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Synthesis of hyacinth, vanilla, and blossom orange fragrances: the benefit of using zeolites and delaminated zeolites as catalysts
Applied Catalysis A: General 263 (2004) 155–161
Synthesis of hyacinth, vanilla, and blossom orange fragrances: the benefit of using zeolites and delaminated zeolites as catalysts Maria José Climent, Avelino Corma∗ , Alexandra Velty Instituto de Tecnologia Qu´ımica, UPV-CSIC, Universidad de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain Received in revised form 31 October 2003; accepted 3 December 2003
Abstract The synthesis of phenylacetaldehyde glycerol acetals, 2-benzyl-4-hydroxymethyl-1,3-dioxolane (1), 2-benzyl-5-hydroxy-1,3-dioxane (2), and vanillin propylene glycol acetal (2-(4-hydroxy-3-methoxyphenyl)-4-methyl-1,3-dioxolane) (3) which are flavoring compounds with hyacinth and vanilla scent fragrances, have been carried out successfully by acetalization of phenylacetaldehyde and vanillin with glycerol and propylene glycol, respectively, using toluene as solvent and zeolite catalysts whose adsorption properties have been optimized. However, in the case of a larger size acetal such as 2-acetonaphthone propylene glycol acetal (4) with blossom orange scent, geometrical constraints make the diffusion of reactants and products inside the micropores more difficult. In this case, a delaminated zeolite (ITQ-2) with very large and structured external surface is an active and selective catalyst. Furthermore, the delaminated zeolite allows the reaction to be carried out in a solvent-free system, opening the possibility for an environmentally friendly process for the synthesis of acetal fragrances. © 2004 Elsevier B.V. All rights reserved. Keywords: Hyacinth fragrance; Vanilla fragrance; Blossom orange fragrance; Zeolites; Delaminated zeolites; Acetals
1. Introduction The acetalization reaction is sometimes a necessary requirement to protect carbonyl groups when reacting multifunctional organic molecules [1]. For instance, the dioxolane group is generally stable to bases, Grignard reagents, hydrogenation reagents, metal hydrides, oxidants, bromination and esterification reagents. Besides the interest of acetals as protecting groups, many of them have found direct applications as fragrances, in cosmetics, food and beverage additives, pharmaceuticals, in detergents, and in lacquer industries [2]. It is known that the conversion of a carbonyl compound to an acetal, profoundly changes its vapor pressure, solubility, aroma characteristics, and generally attenuates or alters its flavor impact. For example, the propylene glycol acetal of vanillin is used to imitate vanilla flavors because it causes flavor attenuation [3]. However, the flavor of the original aldehyde may rapidly be regenerated upon hydration in a high moisture food or beverage [4].
∗
Corresponding author. Tel.: +34-96-3877800; fax: +34-96-3877809. E-mail address: [email protected] (A. Corma).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.007
The most general method for the synthesis of acetals is to react carbonyl compounds with an alcohol or an orthoester in the presence of acid catalysts. A number of acetalization procedures include the use of protic acids, Lewis acids (zinc chloride) [5], alumina [6], montmorillonite [7], zeolites [8,9], mesoporous aluminosilicates [10], and cation exchange resins [11]. Generally speaking, acetalization of aldehydes can be performed in the presence of weak acids, while ketones generally need stronger ones like sulfuric, hydrochloric, or p-toluenesulfonic acid (PTSA) and larger amounts of catalyst. However, many of the methods mentioned above present limitations derived from the use of expensive reagents, tedious work-up procedure, and necessity of neutralization (with the exception of solid catalysts) of the strongly acidic media, with the production of undesired wastes. In this sense, synthetic zeolites appear as promising catalysts with the obvious advantages over conventional Brönsted or Lewis acids [12], of easy separation from the reaction mixture, shape selectivity, reusability and, overall, the possibility to control adsorption, strength, and distribution of acid sites. The use of aromatic chemicals in perfumery has been growing since the chemical developments that allowed their synthesis and commercial production. Among them vanillin
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propylene glycol acetal [4] and phenylacetaldehyde glyceryl acetals [4,13] are flavoring materials with vanilla scent and hyacinth fragrance, respectively, that are prepared by reacting the corresponding aromatic aldehyde with propylene glycol and glycerol, respectively. The commercial process is catalyzed by strong acids such as PTSA, HCl, H3 PO4 , and divinylbenzene–styrene resin [11]. These fragrances are included in the list of the FEMA-GRAS, which offers naturally occurring or synthetically produced flavoring substances regulated by the Food and Drug Administration (FDA) [14]. Phenylacetaldehyde glyceryl acetal is used for food and may be added (at a level of 20 ppm) without hazard to public health [13]. We have shown [9] that zeolites can carry out selectively the synthesis of acetals and fructone fragrancy. Here, we have attempted to optimize the synthesis of phenylacetaldehyde glyceryl acetals (1 and 2), propylene glycol acetal of the vanillin (3), and acetonaphthone (4) using zeolites, delaminated zeolites, and mesoporous molecular sieves (MCM-41) as catalysts. It will be shown that controlling the acidity, accessibility, and hydrophobic–hydrophilic properties of the catalyst, it has been possible to design a successful catalyst that, when coupled with the adequate process, maximizes conversion and catalyst life, allowing the process to be carried out in absence of solvent.
2. Experimental 2.1. Catalysts USY samples were obtained from PQ Zeolites B.V., and they were NH4 + exchanged followed by calcination at 773 K during 3 h. Beta-1 zeolite was supplied by PQ corporation, in the acidic form. Zeolite beta samples (Beta-2, Beta-3, Beta-4, Beta-5, Beta-6) were synthesized in our laboratory by working in fluoride media [15]. The acid form of mordenite (MOR) was obtained by calcination of a Conteka sample, followed by an acid treatment to extract the extra-framework Al. The ZSM-5 sample was calcined at 823 K for 3 h to obtain the acid form. One sample of MCM-41 (pore diameter 3.5 nm and framework Si/Al = 50) was synthesized following the procedure given in Ref. [16], using hexadecyltrimethylammonium (Aldrich) cation as template, and pseudoboehmite (Capatal B, Vista) and Aerosil (Degussa) as the aluminum and silicon source, respectively. The sample was calcined in N2 for 4 h and in air for 6 h at 813 K. The ITQ-2 delaminated and the corresponding MCM-22 zeolites were synthesized following Ref. [17]. Acidity measurements were carried out by adsorption– desorption of pyridine by IR spectroscopy. The infrared spectra were recorded on a Nicolet 710 FTIR using selfsupported wafers of 10 mg cm−2 . The calcined samples were outgased overnight at 673 K and 10−3 Pa dynamic vacuum;
Table 1 Main structural characteristic of the catalysts Zeolite
Si/Al
Brönsted acidity (K)a 523
623
USY-1 USY-2 USY-3 Beta-1 Beta-2 Beta-3 Beta-4 Beta-5 ITQ-2(I) ITQ-2(II) MCM-22 ZSM-5 MCM-41 MOR
19 35 62 13 15 30 100 250 15 25 15 40 50 10
80 43 39 42 45 36 11 8 30 24 55 30 3 80
32 18 15 22 22 28 10 1 20 16 44 26 1 47
Area (m2 g−1 )
Crystallite size (m)
750 730 750 666 518 503 463 460 573 632 453 420 838 507
0.4–0.6 0.4–0.6 0.4–0.6 0.15 0.50 0.25 0.50 0.50 0.30 0.30 0.3–0.5 1.0–3.0 0.05 0.15–0.20
a Acidity (mol pyridine adsorbed per gram of catalyst) calculated using the extinction coefficients.
