碱金属离子交换Y型分子筛CO分子吸附量子化学研究

应用量子化学方法,从微观角度了解一氧化碳（CO）分子吸附在碱金属离子（Li⁺、Na^+、K⁺、Rb⁺、Cs⁺）交换Y型分子筛上的机理,详细研究了吸附时红外光谱的变化情况。研究发现,CO分子能与碱金属离子交换Y型分子筛发生多分子吸附,且其吸附行为和结构相类似。CO发生单分子和双分子吸附后,其红外光谱均会发生蓝移。当CO双分子吸附时,其红外光谱会出现两个新的吸附带,且其振动频率比单分子吸附时有所降低。当CO在Li-Y、Na-Y分子筛上发生双分子吸附时,其振动频率与单分子吸附时有较大差别;而CO在K-Y、Rb-Y、Cs-Y分子筛上发生双分子吸附时,其吸附前后的振动频率变化较小,且两个CO分子的振动频率很接近。能量计算结果显示,CO分子吸附强度会随着离子半径的增加而减弱。     

FTIR spectroscopic evidence of formation of geminal dinitrogen species during the low-temperature N2 adsorption on NaY zeolites

Adsorption of N₂ on NaY zeolites at 85 K and equilibrium pressures higher than 1 kPa results in the formation of geminal dinitrogen complexes characterized by an IR band at 2333.5 cm⁻¹ (2255.4 cm⁻¹ after adsorption of ¹⁵N₂). With decreasing equilibrium pressure the complexes tend to loose one N₂ ligand, thus forming linear species characterized by an IR band at 2336.8 cm⁻¹ (2258.7 cm⁻¹ after adsorption of ¹⁵N₂). All species disappear completely after evacuation. Co-adsorption of N₂ and CO revealed that the dinitrogen complexes are formed on Na⁺ cations. The changes in the concentrations of the linear and geminal N₂ species with the changes in the equilibrium pressure are excellently described by equations of adsorption isotherms proposed earlier for mono- and di-carbonyls. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared spectroscopy of carbon monoxide adsorbed on Pt/L zeolite

A series of 0.6 wt% Pt/MBaL zeolites, where M is Li, Na, K, Rb or Cs, were prepared and characterized by transmission electron microscopy, chemisorption, and infrared spectroscopy of adsorbed carbon monoxide. Greater than 90% of the exposed platinum in the samples is associated with small clusters, less than 7 Å across, inside the zeolite channels. The remaining fraction of exposed platinum is on 100–500 Å crystallites outside the channels. Adsorption of carbon monoxide on the platinum at 25 °C produces a broad infrared band whose maximum shifts from 2065 to 2025 cm^–1 as the alkali cations in the zeolite are changed from Li to Cs. This shift is indicative of electron transfer between the cations and the platinum clusters. Heating the Pt/L catalysts to 225 °C produces new infrared bands at 2020–2015, 1975, and 1935–1920 cm^–1. The appearance of these low-frequency bands strongly suggests that the CO-covered platinum clusters change their structure during heating. We propose that the new structure is one in which the carbon monoxide molecules insert into spaces between the framework atoms of the L zeolite.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

Sorption kinetics study of the diethylbenzene isomers in MFI-type zeolites

The uptake rates of the three diethylbenzene isomers in MFI-type zeolites were studied with a sorption kinetics apparatus of the barometric type. It was observed that the uptake rates depend crucially on the position of the substituted ethyl groups. While p-diethylbenzene was adsorbed with uptake rates of the same order of magnitude as benzene and the monosubstituted benzenes, diffusion of m-diethylbenzene was approximately four orders of magnitude slower. No sorption uptake was observed for o-diethylbenzene. In the experiments with H-ZSM-5, sorption kinetics were partly determined by a secondary process additional to Fickian diffusion. Comparison to the results obtained with silicalite-1 and earlier results proved that the secondary process is correlated with the existence of acidic sites inside the zeolite and can be tentatively ascribed to an isomerization reaction.     

