Extra-framework Aluminum Species Research Articles

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

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.

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.

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.

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst.

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized.

Enhanced catalytic ozonation of toluene using supported MnOx/USY via regulating the distribution of aluminum species in USY by dealumination

Recently, the problem of VOCs pollution has become serious in many regions, especially developing countries. Herein, the distribution of aluminum species in USY was controlled by dealumination of Y zeolite under different conditions, and then the dealuminated samples were used to prepare five different supported catalysts MnOx/USY to achieve high-efficiency catalytic ozonation of toluene at low temperature. It was found that aluminum species were successfully leached from the zeolite framework to the extra-framework, which changed the properties of the manganese oxide nanoparticles loaded on the zeolite. Moreover, the oxygen vacancies and the acid sites in different catalysts showed great differences. As the extra-framework aluminum species increased, the amount of oxygen vacancies in the catalysts decreased, while the content of acid sites exhibited a “volcano-type” trend similar to toluene conversion. The best catalysts (Mn-Y-3) maintain > 90% toluene conversion at 30 °C but the worst catalysts (Mn-Y-5) deactivate rapidly. With the help of in situ DRIFTS, it is found that the activation ability of toluene on the acidic sites of the catalyst is the key to the toluene conversion when the amount of oxygen vacancies is sufficient to decompose ozone into active oxygen species, and the contribution of Lewis acid sites is greater than Brønsted acid sites. It provides some theoretical guidance in designing effective catalysts for ozone catalytic oxidation of VOCs.

DFT and dynamic analysis of glucose alcoholysis conversion to 5-ethoxymethylfurfural and ethyl levulinate

An effective catalytic strategy for the conversion of glucose to 5-ethoxymethylfurfural (EMF) catalyzed by ultrastable Y zeolite (USY) in a co-solvent system was developed. Effects of co-solvent and reaction conditions were first investigated. Tetrahydrofuran (THF) with low dipolarity/polarizability (π*) was screened as the appropriate co-solvent, and a higher EMF yield of 41.46% was obtained as the ratio of ethanol to THF was 7:3. The macroscopic reaction kinetic study indicated that THF can protect EMF from easy degradation to EL. Moreover, the reusability and the characterization of USY verified the feasibility of USY as an efficient solid acid catalyst. In this study, density functional theory (DFT) calculations for the catalytic mechanism and solvent effect at the molecular scale were performed for the first time. The extra-framework aluminum species (EFAL), in which Al3+ played an essential role in the reaction as the preferred EFAL species, and THF can reduce the energy barrier of USY-catalyzed glucose isomerization. Meanwhile, the inhibitory effect of THF on EMF alcoholysis was explored via the frontier molecular orbital theory. The results showed that THF increased the LUMO energy of EMF compared with ethanol and γ-valerolactone, which decreased its susceptibility to nucleophilic attack and reduced the adverse alcoholysis of EMF and humins formation. This study provided an enlightening reference for the efficient synthesis of EMF from carbohydrates catalyzed by solid acid catalyst under solvent effect.

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

Correlating Lewis acid activity to extra-framework aluminum species in zeolite Y introduced by Ion-exchange

Lewis acidity was introduced into zeolite Y (Si/Al = 30) by a facile ion-exchange of aluminum cations followed by calcination. X-ray diffraction and nitrogen physisorption suggest that the zeolitic framework is retained after these treatments. With an increase in the total extra-framework aluminum in the modified zeolites, there was a pronounced increase in the number of Lewis acid sites, as illustrated by FTIR spectroscopy of adsorbed pyridine and carbon monoxide, whereas no significant change in the number of Brønsted acid sites was observed. Likewise, the catalytic activity, i.e. the conversion and rate of reaction for the MPV reduction of 4-tert butyl cyclohexanone using isopropanol over zeolite catalysts, improved considerably after ion-exchange. A quantitative agreement was observed between the concentration of these introduced extra-framework aluminum species with the total Lewis acid content from pyridine-FTIR and with the initial catalytic rates. This illustrates that the newly introduced aluminum Lewis acidic species adopt an octahedral coordination under the conditions of NMR measurement and are responsible for enhanced Lewis acid catalytic activity of the aluminum exchanged zeolites. The preservation of Brønsted acid sites further endorses the charge neutrality of these extra-framework aluminum complexes.

Global optimization of extraframework ensembles in zeolites: structural analysis of extraframework aluminum species in MOR and MFI zeolites.

Metal-modified zeolites are versatile catalytic materials with a wide range of industrial applications. Their catalytic behaviour is determined by the nature of externally introduced cationic species, i.e., its geometry, chemical composition, and location within the zeolite pores. Superior catalyst designs can be unlocked by understanding the confinement effect and spatial limitations of the zeolite framework and its influence on the geometry and location of such cationic active sites. In this study, we employ the genetic algorithm (GA) global optimization method to investigate extraframework aluminum species and their structural variations in different zeolite matrices. We focus on extraframework aluminum (EFAl) as a model system because it greatly influences the product selectivity and catalytic stability in several zeolite catalyzed processes. Specifically, the GA was used to investigate the configurational possibilities of EFAl within the mordenite (MOR) and ZSM-5 frameworks. The xTB semi-empirical method within the GA was employed for an automated sampling of the EFAl-zeolite space. Furthermore, geometry refinement at the density functional theory (DFT) level of theory allowed us to improve the most stable configurations obtained from the GA and elaborate on the limitations of the xTB method. A subsequent ab initio thermodynamics analysis (aiTA) was chosen to predict the most favourable EFAl structure(s) under the catalytically relevant operando conditions.

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.

