Research Article pubs.acs.org/acscatalysis
Role of Manganese Oxide in Syngas Conversion to Light Olefins Yifeng Zhu,† Xiulian Pan,*,† Feng Jiao,†,‡ Jian Li,†,‡ Junhao Yang,†,‡ Minzheng Ding,† Yong Han,§,∥ Zhi Liu,§,∥ and Xinhe Bao*,† †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ∥ School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, People’s Republic of China S Supporting Information *
ABSTRACT: The key of syngas (a mixture of CO and H2) chemistry lies in controlled dissociative activation of CO and C−C coupling. We demonstrate here that a bifunctional catalyst of partially reducible manganese oxide in combination with SAPO-34 catalyzes the selective formation of light olefins, which validates the generality of the OX-ZEO (oxide-zeolite) concept for syngas conversion. Results from in situ ambientpressure X-ray photoelectron spectroscopy, infrared spectroscopy, and temperature-programmed surface reactions reveal the critical role of oxygen vacancies on the oxide surface, where CO dissociates and is converted into surface carbonate and carbon species. They are converted to CO2 and CHx in the presence of H2. The limited C−C coupling and hydrogenation activities of MnO enable the reaction selectivity to be controlled by the confined pores of SAPO-34. Thus, a selectivity of light olefins up to 80% is achieved, far beyond the limitation of Anderson−Shultz−Flory distribution. These findings open up possibilities to explore other active metal oxides for more efficient syngas conversion. KEYWORDS: syngas chemistry, light olefins, manganese oxides, CO dissociation, bifunctional catalysts, heterogeneous catalysis, oxide-zeolite ight olefins containing two to four carbon atoms (C2=− C4=) are important feedstocks for production of a variety of chemicals, such as polymers and drugs.1 Conventionally, light olefins are produced from steam cracking and fluid catalytic cracking of naphtha. With diminishing oil reserves, it is desirable to develop technologies from alternative resources such as coal and natural gas. This can be achieved when these resources are converted first to syngas (a mixture of CO and H2) and then transformed to olefins via methanol, known as methanol to olefins technology (MTO). Alternatively, syngas can be directly converted to light olefins, so-called Fischer− Tropsch to olefins (FTTO), with modified Fischer−Tropsch synthesis (FTS) catalysts, which has been studied for decades.1−3 In FTS, the reaction gives rise to products with a wide spectrum ranging from C1 to C20 and even higher hydrocarbons, which can be described by the Anderson− Schulz−Flory (ASF) model.2,3 Theoretically, the selectivity for C2−C4 hydrocarbons (including light olefins C2=−C4= and paraffins C20−C40) does not exceed 58%. Therefore, extensive efforts have been made over the years to modify the catalyst structures and their compositions, intending to improve the
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© XXXX American Chemical Society
light olefin selectivity beyond the ASF limit. Another nuisance in FTTO is the formation of a large amount of the byproduct CH4. Its selectivity reaches over 25% when the C2−C4 selectivity is 58%. Recently, significant progress was reported by Torres Galviz et al. that a C2=−C4= selectivity of 61% was achieved over an Fe-based catalyst.3 Zhong et al. reported that cobalt carbide nanoprisms enabled syngas conversion to light olefins with a selectivity of 61%.4 Extensive studies have revealed that the challenge of selective conversion of syngas to light olefins lies in the precise control of C−C coupling and the simultaneous suppression of the overhydrogenation of CC and methane formation. We recently reported a concept of oxide-zeolite (OX-ZEO), using a bifunctional composite with partially reduced ZnCrOx and mesoporous SAPO-34 zeolite (denoted as MSAPO, with M representing mesoporous).5 Such a composite catalyzes syngas Received: January 22, 2017 Revised: March 10, 2017
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DOI: 10.1021/acscatal.7b00221 ACS Catal. 2017, 7, 2800−2804
Research Article
ACS Catalysis Table 1. Properties of Manganese Oxide Catalysts and Their Catalytic Performancea
hydrocarbon distribn (%) catalyst
cryst structure (fresh)c
cryst size (nm)b,c
cryst structure (used)d
cryst size (nm)b,d
CO conversn (%)
MnO Mn2O3 MnO2
c-MnO c-Mn2O3 γ-MnO2
18.7 30.3 7.6
c-MnO c-MnO c-MnO
49.6 73.6 69.9
7.3 7.6 8.5
C2=−C4= C20−C40 79.1 78.2 79.2
14.5 14.3 12.9
CH4
othere
CO2 sel (%)
1.4 1.7 2.0
5.0 5.8 5.9
43.4 43.4 41.0
Reaction conditions: 400 °C, 2.5 MPa, H2/CO = 2.5, GHSV = 4800 mL h−1 gcat−1, oxide/zeolite mass ratio 1. bThe crystal sizes estimated by the Scherrer equation. cFresh catalyst. dUsed catalyst. eOther products containing mainly C5+ hydrocarbons with the selectivity of methanol and dimethyl ether
