Langmuir 1994,10, 4357-4360
4357
Zirconium Dioxide Supported on a-Cellulose: Synthesis and Characterization Ubirajara P. Rodrigues Filho, Yoshitaka Gushikem," and Fred Y. Fujiwara Instituto de Quimica, UNICAMP, 13081-970, CP 6154,f Campinas, SP, Brazil
Sandra C. de Castro, Iris C. L. Torriani, and Leide Passos Cavalcanti Instituto de Fisica Gleb Wataghin, UNICAMP, 13081-970, Campinas, SP, Brazil Received March 29, 1994. I n Final Form: June 21, 1994@ Cellulose-supported zirconium dioxide was prepared by reaction of ethanolic zirconium tetrachloride solution with a-cellulose. The treated cellulose was characterized by C-13 cross-polarization magic angle spinning nuclear magnetic resonance, powder X-ray diffraction, X-ray photoelectron spectroscopy,scanning electron microscope, and X-ray fluorescencespectroscopy. The results show that the zirconiumtetrachloride reacts with a-cellulose, giving origin to zirconium dioxide (ZrOz) agglomerates on the surface of cellulose fibers. The reaction increased the crystallinity of cellulose. This was attributed mainly to the reaction of hydrogen chloride, generated during the dissolution of zirconium tetrachloride in ethanol, with the amorphous regions of cellulose. Finally, the interaction of cellulose and the oxide is only through London forces.
Introduction The preparation of organomodified celluloses which can adsorb organic molecules a n d inorganic ions has become a very common practice.'S2 Recently some reports on transition metal oxide modified agarose3 a n d cellulose4,5 have been shown to have interesting properties for ion a n d protein adsorption. However, the nature of the interaction of polysaccharide with t h e oxide has not been reported. The aim of this work is to report the preparation of zirconium dioxide modified cellulose using a method similar to that described elsewhere for silica modification.6 The materials were characterized by X-ray powder diffraction (XRD),X-ray fluorescence (XRF),carbon-13 crosspolarization magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR), X-ray photoelectron speca n d scanning electron microscopy (SEMI. troscopy (XPS), The content of supported zirconium was determined by XRF. The crystallinity of t h e materials was determined by XRD a n d 13C CP/MAS NMR. The nature of the cellulose-oxide interaction was determined by XPS, 13C CP/MAS NMR, a n d SEM. The materials' morphologies were determinated by SEM with a backscattering electron image (BEI).
Experimental Section
ethanolic solutions were prepared by dissolving ZrC4 in 100 mL of dry ethanol in a glovebag under a dry nitrogen atmosphere at 6.00 x and 11.9 x M concentrations,preparation 1 and 2, respectively. Five grams of a-cellulose (Sigma,cotton linter) was suspended with stirringin 100mL of ZrC4 (Aldrich)solution. The suspensions were stirred for 2 hat 355 Kunder a dry nitrogen atmosphere. The resulting suspensionswere fdteredin a Schlenk apparatus under a dry nitrogen atmosphere. The filtered materials were washed with dry ethanol under a dry nitrogen atmosphere and the solvent pumped off under vacuum (10-3 Torr). After a flux of gaseous ammonia was passed through for 15 min, to eliminate the trapped chloride, the cellulose was washed with an ethanollwater solution 50%(v/v). The treated cellulose was dried for 1h at room temperature under vacuum (10-3 TO^).
Zirconium Analysis. The amount of zirconium in the materials was determined by X-ray fluorescence analyses with a TRACOR Northern X-ray fluorimeter. The standards were ZrO, diluted in a-cellulose previously ground and sieved to 200 mesh. Powder X-ray Diffraction. XRD measurements were carried out on a URS-6 diffractometer with the following conditions: power supply of 40 kV, current of 20 mA, detection time of 5 s with step of 0.05" (28). The experimental diffraction patterns were corrected for Lorentz and polarization factors. To determinethe relative crystallinityofthe cellulose samples, deconvolutionsofthe X-ray difEactogramswere made by applying the equation
Preparation. Ethanol (Merck)was initially dried with CaCJ, distilled, and stored on molecular sieves (4 A). T w o ZrClr t e-mail:
[email protected].
