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Diffusion in Zeolites - Impact on Catalysis

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... It is known that heterogeneous catalytic performance in porous materials is determined by a synergistic interplay between molecular transport and reaction kinetics [114]. In [113] with permission from The Royal Society of Chemistry Content courtesy of Springer Nature, terms of use apply. ...
... many zeolite catalysts, the pore diameters are only slightly larger than the molecular diameter of the reactants, leading to slow diffusion and reduced performance. Among various strategies to overcome diffusion limitations, the introduction of mesopores in the microporous crystal bulk phase via dealumination has attracted special attention [4,114]. However, the post-synthesis introduced mesopores can create new geometrical constraints that will ultimately alter the catalytic performance of zeolites due to a different shapeselectivity [114][115][116][117]. Therefore, it is critical to study the effect of dealumination on the diffusion-reactivity interplay at the nanoscale. ...
... Among various strategies to overcome diffusion limitations, the introduction of mesopores in the microporous crystal bulk phase via dealumination has attracted special attention [4,114]. However, the post-synthesis introduced mesopores can create new geometrical constraints that will ultimately alter the catalytic performance of zeolites due to a different shapeselectivity [114][115][116][117]. Therefore, it is critical to study the effect of dealumination on the diffusion-reactivity interplay at the nanoscale. ...
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Nanoporous solids, including microporous, mesoporous and hierarchically structured porous materials, are of scientific and technological interest because of their high surface-to-volume ratio and ability to impose shape- and size-selectivity on molecules diffusing through them. Enormous efforts have been put in the mechanistic understanding of diffusion–reaction relationships of nanoporous solids, with the ultimate goal of developing materials with improved catalytic performance. Single-molecule localization microscopy can be used to explore the pore space via the trajectories of individual molecules. This ensemble-free perspective directly reveals heterogeneities in diffusion and diffusion-related reactivity of individual molecules, which would have been obscured in bulk measurements. In this article, we review developments in the spatial and temporal characterization of nanoporous solids using single-molecule localization microscopy. We illustrate various aspects of this approach, and showcase how it can be used to follow molecular diffusion and reaction behaviors in nanoporous solids.
... One usually tries to circumvent this problem in a series of measurements, with one parameter (notably the diameter of the catalyst) intentionally varied and all others kept constant. This latter requirement, however, is hardly to be validated by direct experimental evidence (Haag et al. 1981, Garcia and Weisz 1993, Avila et al. 2007, van den Bergh et al. 2010. ...
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Microimaging on the basis of, respectively, interference microscopy and IR microscopy permit the observation of the distribution of guest molecules in nanoporous solids and their variation with time. Thus attainable knowledge of both concentration gradients and diffusion fluxes provides direct access to the underlying diffusion phenomena. This includes, in particular, the measurement of transport diffusion under transient, i. e. under non-equilibrium conditions, and of self- or tracer diffusion on considering the rate of tracer exchange. Correlating the difference in guest concentration close to the external surface to its equilibrium value with the influx into the nanoporous solid, microimaging does as well allow the direct determination of surface resistances. Examples illustrating the variety of information thus attainable include the comparison of mass transfer under equilibrium and non-equilibrium conditions, single- and multicomponent diffusion and chemical reactions. They, finally, introduce into the potentials of microimaging for an in-depth study of mass transfer in mixed-matrix membranes. This tutorial review may serve as first introduction into the topic. Further references are linked for the interested reader.
... A difference of this order of magnitude is relatively small in comparison with the usual outcome of comparative studies with seemingly identical systems under application of different measuring techniques and different samples. 78,79 They are, commonly, referred to the differences in the investigated samples (caused by differences in their synthesis and/or pretreatment) which, quite often, appear much more distinctly in their transport patterns than in their structural properties. 80 In the present case, on comparing the results of PFG NMR investigations of intracrystalline diffusion with uptake measurements, one has to be aware that the latter process may, additionally, be retarded by the influence of surface resistances. ...
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Within the family of metal organic frameworks (MOFs), zeolite-imidazolate framework (ZIF) of type ZIF-8 is among the most promising materials with potential application for hydrocarbon separation, notably of alkanes and alkenes. We have applied NMR spectroscopy for exploring the interaction of representative alkane-alkene mixtures with the host framework as well as the mobility of the various components in their mixture. Deviating from, e.g., the separation patterns known for MFI-based membranes, the mobility of the faster molecules (alkenes) is found to remain essentially unaffected by the presence of more slowly diffusing molecules (alkanes). In addition, 2H and 67Zn MAS NMR contribute to the characterization of the ZIF-8 in search for guest-induced changes in the lattice structure.
... Permeation of gases through the zeolitic micropores at moderate temperature is generally controlled by surface (configurational) diffusion; it can be conveniently described in terms of hopping of molecules from one adsorption site to another [51,52]. Microporous transport of non-adsorbing as well as adsorbing molecules at higher temperature, that is when molecules keep the physical properties of the gas to a large extent because of weak adsorption, is defined by activated gaseous diffusion [53][54][55]. In general, excluding flow through defects, the overall flux through the zeolite film is a combination of surface and gaseous diffusion. ...
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The latest advances in the field of zeolitic membranes for gas separation are critically reviewed with special emphasis on new synthetic protocols. After introducing the most relevant aspects to membrane performance, including adsorption trends, permeation mechanisms and support effects, we review recent achievements in membrane synthesis and discuss in detail the effect of zeolite topology and chemical composition on membrane gas separation. We pay special attention to promising 8MR high-silica structures. As the formation of defects during synthesis remains one of the major challenges for large-scale production of such membranes, we review various approaches to either limit defect formation or decrease their adverse effect by post-synthesis modification. Finally, the current challenges for this field of research are summarized and an outlook is offered on approaches to decrease fabrication costs, improve reproducibility and rational design of zeolite membranes.
... uctures. [1] Zeolite Y is a highly versatile zeolite from the faujasite family, the 3 D pore structure and acidic characteristics of which have been used in various applications. However , although the micropores provide for a large internal surface area containing active sites, they induce resistance for mass transport to or from the active sites. [2] In zeolite Y, mass transfer limitations can occur if large reactant molecules are in- volved. [3, 4] To overcome this problem, the synthesis of largepore zeolites [5, 6] and zeolites consisting of nanosized crystals [7, 8] has been reported. However, owing to the low acidity and low thermal stability of the large-pore zeolites as well a ...
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A new composite material consisting of amorphous TUD-1 encapsulating crystalline zeolite Y was synthesized. Samples with different HY zeolite loadings (10, 20, 40, and 60 wt %) were prepared, and the resulting solid products were characterized with elemental analysis, XRD, N2 physisorption, 27Al MAS NMR, IR, pyridine adsorption in combination with FTIR, temperature-programmed desorption of ammonia, HRSEM, and HRTEM. Characterization data confirm the presence of a thin layer of the mesoporous TUD-1 phase with a thickness of 30–100 nm surrounding the zeolite crystals. The catalytic performance of the composite was studied in the Friedel–Crafts benzylation of benzene with benzyl alcohol at 353 K. The catalytic activity of the HY/TUD-1 composite was higher than that of HY zeolite, whereas the composite showed a much slower rate of deactivation. The improved performance of the composite is related to beneficial changes in the acidity of the HY crystal through chemical interactions with TUD-1.
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Nanosized Y zeolite is a material which could potentially be used as an adsorbent or catalyst in the oil industry thanks to its increased high external area and mesopore volume. However, a simple, inexpensive, and environmentally friendly method to synthesize this zeolite has not yet been reported. Currently, the synthesis methods used to obtain nanosized Y zeolite are based on the use of organic structure-directing agents, since the zeolites obtained without these compounds have low Si/Al ratios. Based on these considerations, in this work nanosized Y zeolites were synthesized with high crystallinity and Si/Al ratios between 1.5 and 2.6, without the use of organic compounds and in a relatively short time (3 days). The study focuses on understanding the variation in the crystallization temperature when the amount of sodium aluminate, silica and sodium hydroxide is modified in the synthesis and correlates this variation with the Si/Al ratio and the particle size of the resulting zeolite.
