Al Sites Research Articles

Cuboidal molybdenum sulfur clusters as a platform to construct novel catalysts for propane dehydrogenation

Growing demand for short chain olefins such as propylene for plastic production drives the development of high performing catalysts for propane dehydrogenation (PDH). Here, a confinement strategy originating from metal-sulfur clusters was employed to fabricate defective MoPtSx phase via anchoring by coordinatively unsaturated Al sites present on alumina, followed by reduction under H2 flow at high temperature. This catalyst with a low Pt content exhibits great ability for C–H activation with high intrinsic activity and facile propylene desorption in PDH reaction. More importantly, the catalyst shows excellent stability during long-term tests with both stable conversion and selectivity. Through combined X-ray absorption spectroscopy, electron microscopy and electron paramagnetic resonance measurements it could be affirmed that interaction between isolated and electron-enriched Pt sites as well as their proximity to adjacent sulfur vacancies is vital for the first and second C–H bond activation, while suppressing C–C cleavage which otherwise lead to coking.

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali‐Treatment for Target Acid‐Catalyzed Reactions

Constructing mesoporous structure within zeolites by alkali‐treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid‐catalyzed reactions. In this context, the crucial roles of spatial aluminum distribution were unraveled during alkali‐treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one‐ and two‐dimensional magic‐angle‐spinning (MAS) NMR techniques, were performed to track the alkali‐treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si‐OH in zeolite as Al(OH)4‐. Those re‐deposited Al(OH)4‐ sites were easily transformed into NMR‐invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to“acid‐free”Al species. While most tetrahedral Al sites were preserved after the alkali‐treatment of sample with non‐uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid‐catalyzed reactions, such as catalytic cracking of 1,3,5‐triisopropylbenzene and methanol‐to‐olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated“acid‐free”Al during the precise alkali‐treatment.

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali-Treatment for Target Acid-Catalyzed Reactions.

Constructing mesoporous structure within zeolites by alkali-treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid-catalyzed reactions. In this context, the crucial roles of spatial aluminumdistribution were unraveled during alkali-treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one- and two-dimensional magic-angle-spinning (MAS) NMR techniques, were performed to track the alkali-treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si-OH in zeolite as Al(OH)4-. Those re-deposited Al(OH)4- sites were easily transformed into NMR-invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to"acid-free"Al species. While most tetrahedral Al sites were preserved after the alkali-treatment of sample with non-uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid-catalyzed reactions, such as catalytic cracking of 1,3,5-triisopropylbenzene and methanol-to-olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated"acid-free"Al during the precise alkali-treatment.

Influences of Zr and V Addition on the Crystal Chemistry of θ-Al13Fe4 and the Grain Refinement of α-Al in an Al-4Fe Alloy Based on Experiment and First-Principle Calculations

Fe-containing intermetallic compounds (IMCs) are among the most detrimental second phases in aluminum alloys. One particularly harmful type is θ-Al13Fe4, which exhibits a needle- or plate-like morphology, leading to greater degradation of mechanical properties compared to other Fe-IMCs with more compact structures, such as α-Al15(Fe,Mn)3Si2. The addition of alloying elements is a crucial strategy for modifying the microstructure during the solidification process of aluminum alloys. This study investigates the effects of adding vanadium (V) and zirconium (Zr) on the morphology and crystal chemistry of θ-Al13Fe4 in an Al-4Fe alloy, employing a combination of experimental observations, first-principle calculations, and thermodynamic analysis. Our findings indicate that zirconium significantly refines both the primary θ-Al13Fe4 particles and the α-Al grains. Additionally, a small amount of vanadium can be incorporated into one of the Wyckoff 4i Al sites in θ-Al13Fe4, rather than occupying any Fe sites, under casting conditions, in addition to the formation of binary Al-V phases.

Low-temperature oxidation of methane mediated by Al-doped ZnO cluster and nanowire: a first-principles investigation.

