Supercritical Carbon Dioxide - American Chemical Society

Chapter 18

Dissolving Carbohydrates in CO : Renewable Materials as CO -philes 2

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 13, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch018

Poovathinthodiyil Raveendran and Scott L. Wallen

Department of Chemistry, Kenan and Venable Laboratories and the NSF Science and Technology Center for Environmentally Responsible Solvents and Processes, The University of North Carolina, Chapel Hill, N C 275993290 Corresponding author: email: [email protected] *

The design and synthesis of inexpensive, hydrocarbon based CO -philic materials are of current interest as they are applicable to the utilization of liquid and supercritical CO as an environmentally benign solvent. In this section, we describe the molecular interactions involved in enhancing the solubility of carbohydrates in CO . We propose the peracetylation of these compounds as a simple method for the preparation of inexpensive, environmentally benign CO -philes. Utilization of these materials as CO -philes combines the merits of two important green chemistry principles-environmentally benign solvents and renewable materials. 2

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© 2003 American Chemical Society

In Supercritical Carbon Dioxide; Gopalan, Aravamudan S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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1. Introduction The chemical industry in the U.S. has been tremendously successful as demonstrated by the enormous trade surplus it commands ($20.4 billion in 1995), the fact that it is the largest manufacturing sector in terms of value added product, and the tremendous amount of money (-$17.6 billion) that is reinvested in research and development annually (/). Obviously many of the products that consumers rely on in daily life are a direct result of the success of this manufacturing sector. These include, but are not limited to, pharmaceuticals, medical products, communications technologies, transportation products and an enormous number of other consumer goods. In spite of these phenomenal successes there are major challenges facing the U . S . chemical manufacturing industry in the next century. Foremost among these challenges are the implementation o f new processes which focus on environmental stewardship, sustainability and the recycling and/or reduction of waste. A large percentage of the current environmental problems that our country, and other industrialized nations, face is due to the reliance on standard organic solvents and organicinorganic catalysts for the manufacture of feedstock chemicals, specialty chemicals and a vast array of consumer products. In fact the majority of products made today rely on traditional synthetic and separation technologies that were developed over 40 to 50 years ago (/). A n additional consideration when examining chemical manufacturing processes is the waste generated after the useful lifetime of a product or the packaging that accompanies the product. Green chemistry is defined as, "the utilization of a set of principles that reduces or eliminates the use and generation of hazardous substances in the design, manufacture and application o f chemical products." (2) Over the past decade there has been an increasing emphasis on the introduction of green chemical methods in the analytical, synthetic and process chemistry segments of industry (5-7). One measure of the impact a particular process has on the environment is the environmental factor (E factor) introduced by Sheldon (S). The Ε factor is defined as the amount of waste generated per kilogram of product produced. The pharmaceutical industry has one o f the highest Ε factors with values as high as 100 (9). The majority of waste generated is in the form of inorganic salt by-products and organic solvents for reaction, separation and purification. Two of the most effective approaches to lowering the Ε factor is by increasing reaction efficiency through the incorporation of catalysts or through the replacement and/or elimination of hazardous solvents (9). The latter approach can also have significant economic incentives when one considers the ever-increasing costs associated with waste stewardship and disposal. One of the key areas within the realm of green chemistry is the identification of environmentally benign, alternative solvents that can replace the conventional, toxic, volatile organic solvents. Supercritical fluids (SCF) offer great


272 opportunities in this arena due to the ease o f solvent removal, recyclability and tunability of solvent parameters (4). Among SCFs, supercritical carbon dioxide (scC0 ) is advantageous to the green chemist due to its non-toxicity, nonflammability, low cost, abundance and easily achievable critical conditions. In fact s c C 0 is widely regarded as an environmentally responsible solvent and has become almost synonymous with green chemistry. Investigations to explore the utilization of this solvent system have been widely examined in the past decade (10-25). 2


