Probing zeolites by vibrational spectroscopies

The proximity of aluminium atoms influences the reaction pathway of ethanol transformation over zeolite ZSM-5

The organization of aluminium atoms in zeolites affects their catalytic properties. Here we demonstrate that the aluminium distribution is a key parameter controlling the reaction pathway of acid catalysed reactions over ZSM-5 zeolites. We study ethanol transformation over two ZSM-5 samples with similar Si/Al ratios of ~15, and with aluminium atoms located mainly at the channel intersections but differently distributed in the framework. One of the samples contains mostly isolated aluminium atoms while the other has a large fraction of two aluminium atoms located in one ring. The FT-IR time-resolved operando study, supported by catalytic results, reveals that the reaction pathway in ethanol transformation over ZSM-5 is controlled by the proximity of aluminium atoms in the framework. ZSM-5 containing mostly isolated Al atoms transforms ethanol in the associative pathway, and conversely ZSM-5 containing a dominating fraction of two aluminium atoms in one ring transforms ethanol in the dissociative pathway.

Comparing the Nature of Active Sites in Cu-loaded SAPO-34 and SSZ-13 for the Direct Conversion of Methane to Methanol

On our route towards a more sustainable future, the use of stranded and underutilized natural gas to produce chemicals would be a great aid in mitigating climate change, due to the reduced CO2 emissions in comparison to using petroleum. In this study, we investigate the performance of Cu-exchanged SSZ-13 and SAPO-34 microporous materials in the stepwise, direct conversion of methane to methanol. With the use of X-ray absorption spectroscopy, infrared (in combination with CO adsorption) and Raman spectroscopy, we compared the structure–activity relationships for the two materials. We found that SSZ-13 performed significantly better than SAPO-34 at the standard conditions. From CH4-TPR, it is evident that SAPO-34 requires a higher temperature for CH4 oxidation, and by changing the CH4 loading temperature from 200 to 300 °C, the yield (μmol/g) of SAPO-34 was increased tenfold. As observed from spectroscopy, both three- and four-fold coordinated Cu-species were formed after O2-activation; among them, the active species for methane activation. The Cu speciation in SAPO-34 is distinct from that in SSZ-13. These deviations can be attributed to several factors, including the different framework polarities, and the amount and distribution of ion exchange sites.

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (ρ) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm^-1 are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm^-1 is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm^-1 can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm^-1 is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Brønsted acid sites. The intensity of the characteristic mode at 3611 cm^-1 reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure.

Facile sulfolane-modified resins for enhanced dimethoxymethane carbonylation

A simple resin modification method for 50 h DMM carbonylation with a high STY of 3.15 mmol_MMAc (gcat h)⁻¹ and 100% MMAc selectivity.

Active sites and mechanisms in the direct conversion of methane to methanol using Cu in zeolitic hosts: a critical examination

In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol.

Strong Lewis acidic catalysts for C–F bond activation by fluorination of activated γ-Al2O3

A strong solid Lewis acid catalyst has been successfully obtained by modifying a known procedure of fluorinating γ-aluminium oxide, which was pre-calcined under vacuum.

Direct conversion of syngas into light aromatics over Cu-promoted ZSM-5 with ceria–zirconia solid solution

Syngas conversion with 72.1% selectivity to aromatics and 40.8% CO conversion was achieved over a copper and silica-modified ZSM-5 and ceria–zirconia solid solution bifunctional catalyst.

Understanding the impact of one-dimensional pore containing 10MR and 12MR and aluminium content on MTH reaction pathways: direct synthesis of heteroatom containing UZM-55

We directly synthesized one-dimensional zeolite UZM-55 as an aluminosilicate and catalyzed MTH to understand pore structure influence on catalytic properties.

Kinetic study and effect of water on methane oxidation to methanol over copper-exchanged mordenite

Kinetic experiments show that both methoxy species and carbon monoxide are primary products. Adsorption of one water molecule reversibly blocks at least two copper atoms in active species.

