Hydrogenation Reactions in Supercritical CO2 Catalyzed by Metal

Chapter 27

Hydrogenation Reactions in Supercritical CO Catalyzed by Metal Nanoparticles in a Waterin-Carbon Dioxide Microemulsion

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 13, 2015 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch027

2

Mariko Ohde, Hiroyuki Ohde, and Chien M. W a i

*

Department of Chemistry, University of Idaho, Moscow, I D 83844-2343

Water-in-CO microemulsions with diameters in the order of several nanometers are prepared by a mixture of A O T and a PFPE-PO4 co-surfactant. The CO microemulsions allow metal species to be dispersed in the nonpolar supercritical C O phase. B y chemical reduction, metal ions dissolved in the water core of the microemulsion can be reduced to the elemental state forming nanoparticles with narrow size distribution. The palladium and rhodium nanoparticles produced by hydrogen reduction of Pd and Rh ions dissolved in the water core are very effective catalysts for hydrogénation of olefins and arenes in supercritical CO 2

2

2

2+

3+

2.

© 2003 American Chemical Society

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

419

420 In the past two decades, supercritical fluid extraction technology has attracted considerable attention from chemists and engineers for its potential applications as a green solvent for chemical processing (1-3). One difficulty of using this green solvent for metal dissolution is that metal ions are not soluble in supercritical C 0 because of the charge neutralization requirement and the weak interactions between C 0 and metal ions. Converting metal ions to C0 -soluble metal chelates utilizing organic chelates is one method of extracting them into supercritical C 0 (4, 5). This in situ chelation/supercritical fluid extraction method developed in the early 1990s by Wai and coworkers has been widely used today by many researchers for extracting metal species into supercritical C0 (6-/2). A new development in dissolution of metal species in supercritical C 0 is based on the observation that metal ions can be stabilized in a water-in-C0 microemulsion (13-19). The water-in-C0 microemulsions with diameters typically in the order of several nanometers (nm) allow metal species and high polarity compounds to be dispersed in nonpolar supercritical C 0 . This type of microemulsion may be regarded as a new solvent for dissolution and transport of metal species in supercritical fluids. It has also been shown that the water-inC 0 microemulsion can be used as a reactor for chemical synthesis of nanometer-sized materials in supercritical fluids. The first report on the synthesis of nanometer-sized metal particles using a water-in-C0 microemulsion appeared in 1999 (14). In this report, Wai and co-workers showed that silver ions in the water core of a water-in-supercritical C 0 microemulsion could be reduced to nanosized metallic silver particles by a reducing agent dissolved in the fluid phase. The silver nanoparticles can be stabilized and dispersed uniformly in the supercritical fluid phase by the microemulsion for an extended period of time (15). Further reports from our research group indicate that by mixing two microemulsions containing different ions in the water cores, exchange of ions can take place leading to chemical reactions (17, 18, 20). These reports suggest the possibility of utilizing the water-in-C0 microemulsions as nanoreactors for synthesizing a variety of nanoparticles in supercritical C 0 . More recently, using metal nanoparticles synthesized in the C 0 microemulsion as catalysts for hydrogénation reactions was reported (21, 22). This new development opens the door for a wide range of catalysis utilizing microemulsion dispersed nanoparticles in supercritical fluids. This article describes our recent results of metal nanoparticle synthesis using the water-in-C0 microemulsion as a template and their potential applications as catalysts for chemical reactions in supercritical C 0 . 2

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Synthesizing Metal Nanoparticles In C 0 Microemulsion 2

There has been much interest in recent years to exploit the properties of microemulsion phases in supercritical fluids (23-33). A reverse micelle or microemulsion system of particular interest is one based on C 0 because of its minimum environmental impact in chemical applications. Since water and C 0 2

2


421 are the two most abundant, inexpensive, and environmentally compatible solvents, the application of such a system could have tremendous implications for the chemical industries *of the 21 century. Reverse micelles and microemulsions formed in supercritical C 0 allow highly polar or polarizable compounds to be dispersed in this nonpolar fluid. However, most ionic surfactants with long hydrocarbon tails, such as sodium bis(2-ethylhexyl) sulfosuccinate (AOT), are insoluble in supercritical C O 2 . Nonionic surfactants have a greater affinity for C 0 . However, they have little tendency to aggregate and take up water due to weak electrostatic interactions of the head groups. The use of ionic surfactants with fluorinated tails provides a layer of a weakly attractive compound covering the highly attractive water droplet cores, thus preventing their short-range interactions that would destabilize the system. However, our experiments indicate that these C0 -philic ionic surfactants do not form very stable water-in-C0 microemulsions especially when the water cores contain high concentrations of electrolytes. A recent communication shows that using a conventional A O T and a fluorinated cosurfactant, a very stable water-inC 0 microemulsion containing a relatively high concentration of silver nitrate can be formed (14-18). st

