Real-Time Raman Monitoring and Control of the Catalytic

Article pubs.acs.org/JPCC

Real-Time Raman Monitoring and Control of the Catalytic Acetalization of Glycerol with Acetone over Modified Mesoporous Cellular Foams V. Calvino-Casilda,*,† K. Stawicka,†,‡ M. Trejda,‡ M. Ziolek,‡ and M.A. Bañares† †

Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2, E-28049 Madrid, Spain Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, PL-61-614 Poznań, Poland

‡

S Supporting Information *

ABSTRACT: The acetalization of glycerol with acetone over modified mesoporous cellular foam materials has been widely investigated using in situ Raman spectroscopy during reaction. Mesoporous cellular foams (MCFs) modified by niobium or tantalum and (3-mercaptopropyl)trimethoxysilane (MP) followed by H2O2 treatment were used as catalysts in the acetalization of glycerol with acetone. The influence of the type of catalyst, which determines the solid texture and number of Brønsted acid sites, and different reaction parameters, such as reaction time, reaction temperature, glycerol/ acetone ratio, and catalyst amount on acetalization reaction, were investigated. The results obtained in the characterization of the catalysts show that the materials obtained differ in the number of Brønsted acidic sites. Raman spectroscopy provides noninvasive insight during acetalization of glycerol with acetone in the presence of acid heterogeneous catalysts. The progress of the acetalization reaction was monitored following the variation in intensity of characteristic Raman bands and using chemometric analyses. The results obtained by real-time Raman monitoring confirm the mechanism proposed for the reaction, which proceeds via the formation of the 3-(2hydroxypropan-2-yloxy)propane-1,2-diol intermediate, whose presence is confirmed by Raman spectroscopy. Under optimal reaction conditions, the 5-membered ring ketal 2,2-dimethyl-1,3-dioxolane-4-yl methanol (solketal) was obtained with the highest selectivity (99%). Raman monitoring enables real-time control of the reaction, thus enabling the optimization of reaction conditions for a more efficient reaction. Raman monitoring illustrates the reversibility of the reaction upon evaporation of acetone, even under reflux. oxidation stability.4 Furthermore, the addition of these compounds to the diesel fuel reduces significantly the emissions of carbon monoxide, hydrocarbons, and other unregulated substances.5 Glycerol can be acetalized with acetone (Scheme 1) to produce the 5-membered ring 2,2-dimethyl-1,3-dioxolane-4-yl methanol (solketal) and the 6-membered ring 2,2-dimethyl-1,3dioxan-5-ol.6 Solketal is an important organic compound that acts as a solvent, plasticizer, surfactant, flavor enhancer, pharmaceutical intermediate, and fuel additive.7−9 Homogeneous acid catalysts have been proven as efficient catalysts in the acetalization of glycerol with acetone.10−12 However, the use of homogeneous catalysts presents limitations because of environmental constraints. Therefore, there is a need of sustainable processes that substitute homogeneous catalysts with heterogeneous ones. Zeolites, clays, resins, and silicas, among others materials, have been used as heterogeneous acidic catalysts for the conversion of glycerol to

1. INTRODUCTION Biodiesel is an alternative to traditional fossil fuels thanks to its biodegradability, nontoxicity, and renewable nature. Biodiesel production by transesterification of vegetable oils or animal fats with methanol over acidic or basic catalysts produces nearly 10 wt % of glycerol as a byproduct. To enhance the economic productivity of biodiesel, the chemical conversion of glycerol to useful products and the search for new applications for this byproduct are necessary. Glycerol is a viscous and polar substance that is very wellknown for its useful properties. Its nontoxicity and biodegradability allows using it in a large range of applications, especially in the cosmetic and food industries. More interestingly, glycerol is also an important raw material for the synthesis of many known valuable compounds.1−3 Several processes for the catalytic conversion of glycerol have been investigated, such as hydrogenolysis, dehydration, polymerization, oxidation, steam reforming, esterification, etherification, and acetalization, among others. Glycerol acetals and ketals are widely used in industry as bases, pharmaceutical intermediates, additives, flavors, and scents. It enhances the viscosity and cold properties of biodiesel and satisfies their needs for flash point and © 2014 American Chemical Society

