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Selective hydration of dihydromyrcene to dihydromyrcenol over H-beta zeolite.
Applied Catalysis A: General 203 (2000) 251–258
Selective hydration of dihydromyrcene to dihydromyrcenol over H-beta zeolite. Influence of the microstructural properties and process variables P. Botella a , A. Corma a,∗ , J.M. López Nieto a , S. Valencia a , M.E. Lucas b , M. Sergio b a
Instituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Avda. de los Naranjos s/n, 46022 Valencia, Spain b Laboratorio de Fisicoqu´ımica de Superficies, Facultad de Qu´ımica, Montevideo, Uruguay
Received 23 November 1999; received in revised form 4 February 2000; accepted 6 February 2000
Abstract H-beta zeolite has been tested for the hydration of dihydromyrcene (DHM) to produce dihydromyrcenol (DHM-OH). By using a non-protic co-solvent like acetone, in order to work under a single phase system, reasonable conversion with excellent selectivity to DHM-OH is obtained, although some double bond isomerization of the terpene is also taking place. The reaction is not controlled by intrapore diffusion, and samples with different crystal sizes gave similar activities and selectivities. However, defect free beta samples with high hydrophobic character are preferred for the reaction due to their improved adsorption properties for non-polar molecules. Working with those materials more than 50% yield of DHM-OH was obtained with a selectivity close to 100%. The influence of zeolite hydrophobicity was further proved in a two-phase liquid reaction system. In such conditions, best results were achieved with the most hydrophobic catalysts. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydration; Terpenes; Dihydromyrcene; Dihydromyrcenol; H-beta zeolite
1. Introduction Terpene alcohols and esters are two important groups of fine chemicals used in flavors and perfumes [1]. Among them, dihydromyrcenol (DHM-OH, 2,6-dimethyl-7-octen-2-ol), produced from dihydromyrcene or citronellene (DHM, 3,7-dimethyl-1,6octadiene), is extensively used, due to its excellent stability and powerful lime-like aroma. The most simple pathway for the synthesis of this flavor is the direct hydration of DHM. However, the reaction is extremely unfavorable because the decrease in the ∗ Corresponding author. Tel.: +34-96-3877-801; fax: +34-96-3877-809. E-mail address: [email protected] (A. Corma).
free energy is very small [2,3]. The industrial-scale production involves the use of a concentrated solution of a mineral acid, typically sulfuric acid [4]. In this way, DHM is directly hydrated at low temperature (273–278 K), with high yield and selectivity to DHM-OH, but also produces dilute sulfuric acid as a by-product stream. Disposal of the waste sulfuric acid introduces some environmental problems (corrosion, toxicity) and economical inconveniences (difficult reuse) and, therefore, it would certainly be of interest to develop an alternative process for this synthesis. Davey et al. [5] have recently patented the process of transesterification of dihydromyrcenyl acetate with 4-tert-butylcyclohexanol using sodium methoxide as catalyst to produce dihydromyrcenol and 4-tert-butylcyclohexyl acetate. The system over-
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comes the problem of dilute sulfuric acid waste, but it involves a two-steps process. With respect to alternative acid catalysts Keggin-type heteropolyacids, e.g. H3 PW12 O40 , have been tested for the hydration of DHM [6], but the DHM-OH yields were low. Better results have been presented by Nigam et al. [7] for the hydration of DHM and other terpenes in a ternary azeotropic system containing the hydrocarbon, water and a solvent (methanol or acetic acid), using a cation exchange resin as catalyst. Nevertheless, this work has not been further developed. Zeolites have also shown great utility as solid acid catalysts [8], because of their high activity, non-hazardous character and easy recycling, besides potential shape selective properties. The application of zeolites to the hydration of different olefins [2,3] is not surprising. Thus, Asashi Chemicals has recently industrialized the liquid-phase production of cyclohexanol from cyclohexene using a zeolite as hydration catalyst [9]. Moreover, zeolites have been found useful for the hydration of different terpenes [10–16]. Thus, Nomura et al. have found some activity with Mordenite for the hydration of DHM. However, the yields and selectivities obtained were rather low. In this paper, we report on the application of H-beta for the direct hydration of DHM to DHM-OH and the influence of the physico-chemical properties of the catalyst, i.e. Si/Al framework molar ratio, hydrophilic/hydrophobic character and the crystallite size and outer surface, on the catalytic activity and selectivity to DHM-OH. For this purpose, two types of heterogeneous systems have been considered a biphasic water/terpene system and a homogeneous solution of the reagents using a co-solvent. In the last case, the effect of reaction conditions, i.e. temperature, contact time and water/terpene weight ratio, on the catalytic performance has been studied.
series A were synthesized hydrothermally at 413 K in basic medium within PTFE-lined stainless steel autoclaves under rotation, using amorphous silica (Aerosil 200, Degussa) as silica source. The synthesis of samples A2, A4 and A5 was carried out in the presence of alkali cations according to a reported procedure [17]. Samples A1 and A3 are nanocrystalline beta zeolites synthesized in an alkali-free basic medium [18]. Beta zeolites of the B series were prepared in a similar way but using tetraethylorthosilicate (TEOS, Aldrich) as silica source in a fluoride media at near neutral pH, according to a previously reported procedure [19]. Al content in zeolites was determined by atomic absorption spectrophotometry (Varian spectrAA-10 Plus). Crystallinity was measured by powder X-ray diffraction, using a Phillips PW1710 diffractometer with Cu Ka radiation, and the intensity of the peaks in the diffractograms was compared with that of a highly crystalline standard sample. Acidity of zeolites was established by the standard pyridine adsorption-desorption method [20]. Surface area of the zeolite catalysts was calculated by means of the BET/BJH methodology from the adsorption/desorption isotherm of N2 performed at 77 K in a Micromeritics ASAP 2000 instrument. Crystal size was determined from the SEM images obtained in a JEOL 6300 scanning electron microscope. The most relevant physicochemical properties of these A and B series are given in Tables 1 and 2, respectively. 27 Al MAS NMR spectra were recorded on a Varian VXR 400S WB spectrometer at an 27 Al frequency of 104.214 MHz and a spinning rate of 7 kHz with a 10◦ pulse length of 0.5 s and a recycle delay of 0.5 s. The 27 Al chemical shifts are reported relative to an Al(H2 O)6 3+ solution. 2.2. Reagents and reaction procedures
2. Experimental 2.1. Catalysts Several H-beta zeolites have been tested as catalysts for the hydration of dihydromyrcene. A commercial H-beta was provided by P.Q. Industries (Sample CP-811), and the rest of the beta samples (Series A, B) were prepared in our laboratory. Samples of the
Dihydromyrcene (DHM, mixture of isomers with 92 wt.% purity — the rest are double bond isomers of DHM) and reference samples of dihydromyrcenol (DHM-OH) and its isomer 1-(3,3-dimethylcyclohexyl)ethanol (DMCHE) were kindly provided by Acedesa. Experiments were made in a 25 ml three-necked round bottom flask at 329 K, under magnetic stirring, connected to a reflux cooler system at 273–275 K. Products were analyzed by GC in a Varian 3350 in-
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Table 1 Physicochemical characteristics of commercial and synthesized (in a basic medium, A-series) H-beta zeolites Sample
CP-811 A1 A2 A3 A4 A5 a b
Si/Ala
13 7.5 13 16 27 40
Al/(Al+Si)a
0.0714 0.1176 0.0714 0.0588 0.0357 0.0245
Area BET (m2 g−1 )
570 722 549 532 548 581
Crystallinity (%)
71 67 72 73 71 66
Crystal size (m)
0.15 0.01 0.20 0.05 0.20 0.20
Acidity (mol py)b Brönsted
Lewis
523 K
623 K
673 K
523 K
623 K
673 K
82 34 33 63 22 27
55 15 15 49 14 11
33 6 7 28 8 9
89 63 32 38 22 20
73 51 26 38 18 18
73 51 25 38 16 18
Molar ratio. Determined from the infrared spectra of adsorbed pyridine after desorption at 523, 623 and 673 K.
strument equipped with a 25 m capillary cross-linked 5% phenylmethylsilicone column and a FID detector, and by mass spectrometry in a Varian Saturn II GC-MS model working with a Varian Star 3400 gas chromatograph. Nitrobenzene (Aldrich) was used as internal standard. Comparing the GC-MS results with those of reference samples the different compounds could be identified. Two different reaction procedures have been used for the direct hydration of dihydromyrcene 2.2.1. Hydration in a homogeneous dihydromyrcenewater-co-solvent solution In a typical reaction 0.50 g of catalyst were charged together with 0.55 g (4 mmol) of DHM, 10 ml of co-solvent and 0.02 g of nitrobenzene in a 25 ml batch reactor. When the mixture was at 329 K, which is the boiling point of acetone, 2 ml of water were added.
Samples were taken at regular intervals during 24 h and analyzed. Amounts of catalyst and water were varied when necessary to provide different catalyst concentration and water/terpene weight ratios. 2.2.2. Hydration in a two-phase water/terpene system First, 0.5 g of catalysts and 0.55 g of DHM were introduced in a 25 ml batch reactor with 0.03 g of nitrobenzene. When the mixture was at 332 K, 3 ml of water were added under vigorous stirring. After 24 h the reaction was stopped by cooling the reactor. Then 10 ml of co-solvent were added in order to obtain a homogeneous sample. At the end of the reaction the catalyst was filtered and recovered. Carbon determination in the used catalyst allowed us to quantify the non-extractable organic deposit by thermogravimetric and C-elemental analysis performed, respectively, in a NEST instru-
Table 2 Physicochemical characteristics of H-beta zeolites prepared in a fluoride media (B-series) Sample
B1 B2 B3 B4 B5 B6
Si/Ala
15 27 53 93 183 >10000 a b
Al/(Al+Si)a
0.0625 0.0357 0.0185 0.0106 0.0054 <0.0001
Area BET (m2 g−1 )
519 503 452 463 457 457
Crystallinity (%)
85 97 98 100 100 100
Crystal size (m)
0.40 0.25 0.40 0.25 0.25 0.40
Acidity (mol py)b Brönsted
Lewis
523 K
623 K
673 K
523 K
623 K
673 K
45 36 19 11 6
22 29 9 10 4 ND
14 5 4 2 0
22 27 7 5 2
19 27 4 4 2 ND
14 20 4 2 0
Molar ratio. Determined from the infrared spectra of adsorbed pyridine after desorption at 523, 623 and 673 K.
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ment using calcined Kaolin as reference material and in a FISONS EA 1108 CHSN-O apparatus. Although its percentage was usually very small, it was considered to calculate the DHM mass balance. Following this procedure, it has been possible to obtain mass balances ≥98% when the reaction was carried out in an acetone solution, and ≥90% in the experiments performed in a biphasic water/terpene system.
3. Results and discussion During the hydration of DHM on beta zeolite using acetone as co-solvent, other products besides DHM-OH have also been found. These are isomers of DHM, DMCHE and very small amounts of other alcohols. Both, the isomers of DHM and DHM-OH appear as primary products, while DMCHE and other alcohols detected are secondary products (Figs. 1 and 2). Their formation can be explained from reactions given in Scheme 1. Indeed, DHM-OH (2) would be formed by the nucleophilic attack of H2 O on the tertiary carbocation 1a following Markovnikov’s rule. Moreover, 1a can isomerize to a secondary carbocation which attacks the terminal double bond of the DHM molecule to produce its cyclization, giving 1b and carvomenthol (3). However, this molecule was not found in our case in the reaction products. On the other hand, the less favored non-substituted double bond could also be protonated giving the correspond-
Fig. 1. Yield of primary products obtained in the hydration of DHM over CP-811 zeolite in acetone solution. Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio of 1; water/DHM weight ratio of 4. Symbols: DHM-OH (䉱); DHM double bond isomers (䊉).
Fig. 2. Yield of secondary products obtained in the hydration of DHM over A3 zeolite in acetone solution. Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio of 1; water/DHM weight ratio of 4. Symbols: DMCHE (䉱); unidentified terpenic alcohol (䊏).
ing secondary carbocation that cyclates yielding the observed DMCHE (4). Finally, we have observed double bond isomerization giving a series of isomers, some of which were hydrated in a consecutive step. Nevertheless, it can be said that the catalyst is
Scheme 1. Acid catalyzed hydration of dihydromyrcene. 1: Dihydromyrcene; 2: dihydromyrcenol; 3: carvomenthol; 4: 1-(3,3-dimethylcyclohexyl)-ethanol.
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Table 3 Catalytic results obtained for the direct hydration of DHM in acetone solution on H-beta (A series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
A1 A2 A3 A4 A5
7.5 13 16 27 40
21.7 61.4 67.0 55.1 52.3
17.6 48.5 46.2 48.5 49.9
100 98 94 100 100
a
Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=4. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
highly selective to DHM-OH in the sense that this is practically the only alcohol observed. In preliminary experiments we have tested that by stirring the reaction at 700–800 rpm and using a catalyst particle size <250 m, no control by external or internal diffusion exists. Nevertheless, in the case of zeolite catalyst it is important to study possible pore diffusion limitations. This can be done by decreasing the crystallite size of the zeolite while keeping the same framework composition. To do this, two samples with similar Si/Al ratios, i.e. 13 and 16, but with different crystal sizes, 0.20 and 0.05 m (Samples A2 and A3, respectively), were used as catalysts. They both showed similar activity and selectivity (Table 3) and, consequently, we conclude that the reaction is not controlled by intrapore diffusion. In a second step, and in order to optimize the reaction conditions, we have studied the influence of
temperature, H2 O/DHM weight ratio, and nature of the co-solvent. It is possible by lowering the reaction temperature (Table 4) to avoid the formation of DHM isomers, although this is at the expense of strongly reducing the yield of DHM-OH. The water/DHM ratio has an important impact on the conversion of DHM, which rises when increasing such ratio, while it has no influence on the selectivity to DHM-OH (Table 4). In a first approach, we have avoided to work in a three-phase system which could involve some mass transfer limitations. To do this, the following co-solvents were tested: acetonitrile, butanone, MTBE, isopropanol and acetone. We were expecting with this large variety of solvents to be able to study the influence of proticity and dielectric constant in the hydration of DHM. However, at the concentration levels used here, a single liquid phase could only be obtained with 2-propanol and acetone. Therefore,
Table 4 Influence of the process variables on the catalytic behavior of commercial H-beta CP-811 for the hydration of DHM in an acetone solutiona Process variable
Temperature (K)
TOS (h)
Water/DHM ratio (wt/wt)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
Temperature
313 329
24 24
2 2
13.5 42.9
13.5 31.7
100 98
Water/DHM ratio
329 329 329 329
24 24 24 24
6 4 2 1
55.0 49.3 42.9 38.6
38.1 37.3 31.7 25.0
98 98 98 97
329 329
6 6
2 2
16.0 13.4d
12.0 10.5
98 100
Co-solvent Acetone 2-Propanol a
Experimental conditions: catalyst/DHM weight ratio=1. Hydration and double bond isomerization have been considered. c Coke formation is not considered. d Hydration and double bond isomerization have been considered and also double bond alkylation. b
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further studies were only performed with these two solvents. The results presented in Table 4 show that better results were obtained when using acetone. This is not surprising if one takes into account that isopropanol can compete with water for adsorption on the acid sites. While it can be expected that water has a higher adsorption constant than isopropanol, one has to take into account that the concentration of the alcohol is about five times higher than that of water. It appears then that non protic co-solvents can be more adequate for carrying out the hydration of DHM. 3.1. Influence of the zeolite composition Since the hydration reaction is catalyzed by Brönsted acid sites, it is of interest to optimize this from the point of view of the total number as well as from the acid strength of the sites. This can be done by looking at the influence of the framework Si/Al ratio. Upon increasing this, the total number of Brönsted acid sites decreases while the acid strength of the remaining increases up to the point when all the framework Al atoms supporting the acid sites are completely isolated [21]. By a classical synthesis of beta in OH− media, samples with a Si/Al range between 7.5 and 40 were prepared (Table 1). It can be seen that although sample A1 with a Si/Al ratio of 7.5 should have the maximum number of Brönsted acid sites, the amount detected by pyridine adsorption-desorption was very close to that of A2 sample, which has a much higher Si/Al ratio. The reason for this is the lower stability of A1, which is responsible for the higher dealumination observed [22]. This was demonstrated by 27 Al MAS NMR (Fig. 3), which shows that while no extra framework Al (EFAL) is present in the as-synthesized sample, a large peak at ∼0 ppm associated to EFAL is present in the calcined form of the zeolite A1. In agreement with this, A1 sample has a much larger amount of Lewis acidity (Table 1) than any other zeolite sample, and this Lewis acidity is associated to the presence of highly dispersed EFAL. From the point of view of the global catalytic activity (Table 3), a maximum was observed for sample A3 with a Si/Al ratio of 16, and the activity behavior runs parallel to that of the Brönsted acidity, except for sample A1. In order to explain this, we assume that the presence of large amounts of EFAL in sample
Fig. 3. 27 Al MAS NMR spectra of catalyst A1. The peak at ∼0 ppm is associated with the non-framework aluminum present in sample.
