<p>This work studies propene aromatization reaction on H-MFI (Si/Al = 40) and physical mixtures of H-MFI (Si/Al = 40) and PtZn/SiO2 (2 wt% Pt, 3 wt% Zn) at 723 K - 823 K and 3 kPa C3H6. The influence of PtZn alloy (dehydrogenation function) is investigated on the product distribution and selectivity of metal-acid catalyzed propene aromatization. Typical product distribution consists of methane, ethane, ethene, propane, C4-C6 alkanes and alkenes, and benzene, toluene, xylene (BTX). On comparing the BTX carbon selectivity over the two catalysts at first equivalent space velocity and then equivalent propene conversion, higher BTX selectivities are observed on PtZn+H-MFI than H-MFI in both the cases. The higher BTX selectivities were previously attributed in the literature to the dehydrogenation pathway on the metal function. However, space velocity is an inadequate descriptor of reaction progress because the conversion of reactants can be different at same space velocity. Similarly, propene conversion is an incomplete descriptor for reaction progress because intermediates such as ethene and C4-C6 hydrocarbons react to form higher molecular weight hydrocarbons and subsequent aromatics as the reaction progresses. Such reactive hydrocarbons were lumped together as reactive intermediates and the remaining hydrocarbons were classified as non-reactive species or products. When BTX selectivities over PtZn-H-MFI and H-MFI are compared at equivalent temperature and equivalent conversion of all the reactive intermediates, both the catalysts exhibit similar BTX selectivities, suggesting that the presence of the dehydrogenation metal function doesn’t influence the selectivity towards BTX products. Further, we hypothesize cyclohexene as an intermediate in aromatic formation and use cyclohexene conversion as a probe reaction to understand how aromatics are formed over Brønsted acid sites and PtZn alloy. Cyclohexene conversion results at 723 K and 823 K shows the presence of an alternate route of aromatic formation via dehydrogenation of cycloalkenes, and this dehydrogenation pathway has an order of magnitude higher rates than the hydride transfer route on Brønsted acid sites. Further, we propose dominant reaction pathways of C1 – BTX hydrocarbon formation on H-MFI and bifunctional PtZn+H-MFI. Finally, we discuss the implications of using PtZn+H-MFI on developing a commercial propylene aromatization process and provide our recommendations for chemical and fuel production. In summary, these findings reveal previously unknown mechanistic details of metal bifunctionality for propene aromatization catalysis.  </p>

Surface Organometallic Chemistry (SOMC) is an excellent approach to erase the gap between homogeneous and heterogeneous catalysis by grafting the molecular organometallic complex on various oxide surfaces, forming well-defined and single-site catalysts. This strategy allows for better characterization as well as the improvement and development of existing and new catalysts. These surface species could promote a wide range of catalytic applications (i.e., metathesis of hydrocarbons, hydrogenolysis of alkanes, and olefin polymerization reactions) depending on the metal center and its coordination sphere. In particular, the grafting of alkylated organometallic complexes of groups 4, 5, and 6 metals on the surface oxide is a thermodynamically favored reaction generally leading to strongly bonded well-defined surface species, which are highly reactive catalysts. This thesis has focused on the preparation, characterization, and catalytic investigation of different supported complexes that contain methyl, alkyl, and alkylidyne ligands. The first part compares the catalytic activity of [(≡Si−O−)W(-CH3)5] and [(≡Si-O-)Mo(≡CtBu)(-CH2tBu)2] surface species experimentally and by DFT calculations in the metathesis reactions of linear classical and functionalized olefins. Both pre-catalysts perform almost equally in the α-olefin metathesis reaction. However, in the functionalized olefin metathesis reaction, W pre-catalyst provides selective metathesis products and performs much better than Mo that gives a range of isomerization products. The second part deals with the synthesis and characterization of [(THF)2Zr(-CH3)4] and its grafting on silica support for the first time. The generated surface species [(≡Si−O−)Zr(CH3)3(THF)2] and [(≡Si−O−)2Zr(CH3)2(THF)2] are used for the conversion of CO2 and propylene oxide to cyclic propylene carbonates achieving a TON of 4227. The third part describes the first synthesis and characterization of the highly unstable homoleptic [Ti(-CH3)4] without any coordinating solvent. This complex was stabilized by grafting on SiO2-700, yielding two fully characterized surface species [(≡Si-O-)TiMe3] and [(≡Si-O-Si≡)(≡Si-O-)TiMe3], which were used in the hydrogenolysis reaction of propane and n-butane, with TONs of 419 and 578, respectively. Finally, the fourth part reports the immobilization and characterization of [TiMe2Cl2], an intermediate in the synthesis of [Ti(-CH3)4], on SiO2-700 resulting in [(≡Si-O-)TiMeCl2] and [(≡Si-O-)TiMe2Cl] surface species. These complexes reacted with a demethylating Lewis acid agent (BARF), forming the corresponding cationic Ti species [(≡Si-O-)TiMeCl]+ and [(≡Si-O-)TiCl2]+. Both neutral and cationic complexes were tested in the ethylene polymerization reaction affording linear HDPE with high molecular weights of 500,367 and 486,612 g/mol.

