Is 1-hexene epoxidation in TS-1 diffusion limited in different solvents?

PFG NMR diffusion measurements were carried out to determine the effect of solvent on intracrystalline reactant diffusivities and on 1-hexene epoxidation rates in TS-1 catalyst. Using n-hexane in silicalite as a mimic for the TS-1 system, the self-diffusivity of n-hexane in silicalite was found to be 24% higher in methanol solvent than in acetonitrile solvent and 45% higher than in acetone solvent. The presence of trace Al did not affect n-hexane diffusivity. Based on analysis of the Weisz modulus for a slab morphology, the 1-hexene epoxidation reaction in TS-1 was found to be diffusion limited only if the crystal size is at least 38 μm in the methanol system.     

Characterization and catalytic properties of titanium silicalite-1 catalyst

Titanium silicalite 1 (TS-1) catalyst was prepared by three different hydrothermal synthesis methods. XRD, elemental analysis, SEM, IR,²⁹Si MAS NMR, UV-vis DRS, physical adsorption of nitrogen, and EXAFS analysis were then performed to evaluate its physical properties. For a test of catalytic activity, liquid phase 2-butanol oxidation and 1-hexene epoxidation using hydrogen peroxide as an oxidant were conducted. The crystal size of the TS-1 catalyst was shown to have a considerable effect on the 1-hexene epoxidation, and smaller crystals resulted in higher activity.     

Al-TS-1催化1-己烯环氧化合成1,2-环氧己烷

以H2O2为氧化剂,Al-TS-1为催化剂催化1-己烯环氧化合成了1,2-环氧己烷。研究了Al含量和溶剂对反应的影响,筛选出较合适的催化剂Al-TS-1(SiO2/Al2O3=616)。采用甲醇为溶剂时,Al-TS-1表现出较低的催化活性和选择性;而采用乙腈为溶剂时,Al-TS-1表现出较高的催化活性和选择性。说明掺杂Al3+后,TS-1的疏水性发生了改变,乙腈溶剂的采用可能改变了反应的历程。     

TS-1催化丙烯环氧化反应及动力学研究

研究了丙烯环氧化反应条件,系统考察了反应压力、溶剂中水质量分数、TS-1质量分数以及反应温度对过氧化氢转化率以及环氧丙烷选择性的影响,采用非机理速率方程确定了主反应动力学参数。并采用该反应动力学模型对TS-1质量分数、溶剂中水质量分数对在不同温度下的双氧水转化速率常数进行了关联。为今后建立复杂动力学模型以及该工艺的工业化打下了良好的基础。     

Selective catalytic oxidation of aniline to azoxybenzene over titanium silicate molecular sieves

The oxidation of aniline using aqueous H₂O₂ and titanium silicates, TS-1 and TS-2 as catalysts was carried out in a batch reactor in the temperature range 333–353 K. TS-1 catalyzes aniline selectively to azoxybenzene and is superior to TS-2. The influence of different solvents, concentration of H₂O₂ and the catalyst in the reaction mixture on the conversion and product distribution has been studied. Acetonitrile is a suitable solvent in this reaction, while acetone is not. For the TS-1 catalyzed oxidation reaction,t-butyl hydroperoxide is not a suitable oxidant. At optimum conditions, a H₂O₂ efficiency of about 100% for aniline conversion is obtained with a selectivity of 97% to azoxybenzene in the product.     

The role of the solvent in TS-1 chemistry: active or passive? An early study revisited

The oxidation of n-hexane with hydrogen peroxide, catalyzed by TS-1 in various solvents, is reported. The initial rate of reaction decreases in the order: t-butanol > t-butanol/water > methanol acetonitrile water, whereas the reverse trend was observed for the adsorption of n-hexane in the catalyst. A dual function is postulated for the solvent in relation to its effects on kinetics. It assists the sorption/desorption of reagents/products in TS-1 (passive role) and participates in the catalytic cycle, through interactions with polar species involved in it (active role). A mechanistic pathway for the hydroxylation of paraffinic and aromatic C–H bonds is suggested.     

