Article pubs.acs.org/jced
Adsorption and Desorption of Isoflurane on Carbonaceous Adsorbents and Zeolites at Low Concentrations in Gas Phase Roman Ortmann,*,† Christoph Pasel,† Michael Luckas,† Ruben Heimböckel,∥ Sebastian Kraas,∥ Jürgen Bentgens,§ Michael Fröba,∥ and Dieter Bathen†,‡ †
University of Duisburg-Essen, Lotharstraße 1, D-47057 Duisburg, Germany IUTA Institute of Energy and Environmental Technology, Bliersheimer Straße 58-60, 47055 Duisburg, Germany § Klinikum Duisburg gGmbH, Zu den Rehwiesen 9-11, 47055 Duisburg, Germany ∥ Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany ‡
S Supporting Information *
ABSTRACT: This paper presents adsorption isotherms and desorption data of isoflurane from a carrier gas (nitrogen) on different adsorbents at 25 °C and 1 bar. As adsorbents activated carbons, newly developed carbon adsorbents and dealuminated zeolites were used. The adsorption of isoflurane was studied in trace level concentrations up to 1200 ppmV. Common isotherm equations were fitted to the measured data. The adsorption isotherms show distinctly different capacities depending on the polarity and the pore structure of the adsorbent. The investigation of desorption reveals weak physical interactions between isoflurane and the surface of most adsorbents.
to remove isoflurane.6−9 Because water is more polar than fluranes, flurane adsorption on zeolites from humid exhaled air will be more efficient if the zeolite polarity is reduced by dealumination. With declining aluminum content dealuminated zeolites are more hydrophobic and often reach high isoflurane capacities even at high air humidities.10 However, with very low aluminum content the surface might be not polar enough to efficiently bind the polar isoflurane. Using activated carbon as an adsorbent in respiratory protection systems requires a combination with hydrophilic zeolites to previously adsorb water from the exhaled air. At present no data are available to characterize trace level flurane adsorption on any commercially available adsorbent. Against this background a systematic study of adsorption capacities of isoflurane on activated carbons, newly developed carbon adsorbents, and dealuminated zeolites was carried out. The properties of the adsorbents were varied to describe the influence of surface chemistry and pore geometry on adsorption. Additionally desorption experiments were done to characterize the type of interaction between the adsorbent’s surface and the isoflurane. All experiments were done at 1 bar and 25 °C with isoflurane concentrations varying from 50 ppmv to 1200 ppmv.
1. INTRODUCTION Isoflurane belongs to the chemical group of fluranes, which are halogenated ethers. Globally, isoflurane is one of the most important inhalational anesthetic along with desflurane and sevoflurane.1 Several thousand tons of these fluranes are applied on humans and animals every year. In addition to its narcotic effect, isoflurane has a high greenhouse potential.2 During anesthesia, isoflurane is evaporated and mixed with air to supply a concentration in the range of 2−7 vol %. The mixture is then delivered by an inhalation mask. Because only a small proportion of isoflurane is absorbed by the patient, the largest part remains in the exhaled air and must be removed by masks equipped with adsorptive cleaning systems. However, the room air is permanently contaminated with flurane through small leakages and after removal of the mask in the recovery room. Even trace contaminations may lead to symptoms of fatigue on medical staff if applied over longer periods of time.3 The MAC values of isoflurane in European countries are in the range of 10−50 ppmV.4 Activated carbon filters which are used for many adsorptive applications are ineffective to remove trace concentrations of fluranes from exhaled air. Activated carbon cannot efficiently adsorb fluranes at humidity in excess of 60%, where water blocks the pore system due to capillary condensation.5 Therefore, specially designed adsorbents are required for respiratory protection of medical staff. In respiratory protection systems, dealuminated zeolites as well as combinations of zeolites and activated carbons are used © 2015 American Chemical Society
Received: October 3, 2015 Accepted: November 26, 2015 Published: December 17, 2015 686
DOI: 10.1021/acs.jced.5b00844 J. Chem. Eng. Data 2016, 61, 686−692
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2. EXPERIMENTAL SECTION
