Structure and Reactivity of Al−O(H)−Al Moieties in Siloxide Frameworks: Solution and Gas‐Phase Model Studies

Abstract

Even though aluminas and aluminosilicates have found widespread application, a consistent molecular understanding of their surface heterogeneity and the behavior of defects resulting from hydroxylation/dehydroxylation remains unclear. Here, we study the well‐defined molecular model compound, [Al3(μ2‐OH)3(THF)3(PhSi(OSiPh2O)3)2], 1, to gain insight into the acid–base reactivity of cyclic trinuclear Al3(μ2‐OH)3 moieties at the atomic level. We find that, like zeolites, they are sufficiently acidic to catalyze the isomerization of olefins. DFT and gas phase vibrational spectroscopy on solvent‐free and deprotonated 1 show that the six‐membered ring structure of its Al3(μ2‐OH)3 core is unstable with respect to deprotonation of one of its hydroxy groups and rearranges into two edge‐sharing four‐membered rings. This renders AlIV−O(H)−AlIV units strong acid sites, and all results together suggest that their acidity is similar to that of zeolitic SiIV−O(H)−AlIV groups.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

“…[111] To characterize the acidic nature of the -OH species in more detail, these studies were subsequently extended to include gas-phase clusters. [107] Information on the unperturbed bridging Al-O(H)-Al groups can be obtainedb yr emoving the three THF molecules from 1.C alculations predict that the diprotonation energy of this species is similar to that in the proton forms of well-known zeolite catalysts. Deprotonationt hen yields two characteristically different structures for [Al 3 (m 2 -OH) 2 O(PhSi(OSiPh 2 O) 3 ) 2 ] À , 1* (see Figure 7).…”

“…The IRPD spectrumo fN 2 -tagged anions 1*(N 2 ) 1-6 is shown in Figure 8 N 2 -loss contribution to the IRPD trace stems from 1*(N 2 ) 2 (see Supporting Information of Ref. [107] for additional information). Measurements under different tagging conditions shows that the 1*(N 2 ) 2 dissociation channel indeed defines the overall form of the IRPDs pectrum, whiles ize-dependent tag effects have am ore subtle influence.T herefore, DFT calculations were performed on 1* complexes with two N 2 molecules.…”

“…have predicted that the acidity of the Al‐O(H)‐Al entities is lower than that of the Si‐O(H)‐Al units in zeolites but higher than that of a terminal Si−OH or Al−OH group . To characterize the acidic nature of the ‐OH species in more detail, these studies were subsequently extended to include gas‐phase clusters . Information on the unperturbed bridging Al‐O(H)‐Al groups can be obtained by removing the three THF molecules from 1 .…”

“…The dominant N 2 ‐loss contribution to the IRPD trace stems from 1* (N 2 ) 2 (see Supporting Information of Ref. for additional information). Measurements under different tagging conditions shows that the 1* (N 2 ) 2 dissociation channel indeed defines the overall form of the IRPD spectrum, while size‐dependent tag effects have a more subtle influence.…”

“…Gas phase IRPDspectrum of the N 2 -tagged anion 1*·(N 2 ) 1-6 (top panel).S imulateds pectra of 1*a·(N 2 ) 2 (purple trace), 1*b·(N 2 ) 2 (blue trace) and 1*c·(N 2 ) 2 (red trace)o btained with B3LYP + D2.The two differentA l 3 O 3 core motifs,b ridged ring (middle) and six-membered ring (bottom), are shownt ogether with OH bond distances in .Adapted from Ref [107]…”