A Green and Facile Synthesis of Ordered Mesoporous Nanosilica

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A green and facile synthesis of ordered mesoporous nano-silica using coal fly ash Feng Yan, Jianguo Jiang, Sicong Tian, Zongwen Liu, Jeffrey Shi, Kaimin Li, Xuejing Chen, and Yiwen Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00793 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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ACS Sustainable Chemistry & Engineering

A green and facile synthesis of ordered mesoporous nano-silica using coal fly ash Feng Yan a, Jianguo Jiang

a,b,c *

, Sicong Tian a, Zongwen Liu d, Jeffrey Shi d, Kaimin Li a, Xuejing

Chen a, and Yiwen Xu a a

School of Environment, Tsinghua University, Beijing, China.

b

Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education, Beijing, China.

c

Collaborative Innovation Center for Regional Environmental Quality, Tsinghua University, Beijing, China.

d

School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia

*

Corresponding author:

Prof. Dr. Jianguo Jiang (School of Environment, Tsinghua University, Beijing 100084, China.) Tel./Fax.: +86 01062783548; E-mail address: [email protected]

ABSTRACT: Ordered mesoporous materials have attracted much attention owing to their superior structural properties. In this work, we develop a green and facile method to convert coal fly ash, a cheap, abundant, and silicon-rich industrial waste, into highly ordered mesoporous nano-silica. An energy-efficient technique, the alkali-dissolution process, was systematically studied for the extraction of silica from waste materials, instead of the conventional alkaline fusion method. The extraction efficiency of silica could reach up to 46.62% within 0.5 h at 110°C in 25 wt. % sodium hydroxide solution and the liquid-solid ratio was reduced down to 1.5:1. Subsequently, simulated flue gas was introduced to precipitate the nano-silica with the assistance of a surfactant through a twice-carbonation process. A series of characterization techniques confirmed that the synthetic nano-silica (SiO2-0.16) has a high purity (99.35%), high surface area (1,157 m2 g-1), large pore volume (0.95 cm3 g-1), and a highly ordered hexagonal mesostructure (2.88 nm); similar to the characteristics of the material derived from silicon alkoxide. This strategy significantly decreased the energy consumption and shortened the synthesis process through the utilization of flue gas, and is thus an effective and scalable approach for the synthesis of ordered mesoporous nano-silica from coal fly ash. KEYWORDS: Coal fly ash; mesoporous nano-silica; alkali-dissolution; twice-carbonation; MCM-41; carbon utilization

INTRODUCTION Ordered mesoporous materials have attracted much attention since Mobil scientists first

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announced the successful synthesis of M41S in 1992.1,2 M41S is the generic term for the various types of Mobil Composition of Matter (MCM) materials with a regular mesoporous structure, uniform pore size (2–10 nm), high surface area (> 1,000 m2 g-1), and variable morphologies.3 Therefore, numerous studies have reported their application in the diverse fields, such as in drug delivery,4 gene delivery,5 biosensors,6 composite coating,7 molecular separation,8 catalysis,9 and adsorption.10 The original M41S family was synthesized, in general, by combining appropriate amounts of a silica source (e.g., silicon alkoxide, fumed silica, or soluble silicates), a soft templating (surfactant), a base (e.g., sodium hydroxide [NaOH] or tetramethylammonium hydroxide [TMAOH]), and water.11 The mixture was aged at elevated temperatures (≥ 100°C) for 24–144 h, followed by filtration, washing with water, air-drying, and calcining at 500–550°C. The different surfactants of quaternary ammonium salts and gemini surfactants resulted in different mesophase sequences of hexagonal MCM-41, cubic MCM-48, and lamellar MCM-50.12 To date, significant advances have been achieved in the synthesis of mesoporous materials; however, considering the costly silica sources and expensive surfactants, environmental-benign synthesis techniques and the use of inexpensive raw materials remain a research hotspot. Natural minerals, industrial wastes and agricultural byproducts are cheap and abundant silica resources that could be alternative silica precursors.13 Therefore, studies in recent years have reported the synthesis of mesoporous materials from halloysite, 14 diatomite, 15 kaolin, 16 bentonite, 17 coal fly ash, 18 bottom ash, 19 , 20 iron-ore tailing, 21 copper-ore tailing, 22 photonic waste,23,24 resin ash,25 husk ash,26,27 sedge ash,28 and miscanthus ash,29 using a conventional templating mechanism. The recycling process could efficiently convert useless wastes into high-value silica-based materials as well as solving the problem of waste disposal. Among the industrial wastes, coal fly ash generated in Chinese coal-fired power plants reached 578 million tons in 2014, yet a substantial amount of ash is still disposed of in landfills or is used as road sub-base which contributes to a serious environmental problem.30 Nevertheless, the main component of SiO2 in coal fly ash accounts for 40–60 wt. %,31 making it an extensive, cheap and readily available source of silica. The synthesis procedure starting from coal fly ash involves silicon extraction as a first step before the synthesis of M41S. An alkali-fusion strategy is typically used to extract silica with NaOH at 550°C for 1 h, followed by overnight milling, one-day dissolving, and filtration.32 Considering the 2 ACS Paragon Plus Environment

