Zeolite Topology Research Articles

Cation-exchanged zeolites for the selective oxidation of methane to methanol

Development of an ideal methane activation catalyst presents a trade-off between stability and reactivity of the active site that can be achieved by tuning the transition metal cation, active site motif and the zeolite topology.

Synthesis of New Microporous Zincosilicates with CHA Zeolite Topology as Efficient Platforms for Ion-Exchange of Divalent Cations.

There is growing interest to develop zeolite materials capable of stabilizing divalent cations such as Cu2+ , Fe2+ , and Ni2+ for catalytic applications. Herein the synthesis of a new microporous zincosilicate with CHA zeolite topology is reported for the first time, by particularly focusing on the mixing procedures of the raw materials to prevent the precipitation of zinc oxides/hydroxides and the formation of impurity phases. The obtained zincosilicate CHA products possess remarkably higher ion-exchange ability for catalytically useful, divalent cations, demonstrated here using Ni2+ as an example, compared to that of aluminosilicate and zincoaluminosilicate analogs. It is anticipated that these zincosilicate CHA materials can be an efficient platform for several important catalytic reactions. In addition, the present finding would provide a general guideline for effective substitution of other heteroatoms into the zeolite frameworks.

Active Sites of M(IV)-incorporated Zeolites (M\u2009=\u2009Sn, Ti, Ge, Zr)

M(IV)-incorporated zeolites have recently aroused wide interest due to outstanding catalytic effects while their active sites remain largely elusive. Here periodic density functional theory calculations are conducted finding that active sites are determined jointly by identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites. All M2(IV) active sites in BEA zeolites are penta-coordinated with chemisorption of one water while subsequent water molecules that form only H-bonds promote chemisorption of the first water, especially the second water possessing comparable or even higher adsorption strengths as the first water; Ti(IV) and Ge(IV) active sites at the intersection remain penta-coordinated and Sn(IV) and Zr(IV) active sites prefer to hexa-coordination although potentially expanded to hepta-coordination. Different from other zeolites, Ti(IV) active sites in FER zeolites are hexa-coordinated as Sn(IV) active sites, due to the promoting effect of the first water. Lewis acidic defects expand Ti(IV) active sites to hexa-coordination while inhibit the formation of hepta-coordinated Sn(IV) species. Two forms of Brϕnsted acidic defects exist for Sn(IV) sites instead of only one for Ti(IV) sites, and all M(IV) Brϕnsted acidic defects, regardless of different acidic forms and M(IV) ions, can chemisorb only one water.

Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties.

Interest for functional silver clusters (Ag-CLs) has rapidly grown over years due to large advances in the field of nanoscale fabrication and materials science. The continuous development of strategies to fabricate small-scale silver clusters, together with their interesting physicochemical properties (molecule-like discrete energy levels, for example), make them very attractive for a wide variety of applied research fields, from biotechnology and the environmental sciences to fundamental chemistry and physics. Apart from useful catalytic properties, silver clusters (Agn, n < 10) were recently shown to also exhibit exceptional optical properties. The optical properties and performance of Ag-CLs offer strong potential for their integration into appealing micro(nano)-optoelectronic devices. To date, however, the rational design and directed synthesis of Ag-CLs with specific functionalities has remained elusive. The inability for rational design stems mainly from a lack of understanding of their novel atomic-scale phenomena. This is because accurately studying silver cluster systems at such a scale is hindered by the perturbations introduced during exposure to various experimental probes. For instance, silver possesses a strong tendency to cluster and form ever-larger Ag aggregates while probed with high-energy electron beams and X-ray irradiation. As well, there exists a need to provide a stabilizing environment for which Agnδ+ clusters can persist, setting up a complex interacting guest-host system, as isolated silver clusters are confined within a suitable hosting medium. Fundamental research into Agnδ+ formation mechanisms and their important optical properties is paramount to establishing truly informed synthesis protocols. Over recent years, we have developed several protocols for the ship-in-a-bottle synthesis of highly luminescent Ag-CLs within the microporous interiors of zeolite frameworks. This approach has yielded materials displaying a wide variety of optical properties, offering a spectrum of possible applications, from nano(micro)photonic devices to smart luminescent labels and sensors. The versatility of the Ag-zeolite multicomponent system is directly related to the intrinsic and complex tunability of the system as a whole. There are several key zeolite parameters that confer properties to the clusters, namely, the framework Si/Al ratio, choice of counterbalancing ions, silver loading, and zeolite topology, and cannot be overlooked. This Account is intended to shed light on the current state-of-the-art of luminescent Ag-CLs confined in zeolitic matrices, emphasizing the use of combinatorial approaches to overcome problems associated with the correct characterization and correlation of their structural, electronic, and photoluminescence properties, all to establish the important design principles for developing functional silver-zeolite-based materials. Additionally, examples of emerging applications and future perspectives for functional luminescent Ag-zeolite materials are addressed in this Account.

Co-reaction of methanol and ethylene over MFI and CHA zeolitic catalysts

Co-reaction of methanol and ethylene (CME) have been performed on a number of proton type MFI zeolites and CHA zeotype materials to investigate the influence of zeolite topology and acidities upon the transformation of reactants and the formation of desired product of propylene. Two other reactions of methanol-to-olefins (MTO) and ethylene transformation (ET, with only ethylene as the reactant) were carried out in order to compare the promotion effect for the propylene productivity. The reactions were carried out at 773 K. The addition of methanol remarkably promoted the ethylene transformation over Al-poor HMFI zeolite and, more importantly, the by-products due to hydrogen-transfer reactions were efficiently inhibited, leading to higher propylene selectivity. Such promotion effect was, however, not significantly observed upon the Al-rich HMFI zeolite. Moreover, such promotion effect of methanol upon ethylene transformation was un-pronounced or non-existed upon the CHA zeotype materials including aluminosilicate, gallosilicate and silicoaluminophosphate, possibly owing to the CHA topology.

The Lewis acidity of three- and two-dimensional zeolites: The effect of framework topology

The zeolite activity in processes driven by Lewis acid sites is determined by the distribution of the extra-framework cations in the zeolite and their coordination with the framework (the stronger is the cation coordination, the weaker is its Lewis acidity). This study aims to test how much the transformation from 3D zeolite to its corresponding 2D layered form changes the extra-framework cation distribution and coordination and thus how much it affects the zeolite Lewis acid strength. Zeolites with two distinct topologies, MWW and MFI, which form layers with considerably different silanol density (1.1 and 2.9 silanols/nm2, respectively) were considered. To probe the Lewis acidity of extra-framework Li+ sites the carbon monoxide stretching frequencies and adsorption enthalpies were calculated, employing the dispersion-corrected density functional theory and the results are in good agreement with available experimental data. No or negligible differences in the Lewis acidity of Li+ sites in 2D and 3D form of zeolites with MWW topology are found; both, the extra-framework cation distribution and the coordination with the framework remain the same. On the contrary, there are significant differences in Lewis acidic properties of 3D and 2D zeolites with MFI topology. The Li+ cation coordination in 2D-MFI is considerably improved compared to 3D-MFI due to the large concentration of surface silanols and due to an increased flexibility of the material. Consequently, Lewis acidity of 2D-MFI is lower compared to corresponding 3D-MFI material. This Lewis acidity decrease also reflects the fact that the strongest Lewis acid sites found in 3D zeolite − the channel-intersection sites (coordinated with two framework oxygens only) − are not present in 2D-MFI. The results reported herein suggest that the flexibility of the layer, which increases with silanol density and varies for each zeolite topology, significantly influences its Lewis acidity; the larger is the flexibility of the layer, the better is the cation coordination with the framework and thus smaller is its Lewis acidity.

Strategy to design zeolite catalysts in the presence of biomass

LTA, GIS, *BEA and MFI zeolites have been successfully prepared using sugar cane bagasse as a hard templating agent as well as after alkaline hydrolysis of those biomass residues. Depending on the zeolite topology chosen, different effects could be highlighted: susceptibility to crystal intergrowths (LTA), formation of macroporous micron-sized assemblies (GIS), or significant impact on MFI crystal morphologies.The LTA zeolite series was used in ammonia ionic exchange experiments. A definite enhancement in the total NH4+-adsorption capacity could be evidenced for a biomass hydrolysate structured sample.In the case of MFI zeolites, different catalytic activities and stabilities were achieved in the methanol conversion into hydrocarbons. Likewise, drastic changes were observed in the selectivities, ranging from an expected methanol-to-olefins (MTO) behaviour for highly siliceous zeolites, to a close to 90% selectivity toward C5+ gasoline fraction (MTG) for counterparts with low Si/Al-ratios.

Time- and space-resolved study of the methanol to hydrocarbons (MTH) reaction - influence of zeolite topology on axial deactivation patterns.

