Sieghard E. Wanke

The Sintering of Supported Metal Catalysts

Abstract Metal catalysts are commonly employed in the form of metal dispersed as small crystallites on high surface area supports. The use of these supported metal catalysts increases the utilization of the metal as a catalyst since a large fraction of the metal atoms are at the surface of the small metal crystallites. Another important function of the support is to physically separate the small metal crystallites and thereby hinder the agglomeration of the small metal crystallites into larger crystallites. This agglomeration would decrease the number of surface metal atoms per unit mass of metal, and thereby decrease the utilization of the metal and the activity of the catalyst.

A model of supported metal catalyst sintering: I. Development of model

Abstract An interparticle transport model for the sintering of supported metal catalysts is developed. The model postulates escape of atoms from crystallites to the support surface, rapid migration of these atoms along the surface, and their recapture by crystallites upon collision. A reduction in surface energy provides the driving force for transfer of metal from small to large particles. The model can account for redispersion, for an effect of gas atmosphere on sintering behavior, and for considerable variation in order for a power law fit of sintering. It predicts that the character of the particle size distribution affects the rate of sintering.

The sintering of supported metal catalysts: II. Comparison of sintering rates of supported Pt, Ir, and Rh catalysts in hydrogen and oxygen

Abstract Changes in dispersion of alumina supported Pt, Ir, and Rh catalysts due to thermal treatment (250–800 °C) in oxygen and hydrogen atmospheres were measured. In oxygen atmospheres the sequence of thermal stability was found to be Rh > Pt > Ir, while in hydrogen atmospheres the sequence was Ir > Rh > Pt. Increases in dispersion due to treatment in oxygen were observed for Pt and Ir catalysts. The observed relative stabilities are compared to qualitative predictions based on sintering mechanisms.

The sintering of supported metal catalysts: I. Redispersion of supported platinum in oxygen

The effects of treatment in oxygen at elevated temperatures on the dispersion of five Pt/MAl2O3 catalysts (0.5 to 4.0 wt% Pt) were determined. Treatment temperatures of 300 to 700 °C and treatment periods of 1 to 128 hr were studied. Significant increases in dispersion (at times a two- to threefold increase) were observed at ≤600 °C. At 500 and 550 °C the increases in dispersion were relatively independent of the treatment time. At higher temperatures, rapid, time-dependent decreases in dispersion occurred. The results are in agreement with a molecular migration mechanism.

A model of supported metal catalyst sintering: II. Application of model

Abstract An interparticle transport model for the sintering of supported metal catalysts is solved by finite difference methods and applied to several theoretical particle size distributions (PSD). The model predicts an increase in the rate of sintering as the width of the initial PSD increases. The rate of sintering also increases as the surface velocity and the metal loading increase. Sintering behavior is sensitive to the activation energy and temperature. Under certain conditions substantial redispersion is predicted. The model can account for power-law orders from 13, as observed experimentally. Power-law order increases with PSD width and with increases in mean crystallite size.

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.

Experimental studies of sintering of supported platinum catalysts

Abstract Changes in the dispersion of supported Pt Al 2 O 3 catalysts following reduction and a variety of thermal treatments have been monitored by gas uptake and electron microscopy. Evidence of redispersion was found after sintering of one catalyst in oxygen at 450 to 600 °C. Sintering was found to be sensitive to gas atmosphere and metal loading. Addition of a portion of presintered catalyst containing large Pt particles increased the rate of sintering of a catalyst. From electron micrographs of the same catalyst area before reduction and after reduction and various thermal treatments, it was concluded that Pt agglomeration occurs during all these steps. Some Pt crystallites remain in a fixed location during reduction and thermal treatments.

The limitation of the transmission electron microscope for characterization of supported metal catalysts

Abstract The accuracy of particle size distributions determined from electron micrographs is examined from both theoretical and experimental points of view. Particle detectability and apparent size are found to be sensitive functions of defocus, and hence of elevation of particles in the specimen. Contrast is shown to vary with orientation of both particles and support material. Sources of contrast inherent in the support in the subnanometer range are illustrated. It is concluded that particle size distributions become increasingly subject to error as the fraction of particles with sizes below about 2.5 nm increases.

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.

Solubility of ethylene, 1-butene and 1-hexene in polyethylenes

The gravimetric method was used to measure the solubility of ethylene, I-butene and 1-hexene for four polyethylene samples with different crystallinities and branching structures. Buoyancy corrections were made for polymer swelling; it was found that polymer volume varied approximately linearly with the amount of olefin sorbed. The solubility of ethylene, at temperatures of 30-90 C and pressures up to 3.5 MPa, was well described by Henrys law; Coefficients of Henrys law were in the range of 0.005 -0.014 (g C 2 H 4 )/(g of amorphous PE MPa) The coefficients decreased with increasing temperature and increasing crystallinity. The solubility of 1-butene and 1-hexene, measured over the same temperature range and at pressures up to about 90% of the vapour pressures, did not follow Henrys law; these olefins were much more soluble than ethylene in all four types of polyethylene. Olefin solubility, based on amount of amorphous polyethylene, decreased with increasing polyethylene crystallinity and was also a function of the type of branching.