Abstract published in Advance ACS Abstracts, September 15, 1994. (1)Muzzarelli, R. A. A. Natural Chelating Polymers; Pergamon Press: New York, 1973;pp 1-22. (2)Inagaki, H, Phillips, G. 0. Cellulosics Utilization: Research and Rewards in Cellulosics. Proceedings of Nishimbo International Confer@
ence on Cellulosics Utilization in tne Near Future; Elsevier Applied Science: London, 1989. (3)HjBrten,S.;Zelikman,I.; Lindeberg,J.;Lederer,M. J. Chromatogr. 1989,481, 187-199.
(4)Sabybaeva, B. I.; Sultankulova,A. S.; Vasilikova, T.; Afanasiev, M. A. Cellulose Chem. Technol. 1991,26, 199-210. (5) Suzuki, M.; Fujii, T. New Developments in Zon Exchange. Proceedings of the International Conference on Ion Exchange, Tokyo, Japan, 1991,pp 355-60. (6)Peixoto, C. R.M.; Kubota, L. T.; Gushikem, Y. New Developments in Ion Exchange. Proceedings of the International Conference on Ion Exchange, Tokyo, Japan, 1991,pp 607-12.
where A20) is the function to be adjusted to the experimental diffraction pattern, Gi(28)are the Gaussian distribution functions, and AM(28)is a broad Gaussian function with a maximum at ca. 20"; for the base line function B(28),a straight line was used. The fitting between experimental and calculated curves was obtained using a least squares algorithm proposed by Marq~ardt-Levenberg.',~ The ratio of crystalline peak areas to the total area under the diffractogramsg gives the crystallinity degree. (7)Bevington, P. R. Data Reduction and Error Analysis for the Physical Science; McGraw-Hill: New York, 1969;pp 204-265. (8)Kenedy, W. J., Jr.; Gentle, J. E. Statistical Computing; Marcel Dekker Inc.: New York, 1980,p 483. (9)Kakudo, M.; Kasai, N. X-ray Diffraction by Polymers; Kodansha Ltd.: Tokyo, Japan, 1972;pp 359-367.
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4358 Langmuir, Vol. 10,No. 11, 1994
A Figure 1. Structure of monomeric unit of cellulose, anhydrocellubiose.
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Figure 2. l3C CP/MAS NMR spectra of ZdCel2 (A), Zr/Cel3 (B), and pure cellulose (C). The peaks are marked with the carbon numbers of Figure 1;a and c of carbon 4 indicate the peaks of amorphous and crystalline regions of cellulose. Table 1. Crystallinity Degrees (%) Obtained by XRD and '3C C P m NMR sample XRD C13 CP/MAS NMR a-cellulose 44(U 34(3) Zr/Cell(4% of Zr) 50U) 38(3) Zr/Cel2 (5%of Zr) 57U) 47(3) X-ray Photoelectron Spectroscopy ( I P S ) . XPS measurements were carried out on McPherson-30spectrometer, using Al Ka 1486.6evradiation as the excitation source under a pressure lower than 2 x lo-' Torr. The atomic ratios were estimated from the areas under the binding energies peaks and the Scofield cross section.1° The carbon 2p binding energy peak of hydrocarbon was used to calibrate the instrument. Solid State 13CNuclear MagneticResonance. Magic angle spinning NMR measurements were carried out using a Bruker AC 300P spectrometer. A pulse sequence with contact time of 1ms, with a 2 s interval between pulses, and a 53 ms acquisition time were used. TMS was used as a reference to calibrate the chemical shift scale. The NMR spectra were deconvoluted using the function
where Gi(ppm)are seven Gaussian functions, one for each peak. The fitting offlppm) with the experimental spectra was carried out by the method described in XRD section. The degree of crystallinity was calculated by the ratio of the area under the crystalline C4 peak (where C4 refers to carbon 4 in the structure of anhydrocellubiose shown in Figure 1)at ca. 89 ppm and the area under the peaks at ca. 89 ppm and at ca. (10) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (11) Szerevenyi,N. M.; Sullivan,M. J.;Maciel, G. E. J.Mugn. Reson. 1982, 47, 462-75.
