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.

Highly Selective Carbon‐Supported Boron for Oxidative Dehydrogenation of Propane

“…Alternatively, an MCM‐22 zeolite isomorphously substituted with boron primarily contains B (OSi) 3 species isolated in the zeolite framework and is inactive for ODH. This observation further supports the hypothesis that the clustering of oxidized/hydrolyzed boron gives rise to its ODH catalytic activity [12,14] . However, B/SiO 2 catalysts show worse selectivity for ODH of propane to propylene than h‐BN (B/SiO 2 : 76.5 %, h‐BN: 87.4 %; Figure 2b), which is attributed to oxygen functionalities on the silica surface that induce unwanted side reactions with intermediates [5] .…”

“…Structured carbon surfaces are often functionalized by chemical treatments to induce thermal stability under oxidative conditions (Figure 1). [6–14] This is done by inducing surface functional groups, such as −OH and −COOH, with an oxidizing treatment ( e. g . nitric acid) [6–14] .…”

“… [6–14] This is done by inducing surface functional groups, such as −OH and −COOH, with an oxidizing treatment ( e. g . nitric acid) [6–14] . These functionalities are anchoring sites for boron precursors, thus giving the ability for carbon to support greater quantities of active sites, but also restricting the surface mobility of the species.…”

“…As the 11 B NMR signal at δ iso ( 11 B)=11.5 ppm was never observed, it suggests no B−O−B bonds are present for that species. It is well known that the δ iso ( 11 B) decreases as B−O−B units are replaced with B−O−Si bonds for boron oxide on silica or zeolites, where B (OB) 3 and B (OSi) 3 species typically resonates at δ iso ( 11 B) ∼16–18 ppm and 10–12 ppm, respectively [5,12,14,59,60] . Therefore, based on the high δ iso ( 11 B) and strong correlations observed in the 1 H‐ 11 B HETCOR (with short duration of recoupling) and 11 B homonuclear correlation spectra, we assign the 11 B NMR signal at δ iso ( 11 B)=18.3 ppm to B (OB) x (OH) 3‐ x ( x =1 or 2) (Table 1).…”

Advances in zeolite-supported metal catalysts for propane dehydrogenation

“…Besides single-site metal species, single-site non-metal species (e.g., boron) also possess catalytic activity for PDH reactions. 100,101 Boron-containing catalysts (such as BN, supported boron oxide and B-MWW) have been discovered to have the ability to catalyze the oxidative dehydrogenation of propane (ODHP). 101,[129][130][131] B-O-B oligomers are considered as the active centers, which can suppress the over oxidation of propane and hinder the formation of coke.…”