April 30, 2012
This catalyst makes aromatics from shale gas–derived propane. Over the past few years, the use of inexpensive ethane from shale gas has lowered the cost of ethylene production. The renewed profitability of ethylene caused several ethylene producers to announce their interest in building new ethylene crackers along the US Gulf Coast and even one in the Northeast.
Propane prices are not as low as ethane, but, because of the abundance of shale gas, they are lower than in the recent past. This too has stimulated interest in propane dehydrogenation plants for making propylene. One plant is already up and running in Houston, and another will be built soon in Freeport, TX.
All of the conventional routes to aromatics (benzene, toluene, and the xylenes [BTX]) use petroleum naphtha, not natural gas liquids (NGLs), as the feedstock. Developing routes to aromatics from low-cost NGLs (e.g., propane) would be a significant development and could stimulate US investment in new aromatics capacity. This is especially the case for benzene because its US production capacity falls short of requirements, and large amounts must be imported from Asia.
P. E. Ellis and colleagues disclose a catalyst system that can convert propane to BTX with reasonably good selectivity. Their catalyst is based on a zeolite structure that has germanium incorporated into its framework. Platinum is precipitated from solution onto the germanium–zeolite structure.
One of the better-performing catalysts reported contained 6.06% germanium in the zeolite framework and had a crystallite size of 1–1.5 μm. The zeolite was bound with silica (50 wt% zeolite and 50 wt% SiO2) and sized to 20–40 mesh particles. The zeolite was converted to its acidic form; Pt2+ was deposited on the catalyst by ion exchange; and the combination was calcined at 300 °C.
Before the catalyst was used for BTX production, it was pretreated with hydrogen and then with H2S. The inventors tested the catalyst at 500 °C and 33 psig with 1 h–1 weight-hourly space velocity of propane. Propane conversion was initially ≈50% and settled to ≈42% after 7000 h of operation. Selectivity to BTX leveled out at ≈55% over this period. The only other product was reported as fuel gas. (Saudi Basic Industries [Riyadh, Saudi Arabia]. US Patent 8,153,852, April 10, 2012; Jeffrey S. Plotkin)