Organofunctional porous methyltrimethoxysilane (MTMS)-based aerogels are attractive for various adsorption purposes due to the combination of their unique properties such as low densities and high specific surface areas with tunable and accessible functional groups that can coordinate to, e.g., heavy metals and/or organic dye molecules in polar and non-polar solutions.
Microporous and Mesoporous Materials 312 (2021) 110759 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Carboxylic acid-modified polysilsesquioxane aerogels for the selective and reversible complexation of heavy metals and organic molecules C.R Ehgartner a, V Werner a, S Selz a, N Hüsing a, A Feinle a, b, * a b Paris-Lodron-University of Salzburg, Department of Chemistry and Physics of Materials, Jakob-Haringer-Str 2a, 5020 Salzburg, Austria McMaster University, Department of Chemistry and Chemical Biology, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada A R T I C L E I N F O A B S T R A C T Keywords: Adsorption Carboxylic acid Heavy metal ions Methyltrimethoxysilane Organofunctional Porous polysilsesquioxanes Organofunctional porous methyltrimethoxysilane (MTMS)-based aerogels are attractive for various adsorption purposes due to the combination of their unique properties such as low densities and high specific surface areas with tunable and accessible functional groups that can coordinate to, e.g., heavy metals and/or organic dye molecules in polar and non-polar solutions Furthermore, the MTMS backbone gives these aerogels mechanical strength, the ability to be dried under ambient conditions and ensures their non-degradability in aqueous media and recyclability Herein, we report the preparation of carboxylic acid-modified polysilsesquioxane aerogels via a simple and straightforward acid-base catalyzed sol-gel approach by using MTMS and the novel and stable 5(trimethoxysilyl)pentanoic acid In this surfactant assisted co-condensation approach, all parameters (concen tration, pH, and temperature) have been carefully designed to yield porous (porosities between 82% and 53% and specific surface areas between 345 m2.g− and 36 m2.g− 1), light (bulk densities between 1.38 g.cm− and 1.16 g.cm− 3), and hydrophobic aerogels with accessible and reactive functional carboxylic acid groups (-COOH) (accessible surface loading up to 0.19 mmol.g− 1) for the adsorption of heavy metals ions (Zn2+ and Cu2+) and cationic dyes (methylene blue and rhodamine B) The maximum adsorption capacities obtained from Langmuir isotherms were 154 mg.g− 1, 106 mg.g− 1, 111 mg.g− 1, and 78 mg.g− for RhB, MB, Zn2+, and Cu2+, respectively An increasing content of carboxylic acid groups influences the morphology, specific surface area and adsorption behavior of the synthesized aerogels Optimized functionalized aerogels can be dried ambiently and show high and reversible adsorption abilities of 87% over several cycles towards cationic dyes in aqueous media Moreover, these carboxylic acid-modified aerogels demonstrate excellent adsorption selectivity by adsorbing only positively charged molecules from mixed dye solutions, making them ideal candidates for diverse adsorption processes in polar and non-polar solutions Introduction Freshwater pollution with either heavy metals or organic dyes has been a major concern to human health in the last decade On the one hand the inefficient treatment of wastewater from industry and sanitary leads to pollutant accumulation in ground water and soil and on the other hand precious resources (like gold and silver ions) are lost for further industrial development [1–3] Versatile techniques, like adsorption [4], filtration [5], membrane separation [6], ion exchange [7], and electrolysis [8] have been used in the past for waste water treatment From the mentioned approaches the adsorption technique is considered the most promising and efficient technique regarding ease of operation, cost-effectiveness, and regeneration [9] Nevertheless, adsorbent materials which focus on selective and repeatable adsorption are rarely reported, even though such adsorbents have major advantages in separation and regeneration of dye mixtures and for diverse sensor applications [10–13] Therefore, there is a high need to design suitable adsorbent materials with tunable surface functionalities that show high affinity to specific adsorbates [14–16] Specifically organofunctional mesoporous aerogels with high spe cific surface areas, high porosities, and low densities are very promising materials in the field of separation science and selective adsorption processes [17] The surface chemistry of these aerogels can be tailored with a high number of different organic functional groups (e.g amino, sulfonate, mercapto, and hydroxyl groups) for specific applications ˇ [18–21] Standeker et al synthesized silica aerogels with mercapto * Corresponding author Paris-Lodron-University of Salzburg, Department of Chemistry and Physics of Materials, Jakob-Haringer-Str 2a, 5020, Salzburg, Austria E-mail address: andrea.feinle@sbg.ac.at (A Feinle) https://doi.org/10.1016/j.micromeso.2020.110759 Received 12 August 2020; Received in revised form November 2020; Accepted 10 November 2020 Available online 18 November 2020 1387-1811/© 2020 The Authors Published by Elsevier Inc This is an open (http://creativecommons.org/licenses/by-nc-nd/4.0/) access article under the CC BY-NC-ND license C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 moieties in a co-condensation process for the absorbance of copper (II) and mercury (II) ions from aqueous solution [22] Ali et al grafted aminopropyl groups onto a silica aerogel surface for the adsorption of chromium (III) ions [23] and the group of Nakanishi reported flexible organofunctional mercapto-modified silica aerogels for the adsorption of gold ions [24,25] Another promising functional group for diverse and selective adsorption and other applications is the carboxyl group since this group can form hydrogen bonds with organic and inorganic species Under basic conditions, the carboxyl group is deprotonated and the resulting negatively charged carboxylate entity can then act as a ligand for the complexation of a variety of metal cations and other positively charged molecules [26] Functionalization of mesoporous silica with carboxyl groups is mostly performed via multiple post-synthesis processing steps which is often associated with diffusion problems, limited attachment sites, loss of homogeneity, and pore blocking [27] Anhydride or iso cyanate groups can, for