Assessment of lipase- and chemically catalyzed lipid modification strategies for the production of structured lipids

The purpose of the present study was to devise a two-step reaction to produce partial glycerides, which would subsequently be used as substrates in both lipase-catalyzed and chemically catalyzed esterification reactions with caprylic acid. The yields and kinetics of these two-step reactions were compared to established lipase-catalyzed acidolysis and transesterification as well as to chemical transesterification reactions. Acyl migration did not occur during the hydrolysis or short-path distillation steps in the preparation of free fatty acid-free partial glycerides for esterification reactions. No significant differences in final yields (59.9% to 82.8% w/w of total triacylglycerols) of new structured lipids were detected among lipase-catalyzed (24 h) and chemically catalyzed (5 h) reactions; however, the yield of new structured triacylglycerols (TAG) after 2 h was lower for acidolysis than for the other lipase-catalyzed reactions (P≤0.05). Since no differences in final yields were detected among the reactions, chemical esterification using hydrolyzed oil could represent the best synthetic option, since it offers the advantage of positional distribution control associated with lipase-catalyzed reactions as well as rapid reaction times associated with chemically catalyzed reactions. Attempts to evaluate the positional distribution of caprylic acid using allyl magnesium bromide (Grignard reagent) were hampered by production of unknown species, which prevented accurate determination of the concentration of some key fatty acids.     

Chemistry of ice surfaces. Elementary reaction steps on ice studied by reactive ion scattering

This Account describes a recent study of reactions on ice surfaces with the emphasis on the mechanistic features of elementary reactions steps. Cs(+) reactive ion scattering (Cs(+) RIS) and low-energy sputtering (LES) techniques monitor the reactions by detecting the molecules and ions on the ice surface. The types of reactions include molecule diffusion and migration, proton transfer, and some simple reactions on frozen water and alcohol surfaces. Ice surface reactions exhibit unique behaviors due to a kinetic constraint, resulting in the isolation of reaction intermediates, preferential stabilization of charged species, and diversity of reaction products.     

The Effect of Pressure on Organic Reactions

The utility of high pressure for the understanding of chemical reactions and its application in organic synthesis is shown for cycloadditions (inter‐ and intramolecular Diels‐Alder reactions, 1,3‐dipolar and [2+2] cycloadditions), cheletropic reactions and pericyclic rearrangements (Cope and Claisen rearrangements and electrocyclizations). The origin of the effect of pressure on chemical reactions is discussed. Especially, the change in the packing coefficient during cyclization of chains and the effect of electrostriction on reactions, in which charged species are generated, contribute substantially to a volume contraction leading to a powerful pressure‐induced acceleration of such reactions. Finally, the effect of pressure on free‐radical reactions (homolytic bond dissociations and quinone oxidations) is described.     

Ion exchange membranes and separation processes with chemical reactions

Recent research trends in the development of ion exchange membranes and their use in separation processes with chemical reactions are reviewed. Emphasis in research on the ion exchange membrane is trending toward analysis of micro-structure of the membranes and to development of new functionalized ion exchange membranes in response to industrial requirements. Separation processes with chemical reactions are discussed according to the following classifications: (1) double decomposition of electrolytes; (2) production of acid and base by bipolar ion exchange membrane processes; (3) separators for electrolysis; (4) separators for batteries; (5) use as solid polyelectrolytes; (6) active transport through ion exchange membranes; (7) acceleration of chemical reactions by ion exchange membranes; (8) carrier transport in ion exchange membranes; (9) transducers for electrical signals from chemical reactions; and (10) modified electrodes.     

Model Reactions Involving Ester Functional Groups during Thermo-Oxidative Degradation of Biodiesel

Biodiesel is a renewable fuel used in diesel engines that is typically blended with diesel fuel. However, biodiesel is susceptible to oxidation, which has the potential to produce higher molecular weight materials that may adversely impact vehicle fuel-system performance. To investigate the chemical reactions potentially important in biodiesel oxidation, four different types of chemical reactions involving esters were studied: (1) ester formation (reactions of acids with alcohols), (2) alcoholysis (reactions of alcohols with esters), (3) acidolysis (reaction of acids with esters), and (4) ester exchange (reactions between two esters). Experiments with representative model compounds were used to evaluate these reactions at 90 °C with aeration; conditions previously used to simulate thermo-oxidative degradation during biodiesel aging. Reactions were monitored using gas chromatography, fourier transform infrared (FTIR) spectroscopy, and total acid number (TAN). Evidence is presented suggesting that alcoholysis and ester formation (Reactions 1 and 2), catalyzed by carboxylic acids, are important reactions of esters that could lead to larger molecules. Acidolysis (Reaction 3) proceeded at a comparatively slow rate and ester exchange reaction products (Reaction 4) were not detected under these aging conditions.     

