Zeolites adsorption microcalorimetry

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

NHj adsorption microcalorimetry was used by Shannon et al. [225] to follow the changes in acid sites of a H Y zeolite during dehydroxylation, framework dealumina-tion, and the formation of nonframework aluminum species. 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

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]. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Auroux et al. [251] used adsorption microcalorimetry of different alkanes to investigate Ga and Al substituted MFI zeolites used as catalysts in dehydrogenation and cracking reactions. 

In this chapter, a brief summary of studies that made use of calorimetry to characterize compounds comprising group IIIA elements (zeolites, nitrides, and oxides catalysts) was presented. It was demonstrated that adsorption microcalorimetry can be used as an efficient technique to characterize the acid-base strength of different types of materials and to provide information consistent with the catalytic data. 

Flow adsorption microcalorimetry has been used to measure the heats of adsorption of ammonia in a nitrogen carrier on the H and Na forms of a Y zeolite [21]. The calorimeter was linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure. The authors found that, as surface coverage increased, the sites covered first were not necessarily those with the highest molar heats of adsorption. 

The aim of this paper is therefore to check the consistency of three basic techniques (adsorption manometry, adsorption gravimetry and adsorption microcalorimetry) in the 0-50 bar pressure range, using a standard NaX zeolite adsorbent and selecting three adsorbable gases, namely ... 

The effect of adsorption temperature on metals or supported metals on the mobility of adsorbed probe molecules has not received as much attention as on metal oxides. Gelin and co-workers (96) used adsorption microcalorimetry at 296 and 423 K and IR spectroscopy to study the adsorption of CO on Ir supported on NaY zeolite reduced from 383 to 923 K and on Ir supported on silica. At 296 K it was observed that for intermediate coverages (6 > 0.3) the kinetics of adsorption changed, with the thermograms displaying long tails... 

The acid-base properties of the decationated HY zeolites have been extensively studied with adsorption microcalorimetry. Tables II and III present a summary of calorimetric studies of the adsorption of ammonia and other probe molecules on HY zeolites with different Si/AI ratios, preparation methods, pretreatments, adsorption temperatures, and sodium contents. The large variety of conditions used in these studies complicates the comparison of the materials. For example, the initial differential heat of ammonia adsorption at... 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

A prior study of the adsorption of nitrogen at 77 K on 5A and 13X zeolites using quasiequilibrium, isothermal, adsorption microcalorimetry experiments at 77K [16] has detected a step in the differential enthalpies of adsorption, towards the end of micropore filling. At the time, this was interpreted as a consequence either of the adsorbate-adsorbate interactions, or... 

Groszek [56, 57] studied the adsorption of simple gases (COj, CH4, SOj, O2, He, and Nj) on microporous carbons using flow adsorption microcalorimetry. Shen and Biilow [58] demonstrated that the isosteric adsorption technique (Eqns (3.11) or (3.31)) is a useful and effective tool to obtain highly accurate thermodynamic data for microporous adsorption systems like the heat of adsorption given by Eqn (3.47). They studied the adsorption of CO2 and N2—O2 mixtures on a super-activated, almost entirely microporous, carbon (M-30, from Osaka Gas) and three faujasite-type zeolites. They also estimated the energetic heterogeneity of the solids due to specific interactions between the adsorbate and the solid. 

P-16 - Gas adsorption microcalorimetry on zeolites under supercritical conditions up to 15 bars... 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

The number of studies in which adsorption microcalorimetry has been successfully applied to this end has increased in recent years, especially concerning the determination of the acidic function of molecular sieves, and extensive reviews of the systems investigated using this methodology have been published [1,5-14,19,78-81]. In particular, an extensive review [4] summarizes some of the most recently published results concerning applications of microcalorimetry to the study of the acid-base sites of zeolites and mesoporous materials. The efficiency of thermal analysis techniques for the characterization of the acid-base strength of zeolite materials is also discussed, as well as their ability to provide information consistent with catalytic data [4]. 

