Znd. Eng. Chem. Res. 1996,34, 4508-4513
4608
Supercritical C02 Desorption of Bergamot Peel Oil D. Chouchi and D. Barth LTCA-ENSIC, 1 Rue Grandville, B.P. 451, 54000 Nancy, France
E. Reverchon* Dipartimento di Ingegneria Chimica e Alimentare, Universita di Salerno, Via Ponte Don Melillo, Fisciano (SA) 84084, Italy
G. Della Porta Istituto di Scienze dell'Alimentazione, C.N.R., Via Rubilli, 83100 Avellino, Italy
Supercritical COZ desorption was performed on bergamot peel oil to improve the oil quality through selective elimination of hydrocarbon terpenes (deterpenation) and psoralens. Fractionation of the oil was performed a t 40 "C and a t increasing pressures from 75 to 100 bar. Oil fractions were analyzed by GC-MS, and volatile compounds were identified and divided into two main families: hydrocarbon terpenes and oxygenated terpenes. Also, nonvolatile compounds like waxes, coumarins, and psoralens were systematically identified. Particular attention was focused on the elimination of bergapten, which has been indicated as the main phototoxic agent contained in products of this kind. We also studied the influence of solute loading on bergamot oil desorption, which largely influenced the selectivity of the desorption process. Breakthrough curves for hydrocarbon terpenes and oxygenated terpenes were obtained by weighing the oil fractions desorbed and by quantitation of GC-MS area responses. These results were fairly well modeled by integrating mass balance equations written for a differential element of the desorption column.
Introduction Citrus peel oils are widely used in the perfume and cosmetic industry. Among them, the most valuable is bergamot peel oil, which is a fundamental ingredient in many fragrance formulations. Bergamot oil is obtained, as others citrus oils, by cold pressing (CP). This technique consists of the mechanical expression of the fruits, which allows oil to be released from the subcutaneous peel cells. Cold pressing is simple and cheap; furthermore, it does not produce thermal degradation of the product and does not require solvents. Nevertheless, products obtained by CP are subject t o some problems: i.e., they contain a large quantity of hydrocarbon terpenes that do not contribute t o the characteristic fragrance of the citrus oil and are relatively unstable. Thus, the shelf life of these products is short. Moreover, CP oils contain nonnegligible quantities of coumarins and psoralens. These products can cause turbidity in the oil, and some of them have a wellascertained phototoxic activity (Naganuma et al., 1985; Zaynoun et al., 1977). Bergamot oil shares these problems with other citrus oils. Indeed, it contains the lowest percentage of hydrocarbon terpenes but also a high quantity of bergapten, which is the most powerful phototoxic agent identified in such products (Zaynoun et al., 1977). Therefore, strong limitations have been imposed on its use. IFRA (International Fragrance Association) recommends a maximum content of 0.4% cold-pressed bergamot oil in products that are applied on skin areas exposed to sunshine (IFRA, 1992). For example, the same regulation imposes a maximum content of 2% for lemon peel oil. Supercritical C02 (SC-COZ)can be used as a very selective solvent and does not produce thermal degradation or solvent pollution of products. Therefore, some
* Author to whom correspondence
should be addressed.
