Catalytic synthesis reactor comprising: a catalytic bed with an annular cylindrical shape; an inlet header and an outlet header which are essentially cylindrical shaped and are concentrically arranged so as to form an outer wall and inner wall for delimiting and containing said catalytic bed; said headers being gas-permeable so as to allow the distribution of a gaseous flow containing reactants and the collection of a gaseous flow containing reaction products respectively into and from the catalytic bed; wherein at least one of said inlet header and said outlet header comprises: a first perforated plate which forms an outer wall of the header opposite to the catalytic bed and configured to act as a load-bearing structure of said header; a second perforated plate, which is termed micro-perforated plate and which forms an inner wall of the header facing the catalytic bed; an intermediate gas-permeable wall which is sandwiched between said perforated plate and said micro-perforated plate and said intermediate gas-permeable wall acts substantially as a spacing element between said perforated plate and said micro-perforated plate, wherein the first perforated plate is perforated with holes all having a characteristic size which is equal to or greater than a minimum size and the micro-perforated plate is perforated with holes all having a characteristic size which is not greater than a maximum size, said maximum size being smaller than said minimum size so that the holes of the micro-perforated plate are smaller than the holes of the first perforated plate, wherein the holes of the first perforated plate and the holes of the micro-perforated plate have a circular shape or a regular polygonal shape, wherein said characteristic size is the diameter of circular holes or double the apothem for polygonal holes, and wherein the micro-perforated plate is thinner than the perforated plate, and wherein the holes of said plates are arranged so that each hole of the first plate faces a plurality of the smaller holes of the micro-perforated plate. a catalytic bed with an annular cylindrical shape; an inlet header and an outlet header which are essentially cylindrical shaped and are concentrically arranged so as to form an outer wall and inner wall for delimiting and containing said catalytic bed; said headers being gas-permeable so as to allow the distribution of a gaseous flow containing reactants and the collection of a gaseous flow containing reaction products respectively into and from the catalytic bed; wherein at least one of said inlet header and said outlet header comprises: a first perforated plate which forms an outer wall of the header opposite to the catalytic bed and configured to act as a load-bearing structure of said header; a second perforated plate, which is termed micro-perforated plate and which forms an inner wall of the header facing the catalytic bed; an intermediate gas-permeable wall which is sandwiched between said perforated plate and said micro-perforated plate and said intermediate gas-permeable wall acts substantially as a spacing element between said perforated plate and said micro-perforated plate, wherein the first perforated plate is perforated with holes all having a characteristic size which is equal to or greater than a minimum size and the micro-perforated plate is perforated with holes all having a characteristic size which is not greater than a maximum size, said maximum size being smaller than said minimum size so that the holes of the micro-perforated plate are smaller than the holes of the first perforated plate, wherein the holes of the first perforated plate and the holes of the micro-perforated plate have a circular shape or a regular polygonal shape, wherein said characteristic size is the diameter of circular holes or double the apothem for polygonal holes, and wherein the micro-perforated plate is thinner than the perforated plate, and wherein the holes of said plates are arranged so that each hole of the first plate faces a plurality of the smaller holes of the micro-perforated plate. a first perforated plate which forms an outer wall of the header opposite to the catalytic bed and configured to act as a load-bearing structure of said header; a second perforated plate, which is termed micro-perforated plate and which forms an inner wall of the header facing the catalytic bed; an intermediate gas-permeable wall which is sandwiched between said perforated plate and said micro-perforated plate and said intermediate gas-permeable wall acts substantially as a spacing element between said perforated plate and said micro-perforated plate, wherein the first perforated plate is perforated with holes all having a characteristic size which is equal to or greater than a minimum size and the micro-perforated plate is perforated with holes all having a characteristic size which is not greater than a maximum size, said maximum size being smaller than said minimum size so that the holes of the micro-perforated plate are smaller than the holes of the first perforated plate, wherein the holes of the first perforated plate and the holes of the micro-perforated plate have a circular shape or a regular polygonal shape, wherein said characteristic size is the diameter of circular holes or double the apothem for polygonal holes, and wherein the micro-perforated plate is thinner than the perforated plate, and wherein the holes of said plates are arranged so that each hole of the first plate faces a plurality of the smaller holes of the micro-perforated plate.