Aromatization of ethylene over zeolite-based catalysts

Elucidating the role of metal modification and confined hydrocarbon species in the aromatization of ethylene on zeolite catalysts.

Effects of crystal size on methanol to hydrocarbon conversion over single crystals of ZSM-5 studied by synchrotron infrared microspectroscopy

Operando synchrotron infrared microspectroscopy (OIMS) was used to study the conversion of methanol over coffin-shaped HZSM-5 crystals of different sizes: large (∼250 × 80 × 85 μm3), medium (∼160 × 60 × 60 μm3) and small (∼55 × 30 × 30 μm3). The induction period, for direct alkene formation by deprotonation of surface methoxy groups, was found to decrease with decreasing crystal size and with increasing reaction temperature. Experiments with a continuous flow of dimethylether showed that evolution of the hydrocarbon pool and indirect alkene formation is also strongly dependent on crystal size. These measurements suggest that the hydrocarbon pool formation and indirect alkene generation should be almost instantaneous at reaction temperatures used in practical catalysis with crystal sizes typically ∼1 μm3.

Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol

The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15–407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–180 °C has been explored. We describe and discuss data concerning the strength and concentration of acid sites of H-BEA obtained by techniques of stepwise (quasi-equilibrium) thermal desorption of ammonia, X-ray diffraction, low-temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed pyridine and solid-state ²⁷Al MAS NMR. The average values of the adsorption energy of NH₃ on H-BEA were experimentally determined as 63.7; 91.3 and 121.9 mmol g⁻¹ (weak, medium, and strong, respectively). In agreement with this, a correlation between the rate of ethyl-tert-butyl ether synthesis and the concentration of weak acid sites (E_NH3 = 61.6–68.9 kJ mol⁻¹) has been observed. It was concluded that the active sites of H-BEA for this reaction are Brønsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.

The active sites of H-BEA zeolites for ETBE synthesis are the weak Brønsted acid sites representing internal silanol groups.

Calculating adsorption isotherms using Lennard Jones particle density distributions

The computation of particle density distributions in pore channels is a fundamental post process practice in molecular adsorption simulations. The distributions, although not appropriate for direct experimental interrogation, when expressed in variable temperature, may be used to evaluate thermodynamic properties. As with molecular simulations, we can spotlight any frame or region of interest inside the computational cell, the distributions and subsequently the thermodynamic evaluations can be pore or site specific. This allows us to establish correlations for the adsorption capacity, on different pore partitions and surface textures and compare strengths of explicit interactions. We confirm this assumption using adsorption simulations of Lennard Jones particles such as argon in the dual cylindrical cavity of ZIF-69. We initiate the computations from the density distribution functions and reproduce full curves of adsorption isotherms which correlate well with the direct simulations. The computations are fast and implemented using linear van' t Hoff plots for the surface coverage at a sequence of pressures.

Nature of Synergy between Brønsted and Lewis Acid Sites in Sn-Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis

The role of Lewis and Brønsted acid sites and their potential synergy remains ambiguous for the production of polyoxymethylene dimethyl ethers (OME), which are suitable as a Diesel substitute. Here, this synergistic effect was investigated by using a series of beta polymorph A (BEA) zeolites with various degrees of Brønsted and Lewis acidity. Lewis acidity was introduced in dealuminated zeolites by Sn grafting in dichloromethane. These sites were only active in paraformaldehyde decomposition, OME growth, and acetalization. The Brønsted acid sites arising from bridging hydroxyl groups were active for all reaction steps, and notably for trioxane ring-opening and dissociation to formaldehyde (FA), which did not occur on the Lewis acid sites. Presence of both Lewis and Brønsted acid sites led to a four-fold increase in turnover frequency and a significant decrease of byproduct formation compared with the parent zeolite during OME synthesis from dimethoxymethane and trioxane. The synergistic effect between both types of acid sites is explained by FA insertion on Lewis acid sites leading to OME growth. Interaction between tetrahedral Sn and the carbonyl group of FA resulted in an activated carbonyl bond, which was likely the initial step for insertion of FA into OME.

