Metathesis of methyloleate over methyltrioxorhenium supported on ZnCl2-promoted mesoporous alumina

Applied Catalysis A: General 455 (2013) 155–163

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Metathesis of methyloleate over methyltrioxorhenium supported on ZnCl2 -promoted mesoporous alumina Subha Kumaraswamy Pillai a,b,c , Safia Hamoudi a,c , Khaled Belkacemi a,c,∗ a b c

Department of Soil Sciences and Food Engineering, Laval University, Quebec G1V0A6, Canada Department of Chemical Engineering, Laval University, Quebec G1V0A6, Canada Centre de Recherche sur les Propriétés des Interfaces et la Catalyse (CERPIC), Laval University, Quebec G1V0A6, Canada

a r t i c l e

i n f o

Article history: Received 8 November 2012 Received in revised form 15 January 2013 Accepted 19 January 2013 Available online xxx Keywords: Self-metathesis Methyloleate Methyltrioxorhenium (MTO) ZnCl2 -promoted mesoporous alumina

a b s t r a c t Methyltrioxorhenium impregnated on ZnCl2 -modified mesoporous alumina at Al/Zn ratio of 8–12 was revealed as an effective catalyst for self-metathesis of methyloleate at mild reaction conditions. The use of other promoters either in form of metal chlorides or as other zinc halides did not promote the metathesis of methyloleate as zinc chloride. The enhancement of the activity was not only due to the chloride and/or zinc but could be attributed to the interaction of both elements as Zn–Cl or Zn Cl2 bonds with the mesoporous alumina. The results of this research suggest that it is possible to design a heterogeneous catalyst for the efficient and selective self-metathesis of functionalized olefins such as methyloleate and edible oils, avoiding the utilization of expensive homogeneous catalysts or use of toxic promoters such as SnR4 with heterogeneous catalysts. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Rising global population demands increased production and consumption of petroleum-based products leading to increase the price and the depletion of this resource. To ensure sustainability and a good quality of life for future generation, the necessity for the increased exploitation of renewable raw materials replacing petrochemical-based products arises [1]. The importance of this has been emphasized by UN assembly by declaring 2012 as ‘International year of sustainable energy for all’. Continuous growth of plants on our planet exceeds men’s primary energy requirements many times over. The similarity in structure of the plant oils to petroleum products show their potential as suitable alternatives. Though the idea of using plant based resource was implemented as early as 1900 by Rudolf Diesel, by using peanut oil as fuel [2], it was not exploited until recently. The current environmental concerns, cost and availability encourage focus on plant based resources for various successful applications such as in lubricants [3], cutting fluids [4], fuels [5,6], pharmaceuticals, cosmetics and polymers [7–9]. Conversion of plant-based feedstock to useful chemicals requires its transformation by chemical reactions [10]. One such

∗ Corresponding author at: Department of Soil Sciences and Food Engineering, Laval University, Quebec G1V0A6, Canada. Tel.: +1 418 656 2131x6511; fax: +1 418 656 3723. E-mail address: [email protected] (K. Belkacemi). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.01.026

reaction is olefin metathesis forming new compounds through the cleavage and exchange of carbon–carbon double bonds of two olefins. This alkylidene rearrangement reaction discovered as early as 1931, offers the potential to use simple olefins to complex functionalized molecules in the synthesis of many valuable products with noteworthy commercial applications [11,12]. Fats and oils containing carbon–carbon double bonds can undergo transformation by olefin metathesis to introduce new chemical functionality or to alter the properties of the reactants to enable synthesis of a wide range of reaction products from pharmaceuticals and cosmetics to polymers and fine chemicals [13]. Well-defined homogeneous catalysts are compatible with a wide range of functional groups. These catalysts based on group 6 and 7 transition metals present the advantage of working at rather low temperatures (25–80 ◦ C), giving rise to good selectivity and activity [14]. However, high costs and decomposition hinder their recycling or regeneration. Consequently, this necessitates development of heterogeneous catalysts. Heterogeneous catalysts, typically from group 6–7 oxides supported on large surface area oxide supports, are robust and can be regenerated by simple calcination. However, they are generally not active with functional groups such as olefin esters unless activators are used [15]. The well-known heterogeneous catalytic system Re2 O7 /Al2 O3 promoted by R4 Sn is relatively active but is susceptible to poisoning by Sn, which hampers its regeneration [16]. At this time, the bottleneck for commercial fatty material metathesis using heterogeneous catalysts resides in their lack of stability, insufficient activity and selectivity and poor mass

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transfer features. Methyltrioxorhenium (MTO)/␥-alumina have been proven to be a more efficient catalyst than Re2 O7 /Al2 O3 for metathesis [17,18]. Nonetheless, metathesis of functionalized olefins such as methyloleate and other unsaturated esters such as bulky triglycerides using MTO-based catalysts supported on mesoporous materials remains largely unexplored or never attempted. Recently, we demonstrated for the first time that MTO/ZnCl2 modified mesoporous Al2 O3 is an efficient catalyst for the metathesis of methyloleate, a model molecule of bulky triglycerides present in fats and vegetable oils [18]. The aim of this work is to investigate the effect of Al/Zn molar ratio on the activity and selectivity towards metathesis products of MTO impregnated on ZnCl2 -modified mesoporous alumina for the self-metathesis of methyloleate. It is also question to chemically characterize the formulated catalysts and investigate the activity of the metathesis for functionalized olefins such as methyloleate. The effect of other precursors for modifying mesoporous alumina as support for MTO to carry out self-metathesis of methyloleate is also studied to understand the role of ZnCl2 .

