Cyrus E. Crowder

VPI-5: The first molecular sieve with pores larger than 10 Ångstroms

The adsorption properties of a novel family of aluminophosphate based molecular sieves denoted as VPI-5 are described. These molecular sieves are the first to contain pores larger than 10 A. The large pores of the VPI-5 sieves consist of channels circumscribed by eighteen membered rings and possess free diameters of approximately 12–13 A. The VPI-sieves are capable of adsorbing molecules excluded from other molecular sieves.

Hypothetical structures of magadiite and sodium octosilicate and structural relationships between the layered alkali metal silicates and the mordenite- and pentasil-group zeolites

Hypothetical model structures for magadiite and sodium octosilicate, based on the structure of the zeolite dachiardite, are proposed that consist of layers of 6-member rings of tetrahedra and blocks containing 5-member rings attached to both sides of the layers. The infrared (IR) and nuclear magnetic resonance spectra of magadiite and sodium octosilicate have features in common with spectra of zeolites in the ZSM-5 and mordenite groups. A peak at 1225 cm-1 in the IR spectra of magadiite and sodium octosilicate is characteristic of zeolites containing 5-member rings, such as ZSM-5- and mordenite-type zeolites. The defect structures of pentasil zeolites may therefore be akin to layered alkali metal silicates containing zeolite-like domains, in which part of the silanol groups from adjacent silicate layers are condensed (cross-linked) forming siloxane linkages.

Structure refinement and water location in the very large-pore molecular sieve VPI-5 by X-ray Rietveld techniques

Abstract The structure of the very large pore molecular sieve VPI-5 was solved based on a combination of X-ray power diffraction measurements and modeling techniques. The original structure refinement was hampered by the fact that the water molecules, which represent 25% of the scattering material, could not be located. In the present work, one VPI-5 family member was analyzed using X-ray Rietveld refinement. Rietveld refinement results show that the reported structure is correct. All the water molecules were located and refined in the 18—ring pore. The water molecules form weakly associated layers within the pore, in a manner not unlike that of liquid water. VPI-5, AlPO4·2 H2O, is hexagonal with space group P63cm and cell parameters of a = 18.9777(3), c = 8.1155(1) A ; V = 2531.4(1) A 3 ; D calc = 1.243 g cm −3 . Small errors in the refinement were found to be caused by a second phase of a slightly dehydrated VPI-5. This was the reason for a high final refinement residual, Rwp, of 0.31.

Establishing an Instrumental Peak Profile Calibration Standard for Powder Diffraction Analyses: International Round Robin Conducted by the JCPDS-ICDD and the U.S. National Bureau of Standards

With the explosive growth in the number of highly automated powder diffraction systems, many types of analyses which were previously considered a specialty analysis are now performed on a routine basis. Algorithms have been developed for measuring peak profiles from which crystallite sizes, residual microstrain, and X-ray crystal structure (Rietveld techniques for example) can be determined. However, these techniques require an instrumental peak profile calibration standard to correct the experimental data for instrumental broadening due to the system optics. Significant problems are encountered when laboratories try to cross-correlate or reproduce published data due to the lack of a common reference material for instrumental calibration. This is particularly distressing in microstrain and crystallite size calculations which can be dramatically affected by a poor choice of standard materials. Microstrain and crystallite size measurement are becoming increasingly important for the characterization of advanced materials and catalysts.

Reference materials for the study of polymorphism and crystallinity in cellulosics

Eighty specimens of cellulosic materials were analyzed over a period of several years to study the diffraction characteristics resulting from polymorphism, crystallinity, and chemical substitution. The aim of the study was to produce and verify the quality of reference data useful for the diffraction analyses of cellulosic materials. These reference data can be used for material identification, polymorphism, and crystallinity measurements. Overall 13 new references have been characterized for publication in the Powder Diffraction File (PDF) and several others are in the process of publication.

The effect of stoichiometric variations on the magnetic structure of Y6Mn23

Abstract As a consequence of large variations in previously reported magnetic moments of manganese on the four non-equivalent crystallographic sites of Y6Mn23, it was suggested that stoichiometric variations in composition might be responsible. Accordingly, non-stoichiometric ingots of Y6Mn23 were prepared ranging in composition (precast analysis) from Y6Mn21 to Y6Mn26. On the basis of atomic absorption analysis, high precision back-reflection X-ray diffraction and Rietveld profiling of powder neutron diffraction data, we conclude that extreme variations in stoichiometry cause little change in the magnetic moment magnitudes and that the variable results reported probably arise from the presence of small amounts of a second phase, YMn2 in the manganese-deficient samples and YMn12 in the manganese-rich samples.

Crystal structure of folic acid dihydrate, C29H29N2O6(H2O)2

The crystal structure of folic acid dihydrate has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. Folic acid dihydrate crystallizes in space group P 2 1 2 1 2 1 (#19) with a = 7.275 78(3), b = 8.632 17(4), c = 32.417 19(22) A, V = 2035.985(18) A 3 , and Z = 4. The structure is dominated by a three-dimensional network of hydrogen bonds. The dicarboxylic acid side chain occurs in a bent conformation, helping explain the ability of folate derivatives to coordinate metal cations. The powder pattern has been submitted to ICDD for inclusion in future releases of the Powder Diffraction File™.

A green synthesis of bis[1-(hydroxy-κO)-2(1H)-pyridinethionato-κS2]-(T-4)-zinc (zinc pyrithione) nanoparticles via mechanochemical milling

Particulate bis[1-(hydroxy-κO)-2(1H)-pyridinethionato-κS2]-(T-4)-zinc (zinc pyrithione; ZPT) in the diameter range 0.5–0.7 µm is a US FDA-approved anti-dandruff active widely used in anti-dandruff shampoos. A nanoparticulate form of ZPT is expected to exhibit a higher activity, be distributed more effectively on the scalp, require less thickening agent in the shampoo formulation to ensure its stability against settling than the standard form of ZPT, and would enable clear anti-dandruff shampoo formulations. We demonstrate, for the first time, that a green, mechanochemical nanoparticle synthesis process can be used to prepare nanoparticulate ZPT from zinc chloride and sodium pyrithione monohydrate. Both a Reeves attrition mill and a Retsch MixerMill were found to be effective tools for delivering the mechanical energy needed for the conversion. The infrared spectra and X-ray powder diffraction patterns for the products correspond to those for the known desired material. Transmission electron microscopic analysis indicates that ZPT nanoparticles with primary particle diameters in the range of 20–200 nm (mean diameters of 65–100 nm) can be obtained via this method.

X-ray powder diffraction data for beryllonite from Stoneham, Maine, U.S.A.

Observed and calculated X-ray powder diffraction data are given for the mineral beryllonite, NaBePO 4 , from Stoneham, Maine, U.S.A. Diffractometer data taken with CoK α 1 radiation and Guinier film data registered with CuK α 1 radiation are compared. Two independent procedures were employed to index the patterns.