Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis

Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy

Introduction of a dielectric material in a nuclear magnetic resonance (NMR) probe head modifies the frequency response of the probe circuit, a phenomenon revealed by detuning of the probe. For NMR spectroscopy, this detuning is corrected for by tuning and matching the probe head prior to the NMR measurement. The magnitude of the probe detuning, "the dielectric shift", provides direct access to the dielectric properties of the sample, enabling NMR spectrometers to simultaneously perform both dielectric and NMR spectroscopy. By measuring sample dielectric permittivity as a function of frequency, dielectric permittivity spectroscopy can be performed using the new methodology. As a proof of concept, this was evaluated on methanol, ethanol, 1-propanol, 1-pentanol, and 1-octanol using a commercial cross-polarization magic angle spinning (CPMAS) NMR probe head. The results accurately match the literature data collected by standard dielectric spectroscopy techniques. Subsequently, the method was also applied to investigate the solvent-surface interactions of water confined in the micropores of an MFI-type, hydrophilic zeolite with a Si/Al ratio of 11.5. In the micropores, water adsorbs to Bro̷nsted acid sites and defect sites, resulting in a drastically decreased dielectric permittivity of the nanoconfined water. Theoretical background for the new methodology is provided using an effective electric circuit model of a CPMAS probe head with a solenoid coil, describing the detuning resulting from the insertion of dielectric samples in the probe head.

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

Selective catalytic reduction of NOx with ammonia (NH3-SCR) over copper loaded LEV type zeolites synthesized with different templates

LEV type zeolites were synthesized with four different structure-directing agents and converted to copper loaded NH₃-SCR catalysts. The synthesis recipe was found to impact the respective Al population in the two topologically different framework sites in double and single 6-rings, resolvable by ²⁷Al MAS NMR spectroscopy. Hydrothermal stability was found to be related to the silanol concentration, Si/Al ratio, particle size, crystal morphology, crystal defects, external surface area, and microporosity. Catalytic activity in NH₃-SCR was dependent on preferential Al siting in the double 6-rings. Levinite synthesized using adamantylamine showed the strongest preference for Al atoms sitting in double 6-ring sites, and showed the highest catalytic turnover frequency. Unfortunately, because of the large crystal size, copper loading of this sample was limited to 0.6 wt% while other samples could be loaded with copper up to 3.3 wt%. An optimum combination of hydrothermal stability and catalytic activity was obtained with N,N'-bis-dimethylpentanediyldiammonium dibromide as structure-directing agent.

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.

Surface Fingerprinting of Faceted Metal Oxides and Porous Zeolite Catalysts by Probe-Assisted Solid-State NMR Approaches

Acid catalysis in heterogeneous systems such as metal oxides and porous zeolites has been widely involved in various catalytic processes for chemical and petrochemical industries. In acid-catalyzed reactions, the performance (e.g., activity and selectivity) is closely associated with the acidic features of the catalysts, viz., type (Lewis vs Brønsted acidity), distribution (external vs internal surface), strength (strong vs weak), concentration (amount), and spatial interactions of acidic sites. The characterization of local structure and acidic properties of these active sites has important implications for understanding the reaction mechanism and the practical catalytic applications of acidic catalysts. Among diverse acidity characterization approaches, the solid-state nuclear magnetic resonance (SSNMR) technique with suitable probe molecules has been recognized as a reliable and versatile tool. Such a probe-assisted SSNMR approach could provide qualitative (type, distribution, and spatial interactions) and quantitative (strength and concentration) information on each acidic site. This Account aims to integrate our recent important findings in determining the structures and acidic characteristics of some typical metal oxide and zeolite catalysts by using the probe-assisted SSNMR technique, as well as clarifying the continuously evolving process of each discrete acidic site under hydrothermal or chemical treatments even at the molecular level with multiscale theoretical simulations.More specifically, we will describe herein the development and applications of the probe-assisted SSNMR methods, such as trimethylphosphine (TMP) and acetonitrile-d₃ (CD₃CN) in conjunction with advanced two-dimensional (2D) homo- and heteronuclear correlation spectroscopy, for characterizing the structures and properties of acidic sites in varied solid catalysts. Moreover, relevant information regarding the surface fingerprinting of various facets on crystalline metal oxide nanoparticles and active centers inside porous zeolites, the mapping of relevant spatial interactions, and the verification of structure-activity correlation were investigated as well. Relevant discussions are mainly based on the recent NMR experiments of our collaborating research groups, including (i) determining the acidic characterization with probe-assisted SSNMR approaches, (ii) mapping various active centers (or crystalline facets), and (iii) revealing their influence on catalytic performance of solid acid catalyst systems. It is anticipated that this information may provide more in-depth insights toward our fundamental understanding of solid acid catalysis.

