Aromatisation of propane over Ga/H-ZSM-5: comments on the activation of propane

The aromatisation of propane has been studied using Ga₂O₃, H-ZSM-5 and physical mixtures of Ga₂O₃/H-ZSM-5 as catalysts. Experiments using co-fed reactants of H₂, O₂ and NO are described, together with the use of 2-chloro-propane as a model reactant. The results are discussed in terms of a mechanism for the formation of the initial product propene in which propane is activated at the interface between the gallium oxide and the zeolite. The system is therefore an example of contact synergy and furthermore experiments are described that provide evidence for the reversible formation for the active site for this catalyst system.     

Revisiting Ga2O3/H-ZSM-5 propane aromatization catalysts

Oxidative or reducing treatments at 873 K of Ga₂O₃/H-ZSM-5 (Si/Al = 13 or 60) cause a change in H⁺ concentration as evidenced by IR spectroscopy of OH groups at 3610 cm^–1; this change is nicely correlated to the variation ofm-xylene isomerisation rate.     

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.     

Novel Mo/Ga/H-ZSM-5 catalyst in environmentally important reductive transformation of N2O in presence of CH4

Direct decomposition of N₂O and the reduction of N₂O with CH₄ over Ga/H-ZSM-5 and Mo/Ga/H-ZSM-5 (Si/Al = 40) catalysts in a plug flow reactor under steady-state conditions as well as by temperature programmed surface reaction (TPSR) have been investigated. Ga ions were ion-exchanged from liquid phase while Mo was deposited onto the Ga/H-ZSM-5 sample using incipient wetness technique. The catalysts were characterized by means of XRF, XPS, TPR, CO chemisorption, TEM and EDS. The N₂O forms redox centers in the Mo/Ga/H-ZSM-5 catalysts at elevated temperatures, which are extremely active in the reaction with CH₄ already at around 373 K. Addition of Mo to the Ga/H-ZSM-5 decreased the T₅₀ temperature in the N₂O decomposition and reduction of N₂O with CH₄ from 819 to 787 K and from 755 to 646 K, respectively. The oxidation/reduction of the Mo/Ga/H-ZSM-5 sample is more favoured in the interaction with N₂O/CH₄ as compared to that using O₂/H₂ and the mechanism of the redox reactions might also be different. The reduction of N₂O with CH₄ cannot be described with the Mars–van Krevelen redox mechanism, but by the participation of CH₄ via MoGa–OCH₃ species in a complex oxygen transfer mechanism is proposed at which N₂O does not directly reoxidise the reduced active centers.     

Effect of reductive and oxidative atmospheres on the propane aromatisation activity and selectivity of Ga/H-ZSM-5 catalysts

The aromatisation of propane was investigated at 823 K on two Ga/H-ZSM-5 catalysts differing by their aluminum content (Si/Al = 18 and 128, respectively), i.e. their Brönsted acid site concentration, and having similar gallium content (1.6 wt%) as a function of successive pretreatments by hydrogen and oxygen. Hydrogen treatment increases activity and selectivity to aromatics while decreasing the formation of methane. Subsequent oxygen treatment enhances further the activity and aromatics selectivity and has little effect on methane selectivity. These improvements of catalytic performance are explained by gallium migration in reducing conditions. The concentration of highly dispersed gallium species is thereby enhanced. These species, in association with Bronsted centers, were reported previously to constitute the dual catalytic sites responsible for alkane activation. Hydrogen-oxygen pretreatment of Ga/H-ZSM-5 catalysts is thus a means to ensure rapid catalyst preactivation and stabilisation in large scale light alkane aromatisation units.     

Aromatization of propane over Ga/H-ZSM-5: An explanation of the synergy observed between Ga3+ and H+

The aromatization of propane is investigated for Ga₂O₃, H-ZSM-5 and Ga₂O₃/H-ZSM-5 catalysts, and the results are discussed for a series of ZSM-5 catalysts containing varying SiO₂/Al₂O₃ ratios. It is apparent that on addition of a gallium phase to H-ZSM-5, the yield of methane is significantly decreased. These results are discussed with respect to the mechanism of formation of the initial reaction product from propane. It is proposed that the synergy observed between the gallium compound and the zeolite can be explained in terms of a mechanism in which the role of the gallium phase is to induce C-H bond polarization in the propane, which leads to attack via the Bronsted acid sites of the zeolite, which leads to initial C-H bond cleavage occurring.     

