(706d) Amine Functionalized Hierarchical MIL-101(Cr)/SBA-15 Composites for Enhanced CO2 Capture

Controllable synthesis of amine functionalized mesoporous silica (SBA-15) incorporated metal-organic framework (MIL-101(Cr)) with the target of maximizing CO₂ adsorption efficiency is presented. SBA-15 influenced the growth and crystallization of MIL-101(Cr) resulting in increased mesopore volume of the composite, making them more suitable for CO₂ capture. SBA-15 decorated MIL-101(Cr) composites represented an amalgamation of both type I and type IV adsorption isotherms. This is a clear indication of the hierarchical structure i.e., presence of microporosity and mesoporosity of the composites. The pore size distribution indicated the increased volume of mesopores in MIL-101(Cr)/SBA-15 (1.62 cm³/g) as compared to nascent MIL-101(Cr) (0.65 cm³/g). In contrast, SBA-15 showed higher mesopore volume as compared to the composites, inferring the growth of MOF crystals on both the external surface as well as pores of SBA-15 (Chen et al., 2017). The MIL-101(Cr)/SBA-15 composite was further modified with polyethyleneimine (PEI) to increase the basic sites facilitating chemisorption of CO₂. The basic sites of PEI, unsaturated Cr(III) metal sites, and the hierarchical pore structure of the modified composite impart chemical as well physical adsorption forces, respectively, towards CO₂ uptake.

The pure CO₂ adsorption efficiency of MIL-101(Cr)/SBA (2.1 mmol/g), estimated by gravimetric process at 30 °C and 1 bar, was significantly enhanced compared to the nascent MOF (1.3 mmol/g) or SBA-15 (0.8 mmol/g). The introduction of mesoporosity by addition of SBA-15 resulted in improved CO₂ diffusion and hence enhanced mass transfer from gas phase to solid phase. The synergistic interplay between MIL-101(Cr) and SBA-15 were responsible for the improved adsorption. Almost similar results were obtained using both volumetric and gravimetric methods. The adsorption further increased with increasing PEI loading from 10 to 25 wt.% due to chemisorption of CO₂ on amines. The lower molecular weight of PEI facilitates its uniform dispersion within the pores of the composite and aids in reduction of the diffusion resistance for CO₂ as well as increase accessibility to amine sites (Gaikwad et al., 2021a). However, increasing the amine loading to 50 wt.% drastically reduced adsorption to 0.65 mmol/g, probably due to pore blocking by PEI. The MIL/SBA/PEI-25 composite showed maximum CO2 adsorption capacity of 3.2 mmol/g. For 400 ppm CO_2,the adsorption capacity was found to be 1.6 mmol/g for 180 min of adsorption at 30 °C and 1 bar.. The CO₂ adsorption from the dilute stream was very rapid within the initial period of 20 min. The maximum adsorption capacity estimated from the Langmuir Adsorption Isotherm was estimated to be 5.9 mmol/g. The heat of adsorption (Q_ist) was found to be 52 KJ/mole indicating moderate CO₂ binding on the adsorbent surface (Sun et al., 2019). The composite MIL/SBA-15/PEI-25 displayed around 8% reduction in adsorption capacity after five cycles of reuse and regeneration. Lower amine loading of 25 wt.% resulted in facile CO₂ desorption at 65 °C under nitrogen atmosphere. This guaranteed good reusability of the composite after five cycles of reuse, as evident from their structural and morphological studies. This study envisions the synthesis of MOF based meso/micro-porous composites with tailored properties for environmental remediation applications.

Research Interests

Increased industrialization and urbanization have led to a surge in the (i) concentration of toxic contaminants in water and (ii) energy demand. In this regard, my research interests involve development of materials and membranes and for energy and environmental sustainability. This includes (i) CO₂ capture and conversion, (ii) Catalytic deconstruction of plastic, (iv) Fuel cell applications, and (iii) water and wastewater treatment. The summary of the research have been described below.

1. Capture and Conversion of CO₂

• Carbon di-oxide (CO₂) uptake from ambient atmosphere using composites of metal-organic framework (MOF), zeolites, mesoporous silica.
• Fabrication of PDMS, polyetherimide based mixed matrix membranes for CO₂ separation from N₂, air, CH₄.
• Electrochemical conversion of CO

2. CO₂/CO Hydrogenation to Value Added Products

• Synthesis of transition metals impregnated zeolites as catalysts for (i) CO₂ hydrogenation to fuels and (ii) Fisher-Tropsch reaction for conversion of syngas to hydrocarbons.
• Evaluation of catalytic efficiency in stainless steel microreactor.
• Assessment of hydrocarbon selectivity and CO/CO₂ conversion at various pressure and reactor configurations.

3. Catalytic Deconstruction of Plasma Treated Plastics

• Treatment of plastics using non-thermal DBD plasma
• Synthesis of high zeolite composites for degradation of plasma pretreated plastics at 1 atm pressure.
• Reactor design for thermo-catalytic plastic degradation.

4. Materials for Water Treatment

• Treatment of textile wastewater by adsorption and visible-light mediated Photocatalysis.
• Fluorescence sensing of pollutants from contaminated water.
• Synthesis and characterization of adsorbents, photocatalysts, and sensors (such as quantum dots, MOF composites) and maximization of process efficiency.

5. Membrane for Fuel Cell Applications

• Fabrication of MOF incorporated mixed matrix proton exchange membrane for direct methanol fuel cell application.
• The membrane showed better selectivity than commercial Nafion

6. Membranes for Removal of Pesticides and Pharamaceutical and Personal Care Products (PPCP) from Contaminated Water

• Removal of the contaminants from surface and groundwater via membrane bioreactor and ultrafiltration processes.
• Isolation, culture of bacterial strains, and optimization of biodegradation efficiency of the contaminants.
• Development of tubular composite membranes with varying pore size, morphological properties, and hydrophilic/hydrophobic properties for separation of PPCPs from contaminated water.
• Preparation of ceramic microfiltration membranes from waste tannery sludge collected from tannery industry.
• Statistical analysis of experimental parameters.