Pore selectivity and electron transfers in HZSM-5 single crystals: a Raman microspectroscopy mapping and confocal fluorescence imaging combined study

Electron transfers at the single particle level in HZSM-5 zeolite are followed by combining Raman microspectroscopy mapping and confocal fluorescence imaging. The effects of pore accessibility and guest diffusion on reactivity are investigated.

Maximizing sinusoidal channels of HZSM-5 for high shape-selectivity to p-xylene

The shape-selective catalysis enabled by zeolite micropore’s molecular-sized sieving is an efficient way to reduce the cost of chemical separation in the chemical industry. Although well studied since its discovery, HZSM-5′s shape-selective capability has never been fully exploited due to the co-existence of its different-sized straight channels and sinusoidal channels, which makes the shape-selective p-xylene production from toluene alkylation with the least m-xylene and o-xylene continue to be one of the few industrial challenges in the chemical industry. Rather than modifications which promote zeolite shape-selectivity at the cost of stability and reactivity loss, here inverse Al zoned HZSM-5 with sinusoidal channels predominantly opened to their external surfaces is constructed to maximize the shape-selectivity of HZSM-5 sinusoidal channels and reach > 99 % p-xylene selectivity, while keeping a very high activity and good stability ( > 220 h) in toluene methylation reactions. The strategy shows good prospects for shape-selective control of molecules with tiny differences in size.

Full utilization of ZSM-5 shape-selectivity is restricted by crystal external surface acid sites and co-existence of two different sized channels. Here, the authors synthesize reverse Al zoned ZSM-5 with sinusoidal channel preferentially opened to its inert external surface to achieve 99.3% p-xylene selectivity.

Highly selective conversion of methanol to propylene: design of an MFI zeolite with selective blockage of (010) surfaces

As an important catalyst of methanol-to-propylene (MTP) conversion, the ZSM-5 zeolite has an anisotropic diffusion path and a large pore size, resulting in the formation of undesirable heavy aromatic by-products. Herein, we developed a surface-specific silica deposition method to block straight channels of nanosized ZSM-5 crystals selectively. By such a coating method, we can selectively suppress the yield of aromatics from the original 13% to 2.4% at 100% conversion of methanol. Trapped hydrocarbon pool species are directly confirmed by aberration-corrected S/TEM for the first time. Such a method of trapping and restricting hydrocarbon pool species in a multiscale zeolite with 10-membered rings would significantly increase its catalytic efficiency and olefin diffusion. Moreover, this provides new methodologies for zeolite structure construction and will be greatly beneficial for the industrial MTP process.

Co-site substitution by Mn supported on biomass-derived active carbon for enhancing magnesia desulfurization

Oxidation of magnesium sulfite (MgSO₃) is a crucial step for reclaiming the product in wet magnesia desulfurization processes. Here, for enhancing this reaction, a bimetallic catalyst was developed by loading CoO_x and MnO_x species on a biomass-derived active carbon (AC) support to minimize the costs and potential environmental risks during catalyst application. The substitution effect of Mn to Co sites was investigated, and a comparison of the catalyst with plain cobalt suggested that the ratio of Co/Mn must be greater than 3. A series of catalyst characterizations was performed to reveal the synergistic effect of Co and Mn in the bimetallic catalyst. The introduction of Mn species not only improved the dispersion of CoO_x-MnO_x mixed oxide but also generated abundant Co³⁺ species and surface-adsorbed oxygen, both of which acted as the main active sites for sulfite oxidation. Notably, in the bimetallic catalyst, the presence of Mn⁴⁺ species assisted regeneration of Co²⁺ to Co³⁺ species, further accelerating sulfite oxidation. Besides, the partial substitution of Co sites by Mn also suppressed the losing of Co species during reaction, favoring to decrease the environmental risk, as well as to save the cost of catalyst.