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equipment corrosion and energy consumption, the alkali-dissolution process was used for silicon extraction in NaOH solution at 80–100 °C for 2–4.5 h.33 The reaction mechanisms of the main components (SiO2 and Al2O3) in coal fly ash during the hydrothermal process, however, have not yet been clarified, resulting in a low extraction efficiency with high impurities. Secondly, inorganic acid (sulfuric or hydrochloric acid) was generally used to precipitate mesoporous silica from the extracted solution.34 Because of the long aging time for particle growth and the generating of useless byproduct (sodium salt), carbon dioxide (CO2) which contributes significantly to global warming and climate change,35 was recently proposed as an alternative precipitant in the synthesis of precipitated silica.36 However, to our best of knowledge, this carbonation process has not been certified for the preparation of high-purity nano-silica with an ordered mesoporous structure. In this study, a green and facile approach, the alkali-dissolution and twice-carbonation processes, was firstly proposed for the conversion of coal fly ash into ordered mesoporous nano-silica. The reaction mechanisms of Al2O3 and SiO2 in alkali solution were comprehensively discussed, focusing on the effects of the alkali dosage, alkali concentration, extraction temperature, and extraction time. Then simulated flue gas containing 15 vol. % CO237 was introduced into the extracted liquid with a certain quantity of surfactant, resulting in the high-purity nano-silica and the byproduct (sodium carbonate) after the twice-carbonation process. The superior properties of the synthetic products were subsequently investigated through various characterization techniques. Owing to the high extraction efficiency of silica and the fast aging process with flue gas, this approach could be used both for the high-value utilization of coal fly ash and large-scale production of ordered mesoporous materials.

EXPERIMENTAL SECTION Extraction of silicon from coal fly ash A typical high-alumina coal fly ash from a coal-fired power plant (Tianjin, China) was used as the silicon source to prepare the ordered mesoporous nano-silica. The coal fly ash was first calcined for 2 h at 800 °C in air to remove organic components, after which the preheated ash was ground and sieved to obtain particle sizes of less than 74 µm (FA). The chemical composition of “FA” was determined using X-ray fluorescence analysis (XRF, Shimadzu XRF-1800, Japan, Table S1). The desilication reaction was conducted in a sealed Teflon-lined reactor (BeiLun, China) with a 3 ACS Paragon Plus Environment


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magnetic stirring system, which was heated by electric power and could maintain a consistent temperature of 25–250°C. The “FA” (5 g) was mixed with NaOH solution in the 100-mL Teflon-lined reactor, and then the mixture reacted at 110°C for 0.5 h with stirring (300 rpm). To investigate the influence of NaOH concentration and NaOH dosage, the concentration of NaOH (cNaOH) was chosen as 5–35 wt. %, and the mass ratios of NaOH/FA were set as 0.25, 0.50, 0.75, and 1.00. To investigate the influence of desilication conditions, 5-g “FA” was mixed with 2.5-g NaOH and 7.5-mL ultrapure water (cNaOH = 25 wt. %), and then the desilication reaction was performed at 50–130°C for 0.25, 0.5, 1, and 2 h. After the desilication reaction, the resultant suspension was immediately filtrated several times with ultrapure water to separate the desilicated coal fly ash and the sodium silicate solution. The residue was dried at 105°C for 12 h to further analyze the chemical and mineral composition. The solution was diluted to 100 mL in a volumetric flask to obtain the extracted liquid for the production of nano-silica. An elemental analysis of the extracted liquid was conducted using inductively coupled plasma-atomic emission spectrometry (ICP-AES, Thermo Fisher Scientific, America). The extraction efficiency of SiO2 (ηSiO2) can be calculated with Eq. 1, where MSiO2 and MSi are the molecular weights of SiO2 and Si (g mol-1), respectively; cSi is the concentration of Si in the extracted liquid (mg L-1); V is the volume of the extracted liquid (mL); mFA is the mass of “FA” (g); and WSiO2 is the weight percentage of SiO2 in “FA”.

