Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Experimental data on the self‐diffusivities, D_i,self, of a variety of gases (CH₄, N₂, Kr, C₂H₆, and C₃H₈) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q_i. In LTA, D_i,self appears to increase with q_i for all molecules except N₂. In FAU and in MFI the D_i,self shows a sharp decrease with increasing q_i. In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self‐diffusivities of seven gases (CH₄, N₂, Kr, C₂H₆, C₃H₈, Ar, and Ne) in six different all‐silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D_i,self with q_i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one‐dimensional channels (AFI) and intersecting channel structures (MFI), the D_i,self invariably decreases with increasing q_i. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D_i,self decreases with q_i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D_i,self often exhibits a sharp increase with q_i, reaches a maximum and reduces to near‐zero values at saturation. The sharpness with which D_i,self increases with q_i, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D_i,self with q_i.     

A Molecular Dynamic Investigation of the Diffusion of Methane‐Ethane and Methane‐Propane Mixtures in Zeolites

Molecular Dynamic (MD) simulations were carried out to determine the Maxwell‐Stefan (M‐S) diffusivities, ?_i, and self‐diffusivities, D_i,self, of methane (C1), ethane (C2), and propane (C3) for a variety of molecular loadings, q_i, in three classes of zeolite topologies: (1) intersecting channels (MFI, ISV, BEA), (2) one‐dimensional (1D) channels (AFI, TON, FER, MOR, LTL), and (3) cages separated by windows (FAU, LTA, ERI, CHA, DDR). The ?_i are strongly dependent on loading, decreasing to zero at saturation loading in all cases. For 1D channels, the decrease of ?_i with q_i is severe. For cages separated by narrow windows (LTA, ERI, DDR, CHA), the ?_i increase sharply with q_i before eventually reducing to zero at saturation loading. Correlation effects are reflected in the ratio of the self‐ to M‐S diffusivity, D_i,self/?_i; this ratio is seen to be strongly dependent on the topology. Correlation effects are negligibly small in zeolite structures with cages separated by narrow windows. For binary C1–C2 and C1–C3 mixtures in both intersecting channel structures and 1D channels, the D_i,self of the more mobile species, C1, is reduced significantly due to the presence of the more tardy C2 or C3. The mobility of the tardy species is enhanced due to the presence of the mobile C1. For cages separated by narrow windows, the inter‐cage hops are practically independent and there is no accelerating or decelerating effects during mixture diffusion.     

Impact of different die draw ratio on crystalline and oriented properties of polypropylene cast films and annealed films

Four kinds of polypropylene (PP) cast films with different die draw ratios (DDR) were prepared. The impact of different DDR on the crystalline and oriented properties of PP cast films and annealed films was explored herein. Wide angle X-ray diffraction (WAXD) and fourier transform infrared (FTIR) methods were adopted to examine the orientation degree of crystalline and amorphous phases. Long period distance (L_p) of the crystalline structure was tested by small angle X-ray scattering (SAXS). Crystallization was determined by differential scanning calorimeter (DSC). The oriented and crystalline behaviors of the samples were carried out by the elastic recovery (ER) testing. Then, samples after being annealed were examined by the same methods. The influence of annealing process on the films’ structures and properties was explored. Besides, the final stretched microporous membranes manufactured via stretching the annealed films along machine direction were examined by scanning electronic microscope (SEM). No matter for cast films or for annealed films, it is found that the films’ orientation degree of crystalline and amorphous phases, as well as L_p and crystallinity are larger at higher DDR and relatively lower at lower DDR. When the DDR is overly high (DDR?=?170), both the oriented and crystalline properties will decline. Elastic recovery testing indicates that a film with better orientation of the crystalline and the amorphous phases as well as with higher crystallinity can be obtained at an appropriate DDR. SEM images show that stretched membranes with better microporous structure can be obtained when the precursor film is prepared at a proper DDR.     

Selective oxidation of CO in the presence of H2, CO2 and H2O, on different zeolite-supported Pt catalysts

The selective oxidation of CO in the presence of H₂O and CO₂ has been studied on Pt supported on different zeolitic materials (MOR, ZSM-5, FAU and ETS-10) using a range of operating conditions and a variety of characterization techniques. The behavior of the Pt–ETS-10 and Pt–FAU catalysts has been investigated in more depth and the results obtained have been compared and related to the different characteristics of the supports. The best results in the presence of H₂O and CO₂ were obtained with Pt–FAU catalysts, showing stable catalytic activity and complete conversion of CO (λ = 2) at 439 K.     

