Anionic Tuning of Zeolite Crystallization

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Anionic Tuning of Zeolite Crystallization Chang Sun†, Zhiqiang Liu†, Shuang Wang, Hao Pang, Risheng Bai, Qifei Wang, Wei Chen, Anmin Zheng, Wenfu Yan and Jihong Yu Chang Sun† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 †C. Sun and Z. Liu contributed equally to this work.Google Scholar More articles by this author , Zhiqiang Liu† State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071 †C. Sun and Z. Liu contributed equally to this work.Google Scholar More articles by this author , Shuang Wang College of Chemistry and Chemical Engineering, Henan Province Function-Oriented Porous Materials Key Laboratory, Luoyang Normal University, Luoyang 471934 Google Scholar More articles by this author , Hao Pang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Risheng Bai State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Qifei Wang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Wei Chen State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071 Google Scholar More articles by this author , Anmin Zheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071 Google Scholar More articles by this author , Wenfu Yan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Jihong Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000558 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Zeolites are of great industrial relevance as catalysts, adsorbents, and ion-exchangers and typically synthesized under hydrothermal conditions. Rational regulation of their crystallization process is of great significance for zeolite production. In this work, we systematically investigate the role of anions in tuning zeolite crystallization via anion introduction including SO42−, F−, Cl−, Br−, I−, and SCN− in the sodium form into the SiO2–TPAOH–H2O [tetrapropylammonium hydroxide (TPAOH)] synthetic system of silicalite-1 zeolite. The crystallization of silicalite-1 was accelerated by the sodium salts in the Hofmeister series based on the order of Na2SO4 > NaF > NaCl > NaBr > NaI > NaSCN. The liquid 1H NMR and radial distribution function (RDF) analyses revealed that the anions were located around the template cation with a distance to the N atom of TPA+ in a sequence of SO42− > F− > Cl− > Br− > I− > SCN−. Combining the analyses on 1H NMR and charge density of anions, we discovered that the anions affected the release of water molecules by activating the hydration spheres around the TPA+ cation in a different degree, thus regulating zeolitic crystallization. This work provides new insights into zeolite crystallization via anion additive introduction. Download figure Download PowerPoint Introduction Zeolites are microporous crystalline aluminosilicates whose open-framework structures are built from the connection of TO4 tetrahedra (T = Si or Al). Based on their ordered pores, tunable acidities, and high thermal stabilities, zeolites are the most important solid catalysts in coal chemistry, petroleum refining, and the petrochemical and fine chemical industries.1–3 Zeolites are typically synthesized under hydrothermal conditions, which involve the heating of a mixture at an elevated temperature for a period of time ranging from hours to days. The mixture contains sources of silica and alumina, water, and the structure-directing agent (also known as the “template”) such as alkali or alkaline earth hydroxides, quaternary ammonium bases (or salts), and organic amines, and so forth.4 During the heating treatment, the aluminosilicate gel in the mixture is depolymerized to form fragments of silicates, aluminates, or aluminosilicates, which are assembled and further polymerized around the hydrated template cation. Even though extensive investigations have been conducted on the crystallization mechanism of zeolites, how the source species are assembled under the direction of the “template” to form the long-range ordering of a specific zeolite remains unclear. In 1995, Burkett and Davis5 investigated the crystallization process of silicalite-1, the pure silica form of zeolite ZSM-5, and depicted a picture for the early stage of the crystallization of silicalite-1. First, a hydrophobic hydrated sphere of TPA+ and a hydrophobic hydrated sphere of silicate species collided and overlapped each other in the liquid phase. Subsequently, the water molecules in the hydrophobic hydration sphere were gradually driven away