Claims:WE CLAIM
1. A process for producing aromatic compounds from a feed comprising at least one component selected from the group consisting C7 hydrocarbons, C8 hydrocarbons and C9 hydrocarbons; said process comprising:
a. mixing said feed comprising at least one component selected from the group consisting C7 hydrocarbons, C8 hydrocarbons and C9 hydrocarbons with at least one hydrogen acceptor, at least one base, a Pincer ligated catalyst and at least one fluid medium to obtain a mixture; and
b. heating said mixture to a temperature in the range of 230 ?C to 250 ?C to obtain a product comprising aromatic compounds and having olefin and dimer content less than 20%, preferably less than 15%;
2. The process as claimed in claim 1, wherein the feed comprises naphtha.
3. The process as claimed in claim 1, wherein the Pincer ligated catalyst is at least one selected from the group consisting of compounds Ia and Ib

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups and M is a metal selected from iridium and rhodium.
4. The process as claimed in claim 3, wherein the Pincer ligated catalyst is at least one selected from the group consisting of iPrCz-POCNP-IrHCl (IIa), and iPrInd-POCNP-IrHCl (IIb) and is represented as

5. The process as claimed in claim 1, wherein the weight ratio of the Pincer ligated catalyst to the feed is in the range of 0.001: 1 to 0.01: 1.
6. The process as claimed in claim 1, wherein the hydrogen acceptor is alkene compound.
7. The process as claimed in claim 1, wherein the hydrogen acceptor is at least one selected from the group consisting of 3,3-dimethyl-1-butene (TBE), propylene, and ethylene.
8. The process as claimed in claim 1, wherein the weight ratio of the hydrogen acceptor to the feed is in the range of 1: 1 to 5: 1.
9. The process as claimed in claim 1, wherein the base is an alkali metal alkoxide.
10. The process as claimed in claim 1, wherein the base is at least one selected from the group consisting of potassium t-butoxide, sodium t-butoxide, and lithium t-butoxide.
11. The process as claimed in claim 1, wherein the weight ratio of the base to the feed is in the range of 0.001: 1 to 0.01: 1.
12. The process as claimed in claim 1, wherein the fluid medium is an aromatic compound.
13. The process as claimed in claim 1, wherein the fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
14. The process as claimed in claim 1, wherein the weight ratio of the fluid medium to the feed is in the range of 1: 1 to 5: 1.
15. The process as claimed in claim 1, wherein the step of heating the mixture is carried out for a time period in the range of 1 hour to 5 hours.
16. The process as claimed in claim 1, wherein the conversion of alkane is more than 90% and the yield of aromatic compounds is more than 75%.
, Description:FIELD
The present disclosure relates to a process for producing aromatic compounds.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicates otherwise.
A POCNP Pincer ligand is a tridentate ligand that binds with a metal at three sites; one phosphorous atom of phosphinite group, one phosphorous atom of phosphinous amide group and a carbon of an aromatic framework. The phosphinite group is in the form of a phosphorous-oxygen (P-O) bond and phosphinous amide group is in the form of a phosphorous-nitrogen (P-N) bond.
BACKGROUND
Aromatic hydrocarbons are amongst the most important building blocks in the chemical industry. The high temperature thermolysis of alkanes and the catalytic dehydroaromatization of n-alkanes are reported to produce aromatic compounds. But, these processes for production of aromatics involve the competitive side reactions such as dimerization, isomerization, dealkylation or coking, thereby leading to the drawbacks such as low yields and low selectivity.
Aromatic compounds are also obtained by reforming petroleum feedstock. The petroleum feedstock comprises three major classes of compounds viz. n-alkanes, iso-alkanes and naphthenes. However, n-alkanes present a potential problem in the commercially practiced processes for production of aromatic compounds. n-Alkanes enhance the coke formation, which further leads to reduction in aromatics formation. The yield of aromatic compounds obtained from n-alkanes in the standard reforming processes is comparatively low to the yield of aromatic compounds formed from other classes of compounds viz. iso-alkanes and naphthenes.
Therefore, there is felt a need to provide a process for producing aromatic compounds that mitigates the drawbacks mentioned hereinabove.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
An object of the present disclosure is to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a process for production of aromatic compounds.
Still another object of the present disclosure is to provide a process for selective production of aromatic compounds with reduced dimer content.
Yet another object of the present disclosure is to provide a process for selective conversion of n-alkanes to aromatic compounds among the mixture of alkanes.
Still other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure provides a process for producing aromatic compounds from a feed comprising at least one component selected from the group consisting C7 hydrocarbons, C8 hydrocarbons, C9 hydrocarbons and naphtha.
The process comprises the step of mixing the feed with at least one hydrogen acceptor, at least one base, a Pincer ligated catalyst and at least one fluid medium to obtain a mixture. The mixture is heated to a temperature in the range of 200 ?C to 250 ?C for a time period in the range of 1 hour to 5 hours to obtain a product comprising aromatic compounds and having olefin and dimer content less than 20%, preferably less than 15%.
Typically, the Pincer ligated catalyst is at least one selected from the group consisting of compounds Ia and Ib

