Kinetic study on isomerization of verbenol to isopiperitenol and citral

Thermal isomerization of verbenol in the gas and liquid phases is discussed. The effect of temperature and residence time on the reaction mixture composition was studied (at 460-600°C and a residence time t of about 10^-3 min for the gas phase conditions; at 200-350°С and t = 10-80 min for the liquid phase process). The kinetic behavior and the reaction mechanisms are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic studies of the hydrolysis of furfurylidene benzoylhydrazone

Summary The hydrolysis of furfurylidene benzoylhydrazone,FBH, in hydrochloric acid medium has been found to follow specific acid catalysis, whereas the hydrolysis in universal buffer medium has been found to follow general acid-base catalysis. Mechanisms for the hydrolysis in different acidic media have been postulated. The observed rate constants, secondary rate constants, activation and thermodynamic parameters for the hydrolysis of furfurylidene benzoylhydrazone have been reported. The effect of change in percentage of ethanol in the ethanol-buffer mixture on the rate of hydrolysis has been discussed.

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Kinetische Untersuchungen zur Hydrolyse von Furfurylidenbenzoylhydrazon

Zusammenfassung Es wurde festgestellt, daß die Hydrolyse von Furfurylidenbenzoylhydrazon (FBH) in HCl-Medium einer speziellen Säurekatalyse gehorcht, wohingegen die Hydrolyse in universellen Puffer-Medien nach einer generellen Säure-Base Katalyse abläuft. Es werden Mechanismen für die verschiedenen sauren Medien postuliert. Die beobachteten Geschwindigkeitskonstanten, die sekundären Geschwindigkeitskonstanten, Aktivierungs- und sonstige thermodynamische Parameter für die Hydrolyse von Furfurylidenbenzoylhydrazon werden berichtet. Der Effekt wechselnder Ethanolkonzentration auf die Hydrolysengeschwindigkeitskonstante in Ethanol-Puffer-Mischungen wird diskutiert.

     

Silicon-carbon unsaturated compounds. 77. Thermal behavior of cis- and trans-1-silacyclobut-3-ene formed from pivaloyl[tert-butylbis(trimethylsilyl)]silane and tert-butylacetylene

The reaction of tert-butylbis(trimethylsilyl)silyl potassium with pivaloyl chloride gave pivaloyl[tert-butylbis(trimethylsilyl)]silane (1) in 89% yield. The cothermolysis of 1 with tert-butylacetylene at 140 °C for 24 h produced the mixture consisting of cis- and trans-1,2,3-tri(tert-butyl)-2-(trimethylsiloxy)-1-(trimethylsilyl)-1-silacyclobut-3-ene (cis-2 and trans-2) in a ratio of 0.7: 1, in 88% combined yield. The thermolysis of the mixture, cis-2 and trans-2, at 250 °C for 24 h proceeded to give trans-1,2,4-tri(tert-butyl)-1-(trimethylsiloxy)-2-(trimethylsilyl)-1-silacyclobut-3-ene (4) as a single product in 96% yield. Similar treatment of cis- and trans-2 at 190 °C for 15 h afforded silylcyclopropene 3 quantitatively, which underwent further isomerization at 250 °C to give trans-1-silacyclobut-3-ene 4 in quantitative yield.     

Kinetic migration studies using Porapak as solid-food simulant to assess the safety of paper and board as food-packaging materials

Porapak has been studied as a solid-food simulant in experiments on paper and board packaging. Three samples of paper with different recycled pulp content and surface treatment, and of different grammage and thickness, were used for the studies. Kinetic behaviour from 25 °C to 100 °C and different contact times ranging from 5 min at 100 °C to 10 days at 25 °C were studied using Porapak or, occasionally, Tenax or milk powder. Similar results were obtained with Porapak and Tenax but those from Porapak were more stable with temperature. Porapak behaves as good solid-food simulant even at high temperature.     

