The enhancement of porosity of carbon xerogels by using additives

* Corresponding autor. E-mail: [email protected] (A. Arenillas) Tel. +34 985119090 Fax +34 985297662 The enhancement of porosity of carbon xerogels by using additives Natalia Rey-Raap, Sandra Rodríguez-Sánchez, Isabel D. Alonso-Buenaposada, Esther G. Calvo, J. Angel Menéndez, Ana Arenillas* Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain ABSTRACT Resorcinol-formaldehyde carbon xerogels were synthesized by means of microwave heating by using precursor solutions with pH values ranging from 3 to 7 and adding various amounts of sodium sulfate, hexadecyltrimethylammonium bromide and Span80. It was found that the amount of additive that can be introduced depends to a large extent on the final pH of the precursor solution. Characterization of the porous structure of the carbon xerogels thus synthesized demonstrated that their porosity was modified by interactions between the additives and the polymeric structure of the xerogels. It is worth noting that carbonaceous materials with a pore size that could not be obtained by merely modifying the pH could be synthesized by adding different types of additive, with the result that a significant improvement of the porous properties of the carbon xerogels was achieved. The addition of sodium sulfate increased the size of the clusters and pores due to repulsive forces created between the additive and resorcinol anions. Hexadecyltrimethylammonium bromide gave rise to a dense branched structure with pores of a small size attributable to forces of attraction between the cations of the additive and resorcinol anions. In contrast, the presence of Span80 in the precursor solution produced a condensation reaction between the resorcinol and the additive, as a result of which the amount of resorcinol available for the sol-gel reaction was reduced. 2 Keywords: microwave heating, carbon xerogels, additives, porosity 1. Introduction Carbon gels are porous materials that are obtained by drying and carbonizing organic polymer-based gels [1]. The most common process for synthesizing this organic gel is by the polymerization of resorcinol with formaldehyde in the presence of a solvent following Pekala´s method [2]. The great advantage of these materials is that their final properties can be controlled and tailored to fit specific applications by modifying the process conditions. From a chemical point of view, the pH of the precursor solution is the variable that has the greatest influence, though the concentrations of water, resorcinol and formaldehyde are also key variables that determine the final porous structure of the carbon gels [1, 3-5]. In terms of the process conditions, the drying method also plays a very important role in determining the final properties [1]. Resorcinol-formaldehyde gels can be dried under supercritical conditions, giving rise to materials known as aerogels [2, 6, 7] or by the sublimation of water from the pores by a lyophilization process, resulting in the formation of cryogels [8]. In both of these processes, the surface tension forces are small and hence the gel structure shrinks only slightly, leading to materials with well-developed porous structures. Although these drying methods yield highly porous materials, they involve cumbersome solvent exchange processes and complex drying stages. The solvent can be also removed by direct evaporation, which is a simpler and quicker drying process that yields more competitive materials, known as xerogels [9-11]. However, evaporative drying causes greater shrinkage than supercritical and freezing drying, resulting in materials with less developed porous structures. It is therefore necessary to modify the 3 synthesis process in order to avoid or reduce the effect of collapse of the polymeric structure. As a solution to this problem, some authors have proposed the addition of additives to the precursor solutions of carbon gels to prevent shrinkage during drying and to obtain well developed mesoporous materials [12, 13]. The most widely used additives for synthesizing carbon gels are surfactants, which can be anionic, cationic or non-ionic. Of the possible anionic and cationic surfactants, probably the most commonly used for synthesizing RF gels are sodium dodecylbenzenesulfonate (SDBS) [6, 9, 12, 14] and hexadecyltrimethylammonium bromide (CTAB) [7, 9, 13-16], respectively. Alternatively, the non-ionic surfactants Span and Pluronic are frequently used [9, 14] . However, it is difficult to assess the effectiveness of each of these surfactants as researchers use different experimental conditions that influence the effect of these additives. Most studies on the synthesis of carbon gels by the addition of surfactants employ the emulsion technique [6, 7, 9, 15] which