Axial-radial flow reactor comprising a catalytic bed (1) of a hollow cylindrical shape, having a vertical axis (2), a base (5), a radial gas inlet section (3b), an axial gas inlet section (6) and a radial gas outlet section (4b), wherein the catalytic bed (1) comprises: a first cylindrical annular region (10) containing a layer of a first catalyst (A) and a layer of a second catalyst (B), the layer of the first catalyst being above the layer of the second catalyst; a second cylindrical annular region (9) coaxial to the first annular region and containing only the first catalyst (A).