C A R B O N 4 7 ( 2 0 0 9 ) 1 0 1 2 – 1 0 1 7

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Using alkali metals to control reactivity and porosity duringphysical activation of demineralised kraft lignin

Suhas, P.J.M. Carrott*, M.M.L. Ribeiro Carrott

Centro de Quımica de Evora e Departamento de Quımica, Universidade de Evora, Colegio Luıs Antonio Verney, 7000-671 Evora, Portugal

A R T I C L E I N F O

Article history:

Received 13 October 2008

Accepted 1 December 2008

Available online 10 December 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.12.001

* Corresponding author: Fax: +351 266745303E-mail address: peter@uevora.pt (P.J.M. Ca

A B S T R A C T

Demineralised kraft lignin was impregnated with between 6.2% and 50% NaCl or KCl and

physically activated in CO2 at 750 �C. The results presented show that a considerable reduc-

tion in activation time even at a comparatively low activation temperature could be

achieved, particularly when using KCl. Considering a fixed level of burn-off, the impregna-

tion did not affect the pore volume and only increased the pore width by about 0.1–0.2 nm,

depending on the concentration of NaCl or KCl used. By controlling the conditions it was

possible to obtain predominantly ultramicroporous materials with mean pore widths over

the range 0.53–0.77 nm. On the other hand, at high levels of burn-off there was evidence for

micropore widening into the small mesopore range and also for the formation of a second-

ary mesopore structure. Under these conditions it was possible to obtain materials with

pore volumes as high as 0.82 cm3 g�1.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Lignin is one of a wide variety of bioresources that has been

used to produce activated carbons [1–7]. For a given type of acti-

vation method, the precise properties of the product and the

optimum operating conditions will depend on the molecular

structure of the biopolymers present and also on the presence

of inorganic impurities (ash) [8]. For instance, in the specific

case of physical activation of lignin with CO2, higher micropore

volumes and narrower micropore widths can be obtained from

pure hydrolytic lignin or demineralised kraft lignin than from

impure raw kraft lignin [9]. The disadvantage is that ash free

lignin is extremely non-reactive. For instance, it was found

that a burn-off of 20% could be reached after about 1 h at

750 �C using raw kraft lignin but over 16 h were needed with

demineralised kraft lignin [9]. This difference in reactivity is

probably due to the presence of sodium, which is a known car-

bon gasification catalyst, and is the main impurity in kraft lig-

nin [10–14]. Evidence that this is the case has come from a

recent preliminary study where addition of NaCl to deminera-

lised kraft lignin was found to increase the reactivity again [15].

er Ltd. All rights reserved

.rrott).

Surprisingly, however, the deliberate addition of NaCl did not

adversely affect the porosity of the product. That is, the effect

of sodium seems to be associated not only with the amount

present but also with the manner in which it is incorporated

into the lignin. It occurred to us that impregnating deminera-

lised lignin with sodium under controlled conditions could

be an effective way to obtain activated carbons with good

porosity characteristics but with a much reduced period at

comparatively low activation temperature, thereby reducing

considerably the energy costs associated with the activation

process. A more systematic study of the effect of the alkali

metals sodium and potassium and the alkali earth metal cal-

cium on the activation of demineralised kraft lignin was there-

fore carried out and the results are presented in this paper.

2. Experimental

2.1. Materials and methods

The kraft lignin sample, designated alfa lignin, was obtained

from Lignotech Iberica and is similar to that used in previous

.

C A R B O N 4 7 ( 2 0 0 9 ) 1 0 1 2 – 1 0 1 7 1013

work [9,10,15,16]. In order to eliminate the mineral content,

10 g of alfa lignin was treated with 500 mL 1% H2SO4 with con-

tinuous stirring at 300 rpm for 1 h at room-temperature and

then kept overnight. The dispersion was then filtered and

washed with 500 mL 1% H2SO4 followed by �1 L distilled

water. Finally the treated sample, designated alfa-A, was

dried overnight at 110 �C and stored in sample flasks for fur-

ther use. The ash content, determined according to ISO stan-

dard 1171, was 0.2%.

