Ind. Eng. Chem. Res. 1996, 35, 241-244
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Effect of Aluminum Hydroxide Precipitation Conditions on the Alumina Surface Acidity Janusz T. Trawczyn ´ ski Institute of Chemistry and Technology of Petroleum and Coal, Technical University of Wroclaw, ul. Gdan´ ska 7/9, 50-344 Wroclaw, Poland
The effect of specific ions present during the precipitation of alumina precursor (aluminum hydroxide) on resulting alumina surface acidity has been investigated. It was found that both the method of pH control and the nature of ions introduced to the slurry apparently affected the phase composition of aluminum hydroxide as well as the strength and distribution of alumina active sites measured by means of the temperature programmed desorption of ammonia (NH3 TPD). The results were interpreted in terms of processes taking place during both polycondensation of aquo ions and aging of amorphous hydrogel of aluminum hydroxide. The ions present in the medium of precipitation (particularly at the beginning of aluminum hydroxide precipitation) apparently influenced the course of the reaction. Introduction
Table 1. Parameters of Aluminum Hydroxide Precipitation and Symbols of Prepared Samples
Solid oxide catalysts or their carriers are frequently prepared by the precipitation of suitable salts from aqueous solution followed by their thermal treatment. Depending on the conditions of precipitation, resulting products exhibit substantial differences not only in magnitudes of the specific surface area and pore size distribution but also in the surface properties. The acid-base properties of metal oxide carriers can significantly affect final selectivities of heterogeneous catalysts (Tanabe, 1970). One of the most common supports in the heterogeneous catalysis is alumina, γ-Al2O3. Vast numbers of investigations have been carried out in order to understand the structure of the alumina surface and to determine the nature of the acid-base sites (Kno¨zinger and Ratnasamy, 1978). Essential differences in surface properties have been observed with aluminas that were prepared using various procedures even if the final crystallographic structure was found evidently the same (Jiratova and Beranek, 1982). The differences in surface properties can be partially explained by the diversity in procedures of preparation which can influence the degree of hydration. Also, the role of impurities incorporated into the alumina structure during the preparation procedure due to imperfect removal of ions from the precipitate during washing out (Saad et al., 1993) should be seriously considered. A detailed knowledge of the influence of individual ions would result in determination of the limits of concentration of admixtures. The surface concentration of acidic sites depends on the phase composition of prepared alumina. An amorphous alumina exhibited lower Lewis acidity than a well-crystallized one (Nortier et al., 1990). Therefore, by the intentionally controlled content of specific ions in alumina one would prepare aluminas of desired surface properties. The aim of this work was to examine the effect of ions present in the liquid phase during aluminum hydroxide (AH) precipitation on the surface properties of the obtained aluminas. Modification of the acidic properties of γ-Al2O3 induced by anions present in the liquid phase of the initial AH precipitation was investigated. 0888-5885/96/2635-0241$12.00/0
pH of reaction slurry raised with pH of precipitationa
sodium aluminate
ammonia water
Ammonium Nitrate Poured into Reaction Vessel at Beginning of Precipitation S-1 A-1 6 ()pHk) 9 ()pHk) S-2 A-2 9 (pHk ) 7) S-3 A-3 Only Water Present in Reaction Vessel at Beginning of Precipitation 9 ()pHk) O-2 O-3 9 (pHk ) 7) O-4 O-5 a
pHk, value of the pH at the end of precipitation.
Experimental Section Aluminum hydroxides were precipitated from sodium aluminate solution (27 g of Al/dm3) with nitric acid solution (25%) at a temperature of 343 K. Both solutions were simultaneously added to a reaction vessel equipped with a stirrer to maintain a constant slurry pH. The precipitation parameters and sample denotations are given in Table 1. Prior to the commencement of precipitation either 0.5 dm3 of 25% ammonium nitrate solution or 0.5 dm3 of distilled water was poured into the reaction vessel and then the pH of the solution was elevated by addition of either sodium aluminate solution or aqueous ammonia. The precipitate was subsequently aged for 1 h in the mother liquor, filtered under vacuum, washed out with hot distilled water, dried for 24 h at 383 K, and then calcined for 4 h at 773 K. Specific acidity and acidity strength distribution of obtained aluminas as a function of ammonia desorption temperature were evaluated by the temperatureprogrammed desorption of ammonia (NH3 TPD) method according to the following procedure: heating of the sample (∼2.0 g) at 823 K under helium atmosphere over 1 h; cooling under helium stream to the temperature of 453 K; adsorption of pure ammonia at 453 K for 0.5 h; purging with helium at 453 K over 1 h in order to eliminate physically adsorbed ammonia; TPD measurement (helium atmosphere, heating rate of 10 K min-1, temperature range from 453 to 823 K). The concentration of desorbed ammonia was measured by a thermal conductivity detector. For quantitative analysis, the apparatus was first calibrated with © 1996 American Chemical Society
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Figure 1. Contribution of acid centers of equal strength alumina series S and A.
