David T. Lynch

Effect of molecular structure distribution on melting and crystallization behavior of 1-butene/ethylene copolymers

Abstract Short chain branching (SCB) and methylene sequence length (MSL) distributions were measured by TREF and DSC coupled with successive nucleation/annealing (SNA) for a Ziegler–Natta and a metallocene ethylene–butene copolymer. TREF analysis indicated that the copolymer made with Ziegler–Natta catalyst exhibited a broad bimodal SCB distribution, while the polymer made with the metallocene catalysts had a narrow SCB distribution. SNA-DSC analysis showed that the Ziegler–Natta copolymer had a broad MSL distribution with significant amount of long methylene sequences; the metallocene copolymer had a much narrower MSL distribution and contained a large amount of polymer with short methylene sequences. The melting and crystallization measurements on PTREF fractions of the two polymers showed that the melting temperature, crystallization temperature and enthalpy of fusion of the PTREF fractions for the Ziegler–Natta polymer decreased substantially with increasing SCB content, while these properties varied only slightly for the PTREF fractions of the metallocene polymer. This indicates that the SCB distribution has a more significant effect on melting and crystallization behaviors of polyethylene copolymers than the average SCB content.

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

A new technique for characterization of linear low-density polyethylene (LLDPE) is presented in this report. The molecular structure of two commercial LLDPEs, produced by copolymerization of ethylene with 1-butene over a Ziegler-Natta and a metallocene catalyst, was investigated. The LLDPE resins were fractionated by temperature rising elution fractionation (TREF), and the TREF fractions were further analyzed by size exclusion chromatography and differential scanning calorimetry (DSC) coupled with successive nucleation/annealing (SNA). The cross-fractionation techniques provided detailed information about the molecular structure of different types of LLDPEs; of particular interest is the TREF-SNA-DSC cross-fractionation which allowed a direct observation of methylene sequence distribution and thus short chain branch (SCB) distribution. TREF-size exclusion chromatography cross-fractionation showed that the molar mass of the Ziegler-Natta LLDPE increased monotonically with decreasing SCB, whereas the plot of Mw vs SCB for the metallocene LLDPE showed a maximum. TREF-SNA-DSC cross-fractionation clearly showed that the metallocene LLDPE only had intramolecular heterogeneity in SCB distribution, whereas the Ziegler-Natta LLDPEs exhibited both intermolecular and intramolecular heterogeneity.

Oxydehydrogenation of isobutyric acid with heteropolyacid catalysts: Experimental observations of deactivation

Abstract the heteropolyacid catalysts H 5 Mo 10 V 2 PO 40 and H 6 Mo 9 V 3 PO 40 have been used for producing methacrylic acid via the oxydehydrogenation of isobutyric acid. This reaction proceeds via a redox process involving lattice oxygen from the Keggin structure of the catalyst molecules. Changes in the oxidation state of the catalyst produce a deactivation effect which is reversible by fully reoxidizing the catalyst. An irreversible, long-term type of deactivation related to a loss of molybdenum from the catalyst has also been observed. This occurs through the formation of a volatile Mocontaining gas-phase species. Passage of the IBA feed over a bed of molybdenum trioxide prior to entering the reactor eliminates both types of deactivation.

Chaotic behavior of reaction systems: parallel cubic autocatalators

Abstract A model of a reaction system consisting of two parallel, isothermal autocatalytic reactions in a CSTR has been examined. It is shown that self-sustained chaotic behavior can occur in this system. The region of chaos is entered and exited according to period-doubling and halving sequences, with both sets of bifurcations giving rise to Feigenbaums number. Power spectral density calculations show that the nature of the chaotic behavior depends quite strongly on the parameter values. From a calculation of the Lyapunov exponents it is found that the Lyapunov dimension of the strange attractor is only slightly greater than that of a two-periodic torus.

