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Effect of crystal size on framework defects and water uptake in fluoride mediated silicalite-1 Michelle E Dose, Ke Zhang, Joshua A Thompson, Johannes Leisen, Ronald R. Chance, William J. Koros, Benjamin A McCool, and Ryan P Lively Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm500914b • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 4, 2014

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Chemistry of Materials

1

Effect of crystal size on framework defects and water uptake

2

in fluoride mediated silicalite-1

3 4 5

Michelle E. Dose a, Ke Zhang b, Joshua A. Thompson b, Johannes Leisen c, Ronald R. Chance a,

6

William J. Koros b, Benjamin A. McCool a, and Ryan P. Lively a,b*

7 8 9

a

Algenol Biofuels, 28100 Bonita Grande Dr., Bonita Springs, FL 34315-6220

10

b

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst

11

Dr. NW, Atlanta, GA 30332-0100

12

c

13

Atlanta, GA, 30332-100

School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Dr.,

14 15

*

Email address: [email protected]

16

1

ACS Paragon Plus Environment


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ABSTRACT

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The relationship between framework defects and crystal size in fluoride mediated

19

silicalite-1 was investigated though nitrogen physisorption, X-ray diffraction, and vapor

20

adsorption of ethanol and water on samples with crystal sizes ranging from 0.4 - 30 µm in the b-

21

direction of the silicalite-1 crystals. X-ray diffraction reveals a shift in lattice system from a

22

predominantly monoclinic phase in smaller crystals to an orthorhombic phase in the larger

23

crystals. 29Si MAS studies reveal minimal differences in framework silanol defect concentration.

24

An H-4 type hysteresis in 77 K N2 adsorption isotherm and BdB-FHH pore size analysis reveal

25

the presence of slit-like pores and a larger average pore size as well as a larger volume fraction

26

of “microfissures” in the larger crystals. Pure vapor adsorption measurements show more than a

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2-fold increase in water uptake from 0.21 mmol/g to 0.51 mmol/g from the smallest to largest

28

samples at 308 K near unit activity, while ethanol uptake remains on the order of 2.4 mmol/g for

29

all samples. An increase in desorption hysteresis with crystals size and negligible differences in

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isosteric heats of adsorption of water lend support to the presences of microfissure defects in the

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larger samples. IAST predications for binary adsorption of ethanol and water in the silicalite-1

32

samples reveal a 2-fold ethanol/water selectivity enhancement for dilute (< 5 wt% EtOH)

33

solutions when crystal size is reduced. This systematic study of N2 physisorption, framework

34

composition, and ethanol/water adsorption highlights the critical role crystal size plays in the

35

adsorption process, which can have significant implications for biofuel processes that produce

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dilute aqueous ethanol as a raw product.

37 38

Keywords: Adsorption, Silicalite-1, MFI, Silanol Defects, Framework Defects, Crystal Size .

2


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INTRODUCTION

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Zeolites – porous crystalline materials with well-defined pore structures – have a wide

41

range of industrial applications including adsorption, catalysis, and membrane-based separations

42

(both gas and liquid). In particular, LTA-type zeolites have successfully been applied on the

43

industrial scale for the dehydration of organic solutions. These materials are highly effective due

44

to both their hydrophilic nature and small aperture (< 0.4 nm) capable of rejecting organic

45

molecules larger than water.1 Recently, however, the need for hydrophobic adsorbents capable of

46

concentrating dilute organics (i.e. ethanol, propanol, butanol) from aqueous solutions has

47

increased due to the rapid growth in algae-based biofuel research and development.2, 3

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Among hydrophobic materials, zeolites with the MFI-type structure are among the most

49

widely studied for this purpose due to their large pore size (~0.54 nm) and thermal/structural

50

stability. Silicalite-1 (Al-free MFI), in particular, has received significant attention due to its lack

51

of acidic aluminum sites and ideally hydrophobic nature. The typical synthesis of silicalite-1

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tends to be highly alkaline, using OH- as the mineralizing agent. This synthesis method results in

53

a considerable number of internal silanols (-SiOH) when Si atoms detach from the framework or

54

after

55

tetrapropylammonium.4

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these point defects can result in more than 5% of the Si atoms being hydroxylated.2,4 While

57

considerably less hydrophilic than acidic aluminum sites, the slightly polar nature of the silanol

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defects serve as “seeds” for water adsorption and are unfavorable for extraction of organics from

59

dilute aqueous solutions.

60

silanol defect concentration has on water and ethanol uptake by comparing a wide variety of

61

chemically different MFI-type zeolite. 2

the

removal

of

the

charge

balancing

centers

for

the

structure

template

29

Si MAS NMR (magic angle spinning NMR) studies have revealed

2

Our previous work has shown the significant effect difference in

3



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Near perfect silicalite-1 crystals have been synthesized with extremely low density of

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silanol defects by using F- as the mineralizing agent at near neutral pH conditions.4 The smaller

64

fluoride ion balances the template ions in such a way that few silanol defects are introduced into

65

the framework.2 In a recent publication, the adsorption of ethanol and water in several MFI-type

66

zeolites was studied and the effective elimination of silanol defects from the silicalite-1 was

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found to dramatically reduce the water uptake.2 The total water uptake by silicalite-1 (F-) at near

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unit activity is almost an order of magnitude lower than the uptake in silicalite-1 (OH-) under the

69

same conditions. With minimal differences in ethanol uptake between the samples, the ideal

70

ethanol/water sorption selectivity of silicalite-1 (F-) is an order of magnitude higher than that of

71

silicalite-1 (OH-).2

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Others have studied the effect of silanol defect concentration in silicalite-1 on the

73

adsorption of water and other alcohols and polyols.2,

5-8

74

crystal size on the adsorption properties of MFI-type materials has not been investigated, most

75

likely due to the difficulty of controlling crystal size in fluoride media. In this work, we report

76

for the first time the unexpected decrease in water uptake with a decrease in crystal size in

77

silicalite-1 (F-) samples with crystal dimensions ranging from 0.40 µm to 30 µm. As a result, we

78

propose the presence of a second type of framework defect – dislocations between platelets and

79

crystal subunits resulting from the mismatch of orthorhombic and monoclinic phases – that

80

scales directly with the crystal size.

