SYNTHESIS OF ETHYLENE CARBONATE FROM ETHYLENE OXIDE AND CO2 IN THE PRESENCE OF ZINC PHENOLATES / IONIC LIQUID CATALYSTS Текст научной статьи по специальности «Химические науки»

ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2022 ISSN 0005-2531 (Print)

UDC 547.56.564

SYNTHESIS OF ETHYLENE CARBONATE FROM ETHYLENE OXIDE AND CO2 IN THE PRESENCE OF ZINC PHENOLATES / IONIC LIQUID CATALYSTS

E.F.Nasirli, M.J.Ibrahimova, M.Kh.Mamedov, S.R.Rafiyeva, F.A.Nasirov

Yu.Mamedaliyev Institute of Petrochemical Processes, NAS of Azerbaijan

fizuli_nasirov@yahoo.com

Received 06.01.2022 Accepted 16.02.2022

The synthesis of cyclic carbonates by the cycloaddition reaction of CO2 with epoxy compounds is one of the most promising directions in the utilization of CO2. The present work studies the synthesis of ethylene carbonate by the cycloaddition reaction of ethylene oxide with carbon dioxide in the presence of zinc-phenolate catalysts of the formula ZnY (where, Y - methylene-bis, thio-bis and dithio-bis alkyl phenols), ionic liquids of the formula RLX (where, R - H atom or C1 -C8 alkyl groups; L - cations of [NMP]+, [MIM]+, [Py]+ or alkyl amine; X - anions of Br-, HCOO-, CH3COO), or binary catalysts Zn-phenolate/ionic liquid, using selected optimal zinc phenolate and ionic liquid catalysts, or by using synthesized ionic liquid catalysts of the formula R2L2X2ZnY. In the presence of these catalysts the conversion of ethylene oxide is 32.0-99.0%, the selectivity for ethylene carbonate is 81.0-100.0%, the catalyst productivity is 210.0-1501.0 g EC/g Cat, and TOF is 891.0-12000.0 mol EC/mol Cath. Organic carbonates have a fairly large market (about 1.8 million tons/year) and are widely used as solvents, selective reagents, intermediates (for the synthesis of pharmaceuticals and agricultural preparates), fuel additives, as well as monomers in polymer synthesis.

Keywords: methylene-bis, thio-bis, dithio-bis, zinc alkyl phenolates, ionic liquids, cycloaddition reaction, ethylene oxide, carbon dioxide, and ethylene carbonate.

doi.org/10.32737/0005-2531-2022-2-69-86 Introduction

The use of CO2 (a by-product of many industrial processes) can provide the synthesis of valuable products from non-toxic, renewable, and inexpensive resources, which is a perspective and promising way to reduce the dependence of the chemical industry on fossil raw materials [1-3]. However, the high thermodynamic stability of CO2 creates difficulties in its chemical transformations [4-8]. In this aspect catalysis plays a decisive role, which arouses great interest in carrying out many studies in this direction.

The synthesis of cyclic carbonates by the reaction of cycloaddition of CO2 with epoxy compounds is one of the most promising directions in the utilization of CO2 since this reaction can be realized in one pass with 100% economy of atoms [9-13] (Figure 1).

Fig. 1. Synthesis of cyclic carbonate by reaction of CO2 with epoxides. R - H, CH3, CH2-CH3, CH2-Cl, QH etc.

Today, chemists have at their disposal a significant number of catalytic systems used in the synthesis of cyclic carbonates, and all catalytic systems currently used for the synthesis of cyclic carbonates can practically be attributed to one of two large groups: the first includes metal catalysts, that is salts or metal complexes; the second is made up of organocatalysts, which are most often used as phosphonium and ammonium salts, as well as alcohols and phenols. Most of the catalysts described in the literature have significant drawbacks, among which are low productivity, low stability, the need to use a cocatalyst, high cost, harsh process conditions, etc. [14].

It is known that the simplest and most readily available catalysts for the synthesis of various cyclic alkylene carbonates are zinc compounds. Among them, it should be noted systems zinc hal-ide/quaternary ammonium salts [14], in particular zinc bromide or chloride with tetrabutylammoni-um iodide (ZnBr2/TBAI) [15, 16], homogeneous and heterogeneous catalytic systems zinc (II)-pyridine [17], with the participation of which the reaction proceeds at 25-1400C and a CO2 pressure of 0.1-1.0MPa with a yield of 50-

65%, but not satisfactory TOF values, which are 680-1000 h-1.

It should be noted that despite of the high stability and satisfactory activity of most zinc complexes, the complexity of synthesis and multicomponent leads to their high cost, and, consequently, to the limitation of wide industrial application.

The use of ionic liquid (IL) technology should have solved these problems to some extent. In recent decades, numerous studies have been carried out on the cycloaddition reaction of CO2 with epoxides for the synthesis of cyclic carbonates with the participation of IL as a catalyst. This has greatly improved the understanding of how to effectively activate the CO2 and C-O bonding of epoxides.

It is known that tetrabutylammonium salts and some imidazole-based ILs as orga-nocatalysts are environmentally friendly and alternative metal-free catalysts for CO2 utilization with the formation of cyclic carbonates [17-22]. However, these organocatalysts often only work effectively at high temperatures (>1200C) and pressures (>10 bar CO2), which makes the processes difficult to implement in terms of a positive carbon dioxide balance. To increase the reactivity of organo-catalysts, which are often less reactive than metal complexes, cocatalyst of Lewis acid metals, such as zinc compounds, are added, which have a beneficial effect on the reaction rate [23-26].

