Mechanochemical Synthesis of Lanthanum Oxyfluoride by Grinding

The MC reaction proceeds with an increase in the grinding time, forming lanthanum oxyfluoride in the ground product. The reaction yield reaches about ...
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Ind. Eng. Chem. Res. 2001, 40, 4785-4788

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Mechanochemical Synthesis of Lanthanum Oxyfluoride by Grinding Lanthanum Oxide with Poly(vinylidene fluoride) Jaeryeong Lee,† Qiwu Zhang,* and Fumio Saito‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

A mixture of lanthanum oxide and poly(vinylidene fluoride) (PVDF) was ground in air by a planetary ball mill to investigate the mechanochemical (MC) reaction between the two materials. The MC reaction proceeds with an increase in the grinding time, forming lanthanum oxyfluoride in the ground product. The reaction yield reaches about 98% at 240 min. The organic phases from PVDF in the ground sample after the MC reaction have C-O and C-O-H bindings. 1. Introduction Lanthanum oxyfluoride (LaOF) is one of the functional materials used as an activator or a host material of phosphors, as a catalyst for oxidative coupling of methane, in oxidative dehydrogenation of ethane, and so on.1-5 It has been synthesized by mainly direct solidstate and electrowinning processes.6-10 The former process requires high temperature and pressure under severe gaseous conditions, and the latter needs pretreatment for preparing an aqueous solution or molten salt before the synthesis. Normally, lanthanum fluoride or ammonium fluoride has been employed as a fluoride source; a more economical fluoride source would be, however, expected in the field of LaOF production industries. In the meantime, lots of PVDF wastes have been emitted from industries which deal with electric products as well as PVDF commodities.11,12 These wastes are burned out in disposal facilities, but harmful gases such as HF are emitted, so that another proper method for disposing of PVDF wastes has been required.13-15 It would be useful for such PVDF wastes to reuse as a fluoride source in the synthesis of LaOF. However, little attention has been paid to the reuse of PVDF waste.16-19 The main purpose of this paper is to provide information on the mechanochemical (MC) synthesis of LaOF by grinding a mixture of lanthanum oxide (La2O3) and PVDF. This investigation offers a novel method for disposing of PVDF waste as well as its reuse as a fluoride source in the synthesis of LaOF. 2. Experimental Section La2O3 used in the experiment was a chemical reagent supplied from Wako Pure Chemical Industries, Ltd., Japan, and its purity was 99.9%. Poly(vinylidene fluoride) (PVDF, [-CH2CF2-]n, guaranteed grade) was supplied from Dowa Mining Co. Ltd., Japan. The two reagents were mixed at an equivalent molar ratio of F to La, and it was used as a starting mixture. A planetary mill (model no. Pulverizette-7, Fritsch Gmbh, IdarOberstein, Germany) was used for grinding of the mixture. The mill consists of a pair of pots made of zirconia and a rotating disk. Four grams of the mixture was put into each zirconia pot (45 cm3 inner volume) * To whom correspondence should be addressed. Tel. and Fax: +81-22-217-5137. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail: [email protected].

with seven zirconia balls of 15 mm diameter and subjected to grinding in air (i.e., without evacuating or sweeping with an inert gas) at 700 rpm for various periods of time. After grinding, the resulting product was a free-flowing powder. There was minimal temperature increase, and no gaseous products; therefore, safety concerns did not arise. The ground mixtures were characterized by an X-ray diffraction (XRD) analysis (model no. RAD-B, Rigaku Co., Ltd., Tokyo, Japan) method using Cu KR radiation to identify the phases formed in the products. X-ray photoelectron spectroscopic (XPS) analysis (PHI 5600 ESCA system, UlvacPhi Inc., Chigasaki, Japan) was conducted to obtain the chemical bond information of samples. Infrared spectra of the samples were also measured using an FT-IR spectrometer (Bio-Rad FTS-40A) with the KBr disk method. Thermogravimetric analysis was conducted by a TG/DTA analyzer (Rigaku Denki TAS-200) at a heating rate of 10 K/min in air. The morphology of the samples was observed by a scanning electron microscope (SEM; Hitachi 4100L). 3. Results and Discussion 3.1. MC Reaction between La2O3 and PVDF. Figure 1 shows the XRD patterns of the mixtures of PVDF and La2O3 ground for different periods of time. The peak intensity of La2O3 decreases gradually with an increase in the grinding time. On the contrary, new peaks of LaOF appear in the patterns of the mixtures ground for 30 min or more, and their intensity increases as the grinding progresses. According to the XRD patterns of the mixtures ground for 120 min or more, LaOF (rhombohedral; JCPDS no. 6-281) is formed in the products, and accordingly the amount of La2O3 has been reduced. These suggest that LaOF can be synthesized mechanochemically by grinding, and the reaction can be expressed by eq 1, which is shown as a monomer unit reaction. The chemical bonding states of the resultant organic phases will be discussed in the next section from the viewpoint of XPS and FT-IR analyses.

