Transformations of aromatic hydrocarbons over zeolites

Aromatic hydrocarbons represent an important group of starting materials, intermediates, as well as final products produced in the chemical industry in the range from large processes in petrochemistry up to the synthesis of fine chemicals. This short overview covers the recent achievements in acid-catalyzed transformations of aromatic hydrocarbons with a special focus on alkylation-isomerization-disproportionation reactions in petrochemistry, and acylations or condensations used mainly in synthesis of chemical specialties. In the case of fine chemical synthesis, some zeolite applications in the preparation of important intermediates for pharmaceutical or fragrance products are presented. Advantages and disadvantages of the various types of zeolite are discussed in these reactions from the point of view of their (shape) selective properties, as well as the accessibility of acid sites.     

Catalytic cracking of large molecules over hierarchical zeolites

A hierarchical zeolite catalyst was synthesized by transforming the skeletons of a bimodal pore silica gel into a zeolite through a steam-assisted conversion method, and shows high catalytic activity and a long catalyst lifetime for catalytic cracking of large molecules.     

Isomerization, dehydrogenation and cracking of methylcyclohexane over HNaY zeolites

On HNaY zeolites the fractions of acid sites which are active for cracking, isomerization and dehydrogenation of methylcyclohexane do not increase linearly with increasing degree of exchange. Isomerization and dehydrogenation seem to involve a similar activated complex, while cracking proceeds through a different one and requires stronger acid sites. Activation energies are independent of exchange degree.

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HNaY, , . , . .

     

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。     

Selective transformation of light olefins into aromatic hydrocarbons over pentasil zeolites

1-Pentene and 1-hexene transformation into aromatic hydrocarbons over H-ZSM-5, H-ZBS-11 and H-ZSM-11 modified with Zn(II), Ga(III), Mn(II) and Fe(III) has been studied. Aluminosilicate zeolites exhibited higher activity than borosilicate zeolites. Zn-species are effective in dehydrogenation, generating allylic intermediates from olefins used as feed.     

A quantum chemical study of the influence of an electrostatic field on the reactivity of methanol over zeolites

The INDO method with an electrostatic potential term in its Hamiltonian has been used to estimate the changes in the electronic structure of methanol and methanol dimer due to the electrostatic field of zeolites. It has been shown that fields of the strength F=2–8×10¹⁰V/m change significantly the electronic structure as well as the geometry of both molecules and can cause their decomposition leading to the species H₂O, CH₃, CH₃O, CH₂O, and CH₂.

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INDO . , F=2÷8·10¹⁰ / , , H₂O, CH₃, CH₃O, CH₂O CH₂.

     

Noble metal loaded zeolites for the catalytic oxidation of chlorinated hydrocarbons

Summary A series of metal loaded zeolite catalysts (Pd/H-ZSM-5, Pd/H-BETA, Pt/H-ZSM-5, and Pt/H-BETA) were investigated for their activity and selectivity during oxidation of different chlorinated hydrocarbons, namely dichloromethane and trichloroethylene, at constant gas space velocity (15,000 h^-1) and constant chlorohydrocarbon concentration (1,000 ppm in dry air). It was observed that the two noble metals played a major role in influencing the catalytic performance for complete oxidation of both chlorinated compounds. The acidic properties of the zeolite support in combination with increased oxygen activation owing to the noble metal were responsible for the high chlorocarbon destruction activity exhibited by this type of catalysts.     

不同硅铝比ZSM-5分子筛作为助催化剂在渣油裂化中的应用

采用不同方法表征了硅铝比(SiO2/Al2O3)为33、266和487的质子型ZSM-5分子筛,并研究了ZSM-5分子筛作为助催化剂在渣油裂解中的应用。与USY分子筛基催化剂混合后,在固定流化床上,评价了ZSM-5分子筛助催化剂的催化裂化性能。研究发现,提高ZSM-5分子筛硅铝比,可以有效抑制混合催化剂对汽油烯烃的裂解,从而避免了汽油烷烃的大量损失。加入ZSM-5助催化剂后,伴随着液化气(LPG)产率的增加,异丁烷和异戊烷产率增加,这可能是由USY基催化剂和ZSM-5助催化剂的综合效应引起的。汽油烷烃和芳烃含量的变化,引起了汽油辛烷值的增加。高硅铝比ZSM-5分子筛(硅铝比为266和487)不仅可以显著改善汽油的辛烷值,而且有效避免了汽油的大量损失。催化汽油辛烷值的改善主要是由于高硅铝比ZSM-5分子筛具有适宜的芳构化和异构化活性,这些变化主要源于高硅铝比ZSM-5分子筛小的孔道直径和适宜的酸性。     

