Green Chemistry Dynamic Article Links Cite this: DOI: 10.1039/c1gc15495h www.rsc.org/greenchem PAPER Advantageous heterogeneously catalysed hydrogenation of carvone with supercritical carbon dioxide Catarina I. Melo, a Rafal Bogel-Lukasik, a,b Marco Gomes da Silva a and Ewa Bogel-Lukasik* a Received 3rd May 2011, Accepted 26th July 2011 DOI: 10.1039/c1gc15495h The hydrogenation of carvone was investigated for the first time in high-density carbon dioxide. The hydrogenation over 0.5 wt% Pd, or Rh, or Ru supported on alumina was found to be generally faster in a single supercritical (sc) phase (fluid reagents) than in a biphasic system (liquid + fluid reactants). The reaction with Pd produced fully hydrogenated products (isomers of carvomenthone) and carvacrol. The Rh catalyst was more selective and favoured carvomenthone isomers with higher selectivity and carvotanacetone as a secondary product. Additionally, the rhodium catalysed reaction exhibited high > 84% selectivity of carvotanacetone with the conversion of > 25% after only 2 min of reaction. The less active Ru catalyst gave significantly lower conversion and the product variety was greater as carvomenthone isomers, carvotanacetone and carvacrol were formed. The conversion and selectivity to carvomenthone within 2 h of the reaction starting followed the order: Pd > Rh > Ru and Rh > Pd > Ru, respectively. High conversion, and diverse and high selectivity accompanied by significant reduction in reaction time depending on the catalyst were achieved in supercritical CO 2 compared with hydrogenation occurring in conventional organic solvents. Introduction Selective hydrogenation of organic molecules plays an important role in the synthesis of fine chemicals on a multi-ton scale via heterogeneous catalysis. 1 Supercritical fluids are considered to be green solvents employed in a diverse range of chemical reactions, including both homogeneous and heterogeneous catalytic processes. 2,3 Among scF, supercritical CO 2 (scCO 2 ) has received considerable attention as a useful replacement of organic solvents. The solvent power of scCO 2 can be easily tuned by changes in both temperature and pressure. scCO 2 can reduce the undesirable by-products yielding a higher end product quality. 4,5 Terpenes and terpenoids are an important class of naturally occurring compounds widely employed in organic synthesis. 6 Our previous work published in Green Chem. discussed the successful hydrogenation of terpenes (C 10 H 16 : limonene 7 or myrcene 8 ) in high pressure assisted CO 2 in biphasic or monopha- sic conditions 7,8 due to a favourable solubility relationship between terpenes and carbon dioxide allowing an easy switch from two phases to a one phase system. 9 In the hydrogenation a REQUIMTE, Departamento de Qu´ ımica, Faculdade de Ciˆ encias e Tecnologia, Universidade Nova de Lisboa, 2829-516, Caparica, Portugal. E-mail: ewa@dq.fct.unl.pt; Fax: (+351) 212948550 b Laborat´ orio Nacional de Energia e Geologia, I.P., Unit of Bioenergy, Estrada do Pac ¸o do Lumiar 22, 1649-038, Lisboa, Portugal of terpenes in scCO 2 , the effects of hydrogen pressure 10 and flow rate 11 as well as tuning the selectivity towards an intermediate by use of solid catalysts with ionic liquid layer (SCIL) have been studied. 12 Caraway, a major source of carvone (C 10 H 14 O, 2-methyl-5- (1-methylethenyl)-2-cyclohexenone), is one of the oldest spices cultivated in Europe and is used as an alternative medicine in colic treatment, as a laxative, or a breath freshener. 13 Carvone, a terpenoid, found application as a fragrance and flavour, potato sprouting inhibitor, antimicrobial agent, building block and biochemical environmental indicator. 13 The hydrogenation of carvone has usually been carried out with heterogeneous supported mono- and bimetallic catalysts. It can be noticed that reaction times are very long 14 (generally from 10 h up to even more than 45 h). The main problem met is the conversion of carvone 15 which can vary from 10%, after 40 h for Pt modified by Sn, up to 100% at 20 h reaction time for Pt/C catalyst. 16 The access of reagents to the catalyst bed seems to play a crucial rule in the liquid phase hydrogenation and can be recognized as a limiting factor for conversion. An increase in the reaction time or temperature (40 ◦ C and 90 ◦ C) did not solve the problem of low conversion (i.e. after 45 h conversion was 30% with PtSn/C- HP 17 while the same catalyst modified by Ge after 10 h led to minimal conversion 18 ). For these reasons various modifications of the reaction parameters have been tested, i.e. at 100 ◦ C, high acidic catalysts on zeolites 19 or Pd–Cu. 14 Even reaction at 12 MPa compared to atmospheric pressure 14–19 at 10 ◦ C–50 ◦ C This journal is © The Royal Society of Chemistry 2011 Green Chem. Downloaded by Universidade Nova de Lisboa on 26 August 2011 Published on 26 August 2011 on http://pubs.rsc.org | doi:10.1039/C1GC15495H View Online