Oxidized Carbon Black as an Activator of Transesterification Reactions under Solvent-Free Conditions.

A metal-free procedure for oxidation of carbon black (CB), under mild and ecofriendly conditions, is described. The procedure, based on 5/1 w/w H2O2/H2SO4, when applied to high-surface-area CB, leads to oxidation contents (O/C = 0.66) comparable to those obtained for graphite with the more aggressive and metal-based Hummers method (O/C 0.63). Oxidized nanocarbons are able to activate transesterification reactions under solvent-free conditions. Activation of transesterification reactions is much more effective by oxidized CB than by graphene oxide.

Introduction

The large diffusion of carbon-based materials as heterogeneous catalysts is essentially due to their unique set of properties such as physical stability, accessibility, easy handling, and recyclability.

Recently, the emerging application of carbon materials as cheap and metal-free catalysts has attracted much attention[1−6] mainly to graphite oxide (GO) and exfoliated GO (eGO, i.e., graphene oxide), as a solid acid in Friedel–Crafts[7,8] or aza-Michael additions,[9] Mukaiyama–Michael addition,[10] Mukaiyama aldol reaction,[11] and in esterification reaction,[12] meeting the need of environmentally benign alternatives to current organic chemical processes.

Oxidation procedures, widely explored for graphite,[13−15] are much less studied for cheaper and easily available carbon black (CB), obtained by incomplete combustion of hydrocarbon or as a byproduct of many industrial processes.[16] The well-known Hummers procedure,[15] effective for converting graphite into GO, is also effective for converting CB into oxidized CB (oCB). In particular, high oxidation degrees (O/C > 0.8) can be achieved starting from the high-surface-area CB (SACB ≈ 150 m2/g).[17] oCB exhibits functional groups (epoxides, carboxylic acids, and hydroxyl groups),[17] which are expected to promote many reactions. In particular, the large amount of hydroxyl and carboxyl groups suggests oCB as a candidate as a carbocatalyst for many important reactions. Although the Hummers procedure is effective with both graphite and CB samples leading to high-functionalized carbon materials, the not-negligible amount of metal impurities could affect the oxidized materials, generating new active sites for catalytic reactions.[6]

In the first part of this paper, we present a complete metal-free procedure for oxidation of CB under mild and ecofriendly conditions, which is effective on high-surface-area CB. This procedure leads to oCB, with sulfur contents comparable to those obtained by the Hummers method.

In the second part of the paper, we evaluate the catalytic activity of different oxidized carbon materials in transesterifications (Scheme ), that is, relevant organic reactions often used for laboratory and industrial applications. In fact, the ester-to-ester transformation is particularly useful when the parent carboxylic acids are labile and difficult to isolate. Moreover, some esters are readily or commercially available, more stable, and easy to handle, and thus they conveniently serve as starting materials in transesterification. Transesterification is relevant not only for organic synthesis but also for the production of esters of oils and fats[16] as well as for many kinds of polymerizations, for example, ring opening of lactones[18] or for curing of alkyd resins.[19]

Scheme 1

General Scheme for Transesterification Reaction

To accelerate the reaction, an acid or a base catalyst is often employed and it can be homogeneous[20] or heterogeneous[21−23] with the system. Many disadvantages of the homogeneous catalysts, such as corrosivity, sensitiveness to purity of reactants, and difficulty in their removal from generated wastewaters, limit their application and drive the catalyst choice toward the heterogeneous ones. The use of solid catalysts for chemical transformation has received much attention owing to their properties such as easy recovery, reuse, and environmental friendliness. In particular, the possibility to catalyze the reaction under acidic condition was particularly explored for biodiesel production where the inorganic solid acids, such as zeolite,[21] niobic acid,[22] and sulfonated zirconia,[24] were used. Strongly acidic ion-exchange resins, such as, Amberlyst 15 and Nafion NR50,[25] can also be effective, although their applications are limited by their high cost and low stability.

We herein report the activation, under solvent-free conditions, of many transesterification reactions by several oxidized carbon materials: GO, graphene oxide (i.e., eGO), oCB by Hummers procedure (oCB-1), and oCB by the H2O2/H2SO4 mixture (oCB-2). oCB is revealed to be a highly more efficient transesterification catalyst than eGO.

