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\begin{document}

\title{Spaced-Confined Capsule Catalysts with Tunable Micro-Environments for
Efficient CO2 Conversion}



\author[1]{Hao Wu}%
\author[1]{Lisheng  Guo}%
\author[1]{Xianbiao Wang}%
\author[1]{Wenjie Zhou}%
\author[1]{Fang Chen}%
\author[2]{Da Li}%
\author[3]{Kai Liu}%
\author[4]{Peipei Ai}%
\author[1]{Yuxue Wei}%
\author[1]{Mengdie Cai}%
\author[1]{Song Sun}%
\affil[1]{Anhui University}%
\affil[2]{Linhuan Coking Company Limited}%
\affil[3]{Affiliation not available}%
\affil[4]{Taiyuan University of Technology}%


\vspace{-1em}



  
  \date{May 23, 2024}


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\selectlanguage{english}
\begin{abstract}
CO2 as a greenhouse gas causes a series of issues, and catalytic
utilization of CO2 to fuels is a favorable strategy. Herein, we report
the discovery in CO2 hydrogenation reaction where C5+ yield can be
evidently improved by encapsulating ZnFe2O4 inside ZSM-5, in which the
micro-environments of core-shell components can be tuned. For the
ZnFe2O4, the K promoter makes the Fe-C structure more electron deficient
than the Na, which contributes to the formation of long-chain olefins.
ZSM-5 with K or Ce modification presents enhanced adsorption ability of
alkene, then promoting aromatization and isomerization reactions of
alkenes. Compared with Ce, K-ZSM-5 contributes to isomerization rather
than aromatization, forming more isoparaffins. In this work, regulating
the microenvironment of capsule catalysts provides a new idea for the
design of efficient tandem catalysts, and expands the ability of hybrid
catalysts against other catalysts, thus presenting an excellent
catalytic efficiency for CO2 upgrading.%
\end{abstract}%



\sloppy


Spaced-Confined Capsule Catalysts with Tunable Micro-Environments for
Efficient CO\textsubscript{2} Conversion

Hao Wu \textsuperscript{1}, Lisheng Guo \textsuperscript{1,} *, Xianbiao
Wang \textsuperscript{1}, Wenjie Zhou \textsuperscript{1}, Fang
Chen\textsuperscript{1}, Da Li \textsuperscript{3}, Kai
liu\textsuperscript{3}, Peipei Ai \textsuperscript{2,} *, Yuxue
Wei\textsuperscript{1}, Mengdie Cai \textsuperscript{1}, Song
Sun\textsuperscript{1,} *

\emph{\textsuperscript{1} School of Chemistry and Chemical Engineering,
Anhui University, Hefei 230601, Anhui, People's Republic of China}

\emph{\textsuperscript{2} State Key Laboratory of Clean and Efficient
Coal Utilization, Taiyuan University of Technology, Taiyuan, 030024,
Shanxi, People's Republic of China}

\emph{\textsuperscript{3} Linhuan Coking Company Limited, Huaibei,
235141, Anhui, People's Republic of China}

\emph{E-mail addresses:}

lsguo@ahu.edu.cn (L. S. Guo), aipeipei@tyut.edu.cn (P.P. Ai),
suns@ustc.edu.cn (S. Sun)



\textbf{ABSTRACT}

CO\textsubscript{2}as a greenhouse gas causes a series of issues, and
catalytic utilization of CO\textsubscript{2} to fuels is a favorable
strategy. Herein, we report the discovery in CO\textsubscript{2}
hydrogenation reaction where C\textsubscript{5+} yield can be evidently
improved by encapsulating ZnFe\textsubscript{2}O\textsubscript{4} inside
ZSM-5, in which the micro-environments of core-shell components can be
tuned. For the ZnFe\textsubscript{2}O\textsubscript{4}, the K promoter
makes the Fe-C structure more electron deficient than the Na, which
contributes to the formation of long-chain olefins. ZSM-5 with K or Ce
modification presents enhanced adsorption ability of alkene, then
promoting aromatization and isomerization reactions of alkenes. Compared
with Ce, K-ZSM-5 contributes to isomerization rather than aromatization,
forming more isoparaffins. In this work, regulating the microenvironment
of capsule catalysts provides a new idea for the design of efficient
tandem catalysts, and expands the ability of hybrid catalysts against
other catalysts, thus presenting an excellent catalytic efficiency for
CO\textsubscript{2} upgrading.

\textbf{Keywords}

Capsule catalyst; CO\textsubscript{2} hydrogenation; Zeolite; Tunable
micro-environments; Olefins



\textbf{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.\textsuperscript{1-4} The
widespread use of fossil fuels has led to rising concentrations of
CO\textsubscript{2} in the atmosphere, which brings about prominent
problems such as the destruction of the ecological balance and global
warming.\textsuperscript{5-7} As an easily available source of carbon in
nature, the efficient conversion of CO\textsubscript{2} into industrial
raw materials has attracted the attention of many
researchers.\textsuperscript{6,8} The difficulties of the conversion
from CO\textsubscript{2} to hydrocarbon cause from that the
CO\textsubscript{2} 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.\textsuperscript{9}

The conversion from CO\textsubscript{2} 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.\textsuperscript{3,10-13} In a
methanol-mediated route, mixtures of CO\textsubscript{2} and
H\textsubscript{2} have been reacted to form methanol intermediates,
followed by dehydration to hydrocarbons over a
zeolite.\textsuperscript{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.\textsuperscript{15-17} In terms of FTS route,
CO\textsubscript{2} is firstly transformed into CO via reverse water-gas
shift (RWGS) reaction and then the formed CO is subsequently converted
to hydrocarbons.\textsuperscript{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
CO\textsubscript{2}.\textsuperscript{7,10,21,22} Especially, Fe-based
can \emph{in-situ} form Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{x}C\textsubscript{y} active phase, synergistically
catalyzing RWGS and carbon chain
propagation.\textsuperscript{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,
CO\textsubscript{2} 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
CO\textsubscript{2}.\textsuperscript{22,25-28} The alkali metal can
improve the electronic environment of iron, which increases the surface
basicity and results in an improvement in CO\textsubscript{2}adsorption.
This method improves the conversion of CO\textsubscript{2}indeed, but
the problem of low liquid hydrocarbon selectivity still remains (less
than 55\%).\textsuperscript{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.\textsuperscript{30-32}

In addition to the systems mentioned above, coupling Fe-based catalysts
and zeolite is an alternative prospective
way.\textsuperscript{22,33-35}It is an effective chemical process
intensification strategy by coupling multiple consecutive chemical
reactions in a vessel/catalyst under similar or identical
conditions.\textsuperscript{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.\textsuperscript{21,36-38} Wei et al. reported a
Na-Fe\textsubscript{3}O\textsubscript{4}/HZSM-5 for directly converting
CO\textsubscript{2} to gasoline-range hydrocarbons. Three types of
active sites (Fe\textsubscript{3}O\textsubscript{4},
Fe\textsubscript{5}C\textsubscript{2} and acid sites) achieve a
synergetic catalytic conversion of CO\textsubscript{2} to
gasoline.\textsuperscript{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.\textsuperscript{41}Since zeolite catalysts can directly
participate in the catalytic reaction process, CO\textsubscript{2}
hydrogenation process can be tuned by controlling the acidity of
zeolite. Br\selectlanguage{ngerman}ønsted-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.\textsuperscript{42,43}In 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.\textsuperscript{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.\textsuperscript{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
CO\textsubscript{2}hydrogenation. 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.\textsuperscript{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 CO\textsubscript{2}.

