Carbon-mineral adsorbents — new type of sorbents part II. Surface properties and methods of their modification

Materials

Chemistry

and Physics,

34 (1993)

123

123-141

adsorbents - new type of sorbents Carbon-mineral Part II. Surface properties and methods of their modification R. Leboda Faculty

of Chemistry,

(Received

November

Maria

Curie-Sklodowska

4, 1991; accepted

University,

September

20031 Lublin

(Poland)

9, 1992)

Abstract Chemical structure and morphology of carbon deposits contained in complex carbon-mineral adsorbents prepared by pyrolysis of different substances on the surface of mineral adsorbents such as silica gels, aluminium oxide, aluminosilicates, porous glasses, etc. have been discussed. The morphology of the carbon deposit depends on the course of the pyrolysis reaction and determines the surface properties of complex adsorbents and the possibilities of their practical utilization as sorbents and catalysts. The methods of modification of adsorption properties of such adsorbents prepared on technical and laboratory scales were also presented. Thermal properties of carbon-mineral adsorbents in atmospheres of different gases were also discussed.

Introduction

In the previous work [l] the preparation methods of complex carbon-mineral adsorbents were described. The possibilities of utilization of such adsorbents in adsorption practice depend on the physical and chemical characteristics of these adsorbents. The mechanical properties and energies of the bending atoms of the mineral matrix with a carbon deposit are the important physical features of the complex adsorbent. These characteristics are associated with the morphology of deposited coke which is dependent on the mechanism and conditions of coke formation. The latter factors determine in turn sorption, ion-exchange, catalytic and other properties of the carbon adsorbent. For this reason the above questions are discussed in detail in this paper. Moreover, the data relating to surface properties of carbon-mineral adsorbents prepared on the basis of natural sorbents, carburized catalysts and adsorbent as well as on the basis of silica gels are presented in this paper. Methods to modify the surface properties of these adsorbents with respect to their application in industrial and laboratory techniques have also proved reasonable.

Morphology deposit

and chemical

composition

of carbon

In general the following types of carbon which may be formed during the carburization of adsorbents and/ or catalysts can be distinguished [2]:

0254-0584/93/$6.00

1) polycrystalline graphite; 2) tube-like graphite fibers; 3) dendrites; 4) carbon blacks; 5) carbon films and pyrocarbon layers. The formation of carbon-mineral adsorbents containing carbon deposits in the form of dendrites, whiskers or carbon blacks is not advantageous because of the poor mechanical properties of these deposits. The morphology of the coke depends on the mechanism and conditions of its formation on the mineral surface. Two main mechanisms of formation of carbon deposits can be distinguished: consecutive and carbide forming. The latter mode consists in the thermal decomposition of hydrocarbons. From the viewpoint of the expected properties of carbon-mineral adsorbents the cokes formed in consecutive reactions are more interesting than those formed in a carbide cycle because in such reactions it is mostly the carbon deposits characterized by good mechanical properties that are obtained, although these deposits are less homogeneous and more poorly ordered than the cokes formed during the carbide cycle. However, they are characterized by a stronger adhesion to the mineral matrix. In practice, independently of deposit formation, these deposits are usually mixtures of different types of carbon. As a result, they are too small or to a greater extent morphologically heterogeneous. Thus we will consider coke formation on the surface of the adsorbent/catalyst as the polycondensation of starting organic substances or the products of an earlier

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124

reaction of these substances which are capable of further reactions [3]. The process of formation of polycondensation products cannot be illustrated by only one chemical reaction (Scheme 1 presented in a previous paper [l] illustrates only the general mechanism of coke formation). This process can be imagined as a multitude of different polycondensation reaction processes for hypermolecular structuring of the condensation products. In the case of aluminosilicate sorbents and catalysts the tendency for transformation into coke is observed mainly for unsaturated hydrocarbons and to a significantly smaller extent for saturated hydrocarbons [4]. Formation of the cokes from unsaturated hydrocarbons on the aluminosilicate surface occurs according to the consecutive scheme consisting in continuous cyclization, dehydrogenation and formation of the bonds between the cycles [2]. Figure 1 shows as an example a simplified scheme for the dehydrocondensation reaction of ethylene on aluminium oxide in the temperature range 700-750 “C [2]. During this process aromatic tars and carbon are formed. The hydrogen/carbon mass ratio in the pyrolysis products depends on the type of catalyst, composition of the starting material, temperature and time of the process. During cracking of different starting materials on aluminosilicate catalysts under the same temperature and time conditions the hydrogen/carbon mass ratio in the deposited coke ranged from 0.86 (when benzene was used as a starting material) to 0.38 (for light gas oil) [5]. With an increase in temperature the coke becomes more dense owing to the removal CH*-CH*

.CH2=CH2,CH3_CH2_CH-CH=CH2 *cH~-cH~CH3-CH*-CH=cH~ CH3

Fig. 1. Simplified scheme of the reaction of ethylene on A1,03 in the temperature

I W

*CH =CH 7

of dehydrocondensation range 70&750 “C [Z].

of light hydrocarbons. Then a continuous increase of the molecular weight of coke, an increase in the content of aromatic structure as well as a decrease of the hydrogen/carbon mass ratio are observed. With an increase of time of the cracking process the content of hydrogen in the formed coke decreases. For example cracking of hexane on an aluminosilicate catalyst at 520 “C for 1.5 min leads to the formation of a coke of composition of CH1.61, and after 180 min of the process the product formed is CH,,.48 [6]. In practice the hydrocarbons used as starting materials always contain sulfur-containing compounds. Thus the formed cokes usually contain sulfur. Sulfur deposits are always in the most condensed part of the coke layer [5, 71. Some information about the contents of carbon, hydrogen and sulfur in the coke can be obtained from the stoichiometric formula of the coke formed during cracking of the high boiling fractions of Tumazin (Russia) crude oil on an aluminosilicate catalyst. The formula is CH,,,&$,, [6]. In addition, the carburization process occurs often in an oxygen-containing medium and owing to this fact some amounts of oxygen also deposit in the coke structure [6]. For example, during the oxidizing dehydrogenation of ethyl benzene on a A&O, catalyst a coke is formed. Some components of this coke dissolve partially in the alcohol-benzene mixture. Soluble components are of the formula CH,,,O,,,, and insoluble CH,,,O,,,, [5]. Sulfur and oxygen enter in the skeletons of polycyclic carboids in the form of heteroatoms. These elements also form the OH, COOH, SH and other functional groups saturating the peripheral free valences of polycyclic macromolecules of the carboids. One can suppose that similarly to tarry asphaltene fractions of crude oil, the sulfur atoms form sulfur and sulfur-oxygen bridges between the cyclic arrangements of the coke. Penetration of oxygen into the carbonaceous material takes place also during partial oxidation of this material. For example, it is well known that during heating of the carbon in air or in an oxygen stream at 420-450 “C acidic oxides are formed on the carbon surface [8]. Complexity of the composition of carbon deposits formed according to the consecutive mechanism determines the structural heterogeneity of these deposits. By means of roentgenographic measurements it was stated that the surface layer of the coke contained either pseudographite or roentgenographic structures. In some cases the content of these structures reaches 50% [5-91. The contribution of the pseudographite component increases with the increase of temperature and carburization time. Pseudographite and roentgenographic components are characterized by narrow and wide EPR signals, respectively [lo, 111. The narrow signal is very susceptible to the adsorption of oxygen on the coke, while the wide one is not susceptible to such an adsorption. Broadening of the narrow signal

125

observed during the adsorption of oxygen may be explained by mutual interactions of paramagnetic centres of polycyclic systems in which the system of delocalized electrons and paramagnetic oxygen also participates. Probably the roentgenographic component of the coke giving wide EPR signals corresponds to monoaromatic compounds with branched peripheral aliphatic chains or phenyl radicals connected via aliphatic hydrocarbon chains. Similar conclusions may be drawn on the basis of the data obtained by reaction gas chromatography [12] and IR spectroscopy [12, 131 as well as by electron microscopy [14] of carbon deposits on a silica surface. If we compare such carbon deposits to high molecular kerosene compounds it will appear that the roentgenographic part of the coke (so-called ‘soft coke’ according to the terminology proposed in ref. 11) is very similar to tar as far as the structure and properties are concerned, whereas the pseudographite structure (so called ‘condensed coke’) is similar to the asphaltenes [15]. According to the evaluation presented in ref. 5 the amorphous phase of the coke surface structure is 1.2-1.4 nm. The size of the pseudographite structure of the coke crystallites is greater (c. 4 nm). However, in a macroporous space of an inorganic matrix large crystallites of the coke of 14-25 nm and even 30 nm are observed. The surfaces of porous glasses carburized by the products of pyrolysis of aliphatic alcohols contain crystallites of greater dimensions (50 nm) as well as larger agglomerates [14]. These crystallites exist in the form of spherical particles. The presence of two types of pseudo-graphite surface structure differing in size indicates the possibility of migration of the condensation products along the solid surface. Few data exist relating to bonding energies of carbon deposits with the mineral surfaces of carburized adsorbents/catalysts. The carbon deposits formed in the decomposition reactions of methylene chloride [17] are especially strongly bonded with this surface. The energy of bonding of surface aluminium atoms contained in Al,O, with coke deposits was estimated as 10-16 kJ mol-’ [18, 191. Th ese deposits were characterized by a high percentage of carbon and graphite. Thus it can be expected that less ordered deposits will interact more strongly with the mineral matrix.

Surface

properties

Technical adsorbents

Formation of the porous structure of carbon-mineral adsorbents in the process of carburization of adsorbents/ catalysts is very complex. The course of this process depends on diffusion and kinetic factors as well as on the type (i.e. dimensions of molecules and their properties) of the carburized substance. The primary struc-

ture of a mineral matrix is to a greater or smaller extent modified with the carbon deposit which can have also its own porous structure. It is generally assumed that the coke deposits above all in the peripheral parts of the grains of an adsorbent/catalyst as well as in the inlets of the pores [20]. This fact may be illustrated by the example discussed in an earlier paper [l] on the preparation of carbon-mineral adsorbents by the pyrolysis of divinyl on an aluminum oxide surface [21]. The coke was deposited on the entire kinetically accessible surface of Al,O, in the form of particles of c. 3 nm. The general percentage of the coke ranges from 18-22% w/w. Macropores greater than 10 pm contain 25-30% of the total amount of the coke, pores of radii 20-200 nm contain 15-20%, whereas within the pores of 20 nm 45-50% of the total amount of coke is deposited [23]. Thus a portion of primary narrow pores is blocked by the coke particles, and in other large pores it forms a secondary porous structure which is due in turn to the presence of not too large carbon agglomerates. The above phenomena are very important for preparation of effective carbon-mineral adsorbents. In the case of aluminosilicate catalysts the theoretical amount of the coke needed for filling the whole volume of the pores is about 60-70% wt./wt. [24]. In practice, however, the maximum amount of coke at which the process of formation of the coke disappears is very different for individual hydrocarbons and changes significantly, i.e. from lO-50% wt./wt. [2], which indicates a high degree of pore blocking. The problems presented here are widely described in the literature and are the subject of interest for catalysis and therefore they are not discussed in detail here. Now we will discuss some curiosities of complex adsorbents prepared on the basis of carburized slimes (such as montmorillonites, bentonites, palygorskites, etc.). The analysis of porous structure and properties of carbon-mineral adsorbents prepared on the basis of carburized slimes [25] shows that in respect of their physico-chemical characteristics these adsorbents are close to a hypothetically ideal synthetic carbon-mineral adsorbent. Such an ideal complex sorbent can be imagined as a binary carbon-aluminosilicate material of biporous structure and characterized by strong mutual adhesion of carbon and aluminosilicate layers. In narrow pores of the starting mineral adsorbent (pore radii change from 0.5 to 2 nm) there should be sorbed from aqueous solutions such substances as noniogenic and cationic surface active agents, cationic dyes, proteins, water-soluble polymers, etc. Above all it refers to substances for which removal of aluminosilicates rather than active carbons is recommended. Larger pores present in carbon-mineral adsorbents (i.e. mesopores of 10-20 nm radii) should be covered with a sufficiently thin (2-5 nm) and if possible microporous carbon film.

