STUDY OF THE LIQUID-PHASE HYDROGENATION OF BENZENE IN THE PRESENCE OF METAL-POLYMER COMPLEXES BASED ON POLYVINYLPYRIDINES Текст научной статьи по специальности «Фундаментальная медицина»

ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2023 ISSN 0005-2531 (Print)

UDC 547.52/68

STUDY OF THE LIQUID-PHASE HYDROGENATION OF BENZENE IN THE PRESENCE OF METAL-POLYMER COMPLEXES BASED ON POLYVINYLPYRIDINES

R.H.Suleymanova, N.A.Zeynalov, L.N.Qulubayova, A.R.Guliyeva, N.T.Shixverdiyeva

M.Nagiyev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education

of the Republic of Azerbaijan

suleyman.rena@gmail.com

Received 20.12.2022 Accepted 25.01.2023

The liquid-phase hydrogenation of benzene on metal-polymer complexes, which were prepared on vi-nylpyridine polymers containing functional groups and salts of transition metals such as nickel, palladium, and platinum, was studied. Poly-4-vinylpyridine and poly-2-methyl-5-vinylpyridine were used as vinylpyr-idine polymers. Hydrogenation was carried at a temperature of 20-60° C. The study additionally focussed on the influence of the nature of the medium on the benzene hydrogenation process on poly-4-vinylpyridines. It has been established that in the presence of a platinum-poly-4-vinylpyridine complex, the reaction of liquid-phase hydrogenation of benzene, not only the product of complete hydrogenation, but also products of incomplete hydrogenation, cyclohexene, are formed. It was also found that the nature of the medium affects the yield of cyclohexane and cyclohexene selectively and effectively.

Keywords: hydrogenation, benzene, polyvinylpyridines, metal-polymer complexes, cyclohexane, cyclohexene.

doi.org/10.32737/0005-2531-2023-2-97-103 Introduction

Benzene and the products of its complete and incomplete hydrogenation are of great importance in modern chemical technology. Their predominant application is the manufacture of a small number of monomers used in the production of large-tonnage polymeric materials, plas-ticizers and synthetic fibers [1].

Benzene accounts for more than 50% of the total amount of aromatic hydrocarbons used in chemical industry. More than 80% of benzene is consumed to manufacture just three products: ethylbenzene, isopropylbenzene and cyc-lohexane.

With the complete hydrogenation of benzene, cyclohexane is obtained, which is the main raw material for the production of polyamide fibers [2].

The products of incomplete hydrogenation of benzene are cyclohexadienes and cy-clohexene. The high reactivity of unsaturated cyclic hydrocarbons opens up the possibility of obtaining valuable products such as synthetic rubbers, fibers, drugs, plasticizers, emulsifiers, and solvents [3].

Thus, cyclohexadiene-1,3 oligomers (molecular weight 270-750) are used to prepare stabilizing additives to compositions based on polyethylene in order to increase its resistance to thermal aging Polymers and copolymers of cy-clohexadiene-1,3 are added to light-resistant materials to increase their sensitivity to radiation [4].

Cyclohexadiene-1,4 is used as a chain transfer agent in the synthesis of color enhanced acrylonitrile polymers.

Cyclohexene was one of the first objects of the catalytic epoxidation of olefins with organic peroxides.

It is known that commonly used heterogeneous catalysts require relatively harsh reaction conditions (120-2000C, 3-7 MPa), which does not provide sufficient selectivity of the process.

Incomplete selective hydrogenation of benzene, despite the stepwise course of this process, under traditional conditions of the reaction mixture of cyclohexene and cyclic dienes, is not detected. One of the ways to solve this problem is the development of catalysts capable of selec-

tively activating the aromatic ring in the direction of partial hydrogenation reactions.

