Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate

G Model CATTOD-8750; No. of Pages 7

ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate Bianca L. Oliveira, Victor Teixeira da Silva ∗ Universidade Federal do Rio de Janeiro, NUCAT – Programa de Engenharia Química – COPPE, P.O. Box 68502, 21941-914 Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Article history: Received 4 September 2013 Received in revised form 5 October 2013 Accepted 6 November 2013 Available online xxx The authors would like to dedicate this paper to Professor Alberto Luiz Coimbra, in the 50th anniversary of COPPE (1963–2013), the Graduate School of Engineering of the Universidade Federal do Rio de Janeiro. Keywords: Levulinic acid Ethyl levullinate Sulfonated carbon nanotubes

a b s t r a c t Multiwall carbon nanotubes were sulfonated at different temperatures (150, 180, 210, 230, 250 and 280 ◦ C) and used as catalysts in the esterification of levulinic acid with ethanol. The materials sulfonated between 150 and 230 ◦ C presented almost the same acidity (measured by ammonia chemisorption), which was higher than that obtained for samples sulfonated at 250 and 280 ◦ C. Raman spectroscopy revealed that the treatment with sulfuric acid did not lead to the destruction of the carbon nanotubes structure, even for the higher temperature treatment. The activity results have shown that the nanotubes sulfonated below 250 ◦ C presented a specific activity higher than those sulfonated at 250 and 280 ◦ C. The association of these results with those obtained by temperature-programmed desorption of ammonia indicate that the activity in the esterification reaction is related to the number of acidic sites that desorb ammonia in temperatures around 220 ◦ C. Temperature-programmed desorption experiments suggest that there is a strong adsorption of the levulinic acid on the active sites therefore not allowing the reuse of the materials. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The need for diversification in the global energy matrix has lead to the exploration of alternative energy sources, such as solar, wind and residual biomass. Residual biomass can be used to produce energy, base chemicals and biofuels. Thus, the rational use of residual biomass not only helps to reduce the environmental problems caused by the fossil fuels but also partly meets the increasing demand for energy. Levulinic acid obtained from the acid hydrolysis of lignocellulosic residues is regarded as one of the twelve most promising molecules derived from biomass because it can be transformed into a variety of other compounds important to the chemical industry [1–4]. In particular, ethyl levulinate is a very promising compound produced via the esterification of levulinic acid with bioethanol for use as an oxygenated additive to fuels [5–7]. Esterification reactions normally use inorganic acid catalysts with H2 SO4 as the most widely employed. However, the use of mineral acids in industry is undesirable because they corrode equipment and require separation from the final product, which involves neutralization and waste disposal. Thus, heterogeneous

∗ Corresponding author. Tel.: +55 21 2562 8344; fax: +55 21 2562 8300. E-mail address: [email protected] (V. Teixeira da Silva).

catalysts that can be easily separated from the product and reused are desirable. In particular, the synthesis of ethyl levulinate via esterification of levulinic acid with ethanol using heterogeneous catalysis has attracted the attention in the last two years with many works being published in the literature [8–10]. Pasquale et al. [9] explored the potential of Well-Dawson heteropolyacid incorporated to a silica framework by the sol–gel technique as a catalyst in the esterification of levulic acid with ethanol and observed that the material presented an excellent activity and was reusable. Fernandes et al. [8] studied the use of several zeolites (HBEA, HMOR, HUSY, HMCM-22, HZSM-5) and found out that the zeolite structure plays a more important role than the acidity in the esterification of levulinic acid with ethanol. These authors also studied sulfated stania as catalyst and observed that despite the remarkable performance there was leaching of the sulfate groups into the reaction medium, thus discarding this catalyst. Yan et al. [11] employed H4 SiWO40 /SiO2 for methyl and ethyl levulinate production but the high yields obtained have to be carefully considered due to the large amounts of catalyst used. Finally, Melero et al. [12] have successfully incorporated sulfonic groups to a mesoporous silica (SBA-15) and found out that the resulting material presented an outstanding performance for the esterification of levulinic acid with ethanol. The moderate acid strength and hydrophobicity of these organosulfonic acid-modified mesoporous materials was the key of the catalytic performance,

