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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Amidine-Mediated Zwitterionic Ring-Opening Polymerization of N -Alkyl N -Carboxyanhydride: Mechanism, Kinetics, and Architecture Elucidation

Zwitterionic ring-opening polymerization (ZROP) of N-butyl N-carboxyanhydrides (Bu-NCAs) has been investigated using 1,8-diazabicycloundec-7-ene (DBU), a bicyclic amidine initiator. It was found that poly(N-butylglycine)s (PNBGs) with molecular weight (M n ) in the 3.5–32.4 kg mol –1 range and polydispersity index (PDI) in the 1.02–1.12 range can be readily obtained by systematically varying the initial monomer to initiator feed ratio. The polymerization exhibits characteristics of a controlled polymerization, as evidenced by the linear increase of polymer molecular weight with conversion and the successful enchainment experiments. Kinetic studies revealed that the reaction is first-order dependent on the monomer and the DBU concentration. The rate of initiation is comparable to that of the propagation. Random copolypeptoids of poly[(N-propargylglycine)-r-(N-butylglycine)]s [P(NPgG-r-NBG)s] were also synthesized by DBU-mediated copolymerization of Bu-NCA and N-propargyl N-carboxyanhydride (Pg-NCA). Subsequent grafting with azido-terminated poly(ethylene glycol) (PEG) produces bottlebrush copolymers. Analysis of bottlebrush copolymer samples using atomic force microscopy (AFM) revealed a surface morphology of toroid-shaped nanostructures, consistent with the polypeptoid backbone having cyclic architecture. Here, small-angle neutron scattering (SANS) characterization of the bottlebrush polymer ensemble in solution also confirms the cyclic architecture of the polypeptoid backbones.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Chain flexibility and glass transition temperatures of poly(n-alkyl (meth)acrylate)s: Implications of tacticity and chain dynamics

The relationship between chain microstructure and glass transition temperature (T g ) is complicated for poly(alkyl methacrylate)s and poly(alkyl acrylate)s. Despite intensive studies, relationships between the structures of these polymers and their properties, including solution characteristics and T g , are still controversial. Solution properties, chain conformations, including Flory's characteristic ratio (C ∞ ), persistence length (l p ), and chain diameters, and T g are reported for series of poly(n-alkyl acrylate)s and poly(n-alkyl methacrylate)s having alkyl side chain lengths (n) ranging from 1 to 10 carbons in length, with uniform and well-characterized tacticities. Chain flexibilities of both series of polymers decrease as n increases, reflecting increased hindrances to rotation about backbone bonds as side chains become longer. Conversely, the T g s for both series of polymers decrease substantially as n increases, reflecting the greater side chain mobilities of long alkyl substituents. For shorter alkyl chain lengths, T g s for the poly(n-alkyl acrylate)s are much lower than for the corresponding poly(n-alkyl methacrylate)s, a difference which has been attributed in the past to the presumed reduced chain flexibility of polymethacrylates due to the presence of the α-methyl substituent. However, contrary to such expectations, C ∞ and l p values for these two series of polymers are nearly identical at a given n value, except for the longest n-alkyl substituents. Instead, the differences in T g may be attributed to the differences in tacticity of the two series – almost ideally atactic for the poly(n-alkyl acrylate)s but high in syndiotacticity for the poly(n-alkyl methacrylate)s. The T g s for the two series of polymers approach that of polyethylene at longer alkyl chain lengths. In addition, the effects of tacticity and chain dynamics on C ∞ , l p , and T g of poly(methyl methacrylate) is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

On the prediction of pressure effects for the combination kinetics of two alkyl radicals with the geometric mean rule

