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\begin{document}

\title{Cyclic Extrusion Synthesis of Mesoporous Carbon-Supported Metal
Catalysts for Selective Hydrogenation}



\author[1]{Pengfei Zhang}%
\author[1]{Jialu Xu}%
\author[1]{Shengtai Hou}%
\author[2]{Qiang Niu}%
\author[1]{Zheng-Hong Luo}%
\affil[1]{Shanghai Jiao Tong University}%
\affil[2]{Group Co., Ltd.}%


\vspace{-1em}



  
  \date{June 07, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
The mesoporous carbon-supported metal catalysts are important materials
in heterogeneous catalysis. From the standpoint of process
intensification, the current synthesis methods still suffer from several
issues, such as the excessive use of toxic solvents, the time-consuming
self-assembly process, and the difficulty of continuous production.
Herein, we report a solvent-free and continuous route to synthesize
mesoporous carbon-supported metal catalysts. Interestingly, the use of
cyclic extrusion could promote the mixing, coordination, and assembly of
catalyst precursors (tannin-metal salts-additives) by shearing and
squeeze force. The as-made mesoporous carbon-supported nickel showed
high surface areas (up to 512m2g-1), narrow pore size distribution
(\textasciitilde{}10 nm), and good performance in the selective
hydrogenation of nitrobenzene to aniline (Conv.\textgreater{}99\%;
Sel.\textgreater{}99\%). In addition, the fluid property of catalyst
precursors and the geometry of the twin-screw extrusion channel were
studied, and the essence of this cyclic extrusion process is also
discussed.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Mesoporous carbon-supported metal catalysts are important carbon
materials, which have been proved to be widely used in many areas, such
as cross-coupling reaction,\textsuperscript{1} selective
hydrogenation,\textsuperscript{2-5} selective
oxidation,\textsuperscript{6} and fuel cells.\textsuperscript{7, 8} The
wide pore size and large pore volume of mesoporous carbon could provide
an ideal platform for the dispersion of active metal species, and the
sorption, reaction, and transfer of reactants, especially those big
molecules.\textsuperscript{9} There are usually two methods to prepare
mesoporous carbon supports. The hard templating technology usually uses
ordered mesoporous silica (OMSs) or colloid silica as templates,
followed by the carbonization and the etching of
templates.\textsuperscript{10, 11} In fact, the silica removal process
by HF or other related reagents is environmental-unfriendly and requires
extra cost.\textsuperscript{12} Meanwhile, the evaporation-induced
self-assembly method, which uses amphiphilic nonionic copolymers as soft
templates and phenolic resin as carbon precursors, has also been well
developed in the past few years. Indeed, a family of ordered mesoporous
carbon and related metal catalysts have been developed through this
method.\textsuperscript{13, 14} However, it is a batch process and needs
excessive time for solvent evaporation and a large number of organic
solvents.\textsuperscript{15}

In the past few years, solid-state synthesis such as statically
solvent-free method,\textsuperscript{16}manually
grinding,\textsuperscript{17, 18} ball milling\textsuperscript{19, 20}
have been applied in the synthesis of the mesoporous carbon-supported
metal catalysts. However, those mechanochemical methods still suffer
from several problems, such as the limitation of continuous production,
the poor control of processing temperature, and the lack of related
chemical law models.\textsuperscript{21} To date, it is interesting to
explore continuous and solvent-free methods to prepare mesoporous
carbon-supported metal catalysts.