Carrying out the hydrogenation process under "mild conditions" became possible due to the use of homogeneous metal complex catalysts [5-7]. Almost all transition metal complexes exhibit catalytic activity in benzene hydrogenation reactions. However, despite the advantage of homogeneous metal complex catalysts, their use in industrial conditions is limited by the difficulty of isolation from the reaction mixture. Attempts to eliminate the shortcomings of a homogeneous catalyst led to the creation of complexes containing organic polymers as microligands. In this case, the polymers act mainly as a heterogenizing substrate [8-19].

This work is aimed at creating active and selective hydrogenation catalysts based on pol-yvinylpyridines and salts of transition metals such as Ni, Pd and Pt.

Experimental part and discussion of the results

Heterogenized metal-polymer complexes were prepared on the basis of various polymers and copolymers containing active functional groups.

We used polyvinylpyridines based on 4-and (2-methyl-5)-vinylpyridines as polymeric macroligands. Poly-4- and poly-2-methyl-5-vi-nylpyridine polymers were obtained by polymerization of the respective monomers in a solution of toluene in vacuum at 600C in the presence of 1 wt.% cumene hydroperoxide. NiCl2* 6H2O, PdCl2, and H2PtCl6* 6H2O - platinum hydrochloric acid were used as metal salts for the preparation of complexes.

Hydrogenation of benzene was carried out in an autoclave at a temperature of 20-600C, a hydrogen pressure of 0-10 atm. The autoclave

Table 1. Hydrogenation of benzene on Ni-polymer complexes

was loaded with a magnetic stirrer, a sample of an air-dry catalyst and benzene in an amount of 5-10 ml with a calculated catalyst concentration of 0.01-0.07 g/ml. The autoclave was purged with hydrogen, after which it was filled with gas to a predetermined pressure, and the time was recorded, the pressure drop was noted at certain time intervals.

The reaction mixture was analyzed on a gas chromatograph (Agilent 7890B GC system).

Table 1-4 presents the results of hydrogenation experiments with benzene on various samples of metal-polymer catalysts synthesized in the laboratory.

The legends of the tables where, V0 - is the loading of benzene into the autoclave, ml;

g0 - concentration of the metal-polymer complex, g / ml;

t - reaction temperature, 0C; t - is the duration of the reaction, hour; P0 - initial pressure of hydrogen, atm; AP - is the pressure drop over time t. These data show that Ni-polymer complexes based on poly-2-methyl-5-vinylpyridine do not exhibit catalytic activity in the reaction of benzene hydrogenation under mild conditions. Increasing the temperature to 600C and pressure 4-8 atm also did not give positive results.

Table 2 data on the hydrogenation of benzene on Pd-polymer complexes also show unsatisfactory results. Complexes based on poly-4-vinylpyridine and poly-2-methyl-5-vinylpyridine were tested. This series of experiments was in most cases carried out at a temperature of 20-60°C, a hydrogen pressure of 2-2.5 atm., with a complex concentration of 0.01-0.02 g/ml of the solution.

№ Vo go t t P0, AP, Yield %

ml g/ml 0C hour atm. atm. C6H12 C6H:0

Ni- poly-4-vinylpyridine

1 5 0.0733 20 22 1.80 0 absent

2 5 0.0120 20 70 4.10 0

Ni- poly -2-methyl-5 -vinylpyridine

1 5 0.0400 20 27 2.00 0 absent

2 5 0.0500 60-20 69 8.40 0

Only in the experiment carried out in Pd-poly-2-methyl-5-vinylpyridine complexes at a concentration of 0.0486 g/ml and a pressure of 6.9 atm traces of cyclohexane were found (yield 028%).

Taking into account the results obtained, as well as the data known from the literature[20], according to which Pd shows lower activity than Pt, further research was carried out on Pt-polymer complexes.

Table 3 presented data on the hydrogenation of benzene on the Pt-poly-4-vinyl-pyridine complex. This complex made it possible to obtain not only the product of complete hydrogenation, cyclohexane, but also the intermediate product, cyclohexene.

