hydrothermal preparation boehmite

7/29/2019 Hydrothermal Preparation Boehmite

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January 2000

.Materials Letters 42 2000 3845

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 Al O Px H O, 1-x-1.5 have been prepared hydrothermally from Al NO P9H O and urea. Effect of2 3 2 3 3 2temperature on preparation was studied in the range of 160 82208C. 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 .XRD 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 FTIR spectra of

samples obtained at 1808, 2008 or 2208C showed 2540 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.

q2000 Elsevier Science B.V. All rights reserved.

Keywords: Hydrothermal; Boehmite; Gel-dehydration; g-Alumina

1. Introduction

Boehmites are oxidehydroxides of aluminumw xwith differing water content and crystallite size 1,2 .

Powders of boehmiterpseudoboehmite play impor-

tant roles in the preparation of catalysts, coatings,

alumina and alumina derived materials of desiredw xporosity and mechanical strength 3 6 . Numerous

methods of syntheses of boehmitesrpseudoboeh-

mites have been reported involving the neutraliza-w xtion-aging process of aluminum salt solutions 7 9 .

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.

ization of aqueous acidic aluminum salt solutions are

amorphous hydroxides containing varying water con- .tents up to 5 molrAl O . Depending upon the2 3

chemical environment prevailing during the agingw xprocess 10 , these products are transformed into

.either the crystalline hydroxides, Al OH , or3oxidehydroxides, AlOOH. Since boehmites are

partly dehydrated aluminum hydroxides, these can

also be produced from aluminum hydroxides by

w xcontrolled calcination 11 , or by hydrothermal trans-w xformation at about 17582008C 12 . Though neutral-

ization-aging of aluminum salt solutions at moderatew xtemperatures below 1008C 79 , and hydrothermal

w xtransformation 12 of hydroxides into oxidehy-

droxides are well documented, literature appears to

be somewhat scarce on the high temperature neutral-

ization of aluminum salt solutions, and the products

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. .P I I : S 0 1 6 7 - 5 7 7 X 9 9 0 0 1 5 6 - 1


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( )D. Mishra et al.rMaterials Letters 42 2000 38 45 39

Table 1

Conditions for preparation of various samples while keeping

urearAl mole ratio as 2

Sample Temperature System Steam Time . . . .no. 8C pressure psi pressure psi h

.S1 0 160 130 90 0 .S1 1 160 130 90 1

.S2 0 180 190 150 0 .S2 1 180 190 150 1 .S3 0 200 280 230 0 .S3 1 200 280 230 1 .S4 0 220 385 350 0 .S4 0.5 220 385 350 0.5 .S4 1 220 385 350 1.0 .S4 1.5 220 385 350 1.5 .S4 2 220 385 350 2.0

obtained thereof. The neutralizing agents usually

w xadded are sodium hydroxide 13 , sodium carbonatew x w x w x14 , sodium bicarbonate 15 , ammonia 9 , etc.

Several important studies have also been reported on

the use of urea as a precipitating agent for synthesiswof various alumina precursors by many workers 16

x20 , 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 tem-

peratures using urea as a neutralizing agent.

2. Experimental

The precipitation work was carried out in a 2-l .capacity closed reactor Parr Model 4542 having

temperature controller, agitation and sampling facili-

ties. 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 1608, 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.


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( )D. Mishra et al.rMaterials Letters 42 2000 38 4540

.at 300 revolutions per minute rpm 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 . w xtested qualitatively 21 . The samples were dried

overnight in an air oven maintained at 908958C.Calcination experiments at the specified temperature

.for the desired duration were carried out with such

air-oven-dried samples. All the reagents and chemi-

cals used were of BDH grade. .X-ray diffraction XRD patterns of the samples

were obtained with a Phillips Powder Diffractometer .model PW 1710 in a range of 68708 2Q 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 FTIR P-

500 spectrophotometer. Samples were pressed into

thin transparent discs with KBr 99.5 wt.% KBrq 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 AFCOSET ER-180A with an accu-

racy 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 SI I and S4 I samples at different temperatures of calcination for 1 h.


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lected at various time intervals after attainment of

the appropriate temperature. Conditions for prepara-

tion 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. 1 3 . The excess

pressure decreased with the increase of temperature

indicating redissolution of the gases at higher tem-

perature.

