1H NMR investigation on phenol saturated and unsaturated activated carbon: Quantification and exchange behavior of protons

Carbon Printed

Vol. 26. No. 3. pp. 275-282. in Great Britain.

1988 Copyright

0008-6223/8X $3.00+ Ml D 1988 Pergamon Press plc

‘H NMR INVESTIGATION ON PHENOL SATURATED AND UNSATURATED ACTIVATED CARBON: QUANTIFICATION AND EXCHANGE BEHAVIOR OF PROTONS P. LE CLOIREC and G. MARTIN Laboratoire Chimie des Nuisances et Gtnie de I’Environnement E.N.S.C.R., Avenue du G&&al Leclerc, 35700, Rennes, France and

J. GALLIER Groupe de Physique Cristalline, ERA au CNRS 070015, UniversitC de Rennes I, Campus de Beaulieu, 35042, Rennes. France (Received 3 December

1986; accepted in revised form 12 October 1987)

Abstrart-The authors propose to study activated carbon samples using solid-state ‘H nuclear magnetic resonance (NMR). The objective of this work is to quantify-from a dynamic point of view-the different kinds of protons present in the inner structure of the material. Four carbon samples were employed: fresh, saturated with phenol, desiccated. or not. For the fresh carbon, protons are assigned to water diffusing in the porous volume (136 lO*Oprotons/g) or to C-H from an incomplete carbonization of raw material (108 10Zoprotons/g). For carbon saturated with phenol, the maximum phenol adsorption capacity is found to be 110 mgig. This value is compared to data obtained by other techniques. The protons relaxation time has been measured between 244 and 328 K. The spin lattice relaxation time T, is meaningless for the presence of paramagnetic impurities. The spin spin relaxation time T2 study gives some insight about dynamic behavior of the adsorbed molecules. On the fresh carbon

sample, an exchange mechanism between free water protons and more tightly bound water protons and protons of -OH surface groups is highlighted. When phenol is adsorbed onto carbon, about 60% of the molecules of water are replaced by phenol. Exchange is not really observable on this sample, but we believe that it also takes place between water protons and phenolic groups, thus reducing the T2 value considerably. A conjectural model for phenol adsorption into activated carbon is proposed. KEY WORDS-Adsorption, groups.

activated carbon, nuclear magnetic resonance, phenol, surface functional

1. INTRODUCTION

To study phenomena of compound adsorption in aqueous solutions onto solid media, solvent, solute, and eventually biomass have to be examined. In the particular case of activated carbon used in water and wastewater treatment, a large number of publications have reported, predicted, or modelized the removal of specific organic compounds or complex mixture onto this material. Generally, research workers concerned with such questions study only the aqueous solutions and the variations of solute concentrations. This is easy to understand since a good quality of water has to be obtained. There are not many studies about material or the role of solvent during or after solute adsorption phenomena. Then, we need an analytical method to study the behavior of solids and the role of the solvent when a specific molecule is adsorbed onto activated carbon. Nuclear magnetic resonance (NMR) is a very interesting and powerful analytical method to study solid phases: adsorbents, zeolites, polymers, or colloids[ 11. An analytical chemical approach (identification of sites, species, bonding) and a physical approach (static and dynamic behavior of species) are

possible[2,3]. If several studies using NMR techniques have been developed on carbon to determine structures[4-61 or to approach formation of coke at high pressure and high temperature[7,8], the literature is not very abundant for specific studies of activated carbon. In 1971, Mattson and Mark[9] reported only one publication[lO] in which proton NMR was employed for this material. These authors concluded that the technique could be very interesting for future work on activated carbon. However, at the same time Gradsztajn et al. [ 11,121 published articles on high resolution NMR studies of liquid adsorbed on carbon black. Information on the solid texture-the bond between solid and adsorbed molecules and chemical exchange-are given. UsingC13 NMR, Kaplan et al.[13] studied benzene adsorbed onto powdered activated carbon and onto silicagel. The objective of our work is to obtain with proton NMR, qualitative and if possible quantitative information on protons present in fresh or saturated with phenol carbon as well as on their possible motions in the inner structure of the media. This could give us a better knowledge of this material and help to approach the role of water during adsorption phenomena. 275