then, pyridine was admitted into the cell at room temperature. After saturation, the samples were outgased at 523 K for 1 h under vacuum, cooled to room temperature and the spectra were recorded. The main characteristics of the acid samples used are summarized in Table 1. Phenylacetaldehyde, vanillin, 2-acetonaphthone, propylene glycol, glycerol, and toluene (purity = 99%) were purchased from Aldrich and were used without further purification. 2.2. Reaction procedure Activation of the catalyst (114 mg) was performed in situ by heating the solid at 373 K under vacuum (1 Torr) for 2 h. After this time, the system was left at room temperature and then, a solution of phenylacetaldehyde (11.2 mmol), glycerol (21.5 mmol) in toluene (40 ml) was poured onto the activated catalyst and a Dean–Stark instrument was adapted to remove the water formed. The resultant suspension was heated at 420 K in a silicone oil bath with an automatic temperature control system while magnetically stirring. Samples were taken at regular time periods and analyzed by gas chromatography (GC). At the end of the reaction, the catalyst was filtered and washed with dichloromethane. The organic phase was washed with water in order to remove the excess of glycerol and dried over anhydrous sodium sulfate. The samples with solvent were previously distilled under reduced pressure. Finally, the product was characterized by 1 H NMR (400 MHz Varian VXR-400S). After reaction, the catalyst was submitted to continuous solid–liquid extractions with dichloromethane in a micro-Soxhlet equipment. After removal of the solvent the residue was also weighted and analyzed by GC–MS and 1 H NMR spectroscopy. In all experiments, the recovered material was superior to 95% (wt/wt).
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161
Experiments were also carried out at P = 2 Torr and 330 K, in absence of solvent and continuous distillation of the water formed.
157
60
50
40 Yield/%
3. Results and discussion 3.1. Results of the acetalization of phenylacetaldehyde with glycerol: synthesis of hyacinth fragrance The acetalization of phenylacetaldehyde with glycerol was carried out in the presence of a zeolite USY-2 (Si/Al = 35) as catalyst at 420 K using toluene as solvent. After 2 h reaction time, the conversion of phenylacetaldehyde was 95%. The cyclic 1,2 addition product (2-benzyl-4-hydroxymethyl-1,3-dioxolane, 1) and the 1,3 addition product (2-benzyl-5-hydroxy-1,3-dioxane, 2) along with the two geometrical isomers (cis and trans configurations) were obtained with yields of 64 and 31%, respectively (Fig. 1; Scheme 1). When the yields versus total conversion were plotted (Fig. 2), the selectivity curves clearly indicated that 1,3-dioxolane (1) is a primary and unstable product, while 1,3-dioxane 2 appears as a primary plus secondary product. This indicates that the 1,3-dioxolane (1) is favored kinetically, and isomerizes to the thermodynamically more stable 1,3-dioxane (Scheme 2). Taking into account the size of both isomers (1.12 nm × 0.4 nm × 0.5 nm and
100
Yield/%
80
60
40
20
0 0
60
120
180
240
Time/min
Fig. 1. Time conversion plot of phenylacetaldehyde (䉱) to 1,3-dioxolane 1 (䊉) and 1,3-dioxane 2 (䊊) at 420 K in the presence of USY-2 zeolite.
30
20
10
0 0
60
80
100
Fig. 2. Selectivity curves for the formation of 1,3-dioxolane (䊐) and 1,3-dioxane (䊊) at 420 K in the presence of USY-2.
1.04 nm × 0.4 nm × 0.5 nm) [18] as well as the size of the transition state for the isomerization (1.22 nm × 0.74 nm × 0.83 nm) it appears that the reaction could take place within the pores of large pore zeolites such as USY and beta. In the case of a medium pore zeolite such as ZSM-5 or even for a large pore unidirectional zeolite (MOR), the products will start to feel diffusional restrictions that will be more severe for the formation of the transition state for converting the 1,3-dioxolane into 1,3-dioxane. Indeed, the results from Table 2 indicate that the activity measured as initial rate (r0 ) per acid site (measured by pyridine adsorption) (r0 /Ba) is much lower for ZSM-5 and MOR, than for Y or beta zeolite which are close to that of PTSA. These results show that the reaction is diffusion controlled in ZSM-5 and MOR, which means that not all the acid sites in the crystal are seen by reactants. Moreover, when the ratio of the two isomers (1/2) is calculated, a similar value is obtained for Y, beta, and PTSA, while a much higher dioxolane to dioxane ratio is produced in MOR and ZSM-5. This effect is not only due to the lower level of conversion achieved in the case of MOR and ZSM-5, and consequently to the lower extension of the consecutive isomerization reaction (Scheme 2), but is also due to shape selectivity effects. Indeed, USY gives a lower 1/2 isomer ratio than ZSM-5 even when the results are compared at the same level of conversion (Fig. 1). Also, in the case of MCM-41 (Table 2) with 3.5 nm pores diameter the ratio of 1/2 isomers is lower than for MOR and ZSM-5 at a similar or even lower level of
OH
+ H H
40
Conversion/%
HO
O
20
OH
O
O
+
+ HO
O 1
O
+ H2O
2
HO
Scheme 1. Reaction scheme of the formation of 2-benzyl-4-hydroxymethyl-1,3-dioxolane (1) and 2-benzyl-5-hydroxy-1,3-dioxane (2).
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Ph
Ph
H O
O
+ H
O
OH
Ph
Ph
H
Ph
H
+ O
H + OH
+ O
-H
H
+ O
O
OH HO
HO
OH
OH
OH
1
2
Scheme 2. General reaction mechanism of isomerization of 1,3-dioxolane into 1,3-dioxane catalyzed by acids.
