Adsorption microcalorimetry pyridine

Bulk boron oxide was found to be much more acidic than basic [168], When SO2 adsorption microcalorimetry was used, no basic sites were observed, but some phy-sisorption occurred. Ammonia and pyridine adsorption microcalorimetry were used to characterize the acidity of B2O3. Boron oxide displays an initial heat for NH3 adsorption of 80 kJ/mol and can adsorb irreversibly a large amount of ammonia. The number of active sites determined by pyridine adsorption and the corresponding integral heats were found to be much lower than those determined by using ammonia. 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The acid properties of nondealuminated and dealuminated commercial HY were also determined by Colon et al. [231] and Ferino et al. [58] using pyridine adsorption microcalorimetry at 150°C or by Biaglow et al. [165] and Chen et al. [232] using NHj adsorption microcalorimetry at 150°C. 

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 acid-base properties of V2O5/7-AI2O3 catalysts prepared by the impregnation method have been characterized by ammonia, pyridine and sulfur dioxide adsorption microcalorimetry. Sulfur dioxide adsorption made it possible to differentiate a vanadate layer from free alumina. 

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. 

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. 

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. 

The most promising approach to this problem is the use of suitable probe molecules for the quantitative characterization of site density and strength by means of adsorption microcalorimetry. The best-known appUcations of chemisorption involve the use of bases such as NH3 or pyridine to probe the acidity of zeoUtes. Moreover, it is well known that adsorption influences all phenomena depending on surface properties, since it constitutes the primary step for every catalytic reaction involving sohd catalysts. Adsorption is generally exothermic (AH < 0) and the heat evolved is called heat of adsorption. This heat is related to the ability of the sites to interact with the probe molecule, i.e. to their basic or acidic character. 

The effect of high temperature calcination in air at 1008 K on H-mordenite (Si/Al = 13) has been studied by Chen et al. [55] using adsorption microcalorimetry of pyridine at 473 K. This treatment caused a significant reduction in the total number and in the strength of the add sites. The plateau characteristic of a large number of sites of uniform strength (near 200 kJ mol ), observed on the sample pretreated at 673 K, also disappeared. High-temperature caldnation is known to induce dealiunination and de-hydroxylation, both of which are expected to reduce the munber of add sites [55]. 

Finally, the acidity of commercial mordenites has also been studied using pyridine adsorption microcalorimetry by Ferino et al. [28]. 

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]. 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

Another way to characterize acidity is to study the differential heat of adsorption of a basic probe compound, such as ammonia or pyridine, by microcalorimetry as a function of uptake. This technique yields the distribution of acid strength relative to coverage, but unfortunately does not differentiate between Br0nsted and Lewis... 

Adsorption and wetting Adsorption of gases and vapors, combined with IR spectroscopy, microcalorimetry, and so forth Temperature-programmed desorption of absorbed substances (ammonia, pyridine, and so forth, coupled with mass spectrometry and IR spectroscopy Heats of adsorption, specific surface area, pore size distribution, average pore diameter, and fractal dimension Acidic functional groups, relative acid strength ... 

Lee et al. [174] have used microcalorimetry to measme the differential heats of adsorption of both a series of alkylamines and a series of substituted pyridines in H-ZSM-5. With few exceptions, the differential heats were approximately constant up to coverages close to the expected BrOnsted site concentration. The authors observed a strong correlation between gas-phase proton affinities and differential heats of adsorption of amine and pyridine bases but no useful correlation with aqueous base strengths, this contrast implying that the Hammett Ho value is probably not a meaningful description of zeohte acid strength. 

Three samples of H-ZSM-5 prepared with varying aluminum content and by different synthesis methods were examined by Parrillo et al. [50] using microcalorimetry of ammonia and pyridine adsorption. Two samples with bulk Si/Al ratios equal to 34 and 58 were prepared employing a TPA-Br template in hydrothermal solution, the third sample (Si/Al 26) was synthesized without a template. The results showed that the heats of adsorption for both ammonia and pyridine were constant with values of 145 kj mol for ammonia and 200 5 kJ mor for pyridine up to a coverage of one molecule per framework Al, and were, in fact, independent of the Si/Al ratio. From these results, the authors suggested that the Brbnsted acid sites in unsteamed H-ZSM-5 are independent of the sample and equal in concentration to the framework aluminum content. 

In another study, three H-MCM-22 zeolite samples with total Si/Al ratios of 10,14, and 30, calcined at 823 K, were characterized by microcalorimetry of pyridine adsorption at 423 K [223]. These experiments showed the presence of very strong add sites, particularly on the most dealuminated sample (initial heat of 230 kJ moH) and a significant increase of the add strength with the Si/Al ratio of the zeohte. 

Mitani at al., in their work related to the investigation on the acidity of faujasites with different framework aluminium content, reported that the extent of the decrease in acid sites concentration varies with the kind of basic probe used in microcalorimetry experiments (see Fig. 9.6) [92]. The experimental results were explained by differences in molecular diameters of ammonia and pyridine, and the fact that pore size of zeolite becomes widely distributed as the dealumination proceeds so OH groups in supercages have become available for adsorption of such large molecule as pyridine is. 

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