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Adsorption: Journal of the International Adsorption Society (v.12, #5-6)
Adsorption and structural properties of channel-like and cage-like organosilicas by Rafal M. Grudzien; Bogna E. Grabicka; Mietek Jaroniec (pp. 293-308).
Channel-like and cage-like mesoporous silicas, SBA-15 (P6mm symmetry group) and SBA-16 (Im3m symmetry group), were modified by introducing single ureidopropyl surface groups, mixed ureidopropyl and mercaptopropyl surface groups, and single bis(propyl)disulfide bridging groups. These hexagonal and cubic organosilicas were prepared under acidic conditions via co-condensation of tetraethyl orthosilicate (TEOS) and proper organosilanes using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) amphiphilic block copolymer templates, P123 (EO20PO70EO20) and F127 (EO106PO70EO106). The modified SBA-15 and SBA-16 materials were synthesized by varying the molar ratio of organosilane to TEOS in the initial synthesis gel. The removal of polymeric templates, P123 and F127, was performed with ethanol/hydrochloric acid solution. In the case of SBA-15 the P123 template was fully extracted, whereas this extraction process was less efficient for the removal of F127 template from the SBA-16-type organosilicas; in the latter case a small residue of F127 was retained. The adsorption and structural properties of the resulting materials were studied by nitrogen adsorption-desorption isotherms at −196∘C (surface area, pore size distribution, pore volumes), powder X-Ray diffraction, CHNS elemental analysis and high-resolution thermogravimetry. The structural ordering, the BET specific surface area, pore volume and pore size decreased for both channel-like and cage-like mesoporous organosilicas with increasing concentration of incorporated organic groups.
Keywords: Channel-like mesostructures; Cage-like mesostructures; Ordered mesoporous organosilicas; Nitrogen adsorption; SBA-15; SBA-16; Co-condensation synthesis; Ureidopropyl surface group; Mercaptopropyl surface group; Bis(propyl)disulfide bridging group
Developments and structures of mesopores in alkaline-treated ZSM-5 zeolites by Yousheng Tao; Hirofumi Kanoh; Katsumi Kaneko (pp. 309-316).
ZSM-5 zeolites with SiO2/Al2O3 molar ratio of 24 were treated in 0.05 M aqueous sodium hydroxide solution at 325 K in different periods. The samples were characterized by means of nitrogen adsorption at 77 K, field emission scanning electron microscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy. Analysis of the experimental results showed that the alkaline treatment periods have influence on the developments and structures of mesopores in the alkaline-treated ZSM-5 zeolites. Alkaline treatment initially develops mesopores mainly from the boundary portion of MFI zeolites to the bulk, while prolonged treatment destroys the mesopores, and an optimum mesoporosity is obtained by the treatment for 1.5 h. On the other hand, crystallinities and short-range order in alkaline treated zeolites have remained virtually unchanged according to the examination from X-ray diffractometry and Fourier transform infrared spectroscopy.
Keywords: ZSM-5 zeolite; Post-synthesis treatment; Characterization of structure; Adsorption hysteresis; Nitrogen adsorption
Study on the adsorption of 1,5-, 1,6- and 2,6-dimethylnaphthalene on a series of alkaline and alkaline earth ion-exchanged faujasite zeolites by Natthakorn Kraikul; Pramoch Rangsunvigit; Santi Kulprathipanja (pp. 317-327).
Adsorption of the equilibrium mixture of 1,5-, 1,6- and 2,6-dimethylnaphthalene (DMN) was investigated on a series of alkaline and alkaline earth ion-exchanged faujasite zeolites using pulse test and breakthrough techniques. It was found that the adsorption of the DMNs would proceed through a complex mechanism governing by at least molecular diffusion, acid-base interaction, preferential interaction with cation and molecular displacement. All ion-exchanged faujasite zeolites only provided rejective systems for 2,6-DMN and some of them were potential adsorbents for the 2,6-DMN purification.
Keywords: Dimethylnaphthalene; Adsorption; Pulse test; Breakthrough; Faujasite; Zeolite
Effect of dopant addition on oxygen sorption properties of La-Sr-Co-Fe-O perovskite type oxide by Qinghua Yin; Y. S. Lin (pp. 329-338).
The paper reports on effect of doping Ag+, Ni2+, Ca2+, Ba2+ or Zr4+ in La0.1Sr0.9Co0.9Fe0.1O3−δ (LSCF1991) on its oxygen sorption capacity and desorption rate. The dopant can be incorporated into LSCF1991 matrix causing lattice expansion. The oxygen sorption capacity and tendency for the disordered perovskite to ordered brownmillerite phase transition for these samples decrease in the order: LSCF1991 > LSCF-Ag > LSCF-Ni > LSCF-Ca > LSCF-Ba > LSCF-Zr. The oxygen desorption rate also decreases in the same order for the doped samples. Doping increases the tendency of the disorder to order phase transition and enhances oxygen desorption rate during oxygen desorption step. Doping Ag and Ni provides more pronounced enhancement in oxygen desorption rate. The results suggest that Ag and Ni doped LSCF1991 samples are promising sorbents for use in a high temperature sorption process for air separation.
