Recovery and reutilization of the solvents and catalyst used in the sol

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Recovery and reutilization of the solvents and catalyst used in the sol-gel synthesis of silica xerogel Xiansong Li, and Shiquan Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06848 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Sustainable Chemistry & Engineering

Recovery and reutilization of the solvents and catalyst used in the sol-gel synthesis of silica xerogel Xiansong Li,1 Shiquan Liu*, 1 1 School

of Materials Science and Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, Shandong, PRChina

* to whom all correspondence should be addressed. Corresponding author: Prof. Dr. Shiquan Liu [email protected] [email protected] Tel and Fax: 0086-531-82769106

School of Materials Science and Engineering University of Jinan, West Campus No. 336, West Road of Nan Xinzhuang Jinan 250022 PRChina

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract: Silica xerogels were synthesized via the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under the catalysis of n-butylamine (BA), followed by drying the formed wet gel under ambient pressure at 120oC. The solvents (water and ethanol) and catalyst enclosed in wet gels were recovered during the drying process. It was found that the relative percentage of ethanol in the recovered liquids increased, while those of water and butylamine decreased. The recovered liquids were directly used for four runs, then the contents of BA, H2O and TEOS were adjusted to maintain the reactant ratios to be the same as those in the first run, facilitating a fast sol and gel formation. Totally twelve silica xerogels with high degree of three-dimensional Si-O-Si network were synthesized. The results indicate that all the silica xerogels are composed of aggregated particles sized 20-250nm, resulting in porous networks with specific area, pore volume and pore diameter in the range of 290-690m2/g, 1.54-2.40cm3/g and 17.4-68.2nm, respectively. The compositional change in the recovered liquids, the drying time and the volume of wet gel had influence on the aggregation, degree of condensation and surface area of the cyclically synthesized xerogels. The synthesis route not only saves resources but also does not generate any waste, providing a sustainable way to synthesize gel materials via sol-gel route.

Keywords: sol-gel; silica xerogel; synthesis; recovered liquid; reutilization

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Introduction Silica xerogels have abundant interior pores. They are characterized by high porosity, specific surface area and low density.1-3 These outstanding characteristics make them suitable candidates for catalyst supports, drug delivery systems, adsorbents and sensors, etc.4-8 Sol-gel method is the most widely used technique to synthesize silica gels. During the sol-gel process, hydrolysis and condensation of silica precursors in water lead to the formation of siloxane bonds, followed by the formation of silica network9. The sol-gel synthesis of silica gels includes two main steps: the formation of silica wet gels and the following drying of the wet gels. There are three drying routes commonly used:1) supercritical drying10, 2) freeze-drying11, 3) ambient pressure drying (APD)12. Typically, xerogels are produced by drying wet gels through simple evaporation at ambient pressure, while aerogels are fabricated by supercritical drying technique13. The majority of silica xerogels are synthesized using silicon alkoxides as precursors14. Tetraethyl orthosilicate (TEOS) is the most commonly used silica precursors in scientific research15. It is well known that TEOS is not solvable in water. Therefore, a solvent like ethanol is added to facilitate the dissolution of TEOS in water. In addition, the hydrolysis and condensation of TEOS is very slow at room temperature, acidic (e.g. hydrochloride) or basic catalysts (e.g. ammonia, NaOH solution, amine) were applied to facilitate the reaction. During the sol-gel process, the precursor solution undergoes two main stages described by the following reactions16. Hydrolysis Si(OC2H5)4 + nH2O→(OC2H5)4-nSi(OH)n + nC2H5OH (n=1,2,3,4)

(1)

Condensation ≡SiOH + HOSi≡ → ≡Si-O-Si≡ + H2O

(2)

≡SiOH + (OC2H5)Si- → ≡Si-O-Si≡ + C2H5OH

(3)

The solvents including alcohol and water are not desired in the final solid products. 3



