8680
J. Phys. Chem. B 2010, 114, 8680–8688
Monte Carlo Simulations of Mixtures Involving Ketones and Aldehydes by a Direct Bubble Pressure Calculation Nicolas Ferrando,*,†,‡ Ve´ronique Lachet,† and Anne Boutin§ De´partement Thermodynamique et Mode´lisation Mole´culaire, IFP, 1-4, AVenue de Bois Pre´au, 92 852 Rueil-Malmaison Cedex, France, Laboratoire de Chimie Physique, CNRS, UniVersite´ Paris-Sud, baˆt. 349, 91405 Orsay Cedex, France, ENS, Chemistry Department, UPMC, CNRS, 75005 Paris, France ReceiVed: April 8, 2010; ReVised Manuscript ReceiVed: June 2, 2010
Ketone and aldehyde molecules are involved in a large variety of industrial applications. Because they are mainly present mixed with other compounds, the prediction of phase equilibrium of mixtures involving these classes of molecules is of first interest particularly to design and optimize separation processes. The main goal of this work is to propose a transferable force field for ketones and aldehydes that allows accurate molecular simulations of not only pure compounds but also complex mixtures. The proposed force field is based on the anisotropic united-atoms AUA4 potential developed for hydrocarbons, and it introduces only one new atom, the carbonyl oxygen. The Lennard-Jones parameters of this oxygen atom have been adjusted on saturated thermodynamic properties of both acetone and acetaldehyde. To simulate mixtures, Monte Carlo simulations are carried out in a specific pseudoensemble which allows a direct calculation of the bubble pressure. For polar mixtures involved in this study, we show that this approach is an interesting alternative to classical calculations in the isothermal-isobaric Gibbs ensemble. The pressure-composition diagrams of polar + polar and polar + nonpolar binary mixtures are well reproduced. Mutual solubilities as well as azeotrope location, if present, are accurately predicted without any empirical binary interaction parameters or readjustment. Such result highlights the transferability of the proposed force field, which is an essential feature toward the simulation of complex oxygenated mixtures of industrial interest. 1. Introduction Knowledge of thermophysical properties and phase equilibrium of systems involving ketones and aldehydes is of great interest in many industrial processes. Such molecules are a commonly used class of solvent in petrochemical, pharmaceutical, and food industries. Ketones and aldehydes are also involved in many biomass treatment processes and can, for example, represent up to 25 wt % of a typical bio-oil resulting from decomposition of a lignocellulosic material.1-3 Consequently, to design and optimize such industrial processes, accurate models are required to predict phase equilibrium of not only pure ketones and aldehydes but also mixtures involving these chemical species. For many years, transferable united-atom force fields have been developed to make molecular simulation an efficient tool to predict thermodynamic data and phase equilibria of a large number of pure compounds and mixtures. In such united-atom models, a central atom and its bonded hydrogen atoms are modeled through a unique Lennard-Jones sphere, leading to a significant reduction of the computation time comparing to allatoms approaches. An alternative is the anisotropic united atoms (AUA) model, in which the force center is shifted from the central atom toward the group center of mass to implicitly take into account the presence of the hydrogen atoms.4 Moreover, to ensure the predictive aspect of molecular simulation, the force field used in the simulations should be able to predict phase * Corresponding author. Phone: +33 147526624. Fax: +33 147527025. E-mail:
[email protected]. † IFP. ‡ Universite´ Paris-Sud. § Chemistry Department, UPMC.
