One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al2O3 and ZSM-5 Relay

Article abstract of DOI:10.1007/s10562-021-03617-5

Alkylbenzenes have a wide range of uses and are the most demanded aromatic chemicals. The finite petroleum resources compels the development of production of alkylbenzenes by non-petroleum routes. One-pass selective conversion of benzene and syngas to alkylbenzenes is a promising alternative coal chemical engineering route, yet it still faces challenge to industrialized applications owing to low conversion of benzene and syngas. Here we presented a Cu–ZnO–Al₂O₃/ZSM-5 bifunctional catalyst which realizes one-pass conversion of benzene and syngas to alkylbenzenes with high efficiency. This bifunctional catalyst exhibited high benzene conversion (benzene conversion of 50.7%), CO conversion (CO conversion of 55.0%) and C7&C8 aromatics total yield (C7&C8 total yield of 45.0%). Characterizations and catalytic performance evaluations revealed that ZSM-5 with well-regulated acidity, as a vital part of this Cu–ZnO–Al₂O₃/ZSM-5 bifunctional catalyst, substantially contributed to its performance for the alkylbenzenes one-pass synthesis from benzene and syngas due to depress methanol-to-olefins (MTO) reaction. Furthermore, matching of the mass ratio of two active components in the dual-function catalyst and the temperature of methanol synthesis with benzene alkylation reactions can effectively depress the formation of unwanted by-products and guarantee the high performance of tandem reactions. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03617-5

Catalysis Letters
https://doi.org/10.1007/s10562-021-03617-5
One‑Pass Conversion of Benzene and Syngas to Alkylbenzenes
by Cu–ZnO–Al O and ZSM‑5 Relay
2
3
1
1
1
2
2
1
2
Tengfei Han · Hong Xu · Jianchao Liu · Ligong Zhou · Xuekuan Li · Jinxiang Dong · Hui Ge
Received: 24 December 2020 / Accepted: 28 March 2021
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Alkylbenzenes have a wide range of uses and are the most demanded aromatic chemicals. The ꢀnite petroleum resources
compels the development of production of alkylbenzenes by non-petroleum routes. One-pass selective conversion of ben-
zene and syngas to alkylbenzenes is a promising alternative coal chemical engineering route, yet it still faces challenge to
industrialized applications owing to low conversion of benzene and syngas. Here we presented a Cu–ZnO–Al O /ZSM-5
2
3
bifunctional catalyst which realizes one-pass conversion of benzene and syngas to alkylbenzenes with high eꢁciency. This
bifunctional catalyst exhibited high benzene conversion (benzene conversion of 50.7%), CO conversion (CO conversion of
5
5.0%) and C7&C8 aromatics total yield (C7&C8 total yield of 45.0%). Characterizations and catalytic performance evalu-
ations revealed that ZSM-5 with well-regulated acidity, as a vital part of this Cu–ZnO–Al O /ZSM-5 bifunctional catalyst,
2
3
substantially contributed to its performance for the alkylbenzenes one-pass synthesis from benzene and syngas due to depress
methanol-to-oleꢀns (MTO) reaction. Furthermore, matching of the mass ratio of two active components in the dual-function
catalyst and the temperature of methanol synthesis with benzene alkylation reactions can eꢂectively depress the formation
of unwanted by-products and guarantee the high performance of tandem reactions.
Graphic Abstract
Keywords Benzene · Syngas · Alkylation · Cu–ZnO–Al O · ZSM-5
2
3
*
Hui Ge
gehui@sxicc.ac.cn
1 Introduction
1
College of Chemistry and Chemical Engineering, Taiyuan
As important chemical intermediates, alkylbenzenes have
been broadly employed in chemical industries. Such as,
toluene is used as solvent, p-xylene is the main feed for
the production of polyester ꢀbers and ꢀlms, o-xylene and
University of Technology, Taiyuan 030024, Shanxi Province,
China
2
Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan 030001, Shanxi Province, China
Vol.:(0
1
123456789)
3

T. Han et al.
meta-xylene can be used for production of plasticizers, pes-
ticides, dyes, and medicine intermediates [1, 2]. At present,
catalytic reforming technology of petroleum reꢀnery is the
dominant method for production of alkylbenzenes. However,
the crude resources are depleted increasingly. In 2020, the
COVID-19 epidemic swept the world [3, 4], adding uncer-
tainty to the international economy environment and aꢂect-
ing the supply chain of petroleum industry. Hence there is
necessary to develop non-petroleum routes for the produc-
tion of alkylbenzenes. It is widely accepted that coal is one
of the most abundant and widely distributed conventional
energy, but the disadvantages of coal utilization are high
emission and low eꢁciency. Therefore, the development of
coal-based technologies for the production of alkylbenzenes
chemicals can not only alleviate the dependence on oil, but
also can improve the eꢁciently use of coal resources.
good thermal stability and unique shape-selective cataly-
sis, ZSM-5 has been widely applied to alkylation reaction
and aromatization reaction. Combining Cu based compo-
nent with ZSM-5 component to catalyze the alkylation of
benzene with syngas is a popular strategy. Xuebin [22]
prepared Cu-Al O /ZSM-5 catalyst using impregnation
2
3
method for the alkylation reaction of benzene with syngas.
