The Role of Mesopores in Intracrystalline Transport in USY Zeolite: PFG NMR Diffusion Study on Various Length Scales

Direct Synthesis of Highly Siliceous Faujasite-type Zeolite Enabled by Low Charge Density Organic Structure-directing Agents

Ultrastable Y (USY) zeolites (Faujasite-type, FAU) with high SiO2/Al2O3 ratios (SARs) have been widely applied in fluidized catalytic cracking and hydrocracking processes. However, their preparation typically involves labor-intensive post-treatments that inevitably introduce defects, extra-framework species, and compositional gradients. Herein, we report the direct synthesis of FAU-type zeolite with a record-high SAR up to 21.28, which shows superior catalytic performance in the catalytic cracking of cumene and straight-run diesel. This was achieved by using a highly siliceous initial gel and seed, together with low charge-density organic structure-directing agents (OSDAs) of tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) cations, while minimizing the use of high charge-density inorganic Na+ cations. Comprehensive NMR analyses, including two-dimensional (2D) heteronuclear correlation (HETCOR) experiments (1H-13C, 1H-27Al, 1H-29Si), and synchrotron radiation X-ray diffraction-based Rietveld refinements, revealed that (1) TMA+ and TBA+ preferentially interact with Si species over Al species in the initial gel, promoting Si incorporation into the FAU structure's long-range order; and (2) TMA+ cations were tightly confined within the sodalite (sod) cages, while TBA+ cations occupied the faujasite (fau) supercages, adopting folded butyl chain configurations. The encapsulation of low charge-density OSDA cations, coupled with the exclusion of high charge-density Na+ cations, resulted in a highly siliceous Y zeolite with a SAR of 21.28. The synthesized Y zeolite exhibited improved durability and enhanced activity in cumene cracking and comparable selectivity for the target products [liquefied petroleum gas (LPG) and gasoline] while reducing coke formation by 28% in straight-run diesel catalytic cracking compared to conventional USY zeolites.

Molecular Self-Gating Inside a Zeolite Catalyst

Diffusion is a ubiquitous process that is strongly correlated with concentration. Based on developed three-dimensional free energy and a continuous-time random-walk coarse-graining method, we found the optimal diffusion pathway under confinement, determined all diffusional energy barriers, and identified the major units of zeolite where molecular diffusion is limited. Interestingly, a novel diffusion mechanism was determined in the nanopore of a zeolite catalyst by molecular dynamics simulation, pulsed field gradient, and 2D exchange spectroscopy (EXSY) NMR experiments. We describe a "molecular self-gating effect" that effectively predominates the diffusion process in cage-type (e.g., RHO and MER) zeolites through a "traffic jam" and a "smooth traffic" process. Initially, transport is hindered by molecules forming a gate (traffic jam); then, as the number of molecules reaches a certain threshold, diffusion increases rapidly due to the synergistic collisions of aggregated molecules upon the gate (smooth traffic). This unique diffusion behavior is observed here for the first time and illustrates a microscopic mechanism dictated by the molecular self-gating effect in a confined space. The exploitable diffusion disclosed herein should shed new light on the fundamental understanding of transport, as well as enrich diffusion behavior under confinement.

Generating Beta Zeolite Nanosheets of Intergrown Polymorph B and C Using Polycationic Structure-Directing Agent

