Abstract: To effect a quasi-isothermal catalytic reaction of SO.sub.2 to SO.sub.3, a plurality of consecutive rows of banks of cooling tubes, are provided in the bed of shaped catalyst bodies in a contact reactor in such a manner that the banks of succeeding rows are staggered from the banks of the preceding rows. The distance between the cooling tubes in the banks is so selected that a penetration of entire catalyst bodies into the banks will virtually be prevented. The banks are so designed that the pressure drop of the gas in the banks is approximately the same as the pressure drop in the layer of catalyst between the bank. The distance between the banks is larger than the width of the banks.

Abstract: Acrolein and methacrolein are prepared advantageously by oxidizing propylene or isobutylene, respectively, with an oxygen-containing gas mixture under conventional conditions over a supported catalyst possessing a firmly adhering coating, from 150 to 1,500 .mu.m thick, which contains calcined catalytic material based on molybdenum oxide and has a carrier core with a surface area of less than 15 m.sup.2 /g and a diameter of more than 100 .mu.m, if a coated catalyst is used which has been prepared by applying calcined catalytic material having a particle size of from 0.1 to 300 .mu.m, in an amount of from 1 to 40 g/min/liter of carrier and water in a weight ratio of catalytic material to water of from 1:1 to 8:1, continuously and separately from one another, each at a constant speed, at below 100.degree. C.