with surface spectroscopic investigations. 

Chemisorption of gaseous bases, e.g., ammonia or pyridine, followed by adsorption microcalorimetry, FTIR, and/or TPD, can determine the concentration, strength, and type of surface acid sites. 

Microcalorimetric NH3 adsorption is one of the powerful techniques for energetic characterization of solid surfaces and provides a direct and accurate method for the quantitative determination of the number of acid sites of different strengths. Microcalorimetry invplves the measurement of differential heats evolved updn adsorption of smeill quantities (micromoles) of basic probe molecules on to the catalysts. Such measurement yields information about the acid strength distribution i.e., the number of sites having the particular heat of adsorption for the basic probe molecule. 

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