276

P. LE CLOIRECetal.

Table 2. Preparation of activated carbon samples

2.MATERIALS AND METHODS 2.1 Activated carbon Activated carbon used for this study was Picactif

Sample

Preparation

Desiccation

NC60. Its characteristics are summarized in Table 1, These data, except for the surface functional group data, come from Pica Company, Levaflois, France, Surface functional groups have been determined with Boehm’s method[l4-171. This method has already been published elsewhere[ls]. Only the principle of this analysis will be given here. Different organic acid groups present on the internal surface of the material are neutralized with bases: NaHCO,, Na,CO,, NaOH, and CH&H20Na. Four organic functional groups are determined: GI for carboxilic acids, GII for lactones, GIII for hydroxyl phenolic group, and GIV for carbonyls. For this activated carbon sample, only GIII has been found at 0.45 me * g-’ (i.e., 2.7 x 10” protons per gram of carbon).

A

Fresh Fresh Saturated with phenol Saturated with phenol

No Yes No Yes

2.2 Activated carbon structure [ 191 X-ray study has shown that the crystalline structure unit is probably close to the graphite structure. Graphite is composed of parallel planes of three coordinated carbons. Planes are bound by van der Waals interactions, and the distance between two planes is about 3.35 A. Activated carbon structure would be made with small crystalline units built during the activation phase. These units are in an amorphous phase, consisting of atoms of tetrahedric and disorganized carbons. The diameter of these units would be 150 A and the distance 20 to 30 A. Besides, the interplane length would be slightly more (3.5 A) than for the graphite. The crystallized planes would be aggregates from 3 to 30 planes 10 to 100 8, thick. The structure would include heteroatoms. In this crystallized and amorphous structure, porous surface is developed by activation. 2.3 Sample preparation The two samples (A and B) were fresh activated carbon. The B sample was desiccated in an oven (105°C) for 30 h . Two other samples (C and D) were activated carbon grains saturated with phenol. The phenol came from Merck. In a batch reactor, the media (1 g) were continuously stirred with a phenol solution (1000 mg in 1 liter of deionized distilled water). Numerous works have shown a good adTable 1. Characteristics of activated carbon sample (Pica Company, Levallois, France) Parameters Origin Size (mm) Porosity Specific area (m’ * g-‘) Surface functional groups (me * g-‘) GI GII GIII GIV

Coconut 1-1.5 Mesoporous 1200 0 0 0.45 0

B E

sorption of this solute on this material. With a similar procedure, specific adsorption of phenol on activated carbon has already been studied[20,21]. Carbon grains were then rinsed with deionized distilled water. The C sample was dried at room temperature, the D one was desiccated (ZOS’C, 30 h). Preparation of samples is summarized in Table 2. 2.4 Methods ‘H NMR studies were performed with a Bruker SXP 4-100 spectrometer. Frequencies were 20 and 90 MHz and the temperature range was from 244 to 328 K. The dead time of the spectrometer is 6 ps for 90 MHz. The temperature regulation was stable at 20.5 K; temperature gradient in the sample was less than 1”. To determine absolute values of protons in studied samples, a ~mpa~son was made with some reference sample (a known mass of water having the same volume as the carbon sample). About 400 mg of each activated carbon sample were required for analysis. The spin lattice relaxation time T, were measured by the 180-7-90 pulse sequence. The spin spin relaxation time T2 was determined from the Parr-~rceIl-Meiboom-Gill (CPMG) [30,31] pulse sequence or for the lowest values (~1 ms) from the 90-7-180 sequence. 3.RESULTS AND DISCUSSION

3.1 Atrribu~on and quanri~cafion o~~rofo~ activated carbon

in

First, we propose to determine the number of protons in the porous structure of the material. A typical shape of the free induction decay for every analysis is plotted in Fig. 1. Two parts are observed in this curve: a fast decay during the first 50 ps (a) and a slow decay (b). Although the shape of the free induction decay is very similar for each sample, the absolute and relative amplitudes differ a lot. These amplitudes are determined very carefully according to the following procedure. For the slow component, the slow signal decay is extrapolated at t = 0 or for a more precise determination, this extrapolation is made with the 90-~-180 pulse sequence. This slow decay is exponential and given by M(f)llOW= Mo,,~, exp( - tJ T,) 1

(1)

In this case, the observed protons have interactions averaged by a rapid diffusive motion. For the fast component, the free induction decay is extrapolated at t = 0 by plotting In M(t) versus t2. The fast component is actually gaussian according to

NMR of activated carbon

2. To protons bonded to the carbon structure (d-H) present at the periphery of the crystallized planes of the material. 3. To protons of surface functional groups.

n1 (0

nl0

-.

3 I

I

(a)

MO slow

277

‘-c

I

--

I

--w_

--__

(b)

1

I :i I

10

0

t

(us)

Fig. 1. Characteristic signal of the free induction decay (FID) observed for the activated carbon samples. M(t)r,,t

= Mofast exp( - M,t2/2)

(2)

M, is the second moment of the broad resonance line relative to these protons and the total signal is,

of course.

M(t) = M(t)s,ow + M(t),,, It is close to a gaussian decay at the origin. An approximation of the fast component amplitude is obtained from the difference observed at t = 10 ps, the first measures point (see Fig. 1). The slow component is attributed to the protons that diffuse quickly in the inner structure, the fast component is assigned to atoms fixed on the structure. This measurement is performed for the four activated carbon samples: fresh, saturated with phenol, desiccated, or dried at room temperature. Results are given in Table 3. 3.1.1 Protons of the fast decay. For the fresh activated carbon samples (A and B), this kind of proton can be assigned initially to one of the following: 1. Either to molecules of water strongly bonded to the structure. Resing et al. [22] indicated that different kinds of water could be present on the carbon surface Table 3. Quantification of protons on the material Number of protons (x lo*o)* Samule

Slow FID

Fast FID

A B

136.0 7.7 58.0 1.1

20.0 16.3 58.0 60.0

C

D

*Number of protons per gram of activated carbon.

This last hypothesis is obviously insufficient. The surface functional groups of the material are GIII (i.e., phenolic function) quantified at 0.45 me . g-’ (i.e., about 2.7 x 102”protons per gram of carbon). This value is less than the number of fixed protons quantified on the sample A and B (16.3 and 20 x 10” protons g-l). Besides, it is not sure that these protons (-OH), isolated in the structure because of their weak number, will give a fast decay NMR signal. The first hypothesis: the presence of strongly bound water is unlikely, first because of such a quantity of water (about 14%) cannot be present in the structure after a treatment of desiccation; second the NMR signal of rigidly fixed water, which should have a strong anisotropic motion, is a characteristic doublet (Pake’s doublet)[23] formed of two lines. Their separation is a function of the anisotropy of the rotation of the water molecules. But the observed line is not a doublet, it is a gaussian and unstructured line too narrow with regard to the strong dipolar interaction between protons of water molecules (rHH = 1.58 A). We think that these fixed protons involved in the fast FID are C-H protons of carbon cycles located at the periphery of the crystallized parts of the material. Two arguments plead in favor of this assignment: 1. The first one is the average dimension of crystallized units (about 150 A[19]). The ratio (number of protons nH over the number of carbon atoms n,) is clearly a function of this dimension. This ratio is bigger when the crystallites are smaller since n,, is proportional to the periphery and n, to the surface of the crystalline plane. To simplify, we take a square plane with a side a. The ratio nHlnc = 26/a, b being the distance between two nearest protons in this plane (6 = 2.43 A). From the observed ratio equal to 0.036 (n” = 18 x 10zo protons g-’ and n, = 5 x 10Z2 protons g-l), the a value is deduced a = 135 A, this value is close to the dimension found with the X-ray technique. 2. The second argument comes from the measured value of the broad resonance line second moment derived from eqn (2) (MS = 3.60 t 0.15 G’). In our hypothesis, the protons (56 in average) are 2.43 A apart. The dipolar interactions between the protons in this case give a calculated value for the second moment of 3.53 G2, very close to the experimental one. These protons could come from an incomplete carbonization of the raw material (coconut for these samples) during the manufacture of activated carbon. For the carbon saturated with phenol, the number of fixed protons on the internal surface is about three

278

P. LE CLOIRECet al.

times more important than the number found for the fresh samples. Close data are found for C sample (58 x 10Zoprotons g-l) and the desiccated material (D) (60 x 1020).This result may be easily explained. Adsorbed phenol molecules are fixed on the structure. Their protons give a contribution to the fast decay, strongly increasing its amplitude. The number of phenol protons is about 42 X 10zo protons g-’ (i.e., 7 x 10” molecules of phenol, or 110 mg phenol per gram of carbon). This value is rather less than the maximum adsorption capacity found for phenol (data obtained from Adams, Bohart, and Thomas’ equation (158 mg . g-I)[241 or derived by thermogravimetry (160 mg . g-‘)[21]). The second moment value of the NMR line, measured with a maximum of precision for the desiccated sample, is 3.30 + 0.2 GZ. The intramolecular second moment calculated in the case of motionless phenol molecule, is 2.70 GZ. If we take account of the contribution of EC-H protons fixed to the structure we obtained a calculated value equal to 2.95 G*. The intermolecular contribution between protons of neighboring phenol molecules is about 0.3 G2 for a distance of 8 8, between centers of two neighboring molecules. With this contribution, the calculated value equals the experimental one exactly. This agreement confirms the initial hypothesis that the molecules of phenol are fixed on the structure, with no translational or rotational motion around their pseudo axis; more exactly, these motions are too slow to reduce the proton line width. 3.1.2 Protons of the slow decay. These protons are attributed to protons of water molecules that reorient isotropically and diffuse quickly into the structure. Resing et al.[22] found different kinds of water molecules on the surface of carbon and silicagel. They suggested a surface mobility of this water. These mobile protons are really less numerous (112.5) in the sample saturated with phenol. This result obviously corresponds to molecules of water leaving out the porous volume, driven by phenol molecules coming into it. We tried to confirm this result by a simple calculation: only 40% of molecules of water are present in the carbon structure saturated with phenol. On the fresh carbon, 68 x 10zomolecules of water per gram of carbon occupy about 680 m2 . g-’ (Table 3) (assuming that the molecule of water is disclike with a radius of 1.74 A). This area is likely nearly all available surface in the porous volume of carbon[21]. Similarly, on the saturated carbon the 7 x 102” molecules of phenol occupy a surface of about 410 mz . g-l (the radius of a phenol molecule is 4.3 A[25]) (i.e., 60% of the possible surface). This result indicates that only 40% of area can be covered by the water, thus reducing by the same ratio the water content on the saturated carbon. The freezing of the adsorbed water does not occur at 273 K. This is a result generally observed in the case of adsorbents or zeolites[26,27]. This is due to the absence, in a large scale, of a three-dimensional

