- Email: [email protected]
Dioxybenzene-bridged hydrophobic silica aerogels with enhanced textural and mechanical properties
Microporous and Mesoporous Materials 294 (2020) 109863
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Dioxybenzene-bridged hydrophobic silica aerogels with enhanced textural and mechanical properties Qi Wang a, 1, D.B. Mahadik a, *, 1, Puttavva Meti b, Young-Dae Gong b, Kyu-Yeon Lee a, Hyung-Ho Park a, ** a b
Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea Innovative Drug Library Research Center, Dongguk University, Seoul, 04620, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Insulation Dihydroxybenzene Hydrophobic Textural property Mechanical property
Designing aerogel materials with excellent mechanical robustness while maintaining their basic characteristics, is of great importance for applications in energy saving management systems. Organic modification is required to strengthen silica aerogel, but the modified aerogels show decreased textural properties due to phase separation between organic-inorganic components. We report, the fabrication of organically modified aerogels without minimizing their basic properties, based on tetraethoxysilane (TEOS) and a bifunctional precursor prepared from dihydroxybenzene and TEOS via acid catalyst. Three types of dihydroxybenzene isomers were used to study the effect of the position of the hydroxyl groups on textural properties of aerogels and compatibility of the pre cursors. A simple, efficient, and cost effective sol-gel method was employed to develop porous aerogels with no additives and surfactants. Hydroquinone-TEOS (HT) bridged silica aerogels exhibited improved mechanical strength, low dipole moment, high specific surface area (1154 m2/g), high porosity (96%), low density (0.061 g/ cm3), and ultra-low thermal conductivity of 0.0334 W/m⋅K. In addition, precursor was cheaper than the other isomers. The prepared aerogels show hydrophobic nature without the use of harmful silylating reagents.
1. Introduction
enhances pore size [18,19]. Several studies have reported improved mechanical properties by constructing organic-inorganic composites in several ways [20,21]. Physically or chemically introducing polymers to silica networks adds strength to the overall composite. However, the phase separation between organic and inorganic compounds suppresses the textural properties that enhance its density and thermal conductivity [3]. Moreover, surface modification, which release hazardous byprod ucts is needed to improve its hydrophobic nature [19]. Therefore, it is necessary to develop organically modified composite porous material with small pore size, enhanced mechanical properties with a hydro phobic nature without releasing hazardous byproducts via eco-friendly and cost-effective method. As for industrial application, cost-effective large-scale production of aerogels should be considered together with green chemical process [22–25]. Herein, we report the facile synthesis of dioxybenzene-bridged aer ogels with improved textural and mechanical properties. To the best of our knowledge, dioxybenzene linked aerogels have not been reported so
Silica aerogels have gained wide spread attention due to their porous nature with exceptional properties which endow them versatile appli cations in various fields [1–9]. Among these thermal insulating mate rials can play a vital role in living systems and controlling various domestic and industrial processes by saving energy [10–12]. Further more, due to their nano-structured porous networks, silica aerogels possess a low dielectric constant, which has evoked great attention in semiconductor industries [13–15]. The basic prerequisites for aerogel applications are a small pore size, low density, and sufficient mechanical strength. However, practical applications of aerogels have been restricted, due to their fragility and the formation of hazardous byproducts during surface modification [16]. Generally, silica aerogels are cross-linked with organic monomers or polymers to improve their mechanical properties [17]. Specifically, polymer reinforcement of silica aerogels results in high density and
* Corresponding author. Department of Materials Science and Engineering, 50 Yonseri-ro, Yonsei University, Seodaemun-Ku, Seoul, 03722, Republic of Korea. ** Corresponding author. Department of Materials Science and Engineering, 50 Yonseri-ro, Yonsei University, Seodaemun-Ku, Seoul, 03722, Republic of Korea. E-mail addresses: [email protected] (D.B. Mahadik), [email protected] (H.-H. Park). 1 Qi Wang and D.B. Mahadik contributed equally to this work. https://doi.org/10.1016/j.micromeso.2019.109863 Received 16 August 2019; Received in revised form 21 October 2019; Accepted 31 October 2019 Available online 1 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
far. The objectives of this study were to synthesize and investigate the properties of aerogels based on TEOS with various dihydroxybenzene derivatives fabricated via sol-gel process, to increase their mechanical properties. Organotrialkoxysilane based aerogels exhibit large phase separation in the silica network, which reduces their basic properties. Herein, we propose to minimize phase separation by sandwiching organic molecule between silica clusters which connects two ends similar to a bridge over a river. The bridging of the silica network with small organic molecules helps to develop aerogels with the small pore size, required for high textural properties. The benzene ring was selected for bridging as it is non-polar, and can impart thermal stability and mechanical strength to the aerogel. It also provides scope for bridging at ortho-, meta-, and para-positions. A comparative study of three different dioxybenzene isomers on silica network was carried out to analyze the suitability of dihydroxybenzene derivatives for the desired properties of the bridged silica aerogels. These porous multi-functional aerogels can be potentially used in various fields.
4 h and cooled down to room temperature. The precursor P1 obtained was then analyzed using NMR and LC/MS (Fig. S1). Similar reaction conditions were utilized for preparing P2 and P3. (NMR and LCMS data of P2 and P3 are shown in Figs. S2 and S3, respectively). The NMR and LCMS data shows that the product is confirmed as proposed in Fig. 1. The trifluroacetic acid was used as a reaction catalyst. The mixture is refluxed for 4 h for completion of reaction and also purification. The mixture contain product (yield 96%) in majority and small fraction of precursors and remaining trifluroacetic acid [26]. The unused acid catalyst is further used for hydrolysis of precursor and remaining start ing precursors will be used during sol-gel process. The steps of sol-gel reaction contain hydrolysis and condensation of precursors by acid and base catalyst in the large amount of solvent and finally, supercritical drying. In these steps of sol-gel process, most of the impurities will be removed from the silica network during washing and solvent exchange. The as prepared bridged silica composite aerogels are pure. Therefore purification of precursors is not essential for bridged silica aerogel synthesis. 1,4-Bis(triethoxysilyloxy)benzene (P1): Light orange liquid. 1H NMR (500 MHz, CDCl3) δ 6.89 (s, 2H), 6.71 (s, 2H), 3.9 (q, 12H, –Si–O–CH2–), 1.26 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.20. 1,3-Bis(triethoxysilyloxy)benzene (P2): Light brown liquid, 1H NMR (500 MHz, CDCl3) δ 7.14–7.03 (m, 1H), 6.64–6.52 (m, 1H), 6.43–6.33 (m, 1H), 4.01 (q, 12H, –Si–O–CH2–), 1.23 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.30. 1,2-Bis(triethoxysilyloxy)benzene (P3): Light orange liquid. 1H NMR (500 MHz, CDCl3) δ 6.87 (d, 2H), 6.78 (d, 2H), 3.85 (q, 12H, –Si–O–CH2–), 1.24 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.20.
2. Experimental details 2.1. Materials and instrumentation All the reagents and solvents were purchased from Sigma Aldrich. All chemicals were used as received. TEOS, hydroquinone, resorcinol, and catechol were used as precursors. Milli-Q water was used for preparing aqueous solutions. Oxalic acid was used as acid catalyst for hydrolysis and ammonia solution was used as base catalyst. NMR spectra measurements were carried out using Bruker-500 in strument using tetramethylsilane solvent as an internal reference. Liquid chromatography–mass spectrometry (Triple Quadrapole 6400 series LC/MS/Agilent) study was performed on an electrospray ionization (ESI) mass spectrometer. The DFT calculations were completed on B3LYP/6-31G (d orbital) level to gain deeper insight into the dipole moment, electron distribution and geometrical alignment of the struc tural isomers. FTIR was performed using an IR spectrophotometer (PerkinElmer model No. 1760X). The morphology of aerogels was analyzed using field emission scanning electron microscopy (FESEM, JEOL, Japan, 120 kV accelerating voltage). The surface area of bridged silica aerogels was obtained from N2 adsorption-desorption measure ments at 77 K measured using Brunauer-Emmett-Teller (BET) Surface Analyzer (Quantachrome Instruments (v10.0)) system [26,27]. Pore size and pore volume of aerogels were obtained from Barrett-Joyner-Halenda (BJH) method [28]. The % of porosity of the as prepared silica aerogels were measured using following formula [29]. � � ρb % of porosity ¼ 1 � 100
2.3. Synthesis of dioxybenzene-bridged silica aerogels To prepare the hybrid aerogels, precursor P1 and TEOS were mixed at various molar ratios followed by hydrolysis and condensation re actions via sol-gel process. The synthesized aerogels were designated as hydroquinone-TEOS (HT), resorcinol-TEOS (RT) and catechol-TEOS (CT). The molar ratio of TEOS: methanol: acidic water: basic water was kept constant at 1:30:3.5:3.5 and the reference sample named as “TEOS” was prepared at this molar ratio. The molar ratio of P1/TEOS was varied from 0.25, 0.5, 0.75, and 1.0 and named HT1, HT2, HT3, and HT4, respectively. Similarly, the molar ratios of precursor P2 and P3 with TEOS were varied and the samples named as RT1-4 and CT1-4, respectively. Initially, P1 and TEOS were diluted in methanol and hy drolysis was then carried out using 0.001 M oxalic acid and stirred for 1 h. To complete the hydrolysis, the sol stored at room temperature for 24 h. Afterward, a base catalyst (3 M NH4OH) was added dropwise and the mixture stirred for 5 min and allowed gelation at room temperature. After gelation, the gel was aged at 50 � C for 30 min to complete the syneresis reactions and methanol was added above gel to avoid evapo ration of the solvent. Later, the alcogel was retained in an oven at 50 � C for 2 days to complete the syneresis reaction and strengthen network. After aging, gel was dried using alcohol supercritical drying above 265 � C and 11.58 MPa using methanol [22–25]. After cooling, samples were collected for further analysis. Fig. 2 shows schematic representa tion of the reaction between hydroquinone and TEOS and photographs HT4, RT4, and CT4 gels and aerogels. Also detailed step wise experi mental procedure is presented in SI data (Fig. S4).
ρs
where, ρb is the apparent density and ρs is the skeletal density (~1.9 g/ cm3). The thermal conductivity of aerogels was measured using a C-T meter (Teleph Company, France). The degree of hydrophobicity of aerogels was measured using a rame-hart water contact angle meter, by keeping ~5 μL water droplet on aerogel surface using a microsyringe. The thermal stability of dioxybenzene bridged silica aerogels in air was measured using thermo-gravimetric and differential scanning calorim etry (TG-DSC) using a thermal analyzer (SDT-2790, TA Inc.) within a temperature from 25 to 1000 � C with a heating rate of 5 � C/min. All samples dried in an oven at 100 � C for 1 h prior to thermal measurements.
3. Results and discussion 3.1. Characterization of the precursors, bridged silica aerogels and their DFT analysis
2.2. Synthesis of precursors
The precursors (P1, P2, and P3) were prepared from TEOS using different dihydroxybenzene derivatives [26] (hydroquinone, resorcinol and catechol, respectively, Fig. 1). The reaction mechanism is illustrated in Fig. 2 (supporting information, Fig. S4) [27]. The NMR spectra show
Precursor P1 was synthesized using hydroquinone and TEOS at a 1:2 M ratio with the subsequent addition of catalyst trifluoroacetic acid (10 mol%) to the solution. The mixture was then refluxed at 160 � C for 2
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 1. Molecular structures of the monomers (1–3) and precursor (P1–P3) for making dioxybenzene bridged TEOS monomers.
Fig. 2. Schematic representation of the reaction between Hydroquinone and TEOS and photographs gels and aerogels after drying of HT4, RT4 and CT4.
Fig. 3. The demonstrative imaginings of the HOMO and LUMO localizations of precursors. 3
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
aromatic protons at 6.89 and 6.71 as singlets. Peaks at 3.9 appeared as quadrate corresponding to 12 protons and methyl protons appear as triplets corresponding to 18 protons. Additional peaks observed in the NMR were due to the hydrolyzed product, as hydrolysis is our next target product, the reaction mixture was directly used without further purification (Fig. S1). The exact mass of product is matched with the mass observed in LC/MS spectra confirmed the formation of precursor P1. NMR and LC/MS data of precursor P2 and P3 are also provided in Figs. S2 and S3, respectively. The 29Si-NMR of P1, P2, and P3 measured in CDCl3 are shown in Fig. S5. All samples contain two peaks in their spectra. The peak at 81.2 ppm indicates Si–O bonding and the peak at 88 ppm indicates the presence of C–O bonding [21]. These results indicate the covalent bond between TEOS and the dihydroxybenzene derivatives. A density functional theory (DFT) analysis was conducted to gain deeper insight into the dipole moment, electron distribution and geometrical alignment of the structural isomers including the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, and electron densities of the precursor (P1, P2, and P3). An illustrative visualization of HOMO-LUMO locali zations are depicted in Fig. 3. LUMO is localized mainly on benzene ring, whereas, the HOMO is localized on benzene ring and spread outside over the oxygen, to form a quinonoid-like structure [28]. We can observe delocalization of π-electrons which impart color to the precursor. There is extensive conjugation, leading to delocalization of electrons inside and around the benzene ring due to electronegativity of oxygen, this delocalization of electrons imparts color to aerogels. The dipole moment of P1, P2, and P3 measured from DFT calculations were 0.1508, 1.6955, and 1.114, respectively. As observed in Fig. 2, the alcogels and aerogels possess different colors, even though they are structural isomers, because of the differ ence in the dipole moment of their precursors causes different types of conjugation. Interestingly, the dipole moment of P1 is around 10 times lower than those of P2 and P3 due to its symmetry of hydroxyl group. Hence, the visible transmission of aerogels is in order of HT > CT > RT. Among the prepared aerogels, HT possessed the lowest dipole moment which is of great interest to the semiconductor field for constructing low dielectric materials by selecting symmetric structure [6]. RT sample possessed dark intense color compared with the HT and CT as it had largest dipole moment among all. The molar ratio of P1/TEOS was varied from 0.25, 0.5, 0.75, and 1.0 and named HT1, HT2, HT3, and HT4, respectively. Similarly, the molar ratios of precursor P2 and P3 with TEOS were varied and the samples
named as RT1-4 and CT1-4, respectively. FTIR spectra of the HT aerogels with different P1/TEOS molar ratio are displayed in Fig. 4. The ab sorption peaks at around 1073, 800, and 450 cm 1 are the representa tive of Si–O–Si bond. While the strong absorption peaks at 1608, 1467, and 844 cm 1, correlate to the stretching and out of plane bending vi – C) [29–31]. The two peaks at 2964 and brations of the benzene ring (C– 2855 cm 1 correspond to stretching vibrations of the C–H bonds of benzene ring. These peaks were expected above 3000 cm 1 but appeared at a lower wavenumber as the benzene ring was covalently bonded with TEOS which increased the weight of the molecules and shift vibrational frequency towards a lower wavenumber [32]. These peaks can be observed for HT1-4 samples but are absent in TEOS reference sample (as expected), which the confirms covalent bonding between Si–O–C. FTIR spectra for RT1-4 and CT1-4 were also analyzed (Figs. S6 and S7). It is interesting to note that similar spectra were observed for RT and CT, as they are structural isomers (HT/RT/CT). These results clearly demonstrated that the dioxybenzene was covalently integrated into inorganic silica matrix, and thus bridging with organic component in the silica aerogels with dioxybenzene was realized. 3.2. Surface morphology of hybrid silica aerogels The surface morphology of the dioxybenzene-bridged silica aerogels (HT, RT, and CT) with different molar ratio were observed by using FESEM as shown in Fig. 5. The FESEM micrographs clearly show that all of the sample had open pore structure with a continuous interconnected cage-like network. Here we observed that precursors are hydrolyzed partially due to triflouroacetic acid from LCMS and NMR data, so small amount of acid is needed for completion of hydrolysis. Therefore, we have used weak acid i.e. oxalic acid for hydrolysis. As the OH concen tration in solution is increased, the electronegative surface charge on particles suspended in solution will increase, thereby, delaying gelation. Therefore, the obtained silica network looks cage like structure as observed from Fig. 5. In case of strong acids gelation is rapid and ob tained silica network is like particles [1–5]. For preparing aerogels HT1-4, the amount of hydroquinone-TEOS compound was increased from 25 to 100%, it can be observed from the micrographs of HT1 and HT4 that, the pore size increased with increase in organic content and HT4 sample had a more porous morphology than HT1. In the case of RT1-4, the FESEM micrographs show a porous and interconnected cage-like network. It can also be observed that the size of pores increased with increasing organic content [33]. The surface morphology and particle size of HT1-4 and RT1-4 was almost similar and increase line arly with an increase in the amount of P1 and P2. As expected, the hydroquinone-TEOS composite attained a highly uniform network because both sides of the benzene ring were bonded with TEOS and thus very small portion of the benzene ring was exposed to polar solvent (methanol), which minimized phase separation during gelation. Slight morphological difference can be observed for HT1-4 and RT1-4 as the hydroxyl groups are para- and meta-substituted, respectively. The sur face morphology of CT1-4 differs from their HT and RT counterparts as can be observed from the micrographs. Trifluroacetic acid is medium strong acid, which will suppress the condensation reactions. Therefore, the possibility of the formation of oligomers is very small. Even though it is possible, it may form oligomers of small size. The oligomers conden sation sites will have TEOS molecules/hydroxyl bond, therefore phase separation between organic moieties and silica network will be mini mum in both cases i.e. oligomers or impurities rather than dioxybenzene ligands. As a formation of oligomers/impurities will not affect the phase separation between organic and inorganic moieties, therefore the ob tained silica aerogels will have similar properties whether they have oligomers/impurities or not. A comparative study shows that the particle size of CT was bigger than RT and HT. In addition, the porosity of CT1 is comparable to HT4 and RT4 samples, the size of open pores increased with an increased in organic content. The hydroxyl groups are adjacent in catechol; thus
Fig. 4. FTIR spectra of TEOS and HT1-4 silica aerogels samples with various molar ratio. 4
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 5. FESEM images of the HT, RT and CT aerogels with different molar ratio.
large portion of the benzene ring was exposed to the polar solvent which enhanced the phase separation. This led to a similar morphology to that observed for alkyltrialkoxysilanes based silica aerogels, so similar morphology is expected for CT [34]. The morphological analysis confirmed uniform, open pore structure with tunable pore size could be obtained by varying the precursor amount. The position of the hydroxyl moieties clearly affected the morphology and network of the aerogels. Thus by simply adjusting the amount and type of organic precursors we were able to tune the morphology of the aerogels.
analysis confirmed the symmetric structure of HT samples with uniform interconnected network, a pore size of <10 nm, and higher surface area than the other derivatives of the dioxybenzene precursors. Table 1 gives the thermal conductivity of the samples at room tem perature measured using a C-T meter. The average thermal conductivity of the benzene bridged HT samples is ~0.0334 W/m⋅K, which is lower than the pristine silica aerogels due to smaller pore diameter and a uniform network. Indeed, the thermal conductivity and pore diameter of the HT sample were less than that of RT and CT samples. The average value of porosity of the HT samples was ~96% (Table 1), which is nearly same as the pristine TEOS aerogels and RT, CT sample possess 95% porosity. Thus, HT samples demonstrated low thermal conductivity, high porosity and surface area. The high porosity with high specific surface area and light weight with organic modifi cation and low thermal conductivity is of great interest for thermal insulation applications [2,3]. The comparative study of HT, RT and CT hybrid aerogels suggest that HT samples possess a high surface area with a low pore diameter, low dipole moment, low density and low thermal conductivity due to symmetry. A cost analysis of precursors shows that hydroquinone is 60% cheaper (Sigma aldrich) than the other isomers, and so HT bridged aerogels are of great interest for large scale produc tion with green methodology.