First-principles calculations are performed to investigate the catalytic oxidation of methane by using N2O as an oxidizing agent over aluminum (Al)-doped Zn12O12 cluster and (Zn12O12)2 nanowire. The impact of Al impurity on the geometry, electronic structure, and surface reactivity of Zn12O12 and (Zn12O12)2 is thoroughly studied. Our study demonstrates that Al-doped ZnO systems have a better adsorption ability than the corresponding pristine counterparts. It is found that N2O molecule is initially decomposed on the Al site to provide the N2 molecule, and an Al-O intermediate which is an active species for the CH4 oxidation. The conversion of CH4 into CH3OH over AlZn11O12 and (AlZn11O12)2 requires an activation energy of 0.45 and 0.29eV, respectively, indicating it can be easily performed at normal temperatures. Besides, the overoxidation of methanol into formaldehyde cannot take place over the AlZn11O12 and (AlZn11O12)2, due to the high energy barrier needed to dissociate C-H bond of the CH3O intermediate. Dispersion-corrected density functional theory calculations were performed through GGA-PBE exchange-correlation functional combined with a numerical double-ζ plus polarization (DNP) basis set as implemented in DMol3. To include the relativistic effects of core electrons of Zn atoms, DFT-semicore pseudopotentials were adopted. The DFT + D scheme proposed by Grimme was used to involve weak dispersion interactions within the DFT calculations. The reaction energy paths were generated by the minimum energy path calculations using the NEB method.

Study on flotation separation of chlorite and kaolinite by modified fatty acid collector and its mechanism

The modified fatty acid collector α-bromododecanoic acid (α-BDDA) had a significant effect on the separation of chlorite and kaolinite. In this paper, the flotation separation of the independent lithium bearing mineral chlorite (cookeite) from the main gangue mineral kaolinite in clay-type lithium ore was studied. The results showed that when pH was 9, the concentration of CaCl2 was 1.64×10−4 mol/L, and the concentration of α-BDDA was 5.82×10−4 mol/L, the flotation separation effect of chlorite and kaolinite was the best, and the floating difference between them was 91.12 %. The flotation results of artificially mixed ore also proved that α-BDDA could realize the separation of chlorite and kaolinite. Adsorption amount analysis revealed that α-BDDA could adsorb both on chlorite and kaolinite, however, the amount of α-BDDA adsorbed on chlorite was much higher than that on kaolinite. Zeta potential analysis, FT-IR spectra analysis, and XPS analysis confirmed that α-BDDA could produce chemisorption on the surface of chlorite activated by CaCl2. The Al, Mg, Fe and Ca sites on chlorite could reacted with α-BDDA to form new chemical bonds. However, α-BDDA was adsorbed on kaolinite by weak physical adsorption, which was easily de-adsorbed by strong agitating or water washing.

Accelerator or inhibitor? The effects of H2O on selective catalytic reduction of NOx by methanol over HZSM-5 catalysts with different Lewis/Brønsted ratios

HZSM-5 zeolites are widely regarded as promising catalysts for the selective catalytic reduction of NOx using methanol (CH3OH-SCR). However, the effect of H2O on CH3OH-SCR over HZSM-5 zeolites with varying Lewis and Brønsted acid sites (LAS and BAS) remains unclear. Herein, a series of HZSM-5 catalysts with varying LAS/BAS ratios were synthesized through Ga modification combined with calcination. Notably, these catalysts with varying LAS/BAS ratios exhibit diverse CH3OH-SCR activity. The HZSM-5 catalyst with a higher amounts BAS shows superior SCR performance (93 % NOx conversion at 300 ℃) under dry conditions, while Ga/HZSM-5 with abundant LAS demonstrate better De-NOx activity in the presence of H2O. In situ DRIFTS results, TPSR-MS coupled with DFT calculations demonstrate that the Si(OH)Al sites in HZSM-5 zeolites promote the generation of intermediate formamide under dry conditions. Whereas, the extra-framework Al-OH and Ga species in Ga/HZSM-5 catalysts lead to excessive oxidation of methanol to formate species. When water is present, intense competitive adsorption between CH3OH and H2O occurs at the Si(OH)Al sites, significantly decreasing the denitration activity of HZSM-5. However, H2O not only alleviates the over-oxidation of methoxy species, but also enables the Al-OH and GaO+ sites to adsorb and activate CH3OH to generate formamide species, thereby exhibiting preferable catalytic activity in Ga/HZSM-5 catalysts. Accordingly, a reaction mechanism based on the synergistic effect of BAS and LAS is proposed for Ga/HZSM-5 catalysts.

