Where and how shape selectivity of molecular sieves operates in refining and petrochemistry catalytic processes

Zeolites play an important role in many catalytic processes of modern refining and petrochemistry. Their shape selectivity is always a key factor and here we look at the ways it operates in the principal processes either industrialized or with a good industrial potential: cracking, hydrocracking, isomerization of short or long paraffins and of butene, aromatic transformations by isomerization, disproportionation/transalkylation, and alkylations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis and thermal properties of polythioether‐based liquid‐crystalline polymers containing azobenzene in the side chain

BACKGROUND: Owing to the unusual structural rearrangement of polychloromethylthiirane (PCMT) at room temperature, it has not been used as the main‐chain backbone of side‐chain liquid‐crystalline polymers (SLCPs). However, it has been observed that PCMT has a relatively stable and clear structure under special conditions. Therefore, we attempted to synthesize SLCPs using PCMT as main‐chain backbone and investigated their thermal behavior. RESULTS: New polymers, poly[1‐({(4‐methoxyazobenzene‐4′‐oxy)alkyl}thio)‐2,3‐epithiopropane], in which the number of methylene units in the alkyl group is 4, 5 or 6, were prepared by means of reactions of corresponding (4‐methoxyazobenzene‐4′‐oxy)alkylthiols with PCMT. The structures of these compounds were confirmed using elemental analysis and ¹H NMR spectroscopy. The substitution ratios of the copolymers with 4, 5 and 6 methylene units in the alkyl group were 56, 75 and 80%, respectively. Differential scanning calorimetry measurements and polarized optical microscopy observations showed that the resulting copolymers exhibited thermotropic liquid‐crystalline mesomorphism with nematic phase except for the copolymer with a 56% substitution ratio. The decomposition temperature of all the synthesized copolymers was near 195 °C. CONCLUSION: This investigation has demonstrated that PCMT polymerized for 8 h has the ability to act as a suitable main‐chain backbone for SLCPs. Moreover, SLCPs could be obtained only by the reaction of PCMT with thiolate salt containing mesogenic groups. The substitution ratios increased with increasing number of methylene groups in the spacer. Copyright © 2009 Society of Chemical Industry     

Isomerization of methylcyclopentadiene dimers

A commercial mixture of methylcyclopentadiene dimers mainly consisting of four dimeric compounds with partial structures of 1-methyl- and 2-methylnorbornene, undergoes slow oxidation and isomerization when it is dissolved in CH₂Cl₂, CHCl₃ or CCl₄. Both reactions are simultaneous but independent of each other. Isomerization only concerns those isomers that contain in their structure a 2-methylnorbornene ring. The nature of the isomerization and the influence of different acid media is studied.     

Characterization of brighteners in detergents by high-performance liquid chromatography/mass spectrometry/ultraviolet/fluorescence and three-dimensional high-performance liquid chromatography

A method has been developed for the identification of unknown brighteners in detergents by a high-performance liquid chromatography (HPLC)/mass spectrometry (MS)/ultraviolet (UV)/fluorescence system with MS/MS capability. Ten brighteners from five groups—stilbene, biphenyl stilbene, pyrazoline, oxazole, and coumarin derivatives—were separated on a C₈ reverse-phase HPLC column. The UV and fluorescence detectors positively located the brightener peaks and differentiated betweentrans andcis isomers separated by the chromatographic system. Ion-spray (IS)/MS and IS/MS/MS spectra were then used to identify the structure of unknown brighteners in detergent products. An HPLC with a diode-array detector provided a quick identification check and accurate quantitative data for known brighteners, even on overlapping peaks.     

Supported gold clusters and cluster-based nanomaterials: characterization, stability and growth studies by in situ GISAXS under vacuum conditions and in the presence of hydrogen

This contribution is devoted to study of the thermal stability and growth of gold nanoparticles supported on SiO₂/Si(111) and Al₂O₃/SiO₂/Si(111) as a function of initial cluster size, support material and level of surface coverage. Experimental evidence for “flipping” of two dimensional cluster structures from vertical orientation to horizontal on the support is presented.     

Selective isomerization of butene to isobutene

To identify the elements active for the skeletal isomerization ofn-butene, a search along the periods of the periodic system has been performed. It is shown that the group VIA (Cr, Mo and W) oxides and the Al oxide are potentially active catalysts. The reaction mechanism and the nature of the active sites are also discussed in relation to the information from the literature.     

MOR沸石合成条件及对其性能的影响

利用FT－IR光谱法测定不同条件下合成的MOR沸石的酸性分布，并考察其催化剂的反应活性和选择性。结果表明，MOR沸石酸性与合成条件相关，CS芳烃异构化反应活性与MOR沸石的B酸量成正比，选择性与L酸量成正比。     

Shape Selectivity of HFER Zeolites as Responsible for the Positive Effect of Their Si/Al Ratio on the Selectivity to <Emphasis Type="Italic">n</Emphasis>-Butene Isomerization into Isobutene

The selectivity and turnover frequency for isobutene production was shown to increase with the Si/Al ratio of fresh HFER. This can be explained by considering the zeolite pore system as a series of non-interconnected nanoreactors (the 10-MR channels) in which the molecules diffuse and react bimolecularly without any possibility of desorption before the exit.     

在Pt-SO_4~(2-)-ZrO_2催化剂上正丁烷骨架异构反应

叙述了用脉冲色谱法和流动法考查在不同条件下制备的PtSZ催化剂对正丁烷异构化反应的影响，用XRD，XPS，IR等方法探讨了催化反应机理。结果表明，少量铂的引入可以增强催化剂的稳定性，特别是在临氢情况下，由于金属铂的作用，氢被分解而产生强质子酸位，从而导致积碳减少，提高了选择性，降低了反应温度和担载硫酸的浓度，使催化剂的稳定性增强。     

Surface photochemistry: efficient photocatalysis by CdS supported on a solid surface

The photocatalytic efficiency of a series of CdS samples: (i) a colloidal solution in aqueous CH₃CN; (ii) suspensions of (a) supported colloidal particles, (b) bulk CdS, and (c) commercial powders, was compared with respect to stilbenec-t isomerization. The samples of type (ii a), prepared from cadmium chloride and H₂S showed a high photocatalytic activity, attributable to the combined effects of the catalyst particle size and the presence of chloride ions. Increased activity over the commercial sample was also shown in the conversion of hexamethyl(dewar)benzene to hexamethylbenzene, and in the cleavage of anthracene dimer. It has been found that bulk CdS is activated by grinding with < 10% of certain metal chlorides and bromides.