[CH2CF2] + La2O3 f 2LaOF + [C2H2O]

(1)

Figure 2 shows the integrated intensity ratio (RA) defined by eq 2 as a function of the grinding time. The

RA )

ALaOF(110) ALaOF(110) + ALa2O3(101)

10.1021/ie010086k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001

(0 e RA e 1) (2)

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Figure 3. TG analysis of the La2O3 and PVDF mixture ground for different times.

Figure 1. XRD patterns of the La2O3 and PVDF mixture ground for different times (min): (A) 30, (B) 60, (C) 120, (D) 240, (E) 480.

Figure 2. Integrated intensity ratio (RA) between starting (La2O3) and product (LaOF) calculated based on XRD diffraction planes (101) and (110), respectively.

integrated intensities, ALa2O3 and ALaOF, of La2O3 and LaOF were calculated based on their diffraction planes, (101) and (110), in the XRD pattern, respectively.20 The integrated intensity ratio (RA) is known to be one of the most important variables to evaluate a reaction. As shown in Figure 2, RA increases with an increase in the grinding time, but the correlation between RA and the grinding time is not always linearly expressed at all. In fact, RA increases with an increase in the grinding time and reaches about 0.25 at 60 min. Subsequently, it increases more rapidly from 60 min and reaches about 0.8 at 120 min. The gradient of the curve becomes significantly small in the range over 120 min, and the value reaches about 0.98 at 240 min. No increase in RA is observed for further grinding. Figure 3 shows the TG curves of the mixtures ground for different periods of time. Significant weight loss of the mixtures can be seen in the temperature range from 300 to 400 °C, because of the thermal decomposition of PVDF in the mixture into HF, CO2, H2O, and a like. The weight loss of the mixture ground for 30 or 60 min

Figure 4. SEM pictures of La2O3 and PVDF mixtures ground for different times (min): (A) 30, (B) 60, (C) 120, (D) 240, (E) 480.

is measured as around 20 wt %, implying that most of PVDF remains in the mixture. However, the weight loss tends to decrease with an increase in the grinding time and reaches about 13 wt % at 120 min. The significant change of weight loss in the grinding period from 60 to 120 min indicates that the reaction proceeds quickly in this range. As the F of PVDF in the mixture is transferred into the inorganic phase, the weight change becomes small correspondingly. Figure 4 shows SEM photographs of the mixture ground for different periods of time. In photo A of the mixture ground for 30 min, the particle shape of the original PVDF sample is observed to remain with several tens of microns size, and the size is observed to be slightly small at 60 min, as shown in photo B. In the 120 min ground sample (C), the particle size is changed significantly to submicron size and they look like agglomerates. This suggests that the MC reaction proceeds remarkably in the period ranging from 60 to 120 min, consistent with the results of TG analysis. There is some morphological difference observed in SEM

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Figure 6. FT-IR spectra of the La2O3 and PVDF mixture ground for different times (min): (A) mixture, (B) 30, (C) 60, (D) 120, (E) 240, (F) 480.

Figure 5. XPS patterns of the La2O3 and PVDF mixture ground for different times.

