HBEA Zeolite Research Articles

Alkyl Transfer Processes of Substituted Aromatics within H-BEA Zeolite

Alkyl Transfer Processes of Substituted Aromatics within H-BEA Zeolite

Confined Ionic Environments Tailoring the Reactivity of Molecules in the Micropores of BEA-Type Zeolite.

In the presence of water, hydronium ions formed within the micropores of zeolite H-BEA significantly influence the surrounding environment and the reactivity of organic substrates. The positive charge of these ions, coupled with the zeolite's negatively charged framework, results in an ionic environment that causes a strongly nonideal solvation behavior of cyclohexanol. This leads to a significantly higher excess chemical potential in the initial state and stabilizes at the same time the charged transition state in the dehydration of cyclohexanol. As a result, the free-energy barrier of the reaction is lowered, leading to a marked increase in the reaction rates. Nonetheless, there is a limit to the reaction rate enhancement by the hydronium ion concentration. Experiments conducted with low concentrations of reactants show that beyond an optimal concentration, the required spatial rearrangement between hydronium ions and cyclohexanols inhibits further increases in the reaction rate, leading to a peak in the intrinsic activity of hydronium ions. The quantification of excess chemical potential in both initial and transition states for zeolites H-BEA, along with findings from HMFI, provides a basis to generalize and predict rates for hydronium-ion-catalyzed dehydration reactions in Brønsted zeolites.

Regioselectivity and confinement effects for catalytic conversion of carbohydrates within zeolite H-BEA

Zeolites have established the importance during biomass conversion, and H-form zeolites, although extensively used, produce hydroxymethylfurfural (HMF) with low yields. Herein, dispersion-corrected density functional calculations are employed with aim to comprehensively understand glucose conversion to HMF within zeolite H-BEA and unravel the role of confinement effects. The low HMF yields originate from regioselectivity: The preferentially activated O3 and O6 sites are significantly driven to humin precursors, while less reactive O1 and O2 sites lead favorably to HMF. Glucose-to-fructose isomerization occurs only at O5 site, while seems unnecessary for HMF production. Owing to the peculiar confinement effects, HMF precursors are prone to emerge within H-BEA, both upon dehydration and during catalysis, while may turn to be acyclic within aqueous solutions. Direct conversion to levulinic acid without HMF as intermediate thus becomes difficult within acidic zeolites. Results facilitate catalyst design for biomass conversion, and further understanding of regioselective catalysis and confinement effects.

Phenol removal from aqueous media using BEA zeolite: A molecular simulation study on the effect of zeolite mobile cations

Phenol removal from wastewater is crucial due to its high toxicity at low concentrations. In this study, the effectiveness of BEA zeolites (All-Si-BEA, H-BEA, Ca-BEA, and Na-BEA) for phenol removal was investigated using GCMC and MD simulations. The surface area and pore volume of the BEA structures were analyzed. The results showed that All-Si-BEA and H-BEA zeolites had slightly larger pore volumes (less than 2 %) compared to cationic BEA zeolites, indicating minimal impact of cations on pore volume. Efficient phenol adsorption was observed on all zeolite structures, even at low concentrations. All-Si-BEA and H-BEA exhibited high selectivity (105-106) and phenol adsorption capacity (1.86 mmol/g) at low phenol concentrations. Water adsorption was lower on All-Si-BEA and H-BEA, while cationic zeolites had higher water adsorption. The simulations showed that water molecules gather around mobile cations in the pores, causing variation in water adsorption in All-Si-BEA, H-BEA compared to cationic BEAs. The energy examination confirmed this observation. The diffusion coefficients of phenol in BEA (All-Si-BEA, H-BEA, Na-BEA, and Ca-BEA) were 5.34 × 10−5, 3.49 × 10−5, 5.32 × 10−6, and 1.02 × 10−6 (cm2/s), respectively, indicating higher mobility in All-Si-BEA and H-BEA compared to cationic BEA zeolites. Increasing the number of water molecules in the zeolite structures resulted in a decrease in the diffusion coefficient of phenol due to the hindrance caused by water molecules. Overall, All-Si-BEA and H-BEA showed higher potential for phenol separation from water compared to cationic BEA zeolites.

Insights into the effect of Si/Al ratio on Isobutane/1-butene ratio in the pore of H-BEA zeolite in C4 alkylation process

The isobutane/butene (I/O) ratio in the pore is a crucial factor for enhancing the catalytic stability of zeolitic catalysts in C4 alkylation process. In this work, the influence mechanism of the Si/Al ratio on the I/O ratio in the pore for H-BEA zeolite was investigated via CB-GCMC methods under realistic alkylation conditions. It is found that the I/O ratio in the pore decreases dramatically with decreasing the Si/Al ratio due to the higher adsorption heats of 1-butene over the acid sites than that of isobutane. Therefore, there will be an optimal Si/Al ratio for C4 alkylation, which is in good agreement with the experimental results. Furthermore, the relationship between the I/O ratio in the pore and feedstock over the H-BEA zeolite with different Si/Al ratios was built. We found that increasing the I/O ratio in the pore by improving the I/O ratio in the feedstock is limited, when the Si/Al ratio is much low. The external surface modification of catalyst will be an effective method to improve the I/O ratio in the pore. These results are of great significance to the rational design of high-efficiency C4 alkylation catalysts.

