Continuous catalytic oxidation of solid alcohols in supercritical CO2: A parametric and spectroscopic study of the transformation of cinnamyl alcohol over Pd/Al2O3

Abstract

Cinnamyl alcohol was oxidized to cinnamaldehyde in a continuous fixed-bed reactor with molecular oxygen over an alumina-supported palladium catalyst in supercritical carbon dioxide modified with toluene. A strong dependence of the reaction performance on pressure and oxygen concentration in the feed was found. Optimization of the reaction conditions resulted in a higher catalytic activity than in liquid phase. At 120 bar, 80 °C, and double stoichiometric oxygen concentration, a turnover frequency of 400 h-1 at a selectivity of 60% to cinnamaldehyde was achieved. Spectroscopic investigations and the knowledge of the selectivity pattern turned out to be crucial for a deeper understanding of the reaction allowing a rational optimization. Under almost all experimental conditions (even at high oxygen concentration) hydrogenated byproducts, stemming from internal hydrogen transfer reactions, were detected in the effluent. This indicated that alcohol dehydrogenation is the first reaction step, further confirmed by spectroscopic investigations. In situ XANES and EXAFS uncovered that in the whole experimental range investigated the palladium constituent was mainly in a reduced state and that its surface could be oxidized only in the absence of cinnamyl alcohol in the feed. Bulk phase behaviour studies and investigations at the catalyst/fluid interface, performed by visual inspection and combined transmission and ATR-IR spectroscopy, uncovered that the reaction performed best in the biphasic region. Moreover, cinnamaldehyde and carbon dioxide but hardly any toluene and cinnamyl alcohol were detected inside the porous catalyst, evidencing a strongly different product composition inside the porous catalyst compared to the bulk phase.

Time-Resolved and Operando XAS Studies on Heterogeneous Catalysts — From the Gas Phase Towards Reactions in Supercritical Fluids

“…The latter seems to be more reducing under the conditions applied and therefore does not show any deactivation due to surface oxidation of the Pd-particles. These XANES results are also supported by corresponding EXAFS data and fitting [15,16]. FIGURE 5.…”

Gold catalysed selective oxidation of alcohols in supercritical carbon dioxide

“…But the reaction rates are typically lower, particularly since no base can be used [11,13]. An alternative approach is the employment of supercritical carbon dioxide § as an environmentally benign solvent, which was also recently used for selective alcohol oxidations [18][19][20][21][22][23][24]. In addition, since it allows performing the reaction in an inert and non-flammable medium at potentially high reaction rates [25][26][27], it provides several advantages from a process and catalytic point of view.…”

Mechanistic Studies of Alcohol Selective Oxidation

“…Interestingly, no deactivation was observed on increasing the O 2 concentration under these scCO 2 conditions, suggesting palladium over-oxidation is not detrimental to selox. A parallel operando study of cinnamyl alcohol selox under scCO 2 confirmed that reactant-induced reduction was an important process, and that oxygen supply to the catalyst was mass-transport limited [150]. A related XAS investigation of untreated and Bi-promoted Pd/Al 2 O 3 for phenyl ethanol selox revealed that both the promoter and platinum were in a reduced state when 100 % selectivity to acetophenone was observed [151].…”