Juili Parab – Friederike Jentoft Research Group
Influence of oxygen on ethylene polymerization using the Phillips catalyst
Phillips catalyst is one of the most widely used polyolefin catalyst, accounting for 40-50% of the high-density polyethylene (HDPE) produced worldwide. Unlike the other polyolefin catalyst, the polymerization using Phillips catalyst proceeds without metal cocatalyst.1 This simplifies the catalyst preparation process thereby making it less expensive than other polyolefin catalyst.2 However, just like the other polyolefin catalyst the Phillips catalyst is also susceptible to poisoning which can result in an increase in the operating cost and the amount of residual chromium in the polymer product.3 Hence, to avoid economic burden due to catalyst inefficiency and poor product quality it is crucial to understand the influence of poisons on the polymerization behavior.
Several poisons have been studied in literature.1 Amongst them, oxygen presents a challenging case, being a component of air. It can easily find its way in the system if the established protocols about catalyst handling and transfers are not followed properly or if there is a leakage which goes unnoticed. These reasons also make it difficult to study the oxygen as a poison. A possible source of oxygen is the calcination step. The catalyst is calcined in oxygen/nitrogen environment. While cooling, the oxygen flow is switched off around 350 ℃ and the catalyst bed is purged with nitrogen to prevent traces of oxygen from entering the polymerization reactor.1 This is a part of a well-established protocol based on years of commercial manufacturing experience known to give desirable product characteristics. However, a systematic study to understand the influence of varying the nitrogen purge period and in turn the oxygen concentration is lacking in the literature. In a laboratory based setting, we have analyzed the effluent stream from a fixed bed tubular reactor with an online mass spectrometer and established a relationship between the oxygen concentration before starting the ethylene flow and the time to achieve maximum conversion of ethylene. High oxygen concentration delays the time to reach maximum conversion. Overall polymerization activity obtained under the lab setup is much lower than the commercial setup and hence, it is difficult to comment on the influence of oxygen on the activity.
1. McDaniel, M. P. A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization. Advances in Catalysis 53, (Elsevier Inc., 2010).
2. McDaniel, M. P. Some reflections on the current state of Cr-based polymerization catalysts. MRS Bull. 38, (2013).
3. Purvis, D. The impact of ethylene and propylene impurities upon polyolefins units.In: Proceedings of AICHE 2011 Spring meeting ; 2011 March 14th ; Chicago. Abstract number: 211644.
Oscar Zabala-Ferrera – Peter Beltramo Research Group
Electrostatic effects on lipid bilayer compressibility and vesicle adhesion
The cell membrane is a complex two-dimensional fluid primarily composed of phospholipids, with many other molecules such as membrane proteins, ion channels, and cholesterol embedded within or attached to its surface. The phospholipid bilayer allows for the lateral diffusion of molecules and regulates the transport of material across the interface. Interestingly, there is a tremendous chemical diversity in phospholipid headgroup charge and fatty acid tail saturation, which systematically change on a tissue and healthy or diseased cell basis. For example, the concentration of charged phospholipids has been shown to increase during malignant transformation, altering membrane curvature and eventually triggering apoptosis. These interesting compositional observations raises questions on how the underlying phospholipid membrane properties, such as thickness, fluidity, and compressibility, change and dictate the function of the cell in different scenarios. Can these compositional differences be harnessed to selectively transport material across target membranes? To work towards this end, we must first understand how the properties of a bilayer depend on its phospholipid components.
In this work, we use compositionally controlled, free-standing large area model membranes as an artificial platform to investigate the ramifications of altered phospholipid composition on membrane material properties and membrane fusion. By simultaneously applying an external potential and imaging the planar membrane, we can evaluate membrane thickness and Young’s modulus. We investigate the effect of increasing charge on the membrane by incorporating anionic DOPG with zwitterionic DOPC, and measuring the resultant change in material properties. The membrane’s stiffness increases as a function of DOPG content, likely due to the increased electrostatic repulsion between leaflets. Ongoing work is applying this approach to bilayers consisting of other charged headgroups and different acyl tail lengths/degrees of unsaturation to understand the effect of composition on the electromechanical properties of membranes. This provides the basis for understanding compositionally dependent adhesion and subsequent fusion of vesicles with the membrane. As a test case, we study the electrostatic adhesion of 100 nm diameter vesicles with the large (0.9 mm diameter) planar bilayer introduced earlier. Upon addition of calcium ions, the stable vesicles undergo drastic changes in conformation on the membrane- forming branched aggregates on the surface of the bilayer and in some cases fusing with each other or the membrane substrate. These results effectively demonstrate the utility of systematically determining membrane properties and extending our experimental platform to accommodate increasingly complex biological processes.