Mostly they are remained in the synthesis solutions and are separated from the gels by filtration or centrifugation, becoming a part of wastewater. Inorganic acidic and basic catalysts are in the same situation as the solvents. However, in the cases of using organic amines as the catalysts, amines with long alkyl chains may also act as pore templates and assemble with silica via hydrogen bonding17. Thus, they can be removed by calcination or chemical extraction. In contrast, if amines with short chains are used, they may only act as basic catalysts and mostly are remained in the waste solution after the synthesis as the inorganic bases18. The generation of wastewater not only causes environmental problems, but also wastes resources. How to avoid the formation of wastewater and make cyclic utilization of the solvents and catalysts become a big challenge of the sol-gel synthesis. In this work, TEOS was hydrolyzed and condensed in water with the addition of ethanol as the solvent and n-butylamine as the catalyst. A wet gel was formed very shortly after the reaction. Ambient pressure drying was applied to dry the gel at 120 oC. To avoid the generation of waste liquid, evaporants were condensed and collected. The recovered liquid containing mainly water, solvent and butylamine was reused to synthesize xerogels in repeated cycles. Analysis on the silica xerogels synthesized with the recovered liquid was performed to evaluate the influence of the recycled solvents and catalyst on the synthesis. The results proved that the recovered solvents and catalyst could be reused for the cyclic synthesis of silica xerogels, making the whole process free of waste water and gas emission. This study provides a sustainable route for synthesizing materials via the sol-gel method.

Experimental section Materials Tetraethylorthosilicate (TEOS), butylamine (BA) and ethanol were purchased from Sinopharm Chemical Reagent Company, Aladdin Industrial Corporation and Tianjin Fuyu Fine Chemical Company, respectively. All the chemicals were of analytical grade and used as received. Distilled water was used for all experiments. 4


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Synthesis of silica xerogel The silica xerogel was prepared using reactants with molar ratios of TEOS:H2O:EtOH:BA=1:12.5:30.6:1.67. The synthesis procedures were as follows: ethanol was mixed with water and stirred for 10 min at room temperature. BA was then added into mixture stirring contained for 30 min. Finally, TEOS was added into the solution. The mixture was stirred for 4min and the solution turned turbid. Then, the stirring was stopped and the sol became fully gelled while losing fluidity, followed by aging for 1h. The formed wet gel was dried at 120oC for 6h at ambient pressure in a closed system as shown in Figure 1. The dried gel sample was collected and designated as S1. The evaporant was condensed into liquid and collected for the next synthesis.

Figure 1. Schematic diagram showing the cyclic sol-gel synthesis of silica xerogels

For the second synthesis, TEOS was added into the collected liquid from the first run. The amount of TEOS was determined based on the total weight of the collected liquid so that the percentage of TEOS in the synthesis medium was the same as that in the starting medium. All other synthesis procedures were the same as those used in the first run. The synthesis was repeated three times. The resultant products were encoded as S2, S3, and S4, respectively. According to the composition analysis of the recovered liquid from the fourth 5



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synthesis, the relative content of ethanol in the liquid was higher than that in the first synthesis medium (see later discussion). Therefore, the dosages of water, BA and TEOS in the fifth synthesis were adjusted based on the content of ethanol in the recovered liquid to meet their initially designed ratios. All other synthesis procedures were the same as those in the first synthesis except that the drying time was prolonged to 8h due to the increase of the volume of wet gel caused by the reactant adjustments. The prepared silica gel was encoded as S5. After this, samples encoded as S6, S7 and S8 were respectively synthesized by directly using the recovered liquids form the preceding runs without adjustment. Similarly, the recovered liquid from the eighth synthesis was analyzed and reused in the ninth synthesis, accompanied by the adjustments of water, butylamine and TEOS as in the fifth synthesis. Then, samples S10, S11, and S12, were respectively synthesized as in the cases of the S6~S8. It should be mentioned here that the volume of the recovered liquid used for the ninth synthesis was less than that in the fifth synthesis, because ca.80g of the recovered liquid from the eighth synthesis was consumed in the composition analysis (This did not happen in the fifth synthesis, because parallel synthesis for samples in series I was carried out). However, the drying time was still 8h as in the case of S5. Series I

Series II

TEOS+H2O+EtOH+BA 1

Gel

Drying

Recovered liquid 2

Gel

Recovered liquid 3 Gel

Recovered liquid 4 Gel

TEOS

Drying

Gel

Silica xerogel S3

Gel

Drying

Recovered liquid

Gel

TEOS

12 TEOS+Adjusted H2O,BA

Drying

Drying

Recovered liquid

Silica xerogel S5

5 Silica xerogel S4

Gel

11 Recovered liquid

Drying

Recovered liquid

Silica xerogel S6

6

TEOS+Adjusted H2O,BA

Recovered liquid

Silica xerogel S7 10

Recovered liquid

TEOS

Drying

Drying

Gel

Gel

TEOS

7 Silica xerogel S2

Recovered liquid 9

Recovered liquid

TEOS

Drying

Silica xerogel S8

8 Silica xerogel S1

TEOS

Drying

Drying

Gel

Series III

Gel

Drying

Figure 2. The main steps in the synthesis of silica xerogels in 12 runs

6


Silica xerogel S9

TEOS Silica xerogel S10



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For an easy comparison in the following discussion section, the 12 samples are classified into three series：I (S1~S4）, II (S5~S8), III (S9~S12) using the adjustments of the reactant dosages as the beginning of a new series. Figure 2 summarizes the main steps in the synthesis of silica xerogels in 12 runs. Characterization of silica xerogel The morphology of silica xerogel was analyzed on a scanning electron microscope (QUANTAN FEG250, USA). TEM measurements were carried out on Tecnai G2 F20 (FEI, USA). N2-sorption measurements were performed on an Autosorb-iQ-C automatic gas adsorption instrument (Quantachrome, USA). The pore size distribution of the samples was determined using the Barrett-Joyner-Halenda (BJH) method using the adsorption branches of the isotherms and the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model. The FTIR spectra of the silica xerogels were recorded on a spectrometer (Nicolet iS10, Thermo, USA) in the range of 400-4000cm-1 using thin discs prepared with the addition of KBr powder. Highresolution solid state 29Si Nuclear Magnetic Resonance (NMR) spectra of the xerogels were collected with magic angle spinning on a Bruker AVANCE III 600 system (Germany). The composition of recovered liquid was comprehensively analyzed by IR, 1H

NMR (Bruker 400M, Germany) and gas chromatography-mass spectrometer (GC-

MS) (Trace ISQ, Thermo, USA), etc.

Results and discussion Compositions of the recovered liquids from the wet gels Figure 1 shows the main process of silica xerogel synthesis, which includes three main steps: i) sol formation, ii) gelation and iii) the APD drying of wet gel. During the synthesis, the products lost fluidity after a full gelation occurred. All liquids including the formed water and ethanol (see the equations 2 and 3) were enclosed in the wet gel. Therefore, the following drying of the wet gel in the enclosed flask resulted in evaporants which were further condensed into liquid upon cooling and were recovered. It was observed that the recovered liquids from the gels synthesized in fourth and eighth runs were almost ineffective to generate wet gel. For example, in a trail synthesis 7



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after the fourth run, the recovered liquid was directly used in the fifth synthesis, it was observed that the synthesis reaction became significantly slow. The occurrence of turbid was at ca.2h after the stirring was stopped and the full gel formation was greatly delayed till ca.4days, suggesting a possibly great change of the recovered liquid. The analysis on the compositions of the recovered liquids indicated that besides the existence of methanol in the first one, all the liquids mainly contain ethanol, nbutylamine and water. Some minor components (including ammonium nitrate, ammonium chloride, ammonia water, F-/SO42- and residual TEOS) in the first and eighth recovered liquids were also detected. They may be from the reaction products, or the impurities of reagents, or the experimental error. The mass percentages of the main compositions are presented in Table 1. The relative mass ratios of EtOH/BA/H2O in the first synthesis medium were also listed for comparison. It can be seen that the relative percentages of ethanol (in the first recovered liquid, this refers to the sum of ethanol and methanol) are higher than that in the initial synthesis, while the relative percentages of butylamine and water becomes lower. The latter results suggest that part of the amine and water were tightly enclosed and may partially bond with the formed silica gel network.