3.3. Textural properties of HT/RT/CT composite aerogels The effect of the position of the hydroxyl bond in dioxybenzene de rivatives on specific surface area and pore structure of HT/RT/CT was investigated using BET and BJH sorption analysis. Nitrogen adsorptiondesorption isotherms of samples TEOS, HT1-4, RT1-4, and CT1-4 are shown in Fig. 6(a, c, and e), respectively. All of the adsorptiondesorption isotherms follow type Ⅳ isotherm, as per IUPAC system [35,36], indicating the characteristics of the mesoporous solids as a type H2 hysteresis loop, attributed to the pores with narrow openings. The corresponding BJH measurement for pore size distribution of the silica aerogels with different concentrations of HT, RT and CT are presented in Fig. 6(b, d, and f), respectively. The BJH analysis confirmed the TEOS, HT1-4, and RT1-4 samples possessed a pore size of <10 nm and samples CT1-4 a pore size of ~20 nm. Table 1 shows the textural characteristic values of the aerogels with different molar ratio for HT, RT, and CT. The specific surface areas of the TEOS, HT1, HT2, HT3, and HT4 aerogels are determined as 777.0, 986.6, 1077, 1043, and 1154 m2/g, respectively. Moreover, the specific surface area of HT was higher than RT and CT. The silica network growth was restrained to one direction in organically modified aerogels due to the non-hydrolysable functional group, thus pore diameter increased for the organic-inorganic hybrid composites [25,26]. The dihydroxybenzene bridging in the aerogels helped to grow the network in three dimensions, resulting in well-ordered networks with large surface area. This is because the bridging of the silica network with organic moieties, significantly decreased phase separation. The surface areas of RT samples was lowest when compared with HT and CT (Table 1) and from Fig. 2, RT had a darker color than HT and CT. We expect that the chemical structure of CT will have more disordered than the others. DFT analysis revealed that the dipole moment of RT sample was the largest, ensuring more interactions with polar solvents, which might be the reason for the low surface area of RT samples. The textural
3.4. Hydrophobicity of dioxybenzene-bridged silica aerogels Hydrophobicity is an important characteristic of silica aerogels for their long-term stability. Water contact angles were measured for all samples, the data for which are reported in Table 1 and presented in Fig. 7. Samples HT1–4, RT1–4, and CT1–4 showed hydrophobicity (~145� ) except the TEOS sample. As the aromatic ring possesses a nonpolar nature, it repels the polar solvent (water) [37]. Generally, surface modification is needed for silica aerogels for hydrophobicity. Although phenyltriethoxysilane-based silica aerogels show hydrophobicity due to the benzene ring, their textural properties are very low compared with pristine silica aerogels due to organic and inorganic phase separation [38]. Herein, we present a green synthetic route for a hydrophobic aerogel with a cheap precursor (hydroquinone) and without harmful byproducts. Hence, the bridging of aerogels with dioxybenzene not only instills hydrophobicity but also maintains a high surface area, thus this route is favorable for the bulk production of silica aerogels as it is cost-effective and green. For long-term applications, we studied the ef fect of humidity on the aerogels by storing samples in a humid chamber (80%) at 30 � C for 75 days (Fig. 7) and then measured their contact 5
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 6. The N2 adsorption-desorption isotherms of aerogels with different HT, RT and CT (a, c, and e, respectively) molar ratios; and their pore size distributions from the sorption analysis using the BJH method (b, d and f, respectively). Table 1 Structural parameters of the bridged silica aerogel samples. Sample
Density g/cm3
contact angle (o)a
Conductivity (W/m⋅K)b
Surface area (m2/g)c
Pore Volume (cm3/g)
Pore Size (nm)
Porosity (%)
TEOS HT1 HT2 HT3 HT4 RT1 RT2 RT3 RT4 CT1 CT2 CT3 CT4
0.0451 0.0612 0.0678 0.0685 0.0728 0.0650 0.0755 0.0778 0.0819 0.0590 0.0641 0.0697 0.0623
0 146 148 145 148 142 148 146 145 145 148 149 144
0.0351 0.0312 0.0342 0.0372 0.0312 0.0398 0.0415 0.0406 0.0401 0.0551 0.0565 0.0501 0.0664
777.0 986.6 1077 1043 1154 873.4 903.6 888.3 909.0 932.0 936.2 944.3 893.6
5.031 3.828 5.396 5.568 4.88 3.669 3.576 4.597 4.145 3.420 4.724 5.367 4.976
25.90 15.52 20.03 21.35 16.91 16.80 15.83 20.70 18.24 14.68 20.29 22.74 22.27
98.15 96.78 96.43 96.39 96.17 95.53 95.50 95.38 95.16 95.89 95.63 95.33 95.12
a b c
Water contact angle. Thermal conductivity. Specific Surface area.
6
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 7. Water contact angle images after synthesis (a) and after 75 days exposure in humidity chamber (b).
angles; all of the samples still showed good hydrophobicity, which confirms their hydrophobic stability.
with air which minimized the electron conjugation outside the benzene ring (as confirmed from the DFT analysis). The RT samples possessed a darker, more intense color (Fig. 2) compared with the HT and CT as it had a larger dipole moment, which might be the reason behind the low transparency of the RT aerogels in the visible reason. It can be observed that with an increase in the amount of dioxybenzene in a sample, the transparency decreased, as can be seen in photographs of the HT sam ples (Fig. S9). Depending on the use of the aerogels, colored aerogels can be obtained using this simple way of aerogels synthesis. The effect of decrease in pore size on water contact angle is not observed from contact angle data. Because the pore size is decreased in small range i.e. from 25 nm to 18 nm. The water contact angle changes with surface rough ness but this effect can be observed if change in roughness is in micron range [33].
3.5. Effect of aging of gel on physical properties silica aerogels Interestingly, the prepared alcogels and aerogels had color variation with respect to the positions of the hydroxyl groups (Fig. 2). There is extensive conjugation, causing electron transitions from one orbital to another with the emission of visible light, which corresponds to the coloration of the compounds. It can be observed that after gelation, the alcogels possessed intense colors and with aging slowly became trans parent. To study the effect of gel aging on transparency, 6 gels were prepared under the same conditions as the RT4 sample (the RT samples were a dark brown color and so were a good candidate for the aging test). The 6 gels were dried after various aging days. To analyze the reason behind the transparency, the percentage of visible transmittance at 550 nm was recorded and plotted versus aging days, as shown in Fig. 8. N2 adsorption-desorption profiles of the RT4 samples are presented in Fig. S8, and the obtained surface area and pore diameter of the samples are reported in Table S1. It can be observed that pore size decreased with aging, which enhanced the surface area up to 10 days. The surface area at 20 days decreased due to shrinkage of the gels. From the pore-size data, we correlated that the samples became trans parent with decreasing pore size. In addition, after drying, the aerogels had less color because during supercritical drying, methanol is replaced
3.6. Mechanical strength analysis The mechanical behavior of the pristine TEOS, HT1–4, RT1–4, and CT1–4 silica aerogels were studied using a Universal Mechanical Tester by increasing the load on the sample and measuring the load when it broke into pieces. Fig. 9 shows a plot of applied load when the samples broke versus sample name (TEOS, HT, RT, and CT). It can be observed that the TEOS sample broke at a load of 4.5 Kgf, HT1 at 10.19, and HT4 at 24.7 � 0.1 Kgf. The mechanical strength of the dioxybenzene-bridged silica aerogels increased linearly with organic content in the aerogels, which was also observed for the RT and CT samples. The dioxybenzene-bridged aerogel samples HT4, RT4, and CT4
Fig. 8. Visible transmittance of the TEOS, HT4, RT4 and CT4 silica aerogels and photographs of the HT4 samples dried after 2–20 days.
Fig. 9. Mechanical strength of samples 0 (TEOS), 1, 2, 3 and 4 of HT/RT/CT. 7
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
broke into pieces at applied loads of 24.7, 23.64, and 21.8 � 0.1 Kgf, respectively. The profiles of HT, RT, and CT are shown in Figs. S10 and S11, along with inset photographs of HT4 when it started to break. In the case of the HT4 sample, two TEOS monomers are associated with one dioxybenzene ring, indicating maximized organic content. The morphological data of HT and RT showed homogeneous networks, while CT’s was a little agglomerated due to phase separation between the organic and inorganic components. The samples HT4, RT4, and CT4 possess good mechanical strength as they contain the maximum organic content. Sample HT4 had more mechanical strength than RT4 and CT4 as the positions of the hydroxyl groups are symmetric, giving a uniform network and improved mechanical strength compared with the RT4 and CT4 samples. Hence, the bridging of the silica aerogels with dioxy benzene is highly effective for increasing the strength of the aerogels.
