Journal of Sol-Gel Science and Technology (2020) 96:67–82https://doi.org/10.1007/s10971-020-05383-z

ORIGINAL PAPER: FUNDAMENTALS OF SOL–GEL AND HYBRID MATERIALSPROCESSING

In situ synthesis and characterization of sulfonic acid functionalizedhierarchical silica monoliths

Richard Kohns1,2 ● Ralf Meyer2 ● Marianne Wenzel3 ● Jörg Matysik3 ● Dirk Enke2 ● Ulrich Tallarek 1

Received: 16 April 2020 / Accepted: 30 July 2020 / Published online: 9 August 2020© The Author(s) 2020, corrected publication 2021

AbstractSurface functionalization of porous materials with sulfonic acid (SO3H) groups is of particular interest in applicationsinvolving ion exchange, acidic catalysis and proton conduction. Macro-mesoporous silica monoliths are ideal supportstructures for these applications, as they combine advection-dominated mass transport in the macropores with short diffusionlengths and a large surface area (available for functionalization) in their mesoporous skeleton. Here, we report on SO3Hfunctionalized sol–gel silica monoliths with bimodal pore systems exhibiting macro- and mesoporosity, prepared accordingto a simple, efficient in situ synthesis protocol. Based on the co-condensation approach, thiol groups were introducedhomogeneously into the pore structure, followed by their oxidation to SO3H groups and the simultaneous removal of thetemplate. The macropore size, specific surface area, and coverage with SO3H groups are easily adjusted in this synthesisroute. Importantly, the hybrid monoliths have a substantially narrower mesopore size distribution (relative standard deviation~25%) than conventional sol–gel materials (>40%) and can be engineered crack-free in a robust column design (suitable forhigh-pressure flow-through operation) with mean mesopore size down to ~7 nm. They are characterized by IR spectroscopy,thermogravimetry, and elemental analysis as well as 13C and 29Si solid state NMR to corroborate the simple, efficientcombination of sol–gel-based material synthesis, surface functionalization, and template removal (i.e., polymer extraction).Complementary, inverse gas chromatography is presented as a new approach to characterize the incorporated SO3H groupsvia surface energy analysis and particularly resolve changes in the Lewis acid–base characteristics engendered by thatfunctionalization.

Graphical Abstract

* Ulrich Tallarektallarek@staff.uni-marburg.de

1 Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany

2 Institute of Chemical Technology, Universität Leipzig,Linnéstrasse 3, 04103 Leipzig, Germany

3 Institute of Analytical Chemistry, Universität Leipzig,Linnéstrasse 3, 04103 Leipzig, Germany

Supplementary information The online version of this article (https://doi.org/10.1007/s10971-020-05383-z) contains supplementarymaterial, which is available to authorized users.

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Keywords In situ functionalization ● Co-condensation process ● Sol–gel synthesis ● Sulfonic acid ● Polymer extraction ●

Inverse gas chromatography

Highlights● Co-condensation sol–gel process for hierarchical, sulfonic acid functionalized silica monoliths.● Simultaneous extraction of the template (PEO) and oxidation of thiol to sulfonic acid groups.● Macropore size, specific surface area, and surface coverage with sulfonic acid groups adjusted.● Comprehensive characterization including surface energy analysis by inverse gas chromatography.

1 Introduction

Hierarchically porous silica monoliths with bimodal porestructure are widely used for chemical separation, thermalinsulation, electrochemical processes, CO2 adsorption, andheterogeneous catalysis [1–9]. Macro-mesoporous silicamonoliths, in particular, are ideal support structures inthese applications, because the macropores enable fast,advection-dominated transport through the material andthe mesoporous skeleton provides a large external surface(thus, a large contact area) for efficient mass transferbetween macroporous and mesoporous domains, as well asa large internal surface available for functionalization [10].Silica monoliths are also characterized by more homo-geneous structural features (distribution of pore-size andskeleton thickness) than alternative bimodal support struc-tures like packed beds of mesoporous particles, whichagain enhances transport and overall performance [11]. Dueto their high porosity, low bulk density, pore inter-connectivity, mechanical stability, and large surface, silicamonoliths can be approached with various functionalizationstrategies [12, 13].

Functionalization with organic species leads toorganic–inorganic hybrid materials, whereby the synthesisroutes can be quite versatile. A common strategy is to graftthe surface with functional groups by traditional chemicalbonding (involving the silanol groups of the silica surface),for which alkoxysilanes have proven to be well-suited lin-kers [14–17]. However, this approach faces a number ofdisadvantages, such as the intrinsic drawbacks of multistepsynthesis procedures, the inhomogeneous distribution ofactive sites and a low coverage due to pore-blocking effectsand hindered diffusion issues [18]. Furthermore, it has beenshown that the grafting procedure with a trifunctional pre-cursor may not result in a homogeneous monolayer of thelinker, but rather in the formation of oligomers with ladder-like structures [19]. An alternative is the co-condensationprocess, in which alkoxysilanes are hydrolyzed simulta-neously with suitable silica precursors during sol–gelsynthesis [7, 20]. The hydrolyzed precursors competewith each other during condensation and form a homo-geneous porous hybrid material. The sol–gel process itself

is extremely sensitive to parameter changes, so that evenslight modifications regarding, for example, the polarity orpH affect the properties of the resulting monoliths. Itimplies that the synthesis has to be optimized individuallyfor each material and functionality [14, 21–23].

Functionalization of porous materials with sulfonic acid(SO3H) groups is of special interest in applications invol-ving ion exchange, acidic catalysis, and proton conduction.Marshall et al. [24–26] investigated the synthesis of sulfonicacid modified mesoporous materials (like MCM-41 andSBA-15) and their particular property of a high protonconductivity. Other studies focused on the preparation ofSO3H-functionalized mesoporous silicas (MCM-41, SBA-15, and KIT-6) for application in the heterogeneously cat-alyzed esterification of monoglycerides or fatty acids for thesynthesis of biodiesel [27–29]. Dias et al. [30] examined thedehydration of xylose to furfural with SO3H-modifiedMCM-41 materials. The direct synthesis of sulfonic acidfunctionalized porous silicas based on the sol–gel process,however, has received little attention. Wilson et al. [31]used the co-condensation method to prepare SO3H-functionalized mesoporous materials applied in the ester-ification of butan-1-ol with acetic acid. As one of the first,Xu and Lee [32] adopted the co-condensation approach tosynthesize SO3H-modified, macro-mesoporous sol–gelmonoliths (involving polymer-induced phase separation)intended for the microextraction of anesthetics followed bycapillary electrophoretic separation. Apparently, the poly-mer has not been removed from the monolith prior to thatapplication, which is expected to affect transport propertiesand impair the targeted functionality.

The preparation of sol–gel silica monoliths with bimodalpore-size distribution can be divided into four steps:(1) the sol–gel transition with concurrent, polymer-inducedphase separation to form a macro-microporous gel; (2) thewidening process by hydrothermal treatment to enlargemicropores to mesopores and ripening the pore structure;(3) the solvent exchange and drying, which should yield adry, mechanically stable monolith; and (4) calcination toremove organic matter on the inner surface [21]. However,if the co-condensation method is adapted to incorporateorganic groups into the pore system, then calcination cannot

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be applied, because the functional groups will also beremoved. Therefore, an alternative process that guaranteesefficient removal of the template (polymer) becomes one ofthe keys in a general route to these hierarchical hybridmaterials.