Environmentally benign synthesis of quinazoline-based heterocyclic compounds over zeolite H-BEA catalyst with bimodal porosity

Environmentally benign synthesis of quinazoline-based heterocyclic compounds over zeolite H-BEA catalyst with bimodal porosity

Synthesis of biologically active dihydroquinazolinone catalyzed by the micro–meso-composite of zeolite H-BEA

Synthesis of biologically active dihydroquinazolinone catalyzed by the micro–meso-composite of zeolite H-BEA

DTAB Mediated Post Modification of Zeolite H-BEA, Its Characterization and Catalytic Application for n-Butyl Levulinate Synthesis

DTAB Mediated Post Modification of Zeolite H-BEA, Its Characterization and Catalytic Application for n-Butyl Levulinate Synthesis

Al distribution and structural stability of H-BEA zeolites at different Si/Al ratios and temperatures: a first-principles study.

Beta zeolites have been widely used in acid-catalyzed reactions because of their excellent properties. An in-depth study of the position, quantity, and distribution of beta zeolites substituted by Al is significant to understand the catalytic performance of the active site of zeolite catalysts. The distribution of Al in H-BEA and the structure of silanol nests in dealuminated BEA at different Si/Al ratios and synthesis temperatures were studied by the DFT method. T1, T2, T7, and T9 sites were chosen to be simulated. The synthesis temperature can change the distribution of Al and the proportion of T sites at different Si/Al ratios. The proportion of T7 and T9 is more than 70% at different Si/Al ratios of H-BEA and decreases with the synthesis temperature. T1 and T2 sites begin to appear when Si/Al < 20 and the proportion of T1 and T2 sites is less than 20%. When Si/Al < 8, the substitution energy of the AlSiAl structure, which has Si(2Al, 2Si) species, is obviously lower than that of the normal structure, which indicates that the Al-O-Si-O-Al species will appear in H-BEA. The Al(T7)Si(T5)Al(T9)Si(T5)Al(T7) and Al(T1)Si(T1)Al(T9) groups can not only stabilize H-BEA but also play an essential role in the formation of Si(2Al, 2Si) species. For dealuminated BEA zeolites, the silanol nest forms four hydrogen bonds through four silanols. The orientation of silanol groups in the silanol nest formed after dealumination at different T sites is different. The T7 and T9 sites in H-BEA are more likely to undergo dealumination. By contrast, the dealumination of the T1 and T2 sites is a challenge.

Improved Regioselective Mononitration of Naphthalene over Modified BEA Zeolite Catalysts

HBEA zeolite modified with highly electronegative cations is a highly efficient and reusable catalyst for the nitration of naphthalene with nitric acid, which has been successfully prepared in this work. Catalytic selective mononitration of naphthalene was investigated. The ratio of 1-nitronaphthalene isomer to 2-nitronaphthalene could reach 19.2, with a moderate yield of 68.2%, when the reaction was carried out in 1,2-dichloroethane, with 1.0 mmol naphthalene, 0.22 mL nitric acid (95%), and 0.10 g HBEA-25 at −15 °C. The effects of reaction temperature and the quantity of zeolites on 1-nitronaphthalene were also studied. The catalyst is readily recyclable, and we believe this to be a major step forward in the area of clean technology for aromatic nitration.

Restraining deactivation of β-zeolite by modifying with MgAl2O4 spinel in gas-phase glycerol dehydration

MgAl2O4 dispersed on HBEA zeolite with different spinel contents were successfully synthesized and their glycerol dehydration performances were presented. The different solids were characterized by XRD, infrared (IR) and Raman spectroscopy, N2 physisorption, SEM-FEG, acidity by TPD-NH3 and DRIFTS-pyridine adsorption. It was not possible to observe the spinel phase in the diffractograms due to its high dispersion, while the reflections of the HBEA were evidenced in the different samples. The Raman and infrared spectra suggested the presence of stretches concerning the MgAl2O4 spinel. N2 isotherms and low-angle XRD indicated that the pore structure and textural properties were altered after impregnation, but the micro-mesoporosity of zeolite was maintained. The HBEA morphology did not undergo significant changes by modification with spinel and the metals are evenly distributed according to SEM and EDS spectral mapping. The addition of spinel on zeolite caused a change in the acidic properties of HBEA related to the type, number and strength of the sites according to the TPD profiles and the DRIFTS spectra. The 2% of MgAl2O4 on HBEA catalyst exhibited the best catalytic performance due to the effects of its acidic and textural properties, reaching 60% of glycerol conversion, 66% of acrolein selectivity in 6 h and 16% of coke deposited in 10 h, restricting deactivation compared to pure zeolite. The reaction pathway for the different products formed depending on the type of acid site were detailed in the proposed mechanism and correlated with electrostatic potential maps as well as the reactions for coke formation were described according to compounds identified by GC-MS after coke extraction.