Figure 2. Contribution of acid centers of equal strength alumina series O. Table 2. Properties of Prepared Aluminum Hydroxides and Related Aluminas sample symbola phase composn of AH specific acidityb (×106 mol of NH3/m2)
S-1
S-2
S-3
A-1
A-2
A-3
O-2
O-4
O-3
O-5
pb (ba) 2.7
pb (ba) 1.7
pb+ba 3.0
pb (be) 2.0
pb 1.8
pb 1.8
ba (pb) 2.4
pb+g 1.6
pb+ba 2.6
pb+ba 2.9
a pb, pseudoboehmite; be, boehmite; ba, bayerite; g, gibbsite; symbol in parenthesisscomponent is present at trace amounts. b Data concern related alumina.
the empty reactor. The relative experimental error of the NH3 TPD measurements was evaluated to be less than 3%. Results of NH3 TPD experiments were worked out according to the methodology described elsewhere (Berteau and Delmon, 1989) and are presented in Figures 1 and 2. X-ray diffraction spectra were recorded using Dron 1.5 X-ray diffractometer. Results and Discussion It was found that the phase composition of the prepared aluminum hydroxides as well as the specific
acidity of corresponding aluminas had changed with the conditions of precipitation (Table 2). The applied solution of ammonium nitrate was acidic. Therefore, the samples, S-1, S-2, and S-3, were initially precipitated at a pH below 7. All these samples contained pseudoboehmite (boehmite) and bayerite. On the other hand, samples A-2 and A-3 precipitated in the solution containing ammonium hydroxide at pH above 7 did not contain bayerite, nor did sample A-1, which was precipitated at pH 6. If there was no ammonium nitrate at the beginning of precipitation (samples “O”), prepared
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AH would always contain bayerite (gibbsite). Samples prepared in the medium containing ammonium hydroxide (O-3, O-5) contained more pseudoboehmite and less bayerite than samples O-2 and O-4, which were precipitated from the solution that did not contain ammonium hydroxide at the beginning of precipitation. It is known that bayerite is preferentially formed during precipitation at higher pH (Lippens, 1970; Vishnyakova, 1970). Therefore, it can be concluded that ammonium hydroxide evidently stabilizes the pH at which AH is precipitated (especially its initial quantities) and in this way inhibits further formation of bayerite. The effect of addition of salts on the resulting AH phase composition was observed by Hsu (1967), who reported that the type of salt and its concentration have a critical effect on formation of bayerite versus pseudoboehmite. It is well-known that pseudoboehmite forms from a highly hydrated gel of Al3+ either by dehydrationcondensation and polymerization reactions (Krivoruchko et al., 1978) or by oriented growth of needle-shaped particles (Buyanov et al., 1976). Bayerite is formed at higher pH values (above 9). The ionic species existing in partially hydrolyzed solutions of aluminum salts have been extensively studied (Smith, 1971). Most of experimental data were obtained from pH measurements during the titration of diluted solutions of aluminum salt with basic solutions. In my opinion, the most reasonable clarification of the presented results is hydrolytic polymerization of aluminum ions in a series of polycondensation reactions of aquo ions of Al(III) leading to polynuclear hydroxy complexes (PNH): Al(H2O)63+, Al2(H2O)84+, Al13O4(OH)24(H2O)127+ (Krivoruchko et al., 1978). In the course of subsequent processes, the PNH complexes produce particles of amorphous hydrogel of aluminum hydroxide which consist of two different phases. The contribution of various phases results from the content and the structure of PNH and it is already defined at the polycondensation stage of aquo ions Al(III). I presume that this contribution is apparently influenced by the applied circumstances and parameters of precipitation. Both the structure and properties of aquo ions as well as the properties of solution (pH, composition, concentration) affect the polycondensation reaction path. It can be concluded that the pH of the solution and salts additionally introduced to the suspension influence the formation of PNH. There is close “genetic” relationship between the properties of PNH complexes and the phase composition of the resulting aluminum hydroxide (Krivoruchko et al., 1978). Therefore, it can be also concluded that the composition of the liquid phase during the precipitation evidently influences the resulting properties