Reactivity of isobutane on zeolites: a first principles study

In this work, ab initio and density functional theory methods are used to study isobutane protolytic cracking, primary hydrogen exchange, tertiary hydrogen exchange, and dehydrogenation reactions catalyzed by zeolites. The reactants, products, and transition-state structures are optimized at the B3LYP/6-31G* level, and the final energies are calculated using the CBS-QB3 composite energy method. The computed activation barriers are 52.3 kcal/mol for cracking, 29.4 kcal/mol for primary hydrogen exchange, 29.9 kcal/mol for tertiary hydrogen exchange, and 59.4 kcal/mol for dehydrogenation. The zeolite acidity effects on the reaction barriers are also investigated by changing the cluster terminal Si-H bond lengths. The analytical expressions between activation barriers and zeolite deprotonation energies for each reaction are proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts.     

n‐Butane Monomolecular Cracking on Acidic Zeolites: A Density Functional Study

采用5T簇模型,利用密度泛函理论在B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d)水平下研究正丁烷在酸性分子筛上的单分子催化裂解反应。本文重点详细研究了正丁烷在分子筛表面不同C位的脱氢反应。在B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d)水平下计算所得第一和第二位C-C键裂解的活化能垒分别为 238、217 kJ/mol。而第一第二序位脱氢反应能垒分别为296、242 kJ/mol。正丁烷不同序位脱氢反应的活化能垒相差54 kJ/mol。从计算结果可以看出,正丁烷在分子筛上催化裂解脱氢反应优先发生在第二位C原子上。此外,本文还讨论了簇模型结构与酸性的关系,结果显示改变封端Si-H键的键长的方法可以用来模拟分子筛酸性变化。最后研究了分子筛酸性变化与正丁烷催化裂解反应能垒的关系。     

H-ZSM-5分子筛上环己烯芳构化反应历程的理论研究

基于76T簇模型,采用量子力学和分子力学联合的ONIOM2(B3LYP/6-31G(d,p):UFF)方法研究了H-ZSM-5分子筛上环己烯芳构化反应历程.结果表明,环己烯首先吸附在分子筛酸性位上,与酸性质子共同脱除一个H2分子后,在分子筛骨架氧上生成烷氧配合物中间体;然后再脱质子得到环己二烯,同时酸性位复原;再经历脱氢和脱质子历程,最后得到产物苯,并吸附在复原的分子筛酸性位上.计算得到脱氢的活化能依次为279.64和260.21kJ/mol,脱质子的活化能依次为74.64和59.14kJ/mol.所有脱氢反应都是吸热过程,生成表面烷氧活性中间体,随后的脱质子反应能垒较低,而且是放热过程.此外,比较了环己烯在分子筛酸性位上的三个竞争反应,即脱氢、质子化和氢交换反应的活化能垒,证明环己烯优先发生脱氢反应.     

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O₅Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al₁O₅ sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.     

正辛烷热裂化和催化裂化生成甲烷反应机理

采用脉冲微反装置,在反应温度为550~650℃,低转化率(小于15%)下,研究了正辛烷在石英砂和ZRP分子筛上的热裂化和催化裂化反应,分析了甲烷的生成机理。结果表明,正辛烷热裂化时,乙烯、丙烯和正丁烯是初始产物,甲烷由4种反应路径生成。当反应温度为600℃时,甲基自由基攻击碳链端部C-H键生成甲烷。中部C-H键脱氢形成的辛基自由基在端部C-C键断裂的活化能较高,仅在高温下生成甲烷。正辛烷在ZRP分子筛上主要发生质子化裂化反应,正构烷烃占有相当比重,甲烷由质子化裂化步骤生成。热裂化与质子化裂化对甲烷贡献的对比可知,当反应温度低于600℃时,甲烷由质子化裂化反应生成;在高温下,热裂化反应决定甲烷选择性。     