A Spectroscopic Study of Tautomeric Equilibrium of Salicylideneaniline in ZSM-5 Zeolites

Salicylideneaniline (SA) sorbed in cation-exchanged M-ZSM-5 (M = H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Zn²⁺) zeolites was studied by spectroscopic techniques assisted by quantum-chemical calculations. The nature of extra-framework cations present in the zeolite void was found to affect the spectral signature of the sorbed SA molecule that points to the shift of tautomeric equilibrium between the enol and keto forms. Small size cations, such as H⁺ and Li⁺, stabilize a cis-keto SA tautomer along with a enol one in the zeolite structure. The calculations indicate that the sorbed cis-keto tautomer may have the dipole large enough to be considered as a zwitterion. New features appearing in the spectra with the increase of the cation size were attributed to the presence of trans-keto SA tautomer, which up to now has been observed only in time-resolved spectroscopic experiments. A strong interaction of the molecule with cations in Zn-ZSM-5 zeolite results in the chelation of enol SA with the divalent Zn²⁺ ions. The results of the study suggest that the tautomeric equilibrium of molecules belonging to the Schiff base family can be tuned by the confinement in the nanoporous materials via a choice of topology of zeolite framework and the nature of extra-framework cations.

Scope of surface-modified molecular and nanomaterials in gel/liquid forms for developing mechanically flexible DSSCs/QDSSCs

The advanced lifestyle of the human race involves heavy usage of various gadgets which require copious supplies of energy for uninterrupted functioning. Due to the ongoing depletion of fossil fuels and the accelerating demand for other energy resources, renewable energy sources, especially solar cells, are being extensively explored as viable alternatives. Flexible solar cells have recently emerged as an advanced member of the photovoltaic family; the flexibility and pliability of these photovoltaic materials are advantageous from a practical point of view. Conventional flexible solar cell materials, when dispersed in solvents, are usually volatile and create severe stability issues when incorporated in devices. Recently, non-volatile, less viscous functional molecular liquids/gels have been proposed as potential materials for use in foldable device applications. This perspective article discusses the scope of surface-modified non-volatile molecular and nanomaterials in liquid/gel forms in the manufacturing and deployment of flexible photovoltaics.

Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level

Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host–guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.

Single-Layered Two-Dimensional Metal-Organic Framework Nanosheets as an in Situ Visual Test Paper for Solvents

Through a facile-operating ultrasonic force-assisted liquid exfoliation technology, the single-layered two-dimensional (2D) [Co(CNS)₂(pyz)₂] _n (pyz = pyrazine) nanosheets, with a thickness of sub-1.0 nm, have been prepared from the bulk precursors. The atomically thickness and the presence of abundant sulfur atoms with high electronegativity arrayed on the double surfaces of the sheets are making this kind of 2D MOF (metal-organic framework) nanosheets highly sensitive to intermolecular interactions. As a result, it can be well dispersed in all kinds of solvents to give a stable colloidal suspension that can be maintained for at least one month, accompanied by significant solvatochromic behavior and various optical properties, which thus have shown the potential to be practically applicated as in situ visual test paper for solvent identification and solvent polarity measurements. More importantly, combined with a smartphone, this kind of 2D-MOF nanosheets can be developed into in situ visual test paper to identify isomers and determine the polarity of mixed solvents quantitatively and qualitatively, suggesting the promising application of a portable, economical, and in situ visual test strategy in real world.

Resolving the Framework Position of Organic Structure-Directing Agents in Hierarchical Zeolites via Polarized Stimulated Raman Scattering

The direct synthesis of hierarchically intergrown silicalite-1 can be achieved using a specific diquaternary ammonium agent. However, the location of these molecules in the zeolite framework, which is critical to understand the formation of the material, remains unclear. Where traditional characterization tools have previously failed, herein we use polarized stimulated Raman scattering (SRS) microscopy to resolve molecular organization inside few-micron-sized crystals. Through a combination of experiment and first-principles calculations, our investigation reveals the preferential location of the templating agent inside the linear pores of the MFI framework. Besides illustrating the attractiveness of SRS microscopy in the field of material science to study and spatially resolve local molecular distribution as well as orientation, these results can be exploited in the design of new templating agents for the preparation of hierarchical zeolites.

A nonpolar solvent effect by CH/π interaction inside zeolites: characterization, mechanism and concept

The acidity enhancement induced by the nonpolar solvent effect of naphthalene inside zeolites was unambiguously identified.