- Email: [email protected]
Isomerisation of (+)citronellal over Zn(II) supported catalysts
Applied Catalysis A: General 233 (2002) 151–157
Isomerisation of (+)citronellal over Zn(II) supported catalysts C. Milone, A. Perri, A. Pistone, G. Neri, S. Galvagno∗ Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, I-98166 Messina, Italy Received 26 November 2001; received in revised form 25 February 2002; accepted 28 February 2002
Abstract A series of acid catalysts prepared by impregnation of commercial silicas with a solution of ZnBr2 have been characterised with several techniques (BET, adsorption–desorption porosimetry, scanning electron microscopy (SEM), energy dispersive analysis (EDAX), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS)). The results have shown that ZnBr2 is evenly distributed over the surface of the supports regardless of their surface properties. The catalytic surface contains two different sites: physically adsorbed ZnBr2 and Zn(II) sites, obtained from the partial decomposition of the precursor. No correlation between the surface area of the support and the amount of Zn(II) sites has been observed. The ZnBr2 /SiO2 based catalysts have been also tested in the isomerisation of (+)citronellal. The catalytic activity results have shown that the structural properties of the support have a scarce influence on the catalytic performance. On all the investigated samples, the selectivity to isopulegols is 100%, whereas the stereoselectivity to (−)isopulegol ranges between 70 and 86%. Zn(II) are the most active sites for the catalytic isomerisation while ZnBr2 has a negligible catalytic activity. However, the presence of ZnBr2 is of fundamental importance to achieve higher selectivity toward the formation of (−)isopulegol. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Isomerisation; Silica; (+)Citronellal; (−)Isopulegol; Acid catalysts; ZnBr2
1. Introduction The acid catalysed intramolecular isomerisation of (+)citronellal is the synthetic route for the production of (−)isopulegol (Scheme 1). The latter is the most interesting isomer being the precursor for the synthesis of (−)menthol, a fine chemical widely employed in the manufacture of pharmaceutical, cosmetics and other speciality products. In the industrial process for (−)menthol production (Tagasako process), the isomerisation of (+)citronellal is carried out in the presence of ZnBr2 . Under these conditions, (−)isopulegol yields up to 92% have been ∗ Corresponding author. E-mail address: [email protected] (S. Galvagno).
reported [1]. The cyclic alcohol is then separated from the reaction mixture and hydrogenated to the corresponding saturated alcohol (−)menthol. It is reported that the acid catalyst ZnBr2 is employed as aqueous solution [1], however, powdered anhydrous ZnBr2 has been found to be also very selective towards the formation of (−)isopulegol [2]. One of the main drawbacks in the use of solid ZnBr2 is the difficulty to maintain the salt under anhydrous conditions being highly hygroscopic, hence, precautions are required in the storage and handling ZnBr2 if it is used in the solid state. Therefore, the synthesis of heterogeneous catalysts that combine high stereoselectivity to (−)isopulegol and ease of handling can be a strategic tool for the optimisation of the (−)menthol synthetic process.
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 1 3 6 - 9
152
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
Scheme 1.
Cyclisation of citronellal on several solid acid catalysts has been investigated but few studies deal with the distribution of the stereoisomers formed during the reaction [2–10]. In their study on the isomerisation of (+)citronellal on mixed oxides, such as SiO2 –TiO2 , SiO2 –Al2 O3 , SiO2 –ZrO2 and Cu supported on mixed oxides catalysts, Ravasio et al. [7] suggest that the Lewis sites are the most active and stereoselective to (−)isopulegol. They report stereoselectivity values varying from 63 to 74%. Chuah et al. [8] instead do not observe changes in the stereoisomer distribution with strength and nature of the acidic sites when the reaction is carried out on several catalysts including zirconium hydroxide, phosphated zirconia, sulphated zirconia, etc. The maximum yield to (−)isopulegol was 68%. They suggest a reaction mechanism involving strong Lewis together with weak Bronsted acidity. Several studies concern also the isomerisation of (+)citronellal on silica and silica-supported catalysts. Kropp et al. [9] have studied the isomerisation of (+)citronellal on high surface area silica (S.A. = 675 m2 /g). They report a stereoselectivity to (−)isopulegol of 80% when the reaction is carried out on SiO2 , which strongly decreases to 61% after adsorption of 0.1 mmol of H3 PO4 on the solid.
The presence of H3 PO4 leads to an increase of the stereoselectivity to (+)neoisopulegol. The very high stereoselectivity value to (−)isopulegol reported on silica is noticeable and is a unique result in the literature on this solid. Other authors have reported a stereoselectivity to (−)isopulegol of 62% when the reaction is carried out on high surface area silica (S.A. = 600 m2 /g) and it increases to 73%, lowering the surface area of the oxide to 380 m2 /g [7]. On the other hand, in our previous study, we found that the stereoselectivity to (−)isopulegol on SiO2 with surface area 330 m2 /g does not exceed 60% [10]. As is clearly evidenced, literature data on the cyclisation of (+)citronellal on SiO2 is controversial. It can be argued that differences in the product distribution can be due to the presence of impurities on the SiO2 , but it cannot be ruled out that the morphology of the catalyst (surface area, pore-size and distribution) could also play a role in the stereoisomer distribution, due to a size effect during the cyclisation. We have recently investigated the catalytic activity of Lewis acids, such as ZnBr2 , ZnCl2 , Zn(NO3 )2 , FeCl3 and SnCl2 supported on SiO2 in the isomerisation of (+)citronellal. Among the salts investigated, ZnBr2 was the most effective in terms of catalytic activity and selectivity to (−)isopulegol [10]. The
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
catalytic activity and the selectivity to (−)isopulegol was found to increase with the Zn loading and the temperature of calcination. The maximum selectivity of (−)isopulegol obtained on ZnBr2 supported catalysts was 86% [10]. It is noteworthy that variance to the bulk salt ZnBr2 dispersed onto a support can be stored in air and used several times with no loss in activity and selectivity. Moreover, we have also synthesised the first bi-functional catalyst containing stereoselective isomerisation sites and hydrogenation sites for the onestep synthesis of (−)menthol from (+)citronellal [10]. In a program aimed at improving the stereoselectivity to (−)menthol in the one-step process on ZnBr2 /SiO2 catalysts, we have decided to elucidate the influence of the morphology of the catalysts in the stereoselective isomerisation of (+)citronellal. Therefore, we have carried out a systematic study of ZnBr2 /SiO2 catalysts with different surface area and mean pore diameter. Characterisations of the catalysts have been also performed in order to get information on the nature of the catalytic active sites.
