Characterization of pore structure by porosimetry and sorption on adsorbents produced from novolac-biomass

MATERIALS CHEM;S$W&D ELSEVIER

Materials Chemistry

and Physics 41 ( 1995) 245-250

Characterization of pore structure by porosimetry and sorption on adsorbents produced from novolac-biomass J. Simitzis, J. Sfyrakis, A. Faliagas Nutional Technical University of Athens, Department oj’Chemica1 Engineering, Laboratory ofSpecial Chemicul Technology, 9 Heroon Polytechniou Zogra@ Campus. GR-157 80 Athens. Greece Received

I4 September

str.,

1994; accepted 22 March 1995

Abstract

Mixtures of novolac resin and olive stone biomass (20/80 wt./wt.) were cured, pyrolyzed up to 800 “C (material C2) and 1000 “C (C3) and activated with steam (C4). 100% olive stone biomass was also pyrolyzed up to 1000 “C (Cl). The pore structure of these materials, along with a commercial activated carbon for comparison purposes, was determined by mercury porosimetry and vapour sorption of pentane, cyclohexane and toluene as well as by sorption of methylene blue dye from its aqueous solution. Cl was found to contain mainly mesopores and only a few micropores; this is the reason it adsorbed methylene blue but did not practically adsorb from the vapour phase. C2, C3 and C4 contained more micropores (C2
1. Introduction

Porous solid materials are used as adsorbents for the recovery of substances from the liquid and vapour phases, and adsorption is an important operation in several applications. Activated carbon is a widely utilized material for water and air pollution control in various separation and purification processes, and it is the most convenient adsorbent for the removal of substances in low concentrations [ 1,2]. The feasibility of replacing distillation by adsorption is being pursued in order to reduce energy consumption. Hybrid processes combining adsorption with distillation are promising for the separation of mixtures such as propane and propylene [ 3,4]. The application of adsorption in such hybrid processes or in new fields, for example for the removal of toxic organic contaminants (such as chlorofluorocarbons and other halocarbons) or heavy metals (such as Cd( II) and Hg( II) ) from aqueous solutions, also requires the development of new adsorbents [S-7]. Common commercial carbonaceous adsorbents are produced primarly from raw materials such as coals, wood, peat coconut and petroleum coke using a thermal or chemical activation process [ 1,2]. The demand for activated carbons 0254-0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .SSDIO254-0584(95)01539-7

Novolac-biomass

mixtures

with modified properties has led to the development of commercial activated carbons based on the carbonization of typical synthetic polymers such as styrene/divinylbenzene copolymers used specifically in water treatment [2,8]. Recently, the use of agricultural waste byproducts, e.g., peanut hulls, has led to the production of low-cost adsorbents [ 51. On the other hand, lignocellulosic natural polymers are important as polymeric materials, and are sometimes combined with synthetic polymers such as phenol, epoxy resins, etc. [ 9-121. The adsorptive capacity and selectivity of the adsorbents depends both on the raw material and on the production processes (carbonization, activation), which influence their pore structure [ 13,141. Common activated carbons have a broad pore size distribution with a large proportion of mesoand macropores showing adsorption capacities up to 0.85 cm3 g- ’ [ 151. On the other hand, carbon molecular sieves prepared from polyvinylidene chloride (PVDC) by pyrolysis have adsorption capacities of the order of 0.40 cm3 g-’ and pore sizes between 7 and 8 A [ 161. Carbonaceous adsorbents are less hydrophilic and their capacity and selectivity are relatively unaffected by the presence of water (vapour or liquid) in contrast to zeolites [ 81.

J. Simitzis et al. /Materials Chemistry and Physics 41 (1995) 245-250

246

Mercury porosimetry is appropriate for the investigation of pore size distributions in solids with pore diameters between about 37 and lo6 A. Pore sizes between a few angstroms (3 A or more) and about 5 X lo3 A can be determined by adsorption methods [ 17,181. Gases (e.g., nitrogen, helium) or vapours of liquids (water, alcohol, benzene, hydrocarbons, etc.) can be adsorbed by a solid if the width of its pores is greater than the molecular diameter of the adsorbate. The amount of adsorption depends also on the shape of the adsorbent molecule and on the shape of the pores of the adsorbent [ 17-231. Dye tests can be used to characterize the activity of adsorbents such as charcoal [ 171, Polymeric carbon adsorbents have also been produced by using suitable mixtures of novolac resin with olive stone biomass, which is an agricultural byproduct produced in very large quantities in Mediterranean and other countries [24271. It was shown in Refs. [ 24-271 that the adsorbents produced from mixtures of novolac and biomass in the proportion 20/80 have optimal adsorptive properties. The aim of the present work is to characterize the pore structure of these adsorbents. The following characterization methods were used: (i) mercury porosimetry; (ii) adsorption of organic adsorbates with different chemical structures (pentane, cyclohexane, toluene) from the vapour phase; and (iii) adsorption of methylene blue dye from aqueous solution.

for 30 min. Then the specimens were removed from the molds, put into a horizontal oven, and carbonized under an N2 atmosphere at an average heating rate of 4 “C min-’ up to different maximum temperatures. The residence time at the maximum temperature was 10 min. The material that consisted of 100% biomass was carbonized directly. Some samples (as shown in Table 1) were activated by using steam at 930 “C in a vertical oven. 2.2. Characterization of the carbonaceous materials

2. Experimental 2.1. Production method Carbonaceous adsorbents were produced from novolac resin and biomass [ 24,251. The resin was prepared by polymerization of phenol and formaldehyde (in the molar ratio 1.2211) with oxalic acid as the catalyst (in the proportion 1.5 wt./wt. with respect to phenol). Then the resin was separated, dried and pulverized. The biomass consisted of the residue obtained after removal of the oil from olives (olive stone biomass), which after drying was ground and sifted to yield grains with a diameter of less than 300 pm. Hexamethylenetetramine (hexa) was used as the curing agent for novolac in the weight ratio 7/2 (novolac/hexa) . Mixtures of biomass with novolac and hexa were placed in small cylindrical molds and curing was performed by heating at 170 “C Table 1 Compositions No.

of the raw materials, processes Material

Proportion (wt./wt.) Novolac

1 2 3 4 5

Cl c2 c3 c4 c5

for production

0 20 20 20

of adsorbents

of raw materials

The properties of the materials produced by the abovedescribed carbonization process and a commercial activated carbon were investigated. The samples were degassed under vacuum at 200 “C for 3 h just before characterization. Mercury porosimetry measurements (intrusion, extrusion) were performed at room temperature by using a Carlo Erba Porosimeter 2000 with re-evacuation of the samples to less than 0.5 torr [28]. For the vapour phase adsorption experiments, a quartz spring apparatus was utilized that included a Griffin and George cathetometer [ 29,301. The adsorbate substances were pentane, cyclohexane and toluene (reagent grade). The experiments were conducted at 20 “C and a relative pressure of P/P, = 0.1, where PO is the saturation pressure of the vapour at this temperature. The discoloring ability of the materials was examined against methylene blue dye in aqueous solution (initial concentration 0.0320 g l- ‘) . For this determination a certain amount of a carbonaceous material was mixed with a quantity of the aqueous solution of the dye at 20 “C under continuous stirring. Liquid samples were withdrawn at different time intervals, and after removal of the solid components from the samples, the color of the pure liquid was determined using a Colourimeter Lovibond Tintometer (model E) and a calibration scale.

3. Results Table 1 presents the compositions of the raw materials, the processes used for the production of the carbonaceous adsorbents and their shape. Tables 2 and 3 summarize the results of the mercury intrusion measurements. The volume of mer-

and their shapes Pyrolysis

Activation

W)

W)

(with steam)

Shape of product

Biomass 100 80 80 80 commercial

1000 800 1000 930 1000 activated carbon (mainly for industrial use)

grains small cylinders small cylinders small cylinders granules

J. Simitzis et al. /Materials

Table 2 Summary of the results of the mercury porosimetry Material

No.

570 8,

17.7 13.4 11.5 10.8 17.4

Cl c2 c3 c4 c5

a V, total pore volume; S, specific surface area; r,,, average Table 3 Summary of the results of the mercury porosimetry Mesopores

Material

No.

