All-silica MFI Research Articles

Structure-property relationships that influence platinum stability in all-silica or highly siliceous zeolites

In this study we examine the role of alkali cations and siloxy/silanol nests in stabilizing platinum clusters on all-silica zeolites. This work complements a recent study by the Corma group at Instituto de Tecnología Química (ITQ) that investigated platinum stability within Pt–K/MFI zeolites prepared by one-pot methods [1]. Here we demonstrate that stable platinum particles are likewise produced either by impregnating platinum onto an alkali-cation-containing all-silica MFI or by deboronating a borosilicate to create silanol nests and then co-impregnating with potassium and platinum sources. In this context, stability is assessed by resistance to sintering as measured by hydrogen chemisorption. Platinum impregnation alone onto a deboronated or borosilicate MFI does not produce stable platinum clusters after reduction. Furthermore, a one-pot synthesis of a Pt–K/B-MFI (borosilicate) or exchange of Pt onto a K-containing borosilicate does not lead to stable Pt particles. This further demonstrates the necessity of internal silanol/siloxy nests in stabilizing platinum clusters. In summary, silanols and alkali cations together create silanol/siloxy nests that stabilize Pt clusters. Hence, stable Pt clusters can be achieved by 1) a one-pot synthesis of the all-silica zeolite in the presence of Pt and alkali cations, 2) platinum impregnation on an all-silica sample that has occluded a high enough concentration of an alkali cation during the synthesis, or 3) by co-impregnating platinum and alkali metal cations together onto a calcined zeolite that has a high density of internal silanol nests. These nests can be generated by removal of boron from the framework of the zeolite. In the current study, we have examined these systems by 11B and 29Si NMR spectroscopy.

Structure-property relationships that influence platinum stability in all-silica or highly siliceous zeolites

In this study we examine the role of alkali cations and siloxy/silanol nests in stabilizing platinum clusters on all-silica zeolites. This work complements a recent study by the Corma group at Instituto de Tecnología Química (ITQ) that investigated platinum stability within Pt–K/MFI zeolites prepared by one-pot methods [1]. Here we demonstrate that stable platinum particles are likewise produced either by impregnating platinum onto an alkali-cation-containing all-silica MFI or by deboronating a borosilicate to create silanol nests and then co-impregnating with potassium and platinum sources. In this context, stability is assessed by resistance to sintering as measured by hydrogen chemisorption. Platinum impregnation alone onto a deboronated or borosilicate MFI does not produce stable platinum clusters after reduction. Furthermore, a one-pot synthesis of a Pt–K/B-MFI (borosilicate) or exchange of Pt onto a K-containing borosilicate does not lead to stable Pt particles. This further demonstrates the necessity of internal silanol/siloxy nests in stabilizing platinum clusters. In summary, silanols and alkali cations together create silanol/siloxy nests that stabilize Pt clusters. Hence, stable Pt clusters can be achieved by 1) a one-pot synthesis of the all-silica zeolite in the presence of Pt and alkali cations, 2) platinum impregnation on an all-silica sample that has occluded a high enough concentration of an alkali cation during the synthesis, or 3) by co-impregnating platinum and alkali metal cations together onto a calcined zeolite that has a high density of internal silanol nests. These nests can be generated by removal of boron from the framework of the zeolite. In the current study, we have examined these systems by 11B and 29Si NMR spectroscopy.

Pure silica-supported transition metal catalysts for the non-oxidative dehydrogenation of ethane: confinement effects on the stability

All-silica MFI zeolite was used as a support for the synthesis of promoter-free robust transition metal catalysts. Effects of different physical parameters and catalyst deactivation mechanism were studied for the ethane dehydrogenation reaction.

Ethanol and Water Adsorption in Conventional and Hierarchical All-Silica MFI Zeolites.

Hierarchical zeolites containing both micro- (<2 nm) and mesopores (2-50 nm) have gained increasing attention in recent years because they combine the intrinsic properties of conventional zeolites with enhanced mass transport rates due to the presence of mesopores. The structure of the hierarchical self-pillared pentasil (SPP) zeolite is of interest because all-silica SPP consists of orthogonally intergrown single-unit-cell MFI nanosheets and contains hydrophilic surface silanol groups on the mesopore surface while its micropores are nominally hydrophobic. Therefore, the distribution of adsorbed polar molecules, like water and ethanol, in the meso- and micropores is of fundamental interest. Here, molecular simulation and experiment are used to investigate the adsorption of water and ethanol on SPP. Vapor-phase single-component adsorption shows that water occupies preferentially the mesopore corner and surface regions of the SPP material at lower pressures (P/P 0 < 0.5) while loading in the mesopore interior dominates adsorption at higher pressures. In contrast, ethanol does not exhibit a marked preference for micro- or mesopores at low pressures. Liquid-phase adsorption from binary water-ethanol mixtures demonstrates a 2 orders of magnitude lower ethanol/water selectivity for the SPP material compared to bulk MFI. For very dilute aqueous solutions of ethanol, the ethanol molecules are mostly adsorbed inside the SPP micropore region due to stronger dispersion interactions and the competition from water for the surface silanols. At high ethanol concentrations (C EtOH > 700 g L-1), the SPP material becomes selective for water over ethanol.

