Selectivity Enhancement by Catalyst Deactivation in Three-Phase

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Selectivity Enhancement by Catalyst Deactivation in Three-Phase Hydrogenation of Nerol Pa1 ivi Ma1 ki-Arvela, Narendra Kumar, Ali Nasir, Tapio Salmi, and Dmitry Yu. Murzin* Process Chemistry Centre, Åbo Akademi University, 20500 Turku, Finland

Selectivity enhancement by catalyst deactivation in the liquid phase has been investigated in this work with batchwise nerol hydrogenation over Rh/Al2O3 catalyst in 2-pentanol as a model system. The desired product is an unsaturated alcohol citronellol. Besides hydrogenation, isomerization of nerol to citronellal as well as hydrodeoxygenation reactions also took place. Other possible products were the undesired product of complete hydrogenation, 3,7-dimethyloctanol, and cis-3,7-dimethyl-2-octenol. The selectivity enhancement of citronellol as a function of nerol conversion was observed at higher reaction temperatures, with higher initial nerol concentrations and under lower hydrogen pressures. Under these conditions the final conversions even after prolonged reaction times were lower than 100% due to catalyst deactivation. Additionally, nearly 3-fold selectivity enhancement to citronellol compared to that of the fresh catalyst was achieved in the second reuse of the same catalyst. The main reason for the catalyst deactivation is most probably decarbonylation of nerol. 1. Introduction Catalyst stability and deactivation are industrially very important issues. Most of the bulk chemicals are produced in gas-phase continuous reactors, and catalyst deactivation in the gas phase is quite well recognized. At the same time the majority of the fine chemicals are produced under milder conditions in liquid phase in batch reactors. The occurrence of catalyst deactivation in the liquid phase is recognized,1-4 but due to the difficulties in separating kinetics and catalyst deactivation, there is still a need for more detailed investigation of deactivation in the liquid phase. Not only activity but also selectivity can change with catalyst deactivation, because the catalyst surface can change in several ways during the reaction. In chemoselective hydrogenations the desired product selectivity can be affected by several factors, like reaction temperature, pressure, solvent, and the type of catalyst. Typically, in consecutive and parallel hydrogenations the selectivity to an intermediate product obeys the general pattern, where it goes through a maximum with increasing conversion.5 However, a nontypical selectivity enhancement to an intermediate product with increasing conversion has been reported in some cases, like in the hydrogenation of cinnamaldehyde over Ru zeolites in the liquid phase.6,7 Furthermore, gas-phase deactivation of the Ru/Y catalyst with 1-butene yielded to increased selectivities to cinnamyl alcohol under 50 bar at 100 °C in cyclohexane compared to that of the fresh catalyst.6 In the gas-phase hydrogenation of crotonaldehyde the origin for selectivity enhancement in repeated experiments was the selective poisoning of the most active metal sites.8 The model reaction in this work is hydrogenation of nerol to citronellol, which is industrially used in the production of rose odor.9 The preliminary results in nerol hydrogenation over Rh/Al2O3 catalyst revealed that the * To whom correspondence should be addressed. Tel: + 358 2 215 4985. Fax:+ 358 2 215 4479. E-mail: [email protected].

Figure 1. Reaction scheme in nerol hydrogenation.

selectivity to a desired, intermediate product, citronellol, increased with increasing conversion.10 Selectivity enhancement in nerol hydrogenation was not, however, observed over a Pt/SiO2 catalyst.11 The main aim in this work was to investigate the origin of the selectivity enhancement over Rh/Al2O3 catalyst in nerol hydrogenation by varying the reaction conditions as well as to investigate the catalyst reuse. Additionally, the reaction mechanism was studied, since besides hydrogenation there also occurs isomerization to citronellal (Figure 1). There are several mechanisms describing catalyst deactivation, like poisoning, sintering, leaching, fouling, and mechanical degradation.12 Catalyst poisoning occurs when strong adsorbents, like sulfur, are present in the feed. Sintering needs higher reaction temperatures, and thus, it might be excluded in liquid-phase reactions.