0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.11.028

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

G Model CATTOD-8750; No. of Pages 7

ARTICLE IN PRESS B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

2

with the materials reused up to three times without losing performance. Due to their interesting chemical, physical and textural properties (for example, their high specific area and resistance to both acidic and basic media), carbon nanotubes (CNTs) have shown great potential not only as catalysts but also as supports [13]. Moreover, the potential to modify their surface to incorporate specific functional groups opens new opportunities for their use as an acid catalyst in a variety of reactions. For example, Peng et al. [14] treated multiwall CNTs with concentrated H2 SO4 at high temperatures (250 ◦ C) in order to incorporate sulfonic acid groups ( SO3 H) onto their surface and then allowing their use as acid catalysts. The authors found that this treatment led to a high sulfonic acid density on the nanotubes surface, which showed excellent activity for the esterification of acetic acid with methanol. The authors used a sulfonation temperature of 250 ◦ C but did not assess the influence lower temperatures had on the density and activity of the materials. The main objective of this work was to sulfonate multiwall CNTs using different temperatures to vary the acid site density on their surface and use them as catalysts to convert levulinic acid into ethyl levulinate. The commercial resin Amberlyst-15 was used as a reference catalyst because it also contains sulfonic acid groups on its surface and is widely used in the literature as a solid acid catalyst in reactions conducted in the liquid phase under mild conditions [8]. 2. Experimental 2.1. Catalyst preparation Commercial multiwall CNTs were purchased from Nanocyl and used as received. The ion exchange resin Amberlyst-15 was obtained from Rohm and Haas and also used as received. The CNTs sulfonation consisted of preparing a suspension of 1 g of CNTs in 100 mL of concentrated sulfuric acid, which was heated to the desired temperature and stirred for 18 h. The suspension was then diluted, filtered and washed with distilled water until neutral pH before drying the filtrate at 110 ◦ C to obtain the final material. Six different sulfonation temperatures were employed: 150, 180, 210, 230, 250 and 280 ◦ C. The resulting materials were labeled as CNT-TTT, where TTT represents the sulfonation temperature employed. The commercial nanotubes were labeled CNT. 2.2. Characterization The specific surface area of the materials was determined by N2 physisorption at −196 ◦ C via the B.E.T. method using an ASAP model 2020 (Micromeritics). Before the analysis, the samples were subjected to vacuum (6.7 × 10−6 MPa) at 150 ◦ C for 20 h. X-ray diffraction analyses were performed using a Rigaku Mini˚ in the Bragg angle flex with the K␣ radiation of Cu ( = 1.5418 A) range from 10 to 90◦ with a speed of 2◦ step−1 and acquisition time of 2 s. Raman spectra were obtained at room temperature using a LabRam HR-UV800/Jobin-Yvon spectrometer with a 1 ␮m3 resolution coupled with a He–Ne laser ( = 632 nm), thermal conductivity detector (T = −70 ◦ C) and an Olympus BX41 microscope. Three separate regions of each sample were analyzed to evaluate the homogeneity of the sample. The morphology of the samples was studied using a field emission scanning electron microscope (FEG-SEM from FEI Company, model Quanta 400) operated at 20 kV. For all samples a magnification of 50 000x was used therefore all micrographs are in the scale. High-resolution transmission electron microscopy (HRTEM) micrographs were obtained using a JEOL 3010 transmission