[3] H. Maleki, Recent advances in aerogels for environmental remediation applications: a review, Chem. Eng. J. 300 (2016) 98–119, https://doi.org/ 10.1016/j.cej.2016.04.098. [4] W. Liu, A.K. Herrmann, D. Geiger, L. Borchardt, F. Simon, S. Kaskel, N. Gaponik, A. Eychmuller, High-performance electrocatalysis on palladium aerogels, Angew. Chem. Int. Ed. 51 (2012) 5743–5747, https://doi.org/10.1002/anie.201108575. [5] S. Zhao, Z. Zhang, G. Sebe, R. Wu, R.V. Rivera Virtudazo, P. Tingaut, M.M. Koebel, Multiscale assembly of superinsulating silica aerogels within silylated nanocellulosic scaffolds: improved mechanical properties promoted by nanoscale chemical compatibilization, Adv. Funct. Mater. 25 (2015) 2326–2334, https://doi. org/10.1002/adfm.201404368. [6] D.B. Mahadik, R.V. Lakshmi, H.C. Barshilia, High performance single layer nanoporous antireflection coatings on glass by sol–gel process for solar energy applications, Sol. Energy Mater. Sol. Cells 140 (2015) 61–68, https://doi.org/ 10.1016/j.solmat.2015.03.023. [7] G. Zu, J. Shen, W. Wang, L. Zou, Y. Lian, Z. Zhang, B. Liu, F. Zhang, Robust, highly thermally stable, core-shell nanostructured metal oxide aerogels as hightemperature thermal superinsulators, adsorbents, and catalysts, Chem. Mater. 26 (2014) 5761–5772, https://doi.org/10.1021/cm502886t. [8] Q. Zheng, Z. Cai, S. Gong, Green synthesis of polyvinyl alcohol (PVA)-cellulose nanofibril (CNF) hybrid aerogels and their use as superabsorbents, J. Mater. Chem. 2 (2014) 3110–3118, https://doi.org/10.1039/C3TA14642A. [9] F. Jiang, Y.L. Hsieh, Amphiphilic superabsorbent cellulose nanofibril aerogels, J. Mater. Chem. 2 (2014) 6337–6342, https://doi.org/10.1039/C4TA00743C. [10] Z. Shi, H. Gao, J. Feng, B. Ding, X. Cao, S. Kuga, Y. Wang, L. Zhang, J. Cai, In situ synthesis of robust conductive cellulose/polypyrrole composite aerogels and their potential application in nerve regeneration, Angew. Chem. Int. Ed. 53 (2014) 5380–5384, https://doi.org/10.1002/anie.201402751. [11] D.B. Mahadik, S. Gujjar, G.M. Gouda, H.C. Barshilia, Double layer SiO2/Al2O3 high emissivity coatings on stainless steel substrates using simple spray deposition system, Appl. Surf. Sci. 299 (2014) 6–11, https://doi.org/10.1016/j. apsusc.2014.01.159. [12] S. Zhao, W.J. Malfait, N.G. Alburquerque, M.M. Koebel, G. Nystrom, Biopolymer aerogels and foams: chemistry, properties, and applications, Angew. Chem. Int. Ed. 57 (2018) 7580–7608, https://doi.org/10.1002/anie.201709014. [13] F. Jiang, S. Hu, Y. Hsieh, Aqueous synthesis of compressible and thermally stable cellulose nanofibril-silica aerogel for CO2 adsorption, ACS Appl. Nano Mater. 1 (2018) 6701–6710, https://doi.org/10.1021/acsanm.8b01515. [14] J.L. Gurav, I.K. Jung, H.H. Park, E.S. Kang, D.Y. Nadargi, Silica Aerogel: synthesis and applications, J. Nanomater. 24 (2010) 1, https://doi.org/10.1155/2010/ 409310. [15] A.C. Pierre, G.M. Pajonk, Chemistry of aerogels and their applications, Chem. Rev. 102 (2002) 4243–4266, https://doi.org/10.1021/cr0101306. [16] A.J. Westphal, R.M. Stroud, H.A. Bechtel, F.E. Brenker, Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft, Science 345 (2014) 786–791, https://doi.org/10.1126/science.1252496. [17] H. Maleki, L. Duraes, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74, https://doi.org/10.1016/j.jnoncrysol.2013.10.017. [18] J.P. Randall, M.A.B. Meador, S.C. Jana, Polymer reinforced silica aerogels: effects of dimethyldiethoxysilane and bis(trimethoxysilylpropyl)amine as silane precursors, J. Mater. Chem. 1 (2013) 6642–6652, https://doi.org/10.1039/ C3TA11019B. [19] D.B. Mahadik, A.V. Rao, R. Kumar, S.V. Ingale, P.B. Wagh, S.C. Gupta, Reduction of processing time by mechanical shaking of the ambient pressure dried TEOS based silica aerogel granules, J. Porous Mater. 19 (2012) 87–94, https://doi.org/ 10.1007/s10934-011-9451-3. [20] Y. Kobayashi, T. Saito, A. Isogai, Aerogels with 3D ordered nanofiber skeletons of liquid-crystalline nanocellulose derivatives as tough and transparent insulators, Angew. Chem. Int. Ed. 53 (2014) 10394–10397, https://doi.org/10.1002/ anie.201405123. [21] S. Yun, H. Luo, Y. Gao, Ambient-pressure drying synthesis of large resorcinol–formaldehyde-reinforced silica aerogels with enhanced mechanical strength and superhydrophobicity, J. Mater. Chem. 2 (2014) 14542–14549, https://doi.org/10.1039/C4TA02195A. [22] L. Verdolotti, M. Lavorgna, R. Lamanna, E.D. Maio, S. Iannace, Polyurethane-silica hybrid foam by sol-gel approach: chemical and functional properties, Polymer 56 (2015) 20–28, https://doi.org/10.1016/j.polymer.2014.10.017. [23] M.R. Miner, B. Hosticka, P.M. Norris, The effects of ambient humidity on the mechanical properties and surface chemistry of hygroscopic silica aerogel, J. NonCryst. Solids 350 (2004) 285–289, https://doi.org/10.1016/j. jnoncrysol.2004.06.023. [24] K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, New transparent methylsilsesquioxane aerogels and xerogels with improved mechanical properties, Adv. Mater. 19 (2007) 1589–1593, https://doi.org/10.1002/adma.200602457. [25] D.A. Loy, K.J. Shea, Bridged polysilsesquioxanes. Highly porous hybrid organicinorganic materials, Chem. Rev. 95 (1995) 1431–1442, https://doi.org/10.1021/ cr00037a013. [26] E.N. Khimich, G.S. Buslaev, A.V. Zdravkov, N.N. Khimich, M.G. Voronkov, Polysiloxane structures cross-linked with hydroquinone and phloroglucinol, Russ. J. Appl. Chem. 81 (2008) 1410–1413, https://doi.org/10.1134/ S107042720808017X. [27] D.B. Mahadik, Y.K. Lee, N.K. Chavan, S.A. Mahadik, H.H. Park, Monolithic and shrinkage-free hydrophobic silica aerogels via new rapid supercritical extraction process, J. Supercrit. Fluids 107 (2016) 84–91, https://doi.org/10.1016/j. supflu.2015.08.020.
4. Conclusions Three kinds of novel dioxybenzene-bridged silica aerogels were successfully prepared via a cost-effective sol-gel process. The textural properties of the bridged silica aerogels could be tuned over a wide range by varying precursor and amount. Hydroquinone-based aerogels attained highly ordered networks with open porous morphology and possessed low thermal conductivity compared with the resorcinol- and catechol-based ones. All of the prepared aerogels were hydrophobic without using any silylating reagents, as confirmed from FTIR spec troscopy and contact angle measurements. The position of the hydroxyl groups and content of the organic precursor had a significant influence on the textural properties and mechanical strength of the aerogels. These results suggest that the hydroquinone-based HT4 aerogel possessed the lowest dipole moment due to symmetry forming a uniform cage-like porous network with improved mechanical strength compared to RT and CT. Its fabrication was also cost-effective. Such aerogels have great potential as low dielectric materials in semiconductor industries, and also this green process could lead to the large-scale production of silica aerogels for thermal insulation. Notes The authors declare no competing financial interest. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported (in part) by the Yonsei University Research Fund (Post Doctorate Researcher Supporting Program) of 2018 (Project No.: 2018-12-0015). This work was supported by the Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-TA1703-04. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109863. References [1] L. Ren, S. Cui, F. Cao, Q. Guo, An easy way to prepare monolithic inorganic oxide aerogels, Angew. Chem. Int. Ed. 53 (2014) 10147–10149, https://doi.org/ 10.1002/anie.201406387. [2] J.E. Amonette, J. Matyas, Functionalized silica aerogels for gas-phase purification, sensing, and catalysis: a review, Microporous Mesoporous Mater. 250 (2017) 100–119, https://doi.org/10.1016/j.micromeso.2017.04.055.
8
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
[28] M.H. Stockett, S.B. Nielsen, Transition energies of benzoquinone anions are immune to symmetry breaking by a single water molecule, Phys. Chem. Chem. Phys. 18 (2016) 6996–7000, https://doi.org/10.1039/C5CP06095H. [29] J. Guo, B.N. Nguyen, L. Li, M.A.B. Meador, D.A. Scheiman, M. Cakmak, Clay reinforced polyimide/silica hybrid aerogel, J. Mater. Chem. 1 (2013) 7211–7221, https://doi.org/10.1039/C3TA00439B. [30] D.B. Mahadik, H.N.R. Jung, W. Han, H.H. Cho, H.H. Park, Flexible, elastic, and superhydrophobic silica-polymer composite aerogels by high internal phase emulsion process, Compos. Sci. Technol. 147 (2017) 45–51, https://doi.org/ 10.1016/j.compscitech.2017.04.036. [31] A.V. Rao, R.R. Kalesh, G.M. Pajonk, Hydrophobicity and physical properties of TEOS based silica aerogels using phenyltriethoxysilane as a synthesis component, J. Mater. Sci. 38 (2003) 4407–4413, https://doi.org/10.1023/A:1026311905523. [32] D.B. Mahadik, A.V. Rao, A.P. Rao, P.B. Wagh, S.V. Ingale, S.C. Gupta, Effect of concentration of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents on surface free energy of silica aerogels, J. Colloid Interface Sci. 356 (2011) 298–302, https://doi.org/10.1016/j.jcis.2010.12.088. [33] D.B. Mahadik, A.V. Rao, V.G. Parale, M.S. Kavale, P.B. Wagh, S.V. Ingale, S. C. Gupta, Effect of surface composition and roughness on the apparent surface free
[34] [35] [36] [37]
[38]
9
energy of silica aerogel materials, Appl. Phys. Lett. 99 (2011), https://doi.org/ 10.1063/1.3635398, 104104-3. J.P. Randall, M.A.B. Meador, S.C. Jana, Tailoring mechanical properties of aerogels for aerospace applications, ACS Appl. Mater. Interfaces 3 (2011) 613–626, https:// doi.org/10.1021/am200007n. L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen, C.Y. Su, A synthetic route to ultralight hierarchically micro/mesoporous Al(III)-carboxylate metal-organic aerogels, Nat. Commun. 1774 (2013) 1, https://doi.org/10.1038/ncomms2757. E.P. Barret, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances, J. Am. Chem. Soc. 73 (1951) 373–380, https:// doi.org/10.1021/ja01145a126. D.B. Mahadik, K.Y. Lee, R.V. Ghorpade, H.H. Park, Superhydrophobic and compressible silica-polyHIPE covalently bonded porous networks via emulsion templating for oil spill cleanup and recovery, Sci. Rep. 8 (2018) 16783, https://doi. org/10.1038/s41598-018-34997-1. G. Zu, K. Kanamori, T. Shimizu, Y. Zhu, A. Maeno, H. Kaji, K. Nakanishi, J. Shen, Versatile double-cross-linking approach to transparent, machinable, supercompressible, highly bendable aerogel thermal superinsulators, Chem. Mater. 30 (2018) 2759–2770, https://doi.org/10.1021/acs.chemmater.8b00563.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Dioxybenzene-bridged hydrophobic silica aerogels with enhanced textural and mechanical properties Qi Wang a, 1, D.B. Mahadik a, *, 1, Puttavva Meti b, Young-Dae Gong b, Kyu-Yeon Lee a, Hyung-Ho Park a, ** a b
Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea Innovative Drug Library Research Center, Dongguk University, Seoul, 04620, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Insulation Dihydroxybenzene Hydrophobic Textural property Mechanical property
Designing aerogel materials with excellent mechanical robustness while maintaining their basic characteristics, is of great importance for applications in energy saving management systems. Organic modification is required to strengthen silica aerogel, but the modified aerogels show decreased textural properties due to phase separation between organic-inorganic components. We report, the fabrication of organically modified aerogels without minimizing their basic properties, based on tetraethoxysilane (TEOS) and a bifunctional precursor prepared from dihydroxybenzene and TEOS via acid catalyst. Three types of dihydroxybenzene isomers were used to study the effect of the position of the hydroxyl groups on textural properties of aerogels and compatibility of the pre cursors. A simple, efficient, and cost effective sol-gel method was employed to develop porous aerogels with no additives and surfactants. Hydroquinone-TEOS (HT) bridged silica aerogels exhibited improved mechanical strength, low dipole moment, high specific surface area (1154 m2/g), high porosity (96%), low density (0.061 g/ cm3), and ultra-low thermal conductivity of 0.0334 W/m⋅K. In addition, precursor was cheaper than the other isomers. The prepared aerogels show hydrophobic nature without the use of harmful silylating reagents.
