Direct transformation of syngas to lower olefins synthesis over hybrid Zn-Al2O3/SAPO-34 catalysts

Article abstract of DOI:10.1039/c7nj04734g

Syngas is a key platform chemical for the utilization of non-petroleum carbon resources. Therefore, great attention has been paid to syngas conversion to value added chemicals, such as lower olefins (C₂-C=4), liquefied petroleum gas (LPG), gasoline, and aromatics. Among these value added chemicals, the lower olefins are promising chemicals, and the direct synthesis of lower olefins from syngas was investigated over a series of hybrid catalysts composed of Zn-M catalysts (M = Al₂O₃, ZrO₂, Cr₂O₃, and CeO₂) and SAPO-34 zeolite. The Zn-M catalysts and different Si/Al ratios of SAPO-34 zeolites were synthesized by co-precipitation and hydrothermal methods, respectively. The aim of the present study was to investigate the influence of various supports on the catalytic performance of this reaction. The catalysts were well characterized by X-ray diffraction (XRD), SEM, ICP-AES, BET surface area, NH₃-TPD, CO₂-TPD, FT-IR, TEM-EDS, and H₂-TPR techniques. NH₃-TPD results showed that with the increase in the Si/Al ratio in the SAPO-34 zeolite, the catalysts exhibited higher acidity due to the higher amount of silica content. The H₂-TPR and XRD results suggested that the Zn/Al₂O₃ catalyst had strong metal-support interactions that were favorable for the higher activity. Various factors that affect the catalytic activity, including the reaction pressure, catalysts weight ratio, SiO₂/Al₂O₃ ratio, and the integration manner of both active components, were investigated. These results clearly demonstrated that the Si/Al ratio in the SAPO-34 catalyst had a predominant effect on lower olefins selectivity. The Zn/Al₂O₃ with SAPO-34 (0.043) catalyst showed excellent lower olefins selectivity (80%). In terms of the integration manner of the hybrid catalysts, the highest activity was obtained when the powders of both Zn/Al₂O₃ and SAPO-34 catalysts were ground in a mortar for 10 min and pelletized; while the sequential bed and granule mixing of Zn/Al₂O₃ and SAPO-34 catalyst showed less lower olefins selectivity, indicating that the appropriate proximity of the active components favors the lower olefins selectivity.

Full text of DOI:10.1039/c7nj04734g

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DOI: 10.1039/C7NJ04734G
Direct Transformation of Syngas to Lower Olefins synthesis over
Hybrid ZnꢀAl2O3/SAPOꢀ34 Catalysts
G. Raveendra, Congming Li*, Yang Cheng, Fanhui Meng and Zhong Li*
Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi
Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
Abstract
Syngas is a key platform chemical for the utilization of nonꢀpetroleum carbon
resources. Therefore, a great attention has been paid to the syngas conversion to value added
=
chemicals, such as lower olefins (C2ꢀC4 ), liquefied petroleum gas (LPG), gasoline and
aromatics. Among these value added chemicals the lower olefins is a promising chemicals,
and the direct synthesis of lower olefins from syngas was investigated over a series of hybrid
catalysts composed of ZnꢀM catalysts (M=Al2O3, ZrO2, Cr2O3 and CeO2) and SAPOꢀ34
zeolite. The ZnꢀM catalysts and different Si/Al ratio of SAPOꢀ34 zeolites were synthesized
by coꢀprecipitation and hydrothermal methods, respectively. The aim of the present study the
influence of various supports on the catalytic performance of in this reaction. The catalysts
were well characterized by Xꢀray diffraction (XRD), SEM, ICPꢀAES, BET surface area,
NH3ꢀTPD, CO2ꢀTPD, FTꢀIR, TEMꢀEDS and H2ꢀTPR techniques. NH3ꢀTPD results showed
that with the increase of Si/Al ratio in SAPOꢀ34 zeolite the catalysts exhibit higher acidity
due to higher amount of silica content. From the H2ꢀTPR and XRD results suggested that the
Zn/Al2O3 catalyst shows strong metal support interactions it is favorable for the higher
activity. Various factors that affect the catalytic activity, including reaction pressure, catalysts
weight ratio, SiO2/Al2O3 ratio and integration manner of both active components were
investigated. These results clearly demonstrated that Si/Al ratio in SAPOꢀ34 catalyst had a
predominant effect on lower olefins selectivity. The Zn/Al2O3 with SAPOꢀ34(0.043) catalyst
1