Impregnation of alfa-A with KCl, NaCl or CaCl2 was carried

out by mixing dry lignin and inorganic salt in fixed weight ra-

tios, corresponding to weight percentages of 6.2%, 33% or 50%,

and then adding 25 mL distilled water. The mixture so ob-

tained was stirred, kept overnight for drying, and used for

the preparation of activated carbons.

For the production of the activated carbons about 5 g of

precursor were placed in a ceramic boat and positioned in

the central constant temperature zone of a conventional hor-

izontal tubular furnace. Carbonisation was carried out by

heating to 750 �C at a rate of 8 �C min�1 under a constant

N2 flow of 85 cm3 min�1 and maintaining for 30 min. Physical

activation in CO2 was carried out by switching to a CO2 flow

of 85 cm3 min�1 at the same temperature, maintaining for

the appropriate time at 750 �C in order to obtain samples

with different levels of burn-off, switching back to the N2

flow and allowing to cool to below 50 �C before removing

the product from the furnace. The impregnants were re-

moved by washing with 2 L of distilled water, followed by

drying at 110 �C overnight and were then stored in sealed

sample flasks. The burn-off, X, of the samples was calculated

from

X ¼ 100 ðwo �wÞ=wo ð1Þ

where wo is the weight of carbonised lignin and w is the final

weight of activated carbon after, in both cases, removal of the

impregnant.

2.2. Characterisation

Characterisation of the alfa lignin by thermogravimetric anal-

ysis and FTIR, as well as elemental analyses of C, H, N, S and

O, and humidity and ash determinations according to ASTM

and ISO standards, have been described previously [9,15]. N2

isotherms at 77 K were determined using a CE Instruments

Sorptomatic 1990, using helium (for dead space calibration)

and nitrogen of 99.999% purity supplied, respectively, by Linde

and Air Liquide. Prior to the adsorption measurements all

samples were outgassed for 4 h at 300 �C. The N2 isotherms

were analysed by means of the as method using published

standard data in order to obtain the external surface area,

As, and total pore volume Vs [17]. They were also analysed

by means of the DR equation in order to obtain the DR pore

volume, Vo, and the mean pore width, Lo, from the relation-

ship [18]:

Lo ¼ 10:8=ðEo � 11:4Þ ð2Þ

where Eo is the characteristic energy obtained by application

of the DR equation. The liquid density of N2 was taken as

0.808 g cm�3 and the affinity coefficient as b = 0.34 [19].

3. Results and discussion

3.1. Reactivity

Values of activation time and the corresponding burn-offs are

given in Table 1. The reaction rate, assuming first order

kinetics, is given by –dw/dt = kw. If Eq. (1) is used to replace

the weight, w, by the burn-off, X, it follows that estimates of

the first order rate constants, k, can be calculated as

(ln(1�X/100))/t and these values are shown as a function of

burn-off in Fig. 1. For each sample the value of k obtained is

a mean value averaged over the whole range of burn-off from

0 to X. Nevertheless, the results obtained are in general

agreement with published results from other authors for

pre-carbonised kraft lignin containing 2.1% ash and activated

at the same temperature of 750 �C and determined using a

more rigorous thermogravimetric technique [6].

For the demineralised lignin the value of rate constant ob-

tained was approximately 0.0002 min�1. After impregnation

the rate constant increased by at least 16 times to values

>0.0033 min�1. In a previous study it was found that the k val-

ues were fairly constant over a range of burn-off up to about

55% but then increased significantly at higher burn-offs [6].

The number of data points on Fig. 1 is too small to verify if ex-

actly the same behaviour is observed here. However, it is clear

that the k values are higher for the higher burn-off samples.

In the previous study k values in the approximate range

0.002–0.005 min�1 were reported for activation at 750 �C [6].

It can be seen from Fig. 1 that the range of k values obtained

in this work for burn-offs less than 41% are within the same

range. The results in Fig. 1 also indicate that, considering

the overall trend of the data, the rate constants increase as

the amount of Na or K increases, and that the values for

KCl impregnation are higher than those for NaCl impregna-

tion. Results for CaCl2 are not shown in Fig. 1. However it

was found that the use of this impregnant also resulted in sig-

nificant increases, similar or greater than KCl, in the rate

constants.

These results show that addition of even a relatively low

amount of NaCl, KCl or CaCl2 provokes a significant increase

in the rate constants for the activation and allows degrees of

activation, or burn-offs, to be controlled over a reasonably

short time scale even at the comparatively low activation

temperature of 750 �C.

