Micropore volume determination in activated carbons

Micropore volume determination in activated carbons

Micropore volume activated carbons determination in M. C. M. Alvim Ferraz Faculdade de Engenharia, 4099 Porto Codex, Portugal (Received 75 October ...

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Micropore volume activated carbons

determination

in

M. C. M. Alvim Ferraz Faculdade de Engenharia, 4099 Porto Codex, Portugal (Received 75 October 1987; revised 1 August 7988)

A comparison was made ofthe adsorptive properties ofimpregnated and non impregnated activated carbons, obtained in a similar way from pinewood sawdust. Deviations from linearity were shown in two of the carbons by application of Dubinin-Radushkevitch theory; in these cases the Dubinin-Astakhov theory was applied. As these carbons have a developed meso and macroporous structure, the t method was used to correct the isotherms for adsorption in wider pores. The micropore volumes determined chromatographically by benzene adsorption were compared with those obtained by nitrogen adsorption. This led to the conclusion that it is not reasonable to extrapolate data obtained chromatographically when there are deviations from the Dubinin-Radushkevitch theory. These facts were interpreted by correlating the structures of the different carbons. (Keywords: carbon;

pore

structure; micropore

volume)

Active carbons are finding increasing applications as catalyst supports in the treatment of polluted gaseous effluents, where advantage is taken of the enhanced retention of organic solutes in the pore system of the carbon. The impregnation of a suitable carbon precursor with cobalt and iron nitrate solution produces impregnated active carbons with good characteristics for pollutant destruction’. Activated carbons in general possess a large micropore volume, often associated with a more or less welldeveloped meso and macroporous structures. In this case, the adsorbed volume u, comprises the volume adsorbed in micropores Wand the volume adsorbed in wider pores u, o,= w+0

(1)

Taking ai as the area associated with u, in the pressure range where no capillary condensation exists, this equation will be: D,= w + a;.t

(2)

The thickness of the adsorbed layer t is determined from the values t, published by Lecloux et al.* and corrected using the following equation3 :

CL1+ (C’-

1)PlPol

t%p+(~-~)P~poj

(3)

where C and C’ are the constants of the BET equation for the given solid and the standard, respectively and p/p,, is the relative pressure. The micropore volume W, is determined by application of the Dubinin-Astakhov equation (DA) to the adsorbed volume in micropores, whenever the Dubinin-Radushkevitch equation (DR) cannot be applied4,5. DA equation:

W = W, exp[ - (A/E)“]

(4)

DR equation:

W = W, exp[ - (A/E)*]

(5)

For TdT,,

A=RT

In pO/p

0011%2361/89/05063546$3.00

#(I 1989 Butterworth & Co. (Publishers) Ltd.

(6)

For T>T C?A=RT

[email protected]: PT,*

W=n,v*=itE P where A is the differential molar work of adsorption, E is the characteristic energy of adsorption, n is an adjustable parameter, T and T, are temperature and critical temperature, respectively, R is the gas constant, pc is the critical pressure, n, is the amount adsorbed by unity of mass, v* is the molar volume of the adsorbate, p* is the density in the adsorbed state and M is the molecular weight. To determine the molar volume of the adsorbate v*, the fact that the properties of the adsorbed substances in the micropores are radically different from those of the liquid phase must be noted. However, for temperatures below the boiling point (TbP) the compressibility of the adsorbed phase may be neglected and its molar volume considered equal to the molar volume of the liquid phase. For higher temperatures it is important to keep in mind that the compressibility of the adsorbed phase rises as the critical temperature is reached; in the temperature interval from TbP to T, it is possible to approximate the temperature dependence of the density of adsorbed substances, by a linear function5: (9) (10) where pbp and pm are the density at the boiling point and at critical temperature, respectively, and b is the van der Waal’s constant. This is equivalent to considering that the molar volume of adsorbed substances at the critical temperature and above is equal to the van der Waal’s constant.

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Micropore

volume determination

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carbons: M. C. M. Alvim Ferraz

If these values are used for the molar volume in the determination of the characteristic curve A = f( W), it is possible to verify its invariability with temperature, whenever the forces that take part in the adsorption are dispersive forces. This leads to the conclusion that these o* values reliably represent the state of the adsorbed substance. The analysis of the porous structure of the carbons was based on the nitrogen adsorption isotherms obtained by the conventional volumetric method, and on the isotherms obtained by the chromatographic method6. EXPERIMENTAL The activated carbons tested were prepared from pinewood sawdust of average particle size 0.063 cm. After washing with 10 ~01’4 H,SO, solution and drying at 383 K, the acid was totally removed from the sawdust with distilled water. Then the carbon precursors of SACo-2 and SA-Fe-2 were impregnated with 0.1 M cobalt and iron nitrate solutions, respectively, at room temperature and under vacuum (20 cm3 of solution per g of dry sawdust). After drying at 383 K, the impregnated sawdust carbonization was conducted in a silica reactor placed in a tubular furnace under flowing nitrogen. The heating rate was 10 K min- ’ up to a temperature of 1123 K, which was maintained for 60 min. The activation was carried out by partial gasification of the carbon with CO, at 1098 K for 15 min, the weight loss being about 20 %. The non-impregnated carbon SA-2 was prepared in a similar way but without the impregnation step. The activation time was extended to 60 min to obtain the same degree of burnoff. The carbon SA-Co-2-U is derived from SA-CO-~, which had been used for two months in air at temperatures between 553 and 633 K. A commercial activated carbon Norit PK 1-3, designated by AC, was also tested. Nitrogen adsorption isotherms at 77 K were obtained by the conventional volumetric method in a semiautomatic Micromeritics 2100 D apparatus. It is possible to reach a vacuum of 1.33 x 10e4Pa and pressure detection is made with a digital capacitance manometer. The range of detectable pressures varies between 1.33 x lo-* Pa and 1.33 x lo5 Pa, and it is possible to follow the variation of pressure with time. The liquid nitrogen temperature was measured with a thermistor, with a maximum associated error of 5 x lo-* K. The samples were originally outgassed at 473 K for 15 h, and the results were expressed in relation to unit mass of outgassed carbon. All isotherms were repeated at least once, and the true adsorption equilibrium was checked at regular intervals, by following it over long periods. The adsorption isotherms of benzene, propene (CH,CH=CH,) and ethanethiol (CH,CH,SH) were obtained chromatogTable 1

Parameters

determined

by BET and

raphically in a g.c. fitted with a thermal conductivity detector. The column temperature was measured with an error smaller than 2 K and the variations in temperature during the experiments were not higher than 1 K. The carrier gas used was nitrogen R (Ar Liquido), which is not adsorbed at the temperatures used; its flow rate was measured with a bubble flowmeter (error < 2 %). The stainless steel columns, 0.48cm i.d. and 25.0 to 28.4cm length, were filled with a mass of dry carbon between 0.72 g and 1.55 g. The average size of the carbon particles is 0.034-0.051 cm. It was noticed that the pressure drop along the column was negligible.

RESULTS In spite of all the limitations to the application of the BET theory to microporous solids, as it is a reference method, it was used to analyse the data of nitrogen adsorption at 77 K, as a means of characterizing the different carbons. In the calculations, the area occupied by the adsorbed nitrogen molecule was considered to be 0.162 nm*. Nitrogen vapour pressure at the temperature used’,’ was calculated by the following expression:

(11) where ~b and pg are the vapour pressures at the temperatures T’ and T” and 2 is the latent heat of vaporization. Values of volume equivalent to the monolayer u,, specific area equivalent to u,, a, (BET), constant C and the range of relative pressures where BET theory is applicable, are included in Table 1. In the same table, values corresponding to carbon AC9 are represented for comparison. The t method was used to determine values of the total specific area a,(t) and of the area associated with the larger pores al(t) (Ref. 10). The thickness of the adsorbed layer was determined by Equation (3). These values are listed in Table 1. The application of the DR equation in the pressure range where no capillary condensation exists, leads to the conclusion that its use is acceptable for the carbons SA-2 and SA-CO-~-U, while for carbons SA-Co-2 and SA-Fe-2 some deviations are observed’ ‘, as shown in Figure

1.

With exception of SA-2, the other carbons of the SA series have a well-developed meso and macroporous structure’*. For this reason Equation (2) was used to determine the volume adsorbed in the micropores w to which the DR equation was applied. The influence of this correction is shown in Figure 2 for the carbon SA-CO-~U.

t methods BET method

Carbon

(cm3g-‘)

u&BET) (m’g-‘)

SA-2 SA-Co-2 SA-Co-2-U SA-Fe-2 AC

0.388 0.103 0.109 0.0917 0.278

1084 287 304 256 774

Orn

636

FUEL, 1989, Vol 68, May

c 7262 541 92.1 172

t method Applicability range of p/p0 4.1 x 8.5 x 3.8x 8.6x 5.0x

10-‘-1.1x 10-‘-2.5x 10-2~3.1x 10-‘-2.6x 10-3~1.3x

10-I 10-l 10-l 1OW’ 10-1

a&) (m2K’)

(r;(r) (m’g-‘)

1091 292 313 262

17.4 92.0 126 93.1

Micropore

volume

determination

Considering the deviation from linearity observed in carbons SA-Co-2 and SA-Fe-2 (Ref. 5), the determination of the micropore volumes using the DR equation will give only an approximate value, by extrapolation of the linear intermediate range of pressures. The parameters obtained by this method are given in Table 2, where the applicability range in relative pressures is shown. The non applicability of the DR theory for a reasonably large range of pressures, suggests the application of the DA equation to eliminate the observed deviations. The parameter, n, of this equation was determined by optimizing the fitting of In W uersus A” for different values of n. The parameters calculated with this equation and those determined by the DR equation for SA-2 and SACo-2-U are included in Table 3, where A (%) represents the average percentual error of the fitting. The adsorption isotherms of benzene, propene and ethanethiol were obtained by a chromatographic method; this process is attractive to the study of the low pressure range of adsorption, which for activated carbons

in activated

carbons:

M. C. M. Alvim Ferraz

with a developed microporous structure, is difficult to obtain experimentally using conventional static methods. Ideal chromatography conditions were tested6,’ 2,13. The adsorption isotherms shown in Figure 3 were obtained with the conditions of chromatographic injection of benzene in the SA-2 column given in Tuble 4 (v, is the injected volume, T, and p, are the room temperature and pressures, pe is the pressure at column entry and U is the flow rate of carrier gas). The DA theory was applied to these isotherms optimizing the parameter n. To calculate the molar volume of the adsorbate for temperatures higher than critical, published values of critical properties were considered14. In the temperature interval from the boiling point to the critical temperature, the calculations were

c logw

‘,*

‘16 -

SA-Fe-2

I I

‘,3

I

I

Figure 1 Dubinin-Radushkevitch on SA-Co-2 and SA-Fe-2

I

.

I 2

plot for the adsorption

Table 2 Parameters determined carbons of SA series

by application

l

fog

of nitrogen

of the DR equation

2

I

PO/PI2

Figure 2 Dubinin-Radushkevitch plot considering the correction due to the adsorbed volume in meso and macropores, SA-Co-2-U: 0, without correction; x , with correction

to

B

‘1

Carbon

range of p/p0

w, (cmag’)

E (KJ mole-‘)

WI (cm3 g-r)

E (KJ mole- ‘)

SA-2 SA-Co-2 SA-Co-2-U SA-Fe-2

4.2 8.3 3.8 9.1

0.443 0.140 0.144 0.126

1.13 3.15 2.86 2.87

0.435 0.0973 0.0875 0.0828

1.15 3.05 2.71 2.78

Applicability

x application p application

Table 3

x x x x

of the DR theory of the DR theory

Parameters

10-‘-3.3x 10-l lo-*-2.1 x 10-l 10m2-2.5 x IO-’ W-2.0x 10-l

to the total adsorbed volume to the micropore adsorbed volume

of carbons

of SA series

Carbon

Applicability range of PIPS

w, (cm3 g-r)

E (KJ mole-‘)

n

A (2,)

