Structure Determination of Boron-Based Oxidative Dehydrogenation Heterogeneous Catalysts With Ultrahigh Field 35.2 T ¹¹B Solid-State NMR Spectroscopy

Abstract

Boron-based heterogeneous catalysts, such as hexagonal boron nitride (h-BN) as well as supported boron oxides, are highly selective catalysts for the oxidative dehydrogenation (ODH) of light alkanes to olefins. Previous catalytic measurements and molecular characterization of boron-based catalysts by 11 B solid-state NMR spectroscopy and other techniques suggest that oxidized/ hydrolyzed boron clusters are the catalytically active sites for ODH. However, 11 B solid-state NMR spectroscopy often suffers from limited resolution because boron-11 is an I = 3/2 half-integer quadrupolar nucleus. Here, ultrahigh magnetic field (B 0 = 35.2 T) is used to enhance the resolution of 11 B solid-state NMR spectra and unambiguously determine the local structure and connectivity of boron species in h-BN nanotubes used as an ODH catalyst (spent h-BNNT), boron-substituted MCM-22 zeolite (B-MWW), and silica-supported boron oxide (B/SiO 2 ) before and after use as an ODH catalyst. One-dimensional direct excitation 11 B NMR spectra recorded at B 0 = 35.2 T are near isotropic in nature, allowing for the easy identification of all boron species. Two-dimensional (2D) 1 H-11 B heteronuclear correlation NMR spectra aid in the identification of boron species with B−OH functionality. Most importantly, 2D 11 B dipolar double-quantum single-quantum homonuclear correlation NMR experiments were used to unambiguously probe boron−boron connectivity within all heterogeneous catalysts. These experiments are practically infeasible at lower, more conventional magnetic fields due to a lack of resolution and reduced NMR sensitivity. The detailed molecular structures determined for the amorphous oxidized/hydrolyzed boron layers on these heterogeneous catalysts will aid in the future development of nextgeneration ODH catalysts.

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] .…”

“…The enhanced performance of h‐BN for ODH is attributed to an amorphous oxidized/hydrolyzed boron layer [B 2 (OH) 2x O 3‐x , x=0–3] which resides on the BN sheets [2,7–10] . Solid‐state NMR spectroscopy and other surface characterization techniques have shown that the amorphous oxidized/hydrolyzed boron layer is primarily composed of 3‐coordinate boron oxide/hydroxide species (B 2 (OH) 2x O 3‐x , x=0–3) that form under reaction conditions [2,9,11,12] . Ultra‐high field 11 B NMR spectroscopy revealed that the oxidized/hydrolyzed boron layer grows off the BN sheets through B N 2 O groups, where the O atom is bridging to the oxidized/hydrolyzed boron layer [12] .…”

“…Solid‐state NMR spectroscopy and other surface characterization techniques have shown that the amorphous oxidized/hydrolyzed boron layer is primarily composed of 3‐coordinate boron oxide/hydroxide species (B 2 (OH) 2x O 3‐x , x=0–3) that form under reaction conditions [2,9,11,12] . Ultra‐high field 11 B NMR spectroscopy revealed that the oxidized/hydrolyzed boron layer grows off the BN sheets through B N 2 O groups, where the O atom is bridging to the oxidized/hydrolyzed boron layer [12] . Notably, h‐BN does not appear to deactivate but features an induction period for optimal catalytic performance, suggesting that the oxidized/hydrolyzed boron layer provides the active species [1,5,8,11] .…”

“…Further investigation of oxidized/hydrolyzed boron oxide species for ODH have been conducted through the synthesis and characterization of silica‐supported boron oxide materials (B/SiO 2 ) [5,13] . Detailed characterization of B/SiO 2 ODH catalysts showed that the majority of boron is composed of clustered 3‐coordinate boron oxide/hydroxide species [B 2 (OH) 2x O 3‐x , x=0–3], likely representing similar species observed in h‐BN ODH catalysts [5,12,13] . 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.…”

Dual Active Sites on Molybdenum/ZSM‐5 Catalyst for Methane Dehydroaromatization: Insights from Solid‐State NMR Spectroscopy

“…Both NMR active Mo nuclei 95 Mo and 97 Mo are quadrupolar nuclei of spin 5/2. Due to the low gyromagnetic ratio nature, γ( 95 Mo)=−1.751×10 7 rad T −1 s −1 and γ( 97 Mo)=−1.78×10 7 rad T −1 s −1 , sensitivity‐enhanced WURST‐QCPMG method was employed in combination with ultra‐high magnetic field of 35.2 T (1.5 GHz) which shows advantages for high‐resolution NMR spectroscopy of half‐integer quadrupolar nuclei [44–48] . The 95 Mo WURST‐QCPMG MAS NMR spectrum acquired at 35.2 T of the fresh 5Mo/ZSM‐5 exhibits a broad quadrupolar peak ( C Q ≈5.5 MHz) with an isotropic chemical shift (δ iso ) of ca.…”