Monday, July 26, 2010

J. Phys. Chem. C, vol.114, Issue 29

Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2

Maria Castro†, Valerie R. Seymour†, Diego Carnevale†, John M. Griffin†, Sharon E. Ashbrook*†, Paul A. Wright*†, David C. Apperley‡, Julia E. Parker§, Stephen P. Thompson§, Antoine Fecant and Nicolas Bats

J. Phys. Chem. C, 2010, 114 (29), pp 12698–12710
DOI: 10.1021/jp104120y
Copyright © 2010 American Chemical Society

Abstract: Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO4 form of STA-2, bis-diazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state 13C and 15N MAS NMR, supported by 14N and 1H-15N HMQC solution NMR and density functional calculations. As-prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al12P12O48(OH)2·BDAB, P1, a = 12.3821(2) Å, b = 12.3795(2) Å, c = 12.3797(3) Å, α = 63.3585(8)°, β = 63.4830(7)°, γ = 63.4218(7)°) with the distortion from rhombohedral R symmetry resulting from the partial order of hydroxide ions in bridging Al−OH−Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO4 STA-2 with a pore volume of 0.22 cm3 g−1 (R, Al36P36O144, a = 12.9270(2) Å, c = 30.7976(4) Å). Dehydrated calcined AlPO4 STA-2 has two crystallographically distinct P and Al sites: 31P MAS NMR resolves the two distinct P sites, and although 27Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D 27Al → 31P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via Al−O−P connections in a 3:1 ratio (and vice versa for P linked to different Al). The 27Al and 31P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO4(SAT) framework.

Propane Aromatization on Zn-Modified Zeolite BEA Studied by Solid-State NMR in Situ

Anton A. Gabrienko†, Sergei S. Arzumanov†, Dieter Freude‡ and Alexander G. Stepanov*†
J. Phys. Chem. C, 2010, 114 (29), pp 12681–12688
DOI: 10.1021/jp103580f
Copyright © 2010 American Chemical Society

Abstract:The conversion of propane (propane-1-13C and propane-2-13C) on Zn/H-BEA zeolite at 520−620 K has been studied by 1H and 13C (CP) MAS NMR. Propene adsorption complex with zinc sites (π-complex) and σ-allylzinc species as intermediates have been identified in the course of propane conversion to aromatics. The mechanism leading to the formation of methane and ethane, which are constituents of an undesirable route in propane conversion, has been examined by kinetic modeling of the expected reaction network based on in situ 1H MAS NMR kinetic measurements of the reaction performance. The pathways for propane aromatization and hydrogenolysis have been proposed. Hydrogenolysis of propane has been concluded to occur with the involvement of both Brønsted acid sites and Zn sites.

13C Chemical Shift of Adsorbed Acetone for Measuring the Acid Strength of Solid Acids: A Theoretical Calculation Study

Hanjun Fang†‡, Anmin Zheng*†, Yueying Chu†‡ and Feng Deng*†
J. Phys. Chem. C, 2010, 114 (29), pp 12711–12718
DOI: 10.1021/jp1044749
Copyright © 2010 American Chemical Society

Abstract: Adsorption of basic probe molecules is one of the widely used methods to characterize the acid strength of solid acids. In this contribution, the adsorptions of acetone on various Brønsted and Lewis acid sites (from weak acid to superacid) are theoretically studied, in order to elucidate the quantitative relationships between 13C chemical shifts of acetone and intrinsic acid strength of solid acids. The Brønsted acid sites are represented by a series of 8T zeolite models with varying terminal Si−H bond lengths, and the different extents of acidic proton transfer from these acid sites to acetone are revealed explicitly. We found that three adsorption conformations (hydrogen-bonded, proton-shared, and ion-pair) exist for acetone, and concurrently, a correlation of three-broken lines is obtained for the 13C chemical shift of acetone versus the deprotonation energy (DPE). The correlation can be used as a scale for quantitatively measuring the Brønsted acid strength of solid acids. A threshold of 245 ppm is determined for superacidity, in good agreement with the experimental value (244 ppm). The Lewis acid sites are modeled by tricoordinate framework aluminum species and various extra-framework aluminum cations or neutral species such as Al3+, AlO+, AlOH2+, Al(OH)2+, Al(OH)3, and AlOOH. We found that acetone is coordinately adsorbed on the aluminum atoms of Lewis acid sites and that the 13C chemical shift of acetone is almost linear to the lowest unoccupied molecular orbital (LUMO) energy of the acid sites.

No comments: