Sol–gel iron complex catalysts supported on TiO2 for ethylene polymerization

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Abstract

The Fe((SO4)2(NH4)2)/TiO2 compound was prepared via the sol–gel technique at pH 9 dried at 70 °C and treated thermally at different calcination temperatures (200, 400, 600 and 800 °C). It was characterized by BET, XPS and XRD and it was tested as a catalyst for ethylene polymerization. A strong dependence was found between the structural properties of the support, the iron complex and the annealing temperature. Catalytic activity decreased with increasing heat treatment temperature up to 400 °C and remained practically constant above this temperature. These differences occurred without important changes in the physical, chemical and structural properties of the synthesized polymer.

Desorption pore distribution curves for the fresh catalyst (70 °C) and the catalysts calcined at 200 and 600 °C.

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Introduction

The synthesis of metallic oxides by the use of the sol–gel method provides a way of obtaining in only one step solids with high surface area, controlled porosity and high resistance to deactivation. The sol–gel technique has allowed control of nanosolid parameters such as: acidity, texture, structure, particle size, porosity, oxidation state of the central atom, coordination number of the transition metal and impurities. These properties are planned before the gelation takes place and they make it possible to establish the synthesis parameters and to prepare tailor-made nanomaterials. Titania is a metallic oxide with many applications: due to its electronic properties, it is used in the production of electrodes, capacitors and solar cells; it is also of interest in catalysis, where it can act in oxidation reactions as the support or the catalyst itself. For example this oxide photocatalyzes the oxidation of organic and inorganic matter suspended in water [1], [2].

Although noble metal catalysts supported on titania have been widely studied, their catalytic properties are still of great research interest. Presently, transition metal oxides can be considered as an attractive option because of their low cost and availability; properties like their good activity and stability in the case of iron and copper in the catalytic oxidation of hydrocarbons make them excellent candidates to rival the noble metals. These catalytic properties will depend on many variables such as thermal treatment, the synthesis process, the different crystalline phases of titania, and the doping with cations or anions. Titania is a catalytic support whose properties are modified by the supported cation, e.g. Pt, Pd, Fe or Cu [3].

Titania can be modified, however, by doping with a cation having a valence different from 4+ or by reducing the valence of titanium from 4+ to 3+. The chemical and electronic properties of this non-stoichrometric titania will depend on the defect density and the impurity concentration of the crystal [3].

Another method for preparing non-stoichrometric titania is the sol–gel technique [4]. Here, the resulting titania has a high defect density produced during the dehydroxylation process. Associated with the sol–gel technique is the presence of a large number of hydroxyl groups on the crystal surface that produces oxygen vacancies. Some authors have shown that the sol–gel Pt/TiO2 system is a potential catalyst for acetylene hydrogenation since it has a high resistance to sintering at 800 °C and its activity does not decrease because the particles of Pt redisperse themselves in the support. Also, iron oxide supported on silica has been tested as a catalyst in methane combustion and as a photocatalyst [4], [5], [6].

The sol–gel method has potential applications in catalytic systems since it allows the functionalization and/or surface modification of the supports. Morphologically spherical alumina and silica powder obtained under sol–gel conditions with a basic catalyst are used in organic synthesis.

Recently, a new silica-type material obtained by the sol–gel process has been applied as a carrier for organometallic catalysts in olefin polymerization [7], [8], [9], [10]. Polyolefins such as polyethylene (PE) and polypropylene (PP) produced by homogeneous metallocene catalysts have an irregular morphology and a low bulk density. Also, these polyolefins tend to adhere to the reactor wall surface, thus retarding the heat transfer rate and causing reactor fouling. To overcome this problem, the metallocene catalyst has to be adsorbed on a porous support material to produce an heterogeneous catalyst, and the PE particles should have a granular morphology. A supported metallocene catalyst involves the adsorption of the metallocene on a modified methylaluminoxane (MAO) porous support such as silica, alumina or MgCl2. The method can provide an active catalyst producing PE with satisfactory morphology and bulk density. The metallocene and MAO must be allowed to form an active metallocene species suitable for anchoring onto a support [11], [12].

On the other hand, the type of support is important and influences the catalytic activity. The support pore volume has a strong effect on the yield of the reaction and it can be controlled with the synthesis parameters, improving the possibilities for the polymer chain growing in the active sites located inside the mesoporous [13].

Looking for an alternatives new materials for ethylene polymerization and taken advantage of sol–gel technique about homogeneity, dispersion, and the non traditional oxidation state stability, that can be reach with this sol–gel process. This paper presents a preliminary study of the ethylene homopolymerization using a catalyst of iron complex supported on titania, (Fe(SO4)2(NH4)2)/TiO2, which was prepared by the sol–gel technique at pH 9 and annealed at different temperatures. Titania was choose as a support because, until high temperatures, this material keep a lot of –OH groups that have an important role in the metal complex anchoring and stabilization.

Section snippets

Materials

Ammonium hydroxide (Baker, 98%), absolute ethanol (Baker), iron(II) ammonium sulfate (Baker); titanium(IV) butoxide (Strem Chemicals, 98%) and deionized water were used to prepare the sol–gel catalysts. Commercial toluene was used as a solvent for the polymerization reactions after purification by refluxing over metallic sodium with benzophenone as an indicator. Ethylene (polymerization grade) was treated with BASF R3-11 catalyst and 4 Å molecular sieves in order to remove oxygen and water.

BET

Specific area and average pore diameter were obtained from the nitrogen absorption isotherms. Fig. 1 shows the desorption pore distribution curves for the catalyst treated at 70, 200 and 600 °C. The first two temperatures treatment did not show significant variations in average pore diameter, although surface area was a slightly lower for the higher temperature. In the catalyst treated at higher temperature the pore diameter increased and the specific area decreased. These features maybe

Conclusions

This material thermal treatment decrease its catalytic activity, due to the complex decomposition, the titania phase transformation, the changes in the iron oxidation state as we described; besides, the two iron oxidation state coexistence and the new phases—Ti–O–Fe-formation found in other studies, lead to punctual defects and more than one catalytic sites. Although, the obtained polyethylene have same physical properties analyzed by FTIR, X-ray diffraction, melting temperature and similar

Acknowledgements

The authors wish to thank FONDAP project number #11980002; the Graduated School Department of the Universidad de Chile; the German Scholarship Program DAAD, Professor Victor Fuenzalida and as well as CONACyT-México for their financial support.

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