ZSM-5 Single Crystals Research Articles

Synchrotron Infrared Microspectroscopy of Zeolite Catalysts

This article reviews recent work undertaken at the beamline B22 of the Diamond Light Source using infrared (IR) microspectroscopy to characterise zeolite catalysts and to study their reactivity in real time. The advantage of vibrational microspectroscopic analysis when linked to the brightness and spectral bandwidth of synchrotron IR light are illustrated. The high spatial resolution means that the uniformity of acid site concentrations within individual large crystals of zeolites and between different crystals can be readily checked and changes to acid site concentrations within crystals resulting from steam treatment mapped. When an in situ reaction cell is coupled with mass spectrometric analysis of evolved gases the rapid time response of the method has provided new insight into the initial stages of the conversion of methanol to hydrocarbons over ZSM-5 and SAPO-34 single crystals. Future prospects for applying the method to other types of zeolite catalysed reactions with improved reaction cell design are also discussed.

Catalytic hydrodeoxygenation of lignin enhanced by selectively etching ZSM-5

Cyclanes, a major component of aviation fuel, can be obtained from the catalytic hydrodeoxygenation (HDO) of lignin, which not only reduces the dependence on fossil resources, but also makes lignin refining economically viable. The adsorption capacity and dwell times of reactants and H2 on the catalyst play a key role in the HDO of lignin. It remains a difficult challenge to enhance the adsorption capacity of the catalyst for reactants and H2 and to prolong their extended dwell times on the catalyst. Based on this, a nickel-based catalyst with moderate corrosion was prepared by the dissolution of ZSM-5 single crystals induced by ammonia, and was used for value-added conversion of lignin. It was confirmed by experiments and complementary characterizations that the corrosion of Ni/ZSM-5 for etching 2 h (Ni/ZSM-52) enhanced the adsorption capacity of H2 and extended the dwell times of the reactant and H2. Furthermore, the uniformly dispersed Ni nanoparticles stimulated the intrinsic catalytic activity and efficiently generated H…H and H+, which synergistically promote the HDO of lignin with the cyclanes yield up to 58.6 %. As a result, BOB was completely converted to cyclanes over Ni/ZSM-52 at 140 °C under 2 MPa of H2 for 4 h, suggesting Ni/ZSM-52 exhibits excellent HDO activity under mild conditions.

Hollow ZSM-5 encapsulated Pt nanoparticles for selective catalytic reduction of NO by hydrogen

Pt nanoparticles were successfully encapsulated in hollow ZSM-5 single crystals by tetrapropylammonium hydroxide (TPAOH) hydrothermal treatment with an “dissolution-recrystallization” process. The prepared Pt/hollow ZSM-5 (Pt/h-ZSM-5re) sample exhibited the best activity and a maximum NO conversion of 84% can be achieved at 90 °C with N2 selectivity of 92% (GHSV = 50,000 h−1). Meanwhile, Pt/h-ZSM-5re catalyst exhibited excellent SO2, H2O resistance and durability, which was related to the stabilization of Pt active sites by hollow structure during H2-SCR. It was found that the increase of NO2 concentration in the feed gas mixture led to an activity decline. In addition, the H2-SCR reaction routes over Pt/hollow ZSM-5 catalyst at different temperature were investigated.

Understanding of the dissolution–crystallization fabrication strategy towards macro/microporous ZSM-5 single crystals

Amounts of NaOH, Si/Al ratios and crystallization temperature together determine the dissolution–crystallization balance and thus the generation of macropores in ZSM-5 crystals.

One-pot synthesis of cup-like ZSM-5 zeolite and its excellent oxidative desulfurization performance.

A one-step method to synthesize small size cup-like hollow ZSM-5 single crystals is reported in this paper. Characterization methods show that the product has a typical ZSM-5 structure and most crystals are uniform, mono dispersed crystal shells with distinct hollow structure. The average size of a single crystal is about 350 nm. Every cup-like hollow ZSM-5 crystal can act as a microreactor in the reaction, and the ZSM-5 based catalyst POM-MOF-ZSM-5 (polyoxometalates (POM), metal–organic frameworks (MOF)) shows excellent oxidative desulfurization performance of DBT (dibenzothiophene).

H-ZSM-5 zeolite model crystals: Structure-diffusion-activity relationship in methanol-to-olefins catalysis

Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27Al and 19F MAS-NMR, Rietveld structure refinement and N2- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst.

