Thrust Area D: Chemistry and
Catalysis
Thrust Area D is off to a strong
start. Numerous one-on-one discussions
among the various participants are ongoing. Video-teleconferenced group
meetings have started to trigger cross-fertilization of ideas among the various
researchers, especially among students and post-docs from the various research
groups within Thrust Area D, but more interaction with other Thrust Areas is
needed. We have initiated mechanisms to
foster this type of inter-Area dialog.
Benign Synthesis of
Chemicals Using Novel Heterogeneous
Catalysts in SCF CO2 (23) Adewuyi’s group has completed a literature survey of
zeolites for heterogeneous organic catalysis in conventional solvents and
supercritical media and has shared this data base within the CERSP. Work on the design, acquisition and set-up
of the experimental systems have been initiated. In collaboration with DeSimone’s group, equipment have been
identified which will be used for preliminary batch studies at UNC–CH. We are working with Engelhard Corporation
for supply of commercial zeolite catalysts and possible supply of special
zeolite catalysts. We are developing
collaborative research plans with P. Kropp (UNC-CH). We have ordered a GC/MS system with accessories to analyze
products of reactions and elucidate kinetics and reaction mechanisms.
Biocatalytic
Processes in CO2 Using Hyperthermophylic Enzymes (24) Kelly has focused on biocatalysis in CO2
using enzymes from hyperthermophilic and mesophilic sources, focusing on
nucleotide/nucleoside chemistry in CO2. We have catalyzed the regioselective esterification of thymidine
to the 3’ or 5’ thymidine acetate using an esterase from the hyperthermophilic
archaeon S solfataricus. Other
reactions of potential pharmaceutical intermediates are being considered. The
effect of co-solvents on esterification reactions in water-organic mixtures
were studied to help us formulate co-solvent systems for CO2-based
biotransformations; e.g., hyperthermophilic phospholipase may be used in the
recovery of Naproxen, from racemic mixtures by selective, hydrolytic
modification of one stereoisomer. The gene corresponding to this lipase was
identified in the Pyrococcus furiosus
genome and is being cloned and expressed in E.
coli. Preliminary results look promising. In related work, a mesophilic
lipase is optimally active in the near critical regime. The mechanism for this
phenomenon is being considered. This result will be used to determine optimal
conditions for hyperthermophilic enzymes in CO2.
Surface-Mediated
Reactivity (25) Kropp has found that selective oxidation of amines (I or II) to
hydroxylamines, with little or no over oxidation, can be effected by OXONE over
either silica gel or alumina in the presence of either liquid CO2 or
scCO2. This occurs despite
the fact that amines react rapidly with CO2 to form a carbamic acid
salt. We have found that carbamic acid formation is reversible, even in the
presence of CO2 at 4000 psi. Substitution of the carbamic acid salt
for amine on treatment with OXONE/silica gel in 4000 psi of CO2
affords substantial quantities of the hydroxylamine. When the oxidation is conducted in the presence of CO2,
removal of the hydroxylamine product from the surface of the adsorbent on
treatment with methanol is greatly facilitated relative to oxidation conducted
in the absence of CO2. This
has now been found to be due to residual CO2 adsorbed on the
surface, which reacts with methanol to form the carbonic ester and facilitates
desorption of the hydroxylamine as the amine salt. Removal of methanol was caused decarboxylation and regeneration
of the free hydroxylamine.
Electrochemical
Methods for Basic Studies in CO2 Media (26) Murray’s group set out to adapt existing electrochemical
microelectrodes and microcells to the CO2 environment. The ionic
conductivity strategies include coating the electrode(s) with a swellable
ionically conductive film, using ionic fluoro-surfactants as supporting
electrolyte in the CO2 "bulk" phase, and using
interdigitated array electrodes in steady state mode. An initial working
instrument has been assembled for the first measurement targets, which are
ionic conductivity of liquid and SCCO2. Ionic conductivity is a potentially informative property of
transport in the CO2 medium, and is an essential ingredient for
quantitative voltammetry. A series of
fluorophilic electrolytes and ionic surfactants is undergoing study, both for
solubility and for ionic conductivity of their CO2 solutions. These include variants of siloxane and
fluoropolymers terminated with ionic sites like carboxylate and perfluoronated
sulfonamide, and which have dissociable counter ions. Both dry and
water-containing solutions with measurable and possibly useful ionic
conductivities have been found.
.
Phase
Equilibrium for Polymer/Monomer/Initiator/Supercritical CO2 Systems
(19) Roberts and DeSimone have constructed an
apparatus to measure the phase equilibrium between a small molecule dissolved
in supercritical CO2 (scCO2) and a polymer that is
swollen with scCO2.
Partition coefficients have been measured for the system vinylidene
fluoride (VF2)/poly(vinylidene fluoride) (PVDF)/carbon dioxide. The data are preliminary but show that a
substantial quantity of VF2 monomer is present in the CO2-swollen
polymer particles during a typical polymerization of VF2 in scCO2. This is an important issue because the
partition coefficient may determine the relative importance of the fluid phase
and the polymer phase as loci of polymerization, and may influence the average
molecular weight (MW) and MW distribution.
We are beginning to study the solid-state polymerization of
poly(bisphenol A carbonate). Pre-polymer has been synthesized with varying
concentrations of catalyst and its crystallization with scCO2 is
being studied.
Novel Polymers
and Surfactants (35) DeSimone has focused on synthesis and molecular characterization of
two classes of surfactants: cationic silicone surfactants and non-ionic
fluoro-polymer surfactants. A series of fluorocarbon block copolymers was
synthesized using controlled radical polymerization methods where the second
monomer formed a lyophobic, hydrophilic sequence. These molecules stabilize
heterogeneous suspension and dispersion polymerizations in CO2. Novel PDMS
surfactants having quaternary amine chain end with various anionic counter ions
were synthesized and found to be the first cationic surfactants soluble in CO2. The aggregation behavior of these molecules
in CO2 is being studied with collaborators from Thrust Group A, and
Murray is exploring the ionic conductivity of solutions of these molecules in
CO2.
* in spite of the very low dielectric constant, scCO2
was acceptable for use in reactions involving polar compounds, such as SN2
displacements by anionic nucleophiles;
* in the presence of scCO2, supported PT agents
exhibited an activity even greater than those of conventional onium salts;
* inorganic matrixes (i.e. silica gel) were particularly suitable
for these reactions because they have rigid structures that do not swell,
unlike poorly designed polymeric supports.
Adewuyi plans to (1) acquire and synthesize novel zeolite
catalysts; (2) characterize these catalysts using spectroscopic
techniques; (3) conduct batch studies using existing facilities at UNC-CH; (4)
setup laboratory-scale batch and continuous reactors for catalytic reactions in
supercritical CO2 in the Chemical Engineering Department at
NCA&T.
Kelly’s research focus has changed
slightly in light of interesting initial results. Consequently, plans to construct a GC/reactor system will be
modified to reflect this “mid-course correction”. Plans will be finalized by October 2000 after consulting with
CERSP leadership.
Paul Kropp plans to study the differences in
surface–mediated behavior between liquid CO2 and scCO2 in
detail. In addition, the use of
solvent–free conditions for conducting chemical reactions will be studied. Preliminary studies have shown significant
differences. His studies have also
shown that a number of surface–mediated reactions that do not occur in the
presence of a solvent proceed readily under solvent–free conditions, especially
with accompanying microwave irradiation.
Solvent–free conditions represent an ultimate in pollution prevention
and energy efficiency. The origins of
both the greater driving force for solvent–free conditions and enhancement
under microwave irradiation will be delineated.
Murray will continue ionic conductivity measurements aiming for an
electrolyte medium that permits quantitative microelectrode voltammetry of an
electroactive test solute in CO2, such as [Ru(bpy)3]2+
or ferrocene. Voltammetry will give
access to measurements of transport rates and how these are influenced by
solvent parameters (pressure, temperature) and by the solvent microstructure
(e.g., effects of micellar encapsulation of the redox probe with surfactants). A second target will be exploration of
molecular films (coatings) over the electrodes that are suitable and
informative for measurements of intra-film electron or ionic transport and/or
for partitioning of redox probes from the CO2 phase. Swelling and plasticization of the film by
CO2 and/or its solutes are expected to be important determinants of
transport dynamics.
DeSimone and Roberts plan to
complete the measurement of partition coefficients for the VF2/PVDF/scCO2
system. Various thermodynamic models,
e.g., the SAFT and Sanchez- Lacombe equations of state, will be used to
correlate the data in collaboration with Thrust Group B researchers. The partition coefficient of an initiator
mimic, e.g., diethyl carbonate, will be measured. Equipment to measure the swelling of PVDF in scCO2,
and the rate of sorption of various small molecules into PVDF in the presence
of scCO2, will be designed and purchased. The kinetics and equilibrium of the solid-state polymerization of
poly(bisphenol A carbonate) will also be characterized quantitatively, as a
function of temperature. An apparatus
will be constructed to measure the diffusion coefficient of phenol in
poly(bisphenol A carbonate).
DeSimone will complete the synthesis
of a series of cationic PDMS surfactants in the next couple of months. These molecules will then be thoroughly
characterized as to their aggregation behavior in CO2 with and
without added water. Others will study
these molecules in the Center using PFG NMR and electrochemical methods. It is also planned extend the synthetic
methodology of making PDMS molecules to include block copolymers that have a
labile linkage between the blocks. The
goal would then to use scCO2 to deliver a catalyst to depolymerize the PDMS
block to small cyclics which could then be removed leaving nanometer-sized
voids which would be useful in a number of applications including nanofilters,
interlayer dielectrics, etc.
P. Tundo will complete
construction of an apparatus to use dense CO2. He will identify a
set of test reactions using CO2 as a solvent, in the presence of
both commercial phase-transfer agents and immobilized PT-catalysts (with
diverse salts) over inorganic matrixes such as silica and alumina. He will
explore the behavior of the biphasic system in scCO2-H2O