In order to design more effective catalysts, we deployed X-ray absorption spectroscopy (XAS) to develop structure-activity relationships for supported heterogeneous catalysts. Starting from carefully-designed synthesis, the metal ions are uniformly dispersed on the support. The pre-treatment and other synthetic modifications are strategically chosen to alter the structure and reactivity of the metal sites. Specially, we explored catalytic systems in which the active sites are mononuclear metal complexes grafted on the oxide supports. XAS is essential for probing the local structures of the metal centers, such as electronic properties and coordination geometry. The applications of the catalysts include olefin metathesis, propane dehydrogenation and C-O hydrogenolysis. Monomolybdate sites with relatively uniform first coordination spheres were synthesized by a direct solid-solid reaction between MoO2Cl2 and silica. The resulting Mo(=O)2(OSi)2 sites were characterized and compared to the spectra of several well-defined model molybdate compounds. In extended X-ray absorption fine structure (EXAFS) analysis, the attenuation of the long-range EXAFS features at ca. 3 Å in Mo(=O)2(OSi)2 sites suggests the attachment to the silica surface as molybdasiloxane rings of variable ring size. The partially well-defined Mo(=O)2(OSi)2 sites were treated with a few common activation methods, but failed to catalyze propylene metathesis. Only exposure to olefins at high temperatures (>500 C) leads to promoting propylene metathesis at 23 C and resulting in an active site fraction as high as 36%. However, the co-existence of multiple types of metal sites makes it difficult to identify the active sites. GaiBu3/γ-Al2O3 catalyst promotes propane dehydrogenation at 550 °C, and the Ga species present under reaction conditions were unknown. We used operando XAS to monitor the evolution of Ga K-edge from as-prepared to catalytic conditions. The X-ray absorption near edge structure (XANES) reveals that isobutyl ligands are replaced by oxygen-donor ligands derived from the alumina surface in a single, gradual process, leading to a 3-coordinate mononuclear Ga site at 550 °C. Additional, minor Ga(I) species may form simultaneously. DFT-based simulations elucidate the ligand influence on the Ga K-edge XANES features, such as the white line intensity and edge position. This structural insight sheds light on the correlation between the mononuclear Ga site and the subsequent catalytic activity. The direct synthesis of single-atom Fe catalysts supported on nitrogen-doped carbon gives five-coordinate Fe(III), with four in-plane pyridinic nitrogen donors and one axial hydroxyl ligand. These Fe catalysts exhibit high selectivity, activity, and reusability for C–O bond hydrogenolysis using H2 at temperatures as low as 170 °C. The characterization of the catalyst shows the reduction of Fe(III) to Fe(II) sites with loss of the hydroxyl ligand under reaction conditions. The four-coordinate Fe(II) sites are the active sites for H2-splitting. The interaction between metal sites and the support is critical to promote activity.

Flow chemistry is one of the most efficient ways of practicing chemistry. The development of reliable flow chemistry methods is of great significance from an industrial perspective since safety and ease in scaling up processes are this technology's key advantages. Herein we discuss the development of novel flow methodologies for making azacycles and amides.Azacycles are an essential class of pharmaceutically active compounds whose synthesis is generally conducted in batch by methods involving highly functionalized reagents and a homogeneous catalyst with or without a promoter. A more direct route to azacycles, for example, from ethers, would enable more streamlined, efficient, and sustainable processes. This work synthesized a wide range of N-substituted pyrrolidines, piperidines, and pyrroles by the direct reaction of cyclic ethers or furans with primary amines. Amorphous silica-alumina, a simple and readily available solid acid, catalyzes the cyclocondensation reaction for both aromatic and aliphatic amines and allows the reaction to be conducted continuously in a packed-bed reactor. When the reactant is a simple ether that can also serve as the solvent, N-substituted azacycles are obtained in high yield by simple solvent removal without further workup. The solid acid-catalyzed reaction in flow provides a simple and efficient route to N-substituted azacycles. Amide bond formation is a crucial transformation in pharmaceutical manufacturing. Conventional protocols involve harsh conditions or the stoichiometric usage of toxic coupling agents such as HATU or EDC. Amidation of esters is a promising strategy because it requires cheap, stable, and readily available methyl or ethyl esters and produces volatile alcohol as the only byproduct. In this project, we have developed an efficient continuous flow protocol with short residence times using amorphous silica-alumina as the catalyst.