126

Hydrophobization of mesopores with the carbon layers promotes capturing such substances as dye anions, surface active agents and other compounds from the aqueous solutions which are not adsorbed on the starting mineral adsorbent. The two systems of pores in ideal carbon-mineral adsorbents should be mutually independent, i.e. should possess their own arteries connecting them with the transport macropores. In the macropores large carbon clusters should be localized, characterized by a microporous structure and capable of adsorption of benzene and its derivatives from aqueous solutions (traditionally, active carbons are used for such purposes). In fact, the carbon-mineral adsorbent prepared on the basis of carburized palygorskite-montmorillonite clay has two systems of the pores resulting from the structure of the mineral matrix [28]. Due to a relatively low temperature of activation of the coke layer, i.e. 350 “C, the mineral matrix does not change its primary adsorption properties, above all, the primary (interlayer spaces) and secondary (tube pores) of montmorillonite of 0.8-2 nm; the secondary pores are similar to those, cylindrical (these are the palygorskite pores of the radii 1.8-2.5 nm), and free from carbon clusters. In such pores such substances as e.g. noniogenic surface active agents can be effectively adsorbed (Table 1). The most active centres on hydrophilic surfaces of secondary mesopores in the clay of the radii 8-30 nm are blocked by thin discrete carbon layers of t=3-5 nm. In such hydrophobized pores large molecules of anionic dyes are effectively adsorbed (see Fig. 2). The microporous carbon clusters up to 30 nm distributed in large mesopores and macropores of the sorbent adsorb in turn molecules of n-nitroaniline relTABLE 1. Maximal sorption capacities for n-nitroaniline and OP-7 obtained on the following adsorbents: starting, natural montmorillonite-palygorskite clay (A-4), the same clay consumed in the process of purification of aromatic substances (OA-4), KAD active carbon and on carbon-mineral (C-M) adsorbents obtained by the activation of OA-4 adsorbent in the nitrogen stream and in the temperature range of 300-500 “C (column 1). In columns 2 and 3 the properties of carbon-mineral adsorbents prepared earlier from OA-4 and treated before the thermal activation with copper (2) and cobalt (3) chlorides are listed Adsorbent

am (mg g-i) n-nitroaniline 1

OA-4 A-4 C-M-300 C-M-350 C-M-500 KAD

4 0 8 14 18 280

OP-7

2

3

1

2

3

-

-

170 370

-

-

15 23 21 -

30 28 23 -

210 190 150 160

220 190 150 -

230 210 160 -

\m6 -u

EL 2

0

(4

i:_‘_ A

d

5

10

15

Cp.104mole/l

20

0

(b)

5

10

15

20

Cp'10L,mole/l

Fig. 2. Adsorption isotherms of blue dye (a) and Congo red (b) from aqueous solutions on carbon-mineral adsorbents prepared on the basis of attapulgite (l), a geneticmixture of montmorillonite and palygorskite (2) and on KAD (Russia) active carbon (3).

atively well (Tables 1 and 2). The carbon layer in the carbon-mineral composite is formed on the basis of the olefines which have undergone earlier chemisorption on the acidic centres of the clay surface. For this reason the carbon deposit is strongly bonded with the mineral matrix. The points of contact between the particles of the modified carbon layer are also in the summaric systems of the mineral matrix. This prevents wettability of the mixed adsorbent in water. Moreover, the total mechanical resistance of carbon-mineral adsorbent produced on the basis of palygorskite and palygorskitemontmorillonite clays is one order of magnitude greater than that of KAD charcoal (made in Russia) [27]. Table 2 lists the adsorption characteristics of carbon-mineral adsorbents prepared on the basis of carburized clays, as well as the data obtained for starting and thermally modified sorbents [2&30]. From these data it can be seen that the carbon-mineral adsorbents show, to a significant degree, the adsorption properties of the starting material. In general, their porosity is determined by the porous structure of the sorbent component; the carbon layer deposits mainly in the mesopores of the mineral matrix and its presence practically does not influence the pore distribution according to their effective radii; small (r=2.5-3 nm) mesopores of carbon-mineral adsorbent are accessible to adsorption of water and hexane molecules. By an appropriate selection of starting mineral, the dimensions of carbon-covered pores present in the complex adsorbent and its adsorption properties in relation to different (i.e. differing in molecule sizes) substances dissolved in water can be regulated. To illustrate this fact we will analyze the data presented in Table 1 and Fig. 2 related to the adsorption characteristics of carbon-mineral adsorbents prepared on the basis of a strongly dispersed genetic mixture of palygorskite and montmorillonite (originating from Tcherkask region, Ukraine) and less dispersed attapulgite. The last material is characterized by larger mesopores (Table 2). In this connection the carbon-mineral adsorbent prepared on the basis of such material is characterized by an increased sorption ca-

127 TABLE 2. The characteristics of the porous structure of starting (natural) mineral adsorbents undergoing thermal treatment and complex carbon-mineral sorbents obtained on the basis of the former. S, the surface area; V,, the maximum sorption capacity; r, the effective radii of mesopores V, (cm3 g-‘)

S (m’ g-‘)

Adsorbent

r (nm)

water

hexane

water

hexane

water

310

124

0.44

0.34

2.5, 6

10

6-8

10

palygorskite (500 “C)

123

124

0.29

0.33

8

10

8

10

C-M on the basis palygorskite

139

125

0.22

0.18

6

10

6

10

natural attapulgite attapulgite (500 “C)

190 80

90 70

0.25 0.26

0.16 0.21

3, 6 3

10 10

3, 6 3

10 10

C-M on the basis attapulgite genetic mixture of palygorskite and montmorillonite (A-4)

110

70

0.20

0.15

3, 6

10

3

10

378

211

0.47

0.35

2.5-3

2.5-3

130 128

150 102

0.33 0.22

0.36 0.20

2.5-3 3

2.5-3 3

natural

palygorskite

A-4 (500 “C) C-M on the basis of A-4

pacity in relation to large molecules of anionic blue dye in comparison to the complex adsorbent prepared on the basis of the genetic mixture of the clays and microporous active carbon KAD. It is, however, a poorer material in comparison to the latter in respect of adsorption capacity for small n-nitroaniline molecules from aqueous solutions (Fig. 3) [27]. On the basis of the data presented in Table 2 and in Fig. 3, the authors of ref. 27 have drawn a conclusion that the modifying layer of carbon deposit has a discrete nature and the dimensions of its clusters contained in mesopores change from 5.7 to 9.2 nm. Reference 31 is concerned with structural-sorption investigations of the carbonized active precipitates (clays) originating from the biological purification stations for waste water from cellulose paper factories. Pyrolysis was in the temperature range 500 to 900 “C for 30, 60 and 90

0.2

? f

0,l

z

vihie&

0-

(a)

0 Cp,mmole/

I

(b)

I

Cp,

,

mmole/l

Fig. 3. Adsorption isotherms of n-nitroaniline (a) and blue dye (b) from aqueous solutions on the carbonizates of active clays prepared in the following conditions: 700 “C, 30 min (1); 700 “C, 60 min (2); 500 “C, 30 min (3); 500 “C, 60 min (4, 6) and on KAD active carbon (7).

hexane

min; for each successive operation the temperature was increased by 100 “C. A significant effect of these parameters on the amount of carbonizate obtained has not been observed. The content of carbon in the carbonizates was 45-50%. Sigmoidal adsorption isotherms of water and n-hexane vapours were obtained. Such a shape of adsorption isotherms is typical for sorbents of heterogeneous porous structure. Pyrolysis of the clay at a temperature above 700 “C leads to the formation of carbonizates of decreased ability of adsorption of water in comparison to the adsorbents prepared at lower temperatures. At this temperature the adsorbents of highest specific surface area (SHZO=189 m2 g-l, s CbH14 =99 m2 g-l) are obtained. Specific surface area S of all tested adsorbents measured in relation to nhexane was 2-3 times lower than that measured in relation to water. This is connected with the presence of the hydrophilic mineral component in the carbonizate which adsorbs preferentially the molecules of water and not those of hexane. The above fact also suggests that in the structure of carbonizates there exist ultramicropores accessible for small water molecules. In comparison to the active carbons such adsorbents are characterized by their small volume of micropores. This parameter is practically independent of pyrolysis process conditions. In the porous structure of the adsorbents discussed here, the supermicropores and small mesopores predominate. For adsorption of organic substances (n-nitroaniline, anionic blue dye) and iodine the optimal adsorbents were prepared at a temperature of 600-700 “C for

