is meant the phenomenon by which the surface of the catalyst becomes coated with carbon thus blocking its pores and reducing its effectiveness it has now been found that this coking can be minimized if instead of the phosphoric acid alumina catalyst one uses an aluminosilicate catalyst containing 1 20 silica and 80 99 alumina surprisingly use of such a catalyst according to the invention not only reduces the amount of coking but also significantly increases the rate of the dehydration reaction over that obtained with the phosphoric acid alumina catalyst and greatly reduces the number and amounts of byproducts formed notably hydrogen carbon monoxide methane ethane propane ethylene propylene and various ethers having high boiling points the catalytic dehydration of methanol to form dimethyl ether is well known and is described in detail in u s pat no 2 014 408 to john c woodhouse the product of the reaction is principally a mixture of dimethyl ether unreacted methanol and water the reaction is ordinarily conducted continuously in a column reactor in the vapor phase generally at a maximum reactor bed temperature of 200 u00b0 500 u00b0 c preferably 275 u00b0 420 u00b0 c and a pressure ranging from ambient to 3447 kpa gauge preferably 1034 1723 kpa gauge the catalyst is packed into the reactor in the customary way and the vaporized and preheated 200 u00b0 300 u00b0 c methanol is passed through it preferably downwardly residence time of the methanol in the reactor is determined according to well known chemical engineering principles as are the methods of recovering the dimethyl ether from the reactor effluent and the methods of refining it the catalysts used according.