requires the surfactants to be added after gelation has taken place, while other authors dissolve the surfactant in a solution of resorcinol prior to the sol-gel reaction [14, 16]. Furthermore, to the best of our knowledge, all these carbon xerogels were conventionally synthesized and, there is no evidence in the literature as to what effect these additives would have on the synthesis of carbon xerogels if another heating source, such as microwave radiation, is used. Accordingly, the aim of this work is to elucidate the effect of different additives on the formation of the polymeric structure of carbon xerogels synthesized by means of microwave heating and establish whether they can be used to tailor porous properties. All of the synthesized carbon xerogels were subjected to characterization of their porous 4 structure to establish the effect of these additives when modifying both the initial pH of the precursor solution and the amount of additive. Moreover, possible chemical interactions between the additives and the polymeric structure were also investigated. 2. Experimental 2.1. Materials The resorcinol was purchased from Indspec (99 %). Formaldehyde (37 wt. % in water, stabilized by 10-15 wt. % of methanol), hexadecyltrimethylammonium bromide (CTAB, 98%) and Span 80 were supplied by Merck. Sodium sulfate (Na2SO4, 99%) was purchased from VWR International. Other chemicals used included deionized water and 5M sodium hydroxide (Analar Normapur, VWR International) solution. Sodium sulfate, instead of sodium dodecylbenzenesulfonate (the most common anionic surfactant used to synthesize carbon gels), was used in order to investigate whether the formation of micelles, a phenomenon typical of surfactants, plays a role in the modification of the pore structure of carbon xerogels or whether the repulsive forces produced by sulfate anions are sufficient to prevent the collapse of the polymeric structure during the drying of RF xerogels. 2.2. Preparation of pure RF precursor solutions Resorcinol was first dissolved in deionized water in unsealed glass beakers under magnetic stirring. After dissolution formaldehyde was added and the resulting mixture was stirred until a homogeneous solution was obtained. Five different precursor solutions were prepared with different initial pH values ranging from 3 to 7 by adding sodium hydroxide. 5 All the xerogels were synthesized from 200 ml of precursor solution, using the stoichiometric R/F molar ratio (i.e., 0.5) and a dilution ratio of 5.7. The dilution ratio parameter is defined as the molar ratio between the total amount of solvent and the total amount of reactants. The term total solvent refers to the water and methanol contained in the formaldehyde and the deionized water that is added, whilst the term reactant refers to the resorcinol and formaldehyde. 2.3. Preparation of Na2SO4-RF precursor solutions Resorcinol and sodium sulfate were first dissolved in deionized water in separate unsealed glass beakers under magnetic stirring. After dissolution, both solutions were mixed together and then formaldehyde was added. The resulting mixture was stirred until a homogeneous solution was obtained. Twenty different precursor solutions were prepared with different initial pH values ranging from 3 to 7 by adding sodium hydroxide and with four different concentrations of sodium sulfate (0.5, 2, 4 and 8 wt. %). All the xerogels were synthesized from 200 ml of precursor solution, using the stoichiometric R/F molar ratio (i.e., 0.5) and a dilution ratio of 5.7. 2.4. Preparation of CTAB-RF precursor solutions Precursor solutions containing hexadecyltrimethylammonium bromide (CTAB) were prepared following the same methodology as that described above for the preparation of Na2SO4-RF precursor solutions (section 2.3) but dissolving CTAB instead of sodium sulfate in an unsealed glass with deionized water. 2.5. Preparation of Span80-RF precursor solutions 6 Resorcinol was first dissolved in deionized water in unsealed glass beakers under magnetic stirring. After dissolution formaldehyde was added. Then Span80 was introduced and the resulting mixture was stirred until a homogeneous solution was obtained. The non-ionic surfactant was added to the resorcinol-formaldehyde solution rather than to the dissolved resorcinol due to its low solubility in water (HLB value of 4.3). Twenty different precursor solutions were prepared with different initial pH values ranging from 3 to 7 by adding sodium hydroxide and with four different concentrations of Span80 (0.5, 2, 4 and 8 wt. %). All of the xerogels were synthesized from 200 ml of precursor solution, using the stoichiometric R/F molar ratio (i.e., 0.5) and a dilution ratio of 5.7. 