As one of well-known spinels (typically refers to AB2O4, A2+, B3+ or A4+, B2+), magnesium aluminate (MgAl2O4) has been mainly developed and applied in industrial and military applications, including high-pressure arc lamps, transparent armor, optical heat exchangers, and missile domes      . Among them, appropriate transparent ceramic based materials were employed due to its unique properties, like high thermal stability with high melting point of 2135˚C  , unique optical properties and mechanical strength at high temperature. With increasing utilizations of catalyst and support, MgAl2O4 spinel has been synthesized with high purity, small particle size with uniform pore size distribution and high surface area. Therefore, it is crucial to widely investigate on the synthetic parameters of MgAl2O4  . So far, several approaches were introduced for the preparation of MgAl2O4, namely solid-state   , sol-gel     , spray drying  and co-precipitation   . Especially, sol-gel is considered to be one of the most effective methods focused on the synthesis of MgAl2O4. Any positive ions can be introduced during the hydrolysis process. Besides, the reaction conditions of alkoxide hydrolysis are mild with various advantages, such as simple operation, high purity of product and small particle size, leading to the final high activity. It is known that alkoxide hydrolysis has been extensively used not only on the development of nano-alumina powder but also on the synthesis of MgAl2O4   . To the best of our knowledge, few investigations were reported on the synthetic parameters for preparation of MgAl2O4 via alkoxide hydrolysis  . Great efforts have been made on the features of metal alkoxides, particularly in the preparation of high-purity oxide powders of uniform size, shape as well as the composition.
Herein, discussion on the synthetic parameters on MgAl2O4 through alkoxide hydrolysis was introduced by Mg[Al(OR)4]2 derived from Mg, Al and isopropanol by tuning the reaction conditions and synthetic factors. In this work, several synthetic influenced factors (especially refer to type of organic alcohol, pH, aging time, temperature, etc.) were considered on the effect of the structure and size distribution of as-acquired samples.
It has been known that three reactions simultaneously occur based on hydrolysis theory for alkoxide, namely 1) hydrolysis reaction process ( ); polycondensation reaction of 2) and 3) . It is believed that the hydrolysis of MgAl2(OC3H8)8 is incomplete with much lower content of water. The hydrolysis rate is boosted at the point of too much water. The hydrolyzed product of MgAl2(OH)8 incompletely disperse, resulting in agglomeration and enlargement of the particles. Therefore, moderate of hydrolysis rate is favorable to obtain uniform particle size with well-dispersibility at 40˚C for 24 h with CTAB at pH value of 8.5. As a result, the obtained MgAl2(OH)8 was further calcined at 1200˚C for preparation of MgAl2O4. This study further opens a new avenue to supply an optimal synthetic route to achieve pure metal oxides by hydrolysis, which can be widely applicated in the fields of industrial production and military applications.
Mg foil (99.9%), Al foil (99.9%) and Anhydrous aluminum chloride (AR) were obtained from Tianjin Bodi Chemical Co., Ltd. Cetyltrimethylammonium bromide (CTAB, AR) and polyvinyl pyrrolidone (PVP) were purchased from Shanghai Reagents Factory. AlF3・3.5H2O (CP) was got from Shanghai Reagent Third Factory. Alcohol (AR), isopropanol (AR) and n-butanol (AR) were obtained from Liaodong Chemical Reagent Factory. The deionized water was directly made in the experiment.
2.2. Preparation of MgAl2O4
Preparation of alkoxide: Mg and Al foils were first grinded and cleaned with ethanol. The treated Mg and Al were weighed and added into the reaction kettle based on the stoichiometric ratio of MgAl2O4 with the addition of trace AlF3 and excess isopropanol. After heated at 85˚C under reflux, the product of MgAl2(OC3H8)8 was obtained after distillation. Here, in the experiment organic alcohols of ethanol and n-butanol were selected under reflux at corresponding boiling point for comparison.
