CHA-type zeolites with high boron content: Synthesis, structure and selective adsorption properties

Borosilicate zeolites with CHA-type framework are synthesized hydrothermally by using N,N,N-trimethylcyclohexylammonium hydroxide as structure directing agent. The use of this cation induces an increase of boron content in the CHA-type zeolites, and the Si/B ratios of the as-synthesized samples is in the range of 11.8-6.9. Rietveld refinements of the calcinated samples reveal a contraction of unit cells with the increase of boron content, and the 8-ring opening window of cha cavity becomes narrower. ¹¹B MAS NMR shows that all the boron atoms are incorporated into the framework as tetrahedral BO ₄ units in the as-synthesized samples. The thermal stability of these CHA-type borosilicates decreases with the increase of boron content, and the framework can retain up to 800 °C. These borosilicates, with the BET surfaces of 583-632 m²/g, show a high adsorption capacity for H ₂ at 77 K, 900 mmHg and a preferential adsorption for CO₂ at 273 K. This selective adsorption property enables CHA-type borosilicates to be potential materials as CO₂ adsorbent.