Selective aerobic oxidations using carbon nitride nanotubes

Abstract

The present invention discloses an improved oxidation process using carbon nitride nanotubes as metal free catalyst and molecular O2 as the oxidant to obtain desired adipic acid and other oxygenated hydrocarbons with improved conversion and selectivity.

Claims

1. A single step and metal free oxidation process for the preparation of an oxygenated hydrocarbon which comprises reacting a hydrocarbon substrate with molecular O.sub.2 in presence of C.sub.3N.sub.4 (carbon nitride), nanotubes catalyst and a solvent.

2. The oxidation process as claimed in claim 1, wherein the process is carried out at temperature 100-140 C.

3. The oxidation process as claimed in claim 1, wherein 25-100 mg carbon nitride nanotubes catalyst was used for 0.15 mole of substrate.

4. The oxidation process as claimed in claim 1, wherein the solvent s selected form acetonitrile and acetone.

5. The oxidation process as claimed in claim 1, wherein the oxygenated hydrocarbon is selected from the group consisting of acids, ketones, aldehydes, and lactones.

6. The oxidation process as claimed in claim 1, wherein the oxygenated hydrocarbon is adipic acid and the hydrocarbon substrate is cyclohexane, cyclohexanone, cyclohexanol, or a combination of cyclohexanone and cyclohexanol.

7. The oxidation process as claimed in claim 1, wherein the oxygenated hydrocarbon is 2-hexanone and the hydrocarbon substrate is n-hexane.

8. The oxidation process as claimed in claim 1, wherein the oxygenated hydrocarbon is caprolactone and the hydrocarbon substrate is cyclohexanone in the presence of benzaldehyde.

9. The oxidation process as claimed in claim 5, wherein selectivity of acids, ketones and lactones is in the range of 10-90%.

10. The oxidation process as claimed in claim 1, wherein the hydrocarbon substrate is n-hexane, cyclohexanone, cyclohexanol, and cyclohexane and conversion of the substrate is in the range of 10-70%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts XRD pattern carbon nitride nano tubes.

(2) FIG. 2 depicts XRD spectra of (a) melamine, (b) nano fiber and (c) nano tube.

(3) FIG. 3 depicts Infrared spectra of (a) melamine, (b) nano fiber and (c) nano tube.

(4) FIG. 4 depicts SEM images of carbon nitride nanotubes.

(5) FIG. 5 depicts TEM images of carbon nitride (a & b) and carbon nano fiber (c).

(6) FIG. 6 depicts XPS of nitrogen 1 s carbon nitride.

(7) FIG. 7 depicts UV-Vis spectra of carbon nanotube.

(8) FIG. 8 depicts PI spectra of carbon nanotube.

(9) FIG. 9 depicts TGA plot of melamine nano fibers.

(10) FIG. 10 depicts Temperature programmed desorption of CO.sub.2.

(11) FIG. 11 depicts Cyclic voltametric study (in KOH) of the carbon nitride (a); linear sweep cyclic voltammetry.

(12) FIG. 12 depicts CV of catalyst in 0.5M HClO.sub.4 solution (a); Linear sweep cyclic voltammetry.

(13) FIG. 13 depicts Effect of temperature on adipic acid yield during CH oxidation. Conditions: Acetonitrile solvent 6.1 g, catalyst=50 mg, 4 h, 20 bar O.sub.2, cyclohexane 12.6 g

(14) FIG. 14 depicts Effect of reaction time on adipic acid yield. Conditions: Cyclohexane=12.6 g, Acetonitrile=6.1 g, 130 C., Catalyst=50 mg, 20 bar O.sub.2.

(15) FIG. 15 depicts Effect of catalyst content on adipic acid yield. Conditions: Cyclohexane=12.6 g, Acetonitrile=6.1 g, 130 C., 4 h, 20 bar O.sub.2.