Mesostructure controllable ZSM-5 single crystals supported Pd/transition metal oxides: efficient and reusable catalysts for selective oxidation under aerobic conditions

Pd and transition metal oxides functionalized ZSM-5 single crystals with b-axis aligned mesopores (ZSM-5-OM-PdOx-MOx) were prepared. ZSM-5-OM support was obtained from crystallization of aluminosilicate gels in the presence of cationic polymers. Characterizations indicate abundant nanopores and highly crystalline degree of ZSM-5-OM-PdOx-MOx, and active species of Pd and transition metals were homogeneously dispersed into ZSM-5-OM, which showed unique interactions and enhanced ability for activating oxygen. Catalytic tests showed that ZSM-5-OM-PdOx-MOx were highly active and reusable catalysts for selective oxidation of alcohols under aerobic and solvent free condition, which were much better than those of Pd/transition metal oxides functionalized ZSM-5, mesoporous silica of SBA-15, and activated carbon catalysts.

A Sustainable Template for Mesoporous Zeolite Synthesis

A generalized synthesis of high-quality, mesoporous zeolite (e.g., MFI-type) nanocrystals is presented, based on a biomass-derived, monolithic N-doped carbonaceous template. As an example, ZSM-5 single crystals with desirable large-diameter (12-16 nm) intracrystalline mesopores are synthesized. The platform provides scope to optimize template dimensions and chemistry for the synthesis of a range of micro-/mesoporous crystalline zeolites in a cost-effective and highly flexible manner.

Facile synthesis of nano-sized hollow single crystal zeolites under mild conditions

We report a method to synthesize hollow ZSM-5 single crystals of a size below 100 nm that could function as nanoreactors with access through the zeolite micropores only. In the first step, ZSM-5 is synthesized with the respective crystal size. In the second, the zeolite is base leached and acid washed under mild conditions.

Pd/transition metal oxides functionalized ZSM-5 single crystals with b-axis aligned mesopores: Efficient and long-lived catalysts for benzene combustion

A series of PdO/transition metal oxides functionalized mesoporous ZSM-5 single crystals with b-axis aligned mesopores (x%Pd/x%M-ZSM-5-OMs) have been successfully prepared from self-assembly of the designed cationic amphiphilic copolymer of polystyrene-co-poly4-vinylpyridine (C-PSt-co-P4VP) with ZSM-5 zeolite precursor based on their strong positive-negative charge interaction under high temperature hydrothermal conditions. Characterizations of XRD patterns showed that x%Pd/x%M-ZSM-5-OMs exhibit good crystallinity with typical MFI zeolite characters; N2 isotherms showed that x%Pd/x%M-ZSM-5-OMs have large BET surface areas and mesopore volumes. SEM images showed the single crystal structure of x%Pd/x%M-ZSM-5-OMs, which have abundant and b-axis aligned mesopores, running through the whole zeolite crystals. TEM images further confirmed b-axis aligned mesopores, and good dispersion of active sites with high degree of crystallinity in x%Pd/x%M-ZSM-5-OMs. XPS spectra showed that transition metal oxides and PdO active species have been successfully loaded into ZSM-5-OM support. H2–TPR curves showed the low reduction temperatures of PdO and transition metal species in x%Pd/x%M-ZSM-5-OMs, which was favorable for the enhancement of their abilities for VOCs combustion. Catalytic tests showed that x%Pd/x%M-ZSM-5-OMs exhibit good catalytic activities and long catalytic lives for benzene combustion under rather mild conditions in comparison with that of x%Pd/x%M-ZSM-5, which would be very important for the wide applications of zeolites with controlled mesopores in the areas of VOCs oxidation.

ZSM-5 Zeolite Single Crystals with b-Axis-Aligned Mesoporous Channels as an Efficient Catalyst for Conversion of Bulky Organic Molecules

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with b-axis-aligned mesopores, synthesized using a designed cationicamphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the b-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The b-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications.

Uniform mesoporous ZSM-5 single crystals catalyst with high resistance to coke formation for methanol deoxygenation

In this study, an efficient procedure for controllable synthesis of uniform mesoporous ZSM-5 single crystals with various crystal architectures and high mesoporosity was developed. Compared with conventional ZSM-5 catalyst, mesoporous ZSM-5 single crystals synthesized by this method exhibited significantly higher external surface area and larger mesopore volume than conventional one. The catalytic performance of mesoporous zeolites were evaluated in the conversion of methanol to hydrocarbons using a fixed-bed reactor operating at 370 °C, atmospheric pressure and weight hourly space velocities (WHSV) of 0.16–6.58 h −1. By controlling the ZSM-5 synthesis procedure, the activity and stability of the ZSM-5 catalyst in the conversion of methanol to gasoline-range hydrocarbons can be favorably tuned. We also found that mesoporosity plays a crucial role in catalyst stability. Good correlation was observed between catalyst lifetime and mesoporosity. While the catalyst activity is related to the acid site density, the catalyst stability (deactivation rate) correlates with the measured surface ratio of BET and external surface area ( S BET/ S meso). The novel ZSM-5 catalyst exhibited improved stability due to the faster removal of products with shorter diffusion path-length and lower coke formation. The obtained results indicated that the novel mesoporous ZSM-5 catalyst containing relatively large pores (mostly mesopores) enhances reaction yield towards gasoline-range hydrocarbons. It is therefore concluded that the relatively slow deactivation rate (coke formation) of novel mesoporous ZSM-5 single crystals catalyst makes this zeolite the preferred catalyst for the conversion of methanol to gasoline-range hydrocarbons at mild conditions.