Therefore, the synthesis and application of readily available zinc compounds and ionic liquids with high activity, selectivity and productivity in the reaction of obtaining cyclic carbonates (in particular, ethylene carbonate) from CO2 and epoxides is an urgent task and has a certain scientific and practical significance.

Experimental part

All the starting phenolic compounds used, zinc chloride, paraform (30% aqueous solution of formaldehyde), N-methyl-2-pyrroli-done, pyridine, diethyl amine, methyl- and ethyl imidazoles, formic and acetic acids, and the solvent - methylene chloride, received from Sigma-Aldrich and, if not specified separately, were used without additional purification.

The starting bis-alkyl phenol components of the catalyst of the formula ZnY (where, Y-methylene-bis, thio-bis and dithio-bis alkyl phenols) were synthesized according to a known method according to [27-30], by the Mannich condensation reaction of the corresponding alkyl phenols with CH2O, SCh, or S2Ch, in the presence of KU-2 catalyst. Methylene bis-, thio-bis-or dithio-bis alkyl phenolates of zinc (ZnY) used as a catalyst were synthesized through the reaction of the starting bis-alkyl phenols with NaOH (or KOH) and the exchange reaction of the obtained sodium (potassium) salt with ZnCl2, according to Scheme 1.

Physicochemical parameters of synthesized zinc phenolates are presented in Table 1.

Scheme 1. Reactions of the synthesis of zinc alkyl phenolates (A - CH2, S, S-S groups; R - H, C1-C8 alkyl groups or (C2H5)2-N-CH2 (dimethyl amino methyl) group; R' - H, C1-C8 alkyl groups).

Table 1. Physicochemical properties of methylene-bis alkyl phenolates of zinc

S-H S C H O Zn

№ Chemical formula Name Symbol Empirica formula Molecula mass F/C F/C F/C F/C F/C

1 (gr<"Hg) tert C4H9 tert ¿4H9 2,2'-methylene-bis-4-tert.-butyl phenolate-Zn MB-4-BPh-Zn n Z O VO O 375.0 - 66.2/67.2 7.1/6.9 8.8/8.5 17.8/17.4

2 / Zn\ X X H17C8 CH2 fOJ CSH" 2,2'-methy-lene-bis-6-octyl phenolate-Zn MB-6-OPh-Zn n Z O K 0\ O 487.0 - 70.3/71.5 8.7/8.6 6.5/6.6 13.1/13.6

3 tert-H17C8-jgj-CHrjgj-C8H17-tert tert'c4H9 tert'ciHç 2,2'-methy-lene-bis-4-tert.butyl-6-octyl pheno-late-Zn MB-4-B-6-OPh-Zn n Z o 00 K o 599.0 - 72.5/71.4 9.5/9.7 5.2/5.3 10.6/10.9

4 1 1 (^Hj^N-CHi-jgi-CHj-jg-C^-NiC^ tert'c4H9 tert'ciH, 2,2'-methy-lene-bis-4-tert.butyl-6-aminomethyl phenolate-Zn MB-4-B-6-DEAMPh-Zn n Z o 00 T K o 544.0 - 68.0/68.5 8.5/8.8 5.7/5.8 11.4/11.7

5 0 0 1 1 tert-HjC-j^oj—S-j^Qj-C4H»-tert 2,2'-thio-bis-6- tert.butyl phenolate-Zn TB-6-BPh-Zn n Z o M K o o 393.4 8.0/8.1 60.0/61.0 6.3/6.1 8.2/8.1 16.0/16.1

6 / Zn\ ° X 1 1 tert-C8H17 tert-C8H17 2,2'-thio-bis-4-octyl phenolate-Zn TB-4-OPh-Zn n Z O M o ^ K 00 o 505.4 6.2/6.3 65.9/66.5 7.5/7.9 6.2/6.3 12.5/12.6

7 ? ? tert-H9C4-|gj-S-^Qj-C4H9-tert 1 1 tert-C4H9 tert-QHg 2,2'-thio-bis-4,6-di-tert.-butyl phenolate-Zn TB-4,6-DBPh-Zn n Z o K o ^ K 00 O 505.4 6.4/6.3 66.5/66.3 7.7/7.9 6.3/6.3 12.5/12.6

8 Zn 0 0 1 1 (C2H5)2N-H2C-^-s-^-CHrN(C2H tert-C^ tert-CiHs 2,2'-thio-bis-4-tert.butyl-6-diethylamino-methyl phemo-late-Zn 1 n 6e? -4£ n N O 2? M ffi O CI O 559.4 5.3/5.7 63.1/64.3 7.8/7.5 5.5/5.7 11.5/11.4

9 p x 2,2'-dithio-bis-6-tert.butyl phenolate-Zn DTB-6-BPh-Zn n N O c/? O O 425.4 15.2/15.0 56.0/56.4 5.5/5.6 7.2/7.5 15.1/15.0

10 Zn £ > tcrt-H17C1!-'0^—S-S—i'q j-C8H17-tert 1 1 tert-C8H17 C8H17 -tert 2,2'-dithio-bis-4,6-dioctyl phe-nolate-Zn DTB-4,6-DOPh-Zn n N O c/? ¿H T 0 761.4 8.2/8.4 69.0/69.3 9.3/9.4 4.4/4.2 8.2/8.4

11 Zn / > 1 1 tert-C8H17 CgH17 -tert 2,2'-dithio-bis- 4-octyl-6-tert.butyl pheno-late- Zn DTB-4-O-6-BPh-Zn n N O c/? ffi VO ci 0 649.4 9.5/9.8 66.4/66.5 8.5/8.6 4.7/4.5 9.7/9.8

12 ' 1 (C2H5)2N-H2C^^v-S-S-[^VCH2-N(C2H5 1 1 tert-C8H17 tert-C8H17 2,2'-dithio-bis-^ 4-octyi-6-diethyl ami-nomethyl phe-nolate- Zn DTB-4-O-6-DEAMPh-Zn n N O 2? c/? ffi 00 CI O 706.0 8.9/9.0 64.2/64.5 8.8/8.7 8.9/9.0

Note: F - found; C - calculated.