Keywords: Air separation; Perovskite-type oxides; Phase transition; Kinetics; Oxygen sorption capacity
MFI zeolite membranes from a- and randomly oriented monolayers by Jungkyu Choi; Shubhajit Ghosh; Lisa King; Michael Tsapatsis (pp. 339-360).
c-Oriented columnar MFI films made by secondary growth of randomly oriented seed monolayers, deposited using sonication-assisted covalent attachment, exhibit n-hexane/2,2-dimethylbutane separation factor of up to 104, n-/i-butane separation factor of up to 50, and p-/o-xylene separation factor of up to 2. A MFI film from a-oriented seed layer shows lower separation factors for the linear vs. branched isomers but higher separation factor for p-/o-xylene.
Keywords: Zeolite; MFI; Preferred orientation; Butane; Hexane; Xylene; Permeation; Separation
Trace carbon monoxide and hydrogen conversion prior to cryogenic distillation of air by R. Kumar; S. Deng (pp. 361-373).
Liquid Nitrogen is required in the semiconductor industry. This is generally produced by cryogenic distillation of air. However, trace levels of Carbon Monoxide and Hydrogen need to be removed from Nitrogen prior to its use in the semiconductor industry. This may be accomplished by catalytic conversion of trace Carbon Monoxide and Hydrogen to Carbon dioxide and Water, respectively. These impurities (Carbon dioxide and Water) are then removed by adsorption from air. The latest technology is to incorporate the catalytic conversion into adsorption based thermal swing pre-purification units, which are already used to remove Water and Carbon dioxide from air prior to its cryogenic distillation. Our experiments show that even though Hydrogen is converted to Water by a catalytic reaction, presence of Carbon dioxide in this stream significantly lowers the performance of the catalyst by as much as five-fold. Also, Hydrogen removal by the novel metal Pd catalyst does not follow a typical catalyst behavior but an adsorption breakthrough type behavior, i.e. for a constant inlet concentration the outlet concentration of Hydrogen breaks through at some time and then increases with time. On the other hand, Carbon monoxide conversion on a Hopcalite type catalyst follows typical catalyst behavior, i.e. for a constant inlet concentration the outlet concentration of Carbon monoxide is constant and does not change with time. Experimental data demonstrating these effects followed by a theoretical explanation are presented.
Keywords: Adsorption; Catalysis; Air-purification; Trace removal; Carbon monoxide; Hydrogen
Simulated moving bed technology: old and new by Pedro Sá Gomes; Mirjana Minceva; Alírio E. Rodrigues (pp. 375-392).
The Simulated Moving Bed (SMB) concept has been applied to the separation of different mixtures as a continuous counter current separation process, avoiding several problems related with solid motion. The aim of this work is to present some relevant examples of SMB separations corresponding to the two major ages in the use of the SMB concept, here named “old” and “new” applications. The “old” applications of SMB technology in the petrochemical industry are still important, with large and highly productive units; and the “new” applications of the second “age” of SMB concept are from the fine chemical, pharmaceutical and biochemistry areas, associated with the demand of high purity products during the last 10 years.Different examples are presented for different ages: a UOP Parex ® process for the “old”, modelled with the equivalent True Moving Bed (TMB) approach; and a chiral resolution for the “new”, modelled by the real SMB model. Some of the latest developments are also mentioned: the non conventional techniques as the Varicol ® process, PowerFeed, Modicon, M3C or Enriched Extract-SMB (EE-SMB), MultiFeed (MF), Outlet Streams Swing (OSS) or Pseudo-SMB, involving considerable changes in the SMB concept itself. The use of the last optimization/modelling packages for the development of design techniques, either at the conception stage as well as for performance improvements of existing units is emphasized.
Keywords: Simulated moving bed; True moving bed; Chromatographic separations; p-xylene; Chiral; Modelling; Simulation and optimization
Reliable measurement of near-critical adsorption by gravimetric method by Ronny Pini; Stefan Ottiger; Arvind Rajendran; Giuseppe Storti; Marco Mazzotti (pp. 393-403).