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Using a mixture of A O T and a perfluoropolyether (PFPE) surfactant the resulting water-in-C0 microemulsion is stable with W (ratio of water/AOT) values >40 at 40 °C and 200 atm according to the experiments conducted recently in our laboratory. The perfluorinated surfactant obtained from Ausimont has a general structure of C F 0 [ O C F ( C F 3 ) C F ] n ( O C F 2 ) O C F C H O C H C H 0 - P O ( O H ) and an average molecular weight of 870. The initial experiments for synthesizing silver nanoparticles used a mixture of A O T (12.8 mM) and the perfluoropolyether-phosphate (PFPE-PO4, 25.6 mM) at W = 12. The microemulsion containing A g N 0 (0.33 mM) was optically transparent and stable in supercritical C 0 for hours. To make metallic silver nanoparticles, a reducing agent such as N a B H ( O A c ) or N a B H C N was injected into the superciritcal fluid microemulsion system to reduce Ag* in the water core to A g (14-16, 18). The formation of A g nanoparticls in the microemulsion system was observed within a minute after the introduction of the reducing agent. The formation and stability of the A g nanopartices was monitored in situ by U V - V i s spectroscopy utilizing the 400 nm band originating from the surface plasmon resonance of nano-sized A g crystals. The supercritical fluid solution remained optically clear with a yellow color due to the absorption of the A g nanoparticles. Further spectroscopic studies indicate that the microemulsions are dynamic in nature. Ionic species in the water core of the microemulsion obviously can interact effectively with molecular species dissolved in the fluid phase. Using the same approach copper nanoparticles can be synthesized (15, 18). In a recent communication, Ohde et al. showed the synthesis of palladium nanoparticles by hydrogen reduction of P d ions dissolved in the water core of a C 0 microemulsion (18, 21). The Pd nanoparticles so produced are uniformly dispersed in the supercritical fluid phase and are stable over an extended period of time long enough for catalysis experiments. Reduction of a 2

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m

422 metal ion to its elemental state in supercritical CO2 using hydrogen gas is a simple, clean, and effective method for producing nanometer-sized metal particles in the microemulsion. This method is particularly attractive for studying hydrogénation reactions in supercritical C 0 because H gas is miscible with C 0 and can serve both as a reducing agent for metal nanoparticles formation as well as the starting material for subsequent hydrogénation. The advantages of performing hydrogénation reactions in supercritical C 0 compared with conventional solvent systems are known in the literature (3, 4, 33). High solubility of hydrogen gas and enhanced diffusion in supercritical C 0 relative to conventional solvent systems often result in faster and more efficient processes in the supercritical fluid phase. In addition, tunable solvation strength of supercritical C 0 , easy separation of solvent from products, and minimization of waste generation are other attractive features of conducting chemical synthesis in supercritical C 0 . 2

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Metal Nanoparticle Catalyzed Hydrogénation Reactions The hydrogénation of olefins in supercritical carbon dioxide catalyzed by palladium nanoparticles synthesized in a water-in-C0 microemulsion was reported by Ohde et al (27). The Pd nanoparticles were prepared by hydrogen reduction of P d ions (a P d C l solution) dissolved in the water core of the microemulsion. Effective hydrogénation of both C0 -soluble olefins (4methoxycinnamic acid and trans-stilbene) and a water-soluble olefin (maleic acid) catalyzed by the palladium nanoparticles in the microemulsion was demonstrated. The hydrogénation of 4-methoxy cinnamic acid to 4-methoxy hydrocinnamic acid catalyzed by the Pd nanoparticles was performed first in liquid C 0 at room temperature (20 °C) (Equation 1). The spectra shown in Figure l a were taken at 20-second intervals after the injection of hydrogen and 4-methoxy cinnamic acid into the water-in-C0 microemulsion with P d C l in the water core (W = 20). The first spectrum obtained immediately after the injection (spectrum 1) was identical to that of 4-methoxy cinnamic acid dissolved in C 0 . The broad absorption peak centered around 300 nm decreased gradually and a new absorption peak centered around 270 nm appeared. After about 2 minutes, the absorbance at 300 nm dropped to the baseline level. The absorption peak (270 nm) in spectrum 2 (Figure la) was consistent with that of 4-methoxy hydrocinnamic acid. In the absence of P d C l in the microemulsion, the absorption peak of 4-methoxy cinnamic acid did not show a measurable decrease after the injection of the olefin and hydrogen into the fiber-optic reactor. Also, in the absence of hydrogen, injection of 4-methoxy cinnamic acid into the reactor with P d C l in the water core of the microemulsion did not show any change of absorption at 300 nm either. Figure l b shows the decrease in the 2

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423


C

1 < 230

380

280 330 Wavelength (nm)

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1

(b)

