Synthesis of some silyl derivatives of graphene oxide

Abstract

In this work, we synthesized some organosilicon derivatives of graphene oxide (GO) containing various groups such as trimethyl, triethyl, dimethyl-tert-Bu, and dimethyl-vinyl. Also, some reactions were done in the presence of DES, as catalyst. This green catalyst improved the yields of reactions successfully. Synthesis of methoxy, and ethoxy vinyl silyl ethers of GO were done. In this work, hybrid organic-inorganic was done. In the next step, network polymers of GO with styrene, and methacrylic acid (MAA) were done. Incorporation of organosilicon groups were modified GO properties for example thermal stability or it.

Keywords

Graphene oxide silyl ether organosilicon styrene polymer

1 Introduction

Formation of silyl ether from a hydroxyl function group can be used either to provide a volatile derivative for GC and GC-MS, or as a means of protection in synthesis of drugs, steroids, sugars and natural products. Also, we have used silyl ethers for the synthesis of polymeric prodrugs [1]. Popularity and extensive use of silyl ethers as protecting groups for alcohols come from their ease of formation, resistance to oxidation, good stability toward most non-acidic reagents and easy deprotection to provide free alcohols [2 –4].

Generally, the formation of silyl ethers was carried out by treatment of hydroxyl functional group with silylating agents under the influence of base or catalyst [5 –7]. A literature survey shows that several methods have been reported using various catalysts and bases such as tertiary amine [8], Sc(OTf)3 [9, 10], K-10 montmorillonite [11], iodine [12], zinc chloride [13, 14], lithium perchlorate [15] and Bi(OTf)3 [16]. However, some of these methods frequently suffered from drawbacks such as toxic metal catalysts [17], costly catalysts [18], high temperatures [19], long reaction times [20] and use of excess reagents [21].

On the other hand, the synthesis of the most hindered silyl ethers such as triethylsilyl ethers and t-butyldimethylsilyl ethers are more important for many purposes. Graphene oxide (GO), one of the most important derivatives of graphene, is an inexpensive, stable and extraordinarily versatile carbon material [22]. Specifically, due to the involvement of various functional groups containing oxygen (e.g., hydroxyl, carboxyl, and epoxy groups) attached to carbons or at the edges of the layer [23], the GO surface can easily be modified with compounds containing active groups [24, 25]. Therefore, GO reforms as an excellent supporting material for immobilizing homogenous catalysts. These suggest that mobilizing Karstedt’s catalyst on GO might afford an efficient, reusable and environmentally benign catalyst for hydrosilylation. Herein, we report the preparation and characterization of the intercalated GO composites modified with some silylating agents such as vinyltriethoxysilane (VTEO).

2 Experimental

2.1 Materials and equipment

The IR spectra were recorded on a Bruker PS-15 spectrometer. The melting points were measured on an electrothermal melting point apparatus, model 9100-2. All starting materials and reactants were commercially available and were purchased from Merck. All commercial reagents were used without prior purification. All of reactions were performed in argon atmosphere.

2.2 Synthesis of trimethyl silyloxy graphene oxide

GO (0.3 g), chlorotrimethylsilane (TMSCl) (1 mL), triethylamine (1 mL), in 20 mL THF were added as solvent and mixed at 80°C for 72h in inert atmosphere. Then precipated was separated and washed three times with diethyl ether and deionizer water. DES was synthesized as previous work [1].

FT-IR (KBr) (ν/cm^–1): 3419, 2939, 1722, 1600, 1260, 1035, 803.

2.3 Synthesis of triethyl silyloxy graphene oxide

GO (0.3 g), chlorotriethylsilane (3 mL), triethylamine (1 mL), imidazole (1 mL) in 20 mL THF were added as solvent and mixed at 80°C for 72 h under an inert atmosphere. Then, the obtained precipitate was separated and washed three times with diethyl ether and deionized water.

FT-IR (KBr) (ν/cm^–1): 3419, 2900, 1734, 1650, 1260, 1035, 803.

2.4 Synthesis of dimethyl-tert-butyl-silyloxy graphene oxide

GO (0.3 g), Chlorodimethyl-tert-Bu-silane (TMSCl) (1.5 mL), trimethylamine (1 mL), imidazole (1 mL) in 20 mL THF were added as solvent and mixed at 80°C for 72 h under an inert atmosphere. Then the obtained precipitation was separated and washed three times with diethyl ether and deionized water.