Herein, based on rotation coating method, the capsule catalysts are
fabricated with alkali metal modified spinel-like
ZnFe\textsubscript{2}O\textsubscript{4} as the core and an outer
encapsulated ZSM-5 (molar ratio \selectlanguage{english}[?] 25-30) as the shell. During the
reaction process, olefins are formed on spinel-like
ZnFe\textsubscript{2}O\textsubscript{4}, then the olefins will migrate
on H-ZSM-5 shell proceeding catalytic reforming to high carbon
hydrocarbons over acidic sites, in which K-modified
ZnFe\textsubscript{2}O\textsubscript{4} have a higher heavy olefins
selectivity than Na-modified ZnFe\textsubscript{2}O\textsubscript{4}.
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.



\textbf{2. EXPERIMENTAL SECTION}

\textbf{2.1. Chemical.} All chemicals were purchased from chemical
companies and used without further purification: Iron (III) nitrate
nonahydrate (Fe
(NO\textsubscript{3})\textsubscript{3}*9H\textsubscript{2}O, AR 98.5\%,
Sinopharm Chemical Reagent Co., Ltd. China), Zinc nitrate hexahydrate
(Zn (NO\textsubscript{3})\textsubscript{2}*6H\textsubscript{2}O), AR
99\%, Sinopharm Chemical Reagent Co., Ltd. China), Sodium hydroxide
(NaOH, AR, 96\%, DAMAO CHEMICAL REAGENT FACTORY. China), Potassium
hydroxide (KOH, AR, 85\%, Sinopharm Chemical Reagent Co., Ltd. China),
ZSM-5
((SiO\textsubscript{2})\textsubscript{x}(Al\textsubscript{2}O\textsubscript{3})\textsubscript{y},
SiO\textsubscript{2}/Al\textsubscript{2}O\textsubscript{3} (mole ratio)
\selectlanguage{english}[?] 25-30, Shanghai MackLin Biochemical Technology Co., Ltd, China),
LUDOX\textsuperscript{(r)}SM Colloidal Silica (30 wt\%, Grace Trading
(Shanghai) Co., Ltd. China), Potassium nitrate (KNO\textsubscript{3},
\textgreater{} 99.0\%, Sinopharm Chemical Reagent Co., Ltd. China),
Cerium nitrate hexahydrate
(Ce(NO\textsubscript{3})\textsubscript{3}*6H\textsubscript{2}O, AR,
99.5\%, Shanghai MackLin Biochemical Technology Co., Ltd, China), were
selected to fabricate the samples.

\textbf{2.2} \textbf{. Synthesis of
ZnFe\textsubscript{2}O\textsubscript{4}.} The synthesis of spinel-like
ZnFe\textsubscript{2}O\textsubscript{4} catalyst was referred to our
previous report by a solvent-thermal method. Typically, 2.02g iron (III)
nitrate hexahydrate and 0.74g zinc nitrate hexahydrate (Zn: Fe=1: 2
molar radio) were dissolved in distilled water (40 mL), and 0.1 mol NaOH
was added into aqueous solutions to ensure alkaline solution.
Transferring the solution to a 100 mL Teflon tube being set in a
stainless autoclave and placing it in an oven at 180 degC for 8 h for the
synthesis reaction. After the product was cooled to ambient temperature,
the product was washed with 0.5 L deionized water to control the amount
of Na and dried at 60 degC, denoted as
Na-ZnFe\textsubscript{2}O\textsubscript{4}. Instead of NaOH with KOH,
K-ZnFe\textsubscript{2}O\textsubscript{4} can be obtained by the same
procedure.

\textbf{2.3. Synthesis of
ZnFe\textsubscript{2}O\textsubscript{4}@H-ZSM-5.} The
ZnFe\textsubscript{2}O\textsubscript{4}@H-ZSM-5 composite catalyst with
a core-shell structure coupled modified spinel-like
ZnFe\textsubscript{2}O\textsubscript{4} catalyst and H-ZSM-5 catalyst by
spin coated method. Transferring the 0.2 g
ZnFe\textsubscript{2}O\textsubscript{4} catalyst (Na or K) wetted with
colloidal silica (30 wt \%) into a round-bottom flask. Subsequently,
0.15 g ZSM-5 was added into the flask, and spinning round-bottom flask
made H-ZSM-5 and ZnFe\textsubscript{2}O\textsubscript{4} in intimate
contact\textsubscript{,} forming a composite catalyst with a capsule
structure.

\textbf{2.4. Synthesis of M-ZSM-5 (K, Ce).} Surface acid properties of
parent H-ZSM-5 zeolite (Si/Al = 25-30) was treatment through different
ions-exchange strategies. Prior to treatment, H-ZSM-5 was calcinated at
550 degC to remove adsorbed water molecules. Subsequently, H-ZSM-5 (1.0g)
was directly treated by one of nitrate solution of K or Ce ion (100 mL,
0.2 mol/L) at 80 degC for 12 h. The filtered H-ZSM-5 was rinsed several
times with plenty of deionized water, and then it was calcined at 550 degC
for 5 h. Finally, the obtained products were labelled as M-ZSM-5, in
which M stands for the exchanged ion. 1 g K-ZSM-5 was treated in 100ml
of NH\textsubscript{3}*H\textsubscript{2}O (0.2 mol/L) solution at 80 degC
for 12 h, to obtain H-ZSM-5*.

\textbf{2.5. Catalysis evaluation.} The catalytic reactions were carried
out in a stainless steel fixed-bed reactor. Typically, 0.35 g of
composite catalyst (20-40 mesh),
M-ZnFe\textsubscript{2}O\textsubscript{4}@M-ZSM-5, was used. Prior to
reaction, the catalyst was \emph{in situ} reduced in pure
H\textsubscript{2} with 50 mL/min at atmospheric pressure, 400 degC for 6
h. After reduction, the temperature was cooled to 320 degC and the
reactant gas with H\textsubscript{2}/CO\textsubscript{2} ratio = 3/1
(24.15\% CO\textsubscript{2}, 71.71\% H\textsubscript{2}, and 4.14\% Ar
was employed as an internal standard) was switched into the reactor. The
reactions were conducted at 320 degC, 2.0 MPa with a reaction gas flow
rate of 20 mL/min.

CO\textsubscript{2} conversion and CO selectivity were analyzed by an
online gas chromatograph (GC) using a thermal conductivity detector
(TCD, Anhui Chromatographic instrument GC5190). The light hydrocarbons
were analyzed by an online GC with a flame ionization detector (FID,
Anhui Chromatographic instrument GC5190). N-octane (C\textsubscript{8})
as solvent was equipped to capture the heavy hydrocarbons in the
effluents, and the products were analyzed by an off-line GC using an
FID. The detailed product distribution was calculated as previously
reported.

CO\textsubscript{2} conversion, CO selectivity, and hydrocarbon
selectivity were calculated on a molar carbon basis:

\begin{equation}
\mathrm{\text{CO}}_{\mathrm{2}}\mathrm{\text{\ conversion}}\mathrm{\ }\mathrm{(\%)=}\frac{\mathrm{\text{CO}}_{\mathrm{2\ in}}\mathrm{-}\mathrm{\text{CO}}_{\mathrm{2\ out}}}{\mathrm{\text{CO}}_{\mathrm{2\ in}}}\mathrm{\times 100\%}\nonumber \\
 \end{equation}

Where the CO\textsubscript{2in} and CO\textsubscript{2out} represent the
moles of CO\textsubscript{2} at the inlet and outlet, respectively.

\begin{equation}
\mathrm{\text{CO\ selectivity}}\mathrm{\ }\mathrm{(\%)=}\frac{\mathrm{\text{CO\ }}_{\mathrm{\text{out}}}}{\mathrm{\text{CO}}_{\mathrm{2\ in}}\mathrm{-}\mathrm{\text{CO}}_{\mathrm{2\ out}}}\mathrm{\times 100\%}\nonumber \\
 \end{equation}

Where the CO\textsubscript{out} represent the moles of CO at the outlet.