The ionic liquids of the general formula RLX used in this work (where R - H or Ci-Cg alkyl groups; L - cations of N-methylpyrroli-dinium ([NMP]+), imidazolium ([IM]+), pyri-dinium ([Py]+) or alkylamine ([AlkAm] ); X -anions of Br-, [ZnCl2]-, HCOO-, CH3COO) were synthesized and characterized according to the general scheme [31-41]:

and according to the following procedure: equi-molar amounts of L (NMP, methylimidazole, py-

ridine or alkyl amine) and compound RX (1-alkyl bromide, zinc chloride, formic or acetic acids, where R - H or C1-C8 alkyl groups; X - Br-, [ZnCh]-, HCOO-, CH3COO) in an appropriate solvent (or in the absence of solvent) is placed in a three-necked round-bottom flask, thoroughly mixed and heated to 70-1000C for 4-24 hours in a nitrogen atmosphere. The resulting viscous liquid is cooled to room temperature, washed several times with small portions of ethyl acetate (EtOAc), unreacted starting compounds are

removed, and the main product is dried under vacuum at 800C. The result is an RLX ionic liquid with yields of 85.0-98.0%.

Synthesis of zinc-containing ionic liquids of the formula R2L2X2ZnY (where, R - H or Ci-C8 alkyl groups; L - cations of NMP, IM, Py or AlkAm; X - anions of Br- or [ZnCl2]-; Y - methylene-bis, thio-bis or dithio-bis alkyl phenolate groups) is carried out under a nitrogen atmosphere by the reaction of the corresponding ionic liquids of the general formula RLX [RNMPfX-, [RMIM]+X-, [RPy]+X- or [RAlkAm] X- (where, R - H or C1-C8 alkyl groups; X - Br-, [ZnCl2]-) with the corresponding zinc phenolates of the general formula ZnY (where, Y - methylene-bis, thio-bis or dithio-bis alkyl phenolate groups) according to the scheme [42]:

2RLX + ZnY » R;l:X:ZnY and according to the following procedure: in a

three-necked flask, to a solution of the ionic liquid of the general formula RLX (0.2 mol) in methylene chloride, a solution of the corresponding zinc phenolate of the general formula ZnY (0.1 mol) in toluene is added and stirred for 3 hours at a temperature of 70-1000C. After cooling to room temperature, separation is observed. By separating the resulting phases by filtration, the lower phase is washed several times with ethyl acetate and dried under vacuum at 800C. The output of IL R2L2X2ZnY is 95.0-99.0%.

Physicochemical parameters of synthesized ionic liquids are shown in Table 2.

All experiments on the synthesis of ethylene carbonate (EC) by the cycloaddition reaction of ethylene oxide with carbon dioxide under pressure were carried out in a 1 l stainless-steel autoclave. For this, in order to remove air, the autoclave was filled three times with carbon dioxide to 1.0 MPa, and the pressure was released.

Table 2. Physicochemical properties of used ionic liquids

№

Name

Structural formula

Symbol

Molecular mass

Density, g/sm3

Tm, °C

N-methylpyrrolidinium zinc chloride

[NMP] [ZnCl2]

235.43

1.25

-15

N-methylpyrrolidinium format

[HNMP]For

117.1

1.12

-32

N-methylpyrrolidinium acetate

[HNMP]Ac

131.2

1.05

-49.1

N-methylpyrrolidinium bromide

C< fe

[HNMP]Br

180.03

1.15

-35

1 -methyl-imidazolium bromide

^ Rr

[HMIM]Br

163.0

1.28

41.0

1 -ethyl-3 -methyl-imid-azolium bromide

[EMIM]Br

191.1

1.48

78.9

1-butyl-3-methyl-imidazolium bromide

[BMIM]Br

219.2

1.29-1.30

73-77

1-butyl-3-methyl-imidazolium acetate

[BMIM]Ac

198.3

1.06

-20

Diethyl ammonium formate

[HDEA] [For]

119.2

0.99

4.0

10

Triethyl ammonium bromide

[HTEA]Br

182.1

1.03

11

Pyridinium bromide

r^H Br

<L/H

[HPy]Br

160.0

1.35

-12

1

2

3

4

5

6

7

8

9

5

The autoclave was charged with the required amount of catalyst in a solvent and ethylene oxide in a stream of CO2. Then the required pressure was brought up using CO2. The autoclave was heated to a predetermined temperature for the duration of the reaction. After completion of the reaction, the autoclave was cooled to room temperature, the pressure was released, and the reaction mixture was passed through silica gel to separate the catalyst (eluent CH2Cl2). The reaction products were purified by column chromatography, eluent EtOAc:hexane = 1:3.

The structure of all starting bis-alkyl phenols, synthesized catalysts and target products were confirmed by IR- and 1H-NMR, 13C-NMR spectroscopy.

IR-spectra were recorded on a "Bruker Lumos FT-IR Microscope spectrometer" (Germany) at 400-4000 cm-1. IR-specters of zinc phenolate containing ionic liquid catalyst -R2L2X2ZnCl2 and ethylene carbonate are shown in Fig. 1 and 2, respectively.

NMR-spectra were recorded on a "Bruker-Fourier spectrometer at frequencies of 300 MHz (for 1H-NMR) and at a frequency of 101 MHz (for 13C-NMR). DMSO-d6, CDCl3, CD3OD, D2O, or CD2Cl2 were used as solvents. 1H-NMR- (a) and 13C-NMR- (b) specters of zinc phenolate containing ionic liquid catalyst -R2L2X2ZnCl2 and ethylene carbonate are shown in Figures 3 and 4, respectively.

Fig 1. IR-spectra of zinc phenolate containing ionic liquid catalyst - R2L2X2ZnCl2: [2,2^-thio-bis 4-butyl-6-diethyl-aminomethyl phenolate of zinc]-[diN-methyl-2-pyrrolidinium zinc chloride] - [TB-4-B-6-DEAMPh-Zn]+][NMPZnCl2]2-.

Fig. 2. IR-spectra of ethylene carbonate.

b

Fig 3. 1H-NMR- (a) and 13C-NMR- (b) specters of zinc phenolate containing ionic liquid catalyst - R2L2X2ZnCl2: [2,2-thio-bis 4-butyl-6-diethylaminomethyl phenolate of zinc]-di[N-methyl-2-pyrrolidinium zinc chloride] - [TB-4-B-6-DEAMPh-Zn]+] [NMPZnCl2]2-.

160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm

a b

Fig. 4. 1H-NMR- (a) and 13C-NMR- (b) specters of ethylene carbonate.

The IR spectrum of ethylene carbonate shows peaks at 1770.4 cm-1, 1796.2 cm-1 (C=O stressed vibration), 1155.8 cm-1, 1067.3 cm-1 (C-O-C symmetric stressed vibration), 971.0 cm-1 (C-O symmetrical vibration) (Figure 2). Absorption peaks at 715.9 cm-1 indicate an ester group in ethylene carbonate (ethylene oxide), and absorption peaks at 1481.0 cm-, 893.0 cm-1, 773.0 cm-1 indicate a carbonate group (Figure 2).

Examination of the 1H-NMR spectrum reveals an absorption signal in the chemical shift region of 3.571 ppm, which belongs to ethylene carbonate. This signal indicates an ethylene group between two carbonate fragments (Figure 4, a).

In the 13C-NMR spectrum (Figure 4, b) there are absorption signals in the chemical shift region of 64.13 ppm and 155.46 ppm, belonging to CH2 and the carbonate group in ethylene carbonate.

Results and discussion

In this work, for the synthesis of ethylene carbonate, were applied three different approaches: first, was studied the activity and selectivity of synthesized various methylene-bis, thio-bis, and dithio-bis alkyl phenolates of zinc (ZnY) in the synthesis of cyclic ethylene carbonate by cycloaddition reaction of CO2 with ethylene oxide with the purpose of choosing the optimal zinc phenolate catalyst and optimal reaction conditions; then, in a similar reaction, the

activity and selectivity of various ionic liquids of the formula RLX ([NMP]+[ZnCl2]-, [NMP]+X-, [NMP]+For-, [NMP]+Ac-, [TBA]Br-, [RIM]+X-, [DAlkAm]+Ac-, etc.) were tested and chose of the optimal ionic liquid and optimal conditions; further, in the reaction of ethylene carbonate synthesis, the activity and selectivity of either binary Zn-phenolate/ionic liquid catalysts were tested using the selected optimal zinc phenolate and ionic liquid catalysts and optimal conditions, or by using zinc phenolate containing ionic liquid catalysts of the formula R2L2X2ZnY synthesized on their basis.

Study of the activity of various zinc phenolate catalysts ZnY in the cycloaddition reaction of ethylene oxide and carbon dioxide

For the synthesis of ethylene carbonate have been tested various methylene-bis, thio-bis and dithio-bis alkyl phenolates of zinc of the general formula:

where, X - CH2, S, S-S groups; R - H, C1-C8 alkyl groups or C2H5)2-N-CH2 (diethyl amino methyl) group; R' - H, C1-C8 alkyl groups.

The experimental results are summarized in Table 3.

Table 3. Activity of methylene-bis, thio-bis, dithio-bis alkyl phenolates of zinc in the synthesis of ethylene carbonate based on EO and C02. Reaction conditions: [EO] =3.0 mol/1; [Cat] =1.0-10"3 mol/1; PCO,=5.0 MPa; T=80°C; x =60 min, solvent is methylene chloride_

№ PfltiilvQt EO Conversion, Selectivity on EC, Catalyst productivity, TOF,

% % g EC/g Cat mol EC/mol Cath

1 MB-4-BPh-Zn 85.0 99.0 300.0 1275.0

2 MB-6-OPh-Zn 92.0 99.0 250.0 1380.0

3 MB-4-B-6-OPh-Zn 95.0 99.5 210.0 1426.0

4 MB-4-B-6-DEAMPh-Zn 96.0 99.0 350.0 1424.5

5 TB-6-BPh-Zn 88.0 92.0 543.6 1215.0

6 TB-4-OPh-Zn 92.0 91.0 437.2 1255.2

7 TB-4.6-DTBPh-Zn 90.0 93.0 437.3 1255.1

8 TB-4-B-6-DEAMPh-Zn 99.0 100.0 372.1 1500.0

9 DTB-6-BPh-Zn 82.0 94.0 403.6 976.6

10 DTB-4-0-6-BPh-Zn 91.0 95.0 295.9 1098.0

11 DTB-4,6-DOPh-Zn 94.0 98.0 268.9 1164.0

12 DTB-4-0-6-DEAMPh-Zn 96.0 98.0 296.0 1186.6

As you can see from Table 3, under the reaction conditions: [EO]= 3.0 mol/l; [Cat]= 1.010"3 mol/l; Pcoz = 5.0 MPa; T= 80°C; x= 60 min, the solvent is methylene chloride, all used methylene-bis, thio-bis and dithio-bis alkyl phenolates of zinc exhibit high activity and selectivity in the reaction of ethylene carbonate synthesis based on CO2 and EO. In the presence of these catalysts, the conversion of EO reaches 82.0-99.0%, and the selectivity for EC is 91.0100.0%. In this case, the productivity of the catalyst and the process TOF are in the ranges of 210.0-437.0 g EC/g Cat and 976.6-1500.0 mol EC/mol Cath, respectively.