FT-IR (KBr) (ν/cm^–1): 3418 (O–H stretch), 2978, 2943 (–C–H stretch), 1721 (C = O stretch), 1574 (C = C stretching), 1288 (Si–C stretching), 1172 (Si–O stretching), 806 (Si–C bending).

2.5 Synthesis of trimethoxy-silyloxy graphene oxide

GO (1 mL), chlorotrimethoxysilane (TMSCl) (8 mL), trimethylamine (4 mL), imidazole (1. mL) in 20 mL THF were added as solvent and mixed at 80°C for 72 h in inert atmosphere. Then the obtained precipitation was separated and washed three times with diethyl ether and deionized water.

FT-IR (KBr) (ν/cm^–1 3420 (O–H stretching), 2959 (–C–H stretching), 1726 (C=O stretching), 1588 (C=C stretch), 1258 (Si–C stretching), 1084 (Si–O stretching), 798 (Si–C bending).

2.6 Synthesis of dimethoxy-vinyl silyloxy graphene oxide

First GO (1 mL) was added to water (100 mL) then the reaction mixture was treated with ultrasonic vibrations for one h and trimethoxy-vinyl silane (6 mL), triethylamine (3 mL), imidazole (1 mL) in 20 mL THF as solvent were added and for 24 h in inert atmosphere at 75°C. Then, the obtained precipitate was separated and washed three times with diethyl ether and deionized water.

FT-IR (KBr) (FT-IR (KBr) (ν /cm^-1): 3423 (O–H stretch), 3064, 3025 (=C–H stretch), 2984, 2960 (–C–H stretch), 1739 (C=O stretch), 1604 (C=C stretch), 1278 (Si–C stretch), 1122 (Si–O stretch), 760 (Si–C bend).

2.7 Synthesis of diethoxy-vinyl silyloxy graphene oxide

First GO (1 mL) was added to water (100 mL) then the reaction mixture was treated with ultrasonic for 1h and triethoxy-vinyl silane (6 mL), triethylamine (3 mL), imidazole (1 mL) in 20 ml THF were added as solvent and mixed for 48 h under an inert atmosphere at 75°C. Then, the obtained precipitation was separated and washed three times with diethyl ether and deionized water.

FT-IR (KBr) (ν /cm^-1): 3421 (O–H stretching), 3063–3025 (= C–H stretching), 2984–2960 (–C–H stretching), 1737 (C = O stretching), 1603 (C = C stretching), 1278 (Si–C stretch), 1123 (Si–O stretching), 762 (Si–C bending).

2.8 Synthesis of diethoxy-vinyl siloxy graphene oxide in the presence of DES

First GO (1 mL) was added to water (100 mL). Then, the reaction mixture was treated with ultrasonic for 1 h and triethoxy-vinyl silane (6 mL), triethylamine (3 mL), imidazole (1 mL), and DES in 20 mL THF were added as solvent and mixed for 48h in inert atmosphere at 75°C. Then the obtained precipitation was separated and washed three times with diethyl ether and deionized water. The yield of reaction in DES improved successfully.

FT-IR (KBr) (ν /cm^-1): 3421 (O–H stretch), 3063, 3025 (= C–H stretch), 2984, 2960

(–C–H stretch), 1737 (C = O stretch), 1603 (C = C stretch), 1278 (Si–C stretch), 1123 (Si–O stretching), 762 (Si–C bending).

Stretch), 762 (Si–C bend).

2.9 Synthesis of some homo, and copolymers of vinyl siloxy silane of graphene oxide.

2.9.1 Synthesis of homo polymer:

2.9.1.1Synthesis of homo-poly-ethyl-vinyl-siloxy-graphene oxide. A mixture of resulting vinyl siloxy graphene oxide was dissolved in THF in a pyrex glass and was let down to polymerize at 60–70°C in a thermostat water bath, using 2,2_-azaobis(isobutyronitrile) (AIBN) as initiator (2% mol) for 72 h. After the desired time, the precipitation was collected and washed with methanol for 1 day, while the methanol was changed every 4 h in order to remove any unreacted monomers. Finally, the sample was dried in air and stored in desiccators until use.