Of these publications, the effect of

81

Several works by Agger9, Kortunov10, Karwacki11, and others have reported the internal

82

architecture of MFI-type crystals as being comprised of sub-units that form an hourglass

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structure (Fig. 1b). This sub-unit structure can easily be observed using interference microscopy

84

and confocal fluorescence microscopy techniques.9-11 The source of these different sub-units is

4


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not entirely clear, but many studies have attributed the patterns to regular terrace growth

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mechanisms that possibly result in each constituent growing in different crystallographic

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directions.9, 10 Kortunov et. al. showed that the intersections of the different sub-units serve as

88

mild transport barriers to isobutane within the crystal. Additionally, using intracrystalline

89

concentration profiles of isobutene in silicalite-1 samples, Kortunov et. al. determined that line-

90

like defects along the sub-unit intersections (red surfaces highlighted in Fig. 1b) are “cracks” that

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penetrate into the center of the crystals.10 These cracks allowed for an additional route for

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adsorption and desorption of isobutane. For ease of classification in the remainder of this paper,

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the well-defined silanol point defects are referred to as Type A defects, while the “crack-like”

94

defects (i.e., “microfissures”) are referred to as Type B defects.

95 96

Figure 1. Types of defects present in silicalite-1 crystals; (a) Type A silanol defects resulting

97

from missing/unsatisfied Si atoms – subsection of a MFI unit cell where red indicates oxygen,

98

blue indicates silicone, and white indicates hydrogen atoms; (b) Type B framework dislocation

99

“cracks” along sub-unit intersections highlighted in red.

100 101

In this work, we further support the presence of the Type B defects in fluoride mediated

102

silicalite-1 through XRD, N2 physisorption pore-size analysis, and ethanol/water vapor

5



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adsorption. We also investigate the impact of crystal size on these defect properties.

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Furthermore, the impact of crystal size on the adsorption properties was analyzed using ideal

105

adsorbed solution theory (IAST) to determine the effectiveness of the different samples as

106

ethanol-selective sorbents.

107 108

EXPERIMENTAL

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Zeolite Synthesis

110

A fluoride-mediated synthesis route was used to synthesize nearly defect-free pure-silica

111

silicalite-1 crystals.12 The synthesis conditions were altered to achieve silicalite-1 crystals with

112

varying dimensions. The details of each synthesis procedure are included in Table I with the

113

complete

114

Tetrapropylammonium bromide (TPABr, 99% Sigma Aldrich) and ammonium fluoride (NH4F,

115

>99.99% Sigma Aldrich) were dissolved in deionized water within a Teflon® jar. For secondary

116

growth procedures using crystal seeds (Sample I), sonication was used to disperse the seeds in

117

water prior to dissolving TPABr and NH4F.13 The seeds used were prepared by ball milling 0.5 g

118

of “normal” silicalite-1 (silicalite-1 prepared using the conventional fluoride procedures in the

119

literature) in ~1 mL of DI water for 5 min in a Spex 8000M Mixer/Mill® using a hardened steel

120

grinding vial with two 0.5 inch steel balls.14 The mixture was covered and stirred for 10 minutes

121

at room temperature to ensure the reagents completely dissolved. Cab-O-Sil® M-5 (untreated

122

fumed silica, Cabot Corporation) was slowly added to the mixture, stirring manually for 10

123

minutes to form a highly viscous, opaque gel. Some procedures (Sample III) aged the synthesis

124

gel for 20 h, which was achieved by use of an automated impeller and covering the sample with

125

Parafilm® to limit water evaporation. For all samples, the mixed synthesis gel was transferred to

details

of

each

procedure

given

6

in

the


Supplementary

Information.

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an HF-washed 25 mL Teflon® sleeve and tightly sealed in a stainless steel Parr® reactor. The

127

samples were then reacted for the indicated amount of time (Table I) in a preheated 453 K oven.

128

Other permutations not listed in Table I – including diluting the synthesis gel with water, adding

129

non-solvents (ethanol or methanol), and different combinations of aging, seeding, and diluting –

130

were unsuccessfully attempted. The synthesis procedures discussed in this work resulted in

131

clearly defined crystals with a high yield (>50% is essential for best sample recovery from

132

unreacted silica) and relatively narrow crystal size distributions.

133 134

Table I. Silicalite-1 sample synthesis conditions. Aging occurred at room temperature, while the

135

reaction occurred at 453 K.

I

DI water (g) 13.44

seeds (mg) 25

TPABr (g) 0.81

II

13.44

0

III

13.44

IV V

sample

0.059

Cab-OSil (g) 2.24

aging time (h) 0

reaction time (d) 4

0.81

0.059

2.24

0

4

0

0.81

0.059

2.24

20

14

13.44

0

0.81

0.059

2.24

0

14

13.44

0

0.608

0.059

2.24

0

14

NH4F (g)

136 137

After cooling within the sealed reactor at room temperature for ~8 h, the solids were

138

vacuum filtered and washed with at least 200 mL of DI water. To remove the un-reacted silica,

139

the solids were dispersed in 30 mL of DI water and sonicated for 90 s. The solids were recovered

140

using centrifugation and the sonication-centrifugation cycle was repeated a minimum of two

141

more times. After drying at 383 K under vacuum, the samples were calcined to remove the

142

organic structure directing agent (TPA+) by heating the samples under air to 393 K at 5 K/min,

143

holding for 2 h, heating to 823 K at 5 K/min, holding for 12 h, and then allowing to cool

7



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144

naturally. From this point forward, references are made to the calcined samples unless otherwise

145

noted.