Mechanism of cyclopropane-propene isomerization

A SINDO method was used to investigate the sequence of rearrangements in the cyclopropane-propene isomerization. Geometries of reactant and product were optimized for a complete set of 21 independent internal coordinates. Then the bond angle of the migrating H atom was chosen as reaction coordinate and all other geometrical parameters were energy minimized along this reaction coordinate. In the neighborhood of the transition state a Newton type iteration process is initiated to locate the transition state. The latter is characterized by a CCC bond angle close to 90 ° and an almost finished migration of the H atom. The activation energy was found about 25% lower than the experimental value of 65 kcal/mole. A sequence of rearrangements was established, suggesting the initiation of the reaction by a combination of two normal modes: a torsional vibration of the methylene groups together with an asymmetric stretch of the carbon framework. Our calculations classify the reaction as concerted.     

Kinetic modeling of the ethylbenzene/xylene isomerization reaction over HZSM-5 zeolites revisited

In this article, a binderless dealuminated HZSM-5 zeolite (Si/Al = 41.4) was used as a catalyst for the isomerization of a mixture of ethylbenzene and xylene. The experimental results indicated that at low residence times the catalyst is effective to isomerize the ethylbenzene into xylenes. A comprehensive kinetic model considering chemisorption, surface chemical reactions, and diffusional processes was developed for this reaction. The intrinsic activation energy (71.99 kJ mol⁻¹) for the surface reaction of ethylbenzene into m-xylene was calculated for the first time, and the corresponding intrinsic activation energies for o-xylene to m-xylene and m-xylene to p-xylene surface reactions were calculated to be 59.45 and 50.68 kJ mol⁻¹, respectively. Lower apparent values have been reported in the literature, and we rationalize that they correspond to multistep processes and intrinsically include a negative activation energy pertaining to chemisorption. The results also revealed that the ethylbenzene diffusion within the zeolite channels was four orders of magnitude smaller than p-xylene.     

Theoretical studies on the structures and isomerization of the LiSiF_3 system

Various possible isomers of LiSiF_3 system and isomerization between them have been studied at G2(MP2) level using ab initio calculations. The relative energies of four minimum points on the potential energy surface are -128.6, -194.3, -12.7 and -122.8 kJ/mol (taking the sum of the energies of LiF and SiF_2 as zero) . The structural energy of the four-membered ring that contains three F-Si-F-Li four-membered rings with C_(3v) symmetry is the lowest. The highest potential barrier for the isomerization of the remaining three-or four-membered structure is 12.5 kJ/mol.     

Kinetic studies on the aggregation behavior of phosphatidylcholines and their analogs

Phosphatidylcholines (Ln's) and their neutral analogs 1,2-diacyl-sn-glycerol-3-phospho-ric acid bromoethyl esters (Pn's) have been found to exhibit different aggregation behaviors brought about by hydrophobic-lipophilic interactions (HLI) by means of kinetic probe in MeOH-H2O binary aquiorganic systems. The effects of the structure and the environment on the interesting aggregation of the amphiphilic molecules are discussed.     

Selective isomerization of α-pinene

a-Pinene isomerization was studied in gaseous and liquid phases on g-zirconium phosphate and rhodium g-zirconium phosphate. The first showed higher activity but lower reaction selectivity than the second, that led to tricyclic terpenes formation.     

Negative effect of Ni on PtHY in n-pentane isomerization evidenced by IR and ESR studies

Ni/PtHY with different Ni loadings was prepared by impregnating HY with hexachloroplatinic acid solution and Ni2+/N,N-dimethylformamide solution. An increase in the Ni loading decreased the crystallinity, specific surface area and meso-micropores of the catalysts. Ni interacted with hydroxyl groups to produce IR absorption bands at 3740-3500 cm-1. Increasing Ni loadings resulted in a decrease in the intensities of the broad bands at 3730-3500 cm-1 and the sharp band at 3740 cm-1 with simultaneous development of new absorbance band at 3700 cm-1 that was attributed to (-OH)Ni. The acidity of the samples did not significantly change with Ni loadings up to 1.0 wt%, which indicated that Ni mostly interacts with non-acidic silanol groups (terminal- and structural-defect OH groups). The presence of Ni decreased the activity of PtHY toward the isomerization of n-pentane because of a decrease in the number of active protonic-acid sites that formed from molecular hydrogen. IR and ESR studies confirmed that Pt facilitated the formation of protonic-acid sites from molecular hydrogen, whereas Ni, even when combined with Pt, didn’t exhibit such ability. The absence of protonic-acid sites from molecular hydrogen significantly decreased the yield of iso-pentane and markedly increased the cracking products.     

