The oxosynthesis process (also called hydroformylation) involves the reaction of syngas with olefins (alkenes; CnH2n) to form a mixture of aldehydes (containing 3 to 15 carbons). The short chained aldehydes are subsequently converted into alcohols, acids, or other chemicals used in solvents, synthetic detergents, flavorings, perfumes and other cosmetic products, healthcare products (such as Vitamin A), and other high value commodity chemicals including plasticizers for flexible PVC. Longer chain aldehydes (C11-C14) are typically used to produce surfactants used in the detergen industry.
Syngas is gaseous mixture consisting primarily of carbon monoxide (CO) and hydrogen (H2). Syngas is produced from the gasification of feedstocks at temperatures in excess of 1100°F and under conditions where the amounts of oxygen (from air, pure oxygen, or steam) are less than what is needed for complete combustion. Syngas can be produced from many feedstocks, including natural gas, petroleum products, coal, and biomass.
The basic oxosynthesis reaction produces large quantities of heat and the reaction is favored at ambient pressures and low temperatures. Usually a syngas hydrogen to carbon monoxide mixture of 1 to 1 is needed for oxosynthesis. The reaction rate is independent of total pressure, but higher pressures are usually required to maintain the stability of the catalysts. One of the more important factors in oxosynthesis is the ratio of straight chain to branched aldehydes, with straight chain isomers the desired product.
Oxosynthesis generally uses soluble cobalt or rhodium catalysts. The choice of catalyst depends of the particular starting olefin or desired product. Three complimentary processes have been developed and commercialized. (1) The first oxosynthesis catalysts were cobalt carbonyls (HCo(CO)4). The cobalt catalyzes both double bond isomerization and oxosynthesis. Undesired competing side reactions are generally avoided in the Co-catalyzed process. A straight chain to branched isomeric ratio of 4 to 1 can be achieved with carbonyl catalysts. Lower process temperatures and higher 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. (2) Phosphine-modified cobalt catalysts are used for the production of long chained (detergent range; 11 to 14 carbons) alcohols. They show high selectivity for straight-chain aldehydes and have improved thermal stability relative to unsubstituted cobalt catalysts which allows for the use of lower pressures and higher temperatures. (3) Phosphine-modified rhodium catalysts function under significantly lower operating pressures and temperatures than cobalt catalysts, and demonstrate increased selectivity to linear products. They are used mainly to produce short chained olefins (e.g., propylene to butyraldehyde) as they are unstable at the high distillation temperatures required to separate the long chained products and the catalyst. Rh-based catalysts are expensive relative to other catalysts, and the availability of rhodium is low. The high cost, 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. Catalyst lifetimes are significantly reduced by poisoning from strong acids, HCN, organosulfur, H2S, COS, O2, and dienes.
The Low Pressure OxoTM process (developed by Union Carbide and Davy Process Technology in 1976) is the dominant process used by world producers of oxo alcohols (figure 1). It involves the reaction of propylene with syngas to produce butyraldehydes (conversion rate as high as 97.5% at commercial scale) which are then converted to alcohols. The efficiency is a function of feedstock purity, with higher purity resulting in higher conversion rates. The Shell Process uses a phosphine-modified cobalt catalyst and produces alcohols directly from olefins. This process yields detergent size alcohols.
Today, oxosynthesis processes are the fourth largest commercial use of syngas. Worldwide production of oxo-aldehydes and alcohols was 6.5 million tons per annum in 1997. Announced planned additions will increase production capacity by nearly 1 million tons over the next 5 years. Oxochemical producers in the U.S. include BASF Chemicals, Celanese Chemicals, Dow Chemicals, Eastman Chemicals, ExxonMobile Chemicals, Shell Chemicals, Sterling Chemicals, and Sunoco.
The cost of producing oxosynthesis chemicals depends on the 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 the cost of producing the intermediate chemical. For 2002, the market price for most oxo products ranged from $0.44 to $0.68/lb.