On the reactivity of surface methoxy species in acidic zeolites

The in situ preparation and isolation of surface methoxy species on acidic zeolites are followed by further investigations of their reactivity in heterogeneously catalyzed reactions. For the first time, the following solid-state NMR evidence for the high reactivity of surface methoxy species has been obtained: (i) Surface methoxy species react readily with ammonia on acidic zeolites at room temperature, by which methylamines and methylammonium cations are formed. (ii) The transformation of surface methoxy species to other alkoxy species can be achieved by the reaction of surface methoxy species and corresponding alkyl halides on acidic zeolites. (iii) Surface methoxy species react readily with hydrochloride, giving methyl chloride as the sole product. (iv) The classic Koch carbonylation reaction and Ritter reaction in solution can be performed with surface methoxy species on acidic zeolites. (v) Carbon monoxide and carbon dioxide are produced by the oxidation of surface methoxy species in the presence of oxygen. The stability and reactivity of surface methoxy species are discussed in comparison with other surface alkoxy species (> C(1) species).     

Low-temperature radiation-induced cracking of liquid hydrocarbons

Conditions of chain cracking reactions in liquid hydrocarbons at lowered temperatures are discussed subject to contribution of the long-living radiation-excited molecular states. It is predicted that chain reactions of hydrocarbon degradation can proceed without thermal activation at the high dose rates of ionizing irradiation. In this process, both initiation and propagation of the chain are caused only by the action of radiation. Experimental verification has allowed observation of low-temperature chain cracking reactions in high-viscous oil.     

煤焦表面官能团对甲烷裂解的影响

采用石英管固定床反应器,考察了1 123 K,CH_4/N_2=1∶4的状态下,煤焦表面官能团对甲烷裂解的影响。煤焦表面主要含有羟基、羰基和醚键等官能团,分别通过氢氧化钡、苯肼和碘化氢溶液对煤焦表面进行化学处理,除去煤焦表面相应的官能团,研究处理后煤焦的催化活性,得出煤焦中官能团对甲烷裂解的影响。氢氧化钡处理煤焦后,煤焦中部分羟基被氢氧化钡消耗,甲烷初始转化率和氢气初始收率分别为90.5%和65.2%,说明煤焦中的羟基不利于甲烷的裂解。通过苯肼处理煤焦中羰基并使之转化,甲烷初始转化率和氢气初始收率分别为55.4%和42.9%,说明煤焦中的羰基对甲烷裂解有利。碘化氢处理煤焦后,煤焦中的醚键转化成羟基,甲烷的转化率和氢气收率都明显下降,说明醚键的存在对甲烷裂解同样有利。随着反应时间的延长,甲烷的转化率和氢气的收率降低,催化剂逐渐失活。反应前后煤焦电镜扫描照片表明,甲烷裂解生成的积炭沉积在煤焦表面,堵塞煤焦的孔道,煤焦的比表面积减小,催化活性降低。     

Thermogravimetric characterization of solid-state ion-exchange procedure in zeolites

The solid-state ion-exchange procedures of zeolites with Cu²⁺, Ni²⁺, Fe²⁺ and Co²⁺ salts can be resulted in prosperous catalysts for NO decomposition. Reactions taking place in solid-state between four transition metal cholirides and H-ZS-5 zeolite were investigated by means of a derivatograph. The results showed that irreversible consumption of surface -OH groups occurred with simultaneous bonding of the transition metal in the zeolite channel structure.     

Methanol-based synthesis of hydrocarbons on bifunctional Y-type zeolites

The directions of methanol conversion on Pt, Pd, Zn, Ni metals and Y zeolites and on bifunctional metal-Y-zeolite catalysts have been studied. The increase of the hydrogenation activity of bifunctional catalysts enhances the rate of methanol conversion and decreases the selectivity of hydrocarbon formation.

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- Y, - Y.

     

Isomerization of dimethylnaphthalenes over zeolites

A dimethylnaphthalene (DMN) isomer mixture from a reforming unit was reacted at 350°C and atmospheric pressure over H-mordenites (Si/Al ranging from 5 to 100), a partially decationated NaY (Si/Al=2.4) and a slightly dealuminated HY (Si/Al=5), with the aim of increasing the 2,6- and 2,7-DMN content by isomerization. The best results were obtained on H-mordenite with Si/Al=10, where shape selective effects made possible to double the amount of the valuable isomers while limiting to a negligible extent the side reactions of disproportionation and dealkylation.     

Computer analysis of the cracking patterns of deuterated hydrocarbons

A generalized, statistical model for calculating the mass spectral fragmentation patterns of deuterated hydrocarbons of low molecular weight is presented. Isotopic differences in bond energies and probabilities of the bond breakage of hydrogen and deuterium are included. Only a single fitting parameter is used. This parameter seems to be consistent for similar species of hydrocarbons. Small statistical deviations from observed spectra for deuterated methanes, acetylenes, ethylenes, and ethanes are obtained when this model is used.