Metal substitution effects of aluminophosphate AlPO₄-5 as solid acid catalyst for esterification of acetic acid with ethanol

Metal substitution in aluminophosphates is known to induce unique structural and surface characteristics that can tune catalytic performance. Si, Co, Fe and Mg substituted AlPO₄-5 (MeAPO-5) catalysts were hydrothermally prepared, characterized, and evaluated for ethanol - acetic acid esterification. Textural evaluation revealed that the Co and Si analogues possessed an order of magnitude higher mesopore volume compared to the rest of the studied materials. Divalent metal ions (i.e. Co, Fe, and Mg) substituted preferentially Al ions via the SM1 mechanism, as confirmed by the P/Al being greater than one, while Si ions substituted P ions (SM2) initially, followed by substitution of pairs of Al-P ions (SM3) at higher Si content. The esterification catalytic activity was found to follow the order SAPO-5 > > CoAPO-5 > MgAPO-5 ≈ FeAPO-5. The superior performance of SAPO-5 is ascribed to the higher concentration of Brønsted acid sites and faster mass transfer rates due to the relatively small crystal size and larger fraction of mesopores. Notably, the SAPO-5 with the optimal Si content demonstrated comparable stability against deactivation within the range of reaction conditions studied and sustained catalytic activity at prolonged reaction durations when compared to commercial Zeolite β, despite the significantly higher surface area, pore volume and density of acid sites of the latter, primarily attributed to less strong adsorption of the reaction products. The reported parametric results for various metal substitutions shed light towards optimizing aluminophosphate catalysts for esterification applications.