Sites In Zeolites Research Articles

Effect of P modification on the structure and catalytic performance of Ti-MWW zeolite

Ti-MWW zeolite has attracted much attention because of its excellent catalytic oxidation performance. As one of the essential components in zeolites, phosphorus (P) has been widely used for modification to enhance the hydrothermal stability and catalytic performance of aluminosilicate zeolite. Herein, a series of P-modified Ti-MWW zeolite samples were prepared with a simple way by directly adding various amounts of phosphoric acid into the acid-treated system. The effect of P modification on both structure and catalytic performance of Ti-MWW zeolite was systematically investigated. We demonstrated that some highly active framework TiO4* species with Ti–O–P structure are formed after modification with moderate P, leading to the remarkable improvement of catalytic activity in 1-hexene epoxidation. While introducing excess amounts of phosphoric acid would result in the formation of much inactive titanium phosphate species, which block the channels and finally decrease the catalytic performance significantly. Density functional theory calculations confirmed that the new formed Ti–H2PO4 site is more active than the Ti–OH site in Ti-MWW zeolite for model propene epoxidation, revealing the promotional effect of P modification. This work would deepen our understanding of P modification at the atomic level and benefit to improve the catalytic performance of titanosilicate zeolite.

Tuning Lewis acid sites in TS-1 zeolites for hydroxylation of anisole with hydrogen peroxide

One-step direct hydroxylation of aromatics is an atom-economic way for phenols production. Ti-containing MFI zeolite is considered to be one of the most potential catalysts for hydroxylation of aromatics to phenols under mild condition with aqueous H 2 O 2 as the oxidant. However, the products are often a mixture of mono- or multi-phenols. Designing TS-1 zeolite catalyst with high yield and selectivity to the target phenols remains a challenging task. Here TS-1 zeolites are prepared by direct hydrothermal and post-synthetic methods. Multiple spectroscopic characterizations show that the crystallinity and Ti-Lewis acid sites (Ti-LASs) distribution of TS-1 zeolite can be controlled by the synthetic approach. The structure-activity relationship of TS-1 zeolite is established in the catalytic hydroxylation of anisole, revealing that Ti-LASs concentration determines the conversion of anisole, while both the crystallinity and Ti-LASs distribution significantly influence the selectivity to the desired product para -hydroxyanisole. •The Ti-Lewis acid properties of TS-1 zeolites can be precisely modulated by the sample preparation. •The concentration of Ti-LASs in TS-1 determines the conversion of anisole hydroxylation. •Introducing Ti-LASs into the well-crystalized TS-1 channels facilitates the selective production of para-isomers in the hydroxylation of anisole. •This work provides insights into the understanding of the catalytic properties of TS-1 zeolites towards the selective hydroxylation of anisole to phenols.

Formation and local structure of framework Al Lewis sites in beta zeolites.

Framework AlFR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by 27Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by 1H MAS, 27Al WURST-QCPMG, and 29Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI-OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2MHz. In addition, the AlTRI atom with adsorbed acetonitrile-d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR Lewis sites are formed from Al-OH-Si-O-Si-O-Si-OH-Al sequences located in 12-rings (i.e., close unpaired Al atoms).

Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low-Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation.

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure-performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T3 sites by low-pressure SiCl4 treatment (LPST). High-field 27 Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T3 sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low‐Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation

AbstractControlling the location of aluminum atoms in a zeolite framework is critical for understanding structure–performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T3 sites by low‐pressure SiCl4 treatment (LPST). High‐field 27Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T3 sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Thiophene-functional hierarchical carbons for the removal of ionic and organic mercury in light hydrocarbon liquids: Synthesis, evaluation and adsorption mechanism