Received: January 20, 2014 Revised: May 1, 2014 Published: May 2, 2014 10780

dx.doi.org/10.1021/jp500651e | J. Phys. Chem. C 2014, 118, 10780−10791

The Journal of Physical Chemistry C

Article

Scheme 1. Acetalization Reaction of Glycerol with Acetone over Acid Catalysts

solketal.13−16 Ideally, such a reaction would also be run under less energy-demanding conditions. In this paper, the materials based on mesostructured cellular foams (MCFs) modified with (3-mercaptopropyl)trimethoxysilane (MPTMS) were used as acid catalysts to obtain solketal. We already showed that these materials were very promising catalysts in the esterification of acetic acid with glycerol, giving high yield of triacetylglycerol.17−19 MCF materials are constituted by spherical cells, which are connected by windows.20 The opened MCF structure (cells size around 22−42 nm) make them very attractive for functionalization and provide room for bulky reactants.20 Water production is considered the main problem during glycerol acetalization. Water removal would prevent the reverse reaction.14,16,21 Thus, Silva et al. designed zeolites with hydrophobic character to avoid the diffusion of water molecules into the pores, preserving the strength of the acid sites and limiting the reverse reaction.14 Other authors remove water from the solution during the experiment to hinder the reversibility of the reaction.16 However, Percarmona et al. suggested that the deactivation of the catalysts during the acetalization of glycerol with acetone is caused by reaction residues deposited on the active sites rather than by the water absorbed on the porous structure of the catalysts.21 In the present work, the acetalization of glycerol with acetone was performed over rather hydrophilic MP-MCF materials. Thus, water molecules generated during the acetalization reaction are absorbed by the MP-MCF material during reaction, avoiding the problem of reversibility of the reaction because of the presence of water in the reaction media and not deactivating the catalytic surface of the MP-MCF material. After reaction, the water anchored in the structure of MP-MCF catalysts can be removed simply by drying the material to be reused later in a new cycle of reaction. The MCF catalysts were selected based on the number of acidic sites in the final material. For that purpose, one-third of hydrogen peroxide was added to the synthesis gel (in case of silicates sample). In addition, metal sources (Nb, Ta) were used to enhance the MPTMS oxidation. The introduction of the mentioned metals to the MCF structure allowed oxidation of incorporated organosilane species by hydrogen peroxide in the course of catalyst synthesis.17 Other parameters such as catalyst composition and acetalization parameters (reaction time, glycerol/acetone ratio, catalyst amount, and reaction temperature) were investigated. For that purpose, real-time in situ Raman spectroscopy was employed to monitor the reaction. In the past decade, real-time Raman monitoring of important processes and product features have been carried out because this technique allows rapid real-time nondestructive measurements.22−24 The molecular information provided by this technique leads to a more complete process understanding, which is key for implementing process analytical technologies

(PAT).25,26 Raman monitoring also presents important advantages over offline chromatography.27−29 In summary, the present work studies the acetalization of glycerol with acetone using temperatures from room temperature (RT) to 353 K, based on the experimental conditions reported for this reaction in the literature. Other parameters such as type of catalyst (MP-MCF, MP-NbMCF, MP-TaMCF, and MP-PTaMCF), glycerol/acetone ratio (1:1, 1:2, and 1:4), and amount of catalyst (1, 2, and 5 wt %) were also studied. Real-time Raman monitoring during reaction is used to study the influence of all these parameters, to probe the reaction mechanism, and assess the role of intermolecular interactions.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The mesostructured cellular foam modified with (3-mercaptopropyl)trimethoxysilane (MPTMS) was prepared by hydrothermal one-pot synthesis in a polypropylene bottle according to Stucky et al.30 Pluronic P123 (poly(ethylene glycol)-block-poly(propylene glycol)block-(poly(ethylene glycol)-block) (Aldrich) (8 g, 1.4 mmol) was first dissolved in 300 g of 0.7 M HCl solution at 308−313 K. Next, 1,3,5-trimethylbenzene (Aldrich) (4 g, 33.28 mmol) and NH4F (Aldrich) (0.0934 g, 2.52 mmol) were added together under vigorous stirring. After the mixture was mixed for 1 h, TEOS (Fluka) (17.054 g, 81.99 mmol) was dropwise introduced to the mixture. MPTMS (Aldrich) (Si/MPTMS molar ratio, 10) and H2O2 (32%, Merck) were added together to the synthesis bottle (H2O2/MPTMS molar ratio, 3) after 45 min. The final gel was stirred at 308−313 K for 20 h and heated at 383 K for 24 h in an oven without stirring. The resulting product was separated by centrifugation, washed with distilled water (600 cm3), and dried at RT. The template was removed by constant extraction with ethanol for 24 h using a Soxhlet and finally dried at RT. The niobium- and tantalum-containing samples (MPNbMCF and MP-TaMCF, respectively) were prepared following a procedure similar to that described above for MCF. The metal source (ammonium niobate(V) oxalate or tantalum(V) ethoxide, Aldrich) was added to the synthesis gel 10 min after TEOS addition. The nominal Si/Nb or Si/Ta molar ratio was 64. The H2O2/MPTMS molar ratio was 9 for metal-modified samples. The tantalum-phospho-silicate material (MP-PTaMCF) was prepared using a procedure equivalent to that of the tantalumcontaining catalyst. The phosphorus source (ammonium phosphate dibasic, Aldrich) was added at the same time as the tantalum source. The molar ratio of silicon to phosphorus was 10. 2.2. Characterization of the MP-MCF Materials. The surface area and cell and window diameter of materials obtained were measured by N2 adsorption−desorption at 77 K using Quantachrome Instruments autosorb IQ2. Prior to the adsorption measurement, the samples were outgassed under 10781



