DOI: 10.1002/cssc.201500214
Full Papers
Catalytic Decarbonylation of Biosourced Substrates Jrmy Ternel,[a] Thomas Lebarb,[b] Eric Monflier,[a] and Frdric Hapiot*[a] Linear a-olefins (LAO) are one of the main targets in the field of surfactants, lubricants, and polymers. With the depletion of petroleum resources, the production of LAO from renewable feedstocks has gained increasing interest in recent years. In the present study, we demonstrated that Ir catalysts were suitable to decarbonylate a wide range of biosourced substrates under rather mild conditions (160 8C, 5 h reaction time) in the
presence of potassium iodide and acetic anhydride. The resulting LAO were obtained with good conversion and selectivity provided that the purity of the substrate, the nature of the ligand, and the amounts of the additives were controlled accurately. The catalytic system could be recovered efficiently by using a Kugelrohr distillation apparatus and recycled.
Introduction Linear a-olefins (LAO) are extremely important chemical feedstocks for industrial and consumer products. Although longchain LAO (C10–C14) play an important role in the production of surfactants and lubricants, short-chain LAO (C4–C8) are used in substantial amounts as co-monomers in the production of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). The two main commercial processes to obtain LAO are the oligomerization of ethylene[1–6] and the Fischer–Tropsch process starting from syngas.[7–11] However, these present some drawbacks such as lower yields of longchain LAO (C10–C14) compared to short-chain LAO (C4–C8) coupled with a decrease in selectivity because of the formation of branched olefins and isomerization reactions. The production of LAO with an odd number of carbon atoms appears to be impossible by ethylene oligomerization. Recently, it was shown that LAO can be produced from renewable resources as a means to address environmental and economic concerns. Among them, unsaturated long-chain aliphatic carboxylic acids, esters, and triglycerides are of major interest as they can be derived from abundant feedstocks such as vegetable oils and animal fats. As such, they constitute an eco-friendly alternative to fossil feedstocks. The introduction of a terminal C=C bond within these molecules has been envisaged using several approaches. The first method consisted of the ethenolysis of unsaturated vegetable oils and fatty acid methyl esters (FAME).[12–19] A major drawback of this approach is that besides the expected product, a coproduct is formed in a 1:1 ratio. For example, in the case of glycerol trioleate ethe-
nolysis, the triglyceride of a-decenoic acid is the coproduct.[20–25] Although these coproducts are interesting bi- or trifunctional molecules with potentially valuable applications, the overall yield of a-olefins is low, for example, only 43 % w/w of glycerol trioleate is converted into the desired high-value LAO. Similarly, metathesis has been used widely to cleave fatty acid derivatives to lead to unsaturated long-chain a-olefins.[26] However, numerous byproducts were formed concomitantly, which thus limits the interest of such an approach. Other strategies were also developed to produce long-chain olefins by one carbon degradation. Among them, fatty acids decarboxylation has been investigated extensively.[27] Recent examples demonstrated the use of Lewis-acid catalysts for the selective production of LAO by the decarboxylation of lactones and unsaturated carboxylic acids[28] and described the combination of a P450 fatty acid decarboxylase with a light-driven, in situ H2O2 generation system for the selective and quantitative conversion of fatty acids into terminal alkenes.[29] Concurrently, fatty acid decarbonylation has gained increasing interest recently. For example, the decarbonylation of heptanoic acid over carbon-supported Pt nanoparticles was reported,[30] but low turn-over frequencies (TOF) were measured because of catalyst deactivation.[31–33] Better results were obtained using homogeneous catalysts.[34–39] The Pd-catalyzed decarbonylative dehydration of fatty acids was performed using a low loading of Pd catalyst stabilized by the Xantphos ligands and proceeded under solvent-free conditions.[40, 41] Ir and Fe catalysts were also effective to decarbonylate saturated fatty acids.[42] On using Ir catalysts, the addition of Ac2O and KI allowed the reaction temperature to be decreased and improved the product yield. KI caused ligand exchange between Cl and I, and Ac2O helped to generate an acid anhydride in situ that subsequently underwent oxidative addition onto the Ir catalyst. The resulting acyl iridium complex consecutively underwent decarbonylation to afford the alkyl iridium complex and b-hydride elimination to give a terminal alkene. Except for the oxidative decarboxylation of unsaturated fatty acids,[43] it appears that many of the catalytic systems studied
[a] Dr. J. Ternel, Prof. Dr. E. Monflier, Prof. F. Hapiot Unit de Catalyse et de Chimie du Solide—UCCS Artois University of Artois Facult Jean Perrin, SP18, 62307 Lens Cedex (France) E-mail: [email protected] [b] Dr. T. Lebarb ITERG 11 rue Monge, Parc Industriel Bersol 2, 33600 Pessac (France) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500214.
