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Publicly Available Published by De Gruyter November 21, 2017

The coupling of carbon dioxide with ethene to produce acrylic acid sodium salt in one pot by using Ni(II) and Pd(II)-phosphine complexes as precatalysts

  • Andrea Vavasori EMAIL logo , Loris Calgaro , Luca Pietrobon and Lucio Ronchin

Abstract

The use of CO2 as a feedstock for chemical synthesis is considered as a viable alternative option to some traditional processes. One of the most interesting challenge for the industry is represented by the CO2 coupling with olefins to produce acrylate. Only recently, with the choice of suitable ligands and the use of a sacrificial base, a selective catalytic reaction was established by using Ni(0)-based complexes. The one-pot reaction, which leads to the highest TON (107 mol/mol Ni, in 20 h) reported so far, was successfully developed starting from Ni(0)-based precursors in the presence of disphosphine ligands, a large excess of base and of finely powdered zinc. In the present paper, we carried out the catalytic synthesis of sodium acrylate from CO2 and ethene, in one-pot, by using Ni(II)-chloride and Pd(II)-chloride phosphine-complexes as precatalyst. The reaction occurs under basic conditions and without adding any external reductants. The Ni(II) complexes lead to higher TON than the respective Pd(II) precursors and the best results are obtained by using diphosphines having high bite angles. Such catalysis is favored by aprotic and polar solvents in which a TON of 290 mol/mol Ni is reached by using the [NiCl2(dppp)] precursor in DMSO. Furthermore the TON could be increased by increasing the temperature, the base concentration and by using diphosphine ligands having high bite angle.

Introduction

The human activities related to the power generation and to the combustion of fossil fuels strongly contribute to increasing concentration of CO2 in the atmosphere which is considered one of the most important causes of global warming [1], [2], [3]. With the aim to contribute in the climate protection, several researchers around the world have engaged in a number of strategies to reduce CO2 emissions or alternatively to reduced CO2 concentration in the atmosphere [4], [5]. The most promising results have been obtained with the CO2 capture-storage and the CO2 conversion into useful chemicals. Actually CO2 as a feedstock for chemical synthesis is underutilized, due to the harsh and difficult conditions normally required to activate it [6], [7], [8]. Nevertheless, from an industrial point of view, it is still considered as a viable alternative option to some traditional processes in particular when the target products are value-added, economical and in demand [9], [10], [11], [12], [13], [14], [15]. As a matter of fact, the industrial utilization of CO2 is presently limited to the production of bulk chemicals like salicylic acid, urea, cyclic carbonates and polypropylene carbonate [7], [8], [9], [10], [15], even if one of the most interesting challenge for the industry is represented by the CO2 coupling with olefins to produce acrylate [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. This process would be sustainable and economical, as applications of acrylates are ubiquitous, particularly in the field of coatings, adhesives, construction chemicals, hygiene products and paints.

Currently, industrial synthesis of acrylates is carried out mainly through the SOHIO process, based on the catalytic oxidation of acrolein, at 300–360 °C over molybdenum/vanadium catalysts which, in turn, is produced by oxidation of propene at 300 °C over molybdenum/bismuth catalysts [27], [28], [29], [30]. It is obvious that the replacement of such high-energy two-step process with the one step coupling of CO2 and ethene would be very interesting. However, such reaction needs an efficient and active catalyst in order to reduce the high energy demand for CO2 activation. In the last years, several metal catalysts together with a huge variety of synthetic methods, promoters, and ligands, (Fe [31], [32], Pd [33], [34], Pt [35], [36], Ti [37], Zr [38], [39], [40], Rh [41], [42], Mo [43], [44] and Ni [45], [46], [47], [48]) have been deeply investigated both in experimental and theoretical studies [48]. However, in such papers only metallalactone formation has been mainly reported. Only recently, with the choice of suitable ligands and the use of a sacrificial base, a selective catalytic reaction was established by using Ni(0)-based complexes (reaction 1) [20], [24], [47], [48], [49], [50], [51]. The theoretical studies have allowed to point out that formation of Na-acrylate instead of acrylic acid shifts an overall endergonic reaction (for acrylic acid) [50] to a favorable exergonic process (−59 kJ/mol) [20].

  (1)

The one-pot reaction, which leads to the highest TON (107 mol/mol Ni, in 20 h) reported so far [17], was successfully developed starting from Ni(0)-based precursors in the presence of bisphosphine ligands, a large excess of base (for instance 300 equivalents of Na-phenoxide) and 100 equivalents of finely powdered zinc (100 equiv.). In some papers, however, it has been also pointed out that the monodentate phosphines and small bite-angle diphosphine ligands (i.e. dtbpm, 1,1′-bis(di-terbutilphosphino)methane, or dppm, 1,1′-bis(diphenylphosphino)methane,) gave no product with Ni(0)-based catalysts [49], [51]. A big influence on the TON (turnover number) is also due to the solvent, as the catalytic activity of Ni(0) complexes is favoured by apolar and aprotic solvents [49], [51].

To the best of our knowledge, all the nickel-based (or Pd-based) catalytic systems active in such reaction, include the formation of Ni(0) (or Pd(0)) catalytic active species. The most used route to form such species is the in situ synthesis from Ni(0) complexes, such as Ni(COD)2, Ni(PPh3)4, etc., and the suitable ligands. As the Ni(0) sources would be difficult to handle and manipulate because of their high air sensitivity and thermal instability, in some papers the more stable Ni(II) complexes are used as pre-catalysts, which are then reduced in situ to zerovalent nickel usually by treatment with external reductants, for instance by addition of zinc dust [13], [14], [15], [16], [17], [47]. Also the Pd(0) complexes catalyze the title reaction but they activity is usually lower than by using Ni(0) precursors. Analogously, such active species are formed in situ from Pd(0) precursors or by reduction of Pd(II) species [17].

In the present paper, we carried out the catalytic synthesis of sodium acrylate from CO2 and ethene, in one-pot, by using Ni(II)-chloride and Pd(II)-chloride phosphine-complexes as pre-catalyst. The reaction occurs in the presence of Na-phenoxide without adding any further reducing agent. The influence of phosphine ligands and the influence of some reaction parameters on the catalytic activity have been studied and discussed.

Experimental

Reagents

Carbon dioxide and ethene were supplied by SIAD Company Italy (‘research grade’, purity>99.9 %); PdCl2, NiCl2·6H2O, [PdCl2(PPh3)2] 99.99 %, [NiCl2(PPh3)2] 99.99 %, PPh3 (triphenylphosphine), dppm (1,1′-bis(diphenylphosphino)methane), dppe (1,2-bis(diphenylphosphino)ethane), dppp (1,3- bis(diphenylphosphino)propane), dppb (1,4-bis(diphenylphosphino)butane, Xantphos (4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene), and all the solvents used were Aldrich products used without further purification.

NaOPh 3H2O, 99 % (sodium phenoxide) was a Aldrich product: it was purified by re-crystallization by dissolution in methanol and precipitation of crystals from toluene.

Ni(II) and Pd(II) diphosphine complexes have been synthesized and characterized as described in literature [52], [53], [54], [55].

Equipments and characterization

The catalyst precursors NiCl2 and P-P were weighted on a Sartorious Micro balance (precision 0.001 mg); other reagents were weighted on a Mettler AT261 Delta Range balance (precision 0.01 mg).

Gas-chromatographic (GC) analysis was performed on a Hewlett Packard Model 5890, Series II chromatograph fitted with a HP1, 30 m×0.35 mm×0.53 μm column (detector: FID; carrier gas: N2, 0.2 mL/min; oven: 45 °C (3 min) to 250 °C at 15 °C/min).

GC/MS analyses were performed on a MS Agilent apparatus 5975C Model, interfaced with an Agilent chromatograph 7890 A Model equipped with a HP1 column (30 m×0.25 mm×0.25 μm, oven: 45 °C (3 min) to 250 °C at 15 °C/min).

Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Magna 750 instrument in KBr powder.

The NMR spectra were recorded on a Bruker Avance 300 spectrometer.

Catalytic reactions

The catalysis was carried out in a batch reactor of ca. 100 mL provided with a magnetic stirrer. In a typical experiment, 9.2×10−3 mmol (6.0 mg) of [NiCl2(PPh3)2] (bis(triphenylphosphine)nickel(II) dichloride) was added to 25 mL of THF (solvent) placed in a special cylindrical glass, whereupon it was added the Na-phenoxide (NaOPh, 4.6 mmol (534 mg): Na-phenoxide/Pd=500/1 mol/mol) under vigorous stirring. The cylindrical glass was then placed into the stainless steel autoclave and then the autoclave was pressurized with CO2 (0.2–0.3 MPa) and subsequently degassed at room temperature for three cycles. After that, the reactor was pressurized with 0.5 MPa of a 1/1 CO2/ethene gas mixture and heated up to the final temperature (90 °C) in ca. 10 min without stirring.

The pressure was then adjusted to the desired value (typically 4.5 MPa total pressure) by using a 1/1 CO2/ethene gas mixture, under vigorous stirring.

At the end of each experiment the autoclave was quickly cooled and carefully depressurized.

From the solid obtained by the reaction crude, the acrylic acid sodium salt was separated, purified and characterized following the procedure in Section “Recovery, purification and characterization of acrylic acid sodium salt”.

The TON (Turnover Number) has been calculated as the ratio: (mmol of sodium acrylate)/(mmol of precatalyst). Several experiments have been repeated in order to check the reproducibility. In particular, the reactions carried out in different solvents (see below) have been repeated for 3 times finding an experimental error lower than 3 %, in the TON values (only the average values have been reported in the manuscript).

Moreover, in each experiment carried out by using a constant amount of precatalyst, for instance 6 mg of [NiCl2(PPh3)2] (9.2×10−3 mmol), the weight of the complex really used was slightly different but in the range 6.00 mg±5 %. However, for simplicity, in the run conditions only one weight (6 mg, 9.2×10−3 mmol) it has been reported.

Recovery, purification and characterization of acrylic acid sodium salt

The solid recovered from the crude reaction was weighted (for instance 655.3 mg) and then completely dissolved, under vigorous stirring at 273 K, in a ca. 5 mL of a 2/1 (mL/mL) methanol/glacial acetic acid mixture. The solution was degassed and kept for 24 h, under a nitrogen atmosphere, at room temperature and in the dark, without stirring. The sodium acrylate was precipitated from this solution by adding, under vigorous stirring, 10 mL of a saturated solution of NaOH in methanol, freshly prepared. The solid was separated by the supernatant liquid by centrifugation (8000 rpm for 30 min) and washed for three times with anhydrous methanol. The GC and GC-MS characterizations of the liquid phases have been confirmed the presence of phenol, phosphine and phosphine oxide.

The white solid was dried at 70 °C under reduced pressure, weighted and characterized by FTIR, 1H-NMR, 13C-NMR spectroscopies: 1638 cm−1 (ν C=C), 1561 cm−1as. O=C-O-); 1H-NMR (300 MHz, D2O): δ=5.60 (dd, J=9.9, 2.1 Hz, 1H), 5.96 (dd, J=17.4, 2.1 Hz, 1H), 6.09 (dd, J=17.5, 9.9 Hz, 1H); 13C{H}-NMR (300 MHz, D2O): δ=126.9 (CH), 134.3 (CH2), 176.0 (CO).

The purity of the product was confirmed by the NMR spectra analysis.

In all the experiments reported, the percent of pure Na-acrylate recovered (for instance 250.10 mg corresponding a TON of 290 mol/mol) ranged between 50 and 25 % (in the example 38 % ) of the dried crude solid, in which it was qualitatively detected the presence of unreacted Na-phenoxide, NaCO3, phenol, and other not identified solid compounds.

