Acidity FTIR/pyridine technique

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-situ FTIR of CO (and other molecules as site specific probes) was studied on the H-form and hydrothermally treated samples of H-ZSM-5. These results have been more thoroughly reported elsewhere [10] but are sununarized here for comparison with experimental data from other techniques. Zeolites have been traditionally examined using infrared spectroscopy of N-containing adsorbates such as ammonia and pyridine to assess Bronsted acidity. The use of weaker Lewis bases allows a more discriminating approach to assessing the strength and quantity of Bronsted sites arising from the partial and sequential dealumination of framework A1 in H-ZSM-5. 

Surface acid densities were estimated using pyridine adsomtion monitored by combined thermogravimetric and infi-a-red spectroscopic techniques. Thermogravimetric analyses were carried out using a PC controlled Cl microbalance attached to a conventional vacuum line fitted with rotary and diffusion pumps. Approximately 100 mg of sample as a fragmented disc (prepared as per FTIR experiments, see below) was outgassed for 2 h at 573 K, then exposed to 1 Torr pyridine and cooled to 373 K. A further... 

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