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bioweb.sungrant.org » Technical » Bioproducts » Bioproducts from Syngas » Oxosynthesis

Products from Syngas—Oxosynthesis (Catalyst)	Printer Friendly

In its simplest form, syngas (also called producer gas, town gas, blue water gas, and synthesis gas among others) is composed of carbon monoxide (CO) and hydrogen (H2), which provide the building blocks to produce a number of organic compounds. During the 1800’s, coal gasification was used for lighting and heating. The production of fuels and chemicals from syngas began in the early 20th century. Today, methanol and ammonia are produced from syngas, and in addition to hydrogen, constitute the major uses of syngas.    In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. Under today’s conditions, the least expensive feedstock is natural gas. Using numerous synthesis pathways, a large number of organic compounds can be produced from syngas (figure 1).      The oxysynthesis process uses syngas to hydroformylate olefins and is the principal chemical route to produce C3-C15 aldehydes. These aldehydes are subsequently converted into alcohols, acids, or other derivative chemicals which are used in solvents, synthetic detergents, flavorings, perfumes and other cosmetic products, healthcare products (such as Vitamin A), and other high value commodity chemicals. Longer chain aldehydes (C11-C14) are typically used to produce surfactants used in the detergent industry. An oxosynthesis process is used to produce butyraldehyde (from propylene) which is converted to 2-ethylhexanol, a plasticizer alcohol used to make flexible PVC. Butanediol is made from allyl alcohol using an oxosynthesis process. Today, hydroformylation processes are the fourth largest commercial use of syngas (Wender, 1996).

Oxosynthesis reactions
The oxosynthesis process (also called hydroformylation) involves the reaction of syngas (CO and H2) with olefinic hydrocarbons to form an isomeric mixture of normal- and iso-aldehydes. The basic oxosynthesis reaction is highly exothermic and is thermodynamically favorable at ambient pressures and low temperatures (Whyman, 1985). The reaction proceeds only in the presence of homogeneous metal carbonyl catalysts. One of the more important factors in oxosynthesis is the normal to branched isomeric ratio (n/i). The normal (straight chain) isomer is the desired product, as shown in the equation below.   RCH=CH2 + CO + H2 → RCH2CH2CHO + R(CH3)CHCHO                                       (normal)            (branched)   Usually a 1:1 H2 to CO syngas mixture is required for oxosynthesis. The overall reaction rate has first-order dependence on the hydrogen partial pressure and inverse-first order dependence on CO partial pressure making the reaction rate essentially independent of total pressure (Pruett, 1979). Higher CO partial pressures are usually required, however, to maintain the stability of the metal carbonyl catalysts. The reaction is also first order in olefin and metal concentration at the higher CO partial pressures.   The first step in the oxosynthesis process is to remove the CO from the organometallic catalyst making the catalyst deficient in electrons. The double bond in the olefin attaches to the metal atom at this site (the M-H bond) resulting in an alkyl metal carbonyl complex. A CO molecule is then inserted into the complex at the C-M bond followed by insertion of hydrogen at the same point to yield an aldehyde. The general reaction mechanism is shown in figure 2 (Whyman, 1985).

Oxosynthesis catalysts
Oxosynthesis generally uses soluble cobalt or rhodium catalysts. Three complimentary catalytic hydroformylation processes have been developed and commercialized. The choice of catalyst depends of the particular starting olefin or desired product.   The first hydroformylation catalysts were cobalt carbonyls, specifically, HCo(CO)4 in equilibrium with Co2(CO)8. Cobalt metals and most cobalt salts will form cobalt carbonyl under hydroformylation conditions. The cobalt catalyzes both double bond isomerization and oxosynthesis. Undesired competing side reactions such as the direct hydrogenation of the starting olefin and the condensation of product aldehydes to high boiling products are generally avoided in the Co-catalyzed process. For cobalt carbonyl catalysts, a normal to branched isomeric ratio (n/i) of 4:1 can be achieved with catalyst concentrations of 0.1-1% metal/olefin at 200-300 atm and 110-200°C with a 1:1 H2/CO ratio. Lower process temperatures and higher CO partial pressures favor the formation of the straight chain isomer, however, the overall conversion efficiency decreases. Cobalt carbonyl catalysts are not very stable at high temperatures and tend to deposit on reactor walls decreasing their activity and reducing recovery of the catalyst.   Phosphine-modified cobalt catalysts were developed by Shell Oil Company in the 1960’s, and are used for the production of higher (detergent range) alcohols. The addition of a phosphine ligand to Co results in a trialkylphosphine-substituted cobalt carbonyl catalyst [HCo(CO)3P(n-C4H9)3]. It shows high selectivity for straight-chain aldehydes (n/i = 7:1) and has improved thermal stability compared to the unsubstituted cobalt catalysts. The improved thermal stability allows for lower process pressures but higher process temperatures (50-100 atm and 160-200°C with H2:CO = 1). Even though this catalyst has improved thermal stability, it has a lower hydroformylation activity than cobalt carbonyl catalysts, hence the higher reaction temperature. The higher temperatures also increase the competing olefin hydrogenation reaction. Shell has optimized the process to produce detergent range alcohols (C11-C14) in a single step by capitalizing on the conversion of terminal olefins to alcohols by hydrogenating the aldehyde hydroformylation products. A high n/i ratio results from increased isomerization rates concurrently with hydroformylation.   Phosphine-modified rhodium catalysts are composed of triphenylphosphine rhodium (HRh(CO)(PPh3)3). These catalysts function under significantly lower operating pressures and temperatures (7-25 atm; 60-120°C; n/i ratio of 8-12:1) and demonstrate increased selectivity to linear products compared to other oxosynthesis catalysts. Rh-based catalysts are used mainly for the hydroformylation of lower olefins (e.g., propylene to butyraldehyde) and are typically not used for higher olefins because of their thermal instability at the high distillation temperatures required to separate the product and the catalyst. Rh-based hydroformylation catalysts are also expensive relative to other catalysts, and the availability of rhodium is low. The high cost of rhodium, however, is offset by lower equipment costs, increased activity, and higher selectivity and efficiency. The development of water-soluble Rh-based catalysts avoids some of these issues. Rhone-Poulenc commercialized an oxo process based on a water-soluble Rh catalyst in 1984 (Billig, 2000).   Catalyst lifetimes are significantly reduced by poisoning from strong acids, HCN, organosulfur, H2S, COS, O2, and dienes (Bahrmann, 2000).

Commercial production of oxosynthesis products
In 1976, Union Carbide and Davy Process Technology commercialized the Low Pressure OxoTM process which is the world's leading process for the production of oxo alcohols from olefins and is used by most of the world's oxo alcohols capacity licensed during the last 20 years. A schematic of the LP Oxo Process is shown in figure 3.               The LP Oxo Process involves the reaction of propylene with syngas to produce butyraldehydes (n/i = 10-20:1), which are in turn converted to 2-ethylhexanol. Normal or iso-butyraldehydes are also converted to normal and iso-butanols and other derivatives. Propylene can be converted to mixed butyraldehydes at conversion efficiency rates as high as 97.5% at commercial scale. The efficiency is a function of feedstock purity with a higher purity feed resulting in a higher efficiency due to lower purge losses.   The Shell Process is a commercial oxo synthesis process based on a phosphine-modified cobalt catalyst and used to produce alcohols directly from olefins. The conversion of product oxo aldehydes to alcohols occurs in a single step in the Shell process because of the high hydrogenation activity of the modified catalyst. This process yields detergent range alcohols from a range of higher olefins.   Worldwide production of oxo-aldehydes and alcohols was 6.5 million tons per annum in 1997 (Bahrmann and Bach, 2000). Over the next 5 years, announced planned additions will increase the production capacity of oxo chemicals by nearly 1 million tons (Bitzzari, 2002).   A summary of oxochemical producers in the U.S. (as cited in the Chemical Economics Handbook) is presented in table 1 (Bitzzari, 2002).                The cost of producing oxosynthesis chemicals depends on costs of the initial olefin feedstocks, primarily propylene and ethylene. Oxo aldeyhdes are intermediates for the production of alcohols, acids and other chemical products. The market price for these products also varies with the cost of the feedstocks and intermediates. The Chemical Economics Handbook (CEH) (Bitzzari, 2002) contains estimated costs for a variety of oxo end products, a number of which are summarized in table 2. Although not included, the highest priced oxo chemical listed in the CEH is pelargonic acid ($1.33/lb).  The majority of the oxo products listed in the CEH range in price between 50-70¢/lb.

References
Bahrmann, H., and Bach, H. (2000). "Oxo Synthesis." Ullman's Encyclopedia of Industrial Chemistry, Wiley - VCH Verlag GmbH and Co. Billig, E., and Bryant, D. R. (2000). "Oxo Process." Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons. Bitzzari, S. N.; Gubler, R., and Kishi, A. (2002). "Oxo Chemicals." Chemical Economics Handbook, SRI International, 1-121. Report number 682.7000. Pruett, R. L. (1979). "Hydroformylation." Advances in Organometallic Chemistry, 17, 1-60. Spath, P.L. Spath and D.C. Dayton, Preliminary screening—technical and economic assessment of synthesis gas to fuels and chemicals with emphasis on the potential for biomas-derived syngas, National Renewable Energy Laboratory, NREL/TP-510-34929, December, 2003. Wender, I. (1996). "Reactions of synthesis gas." Fuel Processing Technology, 48(3), 189-297. Whyman, R. (1985). "Industrial Applications of Homogeneous Catalysts." Selected Developments in Catalysis, J. R. Jennings, ed., Blackwell Scientific Publications, Oxford, 128.