Langmuir 1996,11, 931-935
931
Structure of Adsorption Centers on a Carbosil Surface Deduced from lH Nuclear Magnetic Resonance Spectroscopy Data of Adsorbed Benzene and Water Molecules V. V. Turov," R. Leboda,? V. I. Bogillo," and J. Skubiszewska-Ziebat Institute of Surface Chemistry of Ukrainian Academy of Sciences, 252022 Kiev, Ukraine, and Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received January 26, 1994. In Final Form: November 22, 1994@ The structure ofbenzene and water adsorption complexes on a complex carbon-silica adsorbent (carbosil) surface was studied by lH NMR spectroscopy. The adsorbent was obtained by pyrolysis of CHzClz at 500 "C on the surface of pyrogeneous silica (Aerosil-300). Disordered fragments of graphite planes were found to be adsorption centers for benzene molecules. Water molecules are sorbed on the residual hydroxyl groups of silica as well as at oxygen-containingadsorptioncenters of the carbon material. The thicknesses of the solvation shells of benzene and water of initial aerosil and carbon black particles were determined.
Introduction Characterization of the structure of surface active sites provides one of the most important problems in creating new adsorbents and fillers. 'H nuclear magnetic resonance spectroscopy of "probe" adsorbed molecules has much potential for yielding information about structure of centers on the surface of different carbonaceous materials. This results from the fact that the basic structure of any carbon material is a system of condensed benzene ring chains, which show a distinct effect of the local magnetic anisotropy.1,2Getting into the zone of action of the local magnetic fields, molecules adsorbed on a carbon surface undergo additional screening and their chemical shift is defined as the sum of the screening effects in the molecule and of the local magnetic field. The displacement of 'H NMR signal of various molecules adsorbed on the surface of carbonaceous materials to high The detailed studies magnetic field has been of 'H NMR chemical shifts of various organic compounds and water adsorbed on the surface of several synthetic carbon adsorbents, graphite oxide, and thermoexpanded and intercalated graphites were carried O U ~ . ~Measured - ~ values of chemical shifts were compared with those calculated by the ring current methodl for molecules in the adsorption complexes the localization of which either a t a carbon surface or in slit-shaped pores was determined from the computations of adsorption complex energy minimization performed using of interatomic potentials technique1. A qualitative agreement between observed and calculated chemical shifts was found for sorbate molecules localized on the graphite-like surface or in the ~
Maria Curie-Sklodowska University. @Abstractpublished in Advance ACS Abstracts, February 1, 1995.
(1)Emsley, J.W.; Feeney, J.;Sutcliffe, Z. HHigh Resolution Nuclear Magnetic Resonance Spectroscopy; Pergamon Press, New York, 1966. (2)Zschunke, A. Kernmagnetische Resonanzspectroskopie in der Orgunkchen Chemie; Akademie-Verlag: Berlin, 1971. (3)Gradsztajn, S.;Conard, J.;Benoit, H. J. Phys. Chem. Solids 1970, 31, 1121. (4)Tabony, J. In Progress in NMR Spectrosc. 1980,14,11. (5)Boddenberg, B. Neue. G. Mol. Phys. 1989,67, 771. (6)Turov, V. V.; Pogorelyi, K. V.; Burushkina,T. N. React. Kinet. Catal. Lett. 1993,50,279. (7)Turov, V. V.; Kolychev, V. I.; Burushkina, T. N. Teor. Eksp. Khim. (Russ.)1992,28,80. (8)Tur0v.V. V.: Kalpenko, G. A.; Chuiko,A. A. Teor. Eksu. Khim. (Russ.) 1991,27,121. (9) Turov,V. V.; Pogorelyi, K. V.; Kolychev, V. I.; Chuiko, A. A. Ukr. Khim. Zhurn. 1992,52,470.
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slitlike pores formed by graphite planes. One can draw some generalizations from these studies about correlation of the adsorption complex structures and difference in chemical shifts ofmolecules ( 6 )in adsorbed and condensed states (A = &ond For molecules adsorbed on a disordered carbon surface, A = 0. For adsorption on the extended graphite planes, A is variable from -3 to -7 ppm. In slits formed by graphite planes, A ranges from -5 to -18 ppm, depending on the distance between the planes and the molecule location. If adsorbed molecules interact with groups containing oxygen of oxidized carbon atoms through the formation of hydrogen-bonded associates, then A =- 0. By selecting among adsorbates molecules liable to molecular interactions of various types, it is possible to examine the structure ofthe adsorption centers on the adsorbent surface. Carbon-mineral adsorbents represent a new type of material containing two components: mineral and carboneous material. Their properties depend on the amount of carbon deposited on the mineral matrix. In different adsorption processes occuring on the surface of such adsorbents it can utilize the advantages either of carboneous or mineral componens. Silica, porous glass, aluminium oxide, aluminosilicates, zeolites, diatomites, and other natural and synthetic adsorbents are the most popular mineral components of such adsorbents. Because the original surface properties of carbon-mineral adsorbents vary, a promising future can be predicted for them. Carbonized silicas seem to be perspective materials for use in chromatography and various adsorption processes because their properties can be easily regulated by changing the preparation conditions and the type of initial material.13 The characteristic feature of carbosils is that a substance can be adsorbed not only on the carbon surface but also on the silica surface not covered with carbon. The role ofvarious types of adsorption centers can be elucidated by recording signals in the nuclear magnetic resonance spectra of the adsorbed molecules.
