Ethanol dehydrogenation over Cu catalysts promoted with Ni: Stability control
Graphical abstract
Introduction
Cu is a well-known catalyst for several industrially relevant reactions such as alcohol dehydrogenation, low temperature water-gas shift reaction and methanol synthesis [[1], [2], [3], [4], [5], [6]]. All these reactions can be performed under relatively mild conditions when applying Cu as a catalyst. Major advantages, promoting the use of Cu as a catalyst, are its relatively low price and low toxicity [7,8]. Unfortunately, Cu has one significant disadvantage: it is particularly prone to deactivation by sintering [[9], [10], [11]]. Twigg et al. have reviewed the deactivation causes of Cu catalysts in industrial processes and concluded that sintering and poisoning are the two main reasons for deactivation. While poisoning should and can be prevented by tuning the feedstock composition [11], attempts to improve the resistance of Cu catalysts against sintering have typically relied on manipulating the support properties or including promotor elements [10,[12], [13], [14], [15], [16], [17]].
Different support materials have been investigated to improve the stability of Cu-based catalysts [12,13,16]. Marchi et al. tested Cu/SiO2 for isopropyl alcohol dehydrogenation and found that within 2 h on stream, the activity dropped to 20 % of the original level [12]. Guarido et al. used Cu/Nb2O5 for ethanol reforming and reported a strong interaction between Cu and Nb2O5 compared to Cu on Al2O3, as found via TPR [16]. The catalyst was quite active and stable with only 5 % activity loss after 30 h. Freitas et al. tested a Cu/ZrO2 catalyst for ethanol dehydrogenation, which lost only 4 % of activity in 20 h, which is quite remarkable [13].
Alternatively, promoting agents have been added to Cu catalysts to reduce the Cu particle size and increase catalyst stability, e.g. Cr, Fe, B, La [14,15,[18], [19], [20], [21]]. Tu et al. investigated the effect of Cr on the stability of Cu catalysts [21]. They found that Cr improved the stability and an optimum was found for a Cr/Cu molar ratio of 1/10. The catalyst still suffered from deactivation due to sintering however, i.e., 5 % activity loss occurred after 8 h on stream. Zhu et al. used a maximum of 5 wt% B as a promotor for the Cu dispersion and stability and were able to prolong the stable activity regime to more than 56 h compared to 28 h for the non-promoted Cu/SiO2 [14]. However, ultimately, also this catalyst was subject to deactivation due to sintering. Chen et al. added 0.3 wt% Fe as a promotor for the reverse water-gas shift reaction [17]. For the low-temperature water-gas shift reaction, La promotion enhanced the stability of Cu catalysts, as found by Kam et al. [15]. After 25 h, the conversion had only dropped from 45 % to 38 %.
As an alternative to changing the support or adding promoting metals, alloying of the active metal with another metal also presents a route to improve the catalytic properties of a material, i.e., activity, selectivity or stability [[22], [23], [24]]. The major advantage of bimetallic materials is that their properties are not simply the sum of those of the constituent elements, but they often excel the ones of the individual elements [25,26]. In the case of Cu, sintering might be prevented via alloying of the catalyst with other metals [27], such as Au, Pd, Pt or Ni [[28], [29], [30], [31], [32]]. For CO oxidation, AuCu has been reported to exhibit quite stable performance [28]. Alloying Cu with Pd improves the stability for trichloroethylene hydrodechlorination to ethylene, but not all deactivation can be prevented. The latter was not due to sintering, however, but to poisoning of the Cu sites with Cl. Liu et al. recently used AgCu alloys [33]. The interaction between Ag and Cu induced a highly stable and selective catalyst for the hydrogenation of dimethyl oxalate.
For catalytic purposes, Cu is frequently alloyed with Ni metal [[34], [35], [36]]. Ni is a commonly used element in catalysis due to its high activity especially for (de)hydrogenation reactions [37]. However, when using Ni as a monometallic catalyst, it is subject to fast deactivation due to coking [35]. Alloying Cu with Ni might, however, resolve the major deactivation phenomena occurring on the individual, monometallic catalysts through mutual benefit [38,39]. Cu is prevented from sintering by surrounding it with Ni atoms because the higher melting temperature of the latter makes the alloyed particles less mobile [40]. On the other hand, coking of Ni is prevented due to the presence of Cu which avoids the production of carbon at those conditions [32,35,39]. Ni and Cu can be mixed in every concentration with subsequent alloy formation due to their similar atomic radius and their FCC lattice structure [36]. In contrast with Cu, Ni has, apart from a high activity for (de)hydrogenation reaction, also the ability for C-C rupture [41]. Depending on the investigated reaction, alloys can be synthesized with more or less Ni to tune for this C-C rupture.