(16) FIG. 16 depicts Effect of substrate to solvent mole ratio on AA yield. Conditions: Cyclohexane, Acetonitrile, catalyst=50 mg, 130 C., 4 h, 20 bar O.sub.2

(17) FIG. 17 depicts Effect of O.sub.2 pressure on AA yield. Conditions: Cyclohexane=12.6 g, Acetonitrile=6.1 g, Catalyst=50 mg, 130 C., 4 h,

(18) FIG. 18 depicts Recyclability study of the catalyst in CH oxidation. Conditions: Cyclohexane=12.6 g, Acetonitrile=6.1 g, catalyst=50 mg, 130 C., 4 h, 20 bar O.sub.2.

(19) FIG. 19 depicts Effect of reaction temperature on CH conversion and AA yield.

(20) Conditions: Cyclohexane=15.58 g, 4 h, Catalyst=50 mg, 20 bar O.sub.2

(21) FIG. 20 depicts Effect of reaction time on CH conversion and AA yield. Conditions: Cyclohexane=15.58 g, 130 C., Catalyst=50 mg, 20 bar O.sub.2

(22) FIG. 21 depicts Effect of temperature on cyclohexanone oxidation.

(23) Conditions: cyclohexanone 9 g, Acetonitrile 7 g, water=2 g, 4 h, CNNT catalyst 50 mg, 20 bar O.sub.2.

(24) FIG. 22 depicts Effect of time on selective cyclohexanone. Conditions: 9 g cyclohexanone, Acetonitrile 7 g, 120 C., CNNT 50 mg, 20 bar O.sub.2.

(25) FIG. 23 depicts Effect of time on the yield of Caprolactone. Conditions: 10 mmol cyclohexanone, 20 mmol benzaldehyde, 50 mg Catalyst, 50 C., 10bar O.sub.2.

DETAILED DESCRIPTION OF THE INVENTION

(26) The present invention provides an improved oxidation process for the preparation of acids, ketones and lactones with improved conversion and selectivity which comprises reacting the substrate with molecular O.sub.2 in presence of catalytic amount carbon nitride nanotubes and a solvent.

(27) The present invention provides improved oxidation process for the preparation of acids, ketones and lactones with improved conversion and selectivity wherein the acids, ketones and lactones are selected from adipic acid, caprolactone and 2-hexanone.

(28) The present invention provides an improved process for the preparation of adipic acid from cyclohexane using carbon nitride nanotubes (CNNT) as metal free catalyst and molecular O.sub.2 as the oxidant.

(29) The improved process for the preparation of adipic acid from cyclohexane using carbon nitride nanotubes (CNNT) provide better selectivity 72.4% and conversion up to 69.7%.

(30) The present invention provides an improved process for the preparation of adipic acid from cyclohexanone using carbon nitride nanotubes (CNNT) as metal free catalyst and molecular O.sub.2 as the oxidant. The process is same as followed in cyclohexane but the temperature required to get maximum yield of adipic acid is less. The present invention provides an improved process for the preparation of 2-hexanone from n-hexane using carbon nitride nanotubes (CNNT) as metal free catalyst and molecular O.sub.2 as the oxidant.

(31) The improved process for the preparation of 2-hexanone from n-hexane using carbon nitride nanotubes (CNNT) shows 44.3% conversion of n-hexane and 24.9% selectivity at reaction time 8 hrs.

(32) The present invention provides an improved process for the preparation of caprolactone from cyclohexanone using carbon nitride nanotubes (CNNT) as metal free catalyst and molecular O.sub.2 as the oxidant.

(33) The improved process for the preparation of caprolactone from cyclohexanone using carbon nitride nanotubes (CNNT) shows 98% selectivity and conversion 62.8% in conversion of cyclohexanone at reaction time 10 hrs.

(34) The following examples are given by way of illustration of the working if the invention is actual practice and shall not be construed to limit the scope of the present invention in anyway.

