Room Temperature Ionic Liquids as Solvent Media for the Photolytic

Chapter 15

Room Temperature Ionic Liquids as Solvent Media for the Photolytic Degradation of Environmentally Important Organic Contaminants Downloaded by COLUMBIA UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch015

Qiaolin Yang and Dionysios D. Dionysiou* Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221-0071 *Corresponding author: [email protected]

The photolytic degradation of an organic contaminant, naphthalene, in 1 -buty1-3-methylimidazolium hexafluorophosphate has been investigated. The original parent compound could be degraded in this ionic liquid using 253.7 nm UV radiation. Oxygen and the purity of ionic liquid with respect to UV light absorbing compounds had a significant effect on the transformation rate of naphthalene. The addition of oxygen to the naphthalene ring and the rupture of the ring due to direct photolytic effect are discussed as possible reaction pathways that contributed to the transformation of the compound. Intermediate product identification using Electrospray TOF MS, and GC-MS revealed the formation of bicycle[4,2,0]octa-l,3,5-triene, 2'hydroxyacetophenone, and phenol among the stable intermediate products. With the use of non-toxic ionic liquids and further optimization and modification, this process has the potential for further development into a two-step process for extraction of organic pollutants from solid matrices, such as contaminated soils or dredged sediments, using ionic liquids followed by in-situ photodegradation of the organic contaminants in the ionic liquid extractant phase with simultaneous regeneration of the ionic liquids.

182

© 2005 American Chemical Society

In Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

183

Downloaded by COLUMBIA UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch015

Introduction As a new generation of solvents, room temperature ionic liquids (RTILs) have attracted increasing research interest among chemists and engineers during the last few years. A distinct advantage of RTILs is their lack of detectable vapor pressure. As a result, they are currently considered as a promising replacement to volatile organic compounds (VOCs) which are a source of a number of environmental pollution problems. Development of ionic liquids has experienced two different chronological periods. Chloroaluminate-based ionic liquids,frequentlyreferred to as the first generation of RTILs, are a mixture of organic chlorides and aluminum chlorides. These ionic liquids are generally reactive with water and their handling requires an environment with exclusion of moisture. Therefore, the use of such ionic liquids will not be practical in open to the atmosphere environmental and other applications. Since these ionic liquids exhibit high electrical conductivity, enlarged electrochemical window and enhanced thermal stability, they were initially used as electrolytes in high-energy batteries that were sealed and had no contact with the atmosphere. (/) Successful synthesis of the second generation of RTILs in the early 1990s substantially expanded the applications of RTILs. These second generation RTILs share the advantages of their predecessors but also remain stable when exposed to water and air. (1-3) In addition, by fine-tuning their structure, they can be designed to satisfy specific task-specific applications. (4, 5) Since the development of these water and air stable RTILs, the number and diversity of applications of RTILs increased dramatically. Currently they are studied extensively in various applications dealing with electrochemistry, chemical synthesis, catalysis, and liquid-liquid separations. (/-//) However, in the field of photochemistry, ionic liquids received relatively less attention. Only a few photochemical reactions in ionic liquids have been studied to date. Photooxidation of iron(II) diimine complexes to iron(III) diimine complexes in an ionic liquid consisting of aluminum chloride and ethylpyridinium bromide (2:1 mole ratio) is the first of such published processes. (12) This reaction gave a ferric yield of approximately 100%. The proposed reaction mechanism was that ethylpyridinium cations could accept electrons to form the corresponding radicals and subsequent dimerization of these species resulted in the formation of a colored compound. Later, photolysis of anthracene (An) (13) and 9-methylanthracene (9CHaAn) (14) conducted in l-ethyl-3-methylimidazolium chloride


