Protonic Zeolites Research Articles

Carbon dioxide and nitrogen adsorption on porous copolymers of divinylbenzene and acrylic acid

Porous copolymers of divinylbenzene (DVB) and acrylic acid (AA) having DVB:AA ratios of 6:4, 8:2 and 9:1 were prepared following a distillation-precipitation method, using toluene as the porogenic agent. The materials thus obtained, which showed specific surface area in the range of 380–600 m2 g−1 and pore volume in the range of 0.14–0.18 cm3 g−1, were investigated as possible adsorbents for CO2 capture from the flue gas of coal-fired power stations. For that purpose, the isosteric heat of adsorption (and CO2 adsorption capacity) was analysed from N2 and CO2 adsorption equilibrium isotherms obtained over a temperature range. For CO2, qst resulted to be in the range of 27–31 kJ mol−1 (the highest value corresponding to the 6:4 sample), while for N2 a value of qst ≈ 12 kJ mol−1 was obtained. Equilibrium adsorption capacity for CO2 (at ambient temperature and pressure) showed the value of about 1.35 mmol g−1. These results are discussed in the broader context of corresponding literature data for CO2 capture using protonic zeolites.

Deactivation of H-zeolites during catalytic oxidation of trichloroethylene

This paper reports the stability of protonic zeolites (H-ZSM-5, H-MOR and H-BEA) in the catalytic oxidation of trichloroethylene (TCE), focused on determining the causes of the deactivation. The study was conducted by relating the evolution of TCE conversion (with and without water vapor) with the physicochemical properties of the zeolites as a function of time on stream (TOS). The oxidation reactions were carried out in a fixed-bed reactor under 1000ppmv of TCE in air. We found out that both coke formation and chlorine attack were the causes of H-zeolites’ rapid deactivation. Coke was formed from the TCE or the surface intermediates already formed before the TCE oxidation slower step. Chlorine atoms of TCE also caused irreversible deactivation by attacking to Brönsted acid sites, which lead to structural changes. Under wet conditions, H-zeolites were strongly hydrated restoring hydroxyl groups to keep TCE conversion invariably overtime.

Determination of the maximum retention of cobalt by ion exchange in h-zeolites

This work aimed to determine the maximum content of cobalt that can be incorporated by ion exchange in zeolites H-USY, H-Beta, H-Mordenite, and H-ZSM-5. To reach this goal, batch isotherms at 75ºC were constructed after addition of zeolite samples in flasks filled with cobalt nitrate solution. The equilibrium data were fitted to Langmuir, Freundlich, and Tóth adsorption isotherm models. Langmuir was the best model for zeolites H-Beta, H-Mordenite, and H-ZSM-5, whereas experimental data for H-USY were better fitted to the Freundlich isotherm model. From the isotherms, it was possible to determine the maximum cobalt exchange level (q max) that can be incorporated in each zeolite through ion exchange. In this sense, H-USY presented the highest q max value (2.40 meq/g zeol), while H-ZSM-5 showed the lowest one (0.64 meq/g zeol). These results also show the influence of the zeolite framework related to the channel system, pore opening, presence of cavities and secondary porosity and SiO2/Al2O3 ratio (SAR) on the maximum capacity and behavior of cobalt ion exchange in protonic zeolites.

Metal-Exchanged β Zeolites as Catalysts for the Conversion of Acetone to Hydrocarbons.

Various metal-β zeolites have been synthesized under similar ion-exchange conditions. During the exchange process, the nature and acid strength of the used cations modified the composition and textural properties as well as the Brönsted and Lewis acidity of the final materials. Zeolites exchanged with divalent cations showed a clear decrease of their surface Brönsted acidity and an increase of their Lewis acidity. All materials were active as catalysts for the transformation of acetone into hydrocarbons. Although the protonic zeolite was the most active in the acetone conversion (96.8% conversion), the metal-exchanged zeolites showed varied selectivities towards different products of the reaction. In particular, we found the Cu-β to have a considerable selectivity towards the production of isobutene from acetone (over 31% yield compared to 7.5% of the protonic zeolite). We propose different reactions mechanisms in order to explain the final product distributions.

Nanosized Platinum-Loaded H-ZSM-5 Zeolite Catalysts for n-Hexane Hydroconversion

A series of nanosized platinum-containing catalysts was successfully loaded on/in zeolite H-ZSM-5 via exchanging the zeolitic proton with platinum from Pt tetramine dichloride complex. Another series of Pt/H-ZSM-5 catalysts was prepared via wet impregnation of H2PtCl6 solution for comparison. The latter series was found to produce lower Pt dispersion. Pt dispersion was determined by H2 chemisorption. Catalyst characterization via ammonia temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and transmission electron microscopy (TEM) was examined for all catalysts and showed large differences in particle sizes. The data on n-hexane reactions of the catalysts of both series confirmed the formation of Pt nanoparticles in the exchanged catalysts. The relatively lower density and strength of acid sites acquired by Pt-exchanged catalysts contributed to this difference; stronger acid sites in the impregnated catalysts are in favor of hydrocracking reactions, which inhibit isomerization selectivity.

Carbon monoxide, dinitrogen and carbon dioxide adsorption on zeolite H-Beta: IR spectroscopic and thermodynamic studies

Variable temperature IR spectroscopy (VTIR) was used to investigate the adsorption thermodynamics of carbon monoxide, dinitrogen and carbon dioxide on the protonic zeolite H-Beta. Interaction of the adsorbed gases with the zeolite Brønsted acid sites was found to involve an enthalpy change of −27, −19 and −33 kJ mol −1 for CO, N 2 and CO 2, respectively; the corresponding entropy change was −150, −140 and −146 J mol −1 K −1. The adsorbed gases showed also a weak interaction with silanols, which involves a ΔH 0 value in the approximate range of −7 to −10 kJ mol −1. These results were discussed in the context of gas separation and carbon capture and sequestration (CCS) using zeolites.

Reaction mechanism of tert-butylation of phenol with tert-butyl alcohol over H-β zeolite: An ONIOM study

Tert-butylation of phenol with tert-butyl alcohol (TBA) over H-β zeolite was studied using the ONIOM approach with two proposed reaction paths: stepwise and concerted mechanisms. The results obtained by the ONIOM2 (B3LYP/6-31G(d,p):UFF) method reveals that the tert-butylation of phenol preferentially occurs through a co-adsorbed, concerted mechanism without prior dehydration of tert-butyl alcohol, rather than via a stepwise mechanism through dehydration to form tert-butyl carbenium ion as the first step followed by tert-butyl cation attack on the 2- or 4-position on phenol. The kinetic difference between 2- and 4-tert-butylation is more apparent in the concerted path, where 4-tert-butylation proceeds over a lower activation barrier. Decreasing the acid site strength, via substitution of Ga for Al, reduces the H-bonding interaction between the zeolitic proton and tert-butyl alcohol and increases the apparent activation barriers, which slows the overall reaction, and also lowers the selectivity to 4-tert-butyl phenol.

The role of silver species on the hydrothermal stability of zeolite catalysts

Permanent deactivation of acidic zeolite catalysts due to dealumination of the zeolite framework occurs during the regeneration under steaming at high temperature. The stability of zeolite catalysts modified with silver species is investigated. It is found that the stability of the zeolite framework can be improved during steaming by Ag ion-exchange. Silver species can prevent their correlated Al sites in zeolite catalysts from dealuminizing because of the replacement of protonic cations with Ag cations under steaming in air, and the effects of silver species decrease when the steaming treatment is carried out in nitrogen or 100% steam. Moreover, unlike phosphorus or rare earth cations, Ag modified zeolite catalysts can provide the same acid strength as protonic zeolite catalysts during the reaction after the reduction of Ag cations with hydrogen or hydrocarbons. This alternative stabilization method is expected to have serious implications for making stable zeolite catalysts in many applications.

Zeolite Proton Conducting Membrane for Micro Fuel Cell Applications

Supported and self-supporting ZSM-5 membranes (Si/Al = 25) were prepared, assembled and tested for hydrogen fuel cell performance. A Grotthus-like mechanism is proposed for the proton conduction in hydrated zeolites, where the protons hop between neighbouring aluminium sites along a water bridge in the zeolite pores. The supported ZSM-5 membrane was prepared by seeding and secondary regrowth method on cellulose paper, whereas the self-supporting zeolite membrane consists of an array of micromembranes on silicon substrate. The self-supporting HZSM-5 membrane had an open-circuit voltage (OCV) of 0.77 V compared to 0.90 V for supported HZSM-5 and 0.98 V for Nafion 117. The differences are attributed to the contact between the electrode, catalyst and membrane layers in the assembly. The self-supporting HZSM-5 yielded a maximum power density (MPD) of 19.4 mW cm−2 compared to 14.4 mW cm−2 for supported HZSM-5 and 35 mW cm−2 for Nafion 117.

Theoretical Study of Zeolite-Catalyzed Dimethoxymethane Carbonylation to Methyl Methoxyacetate

The gas-phase carbonylation of dimethoxymethane (DMM) to form methyl methoxyacetate (MMAc) can be catalyzed by acid zeolites. This reaction is a critical step in the synthesis of monoethylene glycol (MEG), a widely used chemical, from synthesis gas. The mechanism of DMM carbonylation occurring on H−MFI and H−FAU zeolites has been investigated using density functional theory. We find that the reaction involves three steps: initiation via reaction of zeolite protons with DMM to form methoxymethoxy species, carbonylation of the resulting species, and subsequent methoxylation of the resulting acyl species. Both the carbonylation and methoxylation processes proceed via carbocationic transition states that are stabilized by the framework O atoms of the zeolite. The activation barriers for carbonylation are similar in both zeolites, but the barriers for methoxylation differ significantly. Energy decomposition analysis indicates that a combination of the pore size and of the flexibility of the reactive species gi...

ChemInform Abstract: High-Temperature Interaction of Solid Cu Chlorides and Cu Oxides in Mixtures with H-Forms of ZSM-5 and Y Zeolites.

A high-temperature solid-state ion exchange between separate phases of solid Cu chlorides and hydrogen forms of ZSM-5 and Y zeolites occurs above 570 K and reaches substantial degrees at 770 and 670 K, respectively, i.e. at temperatures at which considerable self-dehydroxylation of the particular zeolite does not take place. The process has been followed using IR, EPR and MS techniques through the evolution of hydrochloric acid into the gaseous phase, the consumption of the bridging OH groups in the zeolite as well as by the formation of isolated Cu ions that exhibit strong electron-accepting properties. The exchange rate is very fast in the initial stage, being proportional to the amount of copper salt in the mixture, and levels off to zero after several hours. Owing to the more difficult deaggregation of Cu oxides compared to Cu chlorides, the interaction of Cu oxides with H-ZSM-5 and H-Y at 770 K and 670 K, respectively, results in a very limited consumption of the bridging OH groups as well as in a decreased formation of isolated Cu ions and electron-acceptor sites compared with the corresponding Cu chloride mixtures. Besides the ability of the copper phases (chlorides or oxides) to deaggregate, a further striking factor affecting the exchange reaction is the mobility of the zeolite protons and exchanged cations, which is controlled by the structure type of the zeolite and by the temperature. This is clearly reflected by the positive effect of the temperature on the exchange process and the higher degree of exchange (at a given temperature) found for H-Y zeolites compared with H-ZSM-5.

ChemInform Abstract: Variable Temperature 1H MAS NMR: A Powerful Tool for the Investigation of Dynamic Properties of Acidic Protons in Zeolites and Heteropoly Compounds

The dynamic nature of the protons in zeolites such as H-ZSM-5, metallosilicates molecular sieves and heteropoly compounds was examined by temperature dependence of 1 H MAS NMR in the range of 298 and 473K. The temperature-dependent lineshape of 1 H MAS NMR spectra of acidic protons in zeolites showed the first and unambiguous evidence for proton migration in zeolite structure. The dynamic property of protons was a more sensitive measure for the activity of protons than the static parameters such as NMR chemical shifi and I R band position of OH bonds. The temperature-dependent 1 H MAS NMR also revealed the mobility of protons in heteropoly compounds (H 3 PW I2 O 40 Ag 3 PW 12 O 40 and palladium metal loaded on H 3 PW 12 O 40 ). This method gave the information of the dynamical process between protons and hydrogen molecules in the gas phase. The variable temperature 1 H MAS NMR is a very useful technique for understanding dynamic properties of protons in solid acids The dynamic property is an important factor infiuencing the catalytic activity of a solid acid catalyst. The various experimental facts described above show that the temperature variable 1 H MAS NMR could reveal the dynamics of protons in solid acids. Information obtained by this technique offers new insights, which could not be obtained by other means, on the acidic properties of solid acids and their catalysis. The significance of proton mobility in catalysts will be able to be discussed in the future.

Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study

A series of H-ZSM-5 crystallites with different framework Si/Al ratios was studied by analyzing the kinetics and reaction mechanism of the oligomerization of 4-fluorostyrene as molecular probe reaction for Brønsted acidity. The formation of carbocationic species was followed by UV-Vis spectroscopy. Three carbocationic products were observed, namely a cyclic dimer, a conjugated linear dimer and a larger, more conjugated carbocation. Rate constants for the formation of all three products show a maximum at a Si/Al ratio of 25. Oligomerization of 4-fluorostyrene within the larger supercages of zeolite H-Y leads solely to cyclic dimers. The experimental observations were rationalized by DFT calculations, which show that the selectivity of the styrene oligomerization is controlled by the steric properties of the intrazeolite micropore voids. Two reaction pathways were considered for the formation of the conjugated linear carbocation. The conventional mechanism involves a hydride transfer between two dimeric hydrocarbons (HCs) in the zeolite pores. We propose an alternative monomolecular path, in which the hydride transfer takes place between a hydrogen atom of a dimeric HC and a zeolitic proton, yielding a conjugated carbocation and molecular H(2). Computed free energies indicate that the preference for a particular reaction mechanism is determined by the local shape of the zeolite micropores.

A theoretical study on the adsorption of an all-metal aromatic molecule Na 2Al 4 on MCM-22 zeolite

In light of the excellent adsorption of zeolites, we firstly propose by means of density functional theory calculations that all-metal aromatic molecules like Na 2Al 4 could be effectively adsorbed into the supercage of MCM-22, which provides a potential way to stabilize the Na 2Al 4 molecule. Our calculations also show that the adsorption of Na 2Al 4 would induce the jumping of the protons in zeolite to other oxygen atoms in the vicinity of Al. Comparing the local mobility of protons before and after the Na 2Al 4-adsorption, the proton migration barrier is significantly reduced from 45.9 to 29.0 kJ/mol. Therefore, the adsorption of the all-metal aromatic molecule Na 2Al 4 can effectively increase the local mobility of proton and improve the acidity of zeolite. Thus, our work provides a new way to improve the acidity of zeolites and the ability of proton transferring by adsorption of guest molecule Na 2Al 4 on zeolites.

Topological Descriptor for Oxygens in Zeolites. Analysis of Ring Counting in Tetracoordinated Nets

Topology of zeolite nets has been studied in terms of rings. A freely available software tool for the calculation and systematic analysis of the rings in zeolite nets is presented. Ring indices, associated to T (tetrahedral) and O atoms, are defined as a new topological concept. Ring count is also proposed as a topological descriptor of a structure which gives a fingerprint of the rings (number and size) in a zeotype. These topological concepts are linked with chemical descriptors and used together to systematize and characterize useful information on zeolites such as ring occurrence and TOT angles between different structures. Ring indices of O-sites can be employed to characterize the location of protons in acid zeolites giving a structured information on the ring environment of each active site.

Mobility of acidic protons in zeolites: A neutron diffraction study of d-heulandite

Neutron Rietveld refinement of a natural heulandite (Si/Al = 3.1) in its deuterium form was performed in space group C2/ m. Two Brønsted acid sites were identified. One was on framework oxygen O1, occupied to 20% and headed toward the center of the 8-membered ring channel running parallel to [1 0 2]; the other was on O6, occupied to 30% and headed toward the 10-membered ring channel running parallel to the c-axis. Three other extraframework sites, located around a distance of 3 Å from the framework oxygens, were attributed to reabsorbed H 2O molecules. On the whole, about 3.2 Brønsted acid sites were located representing about 37% of the value expected on the basis of the aluminium content. This discrepancy is attributed to proton transfer from the Brønsted site to reabsorbed H 2O molecules, forming either a hydroxonium ion (H 3O +) or charged clusters such as (H 5O 2) + and (H 7O 3) +.

