Reports: AC5
48480-AC5 Redox Centers from Brønsted Acid Sites and Defects in H-Zeolites: Formation, Structure and Reactivity in Hydrocarbon Catalysis
We investigate the effect of high-temperature conditions on the surface chemistry and reactivity of catalytic sites inside zeolite crystals. Within this PRF grant we have investigated two cases of thermal transformation of zeolite catalyst. In the first we study the reaction of defective silicalite-1 zeolites at high temperatures, and in the second we investigate the stability and crystal structure of small-pore Cu-exchanged zeolites for selective catalytic reduction of NO.
Upon heating samples of silicalite-1 containing hydroxyl nests to temperatures exceeding 873K, the IR bands characteristic of these nests (~3400 cm-1) decrease in intensity. The disappearance of these signals has been attributed to dehydration of the silanol groups. But using temperature-programmed-desorption (TPD), we have shown that the product of the dehydroxylation of hydroxyl nests is hydrogen and not water. The results of the characterization of the samples using N2 adsorption isotherms, 29Si MAS NMR spectroscopy and optical spectroscopy indicate that the disappearance of hydrogen from the hydroxyl nests is accompanied by the formation of bis-peroxysilyl groups (Si-O-O-Si). Ultraviolet-Visible Light Spectroscopy (UV-Vis) measurements support this model due to an increase in an UV absorption band at ~260 nm with increasing temperatures. Electronic structure calculations were employed to support the feasibility of this reaction mechanism. Silicalite-1 samples made with tetraethyl-orthosilicate show a number of differences in their IR and UV-Vis spectra compared to samples made with colloidal silica (Ludox). It appears that the source of silica plays an important role in the structure of the internal defects. The UV-Vis spectra strongly suggest the formation of O-O bonds inside the zeolite channels.
Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld reÞnement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased signiÞcantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the reÞned diffraction patterns, the atomic positions of framework atoms and copper ion locations were determined as a function of temperature for both zeolites. Copper is found in the cages coordinated to three oxygen atoms of the six-membered rings. As long as the Cu is hydrated, the Cu ions seem to be highly mobile and are not 'found' on a specific site in the structure. Catalytic studies of these samples are currently being conducted.
