Preparation of rGO–mesoporous silica nanosheets as Pickering interfacial catalysts

Abstract

A series of reduced graphene oxide–mesoporous silica nanoflakes (rGO–MSN) with adjustable surface wettability were developed. Detailed SEM, TEM, XRD, BET, and TGA analyses demonstrated that morphologies, structures, porosities of the obtained composite materials were greatly influenced by rGO content. Moreover, micrographs and conductivity measurements were conducted to investigate the performance of rGO–MSN as emulsifier in stabilizing paraffin/water two-phase system. Furthermore, anchored with –SO₃H, the modified hybrid nanoflakes SO₃H–rGO–MSN-x% were employed as solid emulsifier and catalyst (Pickering interfacial catalyst) at solvent free oil–oil interface for acetal reaction and showed excellent dodecyl aldehyde conversion more than 90%.

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Introduction

Pickering emulsions are surfactant-free dispersions containing two incompatible fluids that are stabilized by solid particles¹ including molecular sieves, polystyrene, nanotubes/graphene derivatives, clays, and biopolymers.^2–7 Compared with conventional surfactants such as cetyl trimethyl ammonium bromide (CTAB) and lauryl alcohol phosphate acid ester (MAP), solid particles used for generating stable emulsions can be separated and recycled by conventional filtration and centrifugation after operation. Since their discovery by Pickering and Ramsden, Pickering emulsions have been widely applied in water/oil and oil/water system. Due to the irreversible adsorption of solid particles at L/L interface, Pickering emulsions can afford a large and stable contact area for immiscible reagents. In the meantime, the solid particles with intrinsic active sites can catalyze the reaction of substrates, which is the so-called Pickering Interfacial Catalyst (PIC)-a reaction process encompassed particles acting as emulsifier and interfacial catalyst concurrently for the emulsion.¹ In 2010, Resasco et al. reported the first case of PIC combining carbon nanotubes and silica-supported palladium nanoparticles used as stabilizer and catalyst for biofuel-upgrade reactions through the formation of w/o emulsions.⁸ In 2012, Paula et al. reported the use of surface-modified faujasites as stabilizers of water/oil emulsions and catalysts for the alkylation between phenolics and alcohols.⁹ In a range of pioneering studies, functionalized molecular sieves, silica nanoparticles, nano-TiO₂, and SO₄²⁻/ZrO₂-based, titanium-isopropoxide-based, metal or metal oxide/carbonaceous materials and so forth were used as stabilizer and catalyst for acid/base catalyzed reactions and oxidation/reduction catalyzed reactions.^10–16

As a derivative of graphene, GO was employed as an effective Pickering emulsion stabilizer owing to its amphiphilic nature.^17,18 Moreover, factors that affect the stability of Pickering emulsion stabilized by GO were analyzed in details, such as GO concentration, oil/water ratio, pH value of system,^4,19 the polarity of oil phase.²⁰ On this basis, water–graphene oxide–oil emulsion systems were synthesized for fabricating carbon-based honeycomb-patterned films,²¹ organic (inorganic)-GO hollow hybrid,^22,23 metal organic framework,²⁴ etc. Mesoporous silica, that is, M41S type materials get extensive explorations because of its high specific surface and functionalization advantage.²⁵ Thus, it is attractive to composite the two materials at nanoscopic scale. Yang et al. have reported the synthesis of disordered hexagonal mesoporous silica/graphene composites using CTAB as structure-directing agent.²⁶ Wang et al. have successfully synthesized single-layer graphene oxide–periodic mesoporous silica sandwich nanocomposites with aligned channels vertical to the graphene oxide surface.²⁷ Guardia and his co-workers have reported the selective synthesis and characterization of small-size (a few tens of nanometers) mesoporous silica nanoparticles on graphene sheets and explored the effect of experimental parameters including pH value and the amount of raw materials on the morphology and properties.²⁸ Liu et al. have reported the synthesis of reduced graphene oxide@mesoporous silica sandwich-like nanosheets with size-tunable and vertical funneling mesochannels by an oil–water biphase stratification approach.²⁹

To our knowledge, previous studies on the graphene oxide–mesoporous silica nanoparticles (rGO–MSN) mostly focused on the preparation method^30,31 and mainly applied in heavy metal ion adsorption,³² high-temperature catalysis,³³ drug delivery and release,³⁴ however, there were seldom attempts to employ the material as PIC,¹⁵ especially in solvent-free reaction system.