Characterization and catalytic behaviors of methylamine modified FAU zeolites

The methylamine (MA) modified FAU zeolites with 5.8 and 80 of silica alumina molar ratio were successfully prepared by the treatment of original FAU zeolites with MA. The obtained materials were investigated with diffuse reflectance infrared Fourier transform spectroscopy, X-ray diffraction, magic-angle spinning nuclear magnetic resonance, thermogravimetry, temperature-programmed desorption of CO₂ and NH₃ (CO₂-TPD and NH₃-TPD), nitrogen content analysis, and specific surface area analysis. The formations and thermal stabilities of different Si–N–C groups in the MA modified FAU zeolites were confirmed. The changing of aluminium species due to the interaction of MA and framework/extraframework aluminium (FAL/EFAL) in the modified samples was discovered. The acidity and basicity of the MA modified FAU zeolites were correlated to the hybrid aluminium species and variation of Si–N–C groups, respectively. The catalytic behaviors of the MA modified FAU zeolites were explored by Knoevenagel condensation reaction of benzaldehyde and malonic acid.     

Synthesis and adsorption property of zeolite FAU/LTA from lithium slag with utilization of mother liquid☆

Co-crystalline zeolite FAU/LTA-0 was synthesized by hydrothermal method from lithium slag. To make the most of excess silicon and alkali sources in mother liquid derived from FAU/LTA-0, zeolite FAU/LTA-1b was synthesized in the same method with the use of mother liquid by adding a certain amount of aluminum source. Influences of different adding ways of aluminum source and recycling dosages of mother liquid on synthesis of zeolites FAU/LTA with mother liquid were investigated. The phase, microstructure and thermostability of FAU/LTA-0 and FAU/LTA-1b were characterized by XRD, SEM and TG-DTA. The calcium and magnesium cation exchange capacities (CECs) of the zeolites were determined. The results have shown that co-crystalline zeolite can be synthesized with the use of mother liquid by adding aluminum source after 2 h of reaction. Compared with FAU/LTA-0, the crystal twinning structure of FAU/LTA-1b became weaker, the grain size was smaller, and the thermostability was better. With a lower dosage of mother liquid, the content of P-type impurity in product decreased significantly, and the content of LTA phase increased. The reuse rate of mother liquid can reach 48%. The CECs of FAU/LTA-1b-150 can reach 343 mg CaCO3·g?1 and 180 mg MgCO3·g?1, showing more excellent adsorption capacities than FAU/LTA-0 and commercial zeolite 4A. The full recycling use of mother liquid to synthesize zeolite FAU/LTA which can be applied for detergent not only improves resource utilization but also reduces production cost.     

A fractal-like kinetics equation to calculate landfill methane production

Landfill appears as a convenient choice to get rid of municipal solid waste while providing energy, due to methane generated through anaerobic fermentation. However, without capture and treatment landfill gas is considered an important source of atmospheric methane. The control and use of this gas require knowledge of both, current yield and long-term accumulative production. These values are usually calculated with mathematical expressions that consider 100% of conversion, and homogeneous chemical reactivity inside the fill. Nevertheless, fermentation in landfills is erratic and spatially heterogeneous. This work introduces a fractal-like chemical kinetics equation to calculate methane generation rate from landfill, Q_CH4 (m³/year), in the way: where fermentable wastes are partitioned in readily, moderately and slowly biodegradable categories, L₀ is the potential of methane yield of refuse (m³/tonne under standard conditions), d_s is the solid-phase fracton dimension, k_i is the reaction kinetics constant of waste category i (year⁻¹), and t_j is the time from the year of burying j (year), C_ij⁰ (kg/tonne) and M_ij (kg) are the initial concentration and the mass of waste category i landfilled in year j, respectively. The idea behind this equation is that methane production kinetics is limited by the diffusion of hydrolyzed substrate into a heterogeneous solid-phase towards discrete areas, where methanogenesis occurs. A virtual study for a hypothetical case is developed. The predictions from this fractal approach are contrasted with those coming from two equations broadly used in the industrial work. The fractal-like kinetics equation represents better the heterogeneous nature of the fermentation in landfills.     

Crystallinity of large single crystals of FAU-type zeolites with a wide range of Si/Al ratios

