1. INTRODUCTION
Since nearly two centuries, extensive use of carbon-based fossil energy
sources such as coal, oil and natural gas has rapidly promoted the
development of human economy and society.1-4 The
widespread use of fossil fuels has led to rising concentrations of
CO2 in the atmosphere, which brings about prominent
problems such as the destruction of the ecological balance and global
warming.5-7 As an easily available source of carbon in
nature, the efficient conversion of CO2 into industrial
raw materials has attracted the attention of many
researchers.6,8 The difficulties of the conversion
from CO2 to hydrocarbon cause from that the
CO2 is a stable chemical substance demanding high energy
to break C-O bonding barrier and the C-C coupling follows the
Anderson-Schulz-Flory (ASF) distribution law which limits selectivity of
the target products.9
The conversion from CO2 to hydrocarbons is mainly
achieved by two ways: one is the reaction process of methanol as an
intermediate product, and another is achieved by a modified
Fischer-Tropsch (FT) reaction.3,10-13 In a
methanol-mediated route, mixtures of CO2 and
H2 have been reacted to form methanol intermediates,
followed by dehydration to hydrocarbons over a
zeolite.14 However, the process suffers from many
problems, in particular, high CO by-product selectivity, low catalytic
activity and poor stability, which hinder its
commercialization.15-17 In terms of FTS route,
CO2 is firstly transformed into CO via reverse water-gas
shift (RWGS) reaction and then the formed CO is subsequently converted
to hydrocarbons.18-20 It has been found that the
reactivities of Ni, Ru, Co, Cu and Fe metals are high, and widely used
to catalyze the hydrogenation of
CO2.7,10,21,22 Especially, Fe-based
can in-situ form Fe3O4 and
FexCy active phase, synergistically
catalyzing RWGS and carbon chain
propagation.10,21,23,24 Yet despite, the ASF
distribution law is not conducive to attaining a high selectivity of
target product in single iron-based catalysts. Generally,
CO2 adsorption occurs on basic sites, thus alkali metal
such as K and Na are applied as promoter to enhance the adsorption
capability and/or activation ability of
CO2.22,25-28 The alkali metal can
improve the electronic environment of iron, which increases the surface
basicity and results in an improvement in CO2adsorption. This method improves the conversion of CO2indeed, but the problem of low liquid hydrocarbon selectivity still
remains (less than 55%).26,27,29 Furthermore, the
incorporation of second active metal (Co or Cu) also exhibits a
facilitating effect by pulling RWGS or chain propagation reaction. The
corresponding catalyst also achieves a high selectivity of liquid
hydrocarbon or yield.30-32
In addition to the systems mentioned above, coupling Fe-based catalysts
and zeolite is an alternative prospective way.22,33-35It is an effective chemical process intensification strategy by coupling
multiple consecutive chemical reactions in a vessel/catalyst under
similar or identical conditions.10,33,35 ZSM-5 have
been employed extensively for the isomerization reactions owing to their
unique steric properties such as MFI topology, porosity, and
acidity.21,36-38 Wei et al. reported a
Na-Fe3O4/HZSM-5 for directly converting
CO2 to gasoline-range hydrocarbons. Three types of
active sites (Fe3O4,
Fe5C2 and acid sites) achieve a
synergetic catalytic conversion of CO2 to
gasoline.39,40 Noreen et al. designed a dual-bed
reaction with SAPO-11 and ZSM-5 coupled individually with the NaFe
catalyst, obtaining a high octane gasoline fuel.41Since zeolite catalysts can directly participate in the catalytic
reaction process, CO2 hydrogenation process can be tuned
by controlling the acidity of zeolite. Brønsted-acid site of the zeolite
catalyst is derived from the tri-coordinated Si-OH-Al bridge hydroxyl
groups on the skeleton and in the pores. The acid site of the zeolite
affects the process of its proton transfer or acceptance of electron
pairs, thereby affecting its catalytic activity.42,43In previous study, we found that H-ZSM-5 treated by metal nitrate
solutions presents different surface acid properties, and the
elimination of strong acids is conducive to the formation of high-carbon
hydrocarbons.18,44 Similarly, through the precise
regulation of zeolite acidity and pore size, it is regarded as an
efficient tool for achieving a promoting effect on the selectivity of
the gasoline hydrocarbon product.41,45-47
Despite olefins undergo reactions such as polymerization, isomerization,
disproportionation, etc. over the acid site of zeolite, there has been
no related report mentioning that how the types of olefins species will
impact the selectivity of products on zeolite for CO2hydrogenation. In addition, most of these traditional iron-zeolite
composite catalysts are generally prepared by physical mixing or
impregnating, which results in uneven distribution of active sites or no
preferred order of reactions.48-50 Contrary to a
catalyst fabricated by physical mixing, a composite with core-shell
structure displays distinct advantages. Constructing a catalyst in which
the core catalyst produces different types of alkenes and the zeolite
has different acidity (that is, the core-shell micro-environment
regulation of the catalyst) has potential heuristic significance for the
utilization of CO2.
Herein, based on rotation coating method, the capsule catalysts are
fabricated with alkali metal modified spinel-like
ZnFe2O4 as the core and an outer
encapsulated ZSM-5 (molar ratio ≈ 25-30) as the shell. During the
reaction process, olefins are formed on spinel-like
ZnFe2O4, then the olefins will migrate
on H-ZSM-5 shell proceeding catalytic reforming to high carbon
hydrocarbons over acidic sites, in which K-modified
ZnFe2O4 have a higher heavy olefins
selectivity than Na-modified ZnFe2O4.
H-ZSM-5 treated by different ions (K and Ce) exchange exhibits enhanced
olefins adsorption capacity, further promoting the formation of
gasoline-range hydrocarbons. K-ZSM-5 contributes to the isomerization
reaction, while Ce-ZSM-5 promotes the aromatization reaction.