The selectivity of hydrocarbons (C\textsubscript{i}) in total
hydrocarbons was obtained by the following formula:

\begin{equation}
\mathrm{C}_{\mathrm{\text{i\ }}}\mathrm{\text{hydrocarbon\ selectivity}}\mathrm{\ }\mathrm{(C-mol\%)=}\frac{\mathrm{\text{mol\ of}}\ \mathrm{C}_{\mathrm{i}}\mathrm{\ \times i}}{\sum_{\mathrm{i=1}}^{\mathrm{n}}{\mathrm{\text{mol\ of}}\ \mathrm{C}_{\mathrm{i}}\mathrm{\times i}}}\nonumber \\
 \end{equation}

The space-time yield (STY) of C\textsubscript{5+} hydrocarbons on
catalyst was obtioned by the following formula:

\begin{equation}
\mathrm{STY=}\frac{\mathrm{\text{CO}}_{\mathrm{2}}\mathrm{\ Conversion\times\ }\left(\mathrm{1-CO\ selectivity\ }\right)\mathrm{\times}\mathrm{C}_{5+}\mathrm{\text{hydrocarbon\ selecitivity}}}{\mathrm{Catalyst\ mass\ (g)}}\nonumber \\
 \end{equation}

\textbf{2.6. Catalyst Characterization.} X-ray diffractometer (SmartLab
9kW) with Cu K\selectlanguage{greek}α \selectlanguage{english}radiation was used to obtain diffraction patterns. The
surface morphologies of the catalysts were investigated using scanning
electron microscopy (SEM, Regulus 8230). Transmission electron
microscopy (TEM, JEOL JEM-2100F) was used to observe particle size sand
distribution at an acceleration voltage of 100 kV. The adsorption
behavior of the catalyst was measured by using a Micromeritics AutoChem
II Chemisorption Analyzer 2920. AutoChem II analyzer with a thermal
conductivity detector (TCD) was used to acquire
H\textsubscript{2}temperature programmed reduction
(H\textsubscript{2}-TPR) profiles. A 50 mg sample was first pretreated
with He for 1 h at 200 \selectlanguage{ngerman}°C. When the temperature was cooled to 50 °C, a
5vol \% H\textsubscript{2}/Ar gas mixture was supplied to the reactor
(30 mL/min). Finally, H\textsubscript{2}-TPR profiles were obtained at
temperatures ranging from 50 to 700 °C, with a heating rate of 10
°C/min. The same equipment was also used to investigate the
CO\textsubscript{2} or NH\textsubscript{3} temperature-programmed
desorption (TPD). A 50-mg sample was reduced for 2 h at 400 °C under a
H\textsubscript{2} gas flow (30mL/min). The temperature of the catalysts
was reduced to 50 °C under a He gas flow (30 mL/min) after reduction.
There actor was subsequently filled with a 5\% CO\textsubscript{2}/He or
5\% NH\textsubscript{3}/He gas mixture for 1h. He was then inserted into
the reactor to remove the physically adsorbed CO\textsubscript{2} or
NH\textsubscript{3}. The CO\textsubscript{2}-TPD and
NH\textsubscript{3}-TPD profiles were recorded from 50 to 800 °C with a
heating rate of 10 °C/min. A Thermo Fischer Scientific ESCALAB 250 Xi
instrument with a catalyst pretreatment chamber for altering the gas
composition was used to conduct the X-ray photoelectron spectroscopy
(XPS) analysis. The XAFS was recorded in table XAFS by Chuang Pu
specreation. TG was tested by STA 449C Jupiter\textsuperscript{®}.
N\textsubscript{2} physisorption was performed on a ASAP 2460 surface
area \& pore size analyzer. Prior to texts, the samples were degassed at
200 °C under vacuum conditions. Pyridine adsorption (Py-IR) spectra were
recorded with a Thermo IS50 IR spectrometer from Thermo Scientific to
the measurement, the sample cell was vacuumed to 10\textsuperscript{-2}
Pa at 400 °C for 1 h, and the IR spectra were recorded at 350 °C. The
\textsuperscript{27}Al magic-angle spinning (MAS) NMR was recorded by
Bruker Avance 600 AV. In situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) spectra were recorded on a Thermo iS50
FT-IR spectrometer. Before test, the sample was \emph{in situ} reduced
at 400 °C under 10 vol\% H\textsubscript{2} in Ar (50 mL/min) for 2 h
and then switched to He for 15 minutes (50 mL/min). After that, the
reaction gas was introduced into the reaction cell for reaction.
Finally, at atmospheric and 320 °C, 20 mL/min of mixture gas (24 vol\%
CO\textsubscript{2}, 70 vol\% H\textsubscript{2} and 6 vol\% Ar) passed
through and DRIFT spectra were recorded. To the M-ZSM-5, the sample was
\emph{in situ} heating at 320 °C under He for 15 minutes (50 mL/min) and
then switched to 20 mL/min of mixture gas as 10 vol\%
C\textsubscript{3}H\textsubscript{6}, 90 vol\% Ar passed through and
DRIFT spectra were recorded at atmospheric, 320 °C. After 10minutes, the
mixture gas is stopped and the data is recorded for 20 minutes. Before
performing DFT calculations, the composition and structure of the
smallest monomer can be determined from literature and experimental
results. Based on the determined minimum monomer structure, the smallest
monomer is extracted from the polyatomic structure using the super cell
builder in CASTEP, a sampling tool provided in the calculation software.
This usually requires the setting of periodic boundary conditions and
the adjustment of unit cell parameters to ensure the periodicity and
compatibility of the smallest monomer structure. Structural
optimization: after extracting the smallest monomer structure,
structural optimization can be performed in the calculation software to
determine the most stable zeolite structure. Then the adsorption energy
is calculated.

\textbf{3. RESULTS AND DISCUSSION}

\textbf{3.1. Characterization of
ZnFe\textsubscript{2}O\textsubscript{4}@ZSM-5}

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

\textbf{Figure 1.} \textbf{(a)} XRD patterns of different
ZnFe\textsubscript{2}O\textsubscript{4} catalysts; \textbf{(b)} fresh
and spent ZnFe\textsubscript{2}O\textsubscript{4}@H-ZSM-5,
\textbf{(c)}XRD patterns of H-ZSM-5*, K-ZSM-5, La-ZSM-5, Ce-ZSM-5,
H-ZSM-5.