Alkoxide hydrolysis: Organic alcohol and deionized water were prepared to hydrolysate with a volume ratio of 1:1. NH3・H2O (1.5 mL) and CTAB (0.75 mL) were added into 50 mL above hydrolysate stirring under constant temperature water bath. Colorless transparent sol of MgAl2(OH)8 was obtained by addition of 10 g MgAl2(OC3H8)8 with dropwise at a rate of 0.1 mL・min−1 under continuous stirring in the water bath at 40˚C for 24 h. In this experiment procedure, several synthesis parameters, like alkoxide addition amount, proportion of isopropanol and deionized water in hydrolysate, hydrolysis temperature and time, pH value, the addition of dispersant (refers to CTAB and PVP) were further investigated keeping other parameters consistent.
Preparation of MgAl2O4: The MgAl2(OH)8 sol synthesized via alkoxide hydrolysis was further dried at 80˚C for 4 h and calcined at 1200˚C for 2 h in a box-type resistance furnace. As a result, the sample of magnesium aluminum spinel powder was obtained for further characterization.
The weight loss was tested by TGA (600, USA, TAQ) under N2 with warming speed of 5˚C・min−1. X-ray diffraction (XRD) analysis was recorded on a D/max-3B X-ray diffractometer (CuKα radiation, λ = 1.54056 Å) to detect the structure of samples in a scanning range 2θ of 5 to 80˚ with operating voltage at 40 kV and the current of 35 mA with a scanning rate of 2˚・min−1. The Brunauer-Emmet-Teller (BET) specific surface area and porous properties were measured through N2 adsorption-desorption at −196˚C on a Quadrasorb SI instrument (Quantachrome, USA). A field-emission scanning electron microscope (SEM) was operated on a JSM-6360LV instrument (JEOL Corporation) to describe the morphology of samples at the accelerating voltage of 5.0 kV. A mater sizer 2000 laser particle size analyzer (Malvern Corporation, UK) was used to measure the particle size and its distribution.
3. Results and Discussion
3.1. Effect on Organic Alcohols
Initially, different organic alcohols, including ethanol, isopropanol and n-butanol were selected to investigate its effect on the particle size of precursors. The particle size and surface area of corresponding samples are listed in Figure 1 and Table 1. It can be seen that the particle size distribution of powder prepared with ethanol and isopropanol is relatively concentrated compared to that of powders synthesis with n-butanol. The sample with n-butanol possesses twice as much as that at D90 (the cumulative particle size distribution of a sample reaching 90%) relative to other two samples. It might be the reason that partially negatively charged HOδ− group nucleophilic attack with a partially positively charged metal atom Mδ+ during the hydrolysis process. Meanwhile, positively charged protons are transferred to a negatively charged ORδ−. Finally, the protonated protonated ORδ+ is separated from the metal atom M. The large molecular structure of magnesium n-butoxide results in relatively weak between alkoxy ORδ+. Metal bond and ORδ+ rapidly desorbs from the metal atom to form magnesium hydroxide aluminum particles leading to hydrogen hydroxide in a short time. Besides, the particles are easier to agglomerate and consequently the particle size of corresponding powder with n-butanol has a relatively dispersion. With comprehensive analysis of samples with different alcohols, the sample with isopropanol displays a larger surface area with uniform particle size.
3.2. Effect of Ammonia Content
The concentration of ammonia was also taken accounted in the synthesis of hydrolysis magnesium aluminum alkoxide, in which ammonia was used to adjust
Figure 1. The size distribution of alkoxides with various alcohols of ethanol, isopropanol and n-butanol.
Table 1. The particle size and specific surface area of samples with various alkoxides.
the pH value of the system. Figure 2 displays the ammonia concentration effect on the particle size and size distribution of powders hydrolysis at 20˚C for 2 h. Obviously, the average particle size gradually increases with the increasing ammonia content from 0.5 to 3.0 mL. The well-dispersion of powder with 0.5 mL ammonia content can be acquired by scanning electron microscope (SEM) in
Figure 2. SEM images of powders hydrolysis at 20˚C for 2 h with different addition of ammonia: (a) 0.5 mL, (b) 0.75 mL, (c) 1.0 mL, (d) 2.0 mL and (e) 3.0 mL.
