Conversion of synthesis gas to high octane predominantly olefinic naphtha

In the process of converting synthesis gas by contacting it at a temperature of 450.degree.-750.degree. F., a space velocity of 500-20,000 GHSV, and a pressure of from 50-1000 psig, with a catalyst comprising an intimate mixture of an iron-containing Fischer-Tropsch component and a volume excess of an acidic solid containing a crystalline zeolite, the improvement wherein the crystalline zeolite comprises a zeolite having a composition in its anhydrous state in terms of mole ratios of oxides as follows:

• (a) an Al.sub.2 O.sub.3 :SiO.sub.2 ratio within the range of from 0 to less than 0.005 Al.sub.2 O.sub.3 to more than 1 SiO.sub.2 and
• (b) an (xR.sub.2 O+(l-x) M.sub.2 /.sub.n O):
• SiO.sub.2 ratio of less than 1.1:1
• where M is sodium or sodium in combination with tin, calcium, nickel, or zinc, R is tetraalkylammonium and x is a number greater than 0 but not exceeding 1, said zeolite having the X-ray diffraction lines set forth in Table 1 or Table 2 of the specification and said contact producing an olefinic naphtha having a boiling range of less than 400.degree. F. at 90 percent overhead with an aromatic content of less than about 20 weight percent, an olefin plus aromatic content of at least 50 weight percent, wherein the olefins have a predominantly branched chain structure and an internally-positioned double bond.