in a combination alkylation - methyltertiary butyl ether operation , the unreacted c 4 olefinic hydrocarbons produced by etherification are contacted with molecular sieves to absorb 2 - butenes and the 1 - butenes remaining in the stream are divided so that one portion is subjected to double bond isomerization to form 2 - butenes for alkylation and another portion is subjected skeletal isomerization to form isobutene for etherification .