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Preparation of methyl hydroquinone

    1. Tricresol method
      Using NaOH as catalyst, homocycreol (1) was oxidized to 4-hydroxy-2, 4, 6-trimethyl-2, 5-cyclohexadienone (2) in high-pressure oxygen. TMHQ (Scheme 1) was synthesized by methyl translocation at 250℃ with a total yield of 47%. l can be synthesized by the methylation of phenol or extracted from the by-products of the synthesis of 2, 6-dimethylphenol. Therefore, the homocresol method has great practical value.

    The process of this method is short, but the price of 1 is high, and it is difficult to achieve large-scale production only by the extraction of 2, 6-methyl phenol by-products. At present, the United States and Japan mostly adopt this production route.

    1. Phenol method
      Phenol (3) and methanol [n(phenol):n(methanol)= 1:5] were gasification and mixed, respectively, into a two-stage fixed-bed isothermal reactor. The gas-solid phase catalytic reaction was carried out at 450℃ to 500℃, and 1 was obtained by condensation and distillation. 1. The Bamherger translocation reaction was performed by passing steam into the solution of sodium carbonate and sodium hydroxide to obtain 2. Finally, TMHQ (Scheme 2) was obtained by alkali treatment with a total yield of 60%.

    This method is actually the recovery of the by-product 1 of the production process of 2, 6-dimethylphenol (the monomer for the synthesis of PPO heat-resistant resin). At present, the United States, Japan and other countries use this process to produce TMHQ.

    3, the oxidation method of isophorone
    Acetone (13) is first polymerized to iferketone (14). 14 by rearrangement oxidation, oxyiphorone was obtained (16); 16 was phthalated and rearranged to obtain trimethylhydrodiacetate (17). TMHQ was obtained by saponification and hydrolysis (Scheme 8).

    This method is a kind of green production technology with cheap raw materials, simple process, less environmental pollution and convenient for large-scale production. But the reaction route is long, the operation is tedious and the comprehensive cost is high. In addition, researchers from Germany and the Netherlands have applied for a large number of patents in China, which will also increase the production cost of vitamin E in China.

    The key technology of the process is the oxidation of 14 and the rearrangement and phthalide of 16. At present, the oxidation of moonscreen ferone is mainly based on organic coordination compounds of transition metals and metal-free catalytic system, oxidation with molecular oxygen or air, and adding appropriate additives (such as cosolvent, etc.). The main catalysts used are: transition metal salt catalyst, transition metal Schiff base catalyst, transition metal ethylphthalacetone complex catalyst, ionic liquid supported ethylphthalacetone metal complex catalyst, transition metal leaf or phthalide complex catalyst, all-metal catalyst and metal-free catalytic system catalyst, etc. Martin Jochen Klatt et al., using Schiff base and acetic acid or acetic acid hinged catalysis, this method is easy to produce 3, 5, 5-trimethyl-cyclohexan-2-dilute – 1-ketone and 2, 2, 6-trimethylcyclohexane-1, 4 diketone and other by-products, which greatly reduce the selectivity of the reaction and the yield of oxyisoferone. In addition, the properties of these by-products are similar to products, and it is difficult to separate them from products. Nohuhiko Ito et al. use leaves or phthalide complexes of iron, drill, copper and manganese as catalysts. The yield of this method is very high, but the catalyst is quite expensive and easily deactivated in the reaction, resulting in the high cost of this process.

    Compared with β-isophorone, the enol conjugate system in the structure of A-isophorone has high stability and low reactivity, so it is difficult to directly catalyze the synthesis of 16. So far, the catalytic oxidation of A-isophorone can be divided into two categories according to different catalytic systems: homogeneous catalytic system and heterogeneous catalytic system. Homogeneous catalytic systems include: phosphoplatinate acid or silicplatinate CuSO4, catalytic system, phosphoplatinate/diazin/potassium tert-butanol catalytic system, metal cold enteric light benzaldehyde complex, ethylphthalacetone vanadium, sodium vanadate, tetraphenyl phyllum manganese chloride and n-hydroxyl phthalphthalimide //CuCl2, etc. The heterogeneous catalytic systems include: supported metal ligand, nail supported MG-Al hydrotalcite, Cu/Co/Fe supported Mg-Al hydrotalcite and platinum-vanadium phosphate supported activated carbon, etc. In the presence of the catalyst, 16 and phthalide (such as phthalide, phthalide halide or enol cool) by phthalide reaction to produce TMHQA, and then saponification reaction to trimethylhydroacetic acid cool (TMHQ 1 VIA) or TMHQ. Tmhq-via can directly react with isoplantol to produce the main component of vitamin E, a teratamine E. The traditional catalysts for rearrangement and phthalation are Lewis and Brinson acids, such as HF, trifluoromethyl sulfonic acid, chlorosulfonic acid, polyphosphoric acid, fuming sulfuric acid, and mixtures of these acids. This kind of catalyst has high reactive activity, but it is too corrosive, easy to form acid flow, and generate a lot of salt after neutralization reaction, which is not conducive to product purification and purification. Because solid acid is not easy to corrode equipment, it is easy to separate and recover after reaction, and has been widely concerned. Occluded salt is the most studied solid acid catalyst, trivalent occluded salt is preferred, such as InC13 and perfluorinated sulfonic acid resin. This kind of catalyst has the same high activity as sulfuric acid, can make the raw material conversion of 100 %, but is not resistant to high temperature, weak stability, not easy to reuse. The current research focus is high fluoride ion exchange resin with excellent catalytic performance, such as Nafion NR 50(made by the polymerization of tetrafluoroethylene and CFZCF-S03 H). Its acid content is 0.95 mmol·g-1, its chemical surface area is only 0.02 m2·g-1, and its catalytic performance is stable. However, the reactivity was low in gas phase and non-swelling solvent. At present, a catalyst combining Nafion and soluble Si02: has been developed, which can withstand temperature up to 320℃ and reduce environmental pollution, which is convenient for large-scale industrial production and has a good application prospect.

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