2.4.2
Heterogeneous Catalysts
One problem associated with the above-described peroxotungstate-catalyzed epox-
idation system is the separation of the catalyst after completion of the reaction. To
overcome this obstacle, efforts to prepare heterogeneous tungstate catalysts have
been conducted. De Vos and coworkers employed tungsten catalysts derived from
sodium tungstate and layered double hydroxides (LDH – coprecipitated MgCl
2
,
AlCl
3
, and NaOH) for the epoxidation of simple alkenes and allyl alcohols with
aqueous hydrogen peroxide [32]. They found that, depending on the nature of the
catalyst (either hydrophilic or hydrophobic catalysts were used), different reactivities
and selectivities were obtained for nonpolar and polar alkenes respectively. The
hydrophilic LDH-WO
4
catalyst was particularly effective for the epoxidation of allyl
and homo-allyl alcohols, whereas the hydrophobic catalyst (containing p-toluensul-
fonate) showed better reactivity with nonfunctionalized substrates.
Gelbard and coworkers have reported on the immobilization of tungsten catalysts
using polymer-supported phosphine oxide, phosphonamide, phosphoramide, and
phosphotriamide ligands [33]. Employing these heterogeneous catalysts together
with hydrogen peroxide for the epoxidation of cyclohexene resulted in moderate to
good conversion of the substrate, although in most cases low epoxide selectivity was
observed. A significantly more selective heterogeneous catalyst was obtained by
Jacobs and coworkers upon treatment of the macroreticular ion-exchange resin
Amberlite IRA-900 with an ammonium salt of the Venturello anion {PO
4
[WO-
(O
2
)
2
]
4
}
3
[26, 34]. The catalyst formed was used for the epoxidation of a number of
terpenes, and high yields and good selectivity of the corresponding epoxides were
achieved.
In a different strategy, siliceous mesoporous MCM-41-based catalysts were pre-
pared [34]. Quaternary ammonium salts and alkyl phosphoramides, respectively,
were grafted onto MCM-41, and the material obtained was treated with tungstic acid
for the preparation of heterogeneous tungstate catalysts. The catalysts were employed
in the epoxidation of simple cyclic alkenes with aqueous hydrogen peroxide (35%) as
terminal oxidant, but conversion and selectivity for epoxide formation was rather low.
In the case of cyclohexene, the selectivity could be improved by the addition of
pyridine. The low tungsten leaching (<2%) using these catalysts is certainly
advantageous.
A particularly interesting system for the epoxidation of propylene to propylene
oxide, working under pseudo-heterogeneous conditions, was reported by Zuwei and
coworkers [35]. The catalyst, which was based on the Venturello anion combined with
long-chain alkylpyridinium cations, showed unique solubility properties. In the
presence of hydrogen peroxide the catalyst was fully soluble in the solvent (a 4 : 3
mixture of toluene and tributylphosphate), but when no more oxidant was left, the
tungsten catalyst precipitated and could simply be removed from the reaction
mixture (Scheme 2.5). Furthermore, this epoxidation system was combined with
the 2-ethylanthraquinone (EAQ)/2-ethylanthrahydroquinone (EAHQ) process for
hydrogen peroxide formation (Scheme 2.6), and good conversion and selectivity were
46
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2 Transition Metal-Catalyzed Epoxidation of Alkenes