Microporous carbon spheres derived from resorcinol-formaldehyde solutions. A new approach to coat supports

Abstract Microporous carbon spheres of different morphology and porosity were synthesized from resorcinol-formaldehyde solutions by a simple and fast procedure. Polymeric spheres were shaped by means of microwave heating. Carbonization and activation with carbon dioxide were then applied to obtain the intended final carbon spheres. The influence of the pH, heating time and thermal treatments on the morphology and porosity of the carbon spheres was investigated. It was found that the size of the spheres, can be easily controlled during the synthesis process, specifically by modifying the pH of the precursor solution. An increase in the pH value from 3 to 5 led to carbon spheres with sizes of 4 μm and 3.5 μm, respectively, whereas time seemed to have no effect. These results have been attributed to the chemical mechanisms of the polymerization reaction. On the other hand, microporosity was tailored during the thermal treatments. Carbon spheres with surface areas of 630 m 2 /g and 1500 m 2 /g were obtained by applying carbonization and physical activation, respectively. Furthermore, the synthesis method proposed allows to obtained liquid polymerized inks that can be further used to coat ceramic supports by a simple spray-drying process, which enhances the potential of these materials for several applications.


Introduction
Over the last few decades, there has been an explosive growth in the synthesis, characterization and application of carbon spheres, due to the fact that their size, 2 morphology, porosity and chemical properties can be tailored for specific purposes. The final properties of carbon spheres are strongly influenced by the synthesis conditions under which they are prepared. Chemical vapour deposition (CVD), polymerization using templates, hydrothermal carbonization, inverse emulsion polymerization, extension of the Stöber method, etc. are some of the best-known examples of the wide list of methods studied. The reader is referred to a number of excellent reviews on these synthesis techniques, and on properties and applications of carbon spheres, which have already been published [1][2][3][4][5]. Another aspect worth bearing in mind, as it amends the final properties of carbon spheres, are the precursors employed, which comprise but are not limited to polymers, biomass-derived carbons, phenolic resins, benzene derivatives and other heterocyclic aromatic organic compounds [2,4,6]. Of these, phenolic resins have attracted most attention as carbon sphere precursors [7][8][9][10][11][12][13][14][15][16][17], and particularly resorcinol-formaldehyde (RF) precursor solutions which, as has been extensively reported, generally give rise to carbon gels [18][19][20][21][22][23].
A number of studies can be found in the literature focused on the use of RF solutions as precursors of carbon spheres, for the preparation of which several methods have been reported. The modified Stöber method has been one of the most employed techniques to prepare carbon spheres from RF solutions [8,12,13,15,16]. This method involves the use of additives such as ammonia, ethanol or templates, as well as long processing times. Another method that is widely used is that of inverse emulsion, where not only the use of additives is required but also a full control over the viscosity [11,17].
Recently, hydrothermal treatments has made it possible to synthesize carbon spheres easily without the need for additives [9]. However, this process is still tedious as long times are required. In order to reduce the processing time, ultrasounds and microwave radiation have been employed as suitable techniques to obtain carbon spheres [7,10,14]. Although these methods allow the size and number of carbon spheres to be tailored via a rapid and easy route, the use of surfactants and full control of viscosity also seem to be essential requirements in these processes.
Leaving aside the above mentioned drawbacks, each of these techniques leads to carbon spheres with different final properties, and hence, the choice of the appropriate synthesis method will depend on the application for which the carbon spheres are intended. The widely-recognized advantages of RF solutions as carbon precursors (which include the possibility of tailoring their porous properties or of doping them with metals and 3 heteroatoms [11,24]), confer on them great potential for preparing carbon spheres that offer optimum performances in a wide variety of applications, such as adsorption [1,25], ultrafiltration [26], catalysis [16] or electrochemical systems [9,13,15,17]. In some of these applications carbon spheres are used as coating of surfaces, nanoparticles or supports and, in these cases, spray-drying methods are preferable. This process involves spraying the precursor solution onto a support, which is then dried to give rise to the formation of polymeric spheres on its surface [1]. Finally, the polymer thus obtained is carbonized to yield the intended carbon spheres. In order to use the spraydrying method, liquid precursor solutions are needed, which rules out many of the aforementioned methods. Furthermore, the studies so far published on the spraying method, again refer to the need for additives [27][28][29]. Consequently, although several attempts to achieve a facile synthesis methodology for carbon spheres have been made, there is still an ongoing need for an alternative low-cost and easily scalable method to prepare liquid polymerized precursors that could be used in spray-drying processes for producing carbon spheres.
In the present work, microwave-assisted polymerization of resorcinol and formaldehyde was performed to obtain polymeric spheres, which were then transformed into carbon spheres. The novelty of the method proposed lies in the fact that i) the use of additives is avoided and ii) the synthesis time is greatly reduced. The process is thus simplified and become scalable. Furthermore, the method presented here has demonstrated to be an easy and suitable way to coat surfaces of ceramic solid catalysts, thereby extending the range of applications of carbon spheres prepared from RF precursor solutions.
Resorcinol was first dissolved in deionized water in an unsealed glass beaker under magnetic stirring. After dissolution, formaldehyde was added and the mixture was stirred until a homogeneous solution was obtained. Finally, the pH value was adjusted by adding the NaOH solution. The concentration of each reagent was selected on the basis of results previously reported. These concentrations are reported in the literature to be related to the pH of the precursor solution, the dilution ratio (D) and the molar ratio between the resorcinol and the formaldehyde (R/F) [30]. It should be highlighted that the dilution ratio refers to the molar ratio between the total solvent and the main reagents. Rey-Raap et al. provided an interesting illustration of the combinations of pH-D whereby the synthesis route via microwave heating did not lead to the formation of a solid material [18]. Based on that illustration, in the present work the precursor solutions were therefore prepared with pH values of 3 and 5, using the stoichiometric R/F molar ratio (i.e., 0.5) and a dilution ratio (D) fixed at 17.
Once prepared, each precursor solution was placed in a multimode microwave oven (inlab design and constructed [31]) at 85 ºC. The heating time was selected on the basis of previous experiments performed, in some of which microwave energy consumption was recorded as a function of time [10,19]. The time values selected were 2 and 3 hours to allow a polymeric ink (RF-ink) to be obtained instead of a wet-solid gel. Each polymeric RF-ink was air-dried at 85 ºC for 3 hours until a solid thin-layer (i.e. flake) was obtained. These materials were labelled RFS, in reference to resorcinolformaldehyde spheres, followed by the pH value and the duration of the microwave heating in hours. Corning Incorporated. Briefly, the supports were introduced into a glass flask containing 50 ml of hot RF-ink for 3 minutes. The impregnated support was then centrifuged for 15 seconds at 600 rpm and finally air-dried at 85 ºC for 3h. In order to determine the optimum number of spin-coating cycles required, one and five cycles were performed.