Just recently, reactive extrusion has attracted more and more attention,
after it was listed as one of `Ten Chemical Innovations That Will Change
Our World', put forward by IUPAC in 2019.\textsuperscript{22} Compared
with the traditional solution-based conditions, reactive extrusion can
mix the reactants under solvent-free condition and induce the reaction
and assembly under high pressure and shearing force as
well.\textsuperscript{23} Different from ball milling, twin-screw
extrusion can exactly control the temperature of the whole process and
can satisfy the continuous production, proportionally to
kgh\textsuperscript{-1} on the lab scale.\textsuperscript{24, 25} What's
more, the related chemical law models of reactive extrusion are well
developed. From the 1960s to the 1990s, most of the simulations of
reaction extrusion were reaction engineering models, which needed to
solve numerical equations (e.g., multi-tank reactor
models,\textsuperscript{26, 27} diffusion models,\textsuperscript{28,
29} two parallel plate models,\textsuperscript{30, 31},etc.). Benefiting
from the improvement in computing power and software based on
computational fluid dynamics, computational fluid dynamics models with
discrete meshes (finite volume method\textsuperscript{32, 33} and finite
element method\textsuperscript{34, 35}) are currently used. Moreover,
reactive extrusion has already been applied in the preparation of a
range of functional materials, such as specific organic
compounds,\textsuperscript{21, 25} metal-organic
frameworks,\textsuperscript{36, 37} covalent organic
frameworks,\textsuperscript{38} cages,\textsuperscript{39} and eutectic
salts.\textsuperscript{40} Based on the above investigation, the
extrusion method seems attractive for processing mesoporous
carbon-supported metal catalysts.

Herein, solvent-free and continuous synthesis of mesoporous
carbon-supported metal catalysts, based on cyclic twin-screw extrusion,
is put forward. It is ingenious that the high pressure and shearing
force from the rotation of the twin-screw promote the mixing,
coordination, and assembling of the tannin-metal complex with the
surfactant agent. The as-made mesoporous carbon-supported metal
catalysts have high specific surface areas (up to
512m\textsuperscript{2}g\textsuperscript{-1}), wide pore size, and good
metal dispersion. Moreover, this series of nickel-mesoporous carbon
catalysts show good activity (Conv.\textgreater{}99\%,
Sel.\textgreater{}99\%) and stability in the hydrogenation of
nitrobenzene. At the same time, computational fluid dynamics (CFD) has
been widely used in the simulation of twin-screw extruders. This work
also put forward the geometry of the co-rotating intermeshing taper
twin-screw and the geometry of the fluid channel. It would inspire the
subsequent numerical simulation of the flow and reaction process of the
tannin-surfactant-metal fluid through the CFD method.

\textbf{Experimental details}

\textbf{Preparation of the metal-doped mesoporous carbon sample}

In a typical process by the cyclic twin-screw extrusion method, tannin
(5g), F127(5g), and
Ni(OAc)\textsubscript{2}\(\bullet\)4H\textsubscript{2}O (0.9mmol,
4.5mmol, 9mmol) or Co(OAc)\textsubscript{2}, Mg(OAc)\textsubscript{2},
Zn(OAc)\textsubscript{2}, aluminum isopropoxide (0.9mmol) were added
into the twin-screw extruder (cycling for 15minutes, 30minutes or 60
minutes). The rotational speed of the screw was kept at 15rpm and the
cavity plate temperature was 80. The brown strip-like solid extrudates
were carbonized in an inert N\textsubscript{2} atmosphere
(N\textsubscript{2} 200ml\(\bullet\)min\textsuperscript{-1}) at
600 (5\(\bullet\)min\textsuperscript{-1} for 2h), and then the
final products were obtained.

\textbf{Catalysts characterization}

TGA was carried out in a Thermogravimetric Analyzer Q5000 to provide the
decomposition pattern. The prepared samples were heated in the
N\textsubscript{2} atmosphere from 20\textsuperscript{o}C to
800\textsuperscript{o}C at a heating rate of
20\(\bullet\)min\textsuperscript{-1}. Raw tannin and
tannin-F127-Ni(OAc)\textsubscript{2} were characterized by Fourier
transform infrared spectroscopy (Perkin Elmer Spectrum 100).
N\textsubscript{2} adsorption-desorption isotherms were measured using a
TriStar 3000 volumetric adsorption analyzer at liquid nitrogen
temperature to analyze the BET surface area and BJH pore size of the
prepared samples. Before measurement, the samples were pretreated at
180\selectlanguage{ngerman}°C for 4h. The pore size distribution curves were obtained through
the Barrett--Joyner--Halenda (BJH) method from the desorption branches
of the isotherms. The powder X-ray diffraction (XRD) was collected by a
PANalytical Empyrean diffractometer using a Cu K radiation. The X-ray
tube was operated at 40 kV and 40 mA (scanning step: 0.02° per step).
Transmission electron microscopy (TEM) images were recorded using a
TALOS F200X electron microscope. Raman spectroscopy was done to
determine the degree of graphitization using ThermoFisher DXR.

