Kinetics upon Isomerization of Î±,Î²-Pinene Oxides over Supported

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KINETICS UPON ISOMERISATION OF #,#-PINENE OXIDES OVER SUPPORTED IONIC LIQUID CATALYSTS (SILCAs) CONTAINING LEWIS ACIDS Eero Salminen, Päivi Mäki-Arvela, Pasi Virtanen, Tapio Olavi Salmi, Johan Warna, and Jyri-Pekka Mikkola Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 07 Dec 2014 Downloaded from http://pubs.acs.org on December 8, 2014

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Industrial & Engineering Chemistry Research

KINETICS UPON ISOMERISATION OF α,β-PINENE OXIDES OVER SUPPORTED IONIC LIQUID CATALYSTS (SILCAs) CONTAINING LEWIS ACIDS Eero Salminen1*, Päivi Mäki-Arvela, Pasi Virtanen1, Tapio Salmi1, Johan Wärnå1 and Jyri-Pekka Mikkola1,2

1

Åbo Akademi University, Industrial Chemistry and Reaction Engineering, Department of Chemical Engineering, Process Chemistry Centre, FI-20500 Åbo/Turku, Finland.

2

Umeå University, Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, SE90187 Umeå, Sweden.

* To whom correspondence should be addressed. E-mail: [email protected], Address: Laboratory of Industrial Chemistry, Åbo Akademi University, Piispankatu 8, FI-20500 Turku/Åbo, Finland

Wood extractives such as α-pinene oxide and β-pinene oxide are important renewable chemicals for the synthesis of fine chemicals, pharmaceuticals and value-added intermediates. In this work, catalytic transformations of biomass derived extractives, isomerisation of α,β-pinene oxides were studied over Supported Ionic Liquid Catalysts (SILCAs). SILCAs consist of catalytic species (e.g. metal nano ACS Paragon Plus Environment

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particles or metal complexes) in a thin layer of ionic liquid which is immobilized on a solid support material (e.g. active carbon cloth). The reaction kinetics of the isomerisation reactions over supported ionic liquid catalysts were studied and modeled. Mechanistic kinetic models describing the differences in selectivity and activity of the catalysts containing different ionic liquids were developed and the models described the reaction rates and product distributions very well.

Keywords: Isomerisation, Ionic liquids, Kinetic modeling, Supported Ionic Liquid Catalyst (SILCA)

1. INTRODUCTION

Environmentally friendly catalytic synthesis of fine chemicals is an important topic which is attracting ever more attention in chemical engineering. Renewable terpenes and their corresponding epoxides (e.g. α-pinene oxide and β-pinene oxide) are important precursors for e.g. the fragrance and flavor industry.1,2 Lewis acid promoted isomerisation of α-pinene oxide results in campholenic aldehyde which is an intermediate in manufacturing of a valuable perfume compound santalol. In case of βpinene oxide as a starting material, Lewis acid catalysed isomerisation leads to myrtanal, an anticeptic compound, which is also applied as a perfumery chemical.3 The use of heterogeneous catalysts in the isomerisation of α,β-pinene oxides is preferred because of the feasible catalyst regeneration and separation. Moreover, upon use of heterogeneous catalysts, accumulation of hazardous waste resulting from the use of corrosive mineral acids and homogeneous Lewis acids can be prevented. A number of heterogeneous catalysts for the α-pinene oxide isomerisation reaction have been reported. Zeolites, heteropolyacids, sulfated Al, Ti and Zr oxides and metal modified silicas have been successfully applied as catalysts for the isomerisation of α-pinene oxide to campholenic aldehyde.4-9 In the case of these heterogeneous catalysts, 65–75% selectivity towards campholenic aldehyde has been obtained, at high conversion levels. A complete reaction sequence for α-pinene oxide isomerisation is presented in Scheme 1. ACS Paragon Plus Environment

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O

fencholenic aldehyde (FA) O OH

O -pinene oxide (APO)

campholenic aldehyde (CA)

trans-carveol (CAR)

O

isopinocamphone (IP)

OH

pinocarveol (PC)

p-cymene

Scheme 1. Reaction scheme of α-pinene oxide isomerisation.

