(608a) Composite Polymer/Oxide Hollow Fibers As Scalable Continuous Reactors for Heterogeneous Catalysis in Flow Chemistry
Polymer/Oxide Hollow Fibers as Scalable Continuous Reactors for Heterogeneous
Catalysis in Flow Chemistry Eric G.
Moschetta,1 Solymar Negretti,2 Kathryn M. Chepiga,2
Nicholas A. Brunelli,1,2 Ying Labreche,1 Yan Feng,1
Fateme Rezaei,1 Ryan P. Lively,1 William J. Koros,1
Huw M. L. Davies,2 and Christopher W. Jones1*
Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311
Ferst Drive NW, Atlanta, GA 30332 2Department of
Chemistry, Emory University, 1515 Dickey Drive, Atlanta, GA 30322 *firstname.lastname@example.org Flow
chemistry in organic synthesis is emerging as a viable platform for large-scale
production in the fine chemical and pharmaceutical industries. Presently,
there are numerous flow reactors from μL to L in volume available
commercially worldwide. When compared
to batch reactors, these flow reactors are capable of handling reactions at
higher pressures and temperatures, have superior heat and mass transfer
properties, and provide safer facilitation of toxic and explosive reagents.[3,4] An issue
currently facing these flow reactors, especially microreactors, is the
challenge associated with handling solids such as heterogeneous catalysts in
the flow channels.[3?5] Solid particles
may clog the flow channels and induce excessive pressure drop, reducing the
overall productivity of the flow reactor. Another
challenge in flow chemistry is designing reactors that accommodate expensive supported
organometallic catalysts for low temperature liquid-phase reactions, such as
C?H functionalizations. While
homogeneous C?H functionalization catalysts are extremely active and
enantioselective, retaining these levels of activity and selectivity remains
challenging once the catalysts are tethered to a heterogeneous support. To
address these challenges in flow chemistry, we sought inspiration from advances
in gas separation technology. Hollow fiber membranes are most commonly used to
separate the components of a gas stream with the membrane providing selective
flow from bore to shell or vice versa. Recently, composite polymer/oxide hollow
fibers have found large-scale use in gas adsorption processes, for example
involving CO2 capture from flue gas [7,8] In hollow fiber
sorbent systems, the flue gas flows over the outside of the sorbent-containing fibers
while cooling water flows through the bores to mitigate the heat of adsorption,
with no mass transfer from shell to bore or vice versa.[7,8] These composite
fibers are easy and inexpensive to synthesize, and can be tuned to allow flow
(or no flow) through the fiber walls, making them attractive as alternative
platforms for performing heterogeneously catalyzed reactions in organic
synthesis on a large scale. This work discusses the use and versatility of
hybrid polymer/oxide fibers as flow reactors in organic synthesis.
In the initial work, the reactants flow through the bore and contact the
catalyst via radial flow due to a dead-end configuration, which provides short
reactant-catalyst contact times. Three different inorganic catalysts are
incorporated into the walls of the fibers: ZSM-5 (a Brønsted acid catalyst),
aminopropyl-functionalized silica (a basic organocatalyst), and a
silica-supported analogue of Rh2(S-DOSP)4 (an
organometallic C?H functionalization catalyst). These fibers are used to
catalyze three different reactions in flow: an acid-catalyzed acetal deprotection,
a base-catalyzed C?C bond forming reaction, and dirhodium(II)-catalyzed
cyclopropanations and C?H functionalizations. The yields and enantioselectivities
of the flow reactions are compared to similar batch reactions to assess the
viability of the hollow fibers as scalable flow reactors in organic synthesis. References
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