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samples. To enhance the oxidation of MPTMS species, Nb or Ta was also added to the synthesis gel to incorporate them into the MCF framework. In addition, the sample showing the highest number of Brønsted acid sites (BASs), namely, MPTaMCF, was also prepared with addition of phosphorus species. Figure 1 shows the N2 adsorption−desorption isotherms of the materials. According to IUPAC classification, they can be

vacuum at 423 K. The surface area was calculated using the Brunauer−Emmett−Teller method. Pore volume and diameter was estimated according to Broekhoff−de Boer method with the Frenkel−Halsey−Hills approximation.31 Fourier transform infrared (FTIR) spectra were recorded with a Bruker Vector 22 spectrometer using an in situ cell. Samples were pressed under low pressure into thin wafers of ca. 5−8 mg cm−2 and placed inside the vacuum cell. Prior to pyridine adsorption, catalysts were outgassed at 423 K for 6 h; pyridine was then admitted at 423 K. After saturation with pyridine, the samples were degassed at 423 K for 5 min. The FTIR spectra of the sample activated by outgassing at 423 K (“background spectrum”) was used as background for those acquired upon pyridine adsorption and subsequent outgassing treatments. The reported spectra are the results of this subtraction. The amperometric titration of acidic sites was performed using 100 mg of anhydrous catalyst (dried at 423 K for 12 h). The solid was immersed in a 2 M NaCl solution (60 cm3) and stirred for 18 h. Then the solution was titrated with a 0.005 M NaOH solution for obtaining the total number of acidic sites. The actual molar ratios of Si/Nb and Si/Ta were determined using X-ray fluorescence (XRF; MiniPal-Philips). The measurements were performed using calibration curves based on the XRF measurements of reference mixtures of silica (Degussa) and Nb2O5 (Alfa Aesar) or Ta2O5 (Aldrich). The calibration curves were created using 10 points related to different silica/ oxide mixtures in the 3−300 Si/Nb (Ta) range. The metal concentration in the examined samples was determined by the amount of emitted X-ray radiation related to the values in the calibration curves. Prior to the inductively coupled plasma (ICP) analysis, 0.05 g of anhydrous catalyst (dried at 673 K for 2 h) was solubilized in the mixture of HCl (5 cm3) and HClO4 (5 cm3). After hot water addition the resulting solution was filtered and put into a round-bottomed flask (25 cm3). ICP analyses were performed using an emission spectrophotometer ICP-OES (Varian). 2.3. Synthesis of Solketal. In a typical experiment, glycerol (40 mmol) and acetone (40, 80, or 160 mmol) were taken in a round-bottom flask and mixed under continuous stirring. The mixture was held under reflux at the desired temperature (room temperature, 313, 343, and 353 K) in a temperature-controlled silicone bath. The catalyst (1−5 wt %) was added after 2 min of stirring the mixture, and the reaction time started. The reaction was 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 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

Figure 1. N2 adsorption−desorption isotherms.

assigned as a type IV. These isotherms are similar to those presented for materials with MCF structure.20,32 They show one hysteresis loop at a relatively high pressure (p/p0), which is typical of mesoporous materials; this is usually explained by capillary condensation in mesopores. Interestingly, the hysteresis loop of MP-MCF and MP-NbMCF is of type I, whereas that of MP-TaMCF and MP-PTaMCF is of type II. The difference in this shape is related to the window size, which is smaller for the latter samples. The outgassing of N2 occurs at higher p/p0 pressure in the case of MP-MCF and MP-NbMCF samples. The texture parameters calculated from N2 adsorption isotherms are presented in Table 1. All materials show relatively high surface area, in the 420−600 m2 g−1 range. Table 1. Textural/structural characterisation

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. The preparation of mesostructured cellular foams modified with MPTMS was performed in a way that leads to final materials that vary in the number of acidic sites. This factor proved to remarkably influence the acetalization of glycerol. To differentiate the number of acid sites, the silica MP-MCF material was prepared using an amount of hydrogen peroxide lower than that of other

catalyst

surface area (m g )

pore volume (cm g )

average pore diameter (nm)

MP-MCF MPNbMCF MPTaMCF MPPTaMCF

420 600

1.5a 2.3a

22.6b, 11.3c 27.8b, 11.3c

535

1.2a

37.1b, 6.4c

460

1.4a

22.4b, 6.9c

2 −1

3 −1

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). 10782



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Table 2. Chemical Composition of Materials Obtained Si/M and Si/P molar ratioa

a

catalyst

assumed

real

H+ measured by pyridine adsorption (× 1019 g−1)

H+ measured by NaOH titration (mmol g−1)

MP-MCF MP-NbMCF MP-TaMCF MP-PTaMCF

−/− 64/− 64/− 64/10

−/− 129/− 66/− 99/1027

0.09 4.99 16.05 5.66

− 0.32 0.57 0.28

P (estimated by ICP analysis); M = Nb or Ta (estimated by XRF analysis).