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Figure 3. X-ray diffractogram of pure cellulose (A), Zr/Cel 1 (B)and Zr/Cel2 (C). The points are the experimental data, and the lines are the fitted curves; the broad peaks show the amorphous diffraction for each sample. 84 ppm, which correspond to the C4 peaks of the crystalline and amorphous phases of the cellulose, respectively.12-1s Scanning Electron Microscopy (SEMI. Samples for microscopy were deposited on a double-faced 3M tape adhered on an aluminum sample holder. The samples were coated with a thin film of graphite for X-ray fluorescence analyses with an EDS microprobe. Gold was used to coat the samples for backscattering electron image (BEI)microscopy. The equipment used was a JEOL microscope equipped with a TRACOR Northern EDS microprobe.
Results and Discussion The chemical analyses carried out for Zr incorporated into two samples of treated cellulose presented 4.0 and 5.2%(Zr weightkellulose weight). These samples will be hereafter designated as Zr/Cell and ZrICelz, respectively. The 13CC P W NMR spectra ofboth samples oftreated celluloses are shown in Figure 2. The Gaussian resolved peaks are assigned as follows: C1 at 105ppm, Cz at 72.7, C3 at 75.5, Cq at 83.8 and 89.0 ppm, C5 at 74.4, and CS at 62.9 and 65.6 ppm. As can be observed, there were no significant changes of the carbon chemical shifts with the loading of zirconium atoms on the surface. Presumably, this indicates that there are no stronger interaction forces (12) Atalla, R. H.; Gast,J. C.; Sindorf,D. W.; Bartuska,V. J.; Maciel, G. E. J.Am. Chem. SOC.1980,102, 3249-51. (13) Earl, W. L.; Vanderhart, D. L. J. Am. Chem. SOC.1980, 102, 3251-3252. (14) Hirai,A.; Horii, F.; Kitamura,R. Cellulose Chem. Technol. 1990, 24, 703-711. (15) Hirai, A.; Horii, F.; Kitamuru, R. J.Appl. Polym. Sci. Part C, Polym. Lett. 1990,28, 357-61.
Langmuir, Vol. 10, No. 11, 1994 4359
Zirconium Dioxide Supported on a-Cellulose
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Figure 4. X-ray photoelectron spectroscopy of a-cellulose (A), Zr/Cell (B) and Zr/Celz (C). The carbon 1s peak was deconvoluted into three peaks: C1 (C1in Figure l),C2 (Cp until CSin Figure 1)and C3 (hydrocarbonpeak), leR to right, respectively. The oxygen 1s peak was deconvoluted into two peaks for treated celluloses. The zirconium 3d peaks are the 3d3/2 and 3d5n peaks.
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Figure 5. Scanning electronmicroscopyof Zr/Cel 1(750X-the straight line above the numbers measures 100 pm) using a backscattering electron detector. The bright points are the zirconium dioxide particles.
between cellulose-zirconia than that of the London type. However, a detailed analysis of the areas under the peaks of C4 at 89 ppm due to the crystalline fraction and at 84 ppm due to the amorphousfractionof the cellulose (marked c and a peaks in Figure 2) indicated that there was an increase in the crystallinity of the treated celluloses (Figure2A and B)in relation to untreated cellulose (Figure 2C). As can be seen in Table 1, when the content of ZrO2 in the cellulose increases, the crystallinity degree of the treated cellulose also increases, Le., 38% for Zr/Cell(4% of Zr) and 47% for Zr/Cel2 (5% of Zr). In Figure 3, the XRD of Zr/Cell and Zr/Cela are shown. Comparison of the spectrum in Figure 3A of pure celullose with those of treated cellulose reveals a calculatedincrease in crystallinity degree of ca. 6% and ca. 13% (Table l), respectively, for Zr/Cell (Figure 3B) and Zr/Cel2 (Figure 3C). The increase in the crystallinity as determinated by XRD and 13C CP/MAS NMR was attributed to the action of the acid HC1, liberated by dissolving ZrC4 in ethanol, on the amorphous part of cellulose. No other diffaction
Figure 6. Scanningelectron microscopy of Zr/Cel2 (1000X-the straight line above the numbers measures 100 pm) using a backscattering electron detector. The bright points are the zirconium dioxide particles.
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Figure 7. X-ray fluorescence diagram of the analysis by EDS of bright points of Figure 2.
peaks besides those of cellulose are observed, which indicates that the coating oxide is constituted exclusively of amorphous particles.
4360 Langmuir,Vol. 10, No. 11, 1994
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70 60
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Figure 8. Histogram of average particle size of zirconium dioxide particles on the treatedcellulosefibers: Zr/Cel 1 (white bars) and Zr/Cel 2 (black bars).
