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.
Trang 1Available online 18 November 2020
1387-1811/© 2020 The Authors Published by Elsevier Inc This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
Carboxylic acid-modified polysilsesquioxane aerogels for the selective and
reversible complexation of heavy metals and organic molecules
C.R Ehgartnera, V Wernera, S Selza, N Hüsinga, A Feinlea,b,*
aParis-Lodron-University of Salzburg, Department of Chemistry and Physics of Materials, Jakob-Haringer-Str 2a, 5020 Salzburg, Austria
bMcMaster 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
Keywords:
Adsorption
Carboxylic acid
Heavy metal ions
Methyltrimethoxysilane
Organofunctional
Porous polysilsesquioxanes
A B S T R A C T 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− 1 and 36 m2.g− 1), light (bulk densities between 1.38 g.cm− 3 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− 1 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
1 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 [18–21] ˇStandeker 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)
Contents lists available at ScienceDirect Microporous and Mesoporous Materials
journal homepage: http://www.elsevier.com/locate/micromeso
https://doi.org/10.1016/j.micromeso.2020.110759
Received 12 August 2020; Received in revised form 9 November 2020; Accepted 10 November 2020
Trang 2moieties 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
2 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 1 Stirring was continued for 30 min 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 4 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 8 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 3 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):
Trang 3Bulk Loading
(
mmol
g
)
=[100 − W200− 1000] ×R
the molar ratio of TMPA to MTMS and W 200-1000 (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 5 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 3 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, 5 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 5 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 3 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− 1 for certain time intervals (1 h, 2 h, 4 h, 24 h,
2 d, and 4 d) and 5 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 filtsepa-ration 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 MT80-
T20 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− 1 and 200
mg.L− 1) and 5 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 3 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− 1 and 100 mg.L− 1) and 5 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 3 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− 1 (Zn2+) or 20 mg.L− 1 (Cu2+) and 5 mL EtOH
and kept for certain time intervals (0.5 h, 1 h, 2 h, 4 h, 8 h, 24 h, 48 h, 3
d, and 4 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
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 3 times
Cu2+-complexometric titration: 15 mL of the metal ion filtrate (with
pH 8 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 3 times
The amount of heavy metal ions or dyes adsorbed on the solid samples was calculated based on equation (2):
Qe =(C0− C e)xV
where Q e is the equilibrium adsorption capacity (mg.g− 1), C 0 and C e 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 = 2 mg.L− 1) and 1 mL EtOH or to a mixture of thiazole yellow (ThG) and RhB (1:1, c = 2 mg.L− 1) and 2 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 3 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 5 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 3 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 5 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
Table 1
Starting compositions[a] and selected structural properties of the carboxy group functionalized polysilsesquioxane aerogels
a
b
[%]
ρ bc [g.cm ¡3 ]
ρ sd
e
f
[m 2 g ¡1 ]
aOther components: CTAB 0.6 g, urea 0.5 g, and 10 mM acetic acid 7 g
b Linear shrinkage during drying
cBulk density
dSkeletal density
ePorosity calculated by using the equation P = [1-ρb/ρs]*100
fSpecific surface area determined using the BET model
Trang 42.7 Characterization
The morphology of the samples was analyzed with a scanning
elec-tron microscope (Zeiss ULTRA Plus) operating between 2 and 5 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 acceleroper-ating 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− 1
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
3 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 1 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 acid-
modified 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
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− 3 for the reference sample
(MT100-T0) to 0.55 g cm− 3 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 adsorption- desorption measurements The obtained isotherms are shown in
Fig 1a and the calculated BET specific surface areas are listed in Table 1 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 saturaadsorp-tion 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− 1 and an average pore size of 38 nm was calculated, whereas the specific surface area was decreased to 36 m2.g− 1 and the pore size increased to 119 nm after an increase of the molar ratio of
MTMS and TMPA to 7:3 (MT70-T30)
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− 1 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− 1 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− 1 and 2928 cm− 1, respectively For the carboxylic acid-
modified samples (MT90-T10, MT80-T20, MT70-T30) a new band
appeared at 1712 cm− 1 which is ascribable to the C––O stretching vi-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-creases the band intensity of the C––O vibration proving the successful incorporation Furthermore, a small shoulder at 1735 cm− 1 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
Trang 5◦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− 1 for
MT90-T10 to 0.98 mmol.g− 1 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 2 A partial shift from
