Silica beta synthesized under alkaline conditions

Accepted Manuscript Silica beta synthesized under alkaline conditions Baorong Wang, Min Lin, Jianming Yang, Xinxin Peng, Bin Zhu, Yao Zhang, Changjiu Xia, Weilin Liao, Xingtian Shu PII:

S1387-1811(18)30114-8

DOI:

10.1016/j.micromeso.2018.02.049

Reference:

MICMAT 8812

To appear in:

Microporous and Mesoporous Materials

Received Date: 6 December 2017 Revised Date:

25 February 2018

Accepted Date: 27 February 2018

Please cite this article as: B. Wang, M. Lin, J. Yang, X. Peng, B. Zhu, Y. Zhang, C. Xia, W. Liao, X. Shu, Silica beta synthesized under alkaline conditions, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.02.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Alkaline and Fluorine-containing salt (Mineralization Agent)

Silica Resource

Basic Precursor

Hydrothermal Synthesis

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TEAOH

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Plate-like, hydrophobic silica ß 200×700×700nm

ACCEPTED MANUSCRIPT

Silica beta synthesized under alkaline conditions Baorong Wanga , Min Linb, Jianming Yangc, Xinxin Pengb, Bin Zhub, Yao Zhangb, Changjiu Xiab, *

Weilin Liaoa and Xingtian Shub a National Engineering Research Center for Carbonhydrate Synthesis, Jiangxi Normal University, Nanchang, 330027, China

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b State Key Laboratory of Catalytic Material and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China

c Xi’an Modern Chemistry Research Institute, Xi’an 710065, China

Abstract: Zeolite ß is an important material in catalysis, however the material with

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molar ration of silicon and aluminum more than 200 is difficult to synthesize. Typically, the pure silica ß and heteroatom zeolite ß are prepared at near neutral pH

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using HF as the mineralization agent. However, the HF is highly toxic and corrosive, and the zeolites formed are usually more than 10 μm. In this paper, alkaline and fluorine-containing salts was applied, and pure silica ß was hydrothermally synthesized from a basic precursor. The silica resource and alkali metal cations in the precursor would influence the crystallization process, tetraethyl orthosilicate and

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sodium cations are preferred. The pure silica ß is plate-like, the crystals formed on the third day are about 50×300×300nm, which keep growing with the time, and pure silica zeolite ß with uniform size of about 200×700×700nm are synthesized

hydrophobic.

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in a week. More importantly, the pure silica ß is defects-free, and it’s highly

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Keywords: silica ß, plate-like, alkaline condition, defects-free

1. Introduction

Zeolite ß with *BEA topology is characterized by the three-dimensional

12-ring channels ([100] and [010]: 0.67×0.73nm, [001]: 0.56×0.56nm) and high silica content. For the porosity, acidity, thermal stability and hydrothermal stability, zeolite ß has been used as industrial catalysts in cracking and alkylation. When titanium or stannum was isomorphously substituted into the framework position, the Ti-ß [1,2] and Sn-ß [3,4] obtained could be used in the

ACCEPTED MANUSCRIPT selective oxidation reactions and biomass valorization processes, respectively. Moreover, the polymorph A of zeolite ß is chiral, and it may be applied in the enantioselective reactions [5,6]. Thus, the synthesis of *BEA-type material has attracted lots of attention. Although zeolite ß with the molar ratio of silicon and aluminum (Si:Al) in the

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5~100 range could be effectively synthesized with tetraethyl ammonium hydroxide (TEAOH) as the structure directing agent (SDA) [7], it’s difficult to prepare the material with less aluminum content as the Al3+ and other trivalent cations in the gel seem to be beneficial for the nucleation process [8,9]. Nevertheless, high silica

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zeolite ß (Si:Al > 100) could be synthesized under alkaline conditions in the presence of dimethyldibenzylammonium cations, but de-boronated zeolite ß should be used as

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the seeds, and the boron atoms could also be detected in the products [10]. As fluorine anions was reported to be able to act as a mineralization agent and compensate the charge of the SDAs [11], TEAOH and HF were simultaneously used to synthesize the all-silica zeolite ß, which was firstly truly achieved at near neutral pH without seeding [12,13]. More importantly, the strategy was successfully applied in

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the synthesis of the heteroatom *BEA zeolites, including Ti-ß [1,14], Sn-ß [15,16] and Zr-ß [17,18]. However, the mineralization agent, HF, used in the crystallization process is acid, corrosive and toxic, which would significantly decrease the alkalinity of the precursor, leading to slow nucleation and crystallization rates, the

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crystallization duration is often more than one week [1,9,16], the zeolites prepared are always more than 10μm, and dealuminated seeds should also be used in the

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preparation of Sn-ß and Zr-ß. Then, a more environmental benign and efficient process on the synthesis of *BEA zeolites with high silica content should be developed, the crystal size of the zeolites should also be decreased to improve the accessibility of the active sites.