example, be grafted onto silica in a first step, followed by subsequent hydrolysis reactions [28,29] Only few reports of carboxyl-modified silica materials prepared by a co-condensation route have been published since a limited number of carboxylate group-containing organosilanes is commercially available [30–32] The group of Gaber and the group of Jones synthesized highly ordered carboxyl-modified mesoporous SBA-15 by a co-condensation of tet raethyl orthosilicate (TEOS) and water-soluble carboxyethylsilanetriol sodium salt (CES) [33,34] These materials did not show a significant affinity towards metal ion adsorption (Cu2+), indicating that the carboxyl groups were not available for further chemical modifications Lin et al reported the synthesis of a carboxylic acid-modified, disulfide containing organosilane which was employed in a co-condensation approach with TEOS to yield mesoporous MCM-41 [35] Notwith standing, the synthesis of the precursors was complex and required multiple reaction steps, extraction and purification of the final product led to esterification of the carboxylic acid In our previous work, we reported a simple one-pot synthesis of different stable carboxylic acid derivatized alkoxy silanes [36] The carboxylic acid ligands showed a high affinity for europium(III) ion complexation reactions [37] One of the silanes, namely 5-(triethox ysilyl)pentanoic acid was further employed as a precursor molecule in a co-condensation approach with TEOS to create organofunctional silica particles with high specific surface areas These particles showed excellent adsorption abilities towards the organic dye methylene blue [38] Recently, our group has shown the successful co-condensation of methyltrimethoxysilane (MTMS), 5-(triethoxysilyl)pentanoic acid and the biopolymer silk fibroin to yield mechanically strong and highly porous hybrid aerogels for diverse water-oil separation and thermal applications [39,40] Generally, as illustrated in our previous work, aerogels based on co-condensation of MTMS with other organofunc tional silanes showed promising abilities in terms of hydrophobicity, mechanical stability and flexibility, which makes them ideal candidates as adsorbents for organic pollutants, oil spills and regeneration, and continuous flow processes in polar and non-polar solutions [41] The properties of standard brittle hydrophilic aerogels, with a high number of –OH surface groups would deteriorate in aqueous solutions over time, which limits their practical applicability especially in terms of regen eration of the adsorbents The hydrophobicity of MTMS aerogels, resulting from their stable methyl surface groups ensures their non-degradability when exposed to water in comparison to standard hydrophilic silica aerogels and even allows the aerogels to be dried under ambient condition [42] In this work, we describe a straightfor ward one-pot synthesis of carboxylic acid functionalized silica aerogels from a co-condensation approach of MTMS and 5-(triethoxysilyl)pen tanoic acid followed by either supercritical fluid extraction with CO2 or ambient pressure drying All aerogels were investigated in terms of morphology, structural properties, successful incorporation of the functional (-COOH) group, and the influence of an increasing amount of the functional group The chemical accessibility of the carboxyl group was studied in detail by equilibrium and kinetic adsorption experiments with heavy metal ions (Zn2+ and Cu2+) and organic molecules (methy lene blue and rhodamine B) Special emphasis is also put on the selec tivity and reversibility of the adsorption process and on the ambient pressure drying of the organofunctional silica gels, which makes these materials interesting for diverse applications Experimental details 2.1 Materials Methyltrimethoxysilane (98% purity, MTMS), hexadecyl trimethylammonium bromide (98% purity, CTAB), 4-pentenoic acid (97%), platinum (IV) oxide, zinc sulfate heptahydrate, eriochrome® Black T, ammonium chloride, murexide, methylene blue (hydrate), thiazole yellow G, Titriplex (III), sodium acetate, sulfosalicylic acid dehydrate, sodium carbonate, sodium hydrogen carbonate, and meth anol (99.8%, MeOH) were obtained from Sigma Aldrich Glacial acetic acid (AcOH), copper(II)sulfate (anhydrous), iron(III)nitrate non ahydrate and urea were acquired from Merck Ammonium hydroxide (28% in H2O) and 2-propanol were procured from VWR Trimethox ysilane (95%) was purchased from Acros Organics Rhodamine B was provided by Alfa Aesar 2.2 Synthesis of carboxyl-modified aerogels The detailed synthesis of 5-(trimethoxysilyl)pentanoic acid via a platinum catalyzed hydrosilylation of pentenoic acid with trimethox ysilane is described elsewhere [36] Carboxy-modified poly organosilsesquioxane aerogels were prepared via a co-condensation approach of methyltrimethoxysilane (MTMS) with 5-(trimethoxysilyl) pentanoic acid (TMPA) The amount of hydrolysable silicon centers was kept at a constant value of 34.9 mmol, and an increasing molar% of MTMS (10%, 20%, and 30%) was substituted by TMPA In the first step of the two-step acid-base approach CTAB and urea were dissolved in 10 mM aqueous acetic acid MTMS and TMPA were slowly added to the mixture under stirring and ambient conditions The starting composi tions can be found in Table Stirring was continued for 30 before the sol was poured into tightly sealed PS containers (Ø 17.2 mm; height 57.6 mm) Similar to the approach described by Kanamori et al the containers were placed in a ventilated oven for d at 60 ◦ C to induce gelation and aging [42] For the removal of residual surfactants the aged alcogels were solvent exchanged in methanol (double the volume of the monoliths) with at least h in between three subsequent solvent ex changes cycles For supercritical drying, the alcogels were again solvent exchanged with 2-propanol in the same approach as methanol Super critical drying was conducted in a custom-built autoclave with CO2 at 45 ◦ C and in a pressure range between 80 and 90 bar For ambient pressure drying, the synthesized wet gel was washed three times with methanol and then solvent exchanged with n-heptane three times Af terwards, the gels were dried at room temperature for d The samples are labeled as follows: The first two letters (MT) correspond to the MTMS precursor molecule and the attributed number gives the molar% of the silane used The second letter refers to the second silane used for co-condensation (T for TMPA) and the following numbers correspond to the molar% of