Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

This paper presents a not previously reported catalytic system consisting of transition metals Co2+ and Mn2+ and alkyl nitrites R-ONO for the oxidation of cyclohexanone with oxygen to adipic acid. The influence of type and amount of catalyst, temperature, time, and type of raw material on conversion and product composition were determined. In addition, the oxidation of selected cyclic ketones such as cyclopentanone, cyclohexanone, cyclooctanone, cyclododecanone, 2-methylcyclohexanone, 3-methylcyclohexanone, and 4-methylcyclohexanone in acetic acid as solvent was performed. The results showed that R-ONO systems, under established reaction conditions, form NO·radicals, which oxidize to NO2 under a strong oxidization reaction environment. The Co2+/Mn2+/NO2 system was shown to be highly active in the oxidation of cyclic ketones with oxygen.


Introduction
Aliphatic dicarboxylic acids are used, inter alia, in the production of polyamides, plasticizers, and polyesters.Dicarboxylic acids such as adipic acid (AA), glutaric acid (GA), cork, and 1,12-dodecanedioic acid are mainly produced via the oxidation of cyclic ketones and/or alcohols with HNO 3 [1].1,10-decanedioic acid is obtained by esterification of AA, followed by electrolysis and hydrolysis [2].The methods of dicarboxylic acids production through oxidative cleavage of unsaturated carboxylic acids or hydrogenation of unsaturated diacids are also practiced on industrial scale [3].
Undoubtedly, in industry, AA is considered the most important dicarboxylic acid.In 2014, the AA market was valued at USD 6.5 billion [4], and world production was 2.7 million tons.It has been industrially manufactured since the 1940s via Du Pont technology from cyclohexane [5].In the first stage of this method, cyclohexane is oxidized to a mixture of cyclohexanol (C-OL) and cyclohexanone (C-ON) with air and in the presence of 0.01-1 ppm Co 2+ /Fe 2+ (conversion 4-8%, selectivity 70-80%, 0.5-2.0MPa, 140-180 • C) [6].In the next stage, the obtained mixture is converted to AA via oxidation with HNO 3 (Figure 1) [7,8].
One of the by-products of this process is N 2 O, which is difficult to manage and utilize because it is a powerful greenhouse gas, approx.300 times stronger than CO 2 , with a half-life of approx.120 years [9].As a result of AA production, about 290-310 kg of N 2 O/t AA is obtained [10].
Currently, it is desirable to develop an ecological and economically viable method for the production of dicarboxylic acids [11] that meets specific requirements of sustainable development and principles of green chemistry (high conversion, selectivity, heterogeneous catalysis, use of renewable raw materials [12][13][14][15][16], and elimination of solvents).However, designing technologies that adhere to such restrictions is extremely difficult.As shown previously [17], AA production from biomass is associated with higher greenhouse gas emissions and higher energy consumption compared to traditional processes using benzene as a petrochemical raw material.Therefore, the replacement of the non-ecological oxidizing agent HNO 3 is the most important issue in dicarboxylic acid production.One of the by-products of this process is N2O, which is difficult to manage and utilize because it is a powerful greenhouse gas, approx.300 times stronger than CO2, with a halflife of approx.120 years [9].As a result of AA production, about 290-310 kg of N2O/tAA is obtained [10].
Currently, it is desirable to develop an ecological and economically viable method for the production of dicarboxylic acids [11] that meets specific requirements of sustainable development and principles of green chemistry (high conversion, selectivity, heterogeneous catalysis, use of renewable raw materials [12][13][14][15][16], and elimination of solvents).However, designing technologies that adhere to such restrictions is extremely difficult.As shown previously [17], AA production from biomass is associated with higher greenhouse gas emissions and higher energy consumption compared to traditional processes using benzene as a petrochemical raw material.Therefore, the replacement of the non-ecological oxidizing agent HNO3 is the most important issue in dicarboxylic acid production.
It is of particular interest to replace HNO3 in the oxidation of C-ON and/or C-OL to AA with environmentally friendly oxidizing agents such as oxygen (O2), air, or H2O2, which eliminates problems related to the management or utilization of the generated NOx [38].The use of environmentally friendly oxidizing agents requires the development of catalytic systems capable of obtaining AA with high selectivity.The first patents related to C-ON oxidation under air to AA in the presence of transition metal salts appeared in the 1930s and 1940s [39][40][41][42][43]. Table 1 shows examples of catalysts used in the oxidation of C-ON to AA with O2 or air.The literature has described oxidation processes of cyclohexane directly to AA, with oxygen/air, O 3 [18] , and HNO 3 [19], with the main focus being the use of oxygen/air as the oxidizing agent.The reaction is mainly carried out in acetic acid (AcOH) as a solvent and in the presence of transition metal salts or complexes [20].In order to initiate the process, small amounts of cyclohexanone (C-ON) are introduced [21,22].Additionally, reports have utilized oxidative organocatalysts such as N-hydroxyphthalimide (NHPI) [23][24][25][26][27][28][29] and but-2-one (MEK) [30].Importantly, the oxidation of cyclohexane to AA in the absence of solvent has been studied in the presence of heterogeneous catalysts such as Fe(III)AlPO-31 [31], NHPI/Fe(BTC) [32], Fe(III)T(O-Cl)PP [33] , and salts as well as complexes of transition metals in combination with a lipophilic 4-dodecyloxycarbonyl-N-hydroxyphthalimide (C 12 -NHPI) derivative [34].Oxidation of cyclohexane has been also achieved using a AcOH/H 2 O mixture in the presence of Co/Mn [35] and Mn(III)T(p-Cl)PP [36] clusters in PhCOOH/H 2 O [36] and AcOH/scCO 2 [37].
It is of particular interest to replace HNO 3 in the oxidation of C-ON and/or C-OL to AA with environmentally friendly oxidizing agents such as oxygen (O 2 ), air, or H 2 O 2 , which eliminates problems related to the management or utilization of the generated NO x [38].The use of environmentally friendly oxidizing agents requires the development of catalytic systems capable of obtaining AA with high selectivity.The first patents related to C-ON oxidation under air to AA in the presence of transition metal salts appeared in the 1930s and 1940s [39][40][41][42][43]. Table 1 shows examples of catalysts used in the oxidation of C-ON to AA with O 2 or air.The process of oxidation of C-ON to AA in the presence of O 2 or air was examined previously [53].The oxidation reaction was carried using transition metals, mainly Co 2+ and Mn 3+ acetates or acetylacetonates and para-toluenesulfonic acid (p-TS).The researchers found that the presence of p-TS improved the selectivity of the reaction.For example, when C-ON oxidation was conducted in AcOH (1:7.3 v:v) in the presence of H 2 O and using the Co 2+ /Mn 2+ system with p-TS, AA was obtained with 78% selectivity and 99.3% C-ON conversion.The reaction conditions were 5 h reaction time, 1.21 MPa pressure, at 70 • C using a mixture of O 2 (4.9 l/h) and N 2 (48.1 l/h).Additionally, high AA yields have been obtained via oxidation of C-ON with O 2 in the presence of Mn(NO 3 ) 2 /Co(NO 3 ) 2 [54].
In this study, the influence of alkyl nitrites on the oxidation of cyclic ketones to dicarboxylic acids was demonstrated for the first time.The beneficial effect of the addition of nitrites such as pentyl-(IP), tert-butyl-(TBN), and isopentyl (IPN) nitrite in a system with Co 2+ and Mn 2+ on AA selectivity and C-ON conversion was described.The obtained results show that R-ONO systems form NO• radicals, which oxidize to NO 2 in a strongly oxidizing environment.The proposed Co 2+ /Mn 2+ /NO 2 system provided high activity in the oxidation of cyclic ketones with O 2 .