In basic media, the silica solubility increases with the hydroxyl anion concentration, which facilitate the homogeneous mixing, nucleation and crystallization of the zeolite precursor [19,20]. Typically, almost all of the industrial available zeolites, including zeolite A, Y, ZSM-5 and TS-1, are produced from alkaline precursors. Although it has been reported that

ACCEPTED MANUSCRIPT all-silica *BEA-topology zeolite with tunable morphology was synthesized under alkaline condition using F- as the mineralization agent, the alkalinity was so low (OH-:SiO2 < 0.22) that the precursors may not be homogeneously mixed, resulting in crystals with varied size, the smallest of which was also more than 2 μm, and there were too much framework defects [21,22]. Considering the

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beneficial effects of the fluorine anions and the alkaline conditions, alkaline and fluorine-containing salts, sodium fluoride (NaF) and potassium fluoride (KF), were used in this research. The influences of the silica resource, the mineralilzation agent and the crystallization time are studied,

the

2. Experimental 2.1 Zeolite Synthesis

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physicochemical properties of the as-prepared materials are also analyzed.

In the synthesis of silica β, the reagents used include tetraethyl orthosilicate (TEOS, >99%), silica gel, fumed silica, NaF, KF, aqueous tetraethyl ammonium hydroxide (TEAOH, 25wt%) and deionized water.

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Typically, the silica β was formed following the later process. 0.2 mol silica resource, including TEOS, silica gel and fumed silica, was added to 61.8 g aqueous TEAOH at room temperature. Under stirring, the mixture obtained was heated to 343K, and the hydrolysis was carried out for about 10h to form a clear basic precursor.

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Then, 0.05 mol NaF or KF was added. After stirring for 30 min, a well-distributed basic precursor with the chemical composition of SiO2: TEAOH: NaF/KF: H2O = 1 : 0.55 :

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0.25 : 9 was obtained. Finally, the precursor was transferred into a 100 mL Teflon-lined stainless-steel autoclave, which was then hydrothermally treated under autogenously pressure at 413 K for 3 ~ 7 days. The products were filtrated, washed with deionized water, dried at 383 K for 6 h and calcined at 823 K for 5h. Acid washing was further carried out to remove the sodium cations in the as-prepared silica β. Under stirring, 1 g silica β was added into 20mL HNO3 solution (0.5 mol/L). The acid washing was carried at 333 K for 8h. Then the zeolites were filtrated, washed and calcined to remove the HNO3. This procedure was repeated twice.

ACCEPTED MANUSCRIPT 2.2 Characterization The powder X-ray diffraction (XRD) patterns of the as-synthesized materials were collected on a Philips Panalytical X'pert diffractometer using nickel-filtered Cu Kα radiation (40kV, 250mA). The 2θ scanning was in the 5° 35° range, and the scanning rate was 0.4°/min.

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The inductively coupled plasma (ICP) analysis was carried out on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. The Si:Al in the materials were derived from the results.

The scanning electron microscopy (SEM) images were taken from a Hitachi 4800

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microscope (20kV).

The Fourier transform infrared (FT-IR) spectra in the framework vibration

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and hydroxyl stretching region were recorded on a Thermo Nicolet 750 and Thermo Nicolet 870 infrared spectrometer, respectively. After evacuating at 773 K for 2 h, the crystals were grinded with KBr, which was then pressed into a wafer, the spectra in the region of framework vibration could be recorded in the 400cm-1 ~ 1500 cm-1 range. On the other hand, self-supported wafers were

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also made, and they were outgassed at 723 K and 10-3 Pa for 3h. Then, the spectra in the hydroxyl region (1000 cm-1 ~ 4000 cm-1) were recorded at 363K. The 29Si MAS NMR experiments were performed on a Bruker AVANCE III 500WB spectrometer at a resonance frequency of 99.3 MHz using a 7 mm

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double -resonance MAS probe. The magic-angle spinning speed was 5 kHz in all experiments, and a typical π/6 pulse length of 1.8 μs was adopted for

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Si

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resonance. The chemical shift of 29Si was referenced to tetramethylsilane. The

water

vapor

adsorption

isotherms

were

recorded

on

a

Quantachrome VSTARTM. The samples were firstly outgassed at 573 K for 8h, and the isotherms were recorded at 293 K.