Zeolites representing seven different topologies were subjected to life-time assessment studies as methanol to hydrocarbons (MTH) catalysts at 400 °C, P(MeOH) = 13 kPa and P(tot) = 100 kPa. The following topologies were studied: ZSM-22 (TON), ZSM-23 (MTT), IM-5 (IMF), ITQ-13 (ITH), ZSM-5 (MFI), mordenite (MOR) and beta (BEA). Two experimental approaches were used. In the first approach, each catalyst was tested at three different contact times, all giving 100% initial conversion. The life-time before conversion decreased to 50% at each contact time was measured and used to calculate critical contact times (i.e. the contact time needed to launch the autocatalytic MTH reaction) and deactivation rates. It was found that the critical contact time is strongly correlated with pore size: the smaller the pore size, the longer the critical contact time. The second experimental approach consisted of testing the catalysts in a double tube reactor with 100% initial conversion, and quenching the reaction after 4 consecutive times on stream, representing full, partial, and zero conversion. After quenching, the catalyst bed was divided into four segments, which were individually characterised for coke content (temperature-programmed oxidation) and specific surface area (N2 adsorption). The axial deactivation pattern was found to depend on pore size. With increasing pore size, the main source of coke formation changed from methanol conversion (1D 10-ring structures), to partly methanol, partly product conversion (3D 10-ring structures) and finally mainly product conversion (3D 12-ring structure). As a result, the methanol conversion capacity changed little with contact time for ZSM-5, while it increased with increasing contact time for the catalysts with smaller pore sizes, and decreased with increasing contact time for pore sizes larger than ZSM-5.

Adsorption of Glucose within M(IV)‐Incorporated Zeolites: Insights from Periodic Density Functional Theory Calculations

AbstractAlbeit the isomerization mechanism of glucose to fructose catalyzed by M(IV)‐incorporated zeolites is widely studied, scant attention has been given to the adsorption of related sugars that is critical to catalysis. Here p‐DFT calculations are conducted to have a comprehensive understanding within this context, considering the effects of adsorption modes, identity of framework‐M(IV) ions, pore topology and conformational states of glucose. Monodentate rather than bidentate adsorption structures of glucose are the most energetically favorable within all investigated zeolites except Sn‐CHA. Adsorption performances of different M(IV)‐incorporated BEA zeolites decline as Zr &gt; Sn &gt; Ti &gt; Ge, where Ti‐ and Ge‐BEA, especially the latter, is obviously inferior for sugar adsorption and catalysis. Pore topology of zeolites plays an even more pronounced effect during glucose adsorption. Non‐covalent interactions contribute significantly to the adsorption processes. Dispersion effects of different framework‐M(IV) ions, although close to each other, show a clear opposite trend as adsorption energies. FER rather than other zeolites shows surprisingly high dispersion effects (e. g.; −218 kJ/mol for Sn‐FER vs. −123 kJ/mol for Sn‐BEA). It also shows that dispersion effects for the various conformational states of glucose are closely related with structural flexibilities.

Post-Synthesis Stabilization of Germanosilicate Zeolites ITH, IWW, and UTL by Substitution of Ge for Al.

Germanosilicate zeolites often suffer from low hydrothermal stability due to the high content of Ge. Herein, we investigated the post-synthesis introduction of Al accompanied by stabilization of selected germanosilicates by degermanation/alumination treatments. The influence of chemical composition and topology of parent germanosilicate zeolites (ITH, IWW, and UTL) on the post-synthesis incorporation of Al was studied. Alumination of ITH (Si/Ge=2-13) and IWW (Si/Ge=3-7) zeolites resulted in the partial substitution of Ge for Al (up to 80 %), which was enhanced with a decrease of Ge content in the parent zeolite. In contrast, in extra-large pore zeolite UTL (Si/Ge=4-6) the hydrolysis of the interlayer Ge-O bonds dominated over substitution. The stabilization of zeolite UTL was achieved using a novel two-step degermanation/alumination procedure by the partial post-synthesis substitution of Ge for Si followed by alumination. This new method of stabilization and incorporation of strong acid sites may extend the utilization of germanosilicate zeolites, which has been until now been limited.

An Anionic Interpenetrated Zeolite-Like Metal-Organic Framework Composite As a Tunable Dual-Emission Luminescent Switch for Detecting Volatile Organic Molecules.