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups and M is a metal selected from iridium and rhodium.
Typically, the Pincer ligated catalyst is at least one selected from the group consisting of iPrCz-POCNP-IrHCl (IIa), and iPrInd-POCNP-IrHCl (IIb) and is represented as

The weight ratio of Pincer ligated catalyst to the feed is in the range of 0.001: 1 to 0.01: 1.
The hydrogen acceptor is at least one alkene compound selected from the group consisting of 3,3-dimethyl-1-butene (TBE), propylene, and ethylene. The weight ratio of the hydrogen acceptor to the feed is in the range of 1: 1 to 5: 1.
The base is at least one alkali metal alkoxide selected from the group consisting of potassium t-butoxide, sodium t-butoxide, and lithium t-butoxide. The weight ratio of the base to the feed is in the range of 0.001: 1 to 0.01: 1.
The fluid medium is at least one aromatic compound selected from the group consisting of toluene, xylene and mesitylene. The weight ratio of the fluid medium to the feed is in the range of 1: 1 to 5: 1.
The conversion of alkane by the process of the present disclosure is more than 90% and the yield of the aromatic compounds is more than 75%.
DETAILED DESCRIPTION
Various processes have been explored to produce aromatic compounds from n-alkanes. However, the conventional processes are associated with drawbacks such as low selectivity and low yields due to the undesired side reactions.
The present disclosure, therefore envisages a highly selective process for producing aromatic compounds.
The present disclosure provides a process for producing aromatic compounds from a feed comprising at least one component selected from the group consisting C7 hydrocarbons, C8 hydrocarbons and C9 hydrocarbons. In one embodiment of the present disclosure, the feed comprises naphtha.
Initially, the feed is mixed with at least one hydrogen acceptor, at least one base, a Pincer ligated catalyst and at least one fluid medium to obtain a mixture. The so obtained mixture is heated to a temperature in the range of 200 ?C to 250 ?C to obtain a product comprising aromatic compounds.
In accordance with the process of the present disclosure, the dimer content of the product is less than 20%, preferably less than 15%.
Typically, the Pincer ligated catalyst is at least one selected from the group consisting of compounds Ia and Ib

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups and M is a metal selected from iridium and rhodium.
In accordance with one embodiment of the present disclosure, Pincer ligated catalyst is iPrCz-POCNP-IrHCl (IIa) and is represented as

(IIa)
In accordance with the present disclosure, the iPrCz-POCNP-IrHCl is stable at high temperature, i.e. temperature greater than 200 °C.
In accordance with another embodiment of the present disclosure, Pincer ligated catalyst is iPrInd-POCNP-IrHCl (IIb) and is represented as

