Process for preparing acetic acid by gas-phase oxidation of saturated C4-hydrocarbons and their mixtures with unsaturated C4-hydrocarbons

A process for preparing acetic acid by gas-phase oxidation of saturated C4-hydrocarbons and their mixtures with unsaturated C4-hydrocarbons in a tube reactor comprising

• (1) providing in said tube reactor a coated catalyst comprising an inert nonporous support body having an outer surface and a catalytically active mixed oxide composition coating applied to the outer surface of the support body, said mixed oxide composition comprising
• (a) one or more oxides selected from the group consisting of titanium dioxide, zirconium dioxide, tin dioxide and aluminum oxide as component (a) and
• wherein the coated catalyst further comprises, as component (a), one or more oxides selected from the group consisting of the oxides of boron, silicon, hafnium, niobium, lanthanum and cerium in an amount of from 1 to 30% by weight, based on the total weight of the component (a),
• (b) from 0.1% to 1.5% by weight, based on the weight of the component (a) of vanadium pentoxide as component (b);
• and
• replacing in component (b) of the coated catalyst, part of the vanadium pentoxide by one or more additional oxides selected from the group consisting of molybdenum, chromium, antimony, alkali metals, alkaline earth metals, elements of main groups V and VI of the Periodic Table of the Elements and the transition metals as an additional component (b);
• and
• (2) reacting a gas mixture comprising oxygen or oxygen-containing gas, one or more C4-hydrocarbons selected from the group consisting of a saturated C4-hydrocarbon and a mixture of a saturated C4-hydrocarbon with an unsaturated C4-hydrocarbon and water vapor and having a C4-hydrocarbon/air (oxygen) volume ratio of from 0.2/99.8 to 25/75 and a C4-hydrocarbon/water vapor volume ratio of from 1/1 to 1/60 in contact with the coated catalyst at a temperature of from 100° C. to 400° C. and a gauge pressure of from 0.2 to 50 bar to produce the acetic acid.