Nickel oligomerization catalysts heterogenized on zeolites obtained using ionic liquids as templates

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Nickel oligomerization catalysts heterogenized on zeolites obtained using ionic

liquids as templates

Marcelo L. Mignoni

a

, Miche`le O. de Souza

a

, Sibele B.C. Pergher

b

, Roberto F. de Souza

a

,

Katia Bernardo-Gusma˜o

a,

_*

a

Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves 9500, P.O. Box 15003, CEP 91501-970 Porto Alegre, RS, Brazil

b

Department of Chemistry, Universidade Regional Integrada do Alto Uruguai e das Misso˜es, Av. Sete de Setembro 1621, CEP 99700-000 Erechim, Brazil

1. Introduction

Several zeolites have been used as supports for transition metal catalysts in order to improve their recyclability and selectivity. An important aim in this area is to take advantage of cooperative effects between the shape selectivity due to the zeolite framework and the advantages of coordination compounds to prepare highly selective carbon–carbon bond formation catalysts. A large variety of zeolite structures are currently available, enabling careful optimization of the choice of catalyst and zeolite framework, which can have a striking inﬂuence on the reaction outcome.

Zeolites like ZSM-5 are extensively used in industry as catalysts in the petroleum reﬁning industry[1], as well as for ion-exchange, organic product separations, and for gas adsorption, among other uses[2]. This MFI-type three-dimensional network pore system, containing sinusoidal 10-membered ring channels intersecting straight 10-membered ring channels[3], and its synthesis are still attracting great attention due to their academic and industrial importance. Examples of variations in the starting materials, the choice of synthesis temperature, reaction time[4,5], alkalinity[6],

and choice of the most convenient templates[7]are available in the literature.

One of the most attractive points in the synthesis of these zeolites is the template effect. The use of convenient organic molecules in the synthesis mixture directs nucleation, organizing inorganic tetrahedral units into a particular topology, which provides an initial building block for further crystallization in a particular structure type. A plethora of organic species have been used as templates in the synthesis of ZSM-5. Examples include n-propylamine (n-C3H7NH2), ethylenediamine (NH2C2H4NH2), hydroxyethylamine (OHC2H4NH2), ethanol (C2H5OH) [8], tetra-propylammonium hydroxide (TPAOH), and tetratetra-propylammonium bromide (TPABr)[9].

Another type of zeolite is the

b

-zeolite, which was originally described in a patent of the Mobil Oil Corporation. The

b

-zeolite is a large pore zeolite (7.5 A˚) presenting a three-dimensional interconnected channel system with 12-membered openings[10].

b

-Zeolites have been obtained using different agents as structure directors including di-benzyl-1,4-diazabicyclo[2,2,2]oc-tane chloride [11], tetraethylammonium hydroxide [12] tetra-ethylammonium bromide and 4,4-dimethyl-4-azonia-tricyclo [5.2.2.02,6]undec-8-ene hydroxide[13]. Beta zeolites attract great interest due to their reactivity as catalysts for reactions such as isobutane–n-butene alkylation [14], alkane hydroisomerization

[15,16], aromatic acylation[17,18], etc.

The unusual properties of ionic liquids offer advantages for their use in the synthesis of zeolites [19]. The difﬁculties with

A R T I C L E I N F O

Article history:

Received 24 August 2009

Received in revised form 4 November 2009 Accepted 16 November 2009 Keywords: Zeolite ZSM-5 b-Zeolite Ionic liquid Oligomerization A B S T R A C T

Synthesis of zeolites using 1-butyl-3-methylimidazolium chloride (BMI.Cl) as a template yields highly crystalline materials after a 3-day reaction time. SEM micrographs of material obtained with a Si/Al molar ratio of 50 showed formation of very regular microspherical agglomerates with approximate diameters of 30mm composed of ZSM-5 crystallites with 3–5mm long edges. The regular format of these crystallites has been attributed to the formation of micellar aggregates due to the ionic liquid. Decreasing the Si/Al molar ratio to 20 reduces the diameter of the microspheres to less than 1mm. The ZSM-5 zeolites show a speciﬁc area of 384 m2_g 1_{and a microporous volume of 0.10 cm}3_g 1_{, and the}_b_-zeolite,

obtained after 7 days of crystallization, shows a speciﬁc area of 418 m2_g 1_{and a microporous volume of}

0.11 cm3_g 1_{. The}_b_{-zeolite was used as support for a nickel-}_b_{-diimine complex and the system used in}

ethylene catalytic oligomerization reactions.

