Reports: ND556619-ND5: Catalysts for Direct Conversion of Methane

George Huber, PhD, University of Wisconsin

2017 Report for PRF Grant: Catalysts for Direct Conversion of Methane We have studied the direct, non-oxidative conversion of methane to ethylene and aromatics over mono- and bimetallic catalysts at moderate (973K) and high (1223K) reaction temperatures, and are building a new experimental apparatus (SSITKA) to analyze these catalysts. Our work on these systems has resulted in a publication within ACS Catalysis [1] and we have a second manuscript currently under development [2]. Methane conversion over Pt-Sn/ZSM-5 catalysts For our ACS Catalysis publication [1], we employed a comprehensive experimental and theoretical study on silica- and zeolite-supported Pt and Pt-Sn catalysts for non-oxidative methane conversion at moderate temperatures. We have shown that the addition of Sn to both SiO2- and ZSM-5-supported catalysts led to improved reactivity over their monometallic Pt counterparts. Moreover, the ZSM-5-supported catalysts generally outperformed the SiO2-supported materials. STEM images were used to obtain particle size distributions for mono- and bimetallic SiO2 and ZSM-5 catalysts. The improvements over ZSM-5 were partly attributed to smaller metal particle size compared to the SiO2 support, which was further supported by our microkinetic model. This model used DFT parameters for terrace and step surfaces of Pt and Pt-Sn to predict product formation rates and surface coverages of intermediate species at reaction conditions. The model predicted platinum sites were mainly covered with adsorbed CH* and C* intermediates, whose coverage was drastically reduced with the addition of tin. The Pt-Sn step surfaces have a higher vacancy than terraces, suggesting step sites should be more catalytically active. This supports our observation that higher reactivity of Pt-Sn/ZSM-5 over Pt-Sn/SiO2 could be related to the ZSM-5 catalyst having a larger fraction of smaller nanoparticles. The model also predicted higher catalytic activity with steps of the Pt-Sn surface in terms of ethylene TOF, which qualitatively agrees with our experimental results. Direct conversion of methane over Fe/SiO2 catalysts Fe/SiO2 catalysts were evaluated for high temperature methane coupling as we aimed to understand and replicate the catalytic performance of Guo et al [3], who reported on a Fe©SiO2 catalyst capable of exclusively producing ethylene, benzene, and naphthalene at high methane conversions without coke formation. We prepared quartz- (Q) and TEOS-supported catalysts using nitrate salt (FeNO3), nanopowder (Fe2O3), and fayalite (Fe2SiO4) as iron precursors. Contrary to the work of Guo et al, the main product for all catalysts evaluated in our work was coke, with ethylene and ethane as main gaseous products. Quartz-supported catalysts achieved higher conversions than TEOS-supported materials, suggesting that ordered silica phases were advantageous over disordered phases. When calcined at 1625°C, the cristobalite phase forms and catalysts prepared under these conditions outperform other quartz-supported materials by decreasing the extent of coke formation and increasing the generation of C2 hydrocarbons. This is particularly noticeable with the fayalite catalyst where, although we see an overall lower performance compared to other iron catalysts, the selectivity to coke was significantly lower. Unfortunately, few aromatic products were detected with this catalyst. Therefore, while we observed apparent benefits of this fayalite catalyst associated to coke inhibition, we were unable to match the catalytic performance of Guo et al. Steady-state isotopic kinetic analysis We have constructed a continuous-flow reactor capable of performing steady-state isotopic kinetic analysis (SSITKA). SSITKA is an advanced catalyst characterization technique in which a rapid switch is made between a reactant and its isotopically-labeled analog. A high-speed switching valve allows the switch to occur without disturbing the reaction while a mass spectrometer monitors the concentration of labeled reaction products as a function of time. It is then possible to determine both the number of surface intermediates and the effective surface residence time by tracking the amount of time it takes for the isotopically-labeled reactant to pass through the catalytic system being analyzed. A recent review by Ledesma et al, highlights both the fundamental kinetic parameters that can be obtained by such a system and other similarly complex systems where SSITKA has been applied. [4] The reactor is being used to count active catalytic sites on systems that have remained challenging to characterize with chemisorption or titration methods. Future Work In the final year of this project, we will continue to work on direct methane conversion and develop better analytical tools to understand the catalysis that occurs in this reaction. Impact on Student Researchers Students (Joseph Chada and Keishla Donne Rivera) supported by this research grant have developed and applied skills in catalyst synthesis and characterization, reactor and analytical system design and construction. Moreover, they have assisted in developing manuscripts. References [1] Gerceker, D.; Motagamwala, A. H.; Rivera-Dones, K. R.; Miller, J. B.; Huber, G. W.; Mavrikakis, M.; Dumesic, J. A. ACS Catal. 2017, 7, 2088-2100 [2] Miller, J. B.; Rivera-Dones, K. R.; Vu, H. B.; Novak, J. L.; Mavrikakis, M.; Dumesic, J. A.; Huber, G. W. Manuscript In Progress. [3] Guo, X.; Fang, G.; Li, G; Ma, H.; Fan, H.; Yu, L. Science 2014, 344, 616-619 [4] Ledesma, C.; Yang, J.; Chen, D.; Holmen, A. ACS Catal. 2014, 4 (12), 4527-4547 [5] Shannon, S. L.; Goodwin, J. G. Chem. Rev. 1995, 95 (3), 677-695