\textbf{Selective hydrogenation of Nitrobenzene}

The scheme of the catalytic reaction for selective hydrogenation of
nitrobenzene is given in \textbf{Figure 6} . The specific catalyst, NB,
and ethanol were added to the high-pressure reactor first. Then the
temperature sensor and the stirring mixer were set up in the whole
hydrogenation reactor. The reaction temperature was controlled at
70-110\textsuperscript{o}C, and the stirring speed was kept constant
(1000 rpm). It is necessary to replace the high-pressure reactor
(v=50ml) with pure H\textsubscript{2} three times. In the process of
hydrogenation, the pressure of H\textsubscript{2} was kept at 1-3MPa.
After 4-16 hours, the product was removed from the reaction system and
analyzed by GC-2014c (using a flame ionization detector).

\textbf{Results and discussion}

\textbf{Figure 1} depicts the cyclic twin-screw extrusion route for
mixing, coordinating, and assembling tannins, metal salts, and
PEO--PPO--PEO. Tannins are a kind of natural polyphenols, which widely
exist in diverse plants such as pine bark, chestnut, maple tree, oak,
and grapes.\textsuperscript{41} The self-assembly behavior of the
tannin-metal complex with amphiphilic surfactants in solution or by ball
milling has been confirmed by previous work.\textsuperscript{41-43} It
suggested that mechanochemical ball milling can promote the coordination
between tannin-metal salts and the formation of the hydrogen-bonding
network with PEO--PPO--PEO.\textsuperscript{44} However, the ball
milling method is an intermittent operation, and the chemical model of
the process is still unclear. An accidental observation showed that
triblock-copolymer F127 could melt at \textasciitilde{}70\selectlanguage{english} and could
dissolve other reactants into a viscous fluid. This feature inspired us
to the possibility of mixing and reacting tannin, F127, and metal salts
under the shearing action of the extruder.

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure1.} Cyclic extrusion synthesis of mesoporous
carbon-supported metal catalysts for selective hydrogenation processes.
a) Adding the mixture of tannins, metal salts, and PEO--PPO--PEO into
the extruder; b) Cyclic extruded for 15-60min; c) Carbonized at 600\selectlanguage{english}.

Initially, tannin, F127 and nickel acetate were added to the extruder
and cycled for 15-60 minutes. During this process, the solid mixture
melted as the temperature of the cavity plate increased, forming a
uniform gel composite. The long cylindrical strip mixture was finally
extruded. Tannin-F127-metal composites were thermally treated in the
N\textsubscript{2} atmosphere (600\selectlanguage{english}), during which the F127 micelles
completely decomposed and the tannin-metal-polymer was restructured into
the carbon skeleton. The samples obtained from this method were named
C@Ni-\(x\)(x means\(x\)mmol
nickel acetate per gram tannin, \(x\)=0.18, 0.9, 1.8). In
addition, the metal salts can be extended to cobalt (II) acetate,
magnesium acetate, zinc acetate, and aluminum isopropoxide.

To understand the interaction during the extrusion synthesis of
mesoporous carbon-supported metal catalysts, FTIR was used to study the
tannin-F127-Ni(OAc)\textsubscript{2}mixture after cyclic extrusion for
30 minutes. Compared with tannin, the sorption peak of O--H in
tannin-F127-Ni(OAc)\textsubscript{2} mixture showed a red shift in FTIR,
which revealed a hydrogen-bonding network (PhOH-O) might be formed
between tannin and the hydrophilic PEO block of F127. (\textbf{Figure
2a} ). The mechanochemical coordination behavior between tannins and
transition metal ions has been well known through the formation of
coordination polymer products. Based on the above results, reactive
extrusion can promote the interaction between Tannin, F127, and metal
salts.

