Hydroformylation of Olefins with Cobalt/Phosphonate- and Cobalt/Sulfonate-Phosphines A

Home , Hydroformylation, Phosphonate, Tppts

DGMK/SCI-Conference „Synthesis Gas Chemistry", October 4-6, 2006, Dresden, Germany

Hydroformylation of Olefins with Cobalt/Phosphonate- and Cobalt/Sulfonate-Phosphines A. Martin*, M. Kant*, G. Giuffrida**, S. Rosano**, *Leibniz-Institut fur Katalyse e.V., Berlin, Germany, **Sasol Italy S.p.A., Paderno Dugnano, Italy

Abstract

The hydroformylation of an industrial decene mixture with cobalt/phosphonate- and cobalt/sulfonate-phosphines used as catalysts was carried out. Highest aldehyde yield of ca. 60-65 mol% beside 2-5 mol% decane, 1-5 mol% decenes and 2-5 mol% of other oxo- products was obtained at 170 °C, 160-200 bar syngas pressure and a reaction time of 12-16 h. The reminder is a fraction of non-GC-detectable heavy oligomers (15-20 %). Best olefin conversion was reached with Ph2 P(p-C6H4-SO3 Li) and TPPTS as ligands, best stability of biphasic system with TPPTS and Ph2 P-(CH2 )3-SO3Li. The terminal aldehyde selectivity amounted to 36-42 mol% of the aldehyde pool.

Introduction

In recent years, the progressive development of organometallic chemistry led to an increasing number of applications in the field of homogeneous catalysis that might be used for a high selective synthesis of various chemicals under mild reaction conditions [1]. Hydroformylation (Scheme 1 as shown below, also called as oxo-synthesis or Roelen- reaction) is such a good example. It is a reaction route leading to aldehydes by addition of syngas (i.e. CO and hydrogen) to the double bond of terminal olefins or their derivatives [e.g. 2]. Linear but also branched olefins can be used as feedstock. Due to double bond isomerisation also branched products can be obtained from terminal olefins as shown below.

,O CO/H2, cat. R. R ,H + + R O

Scheme 1

General hydroformylation reaction scheme

DGMK-Tagungsbericht2006-4, ISBN 3-936418-57-8 67 Hydroformylation is an industrially important reaction, 6-7 million tons of oxo-products are manufactured per year. Mostly used feedstocks are C2 -C16 alkenes. The primarily formed aldehydes are subsequently converted by hydrogenation, oxidation, amination etc. Oxo- alcohols are the main product class used as solvents, detergent alcohols or plasticizers.

Approximately 70 % of the above mentioned production volume are C4-aldehydes mainly being the educt for the manufacture of 2-ethyl hexanol, the most used plasticizer alcohol.

An outstanding example for the oxo-synthesis is the manufacture of butyric aldehyde. It is synthesized from propene in a biphasic reaction medium using a water soluble Rh/TPPTS (triphenylphosphine trisulfonate) catalyst according to the so called Ruhrchemie/Rhone- Poulenc-Process [e.g. 3]. Unfortunately, the very effective Rh/TPPTS catalyst system is limited to the selective conversion of short-chain terminal olefins. Only small conversions are obtained using long-chain, branched or internal olefins [4]. However, from industrial point of view (i) long-chain terminal aldehydes are also rewarding chemicals and (ii) it might be very interesting to start the hydroformylation with internal olefins and end up the reaction with terminal aldehydes because internal olefin mixtures are much more cheaper then the terminal olefins.

Recently, the group of van Leeuven reported on an important increase of the yield of terminal aldehydes starting from internal olefins. The used Rh/Xantphos catalyst system showed a terminal aldehyde selectivity of 80-90 % starting from 2- or 4-octene [5], however, the reported activities (TOF = 20-110 h-1) are rather less for industrial applications. Much higher active catalysts were described by Beller and co-workers [6] some years ago using substituted NAPHOS ligands. Selent et al. reported on biphosphonite ligands showing TOF values up to 3000 h-1 in the hydroformylation of internal octene mixtures [7], however, the sensitivity of the ligands against water seems to be the major drawback for further developments.

Up to date, cobalt is the industrially used metal for hydroformylation of technical olefin mixtures in the liquid phase [e.g. 8]. The reaction conditions are rather harsh (150-200 °C,

200-300 bar), pure cobalt carbonyls (e.g. HCo(CO)4) and carbonyl catalysts modified with additional ligands are used as catalysts. The separation of the homogeneously distributed catalysts is mostly done by oxidation or thermal decomposition, but in both the cases the catalyst structure is destroyed. Other separation processes use a catalyst extraction by diluted NaOH, the formed NaCo(CO)4 can be regenerated by adding sulphuric acid, however due to the huge amounts of salts the process is not sustainable.

