Extra-framework Aluminum Research Articles

Effects of adding metals to Beta zeolite on ethanol conversion to hydrocarbons

This study investigated the effects of Beta zeolite ion exchange with Fe, Zn, and Co on ethanol conversion to hydrocarbons. Various characterization techniques such as X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), magic angle rotation nuclear magnetic resonance spectrometry (MAS NMR) of 29Si and 27Al, scanning electron microscopy (SEM), nitrogen physisorption, ammonia temperature-programmed desorption (NH3-TPD) and Fourier transform infrared spectroscopy (FTIR) were employed. Catalytic tests were performed at temperatures from 475 to 550 °C under ambient pressure. The metal addition to Beta zeolite reduced micropore area and volume, decreased the concentration of the extra-framework aluminum, and increased total acid site density and Lewis acid sites, except for Fe. For all catalysts, ethylene was identified as the primary product. However, catalysts containing Zn and Co displayed the formation of propylene and C1 to C3 paraffins. The catalyst with Fe exhibited the highest deactivation and a significant decrease in acidity, attributed to the formation of extra-framework aluminum, resulting in exclusive ethylene production.

Influence of Extra-Framework Aluminum Species on the Catalytic Properties of Acidic USY Zeolite in (Hydro)cracking Reactions

Influence of Extra-Framework Aluminum Species on the Catalytic Properties of Acidic USY Zeolite in (Hydro)cracking Reactions

Solid-state NMR study of the stability of MOR framework aluminum

MOR zeolite has been effectively utilized for dimethyl ether (DME) carbonylation reaction due to its unique pore structure and acidity. During industrial production, the transformation of ammonium type MOR zeolite (NH4-MOR) into proton type MOR zeolite (H-MOR) causes inevitable dealumination. Therefore, understanding the influencing factors and dynamic evolution mechanism of zeolite dealumination is crucial. In this work, the stability of framework aluminum was studied by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, 29Si, 27Al, 1H magic angle spinning nuclear magnetic resonance (MAS NMR), and DME carbonylation performance evaluation. These results indicate that extra-framework cation Na+ and NH4+ could better preserve the aluminum structure of the MOR zeolite framework compared to H+, primarily due to the different ‘attraction’ of the framework to water. Furthermore, the impact of water on the zeolite framework aluminum at high temperature was studied by manipulating the humidity of the calcination atmosphere, revealing the formation of extra-framework six-coordinated aluminum (Al(VI)-EF) and the mechanism of water influence on the zeolite framework aluminum.

Experimental, isotherm, kinetic, and thermodynamic studies of the novel modified zeolite ZSM-5 adsorbent for use in clean fuel processing

Reducing the amount of sulfur in fuels is the simplest way to reduce the emission of sulfur oxide gases into the air. Among various desulfurization methods, adsorptive desulfurization stands out for its mild conditions and affordability. This research used ZSM-5 zeolite as a desulfurization adsorbent. However, adsorption capacity and selectivity are two major challenges that adsorptive desulfurization is facing. Introducing mesoporosity into zeolite is a potential solution. The study focused on mesoporous formation via desilication and dealumination. Desilication involved treating zeolite with 0.2 M and 0.5 M NaOH, while dealumination used 1 M HCl at 80 °C. Adsorbent properties were analyzed using XRD, BET, FT-IR, and FE-SEM. Initial findings indicated dealumination removed only extra-framework aluminum. Conversely, desilication removed silicon, producing a hierarchical structure even at reduced concentrations. Des-0.5M also exhibited the highest adsorption capacity for thiophene compared to the Parent sample, increasing from 4.5 to 11 mgS/g. Moreover, the adsorption capacity for larger sulfur molecules such as dibenzothiophene increased from 2.5 to approximately 6 mgS/g, indicating the significance of the hierarchical structure developed in the adsorbents. In summary, this study highlights the potential of mesoporous formation through desilication to enhance the adsorption capacity of ZSM-5 zeolite for the adsorptive desulfurization of fuels.

