Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

ZSM-5 zeolite was loaded with vanadyl ions (VO²⁺) by treatment of Na–ZSM-5 with aqueous VOSO₄ solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH₃ and 2% O₂ in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g⁻¹ h⁻¹ (≈400 000 h⁻¹) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N₂O formation at NH₃ excess. Admixture of SO₂ (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO₂ conversion was noted at T ≤ 723 K and 450 l g⁻¹ h⁻¹. The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.     

ZSM-5 zeolite films on Si substrates grown by in situ seeding and secondary crystal growth and application in an electrochemical hydrocarbon gas sensor

ZSM-5 zeolite films were grown on Si substrates by a two-step hydrothermal synthesis consisting of in situ seeding and secondary crystal growth. The films were 8–13 μm thick and partly oriented with the c-axis perpendicular to the substrate surface. After ion exchange with sodium ions, one film was applied as solid electrolyte in a potentiometric hydrocarbon gas sensor. A fast and reversible voltage response of the sensor to varying propane concentrations (100 ppm – 10%) was observed in O₂/CO₂/N₂ gas mixtures at 723 K.     

Selective catalytic reduction of NO with C2H4 over Cu/ZSM-5: Influences of oxygen partial pressure and incorporated rhodia

Catalytic activities of copper-exchanged ZSM-zeolites for the reduction of nitric oxide to nitrogen using ethylene as reducing agent in the presence or absence of oxygen are compared with those of Rh-ZSM-5 zeolites and binary Cu/Rh-ZSM-5 catalysts at 623–823 K. Copper ZSM-5 systems with high copper loadings (> 2.1) are shown to be much more active in the presence of oxygen, in marked contrast to Rh ZSM-5 catalysts which were more active in its absence. Binary ZSM-5 catalysts containing both copper and rhodium maintained intermediate activity in both conditions. A sample of Cu ZSM-5 containing only 1.8% copper also evidenced some activity in both conditions, which could be understood on the basis of catalytic action by copper ions within the zeolitic framework, allied to inherent Bronsted acidity of the latter. Comparative catalytic data are presented for the oxygen partial pressure dependence of the conversion to nitrogen over all catalysts at 673 K. These delineate the interplay between selective catalytic reduction (SCR) of NO by C₂H₄ and ethylene oxidation by dioxygen. Techniques utilised to characterise the catalysts include: powder XRD, TPR in H₂, and TPD of O₂.     

Gasoline selective ZSM-5 FCC additives: effects of crystal size, SiO2/Al2O3, steaming, and other treatments on ZSM-5 diffusivity and selectivity in cracking of hexene/octene feed

ZSM-5 catalysts with silica/alumina ratios of 55/1 and 450/1, and with varying crystal size, were studied to better understand the operation of “gasoline selective” FCC additives. Most materials were steamed for 5 h at 1400°F, and some were subjected to other treatments such as mechanical milling, P addition, and HCl reflux. These materials were characterized for total Al content, tetrahedral Al, and sorption capacity and rate. The ratios of octene isomerization to octene cracking, and of octene cracking to hexene cracking, for a mixed hexene/octene feed at 538°C were used to assess the potential FCC gasoline selectivity of these catalysts.

Steaming generally led to a decrease in effective diffusivity, as calculated from uptake rates. HCl reflux of two steamed samples removed substantial quantities of non-framework alumina, resulting in increased diffusivities and higher selectivities. Density of acid sites did not appear to be important in determining the gasoline selectivity of the catalysts examined here. The effects of most catalyst treatments on reaction selectivities could be related to diffusional blockage, especially by non-framework alumina. However, not all results could be explained by a simple reaction/diffusion model. Factors other than diffusion, such as narrowing of available reaction space in the ZSM-5 channels, may also play a role in determining selectivities.     

Predicting locations of non-framework species in zeolite materials

A new grid-based algorithm developed at Molecular Simulations, and molecular dynamics method have been applied to modeling the locations of non-framework species in zeolites. This new method locates the energy minima for various frameworks and populates these sites with the non-framework species. The cation locations were predicted in dehydrated zeolite adsorbents and catalysts, such as Na₈₈X, Ca₄₈X, mixed cation zeolite 3A (K₆₀Na₃₆A), and Cu-mordenite (Si/Al=5.0) using only well-known framework structure models. Furthermore, the locations of benzene molecules in a supercage of zeolite Ca₄₈X were correctly predicted via the application of the Monte Carlo docking and molecular dynamics methods.

