Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
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CHAPTER 2<br />
effective catalyst for styrene epoxidation with TBHP (even better than Au catalysts) but leaching<br />
studies were unfortunately not reported. Since styrene oligomerization was not taken into account,<br />
however, a final conclusion cannot be drawn. Cu2(OH)PO4 as an example for Cu phosphates also<br />
oxidized styrene under aerobic conditions [60] though the selectivity was low. Cu phosphates are in<br />
general good oxidation catalysts (useful e.g. also in benzene hydroxylation [59]) which might be due<br />
to isolation of the Cu ions by interjacent phosphate groups. Cu catalysts are also suitable for<br />
hydroquinone oxidation with oxygen; a good example being Cu/Al(OH) giving full conversion under<br />
mild conditions already at room temperature [216]. Cu3(BTC)2 also exhibited high TOFs [214]. Though<br />
for hydroquinone oxidation a gold catalyst gave the highest TOFs [219], the necessity of chloroform<br />
makes this approach less attractive favoring Cu catalysts especially when taking – once more – the<br />
difference in price into account. As outlined before, alcohol oxidation is certainly a reaction where<br />
gold is highly suitable – under aerobic conditions. Cu affects alcohol oxidation under anaerobic<br />
conditions and among the oxidation reaction this also the only reaction where metallic copper is the<br />
active phase. Therefore, alcohol oxidation is in principal the reaction where the three metals are<br />
most comparable. This reaction requires high temperatures and the reaction rates are usually<br />
significantly lower than in aerobic alcohol oxidation. Cu (and also Ag, vide infra) should thus not be<br />
seen as competitors but rather as a complementary tool to aerobic alcohol oxidation catalyzed by Au<br />
(or Pt‐group metals): oxygen‐free conditions are safer to handle, aldehydes which are highly prone to<br />
overoxidation can be synthesized with high selectivities (e.g. octanal [51]) and most importantely<br />
different chemoselectivities can be obtained with Cu catalysts [52, 100]. Also Pd catalysts catalyze<br />
the anaerobic oxidation of alcohols but both rapid poisoning by CO [114] and the hydrogenation<br />
activity of Pd (giving e.g. toluene from benzyl alcohol) were observed as a problem. The hydrogen<br />
remaining in the system also offers opportunities – thus, Cu catalysts can also catalyze isomerization<br />
reactions under anaerobic, acceptor‐free conditions [101]. On a mechanistic level (Scheme 2‐18),<br />
alcohol oxidation on e.g. palladium catalysts [114] is assumed to occur via a dehydrogenation<br />
mechanism [7] where the O‐H bond is broken upon adsorption followed by β‐C‐H bond scission<br />
affording the carbonyl compound. Oxygen (or another hydrogen acceptor) regenerates the metallic<br />
surface either by removal of hydrogen or oxidative cleansing of the metal surface from adsorpt<br />
catalyst poisons like CO. Pre‐adsorpt oxygen may facilitate breaking of O‐H bonds [242, 243] which<br />
can also be supported by bases as often necessary for the Au catalyzed alcohol oxidation [7]. This<br />
may be the reason why anaerobic alcohol oxidation has not been described over gold catalysts so far.<br />
Both for Au [244] and for Ag [67] a positively polarized transition state was proposed. Since alcohol<br />
oxidation proceeds over Cu (and Ag) in the absence of an oxidant, this also makes the<br />
dehydrogenation mechanism likely. From high‐vacuum experiments it is known that alcohol<br />
60