2.2.3. Synthesis of MXy –Al2 O3 -meso The modification of Al2 O3 -meso by different MXy (M = Ca, Mg, Ga, Zn, Mn; X = Cl, Br, and I; y = 2 or 3) promoters was carried out with calcium chloride, magnesium chloride, gallium chloride, manganese chloride as representatives of different metal chlorides or zinc bromide and zinc iodide as other zinc halide precursors. Modification of Al2 O3 -meso was done by the similar procedure described above for ZnCl2 . The different Al2 O3 -meso modifiers were added at an Al/M molar ratio of 8.

2. Experimental

2.2.5. Catalyst characterization The MTO catalyst was characterized using nitrogen physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and 1 H magic angle spinning nuclear magnetic resonance (1 H MAS NMR) techniques. Nitrogen adsorption/desorption isotherms of calcined samples were obtained using a volumetric adsorption analyzer (model Autosorb-1, Quantachrome Instruments, Boyton Beach, FL) at −196 ◦ C (77 K). Before the adsorption analysis, the samples were degassed for 3 h at 200 ◦ C. Total pore volume was estimated from the amount adsorbed at 0.99 relative pressures. Pore size distributions were calculated using the desorption branch of the N2 adsorption/desorption isotherms and the Barrett–Joyner–Halenda (BJH) method as reported in our previous work [18]. Powder X-ray diffraction (XRD) patterns were obtained using an Ultima III Rigaku Monochromatic Diffractometer using CuK␣ radi˚ Small angle powder diffraction patterns were ation ( = 1.5406 A). acquired at a scanning rate of 0.5◦ /min in the 2 range of 0.6–3◦ . Transmission electron microscopy (TEM) images were taken on a JEM-3010 electron microscope (JEOL, Japan) with an acceleration voltage up to 200 kV. The catalysts samples were suspended in methanol and ultrasonically treated during 10 min. Then, the suspension (5 ␮L) were disposed uniformly and dried on a nickel grid. The catalyst surface and the oxidation state of the elements in the catalysts were studied by X-ray photoelectron spectroscopy (XPS) using an axis-ultra spectrometer from Kratos (U.K.) equipped with an electrostatic analyzer of large ray, a source of double X-rays Al–Mg without monochromator and an Al source with monochromator. The pressure in the XPS room was maintained between 5 × 10−9 and 5 × 10−8 torr during the analysis. All the spectra were recorded with the Al monochromatic source with a power of 300 Watts. The flyover spectrum used to determine the elementary composition was recorded with pass energy in the analyzer of 160 eV and an energy step of 1 eV, using lenses in hybrid mode, which maximizes the sensitivity. The detailed spectra with high resolution were recorded with pass energy of 40 or 20 eV, and step energy of 50 or 100 meV. The spectra with high resolution are used for the chemical analysis. The adjustment of the envelope calculated with the experimental spectrum was carried out using CasaWPS software from Kratos (U.K.). The binding energy (BE) scale was calibrated by measuring C1s peak (BE = 285.0 eV) from the surface contamination. Solid-state NMR spectra were recorded on a Bruker AVANCE 500 NMR spectrometer equipped with a 4-mm broadband MAS probehead. Spectra were acquired at room temperature and at a spinning

2.1. Materials All reagents were of high purity and were used without further purification. Aluminium-tri-sec-butoxide (C12 H27 AlO3 , 97%), zinc chloride (ZnCl2 , 99.9%), calcium chloride (CaCl2 , 96%), gallium chloride (GaCl3 , 99.9%), manganese chloride (MnCl2 , 98%), magnesium chloride (MgCl2 , 99%), zinc bromide (ZnBr2 , 99.9%), zinc iodide (ZnI2 , 98%), methyltrioxorhenium (MTO, CH3 O3 Re 71–76%), methyloleate (CH3 (CH2 )7 CH CH(CH2 )7 CO2 CH3 , 99%) were obtained from Sigma–Aldrich Canada Ltd. (Oakville, ON, Canada). Pluronic P123 (a triblock copolymer (HO(CH2 CH2 O)20 (CH2 CH(CH3 )O)70 (CH2 CH2 O)20 H, Mav = 5900 g/mol) was graciously offered by BASF. All the solvents used were anhydrous. 2.2. Materials synthesis and characterization 2.2.1. Synthesis of Al2 O3 -meso Mesoporous alumina (Al2 O3 -meso) was synthesized according to the procedure described by Ray et al. [19] using Pluronic P123 as templating agent with little modifications. Typically 6.09 g of Pluronic P123 was dissolved in 20.0 g sec-butanol at room temperature. Exact amount of 5.19 g of aluminium tri (sec-butoxide) was added as aluminium source to the pluronic 123 solution and stirred for 6 h at room temperature. Another solution containing 0.76 g water in 8.0 g sec-butanol was added to the obtained mixture and further diluted with 8.0 g sec-butanol. The mixture was stirred at room temperature for 16 h and left static for 4 h. It was then filtered and the solid phase was washed with ethanol and dried at room temperature for 36 h and at 100 ◦ C for 20 h. To remove the occluded P123, it was then heated to 400 ◦ C at ramping rate of 2 ◦ C/min and calcined at 400 ◦ C for 4 h to give Al2 O3 -meso. 2.2.2. Synthesis of ZnCl2 –Al2 O3 -meso Modification of Al2 O3 -meso by incorporation of zinc chloride was carried out according to Oikawa et al. [20]. Addition of ZnCl2 (0.668 g, 4.9 mmol) in ethanol (5 mL) was done drop wise to 2 g Al2 O3 -meso under stirring and the ethanol was allowed to dry. The dried sample obtained was heated under an air flow to 400 ◦ C at ramping rate of 2 ◦ C/min and calcined at 400 ◦ C for 4 h to give ZnCl2 modified Al2 O3 -meso with Al/Zn molar ratio of 8. Similar procedure was followed using different amounts of ZnCl2 to give different Al/Zn molar ratios.

