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Accelerated crystallization of zeolites via hydroxyl free radicals

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Science  11 Mar 2016:
Vol. 351, Issue 6278, pp. 1188-1191
DOI: 10.1126/science.aaf1559

Radically faster synthesis

Zeolite synthesis normally proceeds under basic conditions that allow the oxide bridges between aluminum and silicon atoms to break and reform. Feng et al. show that the formation of hydroxyl radicals, either by irradiation with ultraviolet light or with the Fenton reagent, can speed up the formation of the crystallized zeolite by about a factor of 2.

Science, this issue p. 1188

Abstract

In the hydrothermal crystallization of zeolites from basic media, hydroxide ions (OH) catalyze the depolymerization of the aluminosilicate gel by breaking the Si,Al–O–Si,Al bonds and catalyze the polymerization of the aluminosilicate anions around the hydrated cation species by remaking the Si,Al–O–Si,Al bonds. We report that hydroxyl free radicals (•OH) are involved in the zeolite crystallization under hydrothermal conditions. The crystallization processes of zeolites—such as Na–A, Na–X, NaZ–21, and silicalite-1—can be accelerated with hydroxyl free radicals generated by ultraviolet irradiation or Fenton’s reagent.

Zeolites are microporous crystalline aluminosilicates that are used as ion-exchangers in the detergent industry, catalysts in the petrochemical and chemical industry, and adsorbents in air separation through pressure swing adsorption (1, 2). Zeolite materials are typically synthesized in a strong basic medium in which a high concentration of hydroxide ions (OH) assists in the mineralization of silicate and aluminate species in the reactant gels (3). The crystallization process can be described through the following steps: (i) polymerization, forming an amorphous gel via making Si,Al–O–Si,Al bonds; (ii) depolymerization, forming soluble aluminosilicates and silicates via breaking Si,Al–O–Si,Al bonds; and (iii) repolymerization, remaking Si,Al–O–Si,Al bonds around the hydrated cation species—that is, the structure-directing agents—via condensation reaction (4, 5). These steps comprise the nucleation and crystal growth stage of the crystallization, which are catalyzed by OH involving multiple equilibria. However, the crystallization mechanism of zeolites is still unclear, and the understanding of their formation at a molecular level has not yet been achieved.

Under basic conditions, OH ions increase the coordination of tetrahedral silicon (Si) atoms to pentahedral or octahedral, which weaken and break the Si–O–Si bonds (6). Theoretical calculations show that the dissociation of the Si–O–Si bonds is more favorable when catalyzed by hydroxyl free radicals (•OH) (7, 8); •OH species are highly active in organic synthesis (9), polymerization (10), and modification of proteins (11). Generation of •OH in solution can be achieved by means of, for example, electron pulse radiolysis, ultraviolet (UV) irradiation, Fenton reactions, chemical reactions, and high-voltage electrical discharge (11). We show that hydrothermal zeolite crystallization can be accelerated by introducing •OH via physical or chemical methods.

To investigate the effect of •OH on zeolite crystallization, we used a modified multiparallel reactor that allowed for UV irradiation and controlled heating (fig. S1). The hydrothermal syntheses conducted under UV irradiation are referred to as UV condition, and control syntheses conducted in the oven without the UV irradiation are referred to as dark condition. We initially studied the UV and dark-condition syntheses in the Na2O–Al2O3–SiO2–H2O system at 298 K. Under UV conditions for 24 hours, the experimental and simulated x-ray diffraction (XRD) patterns of Na–X (SiO2/0.21Al2O3/9.36Na2O/85H2O) (Fig. 1A), NaZ–21 (SiO2/0.32Al2O3/10.05Na2O/85H2O) (Fig. 1B), and Na–A (SiO2/0.46Al2O3/4.4Na2O/60H2O) (fig. S2A) and the scanning electron microscopy (SEM) images of the corresponding products (fig. S3) indicate that the products were already crystallized. Under dark conditions for 24 hours, the corresponding XRD patterns (Fig. 1, C and D, and fig. S2B) show that the materials were still primarily amorphous. These results demonstrate that the crystallization rate of zeolites was accelerated by UV irradiation.