10.1021/ie050191k CCC: $30.25 © 2005 American Chemical Society Published on Web 09/15/2005

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Leaching of an active metal has been observed with catalysts, which have relatively weak interactions between the support and the active metal, like Ru/C, or when the pH of the reaction milieu deviates from neutral values, e.g., in the hydrogenation of sugars. Fouling means crystallite encapsulation and pore blocking of a supported metal due to carbon deposition. This phenomenon can occur already at relatively low temperatures, and it is specific for different metals and substrates. Different metals exhibit different electronic structures, and due to their d bandwidth, they can act differently in their activity for breaking, e.g., C-C bonds. Additionally different functional groups in organic compounds adsorb in different ways, and the stabilities of the adsorbed species deviate in the case of aldehydes and alcohols.13 Nerol hydrogenation has been very scarcely reported.10,11,14,15 In the work of Hubaut14 nerol was hydrogenated over a copper chromium oxide catalyst in decalin at 140 °C under atmospheric pressure. Under these conditions the main product was citronellol, and a substantial amount of citronellal (maximal yield of 25%), the isomerization product of nerol was formed. Furthermore, a few percentage of hydrocarbons were formed. The TOF for nerol hydrogenation over Pt/SiO2 catalyst in hexane at 25 °C and under 20 bar of hydrogen was reported by Singh et al.15 and was nearly 8-fold higher that of the TOF reported for citral. On the other hand, at 100 °C the TOF was 214-fold smaller than that at 25 °C, and fast initial deactivation was observed. The main products over Pt/SiO2 catalysts were citronellol, cis-3,7-dimethyl-2-octenol, and 3,7-dimethyloctanol. However, no kinetic curves for nerol hydrogenation were presented.15 Recently Ma¨ki-Arvela et al.11 investigated different catalysts in nerol hydrogenation in 2-pentanol at 70 °C and 10 bar of hydrogen over Pt/ SiO2, Pt/H-Y, and Pt/H-MCM-41 catalysts. The selectivities to citronellol at 30% conversion of nerol were 65%, 55%, and 25% over Pt/SiO2, Pt/H-MCM-41 and Pt/ H-Y, respectively. The selectivity to citronellol was relatively constant with increasing nerol conversion over Pt/SiO2 and over Pt/H-Y, whereas over Pt/H-MCM-41 it decreased due to hydrodeoxygenation of nerol, i.e., elimination of water followed by hydrogenation of the CdC double bond. In the current work nerol hydrogenation was performed over 5 wt % Rh/Al2O3 catalyst. Since the preliminary results over Rh/Al2O3 catalyst revealed a nontypical selectivity-conversion relationship, i.e., the selectivity to citronellol increased with increasing nerol concentration,10 a more detailed kinetic analysis from the product distribution under different conditions was carried out. The main variables were temperature, pressure, initial nerol concentration, and the reuse of a partially deactivated catalyst. The aim was to achieve more detailed information on catalyst deactivation and selectivity dependence from the liquid-phase hydrogenation of multiunsaturated compounds. Besides hydrogenation, hydrodeoxygenation11 and decarbonylation16 also occur. The confirmed hydrodeoxygenation products in nerol hydrogenation are 3,7dimethyloctene and 3,7-dimethyloctane.11 Decarbonylation of an unsaturated alcohol could explain the fast catalyst deactivation during nerol hydrogenation at 100 °C.15 The decarbonylation was observed in the adsorption of allyl alcohol on a Pd surface16 and in the desorption of allyl alcohol from Rh(111).13

2. Materials and Methods 2.1. Materials. Nerol (90%, Fluka 72170) and 2-pentanol (>98%, Merck 807501) were used as received, and 5 wt % Rh/Al2O3 (Lancaster 0544) was used as a catalyst. The kinetic measurements were carried out under hydrogen (AGA, 99.999%) and helium (AGA, 99.9996%) atmospheres. Hydrogen was obtained from AGA (99.999%). 2.2. Methods. The reaction was performed in a stainless steel autoclave with a liquid volume of 200 mL. The reaction temperature and pressure were varied between 50 and 90 °C and 2-30 bar total pressure, respectively. The initial nerol concentration was in a range of 0.01-0.1 M. Typically, 200 mL of solvent, nerol (306 mg), and 100 mg of catalyst were used in the hydrogenation experiments. Prior to the start of the reaction the substrate saturated with hydrogen was injected into the reactor, where the reduced catalyst was located under hydrogen atmosphere. The catalyst was reduced in situ at 175 °C for 60 min, if not otherwise stated. The stirring rate of 1500 rpm was used in experiments. The experiments were carried out in the absence of mass transfer effects due to the use of small catalyst particles and vigorous stirring.17 In the catalyst reuse experiments the catalyst was reduced at 175 °C for 60 min. 2.3. Analysis. The components in the reaction mixture were analyzed with a gas chromatograph equipped with a capillary column (DB-1, length 30 m, i.d. 0.25 mm, film thickness 0.50 µm) and an FI detector. The temperature program for analysis was the following: 120 °C (1 min), then 0.40 °C/min to 130 °C, then 15 °C/ min to 160 °C. The products were identified with GCMS. 2.4. Catalyst Characterization. The mean catalyst particle size of the Rh/Al2O3 catalyst was 53.1 µm, the BET specific surface area was 175 m2/gcat, and the metal dispersion was 18%. The acidity of the catalyst was measured by NH3 desorption to be 0.47 mmol/gcat.18 Temperature-programmed reduction of the spent Rh/ Al2O3 catalyst was performed with a quadrapole mass spectrometer (QTMD). In hydrogen TPR experiments, 50% H2 diluted with 50% N2 (AGA) was used in reducing the carbon species on the catalyst. The formed methane was quantitatively calibrated for methane (AGA). The specific surface area was measured by nitrogen adsorption (Sorptomatic 1900, Carlo Erba Instruments). 3. Results and Discussion 3.1. Reaction Network in Nerol Hydrogenation. The main products in nerol hydrogenation were citronellol, 3,7-dimethyloctanol, and 3,7-dimethyl-2-octenol. Besides hydrogenation, isomerization of nerol to citronellal and hydrodeoxygenation reactions also occur (Figure 1). The former reaction requires the presence of hydrogen, since no reaction occurred (Figure 2), when nerol transformation was carried out at 70 °C in 2-pentanol over Rh/Al2O3 catalyst with 0.1 M nerol under an inert gas, helium, during first 90 min, after which a hydrogen atmosphere (2.3 bar) was used. Isomerization of nerol resulted in citronellal (below 3%), which hydrogenated further to 3,7-dimethyloctanal. Hydrodeoxygenation reaction led to formation of 3,7dimethyloctadiene, 3,7-dimethyl-2-octene, and 3,7-dimethyloctane, which was confirmed by GC-MS. In nerol hydrogenation at 70 °C, 10 bar of hydrogen, and with

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Figure 2. Nerol transformation at 70 °C, 2.3 bar under the first 90 min with helium, after which a hydrogen atmosphere was used. The solvent was 2-pentanol. Symbols: ([) nerol, (b) unsaturated alcohols (cis-3,7-dimethyl-2-octenol, trans-3,7-dimethyl-2-octenol), (+) citronellol, (2) geraniol, (9) 3,7-dimethyloctanol.

Figure 3. Hydrogenation of nerol over Rh/Al2O3 catalyst reduced at 175 °C for 60 min ([) and at 450 °C for 60 min (9). Conditions: 70 °C, 10 bar total pressure, 0.01 M nerol in 2-pentanol.

an initial nerol concentration of 0.02 M, totally 5% of the hydrocarbons were formed after 180 min of reaction time. The product distribution within diunsaturated, monounsaturated, and saturated C8 hydrocarbons was 8%, 46%, and 46%, respectively. This result indicates that it is possible to hydrogenate diunsaturated C8 hydrocarbons over a Rh/Al2O3 catalyst, which is in accord with the literature,19 where it was demonstrated that Rh was the most suitable metal for hydrogenating diolefins with isolated double bonds. Additionally, the presence of diene confirmed that hydrodeoxygenation of nerol is possible prior to hydrogenation. 3.2. Selection of the Catalyst Reduction Temperature. Two different catalyst reduction temperatures were investigated in nerol hydrogenation over Rh/ Al2O3 catalyst, namely, 175 and 450 °C (Figure 3). The catalyst deactivation was very prominent over the latter catalyst, whereas the former reduction temperature was adequate for a commercial, passivated Rh/Al2O3 catalyst. This result was unexpected, since our previous results in citral hydrogenation over the same Rh/Al2O3 catalyst resulted in a higher hydrogenation rate, when the catalyst was reduced at 500 °C than with the catalyst reduced at 350 °C. The Rh/Al2O3 catalyst reduced at 500 °C exhibited a dispersion of 28%, whereas the catalyst reduced at 350 °C had a dispersion of 18%.18 The reactant structure affects the catalyst performance indicating that the catalyst activity could

be kept high in the case of aldehyde hydrogenation compared to that of hydrogenation of an unsaturated alcohol, nerol. According to TPR of a passivated 2.3 wt % Rh/Al2O3, the main peak originating from the metallic rhodium is between the temperatures of -20 and 150 °C, exhibiting a maximum close to 70 °C. About 30% of the hydrogen during TPR was consumed between temperatures of 150-,600 °C corresponding to the more strongly bound spilt over hydrogen.20 3.3. Influence of Pressure. The initial nerol hydrogenation rate increased from 74 to 86 mmol/min/gRh, when the total pressure increased from 10 to 30 bar (Table 1). The initial hydrodeoxygenation rate was independent of the total pressure under the studied pressure range. At higher conversion levels the catalytic activity was higher under the higher hydrogen pressures (Figure 4a). The catalytic activity declined, however, within first 20 min due to catalyst deactivation under 10 and 20 bar, whereas under 30 bar of hydrogen the hydrogenation proceeded very fast. The final conversion was the lowest under 10 bar of total pressure, i.e., 78%, whereas 95% and 100% conversions were achieved under 20 and 30 bar of total pressure, respectively. The selectivity to citronellol increased with increasing conversion under 10 bar of hydrogen (Figure 4b). On the other hand, under 30 bar a typical selectivity conversion relationship was observed for a consecutive reaction indicating that citronellol was hydrogenated further to 3,7-dimethyloctanol. The chemoselectivity ratio between the hydrogenation of the double bond at C3 and C7, i.e., the ratio between 3,7-dimethyl-2-octenol and citronellol, decreased with increasing hydrogen pressure favoring the formation of citronellol. The initial rate for formation of citronellal was independent of the hydrogen pressure, whereas the further hydrogenation of citronellal to 3,7-dimethyloctanal was enhanced with higher hydrogen pressure. The yields of the final product, 3,7-dimethyloctanol, after 180 min of reaction time under 10, 20, and 30 bar of total pressure were 34%, 48%, and 87%, respectively. The lower yields of 3,7-dimethyloctanol under lower (10 and 20 bar) hydrogen pressures indicated substantial deactivation under these conditions. The initial hydrodeoxygenation rate was independent of hydrogen pressure, whereas at higher nerol conversion levels less hydrodeoxygenation products were formed under 30 bar of hydrogen. These higher yields of hydrodeoxygenation products formed at lower hydrogen pressures are in accordance with the negative order for hydrodeoxygenation reported in gas-phase transformations.21 The main hydrodeoxygenation products 3,7dimethyloctadiene, 3,7-dimethyloctene, and 3,7-dimethyloctane originate from the Lewis acidity of Rh/ Al2O3. Additionally, the catalyst was prepared from a chloride-containing precursor,18 which increases the Lewis acidity.22 Both hydrodeoxygenation and isomerization of allylic alcohols are reported to be acid-basecatalyzed reactions.23 The selectivity enhancement toward citronellol with catalyst deactivation was observed under 10 and 20 bar of total pressure, which is explained by the retarded hydrogenation of citronellol and 3,7-dimethyloctenol to 3,7-dimethyloctanol. 3.4. Influence of Temperature. The initial hydrogenation rate increased with increasing temperature from 50 to 70 °C, whereas at a higher temperature, 90

Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9379 Table 1. Initial Reaction Rate of Nerol and Selectivity to Citronellol at 40% and 80% Conversion under Different Pressures at 70 °C in 2-Pentanola pressure (bar)

initial total reaction rate (mmol/min/gcat)

initial hydrodeoxygenation rate (mmol/min/gcat)

initial hydrogenation rate (mmol/min/gcat)

selectivity to citronellol at 40% conversion (%)


10 20 30

3.9 3.2 4.5

0.19 0.19 0.19

3.71 3.01 4.31

18 22 32

27 20 25

a

The amount of catalyst was 100 mg.

Figure 4. (a) Kinetics in nerol hydrogenation and (b) selectivity to citronellol as a function of conversion at 70 °C in 2-pentanol. Symbols: (2) 10 bar, ([) 20 bar, and (9) 30 bar.

°C, the measured initial rates are not reliable due to very fast catalyst deactivation (Table 2 and Figure 5a), which occurred at this temperature within a few seconds. This result is in accordance with the data of Singh et al.,15 where very fast catalyst deactivation was observed in nerol hydrogenation in hexane at 100 °C over Pt/SiO2 catalyst at 20 bar of hydrogen. The catalyst activity remained relatively high within 10 and 30 min at reaction temperatures of 70 and 50 °C, respectively. After prolonged reaction times the obtained conversion levels of nerol decreased from 50 to 70 °C, and the lowest conversion level was obtained at 90 °C (Figure 5a). This result indicates that catalyst deactivation has already, at 70 °C, a prominent effect on the catalytic performance. The highest isomerization rate of nerol was observed at 90 °C, at which the catalyst deactivation was the fastest. Additionally, the selectivity to citronellol increased with increasing nerol conversion at 90 °C (Figure 5b), and at 40% conversion level it was 1.8-fold the selectivity observed at 50 °C (Table 2), because citronellol was very slowly hydrogenated to 3,7-dimethyloctanol at 90° C. The double bond at the C7 position was more easily hydrogenated at lower temperature (50 °C), whereas the higher temperature, 90

°C, favored the formation of citronellol and hydrogenation of the C3 double bond. The highest ratios of 3,7dimethyl-2-octenol to citronellol yields were, at 50, 70, and 90 °C, 2.4, 2.9, and 4.2, respectively. The formation of the final product 3,7-dimethyloctanol was very much affected by the catalyst deactivation, when the reaction temperature was changed. The total yield of 3,7-dimethyloctanol was only 9% at 90 °C after 180 min of reaction time, corresponding to the conversion level of 40%. The yields of 3,7-dimethyloctanol were at 50 and 70 °C after 180 min 44% and 33%, respectively. The highest amount of hydrodeoxygenation products (4%) were formed at 90 °C at the nerol conversion of 30%, whereas at 50 and 70 °C these amounts were below 2%. On the basis of the obtained product distributions at three different temperatures, it can be stated that catalyst deactivation was the most severe at 90 °C followed by that at 70 and 50 °C. This had an effect on the selectivity to citronellol, which was the highest at 90 °C. The total conversion remained, however, at 40% at 90 °C, and the yield of an intermediate product citronellol was maximally only 19%. 3.5. Influence of Initial Nerol Concentration. In these experiments, the initial nerol concentration was varied while the amount of catalyst was kept constant (Table 3 and Figure 6a). The total reaction rates of nerol transformations were slightly increased with increasing initial nerol concentration (Table 3), whereas the initial hydrogenation rates exhibited zero-order dependence on the initial nerol concentration. The latter result indicates strong adsorption of nerol. The initial hydrodeoxygenation and isomerization rate of nerol increased with increasing initial nerol concentration. The formation of 3,7-dimethyloctanal decreased with increasing initial nerol concentration indicating more prominent catalyst deactivation by decreasing the hydrogenation rate of citronellal. The chemoselectivity in hydrogenation of the double bond at the C3 or C7 positions was changed with different initial nerol concentrations. More 3,7-dimethyl2-octenol was formed with higher initial nerol concentrations, i.e., with 0.02 and 0.05 M than with 0.01 M. In fact there were two different opposite effects of initial nerol concentrations visible, namely, less citronellol was formed when C7 hydrogenation was enhanced with increasing initial nerol concentration while more citronellol was formed due to catalyst deactivation. Because of the first effect the chemoselectivity ratio of the yields of 2-dimethyl-2-octenol and citronellol increased slightly from 0.4 to 0.5 with increasing initial nerol concentrations from 0.01 to 0.05 M. This result indicates that the adsorption mode depends on nerol concentration and more citronellol could be initially formed with a lower initial nerol concentration. In the latter case the selectivity to citronellol increased with increasing initial nerol concentration and increasing conversion

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Table 2. Initial Reaction Rate of Nerol and Selectivity to Citronellol at 40% and 80% Conversion at Temperatures under 10 Bar of Total Pressure in 2-Pentanola temp (°C)






50 70 90

2.8 3.9 2.9

0.08 0.19 0.38

2.72 3.71 2.52

15 18 35

20 27 b

a

The initial nerol concentration was 0.01 M. The amount of catalyst was 100 mg. b Conversion remained below 80%.

Figure 5. (a) Kinetics in nerol hydrogenation and (b) selectivity to citronellol as a function of conversion under 10 bar total pressure in 2-pentanol. Symbols: (2) 50 °C, (9) 70 °C, and ([) 90 °C.

due to catalyst deactivation (Figure 6b). The yields of 3,7-dimethyloctanol at 20% conversion level of nerol with 0.01, 0.02, and 0.05 M initial nerol concentrations were 3%, 8%, and 12%, respectively. It can be concluded that the dominating factor, however, in the selectivity for citronellol is the selectivity enhancement due to catalyst deactivation. 3.6. Reuse of the Partially Deactivated Catalyst. The catalyst deactivation was investigated by reusing the same catalyst two times. Before the second and third experiments the catalyst was reduced at 175 °C for 60 min with flowing hydrogen. This was the same reduction procedure used for the fresh catalyst in section 3.2. The total initial reaction rates for nerol are shown in Table 4. The mass ratio of nerol to catalyst was kept constant, and due to the catalyst losses, the initial nerol concentration was decreased in order to keep the nerol to catalyst mass ratio constant. Unexpected results were obtained, as the total initial rate in nerol hydrogenation was the smallest one with the fresh catalyst with 500 mg of catalyst (Table 4). To explain this value, an additional series of experiments with the nerol initial concentration of 0.02 M and with catalyst masses of 100, 300, and 500 mg was

performed at 70 °C and 10 bar of hydrogen. The initial rates decreased with increasing the mass of catalyst from 4 to 2.2 and 1.8 mmol/min/gcat, respectively. The higher amounts of catalysts, however, exhibited higher reaction rates when plotted against normalized abscissa (mass of catalyst × time). If the reaction would occur under conditions of mass transfer limitations, the reactor productivity, e.g., the converted amount of nerol per min should be constant with different catalyst amounts. The reason for these results might be either the very fast catalyst deactivation or impurities in the nerol. According to the nerol manufacturer and the isolation method of nerol, the main impurities in nerol are geraniol and citronellol.24 Additionally, limonene was confirmed by GC-MS as impurity in the nerol. Since the chemical nature of these impurities is very close to the structures of the reactants and products, it is difficult to imagine that impurities in the nerol are responsible for the deactivation. The Rh surface can be deactivated not only due to impurities but via coke formation as well. Moreover, the catalyst contained chloride, inducing acidity, which in turn promotes hydrodeoxygenation. Analogous cases are found in the literature for instance in the hydrogenation of sitosterol over a Pd catalyst.1 The initial total reaction rate with the fresh catalyst (I) in the catalyst deactivation series (Table 4) is thus most probably affected by catalyst deactivation via coke formation. A slight catalyst deactivation was observed between the catalysts in the second (II) and in the third (III) reuse. The specific surface area of the catalyst III was 142 m2/gcat. This value has decreased from its original value by 19% indicating the accumulation of organic material inside the catalyst structure. The accumulation of carbon was additionally confirmed by hydrogen TPR of the spent catalyst (Figure 7). Methane was desorbed from the used catalyst with a temperature maximum at 290310 °C indicating that the catalyst reduction at 175 °C is not enough to remove all the carbon deposits on the catalyst surface. Nearly all the formed methane was removed at much higher temperatures, which if applied in the regeneration treatment might cause sintering of the rhodium. In nerol hydrogenation, accumulation of organic material in the catalyst was observed. The reason for this is most probably decarbonylation of nerol, which has not been studied in detail with advanced surface sensitive methods. In the literature there exists according to our knowledge only one publication where decarbonylation of unsaturated alcohols has been investigated over Rh(111) surface by HREELS and TPD techniques.13 In that work the decarbonylation of allyl alcohol on Rh(111) surface occurred at 31 °C.13 Two adsorbed species were present on the metal surface, namely, ethylidene and CO. The Rh(111) surface is very active to dehydrogenate ethylidenes to acetylides (CCH), which has been confirmed by hydrogen evolution at 132 °C. Between 31 and 146 °C the ethylidene was converted

Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9381 Table 3. Initial Reaction Rate of Nerol and Selectivity to Citronellol at 40% and 80% Conversion with Different Initial Nerol Concentrations at 70 °C and under 10 Bar of Total Pressure in 2-Pentanola initial nerol concentration (M)






0.01 0.02 0.05

3.9 4.0 5.0

0.19 0.60 1.40

3.71 3.40 3.60

18 26 28

18 32

a

The amount of catalyst was 100 mg.

Table 4. Initial Reaction Rate of Nerol and Selectivity to Citronellol at 40% and 80% Conversion under 10 Bar of Total Pressure at 70 °C in 2-Pentanol, When the Catalyst Was Reused Twicea

reuse

initial nerol concentration (M)






I II III

0.02 0.013 0.0086

1.8 2.8 2.7

0.05 0.17 0.12

1.75 2.63 2.58

26 43 51

22 46 56

a

The mass ratio between nerol to catalyst was kept constant at 1.2.

Figure 7. Methane evolution from a spent Rh/Al2O3 catalyst during hydrogen TPR.

