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

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

Dehydrogenation of homocyclic liquid organic hydrogen carriers (LOHCs) over Pt supported on an ordered pore structure of 3-D cubic mesoporous KIT-6 silica

Pt supported on ordered mesoporous silica (KIT-6) catalyst was examined for the dehydrogenation of homocyclic liquid organic hydrogen carriers (LOHCs, 1: MCH, 2: hydrogenated biphenyl-based eutectic mixture (H-BPDM)) conditions. The longer pore-residence time of the MCH molecules in the 3D bicontinuous pore structure of the Pt/KIT-6 catalyst strongly affected the catalytic activity because a higher MCH concentration was achieved in the vicinity of the Pt active sites. Pt/KIT-6 catalyst exhibited a higher surface area, pore volume, and Pt dispersion with narrower particle size distribution (average Pt particle size: ~1.3 nm). Therefore, higher LOHC conversion with faster hydrogen production occurred, with a higher hydrogen selectivity over Pt/KIT-6 compared with Pt/SiO2 and Pt/Al2O3. Long-term experiment results indicated that the Pt/KIT-6 catalytic activity was stable over the reaction time than that of the other catalysts. No significant structural collapse occurred in KIT-6 during the dehydrogenation. Carbon coking was observed for all three samples

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

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

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

Formation of Alternating trans-A-alt-B Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Ring-opening metathesis polymerization (ROMP) is used to prepare trans-poly(A-alt-B) polymers from a 1:1 mixture of A and B where A is a cyclic olefin such as cyclooctene (A[subscript 1]) or cycloheptene (A[subscript 2]) and B is a large norbornadiene or norbornene derivative such as 2,3-dicarbomethoxy-7-isopropylidenenorbornadiene (B[subscript 1]) or dimethylspirobicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate-7,1'cyclopropane (B[subscript 2]). The most successful initiators that were examined are of the type Mo(NR)(CHCMe[subscript 2]Ph)[OCMe(CF[subscript 3])[subscript 2]][subscript 2] (R = 2,6-Me[subscript 2]C[subscript 6]H[subscript 3] (1) or 2,6-iPr[subscript 2]C[subscript 6]H[subscript 3] (2)). The trans configuration of the AB linkages is proposed to result from the steric demand of B. Both anti-MB and syn-MB alkylidenes are observed during the copolymerization, where B was last inserted into a Mo=C bond, although anti-MB dominates as the reaction proceeds. Anti-MB is lower in energy than syn-MB, does not react readily with either A or B, and interconverts slowly with syn-MB through rotation about the Mo=C bond. Syn-MB does not readily react with B, but it does react slowly with A (rate constant ~1 M[superscript -1] s [superscript -1]) to give anti-MA and one trans-AB linkage. Anti-MA then reacts with B (rate constant ~300 M[superscript -1] s[superscript -1] or larger) to give syn-MB and the second trans-AB linkage. The reaction has been modeled using experimental data in order to obtain the estimated rate constants above. The reaction between anti-MA and A is proposed to give rise to AA linkages, but AA dyads can amount to <5%. Several other possible A and B monomers, initiators, and conditions were explored.United States. Dept. of Energy (Grant DE-FG02-86ER13564

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

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>-MeOC₆H₄)­(Cl)₂(py) was prepared through addition of pyridinium chloride to W­(N-<i>t</i>-Bu)₂­(CH₂-<i>o</i>-MeOC₆H₄)₂, while Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(OR_F)₂(<i>t</i>-BuNH₂) complexes (OR_F = OC₆F₅ or OC­(CF₃)₃) were prepared through addition of two equivalents of R_FOH to Mo­(N-<i>t</i>-Bu)₂­(CH₂-<i>o</i>-MeOC₆H₄)₂. An X-ray crystallographic study of Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­[OC­(CF₃)₃]₂­(<i>t</i>-BuNH₂) 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 CF₃ 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>-MeOC₆H₄)­(pyr)₂(2,2′-bipyridine) (pyr = pyrrolide), W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(pyr)­(OHMT) (OHMT = O-2,6-(mesityl)₂C₆H₃), 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)₂, and W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(ODFT)₂ (ODFT = O-2,6-(C₆F₅)₂C₆H₃). Interestingly, W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)₂ 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_CF3)­(pyridine) (Biphen_CF3 = 3,3′-di-<i>tert</i>-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) with B­(C₆F₅)₃ 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_CF3 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_Me) (Biphen_Me = 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_Me) and a neopentyl complex analogous to the one characterized through an X-ray study. The metallacyclobutane complexes W­(N-<i>t</i>-Bu)­(C₃H₆)­(pyrrolide)­(ODFT) and W­(N-<i>t</i>-Bu)­(C₃H₆)­(ODFT)₂ were prepared in reactions involving W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(pyr)₂(bipy), ZnCl₂(dioxane), and one or two equivalents of DFTOH, respectively, under 1 atm of ethylene

Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization.

