Conductive ZSM-5-Based Adsorbent for CO2 Capture: Active Phase vs Monolith

Among microporous adsorbents, zeolites constitute the reference materials in CO2-capture technologies, because of their high CO2 affinity, high chemical and thermal stability, and low cost. Being synthesized in powder form, they need to be shaped in pellets or monolith to be suitable for real applications. The process has a direct effect on CO2-capture properties of the material creating, in this sense, substantial differences between lab-scale (adsorbents) and plant-scale systems (adsorbers). The ability of the honeycomb monolith to efficiently separate gases of interest arises from the properties of its single components such as an active phase and a phase resulting from the decomposition of a binder. Moreover, the textural (i.e., pore distribution and exposed surface) and structural properties (e.g., amorphization) of the active phase can be modified in the conditions adopted during the process that leads to the final artifact. These modifications can affect the CO2-capture performances of the active phase. Nevertheless, often a comparison between the active phase and its corresponding monolith is not possible. In this article, the process to obtain a zeolite/electrical conductive carbon monolith suitable for electric swing adsorption (ESA) process is described. The CO2 adsorption properties of a zeolite (H-ZSM-5) in powder form and its related shaped monolith have been compared to the uptake of other competitive gases (H2O, O-2, and N-2). The difference in the adsorption properties between the powder and the monolith has been analyzed by means of volumetric, spectroscopic, diffractometric, and microscopic techniques. This study underlined the gap between the CO2-capture performances of pure active phases, usually studied at the lab scale, and their related final artifacts, instead conceived for industrial applications. Interestingly, in the present case, the extrusion of a monolith composed by an active phase and a conductive phase had three positive effects (besides those expected) with respect to the pristine powder: (i) increase in the heat capacity of the material, (ii) decrease in the water heat of adsorption, and (iii) increase in the CO2 isosteric heat of adsorption. Whereas the first point is easily correlated to the carbonaceous phase present in the composite, the third can be related to the partial H+/Na+ exchange occurring along with the monolith preparation, as identified by infrared and energy dispersive X-ray spectroscopies. The increase in the hydrophobicity of the monolith was on the contrary related to both these factors.