Heterogenized nickel catalysts for propene dimerization: Support effects on activity and selectivity

Catalysis Communications 32 (2013) 32–35

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Short Communication

Heterogenized nickel catalysts for propene dimerization: Support effects on activity and selectivity Michèle O. de Souza a,⁎, Roberto F. de Souza a, Larissa R. Rodrigues a, Heloise O. Pastore b, Régis M. Gauvin c, Jean Marcel R. Gallo b, Cristiano Favero a a b c

Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre 91501-970, Brazil Instituto de Química, Universidade Estadual de Campinas, CP 6154, Campinas 13083-970, Brazil Laboratoire de Catalyse de Lille, Université des Sciences et Tecnologies, Bât. C3 59655, Villeneuve d'Ascq Cedex, UMR CNRS 8010, France

a r t i c l e

i n f o

Article history: Received 3 September 2012 Received in revised form 20 November 2012 Accepted 26 November 2012 Available online 30 November 2012 Keywords: Oligomerization Nickel Propylene Hybrid material MCM-41 SiO2

a b s t r a c t Ni–SiO2 and Ni-[Si]–MCM-41 are synthesized by grafting Ni(MeCN)6(BF4)2 onto SiO2 (nonporous) and MCM-41 (mesoporous). The catalytic properties of the grafted catalysts after activation with Al2Et3Cl3 are compared with the homogeneous equivalent system in propylene oligomerization. The identification of dimers leads to the determination of preferential reaction steps and the corresponding active species. Differences of the insertion modes of propylene are correlated to the textural properties of the support. Supporting the nickel active species reduces the isomerization side reactions and changes the mode of insertion of the first olefin. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Oligomers of light olefins are largely used by industry as additives for gasoline, co-monomers in the synthesis of specific polymers or as intermediates in the synthesis of detergents, lubricants and plasticizers for example [1]. Synthesis of ethylene and propylene oligomers using organometallic complexes in homogeneous systems is the subject of numerous studies [2,3]. Nickel-containing complexes are specifically interesting since their catalytic activity are exceptionally high and the use of large variety of ligands allows catalyst “tailoring” thus directing the selectivity of the catalytic reaction [4,5]. Grafted organometallic complexes combine the versatility of homogeneous systems with easy feed/catalyst separation thus producing environmentally friendly processes that would allow catalyst recycling and reduce the use of organic solvents. Recent studies have been devoted to investigating the effect of the presence of aluminum in the structure of inorganic mesoporous materials like MCM-41 (Mobil Composition of Matter No 41) using [Si,Al]MCM-41 and [Si]-MCM-41 as support for the complex Ni(MeCN)6 (BF4)2 [6]. The corresponding hybrid materials are active in ethylene oligomerization when associated with an alkylating agent. XPS analyses

⁎ Corresponding author. Tel.: +55 51 3308 7238; fax: +55 51 3308 7304. E-mail address: [email protected] (M.O. de Souza). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.11.023

provided evidence of different active species and have shown that the coordination sphere of the nickel is modified depending on the presence or absence of aluminum in the support structure. Furthermore it has been shown that when Ni(MeCN)6(BF4)2 is grafted onto [Si]-MCM-41 (no aluminum in the support) the hybrid catalyst is more active than those catalysts obtained with a support that contains aluminum. It has also been pointed out that the presence of aluminum in the support is an important parameter since it is responsible for the formation of active species in dimer isomerization, an undesirable side reaction. Overall results showed that Ni/[Si]-MCM-41 remarkably combines high 1-butene selectivity (63%) with high activity (TOF= 3200 h−1). We investigate and compare the catalytic properties of Ni/[Si]MCM-41 and Ni/SiO2, the latter being obtained by grafting Ni(MeCN)6 (BF4)2 onto commercial Aerosil 300 silica, a nonporous material, with the aim of evaluating the effect of the mesoporosity of the support on catalytic performance. Both hybrid systems are compared to the unsupported Ni(MeCN)6(BF4)2 complex in propylene oligomerization using Al2Et3Cl3 as co-catalyst. Based on the methodology introduced by Bogdanović, the use of a dissymmetric olefin such as propylene affords information about the mechanism, the active species and the insertion mode of the first and second olefins involved in the dimerization to be obtained [4]. In this study the selectivities obtained by each system are correlated with the catalytic species responsible for dimerization. Thus the effect of the texture of the support on catalytic performance is highlighted.