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H i e most important limitation o f utilizing C 0 as an industrial solvent has been the extremely poor solubility (26) o f most non-fluorous, polar and amphiphilic materials in C 0 . Thus, die fondamental principles for the design of COrphilic molecules including amphiphiles have attracted great interest, and different molecular level approaches have been used to 'C0 -philize' compounds that are otherwise insoluble in C 0 . The first, and so far the most widely applied method is the introduction o f fluorocarbons (10-12). Though the interaction between C 0 and fluorocarbons are rather weak, fluorocarbons exhibit high solubility in liquid and s c C 0 . Fluorocarbon-based C0 -philes are, however, expensive and have the potential to cause environmental problems on thermal degradation (27). Therefore, there is current interest in the development of inexpensive, hydrocarbon-based C O p h i l e s . It has been identified in the past that the specific interaction o f C 0 molecules with Lewis base groups (28-30), especially carbonyl groups can be used in the design of C0 -philic materials (75, 16). 2

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Another focus in green chemistry currently is the use of renewable materials as a replacement for non-renewable resources (2, 6). Carbohydrates are an important class of inexpensive, abundant, and renewable biomaterials that has been the focus of research recently in a range of chemical, food and pharmaceutical applications. Thus, the utilization o f carbohydrate based materials as C0 -philes as well as the utilization of C 0 for the synthesis, isolation, purification and the processing o f carbohydrate based materials, and for the dispersion o f a wide range of materials in carbohydrate based matrices such as cyclodextrins, are o f great importance in green chemistry today. The C0 -phobic nature o f these polyhydroxy systems, however, makes them insoluble in liquid and s c C 0 . Thus one needs to replace the hydroxyl groups with C0 -philic functionalities to make the carbohydrates miscible with C 0 . Our ultimate vision is the industrial and academic implementation o f chemistry based on environmentally benign solvent and renewable, functionalized solute systems. The following is a presentation o f the fundamental ideas concerning this issue including the exploration o f the nature o f C0 -philic interactions, the application of these ideas to dissolve significant quantities o f renewable materials in C 0 and a brief overview of future research targets in this area. 2

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273 2. Interaction of C 0 with Carbonyl Compounds 2

A n understanding of the charge distribution, self-interaction (31-33) and the solvent-solute interactions of C 0 are essential for the rational design of C 0 philic materials. Our view of C 0 as a solvent comes from quantum mechanical ab initio calculations of C 0 (isolated as well as interacting with itself and other simple carbonyl containing systems). Historically, C 0 was considered as a nonpolar solvent owing primarily to the zero dipole moment and low dielectric constant and it was predicted that supercritical or liquid C 0 should be comparable to hexane in its solvent properties. However, this notion was discarded in later years as the majority of materials that are miscible with hexane were reported to be insoluble in s c C 0 . In fact, at this symposium an excellent historical perspective was provided by Beckman regarding the solubility of materials in C 0 (34). C 0 posseses a large quadrupole moment (35) and from an interaction point of view, C 0 is considered a Lewis acid ( L A ) since the electron deficient carbon atom can interact with a Lewis base (LB) group on a solute molecule (e.g. a carbonyl group). Several experimental and theoretical studies have been carried out to estimate the nature and extent of these interactions. Kazarian et al. (28) used IR spectroscopy to study the specific interaction between C 0 and carbonyl groups. The researchers observed the lifting o f the degeneracy of the C 0 bending modes, v , as a result o f interactions with polymethylmethacrylate. These researchers even suggested that these interactions could be responsible for the swelling of polyacrylates in contact with C 0 . Following these experimental studies, Nelson and Borkman used ab initio calculations to quantify the splitting of the v mode of C 0 upon interaction with carbonyl groups in simple molecules (29). Based on these spectroscopic observations (28) and the work o f McHugh and coworkers (15) concerning the solubility of vinyl acetates in C 0 , Beckman and coworkers synthesized poly-(ether-carbonate) co-polymers having 1-2 wt% solubility in liquid s c C 0 (16). As mentioned previously, C 0 , though having zero dipole moment, is a charge-separated molecule with partial positive charge on the carbon atom and partial negative charges on the oxygen atoms. Thus, C 0 can act as an electron donor as well as an electron acceptor. The situation is analogous to the case of H 0 , which can act both as a Lewis acid and a Lewis base. For a comparison, the Mulliken, as well as the C H E L P G charges (calculated by fitting the electrostatic potentials) on the individual atoms o f C 0 and H 0 , calculated at the MP2/augcc-pVDZ level using Gaussian98 (36) are presented in Table I. It is observed that the charge separation is similar for C 0 and H 0 except for the reversal of the signs of die charges on the end atoms. Although partial atomic charges are only an abstract representation of a more complex electron density picture, at this point it is clear that Table I suggests that, as a solvent, C 0 has the capacity 2

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274 to act as a Lewis acid or a Lewis base and that the oxygen atoms can take part in significant interactions with other chemical moeities. We should make clear that although the charge separation in C 0 and H 0 are similar the intermolecular associations in H 0 make it a unique solvent system due to the extended networks of cooperative hydrogen bonds. 2

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Table I. Comparison of the partial charges on the individual atoms of H 0 and C 0 . Mulliken charges as well as the charges derived by fitting the electrostatic potentials (CHELPG charges) in electrons (e).