The luminescent MOF [(CH3 )2 NH2 ]2 [(Zn2 O)L]⋅5 DMF (NENU-519, NENU=Northeast Normal University) with a zeolite BCT topology was successfully synthesized. It is a rare example of a two-fold interpenetrated framework with a zeolite topology. NENU-519 demonstrates the ability to selectively adsorb cationic dyes. Furthermore we developed Rh6@NENU-519 (Rh6=Rhodamine 6G) as a dual-emitting sensor for probing different volatile organic molecules (VOMs) due to an energy transfer between L and the dye. The composite can be used to distinguish the isomers of o-, m-, and p-xylene and ethylbenzene using the emission-peak-height ratios of L to the dye as detectable signals, in which the readout signals are involved in the interactions between the dye@MOF composite and the guest analytes. Moreover, Rh6@NENU-519 can serve as a luminescent switch for the detection of different aromatic compounds, like benzene, benzene substituted with different groups, and pyridine. In other words, the Rh6@NENU-519 composite can be used as molecular decoder of the structural information of different VOMs into recognizable luminescent signals. Hopefully this work will open a new corridor to develop luminescent guest@MOF composites as sensors for practical applications.

The effect of alkylation route on ethyltoluene production over different structural types of zeolites

Ethyltoluenes (ET) synthesis via different alkylation routes; toluene with ethanol and ethylbenzene (EB) with methanol, was investigated over zeolites with three-dimensional (3D) pore system i.e. SSZ-33, TNU-9, ZSM-5, IM-5, and one-dimensional zeolite MOR in a fluidized-bed reactor in the temperature range 250–350°C for reaction times of 5–20s. Alkylation of toluene with ethanol provided higher yields and selectivities to ethyltoluenes over all zeolites. The maximum ET yield achieved for toluene ethylation was 13.7wt.% using IM-5 and ZSM-5 whereas 12.5wt.% was obtained in EB methylation over SSZ-33. SSZ-33 provided the highest ET-selectivity of ∼50% due to the combine effect of its 3D topology and 12-ring channels. In contrast to ET selectivity, p-ethyltoluene selectivity was almost the same, ∼26%, irrespective of the zeolite topology. Kinetic studies of toluene ethylation over the zeolites were conducted using power law coupled with time-on-stream deactivation model. A satisfactory correlation between experimental data and the model result was achieved. The activation energy of the various zeolites used for the alkylation of toluene to ET decreased in the order: IM-5 (58.2kJ/mol)>SSZ-33 (39.7kJ/mol)>TNU-9 (27.3kJ/mol)>MOR (20.2kJ/mol)>ZSM-5 (17.0kJ/mol) which was explained by different acid strength and nature of the zeolites.

Catalytic Oxidation of Methane into Methanol over Copper-Exchanged Zeolites with Oxygen at Low Temperature.

The direct catalytic conversion of methane to liquid oxygenated compounds, such as methanol or dimethyl ether, at low temperature using molecular oxygen is a grand challenge in C–H activation that has never been met with synthetic, heterogeneous catalysts. We report the first demonstration of direct, catalytic oxidation of methane into methanol with molecular oxygen over copper-exchanged zeolites at low reaction temperatures (483–498 K). Reaction kinetics studies show sustained catalytic activity and high selectivity for a variety of commercially available zeolite topologies under mild conditions (e.g., 483 K and atmospheric pressure). Transient and steady state measurements with isotopically labeled molecules confirm catalytic turnover. The catalytic rates and apparent activation energies are affected by the zeolite topology, with caged-based zeolites (e.g., Cu-SSZ-13) showing the highest rates. Although the reaction rates are low, the discovery of catalytic sites in copper-exchanged zeolites will accelerate the development of strategies to directly oxidize methane into methanol under mild conditions.

First-Principles Kinetic Study on the Effect of the Zeolite Framework on 1-Butanol Dehydration

Understanding the role of zeolite topology in defining its catalytic performance is of prime importance for the development of catalytic processes. Herein, a first-principles-based microkinetic study of 1-butanol dehydration is used to illustrate the effect of different zeolites (i.e., H-FAU, H-ZSM-5, H-ZSM-22, and H-FER) on the dehydration activity and product selectivity. Under identical reaction conditions, microkinetic simulations show significant variation in dehydration rates and butene/ether selectivity profiles within the different zeolites. H-ZSM-5 has the highest catalytic activity, whereas H-FAU and H-FER exhibit a higher butene selectivity. In the large pore H-FAU, the weaker dispersive stabilization of the dimer makes the butene formation by monomolecular direct dehydration via a concerted anti elimination compete with di-n-butyl ether formation. In H-FER, steric constraints due to partial confinement of the protonated di-n-butyl ether in the 8-MR channel decrease its stability, favoring its ...

