Probing the Design Rationale of a High-Performing Faujasitic Zeotype Engineered to have Hierarchical Porosity and Moderated Acidity

Synthesis of the large pore aluminophosphate STA-1 and its application as a catalyst for the Beckmann rearrangement of cyclohexanone oxime

The preparation of stable large pore aluminophosphate (AlPO) zeotypes offers materials for applications in adsorption and catalysis. Here we report the synthesis of the pure AlPO with the SAO topology type (AlPO STA-1) using N,N'-diethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidine (DEBOP) as the organic structure directing agent in the presence of fluoride. The AlPO STA-1 can be rendered microporous (pore volume 0.36 cm3 g-1) via calcination and the calcined form remains stable in the presence of moisture. The structure of the dehydrated form has been established by Rietveld refinement (tetragonal P4̄n2, a = 13.74317(10) Å, c = 21.8131(5) Å, V = 4119.94(16) Å3). Multinuclear 27Al and 31P MAS NMR, together with 2D COSY and CASTEP NMR calculations, enables resolution and assignment of the signals from all crystallographically distinct Al and P framework sites. Structural elucidation of the as-prepared aluminophosphate-fluoride is more challenging, because of the presence of partially protonated OSDA molecules in the 3D-connected channel system and in particular because the fluoride ions coordinate with positional disorder to some of the Al atoms to give 5-fold as well as tetrahedrally-coordinated framework Al species. These are postulated to occupy Al-F-Al bridging sites, where they are responsible for distortion of the framework [P4̄n2, a = 13.3148(9) Å, c = 22.0655(20) Å, V = 3911.9(7) Å3]. Calcination and removal of fluoride ions and OSDAs allows the framework to expand to its relaxed configuration. The SAO topology type aluminophosphate can also be synthesised with small amounts of Si and Ge in the framework, and these SAPO and GeAPO STA-1 materials are also stable to template removal. IR spectroscopy with CO as a probe at 123 K indicates all have weak-to-mild acidity, increasing in the order AlPO < GeAPO < SAPO. These STA-1 materials have been investigated for their activity in the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam at 598 K: while all are active, the AlPO form is favoured due to its high selectivity and slow deactivation, both of which are a consequence of its very weak acid strength, which is nevertheless sufficient to catalyse the reaction.

Defect-Regulated Frustrated-Lewis-Pair Behavior of Boron Nitride in Ambient Pressure Hydrogen Activation

The construction of heterogeneous frustrated Lewis pairs (FLPs) with performance comparable to or surpassing the homogeneous counterparts in H₂ activation is a long-standing challenge. Herein, sterically hindered Lewis acid ("B" center) and Lewis base ("N" center) sites were anchored within the rigid lattice of highly crystalline hexagonal boron nitride (h-BN) scaffolds. The active sites were created via precision defect regulation during the molten-salt-involved (NaNH₂ and NaBH₄) h-BN construction procedure. The as-afforded h-BN scaffolds achieved highly efficient H₂/D₂ activation and dissociation under ambient pressure via FLP-like behavior, and attractive catalytic efficiency in hydrogenation reactions surpassing the current heterogeneous analogues.

Rational Design and Application of Covalent Organic Frameworks for Solar Fuel Production

Harnessing solar energy and converting it into renewable fuels by chemical processes, such as water splitting and carbon dioxide (CO2) reduction, is a highly promising yet challenging strategy to mitigate the effects arising from the global energy crisis and serious environmental concerns. In recent years, covalent organic framework (COF)-based materials have gained substantial research interest because of their diversified architecture, tunable composition, large surface area, and high thermal and chemical stability. Their tunable band structure and significant light absorption with higher charge separation efficiency of photoinduced carriers make them suitable candidates for photocatalytic applications in hydrogen (H2) generation, CO2 conversion, and various organic transformation reactions. In this article, we describe the recent progress in the topology design and synthesis method of COF-based nanomaterials by elucidating the structure-property correlations for photocatalytic hydrogen generation and CO2 reduction applications. The effect of using various kinds of 2D and 3D COFs and strategies to control the morphology and enhance the photocatalytic activity is also summarized. Finally, the key challenges and perspectives in the field are highlighted for the future development of highly efficient COF-based photocatalysts.

Bimetallic PdAu Catalysts within Hierarchically Porous Architectures for Aerobic Oxidation of Benzyl Alcohol

Hierarchically porous (HP) zeotype materials (possessing both micropores and mesopores) offer improved diffusional access to intra-framework active sites, analogous to mesoporous materials, yet retain the high selectivity of the microporous (MP) bulk. We have recently designed crystalline hierarchically porous silicoaluminophosphates (SAPOs) with enhanced mass-transport characteristics, which can lead to significant improvement in catalytic activity and catalyst lifetime. In this study, we have prepared PdAu bimetallic nanostructures supported on HP-SAPO frameworks by an incipient impregnation of metal precursors followed by H₂ reduction at 300 °C, for the aerobic oxidation of benzyl alcohol to benzaldehyde. PdAu NPs supported on HP framework displayed significantly enhanced catalytic activities, when compared with their MP analogues, clearly highlighting the benefits of introducing hierarchical porosity in the SAPO support matrix.

Probing the Design Rationale of a High-Performing Faujasitic Zeotype Engineered to have Hierarchical Porosity and Moderated Acidity

Porosity and acidity are influential properties in the rational design of solid-acid catalysts. Probing the physicochemical characteristics of an acidic zeotype framework at the molecular level can provide valuable insights in understanding intrinsic reaction pathways, for affording structure-activity relationships. Herein, we employ a variety of probe-based techniques (including positron annihilation lifetime spectroscopy (PALS), FTIR and solid-state NMR spectroscopy) to demonstrate how a hierarchical design strategy for a faujasitic (FAU) zeotype (synthesized for the first time, via a soft-templating approach, with high phase-purity) can be used to simultaneously modify the porosity and modulate the acidity for an industrially significant catalytic process (Beckmann rearrangement). Detailed characterization of hierarchically porous (HP) SAPO-37 reveals enhanced mass-transport characteristics and moderated acidity, which leads to superior catalytic performance and increased resistance to deactivation by coking, compared to its microporous counterpart, further vindicating the interplay between porosity and moderated acidity.