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The Role of Brønsted and Lewis Acid Sites in Acetalization of Glycerol over Modified Mesoporous Cellular Foams Katarzyna Stawicka,*,†,‡ Alba E. Díaz-Á lvarez,‡ Vanesa Calvino-Casilda,§ Maciej Trejda,† Miguel A. Bañares,‡ and Maria Ziolek† †

Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, PL-61-614 Poznań, Poland Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2, E-28049 Madrid, Spain § Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Paseo Senda del Rey, 9, E-28040 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: The main objective of this work is to determine which kind of acidic sites, Brønsted (BAS) or Lewis (LAS), are more active and stable in the glycerol acetalization to solketal by means of Raman-monitoring. For this purpose, the mesoporous cellular foams (MCFs) were modified to obtain materials containing exclusively BAS occurring in different number and strength (samples MP-MCF (MP = (3mercaptopropyl)trimethoxysilane) and CS-MCF (CS = 2-(4chlorosulfonylphenyl)ethyltrimethoxysilane)) or the material containing exclusively LAS (NbMCF). Moreover, the materials containing both types of centers in different ratios (MoMCF and TaMCF) were also studied. Real-time Raman monitoring of the catalytic reaction allowed us to observe not only changes in substrates concentration and product yield but also the transformation of acetone−glycerol adduct (O···CO vibrations) formed in the presence of a catalyst. The activity of the catalysts containing BAS depends on the number and strength of these centers, and the best performance was observed for CS-MCF. The maximum of solketal yield and the equilibrium state were achieved after 2 min from the start of the Raman monitoring. NbMCF containing exclusively LAS exhibited both the lowest conversion of glycerol and stability. The role of different recycling treatments was probed.

1. INTRODUCTION

As shown in Scheme 1, the acetalization of glycerol can eventually produce two structural isomers, (2,2-dimethyl-1,3dioxolan-4-yl) methanol and 2,2-dimethyl-1,3-dioxan-5-ol, and water as a byproduct.19,22,23 However, this reaction affords mainly the 1,3-dioxolane ring. Theoretical calculations suggest that, in this particular case, solketal is thermodynamically more stable than the 2,2-dimethyl-1,3-dioxan-5-ol isomer because of steric repulsions provoked by the presence of the methyl group in the axial position of the six-membered ring.24

In the past decade, much effort was made to find substitutes to petrochemical energy sources. Biodiesel is a green alternative to fossil sources for chemicals. As a result of increasing interest in the biodiesel industry, a surplus of glycerol has been obtained; from 10 kg of triglycerides used for biodiesel production, 1 kg of glycerol is produced.1 Even though biodiesel production is an attractive solution, the search for new applications of the glycerol subproduct would revalorize the total process.2 The new applications developed in the last years propose the use of glycerol for green chemistry purposes, as a solvent (substituting water or other organic alcohols/solvents) or as a starting material for other highly value added products.3 The reactions of glycerol transformation into other important substances include glycerol esterification,4,5 oxidation,6,7 reduction,8,9 dehydration,10−12 etherification,13,14 ammoxidation,15 chloridation,16 or acetalization.17−19 The latter is interesting for petrochemical, pharmaceutical, and polymer industry because this process generates a five-membered ring (2,2-dimethyl-1,3dioxolan-4-yl) methanol (solketal), which can be used as a fuel additive, solvent, or plasticizer.20,21 © 2016 American Chemical Society

Scheme 1. Glycerol Acetalization with Acetonea

a

Blue colored molecule, (2,2-dimethyl-1,3-dioxolan-4-yl) methanol named solketal; red colored molecule, (2,2-dimethyl-1,3-dioxan-5-ol).

Received: April 27, 2016 Revised: June 16, 2016 Published: July 19, 2016 16699

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

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The Journal of Physical Chemistry C Scheme 2. Scheme of the Reaction Performed on BAS

materials modified with organosilanes or transition metals was prepared and tested in the acetalization process under the optimal conditions selected in our previous paper.17 Mesoporous cellular foams were used as supports for active centers because of their open structure made of spherical cells connected by windows.33 This type of support structure does not bring diffusion problems, which is very important for reactions performed in the liquid phase. The modifiers selected, that is, organosilanes or metal sources, allowed us to obtain the catalysts showing exclusively BAS or LAS or both types of acidic sites, to verify their role in glycerol acetalization with acetone. Real-time Raman monitoring to observe changes in substrate concentration investigated the reaction and molecular structures, including the formation of acetone−glycerol adduct (O···CO vibrations). Raman spectroscopy can be used in a broad range of experimental conditions,34 providing unique fingerprints for real-time analysis and monitoring of chemical reactions.35−37 Organic reactions are traditionally monitored off-line, such as by chromatography. However, such analyses are time-consuming, require sampling, do not provide real-time analysis of the reaction progress, and do not permit molecular insight into the reaction mechanism.36,38,39