Results and discussion

The Ni(II)-chloride and Pd(II)-chloride preformed complexes efficiently catalyze the one-pot synthesis of Na-acrylate from CO2 and ethene (reaction 1). The catalysis occurs without adding any reducing agent, but a sacrificial base is required to observe Na-acrylate formation (for instance Na-phenoxide [28], [49]).

Under reaction conditions specified in the experimental section, sodium acrylate, phenol and carbonates are formed in large amount (Scheme 1) whereas salycilate salts [56] has been never detected (see reaction 2).

Scheme 1: 
					Formation of sodium acrylate and sodium carbonate.
Scheme 1:

Formation of sodium acrylate and sodium carbonate.

  (2)

Synthesis of Na-acrylate by using as precatalyst Ni(II) and Pd(II) preformed complexes having monodentate phosphine ligands

Compared to the data reported in literature in which the monodentate phosphines (i.e. PPh3, PtBu3) and small bite-angle chelating diphosphines (i.e. dtbpm, dppm) failed in the Ni(0) catalyed synthesis of Na-acrylate [49], we have found that such reaction is efficiently catalyzed starting from [NiCl2(PPh3)2] and [PdCl2(PPh3)2] preformed complexes. The Fig. 1 and Table 1 show that, with Ni(II) complex a TON of 280 mol/mol Ni is reached whereas by using Pd(II) complex the sodium acrylate produced was 222 mol/mol Pd. As the major advantages of Ni-based catalysts compared to the corresponding Pd catalyst systems are their much lower cost, we optimized mainly the Ni(II) system, but in order to better highlight the difference between the two precursors we optimize also the Pd(II) catalysis. Although the TON increases linearly by increasing the amount of catalyst, we used a fixed amount of catalyst (0.0092 mmol, TON 77 mol/mol Ni) in all the experiments.

Fig. 1: 
						Influence of precatalyst concentration on turnover number.
						Run conditions: Na-phenoxide/Pd=500/1 mol/mol; p C2H4: 25 atm; pCO2: 25 atm; THF: 25 mL; 90 °C; 20 h.
Fig. 1:

Influence of precatalyst concentration on turnover number.

Run conditions: Na-phenoxide/Pd=500/1 mol/mol; p C2H4: 25 atm; pCO2: 25 atm; THF: 25 mL; 90 °C; 20 h.

Table 1:

Influence of precatalyst concentration on turnover number.

Precatalyst (mmol) mg Na-acrylate
TONa Na-acrylate
Pd(II) Ni(II) Pd(II) Ni(II)
2.0×10−3 15.57 19.03 18 22
3.5×10−3 25.96 32.88 30 38
8.0×10−3 51.04 / 59 /
9.2×10−3 / 70.94 / 82
18.0×10−3 112.47 / 130 /
25.1×10−3 / 181.69 / 210
38.1×10−3 192.07 242.25 222 280
  1. Run conditions: see Fig. 1.

  2. ammol of Na-acrylate/mmol precatalyst.

Moreover, among the monodentate phosphines we focused on the triphenylphosphine (PPh3), but in some experiments we found that the most hindered phosphines are the most efficient: for instance, under the same experimental conditions the [NiCl2(P(o-tolyl)3)2] complex leads to a TON of 88 mol/mol Ni instead of 77 mol/mol Ni obtained with the [NiCl2(PPh3)2] complex.

Influence of free triphenylphosphine on the catalytic activity

The Fig. 2 and Table 2 show that the TON decreases by using an increasing amount of free ligand (PPh3). The results suggest a competition of PPh3 with the reagents (CO2 and ethene) for the coordination to the metal center.

Fig. 2: 
						Influence of free PPh3 on the activity of [Ni(PPh3)2Cl2] and [Pd(PPh3)2Cl2] precatalysts.
						Run conditions: [NiCl2(PPh3)2] and [PdCl2(PPh3)2] (0.0092 mmol); Na-phenoxide/Pd=500/1 mol/mol; p C2H4: 25 atm; pCO2: 25 atm; THF=25 mL; 90 °C; 20 h.
Fig. 2:

Influence of free PPh3 on the activity of [Ni(PPh3)2Cl2] and [Pd(PPh3)2Cl2] precatalysts.

