Aluminum Species Research Articles

Dealumination of small-pore zeolites through pore-opening migration process with the aid of pore-filler stabilization.

Small-pore zeolites are gaining increasing attention owing to their superior catalytic performance. Despite being critical for the catalytic activity and lifetime, postsynthetic tuning of bulk Si/Al ratios of small-pore zeolites has not been achieved with well-preserved crystallinity because of the limited mass transfer of aluminum species through narrow micropores. Here, we demonstrate a postsynthetic approach to tune the composition of small-pore zeolites using a previously unexplored strategy named pore-opening migration process (POMP). Acid treatment assisted by stabilization of the zeolite framework by organic cations in pores is proven to be successful for the removal of Al species from zeolite via POMP. Furthermore, the dealuminated AFX zeolite is treated via defect healing, which yields superior hydrothermal stability against severe steam conditions. Our findings could facilitate industrial applications of small-pore zeolites via aluminum content control and defect healing and could elucidate the structural reconstruction and arrangement processes for inorganic microporous materials.

Three birds with one stone: Designing a novel binder-free monolithic zeolite pellet for wet VOC gas adsorption

Zeolites are promising materials for the adsorption of volatile organic compounds (VOCs), thanks to their unique pore structure, large specific surface area, and excellent stability. However, the synthesis of monolithic zeolites with a well-defined macroscopic shape, significant mechanical strength, and high VOCs adsorption capacity in humid conditions remains challenging. Here, the synthesis of binder-free monolithic zeolite pellets, possessing robust mechanical strength, high adsorption capacity, and excellent water tolerance, is reported through a citric acid sacrifice strategy. By in situ dealuminization, Y zeolite pellets have greatly improved the specific surface area and mesopore volume, leading to superior acetone and toluene adsorption performance. Moreover, the extracted aluminum species produced during the dealuminization act as a “bridge” to promote the zeolite particles into pelletization with a sufficiently high mechanical strength. Under 90% relative humidity, the acetone and toluene adsorption capacity of the synthesized zeolite pellets reach above 90% of the adsorption capacity under dry conditions, showing potential in practical industrial application. It is anticipated that this synthetic strategy will benefit the forming and development of monolithic zeolite.

Ultrafiltration membrane fouling control by two-stage coagulant dosing with moderate pH adjustment

For the coagulation-ultrafiltration process, it has been recognized that coagulation under acidic condition could greatly alleviate membrane fouling, but the high chemical consumption for pH adjustment and the diminished alkalinity of effluent severely restrict the practical application of this operation. In this study, it was found that two-stage coagulant dosing (separate dosing during rapid mixing and medium speed mixing) with pH adjusted to 6.5 could alleviate membrane fouling as that by one-time coagulant dosing at pH 5.5, which could significantly lower the chemical consumption for pH adjustment. Both the favorable aluminum hydrolysis speciation on the floc surface and the reduced interface interaction between membrane and foulant at pH 6.5 contributed to the effectiveness of this method on membrane fouling control. Compared with the scenarios at pH 5.5 and 7.5, the two-stage dosing at pH 6.5 could promote more aluminum species transformation to amorphous Al(OH)3 on the surface of flocs, resulting in less residual Al in the effluent. The acid-base interaction between foulant and membrane dominated membrane fouling. The degree of membrane fouling by separate dosage ratio of 3:7 at pH 6.5 was as low as that by one-time dosing at pH 5.5 because of the similarly low acid-base interaction energy.