2. Experimental The ZnBr2 /SiO2 samples were prepared by adding the support (SiO2 Grace XWP, grain size 100–200 mesh) to a methanol solution containing a proper amount of the Zn salt. The solvent was slowly removed by rotary evaporation at 35 ◦ C for 1 h. All catalysts were dried at 120 ◦ C for 2 h, then calcined at 350 ◦ C for 2 h, unless otherwise specified. Specific surface area and porosity data were determined by adsorption–desorption of di-nitrogen at 77 K using a Micromeritics ASAP 2010. Pore-size distributions and pore volume were derived from the desorption isotherm at P /P o ≥ 0.3. X-ray diffraction (XRD) patterns were recorded by using an Ital Structure model APD2000 X-ray diffractometer operated at 40 kV and 30 mA using Ni-filtered Cu K␣ radiation at a step scan mode (2θ step, 0.02◦ ; acquisition time, 2 s per step) with a 1◦ divergence and scatter slit and a 0.1 mm receiving slit. Powder samples were mounted on Plexiglas holders. Scanning electron microscopy (SEM) images of powder samples mounted on an aluminium holder were obtained on a JEOL JSM-5600 LV microscope
153
equipped with an EDX (Oxford) analyser. The quantitative analysis was carried out at 20 kV by using the SEMQUANT software applying the ZAF correction procedure. The Zn content was determined by Atomic Absorption Spectrometry using a Perkin-Elmer model 4000 spectrometer. A known amount of solid was dissolved in an aqueous solution of HF. The absorbance of the solution was measured at 213.9 nm. Catalytic experiments were carried out at atmospheric pressure in a 100 ml four-necked flask fitted with a reflux condenser, dropping funnel, thermocouple and a stirrer head. (+)Citronellal and solvent (cyclohexane) were commercial analytical grade products and were used without further purification. The isomerisation of (+)citronellal was carried out under N2 flow at 60 ◦ C. The catalyst (0.1–0.2 g) was added to 25 ml of solvent (cyclohexane) and then the substrate (0.1 ml) was injected through one arm of the flask. The reaction mixture was stirred at 500 rpm. The progress of the reaction was followed by sampling a sufficient number of microsamples. Chemical analyses were performed by means of GC–FID. The gas chromatographic column was a EC–WAX wide-bore capillary column (30 m, 0.53 mm i.d.) connected in series with a RTX-1 column (30 m, 0.53 mm i.d.). Preliminary runs carried out at different stirring conditions, loading and catalyst grain size have demonstrated the absence of external and internal diffusion limitations.
3. Results and discussion 3.1. Characterisation of the catalysts Table 1 shows the BET surface area (SBET ), pore volume and mean pore diameter of the ZnBr2 /SiO2 catalysts and of the parent SiO2 supports. The silicas used show a SBET ranging between 87 and 532 m2 /g and a pore-size distribution varying between 5 and 36 nm (Fig. 1a–d). The addition of ZnBr2 leads to a slight decrease of the surface area and pore volume (Table 1). The Zn loading, determined by atomic absorption spectroscopy (AAS), ranges between 1.7 and 2 wt.%. From Fig. 1a–d it can be seen that the pore-size distribution of the ZnBr2 /SiO2 catalysts shows a similar shape and
154
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
Table 1 Morphological properties of SiO2 supports and ZnBr2 /SiO2 catalysts Catalyst
S.A. (m2 /g)
Mean pore diameter (nm)
Pore volume (cm3 /g)
Zna (wt.%)
SiO2 80 ZnB80 SiO2 175 ZnB175 SiO2 360 ZnB360 SiO2 540 ZnB540
87 81 174 158 330 311 532 465
36 34 20 20 10 10 5 5
0.88 0.93 1.10 0.98 1.52 1.31 0.86 0.76
− 1.74 − 1.84 − 1.96 − 1.79
a
Determined by atomic absorption spectrofotometry.
position as that obtained on the parent supports. This indicates that the salt is evenly distributed on the surface of silicas and no preferential deposition of the precursor into the pores occurs. The decrease of the surface area and pore volume of the ZnBr2 supported
catalysts can be attributed to ZnBr2 filling up the pores of the support [10,11]. XRD analyses of the ZnBr2 supported catalysts have shown the absence of resolved reflections. Since the precursor is present in a high amount, it must be assumed that amorphous ZnBr2 is present on the surface of the support in the form of mono or multilayers [12]. SEM coupled with the elemental analysis (EDAX) of the catalysts have been also performed. SEM micrographs have shown that the morphological structure of the catalysts and of the parent supports is quite similar thus indicating that the ZnBr2 is widespread onto the surface. This conclusion has been confirmed by EDAX analysis of several regions of the samples. The quantitative analysis of zinc and bromide (Table 2) on the samples has shown that the Zn/Br atomic ratio ranges between 0.7 and 0.9, which is higher than 0.5 obtained from the analysis of a mechanical mixture of ZnBr2 and SiO2 .
Fig. 1. Pore-size distributions of the: (a) SiO2 80 (䉬), ZnB80 (䉱); (b) SiO2 175 (䉬), ZnB175 (䉱); (c) SiO2 360 (䉬), ZnB360 (䉱); (d) SiO2 540 (䉬), ZnB540 (䉱).
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
155
Table 2 Quantitative EDAX analysis of ZnBr2 /SiO2 catalysts Catalyst code
Zn (at.%)
Br (at.%)
Zn/Br (at.%/at.%)
ZnB80 ZnB175 ZnB360 ZnB540 ZnBr2 /SiO2 a
40 48 44 41 35
60 52 56 59 65
0.7 0.9 0.8 0.7 0.5
a
Mechanical mixture.
This result suggests that, on the catalysts prepared by impregnation, a partial decomposition of ZnBr2 occurs leading to a release of bromide from the solids. To obtain information on the interaction between ZnBr2 and the support, the catalysts have been washed out until bromide free (AgNO3 test) and the amount of zinc on the solid has been determined by AAS. The Zn loading of the washed sample, reported in Table 3, is almost half that of initial loading. If we assume that during the washing procedure only the weakly interacting ZnBr2 is eliminated because of the high water solubility, it is likely that the residual Zn(II) comes from the decomposition of the precursor, as suggested by the SEM–EDAX analysis which has shown the absence of bromine in the washed sample. It can be argued that the residual Zn(II) can be present as Zn oxy-hydroxide and/or Zn–(O–Si)2 , formed from the reaction between ZnBr2 and the hydroxyl group of SiO2 . In a previous study on ZnCl2 /SiO2 catalysts calcined at 120 ◦ C, Rhodes et al. [13] have reported that the precursor can be easily washed out indicating that no decomposition of the salt occurs. The catalysts of the present study have been obtained following a preparation procedure similar to that reported by Rhodes et al. [13], with the exception of the temperature of activation. In
Fig. 2. Isomerisation of (+)citronellal on ZnB360; (Tr = 60 ◦ C) (䉬) (+)citronellal, (䊏) (−)isopulegol, (䉱) (+)neoisopulegol, (䊉) (+)isoisopulegol.
our case, ZnBr2 /SiO2 catalysts have been calcined at 350 ◦ C, which has been found the optimum temperature to achieve higher selectivity to (−)isopulegol in the isomerisation of (+)citronellal [10]. Based upon the characterisation data, we can conclude that in the ZnBr2 /SiO2 catalysts, weakly adsorbed ZnBr2 and Zn(II) sites are present, almost in the same amount, on the surface of silica catalysts. 3.2. Catalytic activity The isomerisation of (+)citronellal has been carried out both on the “as prepared” ZnBr2 /SiO2 catalysts and on the “washed” samples. “Washed samples” were obtained by washing with water until bromide free. A typical course of the reaction is reported in Fig. 2. Under the experimental condition used
Table 3 Catalytic activities of the “as prepared” and “washed” ZnBr2 /SiO2 catalysts Catalyst code
As prepared Zna
ZnB80 ZnB175 ZnB360 ZnB540 a
1.74 1.84 1.96 1.79
(wt.%)
Washed catalysts Vcat (×
106
mol/g catalyst × s)
1.23 1.39 1.37 1.52
Determined by atomic absorption spectrofotometry.
Zna (wt.%)
Vcat (× 106 mol/g catalyst × s)
0.69 0.74 0.93 0.99
1.24 1.49 1.83 1.98
156
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
(+)citronellal is converted only to the isopulegol stereoisomer in all range of conversion investigated. No other reaction products were detected. The reaction products are: (−)isopulegol, (+)neoisopulegol and (+)isoisopulegol. The less thermodynamically stable stereoisomer, (+)neoisoisopulegol, is not formed under our reaction conditions. Their relative abundance is independent of the conversion levels, thus indicating that they are formed through parallel reactions (Scheme 1). The catalytic activity results, expressed as initial rate of isomerisation of (+)citronellal, of the “as prepared” ZnBr2 catalysts are reported in Table 3. All the SiO2 used as supports are only slightly active for the isomerisation of (+)citronellal. In agreement with our previous findings [14], the catalytic activity increases from 1 × 10−8 to 2 × 10−7 mol/g catalyst/s with increasing the surface area of the supports from 87 to 532 m2 /g. The selectivity towards (−)isopulegol is 60%, regardless of the surface area and the mean pore diameter of silicas; these results indicate that the morphology of the support does not influence the stereoisomer distribution in the isomerisation of (+)citronellal. The presence of ZnBr2 leads to an enhancement of the catalytic activity (Table 3). The initial rate of disappearance of (+)citronellal is quite similar for all the catalysts investigated, regardless of the surface area and mean pore diameter. Moreover, on addition of ZnBr2 an increase of the selectivity towards (−)isopulegol up to 70% has been obtained on all the investigated samples regardless of the silica used. This is a further confirmation that the morphological parameters of the solids do not influence the activity and selectivity of the reactions. It has to be pointed out that, in agreement with previous findings [10] no significant leaching of the modifier was observed under our reaction conditions. This was verified either by analysing the Zn content in the reaction mixture and on the solid. Moreover, no reaction was observed on the solvent after removing of the catalyst. In Table 3, the catalytic activity of the “washed” catalysts is also reported. Notwithstanding the decrease of the amount of zinc upon washing, the catalytic activity per gram of catalyst remains almost constant and in some cases it was found even higher than on the “as prepared” samples. These results suggest that the contribution of the weakly interacting ZnBr2 to