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Full Papers addition of excess PPh3 improved the conversion (91 %; Table 2, entry 18). A 3:1 mixture of PPh3/[Ir(COD)Cl]2 was considered as it was one of the best catalytic systems. The catalytic conditions were further optimized by varying the amount of the additives (Table 3). Conversions were very low in the absence of acetic anhydride (Table 3, entry 1) or potassium iodide (Table 3, entry 2). An increased amount of KI led to an increase in both the conversion and the terminal alkene selectivity (Table 3, entries 3 and 4). However, a slight increase of the amount of KI produced no effect on the catalytic performance (Table 3, entry 8). A decrease of the quantity of acetic anhydride impacted the con-
Scheme 1. Ir-catalyzed decarbonylation of unsaturated fatty acids.
so far have focused on saturated fatty acid substrates. In this context, we assessed the catalytic performance of Ir catalysts in the decarbonylation of biosourced unsaturated fatty acids (Scheme 1). Herein, we demonstrate that a wide range of unsaturated biosourced fatty acids could be decarbonylated efficiently under relatively mild conditions using Ir catalysts under homogeneous catalytic conditions. The conversion and selectivity in terminal olefins are discussed throughout a deTable 1. Selectivity of the Ir-catalyzed decarbonylation reaction for saturated and untailed optimization of the experimental conditions. saturated derivatives.[a]
Results and Discussion
Entry Substrate
Ligand[b] Precursor[b] [mol %] [mol %]
Conversion[c] 2/3 selectivi[%] ty[c]
To begin with, oleic acid (99 wt % purity) was chosen 80 95:5 1 stearic acid 98 % – IrCl(CO)(PPh3)2 as a model substrate. The presence of only one C=C (5) (1 a) 2 oleic acid 99 % (1 b) – IrCl(CO)(PPh3)2 84 89:11 bond on the alkyl chain simplified the analysis of the (5) reaction products greatly. To evaluate the impact of 3 stearic acid 98 % PPh3 [Ir(COD)Cl]2 (2.5) 86 95:5 the unsaturation on the catalytic performance, a com(1 a) (15) parison with stearic acid was performed using IrCl([Ir(COD)Cl]2 (2.5) 95 4 oleic acid 99 % (1 b) PPh3 91:9 (15) CO)(PPh3)2 and [Ir(COD)Cl]2 (COD = cyclooctadiene) as Ir catalysts and potassium iodide and acetic anhy[a] Reaction conditions: 1.0 mmol substrate, 2 mmol acetic anhydride, 0.5 mmol KI, dride as co-reactants (Table 1).[44] The reaction prod160 8C, 5 h. [b] Molar percentage with respect to the substrate. [c] Determined by 1 H NMR spectroscopy and confirmed by GC. ucts were monitored and analyzed by 1H NMR spectroscopy (Supporting Information). At 160 8C, the decarbonylation of oleic acid within 5 h led to higher Table 2. Effect of the ligand and the metallic precursor on the selectivity conversions compared to stearic acid but to lower seof the Ir-catalyzed decarbonylation reaction.