Factors Affecting the Generation and Catalytic Activity of Extra-Framework Aluminum Lewis Acid Sites in Aluminum-Exchanged Zeolites

Factors Affecting the Generation and Catalytic Activity of Extra-Framework Aluminum Lewis Acid Sites in Aluminum-Exchanged Zeolites

Dynamic evolution of HZSM-5 zeolite framework under steam treatment

Understanding the dynamic evolution of zeolite framework structures and the interactions between various hydroxyl groups or aluminum species under different steam conditions at the atomic scale is extremely crucial. Herein, using a series of characterization methods, the framework structures of HZSM-5 zeolites (Si/Al = 36) following exposure to steam in the temperature range of 100 to 500 °C are investigated. Under mild steam conditions (T ≤ 200 °C), dealumination is nearly absent, and the silanol nests directly condense to create new framework Si-O-Si bonds. Conversely, under severe steam conditions (T ≥ 300 °C), the framework tetrahedral aluminum atoms Al(IV)-1 can be sequentially converted to partially coordinated framework aluminum Al(IV)-2 and extra-framework aluminum (EFAL) through partial and complete hydrolysis, which cause an increase in the framework Si/Al ratio and a decrease in crystallinity. Al(IV)-2 is recognized as a significant intermediate species for framework complete dealumination. The Brønsted acid sites on Al(IV)-2 can be perturbed by the framework Al-OH groups due to hydrogen bonding interactions, leading to a shift in 1H chemical shifts to lower fields, appearing at 6.0-9.0 ppm and 12.0-15.0 ppm. The newly generated EFAL and silanol nests further evolve through condensation as well. Meanwhile, during dealumination, the spatial correlations (or interactions) of various hydroxyl groups on structurally distinct aluminum species [Al(IV)-1, Al(IV)-2, and EFAL] and aluminum species become extremely intricate. Based on these findings, the dynamic evolution path of HZSM-5 zeolite framework structures under mild and severe steam conditions is proposed.

Facile synthesis of hierarchical SAPO-56 zeolite as a highly efficient catalyst for CO2 hydrogenation to methanol

Catalytic conversion of CO2 over zeolite-based catalysts has been deemed as a promising route and extensively investigated, while many issues remain intractable. Herein, for the first time, the novel AFX-type zeolite SAPO-56 was developed and impregnated with Cu nanoparticles to construct the bifunctional catalyst for further promoting CO2 transformation. By a systematically comparative analysis on the physicochemical properties and catalytic performance of different catalysts, the corresponding catalytic mechanism for a highly efficient methanol synthesis process from CO2 was comprehensively understood. The results showed that up to 16.4 % of CO2 can be converted into methanol with a relatively high selectivity of 80.9 % via 5% Cu/SAPO-56 catalysts at 280 °C though the formate activation pathway. This may be directly related to the specific Lewis acid sites (LAS) derived from the EFAl (extra framework aluminum) species located in the six-membered rings of SAPO-56 that endows it with distinguished adsorption capability of CO2. Furthermore, the copper nanoparticles that are highly dispersed on the surface or in the porous channel of SAPO-56 zeolite facilitate the ease of CO2 activation. Besides, the introduced hierarchical structure was considered to have played an indispensable role in improving methanol productivity. Meanwhile, all its own multiple superiorities also ensure Cu/SAPO-56 a remarkable stability, indicating the great potential for practical and industrial application.

