Miki Niwa

Measurement of the acidity of various zeolites by temperature-programmed desorption of ammonia

The acidity of H-form zeolites which include mordenite, ZSM-5, Y-faujasite, and those modified by cation exchange or dealumination was measured by temperature-programmed desorption (TPD) of NH3. Ammonia was adsorbed at 373 K to eliminate the contribution of very weak acid sites and improve the spectrum resolution. TPD spectra of mordenites and ZSM-5 zeolites had two desorption peaks named l and h with peak maximum at around 420 and 680 K, respectively, while the spectra of Y-zeolites had a broad peak. From the peak maximum temperature of h-peak, the acid strength was found to be in the order of HM > HZSM-5 > HY. Acid amount was evaluated from the desorption amount of NH3, and correlated with the infrared intensity of the OH band at 3600 cm−1 and the ammonium band at 1447 cm−1, thus indicating that the amount as well as distribution of strength of Bronsted acid sites can be measured by the present method. Cation exchange poisoned the strong acid sites preferentially, whereas dealumination reduced the acid sites in the whole region.

Shape-selectivity over hzsm-5 modified by chemical vapor deposition of silicon alkoxide

Alkylation of toluene with methanol and toluene disproportionation have been carried out over HZSM-5 zeolites modified by chemical vapor deposition of silicon alkoxide. As the silica amount increased, the selectivity to m- and o-xylenes decreased, whereas that to p-xylene increased. The fraction of the para isomer in xylenes increased to more than 98%. From adsorption measurements and test reactions, it was found that this modification resulted in the narrowing of the pore-opening size and the inactivation of the external surface. The high para-selectivity was caused primarily by the narrowing of the pore-opening size.

Toluene combustion over palladium supported on various metal oxide supports

Abstract Metal–support interaction in the catalytic combustion of toluene was studied using metal oxides with different acid–base properties as supports for Pd. The catalytic performance was correlated with XPS data and the reaction order for oxygen. These studies revealed that the affinity for oxygen of Pd surface changed according to the acid–base character of metal oxide over MgO, Al2O3, SiO2, SnO2, Nb2O5, and WO3. However, ZrO2 exhibited exceptional character in that metal Pd was unusually stabilized, which was derived from the weak interaction between Pd and support surface. The reason for high toluene combustion activity of Pd/ZrO2 was ascribed to the stabilization of metal Pd on ZrO2.

Measurements of acidic property of zeolites by temperature programmed desorption of ammonia

The overall view of the TPD of ammonia to measure the acidic property of zeolites is described. The desorption peaks were identified and the significance of readsorption of ammonia was pointed out for the first time. This part of the work was done using reference catalysts of the Catalysis Society of Japan. The theoretical equation for the TPD with free readsorption of ammonia was then derived. Two methods for determining the strength of zeolite acidity based on the derived equation were proposed. A curve fitting method was then proposed to determine the zeolite acidity; based on this method, not only the strength of acidity but also its distribution could be determined. This method was applied to mordenite and ZSM-5 zeolites with different contents of Al and Na cations, and a simple conclusion was reached; namely, the strength of the acidity was not influenced by the number of acid sites but by the structure of the zeolite. Finally, water vapor treatment to rub out the l-peak (lower temperature peak) was briefly mentioned. This method was applied to precisely determine the acidity of Y-zeolite. A case study about the beta zeolite as the catalyst for the amination of phenol was exemplified; the catalytic activity was discussed in terms of the measured acidity.

COMPLETE OXIDATION OF METHANE ON SUPPORTED PALLADIUM CATALYST: SUPPORT EFFECT

Abstract A palladium loaded catalyst was examined in order to reveal the effect of the support on the activity of oxidation of methane. The activity was declined or enhanced with time on stream, and its behavior depended upon the kind of support. The change of activity was accompanied with sintering of palladium metal. The stabilized activity of the palladium was ordered in the sequence; on SiO2 > on Al2O3 > on SiO2-Al2O3. The turn-over frequency of methane oxidation was about 50 times greater than that of platinum supported catalysts, and the activity sequence of support was opposite to that of platinum. The effect of support on the activity of palladium was smaller than on platinum from the viewpoint of activation energy as well as the turn-over frequency. The palladium dispersed on the support was less active, but the large particle which was not interacted strongly with the support was active; the behavior may be related to the small support effect on the palladium activity. Higher activity of the loaded palladium oxide than that of platinum oxide was caused by the higher reducibility of PdO chemical bonds, as confirmed by the temperature-programmed reduction. Silica deposited alumina, prepared by a method of chemical vapor deposition, was designed to create new active sites for the loading of silica-rimmed palladium; the sintering of palladium was suppressed to obtain the durable activity.