With its unrivaled versatility and atom economy, olefin metathesis is arguably the most powerful catalyst methodology now known for the construction of carbon-carbon bonds. When compared to palladium-catalyzed cross-coupling methodologies, however, catalyst productivity lags far behind, even for the “robust” ruthenium metathesis catalysts. Unexpected limitations to the robustness of these catalysts were first widely publicized by reports describing the implementation of metathesis in pharmaceutical manufacturing. Recurring discussion centered on low catalyst productivity resulting from decomposition of the Ru catalysts by impurities, including ppm-level contaminants in the technical-grade solvent. Over the past 7 years, a series of mechanistic studies from the Fogg group has uncovered the pathways by which common contaminants (or indeed reagents) trigger catalyst decomposition. Two principal pathways were identified: abstraction of the alkylidene or methylidene ligand by nucleophiles, and deprotonation of the metallacyclobutane intermediate by Bronsted base. Emerging applications, however, notably in chemical biology, highlight new challenges to catalyst productivity. The first part of this thesis emphasizes the need for informed mechanistic insight as a guide to catalyst redesign. The widespread observation of a cyclometallated N-heterocyclic carbene (NHC) motif in crystal structures of catalyst decomposition products led to the presumption that activation of a C-H bond in the NHC ligand initiates catalyst decomposition. Reducing NHC bulk has therefore been proposed as critical to catalyst redesign. In experiments designed to probe the viability of this solution, the small NHC ligand IMe4 (tetramethylimidazol-2-ylidene) was added to the resting-state methylidene complexes formed in metathesis by the first- and second-generation Grubbs catalysts (RuCl2(PCy3)2(=CH2) GIm or RuCl2(H2IMes)(PCy3)(=CH2) GIIm, respectively). The intended product, a resting-state methylidene species bearing a truncated NHC, was not formed, owing to immediate loss of the methylidene ligand. Methylidene loss is now shown to result from nucleophilic attack by the NHC – a small, highly potent nucleophile – on the methylidene. Density functional calculations indicate that IMe4 abstracts the methylidene, generating the N-heterocyclic olefin H2C=IMe4. The latter is an even more potent nucleophile, which attacks a second methylidene, resulting in liberation of [EtIMe4]Cl. These findings report indirectly on the original question concerning the impact of ligand truncation. The ease with which a small, potent nucleophile can abstract the key methylidene ligand from GIm and GIIm underscores the importance of increasing steric protection at the [Ru]=CH2 site. This chemistry also suggests intriguing possibilities for efficient, selective, controlled methylidene abstraction to terminate metathesis activity while leaving the “RuCl2(H2IMes)(PCy3)” core intact. This could prove an enabling strategy for tandem catalysis applications in which metathesis is the first step. The second part of this thesis, inspired by the potential of olefin metathesis in chemical biology, focuses on the impact of hydroxide ion and water on the productivity of phosphine-free metathesis catalysts. In reactions with the important second-generation Hoveyda catalyst HII, hydroxide anion is found to engage in salt metathesis with the chloride ligands, rather than nucleophilic attack. The resulting Ru-hydroxide complex is unreactive toward any olefins larger than ethylene, while ethylene itself causes rapid decomposition. Proposed as the decomposition pathway is bimolecular coupling promoted by the strong H-bonding character of the hydroxide ligands. Lastly, the impact of the water on Ru-catalyzed olefin metathesis is examined. In a survey of normally facile metathesis reactions using state-of-the-art catalysts, even trace water (0.1% v/v) is found to be highly detrimental. The impact of water is shown to be greater at room temperature than previously established at 60 °C. Preliminary evidence strongly suggests that the mechanism by which water induces decomposition is temperature-dependent. Thus, at high temperature, decomposition of the metallacyclobutane intermediate appears to dominate, but this pathway is ruled out at ambient temperatures. Instead, water is proposed to promote bimolecular decomposition. Polyphenol resin, which can sequester water by H-bonding, is shown to offer an interim solution to the presence of trace water in organic media. These findings suggest that major avenues of investigation aimed at reducing intrinsic catalyst decomposition may likewise be relevant to the development of water-tolerant catalysts.

Olefin metathesis is a powerful tool for the making and breaking of carbon-carbon double bonds. Among well-defined homogenous catalysts for olefin metathesis, ruthenium-based alkylidenes stand out for their robustness and relative ease-of-use. Synthesis of the most active Ru-based metathesis catalysts remains challenging, however, and there is continued interest in new and improved routes to alkylidene installation as metathesis begins to see wide uptake in industry. The first part of this thesis focuses on developing new routes to known catalysts. Magnesium carbenoids are investigated as a potential alkylidene source, and in the process a novel route to benzylmagnesium carbenoids is developed. Initially promising results showing ca. 40% conversion to first generation metathesis catalysts failed to lead to a viable high-yield route to Ru-alkylidenes. A high yield route to RuCl2(H2IMes)(py)4 (previously reported in low yields as a decomposition product of the third-generation Grubbs’ metathesis catalyst) is developed and this complex is investigated as a precursor to indenylidene-based catalysts. Although RuCl2(H2IMes)(py)4 is shown to be substitutionally labile, indenylidene installation could not be achieved. Finally, zinc aryloxides are investigated as an alternative to thallium and silver reagents for the installation of aryloxide ligands. Initial results indicate that zinc aryloxides are kinetically, though not thermodynamically, competent for the installation of the challenging aryloxide C6F5O- on the second-generation Hoveyda catalyst. The second part of this thesis concerns progress towards the development of a new low-valent catalyst platform. Initial experiments involving treating the second-generation Hoveyda catalyst with various reducing agents fail to produce low-valent alkylidenes, leading instead to decomposition of alkylidene. Drawing inspiration from early transition metal systems, the remainder of the second part focuses on alpha-hydride elimination from a RuII alkyl as a means of accessing low-valent alkylidenes. To this end, a novel benzylruthenium complex as well as bis-benzyl and mono-aryloxide derivatives are developed. While attempts to induce benzyl-to-benzyl hydride abstraction or intramolecular deprotonation of the benzyl ligand failed to produce alkylidenes, ligand-induced benzyl-to-aryloxide hydride abstraction appears to be successful, leading to the observation of a broad 1H NMR signal in the region characteristic for low-valent Ru-alkylidenes.

<p>Thermal, non-catalytic conversion of light olefins (C2= - C4=) was originally utilized in the production of motor fuels at several U.S. refineries in the 1920-30’s. However, the resulting fuels had relatively low-octane number and required harsh operating conditions (T > 450 oC, P > 50 bar), ultimately leading to its succession by solid acid catalytic processes. Despite the early utilization of the thermal reaction, relatively little is known about the reaction products, kinetics, and initiation pathway under liquid-producing conditions. </p> <p>In this thesis, thermal ethylene conversion was investigated near the industrial operating conditions, i.e, at temperatures between 320 and 500 oC and ethylene pressures from 1.5 to 43.5 bar. Non-oligomer products such as propylene and/or higher odd carbon products were observed at all reaction temperatures, pressures, and reaction extents. Methane and ethane were minor products (< 1 % each), even at ethylene conversions as high as 74 %. The isomer distributions revealed a preference for linear, terminal C4 and C5. The reaction order was found to be 2nd order with a temperature dependent activation energy ranging from 165 to 244 kJ/mol. The importance of diradical species in generating free radicals during a two-phase initiation process was proposed. The reaction chemistry for ethylene, which has only strong, vinyl C-H bonds starkly contrasted propylene, which possesses weaker allylic C-H bonds and showed preference for dimeric C6 products over C2-C8 non-oligomers. </p> <p>Extending this work further, the thermal oligomerization of ethylene was enhanced using high surface area supports such as silica and alumina. Both supports resulted in order of magnitude rate increases compared to the gas phase reaction, however the ethylene conversion rate with alumina was superior to silica by a factor of between 100 and 1,000. Additionally, the alumina evidently confers a catalytic function, resulting in altered product distributions, notably an increase in branched products such as isobutene and isopentenes. The oligomerization chemistry with alumina appears to reflect the involvement of Lewis acid sites rather than traditional Brønsted acid or transition metal catalysis, which operate via carbenium ion and metal-alkyl intermediates, respectively. </p>

No description available.