M.O. de Souza et al. / Catalysis Communications 32 (2013) 32–35

2. Experimental All experimental operations were performed using standard Schlenk tube techniques under inert atmosphere. Commercial solvents were dried and deoxygenated by refluxing over appropriate drying agents under argon and distilled prior to use. Complex 1 [7] and [Si]-MCM-41 [8] were prepared following literature procedures. The commercial Silica Aerosil 300 was provided by Evonik. The mesoporous material was calcined under oxygen flow (5 °C min−1 heating rate) until 500 °C. Prior to grafting, the silica and [Si]-MCM-41 were activated at 180 °C and 400 °C respectively (5 °C min−1 heating rate) under vacuum (10 −5 Pa) for 6 h. The respective catalytic precursors Ni/SiO2 and Ni/[Si]-MCM-41 were obtained by treating calcined and dehydrated silica and mesoporous material (about 0.4 g) with a solution of 1 in MeCN (4.7 mM) under reflux for 24 h, followed by filtration and drying under vacuum (1 Pa) at room temperature until constant weight. Ni contents in Ni/SiO2 and Ni/[Si]-MCM-41 were determined by atomic absorption spectrometry [9] using an AAS 5EA atomic absorption spectrometer (Analytik Jena AG). Specific surface areas (SSA) were determined from N2 adsorption isotherms measured at 77 K, using a Gemini apparatus and applying the BET method. Before analysis, calcined SiO2 and [Si]-MCM-41 were degassed at 400 °C overnight to a residual pressure of ca. 10−4 Pa, and at 80 °C for the grafted material. To certify that acetonitrile (solvent used in the impregnation experiment) doesn't affect the SSA values obtained for the hybrid systems we performed N2 adsorption isotherms measured at 77 K for the support after its thermal treatment and after being maintained under reflux in acetonitrile during 24 h and dried at 80 °C. No difference was observed between the isotherm nor the SSA values obtained by the BET method. Propylene oligomerization reactions were carried out in a 100 mL stainless-steel batch reactor, magnetically stirred. In a typical run, a suspension of catalytic precursor (about 0.2 g in 20 mL C6H5Cl) was introduced in the reactor, and then a Al2Et3Cl3 solution (at the desired Al2Et3Cl3/Ni molar ratio: 30 or 15 for the homogeneous and the hybrid system respectively) was added under propylene flow at low temperature (−10 °C). The Al2Et3Cl3/Ni molar ratio values have been established according previous studies [10]. The reactor was pressurized, maintained at 8 MPa of propylene, and heated at 50 °C for 1 h using a thermostated jacket. Recycling of the hybrid system has been performed in a glass reactor under 5 MPa of propylene. Three consecutive reactions of 30 min were performed with the same hybrid catalyst at 50° C. After this time, the supernatant was decanted and removed from the solid phase. Before the second and the third reactions 20 mL of C6H5Cl (solvent) and a fresh Al2Et3Cl3 solution (Al/Ni molar ratio = 15) were added to the solid catalyst. The products were then cooled (−10 °C) and analyzed by gas chromatography (Varian 3400CX, FID) using a Petrocol DH capillary column (100 m length). Dimers and trimers were detected allowing selectivity determination. Cyclohexane was used as internal standard to quantify the catalytic activity measured as turnover frequency, TOF (mol of propylene consumed per mole of nickel per hour).

33

Table 1 Materials characterization: weigh % of nickel (Ni), specific surface area (SSA), porous volume (Vp) and decomposition temperature (TD) of Ni[MeCN]6(BF4)2, the hybrid materials and supports. Entry

Sample

Nia (% weight)

SSAb (m2 g−1)

Vp c (cm3 g−1)

TDd (°C)

1 2 3 4 5

Ni [MeCN]6 (BF4)2 [Si]-MCM-41 Ni/[Si]-MCM-41 SiO2 Ni/SiO2

– – 1.32 – 3.83

– 930 220 257 189

– 0.54 0.48 – –

488

a b c d

528 – 553

From atomic absorption spectroscopy analysis. BET method. BJH method. Decomposition temperature obtained from thermogravimetric analysis.

