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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

22 Rare Earth Elements and Materials

as using less expensive starting feedstocks. This leads to
the application of rare earth catalysts for the refinement of
long-chained hydrocarbons to high-end monomers such
as styrene. Cerium (as cerium carbonate) has traditionally
been used in styrene monomer catalyst recipes as a cata-
lyst promoter. Modern styrene monomer catalyst produc-
ers are increasing the levels of cerium in recipes to make
“higher”-activity and “higher”-yield catalysts as well as
using cerium to displace more toxic components such as
chromium.
FIGURE 17 Double activation and cleavage of a diphosphate
Another application of rare earth compounds in catal- substrate by a rare-earth-hydroxo species.
ysis concerns with rare earth organometallics in polymer
synthesis. Modern plastics and synthetic rubber manufac-
ety of new rare earth-containing materials and their appli-
turers are discovering new uses of these compounds to
cations, which extend beyond the traditional technologi-
improve the polymerization process and the selectivity of
cal applications. These new applications are based on the
polymers.
unique 4 f properties of the rare earth elements, and have
become increasingly important. The demand for higher ef-
C. Rare Earths in Catalytic Cleavage ficiency and more environmentally friendly materials and
of DNA and RNA processes has led to a new era for rare earth materials. The
new technologies, along with renovated traditional tech-
Nonenzymatic phosphate ester cleavage is effectively
3+ niques, will continue to drive the rare earth markets well
facilitated by the RE ions. The trivalent charge makes
into the 21st century.
these cations strong Lewis acids for withdrawing electron
density away from a phosphate diester substrate. They
are also known for forming hydroxo species in aqueous SEE ALSO THE FOLLOWING ARTICLES
solution by deprotonating coordinated water molecules.
The requirement of rare earth ions for high coordination ACTINIDE ELEMENTS • CHEMICAL COMPOSITION AND
number allows the simultaneous coordination of the ELEMENT DISTRIBUTION IN THE EARTH’S CRUST •
substrate and hydroxide nucleophiles. The disadvantages LUMINESCENCE • MAGNETIC MATERIALS • PERIODIC
3+
of RE ions are their toxicity and the generally low TABLE (CHEMISTRY) • SUPERCONDUCTIVITY
stability of the complexes with regular ligands. Free RE 3+
3+
ions readily form RE hydroxide precipitates and both
forms are active in phosphate ester cleavage (Fig. 17), BIBLIOGRAPHY
which makes kinetic control complicated. However, some
stable RE 3+ complexes have been successfully applied as Cotton, S. A. (1991). “Lanthanides and Actinides,” MacMillan Educa-
tion, London.
RNA cleavers in antisense oligonucleotides. In addition,
Gschneider, K. A., Jr., and Eyring L. (eds.) (1978). “Handbook on the
other dinuclear lanthanide complexes with macrocyclic Chemistry and Physics of Rare Earths,” North Holland, Amsterdam,
ligands have been demonstrated to accelerate the cleavage Vols. 1–27 (published in 1978 to present).
of supercoiled DNA. Such efforts may eventually lead Hedrick, J. B. (2000). “U.S. Geological Survey, Mineral Commodity
to the discovery of diagnostic and therapeutic agents for Summaries,” pp. 135, 189.
Imamoto,T.(1994).“LanthanideinOrganicSynthesis,”AcademicPress,
genetic diseases.
Harcourt Brace & Company, Publishers, New York.
Over the last few decades, the persistent efforts of the Kaltsoyannis, K., and Scott, P. (1999). “The f Elements,” Oxford Uni-
international community of scientists have led to a vari- versity Press, New York.