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure2.} (a) FTIR spectra of tannin, and
tannin-F127-Ni(OAc)\textsubscript{2} (C@Ni-0.18 before carbonization).
(b) TG curve of Ni-0.18 precursors in N\textsubscript{2} atmosphere.

In order to study the thermal decomposition and carbonization behavior
of the mixture, TGA was studied, and the weight-loss curve with
increased temperatures was shown in \textbf{Figure 2b} . It can be
perceived from the weight-loss curve that the carbonization process had
almost finished at 500\selectlanguage{english} and about 30\% of the pristine weight remain.
Through calculation, the final carbon yield based on tannin is around
61\% at 600\textsuperscript{o}C. The carbonization temperature (600\selectlanguage{english})
was determined by the results of TGA.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure 3.} (a) N\textsubscript{2} adsorption-desorption
isotherms (77 K) and the corresponding pore size distributions of
C@Ni-1.8. (b) N\textsubscript{2} adsorption-desorption isotherms (77 K)
of Co@C, Mg@C, Zn@C. For clarity, the isotherm curves of Mg@C and Co@C
are offset along the axis by 50
cm\textsuperscript{3}g\textsuperscript{-1}and 100
cm\textsuperscript{3}g\textsuperscript{-1}, respectively.

The softening point and viscosity of reactants have a great influence on
the mixed extrusion process. Two kinds of block copolymers were chosen
as soft templates (F127, P123) to explore the self-assembly of tannin
and PPO-PEO-PPO. P123 exhibited as a viscous paste at room temperature,
while F127 was solid sheets at room temperature with a melting point of
53-57degC. The N\textsubscript{2} adsorption-desorption isotherms at 77 K
showed that C@F127 had better performance than C@P123, regarding pore
volume (0.6 cm\textsuperscript{3}g\textsuperscript{-1}) and specific BET
surface area. (\textbf{Table 1,Figure S1} ). The amount of metal salt
precursor was also an important factor affecting the porous structure of
the materials. (Table 1,\textbf{Figure S2} ). Then, different amounts of
nickel acetate were added to explore this effect (tannin: nickel acetate
=1g: 0.18mmol, 1g: 0.9mmol, 1g: 1.8mmol). As suggested by the
N\textsubscript{2} adsorption-desorption isotherm curves at 77K,
C@Ni-1.8 performed the highest BET surface area
(512m\textsuperscript{2}g\textsuperscript{-1}) with the pore volume of
0.5cm\textsuperscript{3}g\textsuperscript{-1}. The N\textsubscript{2}
sorption uptake suddenly increased when the relative pressure
P/P\textsubscript{0} raised above 0.6, and the desorption curve formed a
hysteresis loop. This type IV adsorption-desorption curve was
characteristic of mesoporous structure, and the calculated pore size
distribution was narrow and centered at about 9-10 nm. (\textbf{Figure
3a} ).

