Study of the effect of morphology of nanoporous carbon membranes on permselectivity
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Study of the effect of morphology of nanoporous carbon membranes on permselectivity Ramakrishnan Rajagopalan and Henry C. Foley Department of Chemical Engineering 158, Fenske Laboratory University Park, PA 16802 ABSTRACT The present work is aimed at developing uniform nanoporous carbon membranes on porous stainless steel supports. Thin layers of polyfurfuryl alcohol are applied on the stainless steel supports using spin coating method. Nanoporous carbon is obtained by pyrolyzing polyfurfuryl alcohol films under inert atmosphere. Uniform nanoporous carbon membranes were obtained by repeated spin coating and pyrolysis of polyfurfuryl alcohol on the stainless steel supports. The morphology of the membranes was examined using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). It was shown that there is direct correlation between the morphology and the selectivity of the membranes. SEM and AFM show the presence of globular domains during the formation of carbon membranes. The selectivity increased with the decrease in the size of the globules. It was also shown that the annealing of the membrane affected both the morphology and the selectivity of the membrane. INTRODUCTION Nanoporous carbon materials are interesting class of materials that have excellent size and shape selective properties. The ease of synthesis of these materials in powder form and as well as thin membranes on supports provides the flexibility to process these materials for different applications. These properties make them a promising candidate for applications that include catalysis and gas separation [1-4]. Nanoporous carbon as the name suggests have very small pores in the range of 45 D. This special property of these materials helps them to preferentially separate gases based on their size and thus acts as an excellent molecular sieve. These forms of carbon are typically derived from polymers like polyfurfuryl alcohol (PFA), polyvinylidene chloride (PVDC) and Polyvinylchloride (PVC) [5-8]. The most important factor is the amount of yield of carbon after pyrolysis of these polymers. PFA and PVDC form nongraphitizing carbon while PVC forms graphitizing carbon. Generally, there is about 25 – 50% yield of carbon upon pyrolysis. PFA upon heating under inert conditions reacts to form water, methane, carbon dioxide and hydrogen. PFA is a thermosetting polymer and hence the pyrolysis of the polymer creates lot of crosslinking. This results in a non-graphitizing carbon. The pore sizes of this carbon is of the order of 4-5 D and the density of the carbon is 1.4 – 1.5 g/cm3 [9-12]. There are numerous investigations in the literature, which address the issue of the effect of pyrolysis temperature on the conversion of PFA into nanoporous carbon. Photoacoustic spectroscopy and NMR study on the bulk carbon suggests that pyrolysis of PFA at 600 – 8000 C results in the decrease in the intensity of aliphatic C-H bonds, C=O
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and C=OH groups and on the other hand, the intensity of aromatic C-H bonds increase [13-14]. Our group h
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