Ab initio quantum chemical study on the zeolite catalyzed transformations of para-xylene

Unimolecular 1,2-methyl shift reaction and disproportionation of para-xylene is investigated by means of ab initio quantum chemical calculations. Isolated gas-phase models are used to estimate the optimum volume of the reaction complex. Space filling of the computed models is examined in zeolite Y, Beta and ZSM-5. It is suggested that the shape selectivity effect of zeolite molecular sieves on the disporportionation reaction can be associated with the steric conditions of the intrinsic rearrangement of the reaction complex in the course of the chemical transformation.     

层剥离MWW结构分子筛的直接可控合成

分子筛是一类重要的催化材料,在石油和煤制化学品工业中应用广泛.目前,国际分子筛协会结构委员会确认的248种分子筛骨架结构中的19种已经商业化应用,如FAU,MFI,*BEA,MOR和MWW.具有多层结构的MCM-22是MWW结构分子筛的典型代表,已应用于苯与乙烯液相烷基化制乙苯.然而,烷基化反应主要发生在位于MCM-22分子筛晶体外表面的半超笼内,位于10元环孔道和12元环超笼内的酸性位对乙烯转化率几乎没有贡献.因此,有必要制备能够容纳更多半超笼的具有超大外比表面积的新型MWW结构分子筛.厚度仅2.5 nm的单层MWW结构分子筛ITQ-2具备以上特征,但是其较弱的酸性质及复杂的制备过程使其无法工业应用.受到采用具有长碳链的双子季铵阳离子两亲性有机结构导向剂直接合成层状MFI分子筛的启发,其有机结构导向剂同时起到导向MFI结构和抑制晶体沿b轴方向生长的作用.类似地,层剥离MWW结构分子筛也被直接合成出来,但所得材料的催化性能不理想,且有机结构导向剂不易得.通过对MCM-22分子筛的结构分析发现,其合成态是由多个MWW结构层有序组装而成,可以通过溶胀处理增大层间距,削弱层间相互作用,经超声处理即可实现层剥离.鉴于此,本文采用向合成MCM-22分子筛的常规体系中加入二环己基胺充当有机添加剂构筑'双功能'合成体系的策略,开展层剥离MWW结构分子筛新材料的直接合成.X射线衍射(XRD)结果表明,与MCM-22分子筛相比,在二环己基胺参与下合成的分子筛SCM-1(DCHA/SiO2=0.4)和SCM-6(DCHA/SiO2=0.6)的XRD谱中与MWW结构c轴方向有关的(101)和(102)晶面的衍射峰出现偏移和重叠的现象,表明其沿c轴方向有序性降低.扫描电子显微镜显示,所有样品均为片状形貌,其中SCM-6含有少量无定形物种.氩气及氮气物理吸附结果表明,SCM-1和SCM-6具有超大外比表面积,而微孔体积小于MCM-22分子筛.透射电子显微镜表征结果表明,MCM-22为多层结构,厚度约为20 nm,而SCM-1和SCM-6的厚度分别约为5.0和2.5 nm,对应双层和单层结构,表明在有机添加剂参与下直接合成了层剥离的新型MWW结构分子筛.值得注意的是,通过调节添加剂用量,可以调节剥离程度,即实现可控层剥离,得到完全(单层)或部分(双层)剥离的MWW结构分子筛.该方法所采用的添加剂的尺寸是关键因素,选用尺寸大于有机结构导向剂的添加剂才能有效抑制晶体沿c轴方向的有序生长,实现层剥离MWW结构分子筛的直接合成.采用该方法合成的新型MWW结构分子筛具有超大外比表面积和更多的表面半超笼,在苯与乙烯液相烷基化反应中,SCM-1催化的乙烯转化率明显高于MCM-22分子筛.SCM-6因结晶度低而未能表现出优势.进一步比较SCM-1和MCM-22的催化稳定性发现,SCM-1的稳定性更好,经过64 h连续反应,其吸附的有机物及积碳更少.在1,3,5-三异丙苯裂解反应中,SCM-1的催化性能也明显优于MCM-22.     