Zeolitic nanosheets possess great potential in catalysis due to their enhanced transport property and accessibility toward bulky molecules compared to conventional micron- meter scale crystals. However, the generation of Beta zeolite nanosheets, which are crucial for industrial catalysis, is still challenging for its intergrowth nature. In this work, aluminosilicate Beta nanosheets of ca. 16 nm thick with house-of-cards architecture are generated using a special polycationic organic structure directing agent (OSDA), [-N+(CH3)2-C5H10-N+(CH3)2-C6H12-]n[Br-]2n, in greener hydroxide media. Transmission electron microscopy and electron tomography reconstruction revealed that the nanosheets are composed of unprecedented intergrowth of polymorphs B and polymorph C (i.e., BEC topology), possessing only straight micropore channels. 2D 29Si{1H} and 27Al{1H} heteronuclear correlation NMR spectra reveal that the framework Al atoms are mainly situated close to the ammonium moieties of the OSDA inside straight channels, owing to a strong OSDA-framework attraction. The selectivities of ethylene, propylene and catalyst lifetime have been promoted in n-heptane cracking, due to stronger acidity and enhanced diffusion property. Moreover, the architecture is robust toward deep dealumination and Ti- modification, allowing it to be a superior catalyst in cyclohexene epoxidation. The polycation OSDA design concept and the recipe for Beta nanosheets may find broader applications in catalysis.

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali-Treatment for Target Acid-Catalyzed Reactions

Constructing mesoporous structure within zeolites by alkali-treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid-catalyzed reactions. In this context, the crucial roles of spatial aluminum (Al) distribution were unraveled during alkali-treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one- and two-dimensional magic-angle spinning (MAS) NMR techniques, were performed to track the alkali-treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si-OH in zeolite as Al(OH)4 -. Those re-deposited Al(OH)4 - sites were easily transformed into NMR-invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to "acid-free" Al species. While most tetrahedral Al sites were preserved after the alkali-treatment of sample with non-uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid-catalyzed reactions, such as catalytic cracking of 1,3,5-triisopropylbenzene and methanol-to-olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated "acid-free" Al during the precise alkali-treatment.

Catalytic Cracking of Liquefied Petroleum Gas (LPG) to Light Olefins Using Zeolite-Based Materials: Recent Advances, Trends, Challenges and Future Perspectives

The catalytic cracking of liquefied petroleum gas (LPG) has attracted significant attention due to its importance in producing valuable feedstocks for the petrochemical industry. This review provides an overview of recent developments in zeolite-based catalyst technology for converting LPG into light olefins. Catalytic cracking utilizes zeolite-based catalysts usually associated with stability challenges, such as coking and sintering. The discussion focused on the underlying mechanisms that govern the catalytic cracking process and provided insights into the complex reaction pathways involved. A comprehensive analysis of various strategies employed for improving the effectiveness of zeolite catalysts has been discussed in this review. These strategies encompass using transition metals to modify catalyst properties, treatments involving phosphorous modification, alkaline earth metals, and alkali metals to alter the acidity level of the zeolites. The elucidation of the impact of silica-to-alumina ratios in zeolites and the development of hierarchical zeolite-based catalysts through top-down and bottom-up methodologies are also discussed.

Exploring the effect of molecular size and framework functionalisation on transport in metal-organic frameworks using pulsed-field gradient nuclear magnetic resonance

Molecular transport is an important aspect in metal-organic frameworks (MOFs) as it affects many of their applications, such as adsorption/separation, drug delivery and catalysis. Yet probing the fundamental diffusion mechanisms in MOFs is challenging, and the interplay between the MOF's features (such as the pore structure and linker dynamics) and molecular transport remains mostly unexplored. Here, the pulsed-field gradient nuclear magnetic resonance (PFG NMR) technique is used to probe the diffusion of several probe molecules, i.e., water, xylenes and 1,3,5-triisopropylbenzene (TIPB), within the UiO-66 MOF and its derivatives (UiO-66NH2 and UiO-66Br). Exploiting differences in the size of probe molecules we were able to probe the diffusion rate selectively in the different pore environments of the MOFs. In particular, when relatively small molecules, such as water and small hydrocarbons, were used as probes, the PFG NMR log attenuation plots were non-linear with two distinctive diffusion regions, suggesting faster diffusion in the inter-crystalline space and slower diffusion within crystal aggregates, the latter occurring mostly inside the framework of the MOFs. Conversely, experiments with a larger probe molecule, i.e., TIPB, with a kinetic diameter of 0.95 nm, which makes it unable to access the framework windows of the MOF crystals, showed linear PFG NMR log attenuation plots, which indicates diffusion occurring in a single environment, most likely in the inter-crystalline space. Analysis of the apparent tortuosity values of the systems under investigation highlights the role of linker functionalisation in influencing the molecular diffusion of the probe molecules, which affects both intra-molecular interactions and pore accessibility within the MOF crystals. The findings of this work demonstrate that the diffusion behaviour of probe molecules within MOFs is influenced by the pore size, structure, functionalisation of the MOF linker and molecular interactions. Our study contributes to further advance the understanding of mass transport in MOFs by PFG NMR and provides insights that can inform the design and optimisation of MOF-based materials for various applications.

Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals

We have used confocal laser scanning microscopy on the small, fluorescent resorufin dye molecule to visualize molecular accessibility and diffusion in the hierarchical, anisotropic pore structure of large (~10 μm-sized) zeolite-β crystals. The resorufin dye is widely used in life and materials science, but only in its deprotonated form because the protonated molecule is barely fluorescent in aqueous solution. In this work, we show that protonated resorufin is in fact strongly fluorescent when confined within zeolite micropores, thus enabling fluorescence microimaging experiments. We find that J-aggregation guest-guest interactions lead to a decrease in the measured fluorescence intensity that can be prevented by using non-fluorescent spacer molecules. We characterized the pore space by introducing resorufin from the outside solution and following its diffusion into zeolite-β crystals. The eventual homogeneous distribution of resorufin molecules throughout the zeolite indicates a fully accessible pore network. This enables the quantification of the diffusion coefficient in the straight pores of zeolite-β without the need for complex analysis, and we found a value of 3×10-15  m2  s-1 . Furthermore, we saw that diffusion through the straight pores of zeolite-β is impeded when crossing the boundaries between zeolite subunits.

Dynamics and Structure of a Bitumen Emulsion as Studied by 1H NMR Diffusometry

Self-diffusion in a bitumen emulsion was studied by 1H NMR. The emulsion forms two phases: continuous and dispersed. The continuous aqueous phase contains mainly water, with the energy of activation of the diffusion process equal to that of bulk water, while its diffusivity is smaller than that of bulk water by a factor of 2. The dispersed phase consists of bitumen droplets containing confined water, whose dynamics is characterized by a fully restricted diffusion regime in cavities with sizes of ∼0.11 μm. Therefore, the studied bitumen emulsion can be described by a model of a complex multiple emulsion of the water/oil/water (WOW) type. The suggested model does agree well with data from 1H NMR spectroscopy and diffusometry of the bitumen emulsion doped with paramagnetic MnSO4(aq) as well as with an additional 1H NMR study of the emulsion structure, in which emulsion stability was compromised by freezing at 253 K.

Realizing of ZSM-5 microspheres with enhanced catalytic properties prepared from iron ore tailings via solid-phase conversion method

The comprehensive utilization of iron ore tailings (IOTs) not only resolved environmental problems but also brought huge economic benefits. In this study, the synthetic route presented herein provides a novel method for the synthesis of ZSM-5 microspheres from IOTs. The effects of Si/Al molar ratios and the pH of the precursor solution on the formation of zeolite was evaluated by various analytical methods. The catalytic performance of the catalyst prepared by the solid-phase conversion method (denoted as MP-ZSM-5) was evaluated by methanol-to-propylene (MTP) reaction. Compared with the zeolite catalyst that synthesized via the conventional hydrothermal method (denoted as HM-ZSM-5), MP-ZSM-5 not only prolongs catalytic lifetime from 18.7 to 36.0 h but also has higher selectivity for propylene by MP-ZSM-5 (43.7%) than that for HM-ZSM-5 (38.6%). In addition, Kissinger-Akahira-Sunose (KAS) model is applied to the TG result to study the template removal process kinetics. The average activation energy values required for the removal of CTAB and TPABr are 201.11 ± 13.42 and 326.88 ± 16.91 kJ∙mol^-1, respectively. Furthermore, this result is well coupled with the model-free kinetic algorithms to determine the conversion and isoconversion of the TPABr and CTAB decomposition in ZSM-5, which serves as important guidelines for the industrial production process.

