Methanol conversion of various cation-exchanged forms of high-silica zeolites

Studies of the methanol conversion on various cation-exchanged forms of high-silica zeolites have revealed that the activity and selectivity of the intact pore structure zeolites depend on the cation composition. On decationated forms the selectivity of methanol conversion to aromatic hydrocarbons is much higher than on the hydrogen form, and it decreases with respect to paraffins.

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Selective conversion of methanol to p-xylene on modified high-silica zeolites

A procedure was proposed for preparation of p-xylene from a nonhydrocarbon material, methanol, in the presence of a modified ultrasil. It was shown that ultrasils modified by multiple-charge cations actively catalyze synthesis of xylenes from methanol at atmospheric pressure, and their properties can be controlled by varying the nature, charge, and size, as well as the procedure of introduction of the modifier cation. Modification of H-ultrasil with gallium led to enhanced para-selectivity (70%) of the catalysts, which was attributed to an increase in the ratio of the Lewis to Brønsted acid sites and to a decrease in the effective channel diameters of the zeolite.     

Adsorption energy of water vapor on high-silica and pure-silica zeolites

The high heats of water adsorption on silicalite with the adsorption of 0.4 mmole/g result from an additional interaction with the silicalite OH groups, while the high heats of water adsorption on NaZSM-5 with the adsorption of 0.5 mmole/g result from an additional strong interaction with 0.5 mmole/g sodium cations. Tetraaqua complexes are formed about these sodium cations at the channel intersection sites in NaZSM-5. The first water molecule adds with an energy of about 100 kJ/mole, while the other three molecules add with the same energy of 56 ± 2 kJ/mole. The water adsorption isotherms on silicalite and NaZSM-5 are completely and satisfactorily described by two- and three-term VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2862–2864, December, 1989.     

Studies of coke distribution in high-silica ZSM zeolites

A method for studying coke distribution in zeolites is suggested. It has been established that two forms of coke can form: one is intracrystalline and the other is located on the outer surface of ZSM zeolites.

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Thermodynamic study of water adsorption in high-silica zeolites

The joint use of microcalorimetric and computational approaches has been adopted to describe H2O interaction with cus Al(III) Lewis and Si(OH)+ Al- Br?nsted acidic sites within H-BEA and H-MFI zeolites (both with approximately 6 Al/unit cell). Adsorption data obtained at 303 K were compared to experimental model systems, such as all-silica zeolites, amorphous silica, and silico-alumina, transition alumina. In parallel, ab initio molecular modeling was carried out to mimic, in a cluster approach, Lewis and Br?nsted acidic sites, as well as a variety of Si-OH species either with H-bonding interacting (nests and pairs) or isolated. H-BEA and H-MFI water affinity values were found to be almost equivalent, in both quantitative and energetic terms, in that dominated by Al-containing sites population, more than by nanocavity topology or by acidic site nature. Both H-zeolites, saturated with approximately 5 Torr of H2O vapor, bind approximately 4 H2O per Al site, almost one of which is tightly bound and not eliminated by RT pumping-off. A 160 < q(diff) < 80 kJ/mol interval was measured for the adsorption up to 1H2O/Al. The zero-coverage heat of adsorption (q0 approximately 160 kJ/mol, for both H-zeolites) was assigned to H2O/Lewis complex formation, which dominates the early stage of the process, in agreement with the ab initio computed H2O/Lewis sites binding energy. The rather broad q(diff) interval was interpreted as due to the simultaneous adsorption of H2O on both structural Br?nsted sites and strongly polarized H2O already adsorbed on Lewis sites. For this latter species, BE = 74 kJ/mol was computed, slightly higher than BE = 65 kJ/mol for H2O/Br?nsted sites interaction, showing that H2O coordinated on cus Al(III) Lewis sites behaves as a structural Br?nsted site. The investigated all-silica zeolites have been categorized as hydrophilic in that the measured heat of adsorption (100 < q(diff) < 44 kJ/mol) was larger than the heat of liquefaction of water (44 kJ/mol) in the whole coverage examined. Indeed, polar defects present in the hydrophobic Si-O-Si framework do form relatively stable H2O adducts. Crystalline versus amorphous aluminosilicate q(diff) versus n(ads) plots showed that the measured adsorption heat is lower than expected, due to the extraction work of Al atoms from the amorphous matrix to bring them in interaction with H2O. On the contrary, such an energy cost is not required for the crystalline material, in which acidic sites are already in place, as imposed by the rigidity of the framework. Modeling results supported the experimental data interpretation.     

