Immobilizing Carbon Nanotubes on SiC Foam as a Monolith Catalyst for Oxidative Dehydrogenation Reactions

Abstract

Carbon nanotubes (CNTs) have received widespread attention in the fields of emission displays, high-strength composites, fuel cells, and lithium-ions batteries owing to their unique chemical and physical properties. Recent reports have demonstrated that CNTs are also promising heterogeneous catalysts in the oxidative dehydrogenation (ODH) reactions of n-butane, 1-butene, ethylbenzene, propane, and ethane. Their outstanding catalytic performance makes CNTs candidates for replacing traditional-metal/metal-oxide catalysts in sustainable chemistry. However, commercial CNTs are typically produced in their powdered form by using the chemical vapor decomposition (CVD) method. These CNT powders tend to agglomerate and are difficult to filter in slurry-phase operation. In addition, owing to a high pressure drop, CNT powders cannot be directly used in fixed-bed reactors. Therefore, it is necessary to form CNTs into larger objects. The method of the compaction of powdered CNTs often inhibits the access of reactants to its active sites and may destroy individual CNTs during the compacting process. The immobilization of CNTs onto certain supports is a more practical approach to solving these problems. Some efforts have been made to grow CNTs or carbon nanofibers (CNFs) on carbon felt, activated carbon, and Lava supports for ODH reactions and CNFs have been formed on porous supports (carbon foams, metal foams, and porous ceramic materials) for catalytic applications. Silicon carbide (SiC) foam possesses high thermal conductivity and mechanical stability and it can be used as a support to immobilize catalysts, which could overcome the disadvantage of the pressure drop in fix-bed reactors. Recently, zeolites have been coated onto SiC substrates, thereby forming zeolite/SiC catalysts. Nitrogen-doped CNTs that are grown on SiC foam show high catalytic activity in the selective oxidation of H2S. [13] For the industrial production of alkenes with CNT catalysts, the growth of CNTs on SiC foam as a monolith catalyst is desirable and the strength of the interactions between the CNTs and the support also needs to be considered. Herein, we report a new method for immobilizing CNTs onto SiC foam as a CNT/SiC monolith and its application in ODH reactions. Structural characterization of the as-prepared monolith shows that the CNTs completely and firmly cover the surface of the SiC foam. Moreover, the CNT/SiC monolith exhibits remarkable catalytic performance and stability in the ODH reaction of 1-butene into butadiene. SiC foam was prepared by a combination of macromolecule pyrogenation with chemical bonding. The foam plastic was soaked in a mixed slurry of SiC powder, resin, EtOH, and silicon powder. After drying and solidification, the foam composite was pyrolyzed and sintered under an argon atmosphere, thereby affording the SiC foam. The preparation process of the CNT/SiC monolith is shown in Scheme 1. Firstly, the SiC foam was immersed in an aqueous solution of Fe(NO3)3, Mg(NO3)2,