Yanqiang Shi,Yuetong Xia,Guangtong Xu,Langyou Wen,Guohua Gao,Baoning Zong

State Key Laboratory of Catalytic Materials and Reaction Engineering,Research Institute of Petroleum Processing,Sinopec,Beijing 100083,China

Keywords:Hydrogen peroxide Green chemical process Partial oxidation Nitridation

ABSTRACT Basic organic chemicals and high value–added products are mainly produced by hydrocarbon nitridation and oxidation.However,several drawbacks limit the application of the traditional oxidation and nitridation technologies in the future,such as complex processes,poor intrinsic safety,low atom utilization,and serious environmental pollution.The green nitridation and oxidation technologies are urgently needed.Hydrogen peroxide,a well–known green oxidant,is widely used in green hydrocarbon oxidation and nitridation.But its industrial production in China adopts fixed–bed technology,which is fall behind slurry–bed technology adopted by advanced foreign chemical companies,limiting the development of hydrogen peroxide industry and green hydrocarbon nitridation or oxidation industry.This article reviews the industrial production technologies of hydrogen peroxide and basic organic chemicals such as caprolactam,aniline,propene oxide,epichlorohydrin,phenol,and benzenediol,especially introduces the green production technologies of basic organic chemicals related with H2O2.The article also emphasis on the efforts of Chinese researchers in developing its own slurry–bed technology of hydrogen peroxide production,and corresponding green hydrocarbon nitridation or oxidation technologies with hydrogen peroxide.Compared with traditional nitridation or oxidation technologies,green production technologies of caprolactam,propene oxide,epichlorohydrin,and benzenediol with hydrogen peroxide promote the nitrogen atom utilization from 60% to near 100% and the carbon atom utilization from 80% to near 100%.The waste emissions and environmental investments are reduced dramatically.Technological blockade against the green chemical industry of China are partially broken down,and technological upgrade in the chemical industry of China is guaranteed.

1.Introduction

Chemicals are essential to humankinds.Their production usually involves a variety of reactions.Introducing oxygen–containing or nitrogen–containing functional groups through oxidation or nitridation reactions from petroleum–based hydrocarbons by C-H or C-C bond activation,is crucial to the production of basic organic chemicals,organic intermediates,and fine chemicals.Therefore,the oxidation or nitridation reactions occupy an extremely important position in modern chemical industry.More than 50% of the chemicals’ production involves oxidation reactions,and the production of key monomers such as synthetic fiber,resin,rubber,medicine,pesticide,and fine chemicals all involve oxidation or nitridation reactions[1,2].These synthesized materials now are widely used in civil fields such as electronics,transportation,machinery manufacturing,medical treatment,agriculture,as well as in national defense and military industry fields such as aerospace,thus producing a huge social and economic value [3,4].However,highly toxic reagents (such as dichromate and permanganate),hypochlorite,and nitric acid are the most common oxidants in the current chemical industry.And heavy metal–containing inorganic salt wastes,salt–containing wastewater,or nitrogen oxides are produced at the same time,which results in poor atom economy and serious environmental pollution.The nitric acid,hydroxylamine,azide,and highly toxic cyanide are used as the nitrogen sources in traditional nitridation reactions of hydrocarbon compounds [5].The NH3oxidation process used in the nitric acid production alone emits more than 3 × 105t of nitrogen oxides each year,which accounts for 30% of the total nitrogen oxide emissions in the chemical industry [6].Lots of energy are consumed in nitric acid and hydroxylamine production [7].The nitrogen atom utilization is less than 60% when the above active nitrogen–containing compounds are used as nitrogen sources,and the corresponding salt wastes are produced [8].Association of Plastics Manufacturers in Europe (APME) has calculated the energy consumption,carbon emission,and NOxemission in the full lifecycle of multiple polymeric materials.The converted carbon emission data per unit product is shown in Fig.1 [9].Fig.1 shows that the carbon emission of nylon–6 and nylon–66 involving multi–step oxidation and nitridation reactions are dramatically higher than other polymeric materials.On the other hand,the N2O emission from a single adipic acid production step accounts for 10% of the global N2O emission increment [10].

Green chemical industry is aimed at reducing or eliminating the use and generation of hazardous substances.Therefore,the selection of oxidant or active N–containing agent is the core of green oxidation and nitridation reactions.As a green oxidant,hydrogen peroxide (H2O2) is widely used in chemical industry,especially in hydrocarbon nitridation and oxidation reactions [11–13].For example,the research of cyclohexene selective oxidation with H2O2to produce adipic acid could avoid the production of large amounts of N2O in traditional technologies,which would significantly reduce nitrogen oxide emissions in the chemical industry[14].The demand for H2O2has increased by years with the development of green chemical industry recently.In 2018,China consumed more than 3.2 × 106t of H2O2(100% H2O2,the same below),which accounted for more than 50% of global H2O2consumption.And it is still growing at an annual rate of higher than 5% [15,16].However,what does not match the capacity and demand of H2O2is that the fixed–bed technology that has been eliminated by foreign countries is still used in China for H2O2production.Fixed–bed technology cannot effectively support the effective operation of green chemical plant due to its low single–unit production capacity and high production cost.Yet,the transfer of the advanced slurry–bed technology from foreign countries is prohibited,which seriously restricts the development of green chemical industry in China.

In order to meet the great need of China,Chinese researchers have successfully developed the slurry–bed technology of H2O2production with completely independent intellectual property rights after more than twenty years’efforts.In the meantime,a series of green oxidation or nitridation technologies with H2O2as oxidant were developed,which achieves good economy and social benefits.This article introduces the research work and progress of China in green chemistry and green chemical industry,including the slurry–bed technology of H2O2production and green nitridation and green oxidation with H2O2.

Fig.1.Carbon emission data in the production of different polymeric materials.

2.Slurry–bed Technology of H2O2 Production

The synthesis methods of H2O2include the anthraquinone method,isopropanol method,electrolysis method,oxygen cathode reduction method,oxygen and water synthesis method,and direct synthesis method with hydrogen and oxygen [17,18].With the comprehensive advantages in industrial efficiency,environmental protection,and economy,the anthraquinone method is widely used in industrial production of H2O2.The production process of the anthraquinone method includes anthraquinone hydrogenation,hydrogenated anthraquinone oxidation,H2O2extraction,and anthraquinone working solution purification.The anthraquinone hydrogenation is the key step that affects the production efficiency of H2O2.

The fixed–bed technology is adopted by China to produce H2O2.With palladium black or inorganic carrier–supported palladium as catalyst,the hydrogenation efficiency is only 7.0–7.5 g·L-1.Disadvantages such as reactant accumulation or bypassing,excessive hydrogenation,and severe loss of working solution exist for the fixed–bed technology.In addition,low oxidation yield,high H2O2extraction residue,poor intrinsic safety,and serious pollution are general issues faced in the H2O2factories of China[19,20].Though the catalyst and process are optimized,the unit capacity of the fixed–bed technology has never exceeded 50 kt·a-1,which severely restricts the industrial application of new green chemical technologies related with H2O2in China.For example,a production unit of epoxypropane with a capacity of 300 kt·a-1needs two fixed–bed production units of H2O2,which significantly increases the construction and operation costs of the epoxypropane unit.

It has been long since the slurry–bed technology was applied for H2O2production in foreign countries.Compared with the fixed–bed technology,the slurry–bed technology has obvious advantages in heat and mass transfer,hydrogenation selectivity,hydrogenation efficiency,and large–scale production.The hydrogenation efficiency of the slurry–bed technology can reach 11–18 g·L-1,and the consumptions of working solution and catalysts are significantly reduced[21].Moreover,the capacity of the slurry–bed technology is usually above 100 kt·a-1[22].In order to effectively support the development of the green chemical industry in China,Chinese researchers endeavored to carry out scientific researches on key issues like slurry–bed hydrogenation catalyst,green synthesis of working solution,circulation oxidation,extraction,and catalytic regeneration of working solution.Finally,the slurry–bed technology of H2O2production with independent intellectual property rights was successfully developed.