ηSiO2 =

M SiO2 × cSi × V ×106 ×100% (Eq. 1) M Si × mFA × WSiO2

Preparation of ordered mesoporous nano-silica 40.5-mL extracted liquid derived under the best conditions (5,660 mg L-1 Si), 9.5 mL ultrapure water, and a certain quantity of cetyltrimethylammonium bromide (CTAB, C16H33(CH3)3NBr, Sigma-Aldrich, America) were mixed in the Teflon-lined reactor. A constant flow of 40 mL min-1 simulated flue gas (15 vol. % CO2 and 85 vol. % N2) was injected during the carbonation process through an inlet port located at the bottom of the reactor, and the rest of gas was continuously discharged through an outlet port located on the top of the reactor. The first carbonation process was conducted at 80°C with stirring (300 rpm) until the pH value decreased to 10.8–11.3 (~15 min), and then the purified mixture was obtained through filtration. The second carbonation process was


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conducted using the purified mixture under the same conditions for another 3 h. After cooling to room temperature, the synthetic nano-silica was recovered by filtration, washing, and drying at 105°C for 2 h. Finally, the organic template was removed via a muffle furnace in air at 550°C for 4 h. The molar ratios of CTAB/Si were 0.00, 0.04, 0.08, 0.16, and 0.32, respectively; thus, the products were designated based on their synthesis conditions (e.g., “SiO2-0.16” for the molar ratio of CTAB/Si at 0.16).

RESULTS AND DISCUSSION Extraction of silicon from coal fly ash Figure 1a presented the influence of NaOH concentration and NaOH dosage on the extraction efficiency of SiO2 after alkali dissolution at 110°C for 0.5 h. When the ratio of NaOH/FA was constant, the SiO2 extraction efficiency initially increased as the NaOH concentration increased, and then decreased after reaching the maximum at NaOH concentration of 25 wt. %, which could be explained from two aspects. The increasing of NaOH concentration could increase the chance of molecular collisions38 and thus promote the reactivity of SiO2 (Eqs. S1 and S2). However, it could also promote the reactivity of Al2O3 (Eq. S3), leading to a secondary reaction between the dissolved SiO32- and Al(OH)4- (Eq. S5). The increase of Na2O content in the desilicated coal fly ash (from 2.19 to 9.50 wt. %, Table S1), further confirmed the enhancement of the secondary reaction with the increasing of NaOH concentration. The consumption of NaOH accounted for a large proportion of the costs in the alkali dissolution process. Studies also suggested that the high content of Na+ in extracted liquid might enhance the formation of zeolites39 and impede the formation of mesoporous silica;32 thus, it was crucial to reduce the ratio of NaOH/FA. Figure 1a suggested that the maximum extraction efficiency of 46.62% was achieved at a NaOH/FA ratio of 0.50, while increasing the ratio slightly decreased the extraction efficiency due to the secondary reaction. Particularly, it was more economically efficient at a ratio of 0.25, since the consumption of NaOH per extracted “SiO2” could be decreased from 2.06 g-NaOH/g-SiO2 to 1.23 g-NaOH/g-SiO2 (Table 1). To further illustrate the reaction mechanism of coal fly ash in the alkali solution, the mineral composition of “FA” and the desilicated coal fly ash was characterized based on X-ray diffraction (XRD) patterns (Figure 1b). The major crystalline materials of “FA” were quartz (SiO2, JCPDS 99-0088, 2θ = 20.86, 26.64 and 50.14° for the (1 0 0), (0 1 1) and (1 1 2) reflections, separately), 5 ACS Paragon Plus Environment


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mullite (Al6Si2O13, JCPDS 15-0776, 2θ = 25.97, 26.27, 40.87° for the (1 2 0), (2 1 0) and (1 2 1) reflections, separately), and corundum (Al2O3, JCPDS 99-0036, 2θ = 35.15, 43.36, 57.51° for the (1 0 4), (1 1 3) and (1 1 6) reflections, separately). Moreover, the broad diffraction peak at 2θ = 20–24° indicated the presence of an amorphous silica fraction.40 The standard Gibbs free energy changes (∆GΘ) of reactions (Eq. S1-S5) were calculated and summarized in Table S2 according to the base data in Lang’s Handbook of Chemistry.41 The positive value of ∆GΘ indicates the thermodynamic impossibility of Eq. S4, and thus the diffractions of mullite kept almost unchanged for the desilicated coal fly ash. Although Eqs. S1–S3 could occur spontaneously for ∆GΘ < 0, amorphous silica was more reactive than quartz and corundum due to their different reaction rates. While the broad diffraction of amorphous silica almost disappeared for the desilicated coal fly ash, the diffraction peaks of quartz and corundum only decreased slightly. Moreover, a new crystalline phase, hydroxysodalite (Na8Al6Si6O24(OH)2(H2O)2, JCPDS 41-0009), was detected through the diffraction peaks at 2θ = 13.89, 24.30 and 31.74° for the (1 1 0), (2 1 1) and (3 1 0) reflections, respectively. And the gradual increase in the diffraction peaks confirmed the enhancement of the secondary reaction with an increasing of NaOH concentration.

