Shifting the Paraffin-to-Olefin Ratio and Gasoline Fuel Properties with Bimetallic BEA Zeolite Catalysts in Dimethyl Ether Homologation
Recent research on the production of HOG from DME over BEA zeolites suggests that this process offers an attractive alternative to traditional methanol-to-gasoline (MTG) technologies by operating at milder conditions and providing greater carbon efficiency 1. The HOG product is rich in branched olefins and paraffins, particularly triptane (2,2,3-trimethylbutane) which has high research- and motor-octane numbers (RON 112, MON 101), making it an ideal unleaded fuel or fuel additive for aviation or racing applications 2. RON and MON are important fuel metrics and research shows a correlation between them and paraffin and olefin content 3. Copper-modified BEA (Cu/BEA) is more active than HBEA . Research has identified that ionic Cu(I) species are active for alkane dehydrogenation whereas metallic Cu species responsible for hydrogenation chemistry 4. Here we report on the effect of including ionic Zn and Ni as a means of increasing the dehydrogenation activity of Cu/BEA in DME-to-HOG reactions. Bimetallic Cu-Zn/BEA and Cu-Ni/BEA catalysts were synthesized by first incorporating Ni or Zn by ion-exchange, followed by incipient-wetness impregnation of Cu. Similar metal loadings, acid site densities, and Bronsted/Lewis acid ratios were confirmed across all materials. All catalysts were tested in DME-to-HOG reactions with co-fed H2 at 200 degrees C and 15 psia with 1:1 DME:H-2 and DME weight-hourly space velocity of 2.2 gDME-gcat-1-h-1. Compared at similar turnover numbers (TON), Cu-Ni/BEA displayed similar activity to Cu/BEA however, exhibited greater hydrogenation activity as evidenced by the higher C5-8 P:O ratio. The Cu-Zn/BEA demonstrated decreased activity and the lowest P:O ratio, attributed to enhanced dehydrogenation activity at ionic Zn sites. For the HOG-range products (i.e., C5-8 hydrocarbon species), each catalyst had a 98-99 estimated RON, while the MON decreased with decreasing P:O ratio. Compared to Cu/BEA, these bimetallic catalysts demonstrate a unique balance of hydrogenation and dehydrogenation of the hydrocarbon products to access markedly different P:O product ratios without requiring a separate unit operation or a mixed catalyst bed.