(678b) Tuning the Paraffin-to-Olefin Ratio in High-Octane Gasoline Synthesis Using Bimetallic BEA Zeolite Catalysts

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 efficiency¹. 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 applications². RON and MON are the key fuel properties affecting engine anti-knock performance and are correlated with paraffin and olefin content³.

On copper-modified BEA (Cu/BEA), ionic Cu(I) species are active for alkane dehydrogenation whereas metallic Cu species are responsible for hydrogenation chemistry⁴. Here we report on the effect of including ionic Zn or Ni species as a means of increasing the dehydrogenation activity of Cu/BEA, and thus enabling control over the paraffin to olefin ratio (P/O) 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 °C, 15 psia, 1:1 DME:H­_2, and DME flow of 2.2 g_DME-g_cat^-1-h^-1. Compared at similar turnover numbers (TON), Cu-Ni/BEA displayed similar activity to Cu/BEA however, it exhibited greater hydrogenation activity as evidenced by the higher C_5-8 P/O ratio. 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., C_5-8 hydrocarbon species), each catalyst had an estimated RON of 98-99, 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.

References

1. Tan, E.C., Talmadge, M., Dutta, A., Hensley, J., Snowden-Swan, L.J., Humbird, D., Schaidle, J., Biddy, M. “Conceptual process design and economics for the production of high-octane gasoline blendstock via indirect liquefaction of biomass through methanol/dimethyl ether intermediates” (2016) Biofuels, Bioproducts and Biorefining, 10 (1), pp. 17-35. DOI: 10.1002/bbb.1611
2. Khadzhiev, S.N., Magomedova, M.V., Peresypkina, E.G. “Triptane synthesis from methanol and dimethyl ether: A review” (2016) Petroleum Chemistry, 56 (3), pp. 181-196. DOI: 10.1134/S0965544116030063
3. Ghosh, P., Hickey, K.J., Jaffe, S.B. “Development of a detailed gasoline composition-based octane model” (2006) Industrial and Engineering Chemistry Research, 45 (1), pp. 337-345. DOI: 10.1021/ie050811h
4. Farberow, C.A., Cheah, S., Kim, S., Miller, J.T., Gallagher, J.R., Hensley, J.E., Schaidle, J.A., Ruddy, D.A. “Exploring Low-Temperature Dehydrogenation at Ionic Cu Sites in Beta Zeolite to Enable Alkane Recycle in Dimethyl Ether Homologation” (2017) ACS Catalysis, 7 (5), pp. 3662-3667. DOI: 10.1021/acscatal.6b03582