Relationship between acid amount and framework aluminum content in mordenite

Acid amount of mordenites and dealuminated mordenites with framework aluminum contents of 0.28–2.33 mmol g−1 was measured by temperature-programmed desorption of ammonia. Framework Al content was distinguished from nonframework Al based on 27Al MAS n.m.r. measurements. The relationship between the acid amount and the framework Al content represented a volcano relationship with the maximum acid amount at 1.65 mmol g−1 framework Al content. Below 1.65 mmol g−1 framework Al content, the acid amount was approximately proportional to the framework Al content, whereas the amount decreased as the framework Al content increased above 1.65 mmol g−1. The decrease of acid amount of mordenite with the framework Al content above 1.65 mmol g−1 seems to be due to the generation of nonacidic framework Al atoms owing to paired Al atoms in 4-rings.

Reaction mechanism of ammoxidation of toluene: IV. Oxidation state of vanadium oxide and its reactivity for toluene oxidation

Abstract In order to determine the mechanism of the ammoxidation of toluene, vanadium oxide supported on Al 2 O 3 during the ammoxidation has been characterized by the titrimetric method, ESR, and the temperature-programmed reoxidation. Vanadium oxide was severely reduced during the reaction, and in the state almost equivalent to V 2 O 4 . The oxidation state was influenced profoundly by the atmospheric condition because of the thin layer of vanadium oxide on Al 2 O 3 . Furthermore, the adsorbed hydrocarbon material on the surface of vanadium oxide inhibited the reoxidation, stabilizing the severely reduced surface. Pulse experiments on the definite oxidation state of vanadium revealed that only benzaldehyde could be formed significantly on the V 2 O 4 catalyst. Therefore, the first step of the ammoxidation of toluene could be deduced to be the formation of benzaldehyde on V 2 O 4 .

Support effect of zeolite on the methane combustion activity of palladium

Abstract Catalytic combustion of methane was carried out over Pd loaded on MFI and MOR zeolites. The activity and durability of the catalysts in the methane combustion was dependent on the Al concentration, the kind of cation and the structure of zeolite supports. Especially, Pd loaded on MOR was more active than that on MFI. The catalytic performance of Pd was correlated with the measurement of reaction order, activation energy and local structure of Pd determined by Pd K-edge EXAFS. A significant difference in the state of Pd was observed between MFI and MOR, where the Pd metal and PdO existed during the reaction over MOR and MFI, respectively. The high activity of Pd/MOR for methane combustion was ascribed to the generation of a mixture of Pd metal and PdO during the reaction.

New Method for the Temperature‐ Programmed Desorption (TPD) of Ammonia Experiment for Characterization of Zeolite Acidity: A Review

In this review, a method for the temperature-programmed desorption (TPD) of ammonia experiment for the characterization of zeolite acidity and its improvement by simultaneous IR measurement and DFT calculation are described. First, various methods of ammonia TPD are explained, since the measurements have been conducted under the concepts of kinetics, equilibrium, or diffusion control. It is however emphasized that the ubiquitous TPD experiment is governed by the equilibrium between ammonia molecules in the gas phase and on the surface. Therefore, a method to measure quantitatively the strength of the acid site (∆H upon ammonia desorption) under equilibrium-controlled conditions is elucidated. Then, a quantitative relationship between ∆H and H0 function is proposed, based on which the acid strength ∆H can be converted into the H0 function. The identification of the desorption peaks and the quantitative measurement of the number of acid sites are then explained. In order to overcome a serious disadvantage of the method (i.e., no information is provided about the structure of acid sites), the simultaneous measurement of IR spectroscopy with ammonia TPD, named IRMS-TPD (infrared spectroscopy/mass spectrometry-temperature-programmed desorption), is proposed. Based on this improved measurement, Brønsted and Lewis acid sites were differentiated and the distribution of Brønsted OH was revealed. The acidity characterized by IRMS-TPD was further supported by the theoretical DFT calculation. Thus, the advanced study of zeolite acidity at the molecular level was made possible. Advantages and disadvantages of the ammonia TPD experiment are discussed, and understanding of the catalytic cracking activity based on the derived acidic profile is explained.

Fine control of the pore-opening size of the zeolite mordenite by chemical vapour deposition of silicon alkoxide

Chemical vapour deposition (c.v.d.) of Si(OCH3)4 on the H form of mordenite has been carried out in order to control the pore-opening size without affecting its acidic properties. It has been shown that Si(OCH3)4 is deposited irreversibly on the zeolite. Because the molecular size of the alkoxide is larger than the pore size, the alkoxide does not enter the pore and the silicon compound is deposited on the external surface. The alkoxide may be deposited by reaction with hydroxide, thus covering the external surface of zeolite crystal after subsequent reactions. Calcination with oxygen removes the hydrocarbon residue and produces silica-coated H-mordenite (SiHM). The SiHM thus obtained has been characterized by temperature-programmed desorption (t.p.d.) of NH3, adsorption experiments and X-ray photoelectron spectroscopy. The deposition of the alkoxide does not change the acidity but reduces the size of pore opening. Enrichment of Si on the external surface of the zeolite is confirmed. One can therefore conclude that SiO2 covers the external surface of the zeolite, thus reducing the effective size of the pore opening. The pore size is effectively reduced by ca. 0.1 and 0.2 nm upon formation of 1–2 and 3 molecular layers of silicon oxide, respectively.