Preparation of Micro-Mesoporous Carbons with Potassium Oxalate as Activators

Original Article:
Tannin-derived micro-mesoporous carbons prepared by one-step activation with potassium oxalate and CO2

Jenjira Phuriragpitikhon, et al.

Journal of colloid and interface science 558 (2020): 55-67.

10.1016/j.jcis.2019.09.071

In general, porous materials for adsorbed CO₂ capture require the presence of micropores (<2 nm), especially pores smaller than 1 nm. Because the narrow microporous structure exhibits a strong adsorption affinity for CO₂, resulting in strong retention of these molecules. Microporous structures can be obtained through post-synthetic activation, including physical and chemical activation. However, only the microporous structure may lead to limited mass transfer, which may affect the kinetics of CO₂ adsorption. One solution to enhance gas transport within microporous networks is to add larger pores, such as mesoporous pores (2-50 nm). In this work, the authors employ potassium oxalate (K₂C₂O₄) as an in-situ activator combined with one-step CO₂ activation to prepare condensed tannin-glyoxal-derived micromesoporous carbon (MMC) materials.

Why choose potassium oxalate as activators?

The chemical activation method involves a single-step activation in the presence of oxidation/dehydration chemicals, in which the carbon material is impregnated with a suitable activator, followed by heat treatment under an inert atmosphere. The choice of activator is very important. KOH and ZnCl₂ are the most commonly used activators for the preparation of activated carbons with high porosity and surface area. However, the process involves disadvantages such as corrosive chemicals and secondary pollution to the environment. Potassium oxalate is a less corrosive activator alternative, examples of which include:

• Potassium oxalate was used as an activator for the production of kenaf core activated carbon, and the obtained porous material had a high surface area and showed great potential as an adsorbent for dye removal.
• Using potassium oxalate as an activator, the pore structure of glucose-based porous carbon can be tuned from a microporous network to a hierarchical micro/mesoporous network, resulting in a highly microporous carbon.
• Highly microporous carbon spheres can be synthesized using potassium oxalate as an in-situ activator.

General Procedure of Potassium Oxalate-Assisted Synthesis of MMCs

In this work, the strategies employed for the potassium oxalate-assisted synthesis of MMC from tannin and glyoxal precursors, which were carbonized (Route 1) and one-step CO₂ activation (Route 2), were divided into two.

• Route 1: This route uses a soft template method to incorporate potassium oxalate into three series of phenolic resins. Dissolve mimosa tannins and Pluronic F127 completely in ethanol aqueous solution, and then add glyoxal aqueous solution. Next, add varying amounts of potassium oxalate and continue stirring. The obtained brown viscous mixture was transferred to a Petri dish and dried at 100 °C. The resulting polymer was subsequently carbonized in flowing nitrogen with a specific temperature program.
• Route 2: In this route, the phenolic resin is directly subjected to one-step CO₂ activation instead of separate carbonization in flowing nitrogen. Similar to route 1, first prepare the mixture precursor of mimosa tannin, Pluronic F127, glyoxal and potassium oxalate. The obtained polymer is then directly activated in flowing CO₂ with a specific temperature program.

Effects of Potassium Oxalate as In-Situ Activator

• By changing the amount of potassium oxalate used as an in-situ activator and a one-step CO₂ activation synthesis route, the formation of microporous structures can be effectively achieved.
• Compared with the carbonized samples with the same amount of potassium oxalate added, the specific surface area and microporosity of all activated carbons obtained by one-step CO₂ activation were significantly improved.
• The weight ratio of potassium oxalate to tannin at 4% was optimal for the synthesis of CO₂ carbonaceous sorbents and achieved the highest SBET and the largest porosity through the combination of chemical and physical activation.

Chemicals Related in the Paper:

Catalog Number	Product Name	CAS Number
ACM6487485	Potassium oxalate monohydrate	6487-48-5