Gas-Expanded Liquids and Near-Critical Media - American Chemical

Chapter 7

In Situ Alkylcarbonic Acid Catalysts Formed in C0 -Expanded Alcohols 2

Jason P. Hallett , Charles Α. Eckert , and Charles L . Liotta Downloaded by UNIV OF ARIZONA on December 22, 2012 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch007

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Department of Chemistry, Imperial College, London, United Kingdom School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400 2

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We have developed self-neutralizing acid catalysts formed from carbon dioxide and alcohols for use in chemical synthesis. These alkylcarbonic acid catalysts are generated in situ and can be easily neutralized via depressurization. This technique combines a medium with good organic solubility with acid catalysts that do not require neutralization, thus completely eliminating the solid wastes associated with many acid processes. We examined the origins and characteristics of alkylcarbonic acids by comparing reaction rates of acid– sensitive probes in different CO -expanded alcohols, elucidating the effect of CO pressure on solvent properties and acid formation, and demonstrating the use of alkylcarbonic acids for a variety of reaction types. 2

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© 2009 American Chemical Society

In Gas-Expanded Liquids and Near-Critical Media; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

131

132 Introduction

Acids are the most used catalysts in industry, and they produce more than 1 χ 10 metric ton/year of products.(Z) Some of the most common acids include Bronstead acids such as HC1, H S 0 and H3PO4 and Lewis acids such as A1C1 and BF . Unfortunately, acid-catalyzed processes are plagued by large amounts of waste associated with post-reaction acid neutralization. As an extreme example, for the Friedel-Crafts acylation of methyl benzoate with acetic anhydride, nearly 20 kg of A1C1 are used per kg of product.(2) Acid neutralization produces a contaminated waste salt which must be disposed of carefully (and often expensively). As an alternative, we have developed alkylcarbonic acids, which are acid catalysts generated in situ through the interactions between C 0 and alcohols in gas-expanded liquids (GXLs). Alkylcarbonic acids are a potential replacement for mineral acids which possess all of the solvation and separation advantages inherent in GXLs(5-//) while the acid neutralization step is eliminated in favor of simple depressurization. It is well known that C 0 interacts with water to form carbonic acid, as shown in Scheme 1; an analagous interaction between C 0 and alcohols produces alkylcarbonic acids. The discovery of the catalytic properties of these acids came about as part of our work studying GXLs, when methanol-C0 8

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Downloaded by UNIV OF ARIZONA on December 22, 2012 | http://pubs.acs.org Publication Date: January 6, 2009 | doi: 10.1021/bk-2009-1006.ch007

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133 mixtures were observed to protonate Reichardt's dye. We explained this phenomenon by demonstrating the presence of alkylcarbonic acids in such systems.(/2) These acids are reversibly formed with C 0 pressure, and the easy removal of C 0 forms the basis of our claim of 'self-neutralization'. We have repeatedly used diazodiphenylmethane (DDM) as a probe to measure relative reaction rates of these acids as well as carbonic acid in aqueous-C0 systems.(72,7J). In this report, we review our work with a brief summary of both the characterization (12,13) and utilization (14-16) of alkycarbonic acid as a catalyst in GXL systems. The use of GXLs as a replacement for traditional organic solvents offers many other advantages, which are detailed in previous work (5//) and elsewhere in this volume. Instead, this paper will focus on the acidity of C0 -expanded alcohols, the effect of solvent properties (C0 expansion, dielectric constant) on this acidity, and the application of alkylcarbonic acids to a variety of potential industrial syntheses, including the hydrolysis of β-pinene, the formation of ketals and several diazotization reactions. 2

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Characterization of Alkylcarbonic A c i d s

