A High Pressure Operando Spectroscopy Examination of Bimetal Interactions in 'Metal Efficient' Palladium/In2 O3 /Al2 O3 Catalysts for CO2 Hydrogenation.

CO2 hydrogenation to methanol has the potential to serve as a sustainable route to a wide variety of hydrocarbons, fuels and plastics in the quest for net zero. Synergistic Pd/In2 O3 (Palldium on Indium Oxide) catalysts show high CO2 conversion and methanol selectivity, enhancing methanol yield. The identity of the optimal active site for this reaction is unclear, either as a Pd-In alloy, proximate metals, or distinct sites. In this work, we demonstrate that metal-efficient Pd/In2 O3 species dispersed on Al2 O3 can match the performance of pure Pd/In2 O3 systems. Further, we follow the evolution of both Pd and In sites, and surface species, under operando reaction conditions using X-ray Absorption Spectroscpy (XAS) and infrared (IR) spectroscopy. In doing so, we can determine both the nature of the active sites and the influence on the catalytic mechanism.

Understanding catalytic CO2 and CO conversion into methanol using computational fluid dynamics.

The kinetics of methanol synthesis from a mixture of CO2/CO/H2 have been widely studied in the literature. Yet the role of direct CO hydrogenation is still unclear, in terms of predicting and developing an accurate kinetic model. To investigate, a computational fluid dynamics model has been developed, incorporating two distinct kinetic models, one which includes CO hydrogenation and one which does not. Including CO hydrogenation in the kinetic model provides a more complex interaction between the three involved reactions and can better predict potential inhibitions caused by the presence of H2O. This, however, increases the complexity of the kinetic model. The benefit of applying a fluid dynamics model to study fixed bed reactors is demonstrated, as it offers unique insights into the spatial species concentration, temperature variations, and reaction rate magnitudes. The validated model is shown to be a powerful interrogative tool, capable of supporting system optimization across the catalyst and reactor engineering sectors.

The Significance of Metal Coordination in Imidazole-Functionalized Metal-Organic Frameworks for Carbon Dioxide Utilization.

The grafting of imidazole species onto coordinatively unsaturated sites within metal-organic framework MIL-101(Cr) enables enhanced CO2 capture in close proximity to catalytic sites. The subsequent combination of CO2 and epoxide binding sites, as shown through theoretical findings, significantly improves the rate of cyclic carbonate formation, producing a highly active CO2 utilization catalyst. An array of spectroscopic investigations, in combination with theoretical calculations reveal the nature of the active sites and associated catalytic mechanism which validates the careful design of the hybrid MIL-101(Cr).

Exploring the origins of crystallisation kinetics in hierarchical materials using in situ X-ray diffraction and pair distribution function analysis.

The discovery of novel catalytic materials is predicated on understanding contemporary synthetic processes. With this fundamental knowledge in place it becomes possible to modify the final material with subtle changes to the synthesis process. In this vein, hierarchical materials, formed by the addition of a mesoporogen within the hydrothermal synthesis, have attracted a significant amount of attention due to their catalytic benefits over analogous microporous species. In this work we monitor the hydrothermal synthesis in situ of a hierarchical and a microporous aluminophosphate, for the first time, combining total scattering and pairwise distribution function data. In doing so we observe the local formation of the species, and the longer range crystallisation processes concurrently.

Adsorption Microcalorimetry of CO2 in Confined Aminopolymers.

Aminopolymers confined within mesoporous supports have shown promise as materials for direct capture of CO2 from ambient air. In spite of this, relatively little is known about the energetics of CO2 binding in these materials, and the limited calorimetric studies published to date have focused on materials made using molecular aminosilanes rather than amine polymers. In this work, poly(ethylenimine) (PEI) is impregnated within mesoporous SBA-15, and the heats of CO2 adsorption at 30 °C are investigated using a Tian-Calvet calorimeter with emphasis on the role of PEI loading and CO2 pressure in the compositional region relevant to direct capture of CO2 from ambient air. In parallel, CO2 uptakes of these materials are measured using multiple complementary approaches, including both volumetric and gravimetric methods, and distinct changes in uptake as a function of CO2 pressure and amine loading are observed. The CO2 sorption behavior is directly linked to textural data describing the porosity and PEI distribution in the materials.

Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites.

A combined computational and experimental approach is used to elucidate the effect of silica support morphology on polymer dynamics and CO2 adsorption capacities in aminopolymer/silica composites. Simulations are based on coarse-grained molecular dynamics simulations of aminopolymer composites where a branched aminopolymer, representing poly(ethylenimine) (PEI), is impregnated into different silica mesoporous supports. The morphology of the mesoporous supports varies from hexagonally packed cylindrical pores representing SBA-15, double gyroids representing KIT-6 and MCM-48, and cagelike structures representing SBA-16. In parallel, composites of PEI and the silica supports SBA-15, KIT-6, MCM-48, and SBA-16 are synthesized and characterized, including measuring their CO2 uptake. Simulations predict that a 3D pore morphology, such as those of KIT-6, MCM-48, and SBA-16, will have faster segmental mobility and have lower probability of primary amine and surface silanol associations, which should translate to higher CO2 uptake in comparison to a 2D pore morphology such as that of SBA-15. Indeed, it is found that KIT-6 has higher CO2 uptake than SBA-15 at equivalent PEI loading, even though both supports have similar surface area and pore volume. However, this is not the case for the MCM-48 support, which has smaller pores, and SBA-16, whose pore structure rapidly degrades after PEI impregnation.

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.

Unraveling the Dynamics of Aminopolymer/Silica Composites.

The structure and dynamics of a model branched polymer was investigated through molecular dynamics simulations and neutron scattering experiments. The polymer confinement, monomer concentration, and solvent quality were varied in the simulations and detailed comparisons between the calculated structural and dynamical properties of the unconfined polymer and those confined within an adsorbing and nonadsorbing cylindrical pore, representing the silica based structural support of the composite, were made. The simulations show a direct relationship in the structure of the polymer and the nonmonotonic dynamics as a function of monomer concentration within an adsorbing cylindrical pore. However, the nonmonotonic behavior disappears for the case of the branched polymer within a nonadsorbing cylindrical pore. Overall, the simulation results are in good agreement with quasi-elastic neutron scattering (QENS) studies of branched poly(ethylenimine) in mesoporous silica (SBA-15) of comparable size, suggesting an approach that can be a useful guide for understanding how to tune porous polymer composites for enhancing desired dynamical and structural behavior targeting carbon dioxide adsorption.

Spectroscopic investigation into the design of solid-acid catalysts for the low temperature dehydration of ethanol.

The increased demand for bulk hydrocarbons necessitates research into increasingly sustainable, energy-efficient catalytic processes. Owing to intricately designed structure-property correlations, SAPO-34 has become established as a promising material for the low temperature ethanol dehydration to produce ethylene. However, further optimization of this process requires a precise knowledge of the reaction mechanism at a molecular level. In order to achieve this a range of spectroscopic characterization techniques are required to probe both the interaction with the active site, and also the wider role of the framework. To this end we employ a combination of in situ infra-red and neutron scattering techniques to elucidate the influence of the surface ethoxy species in the activation of both diethyl ether and ethanol, towards the improved formation of ethylene at low temperatures. The combined conclusions of these studies is that the formation of ethylene is the rate determining step, which is of fundamental importance towards the development of this process and the introduction of bio-ethanol as a viable feedstock for ethylene production.

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts.

A combined electronic structure computational and X-ray absorption spectroscopy study was used to investigate the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Probing the nature of the molecular species at the atomic level has led to the identification of a unique Co-O-Ti bond, which serves as the loci for the superior performance of the bimetallic catalyst, when compared with its analogous monometallic counterpart. The structural and spectroscopic features associated with this active site have been characterized and contrasted, with a view to affording structure-property relationships, in the wider context of designing sustainable catalytic oxidations with porous solids.

