Hydrogenation of Phenol in Supercritical Carbon Dioxide Catalyzed by Palladium Supported on Al‐MCM‐41: A Facile Route for One‐Pot Cyclohexanone Formation

Abstract

The hydrogenation of phenol has been carried out in supercritical carbon dioxide (scCO 2 ) under very mild reaction conditions at the temperature of 50 8C over palladium supported Al-MCM-41 (metal loading~1%). This palladium catalyst is shown to be highly active and promotes the selective formation of cyclohexanone (~98%), an industrially important compound, in a "one-pot" way. The effects of different variables like carbon dioxide and hydrogen pressure, reaction time and also silica/alumina ratio of the MCM-41 support along with palladium dispersion are presented and discussed. The pressure effect of carbon dioxide is significantly prominent in terms of conversion and cyclohexanone selectivity. Moreover, the silica/alumina ratio was also found to be an important parameter to enhance the effectiveness of the catalyst as it exhibits a remarkable increase in phenol conversion from 20.6% to 98.4% as the support changes from only silica MCM-41 to Al-MCM-41. A plausible mechanism for the hydrogenation of phenol to cyclohexanone over the palladium catalyst has been proposed. The proposition is validated by transition state calculations using density functional theory (DFT), which reveal that cyclohexanone is a favorable product and stabilized by < 19 kcal mol À1 over cyclohexanol in scCO 2 medium. Under similar reaction conditions, phenol hydrogenation was also carried out with rhodium, supported on Al-MCM-41. In contrast to the palladium catalyst, a mixture of cyclohexanone (57.8%) and cyclohexanol (42.2%) was formed. Detailed characterization by X-ray diffraction and transmission electron microscopy confirmed the presence of metal nanoparticles (palladium and rhodium) between 10-20 nm. Both the catalysts exhibit strikingly different product distributions in solventless conditions compared to scCO 2 . This method can also be successfully applied to the other hydroxylated aromatic compounds.

Hydrogenation of phenol with supported Rh catalysts in the presence of compressed CO2: Its effects on reaction rate, product selectivity and catalyst life

“…It is reported by Chatterjee et al that phenol is hydrogenated to cyclohexanone with a high selectivity > 97% with Pd/Al-MCM-41 catalyst at H 2 and CO 2 pressures of 4 MPa and 12 MPa, respectively [24]. The selectivity to cyclohexanone was found to depend on CO 2 pressure and it was < 60% at 7 MPa or below.…”

“…The Lewis acidity of the MCM-41 supports of Chatterjee's catalysts [24] may contribute to their high selectivity to cyclohexanone observed.…”

“…Our group has also reported that a Rh catalyst supported on carbon nanofiber is active for the phenol hydrogenation in scCO 2 at a low temperature of 323 K and at a lower hydrogen pressure of 4 MPa [23]. Recently, Chatterjee et al have screened several supported catalysts using different noble metals and supports and found that Pd on Al-MCM-41 is effective to the hydrogenation of phenol to cyclohexanone in dense phase CO 2 (12 MPa) [24]. Very recently Liu et al indicated an interesting synergistic effect of supported Pd particles and solid Lewis acid, with which the hydrogenation of phenol to cyclohexanone was achieved in almost 100% selectivity under mild conditions [25].…”

High Efficient Hydrogenation of Lignin-Derived Monophenols to Cyclohexanols over Pd/γ-Al2O3 under Mild Conditions

“…On the other hand, a probable mechanism for the hydrogenation of phenols on Pd-based catalysts indicated that phenols were adsorbed and activated on the support [37], while H 2 was activated on the Pd and induced the hydrogenation of adsorbed phenols. If the adsorption of phenols occurred through the hydroxyl, the methoxy group in ortho-position may have increased steric hindrance of the reaction such that guaiacol had more trouble being hydrogenated than the other phenols did [19,37]. following.…”

“…Additionally, high temperatures require more energy consumption and thus an increase in cost. Recently, much attention has been focused on the liquid phase hydrogenation of phenols [8,[17][18][19][20][21][22][23][24][25]. For example, Huang et al [26] prepared a high-performance PdRu bimetallic catalyst on mesoporous silica nanoparticles for the hydrogenation of phenol in CH 2 Cl 2 solvent.…”