SA-2 SA-Co-2 SA-Co-2-U SA-Fe-2

4.2 7.5 3.8 7.5

4.35 9.91 8.75 8.16

7.75 2.46 2.71 2.17

2.00 2.59 2.00 3.29

0.150 0.517 0.241 0.658

x x x x

10m4’3.3 x 10-l 10-*-3.1x 10-l 10-‘-2.5x 10-l lo-*-3.1 x 10-l

x x x x

10-r 1om2 1O-2 lo-’

FUEL, 1989, Vol68,

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Micropore

volume determination

carbons: M. C. M. Alvim Ferraz

in activated

also based on the determination of the density at the boiling point by the method of reduced densities of liquids’ 4. The characteristic curves of adsorption are shown in Figures 4-7. The parameters determined through benzene and propene adsorption on the carbons are listed in Tuble 5. In Tuble 6 these values are compared with those determined from the nitrogen adsorption isotherms. DISCUSSION The reference isotherms considered in the calculation of a,(r) were chosen as a function of the constant, C, of the BET equation *s3 The good agreement between values of a,(BET) and a,(t) (Table I), shows that the reference isotherm considered was acceptable. This ensures the 1 I

2

3

I

l A'xlO-'

I

(J2moIe-21

I

Figure 3 Benzene adsorption 583 K, 594 K and 606 K

Table 4

Conditions

T(K)

“Flow

rate measured

in SA-2

573 10 292.5 1.013 1.013 60.0

583 10 293.0 1.023 1.023 60.0

at room temperature

I

2

1

I

I

L22K L53K 683K 513K

on SA-2 at 551 K, 573 K,

of benzene injection 551 10 292.5 1.013 1.013 60.0

L’Icut) Ta (K) pax 10-s (Pa) pex lo-’ (Pa) U’ (cm3min~‘)

isotherms

594 10 293.5 1.023 1.023 60.0

606 10 293.5 1.023 1.023 60.0

lcgw +

and pressure

3

L

I

I

Figure 5

I

A*x~O-~ (Jzrnole-2)

,*

Characteristic



I

curves of adsorption

3

2 ’

I

s

I

Table 5

Characteristic

Parameters

curves of adsorption

determined



l(J”m,,(&

(l)-C6H6

-AC

(2)-C3Hg

-AC

5L3 . 558 . 568

I x

of propene

by application

I

on SA-2

A” x 16’

4 ’

. 390K x L33K . 453K I L73K

Figure 4

of ethanethiol

Figure 6 on AC

on SA-2

Characteristic

curves of adsorption

. .

of benzene and propene

of the DA theory _ Propene

Benzene ~_

w, SA-2 SA-Co-2 SA-Fe-2 AC

638

(cm3 g-l)

E (KJ mole-‘)

n

A (%)

w, (cm3 g- ‘)

(KJ mole- ‘)

n

(%)

0.39 0.23 0.17 0.32

28 28 28 24

2.0 2.0 2.1 2.0

1.3 3.6 1.7 1.4

_ _ _ 0.34

_ _ _ 17

2.0

1.2

FUEL, 1989, Vol 68, May

A

E

Micropore Table 6

volume determination

in activated

carbons: M. C. M. Alvim Ferraz

Comparison of W, and n determined by nitrogen adsorption (static method) and benzene adsorption (chromatographic method) Nitrogen adsorption: extrapolation of low pressure range

Nitrogen adsorption

Benzene adsorption WI (cm’g-‘)

n

w, (cm3g-‘)

n

w, (cm3 g _ ’ )

n

SA-2 SA-Co-2

0.39 0.23

2.0 2.0

0.435 0.099 1

2.00 2.59

0.23

2.0

SA-Fe-2 AC

0.17 0.32

2.1 2.0

0.08 16 0.320

3.29 2.00

0.14 _

2.1 _

5 I



-



10 I

A”x 16’

(11-C6 H6 w-c6 H6

SA-2 SA-Co-2 SA-Fe-2

lp,F x I s l

t,

.(J”,,,&j”)

vt(cm3 STPg-

vt (cm3 STPg-‘)

l Adsorption o Desorption

250

R

573K 583K 59&K 606K

250

150

\

. 13)

50

I

logw4 Figure 7 Characteristic curves of adsorption of benzene on SA-2, SACo-2 and SA-Fe-2

Figure8

reliability of the values of a;(t) that were used to calculate the volume adsorbed in the micropores. The application of DR theory to these volumes leads to results significantly different from those obtained when applied to the total adsorbed volume, with the exception of the carbon SA-2. This happens because carbon SA-2 is essentially microporous”. Hence, the necessity to consider the correction for carbons with a wide range of porosity. The optimization of n does not greatly affect the micropore volumes, while values of E show significant differences. The range of pressures where the DA equation is applicable is large in comparison with the DR equation. Carbon SA-2 has the greatest micropore volume and the presence of impregnants during the carbon activation results in a decrease in the volume of the microporous structure’ *. The parameter n is >2 for SA-Co-2 and SA-Fe-2 and this can be related to either the presence of extremely small pores’ 5,1’ or the existence of constrictions, which prevent the access of adsorbed substances to all the adsorbent structure”. As the first hypothesis is considered inadmissible”, it was concluded that the presence of impregnants in the carbonous structure may be responsible for the appearance of constrictions, which prevent the access of the adsorbate and cause the observed deviations. This hypothesis is in accordance with the determination of a higher value of n for SA-Fe-2, which has a greater metal load’. The use of SA-Co-2 in air at high temperatures has as a

N, adsorption

isotherms

(77 K) on SA-Co-2 and SA-Co-2-U

consequence, a second activation that is shown by a weight loss, associated with a simultaneous decrease of micropore volume and increase of meso and macropore volume’*. The decrease of the parameter n leads to the conclusion that this activation is associated with the removal of the obstructions that determine a value of n=2.59 for SA-CO-~. This fact also explains the elimination of low pressure hysteresis (the reproductivity of which was tested) when nitrogen adsorption isotherms at 77 K on SA-Co-2 and on SA-Co-2-U are compared (Figure 8). The phenomenon of low pressure hysteresis18*19, has been associated with stresses induced in the solid by adsorption, which may cause localized fractures leading to changes in the pore structure. Thus, micropores that were previously inaccessible to the adsorbate may be revealed, and/or micropores filled with adsorbate may become blocked. It was concluded that for SA-Co-2 there are obstructions that prevent the access of the adsorbate to part of the microporous structure simultaneous with low pressure hysteresis. For SA-Co-2-U the conclusion was that obstructions were removed by activation at the same time as low pressure hysteresis was eliminated. Therefore, it can be concluded that during the adsorption cycle, obstructions are eliminated by the increase in pressure. Thus, during desorption and after a complete adsorption cycle, the adsorbent structure is different from the

FUEL,

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Micropore volume determination in activated carbons: M. C. M. Alvim Ferraz accessible structure observed during adsorption at low pressures. Application of the DA theory to the adsorption chromatographic data revealed that, with exception of benzene on all carbons and propene on AC, the characteristic curves of adsorption depend on the temperature, Figures 4 and 5. As ideal chromatographic conditions were tested, non-equilibrium cannot be considered6. According to the Dubinin theory, this is the result of the presence of adsorption forces, which differ from the van der Waals forces. In fact, these forces are typical of the attraction between non-polar molecules; for this reason, the observed deviations could be expected, and the applicability of the DA theory to benzene adsorption could have been foreseen, Figures 6 and 7. Comparison of dipolar moments of benzene, propene and ethanethiol, respectively 04.366 and 1.58D (Ref. 20) facilitates the understanding, in accordance with the low polarity of propene, of the application of the DA theory to propene adsorption on some solids of low polarity (AC). The analysis of parameters W, and n determined by nitrogen adsorption (static method) and benzene adsorption (chromatographic method) that are listed in Tuble 6, leads to the conclusion that, if the n values determined by the two methods are similar, the W, values are also very similar (SA-2 and AC). For the other cases @A-Co-2 and SA-Fe-2) the differences are statistically meaningful. As observed, considering n = 2 for nitrogen adsorption in SA-2 and AC, there is no deviation from the DR equation, in contrast to carbons SA-Co-2 and SA-Fe-2. In the case of the chromatographic method, the pressures involved are very low; for this reason it is impossible to observe zone Y in Figure I. This leads to the parameter n being very close to 2, even for the cases in which some type of deviations at higher pressures could eventually be observed. For carbons AC and SA-2 there is no deviation of the DR equation in a large enough range of pressures, the fit in the range of low pressures (obtained in the chromatographic method) is made with the same value of n; so, it is possible to obtain agreement for the values of W, through the extrapolation of these results. In the case of carbons SA-Co-2 and SA-Fe-2 the situation is different. The existence of deviations implies an extrapolation of the low range of pressures which does not take into account the behaviour of the carbon at higher pressures, leading in these circumstances to meaningless values of the micropore volumes. However, values obtained through benzene adsorption are compatible with those obtained through nitrogen adsorption in the low range of pressures; so the

