Preparation and Gas Permeation Properties of FluorineâSilica

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Preparation and Gas Permeation Properties of Fluorine-Silica Membranes With Controlled Amorphous Silica Structures: Effect of Fluorine Source and Calcination Temperature on Network Size Masakoto Kanezashi, Takuya Matsutani, Toru Wakihara, Hiroki Nagasawa, Tatsuya Okubo, and Toshinori Tsuru ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06800 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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ACS Applied Materials & Interfaces

Preparation and Gas Permeation Properties of Fluorine-Silica Membranes With Controlled Amorphous Silica Structures: Effect of Fluorine Source and Calcination Temperature on Network Size

Masakoto Kanezashi,1* Takuya Matsutani,1 Toru Wakihara,2 Hiroki Nagasawa,1 Tatsuya Okubo,2 and Toshinori Tsuru1 1

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University

Higashi-Hiroshima, 739-8527, Japan 2

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,

Tokyo, 113-8656, Japan * Corresponding Author: M. Kanezashi, E-mail: [email protected]

KEYWORDS: Amorphous silica; Fluorine; Molecular sieving; Pore size tuning; Thermal stability

ABSTRACT Triethoxyfluorosilane (TEFS), which is a pendant-type alkoxysilane with a Si-F bond, was utilized for the development of a molecular sieving membrane.

The effect that a source of fluorine

and calcination temperature exerted on gas permeation properties and network pore size was

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evaluated via single-gas permeation properties across a wide range of temperatures.

A TEFS

membrane calcined at 350 oC showed high H2 permeance (2.0 x 10-6 mol m-2 s-1 Pa-1) and high selectivity for H2 over larger molecules (H2/CF4: > 300, H2/SF6: > 18,000), indicating that this network pore size would be suitable for a H2 permselective membrane that could promote the process of methylcyclohexane (MCH) dehydrogenation to produce toluene (TOL).

Based on the

gas permeation properties and the results of XPS and FTIR, network pore size depended on the fluorine concentration incorporated in SiO2 that existed as Si-F bonds, irrespective of the fluorine source.

A TEFS membrane showed approximately the same pore size distribution and level of gas

permeance, irrespective of calcination temperature (350, 550 oC), due to the low Si-OH density in the networks as suggested by the result of FTIR, which can prevent the densification caused by the condensation of Si-OH groups.

The pair distribution function also suggested that densification of

the network structure for TEFS was apparently suppressed compared with that of a tetraethoxysilane (TEOS)-derived structure.

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1. INTRODUCTION Porous silica has an amorphous structure and high levels of chemical and thermal stability, which makes it a promising membrane material for gas and liquid separation.1

Thin microporous

SiO2 layers are generally fabricated on a porous substrate (α-alumina, silica glass) via one of two accepted methods: CVD and sol-gel.1,2

Generally, the pore size of an amorphous SiO2 layer

fabricated via the sol-gel method can be controlled via polymeric and colloidal routes to produce an appropriate pore size for a particular separation system.3 When using the colloidal sol-gel route, tuning the spaces (grain boundaries) to a level of several angstroms is considered difficult. Therefore, a polymeric route was developed for molecular sieving membranes used for gas separation whereby pore sizes would correspond to the spaces within the amorphous silica networks.4 In the 1990s, sol-gel derived amorphous SiO2 membranes with Si-O-Si bonding and Si-OH groups and network structures created by tetraethoxysilane (TEOS) showed excellent separation for either H2 or He under high temperatures.1,2

Using permeation models and positron

annihilation spectroscopy (PALS), several studies have reported network sizes ranging from 0.3 to 0.4 nm,5,6 which shows a high level of H2 selectivity according to molecular sieving properties. However, this suggested that the silica network size was small for separation of CO2/CH4, C1/C2, and olefin/paraffin systems.

Since the silanol (Si-OH) groups are flexible and easily create

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Si-O-Si bonds by condensation reaction during the membrane fabrication (gelation) process,3 controlling the Si-OH density is very important when designing a network size for amorphous SiO2. There are two accepted strategies to control the Si-OH density via a sol-gel method. approach involves changing the calcination temperature.6-8

One

de Vos and Verweij 7 reported drastic

decreases in gas permeance (H2, CO2, O2, N2, and CH4) with enhanced H2 selectivity due to the densification of amorphous silica by increasing the calcination temperature from 400 to 600 oC. Kanezashi et al. 8 reported that amorphous SiO2 membranes calcined at 700 oC showed a high molecular sieving effect for both helium (0.26 nm) and hydrogen (0.289 nm) molecules and water vapor (0.2995 or 0.317 nm), which resulted in improved hydrothermal stability.

Thus, a higher

calcination temperature accelerates the condensation of Si-OH groups with a corresponding decrease in Si-OH groups and a simultaneous formation of Si-O-Si bonds, which improves H2 selectivity and hydrothermal stability.

