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This work describes an experimental kinetic study and the development of a kinetic model for the dimerization of cyclopentadiene (CPD) to dicyclopentadiene (DCPD) in multicomponent reaction mixtures at temperatures ranging from 80 to 160 °C. The reaction mixtures consist of CPD and other reactive C5compounds, including isoprene, trans-1,3-pentadiene, 1-pentene, cis-2-pentene, and trans-2-pentene, which are normally present in the CPD stream derived from steam cracking of naphtha. Reaction rate constants for (a) dimerization of CPD to form exo- and endo-DCPD, (b) the codimerization of CPD with the other reactive components, and the homodimerization of each of the reactive components are presented along with the corresponding activation energies and pre-exponential factors. By including the retro Diels–Alder reaction of endo-DCPD and the formation of CPD trimers, our kinetic model accurately predicts concentrations of CPD, endo-DCPD, exo-DCPD, and codimers with respect to time. The model identifies CPD self-dimerization as the dominant reaction occurring in the multicomponent mixture, followed by the codimerization between cyclopentadiene and isoprene. At 120 °C, codimerization reactivity with CPD was found in the following order: 1-pentene > cis-2-pentene > 2-methyl-1-butene > cyclopentene > 2-methyl-2-butene > trans-2-pentene, with reaction rate constants 1 to 2 orders of magnitude lower than that of exo-DCPD formation. Overall, the model established herein allows an accurate prediction of the DCPD purity that can be obtained from a petroleum pyrolysis stream no matter what the impurity concentrations may be.

Hydrazine, monomethylhydrazine, and ethylenediamine react readily and reversibly with carbon dioxide to form hydrazinium carboxylate, monomethylhydrazinium carboxylate, and ammonium carbamate adducts, respectively. These adducts act as low molecular weight gelators, which self-assemble and in doing so trap unreacted hydrazine or diamine. The properties and molecular structures of the gels were investigated by means of elemental analysis, DSC/TGA, heteronuclear NMR (1H, 15N, and 13C), diffusion-ordered NMR spectroscopy (DOSY), and cryo-immobilized high resolution scanning electron microscopy (cryo-HRSEM). These complementary techniques provided the foundation for developing a pictorial representation of the bulk gel structures on the molecular level.

Cross-linked polyethylene (XLPE) is an important class of materials with critical applications  including pipes and high voltage electric cables. An organic peroxide such as bis(1-methyl-1-phenylethyl) peroxide (BMPEP) is a commonly used radical initiator in the cross-linking process along with a secondary antioxidant such as bis(octadecyloxycarbonylethyl) sulfide (BOCES). Herein is described a detailed investigation into the complex interactions of the radical initiator BMPEP and the secondary antioxidant BOCES in dodecane solvent–a model for the polyethylene polymer. Kinetic and spectroscopic experiments along with DFT calculations provide the foundation for a detailed mechanistic description for the roles of BMPEP, BOCES, atmospheric oxygen, and trace amounts of water in the overall process.

Gas-expanded liquids are tunable media for reaction and separation. We report that gas-expanded liquids under CO2 pressure are unique media for amine formation and separation. In the heterogeneous hydrogenation of benzonitrile and phenylacetonitrile with NiCl2/NaBH4 in CO2-expanded ethanol, the primary amines are protected by CO2 so that the yield of the primary amines is greatly increased and the production of the secondary amines is effectively suppressed. In the homogeneous hydrogenation of benzonitrile and phenylacetonitrile with RhH(P-i-Pr3)3 and benzophenone imine with Rh(1,5-C8H12){P(C6H5)3}2]PF6 in CO2-expanded tetrahydrofuran, the primary amines are separated in situ in the form of solid carbamic acids and/or ammonium carbamates with increased yield while the catalyst remains in the solution. These results demonstrate the potential for using modest pressures of CO2 to facilitate reactions as well as to separate products.

