Isomeric Units Research Articles

Chain Folding Controlled by an Isomeric Repeat Unit: Helix Formation versus Random Aggregation in Acetylene‐Bridged Carbazole–Bipyridine Co‐Oligomers

An unprecedented, positional effect of the isomeric repeat unit on chain folding in donor–acceptor-linked oligomers, which contain alternating bipyridine and carbazole moieties that are connected through an acetylinic linkage, is reported. 4,4′-Linked oligomer 1 adopts an intrachain helical conformation (CD-active) in CHCl3/MeCN (20:80 v/v), whereas oligomer 2, which contains an isomeric 6,6′-linkage, forms interchain randomly coiled aggregates (CD-inactive). The substitution position plays a significant role in controlling the variations in electronic effects and dipole moments around the bipyridyl moiety, which are responsible for this observed phenomenon. Two model compounds of oligomers 1 and 2 (3 and 4, respectively) were prepared and their properties were compared. A systematic investigation of the photophysical and CD properties of these structures, as well as theoretical studies, support our conclusions.

Molar Mass and Microstructure Analysis of PI‐b‐PMMA Copolymers by SEC‐NMR

AbstractThe online coupling of size‐exclusion chromatography and NMR is used to characterize block copolymers consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) regarding their distributions of molar mass (MMD) and chemical composition (CCD). Using the CCD, $\overline{M}_{\rm n}$ and $\overline{M}_{\rm w}$ are calculated on the basis of PI and PMMA homopolymer calibrations. The microstructure distribution of PMMA and the distribution of isomeric units of PI in dependence of molar mass is also demonstrated. Furthermore, a simulation analysis is presented for a bimodal eluting sample. It allows for full separation, quantification and molar mass determination of the coeluting co‐ and homopolymer fractions. The quantification of the fractions is verified by liquid chromatography at critical conditions.

Investigation of Kinetics of Hydroisomerization of C5/C6 and C6/C7 Alkanes and their Binary Mixtures

Modern gasoline quality requirements globally became stricter. These specifications parallel with the applied developments of vehicle technologies contributing to the cleaner environment. Nowadays only the concentration of cyclo- and isoparaffins are not limited in gasoline, because they burn cleaner, have high octane number, lower sensibility and better combustion properties than aromatic or olefinic hydrocarbons. In the last decade because of the applied refinery investments both sulfur and aromatic content of gasolines decreased, but the octane mass of the gasoline pool significantly decreased against the increment of octane number requirement of spark ignition engines. Importance of isomerization of light paraffins has been publicized in many papers. However few of them investigated, studied and explained the individual hydrocarbons (C5/C6/C7) interaction with each other in multicomponent mixtures practically in similar to industrial hydrocarbon feedstocks. Binary and multicomponent hydrocarbon mixture investigations can contribute to the understanding the results of isomerization of real, feedstocks from multiple sources; which contain higher boiling point hydrocarbons (cycloparaffins, benzene, and heptanes). Further these results can help to operate with higher flexibility, safety and economically a light naphtha isomerization units. The hydroisomerization of n-pentane (n-C5), n-hexane (n-C6), cyclohexane (c-C6) and n-heptane (n-C7) and their binary mixture were studied on Pt/sulfated zirconia catalyst at temperature 150–170 °C, total pressure 20 bar, 1:1 H2-hydrocarbon molar ratio. The apparent activation energies of all individual components, further the reaction rates individually and in different composition binary mixtures were specified. Results of our experiments concluded that the rate of reaction of the higher carbon number hydrocarbon or in case of the same carbon number the cycloparaffins (lower volatility component) increased with increasing the concentration in the binary mixtures, while the rate of reaction of higher volatility component decreasing.