The design of loose silica networks simply by controlling

the calcination temperature was possible with amorphous structures containing organic groups, which is referred to as the “organic-template” method.9-11

The decomposition of organic groups

in an amorphous structure at high temperatures leaves spaces, depending on the size and shape of the organic groups, which results in the formation of loose silica networks. Another approach involved the doping of halogen atoms for optical silica glass.12-15

Using

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the sol-gel method, the Si-OH groups in optical silica glasses were generally eliminated via doping with halogen atoms to form Si-Cl and Si-F bonds, because the vibrations of the Si-OH groups quenched the radiative emissions.

Common methods to eliminate dense Si-OH groups have

involved the use of triethoxyfluorosilane (TEFS) containing Si-F bonds, the addition of NH4F and alkali fluorides, and HF-catalyzed reactions.

The replacement of Si-OH groups with Si-F groups

makes it possible to control the hydrophobic/hydrophilic properties, and a hydrophobic surface modification of porous media (zeolite, mesoporous silica (MCM-41)) was successfully achieved in this manner.16-18

Kasinov et al.18 reported that hydrophobic MFI and MEL zeolite membranes

treated with TEFS showed improved ethanol separation from ethanol/water mixtures via pervaporation, and the grafted Si-F groups were hydrothermally stable in aqueous solutions. Incorporated fluorine is known to affect the angle of Si-O-Si bonds and change the density of SiO2.19

However, no researchers have reported tailoring the network pore sizes of microporous

materials via fluorine doping for molecular permeation. Recently, we proposed an innovative strategy to design highly permeable CO2 separation membranes via the fluorine doping of a SiO2 structure.20

Fluorine doping was effective in creating

large network pore sizes when NH4F was utilized as a fluorine source, and F-SiO2 (F/Si=1/9) membranes have shown a CO2 permeance of 4.1 x 10-7 mol m-2 s-1 Pa-1 with CO2/CH4 selectivity of approximately 300 at 35 oC.

The estimated network size was 0.46 nm, which was similar to the

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channel size of DDR-type zeolite.21-24

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It was possible to fabricate the thin layer formation

(100-200 nm), irrespective of fluorine doping.

The pair distribution function characterized via

X-ray Total Scattering (HEXTS) measurements suggested that fluorine doping was effective in preventing the formation of a 2nd Si–O in the 4 membered ring (MR) of the SiO4 tetrahedra, where even small molecules (He, H2) cannot permeate, resulting in the formation of loose and uniform structure compared with SiO2 (F=0), so that F-SiO2 structure contained high fractions of larger rings (> 5MR), that is, the formation of loose structure compared with SiO2 (F=0).20

Moreover, in

previous studies, the order of the network pore size has increased as F concentration increases. In the present study, TEFS, which is a pendant-type alkoxysilane with a Si-F bond, was utilized as a Si precursor for the fabrication of molecular sieving membranes.

The effect of fluorine and

calcination temperature on gas permeation properties and network pore size was evaluated via single-gas permeation properties across a wide range of temperatures.

An intermediate range of

structures for TEOS and TEFS products before and after calcination was evaluated via X-ray Total Scattering (HEXTS).

The effect of the fluorine source and the calcination temperature on network

structure was qualitatively evaluated via FTIR and XPS.

2. EXPERIMENTAL SECTION 2.1 Preparation of TEFS Sol and Membrane Fabrication

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Hydrolysis and polymerization processes were applied to prepare a TEFS sol under acidic conditions.

A simple TEFS sol was prepared by the following procedure.

of TEFS was added to ethanol under vigorous stirring at 25 ºC.

A specified amount

A specified amount of water and

HNO3 was added to maintain the solution composition (TEFS/EtOH/H2O/HNO3 molar ratio: 1/709/200/0.1 TEFS weight%: 0.5 wt%).

These processes were conducted at 25 oC for 30 min to

control the sol size to within a range of 1-3 nm. Porous silica glass tubes (porosity: 60%, pore size: 500 nm) provided by Sumitomo Electric Industries Ltd. were used as supports for the TEFS-derived membranes.

A picture of the porous

silica glass tube is shown in the supplementary information (Figure S1).

Silica glass particles

(average particle diameter: 300 nm) dispersed in SiO2-ZrO2 sol were coated onto porous glass tubes, followed by calcination at 550 oC under air, to form a thermally stable intermediate layer.6,8,20 Then, the TEFS sol was diluted to approximately 0.15 wt% and coated onto the silica glass intermediate layer, which was followed by calcination at 350-550 oC under air for 30 min, for the formation of a separation layer.

These procedures were repeated approximately 4 times to form a

thin defect-free separation layer.

2.2 Characterization of TEFS Gels by FTIR, XPS, and X-ray Total Scattering (HEXTS) Powdered samples of the TEFS gels were prepared by drying at 40 oC, followed by calcination

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at 350-800 oC under air for 30 min, then grinding in a mortar.

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Prior to the FTIR measurement,

prepared samples were kept in a dry container (SD-702-1A, TOYO LIVING Co., Ltd., Japan). The diffuse reflectance FT-IR spectra were recorded using a FT-IR spectrometer (FT/IR-4100, Jasco, Japan) for TEFS samples, which were diluted approximately 10% with KBr (10%: F-SiO2, 90%: KBr).

The status of the fluorine in the network structure and the element composition (F/Si

molar ratio) of the powdered samples was characterized via X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, ESCALAB 250Xi, Al-Ka = 1486.6 eV).

The peak area ratio of

Si 2p at 104.6 eV and Si-F at 688 eV by XPS spectra and Si-O-Si bonds at 1100 cm-1 and Si-OH groups at 3740 cm-1 by FTIR spectra was calculated by Gaussian distribution fitting, respectively. X-ray Total Scattering (HEXTS) measurements were performed on powder samples in a quartz capillary at room temperature using a horizontal two-axis diffractometer at the BL04B2 high-energy X-ray diffraction beamline (SPring-8, Japan). The obtained data were normalized to give the Faber-Ziman total structure factor S(Q). distribution function (PDF; G(r)).

The collected data were used to calculate the pair

Conditions of the measurement and analysis were adopted from

a previously reported paper.20

2.3 Gas Permeation Measurement Figure 1 shows a schematic diagram of the single-gas permeation measurement.

The

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permeation module was made of quartz glass (22φ I.D., 350 length), and the membrane tube was fixed in the module and connected to a permeate stream using heat-shrinkable tubing (FEP-090, Junkosha Inc. Japan).

A picture of the permeation module is shown in Figure S2.

First, TEFS

membranes were heat-treated at 300 oC under a N2 flow of approximately 100 ml min-1 to remove the adsorbed water molecules.

A steady state was confirmed by measuring the time course of N2

permeance at the same temperature.

Then, single gasses (He, H2, CO2, N2, CH4, C2H6, CF4, and

SF6) were fed onto the membrane surface at atmospheric pressure and 35-300 oC, while the permeation side was evacuated using a vacuum pump.

The values for permeance were calculated

from the observed pressure difference across the membrane, and from the permeation rate, which was obtained via a calibrated critical nozzle placed between the permeation cell and the vacuum pump.6,8 It should be noted that a calibrated critical nozzle is a kind of orifice that is used to measure a pressure difference, depending on the permeation rate and gas species.

The pressure

difference can be found in front of and behind a calibrated critical nozzle.

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3 2

1

1 4

5

1. Gas cylinder 2. Pressure controller 3. Pressure gauge 4. Mass flow controller 5. Flow meter 6. Electric furnace

6

7. Permeation module 8. Membrane 9. Temperature controller 10. Stop valve 11. Critical nozzle 12. Vacuum pump 13. Bubble flow meter

3 10

7

11

8

12

13 9

Figure 1 Schematic diagram of the single-gas permeation measurement.

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RESULTS and DISCUSSION

3.1 Effect of Fluorine Source on Network Pore Size and Gas Permeation Properties Figure 2 shows the molecular size dependence of the values for gas permeance at 300 oC for a TEOS and a TEFS membrane, both of which were fabricated at 350 oC.

The results of

fluorine-doped SiO2 membranes (F/Si molar ratio: 1/9, 2/8), where NH4F and TEOS were utilized as fluorine and Si sources, respectively, are also shown.20

H2 permeance and the gas permeance

ratios (H2/N2 and N2/SF6) at 300 oC for TEOS, TEOS-NH4F (F/Si=1/9, 2/8),20 and TEFS membranes are summarized in Table 1.

The TEOS membrane showed H2 permselective properties

(H2 permeance: 1.3 x 10-6 mol m-2 s-1 Pa-1, H2/CH4: 300).

The H2/N2 selectivity clearly decreased

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with increased concentration of F, and each gas permeance increased as F concentration increased.17 For example, a TEOS-NH4F (F/Si=1/9) membrane showed higher H2 permeance compared with that of a SiO2 membrane, and showed high selectivity for H2/CF4 (H2 permeance: 1.6 x 10-6 mol m-2 s-1 Pa-1, CF4: less than10-10 mol m-2 s-1 Pa-1) and moderate selectivity for H2/N2 (~40) and CO2/CH4 (~30) at 300 oC.

Interestingly, a TEFS membrane showed approximately the same value of gas

permeance and pore size distribution as that of F-SiO2 (TEOS-NH4F) membrane (F/Si =2/8), despite having higher F/Si molar ratio of 1/1 in monomer.

10-5 He

Permeance [mol m-2 s-1 Pa-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2

10-6

CO2

N2 CH4

C2H6

10-7 CF4

10

-8

10-9

10-10 0.25

TEOS TEOS-NH4F(F/Si=1/9) TEOS-NH4F(F/Si=2/8) TEFS

0.3

0.35

0.4

SF6

0.45

0.5

0.55

Molecular size [nm]

Figure 2 Molecular size dependence of gas permeance at 300 oC for TEOS, TEOS-NH4F (F/Si=1/9, 2/8) 20, and TEFS membranes fabricated at 350 oC.