Large colorless single crystals of FAU-type zeolites were synthesized from gels with the composition xSiO₂ : 2.0NaAlO₂ : 7.5NaOH : 454H₂O : 5.0TEA, where x = 2.0–6.0. FAU-type zeolite with Si/Al = 1.26(4) was nearly pure and the maximum size of the single crystals was ca. 150 μm. In case of FAU-type zeolites with Si/Al = 1.54(5), the maximum size of single crystals was ca. 200 μm and the ratio of FAU/impurity was 0.07. The framework Si/Al ratio of the as-synthesized FAU-type zeolite tended to increase with the Si/Al ratio of gel composition. All of the large single crystals had good crystallinities for single-crystal X-ray diffraction, leading to enough numbers of significant reflections which have strong intensity. The structure of a single crystal of dehydrated zeolite Na-X (Si/Al = 1.41(4)) with composition |Na₈₀|[Si₁₁₂Al₈₀O₃₈₄]-FAU per unit cell was determined by X-ray diffraction methods in the cubic space group \( Fd \bar{3} m; \) a = 24.9434(6) Å at 294 K. The structure was refined by using all intensities to the final error indices (using only the 771 reflections for which F _o > 4σ(F _o)), R ₁ = 0.048 (based on F) and R ₂ = 0.188 (based on F ²). In the crystallographic studies, the Si/Al ratio of the synthetic FAU-type zeolite is 1.41(4) which is quite consistent with the SEM–EDS analysis.     

TiO2 photocatalyst for degradation of organic compounds in water and air supported on highly hydrophobic FAU zeolite: Structural,sorptive, and photocatalytic studies

Preparation of highly hydrophobic FAU zeolite has been achieved by a two-step preparation method that comprises predealumination treatment using concentrated mineral acids and subsequent calcination treatment. The hydrophilic/hydrophobic character of the zeolites was estimated by detailed adsorption measurements using water and toluene molecules as adsorbates, which showed the strict hydrophobic nature of the prepared FAU zeolites. By means of TG, FT-IR, and ²⁹Si MAS NMR analyses, it was concluded that the enhancement in hydrophobicity originates from the healing of silanol defect sites and the formation of a refined silica surface with fewer adsorption sites. Thus-prepared FAU zeolite, with high crystallinity, structural and thermal stability, and hydrophobicity, significantly improved the photocatalytic degradation rates of 2-propanol in water and acetaldehyde in air on TiO₂ when it was used as a catalyst support. A distinct correlation was found between the hydrophobicity of zeolites and photocatalytic activity of the supported TiO₂.     

Preparation of CPB-modified FAU zeolite for the removal of tannery wastewater contaminants

The preparation of organomodified zeolites with different framework structures (FAU, LTA and MOR) using N-cetylpyridinium bromide (CPB) as tailoring agent was studied. The sorption properties of CPB-modified zeolites were evaluated in the removal of tannery contaminants from aqueous solution. The CPB-modified FAU-type zeolite presented the highest Cr(VI) retention capacity (37 mmol/kg) due to the higher Cr(VI) retention of its unmodified form (larger pore opening) and its high CPB sorption capacity. CPB-modified FAU zeolite also exhibited high thermal stability as consequence of special interactions between the CPB molecules and the zeolite surface. In addition, the intrinsic Cr(III) exchange capacity of FAU zeolite increased with CPB loading (162–527 mmol/kg), which appear to be due to an additional retention mechanism provided by the sorbed cetylpyridinium surfactant layer. On other hand, CPB-modified FAU zeolite also exhibited high toluene retention capacity (62 mmol/kg) due to of the hydrophobic character of its surfactant-modified surface and toluene adsorption on internal sorption sites of FAU zeolite. Thus, CPB-modified FAU zeolite appears as a promising adsorbent for simultaneous removal of Cr(III), Cr(VI) and toluene contaminants from aqueous solution.     

Hydrothermal conversion of FAU zeolite into aluminous MTN zeolite

The hydrothermal conversion of FAU zeolite into aluminous MTN zeolite is described here. In the presence of both benzyltrimethylammonium hydroxide (BTMAOH) and sodium chloride (NaCl) the highly crystalline and pure MTN zeolites with Si/Al ratios of 21-23 could be obtained from the hydrothermal conversion of FAU zeolite. Based on powder XRD refinement and ¹³C CP/MAS NMR spectra, BTMA⁺ ions were not present in cages of the obtained zeolites, but TMA⁺ ions existed instead. It means that BTMAOH underwent degradation during the conversion. Moreover, the effects of Si/Al ratio of starting FAU zeolite, synthesis parameters (BTMAOH/SiO₂ and H₂O/SiO₂ ratios) and the addition of alkali metal chlorides on the hydrothermal conversion of FAU zeolite into MTN zeolite are discussed. As compared to amorphous SiO₂/γ-Al₂O₃, which produced impurity, the hydrothermal conversion of FAU zeolite showed a fast crystallization rate and a high selectivity to MTN zeolite formation. These phenomena indicate that the assembly of locally ordered aluminosilicate species coming from the decomposition or dissolution of FAU zeolite should be taking part in the conversion process.     

