The green and sustainable synthesis of nanometer ZSM-5 without organotemplates via utilizing the recycled mother liquid

Article abstract of DOI:10.1039/d1nj01283e

The development of green and sustainable techniques for synthesizing classes of significant industrial materials, like zeolites, has been recognized as an inevitable trend in recent years. Herein, an efficient and environmentally friendly route was exploited for the synthesis of nanometer ZSM-5 zeolites with recycled mother liquid and seed-assistance. Physical characterization and component analysis revealed that the unutilized species in the raw materials remain in a stable concentration range during the nine recycling experiments, and these recyclable species were successfully applied for the preparation of ZSM-5 zeolites as part of the raw materials. It is noticed that the particle size of the ZSM-5 aggregates (～0.3-0.5 μm) synthesized from the recycled mother liquid sharply decreases compared to that using fresh catalyst (～1.1-1.4 μm), which is attributed to the synergetic effect of the self-made S-1 seeds and the primary structural units with memory effects in the mother liquid, which is helpful for promoting the formation of small-sized particles. Furthermore, the as-obtained samples possess almost the same catalytic performance in cyclohexene hydration as fresh catalyst, even though the mother liquid was recycled nine times. These satisfactory results indicate that this cycled synthesis process effectively utilized the ingredients in the mother liquid and avoided waste liquid discharge to achieve clean and efficient synthesis. Additionally, the green process opens a pathway to sustainable zeolite preparation in industry compared to conventional methods.

Full text of DOI:10.1039/d1nj01283e

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The green and sustainable synthesis of nanometer
ZSM-5 without organotemplates via utilizing the
recycled mother liquid†
Cite this: DOI: 10.1039/d1nj01283e
Lukuan Zong, Hongxia Xu, Xiangyu Wang* and Huijuan Wei
*
The development of green and sustainable techniques for synthesizing classes of significant industrial
materials, like zeolites, has been recognized as an inevitable trend in recent years. Herein, an efficient
and environmentally friendly route was exploited for the synthesis of nanometer ZSM-5 zeolites with
recycled mother liquid and seed-assistance. Physical characterization and component analysis revealed
that the unutilized species in the raw materials remain in a stable concentration range during the nine
recycling experiments, and these recyclable species were successfully applied for the preparation of
ZSM-5 zeolites as part of the raw materials. It is noticed that the particle size of the ZSM-5 aggregates
(B0.3–0.5 mm) synthesized from the recycled mother liquid sharply decreases compared to that using
fresh catalyst (B1.1–1.4 mm), which is attributed to the synergetic effect of the self-made S-1 seeds and
the primary structural units with memory effects in the mother liquid, which is helpful for promoting the
formation of small-sized particles. Furthermore, the as-obtained samples possess almost the same
catalytic performance in cyclohexene hydration as fresh catalyst, even though the mother liquid was
recycled nine times. These satisfactory results indicate that this cycled synthesis process effectively
utilized the ingredients in the mother liquid and avoided waste liquid discharge to achieve clean and
efficient synthesis. Additionally, the green process opens a pathway to sustainable zeolite preparation in
industry compared to conventional methods.
Received 16th March 2021,
Accepted 14th April 2021
DOI: 10.1039/d1nj01283e
rsc.li/njc
To date, there have been numerous reports of green and
1
. Introduction
effective approaches for the preparation of zeolites via utilizing
1
0
Molecular sieves have already been industrially produced and the mother liquid from the previous batch synthesis. Rani has
widely applied in industry for half a century such as in studied the effect of mother liquid on the synthesis of A and Y
petroleum refining, gas adsorption and separation, and ion molecular sieves. It was found that when the mother liquid was
1
–3
exchange.
However, the traditional synthesis routes often added to the synthetic silica-alumina solution, the corres-
suffer from issues such as requiring expensive organic structure ponding skeleton structure of the molecular sieve could be
directing agents (SDAs) and causing the production of harmful directly formed by the primary structural unit in the mother
1
1
x 2
gases like NO and CO in the high temperature calcination liquid. Liu used recycled mother liquid in the synthesis of
process, as well as wasting a large amount of water and hollow titanium silicalite-1, the mother liquid was recycled
unreacted raw materials in the synthesis process, especially in eight times and each time hollow TS-1 was obtained, which
4
,5
industry production.
Therefore, from the viewpoint of showed similar catalytic activity in propylene epoxidation to
sustainable synthesis, it is still a vital challenge to acquire that of the sample prepared by using TPAOH as the desilication
1
2
low-cost environmentally friendly molecular sieves. To alleviate medium. Hu also developed a highly efficient and green
these problems, a variety of synthesis routes have been developed synthesis of TS-1 in a flow system by recycling the mother
1
3
for the preparation of molecular sieves such as free-organic liquid. Pan successfully synthesized a ZSM-5 molecular sieve
6
,7
8,9
template synthesis, solid-state or quasi-solid-state synthesis
with a low Si/Al ratio using the NaY mother liquid and found
that the recovery of the mother liquid is beneficial to increase the
10–17
and mother liquid cycle synthesis methods.
14
relative crystallinity and optimize crystal morphology. Bergk
synthesized a type-A molecular sieve using industrial NaA crystal-
lization mother liquid, in which the method mainly utilizes Al
Green Catalysis Center, College of Chemistry, Zhengzhou University, China.
E-mail: weihuijuan@zzu.edu.cn, wangxiangyu@zzu.edu.cn
2 3
O
15
and Na
2
O in the mother liquid. Bing attempted an economical
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj01283e
and green method for the synthesis of Cu-SAPO-34 catalysts by
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recycling the waste mother liquid and the prepared sample 2. Experimental section
exhibited superior catalytic activity for the selective catalytic
2
.1 Synthesis of ZSM-5 crystals from fresh solution
1
6
reduction of NO with NH . In addition, Bing’s group also
3
The ZSM-5 zeolite was synthesized by a seed-directed template-
free route according to a slight modification of our reported
procedure.
successfully prepared SSZ-13 catalysts by recycling the waste
mother liquid without adding an organic template in the initial
2
8,29
1
7
Colloidal silica (30 wt%) and fumed silica
gel. Duan investigated the syntheses of cobalt-substituted
aluminophosphate molecular sieves in a low-cost way via
recycling the waste mother liquid. According to the reviewed
literature, the reuse of mother liquid to prepare zeolites can be
further improved, such as by avoiding the use of expensive
SDAs, increasing the number of mother liquid recycling times
and reducing the particle size of the sample. Therefore, it is
important to develop a method to extend the number of
recycling times of the mother liquid without affecting the
catalytic performance of the samples obtained by using the
mother liquid. The preparation method via the recycled mother
liquid not only reduces the waste of unreacted raw materials
but also provides the scientific basis for the sustainable
development of zeolite preparation in industry.
(
AEROSIL 200, Evonik degussa) were used as the silicon source,
fumed silica in mixed silicon sources is mainly applied to
balance the amount of water in the next batch feedstock.
Briefly, a specified amount of Al(OH)3 and NaOH (as the
alumina and alkali source) were first dissolved in distilled
water and the resultant solution was heated for several minutes
until the mixture became a clear solution (solution A). Then
colloidal silica and fumed silica were mixed with distilled water
(solution B). After that, solution B was added to solution A
dropwise to obtain a homogeneous solution under stirring.
Finally, the 2.5 wt% self-made silicalite-1 (S-1) seed was added
to the mixture, the as-prepared mother gel with the molar
composition 11.5Na O : 100SiO :3Al O : 2750H O was crystallized
at 443 K for 36 h after aging at ambient temperature for 24 h. The
synthesized solid product was recovered by centrifugation, washed
with distilled water several times, and dried at 373 K for 12 h.
Furthermore, the H-type ZSM-5 was obtained from ion-exchange
with HNO
solid ratio of 11 cm g . Subsequently, the sample was centri-
fuged, washed with distilled water several times, and dried at
3
2
2
2
3
2
MFI-type aluminosilicate zeolites (ZSM-5), a class of well-
known crystalline molecular sieves, have been widely employed
as solid acid catalysts in catalysis reactions such as the
1
8,19
20,21
hydration of cyclohexene,
methanol to olefins,
toluene
À1
2
2
23
3
solution (1.0 mol L ) at 313 K for 3 h with a liquid/
disproportionation and xylene isomerization due to their
uniform microporous channels, tunable acidities, excellent
shape/size selectivity, and outstanding hydrothermal stabilities.
Generally, there are two traditional methods usually adopted to
synthesize ZSM-5 zeolites. The first is using organic amines as
template agents, the obtained ZSM-5 sample must be calcined
and this process will produce a large amount of waste gas
3
À1
73 K in an oven overnight. The resulting solid product was
labelled ZSM-5-0, and the collected mother liquid was stored for
the subsequent recycle. All the crystallized zeolite was dried by
using a conventional oven without calcination.
2
4,25
pollution.
The second is the amine-free method with
2
.2 Synthesis of nano-sized ZSM-5 zeolites with recycled
aluminum sulfate and colloidal silica or sodium silicate as raw
materials, but this process gives off redundant sulfate pollution
mother liquid
2
6,27
and waste discharge still occurs.