Thesis advisor: Amir H. Hoveyda / We have devised a broadly applicable catalytic cross-metathesis method for stereoretentive synthesis of Z- and E-trisubstituted α,β-unsaturated esters. Several new Mo-bisaryloxide complexes were prepared, and they showed superior efficiency in synthesizing the Z-trisubstituted enoates (vs. corresponding mono-aryloxide pyrrolide complexes). Synthetic utility of the method was demonstrated through several concise syntheses of bioactive triterpenoids and value-added derivatives of prenyl-containing compounds such as citronellal, citronellol, and geraniol, all of which are isolated from essential oils. This transformation offers a valuable alternative to carbonyl olefination approaches such as Wittig and Horner-Wadsworth-Emmons reactions. / Thesis (MS) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

<p>Maleation is a common means of modification for many commodity polymers and is used to several ends. In this study, various waxes were functionalized with maleic anhydride through several maleation processes, with the end goal of obtaining a cost effective processes to make emulsifiers to be used in emulsions that impart water-resistance to building products, such as gypsum wallboards. Research was done in collaboration with an industrial partner, in order to replace commercially available emulsifiers currently being used in their processes with a less costly product that could easily be made on-site based on their consumption requirements, through a solvent-free approach. Reactions involving both the free-radical initiated maleation of paraffin waxes and thermal addition of maleic anhydride to alpha-olefins were examined extensively. It was found that emulsions with properties matching or exceeding those of control emulsion formulations were obtainable using experimental emulsifiers made through both maleation methods. When used in gypsum wallboards, emulsifiers made through thermal maleation showed levels of water-repellency that matched or exceeded those of control formulations at lower loading levels, while emulsifiers made through free-radical maleation were subject to performance issues.</p> / Master of Applied Science (MASc)

<p dir="ltr">Zeolites are microporous, crystalline aluminosilicates widely used in catalysis and separation. The substitution of Si⁴⁺ with Al³⁺ ([AlO_4/2]^-) creates a charge imbalance that can be compensated by a metal cation or complex (Mⁿ⁺) or a Brønsted acid proton (H⁺) within microporous voids and at external surfaces. Brønsted acid sites in aluminosilicates of diverse topologies have similar acid strength, but the diffusion of reactants and products can vary depending on the micropore size, tortuosity, and connectivity. The coupled effects of H⁺-site reactivity and diffusional constraints imposed by the inorganic zeolitic framework can be assessed by the diffusion parameter, which depends on the bulk proton density ([H⁺]) and the diffusion pathlength (L), derived from the Thiele modulus expression that relates reaction and diffusion rates within porous catalysts. This motivates synthetic approaches to control zeolite properties that influence diffusion and reactivity such as crystallite size and proton density. Prior synthetic methods have tried to minimize the diffusional constraints by decreasing the diffusion pathlength (L) by synthesizing zeolite crystallites at the nanometer scale or by increasing the effective diffusivity (D_e) by synthesizing hierarchical materials. However, these synthetic approaches may simultaneously influence multiple zeolite properties, such as the spatial distribution of acid sites throughout crystallites or at extracrystalline surfaces, convoluting the influence of these properties on the rates, selectivity, and deactivation of acid-catalyzed reactions.</p><p dir="ltr">Two types of spatial distribution of acid sites could be present within a zeolite. The first is the fraction at unconfined extracrystalline surfaces, and this property is often convoluted with the effect of crystallite size. Assuming acid sites are evenly distributed through the crystallite, as the crystallite size increases, the fraction of external acid sites decreases because the surface area-to-volume decreases. The second type of spatial distribution of acid sites is referred to as “zoning”—a concentration gradient of active sites from the external surface to its core, or vice versa. This type of spatial distribution of acid sites is challenging to quantify accurately. “Zoning” effects may also occur inadvertently during zeolite synthesize using conventional methods. In this work, we synthesize zeolitic materials (i.e., MFI) with controlled spatial distribution of acid sites independently of crystallite size and H⁺-site density to study their effects on propene oligomerization catalysis. A core@shell synthesis approach was used to passivate external MFI zeolite surfaces by an inert shell (Si-MFI) of short thickness relative to the size of the core crystallite. Although other passivation treatments can cause pore blockage or narrowing, transient sorption measurement showed no additional diffusional limitations were introduced by the growth of the Si-MFI shell. Propene dimerization rates (per H⁺, 503 K, 16 – 620 kPa C₃H₆) and transient behavior upon pressure step-changes persist reveal the influence of intrazeolite diffusional constraints on the Al-MFI core due to heavier oligomer products that accumulate inside micropores. On the contrary, dimerization rates did not reach a pseudo-steady-state on Al-MFI@Si-MFI and required high temperature caused by the formation of surface carbonaceous deposits in the absence of acid sites that otherwise assist in the cracking and desorption of coke precursor species. Thus, the passivation of the external surface imposes a transport limitation at the surface due to a carbonaceous layer that forms during the reaction, restricting the diffusion of products out of crystallites and shifting the selectivity towards a lighter product composition.</p><p dir="ltr">An inverted core@shell (Si-MFI@Al-MFI) material was also synthesized to investigate the effect of the spatial distribution of acid sites on the diffusion parameter, where the acid sites are preferentially located at the external surface and the core is inert (i.e. Si-MFI). The spatial distribution of acid sites was varied by growing an Al-MFI shell on a siliceous core and maintaining a similar bulk crystallite size. Mesitylene benzylation was used to quantify the fraction of external acid sites. Differences in measured propene dimerization rates (per H⁺) and product selectivity can be rationalized considering the thickness of the Al-rich shell in the core@shell material to an Al-MFI sample of similar crystallite size, evincing the dominant influence of the diffusion parameter on propene oligomerization catalytic behavior. Overall, this study demonstrated how zeolite synthetic methods can be used to isolate the effects of spatial distributions of Al from crystallite size and H⁺-site density and provide guidance for zeolite catalyst design efforts to control structural properties that influence reactions driven by coupled kinetic-transport phenomena.</p>

