Framework Dealumination Research Articles

Heteroatom‐Containing Zeolites as Solid Lewis Acid Catalysts for the Cycloaddition of CO2 to Epoxides

AbstractThe catalytic cycloaddition of CO2 to epoxides to produce valuable cyclic carbonates represents a simple and promising strategy for CO2 utilization, circumventing the ineffective CO2 reduction process. Despite current progresses, there remains an impending demand for highly‐active, cost‐effective and stable catalysts especially the ideal heterogeneous catalytic systems. Herein, we report the preparation of heteroatom‐containing zeolites through a two‐step process comprising of framework dealumination and subsequent heteroatom incorporation, and their catalytic applications in CO2 cycloaddition to epoxides. Characterization results reveal the successful incorporation of heteroatoms into framework to derive Lewis acidic M‐Beta zeolites (M = Ti, Zr or Hf). The as‐prepared M‐Beta Lewis acids show remarkable performance in the model reaction of CO2 cycloaddition to propylene oxide with the assistance of potassium iodide under solvent‐free conditions. The reaction parameters have been optimized employing Ti‐Beta catalyst and the substrate scope has been investigated. Finally, the impact of Lewis acidity on the cycloaddition reaction is discussed and the actual bifunctional Ti‐Beta/KI catalyst system is proposed, which is of important significance for the understanding of CO2 catalytic cycloaddition to epoxides.

Performance of Li exchange hierarchical X zeolite for CO2 adsorption and H2 separation

Li exchange hierarchical X zeolite (Li-H-X) was prepared by decationization of X zeolite by NH4Cl solution followed by Li exchange and subsequent calcination. NH4-X and Li-X were prepared to compare the results of Li-H-X. XRD, FE-SEM, EDS, N2 adsorption–desorption, and micro-/mesoporous volume were performed. The higher mesoporosity was confirmed in Li-H-X due to the framework dealumination during decationization. Static and dynamic sorption capacity of sorbents was evaluated to identify the performance of sorbent. The CO2 and H2 equilibrium adsorption capacity of Li-H-X was found to be 9.6 mmol g−1 and 0.78 mmol g−1, respectively, at 298 K and 20 bar, which was 25 % (CO2) and 30 % (H2) higher than Li-X. Static experimental data were validated using the Langmuir, Freundlich, and Sips models. The CO2 & H2 dynamic sorption capacity of Li-H-X sorbent for binary gas (CO2/H2, 25/75 %) was 4.145 mmol g−1 and 0.258 mmol g−1 at 303 K and 10 bar. The higher sorption capacity of Li-H-X was obtained due to large micro-/mesoporous volume of sorbent, which may allow to access unoccupied sites at higher pressure. This result reveals that micro-/mesoporous structure of zeolite adsorbs significantly higher CO2 from binary gas stream, which can use to separate pure H2 from gas stream.

Dealumination of Y zeolite through an economic and eco‐friendly defect‐engineering strategy

AbstractA sustainable defect‐engineering strategy for the dealumination of Y zeolite is described. This strategy includes the green synthesis of a well‐crystallized Y zeolite with point defects arising from the incorporation of Fe atoms by using Fe‐containing perlite and the subsequent preparation of ultra‐stable Y (USY) zeolite by efficient steaming dealumination. The systematic characterizations verify that Fe atoms originally existing in the perlite are incorporated into the as‐synthesized Y zeolite and function as point defects, leading to the distortion of framework Al. The step‐by‐step investigation of dealumination process shows that vacancies are formed by the extraction of framework Fe in ammonium exchange, and the framework dealumination is promoted under the combined effect of the distorted framework Al and the formed vacancies during steaming treatment. The resulting USY zeolite owns excellent features in (hydro)thermal stability, pore structure and acid property, and exhibits outstanding catalytic performance in the cracking of n‐octane and 1,3,5‐triisopropylbenzene.

The strong SDA/framework interactions and acidity study of high-silica LTA-type zeolites

The zeolite community spent 50 years in pursuing the synthesis of high-silica LTA-type zeolites, and ultimately succeeded with the pure-silica polymorph by applying specific structure-directing agents (SDA). The high-silica zeolite A has exhibited unique properties and strong potential to be applied as an adsorbent and catalyst in several novel processes. In the present study, the high-silica polymorphs of Si/Al ratios from 15 to ∞ were synthesized and subjected to thorough characterizations using multiple physicochemical methods. It has been revealed that there exists strong interactions between the organic SDAs and the framework. Therefore, removing the SDAs by calcination in air is difficult, and can only be done above 700 °C with a certain degree of framework dealumination. The retained framework structure shows connectivity defects and a unique combination of Brønsted and Lewis acidity. Using methanol-to-olefins (MTO) as a probe reaction, the catalytic performance of high-silica LTA-type zeolites has been evaluated. Compared to the well-studied CHA and MFI-type catalysts, high-silica zeolite A exhibited a shorter life-time and higher selectivity towards C4 olefins, while methane selectivity increased with the deactivation.