Graft copolymers synthesis by dynamic covalent reorganization of polycaprolactone and poly(ethylene‐co‐vinyl alcohol)

Dynamic covalent reorganization of polycaprolactone (PCL) and poly(ethylene‐co‐vinyl alcohol) (EVOH) were realized by solvent free transesterification reactions. Organometallic and organic catalysts effect on these reactions was first evaluated from kinetic studies on small molar mass model reactants. Kinetic constants and activation energies of these second order reverse reactions were calculated. At the higher temperatures, side reactions were observed; they were identified as being principally dehydration reactions. Reactions conducted onto polymers were slower than those on model reactions. This was due to the immiscibility of the used polymers resulting in diffusion controlled reactions. Two competitive types of reactions were detected, since at the catalyst addition, fast induced reorganization of PCL leading to low PCL molar mass decreases the mixing torque, followed by grafting reactions of PCL onto EVOH, resulted in an important increase of the mixing torque. Substitution rate of the EVOH hydroxyl groups were measured up to 14% by ¹H‐NMR spectroscopy. Increasing substitution rate leaded to a decrease of the copolymer crystallinity and the more substituted copolymers were amorphous. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The cure reactions,network structure,and mechanical response of diaminodiphenyl sulfone-cured tetraglycidyl 4,4′diaminodiphenyl methane epoxies

The cure reactions, chemical structure, and network topography of diaminodiphenyl sulfone (DDS)-cured tetraglycidyl 4,4′diaminodiphenyl methane (TGDDM) epoxies are reported. Systematic Fourier transform infrared (FTIR) spectroscopy studies of the cure and degradation reactions of TGDDM-DDS epoxies in the 100–300°C temperature range as a function of DDS and boron trifluoride monoethylamine, BF₃: NH₂C₂H₅ catalyst concentrations are presented. FTIR studies indicate TGDDM epoxide homopolymerizes in the 175–250°C range via epoxide-hydroxyl (E-OH) chain extension reactions. The hydroxyl groups are initially present as α-glycol impurities or are formed by epoxide isomerization and/or oxidation. Three principal cure reactions occur at 177°C for TGDDM-DDS epoxies, namely primary amine-epoxide (PA-E), secondary amine-epoxide (SA-E) and E-OH reaction with the PA-E reaction being an order of magnitude faster than the other two cure reactions. The PA-E reaction dominates the early stages of cure and, hence, composite processing conditions. The three cure reactions are catalyzed to similar extents by BF₃: NH₂C₂H₅. FTIR and molecular modeling studies indicate that the E-OH and SA-E reactions can occur intermolecularly to form crosslinks or intramolecularly to form non-cross-linked internal rings. The complex degradation reactions of TGDDM-DDS epoxies in the 177–300°C range are reported. The principal degradation reactions involve (1) dehydration and/or oxidation to form ether crosslinks and (2) decomposition of the EOH cure reaction products to form propenal. Based on a knowledge of the cure reactions, together with molecular modeling, the chemical structure and network topography of TGDDM-DDS epoxies are reported.     

Chain extending of lactic acid oligomers. 2. Increase of molecular weight with 1,6-hexamethylene diisocyanate and 2,2′-bis(2-oxazoline)

Lactic acid polymers were synthesised from oligomers by the addition of highly reactive 1,6-hexamethylene diisocyanate (HMDI) and 2,2′-bis(2-oxazoline) (BOX) as chain extenders. The effects were studied of adding the extenders simultaneously and sequentially and in different amounts. The reactions were followed and the polymers obtained were structurally characterised by size exclusion chromatography and spectroscopy (NMR and FTIR). High molecular weight poly(lactic acids) were obtained by both simultaneous and sequential addition. The mode of addition and the amounts of extenders had a considerable effect on the branching. The addition of HMDI before BOX in the sequential linking produced more highly branched polymers than did the simultaneous addition. Branching and crosslinking reactions were identified as side reactions of the chain extending reactions. The gel formation was attributed to the reactions of BOX and HMDI with urethane and amide but not with oxamide.     

Long chain branching polylactide: Structures and properties

Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of the end hydroxyl groups of PLA with pyromellitic dianhydride (PMDA) and triglycidyl isocyanurate (TGIC) together. The topological structures of the LCB generated from functional group reactions as well as free radical reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, non-linear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict exact compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the tree-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow, which improves the foaming ability substantially.     