Alkali Earth Catalysts Based on Mesoporous MCM-41 and Al-SBA-15 for Sulfone Removal from Middle Distillates

Mg, Ca, and Ba catalysts supported on structured mesoporous silica oxides types MCM-41 and Al-SBA-15 were synthesized and investigated in sulfone cracking for sulfur removal from oxidized diesel fuel. Functional materials and catalysts were characterized by low-temperature nitrogen adsorption/desorption, transmission electron microscopy, and inductively coupled plasma atomic emission spectroscopy techniques. Catalytic tests were carried out in fixed-bed and batch reactors with a model compound dibenzothiophene sulfone and oxidized diesel fraction as a feed. MgO/MCM-41 and MgO/Al-MCM-41 possess high activity in sulfone cracking. The sulfur content in the diesel fraction decreases from initial 450 up to 100 ppmw. Catalysts can be regenerated for reuse in several cycles and may be potentially scaled up for industrial applications.

3D Raman Spectroscopy of Large Zeolite ZSM-5 Crystals

Hydrothermal treatment is a common method used to modify the physicochemical properties of zeolite‐based catalyst materials. It alters the number and type of acid sites through dealumination and increases molecular diffusion by mesopore formation. Steaming also reduces the structural integrity of zeolite frameworks. In this study, Raman microscopy has been used to map large zeolite ZSM‐5 crystals before and after steaming. 3D elemental maps of T−O (T: Al or Si) sites of the zeolite were obtained. The Raman active vibrational bands were determined, which are indicative of (non‐) framework Al, as well as of structural integrity. Zeolite steaming caused the introduction of additional heterogeneities within the zeolite framework. Al migration and the formation of extra‐framework Al species were observed. The described experiments demonstrate the capability of 3D Raman spectroscopy as a valuable tool to obtain information on the spatial distributions of framework elements as well as defects within a zeolite‐based material.

Synthesis of nanostructured catalysts by surfactant-templating of large-pore zeolites

Zeolites and related crystalline molecular sieves are utilised in a wide range of reactions and processes due to their regular microporous structure, strong acidity, shape selectivity and ion-exchange properties. However, their practical applications can be limited by the small size of the channels and cavities of the microporous structures, and therefore, a great deal of effort has been devoted to enhancing the transport of large-sized molecules in the host pores. Several commercially available zeolites, including faujasite (FAU), mordenite (MOR), beta (BEA), ZSM-5 (MFI) and zeolite L (LTL), have been exposed to a variety of acid and base treatments in the presence of a surfactant (cetyltrimethyl ammonium bromide, CTAB), which led to the controlled introduction of intracrystalline mesoporosity. The detailed characterisation of the obtained mesostructured zeolites has been carried out using FTIR spectroscopy, high resolution TEM, XRD, N₂ adsorption, ²⁹Si and ²⁷Al MAS NMR. This work demonstrates a successful application of the supramolecular templating approach for generating tuneable mesoporosity in a range of zeolites possessing 12-membered ring channels, which has been applied to zeolite L for the first time, thus producing hierarchical meso-microporous materials with improved accessibility of active sites and enhanced catalytic performance in dealkylation of tri-isopropylbenzene.

Selective photocatalytic conversion of methane into carbon monoxide over zinc-heteropolyacid-titania nanocomposites

Chemical utilization of vast fossil and renewable feedstocks of methane remains one of the most important challenges of modern chemistry. Herein, we report direct and selective methane photocatalytic oxidation at ambient conditions into carbon monoxide, which is an important chemical intermediate and a platform molecule. The composite catalysts on the basis of zinc, tungstophosphoric acid and titania exhibit exceptional performance in this reaction, high carbon monoxide selectivity and quantum efficiency of 7.1% at 362 nm. In-situ Fourier transform infrared and X-ray photoelectron spectroscopy suggest that the catalytic performance can be attributed to zinc species highly dispersed on tungstophosphoric acid /titania, which undergo reduction and oxidation cycles during the reaction according to the Mars-van Krevelen sequence. The reaction proceeds via intermediate formation of surface methyl carbonates.

