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Hydrogen spillover in a ceramic membrane reactor and packed-bed reactor Izadi-Najafabadi, Negar

Abstract

The term spillover in heterogeneous catalysis is applied to the transport of active species from one surface to another, in which the second surface does not under the same conditions, sorb or form the active species. Since the first observation of spillover in the 1960's, there has been extensive research in the area of spillover, especially hydrogen spillover. Catalytic reactions such as hydrocracking are explained based on the spillover phenomenon. The role of the catalyst in hydrocracking is thought to be to provide spilt— over hydrogen species. Consequently, there is no need for direct contact between organic reactant(s) and catalyst, provided spilt-over hydrogen can have access to reactant by some means. In conventional hydrocracking systems, the organic reactant(s) and catalyst are in direct contact and this leads to coking and catalyst deactivation. Catalyst deactivation is a major concern in industry and different researchers are focusing on understanding and controlling coking and developing new coke-resistant catalysts. Based on the spillover phenomenon, one possible approach to reduce the impact of coking and catalyst deactivation is to separate the catalyst and organic reactant by a medium which can facilitate hydrogen spillover. A ceramic membrane reactor may be one possible configuration for separation of reactant and catalyst. The membrane tube would act as the separating medium while the ceramic material may facilitate hydrogen spillover. Furthermore, the ceramic material is resistant to the severe conditions present in hydrocracking reactions. The present study reports on an attempt to implement and assess this new approach of hydrocracking in a membrane reactor. In the first part of the work, diphenylmetliane(DPM) hydrocracking was investigated in a ceramic membrane reactor. The. catalyst (sulfided Ni-Mo/Al₂O₃) was separated from the DPM by means of a ceramic membrane tube. The results of different experiments showed the enhancement of the yields of products (benzene and toluene) in the presence of catalyst compared to the yields in the absence of catalyst, that was ascribed to hydrogen spillover. However, the transport of liquid DPM through the membrane and onto the catalyst could not be eliminated and therefore, the extent of hydrogen spillover could not be quantified. To pursue the investigation of hydrogen spillover through the ceramic material of the membrane, and to achieve complete separation of catalyst and reactant, a catalytic reaction with solid reactant was also investigated. Coke was chosen as the solid reactant because it reacts with spilt-over hydrogen to produce methane. Hence for the second part of this work, amorphous silica-alumina was coked by thermal decomposition of propylene. Subsequently, the coked silica-alumina underwent temperature-programmed hydrogenation in the presence and absence of catalyst (2%Co-Si₂). In the former case, the effect of separation of coked silica-alumina and catalyst by low surface area crushed ceramic material (the ceramic membrane used in the first part of the study) and high surface area dried silica-alumina, were investigated. The results confirmed that hydrogen spillover was responsible for at least part of the methane production. Also, hydrogen spillover decreased due to the separation of coked silica-alumina and catalyst by either the ceramic or the dried silica-alumina. Hydrogen transport by spillover through the ceramic membrane was confirmed, however, the extent of spillover was lower for the low surface area ceramic material than the high surface area dried silica-alumina.

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