1. Introduction
enhances pore size [18,19]. Several studies have reported improved mechanical properties by constructing organic-inorganic composites in several ways [20,21]. Physically or chemically introducing polymers to silica networks adds strength to the overall composite. However, the phase separation between organic and inorganic compounds suppresses the textural properties that enhance its density and thermal conductivity [3]. Moreover, surface modification, which release hazardous byprod ucts is needed to improve its hydrophobic nature [19]. Therefore, it is necessary to develop organically modified composite porous material with small pore size, enhanced mechanical properties with a hydro phobic nature without releasing hazardous byproducts via eco-friendly and cost-effective method. As for industrial application, cost-effective large-scale production of aerogels should be considered together with green chemical process [22–25]. Herein, we report the facile synthesis of dioxybenzene-bridged aer ogels with improved textural and mechanical properties. To the best of our knowledge, dioxybenzene linked aerogels have not been reported so
Silica aerogels have gained wide spread attention due to their porous nature with exceptional properties which endow them versatile appli cations in various fields [1–9]. Among these thermal insulating mate rials can play a vital role in living systems and controlling various domestic and industrial processes by saving energy [10–12]. Further more, due to their nano-structured porous networks, silica aerogels possess a low dielectric constant, which has evoked great attention in semiconductor industries [13–15]. The basic prerequisites for aerogel applications are a small pore size, low density, and sufficient mechanical strength. However, practical applications of aerogels have been restricted, due to their fragility and the formation of hazardous byproducts during surface modification [16]. Generally, silica aerogels are cross-linked with organic monomers or polymers to improve their mechanical properties [17]. Specifically, polymer reinforcement of silica aerogels results in high density and
* Corresponding author. Department of Materials Science and Engineering, 50 Yonseri-ro, Yonsei University, Seodaemun-Ku, Seoul, 03722, Republic of Korea. ** Corresponding author. Department of Materials Science and Engineering, 50 Yonseri-ro, Yonsei University, Seodaemun-Ku, Seoul, 03722, Republic of Korea. E-mail addresses: [email protected] (D.B. Mahadik), [email protected] (H.-H. Park). 1 Qi Wang and D.B. Mahadik contributed equally to this work. https://doi.org/10.1016/j.micromeso.2019.109863 Received 16 August 2019; Received in revised form 21 October 2019; Accepted 31 October 2019 Available online 1 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
far. The objectives of this study were to synthesize and investigate the properties of aerogels based on TEOS with various dihydroxybenzene derivatives fabricated via sol-gel process, to increase their mechanical properties. Organotrialkoxysilane based aerogels exhibit large phase separation in the silica network, which reduces their basic properties. Herein, we propose to minimize phase separation by sandwiching organic molecule between silica clusters which connects two ends similar to a bridge over a river. The bridging of the silica network with small organic molecules helps to develop aerogels with the small pore size, required for high textural properties. The benzene ring was selected for bridging as it is non-polar, and can impart thermal stability and mechanical strength to the aerogel. It also provides scope for bridging at ortho-, meta-, and para-positions. A comparative study of three different dioxybenzene isomers on silica network was carried out to analyze the suitability of dihydroxybenzene derivatives for the desired properties of the bridged silica aerogels. These porous multi-functional aerogels can be potentially used in various fields.
4 h and cooled down to room temperature. The precursor P1 obtained was then analyzed using NMR and LC/MS (Fig. S1). Similar reaction conditions were utilized for preparing P2 and P3. (NMR and LCMS data of P2 and P3 are shown in Figs. S2 and S3, respectively). The NMR and LCMS data shows that the product is confirmed as proposed in Fig. 1. The trifluroacetic acid was used as a reaction catalyst. The mixture is refluxed for 4 h for completion of reaction and also purification. The mixture contain product (yield 96%) in majority and small fraction of precursors and remaining trifluroacetic acid [26]. The unused acid catalyst is further used for hydrolysis of precursor and remaining start ing precursors will be used during sol-gel process. The steps of sol-gel reaction contain hydrolysis and condensation of precursors by acid and base catalyst in the large amount of solvent and finally, supercritical drying. In these steps of sol-gel process, most of the impurities will be removed from the silica network during washing and solvent exchange. The as prepared bridged silica composite aerogels are pure. Therefore purification of precursors is not essential for bridged silica aerogel synthesis. 1,4-Bis(triethoxysilyloxy)benzene (P1): Light orange liquid. 1H NMR (500 MHz, CDCl3) δ 6.89 (s, 2H), 6.71 (s, 2H), 3.9 (q, 12H, –Si–O–CH2–), 1.26 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.20. 1,3-Bis(triethoxysilyloxy)benzene (P2): Light brown liquid, 1H NMR (500 MHz, CDCl3) δ 7.14–7.03 (m, 1H), 6.64–6.52 (m, 1H), 6.43–6.33 (m, 1H), 4.01 (q, 12H, –Si–O–CH2–), 1.23 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.30. 1,2-Bis(triethoxysilyloxy)benzene (P3): Light orange liquid. 1H NMR (500 MHz, CDCl3) δ 6.87 (d, 2H), 6.78 (d, 2H), 3.85 (q, 12H, –Si–O–CH2–), 1.24 (t, 18H, CH3). LCMS (ESI) m/z: [MþH]þ of 435.20.
2. Experimental details 2.1. Materials and instrumentation All the reagents and solvents were purchased from Sigma Aldrich. All chemicals were used as received. TEOS, hydroquinone, resorcinol, and catechol were used as precursors. Milli-Q water was used for preparing aqueous solutions. Oxalic acid was used as acid catalyst for hydrolysis and ammonia solution was used as base catalyst. NMR spectra measurements were carried out using Bruker-500 in strument using tetramethylsilane solvent as an internal reference. Liquid chromatography–mass spectrometry (Triple Quadrapole 6400 series LC/MS/Agilent) study was performed on an electrospray ionization (ESI) mass spectrometer. The DFT calculations were completed on B3LYP/6-31G (d orbital) level to gain deeper insight into the dipole moment, electron distribution and geometrical alignment of the struc tural isomers. FTIR was performed using an IR spectrophotometer (PerkinElmer model No. 1760X). The morphology of aerogels was analyzed using field emission scanning electron microscopy (FESEM, JEOL, Japan, 120 kV accelerating voltage). The surface area of bridged silica aerogels was obtained from N2 adsorption-desorption measure ments at 77 K measured using Brunauer-Emmett-Teller (BET) Surface Analyzer (Quantachrome Instruments (v10.0)) system [26,27]. Pore size and pore volume of aerogels were obtained from Barrett-Joyner-Halenda (BJH) method [28]. The % of porosity of the as prepared silica aerogels were measured using following formula [29]. � � ρb % of porosity ¼ 1 � 100
2.3. Synthesis of dioxybenzene-bridged silica aerogels To prepare the hybrid aerogels, precursor P1 and TEOS were mixed at various molar ratios followed by hydrolysis and condensation re actions via sol-gel process. The synthesized aerogels were designated as hydroquinone-TEOS (HT), resorcinol-TEOS (RT) and catechol-TEOS (CT). The molar ratio of TEOS: methanol: acidic water: basic water was kept constant at 1:30:3.5:3.5 and the reference sample named as “TEOS” was prepared at this molar ratio. The molar ratio of P1/TEOS was varied from 0.25, 0.5, 0.75, and 1.0 and named HT1, HT2, HT3, and HT4, respectively. Similarly, the molar ratios of precursor P2 and P3 with TEOS were varied and the samples named as RT1-4 and CT1-4, respectively. Initially, P1 and TEOS were diluted in methanol and hy drolysis was then carried out using 0.001 M oxalic acid and stirred for 1 h. To complete the hydrolysis, the sol stored at room temperature for 24 h. Afterward, a base catalyst (3 M NH4OH) was added dropwise and the mixture stirred for 5 min and allowed gelation at room temperature. After gelation, the gel was aged at 50 � C for 30 min to complete the syneresis reactions and methanol was added above gel to avoid evapo ration of the solvent. Later, the alcogel was retained in an oven at 50 � C for 2 days to complete the syneresis reaction and strengthen network. After aging, gel was dried using alcohol supercritical drying above 265 � C and 11.58 MPa using methanol [22–25]. After cooling, samples were collected for further analysis. Fig. 2 shows schematic representa tion of the reaction between hydroquinone and TEOS and photographs HT4, RT4, and CT4 gels and aerogels. Also detailed step wise experi mental procedure is presented in SI data (Fig. S4).
