Cu-exchanged Zeolite Research Articles

Ethylene adsorption onto natural and transition metal modified Chilean zeolite: An operando DRIFTS approach

Abstract Ethylene is a plant growth regulator that induces accelerated softening and ripening of fruits during transport and storage. Among the most applied methods for ethylene control, adsorption appears as a cheap and efficient technique. In this work, the effect of the incorporation of transition metals into natural Chilean zeolite on ethylene adsorption is investigated. Natural zeolite mainly composed of clinoptilotite and mordenite is modified using copper and zinc nitrate solutions and calcined under oxygen flow at 623 K, generating different transition metal modified zeolites. Parent and modified zeolites were characterised by X-ray diffraction, X-ray fluorescence spectroscopy and nitrogen adsorption. Zeolite surface modifications were assessed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Experimental results reveal the incorporation of Cu2+ and Zn2+ as new compensating cations into the zeolite framework. Ethylene adsorption isotherms show an enhancement on the adsorption capacity of Cu-exchanged zeolite. This result is not only associated to the higher micropore surface area of this sample, but also to the higher affinity of ethylene molecules to copper cations incorporated on this zeolite. DRIFTS operando experiments of ethylene adsorption in the absence and in the presence of moisture reveal a competitive mechanism of water and ethylene molecules toward hydroxyl sites. Si-OH-Al and Si-OH sites are rapidly occupied with water, reducing the adsorption of ethylene.

Reversible and irreversible deactivation of Cu-CHA NH3-SCRcatalysts by SO2 and SO3

Sulfur oxides are a common source for the deactivation of Cu-exchanged CHA zeolite based catalysts used for NOx reduction in diesel exhausts by selective catalytic reduction with NH3 (NH3-SCR). Since water and possible formation of SO3 affect the deactivation of Cu-CHA catalysts, the deactivation in the presence of SO2 or a mixture of SO2 and SO3 was studied by measuring the SCR activity in wet and dry gas at 200 and 550 °C. The estimated S-content in the catalysts before and after 4 h regeneration at 550 °C in NO, NH3, O2 and H2O was related to the deactivation. The deactivation can be divided into two parts: a reversible deactivation that is restored by the regeneration treatment, and an irreversible part. The irreversible deactivation does not affect the activation energy for NH3-SCR and display a 1:1 correlation with the S-content, consistent with deactivation by Cu-sulfate formation. The reversible deactivation results in a lower activation energy and a deactivation that is larger than expected from the S-content. The presence of SO3 at 200 °C leads to higher reversible and irreversible deactivation, but has no significant impact at 550 °C. Furthermore, the irreversible deactivation is always higher when exposed at 200 °C than at 550 °C, and in wet conditions, compared to a dry feed. The deactivation is predominantly reversible, making regeneration at 550 °C a realistic approach to handle S-poisoning in exhaust systems.

Cu-exchanged Al-rich SSZ-13 zeolite from organotemplate-free synthesis as NH3-SCR catalyst: Effects of Na+ ions on the activity and hydrothermal stability

The relatively low activity at lower temperatures and high cost of SSZ-13 zeolite from organotemplate synthesis are two of major problems of presently commercialized Cu-SSZ-13 catalysts for NH3-SCR reaction. Cu-exchanged Al-rich SSZ-13 catalysts with Si/Al=4 from organotemplate-free synthesis have been prepared, and show superior NH3-SCR performance with NO conversions above 85% at wide-temperature window ranging from 150 to 650°C. Cu-Na-SSZ-13 catalysts with varied amount of residual Na+ were prepared by partial ion-exchange of as-prepared Al-rich Na-SSZ-13, and it’s found that Cu-Na-SSZ-13 catalyst with moderate Na+ content can improve both the low-temperature activity and its hydrothermal stability. 27Al and 1H MAS NMR spectra indicate that residual Na+ can stabilize the framework Al in Al-rich SSZ-13 zeolite. UV-Vis-NIR and H2-TPR spectra demonstrate that Na+ co-cations enhance the reducibility of Cu2+, whereas too much amount of Na+ may result in the formation of CuOx during the hydrothermal treatment, and further decrease the hydrothermal stability of Cu-Na-SSZ-13 catalyst. The optimized organotemplate-free Cu-Na-SSZ-13 with 2.7wt% Cu and 1.7wt% Na exhibits almost the same NO conversions as the commercial high-silica Cu-SSZ-13 catalyst at 150–550°C after 750 and 800°C hydrothermal aging, and shows promising practical applications.