Kinetic studies on the reduction of a nickel (III) oxime-imine complex by sulphur(IV) and selenium(IV)

The kinetics of the reduction of [Ni^III(L¹)]²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) by sulphur(IV) and selenium (IV) over the regions pH 2.50–8.02 and 2.01–4.00 respectively have been investigated at 30°C. Attempts were made to evaluate the reactivity of all the reacting species, of sulphur(IV) and selenium(IV) by considering suitable pH ranges. The oxidation of SC₂·H₂O and HSO ₃ ⁻ is proposed to proceed through the formation of a hydrogen-bonded adduct. The reaction with SO ₃ ²⁻ seems to follow a direct outer-sphere route which is well supported by Marcus crossrelation calculation. The oxidation of HSeO _l3 ⁻ is ≈ 10³ times slower than that of H₂SeO₃. The kinetic data indicate that the oxidation of sulphur(IV) by [Ni^IIIL¹)]²⁺ is much more favourable as compared to the corresponding oxidation of selenium(IV).     

On the base-induced isomerization of cyclic propargylamides to cyclic allenamides

The reaction of lactams 4 (n = 1-5) with propargyl bromide affords propargylamides or allenylamides depending on the ring-size. Theoretical calculations support the dependence of the extension of the isomerization on the ring-size.     

Skeletal isomerization of 1-pentene

Series of ZSM-5 and Y zeolite catalysts have been prepared. Zeolites were submitted to ion exchange, drying, formation with 50 wt.% of aluminium hydroxide as a binder and then to final thermal treatment. Determination of catalysts activity in 1-pentene isomerization reaction was carried out in the temperature range of 175–325°C. The highest activity in isomerization reaction was attained in the presence of ZSM-5 zeolite catalysts with strong and medium strength acidity.     

无机锡催化δ-酮腈异构化成吡啶酮反应的研究

本文分别对三种无机锡(SnCl~2,SnO和SnO~2)催化δ-酮腈异构化成吡啶酮的条件进行了考察。实验结果表明:SnCl~2和SnO具有很高的催化活性。     

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.     

A dynamic study of isomerization on MoOx catalyst

The dynamics of n-heptane isomerization on the reduced MoO₃ catalyst have been studied in a fixed bed flow reactor. In the reaction temperature ranging from 523 to 673 K, the apparent energy for n-heptane isomerization obtained from the Arrhenius plot was 49.3 kJ/mol. At 573 K, the reaction orders of 0.33 in n-heptane and 0.35 in H₂ have been obtained.     

松香树脂酸异构化反应研究

以松香为原料、盐酸为催化剂、95%乙醇为重结晶溶剂进行超声波强化松香树脂酸异构化反应研究.通过正交优化实验考察了反应温度、反应时间、超声波频率、搅拌转速、盐酸用量对枞酸单离收率和纯度的影响,确定最佳异构化反应条件为: 超声频率47.2 kHz,反应温度55℃,盐酸用量0.06 mL盐酸·(g松香)-1 ,反应时间40 min,搅拌转速300 r·min-1,枞酸单离收率为31.76%,纯度为97.32%.采用气相色谱仪和圆盘旋光仪对枞酸产品进行了分析鉴定,实验值与文献值吻合.     

Skeletal isomerization of butene over natural ferrierite

The conversion of butenes to isobutene was compared on chemically modified natural and synthetic H-ferrierite catalysts. The proper balance of extra-framework species together with properly chosen reaction conditions allowed 100% isobutene selectivity.