Figure 1. (a) Extraction efficiencies of SiO2 when the mass ratios of NaOH/FA = 0.25-1.00 and the cNaOH = 5-35 wt. %; (b) The XRD patterns of “FA” and the coal fly ash desilicated with 5, 15, 25, and 35 wt. % NaOH solution (NaOH/FA = 0.50). Desilication processes were conducted at 110°C for 0.5h.

Figure 2a plotted the extraction efficiencies of SiO2 under different desilication conditions. The Gibbs free energy changes (∆G) under different reaction temperatures, which were calculated based 6 ACS Paragon Plus Environment

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on classic thermodynamics theory, revealed that the possibility of spontaneously occurring for Eqs. S1–S5 did not change in the range of 50–130°C. However, the reaction rates were considerably influenced by the temperature, and thus the SiO2 extraction efficiency rapidly increased as the temperature increased from 50°C to 110°C. Meanwhile, the increasing temperature also had a positive effect on the dissolution of Al2O3 and the precipitation of hydroxysodalite, leading to a slight decrease in the SiO2 extraction efficiency at temperatures higher than 110°C. Both the XRD patterns (Figure 2b) and the Na2O contents (Table S1) of the desilicated coal fly ash proved that an increase in temperature promoted the reactivity of the SiO2 dissolving and the hydroxysodalite precipitating. With regard to the effect of reaction time, the SiO2 extraction efficiency increased with the extension of reaction time from 0.25 to 2 h at 50–100°C. Nevertheless, the SiO2 extraction efficiency could rapidly reach a maximum within 0.5 h when the temperature ≥ 110°C, and then it decreased as the prolonging reaction time. Thus, this alkali-dissolution technique was more energy-efficient (at 110°C for 0.5 h) compared to the alkali-fusion strategy (at 550°C for 1 h, Table 1), and it also achieved a better extraction efficiency of 0.24 g-SiO2/g-FA compared to 0.21 g-SiO2/g-FA.32 Measurements of N2 physisorption revealed a great structural change in coal fly ash after desilication

(Figure

S1).

Both

“FA”

and

“FA-desilication”

possessed

type

II

N2

adsorption-desorption isotherms, as classified by the International Union of Pure and Applied Chemistry (IUPAC), and the adsorption continued to increase steadily during the medium-pressure (P·P0-1 = 0.2–0.8) stage, revealing the wide size distribution (2–100 nm) of pores.42 The surface area and the pore volume of “FA-desilication” increased from 1.5 m2 g-1 and 0.007 cm3 g-1 to 33 m2 g-1 and 0.16 cm3 g-1, respectively (Table 2). Furthermore, the different morphological contrasts in SEM images visually revealed the structure change (Figure S2). The “FA” particles were spherical in shape and exhibited a relatively smooth surface texture composed of silicate glass, but the glassy surface of these spheres disappeared after desilication and strips of crystal grains, which were identified as mullite and corundum, were totally exposed.43 Since the silicate glass was dissolved in the alkali solution, the pore channels and the surface area of coal fly ash were enlarged, which were beneficial to further utilization of desilicated coal fly ash as an efficient adsorbent.44



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Figure 2. (a) Extraction efficiencies of SiO2 under different reaction temperatures of 50–130°C and different reaction times of 0.25–2 h. (b) The XRD patterns of the coal fly ash desilicated for 0.5 h at 50°C, 70°C, 90°C, 110°C, and 130°C. Table 1. Comparison of the extraction efficiency of SiO2 from coal fly ash in different study NaOH/FA Method

T (mass ratio)

t

Dissolving stage WSiO2 a T

ηSiO2 b

UNaOH c (g-NaOH

t (%)

/g-SiO2)

Reference

(h)

(ºC)

(h)

(wt. %)

0.25

N.A.

f

N.A.

110

0.5

52.00

39.24

1.23

This study

AD

0.5

N.A. f

N.A.

110

0.5

52.00

46.62

2.06

This study

AF e

1.2

550

1

25

24

55.59

11.58

18.64

Ref. 18

AF

1.2

550

1

25

24

67.19

30.87

5.79

Ref. 32

AD

0.8

N.A.

N.A.

100

4.5

62.76

19.12

6.67

Ref. 33

AD

0.8

N.A.

N.A.

125

1

51.64

29.76

5.21

Ref. 34

AF

1.2

550

1

25

24

65.70

35.88

5.09

Ref.

45

AF

1.2

500

1

25

24

50.21

25.84

9.25

Ref.

46

AF

1.2

550

1

25

24

58.50

46.83

4.38

Ref.

47

AD

a

Fusion stage

d

(ºC)

WSiO2 (wt. %), the weight percentage of SiO2 in “FA”; b ηSiO2 (%), The extraction efficiency of SiO2; c UNaOH, the

consumption of NaOH per extracted “SiO2” ( U NaOH =

NaOH / FA , g-NaOH/g-SiO2); d AD, alkali-fusion ηSiO2 × WSiO2 / 10000

strategy; e AF, alkali-dissolution process; f N.A., not available.


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Table 2. Structural characteristics of coal fly ash and the synthetic nano-silica SBET a

Vpore b

DBJH c

a0 d

bp e

DDLS f

(m2 g-1)

(cm3 g-1)

(nm)

(nm)

(nm)

(nm)

1.5

0.007

17.57

N.A.

N.A.

N.A.

N.A.

33

0.16

17.62

N.A.

N.A.

N.A.

N.A.

SiO2-0.00

289

0.85

9.92

N.A.

N.A.

202 ± 46

0.25

SiO2-0.04

404

0.70

6.02

3.95

N.A.

298 ± 46

0.42

SiO2-0.08

792

0.91

4.42

4.11

N.A.

351 ± 41

0.49

SiO2-0.16

1157

0.95

2.88

4.30

1.42

436 ± 51

0.59

SiO2-0.32

1057

0.86

2.94

4.32

1.38

443 ± 48

0.76

Sample FA FA-desilication

a

h

SBET, specific surface area; e

b

Vpore, total pore volume.

c

DBJH, average pore size;

f

parameter; bp, wall thickness; DDLS, average size of silica agglomerates;

g

d

PDI g

a0, hexagonal unit cell

PDI, polydispersity index of silica

agglomerates size; h FA-desilication, the coal fly ash after desilication at 110°C for 0.5 h (5 g FA mixed with 2.5 g NaOH and 7.5 mL H2O).

Characterization of the ordered mesoporous nano-silica The mesoporous nano-silica was synthesized from the extracted liquid, and thus it was crucial to reduce the impurities in the silicon source.22 During the first carbonation process, amber precipitate materials were separated at pH ≈ 11.0, which was confirmed as the co-precipitation of iron oxide, alumina and silicic acid.48 After the removal of impurities in the extracted liquid, the synthetic nano-silica, “SiO2-0.00”, contained a very high purity of SiO2 (99.29 wt. %), with trace amounts of aluminum, sodium, and other components. As a contrast , “SiO2-once”, which was synthesized through the once-carbonation process, contained only 98.35 wt. % of SiO2, 1.00 wt. % of Al2O3 and 0.12 wt. % of Fe2O3 (Table S1). The chemical functional groups of synthetic nano-silica were characterized by infrared spectrograms in the range of 400–4000 cm-1 (Figure S3b). All samples showed the similar absorption bands at ~470, 806, and 1100 cm-1, which corresponded to the rocking vibration, symmetric stretching vibration, and asymmetric stretching vibration of the Si-O-Si bonds, respectively.49 The small band at ~563 cm-1 was linked to a coupled mode in four-member siloxane rings ([SiO]4),50 while the absorption bands at ~1626 and 3441 cm-1 were respectively assigned to the bending and stretching vibrations of the hydroxyl groups of the adsorbed water.40 However, the adsorption band at ~960 cm-1, which corresponded to the bending vibration of the Si-OH bond, decreased as the CTAB/Si ratio increased, and disappeared completely for “SiO2-0.16” and



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“SiO2-0.32”. In addition, the weak absorption band at ~3740 cm-1, which was assigned to the stretching vibration of isolated silanols or terminal silanols,51 was observed in all samples as these silanols were difficult to remove.52 The quantitative analysis of hydroxyl groups was conducted using a thermogravimetric analyzer (TGA) with thermograms (Figure S3a). Accordingly, the weight loss of silica below 100°C and 100-200°C was attributed to the removal of physically and chemically adsorbed water, respectively. When the temperature was higher than 200°C, silica gradually lost different kinds of hydroxyl groups. The weight loss at 200–600°C referred to the removal of geminal and vicinal silanols, and the weight loss above 600°C referred to the removal of isolated and terminal silanols.52 Thus, the hydroxyl content (NOH) and the hydroxyl density (COH) of silica can be calculated with Eqs. 2 and 3, respectively,49 where W200 and W1000 are the weight of silica samples at temperatures of 200°C and 1000°C (wt. %), MH2O is the molecular weight of water (g mol-1), and NA is the Avogadro constant. The calculated results were summarized in Table S3. “SiO2-0.00” possessed a high hydroxyl density of 10.75 OH nm-2, compared to that of fumed silica (3.78 OH nm-2) or precipitated silica (6.40 OH nm-2).49 Since the high hydroxyl density provided plentiful reaction sites, a series of amine agents could be covalently bonded to the hydroxyl groups of “SiO2-0.00” for further utilization in adsorption.8 When CTAB was introduced to the synthesis process, the thermal stability of synthetic materials was enhanced, with a weight loss lower than 3.72 wt. %. The synthetic materials were thus hydrophobic, and had a better compatibility with organic groups and organic phases.40

