theionicliquid[bmim][fecl ]catalyzesthe...

DOI: 10.19185/matters.201803000004 Matters (ISSN: 2297-8240) | 1

[email protected]

DisciplinesNanotechnologySupramolecular ChemistryPhysical Chemistry

KeywordsSilicaIronIonic LiquidIonic Liquid PrecursorHydrogen Peroxide Decom-position

Type of ObservationStandalone

Type of LinkCase study

Submitted Feb 13, 2018 Published Mar 21, 2018

3 x

Triple Blind Peer ReviewThe handling editor, the re-viewers, and the authors areall blinded during the reviewprocess.

Full Open AccessSupported by the VeluxFoundation, the University ofZurich, and the EPFL Schoolof Life Sciences.

4.0

Creative Commons 4.0This observation is dis-tributed under the termsof the Creative CommonsAttribution 4.0 InternationalLicense.

The ionic liquid [Bmim][FeCl4] catalyzes theformation of iron doped mesoporous silicaaerogels for H2O2 decompositionAndreas Taubert, Ruben Löbbicke, Karsten Behrens, Dörte Steinbrück, Lucy Kind, TianZhao, Christoph JaniakChemistry, University of Potsdam; Institut für Chemie und Bioanalytik, Fachhochschule Nordwestschweiz; Institut für Anor-ganische Chemie und Strukturchemie, Universität Düsseldorf

AbstractWe report the synthesis of monolithic porous hybrid materials from tetramethoxysi-lane (TMOS), the bifunctional silane bis(trimethoxysilyl)propylamine (BTMSPA), andthe ionic liquid 1-butyl-3-methylimidazolium tetrachloridoferrate(III), [Bmim][FeCl4],via a one-pot reaction. The reaction products are meso- and macroporous organosil-ica monoliths, the iron is incorporated within the silica matrix, and the materials areefficient catalysts for H2O2 decomposition.

IntroductionIonic liquids (ILs) have attracted interest for many different reasons and they have beeninvestigated for a multitude of applications [1] [2] [3] [4] [5]. A rather recent develop-ment is the use of ILs as precursors for (inorganic) materials. Among others, inorganicnano- and microparticles such as CuCl [6], Ag [7], or Au [7] have been made by usingILs as precursors, where the IL provides at least one component of the final material.Accordingly, these ILs have been termed ionic liquid precursors (ILPs) [8] and an anal-ogous synthesis strategy has also been developed for (doped) carbon materials [9] [10][11], Ni andNiO nanomaterials [12], CuS [13], Fe3C [14], In2O3 [15], ZnO [16], CuO [17].Other studies have focused on the synthesis using fluorine-containing anions as fluo-ride precursors [18] [19] [20]. Alternatively, deep eutectic solvent precursors (DESPs)have also been introduced for the synthesis of such materials [21]. As a result, properchoice of an ILP (or DESP) will lead to materials with interesting and useful propertiesvia synthesis strategies such as carbonization or precipitation, whereby the IL is at thesame time the solvent, template, and the precursor for the (inorganic) material.So far, however, most studies concentrate on the synthesis of rather simple, oftenbinary, compounds from ILPs. The current work focuses on the question whether[Bmim][FeCl4] can act as an ILP for the synthesis of a transition metal containing meso-porous silica material. It has also been demonstrated that ILs are efficient tools for thedesign ofmesoporous silica [22] [23] [24] [25] [26]. As a result, wewondered if a suitableIL may (i) catalyze the hydrolysis of appropriate silanes to form a mesoporous matrixand (ii) at the same time serve as the metal source for the formation of catalyticallyactive metal species within the SiO2 network.

ObjectiveDetermine if the Lewis acidic IL [Bmim][FeCl4] is an all-in-one catalyst-template-precursor for the synthesis of iron-doped mesoporous silica monoliths.