Here, alkyl combination reactions, along with their reverse alkane unimolecular decomposition reactions, are widely present in pyrolysis and combustion systems. These reactions occur mainly at temperatures at which their rate coefficients depend on pressure, and only a few studies have explored this dependence for large size alkyls. The accurate calculation of these kinetic data is limited by the size of the alkyl fragments involved, since the theoretical methods used are computationally expensive. Therefore, little reliable data is available for the modeling of real fuels alkane decomposition kinetics. In this paper, we propose an alternative approach that allows an accurate estimation of pressure-dependent kinetic rates for the combination of long-chain alkyl radicals, based on tabulated reaction models. The rates of combination of linear alkyls radicals forming ethane to pentane were calculated using the CASPT2-F12/cc-pVDZ-F12 level within VRC-TST theory combined with master-equation simulations. The computed high-pressure limit rate coefficients showed that n-propyl could be a minimum-size representative structure for larger n-alkyl chain, but this approach fails to capture the pressure dependence of the reaction that remains size-dependent for larger alkyls. The Geometric Mean Rule (GMR), a simple rule that links the self and cross-reactions of three alkyl radicals, is used to address this issue. Our theoretical calculations show for the first time that the GMR applies not only at the high-pressure limit, but also for every pressure explored. The GMR is shown to be accurate, with a maximum 22 % deviation in the falloff region for the estimation of rates for small alkyl combinations. Using this method, we propose a tabulated reaction rate rule for the combination of alkyl radicals up to C5+C5. The use of computed rate coefficients with the GMR approximation opens ways to develop pressure-dependent reaction rate rules for large n-alkyl combinations, based on accurate and computationally expensive theoretical calculations on smaller alkyl fragments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Effect of alkyl glycerophosphate on the activation of peroxisome proliferator-activated receptor gamma and glucose uptake in C2C12 cells

Highlights: •Alkyl-LPA specifically interacts with PPARγ. •Alkyl-LPA treatments induces lipid accumulation in C2C12 cells. •Alkyl-LPA enhanced glucose uptake in C2C12 cells. •Alkyl-LPA-treated C2C12 cells express increased amounts of GLUT4 mRNA. •Alkyl-LPA is a novel therapeutic agent that can be used for the treatment of obesity and diabetes. -- Abstract: Studies on the effects of lipids on skeletal muscle cells rarely examine the effects of lysophospholipids. Through our recent studies, we identified select forms of phospholipids, such as alkyl-LPA, as ligands for the intracellular receptor peroxisome proliferator-activated receptor gamma (PPARγ). PPARγ is a nuclear hormone receptor implicated in many human diseases, including diabetes and obesity. We previously showed that alkyl-LPA is a specific agonist of PPARγ. However, the mechanism by which the alkyl-LPA–PPARγ axis affects skeletal muscle cells is poorly defined. Our objective in the present study was to determine whether alkyl-LPA and PPARγ activation promotes glucose uptake in skeletal muscle cells. Our findings indicate that PPARγ1 mRNA is more abundant than PPARγ2 mRNA in C2C12 cells. We showed that alkyl-LPA (3 μM) significantly activated PPARγ and increased intracellular glucose levels in skeletal muscle cells. We also showed that incubation of C2C12 cells with alkyl-LPA led to lipid accumulation in the cells. These findings suggest that alkyl-LPA activates PPARγ and stimulates glucose uptake in the absence of insulin in C2C12 cells. This may contribute to the plasma glucose-lowering effect in the treatment of insulin resistance.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Reaction pathways in the liquid phase alkylation of biomass‐derived phenolic compounds

Alkylation is a promising reaction for the upgrading of bio‐oil because it maximizes the retention of carbon in the liquid product. The alkylation of m‐cresol with isopropanol and HY zeolite was studied in a liquid phase system. The experimental results were fitted with two conventional surface kinetic models, Langmuir–Hinshelwood and Eley–Rideal, from which adsorption and rate constants were estimated. Two types of alkylation reactions were observed: C‐alkylation with formation of a CC bond with the ring and O‐alkylation with formation of an ether bond with the hydroxyl group. It was concluded that O‐alkylation products do not undergo intramolecular rearrangement but first decompose into the corresponding phenolic. Alkylation occurs from both isopropanol and propylene, both of them yielding O‐and C‐alkylation to different extents. Isopropanol favors O‐alkylation while propylene favors C‐alkylation. Rate constants for multiple alkylation steps were progressively lower, suggesting the presence of steric hindrance during incorporation of additional isopropyl groups. © 2014 American Institute of Chemical Engineers AIChE J , 61: 598–609, 2015

González‐Borja, Miguel Ángel↗
DOE OSTI2014

Molecular dynamics study of the effect of alkyl chain length on melting points of [CnMIM][PF6] ionic liquids

