Functionalized Olefin Metathesis in Oleochemistry: Self-metathesis of Methyl Oleate on Solid Catalysts

JUAN ZELÍN; ANDRÉS F. TRASARTI; CARLOS R. APESTEGUÍA

Louisville, Kentucky

Congreso; 23rd North American Catalysis Society Meeting; 2013

Norh American Catalysis Society

Fatty acid methyl esters (FAME) are usually obtained from the transesterification of natural oils and fats with a lower alcohol. Most of all oleochemistry reactions of FAME are carried out in the carboxy functions, but the synthesis of products formed by reactions of the C=C bonds is becoming increasingly important at industrial level. In particular, olefin metathesis is a verypowerful tool for the efficient catalytic formation of carboncarbon double bonds allowing the  synthesis of various useful intermediates and compounds for fine chemistry and for the synthesis of polymers. FAME metathesis has been studied in homogeneous catalysis using mainly Ru complexes such as Hoveyda-Grubbs (HG) complexes as recyclable catalysts, but very few papers have investigated this reaction on solid catalysts. Most examples of silica-immobilized Grubbs  catalysts refer to the covalent anchoring of the metal-containing moiety to preformed porous or non-porous silicas. Here, we studied the self-metathesis of methyl oleate (MO) on both Hoveyda-Grubbs complexes supported on silica and methyltrioxorhenium (MTO, CH3ReO3) supported on SiO2-Al2O3. The MO metathesis yields 9-octadecene and 9- octadecene-1,18-dioate (9-OD), a valuable product used in polymer, fragrance, lubricant, and fine chemistry industries.Materials and Methods Four HG/SiO2 samples containing 0.43, 0.87, 1.24 and 1.61%wt HG were prepared. Solutions of HG in toluene were impregnated on commercial silica (Sigma-Aldrich G62, 250 m2/g) at 25ºC. Solutions of MTO (1.64%) in cyclohexane were supported under inertatmosphere on Al2O3 and SiO2-Al2O3 (12% Al2O3). The liquid-phase MO metathesis wascarried out at 30-70ºC and 101.3 kPa in Ar, using cyclohexane as solvent. Product concentrations were followed by ex-situ gas chromatography using n-dodecane as an external standard. Data were collected every 15-20 min for 150-250 min. Results and Discussion Characterization by  DRIFTS showed that the HG structure on HG/SiO2 was stable up to 110ºC. The activity of HG/SiO2 samples increased with both the %HG and the temperature. At 30ºC, the MO  equilibrium conversion (50%) and the 9-OD and 9-octadene equilibrium yields (50%, molar) were reached only on HG(1.24%)/SiO2 and HG(1.61%)/SiO2 catalysts, being the C balance near 100% . For HG(0.87%)/SiO2 and HG(0.43%)/SiO2 the 9-OD yields were 44 and 15%,  respectively. In all the cases, the HG leaching was negligible during the progress of the reaction showing the effective immobilization of the HG complex on the silica surface. MTO is active for olefin metathesis and may also promote the metathesis of functionalized olefins, but only on thesurface of solid acids. Here, we observed that unsupported MTO and MTO/Al2O3 were inactive for MO metathesis. In contrast, MTO/SiO2-Al2O3 yielded 58% of 9-octadecene at 80% MO conversion, thereby confirming that MTO became active for metathesis reactions when supported on acidic carriers. Formation of active carbene species CH2ReO3 would result fromthe C?H activation of the methyl ligand of CH3ReO3 onto reactive surface Lewis acid sites . However, the presence of 9-OD in the reaction mixture was negligible during the MOmetathesis on MTO/SiO2-Al2O3, probably because 9-OD remained strongly adsorbed on the support. This explains also that MO was converted beyond the equilibrium. Thus, the nature, density and strength of surface acid sites determine not only the efficient activation of the MTO complex but also the 9-OD adsorption strength formed in MO metathesis. Significance HG/SiO2 is an active, selective and stable catalyst for methyl oleate metathesis. Methyltrioxorhenium on acid supports is also active for this reaction but the diester formation is hampered by its strong  adsorption on the support.