Bifunctional catalysts based on colloidal Cu/Zn nanoparticles for the direct conversion of synthesis gas to dimethyl ether and hydrocarbons
Graphical abstract
Introduction
A promising route to second generation biofuels comprises the conversion of lignocellulosic biomass to synthesis gas via gasification, and subsequent upgrading to valuable products [1,2]. In this context, the syntheses of DME or hydrocarbons via the methanol route are particularly interesting. Apart from its direct use for domestic applications or as a clean diesel fuel, DME is applied as an intermediate, e.g., in the production of further base chemicals and gasoline range hydrocarbons. Traditionally, DME is produced from syngas in a two-step process, where methanol is generated over Cu-based catalysts in the first stage, followed by subsequent dehydration over inorganic solid acids (e.g., γ-Al2O3 or zeolites) in the second stage. Alternatively, DME can be obtained in a single-step conversion from syngas which involves the following reactions:
In addition, DME production from syngas is thermodynamically favored over methanol and enables higher CO conversions. The overall conversion of syngas-to-DME (STD) is described by Eq. (4).
Gasification of biomass-derived feedstocks provides syngas with a H2:CO ratio in the range of 0.7–1.0 which is below the optimum ratio for methanol synthesis (Eq. (1)) [2,3]. In the present study, CO-rich syngas (CO:H2 ratio 1:1) was employed to simulate biomass-derived feedstocks for the STD process (Eq. (4)).
The STD process has many technical and economic advantages, provided that a suitable catalyst exists. Typically, the two catalysts are combined by simple physical mixing [4,5]. Cu/ZnO (Al2O3) is industrially applied as a catalyst in methanol synthesis and has also been used as the methanol active component in the STD process for several years [[6], [7], [8]]. The ideal solid acid catalyst for methanol dehydration should exhibit appropriate acid sites, high stability, hydrophobic surface, low cost, high activity, and good selectivity for the desired products. In particular, alumina and zeolites have been employed for methanol dehydration to DME [[9], [10], [11], [12], [13]]. γ-Al2O3 is known as a very efficient dehydration catalyst with high DME selectivity, low cost, excellent lifetime, and high mechanical resistance. The surface of γ-Al2O3 remains with an excess of positive charge which is compensated by hydroxyl anions (OH−). The hydroxyl anions form weakly acidic sites but desorption at high temperatures creates coordinatively unsaturated metal cations and oxygen anions that can act as Lewis acid and base sites, respectively. Although the cubic, defect spinel-type γ-Al2O3 reveals a remarkable selectivity to DME, zeolites have been reported to exhibit better catalytic activity and stability. In this context, H-ZSM-5 is of particular interest since it combines strong acidity, high density of acid sites and medium size pores which prevent coking without significantly hindering the diffusion of molecules involved in the DME synthesis. In zeolites, the number of acid sites for methanol dehydration can be further adjusted via the Si/Al ratio. In contrast to γ-Al2O3, molecular sieves (e.g., SAPO-34, HY, H-ZSM-5) can be further employed in hybrid catalysts for the production of hydrocarbons from syngas where, due to a so-called “drain-off” mechanism, no thermodynamic constraints are imposed on the overall syngas feed conversion.
The design of catalysts remains a crucial issue for enhancing catalytic efficiency. In the STD process, the large distance between the two catalytically active sites, a rather low activity, and low long-term stability have been described as the major drawbacks of physically mixed catalytic systems [14]. Diffusion of intermediates (e.g. methanol) through the interface between methanol synthesis catalyst and acid functions is an essential part of the reaction scheme and synergistic effects were observed for single bifunctional entities, if the two catalytically active components were finely dispersed and maintained in close contact [15,16]. A conventional bifunctional catalyst can be prepared by mixed metal impregnation, co-precipitation impregnation, or co-precipitation sedimentation of the dehydration catalyst [[15], [16], [17], [18]]. However, the features and activity of the bifunctional catalyst strongly depend on the preparation history. They often exhibit complex structures with broad size and shape distributions of the active particles impeding fundamental studies on the influence of the various structural parameters on the catalytic performance [19,20]. In this context, catalysts derived from well-defined nanoparticle building units may help to reduce this complexity and to contribute to a more fundamental understanding [20,21]. Colloidal nanoparticles have been employed as quasi-homogeneous catalysts for liquid-phase methanol and DME synthesis by dispersing the nanoparticles in the reaction medium [[22], [23], [24], [25], [26]]. Here, long-chain surfactants or ligands are typically adsorbed on the nanoparticle surface to prevent agglomeration, which influences the catalytic properties of the particles. Following the precursor concept, the nanoparticles are immobilized on the support and subsequently converted into heterogeneous catalysts with a nanoscale proximity of the particles and the support [[27], [28], [29]].