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Zeolite Y and its ultra‐stabilized hierarchical derivative (USY) are the most widely used zeolite‐based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC and hydrocracking. The present study highlights the key difference leading to the exceptional catalytic performance of USY versus the parent zeolite Y in a multi‐technique study combining advanced spectroscopies (IR and solid‐state NMR) and molecular modeling. The set of results highlights a hitherto unreported proton transfer involving inaccessible active sites in sodalite cages that contributes to the exceptional catalytic performance of USY.
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Zeolite Y and its ultra-stabilized hierarchical derivative (USY) are the most widely used zeolite-based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC and hydrocracking. The present study highlights the key difference leading to the exceptional catalytic performance of USY versus the parent zeolite Y in a multi-technique study combining advanced spectroscopies (IR and solid-state NMR) and molecular modeling. The set of results highlights a hitherto unreported proton transfer involving inaccessible active sites in sodalite cages that contributes to the exceptional catalytic performance of USY.
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Zeolite nanoparticles have been widely used to overcome diffusion limitations in heterogeneous catalytic reactions. However, the existence of surface barriers for molecular diffusion in zeolites can limit the benefits of using nanoparticles in catalytic reactions. In this study, a set of silica nanoparticle (SNP)/Silicalite-1 composites with different external surface to micropore surface ratios was synthesized to understand the effects of surface-controlled mass transport on molecular diffusion in zeolite nanoparticles. The Zero Length Column (ZLC) technique was used to evaluate the mass transport of cyclohexane in these materials. It was found that the strong sorbate/sorbent interaction at the external surface of Silicalite-1 nanoparticles can cause diffusing molecules to re-enter into micropores and repeat the micropore diffusion process. This pore re-entry step can lead to an unusually long micropore diffusion length. We also demonstrated that this repeated micropore diffusion process can be effectively reduced by mixing the zeolite nanoparticles with secondary, nonporous nanoparticles. This study provides an alternative way to justify the surface mass transfer resistance, and it also introduces a simple strategy to enhance mass transport in zeolite nanoparticles other than surface modification which can damage the integrity of zeolite crystals. Additionally, previous diffusion results were revisited by adjusting the actual micropore diffusion length. It was concluded that the surface resistance in zeolite nanoparticles is likely due to a combination of pore re-entry of adsorbates and pore blockage.
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Through IR microimaging the spatially and temporally resolved development of the CO2 concentration in a ZIF-8@6FDA-DAM mixed matrix membrane was visualized during transient adsorption. By recording the evolution of the CO2 concentration, it is observed that the CO2 molecules propagate from the ZIF-8 filler, which acts as a transport "highway", towards the surrounding polymer. A high-CO2-concentration layer is formed at the MOF/polymer interface, which becomes more pronounced at higher CO2 gas pressures. A microscopic explanation of the origins of this phenomenon is suggested by means of molecular modeling. By applying a computational methodology combining quantum and force-field based calculations, the formation of microvoids at the MOF/polymer interface is predicted. Grand Canonical Monte Carlo simulations further demonstrate that CO2 tends to preferentially reside in these microvoids, which is expected to facilitate CO2 accumulation at the interface.
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Chapter
Structural aspects of porous systems and the surface chemistry of the most important silicate-based support materials are the focus of this chapter. Typical examples where inorganic materials have been used as supports for various ionic liquids (ILs) are provided. The synthesis of porous materials can follow very different pathways starting from homogeneous or heterogeneous systems applying different principles (e.g. crystallization, aggregation, extraction) with their underlying different physical and chemical roles. Recent developments in the design and tailoring of porous materials include the hierarchically organized zeolites, zeolite-like materials such as aluminumphosphates (AlPOs, SAPOs), the very well-ordered mesoporous materials and periodic mesoporous organosilicates (PMOs), the so-called metal-organic framework materials (MOFs) and COFs, and the macroporous well-structured reactor systems. Layered materials have been applied for various industrial and medical applications. Supported catalysts with an ionic liquid layer (SCILL) on Pd-containing porous glass catalysts have been prepared and tested in the microwave-assisted hydrogenation of citral to citronellal with molecular hydrogen as the reducing agent.
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After base treatment of ZSM-5 crystals below 100 nm in size, TEM shows hollow single crystals with a 10 nm shell. SEM images confirm that the shell is well- preserved even after prolonged treatment. Determination of the Si/Al ratios with AAS and XPS in combination with argon sputtering reveals aluminum zoning of the parent zeolite, and the total pore volume increases in the first two hours of base treatment. In corresponding TEM images, the amount of hollow crystals are observed to increase during the first two hours of base treatment, and intact crystals are visible even after 10 h of leaching; these observations indicate different dissolution rates between individual crystals. TEM of large, commercially available ZSM-5 crystals shows inhomogeneous distribution of mesopores among different crystals, which points to the existence of structural differences between individual crystals. Only tetrahedrally coordinated aluminum is detected with (27) Al MAS NMR after the base leaching of nano-sized ZSM-5. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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The field of metal-organic framework based mixed matrix membranes (M(4)s) is critically reviewed, with special emphasis on their application in CO2 capture during energy generation. After introducing the most relevant parameters affecting membrane performance, we define targets in terms of selectivity and productivity based on existing literature on process design for pre- and post-combustion CO2 capture. Subsequently, the state of the art in M(4)s is reviewed against these targets. Because final application of these membranes will only be possible if thin separation layers can be produced, the latest advances in the manufacture of M(4) hollow fibers are discussed. Finally, the recent efforts in understanding the separation performance of these complex composite materials and future research directions are outlined.
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Diffusional mass transport in porous materials is important for shape-selective catalysis and separation technologies. To maximize turnover and catalytic site accessibility, hierarchical materials are synthesized with length scales as small as single crystal lattices (~2 nm, MFI). While these materials are potentially efficient catalysts, they have been shown to exhibit apparent diffusivities that are orders of magnitude slower than in bulk crystals. To evaluate the dependence of apparent diffusivity with particle size, the kinetics and mechanism have been characterized by frequency response methods for cyclohexane mass transfer into and out of silicalite-1 particles varying in size over two orders of magnitude. Development of a new mass transport model utilizes data obtained by frequency response to characterize two sequential rate limitations: intracrystalline diffusion and asymmetric surface barriers. Activation energy associated with transport into the surface (Ea,s = 20.8 kJ/mol) was observed to be significantly less than intracrystalline diffusion and release (Ea = 53.9 kJ/mol ≈ 54.1 kJ/mol = Ea,-s). Surface pore blockages are proposed to dominate mass transport in small zeolite particles.