Y沸石中不同状态铁离子的还原和催化性质

制备了Fe(Ⅲ)处于骨架四面体内、阳离子位及以氧化物形式分散在沸石表面的含铁Y沸石.用TPR方法测定样品的可还原性,发现不同形态的Fe(Ⅲ)其还原情况不同,在800℃以下均不能被还原成金属Fe.以异丙醇脱水和脱氢为探针,考察了样品的酸碱性,证实在沸石结构内引入不同形态的Fe(Ⅲ)可调变沸石的酸碱性.用乙苯脱氢和裂解反应考察了样品的脱氢活性.当样品表面金属离子和强酸位同时存在时,由于二者的协同作用,裂解反应有可能占主导地位.消除强酸位,增加碱性位,可显著提高脱氢的选择性.     

Theoretical study of the adsorption of ethylene on alkali‐exchanged zeolites

The structures of alkali‐exchanged faujasite (X–FAU, X = Li⁺ or Na⁺ ion) and ZSM‐5 (Li–ZSM‐5) zeolites and their interactions with ethylene have been investigated by means of quantum cluster and embedded cluster approaches at the B3LYP/6‐31G(d, p) level of theory. Inclusion of the Madelung potential from the zeolite framework has a significant effect on the structure and interaction energies of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM‐5 and FAU) that cannot be drawn from a typical quantum cluster model, H₃SiO(X)Al(OH)₂OSiH₃. The Li–ZSM‐5 zeolite is predicted to have a higher Lewis acidity and thus higher ethylene adsorption energy than the Li–FAU zeolites (16.4 vs. 14.4 kcal/mol), in good agreement with the known acidity trend of these two zeolites. On the other hand, the cluster models give virtually the same adsorption energies for both zeolite complexes (8.9 vs. 9.1 kcal/mol). For the larger cation‐exchanged Na–FAU complex, the adsorption energy (11.6 kcal/mol) is predicted to be lower than that of Li–FAU zeolites, which compares well with the experimental estimate of about 9.6 kcal/mol for ethylene adsorption on a less acidic Na–X zeolite. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 333–340, 2003     

改性Y沸石上的氢转移反应

本文试图用环已烯和邻二甲苯两典型反应考察改性Y沸石的氢转移反应性能。发现在改性Y沸石上环已烯的氢转移, 异构化和聚合反应时发生。在反应开始时SY沸石的氢转移反应和异构化反应活性与铝含量之间的关系与沸石强酸量和铝含量的关系一致, 呈火山形曲线, 极大值在Al/(Al+Si)为0.15左右, 结焦以后反应总转化迅速下降, 且氢转移活性和异构化活性随铝含量减少而单调下降。USY沸石的氢转移活性大幅度降低。在改性Y沸石上邻二甲苯异构化和歧化反应同时进行, SY沸石的歧化反应变化规律与环乙烯氢转移反应相似。当沸石样品中有大量非质子酸位存在时, 两个反应的数据会出现予盾。     

Acid-base chemistry of a carbenium ion in a zeolite under equilibrium conditions: verification of a theoretical explanation of carbenium ion stability

The 1,3-dimethylcyclopentenyl carbenium ion (C7H11(+)) was reproducibly prepared on zeolite HZSM-5 using a pulse-quench reactor, and then each of a number of bases was coadsorbed into the catalyst channels to either compete with the cation for protonation or to possibly react with it as a nucleophile. For seven bases with proton affinities (PA) between 142 and 212.1 kcal/mol, there was no reaction with C7H11(+). Coadsorption of smaller amounts of dimethylacetamide (PA = 217 kcal/mol) also produced no reaction, but with a higher loading, a proton was transferred from the carbenium ion to the base to leave 1,3-dimethylcyclopenta-1,3-diene in the zeolite as a neutral olefin. Deprotonation was the primary reaction with coadsorption of either pyridine (PA = 222 kcal/mol) or trimethylphosphine (PA = 229.2 kcal/mol). The estimated experimental deprotonation enthalpy for C7H11(+), approximately 217 kcal/mol in the zeolite, is in excellent agreement with MP4/6-311G gas-phase value of 215.6 kcal/mol. Coadsorption of either NH3 (PA = 204.0 kcal/mol) or PH3 (PA = 188 kcal/mol) does not deprotonate the carbenium ion, but these species do react as nucleophiles to form onium ion derivatives of C7H11(+). Analogous onium complexes with pyridine or trimethylphosphine formed in lower yields due to steric constraints in the zeolite channels. The essential experimental observations were all predicted and explained by density functional calculations (B3LYP/6-311G) and extensions of our recently developed theory of carbenium ion stability in zeolites. In addition, we report theoretical geometries for several complexes which contain unusual C-H- - -X hydrogen bonds.     

Neopentane and solid acids: direct hydron exchange before cracking

The hydrogen/deuterium exchange reaction of 2,2-dimethylpropane (neopentane) over D(2)O-exchanged zeolites (MOR, FAU, BEA, MFI) using a batch recirculation reactor was studied by means of gas chromatography coupled with mass spectrometer. In the temperature range 473-573 K, H/D exchange proceeds without side reaction such as cracking at short contact times. Indeed the C-H bond has appeared favorably involved in the activation of neopentane compared to the less accessible C-C bond. The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between methane and solid acid. The activation energies of the H/D exchange between neopentane and zeolites are the same for all zeolites indicating a common carbonium ion type transition state. On the basis of previous results in the case of the exchange between methane and liquid superacids, the deuterium exchange rates in neopentane were tentatively related to the acidity of the solids. However the order of activity MOR > MFI > BEA > FAU seems to be related to the size of the pores, which may suggest the involvement of a confinement effect in the zeolites cavities. Moreover we found that H/D exchange takes also place between neopentane and deuterated sulfated zirconia (SZ) emphasizing its strong acidity.     

Pt/HZSM-5基催化剂双功能协同性对正丁烷催化裂解反应机制的影响

轻质烷烃结构稳定,化工利用率低.催化裂解轻质烷烃是其高值利用的重要途径,相关研究同时对C-C键和C-H键活化和演变调控具有重要意义.本文在基于原子层沉积法构建高稳定Pt/HZSM-5基双功能模型催化剂的基础上,研究了不同酸性HZSM-5分子筛引入脱氢组分Pt对反应路径及目的产物低碳烯烃的影响规律.研究发现,双功能催化剂中, Pt的脱氢性能与分子筛的酸性裂解性能存在协同作用,不同硅铝比的分子筛上引入Pt,高硅铝比分子筛上Pt的引入对脱氢促进效应更明显.同时,将脱氢组分Pt引入到不同裂解能力的酸性分子筛载体上,会改变其低碳烯烃的生成路径,高硅铝比的分子筛上引入Pt后丁烯的生成路径增强,而低硅铝比的分子筛上则会增强乙烯与丙烯的生成路径,当分子筛硅铝比继续降低时,乙烯的生成路径进一步增强.本研究对多相复杂反应中反应路径调控及高效双功能催化剂设计具有指导意义.     

Ab initioMO calculations on the acidities of water and methanol,and hydrogen bond energies of the conjugate ions with a water molecule

The acidities, deprotonation energies, of water and methanol were calculated by the use of the ab initio self-consistent-field (SCF ) molecular orbital (MO ) method with electron correlation computed by the thirdorder Møller–Plesset perturbation method and configuration interaction with double excitations. Zero-point vibrational energy correction translational energy change, and the PV work term were included to evaluate the accurate acidities. The calculated acidity difference including these corrections was 7 kcal/mol, which is somewhat smaller than the experimental ones (9.5–12.5 kcal/mol) recently determined. The hydrogen bond energies of the conjugate ions (OH^? and CH₃O^?) with a water molecule were calculated to be 2.3 kcal/mol near the Hartree–Fock limit; this energy only amounts to 25% of the (total) hydration energy difference between the two negative ions. The aqueous solvation effect on the acidity scale was discussed.     