Scheme 2. Reaction Mechanism Proposed for the Acetalization of Glycerol with Acetone over Acid Catalyst

band is observed only in the case of MP-TaMCF material and is diminished after introduction of phosphorus species (MPPTaMCF sample). This means that phosphorus species interact with LASs generated by Ta introduction. However, one can also take into account that the amount of Ta incorporated in MP-PTaMCF is lower than that in MP-TaMCF. The MP-MCF series possesses hardly any BASs and no LASs. The spectra after pyridine adsorption are mainly dominated by the presence of weak hydrogen-bonded pyridine (bands at 1597 cm−1 to ν8a mode and 1446 cm−1 to ν19b mode) for MP-MCF material. The presence of this band can be related to the interaction of -SH species from MPTMS, which possess weak acidic character for pyridine. The very low acidity of the MP-MCF sample was confirmed by acid site titration (Table 2). Both MP-NbMCF and MP-TaMCF catalysts show the presence of BASs, which testifies to the efficient oxidation of thiol species in MPTMS to sulfonic ones. Both acidity determinations suggest the number of BASs in MP-TaMCF material is higher than that in MP-NbMCF. The addition of phosphorus species (MP-PTaMCF) decreases the BAS number in comparison to that of MP-TaMCF. Both MP-NbMCF and MP-PTaMCF samples show similar number of BASs. 3.2. Raman Monitoring during Solketal Synthesis. MP-MCF materials were tested in the reaction of glycerol with acetone under conventional thermal activation in a discontinuous reactor. Scheme 2 shows the reaction mechanism proposed for the acetalization of glycerol with acetone.21,33,34 Glycerol can be acetalized with acetone to produce two

MP-NbMCF exhibits the highest pore volume, which is in line with its higher surface area. MP-TaMCF possesses the largest cell size sample. The addition of phosphorus to the synthesis gel decreases the cell size in the final material (MP-PTaMCF), but it hardly affects cell window size. Nb, Ta, and P were added to modulate the number of Brønsted acid sites by increasing of -SH oxidation efficiency (from MPTMS). However, the incorporation efficiency of each element is quite different (Table 2). The Si/Ta molar ratio is almost the same in the synthesis gel and in the final material when phosphorus is not involved in TaMCF preparation. The addition of phosphorus decreases the amount of tantalum incorporated to the MCF structure. The Si/Ta molar ratio is higher than the Si/Nb ratio (99 versus 129, respectively) for the phosphorus-free synthesis of NbMCF. On the other hand, the amount of phosphorus added to mesostructured cellular foam is very small. Table 1 shows that the presence of phosphorus in the synthesis gel strongly influences the texture parameters, namely reducing the size of cells. Table 2 presents the results of acidity measurements (pyridine adsorption followed by FTIR measurements and acidic sites titration). The presence of hydrogen-bonded pyridine does not allow calculating the number of Lewis acid sites (LASs) (overlapping the band related to ν19b mode of vibration in pyridine at ca. 1445 cm−1). However, the presence of LASs can be confirmed by another band (ν8a mode of vibration in pyridine coordinatively bounded to LASs) at ca. 1615 cm−1 (see Figure 1 of Supporting Information). This 10783



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Figure 2. Raman monitoring of the reaction between glycerol and acetone under continuous stirring at 353 K for 3 h without catalyst (40 mmol of glycerol, 80 mmol of acetone). Legend: G, glycerol; A, acetone; G−A, glycerol−acetone interaction, G−G, glycerol−glycerol interaction; S, solketal. (3400 cm−1, G−G; 2946 and 2886 cm−1, G; 788 cm−1, A; 732 cm−1, G−A; 676 cm−1, G; 650, 638, 604 cm−1, S).

the carbon atom of the carbonyl group of acetone while glycerol−acetone intermolecular interactions occur.37 Then, the formation of a bond between the carbonyl oxygen atom and the β-carbon of the glycerol leads to the short-lived reaction intermediate (3-(2-hydroxypropan-2-yloxy)propane-1,2-diol) (Scheme 2, (1)). This short-lived tertiary alcohol intermediate may evolve in the presence of an acid following two possible paths. The path depends on the kind of acid site (LAS or BAS) present in the catalyst (Scheme 2). Five- and six-membered rings can be obtained regardless of the path taken. In the present work, solketal along with water byproduct were produced for all the MP-MCF tested materials as the sole products; no other secondary products were detected. Raman spectra also show strong interactions between glycerol molecules (G−G) at 3324 cm−1 during reaction at this temperature. Glycerol molecules self-associate through intermolecular hydrogen bonds to form dimeric, trimeric, and oligomeric structures. Raman data by Kojima et al. show that the O−H interactions in the G−G interactions are temperature-dependent.38 To characterize the Raman features of these G−G interactions, we monitored glycerol in the absence of acetone and catalyst while vigorously stirring in a discontinuous batch reactor at different temperatures (RT, 313, and 353 K) (Figure 3). Hydrogen-bonded O−H intermolecular glycerol vibrations are apparent at 3324 cm−1. These are apparent after 1 h at 353 K (Figure 3A) and after 2 h at 313 K (Figure 3B), and no apparent self-association of glycerol molecules was apparent at RT after 6 h (Figure 3C). The data in Figure 3 show that strong interactions between reactants molecules are not enough to produce solketal at the temperatures studied and in the absence of a catalyst. Figure 4 shows the reaction between glycerol and acetone at 353 K for 3 h using an excess of acetone (1:2 glycerol/acetone ratio) over MP-NbMCF catalyst (2 wt % of catalyst, based on