Table 1 Main compositions of the original reactants and the first, fourth and eighth recovered liquids Mass ratio (%)

Component

Original

Recovered liquid First Fourth Eighth

ethanol

80.1

74.75 90.5

94.9

butylamine

7

4.75 3.05

3.25

water

12.9

9.75

1.4

methanol

6.5

11.25

It was that the great decrease in butylamine and water as well as the increase of ethanol reduced the hydrolysis and condensation of TEOS, resulting in the great delay in the sol and gel formation during the afore-mentioned trial synthesis after the fourth 8


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run. Therefore, during the fifth and the ninth synthesis, extra water and butylamine were added into the recovered liquids from the fourth and eighth runs, so that the relative ratios of BA and H2O to EtOH were adjusted to close to those in the first synthesis. Also, the amounts of TEOS was accordingly adjusted. Morphology of silica xerogels Figure 3 shows the SEM images of the first synthesized silica xerogel and typical samples synthesized with the recovered liquids. It can be seen that all the samples have very similar morphology and the products consist of aggregated sphere-like particles sized about 20-250nm. The TEM images in Figure 4 indicate that these particles are bridged to some extent, forming a three-dimensional network structure of the SiO2 xerogels. The interparticle voids contribute to the porous texture of the silica xerogels.

Figure 3. SEM images of synthesized silica xerogels 9



Figure 4. TEM images of samples S1 and S9

The SEM images also suggest that the aggregated particles in samples S4, S8, S12 which were directly synthesized without the adjustments of water and butylamine while using the recovered liquids are smaller than those in their correspondent initial ones S1, S5 and S9. This is due to the relative decreases in water and amine in the synthesis media, which could decrease the rate of formation of primary silica particles, reducing the possibility for to form larger aggregated particles. The formation of smaller aggregated particles in samples S4, S8, S12 also can explain why the surface area increases for the samples in the same series, such as from S1 to S4, S5 to S8 and S9 to S12 (see later discussion about N2-sorption results). However, comparing the SEM images of samples S5 and S9 with that of S1, one can find that the aggregation of particles increases, especially from fifth to ninth runs. This was mainly due to the prolonged drying time from 6h in the first to 8h in fifth and ninth synthesis. The greatly increased aggregation from S5 to S9 was due to the reduction in the volumes of the recovered liquids and the resultant wet gel in the ninth synthesis, while both were dried for 8h (see the experimental section). FT-IR results of the silica xerogels The FT-IR spectra of synthesized silica xerogels are shown in Figure 5. The spectra mainly show absorption bands at 3400,1640,1080,800 and 462cm-1, respectively. The broad band at 3400cm-1 is related to the stretching vibration of O-H bonds in water molecules and surface hydroxyl groups of silica (Si-OH) or the N-H 10


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bonds stretching vibrations of BA remained in the gel. The band near 1640cm-1 is attributed to the O-H bending vibration of adsorbed water molecules19. The small bands at 2850-2990cm-1 and 1360-1480cm-1 are respectively ascribed to the C-H symmetric or asymmetric stretching vibration and bending vibration of ethoxy groups (-OC2H5) 20. The weak bands at 1560 cm-1 are assigned to the N-H bending vibrations of amine 21. The results suggest that there were residual ethanol molecules and butylamine remained in the silica xerogels. S8 S7 S6 S5

Transmittance (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


Series Ⅰ

S4 S3 S2 S1 Series Ⅰ

1640 1560

3400

800

560

960 462 1080

4000

3600

3200

2800

2400

2000

1600

1200

800

400

-1

Wavenumber (cm )

Figure 5. The FT-IR spectra of synthesized silica xerogels S1-S8

The bands at 960 cm-1 are due to stretching vibration of Si-OH groups, accompanied by the smaller bands at 560cm-1 which can be attributed to the stretching vibration of the Si-O bonds in the Si-OH groups22,23. These bands become more obvious from sample S1 to samples S2,S3,S4, and from S5 to S8, suggesting that the silica xerogels synthesized directly with the recovered liquids from the preceding runs have more broken Si-O-Si bonds (which were replaced by Si-OH) on the surface. This is in agreement with the SEM results discussed before, which revealed that the latter samples were less aggregated. The strongest bands at 1080cm-1 are assigned to the asymmetric stretching mode of Si-O-Si bonds, while the bands at about 800cm-1 and 462cm-1 are attributed to the symmetric stretching and bending modes of Si-O-Si bonds, 11



respectively19. These are typical characteristic absorption bands of SiO2 gels. And all the samples show a band at 560cm-1 attributed to the Si-O band stretching vibration of the Si-OH23. Textural properties of the xerogels 0.05

1800 1600 S1 S2 S3 S4

1200 1000

0.03

dV (d)

3

Pore volume (cm /g)

S-1 S-2 S-3 S-4

0.04

1400

800

0.02

600 400

0.01

200 0 0.0

0.2

0.4

0.6

0.8

0.00

1.0

20

Relative pressure (p/p0)

40

60

80

100

120

100

120

Pore diameter (nm)

0.05 1400

S5 S6 S7 S8

1000 800

S5 S6 S7 S8

0.04

0.03

Dv (d)

Pore volume (cm3/g)

1200

600

0.02

400

0.01

200 0 0.0

0.2

0.4

0.6

0.8

0.00

1.0

20


40

60

80

Pore diameter (nm) 0.05

1600 1400

S9 S10 S11 S12

0.04

1200

S9 S10 S11 S12

1000 800

dV (d)

Pore volume (cm3/g)

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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0.03

0.02

600 400

0.01

200 0 0.0

0.2

0.4

0.6

0.8

0.00

1.0

20


40

60

80

100

120

Pore diameter (nm)

Figure 6. N2-sorption isotherms and pore size distributions of the synthesized silica xerogels

The N2-sorption isotherms and pore size distribution curves (PSD) of the silica xerogels are shown in Figure 6. Except samples S7 and S8, all other samples display 12


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type II isotherms with hysteresis loop, which indicates that these samples are macropores24. The large uptakes of N2-sorption occur at relative pressure above 0.9 also reflect the existence of macropores related to the irregular interparticle voids as evidenced by the SEM observation. The PSD curves of the samples show almost the same profile except samples S7 and S8, demonstrating that the pores in samples range from mesopores to macropores with maximum values (defined as the pore size listed in Table 2) at 66-68nm for the samples S1-S6, S9-S12. The exception of samples S7 and S8, which are featured as mesoporous according to their isotherms and PSD curves, is not well understood at the moment.

Table 2 Textural properties of the silica xerogels Sample

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

S1

291

2.29

68.2

S2

312

2.25

66.3

S3

413

2.37

66.8

S4

552

2.23

67.8

S5

346

2.04

67.6

S6

449

2.15

67.6

S7

564

2.05

30.5

S8

686

1.54

17.4

S9

303

2.13

67.7

S10

293

2.32

67.5

S11

382

2.40

67.7

S12

457

1.88

67.0

The specific surface area, pore volume and pore size of the samples are summarized in Table 2. It shows that compared with S1, the samples S2 to S4 which were prepared with the recovered liquids possess increasingly larger surface areas (e.g. from 291m2/g for sample S1 to 552m2/g for sample S4). This is due to the relative increase of ethanol and the decrease of water and amine in the recovered liquids. As a result, the hydrolysis and condensation of TEOS was retarded, favoring the formation of smaller particles with more surface broken bonds as evidenced by the FTIR results. Almost the same variation trend of the surface areas is shown for the samples in series 13



