Catalytic oxidation of α-alkenes with hydrogen peroxide to carboxylic acids in the presence of peroxopolyoxotungstate complexes

Catalysis Communications 88 (2017) 45–49

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Catalytic oxidation of α-alkenes with hydrogen peroxide to carboxylic acids in the presence of peroxopolyoxotungstate complexes☆ Z.P. Pai ⁎, N.V. Selivanova, P.V. Oleneva, P.V. Berdnikova, A.M. Beskopyl'nyi Boreskov Institute of Catalysis, Department of Catalytic Processes of Fine Chemical Synthesis, Akad. Lavrentiev Pr. 5, Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 20 July 2016 Received in revised form 29 August 2016 Accepted 14 September 2016 Available online 15 September 2016 Keywords: Phase-transfer catalysis Alpha-alkenes Carboxylic acids Peroxotungstate complexes Quaternary ammonium cations

a b s t r a c t Fine organic synthesis investigation has been performed, focusing on the possibility of efficient oxidation of α-alkenes by hydrogen peroxide under conditions of phase transfer catalysis using bifunctional metal complex catalysts based on peroxotungsten compounds of general formula Q3{PO4[WO(O2)2]4}, where Q is organic cation containing quaternary nitrogen atom. Catalysts screening has been done at oxidation of octene-1, decene-1 and dodecene-1 by 30% aqueous hydrogen peroxide to obtain carboxylic acids: heptanoic, nonanoic and undecanoic acids being of importance since used as precursors in the synthesis of various organic and biologically active compounds. This approach to the synthesis of carboxylic acids may be of interest for the processes of “green chemistry” occurring under mild conditions (Т b 100 °С, Р – atm) in one stage without organic solvents, and providing high target product yields (86–97%). © 2016 Elsevier B.V. All rights reserved.

1. Introduction Carboxylic acids С7, С9 and С11 as well as their derivatives are widely used in various fields such as large scale chemistry, oil chemistry, agro chemistry and food and perfumery industries [1–3]. Thus heptanoic (enanthic) acid is applied in the synthesis of hydraulic liquids and lubricants [4]. Nonanoic (pelargonic) acid is used in production of polyester alkyd resins and synthetic oils, plasticizing agents, dyes, stabilizers as well as in the biosynthesis of polyoxyalcanoates (biodegradable polymers for medicine) [1,3–5]. Undecanoic (undecylic) acid is applied in production of surfactants and emulsifiers. Esters of above mentioned acids are popular as aromatic substrates and taste additives and are actively applied in fine organic synthesis for modification of complex medical preparations [4–6]. Pelargonic acid esters enter biodiesel fuel compositions [7]. Corresponding acid salts, i.e. enanthoates, pelargonates, undecanoates, are also demanded. For example, ammonium pelargonate is used as herbicide and blossom regulator in agriculture. Calcium enanthoate enters testosterone preparations used for healing anemia, trachoma and eye diseases [8]. All ☆ Various α-alkenes (С8, С10 and С12) were oxidized in a two-phase liquid system by a 30% solution of hydrogen peroxide in the presence of metal complex catalysts Q3{PO4[WO(O2)2]4}, Q being organic cation: [Bun4N]+, [MeOctn3N]+, [C5H5NCetn]+. Carboxylic acids (heptanoic, nonanoic and undecanoic) were found to form with high yields (97, 90 and 86% respectively) at temperatures below 100 °С under atmospheric pressure in one stage, no organic solvents being used. This approach towards carboxylic acids synthesis may be of interest for “green chemistry” processes. ⁎ Corresponding author. E-mail address: [email protected] (Z.P. Pai).

http://dx.doi.org/10.1016/j.catcom.2016.09.019 1566-7367/© 2016 Elsevier B.V. All rights reserved.