It was found that increasing the Cu content can increase
the activity of the catalyst, containing 11% Cu catalyst
showed the highest the 16.8% conversion of benzene at
350 °C and 1.5 MPa. They also found that the addition of
a second metal (Zn, Mn or V) to the copper-based catalyst
can increase the selectivity of methylation and decrease
the selectivity of heavy aromatics. Gao [23] used CuO-
ZnO-Al O /ZSM-5 bifunctional composite catalyst to
2
3
catalyze the alkylation reaction of benzene and syngas in
a slurry bed reactor. They modiꢀed ZSM-5 component
with acid–alkali co-processing to produce meso-pores and
reducing strong acid sites, which enhances mass transfer
and increases aromatic selectivity.
At present, there are four main technical routes for obtain-
ing alkylbenzenes from coal, namely: (1) one-step conver-
sion of syngas to aromatics (STA) [5–8], which has the
advantages of single raw material, low cost and high aca-
demic signiꢀcance, but it has disadvantages such as complex
products, low aromatics selectivity, and low CO eꢂective
utilization rate; (2) methanol to aromatics (MTA) [9, 10],
which has the advantages of high methanol conversion rate
and simple operation, but it has disadvantages such as low
aromatic selectivity and poor catalyst stability; (3) alkyla-
tion of benzene with methanol to alkylbenzenes [11–14], the
advantage of this technology is that the selectivity of alkylb-
enzenes is high, but there are disadvantages such as metha-
nol prone to side reactions and low product yield.; and (4)
alkylation of benzene with syngas to produce alkylbenzenes
Although the Cu–ZnO–Al O /ZSM-5 bifunctional cata-
2
3
lyst has showed promising result for the tandem reaction of
benzene with syngas, the lower conversion and selectivity of
benzene impedes the industrial development, and it lacks the
insight understanding of the coupling mechanism of metha-
nol synthesis with benzene alkylation. Here, we presented
a simple prepared Cu–ZnO–Al O /ZSM-5 hybrid catalyst,
2
3
realizing high benzene conversion of 50.7% and high C7–C8
aromatic selectivity by the one-pass conversion of benzene
and syngas to alkylbenzenes in ꢀx-bed reactor. The match-
ing relationship between the two active components in the
bifunctional catalyst and the important role of zeolite’ acid-
ity in the alkylation reaction of benzene with syngas have
been revealed. The coupling mechanism of tandem reaction
was proposed.
[
15–21]. The technology of one-pass conversion of benzene
and syngas to alkylbenzenes has its own unique advantages:
it can eliminate the intermediate step of methanol production
and the liquid products are basically aromatics, which is ben-
eꢀcial to the later separation and transformation. Therefore,
benzene and syngas alkylation technology is preferred in
industrial application. However, until now, the conversion
and selectivity of benzene with syngas are still not satisꢀed.
Recently, the alkylation of benzene with syngas to pro-
duce alkylbenzenes has received increased attention. The
catalytic system are usually composed of dual-functional
components, where the precious metals or metal oxides are
complexed with zeolite. The alkylation reaction of ben-
zene and syngas is proposed as a tandem reaction. First,
syngas is converted into methanol intermediates by the
precious metals or metal oxides, then the generated metha-
nol moved to zeolite, reacted with benzene forming the
alkybenzenes. Among the methanol synthesis catalysts,
Cu–ZnO–Al O catalyst is the most widely researched and
2 Experimental
2.1 Preparation of Bifunctional Catalyst
The Cu–ZnO–Al O /ZSM-5 bifunctional catalysts were pre-
2
3
pared as follows: The two components (CuO–ZnO–Al O
2
3
and HZSM-5) with diꢂerent mass ratios were mixed and
ground in a mortar for 15 min, then pressed, crushed and
sieved to granules of 40–60 meshes. HZSM-5 (framework
Si/Al ratio=15, 31, 84) was obtained from Nankai Univer-
sity Catalyst Corp and CuO–ZnO–Al O component was the
2
3
commercial catalyst of Sud-Chemie company. The prepared
2
3
used, attributing to its low price, high activity and high
selectivity. Meanwhile ZSM-5 is an important industrial-
ized microporous zeolite. Due to its adjustable acidity,
dual-function catalysts were denoted as Cu–ZnO–Al O /
2
3
ZSM-5 (x) = y, x represents the framework Si/Al ratio of
ZSM-5 and y represents the mass ratio.
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One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al O and ZSM-5 Relay
2
3
2
.2 Catalyst Characterization
an inner diameter of 6 mm. Typically, 1 g catalyst (40–60
meshes) was loaded in the middle of the reaction tube and
sealed by the quarts at two end. For each reaction, the ꢃow
The HZSM-5 and CuO–ZnO–Al O componets, as well as
2
3
the hybrid catalysts, were identiꢀed using X-ray diꢂraction
rate of syngas (H /CO=2) and benzene were maintained at
2
(
XRD), patterns were recorded on a Rigaku Ultima IV with
5220 mL/(h g ) and 2.64 g/h g respectively. The syngas
cat
cat
monochromated Cu Kα radiation at a scanning rate of 2°
pressure was controlled at 4.0 MPa and the reaction tempera-
−
1
min from 5° to 80°.
ture were changed between 350 and 450 °C.
