Thermodynamics of Adsorption of Carbon Dioxide on Various Aerogels

The emergence of nanocellulose aerogels in CO2 adsorption

Mitigating the effect of climate change toward a sustainable development is one of the main challenges of our century. The emission of greenhouse gases, especially carbon dioxide (CO2), is a leading cause of the global warming crisis. To address this issue, various sustainable strategies have been formulated for CO2 capture. Renewable nanocellulose aerogels have risen as a highly attractive candidate for CO2 capture thanks to their porous and surface-tunable nature. Nanocellulose offer distinctive characteristics, including significant aspect ratios, exceptional biodegradability, lightweight nature, and the ability for chemical modification due to the abundant presence of hydroxyl groups. In this review, recent research studies on nanocellulose-based aerogels designed for CO2 absorption have been highlighted. The state-of-the-art of nanocellulose-based aerogel has been thoroughly assessed, including their synthesis, drying methods, and characterization techniques. Additionally, discussions were held about the mechanisms of CO2 adsorption, the effects of the porous structure, surface functionalization, and experimental parameters. Ultimately, this synthesis review provides an overview of the achieved adsorption rates using nanocellulose-based aerogels and outlines potential improvements that could lead to optimal adsorption rates. Overall, this research holds significant promise for tackling the challenges of climate change and contributing to a more sustainable future.

Trends on Aerogel-Based Biosensors for Medical Applications: An Overview

Aerogels are unique solid-state materials composed of interconnected 3D solid networks and a large number of air-filled pores. This structure leads to extended structural characteristics as well as physicochemical properties of the nanoscale building blocks to macroscale, and integrated typical features of aerogels, such as high porosity, large surface area, and low density, with specific properties of the various constituents. Due to their combination of excellent properties, aerogels attract much interest in various applications, ranging from medicine to construction. In recent decades, their potential was exploited in many aerogels' materials, either organic, inorganic or hybrid. Considerable research efforts in recent years have been devoted to the development of aerogel-based biosensors and encouraging accomplishments have been achieved. In this work, recent (2018-2023) and ground-breaking advances in the preparation, classification, and physicochemical properties of aerogels and their sensing applications are presented. Different types of biosensors in which aerogels play a fundamental role are being explored and are collected in this manuscript. Moreover, the current challenges and some perspectives for the development of high-performance aerogel-based biosensors are summarized.

The effect of synthesis conditions and process parameters on aerogel properties

Aerogels are remarkable nanoporous materials with unique properties such as low density, high porosity, high specific surface area, and interconnected pore networks. In addition, their ability to be synthesized from various precursors such as inorganics, organics, or hybrid, and the tunability of their properties make them very attractive for many applications such as adsorption, thermal insulation, catalysts, tissue engineering, and drug delivery. The physical and chemical properties and pore structure of aerogels are crucial in determining their application areas. Moreover, it is possible to tailor the aerogel properties to meet the specific requirements of each application. This review presents a comprehensive review of synthesis conditions and process parameters in tailoring aerogel properties. The effective parameters from the dissolution of the precursor step to the supercritical drying step, including the carbonization process for carbon aerogels, are investigated from the studies reported in the literature.

Exploring the Conformational Equilibrium of Mefenamic Acid Released from Silica Aerogels via NMR Analysis

This study examines the influence of mefenamic acid on the physical and chemical properties of silica aerogels, as well as its effect on the sorption characteristics of the composite material. Solid state magic angle spinning nuclear magnetic resonance (MAS NMR) and high-pressure ¹³C NMR kinetic studies were conducted to identify the presence of mefenamic acid and measure the kinetic rates of CO₂ sorption. Additionally, a high-pressure T₁-T₂ relaxation-relaxation correlation spectroscopy (RRCOSY) study was conducted to estimate the relative amount of mefenamic acid in the aerogel's pores, and a high-pressure nuclear Overhauser effect spectoscopy (NOESY) study was conducted to investigate the conformational preference of mefenamic acid released from the aerogel. The results indicate that mefenamic acid is affected by the chemical environment of the aerogel, altering the ratio of mefenamic acid conformers from 75% to 25% in its absence to 22% to 78% in the presence of aerogel.

