The Quantum Chemical Calculation Of Ca?EDTA?^2- Complex And Organic Acid Calcium Compound

At present,high content of calcium element in crude oil is one of characteristics of crude oil in China.It is proved that the calcium element in crude oil can bring the severe damage to the process and equipment of crude oil processing.Therefore,many scholars have completed a lot of research works on the decalcification technology of crude oil in many aspects.It has been proved that injection of complex agent into the crude oil in the electrostatic desalting unit is one of the effective method for the removal of calcium from crude oil.Thus,the selection of the proper complex agent is especially important in the development of decalcifying technology for crude oil.In order to make the development of decalcifying technology for crude oil more effectively,and to explain the decalcification mechanism at the molecular level,the quantum chemical calculation of calcium complex,i.e.Ca(EDTA)2-,and organic acid calcium in crude oil is carried out by the quantum chemistry package,i.e.Gaussian 09 W,in this paper.Based on the geometric configuration of Ca(EDTA)2-in Cambridge Crystallographic Data Centre(CCDC),the B3 LYP method of Density Functional Theory(DFT)is applied,and 6-31 G and LanL2 DZ is chosen as basis set,respectively,the calculation results indicate that the structure of Ca(EDTA)2-is malformed octahedral configuration.And the structures of calcium complexes in which carboxyl is protonized i.e.,Ca(EDTA)-ax and Ca(EDTA)-ra with the axial and radial protonized carboxyl group,respectively,are optimized.At first,the structure of Ca(EDTA)2-with the axial(radial)protonized carboxyl group is simulated,it is found that its length of axial(radial)coordination bond becomes longer and its radial(axial)length of coordination bond is shorter.Then,one and two water molecules are used as the ligand to replace the carboxyl group in Ca(EDTA)2-and Ca(EDTA)-ax,and four kinds of complex isomer,i.e.,Ca(EDTA)2-·H2O,Ca(EDTA)2-·2H2O,Ca(EDTA)·H2O and Ca(EDTA)·2H2O are obtained.The optimized structure for these complex isomers has the geometric configuration with seven ligands,or even eight ligands.From the calculation results for the binding energy and Gibbs free energy change of complex,it is known that the variety of complex isomers could coexist in aqueous solution,and they keep at a dynamic balance.The frontier orbits,i.e.,HOMO and LUMO,of the calcium complex are analyzed,the analysis results present that the energy gap of HOMO and LUMO orbit would enlarge and make complexes more stable due to the effect of water,and the energy gap of Ca(EDTA)2-and its isomers are both more than 500kJ/mol,it means that the electron transition in calcium complexes is difficult to happen.At the B3LYP/6-31G+(d)level,the binding energy and Gibbs free energy change of calcium complex are calculated.The calculation value for the solvation energy of Ca2+ is 1605.19kJ/mol,its relative error with the literature result is only 0.52%,it is verified that the chosen calculation method is suitable.The calculated binding energy of Ca(EDTA)2-is-196.94kJ/mol,the largest absolute value,it demonstrates that the structure of Ca(EDTA)2-is the most stable one among concerned calcium complexes.The calculated Gibbs free energy change is-123.38kJ/mol,it is far larger than the experimental result,but the error between the calculated value in this paper and experimental value is less than that between the calculated value in literature and experimental value,namely closer to the experimental value than the literature.Moreover,it is found that the Gibbs free energy change of these calcium complex isomers is smaller,the experimental value may be a average value of the Gibbs free energy change of these calcium complex isomers.Based on the insight into the literature,it is known that the organic calciumes in crude oil are mainly naphthenic acid calcium and a small amount of phenyl calcium.Therefore,a number of molecular model of organic acid calcium is constructed in this paper.The characteristics of the optimized molecular structural for the organic acid calcium is that the four oxygen atoms in carboxyl group and calcium atom are basically on the same plane,meanwhile two water molecules used as the ligands are perpendicular to above plane.The structure of fatty acid calcium with different length of carbon chain is analyzed.The structure of naphthenate calcium with different types of ring,such as cyclohexane,benzene,and bicyclic,is also determined.On the basis of cyclohexane acetate calcium,the geometric configuration of organic acid calcium molecule in which various effects like the different length of carbon chain for substituent on the ring,the relative position of substituent with carboxyl,different number of carbon atom between carboxyl group and mother ring etc.,has been determined,too.The calculation results display that the bond length of Ca-O would slightly prolong with the enhancing electron donating induced effect of the group connected with the carboxyl group.Overall,length of Ca-O bond is about 0.235 nm.From the calculation for the energy for different organic acid calcium,it is found that the binding energy of Ca-O is in the range of-67.01~-77.72kJ/mol and the Gibbs free energy change is in the range of-41.55~-48.19kJ/mol.These absolute values are smaller than the corresponding energy value of Ca(EDTA)2-.Therefore,it displays that EDTA has the significant performance for the removal of calcium element in crude oil,and it is also verified by the experimental results.Therefore,by mean of the quantum chemical calculation method in this paper,some guidance for the selection of complexing agent used to remove calcium and other metallic element from crude oil could be proposed,and the development efficiency for the demetallization technology could be enhanced and the corresponding development period could be shortened.