We introduce a rapid and efficient synthesis procedurefor the preparation of macro-mesoporous silica monolithswith homogeneously incorporated sulfonic acid func-tionalities by adapting the co-condensation sol–gel pro-cess. Through integration of a mild extraction step, thepolymer can be removed without losing the organicfunctionality. Importantly, full oxidation of the initiallyintroduced thiol groups to sulfonic acid groups occursconcurrently with the polymer extraction, resulting inhybrid monoliths with conveniently adjustable macroporesize, specific surface area, and SO3H coverage. The sim-plicity and efficiency of this novel synthesis route arecorroborated with a portfolio of characterization methods.Notably, inverse gas chromatography (IGC) is a powerfulapproach to quantify by surface energy analysis theimpact of the SO3H groups on the interfacial properties ofthese hybrid materials and address, in particular, thechanges in Lewis acid–base behavior occurring uponSO3H functionalization.

2 Experimental section

2.1 Chemicals

Tetraethoxysilane (TEOS, 99%) came from abcr (Karlsruhe,Germany). Poly(ethylene oxide) (PEO) with an averagemolecular weight of 100,000 was obtained from AlfaAesar (Haverhill, MA). (3-mercaptopropyl)trimethoxysilane(MPTMS, ≥95%) and hydrogen peroxide (30%) were pur-chased from Merck (Darmstadt, Germany). Urea came fromCaesar & Loretz (Hilden, Germany), sulfuric acid (95%)(AnalaR NORMAPUR® ACS, Reag. Ph. Eur.) and nitric acid(65%) from VWR International (Darmstadt, Germany). Allreagents were used without further purification.

2.2 Synthesis

Sulfonic acid modified silica monoliths were prepared by aco-condensation sol–gel synthesis, adapting the synthesisprocedure described in Kohns et al. [14]. In the prepared setof monoliths, only the PEO and MPTMS contents werevaried. With increasing MPTMS content the TEOS contentwas reduced by the respective equivalent. Complementary,a reference material without MPTMS was synthesized.Starting compositions employed for the set of monolithicmaterials are summarized in Table S1 in the ElectronicSupplementary Material (ESM).

First, PEO (1.5–1.8 g) and urea (2.4 g) were added todistilled water (18 g) under vigorous stirring. After stirringfor 30 min at room temperature, sulfuric acid (1.26 g) andTEOS (12.7–14.6 g) were added. After an additional 30 minof vigorous stirring MPTMS (0–2.11 g) was added to thesolution, which was stirred for 10 min. Each mixture waspoured into a PTFE tube (7 mm i.d., filled to 10-cm height).PTFE inlets were placed in a small lab autoclave, which wasput in an oven for gelation (24 h at 55 °C) and subsequenthydrothermal treatment (20 h at 120 °C). After cooling, wetgels were washed by submersion in stirred water, refreshed4–5 times until pH was neutral (~5 h). Each gel was thenplaced in its own plastic vessel, submerged with water, anddried in an oven (24 h at 105 °C). The drying process isbased on the procedure described in Kohns et al. [33]. Driedmonolithic rods had a diameter of ~0.5 cm and a length of~9 cm and were trimmed to a length of 5 cm. Remainingpieces served as reference to evaluate the success of theoxidation/extraction procedure.

The removal of the polymer was performed according tothe procedure described by Patarin [34]. Importantly, thereagents also oxidized the thiol to sulfonic acid groups.Briefly, each MPTMS-functionalized silica rod was stirredin a mixture of hydrogen peroxide (30%) and nitric acid(65%) at a ratio of silica:H2O2:HNO3 of 1 g:37 g:92 g atroom temperature. After 1 h, the solvent was refreshed andthe mixture heated to 60 °C for 18 h. Subsequently, silicarods were placed in a small lab Teflon autoclave withrefreshed H2O2/HNO3 solution and heated in an oven for2 h at 120 °C. After cooling, the wet gels were washed bysubmersion in stirred water, refreshed 5–6 times (~5 h).Each rod was then placed in its own plastic vessel, sub-merged with water, and dried in an oven (24 h, 105 °C) toobtain crack-free monoliths. For later comparison, severalmonolith pieces were calcined in a muffle furnace for 8 h,using a heating rate of 3 °Cmin−1 from room temperatureup to 600 °C.

2.3 Characterization

Silica rods were examined by mercury intrusion por-osimetry (MIP) measurements on a Pascal 140/440 por-osimeter (Thermo Fisher Scientific, Waltham, MA) over thepressure range of 0.015–400MPa. Pore-size distributionswere derived from the MIP data with Pascal softwareaccording to the Washburn equation setting the mercurycontact angle to 141°. The pressure range corresponds topore diameters between 3.7 nm and 100 µm.

Scanning electron microscope (SEM) images wererecorded with a FEI Nanolab 200 (Hillsboro, OR). Sam-ple pieces were vapor-deposited with Au at 10 kV. Thisstep was omitted for energy-dispersive X-ray analysis(SEM-EDX).

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Nitrogen physisorption measurements were carried outat −196 °C on an Autosorb-iQ sorption analyzer (Quan-tachrome Instruments, Boynton Beach, FL). Prior to themeasurements, samples were evacuated for 10 h at105 °C. Total pore volumes (Vmeso) were obtained withthe Gurvich rule at a relative pressure of p/p0= 0.95,specific surface areas (SBET) were determined by means ofthe Brunauer–Emmett–Teller equation in a range of0.05 ≤ p/p0 ≤ 0.3. Pore-size distributions were derivedfrom the adsorption branches of the physisorption iso-therms using the non-local density functional theory(NLDFT) method with a cylindrical pore model [35].

Elemental analysis (CHNS) was conducted on a varioMicro cube (Elementar, Langenselbold, Germany). Ther-mogravimetry (TG/DTA) was carried out in a temperaturerange of 25–600 °C with a 10 °C min−1 heating rate using aNetzsch STA 409 (Selb, Germany). Diffuse reflectanceinfrared Fourier transform spectroscopy (DRIFTS) wasperformed using a Vector 22 FT-IR spectrometer (Bruker,Billerica, MA) in the range of 700–4000 cm−1.

Magic-angle spinning (MAS) 13C and 29Si NMR spectrawere recorded on an Avance-III 400-MHz WB NMRspectrometer (Bruker BioSpin, Rheinstetten, Germany)equipped with a 4 mm MAS BB/1H probe at a Larmorfrequency of 400.15 MHz for protons, 100.62 MHz for 13C,and 79.49MHz for 29Si, respectively. All spectra werecollected at a spinning speed of 12 kHz at 20 °C andreferenced externally to TMS at 0 ppm. For cross-polarization (CP) experiments, 1024 scans were accumu-lated with a recycle delay of 5 s. Polarization transfer fromprotons to 29Si and 13C, respectively, was conducted for acontact time of 8 ms. For high-power decoupled (HPDEC)experiments, 256 scans were accumulated with a 90° pulsewidth of 6 µs (29Si) and a recycle delay of 60 s. The offsetwas set at −50 ppm and the spectral width to 32 kHz in bothexperiments. During acquisition, heteronuclear decouplingwas reached, using a SWf-TPPM at a radio-frequency fieldof 100 kHz [36]. Before conducting the NMR measure-ments, the monoliths had to be crushed.