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H(l) 0 H (2)

0.16 -0.32 0.16

0.36 -0.72 0.36

O(l) C 0(2)

-0.18 0.36 -0.18

-0.36 0.72 -0.36

3. Cooperative C-H—O Interactions The C 0 oxygen can act as a weak Lewis base unit for interaction with Lewis acid groups or acidic hydrogen atoms while the carbon atom, as mentioned previously, can act as a Lewis acid. This observation is of relevance since most o f the carbonyl compounds encountered have hydrogen atoms attached to the carbonyl carbon or to an α-carbon atom. These hydrogen atoms are somewhat acidic (due to the electron withdrawal by the carbonyl group) and carry partial positive charges. Our calculations indicate, in all cases studied to date, that these C - H bonds may be involved in weak C - Η - Ό hydrogen bonds (57, 38) with oxygen atoms of die C 0 molecule while the carbonyl oxygen acts as a Lewis base unit through interacts with the electron deficient carbon atom of the C 0 molecule (39, 40) as shown in Figure 1. These two interactions also act cooperatively, each reinforcing the other, and contribute to further stabilization of the solute-solvent complex. In order to understand which among the carbonyl containing functionalities will be more suitable for the C0 -philization of molecular systems we investigated the interaction energies o f C 0 with simple carbonyl containing molecules such as formaldehyde (HCHO), acetaldehyde (AcH), acetic acid (AcOH) and methyl acetate (MeOAc) in different interaction geometries. The optimized geometries for C 0 dimer and the complexes o f C 0 with H C H O , A c H , and M e O A c are presented in Figures 2 and 3. 2

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For all these systems the optimizations are carried out at the M P 2 level of theory using the 6-31+G* basis set and the depth of the potential energy well are calculated by single point calculations at the optimized geometries using Dun-



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Figure L Typical interaction geometry of C0 complexes involving a cooperative C-H-O interaction associated with a typical LA-LB interaction between CO2 and a Lewis base system (carbonyl group) as in the interaction between CO2 and an acetate group. 2

Figure!* Optimized structures (MP2/ 6-31+G*) for (A) the 'slipped* parallel (C2h symmetry) and (B) T-geometries (C2v symmetry) of the C0 dimer; (C) the T-structure (C2v symmetry) and (D) the proton side (Cs symmetry) configuration of the HCHO-CO2 complexes (39). 2



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Figure 3: Optimized structures (MP2/6-31+G*) for ofAcH-C0 complexfor the methyl side (A) and proton side (B) interaction geometries and the Me0Ac-C0 complexes for the methyl side (C) and ester side (D) configurations (39). 2

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ning's correlation consistent basis sets augmented by diffuse functions (aug-ccpVDZ). Vibrational frequencies and other molecular parameters including bond distances, bond angles were also investigated to understand the nature of the interaction in more detail (39). The interaction energies, corrected for basis set superposition error, are presented in Table II. For a comparison the interaction energies for the C 0 dimer in the T-geometry and the slipped parallel configuration are also calculated at die same level of calculation. In all the complexes studied, the C 0 molecule is bent from its otherwise linear geometry as a result of this interaction. The two O O bonds of the 'bound' C 0 are non-identical. The C = 0 bond involved in the C - H Ο interaction is consistently longer than the 'free' C=0, strongly supporting the presence of these interactions. Charge transfer and electron density changes in the systems upon complexation provides conclusive evidence for the existence o f the C Η - Ό hydrogen bond (39). A molecular orbital diagram of the methylacetateC 0 complex (Figure 4) also supports this view. The question remains as to the relative contribution of the C-H—O interaction to the overall stabilization, but such an estimate is elusive at present considering the intricate nature of the cooperativity of these interactions. 2