* Corresponding author. Tel.: +55 51 33087240; fax: +55 51 33087304. E-mail addresses:m_mignoni@yahoo.com.br(M.L. Mignoni),

michele.souza@ufrgs.br(M.O. de Souza),pergher@uri.com.br(Sibele B.C. Pergher),

rfds@iq.ufrgs.br(R.F. de Souza),katiabg@iq.ufrgs.br(K. Bernardo-Gusma˜o).

Contents lists available atScienceDirect

Applied Catalysis A: General

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

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synthesis of silicates using ionic liquids as template and solvent simultaneously have been discussed in a recent article[20]. Some ionic liquids have been successfully used as templates in the synthesis of MCM-41 [21] and IM-5 zeolites [22], mesoporous alumina[23], silica, and Ru–SiO2[24], as well as aluminophosphate molecular sieves[25,26]. To our knowledge, no description of the synthesis of ZSM-5 or beta zeolites using 1-butyl-3-methylimi-dazolium chloride (BMI.Cl) as a template has previously been described in the literature. A recent publication[27] describes attempts to use 1-methyl-3-ethylimidazolium bromide (EMI.Br) for the synthesis of zeolites, but the authors did not succeed in the formation of an MFI-type framework, probably due to their choice of reaction composition and conditions.

Herein, we describe the use of dialkylimidazolium ionic liquids as templates for the synthesis of ZSM-5 and

b

-zeolites, some of them with particular crystallization structures, and use of the synthesized zeolites as supports for

b

-diimine–nickel catalysts for ethylene oligomerization.

2. Experimental

1-Butyl-3-methylimidazolium chloride (BMI.Cl) has been synthesized according to published procedures[28].

The zeolite synthesis was performed by mixing SiO2(7.23 or 18.5 g of aerosil 200, Degussa), BMI.Cl (8.16 g), Na2Al2O4(0.58 g, Riedel-de-Haen), NaOH (2.44 g Aldrich), and H2O (115 or 130 g), corresponding to the molar ratios Si/OH = 2 or 5, H2O/Si = 53 or 23, Si/Na = 2 or 5, BMI.Cl/Si = 0.39 or 0.15, and Si/Al = 20 or 50. The mixture was stirred for 15 min, then three equal volumes of the gel were poured into 60-ml capacity Teﬂon-lined stainless steel autoclaves and maintained in an oven at 150 or 180 8C for 16 h up to 14 days without stirring or with mechanical stirring. After the reaction time, the solids were ﬁltered out and washed with 50 ml of water and 50 ml of acetone, and the samples dried for 5 h at 105 8C giving 4 g of powder material.

The zeolitic phase of the samples was characterized by X-ray diffraction (XRD) on a Siemens D500 diffractometer with Cu K

a

radiation (

l

= 1.54056 A˚). For the X-ray data collection, after calcination (600 8C), a sample was placed in a glass measurement cell that was quickly closed to avoid contact with moisture. It was then degassed at 300 8C under a vacuum prior to the measure-ments. The morphology and particle size of the products were investigated using a SSZ 550 (Shimadzu) scanning electron microscope (SEM). The speciﬁc surface areas were determined from nitrogen adsorption on Micromeritics Gemini VacPrep 061 using the BET method. Thermogravimetric analysis (TGA) was performed under air on a TA instrument with a heating rate of 5 8C min 1_{up to 700 8C.}

The oligomerization reactions were performed under argon using standard Schlenk tube techniques. Ethylene oligomerization reactions were performed in a 200-ml double-walled glass reactor equipped with magnetic stirring and a thermocouple, with continuous feed of ethylene at 5 bar. The reaction temperature was controlled by a thermostatic circulation bath. A typical reaction run was performed by introducing 100 mg of zeolite (nearly 10

m

mol of the nickel (II) complex) in 30 ml of isooctane. The system was saturated with ethylene, and then 0.3–1.5 ml of methylalumoxane (MAO) was added (Al/Ni molar ratio of 25–200). The ethylene pressure was maintained at the desired value and continuously fed. After 0.5 h, the stirring was stopped and the phases were allowed to separate.

3. Results and discussion

The mixture of SiO2, Na2Al2O4, NaOH, and BMI.Cl in water forms a gel which, depending on the Si/Al molar ratio and use of

appropriate temperature and crystallization time, gives different zeolitic structures.

