No description available.

Cocatalyzed reactions using Brønsted acids and chiral N-heterocyclic carbenes to yield highly enantioselective products have been reported recently in many journals. The development of new chiral N-heterocyclic carbenes is a competitive field among synthetic chemist. In a recent study we found that conjugated trienes could be cyclized using Brønsted acids and chiral N-heterocyclic carbenes. The synthesis of novel chiral N-heterocyclic carbene precursors, and the precursors to novel conjugated trienes are reported herein.

The rapid global climate change associated with increasing planetary CO$_2$ levels is possibly one of the greatest challenges existing currently. In order to address this grave problem, a variety of solutions and approaches have been proposed. It is likely that a combination of these approaches would be required to solve the multi-dimensional problem of climate change. One potential approach to mitigate carbon emissions is the concept of a ‘Circular Carbon Economy’. This approach encompasses the concept of capturing carbon emissions and reusing the captured CO$_2$ to make fuels and chemicals using renewable energy. Use of fuels and chemicals manufactured via this approach would thus avoid ‘new’ CO$_2$ emissions and prevent the accumulation of additional CO$_2$ in the atmosphere as these products will be CO$_2$-neutral. The use of CO$_2$-neutral fuels would especially be beneficial as not only would it cause a significant impact on CO$_2$ emissions in terms of volume but also it would provide a way to store energy from intermittent sources like solar, wind etc. Furthermore, these fuels can be used without requiring a significant overhaul of the energy infrastructure. One of the most promising routes for the synthesis of fuels and chemicals from CO$_2$ is via the thermal hydrogenation of CO$_2$ using multifunctional heterogeneous catalysis. Multifunctional catalysis refers to the combination of catalysts having different functionalities into a single reactor (one-pot). This catalytic route is a powerful tool for tuning the product distribution during a reaction and for enhancing the yield of target products. Thus, this PhD Thesis describes the design of several multifunctional catalyst combinations which have been applied for producing various hydrocarbon products of interest from CO$_2$ ranging from light olefins, aromatics and fuel range paraffins. The catalyst combinations consisted of a metal/metal oxide and a zeolite and depending on the configuration used, enhanced the selectivity to target products. Various advanced characterization techniques have also been utilized in order to reveal the status of active species and the underlying reaction mechanism(s).

Doctor of Philosophy / Department of Chemical Engineering / Bin Liu / In the course of mitigating our dependence on fossil energy, it has become an urgent issue to develop unconventional and innovative technologies based on renewable energy utilization for fuels and chemicals production. Due to the lack of fundamental understanding of catalytic behaviors of the novel chemical compounds involved, the task to design and engineer effective catalytic systems is extremely challenging and time-consuming. One central challenge is that an intricate balance among catalytic reactivity, selectivity, durability, and affordability must be achieved pertinent to any successful design. In this dissertation, density functional theory (DFT), coupled with modeling techniques derived from DFT, is employed to gain insights into molecular interactions between elusive intermediates and targeted functional catalytic materials for novel electrochemical and heterogeneous catalytic processes. Two case studies, i.e., electroreduction of furfural and step-catalysis for cyclic ammonia production, will be discussed to demonstrate the capability and utility of DFT-based theoretical modeling toolkits and strategies. Transition metal cathodes such as silver, lead, and nickel were evaluated for furfuryl alcohol and 2-methylfuran production through detailed DFT modeling. Investigation of the molecular mechanisms revealed that two intermediates, mh6 and mh7 from mono-hydrogenation of furfural, are the key intermediates that will determine the product formation activities and selectivities. Nickel breaks the trends from other metals as DFT calculations suggested the 2-methylfuran formation pathway is most likely different from other cathodes. In this work, the Brønsted–Evans–Polanyi relationship, derived from DFT energy barrier calculations, has been found to be particularly reliable and computationally efficient for C-O bond activation trend predictions. To obtain the solvation effect on the adsorptions of biomass-derived compounds (e.g., furfural and glycerol), influence of explicit solvent was probed using periodic DFT calculations. The adsorptions of glycerol and its dehydrogenation intermediates at the water-platinum surface were understood via various water–adsorbate, water–water, and water–metal interactions. Interestingly, the bond-order-based scaling relationship established in solvent-free environment is found to remain valid based on our explicit solvent models. In the second case study, step-catalysis that relies on manganese’s ability to dissociate molecular nitrogen and as a nitrogen carrier emerges as an alternative route for ammonia production to the conventional Haber-Bosch process. In this collaborative project, DFT was used as the primary tool to produce the mechanistic understanding of NH3 formation via hydrogen reduction on various manganese nitride systems (e.g., Mn4N and Mn2N). Both nickel and iron dopants have the potential to facilitate NH3 formation. A broader consideration of a wide range of nitride configurations revealed a rather complex pattern. Materials screening strategies, supported by linear scaling relationships, suggested the linear correlations between NHx (x=0, 1, 2) species must be broken in the development of optimal step catalysis materials. These fundamental findings are expected to significantly guide and accelerate the experimental material design. Overall, molecular modeling based on DFT has clearly demonstrated its remarkable value beyond just a validation tool. More importantly, its unique predictive power should be prized as an avenue for scientific advance through the fundamental knowledge in novel catalysts design.

Doctor of Philosophy / Department of Chemical Engineering / Bin Liu / Efficient and selective catalysis lies at the heart of many chemical reactions, enabling the synthesis of chemicals and fuels with enormous societal and technological impact. A fundamental understanding of intrinsic catalyst properties for effective manipulation of the reactivity and selectivity of industrial catalysts is essential to select proper catalysts to catalyze the reactions we want and hinder the reactions we do not want. The progress in density functional theory (DFT) makes it possible to describe interfacial catalytic reactions and predict catalytic activities from one catalyst to another. In this study, water-gas shift reaction (WGSR) was used as a model reaction. First-principles based micro-kinetic modeling has been performed to deeply understand interactions between competing reaction mechanisms, and the relationship with various factors such as catalyst materials, structures, promoters, and interactions between intermediates (e.g., CO self-interaction) that govern the observed catalytic behaviors. Overall, in this thesis, all relevant reaction mechanisms in the model reaction on well-defined active sites were developed with first-principles calculations. With the established mechanism, the promotional effect of K adatom on Ni(111) on WGSR compared to the competing methanation was understood. Moreover, the WGSR kinetic trend, with the hydrogen production rate decreasing with increasing Ni particle diameters (due to the decreasing fractions of low-coordinated surface Ni site), was reproduced conveniently from micro-kinetic modeling techniques. Empirical correlations such as Brønsted-Evans-Polanyi (BEP) relationship for O-H, and C-O bond formation or cleavage on Ni(111), Ni(100), and Ni(211) were incorporated to accelerate computational analysis and generate trends on other transition metals (e.g., Cu, Au, Pt). To improve the numerical quality of micro-kinetic modeling, later interactions of main surface reaction intermediates were proven to be critical and incorporated successfully into the kinetic models. Finally, evidence of support playing a role in the enhancement of catalyst activity and the impact on future modeling will be discussed. DFT will be a powerful tool for understanding and even predicting catalyst performance and is shaping our approach to catalysis research. Such molecular-level information obtained from computational methods will undoubtedly guide the design of new catalyst materials with high precision.

Fuel cells represent a promising technology for clean power generation because they convert chemical energy (fuel) into electrical energy with high efficiency and low-to-none emission of pollutants. Direct ethanol fuel cells (DEFCs) have several advantages compared to the most studied hydrogen and methanol fuel cells. First and foremost, ethanol is a non-toxic liquid, which lowers the investment of handling facilities because the current infrastructure for gasoline can be largely used. Second, ethanol can be conveniently produced from biomass, hence is carbon neutral which mitigates increasing atmospheric CO2. Last but not least, if completely oxidized to CO2, ethanol has a higher energy density than methanol since it can deliver 12 electrons per molecule. The almost exclusive oxidation to acetic acid overshadows the attractiveness of DEFCs considerably, as the energy density is divided by 3. The standard potential of acetic acid formation indicates that a reaction path including acetic acid, leads to inevitable potential losses of about 0.4 V (difference between ideal potential for CO2 and acetic acid "production"). The development of alkaline DEFCs had also been hampered by the lack of stable and efficient anion exchange membranes. Fortunately, this challenge has been well tackled in recent years,8,9 making the development of alkaline fuel cells (AFCs) which are of particular technological interest due to their simple designs and ability to operate at low temperatures (25-100 °C). In alkaline conditions, the kinetic of both the cathodic oxygen reduction and the anodic ethanol oxidation is facilitated. Furthermore, the expensive Pt catalyst can be replaced by the lower-cost and more active transition metals such as Pd. The main objectives of this project are: i) to provide detailed fundamental understanding of ethanol oxidation reaction on transition metal surfaces in alkaline media, ii) to propose the best rational catalyst design strategies to cleave the C–C bond during ethanol electrooxidation. To achieve these goals two methodologies are used, i.e., in-situ identification of ethanol electrooxidation products using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and mechanistic investigation using computational studies in the framework of density functional theory (DFT). The PM-IRRAS technique was advanced in this project to the level of distinguishing electrooxidation products at the surface of the nanoparticles (electrode) and in the bulk-phase of the electrolyte. This new PM-IRRAS utility makes it possible to detect molecules such as CO2 which desorbs from the catalyst surface as soon as they are formed. The DFT insights in this project, provides an explanation as to why it is difficult to break the C–C bond in ethanol and is used for screening the top candidate metals for further studies.

Computational chemistry is a branch of modern chemistry that utilizes the computers to solve chemical problems. The fundamental of computational chemistry is Schrödinger equation. To solve the equation, researchers developed many methods based on BornOppenheimer Approximation, such as Hartree-Fock method and DFT method, etc. Computational chemistry is now widely used on reaction mechanism study and new chemical designing. In the first project described in Chapter 3, we designed phosphine oxide modified Ag3, Au3 and Cu3 nanocluster catalysts with DFT method. We found that these catalysts were able to catalyze the activation of H2 by cleaving the H-H bond asymmetrically. The activated catalyst-2H complex can be further used as reducing agent to hydrogenate CO molecule to afford HCHO. The mechanism study of these catalysts showed that the electron transfer from electron-rich metal clusters to O atom on the phosphine oxide ligand is the major driving force for H2 activation. In addition, different substituent groups on phosphine oxide ligand were tested. Both H affinity of metal and the substituent groups on ligand can both affect the activation energy. Another project described in Chapter 4 is the modelling of catalyst with DFT. We chose borane/NHC frustrated Lewis pair (FLP) catalyzed methane activation reaction as example to establish a relationship between activation energy and catalysts’ physical properties. After performing simulation, we further proved the well-accepted theory that the electron transfer is the main driving force of catalysis. Furthermore, we were able to establish a linear relationship for each borane between activation energy and the geometrical mean value of HOMO/LUMO energy gap (ΔEMO). Based on that, we introduced the formation energy of borane/NHC complex (ΔEF) and successfully established a generalized relationship between Ea and geometrical mean value of ΔEMO and ΔEF. This model can be used to predict reactivity of catalysts.

Despite the growing interest in heterogeneous polymerization catalysis, the majority of the polymerization catalysts used industrially are single-use entities that are left in the polymer product. Recoverable and recyclable polymerization catalysts have not reached the industrial utility of single-use catalysts because the catalyst and product separation have not become economical. The successful development of recyclable transition metal polymerization catalysts must take a rational design approach, hence academic and industrial researchers need to further expand the fundamental science and engineering of recyclable polymerization catalysis to gain an understanding of critical parameters that allow for the design of economically viable, recoverable solid polymerization catalysts. Unfortunately, the rapid development of Atom Transfer Radical Polymerization over the past 10 years has not resulted in its wide spread industrial practice. Numerous reports regarding the immobilization of transition metal ATRP catalysts, in attempts to increase its applicability, have extended the fundamentals of recyclable polymerization catalysis. However, for industrial viability, more research is required in the area of how the catalyst complex immobilization methodology and support structure affect the catalyst polymerization performance, regeneration, and recyclability. A comprehensive rational catalyst design approach of silica-immobilized ATRP catalyst was undertaken to answer these questions and are discussed here.

No description available.

No description available.