Base-free conversion of glycerol to methyl lactate using a multifunctional catalytic system consisting of Au–Pd nanoparticles on carbon nanotubes and Sn-MCM-41-XS

Base-free, multifunctional heterogenous catalytic systems for the highly efficient conversion of glycerol into methyl lactate. Multifunctional catalytic systems consisting of physical mixtures of (i) bimetallic Au–Pd nanoparticles (average size of 3–5 nm) supported on functionalised carbon nanotubes (CNTs) and (ii) Sn-MCM-41 nanoparticles (50–120 nm), were synthesised and investigated for the base-free, selective conversion of glycerol to methyl lactate in a batch reactor. The catalysts were characterised by means of transmission electron microscopy, N 2 -physisorption, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and by Boehm titration. The catalyst based on bimetallic AuPd/CNTs showed much higher activity than the monometallic Au or Pd counterparts, thus indicating synergetic effects. Functionalisation of the CNTs by oxidative treatments had a positive effect on catalyst performance, which was correlated to the observed increase in surface acidity and hydrophilicity. The highest yield of methyl lactate achieved in this work was 85% at 96% glycerol conversion (140 °C, 10 h at 30 bar air), which is the highest yield ever reported in the literature so far. Insights in the reaction pathway were obtained by monitoring the conversion-time profiles for intermediates and their possible role as inhibitors. Batch recycling experiments demonstrated the excellent reusability of the catalyst.