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The structure of acid sites in cobalt-substituted aluminophosphates (AlPOs) catalysts has been investigated, with EXAFS spectroscopy. Data obtained, using in situ methods, for the Co K-edge spectra of cobalt-substituted AlPO-5, AlPO-18, AlPO-36, and APSO-44 in as-prepared, calcined, and reduced states yield the local structure of the Co ions. Whereas the as-prepared materials clearly contain Co(II) ions in regular four-coordinated sites, complex behavior is exhibited by their calcined analogues, with essentially complete oxidation of Co(II) to Co(III) in CoAlPO-18, with the local coordination of the high-spin Co(III) being distorted. By contrast incomplete oxidation of the Co(II) is observed for CoAlPO-5 and CoAlPO-36. A combination of EXAFS data analysis with the results of computer modeling studies suggests that this behavior is interpretable in terms of the formation of oxygen vacancies by dehydroxylation which lead to undercoordinated Lewis acid, Co(II) sites. IR data of the reduced samples reveal the presence of Brønsted acid sites which, from the EXAFS analysis, are seen to correspond to Co(II) species in a distorted environment owing to the presence of a neighboring protonated oxygen ion. The implications of the results for the catalytic properties of the systems are considered.

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A novel topology, consisting of a three‐dimensional system of large pore channels, is found for the zeolite ITQ‐7 (see picture). This material has been synthesized by combining the structure‐directing effects of fluoride ions and a new organic cation purposefully designed to promote the crystallization of a low‐density silica phase. ITQ‐7 is the least dense crystalline silica phase known so far.

Pure silica octadecasil (AST) has been synthesized hydrothermally in a fluoride medium using a new structure-directing agent (tert-butyltrimethylammonium). Fis occluded in the material inside the small [4 6 ] cage and is removed completely upon calcination, in sharp contradiction with recent predictions based on computational studies. We propose chemically feasible pathways for the migration of Fout of the [4 6 ] cage that are based on the known chemical reactivity of Fand SiO 2 and on the presence of small cations formed by thermal degradation of the organic species. After calcination to remove the guest species, the structural integrity of the host is completely preserved, yielding a pure SiO 2 framework devoid of connectivity defects. Its structure has been solved by direct methods and refined using synchrotron X-ray powder diffraction data. Despite the removal of guest species, the framework is seen to adopt a tetragonal symmetry rather than the cubic maximum topological symmetry. It is proposed that symmetry lowering in AST is a consequence of the energetically unfavorable angles in the higher space group, in which 20% of the Si present four linear Si-O-Si angles. 29 Si MAS NMR spectroscopy and energy minimization calculations support this notion and the refined structure proposed. At 500 °C calcined octadecasil undergoes a reversible phase transition to pseudocubic symmetry. Finally, the reported AlPO 4 -16 or octadecasil cubic forms are proposed to actually be disordered pseudocubic structures.

The new synthetic form of microporous crystalline silica, denoted as ITQ-12, shows a high potential for the separation of propane and propene from its mixtures.

Pure silica ITQ-4, a zeolitic material with large one-dimensional 12MR channels and an unexpectedly large microporous void volume, can be synthesized in the presence of fluoride ions and N-benzylquinuclidinium cations within a wide range of synthesis pH. Substitution of the tertiary carbon of the quinuclidine moiety by a N atom still renders the organic cation as a suitable structure-directing agent for this material. The role of F- anions and the influence of the pH of the synthesis on both the phase selectivity of the crystallization and the presence of connectivity defects in the final material are discussed. In the presence of F-, defect formation is observed to be pH-dependent, and there appears to be a threshold pH for the generation of a constant defect concentration. A plausible explanation for this observation based upon the ionization state of the condensing species is also presented. The structure of as-made ITQ-4 has been obtained and refined from high-resolution powder XRD data, and the organic and fluoride ions have been located. The sinuosity of the channel of ITQ-4 is found to coincide with the bent geometry observed for the structure-directing agent, while the fluoride ions are found to reside within a small [43526] cage located around the periphery of the central pore space.