催化剂孔道结构设计及孔内反应-扩散耦合模拟

采用硬球-拟颗粒(HS-PPM)耦合方法并结合简化的集总反应模型,模拟了碳四(C4)烯烃催化裂解过程中反应物和产物在孔道内的反应?扩散耦合过程,提出了一种多级孔道结构可控设计方法,实现了对孔隙率、孔径和孔体积占比等参数的独立量化调控,建立了包含三种不同孔径分布的催化剂孔道模型并定义了量化参数对反应?扩散耦合进行表征。结果表明,各组分在复杂孔道内的扩散过程显著受限,反应?扩散耦合性能较差,导致催化剂总体性能降低,达不到本征动力学允许的高反应速率。提出的可控设计孔道模型结合HS-PPM耦合模拟方法有助于催化剂材料的孔尺度结构的设计。     

重质油在催化剂孔道内受限扩散及其关联模型研究进展

重质油分子结构复杂、尺寸较大且容易聚集形成缔合体,致使在加氢及催化裂化催化剂孔道内存在明显的扩散阻力,显著阻碍了反应物分子与催化剂孔道内部活性位的接触,导致反应速率与转化率明显低于轻质原料。因此,研究重油分子在催化剂孔道内扩散受阻情况可以为催化剂的设计优化提供信息与指导。尽管借助仪器或膜材料能获得溶质分子的扩散性能,但与反应条件下扩散规律差别较大,因此在实际反应体系下获得重油分子在催化剂孔道中的扩散传质具有重要的现实意义。综述采用反应动力学手段获得扩散传质数据并建立受限因子关联模型及其影响因素的研究进展,从反应物选择、反应条件及催化剂等因素的优化角度对重油分子受阻扩散研究进行展望。     

多级孔分子筛在重油加氢裂化催化剂的应用进展

分子筛是加氢裂化催化剂关键组分,其性质影响着加氢裂化反应效率和产品分布。微孔分子筛的孔结构降低了大分子反应物的扩散效率和酸中心的可及性,不宜直接用作加氢裂化催化剂的载体。本文从分子筛的孔结构和酸中心可及性的角度出发,介绍了具有多级孔体系的分子筛和具有核壳结构分子筛的加氢裂化性能。与相应的参比剂比较,分子筛的多级孔结构能大幅提高反应物种的扩散效率和酸中心的可及性,呈现出更好的催化活性、稳定性以及目标产物选择性。此外,金属加氢活性中心与分子筛裂化活性中心的合理调配,也是多级孔分子筛在重油加氢裂化应用中面临的挑战。     

载体孔结构对常温COS水解催化剂性能的影响

运用固定床流动态反应装置和压汞仪等手段考察了γ-Al₂O₃载体的比表面和孔结构对常温COS水解催化剂反应性能的影响。结果表明,载体必须具有适当的比表面积和孔结构,以利于反应物分子在催化剂表面吸附并与活性中心接触,同时也有利于产物分子脱附并离开催化剂表面。     

介孔导向剂制备多级孔结构SAPO-34分子筛催化剂及其在甲醇制烯烃反应中的应用

SAPO系列分子筛广泛应用于甲醇制烯烃(MTO)过程,SAPO分子筛的微孔结构仅允许小分子物质进出孔道,限制了反应物在催化剂晶体结构孔道中的扩散,增加了反应物扩散到活性位点的阻力,降低催化剂活性。此外,生成物不易从微孔结构扩散出来,导致过度反应生成积炭。采用聚乙二醇(PEG)作为导向剂,三乙胺(TEA)为模板剂,制备多级孔结构分子筛,在n(Si)∶n(Al)=0.20、介孔导向剂加入量n(PEG)∶n(Al)=0.20、模板剂加入量n(TEA)∶n(Al)=1.5和晶化时间为48 h条件下制备的分子筛0.20P48-SAPO具有良好的物化性质,比表面积和孔径比单一孔结构SAPO-34分子筛大幅提升,表面活性位点数量增加,有利于提高催化活性。微型固定床反应器评价结果表明,甲醇转化率为91%,丙烯选择性为42%;催化剂热稳定性良好,可以进行再生,再生后活性与新鲜催化剂相当,甲醇转化率为90%,丙烯选择性为43%。     

Modeling the reaction of gaseous HCl with CaO in fluidized bed

An integrated mathematical model is developed to evaluate the performance of the reaction of gaseous HCl and CaO in fluidized bed. The model considers initial pore size distribution of solid reactant, pore structure change and attrition caused by particles movement. Bethe network is used to describe the pore space topology, and the percolation theory is used to determine the accessible reaction surface area of the sorbent particles and the effective diffusion coefficient of gaseous HCl. This model prediction accounts for the diffusion of HCl in shrinking pore space as well as in product layer, and clearly demonstrates the increasing diffusion resistance and the isolation of partially reacted pores causing incomplete conversion of solid. The model shows excellent agreement with the experimental data.     