ZSM-5 and ferrierite single crystals with lower Si/Al ratios: Synthesis and single-crystal synchrotron X-ray diffraction studies

Less twinned and high Al-containing ZSM-5 (MFI, Al 3.8Si 92.2O 192, Si/Al = 24.1) and high Al-containing ferrierite (FER, Al 2.0Si 34.0O 72, Si/Al = 16.7) large single crystals were synthesized by hydrothermal methods using mixed organic structure-directing agents and confirmed by single-crystal X-ray diffraction and SEM-EDX techniques. These large zeolite single crystals with an average size of 100 and 280 μm on an edge for ZSM-5 and ferrierite, respectively, were found to exhibit good qualities for potential use in advanced applications.

Direct synthesis of carbon-templating mesoporous ZSM-5 using microwave heating

Carbon-templated mesoporous ZSM-5 zeolites were synthesized directly avoiding a drying process. Carbon nanoparticles were simply mixed into synthesis precursor of ZSM-5 and hydrothermally treated by microwave irradiation. The amount of mesopores formed inside the ZSM-5 single crystals was controllable by adjusting the amount of carbon used. For comparison, mesoporous ZSM-5 zeolites have also been synthesized under hydrothermal conditions. The influence of microwave irradiation on mesoporous ZSM-5 materials was thoroughly investigated by using nitrogen adsorption/desorption studies and 27Al MAS NMR. The nature of acid sites both in the micropores (internal) and on the surface of mesopores (external) was investigated by in situ FTIR spectroscopy using pyridine (Py) and 2′,6′-di-tert-butylpyridine (DTBPy) as a probe molecules. Mesoporous ZSM-5 prepared by microwave synthesis showed higher catalytic activity in the bulky molecular reaction of 2′,4′-dimethoxyacetophenone (2′,4′-DMAP) with 4-methoxybenzaldehyde as a model reaction in comparison with the results obtained over hydrothermally prepared ZSM-5. The further catalytic behavior has been studied in condensation reaction and cracking of substituted benzene.

Optical nonlinearities of Se-loaded zeolite (ZSM-5): A molded nanowire system

Nonlinear optical properties of Se-loaded ZSM-5 single crystals have been studied comparatively with those of glassy Se. Two-photon absorption coefficients and intensity-dependent refractive-index of the Se-zeolite are higher by three orders of magnitude than those of glassy Se. The enhancements can be ascribed to electron confinement in single Se chains.

Selenium-loaded zeolite (ZSM-5): I. Structures and fundamental properties

Atomic structures of Se clusters impregnated into ZSM-5 single crystals have been investigated through structural and optical experiments. Raman-scattering spectroscopy suggests that Se in the zeolite takes single chain structures. X-ray diffraction implies that the chains form fourfold (not threefold as in trigonal Se) helices. The chain length is estimated from weight and electron-spin resonance measurements at 102 atoms. The material shows an optical absorption edge at ℏω≃2.2 eV, which accompanies dichroism, suggesting chain alignment in the zeolite pores.

Selenium-loaded zeolite (ZSM-5): II. Photoinduced phenomena

Photoinduced phenomena in Se-loaded ZSM-5 single crystals have been studied through electron-spin resonance, optical absorption, and photoluminescence spectroscopy. Marked exposure effects appear at room temperature. Upon illumination of visible light, an electron-spin signal at g≈2.00 increases by an order, subgap absorption of 103 cm−1 appears, and photoluminescence at subgap regions weakens to a half. Timescales of these photoinduced changes including recoveries are not the same, which suggests different origins for the three changes. The spin increase evinces bond scission in single Se chains.

Optical properties of chalcogen-loaded zeolite (ZSM-5)

Sulfur, selenium, and tellurium were loaded into sub-millimeter size ZSM-5 single crystals, and the optical properties have been comparatively studied. S and Te show similar features, while Se is unique. S and Te have optical absorption edges at wavelengths of ∼400 nm with transmission dips at ∼450 nm, while Se has the edge at ∼550 nm. The three materials provide photoluminescence at visible wavelengths, with the intensities of S and Te being stronger than that of Se by two orders. These optical properties imply that S and Te in the zeolite form small atomic units such as S 3 and Te 2, while Se condenses into single-chain structures.

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Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27Al and 19F MAS-NMR, Rietveld structure refinement and N2- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst.