Selectivity promotion by halogens in the Ag-catalysed epoxidation of
ethene: the final chapter in a long story
Richard M. Lambert1, Rachael L. Cropley1, Alifiya Husain1 and Mintcho S. Tikhov1
1
Department of Chemistry, University of Cambridge,
Lensfield Rd, Cambridge CB2 1EW, UK.
Introduction
The mechanism of silver-catalysed ethene epoxidation has been intensively
investigated due to the apparently unique chemistry involved and because of the
major technological importance of the process. The mechanism of ethene
epoxidation remained a controversial issue for many years, virtually all the discussion
pivoting on the identity of the oxygen species responsible for epoxidation: O2(a) or
Oa.
Chlorine atoms are very effective promoters. In the absence of chlorine, and at
relatively low ethene conversion, Ag/alumina catalysts typically deliver selectivities
towards epoxide formation of no better than ~ 40%. Adsorbed chlorine, supplied
from the gas phase by continuous addition of ppm levels of chloro-carbons (e.g.
C2H4Cl2) raises the selectivity to ~ 70%.
One school of thought is that they operate by simply blocking the adsorption of single
oxygen atoms (as opposed to diatomic oxygen molecules) on the silver surface1. On
the other hand, our model2 calls for the chlorine promoter to act electronically,
withdrawing valence charge from co-adsorbed Oa. Lower valence charge density on
Oa makes it a better electrophile, leading to electrophilic attack on adsorbed ethene
and hence epoxidation.
So what about F, Br and I? Experiments with these other halogens should provide an
acid test of reaction mechanism and should lay the matter to rest, once and for all.
According to the geometric or steric view, F, Br, I should act in a manner similar to
Cl. At comparable halogen coverages, all three should increase epoxidation
selectivity to about the same extent as chlorine. On the other hand, according to our
electronic hypothesis, given the well-known electron affinities of the halogens, one
might expect a clear selectivity maximum at chlorine (EA = 3.40, 3.61, 3.36, 3.06 eV
for F, Cl, Br, I, respectively). Surprisingly, nobody seems to have tested this issue
directly before. Here we report such a test.
Results and Discussion
For all four halogens, successive injections of halocarbon were applied to the reactor
until all the initial catalyst activity was quenched. Adsorbed fluorine, chlorine,
bromine and iodine all act to enhance selectivity towards epoxide formation in the
Ag-catalysed selective oxidation of ethene. However, chlorine is much more
effective than the other three halogens.
Figure 1a shows the crucial like-with-like comparison – selectivity enhancement at
the 50% activity point where the surface concentration of halogen should be
approximately the same in the four cases. It is very clear that chlorine greatly
outperforms F, Br, and I. Figure 1b shows a plot of the electron affinities of the
halogens. This behaviour correlates with the electron affinities of the halogens,
which show a maximum at chlorine. This constitutes strong evidence that halogen
promotion of ethene epoxidation is overwhelmingly due to an electronic effect, and
that steric or geometric factors are of minor or insignificant importance.
Figure 1 a) EO selectivity enhancement at 50% reactant conversion for each halogen.
b) electron affinities of the halogens
In order to provide additional insight into the catalytic behaviour, XPS measurements
were carried out on ex-reactor freshly-activated and halogen-treated samples. Due to
the large differences in halogen photoelectron ionization cross sections, fluorine
uptake was readily detectable, iodine uptake was just detectable, and the other two
were undetectable. The results are nevertheless revealing. The XPS data strongly
suggests that penetration of Ag by F occurs and, in support of this view, it is
interesting to note that the iodine coverages deduced from XPS are 0.5 ML and 1 ML
at the 50% activity point and the end point, respectively. This is consistent with
iodine, unlike fluorine, being confined to the Ag surface.
Conclusions
Oxygen adatoms on silver surfaces epoxidise alkenes and their efficiency in this
process can be improved by electron withdrawal due to adjacent halogen adatoms.
Chlorine is much the best promoter because it has the highest electron affinity of all
the halogens.
References
1. Sachtler, W. M. H. Catal. Rev., 4, (1970), 27
2. Grant, R. B.; Lambert, R. M. J. Catal., 92, (1985), 364.