Results and Discussion

Carbon Black Oxidation

Graphite and CB samples used in the present study exhibit high surface areas (330 and 151 m2/g, respectively). The elemental analyses of high-surface-area graphite and CB, after oxidation by the Hummers method[15] or by the presently proposed method based on H2O2/H2SO4 (5/1 by volume), are compared in Table . As recently described,[17] the Hummers oxidation method works well with both graphite and CB. The presently proposed method, based on H2O2/H2SO4, even when applied on high-surface-area graphite leads to a rather low oxidation degree (O/C = 0.13), which is much lower than the one obtained by the oxidation of the same graphite by the Hummers method (O/C = 0.71). The proposed method is much more effective with high-surface-area CB and leads to an oxidation degree (O/C = 0.66), while is instead poorly effective on the CB samples of low surface area. For instance, the same procedure applied to a CB sample with SA = 36 m2/g provides an oCB sample with the O/C ratio close to 0.04.

Table 1

Elemental Analysis of the Oxidized Carbon Materials

starting material	oxidation method	sample name	C (wt %)	H (wt %)	O (wt %)	S (wt %)	O/C (w/w)
CBa	Hummers	oCB-1	50.3	2.3	41.7	5.4	0.83
CBa	H2O2/H2SO4 5/1, v/v	oCB-2	55.4	1.7	36.6	6.1	0.66
graphiteb	Hummers	GO	56.1	1.2	39.8	2.7	0.71
graphite	Hummers	eGO	59.4	0.6	37.1	2.6	0.62
graphiteb	H2O2/H2SO4 5/1, v/v		88	0.7	11.6		0.13

CB has a surface area of 151 m2/g.

Graphite has a surface area of 330 m2/g.

The elemental analysis results of Table also indicate that both oxidation procedures leave the oCB sample with a high sulfur content, nearly double than that of graphite.

It is worth adding that although the surface area of CB is noticeably reduced after oxidation by using the Hummers method, going down from 151 to 61 m2/g, the surface area of CB after oxidation with the H2O2/H2SO4 mixture is almost saved around 100 m2/g.

Additional differences emerge from the comparison of infrared (IR) spectra of eGO and oCB (Figure ). In particular, the oCB-1 sample presents well-defined peaks in the range 1300–900 cm–1 corresponding to the vibrations of ether, epoxy, alcoholic, and carboxylic groups which are much better defined than that for graphene oxide[26] (Figure , oCB-1 vs eGO), as well as two peaks at 885 and 849 cm–1 assigned to the asymmetric stretching and deformation vibrations of epoxy groups, respectively, and a 575 cm–1 peak which could be possibly attributed to peroxides.[26]

Figure 1

FTIR spectra in the range 2000–450 cm–1 of oxidized graphene (GO and eGO) and oCB, by the Hummers method (oCB-1) or by the H2O2/H2SO4 mixture (oCB-2).

Figure 2

FTIR spectrum of oCB-2 before (a) and after (b) activation of transesterification reaction.

Moreover, the well-defined peak at 1170 reveals the contribution of an increased amount of the sulfonic group.[17]

Also interesting is a comparison between Fourier transform infrared (FTIR) spectra of the oCB samples as oxidized by the two different methods. As a matter of fact, although the two spectra were substantially superimposable and the 1170 cm–1 peak related to the presence of the sulfonic group is consistent for both the oCB samples as well, the carboxylic peak at 1718 cm–1 is much less intense for the CB sample oxidized by H2O2/H2SO4 (oCB-2) than for the sample oxidized by the Hummers method (oCB-1) (Figure , oCB-1 vs oCB-2).

Transesterification Reactions

In a preliminary study, the ability of different nanocarbons to activate transesterification reactions was tested for ethyl 3-phenylpropanoate 1 and benzyl alcohol 2 under solvent-free conditions using a molar ratio of 1:2 equiv for 1 and 2, respectively. The possible catalytic activity was explored by changing the loading and temperature conditions of nanocarbons (Table ).

Table 2

Transesterification Reaction between Ethyl 3-Phenylpropanoate 1 and Benzyl Alcohol 2

entry	catalyst (wt %)	T (°C)/t (h)	yield (%)a
1		60/24	19
2		80/24	30
3	HSAG (5)	80/48	35
4	GO (5)	60/24	57
5	GO (10)	60/24	53
6	GO (5)	80/24	70
7	GO (5)	80/48	75
8	eGO (5)	80/48	85
9b	eGO (5)	80/48	64
10	oCB-1 (5)	80/18	92
11	oCB-2 (5)	80/18	99

All yields refer to the isolated products.

The reaction was performed using 1/1 molar ratio of reagents 1 and 2.

