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Hydroxylated magnesium fluorides as environmentally friendly catalysts for glycerol acetylation
Applied Catalysis B: Environmental 107 (2011) 260–267
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Hydroxylated magnesium fluorides as environmentally friendly catalysts for glycerol acetylation Simona B. Troncea a , Stefan Wuttke b , Erhard Kemnitz b , Simona M. Coman a , Vasile I. Parvulescu a,∗ a b
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta, 4-12, Bucharest 030016, Romania Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 11 July 2011 Accepted 19 July 2011 Available online 26 July 2011 Keywords: Glycerol Bioadditive Hydroxylated fluorides Microwave Ultrasound
a b s t r a c t The increase of biodiesel production during the next years will result in an overproduction, and therefore, an accumulation of glycerol on the market. The use of glycerol-based additives to improve the properties of biodiesel or gasoline is one of the possibilities currently being explored to utilize this renewable feedstock. In this context, partly hydroxylated magnesium fluorides have demonstrated catalytic activity in the esterification of glycerol with acetic acid to yield diacetylglycerine (DAG) and triacetylglycerine (TAG). The catalytic activity depends on the density of acid sites at the external catalytic surface while the selectivity to different acylated products is influenced by the nature of the acid sites (Lewis and/or Brønsted). Using non-conventional activation methods of the reactant molecules, as microwave or ultrasound irradiation, optimal glycerol conversions (>90%) and selectivities to the desired compounds (over 85%) were obtained. The activities and selectivities achieved using partly hydroxylated fluorides as catalysts are comparable or even superior to those displayed by conventional oxide based acid catalysts under thermal activation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the last years biodiesel became one of the most important alternatives to the petroleum-based diesel fuel, partly due to some important advantages such as high biodegradability, non-toxicity, production from renewable energy sources and less harmful to the environment [1]. Since the main by-product (up to approximately 10 wt% of the total product) of the industrial biodiesel production (FAME production) is glycerol, it is predicted that the increase of the biodiesel production in the future will result in its accumulation creating a glut in the market [2]. This situation has prompted a search for new glycerol uses. Among different alternatives, the use of glycerol-based additives (e.g., di- and triacyl glycerin, obtained through the esterification of glycerol with acetic acid), which would improve the properties of biodiesel (e.g., cold and viscosity properties) or gasoline (e.g., antiknocking properties) is being explored [3]. Up to date several synthetic routes for obtaining organic esters were developed. Among these, the direct esterification of alcohols with carboxylic acids, in the presence of liquid acid catalysts (e.g., sulfuric acid, hydrochloric acid, and para-toluenesulfonic acid), is
∗ Corresponding author. Tel.: +40214100241; fax: +40214100241.. E-mail addresses: [email protected] (E. Kemnitz), [email protected] (S.M. Coman), [email protected] (V.I. Parvulescu). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.021
the most common method [4]. However, the use of conventional homogeneous acid catalysts must be limited in the future to comply with the increasingly strict environmental regulations. Efforts to green chemical reactions focus predominantly on greening the three main components of the reaction system: solvent, reagent/catalyst, and energy input, respectively. Thus, processes are recommended, (i) using benign solvents or solvent-free methodologies, (ii) using alternate, more efficient and less toxic reagents/catalysts as well as reusable catalysts, and (iii) using costeffective, eco-friendly alternative processes. The use of so-called non-classical energy forms (e.g., microwaves, ultrasound irradiation) was reported to lead to an optimization of the reaction time and product yield, thereby also reducing the by-products [5]. In a typical case of esterification, the literature reports a high number of contributions replacing the conventional homogeneous acid catalysts by more benign heterogeneous acid catalysts like ion-exchange organic resins [6], zeolites [6–9] and silica-supported heteropoly acids [6,10–12]. However, many of these catalysts show some limitations such as low thermal stability (resins), mass transfer resistance (zeolites), or loss of active acid sites due to high solubility in a polar medium (heteropoly acids). Moreover, the presence of water can seriously affect the catalytic behaviour of some of these solid acid catalysts because they are highly hygroscopic in nature. To overcome most of these limitations superacidic silica-embedded Nafion (SAC-13) has been suggested as an alternative [13]. There are also studies on sulfonic acid functionalized
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mesoporous silica as SBA-15 [14] but the expensive templating copolymer in the SBA-15 preparation and complex acid functionalized process limits the industrial application. The main objective of this study was to reach a maximum glycerol conversion with maximum selectivities to the most valuable acetylated products, i.e., di- and tri-acetylglycerol. Theoretic studies reported by Melero et al. [14] indicated that high reaction temperatures and acetic acid/glycerol molar ratios favor the evolution of the final product distribution in equilibrium to the highly substituted glycerol acetylated derivatives. However, a very high acetic acid/glycerol ratio is not convenient. In order to achieve this goal the methods being used are not always in concordance with the green chemistry principles. It was thus suggested a two-step method in which the esterification of glycerol with acetic acid is followed by the acetylation with acid anhydride carried out over resins and zeolites [7,9,15]. However, such a procedure has several drawbacks like (i) the use of a relatively large amounts of acetic acid co-reactant may generate high amounts of wastes, (ii) the use of acid anhydride drastically decreases the atom economy of the process. Moreover, the price of acetic anhydride is higher (ca. four times) than that of acetic acid. But the reaction can be carried out with a small acetic acid/glycerol ratio and at lower temperatures by using an appropriate heterogeneous catalyst and activation conditions [16]. Recently developed nanoscopic partly hydroxylated magnesium fluorides (MgF2−x (OH)x with x < 0.1) with both Lewis/Brønsted acid centres have been proved to posses unexpected catalytic properties in several syntheses, which are of great interest in the fine chemistry area (e.g., vitamin E, vitamins K) [17,18]. The catalytic properties are related to their particular structural and chemical features: (i) high surface area with pore diameters in the range of mesopores; (ii) very low solubility in strong polar solvents; (iii) hydrolysis resistance; (iv) mediumstrength Lewis and Brønsted acid sites; (v) the possibility of easily tuning the surface acidity; (vi) nanoscopic particle dimension [19,20]. Taking into account these structural and chemical features with the aim to extend the applicability of this new class of catalytic materials, we applied them in the synthesis of glycerine acetates as valuable transportation fuel additives through the catalytic esterification of glycerol with acetic acid. Another aim was to further optimise the reaction conditions by using non-conventional methods to activate reactant molecules such as microwaves or ultrasound irradiation.
2. Experimental 2.1. Catalyst preparation The catalysts were synthesized as reported elsewhere [19], from metallic Mg using the fluorolytic sol–gel method, as follows: metallic Mg (Aldrich, 99.98% powder) (1.56 g, 64 mmol) was dissolved in dry methanol (50 mL) at room temperature overnight. After heating under reflux conditions for 3 h, a stoichiometric amount of HF (130 mmol) dissolved in different amounts of water (HF solution concentrations: 40, 57, 71, and 87 wt% HF) was added to the formed Mg(OCH3 )2 solution. The mixtures reacted to form highly viscous transparent gels. After ageing for 12 h, each gel was dried under vacuum at room temperature. The solid product thus obtained was then further dried under vacuum at 70 ◦ C for 5 h. The prepared catalysts are referred to hereafter as MgF2 -40, MgF2 -57, MgF2 -71 and MgF2 -87, indicating the different concentrations of HF solutions used. For comparison, commercial H-Beta zeolite (PQ-Valfor Company) with Si/Al of 10.8 (PQ25) was used as reference.
