Generation and reactivity of simple chloro(aryl)carbenes within the cavities of nonacidic zeolites

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the sigma+ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.     

Methanol conversion of various cation-exchanged forms of high-silica zeolites

Studies of the methanol conversion on various cation-exchanged forms of high-silica zeolites have revealed that the activity and selectivity of the intact pore structure zeolites depend on the cation composition. On decationated forms the selectivity of methanol conversion to aromatic hydrocarbons is much higher than on the hydrogen form, and it decreases with respect to paraffins.

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Reduction of NO by C1-C4 hydrocarbons on cation-exchanged zeolites

New data are presented for the reduction of nitric oxide by C₁-C₄ hydrocarbons (alkanes and olefins) on active copper, cerium, and cobalt zeolite catalysts both with and without oxygen in the reaction mixture. The following hydrocarbon reactivity series were obtained. Olefins: C₄H₈ > C₃H₆ > C₂H₄. Alkanes: C₄H₁₀ > C₃H₈ > CH₄.L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, 252039 Kiev, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 4, pp. 238–242, July–August, 1996. Original article submitted September 14, 1995.     

The disappearance of aryl carbenes in a reactive matrix

The disappearance of diphenylmethylene, fluorenylidene and phenylmethylene in isobutylene and fluorolube was monitored by esr spectroscopy at temperatures between 77°K and 171°K.     

Phosphorescence and photochemical properties of benzophenone included within alkali metal cation-exchanged ZSM-5 zeolites

The phosphorescence properties of benzophenone included within alkali metal cationexchanged ZSM-5 zeolites were investigated to clarify the effects of the micro-environment of hostadsorbents on the electronic excited states of guest-molecules included within the restricted void spaces. Benzophenone included within such cation-exchanged ZSM-5 zeolites was found to exist in both a protonated and hydrogen-bonded form. It was found that exchanging the cations dramatically affects the ratio of their contents. Photolysis of these systems revealed that both benzhydrol and benzpinacol were the main products, their yields strongly depending on the kind of the cations exchanged. Especially, the protonated species was found to play a significant role in the photoreactions observed with benzophenone included within zeolite cavities.     

Vibrational analysis of propene adsorbed in zeolites Y

Normal coordinate analysis was performed for propene adsorbed in faujasites NaY, CaY and MgY applying a valence force model. The force fields obtained were compared with that of the free molecule. Only small shifts of the fundamentals are observed due to the weak interaction with the zeolites. Although in general the results are comparable to those of propene adsorbed in different zeolites A, some distinct features such as a stronger influence on the CC stretching vibration, a smaller hindrance of the CH bending vibration and a higher reduction of the CH (CH₃) bond strength are detectable, which must be attributed to both the different composition and structure of the zeolite framework. The frequency shifts and the corresponding changes of the force constants are related to the geometry of the sorption complex of propene on the zeolite surface.     

Photocatalytic isomerization of norbornadiene over Y zeolites

K, Zn, Cs and La ion-exchanged Y zeolites were prepared and used as photocatalysts for the isomerization of norbornadiene. The prepared catalysts were characterized by EDS, XRD, ²⁷Al- and ²⁹Si-NMR, UV-vis and IR. The La-exchanged sample showed the highest degree of exchange, but accompanied with some dealumination. La-exchange generated both B-acid and L-acid sites, whereas other metals produced only L-acid sites. For the photocatalytic isomerization reaction, the activity order was LaY > CsY > ZnY > KY. The heavy atom effect and B-acidity are expected to play important roles in the reaction.     

Kinetic study of the dehydration of modified Y zeolites

The dehydration of ferric exchanged Y zeolites is studied by thermal analysis. Their DTA shows three endotherms in the temperature range 80–450°C. The order of reaction and apparent energies of activation are calculated using various equations. The order of dehydration is nearly one and the apparent energy of activation is 4–8 kcal mole^?1. The effect of heating rate is studied. The energy of activation as determined by the Kissinger and Ozawa method is about 12 kcal mole^?1, which is comparable with the heat of adsorption of water determined by the gravimetric method, and is more acceptable.     

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。     

First principles studies on boron sites in zeolites

A systematic computational investigation on protonated and nonprotonated boron-containing zeolites (boralites), performed by using different periodic density functional theory approximations, is presented. Both minimum energy structures and finite temperature behavior of model boron sodalites were analyzed. All of the adopted computational schemes agree in predicting an acid site composed of a silanol Si-OH group loosely linked to a planar BO(3) structure in the protonated system and a BO(4) tetrahedral site in the sodium-containing zeolite. Calculated structural and vibrational properties are in line with experimental data. Comparisons of the protonated boralite site with Al and Ga zeolitic acid sites are discussed as well. Results indicate that this class of mild acid catalysts is characterized by significant framework flexibility and pronounced thermal effects due to the loosely bound acid site.     

Dehydroxylation and catalytic activity of dealuminated Y zeolites

Dehydroxylation and catalytic conversion of m-xylene over dealuminated zeolites were studied.

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First direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical

Direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical is reported for the first time. The influence of temperature on the observed reaction kinetics has been used to obtain kinetic and thermodynamic parameters relating to the reversible addition of oxygen to the carotenoid radical obtained from reaction of 7,7'-dihydro-beta-carotene (77DH) with phenylthiyl radical (PhS.) in benzene. In addition, the rate constant for oxygen addition to the equivalent beta-carotene (beta-CAR) derived radical is also reported. [reaction: see text]     

含镍Y沸石的硫化及加氢脱硫性能

本文制备了不同形态的含镍Y沸石, 研究了温度, 时间和H~2S浓度对含镍Y沸石硫化的影响, 发现负载型含镍Y沸石最易硫化, 离子交换型次之, 骨架型最难硫化。考察了沸石表面硫化物物种的热稳定性和水热稳定性。指出高温下沸石脱附的水蒸汽能使硫化物发生水解, 导致催化剂失硫。用XRD, IR和化学分析等方法对硫化的含镍沸石催化剂进行表征, 硫化后沸石表面形成的NiS~x有可能是属正交晶系的Ni~7S~6化合物。对噻吩加氢脱硫反应, 硫化的含骨架镍Y沸石无催化活性, 硫化的离子交换型含镍Y沸石活性比负载型含镍Y沸石高5-6倍左右, 因其酸性较强。催化剂表面酸性位和硫化镍活性位共存, 对噻吩加氢脱硫反应是有利的。     

Location of chromium and nickel cations in stabilized Y zeolites

The influence of the hydrothermal treatment of stabilized and nonstabilized Y zeolites on the location of Cr and Ni cations was tested. A significant influence of the above treatment was found.

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Validity of energy distribution derived from heat of adsorption in an ammonia/cation-exchanged Y zeolite system

The validity of- functions (energy level functions) which were derived from experimentalq- functions (heat functions) was examined in an ammonia/cation-exchanged Y zeolite system. The- functions of Na-Y zeolite and Cs-Na-Y zeolite were respectively calculated from theirq- functions measured by ammonia adsorption at 373 K. The heat of adsorption of ammonia was also measured on a mixture of these two zeolites at a certain mixing ratio at 373 K. The- function of the mixture was likewise calculated on the basis of this measurement. The- function thus calculated was in good agreement with the synthesized- function derived by linearly combining two- functions of each zeolite mentioned above. The- function derived from experimental- function thus proved to be valid in an actual adsorption system.     

First example of chiral N-heterocyclic carbenes as catalysts for kinetic resolution

Chiral N-heterocyclic carbenes, which are derived from C2-symmetric 1,3-bis(1-arylethyl)imidazolium salts, catalyze enantioselective acylation of racemic secondary alcohols.