Fig. 1 Crystallization processes under UV irradiation and dark conditions at 298 K.

(A to D) The experimental XRD patterns of Na–X and NaZ–21 synthesized under the UV conditions [(A) and (B), black, 4.0 mW/cm2] and dark conditions [(C) and (D), black] for 24 hours, respectively. The corresponding simulated XRD patterns are plotted for comparison [(A) and (B), red]. (E) Crystallization curves of zeolite Na–A under dark conditions and under UV conditions with irradiance of 2.0, 4.0, and 8.0 mW/cm2, with a Na2O/SiO2 ratio of 4.4.

The crystallization process of zeolite Na–A with the starting molar composition of SiO2/0.46Al2O3/4.4Na2O/60H2O was further investigated at 298 K under dark and UV conditions with different irradiances that varied the •OH concentration (reported as crystallization curves in Fig. 1E). Under dark conditions, long-range ordering of zeolite Na–A, as confirmed with XRD, began to be observed after 40 hours (fig. S4A). After 60 hours, highly crystalline zeolite Na–A was obtained, as confirmed with the SEM and high-resolution transmission electron microscopy (HRTEM) images (fig. S5). In contrast, the XRD patterns (fig. S4, B, C, and D) show that the long-range ordering of zeolite Na–A was already observed at 32, 20, and 16 hours, and highly crystalline zeolite Na-A was obtained at 52, 40, and 36 hours for irradiances of 2.0, 4.0, and 8.0 mW/cm2, respectively, as shown in the SEM and TEM images (figs. S6 to S8).

Reducing the OH concentration could slow down the crystallization of zeolites. We further studied the crystallization behavior of the initial reaction mixture with a reduced Na2O/SiO2 molar ratio (3.08 versus 4.4) at 298 K. Under the dark conditions, the long-range ordering of zeolite Na–A was not observed until 45 hours, and the crystals have been well developed when the reaction time reached 55 hours (fig. S9A, XRD; fig. S10, SEM and TEM images), whereas long-range ordering was already observed at 40 hours when the Na2O/SiO2 ratio was 4.4 (fig. S4A). In contrast, when the initial reaction mixture was irradiated with a UV lamp, highly crystalline Na-A was formed at 40 hours (fig. S9B, XRD; fig. S11, SEM and TEM images). The results presented above clearly demonstrate that reducing OH decreases the crystallization rate of zeolite, whereas •OH can promote the crystallization at the same time. Consequently, the crystallization rate of zeolite Na–A under UV with a Na2O/SiO2 ratio of 3.08 is even faster than that under dark with a Na2O/SiO2 ratio of 4.4. The accelerating effect on crystallization upon UV with the reduced alkalinity is further enhanced as compared with that upon UV with normal alkalinity.

Upon UV irradiation, water can generate •OH (11). We used electron paramagnetic resonance (EPR) to characterize •OH and the derived species formed in the zeolite reaction system. We added 5,5-dimethylpyrroline-N-oxide (DMPO) as the spin-trapping agent of •OH into the initial reaction mixture, and we recorded the EPR signals in situ after the reaction mixture was irradiated for 0, 30, and 90 s. For comparison, we also characterized the initial reaction mixture and the pure water under dark conditions. As anticipated, the EPR signals from •OH captured by the DMPO—a 1:2:2:1 quartet pattern, with a splitting of 1.5 mT characteristic of a DMPO-•OH adduct—were observed after irradiation for 90 s (Fig. 2A) and 30 s (Fig. 2B). The characterized hyperfine coupling constants (HFCs) were aN = a = 1.5 mT. Because DMPO was also photolyzed, the EPR signals of oxidized DMPO radicals featured by a three-line spectrum were also observed. Sextet EPR signals (Fig. 2, A and B) with aN = 1.59 mT and a = 4.5 mT were observed. None of these signals were observed in the spectrum of the initial reaction mixture without the UV irradiation (0 s) in the presence of DMPO (Fig. 2C), indicating that the new sextet EPR signals were not from the inorganic impurities of the initial reaction mixture but from the radicals generated by the UV irradiation.

Fig. 2 Radicals’ identification from UV irradiation.

(A to C) EPR spectra of the initial reaction mixture containing DMPO under the UV irradiation for (A) 90 s, (B) 30 s, and (C) 0 s. (D to F) Comparison of the experimental and the simulated EPR spectra of (D) DMPO–•OH adduct, (E) DMPO–•Si adduct, and (F) oxidized DMPO radicals. (G) EPR spectra of the initial reaction mixture containing the spin-trapping agent of BMPO after 10 hours of dark incubation. The EPR signals are marked as following: red circles, hydroxyl free radicals; green rectangles, oxidized DMPO radicals; and blue arrows, silicon-based radicals.

Because of the large a value (4.5 mT), these sextet EPR signals cannot be attributed to the carbon (C)– and oxygen (O)–centered radicals. For the C-centered radical, the aHrβ is at the range of 1.8 to 2.8 mT, and for the O-centered radical, the a is at the range of 0.7 to 1.4 mT (12, 13). In fact, the large HFCs of these sextet EPR signals are similar to that of the DMPO-•P(O)(OC2H5)2 adduct (12). Because no phosphorous (P) species was involved in the initial reaction mixture, these signals might be attributed to the DMPO-•Si adduct. If this is so, then the slight change in the intensity of these signals in the spectra after 30 and 90 s of irradiation indicated that these Si-based radicals were generated during the zeolite crystallization.

Previous studies show that fumed silica has an intrinsic population of planar three-membered-rings (3MRs) formed at high temperature and “frozen-in” by rapid quenching, which can undergo cleavage forming Si• and Si–O• radicals because of the strain in the Si–O–Si bonds (1416). Aqueous alkaline silicate solutions also contain 3MRs, as confirmed by previous 29Si–29Si correlation spectroscopy nuclear magnetic resonance studies (17). Thus, the Si-based radicals in zeolite synthesis probably form through homolytic cleavage of the strained Si–O–Si bonds in planar 3MRs in the initial reaction mixture. The reason why no Si–O• radicals (another type of oxygen-centered radical) were observed might be caused by their high activity; they can easily react with water to form •OH. Shown in Fig. 2, D to F, is a comparison of the experimental and the simulated EPR spectra of DMPO–•OH adduct, DMPO–•Si adduct, and oxidized DMPO radicals, respectively, which confirms the assignment.

Considering that the concentration of radicals in the non–UV-irradiated initial reaction mixture and in the pure water might be too low to be detected by DMPO, we used a recently developed spin-trapping agent, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), that allows a long-time accumulation of the EPR signals (18). We also observed •OH in the initial synthesis mixture under dark conditions for 10 hours (Fig. 2G). We recorded the EPR signals that arose from •OH captured by BMPO, a characteristic 1:2:2:1 quartet pattern of a BMPO–•OH adduct (18). The complicated spectrum is due to the spectral overlapping of two isomers of BMPO (Fig. 3F, as well). Pure water under dark conditions did not give any EPR signals (fig. S12), suggesting that •OH came from the zeolite reaction system. The observation of •OH in the initial mixture under dark conditions suggests that in addition to OH, •OH was also involved in the zeolite crystallization as mineralizers. Although the reactivity of the •OH is much greater than that of OH, the concentration difference favors the OH mechanism.

Fig. 3 Acceleration processes by Fenton’s reagent.

(A to C) Crystallization processes of silicalite-1 at 343 K under (A) Fenton conditions, (B) UV conditions (4.0 mW/cm2), and (C) dark conditions. (D to F) EPR spectra of the TPAOH–TEOS–H2O system under (D) Fenton conditions, (E) UV conditions (4.0 mW/cm2), and (F) dark conditions. The EPR signals are marked as following: red circles, •OH; green rectangles, oxidized DMPO radicals; blue arrows, silicon-based radicals; asterisks, ethanol radicals; and red rectangles, oxidized BMPO radicals.