2.1.Microsphere hydrogenation catalyst

The hydrogenation catalyst with higher strength and selectivity is required by the slurry–bed reactor.Although the Pd/Al2O3catalyst has high strength,its poor selectivity makes the product contain many degradation products.The Pd/SiO2catalyst has good selectivity,but the palladium loss is serious.To solve these problems,researchers did a lot of work on the modification of catalyst carrier,palladium loading,and catalyst molding.Shi et al.[23]found the type and thickness of oxide layer significantly affect the performance of Pd–based catalyst in 2–ethylanthraquinone hydrogenation,and γ–Al2O3had a higher catalytic performance compared with SiO2and SiO2-Al2O3.After analysis,they thought the larger pore size of γ–Al2O3plays a key role.The larger pore size not only improved the accessibility of active Pd sites,but also reduced the residence time of products in catalyst channels,which reduced the chance of side reactions.Furthermore,the thickness of oxide layer should be in a reasonable range.Thinner layer could not achieve effective dispersion of Pd,while thicker layer made the Pd penetrate too deep to be fully exploited.The thickness of Al2O3with 6 μm exhibited the highest conversion of 2–ethyl–ant hraquinone(99.1%),which suggested the microsphere hydrogenation catalyst should have an appropriated surface.Wang et al.[24]used propyl–triethoxysilane to modify the support of mesoporous silica SBA–15 with different pre–hydrolysis time (from 0.5 h to 4 h).Modified catalysts with 1 h or 2 h pre–hydrolysis time (Pd/P–1 and Pd/P–2) had a higher activity than unmodified catalyst(Pd/SN),while modified catalysts with 0.5 h or 4 h pre–hydrolysis time (Pd/P–0.5 and Pd/P–4) had a lower activity.The hydrogenation activity of Pd/P–2 on ethyl–,tert–butyl–and amyl–anthraquinone were increased by 33.3%,60.0% and 150.0%,compared with Pd/SN.After analysis,the authors found Pd/P–2 possesses the highest hydrophobicity and relatively large pore width,which benefit the adsorption and diffusion of anthraquinone molecules to active sites.Though Pd/P–0.5 and Pd/P–4 also had high hydrophobicity than Pd/SN,their small pore size limit the diffusion of anthraquinone molecules.Zheng et al.[25,26] disclosed the modification catalyst carrier to promote its anti–wear property,and the synergistic effect of metal promoters with palladium to promote its activity and selectivity.They found the anti–wear property of carrier could be improved by adding silica or zirconia into alumina,and the anti–wear property further be improved after boron or lithium modification.The abrasion index was reduced from 3.6% to 0.7%–0.9%.As promoters,Ni or La not only improved the hydrogenation efficiency of Pd–based catalyst from 9 g·L-1to 13–14 g·L-1,but also improved the selectivity from 91% to 98%.Furthermore,the hydrogenation efficiency loss of catalyst after hydrothermal treatment was reduced from 1.1–1.6 g·L-1to 0.3–0.6 g·L-1due to the metal promoters.Finally,Chinese researchers found microsphere hydrogenation catalyst with appropriate surface area (80–300 m2·g-1),larger pore volume (0.2–2 ml·g-1) and better anti–wear property was suitable for anthraquinone hydrogenation reaction in slurry–bed reactor.In 2019,Sinopec carried out industrial test with slurry–bed technology of H2O2production,the result showed the microsphere hydrogenation catalyst with independent intelligent property right is comparable to foreign catalysts.The hydrogenation efficiency of the catalyst is 12–13 g·L-1,and no obvious change of the catalyst is observed after industrial test.

2.2.Synthesis and regeneration of working solution

As an important component of the working solution,2–alkylanthraquinone is produced by phthalic anhydride method,which is shown in Fig.2.This method consumes large amount of catalyst and concentrated sulfuric acid,and a large amount of waste acid is produced,which causes serious corrosion to the equipment,serious pollution to the environment,and outstanding environmental protection problems.The oxidation of 2–alkylanthracene to 2–alkylanthraquinone is a green synthesis route,but no industrial application has been reported due to the deficiency of 2–alkylanthracene.Zheng et al.[27] disclosed that the selectivity of 2–alkylanthracene is significantly improved in the membrane reactor with Y zeolite as the catalyst and 1,3,5–trimethylbenzene as the solvent.At the same time,melt crystallization and multistage vacuum distillation improved the purity of 2–alkylanthracene product.By using the above technologies,Chinese researchers is trying to industrialize the green synthesis of 2–alkylanthraquinone.

Fig.2.The industrial production of 2–alkylanthraquinone by phthalic anhydride method.

Zhang et al.[28] analyzed the composition of working solution and degradation solution in detail with GC–MS.Based on the research result,their team increased the hydrogenation efficiency of working solution by at least 30% with optimizing the ratio of tetrahydro–2–pentylanthraquinone to 2–pentylanthraquinone.Meanwhile,the catalytic regeneration technology of the working solution was developed,which could catalyze the oxidation of anthrone into effective anthraquinone.Compared with traditional regeneration technology,the catalytic regeneration technology increases the conversion of anthrone by 10 times.The use of alkaline alumina is avoided,and therefore H2O2decomposition and solid wastes can also be avoided.The intrinsic safety of H2O2production is significantly elevated.

2.3.Oxygen–enriched cyclic oxidation

Air oxidation is used in traditional oxidation process,which results in a huge amount of exhaust gas.Zhou et al.[29] once reported that more than 98% of exhaust gas in H2O2production comes from the oxidation process.The high boiling–point aromatic hydrocarbons in the tail gas not only pollute the environment,but also have great potential safety hazards.Gao et al.[15]believed the raffinate in the bottom of the oxidation tower mainly comes from the water in compressed air,after calculating,they found a unit with a capacity of 45 kt·a-1produces at least 1000 t of wastewater each year.To solve this problem,Gao et al.[30]developed an oxygen–enriched cyclic oxidation technology,it not only eliminated the tail gas emissions in H2O2production by supplying oxygen and cycling the tail gas,but also reduced the amount of raffinate in the oxidation tower significantly.

In 2019,Sinopec,united with Tianjin University,carried out an industrial demonstration of slurry–bed technology of H2O2production with independent intellectual property rights (Fig.3).The industrial operating data showed the hydrogenation efficiency was 12–13 g·L-1,the oxidation efficiency was 10–12 g·L-1,and the oxidation yield was more than 99%.Compared with a fixed–bed production unit with a 50 kt·a-1,the single–unit capacity of a slurry–bed production unit was increased by 100%,the energy consumption and material consumption could be reduced by about 20%,the discharged wastewater was diminished by 30%,and no tail gas emissions was produced.The slurry–bed technology has obvious advantages in economy,greenness,intrinsic safety,and productivity.The main economic and technical indexes are comparable with the foreign slurry–bed technology.The detailed comparison of the slurry–bed technology developed by China with other companies are shown in Table 1.

Table 1 Comparison of different technologies for H2O2 production

As a green oxidant,H2O2can participate in the hydrocarbon oxidation or nitridation reactions under mild conditions,introducing O or N atoms into hydrocarbons to produce high value–added fine chemicals and intermediates in organic synthesis.Chinese researchers not only exploited the slurry–bed technology of H2O2production,but also carried out a lot of fundamental and applied researches in the green hydrocarbon oxidation and nitridation fields with H2O2.Various green chemical industry technologies have been developed in the production of caprolactam,epoxypropane,epichlorohydrin,and benzenediol.

Fig.3.The 100 kt·a-1 industrial demonstration unit of H2O2 production with slurry–bed technology.