Revamping of isomerization units

The problem of obtaining motor fuels with improved environmental characteristics corresponding to Euro-4 and Euro-5 standards is examined. The experience of Kedr-89 Scientific-Production Company in revamping of naphtha cut isomerization units is presented. The operating parameters of deisohexanizers developed by Kedr-89 Co. are reported. The possibility of including depentanization and deisopentanization towers in the network of isomerization units is considered.

ChemInform Abstract: Synthesis and Anti-HIV Activity of Isonucleosides.

A series of isomeric 2',3'-dideoxynucleosides which contains a modified carbohydrate moiety has been prepared. This class of compounds was designed to mimic the activity of known anti-HIV dideoxynucleosides, while imparting enhanced chemical and enzymatic stability. Isonucleosides containing the standard heterocyclic bases (A, C, G, T) were synthesized via nucleophilic addition of the base to an isomeric sugar unit. Modified derivatives were generated by manipulation of the intact isonucleoside. Two of the compound prepared, iso-ddA (1) and iso-ddG (6), exhibit significant and selective anti-HIV activity, as well as beneficial hydrolytic stability.

Demixing and Remixing Kinetics of Poly(2-isopropyl-2-oxazoline) (PIPOZ) Aqueous Solutions Studied by Modulated Temperature Differential Scanning Calorimetry

The demixing and remixing kinetics of aqueous solutions of poly(2-isopropyl-2-oxazoline) (PIPOZ) with four different chain lengths (number-average molar mass of 3300-13000 g mol -1 )i s studied by meansofmodulatedtemperaturedifferentialscanningcalorimetry(MTDSC)inbothnonisothermalandquasi- isothermal modes. The nonisothermal measurements show that the aqueous solutions of all four PIPOZ samples follow the lower critical solution temperature (LCST) phase behavior. Both the LCST and the correspondingpolymerweightfraction(fw) decreasewithincreasingmolarmass:33.8(0.1Cat29.8wt%for 3300 g mol -1 shifts to 26.2 ( 0.1 C at 19.8 wt % for 13000 g mol -1 . This typical type I phase behavior of aqueoussolutionsofPIPOZisincontrasttothetypical typeIIphasebehaviorofpoly(N-isopropylacrylamide) (PNIPAM) which has isomeric repeat units. The demixing and remixing kinetics throughout the phase transition is studied by quasi-isothermal heat capacity measurements. Overall, the response for PIPOZ solutions is markedly faster than for PNIPAM solutions.

Isomeric pyridyl-thiazole donor units for metal ion recognition in bi- and tri-metallic helicates

Linking two isomeric tridentate N-chelates together produces a hetero-ditopic ligand capable of selectively binding Hg2+ and Zn2+ ions in a double-stranded helicate.

A scenario-based clean gasoline production strategy for China National Petroleum Corporation

Facing increasingly strict environmental regulations on transportation fuels, China National Petroleum Corp. (CNPC), the second largest supplier of petroleum products in China, needs to upgrade its transportation fuels. Using the scenario-based analysis method, this paper analyzes how the emission related properties, including antiknock index, and sulfur, olefin, benzene and aromatics contents of gasoline produced by CNPC, vary with the change in the configuration of gasoline production units in the future 5–15 years. The results showed that for CNPC to upgrade its gasoline, the share of fluid catalytic cracking (FCC) naphtha must be reduced, but just increasing reformate or alkylate and isomerate will result in excessive increase in benzene and aromatics contents or a great loss of gasoline octane number. Therefore, CNPC should suitably control the capacity of its FCC units and increase the capacity of reformer, alkylation and isomerization units. Most importantly, CNPC should dramatically expand the capacity of its hydrotreating or non-hydrotreating gasoline upgrading units to decrease the olefin and sulfur contents in FCC gasoline that takes a dominant share of about 80% in the gasoline pool of China.