IGC experiments were carried out on a Clarus580 GCapparatus (PerkinElmer, Waltham, MA) equipped withflame ionization detector and controlled by IGC software(Adscientis SARL, Wittelsheim, France). To conduct theIGC measurements, the rods had to be transferred into arobust column format. For that purpose, the rods wereembedded in a stainless-steel tubing (10.6 mm i.d./12.6 mmo.d. × 5 cm length; Swagelok, Solon, OH) using UHU®PLUS 300 epoxy resin adhesive (UHU, Bühl, Germany)with a composition of 1:1 (v/v) tube binder and hardener,adapting the method from Kohns et al. [14]. Columns werecompleted by attaching zero-volume reducing unions(Swagelok) to the stainless-steel tubes (Fig. S1 in the ESM).Prior to the investigations, the columns were conditioned

overnight at 80 °C with a helium flow (20 mLmin−1). AllIGC experiments were run at the same temperature and flowrate. Molecular probe molecules (C6–C9 n-alkanes, dichlor-omethane, chloroform, diethyl ether, acetonitrile, and ben-zene) were injected at least three times onto each column toderive retention times. Methane served as inert reference.

3 Results and discussion

3.1 Pore-space properties of the functionalizedmaterials

To present a novel preparation sequence for sulfonic acidfunctionalized sol–gel monoliths with hierarchically struc-tured pore systems, we adapted the strategy of a co-condensation synthesis procedure. The starting sol com-prised a sulfuric acid solution of TEOS as silica precursor,water as solvent, PEO as porogen, urea as pore-size control-ling agent (especially for mesopores), and MPTMS as func-tionalization precursor. MPTMS was added 30min afterstarting the hydrolysis of TEOS, as it hydrolyses faster due tothe methoxy groups. Following the well-known and estab-lished synthesis steps (gelation, hydrothermal treatment,washing, and drying), an extraction and at the same timeoxidation step was implemented in order to obtain hier-archically structured sulfonic acid functionalized silica rods[1, 14, 20, 33]. To confirm the success of the synthesis,samples were preserved untreated in their pristine state and (insome cases) subsequently calcined. The terminology forsample denotation is as follows: P-X stands for the initialweight of PEO and S-X for the initial volume of MPTMS.Abbreviations ext and c denote extracted and calcined. If notstated otherwise, results are presented for the modificationafter the oxidation/extraction procedure, referring to the sul-fonic acid functionalized silica. All monoliths were examinedby SEM and subjected to porosimetry analysis (Table 1).

First, it is important to note that bimodal pore systemswith interconnected pore structures were obtained for allsynthesized samples. SEM images and MIP results forsamples P-1.5-S-1 (lower end of polymer variation), P-1.8-S-1 (upper end of polymer variation), and P-1.8-S-2(highest MPTMS content) are compared in Fig. 1. Theimages visualize that the polymer-induced phase separationconserved an interconnected pore structure resulting fromspinodal decomposition. As the amount of polymer (PEO)increases, macropores and silica skeleton become muchfiner while the general morphology is preserved, as knownfor these sol–gel systems [1, 21]. The sponge-like porestructure is also recognized for the sample with the highestMPTMS content, although no state of spinodal decom-position could be frozen with a lower polymer content. Thiscan be explained by the fact that MPTMS affects the

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polarity of the sol. This parameter (among others) has asignificant influence on the gelation and phase separation[14, 21, 22].

The MIP-based pore-size distributions correlate wellwith a visual analysis of Fig. 1. The mean macropore size(dmacro) decreases with increasing PEO content from ~8.2 to~1.2 µm (Table 1), reflecting a familiar trend [21, 22, 37].However, the macropore size is also influenced by theMPTMS concentration. With increasing amount ofMPTMS, the size of the macropores increases, whichbecomes clear by comparing the respective samples with aPEO content of 1.7 or 1.8 g (Table 1): dmacro increases from~1.1 µm (for the reference sample P-1.7-S-0) to ~2.2 µm

(P-1.7-S-1) to ~4.8 µm (P-1.7-S-1.5). Similarly, with anincrease in MPTMS concentration by a factor of two fromsample P-1.8-S-1 to P-1.8-S-2, dmacro increases from ~1.2 to~2.4 µm. Apparently, the MPTMS influences the sol–gelprocess in such a way (polarity) that a later state of spinodaldecomposition is frozen, allowing larger pore structures toemerge. Interestingly, this behavior is opposite to thatobserved with urea [14]. In that work, we found thatincreasing the urea content of the starting sol comprising asulfuric acid solution of TEOS, PEO, and urea from 3 to24 wt% decreased dmacro from 2.3 to 0.6 μm and the skeletonthickness from 2.0 to 0.4 μm. Supposedly, with increasingurea content, phase separation, and gelation are retarded as

Fig. 1 MIP analysis (mercury intrusion curves and pore-size distributions as cumulative and relative pore volumes, respectively) and SEM imagesfor selected sulfonic acid functionalized silica monoliths with hierarchically structured pore systems (Table 1)

Table 1 Porosimetry data for the sulfonic acid functionalized silica monoliths

Sample dmacroa (nm) Vmacro

a (cm3 g−1) dmesoa (nm) Vmeso

a (cm3 g−1) εtotala (–) SBET

b (m2 g−1) dmesob (nm) Vtotal

b (cm3 g−1)

P-1.5-S-1 8253 1.98 10 0.54 0.85 418 9 0.88

P-1.6-S-1 3547 2.25 10 0.72 0.87 428 9 0.95

P-1.7-S-1 2162 2.33 11 0.81 0.87 469 9 1.04

P-1.8-S-1 1202 2.46 12 0.87 0.88 470 9 1.10

P-1.7-S-1.5 4821 2.09 13 0.52 0.85 546 7 0.96

P-1.8-S-2 2353 2.07 13 0.38 0.84 576 7 1.00

P-1.7-S-0c 1100 2.33 21 1.08 0.88 195 26 1.21

aMacropore size (dmacro) and volume (Vmacro), mesopore size (dmeso) and volume (Vmeso), as well as total porosity (εtotal) calculated from MIP data.dmacro and dmeso correspond to the median pore diameter (corresponding to 50% of the total intrusion volume increment)bBET surface area (SBET), mesopore size calculated via NLDFT (dmeso), as well as total mesopore volume (Vtotal) based on nitrogen physisorptionmeasurementscAll data refer to the calcined material

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well as shifted closer together, so that the formed mono-lithic structures represent a less evolved state of spinodaldecomposition, preserving smaller macropores and a thin-ner skeleton.

The total porosities of the sulfonic acid modified mate-rials are comparable to that of the calcined referencematerial of εtotal= 0.88 (Table 1). However, the samplesprepared with larger amounts of MPTMS (P-1.7-S-1.5 andP-1.8-S-2) do not that closely approach that value, which ismainly caused by their smaller mesopore volumes (Vmeso).Mean mesopore sizes (dmeso) based on the MIP data aresimilar for all samples (dmeso= 10–13 nm), but the actualpore-size distributions suggest that MIP is not appropriatefor the investigation of the mesoporosity in these samples. Itmay be possible that organic matter inside the mesopores iscompressed at the high pressures up to 4000 bar, whichwould bias the results except for the calcined referencematerial. Nitrogen physisorption measurements weretherefore additionally conducted. Figure 2 shows the iso-therms and derived pore-size distributions for selected sul-fonic acid modified, hierarchically structured sol–gelmonoliths and the calcined reference sample P-1.7-S-0.