Selective oxidation of methane by dinitrogen monoxide on FeZSM-5 zeolites. Ab initio quantum chemical analysis

On the basis of ab initio quantum chemical calculations, we propose the structure and energetics of the species taking part in the selective oxidation of methane by dinitrogen monoxide on FeZSM-5 zeolites. The immediate oxidizing site is reasonably represented by a binuclear iron-hydroxide cluster with a peroxo-like fragment located between iron atoms.     

Low temperature infrared study of carbon monoxide adsorption on sulfated titania

Acidic properties of a TiO₂ sample containing approximately 1 wt% S were examined by low-temperature IR spectroscopy of CO and by coadsorption of CO and NH₃. Due to the almost negligible OH content of the sample, Brønsted acidity could not be detected. However, Lewis acid sites with enhanced acid strengths as compared to sulfate-free titania could clearly be detected. It is inferred that these sites (namely cus Ti⁴⁺ sites) are located in close proximity to sulfate groups and that CO (and NH₃) coordinated to the Ti⁴⁺ sites interact with the sulfate groups presumably via electronic inductive effects.     

Ionic clusters in zeolites formed by interaction with sodium solutions in liquid ammonia

An alternative route for the preparation of Na ₄ ³⁺ paramagnetic clusters in Y zeolites is described involving contact of the solid with a solution of sodium in liquid ammonia. Besides the EPR signal of the cluster a second signal, likely due to be a solvated electron in the sodalitic cage, is also observed.     

Infrared study of nitrogen monoxide adsorption on palladium ion-exchanged ZSM-5 catalysts

The adsorption of NO at room temperature on a H-ZSM-5 catalyst exchanged with Pd(NH₃) ₄ ²⁺ complex and activated in oxygen at 773 K has been examined by FTIR spectroscopy. After the oxidizing treatment, the Pd tetrammine complex decomposed into Pd(II) ions and/or Pd(II) hydroxyl complexes dispersed in the zeolite channels. The subsequent adsorption of NO at room temperature led to the reduction of Pd(II) to Pd(I) entities, resulting in the formation and adsorption of NO₂ on H-ZSM-5. The Pd(I) entities were shown to adsorb NO and form mononitrosyl complexes dispersed in the zeolite porosity and characterized by a single infrared absorption band at 1881 cm^–1. The Pd(I) mononitrosyl complex was shown to reversibly coordinate water and NO₂ molecules. The resulting nitrosyl complex was characterized by a single NO vibration band at 1836 cm^–1.     

The transformation of formaldehyde on CoZSM-5 zeolites

The interaction of formaldehyde molecules with Co²⁺ ions in CoZSM-5 zeolites of various compositions containing various proportions between Co²⁺ in exchange positions and in oxide-like clusters, as well as the transformation of formaldehyde sorbed in zeolites was followed by IR spectroscopy. The interaction of formaldehyde with Co²⁺ resulted in a weakening of CO bond, what can be partially due to π-back donation of d electrons of Co²⁺ to π^* antibonding orbitals of formaldehyde. Only Co²⁺ in exchange positions (contrary to Co²⁺ in oxide-like clusters) were able to interact with formaldehyde. IR studies illustrated also that formaldehyde molecules are oxidized to formate ions, by either Co³⁺ or Co²⁺ as oxidants. According to IR results Co³⁺ and Co²⁺ are reduced mostly to Co⁺ and metallic Co.     

Computational studies on the diffusion behaviour of alkylaromatics in large pore zeolites

Alkylation of aromatics over solid acid catalysts such as zeolites, has emerged in the recent past as a viable alternative to conventional Friedel–Crafts alkylation over environmentally hostile catalysts. We studied the diffusion behaviour of ethylbenzene (EB), isobutylbenzene (IBB), o-, m- and p-isobutylethylbenzene (IBEB) in various zeolites such as offretite (OFF), cancrinite (CAN), ZSM-12 (MTW) and ZSM-18 (MEI) by computational procedures. The periodic variations of interaction energy between the molecules and zeolite framework in the calculated diffusion energy profiles are used to predict the energy barrier for diffusion. We analyzed the results to understand the product selectivity in the formation of IBEB in the transalkylation/disproportionation reaction between IBB and EB. The results indicated that the zeolites with channel-like pores are more suitable than those with cage-like pores to achieve better selectivity. The zeolites with channels whose diameters are close to the dimensions of the molecules and those which do not have intersecting channels are better selective catalysts. The efficiency of shape selective production of p-IBEB in these zeolites will be in the order MEI < OFF ∼ MTW < CAN as predicted from their diffusion energy barriers. The detailed analysis of the configurations of the molecules in the most favourable and unfavourable adsorption location, indicate that the p-IBEB has favourable interaction energy in all the four zeolites with different pore architecture, compared to o- and m-IBEB except for MEI. It could be concluded that the pore architecture plays a dominant role in controlling the adsorption and diffusion characteristics of these molecules. The actual values of interaction energy themselves are indication of their adsorption behaviour inside the pores of the zeolite. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alkylation of toluene with ethene over H-ZSM-5 zeolites