Herein, we describe the preparation of rGO–MSN by using a simple method with less cost of time and structure-directing agent compared with above methods. As a type of amphiphilic material, rGO–MSN were used as stabilizer at oil–water interface. Moreover, the wettability of the materials with different addition of GO was compared. To evaluate the catalytic and emulsifying performance of the material, the rGO–MSN anchored by –SO₃H were used as PIC for the solvent-free acetal reaction and the catalytic performance of the prepared materials was investigated.

Experiments

Materials

Sulfuric acid (AR), potassium permanganate (99%), sodium nitrate (99%), hydrogen peroxide (30%), naphthalene (98%), benzaldehyde (98%), p-toluenesulfonic acid (PTSA, 99.5%) and toluene (99%) were purchased from Yuanli Chemical Technology Co., Ltd, China. Graphite powders (99.95%), tetraethyl orthosilicate (TEOS, 98%), 3-(mercaptopropyl)-trimethoxysilane (MPTMS, 95%) were purchased from Aladdin, China. Cetyl trimethyl ammonium bromide (CTAB, 95%) was obtained from Sigma-Aldrich, USA. 1-Heptanal (97%), 1-nonanal (95%), dodecyl aldehyde (95%) were purchased from Damas-beta, China. All regents were used without further purification.

Preparation of graphene oxide (GO) sheets

Graphene oxide sheets were prepared by improved Hummers' method from natural graphite flakes.³⁵ Preliminarily, 2 g graphite and 1 g NaNO₃ were mixed in a flask immersed in ice-water bath, and 50 mL concentrated H₂SO₄ was added under constant stirring for 30 min. Secondly, 12 g KMnO₄ was added into the system and kept agitating for 2 h. Then the reaction flask was transferred to a water bath at 35–40 °C and stirred for another 2.5 h. Next, 100 mL water was added and the suspension was maintained at 98 °C for 1 h. Then, 20 mL H₂O₂ (30%) was added, whereupon the brown solution changed to yellow. After centrifugation at 13 000 rpm for 3 min, the sediment was collected and washed three times with 0.1 M HCl. The sample was then rinsed with deionized water until the pH of the solution was nearly neutral. The product was exfoliated under 300 W ultrasonication and then dried in a vacuum oven at 50 °C.

Synthesis of rGO–mesoporous silica nanosheets (rGO–MSN)

For the synthesis of the hybrids, a set of PIC rGO–MSN-x% (x represents the mass ratio of GO to TEOS, varying from 0 to 5, 10, and 15) was prepared, and a typical optimized process was the following:

50.0 mg GO was firstly dispersed in 48 mL water by sonicating for 2 h. Then, 0.1 g CTAB was added to the GO suspension under mild stirring. The pH value of the solution was adjusted to 11.6–11.8 by NaOH solution. Next, the resulting mixture was stirred at 80 °C for 2.5 h. Afterwards, 0.5 g tetraethoxysilane was added into the solution and the reaction was allowed to continue at 80 °C under stirring for 3 h. The product was recovered by filtration, drying (80 °C, 12 h) and finally subjected to heat treatment at 700 °C for 6 h under nitrogen flow with a twofold purpose (remove the residual surfactant and reduce the graphene oxide to reduced graphene oxide) (Scheme 1).

Scheme 1 Synthesis of rGO–MSN.

Synthesis of sulfonic acid-functionalized rGO–MSN (SO₃H–rGO–MSN)

Sulfonic acid functionalized rGO–MSN was synthesized by “grafting” according to the previous report.²⁵ In a typical grafting synthesis, 0.5 g rGO–MSN was firstly added to 10 mL dry toluene containing 0.5 g 3-mercaptopropyltrimethoxysilane (MPTMS), and above mixture was refluxed at 110 °C for 24 h. The material was recovered by filtration, then washed with toluene and ethanol for several times and dried at 80 °C overnight. Thereafter, dried material was kept with hydrogen peroxide (30%) at room temperature for 12 h to oxidize the –SH to sulfonic groups. After filtration the solid was dried at 80 °C overnight, the process of acidification was accomplished by mixing the solid with sulfuric acid (1 mol L⁻¹) and stirring at 40 °C for 24 h.

Acetal reaction

The reaction was conducted in a 100 mL 3-neck-flask, by taking 30 mg (∼0.3 wt%) SO₃H–rGO–MSN in 5 mL ethylene glycol (EG) at 25 °C and sustaining 0.5 h under ultrasonic condition. Next, 5 mL dodecyl aldehyde was added and the mixture was emulsified at 60 °C under vigorous stirring (2000 rpm, ultra-turax IKA) for 10 minutes. Then, the emulsion was sealed and reacted in an water bath at 60 °C under stirring (270 rpm) for 110 minutes. The product was analyzed by a gas chromatography (SP3420-A, Beijing Beifen-Ruili Analytical Instrument (Group) Co., Ltd containing a hydrogen flame ion detector (FID)). After reaction, the spent solid particles were collected by centrifugation and washing with ethanol.

Characterization

SEM images were obtained on a JEOL S-4800 scanning electron microscope. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 transmission electron microscope. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 X-ray diffractometer with a Cu Kα radiation source (λ = 0.15405 nm), the scans covered the 2θ range from 0.5° to 10°. The specific surface area, average pore diameter, and pore volume were recorded by N₂ adsorption–desorption method using NOVA 2000 instrument at 77 K. Pore size distribution (PSD) was detected by applying the BJH pore analysis that applied to desorption of the N₂ adsorption–desorption isotherm. A NETZSCH-STA449F3 was used for thermogravimetry analysis (TGA) at a heating rate of 5 °C min⁻¹ in air flow. The emulsion phase inversions were observed by a DDS-307 conductometer with a Pt platinized electrode at 25 °C and emulsion morphologies were recorded by an optical microscope with camera (SIGE300C). Contact angles were measured by an optical contact angle & interface tension meter (SL200KS, KINO, USA).

Results and discussion

Synthesis and characterization

Fig. 1 shows the FE-SEM images of rGO–MSN-0% (a), rGO–MSN-5% (b), rGO–MSN-10% (c), and GO (d). As shown in Fig. 1a, rGO–MSN-0% adopts spherical and ellipsoidal morphologies with high degree of uniformity. Meanwhile, both spherical silica and rGO nanoflakes coated by silica are found in Fig. 1b, demonstrating that silica could not grow in graphene oxide surface unrestrainedly once exceeds a certain amount. For rGO–MSN-10%, dissociative silica nanoparticles disappeared and only rGO nanoflakes coated by silica is detected in Fig. 1c.

Fig. 1 Scanning electron microscopy (SEM) images of rGO–MSN-0% (a), rGO–MSN-5% (b), rGO–MSN-10% (c), and GO (d). The thickness of rGO–MSN-10% was measured and labeled roughly in (c).

In this experiment, surfactant CTAB was chose as structure-directing agent, thus, the silica in the rGO–MSN hybrids is expected to possess a mesoporous pore structure.²² Fig. 2 illustrates the pore structure of rGO–MSN-0% (a) and rGO–MSN-10% (b). The pure silica nanoparticle shows a typical ordered hexagonal pore array which is similar to MCM-41 as previously reported (Fig. 2a). TEM image (Fig. 2b) captures the plane character of the rGO–MSN-5% and indicates a porous structure of the nanoflakes with a pore size about 3.5 nm (Fig. 2c). Interestingly, it was found that the pore channels of the silica are vertical to reduced graphene oxide sheet from both sides as previously reported,²³ and the thickness of rGO–MSN-10% is approximately 32 nm, as shown in Fig. 2d and e, bordering on the value measured from the SEM image in Fig. 1c.