These examples demonstrate that when the framework type is known, the new technique can provide a realistic initial structure input for the challenging task of solving crystal structures of zeolites containing non-framework species.     

Analysis of Fe species in zeolites by UV–VIS–NIR, IR spectra and voltammetry. Effect of preparation, Fe loading and zeolite type

Three procedures were employed for the preparation of Fe-zeolites with ZSM-5 (MFI), ferrierite (FER) and beta (BEA) structures: ion exchange from FeCl₃ solution in acetyl acetonate and solid-state ion exchange from FeCl₂ using an oxygen or nitrogen stream. A combination of UV–VIS–NIR spectra, IR spectra of skeletal vibrations and of adsorbed NO, as well as voltammetry provided information on the type of Fe species introduced. Single Fe(III) ion complexes (Fe(H₂O)_6−xOH_x) in hydrated zeolites were reflected in the charge-transfer bands at 33 100, 37 300 and 45 600 cm⁻¹. The single Fe(II) ions at cationic sites in evacuated zeolites yielded (through perturbation of framework T–O bonds) characteristic bands (910–950 cm⁻¹) in the region of the skeletal window. These Fe(II) ions with adsorbed NO were also reflected in vibrations at 1880 cm⁻¹. Dinuclear Fe–oxo complexes yielded the Vis band at 28 200 cm⁻¹. Voltammetry indicated the presence of Fe oxides (hematite) through the reduction peak at −0.7 V. Such oxide-like species were also reflected in the absorption edge at 19 800 cm⁻¹, and a doublet at 11 000 and 11 800 cm⁻¹ in the Vis spectra. Fe(II)–NO vibrations at 1840, 1810 and 1760 cm⁻¹ belonged to the undefined exposed Fe cations, probably originating from supported oxides. Using an ion exchange procedure, employing FeCl₃ in acetyl acetonate, exclusively Fe ions at cationic sites could be introduced at low concentrations (Fe/Al < 0.1). At higher Fe loadings, dinuclear Fe–oxo complexes were formed preferably in Fe-ZSM-5, but were absent in Fe-beta. Exclusively single Fe species could not be prepared at Fe concentrations above Fe/Al > 0.2; all three types of Fe species, single Fe ions, dinuclear Fe–oxo complexes and Fe oxides were formed.     

Effect of the support upon the behavior of Cu in NO decomposition exemplified on Cu-ZSM-5 containing Zr

ZSM-5 containing Zr in the lattice was prepared following the procedure for ZSM-5 using ZrCl₄ as zirconium precursor. Zr was substituted for Si in the ZSM-5 lattice and then this zeolite was loaded with Cu. Loading these catalysts with Cu was carried out following Iwamoto’s procedure. An increase in Zr results in a decrease in Cu loading, although the exchange solutions were the same. The increased Zr loading and, hence, decreased Cu loading resulted in a decreased NO decomposition activity relative to the Zr-free ZSM-5 loaded with Cu. NO conversions, obtained on Cu-ZSM-5 zeolites in which zirconium replaced Si, were below 40%, which is about 30% lower than those recorded on Cu-exchanged pure zeolites. The amount of NO₂ detected in the reaction products proved that, for total conversions lower than 20%, the reaction of NO takes place almost exclusively without decomposition to N₂ and O₂. The catalysts were characterized by DRIFTS and NO-DRIFTS, Raman spectroscopy, O₂ — TPD and XPS. The OH region observed in DRIFTS changed with an increase in Zr loading and, hence, Cu(II)–O–Cu(I) species cannot be formed. This may play a role in the decreased activity. Raman spectra showed that increased Zr also results in increased CuO which may lead to the decreased activity. Although the exposure of the samples with high Zr content to NO results in the apparent redispersion of CuO to isolated sites within the ZSM-5, the Cu moves to locations which are inactive and/or inaccessible to NO and, hence, the activity does not increase even though the Cu is dispersed. The formation of NO₂ follows the mechanism proposed by Shelef.     

Role of oxygen on NOx SCR catalyzed over Cu/ZSM-5 studied by FTIR, TPD, XPS and micropulse reaction

Pulse reaction, TPO/TPD (temperature programmed oxidation/temperature programmed desorption), XPS (X-ray photoelectron spectroscopy) and DRIFT (diffuse reflectance infrared transform spectroscopy) have been used to investigate the role of oxygen on SCR (selective catalytic reduction) of NO over Cu/ZSM-5 zeolites, especially during the early stage. Pulse reaction shows that propene is deposited on Cu ions and inhibits the adsorption of NO on them. It is necessary for oxygen to make Cu sites clean by deep oxidation of propene. In the presence of O₂, SCR at 623 K is accompanied by coke deposition on the zeolite. This coke can be discriminated from carbonaceous species formed on Cu in the absence of O₂ from the result of TPO. XPS results implies that Cu(II)–O species play a crucial role in forming Cu(II)–NO₂ species, and subsequently are well correlated with the activity of NO SCR.     

Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO–H2–O2 reactions

The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H₂ in the presence of excess O₂ (NO–H₂–O₂ reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H₂, 10% O₂, and He balance yielded N₂ with less than 10% selectivity, which could not be increased by changing Pt loading or H₂ concentration in the gas feed. Co-impregnation of NaHCO₃ and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO_x conversion (>90%) and the N₂ selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N₂ selectivity was attained by the post-impregnation of NaHCO₃ onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO₂⁻-type species, which would play a role of an intermediate to yield N₂. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H₂–O₂ reaction at low temperatures.     

Effect of temperature and time in the hydrothermal treatment of HY zeolite

Zeolite HY was steamed at 1 bar water pressure in steps of 100 K from 473 K to 1073 K in dependence of the time (0.5–20 h). Three routes of hydrothermal reactions can be outlined: At low temperature (473 K), a partly framework dissolution occurs accompanied by the formation of X-ray amorphous material. At intermediate temperatures (773–873 K), framework dealumination takes place with the occurrence of different non-framework species. Thereby, with increasing steaming time, saturation of the Si/Al ratio occurs on low levels (Si/Al ratios of about 4 and 5, respectively). At 973 K, a crossover from a low level saturation in Si/Al (6) towards higher values (11) is observed, which is accompanied by a partial amorphization of the zeolite with formation of aluminium oxide and silica gel and a significant decrease in crystal size as well. At high temperatures (1073 K), a fast thermal decomposition occurs. The IR spectra directly reveal an aluminosilicate phase as decomposition product of the framework. At 573 K, the zeolite framework remains unchanged.     

High-temperature adsorption of n-alkanes on ZSM-5 zeolites: influence of the Si/Al ratio and the synthesis method on the low-coverage adsorption properties

The influence of the composition and synthesis method on the low-coverage adsorption properties of C₅–C₉ n-alkanes on ZSM-5 zeolites was studied using the pulse chromatographic technique at temperatures between 200 and 400 °C. Experiments were performed with materials having Si/Al ratios between 12 and 400, synthesized with and without an organic template. For all ZSM-5 samples, the Henry adsorption constants increase exponentially with the carbon number, while zero-coverage adsorption enthalpies increase in a linear way. With decreasing Al content, the Henry constants and adsorption enthalpies decrease. An increase in adsorption enthalpy of 10.1 kJ/mol per added –CH₂– group is observed for an Si/Al ratio of 400, while an increase of 12.1 kJ/mol is found for an Si/Al ratio of 15. The contribution to the adsorption entropy per carbon atom depends on the ZSM-5 composition and varies between 11.2 and 14.4 J/(mol K). A significant effect of the synthesis method on the Henry constants, adsorption enthalpies and entropies is observed. All ZSM-5 samples synthesized using organic templates show the same unique relationship between adsorption enthalpy and entropy, different from that of zeolites synthesized without organic template.     