saturated fatty acids are known to possess antibiotic and antifungi activities [9]. For a long time aliphatic carboxylic acids were obtained from the plant raw materials. At first, nonanoic and azelaic acids were obtained via oleic acid oxidation by concentrated (N 80%) nitric acid in the presence of vanadium compounds [10]. Since that time many methods were suggested for carboxylic acid synthesis improvement such as oxidizer (nitric acid) concentration decrease, process temperature and pressure increase; oxidative decomposition of unsaturated fatty acids of natural origin; application of novel catalysts, allowing process performance under milder conditions [11]; oxidation performance by strong mineral acids combined with other oxidizers such as hydrogen peroxide [12,13]. In 70-s carboxylation was intensively studied as a method for alkenes processing into carboxylic acids via carbon monoxide and water molecules addition along the alkene double bond. This method provided best results, when assisted by Rh and Pd phosphine complexes [14]. However, in this case carboxylation was performed under rather severe conditions such as pressure up to 8 МPa and temperature up to 200 °С. Some industrial processes are based on alkenes hydrocarboxylation at 90–120 °С and under elevated CO pressure (up to 125 atm) in the presence of Ni or Co carbonyls based catalysts such as pyridine modified Со(СО)8 in organic solvent [15]. Aliphatic carboxylic acids are also produced via alkanes oxidation by air oxygen or ozone-oxygen mixture in the presence of manganese salts at 105–120 °С [16,17]. However, all above described technologies have disadvantages: low yields of target acids (b 50%), poor product purity, large amounts of waste waters (up to 8 м3 per 1 ton of product) polluted by Na2SO4 and low molecular

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Z.P. Pai et al. / Catalysis Communications 88 (2017) 45–49

weight acids. Therefore, more feasible and environment friendly technologies for carboxylic acids synthesis are still required. In the present study we focus on new approaches to the synthesis of aliphatic monocarboxylic acids to be used for the up to date ways of their production. For the purpose α-alkenes (octene-1, decene-1, dodecene-1) are oxidized by hydrogen peroxide in a two phase system using bifunctional catalysts based on oxoperoxotungstate complexes Q3{PO4[WO(O2)2]4}, where Q is quaternary ammonium cation [18– 21]. The anion {PO4[WO(O2)2]4}3 − (Scheme 1) is a tetra nuclear peroxopolyoxotungstate known as Venturello complex [18]. It has the C2 symmetry and consists of the central PO4 tetrahedron linked through its oxygen atoms to two pairs of edge-sharing distorted pentagonal bipyramids W(O2)2O3. Each tungsten atom is linked to two peroxo groups – one nonbridging (η2-O2) and the other bridging (μ-η1:η2-O2) – located in the equatorial plane of the pentagonal bipyramid. The oxidizing moiety is the η2-peroxo group. Above mentioned α-alkenes are chosen as substrates, as being inexpensive large scale oil chemistry products obtained at high temperature ethylene oligomerization [22]. 2. Experimental 2.1. Materials Commercially available octene-1 («Acros Organics», 99 + %), decene-1 («Acros Organics», 95%), dodecene-1 («Acros Organics», 93– 95%), hydrogen peroxide (30–33% water solution) («Khimreaktiv», special purity grade), 1,2-dichloroethane («Khimreaktiv», chemical purity grade) were used without preliminary purification. Keggin-type 12-tungstophosphoric heteropoly acid – H3PW12O40 × 6H2O («Acros Organics») and quaternary ammonium salts: [Bun4N]Cl («Fluka Chemie», N 98%), [MeOctn3N]Cl (Aliquat®336, «Acros Organics»), [C5H5NCetn]Cl («Acros Organics») were used for catalysts' synthesis. 2.2. Analytic methods GC analysis of products of α-alkenes catalytic oxidation (octene-1, decene-1, dodecene-1) was performed with gas chromatograph “Khromos-1000” (Russia), equipped with flame ionization detector and capillary column SolGel-Wax 30 m × 0.53 mm «SGE». Reaction mixture analysis was performed in isothermal regime Тcol = 220 °С, detector temperature was 300 °С, evaporator temperature was 230 °С, carrier gas – He. Analysis duration was about 50 min. Absolute calibration was used to determine the quantitative product amount. GCMS analysis of organic phase was performed with GCMS-QP2010 Ultra Gas chromatography mass spectrometer. Analysis conditions were: injection port temperature 250 °C, capillary column GsBP1-MS 30 m × 0.32 mm, programmed heating: 50 °C (7.5 min) – 20 °C/min –

300 °C (10 min), carrier gas – He with linear velocity of 50 cm/s (GC conditions); determined m/z 35–500, detector voltage 0.9 kV, emission current 60 μA, ion source temperature 250 °С (MS conditions). 2.3. Synthesis of the catalytic complexes Catalysts Q3{PO4[WO(O2)2]4} were synthesized according to procedures [23,24], which describe synthesis of tungstate tetra nuclear peroxopolyoxo complexes with using tungstophosphoric heteropoly acid of Keggin-type and 30%-H2O2 as precursors. It has previously been established [24], that bi- and tetra-nuclear tungsten peroxo complexes are formed during the synthesis as a result of a number of reactions, which can be represented by the following summary equation:    H3 PW12 O40 þ 24H2 O2 þ 3QCl→Q 3 PO4 WOðO2 Þ2 4 ↓þ   þ 4H2 W2 O3 ðO2 Þ4 þ 3HCl þ 20H2 O

