The invention relates to a reactor for carrying out an exothermic process comprising a reactor shell, inlets for introducing reactants and coolant into the reactor shell, outlets for removing product and coolant from the reactor shell, at least two reactor tubes, a coolant chamber, and a gas distribution system below the coolant chamber, whereby at least two reactor tubes extend through the coolant chamber to enable fluid communication between the space below the coolant chamber and the space above the coolant chamber, said reactor comprising one or more highly porous catalysts, said catalyst(s) having a size of at least 1 mm and comprising a porous body and a catalyst material, whereby the porous body has a porosity within the range of between 50 and 98 volume %.