β-Pinene oxide can be isomerized to value-added chemicals such as myrtanal, myrtenol and perillyl alcohol which have applications as fragrance and pharmaceutical chemicals.10 Isomerisation of β-pinene oxide to myrtanal is promoted by Lewis acidic species while Brønsted acids give mainly perillyl alcohol and myrtenol (Scheme 2).11-13 However, isomerisation of β-pinene oxide has been scarcely studied. Solid Lewis acid catalysts and homogeneous Lewis acids have been used in the isomerisation of βpinene oxide. Metal-modified Beta zeolites (e.g. Zr-Beta zeolite) are active and selective catalysts for the isomerisation of β-pinene oxide to myrtanal with yields varying from 64% to 92 %.11 With mesoporous molecular sieves such as Sn-MCM-41 and Ti-MCM-41, 64 % and 49 % selectivities towards myrtanal have been obtained, at complete conversions. Previously, tin modified Beta zeolites,

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Y-zeolites and Brønsted acidic resins have also been successfully applied to the isomerisation of βpinene oxide to value-added chemicals.12,13

Scheme 2. Reaction scheme of β-pinene oxide isomerisation.

Ionic liquids (ILs), also called as molten salts, are ionic compounds in which either the cation or the anion (or both) is of organic origin. Ionic liquids have many unique properties such as tunable solvation properties, low vapor pressure and high thermal stability.14 Ionic liquids have potential for developing clean catalytic technologies in the field of electrolytes, batteries and solar cells.14,15 Moreover, catalysis is one of the areas where ionic liquids have been successfully applied.16 However, cost aspects and biodegradability are some of the issues which are still limiting the widespread use of ionic liquids. Nevertheless, supported ionic liquid catalysis is a concept where some of the advantages of ACS Paragon Plus Environment

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homogeneous and heterogeneous catalysis can be combined. In the case of supported ionic liquid catalysts, difficult and costly catalyst separation from the reaction mixture is eliminated. Moreover, only small amounts of ionic liquids are required in SILCAs compared to e.g. biphasic reaction systems. Consequently, the SILCA concept is preferred on the basis of economic criteria.17 In the case of Supported ionic liquid catalysts (SILCAs), the special properties of ionic liquids can be utilized by immobilizing them on solid support (e.g. active carbon cloth). SILCAs consist of catalytic species in a thin layer of immobilized ionic liquid. The catalytically active species can be e.g. solid metal nanoparticles, metal complexes or enzymes which are dissolved in ionic liquids.16,18.19 Ionic liquids can be immobilized by means of grafting, sol–gel method, polymerization or impregnation.20-23 In case of SILCA catalyst, simple impregnation method was applied. The ionic liquid and the primary catalyst (e.g. metal species or enzymes) are both dissolved in a molecular solvent after which the solution is then poured over the solid support material. Finally, the solvent is evaporated.21 The ionic liquid applied in SILCAs can act as a reaction environment where the catalytic species are more active. Ionic liquids can also influence the concentrations of reactive species on the catalyst surface. Consequently, aforementioned characteristics of ionic liquids can lead to enhanced reaction rates.17 With SILCAs, a high surface area between two liquid phases can be stabilized, thus allowing for a rapid mass transfer across the phase boundary.24 As an example, ionic liquids have improved the rates and selectivities for many reactions such as hydrogenation of citral, hexene and cyclooctadiene.17,18,21

2. EXPERIMENTAL

Catalysts for the isomerisation of α,β-pinene oxides were prepared as follows: Ionic liquid (150 mg) and Lewis acid such as ZnCl2 (Fluka, ≥98%) or SnCl2 (Sigma-Aldrich, ≥99%) were dissolved into a suitable solvent (e.g. methanol or acetone). The solution was poured over five pre-dried active carbon cloth (ACC) Kynol® pieces (approx. 1.5 g) followed by evaporation of the molecular solvent in an oven (80 °C) for two hours. As a result, Lewis acidic species in an immobilized ionic liquid was obtained. ACS Paragon Plus Environment

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The active carbon cloth support material is presented in Figure 1. For the catalysts applied in the isomerisation experiments, the molar ratio of Lewis acid to ionic liquid was 2:1. The ionic liquids used in the preparation of SILCAs were N-butyl-4-methylpyridinium tetrafluoroborate ([NB4MPy][BF4], Merck, 98%) and N-(3-hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)imide ([N(3-OHPr)Py][NTf2], Merck, 98%), stored under an inert atmosphere and used without further purification. Isomerisation experiments were performed in a batch reactor. The total volume of the reactor was 600 ml whereas the effective liquid volume was 250 ml. The temperature and stirring rate were controlled by a Parr 4843 control unit (Watlow control series 982). All experiments were performed at a constant pressure and temperature.