The experimental error in each permeance was

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less than 5%.

Table 1 H2 permeance and gas permeance ratio (H2/N2, N2/SF6) at 300 oC for TEOS, TEOS-NH4F (F/Si=1/9, 2/8) 20, and TEFS membranes fabricated at 350 oC. The experimental error in each permeance was less than 5%.

H2 permeance [10-6 mol m-2 s-1 Pa-1]

H2/N2 [-]

N2/SF6 [-]

F=0

1.3

136

-

F/Si = 1/9

1.6

40

-

F/Si = 2/8

2.3

10

125

TEFS

2.0

8.9

2100

Figure 3 (a) shows the temperature dependence of an F-SiO2 (TEOS-NH4F (F/Si=1/9)) membrane for temperatures ranging from 35-300 oC.

He, H2, N2, CH4, and C2H6 all showed an

activated permeation mechanism, and the temperature dependence slope of CH4 was the largest of these molecules; the permeance of CH4 largely increased with temperature (activated diffusion). An F-SiO2 membrane (F/Si=1/9) showed H2/C2H6 selectivity of more than 1000 at 35-300 oC, and the selectivity for H2 (H2/N2, H2/CH4) increased as the temperature decreased.

The slope of

temperature dependence for He, H2, N2, and CH4, corresponding to apparent activation energy,

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increased as molecular size increased.

de Vos and Verweij summarized the values for the heat of

adsorption for H2, N2, and CH4 on microporous silica (5-20 kJ mol-1), and the calculated activation energy of diffusion through microporous silica membranes increased as molecular size increased,7 which is consistent with the results in the present study. that can be achieved by uniform pores with less pinholes.

Thus, these molecules permeated pores The slope of temperature dependence

for C2H6 was smaller than that for CH4 because C2H6 has better affinity than CH4.25 The permeance of CO2 increased with decreasing temperature, which is the result of a surface diffusion mechanism.26

The permeance of CO2 peaked between 35 and 50 oC, which was caused

by a decrease in the number of adsorbed molecules at temperatures higher than the peak, while adsorption, which is enhanced at temperatures lower than the peak, decreased the mobility of molecules.27

The CO2/CH4 permeance ratio largely increased with decreasing temperature, and

was approximately 300 when CO2 permeance was greater than 10-7 mol m-2 s-1 Pa-1at 35 oC.20 Figure 3 (b) shows the temperature dependence of gas permeance for a TEFS membrane at temperatures ranging from 50-300 oC.

The permeance values of CF4 and SF6, both of which have

a larger molecular size, seemed almost independent of temperature, and only the permeance of small molecules slightly increased as temperature decreased (Knudsen diffusion).

Knudsen

diffusion through loose networks dominated the permeation properties of small molecules, resulting in low H2/N2 selectivity.

However, a TEFS membrane showed high H2 permeance and selectivity

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for H2 over large molecules (H2/SF6: > 18000).

These results suggest that network pore size

derived from TEFS would be suitable as a H2 permselective membrane for methylcyclohexane

104 103 102

Temperature [oC] 200 100

300

50 35

Permeance ratio [-]

Permeance ratio [-]

(MCH) dehydrogenation to toluene (TOL).28-30

H2/C2H6 H2/CH4 CO2/CH4

101 100 10

-5

10

-6

105

Temperature [oC] 200 100

300

104 103

H2/CF4

102 H2/CH4

101 100

10-5 H2 -1 -2

N2 CH4

10-8

C2H6

10-9

2

10

-6

CO2

He

-1

CO2

-1 -2

Permeance [mol m s Pa ]

-1

He

10-7

10-10 1.5

50

H2/SF6

H2

Permeance [mol m s Pa ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5

3

3.5

10-7

CH4

C2H6

10-8

CF4

10-9

10-10 1.5

SF6

2

1000/T [K ]

2.5 -1 1000/T [K ]

(a)

(b)

-1

3

3.5

Figure 3 Temperature dependence of F-SiO2 (TEOS-NH4F (F/Si=1/9)) (a) and TEFS (b) membranes for temperatures ranging from 35-300 oC.

The experimental error in each permeance was less than

5 %.

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The apparent activation energy (Ep) and pre-exponential parameter k0 was obtained via a modified gas translation (GT) model (Eq. (1)), and was used to estimate the pore size.6,31 With the pre-exponential parameter k0 expressed using Eq. (2), network pore size, dp, can be obtained by the relationship between the k01/3 of each gas molecule and the molecular size.

The theory for using

the k0 plot to evaluate pore size was described in a previously published paper.6

(d p − d i ) ε Pi = (d p − d i ) 3τL d p2 k 0 ,i =

2

k 0 ,i  E p ,i   E p ,i  8  =  exp − exp − πM i RT M i RT  RT   RT 

ε (d p − d i ) 3 8 = a(d p − d i ) 3 2 π 3τL dp

(1)

(2)

Figure 4 shows the relationship between k0,i1/3 and molecular size (di) for TEOS, TEOS-NH4F (F/Si=1/9, 2/8), and TEFS membranes fabricated at 350 oC.