The best results are obtained when using MB-4-B-6-DEAMPh-Zn and TB-4-B-6-DEAMPh-Zn as catalysts (Table 3, experiments 4 and 8). Considering the performance of the catalyst thio-bis-4-butyl-6-diethyl amino methyl phenolate of zinc (TB-4-B-6-DEAMPh-Zn) was chosen as optimal. In the presence of this catalyst, the best values of the conversion of EO (99.0%), selectivity for EC (100.0%), productivity of the catalyst (372.1 g EC/g Cat), and the process TOF (1500.0 mol EC/mol Cath) are achieved.

Study of the activity of various ionic liquid catalysts RLX in the synthesis of ethylene carbonate by the cycloaddition reaction of ethylene oxide and carbon dioxide

As ionic liquids we used N-methyl-2-pyrro-lidinium zinc chloride ([NMP]+[ZnCl2]-), N-methyl-2-pyrrolidinium acetate [HNMP]+Ac), N-methyl-

2-pyrrolidinium formate ([HNMP]For), N-methyl-2-pyrrolidinium bromide ([HNMP]Br), 1-methyl-imidazolium bromide ([HMIM]+Br ), 1-ethyl-3-methylimidazolium bromide ([EMIM]+Br), 1-bu-tyl-3-methyl-imidazolium bromide ([BMIM]+Br ) and acetate ([BMIM]+Ac ), diethyl ammonium formate ([HDEA]For), triethylammonium bromide ([HTeA]+Br) and pyridinium bromide ([HPy]Br). The results of the synthesis of ethylene carbonate using the synthesized ionic liquids are summarized in Table 4.

As you can see from the Table 4, from a number of synthesized ionic liquids [HNMP]+Ac-, [HNMP]+For-, [EMIM]+Br-, [HDEA]+For-, [BMIM]+Ac-, [HTeA]+Br- and [HPy]+Br- (Table 4, experiments 2, 3, 6, 8-11) exhibit slightly lower catalytic activity and selectivity in the synthesis of ethylene carbonate, than others. In their presence, the conversion of EO is 31.6-77.7%, the selectivity for EC is 80.0-95.0%, the productivity of the catalyst is 478.7-1128.0 g EC/g Cat and TOF is 891.02190.0 mol EC/mol Cath.

The ionic liquids [NMP]+[ZnCl2]-, [HNMP]+Br-, [HMIM]+Br-, [BMIM]+Br- exhibit high activity and selectivity (Table 4, experiments 1, 4, 5, 7). In their presence, the conversion of EO and the selectivity for EC reaches 94.5-99.0% and 98.0-99.0%, respectively. The productivity of the catalyst and TOF are also high: 1077.2-1500.8 g EC/g Cat and 2778.02940.0 mol EC/mol Cath, respectively.

№ Catalyst Conversion of EO, % Selectivity, % Yield of EC, % Catalyst productivity, g EC/g Cat TOF, mol EC/mol Cat h

1 [NMP]+[ZnCl2]- 98.0 98.0 96.0 1077.2 2880.0

2 [HNMP]+Ac- 65.0 86.0 56.0 1128.0 1680.0

3 [HNMP]+For- 56.0 85.0 48.0 1083.4 1440.0

4 [HNMP]+Br- 96.5 99.0 95.5 1402.0 2865.0

5 [HMIM]+Br- 94.5 98.0 92.6 1500.8 2778.0

6 [EMIM]+Br- 77.7 95.0 73.0 1009.2 2190.0

7 [BMIM]+Br- 99.0 99.0 98.0 1181.1 2940.0

8 [HDEA]+For- 52.0 80.0 42.0 930.8 1260.0

9 [BMIM]+Ac- 59.0 81.0 47.8 636.8 1434.0

10 [HTeA]+Br- 34.9 95.0 33.0 478.7 990.0

11 [HPy]+Br- 31.6 94.0 29.7 490.0 891.0

Table 4. Activity and selectivity of various ionic liquids in the reaction of ethylene carbonate synthesis. Reaction conditions: [E0]=3.0 mol/l; [Cat]=1.0-10-3mol/l; Pco=5.0 MPa; T=800C; t=60 min, stirrer rotation speed 350 rpm, solvent -methylene chloride

Study of the synthesis of ethylene carbonate by the cycloaddition reaction of ethy-lene oxide and carbon dioxide in the presence of the optimal catalysts ZnY+RLX or R2L2X2ZnY

In order to clarify the possible synergism in the activity and selectivity of zinc alkyl phenolate and ionic liquid catalysts, we studied binary catalysts RLX+ZnY (where R - H or C1-C8 alkyl groups; L - [NMP]+, [MIM]+, [Py]+ or [Alkyla-mine]+ cations; X - Br-, HCOO-, CH3COO- anions and Zn - metal, Y - methylene-bis, thio-bis and dithio-bis alkyl phenolate groups) in the cycloaddition reaction of EO and CO2. The experiments were carried out under the following conditions: [RLX]= 2.5 0-4 mol/l, [ZnY]= 2.510-4 mol/l, [EO]= 3.0 mol/l; PCO=5.0 MPa; T=800C; t =60 min, stirrer rotation speed 350 rpm, solvent -methylene chloride. The results of the study are summarized in Table 5.