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2.2. Catalyst characterization XRD, MAS-NMR, TEM, thermal analysis, XPS, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials [19,21]. The N2 /Ar adsorption–desorption isotherms, FT-IR photoacoustic pyridine adsorption spectra, and NH3 -TPD have been carried out to investigate the surface properties. The porosity characteristics of the samples were determined by acquiring adsorption–desorption isotherms of N2 at 77 K and of Ar at 87 K on a Micromeritics ASAP 2020 instrument based on the BJH-model. Samples were first degassed in vacuum at 70 ◦ C. Surface areas were calculated according to the BET method. FT-IR photoacoustic pyridine adsorption spectroscopy was carried out according to the following procedure: the sample (75 mg) was pre-heated at 150 ◦ C under N2 flow (35 ml min−1 ) for 15 min, then 60 ml pyridine was injected into the sample tube. The sample was flushed with nitrogen for another 15 min to remove physisorbed pyridine. Spectra of the sample were recorded at room temperature using a MTEC cell and FT-IR system 2000 (Perkin-Elmer). Spectra of the samples without pyridine adsorption were also measured as the background. Temperatureprogrammed desorption of ammonia (NH3 -TPD) was employed to determine the number of acid sites and their distribution based on their relative strength. The sample (about 0.2 g) was first heated under nitrogen up to 300 ◦ C, then at 120 ◦ C exposed to NH3 . After flushing the excess NH3 at 120 ◦ C with N2 for 1 h and cooling to 80 ◦ C the TPD program was started (10◦ /min up to 500 ◦ C, keeping for 30 min). Desorbed NH3 was monitored continuously via IR spectroscopy (FT-IR System 2000, Perkin-Elmer). 2.3. Catalytic tests The catalytic samples were tested using three different activation procedures: thermal (A), microwaves (B) and ultrasound (C) irradiation. (A) In a typical procedure, to 1.8 g of glycerol was added to 3.6 g of acetic acid (corresponding to 3/1 acetic acid/glycerol molar ratio) in free solvent conditions, in a 20 ml glass vial equipped with a magnetic stirrer. To this mixture 50 mg of catalyst were added. After that, the vial was closed, immersed in an oil bath with a temperature of 80–100 ◦ C, and the charged mixture was stirred (1.250 rpm) for 18–22 h. (B) Microwave assisted reactions were carried out with a Milestone Start S system operating at 600 W. The stirring rate inside the reactor was 350 rpm. Typical experiments were carried out using 7.4 g of glycerol and 14.4 g acetic acid (corresponding to 3/1 acetic acid/glycerol molar ratio). To this system were added 150 mg of catalyst and the slurry was maintained at 100 ◦ C, under stirring, between 30 min and 4 h. (C) The ultrasound experiments were carried out with an Elma Transonic 460/H bath working at a frequency of 35 kHz. Typical experiments were carried out using 1.8 g of glycerol and 3.6 g acetic acid (corresponding to a 3/1 acetic acid/glycerol molar ratio). To this system 50 mg of catalyst were added and the slurry was maintained at 24–80 ◦ C for 30 min. The temperature inside the vessel was controlled using either a thermocouple (thermal and ultrasound) or a fibber optic (microwave). 2.4. Product analysis The resulted reaction mixture was cooled, the bulk of the catalyst was removed by filtration and the liquid phase was evaporated under vacuum at 80 ◦ C. The concentrated product was sylilated according to following conditions: at 50 mg of reaction product 1 ml of chloroform and 1 ml of sylilation agent (bis-trimethyl-syliltrifluoroacetamide) was added. The mixture was maintained at 60–70 ◦ C, for 30 min, under stirring. The resulted reaction mixture
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100 90 80 70 60
%
50 40 30 20 10
C (%) S MAG (%)
0 MgF2-87
S DAG (%) MgF2-71
MgF2-57
MgF2-40
Catalyst
S TAG (%) H-BEA
blank
Fig. 1. The catalytic results obtained in the thermal esterification of glycerol, as a function of the catalyst nature (acetic acid/glycerol molar ratio: 3/1; 100 ◦ C, 22 h).
was cooled at room temperature and the solvent was removed under vacuum, at 35 ◦ C. The samples were then diluted with 1 ml of n-hexane and analyzed by GC-FID chromatography (GCShimatzu aparatus in following conditions: column: CP-SIL 8 CB, 30 m, ID = 0.25 mm). The identification of the products was made using a GC-MS Carlo Erba Instruments QMD 1000 equipped with a Factor Four VF-5HT column with the following characteristics: 0.32 mm × 0.1 m × 15 m working with a temperature program at a pressure of 0.38 Torr with He as the carrier gas. 3. Results and discussion Scheme 1 presents the esterification of glycerol with acetic acid, with MAG – monoacetylglycerol, DAG – diacetylglycerol and TAG – triacetylglycerol. It must be noted that MAG and DAG can include several isomers depending on the acetylation position within the glycerol molecule. Among the obtained products, DAG and TAG are the most interesting products from a fuel-production point of view; they can be formulated with petroleum-derived fuels to improve either cold and viscosity properties (biodiesel) or antiknocking properties (gasoline). The product MAG is considered as an undesired product (by-product) due to its relatively high solubility in water. The initial screening of the nanoscopic partly hydroxylated fluorides in the esterification of glycerol with acetic acid using a conventional thermal energy source showed that after 22 h, at 100 ◦ C, the conversion of glycerol was still below 100% (Fig. 1). In order to evaluate the progress of the reaction in the absence of the solid catalyst, a blank run was also carried out. As Fig. 1 shows, the esterification reaction occurs even in the absence of a catalyst – although the glycerol conversion remains much lower comparing with the cases when fluorides have been used. In the absence of the catalyst, the esterification led only to MAG as reaction product. Therefore, the main function of the catalysts in this reaction should be to direct the selectivity to the desired products (DAG and TAG). Comparing to H-BEA zeolite the fluoride-based catalysts exhibit a different behaviour. However, this cannot be related to the acidity (Table 1) or to the acidic strength (Fig. 2) of the two systems. The investigated H-BEA zeolite has a density of acid sites of 0.18 mmol/g that is close to the acidity of some hydroxylated fluorides (i.e., MgF2 -40 – 0.17 mmol/g and MgF2 -71 – 0.16 mmol/g). Nevertheless, differently to hydroxylated fluorides, H-BEA is characterized by a stronger acidity as TPD measurements have shown (Table 1
Fig. 2. NH3 -TPD profiles of the investigated catalysts: (a) MgF2 -40; (b) MgF2 -57; (c) MgF2 -71; (d) MgF2 -87; (e) H-BEA.
and Fig. 2) and different Lewis and Brønsted acid site population as evidenced from Py-IR spectra (Table 1 and Fig. 3). Taking into account these differences and the literature reports the selectivity to DAG + TAG was expected being in favor of H-BEA (e.g., the stronger Brønsted acid sites the higher the selectivity to DAG + TAG [14]). With this respect, Fig. 1 shows that the use of MgF2 -87 (that exhibits the highest Lewis/Brønsted acid ratio, i.e. the lowest amount of Brønsted sites) led to the highest selectivity to DAG and TAG products, due to the consecutive acylation of MAG. With the same sample the conversion of glycerol reached 94.2%. A high conversion (83.2%) was also obtained in the presence of H-BEA zeolite but the selectivity to DAG and TAG products was only 52.5%, and a high amount of MAG still remained in the reaction mixture. This result is in agreement with those of Silva et al. [7] who reported
Fig. 3. The FTIR spectra of pyridine absorbed on hydroxylated fluorides.
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OH
OCOCH3 +
OH
OCOCH3
OCOCH3
OCOCH3 +
OH
CH3COOH
OCOCH3 OH
OCOCH3
OH
DAG
+
OH
- H2O
MAG
OH
OH OH
263
glycerol OCOCH3 TAG OCOCH3 OCOCH3 Scheme 1. The esterification of glycerol with acetic acid.
