Frustrated Lewis Pair in Zeolite Cages for Alkane Activations

Abstract

The frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol-to-olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB + ) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H 2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography-mass spectrometer (GC-MS) catalytic experiments inside FLP-containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO2 catalysts

“…In parallel, the three O atoms in MnO 2 -D process more electrons due to the electron transfer from Mn to O, which could further transfer electrons to reactant molecule (Lewis base property). Hence, the behavior of defective Mn δ+ –O V structure in MnO 2 -D aligns well with the concept of FLPs 30 , 66 – 70 . To probe the FLPs on MnO 2 -D, NH 3 -TPD, and CO 2 -TPD were further performed.…”

Correlating the Nature of Carbenium Ions in Zeolites to the Product Distribution in the Methanol-to-Olefins Process

“…To summarize, the evolution of hydrocarbon species in ZSM-5 and SSZ-13 reflects the behavior of the acidic groups in the ν­(O–H) region, while it is more correlated with the decrease in activity in ferrierite. The correlation between the irreversibly perturbed Bro̷nsted sites and the formation of carbenium ions in ZSM-5 and SSZ-13 implies noncovalent interactions between the positively charged carbenium ions and deprotonated Bro̷nsted sites residing in their proximity. , At this point, an acidic proton, or even a surface methoxy group, has been transferred from the zeolite to a polyunsaturated molecule, generating a carbenium ion. The stabilization of the carbenium ion in the porosity of the zeolite is expected to arise from H-bonding interactions and electrostatic interaction between the positively charged ion and the negatively charged zeolite framework together with diffusion constraints imposed by the topology.…”

“…25 As a consequence, carbenium ions can be described as frustrated Lewis ion pairs when interacting with the negatively charged zeolite framework. 26 Typically, the nature of the carbenium ions and hydrocarbon products in the MTO process is influenced by the confined space induced by the topology of the zeolite framework, the strength, the density as well as the distribution of acid sites, the operating conditions, and the presence of extra-framework metal cations. 5,27,28 Zeolites with medium/large pores and voluminous channel intersections, such as MOR, BEA, MFI, and MEL topology, favor the formation of methylated aromatics, with long-chain aliphatics contributing also significantly to the effluent composition.…”

“…However, steric factors in zeolites become critical to their reactivity if the carbenium ion is exceptionally constrained and hinders the stabilization of the C–O bond with the zeolite framework, thus adopting the alkoxy form . As a consequence, carbenium ions can be described as frustrated Lewis ion pairs when interacting with the negatively charged zeolite framework …”

Intensifying Hydrogen Heterocracking via Regulating the ZnO Overlayer for Enhanced Fatty Acid Ester Hydrogenation

“…At the forefront, the types of hydrogen dissociation, namely homolytic dissociation and heterolytic dissociation, play a crucial role in the reactivity for the hydrogenation of polar functional groups containing CO, CS, and CN bonds. − In particular, heterolytically dissociated H δ+ species display superior activity for the hydrogenation of polar unsaturated groups since polar bonds are ideal acceptors for hydride pairs and protons . Of note, the metal–support interface, as one of the advanced catalysts with Frustrated Lewis acid–base pairs (FLPs), is demonstrated to enable the heterolytic dissociation of H 2 into hydrides (H δ‑ ) and protons (H δ+ ) due to its unique chemical environment caused by the interfacial metal atoms anchored on the surface atoms of the support such as O, N, P, or S. − However, the limited interfacial sites caused by poor contact between metals and oxide support inhibit the heterolytic dissociation of H 2 and substrate activation. − In addition, both the homolytic and heterolytic dissociations of H 2 occur on the metal–metal pairs and the metal-oxide interfacial sites, respectively, promoting the hydrogenation reaction .…”