Fluorous biphasic chemistry enables the recovery of homogeneous catalysts but presents engineering challenges because of issues concerning phase contacting and solvent loss. The addition of gaseous CO2 to fluorous−organic biphasic systems results in a single homogeneous liquid phase at temperatures well below the upper critical solution temperature of the binary liquid mixture. This phenomenon is due to the high solubility of CO2 in both organic liquids and fluorocarbons, and it facilitates reactions and can also reduce fluorous solvent losses. We demonstrate two homogeneously catalyzed reactions, a hydrogenation and an epoxidation, which result in an enhancement in the turnover frequency of 70% and 50%, respectively, for the CO2-merged phase relative to the fluorous biphasic system. This creates new opportunities for the use of fluorous-sequestered catalysts.

Gas-expanded liquids are tunable media for reactions and separations. We report for the first time that CO2-expanded alcohols are catalytic media for cyclohexanone acetal formation. This process offers the opportunity to replace some environmentally harmful acid catalysts, as the CO2-based acid catalysts are self-neutralizing and result in no solid waste. We propose that the alkylcarbonic acids formed in situ from CO2 and alcohols are the acid catalysts and that the catalytic capacity can be tuned with CO2 pressure and temperature. The kinetics of cyclohexanone acetal formation in CO2-expanded methanol was investigated under various CO2 pressures over a temperature range of 25−50 °C without the addition of acid. Pseudo-first- and second-order rate constants were evaluated. These rate constants go through a maximum and the superficial activation energies go through a minimum between 10 and 40 bar of CO2.

A novel class of tunable solvents facilitates recycle of homogeneous catalysts for both economic and environmental advantages. In a mixed organic−aqueous tunable solvent (OATS), reactions between water-soluble catalysts and moderately hydrophobic substrates can be run homogeneously, with subsequent phase separation for product purification and catalyst reuse. One example demonstrated is the dimethyl ether (DME)−water system, which has been employed as a benign alternative to organic solvents for alcohol dehydrogenase (ADH)-catalyzed reduction of hydrophobic ketones coupled with regeneration of the cofactor NADH. We also show the feasibility of a biphasic DME−water separation scheme to couple with the biocatalytic reaction. The subsequent downstream processing offers the advantages of easily isolating water-insoluble products from the aqueous phase and recycling enzyme−cofactor. Other OATS systems are discussed where the preferential dissolution of modest pressures of CO2 causes phase separations, which result in very large distribution coefficients of target molecules in the biphasic organic−aqueous system, with substantial promise as tunable solvents for biocatalysis.

Near-critical water (liquid water between 200 and 374 °C) offers an environmentally benign alternative for the replacement of undesirable solvents and catalysts. This work characterizes the solvent strength of liquid water at saturation pressure from ambient temperature to 275 °C in terms of its Kamlet−Taft dipolarity/polarizability, hydrogen-bond-donating acidity, and hydrogen-bond-accepting basicity using in situ UV−vis spectroscopy. The results suggest that near-critical water exhibits a wide range of polarity and hydrogen-bond-donor ability for tailoring chemical reactions and separations. These Kamlet−Taft solvent parameters can be used to correlate kinetic properties for reactions in water. As model reactions, the temperature-dependent kinetics of the hydrolyses of two nitroaromatic compounds, 4-nitroaniline and N,N-dimethyl nitroaniline, were determined in NCW in the temperature range of 200−275 °C. The hydronium ion dissociated from water promotes the initial hydrolysis reaction without the addition of any acid. Solvent effects on the rate constant were correlated with Kamlet−Taft solvent parameters based on a linear solvation energy relationship (LSER).