Polyethylene is a ubiquitously used commodity plastic due to its low cost, superior physical properties, and chemical inertness. Despite the wide use of polyethylene, current recycling technologies fail to recover sufficient value from waste polyethylene to promote a circular plastic economy. The world urgently needs to develop and implement new effective strategies to decrease the amount of discarded plastic while simultaneously reducing new inputs of fossil carbon feedstock and energy.Advanced recycling involves chemically deconstructing polyethylene to form useful products that can be reused. These chemical processes are often accelerated and facilitated by catalysts. The combination of specific catalysts under the right reaction allows polyethylene to be transformed in novel ways. This dissertation focuses on two catalytic transformations for recycling polyethylene. The first recycling strategy that was investigated involves converting polyethylene to propylene, the chemical precursor needed to make polypropylene, another widely used commodity plastic. The transformation combines three catalytic reactions: transfer dehydrogenation, ethenolysis, and olefin isomerization. Using MTO/Cl-Al2O3 as a bifunctional ethenolysis and isomerization catalyst with mono-unsaturated polyethylene, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture. For this result, the rate of propylene formation was limited by the isomerization catalyst (Cl-Al2O3, 16 times slower than the rate of ethenolysis). By adding a more active and less ethylene-inhibited isomerization catalyst (SiO2-Al2O3), the rate of propylene formation in tandem ethenolysis/isomerization of polyethylene was increased by a factor of five. A preliminary life cycle assessment of the process suggests moderate greenhouse gases emissions reductions could be achieved by adopting this technology. Second, polyethylene was upgraded to value-added α,ω-divinyl-functionalized oligomers with shorter, tunable chain lengths via a sequence of bromination, dehydrobromination, and olefin metathesis reactions. These oligomers can potentially be polymerized to form polyethylene. The three chemical transformations were conducted in series and achieved high yields each individual step. Preliminary technoeconomic analyses demonstrate that this three-step process could be economically viable on an industrial scale. The catalyst synthesis procedures and catalytic reactions were conducted using standard glovebox and Schlenk line techniques to ensure that the catalysts were not deactivated by air and moisture. High pressure batch reactions were performed in Parr reactors. The products of the chemical reactions were analyzed using a variety of techniques including GC-FID, GC-TCD, GC-MS, 1H and 13C NMR, GPC, and elemental analysis. Life cycle assessment and techno-economic analysis were performed using standard procedures.

Plastics, a pivotal product of the last century, have dramatically transformed industries ranging from healthcare and food safety to textiles, electronics, and construction, making consumer goods more accessible worldwide. Plastic production has soared from its early days, and is projected to continue growing significantly. This increase is primarily due to polyolefins like polyethylene (PE) and polypropylene (PP), which make up about half of all plastic production. However, this manufacturing boom has led to a massive surge in plastic waste, with poor waste management practices causing significant environmental issues. A small portion of this waste is recycled, mainly through mechanical methods that degrade quality.

Chemical recycling to monomers is effective for condensation polymers but less so for polyolefins due to their strong carbon-carbon bonds, often resulting in low-value products. Upcycling is a possible alternative for managing polyolefin waste. The aim is to selectively cleave specific C-C bonds in polyolefins, converting them into building blocks that can be used as feedstocks for chemical manufacturing.

Catalytic conversion of waste polyolefins to value-added alkylaromatics could contribute to carbon recycling. Tandem catalytic conversion by platinum supported on g-alumina converts various polyethylene (PE) grades into valuable long chain alkylaromatics and alkylnaphthalenes (average ~C30, dispersity Ð = 1.1) with high yields, in the absence of added solvent or molecular hydrogen. Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280 °C.