Selective dealumination of large pore Zeolite Beta for effective Brønsted acid site utilization

Tuning of Al sites through spatial and geometric distribution is a novel pursuit for deterministic catalytic performance. To design an efficient Zeolite Beta for phenol alkylation with optimal physicochemical attributes, we conducted a systematic study with a series of dicarboxylic acids to selectively extract the framework and/or non-framework Aluminium. Herein, the post-synthetic treatment resulted in dealuminated catalysts of silicon-to-aluminum ratio ∼15–53. Extensive Al and Si NMR studies glean the selective extraction of aluminum from the zeolitic framework along with recomposition in T sites. The potency study alludes to the convoluted role of pH, chelating ability, and/or site accessibility for complexation. The differentiated Al extraction results in the emergence of unique super-strong acid sites. The novelty of our approach was established using phenol alkylation with cyclohexanol wherein we observed the highest conversion and desired CC alkylated product formation for the malonic acid-treated Zeolite Beta.

Insights into the Effect of a Microwave Field on the Properties of Modified γ-Alumina: A DFT Study

γ-Alumina is often used as a support for hydrodesulfurization catalysts due to its excellent performance. During the catalytic reaction, the strong surface acidity of γ-alumina can induce a strong interaction between the active phase and the support. The reaction activity of the catalyst can be affected by changing the present mode of the active phase on the surface of the support. The (110) crystal plane, acting as the strongest acidity plane of γ-alumina, was selected for modification. The supports modified with boron and phosphorus were successfully constructed, and the acid strengths were quantified by simulating the adsorption of the relevant probe molecules: pyridine in correlation with surface electronic properties via density functional theory. The surface adsorption energy calculation shows that the boron-modified surface is able to moderately reduce the adsorption capacity of alumina, while that of the surface modified by phosphorus is found to be enhanced over the sites of a tetrahedral coordination structure; however, at the other unsaturated Al sites, this is obviously reduced. The results of introducing electric fields imply that applying horizontal electric fields changes the surface acidity of alumina under the premise of a stable structure. With the enhancement of the horizontal electric fields, the adsorption capacity of tetra-coordination sites on the original surface gradually decreases, while those of the others gradually increases. However, for the boron-modified surface, introducing horizontal electric fields can reduce the adsorption capacity of all sites. Hence, microwave-electric-field-assisted modification of B further reduces the surface acidity of alumina, making it beneficial for deep hydrodesulfurization reactions.

Discovering novel γ-γ′ Pt-Al superalloys via lattice stability in Pt3Al induced by local atomic environment distortion