As reported in Table , in the absence of a catalyst under solvent-free conditions, the reaction has a low yield (entry 1, Table ) and even increasing the temperature to 80 °C, no significant improvement was detected (entry 2, Table ). The yield remains low also in the presence of high-surface-area graphite (entry 3, Table ), which has instead relevant catalytic activities for other reactions.[10,27]

The reaction, again under solvent-free conditions, is clearly activated by 5 wt % GO both at 60 °C and at 80 °C (entry 4 and 6, Table ) with only a minor yield increase with a prolonged reaction time (entry 7, Table ). It is worth adding that the increase of nanocarbon loading or the reduction of the amount of alcohol does not improve the yields (entry 5 and 9, Table ).

Significant increases of yields are instead obtained by replacing the crystalline GO with derived graphene oxide (eGO). In fact, as reported in Table (entry 8), the use of GO after exfoliation by ball milling positively influence the reactivity by increasing the yield up to 85%.

Additional relevant yield improvements are obtained by conducting the reaction in the presence of oCB-1, as oxidized by Hummers oxidation, providing the product 3a in 92% (entry 10, Table ), and mainly in the presence of oCB-2 powder, as obtained from the new and mild oxidation procedure, providing the product in 99% yield in only 18 h (entry 11, Table ).

To evaluate the generality of the method, some reactions with different alcohols 2a–i and esters 1a–c at 80 °C were performed with some of the oxidized carbon materials under solvent-free conditions (Scheme ).

Scheme 2

General Scope of the Reaction

As reported in Table , although transesterification reaction proceeds quite well already in the presence of eGO (entries 1–4, 7, and 8 of Table ), a reduction of reaction time and higher yields are assured by the presence of oCB-1 and oCB-2.

Table 3

Transesterification Reaction of Alcohol 2a–i with 1a–c Ester Catalyzed by eGO (5 wt %), oCB-1 (5 wt %) and oCB-2 (5 wt %)

All the yields refer to the isolated products.

It has to be noted that although in the presence of aromatic esters the reaction catalyzed by eGO proceeds with a very poor (entries 4 and 8 of Table ) and sometimes no yield (entries 5 and 6 of Table ), the same reactions catalyzed by oCB-1 and oCB-2 proceed with moderate to excellent yields with sensible reduction of reaction time. The more efficient activation of the transesterification reactions is possibly due to the higher concentration on the oCB surface of the sulfonic groups, which are more acidic than carboxylic groups.[28,29]

It has to be noted that although the O/C ratio of oCB-2 is lower than that of oCB-1 (0.66 vs 0.83), the almost same sulfur content, about 6%, associated to a higher surface area of oCB-2 (100 m2/g) than the one reported for oCB-1 (61 m2/g), makes comparable the activity of two catalysts proposed.

Therefore, to meet the growing need of more environmental and benign procedures, the use of catalyst oCB-2 obtained by the new mild and ecofriendly procedure has to be preferred. In this way, the reaction can be performed under solvent-free conditions and by using a completely metal-free catalyst obtained by a green procedure.

It is worth adding that such a great efficiency in the presence of just 5 wt % oCB-2 heating the mixture at 80 °C under solvent-free conditions is a very intriguing result. In fact, the most reported procedures[30,31] need to work for 16–24 h to provide similar results by using metals and in high catalyst loading, and similar efficiency has been previously reported just by using higher catalyst loading, at least 30 wt % sulfonated polypyrene, but in the presence of aromatic and aliphatic solvents as well as using necessarily higher temperature.[31]

Recently a sulfonated graphene oxide monolith was used for the esterification reaction giving good yields in 90 min but in the presence of catalyst loading close to 20 wt % and again in the presence of aromatic solvents.[32]

Finally, we investigated the recyclability of the recovered oCB-2 after the transesterification reaction between 3-phenylpropanoate 1 and benzyl alcohol 2. Recovered oCB-2 has a strongly reduced activation ability, with the yield reduced from 99% down to 45%, possibly because of a thermal desulfonation with consequent reduction of acidity. In fact, as reported in the IR spectrum of Figure , many vibrational peaks of oCB-2 disappear after the reaction (b vs a), indicating a decrease of concentration of the oxidized functional groups, mainly sulfonic groups, as also confirmed by the elemental analysis (sulfur content becoming less than 1.5 wt %).

Interestingly, the recovered oCB-2 sample can be again oxidized under mild conditions by using the H2O2/H2SO4 5/1 mixture. The reoxidized oCB-2 sample can be reused for the activation of transesterification reactions, providing an efficiency very close to that of the first run (95% yield) (Figure ). The activator is cheap and can be recovered and reoxidized by green and ecofriendly procedures.

Figure 3

oCB recovering and recycling.

