The ex- perimental affinity coefficients (&,) for the vapors, with respect to ... In the case of ASC whetlerite, the adsorption of both vapors was much greater than.
Carbon, 1977. Vol. IS. pp. 285-290.
Pergamon Press.
Printed in Great Britain
ADSORPTION OF CYANOGEN CHLORIDE AND HYDROGEN CYANIDE BY ACTIVATED AND IMPREGNATED CARBONS P. .I. REUCROFT and C. T. CAloUt Department
of Metallurgical
Engineering and Materials Science, University of Kentucky, U.S.A.
Lexington,
KY 40506,
(Received 31 January 1977)
Abstract-Equilibrium adsorption and desorption isotherm data at room temperature are reported for cyanogen chloride and hydrogen cyanide vapors on BPL grade activated carbon, ASC whetlerite and several ASB impregnated carbons. Isotherm data have been analyzed in terms of the Dubinin-Polanyi equation. The experimental affinity coefficients (&,) for the vapors, with respect to chloroform, were determined from the slope of the Dubinin-Polanyi plot. The relative adsorptive capabilities of the adsorbents were assessed by comparing the theoretical affinity coefficient (&,) with &,. & was estimated assuming that physical adsorption is the dominant adsorption process. Cyanogen chloride shows about 20% higher adsorption than predicted on BPL activated carbon and about 30% higher than predicted on the ASB impregnated carbons. Hydrogen cyanide shows about 30% higher adsorption than predicted on BPL activated carbon and about 45% higher adsorption than predicted on the ASB impregnated carbons. In the case of ASC whetlerite, the adsorption of both vapors was much greater than predicted and the isotherms displayed a levelling-off trend in the low pressure region indicating strong retention through chemisorptive interactions. No unique fi,, could be determined from the original isotherms. An attempt was made to separate the chemisorotion contribution from the total adsorption, and thus assess /3,, for the physical adsorption contribution.
1.
the predicted values, in comparison to the results obtained for phosgene on the same carbon samples[l]. In the case of hydrogen cyanide on ASC whetlerite impregnated carbon, the amount of hydrogen cyanide adsorbed at equilibrium in the low pressure region is more than ten times the amount predicted based on physical adsorption because of the dominant role of chemisorption. Two types of impregnated carbons have been investigated in addition to the BPL grade activated carbon studied earlier[2b]. ASC whetlerite was obtained by impregnating the activated carbon with CL?‘, Ag’ and Cr04’- ions in the presence of ammonia. In the ASB carbons, borate ion was substituted for the chromate ion in different mole ratios with respect to the copper, and silver was omitted. The studies on the ASB carbons were carried out to evaluate the effect on adsorptivity of replacing chromate ion, a strong oxidizing ion, with borate, an ion which is not subject to redox reactions.
INTRODUCTION
The adsorption studies were undertaken in order to obtain accurate isotherm data for several chemically reactive vapors adsorbed on activated and impregnated carbons and to assess the relative contributions of physical adsorption and chemisorption to the total measured adsorption. In a previous study, which dealt with adsorption of phosgene on several carbon adsorbents, it was shown that impregnating the carbon with certain metallic salts greatly enhances the adsorptive interaction through chemisorption and leads to an increased adsorptive capacity at low pressures[ I]. These data were analyzed in terms of the Dubinin-Polanyi equation [2]. Physical adsorption of a vapor on a (porous) adsorbent generally increases with increasing molecular size of the vapor, in the absence of molecular sieve and other geometric effects[3,4]. If two vapors have similar chemisorptive interactions with an adsorbent, it is anticipated that chemisorption will be more pronounced relative to physical adsorption in the case of the vapor of smaller molecular size. Proceeding in the sequence phosgene, cyanogen chloride, hydrogen cyanide, i.e. decreasing the molecular weight and size, the present study shows that chemisorptive effects on activated and impregnated carbons become more dominant relative to physical adsorption, as expected. The experimental affinity coefficients for cyanogen chloride and hydrogen cyanide with reference to chloroform on the carbon samples studied generally show greater deviation from