不同结构扩孔分子筛催化MTP反应行为及表面积炭物种表征

采用TEAOH溶液处理MFI结构ZSM-5分子筛、MWW结构MCM-22分子筛,NaOH溶液处理TON结构ZSM-22分子筛、CHA结构SSZ-13分子筛得到四种结构的扩孔分子筛。在反应温度480℃、反应压力0.1 MPa、甲醇与水质量比1∶1、甲醇质量空速1.5 h~(-1)的条件下,考察了四种扩孔分子筛的甲醇制丙烯(MTP)催化性能,并采用XRD、N_2吸附-脱附、NH_3-TPD、TG、UV-Raman和GC-M S等方法表征催化剂的物化性质及M TP反应2 h后的分子筛积炭性质。结果表明,四种分子筛扩孔改性后均出现介孔,其中,T-ZSM-5分子筛在MTP反应中寿命最长;T-MCM-22分子筛寿命次之且失活速率慢;而一维孔道结构N-ZSM-22分子筛和八元环尺寸较小的N-SSZ-13分子筛均失活迅速。受拓扑结构和孔道扩散的影响,MTP反应2 h后,分子筛积炭量增加的顺序为T-ZSM-5N-ZSM-22T-MCM-22N-SSZ-13且可溶焦分子质量随积炭量增加而增重,即从五甲基苯增重到菲、芘等多环芳烃。     

Effects of confinement on the adsorption behavior of methane in high-silica zeolites

Adsorption isotherms of methane on high-silica zeolites at ambient temperature and up to high pressure were experimentally measured. The isotherms were analyzed on the basis of thermodynamics and statistical mechanics, and the relationship between microscopic properties and the macroscopic adsorption behavior was investigated. A comparison between the confined and unconfined phases revealed that molecular motion is restricted in the pores. As a result, the adsorbed phase is entropically destabilized, which cannot be neglected in comparison with the energetical stabilization that occurs as a result of the solid-molecule interaction. Our findings also indicate that the smaller slope (drho/d ln p) of the adsorption isotherms compared to that of the isotherm of the bulk at the same density is due to the smaller intermolecular interaction in the pores.The pore-size dependence is indicated not only in solid-molecule interactions but also in intermolecular interactions and molecular motion. Of these, the solid-molecule interactions strongly influence the adsorption behaviors in pores of different sizes. The origin of the restriction of molecular motion in the pores is well-explained by the one-dimensional transition and two-dimensional vibration (1D-trans, 2D-vib) model.     

The chemistry of phase selectivity in the synthesis of high-silica zeolites

In this review we discuss the roles that inorganic and organic components play in the synthesis of high-silica zeolites. We discuss the effects that heteroatom identity, heteroatom substitution, and SDA shape have on synthesis products. Then we summarize recent developments in zeolite synthesis that have been made by performing syntheses in concentrated fluoride media and by using germanium as a tetravalent heteroatom. Finally, we examine the combined roles that framework stability, framework/SDA interactions, and silanol defects may have in determining phase selectivity.     

The nature of active sites in high-silica zeolites containing platinum and cobalt nanoparticles

The calculations of interaction of Co₆ and Pt₆ nanoparticles with silicalite ZSM-5 and zeolite HZSM-5 were performed by the density functional theory method. Introduction of cobalt or platinum to silicalite slightly stabilizes the metal cluster and induces a small positive charge on the Co₆ particle and an appreciable negative charge on the Pt₆ particle. In the case of cobalt, the interaction of the metal particle with the acid zeolite is accompanied by a proton transfer from the Brønsted acid site to the surface of the metal cluster, resulting in the formation of a hydride-like structure, whereas in the Pt-zeolite system a complex cation containing adsorbed hydrogen atom is formed.     

在无有机导向剂条件下采用乙醇为助剂合成高硅沸石

高硅沸石具有优异的热稳定性、水热稳定性、大的微孔体积、高表面积和均匀的微孔孔道,因而广泛应用于催化领域.然而,高硅沸石的合成往往需要使用有机结构导向剂,不但增加了沸石合成成本,而且还产生了大量的三废排放.为了解决这个问题,我们发展了在无有机导向剂存在条件下采用沸石晶种诱导合成沸石的方法,但是该方法合成的沸石产物骨架富铝...     

Densification products in conversion of ethylene on a high-silica zeolite catalyst

Russian Journal of Applied Chemistry - Fundamental aspects of the conversion of ethylene on a high-silica zeolite catalyst in the temperature range 200–400°C were studied. The methods of...     

Influence of thermal and hydrothermal activation on the adsorptive, acidic and catalytic properties of high-silica zeolites

Influence of HS-zeolite calcination up to 1023 K in air and steam on its adsorptive, acidic and catalytic properties in n-hexane cracking has been studied.

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