Physical and chemical properties of obtained tetra nuclear complexes of tungsten are similar to those described in [23,24]. Catalytic complexes: I – [Bu n 4 N] 3 {PO 4 [WO(O 2 ) 2 ] 4 }, colourless crystals, m.p. = 128–129 °C (lit. m.p. = 127–130 °C [23]); III – [C5H5NCetn]3{PO4[WO(O2)2]4} – colourless crystals, m.p. = 130 °C (lit. m.p. = 130–131 °C [23]); II – [MeOctn3N]3{PO4[WO(O2)2]4}, yellowish syrup-like substance, 1Н NMR (300 MHz, C6D6), δ (ppm): 4.29 (s, 1H), 3.20 (m, 5H), 2.94 (m, 10H), 1.39 (m, 78H), 1.03 (m, 20H), 0.29 (s, 2H); 31P NMR (121.49 MHz, C6D6), δ (ppm): 4.41 (m). IR spectra of complexes I–III correspond to anion {PO 4 [WO(O 2 ) 2 ] 4 } 3 − and consistent with our results [23,24b] and the published data [18, 19]: I – [Bu n 4 N] 3 {PO 4 [WO(O 2 ) 2 ] 4 } – (P-O) as 1084, 1063, 1034; (W = O) as 972; (O-O) 853, 845; (W-O-O) s 650; (W-O-O) as 590, 574, 548, 521 cm − 1 ; II – [MeOct n 3 N] 3 {PO 4 [WO(O 2 ) 2 ] 4 } – (P-O) as 1088, 1057, 1032; (W = O)as 976; (O-O) 856, 846; (W-O-O)s 652; (W-O-O)as 591, 577, 549, 523 cm−1; III – [C5H5NCetn]{PO4[WO(O2)2]4} – (P-O)as 1090, 1060, 1033; (W = O) as 985, 960; (O-O) 855, 844; (W-O-O)s 649; (W-O-O)as 590, 572, 548, 523 cm− 1. Samples IR spectra were recorded with IR Fourier spectrometer IRAffinity-1 Shimadzu within 400–4000 cm−1 resolution being 4 cm−1, scans number being 50. Samples were arranged as tablets with KCl (samples of catalysts I, III) or in the form of a film (sample of catalyst II). For tablets preparation 3 mg of compound were mixed with 300 mg of dewatered KCl then carefully ground in agate mortar and pressed. Samples were weighted using Leki Electronic Balance B2104. 1 H and 31P NMR spectra were recorded at a Bruker AV-300 spectrometer in C6D6 (300.13 MHz for 1H, 121.49 MHz for 31P) for solution. Proton chemical shifts were recorded relative to tetramethylsilane external standard. The 31P NMR spectra were recorded using H3PO4 (0 ppm) as an external standard.

3-

2.4. Organic substrate oxidation procedure

η 2−O2

μ− η1: η 2−O2

W

O

Scheme 1. Structure of tetra nuclear peroxopolyoxotungstate {PO4[WO(O2)2]4}3−.

α-Alkenes were oxidized in a thermostat glass reactor (volume 180 ml), equipped with reflux condenser and magnetic stirrer (n = 500 min−1). Temperature 60–95 °С was maintained with water thermostat with an accuracy of ±0.1 °С. For reaction mixture preparation weighted catalyst sample was put into reactor, then, substrate was added and mixed with catalyst. After that 30% aqueous hydrogen peroxide was introduced, and heating was started. Catalysts II and III well dissolved in the substrate. Catalyst I was preliminarily dissolved in a small amount of 1,2-dichloroethane (1–2 ml). Reaction mixture was sampled in definite time intervals. For the purpose stirring was stopped. After organic and aqueous phases complete separation into layers (no longer than 20–30 s) sample from organic phase was taken. Carboxylic acid yield was determined using chromatography analysis via absolute calibration.

