Pinpointing basic sites formed upon incorporation of iron in hierarchical SAPO-11 using catalytic model reactions

Abstract

By utilizing previously established catalytic model reactions, a method for probing the topological location of transition metal sites incorporated in hierarchical silicoaluminophosphates (SAPOs) is presented. For the first time, iron(III)-incorporated hierarchical SAPO-11 (FeCTAB-11) was prepared and thoroughly characterized with conventional iron(III)-incorporated SAPO-11 (FeSAPO-11) as a reference. Initially, inductively coupled plasma – mass spectrometry (ICP-MS) indicated that the FeSAPOs contained similar amounts of metal (∼2.0 wt%), while N₂-physisorption confirmed the bimodal porosity of the hierarchical FeCTAB-11. Furthermore, X-ray absorption spectroscopy (XAS) revealed that iron(III) was isomorphously incorporated into both SAPO-11 samples, whereas CO₂-temperature programmed desorption (TPD) revealed the first reported presence of strong basic sites in the vicinity of a transition metal incorporated into a SAPO framework. The location of the basic sites, and thus the incorporated iron, was subsequently probed by studying the products of the base-catalyzed vapor phase isomerization of cyclohexanone oxime (Beckmann rearrangement, BMR) model reaction. Through an increased lifetime for the base-catalyzed production of aniline, the incorporated iron for FeCTAB-11 was found to be located in highly accessible mesopores, whereas the conventional FeSAPO-11 had incorporated iron located in its micropores. Lastly, the methanol-to-hydrocarbons (MTH) model reaction showed that both FeSAPOs only had Brønsted acid sites in the micropores of the structures. This was used to verify the pore connectivity of the hierarchical FeCTAB-11 by utilizing the base-catalyzed BMR mechanism's dependency on acid sites.