Literature provides examples of glycerol acetalization with acetone in the presence of solid catalysts of different acidic nature, Lewis or Brønsted acid sites, both of them being active in the reaction. However, no further mechanistic studies were developed until now, and several reaction pathways are proposed in the literature.19,25−27 In the mechanism of the reaction taking place on Lewis acid sites, proposed in ref 25, LAS would first activate the carbonyl group in acetone. The resulting electrophilic center formed on the carbon atom of the carbonyl group in acetone then would interact with one of the hydroxyl groups of glycerol. Finally, a dehydration step generates solketal or the six-membered ring 2,2-dimethyl-1,3dioxan-5-ol. In the case of mechanism taking place on Brønsted acid sites, it was proposed in refs 19, 26, and 28 that glycerol acetalization occurs through a two-step mechanism. According to Scheme 2, first, a tertiary alcohol, formed at the beginning of the reaction, as a result of interaction between the terminal hydroxyl group of glycerol and the carbonyl group of acetone, interacts with BAS sites; the formation of such alcohol was confirmed in our previous paper.17 Next, a carbenium ion forms upon alcohol dehydration; this can be quickly attacked by the secondary or terminal hydroxyl group of glycerol to give solketal or six-membered ring 2,2-dimethyl-1,3-dioxan-5-ol, respectively, and a water molecule as a byproduct. Another study27 proposes that the first step of glycerol acetalization occurs in the presence of BAS, which activates the carbonyl group in acetone. Next, one of the free hydroxyl groups of glycerol interacts with the carbonyl carbon to form an intermediate, which is quickly transformed into a five- or sixmembered ring product with the water molecule elimination in the last stage. Solketal synthesis by the catalytic reaction of glycerol and acetone has been reported using both homogeneous and heterogeneous catalytic processes. Traditionally, homogeneous catalysts such as p-toluene sulfonic acid,29 hydrochloric acid, and sulfuric acid have been employed.30,31 In heterogeneous catalysis, different types of catalysts have been used in this acetalization process, including modified silicas,17,22,25 carbons,23,26 zeolites,25,27 oxides,18,32 or heteropolyacids.19 In many papers, the effects of parameters affecting glycerol acetalization such as the reaction temperature, time, mass of the catalyst, and molar ratio of used reactants have been studied.19,26,32 However, to the best of our knowledge, no paper has addressed the role of the nature of acidic sites, Brønsted or Lewis, which is the purpose of this work. Our goal was to prepare catalysts containing exclusively Brønsted acid sites (BAS) or Lewis acid sites (LAS), and test them in glycerol acetalization. For this reason, a group of MCF

2. MATERIALS AND METHODS 2.1. Catalyst Preparation. The synthesis procedure of MCF described in ref 33 was modified by introduction of organosilane (MPTMS, (3-mercaptopropyl)trimethoxysilane; CSPTMS, 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane) or metal sources (Nb, Ta, Mo) at the stage of the catalyst synthesis. The synthesis procedure of MCFs modified with a Nb or Ta source was as follows. At the beginning, Pluronic P123 (poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol)-block) (8 g, 1.4 mmol) (Aldrich) was dissolved in 300 cm3 of 0.7 M HCl (POCH) solution at 308−313 K in a polypropylene bottle. After dissolution, 1,3,5trimethylbenzene (Aldrich) (4 g, 33.28 mmol) and NH4F (Aldrich) (0.0934 g, 2.52 mmol) were added under vigorous stirring. Following 1 h of stirring, TEOS (Aldrich) (17.054 g, 81.99 mmol) as silica source was added. After 10 min of synthesis gel mixing, a metal source Nb(OEt)5 (Aldrich) or Ta(OEt)5 (Aldrich) was introduced (the nominal Si/Nb or Si/ Ta molar ratio was 30). The synthesis mixture was then stirred at 308−313 K for 20 h and heated in an oven at 373 K under static conditions for 24 h. The solid product was filtered, washed with distilled water (600 cm3), and dried at room temperature. The template was removed by calcination at 773 K for 8 h under static conditions. 16700

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

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The Journal of Physical Chemistry C