Measurement of Relative Acidities using Diazodiphenylmethane (DDM) The characterization of the acid properties of gas-expanded alcohols has involved several stages. We first reported preliminary finding (12) verifying the presence of alkylcarbonic acids in C0 -expanded alcohols by using DDM as a reactive probe to trap the acid species. One of the advantages of the DDM probe is that it incorporates the conjugate base of any reactive acids present in the system into the reaction products. Therefore, we could confirm through NMR analysis that alkylcarbonic acids were our proton source and not just the solvent alcohol. Futher, we used the relative rates of reaction of the alkylcarbonic acids with DDM to develop a 'scale' for correlating the relative strengths of different alkylcarbonic acids. Our initial study (12) reported the relative reaction rates of DDM in both methanol and ethanol expanded by 20 mole % C 0 . The rate in methanol was found to be roughly 2.8 times that in ethanol, indicating that there are more available protons in the GXL-methanol system. It should be noted that we cannot distinguish between relative acid strength (pK ) and equilibrium acid concentration using this method, because the expansion of the alcohols with C 0 will increase the equilibrium concentration of acid species but simultaneously decrease the polarity of the solvent mixture and therefore the acid dissociation (concentration offreeprotons). 2

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134 DDM results using acetone as a common diluent To separate these effects, we compared the rates obtained for different alcohols (including water) using acetone as a common diluent to maintain a constant C 0 concentration. Kinetic measurements were conducted at 40 °C using a solution containing 60 mol% acetone, 20 mol% C 0 , and 20 mol% ROH. Our results revealed that the rates in alcohols follow a logical progression, with a monotonie decrease in rate with increasing alkyl chain length and shorterchain alcohols yielding faster rates with the fastest rate in methanol and the relative order 1°>2°>3°. These results are consistent with steric and electronic factors affecting the equilibrium and dissociation of the alkylcarbonic acids. The rate using carbonic acid (from water and C0 ) was found to be slower than either methanol or ethanol. In our second study,(75) we used a wider set of alcohols, including methanol, ethylene glycol, propylene glycol, benzyl alcohol, 4-nitro benzyl alcohol, 4-chloro benzyl alcohol, and 4-methoxy benzyl alcohol. Once more, acetone was used as the dominant component in the liquid phase (60 mol%) in order to control C 0 solubility as closely as possible. Surprisingly, reactions utilizing ethylene glycol were faster than all mono-hydroxy alcohols, including even methanol. The reaction involving propylene glycol was similarly accelerated, with rates similar to those obtained using methanol, and faster than ethanol. It is expected that the relative basicity and nucleophilicity of the oxygen in the hydroxyl functionality will control the formation and acid strength of alkylcarbonic acid, as more basic oxygens will be more likely to react with C 0 . We further believe that the second alcohol fuctonality in glycols stabilizes the alkylcarbonic acid form of the C0 -OH interaction through hydrogen bonding. The results for the reaction of DDM with various C0 -expanded alcohols are summarized in Figure 1. 2


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Effect of CO concentration on DDM reactions 2

In order to determine the effect of C0 -expansion on the DDM reactions, we performed a series of experiments in which the pure alcohol was expanded with C 0 (no diluent). Figure 2 displays the results for C0 -expanded methanol. The most important result from these experiments was the observance of a maximum in rate at 60 bar C 0 . The increase in rate with C 0 expansion at pressures below 60 bar was attributed to an increase in the equilibrium concentration of alkylcarbonic acid (at the expense of free alcohol) with increased C 0 concentration. The decrease in rate after 60 bar results from this increase in concentration being counteracted by a large decrease in acid dissociation resulting from a drop in medium polarity. Any decrease in the ability of the solvent to solvate free protons will reduce the effective acidity of the medium. This decrease in polarity can be explored by examining the 2

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Figure 1. Rate of DDM decomposition in various C0 -expanded solvents. All reactions were performed in 60 mol% acetone/20 mol% ROH/20 mol% C0 at 40 °C. Data from references 12 and 13. 2

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10 8

X 6 4 2 -0 · 0.0

1

1

1

20.0

40.0

60.0

80.0

Pressure (bar) 1

Figure 2. Pseudo first order rate constant (k, s' ) for the reaction of DDM with methylcarbonic acid as a function of C0 pressure at 40°C. 2


136 dielectric constant as a function of C 0 concentration, which is discussed below. It should be noted that the maximum rate is 3 orders of magnitude faster than the rate obtained in the pure alcohol. Thus, alkylcarbonic acids demonstrate a clear catalytic effect on DDM decomposition. 2


