Novel catalytic systems for carbon dioxide activation

This work has developed a series of strategies for the efficient hydrogenation of CO2 to different products with applications as chemicals and fuels. The main focus has been on the synthesis of formic acid (FA). Thermodynamic and kinetic barriers imposed in the hydrogenation of CO2 to formic acid have been overcome by utilizing ionic liquids as additives in the reaction. In the first part the beneficial thermodynamic role of ionic liquids acting as a buffer is described. The buffering properties of ionic liquids are afterwards utilized to develop a structure-activity relationship for a range of catalysts including homogeneous and heterogeneous examples. This enables for the development of highly efficient catalysts in the hydrogenation of CO2 to FA. When CO2 is used as feedstock for carbon-based material, the reduction of the oxidation state at the carbon is critical in order to generate a highly diversified product portfolio. Formic acid is, beneath carbon monoxide, the first product with a reduced oxidation state at the carbon atom. Consequently, formic acid represents a perfect starting point for the development of decarbonised chemical value chains. Furthermore, it is both industrially relevant and has potential to be implemented in future technologies including hydrogen storage schemes, thus underlining the importance for the development for new processes for the synthesis of formic acid. However, the catalytic transformation of CO2 with hydrogen is thermodynamically and kinetically challenging. On the one hand, the formation of formic acid is thermodynamically uphill in gas phase and consequently the equilibrium is commonly shifted to the product side by the addition of stoichiometric amounts of bases. The key here is the formation of formate salts and adducts which would inevitably lead to tedious purifications and to the generation of large amounts of waste. Alternatively, the reaction is carried out in liquid phase which results in an acidic reaction media. On the other hand, CO2 is a kinetically inert molecule, thus necessitating highly nucleophilic, basic metal hydride complexes in order to activate CO2. If no bases are employed, the basicity of the active metal hydride complexes leads inevitably to a fast deactivation of the catalysts. This work addresses the thermodynamic challenges by utilizing ionic liquids (ILs) as additives in the reaction. The ILs act as a buffer during the reaction, thus stabilising the pH of the solution at higher values than that expected by the concentration of FA generated, whilst avoiding the formation of formate salts and adducts. Indeed, the additional stabilisation of formic acid falls into the range of an additional H-bond, compared to the pure solvent system. The buffering properties of ILs allowed also for the screening of different catalysts, including heterogeneous RuFe-nanoalloys. These nanoalloys displayed low activity and selectivity towards the formation of formic acid in comparison to the homogeneous counterparts. One major issue is the preferred formation of carbon monoxide under elevated temperatures, prohibiting efficient transformation rates. Consequently, a wide range of Ru complexes with pyridylideneamide ligands were screened for the hydrogenation of CO2 to formic acid. It was found that the rate of reaction can be enhanced by a) an increased electron density at the metal centre and b) by changing the dominant mechanism for the CO2 insertion into the metal hydride bond. The catalyst durability can be increased by increasing the resistance of the active hydride towards protonation. This is achieved by using pyridinium carbenes as coordinating sites. The catalyst stability and activity with pyridylideneamide ligands is in general good, in line with the literature reports. Based on the experience learned from the pyridylideneamide-based catalysts, a guideline for a highly robust, active and stable catalysts was created. The first example synthesised is a Ru-pincer complexes with N-heterocyclic (NHC) side arms as coordinating sides. Here, the NHC groups, mimic the strong electron donation observed with pyridylideneamides, whilst avoiding side reactions such as electrophilic attacks onto the ligand scaffold. This scaffold allows the performance of the catalyst at demanding reaction conditions without a loss in catalytic activity via the formation of heterogeneous nanoparticles. Furthermore, the structural design of the catalyst avoids the poisoning of the catalyst by the reagents, product or lewis acids. Consequently, the here developed catalyst achieved a TON of 833,800 with an initial turnover frequency (TOF) of 22,000 h-1. These results represent the best catalytic performance reported to date by a factor of 50 (TON) and 22 (TOF), compared to the best performing catalysts reported to date under base free conditions.

