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Institution of Chemical Engineers
Trans IChemE, Vol 79, Part A, September 2001
EFFECT OF ETHERIFYING SPECIES ON
O-ALKYLATION OF PHENETHYL ALCOHOL TO
PERFUMERY ALKYL ETHERS
V. V. BOKADE
Catalysis Division, National Chemical Laboratory, Pune, India.
T
he ethers like phenethyl methyl ether (PEME), phenethyl ethyl ether (PEEE), phenethyl isopropyl ether (PEIPE) and phenethyl isoamyl ether (PEIAE) are major components in the perfume and avour industries. The O-alkylation reaction of phenethyl alcohol (PEA) with alkanols like methanol, ethanol, isopropanol, isoamyl alcohol, in the presence of Dodecatungstophospheric acid, a heteropoly acid supported on K-10 (montmorillonite) catalyst, to the selective formation of PEME, PEEE, PEIPE, PEIAE, respectively, were studied. The initial rate of reaction of phenethyl alcohol with different alkanols was found to be independent of the concentration of the alkanol for an initial phenethyl alcohol concentration of
1.465 ´ 10 -^3 gmol cm-^3. The reaction up to 90 minutes on stream, was also independent of the
concentration of alkanols, despite the fact that different initial concentrations of alkanols were chosen. It is interesting to note that, the Arrhenius plots of ln(rate) versus 1= T for all alkanols are represented by a single line with slope which is almost equal, to give activation energy values of 6.1, 6.6, 6.1 and 6.1 kcal gmol-^1 for methanol, ethanol, isopropanol and isoamyl alcohol, respectively. According to the Eiley–Rideal type of mechanism, the rate of phenethyl alcohol reaction is controlled by its chemisorption on the active sites in the absence of any intraparticle resistances.
Keywords: dodecatugestophospheric acid, K-10, PEA, alkanol; PEME; PEEE; PEIPE; PEIAE; Eiley–Rideal mechanism.
INTRODUCTION
Ethers, acetals, ketals, and hemiacetals form an interesting group of oxygenated compounds in the perfume and avour industry. Substituted phenyl and phenethyl ethers have outstanding attributes and these could be prepared typically by the Williamson synthesis on the laboratory scale, which works best for primary halides and is least successful for tertiary halides. Water, dimethyl formamide, acetone or even alcohols are used as solvents. Alkyl phenethyl ethers such as phenethyl methyl ether (PEME), phenethyl ethyl ether (PEEE), phenethyl isopropyl ether (PEIPE) and phenethyl isoamyl ether (PEIAE) and ethyl-ortho-methoxy benzyl ether (EOMBE) are important perfumery ingredients, whereas methyl-tert-butyl-ether (MTBE), ethyl-tert-butyl- ether (ETBE), tert-amyl-methyl ether (TAME), are important oxygenates in reformulated gasoline (which meets strict environmental anti-pollution measures). The world market for bulk perfumery and avour now exceeds US $ 7 billion. The PEME, PEEE, PEIAE and PEIPE, are signi cantly used in the perfume and avour industry. There are several routes by which ethers can be synthesized1,2, but the O-alkylation of phenethyl alcohol with alkanols, in the presence of solid acid catalyst, appears to be the most attractive, having industrial relevance. It will be necessary to adopt cleaner and safer processes; it would be useful to employ intrinsically safe processes. It was therefore decided to undertake a systematic investigation
on the preparation of different ethers including kinetic aspects by using modi ed metal oxides. The basic raw material for preparation of the ether in this study is phenethyl alcohol, also known as rose oil. This is manu- factured via the Fridel–Crafts reaction of benzene with ethylene oxide, using stoichiometrically excessive amounts of aluminium chloride. This is not a clean route and a better and cleaner process is via epoxidation of styrene and direct hydrogenation of styrene oxide to phenethyl alcohol. This route, via epoxidation, is generic and substance like methyl styrene, p-isobutyl styrene etc, can be used. The various ethers considered in this work are discussed in Table 1. The O-alkylation of PEA with different alkanols using solid acid catalyst is an easy, safe and economical process. It will not create any environmental problem, such as acid waste, which is the major problem when using homoge- neous catalysts. The reaction schemes for acid catalyzed etheri cation reaction are given in Figure 1.