Compared to tandem hydrogenolysis/aromatization of PE catalyzed by Pt/g-Al2O3 at 280°C, both a five-fold enhancement in the rate of C-C bond scission and a doubling of the molar yield of alkylaromatics were realized using a more acidic Pt/F-Al2O3 catalyst instead. Bifunctional (metal/acid) catalysts also generate alkylaromatic products with lower average carbon numbers (ca. C20), similar to conventional anionic surfactants. Since physical mixtures of weakly acidic Pt/g-Al2O3 or non-acidic Pt/SiO2 with strongly Brønsted acidic Cl-Al2O3 or F-Al2O3 are also effective, the tandem reaction does not require nano-scale intimacy between metal and acid active sites. Kinetic studies using triacontane (norm-C30H62) as a model for PE show that the Pt-catalyzed dehydrogenation/hydrogenation reactions are quasi-equilibrated, while the acid-catalyzed C-C bond scission and skeletal transformations (isomerization and cyclization) determine the overall rates of depolymerization and aromatic formation.

Different from hydrocracking, aromatic formation must be conducted without high pressure H2. Depolymerization can be performed without adding any external H2, depending instead on redistribution of PE-derived H2 to value-added alkylbenzenes. However, moderate external H2 pressures (1-8 bar) significantly enhance the depolymerization rate and alkylbenzene yield (maximum yield at 4 bar H2). The rate of C-C bond scission is pseudo-zeroth-order. It increases with external P(H2) at lower pressures (< 10 bar), then decreases at higher pressures. Modifying the traditional hydrocracking mechanism to include competitive adsorption of aromatics explains the P(H2) dependence of the rate. Dynamic control of the reactor atmospheres can further increase the alkylbenzene yield, especially in the desired molecular weight range for anionic surfactants.

Depolymerization and upcycling are promising approaches to managing plastic waste. However, quantitative measurements of reaction rates and analyses of complex hydrocarbon product mixtures arising from the depolymerization of polyolefins constitute significant challenges in this emerging field. The methods for recovery and analysis of all products arising from batch depolymerization of polyethylene are detailed. The quantitative analyses of reaction rates and selectivity for alkanes and aromatic products are described. This protocol can be extended to depolymerization of other plastics such as polypropylene, and to characterization of other product mixtures (including long-chain olefins).

Obtaining molecular-level structural information about heterogeneous catalysts and catalyst supports such as silicas and all-silica zeolites (zeosils) is critical for the rational design of new catalysts. Magnetic resonance techniques, including solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and electron paramagnetic resonance (EPR) spectroscopies, are powerful and non-invasive tools, which were used to acquire detailed structural and dynamic information about these silica-based systems. Thermal dehydroxylation reduces the number of silanol sites on silica surfaces and is typically used to increase the fraction of non-interacting silanols for well-defined active sites, however, several studies have suggested that pairs or clusters of silanols persist even upon extreme heat treatment. The spatial distribution of these silanols was investigated using VCl4 as a paramagnetic probe molecule. EPR signals for the grafted V(IV) were absent at room temperature, but a Lorentzian lineshape characteristic of spin-spin coupled centers was observed below 20 K. The latter finding indicates strong electron spin-spin coupling and implies that the silanols are clustered. 1H double-quantum single-quantum MAS NMR also suggests that many silanols are closely spaced. Varying the V-loading combined with an analysis of the contribution of the coupled V(IV) component to the total EPR spectrum led to the finding that silanols are clustered in groups of 7 or more, and consequently cannot be described as isolated. Zeosils modified with H3PO4, referred to as P-zeosils, selectively catalyze the dehydration of biomass-derived alcohols, but are unstable in the presence of water. The nature of the active P-sites in these catalysts, and how they are impacted by water is not known. The P-site distribution in a P-modified self-pillared pentasil (P-SPP) was probed using solid-state 31P MAS NMR with frequency-selective detection, dynamic nuclear polarization-enhanced 29Si-filtered 31P detection and 31P-31P correlation experiments. The P-sites in the dehydrated material are surface-bound via hydrolytically sensitive P-O-Si linkages, while some are also oligomers containing hydrolytically robust P-O-P linkages. The P-sites evolve rapidly when exposed to water, even at room temperature. Initial cleavage of some P–O–Si linkages results in an evolving mixture of surface-bound mono- and oligonuclear P-sites with acidity due to the generation of POH groups. Eventually all are converted to H3PO4. The effect of the zeosil framework on the stability of the P-sites was determined by comparing the solid-state 31P and ultrafast 1H MAS-NMR of P-SPP to that of a hydrophobic P-modified BEA zeosil (P-BEA). P-BEA contains a higher fraction of hydrolytically-stable P-O-P bonds and a lower accessibility to water compared to P-SPP. Thus, both the water content and the framework play a role in the P-site distribution, which in turn impact the acidity and hence, catalytic activity of P-zeosils. However, elucidating the precise nature of the active P-sites under reaction conditions requires the use of operando NMR. At elevated temperatures (140 °C) and in the presence of 2-propanol, 31P and 13C operando MAS NMR spectra suggest the POH sites are converted to phosphate esters. The ability to identify acidic sites in P-zeosils, and to describe their structure and stability, by combining insights using conventional and operando NMR, will play an important role in controlling the activity of microporous catalysts.