The first example of a phase-transfer-catalyzed alkylation reaction under supercritical fluid conditions is reported. The reaction is that of phenylacetonitrile and ethyl bromide in the presence of tetrabutylammonium bromide and potassium carbonate in supercritical ethane at 45, 60, and 75 °C and 138 bar. Results show that the reaction will go to completion in less than 24 h in the presence of the catalyst but that only a few percent conversion is achieved without it during the same period of time. The effects of catalyst concentration, temperature, and cosolvents are investigated. Catalyst solubility estimates and kinetic analyses suggest that the reaction takes place on the surface of the potassium carbonate particles. When the same reaction is attempted in supercritical carbon dioxide, both carboxylation and alkylation are observed. Cycloalkylation reactions between phenylacetonitrile and dibromoalkanes are also discussed.

An alternative means of epoxidation is reported that uses environmentally benign supercritical carbon dioxide as both a solvent and reactant in combination with aqueous H2O2, which is made possible through the in situ formation of peroxycarbonic acid. Experiments were conducted at 40 °C and 120 bar in which cyclohexene was epoxidized to 1,2-cyclohexene oxide and 1,2-cyclohexanediol in this aqueous−organic biphasic system. Through the addition of NaHCO3 and the hydrophilic cosolvent dimethylformamide, the conversion increased from 0.4 mol % (without additives) to 12.6 mol % (with 0.1 mol % NaHCO3 and 13 mol % dimethylformamide). The results suggest that the reaction occurs within the aqueous phase, which led to investigations using the water-soluble olefin 3-cyclohexen-1-carboxylate sodium salt as a means of verifying the reaction location. Epoxidation of 3-cyclohexen-1-carboxylate sodium salt went to completion in less than 20 h at 40 °C and 120 bar with an epoxide yield of 89 mol % and diol yield of 11 mol %.

Environmentally benign near-critical water offers substantial advantages over traditional organic solvents. In this work we use the hydrolyses of several substituted benzoate esters and a series of substituted anisoles as probes to elucidate the activity of the two ionic species in near-critical water. Each of these hydrolyses can run via both acid- and base-catalyzed pathways, as well as an SN2 pathway. An analysis of Hammett plots suggests that the benzoate esters hydrolyze autocatalytically, following an acid-catalyzed mechanism, whereas the anisoles are hydrolyzed via the SN2 mechanistic pathway. This work suggests that near-critical water offers significant potential, both as a benign solvent and as a reaction medium that does not require neutralization with acid or base after the desired reaction is complete.

The key to sustainable technology is achieving an economic benefit for environmental improvements. The tunability of benign compounds such as CO2 and water as supercritical and near-critical fluids offers improved performance and greater flexibility for separation and reaction processes, leading to economic advantages. Examples of such behavior are supercritical CO2 extractions and separations, CO2-expanded polar liquids for separations, and reactions in near-critical water. In each case the tunability is achieved through density changes or cosolvents and may augment product yield, quality, or rates.

The hetero-Diels−Alder reaction between anthracene and excess 4-phenyl-1,2,4-triazoline-3,5-dione has been investigated in supercritical CO2 at 40 °C and pressures between 75 and 216 bar. Bimolecular reaction rate constants have been measured via fluorescence spectroscopy by following the decrease in anthracene concentration with reaction time. The reaction rate is elevated in the vicinity of the critical pressure. This difference is consistent with local composition enhancement and can be modeled with the Peng−Robinson equation of state.

We report the selective removal of a desired reaction intermediate with a supercritical fluid in a new synthesis route to poly(ethylene terephthalate), avoiding ethylene glycol. We have successfully catalyzed the esterification of terephthalic acid with ethylene oxide in a supercritical fluid with a series of quaternary ammonium salts to form mono(2-hydroxyethyl terephthalate). This desired monoester was removed from the involatile bed of terephthalic acid and catalyst by continuous extraction with supercritical fluid before subsequent reaction to the diester could take place.