N OH =

2(W200 − W1000 ) ×1000 (Eq. 2) M H2O

COH =

2(W200 − W1000 ) NA × (Eq. 3) M H2O S BET ×1018

The small-angle XRD patterns of synthetic nano-silica were shown in Figure 3. The three well-defined diffraction peaks for “SiO2-0.16” and “SiO2-0.32”, associated with the (1 0 0), (1 1 0), and (2 0 0) reflections, indicated the formation of a highly ordered hexagonal mesostructure with a space group of p6mm.10 The intensity of the diffraction peaks was enhanced as the CTAB/Si ratio increased, suggesting a better pore arrangement of “SiO2-0.16” and “SiO2-0.32”. The location of diffraction peaks (2θ) and the corresponding interplanar spacing (d) were calculated and summarized in Table S4. It was noticeable that the diffraction peak of the (1 0 0) reflection shifted 10 Environment ACS Paragon Plus

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to lower degrees as the CTAB/Si ratio increased, which implied that “SiO2-0.16” and “SiO2-0.32” exhibited larger d-spacing values.23 On the other hand, there was a broad peak centered at 22° that was observed in the XRD patterns of synthetic nano-silica (Figure S4), which could confirm the presence of amorphous silica as extra-framework particles. “SiO2-0.00” possessed type II N2 adsorption-desorption isotherms with a H3 hysteresis loop at P·P0-1 = 0.7–1.0 (Figure 4a), caused by capillary condensation of nitrogen in slit-shaped pores between the plate-like particles.3 With the increasing of CTAB/Si, the adsorption-desorption isotherms of “SiO2-0.08”, “SiO2-0.16” and “SiO2-0.32” changed to a type IV mode, and exhibited a steep step for capillary condensation at a relative pressure of 0.25–0.35, which was typical for ordered mesoporous silica with a narrow distribution of mesopores.22 The higher steep condensation with a higher CTAB/Si ratio indicated a higher degree of uniform mesostructure for “SiO2-0.16” and “SiO2-0.32”,23 in accordance with the small-angle XRD results. Figure 4b displayed the corresponding pore size distribution curves of synthetic nano-silica. Generally, nano-silica had a relatively wide mesopore size distribution of 2–100 nm without surfactant;40 however, “SiO2-0.00” exhibited a narrow pore size distribution centered at 11.2 nm. This encouraging result might be ascribed to the CO2 bubble, which played a similar role of a surfactant.36 When CTAB was introduced to the synthesis process, all of the resultant nano-silica showed a narrow pore size distribution centered at 2.3–2.8 nm, suggesting the uniform porosity of the materials. Moreover, “SiO2-0.04” still contained some larger mesopores of 5.5–11.2 nm, indicating that some particles maintained the properties of “SiO2-0.00” due to the limited surfactant. The physical parameters, such as specific surface area (SBET), total pore volume (Vpore), and average pore size (DBJH), were summarized in Table 2. The cell parameter (a0) and the wall thickness (bp) of the silica with a hexagonal mesostructure can be calculated with Eqs. 4 and 5,53 respectively, where d100 refers to the interplanar spacing of the (1 0 0) reflection (nm). The SBET of nano-silica significantly increased with an increase in the CTAB/Si ratio, reaching a maximum of 1157 m2 g-1 for “SiO2-0.16”. Nevertheless, the SBET did not continuously enhance when the CTAB/Si ratio further increased to 0.32, in agreement with Fu et al.22 Commonly, nano-silica synthesized from waste sources has a lower surface area and lower pore volume than those materials synthesized from pure silica precursors, due to the incorporation of aluminum cations and other metal ions derived from waste sources.13 However, the structural properties (SBET and Vpore) of 11 Environment ACS Paragon Plus


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“SiO2-0.16” could be comparable to the material derived from tetraethylorthosilicate (TEOS),54 since a twice-carbonation process was conducted to ensure the purity.

a0 = 2 × d100 / 3 (Eq. 4) bp = a0 − DBJH (Eq. 5)

Figure 3. Small-angle XRD patterns of “SiO2-0.00”, “SiO2-0.04”, “SiO2-0.08”, “SiO2-0.16”, and “SiO2-0.32”.