Efficient carbon dioxide hydrogenation to formic acid with buffering ionic liquids

The efficient transformation of CO2 into chemicals and fuels is a key challenge for the decarbonisation of the synthetic production chain. Formic acid (FA) represents the first product of CO2 hydrogenation and can be a precursor of higher added value products or employed as a hydrogen storage vector. Bases are typically required to overcome thermodynamic barriers in the synthesis of FA, generating waste and requiring post-processing of the formate salts. The employment of buffers can overcome these limitations, but their catalytic performance has so far been modest. Here, we present a methodology utilising IL as buffers to catalytically transform CO2 into FA with very high efficiency and comparable performance to the base-assisted systems. The combination of multifunctional basic ionic liquids and catalyst design enables the synthesis of FA with very high catalytic efficiency in TONs of >8*105 and TOFs > 2.1*104 h−1

Selective CO2 hydrogenation to formic acid with multifunctional ionic liquids

The development of simple, cost-effective and sustainable methods to transform CO2 into feedstock chemicals is essential to reduce the dependence on fossil fuels of the chemical industry. Here, we report the selective and efficient catalytic hydrogenation of CO2 to formic acid (FA) using a synergistic combination of an ionic liquid (IL) with basic anions and relatively simple catalysts derived from the precursor Ru3(CO)12]. Very high values of TON (17000) and TOF have been observed and FA solutions with concentrations of up to 1.2 M have been produced. In this system, the imidazolium based IL associated with the acetate anion acts as precursor for the formation of the catalytically active Ru-H species, catalyst stabilizer and as an acid buffer, shifting the equilibrium towards free formic acid. Moreover, the IL acts as an entropic driver (via augmentation of the number of microstates), lowering the entropic contribution imposed by the IL surrounding the catalytically active sites. The favorable thermodynamic conditions enable the reaction to proceed efficiently at low pressure pressures, and furthermore the immobilization of the IL onto a solid support facilitates the separation of FA at the end of the reaction

Blurring the boundary between homogenous and heterogeneous catalysis using palladium nanoclusters with dynamic surfaces.

Using a magnetron sputtering approach that allows size-controlled formation of nanoclusters, we have created palladium nanoclusters that combine the features of both heterogeneous and homogeneous catalysts. Here we report the atomic structures and electronic environments of a series of metal nanoclusters in ionic liquids at different stages of formation, leading to the discovery of Pd nanoclusters with a core of ca. 2 nm surrounded by a diffuse dynamic shell of atoms in [C4C1Im][NTf2]. Comparison of the catalytic activity of Pd nanoclusters in alkene cyclopropanation reveals that the atomically dynamic surface is critically important, increasing the activity by a factor of ca. 2 when compared to compact nanoclusters of similar size. Catalyst poisoning tests using mercury and dibenzo[a,e]cyclooctene show that dynamic Pd nanoclusters maintain their catalytic activity, which demonstrate their combined features of homogeneous and heterogeneous catalysts within the same material. Additionally, kinetic studies of cyclopropanation of alkenes mediated by the dynamic Pd nanoclusters reveal an observed catalyst order of 1, underpinning the pseudo-homogeneous character of the dynamic Pd nanoclusters

Investigating the impact of copper leaching on combustion characteristics and particulate emissions in HPCR diesel engines

© 2019 Elsevier Ltd Copper leaching in diesel fuel and its impact on combustion and emission characteristics of a Direct Injection High Pressure Common Rail (DI HPCR) diesel engine was investigated. This work was performed using a single cylinder Ricardo Hydra research engine fitted with a cylinder head, piston assembly, and crankshaft from a production 2.2 L DI diesel engine. A fuel conditioning device consisting of a helicoidally shaped copper duct and electromagnetic coils powered from the battery was installed along the fuel line just before the high pressure pump. A diesel fuel with a copper content of less than 0.2 ppm was used. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed an increase of copper content to 1 ppm when fuel flowed through the conditioning device prior to the injection and returned from the engine back to the fuel tank. Copper leaching from the conditioning device was confirmed using a bespoke test rig. Combustion characteristics were analysed via post-processing pressure measurements, while an AVL Smoke Meter was used to monitor particulate emissions. A pilot plus main strategy was used to achieve a target Brake Mean Effect Pressure (BMEP) typical of medium load. Soot reduction in the range of 7–14% was measured when the device was connected to the fuel line, compared to the baseline. The initiation and early development of combustion was also investigated using an unstirred, quiescent combustion chamber with optical access. High-speed photography showed that ignition probability was enhanced in presence of the fuel conditioning device

Unravelling synergistic effects in bi-metallic catalysts: deceleration of palladium–gold nanoparticle coarsening in the hydrogenation of cinnamaldehyde