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Reaction time (sec) Figure 1. (a) Variation ofUV-Vis spectra of 4-methoxy cinnamic acid with time during the hydrogénation process in CO2 at 20 °C and 200 atm. Each spectrum was taken at 20-second intervals starting from zero time, spectrum 1. (b) Variation of the 4-methoxy cinnamic acid absoption at 300 nm with time at 20 "C and 200 atm ( a),35 "C and 200 atm ( 4) and at 50 °C and 200 atm ( 99 % within 30 minutes in supercritical C 0 at 50 °C and 200 atm. The Pd nanoparticles synthesized in the C 0 microemulsion are effective for hydrogénation of C0 -soluble and water-soluble olefins but are not effective for hydrogénation of aromatic compounds. Hydrogénation of arenes is conventionally carried out with heterogeneous catalysts. Bonilla et al. recently reported a Rh catalyzed hydrogénation of arenes in a water/supercritical ethane biphasic system (55). Hydrogénation occurred well in this biphasic system with excellent results obtained for a number of arenes after 62 hours of reaction 2

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425 times. However, this approach did not work for a water/supercritical C 0 biphasic system according to the authors. We have recently explored the possibility of making rhodium nanoparticles in a water-in-C0 microemulsion using the hydrogen gas reduction method. To our surprise, the Rh nanoparticles in the microemulsion are capable of catalyzing hydrogénation of arenes in supercritical C 0 with good efficiencies (22). Naphthalene was selected as a C0 -soluble arene for the hydrogénation study because it absorbs in the U V region that could be monitored in situ by the fiber optic cell. Figure 2 shows the variation of the U V - V i s spectra of naphthalene with time after the injection of hydrogen and naphthalene into the fiber optic cell containing the R h ions dissolved in the water core of the C 0 microemulsion at 50 °C and 240 atm. In this experiment, the amount of naphthalene injected was 1.4 xlO" mmol which is in large excess relative to the amount of Rh in the system. About 5 minutes after the injection, the absorbance of naphthalene in the reaction cell was reduced to about one half of the initial value. The absorbance was decreased to near background level after 20 minutes. After one hour, the system was depressurized and the materials in the C 0 phase were collected in CDC1 for N M R measurements. The N M R results indicated that nearly all of the original naphthalene (> 96 %) injected into the reactor were reacted to form tetralin (Equation 2). 2

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2

3 +

2

1

2

3

(2)

Hydrogénation of a water soluble arene (phenol) was also studied using the Rh nanoparticles formed in a water-in-C0 microemulsion as a catalyst. In this case, phenol (3.3x10'* mmol) was dissolved in the water core of a microemulsion together with R h ( 1 . 7 x l 0 mmol). The microemulsion was made of 18.2 m M A O T , 36.4 m M PFPE-PO4 and W = 30. The hydrogénation experiments were carried out at 50 °C and 240 atm with 10 atm of H gas injected into the microemlsion system. Five minutes after the injection of hydrogen, the fluid phase was expanded into a CDC1 solution for N M R analysis. The *H-NMR spectra showed that phenol was not detectable in the solution (Figure 3). The major product was cyclohexanone according to the N M R spectra with minor amounts of cyclohexane also presented in the spectrum (Equation 3). Rhodium catalyzed hydrogénation depends on the formation of the catalyst and the solvation environment. A possible reaction route for the hydrogénation of phenol using Rh nanoparticles in the C 0 microemulsion is probably by the addition of two moles of hydrogen to form cyclohexene- l-ol, which undergoes tautomerization to form cyclohexanone. Hydrogénation of cyclohexanone to cyclohexanol does not proceed in this reaction system according to the report by Ohde et al. Formation of cyclohexane as a minor product was also observed by hydrogénation of phenol with R h (III) in 1, 2dichloroethane (35). More studies are needed in order to understand the 2

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1.9 -,


J

J

0.9 0.4 -0.1

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300 Wavelength (nm)

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Figure 2. Variation of UV-Vis spectra of naphthalene with time during hydrogénation process in C0 at 50 °C and 240 atm. Each spectrum was taken at zero time (spectrum 1), 10 s, 1 min, 5min,10 min, 20 min and 60 min (spectrum 2) from top spectrum to bottom spectrum, respectively. (Reproduced with permission from reference 22. Copyright 2002 Royal Society of Chemistry.) 2

(a)

Cyclohexane Cyclohexanone

(b) CDCI

3

ppm

^Phenol

AOT

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1

Acetone x

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Figure 3. NMR spectra of (a) the products collected from the phenol hydrogénation experiment in (b) control experiment (phenol, AOT, PFPE-P04 (ind Hi gas) (Reproduced with permission from reference 22. Copyright 2002 Royal Society of Chemistry. )

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427 mechanisms involved in hydrogénation of phenol and other arenes catalyzed by Rh nanoparticles formed in the CO2 microemulsion. OH Ο Rh nanoparticle / \

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Hydrogenation Reactions in Supercritical CO2 Catalyzed by Metal

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