2.10 Synthesis of copolymers

2.10.1 Synthesis of co-poly-ethylene-vinyl-siloxy-graphene oxide with Methacrylic acid (MAA)

A mixture of resulting vinyl siloxy graphene oxide, and MAA with different molar ratio were dissolved by THF in a pyrex glass and was let down to polymerize at 60–70°C in a thermostatic water bath, using AIBN as initiator (2% mol) for 72 h. After the desired time, the obtained precipitation was collected and washed with methanol for 1 day, while the methanol was changed every 4 h in order to remove any unreacted monomers finally the sample were dried in air and stored in desiccators until use.

2.10.2 Synthesis of co-poly-ethylene- -siloxy-graphene oxide with styrene

A mixture of resulting vinyl siloxy graphene oxide, and styrene different molar ratio were dissolved in THF in a pyrex glass and was let down to polymerize at 60–70° in a thermostat water bath, using AIBN as initiator (2% mol) for 72 h. After the desired time, the precipitation was collected and washed with methanol for 1 day, while the methanol was changed every 4 h in order to remove any unreacted monomers; then, the samples were dried in air and stored in a desiccator until use.

3 Results and discussions

In this work we have synthesized some silyl ether of graphene oxide, and investigated properties of them.

3.1 Synthesis of silyl ethers of graphene oxide

Reactions of chlorosilanes (R₃SiCl) (R₃= Me₃, Et₃, t-Bu-Me₂, Me₂-Vinyl), were performed in THF, in the presence of Et₃N as base with GO (Scheme 1, 2) to produce silyl ethers of graphene oxide:

Scheme 1

Synthesis of silyl ethers of GO.

Scheme 2

Schematic representation of GO Synthesis, and reactions with organosilicon compounds.

Trimethylsilyloxy-Go, triethlsilyloxy-GO, Dimethyl-t-Busilyloxy-GO, Dimethylvinylsilyloxy-GO,and Dimethoxysilyloxy-GO.

Also, we were able to increase the reaction yield by changing the reaction conditions, including temperature, time, percentage of reactants, as well as use of a catalyst. Deep eutectic solvent (DES) is green solvent, which is commonly used in synthesis.We chosen choline chloride (ChCl):urea (1:2) as the best catalyst according to our experience and estimations [27]. The mechanism of this catalyst is shown in Scheme 3. The best results were obtained for trimethoxy, and triethoxy silane derivatives.

Scheme 3

Proposed synthesis, and mechanism for the silylation reaction in ChCl/Urea.

3.2 Identification of silyl groups in GO units

By infrared spectroscopy, we obtained evidence to support incorporation of organosilicon units into the GO chain. The peaks at 2974 cm^-1 and 2939 cm^-1 are related to C–H stretching vibrations. The peak at 1722 cm^-1 is related to the C=O stretching vibration. The absorption of 1622 cm^-1 is related to the stretching vibration C=C of the GO aromatic ring. The strong absorption in 1261 cm^-1 is related to Si–C stretching vibrations, absorption in cm^-1 1035 cm^-1 is related to Si–O stretching vibrations and strong absorption in 803 cm^-1 is related to Si–C bending vibrations, which indicates silyl-ether of GO. As shown in Fig. 1, the relatively wide absorption intensity of O–H relative 3419 cm^-1 has been reduced due to the deposition of a large number of hydroxyl groups and formation of O–Si bonds. All of these pieces of evidence suggest that silyl groups were successfully linked to GO (Fig. 1).

Fig. 1

Infrared spectroscopy of silyl ethers of GO.

3.3 XRD Spectrum

In the Spectrum XRD spectrum of trialkylsiloxy graphene oxide, the peak of graphene oxide has become wider and more amorphous because the amount of graphene oxide has decreased due to the reaction with the silicon reagent. The wider the peak of graphene oxide, the more the functional groups react to with the material we have modified. The amorphous peak at higher than 2θ is due to the formation of trialkyl siloxy graphene oxide, which, in addition to indicating that a reaction has occurred and altered the nature of graphene oxide, indicates that crystallization rate of GO has also decreased and it has become more amorphous. In the XRD model, the GO peak is at the angle of θ2 = 11.62, which has been moved to a lower angle after reaction with reagent. According to Bragg’s law, the transfer of a peak from a high angle to a low angle is due to the increasing distance between the layers (Fig. 2).

Fig. 2

XRD of trialkyl-siloxy-GO.

