Cu-H-ZSM

The solids, particularly the 1.77 %-Cu-H-ZSM-5(19) and the parent H-ZSM-5, were characterized before and after steam treatments by various techniques powder X-ray diffraction (Siemens diffractometer with CuKa radiation), SEM (Hitachi S800 with a 10 nm resolution), N2 adsorption (BET and pore volume with a lahoratory-made automatic apparatus), FTIR spectroscopy (framework vibrations with KBR dilution and CO probe molecule in an in situ cell), Si and AI MAS NMR (BRUKER DSX 400). 

Upon addition of water to the stream the activity of the fresh Cu-H-ZSM-5(19) solids clearly decreases (Figure 1) but this effect is fully reversible if the water is suppressed and if the temperature does not exceed 773 K, as already noticed (14). Most probably an competitive adsorption between H2O, NO or CsHg, is invoked. When the temperature reaches 873 K in the presence of water the catalyst is deactivated irreversibly (Table 1). Kharas et al (5) have also noticed that a working temperature of 873 K induces a deactivation contrarily to 773 K. Furthermore, Yan et al (15) have noticed that the deactivation is faster with a complete [hydrocarbon, NO, O2, H2O] mixture than if a component is missing. In fact, we observed more deactivation under catalytic conditions and in the presence of water than under the air + 10 % H2O mixture at the same temperature (873 K). We will explain this phenomenon later on, in paragraph 3.2. 

AgAlBO [28], VMoCrPdOv [29], SbVTi with additives [30], and perovskite systems such as YBaCu30g [31], Cerium oxide (on titania) [32] or nickel oxide (on alumina) have also been used as catalysts in special circumstances. Nowadays, zeolite-supported catalysts, e. g. Cu-exchanged materials such as Cu-H-ZSM-5... 

Fig. 11. ESR spectra of Cu(II)-contain-ingMFI-type zeolite samples. A, CUCI2/ H-ZSM-5 mixture heat-treated in vacuum at 10 75 K B, Cu,H-ZSM-5 obtained by conventional exchange, calcined in air at 1075 K and evacuated at 300 K (after Refs.

Cat. 1 Cu/H-ZSM-5 H-ZSM-5 having Si/Al atomic ratio 40 prepared by the rapid crystallization method (ref. 3) was ion-exchanged by using 0.5 mol aqueous solution of Cu nitrate at room temperature. The Cu loading was 1.00 wt% which corresponded to 80% ion-exchanged. 

Cu/H-ZSM-5 (Cat. 1) and Cu-silicate (Cat. 2) gave the XRD patterns same as that of H-ZSM-5, and no indication for the existence of isolated copper oxides. NO conversion was measured on both catalysts under the 02 absent condition. 4% NO diluted with N2 was fed to the reactor with a SV 2000 h" at 500°C and that temperature was maintained for 8 h. The NO conversions at the steady state on Cats. 1 and 2 were 42% and 12%, respectively. The integrated amount of NO converted till 8 h on stream... 

Upon water vapor removal from the stream, only part of the dry gas-catalytic activity is recovered. This partial activity recovery may be mainly attributed to decomposition of the hydrolyzed copper complexes, and bare ion (due to the dehydration of copper complexes on the exterior surface) migration to active sites inside the zeolite cavities. Another contribution may come fi m tiie fine CuO particles on the zeolite surface, i.e., small part of active Cu cations are slowly restored by solid ion exchange with Bronsted acid sites. This hypothesis is drawn from the observation that Cu(H)-ZSM-5 with low Cu ion exchange level can be obtained by solid ion exchange between H-ZSM-5 and CuO in a vacuum at 5(X)°C as reported by Karge, et al (75). The permanent activity loss, however, is not explained. This may be attributed to irreversible CuO particle formation and deactivation, or dealumination if it happened in this study. 

Fig. 29. Linear relationship between the maximum ESR intensity of in Cu,H-ZSM-5 (obtained by solid-state ion exchange in a CuO/H-ZSM-5 mixture) and the Al content of the ZSM-5 framework (after [21], with permission)...

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

It has been shown that on Cu-ZSM-5 and Cu-ZrOj catalysts, reduction of NO and NOj in the presence oflarge excess of Oj proceed at about the same rate [20,21]. This is because over these catalysts, NO2 is rapidly reduced to NO (and not NO being rapidly oxidized to NOj) [20,22,23]. On the other hand, on catalysts that do not contain transition metal ions, such as Na-ZSM-5 [24], GajOj [25], AljO, [26], and H-ZSM-5 [26], NO2 reduction to Nj proceeds much... 

The basis of the demonstration can be based on already published data on the surface reaction between NOz and adsorbed organic compounds. Yokoyama and Misono have shown that the rates of N02 reduction over zeolite or silica are proportional to the concentration of adsorbed propene [29], whereas Il ichev et al. have demonstrated that N02 reacts with pre-adsorbed ethylene and propylene on H-ZSM-5 and Cu-ZSL-5 to form nitro-compounds [30], Chen et al [2-4] have observed the same nitrogen-containing deposits on MFI-supported iron catalysts. The question on the pairing of nitrogen atoms is not considered here. 

Propane reaction. In a series of experiments propane (760 torr) reacted at 773 K over H-ZSM-5 (Si/Al = 15) and H-ZSM-5 modified with Ga or Pt. The conversion of propane was maintained at around 30% by adjusting the flow rate between 1 and 10 l.h , higher flow rates being used for the most active catalysts. The catalytic activities for the different solids were normalized to that of H-ZSM-5. The data are summarized in Table 1. It is apparent that the addition of Ga, Pt, Pt-Cu to the H-ZSM-5 zeolite increased its activity for the propane conversion. 