Figure 6. (a) Kinetics in nerol hydrogenation and (b) selectivity to citronellol as a function of conversion at 70 °C and 10 bar total pressure in 2-pentanol. Symbols: ([) 0.01 M, (9) 0.02 M, and (2) 0.05 M intial nerol concentration.

to acetylidene and at the same time the adsorbed CO was converted to linear mode. The possibility for the adsorbed allyl alcohol to decarbonylate is very high due to its adsorption mode. The CdC double bond between the β- and γ-carbons is very hydrogen deficient, interacting strongly with the Rh surface. Allyl alcohol is thus adsorbed in a η3(Cγ,Cβ,O) oxametallacycle mode, which is very strained. Since the structure of the adsorbed allyl alkoxide is strained, the R-scission of the C-C bond is very probably leading to decarbonylation of allyl alcohol. Compared to allyl alcohol, the adsorption of γ-carbon on the Rh surface in nerol is sterically more difficult than in the case of allyl alcohol.

The total conversion of nerol was obtained after prolonged reaction times in the catalyst reuse series (Figure 8a), since a much higher catalyst to nerol ratio was used than in experiments reported in the previous section (Figure 6a). The selectivities to citronellol increased in reusing the catalyst (Table 4). The isomerization of nerol to citronellal and the formation of hydrodeoxygenation products were more prominent over the fresh catalyst, followed by catalyst II and III. This result is rationalized by catalyst deactivation, which declines simultaneously the catalyst acidity via coke formation. Citronellal could be further hydrogenated to 3,7-dimethyloctanal, but its formation was additionally retarded by catalyst deactivation. The maximum yields of 3,7-dimethyloctanal over I, II, and III catalysts were 6%, 2.5%, and below 2%, respectively. Chemoselectivity to citronellol was also clearly enhanced by catalyst deactivation, because the chemoselectivity ratio between the maximal yields of 3,7-dimethyl-2-octenol to citronellol was 0.62, 0.12, and 0.03 for catalysts I, II, and III, respectively. This result indicated that it is easier to form citronellol than 3,7-dimethyl-2-octenol over the deactivated catalyst. Catalyst deactivation retarded the consecutive hydrogenation of citronellol and 3,7-dimethyl-2-octenol, since at the 80% conversion level the yields of 3,7dimethyloctanol over I, II, and III catalysts were 32%, 28%, and 22%, respectively.

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Acknowledgment This work is part of the activities at the A° bo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Program (2000-2005) by the Academy of Finland. Literature Cited

Figure 8. (a) Kinetics in nerol hydrogenation and (b) selectivity to citronellol as a function of conversion at 70 °C and under 10 bar total pressure in 2-pentanol. Symbols: (2) fresh catalyst, ([) reuse I, and (9) reuse II.

Overall, it can be concluded, that the selectivity enhancement for citronellol over deactivated catalysts was achieved partly due to higher chemoselectivity to citronellol compared to 3,7-dimethyl-2-octenol, retarded consecutive hydrogenation of citronellol to 3,7-dimethyloctanol, and also because of decreased isomerization and hydrodeoxygenation reactions. 4. Conclusions Nerol transformations in the presence of helium or hydrogen were studied over Rh/Al2O3 catalyst in 2-pentanol. The kinetic analysis revealed that selectivity enhancement with increasing nerol conversion was achieved under reaction conditions where the catalyst deactivation was the most prominent, namely, at higher temperatures, initial nerol concentrations, and lower hydrogen pressures. Additionally, the selectivity to citronellol was systematically increased in consecutive catalyst reuses over the partially deactivated catalysts. The origin for catalyst deactivation in nerol hydrogenation is a decarbonylation reaction leading to catalyst fouling, which was confirmed by a decrease of the specific surface area of the spent catalyst and methane formation during temperature-programmed reduction of the spent catalyst. Lowered final nerol conversions were obtained under reaction conditions where catalyst deactivation was more prominent. Citronellol selectivity was enhanced mainly due to retarded consecutive hydrogenation to 3,7-dimethyloctanol. Other effects which increased the selectivity to citronellol were the declined formation of cis-3,7-dimethyl-2-octenol as well as decreased isomerization reaction.

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Received for review February 17, 2005 Revised manuscript received August 5, 2005 Accepted August 11, 2005 IE050191K

Selectivity Enhancement by Catalyst Deactivation in Three-Phase

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