1 2 3 4 5

’

Pore volume (%) 10 000-72

1 2 3 4 5

measurements

Cl c2 c3 c4 c5

(mm3 gg’)

(m’g-‘)

(A)

44.3 49.1 51.5 50.0 56.4

38.0 31.5 37.0 39.2 26.2

452.4 269.4 287.7 551.9 781.9

74.5 43.0 45.8 96.6 95.8

121.5 125.4 125.7 114.2 163.2

pore radius.

measurements

for the macropores

and mesopores a Macropores

(37 A < r < 1000 A)

( r > 1000 A)

Cum. area (m’g-‘)

Cum. vol. (mm’ g-l)

Cum. area

306.9 176.4 190.7 398.8 457.1

73.86 42.44 45.20 95.73 94.50

145.5 93.0 97.0 153.1 324.8

0.64 0.56 0.60 0.87 1.30

volume; Cum. area, cumulative

.2

.l

C3

C4

(m'gg ‘)

specific surface area.

iv3 X

CS

Fig. 1. Adsorption x/m of pentane (white bars), cyclohexane (black) and toluene (shaded) after equilibrium for the adsorbents Cl-C5, wherex is the amount of vapour adsorbed and m is the amount of carbonaceous adsorbent.

cury penetrating the sample at pressures less than about 3 bar (at about 25 100 A> goes into void spaces among individual particles (i.e., a powder with relatively coarse grains), while the volume at pressures above 3 bar penetrates pores within the powder grains [ 171. The former volume is higher for the material C 1 owing to its shape and much lower for the other materials. All materials have a broad pore distribution in the range that is covered by the mercury porosimetry method (35krr75000A) [18]. Table 2 gives the results determined for the total pore volume, the surface area and the average pore radius, which were calculated according to the relationship r a” =

rw

37-100 A

.4

c2

S

100-10 000 A

ul $

Cl

V

Cum. vol. (mm3 g-‘)

a r, pore radius; Cum. vol., cumulative

.O

241

Chemistry and Physics 41 (1995) 245-250

2v,/s,

where r,, is the average pore radius, V,, is the specific pore volume and S, is the specific surface area; the cylindrical pore

model [ 8,17,31] is assumed. In the same table are also included the percentages of the pore volume that correspond to different groups of pore sizes. In all cases the highest values are observed in the range between 100 and 10 000 A. The total cumulative volume is more than 250 mm3 g- ’ for all materials, and the volume for C 1 and C4 is almost twice that of C2 and C3. The materials examined show hysteresis during the mercury extrusion [ 32-351, and there is also a similarity in the shape of the pore spectrum. Table 3 shows the cumulative volume and cumulative specific surface area of the mesopores and macropores. The pore 0 radius 1000 A has been chosen as the limit between mesopores and macropores, according to Ref. [ 361, for comparison purposes. This limit deviates from that for the IUPAC classification (micropores: r < 10 A; mesopores: 10 A < r < 250 A; macropores: r > 250 A, where 2r is the characteristic pore width) [ 17,181. Fig. 1 shows the pentane, cyclohexane and toluene uptake after equilibrium. Material C4 presents the highest ability for adsorption of vapours, which is higher than that of the commercial material C5; Cl (100% biomass) shows the lowest adsorption; and intermediate adsorption abilities are shown by C2 and C3 (that for C3 is higher). The adsorption of pen&me, cyclohexane and toluene separately, on the pyrolyzed but not activated materials (Table 1), follows the order C3 > C2 > C 1. For all materials, the weight of toluene vapour taken up is higher than the weight of cyclohexane adsorbed, and this is higher than the adsorption of pentane. Material C 1 shows low adsorption abilities from the vapour phase in comparison with the other adsorbents [ 351.

248

J. Simitzis et al. /Materials Chemists and Physics 41 (1995) 245-250

c4

Fig. 2. Specific pore volume VPfor the adsorption cyclohexane (black) and toluene (shaded).