A simple synthesis of thiophene-functional zeolite-templated carbons (ZTCs) is proposed to remove ionic mercury (Hg2+) and organic mercury (Hgorg) from gasoline. Microporous zeolite (Beta) and mesoporous zeolite (MCM-41) were used to prepare porous carbon by impregnating with 2-thiophenemethanol (ThM) to explore the influence of template type on the quality of templated carbon. The amount of strong acidic sites in MCM-41 zeolite was more than Beta zeolite which promoted the catalytic polymerisation of the ThM in the pores structure of the zeolite and highly templated carbon was obtained. The impregnation concentration of ThM on the structure and functional groups of the samples was explored. It was found that increasing the concentration of ThM properly can promote the filling of zeolite pores by ThM and form more ordered templated carbons. In addition, the increased ThM promoted the formation of the S = O and C-O oxygen-containing functional groups. The optimal impregnation concentrations of ThM were 25% for B-Al (25%-B-Al-ZTC) and 75% for M−Al (75%-M−Al−ZTC). The 75%-M−Al−ZTC with sufficient S = O and C-O oxygen-containing functional groups showed excellent mercury adsorption capacity (617.49 μg/g) and a high intra-particle diffusion rate (294.10 h−1). The simultaneous adsorption of Hg2+ and Hgorg was also conducted and it was found that Hgorg had competitive adsorption for Hg2+ which led the total adsorption capacity to decrease. In addition, the 25%-B-Al-ZTC and 75%-M−Al−ZTC adsorbents had good regeneration ability and the adsorption capacity remained over 75% and 71% for Hg2+ and Hgorg after five desorption-sorption cycles, respectively. Thus, the new adsorbents with stable and efficient Hg removal performance have the potential for practical application by oil refineries.

Ammonia-Assisted Light Alkane Anti-coke Reforming on Isolated ReOxSites in Zeolite

The conventional reforming produces H2 with stoichiometric amounts of CO and CO2 from hydrocarbons. Here, we show that COx-free H2 can be produced from ammonia-assisted reforming (ammoreforming) of natural gas liquids (CnH2n+2 + nNH3 = nHCN + (2n + 1) H2, n = 2 or 3) at the same conditions as the steam reforming. Such a process co-produces HCN, which can be easily separated from H2 and used as value-added chemicals or for NH3 recycling through hydrolysis. The ammoreforming of ethane and propane was realized over the Re-modified HZSM-5 zeolite rather than the traditional Pt-based catalyst for the BMA process (methane ammoreforming). The specific activity of the Re/HZSM-5 catalysts at 650 °C is up to 1 molH2/gRe/min (or 180 min–1) during ethane ammoreforming. The catalyst is highly coke resistant and shows only slight deactivation with a time-on-stream up to 20 h. Characterization of the fresh and used catalysts by X-ray absorption and Raman spectroscopies suggested that the isolated ReOx site grafted by AlO4– tetrahedral in the zeolite framework is responsible for the outstanding catalytic activity and coke resistibility.

Probing Acid Sites in MOR Zeolite Using Low-Temperature 13C Solid-State NMR Spectroscopy of Adsorbed Carbon Monoxide

Understanding the structure of Lewis and Brønsted acid sites in zeolites is a key step toward unraveling mechanisms of catalytic reactions, establishing molecular-level structure–activity relationships and thereby rational improvement of their performance. Acid sites are typically studied indirectly, using probe molecules, whose spectroscopic signatures are linked to the nature of specific sites. In this context, carbon monoxide and pyridine (Py) are the most frequently used probe molecules due to their sensitivity toward the nature and strength of surface acid sites. Although infrared and solid-state NMR spectroscopies of adsorbed Py are now widely used, the utilization of carbon monoxide to study surface sites has been mainly limited to infrared spectroscopy. Considering that NMR spectroscopy parameters, such as isotropic chemical shift (δiso) and chemical shift anisotropy (CSA), provide detailed structural information, we have explored the capabilities of low-temperature carbon-13 NMR spectroscopy of adsorbed 13CO on dehydrated mordenite. One-dimensional (1D) and two-dimensional (2D) 13C low-temperature magic-angle spinning NMR spectra show that carbon monoxide interacting with Lewis (LAS) and Brønsted (BAS) acid sites displays distinct 13C NMR signatures, in line with density functional theory (DFT) calculations. Furthermore, {1H}13C HETCOR NMR allows the observation of surface OH groups interacting with CO and points to the spatial proximity (within 1 nm) of BAS to both Si-OH and Al-OH groups, while LAS predominantly comprises the Al-OH group. These observations suggest the formation of defective aluminum sites with one and/or two OH groups bound to aluminum, upon calcination of a NH4-MOR with Si/Al = 6 at high temperature (580 °C). The NMR protocol discussed is perfectly suited to explore the surface sites of other solid materials.