As you can see from Table 5, binary catalysts exhibit rather high activity and selectivity even at low concentrations of the starting components (2.5T0-4 mol/l of each component instead of 1.0 10-3 mol/l in previous experiments, Tables 3 and 4), which is undoubtedly proves the presence of a synergistic effect in their joint action in the reaction of ethylene carbonate synthesis. Thus, in their presence, high conversions of EO and selectivity for EC are achieved in the range of 95.099.0% and 97.0-100.0%, respectively. In this case, high productivity of the catalyst and TOF are also achieved in the range of 1000.0-1428.0 g EC/g Cat and 5238.0-5940.0 mol EC/mol Cath, respectively (Table 5, experiments 1-5).

A similar dependence is also observed when using zinc phenolate containing ionic liquid catalysts of the formula R2L2X2ZnY (where, R - H or C1-C8 alkyl groups; L - NMP, IM, Py or alkylamine cationic groups; X - anions of Br or ZnCl2; Y - methylene-bis, thio-bis or dithio-bis alkyl phenolate groups) synthesized from the ZnY and RLX components.

Under the reaction conditions: [R2L2X2ZnY] = 2.5-10"4 mol/l, [E0]=3.0 mol/l; Pccj:=5.0 MPa; T=80°C; t=60 min, stirrer rotation speed 350 rpm, solvent - methylene chloride, EO conversion is 97.0-100.0%, with an EC selectivity of 98.0-100.0%. Catalyst productivity is 1315.6-1427.0 g EC/g Cat and TOF reaches a value of 11400.0-12000.0 mol EC/mol Cath, which indicates both high conversion of EO and EC selectivity, as well as the high productivity of the catalyst and the process TOF in the presence of a zinc containing ionic liquid catalyst R2L2X2ZnY.

The proposed mechanisms of EC synthesis by the cycloaddition reaction of EO and CO2 in the presence of zinc phenolate, ionic liquid and zinc phenolate/ionic liquid catalysts

Mechanism of the cycloaddition reaction of EO and CO2 in the presence of zinc phenolate catalysts ZnY

The scheme of the proposed reaction mechanism for the synthesis of ethylene carbonate by the reaction of ethylene oxide with CO2 in the presence of zinc phenolate catalysts is shown in Figure 5.

Table 5. Synthesis of ethylene carbonate by the cycloaddition reaction of ethylene oxide presence of the optimal catalysts RLX+ZnY or R2L2X2ZnY. Reaction conditions: [EO] T=80°C; T=60min, stirrer rotation speed 350 rpm, solvent - methylene chloride

and carbon dioxide in the =3.0mol/l; PCOn=5,OMPa;

№ Catalyst Catalyst concentration, [Cat.]^104, mol/l EO Conversion, % Selectivity, % Catalyst productivity, g EC/g Cat. TOF, mol EC/mol Cath

1 [NMP]+[ZnCl2]-+TB-4-B-6-DEAMPh-Zn 2.5+2.5 99.0 100.0 1324.4 5940.0

2 [HNMP]+Br + MB-4-B-6-OPh-Zn 2.5+2.5 98.0 99.0 1314.7 5820.0

3 [BMIM]+Br-+ TB-4,6-DBPh-Zn 2.5+2.5 98.0 100.0 1428.0 5880.0

4 [HPy]+Br + DTB-4,6-DOPh-Zn 2.5+2.5 90.0 97.0 1000.0 5238.0

5 [HTEA]+Br + MB-4-B-6-DEAMPh- Zn 2.5+2.5 95.0 98.0 1354.0 5586.0

6 [NMP][ZnCl2]-[TB-4-B-6-DEAMPh-Zn]+ 2.5 99.0 100.0 1315.6 11880.0

7 ([HNMP]Br)2]-[TB-4-B-6-DEAMPh-Zn]+ 2.5 100.0 100.0 1427.0 12000.0

8 ([BMIM]Br)2]-[TB-4-B-6-DEAMPh-Zn]+ 2.5 99.0 100.0 1342.4 11880.0

9 ([HPy]Br)2]-[TB-4-B-6-DEAMPh-Zn]+ 2.5 98.0 99.0 1423.5 11640.0

10 ([(HTEA)Br)2]-[TB-4-B-6-DEAMPh-Zn]+ 2.5 97.0 98.0 1353.6 11400.0

,_

V)

\ p

0 1 o- JL

r

I Y

II

III

o

II

o

R;

j

>

O

IV

where Zn is a metal atom (acid) and O is an oxygen atom (base) in the Zn-phenolate catalyst.

Fig. 5. The proposed reaction mechanism for the synthesis of ethylene carbonate in the presence of Zn-phenolate catalysts.

In the proposed mechanism, the zinc phe-nolate catalyst contains both an acid fragment (Zn) and a Lewis base (O), providing electro-philic and nucleophilic activation of epoxide and CO2. The metal atom acts as a Lewis acid for electrophilic activation, while the oxygen atom acts as a Lewis base for nucleophilic activation. The cycloaddition reaction is initiated by the recruitment of a CO2 molecule at the main sites of the Lewis base to form a carbox-ylate anion, while the epoxide is activated by inclusion at the acid site of the Lewis. The carbon atom of the epoxy component is attacked by the carboxylate anion, which causes the epoxy to ring open to form the oxy-anion. Dissociation of the metal oxide (catalyst) from the

oxy-anion results in ring closure and removal of the cyclic carbonate as the main product. A few (~1.0-10.0%) by-products resulting from the side reactions of cyclic addition of CO2 and epoxide include epoxide isomers such as ketone and aldehyde and epoxide dimers.

