Studies On Resorcinol Selective Hydrogenation To Prepare 1,3-cyclohexanedione

The selective hydrogenation of resorcinol is an attractive and elegant routine for the production of 1,3-cyclohexanedione, owing to the advantages such as high product yield, good product quality and much less pollution to environment. Resorcinol catalytic hydrogenation in alkali solution is currently the main process to prepare 1,3-clyclohexanedione. But the process reported are not advanced enough and the product purification problem need to be solved. Up to now, however, the mechanism and reaction dynamics of this reaction are rarely reported. Theoretical study about this process lags behind its application. Optimization of the reaction process helps to improve the application of this method. Studies of the mechanism and kinetics are the basis of the process design and industrial optimization.The reaction process, mechanism and kinetics of resorcinol selective hydrogenation have been studied in this paper. A gas chromatography method was established to quantitatively measure resorcinol and 1,3-cyclohexanedione. The method has good linearity in the range of 0.00200 g·ml-1～0.0200 g·ml-1 for both resorcinol (r1=0.9993) and 1,3-cyclohexanedione (r2=0.9978), as well as high repeatability and accuracy (RSD< 2.5%, n=6). The quantification limits (S/N=10) and recovery rate are 1.00 mg·ml-1 and 102%(RSD=2%) for resorcinol, and 50 mg·ml-1 and 99.3%(RSD=2.6%) for 1,3-cyclohexanedione, respectively.The stability of resorcinol and 1,3-cyclohexanedione in organic solvents were studied by GC-MS, showing acetone, ethanol, ethyl acetate are not suitable and acetonitrile is the most favorable solvent for the reaction and post processing. Acetonitrile was used as recrystalization solvent for 1,3-cyclohexanedione and a higher purity was obtained.The base co-catalyzed reaction process was improved and optimized by the single factor method. The optimized reaction conditions are 1.1-1.2 of the mole ratio of sodium hydroxide to RES,15%(w/w) of the catalyst loading,0.830 mol·l-1 of the initiate RES concentration,353 K of the reaction temperature. Under this reaction condition, the conversion of resorcinol and 1,3-cyclohexanedione yield can reach nearly 100%.Based on the report that two hydrogenation pathways such as the simultaneous addition of the two hydrogen atoms while the van der Waals complex is formed between the aromaticπ-bond and the catalyst surface, and the sequential addition of single hydrogen atom while theπ/δcomplex is formed between a single double bond and the catalyst surface control the hydrogenation process simultaneously, a kinetic model was established and the model parameters as well as the activation parameters were estimated. Results showed the reaction is mainly controlled by the sequential pathway, and the addition of the first hydrogen atom is the rate determining step, with the energies of activation of 19.9 KJ·mol-1and 35.0 KJ·mol-1 for the addition of the two hydrogen atoms, respectively. The energy of activation for the addition of two hydrogen atoms simultaneously is 54.1 KJ·mol-1. Besides, the adsorption heats for the conversion of RES and the formation of CHD on the surface of the catalyst are 63.4 KJ·mol-1and 25.7 KJ·mol-1, respectively.Catalytic hydrogenation of resorcinol over Lewis acid-Pd/C was studied. ZnCl2, AICl3, CrCl3, SnCl4, TiCl4 and Pd/C under different reaction conditions were studied and the results show that non-polar solvents should be used as reaction solvent since the polar solvents may inhibit the hydrogenation reaction. Lewis acid facilitate resorcinol hydrogenation, but stable conjugated structure of resorcinol cannot be formed because of the dihydroxyl structure when react with Lewis acid. So Lewis acid cannot inhibit resorcinol from further hydrogenation. Lewis acid-Pd/C cannot catalyse resorcinol to selective hydrogenation to 1,3-cyclohexanedione as like as phenol to cyclohexanone.