Surface polarity may impact the rate and selectivity of heterogeneous catalytic reactions because of the different affinities of the surface for reactants, products, and spectator molecules (e.g., solvent), and may affect the stability of the catalyst under operating conditions. Surface polarity must be varied systematically in order to construct structure-activity correlations. In addition, the lack of methods to assess surface polarity and solid-liquid interfaces makes it difficult to construct such a correlation in heterogeneous catalysis. To explore the effect of surface polarity, a series of ordered mesoporous organosilica materials with similar surface textural properties but a wide range of surface polarities was prepared via the incorporation of oxo, phenylene, and biphenylene bridging groups in various ratios. The SBA-15-type materials were synthesized through co-condensation and were characterized using TGA, powder XRD, and 13C CP/MAS NMR. The surface polarity was probed by measuring the fluorescence of a solvatochromic dye, Prodan, adsorbed onto the organosilica surfaces from water. By comparing the emission maxima of the fluorescence from the dry materials to that of the dye dissolved in various solvents, the surface polarities were observed to range from values similar to methanol for the pure silica material, to DMSO for biphenylene-bridged organosilicas. Surface functionalization with TEMPO allowed us to probe surface polarity using electron paramagnetic resonance (EPR) spectroscopy, since the line broadening of the EPR signal increases with decreasing surface polarity. In order to study the role of silica surface chemistry and polarity on the properties of near-surface water, the translational dynamics of surface water were investigated using Overhauser dynamic nuclear polarization. In pure silicas, condensation of polar silanols to give moderately nonpolar siloxane groups leads to an increase in translational water diffusivity due to the weak interactions between the surface and water. In contrast, increasingly nonpolar surfaces achieved by incorporating organic groups, such as phenylene, biphenylene and ethylene bridges, lead to a gradual decrease in surface water diffusivity. The opposite trend for water diffusivity observed for the nonpolar, organic surfaces is likely due to the formation of a strong hydrogen-bonding network at the organic-water interface. Hydrophilic catalysts with surface silanol groups and hydrophobic catalysts with biphenylene linkers were synthesized by incorporating Pd into the ordered mesoporous silicas. Phenol hydrogenation was studied using operando NMR spectroscopy. In addition to the signals for solution-phase molecules, additional peaks representing each of the key molecules (phenol, cyclohexanone, and cyclohexanol) interacting with the catalyst surface were observed. The latter peaks shifted during the reaction due to the changing composition of the adsorbed layer the surface. Quantitative analysis of the NMR arrays and measurements of adsorption suggest that an increased local phenol concentration on the surface leads to an increase in Pd active site-phenol interactions, accelerating the rate of phenol hydrogenation while suppressing the rate of cyclohexanone hydrogenation despite increased cyclohexanone adsorption onto the support. The effect of solvent polarity on phenol adsorption and hydrogenation was also investigated as a function of surface type. Solvents that do not solvate phenol well and therefore allow appreciable phenol adsorption, such as cyclohexane and water, lead to significantly faster phenol conversion compared to organic solvents that solvate phenol well, such as acetonitrile, THF, and p-dioxane. The use of hydrophobic catalysts in cyclohexane and water further improved the rate of phenol conversion (by a factor of ca. 2) and selectivity to cyclohexanone (by ca. 20 %), due to the increased interaction between phenol and the surface compared with the hydrophilic catalyst. Overall, this research demonstrates how different methods can be used to assess surface properties such as polarity and hydration dynamics at the molecular level, and shows that tuning surface polarity is an effective strategy to modulate molecular adsorption and activity/selectivity in heterogeneous catalysis.