Thermal desorption of trimethylphosphine (TMP) on the HY zeolite followed by FT-IR and 31P MAS NMR

Characterization of the acidic properties of solid acid catalysts is essential to understanding their catalytic performance with respect to activity, deactivation rate and product selectivity. In this work, trimethylphosphine (TMP) was used as a probe molecule for the study of acidity in a HY zeolite activated at 773 ​K. The variations in type, concentration and acid strength with the desorption temperature of the probe molecule were followed by infrared spectroscopy (FT-IR) and solid-state Nuclear Magnetic Resonance (ss-NMR). Oxidation of TMP to trimethylphosphine oxide (TMPO) provided additional information about Brønsted (BAS) and Lewis (LAS) acid sites with different acid strengths. The variation in acid strength of certain species determined by calculating the concentration percentage of the desorption and reabsorption of the majority species by increasing the desorption temperature. The 31P ss-NMR chemical shifts at δ=-62, −32 to −58, 45 and 51 ​ppm correspond to weak acid sites (<423 ​K). The peaks at δ= 54 and 55 ​ppm arose from medium acid sites (423–623 ​K). The strong acid sites at δ= 58, 61, 63, 64, 65, 67, 70, 73, 76 and 80 ​ppm correspond to sites with different acid strengths (>623 ​K). Our results demonstrated differences in the concentrations of acid and the distribution of acid strengths of extra-framework (EFAl) aluminum species in different cavities by varying the desorption temperature of the probe molecule. The developed methodology provides more detailed information about acidic properties and can be used for solid acid catalysts using alkylphosphines and their oxides as probe molecules.

Development of hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline/acid treatments and their catalytic performances for methanol-to-olefins

An efficient and simple method was proposed to synthesize hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline and acid treatment. In the first NaOH alkaline treatment, intracrystalline mesoporosity was introduced into the zeolites by the crystal defect formation and growth of crystal defects due to desilication. The excessive defect growth caused amorphization of the zeolites, dramatically decreasing their surface area and micropore volume. The aluminum-related debris resulting from the desilication deposited on the extra-framework positions and had a dramatic influence on the crystallinity and microporosity of the zeolites. In the subsequent H3PO4 treatment for the alkaline-treated HZSM-5, the deposited extra-framework aluminum species were selectively removed, thus increasing the micropore volume and surface area of the zeolites. Along with the dealumination, the phosphorous species were introduced into the framework to occupy the silicon sites, leading to a crystalline structure repair. The post-treatments could modify the structural and acidic properties of the HZSM-5 zeolite. Catalytic tests showed that the hierarchical and phosphorous-modified HZSM-5 zeolite exhibited higher methanol conversion and propylene selectivity in methanol-to-olefins reactions over a longer period compared with the parent HZSM-5.

Rapid and green synthesis of SAPO-11 for deoxygenation of stearic acid to produce bio-diesel fractions

A grinding synthesis approach is employed to prepare water containing gel. SAPO-11G was successfully prepared by a dry gel conversion method using water containing gel assisted by microwave radiation (120–180 °C, 2 h). Compared to hydrothermal system, approach employed in the present study show green characters such as conservation of energy and time, low cost of organic filler, and reducing wastewater discharge. Deoxygenation of stearic acid shows that Ni/SAPO-11 catalysts have a high selectivity to paraffin and a low yield to cracking products. Ni/SAPO-11G has a comparable activity (conversion> 95%) with Ni/SAPO-11H molecular sieves. Coke deposition in SAPO-11G is lower than that in SAPO-11H due to a lower concentration of extra-framework aluminum species (i.e., Lewis acid sites). These results propose a rapid way to prepare zeolites in an environmental friendly route and are important to produce bio-diesel fractions via a deoxygenation reaction. • A grinding synthesis method is employed to prepare water containing precursor. • SAPO-11 is prepared using dry gel conversion heated by microwave. • Ni/SAPO-11 catalysts exhibit a high selectivity to C15-18 paraffin in deoxygenation of stearic acid.

A Periodic DFT Study of the Synergistic Mechanisms between Extraframework Aluminum Species and Bro̷nsted Acid Sites in HY Zeolites

A series of model probes with different sizes and polarity, such as ammonia, pyridine, and thiophene, have been used to gain insight into the influence mechanisms of extraframework aluminum (EFAl) ...

Synthesis of all-silica hollow zeolites by selective demetallation

Exploiting the protective role of extra-framework aluminum species on the crystal rim, all-silica hollow zeolites are efficiently produced by demetallation.

Understanding structure-function relationships in HZSM-5 zeolite catalysts for photocatalytic oxidation of isopropyl alcohol

The HZSM-5 molecular sieves are a kind of special photocatalyst with photocatalytic activity, while their relationship between structure and function has been a matter of debate. Here, we report that for HZSM-5, the distinguished transformation of coordination environment derived from the removal of tetrahedral framework aluminium by thermal treatment will result in a considerable change in photocatalytic behaviours. The established NMR analysis combined with the theoretical calculations demonstrates the formation of extra-framework aluminium (EFAL) upon calcinations. This leads to a changed reaction path of photocatalytic oxidation of isopropyl alcohol in gas-phase over calcined HZSM-5 catalysts. DFT calculation results show the formed of EFAL species can increase the interaction energy between reactant molecules and zeolites which affords an outstanding photocatalytic reactivity and a controllable photocatalytic selectivity in the oxidation of isopropyl alcohol. This clearly indicates that the framework aluminium (FAL) species are responsible for acetone formation, while the EFAL species are apt to promote the deep mineralization of isopropyl alcohol to inorganic CO2. This study provides new understanding in the issue of photocatalysis for zeolite molecular sieves.