Article pubs.acs.org/IECR
Alumina-Precursor Nanoparticles Prepared by Partial Hydrolysis of AlCl3 Vapor in Tubular Flow Reactor: Effect of Hydrolysis Conditions on Particle Size Distribution Hoey Kyung Park, Kyun Young Park,* and Kyeong Youl Jung Department Chemical Engineering, Kongju National University, 275 Budae-Dong, Cheonan, Chungnam 330-717, Korea S Supporting Information *
ABSTRACT: Aluminum chloride (AlCl3) was evaporated and partially hydrolyzed in water vapor at 300−700 °C in a tubular reactor, 2.4 cm in diameter and 50 cm in length, to form alumina-precursor particles that were more spherical and less agglomerated compared with the fumed alumina produced by flame hydrolysis and oxidation. This study was focused on the effects of the H2O to AlCl3 molar ratio, the reactor temperature, the AlCl3 concentration, and the contact point of AlCl3 with H2O vapors on the morphology, size, and chemical composition of the obtained particles. The primary particle size ranged from 50 to 200 nm depending upon the operating conditions. The particle size increased with increasing AlCl3 concentration but decreased with increasing reactor temperature or with increasing molar ratio of H2O to AlCl3. The particle size became smaller and the particle size distribution narrower as the contact point of AlCl3 with H2O vapors was moved from the inlet of the reactor to a point 10 cm inward toward the center of the reactor. The primary particle sizes were simulated with a discrete-sectional model under different operating conditions. The as-produced particles contained chlorine because of the incomplete hydrolysis; the atomic ratio of Cl to Al in the resulting particles was measured to be 0.21−0.47 or 10−15% of the chlorine in the AlCl3 charged for the hydrolysis. The particles prepared at the reactor temperature of 500 °C were calcined at 1200 °C for 1 h. The calcined particles were chlorine free, α in crystalline phase, 98 nm in surface-area equivalent diameter, and 0.81 g/cm3 in bulk density.
1. INTRODUCTION Alumina (Al2O3) has been widely used for a variety of applications as abrasive, catalyst, coating additives, bioceramics, and advanced ceramics.1 A typical precursor of alumina is Al(OH)3, which is produced by the Bayer process through dissolution of bauxite in concentrated sodium hydroxide and subsequent precipitation from the sodium aluminate solution with gibbsite as a seed.2 The calcination of the Al(OH)3 led to alumina in which a significant amount of sodium was included as an impurity. Sol−gel processing was used to obtain alumina of higher purity and controlled particle size distribution.3−5 In the sol−gel method, aluminum salts or alkoxides are hydrolyzed to produce sol-type precursors which are grown to particles by condensation. The particles are separated, dried, and calcined to produce alumina. The Bayer and sol−gel processes involve a series of timeconsuming steps including filtration, washing, drying, and calcination. In contrast, dry alumina particles could be obtained in one step by using either spray pyrolysis or flame oxidation. In the spray pyrolysis, aluminum salts or alkoxides are sprayed to form liquid droplets that are injected into a flame6,7 or passed through an electric furnace8,9 to produce a mixture of amorphous and γ alumina particles. The spray pyrolysis consumes a large amount of energy to evaporate the solvents in the droplets. In the flame oxidation, aluminum salts or alkoxides are evaporated and injected as vapor into the flame. A flame oxidation of AlCl3 vapor at temperatures as high as 1600−1800 °C has been commercialized to produce fumed alumina through hydrolysis and oxidation. The fumed alumina is aggregated and exhibits a chainlike structure due to sintering © 2014 American Chemical Society
between neighboring particles. The crystalline structure is a mixture of amorphous and γ phases. The fumed alumina has been used as coating and for insulator additives but rarely used for ceramics because the bulk density may be too low for compaction and densification of the alumina.10 A different precursor, AlOxCly(OH)z, for alumina was produced by partial hydrolysis of AlCl3 vapor in a batch reactor at 200 °C for 1 min.11 The precursor was more spherical and less agglomerated than the fumed alumina because of the lower hydrolysis temperature. The calcination of the precursor led to α-alumina particles that are higher in bulk density than the fumed alumina and may be suitable for ceramic applications. In the present study, the partial hydrolysis of AlCl3 was carried out in a tubular flow reactor to examine the possibility of continuous production for industrial application. Not only geometry of the reactor was changed from a batch type to a continuous flow type, but the operating conditions were diversified to study the effects of the H2O to AlCl3 molar ratio, reactor temperature, AlCl3 concentration, and the contact point of the AlCl3 with H2O vapors on the morphology, size, and chemical composition of the obtained particles. Furthermore, the growth of the primary particles based on basic aerosol dynamics is discussed using a discrete-sectional model. The precursor particles obtained under selected conditions were calcined to α-alumina. The particle morphology, size, bulk Received: Revised: Accepted: Published: 10372
April 4, 2014 May 30, 2014 June 3, 2014 June 3, 2014 dx.doi.org/10.1021/ie501400c | Ind. Eng. Chem. Res. 2014, 53, 10372−10379
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Figure 1. Schematic drawing of the experimental apparatus.
