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Heterogeneous Catalyzed Polymer Hydrogenation in Oscillating Systems

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D1. Heterogeneous Catalyzed Polymer Hydrogenation in Oscillating SystemsThis project will: develop new techniques for polymer hydrogenation by incorporatingnovel reactor designs. One such design will implement oscillating conditions toaddress specific limitations of current methods.Primary Faculty Co-Advisors: Dr. Kerry Dooley, Chemical Engineering (Heterogeneous Catalysis) Dr. Carl Knopf, Chemical Engineering (Bubble Column Reactor) Dr. Dmitri Nikitopoulos, Mechanical Engineering (Two-Phase Flow) Off-campus Participant: Miguel Baltanas (University of Santa Fe and INCAPEResearch Institute) Technical Proposal: Some 70% of the world polymer market is devoted to just four parent polymers:polystyrene (PS), polyethylene (PE), polypropylene (PP), and polyvinylchloride (PVC).With the flood of new plastics applications, these four simple polymers cannot satisfythe need for more specialized materials. Innovative uses demand elastomers that arestronger, better able to resist chemical attack, possess a wider temperature range ofusage and better environmental stability, and are easy to produce. One way to produce aderivative with potentially improved attributes is through a post-polymerizationreaction. Hydrogenations are just one type of modification that can “fit the bill” forcreating new plastics. It is convenient because it builds on commercially availablepolymers and components, meaning no new investments are required for monomerproduction. One such example of a useful polymer formed upon hydrogenation of PS ispolyvinylcyclohexane (PVCH). PVCH has a Tg that is 42 K higher than PS, meaning itcan be used at elevated temperatures where PS would pass into its melt state andbecome soft. Another example is hydrogenated nitrile butadiene rubber (HNBR), whichhas superior resistance than standard NBR to degradation from prolonged oil contactand abrasion, even at elevated temperatures. The goal of this research is to advance the field of polymer hydrogenation bydesigning and executing new approaches to the catalysis/reactor design. One of the mostwidely studied hydrogenations is the reduction of a diene polymer, such as polybutadiene(Fig 1). Fig 1: PolybutadieneDuring this reaction, the olefinic double bonds in the parent polymer arehydrogenated until saturation is reached. The exact end product of hydrogenation largelydepends on the configuration of repeating units in the polymer. Current hydrogenation techniques are severely limited in their ability to economicallyand efficiently produce hydrogenated elastomers. While diimide reduction using ahydrazide reagent is one way to hydrogenate certain polymers, almost all reactions arenow carried out in the presence of a catalyst to improve reaction rates and make theprocess economically viable. Homogeneous CatalysisMany polymer hydrogenation reactions are carried out using homogeneous catalysis.The main advantages of this technique are mild reaction conditions (though not alwaysthe case), and the ability to better realize quantitative hydrogenation. The majordisadvantages of this method are incomplete conversion to saturated polymer and theinherent difficulty in post-polymerization separation of catalyst from product. Over thepast several decades, most hydrogenation research has been in homogeneous catalysis.Consequently, each hydrogenation reaction has been studied with many differenttransition metal complexes and solvents. The best choices for each respective reactionare generally understood. In fact, material on homogeneous polymer catalysispractically reads as a recipe book. Given that homogeneous methods are well known,the future of hydrogenation research lies in heterogeneous techniques.Heterogeneous CatalysisThe shortcomings of homogeneous catalysis can be addressed by developingheterogeneous alternatives. Since heterogeneous catalysts do not in general imposeundue separation requirements, they are clearly the desired choice. Unfortunately,several problems must be addressed before polymer hydrogenation by heterogeneouscatalysis can reach its potential. The main problems are:1. Catalyst Selectivity: Obviously, the desired catalyst must selectively catalyzehydrogenation as opposed to chain scission or side chain hydrodealkylation.However, at the extreme temperatures and pressures commonly found in currentreactor systems, the catalyst often operates unselectively. For instance, PShydrogenation to PVCH may require a temperature around 200ºC and pressuresover 5 MPa while still taking about 3 h to achieve 90% conversion. Most of theliterature does not deal with the selectivity issue directly. Instead, people workaround it by finding alternative solvents or reaction conditions (e.g., lowtemperatures and addition of THF solvent) to abate side reactions. Currently, theonly known sure way to limit chain scission is to keep temperatures low.2. Leaching of Precious Metals: Because precious metals like Pt, Pd, and Ru arerequired, solvent leaching can be a serious problem. Constant replacement orregeneration of the catalysts is typically not economically viable. Therefore, thesolvent/catalyst choice must be carefully screened for any corrosive tendencies.Despite its importance, most current research does not address the leaching issue. 3. Solvent Choice: Since the polymer must typically be dissolved in solution for thereaction to take place, the solvent choice is important. Solvents must be relativelycheap, noncorrosive, cannot contaminate the product, and must adequatelydissolve the polymer reactant. By far the most popular solvents aredecahydronaphthlalene (DHN) and tetrahydronaphthlalene (THN). Solventscontaining oxygen, nitrogen, or sulfur are usually avoided as these tend to bedifficult to separate from product. Certain fluorinated solvents will dissolve manypolymers, but prolonged contact with H2 at high pressures can lead to undesiredHF formation and safety concerns. Gehlsen et al. (1995) successfully conductedhydrogenation of PS homopolymers to synthesize PVCH derivatives using dilutecyclohexane solvent. For certain polymers such as PS and poly(α-methylstyrene)(PαMS), 10% (vol.) THF was added to enhance miscibility and prevent the largeincrease in polydispersity that was evident


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