A Quasi-Solid-Phase Approach to Activate Natural Minerals for Zeolite

Research Article pubs.acs.org/journal/ascecg

A Quasi-Solid-Phase Approach to Activate Natural Minerals for Zeolite Synthesis Jinbiao Yang,†,§ Haiyan Liu,†,§ Haiju Diao,† Bingshuang Li,† Yuanyuan Yue,‡ and Xiaojun Bao*,‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, No. 18 Fuxue Road, Changping District, Beijing 102249, P. R. China ‡ National Engineering Research Center of Chemical Fertilizer, College of Chemical Engineering, Fuzhou University, No. 2 Xueyuan Road, Fuzhou 350116, P. R. China S Supporting Information *

ABSTRACT: Synthesis of zeolites or zeolite/clay composites from natural aluminosilicate minerals has received extensive attention because of its great usefulness in greening the zeolite manufacturing process, in which effective activation of natural aluminosilicate minerals is crucially important. Herein, we present an energy saving and green approach, the quasi-solid-phase activation method, to efficiently destruct the natural minerals under the conditions of a temperature as low as 100 °C and an activation time as short as 30 min. Our strategy consists of the following three steps: (1) preparation of a mixture of a natural kaolin mineral, NaOH, and water by kneading, (2) extruding of the mixture into sticks, and (3) low-temperature calcination of the stick, featured by the combined use of mechanochemical actions. The results showed that 84.7% Si species and 69.0% Al species in the kaolin mineral underwent a large degree of depolymerization in the kneading and extruding steps, and following calcination at 100 °C resulted in the complete depolymerization of the kaolin mineral to monomer orthosilicate anions (Q0) and tetracoordinated aluminum (AlIV) species, which are highly active silica and alumina sources for synthesizing aluminosilicate zeolites. Using the activated kaolin mineral as a starting material, pure-phase NaY and NaA zeolites have been successfully synthesized. KEYWORDS: Kaolin mineral, Quasi-solid-phase activation, Mechanical treatment, Low-temperature calcination, NaA, NaY

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INTRODUCTION Zeolites are crystalline aluminosilicates of alkaline or alkaline earth metals with uniform pore size. The frameworks of zeolites are composed of [SiO4]4− and [AlO4]5− tetrahedra connected via shared oxygen atoms that form different three-dimensional network structures. In the past half century, zeolites have been heavily used in the chemical and petrochemical industries as catalysis, adsorption, and ion exchange materials.1−3 At present, the main raw materials for commercially synthesizing aluminosilicate zeolites are silicon- and aluminum-containing chemicals such as aluminum sulfate and water glass.4 Although the various synthesis methods based on these chemicals have matured in terms of both process technology and product quality, they are facing great challenges for their sustainable development. On the one hand, the conventional chemicals such as water glass and aluminum sulfate for synthesizing zeolites are usually obtained from natural quartz and bauxite minerals via complicated reaction and separation processes which are associated with huge energy and material consumptions and waste emissions.5,6 On the other hand, the increasing demands for various zeolites due to their increasingly wide applications require significant reduction in synthesis cost.7,8 This situation calls for alternative feedstocks whose manufac© XXXX American Chemical Society

ture is green yet cost effective. Since the pioneering work of Haden et al.9 who for the first time synthesized zeolite NaY using kaolin, a 1:1 type aluminosilicate clay mineral10 of composition Al4Si4O10(OH)8, numerous efforts have been devoted and promising results have been achieved. It has been recognized that the key to succeeding in synthesizing highquality zeolites or zeolite/clay composites from natural aluminosilicate clay minerals is their effective activation, i.e., the transformation of an aluminosilicate clay mineral to an X-ray amorphous product that provides part or all of active SiO2 and Al2O3 species which can be leached by acidic or basic solutions and contribute Si and Al species for zeolite synthesis. Most natural aluminosilicate clay minerals have highly stable crystalline structures and are intrinsically inactive, so their activation usually involves the use of thermal treatment at high temperature or highly reactive medium or mechanical treatment to destroy their crystalline structures. However, the conventional thermal calcination usually conducted at a temperature as high 600−1000 °C is energy-intensive, even so it can Received: December 12, 2016 Revised: February 2, 2017 Published: February 13, 2017 A

DOI: 10.1021/acssuschemeng.6b03031 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering only destruct part of the aluminum−oxygen bonds in natural aluminosilicate minerals such as kaolin, with silicon−oxygen bonds being intact.11,12 While the recently developed alkali fusion (AF) method could effectively destruct both AlO6 octahedron and SiO4 tetrahedron sheets in aluminosilicate minerals,13,14 it also needs a temperature as high as 600 °C, an energy-consuming process. Mechanical treatment in grinding devices can produce the structural breakdown of aluminosilicate minerals and change them into amorphous substances, but unfortunately, such a mechanic treatment process usually needs dozens or even hundreds of hours and usually gives an incomplete depolymerized product similar to the thermal calcination product.15,16 In previous work, we proposed a unique high-concentration alkali solution (HCAS) activation method that can activate aluminosilicate minerals such as kaolin and rectorite at a temperature as low as 250 °C.17−20 The as-synthesized hierarchical ZSM-5 zeolites from activated rectorite showed significantly higher activity than the conventional ones. The increased activity was ascribed to the improved accessibility of the active sites and better diffusion properties of its micromesoporous structure.21−23 While having significantly decreased the activation temperature compared with the conventional thermal activation and AF activation methods, the HCAS activation method involves the use of a highconcentration alkali solution with a molar ratio of H2O to kaolin at about 110, so a large amount of water needs to be evaporated, incurring additional energy consumption and wastewater emission. More importantly, the HCAS activation is conducted in a batch mode and thus is unsuitable for largescale industrial production. With the aim of developing a highly efficient and greener method to activate natural aluminosilicate clay minerals, here we propose a quasi-solid-phase (QSP) activation method. Compared with the HCAS method we previously developed, the QSP method can be performed in a continuous mode under much milder conditions and only use a small amount of water, thus presenting itself as an energy-saving and greener activation process. Using a QSP-activated kaolin as a starting material, pure-phase zeolites NaA and NaY are successfully synthesized.