Stochastic Lattice Model Simulations of Molecularly Imprinted Polymers

A stochastic computer model was developed that simulates the formation and binding properties of molecularly imprinted polymers (MIPs). This simulation allowed examination of possible mechanisms in the imprinting process. The simulation enabled the rapid study of each variable in the imprinting process, which is difficult to do experimentally because of the interdependence of the variables. In particular, the simulation allowed examination of influence of binding site heterogeneity on the imprinting effect and on the binding properties of MIPs. The simulation was based on a lattice model in which monomer, template, cross-linker, and solvent units occupy positions in a square grid. A stochastic algorithm was utilized to position the monomer and template units in the lattice matrix based on the monomer−template binding constant. The lattice model simulation provides a method to study the imprinting mechanism and also to rapidly optimize the imprinting process. The model was able to simulate the imprinting effect as evidenced by a higher population of high-affinity binding sites in polymers made with the presence of template units. The lattice model simulation was also able to accurately reproduce the structural and energetic binding site heterogeneity of MIPs. Next, the effects of changing the imprinting variables such as the monomer−template stoichiometry and association constant were examined. Again, the simulated MIPs displayed the same trends as experimental MIPs formed under similar imprinting conditions. Finally, the simulation was carried out with isomeric template units to examine the origins of selectivity in MIPs. A clear preference for the imprinted isomer was observed. The ability of this lattice model to accurately replicate the binding properties of MIPs gives credence to the importance of binding site heterogeneity in determining the binding properties of MIPs. The simulation gives support for the hypothesis that the diverse mixture of monomer−template complexes in the prepolymerization mixture is a major source of binding site heterogeneity in MIPs. The simulation suggests new mechanisms and strategies for improving the imprinting effect such as the careful optimization of the statistical ratios of monomer and template or the introduction of blocking groups to reduce the number of low-affinity background binding sites.

Study of Catalysts Deactivation in Isomerization Process to Produce High Octane Gasoline

In this study the isomerization of desulfuerized light Iraqi petroleum naphtha (Al-Dura Refinery) with boiling point range of 37 to 124 °C , 80.5 API specific gravity and 68.2 octane number has been investigated. Two types of catalysts were prepared (Pt/HX and Pt/SrX) by impregnation of 0.8 wt% Pt on l 3X-zeolite. The catalyst activity and selectivity toward isomerization, and catalyst deactivation were investigated.
 The isomerization unit consisted of a vertical tubular stainless steel reactor of 2 cm internal diameter, 3 cm external diameter and 68 cm height. The operating pressure was atmospheric for all experimental runs. The liquid flow of light­naphtha was 0.4 Uh, and the catalyst weight was 50 gm, H/ H.C ratio used was 4 for all experimental runs. The isomerization process was studied at different temperatures of 250, 270, 275, 300, 325, and 350 °C . It was found that, the optimum isomerization temperature is 270 °C.
 The isomerization activities and selectivities as a function of time showed high activity at the beginning of the reaction and were deactivated rapidly. This indicates that the deactivation of Pt/HX and Pt/SrX results from the blocking of pore mouth by the deposited carbon. The following deactivation decreasing order, Pt/HX > Pt/SrX was found. On the other hand, Pt/HX catalyst shows higher activity and selectivity than that of Pt/SrX
 It was concluded that, only an average o/ 90 wt% of the carbon atoms feed into the reactor (light naphtha) is detected " the product stream due to formation of coke deposits which leads to catalyst deactivation. The results clearly showed 1at hydrogen is necessary for the hydrogenation of olefins in order to prevent oligomerization reaction that leads to coke formation and catalyst deactivation.

