Application of computational chemistry methods to obtain thermodynamic data for hydrogen production from liquefied petroleum gas

Abstract

-The objective of this study was to estimate thermodynamic data, such as standard enthalpy, entropy and Gibbs free energy changes of reaction and, consequently, chemical equilibrium constants, for a reaction system describing the hydrogen production from Liquefied Petroleum Gas (LPG). The acquisition of those properties was made using computational chemistry methods and the results were compared with experimental data reported in the literature. The reaction system of steam reforming of LPG was reported as a set of seven independent reactions involving the chemical species n-C 4 H 10 , C 3 H 8 , C 2 H 6 , C 2 H 4 , CH 4 , CO 2 , CO, H 2 O, H 2 and solid carbon. Six computational approaches were used: Density Functional Theory (DFT) employing Becke's three parameter hybrid exchange functional, and the Lee-Yang-Parr correlation functional (B3LYP) using the 6-31G++(d,p) basis set and the composite methods CBS-QB3, Gaussian-1 (G1), Gaussian-2 (G2), Gaussian-3 (G3) and Gaussian-4 (G4). Mole fractions of the system components were also determined between 873.15 and 1173.15 K, at 1 atm and a feed with a stoichiometric amount of water. Results showed that the hybrid functional B3LYP/6-31G++(d,p), G3 and G4 theories were the most appropriated methods to predict the properties of interest. Gaussian-3 and Gaussian-4 theories are expected to be good thermodynamic data predictors and the known efficient prediction of vibrational frequencies by B3LYP is probably the source of the good agreement found in this study. This last methodology is of special interest since it presents low computational cost, which is important when more complex molecular systems are considered.

Theoretical Study of the Oxidation Mechanisms of Naphthalene Initiated by Hydroxyl Radicals: The OH-Addition Pathway

“…In addition, this approach includes empirical corrections for spin contamination. The CBS-QB3 method involves five steps, starting with a geometry optimization at the B3LYP level of theory, , followed by a calculation of vibrational frequencies and canonical partition functions to obtain thermal corrections to enthalpies, zero-point vibrational energies, and entropies . Specifically, the CBS-QB3 method involves the following steps: B3LYP/6-311G­(2d,d,p) geometry optimization B3LYP/6-311G­(2d,d,p) frequency calculation with a 0.99 scale factor for the ZPE CCSD­(T)/6-31+G* energy calculation MP4­(SDQ)/6-31+G­(d­(f),p) energy calculation UMP2/6-311+G­(3d2f,2df,2p) energy calculation and CBS extrapolation …”

“…The CBS-QB3 method involves five steps, starting with a geometry optimization at the B3LYP level of theory, 53,54 followed by a calculation of vibrational frequencies and canonical partition functions to obtain thermal corrections to enthalpies, zero-point vibrational energies, and entropies. 55 Specifically, the CBS-QB3 method involves the following steps: 31 − B3LYP/6-311G(2d,d,p) geometry optimization − B3LYP/6-311G(2d,d,p) frequency calculation with a 0.99 scale factor for the ZPE − CCSD(T)/6-31+G* energy calculation − MP4(SDQ)/6-31+G(d(f),p) energy calculation − UMP2/6-311+G(3d2f,2df,2p) energy calculation and CBS extrapolation The CBS-QB3 approach is known to yield mean absolute deviation of only 1.1 kcal mol −1 on the G2/97 test set 39 for reaction energies, which can hardly be surpassed by any other method these days. A more recent work indicates a maximum error of 2.8 kcal mol −1 for the G2 test set of reaction energies, and average and mean absolute errors of 0.2 and 0.98 kcal mol −1 , 56 which at first glance seems amply sufficient for our purposes.…”

Study on the Application of the Combination of TMD Simulation and Umbrella Sampling in PMF Calculation for Molecular Conformational Transitions

“…Geometry optimizations for a butane molecule were computed at the B3LYP/6-31++G(d,p) [ 60 , 61 ] levels of theory using the Gaussian 09 program [ 62 ]. The structural parameters of butane are shown in Table S1 of Supplementary Materials with the experimental data [ 63 , 64 , 65 ].…”

Oxidation reaction mechanism and kinetics between OH radicals and alkyl-substituted aliphatic thiols: H-abstraction pathways

“…It also includes an energy extrapolation up to the coupled cluster theory including single, double, and perturbative triple excitations (CCSD(T)) level 58260 in complete basis set limits in order to correct second-order Møller-Plesset correlation energies. 59 The five-step CBS-QB3 series of calculations starts with a geometry optimization at the B3LYP theoretical level, 33,38,61 followed by calculation of vibrational frequencies and thermodynamic state functions 62 obtained from canonical partition functions that were computed for an ideal gas using Boltzmann statistical thermodynamics. 63266 A weakness of the B3LYP approach is that it neglects dispersion forces which may have some influence on torsional characteristics.…”

“…The five-step CBS-QB3 series of calculations starts with a geometry optimization at the B3LYP theoretical level, 33,38,61 followed by calculation of vibrational frequencies and thermodynamic state functions 62 obtained from canonical partition functions that were computed for an ideal gas using Boltzmann statistical thermodynamics. 63−66 A weakness of the B3LYP approach is that it neglects dispersion forces which may have some influence on torsional characteristics.…”