Experimental Section In this paper the authors studied the surface structure of the carbosil obtained as a result of methylene chloride pyrolysis at 500 'C13J4 on the surface of pyrogeneous silica (aerosil type (10)Leboda, R.Mater. Chem. Phys. 1992,31, 243. (11)Leboda, R.Mater. Chem. Phys. 1993,34,123. (12)Kamegawa, K.; Yoshida, H . J . Coll. Znterf. Sci. 1993,159,324. (13)Gierak, A.; Leboda, R.Mater. Chem. Phys. 1988,19,110.
0 1995 American Chemical Society
Turov et al.
932 Langmuir, Vol. 11, No. 3, 1995 7,26 7.11
a
b
5.93
30
20
10
0
-10
6,ppm
30
20
10
0
-IO
-20
-30 6,ppm
Figure 1. The effect of temperature on the shape of the 'H NMR spectra for benzene adsorbed on carbosil (a) and carbon black
(b). Table 1. Adsorptional and Structural Characteristics of Adsorbent
Carbosil carbon black Aerosil specific surface area from BET
260
110
295
0.07 2.2
0.31
0.06 2.2
(Nz m2/g)
packing density, ep(g/cm3) density, emat(g/cm3)
1.9
A-300).16The carbon content of the carbosil was 6.4% wlw. Samples of the carbon black (P-245)with a comparable carbonization temperature and of the initial aerosil were studied simultaneously. Some adsorptive and structural properties of the materials studied are presented in the Table 1. Benzene and water were used as adsorbates. The choice of
the adsorbateswas determinedby the fact that benzene molecules are adsorbed on both surface types through physical adsorption and the basic type of interaction of water molecules with the surface is the formation of hydrogen-bonded complexes with the hydroxylic groups of the silica surface and with carbonyl or carboxylic groups of the oxidized atoms of the carbon-containing surface. NMR spectra were made on a high-resolution NMR spectrometer WP-100 SY, Bruker using the method of liquid phase freezing-out.16 This method consists in placing the adsorbent in the liquid to be studied and reducingthetemperature below its freezingpoint. This results in freezing of the essential part of the substance which is not observed in high-resolution spectra due to small spin-spin relaxation times. Then w e observe lH signals of molecules bonded with the surface and not taking part in the formation of the crystal network of the freezing phase. In this way we can record the signal from the maximal number of molecules which are disturbedby the surface. Not only molecules adsorbed at primary adsorption centers are bonded with the surface but also those several molecular diameters away from it. This thickness of unfrozen liquid can be determinedfrom the intensity ratio of signals before and after freezing. (14)Gierak, A.; Leboda, R.; Tracz, E. J.Anal. Appl. Pyrol. 1988,13, 89. (15)Leboda, R.; Skubiszewska-Zigba, J.; Sidorchuk, V. V.; Tertykh, V. A.; Zarko, V. I. J.Non-CrystallineSolids, submitted for publication. (16) Turov, V. V.; Kolychev, V. I.; Bakay, E. A. e t al. Teor. Eksp. Khim. (Russ.) 1990,26,111.
Results and Discussion Figure la,b presents 'Hspectra ofbenzene bonded with the surface of carbosil and carbon black. Chemical shifts were determined in relation to the outer standard of tetramethylsilane. It appears from Figure l a that the signal from the benzene protons on the carbosil surface is observed as a single one, for which the chemical shift (6) is displaced by 1.5 ppm toward a strong magnetic field with a temperature decrease to 230 K. As the temperature decreases so does the signal intensity due to freezing of the substance nearest the surface. The signal width increases with temperature lowering because of the decreased mobility of the adsorbed m01ecules.l~Close 6 values are observed for benzene bonded with the surface of carbon black (Figure lb), but besides the basic signal with the chemical shift 6 = 6.2 ppm, a weak signal of 6 = 0.14 ppm occurs in the spectrum. For carbosil and carbon black surface-bonded water, the character of the changes in the N M R spectra at low temperatures is different (Figure 2a,b). In the temperature range 268 < T < 273 K water on carbosil is observed as a single signal with chemical shift 6 = 5.6 ppm (Figure 2a). At T < 268 K, the signal splits into two signals of different intensity: signal 1 with chemical shift 61 = 7 ppm and signal 2 with chemical shift 8 2 = 4.5 ppm. When the temperature decreases the intensity of signal 1 decreases, and that of the second one is practically unchanged. "he chemical shift of signal 2 does not depend on the temperature, and for signal 1 only a slight displacement toward a weak magnetic field occurs. Water on carbon black is recorded as a single signal with chemical shift 6 = 7 ppm at T > 245 K. For lower temperatures, a second signal occurs on the wing of the basic signal, the chemical shift of which changes from 62 = -3.6 ppm at T = 245 K to 62 = -7 ppm at T = 225 K. In the case ofwater and benzene molecules bonded with the surface of initial aerosil, the chemical shifts of signals of the adsorbed substances are compatible with cor(17) Mank,V. V.; Lebovka, N. I. NMR Spectroscopy in Heterogeneous Systems; Naukova Dumka: Kiev, 1988.
'HNMR Study of Carbosil
Langmuir, Vol. 11, No. 3, 1995 933 b
