CO2 Adsorption on Activated Carbon Honeycomb-Monoliths: A Comparison of Langmuir and Tóth Models

2.1. Preparation and Characterization of Materials

Preparation of monolith structures usually requires additional binding components to assure a better particle adhesion during pressure agglomeration for a higher mechanical resistance. However, the binding components can produce a significant decrease of porosity in the resulting material. For this reason, the preparation of carbon monoliths in this work was carried out without any binder material [ 20 ].

A usual procedure for preparing monolith adsorbents without binding materials is based on the use of an activating agent that can interact with the lignocellulosic precursor, producing dehydration reactions as well as degradation and condensation of the biopolymer molecules, becoming substances able to produce agglomeration of the particles during the pressure treatment of the solid [ 21 ].

The impregnation step was carried out by adding the granular solid precursor to calcium chloride, zinc chloride or phosphoric acid solutions. In these treatments with an excess of liquid solution, part of the organic products resulting from the chemical transformations and present in the solution can be concentrated by partial evaporation of the solvent during heating, resulting in some binding properties similar to a tarr product, and thus becoming a kind of binding component. In this way, the impregnated precursor materials show a slurry-like texture and a plastic behavior as a result of the chemical treatment which helps preparing the monoliths when applying pressure, resulting in a significant reduction of the interparticle space and as a consequence a remarkably consistent final material. The monolith structures thus obtained were stable during the following steps of thermal treatment and washing.

2 adsorption capacity in the experimental conditions studied. Sample ACMP48, and to a less extent sample ACMCa2, show a more open curvature at low relative pressures (

P

/

P

0 < 0.1), which indicates a wider pore size distribution, while sample ACMZn48 shows a steep curve with a more closed curvature in this pressure range, characteristic of highly microporous materials.

P

/

V(P

0

-P)

to

P

/

P

0, where

V

is the adsorbed volume. Correlation coefficients obtained (

R

2) for the three samples were 0.9999, 0.9995 and 0.9998, respectively.

All monoliths samples were characterized by nitrogen adsorption at 77 K. Figure 1 shows the adsorption isotherms corresponding to some of the samples, particularly those of highest adsorption capacity of each series. The isotherms correspond to type I according to IUPAC [ 22 ]. As observed, monolith samples named ACMCa2, ACMZn48 and ACMP48 (numbers “2” and “48” correspond to the percentage concentration of the activating agent used, as explained later) have the highest narrow micropore volume, lower than 0.7 nm, and are therefore those with the largest COadsorption capacity in the experimental conditions studied. Sample ACMP48, and to a less extent sample ACMCa2, show a more open curvature at low relative pressures (< 0.1), which indicates a wider pore size distribution, while sample ACMZn48 shows a steep curve with a more closed curvature in this pressure range, characteristic of highly microporous materials. Figure 1b shows the results of linear fitting to BET equation for samples ACMCa2, ACMP48, ACMZn48, relatingto, whereis the adsorbed volume. Correlation coefficients obtained () for the three samples were 0.9999, 0.9995 and 0.9998, respectively.

2·g−1 and pore volume values of 0.26–0.64 cm3·g−1. As observed, these parameters depend on the activating agent used, increasing in the order ACMZn < ACMP < ACMCa. The treatment with CaCl2 produces a stronger degradation of the precursor, so that after carbonization higher porosity and surface area are obtained for the monolith. In the case of samples ACMP and ACMZn, higher surface areas and pore volumes are reached with increasing concentrations of the activating agent solution. However, for samples treated with CaCl2 (ACMCa) the trend is the opposite, as observed in Table 1 summarizes the textural properties of all samples, corresponding to the 12 carbon monoliths prepared, which have surface area values in the range 660–1700 m·gand pore volume values of 0.26–0.64 cm·g. As observed, these parameters depend on the activating agent used, increasing in the order ACMZn < ACMP < ACMCa. The treatment with CaClproduces a stronger degradation of the precursor, so that after carbonization higher porosity and surface area are obtained for the monolith. In the case of samples ACMP and ACMZn, higher surface areas and pore volumes are reached with increasing concentrations of the activating agent solution. However, for samples treated with CaCl(ACMCa) the trend is the opposite, as observed in Table 1 ; this agent acts as a template in the developed porosity [ 23 ], after which with a washing process the agent is removed. With the increase of the Ca content in the carbonaceous materials a block is generated in the material, due to the increasing difficulty to remove the agent in the washing stage, causing a decrease in the textural characteristics of activated carbon. Likewise, it is possible that at high concentrations there is a sudden drop in both the surface and pore volume due to the partial retraction / destruction of the porous structure generated by the Ca content.

It was determined that the volume of narrow micropores by CO2 adsorption is between 0.23–0.43 cm3·g−1 for all monoliths; this value is more than 50% of the total microporosity, that which promotes the gas adsorption on the materials of interest. It is observed that with increasing surface area, the proportion of narrow microporosity increase, in each series, finding a maximum value of 0.43 cm3·g−1 on the MCa2 honeycomb which has the highest CO2 adsorption capacity.