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J. Phys. Chem. B 2010, 114, 11430–11435
Clathrate Hydrates for Ozone Preservation Sanehiro Muromachi,† Ryo Ohmura,† Satoshi Takeya,‡ and Yasuhiko H. Mori*,† Department of Mechanical Engineering, Keio UniVersity, Yokohama 223-8522, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan ReceiVed: June 1, 2010
We report the experimental evidence for the preservation of ozone (O3) encaged in a clathrate hydrate. Although ozone is an unstable substance and is apt to decay to oxygen (O2), it may be preserved for a prolonged time if it is encaged in hydrate cavities in the form of isolated molecules. This possibility was assessed using a hydrate formed from an ozone + oxygen gas mixture coexisting with carbon tetrachloride or xenon. Each hydrate sample was stored in an air-filled container at atmospheric pressure and a constant temperature in the range between -20 and 2 °C and was continually subjected to iodometric measurements of its fractional ozone content. Such chronological measurements and structure analysis using powder X-ray diffraction have revealed that ozone can be preserved in a hydrate-lattice structure for more than 20 days at a concentration on the order of 0.1% (hydrate-mass basis). 1. Introduction Ozone (O3) is chemically unstable at high concentrations, rapidly decaying to diatomic oxygen (O2). Besides the wellknown role of naturally occurring ozone in the Earth’s stratosphere, there are vast applications of artificially generated ozone in industry, e.g., bleaching substances,1,2 sterilizing municipal drinking water3 or sterilizing perishables,4,5 etc. Recently, the utility of ozone has been extended to the semiconductor industry concerning, for example, various cleaning or surface-conditioning processes and a process of forming ultrathin oxidized films on silicon wafers.6,7 The major inconvenience in using ozone in such applications is the lack of appropriate means for storing and/or transporting ozone. The only means currently available for storing and/or transporting ozone is to dissolve freshly formed ozone in liquid water or to freeze the water thus ozonated in the form of ozonated ice.8 However, the initial ozone concentrations in ozonated water actually measured in previous studies are only of the order of 10 or 1 ppm (see, for example, refs 8b, c, and 9-12). Moreover, ozone molecules rapidly decompose into oxygen molecules in the ozonated water. The halflife of ozone dissolved in ozonated water at a temperature of ∼20 °C is typically 20-30 min.13 The latter disadvantage of ozonated water may be reduced, though not overcome, by freezing it and storing it in the form of ice at a sufficiently low temperature.14 The idea for storing ozone in a clathrate hydrate at a concentration much higher than that available using ozonated water or ice was first presented by McTurk and Waller15,16 in 1964, but it has been overlooked until recently. Clathrate hydrates (abbreviated as hydrates hereafter) are crystalline solid compounds consisting of water molecules hydrogen-bonded in the form of polyhedral cages with guest molecules encaged in them. Generally, each cage is either occupied by one guest molecule or left vacant. This fact indicates that unless the ozone molecules in separate cages in the hydrate structure can interact the O3-to-O2 reaction should not occur within the confines of * Author to whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +81-45-566-1522. Fax: +81-45-566-1495. † Keio University. ‡ AIST.
the hydrate. If a substantial proportion of a particular type of cage in the hydrate is occupied by ozone molecules, the ozone concentration in the hydrate should be higher than that in typical ozonated water or ice by 2 to 3 orders of magnitude. Thus, the idea of storing ozone in a hydrate is in principle very attractive. However, this idea still needs careful evaluation because, as outlined below, the nature of ozone-containing hydrates revealed in previous studies is very limited. McTurk and Waller15,16 reported data for the O3 + CCl4 double hydrate formed from pure liquefied ozone and carbon tetrachloride. The hydrate formation was performed by stirring two liquids, water and carbon tetrachloride presaturated by ozone, and seed crystals of (according to the authors’ description) a simple CCl4 hydrate17 or an O3 + CCl4 hydrate at a temperature around 0 °C and atmospheric pressure. McTurk and Waller15 determined, based on an X-ray diffraction analysis, that the formed hydrate was in the structure II (sII), which is known to be composed of unit cells each consisting of 136 water molecules configured into 16 pentagonal dodecahedron (512) cages and 8 hexakaidecahedron (51264) cages.18 They measured the volume (presumably converted to the condition of 0 °C and 101.325 kPa) of the gas evolved from the hydrate due to its dissociation to be up to 30 cm3 per gram of hydrate. If the evolved gas was pure ozone and the 51264 cages of the hydrate were fully occupied by carbon tetrachloride molecules, it turns out from the above gas-volume measurements that the hydrate formed by McTurk and Waller contained ozone up to 6.4 mass %. This means that ozone molecules occupied up to 37% of the 512 cages in the hydrate.19 The pure ozone initially bubbled into a liquid mixture of water and carbon tetrachloride may have partially decayed to oxygen through the subsequent dissolvingin-liquid, hydrate-forming, hydrate-dissociating, and gas-volume measuring procedures. However, no direct information is available as to how the preserved ozone fraction varied through these procedures. Besides the above-mentioned invention of McTurk and Waller,15,16 we find only two archived information sources on the artificial formation of an ozone-containing hydrate.20,21 In the patent application document, Masaoka et al.20 stated that an ozone-containing hydrate could be formed from an ozone +
10.1021/jp105031n 2010 American Chemical Society Published on Web 08/13/2010
Clathrate Hydrates for Ozone Preservation
Figure 1. Schematic of the experimental setup for forming ozonecontaining hydrates to be subjected to the preservation tests. This setup consists of (a) an oxygen cylinder, (b) a xenon cylinder, (c) an ozone generator, (d) a hydrate-forming reactor, (e) a data logger, (f) a Ptwire resistance thermometer, (g) a pressure gauge, (h) a PID-controlled heater, (i) an immersion cooler, (j) an ozone monitor, (k) a vacuum pump, (l) an exhaust pump, (m) an ozone decomposer, and (n) a magnetic stirrer.
oxygen gas mixture (containing ozone at ∼5 mol %) at a pressure of 2 MPa or above and a temperature of -3 °C or below. In practice, hydrates were formed by spraying water into a reactor charged with the above gas mixture and maintaining the reactor at 13 MPa and -25 °C. By analyzing the gas released from this hydrate while being decomposed, the ozone content in the hydrate was estimated to be 2.3 g/L (≈ 0.2 mass %). It was also described that the hydrate could be preserved for 10 days in a closed container conditioned at 13 MPa and -25 °C, without causing a substantial loss of its ozone content. No information on ozone preservation at other pressure-temperature conditions was provided. The third information source due to Vysokikh et al.21 outlined their qualitative experiment of hydrate formation from an ice film brought into contact, at a temperature increasing from 77 to 100 K, with a low-pressure (0.4 kPa) oxygen gas, which was performed to demonstrate the possibility of ozone hydrate formation in the Earth’s stratosphere. As summarized above, none of the previous studies examined the actual preservation of enclathrated ozone at nearly atmospheric pressure, which is our major concern in relation to the practical application of ozone-containing hydrates. In this article, we show, based on chronological measurements of the ozone content in each hydrate sample formed from an ozone + oxygen gas mixture and either carbon tetrachloride or xenon, that ozone can be preserved in the hydrate for over 10-20 days following its formation. 2. Experimental Section 2.1. Materials. The compounds used as the raw materials for forming the hydrates were deionized and distilled water, carbon tetrachloride of 99.5% (volume basis) certified purity (Junsei Chemical Co., Ltd., Tokyo, Japan), oxygen of 99.9+% (volume basis) certified purity (Japan Fine Products Corp, Kawasaki, Kanagawa Prefecture, Japan), and xenon of 99.999+% (volume basis) certified purity (Taiyo Nippon Sanso Corp., Tokyo, Japan, and Spectra Gases, Inc., West Branchburg, NJ, USA). Except for water, the above materials were used as received from their respective manufacturers.
J. Phys. Chem. B, Vol. 114, No. 35, 2010 11431 2.2. Apparatus for Hydrate Formation. Figure 1 shows a schematic of the experimental setup that we used to form the ozone-containing hydrates that were subjected to the preservation tests. The major portion of the apparatus was a vertically oriented, 500 cm3 (65 mm ID) reactor made of a borosilicate glass cylinder and flange-type stainless-steel lids. The reactor was immersed in a temperature-controlled bath of an aqueous ethylene-glycol solution. A Teflon-coated stirring bar was placed on the bottom lid of the reactor to provide, during each hydrateforming operation, the fluids inside the reactor with sufficient mutual mixing. A strain-gauge pressure transducer (VPMC-A4A-(-100 ∼ 1000)-1, Valcom, Inc., Toyonaka-shi, Osaka, Japan) was inserted into the reactor through the top lid, which allowed for measurements of the pressure inside the reactor with an uncertainty of (1.8 kPa. The ozone generator incorporated in the above setup was a dielectric-barrier-discharge-based machine (ED-OG-R4, EcoDesign Co., Ltd., Saitama Prefecture, Japan) that can generate a continuous supply of ozone up to 8 mol % from pure oxygen. To specify the ozone + oxygen gas mixture produced by the ozone generator, we employed an ozone monitor (PG-620HA, Ebara Jitsugyo Co., Ltd., Tokyo, Japan) to measure the ozone concentration in the mixture in the range up to 8.9 mol % with an uncertainty of (0.1 mol % based on ultraviolet absorptiometry. 2.3. Procedure of Hydrate Formation. The procedure of forming an O3 + O2 + CCl4 hydrate was commenced by charging the reactor with 50 g of water and 30 g of carbon tetrachloride. This mass proportion between the two liquids ensures that as the hydrate formation is completed water no longer remains in a liquid or solid-ice state, while some amount of carbon tetrachloride should remain in the liquid state. The reactor was then immersed in the bath temperature-controlled at 0.1 °C, flushed four times with pure oxygen at a pressure of 0.2 MPa to eliminate any residual air, evacuated again, and charged with the gas mixture generated by the ozone generator (a mixture of ozone and oxygen in the molar ratio of 8:92) until the pressure increased to 0.25 MPa. At this stage, a batch operation for forming a hydrate was started by driving the stirrer. As the pressure decrease inside the reactor due to hydrate formation almost ceased, the operation was interrupted. The residual gas inside the reactor was discharged through its top vent until the pressure decreased to 0.1 MPa,22 and then a fresh ozone + oxygen mixture was supplied from the ozone generator to the reactor to make the pressure recover to its initial level of 0.25 MPa. Such a gas-replacing operation and the subsequent batch hydrate-forming operation were repeated about 20 times through several days until the pressure no longer decreased during each batch operation. The hydrate thus formed in the reactor may be considered to be nearly in equilibrium with the ozone + oxygen gas mixture containing 8 mol % ozone under the pressure-temperature condition set inside the reactors0.25 MPa and 0.1 °C. The procedure of forming an O3 + O2 + Xe hydrate was slightly different from that for the O3 + O2 + CCl4 hydrate because, unless enclathrated, xenon is in the gaseous state, and the critical dissociation pressure for the former hydrate is higher than that for the latter hydrate. The reactor charged with 50 g of water was immersed in the bath controlled at 0.1 °C, then flushed four times by pure oxygen gas. The reactor was then charged with the gas mixture generated by the ozone generator until the pressure in the reactor increased to 0.20 MPa. Subsequently, xenon gas was supplied to the reactor to increase the pressure to 0.40 MPa. The first batch operation for forming a hydrate was immediately started by driving the stirrer. This
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Figure 2. Schematic of the apparatus used for ozone-preservation tests.
operation was interrupted when the pressure decay inside the reactor almost stopped. After discharging the residual gas inside the reactor until the pressure decreased to 0.20 MPa,22 a fresh ozone + oxygen mixture was supplied from the ozone generator to the reactor to make the pressure increase to 0.25 MPa, and then xenon gas was supplied to the reactor until the pressure increased to 0.35 MPa. The reactor was then used for the second batch hydrate-forming operation. Such a gas-replacing operation and the subsequent batch hydrate-forming operation were repeated 30-40 times until the pressure no longer decreased during the batch operation. The hydrate thus formed in the reactor may be considered to be nearly in equilibrium with the gas mixture of (ozone + oxygen) and xenon in the molar ratio of 1:2. 2.4. Ozone Preservation Tests. The mass of the O3 + O2 + CCl4 or O3 + O2 + Xe hydrate available by one hydrateforming operation in our experimental system was about 50-70 g. The formed hydrate was immediately crushed in a chilled vessel into particles ∼5 mm in linear dimension. These particles were then placed in a Pyrex test tube, 35 mm in diameter and 210 mm in height, which was immersed in a constanttemperature bath of an aqueous ethylene-glycol solution, leaving its top 40-50 mm exposed to air (Figure 2). To prevent water condensation from the air onto the hydrate, we inserted a Teflonfilm into the top portion of the test tube that allowed aeration between the inside and the outside of the test tube. The bath was controlled at a prescribed temperature in the range between -20 and 2 °C with a fluctuation less than (0.5 °C throughout each preservation test, which typically lasted 20 days.23 During the test, small samples (∼1-2 g each) were removed from the preserved hydrate at intervals of one day or longer. Each sample was subjected to an iodometric measurement for the ozone content. 2.5. Powder X-ray Diffraction Measurements. An O3 + O2 + CCl4 hydrate sample was subjected to powder X-ray diffraction (PXRD) measurements to confirm its crystallographic structure and, at the same time, to examine the fraction of condensed water and carbon-tetrachloride phases inevitably involved in the sample. The reason for our use of PXRD, instead of single-crystal diffraction,24,25 was a technical difficulty in forming single crystals suitable for diffraction measurements, particularly in such multiple hydrate-guest systems as those we dealt with in this study. The sample was finely ground in a nitrogen atmosphere at a temperature below 100 K and then top-loaded on a copper-made specimen holder. The loaded sample was exposed to Cu KR radiation generated by an Ultima III diffraction system (Rigaku Corp., Tokyo, Japan) in a parallelbeam alignment. Each measurement was performed in the θ/2θ scan mode with a step width of 0.02° at 93 K. The analyses of the crystal structure of the hydrate sample and its cage
Figure 3. Photographs of (a) an O3 + O2 + CCl4 hydrate and (b) an O3 + O2 + Xe hydrate, each filling the hydrate-forming reactor (top view of the reactor with its top lid removed just after completion of the hydrate formation) and (c) lumps of the O3 + O2 + CCl4 hydrate (right side) and an O2 + CCl4 hydrate (left side) just removed from a dry shipper, in which they were once stored at a cryogenic temperature (about -150 °C), and exposed to the room air controlled at -15 °C. Note the ultramarine-blue color of the ozone-containing hydrates. The O3 + O2 + CCl4 hydrate exhibits a deeper color than the O3 + O2 + Xe hydrate. The color of the O3 + O2 + CCl4 hydrate lumps in (c) had somewhat faded from that observed in (a) due to icing on their surfaces but was still in clear contrast to the whiteness exhibited by the O2 + CCl4 hydrate lumps placed on the left side of the former lumps.
occupancies by ozone and oxygen molecules were performed by the Rietveld method using the RIETAN-2000 program,26 and the resolved structures were visualized by the VESTA program.27 Because the measured diffraction patterns of the hydrate sample showed the coexistence of hexagonal ice Ih and solid CCl428 in addition to the O3 + O2 + CCl4 hydrate, a triplephase refinement was performed. For the Rietveld refinement, the rigid-body constraints were used to solve the disorder of the guest molecules. Additionally, the carbon in CCl4 was fixed at the center of a 51264 cage, and the gravity centers of the O3 and O2 molecules were fixed at the center of a 512 cage, consistent with the results of previous neutron diffraction studies of CCl4 and O2 hydrates.29,30 3. Results and Discussion 3.1. Characterization of Hydrate Samples. For both simplicity and safety in the experimental operations, we exclusively used an ozone + oxygen gas mixture (containing ozone at ∼8 mol %), instead of pure ozone, in forming the ozone-containing hydrates. This gas mixture was produced from a commercially available dielectric-barrier-discharge-based ozone generator at a pressure of 0.25 MPa. Either carbon tetrachloride or xenon was also used as a help guest, a secondary hydrate-guest substance with which the hydrate-forming pressure may be substantially reduced. That is, the hydrates that we prepared for our preservation tests were O3 + O2 + CCl4 and O3 + O2 + Xe ternary hydrates. The latter hydrate was estimated to be equilibrated with the gas mixture of (ozone + oxygen) and xenon in a molar ratio close to 1:2 (see section 2.3). Figure 3 shows typical pictures of the formed hydrates. Note the ultramarine-blue color of the ozone-containing hydrates. PXRD measurements (section 2.5) confirmed that the O3 + O2 + CCl4 hydrate crystals sII with a lattice constant of 17.2572(4) Å at
Clathrate Hydrates for Ozone Preservation
Figure 4. Time evolution of ozone concentration in an O3 + O2 + CCl4 hydrate (a) and an O3 + O2 + Xe hydrate (b), each preserved at atmospheric pressure (0.1 MPa) and a constant temperature (-20, -10, 0.5, or 2 °C). The error bar for each data point represents the uncertainty of the ozone-concentration measurement by iodometry.
93 K. The contents of ice Ih and solid CCl4 as impurities were estimated to be ∼15%. 3.2. Preservation Tests. Figure 4 summarizes the results of the preservation tests in the form of a time series of ozone-inhydrate concentration data obtained with the hydrate formed by each hydrate-forming operation. The major findings of the preservation tests are summarized as follows: (1) Ozone can be preserved in an O3 + O2 + CCl4 hydrate held at a temperature of -10 °C or below for more than 20 days, maintaining an almost constant concentration in the range from 0.15 to 0.4 mass %. This ozone concentration level is much less than that suggested by McTurk and Waller15 (6.4 mass %; see Part 1 of the Supporting Information) but still higher than those achieved by the ozonated-water or ozonated-ice technology by 2 to 3 orders of magnitude. The duration of ozone preservation with the hydrate (>20 days) is far longer than those available using ozonated water9-12 or ozonated ice.8c (2) At temperatures above 0 °C, the O3 + O2 + CCl4 hydrate melted away within several days (
