Showing papers on "Equilibrium constant published in 2012"

A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas

Abstract: Synthetic natural gas (SNG) can be obtained via methanation of synthesis gas (syngas). Many thermodynamic reaction details involved in this process are not yet fully understood. In this paper, a comprehensive thermodynamic analysis of reactions occurring in the methanation of carbon oxides (CO and CO2) is conducted using the Gibbs free energy minimization method. The equilibrium constants of eight reactions involved in the methanation reactions were calculated at different temperatures. The effects of temperature, pressure, ratio of H-2/CO (and H-2/CO2), and the addition of other compounds (H2O, O-2, CH4, and C2H4) in the feed gas (syngas) on the conversion of CO and CO2, CH4 selectivity and yield, as well as carbon deposition, were carefully investigated. In addition, experimental data obtained on commercial Ni-based catalysts for CO methanation and three cases adopted from the literature were compared with the thermodynamic calculations. It is found that low temperature, high pressure, and a large H-2/CO (and H-2/CO2) ratio are favourable for the methanation reactions. Adding steam into the feed gas could alleviate the carbon deposition to a large extent. Trace amounts of O-2 in syngas is unfavourable for SNG generation although it can lower carbon deposition. Additional CH4 in the feed gas almost has no influence on the CO conversion and CH4 yield, but it leads to the increase of carbon formed. Introduction of a small amount of C2H4, a representative of hydrocarbons in syngas, results in low CH4 yield and serious carbon deposition although it does not affect CO conversion. CO is relatively easy to hydrogenated compared to CO2 at the same reaction conditions. The comparison of thermodynamic calculations with experimental results demonstrated that the Gibbs free energy minimization method is significantly effective for understanding the reactions occurring in methanation and helpful for the development of catalysts and processes for the production of SNG.

Assessing the Chemical Speciation during CO2 Absorption by Aqueous Amines Using in Situ FTIR

Abstract: During amine scrubbing of CO2 from flue gas, carbamate and bicarbonate species are formed, the amount of which is directly related to the process performance. In this study we present a fast calibration-free spectroscopic technique for determining the speciation of CO2–H2O–alkanolamine systems and in turn the amine protonation and carbamate thermodynamic equilibrium constants. The method is based on in situ infrared monitoring of the liquid phase during CO2 absorption by an aqueous amine solution in a stirred vessel, combined with mathematical hard modeling of the reaction mechanism. The species concentrations are calculated by fitting of a thermodynamic model to multivariate spectroscopic measurements using nonlinear regression. Successful applications include the determination of the amine protonation and carbamate equilibrium constants of one primary (MEA), one secondary (DEA), and one sterically hindered primary (AMP) amine at 40 °C.

Thermodynamics of reactions of ClHg and BrHg radicals with atmospherically abundant free radicals

Abstract: . Quantum calculations are used to determine the stability of reactive gaseous mercury (Hg(II)) compounds likely to be formed in the Br-initiated oxidation of gaseous elemental mercury (Hg(0)). Due to the absence of any evidence, current models neglect the possible reaction of BrHg with abundant radicals such as NO, NO2, HO2, ClO, or BrO. The present work demonstrates that BrHg forms stable compounds, BrHgY, with all of these radicals except NO. Additional calculations on the analogous ClHgY compounds reveal that the strength of the XHg-Y bond (for X = Cl, Br) varies little with the identity of the halogen. Calculations further suggest that HO2 and NO3 do not form strong bonds with Hg(0), and cannot initiate Hg(0) oxidation in the gas phase. The theoretical approach is validated by comparison to published data on HgX2 compounds, both from experiment and highly refined quantum chemical calculations. Quantum calculations on the stability of the anions of XHgY are carried out in order to aid future laboratory studies aimed at molecular-level characterization of gaseous Hg(II) compounds. Spectroscopic data on BrHg is analyzed to determine the equilibrium constant for its formation, and BrHg is determined to be much less stable than previously estimated. An expression is presented for the rate constant for BrHg dissociation.

Determination of ATRP Equilibrium Constants under Polymerization Conditions

Abstract: Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or aceto...

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

Abstract: The highest frequency hydrogen bond fundamental of formic acid dimer, ν24 (Bu), is experimentally located at 264 cm−1. FTIR spectra of this in-plane bending mode of (HCOOH)2 and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three Bu combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D0 = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.

Dynamics of Interfacial Charge Transfer to Formic Acid, Formaldehyde, and Methanol on the Surface of TiO2 Nanoparticles and Its Role in Methane Production

Abstract: Transient absorption and electron paramagnetic resonance (EPR) spectroscopies were used to study reactions of photogenerated electrons and holes on TiO2 with methanol, formaldehyde, and formic acid (compounds that, together with methane, have been observed in the photocatalytic reduction of CO2). The ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO2 than in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO2 with adsorbed CO2 was estimated to be less than 3.2 M–1, regardless of the presence of hole scavengers and product molecules. Formic acid serves as both the hole and the electron acceptor, yielding the protonated radical anions (OC•OH), and formyl radicals, respectively. For methanol and formaldehyde only photooxidation, but no one-electron photoreduction, was observed by EPR spectroscopy; these molecules are either reduced in a two-electron process or act only as hole...

Water effect on the OH + HCl reaction.

Abstract: Hydrochloric acid is a major reservoir for chlorine radicals in the atmosphere. Chlorine radicals are chemically reactivated by the relatively slow attack of OH radical on HCl. Through the formation of hydrogen-bonded complexes, water has a dramatic effect on the rate of this reaction. The introduction of water opens several new reaction pathways with rate coefficients that are faster than the “bare” reaction. Accounting for the low fraction of hydrogen bonded water complexes in the atmosphere, the present results suggest that these new mechanisms involving water can contribute, although modestly, to the total chemical reactivation of chlorine from HCl in the lower troposphere. The first reported value for the equilibrium constant for the formation of H2O·HCl complex, which is important in understanding the removal of HCl from the atmosphere by deposition, is presented.

Identification of the dimethylamine-trimethylamine complex in the gas phase.

Abstract: We have identified the dimethylamine-trimethylamine complex (DMA-TMA) at room temperature in the gas phase. The Fourier transform infrared (FTIR) spectrum of DMA-TMA in the NH-stretching fundamental region was obtained by spectral subtraction of spectra of each monomer. Explicitly correlated coupled cluster calculations were used to determine the minimum energy structure and interaction energy of DMA-TMA. Frequencies and intensities of NH-stretching transitions were also calculated at this level of theory with an anharmonic oscillator local mode model. The fundamental NH-stretching intensity in DMA-TMA is calculated to be approximately 700 times larger than that of the DMA monomer. The measured and calculated intensity is used to determine a room temperature equilibrium constant of DMA-TMA of 1.7 × 10(-3) atm(-1) at 298 K.

Experimental determination of thermodynamic equilibrium in biocatalytic transamination

Abstract: The equilibrium constant is a critical parameter for making rational design choices in biocatalytic transamination for the synthesis of chiral amines. However, very few reports are available in the scientific literature determining the equilibrium constant (K) for the transamination of ketones. Various methods for determining (or estimating) equilibrium have previously been suggested, both experimental as well as computational (based on group contribution methods). However, none of these were found suitable for determining the equilibrium constant for the transamination of ketones. Therefore, in this communication we suggest a simple experimental methodology which we hope will stimulate more accurate determination of thermodynamic equilibria when reporting the results of transaminase-catalyzed reactions in order to increase understanding of the relationship between substrate and product molecular structure on reaction thermodynamics. Biotechnol. Bioeng. 2012; 109:2159–2162. © 2012 Wiley Periodicals, Inc.

Oligomer formation of the bacterial second messenger c-di-GMP: reaction rates and equilibrium constants indicate a monomeric state at physiological concentrations.

Abstract: Cyclic diguanosine-monophosphate (c-di-GMP) is a bacterial signaling molecule that triggers a switch from motile to sessile bacterial lifestyles. This mechanism is of considerable pharmaceutical interest, since it is related to bacterial virulence, biofilm formation, and persistence of infection. Previously, c-di-GMP has been reported to display ar ich polymorphism of various o ligomeric forms at millimolar concentrations, which differ in base stacking and G-quartet interactions. Here, we have analyzed the equilibrium and exchange kinetics between these various forms by NMR spectroscopy. We find that the association of the monomer into a dimeric form is in fast exchange (

Kinetics of esterification of acetic acid with 1-octanol in the presence of Amberlyst 36