The pore-size distributions in Fig. 2 reveal an impact ofthe MPTMS content on the mesopore formation. Thereference material has a very wide distribution of pore sizesafter pore widening, ranging from about 10 to 50 nm (meanpore diameter, dmeso= 26 nm). In contrast, the functiona-lized materials have relatively narrow pore-size distribu-tions after pore widening, with dmeso ≈ 9 nm (decreasing to7 nm as the amount of MPTMS is increased, Table 1). Onereason for this observation may be that the formation ofmicropores is influenced during the sol–gel process, espe-cially during condensation of the primary particles, resultingin smaller micropores with higher MPTMS concentrations[31]. On the other hand, hydrothermal treatment could beaffected, in which urea decomposes to ammonia, resultingin a basic pore widening (syneresis and Ostwald ripening).During this process, micropores formed in the sol–gel stepare widened to mesopores. It makes the pore system morestable and the material can also be easier dried with larger

pores under ambient conditions [33, 38]. The functionali-zation that is covalently bonded to the surface could miti-gate the effects of syneresis and Ostwald ripening, so thatthe pore widening is milder and more homogenous. At thispoint, it should be mentioned that the drying of crack-freemonoliths was possible even with the smallest mesopores(dmeso= 7 nm). The simple drying method of Kohns et al.[33] was adapted here with a lower temperature (105 °C) toprevent damage of the organic component.

Narrowed mesopore size distributions, as observed inFig. 2, are advantageous in applications that rely on size-selective effects, such as size-exclusion chromatography [39]and catalysis under confinement [40, 41]. This has beendemonstrated recently for the ring-closing metathesis ofvarious dienes, where the catalyst was selectively immobi-lized inside the mesopores of the support [42]. A narrowpore-size distribution with carefully adjusted mean pore size(relative to the size of the dienes) is essential here to define anarrow operational window for optimal macrocyclizationselectivity. Larger pores quickly increase the risk of unfa-vored oligomerization (decreasing the selectivity) and smallerones ultimately impede the favored macro(mono)cyclizationas well as diffusive transport into and out of the mesopores,which decreases conversion. For the materials compared inFig. 2, we note a substantial narrowing of the mesopore sizedistribution from the calcined reference material (standarddeviation >10 nm, relative standard deviation >40%) to thefunctionalized materials, characterized by a standard devia-tion of only ~2 nm and a relative standard deviation of ~25%.If the calcined sample were subsequently functionalized viapost-grafting, its wide mesopore size distribution would belargely retained.

The difference between the functionalized and pure silicamaterials is also seen in the isotherms in Fig. 2. All iso-therms belong to type IV, whereby the sulfonic acid func-tionalized materials display a clear H2(a) hysteresis. Theother materials (not included) have a similar characteristic.In contrast, the hysteresis behavior of the pure silicamaterials can be assigned to type H1 (but for a disorderedpore system). For this isotherm no plateau is reached, as the

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

Vad

s (cm

3 g-1

[STP

])p/p0

P-1.5-S-1 P-1.8-S-1 P-1.8-S-2 P-1.7-S-0

0 10 20 30 40 500.00

0.05

0.10

0.15

0.20

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dV/d

d p (c

m3 n

m-1

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dpore (nm)

P-1.5-S-1 P-1.8-S-1 P-1.8-S-2 P-1.7-S-0

Fig. 2 Nitrogen physisorptionisotherms (left) and pore-sizedistributions (right) for sulfonicacid functionalized silicamonoliths and the calcinedreference material (cf. Table 1)

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larger pores could not be completely filled during con-densation [43]. The impact of the functionalization reagenton the pore-space properties is also expressed in the specificsurface areas (SBET). The pure sol–gel material has a sig-nificantly smaller surface area (195 m2 g−1) than the func-tionalized materials (Table 1), for which SBET increases to418–470 m2 g−1 (mainly due to the smaller mesopore dia-meters, dmeso= 9 nm). Although the total pore volume ofthe non-functionalized material (1.21 cm3 g−1) is notreached, Vtotal of the functionalized materials increasessystematically from 0.88 to 1.1 cm3 g−1 with the amount ofpolymer, which is consistent with the SBET-values. Theincrease of the MPTMS content does not have a majoreffect on Vtotal, but SBET increases further (up to 576 m2 g−1

for sample P-1.8-S-2) and becomes similar to valuesreported in the literature for pure and sulfonic acid modifiedbimodal sol–gel monoliths [3, 32, 44].

To understand the impact of the oxidation/extractionstep, sample P-1.7-S-1 was exemplarily examined bynitrogen physisorption before that treatment (in pristineform), after the treatment (sulfonic acid functionalization),and after calcination. Isotherms and pore-size distributionsare summarized in Fig. S2 (ESM). We also recorded iso-therms for the reference material P-1.7-S-0 in untreated andcalcined forms for comparison. For sample P-1.7-S-1,mesopore size increases from dmeso= 8 nm (SH) to 9 nm(SO3H) to 10 nm (calcined), whereas no change is seen forthe reference material (Table S2, ESM). More importantly,the specific surface area of the pristine form is 273 m2 g−1,rising to 469 m2 g−1 after oxidation/extraction. After calci-nation, the specific surface area increases just slightly fur-ther to 479 m2 g−1. By contrast, for the reference materialP-1.7-S-0, SBET only increases from 164 to 195 m2 g−1. Thesharp increase in SBET for sample P-1.7-S-1 after the oxi-dation/extraction step strongly indicates the successfulremoval of the polymer from its pore system.

3.2 Efficiency of the in situ functionalization andextraction procedure

To examine our general synthesis procedure for its useful-ness, various characterization methods were applied.

Qualitative and quantitative techniques were selected toverify the approach and its individual steps. They provideresults, which altogether allow to reach clear conclusions onthe success of the presented synthesis procedure.

3.2.1 Elemental analysis and SEM-EDX measurements

The prepared materials were subjected to elemental analysisfor a first assessment of the amount of incorporated sulfonicacid groups. These data also help to evaluate the extractionprocedure that we implemented in order to remove thepolymer. Table 2 summarizes the carbon, hydrogen andsulfur contents of pristine samples before and after theoxidation/extraction step. Nitrogen is not listed, because itscontent remained below the detection limit.

A first qualitative identification can be based on thesignificant sulfur contents of all materials, except for sampleP-1.7-S-0 (synthesized without MPTMS). As the MPTMScontent increases (P-1.7-S-1.5), also the sulfur contentincreases from ~3 to ~4%. A further increase to twice theinitial MPTMS amount (P-1.8-S-2), however, does notincrease the sulfur content further in the formed material(Table 2). Because the condensation of additional MPTMSis unsuccessful, we assume that the system is saturated withfunctional groups. Based on the carbon contents of thepristine samples, the increasing amount of organic species(PEO, MPTMS) during the synthesis can be easily recog-nized. In comparison, the carbon content of the referencematerial (P-1.7-S-0) is much lower due to the absence of thefunctionalization reagent. Comparing extracted with pristinesamples helps to verify the success of the extraction step.Carbon contents decrease significantly after extraction, asdocumented in a loss of 70–73%. The impact of theextraction step can be evaluated most clearly by inspectingthe reference material, in which the amount of organiccomponent is reduced by 69%. Therefore, the extractedreference material still contains carbon, suggesting that thepolymer has not been completely removed. This is not thatobvious for the functionalized materials, because we alsointroduce carbon with the propyl chain at the sulfur. On theother hand, the sulfur contents differ only slightly beforeand after the extraction step. Here, an exceptional loss of

Table 2 Elemental analysisbefore and after the oxidation/extraction step

Sample Pristine Extracted

C (%) H (%) S (%) C (%) H (%) S (%)

P-1.5-S-1 12.9 2.3 2.9 3.9 1.5 2.5

P-1.6-S-1 13.5 2.4 2.9 3.6 1.6 2.7

P-1.7-S-1 14.1 2.5 2.8 3.9 1.5 2.8

P-1.7-S-1.5 15.8 2.9 4.1 4.8 1.7 3.8

P-1.8-S-2 16.1 3.0 4.0 4.8 1.8 3.8

P-1.7-S-0 3.5 0.8 0.0 1.1 0.7 0.0

Journal of Sol-Gel Science and Technology (2020) 96:67–82 73

14% is observed for sample P-1.5-S-1, but the sulfur lossfor all other extracted samples remains below 6%.

