Gas-phase hydrogenolysis of dimethyl maleate to 1,4-butanediol and gamma-butyrolactone over copper/zinc oxide catalysts

The gas-phase hydrogenolysis of dimethyl maleate has been studied over copp er/zinc oxide catalysts at temperatures between 473 and 513 K in the pressu re range from 2 to 35 bar. A Cu/ZnO catalyst with a low copper content of 1 5 mol % was found to be the most active for the conversion of dimethyl male ate. At very low residence times, significant amounts of dimethyl maleate i somerize to dimethyl fumarate. The reaction then proceeds via the saturated ester, dimethyl succinate, until 1,4-butanediol and gamma-butyrolactone ar e formed together with methanol. Furthermore, the chosen catalyst produces low amounts of tetrahydrofuran and water and only traces of undesirable but anol and CO2. It was found that the production of high amounts of 1,4-butan ediol in one step is not possible, because the simultaneous presence of die sters and butanediol gives rise to the formation of a polymeric material vi a transesterification. Therefore, a two-step process including the complete conversion of dimethyl maleate to gamma-butyrolactone, carried out at high temperature and moderate pressure, without formation of butanediol followe d by the hydrogenation of gamma-butyrolactone to 1,4-butanediol at low temp erature and high pressure was proposed. It could be shown by systematic inv estigation of the hydrogenation of gamma-butyrolactone that the amount of 1 ,4-butanediol formed is limited by thermodynamic constraints. The dependenc e of the equilibrium constant for the reaction between gamma-butyrolactone and 1,4-butanediol was determined as a function of temperature and pressure .