Solid-state magic-angle spinning nuclear magnetic reso-
nance (MAS NMR) spectra were carried out on a Bruker
Avance III 600 MHz Wide Bore spectrometer (14.4 T). The
Exhaust gas were analyzed by an online gas chromato-
graphs (GC-920 from HAIXIN Chromatograph Equipment
Company), which was equipped with 5 A molecular sieve
packed column connected to a the thermal conductivity
detector (TCD) to detect the CO and H , a CH converter
2
1
9 Si spectra were recorded at resonance frequencies of
19.29 MHz with spinning rate of 10 kHz. The framework
2
4
Si/Al ratio of ZSM-5 using the following formula, where
to a the ꢃame ionization detector (FID) to analyze the CH ,
4
I
represents the total area of Si(nAl) [24].
CO and CO , and an activated alumina column connected to
Si(nAl)
2
another FID to detect the hydrocarbon gases. Use CH estab-
4
4
4
ꢀ
ꢀ
Si
n
lished the composition correlation between FIDs and TCD,
and got the composition of the exhaust gas. The condensed
liquid products were collected and analyzed by an Agilent
=
ISi(nAl)∕
[I
]
(1)
Al
4
Si(nAl)
n=0
n=0
The elemental composition ratio of silicon to aluminum
Si/Al) of ZSM-5 is detected by X Ray Fluorescence (XRF).
7
890 B chromatograph equipped with INNOWAX capillary
(
column (60 mX0.32 umX0.25 nm) and FID detector. The
product selectivity and conversion rate of feed were calcu-
lated on a molar carbon basis, and the calculation formulas
are as follows:
The XRF was performed on a Bruker S4 Pioneer.
NH -TPD (temperature programmed desorption of
3
ammonia) of the catalyst was performed on Micromeritics
AutoChem II 2920. The typical test procedure was as fol-
lows: The sample was pretreated in He ꢃow at 350 °C for 1 h
nB,in − nB,out
C =
B
× 100%
(2)
nB,in
and then cooled to 100 °C, then adsorption of NH (40 mL/
3
min) at 100 °C for 0.5 h. After adsorption, the sample was
purged under He for 1 h. Subsequently, the temperature was
raised from 100 to 800 °C at a ramping rate of 10 °C/min.
where C , n
and n represent conversion rate of
(B, out)
B
(B, in)
benzene, mole of benzene at the inlet and outlet, respectively.
nC7
The signal of NH desorption was monitored by a thermal
3
SC7
=
∑
× 100%
(3)
conductivity detector (TCD).
n
(C7, C8, C9+)
Pyridine (Py) adsorbed FT-IR studies of HZSM-5 were
performed with a Bruker INVENIO R instrument. For pyri-
dine adsorption (Py-IR), pyridine vapour was introduced
into the cell at 30 °C for 1 h, after evacuation at 150 °C
for 1 h to removed excess pyridine, then the spectra were
recorded.
where C7, C8, C9 + represent toluene, xylene and ethylb-
enzene, heavy aromatics with carbon number larger than
∑
9
, n (C7, C8, C9+) is the total moles of liquid aromatics
products., respectively. And nC7 represents mole of toluene,
respectively. The calculation formula of S and S
refer
C8
C9+
to the above formula.
Nitrogen adsorption–desorption was carried out on a
Micromeritics ASAP 2020 PLUS HD88. Before testing, all
catalysts are degassed for 5 h under vacuum at 150 °C and
isotherms were recorded at 77 K.
ꢀ
ꢁ × 100%
YC7+C8 = C × S + SC8
B
(4)
C7
where Y
represents the total yield of toluene, xylene
C7+C8
and ethylbenzene.
The residual sodium content of the HZSM-5 and elements
ratio of CuO–ZnO–Al O were determined by ICP-AES
2
3
nCO,in − nCO,out
(
inductively coupled plasma atomic emission spectroscopy)
CCO
=
× 100%
(5)
nCO,in
on a PerkinElmer Avio 500 instrument.
The morphology of catalysts were observed by a scanning
electron microscope (SEM) on a Hitachi SU 8010.
where C , n
and n
represent conversion rate of
(CO, out)
CO (CO,in)
CO, mole of CO at the inlet and outlet, respectively.
n
CO2
SCO2
=
× 100%
(6)
2.3 Catalytic Reaction Tests
nCO,in − nCO,out
The alkylation reaction of benzene with syngas was carried
out on a micro ꢀxed-bed reactor of a stainless-steel tube with
1
3

T. Han et al.
i∗ nCi
3.2 XRD
SCi
=
× 100%
(7)
nCO,in − nCO,out
Figure 2 shows XRD patterns of three fresh hybrid catalysts.
The typical characteristic diꢂraction peaks of MFI (PDF#
44-0003) can be clearly identiꢀed in each XRD pattern.
The three main characteristic diꢂraction peaks of CuO and
one weak characteristic diꢂraction peak of ZnO also can be
identiꢀed in XRD patterns, they corresponding to PDF(#65-
where C represents C1–C5 hydrocarbons, i is the carbon
i
number.
ꢀ S
SA = 1 − S
CO2
−
Ci
(8)
where S represents selectivity of C attached to aromatic
A
2
309) and PDF(#65-3411), respectively. There is a sharp
side chain.
characteristic diꢂraction peak located at 2 theta = 26.4°
belongs to graphite (PDF# 01-0646) which are used as adhe-
sive in the CuO–ZnO–Al O component [26]. No character-
In this work, the total carbon molar balance of products
and feed was calculated between 93 and 100%. Considering
the complexity of the alkylation products of benzene and
syngas reaction, the balancing values are reasonable.
