Conversion of -pinene to terpinyl acetate over H-beta zeolites

Transcription

1 Applied Catalysis A: General 209 (2001) Conversion of -pinene to terpinyl acetate over H-beta zeolites Graeme J. Gainsford, Camila F. Hosie, Roderick J. Weston Industrial Research Ltd., P.O. Box , Lower Hutt, New Zealand Received 19 June 2000; received in revised form 16 August 2000; accepted 16 August 2000 Abstract A low temperature liquid phase process has been investigated by which -pinene can be converted to terpinyl acetate in good yield using commercially available H-beta zeolites at 20 C. The rate of reaction of the -pinene is directly related to the zeolite SiO 2 /Al 2 O 3 ratio. Dealumination of the catalysts by acid treatment slightly improves reaction time. Increasing the temperature is detrimental, leading to an increase in the bornyl acetate concentration. The zeolites can be recycled. The process may be suitable for an industrial operation with the yield as good as that obtained by the current two-stage preparation Elsevier Science B.V. All rights reserved. Keywords: -Pinene; Terpinyl acetate; H-beta zeolites; Liquid phase; Acetic acid 1. Introduction In our laboratories we are attempting to convert monoterpenes, and -pinene in particular, to products of greater value in environmentally benign ways using commercially available microporous and mesoporous solid catalysts, both in solution and in the gas phase. We found that reacting -pinene with methanol using H-beta as a catalyst at room temperature afforded the methyl ether of -terpineol in yields greater than 35%. At this time Hensen et al. [1] published detailed results of similar work of pinene transformations in alcohols. We therefore turned our attention to the preparation of terpinyl acetate, a monoterpene ester used widely in the fragrance industry, especially in soaps. It is one of the 20 most important fragrance chemicals manufactured industrially. Usually it is prepared industrially in two stages: -pinene is treated Corresponding author. address: g.gainsford@irl.cri.nz (G.J. Gainsford). with aqueous sulphuric or phosphoric acid and the resulting -terpineol is then esterified with acetic anhydride and a catalyst. From the results of the previous study, we perceived that it might be possible to prepare -terpinyl acetate in one step in similar or better yields than those obtained in the current two-stage process. Terpinyl acetate can also be prepared from limonene or dipentene using acetic acid in the presence of Fe 2 (SO 4 ) 3 or by reacting -terpineol with acetic anhydride in the presence of a catalyst [5]. Related bornyl acetate can be prepared from -pinene [2,3], and by a continuous process from camphene [4]. We also note that related reactions have been reported for the acetylation of cyclohexenes [6], terpenoids [7] and other aromatics [8 10] using acetic acid and other acetylating agents, at higher temperatures over a range of acid zeolites, and that acetic acid can be esterified by various alcohols, also over a range of zeolites (X, Y [11], H-beta, ZSM-5, etc. [12]) X/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S X(00)

2 270 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) Table 1 Catalyst sources Number of sample Supplier Supplier identifier SiO 2 /Al 2 O 3 ratio Zeolite type 1 Sud Chemie Ag Munchen H-BEA 25 BEA powder 2 Sud Chemie Ag Munchen H-BEA 100 BEA powder 3 Zeolyst International NH + 4 -SAR 25 BEA powder 4 Zeolyst International H-SAR 20 Mordenite 5 Zeolyst International H-beta extrudate 25 BEA 1/6 in. pellets 6 Recycled a sample 1 H-BEA 25 BEA powder 7 Dealuminated sample 3 (with HCl) a H + -SAR 25 BEA powder 8 UOP Molecular Sieves Patent ; Lot Unknown BEA 1/16 in. pellets no Sud Chemie Ag Munchen H-BEA 150 BEA powder 10 Sud Chemie Ag Munchen H-BEA 120 BEA powder a See text. 2. Experimental 2.1. Reactant A commercial -pinene, consisting of 92.4% -pinene, 2.2% camphene and 5.4% -pinene was used in this work together with analytical grade acetic acid Catalysts Table 1 lists details of the catalysts used. The NH + 4 zeolites (catalyst 3) were converted to the H + form by calcination (2 /min heating rate from room temperature then held at 500 C for 4 h). These same conditions were used to recycle catalysts, after filtration, washing with water and drying in air at C for 2 h. Samples were dealuminated by heating while stirring the acid form of catalyst 3 or some of the other catalysts with 0.1 M HCl, 1.5 M HNO 3 or 1.5 M oxalic acid at 80 C for 4 h. Samples were then washed until free of the reacting acid and then dried and calcined as for the recycled catalysts. Retention of the BEA structure was confirmed by XRD; selected samples were also examined by solid state NMR and porosity analysis (ASAP Model 2100) to confirm that the BEA structure framework was unchanged Small scale reactions (1.0 g pinene) -Pinene (1.0 g) in glacial acetic acid (10 ml) was stirred at room temperature for up to 48 h with a catalyst (0.5 g) in a closed flask. Aliquots were withdrawn at set times. The initial set of reactions was run for 24 h. The reaction aliquot was then diluted with dichloromethane (50 ml) and filtered through a Whatman No. 42 paper. The solution was neutralised by shaking with saturated sodium bicarbonate solution. After separation, the organic fraction was washed with water, dried with magnesium sulphate, and then analysed by GC-FID Large laboratory scale reaction (40.0 g pinene) -Pinene (40.0 g) in glacial acetic acid (400 ml) was stirred at room temperature for 30 h in a closed flask with H-beta (catalyst 3, 38 g). The reacted mixture was filtered through a Whatman No. 1 paper and concentrated on a Buchi evaporator under vacuum (15 mm) at 40 C. The concentrated mixture was then distilled initially under a vacuum (of 40 mm) through a Vigreux column and then at 10 mm between fractions 2 and 3; the vapour temperature was recorded at the top of the distillation column. 3. Results Results for the initial small-scale reactions, run for 24 h are shown in Table 2. The two outcomes which were of greatest interest to us were the extent of conversion of -pinene to other products and the percentage of -terpinyl acetate formed in the product