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Abstract  The condensation of silicic acid with aluminate in alkaline environment, the essential reaction of zeolite synthesis, is studied using the density functional theory, with the hybrid functional B3LYP in conjunction with the 6-311++G(d, p) basis set. The Si(OH)₄ monomer and Al(OH)₄⁻ anion are used as the reactant models to study the condensation pathway in basic solution. The solvent effect is included by the COSMO-RS model. The study includes the complete geometry optimization and frequency calculation of reactants, products, reaction intermediates, and transition states, as well as the calculation of the activation energy of the different pathways involved. The intrinsic reaction coordinate method is used to verify the reactant and product corresponding to the transition state. The calculation shows that the formation of Si–O–Al linkage can proceed via two possible reaction pathways. The first is a single-step process, in which the formation of SiO···Al bond and removal of water are synchronous, with the activation energy of 83.7 kJ/mol. The second is a stepwise route, in which the AlO···Si bond is first formed to give a 5-coordianted Si intermediate, and then water is removed to yield a dimer aluminosilicate, with the barriers of 62.7 and 69.3 kJ/mol for the two steps, respectively. Index Abstract   Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites Guo-Feng Jiao, Min Pu, Biao-Hua Chen      

Friedel-Crafts reactions catalyzed by samarium diiodide

Samarium diiodide is an efficient precatalyst for the Friedel-Crafts reaction involving various aromatic substrates and chelating electrophiles. Alkyl 3,3,3-trifluoropyruvates are transformed into α-hydroxyesters in good yields with total regioselectivity. In reactions involving an ethyl glyoxylate or a glyoxylic imine, α-hydroxyesters or α-aminoesters are obtained with variable amounts of products resulting from a double Friedel-Crafts reaction.     

Aza-Michael reactions catalyzed by samarium diiodide

Samarium diiodide catalyzes the Michael addition of aromatic amines onto α,β-unsaturated N-acyloxazolidinones to form β-aminoacid derivatives. Aza-Michael reactions can be followed by an amidation reaction with the aromatic amine, leading to β-aminoamides. β-Amino-N-acyloxazolidinones are selectively obtained with o-anisidine, while amidation reaction is observed with p-anisidine.     

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.     

Influence of temperature inhomogeneity on product profile of reactions occurring within zeolites

In zeolites, diffusion is often accompanied by a reaction or sorption which in turn can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported for the presence of a hot zone presumed to be created by a reaction or chemi- or physi-sorption site. These simulations show that the presence of localized hot regions can alter both kinetic and transport properties such as diffusion. Further, we show that enhancement of diffusion constant is greater for systems with larger barrier height, a surprising result that may be of considerable significance in many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of a hot zone in addition to that which normally exists via concentration. Implications of this coupling for the product profile of a reaction are discussed. We also propose a mechanism by which mobility of ions or diffusion of molecular species within biomembranes may take place. Dedicated to Professor C N R Rao on his 70th birthday     

Tandem reactions catalyzed by lanthanide iodides. Part 2: Tandem iminoaldol-enolisation reactions

Samarium diiodide is a catalyst for the reaction of cyclic and acyclic tert-butyldimethylsilyl enoxysilanes with chelating imines. Reaction products are isolated as β-aminoenoxysilanes instead of β-aminoketones as previously observed with the corresponding trimethylsilyl enoxysilanes. Several mechanistic pathways are discussed.     

Heterogeneous Suzuki reactions catalyzed by Pd(0)-Y zeolite

The Pd(0)-Y zeolite showed high activity in the Suzuki cross-coupling reactions of aryl bromides without added ligands. The type of base and organic solvent were found to be critical for the efficiency of the reaction. The presence of water was essential within the reaction medium. The coupling reactions occurred on the external surface of the zeolite. The catalyst is reusable.     

Tandem reactions catalyzed by lanthanide iodides. Part 1: Tandem Mukaiyama-Michael iminoaldol reactions

Samarium diiodide, as well as lanthanide triiodides catalyze a one-pot procedure allowing to perform sequentially the Mukaiyama-Michael addition of a ketene silyl acetal on a cyclic α,β-unsaturated ketone, followed by the addition of a glyoxylic, aromatic or heteroaromatic imine. According to the nature of the silyl group the adducts resulting from this tandem process are isolated as ketones or as enoxysilanes. The presence of a coordinating group on the imine increases the rate of the reaction.     

Supercritical CO2 as a superior solvent for the cyclization of diallylmalonate catalyzed by palladium-containing zeolites

Palladium-containing zeolites catalyze the cycloisomerization of diethyl diallylmalonate to dimethylcyclopentenes. When the reaction is carried out in toluene, the performance of the palladium catalyst depends on the pore size of the zeolites. At 60 °C, palladium adsorbed on large pore size Beta zeolite (pore size ∼7.4 Å) is more active than medium pore size ZSM-5 (pore size ∼5.4 Å). This lower activity of ZSM-5 compared to Beta is attributable to the restricted diffusion of reagent and products through the ZSM-5 channels as compared to Beta zeolite. However, due to the gas-like diffusion characteristic of the supercritical state, the activity of ZSM-5 increases and becomes identical to that of Beta zeolite using supercritical CO₂ as medium.     