The ionic liqid [Bmim][FeCl4] catalyzes the formation of iron doped mesoporous silica aerogels for H2O2decomposition


a



Figure LegendFigure 1. Representative data obtained from the hybrid materials.(A) Photographs of 1000A1:9, 1000A4:6, and 1000A7:3 monoliths. For sample nomenclaturesee experimental part.(B) Top view of a 1000A1:9 monolith.(C) Low magnification scanning electron micrograph of a 1000A4:6 monolith.(D)Higher magnification scanning electron micrograph of the monolith shown in panelC highlighting the nanometer-sized features described in the text.(E-G) X-ray tomography cross sections of a 1000A1:9 monolith. E: top region, F: centerregion, G: bottom region.(H) N2 adsorption (Ads) and desorption (Des) isotherms of xA1:9 monoliths.(I) N2 adsorption (Ads) and desorption (Des) isotherms of xA7:3 monoliths.(J-K) Photographs of a 1000A1:9 catalyst disk in aqueous H2O2 showing O2 bubble for-mation at the catalyst surface.(L) O2 evolution with and without 1000A1:9 catalyst present vs. time.

Results & DiscussionSynthesis of the monolithic materials was achieved using a published synthesis protocol[27]. In short, the combination of tetramethoxysilane (TMOS), the bifunctional silanebistrimethoxysilylpropylamine (BTMSPA), and the ionic liquid (IL) butylmethylimida-zolium tetrachloridoferrate(III), [Bmim][FeCl4], yields stable macroscopic monoliths viaa one-pot reaction (Fig. 1A and B).We have prepared a library of monoliths with varyingfractions of TMOS and BTMSPA as well as different amounts of [Bmim][FeCl4] presentin the reaction mixture (see experimental part for details and sample nomenclature).The monoliths are white to dark yellow and exhibit large pores. The color is likelyrelated to the presence of some iron species but the exact nature of these iron species(see below) could not be determined. Generally, the higher the fraction of BTMSPA,the softer the materials are. The large pores appear to occur mainly at low fractions ofBTMSPA. At higher fractions of BTMSPA, the (outer) surfaces of the monoliths appearmuch more homogeneous and smoother.Closer inspection of the aerogels with scanning electron microscopy (SEM) reveals thatall materials are macroporous and consist of a silica matrix containing smaller, roughlyspherical nanoparticle like features on their surface (Fig. 1C and D). We assign thesmaller features to the presence of iron-based nanoparticles that have formed duringthe reaction by decomposition of some of the IL. While energy dispersive X-ray spec-troscopy (EDXS) shows the presence of 1–2% of iron, complementary electron param-agnetic resonance (EPR, data not shown) spectroscopy data suggest that the iron maybe present as Fe3+ rather than Fe2+. As the EPR spectra are, however, only very poorlyresolved, a final assignment of the iron species cannot be given.Computer X-ray microtomography further confirms that the entire monoliths are(macro)porous and that the macropores are homogeneously distributed throughout theentire aerogel. Complementary nitrogen sorption experiments (Fig. 1H and I) showthat the 100A aerogels (i.e. silica aerogels produced with 100 mg of IL) produce type IVisotherms with their characteristic H2 hysteresis loop. A type IV isotherm is typical formesoporous adsorbents [28] [29]. The H2 hysteresis loop is associated with capillarycondensation taking place in mesopores and may be attributed to a difference in mech-anism between condensation and evaporation processes occurring in pores with narrownecks and wide cavities (often referred to as ‘ink bottle’ pores). However, also the roleof network effects must be taken into account, e.g., network adjustment to adsorbent.In contrast, silica aerogels made with 500 or 1000 mg of IL show type II isothermswith a slight contribution from H3 hysteresis. The reversible type II isotherm is typ-ically obtained with a non-porous or macroporous adsorbent; it represents unrestrictedmonolayer-multilayer adsorption. The beginning of the almost linear middle section ofthe isotherm is often taken to indicate the stage at which monolayer coverage is com-plete and multilayer adsorption is about to begin. The H3 type loop, which does notexhibit any limiting adsorption at high p/p0, is observed with aggregates, that is, looseassemblies, of plate-like particles giving rise to slit-shaped pores. The surface areas of



