Plate heat exchange unit (12) for isothermal or pseudo-isothermal chemical reactors, for the heat exchange between a fluid circulating inside the plates and an outer flow of reactant gases, wherein the walls (14, 15), of the plates (13) have notches (14s, 15s) surrounded by suitable joining lines (105), obtaining openings (104) passing through the plates (13) themselves, the plates resulting substantially permeable in the transversal direction to the reactant gases.