For example, the mixed multivalent catalyst may include a divalent metal compound, such as hydroxide or carbonate of calcium, magnesium or other alkaline earth metal; a trivalent metal compound, such as, for example, boric acid or other compound of boron or aluminum; and a tetravalent metal compound, such as a compound of formula (3-a): M1-(OR3)4???(3-a) where M1 represent a tetravalent metal, suc