Synthesis of a hierarchical ZSM-5 zeolite from iron-ore tailings by a two-step method
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摘要: 以鐵尾礦為原料替代純化學試劑，采用兩步法制備含有介孔–微孔復合孔的多級孔ZSM-5分子篩。首先在介孔模板劑（CTAB）作用下合成介孔分子篩（MCM-41），然后通過固相轉換法將MCM-41晶化轉變為多級孔ZSM-5分子篩。采用X射線衍射(XRD)、掃描電鏡（SEM）、透射電鏡（TEM）和氮氣吸附脫附測試（BET）等技術對樣品進行表征。實驗結果表明，由于體系中沒有液態水相參與，成功避免了CTAB與微孔結構導向劑（TPABr）在水溶液中相互競爭，從而得到高結晶度的多級孔ZSM-5分子篩。因此，本文為鐵尾礦制備多級孔ZSM-5提供了一種全新方法。Abstract: Iron-ore tailings (IOT) are mineral waste obtained via the iron-ore mining process. This has become a very critical issue in industrial solid waste management. The stacking of IOT occupies a large area and causes serious pollution, which can harm human life. This has become a serious matter of concern for society. IOT are usually rich in SiO2 and Al2O3 and can be used as raw materials in producing zeolitic materials. They have substantial benefits from both economic and environmental perspectives. However, few studies have been reported on the synthesis of zeolite or zeolite-like materials using IOT. As a type of zeolite, ZSM-5 has a regular pore structure and high thermal and hydrothermal stability and is widely used in catalytic materials. However, the application of ZSM-5 is greatly limited in many catalytic reactions owing to the large molecules associated with its inherently small pore sizes (<1.5 nm). Introduction of a hierarchically porous structure into conventional ZSM-5 maintains the crystal structure, acidic active center, and high thermal and hydrothermal stability and accelerates the diffusion/transfer of large molecules and greatly reduces the formation of carbon residue. This prolongs the service life of the catalyst used, which is extremely desirable in catalysis. In this study, hierarchically porous ZSM-5 was prepared via a two-step process using IOT as a silica source instead of pure chemical reagents. First, mesoporous MCM-41 was synthesized using cetyltrimethylammonium bromide as a mesoporous template. Then, hierarchically porous ZSM-5 was fabricated by impregnating a structure-directing agent into the as-synthesized MCM-41, followed by a solid-phase conversion method to transform amorphous silica into a zeolite crystal. To evaluate the textural properties of the zeolites, the as-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption. The results show that phase separation between the surfactant and zeolite crystals is avoided because of the absence of the liquid-water phase during the solid-phase conversion. Therefore, the synthetic route presented herein provides a novel method for the synthesis of hierarchically porous ZSM-5 from IOT.
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圖 1 不同Na2CO3·10H2O/SiO2摩爾比HZSM-5的XRD圖譜（a）和不同堿源合成HZSM-5的XRD圖譜（b）
Figure 1. XRD patterns of HZSM-5 with different Na2CO3·10H2O/SiO2 molar ratios (a) and XRD patterns of HZSM-5 with different alkali (b)
圖 2 不同TPABr/SiO2摩爾比合成HZSM-5的XRD圖譜
Figure 2. XRD patterns of HZSM-5 with different TPABr/SiO2 molar ratios
圖 3 不同晶化時間HZSM-5的XRD圖譜。（a）小角；（b）廣角
Figure 3. XRD patterns of the HZSM-5 synthesized at different crystallization time: (a) small angle XRD; (b) wide angle XRD
圖 4 不同晶化時間HZSM-5的SEM和TEM圖
Figure 4. SEM and transmission electron microscopy images of HZSM-5 synthesized at different crystallization times
圖 5 HZSM-5的氮氣吸附–脫附曲線（a）及BJH孔徑分布（b）
Figure 5. Nitrogen adsorption–desorption isotherms (a) and the BJH adsorption pore distributions (b) of HZSM-5
表 1 IOT化學成分（質量分數）
Table 1. Chemical composition of an iron-ore tailing
% SiO2 Fe2O3 CaO MgO Al2O3 K2O Na2O SO3 P2O5 Cr2O3 68.64 7.67 3.41 2.26 13.14 3.64 0.74 0.36 0.16 0.14 
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表 2 HZSM-5的物化性質
Table 2. Physicochemical properties of the HZSM-5
Sample BET surface area/
(m2·g?1)Micropore surface area/
(m2·g?1)External surface area/
(m2·g?1)Pore volume/
(cm3·g?1)Average pore width/
nmHZSM-5-0 740.92 — — 0.689 3.72 HZSM-5-6 690.28 — — 0.642 3.49 HZSM-5-12 327.01 191.25 135.76 0.197 2.41 HZSM-5-18 336.51 220.03 116.48 0.204 2.32
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