In the late 1970s, it was found that post-modification with a phosphorus precursor can change the acidic and shape-selective properties of zeolites. The most notable phosphorus-containing precursor is H3PO4, which is added to a zeolite by wet impregnation. After this step, the zeolite is thermally treated in air/oxygen, a process better known as calcination, which decomposes phosphorus precursors into (poly)phosphates and phosphorus pentoxide. It has been found that phosphorus can act towards zeolites either as a promoter, e.g., in the fluid catalytic cracking (FCC) process and the methanol-to-hydrocarbons (MTH) reaction, or as a poison, as in ammonia selective catalytic reduction (NH3-SCR) of exhaust gases