Process for dimerizing propylene and for converting hexenes into ethers

Abstract

Hexenes are produced when propylene is catalytically converted under moderate conditions over chosen small pore zeolite catalysts having small 10-membered ring openings. These chosen catalysts are ZSM-22, ZSM-23, ZSM-35, and ZSM-48. They are unexpectedly effective to dimerize a substantially ethylene-free propylene-rich stream to form mainly hexene isomers, a major portion of which isomers are tert-isohexenes, without making a substantial amount of trimer or other byproducts. The feed is preferably at least 60% by weight, the remainder being alkanes, mainly propane, and the olefins being limited to less than 5 mol % ethylene and less than 5 mol %, preferably less than 2 mol %, of C.sub.4.sup.=+ (mainly butenes). Because propane in the feed is essentially unaffected, plural reactors in series may be used to avoid recycling unreacted propylene with propane to the dimerization reactor. The reactors thus allow using a mixed propylene-propane stream without need for a superfractionator to make the desired separation. Etherification is effective because of the high tert-isohexene content of the dimer product. Non-tertiary hexenes from the etherification reactor may be recycled to the dimerization reactor to benefit from the ability of the chosen catalyst to isomerize the non-tert-hexenes to tert-isohexenes on a substantially mol for mol basis without making other products typically in equilibrium in the isomerized effluent of other zeolite catalysts such as ZSM-5.