the overall activity is negligible. The active sites in the isomerisation of (+)citronellal seems to be mainly constituted by the residual Zn(II) sites. The slight increase of catalytic activity observed on some samples could be attributed to a further hydrolysis of the ZnBr2 salt (Table 3). It should be noted that at the Zn loadings reported in Table 3 the selectivity to (−)isopulegol remained almost constant (about 70%) on all samples regardless of the washing procedure and surface area. In a previous paper, we have reported that the catalytic activity and the selectivity toward (−)isopulegol of ZnBr2 supported on SiO2 360 (S BET = 330 m2 /g) depends on the amount of ZnBr2 . The isomerisation activity increases with the salt loading up to 7 mmol/g SiO2 (Zn (wt.%) ∼ = 18), while the selectivity to (−)isopulegol reaches a maximum value (86%) at 4 mmol/g SiO2 (Zn (wt.%) ∼ = 13) [10]. The results reported in the present study have shown that, at low salt loading (Zn (wt.%) ∼ = 2), the activity of the weakly interacting ZnBr2 is negligible. The reaction occurs mainly on the residual Zn(II) sites generated from the decomposition of the precursor. However, the selectivity to (−)isopulegol of the low loaded catalysts is only 70%, which is much lower than the selectivity obtained on the catalysts with higher Zn loading. To clarify this point, the previously investigated catalysts [10], indicated in Table 4 with the code ZnB1/SiO2 and ZnB3/SiO2 , have been washed out until bromide free and the catalytic activity in the isomerisation of (+)citronellal has been measured. As is shown in Table 4, the Zn content of the washed catalysts is much lower than the initial amount and ranges between 3 and 4.3 wt.%. This indicates that the main component of the as prepared catalysts is the weakly adsorbed ZnBr2 . Notwithstanding the lower amount of zinc, the isomerisation activity of the washed catalysts (Table 4) remains practically constant. These results are a confirmation that the contribution of the weakly adsorbed ZnBr2 to the catalytic activity is negligible, even at high loading, and the residual Zn(II) sites are the main active sites in the isomerisation of (+)citronellal. However, the elimination of ZnBr2 causes a decrease of the selectivity towards (−)isopulegol. The washed catalysts show only 70% selectivity to (−)isopulegol, regardless of the Zn loading.
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
157
Table 4 Catalytic activities of the “as prepared” and “washed” ZnB1 and ZnB3 catalysts Catalyst code
As prepareda Znb
ZnB1/SiO2 ZnB3/SiO2 a b
Washed catalysts 106
(wt.%)
Vcat (× mol/g catalyst × s)
Sel (−)isopulegol (%)
Znb (wt.%)
Vcat (× 106 mol/g catalyst × s)
Sel (−)isopulegol (%)
9.53 13.68
3.24 5.19
78 86
3.02 4.34
2.89 5.12
70 71
Data from [10]. Determined by atomic absorption spectrofotometry.
This suggests that large amounts of ZnBr2 are able to modify the reaction stereoselectivity, even though it does not influence the rate of reaction. On the basis of the present results, the role played by ZnBr2 cannot be elucidated and can only be tentatively attributed to a steric effect. Further studies will be necessary to investigate further this point.
4. Conclusions The main results of the present study can be summarised as follows: (1) ZnBr2 salt is evenly distributed on to the surface of the supports, regardless of their surface area and mean pore diameter. (2) The morphology of the catalysts have a scarce influence in the catalytic isomerisation of (+)citronellal. (3) ZnBr2 precursor partially decomposes leading to formation of Zn(II) sites strongly interacting with the support. (4) Zn(II) sites are the most active sites in the isomerisation of (+)citronellal. (5) At high loading (>13%), the presence of ZnBr2 weakly interacting with the support shows
a beneficial effect on the stereoselectivity to (−)isopulegol. References [1] M. Misono, N. Noijri, Appl. Catal. 64 (1990) 1. [2] Y. Nakatani, K. Kawashima, Synthesis (1978) 147. [3] K. Kogami, J. Kumanotani, Bull. Chem. Soc. Jpn. 41 (1986) 2530. [4] M. Fuentes, J. Magraner, C. De Las Pozas, R. RoqueMalherbe, Appl. Catal. 47 (1989) 367. [5] K. Arata, C. Matsuura, Chem. Lett. (1989) 1797. [6] C. Dean, D. Whittaker, J. Chem. Soc. Perkin Trans. 7 (1990) 1275. [7] N. Ravasio, M. Antenori, F. Babudri, M. Gargano, Stud. Surf. Sci. Catal. 108 (1997) 625. [8] G.K. Chuah, S.H. Liu, S. Jaeniche, L.J. Harrison, J. Catal. 200 (2001) 352. [9] P.J. Kropp, W. Breton, S.L. Craif, S.D. Crawford, W.F. Durland, J.E. Jones, J.S. Raleigh, J. Org. Chem. 60 (1995) 4146. [10] C. Milone, C. Gangemi, G. Neri, A. Pistone, S. Galvagno, Appl. Catal. A: Gen. 199 (2000) 239. [11] J.C. Ross, J.H. Clarck, D.J. Macquarrie, S.J. Barlow, T.W. Bastok, Org. Process Res. Dev. 2 (1998) 245. [12] Y.C. Xie, Y.-Q. Tang, Advances in Catalysis, Academic Press, New York 37 (1990) 1. [13] C.N. Rhodes, D.R. Brown, J. Chem. Soc. Faraday Trans. 88 (1992) 2269. [14] C. Milone, C. Gangemi, R. Ingoglia, G. Neri, S. Galvagno, Appl. Catal. A: Gen. 184 (1999) 89.