Z.P. Pai et al. / Catalysis Communications 88 (2017) 45–49

n

2 m

n

2

3

47

2 2 4

3

2

4

(active form)

2 2 3

m

2

(inactive form)

Q+: [Bun4N]+, [MeOctn3N]+, [C5H5NCetn]+ Organic phase

Aqueous phase

H2O2

2

Scheme 2. Scheme of α-alkenes oxidation by hydrogen peroxide in the two-phase system water phase – organic phase in the presence of peroxopolyoxotungstate complexes.

3. Results and discussion Oxidation of α-alkenes by hydrogen peroxide in the presence of catalysts I, II and III at 60–95 °С under atmospheric pressure yields aliphatic carboxylic acids with carbon atoms number less by 1 than that in initial alkene (Scheme 2).

data also confirm results obtained earlier at cycloalkenes [26,27] and fatty acids [28] oxidation by hydrogen peroxide. Despite the fact that complex III has shown the highest activity in oxidation of α-alkenes, for further investigation we have chosen complex II, as showing higher stability with time [23]. 3.2. Temperature effect on the yield of carboxylic acids

3.1. Screening of catalysts Screening of catalytic complexes I, II and III in oxidation of α-alkenes С8, С10 and С12 was done at 80 °С. Ratio [Ox]/[Sub] = 5 was chosen in accordance with reaction stoichiometry (1 mol of substrate/5 mol of oxidizer). The highest yields of heptanoic, nonanoic and undecanoic acids (Fig. 1) were obtained over catalytic complexes II and III, carboxylic acid yields over complex I were lower. This means that lipophilic properties of phase transfer catalyst has essential effect on its distribution between organic and water phases. Activity series [C 5H5 NCetn ] 3 {PO4 [WO(O 2 )2 ] 4 } N [MeOct n 3N] 3 {PO 4 [WO(O 2) 2 ] 4} N [Bun4N]3{PO4[WO(O2)2]4} is in a good agreement with that of extraction constant logarithms experimentally determined for quaternary salts of bromine in system water – organic solvent [25]. Activity

Temperature effect on the yield of carboxylic acids was studied in a range of 60–95 °С. According to the data obtained (Fig. 2), yields of desired acids grow with temperature, and attain maximum at temperatures close to 100 °С. Further temperature increase is limited by the water phase boiling point. Therefore, it is necessary to elevate pressure for higher yields of acids. 3.3. Influence of catalyst and oxidizer concentrations on carboxylic acid yield Oxidation of octene-1 was used to study the influence of oxidizer (Fig. 3) and catalyst II concentrations (Table 1) on the carboxylic acid yield. Heptanoic acid yield versus substrate/catalyst ratio curve shows that [H2O2]/[Sub] = 5 is optimum, which corresponds to reaction

100

100

90

2 3

90

70

85

60

80

50

Yield, %

Yield, %

1

95

80

40 30

75 70

20

65

10

60 55

0 1

2

3

1 - C6H13COOH, 2 - C8H17COOH, 3 - C10H21COOH

50 55

60

65

70

75

80

85

90

95

100

T, ˚C Fig. 1. Diagrams of carboxylic acid yields depending on catalyst type, ([Sub]/[Cat] = 250, – [Bun4N]3{PO4[WO(O2)2]4} (I); [H2O2]/[Sub] = 5, Тreaction = 80 °С, τreaction = 5 h). – [MeOctn3N]3{PO4[WO(O2)2]4} (II); – [C5H5NCetn]3{PO4[WO(O2)2]4} (III).

Fig. 2. Carboxylic acid yield versus reaction temperature (catalyst II, [Sub]/[Cat] = 250, [Ox]/[Sub] = 5, τreaction = 5 h) 1 – C6H13COOH, 2 – C8H17COOH, 3 – C10H21COOH.

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Z.P. Pai et al. / Catalysis Communications 88 (2017) 45–49 Table 1 Heptanoic acid yield versus catalyst concentration in reaction mixture (Sub – 1-C8H16, [H2O2]/[Sub] = 5, Тreaction = 90 °С, τreaction = 90 min).

26

Yield, %

24 22

No

[Cat] × 103 M

[Sub]/[Cat]

Yield C6H13COOH, mol. %

20

1 2 3 4 5 6

63.6 42.4 25.4 12.7 8.47 3.63

100 150 250 500 750 1000

44.9 39.3 25.1 12.3 10.0 0.10

18 16 14

Cat – [MeOctn3N]3{PO4[WO(O2)2]4} (II).

12 10

the substrate. The subsequent regeneration of the peroxo complex with hydrogen peroxide occurs at the interfacial surface (Scheme 2).

