Vol. 30. No.
Copyright 0 1991Pergamon
$5.00 + .OO Press
MICROCALORIMETRIC STUDIES OF SOLUTION ADSORPTION BY ACTIVATED CARBONS HELEN J. COUSINS,
PETER J. GARDNER
STEVE J. MATTHEWS
of Chemistry, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 OEX, U.K. (Received 26 April 1991; accepted in revisedform 18 June 1991)
Abstract-Flow microcalorimetry studies and the determination of solution adsorption isotherms for toluene solutions of triethylenediamine, quinuclidine, and triethylamine at 298.15 K with a commercial activated microporous carbon are reported. The results clarify the mechanism of operation of activated carbon impregnants widely used in the nuclear power industry. Key WordsMicroporous
carbons, calorimetry, TEDA
carbons, usually impregnated with KI or a tertiary amine, are used worldwide to prevent the emission of radioisotopes from nuclear power plants into the environment[l]. Most of the carbons that are used are microporous with high specific surface areas (by B.E.T. N2 adsorption) and the surfaces are extensively contaminated with oxygen-containing functional groups. This surface contamination and the residual ash content (- 10 mass%) arise from the steam activation of the raw material (coal). The efficiency of activated carbon filter beds used in the LJ.K. Advanced Gas-cooled Reactor systems is assessed in terms of their ability to trap “‘I, in the form of CH,I. The efficiency of these filter beds is affected by ageing, surface oxidation, and the presence of pre-adsorbed water. At high relative humidities, often approaching 100% in the gas coolant systems, the pores in the carbon become saturated with water and drastically reduce trapping efficiency. However, this effect has been partially overcome by impregnating the carbon surface with KI, which takes up the radioactive iodine either by chemisorption of CH,I or by isotopic exchange of ‘,‘I for 12’1 from the KI impregnant. Unfortunately, the KI-impregnated carbon appears to deteriorate rapidly when exposed to high relative humidities. In recent years the organic amine 1,4-diazabicyclo[2,2,2]octane (more commonly referred to as triethylenediamine or TEDA) has been used as an alternative impregnant since, under ambient conditions, the rate of ageing is much reduced and so deterioration in trapping efficiency is much slower. Little is known about the nature of the interaction of TEDA with the carbon surface. This paper describes results of flow microcalorimetry and the determination of solution adsorption isotherms of TEDA solutions in equilibrium with a commercial microporous carbon. Corresponding results for quinuclidine and triethylamine are used for comparison. Activated
2.1 Materials Carbon: A commercial activated coal-based carbon (Sutcliffe-Speakman 207A) supplied in the form of 8-12 mesh granules was used. Automated volumetric adsorption (Carlo Erba Strumentazione Sorptomatic 1800) of N2 at 77 K yielded a type I isotherm exhibiting minimal desorption hysteresis, a specific surface area of 900 rn’g-’ (0 < pip” < 0.35) and a specific pore volume of 0.575 cm’g-‘. Mercury porosimetry (Carlo Erba Strumentazione unit 120) indicated the macroporous specific surface area was ca. 20 rn’g-’ with a mean pore dimension = 0.5 km on a log normal distribution. Temperature programmed desorption studies[2,3] show the presence of a significant surface oxygen content. The material has an ash content of 9.85%, of which the major components are Si, Fe, Al, and 0 as (probably) oxides and aluminosilicates. For our experimental studies, carbons were pretreated by heating to 5565°C in a vacuum (< 0.03 mmHg) for ca. 18 hours, during which time between 10% to 12% adventitious water was lost. Toluene: BDH AnalaR grade or M & B Pronalys grade was used and was dried by passing down a 1.0-m x 2.5-cm column containing activated 3 8, molecular sieves. It was de-aerated and stored under N, before use. Amines: Both TEDA and quinuclidine (Aldrich Chemical Company) were purified by vacuum sublimation. The sublimate was stored under dry Ar or N,. The TEDA (and probably quinuclidine) readily converts to an N-oxide in air or aerated solution (a white insoluble precipitate deposits). Triethylamine (TEA) was purified by distillation from KOH pellets, collecting the fraction boiling at ambient pressure between 89”-90°C. 2.2 Apparatus and procedure Flow microcalorimeter: A commercial instrument (LKB 2017A) operated at 25°C in the flow adsorp-
H. .I. COUSINS
