Acid site concentration measurement observation

Further acid site strength and concentration measurements were reported by Morita et al. (164), who related the acidity measurements to various catalytic reactions. Using Y zeolite (Linde SK-40, 90% H form) activated at 450°C, they observed no acid sites stronger than an H0 of -8.2, although the total acid site concentration was almost twice that of the former investigations (Fig. 21, curve 4). They also measured acid site concentration as a function of decomposition temperature for NH4Y, and found that n-butylamine titration values paralleled results obtained from pyridine adsorption studies (41,151). The maximum total acidity occurred... 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

In this paper, we will discuss the use of temperature programmed desorption as a complementary technique for obtaining lattice concentrations for high-silica materials. While TPD of ammonia is commonly used as a measure of the number of acid sites in zeolites, this paper will discuss the use of the reactive probe molecules, 2-propanamine and 2-propanol. It has previously been demonstrated that well-defined adsorption complexes, with a stoichiometry of one molecule/Al atom could be observed in high-silica zeolites for a number of adsorbates. These complexes were found to be independent of Si/Al2 ratio for a series of H-ZSM-5 zeolites and were unaffected by... 

We have examined temperatured-programmed desorption (TPD) and thermogravimetric analysis (TGA) of isopropyiamine on a series of Si-, Co-, and Mg-substituted AIPO-5 samples. The TPD-TGA results on the substituted samples show ammonia and propene desorbing in a well-defined feature between 575 and 650K, a feature not observed on pure AIPO-5. The results suggest that TPD-TGA measurements of isopropyiamine may be useful in determining the framework concentrations and acid site densities for SAPO-5, CoAPO-5, and MAPO-5. 

In addition to demonstrating that discrete acid sites are formed upon substitution into the AIPO-5 framework, we made two other important observations. First, the calcination conditions used for preparing all of the materials was critical. This had been reported previously for CoAPO-5 [6], but we found that it was equally true for SAPO-5. Second, materials prepared with high concentrations of either Co+2 or Mg+2 tended to have many defects. These materials exhibited reasonable pore volumes as measured by 62 uptakes however, larger molecules were unable to enter the structure. 

The samples were synthesized under hydrothermal conditions with a tetraethylammonium hydroxide template. Each sample was characterized by x-ray diffraction and only peaks corresponding to the substituted AIPO-5 structures were observed. The concentrations of Co, Mg, and Si for each sample were determined by ICP and are reported in Table 1, along with the gel concentrations, as Me/(Me+AI+P). All samples were calcined in dry O2, since calcination conditions were found to greatly affect results. Calcining the CoAPO-5 and MAPO-5 samples in air essentially destroyed the samples. Even for SAPO-5, calcination in air decreased the number of acid sites measured by TPD-TGA by more than half and substantially decreased the pore volumes which were measured by O2 adsorption at 78K and 64 torr (P/Po=0.4), even though x-ray diffraction patterns remained excellent and exhibited no measureable changes in peak widths. While some of the pore volumes reported in Table 1 are actually higher than the theoretical value of 0.146 cm3/g, the conditions used for the measurements were the same as those used in previous... 

Strength (Hammett Indicators) is not strictly applicable to microporous solids, although the Hammett scale is a convenient comparative index. As a result, a number of alternative approaches have been taken to determine the concentration, type and strength of acid sites within these solids. Most of these methods give relative scales of acidity, but it is broadly possible to calibrate these acidities to those observed for other acids. The methods fall into three broad categories direct measurements, measurement of interaction with un-reactive probe molecules and the use of catalytic test reactions. 

Contrary to this behavior, various structures with high Si/Al ratio (H-ZSM-5, H-ZSM-12, HY and H-mordenite) were found to exhibit almost the same adsorption energies measured at 480 K, suggesting that the isolated Bronsted acid sites in these materials are identical [76]. Authors observed that the strength of Bronsted acid sites associated with framework A1 atoms does not vary in a given zeolite, and they suggest that Bronsted acid sites in unsteamed zeolites are independent of the sample (what means, independent of Si/ A1 ratio) and equal in concentration to the framework aluminum content [77]. 

---