Vibrational Analysis of Pyridine Adsorption on the Brønsted Acid Sites of Zeolites Based on Density Functional Cluster Calculations

Adsorption of pyridine on Bronsted acid sites of protonic zeolites has been studied using density functional calculations within the cluster approach. Two types of cluster models consisting of 3 (3T: 2Si, Al) and 12 ring (12T: 8Si, 4Al) tetrahedral atoms (with Si/Al ratio of 2) with dangling bonds saturated by either hydrogen atoms or hydroxyl groups have been considered to represent the zeolite acid sites. The B3LYP functional with the 6-31+G* basis set are used to calculate the geometrical structures, adsorption energies, and vibrational frequencies of the complexes formed upon interaction of pyridine on the zeolite clusters. The minimum energy pathways for proton transfer from the Bronsted acid site of zeolite (ZOH) to pyridine molecule are characterized. For all clusters considered, the ion-pair complexes (PYH+···OZ−) are the most stable, while the stability of the hydrogen-bonded complexes (PY···HOZ) depends on the size and nature of the terminating groups of the clusters. Since the proton-transfer b...

Stability of protonic zeolites in the catalytic oxidation of chlorinated VOCs (1,2-dichloroethane)

The deactivation of protonic zeolites in the catalytic oxidation of 1,2-dichloroethane (DCA) was evaluated. DCA oxidation reactions were carried out in a conventional fixed-bed reactor at atmospheric pressure under conditions of lean DCA concentration in air (1000ppm). The outlet composition was analysed by a gas chromatograph, an IR spectroscopy-based analyser and another UV analyser. The effect of the zeolite crystalline structure was examined in order to track the catalytic stability of H-ZSM-5, H-MOR and H-BEA under typical reaction environment and conditions (1000ppm DCA, 300°C, 13,500h−1). With the aim of a better understanding of the deactivation pathway, the influence of the space velocity and temperature on the durability of protonic zeolites was analysed. Since some products formed during reaction could also cause deactivation, H2O and HCl were introduced in the feed stream along with the DCA itself, so as to evaluate their effect. In general terms, coke formation was concluded to be the main reason for zeolite catalyst deactivation. Coke was formed from the intermediate vinyl chloride (VC), which resulted from a first dehydrochlorination step of DCA.

Characterization and catalytic performance of ruthenium sulfide catalysts supported on H-BEA, Na– and Cs–H-BEA zeolites

A series of Ru-X-H-BEA (X = H+, Na+, Cs+) catalysts sulfided at high temperature in H2S (15%)/N2 was characterized and tested in thiophene hydrodesulfurization (HDS) to analyze the effect of zeolite acidity and nature of the alkaline cation on the acid properties, stability of the sulfided phase and HDS catalytic activity. It is shown that after exchange of half of the zeolite protons by alkaline cations (Cs+, Na+) the acid properties of the zeolite are influenced by the nature of the cation suggesting the presence of a remote effect of the alkaline cation over the remaining sites and/or the disappearance of the most acidic sites. The activity of the catalysts and the product distribution in the HDS of thiophene are also influenced by the nature of the exchanged cation. Temperature-programmed reduction of the sulfided catalysts before and after thiophene HDS showed that a sulfided phase with low S/Ru ratio is obtained inside the zeolite channels.

Removal and recovery of nitriles from gaseous streams: An IR study of acetonitrile adsorption on and desorption from inorganic solids

The adsorption and the desorption of acetonitrile over several different inorganic solids (such as silicas, aluminas, modified aluminas, silica-aluminas, metal oxides, protonic zeolites, alkali-metal zeolites, and clays) have been investigated by IR spectroscopy. Different kinds of interactions (weak to strong hydrogen-bonding, coordination on Lewis acid sites, different kinds of surface chemical reactions) have been observed. The temperature needed for acetonitrile desorption from these solids has also been determined. Silicas and alkali metal zeolites have been found to allow molecular adsorption without reactivity and to allow desorption in mild conditions.