Hydrothermal Synthesis of Open-Framework Borophosphates
with Tunable Micropore Sizes, Crystal Morphologies, and Thermal Stabilities
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Abstract
Thermal
stabilities of zeolitic frameworks are important parameters
for many applications. Two decades of research have produced only
a very small number of zeolitic borophosphates such as Na<sub>2</sub>[VO(B<sub>2</sub>O)(PO<sub>4</sub>)<sub>2</sub>(HBO<sub>3</sub>)]·<i>x</i>H<sub>2</sub>O (<i>x</i> <b> ≈ </b> 2.92) (denoted as <b>B</b><sub><b>3</b></sub><b>P</b><sub><b>2</b></sub>), which shows the onset dehydration and
a complete decomposition at 200 and 400 °C, respectively. In
order to enhance thermal stabilities of borophosphate frameworks,
a water-deficient hydrothermal route with phosphoric acid as the sole
solvent has been developed and led to controlled syntheses of <b>B</b><sub><b>3</b></sub><b>P</b><sub><b>2</b></sub> and a new vanadium borophosphate, K<sub>1.33</sub>Na<sub>0.67</sub>[VO(B<sub>2</sub>O)(PO<sub>4</sub>)<sub>2</sub>(HPO<sub>4</sub>)]·<i>x</i>H<sub>2</sub>O (<i>x</i> <b> ≈ </b> 1.63) (denoted as <b>B</b><sub><b>2</b></sub><b>P</b><sub><b>3</b></sub>). The latter is the first-ever borophosphate
possessing the zeolite RHO-type net and is characterized by superlarge
spherical cages, including 16-ring and 8-ring channels along the axes
and 12-ring channels along the diagonals of the cubic cell. The new
compound <b>B</b><sub><b>2</b></sub><b>P</b><sub><b>3</b></sub> has larger structural cages and higher thermal
stability than <b>B</b><sub><b>3</b></sub><b>P</b><sub><b>2</b></sub>, where the enhanced thermal stability is
attributable to different bonding arising from the substitution of
[BO<sub>2</sub>(OH)] by [PO<sub>3</sub>(OH)] in the framework. This
is the first demonstration that the micropore size, crystal morphology,
and thermal stability of zeolitic borophosphates can be tuned by changing
the fundamental building units of their frameworks via adjusting the
B/P ratios in the starting materials