the 500A and 1000A samples are too small to be meaningful. They are in the range of theouter surface of a fine powder.Table 1 (Suppl. Info.) shows the exact values of the active surface and the pore vol-ume from BET analysis. Only the 100A samples show higher surfaces areas from 270 to390 m2/g due to their mesoporous nature. They look and behave like amorphous silicamaterials with high surface areas, pore sizes between 6 and 12 nm, and porosities be-tween 50 and 60 %. The surface area increase correlates with a decreasing amount ofthe organo-silica precursor BTMSPA. The mesoporous structure disappears for higheramounts of IL and the material turns into a macroporous structure with lower surfaceareas (<30 m2/g).Infrared (IR) spectra (Suppl. Info.) of all samples are essentially identical and no signalsfrom the IL are visible. All spectra are dominated by the bands of the silica matrix. Abroad band around 3000–3400 cm-1 can be assigned to Si-OH and H2O vibrations. Bandsat 1110, 1040, 910–940, 790, and 570 cm-1 can be attributed to the asymmetric Si-O-Sistretching vibration, Si-OH vibration, and the symmetric Si-O-Si stretching vibrationand rocking modes, respectively. Bands around 1440–1480 cm-1 can be assigned to thescissor vibration of the CH2 groups of the precursor BTMSPA pretty much as the signalsat 2800 and 2950 cm-1 which can be attributed to N-H and C-H stretching vibrations ofBTMSPA.The signal intensity depends on the ratio of the two precursors TMOS and BTMSPAin the preparation process. Lower fractions of BMTSPA decrease the intensities of thebands at 1440, 2800, and 2950 cm-1 but increases the intensity of the band at 570 cm-1

consistent with the above assignments.Thermogravimetric analysis (TGA, data not shown) of all samples reveals a multistepdecomposition process of all the hybrid materials. A first weight loss of 3–5% can beassigned to the desorption of surface-adsorbed water molecules. Up to 300℃ the systemloses just about 10% of its mass. This mass loss is assigned to condensation of remainingsilanol groups. Between 300 and 600℃ the most significant mass loss due to decomposi-tion of the organic linkers and further silica condensation is observed. Above 600℃ themass of the materials is essentially constant. Generally, higher fractions of the organosi-lane precursor BTMSPA results in a higher mass losses from the material.As the materials, especially the 100A materials, combine a set of interesting properties,namely (1) mesoporosity, (2) resonable surface areas, (3) up to ca. 2% of an iron speciesin the form of nanoparticles, and (4) macroscopic integrity and mechanical stability, thequestion of catalytic activity of these materials arose during the course of the project.To evaluate the catalytic activity of some of the materials, we chose the catalytic de-composition of hydrogen peroxide. Hydrogen peroxide is thermodynamically unstableand decomposes to water and oxygen at room temperature [30]. Many transition met-als catalyze the decomposition and especially Fe2+ can lead to a decomposition pathwayincluding radical species like HO. and HOO. [31].To evaluate the effect of the aerogels, we prepared a sealed reaction vessel containing15 mL of a 10% aqueous hydrogen peroxide solution and recorded the percentage ofoxygen in relation to the air volume inside the vessel (Fig. 1L). The control measure-ments done without the addition of one of the aerogels show a roughly linear increaseof the oxygen concentration in the gas phase above the liquid. This is due to the wellknown decomposition of H2O2 even without any catalyst being present [30]. The sameexperiment was then done with adding the catalyst to the solution after 6 min. Clearly,addition of the catalyst results in a rapid oxygen release. However, after ca. 20 min thecatalyst seems to become inactive and the slope in the oxygen evolution curves of bothreactions, with and without catalyst, are quite similar again.

ConclusionsThe IL [Bmim][FeCl4] is a viable all-in-one catalyst-template-precursor for the synthesisof iron-doped mesoporous silica monoliths. Moreover, the fact that up to 2% of iron isfirmly incorporated into the silica aerogel matrix suggests applications in catalysis.

