Recent investigations by a number of groups have shown that a beta zeolite catalyst (and other solid Brønsted acids) will convert methanol and/or dimethyl ether (DME) to iso-alkanes such as triptane (2,2,3-trimethylbutane), with uniquely high selectivity to C4 and C7 (termed the DME to high octane gasoline process or DTHOG). In comparison, the Exxon-Mobil methanol to gasoline (MTG) process produces a broader mixture of hydrocarbons, which is heavy in benzene, toluene, and xylene (BTX). DTHOG is milder than MTG (350-450 °F vs. 650-950 °F and 130 psia vs. 315 psia), thus offering the benefit of reduced capital and operating costs for the reactor. However, similar to MTG, this new low-temperature, selective production of alkanes is still a hydrogen-deficient process. That is, to produce alkanes, a quantitative production of aromatics is required to maintain stoichiometry—a result of oxygen from methanol or DME being rejected as water.
Highly Active and Selective Metal-modified Zeolite Catalysts for Low Temperature Conversion of Methanol and Dimethyl Ether to Gasoline-range Branched Hydrocarbons
Scientists at the National Renewable Energy Laboratory (NREL) have developed a catalyst formulation and structure that is capable of incorporating hydrogen from gaseous co-fed H2 into methylation and alkylation products without altering aliphatic hydrocarbon selectivity or lowering reaction rates. Volumetric and gravimetric activities are increased and the rate of formation of heavy aromatic residues is decreased. The catalysts are comprised of a beta zeolite modified with Cu, Ga, other non-noble metals, and combinations thereof. The metals provide sites for H2 dissociation, hydrogen addition, and hydrogen abstraction, all of which modulate critical reaction stepsâproviding H for alkane formation and H removal for alkene formation, bringing products back into the carbon chain growth pathway and minimizing side-reactions that produce unwanted byproducts. The metals work with the BrÃ¸nsted acids of the zeolite, which act as the catalyst for alkene methylation and carbon chain growth. Physical mixtures of the metal catalyst and zeolite do not yield the same benefits. Â Metal loadings are low (
For more information, contact Eric Payne at Eric.Payne@nrel.gov.
Applications and Industries
- Methanol/DME to hydrocarbons
- C4 alkylation, butane/butene upgrading
- Gas to Liquids (GTL)
- Octane enhancement/gasoline blending
- Jet/Biojet (via oligomerization of products over acid resin catalysts)
- Lower temperature operation compared to MTG process, resulting in significantly reduced aromatic production and higher yield to gasoline-range branched alkanes and alkenes
- Allows, in principle, recycle of intermediate products that are not activated/upgraded (i.e., are dead-end products) on similar catalysts, improving yield to higher-value products
- Incorporates H2 from the gas phase to ‘fix’ the oxygenate to alkane stoichiometric imbalance instead of expensive hydrogen transfer agents like adamantane while maintaining the attractive product selectivity of similar catalysts
- Increases desired hydrocarbon yield compared to similar catalysts
- Reduces deactivation and the frequency of regeneration cycles by reducing the net rate of formation of heavy aromatic residues