N2O decomposition on Fe- and Co-ZSM-5: A density functional study

Density functional theory (DFT) calculations are employed to study N₂O decomposition on relaxed [(SiH₃)₄AlO₄M] (where M = Fe, Co) cluster models representing Fe- and Co-ZSM-5 surfaces and Fe-ZSM-5 channel cluster. The catalytic cycle steps are completed both for Fe- and Co-ZSM-5 clusters. It is found that the general trend of the results obtained is in agreement with experimental and theoretical literature: Co-ZSM-5 has a lower activation energy barrier than Fe-ZSM-5 and O₂ desorption step is the rate-limiting step for both clusters. The activation barrier for the decomposition of the first N₂O molecule inside a Fe-ZSM-5 channel cluster increases in comparison with that of the cluster model indicating a channel effect on the activation barrier. The activation barrier reported for the channel cluster is 12.63 kcal/mol. This is also in good agreement with experimental literature.     

Dissociation of dihydrogen and hydrogen sulfide over a sulfided NiMo-alumina catalyst as evidenced by D2S-H2 isotopic exchange

To investigate the possible role of surface sulfur in the activation of dihydrogen, the reaction of D₂S with H₂ was carried out at 423 K on a presulfided NiMo-Al₂O₃ catalyst. The incorporation of D atoms in H₂ shows that the catalyst is capable of dissociating H₂S and H₂ into species which by recombination make the exchange of hydrogen atoms possible between these molecules. This can be considered as evidence of the involvement of surface sulfur in the activation of dihydrogen.     

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.     

Catalysis at the toluene/water interface: shape-selective hydrolysis of esters having some rings and lactones promoted by octadecyl immobilized H-ZSM-5 catalyst

Shape-selective properties of octadecyltrichlorosilane-treated H-ZSM-5, abbreviated as H-ZSM-5-C₁₈, have been observed in the hydrolysis of esters having some rings and lactones in toluene-water solvent system. The shape-selectivity for the reaction has been evaluated by the ratio of the relative rate constants in comparison with the rate constant of methyl acetate. The selectivity became higher with increase in bulkiness of the substrate. Substrates having the minimum diameter larger than 6.5 Å, significantly larger in size than the pore openings of ZSM-5, could not react in this system.     

A density functional theory study on the deformation behaviors of fe-si-B metallic glasses

Density functional theory has been employed to investigate the deformation behaviors of glassy Fe-Si-B model systems prepared by ab initio molecular dynamics. The atomistic deformation defects which are closely related to the local dilation volumes or excess volumes and unstable bonding have been systematically analyzed. It has been found that the icosahedral structures are relatively stable under shear deformation until fracture occurs. Plastic flow is indicated by interruption of percolating icosahedral structures, caused by unstable Fe-Si bonding of p-s hybridization in nature.     

The effect of Pd/Al2O3 pretreatment on catalytic activity in cyclopentane/deuterium exchange

Big variations in overall activity and product selectivity in the cyclopentane/deuterium exchange reaction were found in effect of various pretreatments of two chlorine‐free Pd/γ‐Al₂O₃ catalysts. The most important changes are observed when severely prereduced (at 600 °C) Pd/Al₂O₃ catalysts have been reoxidised and mildly rereduced: the multiple type of exchange, typical of mildly pretreated Pd catalysts, is replaced by a stepwise mode, and a big increase in catalytic activity occurs. At this state, the Pd/γ‐Al₂O₃ catalysts retain some water (as surface hydroxyls) generated by reoxidation and mild reduction. Deuterium spillover from Pd onto alumina and changes in acidity of alumina are invoked to rationalize the kinetic results. Changes in the state of Pd after various pretreatments, as probed by temperature‐programmed hydride decomposition, can hardly be correlated with changes in the catalytic behaviour in the exchange reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