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277 Solvation depends on the relative strengths of solvent-solvent, solutesolvent and solute-solute interactions apart from entropie considerations. While the interaction energy of the C0 dimer alone cannot represent exactly the solvent-solvent interaction cross-section in liquid and scC0 due to thepre2

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Table IL BSSE corrected interaction energies (AE ) for the C 0 complexes. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 13, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch018

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Molecular species

Δ£ (MP2) (kcal/mol)

HCHO-C0 (T) H C H O - C 0 (P) A c H - C 0 (M) A c H - C 0 (P) MeOAc-C0 (M) M e O A c - C 0 (E) A c O H - C 0 (M) C 0 dimer (||) C0 dimer (T) H Q—H Q*

-1.92 -2.43 -2.52 -2.69 -2.82 -2.64 -2.80 -1.10 -0.94 -4.11*

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Figure 4. The highest occupied molecular orbital (HOMO) for the optimized geometry of the CO methyl acetate complex (M) at the MP2/6-31+G* level (40). r


278 sence o f many-body interactions, they provide some measure o f the solventsolvent interactions. Our results for the complexes also are an approximation to the enthalpic contribution to solvation. The calculated interaction energies for all systems are presented in Table II (59). The results indicate that for all the simple carbonyl systems studied, the solute-solvent interaction is stronger than the solvent-solvent interaction, enthalpically favoring solvation o f these materials in liquid and s c C 0 . Further, hydrocarbons functionalized with these carbonyl moieties may exhibit good C0 -philicity. However, the solvation also depends on the entropie contribution and in the case o f solids the lattice energies o f materials functionalized with these carbonyl moieties. Table II indicates that among the various carbonyl compounds studied, methyl acetate has the strongest interaction with C 0 . This suggests that acetylation of polyhydroxy systems could be an amenable approach to increase the C0 -philic nature o f a compound. Carbohydrates represent a class o f inexpensive, renewable, abundant md naturally occurring polyhydroxylated molecular systems and these calculations point to acetylation o f carbohydrates as a method to soubilize this C0 -phobic systems in C 0 . Acetylated carbohydrates are quite commonly available and they offer tremendous opportunity for die synthesis o f inexpensive, renewable, environmentally benign C0 -philes. This is the present approach that we are taking in the design of functional C0 -philic systems.


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4 Peracetylated Sugars Based on these design principles, three peracetylated carbohydrate derivatives were selected as model systems (40) for our studies: the a- and βforms of 1, 2, 3, 4, 6-pentaacetyl D-glucose ( A G L U and B G L U , respectively) and 1, 2, 3, 4, 6-pentaacetyl β-D-galactose ( B G A L ) . The chemical structure of these three stereoisomers are given in Figure 5. B G L U is a white solid that melts at 132°C under atmospheric pressure conditions (Figure 6A). However, as B G L U is exposed to C 0 near room temperature (23.0°C) in a conventional high-pressure view cell, it absorbs C 0 and becomes "wetted" with C 0 at a pressure of 35-40 bar. The white solid appears as a salt does in a humid environment. A t a gaseous C 0 pressure o f 55.9 bar a solid to liquid transition (deliquescence) occurs (Figure 6B) that is analogous to the deliquescence of hygroscopic materials absorbing atmospheric moisture. The carbohydrate melt continues to absorb C 0 and swells to many times its original volume with addition of gaseous C 0 pressure. A t the liquid-vapor equilibrium pressure, the liquid C 0 forms a separate layer on top o f the viscous melt containing C 0 . However, the melt easily mixes with the upper layer of liquid C 0 on stirring 2

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Figure 6. Photographs demonstrating the deliquescence of BGLU in C0 at 23.0*C (A) solid material (B) in contact with gaseous C0 at the deliquescence pressure of 55.9 bar undergoing the solid-to-liquid transition as indicated by the black arrow. 2