Influence of various reaction parameters such as catalyst type, catalyst and ethylene oxide concentrations, CO2 pressure, reaction time and temperature, etc. discussed in our previous works [45-47] and it was found that increasing the reaction time increases the EO conversion, EC yield and selectivity. Experimental results have shown that an increase in the reaction temperature above 1200C and a CO2 pressure above 6.0 MPa decreases the EC yield. The op-

I

timal condition for the reaction of cycloaddition of EO with C02 was found: [Cat.]=1.0-10"3 mol/1; [EO] =3.0 mol/1; Pco:= 6.0 MPa; T=800C; t=60 min. and stirring speed 350 rpm in the presence of TB-4-O-6-DEAMPh-Zn catalyst. Under these conditions, the conversion of EO, the yield and selectivity of EC, the productivity of the catalyst and TOF were 96.0%, 99.0%, 95.0%, 372.1 g EC/g Cat and 1424.5 mol EC/mol Cath, respectively.

Mechanism of the cycloaddition reaction of EO and CO2 in the presence of ionic liquid catalysts

In the presence of ionic liquids such as [R2L]+X (R- H, C1-C8 alkyl radicals, X - Cl-, Br-, I-), the coordination of a proton attached to [R2L]+ through a hydrogen bond with the oxygen atom of the epoxide occurs polarization of

epoxy bonds C-O and formation of active epoxide (I). Simultaneously, with the nucleophilic attack of Br- on the less hindered P-carbon atom of the coordinated epoxide, the formation of an intermediate link with the opening of the epoxy ring is provided (II). Then, the interaction of the oxygen anion with CO2 (II) leads to the introduction of CO2 into the halohydrin and the formation of the alkyl carbonate anion (III). Finally, intermediates (III and IV) are further converted to the corresponding cyclic carbonate by closing the intramolecular ring and the catalyst is regenerated. It is assumed that during the entire reaction cycle, the proton in protic ionic liquids should initiate the initial activation of the epoxide and the stabilization of intermediates (II), (III) and (IV) during the reaction (Figure 6).

r2l|+x"+

-R

rzl|+x

V ._/R'

O 'r2l

+ + CO,

r x

II

+ co2

r^O Q o

0[R2L]+ —► \-.

R

O R2L]

x

R

O

R2l|+X"+o//\)

R

III IV

Fig. 6. The proposed mechanism of the cycloaddition reaction of EO and CO2 in the presence of ionic liquid catalysts.

I

Mechanism of the cycloaddition reaction of EO and CO2 in the presence of Zn-phenolate/ionic liquid (ZnY + R LX) catalyst

Based on the obtained experimental data and the proposed mechanisms of the cycloaddition reaction of CO2 with EO, the proposed mechanism of EC synthesis in the presence of the ZnY+R2LX catalyst is considered by us according to the scheme in Figure 7, which includes the following steps: first, the epoxide is coordinated with the Lewis acid Zn to form the

adduct of the metal-epoxy complex I; then the halogen anion of the ionic liquid carries out a nucleophilic attack on the less hindered carbon atom of the epoxide, followed by ring opening to form the oxy-anionic structure II; further, C02 is coordinated with the complex by interacting with Br- and O-, as a result of which complex IV is formed; finally, EC is formed due to intramolecular cyclic elimination (V), with the reduction of the initial components of the catalytic system.

ZnY + R2LX +

,R3

R +

[R2L1+X"+V

X R2L

Zn —

II

+ co2

III

IV

o

+ cf No [R^r

w

R3 X

V

+ i RzL|+x" +

O

A

R

Fig. 7. The proposed mechanism of the cycloaddition reaction of EO and CO2 in the presence of a binary zinc phenolate/ionic liquid catalyst ZnY + R2LX.

I

The influence of the acidity of the cation and the nucleophilicity of the anion of the metal complex can be explained by this mechanism. The coordination of the epoxide is easier if the Lewis acidity of the metal cation is stronger. In addition, the more nucleophilic anion is useful for its interaction with the carbon atom of CO2. Two conflicting factors: nucleophilicity and ste-ric hindrance of halide anions enhance the interaction of X with a carbon atom of CO2, contributing to the formation of a more positive complex with CO2 (complex III).

The proposed mechanism can explain the high yield of ethylene carbonate at a molar ratio of IL/Zn phenolate equal to 2, but an increase in this ratio to 4 does not lead to a significant increase in activity; two epoxide molecules are coordinated with one Zn-phenolate molecule, and two ionic liquid molecules interact with the resulting metal-epoxy complex adduct. Catalytic systems containing ionic liquids with BF4-and PF6- anions show low activity and selectivity in the reaction of synthesis of cyclic carbonates, as was observed in [43, 44], which is associated with the non-nucleophilic nature of the BF4- and PF6- anions.

Mechanism of the cycloaddition reaction of EO and CO2 in the presence of a zinc phenolate containing ionic liquid catalyst R2L2X2ZnY.

The mechanism of EC synthesis in the presence of the R2L2X2ZnY catalyst is similar to the mechanism of the reaction when the binary system ZnY+R LX is used. The difference is that when the binary system ZnY+R LX is used, activation of EO and CO2 molecules occurs on different molecules of zinc phenolate and ionic liquid with the involvement of one EO molecule and one CO2 molecule. In the presence of the R2L2X2ZnY catalyst, at first two moles of EO are activated on one catalyst molecule (I) and complex (II) is formed, after which two CO2 molecules (III) are introduced and complex (IV) is formed (Figure 8).

After that, EC is formed due to intramolecular cyclic elimination, with the reduction of the initial components of the catalytic system. The introduction of two moles of both EO and CO2 in a molecule of one catalyst mole leads to a sharp increase (almost two-fold) in the catalyst productivity.

