B‐MWW Zeolite: The Case Against Single‐Site Catalysis

Abstract

Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B2(OH)xO(3−x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using 11B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO3 units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO3 units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.

Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane

“…[7,8] Thef act that site-isolated BO 3 species with saturated and fully oxidized local environments-as created in az eolite matrix (that is,B -MWW)-are inactive for ODH suggests that the amorphous hydroxy oxide network is actually key. [13] This hypothesis is further supported by the fact that impregnation of boron onto the inactive B-MWW resulted in an active catalyst (B/B-MWW) featuring BÀOÀB connectivity. [13] We emphasize that this amorphous interface is highly dynamic and, as such, it is as uspect for presenting metastable active sites.M etastable species may remain am inority,a nd thus be poorly detectable even by operando characterization.…”

“…[13] This hypothesis is further supported by the fact that impregnation of boron onto the inactive B-MWW resulted in an active catalyst (B/B-MWW) featuring BÀOÀB connectivity. [13] We emphasize that this amorphous interface is highly dynamic and, as such, it is as uspect for presenting metastable active sites.M etastable species may remain am inority,a nd thus be poorly detectable even by operando characterization. [23,24] Such active sites may be studied by leveraging computational tools.P revious theoretical work reveals ad ynamic BO x surface that does indeed contain metastable surface states with distinct geometries,s toichiometries,a nd chemistries,w hich can form on at imescale of picoseconds and become only significantly populated as the temperature is increased from 298 Kto763 K, based on grand canonical simulations.…”

“…[8] In at argeted study of isolated BO 3 species for ODH, it was recently demonstrated that an MWW zeolite with isolated BO 3 units incorporated into its framework is inactive for propane activation. [13] In contrast, BO x clusters supported on silica demonstrate catalytic activity. [8] These gaps,a nd often contradictions,i n knowledge on the ODH reaction mechanism highlight the need to consider additional surface species to rationalize the performance of boron-based ODH catalysts.T he dynamic surface changes during ODH may benefit from computational investigations of metastable surface sites,b eyond thermodynamically stable species such as BO 3 .This approach is akey focus of the present report.…”

Plasma Tuning Local Environment of Hexagonal Boron Nitride for Oxidative Dehydrogenation of Propane

“…[4] With the investigation of various boron-containing catalysts in different alkane atmospheres, it has been generally accepted that the oxygen-containing boron species on the surface of the catalyst is the catalytic origin of the ODH reaction. [5] At present, since the high crystallinity of commercial h-BN leads to a limited amount of active sites, it is necessary to effectively improve the reaction performance of this material to boost industrialization. Although the high surface area h-BN and boron-rich materials have been applied to the ODH reaction, [6] simplifying the synthesis steps remains an ongoing challenge.…”

Porous Materials Confining Single Atoms for Catalysis

“…Ir SACs also showed excellent performances during the butadiene upon reaction with ethylene under mild conditions (80–220°C, 1°bar)( Jaegers et al, 2019 ). Furthermore, subnanometric metal species (single atoms and clusters) Pt or Pt-Sn species confined in zeolites (L. Liu et al, 2020c ; Liu et al, 2020a ) or site-isolated BO 3 units confined in MCM-22 zeolites ( Altvater et al, 2020 ) were also reported to be the catalytically active sites in the dehydrogenation of propane to form propylene ( Liu L. et al, 2019 ; Liu et al, 2020a ) or oxidative dehydrogenation of propane to propene ( Altvater et al, 2020 ), respectively.…”