Multiple important roles of phosphorus modification on the ZSM-5 in ethanol to olefin reaction: Acidity adjustment, hydrothermal stability and anti-coking

The effects of phosphorus doping on the reaction of bioethanol to light olefins catalyzed by HZSM-5 were studied from both catalytic experiments and molecular simulations. P-doping promotes the highest yield of propylene up to 35.0 %. The propylene yield remained above 22.5 % even after 100 h. The introduction of P improves the catalyst acidity and the electronic environment of the active center of HZSM-5, which is beneficial to increase the selectivity of propylene, making the adsorption and conversion reaction of ethanol and the desorption of ethylene and propylene products more favorable. The framework aluminum is stabilized by the phosphate groups outside the framework, so that the framework dealumination is inhibited. The deep polymerization of ethylene is inhibited, and the anti-coking ability is enhanced. The presence of water enhances the hydrothermal stability and inhibit the carbon deposition of P-modified ZSM-5 catalyst, which makes the P-doping ZSM-5 the most promising catalyst for bioethanol conversion applications.

Comparative catalytic study on butene/isobutane alkylation over LaX and CeX zeolites: The influence of calcination atmosphere

Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the CeIV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the CeIV content. This i s believed to be due to the water formed during the oxidation of CeIII, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce3+ was found more benefit for the hydride transfer compared with La3+, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

Ethanol to Aromatics on Modified H‐ZSM‐5 Part I: Interdependent Dealumination Actions

AbstractIn this study a H‐ZSM‐5 zeolite (Si/Al=11) was modified by a stepwise treatment with steam, sodium hydroxide and hydrochloric acid. During progressing dealumination by various treatment methods, the drawbacks of initial post‐synthetic steps, e. g. pore‐filling by steam dealumination, are compensated by subsequent steps, e. g. washing by acid, which leads to a scientifically based preparation of required ZSM‐5 zeolite. Solid‐state properties of as‐synthesized and modified zeolites are determined by structural (XRD, ICP‐OES, NMR), textural (physisorption, laser scattering) and acid sites analysis (TPAD). Consequently, extended dealumination without structural damage is demonstrated. Its origin by framework dealumination and pore cleaning is verified in ethanol to hydrocarbon process (ETH) by shape‐selective formation of coke and aromatics, characterized by “aromatics index” (AI).

Optimizing zeolite stabilized Pt-Zn catalysts for propane dehydrogenation

Zeolite stabilized Pt-Zn catalyst has been developed for propane direct dehydrogenation with propane conversion close to the equilibrium and a very low deactivation rate of 0.0018h −1 at 823 K. Propane dehydrogenation (PDH) provides an alternative route to non-petroleum based propylene and eligible catalysts with good overall performance are still being explored. Herein, we report the construction of zeolite stabilized Pt-Zn catalysts Pt-Zn/Si-Beta for PDH. Characterization results from transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) and Fourier transform infrared (FTIR) spectroscopy reveal that highly-dispersed Zn species are stabilized by the silanols from zeolite framework dealumination, which then act as the anchoring sites for Pt species. The close contact between Pt-Zn species and the electronic interaction thereof make Pt-Zn/Si-Beta robust PDH catalysts. Under optimized conditions, a high propylene production rate of 4.11 m o l m o l P t - 1 s - 1 , high propylene selectivity of 98% and a sustainable deactivation rate of ~ 0.02 h −1 can be simultaneously achieved at 823 K. Coke deposition is not the key reason for the catalytic deactivation, while the loss of Zn species and the resulting aggregation of Pt species under high temperatures are responsible for the irreversible deactivation of Pt-Zn/Si-Beta catalyst in PDH reaction.

Improved performance of mesoporous HZSM-5 designed with selective deactivation of external surface acidity in the isomerization of styrene oxide

Abstract A series of mesoporous HZSM-5 designed by phosphorous modification with or without hexadecyltrimethylammonium bromide (CTAB) pretreatment were prepared and tested in the isomerization of styrene oxide. CTAB pretreatment changed the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Bronsted acid sites caused by a framework dealumination during calcinations. The phosphate species on the external surface could not migrate into the micropores. There were only a few phosphate species formed in the micropores and most of the internal Bronsted acid sites were preserved. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous HZSM-5 was achieved by phosphorous modification with CTAB pretreatment, thus providing a potential industrial application catalyst which remarkably improves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).