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4 272 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) Fig. 1. Product composition with time (catalyst 2). mixture. The inclusion of a base such as NaOAc in the mixture effectively neutralised the activity of the acid sites, so transformation of -pinene was almost negligible (Table 2, catalyst 3). In those cases where Lewis (neutral) bases (Ac 2 O, H 2 O, dioxan) were used, a similar result was seen where the transformation was less than 50% (runs 1 and 3). The reaction rates and product distributions, where acetic acid was used as both solvent and reactant with different catalysts, are summarised in Figs. 1 4 Fig. 2. Product composition with time: comparison of original (catalyst 3) and dealuminated (catalyst 7) catalysts.

5 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) Fig. 3. Product composition with time: comparison of original (catalyst 1) and recycled (catalyst 6) catalysts. (reaction completion is defined as the point when all the -pinene was consumed). The product spread of a typical reaction is shown in Fig. 1 (catalyst 2, products below 2% relative peak area have been excluded for clarity). After all the -pinene has reacted (in this case, 24 h), the ratio (hereafter R tb ) of terpinyl acetate to bornyl acetate yield declines. The increased reaction rate found for H-beta zeolites with high SiO 2 /Al 2 O 3 ratios encouraged us to dealuminate the catalysts with lower ratios, in the hope of Fig. 4. Variation of the % conversion of -pinene and the % yield of -terpinyl acetate, with SiO 2 /Al 2 O 3 ratio after 4 h reaction time.

6 274 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) Table 3 Products with dealuminated H-beta zeolites a,b Dealumination acid (t = 2h) c Dealumination acid (t = 4h) c Dealumination acid (t = 25 h) c RT a HCl Oxalic acid HNO 3 HCl Oxalic acid HNO 3 HCl Oxalic acid HNO 3 -Pinene Camphene Terpinene p-cymene Limonene Terpinolene Unknown <0.3 < <0.3 < <0.3 < Unknown 2 < < < < Fenchyl acetate Unknown 3 < < < < Unknown <0.3 < Bornyl acetate Unknown <0.3 <0.3 < Terpinyl acetate a Listed in retention time (RT) order as relative GC area (%). b Formed from the acid form of catalyst 3 (SAR 25); see text for details. c Time of reaction is denoted by t. achieving an increased reaction rate while holding or improving the R tb values. These dealumination experiments were successful in improving the reaction rate for the slower catalysts, but the product distribution also moved to that observed for the high SiO 2 /Al 2 O 3 zeolites. Table 3 shows the product distribution after reaction of -pinene with three dealuminated catalysts, prepared from the acid form of catalyst 3. The distributions of the key entities for the starting and HCl-dealuminated catalysts (3 and 7) are shown in Fig. 2; here dealumination reduced the estimated completion time from over 52 to 30 h. Recycling of the catalysts, as has been observed before for other zeolite catalysed reactions, improved the reaction rate slightly. In this case, however, it did not change the product distribution significantly. This is demonstrated in Fig. 3 for fresh and recycled catalyst 1. Both catalysts (1 and 3) with SiO 2 /Al 2 O 3 ratios of 25 gave similar reaction rates and product distributions. The effect of both dealumination and recycling on catalyst activity proved to be additive. The fastest completion time of under 5 h was observed using a recycled and HNO 3 -dealuminated catalyst (SiO 2 /Al 2 O 3 = 25) which produced an R tb of 1.20 (23% terpinyl acetate). The results consistently showed that the completion time is inversely related to SiO 2 /Al 2 O 3 ratio of the H-beta catalysts. As shown by the analysis at 4 h (Fig. 4), the faster reactions do not yield more terpinyl acetate; indeed the reaction for catalyst 9 (SiO 2 /Al 2 O 3 = 150) is completed in 1 h, with R tb declining from that point. At completion of the large laboratory scale reaction (2.4 above), the mixture had a composition of -pinene, 21%; camphene, 8%; -terpinene, 11%; p-cymene, 1%; limonene, 12%; -terpinene, 4%; terpinolene, 6%; fenchyl acetate, 1%; unknown acetate, 6%; bornyl acetate, 3% and terpinyl acetate, 24%. These results were almost identical to a reaction using 20 g of pinene and indicated that a change in the ratio of pinene/catalyst from 2:1 to 1:1 had no effect on the composition of the product mixture, which was distilled under vacuum to afford three fractions. Fraction 1 contained the bulk of the solvent (HOAc) and a small proportion of terpene hydrocarbons. Fraction 2 (21.5 g, distillation range C) contained terpene hydrocarbons (66%) and esters (34%) including terpinyl acetate (18%, 3.9 g). Fraction 3 (14.0 g, distillation range C) contained hydrocarbons (11%) and esters and the terpinyl acetate content was