Free fatty acids esterification catalyzed by acid Faujasite type zeolite

The catalytic activity of the protonated form of H-Y(80) zeolite (Faujasite with high Si/Al ratio) was evaluated as an acid catalyst in the esterification step pre-treatment of FFA by means of the esterification reaction of oleic acid with methanol in soybean oil. The zeolite structure was characterized by XRD and FTIR. Textural characterization was carried out by N2 physisorption. The thermal stability was evaluated by TG-DTA and the acidity measured by NH3-TPD and Pyridine-FTIR. The limitations of the use of this zeolite in a pre-treatment for biodiesel production was investigated through oleic acid esterification in soybean oil, as a model reaction, performed with different temperatures, catalyst amounts and molar ratios. The results showed that the amount of remaining FFA decreased to values well below the initial amount. Under the optimal reactional conditions, conversions to methyl esters above 95% were achieved. Results support that such reactions can be performed under H-Y(80) zeolite catalysis and can be applied in a pre-treatment esterification of feedstocks with high contents of FFA. Catalyst reuse is feasible due to its easy separation from reaction products allowing new reaction cycles, as well as the application of the H-Y(80) zeolite in biodiesel production.

What Is Measured When Measuring Acidity in Zeolites with Probe Molecules?

Based on theoretical calculations of CO, NH₃, and pyridine adsorption at different sites in MOR and MFI zeolites, we analyze how confinement effects influence the measurement of acidity based on the interaction of probe molecules with Brönsted acid sites. Weak bases, such as CO, form neutral ZH–CO adducts with a linear configuration that can be distorted by spatial restrictions associated with the dimensions of the pore, leading to weaker interaction, but can also be stabilized by dispersion forces if a tighter fitting with the channel void is allowed. Strong bases such as NH₃ and pyridine are readily protonated on Brönsted acid sites, and the experimentally determined adsorption enthalpies include not only the thermochemistry associated with the proton transfer process itself, but also the stabilization of the Z^––BH⁺ ion pair formed upon protonation by multiple interactions with the surrounding framework oxygen atoms, leading in some cases to a heterogeneity of acidities within the same zeolite structure.

Partial and Complete Substitution of the 1,4-Benzenedicarboxylate Linker in UiO-66 with 1,4-Naphthalenedicarboxylate: Synthesis, Characterization, and H2-Adsorption Properties

We describe the synthesis and corresponding full characterization of the set of UiO-66 metal-organic frameworks (MOFs) with 1,4-benzenedicarboxylate (C₆H₄(COOH)₂, hereafter H₂BDC) and 1,4-naphthalenedicarboxylate (C₁₀H₆(COOH)₂, hereafter H₂NDC) mixed linkers with NDC contents of 0, 25, 50, and 100%. Their structural (powder X-ray diffraction, PXRD), adsorptive (N₂, H₂, and CO₂), vibrational (IR/Raman), and thermal stability (thermogravimetric analysis, TGA) properties quantitatively correlate with the NDC content in the material. The UiO-66 phase topology is conserved at all relative fractions of BDC/NDC. The comparison between the synchrotron radiation PXRD and 77 K N₂-adsorption isotherms obtained on the 50:50 BDC/NDC sample and on a mechanical mixture of the pure BDC and NDC samples univocally proves that in the mixed linkers of the MOFs the BDC and NDC linkers are shared in each MOF crystal, discarding the hypothesis of two independent phases, where each crystal contains only BDC or NDC linkers. The careful tuning of the NDC content opens a way for controlled alteration of the sorption properties of the resulting material as testified by the H₂-adsorption experiments, showing that the relative ranking of the materials in H₂ adsorption is different in different equilibrium-pressure ranges: at low pressures, 100NDC is the most efficient sample, while with increasing pressure, its relative performance progressively declines; at high pressures, the ranking follows the BDC content, reflecting the larger internal pore volume available in the MOFs with a higher fraction of smaller linkers. The H₂-adsorption isotherms normalized by the sample Brunauer-Emmett-Teller specific surface area show, in the whole pressure range, that the surface-area-specific H₂-adsorption capabilities in UiO-66 MOFs increase progressively with increasing NDC content. Density functional theory calculations, using the hybrid B3LYP exchange correlation functional and quadruple-ζ with four polarization functions (QZ4P) basis set, show that the interaction of H₂ with the H₂NDC linker results in an adsorption energy larger by about 15% with respect to that calculated for adsorption on the H₂BDC linker.