density, crystalline structure, and BET surface area of the αalumina were measured and compared with those of a fumed alumina and a sol−gel derived alumina.
particle when the supersaturation reaches a level high enough for nucleation. Under the temperature conditions in this study, the particles exiting the reactor may not be pure Al2O3 but an intermediate on the transition toward alumina, which can be represented by AlOxCly(OH)z.
2. PARTIAL HYDROLYSIS OF ALCL3 VAPOR Aluminum chloride vapor is known to exist as a dimer, Al2Cl6 at temperatures lower than 200 °C, and as a monomer, AlCl3 at temperatures higher than 750 °C.12 In the temperature range of the present study, 300−700 °C, the AlCl3 is assumed to be present as a mixture of AlCl3 and Al2Cl6. By analogy to the mechanism reported for the hydrolysis of SiCl4 vapor,13 we assume that the gas-phase hydrolysis of the aluminum chlorides proceeds as follows: Hydrolysis of AlCl3 AlCl3 + H 2O → AlCl 2(OH) + HCl
(1)
AlCl 2(OH) + H 2O → AlCl(OH)2 + HCl
(2)
AlCl(OH)2 + H 2O → Al(OH)3 + HCl
(3)
3. EXPERIMENTAL SECTION Figure 1 shows a schematic drawing of the experimental apparatus, which consists of a precursor evaporator, a tubular reactor, a filter, and a NaOH scrubber. AlCl3 (Aldrich, 99.99%) was charged to a hopper over the evaporator. The AlCl3, which is a solid at ambient condition, was then loaded in the evaporation boat using a rotary valve. The mass of the AlCl3laden boat was measured in situ with a load cell (Minebea Co., model Ul-20GR) for the control of the evaporation rate of AlCl3. Evaporated AlCl3 was transferred to the tubular reactor with a flow of nitrogen. The transport line was made of three concentric tubes. The AlCl3 vapor flowed through the center tube. Nitrogen saturated with water vapor flowed through the outer tube, and sheathing nitrogen flowed through the tube in between. The saturation with water vapor was carried out by bubbling the nitrogen through a water bottle immersed in a water bath. The concentration of the water vapor was controlled by manipulating the water bath temperature. The saturation was confirmed by measuring the difference in mass of the water bottle before and after bubbling. At the reactor exit, the produced particles were collected in the filter. A 500 mL flask containing 1 M NaOH solution was installed after the filter for removal of the HCl gas produced from the hydrolysis. The morphology of the particles was examined by scanning electron microscopy (SEM) (JEOL, JSM-6700), and the crystalline structure was confirmed by X-ray diffraction with Cu Kα radiation (Scinco, SMD 3000). The chemical composition of the particles was determined by energy dispersive X-ray analysis (EDX). The number-average particle diameter was determined with an image analyzer (Media Cybernetics, Image-Pro Plus 4.0) by measurement of the diameters of about 300 particles. The surface area of the particles was measured by nitrogen adsorption using a Micromeritics ASAP 2010 instrument. The bulk density of the particles was measured by tapping a graduated cylinder containing the powder until little further volume change was observed.