structure for adsorbed water. These molecules, confined in the porous structure of material, seem to form some molecular layer. On the desiccated samples, the NMR signal accounting for the slow decay has become very weak (Table 3): almost all water has left the structure. Attribution of this signal is not sure for two reasons: the number of protons is low, particularly in the D sample saturated with phenol (1.1 x lO*Oprotons per gram), and this value is at the limit of sensitivity of the spectrometer, then the difficulty to identify unambigously these protons. We try to imagine, very schematically, the different adsorbed molecules (water and phenol) in the porous structure and the resulting congestion. The micropores, which constitute a large part of the specific surface, have a diameter from 20 to 100 A[19,21]. Assuming that all the micropores are spheric and with an average diameter (d = 45 A), the surface and the volume of this mean micropores are, respectively, 6400 A2 and 48,000 A’. The number of such micropores is, then, 1.07 x lOI per gram of carbon. There are about 640 water molecules in each micropore of fresh carbon. They occupy all the available surface (6400 A*) and about one-third the volume. In the case of saturated carbon, there are about 65 phenol molecules per micropore. They cover an area of 3900 AZ, but they also occupy one-third of the available volume. Finally, the number of the surface functional group (--OH) is about 25 per micropore, with an average distance of about 16 8, if we assume an uniform distribution on the surface of the micropores. Of course, if another value for the average diameter (d) of the micropores is chosen, the number of adsorbed molecules per micropore may vary, but the following essential results will be similar: 1. Water covers carbon sample. 2. For saturated ered by phenol. 3. The distance surface functional sample is 16 A.

all the available surface in a fresh carbon, 60% of its surface is covbetween the two nearest --OH group in this activated carbon

This last observation shows that the functional protons fixed on the surface are very isolated and thus likely do not contribute to the fast part of the FID. They instead contribute to the slow decay signal. For the fresh dry carbon, proton population of the slow component amounts about 7.7 x 1020protons per gram (Table 3) (i.e., about three times the number of --OH protons on the surface (2.7 x 10Zo)). The most plausible hypothesis is to assume that every proton on the surface traps one molecule of water that stays in the structure after thermic treatment. Every pore or micropore (d = 45 A) traps about 25 molecules of water that move inside the same pore from a -OH site to the nearest one. For the dry saturated carbon, the number of protons involved

NMR of activated carbon

in the slow component in the FID is very low (1.1 x 10zo). Paradoxally, a lower value than the number of -OH protons fixed on the surface is found. This paradox overcome if we take account of the presence of phenol molecules. These molecules may go preferentially near the -OH site and the distance from a surface -OH proton to a proton of the phenol molecule becomes low, less than 3 or 4 A. The dipolar interaction between them becomes high enough so that most of the --OH protons fixed on the surface contribute no more to the slow decay, thus reducing strongly this signal in the case of the desiccated carbon saturated with phenol. This signal could either be due to isolated -OH and/or some few water molecules which remain trapped in the structure. 3.2 NMR relaxation times of the moving protons These protons have been measured on the different samples, except for the D sample where the signal of the moving nuclei is too low (Table 3). 3.2.1 Spin lattice rel~at~on time (T,). Whatever the sample, the measured T, value is found of about 100 ms with the following characteristics:

279

1, Few dependent on temperature (Fig. 2). 2. Undependent on frequency. 3. With a similar value for the two kinds of protons (fixed or moving). 4. Few dependent on treatment of sample (dry or desiccated), therefore, independent of the presence of water in the structure. This observation, and especially (41, allows us to think that the spin lattice relaxation arises from an extrinsic origin, for example, due to paramagnetic impurities as such metallic ions fixed in the structure[28,29] or to free radicals present in the carbons[11,12,32,34]. We have to remember that such elements, even in a low quantity (about 1 ppm) have a huge influence on the relaxation time (T;)[23] OWing to the very high value of the electronic spin. The absence of influence of translational or rotational motion of water molecules on T, indicates that this motion is very fast. 3.2.2 Spin spin relaxation time Tz. This is measured with CPMG pulse sequence[30,31), and its value for the lowest pulse delay (tcp equal 100 PLS) is plotted on Fig. 2.

Sample A 0

0

e

v

v

7

100

r*

V

l

T,

$0 Mhz

0

T,

,20Mhz

n

T,

,90 Mhz

[3

T,

,20 Mhz

Sample n cl

m

A

V

T,

,90 Mhz

v

T,

,20Mhz

A

T,

,90 Mhz

,t+J T,

,20 Mhz

n

10

A

3,5

4

4.3

03/T

Fig. 2. Variations of T, and T2 with frequency and temperature in the case of fresh carbon (A) and after phenol adsorption (C).