A correlation was confirmed in the

obtained k01/3 for each molecule, which suggested a monomodal structure.

The obtained network

pore sizes for TEOS, TEOS-NH4F (F/Si=1/9, 2/8), and TEFS membranes were 0.39, 0.46, 0.58, and 0.57 nm, respectively.

When TEOS and NH4F was utilized for the fabrication of F-SiO2

membranes, the network pore size was increased as F concentration increased.

The network pore

size of a TEFS membrane was approximately the same as that of a TEOS-NH4F (F/Si=2/8)

15



membrane, despite having a monomer F/Si molar ratio of 1/1 that was higher than that of TEOS-NH4F (F/Si=2/8).

0.04

0.03

TEOS-NH4F(F/Si=1/9)

0.03

He

[-]

0.02

1/3

k0,i

k0,i

1/3

[-]

H2 TEOS

0.01

N2

CO2

0.02

0.3 0.4 0.5 Molecular size [nm]

0 0.2

0.6

0.03

H2

He

0.01

CH4

0 0.2

N2 CO2

CH4 C2H6

0.3 0.4 0.5 Molecular size [nm] TEFS

0.01

C2H6

CH4 CF4

0 0.2

H2

0.02 He

CO2

1/3

N2

[-]

CO2

k0,i

He

1/3

[-]

H2

0.02

0.6

0.03 TEOS-NH4F(F/Si=2/8)

k0,i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N2

0.01

CH4

C2H6 CF4

SF6


0.6

SF6

0 0.2


0.6

Figure 4 Relationship between k0,i1/3 and molecular size (di) for TEOS, TEOS-NH4F (F/Si=1/9, 2/8), and TEFS membranes fabricated at 350 oC.

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Figure 5 shows the XPS spectra in the range of 0-1200 eV (a) and from 680-695 eV (b) for TEOS-NH4F (F/Si = 2/8) and TEFS gel powders calcined at 350 ºC.

The F/Si molar ratio for each

sample was calculated from each peak area ratio of Si 2p at 104.6 eV and Si-F at 688 eV,32 respectively, and is summarized in Table 2.

F-SiO2 powder prepared using NH4F and TEOS as the

fluorine and Si sources, respectively, showed approximately the same F/Si molar ratio as the doped fluorine concentration in the sol.

A TEFS powder showed approximately the same F/Si molar

ratio as that of TEOS-NH4F (F/Si=2/8), although it was much smaller than the ideal F/Si molar ratio based on the chemical structure (F/Si=1/1).

The decreased fluorine concentration was unclear at

that moment, but it would have been caused by a reaction between the Si-OH and Si-F groups to create hydrogen fluoride (HF) and/or the hydrolysis of Si-F during the sol preparation and/or the gelation process.33

Both samples showed a peak at around 688 eV assigned to the Si-F bonds,32 so

that the doped F was present in the SiO2 structure as Si-F bonds, irrespective of the fluorine source, according to the XPS narrow spectra shown in Fig. 5 (b).

17


C 2s

TEOS-NH4F (F/Si=2/8)

1200

1000

800

600

Counts [a. u.]

Si 2s Si 2p

F 1s

C 1s

TEFS

F KLL

O KLL

Si-F

Counts [a. u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

O 1s


TEFS

TEOS-NH4F (F/Si=2/8)

400

200

0

695

690

685

Binding energy [eV]

Binding energy [eV]

(a)

(b)

680

Figure 5 XPS spectra ranging from 0-1200 eV (a) and 680-695 eV (b) for TEOS-NH4F (F/Si = 2/8) and TEFS gel powders calcined at 350 ºC.

Table 2 Fluorine concentrations of TEOS-NH4F (F/Si=2/8) and TEFS gel powders calcined at 350 o

C.

Fluorine concentration [-]

Sample

Ideal

Estimated by XPS

TEOS-NH4F

0.2

0.182

TEFS

0.5

0.189

Figure 6 shows the FTIR spectra in the range of 900-1200 cm-1 for TEOS, TEOS-NH4F (F/Si =

18


Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5/9.5, 1/9, 2/8)20, and TEFS gel powders calcined at 350 ºC.

All samples showed the peak in the

range of somewhere between 1037 and 1080 cm-1, which was assigned to the asymmetric vibrations of the Si-O-Si bond,34,35 irrespective of either fluorine concentration or source (NH4F, TEFS), indicating the formation of a SiO2 network via hydrolysis and polymerization of the ethoxy and Si-OH groups.

The peak at 960-980 cm-1 can be assigned to the stretching of Si-OH bonds.34

Fluorine-SiO2 gel derived from TEOS-NH4F and TEFS showed a peak at 940 cm-1 (Si-F bond),14,19,33 which increased with the F concentration.