Fig. 2 Transmission electron microscopy (TEM) of the rGO–MSN-0% (a), rGO–MSN-10% (b), the surface of rGO–MSN-10% (c), the side view of rGO–MSN-10% (d), the model of longitudinal section view illustrate to rGO–MSN-10% (e).

To explore the influence of reduced graphene oxide content on the channel property of the nanoflakes, a series of hybrid nanosheets with various rGO content were prepared and characterized with small-angle XRD patterns, N₂ adsorption–desorption isotherms and pore size distribution (PSD) (Fig. 3). For the XRD of rGO–MSN-0%, the strongest diffraction peak is observed at 2θ = 2.545° represents the (100) lattice plane of a hexagonal unit cell, followed with weak (110) and (200) peaks, which indicate a long-range ordering. However, there is only a broad diffraction peak in the XRD patterns of rGO–MSN-5%, rGO–MSN-10%, and rGO–MSN-15%, which suggests that the introduction of GO has destroyed the well-organized structure, it might be caused by uneven distribution of oxygen-containing groups on GO surface. The sorption isotherms of all samples can be classified as the type IV, which indicates the existence of mesoporous structure. The PSD demonstrates that the increasing amount of GO enlarge the distribution interval of pore size. For example, pore diameter for rGO–MSN-0% centralized in a narrow range of 2.0–4.0 nm whereas, for rGO–MSN-15%, diameter distribution range from 2 nm to even 20 nm. Furthermore, other textural data of materials with different mass ratios of GO is shown in Table 1. Owing to the strong π–π stacking interactions between layers of exposed rGO of materials, the rGO–silica nanosheets tend to restack and aggregate mutually accompanying a serious decrease of S_BET. However, the mechanism why pore diameter increases is still indistinct and needs further research to confirm.

Table 1 Structural properties of rGO–MSN

GO/TEOS (mass%)	SBET (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)	Contact angle (°)
0	930	3.07	0.71	28
5	836	3.72	0.77	32
10	540	3.75	0.50	70
15	260	5.88	0.38	108

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Fig. 3 Small angle XRD patterns, N₂ adsorption–desorption isotherms, and pore size distribution of rGO–MSN (from left to right).

TGA analyses also affirmed that graphene oxide was successfully introduced into mesoporous silica. As proved in Fig. 4 pristine silica only shows a drastic weight loss of about 35% in the temperature range from 30 to 100 °C, which conforms to the loss of physisorbed water. Different from pristine silica, the TGA spectrum of rGO–MSN (b–d) exhibits two steps of weight loss. The early weight loss step is attributed to physisorbed water at the temperature less than 100 °C, which is similar to pristine silica. What should be emphasized is the later one within the scope of 500–630 °C, which was caused by the reduced graphene oxide loss in the heat air flow, and according to that, we can estimate the content of rGO in rGO–MSN-x% (8%, 17%, 25%, corresponding to rGO–MSN-5%, rGO–MSN-10%, rGO–MSN-15%, respectively).

Fig. 4 Thermogravimetric plots under air flow for rGO–MSN-0% (a), rGO–MSN-5% (b), rGO–MSN-10% (c), and rGO–MSN-15% (d).

Particularly, a variety of hybrid nanosheets with different amount of GO were tested by TEM to evaluate the influence of GO amount on wettability of the materials. Fig. 5 shows the TEM images of rGO–MSN-5% (a), rGO–MSN-10% (b), and rGO–MSN-15% (c). Obviously, with the increasing addition amount of GO, the rGO exposed in hybrid nanosheets enlarged. This variation is further confirmed by elemental mapping concisely. For the sample with the lowest content of GO (Fig. 5a), mesoporous silica covered in rGO surface completely and the rGO area can rarely be found, when the GO addition increased to 10%, silica as well as the rGO area appeared and described by Fig. 5b image. In sample rGO–MSN-15%, the entire nanosheets of the multi-layer rGO can be appreciated. This provides a microscopic explanation for discrepancy of wettability among the materials, and macro behaviors in stabilizing emulsions were tested and shown in the next part of this article.

Fig. 5 Transmission electron microscopy (TEM) and elemental mapping images of rGO–MSN-5% (a), rGO–MSN-10% (b), rGO–MSN-15% (c).