The nature of extraframework aluminum species and Brønsted acid site interactions under catalytic operating conditions

A systematic investigation of hydrated extraframework aluminum (EFAl) species interacting with Brønsted acid sites (BAS) in H-ZSM-5 is presented to understand the cooperative active site structure under catalytic operating conditions. Static models of EFAl species confined in the H-ZSM-5 unit cell show that isolated BAS protonate neutral EFAl species to form cations. Ab-initio molecular dynamics (AIMD) simulations and enhanced sampling performed at the temperature for methanol-to-hydrocarbon conversion reveal two regimes of stable EFAl species, namely the [Al(OH)2]+ ion existing with two bonds to the zeolite scaffold or as a pore-guest in the form of [Al(OH)2(H2O)2]+. Our results indicate that hydrogen-bonding plays a significant role in BAS-EFAl active site structure, especially at higher BAS density and that EFAl species can function as both Bronsted and Lewis acidic components to alter proton transfer kinetics as well as shape selectivity within these microporous solids.

A green route of isobutane-butene alkylation catalyzed by Y zeolite modified with synergistic F-containing solution

Fast deactivation is a long-standing problem for zeolite catalyzed isobutane-butene alkylation that hinders its massive industrialization, and the optimization of the zeolites' acidity and pore structure is the key problem. A synergistic F-containing solution modification method was proposed to improve the diffusion performance without damaging the zeolites’ acidity. The low-temperature ammonium fluoride treatment precise partly opened the cage windows of the zeolites to increase the acidity accessibility and diffusion performance. And the following acid treatment of ammonium fluosilicate effectively removed the extra-framework aluminum generated by the former treatment to achieve a high acidity and Brönsted acid amount to Lewis acid amount ratio. The best modified zeolite Y-11 has a lifetime of 60 h, which is 32% higher compared to that of the reference zeolite. Our primary results provide a promising strategy for designing superior zeolite catalysts for alkylation reactions.

Drastic Photoemission Color Alternation from a Single Molecule as a Starting Material Introduced in Acid-Treated Zeolites: From Pure Blue to White.

Since high-purity blue- and white-light emitters are an indispensable group of materials for the creation of next-generation optical devices, a number of light-emitting materials have been developed from both inorganic and organic synthetic chemistry. However, these synthetic chemical methods are far from the perspective of green chemistry due to the multistep synthetic process and the use of toxic reagents and elements. Herein, we demonstrate that the introduction of simple unsubstituted anthracenes into zeolite-like pores can create a wide variety of luminescent materials, from ultrapure blue luminescent materials (emission peak at 465 nm with a full width of half-maximum of 8.57 nm) to efficient white luminescent materials [CIE coordination at (0.31, 0.33) with a quantum efficiency of 11.0% under 350 nm excitation light]. The method for rational design of the luminescent materials consists of the following two key strategies: one is molecular orbital confinement of the anthracene molecules in the zeolite nanocavity for regulating the molecular coordination associated with photoexcitation and emission and the other is the interaction of unsubstituted anthracenes with extra-framework aluminum species to stabilize the 2-dehydride anthracene cation in the zeolite cavity.

Generation of mesoporosity in crystals of zeolite omega by post-synthesis treatments with surfactants

The large-pore zeolite omega (SiAl = 3.1) has been treated with solutions of the surfactant hexadecyltrimethylammonium (CTA) in aqueous ammonia in order to generate intracrystalline mesoporosity. For this purpose, the as-made sample was calcined, transformed in the acid form by ion exchange with ammonium chloride solution followed by calcination, and the resulting acid form subsequently treated with steam at 620 ºC to partially dealuminate the framework. The samples were characterized by XRD, N2 adsorption, TGA, SEM, 29Si and 27Al MAS NMR, pyridine adsorption for acidity determination, and tested in the isomerization/disproportionation of m-xylene. It has been found that partial dealumination of the framework (Si/Al ∼ 7.3) followed by mild acid leaching (HCl = 0.3 M) to remove only the extraframework aluminium species is required for CTA to generate intracrystalline mesopores with a narrow pore diameter of 4 nm. The CTA treatment is ineffective when applied to the acid form (Si/Al ∼ 5.7) or the steamed sample before acid leaching. The concentration of strong acid sites and the average acid strength of the CTA samples are lower than those of the steamed/acid leached sample, which suggests the presence of weak acid sites located at the mesopores. The mesopores greatly enhanced the catalytic activity in the m-xylene conversion by decreasing diffusional constraints. However, the selectivity to the bulky trimethylbenzenes is not improved by the CTA treatment, which suggests that the reaction mainly takes place at the strong acid sites located at the micropores.