Catalytic oxidation of anisole over titanium silicalite-1

The reaction of anisole hydroxylation with hydrogen peroxide to form methoxyphenols over TS-1 was studied. Four C1-C3 alcohols and the two aprotonic solvents acetone and acetonitrile were used. Product conversion was almost proportional to the titanium content of the catalyst sample. The highest conversion and p-selectivity were observed in ethanol. Some speculations, based on the formation of substrate-solvent or titanium-solvent-oxidant complexes, about the role of the solvent are presented. It was found that in all the solvents studied the p/o ratio increased with time.     

TS-1催化氯丙烯环氧化反应过程中的溶剂效应

研究了具有不同官能团的溶剂对TS-1催化氯丙烯环氧化反应性能的影响,发现与无溶剂的反应结果相比,含羟基的溶剂和丙酮对反应有明显的促进作用,在含氰基的乙腈溶剂中,环氧化反应效果变差。试验中还发现,溶剂分子的空间结构影响环氧化的反应速率,并且部分溶剂会与氯丙烯发生竞争性氧化,降低H2O2的有效利用率。在甲醇或丙酮溶剂中混入一定比例的乙腈可提高产物的选择性,但对提高催化剂稳定性效果不显著。     

Oxidative dehydrogenation of propane to propene, 1: Kinetic study on V/MgO

The reaction kinetics of the oxidative dehydrogenation of propane to propene over a V/MgO catalyst were studied. Both propane and propene oxidation kinetics were measured independently to quantify the rates of the parallel and consecutive reactions to propene and carbon oxides. Specific experiments to evaluate reaction products effects showed that water inhibited reaction rates but co‐feeding CO₂ or propene had no measurable effect on selectivity or conversion. Kinetic data generated under integral reactor conditions and over an inert membrane reactor have also been used to estimate the kinetic parameters. Selectivity decreased as the oxygen partial pressure increased; however, propene yield was relatively insensitive to oxygen concentration. A dual site Mars‐van Krevelen model characterizes the reaction kinetics well. The role of lattice oxygen was established by alternating pulses of propane and oxygen. This redox model is able to predict the experimental tendencies observed in the three types of reactor studied.     

Mercury(II) removal from aqueous solutions by adsorption on multi-walled carbon nanotubes

Our aim was to test how MWCNTs can be used as a new adsorbent for mercury(II). Multi-walled carbon nanotubes (MWCNTs) have been used for removal of mercury from aqueous solutions. Mercury removal from aqueous solutions by batch adsorption was investigated. Equilibrium isotherms, such as Freundlich, Langmuir, Temkin, Harkins-Jura, were tested. Kinetic studies based on Lagergren first-order, pseudo-second-order and Elovich rate expressions were done. The batch experiments were conducted at three different temperatures (17, 27 and 37 °C) and different pHs of the initial solution. Error function analysis shows that mercury(II) removal obeys pseudo-second order kinetics and Freundlich isotherm equation. Finally, the effects of solution pH and temperature on the adsorption were studied.     

Desulfurization of Model Oil through Adsorption over Activated Charcoal and Bentonite Clay Composites

Adsorption of dibenzothiophene (DBT) from model oil was investigated using composites of pure activated charcoal and pure bentonite clay. DBT adsorption was carried out in batch mode experiments at laboratory scale, where the developed composite materials showed a synergistic effect in removal of DBT from the model oil in terms of improved surface acidity of the pure activated charcoal and mesoporous structure of the pure bentonite clay. Thermodynamics, kinetics, and optimization of various adsorption parameters were investigated. Kinetic analyses proved that DBT adsorption followed pseudo-second-order kinetics. To study the thermodynamics of the adsorption, different isotherm adsorption models were applied. The Langmuir isotherm best fitted to the adsorption data. Various thermodynamic parameters were evaluated, including Gibbs free energy, entropy, and enthalpy.     

TS-1/堇青石整体催化剂催化苯酚羟基化制苯二酚

以市售TS-1分子筛为活性组分,蜂窝状堇青石陶瓷为载体,硅溶胶(30%水溶液)为黏结剂,采用涂覆法制备TS-1/堇青石整体催化剂,并用于催化苯酚和过氧化氢(30%水溶液)的羟基化反应。在自制强制外循环间歇反应器中考察反应温度、反应时间、TS-1负载量和n(苯酚)∶n(过氧化氢)等对苯酚羟基化的影响。结果表明,添加硅溶胶后,牢固度有了很大提高。在以水为溶剂、n(苯酚)∶n(过氧化氢)=3∶1、m(H_2O)∶m(苯酚)=5∶1、m(苯酚)∶m(TS-1)=10∶1和60℃条件下反应6 h,苯酚转化率为28.3%,苯二酚选择性为93.2%。     