7 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) % (8.0 g). The total yield of terpinyl acetate was 12.0 g (30% w/w). 4. Discussion Methods for the preparation of terpinyl acetate from -pinene were patented early in the 20th century by Zeitschel [13 16] and Simonsen and Owen [17]. More recently, a definitive study of the reaction of -pinene in acetic acid with mineral acids was carried out by Williams and Whittaker [18]. They demonstrated that, after very short periods with perchloric acid, -pinene rapidly forms a carbonium ion which rearranges without capture by nucleophiles to form the hydrocarbons - and -terpinene, terpinolene and camphene. Ester formation occurs very much more slowly by solvation of the hydrocarbon products, rather than from carbonium ion intermediates. With sulphuric acid - and -terpinene are minor products, whereas limonene, terpinolene and camphene are the major hydrocarbons formed. Formation of terpinyl acetate occurs only on prolonged reaction and is derived from limonene and terpinolene. This result supports the conclusion above for ester formation. Williams and Whittaker [18] pointed out that the counter-ion in the above reactions, i.e. perchlorate or bisulphate, influences the course of the reactions and thereby the product distribution. They concluded that the pinene mineral acid ion pair formed in the initial reaction can either fully ionise to allow ester formation with the solvent or, when the counter-ion is nucleophilic, it can abstract a proton with the generation of an olefinic product. Furthermore, rearrangement can take place within the intimate ion-pair with little opportunity for an external nucleophile to capture the ionic intermediate. Anhydrous acetic acid is a medium which does not promote the dissociation of an intimate ion-pair. Williams and Whittaker showed clearly that, when water was added to their reaction mixtures, the yield of terpinyl acetate increased by a factor of 5, but had little effect on the yield of fenchyl and bornyl acetate. Recently, Hensen et al. published details of work on the reactions of -pinene with H-beta zeolites in alcohols [1]. They pointed out that the use of strong acids on an industrial scale leads to corrosion and environmental problems and Lewis acids are difficult to handle on a large scale. These problems have induced much interest in the use of solid acids as replacements for the traditional mineral acid catalysts. Hensen et al. [1] found that reaction of -pinene with H-beta zeolite in alcohols affords the alkyl ethers of -terpineol as the major products. Investigation of the effects of time and temperature on the reaction indicated that the greatest yield (54%) of an ether was obtained at 40 C after 5 h when the conversion of -pinene was 92%. Importantly, a greater yield of ethers was obtained if limonene was used as the substrate, which suggested a selectivity of substrate by the catalyst. They demonstrated that, by increasing the ratio of pinene/catalyst, the percentage conversion of -pinene to products was decreased but with a concomitant increase in the yield of ether. Hölderich pointed out that H-beta zeolites proved more active and selective than any other similar catalyst due to the high surface area and large number of acid sites on the zeolite, and that pore size and shape were responsible for the different results obtained for limonene and -pinene. All but one of the catalysts used in this work were H-beta zeolites, functioning as strong acids. Three other recent papers [2 4] have also discussed the preparation of terpinyl and bornyl acetates from -pinene with the assistance of zeolite catalysts. In each case, the results appear to be similar to ours. Attempts to accelerate the reaction by heating had two detrimental effects on the desired outcome. Firstly, the hydrocarbons -terpinene, terpinolene and limonene are formed rapidly from tertiary carbonium ion intermediates in the absence of (potential) nucleophiles and levels of these products are elevated with heating, in comparison with the reactions carried out at room temperature. Additionally, -terpinene, which is the end-product of this sequence of transformations, is oxidised to p-cymene on prolonged heating in air. The second undesired result was the production of bornyl acetate in preference to terpinyl acetate. This result was observed from the earliest days of terpene chemistry and has been the subject of many patents for the production of bornyl acetate [17]. In all the reactions which we carried out over a 24 h period in glacial acetic acid at room temperature, more than 70% -pinene was transformed and ester formation ranged from 28 to 49%. The selection of a catalyst for possible industrial use was guided by the need for a good yield of terpinyl acetate and a

8 276 G.J. Gainsford et al. / Applied Catalysis A: General 209 (2001) low yield of bornyl acetate, as these substances are inseparable by distillation. The results indicated that the H-beta zeolite manufactured by the Zeolyst International, which had a silica/alumina ratio of 25, produced the marginally greatest yield of terpinyl acetate but also produced one of the highest ratios of terpinyl to bornyl acetate. Interestingly, this ratio increased after the catalyst was recovered from one reaction, recalcined and then reused. A second catalyst (5) also afforded a moderate yield of terpinyl acetate with a higher ratio of terpinyl/bornyl acetate. None of the reactions described here have been optimised, but reduction of the time for ester formation simply resulted in decreased yields (Figs. 1 and 2). Before the reaction can be considered useful for industrial production several factors need to be optimised. These include determining the ratio of pinene/acetic acid and pinene/catalyst, the method of removal of the catalyst from the reaction mixture, so it can be reactivated and reused, and the number of times the catalyst can be used. To produce a pure product the efficiency of a distillation column for isolation of the terpinyl acetate from the reaction mixture must be determined. Consideration of the work described above by Williams and Whittaker [18], Hensen et al. [1] and by ourselves, suggests that the mechanism of the reaction of pinene in acetic acid with H-beta zeolite will be similar to that described by Van der Waal et al. [19]. In our case, however, no water was present and it is possible that pinene attaches directly to the (Lewis) acid sites on the catalyst. Alternatively, these sites might be saturated first with HOAc to generate Brönsted acid sites, before reaction with pinene. We did not observe the formation of the adduct described by Van der Waal et al., when pinene was added to the catalysts (described in our experimental work) in acetone. This observation is an example of the selectivity of H-beta zeolites, as mentioned by Hölderich. 5. Conclusions The process described here afforded a good yield of -terpinyl acetate in one reaction step, comparable with that obtained from the two-step process used industrially. The solvent and the catalyst can be recycled and the process avoids the discharge of acid solutions. It will therefore appeal to those industries which operate within a sphere of environmental regulation. We have shown the following: -Pinene can be converted to -terpinyl acetate at 29% yield in one step, with an H-beta zeolite catalyst in acetic acid at room temperature in 24 h. Distillation of the product mixture affords terpinyl acetate with unoptimized 70% purity. Elevation of the reaction temperature decreases the yield and selectivity of -terpinyl acetate. The rate of the reaction is directly related to the H-beta SiO 2 /Al 2 O 3 ratio. The catalysts can be recycled. Acknowledgements The gifts of zeolites by Sud Chemie, Zeolyst International and UOP, and samples of commercial -pinenes and other monoterpenes by Eka Chemicals (NZ) Ltd. are gratefully acknowledged. This work was funded by the New Zealand Public Good Science Fund Contract number CO8512 and the encouragement of Drs. Neil Milestone and Stephen Bagshaw of the Applied Inorganic Chemistry Team is acknowledged. References [1] K. Hensen, C. Mahaim, W.F. Holderich, Appl. Catal. A: Gen. 149 (1997) 311. [2] S. Lin, S. Fu, K. Zheng, Y. Li, B. Zhan, Fenzi Cuiha 8 (1994) 50; Chem. Abstr. 121 (1994) [3] P. Xiao, H. Wei, P. Xin, Xiangtan Daxue Ziran Kexue Xuebao (18) (1996) 56; Chem. Abstr. 127 (1997) [4] M. Gscheidmeier, R. Gutmann, J. Wiesmuller, A. Riedel, US Patent 5,596,127 (1997), to Hoechst Aktiengesell-Schaft, Germany. [5] T. Yamanaka, Japanese Patent (1976), to Takasago Perfumery. [6] E. Armengol, A. Corma, L. Fernandez, H. Garcia, J. Primo, Appl. Catal. A: Gen. 158 (1997) 323. [7] M. Nomura, S. Hisatomi, T. Hamada, Y. Fujihara, Chem. Abstr. 127 (1997) [8] A.K. Pandey, A.P. Singh, Catal. Lett. 44 (1997) 129. [9] E.G. Derouane, C.J. Dillon, D. Bethell, S.B. Derouaneabdhamid, J. Catal. 187 (1999) 209. [10] U. Freese, F. Heinrich, F. Roessner, Catal. Today 49 (1999) 237. [11] N. Nagaraju, M. Peeran, D. Prasad, React. Kinet. Catal. Lett. 61 (1997) 155.

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