tion mode was used. A stainless steel cell, fitted with stainless steel frits with 30 p,m pores, was packed with the carbon (- 0.2 g) and contained in the calorimeter thermostat. Toluene was passed at 10.1 cmgh-’ over the carbon from a Hamilton syringe (50 cm9) driven by a syringe pump (Raze1 model A99). The thermal output (due to frictional effects, detector imbalance, etc.) was monitored with a strip chart recorder until stable. The flow was then switched externally to a dilute solution of the chosen solute in de-aerated toluene. As the solute was adsorbed preferentially by the carbon, a signal proportional to the rate of heat transfer was recorded and the net thermal output was proportional to the area beneath the power-time curve. Calibration was by electrical substitution. The cumulative method was used to construct enthalpy of displacement vs. concentration curves. Such enthalpies of displacement are accompanied by a heat of dilution which occurs when a new liquid composition enters the cell. This effect comprises: a) dilution due to the preferential adsorption of the solute and b) the dilution that occurs at the interface between the two solutions. Johnson, et a/.[61 have shown that the latter effect is dominant, and it was dete~ined in this work for the TEDAl toluene system in separate blank experiments with the cell part-filled with a teflon plug. The enthalpies of dilution that accompany mole fraction changes of 0.002 (from 0 to 0.002 and from 0.002 to 0.004) are of the order of 5 mJ; this is less than 0.5% of the enthalpy of displacement measured in the presence of carbon. The effect is likely to be the most significant for the TEDA/toluene mixtures and consequently all enthalpies of dilution were ignored in the analysis of the results. Solution adsorption isotherms: Surface excess amounts were measured in a pur~se-built equilibration cell thermostatted at 25°C. About 1.5 g of the carbon was equilibrated in the presence of ca. 25 cm” of solution agitated with a magnetic stirrer. The supernatant liquid was circulated via a peristaltic pump through a vibrating tube densitometer (Paar model DMA55, precise to -t 0.000005 gem-)). The solution density was used with a suitable calibration graph to measure the equilibrium concentration of the solution; additional adsorbate (amine) was introduced via a side arm. The amount adsorbed was expressed in terms of the specific surface excess, np, obtained from eqn (1). a; zz n&i where n, vent, xi xi is the the mass
- x’;) m
is total number of moles of solute and solis the initial mode fraction of the solute, final mole fraction of the solute and m is of carbon. 3. RESULTS AND DISCUSSION
The enthalpies of displacement (Ahf-I,,)are plotted vs. concentration in Fig. 1 for t~ethylenediamine
am5 o.um OF SOLUTEx2
Fig. 1. Enthalpies of displacement of toluene by triethylenediamine (Cl), quinuclidine (A), and triethylamine (0) vs. mole fraction of amine. (TEDA), quinuclidine (QUIN), and triethylamine (TEA) in toluene at 25°C. In each case, the isotherm displayed a steeply rising initial portion where amine and toluene are competing for the surface active sites. At higher concentrations, the amines will be competing for less favourable polar sites, forces between adsorbent and adsorbate will be weaker and hence the enthalpy of displacement decreases steadily producing a shallow plateau region. The TEA generated a less steep rise to the plateau region than the other two amines with plateaux forming 3 11.5 Jg-’ carbon (QUIN), 10.5 Jg-’ carbon (TEDA), and 5.5 Jg-’ carbon (TEA); the plateau was ill-defined for QUIN. A feature of these calorimetric results was that discontinuities were noted at certain mole fractions of TEDA or TEA. The cumulative method was used to obtain these enthalpies of displacement; with this method there is no desorption between successive changes in mole fraction, and the thermal response gets steadily smaller at higher concentrations. At certain mole fractions, desorption was noted in place of the expected adsorption; this effect was reproducible at the same mole fractions with fresh carbon samples, and was not an artefact. The effect is clearly demonstrated in Fig. 1, which shows the curves not to be smoothly monotonic. The mole fractions at which the effect occurs are 0.003,0.011, and 0.030 for TEDA and 0.011 for TEA. The interpretation of this effect is not clear. Certain critical con-
19 Microcalorimetric studies on activated microporous carbons plots for the amine-microporous carbon systems used in this work. These surface energy curves display several interesting features. The TEDA is bound more strongly to the carbon surface than triethylamine or quinuclidine, the difference being most marked at low surface coverage. This is not due to basicity as TEDA (PK, = 5.2 and 9.8) is a weaker base than either QUIN (pK, = 3.4) or TEA (pK, = 3.2); values of pK, are quoted at 2O”C. The TEDA interaction energy at low coverage is far in excess of that associated with physisorption, and the ready formation of N-oxide (or di-N-oxide) supports a chemisorptive interaction with loosely-bound surface oxygen or oxygen functionality. The N-O bond in amine N-oxides is quite strong, about 300 kJ molV’ in pyridine N-oxide, and does not appear to change significantly in other molecular environments. The overall shape of the curves (i.e., convex towards the surface coverage axis and decaying to values comparable with condensation energies at higher surface coverage) is characteristic of a heterogeneous surface[ lo]-both structurally heterogeneous and in terms of energy distribution. The sharp fall in interaction energy at very low surface coverage is o.om o.omo o.oo7s o.omo 0.0126 o.ol5o o.m75 (Law difficult to explain; it is not well-defined for QUIN MOLE FRACTION OF SOLUTE x,, and does not exist for TEA. Because of the very d
Fig. 2. Specific surface excess VS. mole fraction at 25°C for toluene solutions of triethylenediamine (O), quinuclidine (A), and triethylamine (0).