Alkyne cross-metathesis of molybdenum carbyne complex [TolC≡Mo(OCCH3(CF3)2)3]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (&gt;98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e][8]annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followed by ring-opening polymerization of strained ε-caprolactone

Stereoselective Ring-Opening Metathesis Polymerization (ROMP) of Methyl-N-(1-phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate by Molybdenum and Tungsten Initiators

Ring-opening metathesis polymerization (ROMP) of methyl-N-(1-phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate (PhEtNNBE; (S) and racemic) was investigated employing six molybdenum and tungsten imido alkylidene initiators and two tungsten oxo alkylidene initiators. Of the six initiators that we proposed should yield cis,syndiotactic-poly[(S)-PhEtNNBE], two molybdenum OHMT alkylidene initiators, Mo(NR)(CHMe2Ph)(pyr)(OHMT) (R = 1-adamantyl (Ad) or 2,6-Me[subscript 2]C[subscript 6]H[subscript 3] (Ar′); OHMT = O-2,6-mesityl[subscript 2]C[subscript 6]H[subscript 3]; pyr = pyrrolide), and two tungsten oxo alkylidene initiators, W(O)(CHMe[subscript 2]Ph)(2,5-dimethylpyrrolide)(PMe2Ph)(OR) (OR = OHMT or (R)-OBr[subscript 2]Bitet where (R)-Br[subscript 2]BitetOH = (R)-3,3′-dibromo-2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-ol), produced essentially pure cis,syndiotactic-poly[(S)-PhEtNNBE]. Essentially pure cis,isotactic-poly[(S)-PhEtNNBE] was formed when (S)-PhEtNNBE was polymerized by Mo(NAr′)(CHCMe2Ph)(OBiphen[subscript CF3])(thf) or W(NAr′)(CHCMe[subscript 2]Ph)((S)-OBiphenMe) (OBiphenCF3 = 3,3′-di-tert-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate; (S)-OBiphen[subscript Me] = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate). The best initiator for ROMP of rac-PhEtNNBE was Mo(NAd)(CHMe[subscript 2]Ph)(pyr)(OHMT) at 0 °C, which led to a polymer that is biased (∼80%) toward a cis,syndiotactic structure and that contains alternating enantiomers in the chain (cis,syndio,alt-poly[(rac)-PhEtNNBE]).United States. Department of Energy (grant DE-FG0286ER13564)Singapore. Agency for Science, Technology and Research (A*STAR International Fellowship)Alexander von Humboldt-Stiftung (Feodor Lynen Research Fellowship

Formation of Alternating <i>trans</i>-<b>A</b>-<i>alt</i>-<b>B</b> Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Ring-opening metathesis polymerization (ROMP) is used to prepare <i>trans</i>-poly­(<b>A</b>-<i>alt</i>-<b>B</b>) polymers from a 1:1 mixture of <b>A</b> and <b>B</b> where <b>A</b> is a cyclic olefin such as cyclooctene (<b>A</b>_<b>1</b>) or cycloheptene (<b>A</b>_<b>2</b>) and <b>B</b> is a large norbornadiene or norbornene derivative such as 2,3-dicarbomethoxy-7-isopropylidene­norbornadiene (<b>B</b>_<b>1</b>) or dimethyl­spirobicyclo[2.2.1]­hepta-2,5-diene-2,3-dicarboxylate-7,1′-cyclopropane (<b>B</b>_<b>2</b>). The most successful initiators that were examined are of the type Mo­(NR)­(CHCMe₂Ph)­[OCMe­(CF₃)₂]₂ (R = 2,6-Me₂C₆H₃ (<b>1</b>) or 2,6-<i>i-</i>Pr₂C₆H₃ (<b>2</b>)). The <i>trans</i> configuration of the <b>AB</b> linkages is proposed to result from the steric demand of <b>B</b>. Both <i>anti</i>-<b>MB</b> and <i>syn</i>-<b>MB</b> alkylidenes are observed during the copolymerization, where <b>B</b> was last inserted into a MoC bond, although <i>anti</i>-<b>MB</b> dominates as the reaction proceeds. <i>anti</i>-<b>MB</b> is <i>lower</i> in energy than <i>syn</i>-<b>MB</b>, does not react readily with <i>either</i> <b>A</b> or <b>B</b>, and interconverts slowly with <i>syn</i>-<b>MB</b> through rotation about the MoC bond. <i>Syn</i>-<b>MB</b> does not readily react with <b>B</b>, but it does react slowly with <b>A</b> (rate constant ∼1 M^–1 s^–1) to give <i>anti</i>-<b>MA</b> and one <i>trans</i>-<b>AB</b> linkage. <i>anti</i>-<b>MA</b> then reacts with <b>B</b> (rate constant ∼300 M^–1 s^–1 or larger) to give <i>syn</i>-<b>MB</b> and the second <i>trans</i>-<b>AB</b> linkage. The reaction has been modeled using experimental data in order to obtain the estimated rate constants above. The reaction between <i>anti</i>-<b>MA</b> and <b>A</b> is proposed to give rise to <b>AA</b> linkages, but <b>AA</b> dyads can amount to <5%. Several other <b>A</b> and <b>B</b> monomers, initiators, and conditions were explored

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

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