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Diffusion measurements of hydrocarbons in H-MCM-41 extrudates with pulsed-field gradient nuclear magnetic resonance spectroscopy

Mesoporous materials are promising catalysts for production of biofuels. Herein, H-MCM-41 catalysts with different concentrations of the silica Bindzil binder (10-50 wt%) were prepared and characterized using pulsed-field gradient (PFG) NMR in the powder form and as extrudates. Effective diffusion coefficients (De) are measured in all cases. Diffusivities of n-hexadecane were found smaller for extrudates as compared to the powder catalysts. The estimates of diffusive tortuosity were also determined. PFG NMR data showed one major component that reveals diffusion in interconnected meso- and micropores and one other minor component (1-2%) that may correspond to more isolated pores or may represent complex effects of restricted diffusion. Therefore, several approaches including initial slope analysis of spin-echo attenuation curves, two-component fitting and Laplace inversion were used to discuss different aspects of diffusional transport in the studied H-MCM-41 materials. Correlations between De and the amount of Bindzil, the specific surface area, the micropore volume, the particle size, the total acid sites and the Lewis acid sites are discussed.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Unravelling Channel Structure-Diffusivity Relationships in Zeolite ZSM-5 at the Single-Molecule Level

The development of improved zeolite materials for applications in separation and catalysis requires understanding of mass transport. Herein, diffusion of single molecules is tracked in the straight and sinusoidal channels of the industrially relevant ZSM-5 zeolites using a combination of single-molecule localization microscopy and uniformly oriented zeolite thin films. Distinct motion behaviors are observed in zeolite channels with the same geometry, suggesting heterogeneous guest-host interactions. Quantification of the diffusion heterogeneities in the sinusoidal and straight channels suggests that the geometry of zeolite channels dictates the mobility and motion behavior of the guest molecules, resulting in diffusion anisotropy. The study of hierarchical zeolites shows that the addition of secondary pore networks primarily enhances the diffusivity of sinusoidal zeolite channels, and thus alleviating the diffusion limitations of microporous zeolites.

Hydroisomerization of 1-octene utilizing hierarchical SAPO-11-supported Ni catalysts: effect of the alkyl chain length of the mesoporogen

The effects of the alkyl chain length of the mesoporogen on the physicochemical properties and 1-octene hydroisomerization of hierarchical SAPO-11 have been investigated.

Comparative studies on the VOC sorption performances over hierarchical and conventional ZSM-5 zeolites

The sorption behaviors of hexane, toluene and mesitylene as probe volatile organic compounds (VOCs) over hierarchical and conventional zeolite ZSM-5 were investigated by a series of experiments, such as dynamic adsorption, temperature-programmed desorption and cycle adsorption tests. The results showed that hierarchical ZSM-5 exhibited better adsorption capacity for toluene and mesitylene, better diffusion of VOCs and superior cycle adsorption efficiency. As we believe, these findings will offer valuable information for the development of zeolite based adsorbents for VOC elimination or recycling.

Kinetics of n-Hexane Cracking over Mesoporous HY Zeolites Based on Catalyst Descriptors

A simple kinetic model based on the zeolite acid strength, the number of Brønsted acid sites, and the catalyst efficiency was developed for the cracking of n-hexane. A series of HY zeolites with a mesopore volume from 0.04 to 0.32 cm3/g was synthesized and characterized by various physical-chemical methods and tested for n-hexane cracking. The generation of mesoporosity influenced several other important parameters, such as acidity and extra-framework aluminum. Zero-length column diffusion measurements for mesitylene showed a large decrease in the characteristic diffusion time upon the introduction of mesoporosity, which changed only slightly with a further increase in mesoporosity. Similar n-hexane physisorption enthalpies were measured for all samples. The highest initial activity for n-hexane cracking per catalyst volume was observed for the sample with an intermediate mesopore volume of 0.15 cm3/g. The three mesoporous H-USY zeolites showed the same value of the intrinsic rate constant and the same activation energy. The difference in initial activity of the mesoporous zeolites was caused by the difference in the number of Brønsted acid sites. The increase in initial activity for the mesoporous zeolites compared to a microporous zeolite was caused by an increase in the acid strength.