Formation of supramolecular hydrogels with controlled microstructures and stability via molecular assembling in a two-component system

Two isomeric building units, 4-oxo-4-(2-pyridinylamino) butanoic acid (defined as G1) and 4-oxo-4-(3-pyridinylamino) butanoic acid (defined as G2) formed fiber- and tree-like crystals in aqueous solutions, respectively. The crystal formation process of G1 was suggested based on the layered cross section of an individual crystal and the single crystal structure. Through cooling the aqueous solutions of their mixtures under G1/G2 molar ratios ranged from 7/1 to 1/3, a series of supramolecular hydrogels were formed based on hydrogen bonds as the main driving force. As decreasing G1/G2 ratios, the first observed aggregates in solution changed from fiber to particle form, while the gelating time became longer and longer. At the collapsing temperature, the gels formed at G1/G2 ratio ⩾3/1 kept the original gel shape but released water, while at G1/G2 ratio ⩽2/1 they broke into pieces without releasing water. The “dropping ball” experiment indicated that the highest gel-to-sol dissociation temperature ( T gel ) is obtained at G1/G2 ratio of 2/1. As measured by UV–vis spectroscopy, the two building units distributed uniformly within the gel formed at G1/G2 ratio of 1/1, indicating they assembled together in forming hydrogel. The scanning electron microscope (SEM) and infrared spectrometer (FT-IR) analysis of the dried samples indicated that the backbone shape changed from fiber to sheet and the content of free carboxyl groups increased with decreasing G1/G2 ratios, therefore resulting in hydrogels with different stability. The simple gelator structures and the possibility in controlling gel structure and stability make the hydrogels suitable for various uses.

CHEMICAL BOARD BLASTS BP Merritt

SAFETY RECOMMENDATIONS for petroleum refineries, triggered by a fatal accident in March 2005 at a Texas City, Texas, BP refinery, were issued by the Chemical Safety & Hazard Investigation Board (CSB) on Oct. 31. A day earlier, the board ripped into BP for failing to act on long-running plant safety problems. The board's recommendations focus on a refinery pressure-relief system that failed and resulted in an explosion at the BP plant, killing 15 workers and injuring 180 others. A distillation tower and attached blowdown drum were overfilled with highly flammable liquid hydrocarbons during start-up of the plant's octane-boosting isomerization unit, CSB says. The blowdown drumintended to capture excess hydrocarbons—overflowed and exploded in a geyser of flame as it was vented directly to the atmosphere rather than through a flared, controlled safety system. The board urges the American Petroleum Institute and the Occupational Safety & Health Administration (OSHA) to require refineries to eliminate unsafe bl...

New liquid-crystalline homo-and copolymers with phenyl benzoate side groups and a chiral center in a flexible spacer

The phase behavior and structure of liquid-crystalline homo-and copolymers based on structurally isomeric units belonging to the family of phenyl benzoates that differ in orientation of an ester link situated between benzene rings have been studied with X-ray diffraction, scanning calorimetry, and polarization optical microscopy. The orientation of this link in the monomer unit affects the temperature of phase transition, the type of the smectic phase, and the packing mode of side mesogenic groups in smectic layers of homopolymers. When monomer units commensurable with the monomer units of the basic component in terms of geometry but not identical to them in terms of smectogenity are randomly incorporated into macromolecules of the LC polymer, the resulting copolymer is characterized by layered packing not typical of either of the components.

CSB investigation of the explosions and fire at the BP texas city refinery on March 23, 2005

AbstractAt approximately 1:20 p.m. on Wednesday, March 23, 2005 a series of explosions and fire occurred at the BP Texas City oil refinery during the start‐up of an isomerization (ISOM) process unit. Fifteen workers were killed and about 170 other people were injured. Many of those killed were working in or around office trailers located near a blowdown drum and stack open to the atmosphere. The U.S. Chemical Safety and Hazard Investigation Board (CSB) found that because of problems that began during the ISOM process unit start‐up, a sudden, geyser‐like release of flammable hydrocarbon liquid and vapor discharged from the atmospheric vent stack. This release created a flammable vapor cloud, which ignited, causing as many as five explosions and fire in and around the ISOM unit. CSB identified multiple possible ignition sources in and around the blowdown drum area, including idling vehicles; however, the exact ignition source remains unknown.CSB deployed a team of investigators to BP Texas City to conduct a root‐cause investigation immediately after the incident. This team examined blast patterns to determine the explosion origin; reviewed the design of the isomerization unit equipment; and examined plant safety and operating procedures, past accidents, maintenance procedures, and oversight and inspection. Investigators also reviewed the adequacy of applicable regulations and industry standards for the placement of temporary structures such as trailers in refineries. This article presents the preliminary findings from the CSB investigative team regarding the technical and underlying causes of the incident and discusses three urgent recommendations issued by the CSB stemming from the BP incident. © 2006 American Institute of Chemical Engineers Process Saf Prog, 2006

