Fig.1. Proposed models for Brønsted acid sites (BASs) in silica-alumina catalysts[7]. (a) BAS consisting of a bridging silanol site bonded to AlIV site (SiOHAl) in zeolites; (b) BAS consisting of PBS interacting with AlIV site; (c) BAS consisting of the flexible coordination between silanol oxygen and neighbouring AlIV. In the latter two cases, the dotted line does not denote a covalent bond but only the close proximity between O and Al atoms
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