Emulsion properties

The rGO–MSN-x% were dispersed in the paraffin/water system by sonicating and the emulsion was formed by vigorous stirring. The obtained emulsions were conserved in sample bottles and sealed carefully. An obvious difference in the behavior of rGO–MSM with different doping amount of GO was observed. Rapid phase separation was detected with the sample stabilized by rGO–MSN-15% (in the fourth sample bottle, from left to right), while the others can form stable emulsions. The presence of rGO enhances the hydrophobicity of hybrid nanoflakes and is beneficial to the formation of emulsions in a certain range. As a result, the increasing amount of GO from 0 to 10 wt% increased the cream volume (Fig. 6).

Fig. 6 Photographs of emulsions stabilized by rGO–MSN-0%, rGO–MSN-5%, rGO–MSN-10%, and rGO–MSN-15% (from left to right). The cream volume of emulsions was measured after a static process about 12 h.

Owing to the favorable emulsified property of rGO–MSN-10%, the micrographs and conductivities of paraffin/water emulsions stabilized by rGO–MSN-10% were explored. In the experiments, w/o volume ratio was changed from 10% to 90%, the morphologies of above obtained emulsions were detected by optical microscope and shown in Fig. 7a–e. With the increase of the paraffin volume ratios, the average drop diameter increase first and then decrease, which conforms to the general rules of Pickering emulsions. As oil volume fraction increased to 90%, a flocculent internal phase formed, that was difficult to measure the mean drop diameter. Furthermore, the smallest average droplet size (37 μm) was obtained when the volume ratio of water to oil was 3 : 7. Conductivity measurement is exhibited in Fig. 7f, it indicates that when paraffin volume ratio was in a relatively low level, conductivity of the emulsion is rather high, suggesting the oil-in-water emulsion type. On the contrary, the conductivity of the emulsion decreased as the oil phase volume ratio exceeded 70%, meaning the formation of water-in-oil emulsion type.

Fig. 7 Morphologies of emulsions stabilized by rGO–MSN-10% with different oil volume ratio: 10% (a), 30% (b), 50% (c), 70% (d), and 90% (e). The conductivities and average droplet diameter of emulsions (a–e) were summarized in (f). The emulsions were prepared by vigorous stirring (2000 rpm) for 10 minutes. The emulsions were kept under static conditions for 12 h before we measured the properties of emulsions.

The optical microscope images of the Pickering emulsions with varying amounts of rGO–MSN-10% were recorded as well (Fig. 8). Previous work by Binks et al. has reported the increasing of solid emulsifier that added to emulsion systems can lead to the formation of smaller droplet.² It was well conformed to above conclusion as shown in Fig. 8. When the mass fraction of emulsifier in emulsion was 0.2%, we obtained the droplet with an average diameter about 95 μm, in addition, the diameter gradually decreased from 84 μm to 41 μm along with the raised mass fraction of emulsifier from 0.4% to 0.8%. Meanwhile, the emulsions showed favorably stable property. After kept static for 2 weeks, no significant changes were observed in the emulsions except that the diameter of the droplets was slightly larger than before.

Fig. 8 Morphologies of emulsions stabilized by rGO–MSN-10% with different mass fraction of solid emulsifier: 0.2% (a), 0.4% (b), and 0.8% (c) were kept under static conditions for 12 h, 0.2% (a′), 0.4% (b′), and 0.8% (c′) were kept under static conditions for 2 weeks. The emulsions were prepared by vigorous stirring (2000 rpm) for 10 minutes, Φ_water = 0.5.

