Dehydrogenation of Hydroxyl Nest Groups In Silicalite-1
Dustin W. Fickel1, Michael J. Nash2, and Raul F. Lobo2. (1) Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, (2) Chemical Engineering, Univesity of Delaware, 150 Academcy St., Newark, DE 19716
A hydroxyl nest is a common defect in zeolite frameworks where a tetrahedral atom (T=Si, Al, etc.) is missing from the structure. In the absence of the T-atom a cluster of four silanol groups are left forming medium-strength hydrogen bonds with each other. Hydroxyl nests can be formed in aluminosilicate and other zeolite materials by leaching or steaming at high temperatures. Silicalite-1 synthesized in basic media is well known for containing large numbers of silanol nests. The source of silica (tetraethyl-orthosilicate or collodial silica) has proven to be crucial in the number of missing T-atoms leading to varying sizes of these defect sites. The OH region of the IR spectra (3760-3300 cm-1) shows two characteristic peaks that have been assigned to isolated silanol groups in the exterior and interior of the zeolite (3760-3720 cm-1), and to hydrogen-bonded groups of silanol units that give a much broader signal centered at about 3400 cm-1. Upon heating to temperatures exceeding 873K, these bands decrease in intensity, and a new triplet is formed in the 911-896 cm-1 region1. Previously, the disappearance of these signals has been attributed to dehydration of the silanol groups. Using temperature-programmed-desorption (TPD), we show that the product of the dehydroxilation of hydroxyl nests in silicalite-1 (made with colloidal silica) is hydrogen and not water. We propose that the disappearance of hydrogen from the hydroxyl nests is accompanied by the formation of bisperoxysilyl groups (Si-O-O-Si). Ultraviolet-Visible Light Spectroscopy (UV-Vis) measurements support the claim of this bond formation due to an increase in an UV absorption band at ~260 nm with increasing temperatures. Silicalite-1 samples made with tetraethyl-orthosilicate show distinctly different characteristics in their IR and UV-Vis spectra compared to samples made with colloidal silica. It appears that the source of silica plays an important role in the structure of the internal defects and thus hydrogen production capabilities.