Probing the surface of nanosheet H-ZSM-5 with FTIR spectroscopy

Biodiesel Production by Methanolysis of Rapeseed Oil-Influence of SiO2/Al2O3 Ratio in BEA Zeolite Structure on Physicochemical and Catalytic Properties of Zeolite Systems with Alkaline Earth Oxides (MgO, CaO, SrO)

Alkaline earth metal oxide (MgO, CaO, SrO) catalysts supported on BEA zeolite were prepared by a wet impregnation method and tested in the transesterification reaction of rapeseed oil with methanol towards the formation of biodiesel (FAMEs-fatty acid methyl esters). To assess the influence of the SiO2/Al2O3 ratio on the catalytic activity in the tested reaction, a BEA zeolite carrier material with different Si/Al ratios was used. The prepared catalysts were tested in the transesterification reaction at temperatures of 180 °C and 220 °C using a molar ratio of methanol/oil reagents of 9:1. The transesterification process was carried out for 2 h with the catalyst mass of 0.5 g. The oil conversion value and efficiency towards FAME formation were determined using the HPLC technique. The physicochemical properties of the catalysts were determined using the following research techniques: CO2-TPD, XRD, BET, FTIR, and SEM-EDS. The results of the catalytic activity showed that higher activity in the tested process was confirmed for the catalysts supported on the BEA zeolite characterized by the highest silica/alumina ratio for the reaction carried out at a temperature of 220 °C. The most active zeolite catalyst was the 10% CaO/BEA system (Si/Al = 300), which showed the highest triglyceride (TG) conversion of 90.5% and the second highest FAME yield of 94.6% in the transesterification reaction carried out at 220 °C. The high activity of this system is associated with its alkalinity, high value of the specific surface area, the size of the active phase crystallites, and its characteristic sorption properties in relation to methanol.

Generation of Subnanometer Metal Clusters in Silicoaluminate Zeolites as Bifunctional Catalysts

Zeolite-encapsulated subnanometer metal catalysts are an emerging class of solid catalysts with superior performances in comparison to metal catalysts supported on open-structure solid carriers. Currently, there is no general synthesis methodology for the encapsulation of subnanometer metal catalysts in different zeolite structures. In this work, we will show a general synthesis method for the encapsulation of subnanometer metal clusters (Pt, Pd, and Rh) within various silicoaluminate zeolites with different topologies (MFI, CHA, TON, MOR). The successful generation of subnanometer metal species in silicoaluminate zeolites relies on the introduction of Sn, which can suppress the migration of subnanometer metal species during high-temperature oxidation-reduction treatments according to advanced electron microscopy and spectroscopy characterizations. The advantage of encapsulated subnanometer Pt catalysts in silicoaluminate zeolites is reflected in the direct coupling of ethane and benzene for production of ethylbenzene, in which the Pt and the acid sites work in a synergistic way.

Methanol diffusion and dynamics in zeolite H-ZSM-5 probed by quasi-elastic neutron scattering and classical molecular dynamics simulations

Zeolite ZSM-5 is a key catalyst in commercially relevant processes including the widely studied methanol to hydrocarbon reaction, and molecular diffusion in zeolite pores is known to be a crucial factor in controlling catalytic reactions. Here, we present critical analyses of recent quasi-elastic neutron scattering (QENS) data and complementary molecular dynamics (MD) simulations. The QENS experiments show that the nature of methanol diffusion dynamics in ZSM-5 pores is dependent both on the Si/Al ratio (11, 25, 36, 40 and 140), which determines the Brønsted acid site density of the zeolite, and that the nature of the type of motion observed may vary qualitatively over a relatively small temperature range. At 373 K, on increasing the ratio from 11 to 140, the observed mobile methanol fraction increases and the nature of methanol dynamics changes from rotational (in ZSM-5 with Si/Al of 11) to translational diffusion. The latter is either confined localized diffusion within a pore in zeolites with ratios up to 40 or non-localized, longer-range diffusion in zeolite samples with the ratio of 140. The complementary MD simulations conducted over long time scales (1 ns), which are longer than those measured in the present study by QENS (≈1-440 ps), at 373 K predict the occurrence of long-range translational diffusion of methanol in ZSM-5, independent of the Si/Al ratios (15, 47, 95, 191 and siliceous MFI). The rate of diffusion increases slightly by increasing the ratio from 15 to 95 and thereafter does not depend on zeolite composition. Discrepancies in the observed mobile methanol fraction between the MD simulations (100% methanol mobility in ZSM-5 pores across all Si/Al ratios) and QENS experiments (for example, ≈80% immobile methanol in ZSM-5 with Si/Al of 11) are attributed to the differences in time resolutions of the techniques. This perspective provides comprehensive information on the effect of acid site density on methanol dynamics in ZSM-5 pores and highlights the complementarity of QENS and MD, and their advantages and limitations. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Effects of demineralization on food waste biochar for co-firing: Behaviors of alkali and alkaline earth metals and chlorine

