- Email: [email protected]
Hydrothermal preparation and characterization of boehmites
January 2000
Materials Letters 42 Ž2000. 38–45 www.elsevier.comrlocatermatlet
Hydrothermal preparation and characterization of boehmites D. Mishra, S. Anand ) , R.K. Panda 1, R.P. Das Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India Received 16 March 1999; received in revised form 30 June 1999; accepted 1 July 1999
Abstract Boehmites ŽAl 2 O 3 P x H 2 O, 1 - x - 1.5. have been prepared hydrothermally from AlŽNO 3 . 3 P 9H 2 O and urea. Effect of temperature on preparation was studied in the range of 1608–2208C. No precipitation of boehmite was observed until the attainment of ; 1608C at which temperature a partly amorphous gel started precipitating. With the increase in temperature, transformation of the amorphous precipitate into crystalline boehmites took place as indicated by the X-ray diffraction ŽXRD. patterns. From the weight loss studies of the samples prepared at different temperatures and for different reaction time intervals, the value of x was estimated to vary between 1.3 and 1.5. The Fourier transform infrared ŽFTIR. spectra of samples obtained at 1808, 2008 or 2208C showed 25–40 cmy1 upward shift in the –OH stretching and bending vibrations assigned to boehmites. g-Alumina obtained by subsequent calcination of boehmite at 7258C was also characterized by XRD. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal; Boehmite; Gel-dehydration; g-Alumina
1. Introduction Boehmites are oxide–hydroxides of aluminum with differing water content and crystallite size w1,2x. Powders of boehmiterpseudoboehmite play important roles in the preparation of catalysts, coatings, alumina and alumina derived materials of desired porosity and mechanical strength w3–6x. Numerous methods of syntheses of boehmitesrpseudoboehmites have been reported involving the neutralization-aging process of aluminum salt solutions w7–9x. The initial precipitates formed during rapid neutral-
)
Corresponding author. E-mail: [email protected] Materials Science Division, Department of Chemistry, Berhampur University, Berhampur 760007, Orissa, India. 1
ization of aqueous acidic aluminum salt solutions are amorphous hydroxides containing varying water contents Žup to 5 molrAl 2 O 3 .. Depending upon the chemical environment prevailing during the aging process w10x, these products are transformed into either the crystalline hydroxides, AlŽOH. 3 , or oxide–hydroxides, AlOOH. Since boehmites are partly dehydrated aluminum hydroxides, these can also be produced from aluminum hydroxides by controlled calcination w11x, or by hydrothermal transformation at about 1758–2008C w12x. Though neutralization-aging of aluminum salt solutions at moderate temperatures below 1008C w7–9x, and hydrothermal transformation w12x of hydroxides into oxide–hydroxides are well documented, literature appears to be somewhat scarce on the high temperature neutralization of aluminum salt solutions, and the products
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 5 6 - 1
D. Mishra et al.r Materials Letters 42 (2000) 38–45 Table 1 Conditions for preparation of various samples while keeping urearAl mole ratio as 2 Sample no.
Temperature Ž8C.
System pressure Žpsi.
Steam pressure Žpsi.
Time Žh.
S1Ž0. S1Ž1. S2Ž0. S2Ž1. S3Ž0. S3Ž1. S4Ž0. S4Ž0.5. S4Ž1. S4Ž1.5. S4Ž2.
160 160 180 180 200 200 220 220 220 220 220
130 130 190 190 280 280 385 385 385 385 385
90 90 150 150 230 230 350 350 350 350 350
0 1 0 1 0 1 0 0.5 1.0 1.5 2.0
obtained thereof. The neutralizing agents usually added are sodium hydroxide w13x, sodium carbonate w14x, sodium bicarbonate w15x, ammonia w9x, etc. Several important studies have also been reported on
39
the use of urea as a precipitating agent for synthesis of various alumina precursors by many workers w16– 20x, although the formation of boehmites has not been observed in any of the above-cited reports. In the present work, an attempt is made to prepare and characterize crystalline boehemites at elevated temperatures using urea as a neutralizing agent. 2. Experimental The precipitation work was carried out in a 2-l capacity closed reactor ŽParr Model 4542. having temperature controller, agitation and sampling facilities. In all the experiments, mole ratio of urea to aluminum was kept as 2. The required amounts of aluminum nitrate, urea and distilled water were transferred to the reactor. The contents were then heated to different temperatures Ž1608, 1808, 2008 or 2208C. and maintained at that temperature for the required period. The stirring rate was kept constant
Fig. 1. Effect of preparation temperature on weight percent loss on calcination at 7258C for 1 h.
40
D. Mishra et al.r Materials Letters 42 (2000) 38–45
at 300 revolutions per minute Žrpm. and the samples were collected as and when required. The products were cooled to room temperature, filtered, washed with distilled water till they became free of NO 3 — Žtested qualitatively. w21x. The samples were dried overnight in an air oven maintained at 908–958C. Calcination experiments Žat the specified temperature for the desired duration. were carried out with such air-oven-dried samples. All the reagents and chemicals used were of BDH grade. X-ray diffraction ŽXRD. patterns of the samples were obtained with a Phillips Powder Diffractometer model PW 1710 in a range of 68–708 Ž2Q . at a scanning rate of 28rmin using Ni-filtered Cu target. The IR spectra of the samples were obtained on a Perkin Elmer Fourier transform infrared ŽFTIR. P500 spectrophotometer. Samples were pressed into
thin transparent discs with KBr Ž99.5 wt.% KBr q 0.5 wt.% sample. using a 13-mm steel die and applying a load of 10 tons pressure inside the pelletizer. The weight-loss measurements were done using platinum crucibles and weights were taken in an electronic digital balance ŽAFCOSET ER-180A. with an accuracy within q0.1 mg. Some of the measurements were repeated several times in order to ascertain the reproducibility.
3. Results and discussion 3.1. Sample preparation The synthesis of boehmites was carried out at 1608, 1808, 2008 or 2208C and samples were col-
Fig. 2. Cumulative weight percent loss of SIŽI. and S4ŽI. samples at different temperatures of calcination for 1 h.
D. Mishra et al.r Materials Letters 42 (2000) 38–45
lected at various time intervals after attainment of the appropriate temperature. Conditions for preparation of these samples along with the temperature and pressure profile are given in Table 1. Higher pressure than that can be accounted for due to steam may be because of the formation of carbon dioxide and ammonia as shown in Eqs. Ž1. – Ž3.. The excess pressure decreased with the increase of temperature indicating redissolution of the gases at higher temperature.
41
imum weight loss for both the samples was observed around 4008–4508C due to dehydroxylation w22x. From the weight loss results, the value of x in Al 2 O 3 P x H 2 O has been calculated to be in the range between 1.3 and 1.5.
2Al Ž NO 3 . 3 q 3NH 2 –CO–NH 2 q Ž x q 6 . H 2 O
™ Al O P x H O q 6NH NO q 3CO 2
3
2
4
3
Ž 1.
2
2Al Ž NO 3 . 3 q 4NH 2 –CO–NH 2 q Ž x q 7 . H 2 O
™ Al O P x H O q 6NH NO q 2NH q 4CO 2
3
2
4
3
3
2
Ž 2.
2Al Ž NO 3 . 3 q 7NH 2 –CO–NH 2 q Ž x q 14 . H 2 O
™ Al O P x H O q 6NH NO q 4Ž NH . CO 2
3
2
q 3CO 2
4
3
4 2
3
Ž 3.
The evolution of carbon dioxide and ammonia during the above mentioned reactions was confirmed by qualitative analyses w21x. 3.2. Weight loss studies Fig. 1 shows total weight loss at 7008C Žkept for 1 h. of representative samples prepared at different temperatures and time intervals we.g., S1Ž0., S2Ž0., S3Ž0., S4Ž0.; S1Ž1., S2Ž1., S3Ž1., and S4Ž1.x. It is observed that for all the 0-h samples ŽFig. 1., weight percent loss decreased with the increase in preparation temperature. This difference in weight percent loss for 1-h samples was not very significant. From these observations, it can be inferred that initially at 1608C, a poorly crystallized gel was formed and it got transformed to crystalline boehmite with a water content varying between 17% and 20% when the preparation temperature andror time increased. Fig. 2 shows cumulative weight loss at different heating temperatures for two typical samples S1Ž1. and S4Ž1.. Almost no weight loss was observed up to 2008C suggesting the absence of loosely bound water. Max-
Fig. 3. Ža. XRD pattern of S1Ž0. gel, Žb. XRD pattern of S4Ž0. sample, Žc. XRD pattern of S4Ž1. sample, and Žd. XRD pattern of S4Ž1. sample calcined at 7258C for 2.5 h.
D. Mishra et al.r Materials Letters 42 (2000) 38–45
42
3.3. XRD studies Fig. 3 shows the X-ray diffractograms of representative samples, and XRD data for typical powders are compared with those available in literature in Table 2. The XRD patterns of the partly crystallized gel S1Ž0. show broad peaks possessing relative intensities Ž IrI0 . which decrease with decreasing interplanar distance Ž d Br . Žas shown in Fig. 3a, Table 2.; these indicate less ordered and partly amorphous behavior. As can be seen from Fig. 3b and c, the XRD patterns of S4Ž0. and S4Ž1. powders show the reflections of well crystallized boehmite w9,10,12,23x. It is also observed that for the poorly crystallized gel S1Ž0. Žpseudoboehmite. and the well crystallized samples S4Ž0. and S4Ž1. Žboehmite., the three low angle reflections in the 2Q range 68–308 differ slightly in their d Br values and significantly in their IrI0 values. For the pseudoboehmite, the IrI0 of
100% is observed at 2Q ; 9.68, while the maximum IrI0 of 100% for boehmite samples is observed at 2Q ; 14.68 corresponding to the Ž020. plane was reported in literature for boehmite, Refs. w9,10xx. However, the very low angle reflections on the XRD Ž2Q ; 9.68. have to be interpreted with caution, but its very occurrence in all the XRD patterns of the boehmite sample is definite. Moreover, the interchanging of the 100% IrI0 value between the S1Ž0. gel and other boehmite samples is also a pointer to suggest that the S1Ž0. pseudoboehmite gel is only an incompletely crystallized material and that the well crystallized orthorhombic boehmite w23x results with the increase of preparation temperaturerduration. The mean crystallite diameters calculated from the Debye–Scherrer equation are found to be of nearly same size Ž4.9–7.8 nm.. However, all the samples, after calcination at 7258C got converted into galumina. A typical XRD of S4Ž1. heated at 7258C is
Table 2 Comparison of XRD data for the experimentally obtained boehmites with the literature values Žimportant reflections are listed. ?: unassigned, a: Ref. w10x, b: Ref. w12x, c: autoclaved sample of Ref. w12x, and d: from Ref. w23x ŽJCPDS file 1994.. Sample no.rave. MCD
2u
d Br Žnm.
IrI0
dh k l
Standard sample
2u
d Br Žnm.
IrI0
S1Ž0. gel 4.9 " 0.5 nm
9.6 14.61
0.92055 0.60581
100 55.1
? 020
Literature pseudoboehmite
0.31059
30.1
120
39.31 50.20 9.82 14.81
0.22901 0.18159 0.8998 0.59767
20.2 18.0 29.4 100
28.80
0.30974
51.8
39.31 50.20 9.45 14.61 28.42 39.15 50.15 37.41
0.22901 0.18159 0.93513 0.60581 0.31380 0.22991 0.18476 0.24012
57.7 63.5 20.6 100 35.3 33.7 42.4 31.7
– 0.6276 a 0.6272 b 0.3172 a 0.3171b 0.2366 a 0.1864 a – 0.6122 a 0.6156 c 0.3165a 0.3160 c 0.2346 a 0.1860 a – 0.611a 0.3164 d 0.2346 d 0.1860 d 0.2390 a
– –
28.72
– 14.1a 14.11b 28.1a 28.12 b 38.00 a 48.80 a – 14.48 a 13.37 c 28.17 a 27.88 c 38.32 a 48.91a – 14.48 d 28.20 d 38.36 d 48.96 d 37.60 a
– – – 100 a 100 c 65a 65c 55a 33 a – 100 d 65d 55d 30 d 80 a
39.40
0.22843
25.8
222
45.80
0.19872
56.7
400
67.00
0.13954
37.60 d 39.49 a 39.49 d 45.86 a 45.86 d 67.00 a 67.00 d
0.2390 d 0.2280 a 0.2280 d 0.1977 a 0.1977 d 0.1395a 0.1395d
80 d 50 a 50 d 100 a 100 d 100 a 100 d
S4Ž0. 6.8 " 0.5 nm
S4Ž1. 7.8 " 0.5 nm
S4Ž1. 7258C Ž2.5 h. 5.7 " 0.3 nm
100
140r031 051 ? 020
Literature boehmite
120 140r031 051 ? 020 120 140r031 051 311
440
Literature boehmite
Gamma Al 2 O 3
–
D. Mishra et al.r Materials Letters 42 (2000) 38–45
43
Fig. 4. FTIR of Ža. S1Ž0. gel, Žb. S4Ž0. sample, Žc. S4Ž1. sample calcined at 3008C, Žd. S4Ž1. sample calcined at 6008C, and Že. S4Ž1. sample calcined at 7258C.