Hammett-type analysis of DDM reactions In practical terms, the use of the alcohol as a GXL solvent would necessitate an understanding of the susceptibility of the reaction to changing the electronic nature of the alcohol making up the alkylcarbonic acid. A Hammett-type analysis is a very useful method for acquiring such mechanistic information. In the case of our alkylcarbonic acids, we preformed a series of experiments using substituted benzyl alcohols, choosing sustituents for which the Hammett substituent values (σ) are known. Our Hammett correlation is log (k /ko) = σ*ρ, where k is the rate constant for the disappearance of DDM with different substituted benzyl alcohols, k is the rate constant for the reaction involving unsubstituted benzyl alcohol, σ is a constant for the ring substituent based on electron-withdrawing/donating ability, and ρ is the reaction sensitivity to electronic effects. Since unsubstituted benzyl alcohol has a moderate rate of reaction when used as a solvent, a wide range of substituents could be used. 4nitro-, 4-chloro-, and 4-methoxy- benzyl alcohols were chosen in addition to benzyl alcohol (see Table I).(/7) Once again, we equalized C 0 solubility in the system by using 85 mol% acetone, 5 mol% alcohol, and 10 mol% C 0 for all experiments. Additionally, most of our substituted alcohols are solid at room temperature (and some have overlapping bands of UV absorbance with DDM), so low concentrations of alcohol enabled us to examine the electronic effect of alcohol nature on the alkylcarbonic acid activity in the absence of bulk solvent polarity effects or C 0 solubility concerns. x

x

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The resulting Hammett correlation yielded a ρ value for substituted benzyl alcohols of 1.71, which is very similar to the value of 1.93 reported for DDM reacting with benzoic acid in acetone at 30°C.(/5) These relatively high values of ρ indicate that the reaction is very sensitive to electronic effects. The first step is a nucleophilic attack by a benzyl alcohol on the carbon in C 0 , which should be increased by electron-donating groups and decreased by electron-withdrawing groups. On the other hand, the second step of proton dissociation should be increased by electron-withdrawing groups and decreased by electron-donating groups. Meanwhile, the second step, proton dissociation, should be accelerated by electron-withdrawing groups (which increase the acidity of the carboxylic proton) and decelerated by electron-donating substituents. Since the fastest rate occurred with the highly electron-withdrawing nitro substitution, the second step in our reaction scheme (proton dissociation) must be the dominant effect controlling r, and therefore the most sensitive step to electronic effects. 2


137 Table I. Rate constants used in Hammett correlation of para-substituted benzyl alcohols Substituent N0 CI H OCH 2


kxllf/s' 559 62.0 24.5 9.10

σ 0.78 0.23 0.00 -0.27

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Additionally, benzoic acid only undergoes the second step (proton dissociation) since there is no acid formation necessary. The ρ value for the benzoic acid reaction in acetone is similar to our alkylcarbonic acid value, so the acid formation step for alkylcarbonic acids is not likely to be limiting. The Hammett plot also revealed a two order of magnitude difference between the most electron-withdrawing and the most electron-donating groups. Electronwithdrawing substituents enhance the proton transfer step in the reaction with DDM and electron-donating groups decrease the rate of proton transfer. These results are consistent with the proton transfer step being rate-determining.

Analysis of dielectric constant of GXLs One of our primary goals with understanding alkylcarbonic acids is to decouple the relative contributions of acid strength and acid concentration. In order to accomplish this, some simple measure of proton dissociation ability would be beneficial. We chose to do this by measuring the dielectric constant of C0 -expanded methanol as a function of C 0 concentration (or gas expansion). These measurements give some indication of how much the polarity of a GXL (and thus the dissociation of acid species present) is decreased when C 0 is added to the liquid phase. The results of these experiments are displayed in Figure 3. The dielectric constant decreases in a nearly additive manner when compared to volume expansion of the system, resulting in an approximately linear decay of dielectric constant. This supports the concept that proton dissociation will be inhibited as C 0 concentration in the GXL is increased. 2

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Applications of Alkylcarbonic A c i d s

Acetal formation reactions We have also reported several example reactions for which alkylcarbonic acids serve as catalyst. In Xie et al. (16) we used C0 -expanded methanol and 2


