A New Approach for Controlling Mesoporosity in Activated Carbon by the Consecutive Process of Air Oxidation, Thermal Destruction of Surface Functional Groups, and Carbon Activation (the OTA Method)

Abstract

A new and simple method, based entirely on a physical approach, was proposed to produce activated carbon from longan fruit seed with controlled mesoporosity. This method, referred to as the OTA, consisted of three consecutive steps of (1) air oxidation of initial microporous activated carbon of about 30% char burn-off to introduce oxygen surface functional groups, (2) the thermal destruction of the functional groups by heating the oxidized carbon in a nitrogen atmosphere at a high temperature to increase the surface reactivity due to increased surface defects by bond disruption, and (3) the final reactivation of the resulting carbon in carbon dioxide. The formation of mesopores was achieved through the enlargement of the original micropores after heat treatment via the CO2 gasification, and at the same time new micropores were also produced, resulting in a larger increase in the percentage of mesopore volume and the total specific surface area, in comparison with the production of activated carbon by the conventional two-step activation method using the same activation time and temperature. For the activation temperatures of 850 and 900 °C and the activation time of up to 240 min, it was found that the porous properties of activated carbon increased with the increase in activation time and temperature for both preparation methods. A maximum volume of mesopores of 0.474 cm3/g, which accounts for 44.1% of the total pore volume, and a maximum BET surface area of 1773 m2/g was achieved using three cycles of the OTA method at the activation temperature of 850 °C and 60 min activation time for each preparation cycle. The two-step activation method yielded activated carbon with a maximum mesopore volume of 0.270 cm3/g (33.0% of total pore volume) and surface area of 1499 m2/g when the activation temperature of 900 °C and a comparable activation time of 240 min were employed. Production of activated carbon by the OTA method is superior to the two-step activation method for better and more precise control of mesopore development.

The Analysis of Pore Development and Formation of Surface Functional Groups in Bamboo-Based Activated Carbon during CO2 Activation

“…Commercial activated carbon is often produced by a two-step physical activation method, consisting of a char preparation step by the carbonization of a precursor in an inert atmosphere at a mid-high temperature (400–600 °C) [ 3 ] and followed by a char activation step at a relatively high temperature (800–1100 °C) to increase the internal porosity by gasification with an oxidizing agent such as steam, CO 2 , H 2 O 2 , and O 2 [ 17 , 18 , 19 , 20 , 21 ]. Porous properties and surface chemistry of the resulting activated carbon depend on the type of raw materials, the carbonization conditions (time and temperature) and activation conditions (time and temperature) [ 22 , 23 , 24 ].…”

The Use of High Surface Area Mesoporous-Activated Carbon from Longan Seed Biomass for Increasing Capacity and Kinetics of Methylene Blue Adsorption from Aqueous Solution

“…Details for the preparation of longan seed-activated carbon follow those reported in our previous work [25]. In brief, the char was first prepared by carbonizing the longan seed under the flow of N 2 at 500 • C for 90 min.…”

“…The isotherms of N 2 adsorption were shown in Figure S1 of the Supplementary Materials for the microporous-and mesoporous-activated carbons. Discus-sion on the N 2 isotherm behavior of the prepared activated carbons has been presented elsewhere [25]. The specific surface area was calculated from the N 2 adsorption isotherm data by applying the Brunauer-Emmett-Teller (BET) equation [26].…”

“…The mesopore volume was estimated by subtracting the micropore volume from the total pore volume. The pore size distributions of the derived activated carbons were determined from the N 2 adsorption isotherms data by applying the Grand Canonical Monte Carlo (GCMC) simulation [25,28,29]. The average pore size (D av ) was computed based on the equation 4V T /S, where V T is the total pore volume and S is the BET surface area, assuming the pores are cylinder.…”

“…However, to obtain both the low mass transfer resistance in pores (fast kinetics) and a high equilibrium adsorption capacity of relatively large dye molecules, activated carbon with a relatively high proportion of mesopore volume and a high surface area is obviously required [21][22][23][24]. In our previous work, we successfully prepared mesoporous-activated carbon from longan fruit seed with a high surface area (1773 m 2 /g) through a process of modified physical activation with CO 2 , which was referred to as the OTA method [25]. Therefore, the present work was aimed to study and compare the equilibrium and kinetics of methylene blue adsorption from aqueous solution by a microporous-activated carbon, and a high surface area mesoporous-activated carbon prepared from longan fruit seed.…”