11 research outputs found

    Highly selective PtCo bimetallic nanoparticles on silica for continuous production of hydrogen from aqueous phase reforming of xylose

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    Hydrogen (H2) is a promising energy vector for mitigating greenhouse gas emissions. Lignocellulosic biomass waste has been introduced as one of the abundant and carbon-neutral H2 sources. Among those, xylose with its short carbon chain has emerged attractive, where H2 can be catalytically released in an aqueous reactor. In this study, a composite catalyst system consisting of silica (SiO2)-supported platinum (Pt)-cobalt (Co) bimetallic nanoparticles was developed for aqueous phase reforming of xylose conducted at 225 °C and 29.3 bar. The PtCo/SiO2 catalyst showed a significantly higher H2 production rate and selectivity than that of Pt/SiO2, whereas Co/SiO2 shows no activity in H2 production. The highest selectivity for useful liquid byproducts was obtained with PtCo/SiO2. Moreover, CO2 emissions throughout the reaction were reduced compared to those of monometallic Pt/SiO2. The PtCo bimetallic nanocatalyst offers an inexpensive, sustainable, and durable solution with high chemical selectivity for scalable reforming of hard-to-ferment pentose sugars

    Molybdenum and tungsten alkylidene complexes for cis- and trans-selective ring-opening metathesis polymerization

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    Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2015.Cataloged from PDF version of thesis.Includes bibliographical references.Chapter 1 describes the synthesis of tert-butylimido alkylidene complexes for molybdenum and tungsten. A dimer species [chemical formula] served as a bisimido precursor. After alkylation with Grignard reagent, alkylidene formation is accomplished using pyridinium chloride. [chemical formula] crystallizes as a dimer [chemical formula] with a loss of pyridine for each W center. For the case of molybdenum, addition of pentafluorophenol to the diimido dialkyl precursor affords [chemical formula]. Dipyrrolide complexes for both Mo and W are synthesized and isolated as a 2,2'-bipyridine adduct. Addition of a sterically encumbered terphenol along with ZnCl₂(dioxane) affords monoalkoxide pyrrolide (MAP) complexes [chemical formula]. Chapter 2 investigates Z-selective ring-opening metathesis polymerization (ROMP) of 3- substituted cyclooctenes (3-RCOEs) by Mo and W MAP catalysts. [chemical formula], [chemical formula], and [chemical formula] all produced >98% [chemical formula]. The key in forming high molecular weight polymer instead of cyclic oligomer species was to run the reaction neat. Surprisingly, the fastest initiator was [chemical formula] among all three MAP species. Polymerization proceeds via a propagating species in which the R group is of C2 position of the propagating chain, giving HT polymers with high regioselectivity. Chapter 3 describes the synthesis and reactivity of compounds containing a tert-butylimido ligand. Chelating alkylidenes can be synthesized either by alkylidene exchange or by traditional routes in forming alkylidene complexes from diimido dialkyl species. A W MAP complex containing a chelating alkylidene can be synthesized and its reactivity is comparable to that of neopentylidene analogue in 1-octene homocoupling. Complexes with a chelating diolate ligand [chemical formula] and [chemical formula] were synthesized. However, attempts to remove the pyridine ligand induced C-H activation of one tertbutyl group on Biphen ligand to form alkyl complexes. Chapter 4 presents the synthesis of high sequence-regular alternating trans-AB copolymers by ROMP initiated by [chemical formula]. Monomers employed were 2,3-dicarbomethoxy-7-isopropylidenenorbomadiene (B), [chemical formula] (B'), cyclooctene (A), and cycloheptene (A'). All four combinations afford structures containing a high degree of monomer alternation. Evidence suggests a catalytic cycle proceeding through a syn alkylidene arising from insertion of B (syn-MB) reacting with A to form an anti alkylidene (anti-MA) and a trans-AB linkage. A MAP complex [chemical formula] [chemical formula] was also found to form trans-poly[A-alt-B'] with >90% alternating dyad sequences. Variations on imido and alkoxide ligands were surveyed as well as both A and B type monomers.by Hyangsoo Jeong.Ph. D

    Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a tert-Butylimido Ligand

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    National Science Foundation (U.S.) (CHE-1111133)National Science Foundation (U.S.) (CHE-0946721

    Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a <i>tert</i>-Butylimido Ligand