Isomerisation of (+)citronellal over Zn(II) supported catalysts C. Milone, A. Perri, A. Pistone, G. Neri, S. Galvagno∗ Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, I-98166 Messina, Italy Received 26 November 2001; received in revised form 25 February 2002; accepted 28 February 2002
Abstract A series of acid catalysts prepared by impregnation of commercial silicas with a solution of ZnBr2 have been characterised with several techniques (BET, adsorption–desorption porosimetry, scanning electron microscopy (SEM), energy dispersive analysis (EDAX), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS)). The results have shown that ZnBr2 is evenly distributed over the surface of the supports regardless of their surface properties. The catalytic surface contains two different sites: physically adsorbed ZnBr2 and Zn(II) sites, obtained from the partial decomposition of the precursor. No correlation between the surface area of the support and the amount of Zn(II) sites has been observed. The ZnBr2 /SiO2 based catalysts have been also tested in the isomerisation of (+)citronellal. The catalytic activity results have shown that the structural properties of the support have a scarce influence on the catalytic performance. On all the investigated samples, the selectivity to isopulegols is 100%, whereas the stereoselectivity to (−)isopulegol ranges between 70 and 86%. Zn(II) are the most active sites for the catalytic isomerisation while ZnBr2 has a negligible catalytic activity. However, the presence of ZnBr2 is of fundamental importance to achieve higher selectivity toward the formation of (−)isopulegol. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Isomerisation; Silica; (+)Citronellal; (−)Isopulegol; Acid catalysts; ZnBr2
1. Introduction The acid catalysed intramolecular isomerisation of (+)citronellal is the synthetic route for the production of (−)isopulegol (Scheme 1). The latter is the most interesting isomer being the precursor for the synthesis of (−)menthol, a fine chemical widely employed in the manufacture of pharmaceutical, cosmetics and other speciality products. In the industrial process for (−)menthol production (Tagasako process), the isomerisation of (+)citronellal is carried out in the presence of ZnBr2 . Under these conditions, (−)isopulegol yields up to 92% have been ∗ Corresponding author. E-mail address: [email protected] (S. Galvagno).
reported [1]. The cyclic alcohol is then separated from the reaction mixture and hydrogenated to the corresponding saturated alcohol (−)menthol. It is reported that the acid catalyst ZnBr2 is employed as aqueous solution [1], however, powdered anhydrous ZnBr2 has been found to be also very selective towards the formation of (−)isopulegol [2]. One of the main drawbacks in the use of solid ZnBr2 is the difficulty to maintain the salt under anhydrous conditions being highly hygroscopic, hence, precautions are required in the storage and handling ZnBr2 if it is used in the solid state. Therefore, the synthesis of heterogeneous catalysts that combine high stereoselectivity to (−)isopulegol and ease of handling can be a strategic tool for the optimisation of the (−)menthol synthetic process.
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 1 3 6 - 9
152
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
Scheme 1.
Cyclisation of citronellal on several solid acid catalysts has been investigated but few studies deal with the distribution of the stereoisomers formed during the reaction [2–10]. In their study on the isomerisation of (+)citronellal on mixed oxides, such as SiO2 –TiO2 , SiO2 –Al2 O3 , SiO2 –ZrO2 and Cu supported on mixed oxides catalysts, Ravasio et al. [7] suggest that the Lewis sites are the most active and stereoselective to (−)isopulegol. They report stereoselectivity values varying from 63 to 74%. Chuah et al. [8] instead do not observe changes in the stereoisomer distribution with strength and nature of the acidic sites when the reaction is carried out on several catalysts including zirconium hydroxide, phosphated zirconia, sulphated zirconia, etc. The maximum yield to (−)isopulegol was 68%. They suggest a reaction mechanism involving strong Lewis together with weak Bronsted acidity. Several studies concern also the isomerisation of (+)citronellal on silica and silica-supported catalysts. Kropp et al. [9] have studied the isomerisation of (+)citronellal on high surface area silica (S.A. = 675 m2 /g). They report a stereoselectivity to (−)isopulegol of 80% when the reaction is carried out on SiO2 , which strongly decreases to 61% after adsorption of 0.1 mmol of H3 PO4 on the solid.
The presence of H3 PO4 leads to an increase of the stereoselectivity to (+)neoisopulegol. The very high stereoselectivity value to (−)isopulegol reported on silica is noticeable and is a unique result in the literature on this solid. Other authors have reported a stereoselectivity to (−)isopulegol of 62% when the reaction is carried out on high surface area silica (S.A. = 600 m2 /g) and it increases to 73%, lowering the surface area of the oxide to 380 m2 /g [7]. On the other hand, in our previous study, we found that the stereoselectivity to (−)isopulegol on SiO2 with surface area 330 m2 /g does not exceed 60% [10]. As is clearly evidenced, literature data on the cyclisation of (+)citronellal on SiO2 is controversial. It can be argued that differences in the product distribution can be due to the presence of impurities on the SiO2 , but it cannot be ruled out that the morphology of the catalyst (surface area, pore-size and distribution) could also play a role in the stereoisomer distribution, due to a size effect during the cyclisation. We have recently investigated the catalytic activity of Lewis acids, such as ZnBr2 , ZnCl2 , Zn(NO3 )2 , FeCl3 and SnCl2 supported on SiO2 in the isomerisation of (+)citronellal. Among the salts investigated, ZnBr2 was the most effective in terms of catalytic activity and selectivity to (−)isopulegol [10]. The
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
catalytic activity and the selectivity to (−)isopulegol was found to increase with the Zn loading and the temperature of calcination. The maximum selectivity of (−)isopulegol obtained on ZnBr2 supported catalysts was 86% [10]. It is noteworthy that variance to the bulk salt ZnBr2 dispersed onto a support can be stored in air and used several times with no loss in activity and selectivity. Moreover, we have also synthesised the first bi-functional catalyst containing stereoselective isomerisation sites and hydrogenation sites for the onestep synthesis of (−)menthol from (+)citronellal [10]. In a program aimed at improving the stereoselectivity to (−)menthol in the one-step process on ZnBr2 /SiO2 catalysts, we have decided to elucidate the influence of the morphology of the catalysts in the stereoselective isomerisation of (+)citronellal. Therefore, we have carried out a systematic study of ZnBr2 /SiO2 catalysts with different surface area and mean pore diameter. Characterisations of the catalysts have been also performed in order to get information on the nature of the catalytic active sites.