P. LE CLOI~~ et al.

280

Fresh carbon sample. Tz is not temperature and frequency dependent. On this sample, the echo decay time Tzq depends on the pulse repetition rate tcp-‘[32] according to: (3) It is well known[32,33] that such behavior arises from a fast exchange between two protons phases. x: experimental parameter of the sequence x = tcpl2 +r6: lifetime of an exchangeable proton in the b phase A is given by the following equation: A = Pa x T,’ f Pb (Tb + T;,‘)

The parameters A, B, and Toare derived from the linear plot of Tz versus 1 - &.X/X.Their values are given at three different temperature in Table 4. The lifetime 7b is found to be 0.3 to 0.4 ms and varies a little with temperature. Phase a is the most labile and likely corresponds to the water molecules that leave the structure by thermal treatment. Its population amount 128.3 x 10% protons g-’ (Table 3). This phase is presumed to be characterized by one relaxation time Tzoassumed equal to the observed spin lattice relaxation time T,. Phase b is different chemically and dynamically. It can be identified to bound water molecules which stay in the structure after thermal treatment or to fixed surface -OH protons. To simplify, these two kinds of protons are considered to give only one phase, with one TZbvalue of the spin-spin relaxation time. The characteristics of the b phase are measured on the desiccated sample (B): TZb = 1.40 ‘- 0.20 ms at 328K and T,, = 0.56 + 0.10 ms at 244.5 K. Its population equals 7.7 x lOto protons g-’ (Table 3). The deduced value of Pb, the ratio of the number of protons in the b phase over the total number of protons, is 0.057 * 0.03. If the above relations[33] are applied for the two extreme temperature (328 and 244.5 K) with the following data: at 328 K TZa = 250 2 10 ms, rb = 0.396 +: 0.02 ms and at 244.5 K T,, = 74 C 5 ms, TV= 0.299 2 0.02 ms, we obtain the calculated values for A:

T 6) A (s-j) B (10-3, s-2) Tb(ms) Mwm)

of the exchange observed on the carbon samoles

244.5 74.6 149.1 0.299 2.95

273 63.8 191.3 0.308 3.34

and

A2M.SR= 79 3- 7 s-l

The experimental values obtained from the fit are, respectively, A = 48.9 + 5 s-l and 74.6 rf: 6 SS’. With the simplifying hypothesis used in this model, the comparison of the data is satisfying. The study of the spin-spin relaxation shows a fast exchange between two phases: “free” water, which is removed by desiccation and bound water with the --OH protons of the structure still present after desiccation. It is mainly this latter phase that influences the observed relaxation rate. The B parameter is [32]: B = Pa Pb(Gw,)

(4)

It is the TZvalue for the lowest tcp and is plotted on Fig. 2. Pa, Pb, I;,, Tzb: relative populations and true transverse relaxation times of a and b phases.

Table 4. Parameters

A 328K= 35 + 5 s-’

328 48.9 242.6 0.396 3.76

with u0 = 90 Mhz and gb being the relative chemical shift of the b phase. The data of &, are given in Table 4. Such chemical shifts are currently observed with compounds as phenols or hydrogen bounded. Sample saturated withphenol. In the C sample, no exchange is observed by the CPMG sequence[30,31]. It might be too fast and/or there is too little efficiency. The second term in eqn (3) does not become observable if r6 is very low andfor if the first term (A) is very high. Since the measured relaxation rate (T;‘) has a large value (400 s-l), the second solution is possible. Such a low value of T2 (2.5 ms) could be explained two ways: 1. The observed relaxation is the relaxation of protons of the free water molecules (T;,‘); these protons do not exchange with the protons of the b phase, (i.e., -OH protons of phenol). In this case, the low value of T,, could be due to a very strong anisotropy of the motion of the water molecules. The presence of aromatic rings situated on the surface of the micropores would hinder the motion of the water molecules. 2. The nonobservable exchange mechanism is effective and the low value of T2 is attributed, as for the A sample, to a strong contribution of the b phase to spin-spin relaxation rate. Consider this hypothesis. The b phase is mainly -OH protons of molecules of phenol. These protons have strong interactions with the other protons of the same molecules or with the nearest molecules. Their relaxation time (T,,) is low, around some tenth of microseconds (Tzb = 28 ps, if we take the value deduced from the second moment of the gaussian line by the relation: T$ = 2/M,). It is probable that the residence time (TV)is higher than TZb.Equations (3) and (4) give: (5) The relative population of the b phase (Pb) is the ratio between the number of phenolic function protons and the total number of protons. We find Pb = 0.108 (Table 3). The exchange is not observed, 7gcannot be known. We assume that its value