3.Green Hydrocarbon Nitridation

3.1.Caprolactam

Caprolactam is the monomer of nylon–6,which is widely used in the production of polycaprolactam fiber,polycaprolactam resin,and artificial leather.As an important basic organic chemical,its global production exceeded 4.6 × 106t in 2018,and 90% of it was produced by cyclohexanone route [31].This route mainly includes the following four steps:hydrogenation of benzene to cyclohexane,oxidation of cyclohexane to cyclohexanone,ammoximation of cyclohexanone to cyclohexanone oxime,rearrangement of cyclohexanone oxime to caprolactam,and the subsequent multi–step refining processes.The process route is briefly shown in Fig.4.In traditional production processes,cyclohexane oxidation to cyclohexanone,cyclohexanone ammoximation to cyclohexanone oxime,and cyclohexanone oxime rearrangement to caprolactam have a poor atom utilization.Lots of wastewater is produced due to the use of large amounts of sulfuric acid andhydroxylamine salts.At the same time,soda residue,NOxemission,SOxemission,and equipment corrosion are inevitable.

The industrial production methods of cyclohexanone mainly include cyclohexane air oxidation,cyclohexene hydration,and phenol hydrogenation.More than 80% of cyclohexanone in the world is produced by cyclohexane air oxidation,and almost all of cyclohexanone is produced by cyclohexane air oxidation in China[32].However,only 3%–8% of cyclohexane is converted in a single pass,and the selectivity of cyclohexanol and cyclohexanone is only 75%–85%in cyclohexane air oxidation technology[33,34].The utilization of C atom is only 80%,and large amounts of wastewater and soda residue are produced [35,36].For 1 t of cyclohexanone production,1000 m3of exhaust gas,0.57 m3of wastewater,and 0.5 t of soda residue are produced [37].In order to reduce pollution,Asahi Kasei Corporation developed the cyclohexene hydration technology,which includes selective hydrogenation of benzene to cyclohexene and hydration of cyclohexene to cyclohexanone.However,the cyclohexene selectivity is only 75%–80% when benzene conversion is 40%,and it is difficult to separate the mixture of benzene,cyclohexane,and cyclohexene.Although the selectivity of cyclohexene hydration to cyclohexanol is as high as 99%,the single–pass conversion of cyclohexene is only 9%–10%,which is limited by thermodynamics.Considering the cost of phenol,hydrogenation of phenol to cyclohexanone has rarely been used in industrial production.

The esterification reaction rate of cyclohexene with formic acid is very fast,and the selectivity of cyclohexyl formate can be 95%when the conversion of cyclohexene is 95%.Saha et al.[38] investigated the esterification of cyclohexene with formic acid,acrylic acid and methacrylic in the presence of cation–exchange resins.After reacting 6 h,they got a 50% conversion of cyclohexene and 95% selectivity of cyclohexyl formate under 85 °C.The conversion of cyclohexene increased from 50% to 92% by using reactive rectification tower,while the selectivity of cyclohexyl formate retained 96%.They also studied the selective esterification of formic acid with cyclohexene from the mixture of formic acid and acetic acid,and formic acid was easier to react with cyclohexene.In addition,they found the cyclohexyl formate can be hydrolyzed into cyclohexanol and formic acid,which avoided the thermodynamic equilibrium limitation of cyclohexene hydration.Zong et al.[39] also developed a novel process for co–producing cyclohexanol and alkanol,which was shown in Fig.5.They found the cyclohexene esterification reaction was less demanding to the purity of the cyclohexene,such that a crude product containing only 20%(mol) cyclohexene from a benzene partial hydrogenation could be used,which significantly reduced production cost caused by purification or separation step.Macroporous sulfonic acid type ion exchange resins or halogen modified sulfonic acid type ion exchange resins were used as esterification catalyst,a heteropolyacid or zeolite type molecular sieve also could be used as esterification catalyst in their invention.Combined with reactive rectification tower,the single–pass conversion of cyclohexene could be up to 99% or more with a 99.7% selectivity of acetic acid cyclohexyl ester.A zinc–containing or chromium–containing copper–based catalyst was preferred to be used as hydrogenation catalyst to catalyze the conversion of acetic acid cyclohexyl ester to cyclohexanol and ethanol with both the single–pass selectivity and the single–pass conversion were close to 100%.Zhu et al.[40] used Amberlyst 15 to catalyze the cyclohexene esterification in the temperature of 60–100 °C,the selectivity to acetic acid cyclohexyl ester was higher than 95% when the cyclohexene conversion was always ≥68%.Furthermore,they also used the La–promoted Cu/ZnO/SiO2catalyst to catalyze the hydrogenation of acetic acid cyclohexyl ester to cyclohexanol,the selectivity of cyclohexanol was 99.7%when the conversion of acetic acid cyclohexyl ester was 99.5%,which demonstrated a good result from laboratory to industrial test.Compared with the traditional cyclohexane oxidation process,the C atom utilization of cyclohexene esterification–hydrogenation process is close to 100%.The atom economy,environmental friendliness,and intrinsic safety are significantly better than the cyclohexane oxidation process.An industrial demonstration unit with a capacity of 200 kt·a-1is expected to be put into operation in 2021.This work can provide low–cost green cyclohexanone to produce caprolactam and adipic acid.

Fig.5.A new green route for cyclohexanone production.

Cyclohexanone ammoximation is the core step in caprolactam production.Hydroxylamine is often used in traditional cyclohexanone ammoximation process,which involves the oxidation of ammonia to NOx,the reduction of NOxto hydroxylamine,the ammoximation of cyclohexanone with hydroxylamine to cyclohexanone oxime,and the ammonium decomposition steps.The utilization of ammonia is only 60%,and a large amount of NOxis released,which is the main source of NOxemission in caprolactam production.The traditional technology needs harsh reaction conditions and causes serious pollution.Fig.6 shows reactions involved in traditional cyclohexanone ammoximation process.

In 1980s,Marco et al.[41]developed a one–step ammoximation of cyclohexanone to cyclohexanone oxime using H2O2as the oxidant,TS zeolite as the catalyst,and NH3as the nitrogen source.Under a mild reaction condition,the ammonia utilization is near 100%,the conversion of cyclohexanone is more than 99.0%,the utilization of H2O2is 90.0%,and the selectivity of cyclohexanone oxime is higher than 98.2%.The technical route is also shown in Fig.6.The one–step ammoximation method has obvious advantages in energy consumption,production cost,and environmental protection than traditional ammoximation method.

Fig.6.Reactions involved in different cyclohexanone ammoximation methods.