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.

Combined spectroscopic and computational study for optimising catalyst design in hydrocarbon transformations.

Molecular interactions of hydrocarbons within the confined pores of heterogeneous catalysts can influence reaction pathways, which play a crucial role in determining the overall efficacy of catalytic transformations. We probe the interactions of n-butane with a solid-acid zeolite, mordenite, combining inelastic neutron scattering with DFT calculations. This reveals that the solid-acid sites within mordenite induce a conformer change, which could be key in designing optimised catalysts, for hydrocarbon transformations.

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.

Thiol Functionalised Supports for Controlled Metal Nanoparticle Formation for Improved C-C Coupling.

The myriad applications of metal nanoparticle systems have individual demands on their size, shape and electronic states, demanding novel synthetic methods to optimise these properties. Herein we report our method of exploiting strong thiol-Pd binding as a precursor for forming small, uniform Pd nanoparticles on activation. We validate our approach with a range of characterisation techniques and contrast our design strategy with an analogous wetness impregnation method, showing the drastic improvements for catalytic C-C coupling. The presence of the thiol groups offers greater control over nanoparticle formation, particularly temperature resolution on activation, potentially allowing more targeted nanoparticle formation procedures.

Designing Multi-Dopant Species in Microporous Architectures to Probe Reaction Pathways in Solid-Acid Catalysis.

The introduction of two distinct dopants in a microporous zeotype framework can lead to the formation of isolated, or complementary catalytically active sites. Careful selection of dopants and framework topology can facilitate enhancements in catalysts efficiency in a range of reaction pathways, leading to the use of sustainable precursors (bioethanol) for plastic production. In this work we describe our unique synthetic design procedure for creating a multi-dopant solid-acid catalyst (MgSiAPO-34), designed to improve and contrast with the performance of SiAPO-34 (mono-dopant analog), for the dehydration of ethanol to ethylene. We employ a range of characterization techniques to explore the influence of magnesium substitution, with specific attention to the acidity of the framework. Through a combined catalysis, kinetic analysis and computational fluid dynamics (CFD) study we explore the reaction pathway of the system, with emphasis on the improvements facilitated by the multi-dopant MgSiAPO-34 species. The experimental data supports the validation of the CFD results across a range of operating conditions; both of which supports our hypothesis that the presence of the multi-dopant solid acid centers enhances the catalytic performance. Furthermore, the development of a robust computational model, capable of exploring chemical catalytic flows within a reactor system, affords further avenues for enhancing reactor engineering and process optimisation, toward improved ethylene yields, under mild conditions.

The influence of porosity on nanoparticle formation in hierarchical aluminophosphates.

In this work we explore the deposition of gold onto a silicoaluminophosphate, using a variety of known nanoparticle deposition techniques. By comparing the gold particles deposited on a traditional microporous aluminophosphate, with an analogous hierarchical species, containing both micropores and mesopores, we explore the influence of this dual porosity on nanoparticle deposition. We show that the presence of mesopores has limited influence on the nanoparticle properties, but allows the system to maintain porosity after nanoparticle deposition. This will aid diffusion of reagents through the system, allowing continued access to the active sites in hierarchical systems, which offers significant potential in catalytic oxidation/reduction reactions.

Role of Alumina Basicity in CO2 Uptake in 3-Aminopropylsilyl-Grafted Alumina Adsorbents.