the adsorption of nitrogen on the internal surface of the MCM-41 and on the surface of SiO2 providing evidence that the complex takes up space within the mesopores of MCM-41. Decomposition temperatures (TD) of the grafted nickel species are higher compared to that of the ungrafted complex (compare entries 3 and 5 with entry 1) indicating the presence of an interaction between the nickel atoms and the support that stabilize the supported species. Ni (MeCN)6(BF4)2 reacting in the presence of propylene with Al2Et3Cl3 produces a catalytic species of Ziegler–Natta type, a nickel-hydride, that is active in propylene oligomerization [11]. As propylene is a dissymmetric molecule, its insertion into the nickel species may be achieves through carbon 1 (Ni ➔ C1) or carbon 2 (Ni ➔ C2) of the olefin in order for the two insertions to produce a dimer. Combinations of these two insertion modes lead to the formation of several dimers as illustrated in Fig. 1. Each dimer can be related to the insertion mode involved for its production. Therefore factors p and q have been defined by Bogdanović [4]. The direction of propylene addition in the first step may be defined by the quotient p, where: p¼

%1H þ %2H þ %3H þ %2M1P þ %2M2Pa : %2; 3DM1B þ %2; 3DM2B þ %4M1P þ %4M2P þ %2M2Pb

ð1Þ

The average direction of propylene addition in the second step is given by q, where: q¼

%2M1P þ %2M2Pa þ 2; 3DM1B þ 2; 3DM2B : %1H þ %2H þ %3H þ %4M1P þ %4M2P þ %2M2Pb

ð2Þ

Fig. 1 shows that 2M2P is an isomer formed by the reaction pathways (a) and (b) from 2M1P and 4M1P respectively and that for 4M2P cis and trans isomers are produced. It has been assumed, as approximation, that 2M2P is formed proportionally to the amount of residual parent compounds, through both pathways. These considerations give Eqs. (3) and (4). % 2M2P ¼ % 2M2Pa þ % 2M2Pb

ð3Þ

3. Results and discussion

% 2M2Pa % 2M1P : ¼ % 2M2Pb % 4M2P þ % 4M1P

ð4Þ

We have shown recently through analysis by transmission electron microscopy and X-ray diffraction that the organization of the pores of [Si]-MCM-41 is not modified after the immobilization of the complex [6]. Table 1 reports characterization data of the nickel precursor, the supports and the hybrid materials: nickel content determined by atomic absorption analysis, specific surface area (SSA) and porous volume (Vp) determined by nitrogen adsorption and decomposition temperature (TD) obtained during thermogravimetric analysis. The decrease of ASS and Vp data of the nickel hybrid systems in comparison to those obtained for the support (compare entries 2 with 3 and 4 with 5 in Table 1) demonstrates that the immobilized complex hinder

Table 2 reports turnover frequency (TOF), selectivities and the p and q values obtained for propylene oligomerization catalyzed by Ni(MeCN)6(BF4)2, Ni/SiO2 and Ni/[Si]-MCM-41 in the presence of Al2Et3Cl3 as co-catalyst. To certify that the supported nickel species remain on the support during the catalytic reactions, two recycling of the hybrid system employing silica as support were performed. After the last consecutive runs the hybrid system retains its high activity (>400 h−1) and a selectivity in trans-4M2P (main product) of 40.1%, showing that the reaction proceeds heterogeneously. Results reported in Table 2 show that the three systems are active for propylene dimerization since the selectivity to trimer (C9) formation is

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M.O. de Souza et al. / Catalysis Communications 32 (2013) 32–35

Insertion of the 1st propylene

Insertion of the 2nd propylene Ni

Ni

C1

Dimers

Isomers

Ni-H

C2 Ni

1H

3H

Ni 2H

Ni-H Ni

C1

Ni

2M1P

2M2P

4M1P

+

Ni-H

(a)

Ni-H t4M2P

Ni

C2

(b)

Ni c4M2P

Ni Ni

C2

Ni-H Ni

C1

Ni

2,3DM1B

2,3DM2B

Fig. 1. Mechanism of propylene dimerization catalyzed by (MeCN)6(BF4)2, Ni/[Si]-MCM-41 and Ni/SiO2 in the presence of Al2Et3Cl3. The nickel species illustrate the insertion mode of the propylene (through the C1 or C2 carbon).

lower than 10% and that the homogeneous system is the most active among the three systems. The activity values observed for the hybrid systems is lower than that observed for the homogeneous system as observed for ethylene. The values obtained for p and q (Table 2) indicate that for the homogeneous system the insertion of the two olefins occurs preferentially via the Ni➔ C1 and Ni➔C2 pathways. As oppose to, the Ni➔ C2 and Ni➔ C2 pathways which are the preferred routes for the hybrid systems. When comparing the change observed after the first propylene addition for the homogeneous and the hybrid systems an interesting effect due to the support is evident. Bogdanović studied the use of phosphine ligands for homogeneous nickel catalysts synthesized following a Ziegler method, i.e. addition of an alkylation agent to the nickel(II) salt, and concluded that the effect of the phosphines is mainly associated with the changes in the second step evidenced by changes of the q value. For a