General Procedure for Catalytic Oxidation under Pressure in 100 mL Volume
The 100 mL oxidation process was carried out in an Autoclave Engineers Inc. pressure reactor, (Erie, PA, USA), made of Hastelloy C-276 steel and equipped with a high-speed stirrer, heating jacket, internal cooler, and reflux condenser.In a typical process, 2 mL of substrate, 20 mL of solvent, and the catalytic system were introduced into the reactor.The reactor contents were purged with O 2 at 2 l/h for 2 min, then heated to 40-100 • C and stirred at 1000 rpm.Subsequently, O 2 was introduced into the reactor under pressure 0.1-1.5 MPa, and the oxidation process was started.The pressure, the stirring speed and temperature were monitored using a Sentinel control device.During the process, the pressure decreased due to O 2 consumption and was supplemented with additional O 2 .

General Procedure for Catalytic Oxidation under Pressure in 600 mL Volume
The oxidation process on 600 mL scale was carried out in a PARR pressure reactor (Moline, IL, USA) made, of Hastelloy C-276 steel, equipped with a high-speed stirrer, heating oil jacket, internal cooler, and reflux condenser.Air was introduced into the reactor through a bubble placed under the agitator.In a typical process, 30 mL of C-ON, 300 mL of AcOH, and a catalytic system were introduced into the reactor.The reactor contents were purged with air or O 2 at 50 l/h for 2 min, then heated to 60 • C and stirred at 600 rpm.Subsequently, the oxidizing agent was introduced into the reactor at a pressure of 0.5 MPa, the stirring speed was increased to 1200 rpm, the oxidizing agent blow-through was set to 50 l/h, and the oxidation process was started.The pressure, stirring speed and temperature were monitored using a PARR control device.During the oxidation process, the amount of oxidizing agent and gases introduced and oxygen concentration in the gases were monitored.This information was used to calculate the amount of consumed oxygen.