It is shown that the alkylation of toluene with ethylene to give p-ethyltoluene can be conducted selectively on small crystals ( 1μ) of unmodified H-ZSM-5 zeolites to a level higher than that corresponding to a thermodynamic equilibrium composition. A comparison of the ethyltoluene isomers formed with H-Y and H-ZSM-5 zeolites at various conversions and temperatures indicates that p-ethyltoluene is preferentially formed in the channel intersections of the H-ZSM-5 zeolite, even during the initial alkylation step. The final ethyltoluene isomer composition is a result of competition between the rates of alkylation and isomerization and subsequent diffusional transport of individual isomers through the zeolite channels. Zeolite coking causes an apparent increase in the p-ethyltoluene selectivity; however, a more dramatic decrease in toluene conversion results in the formation of a considerably lower amount of p-ethyltoluene. This is caused by a partial blocking of the zeolite channel system.     

Energetic and entropic contributions controlling the sorption of benzene in zeolites

The energetic and entropic contributions controlling the sorption of benzene on acidic (H/ZSM-5) and non-acidic (Silicalite-1) MFI type materials were studied using gravimetry, calorimetry and in situ IR spectroscopy to follow the qualitative and quantitative interactions of benzene with the pores and the functional SiOH and SiOHAl groups. The model derived to describe the sorption isotherms indicates the presence of sterically constrained sorption structures for benzene in MFI type materials. The interaction of benzene with the pore walls controls the sorption energetically, while the localized interaction with the bridging hydroxy groups contributes only to a minor degree. If benzene is located close to SiOHAl groups perturbed hydroxy groups are formed. Their wavenumber reflects the local sorption geometry of benzene as well as the acid strength of the hydroxy group and the base strength of benzene. Two perturbed hydroxy groups were observed for benzene adsorbed, which are assigned to two orientations of the molecules inside the pores, i.e., with the ring parallel to the pore wall and with the ring being oriented towards the bridging hydroxy groups. At higher coverage benzene adsorbs at SiOHAl groups additionally in an unconstrained environment, most probably at the pore openings.     

Influence of crystal size and probe molecule on diffusion in hierarchical ZSM-5 zeolites prepared by desilication

The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17 × 4 × 4 μm³). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m² g⁻¹ of mesopore surface area, respectively, in contrast to the 62 and 5 m² g⁻¹ of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.     

Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules

The use of infrared spectroscopy to probe the surface acidity of oxides and molecular sieves is reviewed. The experimental requirements and the type and nature of probe molecules available are also discussed. Special emphasis is given to the criteria that have to be met to arrive at a characterization of the solid that is useful for its catalytic application.     

Kinetic studies using temperature-scanning: the oxidation of carbon monoxide

We report on an experimental investigation of carbon monoxide oxidation over a 0.05% Pt/γ-Al₂O₃ catalyst using a temperature scanning reactor (TSR) to increase the rate of data acquisition. Temperature scanning (TS) allows us to complete an experimental study of the kinetics of this reaction on one catalyst, at one pressure and feed composition, in less than one working day of fully automated reactor operation. Real-time measurement of CO conversion was done using a quadruple mass spectrometer (MS).

