Framework Dealumination Research Articles

Cation location and migration in lanthanum-exchanged NaY zeolite

LaNH4Y and LaY zeolites were prepared using a double-exchange double-calcination method by La exchange of NaY zeolite. The distribution of the La cations was determined by powder X-ray diffraction with Rietveld refinement. The results indicate that the La cations are initially located in supercages, and then their hydration shells are stripped off and they migrate to small sodalite structures located at SI′ during heating and dehydration. The changes in the T–O–T angles show significant distortion of the flexible framework and instability in LaNH4Y obtained using a one-exchange one-calcination process. For La cations located at SI′ and coordinated with O3, the T–O3 bond distance increased, which indicates that the rare-earth cations not only restrain framework dealumination, suppressing condensation of the unit cell, but also have an effect on the T–O3 bond distance, increasing the unit cell volume. The role of the rare-earth species is to ensure the hydrothermal stability of the zeolite in order to control the acid site density and catalytic activity. The effects of La cations and NH4+ cations on the zeolite acidity were studied using infrared spectroscopy and NH3 temperature-programmed desorption, and the mechanism of rare-earth stabilization of the Y zeolite is described.

Effect of cerium loading on structure and morphology of modified Ce-USY zeolites

This work describes comprehensibly the effect of cerium loading on the structure and morphology of NH4USY zeolite. The Ce-USY (2-25 wt.% of CeO2) was obtained by wet impregnation of CeCl3 followed by calcination at 550ºC for 8 h. At low loadings (2-10%), cerium species are mainly located at ion exchange positions in the framework, whereas at higher loadings (15-25%), small aggregates were formed on the HUSY surface. X-ray diffractograms (XRD) exhibited only the reflections related to HUSY, demonstrating the high dispersion of cerium species, but Fourier transform Raman spectroscopy (FT-Raman) detected CeOx for the materials above 10%. Reaction of CeCl3 with NH4USY produced NH4Cl, which decomposed to form HCl, leading to framework dealumination. The materials showed an increased Lewis/Brønsted ratio with increasing cerium loadings due to the interaction between the excess cerium and the OH groups of USY, and the consequent formation of CeOx species.

Citric acid-assisted hydrothermal method for preparing NiW/USY–Al2O3 ultradeep hydrodesulfurization catalysts

This article presents a novel method for preparing NiW/USY–Al2O3 ultradeep hydrodesulfurization (HDS) catalysts via combined citric acid-assisted hydrothermal dispersion of active metals and hydrothermal modification of HY zeolite. The results showed that the citric acid-assisted hydrothermal method yielded monomeric W species as W precursors that were well dispersed by the interaction between citric acid and WO3 particles and were suitably stacked by the preferential interaction between citric acid and the support, guaranteeing a compromised dispersion and stacking of the supported Ni–W–S phases. The citric acid-assisted hydrothermal modification combined the framework dealumination of the HY zeolite and the removal of nonframework Al species, exposing more Brønsted acid sites on the catalyst. The finely tuned morphology of the Ni–W–S phases and the suitably adjusted Brønsted acidity of the zeolite endowed the resulting NiW/USY–Al2O3 catalyst with outstanding hydrogenation and hydrogenolysis activities for 4,6-dimethyldibenzothiophene and coking diesel HDS.

The Study Of Deactivation And Regeneration Of A Fluid Cracking Zeolite Catalysts

The catalytic activities of modified and unmodified sodium Y-Zeolites catalysts was investigated. Modification by framework dealumination and gallium oxide incorporation led to an increase in catalytic activity. The catalytic activities of both parent and the modified samples were reduced in a time dependent manner due to coke deposition. Incorporation of gallium oxide altered the reaction pathways of C8 intermediates from n-butane reactions, thus producing more aromatic products. However, Ga2O3 in excess of 1% resulted in a large decrease of catalytic activity due to loss in Bronsted acidity. As Bronsted sites are removed, direct protonation of the alkane became difficult and catalytic activity was lowered at high Ga2O3 loading. The oxygen adsorption isotherms of the various coked catalysts showed that the deactivation process involved blockage of the zeolite pore sites by the deposited coke. Interestingly, regeneration of the coked sample was seen to completely restore the catalytic activity of both samples.

A ZSM-5-based Catalyst for Efficient Production of Light Olefins and Aromatics from Fluidized-bed Naphtha Catalytic Cracking

A ZSM-5-based catalyst was prepared by spray-dry method for fluidized-bed naphtha catalytic cracking. Multi-techniques, such as X-ray diffraction, scanning electron microscope, 27Al MAS NMR, and NH3–TPD, were employed for the investigation of ZSM-5 framework stability, framework dealumination, and catalyst acidity variation in hydrothermal treatment. Catalytic performances of fluidized-bed naphtha catalytic cracking at 630–680 °C indicated that light olefins and other value-added products could be more efficiently produced compared with the commercial process of thermal steam cracking. Long-term catalytic evaluation implied that naphtha catalytic cracking over the catalyst prepared with spray-dry method and hydrothermal treatment can be carried out at a variable reaction condition with a relatively high and stable light olefins yield.

Gas-Phase Hydroxylation of Chlorobenzene Over Cu-HZSM-5 and Catalyst Deactivation

Abstract Catalytic activity and deactivation of the Cu-HZSM-5 catalyst used in the gas-phase hydroxylation of chlorobenzene to phenol were studied. Under the conditions of 773 K and water/chlorobenzene volume ratio of 3, a chlorobenzene conversion of 82.4% and a phenol selectivity of 98.2% were obtained. Deactivation of the Cu-HZSM-5 catalyst was observed during the reaction. X-ray diffraction, N2 volumetric adsorption, energy dispersive X-ray spectroscopy, and infrared spectroscopy of adsorbed pyridine were performed on fresh, used, and regenerated catalysts to study the reaction and catalyst deactivation mechanisms. The leaching of copper from the catalyst, framework dealumination of HZSM-5, and coking were the main causes of the catalyst deactivation.

Effect of dealumination severity on the pore size distribution of mordenite

Acid leaching with nitric acid was conducted for the framework dealumination of H-mordenite. The parent as well as the samples obtained after acid treatment were characterized by XRD, surface area, pore volume and pore size distribution for identifying structural changes after dealumination. For the parent (HM) and two acid leached (ATHM-1 and ATHM-3) samples studied, the surface area and micro pore volumes were increased in the order of HM < ATHM-1 < ATHM-3. Nitrogen adsorption studies at 77 K indicated that the increase in micro pore volume is not uniform for the pores of diameter 0–20 Å, where the entire range of micropores were classified into three groups, namely smaller micropores belong to side pockets, micropores belong to main channels, and the secondary micropores that are larger than main channels. The pore volume in small and larger micropores was increased, where as that of the main channels was decreased in the order of HM < ATHM-1 < ATHM-3, and the detailed analysis could provide a direct evidence for the opening of side pockets of mordenite by acid dealumination. The dealuminated samples ATHM-1 and ATHM-3 exhibited enhanced n-hexane isomerization activity.

Effect of temperature and time in the hydrothermal treatment of HY zeolite

Zeolite HY was steamed at 1 bar water pressure in steps of 100 K from 473 K to 1073 K in dependence of the time (0.5–20 h). Three routes of hydrothermal reactions can be outlined: At low temperature (473 K), a partly framework dissolution occurs accompanied by the formation of X-ray amorphous material. At intermediate temperatures (773–873 K), framework dealumination takes place with the occurrence of different non-framework species. Thereby, with increasing steaming time, saturation of the Si/Al ratio occurs on low levels (Si/Al ratios of about 4 and 5, respectively). At 973 K, a crossover from a low level saturation in Si/Al (6) towards higher values (11) is observed, which is accompanied by a partial amorphization of the zeolite with formation of aluminium oxide and silica gel and a significant decrease in crystal size as well. At high temperatures (1073 K), a fast thermal decomposition occurs. The IR spectra directly reveal an aluminosilicate phase as decomposition product of the framework. At 573 K, the zeolite framework remains unchanged.

SCR of NO with methane over Co-HBEA and PdCo-HBEA catalysts: The promoting effect of steaming over bimetallic catalyst

Ion-exchanged Co and PdCo-BEA zeolite-based catalysts were studied for the selective catalytic reduction (SCR) of NO with methane under oxygen excess. Bimetallic samples were submitted to several hydrothermal treatments (steaming) and tested under dry and wet conditions (2% H 2O). These hydrothermal treatments resulted in significant modifications in the amount and distribution of metal species, namely an increase of Co oxides and Co-oxo species, simultaneously with the preservation of Co 2+ isolated exchanged cations. H 2-TPR measurements detected, on these catalysts, a fraction of Co species more easily reducible, eventually interacting with Pd species and possibly related to dispersed nanosized oxide species, placed in more accessible positions. All the steamed catalysts exhibit a higher NO conversion into N 2 and an increased water resistance. The Co and/or Co-Pd species formed during steaming treatment, the balance between them and the framework dealumination are important issues for the high and stable catalyst activity under the presence of water vapor.

Studies on modification and structural ultra-stabilization of natural STI zeolite

A natural STI zeolite, discovered in China, was treated in hot HCl solution with various concentrations for inspecting its acid-resistant property. The framework stability and the adsorption of four types of H-STI zeolite, which were prepared by modification on the natural zeolite with different routes of ion-exchange and calcination, were investigated with X-ray diffraction (XRD), temperature-programmed heating (TPH), 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), in situ FT-IR, N 2 adsorption, X-ray fluorescence scattering spectroscopy (XRF), and energy dispersion X-ray analysis (EDX). The natural zeolite remains stable in hot HCl solution with the concentration ⩽2.0 mol dm −3. The framework compositions of Si and Al were determined and compared with both 29Si MAS NMR and EDX (or XRF) calibrated with 27Al MAS NMR. The modification with different routes leads to the variation in framework de-alumination and the distribution of the framework Si and Al atoms in the cell of the modified zeolites, and changes the contents of extra-framework aluminum (EFAL) and Si–OH defects, resulting in the modified zeolites with different thermal stabilities and adsorption property. A linear plot of Al atomic fraction in sum T (tetrahedral coordinative Si and Al) atoms vs the frequency of the internal asymmetric stretching vibration presents an IR spectra of the de-aluminated H-STI zeolite samples. High silica H-STI zeolites with the framework Si/Al ratios higher than 5.6, prepared with NH 4 + exchange and followed by calcination in fluid air, exhibit thermal stability up to 1273 K with open and perfect channel systems. Series H/M-STI (M = Li, Na, K, Ag, Ca) zeolite samples were prepared by ion-exchange from the high silica H-STI zeolites. The degree of ion-exchange and thermal stability were compared. The series zeolite samples calcined at 1173 K generally remain stable, among which H/Na and H/K form possess the highest thermal stability and H/Ag form shows the lowest thermal stability.

Catalytic degradation of expandable polystyrene waste (EPSW) over mordenite and modified mordenites

Abstract The performance of the several different modified mordenite zeolites in the degradation of expandable polystyrene waste (EPSW) was investigated in this study. The composition of the degradation products, especially the liquid fraction was compared to understand the role of H-mordenite and the effect of its modification through framework dealumination and metal impregnation. The degradation was carried out in a batch reactor at 360 and 400 °C. The dealuminated mordenite with SiO2/Al2O3 ratio of 86 showed enhanced styrene/styrene dimer and styrene/ethylbenzene molar ratios at lower (360 °C) and higher (400 °C) temperature, respectively. This was attributed to the intermediate acid strength distribution generated on removal of framework aluminum by acid leaching. Modification of H-mordenite by metal impregnation substantially changed the acidic and catalytic properties in polystyrene degradation. The 0.64 wt.% phosphorous loading on H-mordenite showed improved EPSW conversion, styrene yield and SM/EB molar ratio at 400 °C. The 0.32 wt.% phosphorous loading facilitated the improvement in SM/EB molar ratio with higher styrene selectivity than thermal degradation at 360 °C.

Low‐Silica [Me]AlMCM‐41 (Me: Na, K, Cs, Ca) Mesophases and Corresponding Mesoporous Materials.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Low-Silica [Me]AlMCM-41 (Me = Na, K, Cs, Ca) Mesophases and Corresponding Mesoporous Materials

An aluminosilicate MCM-41 with a highly ordered mesoscopic structure and Si/Al molar ratio of 1.25 has been synthesized in basic medium in the presence of sodium ions. Standard procedures to exchange Na+ for other cations in calcined MCM-41 led to a gradual collapse of the structure along with a partial dealumination. Various [Me]AlMCM-41, with Me = K+, Cs+, Ca2+, could be obtained by completely exchanging sodium ions prior to calcination, i.e., in the presence of surfactant molecules. These molecules prevent framework dealumination during exchange, while preserving the long-range mesoscopic ordering of the parent [Na]AlMCM-41. The [Me]AlMCM-41 thus obtained possessed all the properties of materials that would have been obtained directly in the presence of the corresponding cations. Moreover, structural and textural properties of the calcined products are very similar to those of the directly calcined [Na]AlMCM-41, suggesting that the nature of the charge compensating cation has little influence on the thermal stability.

Alkylation of biphenyl with t-butylalcohol over modified Y zeolites

The alkylation of biphenyl with t-butylalcohol (TBA) over HY and ultrastable Y zeolites produced by multistep temperature programmed (MSTP) steaming procedure is studied in a stirred batch reactor using cyclohexane as solvent. 4-t-Butylbiphenyl (4-TBB) was observed as the major product with all the HY zeolites studied. However, the selectivity to 4,4′-di-t-butylbiphenyl (4,4′-DTBB) increased with framework dealumination and latter it decreases with the severity of dealumination temperature. The various reaction parameters affecting the activity and selectivity of the products such as temperature, catalyst amount, molar ratio and reaction time were investigated. The isomerization of 4,4′-DTBB over steam-treated HY zeolites is also discussed.

Metakaolin as a reagent for the assembly of mesoporous aluminosilicates with hexagonal, cubic and wormhole framework structures from proto-faujasitic nanoclusters

Metakaolin, formed through the thermal de-hydroxylation of kaolin clay, has been shown to be an effective reagent for the preparation of faujasitic zeolite nanoclusters for framework incorporation into hexagonal, cubic and wormhole aluminosilicate mesophases with Si/Al ratios in the range 5.6–1.6. Single tetrahedral aluminum sites with 27Al NMR chemical shift of 60 ppm are observed for each mesostructure, which is indicative of the incorporation of the protozeolitic nanoclusters into the framework walls. Unlike aluminosilicate mesostructures assembled from conventional precursors, no framework de-alumination was observed for the mesostructures upon calcination at 550 °C. The results suggest that metakaolin is a promising low-cost precursor for the industrial scale production of high quality aluminosilicate mesophases.

Dealumination of ferrierite by ammonium hexafluorosilicate treatment: characterization and testing in the skeletal isomerization of n-butene

Ferrierite zeolite has been treated with ammonium hexafluorosilicate (AHFS). The prepared catalysts were characterized by XRD, atomic absorption spectroscopy, ammonia TPD, FT-IR, SEM, BET/pore size distribution and pyridine chemisorption. Their catalytic activity has been evaluated in the skeletal isomerization of n-butene. Ammonium hexafluorosilicate treatment of ferrierite zeolite leads to a little but progressive framework dealumination, although bulk Si/Al ratio determined by chemical analysis only increases slightly, indicating that most of the extracted aluminum atoms remain in the pores. Higher fluorosilicate amounts decrease crystallinity, micropore area, and volume. Ammonium hexafluorosilicate treatment remove mainly ferrierite external acid sites with a marked improvement in the number of medium strength acid sites. The higher dealumination degree the lower isobutene yield, however, the presence of extraframework aluminum species, medium strength acid sites and lower acid site density reduces the conversion of n-butene through consecutive reactions like n-butene dimerization followed by cracking and hydrogen transfer. These reactions are responsible for the formation of by-products and hence dealuminated catalysts show isobutene selectivity values as high as ferrierite zeolite.

Effect of crystal size on the durability of Co/HZSM-5 in selective catalytic reduction of NO by methane

The effect of crystal size on the durability of Co/HZSM-5 in the selective catalytic reduction (SCR) of NO with methane was studied. The durability of the catalysts subjected to hydrothermal treatment with a He stream containing 10% steam at 600 °C for 24 h. was investigated. After hydrothermal treatment, the decrease of catalytic activity for small crystal sizes of Co/HZSM-5(1.0 and 1.8 μm) was less than for large crystal sizes of Co/HZSM-5(5.6 and 7.6 μm). This infers that the durability of Co/HZSM-5 increased with a decrease in crystal size. Small crystal size catalyst showed a slight decrease in crystallinity and tetrahedral aluminum while the large crystal size catalyst lost crystallinity and tetrahedral aluminum. This indicated that the occurrence of framework dealumination was higher in the large crystal size catalyst.

Zeolite catalysts for Halon conversion

Al and Ni K-edge XANES and EXAFS experiments have been undertaken to characterise the chemical states of aluminium and nickel in NiZSM-5 catalysts used for the pyrolysis and hydrodehalogenation with methane of Halon 1301 (CF 3Br). It is shown that pyrolysis of Halon 1301 over NiZSM-5 involves both extra-framework aluminium fluoride or hydroxyfluoride species and aggregated nickel species, probably fluorided. Reaction products are CF 4 and C 2F 6. The reaction between Halon 1301 and methane over NiZSM-5 produces CHF 3 and CH 3Br as major, but not exclusive products. In the presence of CH 4 framework dealumination and aggregation of nickel is inhibited. Future prospects for utilising these and similar catalysts in reactions of other Halons and CFCs to produce useful fluorocarbon products as an alternative to destruction are briefly discussed.

FT-IR spectroscopic studies of hydrocarbon trapping in Ag+-ZSM-5 for gasoline engines under cold-start conditions

A detailed study of adsorption of automobile exhaust hydrocarbons in Ag+-exchanged zeolites under cold-start conditions (room temperature and in the presence of water) was carried out with FT-IR spectroscopy, using toluene and propylene as probes. The results show that exchanged Ag+ in zeolites is unique for trapping olefin and aromatic hydrocarbons due to its resistance to water adsorption. In contrast, exchanged Cu2+ in zeolites, which has good hydrocarbon trapping properties under dry conditions, does not trap hydrocarbons under wet conditions. Here, solvation of Cu2+ screens the interactions of the cation with adsorbed hydrocarbons. The results also show that, in addition to the nature of the cation, the structure of the zeolite also plays a role in hydrocarbon trapping. Aging at high temperatures, with water vapor (∼10%) and SO2 (∼15ppm), leads to a decrease of the trapping centers in the zeolites due to framework dealumination.

Effect of the degree and type of the dealumination method on the structural, compositional and acidic characteristics of H-ZSM-5 zeolites

A series of H-ZSM-5 zeolites with different framework Si/Al ratios were prepared by hydrothermal synthesis and post-synthesis dealumination by various methods, i.e., HCl, steaming, steaming/HCl and ammonium hexafluorosilicate (AHFS). The degree of framework dealumination and the amount/type of the extra-framework phases formed were greatly dependent on the method of dealumination and the severity of the experimental conditions. The hydrothermal treatment of a parent ZSM-5 sample with Si/Al∼27 was very effective and produced significant amounts of Si–Al extra-framework phases, while the reaction with HCl in relatively strongly acidic environments resulted in very low dealumination. Treatment with AHFS was proven to be an appropriate method for preparing moderately dealuminated H-ZSM-5 samples which are free from extra-framework Al. The relative crystallinity and the microporosity of all dealuminated H-ZSM-5 zeolites were retained to a high degree (>80%), even for the severely steamed samples (∼96% dealumination). The morphology of the crystals/particles was not changed significantly; however, each dealumination method had a different effect on the partial breakdown of the crystals and on the formation of extra-framework/amorphous phases with a mesoporous character. The number of acid sites that corresponded to the high-temperature desorption peak of the ammomia-temperature programmed desorption spectra was found to be equivalent to the framework Al (FAl) content of all H-ZSM-5 samples, irrespective of the degree of dealumination and the amount of extra-framework phases. Theses phases had a low capability of adsorbing ammonia compared to the FAl atoms and were the main source of acidity for the severely steamed samples which had almost no FAl.