Conclusions

A metal-free procedure for oxidation of CB, under mild and ecofriendly conditions, is described. The procedure, based on 5/1 w/w H2O2/H2SO4, leads, for high-surface-area CB, to an oxidation degree comparable to the one obtained with the more aggressive and metal-based Hummers method with the analogous sulfur content. This procedure is, however, poorly effective on low-surface-area CB as well as on graphite, even if exhibiting a very high surface area.

All considered oxidized nanocarbons are able to activate a broad scope of transesterification reactions under solvent-free conditions. Activation occurs already by graphene oxide, but it is much more pronounced by oCB, providing the product in reduced reaction times. The recovered oCB-2 can be again oxidized under mild conditions by using the H2O2/H2SO4 5/1 mixture. The reoxidized oCB-2 sample can be reused for the activation of transesterification reactions, providing an efficiency very close to that of the first run.

Materials and Methods

Materials

High-surface-area graphite, with Synthetic Graphite 8427 as trademark, was purchased from Asbury Graphite Mills Inc., with a minimum carbon wt % of 99.8.

The used CB samples, with surface area of 151 m2/g, were purchased from Cabot.

All reagents were purchased from Sigma-Aldrich and used without any further purification. Thin-layer chromatography was performed on a silica gel 60 F254 0.25 mm glass plates Merck and nonflash chromatography was performed on a silica gel (0.063–0.200 mm) (Merck). All 1H NMR and 13C NMR spectra were recorded with a DRX 400 MHz instrument, by using CDCl3 (δ = 7.26 ppm in 1H NMR spectra and δ = 77.0 ppm in 13C NMR spectra) as a solvent (400.135 MHz for 1H and 100.03 MHz for 13C).

For the products 3aa, 3ac, 3ae, and 3ad, the 1H NMR and 13C NMR match with those reported in the literature.[33−37]

Oxidation of Graphite and CB with the Hummers Procedure

GO and oCB were prepared by the Hummers method.[15] Sulfuric acid (120 mL) and sodium nitrate (2.5 g) were introduced into a 2000 mL three-neck round-bottomed flask immersed into an ice bath, and 5 g of carbon samples was added under magnetic stirring. After obtaining a uniform dispersion, 15 g of potassium permanganate was added very slowly to minimize the risk of explosion. The reaction mixture was thus heated to 35 °C and stirred for 24 h. Deionized water (700 mL) was added in small amounts into the resulting black and dark green slurry of CB and graphite, respectively, under stirring and, finally, gradually adding 5 mL of H2O2 (30 wt %). The obtained sample was poured into 7 L of deionized water and then centrifuged at 10 000 rpm for 15 min with a Hermle Z 323 K centrifuge. The isolated GO and oCB powders were first washed twice with 100 mL of a 5 wt % HCl aqueous solution and subsequently washed with 500 mL of deionized water. Finally, the powders were dried at 60 °C for 12 h. About 7.5 g of GO and 6.5 g of oCB powders were obtained. The obtained oxygen/carbon weight ratio is 0.71 for GO and 0.83 for oCB.

Oxidation of CB by H2O2/H2SO4

Oxidation of commercial CB was carried out in 5/1 H2O2/H2SO4. The CB (500 mg) was added into 500 mL of 30 wt % H2O2 in water under magnetic stirring. Then, H2SO4 was slowly dropped into the uniform dispersion. After 24 h, the mixture was diluted with 1.5 L of deionized water and then centrifuged at 10 000 rpm for 10 min with an Awel centrifuge. The isolated oCB powders were washed with 500 mL of deionized water. Finally, the powders were dried at 60 °C for 12 h. About 1.3 g of oCB (oCB-2) powders was obtained with a oxygen/carbon weight ratio of 0.66.

Exfoliation of GO by Ball Milling

The GO powders were introduced in a 125 mL ceramic jar (inner diameter of 75 mm) together with the stainless steel balls (10 mm in diameter) and were dry-milled in a planetary ball mill (Retsch GmbH 5657 Haan) for 2 h with a milling speed of 500 rpm and a ball-to-powder mass ratio of 10:1.

Transesterification Reactions

The reactions were carried out in a flask. Details were given for the reaction of ethyl 3-phenylpropanoate (1 mmol, 178.2 mg, 176 μL) and benzyl alcohol (2 mmol, 216.3 mg, 208 mL). The reactants were added to the catalyst (5 wt % compared to the ester) at 80 °C. The reaction was stirred at the same temperature for the time indicated. The reaction mixture was extracted with ethylacetate (AcOEt), and the combined organic phase was dried (Na2SO4) and concentrated. The residue was purified by column chromatography (silica gel, petroleum ether/EtOAc, gradient) to obtain the pure product.