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Langmuir 2001, 17, 8188-8192
Thermodynamics and Kinetics of Adsorption of Gaseous Single Cl/Br-VOCs of the Ethane Series onto Siliceous ZSM-5 at 25 °C. Prediction of the Adsorption Selectivity in the Gas Phase Marie-He´le`ne Simonot-Grange* and Be´ne´dicte Garrot Laboratoire de Recherches sur la Re´ activite´ des Solides, UMR 5613, Universite´ de Bourgogne-CNRS, BP 47870, 21078 Dijon Cedex, France Received June 14, 2001. In Final Form: October 8, 2001 This work deals with a thermodynamic and kinetic experimental study of adsorption of gaseous single Cl/Br-VOCs (1,2-dichloroethane, 1-bromo-2-chloroethane, and 1,2-dibromoethane) onto siliceous ZSM-5 at 25 °C. The adsorbed amounts, adsorption heats, and diffusivities were simultaneously obtained. The isotherms were of the type I and verified Henry’s law in a very large domain of filling. The adsorption heats and the diffusivities of the brominated molecules displayed specific behaviors probably owing to a transport resistance. The comparison of the relative position of isotherms and the adsorption heats allowed us to predict that the competitive adsorption of their mixture would be similar to a distillation. The breakthrough curves verified this prediction.
1. Introduction As a general part of a study about the air depollution by adsorption, a fundamental study for better knowledge of the adsorption process of VOCs (volatile organic compounds) onto zeolites was started. After previous works related to C1/C2-chlorohydrocarbons adsorption onto dealuminated faujasite Y and siliceous ZSM-5 zeolites at 25 °C,1-4 this paper deals with a thermodynamic and kinetic experimental study of adsorption of single chlorinated/ brominated VOCs of the ethane series (1,2-dichloroethane, DCA; 1-bromo-2-chloroethane, BCA; and 1,2-dibromoethane, DBA) onto siliceous ZSM-5 at 25 °C. In the literature, the adsorption studies do not deal with the brominated VOCs contrary to chlorinated VOCs. These works are only concerned with X-R diffraction.5 It was then of interest to study the behavior of the substitution of chlorine atoms by bromine atoms. The knowledge of the adsorption properties of single VOCs was analyzed to predict the competitive adsorption of their mixture. 2. Experiment and Methodology The siliceous ZSM-5 (DAZ Degussa, Si/Al > 500) was obtained by direct synthesis and formed into full pellets (L 2 mm) containing about 20% of clay binder. The crystals have the typical coffin-shape form with a length of 8-10 µm according to the well-known morphology of this material (Figure 1). A specific experimental device comprising a Calvet-type differential-flow calorimeter coupled with a volumetric line (manometry) allowed us to obtain simultaneously the adsorbed amounts, the adsorption heats, and the intraparticular diffusivities of gaseous single VOCs, and a fixed-bed column packed with zeolite was used to study the coadsorption of their mixture from breakthrough curves. The experiments were realized at 298 K in the range of pressure 10-1 to 1000 Pa. These apparatus * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Clause, B.; Garrot, B.; Cornier, C.; Paulin, C.; Simonot-Grange, M.-H.; Boutros, F. Microporous Mesoporous Mater. 1998, 25, 169. (2) Garrot, B.; Simonot-Grange, M.-H.; Clausse, B. Stud. Surf. Catal. 1999, 125, 683. (3) Clausse, B. Thesis, Universite´ de Bourgogne, Dijon France, 1997. (4) Garrot, B. Thesis, Universite´ de Bourgogne, Dijon France, 2000. (5) Mentzen, B. Mater. Res. Bull. 1992, 27, 831.
and methodologies were described previously.1,6 However, we must bear in mind that, in the calorimeter, the zeolite was evacuated before adsorption for 5 h in situ, under 10-1 Pa at 300 °C. The experiments were then performed by successive adsorptions of small amounts of gaseous VOC. This device gave only the adsorption branch of the isotherm. Also, isotherms were drawn by TGA under controlled vapor pressure7 to verify the reversibility. In the fixed-bed column, the zeolite was not activated under vacuum, but with an air counter-current flow around 100 °C by reason of device. Preliminary experiments of activation under air and under vacuum as well as adsorption of water vapor, as a function of temperature, showed that the outgassing (2% weight of water) was practically total at this temperature. On the other hand, experiments of coadsorption from samples activated under vacuum at 300 °C showed the same behaviors.4 The VOCs were injected directly into the carrier gas (atmospheric air under 1.2 × 105 Pa) with an automated syringe system and their concentrations were measured at the inlet and outlet of the column by using a gas chromatograph. Flushing the adsorber with an air counter-current flow at about 100 °C regenerated the adsorbent. As the increments of the adsorbed amount were sufficiently small ( 1-bromo-2-chloroethane > 1,2-dichloroethane. Acknowledgment. The Conseil Re´gional de Bourgogne, the Agence de l’Environnement et de la Maıˆtrise de l’Energie (ADEME), sponsored this work. We thank them for their precious assistance and also the Degussa A.G. Society for the supply of the DAZ zeolite. Nomenclature
Figure 7. Breakthrough curves of a DCA/BCA/DBA mixture adsorbed onto ZSM-5.
zeosolubilities vary opposed to the volatilities of compounds and the adsorption heats in the same manner as the vaporization heats (Tables 1 and 5). The breakthrough curves of gaseous mixtures of three VOCs (Figure 7) verify this prediction: the order of breakthroughs occurs from the most volatile compound to the less volatile one and the compound with the higher boiling point displaces from zeolite the compound with the lower boiling point (Table 1). 5. Conclusion The thermodynamic and kinetic study of the adsorption of single chlorinated/brominated VOCs of the ethane series onto the siliceous ZSM-5 at 25 °C showed that specific behavior (adsorption heats and diffusivities) of the brominated compounds probably accounted for a transport or surface resistance owing to a structural constraint of the molecules themselves during adsorption of the first molecule per cavity unit. The thermal effect of this transport resistance was estimated at 8 kJ mol-1 per bromine. The origin of this behavior asks for comple-
L, l, h, m ) length, width, height, mass R ) ratio of the adsorbed amount at time t to the total adsorbed amount at equilibrium r ) radius of adsorbent crystal D0 ) corrected intraparticular diffusivity Na, Na0 ) adsorbed amount, maximum adsorbed amount in molecules per 1/4 of unit cell (molecules/1/4 u.c.) θ ) filling coefficient defined as Na/Na0 W0 ) maximum adsorbed volume per gram of activated zeolite (without binder) calculated from the molar density of liquid VOC p0, s, ∆H°vap, Tb ) vapor pressure, hydrosolubility, and vaporization normal enthalpy at 298 K, normal boiling temperature of VOC θf0 | ) absolute value of real adsorption derivative |∆H˙ ads enthalpy extrapolated at zero filling |∆H˙ θf0 ext | ) absolute value of assumed adsorption derivative enthalpy extrapolated at zero filling ∆H˙ tr ) endothermic effect involved in the transport θf0 ˙ ads | resistance |∆H˙ θf0 ext | - |∆H max |∆H˙ ads | ) maximum adsorption derivative enthalpy max Imax ˙ ads | - |∆H˙ θf0 aa ) |∆H ext | Cout/Cin ) ratio of the VOC concentration at the outlet to the one at the inlet of the column Vinj, d ) total volume of VOC injected in the column per gram of adsorbent, gas flow rate LA0109011