General Procedure for Catalytic Oxidation under Atmospheric Pressure
The oxidation process on 120 mL scale was carried out in a thermostatic bubble reactor made of glass and equipped with a G5 glass bubble and reflux condenser.Air was introduced into the reactor through a bubble placed at the bottom of the reactor.
In a typical process, 8 mL of C-ON, 80 mL of AcOH, and a catalytic system were introduced into the reactor.The reactor contents were heated to 60 • C. Subsequently, the oxidizing agent (air) was added into the reactor at a flow of 12 l/h.During the oxidation process, the amount of oxidizing agent added introduced to the reactor and the amount of oxygen in the off-gas were monitored.

Analytical Methods
The reaction products were analyzed using an Agillent 5890 Series II gas chromatograph equipped with an FID detector, Zebron ZB-5HT column (30 m × 0.25 mm × 0.25 µm) and automatic sample dispenser.Helium was used as the carrier gas.The analysis was performed using the standard method with an internal standard (toluene) (injection port temperature 200 • C, detector temperature 300 • C, split 100:1, injection 1 µL, air 400 mL/min, nitrogen 24 mL/min, hydrogen 30 mL/min, temperature program 70 • C for 10 min, 6 • C/min to 112 • C, 20 • C/min to 212 • C, 8 min at 212 • C).Each sample was analyzed twice, and the substance concentration was calculated on the basis of previously prepared standard curves.The products composition was additionally confirmed by gas chromatography coupled to mass spectrometry (GC-MS) performed using an Agilent gas chromatograph 7890C (Agilent HP-5 MS capillary column, 30 m × 0.25 mm × 0.25 µm, helium 1 mL/min) coupled with an Agilent mass spectrometer 5975C with EI ionization (70 eV) using the NIST/EPA/NIH Mass Spectral Library.
Raw material conversion: First, 1 mL of sample was taken from the reaction products, and 5 mL of toluene in AcOH solution (6 g of toluene in 250 mL of acetic acid) was added.The prepared solution was analyzed by GC.
Content of dicarboxylic acids: Next, 1 mL of sample was taken from the reaction products, and 12 mL of a toluene/methanol solution (12 g of toluene in 1000 mL of methanol) and 1-3 drops of sulfuric acid (VI) were added.The solution was stirred (200 rpm) for 24 h at ambient temperature to form the methanol esters from carboxylic acids.The prepared solution was analyzed by GC, which determined the amount of dimethyl esters of dicarboxylic acids.
It was found that the error resulting from reproducibility and repeatability amounted to 2%.

Oxidation of C-ON with O 2 to AA Using the Co 2+ /Mn 2+ /R-ONO System
Research was carried out on the oxidation reactions of C-ON to AA using alkyl nitrites R-ONO in the Co 2+ /Mn 2+ system and, in polar solvents (AcOH), acetonitrile (MeCN) or benzonitrile (PhCN)).IPN, amyl nitrite (PN), and TBN were used in the research.A reaction was performed using HNO 3 for comparison.In the reaction products, the content of C-ON (AA) and the main by-product (GA) was determined.The reaction was carried out at 50-120 • C, for 2-6 h, under pressure of 0.5-1.5 MPa O 2 (Figure 2).The results of the various experiments are presented in Table 2.
prepared solution was analyzed by GC, which determined the amount of dimeth of dicarboxylic acids.
It was found that the error resulting from reproducibility and repeatability am to 2%.

Oxidation of C-ON with O2 to AA Using the Co 2+ /Mn 2+ /R-ONO System
Research was carried out on the oxidation reactions of C-ON to AA using trites R-ONO in the Co 2+ /Mn 2+ system and, in polar solvents (AcOH), acetonitrile or benzonitrile (PhCN)).IPN, amyl nitrite (PN), and TBN were used in the res reaction was performed using HNO3 for comparison.In the reaction products, the of C-ON (AA) and the main by-product (GA) was determined.The reaction was out at 50-120 °C, for 2-6 h, under pressure of 0.5-1.5 MPa O2 (Figure 2).The resu various experiments are presented in Table 2.The results show that under the tested conditions (60 °C, 0.5 MPa O2), the ad each R-ONO increased C-ON conversion.Addition of IPN and TBN increased A tivity (entries 5, 7), where the IPN/Co 2+ /Mn 2+ system gave 62% selectivity and 97 The results show that under the tested conditions (60 • C, 0.5 MPa O 2 ), the addition of each R-ONO increased C-ON conversion.Addition of IPN and TBN increased AA selectivity (entries 5, 7), where the IPN/Co 2+ /Mn 2+ system gave 62% selectivity and 97% C-ON conversion (entry 5).In the case of the Co 2+ /Mn 2+ system, AA was obtained with 42% selectivity and 68% C-ON conversion (entry 1).
Interestingly, the C-ON oxidation process occurred even under mild reaction conditions (60 • C) and catalyzed solely with IPN (entry 2).This highlights IPN high activity as an organocatalyst.To our knowledge, this is the first example of oxidation of cyclic ketones without the participation of metal catalysts.Examination of Co 2+ and Mn 2+ compounds revealed no significant differences in reaction outcome (entries 9, 10).Furthermore, addition of HNO 3 increased C-ON conversion and AA selectivity (entry 8).However, the IPN/Co 2+ /Mn 2+ system provided AA in higher selectivity.The observed variation in selectivity may stem from NO 2 generation from R-ONO and HNO 3 , where R-ONO at 60 • C decomposes to R-O• and NO•.However, under strongly oxidizing conditions, NO• undergoes spontaneous oxidation to NO 2 , which contributes to the oxidation reaction.In the case of HNO 3 , an equilibrium forms between NO 2 , O 2 , and H 2 O, and under high pressure (0.5 MPa), this equilibrium shifts towards HNO 3 , which results in lower activity of the HNO 3 /Co 2+ /Mn 2+ system.In the case of the IPN/Co(acac) 2 /Mn(acac) 2 catalytic system, which generates AA in the highest selectivity, the influence of basic parameters on the composition of C-ON oxidation reaction products was determined.Figure 3 shows the influence of IPN amount (1-20 mol%) on C-ON conversion and AA and GA selectivity.For comparison, the reaction without IPN addition was conducted.
60 °C decomposes to R-O⸱ and NO•.However, under strongly oxidizing conditions, NO• undergoes spontaneous oxidation to NO2, which contributes to the oxidation reaction.In the case of HNO3, an equilibrium forms between NO2, O2, and H2O, and under high pressure (0.5 MPa), this equilibrium shifts towards HNO3, which results in lower activity of the HNO3/Co 2+ /Mn 2+ system.
In the case of the IPN/Co(acac)2/Mn(acac)2 catalytic system, which generates AA in the highest selectivity, the influence of basic parameters on the composition of C-ON oxidation reaction products was determined.Figure 3 shows the influence of IPN amount (1-20 mol%) on C-ON conversion and AA and GA selectivity.For comparison, the reaction without IPN addition was conducted.The addition of 1 mol% of IPN increased C-ON conversion from 68% to 91% and AA selectivity from 42% to 59%.When the amount of IPN increased to 5 mol%, AA was obtained in 62% selectivity and 96% conversion.Any further increase in IPN amount (20 mol%) had no significant affect the composition of the reaction products.
Table 3 lists the experimental results examining the influence of temperature, O2 pressure and the type of solvent on the composition of the products of C-ON oxidation to AA in the IPN/Co 2+ /Mn 2+ system (Table 3).The addition of 1 mol% of IPN increased C-ON conversion from 68% to 91% and AA selectivity from 42% to 59%.When the amount of IPN increased to 5 mol%, AA was obtained in 62% selectivity and 96% conversion.Any further increase in IPN amount (20 mol%) had no significant affect the composition of the reaction products.
Table 3 lists the experimental results examining the influence of temperature, O 2 pressure and the type of solvent on the composition of the products of C-ON oxidation to AA in the IPN/Co 2+ /Mn 2+ system (Table 3).The obtained results showed that as the temperature increased from 40 to 80 • C (entries 1-4), C-ON conversion increased from 52% to 100% and AA selectivity from 31% to 68%.However, when the temperature increased to 100 • C, AA selectivity deceased (entry 5).At higher temperature, the share of the oxidation reaction to by-products such as lower carboxylic acids was greater, as evidenced by the greater selectivity of the GA obtained.The oxidation was carried out at 60 • C for the first 1 h and at 80 • C for the second h (entry 6).This resulted in similar C-ON conversion and AA selectivity to the reaction conducted at 80 • C (entry 4).Increasing O 2 pressure above 0.1 MPa increased the rate of C-ON oxidation reaction; however, as expected, it did not significantly affect AA selectivity (entries 3, 7-9).According to the results, the role of O 2 was more relevant when considering air or O 2 -enriched air oxidation.Elimination of solvent from C-ON oxidation or oxidation conducted in acetonitrile decreased C-ON conversion and significantly decreased AA selectivity (entries 10-12).C-ON oxidation conducted in PhCN gave a conversion of 99% and 64% AA selectivity (entry 13).This approach can be advantageous for industry due to the reduced effect of the reaction medium on the corrosive effect compared to oxidation in AcOH.Hence, the use of PhCN in an industrial process is more desirable due to less corrosion of equipment.
Figure 4 shows the effect of time on C-ON conversion and AA and GA selectivity (1-6 h).
5).At higher temperature, the share of the oxidation reaction to by-products such as lower carboxylic acids was greater, as evidenced by the greater selectivity of the GA obtained.
The oxidation was carried out at 60 °C for the first 1 h and at 80 °C for the second h (entry 6).This resulted in similar C-ON conversion and AA selectivity to the reaction conducted at 80 °C (entry 4).Increasing O2 pressure above 0.1 MPa increased the rate of C-ON oxidation reaction; however, as expected, it did not significantly affect AA selectivity (entries 3, 7-9).According to the results, the role of O2 was more relevant when considering air or O2-enriched air oxidation.Elimination of solvent from C-ON oxidation or oxidation conducted in acetonitrile decreased C-ON conversion and significantly decreased AA selectivity (entries 10-12).C-ON oxidation conducted in PhCN gave a conversion of 99% and 64% AA selectivity (entry 13).This approach can be advantageous for industry due to the reduced effect of the reaction medium on the corrosive effect compared to oxidation in AcOH.Hence, the use of PhCN in an industrial process is more desirable due to less corrosion of equipment.
Figure 4 shows the effect of time on C-ON conversion and AA and GA selectivity (1-6 h).Under the reaction conditions stated in Figure 4, C-ON conversion was 100% after 3 h.It was found that increasing the reaction time from 1 to 4 h gradually increased AA selectivity from 49% to 71%.Further extension of the reaction time did not affect the composition of the products.

