Copper-exchanged Zeolites Research Articles

A theoretical study of the stability of Cu 2+ ion in IM-5 zeolite and the interaction of Cu-IM-5 with NO

Experimentally, Cu-exchanged IM-5 zeolite has been found to exhibit excellent catalytic activity in NO x reduction. Unfortunately, the origin of such property still remains obscure due to the rather complex structure of IM-5. In this paper, we made the first attempt to provide a theoretical rationalization. By calculating the (Al, Cu) location energies, the stability of both Cu 2+ and hydroxyl-Cu 2+ (i.e., [CuOH] +) was considered and compared to those in ZSM-5. It was shown that [CuOH] + located in IM-5 is comparable to that in ZSM-5. The most favorable location of Cu 2+ cation is in 6-membered ring in the wall of unusual two-dimensional medium-pore channel of IM-5. Moreover, through calculating the interaction of NO with copper-exchanged zeolite (Cu-IM-5), the most active center is Cu 2+ located in the 5-membered ring. The relatively high adsorption energies and numerous active centers in Cu-IM-5 might rationalize why Cu-IM-5 is more active than Cu-ZSM-5 in experiment of NO reduction. Thus, our theoretical results provide support that Cu-IM-5 is a more active and hydrothermically more stable catalyst than Cu-ZSM-5.

A study of copper-exchanged mordenite natural and ZSM-5 zeolites as SCR–NOx catalysts for diesel road vehicles: Simulation by neural networks approach

Copper-catalysts, based on the ZSM-5 (CuZSM5) and Cuban natural Mordenite (CuMORD) zeolites have been prepared by a conventional ion-exchange method and their catalytic activity in the selective catalytic reduction (SCR) of NO was studied using ammonia in presence of H2O and SO2. A commercial catalyst SCR (CATCO) based on V2O5–WO3–TiO2, was also studied as reference. This paper presents experimental results using catalysts without the toxic vanadium and exploits a neural network based approach to predict NOx conversion efficiency of three SCR catalysts. The derived mathematical functions are integrated in a numerical model for diesel road vehicle simulation to simulate diesel vehicles equipped with such SCR catalysts. The main results indicate that despite of toxic vanadium and N2O formation, CATCO shows the better NOx conversion efficiencies. However, CuMORD does not form N2O and have better performance than the CuZSM5. The simulation results show lower level of NOx for heavy-duty and light-duty diesel vehicles compared with homologation load cycles.

Removal of sulfur components from low sulfur gasoline using copper exchanged zeolite Y at ambient temperature

Copper-exchanged zeolite Y has been shown to be an effective material for removal of a variety of sulfur species from hydrocarbon streams, and both monovalent (Cu(I)) and divalent (Cu(II)Y) materials have been claimed to be effective. In this work we discuss experiments aimed at providing a direct performance comparison between the two copper-containing materials. Cu(I)Y zeolite is somewhat more effective than Cu(II)Y in removing thiophene from various fuel blends. Capacity of both materials for thiophene diminishes markedly when aromatics and/or olefins are present, and Cu(I)Y immediately turns dark on exposure to such feeds. Both materials demonstrate ability to convert thiols to disulfides at ambient temperature.

Desulfurization of Tier 2 gasoline by divalent copper-exchanged zeolite Y

The desulfurization of Tier 2 gasoline (<30 ppmw S) by divalent copper-exchanged zeolite Y (Cu(II)Y) zeolite was examined. Sulfur removal was found to be greater at 150 °C than at lower temperatures for Cu(II)Y adsorbents containing as the secondary cation either Na + (CuNaY) or H + (CuHY). Capacities on the order of 60 ml gasoline/g adsorbent were obtained with both materials before the sulfur concentration in the effluent exceeded 3 ppmw. Overall, the CuHY zeolite showed somewhat better desulfurization performance. It is proposed that acid sites (H +) that are present play a role in the desulfurization process, facilitating chemical reactions involving the sulfur-containing molecules. This leads to the generation of sulfur-bearing compounds having molecular weights and sizes greater than those present in the initial feed. Some of these heavier sulfur species are eluted but many do not elute. The net effect is a desulfurization pathway that is the result of entrapment of sulfur-containing molecules in addition to a desulfurization pathway involving direct interaction between the copper and sulfur atoms.