Roles of Zeolite Confinement and Cu–O–Cu Angle on the Direct Conversion of Methane to Methanol by [Cu2(μ-O)]2+-Exchanged AEI, CHA, AFX, and MFI Zeolites

Recent interest in Cu-exchanged zeolite catalysts for methane hydroxylation has been broadened to small-pore Cu-zeolites such as Cu-SSZ-13 (Cu-CHA), Cu-SSZ-16 (Cu-AFX), and Cu-SSZ-39 (Cu-AEI), which were reported to produce more methanol per copper atom than the medium-pore Cu-ZSM-5 (Cu-MFI) and large-pore Cu-mordenite (Cu-MOR) zeolites do. To elucidate the nature of such fascinating catalytic activities, theoretical investigations based on density functional theory (DFT) were performed on the direct conversion of methane to methanol by [Cu2(μ-O)]2+-exchanged AEI, CHA, AFX, and MFI zeolites in periodic systems. DFT computational results show that the important activation energies for C–H bond dissociation by [Cu2(μ-O)]2+-AEI, -CHA, and -AFX zeolites are lower than those for [Cu2(μ-O)]2+-MFI zeolite. Moreover, the rate-determining methanol desorption and N2O decomposition by [2Cu]2+-AEI zeolite are also found to require low barriers, which renders [Cu2(μ-O)]2+-AEI zeolite highly active for the direct conve...

Efficient removal of arsenic using graphene-zeolite based composites.

Removing heavy metals from water can be considered an important problem of global magnitude due to their toxic and carcinogenic properties. The main aim of this presentation was to synthesize different composites of graphene (graphene oxide (GO) or reduced graphene oxide (RGO is referred to reduced graphene oxide, which was obtained using solvothermal method)) with Cu-exchanged zeolite A (Cu-ZEA) and nanoparticles of magnetite (Fe3O4) (including Fe3O4/RGO/, GO/Cu-ZEA, Fe3O4/RGO/Cu-ZEA and Fe3O4/GO/Cu-ZEA) for improving the properties of the individual components of mentioned composite and eventually investigate the composites' efficiency in arsenic adsorption. Among prepared composites, Fe3O4/RGO/Cu-ZEA composite had the highest efficiency in removing arsenic according to atomic absorption spectroscopy (AAS) results due to the high specific surface area, which was provided by the presence of Cu-ZEA and Fe3O4 in the structure of composite. Moreover, the adsorption kinetic investigation revealed that the adsorption of arsenate from aqueous suspension of Fe3O4/RGO/Cu-ZEA and Fe3O4/RGO composites followed a pseudo-second-order kinetic model.

Developing a Thermodynamic Model for the Interactions between Water and Cu in the Zeolite SSZ-13

The Cu-exchanged zeolite SSZ-13 is an efficient catalyst in the selective catalytic reduction of nitrous oxides in the presence of ammonia, and understanding the nature of the active sites under realistic conditions is important in rationalizing its activity. Especially the interactions between Cu and water have drawn significant amounts of attention in recent years. In this work we develop a thermodynamic model for the water coordination of Cu based on static calculations and present occupational probabilities and phase diagrams for different Al distributions at different temperatures and water pressures. We find that only at high temperatures and low pressures is the bare Cu cation the most stable species and that the cation is solvated at lower temperatures and higher pressures. In the following we compare our results to experimental and theoretical work in this field and find good agreement. This work shows that it is possible to construct an accurate thermodynamic model for water interactions with Cu...

Study of the antimicrobial and antifouling properties of different oxide surfaces.