[a] lectivity in terminal alkenes (Table 1). Accordingly, the presence of the C=C bond did not affect the catalytic performance sigPrecursor[b] Conversion[c] 2/3 selectivity[c] Entry Ligand[b] [mol %] [mol %] [%] nificantly. The amount and the nature of the ligand was then varied 1 – [Ir(COD)Cl]2 (2.5) < 5 – 2 PPh3 (5) [Ir(COD)Cl]2 (0.5) 39 77:23 (Table 2). If [Ir(COD)Cl]2 was used as the catalyst, a very small [Ir(COD)Cl]2 (1) 68 84:16 3 PPh3 (10) conversion could be measured in the absence of any ligand [Ir(COD)Cl]2 (2.5) 92 84:16 4 PPh3 (10) (< 5 %, Table 2, entry 1). An increase of the amount of Ir cata5 PPh3 (15) [Ir(COD)Cl]2 (2.5) 90 87:13 lyst and PPh3 favored both the conversion and selectivity [Ir(COD)Cl]2 (2.5) 89 86:14 6 PPh3 (20) [Ir(COD)Cl]2 (2.5) 34 78:22 7 p-Cl-PPh3 (15) greatly, and the best result was obtained for a PPh3/[Ir(COD)Cl]2 (15) [Ir(COD)Cl] (2.5) 56 84:16 8 p-F-PPh 3 2 ratio of 3 (Table 2, entry 5) with a 5 mol % of Ir catalyst with re75 84:16 9 p-tBu-PPh3 (15) [Ir(COD)Cl]2 (2.5) spect to the substrate. The substitution of the aromatic rings 10 p-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) 89 85:15 of PPh3 in the para position by Cl, F, or tBu had a deleterious 86 85:15 11 m-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) – 12 o-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) < 5 effect on the conversion (Table 2, entries 7, 8, and 9). Substitu(15) [Ir(COD)Cl] (2.5) < 5 – 13 o-OMe-PPh 3 2 tion in the para and meta positions by a methyl group showed – 14 dppb (10) [Ir(COD)Cl]2 (2.5) < 5 no change (Table 2, entries 10 and 11), whereas substitution in – 15 Xantphos (10) [Ir(COD)Cl]2 (2.5) < 5 the ortho position deactivated the catalyst (Table 2, entries 12 – 16 DPE-phos (10) [Ir(COD)Cl]2 (2.5) < 5 85:15 17 – IrCl(CO)(PPh3)2 (5) 84 and 13). Bidentate ligands such as 1,4-bis(diphenylphosphino)IrCl(CO)(PPh3)2 (5) 91 87:13 18 PPh3 (10) butane (dppb), Xantphos, and bis-[2-(diphenylphosphino)phe[a] Reaction conditions: 1.0 mmol oleic acid 88 % (1 b’), 2 mmol acetic annyl]ether (DPE-phos) were ineffective (Table 2, entries 14, 15, hydride, 0.5 mmol KI, 160 8C, 5 h. [b] Molar percentage with respect to and 16). If IrCl(CO)(PPh3)2 was used as the catalyst, similar conthe substrate. [c] Determined by 1H NMR spectroscopy. version and selectivity were obtained (Table 2, entry 17). The
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Full Papers 5), very low conversions were obtained. Attempts were also made in aqueous biphasic conditions using sulfonated ligands to retain the Ir catalyst in water. Very low conversions were obtained if the benchmark 3,3’,3’’-phosphanetriyltris(benzenesulfonic acid) trisodium salt (TPPTS) or the more organophilic p-(tris-Me)-TPPTS [sodium 5,5’,5’’-phosphinetriyltris(2-methylbenzene sulfonate)] and p(tBu)-TPPDS sodium (3,3’-{[4-(tert-butyl)phenyl]phosphinediyl}dibenzene sulfonate) were used even in the presence of a surfactant such as cetyl trimethylammonium bromide (CTAB; Table 4, entry 6).
Table 3. Effect of acetic anhydride and additive on the Ir-catalyzed decarbonylation reaction.[a] Entry
Anhydride [mmol]
Additive[b] [mol %]
Conversion[c] [%]
2/3 selectivity[c]
1 2 3 4 5 6 7 8
– Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O
KI (50) – KI (20) KI (50) KI (50) KI (50) KI (50) KI (100)
0
Full Papers
Catalytic Decarbonylation of Biosourced Substrates Jrmy Ternel,[a] Thomas Lebarb,[b] Eric Monflier,[a] and Frdric Hapiot*[a] Linear a-olefins (LAO) are one of the main targets in the field of surfactants, lubricants, and polymers. With the depletion of petroleum resources, the production of LAO from renewable feedstocks has gained increasing interest in recent years. In the present study, we demonstrated that Ir catalysts were suitable to decarbonylate a wide range of biosourced substrates under rather mild conditions (160 8C, 5 h reaction time) in the
presence of potassium iodide and acetic anhydride. The resulting LAO were obtained with good conversion and selectivity provided that the purity of the substrate, the nature of the ligand, and the amounts of the additives were controlled accurately. The catalytic system could be recovered efficiently by using a Kugelrohr distillation apparatus and recycled.
Introduction Linear a-olefins (LAO) are extremely important chemical feedstocks for industrial and consumer products. Although longchain LAO (C10–C14) play an important role in the production of surfactants and lubricants, short-chain LAO (C4–C8) are used in substantial amounts as co-monomers in the production of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). The two main commercial processes to obtain LAO are the oligomerization of ethylene[1–6] and the Fischer–Tropsch process starting from syngas.[7–11] However, these present some drawbacks such as lower yields of longchain LAO (C10–C14) compared to short-chain LAO (C4–C8) coupled with a decrease in selectivity because of the formation of branched olefins and isomerization reactions. The production of LAO with an odd number of carbon atoms appears to be impossible by ethylene oligomerization. Recently, it was shown that LAO can be produced from renewable resources as a means to address environmental and economic concerns. Among them, unsaturated long-chain aliphatic carboxylic acids, esters, and triglycerides are of major interest as they can be derived from abundant feedstocks such as vegetable oils and animal fats. As such, they constitute an eco-friendly alternative to fossil feedstocks. The introduction of a terminal C=C bond within these molecules has been envisaged using several approaches. The first method consisted of the ethenolysis of unsaturated vegetable oils and fatty acid methyl esters (FAME).[12–19] A major drawback of this approach is that besides the expected product, a coproduct is formed in a 1:1 ratio. For example, in the case of glycerol trioleate ethe-
nolysis, the triglyceride of a-decenoic acid is the coproduct.[20–25] Although these coproducts are interesting bi- or trifunctional molecules with potentially valuable applications, the overall yield of a-olefins is low, for example, only 43 % w/w of glycerol trioleate is converted into the desired high-value LAO. Similarly, metathesis has been used widely to cleave fatty acid derivatives to lead to unsaturated long-chain a-olefins.[26] However, numerous byproducts were formed concomitantly, which thus limits the interest of such an approach. Other strategies were also developed to produce long-chain olefins by one carbon degradation. Among them, fatty acids decarboxylation has been investigated extensively.[27] Recent examples demonstrated the use of Lewis-acid catalysts for the selective production of LAO by the decarboxylation of lactones and unsaturated carboxylic acids[28] and described the combination of a P450 fatty acid decarboxylase with a light-driven, in situ H2O2 generation system for the selective and quantitative conversion of fatty acids into terminal alkenes.[29] Concurrently, fatty acid decarbonylation has gained increasing interest recently. For example, the decarbonylation of heptanoic acid over carbon-supported Pt nanoparticles was reported,[30] but low turn-over frequencies (TOF) were measured because of catalyst deactivation.[31–33] Better results were obtained using homogeneous catalysts.[34–39] The Pd-catalyzed decarbonylative dehydration of fatty acids was performed using a low loading of Pd catalyst stabilized by the Xantphos ligands and proceeded under solvent-free conditions.[40, 41] Ir and Fe catalysts were also effective to decarbonylate saturated fatty acids.[42] On using Ir catalysts, the addition of Ac2O and KI allowed the reaction temperature to be decreased and improved the product yield. KI caused ligand exchange between Cl and I, and Ac2O helped to generate an acid anhydride in situ that subsequently underwent oxidative addition onto the Ir catalyst. The resulting acyl iridium complex consecutively underwent decarbonylation to afford the alkyl iridium complex and b-hydride elimination to give a terminal alkene. Except for the oxidative decarboxylation of unsaturated fatty acids,[43] it appears that many of the catalytic systems studied
[a] Dr. J. Ternel, Prof. Dr. E. Monflier, Prof. F. Hapiot Unit de Catalyse et de Chimie du Solide—UCCS Artois University of Artois Facult Jean Perrin, SP18, 62307 Lens Cedex (France) E-mail: [email protected] [b] Dr. T. Lebarb ITERG 11 rue Monge, Parc Industriel Bersol 2, 33600 Pessac (France) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500214.
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Full Papers addition of excess PPh3 improved the conversion (91 %; Table 2, entry 18). A 3:1 mixture of PPh3/[Ir(COD)Cl]2 was considered as it was one of the best catalytic systems. The catalytic conditions were further optimized by varying the amount of the additives (Table 3). Conversions were very low in the absence of acetic anhydride (Table 3, entry 1) or potassium iodide (Table 3, entry 2). An increased amount of KI led to an increase in both the conversion and the terminal alkene selectivity (Table 3, entries 3 and 4). However, a slight increase of the amount of KI produced no effect on the catalytic performance (Table 3, entry 8). A decrease of the quantity of acetic anhydride impacted the con-
Scheme 1. Ir-catalyzed decarbonylation of unsaturated fatty acids.
so far have focused on saturated fatty acid substrates. In this context, we assessed the catalytic performance of Ir catalysts in the decarbonylation of biosourced unsaturated fatty acids (Scheme 1). Herein, we demonstrate that a wide range of unsaturated biosourced fatty acids could be decarbonylated efficiently under relatively mild conditions using Ir catalysts under homogeneous catalytic conditions. The conversion and selectivity in terminal olefins are discussed throughout a deTable 1. Selectivity of the Ir-catalyzed decarbonylation reaction for saturated and untailed optimization of the experimental conditions. saturated derivatives.[a]
Results and Discussion
Entry Substrate
Ligand[b] Precursor[b] [mol %] [mol %]
Conversion[c] 2/3 selectivi[%] ty[c]
To begin with, oleic acid (99 wt % purity) was chosen 80 95:5 1 stearic acid 98 % – IrCl(CO)(PPh3)2 as a model substrate. The presence of only one C=C (5) (1 a) 2 oleic acid 99 % (1 b) – IrCl(CO)(PPh3)2 84 89:11 bond on the alkyl chain simplified the analysis of the (5) reaction products greatly. To evaluate the impact of 3 stearic acid 98 % PPh3 [Ir(COD)Cl]2 (2.5) 86 95:5 the unsaturation on the catalytic performance, a com(1 a) (15) parison with stearic acid was performed using IrCl([Ir(COD)Cl]2 (2.5) 95 4 oleic acid 99 % (1 b) PPh3 91:9 (15) CO)(PPh3)2 and [Ir(COD)Cl]2 (COD = cyclooctadiene) as Ir catalysts and potassium iodide and acetic anhy[a] Reaction conditions: 1.0 mmol substrate, 2 mmol acetic anhydride, 0.5 mmol KI, dride as co-reactants (Table 1).[44] The reaction prod160 8C, 5 h. [b] Molar percentage with respect to the substrate. [c] Determined by 1 H NMR spectroscopy and confirmed by GC. ucts were monitored and analyzed by 1H NMR spectroscopy (Supporting Information). At 160 8C, the decarbonylation of oleic acid within 5 h led to higher Table 2. Effect of the ligand and the metallic precursor on the selectivity conversions compared to stearic acid but to lower seof the Ir-catalyzed decarbonylation reaction.