A1 may partially block the pores of beta and/or can change the adsorption properties of the sample. It is remarkable that while in all cases DHM-OH is practically the only alcohol observed, the selectivity to DHM isomers decreases when increasing the Si/Al ratio of the zeolite, and the total yield of DHM-OH increases. This observation is not surprising, if one takes into account that increasing the framework Si/Al ratio, not only has an effect on the acidity, but also on the adsorption properties of the zeolite. In a reaction system that involves reactants with different polarities, as in the present case, the hydrophobicity/hydrophilicity of the zeolite should be an important variable. If the polarity of the zeolite is responsible for variations in the yield of DHM-OH, then a more hydrophobic beta (higher Si/Al ratio) than those reported in Table 3 would be desirable to maximize the formation of DHM-OH. The classical synthesis procedure in OH− media to prepare the beta zeolites (A series) is not the most appropriate in this case, since it involves the formation of a large number of internal defects or silanols groups, leading to a higher hydrophilic character. A synthesis route that allows the preparation of beta zeolites with less structural defects involves the use of F− as mineralizer [19,23]. Following this synthesis procedure, a series of beta samples within a large range of framework Si/Al ratios were prepared, and their characteristics are given in Table 2 (B series). When their catalytic activity for the hydration of DHM was
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Table 5 Catalytic results obtained for the direct hydration of DHM in an acetone solution on H-beta (B series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
B1 B2 B3 B4 B5 B6
15 27 53 93 183 >10000
61.4 64.6 51.9 38.9 35.7 0
51.3 59.1 50.5 38.7 35.2 0
98 98 100 100 100 0
a
Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=4. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
studied (Table 5), it can be seen that a higher yield of DHM-OH can be obtained regardless of the higher or lower acidity of these samples in comparison with the A series. Moreover, the maximum yield of DHM-OH is obtained at a lower framework Si/Al ratio in the B than in A series of beta zeolites, in good agreement with the higher hydrophobicity of the former. Then, it can be concluded that in the case of the hydration of DHM which involves reactants with different polarities, even more important than the global acidity of the catalysts are their adsorption properties, and more specifically the hydrophobicity–hydrophilicity of the material. By controlling these properties, it is possible to achieve more than 50% yield of DHM-OH with a selectivity close to 100%. When this conclusion was reached, we carried out the hydration of DHM with H2 O without a co-solvent and working, therefore, in a three-phase system. Obviously, here the hydrophobic–hydrophilic properties of the catalyst will be even more critical as the
zeolite has to allow the adsorption of the two reactants from a two liquid phase system formed by H2 O and the terpene. Under these conditions (Table 6) zeolites of the B series are the better catalysts and it is interesting to notice that the optimum yields are obtained with even more hydrophobic samples (B5, Si/Al=183). Nevertheless, the final yield of DHM-OH is much lower than when using a co-solvent. In conclusion, we can say that beta zeolite is a good catalyst for performing the hydration of DHM when using a co-solvent in order to work under a single liquid phase system. Non-protic co-solvents work better than protic ones presumably, since the last ones compete with water for adsorption on Brönsted acid sites. Under these conditions, reasonable conversion with excellent selectivity to DHM-OH is obtained. The hydrophobicity of the zeolite plays an important role in the catalytic performance, and hydrophobic zeolites containing no internal silanol groups are preferred. Thus, a sample with a Si/Al ratio of 27 gave the highest yield of DHM-OH with 98% selectivity. The
Table 6 Catalytic results obtained for the direct hydration of DHM in a two-phase water/terpene system on H-beta (A and B series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
A2 B1 B2 B3 B4 B5 B6
13 15 27 53 93 183 >10000
25.3 18.5 20.7 23.6 21.9 22.5 0
7.0 5.5 10.8 14.9 16.3 17.2 0
75.4 61.2 83 100 100 100 0
a
Experimental conditions: TOS= 24 h; T=332 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=6. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
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influence of zeolite hydrophobicity was further proved by working in a two-phase liquid reaction system. Under these conditions the most hydrophobic zeolites behave better. Nevertheless, the yields to DHM-OH obtained in a two-phase liquid system were lower even if selectivity was 100%. Acknowledgements The authors thank the Comisión Interministerial de Ciencia y Tecnolog´ıa, CICYT, in Spain (Project MAT 97-0561) for financial support. M.E. Lucas also thanks the Minister of Education of Uruguay for a scholarship. References [1] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, 4th Edition, Vol. 23, Wiley, New York, 1992. [2] Y. Izumi, Catal. Today 33 (1997) 371, and references cited within. [3] F. Fajula, R. Ibarra, F. Figueras, C. Gueguen, J. Catal. 89 (1984) 60. [4] J. Ibarcq, B. Lahourcade, French Patent, FR 2597861 A1 (1987). [5] P.N. Davey, C.D. Richardson, C.D. Newman, B.P. Hart, Eur. Pat. Appl. EP 0784043 A1 (1997).
[6] I.V. Kozhevnikov, A. Sinnema, A.J.A. Van Der Weerdt, H. van Bekkum, J. Mol. Catal. A: Chem. 120 (1997) 63. [7] S.C. Nigam, S.N. Bannore, H.N. Subbarao, S. Dev, Proc. 11th Int. Cong. Essent. Oils, Fragrances Flavours 5 (1989) 113. [8] A. Corma, Chem. Rev. 95 (1995) 559. [9] M. Misono, T. Inui, Catal Today, 51 (1999) 369, and references cited within. [10] J.C. van Der Waal, H. van Bekkum, J.M. Vital, J. Mol. Catal. A: Chem. 105 (1996) 185. [11] H. Valente, J. Vital, Stud. Surf. Sci. Catal. 108 (1997) 555. [12] M. Nomura, T. Tada, T. Inoue, Y. Fujihara, Chem. Exp. 8 (1993) 689. [13] M. Nomura, Y. Fujihara, H. Takata, T. Hirokawa, A. Yamada, Nippon Kagaku Kaishi 1 (1992) 63; Chem. Abst. 116:129268. [14] M. Nomura, T. Inoue, Y. Fujihara, Nippon Kagaku Kaishi 4 (1992) 388; Chem. Abst. 117:90513. [15] M. Nomura, T. Hamada, T. Inoue, Y. Fujihara, Nippon Kagaku Kaishi 6 (1992) 657; Chem. Abst. 117:90515. [16] J.Q. Yu, P. Zhou, S.D. Xiao, Chin. J. Chem. 13 (1995) 280. [17] A. Corma, V. Gómez, A. Mart´ınez, Appl. Catal. A: General 119 (1994) 83. [18] M.A. Camblor, A. Corma, A. Mifsud, J. Pérez-Pariente, S. Valencia, Stud. Surf. Sci. Catal. 105 (1997) 341. [19] M.A. Camblor, A. Corma, S. Valencia, J. Mater. Chem. 8 (1998) 2137. [20] C.A. Emeis, J. Catal. 141 (1993) 347. [21] D. Barthomeuf, Mater. Chem. Phys. 17 (1987) 49. [22] M.A. Camblor, A. Corma, S. Valencia, Microporous Mesoporous Mater. 25 (1998) 59. [23] J.L. Guth, H. Kessler, R. Wey, Stud. Surf. Sci. Catal. 28 (1986) 121.