Figure 2(a). In particular, the particle grows a little larger with uniform and well-dispersion at a point of 0.75 mL of ammonia content in Figure 2(b), and the pH value is measured to be 8.5. Clearly, blocks are detected in Figure 2(c) to Figure 2(e), indicating that severe agglomeration occurs in excess alkaline solutions. During the step of MgAl2(OC3H8)8 hydrolysis in an alkaline environment, the negatively charged Mg and Al nuclei appear by directly attacked by small anion OH− radius, leading the cloud shift to the OR group on the other side. This further weakens the M-O bond and breaks out of the OR, and finally completes the hydrolysis reaction. With trace addition of ammonia, the rate of hydrolysis is relatively slow. At this point, it can be immediately spread widely to surroundings. That is, when the forming-speed of particles is smaller than that of the diffusion speed, the hydrolysis of MgAl2(OC3H8)8 acquires a very small particle size with a uniform particle size distribution. Continue to increase the amount of ammonia, the higher concentration of OH− may enhance the hydrolysis rate and rate of colloidal particles. At that time, the agglomerates can be easily observed. Therefore, the controlling of pH in the hydrolysis process not only affects the gelation process but also plays a key role in the colloidal aggregation process. After all, the ideal hydrolysis pH is tuned to be 8.5 based on the formation in Figure 2.
Figure 3. The size distribution of powders with different addition of ammonia from 0.5 mL to 3.0 mL.
Table 2. The particle size powders with different addition of ammonia in the range of 0.5 mL to 3.0 mL.
powder increases. With addition of 0.5 mL ammonia, it shows a main peak with the average particle size of 14.15 μm and a small peak around 2.0 μm, indicating that the higher proportion of small particles in the powder. When the amount of ammonia increases to 0.75 mL, there is a single peak with an average particle size of 21.88 μm. It further confirms that the particle size distribution is uniform with well agreement of SEM images in Figure 2. With continuously increased amount of ammonia, the particle size of the powder is gradually increased. When the amount of ammonia is 2.0 mL, the particle size is observed as two obvious distribution areas. There is no further no difference in the average particle size of the powder but the uniformity of particles deteriorates along with further excessive ammonia.
3.3. Effect of Dispersant during Hydrolysis
Dispersant, like CTAB and PVP, was selected to screen the particle distribution of samples. SEM images in Figure S1 clearly exhibits that the spherical particle is obtained with the dispersant of CTAB, which is very different from that of sample with PVP. It further indicates that only certain dispersant of CTAB is benefit for creation the feature of particles. More importantly, we also compared the difference between the samples with or without CTAB dispersant for preparation of MgAl2O4. The SEM image in Figure 4(a) exhibits that the agglomeration phenomenon is accessible without any dispersant. However, well-dispersion with spherical particles are obtained in Figure 4(b). Based on the tendency described in Figure 4(c) and Table 3, the particle size of sample without any dispersant is centered to be about 43 μm and the distribution is unconcentrated. In contrast, the addition of CTAB greatly changes this phenomenon. The particle size decreases with the increase of CTAB content. Different amount of CTAB
Figure 4. SEM images of MgAl2O4: (a) without CTAB; (b) with 1.5 wt% CTAB and (c) the size distribution of samples with various amount of CTAB.
Table 3. The particle size of samples with different amount of CTAB.
was screened from 0 to 3.0 wt% to further detect the particle size and its distribution in Figure 4(c), indicating that the crucial factor of dispersant amount in the synthesis process. Especially for the point of 1.5 wt%, the particle size is greatly reduced and the distribution begins to concentrate. After that, the continuously increased amount of CTAB results in no more changes for particle size.