2.2.4. Synthesis of the catalysts All MXy –Al2 O3 -meso supports were heated under nitrogen at 540 ◦ C for 2 h before MTO impregnation. Impregnation of 3%MTO (wt.%) was done by dissolving it in hexane and adding it under dry nitrogen onto the modified and treated supports at reaction temperature. MTO was left to react with MXy –Al2 O3 -meso supports for 10 min before addition of reactants. This loading was shown to be optimal for the metathesis of methyloleate as reported in our previous work [18].

2.3. Metathesis reaction and analysis of the products 2.3.1. Metathesis reaction Metathesis reactions were carried out in small quartz batch reactor containing inert atmosphere. Typically, methyloleate (50 ␮L) in 500 ␮L hexane was added to the catalyst (50 mg) in dry conditions at the investigated temperature (45 ◦ C) with stirring at 400 rpm for 5 h. The reaction was then stopped by addition of acetone enabling desorption of the reactants and products from the catalytic surface. The reaction metathesis tests with the homogeneous catalyst were carried out using 2.67 mg of soluble second generation Grubbs catalyst (Sigma–Aldrich Canada Ltd., Oakville, ON, Canada), with exactly 0.89 g of methyloleate in 5 ml of hexane. Identical reaction conditions (temperature, mixing rate and reaction time) were used as mentioned before. 2.3.2. Product analysis Analysis of the reaction samples were carried out by GC and GC–MS. The products and their isomers were identified by GC–MS in HP6890 series gas chromatograph with Agilent 5973 network mass selective detector. The column used was DB-1 measuring 30 m × 0.25 mm ID × 0.25 ␮m film thickness. The injector and detector temperature were set to 250 ◦ C and 1 ␮L sample was injected with a split ratio of 50:1. The oven temperature profile was: initial temperature 100 ◦ C, ramp at 10 ◦ C/min to 250 ◦ C, hold for 5 min; ramp at 10 ◦ C/min to 300 ◦ C, hold for 5 min. Identification of the product and its isomers was achieved by their mass and confirmed both with commercially available and synthesized standards. The product quantification was carried out by Shimadzu GC2010-plus gas chromatograph system equipped with a flame ionization detector using dodecane as internal standard. The column used was BPX-70 60 m × 0.25 mm ID × 0.25 ␮m film thickness with hydrogen as carrier gas. The oven temperature profile was: initial temperature 60 ◦ C, ramp at 10 ◦ C/min to 190 ◦ C, hold for 15 min; ramp at 5 ◦ C/min to 240 ◦ C, hold for 10 min [21]. The collected data was analyzed using Chemstation software. 3. Results and discussion 3.1. Characterization of textural and surface properties of the materials Textural properties of the mesoporous alumina (Al2 O3 -meso) and 3%MTO/ZnCl2 modified mesoporous alumina catalyst (Zn/Al molar ratio of 8) are summarized in Table 1. The isotherms and the corresponding pore size distributions using the BJH theory are depicted in Fig. 1. The shape of these material isotherms corresponds to type IV according to IUPAC classification and displayed a relatively broad H1 type hysteresis loop characteristic of large pore Table 1 Textural properties of Al2 O3 -meso and 3%MTO/ZnCl2 -modified Al2 O3 -meso. Materials

Al2 O3 -meso

3%MTO/ZnCl2 -modified Al2 O3 -meso

BET surface area (m2 /g) Pore size (nm) Total pore volume (cm3 /g)

492 ± 1 8.0 ± 0.1 1.1 ± 0.1

420 ± 1 8.0 ± 0.1 1.0 ± 0.1

-1

157

0.30

-1

800

0.20

3

700

0.10

p

3

Adsorbed volume (cm /g)

600

p

speed of 14 kHz. Single pulse 1 H NMR experiments were performed for Al2 O3 -meso and the ZnCl2 -modified Al2 O3 -meso using a 90◦ pulse length of 2.5 ␮s, an acquisition time of 0.03 s, and a recycle delay of 120 s. Peak areas were calibrated using Si(Si(CH3 )3 )4 as an external standard.