I

О

Y Y

о

R + 2 (/ /°"[R2L]+

С

r3 x

IV

r2L]+?

о

<Л>

R

Fig. 8. The proposed mechanism of the cycloaddition reaction of EO and CO2 in the presence of an ionic liquid catalyst R2L2X2ZnY (where, R - H or Q-Cg alkyl groups; L - NMP, IM or Py groups; X - Cl, Br or I; Y - methylene-bis, thio-bis or dithio-bis alkyl phenolate groups.

Conclusions

This study demonstrates the possibility of successful use of both Zn-phenolate and ionic liquid catalysts, and their combined use as an effective catalytic system for the cycloaddition reaction of ethylene oxide and CO2. The almost quantitative yield of ethylene carbonate is achieved with the highest atom economy under mild reaction conditions. In comparison with the well-known works devoted to the cycloaddition reaction of epoxides and CO2, noteworthy features are:

- synergism in catalytic action with the combined use of Zn phenolate and ionic liquid catalyst, which manifested itself in an increase in both the productivity of the catalyst and the TOF of process;

- simplicity of the reaction system;

- easy separation, regeneration and reuse of the catalyst;

- carrying out the reaction under moderate conditions (relatively low temperature/pressure

in the reaction zone and the duration of the reaction).

The process using the cycloaddition reaction of EO and CO2 has great industrial potential due to its environmental friendliness and high productivity for obtaining the target product - ethylene carbonate.

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ETiLEN OKSiD VO CO2-DON SiNK FENOLYAT/iON MAYE KATALiZATORLARI ΧTÎRAKI

iLO ETiLEN KARBONATIN SiNTEZi

E.F.Nasirli, M.J.ibrahimova, M.X.Mamm3dov, S.RRafiyeva, F.O.Nasirov

Etilen oksidin karbon qazi ils tsiklobirlsçms reaksiyasindan etilen karbonatin sintezi ZnY (burada, Y=metilen-bis, tio,-bis vs ditio-bis alkil fenollar) formuluna malik sink-fenolyat katalizatorlarindan, formulu RLX (burada R = H vs ya C1-C8 alkil qruplari; L = [NMP]+, [MIM]+, [Py]+ vs ya [Alkilamin] + kationlari; X = Br-, HCOO-, CH3COO- anionlari) ion mayelsrindsn, vs ya seçilmiç optimal sink fenolyat vs ion maye (Zn-fenolyat/ion maye) binar katalizatorlarindan, yaxud formulu R2L2X2ZnY olan sintez edilmiç yeni ion maye katalizatorlarindan istifads etmskls apanlmiçdir. Bu katalizatorlarin içtiraki ils etilen oksidin çevrilmssi 32.0-99.0%, etilen karbonata görs seçicilik 81.0-100.0%, kataliza-torun mshsuldarligi 210.0-1501.0 q EK/q Kat, TOF iss 891.0-12000.0 mol EK/mol Katsaat. Üzvi karbonatlar kifayst qsdsr böyük bazara malikdir (tsxminsn 1.8 milyon ton/il) vs polimer sintezinds monomerlsr vs hslledicilsr, selektiv reagentlsr, araliq mshsullan (sczaçiliq vs ksnd tsssrrüfati preparatlarinin sintezinds), yanacaqlara slavslsr vs s. sa-hslsrds geniç istifads olunurlar.

Açar sözlzr: metilen-bis, tio-bis, ditio-bis, sink alkilfenolyatlar, ion mayehri, tsiklobirh§m3 reaksiyasi, etilen oksid, karbon qazi, etilen karbonat.

СИНТЕЗ ЭТИЛЕНКАРБОНАТА ИЗ ОКИСИ ЭТИЛЕНА И СО2 В ПРИСУТСТВИИ ЦИНК ФЕНОЛЯТ/ИОННО-ЖИДКОСТНЫХ КАТАЛИЗАТОРОВ

Э.Ф.Насирли, М.Дж.Ибрагимова, М.Х.Мамедов, С.Р.Рафиева, Ф.А.Насиров

Исследован синтез этиленкарбоната реакцией циклоприсоединения окиси этилена с диоксидом углерода в присутствии цинк-фенолятных катализаторов формулы ZnY (где Y = метилен-бис, тио-бис и дитио-бис алкилфенолы), ионных жидкостей формулы ЯЬХ (где, Я = Н или С1-С8 алкильные группы; Ь = катионы [КМР]+, [М1М]+, [Ру]+ или алкиламина; X - анионы Вг-, НСОО-, СН3СОО-), или бинарных катализаторов Zn-фенолят/ионная жидкость, с использованием выбранных оптимальных цинк фенолятных и ионно-жидкостных катализаторов и оптимальных условий, или же с применением синтезированных новых ионно-жидкостных катализаторов формулы R2L2X2ZnY. В присутствии этих катализаторов достигаются конверсия окиси этилена 32.0-99.0 %, селективность по этиленкарбонату 81.0-100.0%, производительность катализатора 210.0-1501.0 г ЭК/г Кат, и ТОБ 891.0-12000.0 моль ЭК/моль Кат-час. Органические карбонаты имеют довольно большой рынок (около 1.8 млн т/год) и находят широкое применение в качестве растворителей и мономеров в синтезе полимеров, селективных реагентов, промежуточных веществ (для синтеза лекарственных и сельскохозяйственных препаратов), добавок к топливу и т.д.

Ключевые слова: метилен-бис, тио-бис, дитио-бис, алкилфеноляты цинка, ионные жидкости, реакция циклоприсоединения, окись этилена, диоксид углерода, этиленкарбонат.