3.1. Synthesis using Si/Al ratio 50

The synthesis of the zeolite using a Si/Al molar ratio of 50 kept at 180 8C for 16 h to 14 days gives materials with diffractograms shown in Fig. 1. These diffractograms show very characteristic lines of a ZSM-5 framework at 7.978, 8.828, 23.108, 23.358, 23.868, and 24.358[29].

Fig. 1shows that after 3 days at 180 8C the product is almost entirely crystallized. More importantly, it indicates that indepen-dent of the reaction time, the only framework formed under such reaction conditions is of MFI-type. This is an unprecedented demonstration of the effectiveness of BMI.Cl as a template for ZSM-5 crystallization. It is worth noting that the material is almost completely crystalline even after a relatively short crystallization time.

The TGA analysis of sample (d), obtained with a reaction time of 3 days is shown inFig. 2.

Desorption of water appears at temperatures above 95 8C, with subsequent dehydroxylation and decomposition of the ionic liquid present in the cage occurring at 420–490 8C. It is worth noting that

Fig. 1. X-ray diffractograms of the zeolite ZSM-5 obtained in the presence of BMI.Cl with reaction times of (a) 16 h, (b) 1 day, (c) 2 days, (d) 3 days, (e) 7 days, and (f) 14 days at 180 8C at a Si/Al ratio of 50.

Fig. 2. Thermogravimetric analysis of sample (d). (Si/Al = 50, 180 8C, reaction time of 3 days).

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pure BMI.Cl decomposes at 280 8C. The enormous increase in the stability of the imidazolium structure used as a template should be ascribed to its protection inside the zeolitic cage.

The XRD data of the treated material shows that the MFI-type framework maintains its structure even after 5 h in air at 600 8C. The speciﬁc BET surface area and microporous volume values of these ZSM-5 samples obtained by N2adsorption were 384 m2g 1 and 0.10 cm3g 1, in the usual range for ZSM-5 zeolites.Fig. 3

shows the isotherm of sample (d), showing a type I isotherm (as deﬁned by IUPAC) characteristic of a ZSM-5 zeolite.

Purely microporous zeolites ZSM-5 show N2 uptake at low relative pressure (P/P0<0.01) followed by a plateau at 112 cm3_g 1_{as a result of uniform micropores and the absence} of larger pores[30].

The SEM micrographs of these samples are very surprising.

Fig. 4shows morphology of sample (d), but samples (a)–(f) showed the same pattern. As seen inFig. 4a, these materials are composed of highly uniform and regular microspheres with diameters of ca. 35

m

m.

A closer look at individual microspheres inFig. 6b–d shows that these spheres are formed by agglomerates of hexagonal prisms with dimensions of approximately 3

m

m. This kind of structure is observed independent of the crystallization time.

It is remarkable that the synthetic procedure used in order to obtain these ZSM-5 zeolites has engendered a double template effect: a ﬁrst, primary, template induction of formation of the MFI framework with induction of formation of pentasil structures, and a second template effect responsible for formation of regular spheres. This secondary template effect can be ascribed to formation of micelles by the action of the ionic liquids [31]. These micelles are expected to have very regular dimensions and facilitate the growth of hexagonal prisms inside them.

3.2. Synthesis using Si/Al ratio 20

If the synthesis of zeolites is performed as described in Section

3.1but using a Si/Al ratio of 20, i.e. a lower amount of SiO2than in the initial synthesis, the zeolitic materials show the diffractograms presented inFig. 5. The diffractograms of samples (b) and (c) shows

Fig. 3. N2adsorption isotherm of sample (d).

Fig. 4. Representative SEM micrographs of ZSM-5 zeolite synthesized at 180 8C at a Si/Al ratio of 100 with a reaction time of 3 days [sample (d)] a) 500X, b)3000X, c) 5000X and d) 10000X magniﬁcations.

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characteristic lines at 7.78, 8.88 and 23.08. These peaks are characteristic of

b

-zeolites[13].

As can be seen inFig. 5, the total crystallization of the zeolitic phase is attained after 7 days reaction time.

The textural properties of the samples derived from the N2 adsorption isotherms, showing a type IV (as deﬁned by IUPAC) with surprising hysteresis starting in pore volume adsorbed above P/P0 0.68, which is a characteristic of ordered mesoporous materials, are included inFig. 6. The appearance of mesoporous material can be explained by the presence of ionic liquid during zeolite synthesis. For sample (c), the values are in good agreement with the typical ones corresponding to a

b

-zeolite, with a BET surface area of 418 m2_g 1_{, a micropore volume of 0.11 cm}3_g 1_, and mesoporous volume of 0.84 cm3_g 1_[32,33]_.

