Study On Preparation Of2,3,5-trimethylhydroquinone By Catalytic Hydrogenation Of2,3,5-trimethylbenzoquinone

2,3,5-Trimethylhydroquinone (TMHQ) is one of the key intermediates ofsynthetic Vitamin E (VE). VE is more and more widely used. And TMHQ has been ingreat demand for the production of synthetic Vitamin E, especially in the domesticTMHQ market. Therefore, it is imperative to establish new techniques to improve thequality of products, reduce costs and environmental pollution. In this paper, TMHQwas prepared by the catalytic hydrogenation of2,3,5-trimethylbenzoquinone (TMBQ)on Pd/C or Raney-Ni catalyst.Orthogonal experiments were employed to acquire the optimal hydrogenationconditions. The important reaction parameters (e.g. catalyst loading, initialconcentration of TMBQ, hydrogen pressure, temperature) have been investigated. Anexternal standard method of reversed phase HPLC (RP-HPLC) was adopted for thedetermination of TMHQ and TMBQ in the reaction mixture. At800rpm, theoptimized reaction conditions of the process were as follows:(1) using Pd/C ascatalyst: catalyst loading,0.8%(w/w); initial concentration of TMBQ,0.1g/mL;temperature,90°C; hydrogen pressure,0.5~0.6MPa. The TMBQ conversion wasnearly100%, while the isolated TMHQ overall molar yield reached high up to96.7%in5L autoclave.(2) Using Raney-Ni as catalyst: catalyst loading,10%(w/w); initialconcentration of TMBQ,0.1g/mL; temperature,100°C; hydrogen pressure,0.7~0.8MPa. The TMBQ conversion was over99%and the overall molar yield ofTMHQ was above93%.In the process of catalytic hydrogenation on Pd/C, the effects of some importantreaction parameters were also investigated, including temperature, catalyst loading,initial concentration of TMBQ, hydrogen pressure and agitation speed. Fine isolatedproduct TMHQ was obtained with high yield by optimized post-processing. Reuseexperiments were adopted to select the suitable catalyst. The filtrate was firstlydistilled to remove part of solvent. Then the residual solvent was recovered by steamdistillation. Recycling rate of the LBA solvent was above96%. The reuse of Pd/Cindicated that the catalyst activity decreased, but TMHQ selectivity was hardlychanged. Based on various characterizations (e.g. atomic absorption spectroscopy,N2adsorption, XRD, TG/DTA) and regeneration study of catalyst, the deactivation ofPd/C during the reuse of catalysts was ascribed to the loss of active component Pd andthe depositions of TMHQ, TMBQ. The latter would be the main reason. After washing by acetone, and calcination at500°C, the catalyst could be reused withoutany obvious activity loss in hydrogenation of TMBQ to the desired product TMHQ.In the Raney-Ni catalytic hydrogenation process, the effects of solvent, catalysttype and mesh were also investigated. The results indicated that smaller catalystparticle would decrease the TMHQ selectivity. Besides, we found that the trace ofimpurity in TMBQ threatened the hydrogenation process on Raney-Ni catalyst. Toavoid this undesirable feature, two different solutions were proposed: crystallizationand rectification.The possible catalytic hydrogenation mechanism was also discussed. And thereaction process would be: first hydrogenation–isomerization-secondhydrogenation.An efficient and green process of catalytic hydrogenation of TMBQ to TMHQ onPd/C or Raney-Ni catalyst was described. In the reuse of Pd/C catalyst, the catalystactivity decreased, but TMHQ selectivity was almost the same. After simple treatment,the catalyst could be reused without any obvious activity loss. The found optimalreaction conditions made it a desirable industrial process with high yield, fine isolatedproduct and high solvent recovery rate. Although the reuse of Raney-Ni wasunsatisfied, it had a certain practicality with cheap catalyst and high TMHQ yield.