investigators attempted SC-COZdeterpenation and/or psoralen elimination of citrus peel oils. Stahl and Gerard (1985) proposed SC-CO2 deterpenation of orange peel oil by using a continuous countercurrent high-pressure column. A good fractionation was obtained in the pressure range between 70 and 90 bar and a t temperatures below 80 "C. More recently, Sat0 et al. (1994) performed experiments on a supercritical extraction tower in semibatch and continuous mode by using a limonene, linalool, and citral mixture simulating a citrus peel oil. They obtained the fractionation of the model mixture a t pressures of 88 and 95 bar and in the temperature range from 40 to 60 "C. Supercritical C02 adsorption or desorption can also be used to perform very selective processes. Model compounds as well as real systems have been widely studied (Srinivasan et al., 1990; Akgerman et al., 1992; Madras et al., 1993; Iwai et al., 1994). Some authors also attempted modeling of these processes (Recasens et al., 1989; Srinivasan et al., 1990; Akman and Sunol, 1991; Madras et al., 1993; Turgay and Akman, 1994). With a specific reference t o peel oil supercritical desorption, Yamauchi and Saito (1990) proposed the desorption of lemon peel oil from a HPLC column packed with silica gel, obtaining four fractions that they then analyzed by GLC. Cully et al. (1990) proposed peel oil desorption from several adsorbents by using SC-CO2. They operated at temperatures from 50 t o 70 "C and at pressures from 70 to 90 bar. Recently, Barth et al. (1994) proposed SC-CO2 desorption of lemon oil, in which both deterpenation and psoralen elimination were obtained. The process was performed by modulating SC-COZsolvent power and selectivity at a fured temperature of 40 "C and operating the desorption at increasing pressure. A good deterpenation was obtained, and even more interesting, all coumarins and psoralens were eliminated from the cold-
0888-588519512634-4508$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 12,1995 4609 Table 1. Bergamot Peel Oil Desorption Performed at 75 bar and 40 "C" t,
terpenes hydrocarbon,* oxygenated:
t,
terpenes hydrocarbon: oxygenated:
min
g
g
min
g
g
27 37 47 51 55 60
0.91 0.89 0.71 0.20 0.17 0.17
0.06 0.11 0.29 0.15 0.14 0.16
65 75 100 140 160d 185'
0.11 0.16 0.13 0.08 0.11 0.01
0.17 0.29 0.74 0.64 0.77 0.10
The compositions of the fractions collected in the separators is expressed in grams since GC areas were converted into grams by calculating the response factor of the ion trap. Retention times from 11.2 to 22.2 min (see Table 2). Retention times from 23.3 to 43.2 min (see Table 2). Obtained at 85 bar. e Obtained at 100 bar.
pressed lemon oil. Supercritical desorption of other peel oils was discussed in a successive work (Chouchi et al., 1994). In this work, we applied the SC-CO2 desorption technique to bergamot peel oil to assess if it was possible t o deterpenate the oil and eliminate coumarins and psoralens. The desorbed fractions were analyzed by GCMS to identify bergamot oil constituents. We also tested the effect of the solute loading on the performance of the desorption process. The desorption results were expressed in terms of breakthrough curves for the two major families of compounds that constitute the oil. A mathematical model was proposed to fit the experimental data.
Materials and Methods Apparatus. Supercritical C 0 2 desorption was performed in a laboratory apparatus equipped with a membrane pump (Milton Roy, Pont Saint Pierre, France, Model Milroyal D)for supercritical solvent delivery and with a piston pump (Milton Roy, Pont Saint Pierre, France, Model Instrument Minipump A) to charge peel oil into the desorption column. The internal volume of the column measured 0.18dm3 (H= 40 cm). It can be operated at pressures over 250 bar. It was manually filled with 100 g of the adsorbent medium (Kieselgel). A thermostated jacket was used to control the temperature inside the column. Desorbed fractions were precipitated in two highperformance thermostated cyclonic separators operated in series a t 30 bar, 40 "C and at 20 bar, 15 "C, respectively. These separators allowed the continuous discharge of the product during the desorption process. The CO2 flow rate was 1.3 kg/h; it was measured a t the outlet of the apparatus by a calibrated rotameter. More details on the desorption apparatus and on the experimental procedure were provided in a previous paper (Barth et al., 1994). Materials. Cold-pressed bergamot peel oil was supplied by the "Consorzio del Bergamotto" (Reggio Calabria, Italy). Industrial COz (99%purity) was supplied by Carboxyque Francaise (Hauconcourt, France). Analytical Methods. The gas chromatographicmass spectrometric (GC-MS) apparatus consisted of a capillary GC (Varian, San Fernando, CA, series 3400) connected to an ion trap detector (Finnigan Mat. San Jose, CA, Model ITS 40). The column used for separation was a fused silica DB-5 (J&W, Folsom, CA), 30-m x 0.25-mm i.d., with film thickness 0.25 pm. GC conditions for separating the oil fractions were an oven
temperature of 40 "C for 10 min that was then programmed to 50-250 "C (rate 2 "C/min) and subsequently held isothermally at 250 "C for 60 min. The area percentage of compounds was calculated from the GC traces. These percentages were converted into absolute values by measuring the ion trap relative response. The calibration was performed on the ion trap response of some pure standard compounds with the same molecular weight of the compound families contained in bergamot peel oil. The identification of compounds was based on the comparison of the retention times and mass spectra of pure compounds. NIST (National Institute of Standards and Technologies) and WILEY 5 mass spectra libraries were also used.