Abstract: Kinetic data on the esterification of acetic acid with 1-octanol were obtained from both uncatalyzed and heterogeneously catalyzed reactions using a stirred batch reactor in toluene. The equilibrium constant, K c , which is independent of temperature ranging from 333 to 358 K, was found to be 60.7. The uncatalyzed reaction was proved to be a second-order reversible reaction. In the presence of Amberlyst 36, the reaction was found to follow the Eley–Rideal mechanism, and the surface reaction is a rate-limiting step. From this model, the reaction rate can be given by the expression: − r A = k ( m / V ) ( C A C B − ( C E C W / K C ) ) ( 1 + K A C A + K W C W ) . The temperature dependencies of the constants appearing in the rate expressions were also calculated to be: k ( L 2 / g dry resin ⋅ mol ⋅ min ) = exp 1.07 − 2991 T , K A ( L/mol ) = exp 12954 T − 38.28 , and K W ( L/mol ) = exp 25427 T − 76.15 , where T is the absolute temperature in Kelvin.

Density functional computational studies on 2‐[(2,4‐Dimethylphenyl)iminomethyl]‐3,5‐dimethoxyphenol

Abstract: Density functional calculations of the structure, molecular electrostatic potential, and thermodynamic functions have been performed at B3LYP/6-31G(d) level of theory for the title compound of 2-[(2,4-dimethylphenyl)iminomethyl]-3,5-dimethoxyphenol (I). To investigate the tautomeric stability, optimization calculations at B3LYP/6-31G(d) level were performed for the enol and keto forms of I. Calculated results reveal that the enol form of I is more stable than its keto form. The predicted nonlinear optical properties of I are much greater than ones of urea. The changes of thermodynamic properties for the formation of the title compound with the temperature ranging from 200 to 500 K have been obtained using the statistical thermodynamic method. At 298.15 K, the change of Gibbs free energy for the formation reaction of I is 32.973 kJ/mol. The title compound can not be spontaneously produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is computed as 0.868 at 298.15 K for enol-imineketo-amine tautomerization of I. In addition, natural bond orbital analysis of I was performed using the B3LYP/6-31G(d) method. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Uncertainties of geochemical modeling during CO2 sequestration applying batch equilibrium calculations

Abstract: One of the uncertainties in the field of carbon dioxide capture and storage (CCS) is caused by the parameterization of geochemical models. The application of geochemical models contributes significantly to calculate the fate of the CO2 after its injection. The choice of the thermodynamic database used, the selection of the secondary mineral assemblage as well as the option to calculate pressure dependent equilibrium constants influence the CO2 trapping potential and trapping mechanism. Scenario analyses were conducted applying a geochemical batch equilibrium model for a virtual CO2 injection into a saline Keuper aquifer. The amount of CO2 which could be trapped in the formation water and in the form of carbonates was calculated using the model code PHREEQC. Thereby, four thermodynamic datasets were used to calculate the thermodynamic equilibria. Furthermore, the equilibrium constants were re-calculated with the code SUPCRT92, which also applied a pressure correction to the equilibrium constants. Varying the thermodynamic database caused a range of 61% in the amount of trapped CO2 calculated. Simultaneously, the assemblage of secondary minerals was varied, and the potential secondary minerals dawsonite and K-mica were included in several scenarios. The selection of the secondary mineral assemblage caused a range of 74% in the calculated amount of trapped CO2. Correcting the equilibrium constants with respect to a pressure of 125 bars had an influence of 11% on the amount of trapped CO2. This illustrates the need for incorporating sensitivity analyses into reaction pathway modeling.

The mechanism of the photochromic transformation of spirorhodamines

Abstract: We investigate the equilibrium, kinetics, and mechanism of the photochromic transformation of a series of amido spirorhodamine compounds—differing in the nature of the substituents of the amido group and in the rhodamine chromophore—in ethanol at room temperature in the presence of trifluoroacetic acid. A proton participates in the equilibrium between the spiro form and the open rhodamine form. The relaxation times in the dark or under continuous irradiation show a linear dependence on the proton concentration. The slopes of these plots show a linear free energy relation with the equilibrium constant of the transformation. A mechanism involving reversible reaction steps between four states: the two thermodynamically stable isomers, a protonated spiro form, and a deprotonated open form, can account for the kinetic observations in the dark and under irradiation.