With the determined sulfur contents, the loadings of thebulk materials with sulfonic acid groups can be calculated. Formaterials prepared with the standard amount of 1ml MPTMS,the SO3H loading increases slightly with increasing amount ofPEO in the synthesis from 0.78mmol g−1 for sample P-1.5-S-1 to 0.84mmol g−1 (P-1.6-S-1) to 0.87mmol g−1 (P-1.7-S-1).As expected, samples P-1.7-S-1.5 and P-1.8-S-2 have a higherSO3H loading (1.2mmol g−1). Assuming that all sulfur atomsare on the pore surface, the number of SO3H groups/nm2 canbe estimated using the SBET-values from Table 1 and theAvogadro constant. This allows to provide SO3H loadings of1.12 nm−2 (P-1.5-S-1), 1.17 nm−2 (P-1.6-S-1), 1.11 nm−2 (P-1.7-S-1), 1.31 nm−2 (P-1.7-S-1.5), and 1.24 nm−2 (P-1.8-S-2),which are comparable to those for purely mesoporous mate-rials that were also synthesized through the co-condensationprocedure [24]. There, it has been emphasized that co-condensation ensures significantly higher SO3H loadings thanachieved using common grafting methods. Interestingly, themonolithic materials prepared with the standard amount of1ml MPTMS all end up with very similar numbers of SO3Hgroups/nm2, despite their different morphological properties(particularly mesopore volume and specific surface area)caused by the different PEO contents used in their synthesis. Itshould also be noted that the pore-widening process has nosignificant influence on the sulfur content (Table S3 inthe ESM).

Using the SO3H loadings, sulfur and carbon contents canbe interrelated to estimate the fraction of the organic com-ponent belonging to the functionalization. Considering thestoichiometry of the introduced functional chain (S:C=1:3), about 17–19% of the residual carbon content is due topolymer, except for sample P-1.5-S-1 (27%). For materialsprepared with larger amounts of MPTMS, it is only ~10%.Therefore, we started further investigations of the extraction

process by extending the duration of the hydrothermaltreatment with H2O2/HNO3 from 2 h to 4 and 6 h (Table S4in the ESM). These results showed that an extendedhydrothermal treatment further reduced the carbon content,but also the sulfur content significantly (by ~28%). Con-sequently, extending the hydrothermal treatment for poly-mer extraction is no viable alternative, as the associated lossof sulfur (sulfonic acid groups) is undesirable. At this point,it cannot be clarified if the polymer remains on the surfaceof the materials (and prevents access to functional groups)or inside the pores due to diffusion limitations during thewashing process.

SEM-EDX measurements support the data from ele-mental analysis. Figure 3 shows SEM images and EDXmappings for the pristine and extracted forms of sampleP-1.7-S-1. Red dots represent the presence of carbon atomson the surface, which can be attributed to PEO (employedfor the polymer-induced phase separation during the sol–gelsynthesis) and the functionalization. Blue dots indicate thesulfur atoms incorporated by the functionalization reagentMPTMS during the gelation process. From the EDX map-ping, it is seen that the functionalization is homogeneouslydistributed over the surface of the materials, which weexplain with the effect of the in situ co-condensation. Fur-ther, based on Fig. 3 it can be assumed that, while carbonatoms are removed to a substantial degree by the extractionstep, the sulfonic acid functionalization remains largelyintact. This suggests that a carefully timed extraction canindeed be used to selectively remove the polymer from thematerial, without impairing the functionalization.

Calculated compositions demonstrate this effect moreclearly. The untreated sample (top row in Fig. 3) consists of24 at.% C, 1 at.% S, 52 at.% O, and 21 at.% Si (roundedvalues). Here, the ratio between carbon and silicon contentsappears uncharacteristic. However, we have to recall thatEDX is surface sensitive and the bulk of the material (which

Fig. 3 SEM-EDX imagesrecorded for sample P-1.7-S-1 inits pristine form (top) and afterthe oxidation/extraction step(bottom), indicating carbon (red)and sulfur (blue) contents

74 Journal of Sol-Gel Science and Technology (2020) 96:67–82

mainly consists of silicon and oxygen) cannot be detected inthis way. After extraction (bottom row), the atomic com-position is 15 at.% C, 1 at.% S, 56 at.% O, and 26 at.% Si.The loss of organic matter is mainly attributed to theremoval of polymer, as the sulfur content does not change.It is confirmed by the increase of the silicon content. Moresilicon can be detected after removing the polymer, whichno longer obscures part of the surface.

For samples prepared with larger MPTMS amounts,SEM-EDX measurements were carried out to provideinformation particularly on the sulfur content. Thesematerials were examined in the oxidized/extracted form.Sample P-1.7-S-1.5 has a sulfur content of 1.4 at.%(2.3 wt.%), which is an increase compared to sampleP-1.7-S-1 (1.0 at.%, 1.7 wt.%). The increase is evenstronger for sample P-1.8-S-2, which has a sulfur contentof 1.6 at.% (2.5 wt.%). It shows that the sulfur content canbe increased while preserving an interconnected porestructure.

For better evaluation of the polymer extraction step,SEM-EDX measurements were conducted for the referencematerial P-1.7-S-0 in pristine, extracted, and calcined forms(Fig. S3, ESM). The pristine sample reveals 11 at.% C,0.2 at.% S, 59 at.% O, and 28 at.% Si. After extraction, thecarbon content decreases to 1 at.%, while the sulfur contentis constant at 0.2 at.%. The oxygen content increased(slightly) to 61 at.% and the silicon content to 36 at.%. Aftercalcination, the atomic composition did not change withrespect to the extracted sample (1 at.% C, 0.2 at.% S, 61 at.%O, and 36 at.% Si). According to these results, the extractionprocess (which also serves to oxidize the thiol to sulfonicacid groups) is as effective as the calcination step inremoving the polymer. The small amounts of carbon andsulfur remaining after extraction and calcination may be dueto residues physisorbed on the surface. It could not befinally clarified why sulfur was detected. Because quantifi-cation by SEM-EDX is known to be difficult for hetero-geneous porous materials, we rely on the data fromelemental analysis in this regard. The SEM-EDX analysisthen serves for visual representation of the homogeneousdistribution of the functionality.

3.2.2 FT-IR spectroscopy and thermogravimetric analysis

To verify specifically the oxidation of thiol to sulfonic acidgroups (occurring simultaneously during polymer extrac-tion) DRIFTS and TGA were applied. DRIFT spectra werecollected for sample P-1.7-S-1 in pristine, oxidized/extrac-ted, and calcined forms (Fig. 4a). All three samples exhibitvibrations typical for the silica skeleton: Si–O–Si stretchingvibrations in the range of 1000–1250 cm−1 and the corre-sponding deformation vibrations at about 800 cm−1. Inaddition, all spectra show intense absorptions due to O–H

stretching (3400–3600 cm−1) and deformation vibrations(~1640 cm−1) caused by adsorbed water.