Limitations

Currently, there are two main limitations to these materials: (1) the nature of theiron species is unclear and could not be resolved so far, including analysis with EPRspectroscopy, X-ray diffraction, or (high resolution) transmission electron microscopy.Moreover (2), the catalyst appears to be active for only ca. 20 min; pathways to catalystregeneration and the exact reason for deactivation are currently unclear. This is closelyrelated to the fact that the exact nature of the iron species is unknown.

Additional Information

Methods and Supplementary MaterialPlease see https://sciencematters.io/articles/201803000004.

Funding StatementTheUniversity of Potsdam and the DFG projects TA571/2-1, TA571/3-1, and TA571/13-1are thanked for financial support.

AcknowledgementsWe thank Dr. C. Günter for access to the EDX instrument, and Dr. A. Winter and Prof.P. Strauch for EPR measurements.

Ethics StatementNot applicable.

Citations

[1] Natalia V. Plechkova and Kenneth R. Seddon. “Applications ofionic liquids in the chemical industry”. In: Chemical SocietyReviews 37.1 (2008), pp. 123–150. doi: 10.1039/b006677j.url: https://doi.org/10.1039/b006677j.

[2] Andreas Taubert and Zhonghao Li. “Inorganic materials fromionic liquids”. In: Dalton Transactions 7 (2007), pp. 723–727. doi:10.1039/b616593a. url:https://doi.org/10.1039/b616593a.

[3] Zhang Shiguo et al. “Nanoconfined Ionic Liquids”. In: ChemicalReviews 117.10 (2016), pp. 6755–6833. doi:10.1021/acs.chemrev.6b00509. url: https://doi.org/10.1021/acs.chemrev.6b00509.

[4] Watanabe Masayoshi et al. “Application of Ionic Liquids toEnergy Storage and Conversion Materials and Devices”. In:Chemical Reviews 117.10 (2017), pp. 7190–7239. doi:10.1021/acs.chemrev.6b00504. url: https://doi.org/10.1021/acs.chemrev.6b00504.

[5] Ali Eftekhari. “Supercapacitors utilising ionic liquids”. In: EnergyStorage Materials 9 (2017), pp. 47–69. doi:10.1016/j.ensm.2017.06.009. url: https://doi.org/10.1016/j.ensm.2017.06.009.

[6] Kerstin Thiel et al. “On the interaction of ascorbic acid and thetetrachlorocuprate ion [CuCl4]2− in CuCl nanoplatelet formationfrom an ionic liquid precursor (ILP)”. In: Physical ChemistryChemical Physics 13.30 (2011), pp. 13537–13543. doi:10.1039/c1cp20648f. url:https://doi.org/10.1039/c1cp20648f.

[7] Dobbs William et al. “Electrodeposition of Silver Particles andGold Nanoparticles from Ionic Liquid-Crystal Precursors”. In:Angewandte Chemie International Edition 45.25 (2006),pp. 4179–4182. doi: 10.1002/anie.200600929. url:https://doi.org/10.1002/anie.200600929.

[8] Taubert Andreas. “CuCl Nanoplatelets from an IonicLiquid-Crystal Precursor”. In: Angewandte Chemie InternationalEdition 43.40 (2004), pp. 5380–5382. doi:10.1002/anie.200460846. url:https://doi.org/10.1002/anie.200460846.

[9] Fechler Nina, Fellinger Tim-Patrick, and Antonietti Markus.““Salt Templating”: A Simple and Sustainable Pathway towardHighly Porous Functional Carbons from Ionic Liquids”. In:Advanced Materials 25.1 (2012), pp. 75–79. doi:10.1002/adma.201203422. url:https://doi.org/10.1002/adma.201203422.

[10] Zhang Shiguo et al. “Nitrogen-Doped Inverse Opal CarbonsDerived from an Ionic Liquid Precursor for the OxygenReduction Reaction”. In: ChemElectroChem 2.8 (2015),pp. 1080–1085. doi: 10.1002/celc.201500129. url:https://doi.org/10.1002/celc.201500129.