Hydrolysis of Al2Cl6 Al 2Cl 6 + H 2O → Al 2Cl5(OH) + HCl
(4)
Al 2Cl5(OH) + H 2O → Al 2Cl4(OH)2 + HCl
(5)
Al 2Cl4(OH)2 + H 2O → Al 2Cl3(OH)3 + HCl
(6)
Al 2Cl3(OH)3 + H 2O → Al 2Cl 2(OH)4 + HCl
(7)
Al 2Cl 2(OH)4 + H 2O → Al 2Cl(OH)5 + HCl
(8)
Al 2Cl(OH)5 + H 2O → Al 2(OH)6 + HCl
(9)
The chlorines in AlCl3 or Al2Cl6 are replaced with OH successively by hydrolysis to form aluminum chloride hydroxides. The resulting hydroxides are polymerized by condensation. A condensation reaction can be represented by Al 2Cl 2(OH)4 + Al 2Cl 2(OH)4 → Al 2Cl 2(OH)3 −O−Al 2Cl 2(OH)3 + H 2O
(10)
By successive condensation, the product molecules grow with their vapor pressure decreasing, which ultimately leads to a 10373
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4. RESULTS AND DISCUSSION 4.1. Extent of Hydrolysis. The hydrolysis of AlCl3 vapor may not be complete but only partial under the temperature conditions in this study. The atomic ratios of Cl to Al in the particles produced under various hydrolysis conditions were measured by energy dispersive X-ray (EDX) and are shown in Table 1. The ratio of Cl to Al decreased by either increasing the
further increased to 15, the particles became considerably agglomerated and irregular in shape. The average primary particle size was 215 nm at the molar ratio of 1.5 and decreased to 132 nm at the molar ratio of 4.5. The particle size at the molar ratio of 15 was indeterminable because it was difficult to identify the boundaries of the complex particles. For the hydrolysis of AlCl3 the factors that determine particle size are assumed to be nucleation, coalescence, condensation, and surface reaction, as found for other aerosol-synthesized particles.14 The coalescence, condensation, and surface reaction increases the particle size, while the nucleation decreases the particle size by increasing the number of seeds. An increase in the molar ratio or the amount of H2O increased the reaction rate, which resulted in increasing not only the nucleation but also condensation and surface reaction. The nucleation rate turned out to be more influential, and overall particle size decreased. A similar phenomenon was observed in the gasphase synthesis of TiO2 nanoparticles: an increase in the H2O to TiCl4 molar ratio resulted in a decrease in particle size.15 The severe agglomeration observed at the ratio of 15 is probably because the corresponding increase in reaction rate increased the number of particles and consequently the number of collisions leading to agglomeration. The surface reaction and condensation at the neck between particles in contact made the particle boundaries in distinguishable. 4.3. Effect of Reactor Temperature on Particle Morphology and Size. The reactor temperature was varied from 300 to 700 °C at the H2O to AlCl3 molar ratio of 4.5, keeping the other operating conditions constant. The particles remained spherical, insensitive to variation of the reactor temperature. The primary particle size decreased with increasing reactor temperature, as shown in Figure 3. The number-average particle diameters at 300, 500, and 700 °C were 180, 132, and 80 nm, respectively. The particle size distribution became narrower, as the reactor temperature was increased from 300 to 700 °C. A similar dependence of particle size on temperature was observed in the gas-phase synthesis of TiO2 particles,16,17 while the synthesis of alumina particles by thermal decomposition of aluminum tri-sec-butoxide (ATSB) showed a different dependence that the size increased with temperature increase.14 Nakaso et al.18 showed that for TiO2 particles created by thermal decomposition of titanium tetraisopropoxide (TTIP), the particle size decreased with increasing temperature, exhibited a minimum at 700 °C, and increased with further increase in temperature. The increase of particle size at high temperature was attributed to the coalescence by sintering of
Table 1. Atomic Ratios of Cl to Al in Particles Produced at Various Reactor Temperatures and Molar Ratios of H2O to AlCl3 with the AlCl3 Concentration Fixed at 4.7 × 10−4 mol/ L molar ratio of H2O to AlCl3
reactor temp, °C
atomic ratio of Cl to Al
1.5 4.5 15 4.5 4.5 4.5
500 500 500 300 500 700
0.48 0.34 0.29 0.43 0.34 0.21
