Effect of curing catalyst content on the pore structure of porous carbon obtained from phenolic resin and furfuryl alcohol

Materials Letters 110 (2013) 218–220

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Effect of curing catalyst content on the pore structure of porous carbon obtained from phenolic resin and furfuryl alcohol Zhiyong Yuan n, Yumin Zhang, Yufeng Zhou Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 April 2013 Accepted 4 August 2013 Available online 14 August 2013

Homogeneous porous carbons with narrow pore size distributions have been obtained from phenolic resin/furfuryl alcohol/ethylene glycol resin mixtures based on polymer blend carbonization. The effect of curing catalyst content (wb) on the polymerization process of the resin mixtures and the pore structure of the porous carbons has been systematically investigated. The results show that higher wb results in bigger pore size, thicker carbon skeleton and higher apparent porosity. The morphological and pore structural change of the porous carbons is induced by phase separation dynamics and reaction kinetics change on curing of initial resin compositions, by varying wb in the resin–glycol mixtures. More curing catalyst results in chemical reactions between phenolic resin and furfuryl alcohol occurring at a higher speed and the polymerization degree of the cured bodies increases. & 2013 Elsevier B.V. All rights reserved.

Keywords: Porous materials Thermal analysis Curing catalyst content Pore structure

1. Introduction Porous carbons, due to their high specific surface areas, large pore volume and low density, have been applied to water and air purification, gas separation, and catalyst support [1–3]. As applications of porous carbons become more diverse, there is a growing need to control the pore structure within the porous carbons. Compared with the activation method [4,5] and the template method [6,7], polymer blend carbonization (PBC) is simpler and more economic in preparation processes, and easier in controlling pore properties of the porous carbons [8–12]. Many researchers have reported the synthesis of porous carbon with controlled pore structure based on PBC. And the characteristics of the porous carbon synthesized by PBC appear to be defined by many parameters including the specific composition (carbon yielding material, pore-forming agent and solvent, and curing catalyst) and the ratio of the resin mixtures, as well as the thermal history (curing and carbonizing conditions) of the polymer system [10–15]. It has been proved that the morphology of the porous carbons was fully developed after polymerization [9,11]. Thus, in order to exert control over the pore structure of porous carbons, an adequate understanding of the polymerization process is very necessary. For the curing of novolac resin/furfuryl alcohol system with hexamethylenetetramine, the initial reaction produces various intermediates, and further reactions of these first-formed intermediates generate methylene linkages for chain-extension

n

Corresponding author. Tel./fax: +86 451 86403849. E-mail addresses: [email protected], [email protected] (Z. Yuan).

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.015

and cross-linking [15]. However, few studies have addressed the possible polymerization process of the resol-type phenolicformaldehyde resin/furfuryl alcohol/ethylene glycol mixtures under an acid condition, and the possible relations between polymerization process and pore structure of carbonized products. In this work, benzenesulfonyl chloride (BC) was used as a source of catalyst to adjust the polycondensation rate of phenolic resin/furfuryl alcohol/ethylene glycol mixtures, since it could undergo fast hydrolysis in the reaction media. The effect of wb on the polymerization process of the resin mixtures and the effect of polymerization process on the pore structure of the porous carbons were investigated.

2. Materials and experiment Phenolic resin and furfuryl alcohol were used as the carbon precursor, ethylene glycol as the solvent and pore-forming agent, and BC as the curing catalyst. These four materials were first stirred mechanically for 30 min at room temperature. The mixtures were then successively held at 70 1C for 2 h to finish precuring. Then, the samples were heated up to 150 1C and held for 16 h. At last, the cured samples were carbonized in nitrogen atmosphere at 800 1C for 0.5 h. Six samples were prepared, with wb varying from 2 to 12 wt% (with respect to the resin mixture), and the resulting porous carbons were designated as PC1–PC6, accordingly. Surface morphologies of the carbonized products were characterized by a scanning electron microscope (QUANTA 200, FEI, USA). Pore structure (average pore size, apparent porosity, bulk

Z. Yuan et al. / Materials Letters 110 (2013) 218–220

density and skeleton density) of the carbonized products was examined by a mercury porosimeter (Micromeritics, PoreSizer 9500, USA). Weight loss behaviors during polymerization were measured at a heating rate of 5 1C/min in flowing argon using a thermobalance (TGA/SDTA85IE, Switzerland). FT-IR spectra of the cured bodies were obtained from a Fourier transform infrared spectrometer (Spectrum One B, USA) in the wavenumber range of 4000–400 cm 1 at a resolution of 1 cm 1. The remaining weight or volume of samples after curing and pyrolysis was determined from the change in weight or volume between pre-curing and final samples.