146 147

Characterization Methods

148

Scanning electron microscopy (SEM) was used to evaluate the crystal size and

149

morphology. The samples were sputter-coated with a 10-20 nm thick gold coating (Model P-S1,

150

ISI, Mountain View, CA), and transferred to a high-resolution field emission scanning electron

151

microscope (Leo 1530, Leo Electron Microscopy, Cambridge, UK). Images were taken at 10 kV

152

accelerating voltage.

153

X-ray diffraction was used to confirm the samples synthesized had the MFI topology

154

associated with silicalite-1 and to investigate the crystal’s lattice system. Powder x-ray

155

diffraction was performed at room temperature on an X’Pert Pro PAnalytical X-ray

156

Diffractometer using Cu-Kα radiation of wavelength λ = 1.5406 Å. Measurements were carried

157

out from 5 – 40º 2θ, using an X’celerator detector with low-background sample holders.

158

29

Si MAS NMR measurements were performed on a Bruker DSX300 spectrometer

159

operating at 59.64 MHz using a MAS probe operating with 4 mm ZrO2 rotors. Spectra were

160

acquired using spinning rates of 10 kHz and a regular direct polarization experiment with high

161

power decoupling. The excitation pulse length was 2.5 µs corresponding to a 45° pulse. Waiting

162

time between individual scans was 60 s. Up to 1024 scans were accumulated in order to obtain a

163

good signal to noise ratio. 29Si chemical shifts were referenced with respect to 3-(trimethylsilyl)-

164

1propanesulfonic acid sodium salt.

165

N2 isotherms at 77 K, Brunauer-Emmett-Teller (BET) surface areas and micropore

166

volumes (t-plot method) were calculated from N2 physisorption measurements performed on an

8


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167

ASAP 2020 (Micrometrics). The samples were degassed at 473 K for 18 h within the N2

168

physisorption apparatus prior to taking measurements. FT-IR measurements were performed

169

under vacuum with samples prepared using equal amounts of silicalite-1 powder in KBr pellets

170

and analyzed using a Bruker Vertex 80v FT-IR with wavenumbers from 400 to 4000 cm-1.

171

Pure vapor adsorption equilibrium experiments were performed on a VTI-SA vapor

172

sorption analyzer from TA Instruments (New Castle, DE, United States) at 308 K. The vapor

173

activity was controlled automatically by mixing a saturated vapor feed (using N2 as the carrier

174

gas) with a dry N2 stream. The sample “dry mass” was measured under N2 and was at

175

equilibrium before introduction of the vapor to the sample chamber. Isotherms within 0.05 – 0.95

176

activity were obtained.

177 178

RESULTS AND DISCUSSION

179

SEM and XRD

180

Crystal size and morphology of the five samples were compared using scanning electron

181

microscopy. Each of the samples exhibited the typical coffin-shaped crystal morphology which

182

has been previously attributed to the use of TPABr as the structure directing agent.15,

183

shown in the electron micrographs in Fig 2, the five silicalite-1 samples possessed varying a, b,

184

and c dimensions (Fig 2 and Table II). In particular, the Sample I synthesis resulted in the

185

smallest crystals (1.5 µm x 0.4 µm x 9.0 µm), while the Sample V synthesis resulted in the

186

largest crystals (35 µm x 30 µm x 120 µm). Moreover, we observed that a change in the overall

187

crystal size was accompanied by a change in the crystal morphology. In particular, the crystal

188

aspect ratio was found to be approximately 2 for the largest crystals and approximately 13 for the

189

smallest crystals. The ability to decouple the change in crystal size and crystal aspect ratio was

9


16

As


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190

not observed in our work, and may be an intrinsic relationship of the crystal growth mechanism.

191

It should also be noted that distinct “sub-unit” plates and “terraces” within the large Sample V

192

and Sample IV crystals are visible (Fig 2 d,e).

193

194 195

Figure 2. SEM images of fluoride mediated silialite-1 crystals. (a) Sample I, (b) Sample II, (c)

196

Sample III, (d) Sample IV, (e) Sample V, (f) crystal dimensions.

197 198

199

Table II. Silicalite-1 Crystal Dimensions Sample

a (µm)

b (µm)

c (µm)

Aspect Ratio*

I

1.5

0.4

9.0

13

II

15

3.0

50

11

III

15

10

55

3.5

IV

30

20

80

2.8

V

35

30

120

2.6

a

. . /2 ∙

10


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200

X-ray diffraction patterns confirmed that all the silicalite-1 samples are crystalline with a

201

MFI-type framework topology (Fig 3, top). A closer examination and comparison of diffraction

202

peaks between 22 – 25° 2θ with the diffraction patterns of monoclinic and orthorhombic lattice

203

systems indicates that the samples differ slightly in symmetry (Fig 3, bottom).7 Similar to what

204

has been noted by others for silicalite-1 samples with varying silanol defect densities7, the

205

doublet observed between 24.3 – 24.6° 2θ for monoclinic MFI-type crystals weakens in intensity

206

from the smallest Sample I crystals to the largest Sample V crystals where only trace peaks of

207

the monoclinic phase are observed. Additionally, the triplets associated with monoclinic

208

symmetry observed between 23.0 – 23.4° and 23.6 – 24.0° 2θ are more defined for the smaller

209

samples, indicating that these materials contain predominately monoclinic phase, while the larger

210

crystals likely contain a larger portion of orthorhombic phase. This phase shift has been observed

211

in different MFI-type zeolites17-19 and Mallon et. al.7 related this monoclinic/orthorhombic

212

symmetry trend to the relative number of silanol defects within the MFI framework by

213

comparing the diffraction patterns of silicalite-1 samples prepared through alkaline synthesis,

214

alkaline synthesis post-treated with steam, and fluoride mediated routes. While each of the

215

samples analyzed in this work were synthesized via a fluoride mediated route, the work by

216

Mallon et al. suggests a larger defect density exists in the larger crystals synthesized in this work

217

due to the increase in orthorhombic phase contribution within the framework. We directly and

218

indirectly probed this suggestion using

219

experiments to distinguish between Type A point defects and Type B mesopore-scale defects.