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Development of microporous materials with hierarchical structures of both micro/mesopores leads to molecular transport at nanometer length scales. For novel microporous materials including three-dimensionally ordered mesoporous imprinted (3DOm-i) zeolites and zeolite nanosheets, particle dimensions are below 35 nm resulting in surface-dominated structures. At the same time, the existence of surface-controlled mass transport including undefined “surface barriers” has been observed to reduce apparent diffusivity of hydrocarbons by orders of magnitude. This paper systematically characterizes cyclohexane transport in silicalite-1 by zero length chromatography (ZLC) to determine apparent diffusivity varying over 3 orders of magnitude in particles ranging from 35 nm to 3 μm. Three proposed mechanisms for surface barriers including surface pore narrowing, surface pore blockage, or surface desorption are evaluated by comparison with particle-size/diffusivity data. It is concluded that transport control in small particles was likely due to either pore narrowing at the surface or an extension of the diffusional length scale near the surface due to total pore blockages
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Biomass pyrolysis is considered one of the most promising technologies for production of sustainable fuels. Rice husk ash (RHA) contains over 60% silica, 10–40% carbon and minor other mineral composition, which is the by-product during the process of rice husk (RH) gasification/pyrolysis. More researchers become interested in how to use this industrial waste, because RHA is available in abundant, sustainable and almost free of cost. In recent years, RHA has been widely used as a construction material to product concrete, or as an adsorbent to adsorb organic dye, inorganic metal ions and waste gases. Due to its high silica content, RHA can be an economically viable raw material for the production of silicates and silica materials. Biochar and biosilica from high silicon-containing biomass, which can be fabricated into the high value-added porous carbon and silicon materials, such as silica/carbon nanoparticles, mesoporous silica/carbon, have lots of chemical and biological characterization for biomedical and electronical applications. Significantly, RH char, a hybrid composite can be converted in to the homogeneous carbon–silica nanocomposite (C/SiO2) via extraction, which is directly used for synthesis of highly ordered mesoporous carbon and silica materials via a triconstituent co-assembly approach to prepare well-ordered mesoporous polymer–silica and carbon–silica nanocomposites by using resols as a polymer precursor, silicate oligomers as an inorganic precursor, and triblock copolymer F127 as a template. In addition, these materials could be put into extensively use as adsorbents and catalysts for other environmental pollutants treatment. Thus, it has a significant meaning to be engaged in more research works on the physicochemical characteristics of waste biomass to realize the “3R” rules of reducing, reusing and recycling.
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We investigate the influence of the aluminium content on the adsorption of hexane isomers at 293 K on MFI type zeolites (Si/Al ratios – SAR of 25, 100 and ∞). Adsorption isotherms were assessed with a homemade volumetric system. Temporal up-take curves were recorded using a manometric set-up combined with a Rubotherm suspension balance. All three isomers show dual site adsorption behaviour that is well described with a Dual Site Sips (DSS) model. The Si/Al ratio influences strongly the adsorption equilibrium properties. Regardless of the isomer type, the sample with SAR = 100 shows the highest saturation loadings. At the studied pressures the sorption order in the three samples is: n-hexane > 2-methylpentane > 2,3-dimethylbutane > 2,2-dimethylbutane. Generally, we found that the Henry constant increases with aluminium content. Clear diffusional limitations are observed for the di-branched hexane isomers on all three zeolite samples. The sample with SAR = 100 gives the highest diffusion rates for the hexane isomers. Silicalite-1 is the most promising for enhancing gasoline octane numbers at 293 K, as it gives the highest equilibrium 2,3-dimethylbutane/2-methylpentane ratios.
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The combination of metal organic frameworks (MOFs) and polymers in the form of mixed matrix membranes (MMMs) has become an increasingly important field of research over the last few years. The first examples of membranes outperforming state of the art polymers have already been presented, emphasizing the high application potential of these composites. In this paper, the recent progress on the topic is thoroughly reviewed and the main advantages and limitations of the use of MOFs as MMMs fillers are evaluated.
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An investigation of aluminosilicate zeolite L (structure code LTL) nanocrystals converted from rice husk ash in template free system is reported. Crystallization evolution of the zeolite L was monitored and followed by X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and infrared spectroscopy techniques. It was found that fully crystalline zeolite L (Si/Al = 3.0) with square tablet morphology was nucleated at 4 h, and the crystals with a mean particle size of 210 nm was completely transformed from amorphous silica after 24 h of hydrothermal treatment.
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Single component (CO2, CH4, and N2) and equimolar binary mixture (CO2/CH4, N2/CH4, and CO2/Air) permeation data across a disk-shaped all-silica DDR zeolite membrane have been the subject of a thorough modeling study over a challenging broad temperature (220−373 K) and feed pressure (101−1500 kPa) range. The mass transport through the zeolite layer is evaluated for two rival, Maxwell Stefan-based, models: the Relevant Site Model (RSM) and the so-called Reed Ehrlich (RE) approach. Both models have been introduced to account for the strong loading dependency of the diffusivity in small-pore cage-like zeolites like DDR. High pressure adsorption isotherms (up to 7000 kPa) measured on DDR crystals are incorporated to describe adsorption on the zeolite. Both the RSM as the RE approach yield an excellent model fit of the single component permeation data. However, for both models the N2 and CH4 data did not allow an accurate estimation of the model fit parameters. Both models can lead to a good prediction of comparable quality of the mixture permeation data based on the single component model fit parameters. The RE approach is very sensitive toward the model input parameters and the estimated mixture loading, which both can be very hard to determine accurately in practice. The RSM does not suffer from both these issues, which is an evident advantage with respect to application of this model.
Data
An investigation of aluminosilicate zeolite L (structure code LTL) nanocrystals converted from rice husk ash in template‐free system is reported. Crystallization evolution of the zeolite L was monitored and followed by X‐ray diffraction, high‐resolution transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and infrared spectroscopy techniques. It was found that fully crystalline zeolite L (Si/Al = 3.0) with square tablet morphology was nucleated at 4 h, and the crystals with a mean particle size of 210 nm was completely transformed from amorphous silica after 24 h of hydrothermal treatment.
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A microdynamic model is proposed to describe the mass-transfer resistance localized at the surface of zeolite crystals. In addition to the intracrystalline diffusional resistance, this resistance occurs owing to the repulsion and attraction interaction between sorbing molecular species and the crystal surface. Especially for small crystals the surface resistance can exceed the intracrystalline diffusional resistance by several orders of magnitude. In particular, the model explains the non-linear behaviour of Arrhenius plots of uptake data exemplified for the sorption of n-hexane on NaMgA zeolite crystals.
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Diffusion of adsorbed molecules in zeolites plays an important role in the use of zeolites as adsorbents in separation processes and in shape-selective catalysis. Computational chemistry is in a stage where diffusion phenomena, even for multicomponent diffusion, can be treated at a level of high accuracy. The present review presents most of the results on diffusion in zeolites obtained by classical Molecular Dynamics, dynamic Monte-Carlo approaches, Transition-State Theory, and the Maxwell-Stefan approach. Reactive and non-reactive conditions are considered.
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Acylation of different amino derivatives (ethanol amine, diethanol amine, N-aminoethylpiperazine) was carried out under green conditions with oleic acid or tail oil fatty acids. The experiments were carried out using as heterogeneous catalysts mesoporous Al-Meso and UL-ZSM-5 with Si/Al ratios between 100 and 20, in the range of the tempera- tures 80-180 °C without any solvent or in octane or water. The efficiency of the catalysts for the acylation of the investigated substrates appeared to be directly dependent on the Si/Al ratio and the size of the pores. The selectivity toward esters or amides was firstly controlled by temperature, and then by the Si/Al ratios. Additionally, both the conversion and selectiv- ity were controlled by the solvent in which the reaction was carried out. The use of water led mostly to a mixture of esters and amides.
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Iodine indicator technique (IIT) involving light microscopy has been introduced in our laboratory to investigate sorption and mass transport phenomena in zeolites and peculiarities of crystal morphology via coloring of zeolite crystals. The coloring of silicalite-1 90°-intergrowths was performed using pure iodine vapors and binary solutions of iodine in organic solvents (benzene, cyclohexane, toluene, ethylbenzene, p-xylene, and decahydronaphthalene). Coloring from the liquid phase was carried out either under co- or counter-diffusion conditions. The rate of coloring from the vapor phase, and under co-diffusion conditions from the liquid phase, was found to be limited by external mass transport and the coloring was uniform. Under counter-diffusion conditions, the rate of coloring was to about 2 to 3 orders of magnitude lower than in the latter case. Except for decahydronaphthalene, the coloring patterns were non-uniform, and they visualized at least in the beginning stage of the coloring process the interfaces of crystal sections. Also, the cracks were visualized being decorated with iodine. There is a marked difference between the development of coloring patterns of benzene and cyclohexane on one side and their linear alkyl derivatives (toluene, ethylbenzene and p-ethyltoluene) on the other side. In the former case, the iodine enters the bulk of crystal sections via section interfaces. In the latter case, the bulk of the sections appears to be inaccessible for iodine molecules. Other information on channel system accessibility provides IIT for decahydronaphthalene, where the molecular sieve effect of 10-membrane oxygen rings with respect to decahydronaphthalene is indicated.