A significant amount of chlorine, and alkali and alkaline earth metal (AAEM) in food waste has been a major limitation to the utilization of food waste as fuel. The present study aims to investigate the behavior of chlorine and AAEM in food waste biochar during pyrolysis, demineralization, and combustion. Food waste compost (FWC) and food waste feedstock (FWF) were selected as raw materials. Three different pyrolysis temperatures from 300 to 500 °C and two demineralization processes, water and CO_2-saturated water, were employed. As the pyrolysis temperature increased, crystallized salt was removed through demineralization, which further increased the heating value. Effective removal of chlorine was demonstrated in both demineralization methods. During demineralization, re-adsorption of Ca on food waste biochar occurred, which was alleviated by CO₂-water demineralization. The total amounts of volatilized Cl and AAEM after CO₂-water demineralization were reduced by 74.79-99.38% for FWF and 98.34-99.9% for FWC compared to raw biochar. Furthermore, slagging and fouling potentials for all food waste biochar samples were estimated using various indices. The proposed behavior of Cl and AAEM in food waste biochar during various fabrication conditions provides insight into how food waste biochar can be applied in thermos-electric power plant for co-firing with coal.

The Influence of Si/Al Ratio on the Physicochemical and Catalytic Properties of MgO/ZSM-5 Catalyst in Transesterification Reaction of Rapeseed Oil

This work presents the comparative physicochemical and catalytic studies of metal oxide MgO catalysts in a transesterification reaction. The influence of the Si/Al ratio in the catalytic material on their catalytic properties in the studied process was extensively evaluated. In addition, the effect of the type of zeolite ZSM-5 form on the catalytic reactivity of MgO based catalysts was investigated. In order to achieve the main goals of this work, a series of MgO/ZSM-5 catalysts were prepared via the impregnation method. Their physicochemical properties were studied using X-ray diffraction (XRD), BET, FTIR and TPD-CO2 methods. The highest activity in the studied process exhibited MgO catalyst supported on ZSM-5 characterized by the highest ratio between silica and alumina. The most active catalyst system in the transesterification reaction was 10% MgO/ZSM-5 (Si/Al = 280), which showed the highest value of higher fatty acid methyl esters (94.6%) and high yield of triglyceride conversion (92.9%). The high activity of this system is explained by the alkalinity, sorption properties in relation to methanol and its high specific surface area compared to the rest of the investigated MgO based catalysts.

Insights on a Hierarchical MFI Zeolite: A Combined Spectroscopic and Catalytic Approach for Exploring the Multilevel Porous System Down to the Active Sites

The hierarchization of zeolites to overcome the major drawbacks related to molecular diffusion limitation in micropores is a popular concept in heterogeneous catalysis. Despite the constant increase of new synthesis strategies to produce such hierarchical systems, the deep knowledge of their structural arrangement and how the zeolitic lattice is organized in a multilevel porous system is often missing. This information is essential to design a structure, tuning the porosity and the distribution of easily accessible active sites, and successively controlling the catalytic properties. In the present work, the synthesis of one of the most sophisticated forms of the hierarchical ZSM-5 zeolite has been reproduced, obtaining two multilevel porous materials with different crystallinity degrees, with the final aim of investigating and clarifying the finest features of their active sites. For this purpose, an extended characterization step by means of a unique multitechnique approach has been performed, thus revealing the active site nature, abundance, and distribution. IR spectroscopy with different molecular probes and a targeted catalytic test based on the hydroconversion reaction of n-decane were the toolbox for disclosing how the MFI lattice takes part in the hierarchical structure and how it, working in synergy with the mesoporous system, confers to this material a totally new shape-size selectivity. Merging the information obtained for the synthesized hierarchical zeolite with the characterization results of two reference materials (a mesoporous aluminum-containing MCM-41 and a microporous commercial ZSM-5), it was possible to define an internal and external map of the pore network of this complex and unique molecular sieve, where strong Brønsted acidic sites are located at the mouth of the MFI micropores and, at the same time, exposed at the surface of the mesoporous channels. Hence, the possibility of easily releasing bulky products is ensured and the application possibilities of the MFI lattice are expanded beyond cracking reactions.