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II (S5~S8) and III and (S9~S12), except a slight decrease from S9 to S10 (which might be due to experimental error). Although the relative mass ratios of reactants in the synthesis media for samples S5 and S9 were adjusted to be close to that for sample S1, their specific surface areas are still different. The main reason for this is the increases in total volume of reactants after the adjustments of reactant dosages in the cases of S5 and S9 as well as the necessarily prolonged drying time, which induced more evaporation of liquids during the drying process and finally breakage of some weekly bridged network. This may also explain why S4, S8 and S12, which respectively have the smallest volumes in in series I, III to III exhibit smallest pore volumes among their correspondent series samples. High-resolution solid-state 29Si NMR a

Q4

Q3

b

Q4

Q3

c

Q4

Q3 Q2

Q2

Q2

-70

-80

-90

-100

-110

Chemical shift (ppm)

-120

-130 -70

-80

-90

-100

-110

-120

-130 -70


-80

-90

-100

-110

-120

-130


Figure 7. The 29Si NMR spectra of four silica xerogel samples: (a) S1, (b) S5, (c)S9

In order to investigate the microstructure of the synthesized silica xerogels, samples S1, S5 and S9 were characterized by high-solution solid-state spectroscopy measurements. The obtained

29Si

29Si

NMR

NMR spectra and the peak

deconvolution and fitting are shown in Figure 7. It can be seen that three

29Si

NMR

resonances were detected distinctly in the studied silica xerogels. The silicon forms in the synthesized silica xerogels were then identified by referring to possible Si(O1/2)4 tetrahedral units listed in Table 327,28. The chemical shifts at around -94ppm, -103ppm and -113ppm stand for the silicon tetrahedral units of Q2, Q3 and Q4 in the silica network, respectively29. The relative percentages of Qn units were calculated based on the surface areas of the differentiated peaks and the results are presented in Table 4. It is clear that 14


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the units of Q3 and Q4 are dominant in the silica xerogels micro-structure and the relative content of Q2 is low. Table 3 Silicon units found in amorphous silica27,28 Qn

Q0 -

Structure unit

Chemical shift(ppm)

O- OSiO O-

-68─-77

Q1

Q2

Q3

Q4

OOSiOSi O-

OSiOSiOSi O-

Si O SiOSiOSi O-

Si O SiOSiOSi O Si

-78─-86

-87─-95

-96─-106

-107─-120

-

Table 4 Qn results in the four silica xerogel samples Sample S1 S5 S9

Qn

Q2

Q3

Q4

chemical shift

-94.05

-102.45

-112.39

%

16.3

40.1

43.6

chemical shift

-94.74

-103.41

-113.67

%

8.4

33.7

57.9

chemical shift

-94.26

-102.74

-113.96

%

4.2

26.3

69.5

Degree of condensation (%) 81.8 87.4 91.3

The linkage of Si(O1/2)4 tetrahedral units in the structure of silica gels can be quantified by the degree of condensation30, which is defined as: Degree of condensation (%) = [𝑄1(%) +2𝑄2(%) +3𝑄3(%) +4𝑄4(%)]/4 (4) The degree of condensation of the studied samples are listed in Table 4 with value ranging from 81.8 to 91.3%. As 100% is correspondent to silica with a fully threedimensional linked network12, the results indicate that the silica species formed after the hydrolysis have effectively condensed to form cross-linked networks of the silica xerogels. This was also verified by the above FT-IR results. The comparison of the data indicate the degree of condensation of samples is in the order S1

Recovery and reutilization of the solvents and catalyst used in the sol

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