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The new extra-large pore molecular sieve CIT-5 has been synthesised in both hydroxide and fluoride aqueous media. It has been found, contrary to previous reports, that Li+ is not an essential component of the synthesis gel, although it accelerates the crystallisation. The pure silica polymorph synthesised in the F− medium presents almost no connectivity defects, in contrast to that synthesised in the OH− medium. IR and 29Si MAS NMR spectra of the new samples synthesised in the F− medium show very good resolution of bands owing to the high local order which arises from the very low concentration of connectivity defects. The structure of this phase has been reinvestigated and solved successfully by direct methods and refined using synchrotron X-ray powder diffraction data. The framework topology previously proposed for this phase is found to be correct, although the structure is more accurately described in Im2a rather than Imma symmetry. Despite the increase in the number of tetrahedra forming the minimum window of the pore, from 12 to 14, the effective pore diameter of CIT-5 appears to be almost identical to that of the unidimensional 12-ring zeolite SSZ-24.
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Properties of porous, single-crystal membranes have been modelled for Langmuir's isotherm assuming that entry to and exit from the crystal proceeds via an externally adsorbed layer. The potential moderating role of surface processes upon steady flow depends strongly upon temperature and upon the difference in activation energy for escape from the crystal to the externally adsorbed layer and that for intracrystalline diffusion. The moderating role decreases with increasing membrane thickness. Where steady flow is reduced by surface processes the apparent diffusivity found from this flow is less than the true intracrystalline diffusivity by an amount which increases the smaller the crystal. If along one-dimensional channel systems there are occasional partial blockages with associated abnormally high energy barriers, quantitative extension of the model shows potentially large flow reductions, independent of membrane thickness. Provided surface processes have in comparison a negligible effect steady flow will give the correct average intracrystalline diffusivity. Mixture separation has been considered in partially blocked or unblocked crystal membranes in the Henry's law range.
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Three samples of zeolite omega with varying residual contents of sodium have been dealuminated by combined steam and acid treatments. The hydrolysis of the AlO bonds is a fast process that occurs mainly during the initial period of heating above 500° under steam. Sodium ions inhibit the reaction. The steamed solids contain mesopores ≅ 100 Å in diameter, but their porosity is not available to sorbents because of the presence of debris in which aluminum is in tetrahedral and octahedral configurations. Acid leaching removes all the nonframework tetrahedral aluminium and part of the octahedral one. This removal permits access to the microporous and mesoporous voids. Increasing the severity of the steam treatment increases the quantity of the disloged material that cannot be removed by the acid, but has little effect on the texture of the final solids. The TEM observations and the volumetric measurements suggest that the mesopores are not directly linked to the surface of the grains and are connected to each other by narrow necks.
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Hierarchically structured composites (TUD-M) with ZSM-5 nanocrystals embedded in a well-connected mesoporous matrix were synthesized by employing only one organic templating/scaffolding molecule (TPAOH). Micro- and mesopores form separately under different synthesis conditions. Both the size of the zeolite crystals and the mesopore size in the amorphous matrix can be tuned. A lower Si/Al ratio results in a slower growth of zeolite crystals and improves the hydrothermal stability of this new type of composite. Solid state NMR reveals that the aluminium species are all tetrahedrally coordinated, and that silicon species condense further during crystal growth. A carbon replica of TUD-M proves that the mesopores are interconnected, and also hints at the similarities between the meso-structures of TUD-M and TUD-1. The scaffolding mechanism at the basis of the mesostructure is not limited to TPAOH. Other zeolite/meso-structure composites could also be synthesized based on the same concept.
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An aluminium-free Ti-Beta/SBA-15 composite material was prepared by the post-synthesis incipient wetness-deposition of different amounts of Ti-Beta nanoparticles solution on the SBA-15 matrix. The presence of crystalline nanoparticles in the solution, used for impregnation on SBA-15, was confirmed by HRTEM measurements. The hexagonal arrangement of the mesopores of SBA-15 was proven by XRD and TEM measurements. The presence and the deposition of Ti-incorporated zeolitic nanoparticles on the mesopore walls of SBA-15 were proven by nitrogen sorption analysis, IR spectroscopy and TG analysis. The X-ray absorption spectroscopy analysis of local environment of titanium incorporated in the new composite material showed that the product contained five fold coordinated framework titanium. These Ti sites can be oxidation titanium sites.
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Diffusion of n-hexane in large crystals of the ZSM-5 and KVS-5 (V-Sil-1) zeolites (the MFI structure type) is discussed. The theoretical description takes into account morphology of the microporous crystals. Two diffusion models are used to compare theoretical uptake curves with the experimental data and to determine the diffusion coefficient values. The results are related to the classic diffusion model assuming the spherical crystal shape. The model comprising the actual crystal morphology is more realistic and reflects the course of uptake curves clearly better than the classic spherical model does. It yields diffusion coefficient values of ca. 4×10−11m2s−1, which are more than twice higher than the classic ones and, at the same time, somewhat lower than those from the measurements based on the equilibrium-state methods, including the crystal morphology that can bring closer together the results of non-equilibrium and equilibrium methods. This approach may have a more common applicability for the description of diffusion in crystalline microporous solids.
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We report the synthesis and characterization of a series of new mesoporous zeolite and zeotype materials made available by combining new and improved procedures for directly introducing carbon into reaction mixtures with the fluoride route for conventional zeolite synthesis. The mesoporous materials were all prepared by hydrothermal crystallization of gels adsorbed on carbon matrices which were subsequently removed by combustion. The procedures presented here resulted in mesoporous zeolite and zeotypes materials with MFI, MEL, BEA, AFI and CHA framework structures. All samples were characterized by XRPD, SEM, TEM and N2 physisorption measurements. For the zeolite materials it was found that mesoporous MFI and MEL structured single crystals could indeed be crystallized from fluoride media using an improved carbon-templating approach. More importantly, it was found that mesoporous BEA-type single crystals could be crystallized from fluoride media by a newly developed procedure presented here. Thus, we here present the only known route to mesoporous BEA-type single crystals, since crystallization of this framework structure from basic media is known to give only nanosized crystals as opposed to mesoporous single crystals. For the zeotype materials it was found that highly crystalline mesoporous materials of AFI and CHA structure types could be synthesized using a newly developed procedure.
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Macropores act as broad highways for molecules to move in and out of a nanoporous catalyst. The macropore “distributor” network in such a hierarchically structured porous catalyst, containing both nanopores and macropores, is optimized with the aim to find the optimal effectiveness factor, ηopt, of a single reaction with general kinetics in the catalyst. Molecular diffusion is assumed to dominate transport in macropores. It is found that the ηopt−Φ0 relation qualitatively recovers the universal η−Φ relation when the generalized distributor Thiele modulus, Φ0, is defined in a way analogous to the generalized Thiele modulus, Φ, but using the molecular diffusivity in the macropores rather than the effective diffusivity in the nanopores. This is because the concentration gradient inside the optimal hierarchically structured, porous catalyst exists only in one principle direction (e.g., the radial direction in a spherical catalyst particle), and molecular diffusion in the macropores dominates the transport process in this principle direction. The universal ηopt−Φ0 relation is used to design a catalyst for power plant NOx emission control. Overall catalytic activity in a mesoporous catalyst with a median pore size of 32.5 nm could be increased by a factor of 1.8−2.8 simply by introducing macropores (occupying 20−40% of the total volume of the catalyst) with a width of 2−22 µm into the mesoporous catalytic material, so that the remaining mesoporous macropore walls are 5−33 µm thick. In practice, this would correspond to a deNOx catalyst consisting of mesoporous particles with a diameter of 5−33 µm and macropores in between them with a size of around 2−22 µm. Information like this is readily applicable to practical catalyst synthesis.