electron microscope operating at 300 kV. The samples were prepared by ultrasonicating the material in isopropanol and dripping the resultant suspension onto a copper grid covered with a carbon film. Temperature-programmed decomposition experiments were performed to evaluate the thermal stability of the sulfonated CNTs surfaces. A typical experiment consisted in depositing approximately 100 mg of the sample to be analyzed into a quartz U-tube reactor, heating from room temperature to 600 ◦ C under a He stream (100 mL min−1 ) at a rate 20 ◦ C min−1 and maintaining the final temperature for 2 h. During heating the effluent gases from the reactor were continuously monitored by a mass spectrometer (Pfeiffer/Prisma). The total acidity of the materials was determined via the temperature-programmed desorption of NH3 . For this purpose, the samples were first treated at 180 ◦ C for 1 h under a stream of pure He (60 mL min−1 ) before performing the NH3 adsorption using a 20.8% (v/v) NH3 /He (60 mL min−1 ) mixture at room temperature for 30 min. After this adsorption, the mixture was replaced with pure He (60 mL min−1 ) and held for 1.5 h. Finally, the NH3 desorption profile was obtained by heating the sample to 600 ◦ C at a rate of 20 ◦ C min−1 under a He stream (60 mL min−1 ), and the effluent gases from the reactor were continuously analyzed using a mass spectrometer (Pfeiffer/Prisma). The material’s acidity was determined by comparing the NH3 desorption area to that obtained from the injection of a metered amount of gas (22.32 ␮mol). 2.3. Catalytic evaluation The esterification of levulinic acid with ethanol was performed in a three-neck round-bottom flask attached to a reflux condenser cooled to 5 ◦ C with circulating water. The experiments were conducted at 70 ◦ C for 5 h using an agitation speed of 650 rpm, 2.5% (w/w) of catalyst and an excess of ethanol (molar ratio of ethanol:levulinic acid = 5:1). Under the employed conditions there was no mass transfer limitations and these conditions were chosen based on the previous work of the group [8]. During a typical experiment, the desired volume of levulinic acid and ethanol (∼100 mL) was heated to 70 ◦ C in a three-neck round-bottom flask using a glycerol bath with a temperature controller/programmer. When the temperature reached 70 ◦ C the system was homogenized for 5 min before adding the catalyst to the system. The reaction starting time was defined as the instant when the catalyst was added to the system. Samples of the reaction mixture were periodically collected using a 1 mL syringe, filtered and analyzed by HPLC using a Shimadzu (model LC-20AD) equipped with a refractive index detector (model RID-10A) and Shim-pack VP-ODS column (particle size of 4.6 ␮m, 150 mm length and 4.6 mm internal diameter). A 50% (v/v) mixture of acetonitrile in water was used at a flow rate of 0.4 mL min−1 as the mobile phase. The column temperature, injection volume and total analysis time were 40 ◦ C, 1 mL and 10 min, respectively. The levulinic acid conversion was calculated according to Eq. (1): XLA =

NEL 0 NLA

× 100

(1)

where XLA is the percentage of levulinic acid converted, NEL is the 0 is the initial number of moles of ethyl levulinate formed, and NLA number of moles of levulinic acid. The initial specific reaction rate was calculated using Eq. (2): (rEL )0 =

1 S

 dN  EL

dt

(2) t=0

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

ARTICLE IN PRESS

G Model CATTOD-8750; No. of Pages 7

B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

3

Table 1 Chemical and textural properties of the catalysts. Catalyst

Sg (m2 g−1 )

Total acidity ) (0000000000␮molNH3 g−1 solid

Site density (␮mol m−2 )

CNT CNT-150 CNT-180 CNT-210 CNT-230 CNT-250 CNT-280 Amberlyst-15

270 255 252 256 267 305 378 35

91 504 458 491 459 320 236 2360

<1 2 2 2 2 1 <1 64

Fig. 2. Raman spectra for CNT and CNT-TTT.

3. Results and discussion 3.1. Synthesis of the sulfonated carbon nanotubes

Fig. 1. X-ray diffractograms for CNT and CNT-TTT.

where (rEL )0 is the initial specific reaction rate, S is the catalyst surface area (m2 ), NEL is the number of moles of ethyl levulinate produced (mol), and t is the reaction time (min). To calculate the derivative, plots of NEL versus t were made for each catalyst evaluated. Each kinetic curve was fit with a first order decay exponential function (i.e., X(t) = A1 exp(−t/x1 ) + y0 ), and the correlation coefficients (r2 ) obtained ranged from 0.994 to 0.999, which indicates an adequate fit. The function obtained was derived as a function of time and calculated at time zero. The obtained value was divided by the catalyst area (S) in order to obtain the initial specific reaction rate. The turnover rate (TOF) was calculated using Eq. (3):



TOF =

−1 Specific reaction rate (molEL m−2 cat min ) Total acidity of the catalyst (molSO3 H )

   1(min) ×

60(s) (3)

which implicitly assumes that all of the sulfonic acid sites are equally accessible to the reagent molecules.

The specific surface areas and acidities of CNTs sulfonated at different temperatures are shown in Table 1. Similar specific surface area values were obtained for nanotubes sulfonated at 150, 180, 210 and 230 ◦ C and were close to that of the non-sulfonated material (CNT). The slight decrease observed for the specific surface area may be due to the increased interactions between CNTs caused by the sulfonic acid groups, which can form hydrogen bonds and thus reduce the pore size generated by the nanotubes entanglement. In fact, the powder materials became hard, dense solids after the sulfonation step. Nanotubes sulfonated at 250 and 280 ◦ C had an increase in the specific surface area values, which may be a result of the opening the CNT end caps due to the more severe treatment. It can be seen in Table 1 that CNT-150, CNT-180, CNT-210 and CNT-230 had similar acidity values, while the nanotubes sulfonated at 250 and 280 ◦ C had lower values. Furthermore, the original nanotubes had a low acidity that may be due to the functional groups present on its surface (e.g., carboxyl and hydroxyl) that probably have been formed during its purification step after synthesis. Although the acid treatment can damage the CNTs structure, the X-ray diffractograms shown in Fig. 1 indicates that the graphitic structure was preserved regardless of the treatment temperature. All of the diffractions present in this figure are related to graphitic carbon. Fig. 2 shows the Raman spectra of the CNTs both before and after sulfonation at various temperatures. The analysis of the spectra shows that the incorporation the sulfonic groups did not change the D and G band intensities located at 1330 and 1600 cm−1 , respectively. Thus, it can be concluded that the acid treatment did not

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

G Model CATTOD-8750; No. of Pages 7 4

ARTICLE IN PRESS B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

Fig. 3. Scanning electron microscopy (a) and transmission electron microscopy (b) micrographs for CNT.

Fig. 4. Scanning electron microscopy of the CNT-TTT samples.

cause any damage to the CNT structure even when higher temperatures were used. SEM and TEM micrographs of the CNTs samples are shown in Fig. 3. Densely tangled tubular structures with diameters ranging between 10 and 15 nm were observed. A comparison of the micrograph of the original CNT (Fig. 4a) to that of the sulfonated material (Fig. 4b–f) shows no change in morphology. The thermal stability of the sulfonic acid groups incorporated onto the CNTs surface was evaluated via a heat treatment under a He stream based on the evolution of SO2 (m/z = 64) formed from decomposition, i.e., CNT–SO3 H → CNT + SO2 + H2 O. Analysis of Fig. 5

indicates that the maximum decomposition temperature of the sulfonated groups ranged from 280 to 300 ◦ C. This result suggests that samples CNT-250 and CNT-280 have fewer acid sites on their surfaces because at these sulfonation temperatures the decomposition of the sulfonic groups is favored. Thus, the total acidity of samples CNT-250 and CNT-280 may result from the equilibrium between the incorporation to the surface and decomposition of the sulfonic acid groups. In addition to ion fragment m/z = 64 (SO2 ), signals for ions m/z = 28, m/z = 44 and m/z =18 were also observed during the thermal treatment (please see Figs. S1–S3 in supplementary

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

G Model CATTOD-8750; No. of Pages 7

ARTICLE IN PRESS B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

5

Fig. 6. Conversion vs. time plots for levulic acid esterification with ethanol for CNT, CNT-TTT, and amberlyst-15 (T = 343 K, ethanol/acid molar ratio of 5:1 and 2.5 wt% of catalyst). Table 2 Catalytic performance of the CNT, CNT-TTT, and Amberlyst-15.

Fig. 5. Sulfur dioxide (m/z = 64) evolution observed during temperatureprogrammed heating under flow of helium for CNT and CNT-TTT.

