Novel Approaches For Template-free Synthesis Of Kaolin-derived Mesoporous Materials With Large Specific Surface Areas

Nowadays, mesoporous materials are highly attractive for the defined mesoporosity (mesopores of 2-50 nm in diameter), large specific surface area (SBET) and pore volume, which benefit their wide applications in adsorption, separation, catalysis, drug delivery, micro-reactor and so on.The traditional synthesis of mesoporous materials is generally focused on the template strategy. Mesoporous materials obtained via template strategy are characterized by high-order of pore distributions, adjustable pore diameters, large specific surface area, variable morphologies, rich diversity of species, and are among the most popular topics in the field of porous materials at present. However there are still some born shortcomings which are inevitable in the template strategy:(1) High cost. In template strategy, the templates and many organic solvent can not be bypassed and usually have to be of high-purity (Analytical reagent, AR), which is generally of high cost; (2) Long-time production. In the template strategy, plenty of time must be kept for the aging and the crystallization of the surfactant-templated meso-structures and for post-synthetic removal of templates, in order to guarantee the high-degree regularity of surfactant-templated meso-structures and finally to obtain well-defined target mesoporous products; (3) Environment incompatibility. In the template strategy, the templates and volatile solvents employed are generally harmful or poisonous, and the post-synthetic removal of templates and the volatilization of solvents would always discharge lots of tail gas, which would also poison the environment. Considering all the shortcomings above-mentioned, it would be not favorable for the template strategy to be applied in industry production on a large scale. Hence, it would be meaningful for the low-cost production and industry application of mesoporous materials to prepare them through template-free strategies.Natural nano-minerals, especially the phyllosilicate is not only abundant, widely distributed, cheap and easy-accessible, but also exhibits well-defined nano properties, such as nano-sized crystal-unit structure and interlayer space, indicating the potential as raw materials in the preparation of mesoporous materials. Kaolin is a clay mineral with a 1:1 layer structure consisted of a [SiO4] tetrahedral sheet and a [AlO6] octahedral sheet in the layer unit, which is a typical phyllosilicate. Obviously, if kaolin could be used as the major resource to prepare mesoporous materials via facile template-free strategies, it would be beneficial for not only the low-cost production and large-scale application of mesoporous materials, but also the high-added-value utilization of the conventional mineral resources.In this paper, kaolin, as the only raw material, was successfully prepared into high-graded mesoporous materials via a series of newly developed template-free processes. The as-received kaolin sample was purchased from China Kaolin Clay Co., Ltd. At first, the kaolin and its calcined counterparts were characterized by XRF, XRD, TG, FT-IR, SEM and N2 adsorption/desorption. Then, a series of novel template-free strategies to prepare mesoporous materials from kaolin, i.e. "calcination-selective-acid/alkali-etching", "calcination-alkali-activation-acid-etching", "calcination-zeolitization-acid-treatment" and "calcination-free-alkali-treatment-acid-treatment" were put forward through a systematic work. The process condition influencing rules, the processing mechanisms and the properties of the products were systematically studied, and the main results are as follows:1. The physicochemical properties of kaolin and its calcined counterpartsThe kaolin is mainly composed of SiO2 and Al2O3 with a mole ratio of 1.02 of Si/Al, and mainly comprised of flaky kaolinite particles, indicating the high representativeness of the used kaolin. When calcined at 700-900 ℃ for 2 h, the kaolin turned out to be amorphous metakaolin due to the dihydroxylation reaction of kaolinite. When calcined at 950-1100 ℃ for 2 h, some poorly crystallized γ-Al2O3 began to occur in the amorphous product, and the higher calcination temperature (1150-1200 ℃) would benefit the crystallization of mullite (3Al2O3·2SiO2). Besides, it was changeless for the morphology and porosity parameters of kaolin after being calcined below 1100 ℃.2. The novel "Calcination-selective-acid/alkali-etching" processA novel process named as "calcination-selective-acid/alkali-etching" was proposed to synthesis mesoporous silica/alumina form kaolin. In detail, the kaolin firstly underwent a calcination-activation and then submitted to selective-acid/alkali-etching. The related mechanism can be described as follows:during the thermal-treatment, the alumina/silica is in situ activated in the