Oxidation of Cyclohexanol, C-ON, or their Mixtures with O2 using the IPN/Co(acac)2/Mn(acac)2 System
As a result of the industrial process of cyclohexane oxidation, a mixture of C-ON and C-OL was formed.Therefore, the possibility of using C-OL or a mixture of C-OL and C-ON in the AA preparation process was investigated in relation to the tested IPN/Co(acac)2/Mn(acac)2 system (Table 4).Under the reaction conditions stated in Figure 4, C-ON conversion was 100% after 3 h.It was found that increasing the reaction time from 1 to 4 h gradually increased AA selectivity from 49% to 71%.Further extension of the reaction time did not affect the composition of the products.

Oxidation of Cyclohexanol, C-ON, or Their Mixtures with O 2 Using the IPN/Co(acac) 2 /Mn(acac) 2 System
As a result of the industrial process of cyclohexane oxidation, a mixture of C-ON and C-OL was formed.Therefore, the possibility of using C-OL or a mixture of C-OL and C-ON in the AA preparation process was investigated in relation to the tested IPN/Co(acac) 2 /Mn(acac) 2 system (Table 4).On the basis of the above results, the main product of the C-OL oxidation reaction was C-ON, and AA was obtained in relatively low selectivity of 11% and 20% after 2 h and 6 h, respectively (entries 3, 4).Using 1:1 (w:w) mixture of C-ON and C-OL as the substrate, AA was obtained with 58% selectivity, with 100% conversion of raw materials (entry 5).Prolongation of the oxidation reaction to 6 h only gave a slight increase in AA selectivity (entry 6).It has been reported that the oxidation rate of cyclic alcohol is much lower than that of cyclic ketone, mainly due to the large amounts of C-ON in the mixture used to initiate the oxidation of C-OL to AA.The possibility of using the proposed catalytic system in the oxidation of a series of cyclic ketones (C 5 -C 12 ) in order to form the appropriate dicarboxylic acids (C 5 -C 12 ) was examined.The reactions were carried out at 60 • C and 80 • C in the presence and absence of 10 mol% IPN (Table 5).According to the results listed in Table 5, when the temperature was at 80 • C with the addition of 10 mol% IPN, an increase in selectivity was observed for all appropriate dicarboxylic acids.Additionally, the addition of 10 mol% IPN to the oxidation of cycloheptanone, cyclooctanone, cyclododecanone, 2-methylcyclohexanone, 3-methyl cyclohexanone, and 4-methylcyclohexanone increased conversion.