Catalytic features of CuZnZr–zeolite hybrid systems for the direct CO2-to-DME hydrogenation reaction

The direct hydrogenation of CO2 to dimethyl ether (DME) has been studied in a fixed bed reactor, in presence of supported CuZnZr/zeolite hybrid catalysts. To study the effects of zeolite topology on catalyst performance, two different home-made zeolite structures (i.e., FER and MOR) were used as carriers during gel-oxalate coprecipitation of CuZnZr precursors. The hybrid FER-based catalyst showed a very good activity-selectivity pattern at 5.0MPa, also exhibiting an interesting DME productivity (STY, 752gDME/Kgcat/h), with no coke formation under the adopted experimental conditions (TR, 280°C; GHSV, 8800NL/Kgcat/h). The presence of well dispersed metal-oxide clusters of the precursors on the 2-D zeolite framework of FER was indicated as a key factor to realize a more efficient mass-transferring of MeOH from CuZnZr sites to zeolite surface, favoring so the formation of DME with higher yields than obtained using MOR as support.

N-Propylbenzene Disproportionation: An Efficient Tool for Assessing the Framework Topology of Large-Pore Zeolites

The mechanisms of n-propylbenzene (nPB) disproportionation over various large-pore (LaNa-Y, H-Y, H-mordenite, and H-beta) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites were investigated. The GC–MS results from the used zeolite catalysts demonstrate that monopropylated 1,1-diphenylpropane and dipropylated 1,1-diphenylpropane derivatives are serving as the main reaction intermediates of bimolecular nPB disproportionation, whereas 1-phenyl-2-propylphenylpropane, 1-propylphenyl-2-phenylpropane, and 1-propylphenyl-2-propylphenylpropane species are the intermediates of some particular side reactions. It was found that the intrazeolitic buildup of these five different groups of diphenylpropane-based species is strongly influenced by the size and shape of zeolite void spaces, as was also corroborated by DFT calculation results. This allows nPB disproportionation to be quite useful for estimating the framework topology of zeolites with unknown structures, especially those of large-pore materials.

Ammoxidation of ethylene to acetonitrile over vanadium and molybdenum supported zeolite catalysts prepared by solid-state ion exchange

This work reports on the investigation of the influence of the parent zeolite topology (MFI, MOR and USY) on the ammoxidation of ethylene to acetonitrile over vanadium and molybdenum oxide supported zeolite catalysts. The physico-chemical properties were investigated by several characterization techniques such as XRD, N2-adsorption, 27Al MAS NMR, TEM, XPS, DR UV–vis, Raman and DRIFT spectroscopies and H2-TPR. From the catalytic results, V and Mo oxide species in USY and MOR zeolites led to less active catalysts when compared to MFI structure. These results suggest that the catalytic performances depend strongly on the zeolite structure and thus, the size of the formed metal oxide particles. The catalytic activity and selectivity are controlled by the porous structure and the chemical state of V and Mo species. The extent of dispersion and reducibility of supported M-Ox (MV or Mo) species are governed by the chemical identity of the support as detected by TPR analysis and optical absorption spectroscopy. The good catalytic performance of MFI-type zeolite might be related to the high dispersion of metal oxide species and to the internal pore space which permits an effective accessibility of the reactants.

Catalysis of metal supported zeolites for dealkylation–transalkylation of alkyl-aromatics

In search for catalysts with tailorable pore structure for simultaneous dealkylation and transalkylation of C9 aromatics to produce xylenes, the catalytic performance of layered MWW-type zeolites were compared with mordenite (MOR) in terms of the effects of zeolite topology, metal(s) incorporation and preparation method. During the catalytic reaction of trimethylbenzene (TMB)–methylethylbenzene (MEB) model feed, the ion exchanged Pt-MOR catalyzed a significantly higher MEB dealkylation than H-MOR resulting in an increase in xylene yield (from about 27wt% to 34wt%). Impregnation of Re into the Pt exchanged zeolite would moderate Pt hydrogenation activity and reduce MEB conversion. Compared to MOR, MCM-22 and ITQ-2 exhibited lower xylene yield and higher selectivity of C10 aromatics. On the other hand, pillared MCM-36 catalyzed escalated TMB conversion and higher xylene yield at the expense of product selectivity of toluene and C10 aromatics. The improvement in the catalytic performance of MCM-36 is attributed to the enlarged supercage for favorable catalysis for toluene-TMB transalkylation than TMB disproportionation.

Impact of pore topology and crystal thickness of nanosponge zeolites on the hydroconversion of ethylbenzene

Gas-phase hydroconversion of ethylbenzene was investigated using *MRE, MFI and MTW-type zeolite nanosponges with a hydrogenating component (Pt/Al2O3). Spacious channel interconnections in MFI was concluded to remarkably impact product selectivity compared to *MRE and MTW.

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

Ingeniously selecting zeolite topology and acidity, reaction temperature and guest molecule loading enables tuning the reaction mechanism of zeolite-catalyzed methylation reactions.