Activity in acetal reaction

Amphipathic characters of the prepared nanosheets at water–oil interface have been verified. In this part, emulsified property and catalytic performance of the materials in the oil–oil biphasic system will be preliminarily explored. Owing to the mutual immiscible property of dodecyl aldehyde and ethylene glycol (EG), the solvent-free acetal reaction of the two reagents was optimally selected as model reaction. Because acetal reaction requires the presence of acid-catalyzer, we anchored –SO₃H to the rGO–mesoporous silica nanoflakes for stabilizing the system and catalyzing the reaction, in addition, contact angles of rGO–MSN (SO₃H–rGO–MSN) with different GO content were measured and the change of wettability attributed to –SO₃H immobilization were explored (Table 2). However, no obvious difference between rGO–MSM and SO₃H–rGO–MSN was observed. Catalytic activities of pristine rGO–MSN-x% and SO₃H–rGO–MSN-x% in dodecyl aldehyde–EG emulsions are shown in Table 3. The pristine rGO–MSN-x% catalysts all achieved a nearly aldehyde conversion less than 60 wt%. However, with –SO₃H anchored, the further modified materials wholly showed a great improvement on the aldehyde conversion (with an increase more than 30 wt%). It was proved that the introduction of –SO₃H could accelerate the reaction obviously.

Table 2 Contact angle of rGO–MSN and SO₃H–rGO–MSN

	0%	5%	10%	15%
rGO–MSN
SO3H–rGO–MSN

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Table 3 Catalytic activities of different catalysts (reaction time: 2 h, temperature: 60 °C)

Run	Catalyst	Conversion (%)	Selectivity (%)
1	—	28	72
2	GO	67	51
3	SO3H–GO	91	55
4	rGO	30	69
5	rGO–MSN-0%	56	74
6	rGO–MSN-5%	53	77
7	rGO–MSN-10%	55	73
8	rGO–MSN-15%	56	73
9	SO3H–rGO–MSN-0%	87	78
10	SO3H–rGO–MSN-5%	86	75
11	SO3H–rGO–MSN-10%	90	81
12	SO3H–rGO–MSN-15%	92	76

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Comparative advancement of the acetal reaction catalyzed by different SO₃H–rGO–MSN were carried out and shown in Fig. 9 (sample 1, sample 2, sample 3, and sample 4 represents SO₃H–rGO–MSN-0%, SO₃H–rGO–MSN-5%, SO₃H–rGO–MSN-10%, and SO₃H–rGO–MSN-15%, respectively). The emulsions were reacted under vigorous stirring (2000 rpm) for 10 minutes (emulsified at the same time) and followed by a mild stirring (270 rpm). In the process of 10 minutes vigorous stirring for reaction and emulsification, the sufficient oil–oil contact area for reagents was produced, in this case, the mass transfer was unlimited. It must be pointed out that sulfonic content of catalysis can affect the reaction rate simultaneously, in the presence of excess MPTMS,³⁶ the content of loaded sulfonic of materials decrease along with the increasing amount of rGO (MPTMS grafted in silica selectively, while rGO can be hardly loaded by –SO₃H owing to the reduction of oxygen-containing groups on GO surface). Therefore, the reaction rate was supposed to decline accompany with the decrease of silica content: the active site of sample 1 could contact with regents adequately and then exhibited excellent catalytic performance. However, as the stirring speed decreased from 2000 rpm to 270 rpm, the two-phase separation took place quickly in the emulsions stabilized by sample 1 and 2. Consequently, the mass transfer environment degraded and diffusion ascended to be a key inhibition of the reactions. On the contrary, sample 3 and 4 formed stable emulsions for a period of time and provided large contact area for C₁₂-aldehyde and EG. As a result, the rate of reactions catalyzed by sample 3 and sample 4 were relatively faster than sample 1 and sample 2. Moreover, due to the formation of smaller droplet than the sample 4, the emulsion stabilized by sample 3 formed larger oil–oil contact surface and faster reaction rate (Fig. 10a and c). With the mass transfer and composition variation of reaction systems, the droplets broke-up and the stable emulsions collapsed, which also caused the decrease of reaction rate to some extent (Fig. 10b and d). For comparison, PTSA was employed as catalyst for acetal reaction. Owing to the strong acidity of PTSA, the conversion of the reaction reached 79% (20 minutes) when the addition of PTSA is equal to the SO₃H–rGO–MSN, however, the phase separated quickly and no droplets were detected after 20 minutes reaction. As a result, the period of reaction stagnation begins and lasts until the reaction ends.

Fig. 9 Comparative advancement of the acetal reaction of EG and dodecyl aldehyde over SO₃H–rGO–MSN.