EXAMPLES

Example 1

Preparation of Catalyst

(35) Melamine (0.9068 g) was dissolved in ethylene glycol (40 ml) to obtain a saturated solution at 30 C. temperature. To this, aqueous nitric acid (120 ml of 0.12 M) was added drop wise to get white precipitate. This was washed by ethanol to remove residual nitric acid and ethylene glycol. Subsequently, the product was dried at 60 C. for 6 h and calcined at 350 C. for 3 h in air.

(36) Characterization of Catalyst

(37) XRD

(38) The structural aspects of the catalysts were investigated by powder X-ray diffraction. FIG. 1 shows two peaks. The most intense peak at around 27.2 corresponds to interlayer distance d=0.336 nm, close to the characteristic peak of the (002) plane in the g-C.sub.3N.sub.4 structure (d=0.336 nm) which is reported for graphite-like carbon nitride (d=0.321, or 0.328 nm). The peak characteristic to in-plane structural packing motif of the nanotube appears at 17.8 (d=0.49 nm), close to the theoretical d=0.47 nm. So it is evident from XRD that the present nanotube acquires a s-triazine based structure rather than the reported commonly known tri-s-triazine unit.

(39) IR

(40) The IR spectra of melamine, nanofibers and nano tubes are illustrated in

(41) FIG. 3. The strong absorption peaks in the 3330-3550 cm.sup.1 range are attributed to the stretching vibrations of NH.sub.2 and NH groups. The above modes were absent in the nanotubes, suggesting that deamination occurs during calcination of nanofibers, which destroys most of the NH bonds. FIG. 3 (a) represents melamine, (b) its nano fiber and (c) its nanotube.

(42) SEM and TEM

(43) The FE-SEM images in FIG. 4 reveals high yield production of elegant, flexible and ultra-long nanotubes. The nanotube is of a general average outer diameter of 1.5 with lengths up to several millimeter. The magnified TEM image in FIG. 5 reveals that carbon nitride nanotube wall consists of several layers analogous to multiwall carbon nanotubes. The d-spacing's (0.36 nm & 0.49 nm) obtained from TEM images are in agreement with the XRD results.

(44) XPS

(45) Similarly N 1 s spectrum (FIG. 6) has three peaks at around 398.5, 399.95 and 401.0 ev which correspond to pyridinic, pyrrolic and graphitic respectively. The first two peaks may be attributed to sp.sup.3 CN bonds while the third one is due to a N-sp.sup.2C bond, which proves that there is bonding between the nitrogen and carbon atoms.

(46) UV-Visible and Photoluminescence Spectroscopy

(47) FIGS. 7 and 8 illustrates the UV-vis spectra and photoluminescence of carbon nitride. It shows the absorption edge at 440 nm centered around 300 nm, originating from -* electronic transition in the aromatic 1,3,5-triazine compound.

(48) CO.sub.2-TPD

(49) The basicity of the carbon nanofibre catalyst was determined by TPD of CO.sub.2. FIG. 10 shows that significant concentration of basic sites is present on the catalyst. The basic sites are the free NH.sub.2 on surface of the catalyst which were not polymerized.

(50) Cyclic Voltammetry

(51) The electro catalytic activity of the synthesized CNNT's was examined using cyclic voltammetry (CV) and rotating disc electrode (RDE) voltammetry in FIGS. 11 and 12. The onset potential of the material was found to be around 0.6V. The above described CV measurements show that the material has redox sites that can reduce O.sub.2 even in the absence of any metal.

(52) Catalyst testing: Selective oxidation reactions were performed in a 50 ml Parr autoclave. Reactant along with the catalyst was transferred to the haste alloy reactor. After heating the reaction mixture to the desired temperature, reactor was pressurized with oxygen. Conversion of reactant and product selectivity's was calculated based on the GC and HPLC analysis respectively. Products were analyzed using Agilent HPLC, equipped with RI detector and Rezex ROA-Organic Acid H.sup.+ column (300 mm7.8 mm) with 5 mM H.sub.2SO.sub.4 as the mobile phase at a flow rate of 0.6 mL.Math.min.sup.1.