Positive results were obtained at a complex concentration of 0.03-0.07 g/ml, a temperature of 600C and a pressure of 7-9 atm. Experiment 4, under similar conditions, thus yielded cyclohexane (7.6%) and cyclohexene (2.2%).

Table 4 shows the results of hydrogenation of benzene on a Pt-poly-2-methyl-5-

vinylpyridine complex. Taking into account the previous results, the experiments were carried out at a higher pressure, namely 7-9 atm, both on uncross-linked and cross-linked samples.

According to the data presented in the table, it was shown that the only hydrogenation product on the specified complex is cyclohexane. The yield of cyclohexane is 7-10%, and with further addition of hydrogen, the yield increased to 26-42%.

A further goal of the study is to research the conditions which increase the activity and selectivity of platinum-polyvinylpyridine complexes through optimizing their synthesis and activity conditions. For this purpose, benzene was hydrogenated on Pt-poly-4-vinylpyridine catalysts in various media.

To this end, a complex was prepared using the following procedure: a weighed amount of poly-4-vinylpyridine, along with alcohol and water, was placed in a three-necked flask equipped with a magnetic stirrer, a reflux condenser and a thermometer.

Table 2. Hydrogénation of benzene on Pd-polymer complexes

№ Vo ml go g/ml t 0C T hour P0, atm. AP, atm Yield, %

C6H12 C6H10

Pd-poly-2-methyl-5-vinylpyridine

1 10 0.0115 20 42.5 2.4 0 absent

2 3 0.0146 80-20 24 2.4 -

3 5 0.0486 60-20 18 2.6 -

additive H2 - 63-20 72 6.9 0.3 0.28

4 5 0.0200 80-20 120 10.0 -

Pd- poly-4-vinylpyridine

5 5 0.0200 - 6.5 8.8 0 absent

V0 go t, T P0, AP, Yield, %

№ ml g/ml 0C hour atm. atm. C6H12 C6H10

Pt- poly-4-vinylpyridine

1 5 0.018 60-20 18 3.20 - traces -

additive H2 - 60 6 7.00 - abcent

5 0,026 60 3,25 9,70 0,05 - -

2 additive H2 - 20 67 7,95 1,10 0,29 0,03

3 5 0.066 60 4.0 7.83 1.60 7.60 2.2

4 5 0.066 60 5.25 9.65 0 - -

additive H2 - 20 48 9.40 0 1.46 -

Table 3. Hydrogenation of benzene on Pt-poly-4-vinylpyridine complexes

The polymer was completely dissolved in an alcohol-water solution at temperatures ranging from 60 to 200°C. Without stopping the stirring, an aqueous solution of H2PtCl2-6H2O acid was gradually added drop by drop. At the end, an aqueous solution of sodium borohydride (NaBH4), a reducing agent, was introduced. The precipitated complex was washed with diethyl ether. The table presents the results of the study of the benzene hydrogenation reaction in the presence of a complex prepared according to the above-mentioned method.

The data presented in the table demonstrate that the complex exhibits low activity and selectivity in the absence of alcohol and water.

However, the addition of a few drops of water to the medium immediately increases both the activity and selectivity towards cyclohexane. Treatment of the complex with an aqueous solution of sodium borohydride before the experiment also enhances its activity and selectivity. However, the best results were obtained with hydrogenation in ethanol in combination with treatment of the complex with an aqueous solution of a reducing agent before the experiment, as benzene is hydrogenated to cyclohexene (21.18%). This result was obtained at atmospheric pressure, in contrast to the other experiments (7-10 atm).