2.6. Synthesis of organic and carbon xerogels Each precursor solution was placed in a microwave oven (in-lab design and construction) at 85 ºC for 3 hours to allow gelation and curing to take place . After the formation of the polymeric structure, excess water was eliminated by continuing to heat the gel in the microwave oven until a mass loss of 50 wt. % was achieved. This drying step lasted 1-2 hours depending on the final pore structure of the material. After drying, the xerogels were carbonized at 700 ºC under a nitrogen flow of 150 ml/min in a horizontal tubular furnace from Carbolite Ltd. The residence time was 2 hours and the heating rate was set at 50 ºC/min. The carbon xerogels obtained were labelled CX followed by the pH value (from 3 to 7) and by the percentage of additive added (0.5, 2, 4 and 8 wt. %). An ‘S’, “C” or “NI” was appended to the name to indicate the type of additive added: sodium salt, cationic surfactant or non-ionic surfactant, respectively. 2.7. Sample characterization 7 Before being subjected to analysis, all the samples were outgassed at 120 ºC and 0.1 mbar for 8 h using a Micromeritics VacPrep 061. These degassing conditions had been previously optimized for outgassing this type of material. The porous properties of the carbon xerogels were characterized by nitrogen adsorption-desorption isotherm analysis performed at -196 ºC using a Micromeritics Tristar 3020 instrument. It is well known that pore volume measurements by nitrogen adsorption are not precise enough in the case of samples with macropores or even large mesopores. For this reason, mercury porosimetry (Micromeritics AutoPore IV) was used as a complementary technique to determine the pore size distribution. Analysis by mercury porosimetry was based on Washburn’s intrusion theory, following the methodology described elsewhere [17] (see Supporting Information for more details). The morphology of the carbon structure was observed using a Quanta FEG 650 scanning electron microscope. Thermogravimetric analysis was performed by means of a TA instrument DSC Q600 analyzer. The weight loss of each sample was monitored in the temperature range of 25‐1000 ºC by purging nitrogen at 20 ml/min and at a heating rate of 10 ºC/min. 3. Results and discussion 3.1. Na2SO4-RF carbon xerogels The amount of sodium sulfate, Na2SO4, that can be added to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions depends on the final pH value of the precursor solution. The possible pH value and sodium sulfate addition combinations are shown in Figure 1 by the coloured region. 8 As illustrated in Figure 1, 0.5 wt. % of Na2SO4 can be added to the precursor solution when the pH value is set at a value of 4 or higher. The amount of additive can be increased as the pH value increases up to a maximum of 2 wt. % for a pH value fixed at 7. When concentrations of sodium sulfate above the levels specified in Figure 1 are used, the sol-gel reaction still occurs but in an uncontrolled manner since the precursor solution starts to boil. This phenomenon is due to the polymerization reaction between resorcinol and formaldehyde explained in detail elsewhere [3]. First, resorcinol anions are formed by the abstraction of hydroxylic hydrogen [1]. These anions are more reactive than resorcinol and promote the addition of formaldehyde to form hydroxymethyl derivatives, which are the monomers necessary for polymerization to occur [3]. These derivatives combine through methylene and ether bonds to form the polymer chains that produce an interconnected three-dimensional structure [9, 10]. Increasing the initial pH of the precursor solution (i.e. adding more catalyst) leads to the formation of more resorcinol anions, which favors the addition reaction and the formation of more clusters [3]. When no catalyst is introduced into the precursor solution (pH value of 3), the SO4groups originating from the dissociation of the salt in solution contribute to the deprotonation of the resorcinol via the formation of HSO4molecules (Reaction A, Figure 1). In this case, Na2SO4 acts as the catalyst of the reaction. Moreover, the great hygroscopic capacity of this additive leads to the hydration reaction shown in the Reaction B of Figure 1. Both reactions shown in Figure 1 are exothermic and, when there is an excess of additive, the synthesis temperature may increase to above 85 ºC, causing the precursor solution to boil uncontrollably. When NaOH is introduced into the precursor solution it acts as the catalyst, i.e. it promotes the formation of resorcinol anions, preventing the deprotonation caused by 9 SO4groups. An excessive increase in temperature during the reaction is thereby avoided and carbon xerogels can be synthesized under controlled conditions. Thus, the greater the concentration of catalyst used, the greater the amount of sodium sulfate that can be added without