Characterization and activity of Mo supported catalysts for diesel deep hydrodesulphurization

Various highly dispersed Mo supported catalysts with various carriers were prepared for deep hydrodesulphurization of diesel. The carriers included a high surface area and large pore volume γ-Al₂O₃, two types of meso-microporous composite molecular sieves prepared by incipient-wetness impregnation method. A new mesoporous MoSiO_x catalyst synthesized with in situ composite method was also studied. The hydrodesulphurization experiments were carried out in a micro-reactor over different catalysts including Mo supported series and a commercial catalyst. Spectroscopic techniques (FT-IR and UV–vis DRS) were utilized to determine the structure of MoO_x species. The catalyst characterizations of BET, XRD, FT-IR, UV–vis DRS and FTIR pyridine adsorption indicated that the existences of metal active component of Mo in the catalysts were highly dispersed nano MoO₃ clusters and the Mo series catalysts had high surface areas and plenty of large pores which were propitious to the diffusions of reactant and product molecules. Cat_NiMo exhibited the highest B/L acidity ratio and higher total concentration of Brønsted acid sites and Lewis acid sites, and its HDS activity also gave the highest in this study to produce a sulphur-free diesel, which was verified by the sulphur content in products analyzed by GC–MS methods.     

Consideration of the Effect of Irregular Catalytic Active Component Distributions in Mesopores – Extension of a Model for Wall Catalyzed Reactions in Microchannel Reactors

Data available from the literature and experimental results have shown that the distribution of the catalytic active components can be irregular already for fresh catalysts. The determination of the local concentrations of the catalytic active components using wavelength dispersive X‐ray spectroscopy confirms this for microstructured wafers used in microchannel reactors. Considering this nonuniform distribution, the used model gives the relation between the local concentration profiles of the reactants inside the pores and the product yield in the entire pore. These results were used in an equation for the diffusion flux at the pore mouth, which is useful for a microchannel model developed in a recent paper [1]. The theoretical considerations deal with cylindrical pores with known reactant concentrations at the pore mouth and known distribution of the catalytic active component within the pore. Beside numerical results, some analytical solutions with low mathematical expense, applicable to special cases, are discussed. The nonconsideration of the irregular distribution of the catalytic active component can be the reason for difficulties during the extrapolation of experimental results to slightly different conditions and can have a great influence on the reaction results. The regarded examples are typical of wall‐catalyzed reactions in microchannel reactors with mesopores.     

THE KINETICS OF THIXOTROPIC BEHAVIOUR IN CLAY/KAOLIN AQUEOUS SUSPENSIONS

Two possible processes for deactivation in the catalytic hydrolysis of acrytonitriie over Raney copper have been examined at temperatures in the range 40 to 100°C. The first of these is the diffusion limited oxidation of the catalyst surface which produces oxides of copper. The other, more dominant, effect is thought to result from fouling of the catalyst pores by thermally polymerized acrylamide. This mechanism of catalyst poisoning which is independent of both reactant and product concentrations is evidenced by changes in specific surface areas and pore sizes of the catalyst used.     

CATALYST DEACTIVATION IN THE HYDROLYSIS OF ACRYLONITRILE TO ACRYLAMIDE OVER RANEY COPPER

Two possible processes for deactivation in the catalytic hydrolysis of acrytonitriie over Raney copper have been examined at temperatures in the range 40 to 100°C. The first of these is the diffusion limited oxidation of the catalyst surface which produces oxides of copper. The other, more dominant, effect is thought to result from fouling of the catalyst pores by thermally polymerized acrylamide. This mechanism of catalyst poisoning which is independent of both reactant and product concentrations is evidenced by changes in specific surface areas and pore sizes of the catalyst used.     