An Investigation into the Adsorption of Aniline from Aqueous Solution Using H-Beta Zeolites and Copper-Exchanged Beta Zeolites

Zeolite beta, a large-pore zeolite, was investigated in this study with a view to examining it as a potential adsorbent for the removal of aniline from aqueous solutions. Two different metal-loaded zeolites were prepared by exchanging H-beta zeolite (SiO2/Al2O3 = 75:1) with copper. The influence of exchanged copper on the uptake level was assessed. The effect of varying the silica-to-alumina ratio of the H-beta zeolite on the aniline uptake level was also examined, using three different H-beta zeolites with ratios of 25:1, 75:1 and 150:1 as adsorbents. The sorption experiments indicated an uptake level of ca. 110–120 mg/g for each zeolite and this level was also adsorbed by the copper-modified H-beta zeolites (SiO2/Al2O3 = 75:1). In all cases, the adsorption process followed the Langmuir model for adsorption and the level of aniline adsorbed was largely unaffected by a change in temperature or the presence of extra framework copper. The stability of the exchanged copper on these zeolites was then examined by measuring the quantity of copper leached from each zeolite into solution as a function of pH. Minimum copper leaching was observed in the pH range 5–11. This provided a stable pH working range for the adsorbent materials.

Catalytic Asymmetric Heterogeneous Aziridination of Styrene Derivatives Using Bis(oxazoline)‐Modified Cu2+‐Exchanged Zeolite Y.

The copper-catalysed aziridination of styrene derivatives using either Cu(OTf)2 as a homogeneous catalyst or copper-exchanged zeolite Y modified as a heterogeneous catalyst with bis(oxazoline) using [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donor is described and discussed. For the homogeneously catalysed reaction virtually all the substituted derivatives gave lower yields of the aziridine with lower ee when compared with styrene, although 4-chlorostyrene did give enhanced ee. For the heterogeneous catalysed reaction the styrene derivatives often gave enhanced yield, particularly when the substituent was in the 4-position. Particularly high ee was observed for 2-chlorostyrene (95%) and 4-chlorostyrene (94%) and in general the ee observed for the heterogeneously catalysed reaction with the 2- and 4-substituted derivates was significantly higher than that for the equivalent homogeneously catalysed reaction. With copper-exchanged zeolite Y the leaching of Cu2+ into solution during reaction was much lower for the substituted styrene derivatives (0.4-7% of total Cu) than with styrene itself (10.3% of the total Cu). The effect of reaction time on aziridine yield reveals an S-shaped profile that is accentuated by the position of the substituent and this effect is observed for both the homogeneously catalysed pathway and the heterogeneously catalysed reaction. A series of competitive experiments in which styrene was co-reacted with equivalent quantities of 2-, 3- and 4-chlorostyrene revealed that the reaction of styrene enhanced by the other substrate, but the rate of reaction of the substituted styrene was decreased. Co-reaction of 2- and 4-chlorostyrene reveals interesting competitive effects, which are discussed in terms of the nature of the active site.

14 Copper

Highlights of this year's literature include the characterisation of [CuMe], the first diazene-bridged dicopper(I) complex, and illumination of dioxygen binding by dicopper(I) complexes. The observation of {Cu(η2-H2)}+ species has been a major development in studies of copper-exchanged zeolites. Among polynuclear copper(II) species, examples of square planar μ4-bridging hydroxide and trigonal planar μ3-chloride are remarkable.

Density functional study of the interaction of Cu + ion-exchanged zeolites with H 2O and SO 2 molecules

In the present work, we report theoretical calculations of the interaction of H 2O, SO 2, O 2, NO 2 and NO with copper-exchanged zeolite (Cu-ZSM5). Calculations were carried out using ab initio density functional theory (DFT). The DF calculations show that the adsorption energies of NO 2 and NO are larger than the corresponding adsorption energies of water and oxygen. Therefore, neither water nor O 2 can easily displace the adsorbed NO x s. On the contrary, water can displace adsorbed O 2 over the Cu, which indicates that the water could blocks some reaction channels of the catalytic cycle. It was also found, that the adsorption of SO 2 is not strong but can obstructs completely the active site for the reduction of the NO x due to the formation of a CuSO 4 specie. The topological analysis of the molecular charge density shows that the O 2 and SO 2 molecules are activated after the adsorption over the Cu, whereas H 2O is not activated.