Ring-opening metathesis polymerization (ROMP) is a great tool for synthesizing polyolefin materials with different topologies, including hyperbranched polymers—polymers with high degrees of branching and many end groups. However, hyperbranched polymer synthesis via ROMP is challenging due to multifunctional-monomer or multi-polymerization requirements. To simplify the synthesis of hyperbranched ROMP polymers, we developed a new synthetic approach: Self-condensing ROMP. The self-condensing ROMP approach involves a ROMP initiator modification to attach a ROMP-polymerizable group (a ROMP monomer), producing a ROMP "inimer" (initiator + monomer). The ROMP inimer initiates the polymerization and becomes a branching unit in the polymer structure, resulting in single-step hyperbranched polymer synthesis. The key challenge is controlling of this approach the ROMP initiator reactivity to avoid initiating polymerization during the ROMP inimer synthesis. Well-defined ruthenium-based olefin metathesis catalysts are common ROMP initiators due to their high stability, reactivity, and functional group tolerance. Thus, we studied the olefin metathesis catalyst activation temperature to enable ROMP initiator-monomer coupling. Based on the catalyst activity, we designed and synthesized a series of ROMP inimers. Then, we synthesized hyperbranched polymers via self-condensing ROMP. The characterization of hyperbranched polymers indicated the effect of branching density on the physical properties of the polymer. This approach introduced a new class of olefin metathesis complexes, ROMP inimers, containing both the initiator and propagating center. This approach provides a way to synthesize hyperbranched polymers from any known ROMP monomers in a single step. This dissertation also includes the synthesis and characterization of a bimetallic Ru complex that could directly synthesize cyclic polyolefin. We also include the synthesis and characterization of copper-ruthenium bimetallic olefin metathesis catalysts. / Doctor of Philosophy / Hyperbranched polymers are a class of polymers having highly branching structures and functional end-groups, and presenting distinct physical and chemical properties compared with linear polymers. Hyperbranched polymers have been used for many applications including processing additives, cross-linkers, compatibilizers, and catalyst supports. Well-defined ruthenium-based olefin metathesis catalysts enable the synthesis of materials with different topologies, functionalities, and chemical and physical properties via ring-opining metathesis polymerization (ROMP). Ligand modifications on ruthenium catalysts have been applied to improve the catalyst stability and reactivity. However, this dissertation modifies olefin metathesis catalysts to synthesize hyperbranched polymers in a single step. This dissertation illustrates catalyst functionalization with a ROMP monomer moiety to synthesize a ROMP inimer (inimer= initiator + monomer). The ROMP initiator—olefin metathesis catalyst—and ROMP monomer coupling produces an "inimer". The inimer can undergo self-condensing ROMP with a ROMP monomer addition to synthesize hyperbranched polymers. This approach introduced a new class of olefin metathesis complexes containing both the initiator and propagating center. This approach also provides a way to synthesize hyperbranched polymers from any known ROMP monomers in a single step.