causing the precursor solution to boil. When the catalyst concentration is large enough to prevent the deprotonation of the resorcinol by the SO4groups, the Na2SO4, which is in the solution, is in its dissociated form i.e. the solution contains SO4anions and Na+ cations. Repulsive forces are created among the SO4anions and resorcinol anions generated during the addition reaction, and as a result, the formation of methylene and ether bonds is retarded. Thus, the structure of the xerogel forms more slowly and farther away from the additive anions, as shown in Figure 2. It is clear that the addition of sodium sulfate alters the normal course of the sol-gel reaction, leading to changes in the formation of the polymeric structure and, in turn, in the final porous properties of carbon xerogels, as demonstrated in Figure 3, which shows the pore size distributions of carbon xerogels synthesized form precursor solutions with different pH values. Figure 3 also includes the pore size distribution of carbon xerogels synthesized from a precursor solution with a pH value of 7 and with different amounts of sodium sulfate. The average pore size increases from 6 nm (corresponding to sample CX-7 which does not appear in the figure due to its low pore size value) to 25 nm and 150 nm as the amount of sodium sulfate increases from 0.5 to 2 wt. % respectively. In other words, the addition of sodium sulfate to the precursor solution causes the samples to evolve from micromesoporous to micro-macroporous materials, as shown in the nitrogen adsorption- 16 It was also observed that the addition of sodium sulfate causes certain sluggishness in the development of crosslinkages due to the repulsive forces produced between the additive anions and resorcinol anions, resulting in the formation of larger clusters. The addition of a cationic surfactant (CTAB) gives rise to forces of attraction between the surfactant cations and resorcinol anions, leading to chemical interaction between the surfactant and the polymeric structure. When Span80 is added, a polycondensation reaction between this additive and the resorcinol takes place, reducing the amount of resorcinol available. Thus, the effect of adding this surfactant is similar to that of decreasing the molar ratio between the resorcinol and formaldehyde. The addition of both sodium sulfate and Span80 give rise to materials with porous properties that could not be obtained by just modifying the pH value of the precursor solution. Acknowledgements Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733 and IPT-2012-0689-420000) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version. References 17 [1] A.M. Elkhatat, S.A. Al-Muhtaseb, Advances in tailoring resorcinol-formaldehyde organic and carbon gels, Adv. Mater., 23 (2011) 2887-2903. [2] R.W. Pekala, Organic aerogels from the polycondensation of resorcinol with formaldehyde, J. Mater. Sci., 24 (1989) 3221-3227. [3] N. Rey-Raap, J. Angel Menéndez, A. Arenillas, RF xerogels with tailored porosity over the entire nanoscale, Micropor. Mesopor. Mater., 195 (2014) 266-275. [4] J. Laskowski, B. Milow, L. Ratke, Subcritically dried resorcinol–formaldehyde aerogels from a base–acid catalyzed synthesis route, Micropor. Mesopor. Mater., 197 (2014) 308315. [5] R. Zhang, W. Li, K. Li, C. Lu, L. Zhan, L. Ling, Effect of concentration of reactants on porosity of hydrogels, organic and carbon aerogels, Micropor. Mesopor. Mater., 72 (2004) 167-173. [6] M.A. Worsley, J.H. Satcher Jr, T.F. Baumann, Influence of sodium dodecylbenzene sulfonate on the structure and properties of carbon aerogels, J. Non-Cryst. Solids, 356 (2010) 172-174. [7] D. Wu, R. Fu, M.S. Dresselhaus, G. Dresselhaus, Fabrication and nano-structure control of carbon aerogels via a microemulsion-templated sol–gel polymerization method, Carbon, 44 (2006) 675-681. [8] O. Czakkel, K. Marthi, E. Geissler, K. László, Influence of drying on the morphology of resorcinol-formaldehyde-based carbon gels, Micropor. Mesopor. Mater., 86 (2005) 124133. [9] I. Matos, S. Fernandes, L. Guerreiro, S. Barata, A.M. Ramos, J. Vital, I.M. Fonseca, The effect of surfactants on the porosity of carbon xerogels, Micropor. Mesopor. Mater., 92 (2006) 38-46. 18 [10] C. Lin, J.A. Ritter, Effect of synthesis PH on the structure of carbon xerogels, Carbon, 35 (1997) 1271-1278. [11] K. Kraiwattanawong, H. Tamon, P. Praserthdam, Influence of solvent species used in solvent exchange for preparation of mesoporous carbon xerogels from resorcinol and formaldehyde via subcritical drying, Micropor. Mesopor. Mater., 138 (2011) 8-16. [12] M. Haghgoo, A.A. Yousefi, M.J.Z. Mehr, Nano porous structure of resorcinolformaldehyde xerogels and aerogels: Effect of sodium dodecylbenzene sulfonate, Iran Polym J Eng Ed, 21 (2012) 211-219. [13] K.T. Lee, S.M. Oh, Novel synthesis of porous carbons with tunable pore size by surfactant-templated sol-gel process and carbonisation, Chem. Commun. (Cambridge, U. K.), (2002) 2722-2723. [14] H. Jirglová, F.J. Maldonado-Hódar, Chemical interactions of surface-active agents with growing resorcinol-formaldehyde gels, Langmuir, 26 (2010) 16103-16109. [15] M.M. Bruno, N.G. Cotella, M.C. Miras, T. Koch, S. Seidler, C. Barbero, Characterization of monolithic porous carbon prepared from resorcinol/formaldehyde gels with cationic surfactant, Colloids Surf., A, 358 (2010) 13-20. [16] N. Nishiyama, T. Zheng, Y. Yamane, Y. Egashira, K. Ueyama, Microporous carbons prepared from cationic surfactant–resorcinol/formaldehyde composites, Carbon, 43 (2005) 269-274. [17] N. Job, R. Pirard, J.P. Pirard, C. Alié, Non intrusive mercury porosimetry: Pyrolysis of resorcinol-formaldehyde xerogels, Part. Part. Syst. Charact., 23 (2006) 72-81. 19 FIGURE CAPTIONS Figure 1. Combinations of pH values and amount of sodium sulfate that it is possible to add to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions and the chemical behavior of sodium sulfate during the sol-gel reaction between resorcinol and formaldehyde. Figure 2. Effect of Na2SO4 on the pore size of carbon xerogels synthesized by microwave heating. Figure 3. Pore size distribution of carbon xerogels synthesized with and without sodium sulfate. Figure 4. SEM images of sample CX-6 (a) and CX-6-0.5%S (b). Figure 5. Combinations of pH values and amount of CTAB that it is possible to add to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions and the effect of CTAB micelles on the formation of the polymeric structure. Figure 6. SEM images of sample CX-6 (a) and CX-6-0.5%C. Figure 7. Combinations of pH values and amount of Span80 that it is possible to add to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions (a) and condensation reaction between resorcinol and Span80 (b). 20 Figure 8. SEM images of sample CX-6 (a) and CX-6-0.5%NI (b). Figure 9. Pore size distribution of samples synthesized from precursor solutions with pH values of 5 and 6 and different amount of Span80. 21 FIGURES Figure 1. Combinations of pH values and the amount of sodium sulfate that it is possible to add to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions and the chemical behavior of sodium sulfate during the sol-gel reaction between resorcinol and formaldehyde. 22 Figure 2. Effect of Na2SO4 on the pore size of carbon xerogels synthesized by microwave heating. 23 Figure 3. Pore size distribution of carbon xerogels synthesized with and without sodium sulfate. 24 Figure 4. SEM images of sample CX-6 (a) and CX-6-0.5%S (b). 25 Figure 5. Combinations of pH values and amount of CTAB that it is possible to add to the precursor solution in order to obtain carbon xerogels under controlled synthesis conditions and the effect of CTAB micelles on the formation of the polymeric structure. 32 Results Na2SO4-RF carbon xerogels Figure S2. Nitrogen adsorption-desorption isotherm of samples synthesized from precursor solutions with pH 7 with different concentration of sodium sulfate. 33 Table S1. Porous properties of carbon xerogels synthesized from precursor solution with different pH values and different concentration of sodium sulfate. Sample SBETa (m2/g) VDUBa (cm3/g) Vpb (cm3/g) Dpb (nm) Bulk densityb (g/cm3) CX-7 666 0.26 0.02 6 1.22 CX-7-0.5%A 624 0.25 0.90 25 0.59 CX-7-2%A 538 0.21 1.87 150 0.38 CX-6 661 0.26 0.76 16 0.62 CX-6-0.5%A 623 0.25 1.74 86 0.39 CX-5 652 0.25 1.16 75 0.50 CX-5-0.5%A 580 0.22 1.19 1000 0.39 CX-4 623 0.25 1.50 135 0.42 CX-4-0.5%A 560 0.22 1.30 1100 0.36 CX-3 621 0.24 1.40 200 0.34 a Measured by nitrogen adsorption-desorption isotherm. b Measured by mercury porosimetry 34 CTAB-RF carbon xerogels Figure S3. Thermal behavior of the precipitated sample, cationic surfactant, sample OX-6 and sample OX-6-0.5%C. 35 Figure S4. Nitrogen adsorption-desorption isotherm of samples synthesized from precursor solutions with pH 4 with different concentration of CTAB. 36 Span80-RF carbon xerogels Figure S5. Thermal behavior of Span80, sample OX-6 and sample OX-6-0.5%NI. 37 Table S2. Porous properties of carbon xerogels synthesized from precursor solution with different pH values and different concentration of Span80. Sample SBETa (m2/g) VDUBa (cm3/g) Vpb (cm3/g) Dpb (nm) Bulk densityb (g/cm3) CX-6 661 0.26 0.76 16 0.62 CX-6-0.5%NI 655 0.26 0.78 19 0.60 CX-6-2%NI 636 0.25 1.23 28 0.53 CX-6-4%NI 635 0.25 1.27 35 0.48 CX-6-8%NI 570 0.22 130 47 0.47 CX-5 652 0.25 1.16 75 0.50 CX-5-0.5%NI 644 0.25 1.43 51 0.44 CX-5-2%NI 629 0.25 1.25 46 0.49 CX-4 623 0.25 1.50 135 0.42 CX-4-0.5%NI 615 0.25 1.52 75 0.43 CX-3 621 0.24 1.40 200 0.34 a Measured by nitrogen adsorption-desorption isotherm. b Measured by mercury porosimetry