C-ON Oxidation with Air Using the IPN/Co(acac) 2 /Mn(acac) 2 System
Industrial oxidation processes using O 2 as an oxidizing agent are associated with a greater risk of explosion due to the formation of a wide range of explosive mixtures between the raw material, solvent, and O 2 compared to air.The oxidation reaction was carried out at a flow of 50 l/h using the IPN/Co 2+ /Mn 2+ system (Figure 5).Therefore, the oxidation process of C-ON with air and was compared to oxidation using O 2 .The results showed that under the tested reaction conditions, the type of the oxidizing agent used (air or O 2 ) had no significant influence on the composition of C-ON oxidation reaction products.However, when the reaction was conducted in O 2 , the rate of C-ON conversion and AA and GA formation was slightly higher after the first 2 h of the reaction.Differences in the reaction rate depended on the oxidizing agent used (air or O 2 ) and may also have depended on the design of the reactor and distribution system of the oxidizing agent in the liquid reaction mixture.
Figure 6 shows the proposed block diagram of the process for the production of appropriate dicarboxylic acids from cyclic ketones based on our optimized conditions.The figure includes sections for reaction, product isolation and purification, and solvent and catalyst recycling.It is known that the oxidation of cyclic ketones in the presence of O 2 is very exothermic.In the case of 330 mL scale in a 600 mL reactor, it was necessary to cool the reaction mixture in order to maintain a constant temperature.Therefore, a solution to this issue may be the use of a reaction system consisting of a cascade of two reactors.As a result, it was possible to carry out the oxidation at a lower rate in 1onereactor (60 • C), which facilitated the controlled heat removal.The calculated heats of oxidation reactions of C-ON to AA, GA, and SA are 748.9,1367.3, and 2026.9 kJ/mol, respectively.Taking into account the selectivity of respective reactions (AA = 70%, GA = 22%, SA = 8%) and a 100% conversion of C-ON, the total heat of the oxidation process can be estimated at 987.2 kJ/mol.Our research has shown that the purpose of the second reactor was to increase the selectivity of the reaction via oxygenation of the intermediates.It was therefore advantageous to use a higher temperature of about 80 • C.
The next steps were to separate IPN and part of the solvent (AcOH) from the reaction products.According to the results obtained for the oxidation of C-ON to AA, the concentration of the reaction products by evaporation to approx.80% AcOH enables separation of AA with 80% efficiency.Upon reduction of the temperature, a precipitate was formed mainly composed of AA and impurities such as glutaric and succinic acid.AA was further purification by crystallization from AcOH, water, or a mixture thereof.Depending on the type of dicarboxylic acid employed, it was necessary to evaporate different volumes of AcOH due to differences in solubility.The last steps involved the separation and recycling of the Co 2+ and Mn 2+ catalyst and the regeneration of residual AcOH.The current literature has suggested that the addition of alkyl nitrites to the oxidation reaction of cyclic ketones increases the reaction rate, which allows for higher conversions and selectivity at lower temperatures.Figure 7 shows the proposed mechanism of the oxidation of cyclic ketones using the IPN/Co 2+ /Mn 2+ system.a higher temperature of about 80 °C.
The next steps were to separate IPN and part of the solvent (AcOH) from the reaction products.According to the results obtained for the oxidation of C-ON to AA, the concentration of the reaction products by evaporation to approx.80% AcOH enables separation of AA with 80% efficiency.Upon reduction of the temperature, a precipitate was formed mainly composed of AA and impurities such as glutaric and succinic acid.AA was further purification by crystallization from AcOH, water, or a mixture thereof.Depending on the type of dicarboxylic acid employed, it was necessary to evaporate different volumes of AcOH due to differences in solubility.The last steps involved the separation and recycling of the Co 2+ and Mn 2+ catalyst and the regeneration of residual AcOH.The current literature has suggested that the addition of alkyl nitrites to the oxidation reaction of cyclic ketones increases the reaction rate, which allows for higher conversions and selectivity at lower temperatures.Figure 7 shows the proposed mechanism of the oxidation of cyclic ketones using the IPN/Co 2+ /Mn 2+ system.We propose that during the reaction, due to temperature, IPN decomposed to isopentyl radical and NO•.NO• radical was then oxidized to NO2 under O2, which initiated the oxidation of C-ON to form 2-carbonylcyclohexyl radical and HNO2.Then, as a result of oxidation, reduction, and disproportionation, the isopentyl radical transformed into IPN [70].The exact course of AA formation from 2-carbonylcyclohexyl radical was presented in our previous paper [21].
We confirmed the NO2 catalytic activity by carrying out C-ON oxidation in AcOH, 0.2 mol% Mn(acac)2, 0.2 mol% Co(acac)2, and 10 mol% NO2 (60 °C, air 12 l/h).NO2 gas produced in a separate reaction vessel by dropping HNO3 into Cu was redirected to the We propose that during the reaction, due to temperature, IPN decomposed to isopentyl radical and NO•.NO• radical was then oxidized to NO 2 under O 2 , which initiated the oxidation of C-ON to form 2-carbonylcyclohexyl radical and HNO 2 .Then, as a result of oxidation, reduction, and disproportionation, the isopentyl radical transformed into IPN [70].The exact course of AA formation from 2-carbonylcyclohexyl radical was presented in our previous paper [21].
We confirmed the NO 2 catalytic activity by carrying out C-ON oxidation in AcOH, 0.2 mol% Mn(acac) 2 , 0.2 mol% Co(acac) 2 , and 10 mol% NO 2 (60 • C, air 12 l/h).NO 2 gas produced in a separate reaction vessel by dropping HNO 3 into Cu was redirected to the oxidation reactor along with air.After the brown NO 2 was dissolved in the reaction mixture, a color change was observed from yellow to dark brown as well as O 2 consumption, which was monitored by measuring the O 2 concentration in the gases exiting the reactor.The results showed that the O 2 concentration dropped from 21% to 10% in just 5 min.The reaction continued for a further 2 h.
The water present in the reaction system reacted with NO 2 to form HNO 3 and HNO 2 .The produced HNO 2 molecule completed the catalytic cycle, and the presence of HNO 3 (Table 2, entry 10) influenced the oxidation process.The catalytic activity of HNO 3 in the reaction system was largely related to the pressure.As described earlier, HNO 3 formed an equilibrium with NO 2 , O 2 , and H 2 O.Under increased pressure (0.5 MPa), the equilibrium shifted towards HNO 3 , which resulted in lower activity of the HNO 3 /Co 2+ /Mn 2+ system.However, at atmospheric pressure, the activity of the HNO 3 /Co 2+ /Mn 2+ system was significantly higher (Figure 8).
(Table 2, entry 10) influenced the oxidation process.The catalytic activity of HNO3 in th reaction system was largely related to the pressure.As described earlier, HNO3 formed an equilibrium with NO2, O2, and H2O.Under increased pressure (0.5 MPa), the equilib rium shifted towards HNO3, which resulted in lower activity of the HNO3/Co 2+ /Mn 2+ sys tem.However, at atmospheric pressure, the activity of the HNO3/Co 2+ /Mn 2+ system wa significantly higher (Figure 8).Each of the selected catalytic systems was highly active in the C-ON oxidation pro cess.The rate of C-ON oxidation in the presence of HNO3 was initially faster in compari son to IPN, PN, and TBN.However, it was reduced after 1h.As a result, after 4 h, C-ON conversion was similar in all cases, ranging from 91-92%.After 6 h of reaction, 98% con version was obtained for all tested catalytic systems.
GC/MS analysis was utilized to confirm the negligible amounts of isopentyl alcoho in the reaction products, which may prove the detachment of the hydrogen atom from th molecule, e.g., C-ON, by the isopentyl radical (Figure 9).A similar mechanism of the cat alytic effect of IPN was proposed by researchers when examining the oxidation of cyclo hexane directly to AA 55 .The isopentyl radical removes the hydrogen atom from cyclohex ane, producing cyclohexyl radical and isopentyl alcohol, which, as a result of slow con version, generates IPN via the reaction with HNO2.Interestingly, only small amounts o isopentyl alcohol were observed in our reaction products even when they were conducted using 20 mol% IPN.Therefore, only slight conversion of IPN to alcohol occurs under th proposed reaction conditions.Each of the selected catalytic systems was highly active in the C-ON oxidation process.The rate of C-ON oxidation in the presence of HNO 3 was initially faster in comparison to IPN, PN, and TBN.However, it was reduced after 1h.As a result, after 4 h, C-ON conversion was similar in all cases, ranging from 91-92%.After 6 h of reaction, 98% conversion was obtained for all tested catalytic systems.
GC/MS analysis was utilized to confirm the negligible amounts of isopentyl alcohol in the reaction products, which may prove the detachment of the hydrogen atom from the molecule, e.g., C-ON, by the isopentyl radical (Figure 9).A similar mechanism of the catalytic effect of IPN was proposed by researchers when examining the oxidation of cyclohexane directly to AA 55 .The isopentyl radical removes the hydrogen atom from cyclohexane, producing cyclohexyl radical and isopentyl alcohol, which, as a result of slow conversion, generates IPN via the reaction with HNO 2 .Interestingly, only small amounts of isopentyl alcohol were observed in our reaction products even when they were conducted using 20 mol% IPN.Therefore, only slight conversion of IPN to alcohol occurs under the proposed reaction conditions.Additionally, it may be also possible to detach the hydrogen atom from C radical NO• to form 2-carbonylcyclohexyl radical and HNO.The presence of HN reaction system could promote N2O formation as a by-product.Reports have sho HNO can undergo dimerization to form N2O and H2O or react with the NO2 pr the system, which generates NO and HNO2.However, the possible reactions le N2O were insignificant due to the low probability of HNO formation in the highly ing reaction medium.Moreover, HNO dimerization was unlikely due to its trace a in the reaction system.Additionally, it may be also possible to detach the hydrogen atom from C-ON via radical NO• to form 2-carbonylcyclohexyl radical and HNO.The presence of HNO in the reaction system could promote N 2 O formation as a by-product.Reports have shown that HNO can undergo dimerization to form N 2 O and H 2 O or react with the NO 2 present in the system, which generates NO and HNO 2 .However, the possible reactions leading to N 2 O were insignificant due to the low probability of HNO formation in the highly oxidizing reaction medium.Moreover, HNO dimerization was unlikely due to its trace amounts in the reaction system.

