the feed to the present process is preferably pretreated to remove contaminants such as dme , butadiene and sulfur compounds . acceptable levels of these impurities are : dme & lt ; 1 wppm ; total sulfur & lt ; 1 wppm ; and 1 – 3 butadiene & lt ; 10 wppm . the dme can be removed by distillation which may be conveniently combined with the selective hydrogenation of the 1 – 3 butadiene in a distillation column reactor . a hydrogenation catalyst , such as palladium or nickel , is used in a distillation column reactor at mild conditions . the dme is taken as overheads and the remainder is taken as bottoms . the sulfur compounds may be removed by chemisorption on reduced massive nickel catalysts , such as sud - chemie c46 or engelhard d - 4130 . a typical feed to the process of the present invention comprises dilute normal butenes in a mixed c 4 stream which has typically been depleted of isobutene . table ii below lists the components of such a typical stream . the use of the distillation column reactor is known . catalyst is placed on trays or combined into a distillation structure and placed into a distillation column . the selective hydrogenation of diolefins such as propadiene and 1 – 3 butadiene in a distillation column reactor is disclosed in u . s . pat . no . 6 , 169 , 218 which is hereby incorporated by reference . in the present invention a fractional distillation is made simultaneously with the selective hydrogenation of the 1 – 3 butadiene to remove the dme contaminant as overheads . the catalyst , to be effective , must be in the form so as to provide gas liquid contact . there are many forms of catalyst structures available for this purpose and these are disclosed variously in u . s . pat . nos . 5 , 266 , 546 ; 4 , 731 , 229 ; and 5 , 073 , 236 . the most preferred catalyst structure is disclosed in u . s . pat . no . 5 , 730 , 843 which is hereby incorporated by reference . it is believed that in the present reactions catalytic distillation is a benefit first , because the reaction is occurring concurrently with distillation , the initial reaction products and other stream components are removed from the reaction zone as quickly as possible reducing the likelihood of side reactions . second , because all the components are boiling the temperature of reaction is controlled by the boiling point of the mixture at the system pressure . the heat of reaction simply creates more boil up , but no increase in temperature at a given pressure . as a result , a great deal of control over the rate of reaction and distribution of products can be achieved by regulating the system pressure . also , adjusting the throughput ( residence time = liquid hourly space velocity − 1 ) gives further control of product distribution and to a degree control of the side reactions such as oligomerization . a further benefit that this reaction may gain from catalytic distillation is the washing effect that the internal reflux provides to the catalyst thereby reducing polymer build up and coking . internal reflux may be varied over the range of 0 . 2 to 20 l / d ( wt . liquid just below the catalyst bed / wt . distillate ) and gives excellent results . referring now to the figure a simplified flow diagram of the preferred embodiment of the invention is shown . the mixed c 4 stream is fed along with hydrogen via flow line 101 to a first distillation column reactor 10 containing a bed 12 of hydrogenation catalyst . in the distillation column reactor 10 the butadienes are selectively hydrogenated to butenes and at the same time the dme is separated by fractional distillation and removed as overheads via flowline 102 . the bottoms containing the butenes and less than 10 wppm butadiene are removed via flow line 103 and fed to reactor 20 containing a bed 22 of catalyst that chemisorbs the sulfur compounds . the effluent from the reactor 20 containing less than 1 wppm total sulfur compounds is removed via flow line 104 and fed , along with recycle from flow line 108 , via flow line 105 to a second distillation column reactor 30 containing a bed 32 of zsm - 57 zeolite catalyst . a portion of the butenes in the stream are oligomerized to higher olefins , preferably octenes , in the catalyst bed . the higher boiling oligomers and some butenes are removed as bottoms via flow line 107 . some butenes may be taken as overheads and recycled as reflux ( not shown ) with a purge of lighter material taken via flow line 106 to prevent buildup of the lighter material . the bottoms in flow line 107 are fed to a debutanizer column 40 where any c 4 &# 39 ; s are removed as overheads and recycled to the second distillation column reactor 30 for further conversion . product oligomers are removed from the debutanizer as bottoms via flow line 109 for further separation . twenty - one pounds of zsm - 57 zeolite catalyst were loaded in a distillation column reactor utilizing the catalyst structure shown in u . s . pat . no . 5 , 730 , 843 . a typical feed , as shown in table ii above , after treatment to remove the dme , butadiene and sulfur to acceptable levels was fed to the reactor . the reactor conditions and results are shown in table iii below . the catalyst was regenerated in situ by washing with normal heptane under the following conditions : the mixed c 4 feed was restarted to the reactor and a comparison of the regenerated and fresh catalyst is shown in table v below .