Worldwide, more than 85% of all chemical products are
manufactured
with the help of catalysts. Virtually all transition metals of the
periodic table are active as catalysts or catalyst promoters. Catalysts
are divided into homogeneous catalysts, which are soluble in the reaction
medium, and heterogeneous catalysts, which remain in the solid state.
A heterogeneous metal catalyst typically consists of the active metal
component, promoters, and a support material. In some cases, the metallic
state itself forms the active ingredient. However, this situation
is largely restricted to precious metal catalysts and to some base
metals used under reducing conditions. In most cases and especially
in homogeneous catalysis, it is a metal compound or a complex that
forms the active catalyst. Catalysis can be rather puzzling as a given
metal can catalyze a variety of different chemical transformations,
while the same substrate, passed over different catalysts, can give
different products. It is therefore helpful to be familiar with the
fundamentals of catalytic science before being exposed to the uncountable
applications, which form the backbone of industrial chemistry. Examples
of practical importance are used in this paper to highlight important
principles of catalysis.
The catalytic hydrogenation of α,β-unsaturated
carbonyl
compounds can lead to several different products, of which the unsaturated
alcohol (en-ol) is most difficult to obtain. In this regard, cobalt
is known to have a positive influence on platinum catalysts. Little
is known about such an effect on more cost-effective nickel catalysts.
Nickel and cobalt (5 mass % each) were supported on graphite (GRA),
multiwalled carbon nanotubes (MWCNT), and activated carbon (AC). The
catalysts were characterized by N2-physisorption, X-ray
diffraction (XRD), H2-chemisorption, and high-resolution
transmission electron microscopy. XRD indicated the formation of a
Ni–Co alloy. For cinnamaldehyde as the substrate, the en-ol
selectivity and the turnover frequency (TOF) of the catalysts increased
in the order GRA < AC < MWCNT. Ni–Co/MWCNT showed the
highest selectivity over the whole conversion range, and at a conversion
of 63% (TOF: 14.4 h–1) the product contained 62%
en-ol and 38% saturated aldehyde. A positive influence of both cobalt
and the support (MWCNT) on the selectivity of nickel catalysts is
clearly indicated.
The catalyst prepared by nickel(ii) exchange of silica-alumina shows high catalytic activity and long-term stability for ethylene oligomerization at 35 bar, mass hourly space velocity = 2, and 120 "C.
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