text: The reaction requires metal catalysts.  Most commercially important processes employ heterogeneous catalysts.  The heterogeneous catalysts are often prepared by in-situ activation of a metal halide (MCl) using organoaluminium or organotin compounds, e.g. combining MCl–EtAlCl. A typical catalyst support is alumina.  Commercial catalysts are often based on molybdenum and ruthenium. Well-defined organometallic compounds have mainly been investigated for small-scale reactions or in academic research.  The homogeneous catalysts are often classified as Schrock catalysts and Grubbs catalysts. Schrock catalysts feature molybdenum(VI)- and tungsten(VI)-based centers supported by alkoxide and imido ligands.
  Grubbs catalysts, on the other hand, are ruthenium(II) carbenoid complexes. Many variations of Grubbs catalysts are known.  Some have been modified with a chelating isopropoxybenzylidene ligand to form the related Hoveyda–Grubbs catalyst.