the portion of MTMS that was replaced by TMPA For example, the sample MT80-T20 was prepared with 80 mol% MTMS and 20 mol% TMPA and in which the molar% are related to the constant amount of hydrolysable silicon centers (34.9 mmol) 2.3 Determination of the functional group bulk loading The bulk loading of the solids with carboxyl groups were determined by using equation (1): C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 Table Starting compositions[a] and selected structural properties of the carboxy group functionalized polysilsesquioxane aerogels Sample Molar Ratio MTMS:TMPA MTMS [g] TMPAa [g] Lb [%] ρbc ρsd [g.cm¡3] Pe [%] SBET f [m2.g¡1] MT100-T0 MT90-T10 MT80-T20 MT70-T30 10:0 9:1 8:2 7:3 4.80 4.32 3.84 3.36 – 0.80 1.60 2.30 4.7 9.9 23.1 20.9 0.21 0.25 0.46 0.55 1.36 1.38 1.32 1.16 84.4 81.8 65.1 52.5 548 345 145 36 a b c d e f [g.cm¡3] Other components: CTAB 0.6 g, urea 0.5 g, and 10 mM acetic acid g Linear shrinkage during drying Bulk density Skeletal density Porosity calculated by using the equation P = [1-ρb/ρs]*100 Specific surface area determined using the BET model Bulk Loading ( ) mmol [100 − W200− = g M 1000 ] ×R The metal ion content in all experiments was determined via com plexometric titration with EDTA after separation of the metal ions so lution from the solid adsorbent by filtration over a polytetrafluoroethylene syringe filter with a membrane size of 200 nm Zn2+-complexometric titration: 15 mL of the metal ion filtrate (the pH was adjusted to ~10 with an ammonia buffer solution (5.4 g ammonium chloride and 35 mL 25% ammonia solution)) was pipetted into an Erlenmeyer flask and Eriochrome Black T was added as a met alchromic indicator The solution was titrated with a 0.05 M EDTA so lution The titration was repeated times Cu2+-complexometric titration: 15 mL of the metal ion filtrate (with pH from the adsorption experiment) was pipetted into an Erlenmeyer flask and murexide was added as a metalchromic indicator The solution was titrated with a 0.05 M EDTA solution The titration was repeated times The amount of heavy metal ions or dyes adsorbed on the solid samples was calculated based on equation (2): (1) M (g.mol− 1) is the molar mass of the degradable carboxyl group, R is the molar ratio of TMPA to MTMS and W200-1000 (g) is the weight loss between 200 and 1000 ◦ C (determined by thermogravimetric analysis) 2.4 Adsorption experiments Organic Dyes Batch adsorption experiments were conducted at RT to determine the adsorption of methylene blue (MB) and rhodamine B (RhB) on the synthesized carboxylic acid-modified samples In a typical experiment, 10 mg of the solid was mixed with 45 mL of the RhB dye solution (c = 100 mg.L− 1) and mL EtOH The pH was adjusted to ~8 with a diluted ammonia solution (1 M) The mixture was shaken and then left to equilibrate over a period of d In a second step, adsorption equilibrium experiments were performed where 10 mg of MT80-T20 was mixed with 45 mL of the aqueous dye solutions (MB or RhB) of different concentrations (1 mg.L− 1, mg.L− 1, 10 mg.L− 1, 25 mg.L− 1, 50 mg.L− 1, 75 mg.L− 1, and 100 mg.L− 1) and mL EtOH The pH was adjusted to ~8 with a diluted ammonia solution (1 M) The mixture was left to equilibrate over a period of d Furthermore, kinetic adsorption experiments were conveyed, were 10 mg of the silsesquioxane aerogel sample was mixed with 45 mL of the dye solutions (MB or RhB) with a concentration of 50 mg.L− for certain time intervals (1 h, h, h, 24 h, d, and d) and mL EtOH after the adjustment of the pH to ~8 with a diluted ammonia solution (1 M) The dye concentrations in all experi ments were determined via ultraviolet–visible spectroscopy after sepa ration of the dye solution from the solid by filtration over a polytetrafluoroethylene syringe filter with a membrane size of 200 nm Heavy Metal Ions Adsorption equilibrium experiments were con ducted at RT to determine the adsorption behavior of different heavy metal ions (Zn2+, Cu2+) on the synthesized carboxylic acid-modified samples In a typical Zn2+ ion adsorption experiment, 10 mg of MT80T20 was added to 45 mL of an aqueous Zn2+ ion solution of different concentrations (10 mg.L− 1, 25 mg.L− 1, 50 mg.L− 1, 100 mg.L− and 200 mg.L− 1) and mL EtOH The pH was adjusted to ~8 with an ammonia buffer solution (5.4 g ammonium chloride and 35 mL 25% ammonia solution) The mixture was shaken and then left to equilibrate over a period of d In a typical Cu2+ ion adsorption equilibrium experiment, 10 mg of MT80-T20 was added to 45 mL of an aqueous Cu2+ ion solution (5 mg L− 1, 10 mg.L− 1, 25 mg.L− 1, 50 mg.L− and 100 mg.L− 1) and mL EtOH The pH was adjusted to ~8 with a diluted ammonia solution (1 M) The mixture was shaken and then left to equilibrate over a period of d Furthermore, kinetic adsorption experiments were performed 10 mg of MT80-T20 was added to 45 mL of the metal ion solutions with a concentration of 100 mg.L− (Zn2+) or 20 mg.L− (Cu2+) and mL EtOH and kept for certain time intervals (0.5 h, h, h, h, h, 24 h, 48 h, d, and d) The pH was adjusted to ~8 with an ammonia buffer solution for Zn2+ or with a diluted ammonia solution (1 M) for Cu2+ experiments Qe = (C0 − Ce )xV m (2) where Qe is the equilibrium adsorption capacity (mg.g− 1), C0 and Ce are the initial and the equilibrium concentrations of the heavy metal ion solutions/dye solutions (mg.L-1), V is the volume of the heavy metal ion/ dye solution (L) and m is the mass of the polysilsesquioxane aerogel used (g) 2.5 Selective adsorption experiments 0.1 g of MT80-T20 was added to 10 mL of a mixture of either thiazole yellow G (ThG) and MB (1:1, c = mg.L− 1) and mL EtOH or to a mixture of thiazole yellow (ThG) and RhB (1:1, c = mg.L− 1) and mL EtOH The pH was adjusted to ~8 with a diluted ammonia solution (1 M) The mixtures were shaken and then left to equilibrate over a period of d The dye concentrations were determined via UV–vis spectroscopy after separation from the solid adsorbent by filtration over a polytetra fluoroethylene syringe filter with a membrane size of 200 nm 2.6 Reusability experiments 0.01 g of MT80-T20 was added to 45 mL of a MB solution (c = 10 mg L− 1) and mL EtOH The pH was adjusted to ~8 by adding a diluted ammonia solution (1 M) The mixture was shaken and then left to equilibrate over a period of d In a second step, the solid sample was separated from the dye solution by filtration and placed in diluted HCl (pH = 1–2) for 24 h The samples were repeatedly washed afterwards with water This process (adsorption of MB and the consequent washing) was repeated times The dye concentrations in all reusability experi ments were determined via UV–vis spectroscopy after separation from the solid adsorbent by filtration over a polytetrafluoroethylene syringe filter with a membrane size of 200 nm C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 2.7 Characterization cluster structure mentioned in previous reports [42], whereas the car boxylic acid-modified MTMS samples (MT90-T10, MT80-T20, and MT70-T30, Figs S2b–d) were obtained as opaque monoliths The microstructure changed significantly from very small particle-networks (particle size 5–15 nm) with nano-sized voids (4–40 nm) for the sample MT100-T0 and MT90-T10 to the presence of larger structures (15 and 40 nm) and voids in the upper nanometer range (100–300 nm) for MT80-T20 and MT70-T30 Besides changes in the appearance and morphology of the monoliths, the bulk density, linear shrinkage, and the porosities were influenced by the introduction of carboxyl groups as well (Table 1) The bulk density, for example, increased from 0.21 g cm− for the reference sample (MT100-T0) to 0.55 g cm− after the incorporation of 30 mol% TMPA (MT70-T30) and the shrinkage increased from 4.7% for MT100-T0 to 20.9% for MT70-T30 leading to a decrease in the porosity from 84.4% (MT100-T0) to 52.5% (MT70-T30) The specific surface areas of the monoliths and pore sizes were calculated from nitrogen adsorptiondesorption measurements The obtained isotherms are shown in Fig 1a and the calculated BET specific surface areas are listed in Table The isotherms of all prepared monoliths can be classified as Type IV according to IUPAC classification Carboxylic acid-modified MTMS aerogels showed very narrow hysteresis loops (Type H3) and capillary condensation occurred above p/p0 > 0.5 The shape of the hysteresis loops indicate that the samples have a mesoporous character with the possibility of additional macropores in the network system that are not completely filled with the pore condensate [46] The nitrogen adsorp tion intake showed no saturation at the relative pressure close to unity This can be a direct result of pores in the macroporous region, where capillary condensation still takes place at p/p0 ~1 The hysteresis loops of the MTMS/TMPA samples got less pronounced with increasing modification degree of the monoliths with carboxyl groups, indicating broader pore size distributions The average pore diameter (Tables S2 and SI) increased, and the BET specific surface areas (Table 1) decreased with an increasing amount of TMPA For a MTMS-TMPA aerogel with a molar ratio of MTMS to TMPA of 9:1 (MT90-T10) a specific surface area of 345 m2.g− and an average pore size of 38 nm was calculated, whereas the specific surface area was decreased to 36 m2.g− and the pore size increased to 119 nm after an increase of the molar ratio of MTMS and TMPA to 7:3 (MT70-T30) The morphology of the samples was analyzed with a scanning elec tron microscope (Zeiss ULTRA Plus) operating between and kV with an in-lens detector and by a transmission electron microscopy (TEM) JEOL JEM F200 with a cold field emission electron microscope oper ating at an accelerating voltage of 200 kV with a TVIPS F216 2k by 2k CMOS camera Nitrogen adsorption and desorption isotherms were performed using a Micrometrics ASAP 2420 at − 196.15 ◦ C The specific surface area (SBET) was calculated with the Brunauer, Emmett and Teller 5-point method in the relative pressure range of 0.05–0.3 Prior to the measurement, the samples were degassed in vacuum for 12 h at 300 ◦ C Thermogravimetric analyses (TGA) were carried out using a simulta neous thermal analyzer (Netzsch STA 449C Jupiter) The samples were heated from 25 ◦ C to 1000 ◦ C with a heating rate of 10 ◦ C/min and an oxygen flow rate of 30 mL/min Structural characteristics of the aerogel samples were investigated using a FTIR-ATR spectrometer (Bruker Vertex 70) over a wavenumber range from 500 cm− 1–4500 cm− UV–vis spectra were conducted on a PerkinElmer Lambda 750 device The maximum adsorption wavelength was used for further calculations The mass to volume ratio of the cylindrically shaped monoliths was used as a basis to calculate the bulk density (ρb) of samples The skeletal density (ρs) was determined via helium pycnometry (Quantachrome, Micro-Ultrapyc 1200e T) Equation P = [1- ρb/ρs]/100 was used to calculate the porosity (P) of the samples Results 3.1 Preparation of carboxylic acid-modified polysilsesquioxane aerogels The preparation of carboxylic acid-modified polysilsesquioxane aerogels suitable for adsorption purposes requires a careful tuning and understanding of the synthesis conditions, such as the reaction rates of different silanes in co-condensation processes, the choice of suitable catalysts, and the addition of appropriate surfactants [43] The combi nation of a hydrophobic silane (MTMS) with a hydrophilic silane (TMPA) in a sol-gel process is not a trivial task Additionally, steric and charge effects by the large functional moieties of TMPA, competitive cyclization reactions and different hydrolysis and condensation rates have to be overcome [44] In our study we therefore used a surfactant aided two step sol-gel reaction in which the hydrolysis occurred in diluted acetic acid as weakly acidic medium and the polycondensation was initiated by the use of urea as a weak base-releasing agent (at temperatures above 60 ◦ C) [44,45] A ternary phase diagram illustrating the relationship between the functional silane TMPA, the surfactant CTAB, the catalyst for hydrolysis (HOAc) and the resulting gelation behavior and the appearance of the monolith is given in Fig S1 (SI) As seen in the ternary phase diagram, the composition of the sol is crucial for the prevention of phase separation processes and the later appear ance and structural properties of the monoliths There is a critical amount of CTAB (at least >0.5 g under given reaction conditions) that hinders phase separation for all investigated modification with TMPA In this study, we introduced carboxyl groups into MTMS based monoliths and studied the influence of the modification reaction on the morphology and structural properties of the materials as well as on their adsorption behavior towards metal ions and organic dyes The compo sitions of the sol for the preparation of the carboxylic acid-modified MTMS monoliths are given in Table and were chosen to give mate rials with low densities, high porosities, and high specific surface areas (see Tables S1 and SI) MT100-T0 (pure MTMS