flaky particles of kaolin; and during the subsequent selective-acid/alkali-etching, the activated alumina/silica is etched by acid/alkali and finally led to plenty of nanopores in the remained flaky amorphous silica/alumina framework, i.e., the kaolin-like flaky mesoporous silica/alumina with slit-like mesopores. The effects of the process conditions related to calcination temperatures (750-900 ℃/950-1150 ℃), acid/alkali hydrothermal treatment temperatures (60-120℃/60-100℃), acid/alkali concentrations (2-9 mol/L/1-4 mol/L) and the acid/alkali treatment time (0.5-12 h) on the properties of final mesoporous materials were evaluated systematically. In the "calcination-acid-etching" method, the specific surface area increases and then goes down as the calcination temperature and acid-treatment temperature rise, and increase and finally to the maximum as the acid concentration and acid-treatment time increase; meanwhile, the pore size at the maximum probability (DBJH) only depends on the calcination temperature:the DBJH remains-2 nm but turn out to be 3.0 nm only when the calcination temperature comes up to 900 ℃. When calcined at 850 ℃ or 900 ℃ for 2 h, and then acid-treated by 3 mol/L HCl at 80 ℃ for 6 h, the kaolin could give a mesoporous silica with SBET of 426 m2/g and DBJH of 2.0 nm or with SBET of 292 m2/g and DBJH of 3.0 nm. In the "calcination-alkali-etching" method, the specific surface area of the obtained product decreases as the calcination temperature rise, but increases with a limit as the alkali-treatment temperature, alkali concentration and alkali-treatment time arise. When calcined at 1100 ℃ for 2 h, and then alkali-treated by 2 mol/L NaOH at 80 ℃ for 6 h, the kaolin could give a mesoporous alumina with SBET of 178 m2/g and DBJH of 5.3 nm. The as-obtained mesoporous materials from kaolin shows favorable adsorption ability toward methylene blue with the monolayer adsorption capacity up to 361.8 mg/g. Compared with the traditional template strategy, the proposed template-free strategy "calcination-selective-acid/alkali-etching" is cost-effective, facile and friendly to the environment to some degree.3. The novel "Calcination-alkali-activation-acid-etching" processThe previously proposed template-free process "calcination-selective-acid/alkali-etching" has already worked it out that it is feasible to prepare mesoporous materials using kaolin as the only raw material. However, the obtained mesoporous silica/alumina are characterized by much poorer porous properties in comparison with those synthesized by template strategy. In this case, a new template-free strategy, "calcination-alkali-activation-acid-etching", was put forward to obtain mesoporous materials with effectively improved nanoporous properties. In detail, the kaolin firstly underwent a calcination-activation at 850 ℃ and then submitted to 4 mol/L NaOH for 0.5 h, and then submitted to 5 mol/L HC1 for 6 h. The related mechanism can be described as follows:in the calcination process, kaolin is transformed into thermal-activated amorphous Al-O and Si-O; in the "alkali-activation", the NaOH hydrothermal treatment transforms the flaky amorphous aluminum silicate, i.e. MK, into a new amorphous sodium aluminum silicate (AlNaSi2O6) of inherited flaky shapes; in the final "acid-etching", the acid selectively removes the Al3+and Na+of the sodium aluminum silicate with resultant mesopores generated in situ, forming the mesoporous silica with favorable specific surface area (604 m2/g) and pore size (4.4 nm) with slit-like mesopores. It is found that the increase of NaOH concentration and alkali-treatment time would lead to the higher degree of alkali-activation, which would yet cause the large amount dissolution of Al3+of the alkali-activated metakaolin in the following acid treatment and finally result in the disintegratement of the flaky morphology of alkali-activated metakaolin. The optimum mesoporous silica obtained by the "calcination-alkali-activation-acid-etching" route is close to those made by template strategy in nanoporous properties, and shows favorable adsorption ability toward methylene blue with the monolayer adsorption capacity up to 652.9 mg/g, which is as fine as the mesoporous silica made via template route. Though the hydrothermal-alkali treatment is introduced in the "calcination - alkali-activation - acid-etching" route as an additional pre-processing compared with the previous "calcination-acid-selective-etching" method, the obtained mesoporous silica demonstrates highly improved nanoporous properties. And the proposed template-free strategy "calcination-alkali-activation-acid-etching" is still cost-effective, facile and friendly to the environment to some degree in comparison with the traditional template strategy.4. The novel "Calcination-zeolitization-acid-treatment" processFor the sake of further improved specific surface area of the mesoporous materials