三相体系中TS-1分子筛催化环己酮氨肟化制环己酮肟(英文)

An innovative green process of producing ε-caprolactam was proposed by integrating ammoximation and Beckmann rearrangement effectively. As a first part of the new process, TS-1 molecular sieve-catalyzed synthesis of cyclohexanone oxime from cyclohexanone, ammonia and hydrogen peroxide was carried out in a batch plant. Cyclohexane was used as the solvent in the three-phase reaction system. The influences of essential process parameters on ammoximation were investigated. Under the reaction conditions as catalyst content of 2.5% (by mass); H 2 O 2 /yclohexanone molar ratio of 1.10; NH 3 /cyclohexanone molar ratio of 2.20; reaction temperature of 343 K; reaction time of 5 h, high conversion of cyclohexanone and selectivity to oxime (both>99%) were obtained. Thus, the three-phase ammoximation process showed equal catalytic activity as TS-1 but much more convenient and simpler for the separation of catalyst in comparison to the industrial two-phase system with t-butanol used as solvent.     

Silica islands regulated external Ti-site environment of TS-1 for enhanced performance of 1-hexene epoxidation

Rational regulation of the local environment of Ti-sites in TS-1 harbors tremendous industrial and scientific significance to epoxidation reactions. Herein, we report a facile and environment-friendly strategy to boost catalytic selectivity by covering the Ti-sites on an external surface of TS-1 (ITS-1) without sacrificing activity of internal Ti sites. By a quantitative analysis of d₃-acetonitrile and quinoline-DRIFTS, ¹H MAS NMR and XPS, it is found that the percentage of external Ti-sites decreased from 11% to 6% after depositing surface silica islands. This successfully inhibits ring-opening reaction of 1,2-epoxyhexane on catalyst surface, as demonstrated by slower kinetic decomposition rate of 1,2-epoxyhexane. Compared with conventional TS-1 catalyst, selectivity of 1,2-epoxyhexane over ITS-1 catalyst significantly increased from 83.5% to 98.5% while maintaining high 1-hexene conversion. Furthermore, overmuch surface silica coverage only leads to extremely low conversion (2%) due to inhibition of mass transfer. This work paves the way for rational construction of Ti-containing catalysts for 1-hexene epoxidation.     

Nitrite Removal Using Ion Exchange Resin: Batch vs. Fixed Bed Performance

Nitrite is an important pollutant appearing in the large amount of wastewater from flue gas denitrification process. Its removal is essential for wastewater discharge or reuse. In this paper, we made an attempt to remove nitrite using ion exchange resin (IXR) by conducting batch experiments first, in which adsorption equilibrium and kinetics were determined. Fixed bed experiments were then carried out to investigate bed breakthrough and elution behavior. Isotherm data showed that nitrite adsorption on IXR fitted Freundlich model better than Langmuir model. Kinetic data from batch experiments were well correlated with second-order reversible kinetics model. From bed experiments, it was found that increase in flow rate and inlet concentration decreased breakthrough time, but the capacity of saturated adsorption stayed almost the same. It was also found that the capacity of adsorption decreased in the presence of other anions. Breakthrough curves under various operating conditions were well predicted by the Thomas model, and nitrite ions adsorbed in the bed could be completely eluted by using 3.0 wt% NaCl solution at flow rate of 3.0 L?h^?1.     

Kinetics of Reaction of Castor Oil Trimethylol Propane Polyol and 4,4′ Diphenyl Methane Diisocyanate

The kinetics of the uncatalyzed reaction of diphenyl methane diisocyanate and castor oil (CO)/trimethylol propane (TMP) polyol, with xylene as solvent at different temperatures, solvent concentration and NCO/OH ratios were investigated. The polyol was synthesized from castor oil and trimethylol propane (2-ethyl-2-(hydroxymethyl)-1,3-propanediol) with equivalent ratio of 1:3 via transesterification mechanism. Polyol was then characterized using FTIR spectroscopy and liquid chromatography mass spectroscopy (LCMS) under atmospheric pressure chemical ionization mode. All the reactions obeyed second order kinetics. Enthalpy changes, activation energies and entropies of activation were also calculated. Kinetic data from these reactions will facilitate modeling of these reactions in a batch reactor.     