equilibrium of mixtures without any empirical adjustment. Consequently, transferability is a fundamental feature of the force field used. The united-atom OPLS-UA force field (optimized potential for liquid simulations) was used by Jorgensen et al.5 to model acetone at ambient conditions. The OPLS-UA model, however, is known to become less accurate at higher temperatures6 and was not designed to simulate other ketones. The TraPPE-UA force field (transferable potential for phase equilibria) was extended to ketones and aldehydes by Stubbs et al.7 with the introduction of two new groups, the carbonyl carbon atom in ketones and the carbonyl CH group in aldehydes. In this model, the carbonyl oxygen atom was reused from a previous optimization on the carbon dioxide molecule.8 The transferability of this potential allowed the simulation of short and long linear-chain ketones and aldehydes. Saturated properties and critical points are well predicted, except the vapor pressure, which is systematically overestimated. Consequently, systematic deviations are also observed when simulating pressure-composition diagrams of binary mixtures. Finally, a first attempt to extend the AUA4 force field9 to ketones and aldehydes was carried out by Kranias et al.10 With this model, pure compound properties are accurately predicted. However, for each molecule simulated, a preliminary ab initio calculation is required to determine the electrostatic partial charges, which are distributed along the whole molecule. It was not attempted to propose a unique set of charges for the ketone/ aldehyde function, making this potential not directly transferable from one molecule to another. This model will be called “AUAKranias” in this paper. To overcome the lack of a fully transferable and accurate united-atom model able to predict mixtures involving ketones and aldehydes, we propose to
10.1021/jp1031724 2010 American Chemical Society Published on Web 06/14/2010
Monte Carlo Simulations of Mixtures
J. Phys. Chem. B, Vol. 114, No. 26, 2010 8681
develop a new transferable force field for these families of molecules as an extension of the AUA4 potential.9 The proposed force field is parametrized only from pure compound properties and is expected to be sufficiently transferable to be applied without any readjustment for mixture simulations. To minimize the number of parameters to be determined, only one new group is introduced, corresponding to the carbonyl oxygen atom. The π-bonded carbonyl force centers C and CH are transferred from the olefins parametrization of the AUA4 potential.11 All other pseudoatoms involved in the hydrocarbonated part of the molecules are directly transferred without any modification from the AUA4 original model.12 The prediction of binary mixtures involving polar compounds is the main goal of this work. In addition to an accurate force field for ketones and aldehydes, a robust algorithm is also required to calculate the phase diagram of such mixtures. From a practical point of view, two types of flash are usually performed to calculate the pressure-composition diagram of binary mixtures: either by fixing temperature and pressure, and calculating phase compositions (i.e., isothermal-isobaric flash) or by fixing temperature and liquid phase composition, and calculating pressure and vapor phase composition (i.e., bubble point calculation). The first type of flash corresponds to a simulation of the NPT-Gibbs ensemble, a usual way to simulate binary mixtures. The second type of flash is equivalent to the simulation of specific pseudoensembles as those proposed by Escobedo,13 Ungerer et al.,14 or Vrabec et al.15 We use in this work the formulation of the pseudoensemble proposed by Ungerer et al. This method has been successfully applied to predict equilibrium diagrams of n-alkane mixtures,16 and we propose to evaluate it in the case of mixtures with polar compounds for which electrostatic interactions play a preponderant role. This paper is organized as follows: the proposed force field is described in section 2. In the third section, pure component systems are studied, and performances are compared with currently available united-atom force fields for ketones and aldehydes. Section 4 focuses on the simulation of mixtures involving polar + polar or nonpolar + polar compounds: ketone + n-alkane, aldehyde + n-alkane, ketone + aldehyde, and ketone + alcohol mixtures. In this section, the bubble point pseudoensemble is introduced and the advantages of this method are highlighted. Finally, section 5 gives our conclusions. 2. Force Field Development 2.1. Intermolecular Energy. The intermolecular interactions between two force centers, i and j, of different molecules are described through 12-6 Lennard-Jones (LJ) and Coulombic potentials:
[( ) ( ) ]
Uijinter ) 4εij
σij rij
12
-
σij rij
6
+
qiqj 4πε0rij
TABLE 1: Nonbonded Parameters for the AUA4 Force Field atom
ε (K) σ (Å) δ (Å)
CH3 120.15 CH2 (sp3, linear) 86.29 2 CH (sp ) 90.60 C (sp2) 61.90 O (sp2) 96.51
3.607 3.461 3.320 3.020 2.981
0.216 0.384 0.414 0 0
q (e) 0 0 +0.46 +0.49 -0.46 -0.49
1 σij ) (σii + σjj) 2
ref for ε, σ and δ
9 9 in aldehydes 11 in ketones 11 in aldehydes this work in ketones
(3)
All the Lennard-Jones parameters involved in the alkyl part of the studied molecules are reused from the AUA4 potential developed for hydrocarbons12 and summarized in Table 1. In the AUA approach, a force center is shifted by a distance δ from the central atom toward the group center of mass to implicitly take into account the presence of its bonded hydrogen atoms. To reduce the number of new parameters, the LennardJones parameters and the AUA displacement δ of the carbonyl CH and C groups encountered in ketones and aldehydes are assumed to be the same as the CH and C π-bonded groups in olefins.11 Finally, the only new LJ parameters to be fitted are the parameters σO and εO of the carbonyl oxygen atom. For the electrostatic energy calculation, we adopt a charge distribution involving one negative charge located on the carbonyl oxygen atom and one positive charge on the carbonyl carbon atom. As illustrated in Figure 1, the dipole moment of linear ketones does not significantly vary with the chain length. For the ketones studied in this work, the maximum deviation between experimental dipole moments is