Figure 1. A piece of Active Carbon Cloth (ACC) Kynol® support material (approximately 5,5 cm x 2.8cm).

Approximately 0.46 g of pinene oxide was dissolved in 250 ml of n-hexane (Merck, >99%). α-Pinene oxide (Aldrich, 97%) concentration was 0.012 M whereas (+)-β-pinene oxide (80+%, Advanced Technology & Industrial Company Ltd.) concentration corresponded to a pure β-pinene oxide concentration of 0.0096M. A relatively high stirring speed (700 rpm) was applied in order to eliminate external mass transfer limitations. Four pieces of active carbon cloth (1.2 g) containing ionic liquid and metal chloride (1:2 molar ratio of ionic liquid to metal chloride) were applied as a catalyst for the isomerisation experiments. Isomerisation of α,β-pinene oxides were performed at the temperature range of 25-120 °C under argon atmosphere (5 bar). The reaction rates and selectivities were independent of pressure (1-10 bar). The catalysts applied for α-pinene oxide isomerisation were SnCl2/([N(3-OH-


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Pr)Py][NTf2]/ACC

and SnCl2/([NB4MPy][BF4]/ACC.

In case of β-pinene oxide isomerisation,

ZnCl2/[N(3-OH-Pr)Py][NTf2]/ACC and ZnCl2/[NB4MPy][BF4]/ACC were applied as catalysts. α-Pinene oxide and β-pinene oxide isomerisation products were identified by means of gas chromatography (Hewlett Packard 6890 GC equipped with a FI detector). In addition, a gas chromatograph coupled to a mass spectrometer (Agilent 6980N GC with Agilent 5973 MS detector) was used to confirm correct identification of the products. The GC column used was a HP wax bonded polyethylene glycol column, with a length of 30.0 m, inner diameter of 250 µm and film thickness of 0.25 µm (Agilent 19091X-133). The following temperature program was applied: 1 min at 80 °C, then raised 5 °C/min to 200 °C. At the end, the temperature was held 35 min at 200 °C.

3. PARAMETER ESTIMATION AND REACTION KINETICS

The mole fractions of the isomerisation reaction products were plotted as a function of time. Consequently, these values were used in nonlinear regression analysis and the parameter values and their estimated errors were calculated. MODEST 6.1 software was used for the estimation of the kinetic parameters.25 In addition, Levenberg-Marquardt and Simplex methods were applied for the parameter estimation and a personal computer (PC) was used for the calculations. The reactor model applied for the isomerisation reactions was a classical batch model operating within intrinsic kinetic regime and, thus the mass balances can be written for the organic components as follows:

dni dc = ri mcat ⇔ i = ri ρB dt dt

(1)

where ρ B is the catalyst bulk density, ρ B =

mcat . VL


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3.1. Isomerisation of α-pinene oxide

The modeling effort was based on the stoichiometric scheme displayed in scheme 1. The route from α-pinene oxide to p-cymene was neglected since only a small amount (mole fraction < 2 %) of pcymene was detected during the experiments. The reaction steps of α-pinene oxide isomerisation are illustrated in scheme 3.

1.

APO k→ CA

2.

APO k→ FA

3.

APO  k  → CAR

4.

k APO → IP

5.

APO k→ PC

CA

FA

CAR

IP

PC

Scheme 3. α-Pinene oxide isomerisation steps. Abbreviations: APO=α-pinene oxide, CA=campholenic aldehyde, FA=fencholenic aldehyde, CAR= t-carveol, IP= isopinocamphone and PC=pinocarveol.

Ionic liquid acts as a reaction phase in which the homogeneous catalyst (Lewis acid) is dissolved and, thus, the following simple first-order rate equations were obtained:

ri = k j CAPO

(2)

where the index i refers to the reaction number (1-5) and j is an isomerisation product (CA, FA, CAR, IP, PC).

