overview
para-aminophenol, chinese alias: 4-aminophenol, 4-amino-1-hydroxybenzene, foreign name: 4-aminophenol, referred to as pap, is an important chemical widely used in medicine, dyes, antioxidants, and photosensitive materials organic intermediates. para-aminophenol, also known as “para-hydroxyaniline”, is a fine organic chemical intermediate widely used in my country. it is used in the dye industry to synthesize weakly acidic yellow 6g, weakly acidic bright yellow 5g, sulfide deep blue 3r, and sulfide blue. cv, sulfide brilliant green gb, sulfide red brown b3r, sulfide reduction black clg, etc. in the pharmaceutical industry, para-aminophenol is used to synthesize paracetamol, clofibrate, etc. it is also used in the preparation of products such as developers, antioxidants and petroleum additives.
synthesis method[1]
p-aminophenol was first prepared by baeyer and caro in 1874 by reducing p-nitrophenol from tin powder. due to the wide range of uses of para-aminophenol, there are many reports on its synthesis at home and abroad. the synthesis method of para-aminophenol according to the raw material route is summarized as follows:
method 1: p-nitrophenol method
1. iron powder reduction method[2]
p-nitrophenol is reduced by iron filings in an acidic medium to produce crude p-aminophenol, which is then impregnated with sodium metabisulfite solution, filtered and dried to obtain the finished product. the specific reaction formula is:
see table 1 for raw material consumption.
the yield of pap produced by this method is relatively high, 91.8%. however, the process route is long and the production cost is high; at the same time, each ton of product produced requires the discharge of more than 2 tons of iron sludge and a large amount of waste water, resulting in serious environmental pollution. therefore, this method has been eliminated in most countries, but most companies in our country are still using this process. in 1992, the ministry of chemical industry decided to stop expanding production by this method.
2. catalytic hydrogenation method
this method generally uses pt/c and pd/c as catalysts to hydrogenate and reduce p-nitrophenol at about 0.2~0.5m pa and 70~90°c to prepare crude pap. due to the expensive catalyst, difficulty in recycling, and high production costs, there are no reports of industrial production in china.
3.electrolytic reduction method
this method is carried out in 10%～30% h2so4 aqueous solution, electrolytic density 3.14～8.38a/dm2, 40～70℃, t io2/t the i electrode and ti cathode are rotated under the condition of rotation. the yield is about 70%. there are currently no reports of industrialization of this method.
method 2: phenol method
1. phenol nitrosation method
phenol reacts with sodium nitrite and sulfuric acid at 0-5°c to generate p-nitrosophenol, which can then be reduced and acidified to obtain pap. this method has harsh operating conditions and serious environmental pollution, making it difficult to achieve industrial production.
2. phenol coupling method
aniline reacts with sodium nitrite and hydrochloric acid at low temperature (0-5℃) to produce diazonium salt, which is coupled with phenol to form an azo compound. azo compounds are then reduced to generate pap and aniline. the main methods for reducing azo compounds include chemical reduction, electrolytic reduction, and catalytic hydrogenation reduction.
method 3: hydroquinone ammoniation method
use aliphatic ether as a solvent and react hydroquinone with ammonia in the presence of inert gas to prepare pap. this method not only has strict process requirements, but also requiresthe parts are demanding and the production cost is also high, which limits industrial production.
method 4: hydrolysis of p-phenylenediamine
the hydrohalide of p-phenylenediamine can be hydrolyzed by heating at 150~350℃ to obtain pap and hydroquinone.
method 5: p-nitrochlorobenzene method
this method uses p-nitrochlorobenzene as raw material, hydrolyzes it under alkaline conditions to obtain sodium p-aminophenolate, and then acidifies and reduces it to obtain pap. this method is the main method for domestic production of pap. however, the pollution is serious, the production process is long, the overall yield is low, and the product quality is unstable.
method 6: nitrobenzene method
using nitrobenzene as raw material to prepare pap, the raw materials are easy to obtain, there are many process routes, and the potential to reduce costs is great, and it has been a hot research topic in recent years. preparation methods can be divided into three types: metal reduction method, electrolytic reduction method and catalytic hydrogenation reduction method. the main reaction mechanism is that nitrobenzene is hydrogenated to form phenylhydroxylamine, and then bamberger rearrangement is performed to produce pap.
1. metal reduction method
this method uses metal powder such as aluminum powder or magnesium powder to reduce nitrobenzene to pap in one step in dilute sulfuric acid. the metal reduction method was developed earlier and has been reported in domestic and foreign patents and literature. the yield is between 60% and 70%. this method has a simple process, but consumes a lot of metal and has post-processing problems such as recycling, so it is difficult to produce on a large scale.
2. electrolytic reduction method
this method is to use 20% to 30% sulfuric acid as the medium at a temperature of 80 to 90°c, add a small amount of surfactant through electrolysis, and reduce the nitrobenzene placed on the cathode to generate pap. the influencing factors mainly include electrode materials, electrolyte composition, voltage and current density control, etc. using a diaphragm electrolyzer and passing in nitrogen protection can prevent oxidation of pap and reduce the production of azobenzene oxide. it is reported that tio2/t i is the best electrode. this method has simple operation, short process, high product purity, little pollution and low cost. at present, this method is mostly used in large-scale industrial production abroad. domestic shanghai east china institute of chemical technology, tianjin institute of chemical technology, and peking university have all conducted research on this process. this method has high technical requirements for reactor design and process condition control, and consumes high energy.
3. catalytic hydrogenation reduction method
this method uses a suitable catalyst and acidic medium to reduce nitrobenzene to generate the intermediate product hydroxyaniline, and then rearrange it into pap. this method is a new technology in foreign countries in the 1970s. the british harting chemicals company and the american mallin chrodt company successively adopted this method and put it into production. japan’s mitsui toya fine chemicals co., ltd., domestic beijing medical university, tianjin university, changchun institute of applied chemistry, chinese academy of sciences, etc. have all conducted research and development on this process, but there have been no successful industrialization reports in china.
most of the production processes for p-aminophenol use dilute sulfuric acid (concentration 15% to 40%) to maintain the ph value of the reaction. pt and pd are used as catalysts, activated carbon is used as the carrier, and appropriate surfactants are added. react at ~110℃, 0~1mpa. after the reaction is completed, the unreacted nitrobenzene can suspend the catalyst, and the aqueous phase can be separated and processed to obtain pap. the main factors affecting this reaction are catalyst, temperature, acid solution composition and pressure.
in recent years, a lot of research has been done at home and abroad on catalyst selection and composition, and on improving reaction yield. in terms of catalyst composition, norman et al. found that pt～ru/c as a catalyst can prevent excessive hydrogenation of nitrobenzene and improve reaction selectivity[13]. the pt～pd alloy catalyst with npt∶npd=20 has high activity and selectivity. when pt:r u= 5:1 in the pt～ru/c catalyst, the pap yield can reach 80%. in addition, there are literature reports using pts/c and mos2/c as catalysts. catalyst.
the carrier of the catalyst is also a major factor affecting the performance of the catalyst. activated carbon and alumina are generally chosen, the latter is beneficial to the recovery of precious metals. experiments have shown that polymer supported catalysts produced using a certain process will not be deactivated after 500 hours of reaction and are extremely competitive.
henke used the method of adding nitrobenzene multiple times to generate a molar ratio of p-aminophenol and aniline of 3:1. l ain tze et al. added organic acids (such as formic acid) to the reaction system, and the yield and selectivity were improved. adding surfactants such as quaternary ammonium salts will help the aqueous phase and the organic phase fully contact and speed up the reaction. in addition, adding dimethyl sulfoxide, thioether, etc. can also inhibit the formation of by-product aniline. in short, catalytic hydrogenation of nitrobenzene to synthesize pap has the advantages of less pollution and low energy consumption, so it has the most industrial value.
production application
1. in the pharmaceutical industry, pap is mainly used to synthesize n-acetyl para-aminophenol, which is an antipyretic and analgesic for the treatment of colds. it can also be used to synthesize adipine, paracetamol, butalone, vitamin b, complex nicotinamide, etc.;
2. in the rubber industry, p-phenylenediamine antioxidants such as 40/ona, 4020, and 4030 can be synthesized;
3. in the dye industry, it can synthesize the hair dye 4-amino-2-nitrophenol, as well as sulfur dyes and benzoic acid. it is the synthetic azo and sulfur dye intermediate 5-aminosalicylic acid. raw materials;
4.pap can also be used in the production of photographic developer metol (meto l), and can also be directly used as antioxidants and petroleum product additives.
references
[1] gao hong, yuan hua. review on the synthesis and application of para-aminophenol[j]. chemical and biological engineering, 2000, 17(2):1-2.
[2] zhou shibiao, xiong huagao, zhang weiqing, et al. discussion on the synthesis process of p-aminophenol[j]. guangdong chemical industry, 2009, 36(10):50-51.
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