The peak position of the Si-O-Si bond was

clearly shifted to a higher wave number (blue shift) with increased fluorine concentration, which was a trend that was noted in a previously published paper.19,20

A TEFS powder showed the peak

position of a Si-O-Si band at around 1090 cm-1, which was approximately the same as that of TEOS-NH4F gel (F/Si=2/8). A frequency shift in the FTIR spectra corresponded to changes in the bond angle.19,36

A

force-constant model for vibrational properties was adapted to calculate the Si-O-Si bond angle,19,36-38 and the result is shown in the supporting information (Figure S3). angle was increased with F concentration.

A Si-O-Si bond angle of 149

o

The Si-O-Si bond

was obtained in TEFS

gel, which was approximately the same as that of TEOS-NH4F (F/Si=2/8) and was 10 o greater than that of SiO2.

Since the results of XPS established that the F/Si molar ratio of TEFS calcined at 350

o

C was approximately the same as that of TEOS-NH4F gel (F/Si=2/8), the detection of Si-O-Si

19



bonds at the same wavenumber was acceptable and suggested that fluorine concentration incorporated in SiO2 as Si-F bonds could affect the change in the SiO2 structure.

Kim et al. 19 used

spectroscopic ellipsometry to report that the SiOF film density decreased with F concentration because of the blue shift (an increased Si-O-Si bond angle).

Based on the gas permeation

properties and results of XPS and FTIR, it can be concluded that fluorine concentration incorporated into SiO2 as Si-F bonds, irrespective of the fluorine source, could affect the Si-O-Si bond angle, and it is possible that this change is responsible for the formation of a loose structure.

Si-O-Si Si-OH Si-F blue-shift TEFS

K/M [a. u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

F/Si=2/8 F/Si=1/9 F/Si=0.5/9.5 TEOS

1200

1100

1000

900 -1

Wavenumber [cm ]

Figure 6 FTIR spectra in the range of 900-1200 cm-1 for TEOS, TEOS-NH4F (F/Si = 0.5/9.5, 1/9, and 2/8),20 and TEFS gel powders calcined at 350 ºC.

20


Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 Effect of Calcination Temperature on Network Pore Size and Gas Permeation Properties Figure 7 shows the molecular size dependence of gas permeance at 300 oC for TEOS and TEFS membranes fabricated at 350 and 550 oC, respectively.

When a TEOS membrane was calcined at

550 oC, the permeation for each gas was decreased greatly, but He selectivity was increased.

For

example, a TEOS membrane calcined at 550 oC showed a He permeance of 4.0 x 10-7 mol m-2 s-1 Pa-1, which was approximately half that calcined at 350 oC, and the membrane showed He/H2 and He/CH4 selectivities of 2.6 and 600, respectively.

On the other hand, a TEFS membrane showed

approximately the same pore size distribution and gas permeance, irrespective of calcination temperature (350, 550 oC).

A TEFS membrane calcined at 550 oC showed He permeance of 1.3 x

10-6 mol m-2 s-1 Pa-1 with He/H2, He/CH4, and He/SF6 selectivities of 0.7, 6.8, and 3500, respectively.

This indicates that utilizing TEFS as a Si precursor prompted the formation of a

thermally stable and loose amorphous network, which would be suitable for the separation of H2 over large molecules at high temperatures, such as with the dehydrogenation of alkane.39

21



10-5 He


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

H2

10-6

CO2

N2 CH4

C2H6

10-7 TEFS

10-8

CF4

TEOS

10-9

10-10 0.25

SF6

0.3

0.35

0.4

0.45

0.5

0.55

Molecular size [nm]

Figure 7 Molecular size dependence of gas permeance at 300 oC for TEOS and TEFS membranes (open symbols: membrane fabricated at 350 oC; closed symbols: membrane fabricated at 550 oC). The experimental error in each permeance was less than 5 %.

Figure 8 shows the temperature dependence of He and H2 permeance for TEOS (a) and TEFS (b) membranes calcined at 350 and 550 oC, respectively. When a TEOS membrane was calcined at 350 oC, the permeance values for He and H2 were slightly increased as the temperature decreased (activated diffusion), while that for a TEFS membrane calcined at 350 oC were slightly increased as the temperature decreased (Knudsen type diffusion).

A TEOS membrane calcined at 550 oC

showed decreased He and H2 permeance by comparison with those calcined at 350 oC, and the slope

22


Page 23 of 40

of temperature dependence, which corresponds with the activation energy of gas permeation, was greatly increased.

On the contrary, a TEFS membrane calcined at 550 oC showed no large

differences between the slope of temperature dependence and the value of He and H2 permeance. The activation energy based on the modified GT model (Eq. (1)) 6,31 is also shown in the same figure.

It should be noted that the activation energy indicates the repulsive force of gas molecules

for diffusion through network pores, so that a higher activation energy indicates a smaller network pore size.6,8,40,41

Thus, the increased activation energy for SiO2 membranes derived from TEOS

can be reasonably explained by a densification of the SiO2 structure.

