Number range CHAPTEREnvironmentally Beneficial Multiphase Catalysis in
CO2-Based Reaction Media

Bala Subramaniam

Environmental concerns stemming from the use of conventional solvents
and from hazardous waste generation have propelled research efforts
aimed at developing benign chemical processing techniques that either
eliminate or significantly mitigate pollution at the source. This talk
provides an overview of heterogeneous and homogeneous catalysis in dense
phase CO2, considered a green solvent. In addition to solvent
replacement, the advantages of using dense phase CO2 include the
enhanced miscibility of reactants such as O2 and H2 that eliminate
interphase transport limitations, and the chemical inertness of CO2.
Further, the physicochemical properties of CO2-based reaction media can
be pressure-tuned to obtain unique fluid properties (e.g., gas-like
transport properties, liquid-like solvent power and heat capacities).

In TSE-sponsored research, we have exploited the aforementioned
properties of CO2-based reaction media in heterogeneous catalysis in a
variety of ways including: (a) enhanced desorption and transport of
heavy molecules (such as coke precursors) in mesoporous catalysts
alleviating pore-diffusion limitations and improving catalyst
effectiveness; (b) in situ removal of primary products stabilizing
primary product selectivity; and (c) eliminating O2 or H2 solubility
limitations in the liquid phase, and hence interphase mass transfer
resistances, in multiphase reaction systems; and (d) enhanced heat
capacity ameliorating the problem of parametric sensitivity in
exothermic fixed-bed reactors. One or more of these advantages have been
demonstrated for several classes of reactions such as alkylations,
hydrogenations, oxidations and hydroformylations, spanning a wide
spectrum of chemical process industries. In virtually every case, the
CO2-based reaction medium represents an enabling tool used to manipulate
such factors such as catalyst stability, product selectivity and
temperature rise in the reactor. We demonstrated that CO2-based media
can be exploited to prevent the accumulation of catalyst deactivating
species in porous solid acid catalysts. This breakthrough eliminates a
major barrier facing implementation of solid acid catalysts in
industrial practice and has paved the way for research aimed at
designing tailored solid acid catalysts (with the necessary pore
structure and acid site density) as environmentally beneficial
alternatives to liquid acids.

In homogeneous catalysis, the emphasis is on recent developments on
selective oxidations and hydroformylations. Specifically, the use of
mixed solvents (carbon dioxide, water and organic media) as reaction
media is highlighted. In collaboration with Prof. Daryle Busch's
(Chemistry Department, University of Kansas) research group, we
demonstrated that carbon dioxide-expanded liquids (CXLs) represent a
continuum of reaction media with infinite tunability (based on combining
CO2 with any organic solvent) that provide the optimum properties for a
variety of homogeneous catalytic reactions. CXLs constitute a vast array
of media that offer endless opportunities for the improvement of
chemical reactions.

Demonstrated reaction advantages of CXLs include higher oxygen
miscibility compared to neat organic solvents, applicability to
transition metal catalysts without the need for modification (i.e.,
fluorination) to enhance their solubilities, between one to two orders
of magnitude greater turnover frequencies TOFs and either comparable or
better product selectivities than those obtained in neat organic solvent
or in supercritical CO2. Further, the catalyst may be separated from the
reaction mixture by simply adding more CO2. Environmental and safety
advantages are dramatic, including substantial (up to 80 vol.%)
replacement of organic solvents with dense-phase CO2, significantly
lower process pressure (tens of bars) compared to scCO2 (hundreds of
bars), and with such a substantial presence of CO2, it is possible to
eliminate the formation of explosive mixtures in the presence of oxygen
and other potent oxidants. Thus, oxidation in CXLs is inherently safer
when compared to conventional oxidations with traditional solvents.

The challenge for commercializing CO2-based processes will be in
demonstrating processes that simultaneously display the following
attributes: selective, environmentally benign, stable and economical.
The examples discussed in this talk show that the optimum pressure for
CO2-based processes lies closer to or below the critical pressure of the
reaction mixture (typically on the order of tens of bars) which is
advantageous from an economic standpoint. The recently funded
multi-university, multidisciplinary Center for Environmentally
Beneficial Catalysis (CEBC), a National Science Foundation Engineering
Research Center headquartered at the University of Kansas, addresses the
challenge of developing environmentally beneficial yet economically
viable catalytic processes. The CEBC was in part borne out of academic
and industry collaborations and research results that accrued from the
aforementioned NSF/TSE research. CEBC's strategic research plan, based
on input from approximately 25 industry representatives, is driven by
the "engineered systems" based approach that require cross-disciplinary,
multiscale collaborations. This research paradigm and its implementation
for developing novel environmentally beneficial catalytic technologies
will be presented.