Results and Discussion Citrus peel oils are mainly constituted by hydrocarbon terpenes and oxygenated terpenes that form the volatile part of the oil and by nonvolatile compounds like waxes, coumarins, and psoralens. The scope of this work was to selectively desorb hydrocarbon terpenes and oxygenated terpenes and to leave the other compounds undesorbed. Therefore, we assume in what follows that the process can be monitored by taking into account two key compounds: the first consists of the whole contribution of hydrocarbon terpenes; the second one of all oxygenated terpenes. The evaluation of the desorption performance was carried out by discharging the separators at selected times. The yield was measured by weighing the fraction collected into separators. The composition of each fraction was analyzed by GC-MS. The monitoring of the process by means of the two key compounds enables the study of the yield and of the composition evolution during the desorption process. The optimum pressure and temperature used to start the supercritical desorption were the same as the ones used in the work performed on lemon peel (Barth et al., 1994). Indeed, similar compound families had to be desorbed, thus requiring the same operation conditions. However, some tests were performed around the previously optimized conditions to assess the validity of this hypothesis. The best bergamot oil fractionation was obtained by starting desorption at 40 "C and a t 75 bar. At these operating conditions, hydrocarbon terpenes were selectively desorbed during the first 60 min of desorption. After 60 min, the desorption of hydrocarbon terpenes reaches a quasi-asymptotic value. Therefore, all the fractions recovered after this time contain very low percentages of this compound family. Indeed, isobaric desorption from 60 to 140 min produces bergamot oil deterpenated fractions. Detailed results of this part of desorption process are shown in Table 1, in which the desorbed amount of hydrocarbon terpenes and oxygenated terpenes is reported for a charge of bergamot peel oil of 10 g. These data were calculated from GC area contributions of the various compounds that were then converted into grams by measuring the ion trap response factors, as explained in the Analytical Methods. These results confirm that hydrocarbon terpenes are rapidly desorbed, whereas the desorption of oxygenated compounds becomes significant at desorption times higher than 40 min. After 140 min of isobaric process, the desorption of oxygenated compounds tended to be less effective. Therefore, we increased the operating pressure in two steps up to 85 and 100 bar. Small quantities of hydrocarbon terpenes
4510 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 Table 2. Identification and Quantitation of Compounds Found in Bergamot Peel Oil (Crude Oil) and in Its Fractions Produced by SC-CO2 DesorptionD peak area, % compound
RT, min
crude oil
peak area, %
RT, F2
F10
resA res B
compound
min
crude oil F2 F10 resA res B