Temperature [oC] 100 50 300 200

10-6

3.8 kJ mol-1

He 9.4 kJ mol-1 H 2

Temperature [oC] 100 300 200 35

10-5


500

10-5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.3 kJ mol-1

H2

0.7 kJ mol-1

He

0.4 kJ mol-1

10-6

14.6 kJ mol-1

10-7

1

2

-1

3

4

10-7

1

2

-1

1000/T [K ]

1000/T [K ]

(a)

(b)

3

4

Figure 8 Temperature dependence of He and H2 permeance for TEOS (a) and TEFS (b) membranes

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

(open symbols: membrane fabricated at 350 oC; closed symbols: membrane fabricated at 550 oC). The experimental error in each permeance was less than 5 %.

Figure 9 shows the FTIR spectra in the range of 500-4000 cm-1 for TEOS and TEFS gel powders calcined at 350 ºC.

A TEOS powder showed a peak at 3400 cm-1, which can be ascribed

to stretching of the OH groups in chemisorbed H2O molecules29 due to the hydrophilic nature of SiO2.

On the other hand, TEFS powder showed fewer chemisorbed H2O molecules, which

indicated a hydrophobic nature due to the presence of Si-F bonds.13-15 The sharp peak at 3740 cm-1 can be assigned to the isolated Si-OH groups on silica, which is generally used for quantitative evaluation of the Si-OH groups in a silica structure.42

The peaks at around 1100 and 3740 cm-1

were assigned to Si-O-Si bonds and Si-OH groups,29 respectively, and were used to evaluate the Si-OH density normalized with Si-O-Si for TEOS and TEFS powders. Figure 10 shows the Si-OH/Si-O-Si peak area ratio for TEOS and TEFS powders as a function of calcination temperatures. The peak area ratio of Si-OH/Si-O-Si for a TEOS powder was higher than that for a TEFS powder, irrespective of calcination temperatures. The peak area ratio of Si-OH/Si-O-Si for a TEOS powder was greatly decreased as calcination temperature increases. On the contrary, the peak area ratio of Si-OH/Si-O-Si for a TEFS powder decreased slightly within a range of temperatures ranging from 350-500 oC, and was approximately constant above 500 oC.

24


Page 25 of 40

Thus, the degree of densification for the network structure of a TEFS membrane caused by the condensation of Si-OH groups when calcined at high temperatures was much smaller than that of the TEOS version; network pore size derived from TEFS seemed independent of calcination temperature, but that derived from TEOS was greatly dependent on calcination temperatures; that is, the network pore size was decreased as calcination temperature increased.

Si-OH

K/M [a. u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Si-O-Si

TEFS water

TEOS

4000

3000

2000

1000 500 -1

Wavenumber [cm ]

Figure 9 FTIR spectra in the range of 500-4000 cm-1 for TEOS and TEFS gel powders calcined at 350 ºC.

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Si-OH/Si-O-Si peak area ratio [-]


Page 26 of 40

0.3

0.2 TEOS

0.1 TEFS

0 300

400

500

600

700

800

900

Calcination temperature [ ℃]

Figure 10 The Si-OH/Si-O-Si peak area ratios for TEOS and TEFS powders as a function of calcination temperature.

The intermediate-range structures of products were evaluated by HEXTS.

The PDFs, G(r), of

TEOS and TEFS powders before and after calcination at 350 oC are shown in Figure 11. The Faber-Ziman total structure factor S(Q) is shown in the Supplementary Information (Figure S4). The PDFs provide quantitative information on the short- and intermediate-range scale structures in real space.20

The peaks at 1.6, 2.6, and 3.1 Å are related to the nearest Si-O/Si-F (overlapped), O–

O, and Si–Si, respectively.

The peaks between 3.5 and 6.0 Å correspond to the second-nearest Si–

O(F) or Si–Si correlations.

Structural differences between TEOS and TEFS powders before and

after calcination at 350 oC can be understood by examining these peaks.

26


Page 27 of 40

The TEOS samples are assumed to have a different intermediate-range structure before and after calcination.

O-O and Si-Si peaks are less pronounced after calcination, indicating the formation of

a more random structure.

Furthermore, the peaks between 3.5 and 4.5 Å mainly correspond to the

second-nearest Si–O(F) correlations, particularly those around 4.1 Å that are more pronounced; these form a different intermediate-range structure after calcination.

A comparison of the PDFs

for the TEFS samples before and after calcination revealed similar intermediate-range structures (see Figure 11(b)), which reflected fewer structural changes during calcination.

These data

support the differences in density between the Si-OH groups in TEOS and TEFS powders, as shown in Figure 10.

8

8 6

Si-O/Si-F

Si-O/Si-F

6

Before calcination (gelation at 40 oC)

Before calcination (gelation at 40 oC)

4 G(r)

4 G(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2nd Si-O(F)

2

Si-Si

2

Si-Si

O-O

O-O

0

0 -2 1.5

2nd Si-O(F)

After calcination at 350 oC

2

2.5

3

3.5 r [Å]

4

4.5

5

5.5

-2 1.5

After calcination at 350 oC

2

2.5

(a)

3

3.5 r [Å]

4

4.5

5

5.5

(b)

Figure 11 Pair distribution functions of TEOS (a) and TEFS (b) powders before (Blue line) and after

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

(Red line) calcination at 350 oC.

