Preparation of In,H-ZSM-5 for DeNOx reactions by solid-state ion exchange

A simple method is proposed to prepare In,H-ZSM-5 catalyst for DeNOx reactions. This consists of mechanically mixing the fine powders of In₂O₃ and H-ZSM-5 followed by heating in oxygen free inert gas flow to 580 °C where indium undergoes thermal auto-reduction and moves into exchange positions as In⁺ without destroying the crystalline structure of the zeolite.It was evidenced by IR, temperature-programmed reduction (TPR) and reoxidation that, once In⁺ was introduced into the lattice either by reductive solid-state ion exchange (RSSIE) or by thermal auto-reductive SSIE, it can be oxidized by O₂ or in the DeNOx reaction to (InO)⁺. The formed (InO)⁺ can easily be reduced to In⁺ suggesting that In,H-ZSM-5 might be a good catalyst for reactions where a redox cycle in the catalyst is involved in the reaction mechanism.Selective catalytic reduction (SCR) by methane proved that only a small fraction of In exchanged, together with some acid sites of the zeolite formed the active center for the catalytic reaction. XRD, XPS and FT-IR using pyridine proved that the structure of the zeolite and these centers are stable under reaction conditions and In is mainly in the form of (InO)⁺ in the used catalyst.     

Ir/H-ZSM-5 Catalysts in the Selective Reduction of NOx with Hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir⁰ and IrO₂. The presence of Ir⁰ was essential for high DeNOx activity. The ratio of Ir⁰/Ir⁴⁺ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir⁰. Crystallite size and oxidation state of Ir have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.     

Influence of the support on CO2 methanation over Ru catalysts: an FT-IR study

The hydrogenation of CO₂ to methane has been investigated over Ru catalysts supported on zeolite (H-ZSM-5) and on silica. Supported Ru catalysts were very active for the hydrogenation of CO₂. Ru/ZSM-5 was more selective to methane than Ru/SiO₂. On the basis of FT-IR spectra of CO and CO₂ adsorbed on the catalysts, it has been suggested that this behaviour can be related to a higher positive polarization of ruthenium on the zeolite. This leads to a weaker Ru–CO bond on the H-ZSM-5-supported sample with a corresponding increase of the hydrogen surface coverage that favours the transformation of the intermediate CO to methane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective Production of 2-Phenylhexane from Benzene and <Emphasis Type="Italic">n</Emphasis>-Hexane Over Pt- and Ga-Modified Zeolites

Abstract  

Alkylation of benzene with n-hexane was performed over H-ZSM-5 and monometallic Ga- and Pt- and bimetallic Ga- and Pt-modified ZSM-5. The influence of the particle size and the method of incorporation of Ga (during hydrothermal synthesis, by solid-state ion exchange, or by liquid-state ion exchange) was determined. The presence of Pt and well-dispersed extraframework Ga in H-ZSM-5 increased the selectivity in alkylation and suppressed cracking reactions. Well-dispersed Pt particles led to better catalytic performance. The method of Ga incorporation played an important role in obtaining higher selectivity to alkylation products and in the suppression of side reactions. Up to 93% selectivity in alkylation (of which >95% was to 2-phenylhexane) was reached over 2 wt% Pt/H-Ga^fZSM5, in which Ga occupied framework positions. We propose that the close proximity of very small Pt nanoparticles and Ga–(OH)–Si acid sites results in the optimal bifunctional catalyst for selective production of 2-phenylhexane from benzene and n-hexane. During the reaction, the catalyst deactivated, most probably due to the sintering of the Pt particles.     

Influence of the V + Mo/Al ratio on vanadium and molybdenum speciation and catalytic properties of V–Mo–ZSM-5 prepared by solid-state reaction

V–Mo–ZSM-5 catalysts with various composition prepared by solid-state ion exchange were investigated with respect to their physico-chemical characteristics using chemical analysis, XRD, BET, DRIFT, UV–vis, ²⁷Al MAS-NMR spectroscopy, H₂ TPR and TPD of NH₃. It was found that all the preparations leads to either metal ions sitting at the bridging oxygen of Si–OH–Al or anchored at Si–OH groups or deposited as oxide. These different solids were tested in the selective catalytic reduction of NO_x by ammonia. The main result is that upon addition of small amount of Mo to V–ZSM-5, catalytic performances were enhanced.     

Selective Catalytic Reduction of NO with NH3 over Fe-ZSM-5 Catalysts Prepared by Sublimation of FeCl3 at Different Temperatures

Fe-ZSM-5 catalysts were prepared by subliming FeCl₃ into H-ZSM-5. The method used allowed Fe-ZSM-5 catalyst preparation by FeCl₃ exchange at a desired sublimation temperature and was found to be more precise. The sublimation of FeCl₃ into H-ZSM-5 was carried out at 320 and 700 °C. Fe-ZSM-5 prepared by sublimation of FeCl₃ at 320 °C followed by rapid heating to 700 °C and the catalyst prepared by subliming FeCl₃ at 700 °C were found to be more active for NO reduction with NH₃ in the presence of simulated exhaust gases containing water vapor than catalysts prepared by subliming FeCl₃ at 320 °C. To determine the active sites, the catalysts were characterized by H₂-TPR, in situ DRIFTS of NO adsorption, NH₃-TPD, XRD and chemical analysis methods. The observed NO conversion differences in selective catalytic reduction using NH₃ could be correlated to the iron cation species present at different locations determined from diffuse reflectance infrared spectroscopy. Enhanced NO reduction activity was obtained when positions in Fe-ZSM-5, corresponding to Fe²⁺(NO) band at 1877 cm^-1 in DRIFTS, were preferentially occupied.     

13C MAS NMR mechanistic study of the initial stages of propane activation over H-ZSM-5 zeolite

¹³C MAS NMR study of the early stages of propane 2-¹³C activation was performed over H-ZSM-5 catalysts with various content of protonic and aprotonic sites. The reaction mechanism was tested by addition of various probe-molecules (C₃H₆, C₆H₆, H₂, H₂O and CO). The results on tracing the fate of ¹³C label during this experiments conclude to a monofunctional mechanism involving propane protonation on the strong Brønsted sites of H-ZSM-5 and the formation of carbonium ion type transition states, which further evolve in four different ways leading to ¹³C scrambling in propane, cracking, dehydrogenation and disproportionation.     

Direct conversion of cellulose into polyols or H2 over Pt/Na(H)-ZSM-5

The direct conversion of cellulose into polyols such as ethylene glycol and propylene glycol was examined over Pt catalysts supported on H-ZSM-5 with different SiO₂/Al₂O₃ molar ratios. The Pt dispersion, determined by CO chemisorption and transmission electron microscopy (TEM), as well as the surface acid concentration measured by the temperature-programmed desorption of ammonia (NH₃-TPD), increased with decreasing SiO₂/Al₂O₃ molar ratio for Pt/H-ZSM-5. The total yield of the polyols, i.e., sorbitol, manitol, ethylene glycol and propylene glycol, generally increased with increasing Pt dispersion in Pt/H-ZSM-5. The one-pot aqueous-phase reforming of cellulose into H₂ was also examined over the same catalysts. The Pt catalyst supported on H-ZSM-5 with a moderate SiO₂/Al₂O₃ molar ratio and a large external surface area showed the highest H₂ production rate. The Pt dispersion, surface acidity, external surface area and surface hydrophilicity appear to affect the catalytic activity for this reaction.     

Vapour Phase Hydrogenation of Cinnamaldehyde over Ni/γ-Al2O3 Catalysts: Interesting Reaction Network

The vapour phase hydrogenation of cinnamaldehyde over Ni loading γ-Al₂O₃ catalysts was performed at 1 atm and 300 °C in a fixed-bed reactor. The major product was hydrocinnamaldehyde. Unexpected products of styrene and ethylbenzene due to new reaction pathway of hydrodeformylation were observed. The results indicated that Ni metal was the active centers and the catalytic activity was parallel to the Ni surface area. The stability of TOF values implied that the cinnamaldehyde hydrogenation over Ni/γ-Al₂O₃ catalysts was a structure insensitive reaction.     

Gallium-containing zeolites – valuable catalysts for the conversion of cycloalkanes into a premium synthetic steamcracker feedstock

Gallium is incorporated into zeolite H-ZSM-5 by reductive solid-state ion exchange, i.e., by heating a physical mixture of gallium oxide and H-ZSM-5 in a hydrogen flow at 600°C. The samples are characterized by AES/ICP, XRD, FTIR and MAS NMR spectroscopy. Methylcyclohexane is converted over these catalysts in a hydrogen atmosphere at 6 MPa and 250 to 400°C, yielding a high-quality synthetic steamcracker feedstock (consisting mainly of ethane, propane and n-butane). Mechanistic aspects of the reaction are discussed, and the importance of non-classical Haag-Dessau cracking is emphasized.     

催化脱氢合成邻苯基苯酚工艺研究

通过离子交换法制得的载钯ZSM-5分子筛Pd/H-ZSM-5和Pd/Mg-ZSM-5可以作为以环己酮为原料合成邻苯基苯酚的脱氢催化剂,其结果催化剂的活性衰减速度明显降低。由于低硅铝比分子筛表面容易形成多聚物,进而导致催化剂脱氢活性明显降低。实验表明用镁改性后的分子筛为载体制得的催化剂具有较好的催化脱氢性能。     

Aromatization of methane in the absence of oxygen over Mo-based catalysts supported on different types of zeolites

The catalytic performance of Mo-based catalysts supported on various zeolites has been studied for methane aromatization in the absence of oxygen in a fixed-bed continuous-flow quartz reactor, and their catalytic properties are correlated with features of zeolite structure. It was found that H-type silica–alumina zeolites, such as ZSM-5, ZSM-8, ZSM-11 and β possessing two-dimensional structure and pore diameter equaling the dynamic diameter of a benzene molecule (about 6 Å) simultaneously, are fine supports for methane activation and aromatization catalysts. Among them, MoO₃/H-ZSM-11 has the best activity and stability; for instance, a methane conversion of 8.0% and selectivity higher than 90% was obtained at 973 K. The catalytic performance of MoO₃/H-ZSM-8 is somewhat lower than that of MoO₃/H-ZSM-5, while activity of MoO₃/H-β is lower than that of MoO₃/H-ZSM-8. Catalysts supported on H-MCM-41 and H-SAPO-34 exhibit low activity for methane aromatization and those supported on H-MOR, H-X and H-Y give only a little amount of ethylene. Over MoO₃/H-SAPO-5 and MoO₃/H-SAPO-11 no hydrocarbons were detected.     

Identification and Influence of Acidity on Alkylation of Phenol with Propylene over ZSM-5

The alkylation of phenol with propylene has been studied over several H-ZSM-5s with different Si/Al ratios and Cs⁺-ion-exchanged H-ZSM-5s at temperature range 373–623°C. Both O- and C-alkylation, which were closely dependent on the reaction temperature and acidity of the catalysts, were observed. O-alkylated compound is found to be formed preferably at temperature lower than 250°C and over Cs⁺-ion-exchanged H-ZSM-5s. However, at higher temperature, only C-alkylation is observed. The acidic properties of the zeolites were characterized by solid-state ³¹P MAS-NMR of the probe molecule trimethylphosphine oxide and NH₃-TPD (temperature-programmed desorption) and it is suggested that in the case of C-alkylation, moderate acid sites are responsible for the formation of para-isopropylphenol, while ortho-isopropylphenol is favorable for weak acid sites.     

Mesostructured CeO2 as an Effective Catalyst for Styrene Synthesis by Oxidative Dehydrogenation of Ethylbenzene

A new type of mesostructured ceria material was synthesized via template-assisted precipitation method and tested for the oxidative dehydrogenation (ODH) of ethylbenzene to styrene by molecular oxygen. The effect of calcination temperature on the catalytic performances of the ceria catalysts has been investigated. Among the catalysts tested, the CeO₂-450 sample derived by calcination at 450 °C exhibited the highest ethylbenzene conversion (34%) and styrene selectivity (87%). Comparing the reaction rates for ODH of ethylbenzene (ca. 6.1 mmol ST g _cat ^?1 h^?1 at 450 °C) with the highly active nanostructured carbon-based catalysts in the current literature confirmed the very high activity of these new materials. The superior catalytic performance of the CeO₂-450 sample can be attributed to its high specific surface area and enhanced redox properties as revealed by H₂-TPR measurements.     

