Resorcinol-formaldehyde (RF) xerogels are organic materials have been widely studied due to their industrially relevant characteristics, through which, RF gels have significant potential to be tailored to specific applications. Xerogel properties have been tailored, within this study, by altering the synthesis procedure with a focus on monomer concentrations, catalyst to monomer ratio, and the introduction of a nitrogen-rich precursor, thereby incorporating nitrogen into the structure to additionally affect the chemical properties of the final gel. Melamine (M) is used as the source of nitrogen, partially replacing the resorcinol (R) typically used, and resulting in a melamine-resorcinol-formaldehyde (MRF) gel; synthesis was facilitated by a sodium carbonate catalyst (C), as often used in RF gel production. R/C and R/F molar ratios, and M concentration ([M]), were chosen as parameters to study in-depth, as they have previously been shown to markedly influence sol-gel formation. The MRF gels produced were subsequently characterized to determine porous structure and chemical functionality. The results indicate that, texturally, increasing [M] produces a similar effect as increasing R/C values: increasing pore size, while decreasing surface area. Pore volume tends to increase when R/C or M increase individually but pore volume and surface area decrease drastically when both variables increase concurrently. Microporosity also tends to increase as R/C decreases, and as the concentration of M is decreased. Altering the gel matrix, by replacing M for R, results in a weakening of the gel structure, as the bridges formed during curing are reduced in quantity, which indicates a maximum level of substitution that can occur within these materials. Combined, these results suggest that nitrogen can be successfully incorporated into organic gel structures but that the interplay between process variables is crucial in determining final gel characteristics for specific applications

Highlights Concentration of basic functionalities increases as melamine content is increased  Increased melamine lowers fixed carbon, raises volatile matter and weakens structure  CO2 adsorption capacity increases at 0°C and 60°C for higher melamine concentrations  Melamine enhances CO2 adsorption with a low enthalpy of regeneration  CO2 adsorption/desorption cycling measurements study show high sorption stability AbstractSelected melamine-resorcinol-formaldehyde (MRF) xerogels have been synthesised and analysed to determine the influence of nitrogen (N) incorporated into the gel structure, as well as, resorcinol to catalyst (sodium carbonate) and resorcinol to formaldehyde molar ratios. The aforementioned factors were varied, and their effect on gel properties characterised, allowing a better understanding of how gel characteristics can be tailored, and their impact on gel performance. MRF gels, produced in this study, were characterised using volumetric and gravimetric analyses to determine porous structure and quantify CO2 capture capacities and kinetics, as well as allowing determination of heats of adsorption and activation energies for CO2. MRF10_200_0.25 has exhibited the largest CO2 capacity (1.8mmol/g at 0 °C) of the sample tested. Thermal stability was tested by proximate analysis, and MRF xerogels exhibited high thermal stability, however it was found that volatile matter increases as [M] increases, particularly for [M] 20%w/w and higher. Working capacity was determined from a series of cycling studies and capacities of 0.55, 0.58 and 0.56 mmol/g at 60 °C were observed for [M] of 10, 20 and 30%w/w, respectively. The measured heat of adsorption showed that incorporation of nitrogen functionalities results in a low energy penalty demonstrating that the adsorption mechanism is still driven by physical forces. The results obtained indicate that the family of materials studied here offer potential routes for carbon capture materials, through a combination of micropore structure development and incorporation of favourable Lewis acid-base interactions.

Sorptive selectivity of melamine-resorcinol-formaldehyde xerogels, towards CO 2 , CH 4 , N 2 and H 2 , is reported, where all systems demonstrate potential for selective adsorption of CO 2 from corresponding binary gas mixtures. Selected gas mixtures represent important gas separation applications found in industry, i.e. CO 2 removal from power plant flue gases (CO 2-N 2), sour gas sweetening (CO 2-CH 4), and separation of species in the water-gas shift reaction (CO 2-H 2). All materials tested exhibit microporous character, enhancing adsorption of small molecules, however, it is the inclusion of a nitrogen-rich material into the gel matrix that results in enhanced selectivities for these systems. Despite the porous character of the gels, under the test conditions used to simulate industrial parameters, all three balance gases, i.e. H 2 , N 2 and CH 4 , showed low affinities for the xerogels, while CO 2 adsorption was notably higher and increased with the inclusion and increased concentration of melamine. Ideal adsorbed solution theory was used to demonstrate significant differences in adsorption uptake, especially for CO 2-CH 4 , and high selectivities for CO 2 over N 2. In all cases, selected xerogels exhibited industrially relevant adsorption timescales for CO 2 over competitor gases, demonstrating the potential of these materials for the selective adsorption of CO 2 from process streams.

Resorcinol (R) and formaldehyde (F) gel synthesis has been well-studied along with alternative reagents. We present the synthesis of formaldehyde-based xerogels using chemically similar s-triazine precursors, with comparison to traditional analogues. The substitution ranges from tri-hydroxyl to tri-amine, with an intermediate species, allowing changing chemistry to be investigated. Each molecule (X) offers different acid/base properties, known to influence gel formation, as well as differences in crosslinking potential. Varying X/F ratios were selected to recreate the stoichiometry used in RF systems, where one represented higher F to match the increased reaction sites of the additives. X/C ratios were selected to probe different catalyst (C) ratios, while working within the range likely to produce viable gels. Results obtained show little impact for ammeline as an additive due to its similarity to resorcinol (activation sites and pKa); while melamine and cyanuric acid show differing behavior depending on the level of addition. Low concentrations show melamine to have the most impact due to increased activation and competition for formaldehyde; while at high concentrations, cyanuric acid is shown to have the greatest impact as it creates a more acidic environment, which diminishes textural character, possibly attributable to larger clusters and/or weaker cross-linking of the system.