Process for preparing C.sub.4 --C.sub.6 --alkenes having an internal double bond comprising recycling a portion of a product stream

A process for preparing C.sub.4 -C.sub.6 -alkenes having an internal double bond by hydroisomerization of C.sub.4 -C.sub.6 -alkenes having a terminal double bond in the presence of hydrogen and a catalyst comprising a noble metal of group VIII of the Periodic Table of the Elements (Mendeleev), which comprises:

• a) introducing a feed of heated C.sub.4 -C6-alkenes having a terminal double bond, H.sub.2 and a recycle stream having a temperature of 15 to 60.degree. C. into a reactor which is charged with the noble-metal containing catalyst,
• b) isomerizing the C.sub.4 -C.sub.6 -alkenes having a terminal double bond present in the feed in the reactor at an LHSV of 1 to 20 h.sup.-1, 15 to 120.degree. C. and a pressure at which the C.sub.4 -C.sub.6 -alkenes having a terminal double bond and the recycle stream are present in the liquid phase,
• c) dividing the product stream exiting from the reactor into a work-up stream and a recycle stream which in the steady state is 0.1 to 10 times the work-up stream,
• d) recycling the recycle stream to step a) and
• e) isolating C.sub.4 -C6-alkenes having an internal double bond from the work-up stream.