The biphasic hydroformylation, that enables an easy catalyst recovery, of internal olefins was first described in 1979 by Jenk of Rhone-Poulenc [9]. In 1999 Beller and Krauter reported on the biphasic cobalt-catalyzed hydroformylation of short-chain internal olefins using TPPTS as ligand [10]. Aldehyde yields of approximately 70 % were reached at 130-190 °C and pressures up to 100 bar. A cobalt leaching of ca. 1-6 % into the organic phase is reported. However, nothing is known in the literature on the Co-catalyzed hydroformylation of long- chain, internal olefins using TPPTS or other water soluble ligands. The aim of the present work was to check the suitability of such ligands bearing sulfonic acid or phosphonic acid groups in the Co-catalyzed hydroformylation of long-chain olefins.

68 Experimental Ligand and catalyst syntheses

Precursor compounds w-Bromo-alkylphosphonates Br-(CH2 )m-P(O)(OEt)2 (m = 2-12) were synthesized by heating a mixture of Br-(CH2 )m-Br and P(OE% (m = 2-12) and subsequent recovery by distillation according to a literature recipe [11]. Due to high reactivity and thermal instability of w-bromo- alkylphosphonates the distillative work-up was carried out under high vacuum and careful exclusion of moisture and oxygen.

Phosphonic acid diethylesters (PhnP(-(CH2 )m-P(O)(OEt)2 )3-n (n = 1, 2; m = 2-12)) were synthesized according to a general recipe of Ganguli et al. [12].

Phosphine-phosphonate synthesis

The phosphine-phosphonate ligand (PhnP(-(CH2 )m-PO3Na2 )3-n (m = 2-12; n = 1, 2)) synthesis was carried out by heating the corresponding phosphonic acid diethylesters with hydrochloric acid under reflux for 12 h and subsequent treatment with NaOH under non-aerobic conditions. A detailed procedure is given in [13].

PhP(p-C6H4-PO3 Na2 )2 was synthesized by heating a mixture of Li2 PPh and p-F-C6H4-

P(O)(NEt2 )2 in tetrahydrofurane (THF). The acid hydrolysis of the formed amide (PhP(p-C6H4-

P(O)(NEt2 )2 )2 ) led to the phosphonic acid PhP(p-CaH4-PO3H2 )2 after neutralization with NaOH [14].

Phosphine-sulfonate synthesis

Ph2 P-(CH2 )3-SO3Li was synthesized by reaction of Li-diphenyl phosphid with 1,3- propansultone [15]. Ph2 P(p-C6H4-SO3Li) was obtained from a mixture of p-F-C6H4-SO3 Li and Li-diphenyl phosphid in THF at 25 °C for 4 h under stirring.

Catalyst precursor compounds

All catalyst precursor compounds were obtained by careful addition of an aqueous solution of the ligands synthesized according to the above described recipes to a Co2(CO)8/hexane solution under vigorous stirring and Ar atmosphere [16].

Characterization methods

ICP-OES was used to determine Co and P content in the catalyst precursor solutions after repeated HNO3 treatment.

The synthesis of the phosphine ligands and catalyst precursor compounds was checked by 31 P-NMR (Varian UNITYplus-300). The concentration of the ligand solutions was calibrated by using hexamethyl phosphoric acid triamide as inner standard.

The determination of the components of the reaction mixture was carried out using GC-MS (HP 5890 / HP 5972 series), /-octane was added as inner standard. A proportion of higher molecular oligomers that couldn’t be determined by GC were calculated by C-balance, in general. In some reaction runs, oligomers were determined by using gel permeation chromatography (GPC).

69 Catalytic measurements

In general, the hydroformylation runs were carried out in 100 and 250 ml autoclaves, respectively. After loading the aqueous catalyst precursor solution (20 or 30 ml) containing a defined portion of cobalt (0.25 % of cobalt with a ligand/Co-ratio between 2 and 10), an industrial mixture of decenes containing terminal and internal olefins and ca. 10 % decane (10 or 15 ml) was filled. The autoclave was flushed by syngas to remove oxygen. 60 to 200 bar syngas (CO : H2 = 1 : 1) were pressurized and the vessel content was heated under stirring (600 min-1) to the desired reaction temperature. After a defined time, the reaction mixture was cooled down to ca. 90 °C by an internal cooling loop and depressurized. The reaction mixture was transferred to a Schlenk flask, the GC analysis of the organic phase was carried out within the next 1-16 h. A detailed description of the reaction runs is given in [13]. The quality of the phase separation by observation of the phase boundary and the color as well as transparency of both the phases were checked over some weeks.

Results and discussion

The hydroformylation of the decene mixture in the biphasic reaction system always led at temperatures above 150 °C to the complete set of isomeric aldehydes (Scheme 2) as recently reported by Beller et al. [10] for the similar Co-catalyzed reaction of internal pentenes.