Highly CO2-selective and moisture-resistant bilayer silicalite-1/SSZ-13 membranes with gradient pores for wet CO2/CH4 and CO2/N2 separations

Surface hydrophobicity of zeolite membrane which is related to moisture resistance increases with the increase of Si/Al ratio in the framework. An all-silica MFI (silicalite-1) layer was grown on aluminium-containing SSZ-13 membrane as a water-proof shield in this study. The formation process of silicalite-1 layer upon SSZ-13 membrane was investigated. And the moisture-resistant mechanism of bilayer silicalite-1/SSZ-13 membrane was discussed. The gradient pores (5.6 Å @ 3.8 Å) of bilayer silicalite-1/SSZ-13 membranes guarantee the separation efficiency. The bilayer membranes displayed similar CO2 permeances with the original SSZ-13 membranes in the dry mixture because that the modified silicalite-1 layer had larger pores and was CO2-selective. Moreover, the bilayer membrane displayed higher moisture resistance than the original SSZ-13 membrane did at low temperatures. The best bilayer membrane had CO2 permeances of 1.5 × 10-6 and 1.4 × 10-6 mol m-2 s-1 Pa-1 and CO2/CH4 and CO2/N2 selectivities of 153 and 22 in dry and equimolar CO2/CH4 and CO2/N2 mixtures at 298 K, respectively. The CO2/CH4 selectivity of the bilayer membranes in the wet mixture were only 19.4% lower than these in the dry mixtures at 323 K, respectively. The decline rate of the bilayer membrane were much lower than that (39.3%) for the monolayer SSZ-13 membrane. The long-term stability of separation performance of bilayer membranes in the wet mixture was also investigated.

Stable Cr-MFI Catalysts for the Nonoxidative Dehydrogenation of Ethane: Catalytic Performance and Nature of the Active Sites

The nonoxidative catalytic dehydrogenation of ethane allows the production of ethylene at lower temperatures than those applied in steam crackers. This, however, requires stable catalysts that minimize coke production. Here, we report a single-component, promoter-free, low-loading, Cr-based catalyst exhibiting high activity, long-term stability, and improved regeneration properties for the direct dehydrogenation of ethane to ethylene. According to our detailed operando X-ray absorption spectroscopic analysis, the use of all-silica MFI zeolite as support promotes the stabilization of CrII(−O–Si≡)2 species with high coke resistance, even when the dehydrogenation is carried out under high ethane partial pressures (1.5 bar).

Platelike MFI Crystals with Controlled Crystal Faces Aspect Ratio.

Zeolite crystals offering a short diffusion pathway through the pore network are highly desired for a number of catalytic and molecule separation applications. Herein, we develop a simple synthetic strategy toward reducing the thickness along the b-axis of MFI-type crystals, thus providing a short diffusion path along the straight channel. Our approach combines preliminary aging and a fluoride-assisted low-temperature crystallization. The synthesized MFI crystals are in the micrometer-size range along the a- and c-axis, while the thickness along the b-axis is a few tens of nanometers. The synthesis parameters controlling the formation of platelike zeolite are studied, and the factors controlling the zeolite growth are identified. The synthesis strategy works equally well with all-silica MFI (silicalite-1) and its Al- and Ga-containing derivatives. The catalytic activity of platelike ZSM-5 in the methanol-to-hydrocarbons (MTH) reaction is compared with a commercial nanosized ZSM-5 sample, as the platelike ZSM-5 exhibits a substantially extended lifetime. The synthesis of platelike MFI crystals is successfully scaled up to a kilogram scale.

Deep neural network learning of complex binary sorption equilibria from molecular simulation data.

We employed deep neural networks (NNs) as an efficient and intelligent surrogate of molecular simulations for complex sorption equilibria using probabilistic modeling. Canonical (N 1 N 2 VT) Gibbs ensemble Monte Carlo simulations were performed to model a single-stage equilibrium desorptive drying process for (1,4-butanediol or 1,5-pentanediol)/water and 1,5-pentanediol/ethanol from all-silica MFI zeolite and 1,5-pentanediol/water from all-silica LTA zeolite. A multi-task deep NN was trained on the simulation data to predict equilibrium loadings as a function of thermodynamic state variables. The NN accurately reproduces simulation results and is able to obtain a continuous isotherm function. Its predictions can be therefore utilized to facilitate optimization of desorption conditions, which requires a laborious iterative search if undertaken by simulation alone. Furthermore, it learns information about the binary sorption equilibria as hidden layer representations. This allows for application of transfer learning with limited data by fine-tuning a pretrained NN for a different alkanediol/solvent/zeolite system.