structural isomers, (2,2-dimethyl-1,3-dioxolan-4-yl) methanol (Scheme 2, (2)) and 2,2-dimethyl-1,3-dioxan-5-ol (Scheme 2, (3)), and water as a byproduct. However, this reaction affords mainly one of these isomers, the five-membered ring (2,2dimethyl-1,3-dioxolan-4-yl) methanol, commonly known as solketal. Theoretical calculations suggest that the fivemembered ring (2,2-dimethyl-1,3-dioxolan-4-yl) methanol (solketal) is thermodynamically more stable than 2,2-dimethyl-1,3-dioxan-5-ol isomer because of steric repulsions provoked by the presence of the methyl group in axial position of the six-membered ring.35 Other studies have indicated that five-membered ring compounds are kinetically favorable, whereas six-membered ring compounds are favored by thermodynamics.36 3.2.1. Influence of Temperature in the Acetalization of Glycerol with Acetone. 3.2.1.1. Acetalization of Glycerol with Acetone at 343−353 K. The reaction temperature triggers acetone evaporation, reversing solketal formation. For this reason, many authors use an excess of acetone at moderate temperatures, up to 343−353 K, for this reaction.6,16,21,35 Figure 2 shows the Raman spectra during the blank reaction between glycerol and acetone at 353 K in the absence of any catalyst. Besides the Raman bands characteristic of reactants, a Raman band at 732 cm−1 becomes apparent after a 30 min reaction time, which is associated with strong interactions between glycerol and acetone molecules (G−A). This is assigned to O···CO vibrations due to the interaction between the primary hydroxyl group of the glycerol molecule and the carbonyl group of the acetone molecule (Scheme 2). The glycerol−acetone interaction can be of relevance to the reaction. This interaction would reveal an initial interaction between reactants, but its formation hardly delivers any reaction at 353 K in the absence of a catalyst for it affords 13% conversion and 96% selectivity to solketal after 30 min. Mechanistically, the primary alcohol group of glycerol attacks 10784



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consumption because of a shift in the equilibrium of the reaction toward solketal decomposition. Figure 5 shows the concentration profile of Raman band characteristics of reactants, acetone and glycerol, and the solketal product. The solketal Raman bands reach a maximum at 60 min while those of acetone decrease continuously. Acetone Raman signal decrease is due to two processes: it is a consumed in reaction and it also evaporates. As expected, acetone evaporation is increasingly important with temperature. Thus, the reaction progresses normally while the presence of acetone is sufficient. The reaction cannot progress any longer when acetone evaporation creates a reactant deficit. At that moment, solketal formation reaction reverses, thus restoring the presence of glycerol. Gas chromatography confirms that the conversion of the reaction reaches a maximum with time of 70% (98% selectivity) at 30 min and 18% (98% selectivity) at 3 h of reaction time for MP-NbMCF. The reaction remains close to total selectivity. Figure 6 shows the concentration profile of Raman bands characteristic of acetone and glycerol reactants and the solketal product for the reaction between glycerol and acetone at 343 K for 3 h using MP-NbMCF catalyst. The Raman band of solketal increases rapidly for the first 10 min of reaction and then levels off. Gas chromatography results show a 69% conversion (98% selectivity) at 10 min, and then it decreased to 52% conversion (98% selectivity) at 3 h of reaction time. Figure 7 shows the Raman intensity contour plots of the reaction between glycerol and acetone using MP-NbMCF catalyst at 343 K during 3 h. The Raman bands of solketal increase in intensity; first they form readily, then they grow stronger at a slower pace. The 676 cm−1 glycerol Raman band starts to increase slowly after 1 h of reaction time. These results show that the efficiency of the reaction at 343 K is better because acetone volatilization is lower than that at 353 K; thus, the reverse reaction is minimized. High temperatures would be beneficial to increase glycerol conversion and solketal production if acetone volatilization is prevented, such as by the use of autoclave reactors. High reaction temperatures volatilize acetone in an open system, even with the use of reflux. 3.2.1.2. Acetalization of Glycerol with Acetone at 313 K. Figure 8 shows Raman spectra during reaction between glycerol and acetone without catalyst at 313 K for 3 h. The Raman bands of reactants acetone and glycerol are apparent, but not those of the solketal product. As expected, the intensity of acetone Raman bands decreases at this temperature only after long reaction times, with the concomitant increase of the glycerol Raman band intensity, which confirms a minimized volatilization of acetone. Very weak additional Raman bands becoming apparent at 1384, 1190, 1028, 580, 506, 492, 486, and 392 cm−1 (not shown) would evidence the formation of 3(2-hydroxypropan-2-yloxy)propane-1,2-diol, the intermediate (1) of the acetalization reaction of glycerol with acetone (Scheme 2). A detailed analysis of this will be made below. 3.2.1.3. Acetalization of Glycerol with Acetone at Room Temperature: Evidence of a Tertiary Alcohol Intermediate. Figure 9 shows difference Raman spectra (with respect to that at time zero, subtracted using Omnic version 9 software) during the equimolar acetalization of glycerol with acetone at RT for 6 h in the absence of a catalyst in the representative 1400−600 cm−1 range. The positive bands illustrate species that form, whereas the negative bands illustrate species that are consumed or lost. This plot shows the Raman bands characteristics of acetone and glycerol, but most interestingly, it evidences the