However,the activity and selectivity of TS zeolite are unstable,and the preparation reproducibility is poor.The invalid decomposition of H2O2caused by Ti outside the framework of TS zeolite reduces the effective utilization of H2O2.To overcome the above disadvantages,Lin et al.[42] developed a new technology of hydrothermal synthesis plus rearrangement modification to prepare hollow TS zeolite.They firstly used inorganic acid to treat a synthesized TS–1 at 15–60 °C for 10–180 min,and then reacted the acid-treated TS–1 with quaternary ammonium bases in a autoclave at 150–180 °C for 2–120 hours.It not only created hollowness in the crystallites,but also reduced the amount of ex–skeleton TiO2.The stability and activity of hollow TS zeolite were elevated significantly.Sun et al.[43] studied the deactivation of TS zeolite in cyclohexanone ammoximation.They found the deactivation of TS zeolite was due to the dissolution and loss of framework Si,and NH3played the key role.With the increase of system polarity and NH3content,the dissolution of Si increased.This dissolution behavior not only brought higher catalyst consumption,but leaded the migration of skeleton Ti,which finally affected the activation and deactivated catalyst regeneration.Therefore,Sun et al.developed silicon-containing additives to improve the stability of TS zeolite.The single operation time of TS zeolite had been extended from 350 hours to 600 hours with no obvious change in crystallinity.Sun et al.[44] and Wu et al.[45] disclosed the catalyst loss control and regeneration technologies.A liquid silicon–containing assistant was added in to the reaction system to achieve an equilibrium dissolution concentration of silicon in the solution without influencing on the activity and selectivity of catalyst.Any inorganic or organic silicon–containing substances could be used under the premise of being soluble in the reaction system.The concentration of silicon in the system were preferred between 0.1 and 10000 μg·g-1,which could effectively reduce the dissolution of TS zeolite,and the recovery of catalyst was higher than 97%.They also used an acidic solution(pH value ≦1) to treat the deactivated catalyst at 70–90°C for 1–4 hours,wherein the concentration of catalyst in the solution was 3%–15% (mass).An inorganic acid was preferred.Then the treated catalyst was subjected a drying and calcining process.After these treatments,the activity,selectivity and stability of the regenerated catalyst could be recovered to the level of fresh catalyst,while the stability of regenerated catalyst just treated by calcining could only be recovered to about 65%–75%of the level of fresh catalyst.Through the comprehensive application of the above technologies,the catalyst consumption in industry was significantly reduced.With a high–efficiency membrane separation technology,the raw powder of hollow TS zeolite is directly used in one–step cyclohexanone ammoximation.As a result,the mass transfer ability of H2O2,cyclohexanone,and ammonia in the reaction system is improved.Table 2 shows the comparison of Chinese technology with a foreign technology.As shown in Table 2,the one–step cyclohexanone ammoximation technology developed by China is better in the conversion of cyclohexanone,the selectivity of cyclohexanone oxime,the H2O2utilization,and especially the catalyst life and capacity.The N atom utilization has been increased from 60%to over 85%compared with traditional ammoximation method,and no corrosive NOxis produced [46].

Table 2 Comparison of cyclohexanone ammoximation technologies

The production technology of caprolactam developed by China avoided the use of fuming sulfuric acid in rearrangement of cyclohexanone oxime to caprolactam by gas–phase Beckmann rearrangement with silicalite–1 as catalyst.The N atom utilization is increased from 36% to near 100%.China also achieved the first application of magnetically stabilized bed integrated with amorphous nickel in industry in 2003.As a result,the unstable Raney nickel catalyst was eliminated in the refining of caprolactam.The manufacturing costs of the caprolactam was reduced significantly through the above innovation.The green production technology of caprolactam developed by China is constituted by a group of green production technologies with completely independent intellectual property rights.Compared with traditional production technology of caprolactam,the exhaust gas has been reduced by 95%.No low–valued ammonium sulfate is produced.And the overall investment has dropped by 70%.A caprolactam production unit with a capacity of 50 kt·a-1can reduce 2.4 × 108m3of waste gas and 80,000 t of low–valued ammonium sulfate [47,48].Fig.7 shows a 400 kt·a-1industrial production unit of caprolactam adopting China’s technology with independent intellectual property.The green production technology of caprolactam has strongly supported the technology upgrading of caprolactam production in China,giving China a global market share that exceeds 50%,and making it the world’s largest CPL producer—a huge leap from China’s original position of relying almost totally on CPL importation.In 2020,the green production technology of caprolactam was awarded the “China Grand Awards for Industry”.

Fig.7.A 400 kt·a-1 caprolactam industrial production unit.

3.2.Hydroxylamine

Hydroxylamine is an important chemical intermediate.Because of its instability,hydroxylamine usually exists in the form of hydroxylamine salt.Its industrial synthesis methods include Raschig method,catalytic reduction method,nitro–paraffin hydrolysis method,and acetoxime method.The reactions involved are shown in Fig.8.Fig.8 shows existing industrial production methods of hydroxylamine involve ammonia oxidation step that is characterized by complex processes,high energy consumption,and harsh reaction conditions with low atom utilization and serious environmental pollution.

Although ketoxime can be decomposed to hydroxylamine and ketones,its low economic value restricts this route from industrial application,since hydroxylamine is usually used to synthesize ketoxime.However,with the maturity of cyclohexanone oxime synthesis with NH3,H2O2,and cyclohexanone catalyzed by TS zeolite,the hydrolysis of cyclohexanone oxime to hydroxylamine has attracted much attention.Fig.9 shows the reaction route,in which NH3and H2O2are raw materials,and cyclohexanone is a recycling agent [49,50].However,Zhao et al.[51] pointed out that the hydrolysis of cyclohexanone oxime cannot take place spontaneously without Br?nsted acid,and removal of cyclohexanone in time from the system is conducive to the formation of hydroxylamine to break the reaction equilibrium.Therefore,Peng et al.[52] investigated the influence of chemical reaction engineering technologies on the hydrolysis of cyclohexanone oxime.They used continuous reaction–extraction coupling technology with a five–stage series countercurrent reactor to break the hydrolysis equilibrium.Cyclohexane was used as the extraction agent,and the operation conditions were optimized.After four cycles of extraction,the conversion of cyclohexanone oxime could reach 81.9%,much higher than batch reaction,in which conversion of cyclohexanone oxime was only 30%,while no significant change in the selectivity of hydroxylamine.However,the hydrolysis process can’t avoid corrosion and acid–containing wastewater pollution because of the use of acid.Meanwhile,various separation technologies involve high energy consumption.How to overcome these shortcomings becomes the common issue for researchers.

Based on research results,Yang et al.[53] and Zecchina et al.[54]both believe the mechanism of cyclohexanone oxime production is NH3and H2O2are catalyzed by TS zeolite to generate hydroxylamine first,and then hydroxylamine reacts with cyclohexanone to form cyclohexanone oxime,so direct production of hydroxylamine from NH3and H2O2catalyzed by TS zeolite should be the best synthetic route,in which the N atom utilization is near 100%under mild conditions without pollution.The reaction is simple,and the corrosion caused by acid is avoided.The type of Ti in the TS zeolite is the key to the yield of hydroxylamine,and excessive amount of NH3is conducive to the production of hydroxylamine.At present,this technology has already been used in the green production of cyclohexanone oxime,and is one of the core technologies in the green production of caprolactam [46,55,56].Even so,there is no industrial production unit for producing hydroxylamine from NH3and H2O2directly except in the industrial production of cyclohexanone oxime,since the total yield of hydroxylamine is only about 10%.In addition,Fu et al.[57]summarized the research progress of using NOxor SO2in exhaust gas to synthesize hydroxylamine to achieve a win–win result between exhaust gas treatment and hydroxylamine synthesis,but there is no industrial application report.

3.3.Hydrazine hydrate

Hydrazine hydrate is an important intermediate in fields of medicine,pesticide,dye industry,rare metal refining,and rocket propellant.It is also regarded as a promising liquid hydrogen storage material in view of its high hydrogen content and transportation performance [58,59].The industrial production methods of hydrazine hydrate include the Raschig method,the urea method,and the ketazine method,which are shown in Fig.10.NH3and sodium hypochlorite are used as nitrogen source and oxidant,respectively,in Raschig method.As hydrazine is easier to be oxidized than NH3,the yield of hydrazine is low.Hydrazine in the product system is only 3%–4% (mass),and the process is obsoleted in consideration of concentration step,NH3circulation,and salt–containing wastewater emission.In the urea method,urea is used as the nitrogen source to avoid large amounts of energy consumption caused by NH3circulation.But the reaction of hydrazine with sodium hypochlorite still cannot be avoided,so the yield of hydrazine is still low.The N atom utilization of the urea method is only 70%.When 1 t of hydrazine hydrate is produced,5 t of salt–containing residue and 4.6 t of soda residue are produced.In order to improve the yield of hydrazine,ketones are used to react with NH3under the oxidation of sodium hypochlorite to avoid the further reaction of hydrazine with sodium hypochlorite.Then,ketazine is hydrolyzed to form hydrazine hydrate and ketones.The yield of hydrazine is improved significantly,and the N atom utilization is close to 90%.However,the production of 1 t of hydrazine hydrate still produces 4 t of sodium chloride and ammonium chloride [60].In any case,corrosion and salt–containing wastewater cannot be avoided in the above three processes with sodium hypochlorite as the oxidant.

