CO hydrogenation to higher alcohols over CuZnAl catalysts without promoters: Effect of pH value in catalyst preparation

doi:10.1016/j.fuproc.2017.08.012

Fuel Processing Technology

Volume 167, 1 December 2017, Pages 575-581

https://doi.org/10.1016/j.fuproc.2017.08.012 Get rights and content

Highlights

•
pH value had significant influence on the catalytic activity.
•
Catalyst prepared at pH = 8 showed the best catalytic performance.
•
Larger Cu⁰ particle size and a suitable amount of acid favored C_2 + OH formation.

Abstract

CO hydrogenation to higher alcohols (C_2 + OH) was examined in a slurry bed reactor. CuZnAl samples without promoters were prepared by a complete liquid-phase method at different pH value (pH = 7–10), with the aim of studying the influence of pH value on the catalytic performance in this catalytic reaction. The pH value had significantly influence on the Cu⁰ crystallite size, the amount of reducible Cu⁺, surface Cu/Zn atomic ratios and the acidity, which led to the diverse activities toward C_2 + OH synthesis. Characterization results showed that relatively larger crystallite size of Cu⁰ and a suitable amount of acid sites was beneficial to the formation of C_2 + OH. Activity results indicated that the sample prepared at pH = 8 exhibited the best catalytic performance compared with other samples. In addition, the catalytic performance of pH = 8 sample showed that increasing temperature favored to the formation of C_2 + OH.

Graphical abstract

Introduction

The direct catalytic conversion of synthesis gas (syngas, CO/H₂) to higher alcohols (C_2 + 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 C_2 + 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 C_2 + 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 C_2 + OH synthesis from syngas. However, to our best knowledge, CuFe or CuCo catalytic systems are not available due to the low C_2 + 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 C single bond C 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/C_2 + 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 C_2 + 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 C_2 + 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 C_2 + 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 C_2 + 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 Cu^2 + was observed, which was due to the partial decomposition of liquid paraffin, then resultantly produced free carbon and finally reduced Cu^2 + 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 C_2 + 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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Citation 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.
The catalytic conversion of CO hydrogenation is a structure-sensitive reaction over Cu-based catalyst. It is very important to distinguish the structural differentiation and possible reaction pathway of Cu-based catalysts in methanol synthesis and ethanol/C₂₊OH synthesis. Herein, the activity of four types of CuZnAl catalysts is evaluated, and their structural differentiation is investigated. The CuZnAl catalyst (CZA) prepared by a complete liquid-phase (CLP) method after solid–liquid separation directly applied in the fixed bed reactor also shows the ability to synthesize ethanol and higher alcohols (C₂₊OH), with the selectivity of C₂₊OH in liquid product reaching up to 31.6% at the CO conversion of 10.6%. However, commercial methanol synthesis catalyst (CMC) displays the highest CO conversion (28.2%) and the poorest C₂₊OH selectivity (7.8%). After characterized by ICP, XRD, CO-TPD, N₂O adsorption, XPS, SEM, TEM and in-situ DRIFTS techniques, it is discovered that the dispersion of Cu and the exposed Cu surface area significantly influences CO conversion. The selectivity of ethanol and C₂₊OH is related to the size and distribution of Cu, the architectural feature, and the chemical environment of Cu and Al. In-situ DRIFTS indicates that CH_xO* species can be formed over different CuZnAl catalysts. However, the formed CH_xO* species will be rapidly hydrogenated to produce methanol on CMC catalyst, while the step is inhibited on CZA catalyst. Additionally, a bridge-type adsorbed CO is only observed on CZA catalyst, which is beneficial for the formation of CH_x intermediates, leading to the formation of ethanol and higher alcohols.

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Research articleCO hydrogenation to higher alcohols over CuZnAl catalysts without promoters: Effect of pH value in catalyst preparation

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

X-ray diffraction

Conclusions

Acknowledgements

Catal. Today

Appl. Catal. A Gen.

Catal. Commun.

J. Catal.

Appl. Surf. Sci.

Catal. Commun.

Catal. Commun.

J. Mol. Catal. A Chem.

J. Ind. Eng. Chem.

J. Catal.

Appl. Catal. B Environ.

J. Catal.

Appl. Catal. B Environ.

J. Catal.

Catal. Today

Appl. Surf. Sci.

Mater. Sci. Eng. B

Research article
CO hydrogenation to higher alcohols over CuZnAl catalysts without promoters: Effect of pH value in catalyst preparation