The dealuminated zeolites with ceramic binder of types BEA and MFI were supplied by Clariant AG (Bruckmü hl, Germany). The zeolites differ significantly with regard to pore geometry and module. The pores of the types BEA and MFI have a tunneled structure, whereas the zeolite 13X is made of cages. 2.2. Experimental Plant. The experimental plant consists of two coupled magnetic suspension balances (MSB) (Rubotherm GmbH, Bochum)16,17 and a gas chromatograph mass spectrometer (GC-MS, QP 2010, Shimadzu) (see Figures 1 and 2). The magnetic suspension balances have a resolution of 1 μg and a standard deviation of ±4 μg. The liquid isoflurane is vaporized from a diffusion tube suspended in the first magnetic suspension balance (diffusion cell) and dosed into a carrier gas flow. The dosing rate can be determined by the mass loss of the diffusion tube over time. The desired concentrations are obtained by mixing with nitrogen. The diffusion tube consists of a reservoir (V = 1.8 cm3) and a diffusion capillary. The geometry is displayed in Figure 3. The gas mixture is led to the second magnetic suspension balance containing the adsorbent in a suspended sample container at given temperature (sorption cell). The adsorbed mass of isoflurane is gravimetrically determined by the mass change of the adsorbent over time. As an option, the gas flow from the sorption cell can additionally be analyzed by a gas chromatograph mass spectrometer, which is useful in experiments with more than one adsorptive. The exhaust gases of the plant are oxidized in an electrically heated postcombustion chamber. 2.3. Experimental Procedure and Analysis. The experimental procedure comprises the adsorbent activation (a), the adsorption process (b), and the desorption process (c) (see Figure 4 and Table 3). The adsorbent was thermally activated at 150 °C in a nitrogen flow until a constant mass was measured. Then the system was cooled to 25 °C over several hours. During this step, the diffusion cell and the sorption cell were separated. The diffusion cell was cooled to 10 °C to provide a constant diffusion flux of isoflurane. Next, the cells were connected to pipe the gas mixture from the diffusion cell to the adsorption cell and start the adsorption step. Thermodynamic equilibrium is reached as soon as mass constancy of the sample is detected. In trace level adsorption tests, equilibration may be a very slow process, which requires an experimental criterion to define the equilibrium state of the sample in the plant. Equilibrium was assumed as soon as the difference of two mass averages (each averaged over 15 min) at an interval of 60 min was less than 1% of the first measured average. After equilibration the adsorptive concentration was increased to measure the next equilibrium. This was repeated until the isotherm was measured in the concentration range from 50 to 1200 ppmV in a cumulative way.
2.1. Substances. The isoflurane was obtained from Baxter Healthcare Corp. (Deerfield, IL, U.S.A.) and had a purity of >99.9%. Isoflurane is a polar molecule with a molecular dipole moment of 2.15 D and molecular diameter of 5.2 Å. These values were estimated by a quantum mechanical calculation.11,12 The boiling point of isoflurane at 1 bar is 48.5 °C and the vapor pressure at 25 °C is 440 mbar. Table 1. Characteristic Data of Zeolites
manufacturer
name
type
pore diameter/ nm
Clariant Clariant Clariant Clariant Clariant Clariant Zeochem
HCZP800E HCZP200E HCZP27E HCZB150E HCZB25E FeCZB25E Z10−03
MFI MFI MFI BEA BEA BEA 13X
0.64 0.64 0.64 0.67 0.67 0.67 0.74
BET surface/m2 g‑1
module (nSiO2/ nAl2O3)
307 342 366 584 478 467 687
800 230 27 126 25 25 2
The adsorbents used in this study are listed in Table 1 and 2. The module of the zeolites is the molar ratio of SiO2/Al2O3 provided by the manufacturers. BET surfaces were determined by nitrogen sorption at 77 K. Pore size distributions were calculated from nitrogen isotherms by nonlinear density functional theory (NLDFT) assuming cylindrical pores for zeolites and slit pores for activated carbons. The adsorbents D47/3 Extra and C40/4 Extra are activated carbons made of steam activated hard coal and provided by CarboTech GmbH (Essen, Germany). The adsorbent CMK-3 was synthesized at the University of Hamburg using a template method.13,14 In the first step, the silica molecular sieve SBA-15 was impregnated with sucrose and heated to 900 °C under vacuum to completely carbonize the sucrose. The remaining silica template was then removed from the silica carbon composite at ambient temperature by stirring with hydrofluoric acid (5 wt %). The microporous high surface carbon adsorbent (MHSC) was synthesized by University of Hamburg as published by Zheng and Gao.15 A phenol formaldehyde resin with 1.5 mL of tetraethylorthosilicate was carbonized and activated in a single step at 750 °C for 1 h in the presence of potassium hydroxide as a catalyst and chemical activation reactant. The remaining silica was removed at ambient temperature by stirring with hydrofluoric acid (5 wt %). The adsorbent Z10-03 is a customary zeolite of type 13X obtained from Zeochem AG (Uetikon, Switzerland). In spite of this zeolite’s typical hydrophilicity, which presumably renders it unsuable for isoflurane adsorption in a humid atmosphere, it is used as reference zeolite. Table 2. Characteristic Data of Carbonaceous Adsorbents manufacturer
name
type
pore diameter/nm
BET surface/m2 g−1
Hamburg University Hamburg University Carbotech GmbH Carbotech GmbH
MHSC CMK-3 D47/3 Extra C40/4 Extra
microporous high surface carbon adsorbent mesoporous carbon adsorbent commercially available activated carbon commercially available activated carbon
0.4−1.6 (see Figure 5B) 4.1 (see Figure 5B) 0.5−4 (see Figure 5A) 0.5−2.2 (see Figure 5A)
1275 1191 980 1200
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DOI: 10.1021/acs.jced.5b00844 J. Chem. Eng. Data 2016, 61, 686−692
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Figure 1. Flow diagram of the experimental plant.
Figure 2. Experimental plant encased in glovebox18
During the desorption steps, the sorption cell was purged with dry nitrogen and the adsorbent temperature was raised stepwise from 25 to 150 °C. Each desorption step lasted 10 h. The measured data of the magnetic suspension balances, the volume flow meter, and the pressure controller have small statistical errors. For the adsorbent load, a relative error of 5% was determined by a series of replicate measurements.
Figure 3. Engineering drawing of diffusion tube (left) and photo of diffusion tube (right).
D47/3 Extra is significantly higher. For medium and higher concentrations both isotherms have a similar slope. This leads to a higher specific capacity of D47/3 Extra compared to C40/4 Extra over the complete concentration range. The highest loading at small concentrations was found for the purely microporous MHSC. At medium and higher concentrations the load of MHSC does not increase anymore. For concentrations larger than 1000 ppmV the capacity of D47/3 Extra exceeds that
3. RESULTS AND DISCUSSION 3.1. Adsorption and Desorption of Isoflurane on Carbonaceous Adsorbents. Figure 5 shows the adsorption isotherms of isoflurane on the carbonaceous adsorbents. Comparing the isotherms of the activated carbons D47/3 Extra and C40/4 Extra it is evident that the initial slope of 688
DOI: 10.1021/acs.jced.5b00844 J. Chem. Eng. Data 2016, 61, 686−692
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where qc is the loading at equilibrium concentration ceq, K is the Freundlich coefficient, and n is the Freundlich exponent. The model parameters are given in Table 4. Table 4. Isotherm Model Parameters of Isoflurane on Carbonaceous Adsorbents name
isotherm model
qmon/μmol·m−2
b/ppmV−1
R2
MHSC
Langmuir isotherm model
2.341
0.0329
0.94
name CMK-3 D47/3 Extra C40/4 Extra
Freundlich Freundlich Freundlich
K/μmol·m−2·ppmV−n
n/(unitless)
R2
0.259 0.612 0.225
0.248 0.189 0.295
0.99 0.99 0.99
The activated carbons D47/3 Extra and C40/4 Extra differ with respect to the pore size distribution in the mircoporous and mesoporous range (see Figure 6a). The activated carbon
Figure 4. Standard sorption test procedure of isoflurane on a zeolite with activation (a), adsorption (b), and desorption (c) segments.
Table 3. Segments of the Standard Test Procedure segment (see Figure 4)
step
(a)
activation
nitrogen
(b)
adsorption
(c)
desorption
various concentrations of adsorptive nitrogen
gas mixture
temperature/ °C
duration/ min
heating: 150 cooling: 25 25
500
25, 75, 120, 150, 250
600/step
until equilibrium
Figure 5. Isoflurane isotherms on carbonaceous adsorbents at 1 bar and 25 °C.
of MHSC. The mesoporous CMK-3 provides the lowest isoflurane capacity. The isotherm of MHSC was fitted to the Langmuir model (see eq 1) qc = qmon
Figure 6. Pore size distribution plot of (a) D47/3 Extra and C40/4 Extra and (b) MHSC and CMK-3.
b·ceq 1 + b·ceq
D47/3 Extra has a substantially higher pore volume below 1 nm, whereas for pore sizes >1 nm, C40/4 Extra exhibits a higher pore volume. The higher intital slope of D47/3 Extra and the larger total capacity can be attributed to the higher pore volume in the size range 0.5−1 nm, which is accessible for isoflurane with a diameter of 0.52 nm. In the very narrow slit pores, the isoflurane can interact with both surfaces, which leads to a strong bonding. Despite the higher pore volume of C40/4 Extra in the size range >1 nm, there is no difference in the isotherm slopes at medium and higher concentrations. Apparently, isoflurane adsorption in the larger micropores and
(1)
where qc is the loading at equilibrium concentration ceq, qmon is the monomolecular loading according to Langmuir, and b is the Langmuir coefficient. The isotherms of the other adsorbents have a continuous slope and thus were fitted to the model of Freundlich (see eq 2) n qc = K ·ceq
(2) 689
DOI: 10.1021/acs.jced.5b00844 J. Chem. Eng. Data 2016, 61, 686−692
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In the experiments, quick desorption was found for adsorbents with a small proportion of micropores. These results are indicative of reversible physisorption with rather weak interactions like dispersion interactions and interactions of the type dipole−induced dipole between isoflurane and the carbonaceous adsorbents. Desorption from micropores as mainly in case of MHSC and C40/4 Extra proceeds more slowly because of slow diffusion in the narrow channels. Additionally, the bottleneck effect may prevent a quick desorption of isoflurane from mesopores included by microporous structures.19,20 3.2. Adsorption and Desorption of Isoflurane on Zeolites. Figure 8 depicts the isoflurane isotherms on zeolithes
the mesopores is of minor importance compared with adsorption in the small micropores. This is seen as an indication of rather weak interactions between the isoflurane and the surface of the activated carbons. These results are confirmed by the experiments with the tailor-made carbon adsorbents from the University of Hamburg. The adsorbent MHSC reaches the highest capacity of the carbonaceous adsorbents at very small concentrations because it has only micropores