Control of methane chlorination with molecular chlorine gas using zeolite catalysts: Effects of Si/Al ratio and framework type

Abstract CH4 chlorination with Cl2 gas is used for the production of chlorinated products via C H bond activation in CH4. Due to the high reactivity of Cl2, this reaction can occur spontaneously under UV irradiation or with mild thermal energy even in the absence of a catalyst via a free radical-mediated chain reaction mechanism that undesirably causes excessive chlorination of the CH4 and is thus non-selective. In this work, CH4 chlorination is investigated using HY and MFI zeolites with various Si/Al ratios, by which the reaction is catalytically controlled for selective production of the mono-chlorinated product (CH3Cl). Depending on the framework type, Si/Al ratio of the zeolites, and reaction conditions, different degrees of CH4 conversion, CH3Cl selectivity, and hence CH3Cl yield were achieved, by which systematic relationships between the catalyst properties and performance were discovered. A high aluminum content facilitated the production of CH3Cl with up to ∼20 % yield at a high gas hourly space velocity of 2400 cm3gcat−1 h−1 with a CH4/Cl2 ratio of 1 at 350 °C. HY zeolites generally furnished a slightly higher CH3Cl yield than MFI zeolites, which can be attributed to the larger micropores of the HY zeolites that support facile molecular diffusion. With various flow rates and ratios of CH4 and Cl2, the CH4 conversion and CH3Cl selectivity changed simultaneously, with a trade-off relationship. Unfortunately, all zeolite catalysts suffered from framework dealumination due to the HCl produced during the reaction, but it was less pronounced for the zeolites having a low aluminum content. The results shed light on the detailed roles of zeolites as solid-acid catalysts in enhancing CH3Cl production during electrophilic CH4 chlorination.

Ethylene Conversion into Propylene and Aromatics on HZSM-5: Insights on Reaction Routes and Water Influence

Ethanol is an alternative for producing petrochemicals, especially propylene and aromatics (benzene, toluene, and xylenes). To understand ethanol processing routes into olefins and aromatics, it is interesting to use ethylene that is the major primary product of ethanol reaction into hydrocarbons and the intermediate for the formation of olefins and aromatics. In this work, the influence of the operating conditions (ethylene partial pressure, reaction temperature and contact time) in the ethylene conversion into propylene and aromatics, and in the product yield was investigated using HZSM-5 zeolite as catalyst. Lower contact time and ethylene partial pressure, and higher reaction temperature favored propylene yield. Olefin production was based on the formation of carbene species from ethylene that reacts with ethylene to produce propylene and on ethylene dimerization to form butenes. On the other hand, intermediate reaction temperatures and contact times, and higher ethylene partial pressure promote the formation of aromatics, where the dehydrocyclization reaction is favored over hydrogen transfer. The presence of water vapor in long-term reactions deactivated the catalyst. For propylene production, the decrease of ethylene conversion was due to zeolite framework dealumination, while for aromatic formation the reaction mechanism was changed.

Fluoride-modified ZSM-5 for endothermic catalytic cracking of n-decane

Endothermic catalytic cracking of hydrocarbon fuel is a promising technique for active thermal protection of advanced aircrafts. Catalyst with high activity and stability is crucial for obtaining high heat-absorbing capability. Here, a series of fluoride-modified ZSM-5 catalysts (FZ) were prepared and characterized with XRD, SEM, 27Al and 29Si NMR, Ar physisorption, NH3-TPD, XRF and Py-IR. Significant changes in porosity and acidity were revealed on the fluoride-modified samples owing to framework dealumination and fluoride residue. Catalytic and endothermic performances were also investigated in supercritical cracking of n-decane. Good performances with obviously enhanced n-decane conversion and heat sink, and decreased coke deposition were obtained via appropriate fluoride modification.

Thin-felt hollow-B-ZSM-5/SS-fiber catalyst for methanol-to-propylene: Toward remarkable stability improvement from mesoporosity-dependent diffusion enhancement

A hollow-B-ZSM-5/SS-fiber catalyst is developed through alkali leaching of the full-silica core of the silicalite-1@B-ZSM-5 in situ structured onto a thin-felt stainless-steel fiber (20 μm SS-fiber), which is synthesized by a seed-assisted dry-gel vapor-phase transport method. As-obtained catalyst shows marked improvement in the mesoporosity-dependent diffusion (by o-xylene diffusion measurement). Boron incorporation is essential for preventing the framework dealumination during alkali leaching treatment due to the increased framework negative charges. The hollow-B-ZSM-5/SS-fiber catalyst shows remarkable stability improvement in the methanol-to-propylene (MTP) reaction because of the enhanced diffusion of the nano-hollow-structure and the boron-incorporation stabilized framework aluminum. A boron-free hollow-ZSM-5/SS-fiber is also obtainable with the textural properties comparable to the hollow-B-ZSM-5/SS-fiber but shows undesired degeneration of the tetra-coordinated aluminum. The preferential generation of coke in the micropores of the hollow-ZSM-5/SS-fiber causes a rapid deactivation even compared to the parent silicalite-1@ZSM-5/SS-fiber, due to the existence of extra-framework aluminum.

The influence of phosphorus on the catalytic properties, durability, sulfur resistance and kinetics of Cu-SSZ-13 for NOx reduction by NH3-SCR

The influence of phosphorus on the catalytic properties, durability, sulfur resistance and kinetics of Cu-SSZ-13 for NOx reduction by ammonia selective catalytic reduction (NH3-SCR) were systematically investigated to obtain a deeper understanding of the deactivation of Cu-SSZ-13 in the presence of phosphorus-containing impurities in diesel exhaust. An enhanced effect on NO conversion at low phosphorus loading was found above 450 ℃, probably due to the reduction of copper oxides that promote NH3 oxidation at high temperatures. As the phosphorus loading reached 0.4 mmol/gcatal, the framework dealumination and decrease of the BET specific surface area, acidity, and isolated Cu2+ ions accounted for the decrease in activity and hydrothermal stability over the entire temperature range. Simultaneously, the activity of phosphorus-impregnated samples tested in the presence of SO2 indicates that phosphorus accelerates the deactivation of Cu-SSZ-13 in the presence of SO2 at low temperatures, primarily due to the severe decrease of the Cu2+ ion content and partial coverage of the acid sites and active sites by phosphate and sulfate species. Additionally, kinetic analysis further demonstrated that phosphorus has an inhibitory effect on the SCR reaction rates at low temperatures and high phosphorus addition changes the NH3-SCR reaction kinetic parameters. The data presented herein provide a comprehensive picture of the interaction between phosphorus and Cu-SSZ-13.

Influence of Zirconium Modified USY on Coupled Hydrogenation and Ring Opening of Tetralin Over NiW/USY + Al2O3

There is increasing interest in modification USY in hydrotreating catalysts to attain high activity and targeted product selectivity. Zirconium modification on USY (Zr-USY) was designed in this study. Zirconium highly dispersed, and greatly enriched on the surface of USY zeolites. Zirconium modification slightly induced framework dealumination. Introduction of zirconium increased total acid sites of USY zeolites, probably ascribed to Zr4+ polarizing H2O. The corresponding catalysts NiW/Al2O3 + Zr-USY-1 (40.6 wt%) and NiW/Al2O3 + Zr-USY-2 (41.4 wt%) showed higher selective ring opening products yield compared to NiW/Al2O3 + USY (31.7 wt%). The enhanced ring opening products yield may be due to sufficient cooperation between WS2 and acidity. The results provide further insight into USY modification and its role on hydrotreating catalysts.

High performance of H3BO3 modified USY and equilibrium catalyst with tailored acid sites in catalytic cracking

Increasing the cracking ability of catalyst by adjusting its surface acid property is of great significance for oil refining industry. The effects of boric acid on the structure and surface acidity of ultra stabilized Y (USY) and FCC equilibrium catalyst during the modification were investigated. The structural and surface properties of the prepared samples were characterized by low-temperature N2 sorption, XRD, 27Al and 29Si MAS NMR, and FTIR using pyridine probe molecule. Modified USY and equilibrium catalyst possessed reduced Lewis acid sites. The Brönsted/Lewis acid ratios of USY increased from 0.64 to 1.56 after modification, at the same time, the microporous surface areas increased, due to the nonframework and framework dealumination during the modification. The catalytic cracking tests were performed on a bench-scale micro reactor at 500 °C and atmospheric pressure, using n-dodecane as a modeling reactant and vacuum gas oil as conventional feedstock. The results showed that modified samples with high Brönsted acid sites had better cracking abilities and increased hydrogen transfer activity than raw samples.