沙林的密度泛函理论研究

本文应用密度泛函理论的B3LYP方法,在6-31G(d)基组水平上对沙林分子进行了研究,计算得到了沙林分子的稳定构型及其红外光谱。分析后发现,根据分子振动类型的不同,可将红外光谱划分为(0~1300)、(1300~1450)、(1450~3000)和(3000~4000)cm-1 4个区域,且红外光谱中实际振动峰的数目小于简正振动的数目。     

AgCl/H-ZSM-5催化分解NO反应及失活机理

采用水热分散法制备了AgCl/H-ZSM-5催化剂，并对其催化分解NO的反应进行了研究。运用X-射线衍射（XRD）、固体魔角旋转核磁共振（MASNMR）和能量色散X-射线微区分析（EDAX）对Ag-Cl/H-ZSM-5催化剂进行表征，着重阐明AgCl/H-ZSM-5催化剂在NO分解反应过程中活性相、载体的结构变化与催化分解NO反应活性的关系。     

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.     

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.     

Ni-Rh/Al2O3催化剂上甲烷-氘化氢间的氢氘交换性能

采用Ni-Rh/Al₂O₃催化剂,在固定床微型反应器上考察了Ni-Rh/Al₂O₃催化剂对甲烷的氢氘交换的催化性能。结果表明,在进料组成不变的条件下,当温度低于692K时,甲烷的转化率随温度的升高而快速升高,当温度高于692K时,甲烷的转化率不随温度的升高而变化;当温度低于692K时,甲烷的转化率随反应物流量的增加而明显减小,当温度高于692K时,甲烷的转化率基本不随温度和反应物流量的变化而变化;在反应物总流量不变的条件下,当HD/CH₄流量比为1.1～2.5时,甲烷的转化率随着HD/CH₄流量比增加而减小。     

A density functional theory study on the mechanism of dimethyl ether carbonylation over heteropolyacids catalyst

Dimethyl ether(DME)carbonylation is considered as a key step for a promising route to produce ethanol from syngas.Heteropolyacids(HPAs)are proved to be efficient catalysts for DME carbonylation.In this work,the reaction mechanism of DME carbonylation was studied theoretically by using density functional theory calculations on two typical HPA models(HPW,HSiW).The whole process consists of three stages:DME dissociative adsorption,insertion of CO into methoxyl group and formation of product methyl acetate.The activation barriers of all possible elementary steps,especially two possible paths for CO insertion were calculated to obtain the most favorable reaction mechanism and rate-limiting step.Furthermore,the effect of the acid strength of Br?nsted acid sites on reactivity was studied by comparing the activation barriers over HPW and HSiW with different acid strength,which was determined by calculating the deprotonation energy,Mulliken population analyses and adsorption energies of pyridine.     

CO2 adsorption and activation over γ-Al2O3-supported transition metal dimers: A density functional study

Catalytic conversion of CO₂ to liquid fuels has the benefit of reducing CO₂ emission. Adsorption and activation of CO₂ on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO₂ adsorption and activation over the γ-Al₂O₃-supported 3d transition metal dimers (M₂/γ-Al₂O₃, M = Sc–Cu). CO₂ was found to adsorb on M₂/γ-Al₂O₃ negatively charged and in a bent configuration, indicating partial activation of CO₂. Our results showed that both the metal dimer and the γ-Al₂O₃ support contribute to the activation of the adsorbed CO₂. The presence of a metal dimer enhances the interaction of CO₂ with the substrate. Consequently, the adsorption energy of CO₂ on M₂/γ-Al₂O₃ is significantly higher than that on the γ-Al₂O₃ surface without the metal dimer. The decreasing binding strength of CO₂ on M₂/γ-Al₂O₃ as M₂ changes from Sc₂ to Cu₂ was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO₂ for the same metal dimer and weakens the CO₂ chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO₂ adsorption and activation.