ARKEMA uses H2O2as the oxidant to fundamentally solve the corrosion and pollution caused by sodium hypochlorite.The reaction is also shown in Fig.10.Water is the only by–product in ARKEMA method,and it is an intrinsically safe and green chemical reaction.The yield of hydrazine based on H2O2consumption is up to 95%.The energy efficiency is three times of that of traditional methods because of the product is easily separated from water without distillation.The key of ARKEMA method lies in the catalyst of ketazine synthesis and the hydrolysis of ketazine.A catalytic system that is constituted by nitrile and amide as the main catalyst,and ammonium salt and Na2HPO4as the co–catalyst has shown a good effect so far[61].The ARKEMA method is only mastered by a few of foreign companies so far,and technology transfer to China is extremely difficult.

In 1970s,Hayashi et al.[62] began to try to use air as the oxidant to realize the direct reaction of benzophenone with NH3under the catalysis of CuCl and NH4Cl to produce benzophenone azine,which can produce hydrazine hydrate by decomposition with proton acid.However,due to the low catalyst activity,there is no industrial application case till now.

Fig.8.The existing industrial production methods of hydroxylamine.

Fig.9.Production of hydroxylamine by cyclohexanone oxime.

The production of hydrazine hydrate in China is mainly based on the urea method at present,and the industrial equipment for hydrazine hydrate production by the ketazine method increases rapidly,but the green production technology of hydrazine hydrate is still blank in China.

3.4.Aniline

Aniline is widely used in polyurethane,rubber additives,pesticides,and dye industries.More than 70% of aniline is used to produce diphenylmethanediisocyanate (MDI).In 2018,the global production of aniline reached 6.7 × 106t,of which 2.15 × 106t came from China.The reduction of nitrobenzene by iron powder,catalytic hydrogenation of nitrobenzene,and ammoniation of phenol are the main methods used in the industrial production of aniline.Benzene is used as the raw material to prepare aniline through multi–step reactions in the above three methods.The C and N atom utilizations are quite low with serious environmental pollution.More than 85% of aniline comes from the catalytic hydrogenation of nitrobenzene,which is shown in Fig.11.It involves the nitration of benzene and the hydrogenation of nitrobenzene.The mixture containing 60% of sulfuric acid,30% of nitric acid,and 10%of water is used as nitrifying agent in the nitration of benzene,which causes serious corrosion and poor intrinsic safety.High energy consumption is caused by the concentration of diluted acid.Low atom utilizations of C and N are caused by the over–oxidation of benzene.And serious environmental pollution is caused by wastewater containing acid and caustic alkali[63,64].The Cu/SiO2or Pt/C or Pt/Al2O3catalyst is used in the catalytic hydrogenation of nitrobenzene according to different hydrogenation methods [65].The current research progress on the catalytic hydrogenation of nitrobenzene mainly focuses on improving the performance and the green preparation of the catalyst,which does not fundamentally solve the problems of pollution,energy consumption,and safety of the process [66–68].

Fig.10.Different production technologies of hydrazine hydrate.

Fig.11.Different production technologies of aniline.

A new reaction path for green synthesis of aniline is urgent for aniline industry.With the deep–going of the research,Kuznetsova et al.[69] and Zhu et al.[70] found that aniline could be synthesized by reacting hydroxylamine with benzene in an acidic system with comparatively high conversion of benzene and selectivity of aniline.But the industrial production of hydroxylamine involves complex processes,and the shortages are all listed in Section 3.2.Aniline can also be synthesized by reacting NH3with benzene in one–step reaction;the reaction involved is shown in Fig.11.The C atom utilization is 98%,and high–valued H2is the by–product.Due to thermodynamic equilibrium limitations,the conversion of benzene is very low,though the yield of aniline can be promoted by removing the by–product H2.Oxygen or lattice oxygen is usually used as the oxidant to remove H2from the system under harsh conditions.So far,the most successful case was the NiO/ZrO2system developed by Dupont,but the yield of aniline is still only 13.6%under harsh conditions of 350 °C and 30–40 MPa.Hoffmann et al.[71] took temperature–programmed desorption experiments and diffuse reflectance infrared Fourier transform spectra (DRIFT) to study the reaction mechanism of the benzene oxidative amination to aniline over NiO/ZrO2as cataloreactant.The temperature–programmed reaction experiment and DRIFT spectra of adsorbed CO showed metallic nickel was active site,and metallic nickel was essential to the formation of aniline.They also discussed the origin of the extraordinarily high selectivity to aniline in the temperature range 277–377°C.The metallic nickel surfaces present in this temperature range and the catalytic decomposition of NH3determined the upper limit of temperature.The ammonia–derived NHxspecies occupied most of active sites to avoid the unfavorable parallel adsorption geometry of benzene,which would lead to the decomposition of benzene into CHxfragments.Desrosiers et al.[72]pointed out though the NiO/ZrO2system developed by Dupont is the most successful case,its regeneration performance was poor.So,they used high–throughput synthesis and screening methods to screen appropriate catalytic system.Finally,they found noble metals of Rh,Ir,Pd,and Ru were suitable dopants,in which Rh had a best effect.NiO was the most active oxidant,and its regeneration performance would be significant improvement by Mn,as Mn could inhibited the aggregation of Ni particles.The optimized cataloreactant,Rh/Ni–Mn/K–TiO2,achieved stable 10% conversion of benzene with more than 95% selectivity of aniline.Its activity had no significant change after five regenerations,while the unoptimized cataloreactant lost 50% initial activity after five regeneration.

In order to realize the one–step synthesis of aniline under mild conditions,Sichuan University has tried to use H2O2as the oxidant and a variety of transition metals as catalysts for aniline preparation by reacting NH3with benzene.The one–step reaction involving H2O2is also shown in Fig.11.The benzene conversion is only 0.08%.Hu et al.[73] tried to use catalytic distillation technology to reinforce the reaction.After investigating the effects of feed ratio,distillation temperature,reaction time,and the packing manners of catalytic column on the reaction,they found aniline yield was increased when the catalyst was packed segmentally,and the reaction time was prolonged.But the distillation temperature had no obviously influence on the reaction.The authors used V–Ni/Al2O3as catalyst to catalyze the reaction in catalytic distillation reactor under optimized conditions,but the benzene conversion is only increased from 0.08% to 0.15%.An effective catalyst is important to improve the yield of aniline.Based on the results of the hydroxylamine production from NH3and H2O2catalyzed by TS zeolite,and the production of aniline from hydroxylamine and benzene,Guo et al.[74] adopted TS–1 zeolite to catalyze the one–step production of aniline with H2O2.But the TS–1 zeolite had no obvious selectivity to aniline,nitrobenzene and phenol.Then they confirmed the extra–framework Ti was favorable to catalyze the hydroxylation of benzene,while framework Ti was beneficial to activate the N-H bond of NH3.To enhance the selectivity of aniline,they also used different metals (including Ce,V,Co,Ni,Fe and Cu) to modify TS–1 zeolite,only Cu modified TS–1 zeolite(Cu–TS–1)increased the yield of aniline,but Cu species alone could not catalyze the amination of benzene.Experiment data showed Ti-O-Cu species was formed when Cu was doped on TS–1 zeolite,therefore,the chemical adsorption and activation of NH3was enhanced.After the optimized of Cu content,they got 1% conversion of benzene and 88% selectivity of aniline.In order to further increase the yield of aniline,Yu et al.[75] adjusted the pore structure of the TS–1 zeolite,and prepared hierarchical TS–1 zeolite(h–TS–1).Mesoporous structure was created in h–TS–1 zeolite,which was not only advantageous to disperse the copper species in the channels to form more active Ti-O-Cu centers,but also beneficial to improve the accessibility of active sites.Compared with Cu–TS–1,Cu–h–TS–1 could converse 11.1% of benzene,and yield 8.2%(mole) of aniline.After using reactive distillation technology to reinforce the reaction with Cu–h–TS–1 as catalyst,the conversion of benzene was increased to 12.4% with the selectivity of aniline was 84.7%.Nan et al.[76] investigated the effect of introducing K into Cu–TS–1 on the selectivity of aniline.The experiment results showed different introduction methods have huge differences in the selectivity of aniline.The Cu–TS–1 with K introduced by wet impregnation formed Ti-O-K species,which had a harmful effect on Ti site,inhibiting the formation of hydroxylamine,one key intermediate in amination of benzene.The selectivity of aniline catalyzed by Cu–TS–1 with K introduced by wet impregnation was less than 40%.If introducing K into Cu–TS–1 under reaction conditions by adding KNO3into the reaction system,the selectivity of aniline could reach 99.5%.The Si-O-K species were formed when introducing K under reaction conditions.The Si-O-K species could weaken the activity of Br?nsted acid,and enhance the activity of Lewis acid,making it easier to NH3adsorption.Furthermore,K introduced under reaction conditions had no influence on active Ti sites.However,the yield of aniline was still less 2% catalyzed by Cu–TS–1 with K modified.The authors did not study the yield and selectivity of aniline catalyzed by Cu–h–TS–1 with K modified.Whether it has both high yield and selectivity of aniline catalyzed by Cu–h–TS–1 with K modified remains to be investigated.