The temperature dependence of the rate constants were considered to follow the Arrhenius equation,

k j = k 0, j e

− Ea , j RT

(3)


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Consequently, the generation rates can be derived from the reaction stoichiometry as follows: rAPO = – r1 – r2 – r3 – r4 – r5

(4)

(α-PO consumption)

rCA = r1

(5)

(campholenic aldehyde formation)

rFA = r2

(6)

(fencholenic aldehyde formation)

rCAR = r3

(7)

(t-carveol formation)

rIP = r4

(8)

(isopinocamphone formation)

rPC = r5

(9)

(pinocarveol formation)

Equation 10, illustrating α-pinene oxide consumption rate, is obtained from eqs 1, 2 and 4 and it implies first order kinetics with respect to APO, predicting that plotting –ln(1-X) as a function of time should be a straight line (1-X is a degree of consumption of APO and X=conversion of APO). dc APO = (− k CA − k FA − k IP − k PC − k CAR )C APO ρ B dt

(10)

When –ln(1-X) is plotted as a function of time, straight lines for all of the isomerisation experiments were obtained. This indicates that the reaction de facto is of first order with respect to α-pinene oxide concentration (Figure 2). In case of β-pinene oxide, straight lines, were also obtained when –ln(1-X) was plotted as a function of time. β-Pinene oxide isomerisation reaction is a first order reaction with respect to the β-pinene oxide.


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R² = 0.9946 120 °C

2.5 R² = 0.9925

2 -ln (1-X)

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100

R² = 0.9996 1.5

70 °C

R² = 0.9887

1

40 °C

0.5

25 °C R² = 0.9929

0 0

60

120

180

time (min)

Figure 2. –ln(1-conversion) is presented as a function of time over SnCl2/([N(3-OH-Pr)Py][NTf2] catalyst at different temperatures. Correlation factors, calculated by the least square method, are illustrated for each experiment.

3.2. Isomerisation of β-pinene oxide

The reaction scheme for isomerisation of β-pinene oxide is based on scheme 2. The formation of other products was also taken into account. The reaction steps of β-pinene oxide isomerisation are illustrated in scheme 4.

1.

BPO  k  → MAL

2.

BPO  k  → PER

3.

BPO  k  → MOL

4.

BPO  k  → OTH

MAL

PER

MOL

OTH

Scheme 4. β-Pinene oxide isomerisation steps. Abbreviations: BPO=β-pinene oxide, MAL=myrtanal, PER=perillyl alcohol, MOL=myrtenol and OTH= other products. ACS Paragon Plus Environment

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Thus, the following rate equations were obtained:

ri = k j CBPO

(11)

where the index i refers to a reaction number (1-4) and j is an isomerisation product (MAL, PER, MOL, OTH).

The temperature dependence of the rate constants were again considered to follow the Arrhenius equation,

k j = k 0, j e

− Ea , j RT

(12)

Consequently, the generation rates can be derived from the rate equations as follows: rBPO = – r1 – r2 – r3 – r4

(13)

(β-PO consumption)

rMAL = r1

(14)

(myrtanal formation)

rPER = r2

(15)

(perillyl alcohol formation)

rMOL = r3

(16)

(myrtenol formation)

rOTH = r4

(17)

(formation of other products)

Equation 18 illustrates β-pinene oxide consumption rate. dc BPO = (− k MAL − k PER − k MOL − k OTH )C BPO ρ B (18) dt

α,β-Pinene oxide isomerisation products are formed by parallel reactions (no consequtive reactions are occurring). This was confirmed by plotting a concentration of each side product as a function of the concentration of main product (campholenic aldehyde or myrtanal) (Figure 3). Consequently, straight lines were obtained. dc CA = k CA C APO dt

dc MAL = k MAL C BPO dt

(19)


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dc X = k X C APO dt

dc X = k X C BPO dt

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(20)

Equation 21 is obtained from the equations 19 and 20 . X is a isomerisation product.

CX =

kX CCA kCA

CX =

kX CMAL kMAL

(21)

Figure 3. Concentration of different α,β-pinene oxide isomerisation products are presented as a function of the concentration of the main product (CA=campholenic aldehyde and MAL=myrtanal) at different temperatures.