derived from kaolin, a new template-free process "calcination-zeolitization-acid-treatment" was put forward. In detail, the kaolin firstly underwent a calcination-activation at 850 ℃ and then submitted to 4 mol/L NaOH for 6 h, and then submitted to 5 mol/L HCl for 6 h. Both the "zeolitization" and acid-treatment constitute the core process. The related mechanisms can be described as follows:kaolin is transformed into metakaolin (MK) during calcination; in the following "zeolitization", MK as the main Si and Al resources are transformed into the crystalline zeolite LTA; finally, in the acid treatment, zeolite LTA is decomposed by acid-dissolution of Na+ and Al3+ with the crystalline structure and morphology completely destroyed, and the remained acid-resistant Si-O aggregates into mesoporous silica with encouraging specific surface area (750 m2/g) and pore size (4.0 nm) in irregular block-like shapes. It is confirmed that the specific surface area of the obtained mesoporous silica remains unchanged with a little drop as the alkali concentration increase, and increases with a limit as the alkali-treatment time rises. Specially, the obtained mesoporous silica always show newly-formed ink-bottle or cage-like mesopores and the optimal one is equivalent to some of those synthesized by template strategy in nanoporous property. The optimal mesoporous silica obtained also shows favorable adsorption ability toward methylene blue with the monolayer adsorption capacity up to 756.5 mg/g, which is much superior to that of the mesoporous silica made by template strategy. Compared with the "calcination-alkali-activation - acid-etching" method proposed previously, the method "calcination - zeolitization-acid-treatment" only enhances the alkali-treatment time from 0.5 to 6 h but obtains an obvious improvement in nanoporous properties of the final products. And compared with the template strategy, the newly proposed "calcination-zeolitization-acid-treatment" method is still cost-effective, facile and friendly to the environment.5. The novel "Calcination-free-alkali-treatment-acid-treatment" processIt would not be ignored that the thermal-activation of kaolin is essential to all the template-free strategies proposed above, i.e. "calcination-selective-acid/alkali-etching", "calcination-alkali-activation-acid-etching" and "calcination-zeolitization-acid-treatment", for the purpose of weakening the acid/alkali-resistance of kaolin and promoting the reactions between kaolin and acid/alkali. However the calcination of kaolin will not only increase the energy cost but also get the template-free processes complex, both of which could hinder the application. In this case, a new template-free strategy "calcination-free-alkali-treatment-acid-treatment" was proposed. In detail, the kaolin was directly submitted to 6 mol/L NaOH at 100 ℃ for 6 h and then submitted to 5 mol/L HCl at 80 ℃ for 6 h without pre-calcination. In the alkali-treatment, kaolin is transformed into unnamed zeolite (PDF 42-0216); in the acid-treatment, the unnamed zeolite is decomposed into amorphous mesoporous silica with favorable specific surface area (691 m2/g) and pore size (3.9 nm) in irregular block-like shapes, showing ink-bottle-like mesopores. It is revealed that the specific surface area of the mesoporous silica increases with a limit as the alkali-treatment temperature, alkali concentration and alkali-treatment time increase, all of which promote the zeolitization of the unnamed zeolite. The optimal mesoporous silica herein is close to the mesoporous silica prepared by template strategy in nanoporous property. Compared with all the template-free strategies above-mentioned, the "calcination-free-alkali-treatment-acid-treatment" is more cost-effective, facile and friendly to the environment.In summary, a series of template-free strategies, i.e. "calcination-selective-acid/alkali-etching", "calcination-alkali-activation-acid-etching", "calcination-zeolitization-acid-treatment" and "calcination-free-alkali-treatment-acid-treatment" have been put forward to use kaolin as the only raw material to prepare mesoporous silica/alumina, on account of the shortcomings such as high cost, long-time production and environment incompatibility in the template strategy for mesoporous materials production. The optimal mesoporous silica obtain by the template-free strategies shows high specific surface area up to 750 m2/g with 4.0 nm in pore diameter, both of which is equivalent to the nanoporous properties of some mesoporous silica synthesized through template strategy. All the proposed template-free strategies are facile, cost-effective and environment compatible, because it is template-free and uses natural mineral as the only raw material. It would be meaningful not only for the large-scale industry synthesis and application of mesoporous materials, but also for the fine utilization of natural minerals.