Enhanced catalytic activity of phosphorus-modified SSZ-13 zeolite in the ethylene-to-propylene reaction by controlling acidity and intracrystalline diffusivity

A highly selective means of producing propylene via the ethylene-to-propylene (ETP) reaction is required to manage the supply-demand balance of light olefins. To improve the catalytic activity of the ETP reaction, a phosphorus-modified SSZ-13 zeolite (P-SSZ-13) was prepared by introducing phosphorus to the SSZ-13 zeolite via a simple impregnation method. The P-SSZ-13 catalyst exhibited increased propylene selectivity and yield compared to the pristine catalyst. According to solid-state NMR spectroscopy and STEM-EDS observation, the phosphorus species were mainly located on the outer part of the crystal as a form of polyphosphate. To reveal the effect of the phosphorus loading on the enhancement of ETP reactivity, a series of surface-modified SSZ-13 catalysts were prepared and investigated. It was found that the polyphosphate species in the P-SSZ-13 catalyst not only weakened the acidity of the zeolite by bonding with the framework aluminum species but also reduced the diffusion out of the relatively large-sized hydrocarbon species by partially blocking the pore mouths. The change in the diffusivity of the hydrocarbons over the SSZ-13 catalysts also affected the ETP reactivity. The relationship between the amount of phosphorus and the ETP reactivity of the P-SSZ-13 catalysts could also be clearly demonstrated in terms of the change in both acidity and intracrystalline diffusivity. Moreover, 13C MAS NMR measurement with isotopic switch experiment confirmed the alkyl naphthalene-based hydrocarbon pool mechanism over the P-SSZ-13 for the ETP reaction.

Exploring Equilibria between Aluminium(I) and Aluminium(III): The Formation of Dihydroalanes, Masked Dialumenes and Aluminium(I) Species

AbstractThe design of new reductive routes to low oxidation state aluminium (Al) compounds offers the opportunity to better understand redox processes at the metal centre and develop reactivity accordingly. Here, a monomeric AlI compound acts as a stoichiometric reducing agent towards a series of AlIII dihydrides, leading to the formation of new low oxidation state species including symmetric and asymmetric dihydrodialanes, and a masked dialumene. These compounds are formed by a series of equilibrium processes involving AlI, AlII and AlIII species and product formation can be manipulated by fine‐tuning the reaction conditions. The transient formation of monomeric AlI compounds is proposed: this is shown to be energetically viable by computational (DFT) investigations and reactivity studies show support for the formation of AlI species. Importantly, despite the potential for the equilibrium mixtures to lead to ill‐defined reactivity, controlled reactivity of these low oxidation state species is observed.

Exploring Equilibria between Aluminium(I) and Aluminium(III): The Formation of Dihydroalanes, Masked Dialumenes and Aluminium(I) Species.

The design of new reductive routes to low oxidation state aluminium (Al) compounds offers the opportunity to better understand redox processes at the metal centre and develop reactivity accordingly. Here, a monomeric AlI compound acts as a stoichiometric reducing agent towards a series of AlIII dihydrides, leading to the formation of new low oxidation state species including symmetric and asymmetric dihydrodialanes, and a masked dialumene. These compounds are formed by a series of equilibrium processes involving AlI, AlII and AlIII species and product formation can be manipulated by fine‐tuning the reaction conditions. The transient formation of monomeric AlI compounds is proposed: this is shown to be energetically viable by computational (DFT) investigations and reactivity studies show support for the formation of AlI species. Importantly, despite the potential for the equilibrium mixtures to lead to ill‐defined reactivity, controlled reactivity of these low oxidation state species is observed.

Newly generated Cu2O[sbnd]Cu interface for CO2 electroreduction in the presence of reconstructed aluminum hydroxide