a-thujene 11.28 0.25 0.40 elobulol 56.58 0.24 a-pinene 11.46 0.99 1.57 &,cis-Farnesol 57.57 0.33 camphene 12.35 0.11 0.27 cis-/3-santalol 59.25 0.01 0.42 /3-pinene 14.25 6.49 11.48 0.44 truns,truns-farnesol 60.14 0.01 0.75 octen-3-01 15.35 0.08 tr. 0.31 0.08 a-copaenol 65.35 19.76 /3-myrcene 15.46 2.33 2.08 4.26 5.63 0.42 C15H260 67.22 1.88 2-carene 16.22 0.11 0.10 0.31 0.32 nootkatone 0.03 67.50 a-phellandrene 17.15 0.18 0.26 0.08 0.11 octadecanal 71.47 0.19 17.47 1.29 3.89 0.60 1.04 1.24 C15H260 p-cymene 72.56 0.01 1imonene 18.12 32.14 55.33 2.16 2.75 2.29 C15H260 0.07 73.18 cis-ocimene 19.04 0.43 0.21 1.05 1.20 0.08 octadecanol 74.40 0.13 trans-ocimene 19.46 1.06 0.81 2.03 2.32 0.22 C12H1004 74.42 0.05 y-terpinene 20.19 7.54 11.58 0.24 0.20 0.60 CisHz002 75.11 0.32 0.13 citropten 20.46 0.04 75.58 0.10 12.14 C10H16 n-octanol 21.28 0.06 0.65 palmitic acid 76.40 2.51 terpinolene 22.22 0.72 1.08 0.47 0.77 0.45 'methyl palmitoleate 77.39 0.12 terpineyl acetate tr. tr. 0.45 4,7-dimethoxycumarin 23.06 tr. 77.51 0.01 linalool 23.30 11.63 3.36 24.39 11.31 3.69 heicosane 79.18 0.13 nonanal 23.41 0.11 0.13 bergapten 79.33 0.07 5.87 0.12 1,3,8pmenthatriene 25.31 0.17 0.07 0.39 methyl linolenate 79.54 0.03 sabinene hydrate 0.51 0.16 isobergapten 25.29 1.23 80.27 isopulegol 26.17 0.02 0.46 nonadecanol 81.24 0.26 26.54 0.06 0.27 0.30 tr. methyl linoleate menthone 82.13 0.08 isomenthone heneicosane 27.37 0.01 0.04 0.04 0.07 tr. ' 82.38 0.07 27.49 0.02 0.06 0.07 0.05 tr. C15H1603 neom en th o1 83.37 0.02 L-menthol 28.24 0.16 0.60 0.72 0.12 tr. hexadecatriene 83.58 0.01 4-terpineol 28.36 tr. tr. tr. 0.15 1.97 heptadecatriene 85.15 0.01 0.41 1-nonanol 29.14 0.09 0.33 bergaptol 86.41 0.15 a-terpineol 0.10 1.11 12.52 docosane 29.38 0.09 87.11 0.21 31.02 0.05 suberosin dihydrocitronellol tr. 87.38 decanal 31.41 0.16 0.32 0.34 0.60 5-(3-(methylbutenyl)-2-oxy)- 87.59 0.06 nerol 32.33 0.04 1.64 7-methoxycumarin citronellolb 33.01 1.97 isosuberenol . 88.26 0.03 neral 33.19 0.28 0.11 0.45 0.98 8-(3-(methylbutenyl)-2-oxy)- 88.46 0.01 carvoneb 33.26 0.18 7-methoxycumarin piperitoneb 34.06 0.20 methyl heicosenoate 89.14 0.13 0.76 0.11 tricosane cis-sabinene hydrate 34.15 0.07 91.26 0.08 1.47 acetate methyl tricosane 93.36 0.04 0.15 linalyl acetate 34.48 30.06 6.13 56.77 33.02 2.80 docosanol 94.34 0.72 decanol 35.00 0.04 coumarin, MW 202 0.01 95.19 geranial 35.28 0.43 0.31 1.59 tetracosane 0.03 0.64 95.37 perylla aldehyde 35.33 2.23 coumarin, MW 202 0.02 96.06 neomenthyl acetate 35.31 tr. coumarin, MW 202 0.01 97.43 tridecane 0.15 tr. tetracosanol 36.26 0.05 0.02 0.90 98.32 isomenthyl acetate 37.07 0.10 0.31 0.41 psoralen, M W 330 0.03 99.02 carvacrolb 37.39 3.36 pentacosane 99.39 0.07 1.85 carvyl acetate 40.20 tr. 0.83 not identified compd 100.11 0.03 a-terpineyl acetate 40.46 0.18 0.32 0.36 not identified paraffin 102.35 0.85 neryl acetate 42.00 0.47 0.78 1.93 tr. hexacosane 103.32 0.02 0.33 bergamottin 43.16 0.50 0.87 2.57 tr. geranyl acetate 105.40 0.10 2.07 caryophyllene 45.04 0.45 0.08 0.78 0.65 2.13 not identified paraffin 106.00 0.41 aromadendrene 46.23 0.36 0.67 0.59 0.11 heptacosane 107.17 0.04 1.14 a-humulene 47.14 0.03 0.05 0.11 0.21 octacosane 111.55 0.05 0.30 47.54 0.08 0.14 not identified compd cis+farnesene 112.00 0.57 dodecanol 0.51 nonacosane 48.29 0.14 0.54 115.09 y-muurolene 49.00 0.07 0.07 vitamin D 124.05 0.85 0.59 /3-selinene 50.39 0.04 0.12 0.78 vitaminA1 125.30 0.38 /3-bisabolene 50.59 0.52 0.92 1.92 5.07 not identified compd 0.92 129.33 trans-p-farnesene 54.22 tr. 0.63 0.38 C&48@ 134.32 1.56 spathulenol 54.47 0.11 ti-. C29H480 1.81 137.28 caryophyllene oxide 55.01 0.40 0.11 CzgHso@ 12.00 143.37 55.45 C15H260 a Area percentages are expressed without any correction factor. Found only in the residue since these are oxidation products of pinene and limonene. Probably ergosterol, stigmadienol, and stigmastenol, respectively.