The strength of Pt-Al superalloys can be effectively improved using a coherent structure composed of a disordered γ phase and an ordered γ′ phase. However, γ′-Pt3Al has a cubic phase only at high temperatures and a tetragonal phase at low temperatures, thus breaking the coherent interface and deteriorating its mechanical properties. Here, high throughput first-principles calculations and rational experiments were conducted to discover novel γ-γ′ Pt-Al superalloys. The alloying strategy stabilizes the γ′ phase through the local charge distortion (LCD) or the local lattice distortion (LLD). Alloying elements, such as, Ti, Hf, and Ta, occupy the Al site in combination with Pt, making the spherical shape of change density in Pt3Al showing obvious directionality, i.e., LCD. Therefore, the stability of the γ′ phase is proposed at a lower alloying element concentration (3.125 at.%). However, alloying elements near Pt, e.g., Ni, Co, and Ru, tend to occupy the Pt site and induce LLD to stabilize the γ′ phase at higher alloying element concentration (12.5 at.%). The shape of the change density is similar to that of Pt3Al, and no obvious LCD is found when alloying elements occupy the Pt site. LCD-induced phase stability is observed exclusively at the Al site, whereas LLD-induced phase stability is solely present at the Pt site. The Pt-12Al-6X (X = Hf, Ti, Ta, Cr, and Ni in at.%) alloys were prepared, among which Pt-12Al-6Hf possessed a stable γ′ phase, a lower lattice misfit between γ and γ′, and a higher precipitation strengthening, resulting in its hardness (442.1 HV) surpassing that of Ni-based superalloys IN718 (413.6 HV). The γ′ phase in Pt-12Al-6Hf was characterized by high-resolution transmission electron microscopy, suggesting that the Pt site of the γ′ phase was occupied by Pt, while the Al site was shared by Pt, Al, and Hf. This is consistent with the calculated results.

Promotion of La2O3 on Pd/Al2O3 three-way catalyst for passive selective catalytic reduction operation

For three-way catalysts (TWC) applied in lean burn gasoline engines for passive selective catalytic reduction (pSCR) operation, besides the conversion efficiency of HC/CO/NO, the production of NH3 is also an important concern. In this work, La2O3 was introduced into the Pd/Al2O3 catalyst system, the influence on its TWC performance for pSCR operation was investigated, and the physicochemical properties were characterized by a range of techniques. The results reveal that the introduced La2O3 mainly interacts with Al2O3 by preferentially anchoring to the tetra-coordinated Al sites on the surface. When the catalyst is modified by appropriate amount of La2O3 (4 wt%), higher dispersion of Pd species, larger amount of Pd2+ and active oxygen species are achieved, which well contributes to improved low-temperature reduction ability and NH3 desorption property of the catalyst. As a consequence, excellent catalytic conversions of HC/CO/NO are obtained, and meanwhile, higher concentration of NH3 is generated, making the modified catalyst more promising for pSCR operation.

Catalytic Synergy between Lewis Acidic Alumina and Pt in Hydrodechlorination for Plastic Chemical Recycling.

Pyrolysis-based plastic chemical recycling has gained significant industrial attention due to its advantage of eliminating complex plastic sorting processes. However, plastic pyrolysis oil contains various components that require stringent removal before subsequent processes. In particular, Cl compounds originating from the decomposition of poly(vinyl chloride) can cause serious corrosion of reactors and catalyst deactivation in downstream processes. While extensive research has been conducted on the removal of other heteroatoms (S, N, and O) from organic compounds via hydrotreating, studies on the removal of Cl have been scarce. In this study, hydrodechlorination over Pt catalysts on various supports is comprehensively investigated using 1,2-dichloroethane as a model reactant. Our results demonstrate that Pt on γ-Al2O3 can exhibit exceptionally high catalytic activity compared to those on other supports due to a distinct bifunctional mechanism. Rigorous studies reveal that the Lewis acidic pentacoordinated Al sites of γ-Al2O3 activate C-Cl bonds, whereas Pt activates H2 and provides spillover H to remove Cl as HCl. The bifunctional mechanism enables the minimized use of precious Pt (<0.1 wt %) to achieve high activity. Pt/γ-Al2O3 also allows for efficient Cl removal (96.8%) with high stability in treating waste plastic pyrolysis oil containing 7500 ppm of Cl.