Catalyst

Conversion (%)

Blank CNT CNT-150 CNT-180 CNT-210 CNT-230 CNT-250 CNT-280 Amberlyst-15

2 2 55 50 54 52 40 20 54

a b

a Specific reaction rate (×106 )

– – 1,5 1,6 1,7 1,3 0,8 0,3 11,3 −2

Specific reaction rate = molEL molcat min −1 TOF = molEL molSO H s−1 .

−1

b

TOF (×102 )

– – 1,3 1,5 1,5 1,3 1,3 0,8 0,2 .

3

information). Many published studies have shown that several functional groups (e.g., carboxyl, phenols, anhydrides, aldehydes) may be incorporated onto a CNTs surface during their synthesis and/or post-synthesis treatments. These groups decompose under heating to release CO (m/z = 28), CO2 (m/z = 44) and H2 O (m/z = 18) [15–17]. The fact that during the thermal treatment of samples CNT and CNT-TTT it was observed CO, CO2 and H2 O formation indicates that functional groups other than sulfonic are present on the surface of all of the analyzed samples. This result explains the small acidity of the CNT sample shown in Table 1. The sulfonated samples show more intense signals for the ions m/z = 28 (Fig. S1) and m/z = 44 (Fig. S3) than the original nanotubes (CNT) and these findings suggest that a superficial oxidation of the surface occurs in addition to sulfonation during H2 SO4 treatment at high temperatures with the probable formation of carboxylic groups. 3.2. Catalytic activity CNTs sulfonated at different temperatures were used as catalyst in the esterification of levulinic acid with ethanol to produce ethyl levulinate, and their performances were compared to the commercial resin Amberlyst-15 as shown in Fig. 6. Nanotubes sulfonated at 150, 180, 210, 230 ◦ C and Amberlyst-15 achieved similar final conversions, while lower conversion values were attained using nanotubes treated at 250 and 280 ◦ C (see Table 2). Furthermore, a selectivity of 100% was achieved in all tests. Fig. 6 also shows that a low conversion was achieved even in the absence of a catalyst (blank test), which can be explained by the fact that levulinic acid itself is able to catalyze the reaction. The same conversion was obtained using the CNT sample, which indicates that it had no activity for the esterification of levulinic acid. Because

the non-sulfonated sample had a low acidity, as shown in Table 1 and Fig. 5, it can be assumed that the observed activity for the CNTTTT samples was entirely related to the sulfonic groups and not to other groups that can be present in their surface (e.g., carboxylic groups). The kinetic curves obtained for all of the catalysts allowed the specific rates of reaction displayed in Table 2 to be calculated. An analysis of these results reveals that, of the sulfonated CNTs, samples CNT-150, CNT-180, CNT-210 and CNT-230 showed similar values, which are higher than the ones obtained for samples CNT-250 and CNT-280. Thus, an activity pattern CNT-150–CNT180–CNT-210–CNT-230 > CNT-250 > CNT-280 was obtained for these catalysts. However, the Amberlyst-15 showed a much higher activity than those presented by the sulfonated CNTs. Thus, these results demonstrate a correlation between the catalyst acidity and activity because increasing the former increases the latter. The activity pattern observed for the sulfonated samples can be explained taking into account the distribution of the acid sites. Fig. 7 presents the NH3 -TPD profiles obtained for all samples studied in this work. It is possible to observe in this figure that each profile presents several maxima, indicating that sites of different strengths are present in the samples. It is noteworthy that while the maxima located around 220 ◦ C have the highest intensity for samples CNT-150, CNT-180 and CNT-210, the one located at 150 ◦ C becomes more intense for samples CNT-250 and CNT-280. This observation suggests that the acid sites related to the ammonia desorbed at 220 ◦ C are probably associated with the observed trend in catalytic activity. To investigate the hypothesis that the acid sites where ammonia desorbs at 220 ◦ C are indeed related to the activity pattern observed, the experimental NH3 TPD profiles were be

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

ARTICLE IN PRESS

G Model CATTOD-8750; No. of Pages 7

B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

6

Fig. 7. NH3 temperature-programmed desorption profiles of samples CNT-TTT.