4. RESULTS AND DISCUSSION

The performance of supported ionic liquid catalysts applied in the isomerisation of α,β-pinene oxides is illustrated in Figures 4 and 5, respectively. Ionic liquid influences to the product distribution obtained with the catalyst. One of the reasons for this behavior is that ionic liquids can have an effect to the concentrations of reactants on the surface of the catalyst resulting in different selectivities and activities.17 Moreover, the ionic liquid applied in SILCAs can act as a reaction environment where the ACS Paragon Plus Environment

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catalytic species are more active. Consequently, these characteristics of ionic liquids can lead to enhanced selectivities and reaction rates.17,21 With SILCAs, a high surface area between two liquid phases can be stabilized, thus allowing for a rapid mass transfer across the phase boundary.24 As an example, ionic liquids have improved the rates and selectivities for many reactions such as hydrogenation of citral, hexene and cyclooctadiene.17,18,21 In our previous studies, tin chloride (SnCl2) was found to be the most efficient Lewis acid catalyst in campholenic aldehyde production.26 Consequently, two SILCAs containing tin chloride were applied in the α-pinene oxide isomerisation. SnCl2/[N(3-OH-Pr)Py][NTf2] catalysts was the most efficient catalyst for campholenic aldehyde production. In case of β-pinene oxide isomerisation, two different SILCAs containing a zinc chloride as a Lewis acid were applied. ZnCl2/[N(3-OH-Pr)Py][NTf2] catalyst was the most active and selective catalyst towards myrtanal production.27

90%

4 6

80%

20

9

1.E-05

11

1.E-05

70%

21

12

60%

2.E-05 1.40E-05

1.E-05 10

50%

8.E-06

40%

5.21E-06

30%

58

4.E-06

48

20%

6.E-06

Initial rate (mol/(s*g))

100%

Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.E-06

10%

0.E+00

0% [N(3-OH-Pr)Py][NTf2]

[NB4MPy][BF4]

pinocarveol trans-carveol campholenic aldehyde

[N(3-OH-Pr)Py][NTf2]

[NB4MPy][BF4]

isopinocamphone fencholenic aldehyde rate

Figure 4. Product distributions and initial reaction rates in the α-pinene oxide isomerisation over SnCl2/[N(3-OH-Pr)Py][NTf2] and SnCl2/[NB4MPy][BF4] catalysts after 4 hours. The reaction conditions were T=70 °C, p(Ar)=5 bar and Vhexane=250 ml. The initial rates were calculated from the αpinene oxide conversion at 5 min (moles of α-pinene oxide converted to products)/(time x mass of Lewis acid).


13


4.E-05

90%

2.97E-05

27

34

80%

2.68E-05

3.E-05

4

70%

4 12

15

60%

3.E-05

2.E-05

50% 2.E-05

40% 30%

54

1.E-05

50

20%

Initial rate (mol/(s*g))

100%

Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5.E-06

10% 0%

0.E+00 [N(3-OH-Pr)Py][NTf2]

[NB4MPy][BF4]

other products myrtanal

myrtenol rate


[NB4MPy][BF4]

perillyl alcohol

Figure 5. Product distributions and initial reaction rates in the β-pinene oxide isomerisation over ZnCl2/[N(3-OH-Pr)Py][NTf2] and ZnCl2/[NB4MPy][BF4] catalysts after 2 hours. The reaction conditions were T=70 °C, p(Ar)=5 bar and Vhexane=250 ml. The initial rates were calculated from the βpinene oxide conversion at 5 min (moles of α-pinene oxide converted to products)/(time x mass of Lewis acid).

The model fit to experimental data of α-pinene oxide isomerisation was very good. The degree of explanation

was

98.64

%

for

SnCl2/([N(3-OH-Pr)Py][NTf2]

catalyst

and

96.46

%

for

SnCl2/([NB4MPy][BF4] catalyst, respectively. The parameter estimation results for the catalysts applied to the α-pinene oxide isomerisation are presented in Table 1. A model fit for the SnCl2/([N(3-OHPr)Py][NTf2] catalyst at different temperatures (25°C-120°C) is illustrated in Figure 6. In case of the catalyst SnCl2/([N(3-OH-Pr)Py][NTf2], the molar yields towards isopinocamphone increased from 5 % at 25 °C to 26 %, at 120 °C. Increasing temperatures favour the formation of bicyclic species (e.g. isopinocamphone) while lower temperatures yield more monocyclic derivatives, e.g. campholenic aldehyde. The activation energy obtained for campholenic aldehyde was lower than that for isopinocamphone. It is assumed that campholenic aldehyde is the kinetic product and is thus favoured ACS Paragon Plus Environment

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under kinetic control (lower temperatures). Consequently, isopinocamphone is thermodynamic product which is favoured under elevated temperatures (thermodynamic control).