Adsorption of Aniline from Aqueous Solution Using Copper-Exchanged ZSM-5 and Unmodified H-ZSM-5

Three medium-pore aluminosilicates were investigated with a view to examining their potential as adsorbents for the removal of aniline from aqueous solutions. H-ZSM-5 was exchanged with copper to prepare two different metal-loaded zeolites. The aqueous stability of the copper-exchanged zeolites was then examined by assessing the quantity of copper leached from each zeolite into solution as a function of pH. The optimum stability range with minimal copper leaching occurred between pH values of 5 and 9. The adsorption of aniline from aqueous solution onto each of the three zeolites was carried out within this pH window and the influence of the exchanged copper on the uptake level was assessed. The sorption experiments indicated an uptake level of approximately 40 mg/g for each zeolite. In all cases, the process followed the Langmuir adsorption model with the level of aniline adsorbed being largely unaffected by a change in temperature or the presence of extra-framework copper.

On the activity and hydrothermal stability of CuMCM-22 in the decomposition of nitrogen oxides: a comparison with CuZSM-5

A combined spectroscopic and catalytic study on the activity and hydrothermal stability of copper-exchanged MCM-22 and ZSM-5 zeolites is reported; CuMCM-22 is active in the decomposition of NO and N 2O also in the presence of water, whereas CuZSM-5 deactivates rapidly upon steaming. CO adsorption on fresh and steamed samples enlightened that the copper active species are more stable on CuMCM-22.

Catalytic asymmetric heterogeneous aziridination of styrene derivatives using bis(oxazoline)-modified Cu2+-exchanged zeolite Y

The copper-catalysed aziridination of styrene derivatives using either Cu(OTf)2 as a homogeneous catalyst or copper-exchanged zeolite Y modified as a heterogeneous catalyst with bis(oxazoline) using [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donor is described and discussed. For the homogeneously catalysed reaction virtually all the substituted derivatives gave lower yields of the aziridine with lower ee when compared with styrene, although 4-chlorostyrene did give enhanced ee. For the heterogeneous catalysed reaction the styrene derivatives often gave enhanced yield, particularly when the substituent was in the 4-position. Particularly high ee was observed for 2-chlorostyrene (95%) and 4-chlorostyrene (94%) and in general the ee observed for the heterogeneously catalysed reaction with the 2- and 4-substituted derivates was significantly higher than that for the equivalent homogeneously catalysed reaction. With copper-exchanged zeolite Y the leaching of Cu2+ into solution during reaction was much lower for the substituted styrene derivatives (0.4-7% of total Cu) than with styrene itself (10.3% of the total Cu). The effect of reaction time on aziridine yield reveals an S-shaped profile that is accentuated by the position of the substituent and this effect is observed for both the homogeneously catalysed pathway and the heterogeneously catalysed reaction. A series of competitive experiments in which styrene was co-reacted with equivalent quantities of 2-, 3- and 4-chlorostyrene revealed that the reaction of styrene enhanced by the other substrate, but the rate of reaction of the substituted styrene was decreased. Co-reaction of 2- and 4-chlorostyrene reveals interesting competitive effects, which are discussed in terms of the nature of the active site.

Catalytic Asymmetric Heterogeneous Aziridination Using CuHY/bis(oxazoline): Effect of Reaction Conditions on Enantioselectivity

The copper-catalyzed aziridination of styrene with copper-exchanged zeolite HY (CuHY) and copper(II) triflate (trifluoromethanesulfonate) (Cu(OTf)2) as catalysts is described using N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) as the nitrene donor. The effects on the ee and yield of the aziridine when the catalyst is modified by the presence of a chiral bis(oxazoline) are investigated in detail. The heterogeneously catalyzed reaction under these conditions shows a slight, but significant, enhancement in ee with increasing conversion at 25 °C. This is not observed in the more rapid homogeneously catalyzed reaction under identical reaction conditions using PhINTs as the nitrene donor. The enhancement in ee is proposed to result from the preferential reaction of the (S)-aziridine with the Cu2+:bis(oxazoline) complex in the presence of PhI=NTs, leading to an enhancement of the (R)-aziridine in the remaining aziridine product.

