Influence of porous structure of active carbons on the chemical transformation of surface functional groups

J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (Eds.) Characrerization of Porous Solids 111 Studies in Surface Science and Calalysis, Vol. 87 0 1994 Elsevicr Scicncc B.V. All rights rcscrvcd.

705

Influence of porous structure of active carbons on the chemical transformation of surface functional groups Sergey V.Mikhalovsky*, Vladimir G.Glushakov, Anatoliy M.Noscov and Dmitriy B.Rudakov Institute for Sorption and Problems of Endoecology, 32/34 Prospect Palladina, Kiev-142, UKRAINE 252142" Abstract Surface carboxylic groups of oxidized polymer-pyrolized active carbons were converted to surface amides, azides, and nitriles by means of reactions used for transformations of functional groups in organic chemistry. Further reduction of N-containing groups took different courses, resulting in either amine or hydroxylic groups depending on the starting material. Comparing adsorption data of methylene blue, probable mechanisms of surface reactions, and porous structure of the carbons it has been concluded that the phenomenon described is due to the difference in the rate of reagent transport to the surface which, in its turn depends on the porous structure of active carbon. 1. INTRODUCTION Though chemical modification of carbons has certain applications in industrial chemistry, the surface functional groups of these materials scarcely can be considered as well-established discipline. Active carbons with highly developed porous structure and surface heterogeneity are undoubtedly the most complicated objects for study compared to graphite and carbon blacks [l]. Among other problems arising in the work with active carbons there is one concerning their chemical composition. Content of impurities in the carbons obtained from natural sources used to be high therefore one dealing with chemical modification of active carbons cannot be always sure that the resulting functional groups are the consequence of chemical transformation of the carbon surface but not that of admixtures. Polymer-pyrolized active carbons have more regular structure and substantially lower amount of impurities or at least of unexpected impurities. In this study we have chosen polymer-pyrolized active carbons with different pore structure for chemical modification introducing nitrogen-containing surface functional groups by various methods.

'To whom correspondence should be addressed. "The research was supported by the Ukrainian Academy of Sciences grant and the results were presented at COPS-I11due to the support of the Central European University, Budapest.

706

2. MATERIALS AND METHODS 2.1.Active carbons Two sets of carbons have been used for the experiments. One of them is produced by pyrolysis of styrene-divinylbenzenecopolymer (SCS carbon) and the other is produced from phenol-formaldehyde resin (SCP carbon). The procedure includes two-step pyrolysis at 400°C in the inert gas atmosphere with further heating at 900°C either in inert gas (carbonization, samples SCS, and SCP,) or with steam (activation, samples SCS, and SCP3. To activate surface the samples were oxidized with concentrated (93%) or diluted (25%) nitric acid at 95-98"C, duration of the process was 1.5-4.0 h. in the first case and 12-14 h. with diluted acid. Oxidation was carried out with stirring, v/v ratio of carbodacid being 1:4. After oxidation the samples were filtered and washed successively with water, diluted aqueous ammonia, water, diluted hydrochloric acid, and water again to reach neutral pH value in the rinsing solution. Washed samples were dried at 110°C in low vacuum during 3 h. In the Table 1 the sample 4 was obtained by oxidation with concentrated nitric acid, whereas samples 1, 2, 3 were obtained with diluted acid. There is uncertainty in ascribing exact chemical structure to the surface groups of active carbons but, according to the pHpotentiometric data, only those strong acidic appeared to undergo chemical transformations into surface amides and azides resembling reactions of carboxylic acids [ 2 ] . Strong acidic groups were, therefore, assumed to be carboxylic-like groups. If carbon was treated with a mixture of concentrated sulphuric and concentrated nitric acids the -NO2group was attached to the surface by analogy with nitration of aromatic hydrocarbons. This "nitro"carbon was used for ESCA measurements (see below). Table 1. Chemical composition of the oxidized carbons determined by pH-potentiometric titration". Sample N

