Studies on C1 Chemistry Catalytic Reactions

概要

Carbon-one (C1) chemistry refers to the chemistry of synthesis of a series of fuels and high-value-added chemicals from simple molecules containing one-carbon atom, such as methane, carbon monoxide, carbon dioxide, and methanol. The most important resources for C1 compounds are coal, natural gas, organic waste, and biomass. With strongly fluctuating prices and decreasing reserves of crude oil, the increaingly serious problems of environmental pollution, C1 chemistry has become even more important due to the increasing need for the manufacture of fuels and petrochemical commodities.

The conversion of C1 molecules typically relies on the process of catalysis, especially heterogeneous catalysis. The development of highly efficient, low-cost catalyst systems for the targeted synthesis of desired products has always been the core issue in C1 catalysis.

Great advances in basic and applied research have been made for the development of C1 conversion processes on the basis of the important catalytic systems. However, due to these C1 molecules existing with either the relatively inert nature (e.g., CH4 and CO2) or high reactivity (e.g., CO and CH3OH), there is no unified principle for the C1 conversion, leading to great opportunities and formidable challenges in the balance of selectivity and conversion for C1 chemistry. Therefore, it is highly desirable to develop novel and efficient C1 conversion pathways for oriented production of fuels and chemicals, which requires major innovations in the design of catalysts and development of reaction processes.

In this thesis, we mainly concentrate on the design and application of high-performance catalysts for conversion of methane, carbon dioxide, carbon monoxide. The spatial distance of catalytic components and catalysts with unique structure has been precisely controlled and designed. Moreover, these catalysts or catalytic strategies exhibit outstanding performance in C1 chemistry catalytic reaction.

In chapter 1, methane non-oxidative conversion via dehydroaromatization process (MDA) has been realized on Mo/HMCM-22 catalysts. The high-temperature MDA is an alternative process for converting CH4 directly. Since it was first proposed by Wang et al. in 1993, zeolites supporting Mo catalysts have been intensively investigated due to their high activity. The main drawback for the Mo-containing zeolite catalysts is poor lifespan because of the heavy coke deposition. The key problem for the MDA to be solved is to inhibit deep dehydrogenation while retaining high the first C-H bond-cleaving activity. A series of Mo/HMCM-22 catalysts with different spatial distances between the Mo species and HMCM-22 zeolite were designed for the methane dehydroaromatization (MDA) process. The methods and strategies of hydrothermal synthesis (HS), wet impregnation (WI), mechanical milling (MM), physical mixing (PM) and dual catalytic beds (DCB) were employed to control the spatial distances. Our characterization analyses demonstrate that all of the prepared Mo/HMCM-22 catalysts possess the same active components, but the spatial distance plays a key role in determining product selectivity in the MDA process. The MDA performance reveals that Mo/HMCM-22-MM (mechanical milling) catalyst, with a medium spatial distance between Mo species and HMCM-22 zeolite, significantly inhibits carbon deposition and produces high selectivity to benzene. This work shows that spatial distance between molybdenum and zeolite is an important property for suppressing carbon deposition and improving benzene selectivity in MDA process.

In chapter 2, a new EtOH synthesis route from syngas, has been realized via dimethyl ether (DME) carbonylation with CO to methyl acetate (MA) on zeolite and its further hydrogenation to EtOH on Cu-based catalyst. A nano-sized ZSM-35 (NZ35) zeolite combined with CuZnAl (CZAargon) catalyst were applied in this route. The nano-sized NZ35 zeolite prepare by introducing piperidine (Py) as organic structure-directing agent (OSDAs), possessing abundant active sites and porosity and short diffusion path, is found to realize much better activity of DME to MA than that of the conventional ZSM-35 zeolite (CZ35). The CZAargon catalyst is directly prepared by a simple formic-acid-assisted solid-state method without further hydrogenation, which exhibits excellent hydrogenation ability for converting MA to EtOH. In addition, different integration manners of NZ35 zeolite and CZAargon catalyst were investigated. In this reaction system, only when the NZ35 zeolite and CZAargon catalyst separately pack in a dual-catalyst bed, EtOH can be synthesized from DME and syngas. The optimized reaction conditions are found to be 220 ℃ and 2.5 MPa for EtOH synthesis from syngas and DME with the combination of NZ35 zeolite and CZAargon catalyst. For direct EtOH synthesis in a dual-catalyst bed reactor with tandem NZ35 zeolite and CZAargon catalyst, we get a stable DME conversion of 47.0 % and EtOH selectivity of 45.6 % after reaction for 24 h.

In chapter 3, one-pot selective synthesis of LPG from CO2 has been realized by a Pd/SiO2@S1@-H-ZSM-5 capsule catalyst (Pd/SiO2-SZ). The capsule catalyst (Pd/SiO2-SZ) is fabricated through a dual-layer crystal growth method with an auxiliary of hydrothermal reaction. The one-pot LPG synthesis route was completed by the tandem reactions of methanol synthesis on Pd/SiO2 core catalyst and methanol dehydration to hydrocarbons on the H-ZSM-5 shell. And Silicalite-1 was first coated on the surface of Pd/SiO2 to protect the structure of Pd/SiO2 from etching during the hydrothermal synthesis and as a transition layer for growing the H-ZSM-5 zeolite. The Pd/SiO2-SZ capsule catalyst has a similar mesoporous structure, narrow range of Pd particles size distribution, and consistent reduction characteristics to the Pd/SiO2 core catalyst. It maintains the physical and chemical properties of the core catalyst throughout the H-ZSM-5 shell synthesis process. The Pd/SiO2-SZ capsule catalysts exhibited LPG selectivity of up to 35.6 % by CO2 and CO conversions of 5.5 % and 12.8 %, respectively. Compared with the crushed capsule catalyst (Pd/SiO2-SZP), the well-defined-structured Pd/SiO2-SZ catalyst presented stable catalytic performance in a 50 h reaction owing to the well-matched reactions at the Pd/SiO2 core and H-ZSM-5 shell.

Herein, three types of C1 chemistry conversions and catalysts designs have been successfully realized to transform C1 molecules into valuable chemicals or fuels. The physical-chemical properties and catalytic performances of these efficient catalysts were also studied in detail to shed new insights for catalysts design.

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