184 (EMIC)/A1C1 ionic liquids indicated that reactions in basic (55 mol % EMIC) and acidic (45 mol % EMIC) solvents generated different products. Irradiation of An in the basic ionic liquid yielded the dimer formed via 4+4 cycloaddition as in conventional solvents. In the acidic ionic liquid, the result was substantially altered due to the participation of HC1 into the reaction. HC1 was present as impurity in the solvent. Irradiation of 9-CH An in the basic ionic liquid yielded 4+4 dimer as major product and six minor products not previously detected in conventional solvents. Investigation of reaction mechanism indicated that the six minor products were formed via electron transfer from the excited state 9-CH An to the solvent cations followed by a series of subsequent reactions. N-Butylpyridinium chloride/AlCb, an ionic liquid containing better oxidizing cation than EMIC, showed again that the solvent cations could act as electron acceptors, which supported the proposed reaction mechanism. In addition to behaving as electron acceptors, ionic liquids can participate in reactions as hydrogen donors in the presence of highly reactive species. It was found that the triplet excited state of benzophenone ( Bp*) in imidazoliumbased ionic liquids was able to abstract an alkyl chain H atom from the cations of the solvents and produced benzophenone ketyl radical. (15) The activation energy required to initiate the reaction was significantly high, and the reaction rate of Η-abstraction was one order of magnitude lower than that observed in conventional solvents. The reaction might have involved a large change in geometry, which altered the electrostatic interaction between the solvents ions and resulted in raising the activation energy. Although ionic liquids are generally more stable than common solvents, this result indicates that they are not inert under certain extreme conditions. Another study concerning amine mediated photoreduction of benzophenones in imidazolium-based ionic liquids showed that benzhydrol was the only detectable product. (16) This result was in contrast to those observed in non-imidazolium-based ionic liquids and conventional solvents where benzpinacol was the only product. The requirement of imidazolium cation for the formation of benzhydrol suggested that the solvent cation was involved in the reaction mechanism. (16) Besides participating directly in the chemical reactions, ionic liquids can influence the reactions physically. It has been reported that the high viscosity of the ionic liquids could decrease the reaction rates of the bimolecular processes in the photolysis of 9-CH An. (14) In other diffusion controlled photochemical processes of molecular couples such as the quenching of the phosphorescence of 3

3


3

3

3


185 3

Bp* by naphthalene as well as photoinitiated Diels-Alder cyclization reaction between 0 and diphenylbenzofuran (DPBF), the high viscosity of the ionic liquids resulted in higher activation energies. (//) The effect of viscosity on the reactions was also illustrated in the electron transfer process between the ruthenium tris(4,4'-bipyridyl)/methylviologen ([Ru(bpy) ] /MV ) couple in 1 -butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF ]). (17) Although the solvent anions were able to reduce the electrostatic repulsion between the two reaction cations via a charge screening effect and thus to facilitate the reaction, the formation of the encounter complex was diffusion controlled and was significantly affected by the viscous ionic solvent. As a result, the overall reaction rate of the forward electron transfer from [Ru(bpy) ] to M V was not enhanced compared to those in water and acetonitrile. On the formation of the final products ([Ru(bpy) ] and MV ), the electrostatic repulsion between [Ru(bpy) ] and M V ' within the encounter complex could be counteracted by the anions of the solvent, and the diffusion of the products could not compete with the back electron transfer from MV" to [Ru(bpy) ] . Consequently, this should give low cage escape efficiency. However, the latter process involved significant molecular reorientation that was limited by the viscous and significantly ordered environment and was thus the rate-determining step. It was suggested that the influence of the ionic liquid on the back electron transfer and final product formation was of similar extent. Hence, the alteration in the cage escape yield was not significantly different than those in less viscous solvents. The increase of entropy of the system implied that the formation of the encounter complex was accompanied with structure-breaking involving solvent ion freeing. More recently, (i) the energy transfer between xanthone triplet and naphthalene, (ii) hydrogen transfer between xanthone triplet and diphenylmethane, (iii) quenching of singlet excited state 2,4,6triphenylthiopyrylium ion (TPTP ) by neutral, negative and positive quenchers, namely, biphenyl, I" and Co , and (v) quenching of anthracene triplet by methylviologen (MV ) in [bmim]PF illustrated that these diffusion controlled processes were about two orders of magnitude slower than those in conventional solvents due to the higher viscosity of ionic liquids. It was also found that the lifetime of triplet excited state of TPTP was one order of magnitude longer than that in the conventional solvents. (18) These studies provided critical information on the photochemical properties of ionic liquids and helped to better understand other chemical reactions conducted in these solvents. To explain all these results, it is essential to fully understand the nature of ionic liquids. It is also important, however, to further explore the potential of RTILs in new applications. Ionic liquids are capable of solubilizing a number of organic compounds to high concentrations. !

