Process for preparing dimethyl ether and process for preparing a mixture of dimethyl ether and methanol

A process for preparing dimethyl ether or a mixture of dimethyl ether and methanol including passing a raw gas containing carbon monoxide and hydrogen through a catalyst slurry containing a mixture including (a) a medium oil having as a main component, a branched, saturated aliphatic hydrocarbon having 16 to 50 carbon atoms, 1 to 7 tertiary carbon atoms, 0 quaternary carbon atoms, and 1 to 16 carbon atoms in the branched chains bonded to the tertiary carbon atom; and at least one of the tertiary carbon atoms is bonded to hydrocarbon chains with a chain length having 4 or more of carbon atoms in three directions, (b) a methanol synthesis catalyst; (c) a methanol dehydration catalyst; and (d) a methanol shift catalyst or a methanol dehydration/shift catalyst.