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    A variety of molybdenum or tungsten complexes that contain a <i>tert</i>-butylimido ligand have been prepared. For example, the <i>o</i>-methoxybenzylidene complex W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)­(Cl)<sub>2</sub>(py) was prepared through addition of pyridinium chloride to W­(N-<i>t</i>-Bu)<sub>2</sub>­(CH<sub>2</sub>-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>, while Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)­(OR<sub>F</sub>)<sub>2</sub>(<i>t</i>-BuNH<sub>2</sub>) complexes (OR<sub>F</sub> = OC<sub>6</sub>F<sub>5</sub> or OC­(CF<sub>3</sub>)<sub>3</sub>) were prepared through addition of two equivalents of R<sub>F</sub>OH to Mo­(N-<i>t</i>-Bu)<sub>2</sub>­(CH<sub>2</sub>-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>. An X-ray crystallographic study of Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)­[OC­(CF<sub>3</sub>)<sub>3</sub>]<sub>2</sub>­(<i>t</i>-BuNH<sub>2</sub>) showed that the methoxy oxygen is bound to the metal and that two protons on the <i>tert</i>-butylamine ligand are only a short distance away from one of the CF<sub>3</sub> groups on one of the perfluoro-<i>tert</i>-butoxide ligands (H···F = 2.456(17) and 2.467(17) Å). Other synthesized tungsten <i>tert</i>-butylimido complexes include W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)­(pyr)<sub>2</sub>(2,2′-bipyridine) (pyr = pyrrolide), W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC<sub>6</sub>H<sub>4</sub>)­(pyr)­(OHMT) (OHMT = O-2,6-(mesityl)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)­(Cl)­(py) (py = pyridine), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)­(Cl), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(pyr)­(ODFT)­(py), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)<sub>2</sub>, and W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(ODFT)<sub>2</sub> (ODFT = O-2,6-(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>). Interestingly, W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)<sub>2</sub> does not react with ethylene or 2,3-dicarbomethoxynorbornadiene. Removal of pyridine from W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen<sub>CF3</sub>)­(pyridine) (Biphen<sub>CF3</sub> = 3,3′-di-<i>tert</i>-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) with B­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> led to formation of a five-coordinate 14<i>e</i> neopentyl complex as a consequence of CH activation in one of the methyl groups in one <i>tert</i>-butyl group of the Biphen<sub>CF3</sub> ligand, as was proven in an X-ray study. An attempted synthesis of W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen<sub>Me</sub>) (Biphen<sub>Me</sub> = 3,3′-di-<i>tert</i>-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) led to formation of a 1:1 mixture of W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen<sub>Me</sub>) and a neopentyl complex analogous to the one characterized through an X-ray study. The metallacyclobutane complexes W­(N-<i>t</i>-Bu)­(C<sub>3</sub>H<sub>6</sub>)­(pyrrolide)­(ODFT) and W­(N-<i>t</i>-Bu)­(C<sub>3</sub>H<sub>6</sub>)­(ODFT)<sub>2</sub> were prepared in reactions involving W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(pyr)<sub>2</sub>(bipy), ZnCl<sub>2</sub>(dioxane), and one or two equivalents of DFTOH, respectively, under 1 atm of ethylene

    Synthesis of Alternating trans-AB Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Alkylidenes

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    Four alternating AB copolymers have been prepared through ring-opening metathesis polymerization (ROMP) with Mo(NR)(CHCMe₂Ph)[OCMe(CF₃)₂]₂ initiators (R = 2,6-Me₂C₆H₃ (1) or 2,6-i-Pr₂C₆H₃ (2)). The A:B monomer pairs copolymerized by 1 are cyclooctene (A):2,3-dicarbomethoxy-7-isopropylidenenorbornadiene (B), cycloheptene (A′):dimethylspiro[bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate-7,1′-cyclopropane] (B′), A:B′, and A′:B; A′:B′ and A:B′ are also copolymerized by 2. The >90% poly(A-alt-B) copolymers are formed with heterodyads (AB) that have the trans configuration. Evidence suggests that one trans hetero C═C bond is formed when A (A or A′) reacts with the syn form of the alkylidene made from B (syn-MB = syn-MB or syn-MB′) to give anti-MA, while the other trans C═C bond is formed when B reacts with anti-MA to give syn-MB. Cis and trans AA dyads are proposed to arise when A reacts with anti-MA in competition with B reacting with anti-MA.United States. Department of Energy (DE-FG02-86ER13564)United States. National Institutes of Health (GM-59426