\textbf{Table 1.} Calculated pore parameters for the various samples by
N\textsubscript{2} adsorption-desorption sorption curves at 77 K.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Sample\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
S\textsubscript{BET} (m\textsuperscript{2} g\textsuperscript{-1})\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Pore Volume (cm\textsuperscript{3} g\textsuperscript{-1})\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Average Pore Size (nm)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@P123\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
438.7\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.35\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
7.2\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@F127\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
421.9\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.45\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10.2\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-0\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
421.9\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.44\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10.2\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-0.18\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
460.3\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.52\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10.7\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-0.9\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
464.4\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.43\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
8.7\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-1.8\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
512.2\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.49\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
7.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-1.8-Cycle 15min\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
499.2\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.53\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
8.6\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-1.8-Cycle 30min\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
512.2\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.49\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
7.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Ni-1.8-Cycle 60min\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
471.0\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.62\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
8.8\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Co@C\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
418.1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.46\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10.1\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Mg@C\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
446.3\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.37\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
8.4\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Zn@C\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
423.1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.20\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
5.3\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Al-F127-0.8\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
482.1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.85\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
6.8\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Al-F127-1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
474.9\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.40\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
5.6\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Al-F127-1.5\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
435.3\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.43\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
6.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
C@Al-F127-2\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
413.2\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.70\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
14.0\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Then, the effect of TSE parameters on mesoporous carbon-supported metal
catalysts was investigated. Since the twin-screw extruder needs to be
operated in a fluid state, the cavity temperature had to reach the
melting temperature of the soft template. To ensure that the TSE can
rotate well, 80 was finally selected as the working temperature. Then,
different cycle time (15min,30min, and 60min) of the twin-screw extruder
was explored to understand the role of high pressure and shearing forces
during the reactive extrusion process. The 1.8 mmol nickel acetate per
gram of tannin was fixed, and the
N\textsubscript{2}adsorption-desorption isotherms (at 77 K) of the
as-made samples with different cyclic extrusion times were collected.
(\textbf{Figure S3} ). With the cyclic extrusion time prolonged, the BET
surface area of the carbon samples increased firstly and then decreased.
The mechanical force would promote the formation of chemical bonds such
as hydrogen bonds (tannin-P123) and coordination bonds (tannin-Ni), but
it would somewhat affect the self-assembly of porous structures during
long-term extrusion.\textsuperscript{45} The optimized cyclic extrusion
time was 30min, respecting the BET surface area
(512m\textsuperscript{2}g\textsuperscript{-1}). Additionally, this
method could be applied in the construction of mesoporous
carbon-supported different metals, such as Co, Mg, ZnO, and
Al\textsubscript{2}O\textsubscript{3}. The results of
N\textsubscript{2}adsorption-desorption isotherm curves at 77K indicated
that these samples all have relatively high BET surface areas
(>400m\textsuperscript{2}g\textsuperscript{-1}). (\textbf{Figure
3b,Figure S4} ).

\textbf{Figure 4.} (a) XRD pattern of C@Ni-0.18 and C@Ni-1.8. (b)The
Raman spectrum of C@Ni-0.18.

Next, the crystal phases of the supported metal specie within C@Ni-1.8
and C@Ni-0.18 samples were studied by X-ray diffraction. (\textbf{Figure
4a} ). The XRD patterns of C@Ni-1.8 and C@Ni-0.18 both displayed three
diffraction peaks at 44.5\selectlanguage{ngerman}°, 51.8°, and 76.4°, which could correspond to
Ni (111), Ni (200), and Ni (220) planes (PDF\#04-0850). This indicated
that the Ni element in the mesoporous carbon mainly existed in the form
of the metallic nickel phase after calcination, and the diffraction
peaks of C@Ni-0.18 were broader than that of C@Ni-1.8. According to the
Scherrer equation, the average Ni particle sizes in the C@Ni-1.8 and
C@Ni-0.18 were calculated to be 12.9nm and 9.7nm, respectively. The
Raman spectra (\textbf{Figure 4b} ) afforded the typical D-band and
G-band at 1341cm\selectlanguage{english}\textsuperscript{-1} and 1583 cm\selectlanguage{english}\textsuperscript{-1},
respectively. The value (3.3) of the
I\textsubscript{D}/I\textsubscript{G} ratio implied the existence of
abundant defects in as-made porous carbon.

To further characterize the porous morphology of these samples, C@Ni-1.8
and C@Ni-0.18 were observed by transmission electron microscopy (TEM)
and scanning electron microscopy (SEM). The SEM images of C@Ni-0.18
showed a sponge-like structure with apparent pores throughout the whole
surface (\textbf{Figure S5} ). Abundant mesopores were observed in the
SEM images of C@Ni-1.8, with nickel particles dispersed on the surface
(\textbf{Figure S6} ). The typical TEM images of C@Ni-1.8 displayed a
porous structure with thick carbon walls. (\textbf{Figure 5a} ). A large
amount of Ni particles (average particle size: \textasciitilde{}7.6nm)
was well dispersed on the surface or within the pore channels of
mesoporous carbon. (\textbf{Figure 5b-d} ). It should be noted that the
reactive extrusion method can lead to good metal dispersion under
solvent-free conditions. Based on those results, the extrusion pressure
and shearing force generated by the rotating of the twin-screw may
promote the self-assembly behavior of the tannin-metal complex with
amphiphilic surfactants.

