544795 Bifunctional Catalysis Involving Surface Coupling for Low Temperature Oxidative Coupling of Methane

Monday, June 3, 2019: 5:15 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Zhinian Li, Lei He, Shenliang Wang, Shihui Zou, Liping Xiao and Jie Fan, Department of Chemistry, Zhejiang University, Hangzhou, China

Bifunctional catalysis involving surface coupling for low temperature oxidative coupling of methane

Zhinian Li, Lei He, Shenliang Wang, Shihui Zou*, Liping Xiao, Jie Fan*

Since large reserves of natural gas have been explored and mature technologies of producing methane have been developed,1,2 converting methane to chemicals of higher value, such as methanol3, light olefins4-6 and aromatics7,8 is of both scientific and strong economic interest. Currently, the industrial-scale conversion of methane adopts an indirect route, which is energy and capital intensive. The oxidative coupling of methane (OCM) as a direct conversion of methane to C2 hydrocarbons due to its low energy consumption and simple process has been gotten widespread attention since it was firstly reported by Keller and Bhasin in 1982.9

Up to date, several catalyst systems such as Li/MgO, rare-earth oxides, Mn/Na2WO4/SiO2 and perovskite composite oxides have been investigated for this reaction. Among these catalysts, La2O3, a model of rare earth oxides has been widely studied due to its low ignition temperature of methane and long-term stability. However, the C2 selectivity is poor due to inefficient coupling at gas phase.10-12 Mn/Na2WO4/SiO2 discovered by Li group in 1992 with highest C2 yield (23.9%) and stability is taken as the most promising catalyst for industrialization.13,14 One drawback is that it requires as high as above 800 ¡ãC to work. Although close to 40 years extensive research and development has been devoted to this reaction15, it has never met the lowest requirement for commercialization. It is very challenging to gain activity and selectivity at the same time.

All these catalysts involved 68 elements have been developed for OCM reaction in the past 40 years, but it haven't made much progress to its mechanism. It is generally accepted that OCM follows a ¡°heterogeneous-homogeneous¡± mechanism, which involves CH4 initiated on the surface of catalyst to generate methyl radicals16, and CH3⋅ radicals couple to form C2H6 and C2H4 in the gas phase17-20. The homogeneous coupling of methyl radical in gas phase usually need relatively high temperature and was uncontrollable. Besides, the resulting C2 product tend to further oxidize to unwanted CO and CO2. So, there is a limit of 25% to the yield of C2-hydrocarbons in catalytic OCM due to its homogeneous gas-phase coupling. Arutyunov et. al proposed the noticeable exceeding of this limit can be obtained only in the case if catalyst plays significant role not only in heterogeneous generation of methyl radicals but as well in their subsequent transformations.21

Since the contribution from heterogeneous catalysis was restricted to the activation of methane to produce methyl radicals while the subsequent homogeneously coupling of methyl radicals was out of control, which consequently led to limited selectivity to C2 species (ethane and ethylene). To overcome this challenge, we first experimentally confirmed the possibility of coupling of methyl radicals on a Na-W/SiO2 catalyst surface at low reaction temperature (700 oC) by VUV photoionization mass spectroscopy (SVUV-PIMS). Thereafter, we propose to optimize the OCM reaction based on the design of bifunctional catalysts that provide two types of active sites (methane activation site to generate methyl radicals and methyl radical coupling sites to produce ethane and ethylene). Further regulation of the proximity of these two active sites lead to a synergic OCM catalytic system that is capable of achieving high C2 yield at low reaction temperature (reaction temperature <600 oC, C2 yield > 10%).

Figure 1. Mixture effect of temperature on catalytic performance. (a) C2 yield of La2O3 nanoparticles, Na-W/SiO2 and their mixed sample from 550 - 750 oC. (b) The conversion, selectivity and C2 yield of La2O3 nanoparticles, Na-W/SiO2 and their mixed sample at 650 oC. Reaction conditions: catalyst weight, 0.10 g for La2O3 nanoparticles and 0.10 g for Na-W/SiO2; CH4¡ÃO2¡ÃN2 = 3¡Ã1¡Ã2.6, total flow rate 66 mL min-1.

We found that the C2 yield significantly increased as the C2 selectivity enhanced after mixed La2O3 with Na-W/SiO2 below as shown in Figure 1. At lower temperature (< 750 oC), the C2 yield on mixed sample is much higher than La2O3 or Na-W/SiO2 catalyst. The specific comparison of CH4 conversion and C2 selectivity among mixed samples, pure La2O3 and Na-W/SiO2 nanoparticles at 650 oC was shown in Figure 1(b). The substantial increase of C2 selectivity instead of CH4 conversion led to much higher C2 yield on mixed sample than only on La2O3 or Na-W/SiO2 particles. In addition, there is almost no contribution to methane activation for Na-W/SiO2 catalyst at 650 oC. Therefore, the addition of Na-W/SiO2 is beneficial for C2 selectivity, which can offer sites for surface coupling and prevent C2 species from over-oxidization.

Figure 2. Integrated SVUV-PIMS peak intensity of gas-phase methyl radicals, ethylene and carbon oxide and carbon dioxide during OCM reaction catalyzed by various catalysts at 600 oC.

Reaction conditions: CH4 (42 SCCM), O2 (14 SCCM), pressure (2 torr). Mixed sample: 70 mg La2O3 with 70 mg Na-W/SiO2; The intensity is normalized by the amount of La2O3.

As shown in Figure 2, the enhanced C2 selectivity of mixed sample was substantiated by SVUV-PIMS experiment. In Figure 2(a), no detected methyl radical on Na-W/SiO2 even at 700 oC demonstrated Na-W/SiO2 is too inert to activate methane, which is consistent with the experimental results. Compared to La2O3, more ethylene with less methyl radical was produced on mixed sample while almost the same CO and CO2 was generated, which further demonstrated that Na-W/SiO2 could facilitate to turn methyl radical to C2. In other word, the coupling of methyl radical partly was taking place on Na-W/SiO2 surface.

Figure 3. OCM performance of different transition metal oxides and the mixed samples. 0.1 g transition metal oxides nanoparticles (40 - 60 mesh) or mixed sample (0.1g transition metal oxides mixed with 0.1 g Na-W/SiO2 powder), CH4¡ÃO2¡ÃN2=3¡Ã1¡Ã2.6; 500 - 800 oC, total flow = 66 mL min-1.

In addition, significant improvement for C2 yield has been observed for most transition metal oxides after mixing transition metal oxides particles (Cr2O3, MnO2, Fe2O3, Co3O4, NiO and CuO) with Na-W/SiO2 compared with pure transition metal oxides as shown in Figure 3. The higher selectivity and conversation compared to transition metal oxides further verified that coupling of methyl radical can partly occur on the surface of catalyst.

In this study, we found the C2 selectivity of La2O3 nanoparticle at low temperature can further improve after mixing with Na-W/SiO2. These improvements resulted from the coupling site of W-O-Si interface. To summarize, we confirm that the catalyst can play an important role on coupling of methyl radical, which may work as a bifunctional catalysis system exceeding C2 yield limit in traditional heterogeneous-homogeneous mechanism. A new way to explore efficient OCM catalyst is suggested.

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