(IIb)
In accordance with the present disclosure, the iPrInd-POCNP-IrHCl is stable at high temperature, i.e. temperature greater than 200 °C.
In accordance with the embodiments of the present disclosure, the weight ratio of Pincer ligated catalyst to the feed is in the range of 0.001: 1 to 0.01: 1.
In accordance with the embodiments of the present disclosure, the hydrogen acceptor is at least one alkene compound selected from the group consisting of 3,3-dimethyl-1-butene (TBE), propylene, and ethylene. In accordance with exemplary embodiment of the present disclosure, the hydrogen acceptor is 3,3-dimethyl-1-butene (TBE).
The process of the present disclosure further comprises the use of a hydrogen sponge or a hydrogen-permeable membrane. The hydrogen sponge or the hydrogen-permeable membrane is used to remove or absorb hydrogen gas formed during the dehydroaromatization of n-alkanes.
In accordance with the embodiments of the present disclosure, the weight ratio of the hydrogen acceptor to the feed is in the range of 1: 1 to 5: 1.
In accordance with the embodiments of the present disclosure, the base is at least one alkali metal alkoxide selected from the group consisting of potassium t-butoxide, sodium t-butoxide, and lithium t-butoxide. In accordance with one embodiment of the present disclosure, the base is potassium t-butoxide. In accordance with another embodiment of the present disclosure, the base is sodium t-butoxide.
In accordance with the embodiments of the present disclosure, the weight ratio of the base to the feed is in the range of 0.001: 1 to 0.01: 1.
In accordance with the embodiments of the present disclosure, the fluid medium is at least one aromatic compound selected from the group consisting of toluene, xylene and mesitylene. In accordance with exemplary embodiment of the present disclosure, the fluid medium is mesitylene.
In accordance with the embodiments of the present disclosure, the weight ratio of the fluid medium to the feed is in the range of 1: 1 to 5: 1.
The process of the present disclosure involves multiple dehydrogenation reactions. The reaction proceeds via formation of triene. The so formed triene undergo cyclisation and subsequent dehydrogention to form a desired aromatic compound.
In accordance with the process of the present disclosure, the aromatization of C7 compounds is represented below:

The higher reaction temperature, preferably above 200 °C increases the rate of dehydrogenation. As temperature increases, the increase in rate of dehydrogenation is greater than the increase in rate of dimer formation, which results in high selectivity of the desired aromatic compounds.
However, the conventional catalysts used for the dehydroaromatization of n-alkanes are unstable at high temperatures and lead to lower activity due to their rapid degradation at higher temperature. At lower temperature, the rate of dehydrogenation with the conventional catalyst is low and hence the amount of aromatics formed is lower and dimer formed is higher.
Pincer ligated catalysts used in the process of the present disclosure are stable at high temperature and exhibit higher activity. Therefore the amount of the aromatics formed is higher and dimer formed is lower. Pincer ligated catalysts of the present disclosure display a high selectivity for aromatic formation from pure n-alkane. Competition reaction between n-alkane with either iso-alkane or napthenes display a significantly high selectivity for converting specifically n-alkane to aromatics. This is observed specifically with naphtha as the feed where the aromatics obtained is in tune with the n-alkane composition in the initial feed.
Therefore, the process of the present disclosure involves the step of heating the reaction mixture at a higher temperature in the range of 230 °C to 250 °C in the presence of the more stable POCNP Ir-catalysts, wherein the POCNP Ir-catalysts of the present invention exhibit higher selectivity towards the aromatic products compared to the conventional PCP Ir-catalyst.
The present disclosure is further described in light of the following laboratory scale experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. These laboratory scale experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial/commercial scale.
Experimental details
General procedure:
To a Parr reactor, were added an alkane, mesitylene, 3,3-dimethyl-1-butene (TBE), Pincer ligated catalyst, and a base under inert atmosphere to obtain a mixture. The so obtained mixture was rapidly heated to a predetermined temperature for predetermined time period under stirring to obtain a product comprising aromatic compounds. The product was cooled to a temperature in the range of 0 °C to 2 °C and the content was collected, weighed and analyzed by gas chromatography (GC).
Dimer content of the product was estimated independently by distillation.
The % yield of the reaction components was calculated using their relative response factor (RRF) with respect to mesitylene. Their relative response factors are standardized with standard addition method using the same reaction mixture and GC analytical data.
The experimental procedure described herein above was followed to perform the dehydroaromatization of a feed comprising at least one component selected from the group consisting C7 hydrocarbons, C8 hydrocarbons, C9 hydrocarbons and naphtha. The type of feed, type of catalyst and the reaction temperature were varied and the results are provided in Table 1, Table 2 and Table 4 given below.
Experiments 1-9: Dehydroaromatization of n-heptane
Experiments 1-5:
Dehydroaromatization was typically carried out in the presence of iPrCz-POCNP-IrHCl (IIa) catalyst (26±1 mg) and a base (14±1 mg). The type of base was varied and selected from the group consisting of potassium t-butoxide, sodium t-butoxide, lithium t-butoxide. Further the temperature of the reaction was also varied from 210 °C to 250 °C.
The results for aromatization are tabulated below in Table 1.
Comparative Experiments 6-9:
Dehydroaromatization was typically carried out in the presence of conventional catalyst iPrPCP-IrHCl (26±1 mg) and a base (14±1 mg). The type of base was varied and selected from the group consisting of potassium t-butoxide, sodium t-butoxide, and lithium t-butoxide. Further the temperature of the reaction was also varied from 190 °C to 230 °C.
The results for aromatization are tabulated below in Table 1.
Table 1: Dehydroaromatization of n-Heptane.
Examples Mesitylene (g) Heptane
(g) TBE
(g) Base
(mg)
Temperature (°C)/ Time (h) Alkane %Con. Aromatics %Yield Olefins %Yield Dimersa
1 7.0 3.5 13.0
KOtBu
(15) 210/ 4 69.8 25.4 25.4 19.0
2 7.5 3.6 13.2
NaOtBu
(14) 230/ 2 84.5 48.1 17.9 18.5
3 7.4 3.6 13.3
NaOtBu
(14) 230/ 4 96.6 72.5 4.0 20.1
4 7.6 3.5 13.3
KOtBu
(14) 230/ 4 97.2 78.4 2.5 16.3
5 7.4 3.5 13.1
NaOtBu
(14) 250/ 2 93.2 75.5 4.0 13.7
Comparative example 6 7.4 3.5 13.1
NaOtBu
(14) 190/ 7 86.9 56.4 10.0 20.5
Comparative example 7 7.2 3.7 13.3
KOtBu
(15) 210/ 4 83.8 46.8 14.4 22.6
Comparative example 8 7.5 3.6 13.2
NaOtBu
(13) 210/ 4 88.0 54.1 12.0 21.9
Comparative example 9 7.3 3.5 13.1
KOtBu
(14) 230/ 4 79.9 43.9 17.6 18.4
a %Mole of heptane converted into its dimers.
Con. = Conversion.
From Table 1, it is evident that the higher temperature of dehydroaromatization reaction increases the aromatics yield and decreases the dimer production.
Further, it is evident that the process wherein the conventional pincer ligated catalyst is used, the yield aromatics is low and the content of dimer is more when compared to the catalysts of present disclosure.
Experiments 10-12: Dehydroaromatization of Octane and Nonane
Dehydroaromatization was typically carried out for octane in the presence of iPrCz-POCNP-IrHCl (IIa) catalyst (26±1 mg) and a base (14±1 mg) at 230 °C for 4 hours. The type of base was varied and selected from the group consisting of potassium t-butoxide, sodium t-butoxide, and lithium t-butoxide. The type of alkane was also varied and selected from the group consisting of n-octane and n-nonane.
The results for aromatization are tabulated below in Table 2.
Experiment 13: Dehydroaromatization of Nonane
Dehydroaromatization was typically carried out for n-nonane in the presence of iPrInd-POCNP-IrHCl (IIb) catalyst (26 mg), and a base (15 mg) at 230 °C for 4 hours.
The results for aromatization are tabulated below in Table 2.
Comparative Experiment 14:
Dehydroaromatization was typically carried out for octane in the presence of the conventional catalyst iPrPCP-IrHCl (26 mg) and NaOtBu (15 mg).
The results for aromatization are tabulated below in Table 2.
Comparative Experiment 15:
Dehydroaromatization was typically carried out for n-nonane in the presence of the conventional catalyst iPrPCP-IrHCl (51 mg) and NaOtBu (29 mg).
The results for aromatization are tabulated below in Table 2.
Table 2: Dehydroaromatization of Octane and Nonane
Examples Alkane
(g) Mesitylene (g) TBE
(g) Base
(mg) Temperature (°C)/ Time h) Alkane %Con. Aromatics %Yield Olefins %Yield Dimersa
10
Octane
(4.0 ) 7.3 13.2 KOtBu
(15) 230/4 97.4 88.7 3.5 5.2
11
Nonane
(4.2 ) 7.6 13.2 NaOtBu
(15) 230/4 99.3 76.3 7.4 15.6
12
Nonane
(4.2 ) 7.3 13.3 KOtBu
(15) 230/4 98.7 79.5 7.4 11.8
13
Nonane
(4.2 ) 7.6 13.2 KOtBu
(15) 230/4 98.3 82.3 7.3 8.7
Comparative example 14
Octane
(4.0 ) 7.6 13.2 NaOtBu
(15) 210/4 89.6 45.2 21.3 23.1
Comparative example 15
Nonane
(8.2 ) 14.5 26.4 NaOtBu
(29) 210/4 89.4 41.4 15.3 32.7
a %Mole of alkane converted into its dimers.
Con. = Conversion.
From table 2, it is evident that the process wherein the conventional pincer ligated catalyst is used, the yield aromatics is low and the content of dimer is more when compare to the catalysts of present disclosure.
Experiments 17-19: Dehydroaromatization of naphtha feed.
The composition of the naphtha feed used for the dehydroaromatization reaction is given below in Table 3.
Table 3: Representative feed composition of the naphtha used
C6 C7 C8 C9 C10 Total %Wt.
n-Paraffin 0.14 5.5 15.2 11.5 0.7 33.04
iso-Paraffin -- 2.7 12.9 15.9 4.6 36.1
Olefins -- -- -- 0.02 0.14 0.16
Naphthenes 0.32 6.2 10.8 9.2 0.5 27.0
Aromatic -- 0.4 2.2 0.9 0.2 3.7

