Defect-Guided Synthesis of Hierarchical Sn-B-Beta Zeolite with Highly Exposed Sn Sites

Selectively anchoring active centers on the external surface for forming highly exposed acid sites is a highly desirable but challenging task in zeolite catalyst synthesis. Herein, a defect-guided etching-regrowth strategy is rationally designed for facilely positioning Sn Lewis acid sites on the outer surface of the Sn-B-Beta while fabricating a bifunctional hierarchical structure. The synthesis was conducted by hydrothermal treatment of the as-made B-Beta (uncalcined), which has intrinsic defects of the BEA structure, with Sn source and basic organic structure directing agent (SDA). Under a moderate SDA concentration, with blocked micropore channels, such SDA-triggered etching-regrowth will proceed along the defect defined pathway, which ensures Sn selectively anchored on the external surface. Moreover, this methodology has exclusively introduced tetrahedrally coordinated framework Sn with open Sn sites as the predominated species. Mono- and disaccharide isomerizations in ethanol over different Sn-Beta catalysts proved the prominent advantages of the hierarchical structure with highly exposed and synergetic acid sites.

One-step synthesis of hierarchical [B]-ZSM-5 using cetyltrimethylammonium bromide as mesoporogen

One-step facile synthesis of boron containing ZSM-5 microspheres is developed using 1,6-diaminohexane as the structure-directing agent and cetyltrimethylammonium bromide as the mesoporogen. High boron incorporation up to Si/B ratio of 38 is achieved and evidenced by the stretching vibrations of B–O–Si at 667 cm^-1 and 917 cm^-1 using Fourier-transform infrared spectra. The morphology of the crystals resembles berry-like spheres with sizes approximately 15 μm, which is composed of aggregated nanocrystals having sizes around 450 nm, is observed using scanning electron microscopy. The textural properties, i.e. the surface areas and pore volumes are investigated using N₂ adsorption at –196 °C. t-plot micropore volume of 0.11 cm³/g and mesopore volume of 0.14 cm³/g are obtained applying this synthesis method for mesopores having pore diameters within the range of 2–10 nm.

Oxidative dehydrogenation of light alkanes to olefins on metal-free catalysts

Metal-free boron- and carbon-based catalysts have shown both great fundamental and practical value in oxidative dehydrogenation (ODH) of light alkanes. In particular, boron-based catalysts show a superior selectivity toward olefins, excellent stability and atom-economy to valuable carbon-based products by minimizing CO2 emission, which are highly promising in future industrialization. The carbonaceous catalysts also exhibited impressive behavior in the ODH of light alkanes helped along by surface oxygen-containing functional groups. This review surveyed and compared the preparation methods of the boron- and carbon-based catalysts and their characterization, their performance in the ODH of light alkanes, and the mechanistic issues of the ODH including the identification of the possible active sites and the exploration of the underlying mechanisms. We discussed different boron-based materials and established versatile methodologies for the investigation of active sites and reaction mechanisms. We also elaborated on the similarities and differences in catalytic and kinetic behaviors, and reaction mechanisms between boron- and carbon-based metal-free materials. A perspective of the potential issues of metal-free ODH catalytic systems in terms of their rational design and their synergy with reactor engineering was sketched.

Structure Determination of Boron-Based Oxidative Dehydrogenation Heterogeneous Catalysts with Ultra-High Field 35.2 T 11B Solid-State NMR Spectroscopy

Boron-based heterogenous 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 ¹¹B solid-state NMR spectroscopy and other techniques suggests that oxidized/hydrolyzed boron clusters are the catalytically active sites for ODH. However, ¹¹B solid-state NMR spectroscopy often suffers from limited resolution because boron-11 is an I = 3/2 half-integer quadrupolar nucleus. Here, ultra-high magnetic field (B ₀ = 35.2 T) is used to enhance the resolution of ¹¹B solid-state NMR spectra and unambiguously determine the local structure and connectivity of boron species in h-BN nanotubes used as a ODH catalyst (spent h-BNNT), boron substituted MCM-22 zeolite [B-MWW] and silica supported boron oxide [B/SiO₂] before and after use as an ODH catalyst. One-dimensional direct excitation ¹¹B 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 ¹H-¹¹B heteronuclear correlation NMR spectra aid in the identification of boron species with B-OH functionality. Most importantly, 2D ¹¹B 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 heterogenous catalysts will aid in the future development of next generation ODH catalysts.

The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis

At the forefront of global development, the chemical industry is being confronted by a growing demand for products and services, but also the need to provide these in a manner that is sustainable in the long-term. In facing this challenge, the industry is being revolutionised by advances in catalysis that allow chemical transformations to be performed in a more efficient and economical manner. To this end, molecular design, facilitated by detailed theoretical and empirical studies, has played a pivotal role in creating highly-active and selective heterogeneous catalysts. In this review, the industrially-relevant Beckmann rearrangement is presented as an exemplar of how judicious characterisation and ab initio experiments can be used to understand and optimise nanoporous materials for sustainable catalysis.

Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with ¹⁵N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The ¹⁵N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Brønsted (B, Ga, and Al) acidic heteroatoms. The ¹⁵N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H⁺. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental ¹⁵N chemical shift and the calculated metal-nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between ¹⁵N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust method to characterize, and ultimately optimize, metal-reactant binding and activation for Lewis acid zeolites. Direct ¹⁵N MAS NMR detection protocols applied to the Lewis acid-base adducts allowed the differentiation and quantification of framework metal sites in the presence of extraframework oxides, including highly quadrupolar nuclei that are not amenable for quantification with conventional NMR methods.

The curious effects of integrating bimetallic active centres within nanoporous architectures for acid-catalysed transformations

Engineering site-specific interactions, by introducing complementary transition-metal ions within nanoporous architectures, affords tunability in solid-acid catalysed transformations.

Investigating site-specific interactions and probing their role in modifying the acid-strength in framework architectures

The ability to adroitly tailor acid-strength using specifically-engineered bimetallic nanoporous materials has been investigated with a view to exploiting their potential in solid-acid catalysed transformations. Further, it has been demonstrated that through site-specific interactions, extra-framework zinc ions can suitably modify the acidity of Brønsted acid sites, to stimulate diverse catalytic responses, when combined with isomorphously-substituted framework metal cations within porous architectures, for the Beckmann rearrangement of cyclohexanone oxime and in the isopropylation of benzene.

Rationalising the role of solid-acid sites in the design of versatile single-site heterogeneous catalysts for targeted acid-catalysed transformations

Probing multifunctional acid centres in nanoporous architectures through in situ spectroscopy affords a strategy for predictive design of novel catalysts.