2

2+

2+

3

6


2+

2 +

3

3+

,+

3+

3

3

+

+

3+

3

+

2+

2+

6

+



186 (4) In some applications, this may be very beneficial. For example, in photodegradation reactions following first or higher order kinetics, increasing the initial concentration of reactants over a certain range will result in higher reaction rates. This is impractical in some cases when water is used as solvent because the solubility of some substances in water is very low. A good example is that of polycyclic aromatic hydrocarbons (PAHs), which are sparingly soluble in water. In general, the solubility of the PAHs in water decreases as the number of aromatic rings increases. Another concern is the environmental implications of VOCs. The use of VOCs has created major environmental problems due to their volatility. The search for replacements to these solvents is critical for the sustainable development. To be a candidate, ionic liquids need to be fully assessed from all aspects, particularly for their toxicity. While more studies are required to fully examine the environmental implications of ionic liquids, it is worthwhile to investigate such solvents in applications that utilize some of their key advantages. In this study, a representative ionic liquid, [bmim][PF ], is examined as an alternative reaction medium for the photodegradation of naphthalene, a relevant environmental pollutant. This ionic liquid was selected because it has been involved in a number of published research studies concerning RTILs. PAHs are environmental contaminants of great health concern due to their carcinogenic potency. They can be released into the environment as byproducts of the incomplete combustion of fossil fuels and by industrial waste discharge. Natural processes, such as volcanic eruptions, are another source for the formation of PAHs. U.S. EPA is regulating certain PAHs (i.e., benzo(a)-pyrene) and has listed some others (i.e., naphthalene, anthracene and pyrene) as priority pollutants. (19) Among PAHs, naphthalene has the simplest structure. We thus selected this compound as a probe to investigate the potential of photodegrading PAHs in ionic liquids. This experimental approach can be further developed into a two-step process for the remediation of soils or dredged sediments contaminated by organic compounds such as PAHs, polychlorinated biphenyls (PCBs), and pesticides. The first step is extraction of the organic pollutants from the solid matrix using ionic liquids as extractants. The subsequent step is the in-situ photodegradation of the pollutants in the ionic liquid phase with simultaneous regeneration of the extractant phase. Here, we report the photodegradation of naphthalene in [bmim][PF ]. The influence of the impurities present in the ionic liquid and the role of oxygen on the reaction rates were also investigated. 6

6


187 Experimental Naphthalene (99+%) was obtained from Aldrich and used as received. [bmim][PF ] (97%, w/w) was purchased from Sachem, Inc. (Austin, TX). This ionic liquid was either used as received or purified with activated carbon. The purification was achieved using FILTRASORB 400 activated carbon obtained from Calgon Carbon Corp. (Pittsburgh, PA). The activated carbon was washed with water and then dried at 105 °C overnight before use. It has been reported that ionic liquids can absorb moisture from atmosphere, and that equilibrium can be reached given enough time. (20, 21) Aqueous and ionic liquid solutions of naphthalene were prepared by dissolving the compound in double deionized water (18 ΜΩ) and [bmim][PF ]» respectively. Photodegradation was conducted in an approximately 20 mL cylindrical quartz photoreactor with reaction space of 5 mL. Two opposite UV-C sources generated predominantly 253.7 nm UV radiation. The solutions were mixed with a magnetic stirrer during the photodegradation process. To investigate the effect of oxygen on the reactions, the ionic liquid containing naphthalene was first bubbled with pure oxygen (dry; water content 1 ppmv) or nitrogen (dry; water content 3 ppmv) for 1 hour. The solution was subsequently irradiated with UV-C radiation while oxygen or nitrogen bubbling was continued during the irradiation. It should be noted that the 2 hours of bubbling could result in less than 1% loss of naphthalene concentration in the ionic liquid when the initial concentration was 1.11 mM. In water, 1 hour bubbling could reduce the naphthalene concentration to zero when the initial concentration was 0.16 mM. The concentrations of naphthalene in both water and ionic liquid were quantified using a Series 1100 HPLC (Agilent) equipped with a reverse phase amide column (RP-16 Discovery Supelco) and a UV-Vis Diode Array Detector. Water samples were injected directly. Ionic liquid samples were dissolved in acetonitrile prior to injection into the HPLC. The mobile phase used was a mixture of acidic water (0.01N sulfuric acid) and acetonitrile. The flow rate of the mobile phase was 1.5 mL/min. Identification of intermediates was conducted using both Q-TOF II MS (Waters) and HP 6890 series GC-MS (Hewlett-Packard) equipped with a Zebron ZB-5 column (15mx0.25mm>

Room Temperature Ionic Liquids as Solvent Media for the Photolytic

Recommend Documents