monolith without TMPA) was prepared as a reference sample under similar conditions as the carboxylic acidmodified samples to investigate the influence of TMPA on the proper ties of the material Digital, SEM and TEM images are shown in Fig S2 (SI) The pure MTMS reference sample (MT100-T0, Fig S2a) was transparent and showed the typical globular-aggregated mesoporous 3.2 Determination and accessibility of the carboxyl group The successful incorporation of carboxyl groups in the silica network of MTMS/TMPA polysilsesquioxane aerogels was determined via FTIR spectroscopy (Fig 1b) For all samples, the most intense vibration bands were in the range between 1017 and 1105 cm− and can be attributed to asymmetric stretching vibrations of the Si–O–Si bonds The less pro nounced bands at 768 cm− 1, 1408 cm− 1, and 1279 cm− correspond to the vibration of Si–C bonds The antisymmetric and symmetric stretch ing of the methyl C–H bond of the MTMS/TMAP samples was identified at 2972 cm− and 2928 cm− 1, respectively For the carboxylic acidmodified samples (MT90-T10, MT80-T20, MT70-T30) a new band – O stretching vi appeared at 1712 cm− which is ascribable to the C– bration of the carboxyl group [47,48] It can be clearly seen, that an increasing TMPA content and a corresponding increasing number of carboxyl groups attached to the MTMS framework significantly in – O vibration proving the successful creases the band intensity of the C– incorporation Furthermore, a small shoulder at 1735 cm− that can be associated to ester compound formation was detected [36] The thermal stability of the prepared samples was investigated by thermogravimetric analysis (see Figs S3 and SI) The first weight loss occurred in the temperature region between 25 and 200 ◦ C It was almost negligible for the unmodified sample MT100-T0 but increased from 2% for MT90-T10 and MT80-T20 up to 4% for MT70-T30 This weight loss can be attributed to physisorbed water on the poly silsesquioxane surface [49] A further increase in temperature up to 700 C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 Fig Nitrogen adsorption-desorption isotherms (a) and IR-ATR spectra (b) of carboxylic acid-modified MTMS samples C led to oxidation reactions and decomposition of the organic groups (methyl, carbonyl) and to condensation reactions of the residual silanol and alkoxy groups A comparison of the actual and the theoretical weight loss (see Table 2) shows negligible differences for the MTMS/TMPA samples and correlates very well to the molar ratios of the precursor molecules The number of incorporated carboxyl groups (bulk loading) was quantitatively determined and increased with an increasing molar percentage of TMPA from 0.21 mmol.g− for MT90-T10 to 0.98 mmol.g− for MT70-T30 The accessibility of the functional group was shown by a complex ation reaction of the carboxyl group with Zn2+ and adsorption experi ments towards MB The spectra of the sample MT80-T20 before and after complexation to Zn2+ are compared in Fig A partial shift from the carbonyl vibration from 1712 cm− to 1580 cm− was observed and indicates the conversion of the carboxyl group into a carboxylate moi ety The conversion is not complete since the Zn2+ ions are not able to reach every carboxylate group due to the hydrophobic property of the MTMS framework and possible pore blocking The accessible surface loading of the carboxy-modified samples was calculated from adsorption experiments with MB and is summarized in Table It can be clearly seen that MT80-T20 has the highest number of accessible surface groups (0.32 mmol.g− 1) which are almost half of the available bulk surface groups (0.56 mmol.g− 1) The sample MT90-T10 is too hydrophobic for the dye to reach all available adsorption sites and for the sample MT70T30, the low surface loading can be attributed to its low specific surface area The accessibility and successful adsorption of the dyes and metals is also directly related to the pH value of the aqueous solutions and the used acid or base For the adsorption of methylene blue at different pH values (pH = 2, 4, 7, 8, 9, and 10) see Fig S4 (SI) The pH value was adjusted with different concentrations of either ammonia or HCl At a pH above most of the –COOH groups are in their deprotonated form and exhibit a higher adsorption capacity than at a pH below [26] At pH = only parts of the carboxylic acid groups are deprotonated, and the adsorption capacity is lower than at a pH value above The pH value of was chosen for further metal and dye adsorption experiments to ensure mild reaction conditions and due to the fact that higher pH values (pH > 8) lead to the slow dissolution of the polysilsesquioxane backbone, which is not preferable for recyclability of the adsorbent and other intended applications [50,51] Moreover, the complexometric ti trations of Cu2+ solutions is very pH dependent and requires a pH value of Additionally, the adsorption behavior of different bases (ammonia, sodium carbonate and sodium hydrogen carbonate) at pH was analyzed (see Fig S5 (SI)) It can be clearly seen that ammonia and sodium carbonate show similar adsorption behavior, whereas the use of sodium hydrogen carbonate as a base at pH showed a weaker adsorption behavior On this basis ammonia at a pH of was chosen for further adsorption experiments ◦ 3.3 Adsorption studies The above analysis revealed that the functionalized aerogel sample MT80-T20 has a relatively high number of accessible carboxyl surface groups and high specific surface areas, which makes them promising candidates for the adsorption of organic molecules (electrostatic in teractions) and complexation reactions with heavy metals The adsorp tion performance of different samples with increasing –COOH content towards the organic dyes rhodamine B (RhB)/methylene blue (MB) and the metal ions Zn2+ and Cu2+ was tested (Fig 3a) The hydrophobic samples were first wetted with a small amount of ethanol (V = mL) to ensure that the sample does not float on the aqueous dye or metal salt solution and can interact with the respective dissolved molecules and ions The highest equilibrium adsorption capacity was reached for the sample MT80-T20 towards RhB with a value of 104 mg.g− and the lowest for the unmodified sample MT90-T10 towards Cu2+ with 10 mg g− Additional adsorption studies were conducted for the MT80-T20 sample and the adsorption kinetics towards RhB, methylene blue (MB), Zn2+ and Cu2+ was analyzed in detail (see Fig 3b) Initially, the heavy metal/dye removal occurred fast but showed saturation