butyl ether, in the presence of a catalyst constituted by silica, modified by {HEAD ~ the addition} of {TAIL ~ at least one element} or compound of an element preferably chosen from the group constituted by rubidium, cesium, magnesium, calcium, strontium, barium, gallium, lanthanum, cerium, praseodymium, neodymium and uranium and optionally by the addition of at least one element or compound of an element chosen from the group constituted by aluminium, titanium and zirconium 
of
butyl ether , in the presence of a catalyst constituted by silica , modified by { HEAD ~ the addition } of { TAIL ~ at least one element } or compound of an element preferably chosen from the group constituted by rubidium , cesium , magnesium , calcium , strontium , barium , gallium , lanthanum , cerium , praseodymium , neodymium and uranium and optionally by the addition of at least one element or compound of an element chosen from the group constituted by aluminium , titanium and zirconium
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH