IV. CONCLUSIONS AND FUTURE DIRECTIONS

follow reaction kinetics. However, with the exception of one early investigation

(63), these studies were exclusively conducted with ex situ NMR methods. Only

recently have in situ NMR methods been adopted in this research field (64,65).

This research area will in particular benefit from in situ high-pressure NMR

techniques because, as pointed out by Burgemeister et al., “every step of the

catalytic cycle for the rhodium-catalyzed hydrogenation of CO2 to formic acid

can be monitored by 1 H-NMR spectroscopy” (66).

A new development in high-pressure NMR probe design is a multipurpose high-pressure autoclave made from the thermoplastic polyetheretherketone

(PEEK) (67). This NMR autoclave was used for in situ NMR imaging of a

compressed gas system, namely, the exchange of methanol for liquid CO2 in

nanoporous silica-alcogels, reported for the first time.

Magic-angle spinning (MAS) solid-state NMR spectroscopy has for a number of years provided a means to study heterogeneous catalysis reactions by

directly probing the chemical species present on the catalyst surface. Some of

these experiments have been conducted at temperatures in excess of 200◦ C

(68–71) and up to 400◦ C (72). By application of laser (73) or rf heating (74),

fast transient sample heating (temperature jump) can be achieved. However, the

most interesting development is the very recent construction of an isolated flow

MAS NMR probe (75). This development has brought solid-state NMR much

closer for studying heterogeneous catalysis in supercritical fluids.

Another area of potential impact on high-pressure NMR in the future

may come from recent advances in enhancing NMR sensitivity by use of laserpolarized noble gases. The pioneering theoretical and experimental work by

Happer (76) laid the foundation for understanding the physics involved. The major research efforts in this area have focused on (a) applying the laser-polarized

noble gases directly, as in magnetic resonance imaging, or (b) transferring the

129 Xe polarization to another nuclei. Recently, significant progress has been

made in transferring the polarization from laser-polarized 129 Xe to other nuclei.

Signal enhancements of 70-fold in 13 C NMR have been achieved by crossrelaxation from laser-polarized liquid xenon (77). A further breakthrough was

recently reported by the Pines et al. (78), who can routinely polarize supercritical

xenon to enhancements of about 1000 that last for hundreds of seconds. These

reports suggest numerous applications to supercritical fluids, i.e., the study of

the efficiency of cross-relaxation from polarized supercritical xenon to dissolved

solute molecules as a function of temperature and density (pressure). Second,

a spin-labeled reactive functional group may be useful for studying chemical

reactions in supercritical xenon (or eventually in other supercritical fluids).

These are a few of many potential areas were new techniques in combination with high-pressure NMR could make an important contribution to the

fundamental understanding of solution chemistry and physics in supercritical

and high-pressure liquid solvents.



Copyright 2002 by Marcel Dekker. All Rights Reserved.



ACKNOWLEDGMENTS

I express my appreciation to my former colleagues, Drs. S. L. Wallen and

S. Bai, whose contributions to the work in my laboratory (CRY) is represented

in this chapter. The work performed at the Pacific Northwest National Laboratory (PNNL) was supported by the Office of Science, Office of Basic Energy

Sciences, Chemical Sciences Division of the U. S. Department of Energy, under

Contract DE-AC076RLO 1830.

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3

Organic Chemical Reactions

and Catalysis in Supercritical

Fluid Media

Keith W. Hutchenson

DuPont Company, Wilmington, Delaware



I. INTRODUCTION

The development of more efficient and economical chemical transformation

processes remains an important challenge. Organic synthesis reactions in the

polymer, chemical, and pharmaceutical industries are increasingly required to

be highly selective, economical, and environmentally benign. As one means of

developing such processes, the use of supercritical fluids (SCFs) as reaction solvents has been the subject of increasing investigation over the last two decades

because of the host of potential advantages afforded by these media over conventional liquid solvents and gaseous diluents. Specific applications in the materials

area include polymer synthesis as well as the synthesis of monomers and key intermediates. This chapter focuses on the “small-molecule” end of this scale and

includes a comprehensive survey of the major organic reaction classes currently

under investigation in SCF media.