128

30-60 min. Such adsorbents adsorb greater amounts (N 20%) of the dye than typical active carbons which is probably connected with the presence of the mesopores in the porous structure of such adsorbents. These mesopores have greater volume than those present in active carbon. In Fig. 2 the adsorption isotherms of the above organic substances from aqueous solutions on several carbon-mineral adsorbents and on active carbon KAD (Russia) are presented. The relatively low temperature (600-700 “C) of pyrolysis as well as the short time of this process ensure that the complex adsorbents prepared on the basis of the active precipitates (clays) are very economical materials for further purification of waste waters. Ion exchangers Tarasevich [25, 321 suggests that the utilization of carburized sorbents and catalysts for preparation of low-cost carbon-mineral adsorbents which may be utilized, in turn, as ion-exchangers appears to be a very promising scientific undertaking. During removal of ionic substances from water by means of carbon-mineral adsorbents, the ion-exchange properties either of the mineral matrix or of the carbon deposit can be utilized. Natural and synthetic aluminosilicates utilized as the matrices of carbon-mineral adsorbents are characterized by strictly defined cation-exchange properties. However, the sorption capacity of these materials decreases dramatically after their thermal treatment at 500 “C. Moreover, at high temperatures the mineral adsorbents can undergo strong sintering. Thus the problem of the appropriate method of modification of the surface properties of the adsorbent and catalysts permitting the maintenance of their ion-exchange properties emerges. In ref. 33 it was shown that, depending on the type of inorganic matrix and carbonized organic substances as well as on the conditions of thermal treatment, carbon-mineral ,adsorbents characterized by different ratios of the numbers of cationic and anionic centers situated on the surface of the carbon deposit can be obtained. For this reason, the examination of the chemistry of its surface appears to be very important. This is a complex problem because in the sorption, and in ion-exchange processes also, the inorganic support of the carbon deposit can take an active part. Thus in any case the standard method of determination of adsorption capacity of active carbons consisting in the sorption of methylene blue cannot be used to estimate the quality of the modifying carbon layer of a carbonmineral adsorbent because the mineral matrix is also active and adsorbs this dye from the aqueous solution. The nature of ion-exchange centres of carbon adsorbents is determined by the conditions of their preliminary treatment. Active carbon undergoing the hightemperature treatment (900-1000 “C) in an air-free

system shows significant anion-exchange properties [34]. It is capable of chemisorption of oxygen at low temperatures (up to 100 “C), which leads to the formation of surface oxides of a basic nature. The structure of such oxides is relatively little known [35, 361. From the electrochemical data it results, however, that the chemisorbed oxygen is relatively weakly bonded to the carbon surface and passes to a diffusion part of the electrical double layer in the form of hydroxide ions which cause the surface to become positively charged. Acidic surface oxides form intensively due to the interactions between carbon and oxygen at 430+0 “C

[361. The authors of ref. 37 found that a sufficient amount of acidic oxides appeared on the surface of natural carbons when these carbons were oxidized in air at a very moderate temperature (150-200 “C). These differences in temperatures of formation of acidic surface oxides result from the differences in the structures (carbonization degree) of natural and active carbons. However, it should be noticed that at temperatures close to those mentioned above the main product which deposits on aluminosilicate adsorbents and catalysts during the processes of catalytic cracking or adsorptioncatalytic purification of kerosene products, is the coke. Moreover, the activation of the carbon deposit contained in the carbon-mineral adsorbent under limited access of air is realized also in the temperature range 300-350 “C. This means that cation-exchange properties should show on the surface of carbon-mineral adsorbents prepared on the basis of carbonized clays and aluminosilicates. In this connection the comparison of ionexchange properties of active carbons with those of carbon-mineral adsorbents appears to be very interesting. Active carbons are usually anion-exchangers. Their active groups dissociate in water, splitting off hydroxyl ions. For example, from the data of conductometric titration of KAD active carbon it results that this carbon contains only OH groups of concentration 0.63 mol kg-’ [25]. A similar number of basic groups is possessed also by the other active carbons, e.g. those produced by Norit [38]. Carbon-mineral adsorbents produced on the basis of montmorillonite clay [33] contain 0.50 mol kg-’ of carboxyl groups (cation-exchange centers) and 0.47 mol kg-’ of anion-exchange OH centers. The same adsorbent obtained by the carburization of attapulgite clay (Attapulgus, Georgia, USA) possesses 0.165 mol kg-’ of -COOH groups and 0.84 mol kg-’ of OH groups. Both sorbents mentioned above are the carburized sorbents consumed in the adsorption-catalytic process of purification of aromatic substances from unsaturated hydrocarbons. These adsorbents were modified additionally with water vapour at 500 “C according to the procedure described in ref. 30. In the latter

129

adsorbent the number of anion-exchange centres is higher than in the former because the starting attapulgite clay contained significant amounts of calcium carbonate impurities. After a preliminary treatment of the carbon-mineral with acid these adsorbents become capable of cation exchange and are characterized by strictly defined selectivity in relation to Hg(II), Cu(II) and Co(I1) ions 1351. Comparison of sorption properties of natural clays, clays after thermal treatment and carburized, as well as of active carbons is interesting. The results of such a comparison made for l,l’-ethylene-2,2’-dipyrylbromide (d&vat) herbicide are presented in Fig. 4. The above data were obtained from the paper by Tarasevich et al. [33]. At pH values near to neutral the above herbicide exists in solution in the form of an organic bivalent cation [36]. In this connection the dikvat sorbs on the natural minerals according to the cation-exchange mechanism. ,Figure 4(a)-(c) shows the adsorption isotherms of the discussed substance on comparable adsorbents. The individual minerals are characterized by the following cation-exchange capacity E [33]: montmorillonite - 0.18; natural mixture of montmorillonite and palygorskite - 0.53; attapulgite - 0.3 equiv. kg-‘. The existence of a correlation between E values &d sorption am of diquate was observed. A detailed interpretation of the data presented here (Fig. 4) is given in the publication by Tarasevich [33]. Here we will pay some attention to the fact that montmorillonite appears to be the preferred adsorbent for removal of cationic pesticides from water because of its high selectivity and sorption capacity. This capacity is one order of magnitude greater than the adsorption capacity of typical active carbon. The montmorillonite is characterized by high thermostability because heating of this mineral at 500 “C practically does not change its sorption capacity (Fig. 3). Also the carburized montmorillonite containing 20

1

," 2 15

f

2

5 E_ 10 x-4 05 b

3

0

Lziz_&r_

1

(4

1

2'0 Cp.lOL

2

0

I mole/I

@I

1 cp.loL,

2 mole/

i

(cl

Fig. 4. Adsorption isotherms of diquate on unmodified mineral adsorbents (a, b) and carbon-mineral adsorbent (c): (a) montmorillonite (from Tadzykistan) (l), genetic mixture of montmorillonite and palygorskite (3); (b) montmorillonite (from Tadzykistan) (1) and attapulgite (2) thermally treated at 500 “C; (c) carbon-mineral adsorbents prepared on the basis of montmorillonite (from Tadzykistan) (I), attapulgite (3) and on KAD active carbons (4).

9.5% wt./wt. of carbon [33] shows higher (nearly one order of magnitude) sorption capacity (am = 0.38 mol kg-l) than the active carbon (Fig. 4(c)). These differences can be explained by the different surface properties of carbon deposit in complex adsorbents and active carbon which, as was mentioned before, is the anion-exchanger. One of the ways of improvement of sorption properties (selectivity) of active carbons consists in their treatment with sulfur-containing compounds [40-42]. The surface S-H and C=S groups can form stable compounds with the cations of heavy metals. The formation of the coke on the aluminosilicate surface occurs usually in the presence of sulfur and in fact this element enters the porous structure of the surface carbon layer. Thus it can be expected that such adsorbents will show a high selectivity in relation to heavy metal cations. In fact the clays consumed in the processes of purification of feed and technical oils are characterized after thermal treatment at 500 “C by strong adsorption properties in relation to mercury, zinc, cadmium and copper [43]. Similar results are presented in ref. 44 where the thermal treatment of consumed clay was made at 700-900 “C. By mixing charcoal with the solution of metal salts (or metal oxide ~01s) followed by the deposition of 30% of metal oxides on the surface of active carbon, a selective ion-exchanger can be obtained 1451. For example, active carbon with chromium hydroxide sprayed on its surface sorbs vanadium, nickel and lead well from water. The same adsorbent covered with iron(I1) hydroxide removes such elements as vanadium, beryllium, uranium and strontium from water. Laboratory adsorben ts Surface and chromatographic properties of the adsorbents prepared by the pyrolysis of benzene on a silica gel surface were described in the papers by Bebris et aE. [46, 471, Guiochon et al. [48, 491, Leboda [50, Xl], and Gierak [52]. Other papers described properties of the carbosils prepared by pyrolysis of aliphatic and aromatic alcohols, the mixtures of such alcohols [55-781, aliphatic and aromatic hydrocarbons [79, 801, their derivatives [67, 691, and methylene chloride [81-901 as well as of the carbosils prepared in the carburization reactions in the presence of metal catalysts or metal ions [69, 931. The surface properties of carbon-silica adsorbents were investigated by means of the following methods: gas chromatography [50-71,75-931, high pressure liquid chromatography [46-49, 52, 85, 861, gas reaction chromatography [50, 51, 67, 773, IR spectroscopy [50,51,59,77], roentgenography [9], differential thermal analysis [67, 89, 911 and electron microscopy [9, 67, 72-74, 78, 79, 87-901. The main purpose of these works was to examine the morphology and topography of carbon deposits, the effect of these factors

130

on the sorption properties of these materials (either from gas or liquid phase) and on the selectivity of chromatographic separation of the mixtures consisting of substances of different chemical natures. Some surface properties of carbon-silica adsorbents were also presented during the description of the methods of their preparation [l] and in this connection we will pay attention only to some features of these adsorbents. It is known [94] that the theory of adsorption on heterogeneous solid surfaces distinguishes two main models of distribution of energetic centres. The first model of the surface assumes the so-called patch-wise topography distribution of adsorption centres, and in the second model a random topographical distribution of the energetic centres. The carbosils really satisfy the first theoretical model, and some among them also the second one [17], but the first case is more interesting. Figure 5 shows the surface of such a ‘mosaic’ carbosil. The carbon ‘patches’ are so large that they fulfill the assumptions of the theoretical model [94]. The second model photograph (Fig. 6) presents the structure of the carbon deposit removed from the silica surface in a special way [73]. Such deposits are more or less heterogeneous, which is reflected in the photograph (Fig. 6) in the form of darker and brighter regions. Thus the carbosils in general satisfy the patch-wise model with local heterogeneity. Although the significant energetic heterogeneity of the surface of the adsorbent leads to the increase of its sorption capacity [95], this phenomenon is, however, undesirable from the chromatographic point ofview, because it leads to broadening of the chromatographic bands. The ideal complex carbon-silica adsorbent should consist of homogeneous patches and then its selectivity in the process of chromatographic separation will depend on the ratio of areas of the carbon and silica ‘patches’ as well as on the morphology and chemical structure of energetic centres existing on the surface of the carbon deposit.

Fig. 5. Surface of a carbosil obtained by covering of silica gel with a carbon layer formed by the pyrolysis of n-heptanol [X?].

Fig. 6. Morphology of carbon presented in Fig. 5.

layer extracted

from

carbosil

The structure of carbon deposits prepared in the absence of the catalysts is most often globular [17, 781. Good agreement between the course (shape) of the differential function of distribution of adsorption energy of test substances and topography and morphology of the carbon deposits contained in the carbosils prepared by pyrolysis of different substances is found [59, 76, 961. From IR spectroscopy and reaction gas chromatographic data it can be seen that the carbon deposits prepared by pyrolysis of aliphatic and aromatic alcohols and their mixtures contain either aliphatic or aromatic segments and are to a significant degree hydrogenated [12, 571. The larger the percentage of aliphatic substances in the pyrolyzed mixture, the more hydrogenated the carbon deposit. From this there results a simple correlation between the characteristics of the carbosil surface (adsorption isotherm, adsorption heat, etc.) and the composition of the pyrolyzed mixture [76] which is also connected with the amount of carbon obtained. The larger the percentage of aromatic compounds in the carbon deposit, the stranger the adsorption properties [12, 51, 571. The carbosils considered here were prepared in relatively moderate thermal conditions (300-500 “C) and for this reason - regarding the course of the reaction (1) presented in ref. 1 - it can be supposed that these deposits are not the typical strongly condensed cokes. They are called ‘polymeric carbons’ [13,50]. The increase of the temperature of the pyrolysis process causes the increase of the percentage of aromatic compounds in the carbon deposit, which is connected, in turn, with the increase of adsorption energy of hydrocarbons on such carbon deposits [12, 50, 51, 58, 681. The carbon deposits obtained in the decomposition reaction of methylene chloride on the silica surface possess completely different properties. IR measurements showed [12] that such deposits consist of pure carbon and are characterized by a globular structure

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[17, 901. Depending on the reaction conditions the size of these globules can change from 100 to a few thousand A. They can also form agglomerates of 1600-20000 A diameter consisting of small globules of about 100 8, [go]. With a sufficiently high percentage of carbon in the carbosil the spkcific surface area of this adsorbent is determined by the dimensions of the globules deposited on the silica matrix and can be calculated on the basis of the globular model of porous solids [97, 981. The adhesion of carbon deposit to the silica gel surface is in this case very strong. Moreover, such carbosils are characterized by high hardness. The silica matrix tightly screened by the carbon layer is inaccessible for small HF molecules. The colour of the carbon is black or grey-silver depending on the conditions of CH,Cl, pyrolysis. This is the second type of carbon according to the Kinney classification [99]. The carbon deposits prepared in the pyrolysis reaction of methylene chloride show strong energetic heterogeneity and high adsorption heats [17].

Thermal

properties

Knowledge about adsorbents’ thermal properties can be of great importance from the point of view of their practical application, because these adsorbents work often at high temperatures and in atmospheres of different gases which are often aggressive. In such conditions the adsorbents can undergo destruction, or in less dramatic conditions can gradually change their surface properties. It has been stated that the carbosils show relatively high thermal resistance both in the atmosphere of air and nitrogen and in the atmosphere of hydrogen [12, 51, 67, 691. Their properties depend significantly on the nature of the carbonized substance. The process of gasification of the carbon deposit is complex and depends on many factors, among others on the chemical structure of the carbon deposit, its distribution on the surface and on the porous structure of the adsorbent. Aliphatic compounds cause a decrease of thermal stability of carbon deposits. In the previous work [67] the carbosils prepared by the deposition of the products of pyrolysis of cY-phenylethyl alcohol, noctanol, p-chlortoluene and mixtures of these substances on a silica gel surface were tested. The adsorbents underwent the action of hydrogen at 700 “C for a few hours. The reactions were carried out in a special nongradient reactor [lOO] conjugated with a gas chromatograph [12]. This permitted investigations of the products of gasification as well as drawing some conclusions about the chemical structure of the carbon deposit. During this process such gas products as methane, ethane, ethylene, propane and butane were formed. The products form due to hydrogenation and thermal

destruction of individual components of the carbon deposits which are more weakly bonded with the support surface. Unsaturated components of carbon deposits interact more strongly with the silica surface. In the above conditions only a small fraction of the carbon deposit is removed from the surface [12, 671. The reactions occurring during such a treatment can be defined as additional pyrolysis of the carbon deposit, with its partial hydrogenation. The additional pyrolysis occurs also in a nitrogen atmosphere [13]. The carbon deposit contained in carbosil influences significantly the transformation of amorphous silica to crystalline silica owing to heating of the former at high temperature. This appears in the differences in the temperatures of the exothermal effect (T,,) of such transformations occurring in pure silica and in silica covered with the carbon layer. Similar differences in T,,, values are observed in the case of the carbosils untreated and treated with hydrogen. The presence of the carbon layer on the silica surface increases the T,, value of its transformation in the crystalline form. This effect is dependent also on the topography and morphology of the carbon deposit. These factors determine also the course of the process of combustion of the carbon deposit in air. The temperatures corresponding to the maxima of exothermal peaks on DTA curves are relatively high - usually -500 “C or above [67]. The course of the process of combustion of the carbon deposit is in general complex - multistage, which results from the complexity of the chemical and physical structure of this deposit. This fact is reflected in the shape of the DTA curves. The less ‘condensed’ carbon deposit the lower its combustion temperature. Similar conclusions may be drawn from the investigations made by differential thermal analysis in air for the cokes deposited on aluminosilicate catalysts [ 1011. It has been stated that the combustion of the coke on the surface of such a catalyst runs in a few stages. At 450-500 “C only 75% of the coke combusts, at 550-580 “C a further 23%, and the residual 2% of the coke combusts at 600 “C. In the first stage the fractions containing greater amounts of hydrogen combust, and then those containing a small amount of hydrogen. Thermal properties of active precipitates (clays) sampled from the biological purification station for waste water from cellulose-paper factories were also examined [31]. Such carbonizate (pyrolysis temperature at 600 “C and pyrolysis time 1 hour) contained 50% wt./wt. of mineral compounds, 45% of carbon and 5% of humidity. The analysis of the thermograms of this adsorbent has shown that in the temperature range 20-300 “C no thermal effect is observed and removal of humidity occurs gradually. In the temperature range of 300-820 “C on DTA curves of carbonizate a large exothermal peak with a maximum at 430 “C appears. The mass loss of

132

34% observed during this step resulted from intensive combustion of the organic substance. In the region of high temperatures, i.e. 800-900 “C, there appears on the DTA curve the thermoeffects connected with the transformation of a mineral part of a complex adsorbent to the crystalline form. In the case of thermal analysis of active carbon of OU-A type (Russia) the exothermic effects of the combustion are characterized by the maximum corresponding to the temperature of 550 “C. The mass was lost up to 900 “C at a constant rate. The carbon deposits prepared by the pyrolysis of CH,Cl, on the silica gel surface have a globular structure [88, 901. They are chemically homogeneous [12]. In this connection, the size of the globules and their spatial distribution determine the course of the combustion process, which is reflected in the shape of the DTA curves [89]. The more homogeneous globules give the better shaped DTA curves (narrower with sharp maxima and minima). The main exothermal processes connected with the mass loss of the carbon deposit are observed in the following temperature ranges: 500-620 “C (carbosil with a small content of carbon, i.e. 5% wt./wt., irregular globules of diameter 200-800 A) and 500-800 “C (about 25% of carbon deposited on the surface, globules of diameters ranging from 200 to 300 A and from 2500 to 7500 A). The heterogeneity of the carbon deposit may be a reason for oxidation of its surface at relatively low temperatures (140-200 “C) connected with formation of surface compounds. Because of chemical homogeneity and possibilities of formation of globules of different sizes, the carbon deposits prepared by the pyrolysis of methylene chloride may be recommended as comparative model deposits for investigations of other carbon adsorbents, especially those of globular structure [89].

Modification of the surface properties As in the case of the methods of preparation of complex adsorbents [l], it seems reasonable to discuss the problems of modification of surface properties of adsorbents produced on technical and laboratory scales separately, because in the latter case more elaborate methods can be expected because of the relatively specific applications of such adsorbents as well as the lesser significance of economical and technical factors in production of such materials. Technical adrorbents Carburized adsorbents and catalysts which may be considered as potential complex carbon-mineral adsorbents do not always have a suitable chemical nature of their carbon deposits or a suitable porous structure. In this connection their modification in a given process

is very often necessary. Because the surface of carbonmineral adsorbents is mosaic, there exist possibilities of modification either of the layer of carbon deposit or of the mineral matrix, especially in those regions which are not screened by the carbon particles. The carbon deposit is most often the main object of modification. The main purpose of such an operation is the development of an internal porous structure or chemical changes in the carbonaceous substance. In this case the traditional methods usually used for activation of carbon adsorbents are utilized in general. It is known [102] that these methods may be subdivided into physical and chemical. The first group of these methods consists in partial gasification of carbon. The main activating agents used in such operations are water vapour, CO, and air or mixtures of these agents. During the activation process these agents attack first the ‘disordered’ carbon, i.e. tar substances and amorphous carbon and then the crystallites. The burning degree of the carbon substance determines its porous structure. In the case of chemical methods the gasification process is performed in the presence of mineral catalysts (ZnCl,, H3P04, KOH, H$, K,S, KCNS, etc.). When these catalysts are used in the carbonization process then they inhibit the formation of tar substances. In this way during the single operation (carbonization of the organic substance) the product having the properties of good active carbon or at least characterized by a more ordered structure can be obtained. In this way the authors of ref. 103 used the precipitates formed during the physico-chemical treatment of waste water originating from the cellulose factories. It has been stated simultaneously that in the presence of ZnCl, and K2S the temperature of carbonization and activation does not exceed 500 “C. However, it should be taken into account that not all among the agents mentioned above can be used for activation of the carbon-mineral adsorbents because of toxicity and the strong catalytic action of some of them during the gasification reaction and because these agents are added in great quantities. However, the metal ions present in the mineral matrix can fulfill the role of the catalysts in the gasification reaction or in the process of formation of ‘ordered’ carbon deposits. The complex adsorbents usually contain small amounts of carbon. In this connection the burning degree cannot be too high. For this reason any other less aggressive methods of modification of the surface properties of carbon-mineral adsorbents are noteworthy. There is, among others, treatment of complex adsorbents with chemical reagents reacting with the carbon deposits, e.g. hot NaOH solution [104, 1051, as well as thermal [28, 29, 1061 or hydrothermal [33, 107, 1083 treatment in moderate conditions.

133

Chemical treatment

As was shown earlier, the coke contained in consumed carburized catalysts usually consisted of condensed (asphaltene) and low molecular (tar) components. The tar substances are more susceptible to the action of external physical and chemical factors and usually undergo destruction owing to oxidation, hydrogenation, solubilization, etc. One of the methods of chemical treatment of consumed aluminosilicate catalysts was proposed by the Japanese firm Shokubay Kasey Koge [105]. The amount of such catalysts in Japan is estimated to be 7000 tons [104]. Thus utilization of carburized aluminosilicate catalysts becomes a question of great practical importance. Such consumed catalysts were treated with hot NaOH solution and then utilized to remove ammonium nitrate or heavy metal ions 1104, 1051 from waste water. The patents [log] give the conditions of treatment of consumed catalysts with the sodium hydroxide: concentration of NaOH 0.2-0.5 M; temperature 70-105 “C; the contact time 2-48 hours; solid/liquid phase ratio from 1:5 to 1:lO. During such an action of hot NaOH solution on the tar substances the latter undergo the destructive action of oxygen contained in the air and pass in significant amounts to the basic solution [llO]. This facilitates the access to cation-exchange centers situated on the condensed carbon layer and on the surface of the starting mineral matrix. The above procedure is simple from the technical viewpoint but it has also some disadvantages, consisting in the loss of some fraction of the carbon deposit contained in the modified material. The advantage of this procedure is that a great amount of the active carbon layer contained in the complex sorbent affects advantageously its adsorption properties. Thus the content of the coke on the mineral matrix and the selection of appropriate methods of preparation of the adsorbent for work are important. In order to increase the content of a carbon component, spraying of an additional carbon layer on the surface of consumed aluminosilicate catalysts is proposed [ill]. Examples of well realized modification of the porous structure of the mineral matrix of complex adsorbents are given in the papers by Tarasevich et al. [27,28]. Introducing a small amount of soda (playing the role of ‘cultivator’) to the carbonized systems (montmorillonite consumed in the process of purification of cane sugar) an adsorbent of biporous (i.e. micro- and meso-)structure is obtained. Its characteristics from n-hexane adsorption data are: S =220 m2 gg’ (130), V,=O.21 cm3 g-’ (O.lO), effective pore radii r=5-8 8, (3) and > 10 nm. In the parentheses quantities determined for unmodified carbon-mineral adsorbents are given. Both comparable adsorbents contain 15% wt./wt. of the coke. Such a ‘cultivated’ complex adsorbent, as shown in ref. 27, adsorbs effectively large anions of organic dyes from water. In the opinion of

the authors the process of ‘cultivation’ takes place during carbonization at 500 “C. At this temperature adsorption and constitutional water are removed from the montmorillonite surface which leads to relaxation (cultivation) of its texture possessing mainly slit mesopores. The relaxation of direct contacts between the plate-like montmorillonite particles and the appearance of additional secondary slit mesopores and supermicropores leads to the increase of S and V, values. Owing to the reaction of thermolysis of the organic substance and decomposition of inorganic additives (soda) a great amount of gas is evolved. This causes further deeper relaxation of the montmorillonite texture which leads in turn to formation of slit mesopores of r= 5-8 nm and larger.

Thermal treatment

During the thermal treatment of carbon-mineral adsorbents in an inert medium a further pyrolysis of the layer of carbon deposit, which is usually chemically non-homogeneous, is observed. This pyrolysis includes mainly the tar fraction of the carbon deposit which undergoes a further aromatization and condensation which leads to the formation of dense coke [112]. During this process there can evolve hydrogen or low molecular hydrocarbons. The thermal treatment of carburized sorbents and catalysts in an inert atmosphere leads to exposition of small pores of radii 3-5 nm. During the carburization process these pores are plugged by the tar substances. The thermal treatment also plays an important role in the formation of a self-porous structure of large clusters of size up to 30 nm, which are formed on external surfaces of the mineral matrix as well as in macropores and wider pores. The trials of processing of kerosene asphaltene to carbon black, graphite, carbon fibres and other materials show that the most porous material characterized by relatively good adsorption properties is obtained by heating the coke to 400-500 “C [113]. The internal porosity of the products of pyrolysis of furfuryl and phenylformaldehyde polymers is formed in the temperature range 350-600 “C [114]. Thus it can be supposed that the self-porosity of coke deposits on the surface of dispersed silicates will be formed in such a temperature range. In fact, the data obtained by Tarasevich et al. [28, 291 have confirmed this supposition. The above authors stated that the optimal temperature of activation of the coke layer deposited on the consumed aluminosilicate adsorbent in the absence of the catalyst was 500 “C. The increase of the temperature of activation leads on one hand to decrease of the amount of active coke deposited on the surface and on the other hand to structural changes in the inorganic matrix leading, in turn, to a significant decrease of the adsorbent surface. There is

134

no regularity because, as shown by Yuki et al. [106], the consumed clay thermally treated in the temperature range 700-900 “C shows a significant sorption capacity in relation to Cu(II), Cr(V1) and Cd(I1). From the above, the conclusion can be drawn that the method and conditions of activation of the carburized adsorbent should be determined by its origin and destination. In fact, however, at high temperatures, even in the absence of air, a significant amount of the carbon deposit is lost. It was stated for example that during the thermal treatment of carburized aluminosilicate sorbent at 700 “C in a stream of nitrogen for 1 hour, the loss of carbon was 70% [29]. The authors of the mentioned work suppose that in the above conditions the mineral matrix undergoes some structural changes which lead to a decrease of ion-exchange capacity. Similarly, the shortening of the time of thermal treatment of carburized material realised by means of so-called thermoshock [115] does not prevent such changes, consisting in the aggregation of mineral particles leading, in turn, to a general decrease of adsorption capacity. Moreover, the rapid increase of the temperature disturbs the development of the internal porous structure of the carbonized material. The intensive evolution of light products creates a high internal pressure in the granules of the carbonizate, which causes the destruction of these granules [114]. The profitable development of internal porosity takes place in the case of an appropriate rate of increase of temperature (up to optimal) and keeping the adsorbent a sufficiently long time at this temperature. The total activation time, including the accession to the appropriate temperature, is not too long (c. 1 hour). Hydrothermal

treatment

The basic reaction of carbon with water vapour is endothermal and may be illustrated by the following stoichiometric equation: C+H,O-

H,+CO+130

kI

(1)

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ‘ordering’ of the crystallographic structure of the carbon, .which is of significant importance in the case of modification of carbon deposits contained in carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure.

The activation with water vapour is performed usually in the temperature range 750-950 “C. This process is catalyzed by the oxides and carbonates of alkali metals, iron and copper as well as by other compounds [116]. This is useful from the viewpoint of modification of carbon deposits of carbon-mineral adsorbents because such a catalyst causes a significant decrease of the temperature of activation. It is noteworthy that on the surface of complex adsorbents there may be present metal ions e.g. coordinatively unsaturated Fe(II1) cations [107]. Thus the process of activation of carbon-mineral adsorbents prepared on the basis of carburized aluminosilicate catalysts may be performed at the same temperature at which a carbonization process occurs. This temperature is usually about 500 “C and permits aromatization and condensation of tar parts of the coke. Moreover, Tarasevich and Ovcharenko showed 11071 that in many cases the introduction of water vapour to the system in which the hydrothermal activation of the coke layer was performed was not necessary. The water vapour forms during the dehydration and dehydroxylation of the mineral part of the complex adsorbent and/or carbonized substance. Naturally, the amount of water which results from the small amounts of carbon deposit is small. Thus because of the similar conditions of carbonization and activation processes there exists a possibility of a union of these processes. This is shown in the paper by Rudenko et al. [30] who made a special experiment illustrating this question. The carbon-mineral adsorbents were prepared by heating a mixture of different clays (montmorillonite, palygorskite) with saccharose at a temperature of 500-600 “C in an inert atmosphere [30]. For comparative purposes the carbonizate of the saccharose was also investigated (500 “C). It was stated that this carbonizate had not the developed a microporous structure, whereas in complex adsorbents prepared on the basis of the above clays, the same adsorbent showed a strongly developed microporous structure. The authors suggest that in this case Fe(II1) ions act as the catalysts of tbe activation process. From the viewpoint of many properties of mineral adsorbents the decrease of the temperature of activation under the influence of Fe(II1) ions to 500 “C always gives desirable results. Such an operation improves only slightly the adsorption properties of the mineral matrix and in comparison to the conditions of activation used for typical active carbons the amount of residual carbon deposit is relatively small. For this reason, the elaboration of optimal conditions of activation of coke layers at low temperatures to prepare effective carbon-mineral adsorbents is very important. This should decrease the losses of gasified carbon deposit and minimalize the changes in primary adsorption properties of the mineral matrix. There are known cases of preparation of car-

135

bonaceous substances and their activation at low temperatures. During the carbonization of formaldehyde polymers the flat carbon layer appears even after heating to the temperature of 200-350 “C and the active development of the porosity of carbonizate begins at 350 “C [114]. In homogeneous oxidative catalysis of organic substances iron-copper and cobalt-chromium oxides and other complex substances are used as catalysts [117, 1181. In this connection, the efforts of preparation of active carbon-mineral adsorbents at low temperatures were made [log]. Owing to the saturation of carburized clays with copper salts the temperature of activation was decreased to 350 “C. In Table 1 there are listed numerical values of maximal adsorption amax of n-nitroaniline and surfactant OP-7 on several carbon-mineral adsorbents and on active KAD charcoal. (The above data come from the paper by Tarasevich [25].) These adsorbents are as follows: starting (A-4) and montmorillonite clay (O-4) consumed during the process of purification of aromatic hydrocarbons, and the same adsorbent activated in a stream of nitrogen at 300-500 “C (WMA-300, WMA-350 and WMA-500). From the data listed in this Table it can be seen that after the formation of carbon deposit on the mineral surface the adsorbent becomes capable of the sorption of n-nitroaniline. It appears that the treatment of the carburized adsorbent with the copper and cobalt salts (2-3% CuCl, and CoCl, solutions - mass ratio of solid/liquid phase is 1:3) before its activation permits the maximum sorption capacity in relation to n-nitroaniline to be obtained at low activation temperatures, i.e. 350-300 “C. From the data discussed here it also results that the noniogenic surface active agent OP-7 adsorbs better on the consumed OA-4 clay than on the other adsorbents listed in Table 1. The highest amaxvalues for this substance adsorbed on the adsorbents from this group are also obtained on carbon-mineral adsorbents activated at the temperatures mentioned above. This fact as well as the numerical values of amax of OP-7 on KAD active carbon imply that in the temperature range of 300-350 “C there are no significant changes in adsorption capacity of the mineral matrix. A specific feature of the carbon-mineral adsorbents consists in the fact that in many cases they are characterized by higher values of absolute adsorption (adsorption value per unit of specific surface area of the adsorbent) or of the adsorption corresponding to the content of carbon compound in the sample. In the case discussed here the adsorption values of n-nitroaniline corresponding to unit mass of carbon contained in the carbon-mineral adsorbent are significantly higher than the sorption capacity of KAD charcoal [119]. The above examples show that the introduction of catalytic impurities to a carburized aluminosilicate adsorbent causes the decrease of the activation temperature of

the carbon deposit to a theoretically possible level. The chemical processes taking place during the catalytic activation were considered among others by Tarasevich [25]. In this case the activation of molecular oxygen on the catalyst ions is a very important step. For example at 500 “C, under the influence of hydrogen evolved during the condensation of tar-like substances, coordinatively unsaturated Fe(II1) ions reduce to Fe(I1). Fe(I1) ions react, in turn, with oxygen which leads to formation of Fe(II1) and an ionic form of oxygen (O-‘) which oxidizes effectively the carbonizate to gaseous carbon oxide. At lower temperatures of activation and in the presence of Cu(I1) salts, Cu(I1) ions act as the reducers of Fe(II1) because the copper ions have a significantly lower electrode potential E, than the ferric ions (Fe(III) + e- - Fe(II); &=0.771 V and Cu(II)+e-Cu(I); E,=0.167 V). Thus the proper selection of the catalysts permits the process of activation of complex carbon-mineral adsorbents in conditions optimal either for the mineral matrix (,there exist possibilities of its destruction, adsorption deactivation, etc.) or carbon deposit (presence of the compounds of different reactivity). The active precipitates (clays) purchased from the biological purification stations of waste water from cellulose-paper factories are also noteworthy. They contain among others mineral compounds (about 30% wt./ wt.), present mainly in the form of oxides, hydroxides and salts of different metals which promote the activation of carbonizate directly during the pyrolysis process or during additional treatment with water vapour [31, 120-1231.

