New heterometallic copper zinc alkoxides: synthesis, structure properties and pyrolysis to Cu/ZnO composites

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Abstract

The copper compound [(THF)KCu(OtBu)3]∞ 1 was obtained by interaction of a 1:1 mixture of ZnCl2/CuCl2 with KOtBu. Bi- and trifunctional aminoalcohols were used to synthesize the intramolecularly donor stabilized Cu(II) alkoxides Cu(OCH(R)CH2NMe2)2 (3: R=Me, 4: =CH2NMe2) where 4 was structurally characterized. Lewis acid–base adduct formation with (Me3Si)3CZnCl gave the heterodinuclear compounds (Me3Si)3CZnCl · Cu(OCH(R)CH2NMe2)2 (5: R=Me, 6: R=CH2NMe2), which were characterized by X-ray single-crystal structure analysis. The two metal centers Cu and Zn of 5 and 6 are bridged by two oxygen atoms to form a Cu–O–Zn core. Pyrolysis of compounds 5 and 6 in dry argon or a H2/N2 mixture at atmospheric pressure forms metallic copper and zinc oxide, whereas pyrolysis under O2/Ar forms additionally oxidized copper species. Elemental analysis of the pyrolysis products showed carbon and nitrogen contamination. Scanning electron microscopy and energy dispersive X-ray analysis were performed to get information on the morphology and the chemical composition of the pyrolysis products.

Lewis acid–base adduct formation of intramolecularly donor stabilized Cu(II) alkoxides Cu(OCH(R) CH2NMe2)2 with (Me3Si)3CZnCl gave the heterodinuclear compounds (Me3Si)3CZnCl · Cu(OCH(R)CH2NMe2)2 which were characterized by X-ray single-crystal structure and thermal analysis. Pyrolysis studies, SEM/EDX analysis and X-ray powder diffraction showed that these bimetallic alkoxides are suitable organometallic precursors for the preparation of Cu/ZnO composites.

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Introduction

Mixed-metal oxides represent an important class of materials for advanced applications, including catalysts, structural ceramics, sensors and actuators as well as ferroelectric and dielectric materials for microelectronic data processing devices [1], [2], [3], [4], [4](a), [4](b), [5], [6]. The investigation and the use of heterometallic alkoxides as single molecule precursors (SMPs) for the synthesis of mixed-metal oxides have continuously increasing over the past decade. This is particularly connected with the elucidation of the complex sol–gel chemistry of oxide materials processing in general [7] as well as with the related precursor chemistry for the growth of metal oxide thin film materials by chemical vapor deposition techniques [1], [2], [4], [4](a), [4](b), [8]. However, virtually all of these examples refer to single-phased metal oxide materials. Only recently it was shown, mainly by the work of Veith et al. [9], that SMPs can be designed to produce unusual multiphased composite materials directly in one single step depending on both the metal composition and the particular precursor design. Within the context of heterogeneous catalysis the most noteworthy examples are thin films or powders of nanostructured metals homogeneously dispersed in a metal oxide matrix derived from SMPs. Many catalysts are composed of finely dispersed, i.e. nanoscale metal particles bound to a metal oxide support (metal@metal-oxide) [10]. Tailored SMPs may thus represent an interesting alternative approach for catalyst preparation.

We were attracted by this idea and selected the Cu@ZnO system as a study object. The Cu@ZnO system is of eminent importance in large-scale industrial methanol synthesis (107 tonnes per year) from CO/CO2/H2 [11]. The industrial Cu@ZnO-based catalysts for methanol synthesis are generally prepared by a well developed and empirically optimized coprecipitation technique. An aqueous solution of alkali carbonates is mixed with a solution of copper and zinc salts, such as nitrates, resulting in the formation of precipitates, including malachite Cu2(CO3)(OH)2, aurichalcite (Cu,Zn)5(CO3)2(OH)6 or hydrozincite Zn5(CO3)2(OH)6 as well as other related phases with various ratios of zinc and copper ions [12]. Those precursors are then converted into the active Cu@ZnO composite by aging, calcination and reduction. The oxide phases obtained after calcination consist of copper oxide and zinc oxide phases which do not contain multiphased oxides such as a hypothetical CuxZnyOz, but referring to the work of Locmelis and Binnewies [13] there may be a partial dissolution of copper oxide in zinc oxide (2 mol%) and ZnO in CuO (4 mol%). The effects of precursors and preparations on the catalyst activity have been shown to correlate with the copper metal surface area, i.e. with the dispersion of the Cu particles [11], [12]. It was suggested, that particular single phased mixed-metal precursors, such as auricalcite (Cu,Zn)5(CO)3(OH)6 (with a Cu:Zn ratio of 1:1) containing the Cu and Zn in an atomically pre-mixed state, represent superior precursors [14]. However, these inorganic solid state precursors are neither soluble nor volatile. This limits their application as precursors for the impregnation of highly porous supports based on alumina or silica. We were thus led to investigate the possibility to develop soluble or even volatile Cu/Zn metallo-organic SMPs for the preparation of Cu@ZnO catalyst systems. Herein we wish to give a first account on our results on new heterometallic Cu/Zn alkoxide complexes and their solid state pyrolysis to obtain Cu@ZnO composites.

Section snippets

Synthesis

A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic alkoxide ligands (e.g. OMe, OEt, OiPr, OtBu, OCEt3, OCH2CH2NMe2, OCH2CH2OMe, OAr) are known [15], [16], [17], [18], [19], [20], [21], [22], [23]. The steric demand and the coordinative flexibility of the ligand system has a great influence on aggregation and volatility of the complexes, e.g. [Zn(OtBu)2]∞ [17] is an insoluble polymer, while [Zn(OCEt3)2]2.5 [15] is a soluble aggregate which could be distilled

Conclusion

We describe here the synthesis and structural characterization of copper alkoxides as well as new heterobimetallic copper zinc alkoxides. While salt elimination reactions in the system CuCl2/ZnCl2/KOtBu yields only the product [(THF)KCu(OtBu)3]∞ 1, adduct formation using the residual Lewis basic sites of intramolecularly donor stabilized copper(II) alkoxides Cu(OCH(R)CH2NMe2)2 (3: R=Me [21], 4: R=CH2NMe2) with the Lewis acid (Me3Si)3CZnCl yields the heterobimetallic compounds of the general

Experimental

All manipulations were performed using carefully oven-dried reaction vessels and Schlenk techniques under an inert atmosphere of purified argon or in a glove box. Solvents were dried under argon by standard methods and stored over 4 Å molecular sieves (residual water <3 ppm, Karl-Fischer). Starting materials not available from commercial sources were synthesized according to the literature, i.e. Cu(OCH(CH3)CH2NMe2)2 (3) [21], Cu(OMe)2 [19], (Me3Si)3CZnCl [35]. Elemental analysis were provided

Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 159929 for compound 1, CCDC no. 159932 for compound 2, CCDC no. 159930 for compound 4, CCDC no. 159928 for compound 5 and CCDC no. 159931 for compound 6. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

Finacial support was generously provided by the Deutsche Forschungsgemeinschaft (5FB558).

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