that Beta zeolite displays a low selectivity to TAG when acetic acid is used as acylating agent. These authors explained this result in terms of the stability of the acylium ion intermediate. In order to distinguish the catalytic activity of different acid sites, a specific rate per acid site (TOF (h−1 )) has also been calculated. The results are presented in Fig. 4. A comparison between the results presented in Figs. 1 and 4 show that the highest intrinsic activity was displayed by the MgF2 -71 sample. The differences between the activities expressed as the conversion of glycerol and TOF are due to the different density of the acid sites on the catalyst surface. The correlation of TOF (h−1 ) with the total Lewis and Brønsted acid sites density (determined from NH3 -TPD measurements [21]) led to the following order: MgF2 -71 > MgF2 -40 > H-BEA > MgF2 57 > MgF2 -87 (Fig. 2A). This order led to an unexpected conclusion: the higher acid sites density corresponds to the lower TOF. Such behaviour may be explained only if we take into consideration limited accessibility of the acid surface sites as result of a physical blocking of a part of the active sites by the glycerol molecules strongly chemisorbed onto the surface. As a consequence only a relatively small fraction of active sites is effectively involved in the esterification reaction. On the other hand, the reaction data show that the nature and the ratio of the acid sites (Lewis and/or Brønsted) are responsible for the product distribution. Thus, the selectivity to DAG + TAG decreased in the order: MgF2 -87 (94.3%) > MgF2 71 (88.3%) > MgF2 -57 (84.1%) > MgF2 -40 (71.6%) > H-BEA (52.5%) (Fig. 4B). This decrease indicates indeed a direct correlation of the Lewis/Brønsted ratio with the product distribution: a higher
amount of Brønsted acid sites (e.g., MgF2 -40 sample) led to a higher amount of MAG and consequently to small amounts of the desired DAG and TAG products. This behaviour also explains the total selectivity to MAG in the absence of the catalyst (the blank run). In addition, the presence of the acetic acid (acetic acid/glycerol molar ratio = 3/1) introduces a significant source of potentially soft catalytic protons to the reaction mixture (Fig. 1). Taking into account the catalytic behaviour and the large differences between the textural properties of the two catalytic materials (Beta zeolite versus magnesium fluorides) we assume the difference in selectivity can be assigned to mass transfer limitations. It is however evident that DAG and TAG are space demanding and their formation and diffusion inside the zeolite pores would be difficult, thus explaining the poor selectivity to these products. Indeed, the calculation of the Thiele modulus modified by Weisz [23] indicated that the diffusion resistance does not play a role in the case of hydroxylated fluorides (ϕ < 1, i.e. 0.08) while in the case of beta zeolite the diffusion inside the zeolite pores would be difficult (ϕ > 1, i.e. 1.2). These results led to the conclusion that to achieve the optimum catalytic performances in the esterification of glycerol the surface of hydroxylated magnesium fluoride should exhibit a low density of acid sites on the surface and these acid sites should be preponderantly Lewis in nature (i.e. samples with a high Lewis/Brønsted ratio). These properties can be generated during the combined fluorolytic/hydrolytic sol–gel process when the ratio of M(OR)x to HF was kept constant while the water content varied. Under these
Table 1 Textural and acid characteristics of the prepared MgF2 catalysts and commercial H-BEA zeolite. Crt. Nr.
Catalyst
BET surface areaa (m2 /g)
Pore volumea (cm3 /g)
Pore sizea (nm)
Density of acidic sitesb (mmol/g)
B acid sitesc
L acid sitesc
1 2 3 4 5
MgF2 -40 MgF2 -57 MgF2 -71 MgF2 -87 H-BEAd
180 233 264 424 465
0.12 0.10 0.16 0.25 0.19
2.6 2.3 2.2 2.2 <1.0
0.17 0.26 0.16 0.33 0.18
2.7 1.5 1.2 0.4 0.9
1.0 1.1 1.2 2.5 0.7
a b c d
Textural properties measured with N2 at 77 K. Calculated from TPD measurements. Calculated by the ratio of the integrated area of IR adsorption peak to the sample weight. See ref [22].
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A 100 90
MgF2 -71 MgF2 -40
80
H-BEA
TOF (h-1)
70 60
MgF2 -57
50
MgF2-87
40 30 20 10 0
0,16
0,17
0,18
0,26
0,33
Density of acid sites (TPD), mmol/g
B 100 90
MgF -57
DAG + TAG (%)
80
MgF2 -87
Scheme 2. Graphical illustration of possible topological situations on MgF2−x (OH)x surface that could explain the Brønsted acidic nature of the OH-group; (a) OH-group on a MgF2 surface, (b) interaction of an OH-group with undercoordinated magnesium, (c) branched Mg–OH group and (d) hydrogen bonding between the H of the OH-group and a F atom.
MgF -40 2
70 60
MgF2 -71
2
H-BEA
50 40 30 20 10 0
0,7
1,0
1,1
1,2
2,5
L acid sites Fig. 4. The variation of TOF (h−1 ) as a function of the total density of acid sites (A) and of the selectivity to DAG + TAG as a function of the density of the Lewis acid sites (B) (acetic acid/glycerol molar ratio: 3/1; 100 ◦ C, 22 h).
conditions the hydrolysis was favoured by an increased content in water and become a competitive process. The selective solvolysis of metal alkoxide by aqueous HF results preferably in M–F bond formation and only in a minor extent in the M–OH bond formation. However, due to the competitive process the increase of the water content by decreasing the concentration of aqueous HF from 87 to 40 wt% leads to an increase of the density of the hydroxyl groups mainly on the surface as it was detected by 19 F MAS NMR, TEM and XPS [19]. The bulk composition is almost unaffected by the modification of the concentration of aqueous HF. These results suggested a catalyst model consisting of shell-like nanoparticles of MgF2−x (OH)x with inner cores of pure MgF2 . Previous FTIR studies confirmed hydroxyl groups are Brønsted acidic in nature [19–21]. The unexpected Brønsted acid character of the Mg–OH group in these magnesium fluoride surfaces is a result of the combination of different factors as shown in Scheme 2. Based on this state of the art we checked MgF2−x (OH)x catalysts with x < 0.1. As mentioned above, Lewis acidity seems to have a dominant influence to the catalytic behaviour in this reaction. This acidity is generated by the presence of the five- and fourfold coordinated Mg2+ sites on the surface [19]. Thus medium-strength Lewis acid sites (e.g., uncompleted coordinated magnesium sites) and weak Brønsted acid sites (M–OH groups, Scheme 2) coexist in these catalysts. Melero et al. [14] showed a direct correlation between the selectivity to DAG and TAG and the strength of the acid sites of the sulfonic acid functionalized mesostructured materials: the strongest Brønsted acid sites caused the highest selectivity for DAG and TAG. Oppositely, in the case of our partly hydroxylated fluorides the experimental results show that the acid sites determining the formation of the desired reaction product are Lewis sites. We assign this different behaviour to the high difference between the
Brønsted acidity strength of the two classes of catalysts. Nevertheless, in both cases the acid strength is important for the reaction selectivity to DAG and TAG. Moreover, for the blank reaction and the reaction carried out in the presence of Beta zeolite, we can assume that the traditional Fischer esterification mechanism is operating the process (Scheme 3), for hydroxylated magnesium fluorides catalysts the experimental results indicate the existence of a dominant “alcohol effect” (i.e., the yields in the esterification reaction increases as the steric demands of the alcohol decreases) that is in accordance with the reports of Guimarães and co-workers [24]. The increased selectivitites to DAG and TAG with the loading of Lewis sites may be explained if we take into consideration a double role of these sites: firstly as active catalytic sites involved in the formation of the reactive electrophyle intermediate, and secondly as dehydrating sites coordinating the water molecules formed during the reaction (Scheme 4). In this way the equilibrium of the esterification could be faster shifted toward the reaction products with a higher probability of DAG and TAG formation. It is however less probable the so named “two-sites” insertion mechanism [24] in which Mg–OH or Mg–O(H)–Mg species should be able to interact with the glycerol molecules forming an Mg–OR or Mg–O(R)–Mg (where R = CH2 CH(OH)-CH2 OH) intermediate and water molecules. The insertion of HO–C(O)–CH3 over the newly formed Mg–OR group with the formation of the desired ester (mono-, di- or tri-ester) would occur in the next step. In accordance with Guimarães and co-workers [24] this mechanism requires the reaction of glycerol once via the R–OH linkage and then via its RO–H bond. If one assumes that the catalytic site consists of dinuclear Mg–O(R)–Mg bridges the coordination of the next RO–H molecule to one of the metallic sites should lead to a facile elimination of the water molecule, forming on one side the metal glycoholate (Mg–OR) and, on the other, a coordinated Mg–OH2 pair, which should be easily broken down under the reaction conditions reforming the Mg–O(R)–Mg bridge. Nevertheless, it is difficult to accept this “two-sites” insertion mechanism taking into account the breakage-reforming Mg–O– network linkages. Moreover, the intermediate Mg–O(R)–Mg species should generate ethers through a sequence of glycerol insertion–water desorption from the Mg–O(R)–Mg catalytic site, and these compounds were not detected in the reaction product mixtures. The most influential parameters on this equilibrium esterification process are, irrespective of the nature of the solid acid catalyst, the acetic acid to glycerol molar ratio and the reaction tempera-
S.B. Troncea et al. / Applied Catalysis B: Environmental 107 (2011) 260–267
H+
O H3C
O
+
H
H3C
O
H
H
O +
H
O
H3C
O O
H+ O H 3C
H
O
H3C
O
O
O
OH
H 3C
O
H
O
H
O
O
H
H OH
H
+
H3C
OH
+
H3C O
O O
H
O
- H+ OH OH
H
+
H 3C
H
H
O
OH
H
+
H
H
+
OH
H
O
OH
H
O
265
OH
OH OH
OH
Scheme 3. The Fischer esterification mechanism of glycerol with acetic acid to MAG.
ture. Theoretically the increase of both the acetic acid to glycerol molar ratio and the reaction temperature should lead an increased conversion of glycerol and selectivity to TAG (the highest substituted glycerol acetylated derivative). However, data reported by Melero et al. [14] showed that besides a low energy efficiency of the reaction as a result of increasing the reaction temperature a possible superheating may also result with a detrimental effect on the esterification reaction rate, explainable in terms of thermodynamics due to the exothermicity of esterification reactions and its effect on the equilibrium. On the other hand, an increase of the molar acetic acid to glycerol ratio will result in a decrease of the reaction atom economy. Therefore, such an optimisation strategy came in contradiction with two important principles of green chemistry: atom economy and energy efficiency [25].
