Copper-exchanged Zeolites Research Articles

Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature.

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper-containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper-exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di- or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.

C[sbnd]H bond arylation of anilides inside copper-exchanged zeolites

Syntheses of fine-chemicals using heterogeneous catalysts have tremendous industrial potentials, yet CH functionalization studies have been largely focused on homogeneous catalysis. We report here the first meta-selective CH bond arylation of anilides inside copper-exchanged zeolites. Mid- or large-pore zeolite frameworks are selected as supports to access large organic molecules, and atomically distributed copper catalysts exhibit high activities (84–90% conversions) toward direct arylation of anilides with diphenyliodonium salt on 0.5mol% copper concentration. Computational studies indicate the well-fitted copper-aryl complexes inside zeolite frameworks. Electron micrographs, elemental analyses, and reusability study show no observable leaching of catalytically active copper species during the reactions tested. These results demonstrate the practical synthetic potential of copper-exchanged zeolites as promising supported molecular catalysts to afford biaryl motifs-containing compounds with high catalytic activity, chemical stability, and recyclability.

Decontamination of chlorine gas by organic amine modified copper-exchanged zeolite

Removal of chlorine gas (Cl2) from air is of critical requirement in order to address point-source emissions possibly during a terrorist attack or an industrial accident resulting in Cl2 contamination of the atmosphere. In this work, copper (Cu) exchanged zeolite Y (CuY) was functionalised with triethylenediamine (TEDA) and the capacity to remove Cl2 was evaluated. The materials were characterised by nitrogen (N2) adsorption–desorption studies, Fourier Transform Infrared (FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The materials' ability to remove Cl2 was investigated via a dynamic breakthrough test. Copper exchanged zeolite displayed a low adsorption of Cl2 in spite of its large surface area. However, Cl2 removal greatly improved following functionalisation with TEDA. XPS analysis revealed that Cl2 was removed via a catalytic hydrolysis reaction where adsorbed water vapour transformed Cl2 into Cl− which could be further trapped in the zeolite structural framework. Moisture could increase the Cl2 removal capacity, but the competition for adsorption between water and chlorine molecules was also observed. The spent adsorbent after exposure to Cl2 could be easily recycled with an excessive water vapour treatment. The reusability was also investigated and the adsorbent could be used for more than five times. This material can potentially be used in air filters. It may provide an efficient way for decontaminating Cl2 during a terrorist attack or an industrial accident.

Catalytic Removal of Volatile Organic Compounds

The degradation of air quality by the release of volatile organic compounds (VOCs) into the air particularly harms human health and our environment. [...]

A theoretical study of nitric oxide adsorption and dissociation on copper-exchanged zeolites SSZ-13 and SAPO-34: the impact of framework acid-base properties.

The adsorption of nitric oxide as dinitrosyls and the deNOx proton-mediated reaction mechanism are assessed using electronic structure methods and transition state theory. Dinitrosyls bind to copper cations either via a N-atom or via an O-atom, with N-binding being more stable. In their ground states, dinitrosyls reach a planar configuration with the metal cation. The two nitric oxide molecules are kept together in O-bonded dinitrosyls by the N-N bond and the adsorption complex obtains a cyclic planar structure, while N-bonded dinitrosyls have out-of-plane conformations with low energy barriers. An asymmetric structure ZCu(ON)(NO) with one N-bonded nitrosyl and the other O-bonded is of the lowest stability. The cyclic hyponitrite ZCu(ON)2 adsorption complex undergoes O-N bond breaking upon protonation of one oxygen atom and this lowers the energy barrier of the first reaction step of nitric oxide dissociation to yield N2O and a hydroxylated copper site ZCu(OH) by 45 kJ mol(-1) for Cu-SAPO-34 and by 46 kJ mol(-1) for Cu-SSZ-13. The more stable N-bonded dinitrosyl ZCu(NO)2 provides less favorable reaction which passes through the asymmetric ZCu(ON)(NO) intermediate structure. Brønsted acid sites facilitate the reversal of one nitrosyl group. The role of proton transfer from a Brønsted acid site to dinitrosyls is not limited to the initial step of facilitating the N-O bond cleavage, but it also contributes to the stabilization of intermediate oxygen species formed at the copper site as hydroxide ZCu(OH) and hydroperoxide, ZCuOOH. Without protonation, the unstable ZCuO intermediate causes structural deformation with strongly lengthened T-O bonds in the framework. The rate determining step is N2O decomposition to N2 and O2, whether starting with a ZCu(NO)2 or a ZCu(ON)2 adsorption complex, and Cu-SSZ-13 has a clear advantage with an energy barrier of 195 kJ mol(-1)vs. 265 kJ mol(-1) for Cu-SAPO-34. In the final step the Brønsted acid site is restored by proton transfer from the hydroperoxide ZCuOOH to the framework and molecular oxygen is released. The overall energy barrier for the proton-assisted reaction of ZCu(ON)2 decomposition for Cu-SSZ-13 is by 48 kJ mol(-1) lower than the barrier of the proton-free pathway.

Hydrothermal aging effects on Cu-zeolite NH3-SCR catalyst

After-treatment systems for diesel vehicles face a challenge for upcoming emission regulations. Regarding the level of nitrogen oxides (NOx) and soot required, the global catalytic activity of the exhaust line needs to be improved. Hydrothermal aging conditions explored during particulate filter regeneration are very stringent for the deNOx catalyst and can strongly reduce its durability. Ammonia selective catalytic reduction (NH3-SCR) systems mainly based on copper or iron exchanged zeolite catalysts have emerged as effective technologies to reduce NOx emissions with their durability being continuously improved. In this work, we evaluated the impact of different hydrothermal treatments on a commercial copper-exchanged zeolite catalyst. The goal was to link the catalytic material property changes with catalytic performance changes as a function of both aging duration and temperature. The evolution of the zeolite structure as well as the changes of the copper state were experimentally investigated. Based on a detailed material properties analysis of Cu-zeolite, a semi-detailed kinetic model of has been built to quantitatively predict adsorption and desorption of NH3 as a function of hydrothermal aging conditions. This work is a first step to bridge the gap between lab analysis conclusions and global reactivity observed in real conditions.

Encapsulation of copper(II) complexes with three dentate NO$_{2}$ ligands derived from 2,6-pyridinedicarboxylic acid in NaY zeolite

2,6-Pyridinedicarboxylic acid (H$_{2}$dipic) reacts with copper-exchanged zeolite NaY to form [Cu(dipic)(H$_{2}$O)$_{2}$]$_{n}$, which is encapsulated in the pores of the zeolite. In this zeolite-encapsulated form, the copper derivative functions as an efficient catalyst for the oxidation of cyclohexene, toluene, cyclohexane, and ethyl benzene in the presence of hydrogen peroxide (as an oxidant). The catalyst was readily recovered from the reaction mixture, and it could be reused for an additional three runs without perceptible loss of activity. The heterogeneous catalyst exhibited considerably higher activity and selectivity compared with [Cu(dipic)(H$_{2}$O)$_{2}$]$_{n}$ itself.

Solid-State Ion-Exchange of Copper into Zeolites Facilitated by Ammonia at Low Temperature

The effect of the gas phase during solid-state ion-exchange of copper into zeolites was studied by exposing physical mixtures of copper oxides (CuI2O and CuIIO) and zeolites (MFI, *BEA, and CHA) to various combinations of NO, NH3, O2, and H2O. It is shown that heating these mixtures to 250 °C results in active catalysts for the selective catalytic reduction of NO with NH3 (NH3-SCR), indicating that the Cu has become mobile at that temperature. Such treatment allows for a fast (<5–10 h) preparation of copper-exchanged zeolites. Scanning transmission electron microscopy analysis of Cu-CHA prepared using this method shows homogeneous distribution of the Cu in the primary particles of the zeolite. In situ XRD reveals that the Cu ion-exchange is related to the formation of CuI2O. When the zeolite is mixed with CuIIO, addition of NO to the NH3-containing gas phase enhances the formation of CuI2O and the Cu ion-exchange. The mobility of Cu at low temperatures is proposed to be related to the formation of [CuI(NH3)x]+ (x ≥ 2) complexes.

Removal of arsenic (III) and arsenic (V) using copper exchange zeolite‐a

In this work, modification of zeolite‐A was performed by ion exchange method using copper salts and the copper exchanged zeolite (CEZ) was used for removal of Arsenic (III) and Arsenic (V) from water. The adsorption capacity of zeolite‐A was significantly improved after modification by copper. CEZ removes more than 98% of both As (III) and As (V) from monocomponent solutions. The material was characterized by powder X‐ray diffraction (XRD) and scanning electron microscopy (SEM). XRD pattern shows loss in crystalline structure after copper incorporation which is evident from the decrease in intensity of characteristic peaks of zeolite‐A. SEM images shows that cubical morphology characteristic of zeolite‐A has been distorted after copper incorporation. Various isotherm and kinetic models were used to determine adsorption and kinetic parameters and to delineate the probable mechanism of adsorption. The data revealed that adsorption of arsenic on CEZ followed Langmuir model with maximum sorption capacity of 1.37 and 1.48 mg/g at 30°C for As (III) and As (V), respectively. CEZ effectively removes arsenic from water at wide pH range of 3–10 and in the presence of interfering ions namely phosphate, sulfate, nitrate, carbonate, and bicarbonate which is evident from tap water study. CEZ appears to be a promising adsorbent for removing arsenic from water. The water quality after treatment with CEZ also confirms that it is safe for drinking purpose and within the permissible limits as per World Health Organization guideline values and Indian drinking water standards. © 2014 American Institute of Chemical Engineers Environ Prog, 33: 1274–1282, 2014

Influence of lattice stability on hydrothermal deactivation of Cu-ZSM-5 and Cu-IM-5 zeolites for selective catalytic reduction of NOx by NH3

Copper-exchanged zeolites are well-known catalysts for the selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR). To determine the influence of framework stability on catalyst deactivation, two zeolite frameworks, MFI and IMF, were used in this study. The two frameworks have similar window size and connectivities, but the IMF structure is less susceptible towards dealumination. In each zeolite, copper was introduced by aqueous exchange and the catalytic performance in the NH3-SCR reaction compared before and after hydrothermal ageing at 650 and 750°C. The changes in state and local environment of Cu and the degradation of the zeolite structure were characterized using ammonia capacity measurements, solid state nuclear magnetic resonance, X-ray fine structure spectroscopy, temperature programmed reduction with hydrogen, infrared spectroscopy monitoring of adsorbed NO and CO probe molecules as well as the combination of transmission electron microscopy and energy dispersive X-ray spectroscopy to follow copper migration. The catalytic performance of Cu-ZSM-5 and Cu-IM-5 is similar in the fresh state, but after hydrothermal ageing the deactivation of Cu-IM-5 is less severe compared to Cu-ZSM-5 as a consequence of the higher framework stability. The changes in catalyst structure that occur during ageing are (i) partial dealumination of the zeolite, (ii) reversible migration of copper species, and (iii) irreversible formation of catalytically inactive and stable Cu–Al clusters, which have some resemblance to CuAl2O4, but without the symmetry of Cu in the spinel structure. As the Cu–Al clusters only form once Al is detached from the framework, the stability of Al in the zeolite framework is proposed to dictate the overall hydrothermal deactivation behaviour of Cu-ZSM-5 and Cu-IM-5 in the NH3-SCR reaction.

What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?

Recently, the outstanding properties of Cu-SSZ-13 (a zeolite in the chabazite structure) for the selective catalytic reduction of nitrous oxides were discovered. However, the true nature of the active site is still not answered satisfactorily. In this work, we identify the active site for the given reaction from first-principles simulations of the total energy of Cu(II) ions in various positions in combination with previously published catalytic activity as a function of the copper exchange level. This attribution is confirmed by the simulation of vibrational properties of CO adsorbed to the reduced Cu(I) species. The relation between energetic considerations, vibrational calculations, and experiment allows a clear statement about the distribution of active sites in the catalyst. We furthermore discuss the structural properties of the active site leading to the high stability under reaction conditions over a large temperature range. The insights from this work allow a more targeted catalyst design and represent a step toward an industrial application of copper-exchanged zeolites in cleaning car exhaust gases.

CuH-ZSM-5 as Hydrocarbon Trap under Cold Start Conditions

Cold start tests are carried out to evaluate the performance of copper-exchanged zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the role of copper in the performance of these zeolites. It is concluded that the partial substitution of the protons in the parent H-ZSM-5 zeolite is highly beneficial for hydrocarbon trapping due to the formation of selective adsorption sites with specific affinity for the different exhaust components. However, it is also observed that uncontrolled exchanging process conditions could lead to the presence of CuO nanoparticles in the zeolite surface, which seem to block the pore structure of the zeolite, decreasing the hydrocarbon trap efficiency. Among all the zeolites studied, the results point out that a CuH-ZSM-5 with a partial substitution of extra-framework protons by copper cations and without any detectable surface CuO nanoparticles is the zeolite that showed the best performance under simulated cold start conditions due to both the high stability and the hydrocarbon retaining capacity of this sample during the consecutive cycles.

Advantages of a wire gauze structured reactor with a zeolite (Cu-USY) catalyst for NH3-SCR of NOx

Metal wire gauzes as catalyst supports and structured reactor internals offer a number of advantages over monolithic reactors. Structured catalytic reactors based on wire gauzes are explored in this study for the control of NOx emissions from a stationary engine fuelled with gas generated from a biomass gasification process. Simulations are performed on reactors filled with (a) wire gauzes, (b) multi-channel monoliths, and (c) a packed bed with 2mm beads, for the NH3-SCR process using the kinetic data obtained for copper-exchanged ultra-stabilized zeolite Y catalyst (Cu-USY). The purpose of this procedure is to select the most efficient reactor internals and to assess the necessary reactor length allowing maximum conversion of NOx. These simulations show that optimum performance is obtained when the wire gauze support is employed. The results also show that when the Cu-USY catalyst (very active in SCR) is used in combination with a wire gauze structure, the combined effects of higher rates of reaction with higher rates of heat and mass transfer may significantly increase the NOx conversion.

Copper exchanged ultrastable zeolite Y – A catalyst for NH3-SCR of NOx from stationary biogas engines

NOx emissions will need to be controlled from gas engines, which in the future could be using a syngas/producer gas as a fuel (generated from the gasification of biomass). In anticipation of this need, the activity of a copper-exchanged ultrastabilized zeolite Y was investigated, for the selective catalytic reduction of NOx. The catalyst performance and properties were compared with reference samples of Cu-exchanged un-stabilized Y and Cu exchanged ZSM-5 zeolites. Activity was related to the number and nature of active centers, which were determined by in situ FTIR and TPD experiments using CO, NO, and NH3 as probe molecules. In the test reaction the ultrastabilized Cu-USY catalyst showed high activity at low temperatures and offered high hydrothermal resistance, whilst the Cu-ZSM-5 catalyst demonstrated the highest overall activity. The observed low temperature activity of the ultrastabilized Cu-USY catalyst was linked to the high abundance of Cu+ sites, which were detected by in situ FTIR analyses using CO adsorption.

Copper-Silver Bimetallic System on Natural Clinoptilolite: Thermal Reduction of Cu2+ and Ag+ Exchanged

Copper-silver bimetallic system supported on natural clinoptilolite from Tasajeras deposit (Cuba) was studied. Bimetallic samples were prepared by simultaneous ion exchange, and reduced in a wide temperature range in a hydrogen flow. The main goal of the work was analysis of the mutual influence of both metals on their reduction process and the properties of the resultant particles. Analysis was done by combined use of XRD and UV-Vis spectroscopy. The reduction of Cu2+ and Ag+ cations shows existence of notable inter-influence between both cations during this process. The Cu2+ reduction is favored by the presence of Ag+, which should be related with the synergetic influence of silver cations and/or clusters formed on the first stages of reduction on Cu(2+)-framework interaction, facilitating the Cu2+ reduction even at low temperature (25 and 50 degrees C). The aggregation of the reduced highly dispersed species both for copper and silver is limited in this bimetallic system. The introduction of Ag+ as the second cation in the copper-exchanged zeolites favors the copper reduction at lower temperatures (25 and 50 degrees C), and appears to be the efficient tool for the control of the size of the resultant reduced nanoparticles (it means their dispersion).

On the enantioselectivity of aziridination of styrene catalysed by copper triflate and copper-exchanged zeolite Y: consequences of the phase behaviour of enantiomeric mixtures of N-arene-sulfonyl-2-phenylaziridines

By synthesising S-2-phenyl-N-(4-nitrophenyl)aziridine from S-phenylglycinol, it has been demonstrated that the aziridination of styrene by [N-(4-nitrobenzenesulfonyl)imino]phenyliodinane (nosyliminophenyliodinane, PhINNs) in the presence of S,S-2,2'-isopropylidene-bis(4-phenyl-2-oxazoline), catalysed by copper(II) triflate in CH(3)CN solution or heterogeneously by CuHY, has predominantly an R-configuration. The enantioselectivity of the aziridination of styrene by [N-arenesulfonylimino]-phenyliodinanes catalysed by copper-exchanged zeolite Y (CuHY), in conjunction with a chiral bis-oxazoline ligand, has been re-examined. In the case of PhINNs, it is shown that the product mixture of enantiomeric aziridines, on treatment with hexane, gives rise to a solid phase of low enantiomeric excess (ee) and a solution phase of high ee. Separation of the solid phase and recrystallisation afforded a true racemate (racemic compound), which has been confirmed by X-ray crystallography. The aziridine obtained from the solution phase could be recrystallised to produce the pure enantiomer originally in excess. A consequence of the new findings is that previous reports on the enantioselectivity of copper-catalysed aziridination, both in heterogeneous and homogeneous conditions, should be regarded with caution if the analytical procedure involved HPLC with injection of the enantiomeric mixture in a hexane-rich solvent. Such a method has been used in previous work from this laboratory, but has also been used elsewhere, following the procedure developed by Evans and co-workers when the (homogeneous) copper-catalysed aziridination by PhINTs was first discovered. Evidently, the change of substituent in the benzenesulfonyl group reduces the solubility in hexane, affording a solution phase of enhanced ee.

The ammonia selective catalytic reduction activity of copper-exchanged small-pore zeolites

The NH3-SCR activity of the small-pore zeolites, Cu-SSZ-13, Cu-SSZ-16, and Cu-SAPO-34, is investigated using a high-throughput reactor system. These copper exchanged small-pore zeolites have high SCR activity between 150 and 500°C and are shown to be much more hydrothermally stable than the medium-pore zeolite, Cu-ZSM-5. The degree of copper exchange, the dimensionality of the framework, and heteroatom framework substitution all impact the SCR activity and hydrothermal stability of the materials. Of the small-pore zeolites tested, Cu-SSZ-13 and Cu-SAPO-34 display superior SCR performance, both before and after high-temperature hydrothermal treatment.

Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD

Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of atoms, copper locations a...

DFT Calculations of EPR Parameters for Copper(II)-Exchanged Zeolites Using Cluster Models

The coordination environment of Cu(II) in hydrated copper-exchanged zeolites was explored through the use of density functional theory (DFT) calculations of EPR parameters. Extensive experimental EPR data are available in the literature for hydrated copper-exchanged zeolites. The copper complex in hydrated copper-exchanged zeolites was previously proposed to be [Cu(H(2)O)(5)OH](+) based on empirical trends in tetragonal model complex EPR data. In this study, calculated EPR parameters for the previously proposed copper complex, [Cu(H(2)O)(5)OH](+), were compared to model complexes in which Cu(II) was coordinated to small silicate or aluminosilicate clusters as a first approximation of the impact of the zeolitic environment on the copper complex. Interpretation of the results suggests that Cu(II) is coordinated or closely associated with framework oxygen atoms within the zeolite structure. Additionally, it is proposed that the EPR parameters are dependent on the Si/Al ratio of the parent zeolite.

Density Functional Theory Investigation of EPR Parameters for Tetragonal Cu(II) Model Complexes with Oxygen Ligands

Density functional theory (DFT) calculations of the electron paramagnetic resonance (EPR) parameters for a series of tetragonal Cu(II) model complexes were conducted. Model complexes containing four oxygen atoms directly coordinated to a Cu(II) metal center were chosen because of their importance in the Peisach-Blumberg truth tables and their frequent use in the interpretation of EPR spectra of Cu(II) proteins and copper-containing catalysts. Molecular g- and copper A-tensors were calculated using the BP86 and B3LYP density functionals. The DFT calculations reproduce the experimentally observed trends in the parallel components of the A- and g-tensors. Important insight into the structural basis for the empirical trends in g( parallel) and A( parallel) was obtained from the DFT calculations. Notably, g( parallel) systematically increases and A( parallel) systematically decreases with increasing Cu-O equatorial bond length. These results have been used to provide structural insight into copper EPR data for copper-exchanged zeolites.