2. Experimental The ZnBr2 /SiO2 samples were prepared by adding the support (SiO2 Grace XWP, grain size 100–200 mesh) to a methanol solution containing a proper amount of the Zn salt. The solvent was slowly removed by rotary evaporation at 35 ◦ C for 1 h. All catalysts were dried at 120 ◦ C for 2 h, then calcined at 350 ◦ C for 2 h, unless otherwise specified. Specific surface area and porosity data were determined by adsorption–desorption of di-nitrogen at 77 K using a Micromeritics ASAP 2010. Pore-size distributions and pore volume were derived from the desorption isotherm at P /P o ≥ 0.3. X-ray diffraction (XRD) patterns were recorded by using an Ital Structure model APD2000 X-ray diffractometer operated at 40 kV and 30 mA using Ni-filtered Cu K␣ radiation at a step scan mode (2θ step, 0.02◦ ; acquisition time, 2 s per step) with a 1◦ divergence and scatter slit and a 0.1 mm receiving slit. Powder samples were mounted on Plexiglas holders. Scanning electron microscopy (SEM) images of powder samples mounted on an aluminium holder were obtained on a JEOL JSM-5600 LV microscope
153
equipped with an EDX (Oxford) analyser. The quantitative analysis was carried out at 20 kV by using the SEMQUANT software applying the ZAF correction procedure. The Zn content was determined by Atomic Absorption Spectrometry using a Perkin-Elmer model 4000 spectrometer. A known amount of solid was dissolved in an aqueous solution of HF. The absorbance of the solution was measured at 213.9 nm. Catalytic experiments were carried out at atmospheric pressure in a 100 ml four-necked flask fitted with a reflux condenser, dropping funnel, thermocouple and a stirrer head. (+)Citronellal and solvent (cyclohexane) were commercial analytical grade products and were used without further purification. The isomerisation of (+)citronellal was carried out under N2 flow at 60 ◦ C. The catalyst (0.1–0.2 g) was added to 25 ml of solvent (cyclohexane) and then the substrate (0.1 ml) was injected through one arm of the flask. The reaction mixture was stirred at 500 rpm. The progress of the reaction was followed by sampling a sufficient number of microsamples. Chemical analyses were performed by means of GC–FID. The gas chromatographic column was a EC–WAX wide-bore capillary column (30 m, 0.53 mm i.d.) connected in series with a RTX-1 column (30 m, 0.53 mm i.d.). Preliminary runs carried out at different stirring conditions, loading and catalyst grain size have demonstrated the absence of external and internal diffusion limitations.
3. Results and discussion 3.1. Characterisation of the catalysts Table 1 shows the BET surface area (SBET ), pore volume and mean pore diameter of the ZnBr2 /SiO2 catalysts and of the parent SiO2 supports. The silicas used show a SBET ranging between 87 and 532 m2 /g and a pore-size distribution varying between 5 and 36 nm (Fig. 1a–d). The addition of ZnBr2 leads to a slight decrease of the surface area and pore volume (Table 1). The Zn loading, determined by atomic absorption spectroscopy (AAS), ranges between 1.7 and 2 wt.%. From Fig. 1a–d it can be seen that the pore-size distribution of the ZnBr2 /SiO2 catalysts shows a similar shape and
154
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
Table 1 Morphological properties of SiO2 supports and ZnBr2 /SiO2 catalysts Catalyst
S.A. (m2 /g)
Mean pore diameter (nm)
Pore volume (cm3 /g)
Zna (wt.%)
SiO2 80 ZnB80 SiO2 175 ZnB175 SiO2 360 ZnB360 SiO2 540 ZnB540
87 81 174 158 330 311 532 465
36 34 20 20 10 10 5 5
0.88 0.93 1.10 0.98 1.52 1.31 0.86 0.76
− 1.74 − 1.84 − 1.96 − 1.79
a
Determined by atomic absorption spectrofotometry.
position as that obtained on the parent supports. This indicates that the salt is evenly distributed on the surface of silicas and no preferential deposition of the precursor into the pores occurs. The decrease of the surface area and pore volume of the ZnBr2 supported
catalysts can be attributed to ZnBr2 filling up the pores of the support [10,11]. XRD analyses of the ZnBr2 supported catalysts have shown the absence of resolved reflections. Since the precursor is present in a high amount, it must be assumed that amorphous ZnBr2 is present on the surface of the support in the form of mono or multilayers [12]. SEM coupled with the elemental analysis (EDAX) of the catalysts have been also performed. SEM micrographs have shown that the morphological structure of the catalysts and of the parent supports is quite similar thus indicating that the ZnBr2 is widespread onto the surface. This conclusion has been confirmed by EDAX analysis of several regions of the samples. The quantitative analysis of zinc and bromide (Table 2) on the samples has shown that the Zn/Br atomic ratio ranges between 0.7 and 0.9, which is higher than 0.5 obtained from the analysis of a mechanical mixture of ZnBr2 and SiO2 .