Hafnium Reactivity with Boron and Carbon Sources Under Non-Self-Propagating High-Temperature Synthesis Conditions

The homogeneous and heterogeneous reactions of hafnium (Hf) with boron (B) or carbon (C) powders were studied under non-self-propagating high-temperature synthesis (SHS) conditions in searching for chemically aided processes of ultra-high-temperature ceramics at mild and practical conditions. The threshold interactions of the Hf/B and Hf/C powder mixtures consisting of relatively large Hf particles occur at 700° and 800°C, respectively, with no observation of particle melting as previously postulated for igniting SHS reactions. A microscale melting phenomenon at the Hf surface was observed by scanning electron microscopy analysis in heterogeneous reactions between Hf metal strips and B or C powders at the temperature range of 1100°–1200°C, more than 1000°C below the incipient melting point of the metal. The non-SHS conditions allow halting the reactions at intermediate stages for analytical purposes. Kinetic studies of the homogeneous and heterogeneous reactions, heat transfer model analyses, and their association with the investigation of microstructural phenomena were used to postulate the mechanism involved in the observed reactions.     

A model for primary and heterogeneous secondary reactions of wood pyrolysis

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as sutocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.     

A kinetic study on non-catalytic reactions in hydroprocessing Boscan crude oil

Non-catalytic hydrothermal cracking reactions are known to associate with catalytic hydrocracking reactions. In a recent study on hydroprocessing of Boscan crude over a specific catalyst system containing three distinct catalysts, it was found that hydrodesulfurization (HDS) and hydrodemetallation (HDM) reactions continued even when the catalyst is severely deactivated. Since the reactor was packed with considerable amount of inert material besides the three catalysts, it will be advantage to determine if the inert materials can also facilitate hydroprocessing reactions. A series of kinetic experiments for the inert particles was undertaken under different space velocity and temperature conditions.The extent of catalytic and non-catalytic hydroprocessing reactions was assessed. Through statistical analysis, the initial reaction rate constant, reaction order and activation energy for various hydroprocessing reactions were then determined. The absolute average deviations (AAD) of the kinetics values obtained for inert materials are less than 10%.     

Heterogeneous reactions important in atmospheric ozone depletion: a theoretical perspective

Theoretical studies of the mechanisms of several heterogeneous reactions involving ClONO(2), H(2)O, HCl, HBr, and H(2)SO(4) important in atmospheric ozone depletion are described, focused primarily on reactions on aqueous aerosol surfaces. Among the insights obtained is the active chemical participation of the surface water molecules in several of these reactions. The general methodology adopted allows reduction of these complex chemical problems to meaningful model systems amenable to quantum chemical calculations.     

Preparation and rheological characterization of long chain branching polylactide

Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of PLA with pyromellitic dianhydride (PMDA) and 1,4-phenylene-bis-oxazoline (PBOZ) together. The topological structures of the LCB generated from functional group reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, nonlinear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict probable compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the star-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow.     

HOTSPOT DISTRIBUTION WHILE SHORTSTOPPING RUNAWAY REACTIONS DEMONSTRATE THE NEED FOR CFD MODELS

Runaway reactions continuing to be a problem in the chemical industry. A recent study showed that 26.5% of major chemical plant accidents are due to runaways. Runaways are caused by (a) mischarges of the reactants, catalysts, or contaminants or (b) loss of temperature control. Our studies cover the concept of shortstopping the runaway reactions to prevent accident scenarios. Experiments are conducted with CFD (Fluent) models. Shortstopping runaway reactions can be carried out by (a) adding an inhibitor to neutralize the reaction and/or (b) adding a cold diluent to lower the rate of reaction. In this present work we study the characteristics of runaway reactions and inhibition techniques with a full 3-D CFD simulation to explore nonsymmetric addition points for inhibition. Our 3-D simulations are performed using the multiple reference frame method, and reactions are enabled using user defined functions in Fluent. These CFD results show the distribution of hotspots, that characterizes the shortstopping performance. They also clearly demonstrate the value of using CFD simulations in situations that are experimentally prohibitive.     

HOTSPOT DISTRIBUTION WHILE SHORTSTOPPING RUNAWAY REACTIONS DEMONSTRATE THE NEED FOR CFD MODELS

Runaway reactions continuing to be a problem in the chemical industry. A recent study showed that 26.5% of major chemical plant accidents are due to runaways. Runaways are caused by (a) mischarges of the reactants, catalysts, or contaminants or (b) loss of temperature control. Our studies cover the concept of shortstopping the runaway reactions to prevent accident scenarios. Experiments are conducted with CFD (Fluent) models. Shortstopping runaway reactions can be carried out by (a) adding an inhibitor to neutralize the reaction and/or (b) adding a cold diluent to lower the rate of reaction. In this present work we study the characteristics of runaway reactions and inhibition techniques with a full 3-D CFD simulation to explore nonsymmetric addition points for inhibition. Our 3-D simulations are performed using the multiple reference frame method, and reactions are enabled using user defined functions in Fluent. These CFD results show the distribution of hotspots, that characterizes the shortstopping performance. They also clearly demonstrate the value of using CFD simulations in situations that are experimentally prohibitive.     

