Activated Alumina-activated Carbon Composite Material

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User：admin AddDate：2015/5/22 Read：33913Times

ABSTRACT

AlCl3, NH3·H2O, HNO3 and activated carbon were used as raw materials to prepare one new type of activated alumina - activated carbon composite material. The influence of heat treatment conditions on the structure and property of this material was discussed; The microstructures of the composite material were characterized by XRD, SEM, BET techniques; and its formaldehyde adsorption characteristic was also tested. The results showed that the optimal heat treatment temperature of the activated alumina-activated carbon composite material was 45℃, iodine adsorption value was 441.40mg/g, compressive strength was 44N, specific surface area was 360.07 m2/g, average pore size was 2.91nm, and pore volume was 0.26m3/g. According to the BET pore size distribution diagram, the composite material has dual-pore size distribution structure, the micropore distributes in the range of 0.6~1.7nm, and the meso-pore in the range of 3.0~8.0nm. The formaldehyde adsorption effect of the activated alumina-activated carbon composite material was excellent, much better than that of the pure activated carbon or activated alumina, and its saturated adsorption capacity was 284.19mg/g.

1. INTRODUCTION
With its developed pore structure and excellent adsorption performance, activated carbon material has been extensively utilized in different fields such as environmental protection, catalysis, food, pharmacy and so on, but it also has such defects as low strength, difficulty in recycling, flammability and explosibility, etc. As one kind of porous inorganic solid materials with large specific surface area, activated alumina is one kind of fine adsorbent, catalyst and catalyst carrier. However, with the rapid development of different industries, many new requirements are put forward for porous adsorption materials. For example, hydrogen desulfurization of residual oil and demetallization require proper surface area and distribution of macro- and micro-pore in some proportion; hydrogen denitrogenation catalyst is required to even load high metal content, high specific surface area, large pore volume and the proper proportion of mesopore and micropores.
Relevant literature also reported that activated alumina carrier should have sufficiently large pore volume and reasonable pore size distribution. Formaldehyde is one of the main pollutants of indoor air, and has such characteristics as extensive source, high toxicity, and long pollution time and so forth. Presently, researches on the formaldehyde adsorption mainly focus on the activated carbon base materials; activated alumina studiers mainly focus on the catalyst carrier and water treatment, and there are few literatures on air purification. In this paper, AlCl3, NH3 · H2O, HNO3 and activated carbon as raw materials were used to prepare one new type of activated alumina - activated carbon composite materials. The composite material has dual pore size distribution characteristic, excellent property, and obvious formaldehyde adsorption effect. It can not only improve the property of activated alumina catalyst carrier, but also can be used as air purifying material.

2. EXPERIMENTAL
2.1 Preparation of the precursor of aluminum hydroxide
AlCl3 was used as the aluminum source; NH3 · H2O served as the precipitant, and aluminum hydroxide was prepared with alkaline precipitation method. The produced Al(OH)3 precipitation was dried in a drying oven at 120℃, and passed the 100 mesh sieve after ball milling for 6h. The power thus produced was for subsequent usage.
2.2 Preparation of the activated alumina- activated carbon composite material
The experiment was conducted with reference to the preparation method of activated aluminum. The process was as follows. A mixture of 25wt% activated carbon and 75wt% Al(OH)3 power was placed into a planetary ball mill for homogenous mixing, during which, a certain amount of 13wt% HNO3 solution was added for granulation. After banality for 24h, the powder was made into balls of about 5 mm in diameter. After being dried at room temperature for 12h, the samples were dried in the drying oven at 110 ℃. The samples were placed into the computer - controlled electric kin with burial method for sintering at different heat treatment temperature (350, 450, 550, 650 and 750℃) respectively at a heating rate of 200℃/h with the holding time of 2h. After natural cooling, rinsing and drying, samples at different heat treatment temperature were obtained.
2.3 Microstructure characterization
The samples were studied with X-ray powder diffractometer (XD-5A, manufactured by Shimadzu) for phase analysis. Specific surface area of samples was determined with automatic specific surface area and porosity analyzer. Cold field emission-scanning electron microscopy was utilized for micro structure analysis. With reference to GB/T12496.8-1999, the iodine adsorption determination was determined.
2.4 Characterization of the formaldehyde adsorption characteristics Formaldehyde was adsorbed with static adsorption method. 0.50g each of the sample, activated alumina and activated carbon were weighed respectively, and then placed into different weighing bottles which were placed into the air-tight drier full of saturated formaldehyde vapor. After that, the drier was placed into a thermotank under 30℃. The formaldehyde adsorption property and capacity of the samples were tested in different time (6, 24, 36, 48, 60 and 72h) and calculated with mass difference subtraction, respectively.
3. RESULTS AND DISCUSSION
3.1 Heat treatment temperature

Heat treatment temperature is the key factor affecting the adsorption characteristics of composite materials. In the following, iodine adsorption value was taken as the reference with considering XRD pattern, heat loss of activated carbon and so on for discussion and analysis. Fig. 1 shows XRD pattern of samples produced under different

heat treatment temperature. Fig.2 depicts the iodine values of the corresponding According to Fig. 1, the sample had widen diffraction peaks at 13.9, 28.3, 38.5 and 49.2o when the heat treatment temperature was 350℃. Its major component was pseudo boehmite with incomplete crystal. Because of lower calcination temperature and shorter heat treatment process, aluminum hydroxide could only remove some of the crystal water and pseudo boehmite containing very less crystal water was produced. When the calcination temperature was 450℃, the remaining crystal water in pseudo boehmite was completely lost and γ-Al2O3 under transition form was generated with obvious diffraction peaks at 46° and 66.8°; when the heat treatment temperature rose to 650℃, characteristic diffraction peak of γ-Al2O3 strengthened continuously. At the same time, there was slight offsetting, indicating that the γ-Al2O3 content increased, and crystallographic form turned much better. When the heat treatment temperature was 750℃, the diffraction peak of γ-Al2O3 weakened or basically disappeared, and obvious α-Al2O3 characteristic peaks appeared at 25.9, 35.2, 37.8, 43.4 and 57.5°, so α-Al2O3 with fairly complete crystal was mainly produced at 750℃.

