These conversions are catalyzed by zeolites as heterogeneous catalysts. Nitriles are susceptible to hydrogenation over diverse metal catalysts. Typical catalysts are zeolites, i.e. solid acid catalysts, and sulfuric acid. Some trimerization reactions were generated with organoyttrium compounds as catalysts. Reduction of aromatic nitro compounds with hydrogen over metal catalysts gives anilines. Basic substances can be used as insoluble heterogeneous catalysts for chemical reactions. Proteins that act as catalysts in biochemical reactions are called enzymes. Alcohols are also good alkylating agents in the presence of suitable acid catalysts. In the presence of metal catalysts, sodium borohydride releases hydrogen. Enzymes are examples of homogeneous catalysts. Zeolites are also widely used as catalysts and sorbents. Modified heterogeneous silver catalysts are typically employed. This process involves passing the gaseous reactants over a bed of nickel heterogeneous catalysts. In the presence of acid catalysts, ethanol converts to ethylene. Platinum is used in both the reduction and the oxidation catalysts.