A pure silica zeolite molecular sieve was prepared by the Precision Measurement Institute, etc.

Recently, the team of Zheng Anmin from the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences and the team of Xiao Fengshou and Wang Liang from Zhejiang University designed and prepared a catalytic system composed of MFI molecular sieve nanosheets and iron-based catalysts.
Production of light olefins and C ₅ -C ₁₀ olefins
.
July 12
.
The related research results were published in Nature Nanotechnology
.

Olefins are industrially important chemical raw materials, mainly including low-carbon olefins (ethylene, propylene and butene) and long-chain olefins ( _{C 5+} ⁼ ) .
(Fisher-Tropsch synthesis to olefins, FTO) is an important transformation process to obtain fuels and high-value chemicals from coal .
Among them, iron-based catalysts are common FTO catalysts, which are usually carried out at high temperatures above 320 °C .
When the reaction temperature is lower than 300°C, the CO conversion is generally lower, and the product carbon number distribution is broad (C ₁ to C ₂₀₊ hydrocarbons) .

In view of the problems of low low temperature activity and wide product distribution of traditional iron-based catalysts, the scientific research team designed and prepared a catalytic system composed of MFI molecular sieve nanosheets and iron-based catalysts, and realized the low-temperature Fischer-Tropsch high-efficiency production of low-carbon olefins and C5 by iron-based catalysts _.
-C ₁₀ olefin
.
In previous reports, zeolite molecular sieves are often used as tandem catalysts to provide acid sites to participate in reactions, such as cracking, aromatization, isomerization, and carbon-carbon bond coupling processes
.
In this study, it was found that even using pure silicon MFI molecular sieve nanosheets can greatly improve the catalytic activity of iron-based catalysts (CO conversion rate of 82.
6%) at low temperature (260 °C), and obtain high and excellent C ₅ -C ₁₀ Alpha olefin selectivity (74.
0% olefin selectivity, of which 81.
7% of the C ₄₊ olefins were alpha olefins)
.
Under the same conditions, the CO conversion rate of Na-FeC _x
catalyst without zeolite molecular sieve is less than 2% .

To dissect this process, the researchers conducted theoretical simulations to further explore the adsorption and diffusion behavior of olefin molecules on the Na-FeC _x
surface .
Molecular dynamics simulations show that the presence of molecular sieves changes the desorption-resorption equilibrium of olefin molecules, and pulls the reaction forward
.
The slope value of the mean square shift (MSD) quantitatively determines the diffusion coefficient (D _s ) of ethylene molecules in molecular sieve crystals with different layers
.
The results show that the thinner the molecular sieve crystal, the shorter the residence time of the olefin molecules in the molecular sieve pores, which is more conducive to the continuous and rapid transfer of the olefin molecules, thereby improving the reactivity and forming more olefin products
.

It is studied to mix appropriate molecular sieve materials to enable Na-FeC _x catalyst to exhibit high-efficiency low-temperature catalytic activity during FTO process and optimize product distribution
.
The iron carbide and molecular sieve composite catalytic system reported in this work is essentially different from the metal/metal oxide + molecular sieve system reported in the previous literature
.
Experimental data and theoretical studies show that molecular sieves are not used as acid catalysts, but change the desorption-resorption equilibrium of olefin molecules on the surface of iron carbide.
Reasonable control of the morphology and pore environment of molecular sieves can accelerate the desorption of olefin molecules from the Na-FeC _x surface.
In addition, this feature is conducive to the continuous and efficient process of syngas on the Na-FeC _x
surface .
The catalyst system proposed in this study will provide new ideas for the design of heterogeneous catalysts for the synthesis gas conversion process
.

Schematic diagram of FeC _x physical mixed molecular sieve additives to accelerate the desorption of olefin molecules and promote the FTO reaction process

Theoretical simulation reveals the chemical nature of molecular sieve-promoted FTO reaction