to the bulk water and then the inorganic–organic composite species were formed. Hence, it is expected that effective tuning of the water release rate of the hydration sphere to the bulk water may regulate the crystallization of zeolites. It is well known that anions can salt-in or -out the protein suspensions, which are also known as Hofmeister effects.6 Various anion additives have been introduced into zeolite synthetic systems to effectively regulate the crystallization process of zeolites. For example, F−, SO42−, and PO43− can promote the formation of zeolite beta, while Br−, I−, and ClO4− inhibit its formation.7 In addition, anions of ClO4−, PO43−, ClO3−, AsO43−, BrO3−, and IO3− can accelerate the crystallization of NU-1, ferrierite, ZSM-5, ZSM-48, ZSM-12, and beta zeolites.8 However, there remains a lack of in-depth understanding of the role of anions in zeolitic crystallization at the molecular level. In this work, we comprehensively investigated the crystallization of silicalite-1 from an initial mixture containing Na2SO4, NaF, NaCl, NaBr, NaI, or NaSCN additives. Results showed that the crystallization of silicalite-1 was accelerated by the sodium salts in the order of Na2SO4 > NaF > NaCl > NaBr > NaI > NaSCN, which is consistent to that of the Hofmeister series. By combining the analyses on 1H NMR and the radial distribution function (RDF) as well as the charge density of anions, we discovered that anions were located in different spacial regions of the TPA+ hydrated cation and could affect the release of water molecules by activating the hydration spheres around the TPA+ cation at a different degree, thus regulating the zeolite crystallization. This work provides further insights into zeolite crystallization as well as a feasible way to rationally regulate zeolite crystallization in terms of ion-tuning. Experimental Methods Chemicals and reagents The chemicals used in this study were tetrapropylammonium hydroxide (TPAOH; 25% aqueous solution; Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), LUDOX HS-40 colloidal silica (40 wt % suspension in water; Sigma-Aldrich, St. Louis, MO), sodium sulfate (99%; Sinopharm Chemical Reagent Co., Ltd.), sodium fluoride (98%; Tianjin Fuchen Chemical Reagents Company, Tianjin, China), sodium chloride (99.5%; Tianjin Fuchen Chemical Reagents Company), sodium bromide (99%; Tianjin Fuchen Chemical Reagents Company), sodium iodide (99%; Tianjin Fuchen Chemical Reagents Company), sodium thiocyanate (98.5%; Tianjin Huadong Reagent Factory, Wuqing Development Zone, Tianjin), and deionized water (18.2 MΩ*cm). All reagents were used without any further purification. Synthesis of silicalite-1 zeolites The molar composition for the synthesis of silicalite-1 was SiO2: 0.36 TPAOH: 17.91 H2O. Colloidal silica, TPAOH, and deionized water were mixed and agitated at ambient temperature for 6 h. The initial mixtures were transferred into 10 mL vials (each for 8 g) and then shaken well. The molar composition of the initial mixture containing sodium salt was SiO2: 0.36 TPAOH: 17.91 H2O: 0.076 NaX (X = F−, Cl−, Br−, I−, or SCN−) or SiO2: 0.36 TPAOH: 17.91 H2O: 0.038 Na2SO4. The pH of the solution without anions was 12.19, and the pH values of the solution containing SO42−, F−, Cl−, Br−, I−, and SCN− were 12.09, 12.08, 12.08, 12.08, 12.10, and 12.10, respectively. The mixture was heated at 100 °C for varying durations. The products were recovered via centrifugation and thoroughly washed with deionized water, dried at 80 °C for 12 h, and calcined at 550 °C for 6 h. Characterizations Elemental mapping images were taken on an energy dispersive spectrometer (EDS) (Oxford Instruments, High Wycombe, UK). The X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max-2550 diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å) at 50 kV and 200 mA in the range of 2θ value between 4° and 40° at a scan rate of 6° min−1. The crystal morphologies were characterized by scanning electron microscopy (SEM) using a JEOL JSM-7800F microscopy (JEOL Ltd., Tokyo, Japan). 1H NMR spectra were obtained using a Varian 300 MHz NMR spectrometer. 