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FUEL, 1989, Vol 68, May

extrapolation of this zone (X in Figure I) leads to values of W, which can be compared with those determined through benzene adsorption as shown in Table 6. This comparison shows the compatibility of parameters determined through chromatographic and conventional methods. It is evident that, in spite of this, it is not always possible to characterize adsorbents through extrapolation of the range of pressures obtained chromatographically. ACKNOWLEDGEMENTS The author thanks Professor J. Pajares for his helpful discussions. This work was carried out under research contract no. 4578.05, jointly sponsored by the Chemical Engineering Department of Faculty of Engineering and Junta National de Investigacao Cientifica e Tecnologica and supervised by Prof. J. L. Figueiredo.

REFERENCES

I

8

9

10 11 12 13 14 15 16 17 18 19 20

Alvim Ferraz, M. C. Fuel 1988,67, 1237 Lecloux, A. and Pirard, J. P. J. Colloid Interface Sci. 1979, 70, 265 Dubinin, M. M. Izv. Akud. SSSR, Ser. Khim. 1974, 5, 996 Finger, G. and Biillow, M. Carbon 1979, 17, 87 Nikolaev, K. M. and Dubinin, M. M. Bull. Acad. Sci. USSR, Ser. Chim. Sci. 1958, 1124 Conder, J. R. and Young, C. L. ‘Physicochemical Measurement by Gas Chromatography’, John Wiley and Sons, Chichester, UK, 1979, p. 25 Ziegler, W. T. and Mullins, J. C. Report No. IlNational Bureau of Standards, Project A-663, Georgia Institute of Technology, Atlanta, 1963 Johnson, V. J. (Ed.) ‘A Compendium of the Properties of Materials at Low Temperature (Phase I), Wadd Technical Report, 1960, p. 60 Brotas de Carvalho, M., Sales Grade, M. R. and Conceicao, M. A. IV Reunion National de 10s Grupos de Trabajo Relacionados con la Investigation en el Campo de la Adsorcion, Sevilha. 1979 Rives Arnau, V. and Mutiuera, G. Anal. Quim. 1978, Suplt. 1,90 Rand, B. J. Colloid Interface Sci. 1976, 56, 337 Alvim Ferraz, M. C. PhD Thesis, Faculdade de Engenharia da Universidade do Porto, Portugal, 1983 Ferraz, M. C. A., Figueiredo, J. L. and Rodrigues A. E. @Gdud 1983.40, 383 Hougen, 0. A., Watson, K. M. and Ragatz, R. A. ‘Principios de 10s Processes Quimicos’, Editorial Revert& S.A., Barcelona, 1964 Jaroniec, M. and Jaroniec, J. A. Carbon 1977, 15, 107 Dubinin, M. M. and Isirikyan, A. A. Izr. Akod. Nauk USSR. Ser. Khim. 1980, 1, 13 Stoeckli, F., Perret, A. and Mena, P. Curbon 1980, 18,443 McEnaney, B. Trans. Faruduy Sot. 1974, 70, 84 Adams, L. B., Boucher, E. A. and Everett, D. H. Corbon 1970,8, 761 ‘Handbook of Chemistry and Physics’, The Chemical Rubber Co., 48th Edition, 1967/68, E-66167