) are also essentially inef-

Commercial synthetic catalysts are amorphous and contain more silica than is called for by the preceding formulas; they are generally composed of 10-15% alumina (Al20 3) and 85-90% silica (Si02). The natural materials montmorillonite, a nonswelling bentonite, and halloysite are hydrosilicates of aluminum with a well-defined crystal structure and approximate composition Al20 3 · 4Si20 · xH20. Some of the newer catalysts contain up to 25% alumina and are reputed to have a longer active life.