pore windows of about a size such as would be provided by 8 membered rings of oxygen atoms it will be understood of course that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline aluminosilicate the oxygen atoms themselves being bonded to the silicon or aluminum atoms at the centers of the tetrahedra the pores characterizing these zeolites useful in the present process may be substantially circular such as in zeolite zk 5 having uniform pores of about 3 9 angstroms or somewhat elliptical such as in erionite having pores of approximately 3 6 by 5 2 angstroms it will be understood that in any case the small pore zeolites used as catalysts in the process of this invention have a major pore dimension of less than 6 angstroms the pore size dimensions of the above zeolites as well as other feasible zeolites are those specified in 34 zeolite frameworks 34 by w m meier and d h olson appearing in advances in chemistry series vol 101 pages 155 170 1971 the contents of which is incorporated herein by reference a convenient measure of the extent to which a zeolite provides control molecules of varying sizes to its internal structure is the constraint index of the zeolite zeolites which provide a.