The sharp band at ~3750 cm−1 in the calcined materialcan be assigned to Si–O–H stretching vibrations. Thealmost complete disappearance of this silanol vibration inpristine and oxidized samples suggests the successfulincorporation of a covalently bonded functionalization. Forthe pristine material (blue) we note three additionalabsorptions: At ~2580 cm−1 the S–H vibration of the thiolgroup (indicating successful functionalization) and in therange of 2850–2950 cm−1 and 1450–1500 cm−1 the C–Hstretching and deformation vibrations, respectively, by thepropyl chain of the functionalization and by the polymer.These vibrations can also be identified in the oxidizedmaterial (red), but they are much weaker. It suggests that thepolymer has been largely removed, so that absorption eitheroriginates from the propyl chain of the functionalization orresidual polymer. Most importantly, however, the explicitloss of the S–H vibration confirms a successful oxidation ofthiol to sulfonic acid groups. Vibrations typical for the

1000150020002500300035004000

ν Si-O-H

δ C-Hν C-H

ν S-Hδ O-H

ν Si-O-Si

Inte

nsity

(a.u

.)

Wavenumber (cm-1)

calcined SH SO3H

δ Si-O-Si

a

100 200 300 400 500 600

-0,05

-0,04

-0,03

-0,02

-0,01

0,00

SO3Hdm

/dT

(mg

K-1

)

T (°C)

calcined SH SO3HSH

b

Fig. 4 DRIFT spectra (a) and TGA curves (b) for sample P-1.7-S-1 inpristine (blue), oxidized/extracted (red), and calcined (black) forms

Journal of Sol-Gel Science and Technology (2020) 96:67–82 75

sulfonic acid, as shown by Marshall et al. [24] and Wilsonet al. [31], could not be assigned, because they overlap withthe strong Si–O–Si stretching vibrations (1145–1200 cm−1

for –SO2–OR, ~1050 cm−1 for S=O). In comparison, the

calcined sample (black) only shows vibrations typical forsilica and adsorbed water, which indicates completeremoval of organic moieties.

The TGA data, plotted as differential mass loss in atemperature range of 30–600 °C (Fig. 4b), support theconclusions derived from DRIFTS. All modifications ofsample P-1.7-S-1 exhibit a peak below 120 °C caused bythe loss of water, but in different intensity. The pristinesample (blue) reveals a thiol decomposition peak at ~350 °C(no further decompositions are recorded). The actual massloss is high, because the polymer also starts to decomposein this temperature range. For the oxidized/extracted sample(red) a pronounced mass loss is observed, indicating thepresence of the sulfonic acid functionalities. Decompositionbegins at lower temperatures already (~340 °C), whichsuggests the presence of residual polymer. All theseobservations agree with the results of previous studies [45].

3.2.3 MAS NMR spectroscopy

NMR was used to corroborate our previous conclusionsabout the synthesis of the sulfonic acid functionalized sili-cas. These studies were performed exemplarily with sampleP-1.7-S-1 before oxidation/extraction, after this treatment,and after the calcination step in order to compare the dif-ferent surface modifications (thiol, sulfonic acid, and nofunctionalization). For this purpose, 13C and 29Si spectrawere recorded.

Figure 5a shows CP/MAS 13C NMR spectra of thesample before (blue) and after (red) extraction of thepolymer (we did not expect any signals for the calcinedmaterial). The spectrum of the pristine material reveals threepeaks at 11.3, 27.0, and 71.1 ppm. The first resonance at11.3 ppm is caused by the carbon atom C3 of the functio-nalization in direct neighborhood to the silicon atom. Thepeak at 27.0 ppm is due to the central carbon atom of thepropyl chain (C2) and the carbon atom C1 that directly bindsto the thiol group. Both resonances overlap, as known fromthe literature [46]. The peak at 71.1 ppm can be assigned tothe carbon atoms of the polymer (CPEO) that is still on thesilica surface [47]. The treated material (red spectrum) alsoexhibits three signals, localized at 11.8, 17.6, and 53.3 ppm.However, the signal from the polymer is not visible anylonger. It indicates the successful extraction of PEO,whereas our previous study by elemental analysis could notconfirm this. The resonances at 11.8 and 17.6 ppm arecaused by the carbon atom binding to the silicon atom (C3)and the adjacent carbon (C2). The peak at 53.3 ppm can beassigned to the carbon in direct neighborhood to the

sulfonic acid functionality (C1, strongly shielded by theoxygens), which agrees with data for sulfur groups fromthe literature [46]. Based on this insight, we conclude thatthe thiol groups have been converted into sulfonic acidgroups and that the polymer has been removed at the sametime, confirming the goals targeted with the adapted oxi-dation/extraction step.

-140-120-100-80-60-40-200

c

T2

Q4

Q2

29Si chemical shift / ppm

calcined SH SO3H Q3

-140-120-100-80-60-40-200

T1 Q2

29Si chemical shift / ppm

calcined SH SO3H

Q4Q3

T3

T2b

020406080100

C3

C2C1

CPEO

C3

13C chemical shift / ppm

SH SO3H

C1+C2

a

Fig. 5 CP/MAS 13C NMR spectra (a), CP/MAS 29Si NMR spectra (b),and HPDEC/MAS 29Si NMR spectra (c) collected from sample P-1.7-S-1 in pristine (blue), oxidized/extracted (red), and calcined(black) forms

76 Journal of Sol-Gel Science and Technology (2020) 96:67–82

To support this qualitative proof of functionalizationfrom Fig. 5a, CP/MAS 29Si NMR spectra (Fig. 5b) wererecorded in addition. They can be used to define con-nectivities between coupled nuclei and investigate thedynamics in solids. As the technique is based on hetero-nuclear dipolar interactions, it is sensitive to internucleardistances and the mobility of functional groups [48]. Allthree materials exhibit the typical signals of Q4 (SiO4), Q3

(SiO3OH), and Q2 (SiO2(OH)2) silicon species in theirspectra (see Fig. S4 in the ESM for additional illustration).The calcined sample (black spectrum in Fig. 5b) reveals noother signals than from the Q-species, which implies thatthe functionalization has been removed during the calcina-tion step. Silicon species with nearby protons, such as Q2

and Q3, are significantly enhanced compared to Q4 speciesin CP/MAS due to polarization transfer during acquisition.The peak with the Q-species can be broad actually due tothe heterogeneity of the silica skeleton regarding bondlengths and angles [49]. Taking a closer look at the spectraof the untreated functionalized sample (blue) and the oxi-dized/extracted sample (red), T-species can now be identi-fied (cf. Fig. S4) and assigned in agreement with literature[50]. T2-signals (SiO2(OH)(CH2R)) around −66 ppm areprominent in both spectra. A T3-signal (SiO3(CH2R)) isonly detected for the untreated material (−76.6 ppm). Fur-thermore, the T1-signal (SiO1(OH)2(CH2R)) is only weaklydistinctive here (−57.6 ppm). On the other hand, no T3-signal is detected in the treated (SO3H) material, whereasthe T1-signal at −56.9 ppm is much more pronounced. Thismay be caused by bonds on the surface (primarily func-tionalization) that have been broken during the extractionstep, whereby more single (T2) and geminal (T1) silanolgroups were formed, which would be consistent with theloss of some sulfonic acid groups already indicated byelemental analysis and SEM-EDX.

