A multi-structured tubular element for producing a reactor for effecting exothermic/endothermic chemical reactions, comprises two or more monolithic thermoconductive bodies, assembled together so that each has a part of the side surface interfaced with the side surface of one or more monolithic thermoconductive bodies adjacent thereto, so as to form as a whole, a honeycomb structure containing a plurality of longitudinal channels extending from one end to the other of said tubular element, suitable for being filled with a granular catalytic solid.