Preparation and catalytic efficiency of mixed noble metal catalysts on electrochemically activated carbon fibre supports

Mixed noble metal catalytic systems were prepared on electrochemically activated PAN-based carbon fibre supports through a procedure of repetitive cation exchange between the acidic (-OOOH and-OH) groups of the electrooxidized carbon surface and noble metal salts and subsequent cathodic reduction of the exchanged noble metal ions to the metallic state.The whole investigation was carried out for the binary system Ag-Pd. The catalytic efficiency of palladium deposited on silver is more pronounced compared to a Pd-deposition obtained after only one Pd²⁺-exchange procedure and subsequent cathodic reduction, whereas an inhibition of the catalytic activity of palladium is noticed when it is covered by silver deposition. The catalytic efficiency of the obtained mixed noble metal catalysts was studied by means of hydrogen-adsorption and absorption profiles in H₂SO₄, the electroreduction of nitrobenzene in aqueous methanolic H₂SO₄ solutions and the hydrogenation reaction of nitrobenzene to aniline in methanolic solutions. The study was completed by impedance spectroscopic measurements at the potential of hydrogen evolution in H₂SO₄ solutions.Mixed Ag-Pd systems are even more stable than double Pd-Pd depositions on activated carbon fibre supports, as shown by the fact that they retain their mechanical stability and catalytic activity even after prolonged storage in aqueous or methanolic solutions, as well as after ultrasonic treatment.     

Influence of binder on catalytic performance of Ni/HZSM-5 for hydrodeoxygenation of cyclohexanone

The influences of binders (alumina, silica sol, kaolin) on the performance of Ni/H-ZSM-5 for hydrodeoxygenation of cyclohexanone were investigated in a fixed-bed reactor. N₂ sorption, X-ray diffractions, H₂-temperature-programmed reduction, transmission electron microscopy, ²⁷Al MAS NMR, and temperature-programmed desorption of ammonia were used to characterize the catalysts. The obtained results exhibited that porosity and acidity of the catalysts were strongly influenced by the binders. The most outstanding catalytic performance was observed on catalyst with alumina binder, which bears a well-developed pore structure and more acid sites than the others. Thus, alumina was chosen as the optimum binder to Ni/HZSM-5.     

Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts for n-hexane hydroconversion

Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts were hydrochlorinated or hydrofluorinated with 3.0 wt%HCl or HF, respectively. These mono- and bimetallic catalysts were tested for n-hexane hydroconversion in a pulsed microcatalytic reactor in a flow of H₂ gas. The catalysts were characterized via XRD, metal dispersion via H₂ chemisorption and acid site strength distribution using temperature programmed desorption of ammonia (TPD). Although metallic promoters frequently cause inhibition of the catalytic activities of Pt and since iridium is less active than platinum, fortunately, Ir was found to enhance the hydroconversion activities of the current catalysts, particularly after hydrochlorination.     

Direct and transfer hydrogenation of furfural over MOF-derived Pd-Cu@C catalysts

Owing to the self-reducing ability of palladium acetate in solutions, an ethanol solution containing Pd⁰ particles was prepared and coated in-situ into copper metal–organic framework (Cu-MOF), forming Pd@Cu-MOF in a coated structure. The Pd@Cu-MOF was reduced under N₂ or H₂ to form carbon-coated Pd-Cu@C. The pyrolysis and carbonization of Cu-MOF and the reduction of Cu²⁺ were studied. The Cu-MOF under either N₂ or H₂ was pyrolyzed and carbonized, but the Cu²⁺ reduction mechanisms were different. The high-temperature carbothermic reduction of Cu²⁺ under N₂ produced Cu⁰, but during low-temperature reduction under H₂, the reducing H₂ reduced Cu²⁺ to Cu⁰. Furfural hydrogenation experiments showed that compared with H₂, the Pd-Cu@C prepared under N₂ reduction displayed higher furfural hydrogenation activity. The catalytic activity of Pd-Cu@C prepared from in-situ Pd⁰ coating was higher than the Pd/Cu@C prepared from the impregnation method. With i-propanol as the solvent, the catalytic hydrogenation of furfural under H₂ consisted of direct catalytic hydrogenation with molecular hydrogen as the hydrogen source and catalytic transfer hydrogenation with i-propanol as the hydrogen donor. The catalytic activity of direct catalytic hydrogenation is higher than the catalytic transfer hydrogenation.     