Catalytic decomposition of N2O over Cu–Al–Ox mixed metal oxides

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O). Techniques such as XRD, ICP-MS, N₂ physisorption, O₂-TPD, H₂-TPR, in situ FT-IR and XAFS were used to characterize the obtained materials. Physico-chemical characterization revealed the formation of mixed metal oxides characterized by different specific surface area and thus, different surface oxygen default sites. The O₂-TPD results gained for Cu–Al–O_x mixed metal oxides conform closely to the catalytic reaction data. In situ FT-IR studies allowed detecting the form of Cu⁺⋯N₂ complexes due to the adsorption of nitrogen, i.e. the product in the reaction between N₂O and copper lattice oxygen. On the other hand, mostly nitrate species and NO were detected but those species were attributed to the residue from catalyst synthesis.

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O).

The crucial role of clay binders in the performance of ZSM-5 based materials for biomass catalytic pyrolysis

The effect of agglomerating ZrO₂/n-ZSM-5 catalyst with different clays on biomass catalytic pyrolysis is evaluated.

Mild dealumination of template-stabilized zeolites by NH4F

A novel method to remove aluminium from the framework of as-synthesized zeolite crystals is presented.

MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis

Highly active olefin metathesis catalysts were prepared by thermal spreading MoO₃ and/or MoO₂(acac)₂ on MWW zeolites (MCM-22, delaminated MCM-56) and on two-dimensional MFI (all in NH₄⁺ form). The catalysts‘ activities were tested in the metathesis of neat 1-octene (as an example of a longer chain olefin) at 40 °C. Catalysts with 6 wt % or 5 wt % of Mo were used. The acidic character of the supports had an important effect on both the catalyst activity and selectivity. The catalyst activity increases in the order 6MoO₃/HZSM-5(25) (Si/Al = 25) << 6MoO₂(acac)₂/MCM-22(70) < 6MoO₃/2D-MFI(26) < 6MoO₃/MCM-56(13) < 6MoO₃/MCM-22(28) reflecting both the enhancing effect of the supports‘ acidity and accessibility of the catalytic species on the surface. On the other hand the supports‘ acidity decreases the selectivity to the main metathesis product C14 due to an acid-catalyzed double bond isomerization (followed by cross metathesis) and oligomerization. 6MoO₃/2D-MFI(26) with a lower concentration of the acidic centres resulting in catalysts of moderate activity but with the highest selectivity.

Hydroxyl Groups of Exceptionally High Acidity in Desilicated Zeolites Y

The desilication of dealuminated zeolite Y in NaOH/tetrabutylammonium hydroxide mixtures produces hierarchical zeolite Y containing a micropore system as well as mesopores of significant volume and surface. IR studies evidenced that a new kind of hydroxyls was formed if desilication was realized above 318 K. This new kind of acidic hydroxyls is characterized by IR band at 3600 cm^-1 . IR studies showed, that these new hydroxyls showed extremely high acidity. This was evidenced by very high frequency shifts of IR bands of OH interacting with probe molecules: CO and N₂ : Δ ν O H · · · C O =411 cm^-1 and Δ ν O H · · · N 2 =164 cm^-1 , resp. These frequency shift are the highest in all the chemistry of zeolites including very strongly acidic dealuminated mazzite and steamed zeolite USY ( Δ ν O H · · · C O =ca. 380 cm^-1 ) indicating that 3600 cm^-1 hydroxyls are the most acidic in all the zeolites.