UV−vis spectra were recorded using a Varian-Cary 300 Scan UV−visible spectrophotometer. Catalyst powders dried overnight at 373 K, and were placed in the cell (Varian) equipped with a quartz window. The spectra were recorded in the range between 800−190 nm. Spectralon was used as a reference material. UV−vis spectra of CSPTMS and MPTMS (organosilanes were first dissolved in ethanol) were performed with using a Jasco V-550 spectrophotometer. The spectra were recorded in the range between 400 and 200 nm. X-ray photoelectron spectra (XPS) of metallosilicates were obtained on an Ultra High Vacuum (UHV) System (Specs, Germany) equipped with a monochromatic microfocused Al Kα X-ray source (1486.6 eV). Binding energies were referenced to the Cs1 peak from the carbon surface deposit at 284.6 eV. DTA/TGA measurements were performed in an air atmosphere using SETARAM SETSYS-12 apparatus in the temperature range 293−1273 K with a temperature ramp of 5 K min−1. Infrared spectra were recorded with a Bruker Vector 22 FTIR spectrometer using an in situ vacuum cell (homemade). At first, the samples were pressed under low pressure into a thin wafer of ca. 8 mg cm−2 and placed inside the cell. Catalysts were activated before pyridine adsorption at 423 K for 6 h (CS-MCF and MP-MCF samples) or at 673 K for 2 h (metallosilicates). After sample activation, pyridine was admitted at 423 K. After saturation with pyridine, the samples were degassed at different temperatures from 423 to 573 K in a vacuum for 30 min at each temperature. The spectrum without any sample (“background spectrum”) was subtracted from all recorded spectra. The IR spectra of the activated samples (after heating at 423 or 673 K depending on material) were subtracted from those recorded after the adsorption of pyridine followed by various treatments. The reported spectra are the results of this subtraction. The number of Brønsted acidic sites was calculated assuming the extinction coefficient ε 1550 = 1.67 μmol−1 cm, whereas the number of Lewis acid sites was calculated for the extinction coefficient ε 1440 = 2.22 μmol−1 cm.41 Infrared spectra of liquid organosilanes (CSPTMS and MPTMS) were recorded with a Bruker Vertex 70 equipment combined with Platinum ATR diamond F. vacuum type: A225/Q (Bruker). 2.3. Acetic Acid Esterification with Glycerol Probe Reaction. This reaction probes the strength of Brønsted acid sites, delivering monoacetylglycerols (MAGs), diacetylglycerols (DAGs), and triacetylglycerol (TAG), with increasing acidity.5,42 The esterification of acetic acid with glycerol was performed in a liquid phase without any solvents. The reaction was carried out under nitrogen atmosphere at 373 K for 4 h for glycerol to acetic acid molar ratio 1:9. Such reaction conditions were chosen, because the excess of acetic acid is convenient for TAG formation. The yield of TAG is used as indicator for the presence of strong BAS. The amount of catalyst was chosen in such a way that the number of BAS was the same in the reaction medium. It was included in the range 0.1−0.2 g. Reaction products were analyzed by a gas chromatograph (Varian CP 3800) equipped with a 60 m VF-5 ms capillary column, worked at the temperature range of 333−523 K (temperature ramp 10 K min−1), and FID detector. 2.4. Glycerol Acetalization with Acetone. The reaction was performed in a three-necked round-bottom flask fitted with a condenser and the Raman immersion probe. The mixture of glycerol and acetone (molar ratio 1:2) was introduced into the flask and mixed for 2 min of continuous stirring until the desired temperature of reaction was achieved (313 K). Next, 2

For the preparation of molybdenum-modified sample, cetyltrimethylammonium chloride (CTMACl) (2.4 cm3, 25 wt % in H2O solution (Aldrich)) was used as an additional template. CTMACl was added to the synthesis mixture together with Pluronic P123 (molar ratio of P123:CTMACl = 2:1). The metal source, MoO3 (Sigma-Aldrich), was first dissolved in a mixture of 10 cm3 H2O and H2O2 (1:1) at 333 K (Si/Mo molar ratio was 30) and cooled to room temperature before its introduction to the synthesis mixture. The other steps of the synthesis procedure were the same as described above for niobium- or tantalum-modified materials. MCF materials functionalized with MPTMS or CSPTMS were prepared similarly to NbMCF and TaMCF samples. MPTMS (Aldrich) or CSPTMS (50% solution in dichloromethane, ACROS Organics) was slowly added to the synthesis mixture, 45 min after TEOS addition (Si/MPTMS or CSPTMS molar ratio = 10). To MPTMS-modified sample, hydrogen peroxide (35% solution, Merck) was added (17.48 g) together with organosilane (H2O2/MPTMS molar ratio = 20). The organic template was removed by constant extraction with ethanol in Soxhlet apparatus for 24 h and dried overnight at room temperature. 2.2. Catalyst Characterization. The synthesized modified mesoporous cellular foams were examined by several analytical techniques such as N2 adsorption/desorption, XRF, ICP-OES, elemental analysis, UV−vis, DTA/TGA, and FT-IR spectroscopy combined with pyridine adsorption as a probe molecule. N2 adsorption/desorption isotherms were obtained using Quantachrome Instruments Autosorb IQ2. Samples were first outgassed under vacuum at 423 K (CS-MCF and MP-MCF materials) or 573 K (NbMCF, TaMCF, and MoMCF samples). The surface area was calculated using the BET method. The pore volume and diameter were estimated according to the Broekhoff−de Boer method with the Frenkel−Halsey−Hills approximation.40 Transmission electron microscopy (TEM) images were recorded using a JEOL 2000 electron microscope working at 80 kV. First, solid materials were put on a grid covered with a holey carbon film and then transferred to the equipment. The real molar ratio of Si/Nb or Si/Ta was determined by Xray fluorescence (XRF) using MiniPal-Philips equipment. The measurements were performed using the calibration curve based on the XRF analysis for the mixtures prepared from silica (Degussa) and niobium oxide (Alfa Aesar) or tantalum oxide (Aldrich). The calibration curve was drawn on the basis of 10 points corresponding to different SiO2/Nb2O5 or Ta2O5 molar ratios in the range from 3 to 300. The metal concentration in the samples was established by the amount of emitted X-ray radiation linked to the values in the calibration curves. ICP-OES determines the real molar ratio of Si/Mo. First, lithium tetraborate (∼0.5 g, Aldrich) used as a fluxing agent was scattered into a platinum crucible. Next 50 mg of catalyst was introduced on the tetraborate powder. The catalyst then was covered by the second portion of fluxing agent (∼0.5 g). Platinum crucible with the mixture of lithium tetraborate and the catalyst was heated in an oven at 1223 K for 1 h. The resulting colorless alloy was treated with 2 M HCl solution until it was dissolved in acid solution. The final solution was filtered off and put into a round-bottomed flask. ICP analyses of obtained solution were made on an emission spectrophotometer ICP-OES (Varian). Elemental analysis of the solids was carried out with Elementar Analyzer Vario EL III. 16701