The Fenton’s reagent can also produce •OH, and we investigated the crystallization process for silicalite-1 in the reaction system tetrapropylammonium hydroxide (TPAOH)– tetraethyl orthosilicate (TEOS)–H2O with the addition of the Fenton’s reagent (9TPAOH/25SiO2/480H2O/100EtOH/0.015H2O2/0.001FeSO4•18H2O, 343 K). The XRD patterns of the solid products (Fig. 3A) indicate that the long-rang ordering of silicalite-1 was already observed when the reaction time reached 40 hours, which is 10 hours shorter than that under UV conditions (Fig. 3B) and 20 hours shorter than that under dark conditions (Fig. 3C). The greater acceleration effect of the Fenton conditions than UV conditions indicates the higher concentration of •OH generated by the Fenton’s reagent than by the UV irradiation. The SEM and TEM images of silicalite-1 recovered at the end of crystallization under Fenton conditions as compared with the dark and UV conditions are shown in fig. S13. The yields of silicalite-1 at different crystallization periods under Fenton and UV conditions, which are also remarkably improved as compared with the dark conditions, are given in fig. S14. The composition and N2 adsorption data for all samples are included in table S1.

To identify the radicals in the crystallization of silicalite-1, we characterized the initial reaction mixtures at ambient temperature under Fenton, UV, and dark conditions by means of EPR spectroscopy (Fig. 3, D to F). Under dark conditions, signals from the BMPO–•CH(CH3)OH adduct (aN = 1.51 mT, a = 2.1 mT) (Fig. 3F) imply the existence of alkane radicals, which may be formed from the reaction of •OH with the ethanol generated from the hydrolysis of TEOS. This reaction would consume the very limited •OH formed under the dark conditions. Under UV conditions (Fig. 3E), DMPO–•CH(CH3)OH (aN = 1.59 mT, a = 2.27 mT), DMPO–•OH (aN = a = 1.50 mT), DMPO–•Si (aN = 1.59 mT, a = 4.5 mT), and oxidized DMPO radicals were observed and indicate that more •OH formed. The initial reaction mixture under the Fenton conditions (Fig. 3D) contained •OH (aN = a = 1.50 mT), ethanol carbon radicals (aN = 1.59 mT, a = 2.27 mT), Si-based radicals (aN = 1.59 mT, a = 4.5 mT), and oxidized DMPO radicals.

Inhibiting •OH may slow down the zeolite crystallization. Because ethanol is an effective •OH scavenger, we crystallized silicalite-1 at 343 K under UV conditions by evaporating the ethanol in the mixture. We prepared two batches of the initial mixtures with the same molar composition; the batches were stirred overnight at ambient temperature under dark conditions in sealed and open quartz tubes, respectively. The weight loss of the initial mixture in the open quartz tube suggests that most of the ethanol hydrolyzed from TEOS was evaporated from the mixture. Subsequently, the quartz tubes were surely sealed and heated at 343 K for 48 hours under UV irradiation. As anticipated, the crystallization process was accelerated when the ethanol was removed, as indicated by the corresponding XRD patterns (fig. S15).

The zeolite crystallization is a two-step process, including nucleation and crystal growth. The sigmoidal crystallization curves shown in Fig. 1E show that the induction period under UV conditions is much shorter than that under dark conditions, which suggests that •OH generated by UV irradiation plays an important role in accelerating the nucleation stage during the crystallization. However, the slope of the crystallization curve for the UV conditions is similar to that for the dark conditions, implying similar crystal growth rates. To further confirm which stage was accelerated by •OH, we investigated the synthesis of zeolite Na–A at 298 K by means of UV pretreatment in the induction period for 18 hours, followed by crystallization under dark conditions. The XRD patterns of the products (fig. S16) show that the crystallization of the synthetic system under such conditions is similar to that under UV conditions, indicating that •OH mostly influences the nucleation stage but not the crystal growth stage.