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Scheme 1. Schematic Illustration of Quasi-Solid-Phase Activation Processa

a Ext-KNH and QSP-100-3 denote kaolin−NaOH−H2O extrudates with molar ratio of H2O to kaolin of 3 and its calcined product at 100 °C, respectively.

powder-form sample designated as QSP-T-R. In the designation of the above samples, K, N, H, T, and R stand for kaolin, NaOH, H2O, heating temperature, and molar ratio of H2O to kaolin, respectively, and Mix, Ext, and QSP denote mixture, extrudate, and QSP activation, respectively. For instance, sample QSP-100-3 was prepared by sequentially mixing 50 g of the raw kaolin mineral, 75 g of NaOH, and 10.2 mL of H2O, then kneading the above mixture for 3 min, extruding the kneaded mixture into sticks of 1.5 mm in diameter and 2−3 cm in length, heating the sticks at 100 °C for 30 min in a belttype oven, and crushing the dried sticks into particles of 80 mesh in size. For comparison purposes, a HCAS-activated sample, denoted as HCA-200-111, having a molar ratio of NaOH to kaolin identical to QSP-100-3, was obtained by first mixing 50 g of the raw kaolin mineral, 75 g of NaOH, and 375 mL of water in an open-top stainless steel crucible and treating the resulting mixture at 200 °C for 6 h with air recirculation.17 The thermally treated diatomite was obtained by calcinating the raw diatomite mineral at 600 °C for 4 h in an oven with air recirculation. Synthesis of Zeolites NaY and NaA. To synthesize zeolite NaY, a seed solution was first prepared by adding sodium hydroxide, aluminum sulfate, and water glass into deionized water in a molar ratio of 19 Na2O:1 Al2O3:18.5 SiO2:330 H2O, and after gentle agitation for 2 h, this solution was aged for 2 days at room temperature. A typical synthesis procedure of zeolite NaY is as follows. First, 5 g of QSP-100-3, 6.8 g of the thermally treated diatomite, and 6.3 g of the seed solution were added into 48 g deionized water to obtain a mixture. Then, the mixture was aged at 60 °C for 12 h and transferred into a Teflon-lined stainless steel autoclave for crystallization at 100 °C for 24 h. Finally, a crystallization product QSP-Y was collected by filtrating and drying at 100 °C overnight. For comparison, another NaY zeolite HCA-Y was also synthesized by using 5 g of HCA-200-111 to replace 5 g of QSP-100-3, with the other conditions being the same as described above. A typical procedure for synthesizing zeolite NaA is as follows. First, 5 g of QSP-100-3 was added into 48 g of deionized water to obtain a mixture. Second, the mixture was aged at 50 °C for 8 h and then transferred into a Teflon-lined stainless steel autoclave for crystallization at 95 °C for 2.5 h. Third, the solid product QSP-A was collected by filtrating and drying at 100 °C overnight. For comparison, another NaA zeolite HCA-A was also prepared by using 5 g of HCA200-111 to replace 5 g of QSP-100-3, with the other conditions being the same. Detailed information on the characterizations and the preparation and catalytic test of the zeolite Y-derived catalysts can be found in the Supporting Information.

EXPERIMENTAL SECTION

Materials. The well-crystallized raw kaolin mineral with a Hinckley index of 1.224,25 used in the present work was purchased from China Kaolin Clay Company, Ltd. (Jiangsu Province, PR China). The raw diatomite mineral used to adjust the molar SiO2/Al2O3 ratio of the synthesis system was purchased from Qingdao Chuanyi Diatomite Company, Ltd. (Shandong Province, PR China). The chemical compositions of the kaolin and diatomite minerals in term of oxides are given in Table S1 in the Supporting Information. Sodium hydroxide (96.0 wt % NaOH), aluminum sulfate (99.0 wt % Al2(SO4)3·18H2O), and water glass (27.6 wt % SiO2) were purchased from the market and were used without further purification. Commercial zeolites NaA, NaY, and HZSM-5 were purchased from Catalyst Plant of Nankai University (Tianjin, PR China). QSP Activation of the Kaolin Mineral and Thermal Activation of the Diatomite Mineral. As illustrated in Scheme 1, the QSP activation of the kaolin mineral includes the following steps: (1) A certain amount of the raw kaolin mineral is kneaded with NaOH, yielding a mixture designated as Mix-KN, which is further kneaded with water to form a mixture designated as Mix-KNH that has the molar ratio of NaOH, kaolin, and H2O in Mix-KNH at 10:1:3. (2) Mix-KNH is extruded to kaolin−NaOH−H2O extrudates designated as Ext-KNH. (3) The extrudates are heated at 60−100 °C for a certain length of time under air recirculation and crushed into a B


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RESULTS AND DISCUSSION Effects of QSP Activation Temperature on Properties of Activated Kaolins. High-Angle XRD Characterization. The high-angle XRD patterns of the raw kaolin mineral and the activated samples are given in Figure 1A. It can be clearly seen