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Silicalite-1 coatings were synthesized on spherical Pt/TiO2 particles with a diameter of 0.5 mm from an aqueous solution of fumed silica, tetrapropylammonium hydroxide (TPAOH), and ethanol under hydrothermal conditions. SEM images show that the surface of the Pt/TiO2 particles was fully covered by intergrown silicalite-1 crystals with a crystal size of 5 × 2 × 15 μm. From the cross section of crushed silicalite/Pt/TiO2 particles, the estimated thickness of the silicalite-1 layer was about 40 μm. The compactness of the layer to act as a membrane was demonstrated by catalytic tests of the silicalite-1/Pt/TiO2 particles. The hydrogenation of a mixture of linear 1-hexene (1-Hex) and dibranched 3,3-dimethylbut-1-ene (3,3-DMB) (in a molar ratio of 1-Hex to 3,3-DMB of 1:1.68) was performed using a fixed-bed reactor with a continuous flow system. The composite silicalite-1/Pt/TiO2 catalyst showed 1-Hex/3,3-DMB hydrogenation selectivities of 12−20 at 50 °C and 18−30 at 100 °C due to the selective permeation of 1-Hex into the Pt/TiO2 particles through the silicalite-1 layer. Deactivation of the catalyst was also reduced, probably by protection against poisoning impurities in the feed. This work demonstrates the feasibility of the application of catalyst particles coated with a permselective membrane to achieve reactant selectivities on a particle level.
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TS-1 coated Ti-MCF materials have been prepared by the coating procedure using a diluted clear gel solution containing primary TS-1 nano-crystal units. Various techniques such as BET, TEM, FTIR, 29Si MAS NMR were used to monitor the physico-chemical properties of these samples. The coated samples exhibit higher activity and H2O2 selectivity for 1-naphthol hydroxylation compared to those of the parent Ti-MCF and TS-1 samples. Furthermore, a high hydrothermal stability of the mesopore structure and the absence of any Ti leaching during reaction were observed for the coated samples. This could be due to the zeolitic nature and hydrophobicity of the coated samples mesopore surface.
Article
A highly crystalline sample of the extra-large-pore, high-silica zeolite UTD-1 has been prepared in fluoride medium. The resulting product (UTD-1F) has a powder diffraction pattern remarkably different from that published for the calcined form of conventionally prepared UTD-1, and it could only be indexed on a monoclinic unit cell. The crystal structure was determined ab initio in the space group P21/c from synchrotron powder diffraction data collected on a textured sample. The resulting structure, with 69 non-H atoms in the asymmetric unit, is by far the largest structure solved directly from powder diffraction data to date. Subsequent Rietveld refinement of the structure in Pc, with 349 positional parameters and 464 geometric restraints, proved to be surprisingly stable and converged with RF = 0.041 and Rwp = 0.134 (Rexp = 0.101). In contrast to the structure reported for calcined UTD-1, no evidence of disorder was found in the UTD-1F framework (structure type code DON). There is a strict up−down alternation of the orientation of the Si tetrahedra in the 14-rings, and double crankshaft chains link adjacent 14-ring channels. Not only the framework, but also the (Cp*)2Co complex, which appeared very clearly on the difference electron density map, was found to be fully ordered in the channels with the 5-fold axis parallel to the channel direction.
Article
Department of Chemical EngineeringUniVersity of Amsterdam, Nieuwe Achtergracht 1661018 WV Amsterdam, The NetherlandsDepartment of Chemical EngineeringDelft UniVersity of Technology, Julianalaan 1362628 BL Delft, The NetherlandsReceiVed December 22, 1997Understanding the adsorption of hydrocarbons in zeolites is aprerequisite for optimizing catalytic processes using these materi-als.
Article
A large probe molecule, cumene, was chosen to study diffusion and adsorption on mesopore structured ZSM-5, using a high precision intelligent gravimetric analyzer. Compared with ZSM-5, a 2–3 order of magnitude increase in the diffusion coefficient of cumene was observed on the mesopore structured ZSM-5. The increased adsorption rate and the increased adsorption heat with increased cumene coverage, supported a mechanism of diffusion, phase transition, and re-arrangement of cumene molecules during the adsorption process on the mesopore structured ZSM-5. The introduced mesopores also decreased the diffusion–adsorption activation energy by a factor of 4.6. Consequently, although the amount of strong Brønsted acid sites decreased, the conversion of cumene on the mesopore structured ZSM-5 catalyst doubled compared to ZSM-5 catalyst, and the total light olefins yield increased by 2.47 wt% with Daqing heavy oil as a feedstock.
Article
The hydroisomerization of n-hexane on large crystals (50 μm) of Pt-loaded H–mordenite has been used as a test reaction to study the effects of concentration-dependent diffusion on zeolite catalyzed reactions. This concentration dependence was observed in the form of a nonlinear deviation from the relation between the reaction rate and the n-hexane pressure as provided by the intrinsic reaction rate equation. The dependence of the activity on the n-hexane pressure was measured at various temperatures and the results were compared with the results of model calculations. In the model used for these simulations it was assumed that the effective diffusion coefficient was proportional to (1−θ)/θ(θ = fraction of occupied sites) as proposed by K. Hahn and J. Kärger (J. Phys. Chem. B102, 5766 (1998)) and P. H. Nelson and S. M. Auerbach (J. Chem. Phys.110, 9235 (1999)) for single-file diffusion at long times. Furthermore, since it is thought that immobile alkoxy intermediates are present in the pores under reaction conditions, it was assumed that the effective diffusion coefficient was proportional to the fraction of surface species consisting of hexanes and hexenes. Except for the effective single-particle diffusion coefficient, for which no reliable literature value was found, independently obtained input parameters were used in the model calculations. Good agreement between experiment and model was obtained using a value for the effective single-particle diffusion coefficient of ∼10−5 m2/s, which is well within the range of orders observed for the single-particle diffusion coefficients of methane and tetrafluormethane in molecular sieves.
Article
The delamination of the layered precursor of the MCM-22 zeolite (MWW structure) affords monolayers of a crystalline aluminosilicate with more than 700 m2g−1 of a well defined external surface formed by cups of 0.7×0.7 nm. In this layered structure the circular 10-member-ring microporous system is preserved. The resultant material presents the strong acidity and stability characteristic of the zeolites but, at the same time, offers the high accessibility to large molecules characteristic of the amorphous aluminosilicates. The cracking behavior during the process of small and large molecules has been compared with that of the zeolite MCM-22 and pillared laminar precursor MCM-36.
Article
Mass transfer and chemical reaction in channels in which the individual molecules cannot pass each other (single-file systems) are studied by Monte Carlo simulations. Applying a simple jump model for the elementary steps of diffusion, macroscopically observable phenomena like molecular adsorption and desorption, tracer exchange, and counterdiffusion are considered. In the case of chemical reaction, the simulation results are used for a generalization of the Thiele concept to single-file systems.