The Rôle of Iron in Zeolite Beta for deNOx Catalysis

Selective catalytic reduction with ammonia (NH3-SCR) is a widely used deNOx process, employing zeolitic catalysts. Here, we examine framework substituted and extra-framework exchanged transition metal zeolite catalysts, in the extensively studied Fe – zeolite Beta focusing on the influence of the transition metal cation on the NH3-SCR reaction, and aiming to unravel the underlying mechanisms and their impact on catalytic performance. We use hybrid multiscale density functional theory (DFT)/molecular mechanics to investigate the NH3-SCR at various Fe-BEA active sites including systems with both framework and extra-framework Fe cations. The catalytically active sites under investigation include Fe-FeF-BEA, Fe-AlF-BEA, and Cu-FeF-BEA, (where the subscript F indicates framework species) on which the formation and consumption of key intermediates of both oxidation and reduction half cycles species are analysed. We show the distinctive ability of framework Fe sites to promote the formation of key nitrate and nitrosamine intermediates. Furthermore, we find a strong binding affinity of NH3 with framework Fe systems including Cu(II)-FeF-BEA, Fe(III)-FeF-BEA, and H-FeF-BEA compared to framework Al systems (Fe(III)-AlF-BEA). We explore the reduction potential of the framework Fe3+/Fe2+ in BEA zeolite and establish its feasibility in the presence of a Brønsted acid site. Our calculations clearly predict the bimetallic Cu-FeF system to be the best catalyst which supports a recent experimental report by Yue et al. (Chem. Eng. J. 2020, 398, 125515) for a related FeCu-SSZ-13 (CHA) zeolite. We provide a new understanding of the reactivity of Fe-BEA zeolites, suggesting the advantages of framework Fe over Al sites in NH3-SCR reactions, and establishing a foundation for interpretation of the rôle of framework cations in metal-exchanged zeolites.

Comparison of surface physicochemical properties of spodumene and feldspar using different grinding media: Implications for flotation separation

Grinding processing after crushing is a common practice in mineral processing circuits involving flotation, and achieving selective liberation of target minerals by grinding is a prerequisite for successful flotation. In this study, the surface characteristics and the floatability of spodumene and feldspar particles produced by laboratory ball and short cylinder mills were determined by XRD, SEM, AFM, XPS analysis, DFT calculation, and flotation experiment. Compared with ball-milled particles, spodumene ground with short cylinder media had more exposed {110} cleavage planes, while feldspar under the same condition had more exposed {110} and {201} cleavage planes. Additionally, the amount of Al sites and the roughness of spodumene and feldspar after grinding by short cylinder media were higher than those of ball-milled particles. Micro-flotation experiments and XPS analysis further indicated that, although the co-adsorption of NaOL/DDA on the surface of spodumene is stronger than feldspar under both milling media, the flotation separation efficiency of spodumene and feldspar remained higher with short cylinder grinding media. Therefore, this study is expected to provide more reference value and theoretical guidance for the development of selective flotation separation of spodumene and feldspar from the perspective of grinding.

Multifaceted investigation into the impact of Ga substitution for Al in V[formula omitted]MnAl inverse Heusler alloys

This study investigates the impact of Ga substitution for Al in V2MnAl inverse Heusler alloys, focusing on lattice parameter variations. Despite similar valence electrons, Ga’s distinct size and surface energy prompt questions about its role in alloy characteristics. Experimental synthesis of V2Mn1Al1−xGax alloys (x = 0 to 0.60) reveals crystal parameter changes. Theoretical exploration using density functional theory (DFT) probes how Ga doping at the Al site influences the density of states, altering the half-metallic properties of V2MnAl. Additionally, machine learning, with a dataset of 391 entries, predicts lattice constants based on atomic properties, enhancing our ability to maneuver material characteristics. This multifaceted approach aims to deepen our understanding of Ga’s impact on Heusler alloys, bridging experimental and theoretical realms for comprehensive insights into materials design and behavior.

