New Method for the Temperature‐ Programmed Desorption (TPD) of Ammonia Experiment for Characterization of Zeolite Acidity: A Review

In this review, a method for the temperature‐programmed desorption (TPD) of ammonia experiment for the characterization of zeolite acidity and its improvement by simultaneous IR measurement and DFT calculation are described. First, various methods of ammonia TPD are explained, since the measurements have been conducted under the concepts of kinetics, equilibrium, or diffusion control. It is however emphasized that the ubiquitous TPD experiment is governed by the equilibrium between ammonia molecules in the gas phase and on the surface. Therefore, a method to measure quantitatively the strength of the acid site (?H upon ammonia desorption) under equilibrium‐controlled conditions is elucidated. Then, a quantitative relationship between ?H and H₀ function is proposed, based on which the acid strength ?H can be converted into the H₀ function. The identification of the desorption peaks and the quantitative measurement of the number of acid sites are then explained. In order to overcome a serious disadvantage of the method (i.e., no information is provided about the structure of acid sites), the simultaneous measurement of IR spectroscopy with ammonia TPD, named IRMS‐TPD (infrared spectroscopy/mass spectrometry–temperature‐programmed desorption), is proposed. Based on this improved measurement, Brønsted and Lewis acid sites were differentiated and the distribution of Brønsted OH was revealed. The acidity characterized by IRMS‐TPD was further supported by the theoretical DFT calculation. Thus, the advanced study of zeolite acidity at the molecular level was made possible. Advantages and disadvantages of the ammonia TPD experiment are discussed, and understanding of the catalytic cracking activity based on the derived acidic profile is explained.     

Heats of Adsorption of Ammonia and Correlation of Activity and Acidity in Heterogeneous Catalysis

Adsorption microcalorimetry was applied to determine heats of adsorption of ammonia on zeolites Y, mordenite, ZSM-5, heteropolyacid H₃PW₁₂O₄₀, as well as silica gel and amorphous aluminosilicates. The plots of differential heats against coverage served to construct the acidity spectra and, in this way, to determine the number of acid sites with different acidity strengths. The behavior of these materials in acid-catalyzed reactions, primarily, in the transformations of hydrocarbons is discussed. Evidence is presented that heats of adsorption of ammonia can be used to obtain correlation plots that describe relations between acidic and catalytic properties of zeolite catalysts.     

Effect of gamma-irradiation on the acidic properties of Na-Y zeolites

Na-Y zeolite samples were subjected to different doses of γ-irradiation ranging between (10–160 Mrad). The effect of this treatment on the surface acidity was investigated via temperature programmed desorption (TPD) of pyridine and ammonia.The results obtained revealed that γ-irradiation brought about a considerable decrease in the concentration of acidic sites to an extent proportional to the dose employed up to 80 Mrad. A dose as high as 160 Mrad was sufficient to expell completely the labile and moderately bound acidic centres (Bronsted type). These results have been attributed to the stimulation effect of the departure of structural water brought about by γ-irradiation. However, γ-irradiation was found to exert undetectable effect on the Lewis acidity of the investigated zeolites.     

改性Y分子筛的酸碱性能及吸附性能的研究

吡啶、吡咯、苯、甲苯、乙苯作为探针分子，在Li＋, Na＋, K＋, Cs＋改性的Y型分子筛上进行吸附，用TPD及IR方法系统地研究了不同碱金属离子改性的Y型分子筛的酸碱性能和吸附性能的变化. 结果表明，按Li、Na、K、Cs的顺序, 随着碱金属离子半径的增大, 其L酸酸强度依次减弱，L碱的强度逐渐增强.由于改性Y型分子筛所含碱金属离子的不同，其对芳烃的吸附的强弱及吸附量的大小亦不同.随着骨架外的阳离子的半径逐渐增大，碱的强度逐渐增强，与芳烃的作用愈强烈，导致TPD脱附峰温增高及芳烃和侧链上的C－H伸缩振动谱带向低波数位移愈多.由于位阻的原因，对含同一种碱金属离子的分子筛来说，随着芳烃侧链C数的增加，芳烃的吸附量逐渐减小.     

Identification and measurements of strong Brønsted acid site in ultrastable Y (USY) zeolite

By using the IRMS-TPD method in which IR (infrared) and MS (mass spectroscopy) worked together, acid sites of USY (ultrastable Y) zeolite were studied. A new band of OH playing a role of Br?nsted acid was clearly detected on Na2H2-ethylenediaminetetraacetic acid (EDTA)-treated USY at 3595 cm(-1) during an elevation in temperature after the adsorption of ammonia. MS-measured TPD (temperature-programmed desorption) of NH3 and IR-measured TPD of the NH4+ cation coincided well to show that this zeolite consisted of the Br?nsted acid sites. The MS-TPD profile at higher temperatures corresponded to the IR-TPD of the 3595-cm(-1) band, and therefore, this OH was identified as a strong acid site. From comparison between IR-TPD of OH and MS-TPD, numbers of three kinds of Br?nsted OH (i.e., those in super and sodalite cages of a Y zeolite structure) and created strong Br?nsted acid site were quantified. On the other hand, strength of the Br?nsted acid site DeltaH was determined individually by a simulation method, where the corrected IR-TPD of OH was simulated based on the proposed equation. Thus, a new strong Br?nsted acid site was identified in the EDTA-treated USY, and the amount and strength was measured quantitatively.     

Characterization of the acidic properties of zeolites by means of temperature-programmed desorption (TPD) of ammonia

Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of different type by means of regularization. This method does not require any limiting assumptions about the distribution function. It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of the zeolites.     

Analysis of Acidic Properties of Zeolitic and Non-Zeolitic Solid Acid Catalysts Using Temperature-Programmed Desorption of Ammonia

A method of ammonia temperature-programmed desorption (TPD) for analysis of acidic property of a solid was improved by introduction of a water vapor treatment method and development of a theory for calculation of ammonia adsorption heat from the TPD profile. The improved method was applied to various solid acid catalysts to establish relationships between the acidic properties and catalytic performances for various acid-catalyzed reactions. Here, examples of the applications to some important acid catalysts are reviewed. The exact analysis of acidic property of Y zeolite and its change by such modifications as steaming and ethylenediaminetetraacetic acid (EDTA) treatment gave a new interpretation on the generation of alkane (paraffin) cracking activity on an ultra stable Y (USY) zeolite. The surface density and strength of acid sites on WO₃/ZrO₂ and SO₄ ^2–/ZrO₂ catalysts were determined, and their relations with catalytic activities for Friedel–Crafts type alkylation and skeletal isomerization of alkane were found.     

Y沸石的酸性

用^29Si MAS NMR(MAS为Magie Angle Spinning), 统计计算,NH3-TPD 等方法对(NH4)2·SiF6去铝补硅得到的高硅Y型沸石的酸位分布情况进行了研究, 并与典型酸催化反应数据相关联, 证实了Y型沸石的酸位强度取决于A1原子的周围环境, 即与次邻位A1原子数目n-NNN(NNN为Next Nearst Neighbor)有关, 沸石的强酸性来自次邻位无A1原子的AIO^-4四面体。 NH3-TPD法测量的沸石酸量和酸强度数据与^29Si MAS NMR 实验结果和统计计算得到的不同n-NNN A1位的分布是一致的。 随着A1含量减少, Y型沸石表面的总酸量是单调下降的。但强酸量却是先增加后下降, 在A1/(A1+Si)为0.15左右出现极大值。不同硅铝比的Y型沸石对典型的强酸性和弱酸性催化反应的活性变化规律亦与酸性相吻合。     

稀土离子调变Y型分子筛结构稳定性和酸性的机制

采用X射线粉末衍射(XRD)、魔角旋转固体核磁共振(MAS NMR)、程序升温脱附(TPD)以及吡啶-傅里叶变换红外(Py-FTIR)光谱等方法研究了稀土离子对Y型分子筛结构稳定性和酸性的影响.结果表明:稀土离子的引入不仅显著增强了Y型分子筛的结构稳定性,而且调变了分子筛酸性,使分子筛中等强度酸中心数量增多,强酸中心数量减少.结合密度泛函理论(DFT)计算结果,从理论上阐述了这种机制:进入分子筛β笼l′位的稀土离子与骨架O原子相互作用,增加了Al-O作用力,提高了分子筛的结构稳定性,从而避免了骨架Al的脱除和非骨架Al物种的生成,进而导致稀土氧Y(REHY)分子筛的Br(o)nsted (B)酸强度较USY分子筛的弱;但与HY分子筛相比,稀土离子的引入使骨架O1负电荷减弱,Al-O1键长变短,O1-H作用力减弱,导致REHY分子筛的B酸强度较HY分子筛的强;在酸中心数量方面,REHY分子筛的B酸中心数量比USY分子筛的多,但比HY分子筛的少.     