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

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The Journal of Physical Chemistry C wt % of catalyst (activated first at 423 K for 6 h, CS-MCF and MP-MCF materials or at 673 K for 2 h, metallosilicates) or liquid CSPTMS was added, and the reaction time was started. The reaction products were analyzed by gas chromatography in a Bruker 430-GC fitted with a capillary column (BR-5 ms Wcot fused silica, 30 m × 0.25 mm) and a flame ionization detector (FID). The column was heated as follows: at 333 K for 1 min, then at the rate of 5 K min−1 up to 523 K and held at 523 K for 10 min. The reaction was continuously monitored by Raman spectroscopy with an InPhotonics immersion probe fitted to a PerkinElmer Raman Station 400F system using 100 mW of near-infrared 785 nm excitation line. Spectra were acquired continuously; each spectrum consisted of 6 accumulations of 10 s. The spectra were analyzed using PEAXACT software. Recycling of the CS-MCF, MP-MCF, and NbMCF catalysts has also been investigated. The catalyst after the first run was separated from the reactant mixture by centrifugation and then tested in a second run. When the reaction was completed, the catalyst was separated again by centrifugation and dried overnight at 423 K for 6 h (CS-MCF and MP-MCF materials) or at 673 K for 2 h (NbMCF). The same mass of catalyst was applied for the second and third runs.

The size of windows (determined from desorption branches of N2 isotherm) and cells (estimated from adsorption branches of N2 isotherm) in the samples modified with organosilanes strongly depends on the organosilane size, and these values are much smaller for CSPTMS than for MPTMS. For metallosilicates, the smallest cell and window sizes are observed for MoMCF synthesized with the use of the additional surfactant, CTMACl. Among all samples prepared, CS-MCF and MPMCF have the smallest window sizes, which may be related to the organosilanes anchored close to the windows of the mesoporous foams. The materials modified with Ta(OEt)5, Nb(OEt)5, or MPTMS have larger cell diameters as compared to the catalysts modified with Mo and CSPTMS precursors. The common phenomenon in the preparation of all of these three materials is the release of alcohols (as the hydrolysis effect), methanol from MPTMS or ethanol from Nb, Ta sources; this alcohol acts as a cotemplate and has an impact on the MCF structure.45 The UV−vis analyses presented in Figure 1 confirm the anchorage of the organosilane and metal incorporation onto

3. RESULTS AND DISCUSSION 3.1. Characterization of Structure and Surface Properties. Figure S1 shows the transmission electron microscopy images of the samples. All of them present the disordered array of silica struts, typical of mesoporous cellular foams without any impurities of SBA-15 phase. It means that the reaction conditions applied in this study allow us to obtain the samples with pure MCF phase.33 Nitrogen adsorption/desorption isotherms confirmed that all materials have mesoporous character, as indicated by the isotherm shape classified as type IV according to IUPAC classification.43 However, there is a significant difference in the isotherm shapes for the materials modified with organosilanes and those modified with transition metals. The isotherms for the latter samples are typical of mesoporous cellular foams and show a characteristic hysteresis loop at a relatively high pressure (p/p0) (Figure S2).40,44 The hysteresis loops recorded for MPMCF and CS-MCF are different. They are characteristic of inkbottle shape of pores and can be allocated to a smaller size of windows in these materials in comparison to those in metallosilicates. N2 adsorption/desorption results allowed the calculation of structural parameters, which are different depending on the modifier used (Table 1). The BET surface areas of all modified samples are relatively large, ranging from 590 to 670 m2 g−1.

Figure 1. UV−vis spectra of the catalysts activated overnight at 373 K and the spectrum of organosilanes (CSPTMS and MPTMS).