Fig. 10 Optical micrographs of C₁₂-aldehyde/EG emulsions stabilized by SO₃H–rGO–MSN-10% at 30 min (a), 120 min (b), and stabilized by SO₃H–rGO–MSN-15% at 30 min (c), 120 min (d). The emulsions were kept under static conditions for 2 minutes before we measured the property of emulsions. The scale bar is 50 μm.

We finally examined the catalytic performance of SO₃H–rGO–MSN with other substrates. Conversion and selectivity of acetal reaction between EG and benzaldehyde, heptaldehyde, nonanal were investigated (as shown in Table 4). In the presence of SO₃H–rGO–MSN-10%, benzaldehyde was fully miscible with EG and achieved a high conversion. When heptaldehyde was employed as substrate instead of benzaldehyde, the difficulty in forming stable emulsion leads to a low reactivity. While the reaction between C₉ (C₁₂)-aldehyde and EG was just the opposite. In this process, the substrates were partly oxidized to C_n-acid (main by-product) by O₂.

Table 4 Conversion and selectivity of acetal reaction with different substrates^a

Run	Substrate	Conversion (%)	Selectivity (%)
a Cn-acid is main by-product. Reaction temperature: 60 °C, time: 2 h.
1		74	89
2		43	92
3		81	58

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Conclusions

In summary, a series of rGO–MSN with different content of rGO were prepared. It was found that the silica pore channels are vertical to the reduced graphene oxide sheet from both sides. Analyses showed the increasing amount of GO could destroy the long-range ordering, enlarge the pore diameter distribution interval and reduce the BET surface area of the rGO–MSN. Then, the amphiphilic materials we prepared with favorable wettability were demonstrated remarkable property to stabilize emulsions at oil–water interface (before grafted by –SO₃H). Moreover, both emulsified function and catalytic performance for the reaction at solvent free oil–oil interface of C₁₂-aldehyde with ethylene glycol (after grafted by –SO₃H) were indicated. Janus nanoflakes, which have two anisotropic sides with different chemical properties, may have preferable affinity for both internal and external phases of emulsions and superior selective performance of emulsion reactions and, hence, prospective developments in progress may concern the study of the synthesis of Janus nanoflakes as above and employ it as PIC.

Acknowledgements

We appreciatively acknowledge the support of the National Basic Research Program of China (973 Program) (Grant no. 2012CB720302) and program for Changjiang Scholars and Innovative Research Terms in Universities (IRT0936).

