1. Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, GB Marg, Bhavnagar 364 002, India, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Department Chemie, Physikalische Chemie, LMU München, Butenandtstrasse 5-13 (Haus E), 81377 München, Germany