Composite modification of starch and adsorption capacity of starch microspherical aerogel

Starch microspherical aerogel (SMA) prepared by enzymatic hydrolysis of starch with α-amylase was demonstrated to be higher adsorption capacity for methylene blue. Proper cleavage of α-1,4 glycosidic bonds could enhance the adsorption capacity of SMA, while the cleavage of α-1,6 glycosidic bonds showed an opposite effect. Compared with tapioca starch (TS), α-amylase hydrolyzed starch exhibited a 9.46 % decrease in amylose content, a 25.40 % increase in adsorbability, and significant decreases in weight-average molecular weight (Mw) of different amylases. When the Mw of enzymolysis starch was 6.39 × 10⁶ g/mol, it was suitable for the preparation of SMA, and could significantly increase its adsorption capacity. The adsorbability of the crosslinked starch microspherical aerogel (CSMA) was 1.816 ± 0.026 mg/g, which was increased by 100.60 % relative to that of native starch microspherical aerogel (NSMA). CSMA had the best adsorption effect on oil and could be applied to the adsorption and removal of vegetable oil.

Decoding Carbon-Based Materials' Properties for High CO2 Capture and Selectivity

Carbon-based materials are well established as low-cost, easily synthesizable, and low regeneration energy adsorbents against harmful greenhouse gases such as CO₂. However, the development of such materials with exceptional CO₂ uptake capacity needs well-described research, wherein various factors influencing CO₂ adsorption need to be investigated. Therefore, five cost-effective carbon-based materials that have similar textural properties, functional groups, and porous characteristics were selected. Among these materials, biordered ultramicroporous graphitic carbon had shown an excellent CO₂ capture capacity of 7.81 mmol/g at 273 K /1 bar with an excellent CO₂ vs N₂ selectivity of 15 owing to its ultramicroporous nature and unique biordered graphitic morphology. On the other hand, reduced graphene revealed a remarkable CO₂ vs N₂ selectivity of 57 with a CO₂ uptake of 2.36 mmol/g at 273 K/1 bar. In order to understand the high CO₂ capture capacity, important properties derived from adsorption/desorption, Raman spectroscopy, and X-ray photoelectron spectroscopy were correlated with CO₂ adsorption. This study revealed that an increase in ultramicropore volume and sp² carbon (graphitic) content of nanomaterials could enhance CO₂ capture significantly. FTIR studies revealed the importance of oxygen functionalities in improving CO₂ vs N₂ selectivity in reduced graphene due to higher quadruple-dipole interactions between CO₂ and oxygen functionalization of the material. Apart from high CO₂ adsorption capacity, biordered ultramicroporous graphitic carbon also offered low regeneration energy and excellent pressure swing regeneration ability for five consecutive cycles.

Biorefinery Approach for Aerogels

According to the International Energy Agency, biorefinery is "the sustainable processing of biomass into a spectrum of marketable bio-based products (chemicals, materials) and bioenergy (fuels, power, heat)". In this review, we survey how the biorefinery approach can be applied to highly porous and nanostructured materials, namely aerogels. Historically, aerogels were first developed using inorganic matter. Subsequently, synthetic polymers were also employed. At the beginning of the 21st century, new aerogels were created based on biomass. Which sources of biomass can be used to make aerogels and how? This review answers these questions, paying special attention to bio-aerogels' environmental and biomedical applications. The article is a result of fruitful exchanges in the frame of the European project COST Action "CA 18125 AERoGELS: Advanced Engineering and Research of aeroGels for Environment and Life Sciences".