The H-ZSM-5 (Si/Al=25, PQ Corp.) and Silicalite (S-1) zeolites were used to prepare the microporous copper-containing catalysts, Cu-ZSM-5 and Cu-S-1. ( We recall that S-1 has the same framework topology of ZSM-5, but without AP+ ions in the framework, and therefore S-1 is an all silica materials as MCM-41.) S-1 was prepared using TEOS and a 20% aqueous solution of tetrapropylammonium hydroxide (TPA-OH) (Fluka-purum). TEOS was poured in the TPA-OH solution. The resulting mixture was kept at 333 K for 3 h and then heated under autogenous pressure in a 350 mL stainless autoclave in an oven at 448 K for 24 h, without stirring. The solid was washed with water, dried 2 h at 383 K and finally treated in air at 823 K for 5 h. Further details were reported in Ref 4. 

Cu-Co/H-ZSM-5 Catalyst for Total Oxidation Over-additive Rise in the Activity and the Thermostability of the Bicationic System... 

The mono-cationic samples CuH-ZSM-5 and CoH-ZSM-5, with 1.8% wt. Cu and 1.4% wt. Co respectively, were prepared by two-fold incipient wetness impregnation of H-form of the zeolite TsVM (homemade analog of H-ZSM-5, crystallinity > 95%, Si/Al = 21) with aqueous solutions of copper or cobalt nitrates (0.7 - 0.8 cm of solution per 1 g of zeolite). The samples obtained were dried in air at 110°C and calcined at 500°C for 5 h (Samples 1 and 2). The bi-cationic sample Co/CuH-ZSM-5, with (1.0% wt. Co + 1.8% wt. Cu), was prepared by a two-fold impregnation of the Sample 1 with aqueous solution of cobalt nitrate with subsequent sample drying and calcination in an air stream at 500°C for 5 h (Sample 3). To compare the effects of high-temperature treatment the samples were calcined at 750°C for 2 h in an air flow (Samples 1 -3 ). ... 

Therefore, neither impregnation nor subsequent calcination of Co/Cu-ZSM-5 at T < 500°c lead to a noticeable redistribution of Cu ions introduced in cationic positions inside ZSM-5 channels. Earlier for the bi-cationic system Cu-Cr/H-ZSM-5 the mutual influence of different ions, with competition for cationic sites at 500°C, was detected [18]. On the contrary, cobalt ions are not capable of displace Cu from cationic positions in H-ZSM-5 matrix. However, an another type of influence is observed upon the more severe dry calcination or steam-aging of Co/CuH-ZSM-5. 

In each case, 5 m% of metal chloride was intimately mixed with an aliquot of HZSM-5 in an agate mortar. These mechanical mixtures were heat-treated at 873 K for 8 h in air. The products were cooled to ambient temperature, washed chloride-free and then dried at 373 K (these samples are denoted Cu(S)ZSM-5, etc.). The compositions of the samples were determined by classical methods and by X-ray fluorescence analysis. 

Shelef et al. [69] specified the role of the NO2 intermediate when the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon. It was shown that in the absence of oxygen NO2 decomposes into NO and O2, whereas under strongly oxidizing conditions NO2 and NO are reduced to Nt. These authors compared Cu-ZSM-5, H-ZSM-5, and CuO on y-A Os again, Cu-ZSM-5 showed the highest activity. 

Since the discovery by three groups working independently that Cu-ZSM-5 catalyses the catalytic reduction of NO by various hydrocarbons, much research has been carried out with this material [234]. However, these catalysts present major problems in terms of their thermal stabilities and sensitivities to water [5]. Another important system that is also being studied intensively is Pt/Al203, and generally Pt on different metal oxide supports [6], since this is already used in conventional (stoichiometric) exhaust gas cleanup and has proven stability and tolerance to typical potential poisons in the engine exhaust. Other systems such as Co-ZSM-5, Ga-ZSM-5, Cu-Zr02 and H-ZSM-5 have been found to be active [7], but their industrial application has not been achieved yet. 

In order to know whether the Pd ions or complexes are anchored to the zeolite framework or not, the IR framework vibrations of Pd-H-ZSM-5(0.49) were investigated (Figure 5). After activation under O2, a weak band at 930 cm" forms. Upon NO adsorption, the 930 cm band disappear while a new band appears at 980 cm". These bands are attributed to asymmetric internal stretching vibrations of T-O-T bonds (T = Si or Al) perturbed by Pd ions. The higher the perturbation, the lower the frequency. Therefore, the 930 cm band could be related to anchored Pd(II) ions or complexes formed upon decomposition of exchanged complexes, and the 980 cm band could be due to Pd(I) nitrosyl entities formed upon NO contact. Similar observations were found on Cu-ZSM-5 catalysts (34). 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

In this work the catalytic activities of fresh and hydrothermally treated Cu-MFI solids (Si/Al = 19, 78, 130, 151, 319) are measured in the absence and in the presence of water. Fresh and aged Cu-MFI catalysts and a parent H-ZSM-5 solid are characterized by various techniques in order to understand the modifications of the copper ions and of the zeolite itself, as well as the relationship between these modifications. 

Figure 1. Conversion of NO into N2 as a function of the temperature with the Cu(1.96 %)-H-ZSM-5(19) solid. The reaction is performed up to 773 K in the absence and in the presence of water in the feed. The arrows indicate the increase and decrease in the reaction temperature.

The modifications of the Cu(1.77 %)-H-ZSM-5(19) solid upon aging have been compared to those of the parent H-ZSM-5 zeolite. 

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