The tailor-made capsule catalyst consists of two components:
ZnFe\textsubscript{2}O\textsubscript{4} as core catalyst and ZSM-5 as
shell catalyst. XRD patterns of
ZnFe\textsubscript{2}O\textsubscript{4}and capsule catalysts were shown
in Figure 1. As seen, the main phase of core catalyst is mainly
ZnFe\textsubscript{2}O\textsubscript{4} apart from a small amount of
Fe\textsubscript{2}O\textsubscript{3}, when NaOH as an alkaline
thermal-solvent is used. It has been reported that the crystallinity of
spinel species increases as the concentration of Na
decreases.\textsuperscript{28} By contrary, the main Fe phase is
ZnFe\textsubscript{2}O\textsubscript{4} without
Fe\textsubscript{2}O\textsubscript{3} formation for
K-ZnFe\textsubscript{2}O\textsubscript{4}, which indicates that KOH is
more conducive to the formation of spinel structure. These phenomena are
also consistent with our previous findings about
ZnFe\textsubscript{2}O\textsubscript{4}.\textsuperscript{24} As drawn in
Figure 1b, the ZnFe\textsubscript{2}O\textsubscript{4}@ZSM-5 capsule
catalyst and H-ZSM-5 exhibit same diffraction peaks that 2\selectlanguage{greek}θ \selectlanguage{english}value of
8.0, 8.9, 23.3, 23.9, and 24.4\textsuperscript{o} (JCPDS 44-0003),
suggesting that the MFI topology structure of ZSM-5 zeolite is not
destroyed during the encapsulation process of
ZnFe\textsubscript{2}O\textsubscript{4} particles. In addition, XRD
spectra of the capsule catalyst have no obvious peaks of Fe phase,
indicating that the surface of the nuclear catalyst is completely
wrapped by ZSM-5 shell. To verify the structure of the capsule catalyst,
the surface morphology of the
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 capsule catalyst was
analyzed by SEM in Figure 2d-i. An obvious boundary between the two
different components can be observed in the cut section of the capsule
catalyst where having different elements distribution profiles in
mapping images that Zn, Fe belong to
K-ZnFe\textsubscript{2}O\textsubscript{4} and Si, Al belong to K-ZSM-5
presented. Zn and Fe elemental signals are obviously surrounded by Al
and Si elemental from mapping images of the
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 capsule catalyst,
which indicates that the ZnFe\textsubscript{2}O\textsubscript{4} core
catalyst is wrapped inside by K-ZSM-5 shell catalyst (Figure 2e). These
above results certify that the capsule structure catalyst with the
ZnFe\textsubscript{2}O\textsubscript{4} as core componets uniformly
encapsulated by K-ZSM-5 has been successfully synthesized.

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\verb`image2.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

\textbf{Figure 2.} Fe-2p XPS spectra of \textbf{(a)} as-prepared
and\textbf{(b)} spent K-ZnFe\textsubscript{2}O\textsubscript{4} and
Na-ZnFe\textsubscript{2}O\textsubscript{4}; \textbf{(c)} the XANES
spectra of Fe K-edge in spent
ZnFe\textsubscript{2}O\textsubscript{4};\textbf{(d)} SEM images of
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5; \textbf{(e)} the
mapping of all the elements over the above section; \textbf{(f)} Fe
elemental distribution; \textbf{(g)} Zn elemental
distribution;\textbf{(h)} Al elemental distribution; \textbf{(i)} Si
elemental distribution.

Compared with as-preapred ZnFe\textsubscript{2}O\textsubscript{4}@ZSM-5
catalyst, phase composition of spent capsule catalyst still remains the
MFI structure of ZSM-5, which illustrates that the core-shell structure
of the capsule catalyst can maintains excellent physical stability
(Figure 1). After reaction, the Zn and Fe in
ZnFe\textsubscript{2}O\textsubscript{4} are transformed into ZnO into
Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{5}C\textsubscript{2}, respecively. Thereinto, Zn acts
as a structure and electronic promoter can enhance the basicity and thus
increases light olefins selectivity. In general,
Fe\textsubscript{3}O\textsubscript{4}, as the active phase for RWGS
reaction, promotes the CO\textsubscript{2} molecules into CO
intermediates, while the co-existence of
Fe\textsubscript{5}C\textsubscript{2} as crucial active phase makes the
chain propagation to form hydrocarbons. As a consequence, different
configuration compositions of Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{5}C\textsubscript{2} can regulate
CO\textsubscript{2}conversion performance, leading to clear distinction
in activity and selectivity. Besides, the SEM images of
Na-ZnFe\textsubscript{2}O\textsubscript{4} and
K-ZnFe\textsubscript{2}O\textsubscript{4} were shown in Figure S1. As
shown in Figure 2b, spent
ZnFe\textsubscript{2}O\textsubscript{4}catalysts were performed by a
X-ray photoelectron spectroscopy (XPS). The binding energy peaks at
706.4 eV, 710.6 eV, and 712.3 eV are attributed to
Fe\textsubscript{5}C\textsubscript{2} species, Fe(II), and Fe(III),
respecitively. Compared with Fe-2p XPS spectra of as-prepared, highly
valenced iron oxide species are converted to carbides and
Fe\textsubscript{3}O\textsubscript{4}, crucial active phases for
CO\textsubscript{2} conversion (Figure 2a). It is worth noting that the
introduction of Na causes the Fe-C peak shift to a direction with low
binding energy. According to the relative content of Fe-2p in different
XPS, the spent Na-ZnFe\textsubscript{2}O\textsubscript{4} have more
iron-carbon bonds content than spent
K-ZnFe\textsubscript{2}O\textsubscript{4} (Table S1). HR-TEM images of
spent K-ZnFe\textsubscript{2}O\textsubscript{4} and
Na-ZnFe\textsubscript{2}O\textsubscript{4} catalyst were shown in Figure
S2. The 4.64\selectlanguage{ngerman}Å and 2.65 Å are belong to
Fe\textsubscript{3}O\textsubscript{4} (111) and
Fe\textsubscript{5}C\textsubscript{2} (311), which coincides with the
results of XRD and XPS. Obviously, the co-existence of
Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{5}C\textsubscript{2} together push the reaction
forward. The morphologies and structures of the core Fe-based catalyst
before and after the reaction were also explored. After reaction, the
structure of bulk ZnFe\textsubscript{2}O\textsubscript{4} present
uniform dispersion of small particles, assigning to the dynamic
transformation of ZnFe\textsubscript{2}O\textsubscript{4} spinel
structure (Figure S1). Besides, after K ions exchange, the morphologies
and structures of zeolite change slightly (Figure S3). For a zeolite
treated by Ce ions, there are a few crystals on the surface of the
zeolite (Figure S4). Moreover, TEM images of zeolites with different
ions exchange strategies were compared in Figure S5. It can be clearly
found that the exchanged metal ions are evenly distributed in the
zeolite. Moreover, the content of the exchanged ions in the zeolite is
relatively low (Table S2).

Fe K-edge XANES was used to investigate the nature and coordination
properties of Fe species in the spent
ZnFe\textsubscript{2}O\textsubscript{4} catalyst under the relevant
operating conditions (Figure 2c and S6). The normalized XANES spectra of
the Fe K-edge in ZnFe\textsubscript{2}O\textsubscript{4} are given in
Figure 2c; and the data for Fe foil,
ZnFe\textsubscript{2}O\textsubscript{4},
Fe\textsubscript{2}O\textsubscript{3},
Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{5}C\textsubscript{2} are also presented. In the XANES
spectra, the K-ZnFe\textsubscript{2}O\textsubscript{4} shifts to a
higher energy than Na-ZnFe\textsubscript{2}O\textsubscript{4},
illustrating that K promoter is conducive to the transition of Fe phase
to a high valence state of Fe species. The introduction of additives can
enhance the electronic transition between active phases and raw
molecules, and then achieve the regulation of product selectivity during
catalytic reactions. In the wavelet detail of spent
ZnFe\textsubscript{2}O\textsubscript{4} ,the Fe has the same
coordination in Figure S6c and S6d. The difference is that the
introduction of K promoter lengthens the number of wave vectors and
strengthens the degree of transition of the catalyst to Fe-C during the
reaction.