Catalytic Asymmetric Heterogeneous Aziridination of Styrene Using CuHY/bis(oxazoline): Comments on the Factors Controlling Enantioselectivity

The copper-catalyzed aziridination of styrene is described using both heterogeneous, copper-exchanged zeolite HY, and homogeneous, copper (II) triflate catalysts using both [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) and [N-(p-nosylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donors. The key differences observed for the two catalysts when modified by chiral bis(oxazoline) ligands are discussed in detail. In particular, the heterogeneously catalyzed asymmetric reaction can give much higher enantioselection than the comparable homogeneously catalyzed reaction. The structure of the bis(oxazoline) ligand is the critical factor, and bis(oxazoline) ligands that are ineffective with the homogeneous catalysts are highly effective for the Cu2+ cation constrained within the zeolite micropores. The consequences of this observation for the design of chiral ligands for asymmetric heterogeneous catalysis are discussed. The effect of the degree of styrene conversion on the enantioselectivity is described in detail using PhI=NNs as a nitrene donor. The reaction shows a significant enhancement in ee with conversion at 25°C, and the possible origin of this effect is discussed.

Observation of Single-Crystal-Type EPR Spectra from Monolayers of Copper-Exchanged Zeolite Na-A Crystals Assembled on Glass Plates

Single-crystal-type EPR spectra were observed for microcrystals of copper(II)-exchanged zeolite Na-A assembled on glass plates when the magnetic field was perpendicular to the glass surface. Two copper species (H and D) were identified both in the powder sample and in the assembled crystals. The g z direction of species H is parallel to [001] of the cubic crystal, while that of species D is tilted 9° away from [001]. Previously, other authors assumed the g z directions of the two corresponding species to be parallel to [111]. On the basis of the g z directions, we suggest that Cu 2 + in species H is displaced 0.149 nm along [111] from the six-ring plane into the β cage and octahedrally coordinated to three zeolite oxygen atoms in the six-ring plane and three water molecules. The g z direction of Cu 2 + in species D is not compatible with site II in the six-ring plane. Species D is probably a trigonal complex with Cu 2 + displaced into the β cage and coordinated to three zeolite oxygen atoms.

On the hydrothermal stability of CuAPSO-34 microporous catalysts for N 2O decomposition: a comparison with CuZSM-5

The hydrothermal stability of CuAPSO-34 microporous silicoaluminophosphates with chabasite structures has been investigated and compared to that of a copper-exchanged ZSM-5 zeolite by a combined XRD, FTIR, and catalytic study. CuAPSO-34 showed a higher hydrothermal structural stability (steaming 80 h at 600 °C), as found by XRD. Spectroscopic FTIR investigation of adsorbed CO revealed that upon this steaming procedure the concentration of monovalent copper ions in CuAPSO-34 was not significantly affected, whereas, even after a less severe steaming (60 h at 550 °C), these species dramatically decreased in CuZSM-5, and extraframework Al 3+ ions were simultaneously formed. Consequently, the high activity in N 2O decomposition shown by fresh CuZSM-5 was essentially lost after steaming, while CuAPSO-34 exhibited a significantly milder deactivation.

Determination of the oxidation and coordination state of copper on different Cu-based catalysts by XANES spectroscopy in situ or in operando conditions

The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.

