Bifunctional Catalysis for Direct Transformations of Syngas and Carbon Dioxide into Lower Olefins and Aromatics
Xiaoliang Liu, Mengheng Wang, Jincan Kang, Kang Cheng, Qinghong Zhang, Ye Wang*
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China
*Corresponding author: wangye@xmu.edu.cn
Syngas (CO/H2), which can be produced from various carbon resources including natural/shale gas, coal and biomass, has attracted much attention as a key platform for the production of liquid fuels and chemicals.1 At the same time, syngas derived from coal or biomass may contain a considerable concentration of CO2, which has to be converted into useful products along with CO. More broadly, the hydrogenation of CO2 by H2 produced using renewable energy into value-added chemicals would offer a sustainable route for utilization of CO2 as a renewable feedstock, contributing to low-carbon society. Lower (C2-C4) olefins and aromatics are key building-block chemicals. Many studies have been devoted to the conversion of syngas via Fischer-Tropsch (FT) synthesis into olefins (FTO), but the selectivity is limited by the Anderson-Schulz-Flory distribution, which shows a maximum C2-C4 selectivity of ~60%.2,3 The hydrogenation of CO2 to lower olefins is proposed to proceed via CO intermediate by the reverse water-gas shift (RWGS) reaction (CO2 + H2 ¨ CO + H2O) followed by the FTO reaction over many catalysts. The selectivity of C2-C4 olefins is thus also limited to 60%. Aromatics are difficult to be obtained with a considerable selectivity via FT synthesis. The development of new routes for selective synthesis of C2-C4 olefins or aromatics from syngas and CO2 is a highly attractive but challenging task.
Our idea is that the hydrogenation of CO into hydrocarbons involves a series of tandem elementary steps including CO activation and C-C coupling, and thus cannot be precisely controlled by one catalyst as in the conventional FT synthesis. The coupling of tandem steps using well-designed multifunctional catalysts containing components suitable to the respective steps would be a promising methodology. The use of one catalyst to integrate CO to C1 intermediate (e.g., methanol) and methanol to olefins or aromatics (MTO or MTA) in tandem is a challenging task. Early studies using Cu-Zn-Al-O, the typical methanol-synthesis catalyst, in combination with the MTO or MTA catalyst led to the formation of C2-C4 paraffins and almost no olefins.4
Recently, we and a few other groups succeeded in integrating the activation of CO and the selective C-C coupling to lower olefins by designing highly selective bifunctional catalysts (Fig. 1).5,6 We discovered that ZnO-ZrO2 solid solution could catalyze the conversion of CO to CH3OH/CH3OCH3 and was unique in keeping lower olefins from further hydrogenation. The selectivity of C2-C4 olefins reached 70-80% at CO conversion of 10-30% over a bifunctional catalyst composed of ZnO-ZrO2 and zeolite SAPO-34 or SSZ-13.6,7 Furthermore, the ZnO-ZrO2/ZSM-5 catalyzed the direct conversion of syngas into aromatics with a selectivity of 80% at CO conversion of 20%.8 The catalyst was very stable and no deactivation was observed in 1000 h.
Fig. 1. Reaction coupling for direct production of lower olefins and aromatics from syngas or CO2 by the integration of two active components.
We further found that bifunctional catalysts composed of Zn-Al-O or Zn-Ga-O binary oxide and zeolite SAPO-34 were highly selective for the conversion of syngas into C2-C4 olefins. Furthermore, these bifunctional catalysts were also very efficient for the hydrogenation of CO2 into lower olefins. For example, the selectivity of C2-C4 olefins reached 86% at CO2 conversion of 13% at 370 ºC.9 The combination of the Zn-Al-O or Zn-Ga-O oxide with H-ZSM-5 could catalyze the hydrogenation of both CO and CO2 to aromatics with selectivity higher than 70%.
Our studies on the effect of catalyst composition revealed that the catalyst with a Zn/Al or Zn/Ga molar ratio of 1/2 was the best for lower olefin or aromatic formation from either CO or CO2. Characterizations using ESR suggested that the density of oxygen vacancies on the catalyst with Zn/Al or Zn/Ga ratio of 1/2 was the highest. The quantitative correlation suggests the oxygen vacancy site on binary metal oxides plays a crucial role in the activation and conversion of CO or CO2 molecule (Fig. 2). We further investigated the activation of H2 by H2-D2 exchange reaction. Our results indicated that the −Zn−O− domain on the Zn-Al-O oxide surface accounts for H2 activation probably via a heterolytic dissociation mechanism (Fig. 2). We further confirmed that Brønsted acid sites in SAPO-34 or H-ZSM-5 are responsible for C-C coupling to form C2−C4 olefins or aromatics, but the density of Brønsted acid sites should be controlled because Brønsted acid sites also catalyze the hydrogenation of lower olefins to paraffins.
Our kinetic studies reveal that methanol and dimethyl ether are mainly formed from syngas and CO2 on ZnAl2O4, which are transferred into SAPO-34 cages to be selectively converted to C2−C4 olefins or into H-ZSM-5 channels to be converted into aromatics owing to the shape-selective catalysis. In situ infrared spectroscopic studies and DFT calculations suggest that formate and methoxide species are key adsorbed species on ZnAl2O4 during the conversions of both CO and CO2 in the presence of H2 (Fig. 2).
Fig. 2. Proposed reaction mechanism for the activation of CO, CO2 and H2, and the formation of lower olefins and aromatics via methanol intermediate over bifunctional catalysts.
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