Carboxylate-Alumoxanes: Precursors for Heterogeneous Catalysts
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To whom correspondence should be addressed. (http://pcheml .rice.edu/-arb/Barron.html) 659 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
We have previously reported the preparation of carboxylate-alumoxanes, [Al(O)x(OH) (O2 CR)zIn, from the reaction of boehmite, [AI(O)(OH)]n, with carboxylic 3 acids. , ,5 &hese carboxylate-alumoxanes are readily converted to alumina bodies or films upon pyrolysis. Importantly, the physical properties of the resulting alumoxanes may be readily controlled by the organic substituent of the carboxylate group (i.e., R) making them adaptable to a wide range of processing techniques. Reaction of the carboxylate-alumoxane with either a metal acetylacetonate complex, M(acac)n, or a salt, results in transmetallation and the formation of a dopedalumoxane. 6 Upon thermolysis, these doped-alumoxanes result in homogeneously doped-alumina. Our results in the use of doped-alumoxanes as precursors to mixed metal and doped aluminum oxides as alumina based catalysts and catalyst supports led us to investigate the possibility of using carboxlyate-alumoxanes as precursors to heterogeneous catalysts. 6 We proposed that by reacting a catalytically active metal with chemically functionalized carboxlyate-alumoxanes, it should be possible to covalently bond these metals to an inorganic support with great dispersion. The initial studies of this method involve metathesis of higher olefins. Olefin metathesis is an interchange of carbon atoms between a pair of double bonds. These reactions can be grouped into three categories: exchange, ring-opening metathesis polymerization (ROMP), and ring-closing metathesis. 7 We are currently interested in investigating the exchange reaction for higher olefins, i. e., Equation 1. R1CH-CHR 2
RICH
CHR 2
R3CH -CHR 4
R3CH
CHR 4
11+
+
1
(1)
There are many different transition metals that show activity for metathesis reaction, however only three show high activities: molybdenum, tungsten, and rhenium, (see Table 1).8 7 Table 1. Transition metals that are active for metathesis
IV
V
VI
VII
VIIII
X
Nb Ta
Mo W
Re
Ru Os
Ir
Ti
*
Metals having highest activity are in boldface
Traditional methods for the synthesis of MoO 3 supported catalysts for metathesis can be grouped into three categories: (a) Impregnation of the support, 9 (b) treatment of the support with Mo(CO) 6 , 10 and (c) treatment of the support with organomolybdenum compounds [e. g. (n-C 3H 5 )4 Mo]. 1 l Supports are generally metal oxides such as alumina, silica, or titania. After the catalyst is attached to the support, heat treatment and oxidation or reduction is often necessary to bring the catalyst into an active state. 12 Catalysis is initiated and then propagated by the formation of metal carbene complexes.13 It is usually important to pay special attention to how the catalyst systems are prepared due to many factors that affect metathesis activity. These factors include the proportions of components, pre-treatment procedures, order in which the components of the syste
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