44
D. Mishra et al.r Materials Letters 42 (2000) 38–45
given in Fig. 3d which agrees well with those of standard samples w10,23x of g-alumina. Not much difference in MCD of calcined sample was observed ŽTable 2.. 3.4. FTIR studies From the XRD results, it is observed that except S1Ž0. all the rest of 0-h samples showed reflections due to crystalline boehmite. The FTIR of S1Ž0. and S4Ž0. are compared in Fig. 4a and b. The spectrum corresponding to the poorly crystallized S1Ž0. gel ŽFig. 4a. exhibited prominent vibrations at ; 3500 cmy1 Žbr; unsplit n OH in hydroxyl and water., 1381 Žs, d OH . and F 1000 cmy1 Žunsplit, m, br, Al–O vibrations.. The OH vibrations in the gel appear to have shifted upward by ; 200 cmy1 . The Al–O stretching and bending vibrations are unsplit and broad when compared to the corresponding vibrations observed in S4Ž0. ŽFig. 4a and b.. In the latter samples, the nAl – O stretching vibration at 740 cmy1 and the dAl – O bending vibration at ; 400 cmy1 can be assigned to the octahedrally co-ordinated oxygens around aluminum, characteristic of boehmite w24–26x; however, the O–H stretching vibrations around 3309 and 3107 cmy1 and the corresponding bending ŽO– H. vibrations at 1166 and 1071 cmy1 are observed to have shifted upward to an extent of 25–40 cmy1 as compared to the earlier reported spectra of boehmite w10x. All the 1-h samples were identical to S4Ž0.. Small portions of Sample S4Ž1. were separately calcined to 3008, 4008, 5008 and 6008C for 1 h and the IR of these samples were taken. It was observed that the IR of 3008C sample is almost identical to the uncalcined one ŽFig. 4c. with shift in the –OH vibrations towards lower wave-number side. By further increasing the calcination temperature to 500 or 600, the individual peaks of boehmites merged to give broad –OH and Al–O vibration bands with decreasing intensity of the O–H vibrations Žtypical spectra obtained at 6008C is shown in Fig. 4d.. For the sample calcined at 7258C Žabbreviated as S4Ž1.725., the O–H vibrations almost disappeared ŽFig. 4e. suggesting complete dehydrationrdehydroxylation of boehmite and its conversion to g-Al 2 O 3 w27x. The results of the present investigations establish that it is possible to hydrothermally obtain boehmites with varying amounts of water content. The precipi-
tation of crystalline boehmite Žas indicated by the XRD results. takes place almost instantaneously at temperatures G 1808C. The poorly crystallized gel obtained at attainment of 1608C possessed higher water content, therefore, underwent more pronounced weight loss. FTIR spectra conclusively indicated the resulting splitting of the broad –OH vibration bands in gel sample obtained at 1608C into sharper peaks for the samples prepared at higher temperatures, as a result of dehydrationrdehydroxylation. Further dehydrationrdehydroxylation by calcination to 7258C transforms the boehmites into g-Al 2 O 3 as indicated by both XRD and FTIR. Acknowledgements The authors are thankful to the Director, Regional Research Laboratory for his kind permission to publish this paper. The authors are also thankful to Dalmia Institute of Scientific and Industrial Research for helping in XRD work.
References w1x B.R. Baker, R.M. Pearson, J. Catal. 33 Ž1974. 265. w2x R. Tettenhorst, D.A. Holfman, Clay Clay Miner. 28 Ž5. Ž1980. 373. w3x B.E. Yoldas, Am. Ceram. Soc. Bull. 54 Ž1975. 289. w4x D.L. Trimn, A. Stanislaus, Appl. Catal. 21 Ž1986. 215. w5x H.L. Zobowa, K. Becker, H.P. Hennig, U. Kretzschmar, M. Naock, U. Steinike, Chem. Tech. 41 Ž9. Ž1989. 387. w6x K. Strenge, U. Bollmann, Colloids Surf. 57 Ž1991. 139. w7x W.H. Gitzen, American Ceramic Society, Columbus, OH, 1970. w8x K.P. Prodromou, A.S. Povlatou-Ve, Clay Clay Miner. 43 Ž1. Ž1995. 111. w9x E. Morgado Jr., Y.L. Lam, F.L. Nazar, J. Colloid Interface Sci. 188 Ž1997. 257. w10x K. Wefers, C. Misra, Alcoa Tech. Paper 19, Alcoa Laboratories, 1987. w11x M.K.B. Day, V.J. Hill, J. Phys. Kolloid Chem. 57 Ž1953. 946. w12x M. Pyzalski, J. Iwanciw, Light Metals Ž1994. 107. w13x T. Sato, K. Sato, J. Ceram. Soc. Jpn. 104 Ž1996. 377. w14x H. Shi, M. Mao, H. Liang, Y. Kang, C. Tao, Youse Jinshu 42 Ž4. Ž1992. 53, Chinese. w15x T. Iga, Y. Mourase, J. Mater. Res. 10 Ž7. Ž1995. 1579. w16x H.H. Willard, N.K. Tang, J. Am. Chem. Soc. 59 Ž1937. 1190. w17x M.D. Sacks, T.Y. Tseng, Y.S. Lee, Am. Ceram. Soc. Bull. 63 Ž2. Ž1984. 301.
D. Mishra et al.r Materials Letters 42 (2000) 38–45 w18x J.L. Shi, J.H. Gao, Z.X. Lin, Solid State Ionics 32–33 Ž1988. 537, Pt. 1. w19x H. Nagai, S. Hokazono, A. Kato, Br. Ceram. Trans. J. 90 Ž1991. 44. w20x K.J. Kim, H.S. Young, Nonmunjip-Ch’ungnam Taehakkyo Sanop Kisul Yon’guso 7 Ž1. Ž1992. 7. w21x G. Svehla, Vogels’ Qualitative Inorganic Analysis, 6th edn., Orient Longman, 1979, pp. 146, 150, 182. w22x W.J. Smothers, I. Chiang, Differential Thermal Analysis Theory and Practice, Chemical Publishing, New York, 1958.
45
w23x Joint Committee on Powder Diffraction Standard ŽJCPDS., International Center For Diffraction Data, Swathmore, PA, Files 10-425 and 21–1307. w24x Ph. Colomban, J. Mater. Sci. 24 Ž1989. 3002. w25x D. Mazza, M. Vallino, G. Busea, J. Am. Ceram. Soc. 75 Ž1992. 1929. w26x J. Hubert, A.K. Vasudevan, A.D. Damodaran, K.G.K. Warrier, J. Solid State Chem. 131 Ž1997. 181. w27x M.D. Sacks, T.Y. Tseng, S.O. Lee, Am. Ceram. Soc. Bull. 63 Ž2. Ž1984. 301.
Materials Letters 42 Ž2000. 38–45 www.elsevier.comrlocatermatlet
Hydrothermal preparation and characterization of boehmites D. Mishra, S. Anand ) , R.K. Panda 1, R.P. Das Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India Received 16 March 1999; received in revised form 30 June 1999; accepted 1 July 1999
Abstract Boehmites ŽAl 2 O 3 P x H 2 O, 1 - x - 1.5. have been prepared hydrothermally from AlŽNO 3 . 3 P 9H 2 O and urea. Effect of temperature on preparation was studied in the range of 1608–2208C. No precipitation of boehmite was observed until the attainment of ; 1608C at which temperature a partly amorphous gel started precipitating. With the increase in temperature, transformation of the amorphous precipitate into crystalline boehmites took place as indicated by the X-ray diffraction ŽXRD. patterns. From the weight loss studies of the samples prepared at different temperatures and for different reaction time intervals, the value of x was estimated to vary between 1.3 and 1.5. The Fourier transform infrared ŽFTIR. spectra of samples obtained at 1808, 2008 or 2208C showed 25–40 cmy1 upward shift in the –OH stretching and bending vibrations assigned to boehmites. g-Alumina obtained by subsequent calcination of boehmite at 7258C was also characterized by XRD. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal; Boehmite; Gel-dehydration; g-Alumina
1. Introduction Boehmites are oxide–hydroxides of aluminum with differing water content and crystallite size w1,2x. Powders of boehmiterpseudoboehmite play important roles in the preparation of catalysts, coatings, alumina and alumina derived materials of desired porosity and mechanical strength w3–6x. Numerous methods of syntheses of boehmitesrpseudoboehmites have been reported involving the neutralization-aging process of aluminum salt solutions w7–9x. The initial precipitates formed during rapid neutral-
)
Corresponding author. E-mail: [email protected] Materials Science Division, Department of Chemistry, Berhampur University, Berhampur 760007, Orissa, India. 1
ization of aqueous acidic aluminum salt solutions are amorphous hydroxides containing varying water contents Žup to 5 molrAl 2 O 3 .. Depending upon the chemical environment prevailing during the aging process w10x, these products are transformed into either the crystalline hydroxides, AlŽOH. 3 , or oxide–hydroxides, AlOOH. Since boehmites are partly dehydrated aluminum hydroxides, these can also be produced from aluminum hydroxides by controlled calcination w11x, or by hydrothermal transformation at about 1758–2008C w12x. Though neutralization-aging of aluminum salt solutions at moderate temperatures below 1008C w7–9x, and hydrothermal transformation w12x of hydroxides into oxide–hydroxides are well documented, literature appears to be somewhat scarce on the high temperature neutralization of aluminum salt solutions, and the products
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 5 6 - 1
D. Mishra et al.r Materials Letters 42 (2000) 38–45 Table 1 Conditions for preparation of various samples while keeping urearAl mole ratio as 2 Sample no.
Temperature Ž8C.
System pressure Žpsi.
Steam pressure Žpsi.
Time Žh.
S1Ž0. S1Ž1. S2Ž0. S2Ž1. S3Ž0. S3Ž1. S4Ž0. S4Ž0.5. S4Ž1. S4Ž1.5. S4Ž2.
160 160 180 180 200 200 220 220 220 220 220
130 130 190 190 280 280 385 385 385 385 385
90 90 150 150 230 230 350 350 350 350 350
0 1 0 1 0 1 0 0.5 1.0 1.5 2.0
obtained thereof. The neutralizing agents usually added are sodium hydroxide w13x, sodium carbonate w14x, sodium bicarbonate w15x, ammonia w9x, etc. Several important studies have also been reported on
39
the use of urea as a precipitating agent for synthesis of various alumina precursors by many workers w16– 20x, although the formation of boehmites has not been observed in any of the above-cited reports. In the present work, an attempt is made to prepare and characterize crystalline boehemites at elevated temperatures using urea as a neutralizing agent. 2. Experimental The precipitation work was carried out in a 2-l capacity closed reactor ŽParr Model 4542. having temperature controller, agitation and sampling facilities. In all the experiments, mole ratio of urea to aluminum was kept as 2. The required amounts of aluminum nitrate, urea and distilled water were transferred to the reactor. The contents were then heated to different temperatures Ž1608, 1808, 2008 or 2208C. and maintained at that temperature for the required period. The stirring rate was kept constant
Fig. 1. Effect of preparation temperature on weight percent loss on calcination at 7258C for 1 h.
40
D. Mishra et al.r Materials Letters 42 (2000) 38–45
at 300 revolutions per minute Žrpm. and the samples were collected as and when required. The products were cooled to room temperature, filtered, washed with distilled water till they became free of NO 3 — Žtested qualitatively. w21x. The samples were dried overnight in an air oven maintained at 908–958C. Calcination experiments Žat the specified temperature for the desired duration. were carried out with such air-oven-dried samples. All the reagents and chemicals used were of BDH grade. X-ray diffraction ŽXRD. patterns of the samples were obtained with a Phillips Powder Diffractometer model PW 1710 in a range of 68–708 Ž2Q . at a scanning rate of 28rmin using Ni-filtered Cu target. The IR spectra of the samples were obtained on a Perkin Elmer Fourier transform infrared ŽFTIR. P500 spectrophotometer. Samples were pressed into
thin transparent discs with KBr Ž99.5 wt.% KBr q 0.5 wt.% sample. using a 13-mm steel die and applying a load of 10 tons pressure inside the pelletizer. The weight-loss measurements were done using platinum crucibles and weights were taken in an electronic digital balance ŽAFCOSET ER-180A. with an accuracy within q0.1 mg. Some of the measurements were repeated several times in order to ascertain the reproducibility.
3. Results and discussion 3.1. Sample preparation The synthesis of boehmites was carried out at 1608, 1808, 2008 or 2208C and samples were col-
Fig. 2. Cumulative weight percent loss of SIŽI. and S4ŽI. samples at different temperatures of calcination for 1 h.
D. Mishra et al.r Materials Letters 42 (2000) 38–45
lected at various time intervals after attainment of the appropriate temperature. Conditions for preparation of these samples along with the temperature and pressure profile are given in Table 1. Higher pressure than that can be accounted for due to steam may be because of the formation of carbon dioxide and ammonia as shown in Eqs. Ž1. – Ž3.. The excess pressure decreased with the increase of temperature indicating redissolution of the gases at higher temperature.
41
imum weight loss for both the samples was observed around 4008–4508C due to dehydroxylation w22x. From the weight loss results, the value of x in Al 2 O 3 P x H 2 O has been calculated to be in the range between 1.3 and 1.5.
2Al Ž NO 3 . 3 q 3NH 2 –CO–NH 2 q Ž x q 6 . H 2 O
™ Al O P x H O q 6NH NO q 3CO 2
3
2
4
3
Ž 1.
2
2Al Ž NO 3 . 3 q 4NH 2 –CO–NH 2 q Ž x q 7 . H 2 O
™ Al O P x H O q 6NH NO q 2NH q 4CO 2
3
2
4
3
3
2
Ž 2.
2Al Ž NO 3 . 3 q 7NH 2 –CO–NH 2 q Ž x q 14 . H 2 O
™ Al O P x H O q 6NH NO q 4Ž NH . CO 2
3
2
q 3CO 2
4
3
4 2
3
Ž 3.
The evolution of carbon dioxide and ammonia during the above mentioned reactions was confirmed by qualitative analyses w21x. 3.2. Weight loss studies Fig. 1 shows total weight loss at 7008C Žkept for 1 h. of representative samples prepared at different temperatures and time intervals we.g., S1Ž0., S2Ž0., S3Ž0., S4Ž0.; S1Ž1., S2Ž1., S3Ž1., and S4Ž1.x. It is observed that for all the 0-h samples ŽFig. 1., weight percent loss decreased with the increase in preparation temperature. This difference in weight percent loss for 1-h samples was not very significant. From these observations, it can be inferred that initially at 1608C, a poorly crystallized gel was formed and it got transformed to crystalline boehmite with a water content varying between 17% and 20% when the preparation temperature andror time increased. Fig. 2 shows cumulative weight loss at different heating temperatures for two typical samples S1Ž1. and S4Ž1.. Almost no weight loss was observed up to 2008C suggesting the absence of loosely bound water. Max-
Fig. 3. Ža. XRD pattern of S1Ž0. gel, Žb. XRD pattern of S4Ž0. sample, Žc. XRD pattern of S4Ž1. sample, and Žd. XRD pattern of S4Ž1. sample calcined at 7258C for 2.5 h.
D. Mishra et al.r Materials Letters 42 (2000) 38–45
42
3.3. XRD studies Fig. 3 shows the X-ray diffractograms of representative samples, and XRD data for typical powders are compared with those available in literature in Table 2. The XRD patterns of the partly crystallized gel S1Ž0. show broad peaks possessing relative intensities Ž IrI0 . which decrease with decreasing interplanar distance Ž d Br . Žas shown in Fig. 3a, Table 2.; these indicate less ordered and partly amorphous behavior. As can be seen from Fig. 3b and c, the XRD patterns of S4Ž0. and S4Ž1. powders show the reflections of well crystallized boehmite w9,10,12,23x. It is also observed that for the poorly crystallized gel S1Ž0. Žpseudoboehmite. and the well crystallized samples S4Ž0. and S4Ž1. Žboehmite., the three low angle reflections in the 2Q range 68–308 differ slightly in their d Br values and significantly in their IrI0 values. For the pseudoboehmite, the IrI0 of
100% is observed at 2Q ; 9.68, while the maximum IrI0 of 100% for boehmite samples is observed at 2Q ; 14.68 corresponding to the Ž020. plane was reported in literature for boehmite, Refs. w9,10xx. However, the very low angle reflections on the XRD Ž2Q ; 9.68. have to be interpreted with caution, but its very occurrence in all the XRD patterns of the boehmite sample is definite. Moreover, the interchanging of the 100% IrI0 value between the S1Ž0. gel and other boehmite samples is also a pointer to suggest that the S1Ž0. pseudoboehmite gel is only an incompletely crystallized material and that the well crystallized orthorhombic boehmite w23x results with the increase of preparation temperaturerduration. The mean crystallite diameters calculated from the Debye–Scherrer equation are found to be of nearly same size Ž4.9–7.8 nm.. However, all the samples, after calcination at 7258C got converted into galumina. A typical XRD of S4Ž1. heated at 7258C is
Table 2 Comparison of XRD data for the experimentally obtained boehmites with the literature values Žimportant reflections are listed. ?: unassigned, a: Ref. w10x, b: Ref. w12x, c: autoclaved sample of Ref. w12x, and d: from Ref. w23x ŽJCPDS file 1994.. Sample no.rave. MCD
2u
d Br Žnm.
IrI0
dh k l
Standard sample
2u
d Br Žnm.
IrI0
S1Ž0. gel 4.9 " 0.5 nm
9.6 14.61
0.92055 0.60581
100 55.1
? 020
Literature pseudoboehmite
0.31059
30.1
120
39.31 50.20 9.82 14.81
0.22901 0.18159 0.8998 0.59767
20.2 18.0 29.4 100
28.80
0.30974
51.8
39.31 50.20 9.45 14.61 28.42 39.15 50.15 37.41
0.22901 0.18159 0.93513 0.60581 0.31380 0.22991 0.18476 0.24012
57.7 63.5 20.6 100 35.3 33.7 42.4 31.7
– 0.6276 a 0.6272 b 0.3172 a 0.3171b 0.2366 a 0.1864 a – 0.6122 a 0.6156 c 0.3165a 0.3160 c 0.2346 a 0.1860 a – 0.611a 0.3164 d 0.2346 d 0.1860 d 0.2390 a
– –
28.72
– 14.1a 14.11b 28.1a 28.12 b 38.00 a 48.80 a – 14.48 a 13.37 c 28.17 a 27.88 c 38.32 a 48.91a – 14.48 d 28.20 d 38.36 d 48.96 d 37.60 a
– – – 100 a 100 c 65a 65c 55a 33 a – 100 d 65d 55d 30 d 80 a
39.40
0.22843
25.8
222
45.80
0.19872
56.7
400
67.00
0.13954
37.60 d 39.49 a 39.49 d 45.86 a 45.86 d 67.00 a 67.00 d
0.2390 d 0.2280 a 0.2280 d 0.1977 a 0.1977 d 0.1395a 0.1395d
80 d 50 a 50 d 100 a 100 d 100 a 100 d
S4Ž0. 6.8 " 0.5 nm
S4Ž1. 7.8 " 0.5 nm
S4Ž1. 7258C Ž2.5 h. 5.7 " 0.3 nm
100
140r031 051 ? 020
Literature boehmite
120 140r031 051 ? 020 120 140r031 051 311
440
Literature boehmite
Gamma Al 2 O 3
–
D. Mishra et al.r Materials Letters 42 (2000) 38–45
43
Fig. 4. FTIR of Ža. S1Ž0. gel, Žb. S4Ž0. sample, Žc. S4Ž1. sample calcined at 3008C, Žd. S4Ž1. sample calcined at 6008C, and Že. S4Ž1. sample calcined at 7258C.
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D. Mishra et al.r Materials Letters 42 (2000) 38–45
given in Fig. 3d which agrees well with those of standard samples w10,23x of g-alumina. Not much difference in MCD of calcined sample was observed ŽTable 2.. 3.4. FTIR studies From the XRD results, it is observed that except S1Ž0. all the rest of 0-h samples showed reflections due to crystalline boehmite. The FTIR of S1Ž0. and S4Ž0. are compared in Fig. 4a and b. The spectrum corresponding to the poorly crystallized S1Ž0. gel ŽFig. 4a. exhibited prominent vibrations at ; 3500 cmy1 Žbr; unsplit n OH in hydroxyl and water., 1381 Žs, d OH . and F 1000 cmy1 Žunsplit, m, br, Al–O vibrations.. The OH vibrations in the gel appear to have shifted upward by ; 200 cmy1 . The Al–O stretching and bending vibrations are unsplit and broad when compared to the corresponding vibrations observed in S4Ž0. ŽFig. 4a and b.. In the latter samples, the nAl – O stretching vibration at 740 cmy1 and the dAl – O bending vibration at ; 400 cmy1 can be assigned to the octahedrally co-ordinated oxygens around aluminum, characteristic of boehmite w24–26x; however, the O–H stretching vibrations around 3309 and 3107 cmy1 and the corresponding bending ŽO– H. vibrations at 1166 and 1071 cmy1 are observed to have shifted upward to an extent of 25–40 cmy1 as compared to the earlier reported spectra of boehmite w10x. All the 1-h samples were identical to S4Ž0.. Small portions of Sample S4Ž1. were separately calcined to 3008, 4008, 5008 and 6008C for 1 h and the IR of these samples were taken. It was observed that the IR of 3008C sample is almost identical to the uncalcined one ŽFig. 4c. with shift in the –OH vibrations towards lower wave-number side. By further increasing the calcination temperature to 500 or 600, the individual peaks of boehmites merged to give broad –OH and Al–O vibration bands with decreasing intensity of the O–H vibrations Žtypical spectra obtained at 6008C is shown in Fig. 4d.. For the sample calcined at 7258C Žabbreviated as S4Ž1.725., the O–H vibrations almost disappeared ŽFig. 4e. suggesting complete dehydrationrdehydroxylation of boehmite and its conversion to g-Al 2 O 3 w27x. The results of the present investigations establish that it is possible to hydrothermally obtain boehmites with varying amounts of water content. The precipi-
tation of crystalline boehmite Žas indicated by the XRD results. takes place almost instantaneously at temperatures G 1808C. The poorly crystallized gel obtained at attainment of 1608C possessed higher water content, therefore, underwent more pronounced weight loss. FTIR spectra conclusively indicated the resulting splitting of the broad –OH vibration bands in gel sample obtained at 1608C into sharper peaks for the samples prepared at higher temperatures, as a result of dehydrationrdehydroxylation. Further dehydrationrdehydroxylation by calcination to 7258C transforms the boehmites into g-Al 2 O 3 as indicated by both XRD and FTIR. Acknowledgements The authors are thankful to the Director, Regional Research Laboratory for his kind permission to publish this paper. The authors are also thankful to Dalmia Institute of Scientific and Industrial Research for helping in XRD work.
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w23x Joint Committee on Powder Diffraction Standard ŽJCPDS., International Center For Diffraction Data, Swathmore, PA, Files 10-425 and 21–1307. w24x Ph. Colomban, J. Mater. Sci. 24 Ž1989. 3002. w25x D. Mazza, M. Vallino, G. Busea, J. Am. Ceram. Soc. 75 Ž1992. 1929. w26x J. Hubert, A.K. Vasudevan, A.D. Damodaran, K.G.K. Warrier, J. Solid State Chem. 131 Ž1997. 181. w27x M.D. Sacks, T.Y. Tseng, S.O. Lee, Am. Ceram. Soc. Bull. 63 Ž2. Ž1984. 301.