Table 2 Turnover frequency (TOF), selectivities, p and q data obtained for the catalytic systems Ni(MeCN)6(BF4)2, Ni/SiO2 and Ni/[Si]-MCM-41 in propylene oligomerization. Catalytic systema

TOF (h−1)

Homogeneous

Ni/SiO2

Ni/[Si]-MCM-41

36500

630

3500

95.0 5.0 4.65 3.95 7.8 44.7 4.7 3.1 21,4 0.70 8.45 0.55 30/70 10/90

93.0 7.0 4.5 14.2 54.3 5.2 7.3 0.1 14,3 0.0 0.1 0.0 22/78 22/78

Products

Selectivity (%)

C6 C9 4M1P 2,3DM1B c4M2P t4M2P (1H + 2M1P) 3H 2H 2M2Pa 2M2Pb 2,3DM2B p q

90.5 9.5 0.6 0.7 2.3 16.6 5.4 5.8 19,5 9.8 35.2 4.4 40/60 20/80

a Reaction conditions: propylene pressure = 8 MPa, temperature = 50 °C, time = 1 h and Al2Et3Cl3/Ni molar ratio = 30 or 15 for the homogeneous and the hybrid system respectively.

large number of phosphines, the p values is unchanged, meaning that in the first step the nickel hydride species adds to propene in a manner largely independent of the nature of the phosphine. Results presented in this paper show that the grafting of nickel species can influence the insertion mode of the first propylene resulting in significant variation of the p values. For each system the p values obtained are 40/60, 30/70 and 22/78 for the homogeneous, Ni/SiO2 and Ni/[Si]-MCM-41 systems respectively. Based on selectivity data of Table 2, Fig. 2 illustrates specifically the selectivity in the major product obtained for each catalytic system that is different: 2M2Pb, c4M2P and t4M2P are the respective major products obtained when the homogeneous, Ni/[Si]-MCM-41 and Ni/SiO2 catalytic system is employed. Using Fig. 1 it can be seen that 2M2Pb, the main product obtained with the homogeneous system is produced through the isomerization of 2M1P. However formation of cis-4M2P and trans-4M2P, the main products formed by the two hybrid systems, does not involve an isomerization reaction. These results indicate that the hybrid systems are less isomerizing than the homogeneous one. Nickel catalysts are also known to be active in isomerization reactions. It has previously been shown [6] that the more active the nickel catalytic species, the more isomerization is occurring. The behavior of the homogeneous catalyst observed in this study, i.e. production of large quantities of isomers, can be attributed to high activity and in the same way; the low formation of isomers observed in the case of Ni/SiO2 can be explained by its low activity. Knowing that alpha olefins exhibit higher reactivity in double bond isomerization than internal olefins, the isomerization activity observed for the homogeneous system can also be a consequence of the mechanism involved. According to Fig. 2, the homogeneous catalyst leads to 1-C6 as a primary olefin, while the hybrid catalytic systems lead to 2-C6 olefins. When MCM-41 is employed as support, the grafted species show a higher activity (3500 h−1) than that of Ni/SiO2 (630 h−1) and yet the selectivity in the main product, i.e. 4M2P, is 54.3%, a result very close to that obtained with Ni/SiO2 (44.7%). One can thus conclude that nickel species grafted onto the mesoporous MCM-41 coordinate preferentially a new propylene molecule to produce a dimer instead of performing the dimers isomerization. The decrease of the isomerization properties of the active species obtained from Ni/[Si]-MCM-41 is also illustrated by the selectivity results of 3H, 2M2P and 2,3DM2B that are almost not detected (see Table 2).

M.O. de Souza et al. / Catalysis Communications 32 (2013) 32–35

Ni-H Ni-H

Homogeneous

Ni

35

Isomerization

Ni Ni-H

Ni/SiO2

Ni-H

Ni Ni Ni-H

Ni/[Si]-MCM-41

Ni-H

Ni Ni

Fig. 2. Influence of the catalytic system Ni(MeCN)6(BF4)2, Ni-SiO2 and Ni/[Si]-MCM-41/Al2Et3Cl3 on the selectivity towards the major product M4P2 and its isomers.

H H β-H elimination

H

Ni

Ni

Aerosil

Aerosil

Ni

H

H

β-H elimination

MCM-41

+ t4M2P

H Ni MCM-41

+ c4M2P

Fig. 3. Mechanism of β-elimination corresponding to the cis e trans isomers of 4-methyl-2 pentene formation (c4M2P and t4M2P respectively).