pictures of mixtures ground for 240 min or more. Generally, if binder material exists, intensive grinding makes fine particles into composites with relatively large size. In the 240 min ground sample (D), the particles of rupturelike shape with several microns of size were observed; however, some part of the particles with submicron size remains in agglomerates. Therefore, this result implies that fine particles of LaOF and resultant organic materials, after the MC reaction, were transformed to relatively large composites with rupturelike shape, where the resultant organic materials may act as binder materials. As the grinding progresses, the size of the composites increases to several tens of microns and also they are covered with fine particles, as shown in photo (E). Thus, the particle size of the ground product becomes larger, and the shape is changed from irregular particles to rupturelike composites covered with fine particles in the prolonged grinding. 3.2. Structural Changes of PVDF after the MC Reaction. Figure 5 shows the XPS F 1s (A) and C 1s (B) spectra of the original PVDF and the mixture ground for different periods of time. As shown in Figure 5A, the peak due to the C-F bond is observed in the spectra. The peak intensity decreases with an increase in the grinding time and disappears at 240 min. This indicates that the C-F bond has been cut off and the defluorination of PVDF has been achieved. On the other hand, a new peak appears in the pattern of the mixture ground for 120 min or more and increases gradually as the grinding progresses. About a 3 eV difference between the two peaks can be seen in Figure 5A, suggesting that the fluoride-binding state has been transformed from covalent (C-F) to ionic bonds.21 This ionic bond in the fluoride-binding state is attributed to the formation of LaOF. This is well consistent with the result shown in Figure 1. As for the C 1s spectra shown in Figure 5B, the two peaks due to PVDF in the mixture imply the existence of C-F and C-C bindings at 288.5 ( 0.5 and 285 ( 0.5 eV, respectively. The peak at 288.5 ( 0.5 eV decreases rapidly as the grinding progresses up to 120 min; however, it does not change in the prolonged grinding. On the contrary, the change of peak

intensity at 285 ( 0.5 eV is imperceptible in the early stage of grinding up to 60 min. This peak intensity increases rapidly for 120 min of grinding; subsequently, it tends to increase gradually with an increase in the grinding time. Figure 6 shows the FT-IR patterns of the mixtures ground for different periods of time. The two peaks, present at 1233 (4) and 1115 (5) cm-1 in the original mixture, are due to the asymmetric and symmetric CF2 stretches, respectively. These peaks decrease as the grinding progresses and disappear in the mixtures ground for 120 min or more. This suggests that the C-F bond of PVDF in the mixture has been cut off. In addition, peaks appearing at 3744 (1) and 672 (6) cm-1 in the original mixture may be due to the free O-H stretching and La-O-H bands, suggesting that water molecules are absorbed on La2O3 to form La(OH)3. These peaks tend to decrease as the grinding progresses and disappear at 240 min or more. This implies that most of La2O3 in the mixture has reacted with PVDF to form LaOF, and this result is in accordance with those shown in Figures 1 and 2. The broad peak at around 3500 (2) cm-1 in the mixtures ground for 120 min or more may be due to the O-H stretching band of intramolecular H bonding. This means that the O-H band exists in the molecule of defluorinated PVDF. The peak at around 1600 (3) cm-1 in the mixtures ground for 120 min or more may be due to the stretch vibration of double carbon binding (CdC), and its changing trend is similar to that of the O-H stretching band of intramolecular H bonding, as exhibited by a very broad pattern with prolonged grinding for 360 min or more. These changes in the FT-IR spectra of the mixture ground for 120 min or more can be attributed to the disordering, which means the structural irregularity in the organic material. According to the XPS and FT-IR analyses, several MC reactions can be estimated in the organic materials formed during grinding, as given by the following equations expressed as a monomer unit reaction.

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The MC reaction between PVDF and La2O3 in the mixture takes place through not only the degradation of the polymer but also the substitution of F- by O2-, given by eq 3. This substitution reaction implies the defluorination of PVDF, and substituted F- is used as a fluorine source in the synthesis of LaOF. Successively, the structural change of the resultant organic material can be achieved by the grinding, as given by eq 4, where the hydrogen bond is also formed because of the interaction between neighboring oxygen and hydrogen atoms, as indicated by the dotted line. Thus, as the MC reaction proceeds, the resultant organic structure seems to be changed into an O-H bond and a double carbon binding (CdC). Because of the effective and safe separation between LaOF and the resultant organic materials, we are considering and researching with various methods such as thermal decomposition by relatively low temperature, solvent extraction by organic solvent, and so on. 4. Conclusions The following conclusions can be made based on the experimental results. (1) LaOF can be synthesized by the room temperature grinding of La2O3 and PVDF through their solid-state reaction. This reaction through the substitution of Fby O2- results in the defluorination of PVDF and the synthesis of LaOF. This MC reaction proceeds with an increase in the grinding time and is almost completed by about 240 min. (2) The size of the synthesized LaOF powder is submicron order, and the particles look like agglomerates. By prolonged grinding, agglomerated fine particles, consisting of LaOF and resultant organic materials, are transformed to the composites of rupturelike shape, and these particles are covered with residual fine particles, progressively. (3) The new bands, such as C-O-H and CdC, are formed in the resultant organic phase in the ground mixture. Literature Cited (1) Mello, C. D.; Dirksen, G. J.; Folkerts, H. F.; Meijerink, A.; Blasse, G. The vibronic spectroscopy and luminescence concentration quenching of the Pr3+ ion in La2O3, LaOF and LiYF4. J. Phys. Chem. Solids 1995, 56 (2), 267. (2) Chao, Z. S.; Zhou, X. P.; Wan, H. L.; Tsai, K. R. Methane oxidative coupling on BaF2/LaOF catalyst. Appl. Catal. A 1995, 130, 127. (3) Wan, H.; Chao, Z.; Weng, W.; Zhou, X.; Cai, J.; Tsai, K. Constituent selection and performance characterization of cata-