The electrochemical CO2 reduction reaction (eCO2RR) to C2+ products is attractive but challenging. Herein, flower-like Al0.9/CuO composed of modified CuO nanosheets was synthesized by a hydrothermal synthesis method under the conditions of high alkaline concentration, in which aluminum hydroxide was dissolved and regenerated on CuO nanosheets. The as-obtained flower-like Al0.9/CuO presents a higher faradaic efficiency of multicarbon (C2+) products 77.6% at -0.95 V vs. RHE for eCO2RR in an H-type cell than other flower-like Alx/CuO and achieves C2+ product faradaic efficiency of 75.2% and partial current density of 752.5 mA cm−2 in a flow cell. Characterization results show that Cu(I)/Cu(0) interface is formed along with the existence of oxygen defects and d-band center shifting toward low-energy region on flower-like Al0.9/CuO compared with traditional CuO nanosheets. Evenly dispersed Cu(0) and oxygen vacancy on the nanosheet promote the absorption and activation of CO2, while Cu(I) adjacent to Cu(0) regulated by aluminum species improves the adsorption of *CO intermediates, beneficial to the CC coupling to C2+ products in eCO2RR. Therefore, this work provided a strategy to efficiently modulate the distribution of electron and ingredient on the copper based electrocatalyst to improve the performance of C2+ products in eCO2RR.

Direct synthesis of aluminosilicate ITH zeolite in the presence of a small organic template

Despite very successful synthesis of many pure silica or germanosilicate zeolites, there are still challenges for direct synthesis of their aluminosilicate forms. As a typical example, it is easy to synthesize germanosilicate ITH, but difficult for direct synthesis of its aluminosilicate form. Recently, cationic oligomer has been successfully templated for direct formation of aluminosilicate ITH, but relatively complex steps for controlling molecular weight of the cationic oligomer strongly hinder the practical production of zeolite products. Herein, it is shown an alternative route for direct synthesis of aluminosilicate ITH zeolite in the presence of a small organic template with both structural directing ability for the ITH structure and strong interaction ability to aluminum species. Characterizations of the zeolite products with Si/Al ratios from 46 to 148 show high crystallinity, nanosheet-like crystal morphology, large microporous surface area (338 m2/g), high micropore volume (0.16 cm3/g), four-coordinated Al species, controllable acidic sites, and extraordinary hydrothermal stability (hydrothermal aging at 800 °C for 5 h with 10% water steaming). Catalytic tests in methanol-to-propylene (MTP) show that the aluminosilicate ITH zeolite has higher selectivity for propylene than that of commercial ZSM-5 zeolite with similar Si/Al ratio, which provides a candidate catalyst for MTP reaction in the near future.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites.

Selective catalytic reduction of NOx in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NOx reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO2 pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NOx into N2. It was not deactivated during the NOx reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NOx reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NOx from the complex flue gas in practical applications.

The chemistry of aluminum salts in papermaking

Alum and related aluminum salts are used extensively in a variety of papermaking operations. The versatility of these aluminum additives stems from the ability of the various aluminum species to adsorb and interact with the other materials in the papermaking system. Underlying these interactions is the aqueous chemistry of the aluminum ion. A review of the current views on the aqueous chemistry of the aluminum ion and papermaking aluminum chemistry is presented. Application of the aqueous aluminum equilibria to aluminum adsorption and alum-rosin sizing are also discussed.

Anchoring Effect of Organosilanes on Hierarchical ZSM-5 Zeolite for Catalytic Fast Pyrolysis of Cellulose to Aromatics.

As an essential chemical feedstock, aromatics can be obtained from biomass by catalytic fast pyrolysis (CFP) technology, in which diffusion limitation is still a problem. In this study, several ZSM-5 zeolites with intercrystal stacking macropores were synthesized by adding organosilanes (OSAs) with different alkyl chain groups. Due to the structure-directing effect of the OSA, the prepared ZSM-5 zeolites possess a larger external surface area and pore volume than Blank-Z5. Moreover, the pore size is related to the extent of anchoring of the OSA and silicon–aluminum species in the zeolite precursor. Pyridine Fourier transform infrared (Py-FTIR) and NH3-temperature-programmed desorption (TPD) analyses show that the obtained ZSM-5 zeolites have a higher Brønsted acidity and total number of acid sites. In addition, excessive addition of OSA is not conducive to the growth of ZSM-5 zeolites. The catalytic performance of the synthesized ZSM-5 zeolites was evaluated by Py-GC/MS. The larger external surface area and pore volume improve the accessibility of the acid sites and thus promote the conversion of biomass into aromatics.

Comparative study of aluminum (Al) speciation on apoptosis-promoting process in PC12 cells: Correlations between morphological characteristics and mitochondrial kinetic disorder

Aluminum contamination in environment is very serious and the central nervous system is the main target of aluminum toxicity. The neurotoxic of aluminum is closely related to its speciation. PC12 cells were taken as the cell model to compare the morphological characteristics and mitochondrial kinetic disorder of two speciation of aluminum compounds (AlCl3 and aluminum-maltolate (Al(mal)3)). When the concentration of AlCl3 was 3 mM, the intracellular aluminum ion content was 3.87 times that of the 0.5 mM Al(mal)3 treatment group. At the 3 mM AlCl3 treatment group, intracellular ion homeostasis was disrupted. Abnormally elevated Ca2+ levels inhibited protein kinase B (AKT) phosphorylation, resulting in impaired cell morphology. At the 0.5 mM Al(mal)3 treatment group, abnormally high levels of Ca2+ caused mitochondrial kinetic disorder, which led to impaired cellular energy metabolism. Al(mal)3 had shown more cytotoxic in PC12 than AlCl3 at the same concentration. AlCl3 tended to inhibit the phosphorylation of AKT and damages cell morphology. Al(mal)3 mainly affected mitochondrial kinetic disorder, which led to impaired cellular energy metabolism. These findings provided experimental evidence for in-depth research on aluminum-induced neurotoxicity.

Direct synthesis of HZSM-5 zeolites with enhanced catalytic performance in the methanol-to-propylene reaction

HZSM-5 zeolites were directly synthesized via a two-stage crystallization method using ammonia hydroxide as an alkaline source instead of the conventional sodium hydroxide, and their performance in the methanol-to-propylene (MTP) reaction was investigated at 470 °C. The use of ammonia hydroxide as the alkaline source can overcome the inherent drawback of requiring multiple ion exchanges for converting Na-form zeolites to NH4+-form derivatives. Bulk and surface characterization techniques were combined to study the changes in crystallinity, porosity, morphology, aluminum distribution, and acidity with the experimental conditions. It was found that ammonia hydroxide had limited effect on the crystallinity and morphology of HZSM-5 zeolites, whereas the silicon and aluminum sources affected significantly the morphology and aluminum distribution. The acid properties and aluminum distribution induced important changes in the MTP activity and selectivity. Thus, increasing the framework SiO2/Al2O3 ratios in HZSM-5, in other words, reducing the acid density and the amount of framework aluminum species in the channel intersections, could improve the selectivity to propylene by favoring the olefinic-based cycle reaction. The catalyst lifetime was found to depend on not only the acid density but also on the acid distribution (or framework aluminum distribution).

Effect of phosphogypsum application on aluminum speciation in acid pasture soils

Effect of phosphogypsum application on aluminum speciation in acid pasture soils

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.

Inter-zeolite transformation from *MRE to EUO: A new synthesis route for EUO zeolite

The morphology control of zeolite has attracted extensive attention because of its close relationship with catalytic performance. How to control the crystal morphology of EUO zeolite on premise of low silica to alumina ratio is a challenging task. This study provides a novel route for controlling the morphology of Al-rich EUO zeolite via inter-zeolite transformation from Si-rich *MRE zeolite within 24 h. The morphology change from nanocrystals to coffin-like single crystal exhibits the merits of inter-zeolite transformation strategy in morphology control. Characterization tracking reveals that the deposition of aluminum species on parent *MRE zeolite is not uniform and migration between different precursors happens during the crystallization. No obvious mass transfer is observed between the liquid and solids after 2 h indicating it may follow a solid phase crystallization mechanism. Compared with conventional EUO samples, the EUO zeolite obtained by inter-zeolite transformation exhibits higher p-xylene selectivity. This approach not only provides a novel morphology control strategy for Al-rich EUO zeolite, but also promotes the understanding of the zeolite crystallization through inter-zeolite transformation route.