The intrinsic energy of the gating isomerization of a neuromuscular acetylcholine receptor channel.

Abstract: I N T R O D U C T I O N Nicotinic acetylcholine receptors (AChRs) are ligandgated ion channels that regulate the flux of cations across cell membranes by switching between closed- and openchannel conformations. This global change in AChR structure (“gating”) occurs both with and without activating ligands (“agonists”) present at the two extracellular transmitter binding sites. However, the gating equilibrium constant increases in the presence of agonists because these ligands bind with a higher affinity to the open-channel conformation of the protein. To understand the energy that agonists provide for the full AChR gating isomerization, it is essential to link measurements of the equilibrium constants in both unliganded and fully liganded conditions in a thermodynamic cycle (Monod et al., 1965; Karlin, 1967; Jackson, 1986; Auerbach, 2010). The unliganded gating equilibrium constant (E0) quantifies the tendency of the protein to adopt the high affinity/open conformation when no agonists are present at the binding sites, and the logarithm of E0 gives the intrinsic energy of the gating conformational change. The extent to which each agonist molecule increases the gating equilibrium constant above this level is a measure of the energy it contributes toward the global conformational change of the protein. Having accurate estimates of this energy is one key to understanding how ligand binding and conformational change are coupled.

Some Fundamental and Historical Aspects of Phenomenological Kinetics in the Solid State Studied by Thermal Analysis

Abstract: Chemical kinetics provides mathematical models for explaining and predicting the transformation rate of a chemical system. The fundamental concept of chemical kinetics is based on the law of mass action established by Cato M. Guldberg (1836–1902) and Peter Waage (1833–1900) in the latter half of the nineteenth century (Waage P, Guldberg CM, Studies concerning affinity. Forhandlinger: Videnskabs – Selskabet i Christinia: 35, English trans. (1986) J Chem Edu 63(12):1044–1047, 1864; Guldberg CM, Waage P Concerning chemical affinity. J Prakt Chem [2]. 19:69, 1879), where equilibrium constants were derived in terms of kinetic data and rate equations. The two different aspects, that is, equilibrium and kinetics, were encountered by the recognition that chemical equilibrium is a dynamic process in which rates of reaction for the forward and backward reactions must be equal, so that the chemical driving force of the forward reaction is compensated by that of the reverse reaction. Because the respective reaction rates are proportional to the product of active masses of the reactant species, the equilibrium constant K can be represented by the ratio of the affinity constants (rate constants) of the forward and reverse reactions, k and k′: K = k/k′. The law of mass action was lately reintroduced by J.H. van’t Hoff (1852–1911) from the aspect of chemical kinetics (van’t Hoff JH, Etudes de dynamique chimique. Frederik Muller, Amsterdam, 1884).

Revisiting zinc coordination in human carbonic anhydrase II.

Abstract: Carbonic anhydrase (CA, general abbreviation for human carbonic anhydrase II) is a well-studied, zinc-dependent metalloenzyme that catalyzes hydrolysis of carbon dioxide to the bicarbonate ion. The apo-form of CA (apoCA, CA where Zn2+ ion has been removed) is relatively easy to generate, and reconstitution of the human erythrocyte CA has been initially investigated. In the past, these studies have continually relied on equilibrium dialysis measurements to ascertain an extremely strong association constant (Ka ≈ 1.2 × 1012) for Zn2+. However, new reactivity data and isothermal titration calorimetry (ITC) data reported herein call that number into question. As shown in the ITC experiments, the catalytic site binds a stoichiometric quantity of Zn2+ with a strong equilibrium constant (Ka ≈ 2 × 109) that is 3 orders of magnitude lower than the previously established value. Thermodynamic parameters associated with Zn2+ binding to apoCA are unraveled from a series of complex equilibria associated with the in vit...

An Insight into the Reversible Dissociation and Chemical Reactivity of a Sterically Encumbered Diphosphane

Abstract: The known 1,1′,3,3′-tetrahydro-2,2′-bi-1,3,2-diazaphosphole 4a was for the first time synthesized on a preparative scale and characterized by spectroscopy. Equilibrium constants for the dissociation 4a [rlarr2] 2 5a were determined from quantitative variable-temperature electron paramagnetic resonance(VT-EPR) studies and were used to obtain thermochemical data for the P–P bond homolysis. The calculated bond dissociation energy of 79(1) kJ mol–1 is substantially lower than the gas-phase dissociation energies of small diphosphanes. Modeling of the molecular structure of 4a by DFT calculations was feasible if dispersion effects were included, but the calculations still failed to give a reasonable account of the energetics of the dissociation process. Chemical reactions of 4a with alkynes proceeded, depending on substrate structure, either as a radical-induced diphosphanation of the triple bond or by a nonradical reaction to give an unprecedented ring metathesis.