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100 τ

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MeOH/water ratio (volume) ezzz hydrocarbons

100% water

—±— pinene conversion

Figure 5. β-pinene conversion and product distribution variations with different methanol/water ratios (75 °C, 24 hrs, 0.127M β-pinene). (Reproducedfrom reference 10. Copyright 2008 American Chemical Society.

also demonstrated the use of alkylcarbonic acid for a number of synthetic reactions: the protection of ketones and aldehydes, the diazotization of aromatics, and the solvolysis of alkenes. In most cases, an optimum C 0 pressure effect on reaction rate was found. This can generally be attributed to a tradeoff between increased acid concentration and decreased acid dissociation with C 0 pressure. It is not certain how many acid-catalyzed processes could be performed using alkylcarbonic acids. Our successes to date have focused on reactions involving the formation of diazonium or carbonium ions or of protecting groups such as acetals. It is evident that alkylcarbonic acids are strong enough to replace mild acids such as carboxylic acids (the /?Ka's are similar) and will suffice for some mineral acid catalyzed processes. However, alkylcarbonic acids are unlikely to be a suitable replacement for applications requiring strong acids, such as Friedel-Crafts alkylations, unless the starting materials are suitably activated. The full potential of these benign acids has yet to be elucidated. 2

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References

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Corma, A. Solid acid catalysts. Curr. Opin. Solid State Mater. Sci. 1997, 2, 63. Dartt, C. B.; Davis, M. E. Ind. Eng. Chem. Res. 1994, 33, 2887.


144 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21.

Hallett, J. P.; Kitchens, C. L.; Hernandez, R.; Liotta, C. L.; Eckert, C. A. Acc. Chem. Res. 2006, 39, 531. Jessop, P. G.; Subramaniam, B. Chem. Rev. 2007, 107, 2666. Eckert, C. Α.; Liotta, C. L.; Bush, D.; Brown, J. S.; Hallett, J. P. J. Phys. Chem. Β 2004, 108, 18108. Eckert, C. Α.; Bush, D.; Brown, J. S.; Liotta, C. L. Ind. Eng. Chem. Res. 2000, 39, 4615. Wyatt, V. T.; Bush, D.; Lu, J.; Hallett, J. P.; Liotta, C. L.; Eckert, C. A. J. Supercrit. Fluids 2005, 36, 16. Li, H.; Maroncelli, M . J. Phys. Chem. Β 2006, 110, Shukla, C. L.; Hallett, J. P.; Liotta, C. L.; Eckert, C. Α.; Popov, Α. V.; Hernandez, R. J. Phys. Chem. Β 2006, 110, 24101. Hallett, J. P.; Pollet, P.; Liotta, C. L.; Eckert, C. A. Acc. Chem. Res., 2008, in press.. Musie, G.; Wei,M.;Subramaniam, B.; Busch, D. H. Coord. Chem. Rev. 2001, 219-221, 789. West, Κ. N.; Wheeler,C.;McCarney, J. P.; Griffith, Κ. N.; Bush, D.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. A 2001, 105, 3947. Weikel, R. R.; Hallett, J. P.; Levitin, G. R.; Liotta, C. L.; Eckert, C. A. Top. Catal. 2006, 37, 75. Weikel, R. R.; Hallett, J. P.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 2007, 46, 5252. Chamblee, T. S.; Weikel, R. R.; Nolen, S. Α.; Liotta, C. L.; Eckert, C. A. Green Chem. 2004, 6, 382. Xie, X.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 2004, 43, 2605. Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry, Brooks/Cole Publishing Company: Pacific Grove, CA, 1998 p.384. Buckley, Α.; Chapman, N . B.; Dack, M . R. J.; Shorter, J.; Wall, H. M . J. Chem. Soc. Β 1968, 2, 631. Salvatore, R.N.; Flanders, V. L.; Ha, D.; Jung, K. W. Org. Lett. 2000, 2, 2797. Towes, K. L.; Shroll, R.; Wai, C. M.; Smart, N . G. Anal. Chem. 1995, 67, 4040. Wen, D.; Olesik, S. V. Anal. Chem. 2000, 72, 475.


Gas-Expanded Liquids and Near-Critical Media - American Chemical

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