Synthesis of carbon with bimodal porosity by simultaneous polymerization of furfuryl alcohol and phloroglucinol

Highlights

• Furfuryl alcohol and phloroglucinol were simultaneously polymerized to prepare bimodal carbon.

• Varying the surfactant and acid concentration, shifted the mesopore size.

• Monomer composition needs to fall within a specific range, in order to obtain bimodal porosity.

• CO₂ oxidation of the carbon shifts both micro and mesopore size and increases the BET surface area.

Abstract

Carbon materials with bimodal porosity have shown enhanced performance in a wide variety of applications including catalysis, energy storage and fluid separation. Presence of mesoporosity is essential to lower the mass transfer limitation imposed by the microporous nature of the carbons. The synthesis approaches used to prepare bimodal carbons with controlled micro/mesopore size and narrow pore size distribution, usually involve multi step processes and the use of harsh chemicals and solvents. Herein, we present a simple one step method that can be used to synthesize carbon with bimodal pore size distribution. Simultaneous polymerization of furfuryl alcohol and phloroglucinol-formaldehyde in the presence of a structure-directing agent (Pluronic F-127) was carried out and the resultant polymer was pyrolyzed to yield the bimodal carbon. Effect of polymerization conditions such as concentrations of monomer, initiator and surfactant on the bimodal pore size distribution of the carbon was studied in detail. Pyrolyzed precursors form carbons with narrow mean micropore size of 0.5 nm and mean mesopores ranging from 3.5 to 6 nm. The range of the mesopore size could be altered by varying the polymerization parameters (acid and surfactant concentration) as well as selective oxidation using CO₂ gas.

Introduction

Porous carbon materials possess unique properties such as high surface area, large pore volume, and good thermal stability. These characteristics make them suitable candidates for different applications including catalysis, gas separation, adsorption and electrodes in electrochemical capacitors for energy storage purposes [1], [2], [3], [4]. To have acceptable performance in many applications, an interconnected porous structure with both meso and microporosity is necessary. The presence of meso and macropores facilitate mass transfer processes that are the controlling steps in many applications including membrane separation and catalysis. On the other hand, it is the micropores that provide high surface area and size selectivity at the molecular level in the adsorption process. Researchers have been using different methods to engineer the pore size and connectivity in carbon so as tailor its properties and performance for different applications [5], [6], [7], [8], [9], [10], [11], [12], [13].

Carbons derived from thermosetting resins intrinsically contain micropores in the range of 0.4–0.5 nm. The porosity in these carbons is the result of misalignment of the graphitic domains [2] and is created when the thermoset polymer is heat treated at temperatures higher than 300 °C but not above 800 °C. The micropore size of these carbons can be enlarged by either physical or chemical activation while maintaining a narrow pore size distribution [14], [15], [16], [17]. Morphology and size of the carbon particles can also be controlled using structure-directing agents, for example during emulsion polymerization of the carbon precursor [18], [19]. It has been shown that by decreasing the diameter of carbon spheres synthesized by the emulsion polymerization of furfuryl alcohol, mass transport of reactant and products inside the pores can be enhanced. This in turn improves catalytic activity in liquid phase hydrogenation reactions, while maintaining high selectivity due to microporosity [20], [21].

To form pores in the mesopore range (2–50 nm) within polymer-derived carbons, it is necessary to use a template (either soft or hard) during the synthesis of the corresponding polymer [5], [7]. Carbons produced via hard-templating route have interesting structures, but the post synthesis processing involves the use of harsh chemicals and the destruction of the rather expensive mesoporous inorganic oxide [8], [9], [10], [11]. In soft-templating approach, the micellar structure formed by the self-assembly of the template molecules (usually block copolymers) is the origin of the mesoporosity in the final material [7], [22], [23].

In applications such as heterogeneous catalysis, it has been shown that introducing a small amount of mesoporosity can improve the catalyst activity by an order of magnitude, through facilitated mass transport in mesopores. However, it is important to have narrow pore size distributions in both micro- and mesopore region to retain size and shape selectivity at the molecular level [20].

In the present study, we demonstrate that it is possible to use the soft-templating approach to develop carbons with well-defined bimodal pore distributions. This was made possible by simultaneous polymerization of two monomers (furfuryl alcohol and phloroglucinol) in the presence of a structure-directing agent (Pluronic F-127). The effects of polymerization conditions on the formation of micro- and mesopores were studied in detail and the role played by the surfactant molecule was elucidated.