Figure 3. Raman spectra of glycerol solution in discontinuous batch reactor under continuous stirring at different temperatures without catalyst: (A) 353 K for 3 h, (B) 313 K for 3 h, and (C) RT for 6 h. G, glycerol; G−G, hydrogen-bonded OH intermolecular (glycerol) vibrations.

the amount of glycerol used in the reaction). Raman spectra show that MP-NbMCF catalyst readily triggers the reaction between glycerol and acetone. In addition to the characteristic Raman bands of reactants, acetone (3008, 1706, 1432, 1358, 1222, 1068, 532, 494, and 392 cm−1), and glycerol (2946, 2886, 1466, 1252, 1114, 1060, 976, 926, 852, 822, 676, 486, and 418 cm−1), new Raman bands are apparent at 2992, 2940, 1482, 1462, 1440, 1238, 1212, 1120, 1072, 1056, 970, 920, 860, 794, 648, 638, 476, 418, 348, and 316 cm−1, which correspond to the reaction product solketal (see reference spectra in Figure 2 of Supporting Information). The Raman bands at 3324 and 732 cm−1, characteristic of G−G and G−A interactions, respectively, are apparent, too. The G−G interactions band at 3324 cm−1 are apparent after 30 min of reaction time (Figure 4), while the formation of solketal is already apparent from the first few minutes of reaction. Thus, glycerol−glycerol interactions would not be critical for the production of solketal. The G−A interaction Raman band at 732 cm−1 appears as soon as the catalyst is added (Figure 4). It shows a trend quite close to that of solketal. The G−A interactions would be related to the reaction for solketal formation. The Raman bands of solketal at 638 and 648 cm−1 form readily because of the high acidity of the catalyst; these bands become weaker at high reaction time because the reaction reverses because of acetone evaporation. Very fast acetone evaporation is also apparent in the increase of intensity of the Raman band of glycerol at 676 cm−1. Increase of the glycerol band was observed even after extended acetone 10785



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Figure 4. Raman monitoring of the acetalization of glycerol with acetone at 353 K for 3 h using MP-NbMCF catalyst (40 mmol of glycerol, 80 mmol of acetone; 2 wt % catalyst). G−A, glycerol−acetone interaction; G, glycerol; A, acetone; S, solketal.

Figure 5. . Concentration profile of reactantas glycerol and acetone and the product solketal versus time for the acetalization of glycerol at 353 K for 3 h over MP-NbMCF catalyst (40 mmol of glycerol, 80 mmol of acetone; 2 wt % catalyst). Red squares, solketal; blue circles, acetone; green triangles, glycerol.

Figure 6. Concentration profile of reactants glycerol and acetone and the product solketal versus time for the acetalization of glycerol at 343 K for 3 h over MP-NbMCF catalyst (40 mmol of glycerol, 80 mmol of acetone; 2 wt % catalyst). Red squares, solketal; blue circles, acetone; green triangles, glycerol.

appearance of new bands at 1384, 1190, 1054, 1028, 966, and 668 cm−1, characteristics of tertiary alcohol vibrations (see also additional bands at 580, 506, 492, 466, 450, 440, 434, and 304 cm−1 in Figure 3 of Supporting Information). These Raman bands suggest the formation of the intermediate (1) (Scheme 2). Both, Raman spectroscopy and gas chromatography show that there is no formation of solketal in the absence of catalyst at RT. However, solketal is obtained with almost total selectivity (>98%) (16% yield on MP-MCF, 31% yield MPNbMCF, 44% yield on MP-TaMCF, and 37% yield on MPTaMCF-64−10) when the acetalization of glycerol was carried out at RT using equimolar amounts of reactants and MP-MCFbased materials (1 wt % based on glycerol).

The formation of a tertiary alcohol intermediate is apparent from the beginning of the reaction. Neither glycerol−acetone nor glycerol−glycerol interactions were apparent at 313 K in the absence of a catalyst. This is unlike data at 343−353 K. It appears that the acetalization of glycerol with acetone at 313 K in the absence of catalyst does not progress and only an incipient formation of the tertiary alcohol intermediate 3-(2hydroxypropan-2-yloxy)propane-1,2-diol (1) is apparent (this product is not commercial) (Scheme 2). This intermediate could be detected only by real-time Raman monitoring but not by gas chromatography (GC) maybe because of its decomposition in the column or at the column injection port. 10786



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Figure 7. Intensity contour plots of the real-time Raman monitoring acetalization of glycerol with acetone at 343 K over MP-NbMCF catalyst for 3 h in the 3400−2600 cm−1 (left panel) and 740−600 cm−1(right panel) ranges (40 mmol of glycerol, 80 mmol of acetone; 2 wt % catalyst).

Figure 8. Raman monitoring of the reaction between glycerol and acetone under continuous stirring at 313 K for 3 h without catalyst (40 mmol of glycerol, 80 mmol of acetone).

3.2.1.4. Catalyzed Low-Temperature Acetalization of Glycerol with Acetone. Low-energy processes for glycerol acetals production may significantly reduce its cost and minimize evaporation of acetone. For this reason, the performance of acetalization reactions at RT becomes an interesting alternative. Figure 10 shows the Raman spectra

during the reaction between glycerol and acetone at 313 K for 3 h using the MP-MCF catalyst. New bands corresponding to solketal product as well as those corresponding to G−A and G−G molecular interactions become apparent. The addition of the MP-MCF catalyst speeds up the reaction and shows a concomitant increase in the G−A molecular interaction 10787



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Figure 9. Raman monitoring of the reaction between glycerol and acetone under continuous stirring at RT for 6 h without catalyst (40 mmol of glycerol, 40 mmol of acetone).

Figure 10. Raman monitoring of the acetalization of glycerol with acetone in two spectral ranges (3400−2600 cm−1 (left panels) and 740−600 cm−1 (right panels)) at 313 K for 3 h using MP-MCF catalyst (40 mmol of glycerol, 80 mmol of acetone; 2 wt % catalyst) (top panels). Raman intensity versus time of glycerol, acetone, and solketal bands (bottom panels).

(Raman band at 732 cm−1), delivering nearly total selectivity to solketal (gas chromatography results show that the reaction reaches 68% glycerol conversion and 98% selectivity in 3 h with MP-MCF catalyst). This is probably due to the ready conversion of the intermediate in the presence of an acidic catalyst site, which is consistent with the noted absence of Raman features of any reaction intermediate at 353−343 K. 3.3. Influence of the Type of Catalyst in the Acetalization of Glycerol with Acetone. Low reaction temperatures minimize acetone volatilization, enabling efficient production of solketal. Acetone excess has a promoting effect for the reaction nearly doubling conversion values for 1:2 glycerol/acetone ratio (60% of conversion and 99% selectivity

for MP-NbMCF catalyst versus 32% conversion and 98% selectivity for 1:1 glycerol/acetone ratio). The impact of additional excess is weaker upon further additions of acetone, such as for 1:4 ratio, which affords 72% of conversion and 99% selectivity for MP-NbMCF catalyst. Figure 11 shows the influence on conversion and selectivity of the type of MP-MCF material used in the reaction of glycerol with acetone at RT, 313, and 343 K. The results were obtained by gas chromatography. The presence of an MP-MCF material catalyzes the acetalization of glycerol, delivering high selectivity (>97%) at either temperature. In the absence of a catalyst, solketal production is apparent only at 343 K. MPMCF materials dramatically increase glycerol conversion. For 10788



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Table 3. Comparison of the Results Obtained in This Work for the Synthesis of Solketal with Those Obtained by Other Authors reaction temperature 70 °C

Figure 11. Study of the influence of the type of catalyst used in the acetalization of glycerol with acetone for 3 h. (Reaction conditions at RT: 1 wt % catalyst, 40 mmol glycerol, 40 mmol acetone. Reaction conditions at 313 K: 2 wt % catalyst, 40 mmol glycerol, 80 mmol acetone. Reaction conditions at 343 K: 2 wt % catalyst, 40 mmol glycerol, 80 mmol acetone.)

any catalyzed reaction, conversion values reach a maximum at 313 K. The data correspond to a batch reactor after 3 h. The decrease of conversion at higher temperatures is due to acetone evaporation, which shifts the equilibrium to the reverse reaction. At the lowest temperatures, tantalum-containing catalysts afford the best performances, then MP-NbMCF, and then the bare MP-MCF substrate. The maximum yields are obtained at 313 K, where the MP-MCF series shows the reactivity trend MP-NbMCF (80% conversion, 97% selectivity) > MP-TaMCF (77% conversion, 98% selectivity) > MPPTaMCF (73% conversion, 99% selectivity). Table 3 shows the results obtained in this work compared to those obtained by other authors. The results reported for 70 °C by other groups deliver somewhat worse results that those reported here. Similarly, the selectivity obtained at 40 °C is better in this work than in that by Fan and co-workers.33 Finally, at room temperature, the work by Reddy’s group shows values similar to those reported here.34 Figure 12 shows the influence of the acidity (data from Table 2) of the catalysts in the acetalization of glycerol with acetone. All the catalysts exhibit values of selectivity similar to that of solketal (>97%); the hydrophobic character of the catalyst would favor the selectivity to solketal. The acetalization of solketal is promoted by both LASs and BASs (Scheme 2). However, this work shows that the BASs present in the materials lead to better conversion values: MP-TaMCF, with the largest number of BAS, shows the best values of conversion at RT. Figure 13 shows the relationship between the average cell diameter and solketal yield for the MP-MCF series. Not only acidity but also average cell size of the MP-MCF materials plays an important role in the acetalization of glycerol with acetone. A larger average cell size would allow for a larger number of molecules inside the pore structure of the catalyst. MPPTaMCF does not fit into the trendline. The presence of phosphorus to the catalyst decrease has a 2-fold effect: it decreases the average pore size to 22.4 nm but triggers the number of acid sites (Figure 13). As a consequence, the yield to solketal for MP-PTaMCF decreases to 37% compared to 44% yield on MP-TaMCF. This indicates that indeed both availability of acid sites and pore structure are relevant for the reaction. The shrinkage from 37 to 22 nm has an effect that is much less intense than that of the change in the number of acid sites. Thus, these data suggest that acidity would be the critical factor when the system has a pore structure open enough for the reactants to enter. The data for this system do