SCF technology has found widespread use in a number of industrial-scale

processes, primarily in separations through SCF extraction. Examples of commercial implementation of SCF extraction applications include coffee and tea

decaffeination, flavors from hops, cholesterol and fat from eggs, nicotine from

tobacco, acetone from antibiotics, and organics from water (1,2). Other commercial applications include the CO2 -based dry cleaning facilities that are in direct

competition with conventional perchloroethylene systems and Union Carbide’s

Unicarb technology for CO2 -based spraying of paint and other coatings (1,3).



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Commercial implementation of SCF technology for conducting chemical

reactions has been much more restricted, although limited applications have been

in operation for decades. Jessop and Leitner (4) provide an excellent history of

the early industrial applications. These include the well-known ammonia synthesis (1913), methanol synthesis (1923), oxidation of light alkanes (1920s), and

the synthesis of low-density polyethylene (1940s). These early successful applications of SCF technology in the manufacture of bulk chemicals and polymers

demonstrated that the technical challenges associated with large-scale operation at high temperatures and pressures could be resolved. However, despite

the precedence established with these early applications, broad implementation

of SCF technology for conducting chemical reactions has been limited to a

handful of specialized applications. For example, the supercritical water oxidation (SCWO) process for the total oxidative destruction of hazardous organic

compounds in aqueous waste streams has been commercialized (5,6), although

in general these plants are limited to waste streams that minimize the impact

of the two significant technical challenges facing this technology—scaling and

corrosion (7). The Japanese firm Idemitsu Petrochemical Co. commercialized a

40,000 metric ton per year integrated reaction and separation process utilizing

SCF butene in 1985 (2,4,8). The acid-catalyzed reaction of 1- and 2-butene to

2-butanol occurs in an aqueous phase, and the product is extracted into the SCF

butene phase to drive the reversible reaction forward. Cooling and depressurizing the SCF extract phase then isolates the 2-butanol. The Danish firm Paul

Møller Consulting, in conjunction with the Chalmers University of Technology

(Göteborg, Sweden), is developing SCF processes at pilot plant scale for the

hydrogenation of fatty acid methyl esters to fatty alcohols and the synthesis of

hydrogen peroxide (4,9,10). The Swiss company Hoffmann-La Roche has reported the development of an 800 ton per year continuous pilot plant utilizing a

40-liter reactor for the heterogeneously catalyzed hydrogenation of vitamin precursors (4,11–13). Thomas Swan & Co., a fine-chemicals manufacturer in the

United Kingdom, has announced the development of a commercial-scale continuous hydrogenation facility using an SCF solvent (1,14). This facility, which

is being designed in conjunction with the Swedish engineering firm Chemature,

is slated for startup during the second half of 2001 with an annual capacity

of 500–1000 tons of an unspecified product. DuPont has announced (15) the

construction of a 2.5 million pound per year market development facility at its

Fayetteville, North Carolina, site that will evaluate supercritical CO2 (scCO2 ) as

a reaction solvent for the production of tetrafluoroethylene-based fluoropolymers

and copolymers. This work was initiated by DeSimone and coworkers (16–18)

and further developed by DuPont (19). Successful demonstration of this technology in the market development facility will be followed by construction of a

commercial-scale plant. These examples show that SCF reaction technology is

beginning to gain broad commercial acceptance.



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The topic of reactions in SCF media has been the subject of a number of

reviews and surveys in recent years (7,8,12,20–30). Of note is a thorough review

by Savage et al. (31) that provides a comprehensive analysis of this literature

from 1985 to 1994. Several recent reviews have been more narrowly focused

on specific topics. For example, Savage (32) and Baiker (33) present reviews

of heterogeneous catalysis applications in SCF media, and Jessop and coworkers (34,35) focus on the homogeneous catalysis literature. Baiker’s paper (33)

includes a brief discussion on the various batch and continuous reactors that

have been employed in much of the work discussed in these various reviews.

Jessop and Leitner (36) recently edited a comprehensive monograph devoted exclusively to chemical synthesis in SCF media with contributions from a number

of researchers in the field.

This chapter surveys the literature on small-molecule organic chemical

transformations in SCF media. The scope is intended to be comprehensive,

but the focus is on studies published since the extensive review by Savage

(31), which covers the literature up through 1994. The chapter is organized

to first present a fundamental understanding of the features of SCFs and the

corresponding potential advantages for their utilization as reaction media. This

is followed by a survey of the published literature on a variety of applications

in various stages of research and development. This latter section is generally

organized by major reaction classes for ease of reference.

As in conventional catalysis, catalytic chemical transformations utilizing

SCF media can be conducted as both heterogeneous and homogeneous systems.

Heterogeneous solid catalysts offer high reaction rates and simple separation of

the catalyst from products. Hence, use of heterogeneous catalysis is by far the

more common industrial practice in conventional solvents [approximately 85%

of all known commercial catalytic processes use heterogeneous catalysts (37)].

However, the use of homogeneous molecular catalysis is increasingly becoming

a viable alternative because these catalysts offer high selectivity, tunability, and

even chirality for the production of a range of small to large molecules (38).

Both types of systems have been investigated in SCF media, and the system

type will be distinguished in the various cited studies.

The coverage of the patent literature is not exhaustive, although an attempt has been made to include the English language patents for SCF reactions

dating back to 1990. Prior patent literature is covered in two primary sources.

McHugh and Krukonis (39,40) provide a compilation of patent summaries in

the area of SCF technology with an emphasis on extraction applications. Bruno

(41) provides a listing and brief summary of patents in the field issued between

1982 and 1989. The appendix A to this chapter summarizes major patents issued

from 1990 to 1999 in the area of chemistry and catalysis in SCFs within the

restrictions noted below. This listing is believed to be representative, if not comprehensive, of the US, EP, and WO patents for this period along with a selection



Copyright 2002 by Marcel Dekker. All Rights Reserved.



of other national patents. Selected patents from this summary are also cited in

the text.

Three important areas within this general field that are beyond the scope

of the current review include the topics of organic synthesis in supercritical

water, polymer applications, and enzyme-catalyzed reactions. Shaw et al. (42)

present an early review on the use of supercritical water as a reaction medium,

and Savage (43) has updated this in a recent comprehensive review focusing on

organic chemical reactions in supercritical water. Polymerizations and polymer

modifications in SCF media have seen widespread interest in recent years, and

this trend will likely accelerate as industrial motivations, such as solvent replacement, become increasingly important. DuPont’s recent announcement (15)

regarding the building of a scCO2 -based development facility for fluoropolymerizations is an example. Comprehensive reviews of this area are provided by

Scholsky (44), Kiran (45), and, more recently, DeSimone and coworkers (46).

Another rapidly emerging area of importance is that of enzyme-catalyzed reactions in SCFs. Randolph et al. (47), Clifford (23), and Savage et al. (31) provide

reviews of this literature, and general overviews of the field are provided by

Aaltonen and Rantakylä (48), Russell et al. (49), and Nakamura (50). Mesiano

et al. (51) provide the most recent review available on this topic.



Figure 1



Pressure–temperature phase diagram for a pure fluid.



Copyright 2002 by Marcel Dekker. All Rights Reserved.



II. FUNDAMENTALS

A. Properties of SCFs

Figure 1 shows the SCF region for a pure component on a pressure–temperature

diagram. By definition, a fluid is in the SCF state when the system temperature

and pressure exceed the corresponding critical point values defined by the critical

temperature (Tc ) and pressure (Pc ). Most useful applications of SCFs that take

advantage of the unusual physical properties in this region occur in the range

TR (= T /Tc ) ≈ 1.0–1.1 and PR ≈ 1–2 (26). However, in the literature, some

so-called SCF reaction systems actually are conducted at conditions slightly subcritical in temperature or pressure where some of the potential benefits afforded

by SCF media are also present.

To a first approximation, the solvent strength of an SCF can be related

to the solution density (52). One of the primary advantages of SCF reaction

media is that the density can be varied continuously from liquid-like to gas-like

values by varying either the temperature or the pressure. Figure 2 illustrates

this unique feature of an SCF by showing the variation in density as a function



Figure 2 Density–pressure projection of the phase diagram for pure carbon dioxide

(From Ref. 53).



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