Selective hydration of dihydromyrcene to dihydromyrcenol over H-beta zeolite. Influence of the microstructural properties and process variables P. Botella a , A. Corma a,∗ , J.M. López Nieto a , S. Valencia a , M.E. Lucas b , M. Sergio b a
Instituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Avda. de los Naranjos s/n, 46022 Valencia, Spain b Laboratorio de Fisicoqu´ımica de Superficies, Facultad de Qu´ımica, Montevideo, Uruguay
Received 23 November 1999; received in revised form 4 February 2000; accepted 6 February 2000
Abstract H-beta zeolite has been tested for the hydration of dihydromyrcene (DHM) to produce dihydromyrcenol (DHM-OH). By using a non-protic co-solvent like acetone, in order to work under a single phase system, reasonable conversion with excellent selectivity to DHM-OH is obtained, although some double bond isomerization of the terpene is also taking place. The reaction is not controlled by intrapore diffusion, and samples with different crystal sizes gave similar activities and selectivities. However, defect free beta samples with high hydrophobic character are preferred for the reaction due to their improved adsorption properties for non-polar molecules. Working with those materials more than 50% yield of DHM-OH was obtained with a selectivity close to 100%. The influence of zeolite hydrophobicity was further proved in a two-phase liquid reaction system. In such conditions, best results were achieved with the most hydrophobic catalysts. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydration; Terpenes; Dihydromyrcene; Dihydromyrcenol; H-beta zeolite
1. Introduction Terpene alcohols and esters are two important groups of fine chemicals used in flavors and perfumes [1]. Among them, dihydromyrcenol (DHM-OH, 2,6-dimethyl-7-octen-2-ol), produced from dihydromyrcene or citronellene (DHM, 3,7-dimethyl-1,6octadiene), is extensively used, due to its excellent stability and powerful lime-like aroma. The most simple pathway for the synthesis of this flavor is the direct hydration of DHM. However, the reaction is extremely unfavorable because the decrease in the ∗ Corresponding author. Tel.: +34-96-3877-801; fax: +34-96-3877-809. E-mail address: [email protected] (A. Corma).
free energy is very small [2,3]. The industrial-scale production involves the use of a concentrated solution of a mineral acid, typically sulfuric acid [4]. In this way, DHM is directly hydrated at low temperature (273–278 K), with high yield and selectivity to DHM-OH, but also produces dilute sulfuric acid as a by-product stream. Disposal of the waste sulfuric acid introduces some environmental problems (corrosion, toxicity) and economical inconveniences (difficult reuse) and, therefore, it would certainly be of interest to develop an alternative process for this synthesis. Davey et al. [5] have recently patented the process of transesterification of dihydromyrcenyl acetate with 4-tert-butylcyclohexanol using sodium methoxide as catalyst to produce dihydromyrcenol and 4-tert-butylcyclohexyl acetate. The system over-
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comes the problem of dilute sulfuric acid waste, but it involves a two-steps process. With respect to alternative acid catalysts Keggin-type heteropolyacids, e.g. H3 PW12 O40 , have been tested for the hydration of DHM [6], but the DHM-OH yields were low. Better results have been presented by Nigam et al. [7] for the hydration of DHM and other terpenes in a ternary azeotropic system containing the hydrocarbon, water and a solvent (methanol or acetic acid), using a cation exchange resin as catalyst. Nevertheless, this work has not been further developed. Zeolites have also shown great utility as solid acid catalysts [8], because of their high activity, non-hazardous character and easy recycling, besides potential shape selective properties. The application of zeolites to the hydration of different olefins [2,3] is not surprising. Thus, Asashi Chemicals has recently industrialized the liquid-phase production of cyclohexanol from cyclohexene using a zeolite as hydration catalyst [9]. Moreover, zeolites have been found useful for the hydration of different terpenes [10–16]. Thus, Nomura et al. have found some activity with Mordenite for the hydration of DHM. However, the yields and selectivities obtained were rather low. In this paper, we report on the application of H-beta for the direct hydration of DHM to DHM-OH and the influence of the physico-chemical properties of the catalyst, i.e. Si/Al framework molar ratio, hydrophilic/hydrophobic character and the crystallite size and outer surface, on the catalytic activity and selectivity to DHM-OH. For this purpose, two types of heterogeneous systems have been considered a biphasic water/terpene system and a homogeneous solution of the reagents using a co-solvent. In the last case, the effect of reaction conditions, i.e. temperature, contact time and water/terpene weight ratio, on the catalytic performance has been studied.