Table 1. Results of parameter estimation for α-pinene oxide isomerisation over SnCl2/([N(3-OHPr)Py][NTf2] and SnCl2/([NB4MPy][BF4] catalystsa. Parameter


[NB4MPy][BF4]

ko,CA (s-1)

33 (2%)

17 (3%)

ko,FA (s-1)

7 (6%)

3 (11%)

ko,CAR (s )

10 (4%)

8 (5%)

ko,IP (s-1)

6 (8%)

4 (9%)

ko,PC (s-1)

3 (12%)

2 (18%)

Ea,CA (kJ/mol)

42 (1%)

38 (3%)

Ea, FA (kJ/mol)

48 (3%)

42 (9%)

Ea, CAR(kJ/mol)

41 (3%)

40 (4%)

Ea, IP (kJ/mol)

66 (3%)

51 (6%)

Ea, PC (kJ/mol)

43 (7%)

40 (16%)

-1

a

Estimated standard errors are shown in parentheses.


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Figure 6. Model fit to the experimental data of α-pinene oxide isomerisation over SnCl2/[N(3-OHPr)Py][NTf2]/ACC at different temperatures. α-Pinene oxide conversion to products presented as a function of time. The lines represent the model predictions while the symbols are experimental values. Legend: (◦) α-pinene oxide, (●) campholenic aldehyde, (□) t-carveol, (▲) fencholenic aldehyde, (■) isopinocamphone and (x) pinocarveol.

The model fit to experimental data of β-pinene oxide isomerisation was very good, resulting in the degree of explanation of 96.54% for ZnCl2/[N(3-OH-Pr)Py][NTf2]/ACC and 97.42% for ZnCl2/[NB4MPy][BF4]/ACC. The results from the parameter estimation are presented in Table 2. The ACS Paragon Plus Environment

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model predictions of the β-pinene oxide isomerisation reaction with catalysts ZnCl2/[N(3-OHPr)Py][NTf2]/ACC and ZnCl2/[NB4MPy][BF4]/ACC are illustrated in Figure 7.

Table 2. Results of parameter estimation of β-pinene oxide isomerisation over ZnCl2/([N(3-OHPr)Py][NTf2] and ZnCl2/([NB4MPy][BF4] catalysts. Estimated standard errors are shown in parentheses. Parameter


[NB4MPy][BF4]

ko,MAL (s-1)

110 (6%)

104 (5%)

ko,PER (s-1)

30 (13%)

23 (12%)

ko,MOL (s )

9 (41%)

9 (31%)

ko,OTH (s-1)

46 (10%)

56 (6%)

Ea,MAL (kJ/mol)

49 (5%)

27 (6%)

Ea,PER (kJ/mol)

46 (11%)

20 (23%)

Ea,MOL (kJ/mol)

48 (29%)

21 (51%)

Ea,OTH (kJ/mol)

27 (10%)

14 (19%)

-1

Figure 7. Model fit to the experimental data of β-pinene oxide isomerisation over ZnCl2/[N(3-OHPr)Py][NTf2] and ZnCl2/[NB4MPy][BF4] catalysts at different temperatures (70°C-120°C). β-Pinene oxide conversion to products presented as a function of time. The lines represent the model predictions


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and symbols are experimental values. Legend: (◦) β-pinene oxide, (□) myrtanal, (■) perillyl alcohol, (▲) myrtenol and (+) other products.

