The Use of Supercritical Fluids to Reduce the Number of Phases in Catalytic Hydrogenation: The Reaction of Fatty Acid Methyl Esters to Fatty Alcohols

Abstract: Supercritical fluids have unique properties - between those of liquids and gases - making them attractive reaction media. In this study, propane was used to change a heterogeneous catalyst from being substrate-covered in gas-liquid hydrogenation reactions into hydrogen-covered in our experiments. For the investigated process this resulted in an increase in the reaction rate by 2 orders of magnitude and high yields.

The standard high-pressure (i.e. 200-300 bar) alcohol process consists of a multi-phase system: liquid (substrate and product), gas (hydrogen) and solid (catalyst). Hydrogen availability for the catalyst is the rate-determining step in this process, and therefore only a fraction of the catalytic potential is used. Dissolving hydrogen and substrate/product into one supercritical phase, consisting of mainly propane, eliminates the gas-liquid interface and allows a surplus of hydrogen at the catalyst.

In the lab-scale fixed-bed reactor (i.e. 0.05-5 ml), the activity of commercial copper catalysts has been studied as a function of the temperature, hydrogen and substrate concentration in the reaction mixture. The influence of mass transport and catalyst deactivation on the process was also investigated.

At 280 ºC, a system pressure of 150 bar and about 80 mol% propane, reaction times of some seconds were needed to reach high product yields. The reaction rates were comparable to those in gas-phase hydrogenation, and product space velocities of 100 h-1 were reached. In some cases, transport limitation of substrate, not hydrogen, occurred. To improve the mass transport small catalyst particles, preferably between 100-300 µm, were used.

At high substrate concentrations (> 2 mol%, i.e. 15 wt. %.), a rapid fall in the reaction rate and an increase in the pressure drop were observed. This was interpreted as a phase split and shows the necessity of having the reaction mixture in a single phase. From these observations phase-equilibria data were deduced.

It was possible to control overhydrogenation of the fatty alcohols to alkanes. With high hydrogen concentrations (20 mol%), catalyst deactivation was reduced and reached values similar to those in industrial processes.