VAPOR-PHASE HYDROGENATION OF MALEIC-ANHYDRIDE TO GAMMA-BUTYROLACTONE. 3. REACTION PATHWAY AND NEW CATALYST COMPOSITIONS

The reactivities of the main products observed in the vapour phase hydrogenation of maleic anhydride (MA) were investigated separately, using a multicomponent catalyst (Cu/Zn/Mg/Cr=40:5:5:50, atomic ratio %), obtained by reduction of a non-stoichiometric spinel-type precursor. Tetrahydrofuran (THF) reacted only at high temperature forming small amounts of ethanol (EtOH), while gamma-butyrolactone (GBL) gave rise, with high conversion, to overhydrogenation [to butyric acid (BuA) or n-butanol (BuOH)] and/or hydrogenolysis [mainly to EtOH. Furthermore, the formation in the catalytic tests with BuA and BuOH of n-butanal (notwithstanding the high H-2/organic ratio), suggested it as the main intermediate in the hydrogenolysis reaction. Therefore, a new reaction scheme is proposed, evidencing the key role of GBL as the ''intersection'' of two possible reaction pathways, giving rise to THF or unwanted and low-cost by-products. Because of the increasing restrictions on the use of Cr-containing catalysts, the research was extended to include an investigation of the reactivity of Al-containing catalysts, also prepared by reduction of spinel-type precursors. These catalysts, in particular those with low copper contents, gave rise to high productions of GEL, favouring also the formation of THF and reducing the amounts of unwanted and low-cost by-products. It was shown that a Cu/Zn/Al=25:25:50 (atomic ratio %) composition may represent a useful compromise, allowing good yields in GBL to be obtained at lower reaction temperatures.