Based on molecular dynamics simulations, the melting points T-m of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids [CnMIM][PF6] with n = 2, 4, 10, 12, and 14 were studied using the free energy-based pseudosupercritical path (PSCP) method. The experimental trend that the Tm decreases with increasing alkyl chain length for ILs with short alkyl chains and increases for the ones with long alkyl chains was correctly captured. Further analysis revealed that the different trends are the results of the balance between fusion enthalpy and fusion entropy. For the ILs with short alkyl chains (ethyl and butyl groups), fusion entropy plays the dominant role so that [C4MIM][PF6], which has a larger fusion entropy due to its higher liquid phase entropy has the lower melting temperature. As for the ILs with long alkyl chains, due to the enhanced van der Waals interactions brought about by the long non-polar alkyl chains, enthalpy becomes the deciding factor and the melting points increase when the alkyl chain goes from C10 to C14. While the melting points for [C2MIM][PF6] and [C4MIM][PF6] were quantitatively predicted and the trends for the long chain ILs were captured correctly, the absolute melting points for [C10MIM][PF6], [C12MIM][PF6] and [C14MIM][PF6] were systematically overestimated in the simulations. Three possible reasons for the overestimation were studied but all ruled out. Further simulation or experimental studies are needed to explain the difference.

Zhang, Y↗
DOE OSTI2024

Iron( IV ) alkyl complexes: electronic structure contributions to Fe–C bond homolysis and migration reactions that form N–C bonds from N 2

High-valent iron alkyl complexes are rare, as they are prone to Fe–C bond homolysis. Here, we describe an unusual way to access formally iron(IV) alkyl complexes through double silylation of iron(I) alkyl dinitrogen complexes to form an NNSi 2 group. Spectroscopically validated computations show that the disilylehydrazido(2−) ligand stabilizes the formal iron(IV) oxidation state through a strongly covalent Fe–N π-interaction, in which one π-bond fits an “inverted field” description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(II) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe–N bonds. The reactivity of these hydrazido(2−) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(IV) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi 2 group to form a new N–C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η 1 and η 2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe–C bond homolyzes to give a three-coordinate hydrazido(2−) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(IV) centers to form new N–C bonds directly to N 2 , though product selectivity is highly dependent on the identity of the migrating group.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Development of Catalytic Alkylation and Fluoroalkylation Methods

In the early stages of this DOE-funded research project, we sought to prepare and study a well-defined nickel-alkyl complex containing tridentate nitrogen donor ligands after Fu had published his initial report showing that pybox ligands could successfully catalyze alkyl-alkyl cross-coupling reactions. We found that reaction of (TMEDA)NiMe 2 (1) with terpyridine ligand cleanly led to the formation of (terpyridyl)NiMe (2, eq 4), which we also determined to be an active alkylation catalyst. The thermal stability of 2 was unlike that seen for any of the active pybox ligands, and enabled a number of key studies on alkyl transfer reactions to be performed, providing new insights into the mechanism of nickel-mediated alkyl-alkyl cross-coupling reactions. In addition to the mechanistic studies, we showed that the terpyridyl nickel compounds can catalytically cross-couple alkyl iodides in yields up to 98% and bromides in yields up to 46 %. The yields for the bromides can be increased up to 67 % when the new palladium catalyst [(tpy’)Pd-Ph]I is used. The best route to the targeted [(tpy)NiBr] (1) was found to involve the comproportionation reaction of [(dme)NiBr 2 ] and [Ni(COD) 2 ] in the presence of two equivalents of terpyridine. This reaction was driven to high yields of product formation (72% isolated) by the precipitation of 1 from THF solvent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Dual Chemical Looping/Catalytic Process for Alkylation of Benzene With Ethane and Propane Yielding Ethylbenzene and Cumene Over Copper‐Containing Mordenite

Given the sustained demand for alkylated aromatics and the strained olefin market, there is an urgent need to develop efficient one‐step processes for the direct alkylation of aromatics using alkanes instead of olefins. Such technologies offer greater energy efficiency and sustainability by eliminating the need for separate, energy‐intensive alkane dehydrogenation steps. In this work, we report a dual chemical looping / catalytic process that couples alkane dehydrogenation with aromatic alkylation over a copper‐containing mordenite yielding up to 25% of alkylated aromatics with >97% selectivity per cycle. In situ MAS NMR and FTIR spectroscopies combined with DFT calculations showed that the alkylation of benzene with alkanes proceeds via a π‐bounded Cu(I)‐olefin intermediate, which subsequently interacts with benzene, catalyzed by Brønsted acid sites, leading to alkylated products that readily desorb from the active material into the gas phase. DFT calculations show that alkylation mediated solely by Cu(I) has prohibitively high barriers (>1.8 eV), whereas a bi‐functional pathway involving both Cu(I) and Brønsted acid sites can proceed with significantly lower barrier (0.8 eV) through a concerted C–C bond formation and proton transfer step.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