Original Research PaperNano-porous hydrosodalite fabrication via hydrothermal modification of processed kaolin by boehmite: Aluminum source effect on physico-chemical characteristics of product
Graphical abstract
Introduction
The nano-porous hydrosodalite which is member of zeolite family has been found extended applications in the selective adsorption [1], cement mortars [2], hydrogen storage [3], catalyst [4] as well as in advance proposes like fluorescence [5], photochromic materials [6] and optic devices [7]. Hydrosodalite is known as three-dimensional array of sodalite-cages which is formed by altering SiO4 and AlO4 tetrahedrons [8]. The general formula of hydrosodalite is Na6+n(Al6Si6O24)(OH)n, mH2O. If n and m are considered to be 2 and 8, respectively, the composite is known as basic hydrosodalite, otherwise the non-basic hydrosodalite is formed when n is equal to zero [9]. The ultra-small apertures along with great ion exchange capacity create the peculiar ability to employ in the widespread applications originated from the β-cages structure able to host various type of guest species such as cations, anions or molecules [10].
Kaolin or china clay predominantly consists of kaolinite, Al2O3.2SiO2·2H2O, in which the Al2O3/SiO2 mass ratio is about 0.85 [11]. The layered structure of kaolinite provides an appropriate condition to employ it as a starting material in the zeolite fabrication [12]. The kaolin purity is of great importance in production of different industrial zeolites [13]. The commercial kaolin extracted from deposits contains kaolinite as a major phase along with other minerals, especially quartz and muscovite, which adversely influence the properties of clay, causing the unsuitability for production of zeolites [14]. Although the purification of kaolin is of prime importance to manufacture appropriate precursor for engineering employments, the beneficiation of process depends upon the quantity and nature of the impurities [15]. The ultrafine quartz grains are normally remains in the processed clays, creating some difficulties in conversion to zeolites [16]. The suitable Al2O3/SiO2 molar ratio for hydrosodalite synthesis is about 1.0 [17]. On the other hand, the appropriate Na2O/Al2O3 molar ratio to achieve this structure should be exceeded than 3.0 [18].
Different methods were reported for fabrication of hydrosodalite from kaolin including hydrothermal route [19], high-temperature solid-state method [6], alkali-fusion technique [20] and crystal transition method [21]. The common method for manufacture of hydrosodalite from purified kaolin is based on hydrothermal reaction [22]. However, this clay mineral cannot be significantly affected by alkaline treatment, even under strong conditions, due to low reactivity. Therefore, kaolinite should be converted to a metastable phase, metakaolin, by calcination at higher temperatures, > 550 °C, to produce a more reactive phase under hydrothermal treatment [23].
When the purified kaolin is available, the production of zeolites from poor kaolin is of great importance due to renewable, widely available and inexpensive resource. By considering the reported constraints, the direct conversion of poor kaolin to hydrosodalite is impossible due to lack of alumina. Because of difficulty in the separation of quartz from poor kaolin, which causes the reduction in Al2O3/SiO2 ratio below 0.85, the compensation of alumina from other resources like sodium aluminate and boehmite is unavoidable. Sodium aluminate, NaAlO2, which is an inorganic chemical, is employed as an effective resource of aluminum for many technical applications [24]. The commercial sodium aluminate is available as a solution or a solid powder which is extensively used in zeolite synthesis by hydrothermal and sol–gel routes [25]. Boehmite, AlOOH, which is the precursor of most important industrial materials such as γ-Al2O3, widely used as a catalyst support in refining, reforming, isomerization, hydro-treatment, and hydro-conversion of oil chemicals [26]. Boehmite, especially in nano-sized form, exhibits a unique dispersibility, causing excellent property for use in the colloidal systems [27], [28].
It seems that the both of mentioned aluminum resources could provide the AlO4 groups for complete conversion of poor kaolin to hydrosodalite as valuable material. To determine the appropriate aluminum resource for fabrication of nano-porous hydrosodalite from poor kaolin, the characterization of physico-chemical properties is required. It should be noted that the addition of aluminum resources into the starting material to prepare hydrosodalite from poor kaolin is the secondary problem of process. Based on the literature review, the information is needed regarding the reaction between the metakaolin containing quartz and aluminum sources like boehmite or sodium aluminate. Hence, this research was aimed to detail the relation between the crystallinity, crystallite size and particle morphology of hydrosodalite with the inorganic additives used in the hydrothermal alkali reaction for fabrication of nano-porous hydrosodalite.
Section snippets
Materials
The processed kaolin manufactured by Iranian China Clay Company was employed as aluminosilicate precursor to prepare hydrosodalite. To examine the effect of aluminum source on product structure, the sodium aluminate was supplied by Behtaj Company, Iran. The pure boehmite, having a AlOOH grade higher than 99.8%, was purchased from Mineral Research Center of West Country in Iran. Methylene blue powder (MB, 6045, Merck, Germany) were provided to prepare dye solution stock. The deionized water was
Evaluation of structure of starting materials
The mineralogical components of selected kaolin determined based on XRD diffractogram are represented in Fig. 2, indicating the characteristic peaks of kaolinite, quartz and calcite, in which the dominant mineral is kaolinite and negligible content of calcite is observable. Quartz is considered as a main impurity compared to calcite, according to the results obtained by chemical analysis and mineralogical determination. The Al2O3/SiO2 mass ratio of used kaolin is significantly lower than that
Conclusions
The nano-porous hydrosodalite can be fabricated through the modification of natural kaolin, as a cheap precursor, in association with appropriate aluminum sources. Not only the higher crystallinity is preferable for engineering proposes but also the specific surface area is a determining factor. Although there is a direct relation between crystallinity and methylene blue adsorption, it was proven that the MB titration can be taken into account to quantify the hydrosodalite porosity. The
Acknowledgement
The authors thank the Sahand University of Technology (SUT) for supporting this investigation under a Project No: 30-22438.
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