3.4 TGA of trialkyl-siloxy-GO

The thermal stability of trialkyl siloxy graphene oxide was investigated by thermogravimetric analysis of nitrogen gas flow at a velocity of 10°C /min. In the TGA thermogram related to trialkyl siloxy graphene oxide, the first weight loss in the temperature range between 185-100°C is related to the evaporation of free water or hydroxyl groups in the structure of graphene oxide and the second weight loss, approximately 550 °C is associated with complete degradation of the trialkylsiloxy graphene oxide compound. As can be seen from the comparison of the graphs related to graphene oxide and the derivative of trialkyl siloxy graphene oxide, the curve has shifted to the right and the silanization has increased the thermal stability of the silica derivative relative to graphene oxide. These results are noticeable in modification of GO (Fig. 3).

Fig. 3

Typical thermogram TGA of trialkyl-siloxy-GO.

The thermal stability of dimethyl tertiarybutylsiloxy graphene oxide is higher than that of trimethylsiloxy graphene oxide derivatives, as well as other derivatives, triethylsiloxy graphene oxide. This means that this derivative makes graphene oxide more stable than the other two derivatives. The reason for this is the higher crowded of this derivative. One of the factors influencing the stability of the silyl ether is the sterically crowded of organosilicon compounds. The higher it is, the more stable it will be, i.e. the sterically hindered of silicon is major factor in stabilization.

3.5 EDX

Due to the large presence of hydroxy group at layers of graphene oxide, it is not possible to completely silylated. EDX data confirms the presence of the silicon element in the synthesis of the obtained compound and shows that the synthesis of trialkyl siloxy graphene oxide was successfully performed (Fig. 4).

Fig. 4

Typical EDX of trialkylsiloxy-GO.

3.6 Synthesis of some vinyl derivatives of GO silyl ethers (Monomers)

In the next section, we synthesized some monomers of silyl ethers of GO. These compounds are: a) Dimethyl-vinyl-siloxy graphene oxide, b) Dimethoxy-vinyl siloxy graphene oxide, c) Diethoxy-vinyl siloxy graphene oxide. In the first derivative (a), it is possible to form one bond with graphene oxide only on one side, that is, from the carbon-chlorine bond. In the next two combinations due to having three active links from three areas, the number of formed bonds will increase, and increase the yield of reaction.

This derivative is prepared by the reaction of dimethyl vinyl-chlorine silane in the presence of triethylamine as base in THF. The method used is shown in Scheme 4. Our purpose for synthesis of these monomers is synthesis of network polymer of GO. Absorption at 2959 cm^-1 is related to vibrations –C–H. The absorption of 1726 cm^-1 is related to the vibration of C = O. Absorption in 1588 cm^-1 related to stretching vibration of C = C in aromatic ring and strong absorption in 1258 cm^-1 related to Si-C stretching vibration, strong absorption in 1084 cm^-1 related to Si-O stretching vibration, these results to indicate the synthesis of the dimethyl-vinyl-siloxy graphene oxide compound.

Scheme 4

Reaction of GO with dimethyl vinylchlorine silane.

3.6.1 Reaction of trimethoxy-vinyl-and, triethoxy-vinyl-with GO

These derivatives were synthesized similar to previous vinyl derivative. The difference is that the presence of three alkoxy groups in these compounds increases the molecular collisions with graphene oxide, thus increasing the efficiency of the silylation reactions.

Spectral data were similar to previous vinyl derivatives. In the infrared spectrum, absorption of carbon silicon and silicon oxygen bonds was more similar to the previous vinyl compound. Also, other analysis such as EDX, and XRD confirmed the reaction. An infrared analysis is shown as a typical spectrum in Fig. 5.

Fig. 5

Typical FT-IR of trimethoxy-vinyl- siloxy-GO.

3.6.2 EDX

Due to the large presence of hydroxy group at layers of graphene oxide, it is not possible to completely silylate it. EDX data confirms the presence of the silicon element in the synthesis of the obtained compound and shows that, the synthesis of trialkoxyl siloxy graphene oxide was successfully performed (Fig. 6).

Fig. 6

Typical EDX of trimethoxyvinyl- siloxy-GO.

Synthesis of trialkoxy-vinyl-siloxy-GO was done in the presence of ChCl/Urea (DES) as catalyst. The yield of the reaction was improved successfully. This catalyst is green in color, and its usage can be important in industrial chemistry.

Due to the presence of vinyl groups on the surface of graphene oxide, it is possible to form complexes with metals. This issue can be used in various applications, including metal separation. This result is potentially an important new finding.