Особенности каталитической трансформации метанола в углеводороды в условиях микроструктурированных потоков

The methanol into hydrocarbons transformation is a complex catalytic reaction accompanied by the formation of a wide range of hydrocarbons and proceeding on the surface of acid sites of various zeolites. Zeolite H-ZSM-5 considered to be most often used catalyst for this process. H-ZSM-5 is a highly dispersed material with a crystal diameter of 1–20 microns, which complicates its direct use in reactors with a fixed catalyst bed due to the high hydraulic pressure drop of the catalytic bed. Traditionally in industry, this issue is solved by using complex reactor systems with a fluidized bed, which is justified for large-scale production. In small and medium-size plants, the use of fluidized bed systems is not economically feasible. One of the possible solutions to this problem is the use of a monolithic catalyst with a supported layer of H-ZSM-5 zeolite. This article presents a study of the catalytic activity of a zeolite-containing microstructured monolith in methanol into hydrocarbons transformation. The monolith was synthesized by pressing a zeolite-containing mass followed by drying, calcining, and secondary growth of the zeolite on the monolith surface. A sample of a monolith with an average channel diameter of 0.5, 1.0, 1.5, 2.0 mm were synthesized this way. Samples of the microstructured catalyst were tested at varying temperatures from 250 to 450 °C and at varying the specific methanol feed rate from 0.65 to 2.3 kg (MeOH)/(kg (Cat) h). For this purpose, the monolithic catalyst was placed in a reactor for testing microstructured catalysts, which consisted of a pump, a temperature controller, a catalytic reactor, a condenser, a separating funnel, and a chromatograph. Varying the conditions showed that for the preferential production of gaseous C1–C4 hydrocarbons, it is advisable to carry out the reaction under the following conditions: the average diameter of the catalyst channels is 2 mm, the reaction temperature is 350 °C, the methanol feed rate is 1.65 kg (MeOH)/(kg (Cat) h). For the predominant formation of liquid hydrocarbons of the C5–C8 fraction, it is advisable to carry out the transformation of methanol into hydrocarbons under the following conditions: the average diameter of the catalyst channels is 1 mm, the reaction temperature is 350 °C, the methanol feed rate is 0.65 kg (MeOH) / (kg (Cat) h). For the predominant formation of liquid hydrocarbons of the C9–C12 fraction, it is advisable to carry out the transformation of methanol into hydrocarbons under the following conditions: the average diameter of the catalyst channels is 0.5 mm, the reaction temperature is 350 °C, and the methanol feed rate is 0.65 kg (MeOH) / (kg (Cat) h).

HZSM-5 zeolite cross-linked with ultrathin siliceous layer for intensifying catalytic cracking and diffusion of n-butane

A series of HZSM-5 zeolites are fabricated to make it cross-link with an external ultrathin siliceous layer by chemical liquid deposition and the corresponding thickness is measured by TEM and XPS analysis. In situ FTIR experiments disclose that the external surface acidic sites of HZSM-5 zeolite can be covered even by siliceous monolayer (0.7 ∼ 1.3 nm, HZ5-S1). In comparison to parent HZSM-5, the catalytic cracking performance of n-butane over the modified zeolites are evidently enhanced. In particular, the n-butane conversion rate per Brønsted acid site over the HZ5-S2 (16.02 min−1) at 550 °C is 2.82 times than that over the HZSM-5 (5.68 min−1). Meanwhile, the diffusion properties of n-butane over the zeolites are quantified by in situ FTIR spectroscopic analysis. Negative correlation between steady-state diffusivity and n-butane conversion rate per Brønsted acid site can be found, which unravels the origin of the progressive decrease in the catalytic stability. Under comparable conditions, more advantages can be greatly achieved by HZ5-S1 catalyst in the catalytic activity and the yields to light olefins due to the benign diffusivity and the elimination of external surface acidic sites. This study can provide a guidance for the understanding of diffusion intensification and the rational design of superior catalyst for the catalytic cracking reactions of alkanes.

Understanding the roles of different acid sites in beta zeolites with different particle sizes catalyzed liquid-phase transalkylation of diethylbenzene with benzene

Moderate and strong Brønsted acid sites were active sites and external acid sites had more contribution to the reaction.

Property-activity relations of multifunctional reactive ensembles in cation-exchanged zeolites: a case study of methane activation on Zn2+-modified zeolite BEA.

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework ZnEF2+ cations, framework oxygens (OF2-), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R2 = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R2 = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.