2Al NO q 3NH CONH q x q 6 H O . .3 2 2 23

Al O PxH O q 6NH NO q 3CO 1 .2 3 2 4 3 2

2Al NO q 4NH CONH q x q 7 H O . .3 2 2 23

Al O PxH O q 6NH NO q 2NH q 4CO2 3 2 4 3 3 22 .

2Al NO q 7NH CONH q x q 1 4 H O . .3 2 2 23

Al O PxH O q 6NH NO q 4 NH CO .2 3 2 4 3 4 32

q 3CO 3 .2

The evolution of carbon dioxide and ammonia dur-

ing the above mentioned reactions was confirmed byw xqualitative analyses 21 .

3.2. Weight loss studies

Fig. 1 shows total weight loss at 7008C kept for 1.h of representative samples prepared at different

w . .temperatures and time intervals e.g., S1 0 , S2 0 , . . . . . .xS3 0 , S4 0 ; S1 1 , S2 1 , S3 1 , and S4 1 . It is

.observed that for all the 0-h samples Fig. 1 , weight

percent loss decreased with the increase in prepara-

tion 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 S1 1 and S4 1 .

Almost no weight loss was observed up to 2008C

suggesting the absence of loosely bound water. Max-

imum weight loss for both the samples was observedw xaround 40084508C due to dehydroxylation 22 .

From the weight loss results, the value of x in

Al O Px H O has been calculated to be in the range2 3 2between 1.3 and 1.5.

. . . .Fig. 3. a XRD pattern of S1 0 gel, b XRD pattern of S4 0 . . .sample, c XRD pattern of S4 1 sample, and d XRD pattern of

.S4 1 sample calcined at 7258C for 2.5 h.


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3.3. XRD studies

Fig. 3 shows the X-ray diffractograms of repre-

sentative 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 S1 0 show broad peaks possessing relative in-

.tensities IrI which decrease with decreasing in-0 . terplanar distance d as shown in Fig. 3a, TableBr

.2 ; these indicate less ordered and partly amorphous

behavior. As can be seen from Fig. 3b and c, the . .XRD patterns of S4 0 and S4 1 powders show the

w xreflections of well crystallized boehmite 9,10,12,23 .

It is also observed that for the poorly crystallized gel . .S1 0 pseudoboehmite and the well crystallized

. . .samples S4 0 and S4 1 boehmite , the three low

angle reflections in the 2Q range 68308 differ

slightly in their d values and significantly in theirBr

IrI values. For the pseudoboehmite, the IrI of0 0

100% is observed at 2Q;9.68, while the maximum

IrI of 100% for boehmite samples is observed at0 . w2Q;14.68 corresponding to the 020 plane as

w xxreported in literature for boehmite, Refs. 9,10 .

However, the very low angle reflections on the XRD .2Q;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 inter- .changing of the 100% IrI value between the S1 00

gel and other boehmite samples is also a pointer to .suggest that the S1 0 pseudoboehmite gel is only an

incompletely crystallized material and that the wellw xcrystallized orthorhombic boehmite 23 results with

the increase of preparation temperaturerduration.

The mean crystallite diameters calculated from the

DebyeScherrer equation are found to be of nearly .same size 4.97.8 nm . However, all the samples,

after calcination at 7258C got converted into g-

.alumina. A typical XRD of S4 1 heated at 7258C is

Table 2 .Comparison of XRD data for the experimentally obtained boehmites with the literature values important reflections are listed

w x w x w x w x .?: unassigned, a: Ref. 10 , b: Ref. 12 , c: autoclaved sample of Ref. 12 , and d: from Ref. 23 JCPDS file 1994 .

