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Tuning Catalytic Selectivity: Zeolite- and Magnesium Oxide-Supported Molecular Rhodium Catalysts for Hydrogenation of 1,3-Butadiene
Regulation of the catalytic selectivity of rhodium for
the industrially
important hydrogenation of 1,3-butadiene to <i>n</i>-butenes
has been achieved by controlling the structure of essentially molecular
rhodium species bound to supports. The selectivity for <i>n-</i>butene formation increases as the nuclearity of the metal species
decreases from several Rh atoms to one, but these catalysts form the
undesired product <i>n</i>-butane, even at low diene conversions.
The <i>n</i>-butene selectivity increases when the rhodium
is selectively poisoned with CO ligands, and it is highest when the
support is the electron-donor MgO and the rhodium is in the form of
clusters that are well approximated as dimers. The electron-donor
support is crucial for stabilization of the rhodium carbonyl dimer
sites, as it limits the oxidative fragmentation of the clustersî—¸which
is facilitated when the support is HY zeolite (a poor electron donor)î—¸that
leads to decreased catalytic activity and selectivity. The selective
MgO-supported rhodium carbonyl dimers suppress the catalytic routes
that yield butane, limiting the activity for H<sub>2</sub> dissociation
to avoid butane formation via primary reactions and also favoring
the bonding of 1,3-butadiene over butenes to limit secondary reactions
giving butane. With this catalyst, selectivities to <i>n-</i>butene of >99% were achieved at 1,3-butadiene conversions as high
as 97%. This selectivity matches that of any reported for this reaction,
and the catalyst works under milder conditions (313 K and 1 bar) than
others that are selective for this reaction