Many authors mention the importance of the support on the desired catalyst activity [38,[42], [43], [44], [45]]. For (de)hydrogenation reactions, especially for alcohol dehydrogenation, the presence of acid sites on the catalyst should be avoided since this might induce dehydration reactions [38]. Furthermore, a high dispersion of the active metal sites is crucial for achieving a high turn-over frequency [41]. To match the requirements mentioned above, hydrotalcite-based mixed oxides are interesting candidates. Hydrotalcite-supported catalysts currently gain interest due to their promising properties, such as active metal dispersion and stability, both in metal as in base catalysis [22,23,[46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]]. Their composition is denoted as and they form a layered structure, where the metals are linked by OH-groups. Mg and Al are commonly used as divalent and trivalent metals, but they can also be replaced, partially or even completely, by other elements such as transition metals. Such incorporation in the support provides a synthesis route for well-dispersed supported metal catalysts. To compensate for the charge induced by the replacement of a bivalent with a trivalent metal-ion, is present between the layers, as well as excess . Upon calcination, water is released from the lattice, resulting in a collapse of the structure and formation of mixed oxides with a high specific surface area and a small crystallite size. After reduction, well-dispersed metallic particles are obtained from the alternative metals incorporated into the mixed oxide Mg(Al)Ox structure. The main advantages of calcined hydrotalcite-supported catalysts are the uniform distribution of elements by incorporation into the hydrotalcite structure, their moderate basicity which inhibits coking, and their thermal stability towards steam and reaction-oxidation cycling. Furthermore, the presence of Al-cations enhances the resistance to sintering [22,23,47,50,52,57,58].
In this work, Ni-promoted Cu catalysts supported on metal-modified calcined hydrotalcite are investigated to improve the stability of Cu catalysts. Here, a one-pot synthesis route for the production of bimetallic Cu–Ni catalysts based on the single-step formation of Cu,Ni, Mg, Al -containing layered double hydroxides. The effect of the atomic Ni/Cu-ratio and total metal loading on the catalytic activity, selectivity and stability in ethanol dehydrogenation are assessed. Taking into account the activity of Ni for CC bond scission, low Ni/Cu ratios will be tested. A non-promoted Cu/Mg(Al)(Cu)Ox has also been tested as a benchmark.
Section snippets
Catalyst synthesis
Ni-Cu-Mg-Al samples were synthesized in a five-neck 5-liter glass reactor equipped with a steam jacket, stirrer, pH electrode, thermocouple and reflux condenser. The carbonate forms of the hydrotalcite-based support were obtained by co-precipitation at 333 K and a constant pH = 9.5―10, using ‘pro analyze’ purity grade nitrate salts of the corresponding metals and Na2CO3 as a precipitating agent. A certain volume of distilled water was placed in the reactor, heated to 333 K, and adjusted with
Catalyst characterization
Table 1 represents the metal loadings of the catalysts as determined via ICP-OES, indicating that the desired metal loading was reached during catalyst synthesis, with only small deviations. After use in the ethanol dehydrogenation reaction, the nominal metal loadings on the catalysts are still the same, illustrating the evolution of the metals upon incorporation in the support. The surface area of the materials was determined via N2 adsorption after calcination and reduction as well as for the
Conclusions
A series of Cu-based catalysts promoted by alloying with different amounts of Ni were tested for ethanol dehydrogenation. Upon alloying of Cu with Ni, a higher turn-over frequency is obtained as compared to pure Cu-based catalysts. It is attributed to the lower reduction temperature of NiO via H2 spillover from Cu, rendering more Ni atoms available for incorporation in the Cu phase. The acetaldehyde selectivity exceeds 95% for all NiCu catalysts. This suggests that only a limited number of
Author contribution
Jolien De Waele – performed experiments, manuscript preparation
Vladimir V. Galvita and Joris W. Thybaut - results discussion, manuscript preparation
Hilde Poelman, material characterisation, results discussion, manuscript preparation
Margarita Gabrovska, Dimitrinka Nikolova, Sonia Damyanova – catalysts preparation, results discussion
Acknowledgements
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no 615456 and from the ‘Long Term Structural Methusalem Funding by the Flemish Government’.
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