Besides, H\textsubscript{2}-TPR patterns of as-prepared
Na-ZnFe\textsubscript{2}O\textsubscript{4} and
K-ZnFe\textsubscript{2}O\textsubscript{4} were compared in Figure S7a.
Compared with Na modification, the introduction of K promoter is
silghtly conducive to the reduction behavior of iron species. Meanwhile,
the CO\textsubscript{2}-TPD profiles were shown in Figure S7b. It can be
found that all the catalysts of
K-ZnFe\textsubscript{2}O\textsubscript{4} and
Na-ZnFe\textsubscript{2}O\textsubscript{4} have obvious weak adsorption
and moderate adsorption.
K-ZnFe\textsubscript{2}O\textsubscript{4}exhibits a better absorption of
CO\textsubscript{2} than Na-ZnFe\textsubscript{2}O\textsubscript{4},
which because of K has a stronger adsorption capacity for
CO\textsubscript{2} than Na or more
ZnFe\textsubscript{2}O\textsubscript{4} phases, which will adsorb more
CO\textsubscript{2} than single
Fe\textsubscript{2}O\textsubscript{3}phase. It demonstrates that
K-ZnFe\textsubscript{2}O\textsubscript{4}have a high active capability
for CO\textsubscript{2} conversion.
K-ZnFe\textsubscript{2}O\textsubscript{4} also exhibits a better
absorption of CO than Na-ZnFe\textsubscript{2}O\textsubscript{4} (Figure
S7c).

\textbf{3.2. Catalytic performance of
ZnFe\textsubscript{2}O\textsubscript{4}@ZSM-5}

The results of catalytic performances were presented in Figure 3a and
Table S3. Na-ZnFe\textsubscript{2}O\textsubscript{4} catalyst mainly
produces C\textsubscript{2}-C\textsubscript{4} olefins, in which
CH\textsubscript{4} selectivity is 16.6\%,
C\textsubscript{2}-C\textsubscript{4} selectivity is 53.3\% and
C\textsubscript{5+} selectivity reaches 30.1\% at a
CO\textsubscript{2}conversion of 28.3\%. When K promoter is introduced,
C\textsubscript{5+}selectivity increases from 30.1\% to 49.2\%, in which
CH\textsubscript{4} is 11.0\%,
C\textsubscript{2}-C\textsubscript{4}selectivity is 39.8\% at a
CO\textsubscript{2} conversion of 32.7\%. In our previous reports, we
found that Na-modified ZnFe\textsubscript{2}O\textsubscript{4} catalyst
facilitates the formation of low-carbon olefins, while K-modification
one facilitates the formation of high-carbon
hydrocarbons.\textsuperscript{24} Based on the above discussion (Figure
2b, 2c, and S7), the performance difference is due to the improved
adsorption capacity of CO\textsubscript{2}modified with K promoter and
the Fe-C structure of carbides more electron deficient. After
encapsulation by H-ZSM-5 zeolite shell, the conversion of
C\textsubscript{2}-C\textsubscript{4} olefins selectivity decrease,
whereas the selectivities of CO and CH\textsubscript{4}slightly
increase. After Ce ions exchange, C\textsubscript{5+}selectivity of
K-ZnFe\textsubscript{2}O\textsubscript{4}@Ce-ZSM-5 increases from 47.8\%
to 59.9\%. Meanwhile, the C\textsubscript{2}-C\textsubscript{4} olefins
decreased profoundly than the parent H-ZSM-5 (Figure 3a). With the
treatment of K, the performance difference between K and Ce modification
are obvious. The selectivity of C\textsubscript{5+} hydrocarbon climbs
to 71.7\% from 59.9\%, producing more liquid fuels than Ce ions
treatment. It indicates that changing the microenvironment of the
zeolite catalysts by ions exchange is a feasible strategy for regulating
products distribution. Catalytic stability of the
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 catalyst was
investigated and depicted in Figure 3b. As seen, the catalyst exhibits a
benign stability during the 80h reaction period. Liquid hydrocarbons
selectivity (C\textsubscript{5+}) maintains above 70\%, and
CO\textsubscript{2} conversion as well as un-desired CO byproduct almost
keep stable. Comparing with other metal oxide/zeolite composite
catalysts, the designed
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 catalyst present a
record-breaking C\textsubscript{5+}-yield based on per gram catalyst
(Figure S8 and Table S4). It indicates that the capsule catalyst
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 is a promising
catalyst for efficiently catalyzing CO\textsubscript{2} hydrogenation to
liquid fuels.

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\verb`image3.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

\textbf{Figure 3. (a)} Catalytic performances over different
catalysts;\textbf{(b)} the catalytic stability of
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 catalyst;
\textbf{(c)}detailed hydrocarbon distribution over bi-functional
catalysts with different ions-exchange strategies for zeolites
(K-ZnFe\textsubscript{2}O\textsubscript{4},
K-ZnFe\textsubscript{2}O\textsubscript{4}@H-ZSM5,
K-ZnFe\textsubscript{2}O\textsubscript{4}@Ce-ZSM-5 and
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5); \textbf{(d)} effects
of contacting manner on catalytic performance. Reaction conditions,
ZnFe\textsubscript{2}O\textsubscript{4} to ZSM-5 is 0.2g to 0.15g, 2.0
MPa, 320 \textsuperscript{o}C, 6000
mL·g\textsuperscript{-1}·h\textsuperscript{-1} for
ZnFe\textsubscript{2}O\textsubscript{4},
H\textsubscript{2}/CO\textsubscript{2} = 3.

The gasoline-range hydrocarbons refer to high octane number
hydrocarbons, e.g. aromatics and isoparaffins as a highly recognized
octane contributor. Octane rating on isoparaffins increases with the
number of branches, and such multibranched isomers synthesis are
preferred in CO\textsubscript{2} conversion. As shown in the Figure 3c,
the main product of K-ZnFe\textsubscript{2}O\textsubscript{4} is
olefins-rich product, which occupies 64.8\% in all hydrocarbons. After
K-ZnFe\textsubscript{2}O\textsubscript{4} catalyst encapsuled by H-ZSM-5
shell, the selectivity of gasoline hydrocarbons in the product changes
slightly. However, for the types of hydrocarbon product, the reduction
in the proportion of olefins in all hydrocarbons is obvious, while the
selectivity of isoparaffins and aromatics in the gasoline range
increases. The effect can be ascirbed to the introduction of ZSM-5,
which increases the selectivities of isoparaffins and aromatics with the
help of its pore structure and acidic sites. Compared with H-ZSM-5, the
selectivity of C\textsubscript{5+} hydrocarbons increases by 10\% with
the introduction of Ce. Besides, the proportion of isoparaffins and
aromatics still increases in whole C\textsubscript{5+} hydrocarbons.
However, CH\textsubscript{4} selectivity is higher than
K-ZnFe\textsubscript{2}O\textsubscript{4}, which maybe because of the
diffusion of the hydrocarbon product via a core-shell structure. The
products of K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 are
aromatics as main component in C\textsubscript{5+} hydrocarbons (Figure
3c). More importantly, the ratio of isoparaffins to aromatics gradually
increases with the change of M-ZSM-5 (from H-ZSM-5 to Ce-ZSM-5 to
K-ZSM-5). It supports that the olefins generated on the surface of
K-ZnFe\textsubscript{2}O\textsubscript{4} catalyst undergo
polycondensation, isomerization, aromatization reactions through the
acidic site of ZSM-5. Meanwhile, comparing the effects of zeolites with
different ions modifications on the selectivity of target hydrocabon,
verifies that K modified ZSM-5 exhibits evidently promoting effect for
the oriented production of C\textsubscript{5+} hydrocarbons.