Article
Synthetic faujasite, mazzite, and offretite have been dealuminated with ammonium hexafluorosilicate. The products have been characterized by elemental analysis, X-ray diffraction, volumetry, scanning electron microscopy, stepwise thermal desorption of ammonia, solid-state NMR and IR spectroscopy. Their catalytic activity has been evaluated using isooctane cracking as test reaction. The maximum level of dealumination which can be achieved without loss of X-ray crystallinity corresponds to 50% for faujasite and 30% for mazzite and offretite. This result and the fact that the dealumination capability depends on the texture of the crystals suggest that the reaction is diffusion controlled. The comparison of the acidic and catalytic properties of the parent and dealuminated samples shows small differences indicating that the fluorosilicate treatment replaces weakly bound and weakly acidic aluminium atoms.
Article
Using the Maxwell–Stefan approach, expressions have been derived for the diffusion of mixtures of hydrocarbons in zeolites wherein the individual components have different saturation loadings. This Maxwell–Stefan diffusion model, in combination with the Ideal Adsorbed Solution (IAS) theory and the single-component adsorption isotherms, provides a superior, qualitative and quantitative, prediction of the permeation fluxes of ethane/methane and propane/methane mixtures through a silicalite-1 membrane. The difference in adsorption saturation loadings becomes apparent in the entropy effects in the mixture adsorption.
Article
The generation of mesopores in the range of 15–20nm by alkaline treatment of a unidimensional high silica content zeolite, namely ZSM-12 was achieved. The factors that affect the generation of mesopores were thoroughly investigated. For the ZSM-12 with Si/Al ratio of 58, additional mesopores with volume in the range 0.05–0.61cm3g−1 were generated without destroying the microporous framework of ZSM-12 by varying the NaOH solution concentration, operation temperature and time. The concentration of NaOH solution was found to be the most dominant factor affecting the desilication process. Meanwhile, parent ZSM-12s with different Si/Al ratios in the range of 31–500 were alkali treated in order to investigate the influence of Si/Al ratios on the desilication of ZSM-12. It was found that the content of tetrahedrally coordinated aluminum in the zeolite frame work also controls the formation of mesopores by facilitating desilication process in alkaline medium. For the ZSM-12 with relatively low Si/Al ratios ranging from 31 to 58, relatively high NaOH concentration was favorable for the generation of mesopores while preserving the microporous structure and acidic property of ZSM-12. However, for the samples with higher Si/Al ratios, lower NaOH concentrations favored the creation of mesopores. In terms of uniform mesopore generation, ZSM-12s with Si/Al ratios in the range of 31–58 were more appropriate. Desilicating the as synthesized ZSM-12 samples (prior to calcination) is much more difficult than desilicating calcined ZSM-12 samples of comparable Si/Al ratios. The diffusivity of long chain hydrocarbons through the framework of alkaline treated ZSM-12 samples improved compared to the untreated ones. The ease to control the mesoporous network developed in ZSM-12 by alkaline treatment offers a new and economic path for the improvement of diffusion in the zeolite framework.
Article
A new material of zeolitic nature, ITQ-6, has been obtained by delamination of a layered precursor of ferrierite. XRD, Ar and N2 adsorption, and 29Si and 27Al MAS NMR pyridine and 2,6-di-tert-butylpyridine adsorption show that a delaminated material with a very high external surface area has been produced. The silicoaluminate is stable upon calcination and presents strong acid sites catalytically active and highly accessible to bulky reactants. We have also obtained by direct synthesis the titanium silicate version of the laminar precursor of ferrierite and ITQ-6. The Ti is demonstrated to be in framework positions, and in the case of the TiITQ-6 is active and selective in the epoxidation of 1-hexene with H2O2. The TiITQ-6 is stable and remains active upon repeated reaction−calcination processes.
Article
Catalysts based on NiMo and Pt supported on the new delaminated ITQ-2 zeolite have been prepared and their catalytic properties evaluated for the mild hydrocracking (MHC) of vacuum gasoil and aromatic hydrogenation. The results were compared with those obtained using other conventional supports, e.g., silica, γ-alumina, amorphous silica–alumina (25 wt% Al2O3), and USY zeolite, all of which contain the same metal loading as the ITQ-2 material. In the case of MHC of vacuum gasoil, NiMo/ITQ-2 displayed a higher hydrocracking activity than NiMo/SiO2–Al2O3 and NiMoγ-Al2O3, and even higher activity than NiMo/USY in the range 375–425°C. Moreover, NiMo/ITQ-2 had a selectivity to middle distillates intermediate between those of NiMo/USY and NiMo/SiO2–Al2O3. For hydrogenation of naphthalene, Pt/ITQ-2 displayed higher activity than Ptγ-Al2O3 and Pt/SiO2-Al2O3 but lower activity than Pt/USY with similar Pt dispersion. This is explained by considering that some of the Pt centers located in the 10 MR channels of ITQ-2 are not accessible to the naphthalene molecules. Indeed, Pt/ITQ-2 is significantly more active than Pt/USY for the hydrogenation of benzene, which can access the metal sites in the 10 MR channels of the delaminated ITQ-2 zeolite. Furthermore, Pt/ITQ-2 gave the highest aromatic reduction when using a hydrotreated light cycle oil (HT-LCO) feedstock containing ca. 70 vol% total aromatics, 0.40 wt% S, and 480 ppm N. In this case, the larger external surface area of ITQ-2 as compared with USY may favor the hydrogenation of the voluminous aromatic molecules present in the HT-LCO feed. These results can be explained by the peculiar structure of the delaminated ITQ-2 zeolite, which combines the good activity of zeolites with the desired selectivity of amorphous catalysts, while minimizing the diffusional problems often encountered in microporous materials.
Article
The classical analysis of kinetic data for diffusion limited heterogeneous catalytic reactions in terms of effectiveness factors is based upon the assumption that the effective diffusivity is independent of concentration. Zeolitic diffusivities are, however, concentration dependent, so that the classical analysis is not, in general, applicable to molecular sieve catalysts. In this paper theoretical concentration profiles and the corresponding effectiveness factors are calculated from the steady-state solution of the differential equation for diffusion with first order reaction for systems in which the concentration dependence of the effective diffusivity arises from the nonlinearity of a Langmuir equilibrium isotherm. Both the concentration profiles and the effectiveness factors show considerable differences from the classical solutions for systems with constant diffusivity. A simple expression, which gives a useful approximation for the effectiveness factor in the limiting case of high diffusional resistance, is derived. It is suggested that the approximations involved in the analysis should be reasonable for certain molecular sieve catalysts.
Article
Molecular Dynamics (MD) simulations have been carried out to determine the Maxwell–Stefan (M–S) diffusivities Đi for linear alkanes with 5, 6, 7 and 8 carbon atoms in MFI zeolite at 433 K for a range of molecular loadings Θ. The dependence of the MS diffusivities Đi on the loading Θ was found to exhibit strong inflection behaviour; this is caused by the inflection in the corresponding sorption isotherm. The Đi-Θ dependence follows the trend suggested by Krishna et al. [R. Krishna, J.M. van Baten, D. Dubbeldam, J. Phys. Chem. B 108 (2004) 14820]. The inflection behaviour also extends to diffusivities in binary mixtures.
Article
For two decades, the concept of reactivity enhancement by “molecular traffic control” has been controversially discussed. According to this concept, effective reactivity in zeolite catalysis may be enhanced if reactant and product molecules do not interfere with each other on their diffusion paths. As a major deficiency of this concept, so far no clear criteria for the occurrence of this effect are known. Considering a network of channels with single-file confinement and selective adsorption affinity to either the reactant or the product molecules, a model is introduced in which the possibility of reactivity enhancement may be rigorously demonstrated.
Article
For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.
Article
This system illustrates "partial molecular sieve action". Methane has a higher affinity for the sorbent and hence is preferentially sorbed at equilibrium, while nitrogen diffuses through the crystal more rapidly and thus is preferentially taken up during the early stages of sorption. Measurements were made with the pure gases and with mixtures at 0 °and −79.4 °C. The sorption isotherms fit Langmuir equations and the isosteric heats of sorption are essentially independent of concentration. The sorption rates for the pure gases may be characterized by diffusion coefficients, D, calculated in the usual manner assuming the flux of diffusion to be proportional to the concentration gradient. The resultant values for D increase with increasing sorbate concentration. Diffusion of a mixture may be formally characterized by D's for each component. While that for methane is approximately the same as for methane alone, D for nitrogen in mixtures is much larger than for the pure gas and also varies with composition. This, as well as the existence of a temporary maximum in the amount of nitrogen sorbed, may be explained by considering the driving force for diffusion to be the gradient in chemical potential rather than in concentration.
Article
Zeolite IPC-3 - which is isostructural with the extra-large pore germanate zeolite ASU-16 - and its Si, Al and Ti forms were synthesized using 1,6-diaminohexane as the template. It was established that the Si atoms occupy the tetrahedral positions in the IPC-3 framework, the Al atoms occupy the octahedral or tetrahedral positions and the Ti atoms the octahedral positions. The germanate zeolite IPC-3 and Si-, Al- and Ti-IPC-3 are stable up to 200-300°C. Despite the presence of the template, which stabilizes the framework of IPC-3, the adsorption capacities of samples previously degassed at 200°C were up to 0.13 cm3/g.
Article
Desilication in alkaline medium is demonstrated as a general route to synthesize hierarchical metallosilicate zeolite crystalswith preserved crystallinity. The lattice trivalent cation (not only Al3+ but also Ga3+, Fe3+, and B3+) regulates the alkaline-assisted hydrolysis of silicon towards controlled mesoporosity development. This finding widens the family of modified zeolite compositions that integrate in a single crystalline material micropores and mesopores and enables better accessibility in catalytic applications compared to the purely microporous counterparts.
Article
The configurational-bias Monte Carlo (CBMC) technique has been used for computing the adsorption isotherms for linear and branched 2-methylalkanes on silicalite. The carbon numbers of the alkanes ranged from four to nine. For branched alkanes inflection behavior was observed for all carbon numbers studied. The inflection was found to occur at a loading of four molecules per unit cell. Below this loading the branched alkanes are seen to be located predominantly at the intersections of the straight and zigzag channels. To obtain loadings higher than four, the branched alkane must seek residence in the channel interiors which is energetically more demanding and therefore requires disproportionately higher pressures; this leads to the inflection behavior. Linear alkanes with six and more carbon atoms also were found to exhibit inflection behavior. Hexane and heptane show inflection due to commensurate “freezing”; the length of these molecules is commensurate with the length of the zigzag channels. This leads to a higher packing efficiency than for other linear alkanes. Available experimental data from the literature are used to confirm the accuracy of the predictions of the CBMC simulations. Furthermore, the temperature dependency of the isotherms are also properly modeled. For purposes of fitting the isotherms we found that the dual-site Langmuir model provides an excellent description of the simulated isotherms for linear and branched alkanes. In this model we distinguish between two sites with differing ease of adsorption:  site A, representing the intersections between the straight and zigzag channels, and site B, representing the channel interiors. CBMC simulations of isotherms of 50−50 binary mixtures of C5, C6, and C7 hydrocarbon isomers show some remarkable and hitherto unreported features. The loading of the branched isomer in all three binary mixtures reaches a maximum when the total mixture loading corresponds to four molecules per unit cell. Higher loadings are obtained by “squeezing out” of the branched alkane from the silicalite and replacing these with the linear alkane. This “squeezing out” effect is found to be entropic in nature; the linear alkanes have a higher packing efficiency and higher loadings are more easily achieved by replacing the branched alkanes with the linear alkanes. The mixture isotherms can be predicted quite accurately by applying the appropriate mixture rules to the dual-site Langmuir model. This model allows the mixture isotherm to be predicted purely on the basis of the parameters describing the isotherms of the pure components. The sorption selectivity exhibited by silicalite for the linear alkane in preference to the branched alkane in mixtures of C5, C6, and C7 hydrocarbon isomers, provides a potential for the development of a novel separation technique based on entropy-driven sorption selectivity.
Article
Deviations from the ideal single-file behavior in actual systems, such as zeolitic host−guest systems with a one-dimensional channel structure, may occur if (i) there is a possibility of mutual passages of the diffusants and (ii) the influence of the file boundary on the movement of the diffusants cannot be neglected. The conditions under which such deviations may become relevant are investigated both by analytical methods and molecular dynamics simulations. The consequences of these deviations for experimental studies of single-file diffusion are discussed.
Article
Translational diffusion of molecules in one-dimensional channel systems has been measured by quasi-elastic neutron scattering. The scattering function for a single-file diffusion model has been derived in order to differentiate this transport model from normal 1D diffusion. In the AlPO4-5 structure, ordinary 1D diffusion is observed for methane and ethane, whatever the loading, which indicates that the molecules are able to pass each other. The diffusion coefficients for both molecules are on the order of 10-9 m2 s-1. On the other hand, a quite different regime is obtained by varying cyclopropane concentration in the same structure. At low loading, normal 1D diffusion is observed because the molecules can be considered as isolated, but at higher concentration cyclopropane is found to follow single-file diffusion. However, the mobility is too small to be determined. In the less open channel system of ZSM-48, ordinary 1D diffusion is found for small methane concentration, the diffusion coefficient being 2.5 × 10-9 m2 s-1, at 155 K. At medium loading, single-file diffusion is observed with a mobility factor of 2 × 10-12 m2 s-1/2. This is the first time that this technique has provided experimental evidence of single-file diffusion in zeolites.
Article
A new model is introduced to describe the loading dependency of diffusion in zeolites. The model is formulated around the idea of segregated adsorption in cage-like zeolites, that is, that molecules are located either in the cage or its window site. Furthermore, only the molecules located at the window site are able to make a successful jump to another cage. This so-called relevant site model (RSM) is based on the Maxwell−Stefan framework for mass transport but includes one extra parameter that describes the adsorption properties of the “relevant site”. The RSM describes diffusivity data of N2 and CO2 in DDR (eight-ring cage-like zeolite) very well up to saturation. The observed diffusivity loading dependency is explained from the relative low window site occupancy that is typically much lower than the total occupancy at lower loadings. The model is successfully extended to nonisothermal diffusivity data of CO2 and N2 in DDR. Relating intermolecular correlation effects to the relevant site occupancy instead of the total occupancy leads to a quantitative prediction of the observed correlation effects and, consequently, the self-diffusivity. Analysis of the N2 data suggests positional rearrangements in the DDR cages in a certain loading range. These effects have been incorporated in the model successfully.
Article
The elementary steps of the sorption of aromatic molecules such as benzene, toluene, p-xylene, and o-xylene on nonporous amorphous SiO2 (Aerosil) and microporous silicas using HZSM-5 as an example are studied by time-resolved rapid scan IR spectroscopy. Sorption into the zeolite pores proceeds via a weakly bound physisorbed and nonlocalized state on the external surface as the dominating reaction pathway. The weak interaction leads to very low sticking probabilities on the order of 10-7 for porous and nonporous materials alike. Within the molecules investigated, the sticking coefficient increases in the series toluene, o-xylene, benzene, and p-xylene. Using a statistical thermodynamic analysis, this sequence is attributed to the symmetry of the sorbate molecule, the sorption enthalpy of the sorbate increasing with the molar mass, and the space the sorbate molecule occupies on the surface.
