butyl ether, in the presence of a catalyst constituted by silica, modified by 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 {HEAD ~ at least one element} or compound of an element chosen from the group constituted by aluminium, titanium and zirconium 
butyl ether , in the presence of a catalyst constituted by silica , modified by 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 { HEAD ~ at least one element } or compound of an element chosen from the group constituted by aluminium , titanium and zirconium
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