Active site modulation in UiO-66(Ce) MOFs by Al3+ doping for boosting photocatalysis

A series of Al3+-doped UiO-66(Ce) MOFs (U(CexAl1−x)) was developed to reveal the roles of metal doping on active site formation and substrate activation in photocatalytic selective transformation of amines to imines. Al3+ doping induced crystal structure distortion to form coordinately unsaturated metal (Al, Ce) sites as Lewis acid sites to chemisorb and activate benzylamine (BA). In situ FTIR revealed that the Al site with stronger acid strength facilitated BA activation. The activation degree of N–H bonds in BA was evaluated by changes of calculated force constant. The intermediate products were confirmed through time-dependent in situ FTIR. The Al3+-doped sample U(Ce0.90Al0.10), the optimal catalyst, showed a significantly increased BA conversion (97.6 %) than that (47.5 %) of undoped sample, which was attributed to enough active sites and the optimal charge mobility. Finally, the present study proposes a synergistic photocatalytic mechanism associated with molecular activation to demonstrate selective oxidation pathways at the molecular level.

A comprehensive investigation of alloying elements on site preferences, elastic properties and electronic structure of Mg17Al12 by first-principles calculations

The effect of elemental doping on the site preference, elastic properties and electronic structure of Mg17Al12 has been systematically investigated by first-principles calculations. And twenty-seven elements (Li, Na, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Ag, Cd, In, Sn, Sb, Pb, Bi and La) are considered in this study. The calculations of substitution formation energies indicate that the alkali metal (Li), alkaline earth (Ca, Sr) and rare earth (Sc, Y, La) group elements and Ti, Zr favor to occupy Mg sites, and transition group (Fe, Co, Ni, Cu, Ag, Zn, Cd) elements and Ga, Ge, Sn, Sb prefer to occupy Al site, while Na, K, V, Cr, Mn, In, Pb and Bi are unstable in Mg17Al12 phase. Meanwhile, the results of phonon dispersion curves for the preferentially doped systems show that all doped structures are dynamically stable except for Sr, La and Sn doped systems. The elastic constants indicate that all preferential doping models are mechanically stable, and solute atoms with high bulk modulus lead to an increase in the bulk modulus of Mg17Al12 phase. Simultaneously, the calculated shear modulus/bulk modulus ratio (G/B), Poisson's ratio (v) and Cauchy's pressure (C‾12‐C‾44) indicate that the ductility of doped systems is better than Mg17Al12 alloy. All structures are elastic anisotropic, while the doping of Fe, Co and Ni reduces the degree of anisotropy of Mg17Al12 phase. Furthermore, the electronic structures show a strong hybridization between the p-orbitals of Mg and Al and the d-orbitals of alkaline earth, rare earth and Zr elements, resulting in higher structural stability. Finally, the adsorption behavior of O atom on Mg17Al12(110) surface and the structural stability, lattice constants, and elastic modulus of Mg34Al24-xZnx random solid solution alloys are investigated.

Correlation of the properties of quasi-ternary Ni-based superalloys computed from DFT to elemental properties enabling predictions on the behavior of metallic alloying elements

Research on multicomponent superalloys derived from Ni3Al shows that certain elemental additions congregate at the boundary of the γ/γ′ interface while certain others partition into either the γ or the γ′ phases. Such microstructural features have a strong bearing on the performance of the superalloy. However, no established correlations exist as of now, that can help predict such a behavior. The present work addresses this issue by examining the properties of the quasi-ternary alloys of Ni3Al and brings out correlations to selected lumped parameters related to the elemental properties. Computation of thermodynamic properties of 11 alloys of quasi-ternary Ni24Al7X system was carried out from first principles using Wien 2k and Gibbs2 code on a supercell constructed with 32 atoms. Elements X substituted at the Al site are selected from the information derived from the literature. Systematic variations are shown to occur in compressibility and heat of formation ΔH of the alloys with average electron concentration, elemental radii and/or electronegativity of the constituent elements in the alloys. Analysis of the data computed for a supercell on ΔH of quasi-ternary Ni31X alloys of Ni (the γ phase) and the ΔH of Ni24Al7X alloys (the γ′ phase) explains the possible reason for the enrichment of elements like Re and W at the γ/ γ′ interface. Trends in the observed correlations correctly predict the experimental observations on alloying effects reported from spectroscopic studies on quasi-ternary alloys of Ni3Al. The scope of predicting alloying behavior based on basic elemental properties gains significance in the design of superalloys.