Catalytic conversion of ethene to butadiene or hydrogenation to ethane on HY zeolite-supported rhodium complexes: Cooperative support/Rh-center route

Rh(C₂H₄)₂ species grafted on the HY zeolite framework significantly enhance the activation of H₂ that reacts with C₂H₄ ligands to form C₂H₆. While in this case, the simultaneous activation of C₂H₄ and H₂ and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C₂H₆, the results obtained for Rh(CO)(C₂H₄)/HY materials exposed to H₂ convincingly show that the support-assisted C₂H₄ hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C₂H₄ ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO)₂ complexes into Rh(CO)(C₂H₄) species under ambient conditions is not a simple CO/C₂H₄ ligand exchange reaction on Rh sites, as this process also involves the conversion of C₂H₄ into C₄ hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10^-3 s^-1. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H₂, as these species significantly accelerate the formation of the C₄ hydrocarbons from C₂H₄ even without the presence of H₂ in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C₂H₄ into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.

Methanol loading dependent methoxylation in zeolite H-ZSM-5

We evaluate the effect of the number of methanol molecules per acidic site of H-ZSM-5 on the methoxylation reaction at room temperature by applying operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS), which capture the methoxylation reaction by simultaneously probing surface adsorbed species and reaction products, respectively. To this end, the methanol loading in H-ZSM-5 (Si/Al ≈ 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively. The operando DRIFTS/MS data show that the room temperature methoxylation depends on the methanol loading: the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules, as evident from the evolution of the methyl rock band of the methoxy species and of water as a function of time. Significantly, no methoxylation is observed with ≤1 molecule per Brønsted acidic site. However, hydrogen bonded methanol occurs across all loadings studied, but the structure of hydrogen bonded methanol also depends on the loading. Methanol loading of ≤1 molecule per acidic site leads to the formation of hydrogen bonded methanol with no proton transfer (i.e. neutral geometry), while loading ≥2 molecules per acidic site results in a hydrogen bonded methanol with a net positive charge on the adduct (protonated geometry). The infrared vibrational frequencies of methoxy and hydrogen bonded methanol are corroborated by Density Functional Theory (DFT) calculations. Both the experiments and calculations reflect the methoxy bands at around 940, 1180, 2868-2876 and 2980-2973 cm-1 which correspond to ν(C-O), ρ(CH3), ν s(C-H) and ν as(C-H), respectively.

Two-Dimensional Zeolite Materials: Structural and Acidity Properties

Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si⁴⁺) and aluminum (Al³⁺) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2-3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues' structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed.

Effect of surface silicon modification of H-beta zeolites for alkylation of benzene with 1-dodecene

H-beta zeolites of 100–200 nm (named BEA-L) and 20–30 nm (named BEA-S) were treated by chemical liquid deposition (CLD) of tetraethyl orthosilicate (TEOS) to improve the selectivity of 2-phenyl linear alkylbenzene (2-LAB) from benzene alkylation with 1-dodecene. The results indicate that H-beta zeolite with a smaller crystal size has a longer lifetime due to shorter channels and less diffusion limitation. The deposited SiO₂ layers passivated the external surface acid sites of the zeolite and made the pores narrower. BEA-L lost more external Brønsted acid sites than BEA-S with the same added amount of TEOS, which was due to the severe aggregation of BEA-S grains. This increased passivation gave BEA-L increased 2-LAB selectivity. And when the added amount of SiO₂ was 7.20 wt% of the parent zeolite, the selectivity of 2-LAB over BEA-L significantly increased from 41.9% to 54.7% while that of BEA-S only increased by 2%.

β zeolite with large grain size has higher external surface acid passivation efficiency while that of β zeolite with small grain size is very low due to severe agglomeration.

H-ZSM-5 Materials Embedded in an Amorphous Silica Matrix: Highly Selective Catalysts for Propylene in Methanol-to-Olefin Process

