Dealumination of NH(4)-Y zeolite during steaming to 873 K was investigated with in situ, time-dependent, synchrotron radiation XRPD and in situ Al K-edge XAS. Water desorption is complete at 450 K, and ammonium decomposition occurs between 500 and 550 K. Only a small fraction of Al(3+) species (5%) leaves the framework during heating from 710 to 873 K; these species occupy site I' inside the sodalite cage. This fraction increases up to 8% in the first 50 min at 873 K and remains constant for the following 70 min isotherm and during the high-temperature part of the cooling experiment. During cooling from 500 to 450 K, the electron density at site I' increases suddenly, corresponding to a fraction of 30-35% of the total Al, confirmed by ex situ (27)Al MAS solid-state NMR. At that temperature, in situ Al K-edge XAS indicates a change in Al coordination of a large fraction of Al, and thermogravimetric (TG) data show the first water molecules start to repopulate the pores. Such molecules drive the dislodgment of most of the Al from the zeolitic framework. Our data indicate that considerable structural collapse caused by steaming does not occur at the highest temperature; however, defects form, which lead to significant migration of framework Al(3+) to extraframework positions, which occurs only as water is able to enter the pores again, that is, at much lower temperature. Contrary to general opinion, these results demonstrate that zeolite dealumination is not primarily a high-temperature process. The standard Rietveld refinement approach failed to identify extraframework Al species. These new results were obtained by adopting the innovative parametric refinement [J. Appl. Crystallogr. 2007, 40, 87]. Treating all of the XRPD patterns collected during the evolution of temperature as one unique data set significantly reduces the overall number of optimized variables and, thus, their relative correlation, and finally results in a more reliable estimate of the optimized parameters. Our results contribute to a better understanding of the phenomena involved on the atomic scale in the preparation of ultrastable Y zeolites (USY). USY are employed in fluid catalytic cracking (FCC), which is the most important conversion process in petroleum refineries to convert the high-boiling hydrocarbon fractions of petroleum crude oils to more valuable products like gasoline and olefinic gases.

Monomolecular cracking of propane was used to investigate the activity of the Brønsted acid sites in amorphous silica−aluminas with three different Si/Al ratios. The reaction rates increased with increasing aluminum content, but the apparent activation energies were identical. In comparison to zeolite ZSM5, the ASA catalysts showed much lower activity, both per weight and per total aluminum content. However, after correcting for the heat of adsorption, the intrinsic activation energies of ASA and H-ZSM5 were similar. This indicates that the ability of the active Brønsted acid sites to protonate propane is similar in amorphous and crystalline structures and that the much lower activity of ASA is due to the lower heat of adsorption and the small number of active sites. Few Brønsted acid sites were detected by means of pyridine adsorption followed by infrared spectroscopy; a broad band at around 3600 cm-1 was observed in the infrared region of the hydroxyl stretch vibrations. It is unclear whether this band was related to the catalytically active sites.

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