Carbon

TEC'" NaOH meqk

~~~

1

scs,

3

SCP,."

4

SCP.

Strong

Medium

Weak

Basic groups, mq/g

1.9

1.1

0.2

0.6

0.0

1.9

1.1

0.4

0.4

0.2

3.2

2.0

0.6

0.7

0.0

2.8

0.7

1.1

0.0

~

SCSC.0.

2

Acidic groups, meq/g

0.

4.5

0 ~~

~

~

''Determination of acidic and basic surface groups was made according to [3]. "TEC - total exchange capacity. 2.2.Solvents and chemicals Dimethyl sulfoxide (DMSO)was distilled in vacuum after drying with calcium hydride; tetrahydrofuran (THF) was distilled with potassium hydroxide, refluxed with metallic sodium, and finally distilled; ether (diethyl ether) was distilled with lithium aluminum hydride; NaN, and LiAlH, were used without additional purification; thionyl chloride was bidistilled with linseed oil.

707 2.3.Introduction of functional groups on the carbon surface As active carbons are considered as a system of condensed benzene-like rings most researchers carry out reactions on their surface by analogy to the chemistry of polycondensed aromatic hydrocarbons and their derivatives [3]. The same approach was used in this paper. Acyl chloride. Oxidized carbon was treated with excess of SOCl, in a solvent (or without) at 80°C for 10-12 h, solvent and non-reacted reagent having been driven off under reduced pressure. Oxidized sample was dried at 100°C and reduced pressure for 6-8 h. The yield is nearly quantitative as well as in the case of thionyl chloride substitution for oxalyl chloride. Acyl amide. Surface acyl chloride groups were converted to acyl amides by reaction with R'-NH-R", where R', R" are: H or aliphatic radical. Example of protocol: carbon sample (5.0 g ) with 2 meqlg (appr.) of surface acyl chloride groups was mixed with 20 ml of 2.5M diethylamine solution in anhydrous ether and left for 5 h, whereupon the sample was filtered, washed with methanol, water, methanol and dried at low pressure and 80°C for 4h. Acyl azide. Synthesis was carried out via reaction of surface -C(O)Cl groups with sodium azide in anhydrous DMSO in the presence of a crown ether. Example of protocol: carbon sample (5.0g) with appr. 10 meq of surface acyl chloride groups was added to the suspension of 1.6 g sodium azide and 0.3 g 18-crown-6 ether in 40 ml of anhydrous DMSO. The mixture was refluxed with stirring at 3540°C for 6 h, filtered and the sample was washed with water, methanol, and dried under reduced pressure and room temperature for 4 h. Amines. Synthesis of primary amine groups on the carbon surface was carried out by either hydrolysis of surface acyl azides or reduction of nitriles and amides. Typical example is following: 5.0 g of carbon containing surface acyl azide groups was refluxed with 40 ml of distilled water for 10 h. The sample was filtered afterwards, washed with water and dried at reduced pressure and 80°C for 3 h. Secondary and tertiary amine groups were obtained by reduction of corresponding acyl amides with excessive amount of reducing agent (LiAlH,). Example of protocol: 5.0 g of carbon containing 2.0 meq/g of surface acyl amide groups was added to 40 ml of 1.5 M solution of LiAlH, in THF and stirred under argon at 60°C for 40 h. The reacted sample was filtered, washed successively with diluted hydrochloric acid, water, diluted aqueous ammonia, and water again to neutral pH. The sample was dried after at reduced pressure and 80°C for 4 h. The same procedure was used for reducing surface nitriles. Concentration of surface amines was determined by acid-base titration. Nitrifes. Acyl amides are dehydrated quantitatively yielding nitriles. For dehydration 5 .O g of carbon containing 2.0 meq/g of surface acyl amide groups was added to 0.1 mole of SOC1, dissolved in 20 ml of nitromethane. The mixture was refluxed for 10 h, non-reacted thionyl chloride and solvent were distilled off afterwards at reduced pressure, the sample was dried at 110°C and reduced pressure for 5 h , then washed with water, methanol and dried again for 3 h in the same conditions. 2.4.Adsorption measurements and characterization of porous structure Pore size distribution was determined by means of mercury porosimetry technique with "Pore Sizer 9300" ("Micromeritics", USA). The data are listed in the Table 2 and Fig.1. Adsorption of reference substance (methylene blue) from aqueous solutions was carried out in the batch experiments at 20"C, w/w ratio of adsorbent to liquid phase was 1:100. Initial concentration of the dye was lo00 mg/l for samples 1 and 2, 40 mg/l for samples 3 and 4. Concentration of methylene blue in solution was measured by optical absorption at 660 nm (UV-VIS spectrophotometer "SF-46", LOMO, St-Petersburg, Russia).