A review on the characterization of metal active sites over Cu-based and Fe-based zeolites for NH3-SCR.

Cu-based and Fe-based zeolites are promising catalysts for NH3-SCR due to their high catalytic activity, wide temperature window and good hydrothermal stability, while the detailed investigation of NH3-SCR mechanism should be based on the accurate determination of active metal sites. This review systematically summarizes the qualitative and quantitative determination of metal active sites in Cu-based or Fe-based zeolites for NH3-SCR reactions based on advanced characterization methods such as UV-vis absorption (UV-vis), temperature-programmed reduction with H2 (H2-TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), Infrared spectroscopy (IR), Electron paramagnetic resonance (EPR), Mössbauer spectroscopy and DFT calculations. The application and limitations of different characterization methods are also discussed to provide insights for further study of the NH3-SCR reaction mechanism over metal-based zeolites.

Lewis acid Sn-Beta catalysts for the cycloaddition of isoprene and methyl acrylate: a greener route to bio-derived monomers

Framework-Sn sites in Sn-Beta zeolites provide the required Lewis acidity to selectively catalyze the Diels–Alder cycloaddition between methyl acrylate and isoprene, affording a precursor to bio-terephthalic acid.

Effect of rare earths on acidity of high-silica ultrastable REY zeolites and catalytic performance of NiMo/REY+Al2O3 catalysts in vacuum gas oil hydrocracking

A series of low-sodium ultrastable Y zeolites (SiO 2 /Al 2 O 3 ≈ 25) partially exchanged with different amounts of rare earth ions (RE 2 O 3 = 0.55–3.32 wt%) was prepared and characterized by XRD, IR spectroscopy, elemental analysis, N 2 adsorption, as well as 29 Si and 27 Al NMR. The effect of RE content on the number and relative strength of acid sites of REY zeolites was studied by IR spectroscopy of absorbed CO and pyridine. The increase in the RE content in the zeolites leads to a decrease in the concentration of strong Brønsted acid sites (BAS) in faujasite cages (bridged Si–O(H)–Al groups), especially the concentration of stronger BAS associated with Si–O(H)–Al groups polarized by extra-framework aluminum species. At the same time, the concentration of less acidic OH groups connected with extra-framework ions increases possibly due to the formation of additional RE–O(H)–Al groups. The total number of BAS in all the samples remains close. An increase in the RE 2 O 3 content from 0.55 to 3.32 wt% decreases almost by half the ratio of strong BAS in faujasite cages to less acidic extra-framework BAS. The catalytic performance of NiMo/(30 wt% REY + 70 wt% Al 2 O 3 ) catalysts was evaluated in hydrocracking of vacuum gas oil. The enhanced hydrocracking activity and selectivity to middle distillates are mainly achieved due to the optimal acidity of the zeolites. • Synthesis of USY zeolites (Si/Al≈12.5) exchanged with different amount of RE ions. • Strong BAS/less acidic extraframework BAS ratio decreases with RE content increase. • Stronger BAS (Si–O(H)–Al groups polarized by EFAL species) are exchanged first. • Additional BAS are associated with the formation of RE–O(H)–Al groups in supercages. • NiMo/REY + Al 2 O 3 with optimal acidity exhibits improved hydrocracking performance.

IR-Spectroscopic Study of Complex Formation of Nitrogen Oxides (NO, N2O) with Cationic Forms of Zeolites and the Reactivity of Adsorbed Species in CO and CH4 Oxidation.

The formation of complexes and disproportionation of nitrogen oxides (NO, N2O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N2O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν10–2 and the combination bands of the stretching vibrations with other vibrational modes for N2O complexes with cationic sites in zeolites (ν30–1 + ν10–1, ν10–1 + δ0–2) are more significantly influenced by the nature of the zeolite. The presence of several IR bands in the region of 2400–2600 cm−1 (the ν10–1 + δ0–2 transitions) for different zeolite types was explained by the availability of different localization sites for cations in these zeolites. The frequencies in this region also depend on the nature of the cation (its charge and radius). The data can be explained by the specific geometry of the N2O complex formed, presumably two-point adsorption of N2O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH4 on the samples with preliminarily adsorbed N2O at 20–180 °C does not result in any oxidation of these molecules. NO+ and N2O3 species formed by disproportionation of NO are capable of oxidizing CO and CH4 molecules to CO2, whereas NOx is reduced simultaneously to N2 or N2O. The peculiarities in the behavior of cationic forms of different zeolites with respect to adsorbed nitrogen oxides determined by different density and localization of cations have been established.