Previously, different contacting manners of composite catalysts, such as
physical mixing and multiple beds, will influence matching combination
between different active sites, which in turn will affect the catalytic
performance.\textsuperscript{51,52} It has been reported that a catalyst
with a core-shell structure can enhance mass and heat transfer during
the reaction comparing with one fabricated by physical mixing
manner.\textsuperscript{49} The effect of contacting manner between
K-ZnFe\textsubscript{2}O\textsubscript{4} and K-ZSM-5 was investigated.
Results of different contacting manners including core-shell catalysts,
powder mixing, granule mixing, and dual bed were shown and summarized in
the Figure 3d and Table S5. As for a powder mixing one
(K-ZnFe\textsubscript{2}O\textsubscript{4} and K-ZSM-5 are physically
mixed firstly and then the mixtures are granulated to obtain 20-40
mesh), the selectivity of C\textsubscript{5+} is only 20.5\%. When
K-ZnFe\textsubscript{2}O\textsubscript{4} and K-ZSM-5 are integrated by
granule mixing or dual bed, the selectivity of
C\textsubscript{5+}hydrocarbons (about 62\%) over both catalysts are
evidently higher than physical mixing one, but lower than the capsule
catalyst of K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5.
Evidently, the capsule structure of
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 exhibits an excellent
CO\textsubscript{2} hydrogenation performance, especially
C\textsubscript{5+} selectivity. Interestingly, these two kinds
contacting manner both have a slight high CO\textsubscript{2}conversion
than capsule catalyst. It is possible that the direct exposure of the
ZnFe\textsubscript{2}O\textsubscript{4} catalyst to the reaction
atmosphere, and the diffusion influence of reaction gases in zeolite
pore is reduced, which improves the utilization of reaction gases.

As discussed above, K-ZnFe\textsubscript{2}O\textsubscript{4} catalyst
coated by K-ZSM-5 shell presents an improved peformacne for
CO\textsubscript{2} hdyrogenation. Then, the effect of zeolite shell
thickness on catalytic performance were further investigated. With the
increase of shell thickness, the particle sizes of capusle catalysts
increases obviously, which clearly indicates that the core
K-ZnFe\textsubscript{2}O\textsubscript{4} catalyst was coated with more
zeolite (Figure S10). When the mass ratio of zeolite to Fe-based
catalyst is 1:1, the K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5
shows the best performance (Figure 4a and Table S6). With the further
increase of zeolite thickness, the selectivity of long-chain hydrocarbon
significantly decreases, which can be ascribed to the overcracking of
long-chain products (Figure 4b and 4c).

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\textbf{Figure 4.} \textbf{(a)} Effects of zeolite shell thickness on
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 catalytic
performance;\textbf{(b)} Paraffins composition over composite catalysts
with different shell thickness; \textbf{(c)} Detailed product
distribution over composite catalysts with different shell thickness.

TG analysis of spent ZnFe\textsubscript{2}O\textsubscript{4} and
zeolites was shown in Figure S11. The spent
K-ZnFe\textsubscript{2}O\textsubscript{4} have more more obvious degree
of carbonization than spent Na-ZnFe\textsubscript{2}O\textsubscript{4},
and the rising curve is the process by which iron-carbon compounds is
converted to ferrites. When K-ZnFe\textsubscript{2}O\textsubscript{4} is
coupled with different ZSM-5, there are more iron-carbon compounds on
K-ZSM-5 after the reaction, not only carbon deposits in H-ZSM-5. With
the introduction of K-ZSM-5,
Na-ZnFe\textsubscript{2}O\textsubscript{4}has more carbon deposition
resulting in reduced activity, and
K-ZnFe\textsubscript{2}O\textsubscript{4} is not easily sintered.

Among different zeolite treated by K ion exchange startegy, the
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 exhibited an excellent
performance of CO\textsubscript{2} hydrogenation. The catalytic
performances of K-ZnFe\textsubscript{2}O\textsubscript{4} coupled with
different kinds of K-modified zeolites were shown in Figure S12 and
Table S7. The introduction of K-ZSM-5 in composite system displayed the
best C\textsubscript{5+} hydrocarbons selectivity, which indicates that
ZSM-5 with K ion exchange promoted the secondary reaction of olefins to
form gasoline-range products due to its unique acidic and pore
structure.

\textbf{3.3. Exploration of ZSM-5 acid-catalysed reaction process}

By coupling K-ZnFe\textsubscript{2}O\textsubscript{4} core catalyst with
K-ZSM-5 shell catalyst, the selectivity of gasoline hydrocarbons in the
product is improved. To confirm the influences of different zeolites on
the target product selectivity, it is necessary to measure the
intensities and type of acidic sites of zeolites.
NH\textsubscript{3}-TPD patterns of H-ZSM-5, Ce-ZSM-5, K-ZSM-5 and
H-ZSM-5* were shown in the Figure 5a. The surface acidity of ZSM-5
changes obviously after different metal ions modification. After Ce ions
exchange, the surface strong acid of ZSM-5 (above
380\textsuperscript{o}C) decreases, but not as significantly as that of
K-ZSM-5. Besides, the surface weak acid of K-ZSM-5 also decreases
obviously. These phenomena confirm that the introduction of alkali
metals causes the change of acidic sites of ZSM-5. However, the acidic
sites of H-ZSM-5* is also stronger than unprocessed H-ZSM-5. The purpose
of NH\textsubscript{3}·H\textsubscript{2}O treatment for K-ZSM-5 is to
replace K\textsuperscript{+} ions by
NH\textsubscript{4}\textsuperscript{+} ions, and to verify that alkali
metal ions are crucial factors leading to the weakening of acidic sites
of ZSM-5. Zeolite is composed of SiO\textsubscript{2} and
Al\textsubscript{2}O\textsubscript{3}, which is easily to desiliconize
and dealuminum in alkaline solution.
N\textsubscript{2}adsorption-desorption isotherms and pore distributions
(inset) of H-ZSM-5*, K-ZSM-5, Ce-ZSM-5, H-ZSM-5 were shown in Figure S13
and Table S8. Clearly, H-ZSM-5* has a bigger pore volume than H-ZSM-5
with NH\textsubscript{3}·H\textsubscript{2}O treatment. According to
previous reports, appropriate pore reaming of zeolite was beneficial to
improve the selectivity of macromolecular
hydrocarbons.\textsuperscript{46} It explaines that H-ZSM-5* is more
acidic because the specific surface area increases, and the expansion of
pores increases the activity of the catalyst (Figure S9). K-ZSM-5
exhibits a same physical adsorption and desorption results with parent
H-ZSM-5. These findings verify that the introduction of K ions weakens
the acidity of the ZSM-5. Evidently, it is feasible to regulate the
selectivity of the product by regulating the acidity and pores of ZSM-5.

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Figure 5. (a) the NH\textsubscript{3}-TPD patterns and (b) the
pyridine-adsorption Fourier transform infrared (FT-IR) spectra of
H-ZSM-5, Ce-ZSM-5, K-ZSM-5 and H-ZSM-5*.

When zeolite is treated with active components such as metal ions or
oxides, new Lewis acidity site and active centers can be formed. With
alkali metals K and Ce ions introduction, the H\textsuperscript{+} ratio
in the ZSM-5 skeleton decreases and the strength of the acidic site
decreases. By comparing differences in catalytic performance, the
decrease in acidic strength increases the catalytic performance of
CO\textsubscript{2} hydrogenation (Figure 3 and
5a).\textsuperscript{53}Additionally, the pore size distribution and
specific surface area of ZSM-5 change slightly after ions exchange
(Figure S13). This phenomenon can be ascribed that alkali metal ions
occupy a larger space in the pores than H\textsuperscript{+}. The acid
properties of M-ZSM-5 are further investigated by pyridine-adsorption
FT-IR spectra (Figure 5b and Table S9). It is widely accepted that the
Bro\selectlanguage{english}nsted acid sites play a dominant role in isomerization, cracking, and
aromatization reactions.\textsuperscript{53} As shown in the Figure 5b,
the band at 1545 cm\textsuperscript{-1} is ascribed to Bro\selectlanguage{english}nsted acid
sites formed by the framework Al species, while the band at 1456
cm\textsuperscript{-1}is related to the Lewis acid sites formed by the
extra framework Al species. Compared to H-ZSM-5, the intensity of the
band at 1545cm\textsuperscript{-1} decreases obviously with the
introduction of alkali metals. At about 1456 cm\textsuperscript{-1}, the
intensity of Lewis acid site increases slightly. The variation is same
as the result of NH\textsubscript{3}-TPD, and illustrate that the
substitution of the skeleton Al by other metal ions led to a decrease in
Br\selectlanguage{ngerman}önsted acid and an increase in Lewis acid site. From SEM and TEM
images of the ZSM-5, it is observed that crystals appear on the
surfgureace of ZSM-5 after the introduction of Ce, and Ce-ZSM-5 has
large amounts of metal particles that are significantly different from
H-ZSM-5 (Figure S3-S5). Besides, the Al coordination of zeolite
determined by \textsuperscript{27}Al magic-angle spinning (MAS) NMR is
shown in Figure S14 and Table S10. The percentage of extraframework Al
increases (Al\textsuperscript{EF}, at 0 ppm), while that of the
framework Al decreases (Al\textsuperscript{F}, at 55
ppm).\textsuperscript{16,46} The K-ZSM-5 and parent H-ZSM-5 have the
same proportion of Al\textsuperscript{F}, which indicates that K ions
weaken the acidic site strength by weakening Si-OH-Al. Meanwhile, the Ce
ions occupies a large number positions of extraframework Al, resulting
in a larger proportion of Al\textsuperscript{F}, but it still present
weaker acidic strength than H-ZSM-5.

CO\textsubscript{2} hydrogenation behaviors over
K-ZnFe\textsubscript{2}O\textsubscript{4},
K-ZnFe\textsubscript{2}O\textsubscript{4}@H-ZSM5,
K-ZnFe\textsubscript{2}O\textsubscript{4}@Ce-ZSM-5, and
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 were further studied
by \emph{in situ} diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy. As illustrated in Figure 6a, the absorption intensity in
the region between 3800 cm\textsuperscript{-1} and 3500
cm\textsuperscript{-1} continues to increase for
K-ZnFe\textsubscript{2}O\textsubscript{4}, which corresponds to the
hydroxyl vibrational bands belonging to H\textsubscript{2}O. The
absorption intensity in the region from 3000 cm\textsuperscript{-1} to
2940 cm\textsuperscript{-1} gradually decreases, which usually
corresponds to the -CH\textsubscript{2}. The absorption peaks at
wavenumber less than 3000 cm\textsuperscript{-1} shift to higher
wavenumber (\textgreater{}3000 cm\textsuperscript{-1}) with time,
indicating that a component shift from saturated C-H bonds to
unsaturated C-H bonds. Besides, absorption peaks located at 1680
cm\textsuperscript{-1} to 1500 cm\textsuperscript{-1}, and about 1000
cm\textsuperscript{-1} could be attributed to the

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\textbf{Figure 6.} The \emph{in situ} DRIFT spectra of
\textbf{(a)}K-ZnFe\textsubscript{2}O\textsubscript{4},
\textbf{(b)}K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM5 for
CO\textsubscript{2}hydrogenation; \textbf{(c)} H-ZSM5, \textbf{(d)}
K-ZSM-5 in C\textsubscript{3}H\textsubscript{6} atmosphere (0.1 MPa, 320
°C, 20 mL min \textsuperscript{-1}).

double-bond telescopic vibration of the olefins C=C. It indicates that
the presence of olefins in the products is produced over the surface of
K-ZnFe\textsubscript{2}O\textsubscript{4} (\textbf{Figure 3a} ).

When ZSM-5 is introduced, C=C bond of olefins telescopic vibration
absorption peak at 1680-1500 cm\textsuperscript{-1} significantly
decreases, replacing by infrared absorption peaks at about 1000
cm\textsuperscript{-1} wavenumber, which are attributed to the off-plane
bending vibrations of aromatic compounds different from olefins (Figure
6b and S15). The infrared absorption spectra of aromatic compounds are
more pronounced in the
K-ZnFe\textsubscript{2}O\textsubscript{4}@Ce-ZSM-5 and
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 (Figure S15). Besides,
there are many bending vibrations of -CH\textsubscript{3} unlike other
catalysts, which are multiple methyl groups linked to the same carbon
atom. For the K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5, more
bending vibrations of -CH\textsubscript{3} appear than the utilization
of other zeolite catalysts, which ascribes to multiple methyl groups
linked to the same carbon atom in the wavenumbers from 1500
cm\textsuperscript{-1} to 1000 cm\textsuperscript{-1}. The result is
also well consistent with the high content of isoparaffins hydrocarbons
in the product over the
K-ZnFe\textsubscript{2}O\textsubscript{4}@K-ZSM-5 catalyst (Figure 3c).

\textbf{Hosted file}

\verb`image7.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

\textbf{Figure 7.} Optimized periodic structures of a H-ZMS-5, b
Ce-ZSM-5 and c K-ZSM-5; C\textsubscript{2}H\textsubscript{4} \emph{(i)}
, C\textsubscript{3}H\textsubscript{6} \emph{(ii)} and
C\textsubscript{4}H\textsubscript{8} \emph{(iii)} are adsorbed on
zeolite with adsorption energies (\emph{E\textsubscript{ads}} ). The
alkali metal bond with O in Si-O-Al.

With the introduction of zeolite, olefins generated on iron-based
catalysts are further reacted to form long-chain hydrocarbons and
aromatic hydrocarbons. To investigate the process mechanism,
C\textsubscript{3}H\textsubscript{6} was selected as probe reactant to
detect dynamic changes on ZSM-5 by \emph{in situ} DRIFT. As shown in the
Figure 6c, 6d and Figure S16, the wavenumber in 3000
cm\textsuperscript{-1} is attributed to the double-bond telescopic
vibration of the C=C of C\textsubscript{3}H\textsubscript{6}. For
H-ZSM-5, three distinct infrared absorption peaks of benzene rings
appear between 1500 cm\textsuperscript{-1} to 1200
cm\textsuperscript{-1}, which indicates that
C\textsubscript{3}H\textsubscript{6} has been aromatized over H-ZSM-5.
When the H-ZSM-5 catalyst is located in a
C\textsubscript{3}H\textsubscript{6} atmosphere, the absorption peak
belonging to C\textsubscript{3}H\textsubscript{6} gradually decreases,
and the absorption peak of the benzene ring gradually increases. As
expected, the main olefins from
ZnFe\textsubscript{2}O\textsubscript{4}catalyst are reacted at the acid
site of ZSM-5 to produce more gasoline-range hydrocarbons, especially
isomeric and aromatic compounds with high octane numbers. In terms of
K-ZSM-5, there are other infrared absorption peaks between 1700-1500
cm\textsuperscript{-1} and 1000-900 cm\textsuperscript{-1}, which are
attributed to alternatives to benzene or isomeric hydrocarbons. These
evidences show that ZSM-5 treated with K ions can efficiently transform
olefins to gasoline hydrocarbons (Figure 3, 6 and S16). Clearly, the
regulation of the microenvironment of zeolite through ions exchange is
conducive to the oriented generation of C\textsubscript{5+}
hydrocarbons.

In order to understand the intrinsic reason for the enhanced catalytic
selectivity over an alkali metal (K and Ce) modified ZSM-5 catalyst, DFT
calculations were performed to investigate the structure and electronic
properties of the catalysts, as well as the adsorption of intermediates
(C\textsubscript{2}H\textsubscript{4},
C\textsubscript{3}H\textsubscript{6}, and
C\textsubscript{4}H\textsubscript{8}). As shown in Figure 7, the
introduction of alkali metal does not affect the overall structure of
the H-ZSM-5 catalyst, maintaining the MFI zeolite structure. It has been
reported that after ions exchange in ZSM-5, ions often replace Al on the
backbone to change its surface acidity.\textsuperscript{18} By
optimizing the individual ZSM-5 zeolite rings, the adsorption energy of
C\textsubscript{n}H\textsubscript{2n} was calculated.

\textbf{Hosted file}

\verb`image8.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

\textbf{Figure 8.} Main reaction routes for
CO\textsubscript{2}hydrogenation over different composite catalysts.

The adsorption energy of C\textsubscript{2}H\textsubscript{4},
C\textsubscript{3}H\textsubscript{6}, and
C\textsubscript{4}H\textsubscript{8} was -0.10 eV, -0.31 eV and -0.39
eV, respectively, which are effective adsorption. The ZSM-5 structure
with Ce and K modification was optimized, and the adsorption energy of
olefins was calculated for the optimized structure. These detailed
results were shown in Table S11 and S12. With the introduce of Ce and K
into H-ZSM-5, the adsorption capacity of olefins on Ce-ZSM-5 and K-ZSM-5
is enhanced. It is worth noting that the Ce-ZSM-5 has strong adsorption
of olefins. When metal ions bond with O, it has significantly enhanced
adsorption behavior for C\textsubscript{4}H\textsubscript{8} and
stronger adsorption behavior for
C\textsubscript{3}H\textsubscript{6}when it replaces Al (Figure S17).
Strong adsorption and more acid sites lead to enhanced aromatization of
olefins during the secondary reaction on Ce-ZSM-5. Combined with
\emph{in situ} DRIFT spectra of ZSM-5 (Figure 6 and S16), the main
products of olefins are aromatic compounds over Ce-ZSM-5. Meanwhile,
hydrocarbon formation in \emph{in situ} DRIFT spectra of K-ZSM-5 is
different from aromatic compounds. It explains that after the
introduction of Ce, the ZSM-5 has a stronger adsorption behavior for
olefins, which is conducive to aromatization and not conducive to
isomerization, resulting in the formation of more aromatic compounds in
the product (Figure 3c). The introduction of alkaline metal to regulate
acidity changes the adsorption capacity of olefins intermediates over
different ZSM-5 zeolites, which further affects olefins secondary
reaction on ZSM-5 catalysts. As mentioned above, the microenvironment of
H-ZSM-5 can be effectively regulated by alkali metal additives, which
helps to achieve the guided synthesis of catalytic products. The
reaction routes over different composites are also listed in Figure 8.
For the conventional H-ZSM-5 catalyst, direct hydrogenation and
aromatization of light olefins are mainly performed, exhibiting poor
hydrocarbon selectivity. By contrary, olefins mianly occurs
oligomerization and isomerization reaction over K-ZSM-5 zeolite, while
it mainly occurs oligomerization and aromatization reaction over
Ce-ZSM-5 zeolite.



\textbf{4. Conclusion}

In conclusion, we report an iron-based composite catalyst with a capsule
structure, ZnFe\textsubscript{2}O\textsubscript{4}@ZSM-5. The catalyst
has adjustable core-shell microenvironment, that is the microelectronic
environment of the core catalyst is changed by the alkali promoters and
the acid environment of the shell catalyst is changed by ions exchange,
which yield of C\textsubscript{5+} products reaches 60.1\%
g\textsuperscript{-1}, a record-breaking value among composite
catalysts. Thereinto, CO\textsubscript{2} molecules are converted to CO
through RWGS reaction, and then alkenes are synthesized by the FTS
process over the ZnFe\textsubscript{2}O\textsubscript{4} catalyst.
Thereinto, long-chain olefins are synthesized over the
K-ZnFe\textsubscript{2}O\textsubscript{4}. The formed olefins secondary
reactions such as isomerization, oligomerization hydrogenation and
aromatization reactions are initiated at the acid site of ZSM-5 shell.
The strategy of alkali metal ions exchange weakens the strong acidic
site of the zeolite and thus promotes the production of heavy gasoline
products. Especially, the K ions, are effective in reduce the strong
acids and precisely control the acidic sites of ZSM-5, which exhibits
improved chemical adsorption capacity for different types of olefins
intermediates as demonstrated by DFT calculation and \emph{in
situ}characterization. As a consequenece, when
ZnFe\textsubscript{2}O\textsubscript{4} is encapsulated into the K-ZSM-5
shell, it effectively enhances the mass transfer process of long-chain
intermediate olefins and greatly improves the selectivity of gasoline
products. This provides a way to improve the selectivity or yield of the
target product over a tailor-made composite catalyst.

\textbf{AUTHOR INFORMATION}

\textbf{Corresponding Author}

\textbf{Li} -\textbf{sheng Guo} - School of Chemistry and Chemical
Engineering, Anhui University

\textbf{Pei-pei Ai -} State Key Laboratory of Clean and Efficient Coal
Utilization, Taiyuan University of Technology

\textbf{Song Sun} - School of Chemistry and Chemical Engineering, Anhui
University

\textbf{Author Contributions}

\textbf{Hao Wu:} Data curation (Equal); Formal analysis (Equal);
Investigation (Equal)Validation (Equal); Writing - original draft
(Equal). \textbf{Lisheng Guo:} Formal analysis (Equal); Funding
acquisition (Equal); Resources (Equal); Writing - original draft
(Equal); Writing - review \& editing (Equal). \textbf{Wang
Xianbiao:}Data curation (Equal). \textbf{Zhou Wenjie:} Software
(Equal).\textbf{Chen Fang:} Software (Equal). \textbf{Li Da:} Funding
acquisition (Equal). \textbf{Liu Kai:} Funding acquisition
(Equal).\textbf{Ai Peipei:} Formal analysis (Equal); Funding acquisition
(Equal); Project administration (Equal). \textbf{Wei Yuxue:} Data
curation (Equal). Cai, \textbf{Mengdie:} Formal analysis
(Equal).\textbf{Sun, Song:} Funding acquisition (Equal); Project
administration (Equal); Resources (Equal); Supervision (Equal).

\textbf{Notes}

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to
influence the work reported in this paper.

\textbf{ACKNOWLEDGMENT}

Funds from National Natural Science Foundation of China (22102001),
State Key Laboratory of Clean and Efficient Coal Utilization of Taiyuan
University of Technology (SKL2022010), Anhui Provincial Natural Science
Foundation (Grant No. 2008085QB85 and 2108085QB48), and Higher Education
Natural Science Foundation of Anhui Province (KJ2021A0027 and
KJ2021A0029) are greatly appreciated.

Present Addresses

\selectlanguage{english}+If an author's address is different than the one given in the
affiliation line, this information may be included here.

\textbf{Data Availability Statement}

The authors declare that the data that supports the findings of this
study are available in the supplementary material of this article. The
numerical data for size distribution XRD, XPS spectra and XAFS in Figure
1 and* Figure 2, BET, In-situ DRIFTS, and DFT calculations in Figures 5,
Figures 6 and Figure 7, and additional illustrative figures in the main
text are provided in the supplementary information are provided as zip
files in the supplemental material.

\textbf{ORCID}

Lisheng Guo https://orcid.org/0000-0003-0272-6254

Peipei Ai https://orcid.org/0000-0002-2603-6226

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\textbf{Graphical Abstract}

\textbf{Hosted file}

\verb`image9.emf`  available at \url{https://authorea.com/users/753747/articles/723881-spaced-confined-capsule-catalysts-with-tunable-micro-environments-for-efficient-co2-conversion}

The composite catalyst with tuned micro-environments of core-shell
components exhibits ultra-high yield of gasoline hydrocarbons.
Long-chain olefins intermediates from
K-ZnFe\textsubscript{2}O\textsubscript{4} core catalyst mainly occur
oligomerization and isomerization reaction to obtain isoparaffin-rich
hydrocarbon products over K-ZSM-5 shell catalysts, achieving
CO\textsubscript{2} utilization upgrading.

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