Zeolite-catalyzed one-pot sucrose-to-HMF transformation: Ge outperforms Sn, Zr, and Al sites

The conversion of renewable compounds to versatile platform molecules over environmentally friendly heterogeneous catalysts is a major challenge. Zeolites stand as active, selective, and reusable solid catalysts for various acid-catalyzed reactions involved in the one-pot cascade transformation of polysaccharides to 5-hydroxymethylfurfural (HMF), a platform molecule opening the way to various valuable chemicals. However, the acidity-performance relationships of zeolite catalysts in HMF synthesis have not been fully elucidated. Here, we have addressed the effect of acid site nature in zeolite catalysts for sucrose-to-HMF transformation by comparing the performance of conventional Al-substituted IWW zeolite with that of Sn-, Zr-, and Ge-containing zeolite catalysts of the same structure. Ge-associated acid sites were found to exhibit superior HMF selectivity compared to Sn, Zr, and Al acid centers, while experiencing evolution into Brønsted acid centers during the catalytic run. The conversion of sucrose over germanosilicate zeolites enhances with increasing catalyst pore diameter or decreasing crystal size. Specifically, the extra-large pore Ge-UTL catalyst, featuring intersecting 14- and 12-ring pores (crystal size 10 × 20 × <1 μm), and large-pore Ge-IWW with 12-, 10- and 8-ring pores (crystal sizes of 1 μm) showed a yield of targeted HMF comparable to or even exceeding the values previously reported for homogeneous or heterogeneous catalysis (54 % after 3 h at 120 °C). Ge-IWW catalyst demonstrated reusability across a minimum of 3 catalytic runs, while in situ structural transformation precluded stable performance of Ge-UTL catalyst in the repetitive catalytic cycles. The results of this study highlight zeolites with uncharacteristic chemical compositions as active, selective, and reusable catalysts for highly demanding applications in biomass valorization.

Efficient removal of phosphate and fluoride from phosphogypsum leachate by lanthanum-modified zeolite: Synchronous adsorption behavior and mechanism

Phosphogypsum is a common large-tonnage by–product in phosphorous chemicals industry, its stacking not only causes severe environmental problems, but also leads to a serious waste of phosphate and fluoride resources. The objective of this study was to tackle the recycling treatment of phosphogypsum leachate by using a highly efficient adsorbent lanthanum hydroxide-modified zeolite (LMZ). The adsorption behaviors of phosphate and fluoride by LMZ in the coexisting system under static and dynamic conditions were studied. And further, the competitive adsorption mechanism involved was analyzed. The results showed that LMZ possessed excellent adsorption performance toward both phosphate and fluoride in the single system, but the fluoride adsorption capacity decreased by 57.9 % in the coexisting system, with no change in phosphate adsorption. In the competitive adsorption process, ligand exchange interaction on La active sites contributed most to phosphate adsorption, while fluoride could be adsorbed onto LMZ via electrostatic attraction, surface precipitation with Al sites, and hydrogen bonds with hydroxyl groups in La−OH and P−OH. In actual phosphogypsum leachate, the removal of phosphate had priority over the removal of fluorine. Under continuous adsorption condition, the concentration of phosphorus and fluoride could be decreased to below 0.5 and 1.0 mg/L, respectively, meeting the safe concentration in drinking water. This study promotes the application of La–based adsorbents in the treatment of phosphogypsum leachate, and provides a green and sustainable strategy toward practical phosphorus and fluorine recovery.