This work reports the first studies designed to examine the detailed mechanism of a phase-transfer catalyzed reaction between a supercritical CO2 phase and a solid salt phase. The nucleophilic displacement of benzyl chloride with potassium cyanide to form phenylacetonitrile and potassium chloride was carried out with a tetraheptylammonium salt as the phase-transfer catalyst. The effects of various reaction variables on the kinetics were investigated, including the amount of catalyst, the amount of potassium cyanide, the presence of acetone cosolvent, and temperature. The kinetic data, along with catalyst solubility measurements, indicate that the operating reaction mechanism is a three-phase system consisting of a supercritical CO2 phase, a catalyst-rich phase, and a solid salt phase and that the reaction actually occurs in the catalyst-rich phase. Further, the reaction mechanism was investigated with two additional phase-transfer catalysts, 18-crown-6 and poly(ethylene glycol), and these results are consistent with the postulated three-phase mechanism.

The n → π* shift of benzophenone has been used to quantify solute−cosolvent interactions in supercritical ethane. Dilute solutions of benzophenone in cosolvent/supercritical ethane mixtures were studied at 35 °C from 50 to 100 bar over a range of cosolvent concentrations. The following cosolvents were chosen for investigation on the basis of their varying abilities to interact with benzophenone:  2,2,2-trifluoroethanol, ethanol, chloroform, propionitrile, 1,2-dibromoethane, and 1,1,1-trichloroethane. In the supercritical systems investigated here, hydrogen bonding of protic cosolvents to the carbonyl oxygen of benzophenone is the primary mechanism of the n → π* shift. The results of this investigation are consistent with a chemical−physical interpretation of cosolvent effects in supercritical fluids in the presence of strong specific solute−cosolvent interactions. The experimental results for the ethane/TFE/benzophenone system were analyzed by using integral equations in order to study the assumptions of the chemical−physical model. This combination of spectroscopic data with radial distribution function models provides a powerful tool for understanding cosolvent effects.

The first example of a phase-transfer-catalyzed reaction carried out in a supercritical fluid (SCF) is reported. The kinetics of the nucleophilic displacement of benzyl chloride with a bromide ion in SCF solvent CO2 in the presence of acetone cosolvent is presented. In addition, the solubility of tetraheptylammonium bromide (THAB) in SCF CO2 in the presence of acetone cosolvent is reported. The reaction rate was measured at 50 and 75 °C at pressures to 200 bar; the reaction was found to follow first-order, reversible kinetics for the reactions catalyzed by THAB and zero-order kinetics for reactions catalyzed by 18-crown-6.

For any valid study of reaction kinetics the phase equilibria involved must be known. We present an apparatus designed to measure both phase equilibria and kinetics for reactions in near-critical water. Liquid−liquid mutual solubilities are reported for the benzene−water and toluene−water systems from 200 to 275 °C at the three-phase pressures and 172 bar. These data were found to be in good agreement with data reported in the literature. Additionally, the near-critical water reaction apparatus was used to verify one-phase regions for the phenol−water and p-cresol−water systems from 250 to 300 °C at 172 bar.

We report successful alkylation reactions in near-critical water in the absence of added acid catalysts. Water has a strong tendency to ionize as the temperature is increased, allowing water in the near-critical region (250−350 °C) to act as an effective acid catalyst. By simultaneously employing near-critical water as the reaction solvent and catalyst, the need for environmentally hazardous organic solvents and acid catalysts is eliminated. The high hydronium ion content of near-critical water proved to be sufficient to synthesize a variety of substituted phenols with tertiary, secondary, and even primary alcohols. The reaction products for each of these reactions are reported.

The utility of the growth mechanism to influence relative particle growth is investigated in a semi-batch reactor. A dynamic competitive growth model is developed and used to simulate the growth of two monodisperse polystyrene particle populations (bidisperse system) at 50 °C. Validation of the model with on-line density and on-line particle diameter measurements demonstrates that radical absorption into particles is more likely to occur by a collision mechanism than either by a propagation or diffusion mechanism. Experimental and simulation results show that relative narrowing of the bidisperse system increases as the weight fraction polymer inside the particles (Wp) decreases. It is concluded that the ability to alter Wp in a semi-batch reactor makes this mode of operation attractive for particle size distribution control.