ZSM-5-decorated CuO/ZnO/ZrO2 fibers as efficient bifunctional catalysts for the direct synthesis of DME from syngas
Graphical abstract
Introduction
The growing concerns about climate change, environmental pollution and energy consumption have been the driving force in seeking for alternative fuels [1,2]. In this scenario, DME is receiving a great deal of attention as an eco-friendly alternative fuel, since it can be produced from a syngas presenting a low H2 to CO ratio, such as that coming from biomass gasification [[3], [4], [5]]. DME shows a high cetane number (CN = 55–60) when compared to refinery diesel (CN = 40–55), making this compound a suitable alternative to diesel fuel for compression-ignition engines. Apart from its high cetane number, it presents a high oxygen content (34.8 %) and the absence of CC bonds, resulting in a smokeless combustion and low particle emissions [6]. In addition, by adjusting the engine design, CO and NOx emissions have been demonstrated to be also very low [7]. Moreover, the use of DME in compression-ignition engines reduces the noise of the motor to the level of a gasoline spark-ignited engine, keeping the fuel consumption close to diesel on an energy basis [6]. Regarding its properties, DME presents a similar boiling point and density as compared with LPG (mostly C3 and C4). These handling characteristics make its supply and storage feasible by using the already existing infrastructure and equipment as those used for LPG with minor modifications, reducing in this way the investment costs [7].
DME has been traditionally produced by a two-step catalytic process (so called indirect process) using synthesis gas (H2 + CO/CO2) as raw material. The first stage, Eqs. (1) and (2), consists of the methanol synthesis by CO and/or CO2 hydrogenation using a Cu/ZnO-based catalyst [8,9]. In the second one, methanol is selectively dehydrated to DME, Eq. (3), on a solid acid catalyst, being the most studied ones for this process γ-Al2O3 and zeolites, such as HZSM-5 and ferrierite [10,11]. The main drawback of this process is the strong thermodynamic limitation affecting the methanol synthesis stage, which results in a low conversion per-pass and high gas recirculation costs [2].CO + 2H2 ↔ CH3OH (ΔHro = -90.8 kJ·mol−1)CO2 + 3H2 ↔ CH3OH + H2O (ΔHro = -49.5 kJ·mol−1)2CH3OH ↔ CH3OCH3 + H2O (ΔHro = -21.3 kJ·mol−1)H2O + CO ↔ H2 + CO2 (ΔHro = -41.1 kJ·mol−1)
Recently, a one-step process for the direct synthesis of DME has been intensively studied [[12], [13], [14], [15]]. In this process, synthesis gas is fed to a reactor containing a bifunctional catalyst, where both methanol synthesis (on the Cu-based sites of the bifunctional catalyst), Eqs. (1) and (2), in parallel with the water gas shift reaction (WGSR) (Eq. 4), and methanol dehydration (on the acid sites of the catalyst), Eq. (3), take place.
The use of this direct route presents two main advantages when compared to the indirect route. On the one hand, since both methanol synthesis and dehydration occur in only one reactor, there is a reduction in the investment costs related to equipment and infrastructures. On the other hand, as the generated methanol is in-situ consumed, equilibrium constraints related to methanol synthesis are reduced. This thermodynamic advantage makes it possible to work at lower pressure and/or higher temperature achieving a higher synthesis gas conversion per pass and allows the incorporation of CO2 to the reactor feed [16]. In the same way, water generated in methanol dehydration (Eq. (3)) can be removed through the WGSR (Eq. (4)), also favoring the methanol dehydration reaction and yielding more H2, which is of the utmost interest when using a synthesis gas presenting a low H2/CO ratio, such as biomass derived syngas [4].
When preparing bifunctional catalysts, an adequate balance between the two phases (metal and acid sites, in this case) must be achieved, keeping a close proximity among them [17]. The preparation of bifunctional catalysts for this process is usually accomplished by physical mixing of their individual components [15,18]. Although this preparation procedure is simple and easy to scale-up, the catalysts prepared by this procedure present too great a distance between the two active sites involved in the syngas-to-DME process, thus lowering the performance of the catalyst.