2. EXPERIMENTAL Experimental isotherms were measured in the gravimetric adsorption system described previously employing the same procedures for outgassing and adsorption and desorption isotherm determination[l]. Hydrogen cyanide was obtained from Airco Industrial Gases with a purity of 99.5%. Cyanogen chloride was obtained from Scientific Gas Products with a minimum purity of 99%. The vapors were further purified by vacuum distillation until vapor pressure at dry-ice temperature agreed with the literature value [5]. The activated carbon adsorbent was a Calgon ac-
iPresent address: Environmental Health and Sciences Center, Oregon State Univer&y. Corvallis, OR 97331, U.S.i\. 285
286
P. J. REUCROFTand C. T. CHIOU
tivated carbon, type BPL, 12-30 mesh, and was obtained from coal for general gas phase applications[ 1,2b]. It has an internal surface area of -1000 m*g-‘, having approximately 70-75% of the internal surface associated with pores less than 20 a in diameter. X-ray fluorescence analysis showed that the BPL activated carbon contains -0.2% by weight of Fe. The ASC whetlerite carbon was obtained by impregnating BPL activated carbon with an aqueous solution (12% NH,) buffered by 10.9% carbonate ion and containing 8.5% cupric copper 4.1% chromate ion and 0.3% silver nitrate[6]. The ASB carbons were impregnated in a similar manner except that boric acid was substituted for chromate ion and no silver ion was added. The four ASB impregnated carbons studied were obtained from solutions having a molar composition of H,BO, with respect to that of cupric copper of 0.1: 1, 1:1 and 2: 1. Analytical data for the carbons was presented previously [ 11. 3. RESULTSAND DISCUSSION
3.1 Cyanogen chloride Figures l-3 show typical isotherm plots for cyanogen chloride on BPL activated carbon, ASC whetlerite and ASB (lH3B03: 1) impregnated carbons, respectively, in the form of the Dubinin-Polanyi equation: log W=log Wo-ke2
departure from linear behavior. Similar departures from linearity in the low pressure region were found in the isotherms of CNCl on three ASB carbons. The nonlinearity is most likely due to weak chemisorptive effects. In the case of the BPL activated carbon it was 1.00 0.90 0.80
F
0.70 0.60 0.50 040
? 2
ASB carbons> BPL activated carbon, it can be concluded that the impregnants in ASC whetlerite (Cu’+, CrO.,and a trace of Ag’) produce a greater degree of chemisorptive interaction than those in the ASB carbons (CL?’ and H,BO,). The results indicate that the chromate
02
r
\” 010 2 009 g 008 007
1
006 [ 005' 004 003-
002-
00
0
’
2
‘1
4
6
”
8
”
”
‘1’
IO 12 14 16 18 20 22 24 E2XI0-6
Fig. 6. Hydrogen cyanide on BPL activated carbon 0, 22.2”, desorption (sample I); 8, 23”, second desorption isotherm after first run (0) was completed and sample I outgassed; A, 22.2”, adsorption (sample II).
289
Adsorption of cyanogen chloride and hydrogen cyanide
0.10 0.09 y 0.08 2 0.07 < 0.06
-moHb-o.e-o_
0.05 0.04 0.03
0.02
i
0
!
1
3
t
2
4
6
8
/
!
8
IO 12 _$4 CZXIO
t
16
I
I
18 20
/
J
0.01
22 24
Fig. 7. Hydrogen cyanide on ASC Whetlerite carbon 0, 22.5, desorption (sample I); 0, 22.5, adjusted isotherm from 0: El, 22.5, second desorption isotherm after first run (0) was completed and sample I outgassed; A, 23.3”, adsorption isotherm (sample II); 0, 23.5”, desorption isotherm (sample II); 0, 25”, desorption isotherm (sample III); @, 25”,adjusted isotherm from 8: El, 23”, second desorption isotherm after first run (8) was completed and sample III outgassed.