over a course of d Overall, the adsorption of metals ions occurred with a higher rate than the adsorption of dye molecules Pseudo-first and pseudo-second order adsorption models were applied to characterize the adsorption kinetics according to the following nonlinearized and linearized equa tions (3)–(6) [52,53] Pseudo-first order nonlinear adsorption model: ) ( (3) Qt = Qe − exp− K1 t Table Comparison of the actual and calculated weight loss and bulk loading of the carboxylic acid-modified polysilsesquioxanes and accessible surface loading Sample Weight Loss [%] Calc Weight Loss [%] Bulk Loadinga [mmol.g¡1] Surface Loadingb [mmol.g¡1] MT90T10 MT80T20 MT70T30 78.7 79.5 0.21 0.04 71.7 71.4 0.56 0.32 67.2 64.8 0.98 0.19 Pseudo-first order linear adsorption model: log(Qe − Qt ) = log(Qe − K1 (4) Pseudo-second order nonlinear adsorption model: a Calculated from the thermogravimetric data and equation b Accessible surface loading, determined from adsorption experiments with MB at a pH value above Qt = K2 Q2e t + K2 Q2e t Pseudo-second order linear adsorption model: (5) C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 Fig IR-ATR spectra of MT80-T20 before and after the complexation with Zn2+ Fig (a) Comparison of the adsorption perfor mance of the MTMS/TMPA polysilsesquioxane aero gels modified with an increasing percentage of TMPA towards Zn2+, Cu2+, MB, and RhB (b) Adsorption kinetic curve for the metal ions (Zn2+ and Cu2+) and dyes (RhB and MB) sorption on the aerogel sample MT80-T20 (c) Pseudo-first order linear kinetic model fit and (d) pseudo-second order linear kinetic model fit for the experimental data of the adsorbed capacity of RhB by MT80-T20 with increasing adsorption time t t = + Qt K2 Q2e Qe variance and normality assumptions of standard least squares [54] Therefore, the kinetic models were also fitted in their nonlinear forms The nonlinear curve fitting of the pseudo-first order and pseudo-second order equations were solved through the lsqcurvefit user-defined func tion in Matlab until resnorm minimization was achieved and are shown in Fig S7 a-d (SI) for the adsorption of the metal ions/dyes onto MT80-T20 The best-fit sorption kinetic model was analyzed with the statistical error function ‘Chi-square Test’ according to following equa tion [54] ( ) i=n ∑ Qe, exp − Qe,cal χ2 = (7) Qe,cal i=1 (6) Qe (mg.g− 1) is the adsorption capacities at equilibrium and Qt (mg g ) is the adsorption capacities at time t (h) The pseudo-first and second order rate constants are K1 (1.h− 1) and K2 (g.mg− 1.h− 1), respectively The linear fitting of the pseudo-first order and pseudosecond order equations were solved in Origin 6.0 and are shown in Fig c/d (for RhB) and in Fig S6 a-f (SI, for MB, Zn2+, and Cu2+) for the adsorption of the metal ions/dyes onto MT80-T20 The constants of the models were calculated from the slope and intercept of the straight lines and the linear regression coefficient (R2) was applied as an indicative of model fittingness The calculated correlation coefficients R2 (Table 3) clearly indicate that the pseudo-second order model fits well with the experimental data of the metal ion (Cu2+, Zn2+) and dye (MB, RhB) adsorption with high R2 values between 0.991 and 0.999 The calculated adsorption capacity Qe (pseudo-second order model, Table 3) addi tionally showed a good agreement with the experimental values of Qe Nevertheless, the transformation of nonlinear equations to their linear forms changes the error structure and can lead to violation of error − This fittingness test measures the difference between the experi mental adsorption capacities Qe,exp and the and model-calculated adsorption capacities Qe,cal The chi-square values (Table 3) of the pseudo-second order model generally display lower values than the chisquare values for the first-order model and clearly indicate that the pseudo-second order model fits better with the experimental data of the metal ion (Cu2+, Zn2+) and dye (MB, RhB) adsorption with low χ2 values C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 following [55] Linearized Freundlich model: Table Pseudo-first and pseudo-second order linear and nonlinear kinetic parameters for the adsorption of RhB, MB, Zn2+, and Cu2+ on MT80-T20 at room temper ature and a pH value of ~8 Linear Pseudo-First Order Model lnQe = lnKF + lnCe n Linear Pseudo-Second Order Model Linearized Langmuir model: Adsorbate K1 [min¡1] Qe [mg g¡1] R K2 [g.mg¡1 min¡1] Qe [mg gĂ1] R RhB MB Zn2ỵ Cu2ỵ 2.00E-04 3.00E-04 5.00E-04 2.00E-04 50.7 69.5 33.3 18.3 0.999 0.957 0.972 0.996 4.50E-05 2.20E-05 1.30E-04 1.50E-04 63.3 84 75.8 50.3 0.991 0.989 0.999 0.999 Nonlinear Pseudo-First Order Model (8) Ce Ce = + Qe Qm KL Qm Where KF is the Freundlich isothermal constants and KL is the Langmuir constant, respectively 1/n is an indicator, if adsorption is favorable Qm (mg.g− 1) is the maximum adsorption capacity of the adsorbent Table and Fig 4a–d clearly illustrate that the experimental data of all adsorption isotherms fit better with the Langmuir model rather than the Freundlich model Correlation coefficients of R2 higher than 0.980 were achieved for all metals and dye adsorption processes when fitted with the Langmuir model The correlation coefficients when fitting with the Freundlich model were much lower (0.935–0.983) Moreover, the maximum adsorption capacity calculated by the Langmuir equation (Table 4) was close to the experimental results Nonlinear Pseudo-Second Order Model Adsorbate K1 [min¡1] Qe [mg g¡1] χ2 K2 [g.mg¡1 minĂ1] Qe [mg gĂ1] RhB MB Zn2ỵ Cu2ỵ 3.70E-03 1.80E-03 1.39E-02 3.50E-03 52.1 67.6 67.3 44.5 1.820 1.043 1.003 0.572 7.61E-05 2.51E-05 2.56E-04 1.40E-03 57.4 79.2 75.59 45.4 1.345 0.979 0.967 0.425 (9) 3.4 Selective adsorption and regeneration studies between 0.425 and 1.345 The calculated adsorption capacity Qe (pseudo-second order model, Table 3) additionally showed a better agreement with the experimental values of Qe in comparison to the pseudo-first order model For a better understanding of the interaction between the adsorbate and absorbent, the respective adsorption isotherms were investigated at room temperature In Fig a-d the equilibrium adsorption capacity Qe (mg.g− 1) is plotted against the equilibrium concentration Ce (mg.L− 1) of RhB (Fig 4a), MB (Fig 4b), Zn2+ (Fig 4c), and Cu2+ (Fig 4d) With an increasing value of Ce the adsorption capacity gradually increased for all metal ion and dye solutions There is an increased driving force from the concentration gradient, which speeds up the diffusion