2
3
istic diꢂraction peaks of Al O were observed in all XRD
2
3
patterns, suggesting that Al O are highly dispersed in the
2
3
bifunctional catalysts [27]. XRD spectras of dual-functional
catalysts with diꢂerent mass ratios are shown in Fig. S1.
With the increase of the content of CuO–ZnO–Al O in the
2
3
3
Results and Discussion
dual function catalyst, the main characteristic diꢂraction
peaks of CuO, ZnO, graphite C become more obvious.
Figure 3 shows XRD patterns of three corresponding used
catalysts. The typical characteristic diꢂraction peaks of MFI,
ZnO and C remain, and the peaks of CuO disappear yet,
and the peaks of Cu (PDF# 65-9026) appeared clearly. It
reveals that CuO was reduced completely by syngas, during
the reaction, the metal Cu is considered as the active site for
methanol synthesis from syngas [28–30].
3
.1 29Si MAS NMR
The local environment of Si in the ZSM-5 framework were
2
9
examined by Si solid state MAS NMR. It can be seen that
ZSM-5 zeolite has a strong response peak at about − 115
ppm of chemical shift and a small peak located at about
−
105 ppm of chemical shift, which represented the group
of Si(OSi) (denoted as Q4) and (AlO)Si(OSi) (denoted as
4
3
Q3), respectively, as shown in Fig. 1 [25]. Calculated the
framework Si/Al of the three ZSM-5 zeolites by formula (1)
to be 15, 31, 84, respectively. Since Brønsted acid of ZSM-5
is closely related to framework Al of ZSM-5, this article
uses framework Si/Al measured by NMR to link the acidity
of zeolite for research.
3
.3 PY‑IR
The pyridine adsorbed IR spectra of the ZSM-5 samples at
50 °C were presented in Fig. 4 to diꢂerentiate situation
1
Fig. 2 XRD patterns of fresh CuO–ZnO–Al O /ZSM-5 samples with
2
3
diꢂerent framework Si/Al.
a CuO–ZnO–Al O /ZSM-5(84)=1:1
2 3
Fig. 1 29Si MAS NMR spectras patterns of ZSM-5 samples with dif-
(mass ratio); b CuO–ZnO–Al O /ZSM-5(31)=1:1 (mass ratio); c
2
3
ferent Si/Al
CuO–ZnO–Al O /ZSM-5(15) =1:1 (mass ratio)
2 3
1
3

One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al O and ZSM-5 Relay
2
3
Lewis acid decreased simultaneously, suggesting that both
Brønsted and Lewis acid sites decreased.
3
.4 NH ‑TPD
3
The acidity of ZSM-5 with diꢂerent Si/Al were detected by
NH -TPD technology also, as presented in Fig. 5 and the
3
acid amount of the ZSM-5 were summarized in Table 1. Two
distinctive desorption peaks at 120–350 °C and above 350
°
C were shown for the ZSM-5 (15), which can be attributed
to NH desorption from the weak & medium and strong acid
3
sites of zeolite [36–38], respectively. As shown in Table 1,
the Si/Al values of the three zeolites measured by XRF are
1
4, 86 and 140, which is consistent with the law measured by
NMR. The diꢂerence between the results measured by NMR
and XRF due to the presence of non-framework aluminum in
the zeolite. It can be seen from the experimental results: as
the ratio of Si/Al (NMR and XRF) increases, both desorp-
tion peaks became smaller and the peaks position moves to
lower temperature, which indicates the acid amount gradu-
ally decreases and acid strength has also decreased as Si/
Al increases.
Fig. 3 XRD patterns of used CuO–ZnO–Al O /ZSM-5 samples with
2
3
diꢂerent Si/Al. a CuO–ZnO–Al O /ZSM-5(84)=1:1 (mass ratio); b
2
3
CuO–ZnO–Al O /ZSM-5(31)=1:1 (mass ratio); c CuO–ZnO–Al O /
2
3
2
3
ZSM-5(15)=1:1 (mass ratio)
3.5 BET and ICP Analysis
The N adsorption desorption isotherms and pore size
2
distribution curves of ZSM-5 and CuO–ZnO–Al O were
2
3
illustrated in Figure S2. All of the ZSM-5 with differ-
ent Si/Al have a small number of mesoporous pores,
which enhance the diffusion of the products and con-
tribute to highactivity. It can be seen from Table 1 and
Table S1, the BET surface areas were 311, 371, 400 and
2
9
0 m /g for ZSM-5(15), ZSM-5(31), ZSM-5(84) and
Fig. 4 FT-IR spectra of pyridine adsorption on HZSM-5 with diꢂer-
ent Si/Al at 150 °C
of Lewis and Brønsted acid sites of ZSM-5. The peaks
−
1
at 1450 and 1545 cm correspond to Lewis acid sites
and Brønsted acid sites of ZSM-5, respectively [20, 31,
3
2]. Brønsted acids mainly come from the -OH group on
the framework of ZSM-5 where the Si is replaced by Al,
resulting in an imbalance of charge and then -OH pro-
duced by adsorption of proton, while Lewis acid mainly
3
+
originates from the coordinated unsaturated Al [33–35].