Laboratory

adsorbents

For modification of surface properties of carbonmineral adsorbents (carbosils) different methods such as thermal treatment in hydrogen and inert gas atmosphere [12, 13, 51, 71, 77, 124, 1251, hydrothermal treatment [71, 1261, chemical treatment [63, 70, 77, 1271 or additional carburization [71, 81, 84, 861 of the heterogeneous surface of the carbosil were used. Thermal modification

As mentioned above the carbon deposits usually contain oxygen present in the form of different functional groups which can be centres of specific adsorption [7, 128, 1291. The interaction energies of the individual functional groups are different and as a result the adsorbent surfaces are most often not homogeneous, which causes broadening of the chromatographic bands, and the presence of functional groups can increase basic or acidic properties of the adsorbent surface. It is inconvenient because the chromatrographed sub-

136

stance as well as the liquid stationary phase may react with such groups and undergo destruction, polymerization or strong adsorption on them. The surface substances containing oxygen atoms may be removed, e.g. by heating carbonaceous adsorbents at high temperature and in an oxygen-free atmosphere. Gaseous products of destruction such as CO, CO,, H,O and H, can then form. The stability of the individual functional groups is very different and for this reason the effectiveness of their decomposition depends on the temperature [130]. The carbosils were heated in a hydrogen atmosphere at temperatures 500-1000 “C [71]. With the increase of the temperature of the treatment the amount of deposited carbon slightly decreased. Similarly the specific surface area of the carbosil initially decreased (up to 900 “C) and then increased. It is known 11281 that in the temperature range 1000-1100 “C in an inert gas or hydrogen atmosphere all groups containing oxygen atoms can be removed from the carbon surface, which leads to energetic homogenization of the surface of the adsorbent. However, the data relating to the adsorption heats of nhexane, benzene and chloroform are surprising. The adsorption heats of the substances interacting either specifically (benzene, chloroform) or nonspecifically (nhexane) increase with the increase of the temperature of treatment. This suggests some changes in the silica skeleton and in the carbon deposit. It is known [131] that the silica can form different crystalline forms at higher temperatures and that at low temperature morphologically heterogeneous (see Scheme 1 in ref. 1) carbon deposits should become more homogeneous owing to the loss of less ordered elements during the hydrogenation. On the surface of crystalline forms of silica this deposit distributes nonuniformly (Fig. 7(a)), i.e. a major part of this deposit localizes in intercrystalline spaces, these being the amorphous parts of the silica bed (Fig. 7(b)). It can be assumed that the exposed crystalline parts of the support of the carbon deposit are characterized by high energetic heterogeneity. Such heterogeneity is known to be due to the crystal edges, cracks, defects of the crystalline network, inclusion of foreign ions, etc. [94]. At high temperatures aromatization of the carbon deposit is observed [12, 131. All the above factors cause the increase of adsorption heats of test substances. The analogous situation takes place during heating of the carbosils at high temperature in a nitrogen atmosphere [131In contrast to similarly treated carbon adsorbents [132] high-temperature carbosils are not useful for chromatographic purposes [71]. In this respect better results are obtained by hydrothermal treatment of the carbosils [70, 711, especially in the case when the

(a>

(b) Fig. 7. Effect of crystallographic structure of a silica surface on topography of carboti particles in carbosil prepared by the pyrolysis of mixture of benzyl and n-heptyl alcohols [Xl. Carbon deposition (a) on a crystalline part of silica and (b) in intercrystal regions.

products of hydrothermal ified with silanes [70].

treatment are chemically mod-

Hydrothermal modification

In Table 3 the surface characteristics of carbosils modified with water vapour in an autoclave at temperatures of 250 and 500 “C are compared [71]. The data listed in this Table are very interesting because they show the effect of such a treatment on the properties of carbosils obtained by pyrolysis of the substances of different chemical nature and containing different amounts of carbon on the support (silica) surface. The process of hydrothermal treatment of the carbonmineral adsorbent must be considered as complex because either the carbon deposit or silica may react with water vapour (the former according to reaction (1) and the latter to refs. 13 and 33). During the oxidation of the carbon deposit the water vapour mainly attacks the most active elements of this deposit, i.e. hydrogenated parts, small particles of the carbon (globules) and the

137 TABLE No.

3. Surface properties

of carbon-silica

Adsorbents

Pyrolyzed substance

2

Carbosil I-O I-O (H,O) 250 “C

3 4 5

Carbosil V V (H,O) 250 “C V (H,O) 500 “C

CH,Cl,

6 7

Carbosil JZ JZ (H,O) 250 “C

n-octanol

8 9

Carbosil SAC SAC (H,O) 250 “C Carbosil KEAN KEAN (H,O) 250 “C

1

10 11

adsorbents

CH2C12

(carbosils) %C (wt./wt.)

modified with water vapour s (m* g-‘)

Adsorption

[71] heat (kcal mol-‘)

n-hexane

benzene

chloroform

22.0 20.8

217 210

14.2 9.6

12.0 9.7

10.6 8.4

14.5 14.0 13.2

286 200 185

19.7 8.6 10.6

18.5 9.0 8.8

15.0 9.8 7.4

3.75 3.09

389 90

9.0 14.0

9.6 13.6

9.1 11.9

acenaphthene

11.35 -

253 90

11.6 13.6

12.5 15.3

11.2 16.1

acenaphthene

17.6 _

189 81

14.1 13.0

13.3 14.2

12.1 14.4

edges. This causes a partial homogenization of the deposit surface. The treatment with water vapour causes dramatic changes in the surface properties of complex adsorbents. The higher the carbon content in the adsorbent, the smaller the changes of specific surface area S and great changes in the adsorption heats of the test substances are observed. Modification of the adsorbents obtained by pyrolysis of CH,Cl, leads to a significant decrease of adsorption heats and energetic homogenization of the carbon deposit. The deposits formed during such a process are most often very strongly heterogeneous [17]. As they are chemically homogeneous [81] their modification consists mainly in removal of physical heterogeneities. The silica gels modified at temperatures near to 250 “C have a globular spongy structure whereas at temperatures above 300 “C the amorphous silica is converted into crystalline silica. Moreover, the increase of the temperature of hydrothermal treatment causes not only a dramatic decrease of specific surface area and pore size but also a decrease of porous structure uniformity. In consequence, the adsorbent may show physical heterogeneities. This results in the differences in the surface properties of adsorbent nos. 4 and 5 (Table 3). In the case of the carbosils containing a relatively small amount of carbon (adsorbents 6, 8 and 10 listed in Table 3) the active surface of silica (not covered with carbon deposit) is significantly greater than for adsorbents 1 and 3 (Table 3). Moreover, the carbon deposits present in adsorbents 6,8 and 10 are chemically more complex [12]. For this reason the hydrothermal treatment of these adsorbents causes significantly greater changes in the physical (porous) and chemical (hydroxylation) structures of the silica and the carbon deposit (gasification, oxidation). This leads among others to a dramatic change of specific surface area and to

a strong increase of adsorption heats (adsorbents 7, 9, 11, Table 3). From the above data it results that, as in the case of ‘technical’ adsorbents, there exists a problem of a proper choice of method and conditions of modification of the carbon-silica adsorbents. Depending on the temperature of thermal or hydrothermal treatment the activity of carbosils may be increased or decreased. For chromatographic purposes the following ways of treatment are most suitable [71]: a) thermal treatment at a moderate temperature (500-600 “C) in hydrogen, which is confirmed by the conclusions presented in ref. 81; b) hydrothermal treatment at lower temperature (about 250 “C). Seconda y pyrolysis

As shown earlier, pyrolysis of methylene chloride on the surface of a mineral adsorbent is an effective way of preparation of complex adsorbents containing any amount of carbon and characterized by good mechanical properties. However, during the pyrolysis of CH,Cl, different heterogeneous carbon particles of different sizes are formed. Such globules of very different sizes (from tens to a few thousands A) [17] contain geometrically heterogeneous micropores or even ultramicropores. The narrow pores are very strong adsorption centers owing to the increase of the potential of nonspecific (dispersive) interactions. In consequence, such adsorbents are strongly energetically heterogeneous and can adsorb a lot of substances irreversibly. Studies [71, 81, 84, 861 show that these heterogeneities can be eliminated by additional pyrolysis of an organic substance on the surface of a complex adsorbent. This is illustrated by the data listed in Table 4 in which the effects of modification of two adsorbents (X and Y)

138 TABLE 4. Surface properties of unmodified carbon-silica adsorbents (carbosils) (X, H, B, Y) and the same adsorbents modified with the products of pyrolysis of n-heptanol (H) and benzyl alcohol (B) in an autoclave (A) and dynamic reactor (R). The carbosils were prepared by the pyrolysis of methylene chloride (adsorbents X and Y), n-heptyl alcohol (H) and benzyl alcohol (B) on the silica surface Adsorbents

Specific surface area S (m’ g-‘)

Pore radius at maximum pore distribution &m

382

27, 34

Adsorbent Adsorbent Adsorbent Adsorbent Adsorbent Adsorbent Adsorbent Adsorbent Adsorbent

370 300 246 422 331 286 61 19 23

27, 31 34 32 28 39 68 200 260 205

containing different amounts of carbon are compared. The processes of secondary pyrolysis of n-heptanol (H) and benzyl alcohol (B) were carried out in an autoclave (A) or in a flow reactor (R). Modification of the surface of X and Y adsorbents with the products of pyrolysis of these alcohols causes significant changes in the geometrical structure of these pores (Rdom,Fig. 8, Table 4) as well as in the chemical structure of adsorption centres (adsorption heats, Table 4). The products of secondary pyrolysis of the alcohols block the narrow pores of the adsorbents more effectively if the pyrolysis is carried out in static conditions in the autoclave. The carbonized substances penetrate such pores more easily and wet them before a reaction of the whole surface of the modified carbosil. The physico-chemical nature (size and structure of the molecule) of the substance

0.L

:03

2 -Y 2.

n

0.2

0.1

Fig. 8. Differential distribution of pore volumes V according to their radii R for the following adsorbents: 1 -Y, 2-Yn_a, 3 -Yn_=_, (Table 4).

Adsorption heat (kcal mol-r) n-hexane

(A)

Initial unmodified Silica gel X X,, Xa., Xn., H B Y YnA Yu_a

%C (wt./wt.)