Thermal stability of these compounds is higher than GO. Spectral data confirm this result.

3.7 Polymer synthesis

3.7.1 Synthesis of copolymers of vinyl-GO with methacrylic acid (MAA)

In the next step of our work, some network polymers of the GO derivatives was synthesized. Due to the fact that methacrylic acid has several applications in various fields, including in drug delivery systems, we prepared the copolymer of this monomer with GO with ratio of 1:1, 1:3 and 1:5 in the presence of AIBN as radical catalyst.

All of polymers were identified with FT-IR spectroscopy, peaks of functional groups of GO, and MAA, also silyl groups are seen. This evidence confirms the existence of GO, MAA, and silyl group in network polymers. Also, silyl group was identified by EDX spectroscopy, as shown in some of the spectra in Fig. 7.

Fig. 7

Typical FT-IR copolymer of Silyl-GO derivative with MAA.

3.8 Synthesis of copolymers of vinyl-GO with Styrene

In final stage of our work, we synthesized some network polymer of GO derivatives with styrene. Due to the fact that styrene has several applications in various fields, including in industrial systems, we prepared the copolymer of this monomer with GO with ratio of 1:1, 1:3, and 1:5 in the presence of AIBN as radical catalyst. Vinyl derivative that used is dimethyl vinyl GO (Scheme 5).

Scheme 5

Reaction of Dimethyl-Vinyl GO with Styrene.

The absorption in 3424 cm^-1 with low intensity is related to the stretching vibrations of the remaining OH groups of graphene oxide. The absorbance signals at 3026 cm^-1 and 3060 cm^-1 are related to the stretching vibrations of H –C = C and the absorption 2921 cm^-1 and 2963 cm^-1 are related to the stretching vibrations H–C–C. In the area of 1721 cm^-1, adsorption is related to the stretching vibration of the carbonyl group present in the structure of graphene oxide. Absorption of 1576 cm^-1 is related to the stretching vibration of C = C bond in the aromatic ring of styrene and graphene oxide. Absorption in the area of 1261^-1 related to the stretching vibrations of the Si-C bond and in the area of 1093 cm^-1 strong peak related to the stretching vibrations of the Si-O bond and absorption in the area of 799 cm^-1 of the bending vibrations of the Si bond indicates that vinyl dimethyl silicon graphene-oxide has entered to the network (Fig. 8).

Fig. 8

Typical FT-IR copolymers of vinyl-GO with styrene.

3.8.1 DSC spectrum

The destruction of polymer chains and bonded groups occurred in the range of 250–450°C and above 550°C this region is related to the complete destruction of the compound (Fig. 9).

Fig. 9

DSC spectrum copolymers of vinyl-GO with Styrene.

3.9 Comparsion of thermal stability of new material

One of the important results obtained from this research work is to increase the thermal stability as can be provided in Table 1. Bonding of silyl groups to graphene oxide causes the formation of Si-O bonds. This bond is the strongest bonds after Si-F, so the existence of silyl groups in the structure of the raw material has become important due to this effect, which is important result.

Table 1
Thermal stability of new material

Synthetic Material Thermal Stability (°C)

GO 170

Me₃Si-GO 510

t-BuMe₂Si-GO 490

Vinyl(MeO)₃Si-GO 500

(VDMS-GO)_m(Styrene)_n 395

(VTMOS-GO)_m(MAA)_n 550

Synthetic Material	Thermal Stability (°C)
GO	170
Me₃Si-GO	510
t-BuMe₂Si-GO	490
Vinyl(MeO)₃Si-GO	500
(VDMS-GO)_m(Styrene)_n	395
(VTMOS-GO)_m(MAA)_n	550

4 Conclusion

Due to the unique properties of organosilicon compounds, such as thermal stability, flexibility, and lipophilicity we synthesized some silyl ethers of GO. Incorporation of these compounds to GO cause new material that improved the properties GO. Also, network polymer of GO was synthesized. In addition, the vinyl derivatives of this compound were synthesized. The presence of vinyl in the chain of this compound can be used in some cases, including in the formation of complexes with metals, which can be important in the separation of toxic metals.

Footnotes

Acknowledgments

The authors sincerely acknowledge the Research Office of Azarbaijan Shahid Madani University for financial support. These compounds can be investigated as candidates for several works, such as analytical chemistry in separation of various materials, drug delivery system, and as a catalysis in organic synthesis.

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