Figure 4. (a) N2 physisorption isotherms and (b) pore size distributions of “SiO2-0.00”, “SiO2-0.04”, “SiO2-0.08”, “SiO2-0.16”, and “SiO2-0.32”.

Figure S5 showed the changes in morphology of the synthetic nano-silica with increasing CTAB/Si ratio. “SiO2-0.00” showed uniform plate-like agglomerates in the range of 150–250 nm, which formed apparent intergranular gaps. When CTAB was introduced to the synthesis process, the shapes of agglomerates turned to spherical or to spheroid with a smooth surface due to the template effect of the surfactant.13 Besides, the average size of silica agglomerates (DDLS) was 12 Environment ACS Paragon Plus

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obtained using the dynamic light scattering (DLS) method (Table 2), and the results showed that the silica agglomerates gradually grew up with increasing CTAB/Si ratio. Figure S6 revealed that all the synthetic nano-silica presented a relatively narrow particle size distribution with an average size of 200–500 nm. The textural structures of “SiO2-0.00” and “SiO2-0.16” were obtained from transmission electron microscopy (TEM) analysis. “SiO2-0.00” showed plate-like agglomerates composed of uniform primary silica particles of ~10 nm (Figure 5b), consistent with the N2 physisorption results. Because of the absence of a surfactant, “SiO2-0.00” did not possess a well-organized mesostructure, which was also revealed by the small-angle XRD analysis. Nevertheless, “SiO2-0.16” formed a highly ordered hexagonal mesostructure (Figure 5c), following the guidance of CTAB. The high-resolution TEM (HRTEM, Figure 5d) image clearly showed a long-range ordering of the mesopores with a width of ~3.2 nm, close to the parameters of DBJH and d100. The selected-area electron diffraction (SAED) pattern (inset, Figure 5d), which showed the diffraction pattern of single-crystal material along the (1 0 0) zone axis, further confirmed the single crystalline structure. In addition, these confirmed mesopore structures of “SiO2-0.16” were in agreement with the narrow pore size distribution and the well-defined diffraction peaks of the small-angle XRD result. In general, the synthesis of ordered mesoporous nano-silica requires a long aging time (> 24 h) for particle growth, and the synthetic materials prepared from waste sources showed lower structural properties because of the impurities (Table 3). However, the products in this study could be prepared within 3 h and also showed a highly ordered mesostructure with high purity, owing to the alternative precipitant (CO2 gas) and the twice-carbonation process. This result thus provides a facile and promising approach to the scalable production of ordered mesoporous nano-silica from coal fly ash, considering both economic and environmental points of view. Table 3. Comparison of mesoporous nano-silica synthesized from the different silica sources Silica source

Acid

CTAB/Si

ta

Tb

precipitant

(mol/mol)

(h)

(°C)

0.00

3

80

289

0.16

3

Coal fly ash

CO2

Coal fly ash

CO2

TEOS

54, d

Fumed silica

TEAOH 54

Sodium silicate

24

e

0.25

SBET

DBJH

WSiO2 c

(nm)

(wt. %)

0.85

9.92

99.29

Vpore

(m2 g-1) (cm3 g-1)

80

1157

0.95

2.88

99.35

48

f

25

1128

0.95

3.51

N.A.

f

25

1027

0.92

3.87

N.A.

145

1102

1.13

3.00

N.A.

TEAOH

0.25

48

H2SO4

0.20

36

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Coal fly ash 32

CH3COOH

0.16

96

97

842

0.75

3.70

93.24

Coal fly ash

33

H2SO4

0.17

16

25

1149

0.60

2.70

98.50

Coal fly ash

34

H2SO4

0.66

24

90

525

0.71

5.13

95.33

Coal fly ash

45

CH3COOH

0.15

120

100

740

0.42

2.30

N.A.

Coal fly ash

46

H2SO4

0.05

24

100

610

1.03

6.20

88.79

Coal fly ash

47

CH3COOH

0.25

16

25

1020

0.98

2.35

69.25

Halloysite

14

HCl

0.16

24

100

509

0.49

3.80

96.67

Diatomite

15

H2SO4

0.20

72

80

1006

1.27

4.10

N.A.

HCl

N.A.

24

100

1041

0.97

3.70

98.43

CH3COOH

N.A.

48

100

494

0.72

3.80

98.10

CH3COOH

0.33

48

100

847

0.70

3.00

88.86

H2SO4

0.16

48

105

992

0.85

3.44

N.A.

HCl

0.10

48

100

882

1.02

4.20

N.A.