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

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

Mechanistic insights into positional and skeletal isomerization of cyclohexene in the H-BEA zeolite.

The isomerization of cycloalkenes via the formation of carbenium cations assisted by the Brønsted acid site (BAS) in zeolites is the vital reaction step in hydrocracking and hydroisomerization processes of the petrochemical industry. To understand the acid-catalyzed positional isomerization and skeletal isomerization of cycloalkenes via carbenium intermediates, a series of ab initio molecular dynamics (AIMD) simulations of cyclohexene within the H-BEA zeolite have been carried out. AIMD simulations combined with the enhanced sampling technique reveal that the half-chair conformer is the most stable conformation for cyclohexene within H-BEA. Free energy landscapes characterizing protonation/deprotonation, positional isomerization, and skeletal isomerization of cyclohexene have been mapped out at 413 K. The free energy barrier for the formation of carbenium is calculated to be 44 kJ mol-1. The skeletal isomerization of cyclohexene to methylcyclopentylium via the protonated cyclopropane transition state involves four stages with a total free energy barrier of 134 kJ mol-1. Further geometrical analysis provides additional information about the structural origin of free energy barriers.

Nitrogen as a Probe Molecule for the IR Studies of the Heterogeneity of OH Groups in Zeolites.

One of the methods of IR studies of the heterogeneity of Si–OH–Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si–OH–Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si–OH–Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si–OH–Al of various number of Al near the bridge and of Si–OH–Al of various geometry.

Deactivation of Zeolite Catalysts in the Prins Reaction between Propene and Formaldehyde in the Liquid Phase

The Prins reaction between propene and formaldehyde was studied over H-BEA, H-FAU, H-MFI and H-MOR zeolites at 150 °C in liquid phase. It was found that the H-BEA sample is the most active and selective toward buta-1,3-diene; the H-MFI is a potential catalyst for 3-buten-1-ol synthesis, while H-FAU can be used for 4-methyl-1,3-dioxane production. It had been confirmed that zeolite textural and acidic properties influence catalyst behaviour: the acidic properties influence sample activity, while product distribution is controlled by pore volume and effective pore diameter. The sample’s deactivation process had been studied and the kinetic model of deactivation was proposed. It was shown that the deactivation rate for the H-MFI catalyst is four times greater than for the H-BEA catalyst, probably because its strong/weak acid sites ratio is much more high than for the H-BEA.

Comparative assessment of pesticide adsorption capacity and antioxidant activity of Silver Dodecatungstophosphate/HΒEA zeolite composites

Composite materials consisting of the silver salt of dodecatungstophosphoric acid and HBEA zeolite are developed in order to give materials of superior adsorption properties for aqueous pesticide removal. Two-step impregnation and an ion-exchange method with variable constituent mass ratios were employed as synthesis procedures. One of the study goals was to optimize the composite preparation for efficient elimination of glyphosate pesticide and to investigate the modulation of antioxidant activity in the presence of pesticide. Based on comprehensive results, we propose mechanisms for simultaneous glyphosate removal and antioxidant activity. Spectroscopic analysis shows that the applied two-step impregnation method results in advanced composite materials with evenly distributed active sites for glyphosate adsorption and radical-scavenging activity. The original Krishnamurti isotherm is successfully used to fit pesticide adsorption data, pointing to co-operative adsorption of glyphosate on partly saturated adsorbents sites. The amount of glyphosate adsorbed from water suspension was 378 mg per gram of composite material, the highest value reported for glyphosate removal to date.

Greenness Assessment and Synthesis for the Bio-Based Production of the Solvent 2,2,5,5-Tetramethyloxolane (TMO)

2,2,5,5-tetramethyloxolane (TMO) has recently been identified and demonstrated as a safer solvent to replace toluene, THF, and hydrocarbons in a handful of applications. Herein, several bio-based routes to TMO are presented and assessed for greenness, assisted by the CHEM21 Metrics Toolkit and BioLogicTool plots. Using glucose as a common starting point, two chemocatalytic routes and two biochemical routes to TMO were identified and the pathways compared using the aforementioned tools. In addition, bio-based TMO was synthesised via one of these routes; from methyl levulinate supplied by Avantium, a by-product of the sugar dehydration step during the production of 2,5-furandicarboxylic acid. First, methyl levulinate underwent triple methylation using methyl magnesium chloride (MeMgCl) to yield 2,5-dimethylhexane-2,5-diol (DHL) in high yields of 89.7%. Then DHL was converted to high purity TMO (&gt;98.5%) by cyclodehydration using H-BEA zeolites based on the previously reported approach. Bio-based content of this TMO was confirmed by ASTM D6866-20 Method B and found to have 64% bio-based carbon, well above the threshold of 25% set by CEN/TC 411 standards and matching the anticipated content. This study represents the first demonstration of a bio-based synthesis of TMO and confirmation of bio-content by accepted standards.