NMR of activated carbon

is similar to the value for the A sample. The following data for the calculated T2 are thus derived: T2,328K = 3.56 ms The experimental

T2,244,5K = 2.67 ms

and

values are (Fig. 3):

T 2.328K = 3.75 ms

T 2.244 5K - 2.20 ms

and

A good comparison is observed between the experimental and calculated data. We believe that the exchange between the ---GH protons of phenol and the protons of water is responsible for the observed T2 value of the moving proton in this sample.

4.

281

the surface functional group (GIII) (relative population is 0.020). 2. For carbon saturated with phenol, 60% of the available surface is covered by molecules of phenol (110 mg/g) that are fixed preferentially close to the surface functional groups, the molecules of phenol moving a great part of the water molecules out of the pores: only 40% of water is still present in the porous structure. The protons of the -OH phenolic function exchange very quickly with the protons of water. Figures 3a and 3b attempt to describe the positions of these different molecules. 3. Whatever the samples (fresh or saturated carbon) the EC--H protons on the borderline of the crystallites are responsible for a broad resonnance line.

CONCLUSION

Our main results obtained with proton NMR on activated carbon are the following:

Three possibilities of interactions between phenol and activated carbon seem to occur:

1. For the fresh activated carbon, the very mobile adsorbed water seems to cover all the porous surface and exchange quickly (T,, = 0.35 ms at room temperature) with a low amount of bound water (relative population is 0.037) and with the -OH protons of

1. The most frequently observed bond is an exclusive bond between the aromatic ring and the porous surface of the material. This is the case for twothirds of the molecules (case 1). 2. The surface functional group (-OH) is in-

, h/ / O\H

H

f H, ,H 0

‘k .’ \ ONH o;,,*-

b

)

Surface

of

carbon

____ _-;“,p_(g

0

0

Q

OH

with

phenol

4

1 ---fpH

6’.

/ f

(i) c ) j\ctivated

carbon

surface

H

(iii)

(ii) with

adsorbed

phenol.

[‘or

the

models

(ii

of phenol has a peralleie plane with activated FOP the model (ii), It is perpendicular with media surface.

and

(iii),

aromntic

ring

carbon

Fig. 3. Conjectural model for the distribution of adsorbed molecules in the porous volume of carbon samples. -+: motion of molecules. -: exchange.

P. LE CLOIRECet al.

282

volved in the bond: the interaction is either exclusively with the -OH function (case 2) or also with the aromatic ring (case 3). Since the molecules

ofphenolinteract

more strongly

with the support than with the molecules of water -. and moreover, the protons of phenol function is exchanged with the protons of water, it is probable that the second mechanism of interactions (case 2) is margi,nal. This work has shown that the wide-band proton NMR is a very powerful technique to study activated carbon. It has been possible to classify and quantify different kinds of protons: fixed S-H protons, phenolic -OH, protons of “free” and bound water molecules, -OH from surface functional group. This study highlights the presence of exchange mechanisms between protons of different “phases” and helps to deduce the kinetics of exchange. Besides, observations of removal of water during the adsorption of phenol could be explained by a similar site of adsorption for phenol and water.

Acknowledgments-The authors thank Professor M. Maunaye for his help in the preparation of this manuscript.

REFERENCES

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