To avoid these effects and to reach both an optimal activity and a high selectivity to DAG and TAG we took into consideration the use of so-called non-classical energy forms: microwaves and ultrasound irradiation. The results obtained using these energy forms are presented in Table 2. A comparison with the catalytic results obtained by using the classical thermal activation is also presented in the same Table 2. It is well known that the traditional heat transfer techniques (e.g., convection) are much slower and create gradients within the reaction medium. The hot surface of the reaction vessel may often also result in a localized overheating, thus leading to the decomposition of the products, substrate and reagents when heated for prolonged periods of time (this problem provokes high amounts of by-products). Such negative effects may be overcome by using
Scheme 4. The exemplification of the acetylation mechanism in which a Brønsted/Lewis pair of the magnesium fluoride catalyst is involved.
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Table 2 Catalytic results obtained in the esterification of glycerol by activation of reactants with no-classical energy forms versus thermal activation. Energy type Thermal Microwaves Ultrasound
Reaction conditionsa ◦
MgF2 -87, 3/1 molar ratio, 100 C, 22 h Blank reaction, 3/1 molar ratio, 100 ◦ C, 4 h MgF2 -87, 3/1 molar ratio, 100 ◦ C, 4 h MgF2 -87, 3/1 molar ratio, 80 ◦ C, 30 min MgF2 -87, 3/1 molar ratio, r.t., 30 min
C (%)
TOF (h−1 )
SDAG + TAG (%)
94.2 40.6 94.4 63.8 92.2
50.5 – 381 1493 2169
94.3 7.5 72.8 78.6 88.8
a The same molar acetic acid/glycerol ratio involves different amounts of reactants as a function of the experimental procedure. Therefore, for thermal activation were used 1.8 g of glycerol/3.6 g acetic acid (for 50 mg of catalyst), microwaves: 7.4 g of glycerol/14.4 g acetic acid (for 150 mg catalyst) and for ultrasounds were used 1.8 g of glycerol/3.6 g acetic acid (for 50 mg of catalyst)
microwaves energy sources where there is not a direct contact between the energy source and the reaction mixture. Microwave radiation passes through the walls of the vessel heatting the contents directly by transforming electromagnetic radiation into heat. Briefly, there are two main mechanisms by which materials dissipate microwave energy, namely dipole rotation and ionic conduction both of them affecting the reaction rate [26]. Many times, however, the rate enhancements may also result from superheating, non-uniform heating, differential heating and/or transport process, rather than “specific microwave effects” [25]. This is the case for many heterogeneous reaction systems. Unfortunately, in these conditions it is quite difficult to distinguish the different mechanisms of energy dissipation. Usually, the heating rate is important with reactions that proceed to more than one reaction product through separate or consecutive reaction pathways - especially if the reaction that proceeds first (i.e., that with lower activation energy) is the undesired one [27]. In such circumstances, rapid microwave heating is advantageous, as Stuerga et al. [28] demonstrated for the sulfonation of naphthalene in order to obtain 1- and 2-naphthalenesulfonic acids. The esterification of glycerol takes also place through consecutive reaction steps from MAG to DAG and TAG, respectively. As Table 2 shows, under microwave conditions a rate enhancement was obtained and the same level of glycerol conversion was obtained in only 4 h (Table 2, entry 3) comparing with 22 h – the reaction time needed when the thermal activation was used (Table 2, entry 1). The rate enhancement is also rationalized by a TOF increase from 50.5 h−1 to 381 h−1 . However, the consecutive acylation to MAG to DAG and TAG was slower showing again a strong influence of the reaction conditions on the selectivity. Using ultrasound irradiation led to much better results (Table 2, entries 4 and 5). Most physical and chemical effects of ultrasound arise from cavitations without an alteration of the rotational or vibrational states of molecules. It is only the cavitational collapse that releases sufficient kinetic energy to drive the chemical transformations. For decades, the accelerating effect of ultrasonic irradiation has been a useful reactivity paradigm. The analysis of numerous experiments reveals that ultrasounds have no effect on the chemical pathway and often reaction rates are comparable to those of non-irradiated processes [25]. In this case the enhanced yields and reaction rates of the reaction can be due to the mechanical effects of shock waves. The chemical effects of ultrasounds can occur if an elemental process is a sonication-sensitive step, or when the high-energy species released after cavitational collapse does participate as reaction intermediates. Sonochemistry in heterogeneous systems is the result of a combination of chemical and mechanical effects of cavitations, and it is very difficult to ascribe sonochemistry to any single global origin, other than the overriding source of activity, namely, cavitation [5]. Additionally, cavitational collapse at or near of the solid catalyst surface may produce enough energy to cause its frag-
mentation - thereby increasing surface area for the reaction and providing more or/and novel active sites exposed to the reactants. One of the most striking features of sonochemistry is that there is often an optimum value for the reaction temperature. In contrast to classical chemistry it is often not necessary to go to higher temperatures in order to accelerate the chemical process. Such an effect was also observed in this esterification reaction, where at room temperature (Table 2, entry 5) both the reaction rate and the selectivity in the main product were enhanced in comparison with the values obtained at 80 ◦ C (Table 2, entry 4). Whichever the mechanism, the real benefit of using ultrasound lies in its unique reactivity enhancement: the reaction rate increased 43 times when compared to thermal activation and around 6 times when compared to microwaves. Even most important the selectivity to DAG and TAG was after 30 min at almost the same level as those obtaining by thermal heating at 100 ◦ C for 22 h. 4. Conclusions Partly hydroxylated nanoscopic inorganic fluorides have demonstrated a good catalytic behaviour in the acylation of glycerol with acetic acid to yield acylated DAG and TAG compounds, with interesting properties as bioadditives for petrol fuels. Their activity and selectivity have been shown to be influenced by different catalytic properties such as the density of the acid sites at the catalytic surface and the nature of the acid sites (Lewis or/and Brønsted). Very important is also the mesoporosity that brings an advantage of inorganic fluorides over H-BEA zeolite by elimination of the diffusion limitations. To obtain optimal glycerol conversions and selectivity to the desired compounds in a short time it is advantageous to use non-conventional activation methods of the reactant molecules, as microwave or ultrasound irradiation. In this way the reaction time necessary to reach the same level of the conversion (>90%) can be shorted from 22 h to 4 h or 30 min only. Therefore, under microwave conditions, the same high level of conversion can be reached after 4 h, at 100 ◦ C with a molar stoichiometric acetic acid/glycerol ratio of 3/1. Moreover, high conversion degrees (>90%) as well as high selectivity toward di- and triacetylglycerol of over 85%, can be achieved in only 30 min with a molar stoichiometric acetic acid/glycerol ratio of 3/1 already at room temperature under ultrasound irradiation. Similar results are reported in the literature using catalytic materials with much higher acid strength (e.g., SAC-13, sulfonic acid modified mesostructured materials) and under thermal activation [14]. Once again, it seems that the medium-to-strong acidity of partly hydroxylated inorganic fluorides is balanced by the nanoscopic dimension of these materials making them, as we already demonstrated in previously catalytic studies [17,18], more active or, at least, as active as other solids with a very strong acidity. To settle the potential large scale of applicability of these catalytic materials, the use of crude glycerol from FAME production can be used as raw materials.