It can be seen that as the framework Si/Al of ZSM-5
increased from 15 to 84 the peaks referring Brønsted and
Fig. 5 NH -TPD proꢀles of HZSM-5 with diꢂerent Si/Al
3
1
3

T. Han et al.
Table 1 Element composition
and texture properties as well
as the acidity of HZSM-5 with
diꢂerent Si/Al
Si/Alb
Surface area / Pore volume / Acid amount (mmol/g)d
c
c
Sample (frame-
Total acid
a
2
−1
3
−1
work Si/Al)
m ·g
cm ·g
amount
Weak and
Medium
Strong
mmol/g)d
(
e
HZSM-5 (15)
14
86
140
311
371
400
0.22
0.24
0.24
0.18
0.06
0.01
0.23
0.06
0.02
0.41
0.12
0.03
e
HZSM-5 (31)
e
HZSM-5 (84)
a
Framework Si/Al of ZSM-5 was calculated by formula (1)
Si/Al of ZSM-5 was measured by XRF
Surface area and pore volume were measured by nitrogen adsorption–desorption
b
c
d
e
Acid amount of ZSM-5 was determined by NH -TPD
3
The residual sodium content of all ZSM-5 is less than 0.6%, measured by ICP
CuO–ZnO–Al O , with a pore volume of 0.22, 0.24, 0.24
3.6 SEM
2
3
3
and 0.30 cm /g, respectively. With the increase of Si/Al,
the surface of zeolites increase correspondingly. In addi-
tion, the residual sodium content of ZSM-5 were detected
by ICP. The residual sodium content of all ZSM-5 is
minimal, indicating that ZSM-5 with different Si/Al
are indeed H-type zeolite. The Cu/Zn/Al molar ratio of
CuO–ZnO–Al O component is 65.8/26.5/7.7, as detected
Figure 6 showed the SEM images of CuO–ZnO–Al O /
2
3
ZSM-5(31) bifunctional catalyst and two active ingredi-
ents. The ZSM-5 sample has a hexagonal exterior shape
with a particle length of around 0.5 μm (Fig. 6a). And the
CuO–ZnO–Al O (Fig. 6b) showd irregular particles, plates
2
3
and their aggregates, similar to the morphology observed
by Jong-Wook Bae [39], Fang [40], et al. In hybrid catalyst,
some of the CuO-ZnO-Al O particle clusters are scattered
2
3
by ICP (Table S1).
2
3
around the ZSM-5 and some of CuO–ZnO–Al O particles
2
3
are deposited on the surface of ZSM-5.
Fig. 6 SEM images of a
HZSM-531), b CuO–ZnO–
Al O and c CuO–ZnO–Al O /
2
3
2
3
ZSM-5(31)=1:1 (mass ratio)
1
3

One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al O and ZSM-5 Relay
2
3
3.7 Catalysts Evaluation
3.7.1 Eꢀect of Si/Al on Catalytic Performance
Figure 7 showed the catalytic performance of benzene
alkylation with syngas over the Cu–ZnO–Al O /ZSM-5
2
3
with different Si/Al ratios. The benzene conversion of
Cu–ZnO–Al O /ZSM-5(84) was only about 25.2%, while
2
3
those of Cu–ZnO–Al O /ZSM-5 with smaller Si/Al showed
2
3
signiꢀcant improvements of conversions. The benzene con-
version increaseed to 50.7% in Cu–ZnO-Al O /ZSM-5(31)
2
3
and then reduced to 45.6% in Cu–ZnO-Al O /ZSM-5(15).
2
3
The similar result were found by Yu [19], Gao [41] and
Wang [42]. C7 and C8 aromtics, including toluene, ethylben-
zene and xylene, are the most important alkylbenzene chem-
icals. It was observed that for all experiments the total selec-
tivity of C7 and C8 is maintained between 88.7 and 94.4%.
It can be seen from Fig. 7 b that the total yield of C7 and C8
of the Cu–ZnO–Al O /ZSM-5(31) catalyst reached 45.0%
2
3
which is the highest among all catalysts. For the gaseous
phase, CO conversions changes in the same way as benzene
conversion, and the Cu–ZnO–Al O /ZSM-5(31) showed the
2
3
highest CO conversion of 55.0%. It also showed the highest
selectivity of carbon atoms in CO attached to aromatic side
chain, suggesting higher eꢁciency of CO utilization. As for
the selectivity of CO , CH , C2–C5 byproducts, for each
2
4
catalyst, Cu–ZnO–Al O /ZSM-5(84) catalyst has higher
2
3
selectivity of CO , which originated from the water–gas-
2
shift reaction [43–45]. And the Cu–ZnO–Al O /ZSM-5(15)
2
3
catalyst showed signiꢀcantly higher C2–C5 selectivity.
The alkylation of benzene and syngas is considered to
be the coupling of the two reactions of syngas to methanol
and benzene alkylation with methanol. Cu–ZnO–Al O and
2
3
ZSM-5 hybrids can perform relay catalysis, the methanol
formed from syngas over Cu–ZnO–Al O component under-
2
3
goes alkylation reaction with benzene at the Brønsted acid
sites of the ZSM-5 to produce the target product alkylben-
zenes. It was found through detailed experiments carried out
at 400 °C, the alkylation of benzene and methanol requires
suitable strength and density of Brønsted acids [46].
