Use of Eutectic Mixtures for Preparation of Monolithic Carbons with CO2-Adsorption and Gas-Separation Capabilities

With global warming becoming one of the main problems our society is facing nowadays, there is an urgent demand to develop materials suitable for CO2 storage as well as for gas separation. Within this context, hierarchical porous structures are of great interest for in-flow applications because of the desirable combination of an extensive internal reactive surface along narrow nanopores with facile molecular transport through broad “highways” leading to and from these pores. Deep eutectic solvents (DESs) have been recently used in the synthesis of carbon monoliths exhibiting a bicontinuous porous structure composed of continuous macroporous channels and a continuous carbon network that contains a certain microporosity and provides considerable surface area. In this work, we have prepared two DESs for the preparation of two hierarchical carbon monoliths with different compositions (e.g., either nitrogen-doped or not) and structure. It is worth noting that DESs played a capital role in the synthesis of hierarchical carbon monoliths not only promoting the spinodal decomposition that governs the formation of the bicontinuous porous structure but also providing the precursors required to tailor the composition and the molecular sieve structure of the resulting carbons. We have studied the performance of these two carbons for CO2, N2, and CH4 adsorption in both monolithic and powdered form. We have also studied the selective adsorption of CO2 versus CH4 in equilibrium and dynamic conditions. We found that these materials combined a high CO2-sorption capacity besides an excellent CO2/N2 and CO2/CH4 selectivity and, interestingly, this performance was preserved when processed in both monolithic and powdered form.

CuH-ZSM-5 as Hydrocarbon Trap under Cold Start Conditions

Cold start tests are carried out to evaluate the performance of copper-exchanged zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the role of copper in the performance of these zeolites. It is concluded that the partial substitution of the protons in the parent H-ZSM-5 zeolite is highly beneficial for hydrocarbon trapping due to the formation of selective adsorption sites with specific affinity for the different exhaust components. However, it is also observed that uncontrolled exchanging process conditions could lead to the presence of CuO nanoparticles in the zeolite surface, which seem to block the pore structure of the zeolite, decreasing the hydrocarbon trap efficiency. Among all the zeolites studied, the results point out that a CuH-ZSM-5 with a partial substitution of extra-framework protons by copper cations and without any detectable surface CuO nanoparticles is the zeolite that showed the best performance under simulated cold start conditions due to both the high stability and the hydrocarbon retaining capacity of this sample during the consecutive cycles.

Textural Characterization of Micro- and Mesoporous Carbons Using Combined Gas Adsorption and n-Nonane Preadsorption

Porous carbon and carbide materials with different structures were characterized using adsorption of nitrogen at 77.4 K before and after preadsorption of n-nonane. The selective blocking of the microporosity with n-nonane shows that ordered mesoporous silicon carbide material (OM-SiC) is almost exclusively mesoporous whereas the ordered mesoporous carbon CMK-3 contains a significant amount of micropores (25%). The insertion of micropores into OM-SiC using selective extraction of silicon by hot chlorine gas leads to the formation of ordered mesoporous carbide-derived carbon (OM-CDC) with a hierarchical pore structure and significantly higher micropore volume as compared to CMK-3, whereas a CDC material from a nonporous precursor is exclusively microporous. Volumes of narrow micropores, calculated by adsorption of carbon dioxide at 273 K, are in linear correlation with the volumes blocked by n-nonane. Argon adsorption measurements at 87.3 K allow for precise and reliable calculation of the pore size distribution of the materials using density functional theory (DFT) methods.