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Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Hydroxylated magnesium fluorides as environmentally friendly catalysts for glycerol acetylation Simona B. Troncea a , Stefan Wuttke b , Erhard Kemnitz b , Simona M. Coman a , Vasile I. Parvulescu a,∗ a b
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta, 4-12, Bucharest 030016, Romania Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 11 July 2011 Accepted 19 July 2011 Available online 26 July 2011 Keywords: Glycerol Bioadditive Hydroxylated fluorides Microwave Ultrasound
a b s t r a c t The increase of biodiesel production during the next years will result in an overproduction, and therefore, an accumulation of glycerol on the market. The use of glycerol-based additives to improve the properties of biodiesel or gasoline is one of the possibilities currently being explored to utilize this renewable feedstock. In this context, partly hydroxylated magnesium fluorides have demonstrated catalytic activity in the esterification of glycerol with acetic acid to yield diacetylglycerine (DAG) and triacetylglycerine (TAG). The catalytic activity depends on the density of acid sites at the external catalytic surface while the selectivity to different acylated products is influenced by the nature of the acid sites (Lewis and/or Brønsted). Using non-conventional activation methods of the reactant molecules, as microwave or ultrasound irradiation, optimal glycerol conversions (>90%) and selectivities to the desired compounds (over 85%) were obtained. The activities and selectivities achieved using partly hydroxylated fluorides as catalysts are comparable or even superior to those displayed by conventional oxide based acid catalysts under thermal activation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the last years biodiesel became one of the most important alternatives to the petroleum-based diesel fuel, partly due to some important advantages such as high biodegradability, non-toxicity, production from renewable energy sources and less harmful to the environment [1]. Since the main by-product (up to approximately 10 wt% of the total product) of the industrial biodiesel production (FAME production) is glycerol, it is predicted that the increase of the biodiesel production in the future will result in its accumulation creating a glut in the market [2]. This situation has prompted a search for new glycerol uses. Among different alternatives, the use of glycerol-based additives (e.g., di- and triacyl glycerin, obtained through the esterification of glycerol with acetic acid), which would improve the properties of biodiesel (e.g., cold and viscosity properties) or gasoline (e.g., antiknocking properties) is being explored [3]. Up to date several synthetic routes for obtaining organic esters were developed. Among these, the direct esterification of alcohols with carboxylic acids, in the presence of liquid acid catalysts (e.g., sulfuric acid, hydrochloric acid, and para-toluenesulfonic acid), is
∗ Corresponding author. Tel.: +40214100241; fax: +40214100241.. E-mail addresses: [email protected] (E. Kemnitz), [email protected] (S.M. Coman), [email protected] (V.I. Parvulescu). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.021
the most common method [4]. However, the use of conventional homogeneous acid catalysts must be limited in the future to comply with the increasingly strict environmental regulations. Efforts to green chemical reactions focus predominantly on greening the three main components of the reaction system: solvent, reagent/catalyst, and energy input, respectively. Thus, processes are recommended, (i) using benign solvents or solvent-free methodologies, (ii) using alternate, more efficient and less toxic reagents/catalysts as well as reusable catalysts, and (iii) using costeffective, eco-friendly alternative processes. The use of so-called non-classical energy forms (e.g., microwaves, ultrasound irradiation) was reported to lead to an optimization of the reaction time and product yield, thereby also reducing the by-products [5]. In a typical case of esterification, the literature reports a high number of contributions replacing the conventional homogeneous acid catalysts by more benign heterogeneous acid catalysts like ion-exchange organic resins [6], zeolites [6–9] and silica-supported heteropoly acids [6,10–12]. However, many of these catalysts show some limitations such as low thermal stability (resins), mass transfer resistance (zeolites), or loss of active acid sites due to high solubility in a polar medium (heteropoly acids). Moreover, the presence of water can seriously affect the catalytic behaviour of some of these solid acid catalysts because they are highly hygroscopic in nature. To overcome most of these limitations superacidic silica-embedded Nafion (SAC-13) has been suggested as an alternative [13]. There are also studies on sulfonic acid functionalized
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mesoporous silica as SBA-15 [14] but the expensive templating copolymer in the SBA-15 preparation and complex acid functionalized process limits the industrial application. The main objective of this study was to reach a maximum glycerol conversion with maximum selectivities to the most valuable acetylated products, i.e., di- and tri-acetylglycerol. Theoretic studies reported by Melero et al. [14] indicated that high reaction temperatures and acetic acid/glycerol molar ratios favor the evolution of the final product distribution in equilibrium to the highly substituted glycerol acetylated derivatives. However, a very high acetic acid/glycerol ratio is not convenient. In order to achieve this goal the methods being used are not always in concordance with the green chemistry principles. It was thus suggested a two-step method in which the esterification of glycerol with acetic acid is followed by the acetylation with acid anhydride carried out over resins and zeolites [7,9,15]. However, such a procedure has several drawbacks like (i) the use of a relatively large amounts of acetic acid co-reactant may generate high amounts of wastes, (ii) the use of acid anhydride drastically decreases the atom economy of the process. Moreover, the price of acetic anhydride is higher (ca. four times) than that of acetic acid. But the reaction can be carried out with a small acetic acid/glycerol ratio and at lower temperatures by using an appropriate heterogeneous catalyst and activation conditions [16]. Recently developed nanoscopic partly hydroxylated magnesium fluorides (MgF2−x (OH)x with x < 0.1) with both Lewis/Brønsted acid centres have been proved to posses unexpected catalytic properties in several syntheses, which are of great interest in the fine chemistry area (e.g., vitamin E, vitamins K) [17,18]. The catalytic properties are related to their particular structural and chemical features: (i) high surface area with pore diameters in the range of mesopores; (ii) very low solubility in strong polar solvents; (iii) hydrolysis resistance; (iv) mediumstrength Lewis and Brønsted acid sites; (v) the possibility of easily tuning the surface acidity; (vi) nanoscopic particle dimension [19,20]. Taking into account these structural and chemical features with the aim to extend the applicability of this new class of catalytic materials, we applied them in the synthesis of glycerine acetates as valuable transportation fuel additives through the catalytic esterification of glycerol with acetic acid. Another aim was to further optimise the reaction conditions by using non-conventional methods to activate reactant molecules such as microwaves or ultrasound irradiation.
2. Experimental 2.1. Catalyst preparation The catalysts were synthesized as reported elsewhere [19], from metallic Mg using the fluorolytic sol–gel method, as follows: metallic Mg (Aldrich, 99.98% powder) (1.56 g, 64 mmol) was dissolved in dry methanol (50 mL) at room temperature overnight. After heating under reflux conditions for 3 h, a stoichiometric amount of HF (130 mmol) dissolved in different amounts of water (HF solution concentrations: 40, 57, 71, and 87 wt% HF) was added to the formed Mg(OCH3 )2 solution. The mixtures reacted to form highly viscous transparent gels. After ageing for 12 h, each gel was dried under vacuum at room temperature. The solid product thus obtained was then further dried under vacuum at 70 ◦ C for 5 h. The prepared catalysts are referred to hereafter as MgF2 -40, MgF2 -57, MgF2 -71 and MgF2 -87, indicating the different concentrations of HF solutions used. For comparison, commercial H-Beta zeolite (PQ-Valfor Company) with Si/Al of 10.8 (PQ25) was used as reference.