4

CONCLUSIONS Triethoxyfluorosilane (TEFS) was utilized for the development of a molecular sieving

membrane.

The effect that a source of fluorine and calcination temperature exerted on the network

pore size and gas permeation properties was evaluated according to the single gas permeation properties across a wide range of temperatures.

A TEFS membrane calcined at 350 oC showed

high H2 permeance and a high level of selectivity for H2 over large molecules (H2/SF6: > 18000), and showed approximately the same values for gas permeance and pore size as that of a F-SiO2 (TEOS-NH4F) membrane (F/Si=2/8), despite having a higher F/Si monomer molar ratio of 1/1. A TEFS powder calcined at 350 oC showed approximately the same F/Si molar ratio as that of TEOS-NH4F (F/Si=2/8), because of the reaction of Si-OH groups and Si-F groups during the sol preparation and/or gelation process.

Based on the gas permeation properties and results of XPS

and FTIR, the incorporated fluorine concentration in SiO2 in the form of Si-F bonds could have affected the Si-O-Si bond angle, and this change in the SiO2 structure was suggested to have caused the formation of a loose structure compared with that of conventional SiO2. When a TEOS membrane was calcined at 550 oC, the permeance for each gas was greatly decreased, but He selectivity was increased.

The decreased gas permeance and increased

28


Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


selectivity could be caused by densification of the SiO2 structure.

On the other hand, a TEFS

membrane showed approximately the same pore size distribution and gas permeance regardless of the calcination temperature (350 and 550 oC), which was due to the low density of the Si-OH groups as suggested by the result of FTIR.

The results of the pair distribution function suggested

that densification of the network structure for the TEFS membrane during calcination was suppressed compared with that of the TEOS-derived network structure.

ACKNOWLDGEMENTS HEXTS experiments conducted at SPring-8 were approved by the Japan Synchrotron Radiation Research Institute under proposal numbers 2015A0115 and 2015B0115.

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McDonald, R. S. Surface Functionality of Amorphous Silica by Infrared Spectroscopy. J. Phys. Chem. 1958, 62, 1168-1178.

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Figure Captions

Figure 1 Schematic diagram of the single-gas permeation measurement.

Figure 2 Molecular size dependence of gas permeance at 300 oC for TEOS, TEOS-NH4F (F/Si=1/9, 2/8) 20, and TEFS membranes fabricated at 350 oC.

The experimental error in each permeance was

less than 5%.

Figure 3 Temperature dependence of F-SiO2 (TEOS-NH4F (F/Si=1/9)) (a) and TEFS (b) membranes for temperatures ranging from 35-300 oC.

The experimental error in each permeance was less than

5%.

Figure 4 Relationship between k0,i1/3 and molecular size (di) for TEOS, TEOS-NH4F (F/Si=1/9, 2/8), and TEFS membranes fabricated at 350 oC.

Figure 5 XPS spectra ranging from 0-1200 eV (a) and 680-695 eV (b) for TEOS-NH4F (F/Si = 2/8) and TEFS gel powders calcined at 350 ºC.

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Figure 6 FTIR spectra in the range of 900-1200 cm-1 for TEOS, TEOS-NH4F (F/Si = 0.5/9.5, 1/9, and 2/8),17 and TEFS gel powders calcined at 350 ºC.

Figure 7 Molecular size dependence of gas permeance at 300 oC for TEOS and TEFS membranes (open symbols: membrane fabricated at 350 oC; closed symbols: membrane fabricated at 550 oC). The experimental error in each permeance was less than 5%.

Figure 8 Temperature dependence of He and H2 permeance for TEOS (a) and TEFS (b) membranes (open symbols: membrane fabricated at 350 oC; closed symbols: membrane fabricated at 550 oC). The experimental error in each permeance was less than 5%.

Figure 9 FTIR spectra in the range of 500-4000 cm-1 for TEOS and TEFS gel powders calcined at 350 ºC.

Figure 10 The Si-OH/Si-O-Si peak area ratios for TEOS and TEFS powders as a function of calcination temperature.

Figure 11 Pair distribution functions of TEOS (a) and TEFS (b) powders before (Blue line) and after

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(Red line) calcination at 350 oC.

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Table Captions

Table 1 H2 permeance and gas permeance ratio (H2/N2, N2/SF6) at 300 oC for TEOS, TEOS-NH4F (F/Si=1/9, 2/8) 17, and TEFS membranes fabricated at 350 oC. The experimental error in each permeance was less than 5%.

Table 2 Fluorine concentrations of TEOS-NH4F (F/Si=2/8) and TEFS gel powders calcined at 350 o

C.

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Table of Contents

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Preparation and Gas Permeation Properties of FluorineâSilica

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