MCFs materials. The spectrum of CS-MCF sample exhibits two bands at 226 and 267 nm characteristic of the electron charge transfer in anchored CSPTMS molecules. Such assignment is confirmed by the UV−vis spectrum of pure organosilane, although a red shift is observed after CSPTMS anchoring in MCF structure. In the spectrum of MP-MCF, three bands are visible. The first of them at 216 nm corresponds to charge transfer in thiol groups, whereas the two other bands at 271 and 331 nm are typical of charge transfer in sulfonic species in different environments.46,47 The presence of the UV−vis band assigned to the charge transfer in thiol species in the spectrum of MP-MCF sample proved that not all MPTMS anchored thiol groups were oxidized to sulfonic ones by H2O2 during the catalyst synthesis. In the spectrum of the TaMCF sample, only one band at 218 nm is present. This is attributed to the charge transfer in metal Ta (+5) tetrahedrally coordinated with oxygen.48 In the spectrum of NbMCF material, two bands at

Table 1. Texture Parameters Calculated from Nitrogen Adsorption/Desorption Isotherms catalyst

BET surface area, m2 g−1

pore volume, cm3 g−1

CS-MCF MP-MCF TaMCF MoMCF NbMCF

660 590 660 640 670

1.2a 1.9a 2.4a 1.7a 2.4a

average pore diameter, nm 22.5b 37.1b 28.1b 22.5b 37.1b

4.9c 6.9c 14.3c 9.3c 12.7c

a

Determined from adsorption branches of N2 isotherms (BdB−FHH method). bCell diameter as determined from adsorption branches of N2 isotherms (BdB−FHH method). cWindow diameter as determined from desorption branches of N2 isotherms (BdB−FHH method). 16702

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form of sulfonic species, whereas in MP-MCF some sulfur is in the form of thiol groups not fully oxidized to −SO3H as evidenced by DTA data (Figure 2).

217 and 270 nm are detected, which can be assigned to electron charge transfer typical of tetrahedral and pentacoordinated niobium.49,50 These bands were identified in ref 51 as corresponding to Nbδ+ and NbOδ− on NbMCM-41 materials. In the spectrum of the MoMCF sample, two intense bands at 218 and 262 nm are well visible and relate to the presence of two different surroundings of molybdenum in tetrahedral coordination.52,53 Interestingly, all UV−vis spectra of metallosilicate samples do not show the band at ca. 330 nm, which is typical of charge transfer in octahedrally coordinated metal at an extra-framework position.48,49,52 More detailed information about the oxidation states of the metals loaded in MCFs was obtained from the XPS study. The XP spectra shown in Figure S3 reveal the binding energies (BE) in the regions characteristic of Ta(V), Nb(V), and Mo(VI).54−56 The BE values of Nb 3d5/2 and Mo 3d5/2 are lower than the values typical of bulk Nb2O5 (207.1 eV)54 and MoO3 (232.8 eV).55 The decrease of BE was of 0.60 and 0.99 eV, respectively.54,55 The lower binding energies of metals in NbMCF and MoMCF than in their bulk oxide counterparts suggest a slight reduction of Nb and Mo oxidation states. This phenomenon could be related to the dehydroxylation of M− OH species formed after inclusion of metals at higher oxidation states than (IV) into silica matrix. On the contrary, in the case of TaMCF the BE of Ta 4f7/2 is shifted to the greater value (the increase of 0.45 eV) than in the bulk of tantalum oxide (26.1 eV).54 Similar behavior is exhibited by the BE of Ta 4d5/2 and Ta 4d3/2.56 Such shift is typical of metals forming M−O−Si bonds in silica matrix. It is worth noting that the difference in shifts of BE for niobium and tantalum (having the same (V) oxidation states) in MCF materials well corresponds to the differences in Brønsted acidity discussed below. The chemical composition of the synthesized samples is shown in Table 2. Niobium and tantalum concentrations were

Figure 2. DTA analysis of organosilanes-modified MCFs.

Apart from the confirmation of organosilanes anchoring onto MCF surface, DTA analysis was used to evaluate the effectiveness of thiol species oxidation from MPTMS by H2O2 during the MP-MCF synthesis. Knowing that the exothermic effect related to −SH groups decomposition appears at 500−550 K, while the exothermic effect characteristic of −SO3H groups decomposition is observed in the range between 720−900 K,57 it is possible to estimate the effectiveness of thiol groups oxidation. Besides the two exothermic effects mentioned, one endothermic effect at around 300−400 K is also visible. It is related to water desorption.58 In the MP-MCF sample, not all −SH groups from MPTMS anchored were fully oxidized to sulfonic species. The DTA curve of this sample reveals two exothermic effects at ca. 527 and 748 K from thiol and sulfonic species decomposition, respectively. It is worth mentioning that the additional proof for the presence of thiol species on MP-MCF surface was gained from UV−vis analysis (Figure 1). In the DTA curve of CS-MCF sample, two exothermic effects at ca. 691 and 860 K are visible. Their appearance can be correlated with the presence of anchored CSPTMS molecules on the MCF surface.59 For the identification of BAS and LAS on the catalysts surfaces as well as for the determination of their amounts and strength, the method of pyridine adsorption combined with FTIR study was applied. The spectra after pyridine adsorption and its evacuation at different temperatures are shown in Figure 3. The spectra of all samples, except that of NbMCF, show the IR bands at 1638 and 1547 cm−1 from pyridinium ions formed by the abstraction of proton from BAS.60,61 The band at 1547 cm−1 disappears after evacuation of pyridine at 473 K in the spectra of samples modified with organosilanes and at 523 K for TaMCF material, whereas for MoMCF this band is still observed until evacuation at 573 K. Such observation suggests that the BAS centers in the molybdenum-modified sample are stronger. In the spectra of metallosilicate-modified samples, the band at ca. 1611 cm−1 is clearly observed. This band is assigned to vibrations of pyridine chemisorbed on Lewis acid sites.61 It is accompanied by a band at ca. 1450 cm−1, also characteristic of vibrations of pyridine coordinatively bonded to LAS.60,61 This

Table 2. Chemical Composition of the Samplesa catalyst

Si/M molar ratio in the synthesis gel

CS-MCF MP-MCF TaMCF MoMCF NbMCF a

Si/M real molar ratio

S, mmol g−1 0.59 0.58

30 30 30

54 66 65

M = Ta, Mo, or Nb.

evaluated by XRF analysis, whereas the amount of molybdenum was estimated by ICP-OES technique. The data obtained are expressed in Si/M molar ratio, where M stands for Nb, Ta, or Mo. For the calculation of the real amount of anchored organosilanes on MCFs, surface elemental analysis was used. The efficiency of CSPTMS or MPTMS incorporation is determined by the sulfur atoms content [mmol g−1]. Table 2 data show that the real amount of metal incorporated into modified MCFs samples is around one-half of the nominal value. The highest metal concentration was achieved for TaMCF. It confirms earlier results54 showing that tantalum is more efficiently incorporated into silica matrix than niobium or molybdenum. The amount of sulfur introduced by modification of MCFs with organosilanes is almost the same for both MPMCF and CS-MCF materials, ca. 0.59 mmol g−1. However, the number of −SO3H groups (playing the role of BAS) is different for both samples. In CS-MCF all sulfur atoms are present in the 16703

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

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Figure 3. FTIR spectra of materials activated under vacuum (a) and after pyridine adsorption and evacuation for 5 min at 423 K (b) and after evacuation for 30 min at 423 K (c), 473 K (d), 523 K (e), and 573 K (f). Spectra are normalized to 5 mg. The spectra of activated samples were subtracted from all spectra recorded after pyridine adsorption and evacuation. FTIR spectra of MPTMS and CSPTMS are in the right bottom corner.

Table 3. Number of Brønsted and Lewis Acid Sites Calculated from Pyridine Adsorptiona number of BAS, μmol/g catalyst

423 K

CS-MCF MP-MCF TaMCF MoMCF NbMCF

406 124 19 11 0

b

423 K 226 36 13 10 0

c

473 K 0 0 11 4 0

c

number of LAS, μmol/g

523 K

c

573 K

A423_30/A423_5

473 Kc

523 Kc

573 Kc

A573/A523

0 4 0

0.56 0.29 0.68 0.91 0

0 0 20 19

0 0 8 5 5

0 0 5 4 4

0 0 0.62 0.80 0.80

0 2 0

c

a BAS extinction coefficient, ε 1550 = 1.67 μmol−1 cm; LAS extinction coefficient ε 1440 = 2.22 μmol−1 cm.41 bEvacuation for 5 min. cEvacuation for 30 min.

adsorbed on LAS in the spectra of organosilane-modified foams proves that these samples have only BAS, while the absence of IR bands characteristic of pyridine adsorbed on BAS in the spectrum of NbMCF confirms that this sample has only Lewis acidity. Besides the above-mentioned bands, in the spectra of almost all catalysts tested (except NbMCF), the band at ca. 1490 cm−1 also appears, the presence of which is correlated with the existence of both BAS and LAS sites.61 Furthermore, in the spectra of TaMCF and NbMCF, there is also another band at ca. 1597 cm−1, which originates from the hydrogenbonded pyridine.64 The number of BAS and LAS was calculated as the number of pyridine molecules chemisorbed on the catalysts surface after

band is still observed after pyridine evacuation at higher temperatures, indicating high strength of Lewis acid sites. The FTIR spectra after pyridine adsorption on CS-MCF show an additional band at ca. 1600 cm−1. It is not caused by pyridine adsorbed on LAS. It actually corresponds to the ν(CC) vibration in the benzene ring in the anchored organosilane.62 Similarly, the band at ca. 1445−1455 cm−1 observed in the spectra of both CS-MCF and MP-MCF typical of pyridine chemisorbed on LAS is related to the δas(CH3) vibrational mode in methoxy groups in organosilanes.63 The presence of this band indicates that not all methoxy species in CSPTMS and MPTMS participated in the organosilane anchoring on MCFs. The lack of the bands typical of pyridine 16704