Fig. 2 shows that the iodine values of samples slowly increased first and suddenly reduced after the platform stage with continuous rise of heat treatment temperature. The reason can be explained from the prospective of Al(OH)3 phase transition and ablation samples. loss of activated carbon. According to Fig.1, the main crystalline phase of sample was pseudo boehmite when the heat treatment temperature was 350℃. Withthe continuous rise of heat treatment temperature, the main crystalline phase of sample gradually transformed from pseudo boehmite to γ-Al2O3 with larger specific surface area. When the heat treatment temperature was higher than 650℃, the main crystalline phase of the sample transformed to be α-Al2O3 with very small specific surface area. Under lower heat treatment temperature, the oxidation loss of activated carbon was lower. With continuous raising the heat treatment temperature, carbon loss of activated carbon gradually increased. When the heat treatment temperature was above 650℃, oxidation loss of activated carbon and secondary pore enlarging under high temperature caused significant reduction of specific surface area of the activated carbon.
The purpose of heat treatment met the demand of preparing activated alumina carrier. It started to transform to γ-Al2O3 since 450℃. With rising the heat treatment temperature, the sample strength increased, but the heat loss of activated carbon continuously increased and the iodine value reduced. Based on the above experimental results, it can be considered that the heat treatment temperature of sample shall not be too high, which basically complies with the conclusion of the reference. The optimal heat treatment temperature from experiment was 450℃, the iodine value of sample was 441.40mg/g, and the compressive strength was 44N.
3.2 Microstructure

Fig.3 shows the SEM photograph of the sample at the heat temperature of 450℃. According to Fig. 3, there are a great number of nanometer scale micropores distributing homogenously in the sample.

Fig.4 depicts the N2 adsorption-desorption isotherms of the sample at 450℃ heat temperature. According to IUPAC classification, the adsorption isotherm was a type IV adsorption isotherm. From the isotherms, the specific surface area of the sample was calculated to be 360.07m2/g, average pore size was 2.91nm, and pore volume was 0.26m3/g. In addition, there was obvious hysteresis loop in the adsorption isotherm, which revealed a mesoporous structure in the sample.

Fig. 5 shows the pore size distribution of the sample at 450℃ heat temperature. Fig.5a corresponds to the BJH method with the mespore size of the sample distributing in the range of 3.0~8.0 nm,
and Fig.5b to the HK method in the range of 0.6~1.7 nm. This indicated that the sample has a dual pore size distribution structure, which conformed to the expected result.
3.3 Formaldehyde adsorption

Fig.6 shows the formaldehyde adsorption effects of sample, pure activated carbon and activated alumina under different time. According to Fig. 6, it showed that formaldehyde adsorption amount of three materials continuously increased with the extension of adsorption time and became saturated basically after 48h, and their saturated adsorption amount was 284.19, 260.08 and 129.76mg/g, respectively. Fig.6 clearly showed that the formaldehyde adsorption capacity of the sample was excellent, and it was superior to that of pure activated carbon or activated alumina. In carbon content test, the contents of activated alumina and carbon in the sample were 65 and 35wt%, respectively. Through calculation, the formaldehyde adsorption amount of the sample was greatly larger than the sum value of activated alumina and activated carbon. Owing to the molecular size, about 3 ?, of formaldehyde, these small molecules were mainly adsorbed through micropore adsorption, and the micropore had large specific surface area with better adsorption effect. Mespore could play the channel role during formaldehyde adsorption and make the formaldehyde gas enter into the inner surface in a faster way. According to Figs. 4 and 5, the sample had dual pore size distribution that mainly includes micropore and mespore, which was good for formaldehyde adsorption. In addition, during the preparation process, some added HNO3 reacted with Al(OH)3 to generate Al(NO3)3 (3HNO3+Al(OH)3=Al(NO3)3+3H2O). Part of Al(NO3)3 was adsorbed in the pores of the activated carbon, which changed the pore size of the activated carbon; furthermore, part of HNO3 was adsorbed by activated carbon and undergone heat treatment, thus changing the composition of the surface functional group of the activated carbon. A large quantities of oxygen functional groups, such as C=O, O–C=O, Al–O and so on, were introduced, and it could achieve modification effect on the activated carbon. The adsorption property of the modified activated carbon was significantly improved.

4 CONCLUSION
(1) The activated alumina - activated carbon composite material was one new type of porous adsorption materials: iodine value 441.40mg/g, compressive strength 44N, specific surface area 360.07m2/g, average pore size 2.91nm, and pore volume 0.26m3/g.
(2) Heat treatment temperature was the key factor affecting the structure and property of the material. The experimental optimal heat treatment temperature of the sample was 450℃.
(3) Micropore size of the activated alumina-activated carbon composite material distributed in the range of 0.6~1.7 nm, while the mespore size in the range of 3.0~8.0 nm, which indicated that the composite material characterizes a unique dual pore size distribution structure.

(4) The formaldehyde adsorption effect of composite material was excellent, with its saturated adsorption capacity of 284.19mg/g, which was superior to that of pure activated carbon or activated alumina.