Porosity-Activity Relationships in Selective Vapour Phase Hydrogenation of Maleic Anhydride on Al-Containing Catalysts

The relationships between physical and catalytic properties for Cu/Zn/Al catalysts in selective vapour phase hydrogenation of maleic anhydride (MA) to gamma-butyrolactone (GBL) was investigated, with particular attention focused on the role of surface area and porosity. The Al-containing catalysts prepared by reduction of cubic spinel-type precursors can be an interesting alternative to Cr-containing catalysts, the use of which are becoming increasingly difficult due to the toxic nature of the spent catalysts. These catalysts gave rise to high productions of GBL, favouring also the formation of tetrahydrofuran (THF) and reducing the amounts of low-cost by-products. In particular, 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, at which the production of by-products is negligible. However, the Al-containing catalysts show higher irreversible adsorption and lower mechanical strength. These unwanted phenomena can be significantly reduced by pressing the catalyst powder without binders at 3.0 tons cm(-2) for 30 min. This treatment gave rise to a decrease of about 25% in both surface area and pore volume, with a significant decrease in micropores and in macropores larger than 150 nm, with a corresponding increase in the pores in the 50-150 nm range. In the catalytic tests with either GBL or a MA/GBL solution, the pressed catalyst showed a decrease in hydrogenation capacity significantly lower than the decrease in surface area, with, however, higher yields in THF and lower formation of unwanted and low-cost by-products (mainly n-butanol and butyric acid). Furthermore, this catalyst exhibited a considerable reduction in crushing and adsorption phenomena, evidencing a positive effect of the pressure treatment on both physical and catalytic properties.