Synergistically enhance confined diffusion by continuum intersecting channels in zeolites

In separation and catalysis applications, adsorption and diffusion are normally considered mutually exclusive. That is, rapid diffusion is generally accompanied by weak adsorption and vice versa. In this work, we analyze the anomalous loading-dependent mechanism of p-xylene diffusion in a newly developed zeolite called SCM-15. The obtained results demonstrate that the unique system of "continuum intersecting channels" (i.e., channels made of fused cavities) plays a key role in the diffusion process for the molecule-selective pathways. At low pressure, the presence of strong adsorption sites and intersections that provide space for molecule rotation facilitates the diffusion of p-xylene along the Z direction. Upon increasing the molecular uptake, the adsorbates move faster along the X direction because of the effect of continuum intersections in reducing the diffusion barriers and thus maintaining the large diffusion coefficient of the diffusing compound. This mechanism synergistically improves the diffusion in zeolites with continuum intersecting channels.

Hierarchical ISI-1 zeolite catalyst for hydroconversion of long-chain paraffins

The hierarchization via desilication of ISI-1 zeolite, belonging to the TON type family and its impact on catalytic performance in hydroconversion of model long chain n-alkanes are presented.

Diffusion and catalyst efficiency in hierarchical zeolite catalysts

The preparation of hierarchical zeolites with reduced diffusion limitation and enhanced catalyst efficiency has become a vital focus in the field of zeolites and porous materials chemistry within the past decades. This review will focus on the diffusion and catalyst efficiency of hierarchical zeolites and industrial catalysts. The benefits of diffusion and catalyst efficiency at two levels of hierarchies (zeolitic component level and industrial catalyst level) from a chemical reaction engineering point of view will be analysed. At zeolitic component level, three types of mesopores based on the strategies applied toward enhancing the catalyst effectiveness factor are presented: (i) 'functional mesopores' (raising effective diffusivity); (ii) 'auxiliary mesopores' (decreasing diffusion length); and (iii) 'integrated mesopores' (a combination thereof). At industrial catalyst level, location and interconnectivity among the constitutive components are revealed. The hierarchical pore interconnectivity in multi-component zeolite based industrial catalysts is exemplified by fluid catalytic cracking and bi-functional hydroisomerization catalysts. The rational design of industrial zeolite catalysts at both hierarchical zeolitic component and catalyst body levels can be fully comprehended using the advanced in situ and/or operando spectroscopic, microscopic and diffraction techniques.

Creation of Al-Enriched Mesoporous ZSM-5 Nanoboxes with High Catalytic Activity: Converting Tetrahedral Extra-Framework Al into Framework Sites by Post Treatment

ZSM‐5 zeolite nanoboxes with accessible meso‐micro‐pore architecture and strong acid sites are important in relevant heterogeneous catalysis suffering from mass transfer limitations and weak acidities. Rational design of parent zeolites with concentrated and non‐protective coordination of Al species can facilitate post‐synthetic treatment to produce mesoporous ZSM‐5 nanoboxes. In this work, a simple and effective method was developed to convert parent MFI zeolites with tetrahedral extra‐framework Al into Al‐enriched mesoporous ZSM‐5 nanoboxes with low silicon‐to‐aluminium ratios of ≈16. The parent MFI zeolite was prepared by rapid ageing of the zeolite sol gel synthesis mixture. The accessibility to the meso‐micro‐porous intra‐crystalline network was probed systematically by comparative pulsed field gradient nuclear magnetic resonance diffusion measurements, which, together with the strong acidity of nanoboxes, provided superb catalytic activity and longevity in hydrocarbon cracking for propylene production.

PFG-NMR as a Tool for Determining Self-Diffusivities of Various Probe Molecules through H-ZSM-5 Zeolites

The understanding of major zeolite applications is partially based on diffusion of molecules inside or outside microporous networks. However, it is still a challenge to measure such phenomena. The diffusion ordered nuclear magnetic resonance spectroscopy (DOSY) technique has been reported to measure a probe molecule's diffusion inside porous solids. Pulsed-field gradient (PFG)-NMR has been used herein to measure the self-diffusivity of different probe molecules, such as neopentane, benzene, toluene and 1-dodecene with increasing dynamic diameter, respectively, on a series of H-ZSM-5 zeolites. The latter materials exhibit different crystal sizes, Si/Al ratios and the presence (or absence) of crystalline defects. In addition, shaped zeolite bodies representing industrial catalysts were compared with the afore-mentioned samples.

Synthesis of highly active ETS-10-based titanosilicate for heterogeneously catalyzed transesterification of triglycerides

In this contribution, the preparation of hierarchically structured ETS-10-based catalysts exhibiting notably higher activity in the conversion of triolein with methanol compared to microporous titanosilicate is presented. Triolein, together with its unsaturated analog trilinolein, represent the most prevalent triglycerides in oils. The introduction of mesopores by post-synthetic treatment with hydrogen peroxide and a subsequent calcination step results in the generation of an additional active surface with Brønsted basic sites becoming accessible for triolein and enhancing the rate of transesterification. The resulting catalyst exhibits a comparable triolein conversion (≈73%) after 4 h of reaction to CaO (≈76%), which is reportedly known to be highly active in the transesterification of triglycerides. In addition, while CaO showed a maximum conversion of 83% after 24 h, the ETS-10-based catalyst reached 100% after 8 h, revealing its higher stability compared to CaO. The following characteristics of the catalysts were experimentally addressed - crystal structure (X-ray diffraction, transmission electron microscopy), crystal shape and size (scanning electron microscopy, laser diffraction), textural properties (N2 sorption, Hg porosimetry), presence of hydroxyl groups and active sites (temperature-programmed desorption of NH3 and CO2, 29Si magic angle spinning nuclear magnetic resonance (NMR)), mesopore accessibility and diffusion coefficient of adsorbed triolein (pulsed field gradient NMR), pore interconnectivity (variable temperature and exchange spectroscopy experiments using hyperpolarized 129Xe NMR) and oxidation state of Ti atoms (electron paramagnetic resonance). The obtained results enabled the detailed understanding of the impact of the post-synthetic treatment applied to the ETS-10 titanosilicate with respect to the catalytic activity in the heterogeneously catalyzed transesterification of triglycerides.

Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites |Na12|-A and |Na12-xKx|-A

Carbon dioxide must be removed from biogas or natural gas to obtain compressed or liquefied methane, and adsorption-driven isolation of CO₂ could be improved by developing new adsorbents. Zeolite adsorbents can select CO₂ over CH₄, and the adsorption of CH₄ on zeolite |Na_12-xK_x|-A is significantly lower for samples with a high K⁺ content, i.e., x > 2. Nevertheless, we show, using ¹H NMR experiments, that these zeolites adsorb CH₄ after long equilibration times. Pulsed-field gradient NMR experiments indicated that in large crystals of zeolites |Na_12-xK_x|-A, the long-time diffusion coefficients of CH₄ did not vary with x, and the upper limit of the mean-square displacement was about 1.5 μm, irrespective of the diffusion time. Also for zeolite |Na₁₂|-A samples of three different particle sizes (∼0.44, ∼2.9, and ∼10.6 μm), the upper limit of the mean-square displacement of CH₄ was 1.5 μm and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion restriction for CH₄ within the medium- and large-sized zeolite A crystals and possibly of clustering and close contact among the small zeolite A crystals. The upper limit of the long-time diffusion coefficient of adsorbed CH₄ was (at 1 atm and 298 K) about 10^-10 m²/s irrespective of the size of the zeolite particle or the studied content of K⁺ in zeolites |Na_12-xK_x|-A and |Na₁₂|-A. The T₁ relaxation time for adsorbed CH₄ on zeolites |Na_12-xK_x|-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH₄ was hindered.