Additionally, the waste After synthesis of the ZSM-5 zeolite, the mother liquid obtained
mother liquid from these two synthesis processes is hardly ever from the previous batch synthesis was used as H O and as part
2
reused because the organic amine may decompose in the of the raw materials in the next procedure of the samples. The
crystallization and the sulfate ions in the waste mother elemental composition and amount of the mother liquid were
liquid are difficult to remove. Mother liquid consisting of measured to satisfy the molar composition of each batch for
alkali, silicates and aluminates, increases the cost for post- zeolite synthesis. Moreover, the molar composition of the
synthesis separation and leads to waste liquid pollution and initial gel was kept constant by supplementing the fresh raw
health issues. Consequently, all of these issues stimulate the materials without adding any organic templates. The synthesis
great desire for exploring environmentally friendly and low-cost procedure was the same as that of the ZSM-5-0 sample except
ZSM-5 zeolites.
that the mother liquid was added as part of the raw materials.
Herein, a green and economical strategy was developed for The mother liquid of the ZSM-5-n series was recycled nine times
synthesizing ZSM-5 zeolites without an organic template via and the corresponding samples were denoted ZSM-5-1, ZSM-5-2,
using recycled mother liquid. After the synthesis of the ZSM-5 ZSM-5-3, ZSM-5-4, ZSM-5-5, ZSM-5-6, ZSM-5-7, ZSM-5-8 and
zeolite, the elemental composition and amount of the mother ZSM-5-9, respectively. For comparison, ZSM-5 zeolites without
liquid were measured to satisfy the molar composition of each self-made S-1 seeds were prepared via using recycled mother
batch in the zeolite synthesis, and the mother liquid obtained liquid under the same conditions with the above synthesized
from the previous batch synthesis was used as H
2
O and as part ZSM-5-n zeolites, and the obtained samples were named Z-cycle
of the raw materials in the next batch of the ZSM-5 samples. n (n = 1, 2, 3).
Furthermore, to verify the feasibility of this novel approach, the
In a typical synthesis, ZSM-5 zeolite was obtained by filtration
catalytic performances were determined for the hydration of after crystallization. The amount of the mother liquid obtained
cyclohexene over the ZSM-5 samples synthesized with recycled from the batch synthesis accounted for about 80% of the initial
mother liquid, which was carried out in a stainless-steel reactor gel amount (e.g., initial gel = 180 g, mother liquid = 144 g).
of 500 mL capacity.
The pH values of the initial gel and the mother liquid were
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13 and 12, respectively. About 15 g of mother liquid obtained agitation and a temperature control device. In each run,
each time was reserved for characterization analysis, and the specified amounts of catalyst, water and cyclohexene as
remaining mother liquid was all used for the next synthesis.
.3 Characterization of ZSM-5
reactants were first added into the reactor. Then, the reactor
was filled with nitrogen at a pressure of 0.45 MPa because the
hydration reaction is generally considered to occur in the
2
X-Ray powder diffraction (XRD) patterns were collected on a aqueous phase. Thereafter, the reaction mixture was heated
Panalytical X’pert PRO Diffractometer (40 kV, 40 mA) using Cu up to 399 K with slow stirring (600 rpm) to ensure uniform
Ka radiation (l = 1.540598 Å), from 5 to 501 with a scan rate of temperature distribution and avoid the deposition of catalysts.
À1
1
.21 min . The relative crystallinity (RC) of the as-prepared When the mixture was heated to reaction temperature, the
samples was quantified by comparing the sum of the integrated agitation speed of 900 rpm was set at the same time and the
peak areas of the characteristic peaks at 2y = 7.81, 8.81, 23.01, corresponding time was regarded as the zero-reaction time.
3
0
23.91 and 24.41 based on the fully crystallized ZSM-5 zeolite.
Finally, the reactor was cooled to ambient temperature after a
The N sorption isotherms at the temperature of liquid nitrogen fixed reaction time. The obtain samples from the organic phase
2
were performed using the Quantachrome NOVA1000e analyzer were analyzed on a GC 9790 gas chromatograph equipped with
at 77 K and the samples were vacuum degassed at 573 K for 12 h a flame ionization detector and a 30 m capillary column, using
before analysis. The total surface area (SBET) of the ZSM-5 zeolites the internal standard analysis method. Furthermore, the
was calculated by the multi-point Brunauer–Emmett–Teller conversion of cyclohexene (Conv%) and the yield (Yiel%) and
(BET) method based on the adsorption data at relative pressure selectivity (Sel%) of cyclohexanol can be calculated according to
ranging from 0.001 to 0.05. The micropore surface area (Smic), the following equation:
external surface area (Sext) and the micropore volume (Vmic) were
calculated by the t-plot method. The total pore volume (Vtot) was
Con (%) = (n À n )/n  100%
0
1
0
0
determined at P/P = 0.99. Scanning electron microscopy (SEM)
Yiel (%) = n/n
0
 100%
images were observed on a Quanta FEG 250 scanning micro-
analyser to obtain the particle size and morphology of the
2
7
Sel (%) = Yiel (%)/Con (%) Â 100%
synthesized ZSM-5 zeolites. Al solid-state NMR experiments
were performed under magic angle spinning (MAS) on a DSX-300
spectrometer equipped with a 4 mm probe to investigate
the coordination of Al in the framework for the samples.