Table 2 Results obtained in the acetalization of phenylacetaldehyde and glycerol with different solid acid catalysts Catalyst
r0 (×103 mol min−1 g−1 )
r0 /Ba (×10−2 min−1 )
Conversiona
Yield 1 (%)
Yield 2 (%)
Ratio 1/2
USY-2 Beta-2 Mordenite MCM-41 ZSM-5 PTSAb
7.70 4.80 0.65 0.51 1.02 20.00
1.80 1.45 0.08 1.70 0.34 1.38
93 92 33 36 54 97
58 61 28 26 46 66
35 31 5 10 8 31
1.6 1.9 5.6 2.6 7.7 2.1
Ba: Brönsted acidity, mol pyridine per gram of catalyst, measured at 523 K. a Reaction conditions: molar ratio glycerol/phenylacetaldehyde: 2, 7.5 (wt./wt.%) of catalyst with respect to carbonyl compound, ratio toluene/carbonyl compound: 27, 420 K, at 1 h reaction time. b 1.7 wt.% of catalyst.
ro 103/molmin -1g-1
8
4
0 0
20
(a)
40 Si/Al ratio
60
30 25 ro 103/molmin -1g-1
conversion. We have then to conclude that MOR and ZSM-5 present a transition state shape selectivity effect and the isomerization of 2-benzyl-4-hydroxymethyl-1,3-dioxolane to 2-benzyl-5-hydroxy-1,3-dioxane is strongly disfavored within the pores of these zeolites. Controlling the relative yield of the isomers can also be of practical interest when considering the different fragrance intensity of the two molecules. However, at present, the industry accepts both isomers in the commercial fragrance, and therefore total conversion, will be the parameter to be optimized. Taking this into account, we can conclude that both USY and beta zeolite catalysts could be of interest from an industrial point of view. However, when trying to optimize these two catalysts, one should take into account that in the case of the synthesis of acetals in where molecules with different polarities are reacted, the optimum design of a catalyst requires not only the optimization of the active sites, but also the optimization of the adsorption properties of the catalyst [9], i.e. the polarity of the catalyst surface. This could be done in zeolites by changing the framework Si/Al ratio [19]. Thus, by increasing the framework Si/Al ratio, the catalyst becomes more hydrophobic, while the number of acid sites decreases and the acid strength of the remaining increases until reaching a Si/Al ratio approximately of 10, after which all acid sites associated to framework aluminum should become equivalent [19]. When catalyst samples with different framework Si/Al ratios were prepared and tested for the synthesis of hyacinth fragrance, the results from Fig. 3a and b show that the adsorption properties, and more specifically the polarity of the zeolite becomes of paramount importance. In fact, the decrease in the number of acid sites when increasing the
20 15 10
5 0 0
(b)
50
100
150
200
250
Si/Al ratio
Fig. 3. Influence of Si/Al ratio of different (a) USY and (b) beta zeolites on the initial formation rate of glycerol acetals (1 + 2).
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161
159
100
O
HO
O H
H
O
+
O
O H
+
75
+H 2 O
HO OH
3
Scheme 3. Reaction scheme of the formation of vanillin propylene glycol acetal (3).
Yield 3/%
HO
50
25
framework Si/Al ratio of the zeolite from 19 to 35 and from 15 to 100 does not produce a decrease of catalyst activity but an increase of the initial rate and a very large increase in the turnover number. This is a clear indication that more hydrophobic samples achieve a better simultaneous diffusion and a more optimal concentration of the two reactants (phenylacetaldehyde and glycerol) within the zeolite pores. Obviously, a further increase in the framework Si/Al ratio produces catalysts with too few actives sites, and also probably too hydrophobic, with the corresponding decrease of catalyst activity. At this point, and with the knowledge acquired during the synthesis of hyacinth fragrance, we have studied the synthesis of a very interesting flavor and fragrance molecule such as the vanillin propylene glycol acetal. 3.2. Synthesis of the vanillin propylene glycol acetal fragrance (2-(4-hydroxy-3-methoxyphenyl)-4-methyl-1,3-dioxolane) The synthesis of the vanillin propylene glycol acetal (3) is carried out by reacting 4-hydroxy-3-methoxybenzaldehyde with propylene glycol (Scheme 3). The reaction was carried out with the zeolite catalysts described above, and the formation of vanillin acetal 3 is again diffusion controlled within the pores of MOR and ZSM-5 (Table 3). Moreover, when the activity per acid site of the large pore USY and beta zeolites is compared with PTSA catalyst, a similar
Table 3 Results of acetalization of vanillin with propylene glycol using different solid acid catalysts Catalysts
r0 (mol min−1 g−1 ) 103
Yield 3 (%)
r0 /Ba (×10−2 min−1 )
USY-2 Beta-1 Beta-4 Mordenite ITQ-2(II) ZSM-5 PTSA
13.9 12.5 6.0 8.4 9.6 1.6 30.0
81 88 87 67 89 30 91
3.2 2.9 5.4 1.0 4.0 0.5 2.1
Reaction conditions: molar ratio propylene glycol/4-hydroxy-3-methoxybenzaldehyde: 2, 7.5 (wt./wt.%) of catalyst respect to carbonyl compound, ratio toluene/carbonyl compound: 27, 419 K. Ba: mol pyridine per gram of catalyst, measured by the pyridine remaining adsorbed at 523 K. Yield measured at 15 min of reaction time.
0 0
60
120
180
240
Time/min
Fig. 4. Yield of vanillin propylene glycol acetal 3 vs. the reaction time, when the acetalization reaction was carried out in the presence of USY-2 using toluene as solvent (䉫), without solvent (T = 398 K) (䊉), and under reduced pressure (T = 330 K, P = 2 Torr) (䊐).
value is found indicating that the reaction is not diffusion controlled with these two solid catalysts. The excellent catalytic results obtained with the large pore zeolites for the synthesis of vanillin acetal, makes these catalysts a real alternative for the homogeneous acid catalyst (PTSA) used today in industry. However, besides the replacement of a homogeneous acid catalyst by a solid one, it would be of much interest to substitute the azeotropic distillation, that uses toluene as solvent, by a process where water does not need to be removed during the reaction or, alternatively, by a process in where water is removed by direct distillation during reaction without the need of any solvent. This is certainly a non-easy task since in the first case the water present will negatively influence the thermodynamic equilibrium, and in the second alternative the absence of solvent will make the desorption of products from the surface of the catalyst more difficult leading to a faster catalyst deactivation. We have performed experiments on the acetalization of vanillin, using USY-2 and Beta-4 catalysts at 398 K in absence of solvent. The results obtained indicate that under these conditions the conversion does not exceed 30% after 5 h reaction time. As expected, it is necessary to remove the water formed in the reaction media in order to achieve high yields of acetal 3. To do this in absence of toluene and azeotropic distillation, we have carried out the reaction under reduced pressure (2 Torr) with continuous distillation of the water formed. The results presented in Fig. 4 show that when the reaction is carried out with USY zeolite and removing the water formed by continuous distillation at reduced pressure, the initial rate of acetalization is lower than in the case where the water was removed by azeotropic distillation with toluene. Moreover, the final conversion achieved after 4 h was only 63%, while when using toluene
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M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161 100 HO
O
H
O
+
80
O
+H2 O
+
Yield 3/%
OH
4
60
Scheme 4. Reaction scheme of the formation of 2-methyl-2-naphthyl4-methyl-1,3-dioxolane (4).
