烷基化催化剂表面酸性及催化性能的动力学研究

在确定关联升温速率、脱附峰温和脱附峰覆盖率的程序升温脱附动力学模型的基础上，通过TPD实验和模型参数估值，建立了表征催化剂酸密度、酸强度及强度分布情况的方法。研究表明，随着活化温度的提高，固体酸催化剂表面酸中心强度分布先变宽后趋于均匀，350?℃活化催化剂的强度分布最宽；催化剂表面酸强度和酸密度随活化温度提高均呈先增大后降低、分别在350 ℃和250 ℃活化温度达到极大值的变化规律。催化剂酸性与催化性能关联的结果表明，随着活化温度的提高，烷基化反应速率常数与总脱附量的变化趋势相同，而催化剂失活速率常数与脱附活化能变化趋势相同；催化剂活性稳定性随其酸强度的增大而变差，催化剂活性与催化剂酸量和酸强度有关。     

SO_4~(2-)/ZrO_2超强酸催化剂的表面酸性质

本文用红外光谱和程序升温氨脱附对ＳＯ～（２－）＿４／ＺｒＯ＿２超强酸催化剂的 表面酸性特征进行了研究，并结合Ｘ－光电子能谱分析结果和催化剂 上甲醇转化反应的结果得到了如下一些重要信息；１、在焙烧温度不太 高时催化剂表面除存在Ｌｅｗｉｓ酸外还有Ｂｒｏｎｓｔｅｄ酸后者的强酸性是 通过结构（Ⅱ）式所体现出来的．且Ｂ酸随焙烧温度的升高而增多；２、 Ｈ＿２ＳＯ＿４的浓度和焙烧温度都是影响酸强度分布的重要因素．当Ｈ＿２ＳＯ＿４ 浓度为１ｍｏｌ／Ｌ或焙烧温度低于７７３Ｋ时，催化剂表面强酸中心最多； ３、甲醇转化为二甲醚在较弱的酸中心上就可进行，而生成烃的反应不 仅要求强酸中心而且是在Ｂ酸中心上进行的。     

A study of acidity and coke deposits on NiHZSM-5 catalyst in aromatization

The acidity and coke deposits of HZSM-5 and NiHZSM-5 catalysts were studied in the aromatization using NH₃-TPD, IR and TPD techniques. The results showed that both strong acid and B acid sites decreased quickly at the initial stage of reaction for the two catalysts. The coking and reaction rates also dropped quickly at this stage. Yet it was discovered, that adding Ni into HZSM-5 led to weakening B acid sites and strengthening L acid ones. At the same time, the results revealed that metallic Ni decreases the coking rates and slows down the activity decline rates.     

Site dependent dissociation of CO on supported Pd particle surface: A TPD study

CO adsorbs molecularly on smooth Pd surface and dissociates to a limited extent on small supported Pd particles. CO dissociation on Pd particles were investigated by means of TPD method. Pd particles was deposited on 2 mm thick anodicaly oxidized Al crystals in order to obtain a homogeneous distribution of temperature over Pd surface. It was shown that the size dependent CO dissociation activity of small supported Pd particles is caused by the existence of active sites, such as surface steps, whilst the plane sites are not active.     

Determination of the surface acidity of solid catalysts

A novel method has been developed for determining the surface acidity of white, as well as deeply colored porous solids. The method, which was tested on Al₂O₃, SiO₂–MgO–Al₂O₃ and Fe₂O₃ catalysts, is based on the adsorption of pyridine or benzylamine on acidic sites and subsequent displacement of the adsorbed bases by n-butylamine. A linear correlation was found between the concentration of acidic sites on Al₂O₃ and Fe₂O₃ catalysts and their activity in the dehydration oftert-butyl alcohol.

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, . , Al₂O₃, SiO₂–MgO–Al₂O₃ Fe₂O₃, -. Al₂O₃ Fe₂O₃ - .

     

Fine tuning the surface acidity of titanate nanostructures

The effect of protonation on the surface acidic properties of titanate nanowires (TiONWs) was investigated. Nanowires were synthesized by the alkali hydrothermal method which resulted in one dimensional nanostructures of large external surface area and well-defined lamellar interlayer structure. The Na⁺/H⁺ ratio in the structure can be tuned by ion-exchange. Our aim was to characterize the morphology of the as-synthesized nanostructures by HRTEM and SEM measurements and assess their surface acidity using in situ infrared spectroscopic measurements and temperature programmed desorption. It was found that the numbers of Lewis and Brönsted acidic sites in the Na-form and the H-form of the TiONWs is different. The ratio and the nature of acidic sites can be tuned by the ion exchange process. The wire-like morphology and the tunable acidity are features of titanate nanowires that may render them a promising material in various heterogeneous catalytic applications.     

Polymeric catalyst with polymerization-enhanced Lewis acidity for CO2-based copolymers

Ring-opening copolymerization of CO₂ and epoxides is a promising way to manufacture high value-added materials. Despite a variety of catalyst systems have been reported, the reaction is still limited by low activity and polymer selectivity. Herein, a strategy of polymerization-enhanced Lewis acidity is reported to construct a series of highly efficient polymeric aluminum porphyrin catalysts (PAPCs). The characterization of the coordination equilibrium constant (K_eq) showed significantly enhanced Lewis acidity of PAPC (K_eq = 18.2 L/mol) compared to the monomeric counterpart (K_eq = 6.4 L/mol), accompanied with increased turnover frequency (TOF) from 136 h⁻¹ to 5500 h⁻¹. Through detailed regulation of Lewis acidity, the highly Lewis acidic PAPC-OTs displayed a record high TOF of 30,200 h⁻¹ with polymer selectivity of up to 99%.     