3.2. Qualitative isotherm analysis

Representative N2 isotherms of chars and activated carbons

prepared from the alfa lignin have been presented previously

[9,15]. Most of the isotherms determined in the present work

were similar and representative examples for low burn-off

and high burn-off samples are shown in Figs. 2 and 3. All of

the isotherms presented Type I character at low relative pres-

sures indicating the presence of microporosity in all samples.

With the low burn-off samples obtained in this work it was

found that the isotherms were highly rectangular and almost

horizontal at relative pressures above 0.02p�, which

confirms that they were predominantly ultramicroporous

materials.

Table 1 – Textural characteristics of the activated carbons obtained from alfa-A lignins on activation in CO2 at 750 �C beforeand after impregnation with NaCl, KCl or CaCl2.a

(%) tact h Burn-off (%) ABET m2 g�1 As m2 g�1 Vs cm3 g�1 Vo cm3 g�1 Lo nm

No impregnation

0 0.5 0.2 520 4 0.17 l.p.h.b l.p.h.b

8 9 742 2 0.24 0.25 0.53

16 19 914 1 0.31 0.30 0.63

20 19 890 3 0.30 0.29 0.59

24 19 889 4 0.30 0.29 0.60

28 22 n.d.c n.d.c n.d.c n.d.c n.d.c

NaCl

6.2 0 0 442 6 0.15 l.p.h.b l.p.h.b

0.5 12 654 9 0.22 0.21 0.58

1 18 797 8 0.28 0.27 0.70

2 40 1111 26 0.45 (0.37)d (0.98)d

3 64 1325 47 0.82 (0.43)d (1.27)d

33 0 0 564 4 0.19 0.19 0.71

0.5 12 810 5 0.28 0.26 0.75

1 22 850 10 0.32 0.28 0.77

2 70 1199 29 0.81 (0.39)d (1.28)d

50 0.5 15 817 4 0.30 0.27 0.74

KCl

6.2 0.5 11 868 11 0.29 0.29 0.70

2 62 1907 19 0.68 (0.60)d (1.78)d

33 0.5 15 903 19 0.29 0.30 0.76

1 56 1454 41 0.61 (0.44)d (1.50)d

CaCl26.2 0.5 – 594 172 0.14 0.20 0.89

33 0.5 17 453 26 0.14 0.15 0.72

2 81 379 52 0.11 0.13 1.11

a tact = activation time, ABET = apparent surface area obtained by BET method, As and Vs = external surface area and total pore volume (in terms

of equivalent liquid volume) obtained by as method, Vo and Lo = micropore volume and mean pore width from DR plot.

b l.p.h. = low pressure hysteresis (see text).

c Not determined.

d Overestimated (see text).

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0 10 20 30 40 50 60 70X / %

k / m

in-1

Fig. 1 – Variation of first order rate constant, k, for activation

at 750 �C to burn-off of X% of alfa-A lignin impregnated with

NaCl (open symbols) or KCl (full symbols). Circular buttons –

no impregnation. Squares – 6.2%. Triangles – 33%. Diamond

– 50%.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8p/pº

nads

/ m

mol

g-1

1

Fig. 2 – Representative N2 at 77 K adsorption–desorption

isotherms determined on activated carbons prepared at

750 �C from alfa-A lignin impregnated with 33% NaCl.

Squares – 0.5 h activation. Circles – 2 h activation. Open

symbols – adsorption. Full symbols – desorption.

1014 C A R B O N 4 7 ( 2 0 0 9 ) 1 0 1 2 – 1 0 1 7

With regard to the high burn-off samples, it is clear from

Figs. 2 and 3 that their pore size distributions were consider-

ably broader than those of the low burn-off samples and ex-

tended into the mesopore region. Furthermore, the extent of

pore widening was clearly much greater when the lignin

was impregnated with NaCl rather than KCl.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8p/pº

nads

/ m

mol

g-1

1

Fig. 3 – Representative N2 at 77 K adsorption–desorption

isotherms determined on activated carbons prepared at

750 �C from alfa-A lignin impregnated with 33% KCl.

Squares – 0.5 h activation. Circles – 1 h activation. Open

symbols – adsorption. Full symbols – desorption.