40
20
0 0
60
120
180
240
Time/min
Fig. 5. Yield of vanillin propylene glycol acetal 3 vs. the reaction time, when the acetalization reaction was carried out in the presence of Beta-4 (䊉) and ITQ-2 (+) using toluene as solvent and by direct distillation under reduced pressure (T = 330 K, P = 2 Torr) Beta-4 (䊐) and ITQ-2 (×).
as solvent and carrying out the azeotropic distillation, 99% conversion was achieved after 1 h reaction time. The lower conversion achieved by continuous distillation with the toluene-free reaction media can be due either to the less efficient water removal achieved by direct distillation, and/or because of the lower rate of product desorption from the catalyst surface owing to the absence of solvent. Then, in order to discuss this issue, we have carried out the reaction by direct distillation using a more hydrophobic sample Beta-4. The results presented in Fig. 5 show that also in this case a lower conversion was achieved by the continuous distillation at lower pressure, being the conversion after 4 h in the order of 72%. It is clear that a more hydrophobic zeolite sample helps to improve water removal from the surface, but in any case the conversion achieved in this case is still far from the desired values. As was said above, the absence of solvent may lead to a less efficient removal of the products remaining adsorbed within the pores, and a larger amount of organic on the catalyst after the reaction when working in a solvent-free system could be expected. Indeed, elemental analysis showed that the organic material in Beta-4 zeolite was 2.36 and 3.90% when the acetalization was carried with or without solvent, respectively. From the above results, we can conclude that toluene has not only a positive effect due to the more efficient water removal from the reaction media, but it has also the benefit of helping to remove the reaction products that remain adsorbed within the micropores of the catalyst blocking the access of reactants to active sites. Thus, if the lower efficiency of the direct distillation process is due to low diffusion–desorption of products, one could expect this to be easier, and therefore to achieve a larger conversion, with a zeolitic catalyst with larger pore
diameter and/or with a zeolite with a higher ratio of external to internal surface. We have recently shown that delaminated zeolite materials [17] and more specifically ITQ-2 presents a very high external surface that allows fast desorption and diffusion of products. Thus, taking into account all the above, the delaminated molecular sieve should be an excellent catalyst choice to carry out the synthesis of vanillin acetal by a solvent-free direct distillation procedure. ITQ-2 is a material obtained by delamination of a laminar precursor of the MWW zeolite [17]. The sample of ITQ-2 produced has a Si/Al ratio of 25, microporous surface of 259 m2 g−1 and an external surface area of 373 m2 g−1 . The catalytic results obtained are given in Fig. 5 in where a clear improvement with respect to the results obtained with Beta-4 zeolite is observed when working with a solvent-free direct distillation. The benefit of delamination was not observed, however, when azeotropic distillation was performed instead, since in this case both Beta-4 and ITQ-2 catalysts gave 99% conversion. In conclusion, it can be said that large pore zeolites are very good catalysts for the synthesis of fragrance acetals when using toluene as solvent and azeotropic distillation. However, they are limited if a more environmentally friendly process based on direct distillation of water is employed. On the other hand, the delaminated ITQ-2 zeolite with faster product desorption and diffusion is a very promising catalysts to work in a solvent-free process. The benefit of delaminated zeolitic catalyst due to easy accessibility of sites to reactants and fast product desorption and diffusion can also be found when the fragrance acetal involves large reactant molecules and products that can not diffuse in or diffuse out of the pores and cavities of the zeolites. This is the case of 2-methyl-2-naphthyl4-methyl-1,3-dioxolane (4) that has a blossom orange fragrance (Scheme 4). In this case, the reactant molecule (2-acetonaphthone) has dimensions of 1.35 nm × 0.74 nm × 0.56 nm, while the acetal formed with propylene glycol has 1.40 nm × 1.00 nm × 0.75 nm. Taking into account the size of the reactant ketone and acetal product, it is not surprising that the reaction is diffusion controlled even with large pore zeolites [20]. This reaction is then a clear case in where ITQ-2 material can show an advantage with respect to zeolites. Indeed, when the activity of ITQ-2 is compared with that of the commercial beta zeo-
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161 Table 4 Results of acetalization of 2-acetonaphthone with propylene glycol using different solid acid catalysts Catalysts
Si/Al
r0 /Ba (min−1 )
Yield 4 (%)
Beta-1 MCM-22 ITQ-2 (I)
13 15 15
2.3 1.8 12.3
5 20 63
Ba: Brönsted acidity, mol pyridine per gram of catalyst, measured at 523 K. Yield at 3 h reaction time.
lite (Table 4) for the named reaction, the former presents an initial activity per active acid site more than five times higher than either the counterpart MCM-22 or the beta zeolite.
4. Conclusions It has been shown that tri-directional zeolites are able to carry out successfully the acetalization of phenylacetaldehyde and vanillin by glycerol and propylene glycol, respectively, in good yields for producing commercial fragrances. For this reaction, the hydrophobic properties of the catalysts are as important as the concentration of the active acid sites, and this is shown during the acetalization of phenylacetaldehyde with glycerol where beta samples with higher Si/Al ratio (close to 100) and USY between Si/Al = 35 and 50, were the most active catalysts. It appears that when acetalization is performed with small size carbonyl compounds as reactants, zeolites are intrinsically more active than the delaminated and mesoporous materials, however, when the reactants present larger sizes as in the synthesis of propylene glycol acetal of 2-acetonaphthone, geometrical constraints decrease the rate of diffusion of reactants. When this occurs, the initial activity per acid site of delaminated ITQ-2 zeolite is more than five times higher than that of MCM-22 or the commercial beta zeolite. From the catalytic process point of view, the acetalization of vanillin was successfully performed, by direct distillation working in a vacuum of 2 Torr and in absence of solvent. This was achieved when working with the delaminated ITQ-2 zeolitic material that allows better and faster product desorption than larger pore zeolites.
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References [1] T.W. Green, P.G.M. Wuts, Protective Groups in Organic Synthesis, second ed., vol. 4, Wiley, New York, 1991, p. 212. [2] K. Bauer, D. Garbe, H. Surburg, Common Fragrances and Flavors Materials, second ed., VCH, New York, 1990. [3] K. Woelfel, T.G. Hartman, ACS Symp. Ser. 705 (1998) 193. [4] R. Suffis, M.L. Barr, K. Ishida, K. Sawano, A.G. Van Loveren, T. Nakatsu, C.B. Green, G.A. Reitz, R.K.L. Karg, T. Sato, US Patent 5,626,852 (1997). [5] M.J. Ashton, C. Lawrence, J.A. Karlsson, K.A.J. Stuttle, C.G. Newton, B.Y.J. Vacher, S. Webber, M.J. Withnall, J. Med. Chem. 39 (1996) 4888. [6] T. Vu Moc, P. Maitte, Bull. Soc. Chim. Fr. 9 (1975) 2558. [7] L. Tong-Shuang, L. Sheng-Hui, L. Ji-Tai, L. Hui-Zhang, J. Chem. Res. Synop. 1 (1997) 26; M. Csiba, J. Cleophax, A. Loupy, J. Malthete, S.D. Gero, Tetrahedron Lett. 34 (1993) 1787. [8] A. Corma, M.J. Climent, H. Garc´ıa, J. Primo, Appl. Catal. 59 (1990) 333; R. Ballini, G. Bosica, B. Frullanti, R. Maggi, G. Sartori, F. Schroer, Tetrahedron Lett. 39 (1998) 1615; I. Rodriguez, M.J. Climent, A. Corma, S. Iborra, V. Fornés, J. Catal. 192 (2000) 441. [9] M.J. Climent, A. Corma, A. Velty, M. Susarte, J. Catal. 196 (2000) 345; A. Corma, M.J. Climent, M. Susarte, A. Velty, SP 9,902,439 (1999). [10] M.J. Climent, A. Corma, S. Iborra, M.C. Navarro, J. Primo, J. Catal. 161 (1996) 783; Y. Tanaka, N. Sawamura, M. Iwamoto, Tetrahedron Lett. 39 (1998) 9457; Y. Tanaka, N. Sawamura, M. Iwamoto, Tetrahedron Lett. 39 (1998) 9457. [11] G. Cherkaev, S.A. Timonin, G.F. Yakovleva, L. Shutikova, A.S. Mikhailova, L.D. Shapiro, SU 1,337,384 (1987). [12] A. Corma, L.T. Nemeth, M. Renz, S. Valencia, Nature 412 (2001) 423; A. Corma, M. Domine, L.T. Nemeth, S. Valencia, J. Am. Chem. Soc. 124 (2003) 3194. [13] D.L.J. Opdyke, Food Cosmet. Toxicol. 4 (1976) 829. [14] R.L. Smith, P. Newberne, T.A. Adams, Food Technol. 52 (1993) 104. [15] S. Valencia, Ph.D. Thesis, Valencia, 1997. [16] J.S. Beck, C. Chu, J.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, WO 9,111,390 (1992); C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, J.S. Beck, Nature 359 (1992) 710. [17] A. Corma, V. Fornés, S.B. Pergher, T.L.M. Maesen, J.G. Buglass, Nature 376 (1998) 353; A. Corma, U. Diaz, V. Fornés, J.M. Guil, J. Martinez-Triguero, E.J. Creyghton, J. Catal. 191 (2000) 218. [18] Cerius 2 Software Package, version 4.6, Accelrys, San Diego (2003). [19] D. Barthomeuf, Mater. Chem. Phys. 17 (1987) 49; D. Barthomeuf, A. Corma, Chem. Rev. 95 (1995) 559. [20] M.J. Climent, A. Corma, A. Velty, Green Chem. 4 (2002) 565.
Synthesis of hyacinth, vanilla, and blossom orange fragrances: the benefit of using zeolites and delaminated zeolites as catalysts Maria José Climent, Avelino Corma∗ , Alexandra Velty Instituto de Tecnologia Qu´ımica, UPV-CSIC, Universidad de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain Received in revised form 31 October 2003; accepted 3 December 2003
Abstract The synthesis of phenylacetaldehyde glycerol acetals, 2-benzyl-4-hydroxymethyl-1,3-dioxolane (1), 2-benzyl-5-hydroxy-1,3-dioxane (2), and vanillin propylene glycol acetal (2-(4-hydroxy-3-methoxyphenyl)-4-methyl-1,3-dioxolane) (3) which are flavoring compounds with hyacinth and vanilla scent fragrances, have been carried out successfully by acetalization of phenylacetaldehyde and vanillin with glycerol and propylene glycol, respectively, using toluene as solvent and zeolite catalysts whose adsorption properties have been optimized. However, in the case of a larger size acetal such as 2-acetonaphthone propylene glycol acetal (4) with blossom orange scent, geometrical constraints make the diffusion of reactants and products inside the micropores more difficult. In this case, a delaminated zeolite (ITQ-2) with very large and structured external surface is an active and selective catalyst. Furthermore, the delaminated zeolite allows the reaction to be carried out in a solvent-free system, opening the possibility for an environmentally friendly process for the synthesis of acetal fragrances. © 2004 Elsevier B.V. All rights reserved. Keywords: Hyacinth fragrance; Vanilla fragrance; Blossom orange fragrance; Zeolites; Delaminated zeolites; Acetals
1. Introduction The acetalization reaction is sometimes a necessary requirement to protect carbonyl groups when reacting multifunctional organic molecules [1]. For instance, the dioxolane group is generally stable to bases, Grignard reagents, hydrogenation reagents, metal hydrides, oxidants, bromination and esterification reagents. Besides the interest of acetals as protecting groups, many of them have found direct applications as fragrances, in cosmetics, food and beverage additives, pharmaceuticals, in detergents, and in lacquer industries [2]. It is known that the conversion of a carbonyl compound to an acetal, profoundly changes its vapor pressure, solubility, aroma characteristics, and generally attenuates or alters its flavor impact. For example, the propylene glycol acetal of vanillin is used to imitate vanilla flavors because it causes flavor attenuation [3]. However, the flavor of the original aldehyde may rapidly be regenerated upon hydration in a high moisture food or beverage [4].
∗
Corresponding author. Tel.: +34-96-3877800; fax: +34-96-3877809. E-mail address: [email protected] (A. Corma).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.007
The most general method for the synthesis of acetals is to react carbonyl compounds with an alcohol or an orthoester in the presence of acid catalysts. A number of acetalization procedures include the use of protic acids, Lewis acids (zinc chloride) [5], alumina [6], montmorillonite [7], zeolites [8,9], mesoporous aluminosilicates [10], and cation exchange resins [11]. Generally speaking, acetalization of aldehydes can be performed in the presence of weak acids, while ketones generally need stronger ones like sulfuric, hydrochloric, or p-toluenesulfonic acid (PTSA) and larger amounts of catalyst. However, many of the methods mentioned above present limitations derived from the use of expensive reagents, tedious work-up procedure, and necessity of neutralization (with the exception of solid catalysts) of the strongly acidic media, with the production of undesired wastes. In this sense, synthetic zeolites appear as promising catalysts with the obvious advantages over conventional Brönsted or Lewis acids [12], of easy separation from the reaction mixture, shape selectivity, reusability and, overall, the possibility to control adsorption, strength, and distribution of acid sites. The use of aromatic chemicals in perfumery has been growing since the chemical developments that allowed their synthesis and commercial production. Among them vanillin
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propylene glycol acetal [4] and phenylacetaldehyde glyceryl acetals [4,13] are flavoring materials with vanilla scent and hyacinth fragrance, respectively, that are prepared by reacting the corresponding aromatic aldehyde with propylene glycol and glycerol, respectively. The commercial process is catalyzed by strong acids such as PTSA, HCl, H3 PO4 , and divinylbenzene–styrene resin [11]. These fragrances are included in the list of the FEMA-GRAS, which offers naturally occurring or synthetically produced flavoring substances regulated by the Food and Drug Administration (FDA) [14]. Phenylacetaldehyde glyceryl acetal is used for food and may be added (at a level of 20 ppm) without hazard to public health [13]. We have shown [9] that zeolites can carry out selectively the synthesis of acetals and fructone fragrancy. Here, we have attempted to optimize the synthesis of phenylacetaldehyde glyceryl acetals (1 and 2), propylene glycol acetal of the vanillin (3), and acetonaphthone (4) using zeolites, delaminated zeolites, and mesoporous molecular sieves (MCM-41) as catalysts. It will be shown that controlling the acidity, accessibility, and hydrophobic–hydrophilic properties of the catalyst, it has been possible to design a successful catalyst that, when coupled with the adequate process, maximizes conversion and catalyst life, allowing the process to be carried out in absence of solvent.