29

Si NMR, N2 physisorption, and water vapor sorption

11


increasing crystal size

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increasing crystal size


220 221

Figure 3. XRD diffraction patterns of Sample I (red), II (blue), III (green), IV (purple), and V

222

(orange) from 5 – 40° 2θ (top) with the 22 – 25° 2θ region magnified and normalized to the 23°

223

2θ peak (bottom). Orthorhombic and monoclinic MFI diffraction patterns7 have been included

224

for comparison. The indicated b-dimensions are shown in µm.

225 226 227 228

29

Si NMR The

29

Si NMR spectra of Samples I, II, IV, and V (Fig. 4) exhibit 8 to 10 sharp peaks

229

between -110 and -120 ppm, corresponding to the Q4 groups (Si – [(OSi)4]) and are assigned to

230

the 24 distinct crystallographic silicon sites.20 On each of the spectra, including the highly

231

defective (OH-) Silicalite-1 sample, no signal assigned to Q3 silanol defects (≡Si–OH) was

12


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232

observed at the expected -103 ppm shift (Fig. S.20) An increasing amount of defects or even the

233

loss of crystallinity will lead to broadening of the peaks. One of the major factors affecting the

234

width of peaks in

235

sites – i.e. highly ordered crystalline sites associated with a defined chemical environment will

236

correspond to narrow peaks. The clearly defined peaks displayed by Samples I, II, IV, and V, as

237

compared to the single broad peak exhibited by (OH-) Silicalite-1, indicate very low silanol

238

defect (Type A defects) concentrations with no distinguishable dependence on crystal size. These

239

results lend support to the presence of a second type of framework defect (Type B), rather than

240

larger concentrations of Type A defects in the larger crystals. This is also supported by the FTIR

241

spectra (Fig. S12) showing only a small peak at 3690 cm-1, corresponding to the silanol –O-H

242

stretch9, with minimal differences between the samples.

243 244

Figure 4. 29Si MAS NMR spectra of Sample I - V. Broad peaks in the Q3 region (-110 ppm to -

245

120 ppm) indicate a larger silanol (Type A) defect density in the OH- silicalite-1 sample while

246

there is no detectable difference in peak resolution of Samples I, II, III, IV, and V.

29

Si spectra is the conformational order experienced by associated chemical

247 248

N2 Physisorption

13



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249

Further indication of the difference between the silicalite-1 samples is supported by the

250

hysteresis observed in the nitrogen adsorption isotherms (Figure 5). As according to the IUPAC

251

classification of physisorption isotherms, each sample exhibited a Type IV isotherm with an H-1

252

hysteresis occurring at ~0.15 relative pressure. Additionally, Samples III, IV, and V exhibit an

253

H-4 type hysteresis loop occurring at about 0.45 relative pressure.21

254 255

Figure 5. Nitrogen adsorption (closed circles) and desorption (open circles) isotherms on

256

Samples I - V measured at 77 K. For clarity, the isotherms have been offset by 20 cm3/g STP on

257

the y-axis. The isotherms for each individual sample can be found in the Supplementary

258

Information. The indicated b-dimensions are shown in µm.

259 260

The cause of the predominate adsorption step occurring for each of the samples between

261

0.10 – 0.15 relative pressure has been the subject of significant debate within the literature. Some

262

have attributed this to the adsorbate transition from a “localized fluid-like phase” (α phase) to a

263

“crystalline-like solid phase” (β phase)22-26 while others suggest the adsorbent itself undergoes a

264

reversible monoclinic-orthorhombic transition allowing for the adsorption step to occur.7, 26, 27

14


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265

Our experiments reveal that the extent of the H-1 hysteresis at this step appears to

266

increase with a decrease in crystal size (coupled with a decrease in the presence of orthorhombic

267

MFI phases with decreasing crystal size), although we cannot definitively indicate the source of

268

this phenomenon. While in-situ XRD measurements on the nitrogen filled samples at 77K were

269

not obtained for this work, the presence of these steps lends support to a previously noted

270

reversible orthorhombic to monoclinic phase transition occurring.7, 26, 27

271

The H-4 type hysteresis occurring at a higher relative pressure of ~0.45 can be correlated

272

with an increase in crystal size.23-25 Samples III, IV, and V exhibit a clear hysteresis step while

273

the smaller Samples I and II show no hysteresis in the same region (Figure 5). A similar trend

274

was also observed for nitrogen adsorption on fluoride mediated silicalite-1 and MFI-type zeolites

275

with varying Si/Al ratios.23-25 The increase in hysteresis was attributed to two different effects: 1)

276

the sorbate molecule induced framework swelling within flexible fissured regions in the crystals

277

(likely resulting from synthesis or calcination) and 2) to additional adsorption between two

278

crystal faces.23-25 This conclusion is consistent with the common association of H-4 hysteresis

279

with the presence of narrow slit-like pores or “microfissures” – much like the “cracks”

280

previously observed between the subunits of the silicalite crystals.

281

these fissures are dislocations between monoclinic lattices and orthorhombic lattices within the

282

MFI crystals (Type B defects). The likelihood of these dislocations appears to increase with an

283

increase in crystal size, corresponding to the larger H-4 hysteresis. Additionally, these

284

dislocations likely result in a slight, undetectable increase in silanol concentration within the MFI

285

crystals due to the high likelihood of terminal or geminal silanol groups within the dislocation

286

regions. We probed this hypothesis using pore size analysis and ethanol/water vapor sorption.

15


9-11,21

We hypothesize that


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Page 16 of 33

287

Using a simplified Broekoff–de Boer (BdB) pore size analysis method and the Frenkel-

288

Halsey-Hill (FHH) theory for determining the statistical thickness of an adsorbed gas layer28, the

289

pore size distribution of each sample was determined. As shown in Fig 6, both an increase in the

290

average pore size and distribution correlate well with an increase in crystal size. The larger

291

differential pore volume of Samples I, as compared to Sample II, is likely related to the minor

292

differences in the shape of the N2 physisorption isotherm and the accuracy of the data. The BdB

293

and FHH pore size analysis—which tends to be more reliable for pores over 20 Å—indicates that

294

the microfissures in the larger silicalite-1 crystals are on the order of ~50Å. Moreover, the pore

295

size analysis indicates that the larger crystals have a larger volume fraction of these microfissures

296

(Type B defects) with mesopores comprising >2.5 % of the pore volume in the smallest crystals

297

and 15% in the largest (Table II, Supporting Information).

298 299

Figure 6. Pore size distribution of fluoride mediated silicalite-1 Samples I – V

300

with the b-dimentions labeled in µm. (Note that the curve for Sample II runs very

301

near the x-axis.

302 303 304

Water Vapor Sorption

16


Page 17 of 33

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305

Our characterization experiments revealed the possibility of two types of silanol defects:

306

Type A – framework defects (similar to framework defects in alkaline-mediated silicalite-1) –

307

and Type B – microfissures caused by dislocations between monoclinic and orthorhombic

308

phases. We used water vapor sorption experiments to investigate the extent of these defects and

309

their relation to overall crystal size. Water vapor adsorption isotherms at 35°C for each of the

310

fluoride mediated silicalite-1 samples are illustrated in Fig 7. As according to BDDT

311

classifications, each of the samples exhibit a Type V sorption isotherm formed due to stronger

312

sorbate–sorbate interactions (i.e. water–water interaction), rather than sorbate–sorbent

313

interactions (water – silicalite-1).2 The small initial uptake is related to the weak sorbate-sorbent

314

interaction, while the continued uptake at higher activities is due to stronger sorbate-sorbate

315

interactions which lead to the formation of water clusters.

316 317

Figure 7. Water vapor adsorption at 35°C for Samples I (red circles), II (blue squares), III (green

318

triangles), IV (purple diamonds), and V (orange lines) silicalite-1 with the 0.80 – 1.00 activity

17



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Page 18 of 33

319

range magnified in the inset. Error bars represent a standard percent error at each activity

320

determined by using multiple runs on Sample I to determine the experimental error. The

321

indicated b-dimensions are shown in µm.

322 323

While at lower activities, < 0.6 p/p*, the difference in water uptake for each of the

324

samples is not definitive (i.e. all samples fall within instrument error of each other), a noticeable

325

increase in water uptake for the larger crystals is evident at higher activities, > 0.8 p/p*, (Fig 7

326

inset). The water uptake in the low-activity regime is dominated by water interactions with

327

framework defects within the crystal (the Type A defects).2 The overwhelming similarity of the

328

water loading in our silicalite-1 samples at “infinite dilution” and the similarity of the 29Si-NMR

329

spectra lead us to conclude that the concentration of Type A framework defects within the

330

silicalite-1 crystals is independent of crystal size.

331

formation mechanism of framework defects in pure-silica MFI has been attributed to -Si-O-

332

formation in alkaline media and subsequent protonation to -Si-OH after calcination and exposure

333

to atmosphere.19 The essentially neutral pH conditions of the fluoride media used to synthesize

334

our silicalite-1 samples relative to the typical alkaline zeolite synthesis gels allows for very

335

minor differences in the formation of framework defects, thus very minor (i.e., negligible)

336

differences in Type A defect concentration would be expected for the 5 samples considered here.

This is to be somewhat expected—the

337

The smallest Sample I crystals had a minimal water uptake of only 0.21 mmol/g at 35°C

338

near unit activity, while the uptake in the largest Sample V crystals was more than twice as much

339

(0.51 mmol/g) under the same conditions. Typically, assuming identical pore volumes and

340

framework chemistry, the overall water uptake should increase with decreasing crystal size due

341

to the increased prominence of the external surface area per sorption volume in the smaller

18


Page 19 of 33

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342

crystals. Assuming a monolayer coverage on the outer surface, this expected phenomenon is

343

illustrated in Fig 8 by the red circles and is compared with experimental water uptakes into the

344

silicalite-1 samples. Note that the calculated values do not account for any water uptake by the

345

internal surface area of the samples and are used merely to represent a trend. This trend,

346

however, was not observed in the water vapor sorption measurements and, as indicated by the

347

blue circles in Fig 8, water uptake is in fact less for smaller crystals (larger surface area to

348

volume ratio). These results indicate that the external surface silanols (i.e., terminal silanols) do

349

not significantly influence the water uptake and reveals that there is an additional source of water

350

adsorption in the larger crystals that outweighs the effects of surface silanols.

351 352

Figure 8. Measured water uptake at 95% activity in silicalite-1 (red triangles) and estimated

353

external monolayer water sorption (blue circles) as a function of the ration of external surface

354

area per gram of sample. The lines are used to guide the eye.

355 356

Water vapor desorption isotherms and isosteric heat of adsorption provide further

357

evidence into the differences in defect character between the samples. The isosteric heat of

358

adsorption was calculated by measuring adsorption isotherms at different temperatures (35, 45,

19



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Page 20 of 33

359

and 55°C) and employing the thermodynamic relationship derived from the Clausius–Clapeyron

360

equation, Δ

(1)

361

where ∆Hiso is the isosteric heat of adsorption, T is the temperature, p is the vapor partial

362

pressure, and q is the corresponding equilibrium adsorption amount.29 As was observed

363

previously2, the near-defect-free fluoride mediated silicalite-1 samples (Samples I and IV) are

364

highly hydrophobic (Fig 9). At low loadings, the initial adsorbed water is attributed to water

365

sorbing on the hydrophobic framework (as indicated by the lower heats of adsorption). These

366

“sorbate-sorbent” interactions at infinite dilution are primarily water molecule interactions with

367

any framework defect sites. The presence of a significant amount of framework defects would

368

result in more exothermic interactions between sorbate and sorbent (“(OH-) Silicalite-1” sample

369

is a sample containing ~8.5 Si-OH per unit cell).7 These initial adsorbed water molecules then

370

act as “seeds” for additional water adsorption, leading to the larger isosteric heat of adsorption –

371

similar to the condensation of water, indicating strong “sorbate-sorbate” interactions – at

372

increased loadings. The steep initial slopes of the fluoride mediated silicalite-1 samples are

373

associated with very little interaction between water and the surface, which indicates very few

374

framework silanol defects (i.e., Type A defects) present to serve as sorption sites. As shown in

375

Fig. 9, there is no detectable difference in isosteric heat of adsorption between the smallest

376

silicalite-1 crystal sample (Sample I) and one of the largest crystals (Sample IV). This further

377

leads us to conclude that both samples contain approximately the same concentration of Type A

378

defects (i.e., silanol framework defects).

20


Page 21 of 33

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379 380

Figure 9. Isosteric heat of adsorption for Samples I (red circles) and Sample IV (purple

381

diamond). The heat of condensation of water has been highlighted by the dashed line. (inset)

382

Isosteric heat of adsorption of alkaline synthesize silicalite-1 (green squares) compared to

383

Sample I and Sample IV.

384 385

In the desorption isotherms (Fig 10), a significantly larger type H-1 hysteresis loop is

386

observed for Sample IV, as compared to Sample I. While desorption from Sample I occurs at

387

activities between 0.8 – 0.9, the desorption step from Sample IV does not occur until 0.4 – 0.6

388

activity. This hysteresis loop has been attributed to a capillary condensation-like transition

389

occurring within the pores of the material and/or on surface defects.20 Water condenses around

390

hydrophilic defects (both internal and surface), eventually forming water clusters due to the more

391

favorable sorbate – sorbate interactions. As pressure is increased, the clusters grow and

392

eventually fill the pores (for internal clusters). Upon desorption, the larger pores with more

393

condensed water desorb at lower pressures than smaller pores. As was calculated by BdB-FHH

394

theory using nitrogen physisorption, the increasing hysteresis in the water desorption step also

395

indicates a significant and anomalous concentration of mesopore-scale defects, which we

21



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Page 22 of 33

396

attribute to the Type B defects (i.e., not the zeolite micropores) that arise with an increase in

397

crystal size.

398 399

Figure 10. Water sorption (filled) and desorption (open) on Samples I (red circles) and Sample

400

IV (purple diamonds) showing the larger hysteresis for the larger crystals. The b-dimensions

401

have been noted in µm.

402 403

Using the results from sorption & desorption isotherms, heats of adsorption,

29

Si-NMR,

404

N2 physisorption and X-ray diffraction, we hypothesize that the increase in water uptake for the

405

larger crystals is caused primarily by capillary condensation occurring in the “microfissures”

406

between dislocated unit cells (Type B defects). Due to the growth mechanism that has been

407

reported for this type of zeolite, it is postulated that these dislocations occur along

408

intracrystalline barriers that result from the faceted-terrace growth mechanism, twinned

409

substructures, and “fused” platelets.9,30 Our results indicate that the microfissure/dislocation

410

defects must be present in higher concentrations in the larger crystals (most likely due to

411

increased framework strain in the aforementioned regions).

22


Additional studies would be

Page 23 of 33

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412

necessary to further define the structure and cause of these microfissures, but our current results

413

reveal the importance of crystal size for dilute biofuel recovery from aqueous streams.

414 415

Ethanol Vapor Sorption

416

The ethanol vapor isotherms at 35°C for the samples I and IV (Fig. 11) were used to

417

compare the effectiveness of the samples as organophillic sorbents for dilute aqueous solutions.

418

Each of the samples exhibited an overall organophillic nature, achieving near-saturation

419

adsorption coverage far below unit activity. Unlike the water uptake, these results reveal that

420

there is little difference between the total ethanol uptake of Sample I and IV within the bounds

421

of our experiments. This effect is likely due to ethanol and water undergoing a pore filling step

422

of the microfissues (Type B defects) at different activities. As shown previously in Fig. 7, water

423

does not fill the microfissues until near unit activity is reached, leaving the system very dilute

424

until about 0.8 activity. Ethanol pore filling, on the other hand, is believed to occur at much

425

lower activity – lower than the capabilities of the instrumentation used – resulting in a “liquid-

426

like” ethanol phase.7 Due to this effect, the additional pore filling of the micofissures in the

427

larger crystals has a significantly smaller (within the measurement error) impact on the total

428

ethanol uptake.

429

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

430 431

Page 24 of 33

Figure 11. Ethanol uptake at 35°C in Sample I (red circles) and Sample IV (purple diamonds).

432 433

Ethanol/Water Selectivity

434

The ethanol and water vapor adsorption isotherms reported here indicate that crystal size

435

(and defects that arise as a result of crystal growth) has a significant impact on the

436

hydrophobicity of fluoride-mediated silicalite-1, but little effect on the organophilicity of the

437

crystal. The ideal vapor phase ethanol/water sorption selectivity at 35°C was estimated using the

438

pure-vapor isotherms for samples I and II (Fig 12). Vapor pressures of water and ethanol were

439

estimated using Raoult’s law and Henry’s law, respectively, as described in our previous work.2

440

24


Page 25 of 33

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441 442

Figure 12. Ideal ethanol/water sorption selectivity at 35°C for Sample I (red circles) and IV

443

(purple diamonds).

444 445

The ideal selectivity predictions show a markedly enhanced selectivity in the small

446

Sample I crystals relative to the largest Sample V crystals as a result of reduction of Type B

447

defects in the former sample. Moreover, the pure vapor measurements and ideal selectivity

448

calculations reveal a significant dependence of selectivity on silicalite-1 crystal size. The largest

449

crystals have a predicted selectivity of approximately 30 with an ethanol mole fraction of 0.05,

450

whereas the smallest crystals have a nearly 2x improvement in selectivity. The fact that a

451

reduction in crystal size can have such positive effects on the selectivity of the same material is

452

unusual, and has significant implications for ethanol recovery applications.

453

Ideal adsorbed solution theory (IAST) was used to estimate the adsorption amounts for

454

binary ethanol−water vapor mixtures in the fluoride mediated silicalite-1 crystals. The adsorbed

455

phases are assumed to behave as ideal solutions and the standard state is assumed to be the pure

456

adsorbed species at the same temperature and spreading pressure as that of the mixture (see

457

Supporting Information for details). Several groups have reported sorption results for ethanol-

25



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Page 26 of 33

458

water mixtures in silicalite-1.6,31 These papers revealed that alcohol uptake from the water-

459

alcohol mixture were well-predicted by IAST derived from the pure component isotherms. It

460

should be noted that the predictions for water sorption within the zeolites were generally lower

461

than the experimental findings at low water relative pressures (p/po), but matched reasonably

462

well near unit activity. As a result of these studies, we believe that IAST predictions for water-

463

alcohol mixtures are quite reliable for dilute mixtures of alcohol in water.

464

Figure S13 shows the IAST-predicted phase diagrams for vapor phase ethanol-water

465

mixtures in equilibrium with Sample I and Sample V (a and b), and the IAST-predicted phase

466

diagrams for liquid phase ethanol-water mixtures in equilibrium with the same samples (c and

467

d). The latter figures were determined by assuming that the sorbents are non-wetting when in

468

contact with the liquid mixture.20 For aqueous feeds dilute in ethanol—which are the most

469

reliably predicted by IAST—the IAST predictions illustrate the potential for significant ethanol

470

purity enhancements in an adsorption process. For instance, a vapor feed with an ethanol content

471

of 10-3 mol/mol is predicted to have an adsorbed phase ethanol mol fraction of 0.23 mol/mol

472

(230x improvement for Sample I, 180x improvement for Sample IV), while a liquid phase feed

473

with an ethanol content of 10-3 is predicted to have an adsorbed phase ethanol mol fraction of

474

0.56 (560x improvement for Sample I, 450x improvement for Sample IV). The impact of size on

475

the hydrophobicity of the smallest sample and one of the largest also plays a role in the

476

selectivity of the process; the ethanol product purity of Sample I (b = 0.4 µm) is predicted to be

477

25% higher than the larger Sample IV (b = 20 µm) crystals.

478 479

CONCLUSIONS

480

Silicalite-1 samples with crystal sizes ranging from 0.4 µm to 30 µm were synthesized via

481

a fluoride mediated route. XRD measurements showed an increase in orthorhombic MFI

26


Page 27 of 33

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29

482

structure symmetry with increasing size while

483

silanol defect concentration (labeled as Type A defects in this work) for all samples of varying

484

sizes.

Si MAS NMR showed a relatively constant

485

The H-4 type hysteresis at 0.45 p/po in the 77K N2 physisorption isotherm, which is

486

attributed to slit-like pores, supports the presence of “microfissures” along the previously

487

observed sub-unit boundaries of the crystals (labeled as Type B defects in this work). BdB-HFF

488

pore size analysis indicates the volume fraction and size of these microfissures increases with

489

increasing crystal size.

490

Pure vapor adsorption isotherms for ethanol and water showed a significant increase in

491

water uptake with crystal size – more than a 2-fold increase from 0.21 mmol/g to 0.51 mmol/g

492

from the smallest to largest samples at 35°C near unit activity – while ethanol uptake was on the

493

order of 2.4 mmol/g for all samples. An increase in desorption hysteresis with crystals size

494

indicates an increasing likelihood of microfissure defects (Type B) in the larger samples.

495

Using the pure vapor isotherms, IAST predictions for binary adsorption of ethanol and

496

water in the silicalite-1 samples revealed a 2-fold ethanol/water selectivity enhancement for

497

dilute (< 5 wt% EtOH) solutions for the smallest sample – leading to ethanol product purity

498

predicted to be 25% higher than the larger samples. From these results, it is clear that crystal size

499

has a significant impact on the hydrophobicity of fluoride mediated silicalite-1. This systematic

500

study of N2 physisorption, framework composition, and ethanol/water adsorption in fluoride

501

mediated silicalite-1 highlights the importance of considering the role crystal size has on

502

adsorption properties, which can have significant implications for processes that produce dilute

503

aqueous alcohols as raw product. Moreover, the impact of crystal size on adsorption properties

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

504

shows the importance of considering not only material selection, but also the material

505

morphology when designing systems for specific applications.

506 507

ASSOCIATED CONTENT

508

Supporting information available: Detailed synthesis procedures, nitrogen physisorption plots,

509

BET surface areas, full

510

spectra, water uptake tables, and details of IAST analysis are available.

511

ACKNOWLEDMENT

512

This material was supported by the Department of Engery under the award no. DE-FOA-

513

0000096.**

514

**Disclaimer: This report was prepared as an account of work sponsored by an agency of the

515

United States Government. Neither the United States Government nor any agency thereof, nor

516

any of their employees, makes any warranty, express or implied, or assumes any legal liability or

517

responsibility for the accuracy, completeness, or usefulness of any information, apparatus,

518

product, or process disclosed, or represents that its use would not infringe privately owned rights.

519

Reference herein to any specific commercial product, process, or service by trade name,

520

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement,

521

recommendation, or favoring by the United States Government or any agency thereof. The views

522

and opinions of authors expressed herein do not necessarily state or reflect those of the United

523

States Government or any agency thereof.