V / d (cm nm g )

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0.00 0

500

4 8 12 16 Pore size (nm)

20

MTO/ZnCl2-Al2O3-Meso(16)

400

ZnCl2-Al2O3-Meso Al2O3-Meso

300

Dashed lines stand for desorption

200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 1. Nitrogen adsorption/desorption isotherms of mesoporous alumina and 3% MTO/ZnCl2 -modified mesoporous alumina. The inset figure represents the pore size distribution.

mesoporous solids [22]. The initial increase in adsorption capacity at low relative pressure is due to monolayer adsorption on mesopores. The upward deviation in the range of P/P0 = 0.6–0.9 is associated with progressive mesopores filling. As the relative pressure increases, the adsorption isotherm for the mesoporous support displays a relatively sharp increase characteristic of capillary condensation inside uniform mesopores. The nitrogen physisorption of the Al2 O3 mesoporous support and MTO/ZnCl2 -modified mesoporous alumina catalyst reveals also that they have a structure of mesoporous materials with high BET surface area up to 492 m2 /g with narrow pore size distribution with nearly 8 ± 0.1 nm of average pore size and a total pore volume of ≈1.1 cm3 /g, as depicted in Fig. 1 and Table 1. By incorporation of MTO and after modification with ZnCl2 , the BET surface area slightly decreased to ±420 m2 /g. However the total pore volume and the average pore size remain in the vicinity of 1 cm3 /g and 8 nm, respectively for both materials. Therefore, modification of mesoporous alumina with ZnCl2 and MTO incorporation has a very slight impact on the BET specific surface area. However, the pore and structural morphology of the support remain unchanged. This can be further verified by transmission electron microscopy (TEM). The small angle XRD patterns of the mesoporous alumina support, ZnCl2 -modified mesoporous alumina as well as 3 wt% MTO/ZnCl2 –Al2 O3 -meso catalyst are shown in Fig. 2. It can be noticed that all these materials exhibited a peak at about 2 = 0.7 degree. This signifies the presence of a mesoporous material with no long range order pore structure. Similar low angle XRD patterns were previously reported by Ray et al. [19] using aluminium tri-secbutoxide and TEOS in the presence of Pluronic 123 as a templating agent. Also very similar small angle XRD patterns were obtained when organized mesoporous alumina with wormhole-like interconnected pore structure and narrow pore size distribution were used for supporting of rhenium (VII) oxide utilized for simple olefin metathesis [23,24]. These patterns signify the presence of a correlated distribution of framework interconnected pores instead of a regular cylindrical pore structure. This is in agreement with the TEM findings presented beneath. Indeed, the TEM image of the mesoporous Al2 O3 material, as depicted in Fig. 3, clearly shows the wormhole pore arrangement structure. It can be understood from these results that the present

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120000

R2

R1

Al2O3-meso ZnCl2-Al2O3- meso

105000

R1

Catalyst

R1

R2

R2

R2

R2

+

2 R1

R1

R2

R1

3 wt% MTO/ ZnCl2-Al2O3-meso Scheme 1. Catalytic exchange of alkylidene groups between two olefins during self metathesis.

Intensity (a. u.)

90000 75000

R

M +

60000 R1

45000

R

M

+

R1

R2

R

M

R2

R1

R2

Scheme 2. Accepted Chauvin’s mechanism for metathesis reaction [26,27].

30000 15000 0 0.2

0.7

1.2

1.7

2.2

2.7

3.2

2 -degrees Fig. 2. Small angle XRD profiles of Al2 O3 -meso, ZnCl2 -modified Al2 O3 -meso and 3%MTO/ZnCl2 –Al2 O3 -meso.

mesoporous materials present a wormhole-like framework structure containing interconnected 3D-mesopores that can be ideal for the minimization of diffusion limitation phenomena often encountered in adsorption and catalytic reactions. 3.2. Self-metathesis of methyl oleate The self-metathesis of methyl oleate over MTO/ZnCl2 –Al2 O3 meso catalyst results in formation of dimethyl-9-Octadecene-1,18dioate and 9-Octadecene, as primary metathesis products [25]. Metathesis is a catalytic exchange reaction between two molecules during which two alkylidene groups are interchanged (reaction Scheme 1). Metathesis of unsaturated fatty methylesters allow single step synthesis of mono- and bi-functional derivatives of hydrocarbons of well defined structures like the unsaturated diesters that act as intermediate in polymer and specialty chemical synthesis. The generally accepted mechanism of this transformation was first proposed by Chauvin [26,27] and proceeds via a [2+2]

cycloaddition to form a metallacyclobutane intermediate followed by a cycloelimination to form the transalkylidenated products in a reversible fashion, as depicted in reaction Scheme 2. Self metathesis is the reaction occurring between the double bonds of same molecule. The self-metathesis of methyl oleate gives the two industrially important products, 9-Octadecene and dimethyl-9-Octadecene-1,18-dioate in both cis and trans configuration (reaction Scheme 3). Three catalysts were comparatively tested to evaluate their activity for the self-metathesis of methyloleate, among them, two heterogeneous (3%MTO/Al2 O3 -meso and 3%MTO/ZnCl2 –Al2 O3 meso, with Zn/Al = 12) and one homogeneous (second generation Grubbs) catalysts. The reaction was carried out at temperature of 45 ◦ C for a determined reaction time of 5 h. Table 2 summarizes the results obtained for these catalysts. As expected, the MTObased catalysts and the Grubb’s second generation catalyst yielded the primary metathesis products, namely the 9-Octadecene and dimethyl-9-Octadecene-1,18-dioate in cis and trans configurations [28] as well as methylelaidate as a side-product. The reaction continues towards the metathesis product formation until equilibrium is attained. The MTO catalyst supported on Al2 O3 -meso modified with ZnCl2 resulted in a conversion of 92.0%. The yield of total metathesis products including methyloleate was ±87% while that of the desired metathesis products reached about 53%. These results represent a substantial improvement on both metathesis activity and production of the desired metathesis products in comparison to those of the MTO supported on non-promoted mesoporous alumina where only a weak conversion of 11.4% was obtained after 5 h of reaction with 1.03% of desired metathesis products formation. In terms of turnover number (TON), 3%MTO/ZnCl2 –Al2 O3 -meso registered high TON number of 3314 which is the highest one ever obtained for MTO-based catalyst for the methyloleate metathesis. The nonpromoted 3% MTO/Al2 O3 -meso catalyst reached only a modest TON

OCH3 2 O Methyloleate

O

Catalyst

O

H3CO

OCH 3 Dimethyl-9-Octadecene-1,18-dioate

+

9-Octadecene Fig. 3. TEM micrographs of mesoporous Al2 O3 .

Scheme 3. Global reaction scheme of catalytic self-metathesis of methyl oleate.

159

number of 420. This testifies for the high activity of the MTO supported on ZnCl2 -promoted mesoporous alumina. The product distribution yields for the homogeneous second generation Grubbs catalyst were quasi-similar to that of 3%MTO/ZnCl2 –Al2 O3 -meso using identical reaction conditions after 5 h of reaction time in spite of its huge activity. Effectively, this catalyst attained TON number of 88,000, ∼27 higher than that of 3%MTO/ZnCl2 –Al2 O3 -meso which is expectable since Grubbs catalyst is very active and is homogeneous (no mass transfer phenomena). A cautious look at the results presented in Table 2 reveals more or less pronounced olefin isomerization side-product formation obtained for both highly active catalysts 3%MTO/ZnCl2 –Al2 O3 meso and the homogeneous second generation Grubbs catalyst. As already discussed in the literature, the double bond cis/trans isomerization activity occurs concurrently with metathesis [29–31] or such side reactions could be the result of the metathesis reaction itself [32]. It was also shown that the cis–trans isomerization which occurs to a great extent during metathesis is a result of the metathesis itself and not a cationic side reaction [33]. This was also mentioned in early studies by Calderon et al. [34]. In the present investigation, methylelaidate is considered as a non-productive metathesis product. It is worth mentioning that the MTO on zinc chloride-promoted alumina catalyst was relatively more selective towards the desired metathesis products since its ratio of desired metathesis products -to- methylelaidate is 1.53 in comparison to 1.22 obtained for the second generation Grubbs catalyst. The later is circa 25% less selective towards the desired products under similar reaction conditions. Encouraged with these results, it was decided to optimize the ratio Al/Zn for 3%MTO/ZnCl2 –Al2 O3 -meso catalyst. It is worth to mention that the MTO loading was already optimized in our previous works where 3% MTO loading was an optimum [18]. 3.3. Effect of Al/Zn ratio The effect of Al/Zn molar ratio was studied by varying the amount of zinc chloride precursor added in the catalyst to get the desired Al/Zn molar ratios between 4 and 30. Fig. 4 and Table 3 highlight the effect of Al/Zn molar ratio on the metathesis performance of 3%MTO/ZnCl2 –Al2 O3 -meso catalyst. The metathesis activity and the primary desired metathesis product formation increased gradually with increasing Al/Zn ratio and reached a maximum of TON = 3400 (corresponding to a conversion of 92%) and 53%

4000

100 Desired metathesis products Methylelaidate TON

90

3500

80 3000 70 60

2500

50

2000

40

TON

Product yields (%)

a The conversion is evaluated by the following formula: Conversion (%) = ([Methyloleate]0 − [Methyloleate]t ) /[Methyloleate]0 ∗ 100, where [Methyloleate]0 is the concentration of methyloleate at t = 0and [Methyloleate]t is the concentration of methyloleate at a given time t. b Desired metathesis product yield is the summation of trans-9, cis-9-Octadecene, trans- and cis-dimethyl-9-Octodecene-1,18 dioate yields. These product yields were evaluated as follows: Product yield (%) = [Product]t / ([Methyloleate]0 − [Methyloleate]t ) ∗ 100, where [Product]t is the concentration of the specified product at a given time t. c TON represents the average moles of methyloleate molecules converted into all metathesis products per mole of active organometallic catalyst.