EXPERIMENTAL
Chemicals
Phenethyl alcohol and the ethers (99% plus purity) were used without further puri cation. The alkanols and cyclo- hexane were analytical reagents, obtained from M=s s.d. Fine Chemicals.
Catalyst The HPA=clay was prepared as per the procedure. An appropriate quantity of montmorillonite clay (K-10), (Fluka grade) was taken and evacuated in an oven for 20 minutes, dodecatungstophospheric acid dissolved in methanol was added dropwise, and was stirred using the incipient wetness technique. The catalyst was dried at 120°C for 2 hours and
calcined at 260°C for an additional 3 hours. It was then stored in sealed bottles and dried prior to use.
Experimental Set Up
The reactor consisted of a at bottom glass vessel of 5 cm in diameter with a capacity of 250 cm^3 , equipped with four
CH 2 CH 2 OH
HPA\CLAY
CH 2 CH 2 O
PEME
CH 3
CH 2 CH 2 OH
HPA\CLAY
CH 2 CH 2 O
PEEE
CH 3 CH 3 OH
CH 2
CH 2 CH 2 OH
HPA\CLAY
CH 2 CH 2 O
PEIPE
CH 3 CH OH
CH
CH 3
CH 2
CH 2 CH 2 OH
CH 2 CH 2 O
PEIAE^ (d)
CH 2 CH OH
CH 3
CH 2
CH 2 CH 2
CH 2 CH
CH 3
Figure 1. Reaction scheme for acid catalyzed etheri cation reactions.
Table 1. Phenyl ethyl alkyl ethers. PEME—Phenyl ethyl methyl ether; PEEE—Phenyl ethyl ethyl ether; PEIPE—Phenyl ethyl isopropyl ether; PEIAE—Phenyl ethyl isoamyl ether.
Name Molecular formula Structural formula Molecular weight Boiling point Order of importance in perfumery
PEME C 9 H 12 O
O CH 3 136.20 187 °C i) As a powerful and lifting ingredient in Rose, Hyacinth, Jasmine, Lilac, Pikake, etc. ii) In ‘Kewda oil’.
PEEE C 10 H 14 O
O C 2 H 5 150.22 192 °C i) In Kewda oil, Hyacinth, Narcissns, Wisteria or in fancy oriental bases.
PEIPE C 11 H 16 O
O CH 3
CH 3
164.24 227 °C i) Gives powerful heavy oral fragrances, oriental balsamic fragrances, opopanax etc. ii) Gives variation to lavender.
PEIAE C 13 H 20 O
O CH^3 CH 3 192.30^277
°C i) In Rose, Lilac, Gardenia, Hyacinth, Lily, Narcissn, etc. ii) In soap and household products.
626 BOKADE
Effect of Temperature on the Reaction of Phenethyl Alcohol with Different Alkanols
As explained earlier, the initial rates of reaction of phenethyl alcohol with methanol, ethanol, isopropanol, isoamyl alcohol were found to be independent of the concentration of the alkanol for an initial PEA concentration
of 1.465 ´ 10 -^3 gmol cm-^3 and were also independent of
the concentration of alkanols for reactions up to 90 minutes, despite the fact that different initial concentrations of alkanols were chosen. However, Figure 5 could show that the conversion is independent for reaction times as low as ~ 30–40 minutes. This prompted the author to study the effect of temperature on the rates. The experiments with different alcohols were carried out at different temperature ranges, to increase the phenethyl alcohol conversion without liquid phase transformation. Figures 7, 8, 9 and 10 depict the conversion versus time plots for methanol, ethanol, isopropanol and isoamyl alcohol respectively with tempera- ture as a parameter. The Arrhenius type plots of ln(rate) against 1= T were made as shown in Figure 11. It is inter- esting to note that these points are represented by a single line with a slope which is almost equal to give activation energy values of 6.1, 6.6, 6.1 and 6.1 kcal gmol-^1 for methanol, ethanol, isopropanol, isoamyl alcohol, respec- tively. This is not a fortuitous coincidence but calls for a detailed analysis of experimental data.