Hierarchical Y zeolite with accessible intracrystalline mesopores and enhanced hydrocracking performances via sequential multi-carboxyl acid and fluoroborate post-treatments

Hierarchical Y zeolite that combines the intrinsic microporosity with mesoporosity can alleviate diffusion limitation in hydrocracking catalyst. In this study, hierarchical Y zeolites with abundant mesopores that connect the interior of the zeolite crystals to the external surface were successfully synthesized by post-synthesis dealumination using citric acid (C6H8O7) and/or ammonium tetrafluoroborate (NH4BF4) as agents. The results show that C6H8O7 can effectively extraction framework aluminum species (FALs), while NH4BF4 mainly interacts with extra framework aluminum species (EFALs). Via sequentially adopting C6H8O7 and NH4BF4 (denoted as DAY-C-F), such interconnected mesopores can be effectively introduced without sacrificing the zeolite crystallinity. The results of the scaled-up hierarchical Y zeolite reveal that the above mentioned post-treatment strategy is feasible and reproducible for potential industrial manufacturing and applications. The hydrocracking test results reveal that the naphtha yield for the catalyst derived from the C6H8O7-NH4BF4 sequentially treated USY is higher than that of the counterpart made from commercial USY.

Cu and Zn Bimetallic Co-Modified H-MOR Catalyst for Direct Oxidation of Low-Concentration Methane to Methanol.

The direct oxidation of low-concentration methane to value-added chemicals can not only reduce carbon emission but also provide an alternative production route for fossil fuels. Herein, we proposed a novel catalyst for the direct oxidation of low-concentration methane to methanol via the impregnation method, which selected copper and zinc as co-modifiers to modify the MOR catalyst. The highest methanol yield of 71.35 μmol·gcat-1·h-1 was obtained over a bimetallic Cu0.5Zn0.35-MOR catalyst. The catalyst retained good activity after three cycles of testing experiments, indicating good recyclability. Based on the results of performance tests and characterization studies, it was confirmed that Cu species bound to the zeolite framework were the main active sites for methane oxidation. The introduction of Zn decreased the generation of the octahedrally coordinated extra-framework aluminum, which promoted the dispersion of Cu within the zeolite framework. In other words, more tetrahedrally coordinated FAl-stabilized Cu species were presented in our CuZn-MOR catalyst system in comparison to the monometallic Cu-MOR catalyst. Benefiting from the aforementioned modification, the agglomerative sintering of the metal during the reaction was effectively prevented. This work may provide a feasible guide for the future optimization of Cu-based catalysts designed for the selective oxidation of methane.

HPW supported on ammonium hexafluorosilicate modified Hβ: Efficient catalysts for ethylene glycol and methyl tert‐butyl ether reaction with high selectivity and stability

A series of HPW supported on ammonium hexafluorosilicate (AHFs) modified Hβ (30HPW/Hβ‐Si(x)) was successfully prepared. After Hβ zeolite was pretreated with 0.1 mol·L−1 AHF's solution, the Brønsted/Lewis ratio of Hβ‐Si(0.1) increased compared with those of Hβ, and the strong acid density of 30HPW/Hβ‐Si(0.1) decreased compared with those of 30HPW/Hβ respectively. Strong acidic sites and Lewis acid sites are catalytic sites for by‐products diisobutene and ethylene glycol oligomers, which deposited on the catalyst result in the deactivation of catalysts. According to the results of characterization, the internal Si‐OH groups of Hβ (strong acidity sites) were filled with silicon of AHFs. The extraframework aluminum (strong acid sites) was removed by the H+, which is the hydrolysis of AHFs. In addition, the hydrophobicity of 30HPW/Hβ‐Si(0.1) was also improved compared with that of 30HPW/Hβ due to the SiO2/Al2O3 ratio of Hβ increased after treatment by 0.1 mol·L−1 AHFs solution. The 30HPW/Hβ‐Si(0.1) maintained an 80% conversion of methyl tert‐butyl ether and 100% selectivity to ethylene glycol mono‐tert‐butyl ether and DBE ethylene glycol bis‐tert‐butyl ether with 336 h of time on stream.