In this work, we demonstrate that the synergistic effect of PdAu nanoparticles (NPs) in hydrogenation reactions is not only related to high activity but also to their stability when compared to Pd mono-metallic NPs. To demonstrate this, a series of mono- and bi-metallic NPs: Pd, Pd0.75Au0.25, Pd0.5Au0.5, Pd0.25Au0.75 and Au in ionic liquid [C4C1Im][NTf2] have been fabricated via a magnetron sputtering process. Bi-metallic NPs possess external shells enriched with Pd atoms that interact with [NTf2]− of the ionic liquid resulting in enhanced catalytic performance in hydrogenation of cinnamaldehyde compared to their mono-metallic counterparts. This is ascribed to their higher stability over 24 h of reaction, whilst the catalytic activity and selectivity are comparable for both catalysts. Using a bespoke kinetic model for in situ catalyst deactivation investigations and electron microscopy imaging at the nanoscale, we have shown that PdAu has a deactivation rate constant of 0.13 h−1, compared to 0.33 h−1 for Pd NPs, leaving 60% and 40% of available sites after the reaction, respectively. Beyond that, the kinetic model demonstrates that the reaction product has a strong stabilizing factor for bimetallic NPs against coarsening and deactivation, which is not the case for Pd NPs. In summary, our kinetic model enables the evaluation of the catalyst performance over the entire chemical reaction space, probing the contribution of each individual component of the reaction mixture and allowing the design of high-performance catalysts

Imaging and analysis of covalent organic framework crystallites on a carbon surface: a nanocrystalline scaly COF/nanotube hybrid

Synthesis of covalent organic frameworks (COFs) is well-advanced but understanding their nanoscale structure and interaction with other materials remains a significant challenge. Here, we have developed a methodology for the detailed imaging and analysis of COF crystallites using carbon nanotube substrates for COF characterisation. Detailed investigation using powder X-ray diffraction, infrared spectroscopy, mass spectrometry and scanning electron microscopy in conjunction with a local probe method, transmission electron microscopy (TEM), revealed details of COF growth and nucleation at the nanoscale. A boronate ester COF undergoes preferential growth in the a–b crystallographic plane under solvothermal conditions. Carbon nanotubes were found to not impact the mode of COF growth, but the crystallites on nanotubes were smaller than COF crystallites not on supports. COF crystalline regions with sizes of tens of nanometres exhibited preferred orientation on nanotube surfaces, where the c-axis is oriented between 50 and 90° relative to the carbon surface. The COF/nanotube hybrid structure was found to be more complex than the previously suggested concentric core–shell model and can be better described as a nanocrystalline scaly COF/nanotube hybrid

Elwert (Georg) : Wirtschaft und Herrschaft von « Däxome » (Dahomey) im 18. Jahrhundert. Ökonomie des Sklavenraubs und Gesellschaftsstruktur 1724 bis 1818. Verbunden mit Untersuchungen über Verwendung und Bestimmung der Begriffe Klasse, Macht und Religion in diesem Kontext

Weilhard Werner. Elwert (Georg) : Wirtschaft und Herrschaft von « Däxome » (Dahomey) im 18. Jahrhundert. Ökonomie des Sklavenraubs und Gesellschaftsstruktur 1724 bis 1818. Verbunden mit Untersuchungen über Verwendung und Bestimmung der Begriffe Klasse, Macht und Religion in diesem Kontext. In: Revue française d'histoire d'outre-mer, tome 62, n°229, 4e trimestre 1975. pp. 689-691

Elwert (Georg) : Wirtschaft und Herrschaft von « Däxome » (Dahomey) im 18. Jahrhundert. Ökonomie des Sklavenraubs und Gesellschaftsstruktur 1724 bis 1818. Verbunden mit Untersuchungen über Verwendung und Bestimmung der Begriffe Klasse, Macht und Religion in diesem Kontext

Weilhard Werner. Elwert (Georg) : Wirtschaft und Herrschaft von « Däxome » (Dahomey) im 18. Jahrhundert. Ökonomie des Sklavenraubs und Gesellschaftsstruktur 1724 bis 1818. Verbunden mit Untersuchungen über Verwendung und Bestimmung der Begriffe Klasse, Macht und Religion in diesem Kontext. In: Revue française d'histoire d'outre-mer, tome 62, n°229, 4e trimestre 1975. pp. 689-691