HETEROATOM ENRICHED HIGH SURFACE AREA NANOPOROUS MATERIALS AS METAL-FREE CATALYSTS FOR CO2 UTILIZATION

Abstract

Unprecedented rise in the consumption of fossil fuels for the energy production has led to substantial increase in the CO2 content in the atmosphere causing the devastating environmental phenomenon “the global warming”. Even with all-out effort put together by most of the developed and developing countries could hardly make an appreciable difference to this deadly effect. Among several approaches explored in the recent time, the carbon dioxide capture and sequestration (CCS) along with carbon dioxide capture and utilization (CCU) are considered as the most promising pathways that could provide some relief to the management of CO2, while providing sustainable energy and solutions to the environment. In this direction, utilizing CO2 to make molecules/materials for energy production and also products of industrial importance can definitely contribute towards the sustainability. The thorough investigation of the literatures by the author in “CO2 utilization” has led to the interest in the development of heteroatom enriched high surface area nanoporous materials as metal free heterogeneous catalysts for this purpose. Theoretical investigations as well as experimental studies confirm the dependency of catalytic activity of these nanoporous materials on the textural properties as well as heteroatoms enrichment. In this research, heteroatoms enriched high surface area nanoporous materials (designated as MNENP, HNM, CHNM & NENP) have been synthesized using conventional as well as non-conventional synthetic approaches. These materials are characterized using state of art analytical techniques. Further, these specimens were utilized as a metal-free organocatalysts for the conversion of CO2 and epoxides to cyclic carbonates thermochemically and CH4, CH3OH and C2H5OH electrochemically, respectively. For the synthesis of cyclic carbonates traditionally, a combination of catalyst and co-catalyst systems are used. However, in this research, the use of co-catalyst can be excluded by introducing heteroatom driven multifunctionalities (OH in MNENP, P in cyclophosphazene ring of HNM & CHNM) in the framework of the high surface area nanoporous materials. The synthesized materials have high SABET of 304, 807, 1283 and 707 m2g-1 in MNENP, HNM, CHNM and NENP, respectively, with hierarchical pore structure in their frameworks. Due to the high SABET and large heteroatom (N) content (MNENP-32.6 wt%, HNM-40 wt%, CHNM-8 wt% and NENP-52 wt%), these catalysts could activate the synthesize cyclic carbonates with upto ca. 100% conversion and selectivity at 100 C temperature, 4 bar of CO2 pressure within a reaction time of 12-36 h. The high catalytic activity of these materials attributed to the presence of large no. of heteroatoms as well as high surface area. Among HNM and CHNM, the 100 % conversion was obtained by using HNM while CHNM shows only 64.5% conversion under identical experimental conditions. From this observation, it was proved that among the textural properties and large heteroatom content, the catalytic activity depends on the amount of heteroatoms content present in the framework in the nanoporous materials with moderately high specific surface area. The specific surface area may not play very significant role beyond a certain limit. A superior recyclability with a retention of 79%, 84% and 89.8% of their initial activity after five cycles in the MNENP, HNM and NENP, respectively, has further justified the use of these catalysts for the CO2 utilization. Further, due to the large heteroatoms content and high surface area, the materials MNENP and NENP are utilized as electrocatalysts for electrochemical reduction of CO2. The MNEP is found to be active for the conversion of CO2 to CH4 and CH3OH. However, NENP could convert CO2 to C2H5OH with faradaic efficiency (FE) of 68% at potential of -0.5 vs RHE and CH4 with FE of 16% at potential -0.6 vs RHE. There are very few reports available on metal free catalysts for such conversions. The superior performance of the catalysts for both thermochemical and electrochemical conversion of CO2 not only proves the versatility of the catalysts but also the applicability of these synthesized specimens for energy and environment purposes.