H-ZSM-5 Catalysts for the catalytic upcycling of polypropylene glycol

Without effective management, the steady increase of waste plastics threatens environmental well-being and ecological balance. Plastic up/recycling is a promising solution but has many challenges. In this work, catalytic cracking of polypropylene glycol (PPG) was investigated at varying reaction temperatures of 350–550 °C under nitrogen and steam, using H-ZSM-5 zeolites with different silica-to-alumina (SiO2/Al2O3) ratios of 23:1 and 50:1. The catalysts were assessed through physisorption, chemisorption, solid-state magic-angle spinning nuclear magnetic resonance spectrometry, thermogravimetry, electron microscopy, and positron annihilation lifetime spectroscopy. Extra framework aluminum and Lewis-to-Bronsted acid site ratios were found to play a significant role in the selectivity towards propionaldehyde, where values ∼ 80 % could be reached. In addition, a possible pathway for the PPG cracking reaction was proposed, which may lead to a better understanding of PPG and waste plastic decomposition via zeolite-based catalysts.

Facilely adjusting acidity features of zeolites by steaming treatment for enhanced methanol conversion to aromatics: A mechanism study

In this contribution, the effects of the steaming treatment on the acidity and aluminum structure of ZSM-5 were systematically investigated by using different types of zeolite catalysts. The relationships between the acid properties and the catalytic performance of the methanol-to-aromatics (MTA) reaction were also elucidated. The results revealed that different kinds of acid sites showed varied stability during the steaming treatment. Especially for Brønsted acid sites (BAS), the reduction of the acid site was highly related to the stability of different distributed framework Al sites (Alsingle > Alpairs) and the existence of non-framework species (“neutralization” by the EFAL and ZnO). Additionally, the presence of the extra-framework aluminum (EFAl) and metal compensation ions could enhance the stability of the zeolite framework Si-O(H)-Al. Besides, it was found that the reduction degree in the content of the Zn-LAS (LAS = Lewis acid sites) was less than the Al-LAS. The effects of steaming treatment on acid properties over zeolites with different particle sizes were also explored. Furthermore, the catalytic performance for the MTA reaction showed that the stability of the ZSM-5 catalyst was significantly enhanced after the steaming treatment. Moreover, the reduction in the proportion of the Alpairs sites may inhibit the hydrogen-transfer aromatization process of alkenes, while the increase of the Alsingle species enhanced the cracking process of higher olefins, leading to the reduction of aromatics selectivity and the increase of light olefins selectivity, respectively.

Fast synthesis of highly crystallized LSX zeolite with boosted carbon dioxide hydrogenation activity

Fabrication of suitable zeolite-based multifunctional catalysts has been perceived as a potential method for CO2 conversion to value-added chemicals and fuels. Thus, a highly crystallized LSX zeolite was prepared directly for the first time without aging and strong alkali sodium/potassium salt. Besides, the hierarchical structure was also successfully introduced by template-free method. The resulted LSX zeolite-based catalyst can significantly boost CO2 hydrogenation with a preferential production of methanol (100% selectivity). This can be owing to the electron transfer between extra framework aluminum (EFAl) and ZnO species that drives the remarkable adsorption capability of CO2 and the formate activation pathway. Furthermore, the highly dispersive Cu nanoparticles that interplay with zeolite host and ZnO species render convenient access to the activated CO2 * species for those dissociated H after their spillover. More importantly, an exceptional stability can also be obtained, indicating the promising prospect in practical and industrial application.

Active Sites in H-Mordenite Catalysts Probed by NMR and FTIR

Mordenites are widely used in catalysis and environmental protection. The catalytic properties of mordenite are largely determined by the composition of its crystal framework, i.e., the SiO2/Al2O3 molar ratio (MR), and the cationic form. In H-mordenites, the most important characteristic becomes the structure and distribution of acid sites, which depends on the number and distribution of Al tetrahedra in the framework. In the present work, the local structure of these centers in H-mordenite catalysts with a nominal MR varied from 9.9 to 19.8 was studied in detail using a combination of magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier transform infrared spectroscopy (FTIR). 27Al MAS NMR indicates the presence of extra-framework Al in most of the studied samples that results in a higher real MR of the zeolitic framework compared to the nominal value. Concentrations of Lewis and Brønsted acid sites, as well as of silanol groups were estimated by elemental analysis, NMR, and FTIR spectroscopy. The values of site concentrations obtained from band intensities of adsorbed CO and those of OH groups are compared with the amount of framework and extra-framework aluminum. The advantages and restrictions of different methods of active site characterization are discussed.

High-Temperature-Treated LTX Zeolites as Heterogeneous Catalysts for the Hock Cleavage

Hydroxybenzene, commonly known as phenol, is one of the most important organic commodity chemicals. To produce phenol, the cumene process is the most used process worldwide. A crucial step in this process is the Hock rearrangement, which has a major impact on the overall cumene consumption rate and determines the safety level of the process. The most used catalyst for the cleavage of cumene hydroperoxide (CHP) is sulfuric acid. Besides its strong corrosive property, which increases plant investment costs, it also requires neutralization after the decomposition step to prevent side reactions. In this study, we show that high-temperature-treated Linde Type X (LTX) zeolites exhibit a high activity for the peroxide cleavage step. In addition, the structure–activity relationship responsible for this good performance in the reaction system of the HOCK rearrangement was investigated. XRPD analyses revealed the formation of a new phase after temperature treatment above 900 °C. The Si/Al ratio determined by EDX suggested the formation of extra-framework aluminum, which was confirmed by solid-state NMR analysis. The newly formed extra-framework aluminum was found to be responsible for the high catalytic activity. BET analyses showed that the surface area drops at higher calcination temperatures. This leads to a lower catalytic activity for most known reactions. However, for this study, no decrease in activity has been observed. The newfound material shows extraordinarily high activity as a catalyst in the HOCK cleavage and has the potential to be a heterogeneous alternative to sulfuric acid for this reaction.

Steam catalytic cracking and lump kinetics of naphtha to light olefins over nanocrystalline ZSM-5 zeolite.

This study investigates the reaction pathways and kinetics to comprehend the catalytic cracking of dodecane, a heavy naphtha model compound, over the nanocrystalline ZSM-5 catalyst in the presence and absence of steam with the aim of increasing olefin production. The nanocrystalline zeolite was characterized using XRD and BET, and the surface acidity was measured by NH3-TPD and Py-FTIR. The steam treated ZSM-5 contributed to an increase in pore volume with extra-framework alumina, resulting in highly catalytic active sites and hence higher olefin selectivity. The high conversion of dodecane (>90%) was achieved during catalytic cracking in the presence and absence of steam. In the presence of steam, the short pores of nano ZSM-5 led to an increase in the naphtha-to-olefin conversion with lesser dry gas and coke formation. The activation energies of primary cracking in the presence and absence of steam were slightly different. Lower activation energies through secondary cracking routes and higher reaction rate constants were obtained via assisted-steam catalytic cracking, promoted the selectivity towards light olefin products. Meanwhile the hydrogenation and alkylation reactions toward LPG and C5+ were favored in the absence of steam. Moreover, the ZSM-5 nano zeolite pores promoted more β-scission reactions, resulting in higher selectivity towards ethylene and dry gas.