The role of extra-framework chromium species for the catalytic activity of [Cr]APO-5-type materials

The properties of chromium-substituted aluminophosphate-5 ([Cr]APO-5) with and without extra-framework chromium generated during hydrothermal synthesis are compared with those of AlPO-5 and [Cr]APO-5 into which chromium species were partly or exclusively introduced by impregnation. The prepared materials were characterized by elemental analysis, XRD, UV–Vis spectroscopy and nitrogen adsorption. Extra-framework chromium species exhibit a slight activity for the acid-catalyzed gas-phase conversion of 2-propanol to propene. However, their major effect consists in a reduction of the specific pore volume and, thus, a decreased accessibility of acid sites in [Cr]APO-5 or AlPO-5. This pore volume reduction is more pronounced for materials with extra-framework chromium species from impregnation than from generation during synthesis.