Example 2

(53) Synthesis of Adipic Acid from Cyclohexane

(54) Initially 12.6 of cyclohexane, 6.1 g of acetonitrile and 50 mg of catalyst was placed in 50 mL parr autoclave. The mixture was heated to desired temperature, reactor was pressurized with oxygen. After the completion of reaction products are identified by HPLC and GC:

(55) Selective oxidation of cyclohexane (CH) to adipic acid was carried out using carbon nitride nano tube catalysts. Optimization of various experimental parameters was carried out to optimize the adipic acid yields.

(56) Effect of Temperature: The effect of temperature on selective oxidation of cyclohexane is shown in FIG. 13. It is seen that CH conversion as well as AA selectivity increased initially with temperature, but AA yield drops after reaching a maximum. On the other hand, CH conversion increased continuously with temperature with about 24% at 125 C. to 86 mol % at 140 C. Cyclohexanol and cyclohexanone were also found in the product, particularly at low reaction temperatures, implying that the K-A (Ketone-Alcohol) oil formed as primary product, which is oxidized to AA in a subsequent step. When the temperature is raised, the rate of consecutive oxidation increased, accelerating the conversion of cyclohexanol to cyclohexanone and then to AA. Hence, the AA selectivity increased with increasing temperature up to 130 C. However, further rise in temperature to 140 C. led to the formation of undesired products, mainly the glutaric and succinic acids. These results guide us to an optimum temperature of 130 C., where we can get reasonably high conversion along with very good selectivity to AA. Hence, this temperature was chosen for further investigations.

(57) Effect of Time: Effect of reaction time on conversion of cyclohexane and AA selectivity is shown in FIG. 14. The AA selectivity increased with time, reaching a maximum after 4 hours. The selectivity's of cyclohexanol (OI) and cyclohexanone (One) decreased with time. Between 2-4 h, the selectivity of AA increased at the expense of K-A oil. After 4.sup.th hour, the AA selectivity decreased due to AA degradation to glutaric acid (GA) and succinic acid (SA). This data shows that the KA concentration was high in the initial period (induction period) of the reaction, which subsequently converts in to AA with increasing TOS.

(58) Effect of catalyst concentration: Influence of catalyst concentration in the reaction mixture is shown in FIG. 15. With increasing catalyst amount, conversion of cyclohexane has increased, but reached a maximum at around 50 mg for 12.6 g of substrate (CH). Though the AA selectivity was related to catalyst content at least upto 50 mg, the selectivity to degradation byproducts was almost independent to the catalyst content. With the increase in the catalyst content from 25-50 mg, the AA selectivity increased corresponding to a reduction in K-A oil. Increasing the catalyst content further to 75-100 mg led to a marginal drop in AA selectivity because of its transformation to by-products like GA and GA. Therefore, it is possible to conclude that the selectivity of byproducts depends more on contact time but not on catalyst content.

(59) Effect of substrate to solvent ratio: As shown in FIG. 16, with increase in mole ratio of substrate to solvent, there was a negative influence on conversion. At 1:1 molar ratio, the selectivity of AA was found to be maximum. At lower or higher mole ratios, selectivity of AA was affected. So, there was a considerable effect of solvent on the above reaction.

(60) Effect of oxygen pressure: FIG. 17 demonstrates the influence of oxygen pressure on the performance. It is evident that with increase in oxygen pressure from 5 to 20 bar, conversion of cyclohexane has increased significantly from 10 to 70 mol %. At lower oxygen partial pressure, K-A oil was found in higher concentration. However, higher pressure of O.sub.2 led to the over-oxidation resulting in the formation of other dicarboxylic acids (GA and SA). At 20 bar, AA has formed as major a product. This clearly shows that oxygen pressure plays vital role in AA yield.

(61) Recyclability of the Catalyst:

(62) To check the reusability and stability of the catalysts, recycling tests were carried out, after washing the catalyst with acetonitrile prior to its re-use. The catalyst was used for five such cycles. There was no significant change either in catalyst activity or AA selectivity even after 5th recycle as can be seen from FIG. 18.