Studies on Cage‐Type Tetranuclear Metal Clusters with Ferrocenylphosphonate Ligands

Reaction of FcCH(2)PO(3)H(2) [Fc=(eta(5)-C(5)H(5))Fe(eta(5)-C(5)H(4))] (H(2)FMPA) and 1,10-phenanthroline (phen) with Cd(OAc)(2).2 H(2)O or ZnSO(4).7 H(2)O in methanol in the presence of triethylamine resulted in the formation of two new ferrocenylphosphonate metal-cage complexes [M(4)(fmpa)(4)(phen)(4)] 7 CH(3)OH (M=Cd 1, M=Zn 2). Both structures contain two kinds of isomeric tetranuclear metal phosphonate cages, which are linked to one another by pi-pi interactions between the phen molecules. In 1, the Cd1, Cd3, and Cd4 atoms are all pentacoordinate, while the Cd2 atom is coordinated by four oxygen atoms from three phosphonate ligands and two nitrogen atoms from the chelating phen in a distorted octahedral geometry. Four Cd atoms from each unit are interconnected through bridging phosphonate ligands with different coordination modes, such as 5.221, 4.211, and 2.11 (Harris notation), yielding a {Cd(4)} cage. In 2, each Zn atom is coordinated by three oxygen atoms from three phosphonate ligands and two nitrogen atoms from phen, leading to a distorted square-pyramidal geometry. The four Zn atoms of each isomeric unit are also interconnected through four bridging phosphonate ligands to yield a {Zn(4)} cage. Fluorescent studies indicate that ligand-to-ligand charge-transfer photoluminescence is observed for 1, while the emission bands of 2 can be assigned to an admixture of ligand-to-ligand and metal-to-ligand charge transfer. Solution-state differential pulse voltammetry indicates that the half-wave potentials of the ferrocenyl moieties in 1 and 2 have different deviations relative to the relevant H(2)FMPA ligand. This may be because the highest occupied molecular orbital (HOMO) in 1 is located in the FMPA(2-) groups, while in 2 the HOMO is located in the phen and Zn(II) groups, so the Fe(II) centers in complex 1 are more easily oxidized to Fe(III) centers than those of 2. The third-order nonlinear optical (NLO) measurements show that both 1 and 2 exhibit strong third-order NLO self-focusing effects; hence, they are promising candidates for NLO materials. By calculating the component of the lowest unoccupied molecular orbitals of 1 and 2, we confirmed that the co-planar phen rings control their optical nonlinearity, while the H(2)FMPA ligands and metal ions have only a weak influence on their NLO properties.

Isomeric Molecular Rectangles Resulting from Self-Assembly of Dicopper Complexes of Macrocyclic Ligands

Dinuclear copper complexes containing hexaazacyclophane macrocyclic ligands react with the disodium salt of terephthalic acid resulting in the self-assembly of rectangular molecules with the general formula [(Cu2L)2(p-(O2CC6H4CO2)2)]X4, where X = CF3SO3 and ClO4 (3X4, L = Me2p and 4X4, L = Me2m). Tetranuclear complexes 3(CF3SO3)4 (as polymorphs 3a and 3b) and 4(ClO4)4 have been characterized by single-crystal X-ray diffraction analysis providing definitive proof of their structure as well as their metrical parameters. 3a contains, in its unit cell, two isomeric cationic units (3asyn and 3aanti) that differ in the relative position of the two O carboxylate atoms which bind to the Cu atoms of the different macrocyclic complexes, leading to boxes with different metrical parameters. ESI-MS analyses of solutions of the tetranuclear complexes 3(CF3SO3)4 and 4(CF3SO3)4 exhibit cluster ions which match the solid state formulation, thus demonstrating that the cages are retained in solution.

SAFETY BOARD DETAILS BP FAILURES

BP PRODUCTS NORTH AMERica Inc. failed to identify and remedy half-a-dozen key safety problems that ultimately led to the March 23 explosions and fire at the company's Texas City, Texas, refinery, which killed 15 workers and injured 170 others, federal investigators said on Oct. 28. The Chemical Safety & Hazard Investigation Board (CSB) says the blast could have been avoided if BP had installed a flare system on the unit that exploded. The incident involved a release of flammable hydrocarbon liquid and vapor from an atmospheric vent stack in the refinery's isomerization unit. Workers in nearby trailers were killed and injured in the subsequent explosions. CSB says the vent stack, which dated from the 1950s and was not tied to a safety flare system, was overfilled with hydrocarbons during the start-up of a raffinate splitter tower, a distillation column that became flooded with at least 120 vertical feet of liquid. The tower experienced a ...

Conversion of a Naphtha Reformer Unit to Butane Isomerization

A study was carried out for the conversion of a naphtha reformer unit to a butane isomerization plant, where a normal butane rich stream is isomerizated to isobutane. This compound has a bigger added value when it is used in an alkylation process to produce a high octane alkylated fuel, which is used as an adjustment component in high octane gasoline blending. Basically the new isomerization unit will work with a conventional process scheme. Nevertheless, some adaptations in the loading preparation section are included in this study, in order to fit up the process load because its high light components content, specifically propane. An equipment pre-sizing and current equipment reusing analysis at the process condition specifications are also included. Technically, it is concluded that the conversion is feasible because most of the existing equipment can be used. Finally, the analysis was complemented with an economic study in order to determinate the profitability of the existing plant conversion against the construction of a new one. Conversion resulted in being the most profitable option, requiring an investment equivalent to 70% from that corresponding for a new plant construction, with a 48.14 internal rate of return, and 1 year and 5 months investment recovery period.

Pseudorotaxanes and Rotaxanes Incorporating Diarylcycloheptatriene Stations

AbstractThe nature of the interaction between the tetracationic cyclophane cyclobis(paraquat‐4,4′‐biphenylene) and molecular threads incorporating arylcycloheptatriene units as stations was studied through the differences between the proton resonances observed in the NMR spectra of free molecular threads and in those of corresponding pseudorotaxanes and rotaxanes (CIS values). Molecular threads of different lengths possessing one or two recognition stations and incorporating two different isomeric arylcycloheptatriene units were used for pseudorotaxanes and rotaxanes. The main contribution to the driving forces behind the complexation of pseudorotaxanes and the co‐conformation of rotaxanes was deduced from the CIS values of distinct parts of the molecular threads. The unusual signal dispersion of the cyclophane proton resonances is attributed to the asymmetry caused by the molecular thread incorporating the asymmetric cycloheptatriene ring. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Sasol opens isomerization unit in S Africa

Sasol is constructing a second 1-octene production plant at its Secunda site in South Africa to meet growing global demands from plastics manufacturers. The new multi-million dollar plant, which is expected to come on-stream by the end of 2004, will produce 48 000 tons/year of 1-octene, pushing overall annual capacity to 96 000 tons, double existing output, says Sasol chief executive Pieter Cox. The two plants are operated by Sasol Olefins and Surfactants, which forms part of Sasol Chemical Industries, and first delivery from the new plant is expected to reach global customers by January 2005.This is a short news story only. Visit www.adcomp.com.com for the latest additives and compounding industry news.