Determination of adsorption and diffusion parameters in zeolites through a structured approach

Temporal analysis of products (TAP) is a transient pulse-response technique that allows to extract kinetic information from reacting and adsorbing systems. In a previous work (Chem Eng Sci 57(2002) 1835), a detailed-transport model for the Multitrack set-up, a TAP-like system, was developed, which allows studying systems with a low bed resistance. The use of structured beds, having both a low bed resistance and small sorbent particles, is required to determine adsorption and diffusion parameters when strong adsorption and slow diffusion occurs. A method is presented to extend the range of measurable adsorption and diffusion parameters in zeolitic sorbents in the TAP technique by a structured approach. Small zeolite crystals are coated on larger non-porous glass beads. Adsorption and diffusion parameters for n-butane, SF₆ and 3-methylpentane have been determined in MFI-type zeolites. Absolute diffusivity values in the zeolite coating are estimated by using a well-defined silicalite sample as a reference to determine the effective diffusion length in the coating.Two criteria have been derived, one for the characteristic time for transport through the bed, , and one for the ratio of the latter and the characteristic diffusion time in the zeolite crystal, 0.01<α<200t_bed/(1+t_bed), which should be satisfied both to be able to determine values of the zeolite diffusivity.     

CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries

Computational Fluid Dynamics (CFD) is used to investigate mass transfer from Taylor bubbles to the liquid phase in circular capillaries. The liquid phase volumetric mass transfer coefficient k_La was determined from CFD simulations of Taylor bubbles in upflow, using periodic boundary conditions. The separate influences of the bubble rise velocity, unit cell length, film thickness, film length, and liquid diffusivity on k_La were investigated for capillaries of 1.5, 2 and diameter. The mass transfer from the Taylor bubble is the sum of the contributions of the two bubble caps, and the film surrounding the bubble. The Higbie penetration model is used to describe the mass transfer from the two hemispherical caps. The unsteady-state diffusion model of Pigford is used to describe the mass transfer to the downward flowing liquid film. The developed model for k_La is in good agreement with the CFD simulated values, and provides a practical method for estimating mass transfer coefficients in monolith reactors.     

Synthesis and physical properties of single crystals of zeolite-like aluminophosphates of the AFI type

The improvement of the procedure employed for hydrothermal synthesis of aluminophosphates of the AFI type (AlPO₄) with the use of triethanolamine as a structure-forming template made it possible to grow large-sized AFI single crystals of high optical quality in the form of hexagonal prisms with a size of up to 2–3 mm along the c axis. The dc electrical conductivity and infrared spectra of the AFI crystals thus obtained were measured.     

Iron exchanged ZSM-5 and Y zeolites calcined at different temperatures: activity in N2O decomposition

ZSM-5 and Y zeolites were modified with iron by an ion-exchange method and then calcined at 773, 873, 973 and 1,073 K. The obtained materials were characterized with respect to textural parameters (low-temperature N₂ sorption), structure (X-ray diffraction, UV–vis–DRS), redox properties (H₂-temperature programmed desorption, TPD) and surface acidity (NH₃-TPD). The obtained results have shown that the structure of zeolites influenced form, aggregation and content of the introduced iron species. In case of the FAU type structure characterized by wide pores (max. ring size, T-atoms—12) mainly iron in form of mononuclear Fe³⁺ cations and Fe _x ³⁺ O_y oligonuclear clustered species was found. On the other hand for the MFI type structure characterized by smaller pores (max. ring size, T-atoms—10) significant contribution of iron in the form of bulky Fe₂O₃ clusters located possibly on the outer surface of ZSM-5 was detected. Such significant differences in distribution of iron species is probably related to various mobility of iron species in the pore systems of both zeolites. The obtained materials were tested as catalysts in the process of N₂O decomposition. Calcination of zeolites at different temperatures influenced neither the properties nor the activity of the obtained catalysts.     

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

In this work, the effects of surface-active contaminants on mass transfer coefficients k_La and k_L were studied in two different bubble contactors. The oxygen transfer coefficient, k_L, was obtained from the volumetric oxygen transfer coefficient, k_La, since the specific interfacial area, a, could be determined from the fractional gas holdup, ε, and the average bubble diameter, d₃₂. Water at different heights and antifoam solutions of 0.5- were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k_La and k_L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k_L data within ±30%.     

Untersuchungen zur Kinetik der Diffusion von Ethen in 4A-Zeolithen

Investigation on the Kinetics of Diffusion of Ethene on 4A Zeolites The kinetics of diffusion of ethene on 4A zeolites was examined. Conclusions on the transport mechanism were obtained by the loading dependence on the diffusion coefficients. Contrary to propene and the n-butene isomers, ethene can cross the windows to the cavities blocked up by Na⁺ ions in the NaA and MeNa_0,8A zeolites (Me = Mg, Ca, Zn, Ba) by a translatory jump motion. In NaKA zeolites – by reason of additional blocking up of windows with K⁺ ions – the ethene molecules overcome the barrier only through the ethene/Na⁺(1) addition complexes.