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

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The Journal of Physical Chemistry C

reaction conditions established in our previous paper.17 As earlier proposed,18,19,22,25,27 the acetalization of glycerol with acetone can run in the presence of both LAS and BAS centers producing two isomers (2,2-dimethyl-1,3-dioxolan-4-yl)methanol and 2,2-dimethyl-1,3-dioxan-5-ol and water as a byproduct (Scheme 1). Nevertheless, in this reaction, only the five-membered ring (2,2-dimethyl-1,3-dioxolan-4-yl) methanol, named solketal, is formed, which is more thermodynamically stable.24 Such a behavior was previously described by several authors, who do not detect the six-membered ring by GC and/ or NMR techniques.25,65 Before starting the catalytic experiments, a blank reaction was performed by adding in the reaction media glycerol and acetone at the optimal molar ratio of reactants (glycerol:acetone = 1:2). The mixture was heated at 313 K and real-time monitored in situ by Raman spectroscopy for 3 h. As shown in Figure 4, the

evacuation of pyridine at different temperatures. It is clear that such numbers depend on the treatment conditions and are not absolute. Therefore, for the CS-MCF sample, the number of BAS shown in Table 3 (from pyridine + FTIR results) is lower than the number of sulfur atoms in this material calculated from the elemental analysis (Table 2). However, for the same evacuation conditions, the number of chemisorbed pyridine molecules describes well the differences between the catalysts. Moreover, the ratio of absorbance of IR bands assigned to pyridinium cations (at ca. 1547 cm−1 related to BAS number) observed after desorption for 30 min at 423 K and for 5 min at the same temperature (A423_30/A423_5) can be a measure of the relative strength of BAS. It can be expressed as the ratio of the number of BAS occupied by pyridinium after 30 and 5 min evacuation at 423 K (Table 3). Similarly, one can estimate the strength of LAS, but in this case, the evacuation conditions for such comparison are different. Higher temperature is needed for the removal of pyridine engaged in the hydrogen bond, the band of which overlaps that present at ca. 1450 cm−1, originating from the vibrations of pyridine chemisorbed on LAS. Therefore, the ratio of absorbance of the IR band assigned to pyridine adsorbed on LAS observed after desorption at 523 K for 30 min and at 573 K also for 30 min (A573/A523) was calculated to estimate the relative strength of LAS. According to Table 3, CS-MCF material shows the highest number of BAS. However, the strongest Brønsted acid sites are present in MoMCF, as concluded from the highest value of A423_30/A423_5 = 0.91. As far as Lewis acidity is concerned, the highest number of LAS is present on TaMCF sample, while MoMCF and NbMCF materials have stronger Lewis acid sites as follows from the higher value of A573/A523, that is, 0.80 versus 0.62 for TaMCF. The acetic acid esterification with glycerol probe reaction was performed to further characterize BAS strength on MP-MCF and CS-MCF. This probe reaction may generate several products, depending on the strength of BAS sites: monoacetylglycerols (MAGs), diacetylglycerols (DAGs), and triacetylglycerol (TAG).5,42 The higher is the BAS strength, the higher TAG yield is achieved. The results presented in Table 4

Figure 4. Real-time Raman monitoring of blank (no catalyst) reaction between glycerol and acetone at 313 K. A total of 113 spectra are presented during 180 min runtime. Color code indicates spectrum number in the sequence (A, acetone; G, glycerol).

Raman bands corresponding to the glycerol (G) at 1466 cm−1 (CH2 scissor), 1252 cm−1 (in plane O−H deformation vibration), 1114 cm−1 (C−O stretching), 907 cm−1 (CH2 rocking), 845 cm−1 (C−C−O stretching), 493, and 393 cm−1 (C−O deformation vibration), and the acetone A at 1711 cm−1 (CO stretching), 1429 cm−1 (asymmetric CH3 deformation vibration (CH3−C(O−)), 1223 cm−1 (C−C stretching), 1067 cm−1 (rocking vibration (CH3−C(O−)), 787 cm−1 (C−C(O)-C symmetric stretching), and 531 cm−1 (−C− (O)-C in plane deformation),17 remain constant over time in the absence of catalyst, and no changes in the reaction mixture were observed. Figure 5 illustrates the real-time Raman monitoring of the reaction between glycerol and acetone over modified MCFs materials. The most representative bands to study the reaction progress are 676 cm−1 from glycerol (G), 638 and 648 cm−1 belonging to the solketal product formation (S), and 732 cm−1 characteristic of acetone−glycerol adduct (O···CO vibrations, A−G).17 For all materials tested, the Raman band of glycerol disappears very fast at the beginning of the reaction, while the other features increase at different rates. A lower rate is apparent for NbMCF, which allows observing the complementary evolution of Raman bands for decreasing glycerol and increasing solketal. This case provides a convenient insight on the reaction mechanism because the time evolution of these two adjacent Raman bands does not form an isosbestic point. This is indicative that the conversion is not direct but that there is a stable intermediate between glycerol substrate and solketal product, which is the acetone− glycerol adduct intermediate we observe at 732 cm−1. The samples having only BAS (MP-MCF and CS-MCF) are more active in glycerol acetalization than NbMCF that contains only

Table 4. Acetic Acid Esterification with Glycerol: Results from GC Analysis selectivity, % catalyst blank reaction MP-MCF CS-MCF

glycerol conversion, %

MAGs DAGs

TAG

yield of TAG, %

46

68

30

2

0.9

59 59

6 5

56 53

38 42

22.4 24.8

show that CS-MCF affords a higher TAG yield than MP-MCF, confirming the results obtained from the study of pyridine adsorption and desorption at different temperatures reported above (Table 3). The data presented above show that we obtained the catalysts containing exclusively BAS (CS-MCF and MP-MCF), both BAS and LAS (TaMCF and MoMCF), and solely LAS (NbMCF). The catalysts showing only BAS reveal different strength of acidic centers, higher in CS-MCF than in MP-MCF. 3.2. Catalytic Activity. The catalytic activity of mesoporous cellular foams synthesized within this work was tested in glycerol acetalization with acetone under the most convenient 16705

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

Article

The Journal of Physical Chemistry C

Figure 6. Raman intensity ratio of solketal (band at 638 cm−1) with respect to glycerol (band at 1114 cm−1) Isolk/(Isolk+Igly) with reaction time. *: In the brackets glycerol conversion analyzed by GC after 60 min of the reaction.

agreement with the number of BAS centers present on the samples, according to data in Table 3. The Raman band at 732 cm−1, characteristic of the acetone− glycerol adduct, is less intense for the samples containing the higher number of BAS (Figure 5). This observation suggests that Brønsted acid sites are more favorable for adduct transformation into solketal than Lewis ones. In fact, Raman monitoring of reactions catalyzed by MoMCF, which has mostly LAS, and NbMCF, which contains only LAS, shows that the Raman band intensity of the adduct is even higher than that of solketal. Surprisingly, the intensity of adduct and solketal Raman bands for NbMCF sample decreased after 90 min of the reaction. It can be caused by the change of Nb coordination (documented by UV−vis spectra after the reaction discussed later) that resulted from the interaction with reagents. Maybe the new Nb species make the perturbation in Raman monitoring. The materials containing mostly BAS, that is, MP-MCF, CSMCF, and TaMCF, afford higher glycerol conversion with 99% selectivity to solketal (Figure 6, Table 5). The highest glycerol Table 5. Glycerol Acetalization with Acetone: Results from GC Analysis after 180 min of the Reaction Figure 5. Real-time Raman monitoring of glycerol acetalization with acetone over modified MCFs materials. (Left) Spectra during reaction; color code indicates spectrum number of the 113 spectra taken during the 180 min runtime in each experiment. (Right) Evolution of the intensity of representative Raman bands of solketal (S, ▲), at 638 cm−1, and acetone−glycerol adduct (A−G, □), at 732 cm−1 (A, acetone; G, glycerol; S, solketal).

catalyst

glycerol conversion, %

selectivity to solketal, %

blank reaction CS-MCF MP-MCF TaMCF MoMCF NbMCF

MoMCF (BAS +LAS) > NbMCF (LAS). These observations are in total 16706

DOI: 10.1021/acs.jpcc.6b04229 J. Phys. Chem. C 2016, 120, 16699−16711

Article

The Journal of Physical Chemistry C

Table 6. Glycerol Conversion and Solketal Selectivity in the Reused Tests of Glycerol Acetalization with Acetone: Results from GC Analysis after 180 min of the Reaction

Figure 7. Relationship between the number of BAS and glycerol conversion.

the increase in glycerol conversion. However, this relationship is not linear because the strength of BAS is also important, along with its number. The difference in activity between CS-MCF and MP-MCF is not significant, and therefore the experiments with the use of lower amounts of catalysts were performed. The smaller amount of the catalyst reduces the total number of BAS accessible to the reagents. In this test, we decided to decrease by 40 times the amount of catalysts used, that is, MP-MCF and CS-MCF, introduced into the reaction mixture. Figure 8 indicates that the materials having only BAS are still more active than NbMCF, which has only LAS. The glycerol

catalyst

glycerol conversion, %

selectivity to solketal, %

blank reaction CS-MCFfresh CS-MCFwet CS-MCFsecond act. MP-MCFfresh MP-MCFwet MP-MCFsecond act. NbMCFfresh NbMCFwet NbMCFsecond act.