Conclusions
Herein, we found a positive effect of the addition of IPN, TBN, PN, and HNO 3 towards the composition of the products resulting from C-ON oxidation to AA in air.In the presence of the IPN/Co 2+ /Mn 2 + catalytic system, AA was obtained in the highest selectivity.Additionally, the anion type in the salt as well as the Co 2+ and Mn 2+ complexes did not affect the composition of the reaction products.Through examination of selected parameters, it was revealed that the C-ON oxidation reaction for 4 h at 60 • C was satisfactory, producing AA with 71% selectivity and 100% C-ON conversion.A similar effect was observed when the reaction was conducted for 2 h at 60 • C and then for an additional 2 h at 80 • C.This approach was more advantageous for industrial processes due to dispersion of potentially hazardous energy.Shorter-chain acids, mainly GA and BA, were observed as the main by-products.
The developed catalytic system IPN/Co(acac) 2 /Mn(acac) 2 successfully increased the selectivity of dicarboxylic acids determined from cyclic ketones.The type of oxidizing agent used (air or O 2 ) did not affect the composition of the reaction products.However, on a larger scale, it was necessary to carry out the oxidation reaction in air due to safety reasons.The results indicate that the oxidation proceeds with a high energy effect, which requires a reactor with efficient cooling.
A block diagram was presented describing the preparation of dicarboxylic acids as a simple method requiring only a limited number of unit operations.The raw materials, solvents, and catalysts used were commercially available, and their prices were relatively low.The proposed method for the obtainment of AA from C-ON, using air as the oxidizing agent, was more environmentally friendly than other industrial methods, which employ HNO 3 as an oxidant.
It is highly probable that in the next 5-10 years, the use of HNO 3 as an oxidizing agent will no longer be necessary.The market report [71] for AA for 2020-2025 clearly emphasizes the expectations of stringent environmental regulations regarding the production process.These regulations are likely to inhibit market development and increase interest in technologies for the AA production, e.g., bio-based.It is estimated that new technologies will be implemented in the coming years.The chemical industry recognizes this threat and therefore conducts intensive research to improve the existing production methods or select new alternative production methods.
The proposed method for AA production is an interesting alternative to current industrial production methods based on HNO 3 .Certainly, the optimization of C-ON oxidation using the Co/Mn/IPN system can increase selectivity, which has economic benefits.Moreover, legal and environmental conditions related to the emission of greenhouse gases significantly influence the constantly growing price of AA on the world markets.Therefore, it is possible that current technologies for dicarboxylic acids formation may be considered unprofitable in the future and our method an interesting alternative to the traditional, non-ecological method.