the carbonyl vibration from 1712 cm− 1 to 1580 cm− 1 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 2 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 MT70-
T30, 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 7 most of the –COOH groups are in their deprotonated form
and exhibit a higher adsorption capacity than at a pH below 7 [26] At
pH = 7 only parts of the carboxylic acid groups are deprotonated, and
the adsorption capacity is lower than at a pH value above 7 The pH
value of 8 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 8 Additionally, the adsorption behavior of different bases (ammonia, sodium carbonate and sodium hydrogen carbonate) at pH 8 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 8 showed a weaker adsorption behavior On this basis ammonia at a pH of 8 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 = 5 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− 1 and the
lowest for the unmodified sample MT90-T10 towards Cu2+with 10 mg
g− 1
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
3 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:
Q t=(Q e− exp−K1t)
(3) Pseudo-first order linear adsorption model:
Pseudo-second order nonlinear adsorption model:
Q t= K2Q2
e t
1 + K2Q2
Pseudo-second order linear adsorption model:
Fig 1 Nitrogen adsorption-desorption isotherms (a) and IR-ATR spectra (b) of carboxylic acid-modified MTMS samples
Table 2
Comparison of the actual and calculated weight loss and bulk loading of the
carboxylic acid-modified polysilsesquioxanes and accessible surface loading
Loss
[%]
Calc Weight Loss [%]
Bulk Loading a [mmol.g ¡ 1 ]
Surface Loading b [mmol.g ¡ 1 ] MT90-
MT80-
MT70-
aCalculated from the thermogravimetric data and equation 1
b Accessible surface loading, determined from adsorption experiments with
MB at a pH value above 7
Trang 6Q t
K2Q2
e
+ t
Q e (mg.g− 1) is the adsorption capacities at equilibrium and Q t (mg
g− 1) is the adsorption capacities at time t (h) The pseudo-first and
second order rate constants are K 1 (1.h− 1) and K 2 (g.mg− 1.h− 1),
respectively The linear fitting of the pseudo-first order and pseudo-
second order equations were solved in Origin 6.0 and are shown in
Fig 3 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
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 minimizafunc-tion 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 i=1
(
Q e, exp− Q e,cal
)
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 chi- square 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
Fig 3 (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
Trang 7between 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 4 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
following [55]
Linearized Freundlich model:
lnQ e=lnK F+1
Linearized Langmuir model:
C e
Q e
Q m K L
+C e
Where K F is the Freundlich isothermal constants and K L is the Langmuir
constant, respectively 1/n is an indicator, if adsorption is favorable Q m
(mg.g− 1) is the maximum adsorption capacity of the adsorbent Table 4
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
3.4 Selective adsorption and regeneration studies
The ability of MT80-T20 to selectively adsorb RhB or MB of a
cationic/anionic dye mixture was tested by two selective adsorption
Table 3
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 Linear Pseudo-Second Order
Model Adsorbate K 1
[min ¡ 1 ] Q [mg e
g ¡1 ]
R 2 K 2
[g.mg ¡ 1
min ¡1 ]
Q e
[mg
g ¡1 ]
R 2
RhB 2.00E-04 50.7 0.999 4.50E-05 63.3 0.991
MB 3.00E-04 69.5 0.957 2.20E-05 84 0.989
Zn 2þ 5.00E-04 33.3 0.972 1.30E-04 75.8 0.999
Cu 2þ 2.00E-04 18.3 0.996 1.50E-04 50.3 0.999
Nonlinear Pseudo-First Order Model Nonlinear Pseudo-Second Order
Model Adsorbate K 1
[min ¡ 1 ] Q [mg e
g ¡ 1 ]
[g.mg ¡ 1 min ¡ 1 ]
Q e
[mg
g ¡ 1 ]
χ
RhB 3.70E-03 52.1 1.820 7.61E-05 57.4 1.345
MB 1.80E-03 67.6 1.043 2.51E-05 79.2 0.979
Zn 2þ 1.39E-02 67.3 1.003 2.56E-04 75.59 0.967
Cu 2þ 3.50E-03 44.5 0.572 1.40E-03 45.4 0.425
Table 4
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
Adsorbate K L
[L
mg ¡ 1 ]
Qm [mg
g ¡ 1 ]
R 2 K F
[mg 1-n L n
g ¡ 1 ]
1/n R 2
Trang 8experiments (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 3
d
Regeneration and recyclability of the adsorbent material were tested
by washing the MTMS monolith (MT80-T20) with 1 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)
3.4 Comparison to ambiently dried samples
To broaden the field of application of the carboxylic acid-
functionalized gels and to reduce the cost of the preparation, MT80-
T20 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− 1 (aerogel) to 25 m2.g− 1
(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− 1 for RhB (aerogel 154 mg g− 1)
4 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
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 MT80-
T20 follows a pseudo-second order adsorption reaction in good
agree-ment of the calculated and the experiagree-mental 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
Fig 5 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)
Trang 9corresponds 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 MT80-
T20 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 do 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]
5 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 5
Comparison of maximum adsorption capacity Qm of RhB, MB, Zn2+and Cu2+with various adsorbents reported in literature
[mg.g ¡ 1 ] MB Q [mg.g m ¡ 1 ] Cu
2þ Q m
[mg.g ¡ 1 ] Zn
2þ Q m
[mg.g ¡ 1 ] S BET[
a ]
[m 2 g ¡ 1 ] Reference
aSpecific surface area determined using the BET model
Trang 10Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper
Acknowledgements
The authors thank M Suljic for nitrogen adsorption/desorption
measurements and R Torres for TEM measurements performed at the
University of Salzburg N H gratefully acknowledges financial support
from Interreg ¨Osterreich-Bayern 2014–2010 Project AB29 Synthese,
Charakterisierung und technologische Fertigungsans¨atze für den
Leichtbau “n2m” (nano-to-macro)
Appendix A Supplementary data
Supplementary data to this article can be found online at https://doi
org/10.1016/j.micromeso.2020.110759
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