A novel approach to borosilicate zeolite synthesis in the presence of fluoride

A synthetic route to borosilicate zeolites is described which yields relatively boron-rich products. The reaction takes place under high reactant concentrations and utilizes fluoride delivered as the ammonium fluoride salt rather than as hydrofluoric acid. The reaction is an adaptation of the conditions (HF) initially used to discover zeolite SSZ-70. Exploring a range of structure-directing agents (SDA), several boron-rich phases crystallized. Reactions are generally quite slow, however. A more in-depth study with an adamantyl SDA finds zeolite SSZ-23 favored over SSZ-13. Multinuclear NMR experiments were carried out to better understand the boron substitution in SSZ-23 and to probe if there are key B–F interactions in its formation. Finally, providing alkali cation sources of fluoride shifts the product selectivity away from SSZ-23 and back toward SSZ-13.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The use of HF can be successfully be replaced by NH4F in borosilicate syntheses. ► F anions played a role on product selectivity in the boron-rich syntheses. ► Using an adamantyl cation, the product selectivity favored zeolite SSZ-23 (STT).