September 5, 2011
Will acetaldehyde make a comeback? Direct and selective production of acetaldehyde (MeCHO) from acetic acid (AcOH) has the potential to renew interest in this intermediate. MeCHO formerly was used as an intermediate to make a variety of products, but demand diminished as lower cost routes to these products were developed. For example, MeCHO was used as a raw material for making 1-butanol, but this process has been replaced by the hydroformylation of propylene. Similarly, MeCHO was used as a feedstock for AcOH, but this was replaced by less expensive methanol carbonylation.
MeCHO is still used today as an intermediate in the synthesis of lower volume chemicals such as pentaerythritol and ethyl acetate. The conventional method for making MeCHO is the Wacker process, which oxidizes ethylene directly to MeCHO. But, because ethylene prices rise in parallel with oil prices, the cost of making MeCHO also increases.
V. J. Johnston and co-inventors disclose catalysts and operating conditions that allow AcOH to be hydrogenated to MeCHO in good selectivities and yields. In one example, a catalyst composed of 5 wt% Fe/SiO2 is loaded into a stainless steel tubular reactor. AcOH vapor and hydrogen in a 1:5 mol ratio are passed through the catalyst bed at a gaseous hourly space velocity of 2500 h–1 at 350 °C. AcOH conversion is 75%, and selectivity to MeCHO is 70%.
Higher flow rates used in other experiments resulted in lower AcOH conversion (e.g., 4%) but much higher MeCHO selectivities (e.g., 91%). Using less costly AcOH instead of expensive ethylene could bring down the cost of making MeCHO and rejuvenate interest in it as a versatile intermediate. (Celanese International [Dallas]. US Patent 7,994,368, Aug 9, 2011; Jeffrey S. Plotkin)