\textbf{Hosted file}

\verb`image5.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure5.} (a-c) The TEM images of C@Ni-1.8. (d)The corresponding
particle size distributions. (The particle size distribution was
calculated based on 100 particles randomly selected. APS: average
particle size.)

By the combination of wide pore size, high surface area, and good
metallic Ni dispersion, the nickel-mesoporous carbons prepared by
extrusion were potential candidates for catalytic processes. The
catalytic performance of the as-made C@Ni-1.8 sample was evaluated by
selective hydrogenation of nitrobenzene (NB) to aniline (AN). As an
important chemical process, functional carbon materials such as carbon
nanotubes or nitrogen-doped graphene-supported transition metals, or
noble metals have been explored for selective hydrogenation of
nitrobenzene or halogenated nitrobenzene.\textsuperscript{46-50}However,
the wet chemistry synthesis of these catalysts needs excessive solvents
along with the production of liquid wastes, and additional drying
processes.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6.} Scheme of the catalytic reactor for selective
hydrogenation of NB to AN.

\textbf{Table2.} Exploration of reaction conditions of reaction
temperature for nitrobenzene hydrogenation.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Entry & Pressure & Reaction time & Temperature & Conversion &
Selectivity\tabularnewline
\midrule
\endhead
1 & 1MPa & 4h & 110 & 42.0\% & 21.1\%\tabularnewline
2 & 2MPa & 4h & 110 & 34.7\% & >99\%\tabularnewline
3 & 3MPa & 4h & 110 & 79.2\% & 73.1\%\tabularnewline
4 & 2MPa & 8h & 110 & 42.7\% & >99\%\tabularnewline
5 & 2MPa & 12h & 110 & 88.8\% & >99\%\tabularnewline
6 & 2MPa & 16h & 110 & >99\% & >99\%\tabularnewline
7 & 2MPa & 16h & 90 & 66.1\% & 76.2\%\tabularnewline
8 & 2MPa & 16h & 70 & 36.5\% & 53.4\%\tabularnewline
\bottomrule
\end{longtable}

Initially, ethanol was chosen as the solvent to carry out the
nitrobenzene hydrogenation reaction in a high-pressure reactor.
\textbf{(Figure 6)} . The NB conversion was very low (0.3\%) after 4
hours when the control hydrogenation reaction was performed without
catalysts at 110\selectlanguage{ngerman}°C. Then, 100mg catalysts (C@Ni-1.8) were chosen as the
amount of catalyst to optimize the action conditions. The
H\textsubscript{2}spatial pressure, reaction time, and reaction
temperature for the selective hydrogenation of nitrobenzene were
explored. (\textbf{Table2} ). With the H\textsubscript{2} spatial
pressure increased, the conversion of NB increased. Based on previous
studies, it is known that the hydrogenation of nitrobenzene is a
first-order reaction.\textsuperscript{50}Therefore, its reaction rate is
proportional to the starting concentration of reactants, and the
reaction rate is increased by the increase of hydrogen
pressure.\textsuperscript{51} When the H\textsubscript{2} spatial
pressure was 2MPa, the highest selectivity of AN (\textgreater{} 99\%)
would be obtained. Although the conversion rate still raised when the
H\textsubscript{2} spatial pressure increased to 3MPa, 2MPa was selected
due to the poor selectivity under 3MPa H\textsubscript{2} spatial
pressure. Moreover, with the reaction time being prolonged, the
conversion of nitrobenzene gradually increased. When the reaction time
was prolonged to 16 hours, the conversion of NB and the selectivity of
AN were both above 99\%. Finally, the reaction temperature (70°C, 90°C,
and 110°C) was optimized under 2MPa H\textsubscript{2} spatial pressure.
Under the same reaction time (16 hours), the conversion of NB and the
selectivity of AN gradually decreased with the decrease of reaction
temperature. To sum up, the best reaction condition is 2MPa
H\textsubscript{2} spatial pressure, 110°C, and reaction for 16 hours.