Although the one–step synthesis of aniline can be achieved by reacting NH3with benzene in the presence of H2O2,there are still issues about how to promote the conversion of benzene and the selectivity of aniline by inhibiting the hydroxylation of benzene,and avoiding the excessive oxidation of aniline to nitrobenzene.New catalysts and reaction engineering technologies are the key to solve the poor selectivity of aniline.Among them,photocatalysis has also attracted the attention due to it could catalyze organic syntheses in mild conditions.Yuzawa et al.[77] firstly came true the direct amination of benzene with NH3catalyzed by Pt loaded on TiO2at room temperature.They studied the influence of different TiO2phases on amination,Pt loaded on rutile phase TiO2given a 0.028% yield of aniline with a selectivity of 97%,while Pt loaded on anatase phase TiO2given a higher yield of aniline with a lower selectivity.Furthermore,they also investigated the mechanism of photocatalysis amination.Results shown NH3was firstly activated,and then the activated species attacked benzene to form aniline.So far,the yield of direct synthesis of aniline by photocatalysis is too low to realize industrial production.

4.Green Hydrocarbon Oxidation

4.1.Epoxypropane

Epoxypropane is the third largest propylene derivative.It is widely used in unsaturated resins,surfactants,and polyurethanes.In 2018,the actual output of epoxypropane in China was 2.75× 106t,while the consumption reached 3 ×106t.The industrial production methods of epoxypropane mainly include the chlorohydrin method,the co–oxidation method,and the H2O2method(HPPO),which are all shown in Fig.12.The direct epoxidation of propylene with air or oxygen is still under research [78].

As the main industrial production method of epoxypropane,the chlorohydrin method uses hypochlorous acid as the oxidant.Though the C atom utilization is 95%,serious corrosion and pollution are still caused using chlorine.For every 1 t of epoxypropane production,1.35–1.85 t of chlorine gas is consumed,40–80 t of chlorine–containing wastewater and more than 2 t of CaCl2waste are produced[79].China began to strictly control the construction of new plants adopting the chlorohydrin method in 2011.In order to overcome the disadvantages caused by chlorine gas,the co–oxidation method is developed.However,the process is complicated,and the reaction condition is harsh.The co–oxidation route with ethylbenzene or isobutane as co–reductant requires strictly with the quality of raw materials,and the economic benefit is restricted by the co–products.The co–oxidation route with cumene as co–reductant consumes lots of energy due to the separation and conversion procedures of the intermediate products.

Fig.12.Industrial production methods of epoxypropane.

The HPPO method developed in recent years uses H2O2as the oxidant to realize the direct epoxidation of propylene.The reaction is also shown in Fig.12.For the HPPO method,water is the only by–product,and the reaction conditions are mild.Compared with the chlorohydrin method,the C atom utilization is close to 100%,with a reduction of 25% in the equipment investment,70%–80%in the wastewater discharge,and more than 35%in the energy consumption.HPPO is a green production method of epoxypropane,but it was only mastered by a few foreign companies previously.China started the research on the HPPO method in 2000s.Xi et al.[80] reported a “reaction–controlled phase transfer catalyst” to catalyze the epoxidation of propylene with H2O2,in which the heteropolyphosphatotungstate could form soluble active species under the action of H2O2,and it precipitated from the reaction system after H2O2was used up.Their research results got an 89%conversion of propylene and 95%selectivity of epoxypropane based on 2–ethylanthrahydroquinone.Zhu et al.[81] carried out the research of HPPO method with TS zeolite as catalyst.Based on the consideration of the acidity of the catalyst,which comes from the active centers of Ti,trace Al,and defects of surface and lattice of the TS zeolite,is the main factor that catalyze the solvolysis of epoxypropane,they modified the synthesis process of hollow TS zeolite and succeeded in preparing hollow TS zeolite with silicon enriched on the surface,which is different from the TS zeolite used in the cyclohexanone ammoximation reaction.As a result,multi–hollow TS zeolite with Si enriched on the surface reduces the skeleton defects,and the surface of TS catalyst is acid–deficient.Thereby,the solvolysis side reaction of epoxypropane is inhibited,and the selectivity of epoxypropane is increased to higher than 95%based on H2O2consumption.The HPPO method with TS zeolite as catalyst usually takes place in fixed–bed reactor which requires high–strength catalyst.The traditional way to improve the strength of the catalyst is by increasing the amount of binder,resulting in the decrease of the effective components of TS zeolite in the catalyst and further decrease of the catalyst activity and reactor utilization.To enhance the catalyst strength,Lin et al.[82]and Wang et al.[83]used amorphous SiO2and added additives to modify the catalyst,increasing the crushing strength of the catalyst to higher than 120 N·cm-1without decreasing the catalyst activity and reactor utilization.Based on the understanding of the catalyst deactivation mechanism in the HPPO method,Li et al.[84]developed a catalyst regeneration method with methanol as extracting solvent.After regeneration,the life span of the regenerated catalyst is prolonged to 2000 h,and no significant decrease in catalytic performance.Lin et al.[85] also adopted a two–stage reactor with fixed–bed reactor and slurry–bed reactor in series to improve the conversion of H2O2.The reactant was firstly introduced into fixed–bed reactor,and catalyzed by the catalyst containing TS–1 zeolite,getting more than 50% conversion of H2O2.After separating epoxypropane and propene from product,the left was introduced into slurry–bed reactor with propene and TS–1 zeolite.The combining of fixed–bed reactor with slurry–bed reactor not only overcame the low conversion of H2O2in fixed–bed reactor alone,but also solved the low selectivity of epoxypropane in slurry–bed reactor alone.At last,they conversed 99.5% of H2O2with a 99.1%selectivity of epoxypropane.The technology comparison of Sinopec with foreign companies is shown in Table 3.

Table 3 Comparison of the HPPO technologies developed by different companies

China has successively developed the synthesis and modification technology of multi–hollow TS zeolite with Si enriched on the surface,the preparation and regeneration technology of high–selectivity catalyst,the tandem reaction process and system control technology of epoxidation,the design and manufacturing technology of large–scale tubular reactor,and the safety control technology of whole epoxidation process.The pilot–scale test data showed that the HPPO technology developed by China affords a 96%–99% conversion of H2O2and a 96%–98% selectivity of epoxypropane when the unit runs for over 6000 hours with a nearly unchanged catalytic activity [86].The first industrial demonstration unit of HPPO with a capacity of 100 kt·a-1in China has been completed and put into operation by Sinopec in 2014 (Fig.13).A comprehensive intellectual property system covering TS zeolite,catalyst,process,equipment,safety control,and environmental protection makes China be the third country grasping HPPO technology in the world.In 2020,the technology package of HPPO method by Sinopec with a capacity of 300 kt·a-1passed the technical appraisal,which explores a green path for the epoxypropane industry in China and guarantees the continuous supply of epoxypropane.