Control of the properties of catalysts for methane aromatization by synthesizing ZSM-5 zeolites with different crystallite sizes

MFI zeolite materials (ZSM-5) with crystal sizes in the range from 0.10 to 1.70 μm have been synthesized. Acidic and surface properties, phase and morphological composition of the prepared zeolites have been studied by the IR sprectroscopy, nitrogen porosimetry, XRD, and scanning electron microscopy. Increasing crystal size was shown to decrease the general acidity of the zeolite. Synthesized zeolites served as supports for molybden-containing catalysts for methane aromatization prepared by using the solid phase synthesis approach. Diffuse reflectance IR spectroscopy, thermoprogrammed desoption of ammonia and ²⁷Al NMR spectroscopy were used to characterize the physicochemical properties of the catalysts. An increase in the crystallite size of the zeolite favors a decrease in the acidity of the catalysts and inhibits the formation of alumina molybdate during the catalyst preparation. As a result, a tendency to coke formation is suppressed and the performance of the catalysts in methane aromatization improved: methane conversion and aromatic hydrocarbon yield increase.     

建筑形象那些事

含铜的SSZ-39分子筛（AEI拓扑结构）在机动车尾气氨气选择性催化还原(NH3-SCR)反应中性能优异,其中SSZ-39分子筛的骨架铝分布与对应的Br?nsted酸性质对反应性能影响至关重要。本文通过密度泛函理论计算同时结合固体核磁共振谱学实验探究了高硅和富铝SSZ-39分子筛骨架Al位置以及与相应Br?nsted酸强度之间的关系。通过比较骨架Al在不同位置的替代能发现,高硅H-SSZ-39分子筛的骨架铝主要以孤立Al形式存在,同晶取代后落位在T3位上,其相应的Br?nsted酸质子与O7结合时最稳定。而富铝SSZ-39分子筛的骨架铝主要以NNNN与NNN序列的2Al形式存在,当两个骨架铝原子分别位于六元环和四元环对位的T3位上时体系能量最低,此时两个Br?nsted酸质子指向分子筛的超笼和八元环孔道。在最优构型下计算质子亲核势、NH3吸附态微观结构与脱附能以及吸附氘代乙腈后1H NMR化学位移来表征Br?nsted酸性,发现随着SSZ-39分子筛铝含量增加相应的Br?nsted酸含量增加,而Br?nsted酸强度趋于减弱。这些理论计算结果与NH3-TPD及吸附氘代乙腈的1H MAS NMR实验结果一致。本文为调控SSZ-39分子筛酸性以及合理设计高效催化剂提供了依据。     

Catalytic activity of phosphorus and steam modified HZSM-5 and the theoretical selection of phosphorus grafting model

The modification of HZSM-5 zeolite with phosphorus and steam has been studied. Results show that 1% phosphorus and steam modified HZSM-5 has the highest catalytic activity for n-heptane. Physicochemical and catalytic properties of 1% phosphorus and steam modified HZSM-5 zeolites have been investigated. The X-ray diffraction （XRD） results exhibit that there is considerable variation in the relative intensity of the individual diffraction peaks. The acidity of the samples decreases with an increase in the steaming temperature, which is determined by the IR of adsorbed pyridine and temperature programmed desorption （TPD） of ammonia. The oxidation state of phosphorus shown by XPS is ＋5, and a model for surface structure modification is proposed. The nitrogen adsorption isotherm for all samples is a combination of type I and type IV, all hysteresis loops resemble the H4-type. The density functional and cluster model methods have been invoked to select the phosphorus grafting model, and it was found that the phosphorus grafting model were more probable in the form of the terminal oxygen coordinating with aluminum.     

