Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

Leaching
2+
[Cu ] solv
Al O
2 3
Al O
2 3
CHAPTER 2
Al O
2 3
Al O
2 3
Isolated Cu 2+
Figure 2‐2: Isolated Cu sites and CuO present simultaneously in a catalyst where CuO domains are often
prone to leaching; schematic model of Cu‐doped montmorillonite consisting of regular layers and Al2O3
pillars [33].
Sol‐gel, hydrothermal and coprecipitation synthesis methods allow simultaneous synthesis of
support and incorporation of the catalytically active metal and also afford Cu species with high
dispersion when Cu is applied in low concentrations. All three methods are based on precipitating
the catalyst from a homogeneous solution. Sol‐gel methods are based on the hydrolysis and
subsequent condensation of a precursor (e.g. tetraethyl orthosilicate) often under controlled pH. The
solvent can be removed by simple air‐drying but also more complex strategies like supercritical
drying are used. High surface areas can be achieved by this approach. Addition of other metal ions
allows doping; pore structures can be induced by bulky organic additives. Sol‐gel synthesis is often
used for synthesizing Cu‐doped silica materials. Fast hydrolysis of the copper precursor compared to
the SiO2 precursor may be problematic. Cu additives have a negative influence on MCM‐41 materials
decreasing the surface area [37, 38]. This can partly be compensated by addition of Al ions [31, 38]
forming acidic sites with ion exchange properties binding to Cu(II) [39] which also stabilizes Cu
species against leaching (Figure 2‐3). Sol‐gel synthesis is of course not limited to doped silicas [40].
Hydrothermal synthesis, i.e. reaction of precursors to the desired product in an autoclave at elevated
temperatures and pressures, is also useful for preparing homodisperse Cu species in a
heterogeneous matrix as was done for the synthesis of copper‐substituted aluminophosphate
molecular sieves [38, 41]. (Cu‐based) metal‐organic frameworks are also frequently synthesized via a
solvothermal approach [42]. Cu incorporated in alumina can be synthesized by coprecipitating Cu
and Al [43]. Coprecipitation requires no special (e.g. high‐pressure) equipment and starts from a
solution of two (or more) metal oxides. Precipitation is (most often) induced by a change in pH.
12
Al2O3 Al O
Al O
Al O
2 3
Al O
2 3
2 3
Al O
2 3
CuO domains
Al O
2 3
2 3