Theoretical calculations provide more insight into how •OH accelerates the nucleation stage. Under the very basic conditions used in the synthesis, anionic [Si2O1+x(OH)6–x]x species stabilized by Na+ cations will be preferentially formed via successive and clearly exothermic deprotonation steps (fig. S17). We first considered the depolymerization of the gel by breaking of the Si–O–Si bonds under very basic conditions and studied the attack of either OH or •OH to a highly deprotonated [SiO2(OH)–O–SiO3]Na5 model of the gel. In the first case (Fig. 4A), water reacts with the dimeric silicon species, forming a pentacoordinated intermediate I1, this being the most energy-demanding step (Fig. 4C, blue line). A H transfer step converting intermediate I1 into I2 is necessary to have one OH group on each Si atom, which facilitates the rapid dissociation of the Si–O bond, generating the monomeric species P. Similar structures are involved in the reaction of the [SiO2(OH)–O–SiO3]Na5 model with a •OH radical (Fig. 4B). However, all the activation energies obtained in the reaction with •OH (Fig. 4C, red line) are much lower, indicating that the presence of •OH in the reaction medium will considerably increase the rate of Si–O bond-breaking and depolymerization of the gel.

Fig. 4 Reaction of [SiO2(OH)–O–SiO3]Na5 system and Gibbs free-energy calculation.

(A and B) Reaction of [SiO2(OH)–O–SiO3]Na5 system with (A) OH and (B) •OH. (C) Calculated Gibbs free-energy profiles for the reaction of [SiO2(OH)–O–SiO3]Na5 model with OH (blue) and •OH (red). Electronic (ΔE) and Gibbs free energies (ΔG) are given in kilocalorie per mole.

The second step in the nucleation process is the formation of new Si–O–Si bonds, which could be considered to be the reverse of the process investigated above, starting from products P and ending with reactants R. The high stability of the products when using the [SiO2(OH)–O–SiO3]Na5 model of the gel results in very high activation energies: 30 kcal/mol in the presence of only OH, and somewhat lower—23 kcal/mol—when •OH is also present in the media.

In order to check the influence of decreased alkalinity on the mechanism, a monodeprotonated [Si(OH)2–O–Si(OH)3]Na model was also considered. The theoretical results clearly prove the enhanced positive effect of •OH as compared with OH in breaking of the Si–O–Si bonds (activation barrier, 29 versus 4 kcal/mol) and promoting the formation of new Si–O–Si bonds (activation barrier, 17 versus 8 kcal/mol) in the gel with reduced alkalinity (figs. S18 and S19). This is consistent with our experimental results of the enhanced accelerating effect upon UV with reduced alkalinity.

The discovery that zeolite synthesis mechanism can be promoted through free radicals sheds a new light on zeolite crystallization and will open new perspectives for the synthesis of zeolite materials that are largely demanded in the chemical industry.

Supplementary Materials

www.sciencemag.org/content/351/6278/1188/suppl/DC1

Materials and Methods

Figs. S1 to S19

Table S1

References (1922)

References and Notes

  1. Acknowledgments: This work was supported by the 973 Project (grants 2014CB931802 and 2013CB921802) and the National Natural Science Foundation of China (grants 21320102001, 91122029, and 21571075). A.C. thanks the Program Severo Ochoa for financial support and ERC-AdG-2014-671093—SynCatMatch. J.Y. designed and supervised the project; W.Y., A.C., and R.X. involved the design of the experiments; G.F., P.C., and J.W. performed the experiments; J.-H.S. performed the EPR analyses; M.B., X.L., and Y.L. contributed to the calculations; J.Y. and W.Y. analyzed the data; G.F. wrote the first draft; and J.Y. and W.Y. deeply revised the manuscript. A Chinese patent about the method for UV-assisted synthesis of zeolite materials has been applied for.
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