lysts for oxidative coupling of methane and oxidative dehydrogenation of ethane. Catal. Today 1996, 30, 67. (4) Au, C. T.; Zhang, Y. Q.; He, H.; Lai, S. Y.; Ng, C. F. The characterization of BaCO3-modified LaOF catalysts for the OCM reaction. J. Catal. 1997, 167, 354. (5) Jorma, H.; Eija, S.; Pia, Y.; Elisabeth, A. F.; Michele, L. B.; Pierre, P. Analysis and simulation of the optical spectra of the stoichiometric NdOF. J. Chem. Soc., Faraday Trans. 1998, 94 (4), 481. (6) Klemm, W. Lanthanoxyfluorid. Z. Anorg. Chem. 1941, 248, 167. (7) Zachariasen, W. H. Crystal Chemical studies of the 5f Series of Elements. 14. Oxyfluorides, XOF. Acta Crystallogr. 1951, 4, 231. (8) Achary, S. N.; Ambekar, B. R.; Mathews, M. D.; Tyagi, A. K.; Moorthy, P. N. Study of phase transition and volume thermal expansion in a rare-earth (RE) oxyfluoride system by hightemperature XRD (RE ) La, Nd, Sm, Eu and Gd). Thermoc. Act. 1998, 320, 239. (9) Mu¨ller, J. H.; Petzel, T. High-temperature X-ray diffraction study of the rhombohedral-cubic phase transition of ROF with R ) Y, La, Pr, Nd, Sm, Er. J. Alloys Compd. 1995, 224, 18. (10) Fergus, J. W. Crystal structure of lanthanum oxyfluoride. J. Mater. Sci. Lett. 1997, 16, 267. (11) Carlson, D. P.; Schmiegel, W. Ullmann’s Encyclopedia of Industrial Chemistry; Willey: New York, 1988. (12) Gangal, S. V. Encyclopedia of Polymer Science and Engineering; Wiley: New York, 1989. (13) Tukuda, T. Technology for waste plastics recycling. Shigen Shori Gijutsu 1999, 46, 187. (14) Yoshioka, T.; Fukukawa, K.; Sato, T.; Okuwaki, A. Chemical recycling of flexible PVC by oxygen oxidation in NaOH solutions at elevated temperatures. J. Appl. Polym. Sci. 1998, 70, 129. (15) Gupta, M. C.; Viswanath, S. G. Role of metal oxides in the thermal degradation of poly(vinyl chloride). Ind. Eng. Chem. Res. 1998, 37, 2707. (16) Johnson, J. Destruction of toxic materials. Nature 1994, 367, 223. (17) Hall, A. K.; Harrowfield, J. M.; Hart, R.; Mccormick, P. G. Mechanochemical reaction of DDT with calcium oxide. Environ. Sci. Technol. 1996, 30 (12), 3401. (18) Loiselle, S.; Branca, M.; Mulas, G.; Cocco, G. Selective mechanochemical dehalogenation of chlorobenzenes over calcium hydride. Environ. Sci. Technol. 1997, 31, 261. (19) Cao, G.; Doppiu, S.; Monagheddu, M.; Orru, R.; Sannia, M.; Cocco, G. Thermal and mechanochemical self-propagating degradation of chloro-organic compounds. Ind. Eng. Chem. Res. 1999, 38, 3218. (20) Cullity, B. D. Elements of X-ray diffraction; AddisonWesley Publishing Co.: Reading, MA, 1977. (21) Briggs, D.; Seah, M. P. Practical surface analysis, 2nd ed.; Wiley: New York, 1990.

Received for review January 29, 2001 Revised manuscript received July 10, 2001 Accepted July 22, 2001 IE010086K