H-ZSM-5 materials embedded in an amorphous silica were successfully synthesized with three different Si/Al ratios (i.e., 40, 45, and 50). The presence of the MFI structure in the synthesized samples was confirmed by X-ray diffraction (XRD), Fourier transform infra-red (FT-IR), and solid state-nuclear magnetic resonance (SSNMR) techniques. The morphology and textural properties of the samples were investigated by scanning electron microscopy (SEM), TEM, and N2-physisorption measurements. Furthermore, acidic properties of the synthesized catalysts have been studied by NH3-TPD and FT-IR spectroscopy of CO adsorption studies. Variation of the Si/Al ratio affected the crystal morphology, porosity, and particle size, as well as the strength and distribution of acid sites. The synthesized zeolite materials possessed low acid-site density and exhibited high catalytic activity in the methanol-to-olefin (MTO) reaction. To study the intermediate species responsible for catalyst deactivation, the MTO reaction was carried out at high temperature (500 °C) to accelerate catalyst deactivation. Interestingly, the synthesized catalysts offered high selectivity towards the formation of propylene (C3=), in comparison to a commercial microporous crystalline H-ZSM-5 with Si/Al = 40, under the same reaction conditions. The synthesized H-ZSM-5 materials offered a selectivity ratio of C3=/C2= 12, while it is around 2 for the commercial H-ZSM-5 sample. The formation of hydrocarbon species during MTO reaction over zeolite samples has been systematically studied with operando UV-vis spectroscopy and online gas chromatography. It is proposed that the strength and type of acid sites of catalyst play a role in propylene selectivity as well as the fast growing of active intermediate species. The effective conversion of methanol into propylene in the case of synthesized H-ZSM-5 materials was observed due to possession of weak acid sites. This effect is more pronounced in H-ZSM-5 sample with a Si/Al ratio of 45.

Catalytic combustion of VOC on sandwich-structured Pt@ZSM-5 nanosheets prepared by controllable intercalation

A sandwich-structured Pt@ZSM-5 nanosheets (Pt@PZN-2) was fabricated by controllable intercalating Pt nanoparticles between ZSM-5 single-layer sheets via electrostatic adsorption with ammonium ions in diquaternary ammonium surfactant, and the following calcinations. Pt clusters (ca. 4.3 nm) confined between nanosheet layers (thickness of ca. 2.9 nm) exhibits better dispersion and higher thermal stability even after high-temperature treatment, while the Pt clusters also served as pillars which well protected the inter-layers mesopores. Catalytic combustion of toluene (a model compound of large molecules VOCs) shows catalytic activity of Pt@PZN-2 is significant higher than that of Pt-loaded nanosheet ZSM-5 zeolite prepared by conventional impregnation (Pt/ZN-2). For example, the temperature at 98% toluene conversion of Pt@PZN-2 is only 176 °C which is 24 °C and 34 °C lower than that of Pt/ZN-2 and bulky ZSM-5 supported Pt by impregnation (Pt/CZ-500), respectively. Meanwhile, Pt@PZN-2 also gives 100% toluene conversion for more than 360 h at 230 °C with a higher metal stability. The good performance of Pt@PZN-2 was attributed to the high accessibility and enhanced diffusion of the bulky reactant to active site confined in the stable inter-layer mesopores resulting from the pillar of Pt nanoparticles.

Room temperature methoxylation in zeolite H-ZSM-5: an operando DRIFTS/mass spectrometric study

At high loading, methanol reacts under ambient conditions with acidic hydroxyls of H-ZSM-5 to methoxylate framework oxygen; while a significant proportion remains hydrogen bonded to the framework with protonated geometry.

Poly(ethylene glycol) (PEG)-crosslinked poly(vinyl pyridine)–PEG–poly(vinyl pyridine)-based triblock copolymers prepared by RAFT polymerization as novel gel polymer electrolytes

A series of triblock copolymers based on poly(vinyl pyridine)–PEG–poly(vinyl pyridine) (PVP–PEG–PVP) with different PEG-to-PVP ratios (1 : 200, 1 : 250, and 1 : 500) were prepared using the RAFT polymerization.

Plausibility of potassium ion-exchanged ZSM-5 as soot combustion catalysts

Potassium (K) ion-exchanged ZSM-5 zeolites were investigated for catalytic soot combustion. X-ray absorption fine-structure (XAFS), Raman, in situ IR and NH₃-temperature programmed desorption (NH₃-TPD) confirmed the location of K⁺ at the ion-exchanged sites. Temperature-programmed oxidation (TPO) reactions showed that K-ZSM-5 decreased ignition tempeatures of soot combustion and increased selectivity to CO₂. The improved activity for soot combustion by increasing K⁺-exchanged amounts via decreasing the Si/Al ratio reinforced the K⁺ ions participating in soot combustion. ¹⁸O₂ isotopic isothermal reactions suggested the activation of gaseous oxygen by the K⁺ ions. This demonstrated a new appliction of alkali metal exchanged zeolites and the strategy for enhancement of catalytic soot combustion activity.

Effect of Al content on number and location of hydroxyl acid species in zeolites: a DRIFTS quantitative protocol without the need for molar extinction coefficients

Collidine is able to interact only with hydroxyl species on the external surface or at the pore mouth of the zeolite as it is not able to access the internal pore structure.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Ethene oligomerization on H-ZSM-5 in relation to ethoxy species

The oligomerization of ethene over H-ZSM-5 was studied by infrared (IR) spectroscopy.