It is known that silicates have complex structures and low symmetry, as reflected by their complicated FTIR spectra. Despite this, FTIR spectra of different silicate minerals share one common feature, i.e., there exist two strong Si−O absorption bands at 1200−850 and 500−400 cm−1. With the increasing polymerization degree of Si in silicates, i.e., with the transformation of silicates from ortho-form to cyclo-, ino-, layer-, and finally to tecto-form, the strong absorption bands gradually shift to higher frequency.27 Compared to those in the spectrum of the raw kaolin mineral, the adsorption band in the region of 1150−950 cm−1 for the QSP-activated kaolin samples shifts to a lower frequency due to the depolymerization of layered silicates originally existing in the raw kaolin mineral; particularly, the bands at 1009, 1038, and 1115 cm−1 in the FTIR spectrum of QSP-100-3 dramatically shift to 960, 1012, and 1046 cm−1. It is also noted that, in the FTIR spectra of QSP-60-3, QSP-70-3, and QSP-80-3 that were obtained by activation at temperatures no higher than 80 °C, the bands at 431, 470, and 696 cm−1 attributed to the Si−O bending vibrations in the kaolinite structure are still visible, indicating the existence of the kaolinite structure, but they disappear in the spectra of the samples activated at temperatures higher than 90 °C.28 These results are consistent with the XRD characterization results. Furthermore, we can see that the bands at 794, 914, and 935 cm−1 assigned to (AlVI−O)−H and at 540 cm−1 assigned to AlVI−O in octahedral Al[O(OH)]6 units appearing in the FTIR spectrum of the raw kaolin mineral completely disappear, and new bands at 860 and 879 cm−1 assigned to AlIV−O appear in the spectra of the QSP-activated products. This suggests that, after the activation at temperatures higher than 60 °C, the AlVI species in the raw kaolin mineral have been completely transformed into AlIV species in the activated products. It is also noticed that QSP-100-3 and HCA-200-111 have almost the same FTIR absorption bands but with different absorption strengths. The presence of the stronger bands at 431, 470, and 696 cm−1 ascribed to Si−O bending vibrations and at 540 cm−1 to AlVI−O bending vibrations indicates the existence of a small amount of kaolinite structure in HCA-200111, thus incomplete depolymerization of the kaolin mineral in the HCAS activation. 29 Si and 27Al MAS NMR Characterizations. The 29Si and 27 Al MAS NMR spectra of the raw kaolin mineral and its activated products are given in Figure 1C and D. It is known that the NMR technique can provide important information on the structure and arrangement of natural aluminosilicate minerals because the chemical shifts of Si and Al atoms in aluminosilicates are sensitive to their chemical and structural surroundings. For the presentation of the structure of building units or silicate anions in aluminosilicates, the Qn notation is widely adopted. In this notation, Q represents a silicon atom bonded to four oxygen atoms forming a tetrahedron; the superscript n indicates the connectivity, i.e. the number of other Q units attached to the SiO4 tetrahedron. Thus, Q0 denotes a monomer orthosilicate anion SiO44−, Q1 end groups in chains, Q2 middle groups in chains or rings, Q3 chain branching sites, and Q4 three-dimensionally cross-linked groups. For aluminosilicates, the number of AlO4 tetrahedra bound to the central silicon of a Qn unit is given in parentheses; e.g., Qn(mAl) signifies a SiO4 group connected via oxygen bridges to m Al and other n−m Si atoms, where n = 0−4 and m ≤ n.29,30 From Figure 1C, it can be seen that the 29Si MAS NMR spectrum of the raw kaolin mineral has a single peak at −90.8 ppm,

Figure 1. (A) High-angle XRD patterns, (B) FTIR spectra, and (C) 29Si and (D) 27Al MAS NMR spectra of the raw kaolin mineral and the different activated samples. (a) Raw kaolin mineral, (b): QSP60-3, (c) QSP-70-3, (d) QSP-80-3, (e) QSP-90-3, (f) QSP-100-3, and (g) HCA-200-111. The asterisks in the figure designate the spinning side bands.

that the two prominent peaks at 2θ = 12.3° and 2θ = 24.9° attributed to the kaolinite phase in the XRD pattern of the raw kaolin mineral are absent in those of the samples after the QSP activation at temperatures higher than 80 °C, demonstrating the collapse of the kaolinite structure. It is noted that, in the XRD patterns of the sample activated at temperatures between 60 and 100 °C, the intensity of the peaks at 2θ = 30.1° and 35.2° ascribed to NaOH decreases with the increasing activation temperature, but the two peaks are still discernible because of the excessive addition of NaOH than the stoichiometric ratio. Meanwhile, the intensity of those peaks at 2θ = 29.8°, 30.9°, 34.8°, and 35.6° becomes stronger indicating the increased content of highly active aluminosilicate species in the QSP-activated samples. When the activation temperature is 100 °C, the high-angle XRD pattern of the resultant sample QSP-100-3 is exactly consistent with that of HCA-200-111. This demonstrates that QSP-100-3 which was obtained by the QSP activation at 100 °C has the same phase structure as HCA-200-111 which was obtained through the HCAS activation at 200 °C. FTIR Characterization. Figure 1B illustrates the FTIR spectra of the raw kaolin mineral and its derived products. In the spectrum of the raw kaolin mineral, there exists one strong peak at 3696 cm−1 ascribed to the stretching vibration of “inner surface hydroxyls” that are located on the surface of octahedral sheets opposite to the tetrahedral oxygens of the adjacent kaolinite layer, whereas another strong peak at 3619 cm−1 is related to the stretching vibration of “inner hydroxyls” and refers to OH groups located in the plane common to octahedral and tetrahedral sheets.26 With the removal of these hydroxyl groups in the kaolin mineral after activation, the corresponding absorption peaks disappeared in the FTIR spectra of the activated products. C


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ACS Sustainable Chemistry & Engineering attributed to Si species in the Q3(0Al) environment and typical for layer-type silicates;30 in the 29Si MAS NMR spectrum of QSP-60-3 obtained by the QSP activation at 60 °C, this peak shifts to −85.8 ppm, suggesting that the Si species in QSP-60-3 have been depolymerized from the Q3 structure into the Q2 structure. Interestingly, in the 29Si MAS NMR spectrum of the sample obtained by the QSP activation at 80 °C, there are two small peaks at −81.2 and −77.3 ppm assigned to Q1 Si species and one peak at −69.5 ppm assigned to Q0 Si species. The comparison of the spectra of the different products activated at different temperatures shows that, with the increasing activation temperature, the peak at −72.1 ppm gradually becomes stronger and the others gradually become weaker and finally disappear, indicating the increased Q0 Si species in the activated products. Importantly, in the spectra of QSP-100-3 and HCA-200-111, there is only a single resonance peak assigned to Q0 Si species, indicating that all of the Q3 Si species in the raw kaolin mineral have been depolymerized into Q0 Si species, i.e., monomer orthosilicate anions (SiO44−) that have the highest reactivity and therefore are the ideal silica source for zeolite synthesis.31 Figure 1D shows that the 27Al MAS NMR spectrum of the raw kaolin mineral has a single peak at 1.2 ppm corresponding to AlVI species.30 In the spectrum of QSP-60-3 activated at 60 °C, this resonance peak shifts to higher frequency (δ = 63.6 ppm), suggesting that the Al species in QSP-60-3 are AlIV ones and are in good agreement with the FTIR characterization results. Compared to the Si species, the Al species can be depolymerized at lower temperature because Al−O bonds are weaker than Si−O bonds. With the increasing activation temperature, the peak width of the AlIV species signal increases, possibly due to the presence of distorted AlIV.32 Meanwhile, a small peak at 9.6 ppm assigned to AlVI atoms is observed in the spectra of the samples obtained at lower activation temperatures, but it is absent in the spectra of the samples at higher activation temperatures (e.g., 100 °C). On the basis of the above analyses, we can conclude that, after the QSP activation at 100 °C, the raw kaolin mineral suffers from complete structure collapse, resulting in the formation of monomer orthosilicate anions and AlIV species in the activated product. Al 2p, Si 2p, and O 1s XPS Characterizations. Figure 2a−c shows the XPS spectra of the raw kaolin mineral and the different activated samples. In the XPS spectra of the raw kaolin mineral, the Al 2p peak with a binding energy of 74.5 eV is assigned to the AlVI species,33 the Si 2p peak with a binding energy of 102.8 eV is typical for layer-type silicates,34 and the O 1s peak with a binding energy of 531.8 eV is characteristic of bridging oxygen atoms (BOs).35 In the spectra of HCA-200-111 and QSP-100-3, both the Al 2p and Si 2p peaks shift toward lower binding energies because of the lower polymerization degree of the Al and Si species,34 suggesting that octahedral sheets and tetrahedral sheets have been depolymerized into lowly polymerized aluminosilicates. Specifically, the Al 2p peak at 73.4 eV can be assigned to the AlIV species, and the Si 2p peak at 101.3 eV can be assigned to Q0 species.33,34 In addition, compared to those in HCA-200-111, the binding energies of the silicon and aluminum species in QSP-100-3 are both lower, suggesting that the depolymerization degree of the kaolin mineral achieved via the QSP activation at 100 °C is higher than via the HCAS activation at 200 °C.

Figure 2. (a) Al 2p XPS spectra, (b) Si 2p XPS spectra, and (c) O 1s XPS spectra of the raw kaolin mineral and samples obtained via the different activation methods.

The O 1s spectra can be employed to study the structure of clay minerals36,37 and their activated products, which is especially useful for distinguishing BOs and nonbridging oxygen atoms (NBOs), with NBOs usually having lower binding energies than BOs. The O 1s spectra can be divided into four parts: NBOs, BOs, hydroxide species, and bound water with binding energies of 530−530.5, 531.5−532.7, 533− 533.5, and 534 eV, respectively.35 Therefore, the shift of the O 1s peak from 531.8 eV for the raw kaolin mineral to 530.7 eV for HCA-200-111 and to 530.4 eV for QSP-100-3 indicates the different degrees of transformation of BOs in the raw kaolin mineral to NBOs in the different activated samples. In QSP100-3, all of the BOs originally existing in the raw kaolin mineral have been broken to NBOs; in HCA-200-111, however, there still exists a small number of BOs because their binding energy (530.7 eV) is not in the range of NBOs (530− 530.5 eV). Therefore, this demonstrates again that a higher depolymerization degree of the raw kaolin mineral can be achieved by the QSP activation at a dramatically lower temperature (100 °C) than by the HCAS activation that needs a temperature of at least 200 °C. FESEM and HRTEM Characterizations. The FESEM images of the raw kaolin mineral, HCA-200-111 and QSP-100-3 and the HRTEM image of QSP-100-3 are displayed in Figure 3. The raw kaolin mineral exists typically in the form of pseudohexagonal granules accompanied by some plates, larger books, and vermicular stacks (Figure 3a). Figure 3b shows that the crushed QSP-100-3 granules exhibit a typical “ant nest”-like structure consisting of particles of 50−200 nm in size; more importantly, Figure 3c illustrates that in the crushed QSP-100-3 granules there exist abundant mesopores of ca. 5−30 nm in size which are formed possibly due to water evaporation during the QSP activation. This suggests that the QSP activation can produce large amounts of mesopores in the resultant products and thus make it possible to synthesize hierarchical zeolites or zeolite/clay composites via the so-called “pseudo-morphic crystallization”38 or “quasi-solid-state”21−23 crystallization routes without using any organic mesoporous template. Figure 3d shows that the crushed HCA-200-111 granules consist of particles of the size ca. 5 μm, dramatically larger than the size of the particles in the crushed QSP-100-3 granules. D


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Figure 3. FESEM images of the raw kaolin mineral (a) QSP-100-3 and (b) HCA-00-111. (d) HRTEM image of QSP-100-3 (c). (e) N2 adsorption− desorption isotherms and (f) low-angle XRD patterns of the samples obtained via the different activation methods. Inset of (e) shows the pore size distribution curves of QSP-100-3 and HCA-200-111.

Syntheses of Zeolites from Activated Kaolin Minerals. Synthesis of NaY. To compare the reactivity of the kaolin minerals activated by the two methods, two NaY zeolites were hydrothermally synthesized using QSP-100-3 and HCA-200111 as the main provider of alumina source and part provider of silica source, respectively. The silica-rich diatomite mineral was selected as a makeup silica source of the synthesis system. Figure 4a compares the XRD patterns of the two as-synthesized products QSP-Y and HCA-Y and a commercial NaY zeolite used as the reference. It can be seen that the diffraction peaks in all of the three patterns are exclusively characteristic of the typical FAU structure, evidencing that the as-synthesized samples are all pure-phase NaY zeolites. The relative crystallinities of QSP-Y and HCA-Y are 91% and 86% (Table S2), respectively. From the N2 adsorption−desorption isotherms of the three samples shown in Figure 4b, it can be seen that the isotherms of QSP-Y have a hysteresis loop owing to the capillary condensation of N2 within the relative pressure P/P0 range from 0.4 to 0.9, indicating the coexistence of micro- and mesopores. From the desorption branch of the isotherms, the size of mesopores in QSP-Y was estimated to be 10−50 nm by using the BJH method (inset, Figure 4b). Compared to HCA-Y (Sext = 42 m2/g and Vmeso = 0.05 cm3/g) and the commercial Y (Sext = 34 m2/g and Vmeso = 0.02 cm3/g), QSP-Y has the largest

N2 Adsorption−Desorption Isotherms and Low-Angle XRD Characterizations. The N2 adsorption−desorption isotherms of QSP-100-3 and HCA-200-111 are shown in Figure 3e. QSP-100-3 has a dramatically larger BET surface area (4.8 m2/g) and a significantly larger pore volume (0.04 cm3/g) than HCA200-111 (0.4 m2/g and 0.01 cm3/g) owing to the presence of mesopores in the former. From the desorption branch of the isotherm, the size of the mesopores in QSP-100-3 was estimated to be 30 nm by the Barret−Joyner−Halenda (BJH) method (inset, Figure 3e); also, the low-angle XRD pattern (Figure 3f) of QSP-100-3 has a sharp peak at 0.9° and a broad peak at 1.5°, also indicating the presence of mesopores with a hole-like pore structure.39 Using the material balance and energy consumption analysis method proposed by Li et al.,17 it was estimated that the QSP activation method has an atom economy efficiency40 of 74.72% (Table 1), much higher than that (20.02%) of the HCAS activation method, and further analyses on energy consumption41 also reveal that the QSP activation method is significantly advantageous over the HCAS activation method, as shown in Table 1. So far, we can conclude that the QSP activation is more effective, more energy saving, and more atom efficient for depolymerizing Si and Al species in the raw kaolin mineral to active silicon- and aluminum-species for zeolite synthesis. E


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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Theoretical Energy Consumptions and Atom Economies of Two Activation Methods material and energy consumptions/mol kaolin entry

item

1 2 3 4 5

raw materials kaolin (kg) NaOH (kg) H2O (kg) energy consumption and atom economy energy consumption of reactiona (kJ) actual energy consumptionb (kWh) atom economyc (%)

6 7 8

QSP activation

HCAS activation

0.258 0.400 0.054

0.258 0.400 2.000

81.179

725.976

≈1.25

≈6.5

74.72

20.02

a

The calculation of energy consumptions of the QSP and HCAS activations is based on the data of Barin41 and Li et al.17 bThe calculation of actual energy consumptions of the QSP and HCAS activations is based on the experimental results of 500 g kaolin/per batch. The rated powers of the kneader, extruder, and oven are 2.0, 2.0, and 1.5 kW, respectively. cAtom economy is calculated by dividing the molecular weight of the desired product by the sum of the molecular weights of all substances produced in the stoichiometric equation.40

Figure 5. FESEM images of QSP-Y (a) and HCA-Y (b). (c and d) HRTEM images of QSP-Y.

their size narrowly distributed between 300 and 500 nm, significantly smaller than those of the commercial one (Figure S2). This suggests that QSP-Y should contain intercrystalline mesopores which are actually the interstitial spaces among the stacked crystallites. This inference is also supported by the N2 adsorption−desorption results (Figure 4b and Table S2). The formation of the bimodal micromesoporous structure of QSP-Y can be attributed to the smaller size and “ant nest”like structure of the QSP activated product, which plays a role similar to “crystalline seeds”18,23 in the synthesis system. In comparison, HCA-Y is in the form of particles of the size ca. 700 nm with irregular morphology and contains a certain amount of phase impurities typically some unreacted mineral flakes, as clearly observed in the areas marked by red circles in Figure 5b. Furthermore, the HRTEM images in Figure 5c and d clearly show that the lattice fringes run through the entire crystal, demonstrating the high crystallinity of QSP-Y. To investigate the industrial potential of QSP-Y as the main active component of the FCC catalyst, catalytic cracking tests were conducted in a laboratory-scale confined fluidized-bed reactor using an industrial FCC feedstock Xinjiang vacuum oil whose properties are given in Table S3. The testing method is fully in compliance with the ASTM D-3907 standard. Table 2 summarizes the catalytic results of the three model catalysts prepared from the as-synthesized and commercial Y zeolites. It can be seen that, among the three model catalysts, the QSP-Y derived catalyst showed the best catalytic performance in terms of the yields of liquid, dry gas, and coke, with the HCA-200derived catalyst standing in the middle of the three model catalysts. It is interesting to note that, despite their slightly lower crystallinity, the two zeolites synthesized from the natural aluminosilicate minerals make model FCC catalysts superior to that derived from the commercial zeolite Y that has high crystallinity. This can be explained by the shorter diffusion path and more accessible acid sites (due to the smaller crystal sizes, larger external specific surface areas, and mesopore volumes) of the two aluminosilicate mineral-derived zeolites, as reported in the previous research.17,22 Synthesis of NaA. By using QSP-100-3 as the sole starting materials, we have also successively synthesized zeolite NaA which is now widely used as adsorption materials in the

Figure 4. (a) High-angle XRD patterns, (b) N2 adsorption−desorption isotherms, and (c) 27Al MAS NMR spectra of the commercial NaY zeolite and the NaY zeolites synthesized from the different activated products. (d) 29Si MAS NMR spectra of QSP-Y. Inset of (b) shows the pore size distribution curves.

external surface area (Sext = 51 m2/g) and mesopore volume (Vmeso = 0.08 cm3/g) (Table S2). Figure 4c shows that the27Al MAS NMR spectra of QSP-Y, HCA-Y, and the commercial NaY zeolite have no other peak except for the strong peak at a chemical shift of ca. 61 ppm ascribed to AlIV, thus indicating no extra framework aluminum species (AlVI) in all of the three samples. Besides, the NH3TPD results indicate that the two zeolites synthesized from the natural minerals have slightly stronger acid strength than the commercial one (Figure S1). Furthermore, the framework SiO2/Al2O3 molar ratio of QSP-Y calculated from the 29Si MAS NMR spectrum (Figure 4d) is 5.03, in good agreement with that (5.33) obtained by XRF (Table S1). The FESEM image in Figure 5a shows that QSP-Y is in the form of stacks consisting of well-defined octahedral crystallites with F


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of the (001) plane in the kaolinite structure and thus the collapse of the layer structure. Therefore, in the XRD pattern (Figure 6a and b) of the mixture (sample Mix-KNH) of the raw

Table 2. Product Distributions of Heavy Oil Cracking over Various Model FCC Catalysts 1 2 3 4 5 6 7 8 9

catalyst

QSP-Cat

HCA-Cat

Com-Cat

product yield (wt %) dry gas liquefied petroleum gas (LPG) gasoline diesel heavy oil coke liquid yielda (wt %) conversionb (wt %)

5.34 22.00 42.98 13.32 10.35 4.46 78.30 76.33

5.77 22.80 41.42 13.44 10.45 4.66 77.66 76.11

6.45 22.28 40.78 12.73 11.25 4.94 75.79 76.02

a The liquid yield is defined as LGP yield% + gasoline yield% + diesel yield%, with gasoline being the fraction of boiling point between 30 and 210 °C, diesel being the fraction with boiling point between 210 and 350 °C, and heavy oil being the fraction with boiling point higher than 350 °C. bThe conversion is defined as (100 − diesel yield% − heavy oil yield%)/100.

petrochemical industry. By controlling the feeding proportion of the synthesis system, a pure-phase zeolite NaA sample QSP-A (Figure S3) with a relative crystallinity of 94% was obtained under the conventional synthesis conditions. The chemical composition of QSP-A is similar to that of the commercial NaA zeolite (Table S1). As shown in Figure S4a, the particles of QSP-A have a perfect cube morphology with uniform size of ca. 1 μm. Compared to QSP-A, HCA-A derived from HCA-200-111 has a much lower relative crystallinity (85%) (Figure S3) because of the existence of a significant amount of unreacted kaolin debris and presents an irregular morphology (Figure S4b). This indicates the poorer depolymerization ability of the HCAS activation method than the QSP one. Up to now, we can ensure that a greener pathway to activate natural aluminosilicate minerals for zeolite synthesis has been established, which has led to the successful synthesis of two important zeolites NaY and NaA. Mechanistic Analyses of QSP Activation Process. The above results demonstrate that, compared with the HCAS activation method previously reported, the QSP activation method we present here can dramatically decrease the activation temperature, significantly reduce the water usage, and substantially shorten the activation time, and the resulting activated product can provide highly reactive silica- and aluminum-species for zeolite synthesis. This raises the question of why the QSP method can effectively destruct the kaolinite structure under the conditions of much lower temperature and shorter activation time than the HCAS method? To answer this question, systematic characterizations were made to analyze the intermediates during the different steps of the QSP activation process. The Kneading Step of Kaolin, NaOH, and H2O. It is well known that kaolinite is a two-layer-structured aluminosilicate, consisting of alternating layers of one tetrahedral sheet of SiO4 covalently bound together through common oxygen atoms to one octahedral sheet of AlO6, forming a layer-structured repeating unit. Hydrogen bonds between the hydroxyl ions of AlO6-octahedra and the oxygen atoms of SiO4-tetrahedra hold the repeating units together. Therefore, the layers are bonded together by only weak interatomic forces, and a plane parallel to the bilayers is distinguished as perfect and the only cleavage one in kaolinite. This cleavage plane corresponds to the basal (001) face of a kaolinite structure.42 When a mechanical force is exerted on a kaolin particle, it will result in the cleavage cracks

Figure 6. High-angle XRD patterns of the intermediate products at the different steps of the QSP activation. (a) 5° ≤ 2θ ≤ 50°, (b) 18° ≤ 2θ ≤ 24°, 29Si MAS NMR spectra of Mix-KNH (c) and Ext-KNH (e), and 27Al MAS NMR spectra of Mix-KNH (d) and Ext-KNH (f).

kaolin mineral, NaOH, and H2O after kneading at a pressure of 0−0.4 MPa,43 which is the typical working pressure for most industrial kneaders, all of the diffraction peaks attributed to the kaolinite phase become completely invisible. Specifically, the disappearance of the two peaks at 2θ = 12.3° and at 2θ = 24.9° ascribed to the (001) and (002) surfaces of kaolinite indicates that all hydrogen bonds between tetrahedral and octahedral sheets inside the kaolin particles have been destroyed by kneading.44,45 Meanwhile, the (020), (110), and (111) peaks in the XRD pattern, which can be used to calculate the crystallinity of a raw kaolin mineral (Hinckley index),21 also become almost invisible, signifying the transformation of the long-range ordered kaolinite structure into disordered structure.46,47 The changes in the chemical shifts of Si and Al species after kneading were obtained by NMR analysis, and the results are given in Figure 6c and d and Table 3. From the 29Si MAS NMR spectrum of Mix-KNH, it was estimated that 60.2%, 14.1%, and 8.3% of the Q3 species (δ = −90.8 ppm) are transformed to Q0 (δ = −71.6 ppm), Q1 (δ = −81.5 ppm), and Q2 (δ = −87.8 ppm) species (Figure 6c and Table 3), respectively. Similarly, the 27 Al MAS NMR spectrum of Mix-KNH (Figure 6d) shows that after kneading about 66.2% of AlVI species (δ = 1.2 ppm) in the raw kaolin mineral have been transformed to AlIV species (δ = 70.3 ppm). The above results reveal that the kneading step causes the cleavage of the kaolinite structure in the raw kaolin particles, leading to the formation of cleaved and ruptured kaolinite crystals even unit crystallites between the tetrahedral and octahedral sheets and thereby the increases specific surface G


Research Article

ACS Sustainable Chemistry & Engineering Table 3. Different Polymerization Degrees of Si and Al Species in the Different Samplesa 29

1 2 3 4 a

0

27

Si MAS NMR

1

Al MAS NMR

2

3

4

Component (%)

Q

Q

Q

Q

Q

Mix-KN Mix-KNH Ext-KNH QSP-100-0

27.9 60.2 62.8 83.7

17.3 14.1 15.5 16.3

− 8.3 6.4 −

54.8 17.4 15.3 −

− − − −

IV

AlV

AlVI

23.7 66.2 69.0 67.1

− − − 18.0

76.3 33.8 31.0 14.9

Al

The values of the different species were calculated by the peak areas.

two peaks at 2θ = 12.3° and at 2θ = 24.9° ascribed to the (100) and (001) surfaces of kaolinite are visible, and the (020), (110), and (111) peaks are much stronger than those of Mix-KNH. This indicates that addition of free water enhances the depolymerization of the kaolin mineral in the kneading step. The NMR analysis results in Figure 7a and b show that the

and distorted lattices. With the prolonged kneading time, the kaolinite crystallites gradually decompose to Si and Al species with lower polymerization degree due to the rupture of BO linkages such as Al−OH and Si−O−Al, forming NBO linkages of T−O··· where “T” indicates Si or Al and “···” indicates the electrostatic interaction between NBOs and other cations. The Extruding Step of the Kaolin−NaOH−H2O Mixture Mix-KNH. In the XRD pattern (Figure 6a) of Ext-KNH that was obtained by extruding Mix-KNH at a pressure of 1−3 MPa, the peak at 2θ = 29.8° ascribed to highly active aluminosilicate species becomes stronger compared to that of Mix-KNH. In the 29Si MAS NMR spectrum of Ext-KHN (Figure 6e), the resonance peaks of Q0, Q1, and Q2 are obviously stronger than those of Mix-KNH, with only 15.3% of Q3 being not depolymerized in Ext-KNH. The 27Al MAS NMR spectrum (Figure 6f) of Ext-KHN shows that 69.0% of AlVI species in the raw kaolin mineral have been transformed to AlIV species (δ = 70.3 ppm) after the extruding step (Table 3). These results suggest that the increased extruding pressure compared with that of the kneading step further increased the depolymerization of the kaolin mineral, with only 15.3% of Si species and 31% Al species in the kaolinite structure being preserved after kneading and extruding. From the above results, we can see that the shearing, squeezing, and mixing actions during the kneading and extruding steps can reduce the particle size and increase the specific surface of the kaolin mineral, destruct the kaolinite structure to a great degree, and enhance the contact between NaOH and the mineral particles, which benefits the complete depolymerization of the raw kaolin mineral in the following heating step. The Low-Temperature Calcination Step of Kaolin− NaOH−H2O Extrudates. The extrudates in which the kaolin mineral and NaOH are in intimate contact undergo further low-temperature calcination, during which abundant surface defects formed by kneading and extruding are easily attacked by OH− to produce the corresponding aluminosilicate species. On the one hand, the T−O···NBO linkages formed during mechanical treatments can interact with Na+ to form T−O···Na+ directly. On the other hand, the preserved BO linkages continue to react with NaOH, which can be expressed as T−O− T + (Na+···O2−···Na+) → 2T−O−···Na+, where “T” indicates Si or Al and “···” indicates the electrostatic interaction between Na+ and NBOs. The QSP-activated kaolin has much weaker bonding between one tetrahedral cation (Si or Al) and four Na+ cations and thus can hydrolyze rapidly in the synthesis system, providing highly reactive silica- and aluminum- species for zeolite synthesis.17,48 The Important Role of Water in QSP Activation. During the above three steps, the free water contained in the mixture plays a crucial role, as proved by the following experiments. From Figure 6a and b, we can see that in the XRD pattern of Mix-KN, the mixture of the raw kaolin mineral and NaOH, the