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280 and forms a single-phase liquid mixture in contact with the gaseous C 0 phase. Further addition of C 0 dilutes this liquid phase until beyond the liquid-vapor equilibrium a single phase is observed. A n approximate estimate o f the B G L U concentration in the final melt reveals that the system contains more than 80 wt% o f B G L U and can be diluted with liquid or s c C 0 in any proportion desired. This suggests the use of these systems or other derivatives (e.g. peracetylated trehalose) as viscosity enhancers in liquid and supercritical C 0 solutions. The deliquescence point of A G L U is lower than that o f B G L U by about 6-7 bar. B G A L does not exhibit deliquescence though it is readily soluble in liquid C 0 . These observations can be directly correlated to the differences in lattice energies as reflected in the melting points of the three compounds, A G L U , B G L U , and B G A L which are 109,132 and 142°C respectively. The cloud-point pressures of these systems dissolved in s c C 0 were examined at 40.0°C. As in the subcritical case, initially the solid melts and swells (for A G L U and B G L U ) and all three peracetylated sugars readily go into a single phase, s c C 0 system. A plot of die cloud-point pressure versus the wt% for A G L U , B G L U and B G A L dissolved in supercritical C 0 at 40°C is given in Figure 7 (40). 2

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10 20 30 Wt% of Peracetylated Sugar

Figure 7. Plot of the cloud-point pressure versus the weight percentage of the carbohydrate-derivative for AGLU (* ); BGLU (·) and BGAL (A) in supercritical C0 at a temperature of 40.0 X: (40). (Reproduced from reference 40. Copyright 2002 American Chemical Society.) 2

At the cloud-point pressure, phase separation begins between s c C 0 and the sugar ester. Upon lowering the pressure, the material reappears in die solid state. No cloud-point measurements were made above 30 wt% due to limitations arising from the volume of the view cell and the rapid swelling of the sample in the cases of A G L U and B G L U . Considering this cloud-point data and the data presented in Figure 6, it is apparent that the mixtures of A G L U and B G L U show complete miscibility at relatively low pressures with the 3-phase line being 2



281 shifted to extremely low pressures. It is interesting to note that in Figure 7 there are substantial differences in the cloud points of these three stereoisomers. For the three peracetylated sugars shown in Figure 5 an analysis of the minimum energy structures calculated using density functional theory (DFT) shows an increase in the number of intramolecular C-H- Ο contacts going from A G L U < B G L U < B G A L (40). This increase is correlated with an increase of the cloud point pressure for these compounds suggesting that the intramolecular C-H* 0 contacts, involving the acetate carbonyl groups and methyl hydrogens, interfere with the ability o f C 0 to form a full solvation sphere. It is important to point out that this is a hypothesis and that one must fully consider the lattice energy as well as the enthalpic and entropie interactions in determination of solvation structures. Nevertherless the understanding of the stereochemical aspects revealed here may provide important insights toward the design of larger C 0 philic molecules since there is a definite dependence on the configuration of the individual isomers. Additional experiments were performed on two disaccharides, sucrose octaacetate (linked glucose and fructose) and cellobiose octaacetate (linked glucose and glucose). The former was highly soluble in s c C 0 (40.0°C) with a 30 wt% solution having a cloud point of 95.1 bar and the latter being completely insoluble. These experiments were extended to a polysaccharide (peracetylated-P-cyclodextrin) in which a 0.5 wt% solution was solubilized in s c C 0 (40.0°C) with a cloud point o f 193.1 bar. This system had solubilities that were limited to a few weight percent in contrast to the mono- and disaccharide systems. However, the compound is rich in the number of possible applications opening up a new realm of nontoxic systems for use in C 0 . 2

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5. Summary Based on our computational studies, we proposed that acetylation of polyhydroxy systems such as carbohydrates can be used as a viable method for the preparation o f environmentally benign C0 -philes. The ideas put forth regarding cooperative C-H—O interactions could lead to other C0 -philes in which C 0 concurrently acts as a Lewis acid and a Lewis base. The high solubility o f peracetylated carbohydrates has several important consequences, namely, the first demonstration of renewable, environmentally benign, nontoxic hydrocarbon-based C0 -philic systems. The future for these systems appears bright with regard to enabling green chemistry applications using C 0 in a wide range of processes including food, pharmaceutical, carbohydrate synthesis and materials processing. Use of C 0 as a solvent is ideal for food and pharmaceutical processes not only due to the non-toxicity of the solvent, but also due to the low critical temperature o f C 0 (31.1°C) o f C Q . Among the carbohydrate systems it is of special importance to mention the use of 2

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282 cyclodextrins which forai complexes and have many applications including the sustained release and degradation protection of pharmaceuticals.

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Supercritical Carbon Dioxide - American Chemical Society

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