Enhanced catalytic properties of mesoporous mordenite for benzylation of benzene with benzyl alcohol

Zeolite mordenite has been treated with nitric acid at different severities so as to facilitate the framework dealumination and optimization of the textural properties such as acidity and porosity. The samples obtained have been characterized by X-ray diffraction, FTIR, SEM, TEM, surface area, porosity by N2 adsorption and ammonia TPD. The resultant samples have been evaluated towards the bulky alkylation reaction of benzylation of benzene with benzyl alcohol. The studies indicated the improvement in the textural properties such as surface area, pore volume and acidity of the samples after the acid treatment. While, the phenomenon of enhancement in properties was exhibited by all the acid treated mordenite samples, the highest improvement in properties was observed at a particular condition of acid treatment (SM-2 sample). This particular sample also exhibited highest acidity and the presence of ∼10nm size pores that resulted in the effective catalytic activity towards the bulky alkylation reaction of benzene with benzyl alcohol to produce high yields of di-phenyl methane.

Preparation and characterization of hierarchical USY by post-treatment

Two modifying agents, citric acid and EDTA-2Na, were used to modify USY zeolite to obtain the hierarchical USY with high crystallinity. XRD, N2 isothermal sorption, 27Al and 29Si NMR, and TEM were adopted to characterize the dealumination process in the post-treatment of USY. The results showed that all modified USY have increased surface areas and pore volume due to the removal of non-framework Al species. Besides, framework dealumination occurred in the modification process. USY-EC, which was modified by the combined effects of citric acid and EDTA-2Na, has the highest SiO2/Al2O3 ratio of 19.40 and relatively high crystallinity.

Interaction of coupled titanium and phosphorous on USY to tune hydrodesulfurization of 4,6-DMDBT and FCC LCO over NiW catalyst

There is growing interest in modifying USY in hydrotreating catalysts to obtain higher activity. This study provides a preliminary investigation of the interaction of titanium-phosphorus modification on USY (TiPY) and their influence on NiW-based catalyst containing USY and hydrodesulfurization (HDS) performances of 4,6-DMDBT (4,6-dimethyldibenzothiophene) and FCC LCO (fluid catalytic cracking diesel oil). Single titanium (TiY) and single phosphorus (PY) modified USY are studied simultaneously to provide further insight into how the interaction influence performance. The interaction between titanium and phosphorus on USY significantly enriched surface aluminum, and promoted the acidity and framework dealumination compared to TiY, PY and parent USY. NiW-based catalyst containing TiPY (NiW/TiPYA) had higher WS2 stacking numbers and more acidity than the catalysts containing TiY or PY. The higher HDS performance was observed with NiW/TiPYA due to the synergistic effect of WS2 stacking numbers and acidity among the catalysts, where the conversion of 4,6-DMDBT and sulfur in FCC LCO was 74.3% and 98.3%, respectively. The results can provide further insight into USY modification and its influence on hydrotreating catalysts.

Pilot-scale validation of Co-ZSM-5 catalyst performance in the catalytic upgrading of biomass pyrolysis vapours

The main objective of the present work was the evaluation of commercial ZSM-5 catalysts (diluted with a silica–alumina matrix) in the in situ upgrading of lignocellulosic biomass pyrolysis vapours and the validation of their bench-scale reactor performance in a pilot scale circulating fluidized bed (CFB) pyrolysis reactor. The ZSM-5 based catalysts were tested both fresh and at the equilibrium state, and were further promoted with cobalt (Co, 5% wt%) using conventional wet impregnation techniques. All the tested catalysts had a significant effect on product yields and bio-oil composition, both at bench-scale and pilot scale experiments, producing less bio-oil but of better quality. Incorporation of Co exhibited no additional effect on water or coke production induced by ZSM-5, compared to non-catalytic fast pyrolysis. On the other hand, Co addition significantly increased the formation of CO2 compared to the CO increase which was favored by the use of ZSM-5 alone. These changes in CO2/CO yields are indicative of the different decarbonylation/decarboxylation mechanism that applies for Co3O4 compared to ZSM-5 zeolite, due to the differences in their acidic properties (mainly type of acid sites). Co-promoted ZSM-5 catalysts simultaneously enhanced the production of aromatics and phenols with a more pronounced performance in the pilot-scale experiments resulting in the formation of a three phase bio-oil, rather than the usual two phase catalytic pyrolysis oil (aqueous and organic phases). The third phase produced is even lighter than the aqueous phase and consists mainly of aromatic hydrocarbons and phenolic compounds. Addition of Co in ZSM-5 is thus suggested to strongly enhance aromatization reactions that result in selectivity increase towards aromatics in the bio-oil produced. Possible routes of catalyst deactivation in the pilot plant's continuous operation process have been suggested and are related to pore blocking and masking of acid sites by formed coke (reversible deactivation), partial framework dealumination of the fresh zeolitic catalyst, and accumulative ash deposition on the catalyst that depends on the nature of biomass (content of ash).