2. Experimental 2.1. Catalysts USY samples were obtained from PQ Zeolites B.V., and they were NH4 + exchanged followed by calcination at 773 K during 3 h. Beta-1 zeolite was supplied by PQ corporation, in the acidic form. Zeolite beta samples (Beta-2, Beta-3, Beta-4, Beta-5, Beta-6) were synthesized in our laboratory by working in fluoride media [15]. The acid form of mordenite (MOR) was obtained by calcination of a Conteka sample, followed by an acid treatment to extract the extra-framework Al. The ZSM-5 sample was calcined at 823 K for 3 h to obtain the acid form. One sample of MCM-41 (pore diameter 3.5 nm and framework Si/Al = 50) was synthesized following the procedure given in Ref. [16], using hexadecyltrimethylammonium (Aldrich) cation as template, and pseudoboehmite (Capatal B, Vista) and Aerosil (Degussa) as the aluminum and silicon source, respectively. The sample was calcined in N2 for 4 h and in air for 6 h at 813 K. The ITQ-2 delaminated and the corresponding MCM-22 zeolites were synthesized following Ref. [17]. Acidity measurements were carried out by adsorption– desorption of pyridine by IR spectroscopy. The infrared spectra were recorded on a Nicolet 710 FTIR using selfsupported wafers of 10 mg cm−2 . The calcined samples were outgased overnight at 673 K and 10−3 Pa dynamic vacuum;
Table 1 Main structural characteristic of the catalysts Zeolite
Si/Al
Brönsted acidity (K)a 523
623
USY-1 USY-2 USY-3 Beta-1 Beta-2 Beta-3 Beta-4 Beta-5 ITQ-2(I) ITQ-2(II) MCM-22 ZSM-5 MCM-41 MOR
19 35 62 13 15 30 100 250 15 25 15 40 50 10
80 43 39 42 45 36 11 8 30 24 55 30 3 80
32 18 15 22 22 28 10 1 20 16 44 26 1 47
Area (m2 g−1 )
Crystallite size (m)
750 730 750 666 518 503 463 460 573 632 453 420 838 507
0.4–0.6 0.4–0.6 0.4–0.6 0.15 0.50 0.25 0.50 0.50 0.30 0.30 0.3–0.5 1.0–3.0 0.05 0.15–0.20
a Acidity (mol pyridine adsorbed per gram of catalyst) calculated using the extinction coefficients.
then, pyridine was admitted into the cell at room temperature. After saturation, the samples were outgased at 523 K for 1 h under vacuum, cooled to room temperature and the spectra were recorded. The main characteristics of the acid samples used are summarized in Table 1. Phenylacetaldehyde, vanillin, 2-acetonaphthone, propylene glycol, glycerol, and toluene (purity = 99%) were purchased from Aldrich and were used without further purification. 2.2. Reaction procedure Activation of the catalyst (114 mg) was performed in situ by heating the solid at 373 K under vacuum (1 Torr) for 2 h. After this time, the system was left at room temperature and then, a solution of phenylacetaldehyde (11.2 mmol), glycerol (21.5 mmol) in toluene (40 ml) was poured onto the activated catalyst and a Dean–Stark instrument was adapted to remove the water formed. The resultant suspension was heated at 420 K in a silicone oil bath with an automatic temperature control system while magnetically stirring. Samples were taken at regular time periods and analyzed by gas chromatography (GC). At the end of the reaction, the catalyst was filtered and washed with dichloromethane. The organic phase was washed with water in order to remove the excess of glycerol and dried over anhydrous sodium sulfate. The samples with solvent were previously distilled under reduced pressure. Finally, the product was characterized by 1 H NMR (400 MHz Varian VXR-400S). After reaction, the catalyst was submitted to continuous solid–liquid extractions with dichloromethane in a micro-Soxhlet equipment. After removal of the solvent the residue was also weighted and analyzed by GC–MS and 1 H NMR spectroscopy. In all experiments, the recovered material was superior to 95% (wt/wt).
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161
Experiments were also carried out at P = 2 Torr and 330 K, in absence of solvent and continuous distillation of the water formed.
157
60
50
40 Yield/%
3. Results and discussion 3.1. Results of the acetalization of phenylacetaldehyde with glycerol: synthesis of hyacinth fragrance The acetalization of phenylacetaldehyde with glycerol was carried out in the presence of a zeolite USY-2 (Si/Al = 35) as catalyst at 420 K using toluene as solvent. After 2 h reaction time, the conversion of phenylacetaldehyde was 95%. The cyclic 1,2 addition product (2-benzyl-4-hydroxymethyl-1,3-dioxolane, 1) and the 1,3 addition product (2-benzyl-5-hydroxy-1,3-dioxane, 2) along with the two geometrical isomers (cis and trans configurations) were obtained with yields of 64 and 31%, respectively (Fig. 1; Scheme 1). When the yields versus total conversion were plotted (Fig. 2), the selectivity curves clearly indicated that 1,3-dioxolane (1) is a primary and unstable product, while 1,3-dioxane 2 appears as a primary plus secondary product. This indicates that the 1,3-dioxolane (1) is favored kinetically, and isomerizes to the thermodynamically more stable 1,3-dioxane (Scheme 2). Taking into account the size of both isomers (1.12 nm × 0.4 nm × 0.5 nm and
100
Yield/%
80
60
40
20
0 0
60
120
180
240
Time/min
Fig. 1. Time conversion plot of phenylacetaldehyde (䉱) to 1,3-dioxolane 1 (䊉) and 1,3-dioxane 2 (䊊) at 420 K in the presence of USY-2 zeolite.
30
20
10
0 0
60
80
100
Fig. 2. Selectivity curves for the formation of 1,3-dioxolane (䊐) and 1,3-dioxane (䊊) at 420 K in the presence of USY-2.
1.04 nm × 0.4 nm × 0.5 nm) [18] as well as the size of the transition state for the isomerization (1.22 nm × 0.74 nm × 0.83 nm) it appears that the reaction could take place within the pores of large pore zeolites such as USY and beta. In the case of a medium pore zeolite such as ZSM-5 or even for a large pore unidirectional zeolite (MOR), the products will start to feel diffusional restrictions that will be more severe for the formation of the transition state for converting the 1,3-dioxolane into 1,3-dioxane. Indeed, the results from Table 2 indicate that the activity measured as initial rate (r0 ) per acid site (measured by pyridine adsorption) (r0 /Ba) is much lower for ZSM-5 and MOR, than for Y or beta zeolite which are close to that of PTSA. These results show that the reaction is diffusion controlled in ZSM-5 and MOR, which means that not all the acid sites in the crystal are seen by reactants. Moreover, when the ratio of the two isomers (1/2) is calculated, a similar value is obtained for Y, beta, and PTSA, while a much higher dioxolane to dioxane ratio is produced in MOR and ZSM-5. This effect is not only due to the lower level of conversion achieved in the case of MOR and ZSM-5, and consequently to the lower extension of the consecutive isomerization reaction (Scheme 2), but is also due to shape selectivity effects. Indeed, USY gives a lower 1/2 isomer ratio than ZSM-5 even when the results are compared at the same level of conversion (Fig. 1). Also, in the case of MCM-41 (Table 2) with 3.5 nm pores diameter the ratio of 1/2 isomers is lower than for MOR and ZSM-5 at a similar or even lower level of
OH
+ H H
40
Conversion/%
HO
O
20
OH
O
O
+
+ HO
O 1
O
+ H2O
2
HO
Scheme 1. Reaction scheme of the formation of 2-benzyl-4-hydroxymethyl-1,3-dioxolane (1) and 2-benzyl-5-hydroxy-1,3-dioxane (2).
