Preparative Scale Gel Permeation Chromatography Research Articles

Structural Identification of the Monomeric Constituents of Petroleum Pitch

The monomer fraction of a representative petroleum pitch, M-50, was characterized in terms of both the predominant polycyclic aromatic hydrocarbon (PAH) backbones and their extent of alkylation. Such an identification of individual molecular species and their relative abundance was greatly facilitated by the use of a series of separation processes prior to analysis. In particular, low (up to 270), medium (270−335), and high (335−388) molecular-weight cuts of the monomeric fraction of the M-50 pitch were isolated by dense-gas extraction (DGE). The resultant three cuts were then further fractionated by preparative-scale gel permeation chromatography (prep-scale GPC) to obtain narrow molecular-weight fractions suitable for individual species identification by both matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry and reversed-phase high-performance liquid chromatography with photodiode array detection (HPLC/PDA). Results indicate that the monomer fraction consists of five dominant series, with each series consisting of 1−3 PAH backbones typically substituted with 0−4 methyl groups. The dominant PAH backbones include pyrene (molecular weight = 202); chrysene, benz[a]anthracene, and triphenylene (molecular weight = 228); benzo[a]pyrene and benzo[e]pyrene (molecular weight = 252); and benzo[ghi]perylene (molecular weight = 276). The highest molecular-weight series beginning at 326 could not be unambiguously identified but may in fact be a small dimer formed by the condensation reaction of pyrene and naphthalene in the starting decant oil, because the UV−vis spectra indicate the presence of a five-membered ring, something not observed in any of the other identified series.

EFFECTS OF H/C RATIO AND MOLECULAR WEIGHT ON MICROCARBON RESIDUE OF VACUUM RESID SUB-FRACTIONS

ABSTRACT Studies in the literature have reported correlations of Microcarbon Residue (MCR) either with hydrogen to carbon atomic ratio (H/C) or with the number average molecular weight (Mn). The MCR value can be an indication of die coke yield in a commercial coking process. In this work a single correlation of MCR widi both H/C and Mn has been presented. Vacuum residue distillation fractions from hydroprocessing experiments were separated into five sub-fractions by preparative scale gel permeation chromatography, and characterized in terms of H/C, M„and MCR. In accordance with other correlations it was confirmed that increasing the H/C atomic ratio will decrease coke formation (MCR)- In addition this work has shown that for identical H/C ratio, molecules with smaller Mn values will produce less coke than those with larger Mn; values.

Coprocessing: Elemental and molecular weight distributions in unconverted vacuum residues

The difference between molecules that are converted to distillates during coprocessing (Cold Lake vacuum bottoms and Forestburg subbituminous coal) and those that are not has been studied. The following procedure was used. The fraction of the liquid product boiling above 525 °C was separated into five subfractions by preparative scale gel permeation chromatography (GPC). Each subfraction was weighed. Then the following measurements were made on each subfraction ; molecular weight, elemental analyses (C, H, N, S, V, and Ni), and carbon-13 nuclear magnetic resonance. Five significant observations were made. (1) The molecules that were converted had larger H/C atomic ratios and smaller N/C atomic ratios than the feedstock molecules. (2a) S, V, and Ni heteroatoms could be removed without causing much change in molecular weight. (2b) It was not possible to increase the H/C atomic ratio or decrease the N/C atomic ratio without decreasing the molecular weight. (3) Generally, the unconverted +525 °C residue molecules became smaller as the processing severity increased. (4) The unconverted molecules retained their side chains at mild processing severities (425 °C). When the processing severity increased (450 °C), side chain carbon began to be removed. (5) If there was insufficient reaction time to provide enough hydrogen, then molecules which were larger than the feedstock molecules were formed. Feedstock molecules that lost their hydrogen rich fragments (by cracking or by hydrogen transfer) without contacting enough hydrogen to remove their nitrogen or metal heteroatoms may have oligomerized. If the reaction time was increased to allow more contact time with hydrogen, the oligomerized molecules which were larger than the feedstock molecules disintegrated. Finally, conversion of residue molecules to distillate molecules appeared to be limited by hydrogen addition. For conversion, hydrogen was required either to hydrogenate aromatic rings or to remove nitrogen heteroatoms. There are other important requirements for hydrogen which are not primary steps in the conversion of large molecules to small ones. They include capping pyrolysis fragments and the removal of other heteroatoms (sulfur, metals).