‘ O Undecanal H_ ,O

2-Methyl-decanal

2-Ethyl-nonanal

2-Propyl-octanal

2-Butyl-heptanal

Scheme 2

Isomeric aldehydes expected by hydroformylation of a decene mixture

Table 1 shows the influence of the reaction temperature and pressure on the aldehyde yield using two different ligands. It can be stated that increasing reaction temperatures up to 170 °C and pressures of 100-125 bar are favorable for higher aldehyde yield. Furthermore, it must be noticed that Ph2P-(CH2)3-SO3Li used as ligand shows much higher yields. A further increase in temperature should be avoided because an increasing formation of alcohols due to hydrogenation of the aldehyde group was observed.

70 Table 1 ligand temperature pressure aldehyde Two phase system (°C) (bar) yield (%) behavior Ph2 P-(CH2 )3 -P03 Na2 150 100 5 stable 170 75 11.1 stable 170 100 20.2 stable 190 75 22.4 instable (decomposition) Ph2 P-(CH2 )3 -S03 Li 160 125 49.6 stable 170 125 67.5 stable 180 125 65.8 stable Reaction conditions: [Co] = 0.25 %, t = 16 h, agitation = 600 min"1, 10 ml decenes, 100 ml autoclave

Figures 1-4 depict the reaction results (170 °C, 16 h) from GC analysis in dependence on the reaction pressure using 1. Ph2 P-(CH2 )3 -P03 Na2 , 2: P(m-C6H4-S03 Na)3 (TPPTS), 3: Ph2 P- (CH2 )3-S0 3 Li, 4: Ph2 P(p-C6H4-S03 Li) as ligands. As expected, it can be clearly seen that with increasing pressure decene conversion is increased (empty columns show unconverted decene plus decane). The black columns characterize all found oxo-products, i.e. aldehydes and alcohols, whereas the dark-grey column stands for aldehydes. The light grey columns show the n-aldehyde formation

71 yield (%) 40

125 160 200

Fig. 3 P (bar)

72 yield (%)

P (bar)

Figures 1-4

Unconverted decene mixture amount (empty column, including a distinct amount of decane), sum of all oxo-products (black columns, aldehydes and alcohols), proportion of aldehydes (dark-grey columns) and n-aldehydes (light-grey columns) obtained from GC analysis during catalytic tests using different ligands (1: Ph2 P-(CH2 )3 -P03 Na2 , 2: P(m-C6H4-S03 Na)3 (TPPTS), 3: Ph2 P-(CH2)3-S0 3 Li, 4: Ph2 P(p-C6H4-S03 Li)) at 170 °C for 16 h.

As mentioned above, the empty columns in Figs. 1-4 represent unconverted decene and decane (the feed already contains 10 % decane). Due to a very difficult GC separation of decene and decane, i.e. only at high decene conversion decane can be reliably determined. At highest pressures studied, i.e. 160-200 bar, decene conversion is nearly complete but also ca. 20 % of oligomers are formed (as determined independently using GPC analysis). Taking the oligomer formation into account, for example, in Fig. 4 all values at 200 bar have to be multiplied by 0.8. Doing so, ca. 62 % aldehydes, ca. 15 % decane (including the fed decane) and 20 % oligomers are formed, i.e. the C-balance is close to 100. From this calculation it can be concluded that decane formation due to hydrogenation of the used olefins is less than 5 %. The proportion of the formed alcohols is in the same order of magnitude. From these results it can be also concluded that the hydrogenation activity of the catalysts is rather low.

As expected, the yield of oxo-products increases with increasing pressure. In general the n : iso ratio accounts to ca. 0.3-0.4 and it can be stated that nearly complete conversion is only reached with sulfonated ligands. It is striking that the distribution of the formed aldehydes is independent on the structure of the sulfonated ligands at high conversion and high pressures. At 200 bar, in all cases (Figs. 2-4) ca. 30 % n-aldehyde are formed. This could mean that the catalytic active component is in all these cases the same compound, probably strong acidic ligand-free HCo(CO)4 formed in situ. Additional tests have shown a distinct lower conversion level when working with basic buffer solutions (pH = 7-8, NaHC03 or pH = 10, Na2 C03 ). Furthermore, the n-aldehyde formation in relation to the /so-aldehyde is independent on dropping conversion due to increased pH value supporting the above made assumption of one and the same catalytic active species.

73 Conclusion

Highest aldehyde yield of ca. 60-65 mol% beside 2-5 mol% decane, 1-5 mol% decenes and 2-5 mol% of other oxo-products was obtained at a reaction temperature of 170 °C, a syngas- pressure of 160-200 bar and a reaction time of 12-16 h. The reminder is a fraction of non- GC-detectable heavy oligomers (15-20 %). Best olefin conversion was reached with Ph2 P(p- C6H4-SO3 Li) and TPPTS as ligands, best stability of biphasic system with TPPTS and Ph2 P- (CH2 )3 -SO3 Li. The activity of the catalysts rises with increasing reaction pressure and temperature whereas the differences between 160 and 200 bar starting pressure were only small. At these higher pressures the selectivity is not significantly affected by the structure of the ligand with regard to the terminal aldehyde. The terminal aldehyde selectivity amounted to 36-42 mol% of the aldehyde pool.

References

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