3.4. Effect of Alkoxide Concentration
SEM images in Figure 5 illustrate the influenced concentration of MgAl2(OC3H8)8 on the morphology of hydroxides. The MgAl2O4 powder by introduction of 5 g alkoxide (Figure 5(a)) leads to a mixture of spherical particles and flocculent agglomerates. The average size of particles is determined to be about 400 nm. Some spherical particles are damaged and single spherical particle is found to aggregate with floc on the surface. Increasing the amount of alkoxide to 10 g in Figure 5(b), it is found that spherical particles gradually decrease and agglomerate. The size distribution of floc aggregates is uneven and the boundary is irregular. With the addition of 15 g alkoxide in Figure 5(c), spherical particles substantially disappear resulting in a white floc hydrolysate with a relatively uniform distribution with well dispersion, in which several opaque particles intersperse. It can be seen that alkoxide concentration has a great influence on the formation of colloids and particles during the hydrolysis reaction. It believes that it is mainly attributed to the extremely low concentration of alkoxide solution results in the restricted growth of nucleus to form sol. With increasing high concentration of alkoxide solution, the rapid formation of the nucleus under the state of supersaturation causes the crystal grains to have insufficient growth leading to small particle colloid. It is favorable for the growth of particles to form precipitates with moderate amount. In this work, it demonstrates that the sample with 10 g alkoxide possesses better morphology of spherical particles with well dispersion and uniform size distribution.
3.5. Effect of Hydrolysis Time
Figure 5. SEM images of MgAl2(OH)8 with different amount of MgAl2(OC3H8)8: (a) 5 g, (b) 10 g and (c) 15 g. The hydrolysis reaction time is 24 h.
Figure 6. SEM images of MgAl2(OH)8 prepared with different hydrolysis time: (a) 1 h; (b) 2 h; (c) 8 h; (d) 24 h; (e) 48 h and (f) 72 h.
angular, irregular shapes with a particle size of approximately 400 nm are observed under hydrolysis for 2 h. Particles gradually grow to flocculated agglomerates under hydrolysis for 2 h in Figure 6(b). Extend the hydrolysis time directly to 8 h in Figure 6(c), it is found that most hydrolysate is detected to be agglomerates with various particle sizes and irregular shapes. But it still can be observed that a small amount of spherical or near-spherical particles are attached to the agglomerates. Along with the hydrolysis time for 24 h in Figure 6(d), the as-prepared spherical particles gradually increase with uniform particle size of 300 to 400 nm and well-distribution, meanwhile, the agglomerates are gradually reduced. Continue to extend the reaction time to 48 h in Figure 6(e), spherical particles are found to gradually reduce with larger size and the irregularly agglomerates increase. Here, agglomeration occurs between particles and agglomerates. Until to 72 h the spherical particles disappeared, and all the substances obtained are flocculent agglomerates in Figure 6(f).
3.6. Effect of Hydrolysis Temperature
The hydrolysis temperature is considered to be another important factor that may influence the property of magnesium aluminum spinels. Table 4 and Figure 7 show the particle size, surface area and size distribution of samples hydrolysis at various temperatures from 20˚C to 60˚C. The crystal growth rate and particle size greatly increase as the increasing hydrolysis temperature below 40˚C. The higher temperature results in the wider the particle size distribution. Analysis demonstrates that such phenomenon is caused by hydrolysis temperature, particularly at 50˚C and 60˚C. Such high hydrolysis temperature causes
Table 4. The particle size and surface area of samples at different hydrolysis temperature from 20˚C to 60˚C.
Figure 7. The particle size distribution of the samples with isopropanol as precursor solvent at hydrolysis temperature from 20˚C to 60˚C.
rapid formation of numerous crystals and later larger crystals become to generate. As a consequence, the optimal hydrolysis temperature is acquired to be 40˚C.
3.7. Effect on Calcination Temperature
Thermogravimetry and differential thermogravimetry (TG-DTG) curves were conducted on obtained powders with different types of alkoxides from 50˚C to 900˚C in Figure 8. In Figure 8(a), there are two stages for sample with ethanol as precursor with a total mass loss of 35% during the whole warming interval. During the lower temperature from 50˚C to 200˚C, the weight loss is about 20% due to physical adsorption of water as well as residual solvent residues in the reaction. The weight loss of 15% is ascribed in the range of 200˚C to 600˚C as desorption of dihydroxylation from ethanol and chemical bond break between a trace amount of ethyl and magnesium or aluminum ions. After that, it turned to be stable without any weight loss. As shown in Figure 8(b) and Figure 8(c), there are three stages for samples with n-butanol and isopropanol as precursors
Figure 8. The TG-DTG curves of samples with different organic alcohols of (a) ethanol, (b) isopropanol and (c) n-butanol and (d) particle size distribution of corresponding samples with various magnesium aluminum bimetal alkoxide after dried 80˚C and further hydrolysis; XRD patterns (e), (f) and (g) of samples with different organic alcohols treated at various temperature (600˚C, 1000˚C and 1200˚C).