Then, various nitrobenzene derivatives were investigated to explore the
scope of the mesoporous carbon-supported nickel catalyst (C@Ni-1.8). The
data was collected in \textbf{Table 3.} The as-made C@Ni-1.8 catalyst
afforded high AN selectivity(\selectlanguage{english}>99\%) both on the nitroaromatic
containing methyl or methoxy functional groups and nitrobenzene
derivative with an electron-withdrawing functional group, while the
conversion of those NB derivatives was case by case (69\%-99\%). It's
worth noting that the nitro group on 2-nitrotoluene has steric hindrance
with the\emph{ortho-} methyl group and cannot realize this type of
hydrogenation reaction easily.\textsuperscript{52} However, the
hydrogenation reaction of 2-nitrotoluene catalyzed by the C@Ni-1.8
catalyst performed high conversion and selectivity, while the mesoporous
structure with exposed Ni may contribute to the adsorption of nitro
group on 2-nitrotoluene.

\textbf{Table 3} Summarized the selective hydrogenation reaction of NB
derivatives and selectivity of corresponding aniline derivative
catalyzed by C@Ni-1.8.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Reactants & Reaction time & Conversion & Selectivity\tabularnewline
\midrule
\endhead
& 16h & 77.1\% & >99\%\tabularnewline
& 20h & 85.4\% & >99\%\tabularnewline
& 20h & 99.5\% & >99\%\tabularnewline
& 16h & 67.3\% & >99\%\tabularnewline
& 20h & 87.1\% & >99\%\tabularnewline
& 20h & 69.9\% & >99\%\tabularnewline
\bottomrule
\end{longtable}

The stability of the supported catalysts is important in heterogeneous
hydrogenation. The catalytic stability of the as-made C@Ni-1.8 was
carefully studied in selective hydrogenation of NB (\textbf{Figure7)} .
After the pristine hydrogenation, the catalysts were separated by
centrifugation, and then ethanol was used to wash the recycled powder
for the next run. During five runs, the conversion of NB and the
selectivity of AN were well preserved, suggesting the good stability of
the current mesoporous carbon-supported Ni catalyst.

\textbf{Hosted file}

\verb`image13.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure7.}(a) Recycling tests of selective hydrogenation of
nitrobenzene over C@Ni-1.8. (b) N\textsubscript{2} adsorption-desorption
isotherms (77 K) of C@Ni-0.18 catalyst after five runs and the
corresponding pore size distribution (inside).

The used catalysts after five runs were characterized by an
N\textsubscript{2} adsorption-desorption isotherm curve at 77K. Its
N\textsubscript{2} adsorption-desorption curve maintained a type IV
isotherm with a clear H\textsubscript{1} hysteresis loop as well
(\textbf{Figure 7b} ), which maintains the character of the mesoporous
structure. Compared with the fresh C@Ni-1.8, the BET surface area of the
catalysts after five runs decreased slightly (from
512m\textsuperscript{2}g\textsuperscript{-1} to
302m\textsuperscript{2}g\textsuperscript{-1}), and its calculated pore
size distribution remained narrow and concentrated at about 10nm. The
mesoporous structure was well preserved after five runs.

\textbf{Figure 8.} The viscosity of C@Ni-1.8 precursors in terms of
shear rates at 80\textsuperscript{o}C.

Actually, the mixing process by twin-screw extrusion is different from
the solution methods or solid-state ball milling methods. The physical
and chemical changes of tannin, nickel acetate, and F127 during
twin-screw extrusion would finally lead to the precursor composition for
Ni-mesoporous carbon catalyst. It is interesting to figure out the
mechanochemical effects of twin-screw extrusion on the fluid. As we
know, the properties of the fluid have a great influence on the
extrusion effect of the extruder.\textsuperscript{53} To simplify the
model, the three feedstock mixtures were assumed as single-phase
non-Newtonian fluid at 80\selectlanguage{ngerman}°C. The relationship between the viscosity and
shear rate of the fluid was measured by a rotational rheometer at 80°C.
The experiment and the fitting results of the Cross model was shown in
\textbf{Figure 8} , its rheological model can be expressed as
(\textbf{Eq. (1)} ):

