Alkylation is a chemical process by which an alkyl group is attached to an organic substrate molecule via addition or substitution. An alkyl group is an alkane molecule that is missing a hydrogen atom. For example, methyl groups are the simplest alkyls and result from the removal of a hydrogen atom from methane. Alkyl groups substitute or add to molecules, such as carbocations, carboanions, radicals or carbenes. Alkyl groups may bond to a number of atoms including carbon, nitrogen and oxygen atoms in substrate molecules. Alkylating agents are typically olefin, alcohols, sulfates halides and various nitrogen containing compounds that promote alkylation by enabling alkyl groups to selectively bond to molecules. Often, alkylation requires a catalyst. Common alkylating catalysts are acids, such as HF or H2SO4. In certain processes, zeolites or Lewis acids are used as alkylation catalysts.