Preparation and crystallinity of a large unsubstituted crown ether, cyclic heptacosa(oxyethylene)(cyclo-E27, 81-crown-27), studied by Raman spectroscopy, X-ray scattering and differential scanning calorimetry

Cyclic heptacosa(oxyethyelene)(cyclo-E27, 81-crown-27) was prepared from linear α-hydro, ω-hydroxy-heptacosa(oxyethylene) by reaction with tosyl chloride under alkaline conditions, purified by preparative-scale gel permeation chromatography, and studied by laser-Raman spectroscopy, wide-angle and small-angle X-ray scattering, and differential scanning calorimetry. Comparison was made with the properties of linear oligo(oxyethylene) dimethyl ethers (including C1E27C1). The sub-cell of the crystalline cyclic oligomer was the same as that of its linear counterparts, i.e. the same as that of high-molar-mass poly(oxyethylene). However, the cyclic oligomer crystallised as a twice-folded ring, as confirmed by its long spacing and the frequency of its single-node longitudinal acoustic mode (LAM-1). Enthalpies of fusion and melting temperatures were analysed to provide estimates of the enthalpy and entropy of formation of a fold in an oligo(oxyethylene) layer crystal.

Coprocessing: Elemental and Molecular Weight Distributions in Unconverted Vacuum Residues

The difference between molecules that are converted to distillates during coprocessing (Cold Lake vacuum bottoms and Forestburg subbituminous coal) and those that are not has been studied. The following procedure was used. The fraction of the liquid product boiling above 525 °C was separated into five subfractions by preparative scale gel permeation chromatography (GPC). Each subfraction was weighed. Then the following measurements were made on each subfraction ; molecular weight, elemental analyses (C, H, N, S, V, and Ni), and carbon-13 nuclear magnetic resonance. Five significant observations were made. (1) The molecules that were converted had larger H/C atomic ratios and smaller N/C atomic ratios than the feedstock molecules. (2a) S, V, and Ni heteroatoms could be removed without causing much change in molecular weight. (2b) It was not possible to increase the H/C atomic ratio or decrease the N/C atomic ratio without decreasing the molecular weight. (3) Generally, the unconverted +525 °C residue molecules became smaller as the processing severity increased. (4) The unconverted molecules retained their side chains at mild processing severities (425 °C). When the processing severity increased (450 °C), side chain carbon began to be removed. (5) If there was insufficient reaction time to provide enough hydrogen, then molecules which were larger than the feedstock molecules were formed. Feedstock molecules that lost their hydrogen rich fragments (by cracking or by hydrogen transfer) without contacting enough hydrogen to remove their nitrogen or metal heteroatoms may have oligomerized. If the reaction time was increased to allow more contact time with hydrogen, the oligomerized molecules which were larger than the feedstock molecules disintegrated. Finally, conversion of residue molecules to distillate molecules appeared to be limited by hydrogen addition. For conversion, hydrogen was required either to hydrogenate aromatic rings or to remove nitrogen heteroatoms. There are other important requirements for hydrogen which are not primary steps in the conversion of large molecules to small ones. They include capping pyrolysis fragments and the removal of other heteroatoms (sulfur, metals).

Oxidative degradation of Biomer fractions prepared by using preparative-scale gel permeation chromatography.

The possibility that some macromolecular chains within a chemically heterogeneous polyether-urethane (PEU) may be more susceptible to degradation than others has been investigated. Preparative scale gel permeation chromatography has been used to separate chemically different fractions of a sample of the commercial PEU, Biomer. The fractions were characterized and then tested for susceptibility to oxidative degradation by exposing them to hydrogen peroxide. After exposure to hydrogen peroxide, the samples were analyzed using high pressure gel permeation chromatography (HPGPC), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). By using these methods, we were able to identify chemical changes in some of the Biomer fractions but not in others. Clear differences in the chemistry and reactivity of the fractions were observed. Changes in the weight average molecular weight varied from a decrease of 55.8% to an increase of 3.9%. A decrease in hard segment content at the surface and in the bulk was observed in some samples, but opposite trends were observed in others. The evidence suggests that there may be a number of mechanisms by which hydrogen peroxide can react with PEUs. Some fractions separated from the Biomer were not significantly affected by concentrated hydrogen peroxide solutions. This suggests an intrinsic stability in some PEUs and points the way to the development of more degradation-resistant PEUs.

Dextran fractionation using preparative scale continuous chromatography

AbstractThe continuous fractionation of dextran feedstocks using preparative scale gel permeation chromatography is reported. A semi‐continuous counter‐current chromatographic refiner has been used, which consisted of ten 5.1 cm i.d. × 70 cm long stainless steel columns, packed with Spherosil XOB075 porous silica beads. The system has proved successful in removing the very low and the very high molecular weight fractions and obtaining products of the required specification. Over 90% recoveries have been achieved at throughputs of 0.470 kg dextran h−1.

Fractionation of dextran using repetitive batch chromatography

AbstractThe fractionation of broad dextran fractions using preparative scale gel permeation chromatography has been carried out. A batch system was used, consisting of ten borosilicate columns, each of 5.1 cm i.d. and 70 cm long and packed with Spherosil XOB075 porous silica beads.The repetitive injection technique was employed and the effects of feed, charge volume and concentration on the molecular weight distribution of the products were evaluated.

Fractionation of Poly(vinyl chloride)s by Preparative Scale Gel-Permeation Chromatography

Three poly(vinyl chloride) (PVC) samples were fractionated by a preparative-scale GPC (gel-permeation chromatography) with Styragel. The carrier solvent used was tetrahydrofran. The fractionation of one sample whose average molecular weight was the lowest among the three samples was not affected much by the injected sample concentration and all the fractions obtained gave fairly narrow distributions of molecular weight. The fractionation of the remaining two samples having higher average molecular weights showed an overload effect and the polydispersity of each fraction of these samples was greater than that of the former sample. The relationship between the intrinsic viscosity and weight-average molecular weight for PVC fractions revealed that the fractions of the latter two samples contained long chain branches. The GPC calibration curve for the hydrodynamic volume of PVC agreed with that of the polystyrene standards in the low molecular-weight range, but not in the high molecular-weight range where the effect of chain branching was dominant.

Recent advances in reparative scale gel permeation chromatography

Recent applications of preparative scale gel permeation chromatography are reviewed and examles of tyical performance are reviewed and examples of typical performance are discussed. In priciple, any separation which can be carried out on an analytical scale may have showen that there is little, if any, loss in resolution upon going to the larger size preparative columns compared with the analytical scale columns.

Chemical heterogeneity and composition of a modified poly(ethylene terephthalate)

A series of samples of a modified poly(ethylene terephthalate) has been characterized by gel permeation chromatography, ultraviolet spectrophotometry, differential thermal analysis, and nuclear magnetic resonance techniques. These samples were prepared in the melt phase by reaction of dimethyl terephthalate, ethylene glycol, 1,4-butanediol, and an ultraviolet absorbing monomer, dimethyl Z. To determine if there were heterogeneity in composition as a function of molecular weight, samples were fractionated by preparative scale gel permeation chromatography, and selected fractions were analyzed by ultraviolet spectrophotometry. The composition was found to be relatively constant over the bulk of the distribution; however, the very low-molecular-weight region is appreciably richer in moiety Z. A portion of one of the samples, of ca. 1000 (polystyrene equivalent molecular weight), was submitted for mass spectral examination and found to be a mixture of cyclic dimers of moiety Z and the glycols. The glass transition temperature of the highest molecular weight fraction from the preparative gel permeation chromatography fractionation had the same value (within experimental error) as that of the bulk polymer. This is interpreted as indicating (1) the cyclic dimer material does not act as a plasticizer lowering the glass transition temperature, (2) there is no dependence of glass transition temperature on molecular weight, and (3) no major variation exists in the glycol composition with molecular weight. Two samples of this polymer were prepared using the same monomer feed ratios, temperature, catalyst, etc. These samples were found to have approximately the same viscosity, and molecular weight distributions; however, their glass transition temperatures differed by ca. 10°. This difference was interpreted as indicating that one sample contained more butanediol than expected from the monomer feed concentration. This postulate was confirmed by nuclear magnetic resonance measurements on the bulk samples.