A reactor for exothermic/endothermic chemical reactions which are effected in the presence of a solid catalyst, which comprises: a. a substantially cylindrical vessel equipped with feeding and discharge means of a thermoregulation fluid of the chemical reaction; b. an upper closing element and a lower closing element respectively positioned at the top and at the bottom of the vessel; c. feeding means of the reagent gaseous phase positioned in the upper closing element and/or in the lower closing element; d. discharge means of the reacted phase positioned in the upper closing element and/or in the lower closing element; e. a plurality of honeycomb monolithic structures, as such or coaxially inserted inside tubular structures, inserted inside the substantially cylindrical vessel (a) between the upper closing element and the lower closing element, each monolithic structure consisting of a substantially cylindrical continuous body having a diameter from 1 to 10 cm, inside which there is a series of parallel axial channels, with a square or rectangular cross section having a length of the sides ranging from 0.5 to 5 mm, which extend from one end to the other, separated by the continuous honeycomb matrix, and filled with the catalyst, consisting of packed bodies or structured bodies with an average dimension of from 0.3 mm to less than 3 mm positioned inside each channel, so as to fill the volume for a filling factor, wherein the filling factor is defined as catalyst volume/volume of the longitudinal channel, ranging from 0.2 to 0.8, wherein said monolithic structure is made of a material having an intrinsic thermal conductivity higher than 10 W/m/K; wherein said monolithic structures are in the form of a tube bundle, distanced between each other, and immersed in said thermoregulation liquid. a. a substantially cylindrical vessel equipped with feeding and discharge means of a thermoregulation fluid of the chemical reaction; b. an upper closing element and a lower closing element respectively positioned at the top and at the bottom of the vessel; c. feeding means of the reagent gaseous phase positioned in the upper closing element and/or in the lower closing element; d. discharge means of the reacted phase positioned in the upper closing element and/or in the lower closing element; e. a plurality of honeycomb monolithic structures, as such or coaxially inserted inside tubular structures, inserted inside the substantially cylindrical vessel (a) between the upper closing element and the lower closing element, each monolithic structure consisting of a substantially cylindrical continuous body having a diameter from 1 to 10 cm, inside which there is a series of parallel axial channels, with a square or rectangular cross section having a length of the sides ranging from 0.5 to 5 mm, which extend from one end to the other, separated by the continuous honeycomb matrix, and filled with the catalyst, consisting of packed bodies or structured bodies with an average dimension of from 0.3 mm to less than 3 mm positioned inside each channel, so as to fill the volume for a filling factor, wherein the filling factor is defined as catalyst volume/volume of the longitudinal channel, ranging from 0.2 to 0.8, wherein said monolithic structure is made of a material having an intrinsic thermal conductivity higher than 10 W/m/K; wherein said monolithic structures are in the form of a tube bundle, distanced between each other, and immersed in said thermoregulation liquid.