(668d) Dehydrogenative Coupling of Light Alkanes to SAF-Range Hydrocarbons in a Single Reactor

The syngas-to-hydrocarbons process can effectively turn biomass-derived syngas into isoalkane feedstocks, which can be converted into jet-range C₈₊ hydrocarbons (HCs) through sequential dehydrogenation and olefin coupling steps. Unifying dehydrogenation and coupling in a single reactor is an attractive approach to reduce capital and operating expenses in a biorefinery. To date however, no commercial process exists to sequentially perform these steps in a single reactor, likely due to the mismatched conditions that promote dehydrogenation and coupling. Alkane dehydrogenation is performed at high temperatures (350-500°C) and atmospheric pressure to overcome thermodynamic limitations. Olefin coupling in contrast requires intermediate temperatures (<300°C) and high operating pressures to minimize coupled product cracking.

Using isobutane as a model isoalkane feed, we investigate the feasibility of an integrated dehydrogenative coupling process in a single reactor. Composite catalyst systems, including an industrial dehydrogenation catalyst (PtSn/Al₂O₃) and various solid acid catalysts (zeolites and amorphous SiO₂-Al₂O₃) for olefin coupling were investigated. The strength and acidity of the acid catalysts were tuned to alter the coupling and cracking activity. This was done through H₃PO₄ treatments, which introduced PO_X groups that can coordinate with Brønsted sites. Characterization confirmed that PO_X modification decreased acid strength and acid site density, which is expected to decrease cracking chemistry. The dehydrogenative coupling activity of the composite catalyst systems were evaluated in a broad range of temperature (350-450°C), pressure (0-150 psig), alkane concentration (10-90%) and reactant space velocity (0.65-3h^-1) to identify suitable conditions for C₈₊ HC production. We demonstrate that mixed beds composed of PtSn/Al₂O₃ and PO_X-modified coupling catalysts can produce C₈₊ HCs with selectivity over two times greater than mixed beds containing unmodified coupling catalysts. Altogether, this talk will discuss the relationship between acid site modification and coupling activity, which subsequently enables greater compatibility between dehydrogenation and coupling steps.