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2.2. Catalyst characterization XRD, MAS-NMR, TEM, thermal analysis, XPS, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials [19,21]. The N2 /Ar adsorption–desorption isotherms, FT-IR photoacoustic pyridine adsorption spectra, and NH3 -TPD have been carried out to investigate the surface properties. The porosity characteristics of the samples were determined by acquiring adsorption–desorption isotherms of N2 at 77 K and of Ar at 87 K on a Micromeritics ASAP 2020 instrument based on the BJH-model. Samples were first degassed in vacuum at 70 ◦ C. Surface areas were calculated according to the BET method. FT-IR photoacoustic pyridine adsorption spectroscopy was carried out according to the following procedure: the sample (75 mg) was pre-heated at 150 ◦ C under N2 flow (35 ml min−1 ) for 15 min, then 60 ml pyridine was injected into the sample tube. The sample was flushed with nitrogen for another 15 min to remove physisorbed pyridine. Spectra of the sample were recorded at room temperature using a MTEC cell and FT-IR system 2000 (Perkin-Elmer). Spectra of the samples without pyridine adsorption were also measured as the background. Temperatureprogrammed desorption of ammonia (NH3 -TPD) was employed to determine the number of acid sites and their distribution based on their relative strength. The sample (about 0.2 g) was first heated under nitrogen up to 300 ◦ C, then at 120 ◦ C exposed to NH3 . After flushing the excess NH3 at 120 ◦ C with N2 for 1 h and cooling to 80 ◦ C the TPD program was started (10◦ /min up to 500 ◦ C, keeping for 30 min). Desorbed NH3 was monitored continuously via IR spectroscopy (FT-IR System 2000, Perkin-Elmer). 2.3. Catalytic tests The catalytic samples were tested using three different activation procedures: thermal (A), microwaves (B) and ultrasound (C) irradiation. (A) In a typical procedure, to 1.8 g of glycerol was added to 3.6 g of acetic acid (corresponding to 3/1 acetic acid/glycerol molar ratio) in free solvent conditions, in a 20 ml glass vial equipped with a magnetic stirrer. To this mixture 50 mg of catalyst were added. After that, the vial was closed, immersed in an oil bath with a temperature of 80–100 ◦ C, and the charged mixture was stirred (1.250 rpm) for 18–22 h. (B) Microwave assisted reactions were carried out with a Milestone Start S system operating at 600 W. The stirring rate inside the reactor was 350 rpm. Typical experiments were carried out using 7.4 g of glycerol and 14.4 g acetic acid (corresponding to 3/1 acetic acid/glycerol molar ratio). To this system were added 150 mg of catalyst and the slurry was maintained at 100 ◦ C, under stirring, between 30 min and 4 h. (C) The ultrasound experiments were carried out with an Elma Transonic 460/H bath working at a frequency of 35 kHz. Typical experiments were carried out using 1.8 g of glycerol and 3.6 g acetic acid (corresponding to a 3/1 acetic acid/glycerol molar ratio). To this system 50 mg of catalyst were added and the slurry was maintained at 24–80 ◦ C for 30 min. The temperature inside the vessel was controlled using either a thermocouple (thermal and ultrasound) or a fibber optic (microwave). 2.4. Product analysis The resulted reaction mixture was cooled, the bulk of the catalyst was removed by filtration and the liquid phase was evaporated under vacuum at 80 ◦ C. The concentrated product was sylilated according to following conditions: at 50 mg of reaction product 1 ml of chloroform and 1 ml of sylilation agent (bis-trimethyl-syliltrifluoroacetamide) was added. The mixture was maintained at 60–70 ◦ C, for 30 min, under stirring. The resulted reaction mixture
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100 90 80 70 60
%
50 40 30 20 10
C (%) S MAG (%)
0 MgF2-87
S DAG (%) MgF2-71
MgF2-57
MgF2-40
Catalyst
S TAG (%) H-BEA
blank
Fig. 1. The catalytic results obtained in the thermal esterification of glycerol, as a function of the catalyst nature (acetic acid/glycerol molar ratio: 3/1; 100 ◦ C, 22 h).
was cooled at room temperature and the solvent was removed under vacuum, at 35 ◦ C. The samples were then diluted with 1 ml of n-hexane and analyzed by GC-FID chromatography (GCShimatzu aparatus in following conditions: column: CP-SIL 8 CB, 30 m, ID = 0.25 mm). The identification of the products was made using a GC-MS Carlo Erba Instruments QMD 1000 equipped with a Factor Four VF-5HT column with the following characteristics: 0.32 mm × 0.1 m × 15 m working with a temperature program at a pressure of 0.38 Torr with He as the carrier gas. 3. Results and discussion Scheme 1 presents the esterification of glycerol with acetic acid, with MAG – monoacetylglycerol, DAG – diacetylglycerol and TAG – triacetylglycerol. It must be noted that MAG and DAG can include several isomers depending on the acetylation position within the glycerol molecule. Among the obtained products, DAG and TAG are the most interesting products from a fuel-production point of view; they can be formulated with petroleum-derived fuels to improve either cold and viscosity properties (biodiesel) or antiknocking properties (gasoline). The product MAG is considered as an undesired product (by-product) due to its relatively high solubility in water. The initial screening of the nanoscopic partly hydroxylated fluorides in the esterification of glycerol with acetic acid using a conventional thermal energy source showed that after 22 h, at 100 ◦ C, the conversion of glycerol was still below 100% (Fig. 1). In order to evaluate the progress of the reaction in the absence of the solid catalyst, a blank run was also carried out. As Fig. 1 shows, the esterification reaction occurs even in the absence of a catalyst – although the glycerol conversion remains much lower comparing with the cases when fluorides have been used. In the absence of the catalyst, the esterification led only to MAG as reaction product. Therefore, the main function of the catalysts in this reaction should be to direct the selectivity to the desired products (DAG and TAG). Comparing to H-BEA zeolite the fluoride-based catalysts exhibit a different behaviour. However, this cannot be related to the acidity (Table 1) or to the acidic strength (Fig. 2) of the two systems. The investigated H-BEA zeolite has a density of acid sites of 0.18 mmol/g that is close to the acidity of some hydroxylated fluorides (i.e., MgF2 -40 – 0.17 mmol/g and MgF2 -71 – 0.16 mmol/g). Nevertheless, differently to hydroxylated fluorides, H-BEA is characterized by a stronger acidity as TPD measurements have shown (Table 1
Fig. 2. NH3 -TPD profiles of the investigated catalysts: (a) MgF2 -40; (b) MgF2 -57; (c) MgF2 -71; (d) MgF2 -87; (e) H-BEA.
and Fig. 2) and different Lewis and Brønsted acid site population as evidenced from Py-IR spectra (Table 1 and Fig. 3). Taking into account these differences and the literature reports the selectivity to DAG + TAG was expected being in favor of H-BEA (e.g., the stronger Brønsted acid sites the higher the selectivity to DAG + TAG [14]). With this respect, Fig. 1 shows that the use of MgF2 -87 (that exhibits the highest Lewis/Brønsted acid ratio, i.e. the lowest amount of Brønsted sites) led to the highest selectivity to DAG and TAG products, due to the consecutive acylation of MAG. With the same sample the conversion of glycerol reached 94.2%. A high conversion (83.2%) was also obtained in the presence of H-BEA zeolite but the selectivity to DAG and TAG products was only 52.5%, and a high amount of MAG still remained in the reaction mixture. This result is in agreement with those of Silva et al. [7] who reported
Fig. 3. The FTIR spectra of pyridine absorbed on hydroxylated fluorides.
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OH
OCOCH3 +
OH
OCOCH3
OCOCH3
OCOCH3 +
OH
CH3COOH
OCOCH3 OH
OCOCH3
OH
DAG
+
OH
- H2O
MAG
OH
OH OH
263
glycerol OCOCH3 TAG OCOCH3 OCOCH3 Scheme 1. The esterification of glycerol with acetic acid.