A gravimetric apparatus is used to measure the excess adsorption at high pressure. The equipment consists of a Rubotherm magnetic suspension balance, which allows to measure also the density of the fluid. In order to obtain the excess adsorbed amount, the measured weight has to be corrected with a buoyancy term, for which the density of the adsorbing fluid has to be known at each experimental conditions. Therefore the homogeneity of density in the high-pressure cell plays a fundamental role in determining the accuracy of the measured excess adsorbed amounts. This paper is intended to show the impact of the actual approach to thermostating the unit on the density distribution of the adsorbing fluid inside the high-pressure cell. Namely, by changing the inlet position of the heating fluid, large differences in the measured excess adsorption are produced. The closer to the critical point of the fluid, the stronger are these differences. An optimum configuration for our measuring device has been found and it has been used to study the adsorption of carbon dioxide (CO2) on Filtrasorb 400 activated carbon at supercritical and near-critical conditions.
Keywords: Supercritical CO2 ; Gravimetric method; Near-critical adsorption; Activated carbon
Interpretation of chromatographic retentions of simple solutes with an amylose-based sorbent using infrared spectroscopy and DFT modeling by Rahul B. Kasat; Chim Y. Chin; Kendall T. Thomson; Elias I. Franses; Nien-Hwa Linda Wang (pp. 405-416).
The interactions of amylose tris(3,5-dime- thylphenylcarbamate) (ADMPC, commercially “Chiralpak AD”) with 10 simple solutes—1-propanol, heptane, heptanol, benzene, propylbenzene, benzyl alcohol, pyridine, tetrahydrofuran, diethylamine, and aniline—are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin polymer films, DFT modeling, and high performance liquid chromatography (HPLC). ATR-IR is used to determine the changes in the hydrogen bonding states of the C=O and NH groups of the polymer amide I and II bands upon absorption of each of the solutes at 25∘C. DFT modeling with B3LYP/6-311+g(d,p) level of theory is used to predict the IR wavenumbers, the H-bonding interaction energies, and the hydrogen bonding distances of the polymer side-chains with certain solute molecules. The capacity factors of these solutes with ADMPC have been measured at 25∘C in hexane/isopropanol (95/5, v/v) solvent. From IR data and DFT modeling, we conclude that the C=O and NH are key binding sites of the polymer and interact with the functional groups of various solutes. The capacity factors are understood on the basis of hydrogen bonding, hydrophobic, and dipole-dipole interactions of the C=O, NH, and phenyl groups of the sorbent with OH, NH, NH2, O, phenyl, and N functional groups of the solutes.
Keywords: Capacity factors; Chiral separation; Amylose tris(3,5-dimethylphenylcarbamate); Hydrogen bonding; Hydrophobic interactions; Dipole-dipole interactions
Guest-specific diffusion anisotropy in nanoporous materials: Molecular dynamics and dynamic Monte Carlo simulations by P. Bräuer; A. Brzank; L. A. Clark; R. Q. Snurr; J. Kärger (pp. 417-422).
For anisotropic nanoporous materials, guest diffusion is often reflected by a diffusion tensor rather than a scalar diffusion coefficient. Moreover, the resulting diffusion anisotropy may notably differ for different guest molecules. As a particular class of such systems, we consider an array of two types of channels, mutually intersecting each other, where the rates of diffusion in the different directions depend on the nature of the guest molecules. The simultaneous adsorption of two types of guest molecules is considered, as in technical applications of porous materials such as catalysis. A case study is presented in which atomistic molecular dynamics (MD) and coarse-grained dynamic Monte Carlo (DMC) simulations are compared and shown to yield qualitatively similar results for non-steady-state diffusion. The two techniques are complementary. MD simulations are able to predict the details of molecular propagation over distances of a few unit cells, whereas the evolution of sorption profiles over distances comparable with entire crystallites can be studied with DMC simulations. Consideration of these longer length and time scales is necessary for applications of such systems in chemical separations and heterogeneous catalysis.
Keywords: Diffusion; Molecular traffic control; Zeolites; Molecular simulation
Ortho-selectivity in aluminophosphate molecular sieves: A molecular simulation study by S. Mardônio P. Lucena; Célio L. Cavalcante Jr; João A. F. R. Pereira (pp. 423-434).
Monte Carlo adsorption simulations of xylenes have been performed in aluminophosphate molecular sieve structures. A new force field fitted for o-xylene in AlPO4-5 was used. It is shown that force fields have good transferability among the aluminophosphate sieves series and the new force field adequately describes the experimentally observed adsorption isotherms for xylene/AlPO4-5. A previous investigation of adsorption isotherms and structural analysis has been extended to AlPO4-8 and VPI-5 sieves. In AlPO4-8, like in AlPO4-5, the variations in the channels diameters and the corresponding interaction energy of the molecule-crystal lattice drive all molecular positioning. In VPI-5, the modulation between wide and narrow regions becomes negligible due to the larger pore diameter, so no ortho-selectivity was observed. The simulations confirm the ortho-selectivity mechanism proposed to aluminophosphates.
Keywords: Aluminophosphates; Adsorption; Xylenes; Molecular simulation