A plentiful material found in nature, nonfood lignocellulosic biomass has emerged as a promising renewable source of liquid fuels and valuable chemicals due to its high energy content and abundance of aromatics in the lignin fraction. However, the recalcitrant nature of lignocellulosic biomass hinders its efficient transformation to platform chemicals and fuels. To gain kinetic and mechanistic insight about organic transformations relevant to the valorization of lignin, the reductive cleavage of the aryl ether linkage and the hydrogenation of phenol, catalyzed by the heterogenous catalyst nickel on gamma-alumina, were investigated.Operando Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS-NMR) is a powerful and non-invasive tool for acquiring detailed kinetic information. Recent advances in the design of MAS NMR rotors allows for internal rotor pressures up to 400 bar and temperatures up to 250 °C. The new high temperature and pressure rotors were used to collect operando 1H and 13C spectra during the hydrogenolysis of benzyl phenyl ether (BPE), a lignin model compound, at temperatures up to 250 ºC, both with and without H2 gas. In the absence of H2, the 2-propanol solvent can serve as a source of H2. The reaction generates toluene and phenol with an apparent activation barrier of (80 ± 8) kJ mol-1 at temperatures of 150-170 ºC. Toluene does not undergo further hydrogenation at these temperatures, but phenol undergoes subsequent, slow hydrogenation to cyclohexanol. Benzylic C-O bond cleavage is faster than H/D exchange in the methyl group of toluene, which in turn is faster than phenol hydrogenation. The source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H2/D2 gas rather than the solvent at these temperatures. Although the rate of BPE hydrogenolysis is not strongly solvent-dependent, the selectivity to phenol changes with the choice of solvent: the rate of hydrogenation is significantly faster in solvents with lower polarity. The rates are strongly dependent on the extent of phenol adsorption on the catalyst surface, implicating the balance between phenol solvation and competitive adsorption. For polar protic solvents (typically, alcohols), the rate decreases with the strength of the hydrogen bonding between phenol and the solvent, presumably due to better solvation of phenol. In aprotic solvents (typically, alkanes and aromatics), the rate of phenol hydrogenation correlates inversely with the strength of solvent adsorption on Ni active sites. Consequently, selectivity in aryl ether hydrogenolysis to phenolics can be tuned by solvent selection with little impact on activity.

With the increasing awareness of human society on climate change caused by greenhouse gas emission, and the continuous discovery and exploitation of abundant low-cost natural gas resources, methane pyrolysis (CH4 →C + 2 H2) is considered as a reaction with a great potential in providing an economical way for CO2-free hydrogen production and natural gas utilization. Some of the major barriers for the commercialization of methane pyrolysis are: (1) conventional solid catalysts used to accelerate methane pyrolysis are deactivated by surface carbon deposition and no efficient non-oxidative method for activity recovery has been reported; (2) the prevention of reactor clogging and removal of carbon from the reactor are challenging; (3) the heat transfer into the reactor to maintain the high temperature and supply heat required for this endo-thermic, equilibrium-limited reaction is non-trivial. One strategy to solve these problems is to introduce non-solid phases (liquid or gas) into the reaction for catalysis, heat-transfer and/or carbon removal. In this thesis research, two new types of catalysts are investigated and found to be active for methane pyrolysis. Tellurium, an element with high electron affinity, is an active methane pyrolysis catalyst in both its liquid and vapor form. Zinc chloride, a Lewis acidic salt, is also an active vapor phase catalyst for methane pyrolysis. The activities, active phase and long-term stability of these catalysts and their performance under different hydrogen partial pressures are investigated. The morphology of the carbon produced and the contamination level of these two catalysts are also investigated. The pathway of C-C bond formation in methane pyrolysis is studied using solid Ni and Cu as two model cases. Using methane-deuterium exchange as a probe reaction and with density function theory calculations, two different C-C bond formation pathways are discussed. Similar methods are adopted to study methane pyrolysis with four different molten liquids (Ni-Bi alloy, Sn, KCl and MnCl2-KCl eutectic) as well, and the similarity and difference between these systems are identified. A three-phase reactor was designed to avoid catalyst coking and allow efficient carbon removal and heat transfer is presented in this work. Tungsten carbide as a solid packed bed is shown to be active in molten KCl and molten Sn. The removal of carbon from coked WC surface and the recovery of WC activity with molten KCl treatment is demonstrated. The performance of WC in molten Sn under high H2 partial pressures is also studied, and a high single pass CH4 conversion overcoming pseudo-equilibrium limitation is achieved.