Chaotic behavior of reaction systems: mixed cubic and quadratic autocatalysis

Abstract A family of parallel and consecutive autocatalytic reaction systems has been examined. It is shown that chaotic behavior is possible for several members of this family . In particular, systems of two parallel or consecutive reactions based on cubic, or mixed quadratic and cubic, autocatalysis give rise to chaos. Lyapunov exponents are used to delineate ranges of parameter values for which chaotic behavior is possible. It is found that chaos can occur over wide ranges of values of certain of the parameters, with very complex transitions between chaos and periodicity occuring as the parameters are varied. The transitions to chaos can occur either via a period-doubling route or via a process involving intermittency. A comparison is made between these autocatalytic systems and the analogous nonisothermal reaction systems for which chaotic behavior has previously been shown to be possible.

Oscillations during CO oxidation over supported metal catalysts. I. Influence of catalyst history on activity

Abstract The oxidation of CO over one (PtPd) Al 2 O 3 and three Pt Al 2 O 3 catalysts was studied in a recycle reactor. The objective of the study was to investigate the occurrence of self-sustained oscillatory behavior during CO oxidation over supported metal catalysts. Oscillatory behavior was observed for all the catalysts studied; however, the oscillatory behavior and the stable steady-state activities were strongly affected by the history of the catalyst, i.e., prior treatment and reaction conditions. The influences of treatment and reaction conditions on oscillatory and catalytic behavior are described. Hydrogen and CO chemisorption, COO 2 titrations, and transmission electron microscopy were used to gain insight into the changes in metal surface area resulting from treatment and use.

Oscillations during CO oxidation over supported metal catalysts: III. Mathematical modeling of the observed phenomena

A detailed mathematical model has been developed for the prediction of oscillatory behavior during CO oxidation on supported metal catalysts. This model is based on the hypothesis that the adsorption of CO causes reversible changes in the surface structure of the metal. The changes in the surface phase result in changes in the rate of O2 adsorption. A detailed comparison has been made between the predictions of this model and experimental observations. Excellent model-experimental agreement has been obtained for a wide range of operating parameters. During an oscillatory cycle the model predicts that the system alternates between a region in which oxygen adsorption is rate controlling and one in which there is a sensitive balance between the rates of the adsorption and the reaction steps.

Chaos in a continuous stirred tank reactor

Abstract A model is derived for a nonisothermal, continuous, stirred tank reactor in which two, exothermic, first order, irreversible, reactions are occuring in parallel. A numerical examination of this model reveals that complex periodic and chaotic oscillations are possible. This behavior is studied using linear stability analysis, autocorrelation analysis, and next-amplitude plots. The next-amplitude plots are found to be fractal curves. An examination of the system trajectories shows that in some circumstances the trajectories follow a Mobius-band attractor in state space.

Oscillations during CO oxidation over supported metal catalysts: II. Effects of reactor operating conditions on oscillatory behavior for a (Pt-Pd)Al2O3 catalyst

Abstract The oscillatory behavior occurring during CO oxidation over a (PtPd) γ-Al 2 O 3 catalyst was studied in an isothermal recycle reactor. The effects of recycle ratio (0–50), gas phase temperature (373–423 K), feed composition (0.053–0.43 mole% CO, 50–99% O2), feed flow rate (78–445 μmol/s), and reactor pressure (0.1–0.2 MPa) were examined. Care was taken to ensure that the catalytic activity remained constant throughout the investigation. The results indicate that oscillations are caused by changes in the rate controlling step coupled with heating of the metal crystallites to temperatures above those of the support and gas phase. It is proposed that the rate controlling step changes from oxygen adsorption to surface reaction followed by rapid burn-off (ignition) of adsorbed CO and oxygen.

Chaotic behavior of reaction systems: consecutive quadratic/cubic autocatalysis via intermediates

Abstract The chaotic behavior produced by a reaction system composed of consecutive quadratic and cubic autocatalytic elementary steps has been described. It is shown that the existence of chaotic behavior does not depend on the presence of a reaction step involving cubic autocatalysis because chaotic behavior is preserved when the cubic step is replaced by successive bimolecular steps involving an intermediate. The chaotic behavior, as characterized by the Lyapunov exponents, displayed by the model based on the bimolecular approximation approaches, in the limit, that of the model based on cubic autocatalysis. Chaos also occurs for the bimolecular approximation model for situations in which the limiting situation of cubic autocatalysis is not approached.