29Si NMR spectra were recorded on a Bruker AVANCE III 500 HD spectrometer (Bruker BioSpin AG, Fällanden, Switzerland) with a 5 mm Broad Band Fluorine Observation (BBFO) smart probe, operating at 99.36 MHz. Computational methods To better understand the role of anions for the effect of chemical shift based on structure, molecular dynamics (MD) simulation was performed to simulate the RDFs. The systems consisted of six Na+ cations, six X− (X− = F−, Cl−, Br−, I−, or SCN−) or three SO42− anions, 30 TPAOH molecules, and 1500 H2O molecules. An NPT ensemble (constant number of particles N, pressure P, and temperature T) was used for equilibration so that the volumes were stable, and then the canonical (NVT; constant number of particles N, volume V, and temperature T) ensemble was used for studying the structure. The simulated temperature was held at 298 K and controlled by a Nosé–Hoover thermostat. Periodic boundary conditions were used in all three directions. The Verlet velocity algorithm was applied to integrate Newton’s equations of motion with 1.0 fs time steps. The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) II force field, which has been proven to be able to accurately predict the structure of small molecules, was also adopted in this work.9,10 All charges are force field assigned except NaSCN (Na: +1.000e; S: −0.724e; C: +0.208e; N: −0.484e) as to keep the total charge neutral. The electrostatic interactions were calculated using the Ewald summation method, and Lennard–Jones (LJ) interactions were calculated with a 12.5 Å cutoff radius. The total simulation time was 51 ns for each system. Data analysis was conducted over the last 50 ns trajectory after 1 ns of equilibration was completed. The trajectory was recorded every 100 steps to analyze the RDF. Simulations were performed with Forcite module in the Material Studio 7.0 (Accelrys Software Inc., San Diego, CA). Results and Discussion Influence of salts on the crystallization kinetics of silicalite-1 Highly crystalline silicalite-1 was crystallized from an initial mixture with a molar composition of SiO2: 0.36 TPAOH: 17.91 H2O at 100 °C for 22 h. When the sodium salt of Na2SO4, NaF, NaCl, NaBr, NaI, or NaSCN is introduced into the reaction mixture with a molar composition of SiO2: 0.36 TPAOH: 17.91 H2O: 0.076 NaX (X = F−, Cl−, Br−, I−, or SCN−) or SiO2: 0.36 TPAOH: 17.91 H2O: 0.038 Na2SO4 (Note: higher NaX/SiO2 ratio will result in the formation of gel.), the crystallization of silicalite-1 was remarkably quicker based on the salt additive in the order of Na2SO4 > NaF > NaCl > NaBr > NaI > NaSCN. Note that these anions are not included in the product of silicalite-1 as confirmed by EDS analysis. Strikingly, such a sequence is exactly identical to that of the Hofmeister series. Taking into account that Na+ concentrations in all cases remained the same, the Hofmeister series observed here must be related to the nature of anions. Figure 1 shows the crystallization kinetics curves (yield vs crystallization time) of the silicalite-1 formed from the initial mixtures in the absence/presence of sodium salts. The powder XRD patterns of the products as a function of crystallization time are provided in Supporting Information Figures S1–S7. All of the as-synthesized silicalite-1 samples show a spherical morphology as observed based on SEM ( Supporting Information Figures S8–S14). In the absence of sodium salt (“0” curve in Figure 1), XRD detectable silicalite-1 starts to form at 10 h and the yield reaches ca. 45% at 22 h. However, XRD detectable silicalite-1 starts to form at 7 h when the sodium salt is introduced into the initial mixture. A plateau of ca. 57% in yield is reached at 14 h for the initial mixture containing Na2SO4, NaF, NaCl, or NaBr and at 16 h for the initial mixture containing NaI or NaSCN. Due to the strong dissolution of silicate in the basic solution, the maximum yield for this synthesis system is ca. 57%, which is consistent with previously reported result.11 Although all curves reach a plateau at 16 h, an obvious difference in the yield at 12 h is observed. At a crystallization time of 12 h, the yield of the initial mixture containing Na2SO4, NaF, NaCl, NaBr, NaI, and NaSCN is 39.5%, 31.5%, 25.1%, 21.3%, 16.5%, and 15.1%, respectively. If there is no sodium salt in the initial mixture (“0” curve in Figure 1), the yield at 12 h is only as low as 1.4%. The results in Figure 1 clearly show that the sodium salts remarkably shortened the induction period of the crystallization of silicalite-1 by promoting the formation of nuclei. According to Figure 1, the accelerating sequence of the sodium salts for the crystallization of silicalite-1 is Na2SO4 > NaF > NaCl > NaBr > NaI > NaSCN. In addition, the influence of the content of Na2SO4 on the crystallization of silicalite-1 was also investigated. The Na2SO4/SiO2 ratios are set as 0.0168, 0.0122, and 0.0056. The corresponding crystallization curves are shown in Supporting Information Figure S15, which clearly show that at lower levels of Na2SO4, the crystallization rate slows. We further conducted the seed-assisted synthesis combining the addition of Na2SO4. The results clearly show that the combination of seed and Na2SO4 can further shorten the crystallization time ( Supporting Information Figure S16). Figure 1 | Crystallization kinetics curves of silicalite-1 formed from the initial mixtures in the absence/presence of sodium salts of Na2SO4, NaF, NaCl, NaBr, NaI, and NaSCN. Download figure Download PowerPoint Spatial position relationship between anions and TPA+ As the concentration of Na+ in all initial mixtures is kept consistent, the difference in accelerating the crystallization of silicalite-1 should be attributed to the nature of anions. To understand how the anions affect the crystallization of silicalite-1, we conducted liquid 29Si NMR and 1H NMR analyses for the initial mixtures with/without sodium salts. Supporting Information Figure S17a shows the liquid 29Si NMR spectrum of the initial mixture without sodium salt; four typical resonances at ca. −79, −87, −96, and −105 ppm were observed, which correspond to the Q1, Q2, Q3, and Q4 groups in silicate, respectively [Qn represents the Si(OSi)n(OH)4-n].12–14 As all Si atoms in silicalite-1 are tetrahedrally connected to four other Si atoms via bridging oxygen atom, Q4 groups are believed to be the final species before the formation of silicalite-1 nuclei.5 Therefore, the signal of Q4 is specially analyzed. The resonances of Q4 for all solutions are shown in Supporting Information Figure S17b, which clearly show that the presence of sodium salts has almost no influence on the state of Si species in the solutions. Since there is a large amount of H2O in the silica sol (LUDOX HS-40 colloidal silica (40 wt % suspension in water; Sigma-Aldrich, St. Louis, MO), 40%) and the 1H NMR signals of the H2O may seriously interfere the weak 1H NMR signals from TPA+, the 1H NMR analyses are just applied to the solutions without silicon source. The concentrations of Na+ in all solutions are reduced upon the addition of large amounts of D2O in the 1H NMR analyses. In Figure 2a, an extremely intense resonance at ca. 1437.46 Hz and three relative weak resonances at ca. 941.28, 503.29, and 277.69 Hz are observed for the mixture with a composition of TPAOH: 0.21 NaX: 49.75 H2O: 352.14 D2O (X = F−, Cl−, Br−, I−, or SCN−) or TPAOH: 0.105 Na2SO4: 49.75 H2O: 352.14 D2O, which correspond to the hydrogens of H2O (denoted as H2O-H) and the α carbon (denoted as α-H), β carbon (denoted as β-H), and γ carbon (denoted as γ-H) of the TPA+, respectively.15 To distinguish the subtle differences among the signals due to the presence of sodium salts, the resonances corresponding to the H2O-H, α-H, β-H, and γ-H are shown in Supporting Information Figure S18 and Figures 2b–2d, respectively. Figure 2 | The liquid 1H NMR spectra of the TPAOH solution (TPAOH: 49.75 H2O: 352.14 D2O) and TPAOH solutions containing sodium salts [TPAOH: 0.21 NaX: 49.75 H2O: 352.14 D2O (X = F−, Cl−, Br−, I−, or SCN−) or TPAOH: 0.105 Na2SO4: 49.75 H2O: 352.14 D2O] (a); α-H (b), β-H (c), and γ-H (d) of the TPAOH. The concentration of Na+ in all solutions is 0.017 M. Download figure Download PowerPoint The results in Supporting Information Figure S18 clearly show that the sodium salts have no observable influence on the resonances of H2O-H. Notably a shift to the downfield caused by anions for the three 1H resonances from TPA+ is observed in the sequence of SCN− > I− > Br− > Cl− > SO42− > F−. Interestingly, such sequence is almost the reverse one representing the accelerating ability of the sodium salts for the crystallization of silicalite-1 (i.e., Na2SO4 > NaF > NaCl > NaBr > NaI > NaSCN) except the sequence of SO42− and F−, which suggests that the difference in accelerating the nucleation stage of silicalite-1 by the sodium salts might be from the influence of anions on the TPA+ cation. The exact shift values for the three H resonances (Δδ = δsalt − δ0) are summarized in Table 1. According to the maximum shift (highlighted as bold in Table 1) of the 1H NMR signal for the α-H, β-H, and γ-H of TPA+ caused by the anions, it can be concluded that F− and SO42− mainly affect the γ-H of