2.46 10.63 15.95 3.76 2.30 4.60 30.46 32.46 31.68

CH,Cl

7.9

7.3

8.7 8.5 13.1 8.0 8.0 10.4

7.3 7.3 11.0 7.6 8.6 10.0 very high

12.0 10.0

10.2 7.3

used also plays an important role in the creation of the final properties of the surface of the modified adsorbent. In dynamic conditions blocking of narrow pores is less effective because of diffusion-kinetic reasons, an additional pyrolysis occurring mainly in larger pores. Moreover, in large pores the self-structure of the deposited pyrolyzates can be formed which leads to the increase of specific surface area S of the modified adsorbent. In general, however, the additional pyrolysis of the substance on the surface of the complex adsorbent leads to homogenization of this surface. This has been shown [81] to be an effective method of enrichment of carbon silica adsorbents prepared by decomposition of CH$l, because application of this method leads to significant improvement of selectivity and effectiveness of chromatographic columns packed with such adsorbents. Chemical modification

From the investigations of carbon-mineral adsorbents by the IR spectroscopy method it results that the modification of silica gel with the carbon layer (the amount of carbon changes from a few to anywhere from ten to twenty percent) does not cause a complete screening and deactivation of all hydroxyl groups present on the silica surface. In this way a chemical modification of mosaic surfaces of carbosils becomes possible [127]. One of the most popular methods of modification of hydroxylated silica gels consists in their silanization [134]; silanization is also used for chemical treatment of carbosils [12, 63, 70, 861. The following silanes were used: trichlorooctadecylsilane, trimethylchlorosilane and hexamethyldisilazane. The silanization of carbosils leads to a significant decrease of adsorption heats of hydrocarbons, to improvement of resolution of the

139 TABLE 5. Properties of partially dehydroxylated silica gel (S = 470 m* g-r, adsorbent A) and carbon-silica adsorbent (adsorbent B) silanized with ODS (I) and with ODS+HMDS (II). Pyrolyzed substance - n-heptyl alcohol (500 “C, 6 hours, 0.03 mol of alcohol per 15 g of silica gel). ODS - trichlorooctadecylsilane; HMDS - hexamethyldisilazane Elemental analysis

Adsorbent

adsorbent adsorbent adsorbent adsorbent

AI AI1 BI BII

Heat of adsorption (kcal mol-‘)

References

qsl

%C

%H

n-hexane

chloroform

9.38 11.82 2.80 6.1

2.16 2.60 0.76 1.42

7.45 7.01 7.1 6.7

8.42 6.1 7.5 6.1

4

5

6

adsorbent used as the packing of chromatographic columns and to a significant increase of adsorption capacity. Table 5 lists the exemplary characteristics of silanized carbosils. Interesting results were obtained also for the hydrothermally treated carbosils then undergoing silanization [70].

7 8 9 10 11

Conclusions

From the presented literature data it results that the known carbon-mineral adsorbents and potential starting materials used for their preparation are characterized by the relatively complex morphology of carbon deposits. This means that there exists a very large range of adsorbents of such a type characterized by very different physico-chemical properties. This creates great possibilities of investigation both in theory and in practical respects. In general, each carburized adsorbent or catalyst can be used as a starting material for production of carbon-mineral adsorbents because the methods of modification of texture and chemistry of the surfaces or the carbon deposits and the mineral matrix have been elaborated. Depending on need the complex adsorbents may be activated or deactivated. Considering a variety of known carbon-mineral adsorbents, the complexity of chemical and physical structures of the carbon deposits, the possibilities of new so far unknown complex adsorbents appears, as well as elaboration of new methods of modification of the properties of such materials.

12 13 14 15

16 17 18 19 20 21 22 23 24 25 26 27 28

Acknowledgement

Financial support from the State Committee for Scientific Research, Project No. 205119101, is gratefully acknowledged.

29 30

R. Leboda, Mater. Chem. Phys., 31 (1992) 243. R. A, Buyanov, Zakoksovanie katalizatorov, Nauka, Novosibirsk, 1983. A. P. Rudenko, Rol’ uglekishlyh otlozhenij na katalizatorah w organicheskoj katalizie, Sovriemiennoe probliemy jizicheskoj khimii, Vol. 13, Moskovskovo Universiteta, Moscow, 1968, pp. 263-333. R. A. Buyanov, Zakohxowanie i regeneracija katalizatorov degridirovanija pri polucheniyu monomerov SK Nauka, Novosibirsk, 1968, p. 64. R. M. Masagutov, B. F. Morozov and B. J. Kutiepov, Regeneracija katalizatorov w niefrepiererabotkie i nieftiekhimii, Khimiya, Moscow, 1987, p. 144. R. M. Masagutov, Aluminosilikatnyje katalizatory i izmienieije ich swolstow pti kriekingie nieftieproduktov, Khimiya, Moscow, 1975, p. 272. B. R. Puri, in P. L. Walker, Jr. (ed.), Chemistry and Physics of Carbon, Vol. 6, Marcel Dekker, New York, 1970, p. 191. J. A. Tarkovskaya, Okislennyj ugol’, Naukovaya Dumka, Kiev, 1981. A. Gierak and R. Leboda, J. Chromatogr., 483 (1989) 197. N. G. Kalinina, W. A. Poloboyarov, W. F. Anufrienko and K. G. Yone, Kinet. Katal., 27 (1986) 237. N. G. Kalinina, Yu. W. Ryabov, L. L. Korobincyna, W. A. Poluboyanov, W. I. Erofeev, L. N. Kurina and W. F. Anufrienko, Kinet. Katal., 27 (1986) 240. R. Leboda, Chromatographia, I3 (1980) 703. R. Lcboda, Pol. J. Chem., 54 (1980) 2305. E. Tracz, R. Leboda and E. Mizera, J. Chromatogr., 355 (1986) 412. Yu. W. Pokonova, Khimija wysokomoliekuliarnykh soedinieny niefti, Leningradskovo (Saint Petersburg) Universiteta, Leningrad (Saint Petersburg), 1980, p. 172. A. L. Dawidowicz, Habilitation Thesis, UMCS, Lublin, 1986. A. Gierak and R. Leboda, Mater. Chem. Phys., 19 (1988) 503. R. A. Buyanov, A. D. Afanasyev, W. N. Kolomiychuk and W. A. Melinona, Kinet. Katal., 16 (1975) 802. R. A. Buyanov, A. D. Afanasyev and W. N. Kolomiychuk, Kinet. Katal., 18 (1977) 749. 0. van Zoonen, Third Int. Congr. of Catalysis, Preprint 119, North-Holland, Amsterdam, 1964, pp. l-18. L. N. Rachkovsakaya, E. M. Morozov and W. F. Anufrienko, IN. Sib. Otd. Akad. Nauk SSSR, Ser. Khim., 5 (1982) 34. W. Yu. Gavrilov, W. B. Fenelonov and L. N. Rachkovskaya, Kinet. KataZ., 24 (1983) 1149. W. Yu. Gavrilov, W. B. Fenelonov and L. N. Rachkovskaya, K&et. Katal., 27 (1986) 68. M. E. Levinter, G. M. Panchenkov and M. A. Tanatarov, Khim. Tekhnol. Topl. Masel, 3 (1966) 9. Yu. I. Tarasevich, Khim. Tekhnol. Vody, II (1989) 789. Yu. I. Tarasevich and W. M. Rudenko, Khim. Tekhnol. Vody, II (1989) 568. Yu. I. Tarasevich, W. E. Doroshenko, W. M. Rudenka and Z. G. Ivanova, Khim. Tekhnol. Vody, 10 (1988) 315. Yu. I. Tarasevich, W. M. Rudenko, Z. G. Ivanova, W. E. Doroshenko and W. W. Kulik, Khim. Tekhnol. Vody, 9 (1987) 510. Yu. I. Tarasevich, W. M. Rudenko, G. M. Klimova and I. Ya. Pischchay, I&m. Tekhnol. Vody, 2 (1980) 392. W. M. Rudenko, Yu. I. Tarasevich and Z. G. Ivanova, Khim. Tekhnol. Vody, 5 (1983) 499.

140 31 L. A. Stiecenko, Yu. I. Tarasevich and Z. G. Ivanova, Khim. Tekhnol. Vody, 12 (1990) 513. 32 Yu. I. Tarasevich, Prirodnye sorbienty w processach ochistki wod, Naukovaya Dumka, Kiev, 1981, p. 207. 33 Yu. I. Tarasevich, E. A. Subbotina and A. E. Orazmuradov, Khim. Tekhnol. Vody, 10 (1988) 108. 34 Yu. I. Tarasevich, W. A. Ruppa, W. E. Poljakov and N. W. Prishchep, K&n. Tekhnol. Vody, II (1989) 393. 35 M. R. Tarasevich, Elektrokhimtja uglierodnykh materialov, Nauka, Moscow, 1984, p. 2.54. 36 H. P. Bohem, in D. D. Eley, H. Pines and P. B. Weisz (eds.), Advanced Catalysis and Related Subjects, Academic Press, Vol. 16, New York, 1966, pp. 179, 274. 37 R. W. Kucher, W. A. Komnanec and L. F. Butuzova, Struktura iskopajemykh ugliej i ich sposobnost’ k okislenju, Naukovaya Dumka, Kiev, 1980, p. 168. 38 J. van Driel, in A. Capelle and F. de Vooys (eds.), Activated Carbon . , a fascinating material: Some thoughts on activated carbon, Norit N.V., Amersfoort, 1983, p. 40. 39 A. A. Shamshurin and M. Z. Krimer, Fiziko-khimicheskije svojstwa pesticidov, Khimiya, Moscow, 1976, p. 328. 40 T. Singo, I. Yosiro, M. Siego and I. Wasi, Jpn. Pat. 35399, 13(9) F2. 41 A. Jankowska, M. Gajeski, A. Swiatkowski and S. Zietek, Adsorbtsiya Adsorbenty, 7 (1979) 10. 42 C. Moreno-Castilla, J. Fernandez-Moralez, M. Ddmingo-Garcia and F. J. Loez-Garcia, Chromatographia, 20 (1985) 709. 43 New type collection agent for heavy metal ions, Technocrat, 8 (1975) 85. 44 N. Yuki, A. Yauchi and H. Ogawa, Pollut. Contr., 12 (1976) 56. 45 S. Kadzuhiko, F. Ajako and N. Aio, Jpn. Pat. 37035, 13(9) F2. 46 N. K. Bebris, R. G. Vorobieva, A. V. Kiselev, Yu. I. Nikitin, L. V. Tarasova, 3. J. Frolov and Ya. I. Yashin,J. Chromatogr., 117 (1976) 257. 47 N. K. Bebris, A. V. Kiselev, Yu. I. Nikitin, I. I. Frolov, L. V. Tarasova and Ya. I. Yashin, Chromatographia, 4 (1978) 206. 48 H. Cohn and G. Guiochon, J. Chromatogr., I26 (1976) 43. 49 G. Guiochon and H. Cohn, Chromatography Review, Spectra Physics, Santa Clara, Vol. 4, 1978, p. 2. 50 R. Leboda, Habilitation Thesis, UMCS, Lublin, 1980. 51 R. Leboda, Doctoral Thesis, UMCS, Lublin, 1974. 52 A. Gierak, Doctoral Thesis, UMCS, Lublin, 1985. 53 R. Leboda, Z. Suprynowicz and M. Waksmundzka-Hajnos, Chem. Anal. (Warsaw), 21 (1976) 437. 54 R. Leboda, A. Waksmundzki, Z. Suptynowicz and M. Waksmundzka-Hajnos, Chem. Anal. (Warsaw), 21 (1976) 165. 55 R. Leboda, A. Waksmundzki, J. Skubiszewska and Z. Suprynowicz, Chem. Anal. (Warsaw), 23 (1978) 397. 56 R. Leboda, Chem. Anal. (Warsaw), 2.5 (1980) 69. 57 R. Leboda, Chem. Anal. (Warsaw), 25 (1980) 783. 58 R. Leboda, Chem. Anal. (Warsaw), 25 (1980) 979. 59 R. Leboda, Chem. Anal. (Warsaw), 26 (1981) 19. 60 R. Leboda, Chem. Anal. (Warsaw), 26 (1981) 999. 61 R. Leboda, Chem. Anal. (Warsaw), 27 (1982) 25. 62 R. Leboda, J. Skubiszewska, A. Waksmundzki, A. Wojcik and J. Nawrocki, Chem. Anal. (Warsaw), 27 (1982) 417. 63 R. Leboda, J. Skubiszewska and B. Buszewski, Chem. Anal (Warsaw), 229 (1984) 267. 64 R. Leboda, J. Skubiszewska and E. Mendyk, Chem. Anal. (Warsaw), 330 (1985) 615. 65 J. Skubiszewska and R. Leboda, Pol. J. Appl. Chem., XXXVIII (1984) 339.