Controlling Mechanism and Kinetics of Reaction The theory developed3–8^ could now be utilized to discern the controlling mechanism. Figure 12 is a typical plot of the integrated rst order equation, - ln(1 7 XA ) versus time, for different initial concentrations of PEA. Figure 6 shows a plot of the initial rate of reaction versus the initial concen- tration of different alkanols, where the weight ratio of PEA
Figure 6. Effect of initial concentration of alkanols on initial rate. Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
Figure 7. Effect of temperature (reaction of PEA with methanol). Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
Figure 8. Effect of temperature (reaction of PEA with ethanol). Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
Figure 9. Effect of temperature (reaction of PEA with isopropyl alcohol). Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
Figure 5. Plot of conversion of PEA, XA (%) versus time (minutes) for different alkanols. Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
628 BOKADE
to alkanol was 3 : 10, but the initial concentration of each alkanol is different. This shows an extraordinary behaviour in that the initial rates are independent of the type of alkanol chosen. This suggests that the rate is zero order in alkanol concentration upto a certain value, but it is rst order in PEA concentration. Three possibilities exist:
(i) There is an external resistance to the transfer of PEA from the bulk liquid phase to the exterior surface of the particle and no matter what alcohol is chosen, the rate would be the same for the initial concentration of PEA. This possibility has been discounted because there was no effect of speed of agitation and particle size beyond 1500 rpm and 82.5 mm, respectively.
(ii) The in uence of intraparticle resistance is discounted due to the following: The same rate of reaction of PEA is obtained although different alkanols are used. It is tempting to assign the rst order dependence of the rate of PEA concentration alone, to the pseudo- rst order behaviour due to excess of alkanol taken. It is expected that the surface rate constants should be different for these alkanols and also the values of effective diffusivity De for PEA into the reactive alkanol solution. This would render differ- ent values of Thiele modulus, j (i.e. dp = 6 * (^) Ö k 1 = De ; where k l is the pseudo rst order constant) and hence leading to different effectiveness factors, thereby lead- ing to different rates of reaction. But the facts are contrary; this argument also suggests that the surface reaction controlled mechanism does not become opera- tive. The values of energy of activation are also lower for all alkanols. (iii) The rate of reaction of PEA is controlled by its chemisorption on the active sites in the absence of any intraparticle resistances. For this case, the Eiley– Rideal mechanism is operative3,6. When only the rate of adsorption is controlling:
RA = ktwCA =( 1 + KACA )
where RA is the rate of reaction for A i.e. phenethyl alcohol, gmol-^1 cm-^3 s; k l is the adsorption constant; w is the catalyst loading, g-^1 cm-^3 ; CA is the concentration of phenethylalcohol, gmol-^1 cm-^3 ; KA is the adsorption equilibrium constant for A i.e. phenethyl alcohol.
Thus, the plots were made of w = RA versus 1= CA for different values of w , and the initial concentration of PEA for all alkanols. Figure 13 depicts this type of plot. It passes through origin for alcohols, thereby indicating that: RA = ktwCA i:e: KACA < 1 :
Thus, it is chemisorption of PEA, which is the rate-deter- mining step for the present case.
CONCLUSION
The phenethyl methyl ether (PEME), phenethyl ethyl ether (PEEE), phenethyl isopropyl ether (PEIPE) and phenethyl isoamyl ether (PEIAE) etc, which are signi cantly used in the perfume and avour industry, were system-
Figure 12. Kinetics of reaction for different initial concentrations of PEA with alkanols. Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst load- ing: 10% w=w, speed: 1500 rpm. Figure 13. Plot of w = RA 0 versus 1= CA 0.
Figure 10. Effect of temperature (reaction of PEA with isoamyl alcohol). Catalyst used: 20% HPA=Clay, Temp: 70°C. Catalyst loading: 10% w=w, speed: 1500 rpm.
Figure 11. Arrhenius plot for the reaction of PEA with alkanols.
EFFECT OF ETHERIFYING SPECIES ON O-ALKYLATION 629
