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61.
Brit Gunn Ersland Rune Teigland 《Numerical Methods for Partial Differential Equations》1993,9(3):265-283
Two different schemes for constructing coarse-grid operators are implemented in a linear multigrid code. In the first scheme, the construction of the coarse-grid operators is done using a variational approach. Certain conservation properties of the fine-grid matrices are shown to be preserved on the coarser grids by the variational construction. In the second scheme, the diffusion coefficients for the coarse grids are calculated by a simple restriction of the coefficient from the fine grid, using a flux conservation principle. The multigrid codes are then applied to solve the linear equations from an IMPES formulation of a two-phase porous-media flow model. A standard elliptic model problem with jump discontinuous coefficients is also solved using the two multigrid schemes. In simple cases of particular elliptic equations these two schemes are identical. However, in more general cases, such as in reservoir problems, these schemes differ. It is shown that multigrid efficiency typical of the constant coefficient cases is obtained for these problems involving discontinuous coefficients. © 1993 John Wiley & Sons, Inc. 相似文献
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Study of Fourier transform infrared-temperature-programmed desorption of benzene, toluene and ethylbenzene from H-ZSM-5 and H-Beta zeolites 总被引:1,自引:0,他引:1
In the present study, an infrared (IR) high temperature cell was used, in combination with a Fourier transform infrared (FTIR) spectrometer for the development of an alternative temperature-programmed desorption (TPD) procedure. Three different adsorbates, i.e., benzene, toluene and ethylbenzene were non-isothermally desorbed from two zeolites H-ZSM-5 and H-Beta. The FTIR-TPD profiles were fitted with the help of the complementary error function. The fitting process was carried out with the help of a computer program which allows us to calculate two parameters, the temperature, T0 (K) and the temperature range ΔT (K), which, in conjunction with the complementary error function, characterizes the FTIR-TPD profile. Was found that the parameter T0 is linked with the adsorption energy of the adsorbate in the zeolite and the parameter ΔT was correlated with the transport process of the desorbed molecules inside the zeolites during the desorption process and with the presence of more than one type of adsorption sites. In conclusion, was confirmed that the FTIR-TPD methodology is appropriate for in situ observation of adsorbed molecules on zeolites, and that this technique makes available information concerning the adsorbed state of guest molecules in non-isothermal desorption. 相似文献
66.
This work presents modeling results on the cleavage face of calcite as well as on steps and isolated kinks on this face. We used static lattice energy minimization and interatomic potentials fitted to bulk properties. The energy needed to cleave a bulk calcite crystal along the [1 0 (-)1 4] plane was calculated to be 0.59 J m(-2) in agreement with previous studies using the same potentials. The perfect surface reconstructs in the top few atomic layers, but its symmetry corresponds to the bulk termination. By contrast, the (1 0 (-)1 4) surface with cleavage steps present reconstructs to form a (2 x 1) super cell. This may help explain experimental observations of (2 x 1) symmetry on calcite surfaces. The energy required to form a monatomic obtuse step is calculated to be 1.3 x 10(-10) J m(-1) and for the acute step, 2.4 x 10(-10) J m(-1), suggesting that obtuse steps dominate on cleaved surfaces. Along the two types of steps, a total of 16 kink geometries exist. We calculated kink defect energy with two different approaches: one where kink pairs were added onto infinitely long steps and one where kinks were placed inside pits on a cleavage surface. Calculations on infinitely long steps show that for vacuum conditions, kink pairs possess roughly identical formation energy, about 1.2-2.2 eV, so based on energetics one cannot expect significant differences in kink site frequency 相似文献
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Dietzel PD Johnsen RE Fjellvåg H Bordiga S Groppo E Chavan S Blom R 《Chemical communications (Cambridge, England)》2008,(41):5125-5127
The microporous metal-organic framework Ni(2)(dhtp) (H(4)dhtp=2,5-dihydroxyterephthalic acid) shows distinct end-on CO(2) coordination to coordinatively unsaturated nickel sites giving rise to high CO(2) adsorption capacity at sub-atmospheric pressures and ambient temperatures. 相似文献
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Complexation of the bile salts (BS) taurocholate, tauro-beta-muricholate, taurodeoxycholate, taurochenodeoxycholate, glycocholate, glycodeoxycholate, and glycochenodeoxycholate common in rat, dog, and man with natural beta-CD and the chemically modified beta-CDs 2-hydroxypropyl-beta-CD and 2-O-methyl-beta-CD was studied using mobility shift ACE. The CDs were selected due to their frequent use in preformulation and drug formulation as oral excipients for the solubilization of drug substances with low aqueous solubility. ACE was demonstrated to be a feasible and efficient technique for investigation of the interactions between BS and beta-CDs. All the investigated BS possessed affinity for the three CDs with stability constants ranging from 2x10(3) to 4x10(5) M(-) (1). The requirements and assumptions related to the use of ACE for estimating high affinity stability constants were discussed. The extent and pattern of hydroxylation significantly influenced the affinity of the glyco- and tauro-conjugated BS toward the beta-CDs (chenodeoxycholates > deoxycholates > cholates) whereas the nature of the beta-CD derivatization and BS conjugation played a minor role only. The results indicate that displacement of drug substances from beta-CD inclusion complexes is likely to occur in the small intestine where BS are present potentially influencing drug bioavailability. 相似文献
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The complex marine alkaloid norzoanthamine (2) was envisioned to be assembled from three key building blocks: the C1-C5 fragment A, the C6-C10 fragment B, and the C11-C24 fragment C. The synthesis of fragment A was achieved in 14 steps and 33% overall yield from (R)-gamma-hydroxymethyl-gamma-butyrolactone. Fragment B was made in two steps from PMB-protected 4-pentynol in 76% yield. The C11-C24 fragment C was made from (S)-carvone via (R)-isocarvone in 18 steps (6% overall yield). The convergent stereoselective synthesis of the entire carbon framework (C1-C24) of the target molecule was achieved via the following assemblage. Alkenyl iodide 20 derived from the C11-C24 fragment C was coupled to fragment B (C6-C10) through a high-yielding Stille coupling reaction of these two sterically very demanding coupling partners, affording the key Diels-Alder precursor 24. The intramolecular Diels-Alder reaction proceeded smoothly in excellent yield and diastereoselectivity, generating the tricyclic trans-anti-trans perhydrophenanthrene motif of norzoanthamine (C6-C24). The final fragment coupling between lithiated fragment A (C1-C5) and aldehyde 40 (C6-C24) has also been successfully accomplished affording the entire carbon framework of the natural product. 相似文献
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