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M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161
Ph
Ph
H O
O
+ H
O
OH
Ph
Ph
H
Ph
H
+ O
H + OH
+ O
-H
H
+ O
O
OH HO
HO
OH
OH
OH
1
2
Scheme 2. General reaction mechanism of isomerization of 1,3-dioxolane into 1,3-dioxane catalyzed by acids.
Table 2 Results obtained in the acetalization of phenylacetaldehyde and glycerol with different solid acid catalysts Catalyst
r0 (×103 mol min−1 g−1 )
r0 /Ba (×10−2 min−1 )
Conversiona
Yield 1 (%)
Yield 2 (%)
Ratio 1/2
USY-2 Beta-2 Mordenite MCM-41 ZSM-5 PTSAb
7.70 4.80 0.65 0.51 1.02 20.00
1.80 1.45 0.08 1.70 0.34 1.38
93 92 33 36 54 97
58 61 28 26 46 66
35 31 5 10 8 31
1.6 1.9 5.6 2.6 7.7 2.1
Ba: Brönsted acidity, mol pyridine per gram of catalyst, measured at 523 K. a Reaction conditions: molar ratio glycerol/phenylacetaldehyde: 2, 7.5 (wt./wt.%) of catalyst with respect to carbonyl compound, ratio toluene/carbonyl compound: 27, 420 K, at 1 h reaction time. b 1.7 wt.% of catalyst.
ro 103/molmin -1g-1
8
4
0 0
20
(a)
40 Si/Al ratio
60
30 25 ro 103/molmin -1g-1
conversion. We have then to conclude that MOR and ZSM-5 present a transition state shape selectivity effect and the isomerization of 2-benzyl-4-hydroxymethyl-1,3-dioxolane to 2-benzyl-5-hydroxy-1,3-dioxane is strongly disfavored within the pores of these zeolites. Controlling the relative yield of the isomers can also be of practical interest when considering the different fragrance intensity of the two molecules. However, at present, the industry accepts both isomers in the commercial fragrance, and therefore total conversion, will be the parameter to be optimized. Taking this into account, we can conclude that both USY and beta zeolite catalysts could be of interest from an industrial point of view. However, when trying to optimize these two catalysts, one should take into account that in the case of the synthesis of acetals in where molecules with different polarities are reacted, the optimum design of a catalyst requires not only the optimization of the active sites, but also the optimization of the adsorption properties of the catalyst [9], i.e. the polarity of the catalyst surface. This could be done in zeolites by changing the framework Si/Al ratio [19]. Thus, by increasing the framework Si/Al ratio, the catalyst becomes more hydrophobic, while the number of acid sites decreases and the acid strength of the remaining increases until reaching a Si/Al ratio approximately of 10, after which all acid sites associated to framework aluminum should become equivalent [19]. When catalyst samples with different framework Si/Al ratios were prepared and tested for the synthesis of hyacinth fragrance, the results from Fig. 3a and b show that the adsorption properties, and more specifically the polarity of the zeolite becomes of paramount importance. In fact, the decrease in the number of acid sites when increasing the
20 15 10
5 0 0
(b)
50
100
150
200
250
Si/Al ratio
Fig. 3. Influence of Si/Al ratio of different (a) USY and (b) beta zeolites on the initial formation rate of glycerol acetals (1 + 2).
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161
159
100
O
HO
O H
H
O
+
O
O H
+
75
+H 2 O
HO OH
3
Scheme 3. Reaction scheme of the formation of vanillin propylene glycol acetal (3).
Yield 3/%
HO
50
25
framework Si/Al ratio of the zeolite from 19 to 35 and from 15 to 100 does not produce a decrease of catalyst activity but an increase of the initial rate and a very large increase in the turnover number. This is a clear indication that more hydrophobic samples achieve a better simultaneous diffusion and a more optimal concentration of the two reactants (phenylacetaldehyde and glycerol) within the zeolite pores. Obviously, a further increase in the framework Si/Al ratio produces catalysts with too few actives sites, and also probably too hydrophobic, with the corresponding decrease of catalyst activity. At this point, and with the knowledge acquired during the synthesis of hyacinth fragrance, we have studied the synthesis of a very interesting flavor and fragrance molecule such as the vanillin propylene glycol acetal. 3.2. Synthesis of the vanillin propylene glycol acetal fragrance (2-(4-hydroxy-3-methoxyphenyl)-4-methyl-1,3-dioxolane) The synthesis of the vanillin propylene glycol acetal (3) is carried out by reacting 4-hydroxy-3-methoxybenzaldehyde with propylene glycol (Scheme 3). The reaction was carried out with the zeolite catalysts described above, and the formation of vanillin acetal 3 is again diffusion controlled within the pores of MOR and ZSM-5 (Table 3). Moreover, when the activity per acid site of the large pore USY and beta zeolites is compared with PTSA catalyst, a similar
Table 3 Results of acetalization of vanillin with propylene glycol using different solid acid catalysts Catalysts
r0 (mol min−1 g−1 ) 103
Yield 3 (%)
r0 /Ba (×10−2 min−1 )
USY-2 Beta-1 Beta-4 Mordenite ITQ-2(II) ZSM-5 PTSA
13.9 12.5 6.0 8.4 9.6 1.6 30.0
81 88 87 67 89 30 91
3.2 2.9 5.4 1.0 4.0 0.5 2.1
Reaction conditions: molar ratio propylene glycol/4-hydroxy-3-methoxybenzaldehyde: 2, 7.5 (wt./wt.%) of catalyst respect to carbonyl compound, ratio toluene/carbonyl compound: 27, 419 K. Ba: mol pyridine per gram of catalyst, measured by the pyridine remaining adsorbed at 523 K. Yield measured at 15 min of reaction time.