Alkylation of dichlorobenzene isomers by olefins on zeolites Y and pentasil

Catalytic alkylation of dichlorobenzene isomers by ethylene and propylene on zeolite catalysts has been studied. It has been determined that using catalysts based on zeolite Y makes it possible to change the ratio of 1-alkyldichlorobenzenes formed within a wide range by varying the acidic properties of the catalysts.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1486–1488, August, 1995.     

Catalytic activity and acidity of Al pillared clays and zeolites in different hydrocarbon reactions

In comparison with zeolites, a variety of Al pillared clays did not prove as promising cracking catalysts, possibly because their Br?nsted sites are hidden. In the less demanding double bond shift isomerization, both catalyst types were comparable probably due to the presence of Lewis sites on both catalysts.     

Theoretical study on the mechanism of N-heterocyclic carbene catalyzed transesterification reactions

The mechanism of N-heterocyclic carbene (NHC) catalyzed transesterification reactions have been studied using density functional theory. Our study shows that the role of NHC is to assist proton transfer from alcohol to the carbonyl oxygen, forming the tetrahedral intermediate, which then decomposes to the acylated product. Our predicted activation energies are in fine agreement with the observed reaction rates. An alternative approach, which uses the tetrahedral intermediate as the transition state mimic, provides satisfactory predictions.     

Efficient aldehyde olefination reactions catalyzed by an iron porphyrin complex in an ionic liquid

The commercially available complex Fe(TPP)Cl is an active and highly (E)-selective catalyst for the olefination of a variety of aldehydes in the presence of PPh₃ and diazoacetate in the ionic liquid (bmim)(PF₆). Dependent on the reactivity of the applied aldehyde, the reaction can be carried out at a reaction temperature of 50-80 °C. After 0.5-24 h quantitative olefin yields are reached with a broad variety of different aldehydes. Due to the application of an ionic liquid as reaction medium the products can be easily removed from the catalyst by a simple extraction and the catalyst is conveniently reusable without significant activity loss. Spectroscopic investigations indicate that the reaction mechanism includes the quantitative formation of a phosphorus ylide, which then reacts further in a Wittig reaction under formation of an olefin.     

Synthetic routes to seven and higher membered S-heterocycles by use of metal and nonmetal catalyzed reactions

Nitrogen, oxygen, and sulfur containing heterocycles have a wide range of biological activities. Metal and nonmetal catalysts are used in organic reactions with high activity. New strategies have been developed for the preparation of heterocycles in the last decades. The metal and nonmetal catalyzed synthesis of heterocycles is becoming an important and highly rewarding protocol in organic synthesis. In this review article, the synthesis of seven and higher-membered S-heterocycles is presented with the application of metal and nonmetal catalysts for the period from 1968 to 2018.     

The effect of zeolites on copper uptake by plants growing in contaminated soils

Three-year pot experiments were carried out in a greenhouse with the aim of reducing the copper content in the 2nd link of the soil-plant-animal (man) trophic chain. For this purpose, synthetic zeolites of the 3A, 4A, 5A and 13X type were introduced into copper contaminated soils at levels of 1, 2 and 3% by weight in relation to the soil mass. Mono- and dicotyledonous plants were cultivated in mixtures of soils with the zeolites. The copper content in lettuce grown in the contaminated soil decreased in the presence of zeolites by 29–77%, in grass by 41–78%, in oats by 45–64% and in beets by 21–41%, as compared to the control.     

Factors affecting drug adsorption on beta zeolites

The adsorption behaviour of three commonly used drugs, namely ketoprofen, hydrochlorothiazide and atenolol, from diluted aqueous solutions on beta zeolites with different SiO₂/Al₂O₃ ratio (i.e. 25, 38 and 360) was investigated by changing the ionic strength and the pH, before and after thermal treatment of the adsorbents. The selective adsorption of drugs was confirmed by thermogravimetry and X‐ray diffraction. The adsorption capacity of beta zeolites was strongly dependent on both the solution pH and the alumina content of the adsorbent. Such a remarkable difference was interpreted as a function of the interactions between drug molecules and zeolite surface functional groups. Atenolol was readily adsorbed on the less hydrophobic zeolite, under pH conditions in which electrostatic interactions were predominant. On the other hand, ketoprofen adsorption was mainly driven by hydrophobic interactions. For undissociated molecules the adsorption capability increased with the increase of hydrophobicity.