This assumption is confirmed by the HPDEC spectra inFig. 5c. That technique can be used to derive quantitativeevidence about the respective silicon species by a directstimulation of each silicon atom. For this purpose, areas ofthe individual peaks in a spectrum were calculated byintegration (exemplarily shown for the untreated sampleP-1.7-S-1 in Fig. S4, ESM) and related to each other. Fromthese relations, conclusions can be derived regarding theactual bonding in the materials that improve verification ofthe synthesis procedure. Based on the HPDEC spectra inFig. 5c, it is clear that the Q4 species is the main constituentin all three modifications. This was expected, as it repre-sents the bulk backbone of the monoliths. Furthermore, inboth pristine (blue) and oxidized/extracted (red) materials,only T2-species are detected. Here, the signal for the sul-fonic acid functionalized sample is weak, which furthersupports our assumption about the partial loss of functio-nalization in the extraction step. In the (blue) spectrum of

the untreated material, two peaks are visible side by side (cf.Fig. S4). As they appear exactly in the range for T2-species,we took them as one T2 (SiO2(OH)(CH2R)) signal [50].

Quantification becomes more meaningful with the per-centages of the individual silicon species, as calculated fromthe peak areas and summarized in Table 3. As mentioned,the Q4 is the most prominent species. It dominates with 91%in the calcined material compared to 72% and 71% in SH-and SO3H-functionalized samples. The calcination stepenhances the formation of siloxane bonds (Q4) by dehy-dration of surface silanols. When cooling to room tem-perature, this process is somewhat reversed by atmospherichumidity, resulting in the low Q3 and Q2 quantities forsample P-1.7-S-1_ext_c. A comparison of the samplesbefore and after the oxidation/extraction step shows that Q4

quantities do not differ much. It should be noted that foreach T-signal a Q4-signal is detected, as a siloxane bond isadded due to the functionalization reagent. Furthermore, theT2-species decrease from 6 to 4% after the extraction, assuggested by our previous results. This is accompanied by asignificant increase in Q2-species (from 4 to 9%), which isconsistent, because the silicon atom most likely replaces thecleaved functionalization by a hydroxyl group. As a con-sequence, a Q2-species is formed from a T2-species. Still,the T2-signal is detected for the oxidized/extracted material(sample P-1.7-S-1_ext), which supports the presentedsynthesis scheme. It was not possible to quantify the siteoccupancy with respect to the condensation, as described byIde et al. [51], because the surfaces of the materials weremanipulated by the sample preparation (grinding) requiredfor the NMR measurements. The generation of small par-ticles increases the number of surface silanol groups (Q3-and Q2-species), because additional surface (previouslybulk) is created and the ratio of T- to Q3-species shifts.Nevertheless, the results of the NMR investigations are veryinstructive and demonstrate clearly that sulfonic acid func-tions were covalently bound to the surfaces of the preparedhierarchically structured silica monoliths to a significantextent.

The NMR investigations generally substantiate ourapproach. The 13C spectra demonstrate that it was possibleto extract the polymer and at the same time oxidize thiol tosulfonic acid groups. Complementary, the 29Si spectracorroborate this important step by detecting silicon

Table 3 Relative peak areas of silicon species as quantified byHPDEC/MAS 29Si NMR

Sample Q4 (%) Q3 (%) Q2 (%) T2 (%)

P-1.7-S-1 72 18 4 6

P-1.7-S-1_ext 71 16 9 4

P-1.7-S-1_ext_c 91 7 2 –

Journal of Sol-Gel Science and Technology (2020) 96:67–82 77

T-species, which represent the attachment of carbon atomsto silicon.

3.3 Inverse gas chromatography (IGC)

IGC is a source of physicochemical data for a variety ofmaterials. The term indicates that the stationary phase of achromatographic column is the material of interest and in turnexamined through its interactions with well-known probemolecules. It was therefore necessary to prepare the mono-lithic rods in a column format suitable for the IGC equip-ment. The cladding approach of Kohns et al. [14] was used,in which the rods were first inserted into a stainless-steel tubeand then embedded by epoxy resin. The attachment of zero-volume reducing unions completed the setup (Fig. S1 in theESM). For the IGC investigations, a series of probe mole-cules (C6–C9 n-alkanes, dichloromethane, chloroform,diethyl ether, acetonitrile, and benzene) were injected indi-vidually and at least three times onto all columns for thedetermination of retention times. Sulfonic acid functionalizedmaterials with different SO3H-loadings (samples P-1.7-S-1and P-1.7-S-1.5) and the reference material without functio-nalization (P-1.7-S-0) were examined.

IGC characterizes the strength of interaction between anadsorbate and a solid surface through the total surfaceenergy γs

t, covering dispersive (Van der Waals) and polar/specific (Lewis acid–base) interactions. To describe surfaceproperties (e.g., acidity, wettability) comprehensively, bothterms (dispersive component γs

d and specific component γssp

of the surface energy) have to be taken into account. Thedispersive component can be calculated from interactionsbetween n-alkanes and the solid surface. Dorris and Gray[52] introduced calculations based on the linear relationshipbetween free energy of adsorption and molecular weight ofn-alkanes, the so-called n-alkane line, to access the dis-persive component of the surface energy. The specificcomponent is determined from interactions of polar mole-cules with the surface. These interactions involve specific aswell as dispersive contributions, which makes a distinctionmore difficult. Here, the dispersive component of the freeadsorption enthalpy (as obtained from the n-alkane refer-ence line) can be subtracted from the experimental freeadsorption enthalpy to isolate the specific component.Details about these calculations are given in the ESM.

Free energies of adsorption (ΔGads) obtained by IGCreflect the strength of adhesive interaction with dispersiveand specific contributions for each individual probe. Byplotting ΔGads against the number of carbon atoms(expressed with the topological index χT), the interactionscan also be represented graphically (Fig. 6). The topologicalindex was introduced to compare retention data of differentprobes despite their different molecular properties (such asadsorption area and interaction strength). Brendlé and

Papirer [53, 54] developed the χT-parameter based on theWiener index for linear, branched, and cyclic alkanes, alsotaking polar eluents into account.

The data plotted in Fig. 6 show that both dispersive andspecific interactions are intensified with increasing amountof MPTMS in the synthesis and thus, with increasing sul-fonic acid loading in the formed material. For example, then-alkane line shifts significantly to higher ΔGads values(accompanied by an increasing slope), indicating strongeradhesion of the alkanes to the surface as we progress fromsample P-1.7-S-0 to P-1.7-S-1.5. A clear influence of theintroduced SO3H groups is also seen for the polar probemolecules, ΔGads for sample P-1.7-S-1 increases stronglycompared to the reference material. Interactions are inten-sified for Lewis-base (benzene, diethyl ether, and acetoni-trile) as well as Lewis acid probes (DCM, chloroform). Thisindicates an increase in electron-donor and -acceptor cap-abilities of the materials upon silane incorporation. From thedata in Fig. 6, dispersive and specific components and theresulting total surface energy of the respective material canbe calculated (Table 4).