4.2.Epichlorohydrin

The epichlorohydrin (ECH) is mainly used in the synthesis of epoxy resin.China’s ECH production capacity reached 1.4 × 106t in 2017.The industrial production methods of ECH include propylene chlorination method,allyl acetate method,and glycerin method,which accounte for 45%,48%,and 7%of the total ECH production capacity in 2017,respectively.The reactions involved are shown in Fig.14.The Cl atom utilization is only 25% in propylenechlorination method,and a large amount of chlorine–containing wastewater and CaCl2are produced as a result.The allyl acetate method and the glycerin method can improve the utilization of the Cl atom by changing the synthesis method of dichloropropanol.Especially in glycerin method,bio–based glycerol is used as raw material to get rid of the dependence on fossil resources.Among the above production methods,the glycerin method has the least waste emissions and investment,but it still needs the saponification step,which produces a large amount of chlorine–containing wastewater and 1/2 equivalent molar content of CaCl2.Though different in the raw materials and chlorination methods,the above three methods all generate dichloropropanol,and the dichloropropanol then is saponified to produce ECH.Therefore,they can all be referred to as the dichloropropanol saponification technology.In the chlorohydrination step,equipment corrosion is serious,and large amounts of chlorine–containing wastewater and residue are produced in the process of dichloropropanol saponification[87].

Fig.13.A 100 kt·a-1 epoxypropane industrial production unit.

To achieve the green production of ECH,dichloropropanol production and its saponification should be avoided.The direct epoxidation of chloropropene can completely solve the corrosion and pollution problems caused by dichloropropanol production and saponification.Direct epoxidation of chloropropene with H2O2as oxidant is green for ECH production,as water is the only by–product [88].The reactions involved are also shown in Fig.14.The C atom utilization is close to 100%,and the Cl atom utilization is near 50%.Compared with the propylene chlorination method,the wastewater discharge is reduced by 95%,the investment is reduced by 65%,and the environmental protection investment is reduced by 99%.Compared with the glycerin method,the catalyst consumption decreases by 70%,the wastewater discharge decreases by 90%,the investment decreases by 60%,and the environmental protection investment decreases by 90%.A detailed comparison is shown in Table 4 [89,90].The research on direct epoxidation of chloropropene to ECH mainly focuses on the selection and modification of the catalysts,the selection of solvent composition,product purification,reactor type,and reaction kinetics [80,91–95],but there is no report on industrial application of direct epoxidation of chloropropene to produce ECH until now.

China began to carry out the research on direct epoxidation of chloropropene with H2O2in 2000s.Since Xi et al.[80] developed a new reaction–controlled phase–transfer catalyst for oxidation with H2O2,their team carried out a series of related research work on direct oxidation of olefins with H2O2.Gao et al.[96,97] took advantage of the heteropolyphosphatotungstate to achieve the direct epoxidation of chloropropene with H2O2.They conducted a 5 kt·a-1pilot plant test in 2018.The average yield of epichlorohydrin(based on H2O2)is ≥90%,the average selectivity of epichlorohydrin (based on chloropropene) is ≥96%.It consumes 0.85 t of chloropropene(99.2%),0.4 t of H2O2,2.88 kg of catalyst when producing 1 t of production,and only 0.59 t of wastewater is produced.At the same time,Sinopec conducted the research of direct epoxidation of chloropropene with H2O2catalyzed by TS zeolite.Du et al.[98,99] supported TS zeolite on silica–alumina materials to catalyze the direct epoxidation of chloropropene with H2O2,achieving more than 97% conversion of H2O2and 97%–99%selectivity of ECH.They also adjusted the composition and preparation method of the catalyst to obtain high–strength catalyst containing TS zeolite to meet the requirements of fixed–bed reactors.Lin et al.[100]modified the TS zeolite with transition metal oxides to improve the conversion of chloropropene.Heteropolyacid such as phosphotungstic acid or phosphomolybdic acid was used to catalyze the reaction with hollow TS–1 zeolite,the conversion of chloropropene was increased from 47% to 86%.Zhang et al.[101]studied the deactivation mechanism of TS–1 catalyst in direct epoxidation of chloropropene with H2O2in methanol.They found the deactivation of TS–1 zeolite came from the block of byproducts in the pore.The pH of reaction system had an obvious influence on byproducts production.When pH was less than 2,the side reaction of alcoholysis or hydrolysis of ECH was severe,which produced more byproducts.While pH was greater than 5,no transition state with epoxidation activity would be produced.As a result,TS–1 had the best stability when the pH was 3.They also found the activity of deactivated TS–1 zeolite could be restored by washing with methanol at high temperature.Therefore,online catalyst regeneration process was developed to shorten the shutdown period and improve the production capacity of the unit.Zhang et al.[102]disclosed a continuous long term stable operation of the ECH production technology by combining two fixed–bed reactors with online catalyst regeneration technology.After six regenerations,the unit had run 3121 hours continuously,the conversion of H2O2was still higher than 97%,and the selectivity of ECH was still higher than 95%.Moreover,Zhang et al.[103] also developed a separation and purification system to acquire high–purity ECH continuously.They used water to extract methanol,and 3–chloropropene to extract ECH from product.Compared with the separation which only one extraction solvent was used,the separation method used in their invention increased the yield of ECH from 96.5% to 99.1%,while the unit consumption of steam was reduced from 25.6 t to 9.8 t.Furthermore,Liu et al.[104] developed a catalytic decomposition technology of residual H2O2in ECH production to eliminate the potential explosion risk.The catalyst contained 2%–10%(mass)of manganese oxide,2%–10% (mass) of chromium oxide,2%–10%(mass) of zirconium oxide,and 70%–96% (mass) of aluminium oxide.Sinopec conducted the 600 t·a-1pilot experiment of direct epoxidation of chloropropene with H2O2catalyzed by TS zeolite.The results showed that the conversion of H2O2is no less than 97%,and the selectivity of ECH is no less than 95%.The purity of ECH can reach 99.99%.Compared with the traditional technologies of ECH production,the wastewater discharge is reduced by more than 90%.No solid waste is produced.And the investment and environmental protection cost of the device are reduced.In August 2019,a capacity of 50 kt·a-1process package was completed,which steps further towards the industrialization of green production technology of ECH.

Fig.14.Different production methods of ECH.

4.3.Phenol and benzenediol

Phenol is used as a raw material to produce aniline,cyclohexanone,bisphenol A,and phenolic resin,which are widely used in the fields of medicine,chemicals,and synthetic fibers.In 2019,the global production capacity exceeded 13 × 106t,of which the production capacity in China reached 3.2 × 106t.Moreover,the demand for phenol will continue to grow with the increasing demand for bisphenol A in the future.More than 90% of phenol in industry is produced by the cumene method,which involves the alkylation of benzene with propylene to cumene,oxidation of cumene to cumene hydroperoxide,and the decomposition of cumene hydroperoxide to phenol and acetone.The whole process is shown in Fig.15.The multi–step reaction process results in high energy consumption and low yield of phenol,and the C atom utilization is only 70%.The overall benefits of the process are seriously restricted by acetone,and the equipment is corroded due to the use of acid in the peroxide decomposition.At last,the cumene method has a poor intrinsic safety due to the instability of cumene hydroperoxide existed in the system [105].