Design of a Small Organic Template for the Synthesis of Self-Pillared Pentasil Zeolite Nanosheets.

Zeolite nanosheets with excellent mass transfer are attractive, but their successful syntheses are normally resulted from a huge number of experiments. Here, we show the design of a small organic template for the synthesis of self-pillared pentasil (SPP) zeolite nanosheets from theoretical calculations in interaction energies between organic templates and pentasil zeolite skeletons. As expected, the SPP zeolite nanosheets with the thickness at 10-20 nm have been synthesized successfully. Characterizations show that the SPP zeolite nanosheets with about 90% MFI and 10% MEL structures have good crystallinity, the house-of-card morphology, large surface area, and fully four-coordinated aluminum species. More importantly, methanol-to-propylene tests show that the SPP zeolite nanosheets exhibit much higher propylene selectivity and longer reaction lifetime than conventional ZSM-5 zeolite. These results offer a good opportunity to develop highly efficient zeolite catalysts in the future.

Enhanced light olefins production via n-pentane cracking using modified MFI catalysts

n-pentane catalytic cracking was studied over a series of MFI zeolites with varying SiO2/Al2O3 ratios (30, 80, 280, 500, and 1500) using a fixed-bed reactor operated at temperature 550–650 °C. Other MFI zeolites (SiO2/Al2O3 = 280) with various crystal morphology and size (such as large crystal and nano size) were also synthesized and tested for n-pentane cracking. The effects of MFI zeolite modification with ammonia and phosphorus on its physiochemical properties and catalytic activity were investigated. Among the parent MFI zeolites, MFI (280) demonstrated high selectivity (51%) towards light olefins (C3=/C2= = 0.7) at 650 °C with undesired C1–C4 alkanes (38%). Surface modified MFI (280) zeolites of different crystal size and morphology showed improvement towards propylene selectivity by suppressing undesired reactions. Phosphorous-modified MFI zeolite with a large crystal size was found to improve light olefin selectivity (52.2%) with C3=/C2= = ∼1.3 and reduce undesired C1–C4 alkanes (8%) formation due to suppressed strong acidic sites. The characterization and evaluation results for the modified MFI (280) revealed that the incorporation of phosphorous created moderate acidic sites, which were stabilized by some non-framework aluminum species, thereby leading to suppressing the formation of undesired C1–C4 alkanes with improved light olefins selectivity.

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.

Surfactant‐Free and Microporous AlOOH/Al2O3 Nanosheets on TiO2‐Based Nanofibers: A Sustained‐Release Dominated Topotactic Transformation

AbstractUp to now, the synthesis of non‐ligand‐capped 2D transition‐metal nanostructures remained big synthetic challenges. Herein, ultrathin and microporous AlOOH/Al2O3 nanosheets were topologically grown on the surface of TiO2‐based nanofibers by a water‐only hydrothermal strategy starting from electrospun Al2O3/TiO2 nanofibers. The TiO2 nanofibers lowered the surface energy for the sustained‐released aluminum species and thereby allowed them in situ hydrated to laminar AlOOH. The metastable AlOOH tended to dehydrate to Al2O3 simultaneously, leaving a well‐maintained 2D structure with sharp corners and steps, as well as enriched micropores. The nanosheets‐on‐nanofibers exhibited 2856–3370 mg/g of adsorption capacity toward RhB and an efficient removal efficiency over 70% at high throughput (30 mL/h). The promoted light‐harvesting by the perpendicularly attached nanosheets further endowed a significant photodegradation for RhB within 1 h. The presented green and facile approach toward the fabrication of 2D metal oxide nanostructures paves the way to a new variety of oxide nanomaterials.