Abstract: Apparatus and process for the catalytic gaseous phase reaction of a feed gas mixture. The apparatus includes a reactor having two ends which alternately serve as an inlet for the feed gas mixture and as an outlet for reacted gas and two stationary heat exchange/reaction zones connected by a center zone. Each of the heat exchange/reaction zones contains a layer of catalyst and the center zone contains a feed gas mixture inlet. A first distribution/collection zone is between one of the heat exchange/reaction zones and one of the ends of the reactor and a second distribution/collection zone is between the other heat exchange/reaction zone and the other end of the reactor. In the process, the heat exchange/reaction zones are alternately heated and cooled by periodically reversing the direction of flow of gas through the reactor.