that Beta zeolite displays a low selectivity to TAG when acetic acid is used as acylating agent. These authors explained this result in terms of the stability of the acylium ion intermediate. In order to distinguish the catalytic activity of different acid sites, a specific rate per acid site (TOF (h−1 )) has also been calculated. The results are presented in Fig. 4. A comparison between the results presented in Figs. 1 and 4 show that the highest intrinsic activity was displayed by the MgF2 -71 sample. The differences between the activities expressed as the conversion of glycerol and TOF are due to the different density of the acid sites on the catalyst surface. The correlation of TOF (h−1 ) with the total Lewis and Brønsted acid sites density (determined from NH3 -TPD measurements [21]) led to the following order: MgF2 -71 > MgF2 -40 > H-BEA > MgF2 57 > MgF2 -87 (Fig. 2A). This order led to an unexpected conclusion: the higher acid sites density corresponds to the lower TOF. Such behaviour may be explained only if we take into consideration limited accessibility of the acid surface sites as result of a physical blocking of a part of the active sites by the glycerol molecules strongly chemisorbed onto the surface. As a consequence only a relatively small fraction of active sites is effectively involved in the esterification reaction. On the other hand, the reaction data show that the nature and the ratio of the acid sites (Lewis and/or Brønsted) are responsible for the product distribution. Thus, the selectivity to DAG + TAG decreased in the order: MgF2 -87 (94.3%) > MgF2 71 (88.3%) > MgF2 -57 (84.1%) > MgF2 -40 (71.6%) > H-BEA (52.5%) (Fig. 4B). This decrease indicates indeed a direct correlation of the Lewis/Brønsted ratio with the product distribution: a higher
amount of Brønsted acid sites (e.g., MgF2 -40 sample) led to a higher amount of MAG and consequently to small amounts of the desired DAG and TAG products. This behaviour also explains the total selectivity to MAG in the absence of the catalyst (the blank run). In addition, the presence of the acetic acid (acetic acid/glycerol molar ratio = 3/1) introduces a significant source of potentially soft catalytic protons to the reaction mixture (Fig. 1). Taking into account the catalytic behaviour and the large differences between the textural properties of the two catalytic materials (Beta zeolite versus magnesium fluorides) we assume the difference in selectivity can be assigned to mass transfer limitations. It is however evident that DAG and TAG are space demanding and their formation and diffusion inside the zeolite pores would be difficult, thus explaining the poor selectivity to these products. Indeed, the calculation of the Thiele modulus modified by Weisz [23] indicated that the diffusion resistance does not play a role in the case of hydroxylated fluorides (ϕ < 1, i.e. 0.08) while in the case of beta zeolite the diffusion inside the zeolite pores would be difficult (ϕ > 1, i.e. 1.2). These results led to the conclusion that to achieve the optimum catalytic performances in the esterification of glycerol the surface of hydroxylated magnesium fluoride should exhibit a low density of acid sites on the surface and these acid sites should be preponderantly Lewis in nature (i.e. samples with a high Lewis/Brønsted ratio). These properties can be generated during the combined fluorolytic/hydrolytic sol–gel process when the ratio of M(OR)x to HF was kept constant while the water content varied. Under these
Table 1 Textural and acid characteristics of the prepared MgF2 catalysts and commercial H-BEA zeolite. Crt. Nr.
Catalyst
BET surface areaa (m2 /g)
Pore volumea (cm3 /g)
Pore sizea (nm)
Density of acidic sitesb (mmol/g)
B acid sitesc
L acid sitesc
1 2 3 4 5
MgF2 -40 MgF2 -57 MgF2 -71 MgF2 -87 H-BEAd
180 233 264 424 465
0.12 0.10 0.16 0.25 0.19
2.6 2.3 2.2 2.2 <1.0
0.17 0.26 0.16 0.33 0.18
2.7 1.5 1.2 0.4 0.9
1.0 1.1 1.2 2.5 0.7
a b c d
Textural properties measured with N2 at 77 K. Calculated from TPD measurements. Calculated by the ratio of the integrated area of IR adsorption peak to the sample weight. See ref [22].
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A 100 90
MgF2 -71 MgF2 -40
80
H-BEA
TOF (h-1)
70 60
MgF2 -57
50
MgF2-87
40 30 20 10 0
0,16
0,17
0,18
0,26
0,33
Density of acid sites (TPD), mmol/g
B 100 90
MgF -57
DAG + TAG (%)
80
MgF2 -87
Scheme 2. Graphical illustration of possible topological situations on MgF2−x (OH)x surface that could explain the Brønsted acidic nature of the OH-group; (a) OH-group on a MgF2 surface, (b) interaction of an OH-group with undercoordinated magnesium, (c) branched Mg–OH group and (d) hydrogen bonding between the H of the OH-group and a F atom.
MgF -40 2
70 60
MgF2 -71
2
H-BEA
50 40 30 20 10 0
0,7
1,0
1,1
1,2
2,5
L acid sites Fig. 4. The variation of TOF (h−1 ) as a function of the total density of acid sites (A) and of the selectivity to DAG + TAG as a function of the density of the Lewis acid sites (B) (acetic acid/glycerol molar ratio: 3/1; 100 ◦ C, 22 h).
conditions the hydrolysis was favoured by an increased content in water and become a competitive process. The selective solvolysis of metal alkoxide by aqueous HF results preferably in M–F bond formation and only in a minor extent in the M–OH bond formation. However, due to the competitive process the increase of the water content by decreasing the concentration of aqueous HF from 87 to 40 wt% leads to an increase of the density of the hydroxyl groups mainly on the surface as it was detected by 19 F MAS NMR, TEM and XPS [19]. The bulk composition is almost unaffected by the modification of the concentration of aqueous HF. These results suggested a catalyst model consisting of shell-like nanoparticles of MgF2−x (OH)x with inner cores of pure MgF2 . Previous FTIR studies confirmed hydroxyl groups are Brønsted acidic in nature [19–21]. The unexpected Brønsted acid character of the Mg–OH group in these magnesium fluoride surfaces is a result of the combination of different factors as shown in Scheme 2. Based on this state of the art we checked MgF2−x (OH)x catalysts with x < 0.1. As mentioned above, Lewis acidity seems to have a dominant influence to the catalytic behaviour in this reaction. This acidity is generated by the presence of the five- and fourfold coordinated Mg2+ sites on the surface [19]. Thus medium-strength Lewis acid sites (e.g., uncompleted coordinated magnesium sites) and weak Brønsted acid sites (M–OH groups, Scheme 2) coexist in these catalysts. Melero et al. [14] showed a direct correlation between the selectivity to DAG and TAG and the strength of the acid sites of the sulfonic acid functionalized mesostructured materials: the strongest Brønsted acid sites caused the highest selectivity for DAG and TAG. Oppositely, in the case of our partly hydroxylated fluorides the experimental results show that the acid sites determining the formation of the desired reaction product are Lewis sites. We assign this different behaviour to the high difference between the
Brønsted acidity strength of the two classes of catalysts. Nevertheless, in both cases the acid strength is important for the reaction selectivity to DAG and TAG. Moreover, for the blank reaction and the reaction carried out in the presence of Beta zeolite, we can assume that the traditional Fischer esterification mechanism is operating the process (Scheme 3), for hydroxylated magnesium fluorides catalysts the experimental results indicate the existence of a dominant “alcohol effect” (i.e., the yields in the esterification reaction increases as the steric demands of the alcohol decreases) that is in accordance with the reports of Guimarães and co-workers [24]. The increased selectivitites to DAG and TAG with the loading of Lewis sites may be explained if we take into consideration a double role of these sites: firstly as active catalytic sites involved in the formation of the reactive electrophyle intermediate, and secondly as dehydrating sites coordinating the water molecules formed during the reaction (Scheme 4). In this way the equilibrium of the esterification could be faster shifted toward the reaction products with a higher probability of DAG and TAG formation. It is however less probable the so named “two-sites” insertion mechanism [24] in which Mg–OH or Mg–O(H)–Mg species should be able to interact with the glycerol molecules forming an Mg–OR or Mg–O(R)–Mg (where R = CH2 CH(OH)-CH2 OH) intermediate and water molecules. The insertion of HO–C(O)–CH3 over the newly formed Mg–OR group with the formation of the desired ester (mono-, di- or tri-ester) would occur in the next step. In accordance with Guimarães and co-workers [24] this mechanism requires the reaction of glycerol once via the R–OH linkage and then via its RO–H bond. If one assumes that the catalytic site consists of dinuclear Mg–O(R)–Mg bridges the coordination of the next RO–H molecule to one of the metallic sites should lead to a facile elimination of the water molecule, forming on one side the metal glycoholate (Mg–OR) and, on the other, a coordinated Mg–OH2 pair, which should be easily broken down under the reaction conditions reforming the Mg–O(R)–Mg bridge. Nevertheless, it is difficult to accept this “two-sites” insertion mechanism taking into account the breakage-reforming Mg–O– network linkages. Moreover, the intermediate Mg–O(R)–Mg species should generate ethers through a sequence of glycerol insertion–water desorption from the Mg–O(R)–Mg catalytic site, and these compounds were not detected in the reaction product mixtures. The most influential parameters on this equilibrium esterification process are, irrespective of the nature of the solid acid catalyst, the acetic acid to glycerol molar ratio and the reaction tempera-
S.B. Troncea et al. / Applied Catalysis B: Environmental 107 (2011) 260–267
H+
O H3C
O
+
H
H3C
O
H
H
O +
H
O
H3C
O O
H+ O H 3C
H
O
H3C
O
O
O
OH
H 3C
O
H
O
H
O
O
H
H OH
H
+
H3C
OH
+
H3C O
O O
H
O
- H+ OH OH
H
+
H 3C
H
H
O
OH
H
+
H
H
+
OH
H
O
OH
H
O
265
OH
OH OH
OH
Scheme 3. The Fischer esterification mechanism of glycerol with acetic acid to MAG.
ture. Theoretically the increase of both the acetic acid to glycerol molar ratio and the reaction temperature should lead an increased conversion of glycerol and selectivity to TAG (the highest substituted glycerol acetylated derivative). However, data reported by Melero et al. [14] showed that besides a low energy efficiency of the reaction as a result of increasing the reaction temperature a possible superheating may also result with a detrimental effect on the esterification reaction rate, explainable in terms of thermodynamics due to the exothermicity of esterification reactions and its effect on the equilibrium. On the other hand, an increase of the molar acetic acid to glycerol ratio will result in a decrease of the reaction atom economy. Therefore, such an optimisation strategy came in contradiction with two important principles of green chemistry: atom economy and energy efficiency [25].