低温费托合成蜡油加氢裂化精制技术的研究进展

低温费托合成技术因具有产品质量性好、反应耗能低、生产能力大且催化剂种类广泛等优点在煤化工领域备受关注,低温费托合成的蜡油产品可通过加氢裂化精制获取高品质清洁油品,具有巨大的应用价值。本文首先阐述了费托合成的产物特性,分析了加氢裂化过程的反应特点、双功能催化剂的碳正离子反应机理及蜡油主要反应历程。在此基础上,着重综述了蜡油加氢裂化双功能催化剂的研究进展,讨论了活性金属组分、载体以及助剂对加氢裂化过程的影响,分析表明活性金属的负载量、载体的酸量和孔道结构对催化性能有极大影响,合理优化和平衡加氢金属活性位和裂解酸性位是确保蜡油加氢裂化催化剂活性的关键。更为重要的是,基于分子筛载体的择形效应,实现载体多级孔结构和活性位的理性集成无疑会促进蜡油加氢产物的合理分布。     

Characterization of cancrinite synthesized in 1,3-butanediol by Rietveld analysis of powder neutron diffraction data and solid-state Na NMR spectroscopy

The framework structure and extraframework atoms of calcined and dehydrated cancrinite synthesized in 1,3-butanediol are characterized by powder neutron diffraction and ²³Na nuclear magnetic resonance (NMR) spectroscopy. The cancrinite structure is refined in the hexagonal space group P6₃ (No. 173) with lattice parameters a=12.659 Å and c=5.153 Å. Carbonate anions are found occluded in the pores of the cancrinite structure. Although there are two different crystallographic cation sites found by the Rietveld refinement, there are three peaks in the ²³Na magic-angle spinning (MAS) NMR spectrum. These peaks correspond to sodium cations found in site I inside the cancrinite cages, cations in site II inside the cancrinite pore without neighboring carbonates, and cations in site II with neighboring carbonates. Quadrupole coupling constants (QCC) obtained by a simple point-charge model agree well with the simulation of the ²³Na MAS NMR spectra.     

A study on structural suitability of immobilized aluminum chloride catalyst for isobutene polymerization

Catalytic performance and structural suitability of immobilized AlCl₃ catalyst for isobutene polymerization have been studied. It was found that the activity, selectivity and number-average molecular weight of the product polyisobutene depend, to a certain extent, on the pore structure and the granulation of the catalyst. AlCl₃ on γ-Al₂O₃ support having macro- and meso-pore bimodal structure show excellent catalytic activity and high stability, while those on γ-Al₂O₃ with micro (d 15.6 Å) and meso-pore (d 28.6 Å) structure exhibit low stability and rapid fall of conversion with time. Granulation of the catalyst is also an important factor which affects activity and selectivity of the catalyst and average molecular weight of the product. Increasing granulation of the catalyst (particles become finer) brings about an increase in isobutene conversion, but a decrease in selectivity, resulting in lower average molecular weight and broader distribution.     

Performance Enhancement by Unsteady‐State Reactor Operation: Theoretical Analysis for Two‐Sites Kinetic Model

Theoretical analysis of the reactor performance under unsteady‐state conditions was carried out. The reactions are described by two kinetic models, which involve the participation in catalytic reaction of two types of active sites. The kinetic model I assumes the blocking of one of the active sites by a reactant, and the kinetic model II suggests a transformation of active sites of one type into another under the influence of the reaction temperature. The unsteady‐state conditions on the catalyst surface are supposed to be created (i) by forced oscillations of temperature and concentration in the reactor inlet (periodic operation of reactor) and (ii) by catalyst circulation between two reactors in a dual‐reactor system (spatial regulation). The influence of various parameters like concentration of reactant, cycle split, length of period of forced oscillations, temperatures and the ratio of catalyst volumes in the dual‐reactor was investigated with respect to the yield of the desired product. It is shown that for both cases of unsteady‐state conditions (periodic reactor operation as well as in a dual‐reactor system), a mean reaction rate predicted by the kinetic model I was up to two times higher than the steady‐state value. The kinetic model II shows a 20 % increase of the selectivity towards the desired product.     

Oxidative carbonylation of phenol over the mesoporous palladium-copper-oxide/silica catalyst prepared by sol–gel coupling with W/O microemulsion

A novel mesoporous catalyst Pd-Cu-O/SiO₂ were prepared by sol–gel method coupling with W/O microemulsion for oxidative carbonylation of phenol to diphenyl carbonate (DPC). It has larger specific surface area, pore volume and rather narrow pore size distribution in the range of mesopore. The catalyst shows a comparatively good activity for the synthesis of DPC and the yield reaches to 20.3%, the turnover frequency (TOF) is 15.1 mol-DPC/mol-Pd h, at certain conditions. The less dispersed active centers and the diffusion resistance caused by the numerous pores influence the reaction negatively.     

Forced oscillations of concentration in transport reactors

The performance of a transport catalytic reactor is analysed for an adsorption/desorption type model with Eley-Rideal surface kinetics with square wave oscillations in the feed concentration. The inlet average concentration of both the reactants over a period is assumed to be constant. Significant improvement in the yield of product is obtained by increasing the feed concentration of both the reactants in the first fraction of a period. The effect of adsorption capacity of the catalyst, reaction rate constants, and inlet feed mean concentration of the reactants are evaluated. The deactivation of active sites, due to product inhibition, introduces resonance in the yield versus cycle split. The yield of product shows a minimum with respect to cycle split for the type of forcing where one of the reactant concentrations increases and the second reactant concentration decreases in the first fraction of a period.     