Run conditions: [NiCl2(PPh3)2] and [PdCl2(PPh3)2] (0.0092 mmol); Na-phenoxide/Pd=500/1 mol/mol; p C2H4: 25 atm; pCO2: 25 atm; THF=25 mL; 90 °C; 20 h.

However, it is known that such Pd(II) and Ni(II) complexes could be reduced in situ by reaction with the added ligands, the solvent or the bases forming Pd(0) or Ni(0) species, which are in this case less active in the formation of acrylate [57].

Different from Pd(II) complexes, which can readly form Pd(0) complexes [57], [58], [59], the possible reactions with the solvent or the ligands are normally insufficient to reduce Ni(II) to Ni(0) complexes, even if it has been reported cases in which this happens [56] favored also by the alkaline conditions [ref]. Nevertheless the in situ generation of Ni(0) species starting from Ni(II) complexes are frequently used in catalysis usually in the presence of reducing agents such as organometallic species [60], [61], [62], [63], [64], [65], [66], [67] or Zn dust [28], [49]. As a matter of facts, in the present case, according to Fig. 2 the Ni(0) or Pd(0) species, eventually formed in situ, result stabilized by the free phosphines resulting less active in the catalysis. This has been confirmed carrying out the title reaction starting from the [Pd(PPh3)4] complex which leads sodium acrylate only in trace amount. On the other hand it is also widely reported that the Ni(II) and Pd(II) phosphine and diphosphine [68], [69] precatalysts can be readily reduced in situ to the corresponding zero valent active species due exclusively to a intramolecular reaction with the phosphine ligands. The dispropornation reaction, which forms active Pd(0) species and phosphine oxide [70], [71], [72], [73], is promoted by water [68], [74] (present in the system due to not-anhydrous conditions), and by the presence of alkaline species (NaOPh) [75], [76], [77].

Table 2:

Influence of free PPh3 on the activity of [Ni(PPh3)2Cl2] and [Pd(PPh3)2Cl2] precatalysts.

PPh3/M(II) (mmol/mmol) mg Na-acrylate
TONa Na-acrylate
Ni(II) Pd(II) Ni(II) Pd(II)
0 66.24 53.64 77 62
1.17 66.52 51.91 77 60
2.01 45.85 28.55 53 33
3.08 35.47 19.90 41 23
4.07 21.63 12.98 25 15
  1. Run conditions: see Fig. 2.

  2. ammol of Na-acrylate/mmol precatalyst.

Moreover, such Pd(0) species generate in situ are low-ligated Pd(0) active species which has been proven to have a catalytic activity superior to Pd(PPh3)4 in most cases [73], [78].

Synthesis of Na-acrylate by using Ni(II) and Pd(II) preformed precatalysts having chelating diphosphine ligands

Similar to that reported for the reactions catalyzed by Ni(0) complexes, we tested the diphosphines, with different bridge lengths [(-CH2-)1−3], as chelating ligand for Ni(II)-chloride and Pd(II)-chloride complexes. The Fig. 3 and Table 3 show that such preformed complexes efficiently catalyze the formation of Na-acrylate. The small bite angle ligand, dppm (inactive with Ni(0) complexes [28], [49], [51]), leads to an increase of the TON from 77 mol/mol Ni (obtained with the monodentate ligands in the [NiCl2(PPh3)2]) up to 129 mol/mol Ni (with [NiCl2(dppm)]). By increasing the bite angle of the diphosphines the TON increases linearly from 129 (mol/mol Ni), up to 205 mol/mol Ni with dppb. Regarding the Pd(II)-based complexes, the results in the Fig. 3 and in Table 3 confirm that, though the TON linearly increases from 80 mol/mol Pd with [PdCl2(dppm)], up to 118 mol/mol Pd obtained with the [PdCl2(Xantphos)] complexes, they are less efficient than the respective Ni(II)-based complexes.

Fig. 3: 
						Influence of diphosphine’s bite angle on the catalytic activity.
						Run conditions: [NiCl2(dppX)] and [PdCl2(dppX)]=0.0092 mmol; Sodium phenoxide/metal=500/1 mol/mol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 90 °C;20 h.
Fig. 3:

Influence of diphosphine’s bite angle on the catalytic activity.

Run conditions: [NiCl2(dppX)] and [PdCl2(dppX)]=0.0092 mmol; Sodium phenoxide/metal=500/1 mol/mol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 90 °C;20 h.

Table 3:

Influence of diphosphine’s bite angle on the catalytic activity.

Bite angle (°) mg Na-acrylate
TONa Na-acrylate
Ni(II) Pd(II) Ni(II) Pd(II)
71.5 111.61 69.21 129 80
82.6 141.02 76.13 163 88
91.6 160.92 83.06 186 96
97.1 173.90 87.38 201 101
111.0 / 102.09 / 118
  1. Run conditions: see Fig. 3.

  2. ammol of Na-acrylate/mmol precatalyst.

The same trend has been observed by varying the temperature. For instance focusing on the dppp ligand the TON increases from 33 mol/mol Ni (at 30 °C), up to 290 mol/mol Ni (at 130 °C) for the [NiCl2(dppp)] complex, whereas it increases from 3 mol/mol Pd (at 30 °C), up to 111 mol/mol Pd (at 130 °C) for the [PdCl2(dppp)] complex (see Fig. 4 and Table 4).

Fig. 4: 
						Influence of temperature on the catalytic activity.
						Run conditions: [NiCl2(dppp)] and [PdCl2(dppp)]=0.0092 mmol; Sodium phenoxide/metal=500/1 mol/mol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 20 h.
Fig. 4:

Influence of temperature on the catalytic activity.

Run conditions: [NiCl2(dppp)] and [PdCl2(dppp)]=0.0092 mmol; Sodium phenoxide/metal=500/1 mol/mol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 20 h.

Table 4:

Influence of temperature on the catalytic activity.

T (°C) mg Na-acrylate
TONa Na-acrylate
Ni(II) Pd(II) Pd(II) Ni(II)
30 31.15 4.33 36 5
50 62.29 19.90 72 23
70 126.57 59.02 146 68
90 160.58 82.87 186 96
110 196.57 109.30 227 126
130 249.34 / 288 /
  1. Run conditions: see Fig. 4.

  2. ammol of Na-acrylate/mmol precatalyst.

Influence of the base

The influence of the base on the catalytic activity of the [NiCl2(dppp)] complex has been studied by comparing sodium- and potassium phenoxide. The Fig. 5 and Table 5 show that the sodium salt leads higher TON’s respect to potassium salt. In both cases a large excess of phenoxide salts is needed and the TON increases linearly by increases they concentrations.

Fig. 5: 
						Influence of base concentration on the catalytic activity.
						Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 90 °C; 20 h.
Fig. 5:

Influence of base concentration on the catalytic activity.

Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; THF=25 mL; 90 °C; 20 h.

Table 5:

Influence of base concentration on the catalytic activity.

Base/Pd (mmol/mmol) mg
TONa
K-acrylate Na-acrylate K-acrylate Na-acrylate
0.460 15.61 21.70 18 25
1.380 19.90 51.91 23 60
2.300 23.40 87.40 27 101
3.220 32.91 115.12 38 133
4.600 51.89 155.70 60 180
6.900 69.21 207.61 80 240
  1. Run conditions: see Fig. 4.

  2. ammol of Na-acrylate/mmol precatalyst.

This suggests that the phenoxide is a limiting agent in the reaction mechanism being the conversion and the activity of the catalyst itself dependent by its concentration. Moreover, the solubility and the degree of dissociation of the phenoxides in the solvent under investigation are different [79] and may be expected to influence they reactivity towards the Ni(II)-based precatalysts and the efficiency of the active species formation.

Influence of the solvent on the catalytic activity

The influence of several protic and aprotic solvents with increasing dielectric constant have been tested on the title reaction by using the [NiCl2(dppp)] complex.

The solvent used in the reaction has a big influence on the TON. The Figs. 6 and 7 (together with Tables 6 and 7) show that the reaction is favoured by aprotic solvents. However, among the aprotic solvents tested, the highest catalytic activity has been obtained by using the more polar ones such as acetone and DMSO.

Fig. 6: 
						Influence of dielectric constant of aprotic solvents on the catalytic activity.
						Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; volume=25 mL; 90 °C; 20 h.
Fig. 6:

Influence of dielectric constant of aprotic solvents on the catalytic activity.

Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; volume=25 mL; 90 °C; 20 h.

Fig. 7: 
						Influence of dielectric constant of protic solvents on the catalytic activity.
						Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; volume=25 mL; 90 °C; 20 h.
Fig. 7:

Influence of dielectric constant of protic solvents on the catalytic activity.

Run conditions: [NiCl2(dppp)]=0.0092 mmol; p C2H4=25 atm; pCO2=25 atm; volume=25 mL; 90 °C; 20 h.

Table 6:

Influence of dielectric constant of aprotic solvents on the catalytic activity.

Solvent Dielectric constant Na-acrylate (mg) Na-acrylate TONa
1,4 dioxane 2.25 112.66 136
THF 7.58 154.08 186
acetone 20.7 226.14 273
DMSO 46.7 250.10 290
  1. Run conditions: see Fig. 6.

  2. ammol of Na-acrylate/mmol precatalyst.

Table 7:

Influence of dielectric constant of protic solvents on the catalytic activity.

Solvent Dielectric constant Na-acrylate (mg) Na-acrylate TONa
Phenol 4.30 18.22 22
1-phenylethanol 8.90 84.49 102
2-phenylethanol 13.00 127.57 154
2-butanol 16.70 168.16 203
1-butanol 17.50 191.35 231
2-propanol 19.92 202.12 244
1-propanol 20.33 216.20 261
Ethanol 24.60 112.66 136
Methanol 33.00 35.62 43
  1. Run conditions: see Fig. 7.

  2. ammol of Na-acrylate/mmol precatalyst.

This suggests that the ionic species, such as phenoxide or active intermediates formed in situ during the reaction, are stabilized by the polar solvents.

The Ni(II) complex in protic solvents (alcohols) lead to TON’s lower than in the corresponding aprotic solvent. The TON linearly increases by increasing the dielectric constant, reaching 261 mol/mol Ni with 1-propanol. The trend confirming the stabilization of ionic species as key step to increase the catalytic activity. On the other hand, the Fig. 7 shows that the activity passes through a maximum and decreases by using some alcohols as solvents, even if the dielectric constants increase (ROH=CH3OH>CH3CH2OH >>CH3CH2CH2OH). This can be justified by considering that the reaction rate of such alcohols with the M(II)-chloride complexes, to form inactive or less active M(II)-OR species, increases in the order CH3OH>CH3CH2OH >>CH3CH2CH2OH [80], [81].

Conclusions

The catalyzed one-pot synthesis of Na-acrylate from CO2 and ethene is efficiently carried out by using Ni(II)-chloride and Pd(II)-chloride preformed phosphine and diphosphine complexes. The catalysis is effective only in the presence of a large excess of Na-phenoxide and without adding any reducing agent. Although all the metal complexes tested show high catalytic activity under the conditions reported, those based on Ni(II) lead to higher TON’s than the respective Pd(II) ones. Moreover, the best results are obtained by using diphosphines having high bite angles. Such catalysis is favored by aprotic and polar solvents in which a TON of 290 mol/mol Ni is reached by using the [NiCl2(dppp)] precursor in DMSO. The results further suggest that the TON could be increased by increasing the temperature, the base concentration and by using diphosphine ligands having high bite angle. Although the discussion on the reaction mechanism was not in the aim of this paper, the experimental results suggest an in situ activation of the precursors probably through the formation of low coordinated zero valent intermediates, according to the literature.

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Published Online: 2017-11-21
Published in Print: 2018-02-23

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