Ammonia adsorption microcalorimetry

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 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. 

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... 

Calorimetric measurements of adsorption of CO2 at 303 K on different titania samples have provided evidence of their surface heterogeneity, as expected for oxides, with heats of adsorption ranging from -100 to 30 kj moT. Acidity measurements by ammonia adsorption microcalorimetry on the same samples gave rise to adsorption heats ranging between 150 and 60 kJ moT [47]. 

The acidity of mesoporous expanded clay catalysts has been studied using ammonia adsorption microcalorimetry [110]. The inclusion of Si02-Ti02 clusters... 

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]. 

The acidity and strength distribution of acid sites of a ZSM-5 prepared by hydrothermal synthesis, and zinc and gallium incorporated ZSM-5 catalysts prepared by incipient wet impregnation were compared to those of homologues prepared by a co-synthesis method. Ammonia adsorption microcalorimetry showed that samples prepared by co-synthesis exhibited lower acidity values when compared to the others [191]. 

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]. 

Fig. 22 Scale of acid strength of various zeolites as determined by ammonia adsorption microcalorimetry at 423 K (from [296])...

C.P. Nicolaides, H.H. Kung, N.P. Makgoba, N.P. Sincadu, M.S. Scurrell et til.. Characterization by ammonia adsorption microcalorimetry of substantially timorphous or partially crystalline ZSM-5 materials and correlation with cattilytic activity. Appl. Catal. A Gen. 223,29-33 (2002)... 

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. 

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. 

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 effect of temperature on ammonia adsorption by ZSM5 samples has been investigated by microcalorimetry, varying the adsorption temperature from 150 to 400°C [235]. The initial heats of adsorption were independent of temperature up to 300°C. When the adsorption temperature increased, there was a competition between the formation of ammonium ions on Brpnsted sites and their decomposition. The total number of titrated sites decreased with increasing adsorption temperature. It appeared that an adsorption temperature between 150 and 300°C is appropriate for these calorimetric experiments. 

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]. 

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 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. 

Ammonia accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry... 

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. 

The passivation by oxygen of a commercial ammonia synthesis catalyst was studied with adsorption microcalorimetry by Tsarev and co-workers (240). Two types of adsorbed oxygen at 293 K were found to participate in the formation of a passivating layer. One type was characterized by differential heats of adsorption near 420 kJ mol" that were close to the heat of iron oxidation and which were independent of surface coverage for several mono-layers. The other form was obtained after a large dose, sufficient for coverage of the entire metal surface with a molecular monolayer. Subsequent adsorp-... 

---