Structural Characterization of the Redox Behavior in Copper-Exchanged Sodium Zeolite Y by High-Resolution Powder Neutron Diffraction

The results of structural refinements on high-resolution powder neutron diffraction data for copper(II)- and copper(I)-exchanged zeolite Y and for the parent sodium zeolite Y, with composition Na62Si128Al64O384, are presented. Rietveld refinement of ambient-temperature data shows that the Cu(I) and Cu(II) cations are located on different sites in the two materials and that the structures differ considerably from that adopted by NaY. In CuIIY, with composition Cu33Na12Si130Al62O384, the copper cations are located on the IA‘, IB‘, and II‘ sites with occupancies of 0.552(4), 0.145(7), and 0.162(8), respectively, while some sodium, 0.36(2), remains on site II. For CuIY, there is a redistribution of the cations on sites IA‘, IB‘, II‘, and II to 0.39(2), 0.21(1), 0.24(1), and 0.18(2), respectively. In both zeolites, site I is vacant. The high-quality diffraction data provide direct evidence of structural changes associated with the redox behavior of the zeolite, including distortion of the framework and the location of charge-balancing species.

Heterogeneous aziridination of styrene using [ N-( p-nitrophenylsulfonyl)imino]phenyliodinane as nitrene donor: influence of the reaction parameters on yield and enantioselectivity

The copper-catalysed aziridination of styrene with copper-exchanged zeolite Y (CuHY) and copper(II) triflate (Cu(OTf) 2) as catalysts is described and discussed. In particular, the effects of reaction conditions on the yield and enantiomeric excess of the aziridine product are described using [ N-( p-nitrophenylsulfonyl)imino]phenyliodinane (PhINNs) as nitrene donor. By careful control of the styrene:nitrene donor molar ratio and the solvent, an ee of 95% can be obtained for the heterogeneously catalysed bis(oxazoline)-modified zeolite CuHY. The ee achieved with the zeolite immobilised catalyst is significantly higher than that achieved for the non-immobilised homogeneous catalyst under comparable reaction conditions.

Aziridination of styrene: comparison of [ N-( p-tolylsulfonyl)imino]phenyliodinane and chloramine-T as nitrene donors

Chloramine-T and [ N-( p-tolylsulfonyl)imino]phenyliodinane (PhINTs) are contrasted as nitrene donors for the aziridination of styrene using copper(II) triflate, and copper-exchanged zeolite Y (CuHY) as catalysts. For both catalysts, PhINTs is found to give significantly higher yields of the aziridine both in the presence and absence of a chiral bis(oxazoline) modifier. In addition, chloramine-T is found to induce leaching of most of the Cu from CuHY, and with this nitrene donor CuHY does not function as a heterogeneous catalyst. In contrast, PhINTs causes negligible leaching of Cu from CuHY and, consequently, for the heterogeneous CuHY catalyst, PhINTs is the preferred nitrene donor. With chloramine-T, the beneficial effects observed on addition of copper powder are shown to be due to the copper powder acting as a reservoir for Cu 2+ in solution, since Cu 2+ in solution is rapidly deactivated by toluenesulfonamide, a degradation product of the nitrene donor.

Selective catalytic reduction of NOx, by propene over copper-exchanged pillared clays

Copper-exchanged pillared clays were examined as an SCR catalyst for NOx, removal by propene. Both micropores and mesopores were simultaneously developed by pillaring a bentonite with TiO2. Therefore, TiO2-pillared clay has about 8 to 9 times higher surface area and 3 times higher pore volume than the parent unpillared bentonite. The presence of water in the feed gas stream caused a small and reversible inhibition effect on NO removal activity of Cu/Ti-PILC. The water tolerance of Cu/Ti-PILC was higher than copper-exchanged zeolites such as CuHM and Cu/ZSM-5 due to its high hydrophobicity as confirmed by H2O-TPD experiment. Copper-exchanged PILC was confirmed to be an active catalyst for NOx, removal by propene. The addition of copper to TiO2-pillared clay greatly enhanced the catalytic activity for NO removal. Cupric ions on Ti-PILC were active reaction sites for the present reaction system. The state of copper species on the surface of Ti-PILC varied with the content of copper and TiO2. The catalyst having more easily reducible cupric ions showed maximum NO conversion at relatively lower reaction temperatures. It indicates that the redox behavior of cupric ions is directly related to NO removal mechanism. The redox property of cupric ions depended on the copper content and dehydration temperature of PILC.