To avoid these effects and to reach both an optimal activity and a high selectivity to DAG and TAG we took into consideration the use of so-called non-classical energy forms: microwaves and ultrasound irradiation. The results obtained using these energy forms are presented in Table 2. A comparison with the catalytic results obtained by using the classical thermal activation is also presented in the same Table 2. It is well known that the traditional heat transfer techniques (e.g., convection) are much slower and create gradients within the reaction medium. The hot surface of the reaction vessel may often also result in a localized overheating, thus leading to the decomposition of the products, substrate and reagents when heated for prolonged periods of time (this problem provokes high amounts of by-products). Such negative effects may be overcome by using
Scheme 4. The exemplification of the acetylation mechanism in which a Brønsted/Lewis pair of the magnesium fluoride catalyst is involved.
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Table 2 Catalytic results obtained in the esterification of glycerol by activation of reactants with no-classical energy forms versus thermal activation. Energy type Thermal Microwaves Ultrasound
Reaction conditionsa ◦
MgF2 -87, 3/1 molar ratio, 100 C, 22 h Blank reaction, 3/1 molar ratio, 100 ◦ C, 4 h MgF2 -87, 3/1 molar ratio, 100 ◦ C, 4 h MgF2 -87, 3/1 molar ratio, 80 ◦ C, 30 min MgF2 -87, 3/1 molar ratio, r.t., 30 min
C (%)
TOF (h−1 )
SDAG + TAG (%)
94.2 40.6 94.4 63.8 92.2
50.5 – 381 1493 2169
94.3 7.5 72.8 78.6 88.8
a The same molar acetic acid/glycerol ratio involves different amounts of reactants as a function of the experimental procedure. Therefore, for thermal activation were used 1.8 g of glycerol/3.6 g acetic acid (for 50 mg of catalyst), microwaves: 7.4 g of glycerol/14.4 g acetic acid (for 150 mg catalyst) and for ultrasounds were used 1.8 g of glycerol/3.6 g acetic acid (for 50 mg of catalyst)
microwaves energy sources where there is not a direct contact between the energy source and the reaction mixture. Microwave radiation passes through the walls of the vessel heatting the contents directly by transforming electromagnetic radiation into heat. Briefly, there are two main mechanisms by which materials dissipate microwave energy, namely dipole rotation and ionic conduction both of them affecting the reaction rate [26]. Many times, however, the rate enhancements may also result from superheating, non-uniform heating, differential heating and/or transport process, rather than “specific microwave effects” [25]. This is the case for many heterogeneous reaction systems. Unfortunately, in these conditions it is quite difficult to distinguish the different mechanisms of energy dissipation. Usually, the heating rate is important with reactions that proceed to more than one reaction product through separate or consecutive reaction pathways - especially if the reaction that proceeds first (i.e., that with lower activation energy) is the undesired one [27]. In such circumstances, rapid microwave heating is advantageous, as Stuerga et al. [28] demonstrated for the sulfonation of naphthalene in order to obtain 1- and 2-naphthalenesulfonic acids. The esterification of glycerol takes also place through consecutive reaction steps from MAG to DAG and TAG, respectively. As Table 2 shows, under microwave conditions a rate enhancement was obtained and the same level of glycerol conversion was obtained in only 4 h (Table 2, entry 3) comparing with 22 h – the reaction time needed when the thermal activation was used (Table 2, entry 1). The rate enhancement is also rationalized by a TOF increase from 50.5 h−1 to 381 h−1 . However, the consecutive acylation to MAG to DAG and TAG was slower showing again a strong influence of the reaction conditions on the selectivity. Using ultrasound irradiation led to much better results (Table 2, entries 4 and 5). Most physical and chemical effects of ultrasound arise from cavitations without an alteration of the rotational or vibrational states of molecules. It is only the cavitational collapse that releases sufficient kinetic energy to drive the chemical transformations. For decades, the accelerating effect of ultrasonic irradiation has been a useful reactivity paradigm. The analysis of numerous experiments reveals that ultrasounds have no effect on the chemical pathway and often reaction rates are comparable to those of non-irradiated processes [25]. In this case the enhanced yields and reaction rates of the reaction can be due to the mechanical effects of shock waves. The chemical effects of ultrasounds can occur if an elemental process is a sonication-sensitive step, or when the high-energy species released after cavitational collapse does participate as reaction intermediates. Sonochemistry in heterogeneous systems is the result of a combination of chemical and mechanical effects of cavitations, and it is very difficult to ascribe sonochemistry to any single global origin, other than the overriding source of activity, namely, cavitation [5]. Additionally, cavitational collapse at or near of the solid catalyst surface may produce enough energy to cause its frag-
mentation - thereby increasing surface area for the reaction and providing more or/and novel active sites exposed to the reactants. One of the most striking features of sonochemistry is that there is often an optimum value for the reaction temperature. In contrast to classical chemistry it is often not necessary to go to higher temperatures in order to accelerate the chemical process. Such an effect was also observed in this esterification reaction, where at room temperature (Table 2, entry 5) both the reaction rate and the selectivity in the main product were enhanced in comparison with the values obtained at 80 ◦ C (Table 2, entry 4). Whichever the mechanism, the real benefit of using ultrasound lies in its unique reactivity enhancement: the reaction rate increased 43 times when compared to thermal activation and around 6 times when compared to microwaves. Even most important the selectivity to DAG and TAG was after 30 min at almost the same level as those obtaining by thermal heating at 100 ◦ C for 22 h. 4. Conclusions Partly hydroxylated nanoscopic inorganic fluorides have demonstrated a good catalytic behaviour in the acylation of glycerol with acetic acid to yield acylated DAG and TAG compounds, with interesting properties as bioadditives for petrol fuels. Their activity and selectivity have been shown to be influenced by different catalytic properties such as the density of the acid sites at the catalytic surface and the nature of the acid sites (Lewis or/and Brønsted). Very important is also the mesoporosity that brings an advantage of inorganic fluorides over H-BEA zeolite by elimination of the diffusion limitations. To obtain optimal glycerol conversions and selectivity to the desired compounds in a short time it is advantageous to use non-conventional activation methods of the reactant molecules, as microwave or ultrasound irradiation. In this way the reaction time necessary to reach the same level of the conversion (>90%) can be shorted from 22 h to 4 h or 30 min only. Therefore, under microwave conditions, the same high level of conversion can be reached after 4 h, at 100 ◦ C with a molar stoichiometric acetic acid/glycerol ratio of 3/1. Moreover, high conversion degrees (>90%) as well as high selectivity toward di- and triacetylglycerol of over 85%, can be achieved in only 30 min with a molar stoichiometric acetic acid/glycerol ratio of 3/1 already at room temperature under ultrasound irradiation. Similar results are reported in the literature using catalytic materials with much higher acid strength (e.g., SAC-13, sulfonic acid modified mesostructured materials) and under thermal activation [14]. Once again, it seems that the medium-to-strong acidity of partly hydroxylated inorganic fluorides is balanced by the nanoscopic dimension of these materials making them, as we already demonstrated in previously catalytic studies [17,18], more active or, at least, as active as other solids with a very strong acidity. To settle the potential large scale of applicability of these catalytic materials, the use of crude glycerol from FAME production can be used as raw materials.
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