66 R. Leboda, J. Skubiszewska, E. Mendyk and A. N. Korol, Chem. Anal. (Warsaw), 30 (1985) 737. 67 R. Leboda, J. Therm. Anal., 32 (1985) 1435. 68 R. Leboda, Inz. Chem. Proces. (Wroclaw), 3 (1982) 343. 69 R. Leboda, D. Nazimek, J. Skubiszewska, A. Waksmundzki and W. Wasiak, Pal. J. Appl. Chem., Xxyv (1981) 507. 70 R. Leboda, A. Gierak and A. Lodyga, Pol. J. Appl. Chem., XXXZZ (1989) 487. 71 R. Leboda, A. Gierak, E. Mendyk and A. Lodyga, Pol. J. Appl. Chem., mII (1988) 219. 72 E. Tracz, J. Skubiszewska and R. Leboda, J. Chromatogr., 287 (1984) 136. 73 E. Tracz and R. Leboda, J. Chromatogr., 346 (1985) 346. 74 A. L. Dawidowicz, S. Pikus and D. Nazimek, J. Anal. Appl. Pyrol., 10 (1986) 59. 75 R. Leboda and A. Waksmundzki, Chromatographia, 12 (1979) 207. 76 R. Leboda, Chromatographia, 13 (1980) 549. 77 A. Dawidowicz, D. Nazimek, S. Pikus and J. Skubiszewska, J. Anal. Appl. Pyrol., 7 (1984) 53. 78 R. Leboda and A. Lodyga, J. Anal. Appl. Pyrol., 14 (1988) 203. 79 R. Leboda, Pol. J. Appl. Chem., XXXII (1988) 229. 80 R. Leboda, E. Mendyk and A. Lodyga, Pol. J. Appl. Chem., 32 (1988) 423. 81 R. Leboda, Chromatrographia, 9 (1991) 524. 82 K. Grob, Helv. Chim. Acta, 48 (1965) 1362. 83 L. Zoccolillo and A. Liberti, J Chromatogr., 77 (1973) 69. 84 R. Leboda, Chem. Anal. (Warsaw), 29 (1984) 61. 85 A. Gierak and R. Leboda, Chem. Anal. (Warsaw), 31 (1986) 221. 86 A. Gierak and R. Leboda, Chem. Anal. (Warsaw), 33 (1988) 735. 87 A. Gierak, R. Leboda and A. Lodyga, Folia Sot. Sci. Lublinensis, l-2 (1983) 28. 88 A. L. Dawidowicz, A. Gierak and A. Waksmundzki, Chromatographia, 17 (1983) 627. 89 A. Gierak and R. Leboda, J. Therm. Anal., 35 (1989) 2213. 90 A. Gierak, R. Leboda and E. Tracz, J. Anal. Appl. Pyrol., 13 (1988) 89. 91 A. Gierak, R. Leboda and D. Nazimek, Chem. Anal. (Warsaw), 32 (1987) 957. 92 J. Rayss, Chromatographia, 15 (1982) 517. 93 R. Leboda, A. Waksmundzki, A. Gierak and Z. Suprynowicz, Chem. Anal. (Warsaw), 22 (1977) 683. 94 S. Ross and J. P. Oliver, On Physical Adsorption, WileyInterscience, New York, 1964. 95 M. Jaroniec and R. Madey, Physical Adsorption on Heterogeneous Solids, Elsevier, Amsterdam, 1988. 96 R. Leboda, S. Sokolowski, A. Lodyga and A. N. Korol, Teor. Eksp. Khim., 6 (1990) 698. 97 A. P. Kamaukhov, Kinet. Katal., 12 (1971) 1235. 98 A. P. Karnaukhov, in S. I. Gregg, K. W. Sing and H. F. Stoekh (eds.), Characterisation of Porous Solids, Society for Chemical Industry, London, 1979, p. 301. 99 C. R. Kinney, Proc. Con& Carbon, lst, Buffalo, Nk: 1953, p. 83. 100 J. Barcicki, D. Nazimek, W. Demidziuk and R. Frak, Int. Chem. Eng., 15 (1975) 159. 101 I. Ya. Tyuryaev, A. N. Bushin and T. M. Trockaya, Biul poobmienu opytom wprom-ti sin&. kautchuka i sintiet. spirta., 6 (1957) 27. 102 T. Wigmans, Carbon, 27 (1983) 13. 103 Cho Byung Rin and S. Motoyuki, Jpn. J. Chem. Eng., 13 (1980) 463.

141 104 S. Sanga, A. Kurata, Y. Nishimura and H. Takahashi, Purif 105

Liquid

Wastes

Treat.,

16 (1976)

Wafer

63.

Using waste FCC catalyzers in treating liquid waste, Tech8 (1975) 74. N. Yuki, A. Yauchi and H. Ogawa, Pollut. Contr., II (1976) 56. Yu. I. Tarasevich and F. D. Ovcharenko, in Adsorbienty ich poluchenije swojstwa iprimienienie, Nauka, Leningrad (Saint Petersburg), 1985, pp. 148-153. W. M. Rudenko and Yu. I. Tarasevich, Russian Pat 952315, M.Kl3 BOl J20/12, SO1 W33/30. S. Seiji and N. Yoichi, USA Put. 3 960 760, 252-412 (BOlJ29/ 06). Yu. B. Pokonova, Chimija wysokomoleklitdjarnych sojedinienij niefti, Leningradskovo Universiteta, Leningrad (Saint Petersburg), 1980, p. 172. A. G. Nemchenko, Yu. A. Kulibabich, K. S. Danilov and L. S. Yakimov, Sov. Pat. 404 778, CO2 5/02 BOl 15/08. R. Luazon, P. Fosh and A. Buaye, Koks, Metallurgiya, Moscow, 1975, p. 520. Z. 1. Snyuyaev, Niefiunnoj uglerod, Khimiya, Moscow, 1980, p. 271. L. G. Gladkova, E. F. Kolpikova, Ya. S. Wygodskiy and A. S. Fyalkov, Usp. Khim., 57 (1988) 1742. Yu. I. Tarasevich and W. M. Rudenko, Khim. Tekhnol. Vody, nocrat,

106 107

108 109 110

111 112 113 114 115

11 (1989)

II

(1989)

568.

120

L. A. Stiecenko and Yu. I. Tarasevich, Khim. Tekhnol. Vody,

121

U. Fasoli, G. Gozzelino

II

(1989)

175.

(1984) 7. N. J. Sogdanovich, IN. Vyssh. Uchebn.

and M. Onofrio, Riv. Combust.,

38

L. N. Kuznecova and E. 0. Gelfand, Lesn. Zh., (2) (1985) 75. 123 S. W. Yakovlev, M. A. Pierederij and L.. G. Pirogov, Vo-

122

dosnabzh. 124 125 126

127 128

129

130

568.

H. Jankowska, A. Swiatkowska and J. Choma, Active Carbon, Ellis Hot-wood, Chichester, 1991. 117 A. Y. Sychev, S. 0. Travin, G. G. Duka and Y. S. Skurlatov, Kataliticheskije reakcji i ochrona okruzajushchej sredi, Shtijnca, Kishinev, 1983, p. 272. 116

118 T. G. Alhazov and L. Ya. Morgolis, Glubokoe kataliticzeskije okieslenije or gunicheskikh wieshchestw, Khimiya, Moscow, 1985, p. 187. 119 Yu. I. Tarasevich and W. M. Rudenko, Khim. Tekhnol. Vody,

131 132 133 134

Sunit.

Zaved.

Tekh.,

(4) (1978)

19.

R. Leboda, J. Skubiszewska, D. Nazimek and A. Waksmundzki, Pol. Pat. 133 031. R. Leboda, J. Skubiszewska, D. Nazimek and A. Waksmundzki, Pol. Pat. 135 973. R. Leboda, A. Gierak and A. Lodyga, 1’01. Pat. 270211. R. Leboda, B. Buszewski, J. Skubiszewska and A. Waksmundzki, Pol. Pat. 123303. A. V. Kiselev and Ya. I. Yashin, Gazovaja chromatogrufiu, Nauka, Moscow, 1967. A. C. Zettlemoyer and K. S. Narayan, in P. L. Walker, Jr. (ed.), Chemistryand Physics of Carbon, Vol. 2, Marcel Dekker, New York, 1966, p. 197. N. N. Avgul and A. V. Kiselev, Chemistry and Physics of Carbon, Vol. 6, Marcel Dekker, New York, 1972, p. 1. R. K. Iler, 7’he Chemistry of Silica, Wiley, New York, 1979. A. Di Corcia and F. Bruner, Anal. Chem., 43 (1971) 1634. R. Leboda, E. Mendyk, Muter. Chem. Phys., 27 (1991) 189. G. V. Lisichkin, Modijicirovannye kremnezemy v sorbcii, katalize i khromatgrufii, Khimiya, Moscow, 1986.