Table 3 Percentage of the areas of the peaks obtained after de-convolution of the NH3 -TPD profiles of the CNT-TTT samples, representing acid sites of different strengths. Sample

CNT-150 CNT-180 CNT-210 CNT-230 CNT-250 CNT-280

Area (%) Peak 1 (∼150 ◦ C)

Peak 2 (∼220 ◦ C)

Peak 3 (∼280 ◦ C)

Peaks (4 + 5) (∼400 ◦ C/450◦ )

12.1 6.8 7.2 13.4 14.2 13.0

35.6 35.8 38.0 34.7 29.7 17.5

12.4 8.3 11.3 8.2 22.5 24.9

39.8 49.0 44.6 43.7 53.5 44.5

The values between parenthesis are the maximum for each de-convoluted peak.

de-convoluted into five peaks, each one related to sites of different strength. Fig. 8 shows the peaks obtained for the de-convolution of the NH3 -TPD profile of sample CNT-150 with maxima located at 150, 220, 260, 450, and 500 ◦ C. When the same procedure was applied to the other samples, similar results were obtained. Because the area below each one of the peaks obtained by de-convolution is proportional to the number of sites, calculating the percentage of the areas is possible to conclude after analysis of Table 3 that the sites related to the peaks with maxima around 220 ◦ C do indeed follow the same trend as that observed for the activity. This result strongly indicates that these are the sites responsible for the observed trend in catalytic activity. If this supposition is correct, the sites related to desorption of ammonia below 220 ◦ C are too week and the ones above 220 ◦ C are too strong. This conclusion is in agreement with previous work of our group [8] and others [12] where correlations between the strength of the sites and activity were observed for the esterification of levulinic acid with ethanol.

Fig. 8. De-convolution of the NH3 -TPD profile of sample CNT-150, representative of all CNT-TTT samples.

Table 2 also shows the TOF values of the catalysts employed during the esterification of levulinic acid. These results show that Amberlyst-15 had the lowest TOF despite the specific rate values obtained for this catalyst. This result is most likely because Amberlyst-15 has a very small specific surface area despite its larger number of acid sites. Thus, steric hindrance prevents the reactants from accessing all of the surface sulfonic acid groups, which lowers the activity per active site below the expected value because TOF calculations implicitly assume that all of the acid sites are accessible. The same problem may also occur for the CNT-250 sample. Although CNT-250 showed a lower specific reaction rate than CNT150, CNT-180, CNT-210 and CNT-230, it had a similar TOF. Table 2 shows that the CNT-250 sample had a lower site density than CNT150, CNT-180, CNT-210 and CNT-230 and may indicate that the sulfonic acid groups are better dispersed across the CNT surface, which facilitates the reactant molecules accessing the acid sites. Because catalyst reuse is an important economic factor, CNT150 was separated after the first reaction cycle by vacuum filtration, washed with ethanol, dried and used again in a second cycle. The recycled catalyst showed no activity. After filtering, washing with ethanol and drying, the CNT-150 sample was heated under a He stream and the resulting profiles are shown in Fig. 9. The profiles observed in that figure indicate that the sulfonic acid groups (m/z = 64) were still present on the material and therefore the lack of activity could not be associated to leaching; however, fragments characteristic of levulinic acid appear (m/z = 43, 45 and 29) and desorb at the same temperature as the sulfonic groups. This result strongly suggests that the activity loss of the CNT-150 was related to a strong adsorption of the reactant to the active sites. Thus, despite the CNT-150 sample having an improved activity per site than Amberlyst-15, the latter has the

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028

G Model CATTOD-8750; No. of Pages 7

ARTICLE IN PRESS B.L. Oliveira, V. Teixeira da Silva / Catalysis Today xxx (2013) xxx–xxx

7

Fig. 9. Temperature-programmed desorption profiles for sample CNT-150 before (a) and after (b) levulinic acid esterification reaction. The dotted line specifies the temperature of 285 ◦ C.