NMR, PY-IR and NH -TPD all showed that
3
Cu–ZnO–Al O /ZSM-5(31) catalyst contains the appropri-
2
3
ate Brønsted acidity, which is the key reason why conversion
rate of benzene is the highest and selectivity of C2–C5 by-
products is the lowest. It has been proposed that Brønsted
acids played a vital role in the alkylation reaction of benzene
and methanol, meanwhile strong Brønsted acids will inevita-
bly catalyze the side reaction of methanol to produce C2–C5
hydrocarbons [47, 48].
Fig. 7 Eꢂect of Si/Al of ZSM-5 on a the liquid products selectivities,
b the total yield of C7 and C8 and c the gas phase products selec-
tivities of Cu–ZnO–Al O /ZSM-5 catalyst with the mass ratio of
Cu–ZnO–Al O /ZSM-5(15) preferentially catalyzes the
2
3
2
3
side reaction, such as methanol to oleꢀns (MTO) due to its
too strong and rich Brønsted acid sites. And the oleꢀns can
be hydrogenated to C2–C5 hydrocarbons, or alkylated with
two components of 1. Experiment was carried out under 4.0 MPa
and 400 °C. Syngas (H :CO=2:1) GHSV=5220 mL/h g . Benzene
2
cat
WHSV=2.64 g/h g
cat
1
3

T. Han et al.
benzene to long-chain alkylated benzenes, thus reduced the
amount of methanol directly attached to the aromatic side
chain, which leads to a decrease in the conversion of ben-
zene and an increase in the selectivities of ethylbenzene and
C2–C5. The increase in ethylbenzene leads to a decrease
in the selectivity of para-xylene in C8, which is the most
demanded of alkylbenzenes. When the Si/Al of the dual-
function catalyst increases to 84, too weak Brønsted acid
will limit the alkylation reaction, which leads to the syngas
to be converted into CH , CO , C2–C5 under the catalysis
4
2
of Cu–ZnO–Al O . This is due to that if the methanol can-
2
3
not be consumed by the alkylation immediately the syngas
will be favored to produce more byproducts, such as CO
2
and hydrocarbons, especially at high temperature. By con-
structing the relationship between the catalyst acid density
and the activity (Table S2), it was found that the appropri-
ate increase of the catalyst acid density was conducive to
the rapid alkylation of methanol intermediates and benzene,
improving the activity, but too high acid density would give
priority to the conversion of methanol into hydrocarbons,
leading to the decrease of the alkylation activity.
Based on above discussion, it was reasonable to conclude
that the catalyst with appropriate Brønsted acid strength and
acid density can exhibit the highest activity due to minimize
the occurrence of the side reaction of methanol.
3.7.2 Eꢀect of Reaction Temperature on Catalytic
Performance
Figure 8 showed the catalytic performance of
Cu–ZnO–Al O /ZSM-5(31) catalyst in the alkylation of
2
3
benzene with syngas at diꢂerent reaction temperatures.
As the reaction temperature increased from 300 to 450 °C,
the conversion of benzene showed an increase from 23.9
to 50.7% at 400 °C, and then dropped to 39.5% at 450 °C,
the conversion of benzene reached the highest maximum at
Fig. 8 Eꢂect of reaction temperature on a the liquid products selec-
tivities and b the total yield of C7 and C8 of Cu–ZnO–Al O /
2
3
ZSM-5(31) catalyst with the mass ratio of two component of 1. All
experiment was carried out under 4.0 MPa. Syngas (H :CO=2:1)
2
GHSV=5220 mL/h g . Benzene WHSV=2.64 g/h g
cat
4
00 °C. The change trend of the total yield of C7 and C8 is
cat
consistent with that of the benzene conversion, reaching the
maximum of 45.0% at 400 °C. As shown in Fig S3, when the
reaction temperature is 400 °C, the CO conversion rate is the
highest and the selectivity of CO participating in the alkyla-
tion reaction is also the highest. No methanol intermediate
were detected in product, except at 300 °C, 0.5% of methanol
was detected in the liquid phase product, suggesting that
the methanol was consumed immediately by alkylation of
benzene at higher reaction temperature.
reaction of benzene and syngas, when the temperature is
lower than 400 °C, although it is beneꢀcial to the production
of intermediate methanol from syngas on Cu–ZnO–Al O ,
2
3
the ability of ZSM-5 to activate benzene is limited due to
the low reaction temperature. Therefore, when the reaction
temperature is lower than 400 °C, the benzene alkylation
rate is low and the selectivity of by-products increase. As
an active phase of high temperature methanol synthesis,
ZnO component has a synergistic eꢂect with Cu at over 400
°C to produce methanol [52, 53]. In addition, the generated
methanol will be continuously consumed by the alkylation
reaction, breaking the thermodynamic balance of methanol
synthesis, resulting in an increase in the methanol synthesis
capacity of Cu–ZnO–Al O at high temperatures. Thus the
There is a temperature matching relationship between
the two components in the Cu–ZnO–Al O /ZSM-5(31)
2
3
dual-function catalyst [20, 49]. Generally, the tempera-
ture of Cu–ZnO–Al O catalyzed synthesis of methanol at
2
3
2
30–280 °C [50–52], while the HZSM-5 catalyzed benzene
and methanol alkylation needs to be higher than 400 °C [20,
9]. In the Cu–ZnO–Al O /ZSM-5(31) catalyzed alkylation
4
2
3
2
3
1
3

One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al O and ZSM-5 Relay
2
3
coupling reaction reached its maxium activity at 400 °C. In
addition, due to the optimal coordination of the two-stage
reaction at this temperature, the maximum alkylation uti-
lization rate of CO is achieved. But when the temperature
reaches 450 °C, the excessively high reaction temperature
caused the Cu to sinter quickly, suppressing the synthesis of
methanol. Without the suꢁcient methanol produced, and the
catalyst activity was decreased.