of the metal ions and dye molecules towards the aerogel [13] Two most common line arized model equations for adsorption isotherms, namely Freundlich and Langmuir were used to fit the experimental data as shown in the The ability of MT80-T20 to selectively adsorb RhB or MB of a cationic/anionic dye mixture was tested by two selective adsorption Table Langmuir and Freundlich adsorption isotherm parameters for the adsorption of RhB, MB, Zn2+, and Cu2+ on MT80-T20 at room temperature and a pH value of ~8 Langmuir Freundlich Adsorbate KL [L mg¡1] Qm [mg g¡1] R2 KF [mg1-n.Ln g¡1] 1/n R2 RhB MB Zn2ỵ Cu2ỵ 0.015 0.141 0.011 0.185 154 106 111 78 0.981 0.993 0.989 0.980 4.3 1049.5 7.8 800.8 0.872 0.386 0.692 0.355 0.944 0.983 0.968 0.935 Fig Adsorption isotherms of (a) RhB, (b) MB, (c) Zn2+ and (d) Cu2+ on MT80-T20, including Langmuir and Freundlich model fit C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 experiments (Fig 5) Fig 5a and b display that the color of a RhB/TyG (orange) or MB/TyG (green) solution changes after the adsorption process and resembles the yellow color of the anionic TyG solution The molecular structures of the cationic/anionic dyes are displayed in Table S3 (ESI) The UV–vis measurements underline this observation, showing that the peak of RhB (λ = 554 nm, Fig 5c) and MB (λ = 665 nm, Fig 5d) almost disappear after the adsorption process The removal rate of MB reaches 97% and RhB 96% after processing with MT80-T20 for d Regeneration and recyclability of the adsorbent material were tested by washing the MTMS monolith (MT80-T20) with M HCl after the dye adsorption treatment with MB The low pH value of the HCl washing step protonates the carboxyl groups of the samples, which releases the MB dye out of the monolithic framework The adsorption efficiency was around 92% for three consecutive circles of adsorption and desorption of MB and the removal efficiency remained at 88% and 87% for the 4th and 5th cycle (Figs S8 and SI) increasing particle sizes and the formation of macropores Nevertheless, the obtained carboxy-modified aerogels still possess low bulk densities and high porosities for diverse catalytic and adsorption applications in different media The pure MTMS sample mainly bear methyl (-CH3) groups on the surface which render the monolith hydrophobic and prevent phys isorption of water on the surface On the one hand, the methyl modified framework gives the aerogel excellent mechanical abilities, making the gels easier to handle, to dry ambiently, and to undergo repeated adsorption processes in aqueous media On the other hand, the hydro phobic character makes it harder for intended adsorption purposes Functionalization with carboxyl groups renders the hydrophobic MTMS framework more hydrophilic This has a direct influence on the adsorption behavior of the functional materials The still very strong hydrophobic character of MT90-T10 prevents the adsorption of RhB from aqueous solution, even though the aerogel has a relatively high specific surface area With a higher degree of modification, the interplay between an increasing water wettability and the presence of an increasing number of coordination sites enhances the adsorption per formance of the aerogels Starting from the MT90-T10 sample, the adsorption capacity increases up to a molar amount of TMPA of 20% (MT80-T20) and then decreases for MT70-T30 Although MT70-T30 possess a higher surface loading with carboxyl groups, the capacity difference is related to the lower specific surface area of MT70-T30 (36 m2.g− 1) compared to MT80-T20 (145 m2.g− 1) The adsorption of metal ions occurs fast compared to the adsorption of organic molecules This can be attributed to the different sizes and steric demands The metal ions are relatively small in comparison to the large MB and RhB molecules and are more easily transported to interior adsorbent sites The adsorption of RhB, MB, Zn2+, and Cu2+ on MT80T20 follows a pseudo-second order adsorption reaction in good agree ment of the calculated and the experimental values of the adsorption capacity Qe This is an indication that the adsorption on the aerogel sample is controlled by chemical adsorption with a direct sharing and exchange of electrons between adsorbents and adsorbate [56] The adsorption rate is therefore dominated by the availability of adsorption sites and not by the concentration of the adsorbate [57,58] Neverthe less, the linear adsorption kinetics of RhB corresponds slightly better to the pseudo-first order model with a correlation coefficient of 0.999, suggesting that in this case the adsorption depends on the adsorbate as well as the sorbent and on chemisorption and physisorption processes [52] This behavior cannot be confirmed when applying the nonlinear adsorption kinetic model, where the kinetic adsorption of RhB 3.4 Comparison to ambiently dried samples To broaden the field of application of the carboxylic acidfunctionalized gels and to reduce the cost of the preparation, MT80T20 was also prepared via ambient pressure drying Fig S9 (SI) shows a picture of the xerogel (MT80-T20x), which is translucent in appearance The bulk density (0.81 g.cm− 3) was higher and the porosity (34%) was lower than the corresponding aerogel (0.46 g.cm− 3, 65%) The specific surface area also decreased from 145 m2.g− (aerogel) to 25 m2.g− (xerogel) This is due to a higher degree of irreversible shrinkage due to higher capillary forces experience during the drying step Nevertheless, the maximum adsorption capacity Qm (calculated by applying the Langmuir model, Figs S10 and SI) of the xerogel is still high with a value of 85 mg.g− for RhB (aerogel 154 mg g− 1) Discussion Carboxylic acid-modified MTMS polysilsesquioxane aerogels are successfully prepared by a co-condensation approach of MTMS and TMPA The employment of the surfactant CTAB is essential to synthesize stable monolithic aerogels since there is a great polarity difference be tween the hydrophobic MTMS and the hydrophilic TMPA Additionally, different hydrolysis and condensation rates of the silanes and an increasing content of large functional moieties lead to a loss of homo geneity in the polysilsesquioxanes framework This directly leads to Fig Images of the selective adsorption of RhB from RhB/TyG (a) and MB from MB/TyG (b) mixed solutions using MT80-T20 and their corresponding UV–vis spectra of the solutions before and after adsorption (c, d) C.R Ehgartner et al Microporous and Mesoporous Materials 312 (2021) 110759 corresponds better to the pseudo-second order model This is a clear indication that the nonlinear kinetic model is more accurate in describing the kinetics of adsorption of the analyzed sample MT80-T20 Nevertheless, the linear kinetic models are slightly better for predicting the adsorption at equilibrium Adsorption isotherms of MT80-T20 against RhB, MB, Zn2+, and Cu2+ follow the Langmuir model rather than the Freundlich model This suggest a monolayer adsorption of metal ions and organic molecules on the surface of the polysilsesquioxane [59] Selective adsorption experi ments indicate that the functionalized polysilsesquioxane (MT80-T20) displays excellent adsorption selectivity toward positively charged MB and RhB ions from a mixed cationic/anionic solution Additionally, the carboxylic acid-modified samples are simple to regenerate and can be repeatedly used for further adsorption experiments without significant decrease in the removal efficiency and their form stability The hydro phobicity and the mechanically stable network of the carboxy-modified MTMS samples prevent the degradation during the repeated adsorption processes in aqueous media The loss in the adsorption capacities is negligible and may be caused by MB being trapped inside some pores Comparison of the maximum adsorption capacity Qm of the MT80T20 aerogel towards RhB, MB, Zn2+, and Cu2+ with results reported in literature show that the results of this study exceed the adsorption ca pacities of conventional adsorbents (like activated carbon and standard silica aerogels) and are comparable to or succeed other functionalized silica aerogels and other adsorption aerogels/materials (Table 5) The maximum adsorption capacity Qm of the ambient dried MT80-T20x xerogel towards RhB is also comparable to good adsorption materials (Table 5) This indicates that the functionalization with 5-(trimethox ysilyl)pentanoic acid has a huge impact on the adsorption capabilities of the xerogels and aerogels, whereas the surface area is not as relevant in comparison The highest adsorption capacity in this study was obtained for RhB with Qm being 154 mg.g-1 The positively charged RhB ion (which also has carboxyl groups) has a higher positive charge density than the respective MB molecule and metal ions In addition, the adsorption ef fect of RhB towards the polysilsesquioxane is enhanced by hydrogen bonds due to the presence of electric donors and acceptors (from its carboxyl groups) The positively charged MB molecule and the metal ions not have electronic donors or receptors and the adsorption is mainly dominated by weaker Van der Waals electrostatic interactions [53] The general good values for the adsorption capacity indicates that the carboxylic acid-modified polysilsesquioxane aerogels are efficient adsorbents for organic dyes and can be used for heavy metal complex ation reactions The adsorption performance depends on the adsorbate molecule/ion size, the specific surface area of the adsorbents as well as on electrostatic/ionic interactions between the carboxylic surface groups and the metal ions and dyes [60] Conclusion Tunable carboxylic acid-modified aerogels have been synthesized from a simple co-condensation approach of MTMS with 5-(trimethox ysilyl)pentanoic acid via an acid-base catalyzed sol-gel process employing CTAB as a phase separation suppressing surfactant The one pot approach at RT is time and energy efficient in comparison to tedious grafting procedures The mesoporous functionalized silica aerogels possess low densities, high porosities and large specific surface areas that are dependent on the degree of functionalization Moreover, a high density of active and available carboxyl groups is present which makes the monoliths suitable for adsorption of organic molecules and complexation reactions with metal ions Carboxylic acid-modified MTMS aerogel samples showed maximum adsorption capacities for RhB, MB, Zn2+, and Cu2+ of 154 mg.g− 1, 106 mg.g− 1, 111 mg g− 1, and 78 mg.g− 1, respectively The adsorption capacity is comparable or ex ceeds commercial adsorbents (like activated carbon and standard silica aerogels) with the main advantage of being selective The aerogels selectively adsorb positive charged molecules and metal cations, which makes them ideal candidates for selective adsorption processes in different media Furthermore, the materials have a hydrophobic char acter and are stable for adsorption processes in water and can be easily regenerated by a simple acid washing process where both the adsorbate and the absorbent can be recovered The hydrophobic abilities make the aerogels also ideal adsorbents for oil spillage and organic solvents Additionally, the aerogels can also be dried under ambient conditions, with good and comparable adsorption capacities, which greatly reduces the cost of production and broadens their field of application CRediT authorship contribution statement C.R Ehgartner: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft V Werner: Validation, Investigation S Selz: Investigation N Hüsing: Resources, Writing - review & editing A Feinle: Conceptualization, Writing - re view & editing, Supervision Table Comparison of maximum adsorption capacity Qm of RhB, MB, Zn2+ and Cu2+ with various adsorbents reported in literature Adsorbent RhB Qm [mg.gĂ1] MB Qm [mg.gĂ1] Cu2ỵ Qm [mg.gĂ1] Zn2ỵ Qm [mg.gĂ1] SBET[a] [m2.g¡1] Reference Carboxy-functionalized MTMS aerogel MT80-T20 Carboxy-functionalized MTMS xerogel MT80-T20x Hydrophobic surface modified silica aerogel Amine-functionalized silica aerogel Amine-functionalized silica aerogel Mercapto-functionalized silica aerogel Mercapto-functionalized silica aerogel EDTA-functionalized silica Magnetic amine-functionalized mesoporous Silica Phenyl-functionalized silica aerogel Silica–titania composite aerogel Carboxy-functionalized mesoporous silica Carboxy-functionalized indole-based aerogel Poly(methacrylic acid-co-maleic acid) grafted nano fibrillated cellulose aerogel Microcrystalline cellulose aerogel modified with PDA Sodium alginate aerogel Activated carbon Carbon aerogel Graphite aerogel 154 85 134 – – – – – – – 26 – – – – – 61 ~100 281 106 – 65 – – – – – – 49 – 113 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dyes and metals is also directly related to the pH value of the aqueous solutions and the used acid or base For the adsorption of methylene... and Ce are the initial and the equilibrium concentrations of the heavy metal ion solutions/dye solutions (mg.L-1), V is the volume of the heavy metal ion/ dye solution (L) and m is the mass of