Fig. 1. Pore-size distributions of the: (a) SiO2 80 (䉬), ZnB80 (䉱); (b) SiO2 175 (䉬), ZnB175 (䉱); (c) SiO2 360 (䉬), ZnB360 (䉱); (d) SiO2 540 (䉬), ZnB540 (䉱).
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
155
Table 2 Quantitative EDAX analysis of ZnBr2 /SiO2 catalysts Catalyst code
Zn (at.%)
Br (at.%)
Zn/Br (at.%/at.%)
ZnB80 ZnB175 ZnB360 ZnB540 ZnBr2 /SiO2 a
40 48 44 41 35
60 52 56 59 65
0.7 0.9 0.8 0.7 0.5
a
Mechanical mixture.
This result suggests that, on the catalysts prepared by impregnation, a partial decomposition of ZnBr2 occurs leading to a release of bromide from the solids. To obtain information on the interaction between ZnBr2 and the support, the catalysts have been washed out until bromide free (AgNO3 test) and the amount of zinc on the solid has been determined by AAS. The Zn loading of the washed sample, reported in Table 3, is almost half that of initial loading. If we assume that during the washing procedure only the weakly interacting ZnBr2 is eliminated because of the high water solubility, it is likely that the residual Zn(II) comes from the decomposition of the precursor, as suggested by the SEM–EDAX analysis which has shown the absence of bromine in the washed sample. It can be argued that the residual Zn(II) can be present as Zn oxy-hydroxide and/or Zn–(O–Si)2 , formed from the reaction between ZnBr2 and the hydroxyl group of SiO2 . In a previous study on ZnCl2 /SiO2 catalysts calcined at 120 ◦ C, Rhodes et al. [13] have reported that the precursor can be easily washed out indicating that no decomposition of the salt occurs. The catalysts of the present study have been obtained following a preparation procedure similar to that reported by Rhodes et al. [13], with the exception of the temperature of activation. In
Fig. 2. Isomerisation of (+)citronellal on ZnB360; (Tr = 60 ◦ C) (䉬) (+)citronellal, (䊏) (−)isopulegol, (䉱) (+)neoisopulegol, (䊉) (+)isoisopulegol.
our case, ZnBr2 /SiO2 catalysts have been calcined at 350 ◦ C, which has been found the optimum temperature to achieve higher selectivity to (−)isopulegol in the isomerisation of (+)citronellal [10]. Based upon the characterisation data, we can conclude that in the ZnBr2 /SiO2 catalysts, weakly adsorbed ZnBr2 and Zn(II) sites are present, almost in the same amount, on the surface of silica catalysts. 3.2. Catalytic activity The isomerisation of (+)citronellal has been carried out both on the “as prepared” ZnBr2 /SiO2 catalysts and on the “washed” samples. “Washed samples” were obtained by washing with water until bromide free. A typical course of the reaction is reported in Fig. 2. Under the experimental condition used
Table 3 Catalytic activities of the “as prepared” and “washed” ZnBr2 /SiO2 catalysts Catalyst code
As prepared Zna
ZnB80 ZnB175 ZnB360 ZnB540 a
1.74 1.84 1.96 1.79
(wt.%)
Washed catalysts Vcat (×
106
mol/g catalyst × s)
1.23 1.39 1.37 1.52
Determined by atomic absorption spectrofotometry.
Zna (wt.%)
Vcat (× 106 mol/g catalyst × s)
0.69 0.74 0.93 0.99
1.24 1.49 1.83 1.98
156
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
(+)citronellal is converted only to the isopulegol stereoisomer in all range of conversion investigated. No other reaction products were detected. The reaction products are: (−)isopulegol, (+)neoisopulegol and (+)isoisopulegol. The less thermodynamically stable stereoisomer, (+)neoisoisopulegol, is not formed under our reaction conditions. Their relative abundance is independent of the conversion levels, thus indicating that they are formed through parallel reactions (Scheme 1). The catalytic activity results, expressed as initial rate of isomerisation of (+)citronellal, of the “as prepared” ZnBr2 catalysts are reported in Table 3. All the SiO2 used as supports are only slightly active for the isomerisation of (+)citronellal. In agreement with our previous findings [14], the catalytic activity increases from 1 × 10−8 to 2 × 10−7 mol/g catalyst/s with increasing the surface area of the supports from 87 to 532 m2 /g. The selectivity towards (−)isopulegol is 60%, regardless of the surface area and the mean pore diameter of silicas; these results indicate that the morphology of the support does not influence the stereoisomer distribution in the isomerisation of (+)citronellal. The presence of ZnBr2 leads to an enhancement of the catalytic activity (Table 3). The initial rate of disappearance of (+)citronellal is quite similar for all the catalysts investigated, regardless of the surface area and mean pore diameter. Moreover, on addition of ZnBr2 an increase of the selectivity towards (−)isopulegol up to 70% has been obtained on all the investigated samples regardless of the silica used. This is a further confirmation that the morphological parameters of the solids do not influence the activity and selectivity of the reactions. It has to be pointed out that, in agreement with previous findings [10] no significant leaching of the modifier was observed under our reaction conditions. This was verified either by analysing the Zn content in the reaction mixture and on the solid. Moreover, no reaction was observed on the solvent after removing of the catalyst. In Table 3, the catalytic activity of the “washed” catalysts is also reported. Notwithstanding the decrease of the amount of zinc upon washing, the catalytic activity per gram of catalyst remains almost constant and in some cases it was found even higher than on the “as prepared” samples. These results suggest that the contribution of the weakly interacting ZnBr2 to