0.33 3.56 4.17 0.22 16.20 20.97 0.26 8.29 3.94 0.22 24.39 20.14 1.03 52.44 49.22 3%MTO/Al2 O3 -meso 3%MTO/ZnCl2 –Al2 O3 -meso 2nd Generation Grubbs catalyst

11.43 91.86 91.92

420 3314 88,000

1.40 34.30 40.41

cis-9-Octadecene yield (%) trans-9-Octadecene yield (%) Methyl elaidate yield (%) Desired metathesis product yield (%)b TONc Conversiona (%) Catalyst

Table 2 Comparative results of the metathesis of methyloleate with MTO-based catalysts and homogeneous Grubbs catalyst.

trans-Dimethyl-9Octadecene-1,18-dioate yield (%)

cis-Dimethyl-9Octadecene-1,18-dioate yield (%)

S.K. Pillai et al. / Applied Catalysis A: General 455 (2013) 155–163

1500

30 1000 20 500

10 0

0 4

6

8

10

12

14

16

18

20

22

24

26

28

30

Al/Zn molar ratio

Fig. 4. Effect of Zn/Al ratio on the trans/cis mass ratio of 9-Octadecene and 9-octadecenoate methyl ester during the metathesis of methyloleate. Reaction conditions: T = 45 ◦ C, time = 5 h.

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Table 3 The effect Al/Zn molar ratio on the distribution product yields during methyloleate self-metathesis. Al/Zn ratio

trans-9-Octadecene yield (%)

cis-9-Octadecene yield (%)

trans-Dimethyl-9-Octadecene1,18-dioate yield (%)

cis-Dimethyl-9-Octadecene1,18-dioate yield (%)

4 6 8 12 14 16 18 20 30

8.43 12.69 21.45 24.39 16.71 22.87 14.57 12.98 8.77

6.01 6.86 7.35 8.29 7.19 11.36 7.91 7.7 6.35

6.97 16.69 17.45 16.2 16.48 12.45 15.3 13.33 15.3

3.28 4.99 4.94 3.56 4.13 3.09 3.62 3.64 3.62

yield accounting for the desired metathesis products at the Al/Zn ratio of 12. Further increment in the Al/Zn ratio resulted in a significant decrease of both catalyst activity (monitored by the turnover number, TON) and the desired metathesis products, while methylelaidate stabilized somewhat at a value around 30% (Fig. 4). This is consistent with the previous reported results in case of metathesis of functionalized terminal olefin, methyl-10-undecenoate, where the metathesis activity increased with increasing Al/Zn ratio up to 16 and then decreased for higher ratios [20]. But methyloleate being a functionalized olefin with an internal double bond, the pattern followed is slightly different. The metathesis activity of MTO/ZnCl2 –Al2 O3 -meso with Al/Zn = 4 gave a TON of 1500 corresponding to a conversion of 43% and a yield of 24% accounting to the desired metathesis products. It can be pointed out that both the desired metathesis product as well as the non-productive metathesis product namely methylelaidate are less at Al/Zn ratio of 4. At very high Al/Zn ratio, i.e. at ratio of 30, the yields of desired metathesis products (26%) is quite similar to that of methylelaidate (31%), while the activity stabilized at TON value of circa 2400, ∼30% less that the maximum activity. As shown in Table 3, the distribution of the different product yields is also greatly affected by Al/Zn molar ratio. It is observed that trans-9-Octadecene and trans-dimethyl-9-Octadecene-1,18dioate are the major primary metathesis products where the mass ratio of trans/cis products varies between 1.4–3.0 and 2.1–4.6 for 9Octadecene and dimethyl-9-Octadecene-1,18-dioate, respectively, as depicted in Fig. 5. This figure shows also that the trans/cis mass ratio for 9-Octadecene reached a maximum value of ∼3.0 within the interval of Al/Zn molar ratio situated between 8 and 12, and began to decrease for higher Al/Zn ratios until its stabilization to 1.4 at

trans/cis octadecene

6.0

trans/cis Octodecenoic

Trans/Cis mass ratio

5.0 4.0 3.0 2.0 1.0 0.0 0

4

8

12

16

20

24

28

32

Al/Zn molar ratio Fig. 5. Effect of Zn/Al ratio on the metathesis of methyloleate. Reaction conditions: T = 45 ◦ C, time = 5 h.

Al/Zn ratio of 30. The tendency in the evolvement of the trans/cis mass ratio with Al/Zn ratio is quite different in the case of dimethyl9-Octadecene-1,18-dioate. The trans/cis mass ratio increased with Al/Zn molar ratio up to a value of 4.5 and tended to reach a plateau from Al/Zn molar ratio of 12. As the metathesis also brings about cis–trans isomerization (‘non-productive’ metathesis), the products as well as the starting material end up in their thermodynamic trans/cis equilibrium ratio, in which the trans- chemical conformation is thermodynamically favoured. This phenomenon was reportedly addressed [35]. However, it’s not clear whether the double bond isomerization is promoted by the metathesis catalyst itself, decomposition products, or by impurities from the catalyst synthesis. In the case of the present study, it is clear that the mesoporous Al2 O3 modification with ZnCl2 tends to minimize the formation of trans chemical conformation for both desired metathesis products, namely 9Octadecene and 9-octadecenoate methyl ester at Al/Zn molar ratio below 12. However, the mechanism involved to explain this tendency is yet not known. For these two products, Zn/Al ratio affected similarly their profiles as observed for the activity (TON) profile. Their yields reached a maximal values ∼24% and ∼17%, respectively at Al/Zn ratio of 12. The yields of the same products in cis-configuration remained fairly unchanged at 7.7 ± 1.6% and 3.9 ± 0.7% for cis-9-Octadecene and cis-dimethyl-9-Octadecene-1,18-dioate, respectively. The increased metathesis activity clearly resulted from ZnCl2 modification of the mesoporous alumina support and it may arise from an enhancement in either the site activity, or the increase of the number of active sites. However, a question should be addressed about the reason of this tremendous enhancement in the catalyst activity. Several experimental evidences allow believing that MTO-based heterogeneous catalysts possess Lewis acid sites in the active metal centre [36–39]. The acidity of the support was thought to play a role in enhancing the reaction with MTO-based heterogeneous catalysts. Increase in the catalytic activity with substantial increase in the support’s Lewis acidity was observed for metathesis of 1pentene over niobia supported system [40]. This was confirmed by the studies that showed the Lewis acid sites are favourable chemisorption sites for MTO binding [41]. In the case of the present work, the 1 H MAS NMR analysis of ZnCl2 -modified Al2 O3 and unmodified Al2 O3 was performed to study the effect of ZnCl2 on modifying the surface property of mesoporous alumina. As depicted in Fig. 6, in both samples, two small peaks are observed indicating the presence of residual organic species into the mesoporous alumina. In the unmodified alumina, the 1 H MAS NMR spectrum exhibited two broad signals, with maxima at – 0.4 and circa 2 ppm, assigned to terminal (basic) hydroxyl groups and bridging hydroxyl (Lewis-type acid) groups, respectively [24]. This is in agreement with the literature reporting that different types of hydroxyl groups are present in the surface of alumina according to the well-known Knözinger model [25].

S.K. Pillai et al. / Applied Catalysis A: General 455 (2013) 155–163

Fig. 6.

1

H MAS–NMR analysis of ZnCl2 -modified and unmodified mesoporous Al2 O3 .

For ZnCl2 -modified Al2 O3 -meso, only one major peak is observed and this corresponds to the Lewis type acidic hydroxyl groups present in alumina. The terminal hydroxyls are completely eliminated. This shows that ZnCl2 when added to the alumina neutralizes the terminal hydroxyl groups resulting in increased number of Lewis acidic sites, responsible for the activity of MTO-based catalyst. These analytical observations suggest that terminal OH groups of mesoporous alumina react with ZnCl2 by the formation of favourable catalytic site structures where MTO is grafted, thus enhancing the overall catalytic activity towards methyloleate metathesis in comparison to those generated by MTO grafting on non-modified mesoporous alumina. Until now, the interaction responsible for the increase in olefin metathesis activity was not investigated or known. Furthermore there is no mechanistic evidence about the chemical and structural nature of the MTO/ZnCl2 -modified Al2 O3 meso sites responsible of the increased metathesis activity. In contrast, it is largely admitted that the binding of MTO on non-modified Al2 O3 occurs by coordination of oxo-ligand or heterocyclic splitting of C H bond on Lewis acidic Al sites, the latter resulting in the metal carbene forming the active species for olefin metathesis according to Scheme 4 [42,43]. It is thought that Zn and Cl are present in molecular forms that interact strongly with the support, rather than as ZnCl2 crystallites. In order to understand and situate the significance of the Zn and Cl interactions effect in the enhancement of the overall catalyst activity, it is proposed to investigate other types of metal halide compounds among them metal chlorides M Clx (M = Ga, Mg, Mn, Ca; x = 2 or 3) and Zinc halides (Zn Y2 (Y = Br and I) to see if they possess the same promoting effect as ZnCl2 . 3.4. Effect of other halide promoters As the highest activity was obtained for Al/Zn ratio between 8 and 12, the other halide promoters tested were compared to ZnCl2 in the same basis using similar mesoporous alumina modification,

CH3 O

O

Re O

161

O O

O

CH3 Re

O

O Re

O

O O Al (1)

O

O Al (2)

CH2 Re

CH2

H O

O

O O Al (3)

H O

Scheme 4. Methyltrioxorhenium and its binding with the Al2 O3 through oxo-ligand (1) and by heterocyclic splitting of methyl group (2) resulting in the carbene (3) formation [43].