series A were synthesized hydrothermally at 413 K in basic medium within PTFE-lined stainless steel autoclaves under rotation, using amorphous silica (Aerosil 200, Degussa) as silica source. The synthesis of samples A2, A4 and A5 was carried out in the presence of alkali cations according to a reported procedure [17]. Samples A1 and A3 are nanocrystalline beta zeolites synthesized in an alkali-free basic medium [18]. Beta zeolites of the B series were prepared in a similar way but using tetraethylorthosilicate (TEOS, Aldrich) as silica source in a fluoride media at near neutral pH, according to a previously reported procedure [19]. Al content in zeolites was determined by atomic absorption spectrophotometry (Varian spectrAA-10 Plus). Crystallinity was measured by powder X-ray diffraction, using a Phillips PW1710 diffractometer with Cu Ka radiation, and the intensity of the peaks in the diffractograms was compared with that of a highly crystalline standard sample. Acidity of zeolites was established by the standard pyridine adsorption-desorption method [20]. Surface area of the zeolite catalysts was calculated by means of the BET/BJH methodology from the adsorption/desorption isotherm of N2 performed at 77 K in a Micromeritics ASAP 2000 instrument. Crystal size was determined from the SEM images obtained in a JEOL 6300 scanning electron microscope. The most relevant physicochemical properties of these A and B series are given in Tables 1 and 2, respectively. 27 Al MAS NMR spectra were recorded on a Varian VXR 400S WB spectrometer at an 27 Al frequency of 104.214 MHz and a spinning rate of 7 kHz with a 10◦ pulse length of 0.5 s and a recycle delay of 0.5 s. The 27 Al chemical shifts are reported relative to an Al(H2 O)6 3+ solution. 2.2. Reagents and reaction procedures
2. Experimental 2.1. Catalysts Several H-beta zeolites have been tested as catalysts for the hydration of dihydromyrcene. A commercial H-beta was provided by P.Q. Industries (Sample CP-811), and the rest of the beta samples (Series A, B) were prepared in our laboratory. Samples of the
Dihydromyrcene (DHM, mixture of isomers with 92 wt.% purity — the rest are double bond isomers of DHM) and reference samples of dihydromyrcenol (DHM-OH) and its isomer 1-(3,3-dimethylcyclohexyl)ethanol (DMCHE) were kindly provided by Acedesa. Experiments were made in a 25 ml three-necked round bottom flask at 329 K, under magnetic stirring, connected to a reflux cooler system at 273–275 K. Products were analyzed by GC in a Varian 3350 in-
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Table 1 Physicochemical characteristics of commercial and synthesized (in a basic medium, A-series) H-beta zeolites Sample
CP-811 A1 A2 A3 A4 A5 a b
Si/Ala
13 7.5 13 16 27 40
Al/(Al+Si)a
0.0714 0.1176 0.0714 0.0588 0.0357 0.0245
Area BET (m2 g−1 )
570 722 549 532 548 581
Crystallinity (%)
71 67 72 73 71 66
Crystal size (m)
0.15 0.01 0.20 0.05 0.20 0.20
Acidity (mol py)b Brönsted
Lewis
523 K
623 K
673 K
523 K
623 K
673 K
82 34 33 63 22 27
55 15 15 49 14 11
33 6 7 28 8 9
89 63 32 38 22 20
73 51 26 38 18 18
73 51 25 38 16 18
Molar ratio. Determined from the infrared spectra of adsorbed pyridine after desorption at 523, 623 and 673 K.
strument equipped with a 25 m capillary cross-linked 5% phenylmethylsilicone column and a FID detector, and by mass spectrometry in a Varian Saturn II GC-MS model working with a Varian Star 3400 gas chromatograph. Nitrobenzene (Aldrich) was used as internal standard. Comparing the GC-MS results with those of reference samples the different compounds could be identified. Two different reaction procedures have been used for the direct hydration of dihydromyrcene 2.2.1. Hydration in a homogeneous dihydromyrcenewater-co-solvent solution In a typical reaction 0.50 g of catalyst were charged together with 0.55 g (4 mmol) of DHM, 10 ml of co-solvent and 0.02 g of nitrobenzene in a 25 ml batch reactor. When the mixture was at 329 K, which is the boiling point of acetone, 2 ml of water were added.
Samples were taken at regular intervals during 24 h and analyzed. Amounts of catalyst and water were varied when necessary to provide different catalyst concentration and water/terpene weight ratios. 2.2.2. Hydration in a two-phase water/terpene system First, 0.5 g of catalysts and 0.55 g of DHM were introduced in a 25 ml batch reactor with 0.03 g of nitrobenzene. When the mixture was at 332 K, 3 ml of water were added under vigorous stirring. After 24 h the reaction was stopped by cooling the reactor. Then 10 ml of co-solvent were added in order to obtain a homogeneous sample. At the end of the reaction the catalyst was filtered and recovered. Carbon determination in the used catalyst allowed us to quantify the non-extractable organic deposit by thermogravimetric and C-elemental analysis performed, respectively, in a NEST instru-
Table 2 Physicochemical characteristics of H-beta zeolites prepared in a fluoride media (B-series) Sample
B1 B2 B3 B4 B5 B6
Si/Ala
15 27 53 93 183 >10000 a b
Al/(Al+Si)a
0.0625 0.0357 0.0185 0.0106 0.0054 <0.0001
Area BET (m2 g−1 )
519 503 452 463 457 457
Crystallinity (%)
85 97 98 100 100 100
Crystal size (m)
0.40 0.25 0.40 0.25 0.25 0.40
Acidity (mol py)b Brönsted
Lewis
523 K
623 K
673 K
523 K
623 K
673 K
45 36 19 11 6
22 29 9 10 4 ND
14 5 4 2 0
22 27 7 5 2
19 27 4 4 2 ND
14 20 4 2 0
Molar ratio. Determined from the infrared spectra of adsorbed pyridine after desorption at 523, 623 and 673 K.
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ment using calcined Kaolin as reference material and in a FISONS EA 1108 CHSN-O apparatus. Although its percentage was usually very small, it was considered to calculate the DHM mass balance. Following this procedure, it has been possible to obtain mass balances ≥98% when the reaction was carried out in an acetone solution, and ≥90% in the experiments performed in a biphasic water/terpene system.
3. Results and discussion During the hydration of DHM on beta zeolite using acetone as co-solvent, other products besides DHM-OH have also been found. These are isomers of DHM, DMCHE and very small amounts of other alcohols. Both, the isomers of DHM and DHM-OH appear as primary products, while DMCHE and other alcohols detected are secondary products (Figs. 1 and 2). Their formation can be explained from reactions given in Scheme 1. Indeed, DHM-OH (2) would be formed by the nucleophilic attack of H2 O on the tertiary carbocation 1a following Markovnikov’s rule. Moreover, 1a can isomerize to a secondary carbocation which attacks the terminal double bond of the DHM molecule to produce its cyclization, giving 1b and carvomenthol (3). However, this molecule was not found in our case in the reaction products. On the other hand, the less favored non-substituted double bond could also be protonated giving the correspond-
Fig. 1. Yield of primary products obtained in the hydration of DHM over CP-811 zeolite in acetone solution. Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio of 1; water/DHM weight ratio of 4. Symbols: DHM-OH (䉱); DHM double bond isomers (䊉).
Fig. 2. Yield of secondary products obtained in the hydration of DHM over A3 zeolite in acetone solution. Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio of 1; water/DHM weight ratio of 4. Symbols: DMCHE (䉱); unidentified terpenic alcohol (䊏).
ing secondary carbocation that cyclates yielding the observed DMCHE (4). Finally, we have observed double bond isomerization giving a series of isomers, some of which were hydrated in a consecutive step. Nevertheless, it can be said that the catalyst is
Scheme 1. Acid catalyzed hydration of dihydromyrcene. 1: Dihydromyrcene; 2: dihydromyrcenol; 3: carvomenthol; 4: 1-(3,3-dimethylcyclohexyl)-ethanol.
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Table 3 Catalytic results obtained for the direct hydration of DHM in acetone solution on H-beta (A series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
A1 A2 A3 A4 A5
7.5 13 16 27 40
21.7 61.4 67.0 55.1 52.3
17.6 48.5 46.2 48.5 49.9
100 98 94 100 100
a
Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=4. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
highly selective to DHM-OH in the sense that this is practically the only alcohol observed. In preliminary experiments we have tested that by stirring the reaction at 700–800 rpm and using a catalyst particle size <250 m, no control by external or internal diffusion exists. Nevertheless, in the case of zeolite catalyst it is important to study possible pore diffusion limitations. This can be done by decreasing the crystallite size of the zeolite while keeping the same framework composition. To do this, two samples with similar Si/Al ratios, i.e. 13 and 16, but with different crystal sizes, 0.20 and 0.05 m (Samples A2 and A3, respectively), were used as catalysts. They both showed similar activity and selectivity (Table 3) and, consequently, we conclude that the reaction is not controlled by intrapore diffusion. In a second step, and in order to optimize the reaction conditions, we have studied the influence of
temperature, H2 O/DHM weight ratio, and nature of the co-solvent. It is possible by lowering the reaction temperature (Table 4) to avoid the formation of DHM isomers, although this is at the expense of strongly reducing the yield of DHM-OH. The water/DHM ratio has an important impact on the conversion of DHM, which rises when increasing such ratio, while it has no influence on the selectivity to DHM-OH (Table 4). In a first approach, we have avoided to work in a three-phase system which could involve some mass transfer limitations. To do this, the following co-solvents were tested: acetonitrile, butanone, MTBE, isopropanol and acetone. We were expecting with this large variety of solvents to be able to study the influence of proticity and dielectric constant in the hydration of DHM. However, at the concentration levels used here, a single liquid phase could only be obtained with 2-propanol and acetone. Therefore,
Table 4 Influence of the process variables on the catalytic behavior of commercial H-beta CP-811 for the hydration of DHM in an acetone solutiona Process variable
Temperature (K)
TOS (h)
Water/DHM ratio (wt/wt)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
Temperature
313 329
24 24
2 2
13.5 42.9
13.5 31.7
100 98
Water/DHM ratio
329 329 329 329
24 24 24 24
6 4 2 1
55.0 49.3 42.9 38.6
38.1 37.3 31.7 25.0
98 98 98 97
329 329
6 6
2 2
16.0 13.4d
12.0 10.5
98 100
Co-solvent Acetone 2-Propanol a
Experimental conditions: catalyst/DHM weight ratio=1. Hydration and double bond isomerization have been considered. c Coke formation is not considered. d Hydration and double bond isomerization have been considered and also double bond alkylation. b
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further studies were only performed with these two solvents. The results presented in Table 4 show that better results were obtained when using acetone. This is not surprising if one takes into account that isopropanol can compete with water for adsorption on the acid sites. While it can be expected that water has a higher adsorption constant than isopropanol, one has to take into account that the concentration of the alcohol is about five times higher than that of water. It appears then that non protic co-solvents can be more adequate for carrying out the hydration of DHM. 3.1. Influence of the zeolite composition Since the hydration reaction is catalyzed by Brönsted acid sites, it is of interest to optimize this from the point of view of the total number as well as from the acid strength of the sites. This can be done by looking at the influence of the framework Si/Al ratio. Upon increasing this, the total number of Brönsted acid sites decreases while the acid strength of the remaining increases up to the point when all the framework Al atoms supporting the acid sites are completely isolated [21]. By a classical synthesis of beta in OH− media, samples with a Si/Al range between 7.5 and 40 were prepared (Table 1). It can be seen that although sample A1 with a Si/Al ratio of 7.5 should have the maximum number of Brönsted acid sites, the amount detected by pyridine adsorption-desorption was very close to that of A2 sample, which has a much higher Si/Al ratio. The reason for this is the lower stability of A1, which is responsible for the higher dealumination observed [22]. This was demonstrated by 27 Al MAS NMR (Fig. 3), which shows that while no extra framework Al (EFAL) is present in the as-synthesized sample, a large peak at ∼0 ppm associated to EFAL is present in the calcined form of the zeolite A1. In agreement with this, A1 sample has a much larger amount of Lewis acidity (Table 1) than any other zeolite sample, and this Lewis acidity is associated to the presence of highly dispersed EFAL. From the point of view of the global catalytic activity (Table 3), a maximum was observed for sample A3 with a Si/Al ratio of 16, and the activity behavior runs parallel to that of the Brönsted acidity, except for sample A1. In order to explain this, we assume that the presence of large amounts of EFAL in sample
Fig. 3. 27 Al MAS NMR spectra of catalyst A1. The peak at ∼0 ppm is associated with the non-framework aluminum present in sample.