0.0

0.2

0.4

0.6

0.8

1.0

0 10 15 20 25

X / %

L o /

nm

5

Fig. 4 – Variation of mean micropore width estimated by the

DR method with increasing burn-off, X, for activated carbons

prepared from alfa-A lignin impregnated with NaCl (open

symbols) or KCl (full symbols). Circular buttons – no

impregnation. Squares – 6.2%. Triangles – 33%. Diamond –

50%. Solid lines – trend lines.

C A R B O N 4 7 ( 2 0 0 9 ) 1 0 1 2 – 1 0 1 7 1015

It can be seen from Fig. 2 that the hysteresis loop on the

isotherm of the high burn-off NaCl impregnated sample has

a step which may be an indication of a bimodal pore size dis-

tribution. On the one hand, increasing burn-off widens the

micropores into the small mesopore range and this gives rise

to a Type H4 hysteresis loop. On the other hand, a secondary

mesopore structure may form at high burn-off giving rise to a

Type H1 or H2 hysteresis loop. Overlapping of the two hyster-

esis loops would give rise to the stepped character seen in

Fig. 2. Additional evidence for the formation of a secondary

mesopore structure is referred to below.

N2 isotherms were also determined on samples impreg-

nated with CaCl2 but are not shown as the uptake was always

significantly lower which indicates that CaCl2 is not a very

interesting impregnating agent in the present context.

3.3. Quantitative isotherm analysis

The results of the analysis of the isotherms by the as and DR

methods are given in Table 1. For the two samples indicated

by l.p.h. in Table 1 it is not appropriate to apply the DR meth-

od due to the presence of low pressure hysteresis. This is

indicative of non-equilibrium conditions at very low relative

pressures and equilibrium equations such as the DR equation

should not therefore be applied over this range of the

isotherm.

For the low burn-off samples, with burn-off <40%, the cor-

responding values of Vs and Vo are in close agreement (within

0.04 cm3 g�1), the values of external surface area (As) are low

and the values of mean micropore width, Lo, are all less than

or close to 0.7 nm. These features provide further confirma-

tion that the low burn-off samples were predominantly

ultramicroporous.

For these low burn-off samples, Fig. 4 shows the variation

of mean micropore width with burn-off and it can be seen

that NaCl or KCl addition causes a small increase. Addition

of 6.2% increases the Lo values by about 0.1 nm, while

addition of 33% or 50% increases Lo by a further 0.1 nm. The

results in Fig. 4 suggest that, in the case of these low burn-

off ultramicroporous materials, there is no significant differ-

ence between NaCl and KCl even though, as already pointed

out above, NaCl appears to provoke significantly more pore

widening in high burn-off samples. In comparison with other

precursor materials, the overall porosity of these low burn-off

activated carbons is similar to that found with activated car-

bons prepared from PAN [20] or cork [21] but slightly inferior

to what it is possible to achieve using Kevlar [22] or phenolic

fibres [23] as precursor where, for a fixed narrow pore size, the

micropore volume is somewhat higher.

For the samples with burn-off >40% larger differences,

especially for the NaCl impregnated samples, were found be-

tween corresponding values of Vs and Vo. In all cases, the for-

mer gives the total pore volume including micropores and

mesopores when they are present. Vo, on the other hand,

gives an estimate of a volume of micropores which includes

ultramicropores and may include some, but probably not all,

supermicropores. It does not include mesopores. Before

applying the DR Equation a correction should be made for

adsorption on the non-microporous surface area [24,25]. If

this is not done, then both Vo and Lo will be overestimated

[26]. A very large error can be involved when mesopores are

present as, even at very low relative pressures, the amount

adsorbed on the high surface area of the mesopore walls

can be comparable to the amount being adsorbed in the

micropores. This is the case with the high burn-off samples

prepared in this work and for this reason the calculated val-

ues of Vo and Lo for these samples are given in parentheses

in Table 1.

As the high burn-off samples contain mesopores we can

obtain an estimate of the maximum pore size of these sam-

ples by applying the Kelvin equation with correction for mul-

tilayer adsorption. In the case of the sample impregnated

with 33% KCl and activated for 1 h the linear region of the cor-

responding as plot began at about 0.6p� and we can assume

that all pores were filled at or below this relative pressure.