In general, catalyst studies can often suffer from limited comparability due to differences in their preparation history. Preparation of catalysts typically requires individually optimized synthetic recipes, and the resulting differences in homogeneity, dispersion etc. may additionally complicate the comparison of the catalyst performance. In our approach, the bifunctional catalysts were prepared based on well-defined, colloidal nanoparticle building units, thus providing a flexible toolkit to minimize the influence of differences in preparation histories. Uniform nanoparticles obtained via a colloidal, organometallic approach were employed as a precursor for the methanol active component and subsequently supported on a solid acid, namely γ-Al2O3 and zeolites of different acidity (i.e., HY and H-ZSM-5 with varying Al/Si ratio). This versatile approach to produce bifunctional model catalysts allowed a comparative study of the respective dehydration catalysts in the conversion of syngas. In particular, the effects of Cu loading, pore structure and Si:Al ratio were investigated. The catalysts were characterized by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), ammonia desorption (NH3-TPD), N2-physisorption, temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), and in situ and operando X-ray absorption spectroscopy (XAS). The catalytic performance was evaluated in a continuously operated laboratory scale plant using simulated biomass-derived synthesis gas between 250 and 300 °C and correlated with structural parameters of the bifunctional catalysts.
Section snippets
Materials
All chemicals were used without further purification. Unless specified otherwise, all nanoparticle synthesis steps were carried out under argon atmosphere and water-free conditions. Copper(II) acetylacetonate (Cu(acac)2, Sigma Aldrich, ≥99.9% trace metal basis), diethylzinc ((C2H5)2Zn, Sigma Aldrich, ≥52 wt.-% Zn), toluene (Sigma Aldrich, 99.8%), ethanol (VWR, absolute) were used for nanoparticle synthesis. Zeolites (ZSM-5: CBV3024E, CBV 5524G, CBV 28,014 (nominal cation form ammonium); HY: CBV
Catalyst preparation and characterization
Well-defined Cu/Zn-based nanoparticles were employed as precursors for the methanol active component and successively supported on a solid acid to yield a series of bifunctional catalysts exhibiting close proximity of the two catalytically active sites.
By this model kit principle, a high comparability of the respective bifunctional catalysts was ensured (Fig. 1). The reaction of (C2H5)2Zn with Cu(acac)2 yielded well defined, uniform Cu/Zn-based nanoparticles with a narrow size distribution. The
Conclusions
Bifunctional model catalysts were developed for the conversion of syngas to DME and hydrocarbons using the precursor concept. Well-defined and uniform Cu/Zn nanoparticles were prepared and successively immobilized on different dehydration catalysts to study the influence of the acidic properties on the catalytic performance. In particular, the Cu loading, the type of acidic sites (γ-Al2O3 vs. zeolites), the type of acid catalyst (HY vs. HZSM-5) and different Si/Al ratios were investigated.
Acknowledgements
M.G. kindly acknowledges financial support of the Helmholtz Research School Energy-Related Catalysis. Further acknowledgments are given to the synchrotron radiation source at Karlsruhe Institute of Technology for providing beamtime at CAT-ACT beamline and Dr. Anna Zimina and Dr. Tim Prüßmann for their support during the measurements. Doreen Neumann-Walter and Dr. Thomas Otto are acknowledged for their support with the BET measurements.
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