Solvent Free Oxidation of Cyclohexane

(63) For a selective oxidant process to be called as green, in addition to use environmental friendly oxygen source such as O.sub.2, no solvent should be used for carrying out the reaction. If no solvent is used, energy saving also occurs, as there is no need to separate the solvent from products or un-reacted substrates. Hence, we have conducted selective oxidation of cyclohexane in solvent free conditions. Effect of various parameters has been investigated; results of these experiments are illustrated below.

(64) Effect of Temperature:

(65) Effect of temperature on the yield of AA in solvent free condition is show in FIG. 19. It can be clearly seen, as was the case with solvent based reaction, CH conversion increased with temperature upto 71 mol % at 140 C. The AA selectivity also increased, but it reached to a maximum at 130 C., there after it decreased with temperature. Cyclohexanol and one were present in significant quantities at low reaction temperature, as they are the primary products of the reaction. Since, at higher temperatures, sequential oxidation is favoured, it leads to the formation of AA from cyclohexanol and cyclohexanone. As a result, the AA selectivity increased with increasing temperature up to 130 C. Further increase in temperature to 135-140 C. leads to the formation of undesired GA and SA products. These results shows that even in the absence of solvent, reaction temperature of 130 C. seems to be optimum for achieving high CH conversion and AA selectivity. However, both these values are on lower side as compared to the conversion and yields achieved with solvent.

(66) Effect of Time:

(67) The effect of reaction time on CH conversion and AA yield in solvent free conditions is depicted in FIG. 20. With TOS, conversion of cyclohexane has increased monotonously, but the AA selectivity increased to a maximum after 4.sup.th hour and dropped further on stream. The selectivity's of One and OI were also decreased with increasing time on stream. Between 1-4 h, the AA selectivity increased at the expense of KA oil. This result indicates that the AA formation is favoured with increasing reaction time. But, after 4.sup.th hour, the AA selectivity decreased due to its degradation to by products such as GA and SA.

Example 3

Comparison of Various Catalysts in the Selective Oxidation of Cyclohexane

(68) TABLE-US-00001 TABLE 1 Performance of various catalysts in selective oxidation of cyclohexane Selectivity (mole %) Sr. Cyclohexane: Conv. Glutaric Succinic Adipic No Catalyst Acetonitrile mol % ol one acid acid acid 1 g-C.sub.3N.sub.4 1:1 19.36 23.3 26.5 8.36 5.21 30.23 (12.6:6.1 g) 2 Melamine 1:1 10.2 7.32 21.0 8.78 3.22 61.24 carbon nano (12.6:6.1 g) fibers(after HNO.sub.3 treatment) 3 Carbon nitride 1:1 89.5 10.2 18.23 13.45 5.96 45.3 nano tubes.sup.$ (12.6:8.7 g) 4 Mesoporous 1:1 24.2 8.3 22.5 4.0 2.1 58.4 Carbon nitride.sup.# (12.6:6.1 g) 5 Carbon nitride 1:1 0.6 nano tubes* (12.6:6.1 g) 6 Carbon nitride 1:1 69.7 4.78 9.95 6.06 3.54 72.4 nano tubes.sup.@ (12.6:6.1 g) Conditions: 130 C., 4 h, 20 bar O.sub.2, 50 mg catalyst .sup.$Acetone as solvent, .sup.@Acetonitrile as solvent .sup.#using P123 polymer and H2SO4 *butylated hydroxytoluene (10 mol %)

(69) Under blank reaction conditions, the conversion of cyclohexane was bare minimum. With g-C.sub.3N.sub.4 as catalyst, CH conversion and AA selectivity's were low. The reaction rate increased with acetone as solvent, but AA selectivity was low. Mesoporous carbon nitride was found to be inefficient for the above catalyst and the usage of radical scavenger BHT terminates the oxidation process by capturing the superoxide radical which shows the reaction is initiated by superoxide radical.