of aluminum hydroxide at the stage of the polycondensation of aquo ions. I additionally presume that the composition of the solution during precipitation also affects the phase composition of precipitated aluminum hydroxide at the stage of the precipitate aging. The aging of precipitated hydrogel in the mother liquor leads to a decrease in the content of the amorphous phase, water, and basic salts in hydrogel and to an increase in its specific surface area. The chemical composition of the amorphous hydrogel of aluminum hydroxide (precipitated in the medium containing nitrate ions) can be expressed by the following equilibrium reaction (Dzisko, 1979):
[Al4O2(OH)7(NO3)]n + nx[OH-] a [Al4O2(OH)7+x(NO3)1-x]n + nx[NO3-] (1) Dzisko found a large content of nitrate ions in the
freshly precipitated aluminum hydroxides. As long as ratio of NO3/Al g 0.15 and x < 0.7, the complex was stable. Whereas, when these requirements were not fulfilled, the complex was observed to lose its stability and amorphous precipitate initiated to crystallize (Dzisko, 1979) +(OH-)-(NO3)
[Al4O2(OH)7+x(NO3)1-x]am {\} -H2O
[Al4O2(OH)8]am 98 [Al4O3(OH)6]cryst (2) As seen from eqs 1 and 2, both reactions are influenced by the composition of the liquid phase during precipitation. Consequently, it can be concluded that ions introduced to the aqueous medium in the course of precipitation affect the process of aging-crystallization and change the phase composition of the resulting aluminum hydroxide. The specific acidity of the prepared aluminas depends on the pH of precipitation of the corresponding AH and on the composition of the medium of precipitation (Table 2). The specific acidity of alumina series “S” (pH at the beginning of the precipitation was raised using sodium aluminate) strongly diminishes with the increase of the pH of precipitation. Debasement of the pH at the end of precipitation (S-3) generated an increase in specific acidity. While pH was elevated with ammonium hydroxide (samples A-1, A-2, and A-3) alumina’s specific acidity was less sensitive to the pH of precipitation. A similar relationship between specific acidity and pH of precipitation was observed for samples Osstrong effect of the pH on the specific acidity of samples precipitated after raising pH using sodium aluminate (O-2, O-4) and less distinct effect for samples O-3 and O-5 for which pH was elevated with ammonia. Therefore, it can be concluded that ions present in the aqueous medium during the AH precipitation influence the concentration of acidic centers of the resulting aluminas. The initial composition of the aqueous medium in the course of AH precipitation additionally affected the acid strength distribution of aluminas. Aluminas of series S (Figure 1) exhibited a higher contribution of strong acid centers (desorption temperatures Td ) 723-823 K) and a smaller contribution of weak and medium acid centers (Td ) 523-623 K) than aluminas of series “A”. Alumina S-3 (pHk ) 7) exhibited very low contribution of the weakest acid centers (Td ) 453-523 K). Contributions of acid centers of equal strength for aluminas obtained from AH precipitated in the solution containing no ammonium nitrate (Figure 2) were more complexsonly rough correlations exist for samples prepared at the same pH condition (pairs O-2-O-3 and O-4-O-5). It was observed that decrease of pH at the end of precipitation of precursors of aluminas resulted in a lower contribution of medium acid centers (Td ) 573-723 K) and in a higher contribution of weak acid (Td ) 453-573 K) as well as strong acid centers (Td ) 773-823 K) than was observed for aluminas O-2 and O-3 (pH of precipitation equals pHk). The effect of the method of raising the pH at the beginning of precipitation on the contribution of acid centers of equal strength is less noticeable. It seems that the absence of ammonium nitrate results in the distribution of acid centers of equal strength aluminas series O that is less sensitive to the method of pH raising than aluminas prepared from precursors series A and S with ammonium nitrate present at the beginning of the precipitation. It can be concluded that ions introduced to the solution during the AH precipitation affect the distribu-
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tion of acid center strength on the surface of corresponding alumina. On the basis of the presented results, it can be stated that the method of pH control prior to the commencement of precipitation and the nature of ions introduced to the aqueous medium during precipitation as well as the phase composition of AH influence the concentration of acidic centers on the surface of related alumina. Kno¨zinger proposed a model that provides five different types of hydroxyl groups on the alumina surface (Kno¨zinger and Ratnasamy, 1978). Configuration and net charge of a particular hydroxyl result from the coordination of Al3+ cations to which the hydroxyl group is being coordinated. It is also suggested that there is a relationship between the type of aluminum coordination and Lewis acidity of alumina (Chen at al., 1992). The presented results suggest that the relationship between coordination of Al3+ cations and phase composition of aluminum hydroxide is influenced by the composition of the aqueous medium during the precipitation and circumstances of precipitation as well. On the basis of the presented results, one can conclude that ions present at the early stages of precipitation of AH affect the processes proceeding in a highly hydrated gel of Al3+ as is expressed by eqs 1 and 2 and in this way change the phase composition of the precipitate as well as the acidity and acidity strength distribution of aluminas prepared from these AH. Dehydration of some of the Al(OH)2+ ions to AlO+ by NH4+ and NO3- ions followed by the process of polymerization cannot be excluded (Hsu, 1967). The second statement resulting from the presented investigation is that the way the pH is increased before AH precipitation affects the surface properties of the corresponding aluminas. This effect can be explained on the basis of the relationship between the phase composition of aluminum hydroxide (affected by means of pH arising) and acidity of the corresponding alumina. A different phase composition implies a different configuration of OH groups coordinated with Al3+ cations at the alumina surface, which results in different concentration and strength of the acidic centers on the alumina surface.
Buyanov, R. A.; Krivoruchko, O. P. Rasrabotka tieorii cristallisatsii malorastvorimych gidrookisiey mietallov i nautchnich osnov prigotovleniya catalisatorov is viestchestv etovo classa. Kinietika i Kataliz, 1976, 17, 765-774. Chen, F. R.; Davis, J. G.; Fripiat, J. J. Aluminium Coordination and Lewis Acidity in Transition Aluminas. J. Catal. 1992, 133, 263-278. Dzisko, V. A. Osnovi polucheniya activnoy okisi alumina osashdieniyem is rostvorov. Kinietika i Kataliz 1979, 20, 1526-1532. Hsu, P. H. Effect of salts on the formation of bayerite versus pseudo-boehmite. Soil Sci. 1967, 103 (2), 101-110. Jiratova, K.; Beranek, L. Properties of Modified Aluminas. Appl. Catal. 1982, 2, 125-138. Kno¨zinger, H.; Ratnasamy, P. Catalytic Aluminas: Surface Models and Characterization of Surface Sites. Catal. Rev.-Sci. Eng. 1978, 17, 31-70. Krivoruchko, O. P.; Buyanov R. A.; Fedotov M. A.; Plyasova L. A. O mechanismie formirovaniya bayerita i psevdobemita. Zh. Neorg. Khim. 1978, 23, 1798-1803. Lippens, B. C. Physical and Chemical Aspects of Adsorbents and Catalysts; Linsen, B. G., Ed.; Academic Press: London, 1970; p 171. Nortier, P.; Fourre, P.; Saad, A. B. M.; Saur, O.; Lavalley, J. C. Effects of Crystallinity and Morphology on the Surface Properties of Alumina. Appl. Catal. 1990, 61, 141-160. Saad, A. B. M.; Ivanov, V. A.; Lavalley, J. C.; Nortier, P.; Luck, F. Comparative study of the effects of sodium impurity and amorphisation on the Lewis acidity of γ-alumina. Appl. Catal. 1993, 94, 71-83. Smith, R. In Nonequilibrium System in Natural Water Chemistry; Hem, J. D., Ed.; Advances in Chemistry Series 106; American Chemical Society: Washington, DC, 1971; p 250. Tanabe, K. Solid Acids and Bases and their Catalytic Properties; Academic Press: New York, 1970; pp 103-158. Vishnyakova G. P.; Dzisko V. A.; et al. Vliyaniye usloviy polucheniya na udielnuyu povierhnost catalisatorov i nositieliey. Kinietika i Kataliz 1970, 11, 1545-1551.
Received for review November 29, 1994 Revised manuscript received June 23, 1995 Accepted July 6, 1995X IE940703N
Literature Cited Berteau, P.; Delmon, B. Modified Aluminas: Relationship Between Activity in 1-Butanol Dehydration and Acidity Measured by NH3 TPD. Catal. Today 1989, 5, 121-137.
X Abstract published in Advance ACS Abstracts, September 15, 1995.