催化裂化条件下噻吩与改性Y分子筛的作用机制

以HY、NiY和稀土离子改性的Y分子筛(REY)为研究对象,采用固定床装置评价噻吩模拟油催化裂化性能;运用气相色谱-氢火焰离子发光检测器(GC-FID)、气相色谱-硫化学发光检测器(GC-SCD)和原位红外光谱技术分析产物,关联分子筛的酸性,研究催化裂化条件下噻吩与改性Y分子筛的作用机制。实验结果表明,催化裂化条件下,噻吩与分子筛的作用机制差异主要取决于与B酸或L酸相关的非骨架铝物种或金属离子物种的存在形式。其中,NiY分子筛中,噻吩主要是吸附在与NiOH⁺物种相关的L酸中心,而Ni₄AlO₄³⁺等物种减弱B酸性中心从而降低其裂化性能。对HY来说,噻吩易在与AlO⁺等物种相邻的B酸中心上聚合形成三联噻吩,并发生一定的氢转移和裂化反应;而对REY而言,分子筛中与RE物种相关的L酸位会促进噻吩在与非骨架铝羟基等物种(如Al(OH)₂⁺、Al(OH)²⁺等)相邻的B酸中心形成的二联噻吩发生氢转移和裂化反应。     

On the possibility of the detachment of hydrogen as a result of electron capture by a Broensted center on zeolites

The structure and reactivity of the radical anion center in ZSM-5 zeolite were studied by the density functional theory method. It was shown that the interaction of the hydrogen zeolite form with adsorbed olefins and aromatic hydrocarbons could be accompanied by electron transfer from hydrocarbon molecules to the Broensted acid center with the formation of a radical anion fragment. The radical anion fragment formed is unstable, which contributes to the probability of the exothermic process (ΔE = ?21 kcal/mol) of atomic hydrogen detachment with the activation energy not exceeding 10–12 kcal/mol. The atomic hydrogen split off can initiate hydrocarbon transformations, and such a radical anion center can play the role of a catalytic activity carrier on acid zeolites.     

Enantioselective lithiation of O-alkyl and O-alk-2-enyl Carbamates in the presence of (-)-sparteine and (-)-alpha-isosparteine. A theoretical study

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the enantioselective lithiation/deprotonation of O-alkyl and O-alk-2-enyl carbamates in the presence of (-)-sparteine and (-)-alpha-isosparteine. Complete geometry optimization of the precomplexes consisting of the carbamate, the chiral ligand, and the base (iPrLi), for the transition states of the proton-transfer reaction, and for the resulting lithio carbamates have been performed in order to quantify activation barriers and reaction energies. For the lithiodeprotonation of ethyl carbamate 12 in the presence of (-)-sparteine (5) the preferred abstraction of the pro-S proton (by 2.75 kcal/mol) gives the (S)-lithio derivate (S)-14, which is in good agreement with experimental observations. (-)-alpha-Isosparteine (6) is predicted to be significantly less selective favoring the abstraction of the pro-R proton by 1.2 kcal/mol. The O-alkenyl carbamate 17 as an example of an allylic carbamate is more easily lithiated than 12. As for 12 (-)-sparteine (5) favors the abstraction of the pro-S proton, although with smaller preference (0.7 kcal/mol). Structural parameters are discussed to rationalize the theoretical results.     