\(\eta=\frac{\eta_{0}}{1+{(\lambda\gamma)}^{n}}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\)(1)

where \selectlanguage{greek}η\selectlanguage{ngerman} is apparent viscosity,\(\eta_{0}\) is
zero-shear viscosity, \(\lambda\) is relaxation time,
\selectlanguage{greek}γ\selectlanguage{ngerman} is the rate of shear.\textsuperscript{54} A good
fitting result was obtained and \(\eta_{0}\)=1.18
KPa\selectlanguage{english}[?]s\textsuperscript{-1}, \(\lambda\)=0.25 s, n=3.12,
correlation coefficient of fit R\textsuperscript{2}=0.998. The results
showed that at a specific shear rate (corresponding to the rotating
speed of the extruder), the viscosity of the mixture decreases with
increasing shear rate and exhibits the characteristics of a
non-Newtonian fluid.

Many previous literatures showed that the particles in the twin-screw
extruder move around the twin-screw in an ''\selectlanguage{english}[?]''
shape.\textsuperscript{55, 56} As time goes on, different particles move
forward according to the trajectories and the residence time of those
particles.\textsuperscript{57, 58} At the same time, the physical force
of each position in the twin-screw is also different, so the shear
mixing effect is also different. Therefore, the geometry of the twin
screw is important for further simulation.

\textbf{Hosted file}

\verb`image15.emf`  available at \url{https://authorea.com/users/504150/articles/583610-cyclic-extrusion-synthesis-of-mesoporous-carbon-supported-metal-catalysts-for-selective-hydrogenation}

\textbf{Figure 9.} (a)Schematic picture of the twin-screw extruder and
the fluid channel (b)The geometry of the co-rotating intermeshing taper
twin-screw (c)The geometry of the fluid channel.

The shape and size of the channel would change continuously with the
rotation of the twin screw, which has a great influence on the flow
speed of the material. To ensure the reliability of the simulation, a
three-dimensional geometric model of the real screw and the fluid
channel shape were established by Solidworks (\textbf{Figure 9} ). The
length of the runner of the extruder is 150mm, and the screw pitch is
15mm. The cross-sectional shape of the flow channel is the ''\selectlanguage{english}[?]'' shape.
The measurement of fluid viscosity and the proposal of the geometric
model of the extruder screw and flow channel are of great significance
for the subsequent use of the CFD method to simulate the fluid state
during the extrusion process. Those CFD calculations were ongoing in our
lab.

\textbf{Conclusion}

In this work, a cyclic twin-screw extrusion route for the self-assembly
of tannins, metal salts, and PEO--PPO--PEO is put forward. Because the
block copolymer(F127) would melt below 80\selectlanguage{english}, the melted F127 can be mixed
with other reagents and come into a viscous fluid. That feature provides
necessary conditions for reactive extrusion. It was confirmed that the
formation of hydrogen bond and coordination network between tannins,
metal salts, and PEO--PPO--PEO were promoted by cyclic twin-screw
extrusion. After carbonization, mesoporous carbon-supported metal
materials with high surface areas up to 512
m\textsuperscript{2}g\textsuperscript{-1} and narrow pore size
distribution were obtained. Metal nanoparticles were confirmed to
distribute on the surface of mesoporous carbon uniformly. Nickel-doped
mesoporous carbon (C@Ni-1.8) showed good catalytic performance and
stability in selective hydrogenation of nitrobenzene (NB) to aniline
(AN). At the same time, it is worth to further exploring the flow field
and mixed numerical simulation of the reactive extrusion. The fluid
property and the geometry of the twin-screw and fluid channel were
proposed, which would be the base for further research. The reactive
extrusion method may inspire more works on the continuous production
synthesis of mesoporous carbon-supported metal catalysts under
solvent-free conditions.

\textbf{Acknowledgements}

The work is supported by the National Natural Science Foundation of
China (Grant No. 22178219), Shanghai Rising-Star Program (20QA1405200),
Shanghai Jiao Tong University Scientific and Technological Innovation
Funds (No.2019QYB06), National Key R \& D Program Plan (2020YFB0606400)
and Inner Mongolia R \& D Program Plan (2021ZD0042, 2021EEDSCXSFQZD006).

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