ρs
where, ρb is the apparent density and ρs is the skeletal density (~1.9 g/ cm3). The thermal conductivity of aerogels was measured using a C-T meter (Teleph Company, France). The degree of hydrophobicity of aerogels was measured using a rame-hart water contact angle meter, by keeping ~5 μL water droplet on aerogel surface using a microsyringe. The thermal stability of dioxybenzene bridged silica aerogels in air was measured using thermo-gravimetric and differential scanning calorim etry (TG-DSC) using a thermal analyzer (SDT-2790, TA Inc.) within a temperature from 25 to 1000 � C with a heating rate of 5 � C/min. All samples dried in an oven at 100 � C for 1 h prior to thermal measurements.
3. Results and discussion 3.1. Characterization of the precursors, bridged silica aerogels and their DFT analysis
2.2. Synthesis of precursors
The precursors (P1, P2, and P3) were prepared from TEOS using different dihydroxybenzene derivatives [26] (hydroquinone, resorcinol and catechol, respectively, Fig. 1). The reaction mechanism is illustrated in Fig. 2 (supporting information, Fig. S4) [27]. The NMR spectra show
Precursor P1 was synthesized using hydroquinone and TEOS at a 1:2 M ratio with the subsequent addition of catalyst trifluoroacetic acid (10 mol%) to the solution. The mixture was then refluxed at 160 � C for 2
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 1. Molecular structures of the monomers (1–3) and precursor (P1–P3) for making dioxybenzene bridged TEOS monomers.
Fig. 2. Schematic representation of the reaction between Hydroquinone and TEOS and photographs gels and aerogels after drying of HT4, RT4 and CT4.
Fig. 3. The demonstrative imaginings of the HOMO and LUMO localizations of precursors. 3
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
aromatic protons at 6.89 and 6.71 as singlets. Peaks at 3.9 appeared as quadrate corresponding to 12 protons and methyl protons appear as triplets corresponding to 18 protons. Additional peaks observed in the NMR were due to the hydrolyzed product, as hydrolysis is our next target product, the reaction mixture was directly used without further purification (Fig. S1). The exact mass of product is matched with the mass observed in LC/MS spectra confirmed the formation of precursor P1. NMR and LC/MS data of precursor P2 and P3 are also provided in Figs. S2 and S3, respectively. The 29Si-NMR of P1, P2, and P3 measured in CDCl3 are shown in Fig. S5. All samples contain two peaks in their spectra. The peak at 81.2 ppm indicates Si–O bonding and the peak at 88 ppm indicates the presence of C–O bonding [21]. These results indicate the covalent bond between TEOS and the dihydroxybenzene derivatives. A density functional theory (DFT) analysis was conducted to gain deeper insight into the dipole moment, electron distribution and geometrical alignment of the structural isomers including the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, and electron densities of the precursor (P1, P2, and P3). An illustrative visualization of HOMO-LUMO locali zations are depicted in Fig. 3. LUMO is localized mainly on benzene ring, whereas, the HOMO is localized on benzene ring and spread outside over the oxygen, to form a quinonoid-like structure [28]. We can observe delocalization of π-electrons which impart color to the precursor. There is extensive conjugation, leading to delocalization of electrons inside and around the benzene ring due to electronegativity of oxygen, this delocalization of electrons imparts color to aerogels. The dipole moment of P1, P2, and P3 measured from DFT calculations were 0.1508, 1.6955, and 1.114, respectively. As observed in Fig. 2, the alcogels and aerogels possess different colors, even though they are structural isomers, because of the differ ence in the dipole moment of their precursors causes different types of conjugation. Interestingly, the dipole moment of P1 is around 10 times lower than those of P2 and P3 due to its symmetry of hydroxyl group. Hence, the visible transmission of aerogels is in order of HT > CT > RT. Among the prepared aerogels, HT possessed the lowest dipole moment which is of great interest to the semiconductor field for constructing low dielectric materials by selecting symmetric structure [6]. RT sample possessed dark intense color compared with the HT and CT as it had largest dipole moment among all. The molar ratio of P1/TEOS was varied from 0.25, 0.5, 0.75, and 1.0 and named HT1, HT2, HT3, and HT4, respectively. Similarly, the molar ratios of precursor P2 and P3 with TEOS were varied and the samples
named as RT1-4 and CT1-4, respectively. FTIR spectra of the HT aerogels with different P1/TEOS molar ratio are displayed in Fig. 4. The ab sorption peaks at around 1073, 800, and 450 cm 1 are the representa tive of Si–O–Si bond. While the strong absorption peaks at 1608, 1467, and 844 cm 1, correlate to the stretching and out of plane bending vi – C) [29–31]. The two peaks at 2964 and brations of the benzene ring (C– 2855 cm 1 correspond to stretching vibrations of the C–H bonds of benzene ring. These peaks were expected above 3000 cm 1 but appeared at a lower wavenumber as the benzene ring was covalently bonded with TEOS which increased the weight of the molecules and shift vibrational frequency towards a lower wavenumber [32]. These peaks can be observed for HT1-4 samples but are absent in TEOS reference sample (as expected), which the confirms covalent bonding between Si–O–C. FTIR spectra for RT1-4 and CT1-4 were also analyzed (Figs. S6 and S7). It is interesting to note that similar spectra were observed for RT and CT, as they are structural isomers (HT/RT/CT). These results clearly demonstrated that the dioxybenzene was covalently integrated into inorganic silica matrix, and thus bridging with organic component in the silica aerogels with dioxybenzene was realized. 3.2. Surface morphology of hybrid silica aerogels The surface morphology of the dioxybenzene-bridged silica aerogels (HT, RT, and CT) with different molar ratio were observed by using FESEM as shown in Fig. 5. The FESEM micrographs clearly show that all of the sample had open pore structure with a continuous interconnected cage-like network. Here we observed that precursors are hydrolyzed partially due to triflouroacetic acid from LCMS and NMR data, so small amount of acid is needed for completion of hydrolysis. Therefore, we have used weak acid i.e. oxalic acid for hydrolysis. As the OH concen tration in solution is increased, the electronegative surface charge on particles suspended in solution will increase, thereby, delaying gelation. Therefore, the obtained silica network looks cage like structure as observed from Fig. 5. In case of strong acids gelation is rapid and ob tained silica network is like particles [1–5]. For preparing aerogels HT1-4, the amount of hydroquinone-TEOS compound was increased from 25 to 100%, it can be observed from the micrographs of HT1 and HT4 that, the pore size increased with increase in organic content and HT4 sample had a more porous morphology than HT1. In the case of RT1-4, the FESEM micrographs show a porous and interconnected cage-like network. It can also be observed that the size of pores increased with increasing organic content [33]. The surface morphology and particle size of HT1-4 and RT1-4 was almost similar and increase line arly with an increase in the amount of P1 and P2. As expected, the hydroquinone-TEOS composite attained a highly uniform network because both sides of the benzene ring were bonded with TEOS and thus very small portion of the benzene ring was exposed to polar solvent (methanol), which minimized phase separation during gelation. Slight morphological difference can be observed for HT1-4 and RT1-4 as the hydroxyl groups are para- and meta-substituted, respectively. The sur face morphology of CT1-4 differs from their HT and RT counterparts as can be observed from the micrographs. Trifluroacetic acid is medium strong acid, which will suppress the condensation reactions. Therefore, the possibility of the formation of oligomers is very small. Even though it is possible, it may form oligomers of small size. The oligomers conden sation sites will have TEOS molecules/hydroxyl bond, therefore phase separation between organic moieties and silica network will be mini mum in both cases i.e. oligomers or impurities rather than dioxybenzene ligands. As a formation of oligomers/impurities will not affect the phase separation between organic and inorganic moieties, therefore the ob tained silica aerogels will have similar properties whether they have oligomers/impurities or not. A comparative study shows that the particle size of CT was bigger than RT and HT. In addition, the porosity of CT1 is comparable to HT4 and RT4 samples, the size of open pores increased with an increased in organic content. The hydroxyl groups are adjacent in catechol; thus
Fig. 4. FTIR spectra of TEOS and HT1-4 silica aerogels samples with various molar ratio. 4
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 5. FESEM images of the HT, RT and CT aerogels with different molar ratio.
large portion of the benzene ring was exposed to the polar solvent which enhanced the phase separation. This led to a similar morphology to that observed for alkyltrialkoxysilanes based silica aerogels, so similar morphology is expected for CT [34]. The morphological analysis confirmed uniform, open pore structure with tunable pore size could be obtained by varying the precursor amount. The position of the hydroxyl moieties clearly affected the morphology and network of the aerogels. Thus by simply adjusting the amount and type of organic precursors we were able to tune the morphology of the aerogels.
analysis confirmed the symmetric structure of HT samples with uniform interconnected network, a pore size of <10 nm, and higher surface area than the other derivatives of the dioxybenzene precursors. Table 1 gives the thermal conductivity of the samples at room tem perature measured using a C-T meter. The average thermal conductivity of the benzene bridged HT samples is ~0.0334 W/m⋅K, which is lower than the pristine silica aerogels due to smaller pore diameter and a uniform network. Indeed, the thermal conductivity and pore diameter of the HT sample were less than that of RT and CT samples. The average value of porosity of the HT samples was ~96% (Table 1), which is nearly same as the pristine TEOS aerogels and RT, CT sample possess 95% porosity. Thus, HT samples demonstrated low thermal conductivity, high porosity and surface area. The high porosity with high specific surface area and light weight with organic modifi cation and low thermal conductivity is of great interest for thermal insulation applications [2,3]. The comparative study of HT, RT and CT hybrid aerogels suggest that HT samples possess a high surface area with a low pore diameter, low dipole moment, low density and low thermal conductivity due to symmetry. A cost analysis of precursors shows that hydroquinone is 60% cheaper (Sigma aldrich) than the other isomers, and so HT bridged aerogels are of great interest for large scale produc tion with green methodology.