were further desorbed together with large quantities of oxygenated terpenes (see Table 1for desorbed quantities and Table 2 for compound identification). These two steps allowed a fast and quasi-complete recovery of all oxygenated compounds contained in bergamot oil. The proposed fractionation scheme is not the only one that it is possible to use. For example, a larger pressure
increase after 60 min of desorption could produce faster processing. During all the desorption process, neither coumarins and nor psoralens were detected in the GC traces of the desorbed fractions. The identification of compounds constituting the crude bergamot peel oil is reported in Table 2; the
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4511
r
sise
33 :sa
4W8
66 :a
6088 188 :Be
S i 8
133 :2e
Figure 1. GC trace of untreated bergamot peel oil. (1)Limonene, (2) linalool, and (3) linalyl acetate. 180y:
1
. A
TOT-
'
2
3
Figure 2. (a) GC trace of desorbed fraction F2 (terpenic fraction). (b) GC trace of desorbed fraction F10 (a deterpenated fraction). (1)Limonene, (2) linalool, and (3) linalyl acetate.
corresponding GC trace is shown in Figure 1. As expected, limonene (32.14%), linalool (11.63%), and linalyl acetate (30.06%)are the major components (Dugo et al., 1991). Among the nonvolatile compounds, only citropten (0.10%)and bergapten (0.07%)were detectable in small amounts in the crude oil. Waxes and other high molecular weight compounds were not detectable in the crude oil, since the latter was previously submitted to a "winterization" process (Hendrix et al., 1992), which is a cool storage procedure that induces the precipitation of a part of these compound families. In Table 2, we also reported the detailed composition of two desorbed fractions. They were collected after 37 and 140 min of desorption (Table 2, columns F2 and FlO), i.e., at the beginning and at the end of the isobaric desorption step. The different compositions of these two fractions can be qualitatively evaluated in Figure 2 where their GC traces are reported. The corresponding columns in Table 2 quantitatively illustrate the large composition change in the desorbed oil during the process. Fraction F2 contained limonene (55.33%) and y-terpinene (11.58%)as major compounds and had a 10.88%
area of oxygenated terpenes. Fraction F10 contained linalool (24.39%) and linalyl acetate (56.77%) as major compounds and had a 85.96% area of oxygenated terpenes. The deterpenated bergamot oil was obtained by mixing together all the fractions collected after 50 min of desorption. Therefore, the overall oxygenated terpene percentage was lower than that of fraction F10 and had a value of about 72%. This is, however, a good result since the content of oxygenated compounds in the bergamot oil was approximately 1.85 times higher compared to the crude oil, which contained about 39% of oxygenated compounds. Bergamot oil contains a very high percentage of oxygenated compounds compared to other citrus peel oils. Indeed, these have oxygenated percentages which, as a rule, are lower than 5%. Therefore, in these cases, it is possible t o obtain higher enrichment ratios of oxygenated compounds. For example, the ratio of enrichment was about 20 times higher in the case of lemon peel oil (Barth et al., 1994). The final content of oxygenated compounds can be modified by a different selection of the fractions that are mixed together. Of course, an optimum value will exist between the oxygenated content and the yield of the process. The higher the required oxygenated content is, the lower the process yield will be. In Table 2, we also reported the composition and identification of the residue collected in the desorption column &r the fractionation process. This residue was collected by washing the column with warm ethanol. Its weight was 0.87 g. Residue A corresponds to that part of residue that is soluble in dichloromethane. It contained undesorbed terpenes and also coumarins and psoralens. We identified citropten (12.1%), bergapten (5.9%), bergaptol (0.1%),and bergamottin (0.1%),which are characteristic of the coumarinic and psoralenic fractions of bergamot oil. Some other psoralens were found too, though in smaller quantities. Residue B corresponds to that part of residue that precipitated from the dichloromethane solution. It was then, solubilized with hexane. It also contained undesorbed terpenes, but it was mainly characterized by the content of waxes and sterols. The percentage content of terpenes in both parts of residue indicates that perhaps longer processing times were necessay to obtain a more complete desorption of the volatile compounds. Moreover, as indicated