Comparison between the two hybrid systems regarding the formation of the major product shows that different isomers of 4M2P is produced: the trans-4M2P and the cis-4M2P for Ni/SiO2 and Ni/[Si]-MCM-41 respectively. The thermodynamic (trans-4M2P/ cis-4M2P) ratio obtained through a semi-empirical determination is equal to 1.25, corresponding to 56% of trans-4M2P. This ratio is 5.7 and 0.1 for Ni/SiO2 and Ni/[Si]-MCM-41 systems, respectively (from Table 2). Thus the proportion of trans-4M2P is 85% and 9% in the 4M2P mixture produced with Ni/SiO2 and Ni/[Si]-MCM-41 respectively. This result illustrates an effect of the support that influences the selectivity of the grafted Ni(MeCN)6(BF4)2. As Fig. 3 shows, the formation of these two isomers occurs through a β-elimination that results from different Ni-alkyl conformations depending on whether the nickel species is grafted onto silica or MCM-41. It seems that the rotation of the coordinated molecule is not free and that β-elimination is not statistical. This may result from the textural properties of the host support: in the Aerosil case, catalysis takes place on the particle surface which is mostly convex, whereas in the case of MCM-41, where the metal centers are located in the mesoporous channels (see BET values), that features concave surface. These results illustrate the effect of the ligands surrounding the nickel center and its environment i.e. the support. 4. Conclusion Propylene oligomerization is carried out with Ni(MeCN)6(BF4)2 in the presence of the same alkylation agent under homogeneous conditions and with this complex grafted onto nonporous (SiO2) and mesoporous ([Si]-MCM-41) supports. Significant differences are observed in terms of catalytic activity and selectivity. The latter strongly depends on the pore characteristics of the support used for catalyst immobilization. The homogeneous system is the most catalytically active and the most isomerizing among all the systems investigated. The

major product for the homogeneous, Ni/[SiO2] and Ni/[Si]-MCM-41 catalysts are 2M2P, trans-4M2P and cis-4M2P respectively indicating a support and pore effect on the selectivity. In the case of the supported catalysts, this is assigned to textural properties, demonstrating the non-innocent character of the support in terms of both activity and selectivity. Acknowledgments The authors thank CAPES, CNPq and PRONEX/FAPERGS for their financial support and Mario Roberto Meneghetti for carrying out theoretical calculations. References [1] H.A. Wittcoff, B.G. Reuben, Industrial Organic Chemicals in Perspective, John Wiley, New York, 1980. [2] C. Janiak, Coordination Chemistry Reviews 250 (2006) 66–94. [3] C. Bianchini, G. Giambastiani, L. Luconi, A. Meli, Coordination Chemistry Reviews 254 (2010) 431–455. [4] B. Bogdanovi'c, in: F.G.A. Stone, R. West (Eds.), Advances in Organometallic Chemistry, vol. 17, Academic Press, New York, 1964, pp. 105–140. [5] G. Lefebvre, Y. Chauvin, in: R. Ugo, C. Manfredi (Eds.), Aspects Homog. Catal., vol. 1, 1970, pp. 107–201. [6] M.O. de Souza, L.R. Rodrigues, R.M. Gauvin, R.F. de Souza, H.O. Pastore, L. Gengembre, J.A.C. Ruiz, J.M.R. Gallo, T.S. Milanesi, M.A. Milani, Catalysis Communications 11 (2010) 597–600. [7] R.F. de Souza, A.L. Monteiro, M. Seferin, M.O. de Souza, F.C. Stedile, C.N. Wyrvalski, I.J.R. Baumvol, Journal of Coordination Chemistry 40 (1996) 311–318. [8] H.O. Pastore, M. Munsignatti, D.R.S. Bittencourt, M.M. Rippel, Microporous and Mesoporous Materials 32 (1999) 211–228. [9] M.G.R. Vale, I.C.F. Damin, A. Klassen, M.M. Silva, B. Welz, A.F. Silva, F.G. Lepri, D.L.G. Borges, U. Heitmann, Microchemical Journal 77 (2004) 131–140. [10] C. Wyrvalski, M.O. de Souza, R.F. de souza, Química Nova, 19 (1996) 493-495. L.R. Rodrigues, PhD thesis 2010, UFRGS, Brazil. [11] R.F. de Souza, B.C. Leal, M.O. de Souza, D. Thiele, Journal of Molecular Catalysis A: Chemical 272 (1–2) (2007) 6–10.