Membrane separation processes find applications in an array of fields as they use far less energy and chemical agents than competing processes. However, a major drawback of membrane technology is that biofilm formation alters membrane performances. Preventing biofilm formation is thus a pivotal challenge for larger-scale development of membrane processes. Here, we studied the comparative antibacterial activities of different inorganic membranes (ceramic and zeolite-coated ceramic with or without copper exchange) using several bacterial strains (Escherichia coli, Staphylococcus aureus, and Bacillus subtilis). In static conditions, alumina plates coated with Cu-exchanged zeolite showed significant bactericidal activity. In dynamic mode (circulation of a contaminated nutrient medium), there was no observable bacterial adhesion at the surface of the Cu-exchanged material. These results confirm the antifouling properties of the Cu-mordenite layer due to both the increased hydrophilicity and antibacterial properties of the active layer.Tests performed with tubular filtration membranes (without copper exchange) showed a significant decline in membrane hydraulic properties during filtration of culture media containing bacteria, whereas copper-exchanged membranes showed no decline in hydraulic permeability. Filtration tests performed with concentrated culture media containing spores of B. subtilis led to a significant decrease in membrane hydraulic permeabilities (but less so with Cu-exchanged membranes). The surfaces showed less effective global antifouling properties during the filtration of a concentrated culture medium due to competition between bacterial growth and the bactericidal effect of copper. Analyses of copper leached in solution show that after a conditioning step, the amount of copper released is negligible.

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites.

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

Cu and Co modified beta zeolite catalysts for the trichloroethylene oxidation

In this work we have studied for the first time the catalytic activity for the oxidation of trichloroethylene (TCE) of Cu and Co beta zeolites. The results show that they are active and selective towards CO2, obtaining a better selectivity than that reached with conventional H-zeolites. The copper and cobalt zeolites have been prepared by different methods. It was found that their activity depends on the metal and on the preparation procedure. The most active catalyst was the Cu-BEA prepared by ion exchange (T50%=310°C and T90%=360°C). This catalyst has the highest ammonia adsorption capacity (as a measurement of the acidity) and it was the only tested material in which the Me2+ was completely reduced in a standard H2-TPR experiment (indicative of its important redox properties). Thus, the enhanced activity of the Cu-exchanged zeolite was associated to the presence of strong acid sites in the zeolite and to the redox properties of the copper ion exchanged. The catalyst was stable at 300°C for almost 70h without any important deactivation. This was related to the oxidative properties of the copper that avoid the formation of coke on the strong acid sites of the zeolite. On the other hand, zeolites with the transition metal incorporated into the zeolite framework by hydrothermal synthesis showed lower catalytic activity, probably because the formation of small oxide particles with much less interaction with the silicate framework, that results in a lower redox activity of the transition metals. It has been shown that a proper combination of acidity, redox properties and metal-zeolite interaction is necessary in order to prepare an active and selective zeolite catalyst for the TCE oxidation.

Synthesis of single-site copper catalysts for methane partial oxidation.

Cu-Exchanged zeolites are known as active materials for methane oxidation to methanol. However, understanding of the formation of Cu active species during synthesis, dehydration and activation is fragmented and rudimentary. We show here how a synthesis protocol guided by insight in the ion exchange elementary steps leads to highly uniform Cu species in mordenite (MOR).

The impact of finite temperature on the coordination of Cu cations in the zeolite SSZ-13

Understanding the fundamental properties of the Cu-exchanged zeolite SSZ-13, a catalyst for the selective catalytic reduction of nitrous oxides in the presence of ammonia, is a major challenge. Recent results for vibrational spectroscopy show that the active Cu centers are not static at room temperature, but change their coordination regularly when probe molecules are adsorbed. In this work we model the movement of bare Cu cations in SSZ-13 at room temperature (300K) using ab-initio molecular dynamics simulations. We find that Cu cations also change their coordination without adsorbent. We identify different coordinations for different oxidation states and analyze their distribution in dependence on the local Al position. The results in this work show that this thermal motion of the cation is a general feature, which probably influences the interpretation of various characterization methods as well as reaction path analysis. In the future it will be interesting to see which conditions have to be fulfilled that similar phenomena can also be observed for other systems.

Synthesis and characterization of RGO/zeolite composites for the removal of arsenic from contaminated water

The novel composite RGO/Cu-ZEA (1 : 1) (prepared from reduced graphene oxide (RGO) and Cu-exchanged zeolite A) revealed the highest efficiency for arsenic removal from contaminated water.

Combined experimental and kinetic modeling study of the bi-modal NOx conversion profile on commercial Cu-SAPO-34 catalyst under standard SCR conditions

Bi-modal NOx conversion profile has been consistently reported for selective catalytic reduction (SCR) of NOx with NH3 on certain Cu-exchanged zeolite catalysts under the so-called standard SCR conditions, with two distinct NOx conversion peaks located around 250 and 450°C. However, the nature of this practically important phenomenon remained unexplained. In this work, we have used a combination of experimental studies and kinetic modeling to characterize and de-convolute several side reactions, allowing us to quantitatively reconstruct the overall bi-modal conversion profile. In particular, we have shown that the first peak can be represented as a competition between the standard SCR reaction and parasitic NH3 oxidation. The subsequent increase in the NOx conversion can be attributed to the onset of in-situ NO oxidation to NO2 that enhances NOx reduction via fast SCR reaction route. Specifically, our modeling study indicates that NO oxidation reaction accelerates by orders of magnitude under the standard SCR conditions and becomes the governing step for NO reduction at high temperature.

Enhancement of low-temperature activity over Cu-exchanged zeolite beta from organotemplate-free synthesis for the selective catalytic reduction of NOx with NH3 in exhaust gas streams

A series of Cu-exchanged Al-rich Beta zeolites from organotemplate-free synthesis was prepared and investigated for selective catalytic reduction (SCR) of NOx with NH3 in exhaust gas streams. In comparison to conventional Cu-Beta zeolite with Si/Al ratio of 19, Cu-Beta zeolite with Si/Al ratio of 4 is a superior low-temperature NH3-SCR catalyst. Very high NO conversion (>95%) can be achieved at temperatures as low as 150 to ∼400°C. XRD, UV–Vis-NIR and NH3-TPD measurements show that more isolated Cu2+ ions are present at the exchange sites of Al-rich Beta zeolite. The combination of CO-FTIR and H2-TPR analysis demonstrates that Cu2+ ions could be reduced more readily on the Al-rich Beta than on the conventional Beta probably due to the proximity of the isolated Cu2+ ions. These can be correlated to the enhancement of NO conversion at lower temperatures over Cu-exchanged Al-rich Beta zeolite.

Impact of different forms of feed sulfur on small-pore Cu-zeolite SCR catalyst

Cu-exchanged small-pore zeolite catalysts, belonging to the structural family of chabazite (CHA), have achieved substantial commercial importance in the recent years as the materials of choice for selective catalytic reduction of oxides of nitrogen with ammonia in diesel exhaust. Their successful application was enabled by the understanding and management of the interactions of these catalysts with sulfur species present in the exhaust, mainly SO2 and SO3. While the latter was shown to be of particular importance, there remain very few literature studies on the impact of SO3 on the sulfation–desulfation behaviors of the Cu-zeolite catalysts of this class. This is due to the substantial experimental difficulties associated with using SO3 as a gas feed constituent and especially its accurate quantification. In this work, we have developed two specialized bench flow reactor systems, dedicated to generating controlled amounts of SO3, characterizing its impact on several facets of catalyst behavior, and quantifying the amount of stored sulfur species. Using the tools and methodology thus developed, we have investigated the interactions of a state-of-the-art Cu-zeolite catalyst with SO2 alone and with a mixture of SO2 and SO3, in the presence of water vapor. We have compared their impact at the lower- and upper-end of the practically relevant temperature range, namely at 200 and 400°C. At the low temperature, the effect of sulfur poisoning was found to be modest and species-independent. However, at the higher temperature, presence of SO3 resulted in a substantially more significant impact on the catalyst performance, which was also more difficult to reverse. These findings indicate that sulfur poisoning of the Cu-zeolite catalyst can occur via different mechanisms, from indiscriminate adsorption of SOx species on Cu sites at lower temperatures, to a reversible chemical reaction of SO3 or H2SO4 produced in the wet feed, with the catalyst material at elevated temperatures.

Prospective Catalytic Structured Converters for NH3-SCR of NOx from Biogas Stationary Engines: In Situ Template-Free Synthesis of ZSM-5 Cu Exchanged Catalysts on Steel Carriers

The main objective of this study is to develop highly active catalyst and its preparation method that would meet the requirements of steel carriers for short-channel structured converters for NOx abatement from stationary biogas engines. The in situ synthesis was applied to deposit a series of Cu-exchanged MFI zeolite (ZSM-5) on kanthal sheets. The samples differ in preparation conditions: organic template assisted and template-free synthesis, Si/Al ratio and catalyst carrier pretreatment (calcined vs. non-calcined). Dip-coating method was used as a reference to compare loading efficiency. In order to evaluate preparation quality and purity of resulting structure the samples were examined by XRD and SEM/EDS at various stages of preparation. For the assessment of mechanical endurance of the deposited catalyst layers the ultrasonication method was used. The results demonstrated high depositing efficiency of the in situ synthesis as well as high activity and selectivity of the Cu-exchanged MFI samples prepared without costly organic template.

Ammonia Oxidation on Structured Composite Catalysts

The NH3-based selective catalytic reduction of NO x on monolithic zeolite catalysts has emerged as the technology of choice for heavy-duty diesel vehicles. A class of Cu-exchanged zeolite catalysts has been developed that have very high ammonia sorption capacity and can achieve high NO x conversion to N2 for a variety of transient conditions. In order to fully exploit the latest generation of SCR catalysts, an active, selective and robust post-SCR ammonia conversion system is needed to minimize the breakthrough of ammonia into the environment [1]. The goal of this study is to better understand the steady-state catalytic mechanism of post-SCR ammonia oxidative conversion and product selectivity on low-loading Pt-based catalysts and in so doing provide guidance in the development of a new class of ammonia slip catalysts.

Reaction mechanism of dimethyl carbonate synthesis on Cu/β zeolites: DFT and AIM investigations

The mechanism of dimethyl carbonate (DMC) synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set. The adsorption energy (ΔE) and decomposition activation energy (Ea) for O2 are −1.84 and 1.72 eV, respectively, suggesting that the decomposition of O2 occurs readily under reaction conditions on the Cu site. The formed O atom further reacts with methanol to form surface-bound (CH3O)(OH)–Cu(I)/β, in which CH3O and OH were coadsorbed on the Cu+ of the catalyst; this process proceeds without an activation barrier and with an energy release of 1.23 eV. The (CH3O)(OH)–Cu(I)/β species then reacts with another methanol molecule and carbon monoxide to produce DMC through two different reaction pathways. In path I, insertion of carbon monoxide into the (CH3O)(OH)–Cu(I)/β leads to the formation of monomethyl carbonate species (CH3OCOOH), which then reacts with methanol to produce DMC and H2O. The activation energies for both steps are 0.97 and 0.65 eV, respectively. In path II, (CH3O)(OH)–Cu(I)/β reacts with methanol first to produce a dimethoxide species ((CH3O)(CH3O)–Cu(I)/β), and the formation of DMC is via the insertion of carbon monoxide into the (CH3O)(CH3O)–Cu(I)/β. The activation energies for these elementary reactions are 0.65 and 0.70 eV, respectively. The topological properties of electron density distributions for all the related stationary points involved in this reaction have also been examined using the atoms in molecule (AIM) theory for the illustration of the bond paths and weak interactions of all the stationary points in the reaction path.

Cu–SSZ-39, an active and hydrothermally stable catalyst for the selective catalytic reduction of NOx

A Cu-exchanged SSZ-39 zeolite has been synthesized and tested for the selective catalytic reduction (SCR) of NOx. This material shows an excellent catalytic activity, and most importantly, an extraordinary hydrothermal stability.

Influence of the Substrate Properties on the Performances of NH3-SCR Monolithic Catalysts for the Aftertreatment of Diesel Exhaust: An Experimental and Modeling Study

The effects of structural and geometrical characteristics of wash-coated monolith catalysts on the NO-NO2/NH3 selective catalytic reduction (SCR) activity were experimentally investigated over the same Cu-exchanged zeolite system coated onto honeycomb cordierite substrates with different cell densities, lengths, washcoat loads, and channel shapes. A stacked configuration was also tested. Contrary to previous reports, it was found that both interphase and intraphase diffusional limitations appreciably affected the deNOx efficiency at intermediate to high temperatures, whereas entrance effects did not play a noticeable role in enhancing the NOx conversion. A two-phase 1D+1D dynamic mathematical model of SCR monolithic converters, which explicitly accounts for both gas/solid and intraporous mass-transfer resistances, successfully predicted all of the observed effects using a single set of rate parameters estimated from intrinsic kinetic runs performed over the same catalyst in powdered form, under diffusion-free conditions.