8 6 2

4

6

8

10

[H2O2] / [Sub] Fig. 3. Heptanoic acid yield versus molar ratio [H2O2]/[Sub] (catalyst II, Sub – 1-C8H16, Ox – 30% aqueous H2O2, [Sub]/[Cat] = 250, Тreaction = 80 °С, τreaction = 90 min).

stoichiometry. This fact indirectly confirms that hydrogen peroxide decomposition does not occur under reaction conditions, or that its contribution is negligible. Therefore, such process may proceed in the minimum excess of oxidizer (about 10–12% from theoretical one). According to Table 1 essentially high catalyst concentrations are needed for octene-1 oxidation to heptanoic acid, when [Sub]/ [Cat] ≤ 250. At the same time molar ratio [Sub]/[Cat] ≥ 1000 is required when unsaturated fatty acids and their methyl esters are oxidized to produce epoxides or when their double bond С_С is cleaved at oxidation [28]. When alcohols or cycloalkenes are oxidized to mono- and dicarboxylic acids [23] required ratio [Sub]/[Cat] ranges from 500 to 100. The latter data are in good agreement with results obtained in our experiments.

3.4. Intermediate and side reaction products Analyzing data given on Fig. 4, one may see carboxylic acid yield versus time curves being S-shaped. Let us note that as temperature decreases, induction period increases. Induction period may signify that, first, carboxylic acids (C6H13COOH, C8H17COOH, C10H21COOH) form via a complex mechanism. Second, some intermediate products are forming and accumulating, thus limiting carboxylic acids formation in whole. Separate experiments with octene-1 oxidation to determine of intermediates were performed at 60 °С using chromatography plus mass spectrometry1 for reaction mixture analysis. It has been shown that 1,2-epoxyoctane, 1,2-octanediol and heptanal as well as two more not yet identified compounds (in quantities not larger than 1.0%) are present beside substrate (octene-1) and product (heptanoic acid). Attempts to isolate these components from the reaction mixture for characterization by NMR, were unsuccessful. Thus, based on these results, it can be confirmed that the reaction includes the step of forming the intermediate epoxide (limiting stage), which is most likely exposed to acid hydrolysis to form a diol. Further oxidation leads to the cleavage of C\\C bond of the diol to form of the desired product – heptanoic acid as well as three byproducts including heptanal. On the whole, the oxidation reaction of α-alkenes by hydrogen peroxide under the phase-transfer catalysis occurs in the organic phase (the role of organic phase performs a substrate) via the transport of oxygen atom from the tetra(oxodiperoxotungstato)phosophate (3−) to 1 Authors express gratitude to senior researcher Prikhod'ko S.A. for mass spectrometry analysis and data interpreting.

4. Conclusions Our investigation focused on oxidation of several α-alkenes (octene1, decene-1, dodecene-1) by 30% aqueous hydrogen peroxide in the two phase system using metal complex bifunctional catalysts based on oxoperoxotungsten complexes Q3{PO4[WO(O2)2]4}, containing organic cation (Q) have shown that: - corresponding carboxylic acids such as heptanoic, nonanoic and undecanoic acids form with high yields 97, 90, 86%, respectively, at temperatures b100 °С under atmospheric pressure in one stage, no organic solvents being required; - tested complexes [C5H5NCetn]3{PO4[WO(O2)2]4} and [MeOctn 3N]3{PO4[WO(O2)2]4} have comparable activities, when catalyze oxidation of above mentioned α-alkenes; - oxidizer amount required for reaction performance is close to stoichiometry and its excess should not exceed 10–12%; - in order to provide higher target product yields it is necessary to provide catalyst concentration at which 50 ≤ [Sub]/[Cat] ≤ 250.

Therefore presented method of carboxylic acids synthesis may be of interest as a “green chemistry” process performed in one stage at low temperatures and atmospheric pressure. Regarding all above mentioned and accounting for obvious practical perspectives [29], we plan further detailed investigation of reaction optimization and detailed revelation of its mechanism.

1 2 3

100 90 80 70

Yield, %

0

60 50 40 30 20 10 0 0

30

60

90

120

150

180

210

240

270

300

330

360

t, min Fig. 4. Carboxylic acid yield versus reaction time (catalyst II, [H2O2]/[Sub] = 6, [Sub]/ [Cat] = 200, Т = 90 °С) 1 – C6H13COOH, 2 – C8H17COOH, 3 – C10H21COOH.

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