Separation of propene/1-alkene and ethylene/1-alkene copolymers by high-temperature adsorption liquid chromatography

A high performance liquid chromatography column (HPLC) Hypercarb^® packed with porous graphite has proven to discriminate polyolefin molecules due to differences in their adsorption and desorption behaviour. While linear polyethylene (PE) and syndiotactic polypropylene (sPP) are adsorbed on the graphite packing, isotactic polypropylene (iPP) is not adsorbed. The column operates at 160 °C with 1-decanol as sample solvent and mobile phase. We have now tested this HPLC system for separations of random propene/1-alkene and ethylene/1-hexene copolymers: While copolymers of propene with 1-butene, 1-hexene and 1-octene copolymers eluted in size exclusion mode without adsorption, propene/1-octadecene and ethylene/1-hexene copolymers are strongly retained and eluted only after application of a linear gradient starting from 1-decanol and ending with pure 1,2,4-trichlorobenzene. The retention of propene/1-alkene (>11 carbons in the side chain) copolymers increases with the concentration of comonomer, making this HPLC system suitable to separate these copolymers according to their chemical composition. In contrast, the retention of ethylene/1-hexene samples decreases with increasing 1-hexene content. Branching in this case shortens the length of continuous methylene sequences of the polymer backbone, which are expected to adsorb in a planar conformation to the graphite layers. This is the first report on the separation of short chain branched polyolefins by high-temperature adsorption liquid chromatography.     

Effect of comonomer partitioning on the kinetics of mesophase formation in random copolymers of propene and higher α-olefins

The effect of counit type on the kinetics of mesophase formation has been investigated by means of chip-calorimetry in propene/α-olefin random copolymers, containing counits which show large differences in their co-crystallization behavior with propene, i.e. 1-butene and 1-hexene. Non-isothermal crystallization experiments indicated that the minimum cooling rate at which mesophase formation is observed is directly related to the kinetic of α-phase crystallization, which is lower for the copolymer with the bulkier 1-hexene counit. Isothermal structuring was probed in a wide temperature range, revealing that a double bell-shaped curve is required to describe the temperature dependence of crystallization times of the two polymorphs. The ordering kinetics of the mesophase is the fastest in i-PP homopolymer and decreases with increasing comonomer bulkiness, analogous to what happens for the monoclinic phase. The results are discussed by considering the effect of comonomer on the driving force for mesophase formation, also at the light of new WAXD and density evidences, which prove different extents of inclusion of 1-butene and 1-hexene in the ordered phases.     

Preliminary study on the TS-1 deactivation during styrene oxidation with H2O2

The deactivation of the TS-1 zeolite during styrene oxidation with H₂O₂ has been investigated by a series of kinetic experiments and further characterisation of the spent catalysts. A decline of the TS-1 activity with time has been observed, especially during the first hours of reaction. TG and TPD–MS analyses of the spent catalysts show that the main products occluded within the zeolite pores are styrene, phenylacetaldehyde and benzaldehyde. The presence of styrene oligomeric compounds has also been detected, although it is postulated they are formed mainly in the solution outside the zeolite pores. Diffusional hindrances due to the high degree of occupancy of the TS-1 pores, as well as, a strong adsorption of styrene, phenylacetaldehyde and benzaldehyde on the Ti sites are proposed as the main reasons for the TS-1 deactivation. These phenomena are enhanced at lower reaction temperatures, which cause a faster initial deactivation. Likewise, longer reaction times favour preferential chemisorption of aldehydes versus styrene.     

The kinetics of photoinitiated polymerization of 1-vinyl-2-methylimidazole

The kinetics of isothermal, photoinitiated, solution polymerization of 1-vinyl-2-methylimidazole (MVI) was studied using the standard dilatometric technique. The photoinitiators used were aromatic carbonyl compounds. Most of the experiments were carried out using methanol as solvent, but other solvents were also used. The reaction follows the classical kinetic scheme for free radical polymerization. The activation energy for the polymerization is 3.9 kcal/mol. The polymerization is greatly accelerated by the presence of water in reaction medium. Among the three photoinitiators tried, 2,2′-diethoxyacetophenone (DEAP) has the highest quantum efficiency for the polymerization of MVI.