Article
A new titanosilicate, named Del-Ti-MWW, has been prepared by delaminating the lamellar precursor of Ti-MWW, which is postsynthesized from highly deboronated MWW zeolite with the assistance of cyclic amine. The amount of organic base used for supporting surfactant in swelling the layered structure should be controlled carefully to delaminate efficiently without the collapse of the structure. Ultrasound treatment is demonstrated to delaminate the swollen material more completely. Del-Ti-MWW materials have a large surface area, which mitigates effectively the steric restrictions imposed by conventional microporous titanosilicates to bulky molecules. Del-Ti-MWW, maintaining the fundamental structure of MWW zeolite and tetrahedral Ti species in the framework position, proves to be superior to TS-1, Ti-Beta, three-dimensional Ti-MWW and even mesoporous Ti-MCM-41 in the epoxidation of a wide range of bulky alkenes with hydrogen peroxide.
Article
We have used equilibrium molecular dynamics (EMD) to study the influences of pore shape and connectivity on single component diffusion of several gases in silica zeolites using atomically detailed models of these materials. Results are presented for CH4, CF4, SF6, Ar, and Ne in silicalite, CH4, Ar, and Ne in ITQ-3, CH4, CF4, Ar, and SF6 in ITQ-7, and CH4, CF4, Ar, and H2 in ZSM-12 at room temperature. This set of four silica zeolites includes one, two, and three-dimensional pore topologies and pore volumes of several different shapes. EMD can be used to simultaneously determine the self-diffusivities and corrected diffusivities as a function of pore loading, and this has been done for every example. In combination with adsorption isotherms computed using grand canonical Monte Carlo, EMD results can also determine the transport diffusivity as a function of pore loading. The resulting transport diffusivities are reported for every example. The broad data set presented here is useful for considering the variety of diffusion behaviors that can occur for small molecules adsorbed in zeolite pores.
Article
Novel delaminated ITQ-2 zeolite has a remarkably large accessible external surface area (800 m2 g-1) and reduced microporosity (0.009 cm-3 g-1) and has attracted interest as an alternative to conventional zeolites or mesoporous MCM-41 (Corma, A.; Fornés, V.; Pergher, S. B.; Maesen, T. L. M.; Buglass, J. G. Nature 1998, 396, 353−356). α,ω-Diphenyl polyenes have been used as molecular probes to check the ability of ITQ-2 zeolite to generate organic radical cations. Of these probes, only t,t-1,4-diphenyl-1,3-butadiene (DPB) is transformed into a persistent reaction intermediate upon adsorption on ITQ-2. The process occurs in the “cup”-like cavities open to the exterior since selective silylation of the cups inhibits completely the generation of this reaction intermediate. Detection of 1-phenylnaphthalene as reaction product, EPR spectroscopy, and alternative laser flash photolysis generation strongly support 1-phenylnaphthalene radical cation as the intermediate formed after the adsorption of DPB onto ITQ-2. This contrasts with the behavior of conventional zeolites ZSM-5 and mordenite in which DPB•+ is the only species formed and demonstrates the uniqueness of the behavior of ITQ-2 as result of its unprecedented topology.
Article
Interference microscopy is applied to carry out investigations of the influence of the regular intergrowth effects commonly observed in large silicalite-1 crystals on adsorption/desorption of isobutane. The intracrystalline concentration profiles measured by interference microscopy during adsorption of isobutane are compared with those simulated using the Monte Carlo method. The comparison of the simulated and the measured profiles allows the possibility that the uptake of isobutane from the gas phase into silicalite-1 crystals proceeds through the internal interfaces separating the intergrowth sections of the crystals to be ruled out. The diffusion coefficient of isobutane obtained by comparing the simulated and the measured concentration profiles is found to be in agreement with the previously reported diffusivities of isobutane in silicalite-1.
Article
The molecular sieve UTD-1 is investigated using scanning and transmission electron microscopies (TEM), solid-state NMR spectroscopy, electron (ED) and X-ray diffraction (XRD), adsorption studies, and catalytic test reactions. The results confirm that UTD-1 is the first high-silica zeolite to contain a one-dimensional, extra-large 14-ring pore system. TEM and ED show that UTD-1 is faulted along the (002) planes. Simulations of XRD patterns of faulted structures using DIFFaX indicate that the XRD pattern of a framework containing the so-called double crankshaft chains is in better agreement with the experimental pattern than a framework with the narsarsukite chains previously reported. Thermal/hydrothermal stability studies show that UTD-1 has similar stability to other medium- and large-pore, high-silica zeolites. The ratio of isomerization to disproportionation, and the distribution of trimethylbenzene isomers in the m-xylene isomerization test reaction from UTD-1 are similar to those obtained from other large-pore zeolites (zeolites Y or L). However, UTD-1 shows a p-/o-xylene ratio of products below one.
Article
A precursor to fluoride silicalite was crystallized from a hydrothermal system containing silica, tetrapropylammonium ion (TPA+), and fluoride (F-). Single-crystal X-ray diffraction data for a twinned crystal (180 × 50 × 50 μm) of the precursor were refined in space group Pnma (a = 20.044 (2), b = 19.918 (4), and c = 13.395 (2) Å). The a axis of one twin component is parallel with the b axis of the other. Combined X-ray intensities (hkl and khl) were separated in a least-squares refinement. The silica framework has the same topology as that of silicalite and ZSM-5 zeolite. A weak negative correlation between the secant of angle (Si-O-Si) and Si-O is consistent with molecular-orbital models. The TPAF has similar geometry to tetrapropylammonium bromide and lies at the intersection of the 10-ring channels of the framework in a position consistent with a template mechanism of crystallization. The end carbon atoms of propyl groups lie 2.7 and 3.1 Å away from the end carbon atoms of adjacent propyl groups, and there is insufficient room for replacement of propyl by n-butyl. Oblique optical extinction and slight anomalies in high-angle X-ray diffractions indicate monoclinic symmetry, but the sharpness of X-ray powder diffractions limits angular deviations to less than 0.1° from orthogonal geometry. Slight collapse of the framework and positional disorder of the TPAF may be responsible.
Article
This study examines the roles of intracrystallite diffusion and reaction phenomena during the catalytic cracking of vacuum gas oil. Catalytic cracking experiments on FCC-type catalysts were performed in a fluidized bench-scale CREC riser simulator. This reactor was operated under close-to-industrial FCC conditions in terms of temperature, reaction time, partial pressures of reactant and products, and catalyst-to-oil ratio. The activity and selectivity of two USY zeolite catalysts, with very similar properties but varying zeolite crystallite sizes, were determined. A five-lump kinetic model describing the catalytic cracking of gas oil to light cycle oil, gasoline, light gases, and coke and accounting for diffusional constraints experienced by hydrocarbons while evolving in the zeolite pore network was considered. The results show that the catalyst with the smaller crystallites provided higher activity and selectivity toward desirable intermediate products (gasoline with low aromatics) and lower selectivity for terminal products (coke), indicating that diffusion plays a significant role in catalytic cracking. Diffusivity and kinetic parameters, including modified Thiele modulus and effectiveness factor, were established to determine the effects of crystallite size and temperature on the operating regime of the catalyst. It was found that, in the 510−530 °C range, the overall cracking rate is controlled by the highly temperature-sensitive intracrystalline gas oil transport, whereas in the 550−570 °C range, the overall cracking rate is dominated by a mildly temperature-sensitive intrinsic cracking rate.
Article
A new class of acidic catalysts, useful in catalytic cracking of hydrocarbons, has been discovered: crystalline aluminosilicates of the faujasite type into which hydrogen ions have been introduced to replace a portion of the original sodium ions. Polyvalent ions replace the major part of the remaining sodium ions. Thus, the catalysts are classified as "metal-acid aluminosilicates." Surprising properties of these catalysts are their extraordinary activity and selectivity. A rare earth-acid faujasite was shown to have an activity for gas oil cracking more than 100 times as great as a conventional amorphous silica-alumina catalyst. Moreover, up to 20% more gasoline has been obtained with these catalysts than with conventional silica-alumina catalysts at the same conversion level. These materials have been used in the preparation of the new commercial cracking catalyst called Durabead 5.