Co-ZSM-5 catalysts for N2O decomposition

The catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. TPR, XRD, N₂-BET, XPS, FTIR and UV–vis spectroscopy were used for characterizing the samples. Well-dispersed cobalt oxide-like species and isolated Co²⁺ ions in charge compensation positions were found in the zeolite. Catalysts prepared using a single step cation exchange method showed high activity for N₂O decomposition in a temperature range 300–550°C, in the presence of 0–5% O₂, and high stability in the presence of 10% H₂O to the feed. UV–vis spectra and TPR experiments indicated the presence of some cobalt oxides, not detected by DRX, in a Co-ZSM-5 catalyst containing 3.76 wt% Co, prepared by a solid-state reaction procedure. The N₂O conversion over this catalyst was strongly affected by addition of both O₂ and H₂O to the feed.     

Determination of active palladium species in ZSM-5 zeolite for selective reduction of nitric oxide with methane

The active site in ZSM-5 zeolite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K, and almost all the NO was desorbed at around 673 K, while the palladium species on PdO/SiO₂ hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na⁺ in NaCl solution, indicating that they exist in a cationic state of an isolated Pd²⁺. This method for quantitative analysis of the isolated Pd²⁺ cations is named as ‘NaCl titration’. The amount of the isolated Pd²⁺ cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd²⁺ cation was unchanged after the reaction of NO₂–CH₄, NO₂–CH₄–O₂, or CH₄–O₂ at 673 K, while the adsorbed amount of NO per the Pd²⁺ as determined by NO-TPD decreased after the NO₂–CH₄–O₂ reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO₂–CH₄–O₂ reaction increased with increasing amount of the isolated Pd²⁺ cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO₂ reduction. The palladium species that are active and selective for NO reduction with CH₄ will be proposed.     

脱水处理对HZSM-5分子筛甲苯择形歧化性能的影响

通过²⁷Al MAS NMR、NH₃-TPD和FT-IR等表征手段研究了脱水处理对HZSM-5分子筛催化剂的结构和催化性能的影响。HZSM-5分子筛经过371 ℃水蒸汽处理后,分子筛的部分骨架铝脱出,形成非骨架铝物种,该非骨架铝物种对甲苯择形歧化反应具有较大影响。HZSM-5分子筛的酸性变化主要是由于非骨架铝物种SiAl（OH）的变化而引起,随着脱水温度的升高,HZSM-5分子筛酸中心的强度下降,但酸中心数量增加,当脱水温度为400 ℃时,HZSM-5分子筛的酸中心强度和数量达到最佳,甲苯转化率为28.2%。脱水过程中,甲苯的引入可以阻止HZSM-5分子筛催化剂进一步脱水,起到稳定催化剂活性中心的作用,使催化剂的性能较为稳定。     

ZSM-5分子筛形貌及硅铝比对甲缩醛气相羰基化反应性能的影响

通过调控合成方式、改变原料比例,制备纳米棒状、纳米球状、椭球状、圆柱状及棋子状等不同形貌及硅铝比的ZSM-5分子筛,并对其催化甲缩醛气相羰基化反应性能进行详细考察。在110℃、0.6 MPa、CO与甲缩醛流速分别为100 mL·min-1和0.035 mL·min-1条件下,硅铝物质的量比为30的棋子形ZSM-5分子筛表现出最佳的催化活性,甲缩醛转化率达31.9%,目标产物甲氧基乙酸甲脂选择性为21.4%。通过XRD、SEM、XRF、Py-FTIR、NH 3-TPD以及27 Al MAS NMR等对合成的分子筛进行详细表征,发现调控分子筛形貌及硅铝物质的量比可改变ZSM-5分子筛的酸性特征,并改变分子筛骨架中活性铝物种分布。适量的中强B酸酸位及分子筛交叉孔道内较高比例的活性铝物种分布可能是硅铝物质的量比30的棋子形ZSM-5分子筛表现出较好催化活性的原因。     

Shape selectivity of trace by-products for supercritical water oxidation of 2-chlorophenol effected by CuO/ZSM-48

