Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

5.1 Introduction
CHAPTER 5
The selective oxidation of alcohols to aldehydes or ketones in the liquid phase has been intensively
investigated in the recent years [1, 2]. Toxic and expensive oxidizing agents were replaced by oxygen
being environmentally benign [3]. Besides the coinage metals described in Chapter 2, other noble
metal based heterogeneous catalysts have been used successfully, especially those based on
ruthenium, platinum and palladium. A problem associated with the aerobic alcohol oxidation is the
low oxygen solubility and diffusion coefficient in organic solvents frequently used for these reactions.
Thus, the use of gaseous oxygen introduces an additional phase boundary to the liquid/solid reaction
system inducing further mass transport limitations. These limitations could be overcome by using
pressurized CO2 as an alternative solvent having a high solubility for oxygen where Pd as a
catalytically active metal has received high attention [4‐10]. The performance of other frequently
used metals like gold [11, 12], platinum and ruthenium [10] did not profit from the use of pressurized
CO2 giving poor conversions. This might be connected to overoxidation of the metal due to the high
oxygen availability or blocking of surface sites [10]. In general, dense CO2 features the advantage of
being environmentally benign, safer in combination with organics and oxygen and being chemically
stable with respect to oxidation [13‐15]. The balance between liquid and gas‐like properties results in
a fast mass and heat transfer and a high solubility for low‐functionalized organic molecules. A unique
feature is the tunability of the solvent properties of CO2 by adjusting the pressure and temperature.
Exploiting the beneficial properties of CO2, higher reaction rates were observed in alcohol oxidation
than in standard solvents [6, 15‐17]. Work especially from the Baiker group and co‐workers
demonstrated the potential of tuning of the solvent properties using the same catalyst (0.5%
Pd/Al2O3), temperature and continuous experimental setup: thus, in one catalytic study, benzyl
alcohol was oxidized with a surface TOF of 430 h ‐1 [16]. By adjusting the pressure in a later study,
TOFs close to 2000 h ‐1 were achieved [6, 7]. In order to optimize catalytic processes involving CO2 as a
solvent, knowledge about the phase behavior is important [15, 18]. At conditions relevant to
catalysis, two different scenarios can occur: (1) all the compounds (except the solid catalyst) are in a
single phase at high pressures and (2) formation of two phases, one being rich in CO2 and another
consisting mainly of the organic substrate. The reaction rate may differ greatly when working under
single phase or two phase conditions [7, 8]. Determining optimal reaction conditions in CO2 is
experimentally elaborate and requires special equipment such as a high pressure view cell.
Therefore, catalytic reactions in supercritical CO2 reported in the literature are often only optimized
to a small extend for a very limited set of reaction parameters. This may result in missing close‐to‐
optimal reaction conditions. In contrast to this trial‐and‐error approach, modeling the phase
behavior offers a great chance of conducting experiments in a more controlled way.
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