Fig. 7. Effect of different metal halide promoters on the metathesis of methyloleate. Reaction conditions: T = 45 ◦ C, time = 5 h. Al/M molar ratio = 8.

same MTO catalyst loading, and identical metathesis conditions as used for 3%MTO/ZnCl2 –Al2 O3 -meso at Al/Zn = 8. The use of different promoters such as gallium chloride, magnesium chloride, manganese chloride, calcium chloride show that among these promoters, gallium chloride promotes highest metathesis activity with a TON of 3110 corresponding to a conversion of 84.63% and 46.28% desired metathesis products whereas the calcium chloride has the lowest activity in terms of TON (TON = 521) corresponding to a conversion of 14.17% and yield of 4.25% desired metathesis product formation. But compared to zinc chloride all these promoters are less active for self-metathesis of methyloleate (Fig. 7). The generation of Lewis acidity by these metal chlorides (Ga3+ , 2+ Zn , Mg2+ , Mn2+ , and Ca2+ ) was found to be due to the presence of coordinative unsaturated dopant cations on the surface of the support. The addition of these metal ions decreases the basic sites and thereby increasing the Lewis acidic sites, knowing that the Lewis acidity increases with increasing electronegativity of the cation [44,45]. The formation of Lewis acid sites on dehydroxylated surface of support by gallium chloride was found to be due to the tetrahedral position of Ga3+ [46,47]. The use of other zinc halides as promoters instead of zinc chloride shows that the zinc bromide has higher metathesis activity than zinc iodide but it is less than that of zinc chloride (Fig. 7). The use of zinc bromide as promoter gave a TON of 2647, corresponding to 72% of conversion with 42% desired metathesis product yield and compares somewhat with zinc chloride. Similar observations were obtained for the metathesis of methyl-10-undecenoate with MTO supported on alumina modified with zinc halides, where zinc bromide was effective in promoting metathesis but little less than zinc chloride [20]. It can be observed that the zinc chloride is a more efficient molecule in modifying mesoporous alumina to act as support for MTO catalysts for metathesis of methyl oleate. Neither the other metal chlorides nor the zinc halides were as active as zinc chloride. These results allow understanding that the enhancement of the catalytic activity is not only due to the presence of chloride and/or zinc. It is the combined Zn Cl bond that could contribute to the active site formation. This result gives a rough idea about the nature of interaction of the ZnCl2 with Al2 O3 -meso and it is consistent with the XPS analysis as discussed beneath. The analysis of the catalyst by XPS shows the oxidation states of the different elements present in the active catalyst. The XPS spectrum (Fig. 8) shows that the energy corresponding to Zn 2p3/2 is

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Cl elements are bonded at the catalyst surface as Zn Cl or Zn Cl2 [23]. Meanwhile it is worth to note that the stoichiometry [Cl]/[Zn], as measured with the surface concentrations of these elements on the entire XPS survey spectra (results not shown), is close to 2 which probably could confirm the Zn Cl2 configuration on the mesoporous alumina. This shows that the enhancement of the catalytic activity of the MTO catalyst supported on the ZnCl2 –Al2 O3 -meso is due to the presence of both the Zn and Cl elements. The deconvolution of Re4f core level gives 4 peaks: Re4f 7/2 (1) , Re4f 5/2 (1) , Re4f 7/2 (2) , Re4f5/2 (2) with binding energies of 44.6, 47.6, 47.5 and 49.8, respectively. The Re4f7/2 binding energies show the presence of Re6+ and Re7+ species which are known to be the active species in the metathesis reaction. 4. Conclusion Methyltrioxorhenium incorporated on the ZnCl2 -modified mesoporous alumina was found to be an efficient catalyst for the self-metathesis of methyl oleate. The Al/Zn = 8–12 was found to be an optimal ratio range leading to maximum metathesis activity. The use of other promoters either in the form of metal chlorides or other zinc halides did not promote the metathesis of methyloleate as zinc chloride. So the enhancement of the activity is not only due to the chloride and/or zinc but could be attributed to the interaction of both elements as Zn Cl or Zn Cl2 bonds with the mesoporous alumina. The self-metathesis of methyl oleate in presence of MTO/ZnCl2 Al2 O3 -meso gives the primary metathesis products, 9-Octadecene and dimethyl-9-Octadecene-1,18-dioate, of industrial importance. These results pave the way for the development of an active catalyst that can be used for the functionalized olefins and the vegetable oils to give rise to a number of industrially important products and polymer building blocks. Acknowledgements The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada who provided funds for this research. Alain Adnot (CERPIC) is acknowledged for XPS characterization. They also acknowledge Susannah Scott group at UCSB California, particularly, Trenton Tovar for Solid-state NMR caracterizations. References [1] [2] [3] [4] [5] [6] [7] Fig. 8. The XPS characterization of 3%MTO/ZnCl2 –Al2 O3 -meso catalyst. (A) Zn 2p photoelectron peak; (B) Cl 2p photoelectron peak; (C) Re 4f photoelectron peak.

compatible with Zn association with halides. Similarly, the deconvoluted Cl2p core level also is characterized by binding energies of 200 and 201.5 eV corresponding to Cl2p3/2 and Cl2p1/2 , respectively which signify the presence of metal chloride, namely zinc chloride, in this case. Data on pure ZnCl2 XPS binding energies for Zn 2p and Cl 2p core levels is hardly reported in the open literature. However it seems that Zn2p3/2 of halides, particularly for zinc chloride, have significant chemical shifts [22]. This allows concluding that Zn and

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