Redox-active silica-supported catalysts can be used to create alternatives to stoichiometric (i.e., non-catalytic) processes that are widely used in oxidation reactions and often require toxic and/or environmentally-unfriendly reagants. Catalytically active metal sites can be introduced to a solid support either as part of the framework of porous materials during their synthesis (co-condensation), or through post-synthetic attachment. The strategy of catalyst immobilization emphasized in this work involves linking a metal complex to a silica material via a grafted spacer or tether. This approach requires modifying one of the ligands of the catalyst precursor with a functional group or linker capable of creating the anchor, via a covalent bond, to the silica support. Much of the synthetic effort is devoted to preserving structure and reactivity of the molecular complex upon anchoring.

In this work, three silica-supported complexes were prepared. Each was characterized in detail at each step of the attachment procedure, to verify the reaction and its completeness. The corresponding homogeneous catalysts were also prepared in order to make structural and reactivity comparisons.

First, we report a bis(8-hydroxyquinolinato)(2-propanato)oxovanadium(V) catalyst, V(O)(HQ)2(OiPr), that selectively oxidizes alcohols under mild reaction conditions (60 °C,

air) and operates via an unusual base-assisted mechanism. A silica-supported analog, V(O)(HQ)2(OiPr)x-A380, was prepared via a multi-step grafting procedure and tested for catalytic aerobic oxidation under similar conditions as the homogeneous system. The activity of V(O)(HQ)2(OiPr)x-A380 was much lower than that of V(O)(HQ)2(OiPr), suggesting that the two are not precisely structurally equivalent. Further analysis reveals that during the grafting procedure, unexpected oxidation of the ligand tether occurred, giving rise to a V(IV) product.

In a separate study, we prepared two silylpropyl bis(pentafluorophenyl)boranes as molecular models for the Lewis acid component of supported frustrated Lewis pairs. The borane precursors were thoroughly characterized using NMR to confirm their structures, and to provide reference signals for silica-supported analogs. Three types of supported propyl bis(pentafluorophenyl)boranes, were prepared using a two-step methodology. First, allyl propylsilanes were installed, followed by hydroboration using Piers' borane to give tethered Lewis acid sites. As for molecular FLPs, combination with a Lewis base could lead to applications as heterogeneous metal-free catalysts for organic synthesis.

Lastly, two periodic mesoporous organosilicas (PMO) were synthesized by co- condensing either phenylpyridyl (ppy) silane or bipyridyl (bpy) silane, into the silica framework. The two materials were metalated using Cp*Ir(OH2)3(OTf)2 to give Cp*Ir-ppy- PMO and Cp*Ir-bpy-PMO, respectively, as supported analogs for the molecular catalysts Cp*Ir(ppy)(OTf) and Cp*Ir(bpy)(OTf)2. When tested in the catalytic oxidation of propionaldehye using water as the oxygen source (aldehyde-water shift catalysis, AWS), the

activities of the supported catalysts were comparable to those of the molecular systems, although all had the activities that are rather low. In spite of the low activity seen for AWS catalysis, Cp*Ir-ppy-PMO proved competent in the hydrogenation of levulinc acid.

We describe approaches for the design and synthesis of silica-supported molecular catalysts, using ligand anchoring techniques to create site-isolated, discrete catalyst.