4. Catalyst performance in ethylene oligomerization

The

b

-zeolites described herein have a larger pore diameter than ZSM-5 (7.5 A˚ versus 6 A˚, respectively) and present meso-porosity suitable for improving catalyst effectiveness in chemical reactions. The mesopores provide improved access to the micropores and shorten the diffusion path length, thereby enhancing the rate of diffusion and thus the catalytic performance

[34]. Consequently, the

b

-zeolite was the best choice to be evaluated as support for a nickel-

b

-diimine complex (1), which is a convenient subject for the evaluation of the occurrence of shape selectivity due to the zeolitic framework. The resulting associated material has been tested as a catalyst for ethylene oligomerization and compared with homogeneous conditions.Table 1shows the relevant data for the performance of 1 (Fig. 7) in the homogeneous phase and as heterogeneous catalysts.

Under mild conditions, homogeneous and heterogenized catalysts displayed high catalytic activities. The homogeneous system (entry 1) showed lower activity than its heterogeneous analog (entry 3) for the ethylene oligomerization. Additionally, the nickel complexes incorporated into the

b

-zeolite framework showed, under the same reaction conditions, high C4 fraction contents (up to 93.8%) and high 1-butene selectivities (85.7% of the C4 fraction). These data suggest that the beta zeolite structure works as a shape-selective support that inhibits re-coordination of 1-butene, thereby preventing isomerization and growth of the oligomer chain.

This is a case in which the zeolitic support strongly contributes to the performance of the nickel complex, giving rise to enhanced activity and selectivity.

5. Conclusion

ZSM-5 and

b

-zeolites were successfully synthesized using BMI.Cl as a template. Depending on the Si/Al ratio the system crystallizes differently, giving selectively ZSM-5 or

b

-zeolite structures.

The synthesis of the ZSM-5 zeolite using BMI.Cl as a template has shown very unusual characteristics: easy crystallization of the pentasil structure and, more remarkably, observation of a second

Fig. 7. Nickelb-diimine complex (1).

Fig. 5. Diffractograms of the beta zeolite obtained with reaction times of (a) 3, (b) 7 and (c) 14 days at 150 8C at a Si/Al ratio of 20, with mechanical stirring.

Fig. 6. N2adsorption isotherm of sample (c).

Table 1

Ethylene oligomerization with complex under homogeneous (1) and heterogenized conditions (2).

Entry Complex [Al]/[Ni] TOFa

/h1 SC4% (a%b) SC6% (a%) S>C8% 1 1 50 739 44.3 (55.2) 33.7 (29.6) 17.9 (n.d.) 2 2 25 1430 93.0 (86.5) 7.0 (n.d.) – 3 2 50 1297 93.8 (87.1) 6.2 (n.d.) – 4 2 100 1553 91.3 (85.7) 8.7 (n.d.) – 5 2 200 1576 93.3 (85.7) 6.7 (n.d.) –

Reaction conditions: Mechanically stirred 200 mL double-walled glass reactor, constant ethylene pressure at 5 bar, 10mmol of nickel complex, 30 mL of isooctane, 23 8C, 0.5 h.

a

TOF = turnover frequency: mol of ethylene converted per mol of Ni-complex per hour.

b _a

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template effect consisting of crystallization of primary structures of regular spheres of ca. 35

m

m diameter.

The new method is a simple and highly efﬁcient protocol for the synthesis and crystallization of ZSM-5 zeolites and beta zeolites and opens up the possibility for preparation of new zeolitic phases and materials.

The synthesized

b

-zeolite has been used as support for nickel–

b

-diimine catalysts that are highly active in oligomerization of ethylene. The heterogenized system combines the activity charac-teristic of homogeneous nickel catalysts with a shape selectivity resulting from its placement inside the cage of the zeolitic support. This procedure enables the preparation of new highly active and selective nickel catalysts for ethylene oligomerization.

Acknowledgment

The authors would like to acknowledge CNPq/PRONEX/ FAPERGS for their ﬁnancial support.

References

[1] W. Song, R.E. Justice, C.A. Jones, V.H. Grassian, S.C. Larsen, Langmuir 20 (2004) 8301–8306.

[2] A. Soualah, M. Berkani, M. Chater, C.R. Chim. 7 (2004) 713–720. [3] J.M. Newsam, Science 231 (1986) 1093–1099.

[4] N. Kumar, V. Nieminen, K. Demirkan, T. Salmi, D.Yu. Murzin, E. Laine, Appl. Catal. A: Gen. 235 (2002) 113–123.

[5] A.L. Bugaev, J.A. Bokhoven, P.A. Sokolenko, Y.V. Latokha, L.A. Avakayan, J. Phys. Chem. B 109 (2005) 10771–10778.

[6] J.C. Torres, D. Cardoso, Micropor. Mesopor. Mater. 113 (2008) 204–211. [7] A. Chatterjee, T. Iwavaki, J. Phys. Chem. A 105 (2001) 6187–6196.

[8] G. Gianetto, A. Montes, G. Rodrı´guez, Zeolitas, Caracterı´sticas, Propiedades y Aplicaciones Industriales, Editorial Innovacio´n Tecnolo´gica, Caracas, 2000.

[9] T. Zhong, K. Seok-Jhin, G. Xuehong, D. Junhang, Micropor. Mesopor. Mater. 118 (2009) 224–231.

[10] R.L. Wadlinger, G.T. Kerry, E.J. Rosinski, US Patent 3,308,069 (1967). [11] M.K. Rubin, US Patent 4,554,145 (1985).

[12] M.K. Rubin, US Patent 5,164,170 (1992).

[13] A. Corma, M. Moliner, A. Cantıń, M.J. Dıáz-Cabanãs, J.L. Jorda´, D. Zhang, J. Sun, K. Jansson, S. Hovmo¨ller, X. Zou, Chem. Mater. 20 (2008) 3218–3223.

[14] A. Corma, V. Gomez, A. Martınez, Appl. Catal. A: Gen. 119 (1994) 83–96. [15] J. Perez-Pariente, E. Sastre, V. Forne´s, J.A. Martens, P.A. Jacobs, A. Corma, Appl.

Catal. 69 (1991) 125–137.

[16] E. Blomsma, J.A. Martens, P.A. Jacobs, J. Catal. 159 (1996) 323–331.

[17] H. Van Bekkum, A.J. Hoefnagel, M.A. Van Koten, E.A. Gunnewegh, A.H.G. Vogt, W. Kouwenhoven, Stud. Surf. Sci. Catal. 83 (1994) 379–390.

[18] G. Harvey, B. Binder, R. Pris, Stud. Surf. Sci. Catal. 94 (1995) 397–404. [19] L.X. Yang, Y.J. Zhu, W.W. Wang, H. Tong, M.L. Ruan, J. Phys. Chem. B 110 (2006)

6609–6614.

[20] R.E. Morris, Chem. Commun. 21 (2009) 2990–2998.

[21] C.J. Adams, A.E. Bradley, K.R. Seddon, Aust. J. Chem. 54 (2001) 679–681. [22] E. Benazzi, J.L. Guth, L. Rouleau, US Patent 6,136,290 (2000).

[23] N. Zilkova, A. Zukal, J. Cejka, Micropor. Mesopor. Mater. 95 (2006) 176–179. [24] K. Zhu, F. Pozgan, L. D’Souza, R.M. Richards, Micropor. Mesopor. Mater. 91 (2006)

40–46.

[25] E.R. Cooper, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Nature 430 (2004) 1012–1016.

[26] E.R. Parnham, R.E. Morris, J. Mater. Chem. 16 (2006) 3682–3684. [27] X. Sun, J. King, J.L. Anthony, Chem. Eng. J. 147 (2009) 2–5.

[28] J. Dupont, C.S. Consorti, P.A.Z. Suarez, R.F. de Souza, Org. Synth. 79 (2002) 236– 243.

[29] M.M.J. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, 4th ed., Elsevier, Amsterdam, 2001.

[30] J. Perez-Ramırez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc. Rev. 37 (2008) 2530–2542.

[31] O.A.E. Seoud, P.A.R. Pires, T.A. Moghny, E.L. Bastos, J. Colloid Interface Sci. 313 (2007) 296–304.

[32] J. Aguado, D.P. Serrano, J.M. Rodriguez, Micropor. Mesopor. Mater. 115 (2008) 504–513.

[33] L. Ding, Y. Zheng, Mater. Res. Bull. 42 (2007) 584–590.

[34] J.C. Groen, S. Abello, L.A. Villaescusa, J. Perez-Ramırez, Micropor. Mesopor. Mater. 114 (2008) 93–102.