molar ratio of H2O to AlCl3 or increasing the reactor temperature. At the reactor temperature of 500 °C, the atomic ratio was 0.47 at the molar ratio of 1.5 but decreased to 0.29 as the molar ratio was increased to 15. With the molar ratio of H2O to AlCl3 constant at 4.5, the atomic ratio of Cl to Al decreased from 0.43 to 0.21 as the reactor temperature was increased from 300 to 700 °C. Such a decrease in the atomic ratio of Cl to Al is due to an increase in the extent of hydrolysis. The Cl to Al ratios shown in Table 1 indicate that the residual chlorine after the hydrolysis amounted to 10−15% of that present in the starting material, AlCl3. 4.2. Effect of H2O to AlCl3 Molar Ratio on Particle Morphology and Size. The overall reaction of the hydrolysis of AlCl3 can be represented by AlCl3 + 3/2H2O = 1/2Al2O3 + 3HCl. The molar ratio of H2O to AlCl3 was varied from the stoichiometric ratio of 1.5 to 4.5 and 15 with H2O in excess, holding the reactor temperature at 500 °C, the evaporation temperature at 230 °C, and the total gas flow rate at 1.8 L/min (STP). The AlCl3 concentration was calculated to be 4.7 × 10−4 mol/L by dividing the AlCl3 evaporation rate by the gas flow rate. Figure 2 shows the scanning electron microscopic images of the produced particles. At a molar ratio of 1.5, the particles were spherical, loosely agglomerated, and with a broad size distribution. As the molar ratio was increased to 4.5, the size distribution of the particles became narrower while maintaining their spherical shapes. As the molar ratio was
Figure 2. Scanning electron microscopic images of primary particles at H2O to AlCl3 molar ratios varying from 1.5 to 15 (reactor temperature, 500 °C; AlCl3 concentration, 4.7 × 10−4 mol/L; total gas flow rate, 1.8 L/min). 10374
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Figure 3. Primary particle size distributions at various reactor temperatures (H2O to AlCl3 molar ratio, 4.5; AlCl3 concentration, 4.7 × 10−4 mol/L; total gas flow rate, 1.8 L/min).
Figure 4. Primary particle size distribution at various AlCl 3 concentrations (H2O to AlCl3 molar ratio, 4.5; reactor temperature, 500 °C; total gas flow rate, 1.8 L/min).
particles in contact. In the temperature range of the present study, the particle growth by sintering between neighboring particles may be negligible. As the reactor temperature was increased, the reaction rate increased to produce more nuclei. The primary particles became smaller because of the lower amount of precursor vapor available per nucleus for subsequent growth by surface reaction, condensation, or scavenging. 4.4. Effect of AlCl 3 Concentration on Particle Morphology and Size. The AlCl3 concentration was varied from 2.5 × 10−4 to 7.5 × 10−4 mol/L by controlling the evaporation rate, holding the reactor temperature at 500 °C. At the AlCl3 concentration of 2.5 × 10−4 mol/L, the average particle size was 72 nm. As the AlCl3 concentration was increased, the particles size increased to 132 nm at 4.7 × 10−4 mol/L and further to 203 nm at 7.5 × 10−4 mol/L. An increase in the AlCl3 concentration increased the nucleation and the growth rates by condensation and surface reaction. The increase in the nucleation rate acted to decrease the particle size, while the increase in the particle growth rate increased particle size. The ultimate particle size is determined by these two opposing effects. Under the operating conditions of the present study, the growth by condensation and surface reaction dominated to increase the particle size. The particle size distribution at the highest concentration of 7.5 × 10−4 mol/L was broader than at the two lower concentrations, as shown in Figure 4. Figure 5 shows the SEM images of the particles produced at two concentrations, 4.7 × 10−4 and 7.5 × 10−4 mol/L. A presence of unexpectedly small particles is seen at the higher concentration, which broadened the size distribution. These small particles were formed probably because of secondary nucleation, as was discussed earlier by Park et al.17 In such cases, the concentration may be too high for the nucleation to occur in one step. 4.5. Effect of Contact Point of AlCl3 with H2O Vapors on Particle Morphology and Size. Keeping the H2O to AlCl3 molar ratio at 4.5, the reactor temperature at 500 °C and the AlCl3 concentration at 4.7 × 10−4 mol/L, the contact point of AlCl3 with H2O vapors was varied from the reactor inlet to a point 10 cm inward. Figure 6 shows the configurations for the two different contacts, types A and B. For type A, the AlCl3 vapor met with water vapor right at the reactor inlet, while for type B, they met at a point 10 cm inward from the reactor inlet.
As shown in Figure 7, the primary particle size decreased from 132 to 78 nm as the contact point was moved from the inlet toward the center of the reactor. The particle size for type B at the reactor temperature of 500 °C was similar to that of type A at the higher temperature of 700 °C. The size distribution was even narrower with the former, as indicated by the geometric standard deviations of 1.14 with type B at 500 °C and 1.27 with type A at 700 °C. The graphs used to determine the geometric standard deviations are shown in the Supporting Information. Figure 8 shows the temperature profiles inside the reactor for two set temperatures, 500 and 700 °C. The temperatures were lower at the inlet and increased to the set temperatures over a distance of 10 cm. With type A, the reactants went through the transient temperature zone as a mixture, and the nucleation and particle growth occurred over the gradual increase in temperature. By comparison, with type B the reactants were preheated separately and then brought into contact right at the set temperature. The nucleation rate, which is highly dependent upon temperature, was higher with type B at the same set temperature, producing more nuclei and yielding smaller particles. The temperature at the contact point was rather higher for type B at the lower set temperature of 500 °C than for type A at the set temperature of 700 °C. This is the reason why the particle size with type B at 500 °C was as small as that of type A at the higher set temperature of 700 °C. Thus, the contact point of reactants is an important factor for controlling the particle size and distribution. In a synthesis of TiO2 by gasphase reaction of TiCl4 with water at 1473 K, a contact point 8 cm inward gave primary particles smaller than the contact at the reactor inlet.19 Although the materials are different, the effect of the contact point on the particle size is similar. 4.6. Comparison between Experimental and Simulated Primary-Particle Sizes. In order to support the hypothesis of growth dominated by nucleation and condensation in the preceding section, the growth of primary particles was studied using a discrete sectional model,20 which was developed in our group and validated against the models of Landgrebe and Pratsinis21 and Graham and Robinson.22 In the discrete-sectional model, a product monomer was assumed to be a nucleus, and the particle size domain was divided into 20 discrete sizes and 30 sections. The model considers the sintering-induced coalescence between neighboring particles 10375
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Figure 5. Comparison of scanning electron microscopic images of primary particles at different AlCl3 concentrations (H2O to AlCl3 molar ratio, 4.5; reactor temperature, 500 °C; total gas flow rate, 1.8 L/min).
Figure 6. Illustration of the two types of reaction tubes: type A, AlCl3 vapor meets with H2O vapor at the reactor inlet; type B, vapors meet at a point 10 cm inward from the reactor inlet.
Figure 8. Axial temperature profiles in the reactor at various set temperatures.
Figure 7. Effect of the contact point of H2O with AlCl3 on primary particle size distribution (type A, at the reactor inlet; type B, at a point 10 cm inward from the inlet). The particles were prepared at 500 and 700 °C with the AlCl3 concentration at 4.7 × 10−4 mol/L, the molar ratio of H2O to AlCl3 at 4.5, and the total gas flow rate at 1.8 L/min.
applied to the present study to calculate the conversion of AlCl3 at the reactor exit. The calculated conversion was 0.16 lower than the actual conversion of 0.86, which was determined by comparing the initial content of chlorine in AlCl3 and from the residual chlorine content in Table 1. The pre-exponential factor of the rate equation was adjusted from 1.85 × 109 to 9.26 × 1010 L2.27/mol2.27·s to fit the observed conversion. A rationale for this adjustment is based on the difference in the molecular structure of aluminum chloride that most aluminum chloride exists as dimer at the temperature under which the rate equation was derived, while under the higher operating temperatures in the present study a significant fraction of the chloride should exist as monomer, which is less stable. Using
but not the surface reaction. A description of the population balance equations for particle size segments and the method of solving those equations to obtain the particle size distribution are given elsewhere.20 The discrete-sectional model was applied to the hydrolysis of AlCl3 at the reactor set temperature of 500 °C with the AlCl3 inlet concentration at 4.7 × 10−4 mol/L and the molar ratio of H2O to AlCl3 at 4.5. The axial temperature profile in Figure 8 was used. The rate equation23 proposed for the hydrolysis of AlCl3 vapor in the temperature range of 150−210 °C was 10376
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the simulation conditions and the adjusted kinetic parameter in Table S1 of the Supporting Information, the number-average primary particle size was calculated to be 17.8 nm, an order of magnitude lower than the observed size, 132 nm. The underestimation of the particle size may be due to the assumption of a monomer being a nucleus. This assumption has been used for the flame syntheses of TiO2, SiO2, and Al2O3, the vapor pressures of which are very low,14,24 but may be misleading in the present reaction system because what is produced is not Al2O3 but AlOxCly(OH)z more volatile than the oxide. The discrete sectional model was modified to consider the nucleus larger than a monomer and to include the fraction of the product monomers leading to nucleation. The fraction of 0.1 means that 10% of the product monomers generated from the hydrolysis is used to create new particles or nuclei and the balance or 90% of them are condensed on the existing particles. The nucleus size and the fraction were treated as parameters because of the unavailability of the vapor pressure and the surface tension of the product molecules that are required for the calculation of the nucleus size and the nucleation rate.25 Figure 9 shows the variation of the primary
Figure 10. Comparison between experimental and simulated distributions in primary-particle size for the operating condition used for Figure 9, with the nucleus size at 5 and the fraction for nucleation at 10−4.
section, might provide more realistic primary particle size distribution but would require enormous computation time to calculate the quadruple integrals for coagulation. The fraction of 10−4 and the nucleus size of 5, which gave a good match between experimental and simulated sizes for the reactor temperature of 500 °C with the contact point at the reactor entrance, were applied for the same reactor set temperature but with the contact point of AlCl3 and H2O vapors moved 10 cm inward and also for the higher set temperature of 700 °C with the same contact point. As shown in Figure 11, the primary particle sizes were decreased from 132 to 103 and 113 nm, respectively, by the change in contact point at the same set temperature, 500 °C, and by the temperature increase to 700 °C with the same contact point. These simulation results qualitatively agree with experimental data; however, the decrements in size were smaller than those experimentally observed. By increase of the fraction for nucleation from 10−4 to 3.0 × 10−4, the primary particle sizes further decreased to match experimental values. The simulations based on aerosol dynamics with the two adjustable parameters support the hypothesis of primary particle growth controlled by condensation and nucleation that was used to explain experimental observations. The determination of the physical properties of the product molecules to determine the nucleus size and the nucleation rate and the inclusion of the surface reaction in the simulation may be necessary for better understanding of the particle growth mechanisms. 4.7. Application as Precursor for α-Alumina. The particles obtained with the molar ratio at 4.5, the reactor temperature at 500 °C, the AlCl3 concentration at 4.7 × 10−4 mol/L, and the contact point of AlCl3 with H2O vapors at the reactor inlet were tested as a precursor for α-alumina through calcination of the particles at 1200 °C for 1 h. The hydrolysis condition for the precursor used for the calcination was chosen to yield more spherical particles with narrower size distribution at as low a temperature and high a production rate as possible. The residual chlorine content was not considered seriously in choosing the hydrolysis condition because the residual chlorine content differs little between hydrolysis conditions, as shown in
Figure 9. Effect of the fraction of product monomers consumed for nucleation on primary particle size with varying nucleus sizes for an operating condition: reactor set temperature, 500 °C; AlCl3 inlet concentration, 4.7 × 10−4 mol/L; molar ratio of H2O to AlCl3, 4.5. The contact point of H2O with AlCl3 is at the entrance of the reactor.
particle size with the fraction for nucleation varied from 10−5 to 1.0 for three nucleus sizes, 5, 10, and 15 monomers composing a nucleus. The primary particle size increased with decreasing fraction for nucleation and with increasing the nucleus size. It is more sensitive to the fraction than to the nucleus size. A decrease in the fraction for nucleation must reduce the number of new particles, while increasing the amount of the monomers available for condensation, and consequently increased the primary particle size. The simulated particle size matched the experimental size, 132 nm at the fraction of 10−4 and the nucleus size of 5. Figure 10 shows that the primary particle size distribution predicted by the model is significantly narrower than that experimentally observed. Such discrepancy may be due to the assumptions in the model that the residence time of a fluid element is uniformly distributed in the reactor and that the primary particles within a section are uniform in diameter. The two-dimensional model of Xiong and Pratsinis,26 which allows for the variation of primary particle diameter in each 10377
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an important specification for the aluminas to be used as ceramics. The BET surface area of the present alumina was 16.4 m2/g. This surface area is higher than that of the alumina by a hydrothermal treatment of bohemite powder at 430−450 °C and 10.3 MPa 27 but comparable to that of the alumina derived from a carbon-covered alumina by utilizing the combustion energy of the carbon for the α-phase transformation28 and the alumina produced by thermally treating a sol−gel derived precursor at 1200 °C.29 The surface-area equivalent diameter was calculated to be 98 nm using the alumina density of 3.98 g/ cm3, while the surface areas of the fumed and Sumitomo aluminas were 83.4 and 28.9 m2/g, respectively. A comparison of their morphologies is shown in Figure 12. The morphology differs between aluminas. Agglomerated powders contain large interagglomerate pores and consequently lower bulk density, which poses increased difficulty in both compaction and sintering processes.30 Less agglomerated particles sinter at lower temperatures.31 Compared with the fumed alumina, the present alumina precursor is larger in primary particle size, less agglomerated, and lower in void volume. This improvement in morphology and size is expected to provide an advantage for application as raw materials for ceramic bodies, and future work on compaction and sintering must follow for verification.