The kinetics of this reaction were quantified by using 12,300 conversion–rate–temperature (X, r, T) triplets calculated from raw data obtained using our TS-PFR, and fitting those with two proposed mechanistic rate equations. Due to the large volume of data and the methods of interpretation used in temperature scanning, we will present our results not only in terms of the traditional curves in the conversion–space time (X, τ) plane but also nitroduce the concept of three-dimensional kinetic surfaces, such as the (X, r, T) surface.

The experimental rate data were correlated using two mechanistic rate models:

1. the Langmuir–Hinshelwood dual site molecular adsorption model (MAM), and

2. the Langmuir–Hinshelwood dual site dissociative adsorption model (DAM).

The models differ only in their view of the state of the adsorbed oxygen. The DAM model, which involves the reaction of oxygen atoms with carbon monoxide molecules, both adsorbed on the same type of site, was found to fit our experimental data over a broader range of reaction conditions and feed compositions. The MAM model, which presumes a reaction of carbon monoxide molecules with adsorbed molecular oxygen, can fit experimental rates well for individual feed ratios but not over the range of feed ratios used in our study. We therefore, propose that the rate-controlling step in this mechanism consists of a reaction between adsorbed carbon monoxide molecules and adsorbed oxygen atoms.     

Methanol adsorption and dehydration on alkali metal exchanged NaY zeolites

FT-IR spectroscopy has been applied in a study of methanol adsorption on M^INaY zeolites (M=Li, K, Rb, Cs). Coordinatively and/or hydrogen bonded methanol was registered in the temperature range 300–473 K. Dehydration of methanol to dimethyl ether occurred at 573 and 623 K. It was found that the activity of the catalyst for methanol dehydration strongly depends on the kind of alkali metal cation, and well correlates the IR data relative to methanol adsorption.     

FT-IR characterization of carbenium ions,intermediates in hydrocarbon reactions on H-ZSM-5 zeolites

A series of N-alkylacetonitrilium ions were characterized by IR spectroscopy on an activated H-ZSM-5 sample pretreated by CD₃CN with various adsorbed olefins. Two kinds of(CN) band were evidenced according to whether the carbenium ion is secondary (2387 cm^–1) or tertiary (2376 cm^–1). The method is able to detect the isomerization intermediates of cyclohexene into methyl-cyclopentene.     

Palladium-exchanged MFI-type zeolites in the catalytic reduction of nitrogen monoxide by methane. Influence of the Si/Al ratio on the activity and the hydrothermal stability

Pd-exchanged MFI-type zeolites containing 3.7 and 0.7 framework aluminium atoms per unit cell (corresponding to Si/Al ratios of 25 and 131) were found active in the selective reduction of nitrogen monoxide in the presence of excess oxygen. Upon steaming at 800°C, both catalysts exhibited the total loss of their catalytic activity in the reduction of NO. Such a behaviour was ascribed to the complete aggregation of Pd ions into large metal particles on the external surface of the zeolite crystals. Both supports, although maintaining their crystallinity, are shown to experience extended dealumination upon steaming. Although the loss of Pd exchange capacity could partially explain the Pd migration and sintering, a mechanism involving the formation of mobile Pd hydroxyl entities condensing into PdO particles outside the zeolite crystallites is preferred.     

Transalkylation of toluene with C9 aromatic hydrocarbons over ZSM5 zeolites

The transalkylation reactions of a mixture of toluene and C₉⁺ aromatics were studied over HZSM5 catalysts of varying SiO₂:Al₂O₃ ratios (30–800) as well as over a NiHZSM5 catalyst. As the SiO₂: Al₂O₃ ratio is increased, the rate of transalkylation is decreased. Among the C₉ aromatics that were present in the feed, the ethyl toluenes were extensively de-ethylated to toluene. The shape-selective characteristics of the ZSM5 zeolite were revealed through the relative order of reactivity of the three trimethyl benzene isomers 1.2.4>1.2.3>1.3.5. The stable life of the catalyst can be prolonged by the incorporation of nickel. The influences of process parameters such as temperature, pressure, space velocity and H₂: hydrocarbon ratio are also presented. It was observed that the transalkylation of C₉ aromatics with toluene to yield xylenes occurs mainly on the external surface of the ZSM5 crystals. The disproportionation of toluene to yield benzene and xylene, however, takes place predominantly in the pore system.