708 2.S.Analysis of the chemical nature of surface groups ESCA spectra were obtained with "Varian IEE-15" spectrometer ("Variant',USA) equipped with an A1 K, X-ray source. The reference line was that of Cls of the hydrocarbon layer which forms on the sample inside the spectrometer. Thermogravimetric analysis was carried out with DTG/DTA analyzer "Q-1500D" (Paulik & Paulik, Hungary) under argon. 3. RESULTS AND DISCUSSION 3.1.Evidence of chemical transformations It is very difficult to give direct evidence that certain chemical modification of carbon surface does occur. ESCA is one of the few methods that can reveal the change of surface composition [4]. Though it is impossible to determine exactly the functional group among azide, amide, nitrile, and amine by means of ESCA one can distinguish low valence state of nitrogen in the carbons supposed to possess the mentioned groups (Fig.2, spectrum 1). In all the samples tested N,,-electrons have an energy peak about 399+1 eV, whereas on the surface of the carbon nitrated with conc. HNO, and conc. H,SO, mixture chemical shift of N,,-electrons is considerably higher (Fig.2, spectrum 2) indicating highly oxidized state of nitrogen. It is worthwhile to notice that existence of the peak shoulder shows that other oxidative states of nitrogen are obviously present too. Thermogravimetry appeared to be less informative in determining functional groups. All the samples were gradually losing their weight on heating with endothermic effect at 100120°C probably due to the evaporation of water. Only in the case of azides a low exothermic effect was recorded at 330-360°C which might be ascribed to the decomposition of azide groups (Fig. 3). 3.2.pH-potentiometric data Hydrolysis of surface acyl azide groups as well as reduction of acyl amides appears to give different results depending on the carbon sample taken, whereas yield of basic (amine) groups obtained by the reduction of nitriles is analogous for all the samples (Table 3). Reaction pathways of the first sample differ significantly from the other three carbons, producing substantially lower concentration of surface amines from acyl amide and, moreover, in the case of acyl azide total exchange capacity of the sample N1 after hydrolysis coincided with that of the initial SCS,,,,. 3.3. Role of porous structure To explain these observations porous structure of the samples should be compared. Among the four carbons N 3 and 4 are macroporous, whereas N 1 and 2 have well-developed mesopores (Fig. 1, Table 2). At the same time activated SCS carbon (sample N 2) has very broad mesopore distribution in the range 10-200 nm and certain amount of macropores; sample N 1 possesses only narrow mesopores in the range 10-15 nm. Independent evidences that the difference in porous structure is responsible for different course of surface chemical transformation have been obtained from the adsorption kinetics data (Fig.4). (To simplify the appearance data for the sample N 4 are not presented in Figs.3 and 4 as they are quite similar to the sample N 3). Despite the great difference of adsorptive

709

Table 2. External surface parameters of active carbons’. ~~

Sample N

v,*,

s,-,

VHgma,

cm3/g

cm’lg

sng,a,

m2/g

m2/g

1

0.32

0.01

94

0.0

2

0.98

0.13

250

2.1

3

0.89

0.86

14

2.4

4

0.62

0.56

24

2.1

*) V corresponds to pore volume; S - pore surface area; C - total; ma - macro, as determined by mercury porosimetry (Hg).

2

k M

d

a

\

> a

1:

I

!

I

lo1

\

lo2

lo3

lo4

r, nm Figure 1. Pore size distribution in carbon samples determined by means of mercury porosimetry . r - pore radius, V - pore volume. Curve numbers correspond to the sample numbers from Table 1.

710

410

405

400

395

Energy, e V