Figure 11. Comparison between experimental and simulated primaryparticle sizes for three different temperature profiles in the reactor with the AlCl3 inlet concentration at 4.7 × 10−4 mol/L and the molar ratio of H2O to AlCl3 at 4.5. (A) Reactor temperature was set at 500 °C with the contact point of H2O with AlCl3 at the entrance of the reactor. (B) Reactor temperature was set at 500 °C with the contact point moved 10 cm inward. (C) Reactor temperature was set at 700 °C with the contact point of H2O with AlCl3 at the entrance of the reactor. α represents the fraction of product monomers consumed for nucleation, and n is the nucleus size or the number of product monomers composing a nucleus.
5. CONCLUSION A solid precursor, AlOxCly(OH)z, for α-alumina was prepared by partial hydrolysis of AlCl3 vapor in a tubular flow reactor. The precursor particles were spherical and softly agglomerated compared with the fumed alumina, hardly agglomerated and poor in sphericity. The primary particle sizes obtained ranged from 50 to 200 nm under the operating conditions of the present study. The particle size increased with increasing AlCl3 concentration but decreased with increases in either the H2O to AlCl3 molar ratio or the reactor temperature. The particle size decreased and the particle size distribution became narrower as the contact point of AlCl3 with H2O vapors was moved from the reactor inlet to a point 10 cm inward. The variation of experimentally observed primary particle size with varying temperature profiles in the reactor could be simulated with a discrete-sectional model by controlling the nucleus size and the fraction of the product monomers consumed for nucleation. The as-produced particles contained 10−15% of the chlorines present in the starting material, AlCl3. The precursor was calcined at 1200 °C for 1 h to obtain chlorine-free α-alumina. The bulk density was 0.81 g/cm3, and the surface-area equivalent diameter was 98 nm. Further studies on compaction and densification are necessary to investigate the possibility of
Table 1, and any residual chlorine could be removed in the calcination step. By the calcination, the residual chlorine was completely removed and the crystalline structure was transformed from amorphous to α-alumina, as shown in the Supporting Information. The loss in weight by the calcination depends upon the temperature of the hydrolysis. The loss was 31% with the hydrolysis temperature at 500 °C and decreased to 25% as the hydrolysis temperature was further increased to 700 °C, as shown in Figure S3, Supporting Information. This level of loss is lower than 35% for the alumina derived from Al(OH)3, the major precursor for commercial alumina. The particle morphology, size, bulk density, and surface area of the present α-alumina was compared with that of a fumed alumina (Degussa, Alu 65) and a sol−gel derived alumina (Sumitomo, HIT-70). The bulk density of the present alumina was 0.81 g/ cm3, comparable to that of the Sumitomo alumina but higher than that of the fumed alumina, 0.13 g/cm3. The bulk density is
Figure 12. Comparison of scanning electron microscopic images between the present alumina, a fumed alumina (Deggusa, Alu 65), and a sol−gel derived alumina (Sumitomo, HIT-70). 10378
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using the alumina derived from the aluminum chloride as raw materials for ceramic bodies.
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ASSOCIATED CONTENT
S Supporting Information *
Graphs to determine the geometric standard deviations of the particles, X-ray diffraction patterns of the alumina particles obtained at different calcination temperatures, and simulation conditions for the primary particle size. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge and Economy, Republic of Korea.
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REFERENCES
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dx.doi.org/10.1021/ie501400c | Ind. Eng. Chem. Res. 2014, 53, 10372−10379