. .Sample no.rave. MCD 2u d nm IrI d Standard sample 2u d nm IrIBr 0 h k l Br 0

.S1 0 gel 4.9"0.5 nm 9.6 0.92055 100 ? Literature pseudoboehmite a a14.61 0.60581 55.1 020 14.1 0.6276

b b14.11 0.6272a a28.72 0.31059 30.1 120 28.1 0.3172

b b28.12 0.3171a a39.31 0.22901 20.2 140r031 38.00 0.2366 a a50.20 0.18159 18.0 051 48.80 0.1864

.S4 0 6.8"0.5 nm 9.82 0.8998 29.4 ? Literature boehmite a a a14.81 0.59767 100 020 14.48 0.6122 100c c c13.37 0.6156 100a a a28.80 0.30974 51.8 120 28.17 0.3165 65c c c27.88 0.3160 65a a a39.31 0.22901 57.7 140r031 38.32 0.2346 55a a a50.20 0.18159 63.5 051 48.91 0.1860 33

.S4 1 7.8"0.5 nm 9.45 0.93513 20.6 ? Literature boehmite d a d14.61 0.60581 100 020 14.48 0.611 100d d d28.42 0.31380 35.3 120 28.20 0.3164 65

39.15 0.22991 33.7 140r031 38.36d 0.2346 d 55d

d d d50.15 0.18476 42.4 051 48.96 0.1860 30a a a . .S4 1 7258C 2.5 h 37.41 0.24012 31.7 311 Gamma Al O 37.60 0.2390 802 3

5.7"0.3 nmd d d37.60 0.2390 80a a a39.40 0.22843 25.8 222 39.49 0.2280 50d d d39.49 0.2280 50a a a45.80 0.19872 56.7 400 45.86 0.1977 100d d d45.86 0.1977 100a a a67.00 0.13954 100 440 67.00 0.1395 100d d d67.00 0.1395 100


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. . . . . . . . . .Fig. 4. FTIR of a S1 0 gel, b S4 0 sample, c S4 1 sample calcined at 3008C, d S4 1 sample calcined at 6008C, and e S4 1

sample calcined at 7258C.


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given in Fig. 3d which agrees well with those ofw xstandard samples 10,23 of g-alumina. Not much

difference in MCD of calcined sample was observed .Table 2 .

3.4. FTIR studies

From the XRD results, it is observed that except .S1 0 all the rest of 0-h samples showed reflections

.due to crystalline boehmite. The FTIR of S1 0 and .S4 0 are compared in Fig. 4a and b. The spectrum

.corresponding to the poorly crystallized S1 0 gel .Fig. 4a exhibited prominent vibrations at ;3500

y1 .cm br; unsplit n in hydroxyl and water , 1381OH . y1 s, d and F 1000 cm unsplit, m, br, AlOOH

.vibrations . The OH vibrations in the gel appear to

have shifted upward by ;200 cmy1. The AlO

stretching and bending vibrations are unsplit and

broad when compared to the corresponding vibra- . .tions observed in S4 0 Fig. 4a and b . In the latter

samples, the n stretching vibration at 740 cmy1A l Oand the d bending vibration at ;400 cmy1 canAl Obe assigned to the octahedrally co-ordinated oxygens

w xaround aluminum, characteristic of boehmite 2426 ;

however, the OH stretching vibrations around 3309y1 and 3107 cm and the corresponding bending O

. y1H vibrations at 1166 and 1071 cm are observed

to have shifted upward to an extent of 2540 cm y1

as compared to the earlier reported spectra of

w xboehmite 10 . All the 1-h samples were identical to . .S4 0 . Small portions of Sample S4 1 were sepa-

rately 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 Fig. 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 AlO vibration bands withdecreasing intensity of the OH vibrations typical

.spectra obtained at 6008C is shown in Fig. 4d . For .the sample calcined at 7258C abbreviated as S4 1 -

. 725 , the OH vibrations almost disappeared Fig..4e suggesting complete dehydrationrdehydroxyla-

w xtion of boehmite and its conversion to g-Al O 27 .2 3The 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 as 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 pro-

nounced weight loss. FTIR spectra conclusively indi-

cated the resulting splitting of the broad OH vibra-

tion bands in gel sample obtained at 1608C into

sharper peaks for the samples prepared at higher

temperatures, as a result of dehydrationrdehydrox-

ylation. Further dehydrationrdehydroxylation by

calcination to 7258C transforms the boehmites into

g-Al O as indicated by both XRD and FTIR.2 3

Acknowledgements

The authors are thankful to the Director, Regional

Research Laboratory for his kind permission to pub-lish this paper. The authors are also thankful to

Dalmia Institute of Scientific and Industrial Research

for helping in XRD work.

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hydrothermal preparation boehmite

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