with total mass loss of 45% and 38%, respectively. As for the first stage, the weight loss of 18% and 16% for MgAl2(OH)8 prepared with n-butanol and isopropanol from 50˚C to 200˚C can be ascribed as physical adsorption of water as well as residual solvent residues in the reaction. For the second step, the weight loss of 20% and 18% for MgAl2(OH)8 with n-butanol and isopropanol during 200˚C to 400˚C are due to dehydroxylation from the precursors. In the third period, slightly mass loss happens of 7% and 4% for samples with n-butanol and isopropanol from 400˚C to 600˚C, which may be caused by a chemical bond break between a trace amount of residual butyl or isopropyl group and magnesium or aluminum ions. After that, no more weight loss occurs. An endothermic peak of precursor with n-butanol is significantly larger than that of sample with isopropanol, indicating that preparation of sample with n-butanol needs more energy than that with isopropanol. Besides, both of precursors with n-butanol and isopropanol need more energy than that of ethanol. Additionally, the structure of obtained powders with various precursors of alkoxides was detected by XRD in Figure 8 and Table 5. As shown in Figures 8(d)-(f), XRD was conducted on samples prepared with various precursors of alkoxides and calcinated at a wide range temperature of 600˚C, 1000˚C and 1200˚C for 3 h. The result of XRD patterns clearly indicates that powders with various alkoxides are consistent well with pure magnesium aluminum spinel structure without too much change in the position of the diffraction peak. The lattice intensity enhances with the increasing temperature from 600˚C to 1200˚C, indicating that the degree of crystallization of spinel increases with the increasing calcinated temperature. Based on the data in Figure 8(e), the intensity of diffraction peaks of sample with ethanol as alkoxide is significantly higher than that of n-butanol and isopropanol. It demonstrates that the required temperature for spinel phase with ethanol is low, which is consistent well with the data in Figure 8. On the other hand, the relative intensities of the diffraction peaks of each sample become smaller with the increasing temperature. Each sample displays the same relative intensities at 1200˚C in Figure 8(f). Considering avoiding the severe aggregation of large particles at too much higher temperature, thus the optimal calcination temperature is selected to be 1200˚C.
As shown in Table 5, there is an obvious change of particle size among powders with ethanol, isopropanol and n-butanol as organic alcohol solvent in precursor preparation. The D50 of size distribution are 14.15 μm, 17.70 μm and 24.83 μm for sample with ethanol, isopropanol and n-butanol in Table 5, indicating that
Table 5. The particle size of samples with various alkoxides after treated at 1200˚C.
the particle size of powders increases with the increasing carbon chain of organic alcohol. Among them, the average particle size of the corresponding powder of n-butanol is the smallest. Based on the analysis of particle size distribution, the particle size of isopropanol is relatively concentrated with largest specific surface area. As a result, MgAl2O4 with isopropanol as solvent for synthesis of precursor was better than that of ethanol and n-butanol in terms of particle size and corresponding distribution.
All in all, various synthetic parameters were deeply investigated on the preparation of MgAl2O4 via sol-gel followed by high temperature pyrolysis. The optimal preparation factors are as follows: 1) The alkoxide prepared with isopropanol possesses a larger surface area with uniform particle size; 2) Addition of ammonia with a pH of 8.5 can enhance the hydrolysis rate and rate of colloidal particles; 3) The point addition of 1.5 wt% of CTAB results in greatly reduced particle size with concentrated distribution of particles; 4) The moderate amount of alkoxide (10 g) with hydrolysis at 40˚C for 24 h shows uniform particle and well-distribution; 5) the optimal calcination temperature of 1200˚C introduces a pure magnesium aluminum spinel.
This work was partially supported by National Natural Science Fund of China (Grant Nos. 21606219).
Figure S1. SEM images of MgAl2(OC3H8)8 with dispersant of (a) CTAB and (b) PVP.