3. Results and Discussion The influences of the silica resource and mineralization agent were firstly studied via different synthesis formulations (Table 1). The XRD patterns of the crystallization products are shown in Figure 1 and Figure 2. In the XRD patterns, the characteristic peak of zeolite β is at 2θ = 22.4°, while the bands at 2θ = 7.9°, 8.8°,

ACCEPTED MANUSCRIPT 23.1°, 23.9°and 24.4° are assigned to zeolite with MFI topology. Moreover, the cristobalite exhibits a diffraction peak at 2θ = 21.8°. Although it has been reported that silica β could be synthesized in the presence of potassium cations and fluorine anions [21,22], the crystallization products (sample 7-9) obtained in this research are all cristobalite (Supplementary Information, Figure S1) whether TEOS, fumed silica

the precursor.

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and silica gel was used, which should be a result of the difference in the alkalinity of

Table 1 The influences of silica resource and mineralization agent

Silica

Mineralization agent

NaF/KF:SiO2

1

TEOS

NaF

0.25

2

TEOS

NaF

0.25

3

TEOS

NaF

4

Fumed silica

NaF

5

Fumed silica

NaF

6

Silica gel

NaF

7

TEOS

KF

8

Fumed silica

9

Silica gel

Time/d

Structure

Si:Al

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Sample

*

BEA

>48000

5

*

BEA

>48000

0.25

7

*

BEA

>48000

0.25

7

MFI

--

0.50

7

MFI

--

0.25

7

*

BEA

4400

0.25

7

cristobalite

--

KF

0.25

7

cristobalite

--

KF

0.25

7

cristobalite

--

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3

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When TEAOH was used in the preparation of silica zeolites, MFI or MTW topology material may also be synthesized [23]. The spectra in Fig 1 (b) illustrate that

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the zeolites formed from fumed silica (sample 4 and sample 5) are truly MFI topology when NaF was applied. However, the characteristic peak at 2θ = 22.4° in Figure 1 (a) indicates that *BEA zeolite was achieved by using NaF and TEOS/silica gel as the mineralization agent and silica resource, respectively. Denoting the crystallinity of the all-silica zeolite ß synthesized at near neutral pH as 100%, the relative crystallinity (R.C.) was calculated according to the integrated area of the band at 2θ = 22.4° . After crystallization under alkaline condition for 3 days, the R.C. of the silica ß obtained from TEOS is about 31.3%. With increasing crystallization duration, the crystals keep growing, the R.C. increases, and it’s about 79.1% on the seventh day.

ACCEPTED MANUSCRIPT For the zeolite ß synthesized using silica gel, a R.C. of about 72.1% is calculated. Moreover, the first peak at 2θ = 7.6° in the XRD patterns is identical with that of the conventional zeolite ß, which illustrates that the stacking probability of the silica zeolite ß synthesized following this strategy is about 0.6 [24]. After acid washing, the chemical composition of the zeolite β was analyzed by

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ICP, and the results are shown in Table 1. Due to the little aluminum in the silica gel, the Si:Al in the as-synthesized zeolite β is about 4400. On the other hand, there are almost no aluminum in the zeolite β prepared from TEOS, the Si:Al in which is more

(a)

R.C.=72.1%

Sam. 6

R.C.=79.1%

Sam. 3

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(b)

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than 48000, and the materials could be denoted as pure silica zeolite β.

Sam. 5

Sam. 4

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simulated XRD pattern of MFI topology

R.C.=69.5%

Sam. 2

R.C.=31.3%

Sam. 1

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5 10 15 20 25 30 35 2θ/°

5

10 15 20 25 30 35 2θ/°

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Figure 1 The XRD spectra of (a) sample 1-3 and sample 6, (b) sample 4 -5 and the simulated XRD pattern of MFI topology

The results mentioned above illustrate that the crystallization process may be

influenced by the silica resource and the alkali metal ions in the precursor. TEOS is preferred for the synthesis of pure silica zeolite β, and the sodium cations should be beneficial for the nucleation and crystallization of zeolite β. Figure 2 shows the FT-IR spectra in the framework vibration region of the pure silica β synthesized on different crystallization duration. In the FT-IR spectra, the peaks at 525 cm-1 and 575 cm-1 are regarded as fingerprints of zeolite with *BEA topology. The spectra in Figure 2 further illustrate that the materials synthesized on

ACCEPTED MANUSCRIPT the 3 ~ 7 days using TEOS as the silica resource are all BEA topology. *

100

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80

sam. 2 60

sam. 1 525

575

40

600

800 1000 1200 -1 Wavenumber/cm

1400

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400

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Transmittance/a.u.

sam. 3

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Figure 2 The FT-IR spectra of the pure silica ß synthesized using TEOS as the silica resource

Figure 3 The SEM images of (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 6