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showed excellent lower olefins selectivity (80%). For the integration manner of hybrid
catalysts, the highest activity was obtained when the powders of both Zn/Al2O3 and SAPOꢀ34
catalysts was grinded in a mortar for 10 min and pelletized. While the sequential bed and
granule mixing of Zn/Al2O3 and SAPOꢀ34 catalyst showed less amount of lower olefins
selectivity, indicating that appropriate proximity of active components favor the lower olefins
selectivity.
Keywords: Lower olefins, Syngas, ZnꢀAl2O3, SAPOꢀ34
Corresponding authors: Congming Li; Zhong Li, Email:licongming0523@163.com;
lizhong@tyut.edu.cn
1. Introduction
The lower olefins such as ethylene, propylene and butylenes are very important
chemicals and these are used in petrochemical industry as key building blocks. This can be
used for to synthesize value added chemicals such as polymers, solvents and drugs. The
lower olefins are mainly synthesized by catalytic cracking of naphtha, gas oil and light
2
alkanes.1 The rapid depletion of petroleum resources that serve as the source of these
,
hydrocarbons, there is an urgent need for the development of direct synthesis processes which
can produce lower olefins from alternative feedstock’s.3 4 The production of lower olefins is
,
more than 120 million metric tons per year.1 Syngas converted into lower olefins by three
different routes: (1) FischerꢀTropsch (FT) Synthesis, (2) Syngs to olefins via ketene
intermediate (3) Syngas to olefins via methanol intermediate. FTꢀSynthesis route is a well
known process for the transformation of syngas to olefins synthesis. Long time onwards
many scientists have studied for FT synthesis route mostly on Fe based catalysts.5 6 However,
,
the lower olefins selectivity is limited because of Anderson–Schulz–Flory distribution(ASF).
Another indirect route is syngas to lower olefins synthesis via dimethyl ether or methanol
intermediate.7 The production of lower olefins roared for the development of efficient and
2

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applicable strategies to achieve high selectivity, thermal stability and longer life time.8, 9
Indirect synthesis route, the development of a hybrid catalyst, combining a Zn based
methanol synthesis catalyst and SAPOꢀ34 as a methanol to lower olefins synthesis catalyst,
are well suited for the lower olefins synthesis, in this process first syngas converted into
CH3OH and then methanol selectively converted into lower olefins. This is a promising route
for selective conversion of syngas to lower olefins synthesis in a single step process. Such a
direct synthesis route could take advantage for lower olefins synthesis, as a result of high
selectivity of lower olefins at lower reaction pressure.10 Generally most of the previous
publications from syngas to methanol synthesis over copper based catalysts used at lower
reaction temperatures. As we well known, Cu based catalysts are sensitive to reaction
temperature easy to sintering and leading to deactivation of catalyst particularly at higher
reaction temperatures of >300oC.11 Generally most of the mixed metal oxides are inactive and
rarely been used for CO hydrogenation, they are mainly used as a promoters and supports for
metal catalysts.12 13 The above mentioned drawbacks of Cu based catalysts, first Wang et.al
,
selected Zn metal as an active site component for the one pot synthesis of lower olefins from
syngas at high reaction temperatures.18 In methanol synthesis ZnO as an active site, and the
role of Al2O3was attributed to spinel formation, which can be prevented the sintering of ZnO
crystallites with high surface area and better distribution of ZnO particles.14 The direct
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synthesis of lower olefins (C2 –C4 ) from syngas with high selectivity (>60%) is a big
challenge. But recently, some of the researchers they break through the ASF distribution to
achieved high selectivity of lower olefins. For eg. (1) FTꢀSynthesis route, Torres Galviz et al.
=
=
reported that Fe based catalysts gave lower olefins (C2 –C4 )selectivity of 61%.3 Zhong et al.
found that cobalt carbide nanoprism catalysts gave lower olefins with a selectivity of
61%15,(2) Ketene intermediate route, Zhu et al16 reported MnO/MSAPOꢀ34 catalyst showed
=
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the selectivity of C2 –C4 is 79.2%. Jiao et al17 reported ZnꢀCr2O3/SAPOꢀ34 catalyst gave the
3