[11] Ranjbar Sahraie Nastaran et al. “Noble-Metal-FreeElectrocatalysts with Enhanced ORR Performance byTask-Specific Functionalization of Carbon using Ionic LiquidPrecursor Systems”. In: Journal of the American Chemical Society136.41 (2014), pp. 14486–14497. doi: 10.1021/ja506553r.url: https://doi.org/10.1021/ja506553r.

[12] Alsalme Ali et al. “Facile synthesis of nickel based nanostructuresfrom Ni[EMIM]Cl2 ionic liquid precursor: effects of thermal andchemical methods on the properties of nanoparticles”. In: RSCAdvances 6.89 (2016), pp. 86340–86345. doi:10.1039/c6ra16894a. url:https://doi.org/10.1039/c6ra16894a.

[13] Zhang Jing et al. “Solvothermal Synthesis of Three-DimensionalHierarchical CuS Microspheres from a Cu-Based Ionic LiquidPrecursor for High-Performance Asymmetric Supercapacitors”.In: ACS Applied Materials and Interfaces 7.39 (2015),pp. 21735–21744. doi: 10.1021/acsami.5b04452. url:https://doi.org/10.1021/acsami.5b04452.

6

[14] Göbel Ronald et al. “Synthesis of mesoporous carbon/iron carbidehybrids with unusually high surface areas from the ionic liquidprecursor [Bmim][FeCl4]”. In: CrystEngComm 14.15 (2012),pp. 4946–4951. doi: 10.1039/c2ce25064k. url:https://doi.org/10.1039/c2ce25064k.

[15] Li Zhonghao et al. “In(OH)3 particles from an ionic liquidprecursor and their conversion to porous In2O3 particles forenhanced gas sensing properties”. In: CrystEngComm 15.9 (2013),pp. 1706–1714. doi: 10.1039/c2ce26733k. url:https://doi.org/10.1039/c2ce26733k.

[16] Li Rui et al. “Synthesis and gas-sensing properties of ZnOparticles from an ionic liquid precursor”. In: RSC Advances 2.7(2012), pp. 3049–3056. doi: 10.1039/c2ra00907b. url:https://doi.org/10.1039/c2ra00907b.

[17] Li Rui et al. “Ionic liquid precursor-based synthesis of CuOnanoplates for gas sensing and amperometric sensingapplications”. In: Sensors and Actuators B: Chemical 168 (2012),pp. 156–164. doi: 10.1016/j.snb.2012.03.079. url:https://doi.org/10.1016/j.snb.2012.03.079.

[18] Schütte Kai et al. “Synthesis of Metal Nanoparticles and MetalFluoride Nanoparticles from Metal Amidinate Precursors in1-Butyl-3-Methylimidazolium Ionic Liquids and PropyleneCarbonate”. In: ChemistryOpen 6.1 (2016), pp. 137–148. doi:10.1002/open.201600105. url:https://doi.org/10.1002/open.201600105.

[19] Schmitz Alexa et al. “Synthesis of metal-fluoride nanoparticlessupported on thermally reduced graphite oxide”. In: BeilsteinJournal of Nanotechnology 8 (2017), pp. 2474–2483. doi:10.3762/bjnano.8.247. url:https://doi.org/10.3762/bjnano.8.247.

[20] Olchowka Jacob, Suta Markus, and Wickleder Claudia.“Frontispiece: Green Synthesis of A2SiF6 (A=Li–Cs)Nanoparticles using Ionic Liquids as Solvents and as FluorineSources: A Simple Approach without HF”. In: Chemistry - AEuropean Journal 23.50 (2017), p. 12092. doi:10.1002/chem.201785067. url:https://doi.org/10.1002/chem.201785067.

[21] Zhang Tao, Doert Thomas, and Ruck Michael. “Synthesis of MetalSulfides from a Deep Eutectic Solvent Precursor (DESP)”. In:Zeitschrift für anorganische und allgemeine Chemie 643.23 (2017),pp. 1913–1919. doi: 10.1002/zaac.201700206. url:https://doi.org/10.1002/zaac.201700206.

[22] Donato R. K. et al. “Synthesis of silica xerogels with highlydistinct morphologies in the presence of imidazolium ionicliquids”. In: Journal of Sol-Gel Science and Technology 49.1 (2008),pp. 71–77. doi: 10.1007/s10971-008-1829-6. url:https://doi.org/10.1007/s10971-008-1829-6.

[23] Pujari Ajit A. et al. “The use of acidic task-specific ionic liquids inthe formation of high surface area mesoporous silica”. In: NewJournal of Chemistry 33.10 (2009), pp. 1997–2000. doi:10.1039/b907077j. url:https://doi.org/10.1039/b907077j.

[24] Wang Tongwen et al. “Templating Behavior of a Long-ChainIonic Liquid in the Hydrothermal Synthesis of MesoporousSilica”. In: Langmuir 23.3 (2007), pp. 1489–1495. doi:10.1021/la062470y. url:https://doi.org/10.1021/la062470y.

[25] El Hankari Samir and Hesemann Peter. “Guanidinium vs.Ammonium Surfactants in Soft-Templating Approaches:Nanostructured Silica and Zwitterionici-Silica fromComplementary Precursor-Surfactant Ion Pairs”. In: EuropeanJournal of Inorganic Chemistry 2012.32 (2012), pp. 5288–5298. doi:10.1002/ejic.201200419. url:https://doi.org/10.1002/ejic.201200419.

[26] Yuan Bin et al. “Transparent Cellulose–Silica Composite Aerogelswith Excellent Flame Retardancy via an in Situ Sol–Gel Process”.In: ACS Sustainable Chemistry and Engineering 5.11 (2017),pp. 11117–11123. doi:10.1021/acssuschemeng.7b03211. url: https://doi.org/10.1021/acssuschemeng.7b03211.

[27] Taubert Andreas et al. “First examples of organosilica-basedionogels: synthesis and electrochemical behavior”. In: BeilsteinJournal of Nanotechnology 8 (2017), pp. 736–751. doi:10.3762/bjnano.8.77. url:https://doi.org/10.3762/bjnano.8.77.

[28] Polarz Sebastian and Smarsly B. “Nanoporous Materials”. In:Journal of Nanoscience and Nanotechnology 2.6 (2002),pp. 581–612. doi: 10.1166/jnn.2002.151. url:https://doi.org/10.1166/jnn.2002.151.

[29] Thommes Matthias et al. “Physisorption of gases, with specialreference to the evaluation of surface area and pore sizedistribution (IUPAC Technical Report)”. In: Pure and AppliedChemistry 87.9-10 (2015), pp. 1051–1069. doi:10.1515/pac-2014-1117. url:https://doi.org/10.1515/pac-2014-1117.

[30] C. K. McLane. “Hydrogen Peroxide in the Thermal HydrogenOxygen Reaction I. Thermal Decomposition of HydrogenPeroxide”. In: The Journal of Chemical Physics 17.4 (1949),pp. 379–385. doi: 10.1063/1.1747263. url:https://doi.org/10.1063/1.1747263.

[31] Tachiev et al. “Kinetics of hydrogen peroxide decomposition withcomplexed and “free” iron catalysts”. In: International Journal ofChemical Kinetics 32.1 (2000), pp. 24–35. doi:10.1002/(sici)1097-4601(2000)32:1<24::aid-jck4>3.0.co;2-a. url:https://doi.org/10.1002/(sici)1097-4601(2000)32:1%3C24::aid-jck4%3E3.0.co;2-a.

[32] Dörte Steinbrück et al. “An Innovative Laser-Based SensingPlatform for Realtime Optical Monitoring of Oxygen”. In:Proceedings of the 14th Joint International IMEKO TC1 + TC7 + TC13 Symposium: Intelligent Quality Measurements - Theory andEducation and Training 56 (2011).

theionicliquid[bmim][fecl ]catalyzesthe...

Documents