富氧条件下氢气选择催化还原汽车尾气氮氧化物的研究进展

汽车尾气中的氮氧化物（NO _x ）是大气主要污染物之一,严重危害了环境和人类健康。选择催化还原（SCR）消除技术是一种在富氧条件下能够高选择性消除NO _x 的技术。氢气选择催化还原氮氧化物（H₂-SCR）由于其低温高活性的优势备受关注。目前H₂-SCR研究中以Pt和Pd作活性组分催化剂使用较多,其中Pt基催化剂的研究最为广泛,本文介绍了催化剂载体类型、载体性质（如酸碱性、比表面积、孔道结构）、助剂和预处理方式等对催化活性的影响及影响机制,综述了反应条件参数（如反应气中O₂和NO₂浓度,杂质气体H₂O和SO₂）对催化剂催化活性的影响,认为反应气组分的影响在于各组分在催化剂上的竞争吸附以及对催化剂活性位的影响,最后展望了面向实际应用H₂-SCR技术未来的研究方向。     

On the effects of confinement within a catalyst consisting of platinum embedded within nanoporous carbon for the hydrogenation of alkenes

In heterogeneous catalysis, pore size exerts an influence on reaction pathway, selectivity, equilibrium and adsorption constants. This effect can in principle cause noticeable changes in selectivity. Here, we present an analysis of a diffusion–reaction process in the pores of a catalyst comprised of platinum nanoparticles embedded within a molecular sieving carbon. When the alkenes are hydrogenated over this catalyst, the reaction takes place within the ultramicropores of the carbon. Experimental data for the liquid phase hydrogenation of different alkenes over platinum supported on the carbon versus platinum embedded within the same carbon were collected. From these data kinetic parameters and diffusion coefficients for reactions were evaluated. The forward rate constant for 2-methyl-1-pentene hydrogenation was found to be almost one order of magnitude larger within the embedded platinum catalyst versus the supported platinum catalyst. The variation in pore size and reactant molecule dimension, were also found to affect the adsorption equilibrium constants and diffusion coefficients. For 2-methyl-1-pentene molecule with the highest steric hindrance, K increased to 500 g/mol in embedded catalyst compared to 100 g/mol on supported catalyst. At the same time the diffusion coefficient for 2-methyl-1-pentene was one order of magnitude smaller than 1-hexene.     

Preparation of alumina–silica bimodal pore catalysts for Fischer-Tropsch synthesis

The bimodal pore structure support has excellent advantages in solid catalysis reaction because the large pores provide pathways for rapid molecular transportation and small pores provide a large active surface, contributing to high diffusion efficiency and high dispersion of supported metal simultaneously, A multi-functional bimodal pore catalyst support, alumina-silica bimodal pore support, was prepared from polymer complex solution and silica gel. The obtained bimodal pore support had two kinds of main pores, decreased pore volume and enlarged specific surface area, comparing the original silica gel. This kind of bimodal pore support was applied in slurry phase Fischer-Tropsch synthesis, where cobalt was supported as active metal. Alumina-silica bimodal pore catalyst exhibited high catalytic activity and favorite selectivity, due to the spatial effects of bimodal pore structure and chemical effects of coexisting alumina, which formed the new small pores of bimodal pore support.     

Dynamics of internal diffusion during the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al2O3

Dynamic and pseudo-steady state diffusion–reaction models were simulated for the three-phase consecutive hydrogenation of 1,5,9-cyclododecatriene on a shell Pd/Al₂O₃ catalyst in order to examine the time evolution of concentration profiles inside the catalyst pellet: a model accounting only for the dynamics of the active layer and a model taking also into account the inert part of the catalyst were compared.

In the conditions of the semibatch experiments (T=433 K, p_H2=1.2 MPa), all the models lead to the same bulk concentration–time curves, but the hydrocarbon concentration profiles in the pores are dependent on the model. The influence of the diffusion in the inert part of the catalyst on the bulk concentrations becomes nonnegligible only when the external liquid volume (out of the catalyst) is reduced.

The transient evolution of the concentration profiles in the pores show that hydrogen concentration reaches its steady state within a few seconds, while the evolution of the organic concentration profiles is slower.

Furthermore, the reaction rate has been found to be only affected by the hydrogen diffusion. The diffusion of organics can control the reaction rate only for low values of organic concentration and higher pressure in hydrogen.