In situ XAFS Study of Catalytically Active Species on Brown Coal During Coal Conversion Process

The catalysis and structure of nickel species supported on Loy Yang brown coal during coal conversion processes were studied. The gas evolution profile from the nickel-loaded coals depends on the amount of nickel loaded. The evolution of carbon monoxide was promoted when small nickel metal particles were formed during pyrolysis. Methane was selectively formed on large nickel metal particles during hydrogasification.     

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with¹³C in CH₃ or C-O groups (t-BuOH[2–¹³C₂] andt–BuOH[1-¹³C]), as well as selectively deuterated in methyl groups (t-BuOH[2-²H₉]), was studied on H-ZSM-5 zeolite simultaneously with¹³ C CP/MAS and²H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2–¹³C₁] andt-BuOH[1–¹³C] give rise to identical₁₃C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the side reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.     

Methanol to Hydrocarbon Catalysis on Sulfated Zirconia Proceeds Through a Hydrocarbon-Pool Mechanism

Aromatic hydrocarbons and other cyclic organic species have recently been identified as the locus of carbon--carbon bond forming and breaking in the conversion of methanol to olefins (MTO) on microporous solid acids such as HZSM-5 and HSAPO-34. In order to ascertain whether or not this hydrocarbon-pool mechanism for MTO catalysis is unique to zeotype solid acids, we studied the stronger solid acid sulfated zirconia. This study establishes a greater generality for the hydrocarbon-pool mechanism.     

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to²H NMR, the proton of the acidic Al-OH-Si group of the zeolite is transferred into the CH₂= group of the octene-1 molecule. (2) According to¹³C NMR the¹³C label inserted into the terminal CH₂= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.     

13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a¹³C nucleus in the CH₂ group (i-BuOH[1–¹³C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using¹³C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.     

13C solid state NMR evidence for the existence of isobutyl carbenium ion in the reaction of isobutyl alcohol dehydration in H-ZSM-5 zeolite

Using two-dimensionalJ-resolved and CP/MAS¹³C NMR, the pathway for the transfer of the¹³C label from the CH₂ group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydration inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH₂ group of the isobutyl silyl ether reaction intermediate (IBSE), and then into the CH and CH₃ groups of the isobutyl fragment (-CH₂CH(CH₃)₂) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl carbenium ion is formed as a reaction intermediate or transition state during the transformation of isobutyl silyl ether into butene oligomers.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

In situ MAS NMR spectroscopic investigation of the conversion of methanol to olefins on silicoaluminophosphates SAPO-34 and SAPO-18 under continuous flow conditions

Applying in situ ¹³C MAS NMR spectroscopy under continuous flow (CF) conditions and on-line gas chromatography, the conversion of methanol on silicoaluminophosphates SAPO-34 and SAPO-18 was studied. At reaction temperatures of 573 to 673 K, a strong increase of the yield of propylene and ethylene was observed by on-line gas chromatography. Simultaneously with the occurrence of in situ ¹³CC CF MAS NMR signals at 126–135 ppm indicates the formation of a mixture of aliphatic and/or cyclic olefins and alkylated aromatics with carbon numbers of C₆–C₁₂. All these support the hydrocarbon pool mechanism for the formation of light olefins on the SAPO-34 and SAPO-18.     

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.     

Methanol to Hydrocarbons: Enhanced Aromatic Formation Using Composite Group 13 Oxide/H-ZSM-5 Catalysts

The conversion of methanol to hydrocarbons using composite catalysts comprising physical mixtures of the zeolite H-ZSM-5 with group 13 oxides (-Al₂O₃, -Ga₂O₃, In₂O₃, Tl₂O₃) is reported and discussed. The addition of -Ga₂O₃ at 400 °C gives a marked enhancement in the yield of C₈ and C₉ aromatic compounds, whereas the addition of -Al₂O₃ has no effect and both the In₂O₃/H-ZSM-5 and Tl₂O₃/H-ZSM-5 are inactive. At 300 °C, a marked enhancement in the yield of aromatic hydrocarbons is observed for -Ga₂O₃ and In₂O₃, and a less marked enhancement is observed with Tl₂O₃ and -Al₂O₃. In particular, the addition of In₂O₃ to H-ZSM-5 as a simple physical mixture gives a significant enhancement in catalyst activity at 300 °C. The effect of the Si:Al atomic ratio of H-ZSM-5 is also investigated for the -Ga₂O₃/H-ZSM-5 composite catalysts and the enhancement in aromatic yield is observed with all the ratios investigated but the optimal -Ga₂O₃/H-ZSM-5 ratio is dependent upon the Si:Al ratio. Pretreatment or co-feeding of hydrogen decreases the yield of the aromatic products. The results are explained in terms of an active site formed by the interaction between the oxide and the zeolite.     

The Effect of Different Active Sites on the Catalytic Activity of Fe-ZSM-5 Zeolite for N2O Direct Decomposition

Fe-modified ZSM-5 zeolites (Si/Al = 25) were prepared by adopting the liquid ion-exchange method with nitrate and oxalate of iron as Fe precursors and their catalytic performance was studied in the N₂O decomposition reaction. The results of FT-IR and H₂-TPR investigations indicated that (i) part of the iron ions could replace Brönsted acid protons at the straight channel wall (α sites), intersection of straight and sinusoidal channels (β sites), and sinusoidal channel wall (γ sites) within the ZSM-5 zeolite; and (ii) different Fe precursors gave rise to various distributions of α, β, and γ sites. We observed that the Fe-ZSM-5 catalyst prepared with iron oxalate as Fe precursor outperformed the ones prepared with iron nitrate as Fe precursor in the direct decomposition of N₂O. Furthermore, the catalytic activity of iron ions located at the α sites was higher than those of iron ions located at the β and γ sites.     

Modification of the ZSM-5 zeolite using Ga and Zn impregnated silica fibre for the conversion ofn-butane into aromatic hydrocarbons

The modification of ZSM-5 by metals is generally performed by methods such as ion-exchange, impregnation, chemical vapour deposition and physical mixing. In this work a silica fibre pre-impregnated with Ga and Zn has been used during the synthesis by which Ga-ZSM-5 and Zn-ZSM-5 zeolite catalysts were prepared. The synthesised zeolite catalysts were characterised by an X-ray powder diffractometer, a scanning electron microscope, an X-ray fluorescence spectrometer, a solid state NMR, an energy dispersive X-ray analyser and a sorptomatic 1900. The acidity of the catalysts was determined by temperature programmed desorption of ammonia. The reaction ofn-butane to aromatic hydrocarbons was carried out over the H-ZSM-5, Ga-ZSM-5 and Zn-ZSM-5 catalysts and it was observed that these catalysts exhibited high catalytic activity in the conversion ofn-butane and formation of aromatic hydrocarbons. The effect of temperature on the conversion ofn-butane and formation of aromatic hydrocarbons was studied at 713, 743, 773 and 803 K with a space velocity (WHSV) of 2.5 h^–1. The effect of time on stream on then- butane conversion and formation of aromatic hydrocarbons over the synthesised catalysts was investigated at 803 K for 4.5 h.     

Synthesis, characterization and evaluation of a novel resid FCC catalyst based on in situ synthesis on kaolin microspheres

A new way was provided for in situ synthesized FCC catalyst. Characterization and evaluation results indicated the catalyst is more suitable for cracking resid feed because of its unique manufacturing process which makes this catalyst has appropriate pore structures in which large resid molecules are more accessible to the active sites. The high zeolite content (above 40%) keeps the catalyst having higher activity and selectivity. These advantages ensure the in situ catalyst developed more adaptable for cracking resid and resid-containing feedstocks, particularly the feed containing large amounts of contaminant metals.     

Combined reforming of methane over supported Ni catalysts

The hydrogen exchange for propane-d ₈ adsorbed on zeolite Zn/H-MFI has been studied by¹H MAS NMR spectroscopy in situ within the temperature range of 420–490 K. Kinetic measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that only methyl groups of the alkane are involved in the exchange. Two mechanisms are proposed to rationalize the regioselectivity of the exchange: (i) propane dehydrogenation on Zn-sites followed by protonation of propene by acidic OH groups in accordance to the Markovnikov’s rule and abstraction of deuteride ion from another propane molecule; (ii) the reversible heterolytic dissociative adsorption of propane to form Zn-propyl species and acidic OH groups.     

Studies on the mechanism of ZSM-5 formation

The nucleation of (Al-free) zeolite precursor gels was studied using X-ray diffraction,²⁹Si FT-NMR, and ion exchange. Results suggest that in ZSM-5 nucleation, the channel intersections are first formed. These clathrate-like units, each containing essentially one TPA⁺ cation, are initially randomly connected, but progressively anneal with rearrangement under the influence of OH^– ions to form the ZSM-5 framework.     

Influence of the Oxidation State of Rhodium in Three-Way Catalysts on Their Catalytic Performances: An in situ FTIR and Catalytic Study

Simultaneous IR spectroscopic and catalytic measurements have been performed in order to investigate the nature of adsorbed species involved in the formation of N₂O on Rh/Al₂O₃ in the course of the CO + NO reaction. Only nitrosyl species have been isolated that could be involved in the formation of N₂ and N₂O in accordance with previous kinetic investigations [Granger et al. J. Catal. 175(1998) 194]. Sequential and simultaneous NO and CO exposures lead to the observation of different nitrosyl species that could act as intermediates in the formation of N₂ and N₂O. Correlations between the appearance/disappearance of Rh(NO) ⁺ species and an extra formation of N₂O have been established.     

Dehydrogenation of Cyclohexane Over Ni Based Catalysts Supported on Activated Carbon using Spray-pulsed Reactor and Enhancement in Activity by Addition of a Small Amount of Pt

The polycrystalline silver after pre-treated with oxygen at high temperature was found to show excellent activity and selectivity in the process of direct dehydrogenation of methanol to anhydrous formaldehyde and the active oxygen species for this reaction was observed and proposed as the active centeres with in situ Raman scattering.     

In situ nanoscale wet imaging of the heterogeneous catalyzationof nitriles in a solution phase: novel hydrogenation chemistry through nanocatalysts on nanosupports

Wet-environmental transmission electron microscopy studies of heterogeneous hydrogenation of complex nitriles in a liquid phase over new mesoporous cobalt-promoted ruthenium nanocatalysts on reducible nanotitania supports are presented. The desorbed organic products in the dynamic liquid phase hydrogenation are imaged situ on the nanoscale. The direct studies on the “nanocomposite” catalysts are correlated with parallel reaction chemistry measurements. They demonstrate high hydrogenation activity at low operating temperatures in the presence of atomic scale anion vacancy defects associated with Lewis acid sites at the nanosupport surface and an electronic and synergistic contribution to the promoter mechanism. The combined synergistic effect between the two metals and the interaction with the reduced nanosupport leading to an electronic modification lead to highly reactive site for the hydrogenation catalysis. The results illustrate novel selective hydrogenation chemistry with mesoporous nanocatalyst systems on nanosupports.     

Mechanism for the reduction of NOx on Rh/sulfated titanias

Titanias of different surface areas have been sulfated and used as supports of Rh oxide for the selective catalytic reduction of nitrogen oxides. Only the sulfation of TiO₂ of large surface areas gives strong Br?nsted acid sites, retaining pyridine up to 773 K. Low surface area anatase is unable to retain sulfates. The catalytic activities measured at 523 K, increase with the number of acid sites, and then reach a plateau, showing the intervention of acidity in the SCR. This is true only for Rh but not for Pt. The investigation of the elemental steps on Rh/sulfated TiO₂ by in situ diffuse reflectance spectroscopy permits to clarify a few points: the oxidation of propene, presumably to acetaldehyde, occurs by the reaction with nitrates adsorbed on the support. The further oxidation of this intermediate by NO₂ yields an isocyanate, which can be hydrolysed to ammonia.     

Infrared Evidence for the Existence of Nitrate Species on Cu-ZSM5 During Isothermal Rate Oscillations in the Decomposition of N2O

Isothermal oscillations in the rate of decomposition of N₂O were studied on an over-exchanged Cu-ZSM-5 catalyst by mass spectroscopy and in situ transient FTIR. Oscillations in the production of O₂ and N₂ were observed to occur in a temperature range of 410–490°C at a total pressure of 1.0 Torr pure N₂O. FTIR has provided the first spectroscopic evidence that surface nitrate species are present under oscillatory conditions. This study confirmed a previously proposed model that predicts a slow build-up of surface nitrates, followed by a rapid nitrate decomposition coupled with an increase in the rate of N₂O decomposition. The IR signature of the surface nitrates suggests they are monodentate nitrate species bound to Cu²⁺ ions. Temperature-programmed desorption studies reveal a strong correlation between the stability of the surface nitrate species and the temperature range in which oscillations occur.