in a previous paper (Barth et al., 19941, it is possible that small quantities of oil could not be desorbed since waxes can block some pores of the adsorbent medium. The large number of high molecular weight compounds that were identified in the residue suggests that the desorption process could also be useful as a concentration method to study the higher molecular weight compounds contained in citrus peel oils. We studied the selectivity of the desorption process with different solute loadings into the adsorbent medium. We loaded from 5 t o 30 g of bergamot peel oil in 100 g of Kieselgel and then performed the desorption at the previously indicated conditions. The results of this analysis are reported in Figure 3 in terms of oxygenated compounds recovered in the overall deterpenated fraction against the quantity of solute adsorbed in the column. The straight line in Figure 3 indicates the ideal behavior of the process: i.e., the complete recovery of all the oxygenated compound charged into the desorber. A good performance is obtained for
4512 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
+
the desorption constant (l/min). We chose SO= SI S2 where SIand S2 are the concentrations of hydrocarbon terpenes and oxygenated terpenes, respectively, at the beginning of the desorption process. Equation 2 expresses the lack of information on the mass-transfer process and on the equilibrium relationship through the kinetic constant k. The system of equations (1)and (2) with the proposed initial and boundary conditions can be analytically solved for each key compound family, giving
12
/
10
solute loading, g
(3)
Figure 3. Effect of different solute loadings on the selectivity of the desorption process. 4,
1
where cexis the fluid-phase concentration at the exit of the desorber and H is the bed height (cm). The desorbed amount (y> can then be calculated as
Y = Jc,,Q dt
20
40
60
80
100
120
140
100
lime, min
Figure 4. Desorption breakthrough curves. Continuous curves: eq 4. 0 = hydrocarbon terpenes, 0 = oxygenated terpenes.
loadings up to 15 g. Higher quantities of bergamot peel oil charged into the desorber produce a strong decrease of the process performance. This behavior could be explained by the overloading of the adsorption column. The separation can still be feasible because of the different adsorption equilibria of the oil components, but its efficiency will be lower (Zetzl et al., 1994). Yield data at 75 bar and 40 “C in Table 1 are also reported in Figure 4 against the desorption time. This representation is referred to as the breakthrough curve for desorption of the two key compound families. The data can be fitted by developing an adequate model of the process. When axial dispersion is neglected, the overall mass balance on a bed section of height dz can be written as
a c u-= a c at az
E - +
-(1-
as % t
where 6 is the void fraction in the packed bed, c is the concentration of solute in the fluid phase (g/cm3),U is the solvent velocity (cdmin), S is the concentration of the solute in the adsorbent medium (g/cm3),t is the time (min), and z is the position along the bed (cm). The initial and the boundary conditions are at t = 0, c = 0 and a t z = 0, c = 0. To express the mass balance in the solid medium (Kieselgel), information about the mass-transfer coefficient is needed. Moreover, an equilibrium relationship is required to connect oil concentration between the fluid and solid phases. For a complex system like the one constituting the bergamot oil, this information is missing. Nevertheless, it is possible to adopt the approach suggested by Tan and Liou (1988, 1989). They suggested a mass balance in the solid phase written in terms of a linear desorption kinetics:
as = -kS at with the initial condition at t = 0, S = SO,where k is
(4)
where Q is the COz volumetric flow rate (cm3/min). Two values of k will be obtained from the model, one for each key compound. The desorption constant k for hydrocarbon terpenes and oxygenated terpenes can be obtained by fitting eq 4 to the experimental data. The best fit was calculated by using a nonlinear leastsquares routine that implemented the Marquardt method (Press et al., 1992). A fairly good agreement between the model curves and the experimental results was obtained, as shown by Figure 4. The best fit values for k were 0.037 and 0.004, for hydrocarbon terpenes and oxygenated terpenes, respectively.
Acknowledgment This work was partly developed in the framework of Progetto Strategic0 “Tecnologie Chimiche Innovative”, C.N.R., Rome, Italy.
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* Abstract published in Advance A C S Abstracts, September 15, 1995.