The SEM images of the prepared zeolite ß are illustrated in Figure 3. When HF

was used as the mineralization agent, the precursor turned into a white solid for the significantly decreased alkalinity, leading to decreased diffusion and nucleation rates, and the silica ß prepared was truncated square bipyramids, the crystal size of which was 15 ~ 20 μm (Figure S2). On the other hand, NaF is an alkali salt, the alkalinity of the precursor remained high when NaF was added, both the nucleation and crystallization process may be more fast, and zeolite crystals with smaller size should be synthesized. As the *BEA zeolites prefer to

ACCEPTED MANUSCRIPT grow along the [h0l] face under alkaline condition [21,22], the crystals achieved are all plate-like. When TEOS is the silica resource, pure silica ß with the crystal size of about 50×300×300 nm is synthesized on the 3rd day. However, the crystallization was not finished, the crystal size is varied, and amorphous silica species are also found in the products. Be consistent with the changes of the R.C.,

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the crystals keep growing with the time. The crystals achieved on the 7th day are more uniformly distributed, the size of which is about 200×700×700 nm. Although a small quantity of aluminum could be detected in the silica gel, which

the crystal size is about 0.5×2×2 μm. The

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is supposed to help the nucleation process, the silica ß formed is much larger,

Si MAS NMR spectra and the FT-IR spectra in the hydroxyl region of the

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zeolite ß synthesized on the 7th day are shown in Figure 4. In the

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Si MAS NMR

spectrum, the characteristic lines in the -105 ～ -125 ppm range are often attributed to the framework Si(OSi)4 species, while the signals between -90 ～ -100 ppm are associated with the Si(OSi)3(OH) sites [25]. When quaternary ammonium cations are used as the SDAs for zeolite synthesis, the positive electricity of which is

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supposed to be balanced by the Si-O- groups and trivalent elements, such as Al3+ and B3+, in the framework position, and the zeolites prepared are typically characterized with numerous framework defects. However, the fluorine anions could also balance the SDAs, and there are almost no defects in the materials synthesized in the

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presence of F- [26]. For the zeolite ß synthesized following this strategy, lines at -111.4, -112.8 and -114.9 ppm are observed in Figure 4, and no signals are found in

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the -90 ～ -100 ppm range, which indicate that the connectivity defects are absent in the as-prepared silica ß. Moreover, weak bands at bout 3720 cm-1 and 3680 cm-1 could be observed in the hydroxyl stretching region of the FT-IR spectra (Figure S3), while the peak at about 3515 cm-1 is absent, which give direct information that there are no nest silanol group, and the terminal and vicinal silanols are very little [27].

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sam. 6

-150

-140

-130

-120 -110 ppm

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sam. 3

-100

-90

-80

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Figure 4 The 29Si MAS NMR spectra of the silica zeolite ß synthesized on the seventh day

The water vapor adsorption isotherms of sample 3 and sample 6 are shown in Figure 5. During the water vapor adsorption process, the water molecular is supposed to interact firstly with the functional groups, such as the silanols, on the surface, then the water adsorption proceeds on the adsorbed water via the hydrogen

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bonding [28-29]. Thus, the hydrophobicity/hydrophilicity properties are related to the water adsorption on the surface, which should be finished in the initial adsorption stage. As there is no standard definition of hydrophobicity [30], the adsorption volume at the lowest relative pressure is selected to illustrate the

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hydrophobicity in this paper. Due to the little surface silanols in Sample 3 and Sample 6, the adsorption volume at P:P0 ≈ 0.01 is respectively 0.0037 mL/g and 0.8075 mL/g,

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which indicate that the pure silica zeolite ß is highly hydrophobic.

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5

140

4 3

120

2

0 0.00

0.05

0.10

0.15

0.20

80

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Volume(mL/g)

1

100

60 Sam. 3 Sam. 6

40

0 0.0

0.2

0.4

0.6 P:P0

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20 0.8

1.0

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Figure 5 The water vapor adsorption isotherm of Sample 3 and Sample 6

4. Conclusion

Pure silica zeolite ß with uniform crystal size was synthesized for the first time using sodium fluorine as the mineralization agent. Both the silica resource and the alkali metal ions in the precursor would influence the

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crystallization process, tetraethyl orthosilicate and sodium cations are preferred for the synthesis of pure silica zeolite with *BEA topology. The silica ß synthesized are plate-like, the crystal size increases with the time, and crystals with uniform size of about 200×700×700 nm are formed on the

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seventh day. Moreover, there are almost no framework defects, and the material is highly hydrophobic. The results mentioned above are encouraging, as more

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important materials, such as Ti-ß, Sn-ß and Zr-ß, may also be prepared following the strategy reported in this paper to improve the accessibility of the active sites inside the crystals.

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ACCEPTED MANUSCRIPT 1 Pure silica β was hydrothermally synthesized using NaF as the mineralization agent. 2 The crystals of the silica β are plate-like, the crystal size is about 200×700×700 nm.

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3 The silica β is defects-free, and it’s hydrophobic.