Figure 2. ESCA spectra of N,, electrons in the surface layer of carbons. 1 - acyl amide, sample N 3; 2 - "nitro"carbon, sample N 3 (see Materials and Methods). capacity towards methylene blue between the two sets of carbons N 2 and 3 demonstrate fast adsorption kinetics with saturation after 2-6 h, whereas sample N 1 has not been saturated even after 12 h and obviously slow rate of adsortion probably due to the diffusion limitation of methylene blue transport in the narrow mesopores and micropores. It is well-known fact in organic chemistry that course of the reduction of acyl amides by LiAlH, depends on concentration of the reducing agent. If it is sufficient, reduction is complete producing amine, but the lack of LiAlH, leads to a cleavage of C-N bond forming alcohol [2]. Provided that reduction of surface groups of active carbons occurs via the same mechanism as established for organic compounds, following reactions describing formation of surface amine (scheme 1) and surface alcohol-like structure (scheme 2) can be proposed (s corresponds to surface): LiAlH, scheme 1: c S-CH,-NR'R" - HZO

71 1

0

1

20

3

40

Ei

a

60 80

0

200

40 0

600

800

1000

800

1000

t o ,c

0 40

80 120

160 0

200

40 0

600

t o ,c

Figure 3. Thermogravimetry of active carbons. Upper: azide derivative of the sample N 3. Lower: sample N 2 reduced with LiAlH,. 1 - T, 2 - TG, 3 - DTA.

scheme 2:

LiAlH4

s-c do/R’ ‘N

w

\R’

- NHR’R”

S-CH,OH

9

In the case of nitrile reduction there is no alternative pathway and amine is the only

712

40

100

120

80

I

I

I

I

ao

1.6

60

1.2

40

0.8

20

0.4

M

\

?

.3

4

a

0 ffl

V

d

0

0

120

360

240

720

Time, m i n

Fig. 4. Kinetic of adsorption of methylene blue on active carbons. Curve numbers correspond to the sample numbers from Table 1. possible reduced product of the reaction with LiAlH, (Table 3). Reagent transport limitation can manifest itself only by the reaction rate but not by the final yield. The carbinol-like groups of the sample N 1 can be converted to the amines by treating carbon successively with thionyl chloride and diethylamine dissolved in DMSO (scheme 3). The final concentration of NH, is 0.42 mq/g corresponding to 37% yield from initial carboxylic groups.

soc1,, scheme 3: S-CH,OH

-

caH,

60"C, 10 h

NH(Et),, DMSO S-CH,Cl 60"C, 10 h

* S-CH,N(Et),

Hydrolysis of acyl aides is a reaction well known in organic chemistry as Kurtius rearrangement which includes migration of alkyl/aryl groups to the electron deficient nitrogen atom [2]. This mechanism is unbelievable for acyl azide attached to the carbon surface

713

because in this case the whole surface should have migrated. It seems reasonable that in this case different reaction paths are also due to the different rates of reagent transport to the surface as it happens in the reduction of acyl amide groups though exact mechanism of hydrolysis remains unknown. Table 3. Concentration of surface amines obtained from azides by hydrolysis and from amides and nitrils by reduction with LiAlH.,.). Sample N

Amines from nitrils

Amines from azides

Conc. meq/g

Aminel (strong acidic), %

Conc. meqlg

Amines from amides

Aminel (strong acidic), %

Conc. meq/g

Aminel (strong acidic), %

1

0.36

31

0.00

0

0.12

10

2

0.50

43

0.45

39

0.60

39

3

1.10

39

0.78

40

0.79

40

1.15

41

1.15

41

4

*) Concentration of amines is attributed to the strong acidic group transformation (see Materials and Methods).

CONCLUSIONS Chemical transformations of functional groups on the surface of carbons are not quite similar to the reactions of organic compounds in solution. Different reaction pathways may occur on the adsorbent surface if the reaction mechanism depends on the concentration of dissolved reagent which in its turn is relevant to the porous structure of adsorbent.

REFERENCES 1. N.Tsubokawa. J.Polym.Sci., Polym.Chem.Ed., 22 (1984) 1515. 2. J.March. Advanced Organic Chemistry. Reactions, Mechanisms, and Structure. 4th ed., J.Wiley & Sons, NY (1992). 3. H.P.Boehm. Chemical identification of surface groups. In: Adv. in Catalysis and Related Subjects, v. 16, D.D.Eley, H.Pines and P.B.Weisz (eds.), Academic Press, NY - London, 1966, 179. 4. ESCA: Atomic, Molecular and Solid State Structure Studied by Means of EIectron Spectroscopy/K.Siegban, C.Nordling, A.Fahlman et al. Almquist and Wiksells, Uppsala, 1967.