The crystallinity degrees obtained by XRD are, in relative terms, the same as those observed above by 13C CP/MAS NMR. The higher values obtained by XRD are attributed to differences in the techniques used, and the absolute value of the crystallinity degree in the NMR is related to the delay time.14 Figure 4 shows the XPS spectra of Zr/Cell and Zr/CelZ. The oxygen 1s binding energy peaks appear as doublets; the one at 532.1 eV is assigned to the cellulose oxygen16 and the other at 530.1 eV to the zirconium oxygen.17Two peaks observed at 184.8 and 182.4 eV are due to the Zr 3d312 and 3d512 binding energy peaks in Zr02.17 The C 1s binding energy peaks of the cellulose were assigned as C1 (c1in Figure 1)at 288.3 e v and c 2 (c2until c6 in Figure 1)at 286.3 eV, and the peak at 284.6 eV is due to the standard used for calibration of the equipment.16 The C1 and C2 binding energy peaks in Zr/Cell and Zr/Cel2 did not show any change in relation to those observed in unmodified a-cellulose, a clear indication of a weak cellulose-metal oxide interaction which confirms the conclusion obtained from 13C CP/MAS NMR spectra. The micrographs obtained by SEM backscattering electron imaging, for samples of Zr/Cell (Figure 5) and Zr/Cel2 (Figure 6),clearly show agglomerations of the oxide particles, which appear in the figures as bright points on the fiber surface. Analysis of these bright points by EDS (Figure 7) confirmed that they are agglomerates of ZrO2 (16) Dilks, A. In Characterization of Polymers by ESCA in Deuelopments in Polymer Characterization;Dawkins, J. V.,Ed.;Applied Science Publisher, Ltd.: London, 1980; Vol. 2, pp 145-182. (17) Nefedov, V.I.; Solyn,Y.V.;Chertkov, A. A.; Padurets,L. N. Zh. Neorg. a i m . 1974,19, 1443-5 (in Russian).
particles. A more homogeneous distribution of these points in Figure 5 contrasts with a more heteregeneous distribution observed in Figure 6. A distribution of the particle sizes on the surface for both samples of treated cellulose is shown in Figure 8. Predominance of oxide particles having an average size between 2 and 3 pm is observed for both samples, about 50%for Zr/Cell and about 70% for Zr/Cel2. The occurrence of particles size higher than 3 pm is not significant and, for dimensions between 1and 2 pm, is significant for Zr/Cell(33%). The formation of agglomerates in both preparations is certainly related to the nature of the ZrC4 solution. The ZrC4 and Zr(OEt), are known to be very associated in alcoholicmedium, formingoligomericalkoxychlorides.l8*l9 These oligomeric alkoxy chlorides probably adsorb on the surface of suspended cellulose through hydrogen bonds, as observed when polyamide is treated with silicon alkoxides.20 These oligomeric alkoxy chlorides are responsible for the formation of agglomerates. When the filtered solid material reacts with gaseous ammonia, the alkoxy chlorides are converted to alkoxy hydroxides of zircori _____The treatment with ethanoywater solution hydrolyzes the alkoxy hydroxides to zirconium dioxide which is deposited as islands instead of monolayers on the fiber surface. The predominance of oxide particles having an average size among 2-3pm in Zr/Cel2 in relation to Zr/Cell confirms this proposed mechanism. At higher concentrations of ZrCl4, the oligomeric alkoxy chlorides have a higher molecular weightlsJgand so result in larger agglomerates as shown in Figure 8.
Conclusion The results show formation of an amorphous zirconium oxide agglomerate on the cellulose surface. The nature of the cellulose-metal oxide interaction is of the London type as shown by XPS C l s binding energy, C13 chemical shifts, and electron microscopy of treated celluloses. Formation of the composite occurs with a crystallinity increase of the treated cellulose fibers in relation to that of a-cellulose,which was attributed to the acid ethanolysis of the amorphous part of cellulose. (18) Bradley, D. C.;Mehrota, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: London, 1978; pp 13-24 and 74-122. (19) Hiberg, M.; Glaser, J. Inorg. Chim. Acta 1993,206,53-61. (20) Saegusa,T.;Chujo,Y.Makromol. Chem.,Mucromol.Symp. 1992, 64, 1-9.