Therefore, the optimal reaction temperature of
Cu–ZnO–Al O /ZSM-5(31) catalyzed benzene and syngas
2
3
alkylation is 400 °C. At this temperature, the two active com-
ponents of bifunctional catalysts can exploit their respective
advantages to the full and achieve the best catalytic eꢂect.
3
.7.3 Eꢀect of Mass Ratio of Cu–ZnO‑Al O and ZSM‑5
2 3
on Catalytic Performance
The effects of different mass ratios of Cu–ZnO–Al O
2
3
and ZSM-5 on the catalytic performance of dual-func-
tion catalysts can be seen in Fig. 9. When there was only
Cu–ZnO–Al O component, the benzene conversion rate is
2
3
only 14.2%, and there is only a small amount of toluene in
the product, no C7+product is detected. At the same time,
there are 10.7% cyclohexane in the liquid phase product. So
it is reasonable that pure Cu–ZnO–Al O is hardly active
2
3
to catalyzing the alkylation reaction of benzene, and pref-
erentially catalyzes the hydrogenation reaction of benzene;
When the catalyst only contain ZSM-5 component, benzene
does not react, the CO conversion rate is only 2.0%, and
the main gas phase product is CH , this result is similar
4
to Fan’s[49]. The speciꢀc gas phase product distribution is
shown in Fig. S4, the CO conversion rate reaches the maxi-
mum when the mass ratio of the two components is 1. As the
mass ratio of Cu–ZnO–Al O /ZSM-5 gradually decreases
Fig. 9 Eꢂect of mass ratio of Cu–ZnO–Al O and ZSM-5 on a the
2
3
2
3
liquid products selectivities and b the total yield of C7 and C8 of
from 5:1 to 1:5, the conversion rate of benzene and the total
yield of C7&C8 are volcano curves, namely ꢀrst increases
and then decreases. When the mass ratio of the two compo-
nents is 1:1, activity and yield reach their maximum values
simultaneously.
Cu–ZnO–Al O /ZSM-5(31) catalysts. All experiment was carried out
2
3
under 400 °C, 4.0 MPa. Syngas (H2:CO=2:1) GHSV=5220 mL/h
g
. Benzene WHSV=2.64 g/h g
cat
cat
too high, more methanol intermediates will be generated.
On the contrary, the less ZSM-5 leads to insuꢁcient cata-
lytic alkylation, formed methanol cannot be alkylated, or be
converted into by-products. Similarly, when the proportion
of Cu–ZnO–Al O is too small, too few methanol interme-
The two active components of Cu–ZnO–Al O and
2
3
ZSM-5 in the dual-function catalyst perform relay cataly-
sis. In the ꢀrst step, Cu–ZnO–Al O catalyzed synthesis
2
3
of methanol, and then ZSM-5 catalyzed the alkylation of
benzene and methanol to alkylbenzenes in the second step.
The most suitable ratio of Cu–ZnO–Al O with ZSM-5 can
2
3
diates are produced, which limits the alkylation of benzene
and methanol. Therefore, when the two active components
of Cu–ZnO-Al O /ZSM-5 are mechanically mixed with the
2
3
generate an appropriate amount of methanol intermediates
to react with benzene, minimize side reactions of metha-
nol. In addition, the proper amount of ZSM-5 can rapidly
catalyze the alkylation reaction of methanol and benzene
at an appropriate temperature, breaking the thermodynamic
equilibrium of methanol synthesis, and making the entire
reaction system proceed in the direction required. When the
proportion of Cu–ZnO–Al O in the bifunctional catalyst is
2
3
mass ratio of 1:1, the catalytic eꢂect of benzene and syngas
is the optimal.
3.8 Stability Tests
The 24 h stability tests of Cu–ZnO–Al O /ZSM-5 is shown
2
3
in Fig. 10. It can be seen from the Fig. 10 that the initial
2
3
1
3

T. Han et al.
achieving excellent catalytic eꢂects. Therefore, the hybrid
catalyst will further improves the reaction activity,selectivity
and stability through the optimal matching of the two com-
ponents in the future.
3.9 Reaction Pathway and Mechanism
The alkylation of benzene and syngas is a very complex
reaction system, which involves multiple reactions occurring
simultaneously. By studying the eꢂect of Si/Al of ZSM-5 on
the catalytic performance, it is found that when the Brøn-
sted acidity of zeolite is too weak, the benzene conversion
rate decreases while the gas by-products increase. When
the zeolite has too strong Brønsted acidity, the side reaction
of methanol intermediate increase. Therefore, we infered
the reaction paths of catalysts with diꢂerent acid properties
based on the product distribution and showed in Fig. 11.