沸石分子筛上程序升温脱附谱的Monte Carlo模拟研究Ⅱ．不同晶粒的沸石分子筛

沸石分子筛上程序升温脱附谱的ＭｏｎｔｅＣａｒｌｏ模拟研究＊Ⅱ．不同晶粒的沸石分子筛王建国李永旺陈诵英彭少逸（中国科学院山西煤炭化学研究所，太原０３０００１）关键词沸石分子筛，程序升温脱附，ＭｏｎｔｅＣａｒｌｏ模拟沸石分子筛被广泛用作吸附剂和催化剂的根...     

ZSM-5型分子筛的表面酸性与催化活性

合成了一批不同硅铝比的HZSM－5沸石分子筛，并用CO62 ,Fe^3 离子对其进行改性，测定了它们在醛氨缩合反应合成吡啶中的活性和选择性。对照它们的NH3－TPD，吡啶吸附红外光谱，研究了催化剂的表面酸性与醛氨缩合催化活性的关系。研究表明，硅铝比较小时，HZSM－5的酸中心较多，但过多的酸中心会引发其它的裂解反应和缩合反应，从而降低催化剂的活性和选择性；当硅铝比为120左右时，催化活性达到最大，吡啶碱产率达60％；若继续增大硅铝比，则无足够的酸中心进行反应，用Co^2 ,Fe^3 离子对HZSM进行改性，其B酸中心变化不明显，L酸中心减少，对氨气的吸附能力有所下降，从而保证了适当的酸中心暴露，有利于反应，采用CoZSM-5催化剂，吡啶碱产率可达78％。     

Characterization of short-time dealumined HZSM-5 zeolites

Temperature-programmed desorption (TPD) of water was applied to characterize short-time dealuminated HZSM-5 zeolites. Using a regularization method, distribution functions of the effective desorption energy of water were calculated. The results clearly show that during dealumination a new adsorption site is formed which can be attributed to non-framework or transient aluminium species. The highest concentration of these sites was observed for a dealumination time of 25-30 min. NO adsorption studies support this result. Furthermore, it could be concluded that the heterogeneity and the average acid strength of the remaining Si-OH-Al groups of the dealuminated samples do not change compared to the Si-OH-Al groups of the parent HZSM-5 zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

丙烯在ZSM-5沸石上齐聚反应的研究 Ⅰ. 模板剂用量对丙烯齐聚反应的影响

考察了不同正丁胺（NBA）模板剂用量合成ZSM-5沸石的物化特性和催化性能。采用XRD、SEM、NH3-TPD和BET等手段对合成样品的物化特性进行了表征。结果表明，模板剂与SiO2摩尔比在0.67～0.22时合成的ZSM-5沸石结晶度高于90%；随着模板剂用量的减少，ZSM-5沸石的平均粒径减小，强酸量也存在相同的趋势。丙烯齐聚反应评价结果显示,模板剂用量对合成ZSM-5沸石的催化活性有显著影响，模板剂与SiO2摩尔比在0.67～0.45之间合成的沸石催化性能较好。     

Study of Brønsted acid site in H-MCM-22 zeolite by temperature-programmed desorption of ammonia

Interaction of ammonia with H-MCM-22 zeolite (Si/Al = 24.5) was investigated by temperature-programmed desorption technique in order to obtain information on thermodynamics of the process. Average activation energy for desorption of ammonia from Brønsted acid sites of H-MCM-22 zeolite was estimated from the data obtained under conditions varying in heating rate and also flow rate of carrier gas. It resulted in value of E _d = 127 kJ mol^?1 for heat rate variation method, whereas flow rate variation led to E _d value of 111 kJ mol^?1. Obtained E _d values are compared with those reported in the literature for other zeolitic materials and discussed in the broader context of zeolite acidity. Comparison of E _d values estimated here for H-MCM-22 zeolite with corresponding data for other protonic zeolites shows that H-MCM-22 displays mediocre/lower activation energy for ammonia compared with other high-silica zeolites.     

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture

A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.     

CF-2沸石的表面酸性及催化性能

CF-2是一种新型的高硅沸石。1981年我们实验室首先在二乙醇胺-甘油-Na₂O-SiO₂-Al₂O₃-H₂O体系中制备成功^[1],以后才见到关于Theta-1^[2],ISI-1^[3],KZ-2^[4],NO-10^[5]和ZSM-22^[6]沸石的报道。