On the basis of the data in Ref. [27] and assuming slit-shaped

1016 C A R B O N 4 7 ( 2 0 0 9 ) 1 0 1 2 – 1 0 1 7

pores, this gives a value slightly greater than 3 nm for the

upper limit of pore width. For the sample impregnated with

33% NaCl and activated for 2 h the linear range of the corre-

sponding as plot began at about 0.8p� (including desorption

points) or 0.9p� (only adsorption points). These relative pres-

sures correspond to slit-shaped pore widths of 6 nm or

11 nm, respectively. If cylindrical pores are assumed, instead

of slit pores, these values increase to pore diameters of

10 nm or 20 nm, respectively. It is difficult to imagine that

such large pores would be formed by a normal process of

micropore widening and this therefore seems to provide addi-

tional evidence for the formation of a secondary mesopore

structure at high burn-offs in, at least, the samples impreg-

nated with NaCl.

For both low and high burn-off samples the total pore vol-

ume, Vs, is plotted as a function of burn-off in Fig. 5. It can be

seen that up to about 40% burn-off all of the data lies on a

common line. That is, although the addition of NaCl or KCl

significantly decreased the time needed to achieve a given le-

vel of burn-off, it did not affect the corresponding pore vol-

ume, which increased from 0.19 cm3 g�1 at 0% burn-off up

to 0.45 cm3 g�1 at 40% burn-off. On the other hand, if the line

on Fig. 5 is extrapolated to 70% burn-off, a value of

0.65 cm3 g�1 is predicted, which is lower than the values

determined for 3 of the 4 higher burn-off samples on the fig-

ure. This difference, and the corresponding upward devia-

tions seen on Fig. 5, may be a consequence of the formation

of the secondary mesopore structure.

The results in Fig. 5 are in marked contrast to the behav-

iour of raw alfa lignin, where the micropore volume was

found to reach a maximum of about 0.3 cm3 g�1 at about

20% burn-off and then decreased at higher burn-offs [9]. In

the present work, the total pore volumes always increased

and values greater than 0.8 cm3 g�1 were obtained under

appropriate conditions. These differences confirm that the ef-

fect of alkali metals on the activation does not depend just on

the amount present, but also on the manner in which they are

incorporated in the lignin structure.

0

0.2

0.4

0.6

0.8

0 10 20 30 40 50 60 70X / %

V s /

cm3 g-1

Fig. 5 – Variation of micropore volume estimated by the as

method with increasing burn-off, X, for activated carbons

prepared from alfa-A lignin impregnated with NaCl (open

symbols) or KCl (full symbols). Circular buttons – no

impregnation. Squares – 6.2%. Triangles – 33%. Diamond –

50%. Solid line – trend line for data from 9% to 40% burn-off .

4. Conclusions

The results presented show that a considerable reduction in

activation time even at a comparatively low activation tem-

perature could be achieved by impregnating demineralised

kraft lignin with relatively small amounts of NaCl or KCl.

Higher activation rates were observed when using KCl. Con-

sidering a fixed level of burn-off, the impregnation did not af-

fect the pore volume and only increased the pore width by

about 0.1–0.2 nm, depending on the amount of NaCl or KCl

used. By controlling the conditions it was possible to obtain

predominantly ultramicroporous materials with mean pore

widths over the range 0.53–0.77 nm. On the other hand, at

high levels of burn-off there was evidence for micropore wid-

ening into the small mesopore range and also for the forma-

tion of a secondary mesopore structure. Under these

conditions it was possible to obtain materials with higher

pore volumes up to 0.82 cm3 g�1. The presence of a secondary

mesopore structure in these materials could be advantageous

as it should improve accessibility of adsorptives to the micro-

pore entrances and hence increase rates of adsorption from

the gas or liquid phase.

Acknowledgements

This work was supported by the Fundacao para a Ciencia e a

Tecnologia (Plurianual Finance Project Centro de Quımica de

Evora (619) and post-doctoral Grant SFRH/BPD/20535/2004)

with national and European community (FEDER) funds and

by the Lignocarb Project funded by the European Commission

(Project ALFA-II-0412-FAFI). The authors thank Dr. V. Fierro for

providing the alfa lignin.

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