40 °C

RT

other authors

this work

33

Fan et al. (2012) cat.: TiO2−SiO2 ratio: 1:2, 2.2 wt % catalyst conv. ∼73% (∼88% select.) (180 min) Vicente et al. (2010)16 cat.: Amberlyt 15 ratio: 1:6, 5 wt % catalyst conv. = 84% (99% select.) (30 min) Ferreira et al. (2010)6 cat.: heteropolyacids PW_S ratio: 1:6, 5 wt % catalyst conv. = 55% (97% select.) (45 min) Fan et al. (2012)33 cat.: TiO2−SiO2 ratio: 1:4, 2.2 wt % catalyst conv.∼ 78% (∼75% select.) (180 min) Reddy et al. (2013)34 cat.: Mo and W-promoted SnO2 ratio: 1:1, 5 wt % catalyst conv. = 61% (96% select.) (90 min)

cat.: MP-PTaMCF ratio: 1:2, 2 wt % catalyst conv. = 68% (99% select.) (30 min)

cat.: MP-NbMCF ratio: 1:2, 2 wt % catalyst conv. = 80% (97% select.) (180 min)

cat.: MP-NbMCF ratio: 1:4, 1 wt % catalyst conv. = 72% (99% select.) (180 min) cat.: MP-TaMCF ratio: 1:1, 1 wt % catalyst conv.=44% (98% select.) (60 min)

not allow us to assess the value of the critical pore size necessary for this reaction.

4. CONCLUSIONS The acetalization of glycerol with acetone over modified mesoporous cellular foams materials can be run at room and very moderate, 313 K, temperatures with nearly total selectivity to solketal and conversion values approaching 80%. The results show that there is a relationship between the number of Brønsted acid sites and pore structure with solketal productivity. Modification of MCFs with Nb and Ta species leads to the enhancement of the number of BASs formed by oxidation of SH groups in (3-mercaptopropyl)trimethoxysilane. Tantalum addition maximizes the number of BASs, and the cell size significantly increases. These effects lead to higher solketal productivity. Phophorous addition to the system triggers acidity but significantly decreases the pore structure. Real-time Raman monitoring provides evidence of the existence of a tertiary alcohol, which is a proposed reaction intermetiade: the 3-(2-hydroxypropan-2-yloxy)propane-1,2diol. Under optimal reaction conditions (type of catalyst and reaction parameters), the 5-membered ring ketal 2,2-dimethyl1,3-dioxolane-4-yl methanol (solketal) was obtained with the highest selectivity (99%). Raman monitoring also shows that glycerol−glycerol interaction is not directly related to solketal acetylation, but that the formation of a glycerol−acetone adduct 10789



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ASSOCIATED CONTENT

S Supporting Information *

FTIR measurements (Figure 1); Raman spectra of raw materials glycerol, acetone, and solketal (Figure 2); and Raman monitoring of the reaction between glycerol and acetone at RT without catalyst in the 200−1800 cm−1 and 600−300 cm−1 windows (Figure 3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 913 98 73 46. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from Spanish Ministry Project CTQ2011-13343-E. National Science Centre in Poland is acknowledged for financial support (Projects 2011/01/B/ ST5/00847, 2011/03/N/ST5/04772, and 2013/08/T/ST5/ 00010). K.S. thanks the Adam Mickiewicz Foundation in Poznan for scholarship in 2013.

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REFERENCES

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Figure 13. Influence of the average cell size of the catalyst in the acetalization of glycerol (molecular size, 0.470 nm) with acetone (molecular size, 0.550 nm). Cell diameters were determined from adsorption branches of N2 isotherms (BdB−FHH method). (Solketal reaction conditions: RT, Gly/acetone ratio 1:1, 1 wt % catatyst, 3 h). ▲, MP-MCF; ○, MP-NbMCF; ■, MP-TaMCF; X, MP-PTaMCF.

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