advantage of being reusable by simply washing with ethanol after each reaction cycle as shown by Fernandes et al. [8]. Because the acid groups present in the CNT-TTT are the same as those of the organosulfonic acid-modified mesoporous materials synthesized by Melero et al. [12], that is, SO3 H groups, then it can be reasoned that the strong adsorption of levulinic acid to CNT150 was most likely due to an interaction between the sulfonic groups and the CNTs. This interaction between the CNTs surface and the sulfonic groups probably changes the nature of the later, thus rendering an inadequate material to be used as catalyst for the production of ethyl levulinate. 4. Conclusions Sulfonic acid groups can be incorporated onto CNT surfaces at different temperatures. At higher temperatures, a lower concentration of acid sites is formed on the nanotube surface due to the equilibrium between the incorporation and decomposition rates of the sulfonic acid groups. Because the CNTs sulfonated at higher temperatures have a lower acid site density, their specific catalytic activity is lower than that of Amberlyst-15 for the esterification of levulinic acid with ethanol. Additionally, levulinic acid strongly chemisorbs in the sulfonic groups incorporated to the CNTs surface, which prevents the reuse of the catalyst. Acknowledgements This work was performed within the DIBANET Network, a project within the European Community’s Seventh Framework Program (FP7/2007–2013) under grant agreement no: 227248-2. B.L.O is grateful to CNPq and FAPERJ for the scholarship received. The authors also thank LN Nano for the HRTEM images.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2013.11.028. References [1] T. Werpy, G. Petersen, Top Value Added Chemicals from Biomass, US Department of Energy, Office of Scientific and Technical Oak Ridge, TN, 2004, No. DOE/GO-102004-1992 Information, http://www.nrel.gov/docs/fy04osti/35523.pdf [2] D.J. Hayes, Catal. Today 143 (2009) 138–151. [3] J.J. Bozell, L. Moens, D.C. Elliott, Y. Wang, G.G. Neuenscwander, S.W. Fitzpatrick, R.J. Bilski, J.L. Jarnefeld, Resour. Conserv. Recycl. 28 (2000) 227–239. [4] J.C. Serrano-Ruiz, A. Pineda, A.M. Balua, R. Luque, J.M. Campelo, A.A. Romero, J.M. Ramos-Fernández, Catal. Today 195 (2012) 162–168. [5] H. Joshi, B.R. Moser, J. Toler, W.F. Smith, T. Walker, Biomass Bioenergy 35 (2011) 3262–3266. [6] E. Christensen, A. Williams, S. Paul, S. Burton, R.L. McCormick, Energy Fuel 25 (2011) 5422–5428. [7] Z. Wang, T. Lei, L. Liu, J. Zhu, X. He, Z. Li, Bioresources 7–4 (2012) 5972–5982. [8] D.R. Fernandes, A.S. Rocha, E.F. Mai, C.J.A. Mota, V. Teixeira da Silva, Appl. Catal. A: Gen. 425–426 (2012) 199–204. [9] G. Pasquale, P. Vázquez, G. Romanelli, G. Baronetti, Catal. Commun. 18 (2012) 115–120. [10] Z. Li, R. Wnetrzak, W. Kwapinski, J.J. Leahy, Appl. Mater. Interfaces 4 (2012) 4499–4505. [11] K. Yan, G. Wu, J. Wen, A. Chen, Catal. Commun. 34 (2013) 58–63. [12] J.A. Melero, G. Morales, J. Iglesias, M. Paniagua, B. Hernández, S. Penedo, Appl. Catal. A: Gen. 466 (2013) 116–122. [13] P. Serp, Carbon nanotubes and nanofibers in catalysis, in: P. Serp, J.L. Figueiredo (Eds.), Carbon Materials for Catalysis, John Wiley & Sons, New Jersey, 2009, pp. 309–372. [14] F. Peng, L. Zhang, H. Wang, P. Lv, H. Yu, Carbon 43 (2005) 2397–2429. [15] J.L. Figueiredo, M.F.R. Pereira, Catal. Today 150 (2010) 2–7. [16] M. Mühler, S. Kundu, Y. Wang, W. Xia, J. Phys. Chem. 112 (2008) 16869–16878. [17] R.V. Hull, L. Li, Y. Xing, C.C. Chusuei, Chem. Mater. 18 (2006) 1780–1788.

Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028