desorption isotherm in the low pressure region. Departures from linearity are observed in the low pressure region that are more marked than in the case of cyanogen chloride and phosgene on the same carbon. This indicates the presence of stronger chemisorptive interactions relative to physical adsorption effects. Polymerization of HCN was observed in the condensed bulk liquid phase, but none occurred in the bulk gas phase over a period of several months. Polymerization of the adsorbed (condensed) HCN inside the adsorbent pore space could lead to higher adsorption, particularly in the low pressure region. However, no significant retention of HCN was observed on BPL activated carbon when the sample was outgassed at 250°C under vacuum at the end of an experimen~l run. The departures from linearity are thus not likely to be due to polymerization of hydrogen cyanide in the case of BPL activated carbon. In the case of ASC whetlerite and the ASB carbons, where significant adsorbate retention occurred, polymerjzation of hydrogen cyanide adsorbate could not be ruled our. The chemisorptive effect, relative to physical adsorption, becomes more pronounced in the case of ASB carbons (Fig. 8) and ASC whetlerite (Fig. 7). This is indicated both by the departures from linear isotherm behavior and the degree of adsorbate retention at the end of an experimenta run. In the case of HCN on ASC whetlerite, the adsorption and desorption isotherms show no appreciable difference even at pressures as low as
~2468
IO
12
14 !6
18 20 22 24
c2 x c6
Fig. 8. Hydrogen cyanide on an ASB (I H,BO,: I) carbon 0, 22.5”,desorption.
0
002 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.180.20 P/P,
Fig. 9. Desorption isotherms for hydrogen cyanide on several carbons 0, BPL activated carbon: 9, AX whetlerite carbon; A. ASB (1 H,BO,: 1) carbon; A, ASB (2 H,BO,: 1) carbon.
P = 1 mm, and the isotherms start to level off at higher pressure than in the cases of HCN on the ASB and BPL carbons. This indicates that a much stronger chemisorptive interaction exists between HCN and ASC whetlerite. The chemisorptive effect is reduced in the sequence ASC whetlerite > ASB > BPL activated This carbon. sequence was also observed in the adsorption of phosgene [ I] and cyanogen chloride on the same carbons. After exposing ASC whetlerite and the ASB carbons to hydrogen cyanide vapor, it was not possible to regenerate the original sample weight by out-gassing the sample at 150°C under vacuum conditions. The amount retained on ASB carbons was less than in the case of ASC whetlerite. The amount of HCN retained by ASC whetlerite showed very little dependence on the pressure of HCN to which the sample was exposed. Several desorption isotherms which were obtained by starting at different highest exposing pressures fell on the same
290
P. J. REKJCRO~ and C. T. CHIOU
common curve. The amount of strongly retained hydrogen cyanide vs the highest exposing pressure is shown in Fig. 10 for ASC whetlerite. It is apparent that chemisorption of HCN takes place along with physical adsorption inside the adsorbed pore volume space and as the pressure increases the effect of chemisorption becomes less noticeable since the contribution from physical adsorption becomes greater in the high pressure region. The isotherms then approximate to linear behavior. In other words, some portions of HCN adsorbed in the immediate vicinity of the pore space of ASC whetlerite and ASB impregnated carbons are also bonded through strong chemical interactions. The maximum adsorption capacities, W,,, determined from the HCN isotherms on different adsorbents are in agreement with those values determined from chloroform isotherms[l]. In order to assess relative contributions to the total adsorption from chemisorption and physical adsorption, adjusted isotherms were obtained by subtracting the (constant) amount of tenaciously retained hydrogen cyanide from the corresponding desorption isotherms. The adjusted isotherms thus obtained represent the contribution to the total adsorption from physical adsorption forces and other weak chemical interactions. The adjusted isotherms are more linear in the high pressure region but depart significantly from a linear plot in the low pressure region indicating that contributions to the adso~tion from other than physical adsorption are still included. Also shown in Fig. 7 are desorption isotherms that were obtained on AX whetlerite that was previously exposed to hydrogen cyanide and out-gassed. The results indicate that, based on the initial dry sample weight, ASC whetlerite carbon shows a small reduction in its adsorptive capacity after adsorbing HCN for a desorption run. No such effect was observed in the case of BPL activated carbon. This is probably caused by reaction between adsorbed HCN and the impregnants on the ASC whetlerite carbon (Cu’+, CrO,*- and trace of Ag’f which causes deterioration of the adsorbent. The molar refraction of hydrogen cyanide was obtained from bond refractivities using Vogel’s bond refractivity table[S]. The theoretical affinity coefficient was calculated to be 0.305[l, 2b]. Experimental affinity coefficients /3_ for HCN with respect to chloroform were determined from the measured isotherms of these vapors on each adsorbent using the linear region of the isotherms to determine the experimental siopes.