A detailed TPD study of H2O and pre-adsorbed O on the stepped Pt(553) surface

Water molecules desorbing from the bare Pt(553) surface desorb in a three peak structure, associated with, respectively, desorption from step and terrace sites and the water multilayer. Upon pre-covering the step sites with O(ad) we mainly observe OH formation on step sites. When terrace sites are also pre-covered with O(ad), OH(terrace) formation is favored over OH(step) formation, presumably because OH formed at terrace sites is more easily incorporated in a hydrogen bonded network of OH/H(2)O. This is a gradual process: with increasing θ(O) less OH(step) is formed. Thus, in spite of the fact that OH at step sites has a higher binding energy than OH at terrace sites, the possibility of the formation of OH at terrace sites actually inhibits the formation of OH at step sites, leaving O(step) as the most stable water dissociation product on the step.     

Ir-in-CeO_2催化剂上CO氧化过程中表面碳酸盐的量热研究

应用Tian-Calvet热流量热仪与脉冲微反系统相结合的脉冲量热装置研究了Ir-in-CeO2(Ir主要包裹在CeO2里)催化剂上CO氧化过程表面碳酸盐的形成及其转化.结果表明,碳酸盐的吸附强度与CeO2表面价态直接相关.在反应中首先形成约0.07个单层覆盖度的不可逆吸附碳酸盐,随后形成约0.02个单层覆盖度的可逆吸附碳酸盐并随着Ce3+转变为Ce4+而分解.     

Comprehensive study combining surface science and real catalyst for NO direct decomposition

The catalytic activity of Pd/Al2O3 prepared from various palladium precursors for direct NO decomposition is closely related to the fraction of surface step sites capable of dissociating NO, on the basis of a surface science study using single-crystal model catalyst.     

Temperature effects on the surface acidity properties of zirconium diphosphate

As part of the temperature effects study on the sorption of metallic cations onto zirconium diphosphate, we have first investigated the intrinsic surface properties of this synthetic compound for different temperatures (25, 50, 75 and 90 degrees C). A physico-chemical study (IR, XRD) assessed its purity, and the measured N(2)-BET specific area was 13.4+/-0.2 m(2)g(-1). Mass and potentiometric titrations showed that the experimental point of zero charge (pH(pzc)=2.6+/-0.2) and the surface site density remained constant between 25 and 90 degrees C. The potentiometric titration data were simulated with the constant capacitance model, considering two reactive surface sites, with a total surface site density equal to 7.2 sites nm(-2). The intrinsic protonation and deprotonation constants were found to increase with the temperature, as well as the calculated apparent constants. The simulation results showed that the capacitance increased with the temperature. The proportions of the neutral, protonated and deprotonated forms for each site type were quantified thermodynamically by application of the Van't Hoff relation.     

Evaluation of catalyst acidity and substrate electronic effects in a hydrogen bond-catalyzed enantioselective reaction

A modular catalyst structure was applied to evaluate the effects of catalyst acidity in a hydrogen bond-catalyzed hetero Diels-Alder reaction. Linear free energy relationships between catalyst acidity and both rate and enantioselectivity were observed, where greater catalyst acidity leads to increased activity and enantioselectivity. A relationship between reactant electronic nature and rate was also observed, although there is no such correlation to enantioselectivity, indicating the system is under catalyst control.     

IR spectroscopy of N-methylpyrrole adsorbed on oxides--a probe of surface acidity

IR spectra of N-methylpyrrole (NMP) have been measured following adsorption on, and subsequent desorption from, SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3), H-mordenite, and sepiolite. Three modes of adsorption have been observed: (i) hydrogen bonding to surface hydroxyl groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Br?nsted acidic surface sites. Protonation of NMP was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Br?nsted acidic sites with pK(a) values 相似文献   

Spin probe study of acidity of inorganic materials

Acidic characteristics of various inorganic materials, such as silica gel, aluminum, and titanium(IV) oxides, original and modified with F^? and SO ₄ ^2? ions, in the form of hydrogels and nanostructured powders doped by copper(II) ions, are studied. It is shown that pH values inside pores of the studied sorbents (pH_intr) differ from the pH of external solutions by 0.5–1.1 units. In the pores of original γ-Al₂O₃, the pH_intr values are higher, while, in pores of γ-Al₂O₃ samples modified with F^? и SO ₄ ^2? , as well as nonmodified α-Al₂O₃, SiO₂ gel, and TiO₂ hydrogels, pH_intr are lower compared to the pH of the external solution. It is found that pH_intr decreases, as the content of Cu²⁺ ions in the TiO₂ phase grows, and, beginning with a certain moment, it coincides with pH of an equilibrium solution. The pK _a values of functional group dissociation in some materials are determined. It is shown that, using spin probes, that is, pH-sensitive nitroxyl radicals, the electrostatic potential of the surface of nanostructured TiO₂ can be estimated.     

Correlation between the surface acidity and activity of bismuth tungstate catalysts

The surface acid strength distribution in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 has been investigated. The results are interpreted attributing higher acidities to W–O–W bonds and lower acidities to W=O bonds. An apparent correlation is observed between the surface acidities of the catalysts and their activities in the oxidation and ammoxidation of propylene and 1-butene.

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Bi/W, 2/3, 1/1, 2/1 6/1. , W–O–W W=O. 1-.