3.3. Textural properties of HT/RT/CT composite aerogels The effect of the position of the hydroxyl bond in dioxybenzene de rivatives on specific surface area and pore structure of HT/RT/CT was investigated using BET and BJH sorption analysis. Nitrogen adsorptiondesorption isotherms of samples TEOS, HT1-4, RT1-4, and CT1-4 are shown in Fig. 6(a, c, and e), respectively. All of the adsorptiondesorption isotherms follow type Ⅳ isotherm, as per IUPAC system [35,36], indicating the characteristics of the mesoporous solids as a type H2 hysteresis loop, attributed to the pores with narrow openings. The corresponding BJH measurement for pore size distribution of the silica aerogels with different concentrations of HT, RT and CT are presented in Fig. 6(b, d, and f), respectively. The BJH analysis confirmed the TEOS, HT1-4, and RT1-4 samples possessed a pore size of <10 nm and samples CT1-4 a pore size of ~20 nm. Table 1 shows the textural characteristic values of the aerogels with different molar ratio for HT, RT, and CT. The specific surface areas of the TEOS, HT1, HT2, HT3, and HT4 aerogels are determined as 777.0, 986.6, 1077, 1043, and 1154 m2/g, respectively. Moreover, the specific surface area of HT was higher than RT and CT. The silica network growth was restrained to one direction in organically modified aerogels due to the non-hydrolysable functional group, thus pore diameter increased for the organic-inorganic hybrid composites [25,26]. The dihydroxybenzene bridging in the aerogels helped to grow the network in three dimensions, resulting in well-ordered networks with large surface area. This is because the bridging of the silica network with organic moieties, significantly decreased phase separation. The surface areas of RT samples was lowest when compared with HT and CT (Table 1) and from Fig. 2, RT had a darker color than HT and CT. We expect that the chemical structure of CT will have more disordered than the others. DFT analysis revealed that the dipole moment of RT sample was the largest, ensuring more interactions with polar solvents, which might be the reason for the low surface area of RT samples. The textural
3.4. Hydrophobicity of dioxybenzene-bridged silica aerogels Hydrophobicity is an important characteristic of silica aerogels for their long-term stability. Water contact angles were measured for all samples, the data for which are reported in Table 1 and presented in Fig. 7. Samples HT1–4, RT1–4, and CT1–4 showed hydrophobicity (~145� ) except the TEOS sample. As the aromatic ring possesses a nonpolar nature, it repels the polar solvent (water) [37]. Generally, surface modification is needed for silica aerogels for hydrophobicity. Although phenyltriethoxysilane-based silica aerogels show hydrophobicity due to the benzene ring, their textural properties are very low compared with pristine silica aerogels due to organic and inorganic phase separation [38]. Herein, we present a green synthetic route for a hydrophobic aerogel with a cheap precursor (hydroquinone) and without harmful byproducts. Hence, the bridging of aerogels with dioxybenzene not only instills hydrophobicity but also maintains a high surface area, thus this route is favorable for the bulk production of silica aerogels as it is cost-effective and green. For long-term applications, we studied the ef fect of humidity on the aerogels by storing samples in a humid chamber (80%) at 30 � C for 75 days (Fig. 7) and then measured their contact 5
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 6. The N2 adsorption-desorption isotherms of aerogels with different HT, RT and CT (a, c, and e, respectively) molar ratios; and their pore size distributions from the sorption analysis using the BJH method (b, d and f, respectively). Table 1 Structural parameters of the bridged silica aerogel samples. Sample
Density g/cm3
contact angle (o)a
Conductivity (W/m⋅K)b
Surface area (m2/g)c
Pore Volume (cm3/g)
Pore Size (nm)
Porosity (%)
TEOS HT1 HT2 HT3 HT4 RT1 RT2 RT3 RT4 CT1 CT2 CT3 CT4
0.0451 0.0612 0.0678 0.0685 0.0728 0.0650 0.0755 0.0778 0.0819 0.0590 0.0641 0.0697 0.0623
0 146 148 145 148 142 148 146 145 145 148 149 144
0.0351 0.0312 0.0342 0.0372 0.0312 0.0398 0.0415 0.0406 0.0401 0.0551 0.0565 0.0501 0.0664
777.0 986.6 1077 1043 1154 873.4 903.6 888.3 909.0 932.0 936.2 944.3 893.6
5.031 3.828 5.396 5.568 4.88 3.669 3.576 4.597 4.145 3.420 4.724 5.367 4.976
25.90 15.52 20.03 21.35 16.91 16.80 15.83 20.70 18.24 14.68 20.29 22.74 22.27
98.15 96.78 96.43 96.39 96.17 95.53 95.50 95.38 95.16 95.89 95.63 95.33 95.12
a b c
Water contact angle. Thermal conductivity. Specific Surface area.
6
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
Fig. 7. Water contact angle images after synthesis (a) and after 75 days exposure in humidity chamber (b).
angles; all of the samples still showed good hydrophobicity, which confirms their hydrophobic stability.
with air which minimized the electron conjugation outside the benzene ring (as confirmed from the DFT analysis). The RT samples possessed a darker, more intense color (Fig. 2) compared with the HT and CT as it had a larger dipole moment, which might be the reason behind the low transparency of the RT aerogels in the visible reason. It can be observed that with an increase in the amount of dioxybenzene in a sample, the transparency decreased, as can be seen in photographs of the HT sam ples (Fig. S9). Depending on the use of the aerogels, colored aerogels can be obtained using this simple way of aerogels synthesis. The effect of decrease in pore size on water contact angle is not observed from contact angle data. Because the pore size is decreased in small range i.e. from 25 nm to 18 nm. The water contact angle changes with surface rough ness but this effect can be observed if change in roughness is in micron range [33].
3.5. Effect of aging of gel on physical properties silica aerogels Interestingly, the prepared alcogels and aerogels had color variation with respect to the positions of the hydroxyl groups (Fig. 2). There is extensive conjugation, causing electron transitions from one orbital to another with the emission of visible light, which corresponds to the coloration of the compounds. It can be observed that after gelation, the alcogels possessed intense colors and with aging slowly became trans parent. To study the effect of gel aging on transparency, 6 gels were prepared under the same conditions as the RT4 sample (the RT samples were a dark brown color and so were a good candidate for the aging test). The 6 gels were dried after various aging days. To analyze the reason behind the transparency, the percentage of visible transmittance at 550 nm was recorded and plotted versus aging days, as shown in Fig. 8. N2 adsorption-desorption profiles of the RT4 samples are presented in Fig. S8, and the obtained surface area and pore diameter of the samples are reported in Table S1. It can be observed that pore size decreased with aging, which enhanced the surface area up to 10 days. The surface area at 20 days decreased due to shrinkage of the gels. From the pore-size data, we correlated that the samples became trans parent with decreasing pore size. In addition, after drying, the aerogels had less color because during supercritical drying, methanol is replaced
3.6. Mechanical strength analysis The mechanical behavior of the pristine TEOS, HT1–4, RT1–4, and CT1–4 silica aerogels were studied using a Universal Mechanical Tester by increasing the load on the sample and measuring the load when it broke into pieces. Fig. 9 shows a plot of applied load when the samples broke versus sample name (TEOS, HT, RT, and CT). It can be observed that the TEOS sample broke at a load of 4.5 Kgf, HT1 at 10.19, and HT4 at 24.7 � 0.1 Kgf. The mechanical strength of the dioxybenzene-bridged silica aerogels increased linearly with organic content in the aerogels, which was also observed for the RT and CT samples. The dioxybenzene-bridged aerogel samples HT4, RT4, and CT4
Fig. 8. Visible transmittance of the TEOS, HT4, RT4 and CT4 silica aerogels and photographs of the HT4 samples dried after 2–20 days.
Fig. 9. Mechanical strength of samples 0 (TEOS), 1, 2, 3 and 4 of HT/RT/CT. 7
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
broke into pieces at applied loads of 24.7, 23.64, and 21.8 � 0.1 Kgf, respectively. The profiles of HT, RT, and CT are shown in Figs. S10 and S11, along with inset photographs of HT4 when it started to break. In the case of the HT4 sample, two TEOS monomers are associated with one dioxybenzene ring, indicating maximized organic content. The morphological data of HT and RT showed homogeneous networks, while CT’s was a little agglomerated due to phase separation between the organic and inorganic components. The samples HT4, RT4, and CT4 possess good mechanical strength as they contain the maximum organic content. Sample HT4 had more mechanical strength than RT4 and CT4 as the positions of the hydroxyl groups are symmetric, giving a uniform network and improved mechanical strength compared with the RT4 and CT4 samples. Hence, the bridging of the silica aerogels with dioxy benzene is highly effective for increasing the strength of the aerogels.
