Reduced graphene oxide supported cobalt catalysts for hydroformylation and Fischer-Tropsch synthesis

Abstract

Reduced graphene oxide (RGO) supported cobalt based catalysts with or without rhodium promotion, Rh-Co/RGO (RCG) and Co/RGO (CG), were successfully synthesized and evaluated in both Fischer Tropsch synthesis (FTS) and ethene hydroformylation (EH). The morphology, particle size and reducibility were different for the CG and RCG catalysts. (1) With the fresh catalysts, after calcination, CoO nanoparticles were present in the CG catalyst, while CoO nanorods were formed in the Rh promoted RCG catalyst. (2) With H2 reduction, the reducibility of RCG was much higher than with the CG catalyst. (3) When exposing both the catalysts in syngas or syngas co-fed with ethene, a multi-phase of Co-Co2C (or CoO) with smaller nanoparticles were presented in spent CG, while a pure Co2C with a larger flat plane structure was formed in spent RCG. It was concluded that the small amount of Rh changed the interaction between the cobalt species and RGO, which enhanced the density of Co2C on the surface of RGO during the FTS and EH reactions. The catalyst with a Co-Co2C phase exhibited low FT activity with mainly short chain hydrocarbons and oxygenates. However, the catalyst with the Rh-Co2C phase was completely inactive for FTS. Co2C/RGO derived from CoO/RGO was highly active for heterogeneous EH and offers potential for use in industrial hydroformylation processes, especially with the catalyst promoted with Rh. It was concluded that Co2C could suppress the CO from attending the chain growth reaction, while it promoted the CO insertion reaction to form oxygenates. As the RCG catalyst showed outstanding performance for heterogeneous EH, optimization of reaction conditions was carried out. The influence of the feedgas ratio, pressure, space velocity and temperature on the EH over a Rh-Co/RGO catalyst was investigated in a tubular fixed bed reactor (TFBR). A higher CO partial pressure and a lower H2 partial pressure with a temperature range of 210 °C to 230 °C increased the selectivity of the C3 oxygenates, and decreased the selectivity of the C2H6 (the main by-product of C2H4 hydrogenation). The mechanisms of hydroformylation and chain growth reaction for the RCG catalyst were studied. The potential reaction pathways are discussed based on a comparison of the changes in the product spectrum at different reaction temperature rates (140-290 °C). The C-C bond formed between CO and C2H4 was assumed to be the fastest elementary step, and it may be even faster than C-H bond formation. C2H4 induced the reaction of CO and C2H4 to form C-C coupling with an intermediate of *C2H4CO, which could further react with H2 to form oxygenates and hydrocarbons. When the reaction temperature was higher than 210 °C, CO could induce the formation of ethylidyne (≡C-CH3), which was a very important intermediate that triggered the chain growth reaction to form normal FT products. This work is of guiding the design of the heterogeneous catalysts for conversion of syngas to oxygenates by combining the FTS reaction and the olefin hydroformylation reaction.