Example 3

Synthesis of Adipic Acid from Cyclohexanone

(70) Initially 9 g of cyclohexanone, 7 g of acetonitrile, 2 g of water and 50 mg of catalyst was placed in 50 mL parr autoclave. The mixture was heated to desired temperature, reactor was pressurized with oxygen. After the completion of reaction products are identified by HPLC:

(71) ##STR00001##

(72) Effect of temperature: Effect of the reaction temperature on catalytic activity in the selective oxidation of cyclohexanone to AA is shown in FIG. 21. Increasing the reaction temperature promotes conversion of cyclohexanone. Selectivity of AA reaches maximum at 120 C., but decreases beyond this temperature. The decrease in AA selectivity is attributed to the decarboxylation at higher temperatures to form C.sub.2, C.sub.4 dicarboxylic acids.

(73) Effect of reaction time on catalytic activity: Effect of reaction time on catalytic activity of CNT in the selective oxidation of cyclohexanone is given in FIG. 22. Conversion of cyclohexanone has increased with increasing time on stream, while AA yield was found to decrease with time as a result of increased decarboxylation of AA. So in order to achieve higher amount of AA, choosing a particular reaction time is important in this reaction.

Example 4

Synthesis of Caprolactone from Cyclohexanone

(74) The CNNT catalyst was also tested for Bayer-Villiger oxidation of cyclohexanone to Caprolactone. About 10 mmol of cyclohexanone, 20 mmol of benzaldehyde and 50 mg of catalyst was placed in 50 mL parr autoclave. The mixture was heated to desired temperature, reactor was pressurized with oxygen. After the completion of reaction, products are identified by GC

(75) ##STR00002##

(76) Effect of time on caprolactone yield: FIG. 23 shows the effect of reaction time on Bayer-Villiger oxidation of cyclohexanone to caprolactone cyclohexanone to E-caprolactone. With increase in time, the conversion of cyclohexanone has increased.

Example 5

Synthesis of 2-hexanone from n-hexane

(77) Initially 2.5 g of n-hexane, 12.5 mL of acetonitrile and 100 mg of catalyst was placed in 50 mL parr autoclave. The mixture was heated to desired temperature, reactor was pressurized with oxygen. After the completion of reaction, products are identified by HPLC and GC:

(78) The oxidation of n-hexane with the carbon nitride nanotube catalyst was conducted with H.sub.2O.sub.2 and O.sub.2 as oxidants, the results of which are shown in Table 3.

(79) TABLE-US-00002 TABLE 3 Selective oxidation of n-hexane using CNNT catalysts Selectivity % Sr. No Oxidant Time (h) Conversion 2-hexanone 1 H.sub.2O.sub.2 (3 moles) 4 37.8 24.3 8 57.7 26.9 2 H.sub.2O.sub.2 4 16.3 31.6 (2 moles) 8 30.8 30.4 3 H.sub.2O.sub.2 4 6.9 32.5 (1 mole) 8 15.1 33.7 4 O.sub.2 (15 bar) 4 26.9 22.6 8 44.3 24.9
Conditions: 2.5 g n-hexane, 12.5 ml acetonitrile, 100 mg CNNT, 100 C.

(80) In the case of H.sub.2O.sub.2 oxidations, conversion of n-hexane has increased with increasing H.sub.2O.sub.2/substrate ratio. Conversion of n-hexane also increased with increasing reaction time. When oxidation was performed with molecular oxygen, reasonable conversion of n-hexane was achieved with 2-hexanone as the product.

Advantages of Invention

(81) a. The present process facilitates selective oxidation of hydrocarbons and alcohols, particularly to get adipic acid from cyclohexane in a single step. b. Metal free catalysts, hence there won't be any metal leaching problems. c. Process uses non-corrosive solvents like acetonitrile. d. Process also can be conducted without any solvent. Green and economic process. e. Recyclable heterogeneous catalyst.