Acylation of Anisole With Benzoyl Chloride Over Rapidly Synthesized Fly Ash-Based HBEA Zeolite.

Stable HBEA zeolite with high surface area and strong acid sites was synthesized from coal fly ash–based silica extract via indirect hydrothermal synthesis. The rapid HBEA hydrothermal crystallization times of 8, 10, and 12 h were achieved through a reduced molar water fraction in the synthesis composition. The HBEA zeolites prepared from fly ash silica extract exhibited well-defined spheroidal-shaped crystal morphology with uniform particle sizes of 192, 190, or 239 nm obtained after 8, 10, or 12 h of synthesis time, respectively. The high surface area and the microporous area of 702 and 722 m2/g were achieved as a function of shorter hydrothermal synthesis durations (10 and 24 h, respectively) compared to 48 or 72 h, which resulted in HBEA zeolites with lower surface areas of 538 and 670 m2/g. Likewise, temperature-programmed desorption measurements of fly ash–based HBEA zeolites revealed the presence of weak and strong acid sites in the zeolite. The submicron crystal sizes with a well-defined porosity of HBEA zeolites enhanced the diffusion of anisole and benzoyl chloride molecules toward the active acid sites and hence showed better conversion and selectivity in acylation products. High conversion of benzoyl chloride with anisole was achieved, reaching up to 83% with a 93–96% selectivity toward 4-methoxyacetophenone.

Synergic role of platinum (Pt) and molybdenum trioxide (MoO3) promoted HBEA zeolite towards n-heptane isomerization

The potentiality of Pt and MoO3 loading on HBEA zeolite in enhancing n-heptane isomerization was investigated. The multiple characterization techniques revealed the engagement of Pt and MoO3 on the lattice or defect structures of the HBEA framework. Furthermore, the occurrence of dealumination was confirmed by the existence of hydrated aluminium-molybdate, Al2-(MoO4)3. These findings proposed the presence of strong metal-support interactions. The n-heptane isomerization was comparatively investigated over Pt and MoO3 modified catalysts, and significantly higher rate of n-heptane conversion (0.059 μmol/g-cat m2 s) and rate of iso-heptane formation (0.026 μmol/g-cat m2 s) were achieved with Pt/MoO3-HBEA catalyst. The catalytic performance followed the order of Pt/MoO3-HBEA > MoO3-HBEA > Pt-HBEA > HBEA. It is anticipated that the results of in-situ FTIR studies of low-temperature hydrogen adsorption and n-heptane adsorption analyses help elucidate the isomerization mechanism and indicated there was a prerequisite for the presence of both Pt and MoO3 as dissociation and adsorption active sites for enhanced n-heptane isomerization. This study sheds light on the design of new type of MoO3 based catalysts for isomerization reaction.

N-Butane transformation on Zn/H-BEA. The effect of different Zn species (Zn2+ and ZnO) on the reaction performance

Using solid-state 1H and 13C MAS NMR spectroscopy, the performance of H-BEA zeolite, modified with either Zn2+ cations (Zn2+/H-BEA) or ZnO clusters (ZnO/H-BEA), has been investigated with respect to n-butane transformation by aromatization and hydrogenolysis pathways. 13C-labeled n-butane has been used to follow the main stages of n-butane transformation on both Zn2+/H-BEA and ZnO/H-BEA with 13C MAS NMR at 298–623 K. Similar surface species, including n-butylzinc, n-butene, allyl-like oligomers, are formed as the intermediates on both zeolites. The kinetics of n-butane transformation has been monitored with 1H MAS NMR in situ at 543–573 K. Kinetics modeling reveals that Zn2+/H-BEA is more active for n-butane transformation than ZnO/H-BEA. A remarkable difference in the rates and the pathways of hydrogenolysis for Zn2+/H-BEA and ZnO/H-BEA has also been established. Propane and methane are hydrogenolysis products on ZnO/H-BEA whereas ethane is produced by the reaction on Zn2+/H-BEA. 13C NMR data and the kinetics analysis provide an insight on the occurrence of joint methane and n-butane conversion on Zn-modified zeolites under non-oxidative conditions.