Fig. 10, Chemisorption isotherm, hydrogen cyanide on AX whetlerite carbon.
However, in the case of ASC whetlerite, the non-linear behavior of the isotherm did not allow determination of BtX from the original isotherm plot. The results are tabulated in Table 1 along with other adsorption parameters. By comparing &, with fieXfor each adsorbent, hydrogen cyanide shows about 32% higher adsorption than predicted in the case of BPL activated carbon and about 45% higher then predicted in the case of the ASB carbons investigated. Because of the strong chemisorptive affects, the amount of hydrogen cyanide adsorbed on ASC whetlerite in the low pressure region was found to be about ten times greater than that predicted assuming physical adsorption. When the linear highpressure region of the adjusted isotherm was employed to obtain &, the experimental affinity coefficient for hydrogen cyanide on ASC whetlerite was found to be about 45% higher than the theoretical affinity coefficient. Thus significant amounts of chemisorptive interaction are obtained even in the reversible part of the adsorption isotherm.
In summary, it can be concluded that the impregnants in ASC whetlerite (Cu” CrO:- and a trace of Ag+) produce a greater degree of chemisorptive interaction toward hydrogen cyanide, cyanogen chloride and phosgene than the impregnants in the ASB carbons (C$’ and H,BO,). This suggests that the chromate ion (or a complex form of CrO,*- with Cu*” and other species on whetlerite carbon) is primarily responsible for the strong adsorptive capability of ASC whetlerite towards these vapors. Since chemisorptive effects tend to be a major part of the total adsorptive interaction, it can be concluded that predicted isotherms for vapors such as the ones investigated, based on theoretical affinity coefficients, will be underestimated by up to 30-40%, in the case of BPL activated and ASB carbons in the low pressure region. In the case of ASC whetlerite, predictions based on Dubinin-Polanyi considerations will seriously underestimate the adsorption isotherm in the low pressure region. At higher pressures, e.g. approaching P/P,= 1, predictions based on theoretical affinity coefficients are in reasonable accord with experimental isotherms. Acknowledgements-The research was supported by the ~p~tment of the Army through Edgewood Arsenal Contract No. DAAA15-74-C-0163.The authors wish to acknowledge informative technical discussions with Dr. L. A, Jonas, Edgewood Arsenal. REFERENCES 1. C. T. Chiou and P. J. Reucroft, Carbon 15,49 (1977). 2. (a) M. M. Dubinin, E. D. Zaverina and L. V. Radushkevich, Zh. Fiz. Khim. 21, 1351 (1947). fb) P. J. Reucroft, W. H. Simpson and L. A. Jonas, J. Phys. Chem. 753526 (1971). 3. C. T. Chiou and M. Manes, J. Phys. Chem. 77,809 (1973). 4. C. T. Chiou and M. Manes, J. Phys. Chem. 78,662 (1974). 5. Handbook of Chemistrv and Phvsics. 49th Ed. Chemical Rubber Co. (i%S). . _ 6. L. A. Jonas and J. A. Rehrmann, Carbon 10,657 (1972). 7. N. K. ~aljnin, M. G. Slin’ko, Y. S. Matros and V. G. Gorskii, Dokl. Akad. h&k. USSR, 199(l), 146(1971).