Carbonization and physical activation
The RFS samples and coated supports were heated for 2h under a nitrogen atmosphere at 700 ºC and under a CO 2 atmosphere at 1000 ºC in order to obtain carbonized (CS) and activated (ACS) carbon spheres, respectively.

Sample characterization
All the samples, RFS and supports (dried, carbonized and activated), were first outgassed at 0.1 mbar and 120 ºC overnight in a Micromeritics VAcPrep 0.61 in order to remove humidity and other physisorbed gases. Their porous properties and morphology were then analyzed.
Nitrogen adsorption-desorption isotherm analysis were performed at -196 ºC using a Micromeritics Tristar 3020 instrument. The BET surface area (S BET ) and micropore volume (V DUB-N2 ) of the samples were determined by applying the BET equation and the Dubinin-Radushkevic method to the N 2 adsorption isotherms, respectively. Envelope pycnometry (Geopyc1360 envelope density analyzer from Micromeritics) was also employed to further analyze other porous properties such as bulk density and porosity.
All samples were synthesized and characterized in duplicate to ensure reproducibility The morphology of all the materials was examined using a Quanta FEG 650 scanning electron microscope. The samples were previously attached to an aluminum pin using conductive double-sided adhesive tape. An accelerating voltage of 25 kV and a secondary electron detector EDT (Everhart-Thornley) were used in all the analyses.
This technique was also employed to determine the size of the sphere. However, coulter analysis (LS13320 Beckman fitted with an ALM water module) was also used to verify these data. Coulter analysis were performed in triplicate. Initially, resorcinol anions are formed due to the abstraction of hydroxyl hydrogens, which generally allows the addition of formaldehyde in positions 2 and 4 (addition reaction) [30]. At the same time, the hydroxymethyl derivatives lose OH groups to form benzyl-type cations (condensation reaction). Each cation reacts with a benzene ring of another molecule giving rise to methylene and ether bonds. As the reaction proceeds, the number of bonds between the rings increases to form the polymer backbone, which leads to cross-linked polymer clusters, also named primary particles [18,24]. Unlike in the synthesis of organic gels, these primary particles do not aggregate and crosslink with each other due to the high D value used to prepare the precursor solutions. Indeed, the large amount of water (high dilution ratio value, D = 17) increases the distance between primary particles, and hence, prevents the solution from reaching the gelation point [30]. Therefore, isolated organic spheres are formed, i.e. RF-inks (Figure 3a) instead of organic wet-solid gels (Figure 3b and 3d   Furthermore, the process proposed in the present study allows the preparation of carbon spheres with a controlled morphology and porosity via a simple and easily scalable method. The time required to prepare carbon spheres is shorter in comparison with previous methods reported in literature. The synthesis method proposed also yields RF inks with a fluidity suitable for coating different supports, not only by impregnation but also by spraying methods. These results evidence the significance of the method as this coating procedure can be applied to a wide range of supports regardless of their shape.

Resorcinol-formaldehyde spheres (RFS)
Accordingly, it is to be hoped that the use of carbon spheres increases in the near future, taking advantage of the improvements developed in the presented work.