A1 may partially block the pores of beta and/or can change the adsorption properties of the sample. It is remarkable that while in all cases DHM-OH is practically the only alcohol observed, the selectivity to DHM isomers decreases when increasing the Si/Al ratio of the zeolite, and the total yield of DHM-OH increases. This observation is not surprising, if one takes into account that increasing the framework Si/Al ratio, not only has an effect on the acidity, but also on the adsorption properties of the zeolite. In a reaction system that involves reactants with different polarities, as in the present case, the hydrophobicity/hydrophilicity of the zeolite should be an important variable. If the polarity of the zeolite is responsible for variations in the yield of DHM-OH, then a more hydrophobic beta (higher Si/Al ratio) than those reported in Table 3 would be desirable to maximize the formation of DHM-OH. The classical synthesis procedure in OH− media to prepare the beta zeolites (A series) is not the most appropriate in this case, since it involves the formation of a large number of internal defects or silanols groups, leading to a higher hydrophilic character. A synthesis route that allows the preparation of beta zeolites with less structural defects involves the use of F− as mineralizer [19,23]. Following this synthesis procedure, a series of beta samples within a large range of framework Si/Al ratios were prepared, and their characteristics are given in Table 2 (B series). When their catalytic activity for the hydration of DHM was
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Table 5 Catalytic results obtained for the direct hydration of DHM in an acetone solution on H-beta (B series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
B1 B2 B3 B4 B5 B6
15 27 53 93 183 >10000
61.4 64.6 51.9 38.9 35.7 0
51.3 59.1 50.5 38.7 35.2 0
98 98 100 100 100 0
a
Experimental conditions: TOS=24 h; T=329 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=4. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
studied (Table 5), it can be seen that a higher yield of DHM-OH can be obtained regardless of the higher or lower acidity of these samples in comparison with the A series. Moreover, the maximum yield of DHM-OH is obtained at a lower framework Si/Al ratio in the B than in A series of beta zeolites, in good agreement with the higher hydrophobicity of the former. Then, it can be concluded that in the case of the hydration of DHM which involves reactants with different polarities, even more important than the global acidity of the catalysts are their adsorption properties, and more specifically the hydrophobicity–hydrophilicity of the material. By controlling these properties, it is possible to achieve more than 50% yield of DHM-OH with a selectivity close to 100%. When this conclusion was reached, we carried out the hydration of DHM with H2 O without a co-solvent and working, therefore, in a three-phase system. Obviously, here the hydrophobic–hydrophilic properties of the catalyst will be even more critical as the
zeolite has to allow the adsorption of the two reactants from a two liquid phase system formed by H2 O and the terpene. Under these conditions (Table 6) zeolites of the B series are the better catalysts and it is interesting to notice that the optimum yields are obtained with even more hydrophobic samples (B5, Si/Al=183). Nevertheless, the final yield of DHM-OH is much lower than when using a co-solvent. In conclusion, we can say that beta zeolite is a good catalyst for performing the hydration of DHM when using a co-solvent in order to work under a single liquid phase system. Non-protic co-solvents work better than protic ones presumably, since the last ones compete with water for adsorption on Brönsted acid sites. Under these conditions, reasonable conversion with excellent selectivity to DHM-OH is obtained. The hydrophobicity of the zeolite plays an important role in the catalytic performance, and hydrophobic zeolites containing no internal silanol groups are preferred. Thus, a sample with a Si/Al ratio of 27 gave the highest yield of DHM-OH with 98% selectivity. The
Table 6 Catalytic results obtained for the direct hydration of DHM in a two-phase water/terpene system on H-beta (A and B series) zeolitesa Sample
Si/Al ratio (molar)
DHM conversion (wt.%)b,c
DHM-OH yield (wt.%)c
DHM-OH/alcohols (molar ratio)
A2 B1 B2 B3 B4 B5 B6
13 15 27 53 93 183 >10000
25.3 18.5 20.7 23.6 21.9 22.5 0
7.0 5.5 10.8 14.9 16.3 17.2 0
75.4 61.2 83 100 100 100 0
a
Experimental conditions: TOS= 24 h; T=332 K; catalyst/DHM weight ratio=1; water/DHM weight ratio=6. Hydration and double bond isomerization have been considered. c Coke formation is not considered. b
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influence of zeolite hydrophobicity was further proved by working in a two-phase liquid reaction system. Under these conditions the most hydrophobic zeolites behave better. Nevertheless, the yields to DHM-OH obtained in a two-phase liquid system were lower even if selectivity was 100%. Acknowledgements The authors thank the Comisión Interministerial de Ciencia y Tecnolog´ıa, CICYT, in Spain (Project MAT 97-0561) for financial support. M.E. Lucas also thanks the Minister of Education of Uruguay for a scholarship. References [1] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, 4th Edition, Vol. 23, Wiley, New York, 1992. [2] Y. Izumi, Catal. Today 33 (1997) 371, and references cited within. [3] F. Fajula, R. Ibarra, F. Figueras, C. Gueguen, J. Catal. 89 (1984) 60. [4] J. Ibarcq, B. Lahourcade, French Patent, FR 2597861 A1 (1987). [5] P.N. Davey, C.D. Richardson, C.D. Newman, B.P. Hart, Eur. Pat. Appl. EP 0784043 A1 (1997).
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