Organotemplate-Free β Zeolites: From Zeolite Synthesis to Hierarchical Structure Creation

Interest in the production of β zeolites in the absence of organic structure-directing agent (OSDA) has continued to grow consistently during the past decade. During this time, numerous strategies have been proposed to manipulate the hierarchy of zeolite pore structures in order to facilitate the transport of bulky reactants and to improve the accessibility of active sites in zeolite catalysts. In this work, we describe an organotemplate-free route to produce hierarchical β zeolites. Using OSDA-free β as the starting zeolites, we explored the applicability of various postsynthetic approaches to create hierarchical structures with mesoporosity, including framework stabilization, dealumination, conventional desilication, and hydrothermal desilication. While framework stabilization and dealumination were not effective in generating mesoporosity, they were necessary as modification steps to determine the efficacy of hierarchical structure creation. Compared to conventional desilication, hydrothermal desilication produces larger mesopores and much better preservation of microporosity and acidity because of the occurrence of recrystallization. The cost-effective, scalable production of organotemplate-free hierarchical β zeolites could greatly enhance the adoption of β zeolites in oil refining and petrochemical industries, where the advantages of hierarchically structured zeolites can dramatically improve catalytic performances in formulated catalysts.

Revelation on the Complex Nature of Mesoporous Hierarchical FAU-Y Zeolites

The texture of mesoporous FAU-Y (FAUmes) prepared by surfactant-templating in basic media is a subject of debate. It is proposed that mesoporous FAU-Y consists of: (1) ordered mesoporous zeolite networks formed by a surfactant-assisted zeolite rearrangement process involving local dissolution and reconstruction of the crystalline framework, and (2) ordered mesoporous amorphous phases as Al-MCM-41, which coexist with zeolite nanodomains obtained by a dissolution-reassembly process. By the present systematic study, performed with FAU-Y (Si/Al = 15) in the presence of octadecyltrimethylammonium bromide and 0 < NaOH/Si ratio < 0.25 at 115 °C for 20 h, we demonstrate that mesoporous FAU zeolites consist, in fact, of a complex family of materials with textural features strongly impacted by the experimental conditions. Two main families have been disclosed: (1) for 0.0625 < NaOH/Si < 0.10, FAUmes are ordered mesoporous materials with zeolite walls, which coexist with zeolite nanodomains (100-200 nm) and (2) for 0.125 < NaOH/Si < 0.25, FAUmes are ordered mesoporous materials with amorphous walls as Al-MCM-41, which coexist with zeolite nanodomains (5-100 nm). The zeolite nanodomains decrease in size with the increase of NaOH/Si ratio. Increasing NaOH/Si ratio leads to an increase of mesopore volume, while the total surface area remains constant, and to a decrease of strong acidity in line with the decrease of micropore volume. The ordered mesoporous materials with zeolite walls feature the highest acidity strength. The ordered mesoporous materials with amorphous walls present additional large pores (50-200 nm), which increase in size and amount with the increase of NaOH/Si ratio. This alkaline treatment of FAU-Y represents a way to obtain ordered mesoporous materials with zeolite walls with high mesopore volume for NaOH/Si = 0.10 and a new way to synthesize mesoporous Al-MCM-41 materials containing extralarge pores (50-200 nm) ideal for optimal diffusion (NaOH/Si = 0.25).

Manipulating the mesostructure of silicoaluminophosphate SAPO-11 via tumbling-assisted, oriented assembly crystallization: a pathway to enhance selectivity in hydroisomerization

A house-of-cards architecture of silicoaluminophosphate was fabricated to enhance selectivity for hydroisomerization.