Patents
The research results described in this article have become the subject of a Polish patent with the number PL 239347.

Figure 1 .
Figure 1.Industry method of AA production from cyclohexane.

Figure 6
Figure6shows the proposed block diagram of the process for the production of appropriate dicarboxylic acids from cyclic ketones based on our optimized conditions.The figure includes sections for reaction, product isolation and purification, and solvent and catalyst recycling.

Figure 6
Figure 6 shows the proposed block diagram of the process for the production o propriate dicarboxylic acids from cyclic ketones based on our optimized conditions figure includes sections for reaction, product isolation and purification, and solvent catalyst recycling.

Figure 6 .
Figure 6.Flowchart for the preparation of carboxylic acids from cyclic ketones.

3. 5 .
Proposed Mechanism for the Oxidation of C-ON with O 2 Using the IPN/Co(acac) 2 /Mn(acac) 2 System

Figure 7 .
Figure 7. Probable mechanism for the oxidation of C-ON to AA.

Figure 7 .
Figure 7. Probable mechanism for the oxidation of C-ON to AA.

Materials 2023 ,Figure 9 .
Figure 9. Removal of the hydrogen atom from C-ON via isopentyl radical.

Figure 9 .
Figure 9. Removal of the hydrogen atom from C-ON via isopentyl radical.

Table 1 .
Review of used catalysis and conditions in the oxidation of cyclohexanone to AA with O2 or air.

Table 1 .
Review of used catalysis and conditions in the oxidation of cyclohexanone to AA with O 2 or air.

Table 3 .
Influence of selected parameters on the composition of C-ON oxidation reaction products.

Table 3 .
Influence of selected parameters on the composition of C-ON oxidation reaction products.

Table 5 .
Oxidation of cyclic ketones to dicarboxylic acids.