Competition between hydrotreating and polymerization reactions during pyrolysis oil hydrodeoxygenation

Hydrodeoxygenation (HDO) of pyrolysis oil is an upgrading step that allows further coprocessing of the oil product in (laboratory‐scale) standard refinery units to produce advanced biofuels. During HDO, desired hydrotreating reactions are in competition with polymerization reactions that can lead to unwanted product properties. To suppress this polymerization, a low‐temperature HDO step, referred to as stabilization, is typically used. Small batch autoclaves have been used to study at near isothermal conditions the competition between hydrotreating and polymerization reactions. Although fast polymerization reactions take place above 200°C, hydrogen consumption was already observed for temperatures as low as 80°C. Hydrogen consumption increased with temperature and reaction time; however, when the end temperature exceeded 250°C, hydrogen consumption achieved a plateau. This was thought to be caused by the occurrence of fast polymerization reactions and the refractivity of the products to further hydrotreating reactions. The effect of the gas–liquid mass transfer was evaluated by using different stirring speeds. The results of these experiments (carried out at 300°C) showed that in the first 5 min of HDO, gas–liquid mass transfer appears to be limiting the overall rate of hydrotreating reactions, leading to undesired polymerization reactions and product deterioration. Afterward, intraparticle mass transfer/kinetics seems to be governing the hydrogen consumption rate. Estimations on the degree of utilization (effectiveness factor) for industrially sized catalysts show that this is expected to be much lower than 1, at least, in the early stage of HDO (first 30 min). Catalyst particle size should, thus, be carefully considered when designing industrial processes not only to minimize reactor volume but also to improve the ratio of hydrotreating to polymerization reactions. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Exo-mechanism proximity-accelerated alkylations: investigations of linkers, electrophiles and surface mutations in engineered cyclophilin-cyclosporin systems

Investigations on the scope and utility of exo-mechanism proximity-accelerated reactions in engineered receptor-ligand systems are reported. We synthesized a series of electrophilic cyclosporin (CsA) derivatives by varying electrophiles and linker lengths, prepared a series of nucleophilic cysteine mutations on the surface of cyclophilin A (Cyp), and examined their reactivity and specificity in proximity-accelerated reactions. Acrylamide and epoxide electrophiles afforded useful reactivity and high specificity for alkylation of engineered receptors in Jurkat cell extracts. We found that remote cysteines (>17 A from the ligand) could be alkylated with useful rates under physiological conditions. The results from mutations of the receptor surface suggest that the dominant factors governing the rates of proximity-accelerated reactions are related to the local environment of the reactive group on the protein surface. This study defines several parameters affecting reactivity in exo-mechanism proximity-accelerated reactions and provides guidance for the design of experiments for biological investigations involving proximity-accelerated reactions.     

Influence of preflame reactions on combustion of hydrocarbons in shock-heated air

The reactions leading to the propagation of a flame have been studied in shock-heated air containing small amounts of either (normal) octane or 1,1-dimethyl hexane (isooctane). Preflame reactions, associated with the production of excited hydroxyl radicals, have been shown to occur with both octanes. Such reactions would account for the observed acceleration of the flame with octane and its retardation with isooctane, provided that the products from octane were more reactive (as found here) and the products from isooctane less reactive (as found by Cullis⁹ for heptane) than the parent hydrocarbons. Isooctane, as expected, was more resistant to preflame reactions; nevertheless they were observed at temperatures as low as 1050 K. The temperature coefficients of the reactions governing the growth of chemiluminescent radiation are similar for the two octanes: 330 and 350 kJ mol^?1 for octane and isooctane respectively. The temperature coefficients for the reactions controlling the overall burning are lower, falling to about 190 kJ mol^?1. The marked differences between the growth of pressure in what is essentially a premixed flame, and the growth of pressure in the combustion of droplets of (normal) hexadecane, are noted.     

Reactions of Acenaphthenequinone and Aceanthrenequinone with Arenes in Superacid

The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Br?nsted superacid CF(3)SO(3)H (triflic acid), the condensation products are formed in good yields (58-99%, 10 examples) with high regioselectivity. Computational studies were also done to examine the structures and energies of mono- and diprotonated species from 6 and 7. The results from the condensation reactions are consistent with the formation of superelectrophilic species involving protosolvation of carboxonium ion intermediates.