TPA+, while Cl−, Br−, I−, and SCN− mainly affect the α-H of TPA+. Accordingly, F− and SO42− might be spatially close to the γ-H of TPA+ (SO42− appears to be closer to γ-H than F− according to the data in Table 1), while Cl−, Br−, I−, and SCN− are spatially close to the α-H of TPA+. Table 1 | Shifiting of 1H NMR Signals for α-H, β-H, and γ-H of the TPA+ in the presence of F−, SO42−, Cl−, Br−, I−, or SCN− Anions H Δδ/Hz (Δδ = δsalt − δ0) α β γ F− 0.13 0.15 0.19 SO42− 0.41 0.40 0.50 Cl− 0.93 0.89 0.89 Br− 1.49 1.36 1.36 I− 2.05 1.84 1.78 SCN− 2.33 2.28 2.10 To better understand the influence of anion locations on the 1H chemical shifts, the RDFs between anions and different hydrogen atoms of TPA+ (i.e., α-H, β-H, and γ-H) based on MD simulation (see “Computational Methods” section and Supporting Information Figure S19), which are widely used in solution systems to quantifiably describe the space distribution,16,17 have been calculated and displayed in Figure 3. It is well known that the NMR signal shift of one species caused by another one can be used as an indicator to evaluate the spatial distance between the two adjacent species. Thus, the analysis of RDF is focused on the short distance (about several angstroms) that may induce major changes in the chemical shift.18 In the calculation, we initially assume that the chemical environments are the same for all hydrogen atoms of TPA+ (i.e., α-H, β-H, and γ-H). As shown in Figure 3a, maximum proximity between the SO42− anion and γ-H is observed, which will cause the largest shift of 1H NMR. The same phenomenon has also been observed for the F− anion (see Figure 3b). In contrast to F− and SO42− anions, the shortest spatial distance between α-H and other anions (i.e., Cl−, Br−, I−, and SCN−) is observed in Figures 3c–3f, and thus has obvious influence on the chemical shift of α-H atoms. Notably, the calculation result is consistent with that of 1H NMR results that SO42− and F− mainly affect the γ-H of TPA+, while Cl−, Br−, I−, and SCN− mainly affect the α-H of TPA+ (see Table 1). It should be noted that the second maximum shift is not completely consistent with the results of NMR measurements. For example, the change of β-H chemical shift is the second largest for F−, while a staggered pattern of the distances between F−/β-H and F−/α-H is observed. This may be caused by the nonidentical chemical environments of the three H atoms. As β-H is located in the β-carbon connected to two carbon atoms, while others are only connected with one carbon atom, the shift of β-H is larger than that of α-H and γ-H with the same distance. Overall, SO42− and F− mainly affect the γ-H of TPA+, while Cl−, Br−, I−, and SCN− mainly affect the α-H of TPA+. Figure 3 | RDFs between different H atoms (α-H, β-H, and γ-H) of TPA+ and anions based on the MD simulation: (a) SO42−, (b) F−, (c) Cl−, (d) Br−, (e) I−, and (f) SCN−. The nearest function is highlighted in bold. •: anion. Download figure Download PowerPoint Furthermore, the RDF between anions and N atom of TPA+ was performed and the results are shown in Figure 4a. The results indicate that the distance between anions and N atom of TPA+ follows the order of SO42− > F− > Cl− > Br− > I− > SCN−, where Cl−, Br−, I−, and SCN− anions are close to the N atom in TPA+. On the basis of RDF and the fact that F− and SO42− are spatially close to the γ-H of TPA+ (SO42− should be closer to γ-H than F− according to the data in Table 1), while Cl−, Br−, I−, and SCN− are spatially close to the α-H of TPA+, the distribution of varied anions around TPA+ can be schematically described in Figure 4b. Figure 4 | (a) Anions-N RDF. (b) Schematic diagram of the position for anions located around TPA+. Download figure Download PowerPoint Influence of anions on the structure of the first hydration shell In the present reaction mixture, ions from sodium salts along with silicates and TPA+ are distributed in the aqueous solution, which are unavoidably surrounded by water molecules, and hydrogen bonding formation among the first-shell water molecules is highly possible if the distance and configuration of two neighboring water molecules are appropriate. Previous studies show that charge densities of ions govern their interactions with water in the first hydration shell and a balance of forces of electrostatics and hydrogen bonding determines the water structure of the first hydration shell.19 Small ions having high charge densities cause strong electrostatic ordering of nearby waters, which breaks the hydrogen bonds in the first hydration shell. In contrast, large ions having low charge densities cause less strong electrostatic ordering of nearby waters. Thus, the ion effects on water structure could be explained by a competition between ion–water interactions and water–water interactions. The former is dominated by charge density effects, while the latter is dominated by hydrogen bonding.20 A water molecule has a stronger interaction with a small ion than with a neighboring water molecule, resulting in the activation of the first hydration shell, while water molecules next to big ions are more mobile than bulk water molecules, stabilizing the first hydration shell.21,22 To understand how the anions of F−, SO42−, Cl−, Br−, I−, and SCN− affect the water molecules, we performed 1H NMR analyses of H2O and sodium salt solutions dominated by D2O. Due to the large amount of water in TPAOH solution, a large amount of D2O will be needed in the 1H NMR analyses to obtain better resolution, which will significantly dilute the concentration of sodium salts. Therefore, we conducted the 1H NMR analyses only on pure water and sole sodium salt solution with a Na+ concentration of 0.44 M. Figure 5a shows the liquid 1H NMR spectra of pure water, and sodium salt solutions with a molar composition of Na+: 14.81 H2O: 104.80 D2O and the corresponding H signals are provided in Figure 5b. As shown in Figure 5a, the intense signals at ca. 1436.31 Hz are observed for H2O. However, a close look at these signals in Figure 5b shows that anions cause the shift of the H signal to either upfield or downfield compared with the pure water, and the details on the shift as well as the areal charge density of anions are summarized in Table 2.23–25 As shown in Table 2, F− and SO42− cause shifts of 0.33 and 0.22 Hz (to the downfield), respectively, while Cl−, Br−, I−, and SCN− cause shifts of −0.22, −0.34, −0.78, and −0.34 Hz (to the upfield), respectively; this is consistent with the previous results that F− and SO42− cause the shift of the signal of H2O to the downfield, while Cl−, Br−, I−, and SCN− cause the shift of the signal to the upfield.26 According to the theoretical and experimental investigations on the influence of ions on the structure of water,19,20 ions with high charge density (e.g., F−) or multivalence (e.g., SO42−) will order the water molecules of the first hydration shell due to the strong electrostatic interaction between ions and the dipole of H2O, which will break most of the hydrogen bonds in the first hydration shell, activate the first hydration shell, and cause a shift of the H signal of H2O to the downfield.26–28 In contrast, the ions with low charge density (e.g., Cl−, Br−, I−, and SCN−) will stabilize the first hydration shell due to the weak electrostatic interactions between ions and the dipole of H2O, which causes the shift of the H signal of H2O to the upfield.19 Figure 5 | The liquid 1H NMR of H2O and sodium salt solutions (Na+: 14.81 H2O: 104.80 D2O) (a) and the fragment corresponding to the 1H signal (b). Download figure Download PowerPoint Table 2 | The Pauling Radii and Areal Charge Density of Anions: F−, SO42−, Cl−, Br−, I−, or SCN− Anions Δδ/Hz Pauling Radius (nm) Areal Charge Density (mC/m2)a F− 0.33 0.13 −720 SO42− 0.22 0.23 −481 Cl− −0.22 0.18 −389 Br− −0.34 0.20 −331 I− −0.78 0.22 −263 SCN− −0.34 0.21 −281 aAreal charge density calculated as σ = Ze/(4πRP2) Influence of anions on the overall crystallization of silicalite-1 The effect of cations on zeolite crystallization should also be taken into account. Pure silica version of zeolites can be synthesized in the absence of alkali metal cations and only in the presence of organic structure-directing agent.29–33 It has been experimentally proven that the rate of crystallization of pure silica version of zeolites such as ZSM-12, ZSM-35, ZSM-48, and SSZ-24 could be increased by the addition of alkali metal cations to the initial mixture.34 Therefore, Na+ ions presented in the reaction mixture in this study due to the introduction of sodium salt may also accelerate the crystallization of silicalite-1. However, the accelerating effect of Na+ will not be specially discussed in this study because the concentration of Na+ in each synthetic system is maintained the same, and the accelerating effect from Na+ for all synthetic systems would be assumed to be the same. Besides Na+, a