[a] lectivity in terminal alkenes (Table 1). Accordingly, the presence of the C=C bond did not affect the catalytic performance sigPrecursor[b] Conversion[c] 2/3 selectivity[c] Entry Ligand[b] [mol %] [mol %] [%] nificantly. The amount and the nature of the ligand was then varied 1 – [Ir(COD)Cl]2 (2.5) < 5 – 2 PPh3 (5) [Ir(COD)Cl]2 (0.5) 39 77:23 (Table 2). If [Ir(COD)Cl]2 was used as the catalyst, a very small [Ir(COD)Cl]2 (1) 68 84:16 3 PPh3 (10) conversion could be measured in the absence of any ligand [Ir(COD)Cl]2 (2.5) 92 84:16 4 PPh3 (10) (< 5 %, Table 2, entry 1). An increase of the amount of Ir cata5 PPh3 (15) [Ir(COD)Cl]2 (2.5) 90 87:13 lyst and PPh3 favored both the conversion and selectivity [Ir(COD)Cl]2 (2.5) 89 86:14 6 PPh3 (20) [Ir(COD)Cl]2 (2.5) 34 78:22 7 p-Cl-PPh3 (15) greatly, and the best result was obtained for a PPh3/[Ir(COD)Cl]2 (15) [Ir(COD)Cl] (2.5) 56 84:16 8 p-F-PPh 3 2 ratio of 3 (Table 2, entry 5) with a 5 mol % of Ir catalyst with re75 84:16 9 p-tBu-PPh3 (15) [Ir(COD)Cl]2 (2.5) spect to the substrate. The substitution of the aromatic rings 10 p-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) 89 85:15 of PPh3 in the para position by Cl, F, or tBu had a deleterious 86 85:15 11 m-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) – 12 o-tolyl-PPh3 (15) [Ir(COD)Cl]2 (2.5) < 5 effect on the conversion (Table 2, entries 7, 8, and 9). Substitu(15) [Ir(COD)Cl] (2.5) < 5 – 13 o-OMe-PPh 3 2 tion in the para and meta positions by a methyl group showed – 14 dppb (10) [Ir(COD)Cl]2 (2.5) < 5 no change (Table 2, entries 10 and 11), whereas substitution in – 15 Xantphos (10) [Ir(COD)Cl]2 (2.5) < 5 the ortho position deactivated the catalyst (Table 2, entries 12 – 16 DPE-phos (10) [Ir(COD)Cl]2 (2.5) < 5 85:15 17 – IrCl(CO)(PPh3)2 (5) 84 and 13). Bidentate ligands such as 1,4-bis(diphenylphosphino)IrCl(CO)(PPh3)2 (5) 91 87:13 18 PPh3 (10) butane (dppb), Xantphos, and bis-[2-(diphenylphosphino)phe[a] Reaction conditions: 1.0 mmol oleic acid 88 % (1 b’), 2 mmol acetic annyl]ether (DPE-phos) were ineffective (Table 2, entries 14, 15, hydride, 0.5 mmol KI, 160 8C, 5 h. [b] Molar percentage with respect to and 16). If IrCl(CO)(PPh3)2 was used as the catalyst, similar conthe substrate. [c] Determined by 1H NMR spectroscopy. version and selectivity were obtained (Table 2, entry 17). The
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Full Papers 5), very low conversions were obtained. Attempts were also made in aqueous biphasic conditions using sulfonated ligands to retain the Ir catalyst in water. Very low conversions were obtained if the benchmark 3,3’,3’’-phosphanetriyltris(benzenesulfonic acid) trisodium salt (TPPTS) or the more organophilic p-(tris-Me)-TPPTS [sodium 5,5’,5’’-phosphinetriyltris(2-methylbenzene sulfonate)] and p(tBu)-TPPDS sodium (3,3’-{[4-(tert-butyl)phenyl]phosphinediyl}dibenzene sulfonate) were used even in the presence of a surfactant such as cetyl trimethylammonium bromide (CTAB; Table 4, entry 6).
Table 3. Effect of acetic anhydride and additive on the Ir-catalyzed decarbonylation reaction.[a] Entry
Anhydride [mmol]
Additive[b] [mol %]
Conversion[c] [%]
2/3 selectivity[c]
1 2 3 4 5 6 7 8
– Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O
KI (50) – KI (20) KI (50) KI (50) KI (50) KI (50) KI (100)
0