Cover Feature: DFT Analysis of Methane C−H Activation and Over‐Oxidation by [Cu2O]2+ and [Cu2O2]2+ Sites in Zeolite Mordenite: Intra‐ versus Inter‐site Over‐Oxidation (ChemPhysChem 24/2021)

The Cover Feature illustrates the inter-site nature of methane over-oxidation by dicopper sites in zeolites used for direct methane-to-methanol conversion. Intermediates from methane oxidation can be stabilized at the immediate active site or at Brønsted acid sites, without over-oxidation. This ensures high methanol selectivities. More information can be found in the Article by Samuel O. Odoh and co-workers.

Hydrogen Bonds Dominate Brønsted Acid Sites in Zeolite SSZ-42: A Classification of Their Diversity.

The zeolite catalyst SSZ‐42 shows a remarkable high abundance (≈80 %) of hydrogen‐bonded Brønsted acid sites (BAS), which are deshielded from the 1H chemical shift of unperturbed BAS at typically 4 ppm. This is due to their interaction with neighboring oxygen atoms in the zeolite framework when oxygen alignments are suitable. The classification and diversity of hydrogen bonding is assessed by DFT calculations, showing that oval‐shaped 6‐rings and 5‐rings allow for a stronger hydrogen bond to oxygen atoms on the opposite ring side, yielding higher experimental chemical shifts (δ (1H)=6.4 ppm), than circular 6‐rings (δ(1H)=5.2 ppm). Cage‐like structures and intra‐tetrahedral interactions can also form hydrogen bonds. The alignment of oxygen atoms is expected to impact their role in the stabilization of intermediates in catalytic reactions, such as surface alkoxy groups and possibly transition states.

Catalytic cracking of low-density polyethylene over zeolite-containing hierarchical two-layered catalyst with different mesopore size using Curie point pyrolyzer

Zeolite-containing hierarchical two-layered (ZC2L) catalysts were prepared by the gel skeletal reinforcement method. Their reactivity was estimated in catalytic cracking of low-density polyethylene (LDPE) at 590°C using a Curie point pyrolyzer method. In addition, the “relative activity” per amount of zeolite in the catalyst was calculated, and the relationship between the pore and acid properties of each catalyst was investigated. The results suggested that the presence of both mesopores and the strong acid sites of zeolite would be closely related to the increase in activity. ZSM-5-ZC2L catalysts showed much higher aromatics selectivity especially for toluene and p-xylene than ZSM-5 single and kaolin-mixed catalysts. In contrast to the cases of ZSM-5-containing catalysts, β-zeolite-containing catalysts and Y-zeolite-containing catalysts selectively produced i-olefins and i-paraffins, respectively. The product selectivity of the ZC2L catalyst was influenced by the size of the micropores of the zeolite while the hierarchical ZC2L catalysts with a small amount (26 wt%) of zeolite exhibited the higher activity than zeolite single catalysts.

Defect Sites in Zeolites: Origin and Healing.

This paper deals with the synthesis conditions–defect formation relationship in zeolites. Silicalite‐1 (MFI‐type) is used as a model material. Samples synthesized from a system with high basicity (at 100 °C), a system with moderate basicity (at 150 °C), and a fluoride‐containing system in neutral medium (at 170 °C) are compared. Well‐crystallized materials with sizes ≈0.1, 1–10, and 30–40 µm are obtained. The samples are analyzed by complementary methods providing information on the short‐ and long‐range order in the zeolite framework. A strong correlation between the number of point defects in the zeolite framework and preparation conditions is established. Silicalite‐1 synthesized under mild synthesis conditions from a highly basic system exhibits a larger number of framework defects and thus low hydrophobicity. Further, the calcined samples are subjected to aluminum and silicon incorporation by postsynthesis treatment. The Al/Si incorporation in the zeolite framework and its impact on the physicochemical properties is studied by XRD, TEM/SEM, solid‐state NMR, FTIR, and thermogravimetric analyses. The defects healing as a function of the number of point defects in the initial material and zeolite crystal size is evaluated. The results of this study will serve for fine‐tuning zeolite properties by in situ and postsynthesis methods.