Shape selectivity of trace by-products in supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) catalyzed by CuO/ZSM-48 has been investigated. Experimentally, destruction efficiency of 2CP in the SCWO process is effectively enhanced by CuO/zeolite catalysts. In the two-dimensional (2D) channels of ZSM-48, formation of undesired by-products such as polycyclic aromatic hydrocarbons (PAHs) and higher chlorinated phenols (via the Cl-reinsertion) is extremely suppressed in the SCWO of 2CP compared to those observed for zeolites ZSM-5 and Y with three-dimensional (3D) channel structure and larger pore sizes, respectively. The main oxidation active species on CuO/ZSM-48 surfaces were CuO and Cu₂O determined by X-ray photoelectron spectroscopy (XPS). Ratios of the surface species CuO/Cu₂O are between 13.4 and 14.1 in the SCWO process. However, the existence of the Cu–O and Cu–Cu species with a Cu–O/Cu–Cu ratio of 3.5 in the copper catalyst is also observed by extended X-ray absorption fine structure (EXAFS) spectroscopy. Furthermore, since the absence of Cu–Cl species in the XPS and EXAFS spectra, one would suggest that copper in the channels of ZSM-48 is unlikely involved in the abstraction of Cl species from 2CP in the SCWO process.     

Conventional and microwave hydrothermal synthesis of zeolite ZSM-5 from the cupola slag

ZSM-5 type zeolites have been prepared from cupola slag waste using both conventional hydrothermal and microwave syntheses at 130–200 °C. The ZSM-5 was synthesized by conventional heating by taking advantage of the high silica content of cupola slags. Microwave heating increased the rate of ZSM-5 formation by 4 times at 150 °C compared with conventional heating. The Si/Al ratio of the ZSM-5 produced by the conventional heating and the microwave crystallization were similar 28 and 29, respectively. The conventional-heating produced ZSM-5 particles 3 μm in diameter, while, microwave-heating produced smaller ZSM-5 particles only 0.3 μm in size.     

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH₃ in the presence of O₂ has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T_R<513 K, 2503 (ppm) < 4000, 12 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O₂, and essentially 0 with respect to NH₃. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH₃ was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu^I is oxidized by O₂ to “Cu^II-oxo”, (ii) “Cu^II-oxo” reacts with NO to yield “Cu^II-N_xO_y”, and (iii) finally “Cu^II-N_xO_y” is reduced by NH₃ to give N₂, H₂O, and the regeneration of Cu^I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu^I or “Cu^II-oxo” species constitute the rate-determining active center.     

Catalytic combustion of methane over palladium exchanged zeolites

Palladium cation exchanged zeolites (ZSM-5, mordenite and ferrierite) were studied as catalysts for methane combustion. Pd-zeolites showed much higher activities than PdO/Al₂O₃. For comparable palladium loadings, PdO/Al₂O₃ requires a reaction temperature of ca. 70–80°C higher than Pd-ZSM-5 for conversions between 50–100%. The catalytic activity of Pd-ZSM-5 seems to be related to its reducibility. Temperature-programmed reduction experiments with carbon monoxide showed a lower reduction temperature (ca. 157°C) for Pd-ZSM-5 than for PdO/Al₂O₃ (225°C). Further, the positioning of the palladium by ion exchange offers a highly dispersed form of Pd^II supported on the high surface area zeolite.     

Characterization of the textural properties of metal loaded ZSM-5 zeolites

The textural properties of metal ion over-exchanged zeolites, Co–, Cu–, Ni–ZSM-5, were compared with those of the parent ZSM-5 by nitrogen adsorption measurements and photoelectron spectroscopy. The treatment of the adsorption isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the Dubinin–Radushkevich method. The “t-plot” method was used to determine the external surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath–Kawazoe approach. The Dubinin–Radushkevich method overestimated the microporous volume of the zeolites, in particular when the solid had an important external surface. It is shown that Cu–ZSM-5 had micropore size and micropore size distribution very close to those of ZSM-5, whereas Co–ZSM-5 and Ni–ZSM-5 had lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, r_s, and the mesoporous to microporous volume ratios, r_v, are proposed as practical parameters to classify the solids with respect to their microstructure. ZSM-5, a pure microporous solid, has low values of both r_s and r_v parameters. Ni– and Co–ZSM-5 have high r_s ad r_v parameters, indicative of some degree of mesoporous character. Cu–ZSM-5 has intermediate characteristics, typical for micro-mesoporous solid. XPS measurements confirmed that copper ions penetrated to a greater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, which merely deposited on the external surface of ZSM-5 zeolite as oxidic aggregates.