(694b) Catalytic Upgrading of Olefins Under Methane Environment: Effect of Sulfur Poisons on Catalyst Performance and Reusability

Canada has one of the most extenisive reserves of raw heavy oil and bitumen in the world, infact, it has been forecasted by the National Energy Board of Canada that bitumen production will reach a high of 4.3 million barrels per day (bpd) by 2040. However, the extracted bitumen must be upgraded to facilitate transportation to downstream refineries. Traditionally, such procedures are carried out either by the use of diluents derived from naphtha feedstocks and gas condensates or through hydroconversion processes. The elimination of solvent diluents from this process is beneficial from both a green chemical and financial perspective. Furthermore, the use of H₂ as a refining gas in the upgrading of bitumen is also undesirable due to the high operating pressures (8MPa and above) required by hydroconversion processes even in the presence of optimised heterogeneous catalysts. Industrially, H₂ gas is obtained via the steam reforming process which involves the reaction of high temperature steam (typically 700 – 1000 °C) with methane (CH₄), the main component of natural gas, over a supported metal catalyst to produce carbon monoxide (CO) and H₂. An attractive alternative to hydrotreating would be to bypass the steam reforming process and use methane directly as hydrogen donor for the catalytic upgrading of bitumen and crude oils. Furthermore, the worldwide reserves of natural gas are extensive, currently estimated at 6.95 quadrillion cubic feet.¹ However, the stable tetrahedron structure of CH₄ makes it a highly inert chemical. Notably, the activation of methane requires an initial cleavage of C-H bond, triggering the formation of highly reactive methyl radical. The initial C-H cleavage step is rate limiting and is reported to have an activation energy of approximately 435 kJ/mol, the employment of an effective heterogeneous catalyst to lower this activation energy barrier both under oxidative and non-oxidative conditions has been reviewed extensively and the subject of research for many years. Choudhary et al.² famously reported that the influence of higher hydrocarbon co-reactants can significantly improve the conversion of methane over H-galloaluminosilicate ZSM5 zeolite catalyst at low temperatures (400 – 600 °C) and atmospheric pressure. We have since shown that this process may be applied to the catalytic upgrading of bitumen derived oils, facilitating the effective upgrading of heavy oils under methane environment at low temperatures (400 °C) and pressures (3 MPa) with high selectivity towards commercially valuable BTX and paraffinic compounds.³ Another major issue associated with oil transportation is the presence of unstable olefins which can lead to the formation of carbocaneous and polymeric deposits in both the pipelines and product compounds such as paraffinic fuels. Therefore, it is important to reduce the olefin content as much as possible prior to sending the upgraded product oil to refineries. Previously, we reported the successful transformation of 1-decene to aromatics and paraffins over a AgGa/H-ZSM5 catalyst under methane environment.⁴ Futhermore, it is essential that commercial catalysts are able to operate for long periods on stream without deactivating and display a high resistance towards poisons such as sulfur compounds, which are found in an abundance in bitumen derived oils. In light of this, we now present a comprehensive study into the upgrading of olefins, using 1-decene as model compound in the presence of sulfur containing compounds. The activity of a series of transition metal modified aluminosilicates for the transformation of 1-decene under methane environment is reported and its catalytic performance is further evaluated in the presence of sulfur contaminants. Notably, we show that the catalyst is remarkably resistant to sulfur poisons at temperatures greater than 330 °C and that selectivities >80% towards valuable aromatic compounds and total olefin reduction >85% are sustained despite the presence of 10 wt.% sulfur poisons. A mechanistic study into the mode of sulfur effect on olefin reduction of the developed catalysts is presented using a range of characterization techniques such as isotopic labelling, Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS), Transmission Electron Microscopy (TEM), Temperature Programmed Desorption (TPD), X-Ray Photoelectron Spectroscopy (XPS) and Solid-State Nuclear Magnetic Resonance Spectroscopy (NMR). A hypothesized mechanism for the catalytic olefin conversion in the presence of sulfur contaminants is also proposed.

References:

1. Xu, C.; Bell. L.; Oil Gas J. 2017, 115 (12), 18.
2. Choudhary, V. R.; Kinage, A. K.; Choudhary, T. V. Science 1997, 275, 1286.
3. Guo, A.; Wu, C.; He, P.; Luan, Y.; Zhao, L.; Shan, W.; Cheng, W.; Song, H. Catal. Sci. Technol. 2016, 6 (4), 1201–1213.
4. He, P.; Lou, Y.; Song, H. Fuel 2016, 182, 577.