Research articleCO hydrogenation to higher alcohols over CuZnAl catalysts without promoters: Effect of pH value in catalyst preparation
Graphical abstract
Introduction
The direct catalytic conversion of synthesis gas (syngas, CO/H2) to higher alcohols (C2 + OH) is one of the most challenging and attractive projects in the field of C1 chemistry because the low yield, poor stability and no commercial process exist till now. The resultant C2 + OH can be applied as a clean alternative fuel, fuel additives and other intermediates for value-added chemicals such as medicine, cosmetic and polyester [1], [2]. Currently, two types of heterogeneous catalysts are used for higher alcohols synthesis (HAS) from syngas [3]: noble metal and non-noble metal catalysts. The noble metal catalysts, mainly Rh-based catalyst [4], [5], [6], exhibit high activity and selectivity of ethanol and other C2 + oxygenates due to its ability to catalyze both CO insertion and dissociation but Rh metal is too expensive for commercial application. Among these non-noble metal catalysts [7], [8], [9], Cu-based catalysts (e.g., alkalis promoted Cu-based, CuFe and CuCo catalysts) are regarded as the promising candidates for C2 + OH synthesis from syngas. However, to our best knowledge, CuFe or CuCo catalytic systems are not available due to the low C2 + OH selectivity and the poor stabilization with long-term run for pilot scale application. In addition, K or Cs is believed to play a key role to neutralize the acidity on catalysts surface and Fischer-Tropsch (F-T, such as Fe, Co, or Ni) elements are indispensable sites for CO dissociation and CC chain propagation in the HAS from syngas in those catalytic systems. The liquid products will mainly compose of methanol if alkalis and F-T elements are absent in Cu-based catalysts [10], [11].
It is well known that the catalytic conversion of syngas to methanol over Cu/ZnO supported catalysts is an extremely efficient industrial process with the selectivity of over 99% [12], [13]. And few studies focus on the ethanol/C2 + OH synthesis over much less expensive Cu-based catalysts without modified by the addition of alkalis or F-T elements. Nonetheless, in our previous researches, it is found that the syngas could be direct catalyzed to C2 + OH on CuZnAl catalysts without promoters by a complete liquid-phase method in slurry bed reactor [14]. Previous research indicated that a suitable amount of alkaline triethanolamine is favorable for HAS [15]. And it was found that the acidity/basicity of the solution has a significant influence on the hydrolysis of aluminum isopropylate (AIP) in catalyst preparation, which will further affect the formation of C2 + OH [16]. Lin et al. [17] studied the mixed alcohols synthesis over co-precipitated Cu-Fe catalysts which were prepared in a constant pH of 7–8. Heracleous et al. [9] prepared K-promoted CuZnAl catalysts and the pH of suspension was maintained at 6–7. Obviously, the pH value in catalyst preparation is a key parameter to design catalyst for HAS. However, since C2 + OH was an unexpected product over this kind of CuZnAl catalyst prepared by complete liquid-phase method, any systematic investigation to see the effect of pH value on the catalytic activity in HAS was not attempted yet. Thus, it is imperative to investigate the effect of pH value in catalyst preparation on the performance of HAS.
Therefore, in the present work, the CuZnAl catalysts without promoters were prepared by a complete liquid-phase method for the syngas to C2 + OH. The effect of pH value in the process of catalyst preparation on performance of CuZnAl catalysts was investigated and characterized comprehensively by X-ray powder diffraction (XRD), N2 adsorption-desorption, H2 temperature programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy(XPS) and Transmission electron microscopy (TEM).
Section snippets
Catalyst preparation
Four CuZnAl catalysts with Cu:Zn:Al molar ratios of 2:1:0.8 were prepared at different pH values by a complete liquid-phase method developed by our group [18]. Firstly, a certain amount of citric acid was dissolved in deionized water, then aluminum isopropoxide [(C3H7O)3Al] was added into the above solution and stirred at 323 K for 3 h. Next, the bath temperature was raised to 368 K and kept for 0.5 h. After that, a solution of Cu(II) and Zn(II) nitrates dissolved in 100 mL glycol was added dropwise
X-ray diffraction
The XRD patterns of different samples were exemplarily shown in Fig. 1. It could be seen that all the samples shared similar structural characteristics, with Cu0 and weak ZnO being the dominant phases, revealing that pH values had little effect on the crystalline phases of samples. No Cu+ or Cu2 + was observed, which was due to the partial decomposition of liquid paraffin, then resultantly produced free carbon and finally reduced Cu2 + to Cu0 [14], [15]. In addition, although the peaks of Al
Conclusions
The pH value in catalyst preparation had a significant influence on the physicochemical properties and catalytic performance of CuZnAl catalysts for HAS. Increasing pH value reduced the crystallite size of Cu0, increased the amount of reducible Cu+, surface Cu content and total number of acid sites. It was concluded that relatively larger crystallite size of Cu0 as well as a suitable amount of acid sites were beneficial to the synthesis of C2 + OH. The optimal catalytic performance was obtained
Acknowledgements
This work was received funding by the Key Program of National Natural Science Foundation of China (No. 21336006), the National Key Technology R&D Program (Grant No. 2013BAC14B04), and the National Natural Science Foundation of China (No. 21176167).
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2022, FuelCitation Excerpt :The diffraction peaks of Cu+/Cu2+ are barely visible, indicating the poor crystallization or low content of Cu2O/CuO. This is due to the fact that Cu salt is firstly reduced to Cuδ+(0 < δ < 2) by ethylene glycol during the preparation of sol, and the Cuδ+ will further be reduced to Cu0 when it is placed into the liquid paraffin [30,31]. After pyrolysis in N2 atmosphere, the intensity of diffraction peaks assigned to Cu0 slightly reduces for the CZA-P catalyst, which may be assigned to the relative higher pyrolysis temperature.