References

1. M. Pera-Titus, L. Leclercq, J. Clacens, F. De Campo and V. Nardello-Rataj, Angew. Chem., Int. Ed., 2015, 54, 2006–2021.
2. B. P. Binks and S. O. Lumsdon, Langmuir, 2001, 17, 4540–4547.
3. J. Shao, W. Lv and Q. Yang, Adv. Mater., 2014, 26, 5586–5612.
4. Y. He, F. Wu, X. Sun, R. Li, Y. Guo, C. Li, L. Zhang, F. Xing, W. Wang and J. Gao, ACS Appl. Mater. Interfaces, 2013, 5, 4843–4855.
5. Y. Hu, J. Huang, Q. Zhang, Y. Yang, S. Ma and C. Wang, RSC Adv., 2015, 5, 103394–103402.
6. N. P. Ashby and B. P. Binks, Phys. Chem. Chem. Phys., 2001, 2, 5640–5646.
7. I. Kalashnikova, H. Bizot, B. Cathala and I. Capron, Langmuir, 2011, 27, 7471–7479.
8. S. Crossley, J. Faria, M. Shen and D. E. Resasco, Science, 2010, 327, 68–72.
9. P. A. Zapata, J. Faria, M. P. Ruiz, R. E. Jentoft and D. E. Resasco, J. Am. Chem. Soc., 2012, 134, 8570–8578.
10. Z. Fan, A. Tay, M. Pera-Titus, W. Zhou, S. Benhabbari, X. Feng, G. Malcouronne, L. Bonneviot, F. De Campo, L. Wang and J. Clacens, J. Colloid Interface Sci., 2014, 427, 80–90.
11. L. Fu, S. Li, Z. Han, H. Liu and H. Yang, Chem. Commun., 2014, 50, 10045.
12. G. Nawaratna, S. D. Fernando and S. Adhikari, Energy Fuels, 2010, 24, 4123–4129.
13. W. Li, F. Ma, F. Su, L. Ma, S. Zhang and Y. Guo, ChemSusChem, 2011, 4, 744–756.
14. H. Tan, P. Zhang, L. Wang, D. Yang and K. Zhou, Chem. Commun., 2011, 47, 11903.
15. X. Wei, J. Liu, Y. Yang and L. Deng, RSC Adv., 2014, 4, 35744.
16. Y. Zhao, X. Zhang, J. Sanjeevi and Q. Yang, J. Catal., 2016, 334, 52–59.
17. J. Kim, L. J. Cote, F. Kim, W. Yuan, K. R. Shull and J. Huang, J. Am. Chem. Soc., 2010, 132, 8180–8186.
18. J. Shao, W. Lv and Q. Yang, Adv. Mater., 2014, 26, 5586–5612.
19. T. M. McCoy, M. J. Pottage and R. F. Tabor, J. Phys. Chem. C, 2014, 118, 4529–4535.
20. S. C. Thickett and P. B. Zetterlund, J. Colloid Interface Sci., 2015, 442, 67–74.
21. L. Li, Y. Zhong, J. Li, J. Gong, Y. Ben, J. Xu, X. Chen and Z. Ma, J. Colloid Interface Sci., 2010, 342, 192–197.
22. G. H. Teo, Y. H. Ng, P. B. Zetterlund and S. C. Thickett, Polymer, 2015, 63, 1–9.
23. S. C. Thickett, N. Wood, Y. H. Ng and P. B. Zetterlund, Nanoscale, 2014, 6, 8590.
24. Z. Bian, S. Zhang, X. Zhu, Y. Li, H. Liu and J. Hu, RSC Adv., 2015, 5, 31502.
25. S. Shylesh, S. Sharma, S. P. Mirajkar and A. P. Singh, J. Mol. Catal. A: Chem., 2004, 212, 219–228.
26. S. Yang, X. Feng, L. Wang, K. Tang, J. Maier and K. Müllen, Angew. Chem., Int. Ed., 2010, 49, 4795–4799.
27. Z. Wang, W. Wang, N. Coombs, N. Soheilnia and G. A. Ozin, ACS Nano, 2010, 4, 7437–7450.
28. L. Guardia, F. Suárez-García, J. I. Paredes, P. Solís-Fernández, R. Rozada, M. J. Fernández-Merino, A. Martínez-Alonso and J. M. D. Tascón, Microporous Mesoporous Mater., 2012, 160, 18–24.
29. Y. Liu, W. Li, D. Shen, C. Wang, X. Li, M. Pal, R. Zhang, L. Chen, C. Yao, Y. Wei, Y. Li, Y. Zhao, H. Zhu, W. Wang, A. M. El Toni, F. Zhang and D. Zhao, Chem. Mater., 2015, 27, 5577–5586.
30. C. Lee, K. C. Roh and K. Kim, Nanoscale, 2013, 5, 9604.
31. J. Q. Œ. E. Dalagan, Bull. Mater. Sci., 2014, 37, 589–595.
32. X. Li, Z. Wang, Q. Li, J. Ma and M. Zhu, Chem. Eng. J., 2015, 273, 630–637.
33. L. Shang, T. Bian, B. Zhang, D. Zhang, L. Wu, C. Tung, Y. Yin and T. Zhang, Angew. Chem., Int. Ed., 2014, 53, 250–254.
34. P. Yin, Y. Wang, Y. Li, C. Deng, X. Zhang and P. Yang, Proteomics, 2012, 12, 2784–2791.
35. D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour, ACS Nano, 2010, 4, 4806–4814.
36. L. M. Yang, Y. J. Wang, G. S. Luo and Y. Y. Dai, Microporous Mesoporous Mater., 2005, 84, 275–282.

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