the overall activity is negligible. The active sites in the isomerisation of (+)citronellal seems to be mainly constituted by the residual Zn(II) sites. The slight increase of catalytic activity observed on some samples could be attributed to a further hydrolysis of the ZnBr2 salt (Table 3). It should be noted that at the Zn loadings reported in Table 3 the selectivity to (−)isopulegol remained almost constant (about 70%) on all samples regardless of the washing procedure and surface area. In a previous paper, we have reported that the catalytic activity and the selectivity toward (−)isopulegol of ZnBr2 supported on SiO2 360 (S BET = 330 m2 /g) depends on the amount of ZnBr2 . The isomerisation activity increases with the salt loading up to 7 mmol/g SiO2 (Zn (wt.%) ∼ = 18), while the selectivity to (−)isopulegol reaches a maximum value (86%) at 4 mmol/g SiO2 (Zn (wt.%) ∼ = 13) [10]. The results reported in the present study have shown that, at low salt loading (Zn (wt.%) ∼ = 2), the activity of the weakly interacting ZnBr2 is negligible. The reaction occurs mainly on the residual Zn(II) sites generated from the decomposition of the precursor. However, the selectivity to (−)isopulegol of the low loaded catalysts is only 70%, which is much lower than the selectivity obtained on the catalysts with higher Zn loading. To clarify this point, the previously investigated catalysts [10], indicated in Table 4 with the code ZnB1/SiO2 and ZnB3/SiO2 , have been washed out until bromide free and the catalytic activity in the isomerisation of (+)citronellal has been measured. As is shown in Table 4, the Zn content of the washed catalysts is much lower than the initial amount and ranges between 3 and 4.3 wt.%. This indicates that the main component of the as prepared catalysts is the weakly adsorbed ZnBr2 . Notwithstanding the lower amount of zinc, the isomerisation activity of the washed catalysts (Table 4) remains practically constant. These results are a confirmation that the contribution of the weakly adsorbed ZnBr2 to the catalytic activity is negligible, even at high loading, and the residual Zn(II) sites are the main active sites in the isomerisation of (+)citronellal. However, the elimination of ZnBr2 causes a decrease of the selectivity towards (−)isopulegol. The washed catalysts show only 70% selectivity to (−)isopulegol, regardless of the Zn loading.
C. Milone et al. / Applied Catalysis A: General 233 (2002) 151–157
157
Table 4 Catalytic activities of the “as prepared” and “washed” ZnB1 and ZnB3 catalysts Catalyst code
As prepareda Znb
ZnB1/SiO2 ZnB3/SiO2 a b
Washed catalysts 106
(wt.%)
Vcat (× mol/g catalyst × s)
Sel (−)isopulegol (%)
Znb (wt.%)
Vcat (× 106 mol/g catalyst × s)
Sel (−)isopulegol (%)
9.53 13.68
3.24 5.19
78 86
3.02 4.34
2.89 5.12
70 71
Data from [10]. Determined by atomic absorption spectrofotometry.
This suggests that large amounts of ZnBr2 are able to modify the reaction stereoselectivity, even though it does not influence the rate of reaction. On the basis of the present results, the role played by ZnBr2 cannot be elucidated and can only be tentatively attributed to a steric effect. Further studies will be necessary to investigate further this point.
4. Conclusions The main results of the present study can be summarised as follows: (1) ZnBr2 salt is evenly distributed on to the surface of the supports, regardless of their surface area and mean pore diameter. (2) The morphology of the catalysts have a scarce influence in the catalytic isomerisation of (+)citronellal. (3) ZnBr2 precursor partially decomposes leading to formation of Zn(II) sites strongly interacting with the support. (4) Zn(II) sites are the most active sites in the isomerisation of (+)citronellal. (5) At high loading (>13%), the presence of ZnBr2 weakly interacting with the support shows
a beneficial effect on the stereoselectivity to (−)isopulegol. References [1] M. Misono, N. Noijri, Appl. Catal. 64 (1990) 1. [2] Y. Nakatani, K. Kawashima, Synthesis (1978) 147. [3] K. Kogami, J. Kumanotani, Bull. Chem. Soc. Jpn. 41 (1986) 2530. [4] M. Fuentes, J. Magraner, C. De Las Pozas, R. RoqueMalherbe, Appl. Catal. 47 (1989) 367. [5] K. Arata, C. Matsuura, Chem. Lett. (1989) 1797. [6] C. Dean, D. Whittaker, J. Chem. Soc. Perkin Trans. 7 (1990) 1275. [7] N. Ravasio, M. Antenori, F. Babudri, M. Gargano, Stud. Surf. Sci. Catal. 108 (1997) 625. [8] G.K. Chuah, S.H. Liu, S. Jaeniche, L.J. Harrison, J. Catal. 200 (2001) 352. [9] P.J. Kropp, W. Breton, S.L. Craif, S.D. Crawford, W.F. Durland, J.E. Jones, J.S. Raleigh, J. Org. Chem. 60 (1995) 4146. [10] C. Milone, C. Gangemi, G. Neri, A. Pistone, S. Galvagno, Appl. Catal. A: Gen. 199 (2000) 239. [11] J.C. Ross, J.H. Clarck, D.J. Macquarrie, S.J. Barlow, T.W. Bastok, Org. Process Res. Dev. 2 (1998) 245. [12] Y.C. Xie, Y.-Q. Tang, Advances in Catalysis, Academic Press, New York 37 (1990) 1. [13] C.N. Rhodes, D.R. Brown, J. Chem. Soc. Faraday Trans. 88 (1992) 2269. [14] C. Milone, C. Gangemi, R. Ingoglia, G. Neri, S. Galvagno, Appl. Catal. A: Gen. 184 (1999) 89.