In Situ Aluminum Migration into Zeolite Framework during Methanol-To-Propylene Reaction: An Innovation To Design Superior Catalysts

This work involves methanol-to-propylene (MTP) conversion over aluminosilicate MFI (ZSM-5), borosilicate MFI (B-ZSM-5), and all-silica MFI (Silicate-1). Both B-ZSM-5 and Silicate-1 were inactive in MTP reaction. However, extruded Silicate-1 sample prepared by extrusion with Al2O3 binder, followed by acid washing treatment, displayed a rapid increase in methanol conversion in the initial 30 h time on stream (TOS), and then retained 99% methanol conversion, higher propylene selectivity (52.2%), and higher propylene/ethylene ratio (11.3) for 400 h TOS. Silicate-1 modified with AlCl3 or Al(NO3)3 showed similar catalytic performance as the extruded samples. Several AlCl3-modified Silicate-1 samples after various TOS values were regenerated and characterized by NH3-termperature-programmed desorption (TPD), Fourier transform infrared (FT-IR) spectroscopy, and 27Al magic angle spinning nuclear magnetic responance (MAS NMR). The results point to continuous aluminum insertion into Silicate-1 framework during reacti...

One-pot synthesis of silanol-free nanosized MFIzeolite.

The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Here, we report and characterize, using a combination of experimental and theoretical techniques, the one-pot synthesis of silanol-free nanosized MFI-type zeolites by introducing atomically dispersed tungsten; this prevents silanol group occurrence by forming flexible W-O-Si bridges. These W-O-Si bonds are more stable than Si-O-Si in the all-silica MFI zeolite. Tungsten incorporation in nanosized MFI crystals also modifies other properties such as structural features, hydrophobicity and Lewis acidity. The effect of these is illustrated on the catalytic epoxidation of styrene and separation of CO2 and NO2. Silanol-free nanosized W-MFI zeolites open new perspectives for catalytic and separation applications.

Molecular modeling of adsorbed NDMA and water in MFI zeolites

N-nitrosodimethylamine (NDMA), which is a carcinogenic and toxic N-nitrosamine, can be found in water resources associated with a multitude of processes in various industrial facilities or merely as a by-product of water or wastewater treatment. Therefore, the removal of NDMA from drinking water represents an important human safety and public health concern. The present paper presents a density functional theory study of NDMA adsorption in all-silica MFI, Na-ZSM-5 and H-ZSM-5 zeolites. The stability of NDMA inside the zeolite pores was investigated by calculating the amount of energy released during adsorption. Various configurations of adsorbed NDMA to the zeolites were investigated, predominantly at the intersection of straight and sinusoidal channels. The strength of the adsorption energies followed the order H-ZSM-5>Na-ZSM-5>all-silica MFI. NDMA has a dipole moment and the strongest binding of NDMA occurred through the interactions of the negatively charged O atom of the molecule to positive atoms of the zeolite. For comparison, similar calculations were performed for water adsorption in these three zeolites. The adsorption energy of water in these three structures followed the order Na-ZSM-5>H-ZSM-5>all-silica MFI. We also incorporated van der Waals corrections in the simulations, which had the effect of stabilizing NDMA within the zeolite channels, but did not significantly change the relative stability of the different adsorption geometries. It was concluded that H-ZSM-5 is a good candidate to remove NDMA. Finally, it should be pointed out that H-ZSM-5 has the largest difference in binding energy between NDMA and water of the studied zeolites, suggesting that separation of the two molecules is indeed feasible due to preferential interactions between zeolite and adsorbate molecule.

Highly Selective Lewis Acid Sites in Desilicated MFI Zeolites for Dihydroxyacetone Isomerization to Lactic Acid

Desilication of commercial MFI-type (ZSM-5) zeolites in solutions of alkali metal hydroxides is demonstrated to generate highly selective heterogeneous catalysts for the aqueous-phase isomerization of biobased dihydroxyacetone (DHA) to lactic acid (LA). The best hierarchical ZSM-5 sample attains a LA selectivity exceeding 90 %, which is comparable to that of the state-of-the-art catalyst (i.e., the Sn-beta zeolite); this optimized hierarchical catalyst is recyclable over three runs. The Lewis acid sites, which are created through desilication along with the introduction of mesoporosity, are shown to play a crucial role in the formation of the desired product; these cannot be achieved by using other post-synthetic methods, such as steaming or impregnation of aluminum species. Desilication of other metallosilicates, such as Ga-MFI, also leads to high LA selectivity. In the presence of a soluble aluminum source, such as aluminum nitrate, alkaline-assisted alumination can introduce these unique Lewis acid centers in all-silica MFI zeolites. These findings highlight the potential of zeolites in the field of biomass-to-chemical conversion, and expand the applicability of desilication for the generation of selective catalytic centers.

Diffusion of alkane mixtures in MFI zeolite

The proper description of mixture diffusion in zeolites is important in several separation and reaction applications [1,2]. In recent years molecular dynamics (MD) simulations have been used to investigate diffusion of mixtures of molecules in a variety of zeolite topologies such as AFI, FER, MFI, BEA, ISV, FAU, LTA, CHA, MOR, and DDR [3– 8]. These studies show that the zeolite structure, topology and connectivity have a strong influence on the mixture diffusion characteristics. For zeolites with intersecting channel structures, such as MFI, ISV and BEA the more mobile species is slowed down and the more tardy species is speeded up. The speeding up and slowing down phenomena also manifests for diffusion of mixtures in zeolites such as AFI, MOR and FER in which the transport is predominantly one dimensional in character. For cage type zeolites with narrow windows connecting cages, such as CHA, DDR and LTA diffusion of components occur practically independently on one another. There is also experimental evidence to support the broad picture on mixture diffusion characteristics arising from MD simulation studies [2,9– 13]. It is in the context of the above mentioned studies that the MD simulation results of Hussain and Titiloye [14] for mixtures containing alkanes (methane (C1), ethane (C2), and propane (C3)) in all-silica MFI zeolite deserve comment and critical analysis. We have summarized their data on self-diffusivities in Fig. 1 for equimolar binary mixtures of (a) C1–C2, (b) C1–C3, and (c) C2–C3 at 300 K as a function of the total loading in the mixtures; their data on pure component diffusivities are also plotted for comparison with mixture diffusivity. Consider their C1–C2 data; see Fig. 1a. While the self-diffusivity of pure C1 is consistently higher than that of pure C2, the self-diffusivities in

Characterisation of Lewis and Brønsted acidic sites in H-MFI and H-BEA zeolites: a thermodynamic and ab initio study

Adsorption enthalpies of N 2, CO, CH 3CN and NH 3 on H-BEA and H-MFI zeolites have been measured calorimetrically at 303K in order to assess the energetic features of the various interactions occurring within the zeolite nanocavities, namely: (i) specific adsorption on Lewis and Brønsted acidic sites; (ii) H-bonding interactions with hydroxyl nests; (iii) dispersive forces interactions with the walls of the cavities ( confinement effects). Confinement effects have been investigated on an all-silica MFI zeolite (silicalite). The interaction of the molecular probes with model clusters mimicking Lewis and Brønsted sites has been simulated at ab initio level. The combined use of the two different approaches allowed to discriminate among the different interactions contributing to the measured heat of adsorption (−Δ ads H). Whereas CO and N 2 single out contributions from Lewis and Brønsted acidic sites, CH 3CN and NH 3 are not preferentially adsorbed on Lewis sites, suggesting that the adsorption on Brønsted sites is competitive with Lewis sites. The zero-coverage heats of adsorption for the different probes on the various systems correlate well with the proton affinity (PA) of the molecular probes.

Van der Waals interactions on acidic centres localized in zeolites nanocavities: a calorimetric and computer modeling study

Adsorption enthalpies of CO, N2 and Ar on H-BEA and H-MFI zeolites have been measured calorimetrically at 303 K in order to assess the energetic features of Lewis and Brønsted acidic sites. The interaction of the molecular probes with model clusters mimicking Lewis and Brønsted sites has been simulated at ab initio level, and a good agreement between the experimental and the calculated binding energies (BE) has been found. The combined use of the two different approaches has allowed to discriminate among the different interactions (Lewis, Brønsted and dispersive forces) contributing to the measured heat of adsorption. Confinement effects have been investigated on an all-silica MFI zeolite in order to measure the dispersive forces contribution due to the zeolite cavities. For the acidic zeolites both uptake and enthalpy changes correlate well with proton affinity and polarizability of the molecular probes. Whereas CO and N2 single out contributions from Lewis and Brønsted acidic sites, Ar is only sensitive to confinement effects.

Molecular mechanics studies on MFI-typezeolites: Part 3. The monoclinicorthorhombic phase transition

The transition of the all-silica MFI structure from monoclinic toward orthorhombic symmetry isexamined with our molecular mechanics force field for all-silica zeolites. From experimental measurements, it is known that the symmetry of the framework changes from monoclinic (space group P2 1/n.1.1.) toward orthorhombic (space group Pnma) by increasing the temperature above 340 K. The experimentally observed change in the geometry of the zeolite is compared with that calculated by molecular mechanics. The phase transition is accompanied by a shift of (100) pentasil layers.