The data in Table 4 confirm our expectations thatthe specific component (γs

sp) dominates over the dispersivecomponent (γs

d) for silica due to the surface silanols (sampleP-1.7-S-0). The total surface energy (122 mJ m−2) is

2 3 4 5 6 7 8 95

10

15

20

25

30

diet

hyl e

ther

benz

ene

chlo

rofo

rm

acet

onitr

ileD

CM

P-1.7-S-0 P-1.7-S-1 P-1.7-S-1.5

ΔGad

s (kJ

mol

-1)

χT

n-alkane lines

Fig. 6 Free energy of adsorption for different polar and n-alkane probemolecules on non-functionalized silica P-1.7-S-0 (black) and sulfonicacid modified silicas P-1.7-S-1 (blue) and P-1.7-S-1.5 (red), tem-perature: 80 °C

Table 4 Dispersive (γsd) and specific (γs

sp) components of the totalsurface energy (γs

t) and Gutmann constants KA and KD obtained byIGC at 80 °C

Sample γsd [mJ m−2] γs

sp [mJ m−2] γst [mJ m−2] KA KD KA/KD

P-1.7-S-0 35 87 122 0.57 0.38 1.5

P-1.7-S-1 47 168 215 0.77 0.48 1.6

P-1.7-S-1.5 67 179 246 0.77 0.48 1.6

78 Journal of Sol-Gel Science and Technology (2020) 96:67–82

composed of approximately one-third dispersive and two-thirds specific interactions. Upon functionalization (sampleP-1.7-S-1), γs

sp increases significantly, but also the dis-persive part increases from 35 to 47 mJ m−2, which can beattributed to the nonpolar propyl chain of the introducedgroup. Rückriem et al. [55] have described the gain inpolarizability for modified silica surfaces with each addi-tional methylene unit. Nonetheless, the specific partincreases much stronger and this also raises the total surfaceenergy drastically to 215 mJ m−2. By increasing the SO3Hloading (sample P-1.7-S-1.5), γs

t rises further to twice thevalue of the reference material (246 mJ m−2), but the spe-cific component does not increase as much as the dispersivecomponent. Thus, the surfaces of the functionalized mate-rials can be still described as polar, even as the interactionwith nonpolar probe molecules is enhanced. To conclude,and with respect to characterizations already discussed, wecan assume a covalently bonded sulfonic acid modificationwith different coverages on the surface of these hier-archically structured sol–gel silica monoliths.

Lewis acid–base characteristics of the monolith surfaceswere subsequently analyzed using the concept of Gutmann[56]. Accordingly,

ΔGspads � ΔHsp

ads ¼ KA � DN þ KD � AN; ð1Þ

where ΔHspads is the enthalpy of sorption of a polar probe

molecule, KA is the electron-acceptor number (acid) of theinvestigated surface and KD is the corresponding electron-donor number (base). DN and AN are Gutmann’s donor andacceptor numbers used to quantify the acid–base propertiesof the probe molecules. DN-values are calculated bycalorimetric measurements from the sorption of the probemolecules (electron donor) onto SbCl5 (electron acceptor).AN-values are derived by 31P NMR measurements from thechemical shift of Et3PO (electron donor) in the presence ofan electron acceptor [57–60]. The enthalpy of sorptionΔHsp

ads is obtained by IGC (using at least three differenttemperatures) with the following equation:

ΔGads ¼ ΔHads � TΔSads: ð2ÞRepeating this procedure for each probe molecule to

estimate KA and KD values would be very time-consuming.During these periods aging effects can occur for labilesamples at the elevated temperatures required for themeasurements [61]. We therefore made a common sim-plification by using the specific interaction parameterΔGsp

ads for calculation of KA and KD. Both temperaturedependent values can be received from the slope and theintercept of a linear plot of ΔGsp

ads/AN against DN/AN, asillustrated in Fig. S5 (ESM) [62, 63]. It should be notedthat acid–base parameters for solid surfaces derived byIGC measurements depend on the employed approach and

the DN and AN reference values [57, 59]. In that respect,all measurements were performed under identical condi-tions and with the same evaluation routines. KA and KD

values resulting from IGC analysis are summarized inTable 4. The acid–base character of the monolith surfacescan be described by the ratio KA/KD. A ratio larger than 1.1indicates acidic character of the surface, KA/KD < 0.9 reflectsbasic character [64].

The Lewis acid parameter KA increases from 0.57 to 0.77after introduction of the sulfonic acid functionality into thepore network (Table 4), which is a clear sign that the surfacehas become more acidic. Generally, the density of surfacesilanol groups should decrease after binding of a functiona-lization, which should also cause KA to drop. However, inour synthesis procedure KA increases significantly, whichindicates a more acidic surface and corroborates the intro-duction of the sulfonic acid groups, which are more acidicthan the silanol groups. KA/KD ratios for the functionalizedmaterials are just slightly elevated with respect to the non-functionalized sample. Apparently, the increase in acidity iscompensated by the simultaneously increasing Lewis-baseparameter KD, as already noted with the ΔGads values foracidic probe molecules.

Bauer et al. [63] demonstrated a significant change inKA/KD value for porous glass upon MPTMS functionaliza-tion (leading to the SH modification), where a strong basiccharacter of the surface was observed. With respect to oursynthesis, it is not possible to analyze the materials with theirnon-oxidized (SH) modification, because the polymer that isstill in the pores prevents unbiased interaction of probemolecules with the surface. However, it can be concludedthat the increase in MPTMS concentration does not affectacidity, because the KA/KD value for sample P-1.7-S-1.5remains at 1.6 and KA at 0.77 (Table 4). It may be assumedthat the surface is saturated with functional groups. Eventhough smaller changes are noticeable for these SO3Hloadings of 1.11 nm−2 (P-1.7-S-1) and 1.31 nm−2 (P-1.7-S-1.5), as detected by surface energy analysis (Table 4), Lewisacid–base parameters of these samples remain constant. Theincrease in polar interaction does not contradict constantacidity, because both acidic and basic polar probe moleculesshow stronger adhesive interactions for the higher SO3Hloading (cf. Fig. 6).

4 Conclusions

Organic–inorganic hybrid sol–gel materials with hierarchicalpore systems were synthesized by a simple, efficientsynthesis route via co-condensation of TEOS and MPTMS,resulting in SO3H functionalized macro-mesoporous silicamonoliths. The polymer (template) used for generation ofthe macroporosity was removed with an extraction step, in

Journal of Sol-Gel Science and Technology (2020) 96:67–82 79

which the introduced thiol groups were simultaneouslyoxidized to sulfonic acid groups, leading to hybrid materialswith uniform distribution of the SO3H groups inside themesopores. The macropore size, specific surface area, andcoverage with SO3H groups are conveniently adjusted in thissynthesis route. For example, macropores formed by thepolymer-induced phase separation were generated withmedian sizes from 1.2 to 8.2 µm at conserved mesoporediameters of 7–9 nm, and the materials offer specific surfaceareas of up to 576m2 g−1. Using the co-condensation pro-cess, SO3H loadings of 1.1–1.3 groups/nm2 were realized byvarying the initial MPTMS content. Higher MPTMS contentalso resulted in larger macropore sizes due to its influence onthe phase separation. Importantly, the extraction of thepolymer was facilitated while keeping most of the organicfunctionality intact. Altogether, it demonstrates that the co-condensation approach offers a simple, efficient route tofunctionalized sol–gel materials. Elemental analysis, SEM-EDX, thermogravimetry, DRIFTS, 13C and 29Si solid stateNMR, and IGC provided detailed qualitative and quantita-tive evidence on the formation of surface-bound SO3Hgroups. With IGC, in particular, the effect of the SO3Hgroups could be quantified by analyzing dispersive andspecific components of the total surface energy as well asLewis acid–base parameters of these surfaces. Notably, thechanges in surface acidity upon SO3H functionalizationcould be excellently monitored.

The prepared hybrid monoliths are primarily useful forprocesses involving ion exchange, acid catalysis, or protonconduction, where morphological properties like macro-pore size and surface area will be easily adjusted inaddition to the density of the SO3H groups. However, theirnarrow mesopore size distribution further helps in appli-cations that benefit from size-selective transport likechromatographic (e.g., biomolecular) separations or cata-lysis under spatial confinement. In this respect, the clad-ding approach applied to the monolithic rods in this workfor IGC analysis also offers a robust column designfor their convenient future use as a fixed-bed adsorber and/or reactor.

Acknowledgements The authors thank Niklas Olinski for preliminarylab studies.

Funding Open Access funding enabled and organized by Projekt DEAL.

Compliance with ethical standards

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