Firstly, Cu–ZnO–Al O catalyzes the syngas to generate
2
3
methanol, then the methanol moves to Brønsted acid sites
of the zeolite and reacts with benzene to generate toluene,
the alkylated benzene can reacts secondly to produce xylene
and C9+aromatics. It is worth noting that the role of ZnO
and Al O in the alkylation reaction of benzene and syngas
2
3
is very important, ZnO alone has the ability to synthesize
methanol [58] and can produce a synergistic eꢂect with Cu,
moreover Al O is often used as an additive to increase the
2
3
surface area of the catalyst and improve the catalyst active
[
30, 59–61].
Simultaneously, the methanol intermediate produced is
prone to many reactions such as MTO [62]., MTG [63, 64]
and MTA [65] on the acid sites of the zeolite. As shown in
Table S3, very high alkanes content, due to (1) reactions
such as MTG and MTA occur on the acid sites of zeolite, (2)
oleꢀn hydrogenation reaction on Cu–ZnO–Al O surface.
Fig. 10 Stability of Cu–ZnO–Al O /ZSM-5(31) in alkylation of
2
3
2
3
benzene with syngas. All experiment was carried out under 400 °C,
The zeolite has too weak Brønsted acid sites, methanol can-
not be consumed in time through the alkylation reaction,
which causes the syngas to generate CH , C2–C5, CO under
4
.0 MPa. Syngas (H :CO=2:1) GHSV=5220 mL/h g . Benzene
2 cat
WHSV=2.64 g/h g
cat
4
2
the catalysis of Cu–ZnO–Al O , as evidenced by the reac-
2
3
conversion rate of benzene and CO is the highest. But as
the reaction runs, the conversion rate drops rapidly, and the
catalyst is deactivated after the reaction runs stably for 18 h.
The experimental results show that the Cu–ZnO–Al O /
tion on pure Cu–ZnO–Al O (Fig. S4). CO in gas phase
2 3 2
productsis considered to be mainly formed by the water gas
shift reaction [66, 67]. Based on the above discussion, we
proposed the reaction paths on hybrid catalysts with diꢂer-
ent acidity, as shown in Fig. 11. When the Si/Al of ZSM-5
is smaller and ZSM-5 contains more strong Brønsted acid,
the entire reaction system will be biased towards the reac-
tion path 1, the content of toluene and xylene will decrease
2
3
ZSM-5 has a high initial conversion rate, which illustrates
that Cu–ZnO–Al O component has a strong ability to cata-
2
3
lyze methanol synthesis, even at relatively high tempera-
tures. However, Cu is easy to sinter at high temperature, the
high reaction temperature causes the methanol synthesis
capacity of hybrid catalyst to decrease with time [22]. In
addition, as the reaction progresses, carbon deposition on
zeolite is also an important cause of catalyst deactivation
while ethylbenzene, cumene, CH , and C2-C5 content will
4
increase due to mainly the side reaction of methanol; When
the Si/Al of ZSM-5 is suitable, the entire reaction system
will be proceed towards the reaction path 2, and more tolu-
ene and xylene alkylated products will be generated; when
the Si/Al of ZSM-5 is large, the Brønsted acid of ZSM-5 is
too weak, the entire reaction system will be biased towards
[
54]. The studies of Kang [55], Zhou [56] and Zhang [57]
have shown that the coordination of various components in
the catalyst in the tandem reaction is an important reason for
1
3

One-Pass Conversion of Benzene and Syngas to Alkylbenzenes by Cu–ZnO–Al O and ZSM-5 Relay
2
3
Fig. 11 The reaction paths over
diꢂerent active sits in the ben-
zene alkylation with syngas
the reaction path 3, generating less toluene and more CO ,
strong Brønsted acid will preferentially convert methanol
intermediate to hydrocarbons, resulting in a decrease of
activity, and the optimal framework Si/Al is 31.
4
CO , C2-C5. Therefore, we can control the acidity of the
2
zeolite in the dual-function catalyst to achieve diꢂerent reac-
tion paths according to product requirements.
According to the product distribution, the reaction
paths are inferred. Among them, ethylbenzene and cumene
mainly come from the alkylation of benzene with ethyl-
4
Conclusion
ene and propylene which are produced by the MTO, CO
2
mainly comes from the water gas shift reaction, and CH ,
4
A high-activity dual-functional Cu–ZnO–Al O /ZSM-5
C2–C5 mainly come from the side reaction of methanol
2
3
catalyst is developed for the alkylation of benzene with
syngas. The catalyst realizes the one-step manufacture
of alkylbenzenes from benzene and syngas through the
coordination of two active components and methanol is
deemed as the reaction intermediate. By studying the
inꢃuence of Si/Al on the catalytic performance, we found
that the Brønsted acid acid of the dual-functional catalyst
is indispensable for alkylation of benzene, but too much
intermediates.
Benzene conversion of 50.7%, CO conversion of 55.0%
and total yield to C7 and C8 of 45.0% are achieved over
Cu–ZnO–Al O /ZSM-5(31) = 1:1 (mass ratio) at 400 °C
2
3
and 4 MPa. Our optimal catalyst can make the two active
components of the dual function catalyst achieve a better
match, achieve higher activity.
1
3

T. Han et al.
Supplementary Information The online version contains supplemen-
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(1988) ChemInform 19:1–65
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Acknowledgements This project was sponsored ꢀnancially by the
National Natural Science Foundation of China (No. 22078351)
3
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