Multi-structural variational transition state theory: kinetics of the 1,5-hydrogen shift isomerization of the 1-butoxyl radical including all structures and torsional anharmonicity

Abstract: We investigate the statistical thermodynamics and kinetics of the 1,5-hydrogen shift isomerization reaction of the 1-butoxyl radical and its reverse isomerization. The partition functions and thermodynamic functions (entropy, enthalpy, heat capacity, and Gibbs free energy) are calculated using the multi-structural torsional (MS-T) anharmonicity method including all structures for three species (reactant, product, and transition state) involved in the reaction. The calculated thermodynamic quantities have been compared to those estimated by the empirical group additivity (GA) method. The kinetics of the unimolecular isomerization reaction was investigated using multi-structural canonical variational transition state theory (MS-CVT) including both multiple-structure and torsional (MS-T) anharmonicity effects. In these calculations, multidimensional tunneling (MT) probabilities were evaluated by the small-curvature tunneling (SCT) approximation and compared to results obtained with the zero-curvature tunneling (ZCT) approximation. The high-pressure-limit rate constants for both the forward and reverse reactions are reported as calculated by MS-CVT/MT, where MT can be ZCT or SCT. Comparison with the rate constants obtained by the single-structural harmonic oscillator (SS-HO) approximation shows the importance of anharmonicity in the rate constants of these reactions, and the effect of multi-structural anharmonicity is found to be very large. Whereas the tunneling effect increases the rate constants, the MS-T anharmonicity decreases them at all temperatures. The two effects counteract each other at temperatures 385 K and 264 K for forward and reverse reactions, respectively, and tunneling dominates at lower temperatures while MS-T anharmonicity has a larger effect at higher temperatures. The multi-structural torsional anharmonicity effect reduces the final reverse reaction rate constants by a much larger factor than it does to the forward ones as a result of the existence of more low-energy structures of the product 4-hydroxy-1-butyl radical than the reactant 1-butoxyl radical. As a consequence there is also a very large effect on the equilibrium constant. The neglect of multi-structural anharmonicity will lead to large errors in the estimation of reverse reaction rate constants.

Effect of Pressure on Activation–Deactivation Equilibrium Constants for ATRP of Methyl Methacrylate

Abstract: Atom transfer radical polymerization (ATRP) rate for methyl methacrylate (MMA) in acetonitrile solution has been measured up to 2500 bar. The increase in rate, by up to two orders of magnitude for the CuBr/Me6TREN system, is not compromised by an increase in dispersity. Activation–deactivation equilibrium constants were measured for MMA, KATRP, and for two monomer-free model systems, KEBiB, and KPMMABr, with Me6TREN, TPMA, PMDETA, and HMTETA being the ligands. KATRP and KEBiB agree within one order of magnitude for Me6TREN and TPMA, but differ by about two orders of magnitude for PMDETA and HMTETA, whereas KATRP and KPMMABr are close to each other for PMDETA and HMTETA. The observations indicate an important role of back-strain effects in ATRP of MMA.

Chemical equilibria in the binary and ternary uranyl(VI)-hydroxide-peroxide systems

Abstract: The composition and equilibrium constants of the complexes formed in the binary U(VI)–hydroxide and the ternary U(VI)–hydroxide–peroxide systems have been studied using potentiometric and spectrophotometric data at 25 °C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO2(OH)(O2)]− and [(UO2)2(OH)(O2)2]−. Under our experimental conditions the former is predominant over a broad p[H+] region from 9.5 to 11.5, while the second is found in significant amounts at p[H+] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO2)2(OH)(O2)2]− was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M−1 cm−1, respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO2)(OH)(O2)]− at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO22+(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO2)(OH)(O2)]− might be the building block in cluster compounds such as [UO2(OH)(O2)]6060− studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173–2177). Speciation calculations using the known equilibrium constants for the U(VI) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by α-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.