NHs adsorption microcalorimetry has been used to characterize the acid sites of a H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. Poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, or the distribution of cracked products during 2-methylpentane cracking. Thus, strong Lewis acid sites do not seem to play any important role in cracking reactions [148]. 

The acid-base properties of zeolites or oxides are often studied by measuring the selectivities to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined by ammonia adsorption. A relationship has been found between the strength of the acid sites of bulk oxides, as determined by ammonia adsorption microcalorimetry [95], and the activation energy of dehydration, while the activation energy of dehydrogenation was independent of the strength of the sites [149]. 

Llewellyn, P.L. and Maurin, G. 2005. Gas adsorption microcalorimetry and modelling to characterize zeolites and related materials. C. R. Chim. 8 283-302. 

Abstract We review the use of adsorption microcalorimetry for the determination of the surface acidity and basicity of various types of zeolites and related materials, as well as the relationship between the information gathered by this technique and catalytic activity. 

Another interesting comparison of various thermal analysis techniques, namely adsorption microcalorimetry, thermoprogrammed desorption, and thermoprogrammed reaction using constant rate thermal analysis (CRTA), has been performed by Fesenko et al. in order to study the reactivity of zeolites in terms of the adsorption or desorption of base probe molecules [24]. As an example, CRTA was applied to the desorption of isopropylamine from Na-Y zeolite and its acidic form HY. 

The reaction of NH3 with various kinds of acidic centers on Y zeolites (Si/Al = 2.4) at 573 K was also studied by Kapustin et al. [101] by adsorption microcalorimetry. It was determined that the heats of adsorption of ammonia on the Bronsted acid centers of Y zeolites were 110-90 kj moh. These values were closer to those obtained by Tsutsumi et al. [102,103], and can also be compared with those of Auroux et al. [23,104], Huang et al. [105], van San-ten [106], Stach et al. [107], and Lohse et al. [108], which can be found in Table 2. 

Solid basic catalysts such as K-X zeolite and Cs-loaded K-X zeolite have been studied by CO2 adsorption microcalorimetry [138]. Carbon dioxide adsorbed on K-X with a Affads of about 70 kj mol up to a loading of about 80 xmolg The addition of cesium to K-X zeolite increased both the heat of adsorption (to more than 100 kJ mor ) and the maximum CO2 uptake. Nonetheless, the heats of carbon dioxide adsorption on Cs/K-X zeolites remained 50 kJ mol lower than on, for example, MgO or CS/AI2O3. The stronger basicity of Cs/K-X compared to K-X can be attributed to the occluded alkali metal oxide species, which may be either cesium or potassium oxide. The values of AHads on Cs/K-X samples were similar to those reported previously for Cs-loaded into Cs-X using analogous synthesis procedures [139]. 

A calorimetric and IR study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [192]. The active sites for both N2O and CO are Cu(I) ions, which are present as a result of the pre-treatment in vacuum at 673 K. The measured amounts of chemisorbed species in the investigated systems and the values of differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mol ) and carbon monoxide (between 140 and 40 kJ mor ) demonstrate the dependence of the adsorption properties on the copper content. The samples were additionally characterized by ammonia adsorption microcalorimetry at 423 K [192]. 

A calorimetric and spectroscopic study of the coordinative unsaturation of copper(I) and silver(I) cations in ZSM-5 zeolite has been performed by Bolis et al. [193] using adsorption of NH3 at room temperature. Adsorption microcalorimetry made it possible to characterize both quantitatively... 

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]. 

Ga- and Fe-substituted MFl zeolites have been investigated using adsorption microcalorimetry of different alkanes at 353 K by Auroux et al. [244]. The acid strength of the zeolite protons decreased following the sequence H-[A1]MF1 >H-[Ga]MFl >H-[Fe]MFl. The heats of adsorption decreased with the basicity of the alkane in the order n-butane > isobutane > propane. 

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