A process for upgrading aliphatic feedstocks containing lower olefins employing new synthetic catalyst of ultra-large pore crystalline material. The new crystalline material exhibits unusually large sorption capacity demonstrated by its benzene adsorption capacity of greater than about 15 grams benzene/100 grams at 50 torr and 25.degree. C., a hexagonal electron diffraction pattern that can be indexed with a d.sub.100 value greater than about 18 Angstrom Units and a hexagonal arrangement of uniformly sized pores with a maximum perpendicular cross section of at least about 13 Angstrom units. A new process is provied for catalytic oligomerization of lower olefin component in paraffin-containing mixed aliphatic feedstock which comprises contacting the feedstock under catalytic conversion conditions with acid metallosilicate solid catalyst having the structure of MCM-41 with hexagonal honeycomb lattice structure consisting essentially of uniform pores in the range of about 20 to 100 Angstroms. The oligomerization reaction is very selective, especially when conducted at temperature of about 40.degree. to 250.degree. C., yielding branched intermediate olefins. Low severity reaction permits execellent conversion of lower olefins at pressure of about 100-13,000 pKa range and moderate space velocity. Oligomers of propene produced over MCM-41, when reacted under cracking/disproportionation conditions yield a propylene-rich mixture which is separated as a C.sub.3 stream, and C.sub.6 + isoalkenes are recovered in good yield.