In situ MAS NMR investigations of molecular sieves and zeolite-catalyzed reactions

Solid-state MAS NMR is a powerful technique to study heterogeneous catalysts and the way by which they operate. In situ MAS NMR has been demonstrated to be a powerful method to understand reaction mechanisms, to study the nature, dynamics and reactivity of surface intermediates and active sites, and to characterize structural modifications in the catalyst itself, in particular when using ¹³C strategically labelled substrates. In this paper, three examples selected from our own work are used to illustrate the potential of in situ MAS NMR. They are the formation of cumene and its isomerization to n-propylbenzene on zeolite H-ZSM-11, the activation of propane at low temperature and the alkylation of benzene with propane on zeolite H-ZSM-5, and the characterization of the aluminophosphate molecular sieve VPI-5 structure with temperature. Studies of the alkylation of benzene with propene confirmed that cumene was the primary reaction product. The undesired n-propylbenzene by-product results from the intermolecular reaction between cumene and benzene, enhanced by molecular shape-selective effects in medium pore size zeolites (e.g., H-ZSM-11). It explains why large pore zeolites, e.g., zeolite Beta, are used commercially today for this process. Propane can be activated at low temperature (ca. 573 K) on bifunctional medium pore size zeolites possessing intimately related acidic Brønsted sites and a dehydrogenation function provided by Ga or Zn species. In Ga/H-ZSM-5 catalysts, at 573 K, the activation of propane was shown to occur via a protonated pseudocyclopropane (PPCP) intermediate (or transition state). The latter evolves in a manner that can be formally described by the formation of CH ₃ ⁺ , C₂H ₂ ⁺ , and C₃H ₇ ⁺ carbenium ion intermediates. These species can react with olefins, alkanes, or other electron-rich molecules such as benzene. The primary reaction products of the reaction of propane with benzene are n-propylbenzene (in small amount), ethylbenzene and toluene. Their subsequent reactions lead eventually to toluene and xylenes as the final products. In the structural characterization of VPI-5, ²⁷Al, ³¹P, and ²⁷Al nutation MAS NMR spectra show that, at 294 K, fully hydrated VPI-5 contains three equally populated Al and P crystallographic sites and that one-third of Al is 6-coordinate. The VPI-5 structure then belongs to the P6₃ space group. Above 353 K, VPI-5, fully or partially hydrated, undergoes a structural transformation to a higher framework symmetry, i.e., the P6₃cm space group. The transformation occurs at nearly the same temperature in both cases, indicating that the breakdown of the hydrogen-bonded helical water structure inside the VPI-5 pores is not a factor in the process.     

Pd/Fiber glass and Pd/5% γ-Al2O3/Fiber glass catalysts by surface self-propagating thermal synthesis

The technique of Surface Self-propagating Thermal Synthesis (SSTS) was used to prepare Pd/??-Al₂O₃/fiber glass (FG) catalysts for selective liquid-phase hydrogenation of acetylene in the presence of CO. The results of XRD SR analysis (in synchrotron radiation) in combination with the technique of arrested combustion shed new light on the dynamic of phase transformations in the systems under study and variation in the size of diffraction-active crystallites. The catalytic performance of synthesized catalysts was found to be close to that of similar conventionally prepared catalysts. The EXAFS and TEM data afforded to estimate the variation in relative amounts of Pd⁰ and PdO in synthesized catalysts. In the course of selective hydrogenation, PdO rapidly (<15 min) reduced to Pd⁰.