Ama-catalyst asekelwe ku-graphene oxide (RGO) asekelwe ku-cobalt noma ngaphandle kokukhuthazwa kwe-rhodium, i-Rh-Co/RGO (RCG) ne-Co/RGO (CG), ahlanganiswe ngempumelelo futhi ahlolwa kukho kokubili i-Fischer-Tropsch synthesis (FTS) ne-ethene hydroformylation. I-morphology, usayizi wezinhlayiya kanye nokuncipha kwe-reducibility kwakuhlukile kuma-catalysts e-CG kanye ne-RCG: (1) kuma-catalysts amasha ngemva kokubala, ama-nanoparticles e-CoO spherical anikezwe ku-catalyst ye-CG, kuyilapho ama-nanorods e-CoO akhiwa ku-Rh ekhuthaza i RCG catalyst; (2) ngokunciphisa kwe-H2, ukunciphisa kwe-RCG kwakuphakeme kakhulu kunalokho kwe-CG catalyst; (3) ukuveza womabili ama-catalysts ku-syngas noma kuma-syngas e-co-feeding ne ethene, izigaba eziningi ze-Co-Co2C (noma i-CoO) ezinama-nanoparticles amancane zethulwe ku CG yokuchitha, kuyilapho i-Co2C ehlanzekile enesakhiwo esikhulu sendiza eyisicaba. yakhelwe ekusetshenzisweni kwe-RCG. Kwaphethwa ngokuthi inani elincane le-Rh lishintshe ukusebenzisana phakathi kwezinhlobo ze-cobalt ne-RGO, okuthuthukise ukuminyana kwe-Co2C ebusweni be-RGO ngesikhathi se-FTS kanye nokusabela kwe-ethene hydroformylation. I-catalyst enesigaba se-Co Co2C ibonise umsebenzi ophansi we-FT onama-hydrocarbon amaketango amafushane kanye nama oxygen. Nokho, i-catalyst enesigaba se-Rh-Co2C ayisebenzi ngokuphelele ku-FTS. Ngaphezu kwalokho, i-Co2C/RGO ethathwe ku-CoO/RGO ibisebenza kakhulu ku-ethene hydroformylation ehlukahlukene futhi inikeza amathuba okusetshenziswa ezinqubweni ze-industrial hydroformylation, ikakhulukazi i-catalyst ekhuthazwa nge-Rh. Kwaphethwa ngokuthi i-Co2C ingacindezela i-CO ekuhambeleni ukusabela kokukhula kweketango, kuyilapho ikhuthaza ukusabela kokufakwa kwe-CO ukwenza ama-oxygen. Njengoba i-RCG catalyst ibonise ukusebenza okuvelele kwe-ethene hydroformylation ehlukahlukene, ukuthuthukiswa kwezimo zokusabela kwenziwa. Umthelela wesilinganiso se-feedgas, ingcindezi, isivinini sesikhala nezinga lokushisa ku-ethene hydroformylation phezu kwe-Rh-Co/RGO catalyst iye yaphenywa ku-rector yombhede ongaguquki weshubhu. Ukucindezela kwengxenye ye-CO ephakeme kanye nokucindezela okuncane kwe-H2 okuphansi ngebanga lokushisa elingu- 210°C, 230 °C kungase kukhulise ukukhethwa kwama-oxygen e-C3, futhi kwehlise ukukhethwa kwe-C2H6 (okuyinhloko ngomkhiqizo ovela ku-C2H4 hydrogenation) . Izindlela ze-hydroformylation kanye ne-chain growth reaction ye-RCG catalyst yafundwa. Izindlela zokusabela ezingaba khona kwakuxoxiwe ngazo ngokusekelwe ekuqhathaniseni izinguquko ze spectrum yomkhiqizo emazingeni okushisa ahlukene okusabela (140-290 °C). Ukwakhiwa kwebhondi ye-C-C phakathi kwe-CO ne-C2H4 kwakuyisinyathelo sokuqala esilula ukwenzeka kulesi simiso esilula nakakhulu kunokwakheka kwebhondi ye-C-H. I-C2H4 iyenge ukusabela kwe-CO ne-C2H4 ukuze kwakheke i-C-C ukuhlangana nokuphakathi kwe-*C2H4CO, okungase kuqhubeke kusabela nge-H2 ukuze kwakhe ama-oxygen nama-hydrocarbon. Lapho izinga lokushisa lokusabela liphakeme kune 210 °C, i-CO ingabangela ukwakheka kwe-ethylidyne (≡C-CH3), okwakuyindawo ebaluleke kakhulu ephakathi eyabangela ukusabela kokukhula kweketango ukuze kwakhiwe imikhiqizo evamile ye-FT. Lo msebenzi ungowokuqondisa ukwakheka kwama-catalysts ahlukahlukene okuguqulwa kwama syngas abe ama-oxygen ngokuhlanganisa ukusabela kwe-FTS kanye nokusabela kwe-olefin hydroformylation.