Catalytic transformation of biomass-derived carvone over supported metal catalysts and nitrides
Benavente Donayre, Pedro
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The main focus of this thesis is the investigation of the catalytic transformation of biomass-derived carvone to valuable fine chemical products over supported metal catalysts and nitrides. In the continuous gas phase carvone hydrogenation (P = 1 atm, 423 K ≤ T ≤ 573 K) over Al2O3 supported nanoparticle Au was found to be active towards dihydrocarvone (endocyclic -C=C-), whereas Pt/Al2O3 was selective towards carvotanacetone (exocyclic -C=C-). Temperature had an impact on carvone adsorption at metal/support interface and products distribution. Carvacrol was found to form through carvotanacetone or directly from carvone. Exclusive carvacrol formation on Ag/Al2O3 was linked to the catalyst lowest recorded H2 uptake. The selective conversion of carvone in H2 was studied over (Al2O3, C and CeO2) supported Pd (mean size 2.8-3.0 nm), taking bulk Pd as benchmark. 100% carvacrol yield was achieved over Pd/Al2O3, Pd/C and bulk Pd at an inlet H2/Carvone = 1/6, with appreciably higher rates for the supported catalysts. Carveol formation over Pd/CeO2 was attributed to -C=O activation at surface oxygen vacancies (confirmed by O2 titration) generated during TPR. Carvotanacetone and carvomenthone formation were observed at H2/Carvone > 1/6. The incorporation of water (H2O:Carvone ≤ 1:3) with hydrogen (H2:Carvone = 1:6-2:1) increased carvacrol production rate to deliver 100% yield. A higher H2O content lowered rate due to competition with carvone for surface active sites. Our results demonstrated the benefits of carvacrol production in continuous flow relative to conventional batch processes allowing full selective transformation under mild reaction conditions. Liquid phase hydrogenation of carvone over model Pd/Al2O3 catalyst was also investigated. Dependence of reaction kinetic on range of temperatures, pressures and solvent(less) was studied. Carveol formation was observed by lowering polarity of solvents. On the contrary, promotion in the hydrogenation rates of -C=C- double bonds associated with increase in polarity of solvents. Increasing temperature favoured formation of carvacrol, whereas raising pressure inhibits its production according to the proposed mechanism of hydrogen migration/keto-enol tautomerization. The ongoing study with nitrides (B− and y−Mo2N) prepared by temperature programmed treatment of MoO3 in flow of H2+N2 demonstrated full selectivity towards carvacrol in hydrogen free condition. Variation in surface Mo/N molar ratio of prepared samples did not affect production rate which is controlled by surface area of Mo2N.