The simplest and greenest synthesis method for phenol production is direct oxidation of benzene to phenol in one step.The C atom utilization is close to 100%.The reaction is also shown in Fig.15.Based on different oxidants,it can be divided into the N2O oxidation method,the H2O2oxidation method,and the O2oxidation method.Though the N2O oxidation method has already been implemented in industry,it is difficult to be widely used due to the needs of preparation,storage,and transportation of N2O.Its industrial application value is only embodied when combining with the adipic acid production unit which releases large amount of N2O [106].Although the O2oxidation method is the most economical and environmentally friendly route,the lack of stable and efficient catalysts in liquid phase oxidation and the corrosion caused by reaction solvents make it impossible for the liquid phase oxidation with O2to be industrialized [107].And the temperature in the gas–phase oxidation is comparatively high,which will lead to serious peroxidation of phenol and rapid deactivation of the catalyst.

Many kinds of catalysts are available in the H2O2oxidation method.Bianchi et al.[108] used FeSO4as catalyst to catalyze the oxidation of benzene to phenol,8.6% conversion of benzene and 97% selectivity of phenol (based on benzene) were achieved.Zhang et al.[109] researched the effect of molybdovanadophosphoric heteropoly acid catalysts on the hydroxylation of benzene.They got a result of 34.5%conversion of benzene with a 100%selectivity of phenol (based on benzene).Reis et al.[110] also reported the oxidation of benzene catalyzed by amavadine at room temperature.However,the above catalysts are all homogeneous catalysts,which restrict their application in industry.As a heterogeneous catalyst,TS zeolite is commonly used in catalytic oxidation with H2O2.Jiang et al.[111] believed the environment of the Ti atom,the content of the defect hydroxyl,grain size,pore structure,spacial structure,and purity of TS zeolite strongly influence the oxidation of benzene to pheno.Tanev et al.[112]optimized the reaction conditions with TS zeolite as catalyze,the benzene conversion of 31% and the phenol selectivity of 95% are achieved.In addition to TS zeolite,heterogeneous catalysts containing transition metals such as Fe,V,and Cu also show excellent performance in benzene oxidation to phenol with H2O2.For example,Borah et al.[113]converted 33% of benzene to phenol with the phenol selectivity of 100% under the catalyzation of VOPO4·2H2O encapsulated in graphene oxide.They also tried a heterogeneous catalyst which is prepared by supporting homogeneous catalysts containing V or Fe on a heterogeneous carrier to catalyze the hydroxylation of benzene,the benzene conversion of 27%and phenol selectivity of 100%were achieved[114].Besides the catalysts,Peng et al.[115]also tried to use reaction engineering to extract phenol into the water phase in time to avoid the excessive oxidation of phenol.Although the conversion and selectivity of direct oxidation of benzene to phenol with H2O2have already met the industrial requirements,it cannot take place of the cumene method in industry due to the cost of H2O2.Therefore,how to effectively make use of H2O2as oxidant and reduce its cost are the major problems faced in the future.For this reason,the in–situ production technology of H2O2is developed for direct oxidation of benzene to phenol.The most representative work is done by Niwa et al.[116],who used inorganic palladium film to produce H2O2in–situ in the presence of H2and O2,and then the H2O2was used to oxidize benzene to phenol.They cleverly used inorganic palladium membrane to design a shell–and–tube reactor,in which benzene was fed into the tube together with a mixed gas of oxygen and inert gas.At the same time,a mixture of hydrogen and inert gas was also fed into the shell.Experiment shown dissociated hydrogen penetrated the membrane immediately and reacted with oxygen to produce H2O2.This reactor converted 10%–15% of benzene to phenol with a selectivity of phenol more than 80%below 250°C.If they control the conversion of benzene below 3%,the selectivity of phenol could reach 97%.Because of hydrogen and oxygen were separated by the palladium membrane,the explosion danger caused by the mixing of H2and O2is also avoided efficiently.With the breakthrough of the slurry–bed production technology of H2O2in China,the cost of direct hydroxylation of benzene to phenol with H2O2is expected to decrease further,which benefits the development of China’s green chemical industry.

Fig.15.Reactions involved in different methods of phenol production.

Catechol and hydroquinone are also important chemical intermediates.For decades,China has been relying on import.In China,the main production processes of hydroquinone are hydrolysis of chlorinated phenol,oxidation of aniline,and oxidation of diisopropylbenzene,which have low C atom utilization and cause serious pollution.Direct hydroxylation of phenol to benzenediol with H2O2is a green process with high C atom utilization and little pollution.The industrialization routes mainly include the Rhone–Poulene route,Brichima route,UBE route,and Enichem route.The Rhone–Poulene route and UBE route use strong acid,and the conversion of phenol in single–pass is low.They also require high–concentration H2O2and produce a large amount of acid–containing wastewater.The selectivity of benzenediol in the Brichima route is only 80%.The Enichem route is the most efficient production technology with TS zeolite being used as the catalyst.

In China,Lin et al.[42]used to synthesis hollow TS zeolite which has already been used in the production of caprolactam and epoxypropane.Their team also carried out the application research of TS zeolite on catalyzing the direct hydroxylation of phenol to benzenediol with H2O2.Xia et al.[117] found the selectivity of hydroquinone can be significantly increased by introducing acidic–alkaline sites of MgO–Al2O3into the hollow TS zeolite,they believed the steric hindrance of hollow TS zeolite,the synergistic effect of acid–alkaline sites,and the tetrahedral structure of Ti in the hollow TS zeolite affect the selectivity of hydroquinone and catechol.Furthermore,Xia et al.[118] also studied the irreversible deactivation of hollow TS zeolite.With the treatment of secondary hydrothermal crystallization to embed the acidic Ti–containing particles into the crystal,the performance of deactivated TS catalyst was recovered.Industrial data shows phenol conversion catalyzed by hollow TS zeolite is 20%,much higher than 12% by using conventional TS zeolite.

Due to the high cost of TS zeolite,developing new catalysts with good performance and low cost and new technology with high efficiency and greenness are still the focus in the production of phenol and benzenediol.Supported metal catalysts containing Fe,Cu,or V,heteropolyacid catalysts,and molecular sieves are the research hotspots in this field.Recently,Hai et al.[119] summarized relevant research progress on catalysts in this field.

5.Conclusions

China successfully developed its own slurry–bed technology of H2O2production.As a result,the implementation cost of many green hydrocarbon nitridation or oxidation technologies related with H2O2are reduced.A variety of green hydrocarbon nitridation or oxidation technologies such as caprolactam,epoxypropane and benzenediol have been put into industrialization.However,the cost of H2O2is still high for many green hydrocarbon nitridation or oxidation technologies.More inexpensive H2O2production technology still has huge market demand.Hydrocarbon nitridation or oxidation with H2O2in–situ production or more efficient oxidation technology with air or O2is urgently needed.In addition,green hydrocarbon nitridation or oxidation technologies with H2O2usually use TS zeolite as catalyst,its high price also increased the technology implementation cost.Therefore,high–efficiency and low–cost earth–enriched metal catalysts in green hydrocarbon nitridation or oxidation using H2O2as oxidant should attract more attention.

Furthermore,though China has developed some green hydrocarbon nitridation or oxidation technologies with independent intellectual properties,the gap of chemical industry in China with developed countries still exists.Most of basic organic chemicals production technologies still cause serious environmental pollutions.Some key technologies and core materials still face the transferring barrier set by foreign companies,which greatly restrict the technology upgrading and green transformation of chemical industry in China.

To support the green transformation and improve the development quality of the chemical industry in China,the original innovation should be further promoted to support the development of the green production technologies.The channel between scientific research and industry should be unblocked.Universities and research institutes should be united to carry out basic research on new green reaction paths,while enterprises and engineering design institutes should make joint efforts to solve the amplification and engineering problems.In this way,it is promising to efficiently integrate the green chemical reactions into practical production technologies,and then satisfy the great demand of China.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China(U19B6002)and National Key Research and Development Program of China (2016YFB0301600).