[3] H. Maleki, Recent advances in aerogels for environmental remediation applications: a review, Chem. Eng. J. 300 (2016) 98–119, https://doi.org/ 10.1016/j.cej.2016.04.098. [4] W. Liu, A.K. Herrmann, D. Geiger, L. Borchardt, F. Simon, S. Kaskel, N. Gaponik, A. Eychmuller, High-performance electrocatalysis on palladium aerogels, Angew. Chem. Int. Ed. 51 (2012) 5743–5747, https://doi.org/10.1002/anie.201108575. [5] S. Zhao, Z. Zhang, G. Sebe, R. Wu, R.V. Rivera Virtudazo, P. Tingaut, M.M. Koebel, Multiscale assembly of superinsulating silica aerogels within silylated nanocellulosic scaffolds: improved mechanical properties promoted by nanoscale chemical compatibilization, Adv. Funct. Mater. 25 (2015) 2326–2334, https://doi. org/10.1002/adfm.201404368. [6] D.B. Mahadik, R.V. Lakshmi, H.C. Barshilia, High performance single layer nanoporous antireflection coatings on glass by sol–gel process for solar energy applications, Sol. Energy Mater. Sol. Cells 140 (2015) 61–68, https://doi.org/ 10.1016/j.solmat.2015.03.023. [7] G. Zu, J. Shen, W. Wang, L. Zou, Y. Lian, Z. Zhang, B. Liu, F. Zhang, Robust, highly thermally stable, core-shell nanostructured metal oxide aerogels as hightemperature thermal superinsulators, adsorbents, and catalysts, Chem. Mater. 26 (2014) 5761–5772, https://doi.org/10.1021/cm502886t. [8] Q. Zheng, Z. Cai, S. Gong, Green synthesis of polyvinyl alcohol (PVA)-cellulose nanofibril (CNF) hybrid aerogels and their use as superabsorbents, J. Mater. Chem. 2 (2014) 3110–3118, https://doi.org/10.1039/C3TA14642A. [9] F. Jiang, Y.L. Hsieh, Amphiphilic superabsorbent cellulose nanofibril aerogels, J. Mater. Chem. 2 (2014) 6337–6342, https://doi.org/10.1039/C4TA00743C. [10] Z. Shi, H. Gao, J. Feng, B. Ding, X. Cao, S. Kuga, Y. Wang, L. Zhang, J. Cai, In situ synthesis of robust conductive cellulose/polypyrrole composite aerogels and their potential application in nerve regeneration, Angew. Chem. Int. Ed. 53 (2014) 5380–5384, https://doi.org/10.1002/anie.201402751. [11] D.B. Mahadik, S. Gujjar, G.M. Gouda, H.C. Barshilia, Double layer SiO2/Al2O3 high emissivity coatings on stainless steel substrates using simple spray deposition system, Appl. Surf. Sci. 299 (2014) 6–11, https://doi.org/10.1016/j. apsusc.2014.01.159. [12] S. Zhao, W.J. Malfait, N.G. Alburquerque, M.M. Koebel, G. Nystrom, Biopolymer aerogels and foams: chemistry, properties, and applications, Angew. Chem. Int. Ed. 57 (2018) 7580–7608, https://doi.org/10.1002/anie.201709014. [13] F. Jiang, S. Hu, Y. Hsieh, Aqueous synthesis of compressible and thermally stable cellulose nanofibril-silica aerogel for CO2 adsorption, ACS Appl. Nano Mater. 1 (2018) 6701–6710, https://doi.org/10.1021/acsanm.8b01515. [14] J.L. Gurav, I.K. Jung, H.H. Park, E.S. Kang, D.Y. Nadargi, Silica Aerogel: synthesis and applications, J. Nanomater. 24 (2010) 1, https://doi.org/10.1155/2010/ 409310. [15] A.C. Pierre, G.M. Pajonk, Chemistry of aerogels and their applications, Chem. Rev. 102 (2002) 4243–4266, https://doi.org/10.1021/cr0101306. [16] A.J. Westphal, R.M. Stroud, H.A. Bechtel, F.E. Brenker, Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft, Science 345 (2014) 786–791, https://doi.org/10.1126/science.1252496. [17] H. Maleki, L. Duraes, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74, https://doi.org/10.1016/j.jnoncrysol.2013.10.017. [18] J.P. Randall, M.A.B. Meador, S.C. Jana, Polymer reinforced silica aerogels: effects of dimethyldiethoxysilane and bis(trimethoxysilylpropyl)amine as silane precursors, J. Mater. Chem. 1 (2013) 6642–6652, https://doi.org/10.1039/ C3TA11019B. [19] D.B. Mahadik, A.V. Rao, R. Kumar, S.V. Ingale, P.B. Wagh, S.C. Gupta, Reduction of processing time by mechanical shaking of the ambient pressure dried TEOS based silica aerogel granules, J. Porous Mater. 19 (2012) 87–94, https://doi.org/ 10.1007/s10934-011-9451-3. [20] Y. Kobayashi, T. Saito, A. Isogai, Aerogels with 3D ordered nanofiber skeletons of liquid-crystalline nanocellulose derivatives as tough and transparent insulators, Angew. Chem. Int. Ed. 53 (2014) 10394–10397, https://doi.org/10.1002/ anie.201405123. [21] S. Yun, H. Luo, Y. Gao, Ambient-pressure drying synthesis of large resorcinol–formaldehyde-reinforced silica aerogels with enhanced mechanical strength and superhydrophobicity, J. Mater. Chem. 2 (2014) 14542–14549, https://doi.org/10.1039/C4TA02195A. [22] L. Verdolotti, M. Lavorgna, R. Lamanna, E.D. Maio, S. Iannace, Polyurethane-silica hybrid foam by sol-gel approach: chemical and functional properties, Polymer 56 (2015) 20–28, https://doi.org/10.1016/j.polymer.2014.10.017. [23] M.R. Miner, B. Hosticka, P.M. Norris, The effects of ambient humidity on the mechanical properties and surface chemistry of hygroscopic silica aerogel, J. NonCryst. Solids 350 (2004) 285–289, https://doi.org/10.1016/j. jnoncrysol.2004.06.023. [24] K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, New transparent methylsilsesquioxane aerogels and xerogels with improved mechanical properties, Adv. Mater. 19 (2007) 1589–1593, https://doi.org/10.1002/adma.200602457. [25] D.A. Loy, K.J. Shea, Bridged polysilsesquioxanes. Highly porous hybrid organicinorganic materials, Chem. Rev. 95 (1995) 1431–1442, https://doi.org/10.1021/ cr00037a013. [26] E.N. Khimich, G.S. Buslaev, A.V. Zdravkov, N.N. Khimich, M.G. Voronkov, Polysiloxane structures cross-linked with hydroquinone and phloroglucinol, Russ. J. Appl. Chem. 81 (2008) 1410–1413, https://doi.org/10.1134/ S107042720808017X. [27] D.B. Mahadik, Y.K. Lee, N.K. Chavan, S.A. Mahadik, H.H. Park, Monolithic and shrinkage-free hydrophobic silica aerogels via new rapid supercritical extraction process, J. Supercrit. Fluids 107 (2016) 84–91, https://doi.org/10.1016/j. supflu.2015.08.020.
4. Conclusions Three kinds of novel dioxybenzene-bridged silica aerogels were successfully prepared via a cost-effective sol-gel process. The textural properties of the bridged silica aerogels could be tuned over a wide range by varying precursor and amount. Hydroquinone-based aerogels attained highly ordered networks with open porous morphology and possessed low thermal conductivity compared with the resorcinol- and catechol-based ones. All of the prepared aerogels were hydrophobic without using any silylating reagents, as confirmed from FTIR spec troscopy and contact angle measurements. The position of the hydroxyl groups and content of the organic precursor had a significant influence on the textural properties and mechanical strength of the aerogels. These results suggest that the hydroquinone-based HT4 aerogel possessed the lowest dipole moment due to symmetry forming a uniform cage-like porous network with improved mechanical strength compared to RT and CT. Its fabrication was also cost-effective. Such aerogels have great potential as low dielectric materials in semiconductor industries, and also this green process could lead to the large-scale production of silica aerogels for thermal insulation. Notes The authors declare no competing financial interest. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported (in part) by the Yonsei University Research Fund (Post Doctorate Researcher Supporting Program) of 2018 (Project No.: 2018-12-0015). This work was supported by the Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-TA1703-04. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109863. References [1] L. Ren, S. Cui, F. Cao, Q. Guo, An easy way to prepare monolithic inorganic oxide aerogels, Angew. Chem. Int. Ed. 53 (2014) 10147–10149, https://doi.org/ 10.1002/anie.201406387. [2] J.E. Amonette, J. Matyas, Functionalized silica aerogels for gas-phase purification, sensing, and catalysis: a review, Microporous Mesoporous Mater. 250 (2017) 100–119, https://doi.org/10.1016/j.micromeso.2017.04.055.
8
Q. Wang et al.
Microporous and Mesoporous Materials 294 (2020) 109863
[28] M.H. Stockett, S.B. Nielsen, Transition energies of benzoquinone anions are immune to symmetry breaking by a single water molecule, Phys. Chem. Chem. Phys. 18 (2016) 6996–7000, https://doi.org/10.1039/C5CP06095H. [29] J. Guo, B.N. Nguyen, L. Li, M.A.B. Meador, D.A. Scheiman, M. Cakmak, Clay reinforced polyimide/silica hybrid aerogel, J. Mater. Chem. 1 (2013) 7211–7221, https://doi.org/10.1039/C3TA00439B. [30] D.B. Mahadik, H.N.R. Jung, W. Han, H.H. Cho, H.H. Park, Flexible, elastic, and superhydrophobic silica-polymer composite aerogels by high internal phase emulsion process, Compos. Sci. Technol. 147 (2017) 45–51, https://doi.org/ 10.1016/j.compscitech.2017.04.036. [31] A.V. Rao, R.R. Kalesh, G.M. Pajonk, Hydrophobicity and physical properties of TEOS based silica aerogels using phenyltriethoxysilane as a synthesis component, J. Mater. Sci. 38 (2003) 4407–4413, https://doi.org/10.1023/A:1026311905523. [32] D.B. Mahadik, A.V. Rao, A.P. Rao, P.B. Wagh, S.V. Ingale, S.C. Gupta, Effect of concentration of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents on surface free energy of silica aerogels, J. Colloid Interface Sci. 356 (2011) 298–302, https://doi.org/10.1016/j.jcis.2010.12.088. [33] D.B. Mahadik, A.V. Rao, V.G. Parale, M.S. Kavale, P.B. Wagh, S.V. Ingale, S. C. Gupta, Effect of surface composition and roughness on the apparent surface free
[34] [35] [36] [37]
[38]
9
energy of silica aerogel materials, Appl. Phys. Lett. 99 (2011), https://doi.org/ 10.1063/1.3635398, 104104-3. J.P. Randall, M.A.B. Meador, S.C. Jana, Tailoring mechanical properties of aerogels for aerospace applications, ACS Appl. Mater. Interfaces 3 (2011) 613–626, https:// doi.org/10.1021/am200007n. L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen, C.Y. Su, A synthetic route to ultralight hierarchically micro/mesoporous Al(III)-carboxylate metal-organic aerogels, Nat. Commun. 1774 (2013) 1, https://doi.org/10.1038/ncomms2757. E.P. Barret, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances, J. Am. Chem. Soc. 73 (1951) 373–380, https:// doi.org/10.1021/ja01145a126. D.B. Mahadik, K.Y. Lee, R.V. Ghorpade, H.H. Park, Superhydrophobic and compressible silica-polyHIPE covalently bonded porous networks via emulsion templating for oil spill cleanup and recovery, Sci. Rep. 8 (2018) 16783, https://doi. org/10.1038/s41598-018-34997-1. G. Zu, K. Kanamori, T. Shimizu, Y. Zhu, A. Maeno, H. Kaji, K. Nakanishi, J. Shen, Versatile double-cross-linking approach to transparent, machinable, supercompressible, highly bendable aerogel thermal superinsulators, Chem. Mater. 30 (2018) 2759–2770, https://doi.org/10.1021/acs.chemmater.8b00563.