(C4), however, leads to a remarkable improvement in V, S and r,,. Material Cl has higher values of V and S and almost the same value of r,, as compared with C3, which is also pyrolyzed up to 1000 “C. The pyrolysis of 100% biomass shows larger weight losses than the pyrolysis of the novolacbiomass mixture for the production of C3 [ 371, and the material obtained (C 1) contains more macropores in the range 10 000-72 570 A (Table 2). Mercury porosimetry is generally regarded as the best method available for the routine determination of pore sizes in the macropore and upper mesopore ranges [ 181. The mesopore range (including the low limit) as well as the micropore range can be investigated by the vapour adsorption method. The adsorption depends on many parameters, such as the pore sizes of the adsorbent and their distribution, the shape of the pores, the molecular size and shape of the adsorbate, the polarity of the adsorbent and the adsorbate, etc. In general it is difficult to evaluate the significance of these parameters, especially for carbonaceous adsorbents, which do not have a uniform pore structure, in contrast to other adsorbents such as zeolites. With respect to the molecular diameters of the adsorbate and the cross-sectional area a, of adsorbed molecules, the following values have been reported in the literature: cyclohexane: 4.8 X 6.8 A, a, = 0.38 nm’; toluene: 6.6 A, a, = 0.46 nm2; and pentane: a,,, = 0.37 nm2 [ 1,17,30,38]. Cyclic compounds, such as benzene and cyclohexane, which have large planar molecules are better suited to being adsorbed into slitshaped pores [ 20,221. Carbon molecular sieves prepared by the pyrolysis of polyvinylidene chloride, polyfurfuryl alcohol, etc., should have slit-shaped pores, which give them their molecular sieve properties [ 81. The adsorbates used in this work differ not only in molecular diameter but also in the shape of their molecules and their polarity. Both toluene and cyclohexane have flat mole-

Llii C5

of pentane (white bars),

Fig. 2 shows the results of the specific pore volume VP for the adsorption of pentane, cyclohexane and toluene. The order of the adsorbents with respect to their pore volume VP is similar to their order with respect to their uptake (Fig. 1). However, the order of the pore volumes with respect to the three vapours for each adsorbent differs from the corresponding orders of the weight uptakes. This feature is discussed further in the next section. Table 4 presents the results for the adsorption of methylene blue from aqueous solution. Comparing the pyrolyzed but not activated materials, C3 shows better adsorption characteristics than Cl: larger amounts adsorbed for the same parameter m and shorter times required to reach equilibrium. The activated C4 shows better adsorption characteristics than C 1, C2 and the commercial C5. C2 shows the poorest adsorption characteristics of all the materials.

4. Discussion The weight losses due to the reactions taking place during the carbonization process are very high between about 200 and 600 “C, while they are low above 800 “C [37]. The material pyrolyzed at 1000 “C is more stable during the pyrolysis. New reactions take place during activation of the latter material with steam. All these reactions also influence the formation of the pores in the material. The percentages of the pore volumes of C2, C3 and C4 differ only slightly (Table 2)) being about 50% in the range 100-10 000 A. A continuous decrease in the pore volume in the range 10 000-72 570 A from C2 to C3 and to C4 is observed. The pore volume of C3 in the range 37-100 A decreases slightly (and increases slightly in the range lo@-10 000 A), while the pore volume of C4 increases in the range 37-100 A. This means that during the pyrolysis, as the temperature rises from 800 to 1000 “C, larger pores are transformed into smaller ones ( 100-10 000 A), and then, during the activation stage, the pores are transformed into even smaller ones (37-100 A). According to the values of V, Sand r,, observed (Table 2)) the reactions taking place from 800 “C (C2) to 1000 “C (C3) lead to only a minor improvement in these parameters. The activation process

Table 4 Adsorption of methylene materials a No.

1 2 3 4 5 6 7 8 9 10 11 12

Material

Cl

c3

c4

c5

blue

from aqueous

Adsorption

solution

on carbonaceous

of methylene blue

m (gl-‘)

100x/x,

1.00 0.50 0.25 1.00 0.50 0.25 0.50 0.25 0.10 2.00 1.00 0.50

100.0 83.5 50.5 100.0 85.0 51.5 100.0 100.0 41.5 100.0 95.0 55.0

(%)

t(h) 2.50 24.00 24.00 1.oo 24.00 24.00 0.08 6.00 24.00 2.00 24.00 24.00

ax. amount of methylene blue adsorbed; x0, initial amount of methylene blue in the solution before adsorption; m, amount of carbonaceous material per volume solution; t, time to equilibrium.