Table 4. Hydrogénation of benzene on Pt-poly-2-methyl-5-vinylpyridine complexes

№ Vo, go, t, T, P0, AP, Yield, %

ml g/ml 0C hour atm. atm. C6H12 C6H10

1 3 0.033 60-20 60 7.24 3.2 ~7 absent

2 5 0.027 20 17 7.36 2.6 traces -"-

5 0.027 60 5.5 7.20 5.6 _"_

additive H2 - 60-20 21.5 7.20 6.0 16

3 - 60-20 22 6.8 3.2 27

- 60 11 8.40 1.5 33

- 60-20 90 7.20 - 42

4 5 0.027 60-20 72 8.40 0 absent

5 0.024 60 4.3 6.80 4.60 9.6

5 additive H2 - 60-20 67 8.72 3.52 traces

- 60-20 23 8.83 3.20

- 60 6 9.20 0.72 26

6 5 0.022 23 2.75 8.24 0.46 absent

7 5 0.023 24 4.0 8.20 - traces

8 5 0.024 24 4.6 8.3 0.15

№ Samples Reactants NaBH4 V0 t. T. P0 AP Yield.%

VP:Pt:NaBH4 treatment ml 0C hour atm. atm. C6H10 C6H12

1 3.2:1:1 catalyst benzene - 5 60 4 7.85 1.6 2.2 7.6

2 3.2:1:1 catalyst benzene water 5 85 4 9.9 0.2 0 10.0

3 3.2:1:1 catalyst benzene + 5 80 23 10.24 3.4 0 12.6

4 3.2:1:1 catalyst benzene ethanol + 2 2 19 2.75 atm. 16 15.58 0

5 3.2:1:1 catalyst benzene ethanol + 2 2 15 48 atm. 51 21.18 0

6 3.4:1:1 catalyst benzene methanol water 0.2 3 3 30 24.6 atm. 137.8 76.2 1.8

Table 5. Hydrogenation of benzene in an aqueous-alcoholic medium on a Pt-poly-4-vinylpyridine complex

The results obtained led to the conclusion that alcohols and water are the components of the medium that activate the hydrogenation process.

Having obtained positive results both in methanol and ethanol, a series of experiments on the hydrogenation of benzene in various media were subsequently carried out.

Table 6 shows the hydrogenation of benzene in ethanol, methanol, propanol, butanol, and tetrahydrofuran. The data allow some comparative analysis of the influence of various media on the activity and selectivity of the process.

In the process of benzene hydrogenation, a water-and-ethanol-based mother liquor with hydrogen bubbling was tested. The absence of traces of benzene and cyclohexane in hydro-

genase initially led to the conclusion that the yield consisted of 100% cyclohexene. The change in the composition of the complex and the absence of hydrogen bubbling did not confirm such a high result as only 23% of cyclohexene was obtained. Ethanol on a quaternized catalyst sample favours selective formation of cyclohexene (52.7%).

The sample tested in an aqueous methanol medium also resulted in a high selective yield of cyclohexene (76.2%). The table further shows the effect of methanol on the result of benzene hydrogenation.

Quaternization in combination with methanol leads to a selective yield of cyclohexane (56.3%). The unquaternized sample in methanol is selective with respect to the cyclohexene (32.6%).

Table 6. Hydrogenation of benzene on a Pt-poly-4-vinylpyridine complex in various media

№ Samples reactants V0 t. T P0. AP. Yield. %

VP:Pt:NaBH4 ml 0C hour atm atm C6H10 C6H12

1 4.8:1:0.8 quater.-25% catalyst benzene ethanol water 0.15 3 3 30 2.7 atm - 100 0

2 2.5:1:1.4 quater.-25% 55 0.2 3 3 30 4.5 atm 25 23 0

3 4.75:1:4 quater.-30% 55 0.5 3 3 30 28 atm 62.4 52.7 0

4 3.4:1:1.2 quater.-25% catalyst benzene methanol water 0.2 3 3 30 24.6 atm 137.8 76.2 1.8

5 4.75:1:4 quater.-30% catalyst benzene methanol 0.5 3 30 65 atm 42 0 56.3

6 3.7:1:4.1 55 0.5 3 30 48 atm 248 32.6 8.8

7 4.75:1:4 quater.-30% catalyst benzene methanol propanol 0.5 3 30 4.5 atm 70 11 0

8 3.7:1:4.1 catalyst benzene butanol 0.5 3 30 3 atm 30 - -

9 4.75:1:4 quater.-30% catalyst benzene TGF 0.5 3 30 68.5 atm 25 1.25 0

The effect of butanol, methanol and tetra-hydrofuran on the activity of benzene hydrogenation is insignificant.

