Dynamic microbalance studies of RbOx/SiO2 catalyst deactivation/regeneration for α-methylene γ-valerolactone synthesis Original Research Article

The deactivation-regeneration modes of RbOx/SiO2 catalyst for the synthesis of α-methylene-γ-valerolactone (MeMBL) from γ-valerolactone (GVL) and formaldehyde (HCHO) mixtures were studied with a novel microbalance reactor, the TEOM® (Tapered Element Oscillating Microbalance). An ethanolic solution of GVL and HCHO was fed after vaporization to the microbalance reactor packed with catalyst and operated at 340 °C. The catalyst mass change was monitored continuously during reaction and regeneration steps. It was observed that within the first 60 s of reaction the mass gain was very rapid, this phase was followed by a slower mass gain with constant rate up to 10 h time on stream (TOS). Conversion decreased slowly as determined by mass spectroscopy. Increasing the reaction temperature affected the slow mass gain regime, with higher mass gains observed at higher temperatures. Higher conversion was achieved at higher temperatures, but at a cost in MeMBL selectivity (>90% selectivity at 340 °C versus 20% at 400 °C). Studies of catalyst regeneration in air at different temperatures led to the conclusion that there are two types of deposits on the catalyst surface: (i) organic deposits loosely bound to the surface that burn-off at low temperature (340 °C) and (ii) coke with low H/C ratio that burns-off at higher temperature (450 °C). XPS measurements of fresh and deactivated catalysts indicated that the species causing deactivation are mainly located on the silica support. Partial removal of deposited material in air at 340 °C was found to restore a significant fraction of the catalyst activity while greatly reducing the unproductive loss of reactants due to rapid deposition on the catalyst when the reactant feed was resumed. Thus cyclic operation involving reaction followed by a short regeneration step (∼1 min for 1 h reaction) may be preferable, as the penalty in activity for partial versus complete regeneration is small, and there is no significant loss in MeMBL selectivity.