Basic Study On Crystallization Separation Of Nitrobenzonitrile Isomers

P-Ntrobenzonitrile and m-nitrobenzonitrile are all important aromatic nitriles,which are widely used in organic synthesis,medicine,dyestuffs,pesticides and other industries.And there is a great market demand at home and abroad.Nowadays,P-Nitrobenzonitrile is synthesized using traditional methods of organic chemistry,for instance,the nitration of benzonitrile using HNO3 or another nitrating agent in the presence of a catalyst.However,isomer(m-nitrobenzonitrile)is also produced as a byproduct.The separation of isomers of nitrobenzonitrile has become a hot topic because of its wide application and good market prospects.Solvent crystallization has the advantages of low energy consumption and simple operation,which can effectively remedy the shortcomings of the existing steam distillation separation methods.Solvent crystallization is based on solubility datas.In this paper,the solid-liquid phase equilibria in different systems was studied because of low energy and high efficiency of solvent crystallization separation method,and the separation process of m-nitrobenzonitrile and p-nitrobenzonitrile was also discussed.The solubility data of m-nitrobenzonitrile and p-nitrobenzonitrile in eleven pure organic solvents including methanol,ethanol,n-propanol,isopropanol,n-butanol,isobutanol,acetone,acetonitrile,ethyl acetate,cyclohexane and toluene were determined by isothermal dissolution equilibrium method.The solubility of m-nitrobenzonitrile and p-nitrobenzonitrile in pure solvents increased with the temperature increasing.At the same temperature,the order of solubility of m-nitrobenzonitrile was acetone>(acetonitrile,ethyl acetate)>toluene>methanol>ethanol>n-propanol>n-butanol>isopropanol>isobutanol>cyclohexane;the order of solubility of n-nitrobenzonitrile was acetone>ethyl acetate>(acetonitrile,toluene)>methanol>n-propanol>n-butanol>isopropanol>isobutanol>cyclohexane.The solubility data was correlated by Apelblat model,?h model,Wilson model and NRTL model.The results show that the four models can well correlate the solubility data of nitrobenzonitrile.Apelblat model provided better results than the other three models.In addition,the solvent effect was discussed and the mixing properties of m-nitrobenzonitrile and p-nitrobenzonitrile in solution such as mixed Gibbs free energy,mixing enthalpy,mixing entropy,infinite dilution activity coefficient and infinite dilution excess enthalpy were calculated.The solubility data of m-nitrobenzonitrile and p-nitrobenzonitrile in mixed solvents(ethyl acetate+methanol),(ethyl acetate+ethanol),(ethyl acetate+n-propanol)and(ethyl acetate+isopropanol)were determined by isothermal dissolution equilibrium method.The solubility of m-nitrobenzonitrile and p-nitrobenzonitrile in four mixed solvents increased with the increasing temperature.At the same temperature,the solubility of m-nitrobenzonitrile and p-nitrobenzonitrile increased with increasing mass fraction of ethyl acetate.No solubilization phenomenon was found in the four binary systems.The experimental datas were correlated by Jouyban-Acree model,van't Hoff-Acree model and Apelblat-Acree model The calculated values were in good agreement with the experimental ones and the Jouyban-Acree model had better correlation results.At the same time,the preferential solvation parameters of m-nitrobenzonitrile and p-nitrobenzonitrile in the four mixed solvents were obtained.When the content of ethyl acetate is 0?0.44(0.50),the value of ?x1,3 is negative,which indicates that the local molar fractions of methanol,ethanol,n-propanol or isopropanol around the intermediate and p-nitrobenzonitrile molecules are higher,and the solutes are solvated by methanol,ethanol,n-propanol or isopropanol.When the content of ethyl acetate is 0.44(0.50)?1,the value of ?x1,3 is positive,indicating that m-nitrobenzonitrile and p-nitrobenzonitrile molecules are solvated by ethyl acetate.Based on the solubility data of pure solvents and mixed solvents,ethyl acetate and acetone were initially selected as solvents for separating m-nitrobenzonitrile and p-nitrobenzonitrile mixtures.The phase equilibria of m-nitrobenzonitrile+p-nitrobenzonitrile+ethyl acetate ternary systems at 298.15 K,308.15 K and 318.15 K and m-nitrobenzonitrile+p-nitrobenzonitrile+acetone ternary systems at 288.15 K,298.15 K and 308.15 K were determined by Schreinemaker's method,and the corresponding ternary phase diagrams were constructed.The NRTL model and Wilson model are used to correlate the experimental data,and the calculated values are well correlated with the experimental values.From the calculation results,the Wilson model has a good correlation between m-nitrobenzoquinone+p-nitrobenzonitrile+acetone.NRTL model has a good correlation between m-nitrobenzonitrile+p-nitrobenzonitrile+ethyl acetate.According to the phase diagram of ternary system,the flow chart of crystallization separation of m-nitrobenzonitrile and p-nitrobenzonitrile was designed,which provided a basis for industrial crystallization separation.