The models were successfully applied to the isomerisation of α,β-pinene oxides over supported ionic liquid catalysts (SILCAs). The catalysts contained different ionic liquids which resulted in different reaction rates. A plausible reason for the different reaction rates and selectivities obtained is that ionic liquids can have an effect on the concentrations of reactants and products accessing the catalyst surface.17 Moreover, ionic liquids can also act as catalysts. Isomerisation of α,β-pinene oxides to campholenic aldehyde and myrtanal is catalyzed by Lewis acids. Tin and zinc metals derived from e.g. tin chloride or zinc chloride have a high Lewis acidity compared to many other transition metals.28 Consequently, conventional homogeneous α-pinene oxide isomerisation processes are based on zinc bromide. Moreover, no leaching of metal species (Sn or Zn) to reaction medium was detected upon ICP-MS analysis. Deactivation of SILCA catalysts was studied and it was illustrated that SILCAs are indeed reusable catalysts for the isomerisation reactions.26,27 Furthermore, the product distribution did not change during the deactivation experiments. In case of βpinene oxide, no deactivation was observed during four consecutive experiments. Most likely, the primary reason for the slight catalyst deactivation in the case of α-pinene oxide isomerisation is the accumulation of isomerisation products into the ionic liquid as suggested by nitrogen physisorption measurements and analysis of catalyst washing extractant. Finally with the assistance of kinetic models, the experimentally recorded activities and selectivities of the SILCAs could be explained. Moreover, the effect of temperature on the outcome of α,β-pinene oxide isomerisation reactions could be modeled and explained. In the case of α-pinene oxide, low temperatures favor the formation of campholenic aldehyde (monocyclic compound) whereas high temperatures enhance the production of bicyclic compounds such as isopinocamphone. Similar observations were made for β-pinene oxide. High temperatures favour the formation of myrtanal (bicyclic compound) whereas at low temperatures the yield of myrtanal is supressed. This phenomenon ACS Paragon Plus Environment

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has also been observed for Sn-modified zeolites at a temperature range of 27–70 °C.13 On the basis of these results it can be deducted that short reaction times and high temperatures favour the formation of myrtanal. Furthermore, it was observed that α,β-pinene oxide isomerisation products are formed by parallel reactions with the exeption of p-cymene. Isomerisation studies of β-pinene oxide and myrtenol have also illustrated that β-pinene oxide isomerisation products are formed in parallel/competitive reactions.13

4. CONCLUSIONS

Transformation of biomass extractives, isomerisation of α,β-pinene oxides, to industrial cosmetic, fragrance and pharmaceutical chemicals was studied by applying supported ionic liquid catalysts (SILCAs). It was demonstrated that isomerisation of α-pinene oxide to campholenic aldehyde and βpinene oxide to myrtanal could be succesfully carried out applying SILCAs containing Lewis acids. Supported ionic liquid catalysts are efficient catalysts for the isomerisation reactions while the product distribution and activity is dependent on the nature of the ionic liquid. Ionic liquid act as a reaction phase in which the homogeneous catalyst (Lewis acid) is dissolved. Consequently, the kinetic models were based on homogeneous reactions in immobilised ionic liquids. Mechanistic kinetic models were developed and the models were successfully applied to the isomerisation of α,β-pinene oxides over SILCAs containing different ionic liquids and Lewis acids. Kinetic models were able to describe the reaction rates and product distributions very well.

NOMENCLATURE

Variables As = specific surface area, m2 g-1 c = concentration, M, mol dm-3 ACS Paragon Plus Environment

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Cj,x = concentration of compound, mol m-3 Ea,j = activation energy, kJ mol-1 k0j = rate constant of compound j, s-1 m = mass, g, kg ri = rate for different isomerisation steps t = time, min, h T = temperature, K V = volume, ml, µl, cm3, m3 

ρ B = catalyst bulk density  ρ B =



m cat VL

 3  , kg/m 

Abbreviations [NB4MPy][BF4] = N-butyl-4-methylpyridinium tetrafluoroborate [N(3-OH-Pr)Py][NTf2] = N-(3-hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)imide ACC = activated carbon cloth IL = ionic liquid SILCA = supported ionic liquid catalyst ΑPO = α-pinene oxide CA = campholenic aldehyde FA = fencolenic aldehyde CAR = t-carveol IP = isopinocamphone PC = pinocarveol BPO = β-pinene oxide MAL = myrtanal MOL = myrtenol ACS Paragon Plus Environment

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PER = perillyl alcohol OTH = other products

ACKNOWLEDGEMENTS

This work is part of the activities at the Åbo Akademi Process Chemistry Centre, a center of excellence financed by the Åbo Akademi University. The Academy of Finland and Tekniikan edistamissaatio (TES) are gratefully acknowledged for financial support. In Sweden, the Bio4Energy programme, Kempe Foundations and Wallenberg Wood Science Center under auspices of Knut and Alice Wallenberg Foundation are acknowledged. The Cost action CM1206 (EXIL) is also acknowledged.

LITERATURE CITED

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Kinetics upon Isomerization of Î±,Î²-Pinene Oxides over Supported

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