The effect of gaseous ammonia treatment of AM-6 crystallized in the presence of tetramethylammonium cations on their removal from the crystal pores, and the crystal quality and structure of AM-6

Abstract

Vanadosilicate AM-6 products with different crystal quality (i.e., disorder, or average length of the V-O-V chains in the AM-6 framework), as determined from the FWHM of ∼868 cm−1 Raman band, but with identical crystallinity/long range order and purity, as determined from the XRD data, were hydrothermally synthesized from reaction mixtures with different molar compositions at 448–503 K using tetramethylammonium (TMA+) ions. The combined SEM, EDX, XRD, Raman, TGA, XPS, and nitrogen adsorption isotherm data showed that by treating these as-synthesized materials in gaseous ammonia at 673 K it is possible to completely remove TMA+ ions from the AM-6 pores without adversely affecting product crystallinity and the micropore structure of AM-6, and with only minimal degradation of crystal quality of the resulting products. However, the success in a complete removal of TMA+ ions without affecting other crystalochemical characteristics of the product depended on its initial crystal quality. TMA+ ions could not be completely removed from the lowest crystal quality product (FWHM = 20.3 cm−1) without completely decomposing the V-O-V chains. The intermediate crystal quality product (FWHM = 15.0 cm−1) could be successfully ammonia-treated only at 673 K to remove TMA+ ions, but higher temperatures resulted in a complete decomposition of the V-O-V chains. The highest crystal quality product (FWHM = 12.7 cm−1) retained the integrity of V-O-V chains as well as the long range order and the micropore structure when ammonia-treated up to 748 K.