Starch-Based Aerogels Obtained via Solvent-Induced Gelation

In this work, the ability of several solvents to induce gel formation from amylomaize starch solubilized in dimethyl sulfoxide (DMSO) was investigated. The formed gels were subjected to solvent exchange using ethanol and dried with supercritical carbon dioxide (sc-CO₂) to obtain the aerogels. The influence of starch concentration (3–15 wt%) and solvent content (20–80 wt%) on gel formation was also studied. It was demonstrated that the gelation of starch in binary mixtures of solvents can be rationalized by Hansen Solubility Parameters (HSP) revealing a crucial hole of hydrogen bonding for the gel’s strength, which is in agreement with rheological measurements. Only the addition of water or propylene glycol to starch/DMSO solutions resulted in strong gels at a minimum starch and solvent content of 7.5 wt% and 50 wt%, respectively. The resulting aerogels showed comparably high specific surface areas (78–144 m² g⁻¹) and low envelope densities (0.097–0.203 g cm⁻³). The results of this work indicate that the HSP parameters could be used as a tool to guide the rational selection of water-free gelation in starch/DMSO systems. In addition, it opens up an attractive opportunity to perform starch gelation in those solvents that are miscible with sc-CO₂, avoiding the time-consuming step of solvent exchange.

Cost-Effective Monolithic Hierarchical Carbon Cryogels with Nitrogen Doping and High-Performance Mechanical Properties for CO2 Capture

Cost-effective nitrogen-doped monolithic hierarchical carbon cryogels with excellent mechanical properties and carbon dioxide (CO₂) adsorption performance were prepared from phenol, melamine, and formaldehyde (PMF) by the sol-gel, freeze-drying, and then, pyrolysis processes under an inert atmosphere. The morphology, mechanical properties, pore structure, and chemical characteristics of these cryogels were investigated. The results showed that the dilution ratio played a crucial role in the preparation of nitrogen-doped PMF carbon cryogels with controlled structures. The prepared carbon cryogels were a kind of monolithic materials composed of a hierarchical pore structure and had high compression properties (0.67 and 9.4 MPa for strength and modulus), porosity (97.6%), surface area (1406 m²/g), and heteroatom nitrogen content (0.98-2.09%). CO₂ adsorption capacities up to 5.75 mmol/g at 0 °C and 4.50 mmol/g at 25 °C under 1 bar were obtained, which is at a high level among N-doped carbon materials and far better than resorcinol-based carbon gels reported. These superior CO₂ adsorption capacities, high isosteric adsorption heat (Q_st), and good CO₂/N₂ adsorption selectivity were ascribed to the synergistic effect of high surface area, appropriate pore size, and also heteroatom doping.

Nanostructured ZnO as Multifunctional Carrier for a Green Antibacterial Drug Delivery System-A Feasibility Study

The physico–chemical and biological properties of nanostructured ZnO are combined with the non-toxic and eco-friendly features of the scCO₂-mediated drug loading technique to develop a multifunctional antimicrobial drug delivery system for potential applications in wound healing. Two nanostructured ZnO (NsZnO) with different morphologies were prepared through wet organic-solvent-free processes and characterized by means of powder X-ray diffraction, field emission scanning electron microscopy (FESEM), and nitrogen adsorption analysis. The antimicrobial activity of the two samples against different microbial strains was investigated together with the in vitro Zn²⁺ release. The results indicated that the two ZnO nanostructures exhibited the following activity: S. aureus > C. albicans > K. pneumoniae. A correlation between the antimicrobial activity, the physico–chemical properties (specific surface area and crystal size) and the Zn²⁺ ion release was found. Ibuprofen was, for the first time, loaded on the NsZnO carriers with a supercritical CO₂-mediated drug impregnation process and in vitro dissolution studies of the loaded drug were performed. A successful loading up to 14% w/w of ibuprofen in its amorphous form was obtained. A preliminary drug release test showed that up to 68% of the loaded ibuprofen could be delivered to a biological medium, confirming the feasibility of using NsZnO as a multifunctional antimicrobial drug carrier.