Oxide-supported amine materials are widely known to be effective CO2 sorbents under simulated flue-gas and direct-air-capture conditions. Most work has focused on amine species loaded onto porous silica supports, though potential stability advantages may be offered through the use of porous alumina supports. Unlike silica materials, which are comparably inert, porous alumina materials can be tuned to have substantial acidity and/or basicity. Owing to their amphoteric nature, alumina supports play a more active role in CO2 sorption than silica supports, potentially directly participating in the adsorption process. In this work, primary amines associated with 3-aminopropyltriethoxysilane are grafted onto two different mesoporous alumina materials having different levels of basicity. Adsorbent materials with different amine loadings are prepared, and the CO2 -adsorption behavior of similar amines on the two alumina supports is demonstrated to be different. At low amine loadings, the inherent properties of the support surface play a significant role, whereas at high amine loadings, when the alumina surface is effectively blocked, the sorbents prepared on the two supports behave similarly. At high amine loadings, amine-CO2 -amine interactions are shown to dominate, leading to adsorbed species that appear similar to the species formed over silica-supported amine materials. The sorbent properties are comprehensively characterized using N2 physisorption analysis, in situ FTIR spectroscopy, and adsorption microcalorimetry.

Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study.

Composite gas sorbents, formed from an active polymer phase and a porous support, are promising materials for the separation of acid gases from a variety of gas streams. Significant changes in sorption performance (capacity, rate, stability etc.) can be achieved by tuning the properties of the polymer and the nature of interactions between polymer and support. Here we utilize quasielastic neutron scattering (QENS) and coarse-grained molecular dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), both in bulk form and when supported in a mesoporous silica framework. The polymer chain dynamics (rotational and translational diffusion) are characterized using two neutron backscattering spectrometers that have overlapping time scales, ranging from picoseconds to a few nanoseconds. Two modes of motion are detected for the PEI molecule in QENS. At low energy transfers, a "slow process" on the time scale of ∼200 ps is found and attributed to jump-mediated, center-of-mass diffusion. A second, "fast process" at ∼20 ps scale is also found and is attributed to a locally confined, jump-diffusion. Characteristic data (time scale and spectral weight) of these processes are compared to those characterized by MD, and reasonable agreement is found. For the nanopore-confined PEI, we observe a significant reduction in the time scale of polymer motion as compared to the bulk. The impacts of silica surface functionalization and of polymer fill fraction in the silica pores (controlling the portion of polymer molecules in contact with the pore walls), are both studied in detail. Hydrophobic functionalization of the silica leads to an increase of the PEI mobility above that in native silanol-terminated silica, but the dynamics are still slower than those in bulk PEI. Sorbents with faster PEI dynamics are also found to be more efficient for CO2 capture, possibly because sorption sites are more accessible than those in systems with slower PEI dynamics. Thus, this work supports the existence of a link between the affinity of the support for PEI and the accessibility of active sorbent functional groups.

Design of Highly Selective Platinum Nanoparticle Catalysts for the Aerobic Oxidation of KA-Oil using Continuous-Flow Chemistry.

Highly active and selective aerobic oxidation of KA-oil to cyclohexanone (precursor for adipic acid and ɛ-caprolactam) has been achieved in high yields using continuous-flow chemistry by utilizing uncapped noble-metal (Au, Pt & Pd) nanoparticle catalysts. These are prepared using a one-step in situ methodology, within three-dimensional porous molecular architectures, to afford robust heterogeneous catalysts. Detailed spectroscopic characterization of the nature of the active sites at the molecular level, coupled with aberration-corrected scanning transmission electron microscopy, reveals that the synthetic methodology and associated activation procedures play a vital role in regulating the morphology, shape and size of the metal nanoparticles. These active centers have a profound influence on the activation of molecular oxygen for selective catalytic oxidations.

An efficient low-temperature route to nitrogen-doping and activation of mesoporous carbons for CO2 capture.

An innovative strategy for post-synthesis nitrogen-doping of mesoporous carbons (MCs) with high yields (>90%) at low temperatures (230-380 °C) by using a strong base, sodium amide (NaNH2), was developed. The as-prepared N-doped MCs exhibit a significantly enhanced CO2 adsorption performance in terms of capacity and selectivity when compared to their parent MCs.