Conclusions

The result of the study is the discovery of catalytic activity in platinum-polyvinylpyridine complexes in the reaction of liquid-phase hydrogenation of benzene. The possibility of obtaining on their basis the intermediate product cyclohexene and the product of complete hydrogenation of benzene, cyclohexane, has been established. In addition, water-alcohol media (methanol, ethanol) affect the selectivity of the hydrogenation of benzene into cyclohexene.

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POLÍVÍNÍLPÍRÍDÍN OSASLI METALPOLÍMER KOMPLEKSLOR MÜHÍTÍNDO MAYE FAZADA BENZOLUN HÍDROGENLO§MO REAKSÍYASININ TODQÍQÍ

R.H.Süleymanova, N.A.Zeynalov, L.N.Qulubayova, A.R.Quliyeva, N.T.§ixverdiyeva

Tarkibi nikel, palladium, platin kegid metallarinin duzlari va funksional qruplar ta§kil edan vinilpiridin polimerlari ila hazirlanan metalpolimer komplekslarinin i§tiraki ila benzolun maye fazada hidrogenla§ma reaksiyasi öyranilmi§dir. Reaksiyada poli-4-vinilpiridin va poli-2-metil-5-vinilpiridin istifada olunmu§dur. ffidrogenla§ma reaksiyasi 20-600C temperaturda aparilir. Poli-4-vinilpiridin i§tirakinda benzolun hidrogenla§ma prosesina mühitin tasiri öyranilmi§dir. Müayyan edilmi§dir ki, platin-poli-4-vinilpiridin kompleksi i§tirakinda benzolun maye fazada hidrogenla§masi reaksiyasi zamani tam hidrogenla§ma mahsulu ila yana§i, natamam hidrogenla§ma mahsulu - tsikloheksen alinir. Hamginin mühitin tabiatinin tsikloheksan va tsikloheksenin giximina selektiv va effektiv tasiri öyranilmi§dir.

Agar sözlzr: hidrogenh§m3, benzol, polivinilpiridinbr, metalpolimer kompleksbr, tsikloheksan, tsikloheksen.

ИССЛЕДОВАНИЕ РЕАКЦИИ ЖИДКОФАЗНОГО ГИДРИРОВАНИЯ БЕНЗОЛА В ПРИСУТСТВИИ МЕТАЛЛОПОЛИМЕРНЫХ КОМПЛЕКСОВ НА ОСНОВЕ ПОЛИВИНИЛПИРИДИНОВ

Р.Г.Сулейманова, Н.А.Зейналов, Л.Н.Кулибекова, А.Р.Кулиева, Н.Т.Шихвердиева

Изучено жидкофазное гидрирование бензола на металлополимерных комплексах, которые готовились на винилпиридиновых полимерах, содержащих функциональные группы и солей переходных металлов, таких как никель, палладий, платина. В качестве винилпиридиновых полимеров были использованы поли-4-винилпиридин и поли-2-метил-5-винилпиридин. Гидрирование проводили при температуре 20-600С. А также были изучены влияние природы среды на процесс гидрирования бензола на поли-4-винилпиридине.Установлено, что в присутствии платино-поли-4-винилпиридинового комплекса в реакции жидкофазного гидрирования бензола образуются не только продукт полного гидрирования, но и продукты неполного гидрирования - циклогексен. Также установлена, что природа среда селективно и эффективно влияет на выход циклогексана и циклогексена.

Ключевые слова: гидрирование, бензол, поливинилпиридины, металлополимерные комплексы, циклогексан, циклогексeн.