0 0
60
120
180
240
Time/min
Fig. 4. Yield of vanillin propylene glycol acetal 3 vs. the reaction time, when the acetalization reaction was carried out in the presence of USY-2 using toluene as solvent (䉫), without solvent (T = 398 K) (䊉), and under reduced pressure (T = 330 K, P = 2 Torr) (䊐).
value is found indicating that the reaction is not diffusion controlled with these two solid catalysts. The excellent catalytic results obtained with the large pore zeolites for the synthesis of vanillin acetal, makes these catalysts a real alternative for the homogeneous acid catalyst (PTSA) used today in industry. However, besides the replacement of a homogeneous acid catalyst by a solid one, it would be of much interest to substitute the azeotropic distillation, that uses toluene as solvent, by a process where water does not need to be removed during the reaction or, alternatively, by a process in where water is removed by direct distillation during reaction without the need of any solvent. This is certainly a non-easy task since in the first case the water present will negatively influence the thermodynamic equilibrium, and in the second alternative the absence of solvent will make the desorption of products from the surface of the catalyst more difficult leading to a faster catalyst deactivation. We have performed experiments on the acetalization of vanillin, using USY-2 and Beta-4 catalysts at 398 K in absence of solvent. The results obtained indicate that under these conditions the conversion does not exceed 30% after 5 h reaction time. As expected, it is necessary to remove the water formed in the reaction media in order to achieve high yields of acetal 3. To do this in absence of toluene and azeotropic distillation, we have carried out the reaction under reduced pressure (2 Torr) with continuous distillation of the water formed. The results presented in Fig. 4 show that when the reaction is carried out with USY zeolite and removing the water formed by continuous distillation at reduced pressure, the initial rate of acetalization is lower than in the case where the water was removed by azeotropic distillation with toluene. Moreover, the final conversion achieved after 4 h was only 63%, while when using toluene
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M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161 100 HO
O
H
O
+
80
O
+H2 O
+
Yield 3/%
OH
4
60
Scheme 4. Reaction scheme of the formation of 2-methyl-2-naphthyl4-methyl-1,3-dioxolane (4).
40
20
0 0
60
120
180
240
Time/min
Fig. 5. Yield of vanillin propylene glycol acetal 3 vs. the reaction time, when the acetalization reaction was carried out in the presence of Beta-4 (䊉) and ITQ-2 (+) using toluene as solvent and by direct distillation under reduced pressure (T = 330 K, P = 2 Torr) Beta-4 (䊐) and ITQ-2 (×).
as solvent and carrying out the azeotropic distillation, 99% conversion was achieved after 1 h reaction time. The lower conversion achieved by continuous distillation with the toluene-free reaction media can be due either to the less efficient water removal achieved by direct distillation, and/or because of the lower rate of product desorption from the catalyst surface owing to the absence of solvent. Then, in order to discuss this issue, we have carried out the reaction by direct distillation using a more hydrophobic sample Beta-4. The results presented in Fig. 5 show that also in this case a lower conversion was achieved by the continuous distillation at lower pressure, being the conversion after 4 h in the order of 72%. It is clear that a more hydrophobic zeolite sample helps to improve water removal from the surface, but in any case the conversion achieved in this case is still far from the desired values. As was said above, the absence of solvent may lead to a less efficient removal of the products remaining adsorbed within the pores, and a larger amount of organic on the catalyst after the reaction when working in a solvent-free system could be expected. Indeed, elemental analysis showed that the organic material in Beta-4 zeolite was 2.36 and 3.90% when the acetalization was carried with or without solvent, respectively. From the above results, we can conclude that toluene has not only a positive effect due to the more efficient water removal from the reaction media, but it has also the benefit of helping to remove the reaction products that remain adsorbed within the micropores of the catalyst blocking the access of reactants to active sites. Thus, if the lower efficiency of the direct distillation process is due to low diffusion–desorption of products, one could expect this to be easier, and therefore to achieve a larger conversion, with a zeolitic catalyst with larger pore
diameter and/or with a zeolite with a higher ratio of external to internal surface. We have recently shown that delaminated zeolite materials [17] and more specifically ITQ-2 presents a very high external surface that allows fast desorption and diffusion of products. Thus, taking into account all the above, the delaminated molecular sieve should be an excellent catalyst choice to carry out the synthesis of vanillin acetal by a solvent-free direct distillation procedure. ITQ-2 is a material obtained by delamination of a laminar precursor of the MWW zeolite [17]. The sample of ITQ-2 produced has a Si/Al ratio of 25, microporous surface of 259 m2 g−1 and an external surface area of 373 m2 g−1 . The catalytic results obtained are given in Fig. 5 in where a clear improvement with respect to the results obtained with Beta-4 zeolite is observed when working with a solvent-free direct distillation. The benefit of delamination was not observed, however, when azeotropic distillation was performed instead, since in this case both Beta-4 and ITQ-2 catalysts gave 99% conversion. In conclusion, it can be said that large pore zeolites are very good catalysts for the synthesis of fragrance acetals when using toluene as solvent and azeotropic distillation. However, they are limited if a more environmentally friendly process based on direct distillation of water is employed. On the other hand, the delaminated ITQ-2 zeolite with faster product desorption and diffusion is a very promising catalysts to work in a solvent-free process. The benefit of delaminated zeolitic catalyst due to easy accessibility of sites to reactants and fast product desorption and diffusion can also be found when the fragrance acetal involves large reactant molecules and products that can not diffuse in or diffuse out of the pores and cavities of the zeolites. This is the case of 2-methyl-2-naphthyl4-methyl-1,3-dioxolane (4) that has a blossom orange fragrance (Scheme 4). In this case, the reactant molecule (2-acetonaphthone) has dimensions of 1.35 nm × 0.74 nm × 0.56 nm, while the acetal formed with propylene glycol has 1.40 nm × 1.00 nm × 0.75 nm. Taking into account the size of the reactant ketone and acetal product, it is not surprising that the reaction is diffusion controlled even with large pore zeolites [20]. This reaction is then a clear case in where ITQ-2 material can show an advantage with respect to zeolites. Indeed, when the activity of ITQ-2 is compared with that of the commercial beta zeo-
M.J. Climent et al. / Applied Catalysis A: General 263 (2004) 155–161 Table 4 Results of acetalization of 2-acetonaphthone with propylene glycol using different solid acid catalysts Catalysts
Si/Al
r0 /Ba (min−1 )
Yield 4 (%)
Beta-1 MCM-22 ITQ-2 (I)
13 15 15
2.3 1.8 12.3
5 20 63
Ba: Brönsted acidity, mol pyridine per gram of catalyst, measured at 523 K. Yield at 3 h reaction time.
lite (Table 4) for the named reaction, the former presents an initial activity per active acid site more than five times higher than either the counterpart MCM-22 or the beta zeolite.
4. Conclusions It has been shown that tri-directional zeolites are able to carry out successfully the acetalization of phenylacetaldehyde and vanillin by glycerol and propylene glycol, respectively, in good yields for producing commercial fragrances. For this reaction, the hydrophobic properties of the catalysts are as important as the concentration of the active acid sites, and this is shown during the acetalization of phenylacetaldehyde with glycerol where beta samples with higher Si/Al ratio (close to 100) and USY between Si/Al = 35 and 50, were the most active catalysts. It appears that when acetalization is performed with small size carbonyl compounds as reactants, zeolites are intrinsically more active than the delaminated and mesoporous materials, however, when the reactants present larger sizes as in the synthesis of propylene glycol acetal of 2-acetonaphthone, geometrical constraints decrease the rate of diffusion of reactants. When this occurs, the initial activity per acid site of delaminated ITQ-2 zeolite is more than five times higher than that of MCM-22 or the commercial beta zeolite. From the catalytic process point of view, the acetalization of vanillin was successfully performed, by direct distillation working in a vacuum of 2 Torr and in absence of solvent. This was achieved when working with the delaminated ITQ-2 zeolitic material that allows better and faster product desorption than larger pore zeolites.
161
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