3. Results and discussion Fig. 1 shows the morphologies of the carbonized products obtained from resin mixtures with different wb. It is clear that with wb ranging from 2% to 12%, homogeneous porous carbon with interconnected pores of nanometer level can be obtained, indicating that continuous ethylene glycol-rich phase, which endowed effective channels for removing the solvent and volatile polycondensation products during curing, was formed in all the samples during phase separation [10]. After carbonization, the polymeric resin-rich phase becomes a carbon matrix and the ethylene glycolrich phase is removed to leave pores in the carbon matrix. Fig. 1 also shows that porous carbons obtained from resin mixtures with higher wb have bigger pore size and thicker carbon skeleton, which can be proved by mercury porosimetry, as show in Table 1. Table 1 summarizes the pore structure of PC1–PC6 obtained from resin mixtures with different wb. It is clear that the average pore size and apparent porosity of the porous carbons increase from 9.3 to 210.9 nm and from 27.5% to 55.9%, respectively, with wb increasing from 2.0% to 12%, while their bulk density decreases from 0.95 to 0.57 g/cm3, accordingly. We believe that varying wb in the resin–glycol mixtures results in phase separation dynamics and polymerization reaction kinetics change on curing of initial resin compositions, which leads to morphological and pore structure change of the porous carbons [9,11]. It is found that without curing catalyst, polymerization of the resin–glycol mixture cannot occur at an appreciable rate, and the

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mixture remains as fluid after pre-curing at 40 1C for 2 h, whereas rigid body is already obtained when the resin mixtures with 12% BC are held at the same temperature for 30 min. Fig. 2a demonstrates the TG curves of the resin mixtures with different wb. It is found that the weight loss rate of the resin mixture has four distinct peaks. The second peak at about 110 1C is due to the reactions between phenolic resin and furfuryl alcohol [15], and the third and fourth peaks are due to evaporation of continuous and dispersed ethylene glycol, respectively. When more curing catalyst is used, the second peak increases in intensity, indicating that the chemical reaction between phenolic resin and furfuryl alcohol increases with increasing wb, and the peaks due to ethylene glycol evaporation shift to lower temperatures. According to Xu [12], continuous and dispersed EG-rich phases were both formed during phase separation, and the continuous EG-rich phase was nearly removed during curing, while the dispersed one was removed during pyrolysis (the fourth peak in Fig. 2a). Fig. 2b illustrates the weight and volume change of samples with different wb after curing and pyrolysis respectively. It is clear that the remaining weight of the samples after being cured at 150 1C for 16 h decreases gradually from 62.1% to 49.2% with increasing wb, while that after being carbonized at 800 1C can be steadied in the range 28.0–29.6%. It is concluded that the amount of dispersed EG-rich phase in the cured bodies with higher wb was less than that with lower wb. Fig. 2b also shows that less catalyst leads to more serious volume shrinkage after pyrolysis.

Table 1 Effect of wb on the properties of porous carbons. Sample Average pore size (nm)

Apparent porosity (%)

Bulk density (g/cm3)

Skeleton density (g/cm3)

PC1 PC2 PC3 PC4 PC5 PC6

27.5 33.2 42.9 46.0 51.6 55.9

0.95 0.87 0.76 0.71 0.65 0.57

1.31 1.32 1.31 1.31 1.34 1.29

9.3 23.9 41.9 62.2 127.7 210.9

Fig. 1. Effect of wb on the morphologies of porous carbons: (a) PC1; (b) PC2; (c) PC3; (d) PC4; (e) PC5; and (f) PC6.

Z. Yuan et al. / Materials Letters 110 (2013) 218–220

8%

60

0.03

12%

40

0.02

20

0.01

0

0.00

0

50

100

150

200

250

60

Weight remaining (%)

0.04

80

Weight loss rate (min-1)

Weight remaning (%)

60

0.05

100

50

50 After curing After pyrolysis

40

40

30 2

4

6

8

10

12

Volume remaining (%)

220

30

wc (%)

Temperature (°C)

Fig. 2. The effect of wb on the (a) TG curves of the resin mixtures and (b) weight and volume change of samples after curing and pyrolysis respectively.

4. Conclusions 2%

Transmittance

4% 8%

12%

Porous carbons with interconnected pores and narrow pore size distributions are obtained. With increasing of wb, the skeleton of the porous carbons thickens, and the average pore size and apparent porosity of the porous carbons increase from 9.3 to 210.9 nm and from 27.5% to 55.9%, respectively. Pore structure change of the porous carbons is expected to be induced by polymerization reaction kinetics change on curing of initial resin compositions, by varying wb in the resin–glycol mixtures.

Acknowledgments

4000

3000

2000

1000

400

Wavenumber (cm-1)

This work was supported by Postdoctoral Science Foundation of China (20090460900) and Key Laboratories Foundation of China (9140C49203110C4901).

Fig. 3. FT-IR spectra of the cured bodies with different wb.

Fig. 3 shows FT-IR spectra of the samples from resin mixtures with different wb. The O–H stretching (3400 cm 1) and the aliphatic C–H stretching vibrations (2920 cm 1) [16] become gradually smaller in transmittance with an increase of wb, because more continued ethylene glycol-rich phase is removed in the cured bodies with higher wb during curing, according with TG analysis. The CQO stretching (1710 cm 1), the C–O–C breathing (1204 cm 1) and the aromatic C–H out-of-plane deformation vibrations (823, 758 and 730 cm 1 respectively) [16,17] become gradually stronger in transmittance with rising wb, indicating an increase of the polymerization degree of the skeleton. An analysis of Figs. 2 and 3 shows that resin mixtures with higher wb have a relatively high rate of removing volatile polycondensation products and the solvent (ethylene glycol) during the formation of the cured samples, which results in bigger pore size and higher apparent porosity [18]. Moreover, cured samples obtained from resin mixtures with higher wb have the skeleton of higher polymerization degree resin, which strengthens the ability to withstand shrinkage during pyrolysis, which further leads to bigger pore size and higher apparent porosity [11].

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