Figure 7. 29Si MAS NMR spectra of Mix-KN (a) and QSP-100−0 (c) and 27Al MAS NMR spectra of Mix-KN (b) and QSP-100−0 (d).

polymerization degree of Si and Al species in Mix-KN was almost unchanged compared to those in the raw kaolin, consistent with the XRD characterization results. Although the kaolin mineral is a relatively nonswelling clay mineral, water molecules cannot directly intercalate between the layers of kaolinite.49 Under the experimental conditions dealing with mechanical stress and high pH, however, the attractive force between the Si−O (Al−O) layer and water is much stronger than the hydrogen bond force. Therefore, in the kneading step, the water added can easily get into the space between the kaolinite unit layers as the interlayer water.50 This leads to the gradually increasing basal spacing and the continuously losing stacking of the unit layers as kneading progresses, which facilitates the destruction or structural alteration of the originally ordered kaolinite crystals. In order to understand the role of water in the heating process, no extra H2O was added throughout the entire process. That is, only the mixture of the raw kaolin and NaOH went through the same kneading, extruding, and heating steps as QSP-100-3 did, and the resulting product QSP-100-0 was characterized by NMR (Figure 7c and d). From the 29Si and 27Al MAS NMR spectra of QSP-100-0, it was calculated that the mixture contained 83.7% Q0 species, 16.3% Q1 species, 67.1% AlIV species, 18.0% pentacoordinated AlV species, and 14.9% AlVI species (Table 3). This indicates that the depolymerization degree of QSP-100-0 is significantly lower than that of H


Research Article

ACS Sustainable Chemistry & Engineering Notes

QSP-100-3. Taking into account the humidity of the raw kaolin (Figure S5) and the absorbed moisture by NaOH from the air in the process, the activation effect of kaolin without water is far worse than that with water. The reason is that without sufficient free water, the diffusion of NaOH into the kaolin mineral is very slow, and the intimate contact between the kaolin mineral can be hardly achieved. Therefore, it is necessary to add a small amount of free water in the QSP activation of kaolin. Here, it must be emphasized that adding much water will result in the formation of a slurry-like mixture, decreasing the operating pressure of the kneading and extruding steps and thereby weakening the mechanic actions. The above analyses lead to the conclusion that kneading and extruding exert fracturing and cleaving forces on the kaolin mineral in the QSP activation and play a significant role similar to that reported in the mechanochemical activation of the raw kaolin mineral,47,51,52 and the involvement of NaOH and water during the QSP activation introduces chemical effects and thus results in the complete depolymerization of the kaolin mineral. The combined use of the mechanochemical actions leads to the successful development of the QSP activation method for preparing a highly reactive feedstock from a natural kaolin mineral for zeolite synthesis.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China for providing the financial support through grants 91434206, 21506034 and 21276270.

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CONCLUSION In summary, we have successfully developed a green, highly efficient, and commercially potential method, the QSP activation method, to activate the natural kaolin mineral for zeolite synthesis purposes. The results show that with the help of mechanical forces in the kneading and extruding steps, 84.7% Si species and 69.0% Al species in the kaolin mineral were depolymerized, and the following calcination at a temperature as low as 100 °C within 30 min led to the complete depolymerization of the Si and Al species into highly reactive Q0 and AlIV species. Using the QSP-activated kaolin as the main alumina source and part of the silica source and the thermally activated diatomite as a supplementary silicon source, a purephase NaY zeolite was successfully synthesized, and the resultant FCC catalyst showed outstanding catalytic performance. Using the QSP-activated kaolin as the sole alumina and silica sources, a NaA zeolite with high crystallinity was also successfully synthesized. From the green chemistry perspective, the QSP activation method we report here provides a new route for greening the zeolite manufacturing industry and thus demonstrates a great industrial potential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03031. Characterizations, catalyst preparation, catalytic tests, Tables S1−S3, and Figures S1−S5. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax: +86 591 22865220. E-mail: [email protected]. ORCID

Xiaojun Bao: 0000-0001-7589-5409 Author Contributions §

Jinbiao Yang and Haiyan Liu contributed equally to this work. I


Research Article

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J


A Quasi-Solid-Phase Approach to Activate Natural Minerals for Zeolite

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