centrations of adsorbate may precipitate either some endothermic rearrangement of the adsorbate on the carbon surface, not accompanied by a change in specific surface excess, or a surface reorganisation which initiates a small desorptive effect. Figure 2 shows the toluene solution adsorption isotherms for the three amines. The amine was preferentially adsorbed in each case (i.e., n: > 0, but for the two amines that are solid at room temperature (TEDA and QUIN) a limiting value (- 200 umol go ‘) was reached at a mole fraction = 0.02; the solubility limit for the amines in toluene at 25°C is about mole fraction 0.05. Dividing the enthalpies of displacement, AH&?), at a given mole fraction by the specific surface excess, n;(x,) at the same mole fraction, gives a differential enthalpy of displacement, h&,), per mole of component 2. In this work 2 is taken as the preferentially adsorbed component (the amine) and 1 is taken as toluene. Hence h&J
A plot of hz against nq reveals the changing adsorbate-adsorbent interaction energy as a function of surface coverage-this plot may be loosely described as a “surface energy spectrum.” Figure 3 shows such
Fig. 3. Differential surface
enthalpy of displacement VS. specific excess for toluene solutions of triethylenediamine (O), quinuclidine (A), and triethylamine (0).
COUSINS ef ai.
steep rise in the AH,, vs. xz curves (see Fig. l), particularly for QUIN, the low coverage points on the hz VS.np graph (see Fig. 3) were obtained by interpolation on a straight line joining the origin and the first point on the AH,, vs. x2 curve; points so constructed are joined by dotted lines in Fig. 3. Only in the case of TEDA is the sharp decrease in low surface coverage interaction energy unambiguous. It is important to establish that the magnitude of this unusual effect lies outside the range of experimental error in the measurements. The proportional random error in the enthalpies of displacement varies with mole fraction of solute and is = zt 10% at x2 =: 0.0001 decreasing to + 1% at x2 2 0.004. For measurements of specific surface excess the corresponding uncertainty is ~10% at all mole fractions in our experimental range. These combined errors give absolute uncertainty intervals of + 17 kJmol-’ in hl (at n; = 50 pmol g-‘) and 2 7 kJmoll’ (at ny = 150 prnol g-l). This confirms that the anomalous low differential enthalpies noted for TEDA at Iow surface coverage are a real effect and not due to random error in exFerimenta1 measurement. From Fig. 2 it is clear that the limiting specific surface excess for TEDA occurs at a mole fraction = 0.02. At this mole fraction, the specific surface excess of TEA is twice that for TEDA; this suggests that TEDA may be bound to the surface by both N atoms. The effect of the 10% ash content present in the carbon is not clarified by these results. It has been reported[ll] that both the 13’1trapping efficiency and
the specific surface area of the carbon are markedly dependent on the ash content in the 8 to 15 mass% range, becoming independent at higher ash content. Further studies, with high and low ash fractions, are in hand to explore the effect of ash content on amine adsorption. Acknowledgements-The authors are indebted to the S.E.R.C. and National Power ptc (formerly CEGB) for financial support, and to Dr. Biilinge (now deceased) and Drs. Bowles and Mellor for helpful discussion. REFERENCES 1. F. Kepak, J. Radioanal. Nucl. Chem. 142,215 (1990). 2. B. H. M. Billinge and M. G. Evans, .r. de chimie physique 81, 31 (1984). 3. B. H. M. Billinge, J. B. Docherty, and M. J. Bevan, Carbon 22, 83 (1984). 4. A. Farkas, R. L. Mascioli, F. Miller, and P. F. Strohm, J. Gem. Erzg. Data 13, 278-283 (1968). 5. G. H. Findenegg, C. Koch, and M. Liphard. In Adsorption From Solution (Edited by R. H. Ottewill, C. H. Rochester, and A. L. Smith), p. 87. Academic Press, London (1983). 6. I. Johnson, R. Denoyel, J. Rouquerol, and D. H. Everett, ColIoi~ and Surfaces 49, 133 (1990). 7. D. D. Perrin. In i~n~atio~ constantsof organic acids and bases in aqueous solution (compiled by D. D. Perrin), 2nd ed., Pergamon Press, Oxford (1982). 8. Li Shaofeng and G. Pilcher, J. Chem. Therm. 20,463 (1988). 9. W. E. Acree, J. J. Kirchner, and S. A. Tucker, 1. Chem. Therm. 21, 443 (1989). 10. A. C. Zettlemoyer, Ind. Eng. Chem. 57,27 (1965). 11. B. H. M. Billinge and R. Bowles, Central Electricity Research Laboratories. Unpublished internal report (1985).