J. Simitzis et al. /Materials

Chemistry and Physics 41 (1995) 245-250

cules, but cyclohexane occurs in two different conformations, i.e., armchair and boat forms, the former being more stable [ 391. Furthermore, the dipole moment of toluene is 1.13 ( 103’ Xp in Cm), while that for cyclohexane and pentane is zero [ 401. The dispersion forces between the aromatic rrelectron system of toluene and the n band of the graphitelike planes of the carbon are also responsible for adsorption [ 411. Cyclohexane, unlike benzene or toluene, is not capable of specific adsorption. In the nonspecific adsorption, only dispersion and repulsive forces are involved, while in the specific adsorption, coulombic contributions are also present [ 181. The adsorption of linear molecules such as pentane on the adsorbents used is lower than that for toluene and cyclohexane. The selective adsorption capacity follows the order planar> linear molecules, which is also observed in carbon molecular sieves [ 81. The very low adsorption capacity and pore volume of Cl for pentane, cyclohexane and toluene indicates that this material contains few micropores. On the other hand, the adsorption capacity and the pore volume (Figs. 1 and 2) increase in the order C2 < C3 < C4, where C2 shows a higher adsorption than Cl. This indicates that micropores are also contained in the pyrolyzed solid at 800 “C (C2) and more micropores are formed during the pyrolysis at 1000 “C (C3). The activation process (C4, CS) specifically favours the formation of micropores. The adsorption of different adsorbates on the same adsorbent (C2, C3, C4) increases in the order pentane < cyclohexane < toluene. The differences between the values of VP-toluene and those for Vi,-pentane or Vr-cyclohexane for C3 and C4 could be useful for the selective separation of toluene/pentane or toluene/cyclohexane mixtures. The commercial material, C5, reveals the same VP value for all adsorbents and therefore shows no selectivity for these substances. In spite of the very low adsorption capacity of Cl for all vapours used, it strongly adsorbs methylene blue from aqueous solution. Methylene blue, and dyes in general, have larger molecules than those of the vapours used in this work. The molecular area of methylene blue has been determined to be between 1.30 and 1.35 nm’ [ 171. It has been established that adsorption of such particles requires pores about 65 times larger than the particle [ 3 11. For the adsorption of dyes on activated carbons, their pore diameters should range from 20 to 500 A; active carbons with a high mesopore volume are usually used for decolorizing solutions by removing coloring impurities [36,42]. Cl has a high content of mesopores (Tables 2 and 3)) which favours the adsorption of methylene blue. The other materials contain mesopores, which favor the adsorption of methylene blue, and micropores (this is a conclusion of the above discussion and Figs. 1 and 2), which favor the adsorption of vapours. The results of Table 3 can be evaluated for practical applications according to Ref. [ 321. The volumes of macropores of active carbons usually fall within the range 200-800 mm3 g -I, and the specific surface areas of macropores are equal to 0.5-2 m* g- ‘. In common active carbons the volume of

249

mesopores is relatively small and lies between 20 and 100 mm3 g-l, and the specific surface area of mesopores is between 20 and 70 m* g- ‘. For active carbons with developed mesopores, however, the volume of mesopores may reach 700 mm3 g- ’ and their specific surface area may be in the range 200-450 m* g-‘. The effective radii of mesopores for the maxima of the distribution curves usually fall within 40200 A. The volume of micropores of active carbons is usually within the range 200-600 mm3 g - ’ . The most typical micropore volume for commercial active carbons is close to 400 mm3 gg’ . The pore characteristics of materials Cl-C5 (Table 3) with respect to the mesopore and macropore ranges are within these limits or have higher values, with the exception of the cumulative volume of the macropores. The macropores are transport arteries, making the internal parts of carbon grains readily accessible to the molecules adsorbed, while the mesopores and micropores are the adsorption sites of the adsorbent. The effective radii of the materials (Table 2) also fall within the limits mentioned above. The volume of micropores of C4 falls within the limits, while all others, including the commercial material C5, have lower values.

5. Conclusions The pore structure of adsorbents produced from novolacbiomass mixtures can be characterized satisfactorily by mercury porosimetry and vapour sorption. The adsorbent that consisted of 100% biomass and was pyrolyzed up to 1000 “C developed mainly mesopores, with only a few micropores, and for this reason it adsorbed methylene blue but did not practically adsorb from the vapour phase. The adsorbents prepared from novolac/biomass in the proportion 20/80, pyrolyzed up to 800 and lOOO”C, respectively, and subsequently activated, contained more micropores (C2 < C3 < C4), but they also contained mesopores, so that they were able to adsorb both vapours and methylene blue. The activated material C4 shows better pore characteristics compared with the commercial activated carbon C5. By a suitable choice of the novolac-to-biomass ratio, activated carbons with a controllable pore structure can be produced according to the application for which they are destined.

Acknowledgements The authors would like to thank Ms. Dipl. Chem. Welker of the Institute of Nonmetallic Materials, TU Berlin, for her contribution to the Hg porosimetry measurements.

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