Kaolin

16 17

Bentonite

Bottom ash

19

Bottom ash

20

Iron-ore tailing

21

Copper-ore tailing

22

HCl

0.20

48

100

947

0.76

3.24

96.82

Photonic waste

23

HF

0.20

8

25

790

1.10

3.10

N.A.

Photonic waste

24

H2SO4

0.20

36

145

1082

0.99

2.95

N.A.

Resin ash

25

H2SO4

0.09

48

100

1033

0.89

2.70

99.80

Husk ash

26

HCl

0.00

2

100

164

N.A.

4.20

N.A.

Husk ash

27

H2SO4

0.20

36

145

1063

1.00

3.40

N.A.

CH3COOH

0.13

72

100

1174

0.98

4.10

98.05

H2SO4

0.27

24

25

1020

1.09

4.2

N.A.

Sedge ash

28

Miscanthus ash a

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b

t, aging time of silica preparation;

products characterized by XRF;

d

T, aging temperature of silica preparation;

TEOS, tetraethylorthosilicate (Si(OC2H5)4);

e

c

WSiO2, content of SiO2 in

TEAOH, tetraethyl ammonium

hydroxide (N(C2H5)4·OH); f stirring at 70°C for 2 h before the aging process.

Figure 5. TEM images of (a) “SiO2-0.00”, and (c) “SiO2-0.16”; HRTEM images and SAED patterns (insert) of (b) “SiO2-0.00”, and (d) “SiO2-0.16”.

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Supporting Information Experimental characterization; calculation of ∆G, weight loss and hydroxyl content; chemical composition, N2 physisorption isotherms and SEM images of coal fly ash; cell parameters, TGA analysis, FTIR spectra, XRD patterns, particle size number distribution, and SEM images of the synthetic nano-silica; This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Telephone/Fax: +8610-62783548; E-mail address: [email protected].

Present Addresses * School of Environment, Tsinghua University, Beijing 100084, China.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (grant no. 21576156) and the Tsinghua University Initiative Scientific Research Program (grant no. 2014z22075). The authors also thank the support of the Beijing National Center for Electron Microscopy at Tsinghua University.

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For Table of Contents Use Only Title: A green and facile synthesis of ordered mesoporous nano-silica using coal fly ash Feng Yan, Jianguo Jiang, Sicong Tian, Zongwen Liu, Jeffrey Shi, Kaimin Li, Xuejing Chen and Yiwen Xu Brief synopsis: This strategy provides a facile and promising approach to the scalable production of ordered mesoporous nano-silica from coal fly ash, considering both economic and environmental points of view.

Table of Contents (TOC)



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Table of Contents 47x26mm (300 x 300 DPI)

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Figure 1. (a) Extraction efficiencies of SiO2 when the mass ratios of NaOH/FA = 0.25-1.00 and the cNaOH = 5-35 wt. %. 119x119mm (300 x 300 DPI)



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Figure 1. (b) The XRD patterns of “FA” and the coal fly ash desilicated with 5, 15, 25, and 35 wt. % NaOH solution (NaOH/FA = 0.50). Desilication processes were conducted at 110°C for 0.5h. 119x119mm (300 x 300 DPI)


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Figure 2. (a) Extraction efficiencies of SiO2 under different reaction temperatures of 50–130°C and different reaction times of 0.25–2 h. 119x119mm (300 x 300 DPI)



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Figure 2. (b) The XRD patterns of the coal fly ash desilicated for 0.5 h at 50°C, 70°C, 90°C, 110°C, and 130°C. 119x119mm (300 x 300 DPI)


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Figure 3. Small-angle XRD patterns of “SiO2-0.00”, “SiO2-0.04”, “SiO2-0.08”, “SiO2-0.16”, and “SiO20.32”. 119x83mm (300 x 300 DPI)



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Figure 4. (a) N2 physisorption isotherms and (b) pore size distributions of “SiO2-0.00”, “SiO2-0.04”, “SiO20.08”, “SiO2-0.16”, and “SiO2-0.32”. 119x84mm (300 x 300 DPI)


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Figure 4. (a) N2 physisorption isotherms and (b) pore size distributions of “SiO2-0.00”, “SiO2-0.04”, “SiO20.08”, “SiO2-0.16”, and “SiO2-0.32”. 119x84mm (300 x 300 DPI)



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Figure 5. TEM images of (a) “SiO2-0.00”, and (c) “SiO2-0.16”; HRTEM images and SAED patterns (insert) of (b) “SiO2-0.00”, and (d) “SiO2-0.16”. 66x44mm (300 x 300 DPI)


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A Green and Facile Synthesis of Ordered Mesoporous Nanosilica

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