As an alternative to these physical mixtures, the use of supported catalysts, in which the solid acid phase acts as support for the metallic phase, has been studied. In this way, the tandem process from synthesis gas to DME is achieved on the surface of one particle of the catalyst [19]. Conventional supported catalysts have been prepared by coprecipitation [20], impregnation [21] and sol-gel [22] methods. Although the preparation of bifunctional catalysts by these ways have demonstrated a better catalytic performance as compared to physical mixtures [23], some authors have reported a decrease in the catalytic performance with time on stream due to the migration of copper to the acidic site centers [24,25]. In addition to these methods, other preparation procedures have been reported for the preparation of supported bifunctional catalysts. Along this line, Gentzen et al. prepared this type of catalysts by supporting pre-synthesized Cu and ZnO nanoparticles on the acid support [26]. Zeng et al. used a physical sputtering technique for the preparation of the supported catalysts [27]. The advantages of using supported catalysts reside in the higher metallic surface area, the higher metal dispersion and a higher number of acid sites, allowing a better catalytic activity and selectivity to DME [27,28]. However, they present some drawbacks, such as their complex preparation methodologies.
All of the aforementioned catalysts are mostly prepared in powder form and, thus, for their application in industrial catalytic reactors, they need to be pelletized to a particle size in the range of 4–6 mm [29] in order to avoid high pressure drops. However, the use of such particle size gives rise to intraparticle diffusion limitations, as observed by Graaf et al. [30] for the methanol synthesis reaction, being the efficiency of the catalysts reduced in this way.
The use of structured catalysts could overcome the problems related to high pressure drops and enhance transport phenomena. In this line, electrospinning is a simple and straightforward technique that has been successfully applied for the preparation of carbonaceous and polymeric fibers in the submicro and nanoscale [31]. In this process, a viscous polymer solution held by its surface tension at the end of a capillary tube is subjected to a high voltage electric field, inducing the ejection of a charged liquid jet. As a result of the electrostatic repulsions between the surface charges, a whipping phenomenon occurs in the space between the tip of the spinneret and the collector, causing the solvent evaporation, stretching the polymeric filament and leading to the deposition of the polymer fibers on the collector [32]. The preparation of different catalysts by using the electrospinning technique has also been reported [33,34]. The use of fibrillar structured catalysts in the submicrometric scale for the direct DME synthesis from syngas could be very efficient in terms of intraparticle mass and heat transport, avoiding, at the same time, the problems of pressure drops of fixed-bed reactors working with such a reduced particle size.
In this work, we show the results on the preparation, characterization and application of one-dimensional structured bifunctional catalyst, presenting two well-defined, dispersed and connected catalytic phases, metallic and acid. The use of the electrospinning technique has been highlighted as a straightforward solution to prepare bifunctional fibrillar catalysts in one step. A detailed parametric study of the direct synthesis of DME from syngas on these bifunctional fibrillar catalysts has been carried out in a continuous fixed-bed reactor. These bifunctional fibrillar catalysts, which have not been described in the literature up to now (to the best of our knowledge), have shown a high efficiency for the direct DME synthesis from syngas and the catalytic activity has been well correlated with the textural, structural and surface chemistry of the bifunctional fibrillary catalysts studied.
Section snippets
Fibrillar catalysts preparation
The experimental procedure carried out for the preparation of the fibrillar materials presented in here involved three stages: the preparation of a polymer-based solution, the electrospinning of the resulting solution and the calcination of the fibrillar materials.
Effect of CuO-ZnO content and precursors of fibrillar catalysts on the synthesis of methanol
The morphology of the materials was examined by scanning electron microscopy (SEM). Fig. 1 presents the SEM micrographs of the samples prepared using the two polymeric solutions (Cu and Zn acetate (A) vs nitrate (N)) and containing different % (w/w) of CuO + ZnO. As it can be observed, the mats obtained consisted of a non-woven fabric of fibers, showing a high aspect (length to diameter) ratio and the absence of fused zones, with diameters ranging from 0.7 to 1.7 μm (Fig. 1a–d). For both
Conclusions
A simple and straightforward methodology for the preparation of CuO/ZnO/ZrO2-ZSM-5 fibrillar bifunctional catalysts by using the electrospinning technique has been described in the present work. The fibrillar bifunctional materials were used as catalysts for the direct DME synthesis from syngas and the catalytic activity was well correlated with the textural, structural and surface chemistry of the bifunctional fibrillary catalysts studied.
A comparison between the fibrillar catalysts here
CRediT authorship contribution statement
José Palomo: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Miguel Ángel Rodríguez-Cano: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. José Rodríguez-Mirasol: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Spanish Ministry of Economy and Competitiveness, Spanish Ministry of Sciences Innovation and Universities and European Regional Development Fund (ERDF) through CTQ2015-68654-R and RTI2018-097555-B-I00 projects. J. Palomo acknowledges the assistance of the Spanish Ministry Education for the award of FPU grant (FPU13/02413). M. A. Rodríguez-Cano also acknowledges the assistance of the Spanish Ministry of Sciences Innovation and Universities for the concession of a
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