Experiment 17:
Dehydroaromatization was typically carried out in the presence of iPrCz-POCNP-IrHCl (IIa) catalyst (25 mg) and KOtBu (13 mg) at 230 °C for 4 hours.
Experiment 18:
Dehydroaromatization was typically carried out in the presence of iPrCz-POCNP-IrHCl (IIa) catalyst (54 mg) and NaOtBu (29.mg) at 250 °C for 2 hours.
Comparative Experiment 19:
Dehydroaromatization was typically carried out in the presence of conventional iPrPCP-IrHCl catalyst (53 mg) and NaOtBu (28 mg) at 210 °C for 4 hours.
The results for dearomatization of naphtha feed are tabulated below in Table 4.

Table 4: Dehydroaromatization of naphtha feed.
Examples. Naphtha
(g) Mesitylene (g) TBE
(g) n-Paraffin % Wt. iso-
Paraffin
%Wt. Olefins
%Wt. Naphthene %Wt. Aromatic %Wt. Dimers %Wt.
17 4.3
7.6 13.3 1.0 17.3 11.1 18.5 39.7 12.4
18 8.3 14.7 26.8 2.0 18.7 5.4 20.5 32.0 21.4
Comparative example19 8.2 14.6 26.5 2.1 18.3 6.2 24.1 18.0 31.3

TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process for producing aromatic compounds, which:
- provides aromatic compounds with high selectivity and high yield;
- decreases formation of a dimer.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.