29

Si MAS NMR spectra of alkaline silicalite-1 and sample IV, FTIR

524 525

28


Page 28 of 33

Page 29 of 33

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526

REFERENCES

527

(1) Carmo, M. J.; Gubulin, J.C. Ethanol-water adsorption on commercial 3A zeolites: kinetic and

528 529 530

thermodynamic data. Braz. J. Chem Eng. 1997, 14 (3). pp 1-10. (2) Zhang, K.; Lively, R.P.; Noel, J. D.; Dose, M. E.; McCool, B. A.; Chance, R. R.; Koros, W.J. Adsorption of Water and Ethanol in MFI-Type Zeolites. Langmuir. 2012, 28, pp 8664-8673.

531

(3) Zhang, K.; Lively, L. P.; Dose, M. E.; Liwei, L.; Koros, W. J.; Ruthven, D. M.; McCool, B.

532

A.; Chance, R. R. Diffusion of water and ethanol in silicalite cryastals synthesized in fluoride

533

media. Micropor. Mesopor. Mat. 2012, 170, pp 259 – 265.

534 535 536 537

(4) Camblor, M. A.; Villaescusa, L. A.; Diaz-Cabanas, M. J. Synthesis of all-silica and highsilica molecular sieves in ﬂuoride media. Top. Catal. 1999, 9 (1−2), pp 59−76. (5) Milestone, N. B.; Bibby, D. M. Concentration of alcohols by adsorption on silicalite. J. Chem. Technol Biot. 1981, 31 (1), pp 732-736.

538

(6) Bai, P.; Tsapatsis, M.; Siepmann, J. I. Multicomponent Adsorption of Alcohols onto

539

Silicalite-1 from Aqueous Solution: Isotherms, Structural Analysis, and Assessment of Ideal

540

Adsorbed Solution Theory. Langmuir. 2012, 28 (44), pp 15566-15576.

541

(7) Mallon, E. E.; Jeon, M. Y.; Navarro, M.; Bhan, A.; Tsapatsis, M. Probing the Relationship

542

between Silicalite-1 Defects and Polyol Adsorption Properties. Langmuir. 2013, 29 (22), pp

543

6546-6555.

544 545

(8) Xiong, R.; Sandler, S. I.; Vlachos, D. G. Molecular Screening of Alcohol and Polyol Adsorption onto MFI-Type Zeolites. Langmuir. 2012, 28 (9), pp 4491-4499.

546

(9) Agger, J. R.; Hanif, N.; Cundy, C. S.; Wade, A. P.; Dennison, S.; Rawlinson, P. A.;

547

Anderson, M. W. Silicalite Crystal Growth Investigated by Atomic Force Microscopy. J.

548

Am. Chem. Soc. 2003, 125, pp 830- 839.

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(10) Kortunov, P.; Vasenkov, S.; Chemlik, C.; Karker, J.; Ruthven, D. M.; Wloch, J. Influence of

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Defects on the External Crystal Surface on Molecular Uptake into MFI-type Zeolites. Chem.

551

Mater. 2004, 16, pp 3552-3558.

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(11) Karwacki, L.; Kox, M. H. F.; Matthis de Winter, D. A.; Drury, M. R.; Meeldijk, J. D.;

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Stavitski, E.; Schmidt, W.; Mertens, M.; Cubillas, P.; John, N.; Chan, A.; Kahn, N.; Bare,

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S.; Anderson, M.; Kornatowski, J.; Weckhuysen, B. M. Morphology-dependent zeolite

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intergrowth structures leading to distinct internal and outer-surface molecular diffusion

556

barrier. Nat. Mater. 2009, 8, pp 959-965.

557

(12) Chexeau, J. M.; Delmotte, L; Guth, J. L.; Soulard, M. Zeolites. 1989, 9, pp 78.

558

(13) Xomeritakis, G.; Gouzinis, A.; Nair, S.; Okubo, T.; He, M.; Overney, R. M.; Tsapatsis, M.

559

Growth, microstructure, and permeation properties of supported zeolite (MFI) films and

560

membranes prepared by secondary growth. Chem. Eng. Sci. 1999, 54, pp 3521- 3531.

561 562

(14) Akcay, K.; Sirkecioglu, A.; Tatier, M.; Savasci, O. T.; Erdem-Senatalar, A. Wet ball milling of zeolite HY. Power Technology. 2004, 142, pp 121-128.

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(15) Park, S.; Jiang, N. Morphological Synthesis of Zeolites. In Zeolites and Catalysis. J Cejka,

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A Corma, S Zones.Weinheim, Germany: Wiley-VCH Verlag GmbH, 2010, pp 131-150.

565

(16) Bonilla, G.; Diaz, I.; Tsapatsis, M.; Jeong, H.; Lee, Y.; Vlachos, D. G. Zeolite (MFI) Crystal

566

Morphology Control Using Organic Structure-Directing Agents. Chem. Mater. 2004, 16, pp

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5697-5705.

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(17) Hay,

D.

G.;

Jaeger,

H.

Orthorhombic-Monoclinic

Zeolite/Silicalite. Chem. Comm. 1984, 21, pp 1433.

30


Phase

Changes

in

ZSM-5

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(18) Hay, D. G.; Jaeger, H.; West. G. W.; Examination of the Monoclinic/Orthorhombic

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Transition in Silicslite Using XRD and Silicon NMR. J. Phys. Chem. 1985, 89. pp 1070-

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(19) Cheng, C. H.; Bae,

T. H.; McCool, B. A.; Chance, R. R.; Nair, S.; Jones, C. W.

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Functionalization of the Internal Surface of Pure-Silica MFI Zeolite with Aliphatic

575

Alcohols. J. Phys. Chem. C. 2008, 112, pp 3543-3551.

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(20) Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.;

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Fuchs, A. H. The Effect of Local Defects on Water Adsorption in Silcalite-1 Zeolite: A Joint

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Experimental and Molecular Simulation Study. Langmuir. 2007, 23, pp 10131-10139.

579

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ABSTRACT GRAPHIC

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Effect of Crystal Size on Framework Defects and ... - ACS Publications

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