A new method of theoretical analysis for temperature-programmed desorption (TPD) of ammonia to determine the acid amount and strength and its distribution from a one-time experiment is proposed on the basis of the equilibrium between gaseous and adsorbed ammonia, i.e., free readsorption of ammonia. The entropy change was assumed to consist of the constant phase-transformation term and the gaseous mixing term as a function of gaseous concentration of ammonia. The enthalpy change, namely adsorption heat, was assumed to have several kilojoules per mole of the distribution. Thus a simulated TPD curve could be fitted well with the experimental data observed on mordenite and ZSM-5 zeolites. From the parameter set that gave the best fitted curve, the acidic properties of zeolite were determined. The determined acid amount was close to the difference between the aluminum and sodium contents, [Al] − [Na], in most cases. This confirms a simple principle that one acid site is generated by isomorphous substitution of one aluminum atom into the silicate matrix, and one sodium atom blocks one acid site. On the other hand, the mordenite and ZSM-5 had the acid strength, ca. 145 and 130 kJ mol-1, of the adsorption heat of ammonia, respectively, with several kilojoules per mole of the distribution, irrespective of the acid amount. Another simple principle is therefore proposed:  the acid strength of zeolite is determined by the crystal structure.

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were used to investigate the local structure of Pd supported on ZSM-5, which was affected by the acid sites of support and the adsorption of nitrogen oxide. After the thermal treatment of the initially ion-exchanged Pd amine complex, the formation of metal Pd particles was found, whose size was larger than zeolite pore. The subsequent oxidation led to the disruption of Pd particles and the formation of the dispersed PdO, where the degree of dispersion was dependent on the acid amount of ZSM-5. It was also found on Pd/HZSM-5 (Si/Al 2 ) 24) that the formation of metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. These facts prove the presence of strong interaction between acid sites and PdO. The role of acid sites of zeolite was considered to keep the dispersed state of PdO. The adsorption of NO on highly dispersed PdO induced a significant change in the local structure of Pd at room temperature. At the same time, PdO was reduced to Pd I upon the adsorption of NO. In contrast to Pd/HZSM-5, highly aggregated PdO was found in Pd/NaZSM-5. The deactivation of Pd/HZSM-5 for selective NO-CH 4 -O 2 reaction due to the presence of H 2 O vapor was considered to be caused by sintering of PdO.

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The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm -2 , and the adsorption heat of ammonia was ca. 200 kJ mol -1 . This corresponds to -19 of the H 0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm -2 ), an adsorption heat of ca. 160 kJ mol -1 , an H 0 of -12, and Brønsted nature. The former was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butane in gas phase.

As an index of acid strength, ammonia adsorption energies (E ads ) were calculated with density functional theory on cluster models of Brønsted acid sites belonging to FAU, BEA, MFI, FER, MWW, and MOR structures, which were selected because of the availability of experimental data and industrial importance.The calculated E ads were reasonably consistent with experimental results from the ammonia IRMS-TPD (infrared mass spectroscopy-temperature-programmed desorption) method. The calculated value was slightly (10-20 kJ mol -1 ) lower than the observed value, and its change with varying structure was approximately in agreement with the experiments. A thorough study was carried out to find the geometric parameters of the zeolite clusters (in the H and NH 4 forms) relevant to E ads and to discuss parameters controlling the acidic property. Hydrogen bonding interactions between ammonium cations and neighboring zeolitic oxygens were found to affect E ads observed in small cavities. When NH 4 + was stabilized in relatively open spaces (large cavities), acid strength was controlled by the local geometry of the Brønsted acid site, indicating a contribution of strain around Si(OH)Al to acid strength. In these cases, a shorter Al-O distance (a) gave a higher E ads . This is consistent with the explanation that Lewis acidic Al withdraws the electron charge of the SiOH contributing to Brønsted acid strength. A relationship was found between a and the distance (b) and planar angle (ω) between two triangles consisting of three oxygens each, which surrounded the Si(OH)Al unit, and finally, a relationship was found in which a smaller b and ω brought a higher E ads . The strain (compression) on atoms surrounding the Si(OH)Al unit is reflected in the extent of b and ω, and this contributes to vary Brønsted acid strength.

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Using an IRMS-TPD (temperature programmed desorption) of ammonia, we studied the nature, strength, crystallographic location, and distribution of acid sites of mordenite. In this method, infrared spectroscopy (IR) and mass spectroscopy (MS) work together to follow the thermal behavior of adsorbed and desorbed ammonia, respectively; therefore, adsorbed species were identified, and their thermal behavior was directly connected with the desorption of ammonia during an elevation of temperature. IR-measured TPD of the NH4(+) cation was similar to MS-measured TPD, thus showing the nature of Brønsted acidity. From the behavior of OH bands, it was found that the Brønsted acid sites consisted of two kinds of OH bands at high and low wavenumbers, ascribable to OH bands situated on 12- and 8-member rings (MR) of mordenite structure, respectively. The amount and strength of these Brønsted hydroxyls were measured quantitatively based on a theoretical equation using a curve fitting method. Up to ca. 30% of the exchange degree, NH4(+) was exchanged with Na+ on the 12-MR to arrive at saturation; therefore, in this region, the Brønsted acid site was situated on the large pore of 12-MR. The NH4(+) cation was then exchanged with Na+ on 8-MR, and finally exceeded the amount on 12-MR. In the 99% NH4-mordenite, Brønsted acid sites were located predominantly on the 8-MR more than on the 12-MR. Irrespective of the NH4(+) exchange degree, the strengths deltaH of Brønsted OH were 145 and 153 kJ mol(-1) on the 12- and 8-MR, respectively; that is, the strength of Brønsted acid site on the 8-MR was larger than that on the 12-MR. A density functional theory (DFT) calculation supported the difference in the strengths of the acid sites. Catalytic cracking activity of the Brønsted acid sites on the 8-MR declined rapidly, while that on the 12-MR was remarkably kept. The difference in strength and/or steric capacity may cause such a difference in the life of a catalyst.

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