Ab Initio Study of the Excited State Decomposition Reactions of Monothioformic Acid

IntroductionTherehasbeenconsiderableinterestinrecentyearsinthetheoreticalstudyofthemonothioformicacid1ed.However,mostofthemhaveconcentIatedontheelectronicproperties,isomerizationandintramolecularhydrogenshiftreactionsoftheacid,andstudiesonitschemicalreactionshaverarelybeenrepofted.Recently,wereportedonthetheoreticalstudyofthegroundstateunimolecularpyrolysisofmonothioforndcacid,andtheresultsshowedthathightemperatureisneededforitsgroundstatedecompositionreactions.Asapotentialinterstellarcompoun…     

十氢萘在分子筛催化剂上的开环反应研究

在小型固定流化床(FFB)装置中研究了Y分子筛与ZSM-5分子筛催化剂上的十氢萘裂化开环反应性能,考察了温度和剂油比对Y分子筛开环反应催化性能的影响。结果表明,十氢萘在分子筛催化剂上通过环烷环开环反应生成丙烷、丙烯、丁烷、丁烯、甲基戊烷和环戊烷、环己烷等非芳烃以及苯、C_1~4烷基取代苯等单环芳烃,并通过脱氢缩合反应生成四氢萘、萘、甲基萘和菲、芘等多环芳烃甚至焦炭等。由于扩散和吸附性能的影响,ZSM-5分子筛催化剂的裂化开环反应选择性比Y分子筛催化剂的高,因此,十氢萘环烷环开环与脱氢缩合反应的相对比例(NRO/DHC)在ZSM-5分子筛催化剂上较高。在Y分子筛催化剂上,温度为450~550 ℃、剂油比为3~9,反应温度升高或者剂油比增加,双分子氢转移以及脱氢缩合反应增强,从而导致环烷环开环产物选择性降低。     

Theoretical study of modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite zeolites

Modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite (H-FAU) zeolites have been investigated by a quantum embedded cluster approach, using the hybrid B3LYP density functional theory. The results indicate that there are two possible adsorption pathways, namely the stepwise process where only one water binds strongly to the (-O)3-Al-O(H) tetrahedral unit while the other weakly binds to the zeolite framework and the concerted process where both water molecules form a large ring of hydrogen-bonding network with the Br?nsted proton and an oxygen framework. With inclusion of the effects of the Madelung potential from the extended zeolite framework, for adsorption on H-ZSM-5 zeolite, both the neutral and ion-pair complexes exist with adsorption energies of -15.13 and -14.73 kcal/mol, respectively. For adsorption on the H-FAU, only the ion-pair complex exists with the adsorption energy of -14.63 kcal/mol. Our results indicate that adsorption properties depend not only on the acidity of the Br?nsted acidic site but also on the topology of the zeolite framework, such as on the spatial confinement effects which lead to very different adsorption structures for the ion-pair complexes in H-ZSM-5 and H-FAU, even though their adsorption energies are quite similar. Our calculated vibrational spectra for these ion-pair complexes support previous experimental IR interpretations.     

Conformational study of protonated,neutral, and deprotonated formamide

Neutral, protonated, and deprotonated formamide isomers were studied at the 3-21G SCF level with complete geometry optimization. Ten stable structures, ten first-order saddle points, and three second-order saddle points (conformational maxima) are reported. [Total energies are reported in hartrees (1 hartree = 627.51 kcal/mol = 2625.5 kJ/mol) and energy differences are reported in kJ/mol (1 kJ/mol = 0.239 kcal/mol).] Rotational barriers and proton affinities are discussed and compared to isoelectronic amidine species.     

Theoretical study on the dehydrogenation reaction of alkanes catalyzed by zeolites containing nonframework gallium species

The dehydrogenation reaction of light alkanes in gallium-containing zeolites was studied by using density functional theory (DFT) and a nonframework gallium species in the dihydridegallium ion form. Two different mechanisms were considered: a 3-step mechanism and a 1-step concerted mechanism. The reactions occurring through the 3-step mechanism showed smaller activation barriers than the ones following the concerted mechanism. However, the energy barrier for the 3-step mechanism seems to be more influenced by the size and type (linear or branched) of the hydrocarbon chain and demands major conformational rearrangement, which could be hampered by the zeolite framework, especially for larger and/or branched hydrocarbons. On the other hand, the concerted mechanism seems to be much less dependent on the substrates geometry. Therefore, the concerted mechanism could be preferential when dealing with larger and/or branched alkanes.