The amount of Si and Al in the zeolites was determined by
PANalytical Axios-3600 X-ray fluorescence (XRF) spectroscopy.
X-Ray photoelectron spectroscopy (XPS) measurements were
carried out on an AXIS supra spectrometer to monitor the
valence states of the metal ions. The ammonia temperature
where n is the initial number of moles of cyclohexene. The n
and n values are the final number of moles of cyclohexanol
and cyclohexene in the products after reaction, respectively.
0
1
3
3
. Results and discussion
.1 Elemental composition of the mother liquid
The elemental composition of the mother liquid was detected
and is listed in Table 1. It can be seen that with the increase in
the number of recycling times of the mother liquid, the
concentration of Si in the mother liquid increases first and then
tends to stabilise after the fifth cycle. The Al and Na contents
basically remain stable from the first cycle and the alkalinity of the
mother liquid remains in a stable concentration range during the
nine cycle experiments, indicating that these components in the
mother liquid were found to be stable and recyclable. Therefore,
the concentration stability of each component in the mother
liquid provides a basis for preparing the ZSM-5 via recycled
mother liquid as part of the raw materials.
3
programmed desorption (NH -TPD) was carried out with a
Zhejiang Fantai Corp detector to evaluate the acid strength
and amount. In a typical run, all samples were pretreated at
À1
6
00 1C for 2 h in an argon stream with a rate of 1.0 mL s in
order to remove the physisorbed water. Then, ammonia was
adsorbed at 100 1C until saturated, followed by flushing with He
for 120 min at 100 1C. The desorption of NH was recorded by
3
programmed heating in a range from 100 to 600 1C with a rate of
À1
1
0 1C min . Fourier-transform infrared (FT-IR) spectra were
performed on a Nicolet IR 380 Spectrometer using KBr discs to
determine the characteristic vibration bands of the zeolite
framework. Pyridine-chemisorbed FT-IR analysis (Py-IR) was
performed using a Thermo Nicolet IR 2000 Spectrometer to
study the nature of the acid sites (Brønsted and Lewis) of the
3.2 The characterization of different zeolites
prepared samples. The Na, Si and Al contents of the mother As shown in Fig. 1a and b, the XRD patterns of all the samples
liquid were determined by using a flame photometer FP6410 and display main characteristic peaks at 7.81, 8.81, 23.01, 23.91 and
Thermo Scientific iCAP6500 inductive coupled plasma emission, 24.41, which possess a set of typical MFI Bragg reflections (PDF
respectively. The alkalinity of the mother liquid was analyzed by 00-085-1208). This result indicates that the ZSM-5 samples were
acid–base titration.
successfully synthesized by using the mother liquid from the
previous batch synthesis. In addition, the low relative crystal-
linities of the samples obtained by the mother liquid was due to
2.4 Catalytic performance evaluation
3
1
The hydration reaction of cyclohexene was carried out in a the obvious reduction of particle size for ZSM-1–ZSM-5-9.
stainless-steel reactor of 500 mL capacity, equipped with The poor reproducibility is also related to the uneven particle
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Table 1 The elemental composition of the mother liquid and the prepared samples
Mother liquid
Products
À1
À1
À1
À1
a
b
Sample
Si (g L
)
Al (g L
)
Na (g L
)
Alkalinity (mol L
)
(SiO
2
/Al
2
O
3
)
2 2 3
(SiO /Al O )
ZSM-5-0
ZSM-5-1
ZSM-5-2
ZSM-5-3
ZSM-5-4
ZSM-5-5
ZSM-5-6
ZSM-5-7
ZSM-5-8
ZSM-5-9
0
0
0
0
25
31
28
28
32
28
28
35
31
34
27
36
28
28
29
32
31
32
34
35
8.36
10.2
14.2
12.9
20.4
22.5
22.3
22.3
16.7
0.15
0.15
0.16
0.15
0.16
0.15
0.17
0.16
0.16
7.61
9.20
9.64
9.24
10.5
9.61
11.0
9.51
9.48
0.341
0.348
0.375
0.352
0.402
0.358
0.375
0.364
0.348
a
b
Measured via EDS. Measured via XRF.
size distribution of the sample (as can be seen from the SEM
Besides, the elemental distribution in the samples was
images of ZSM-5-0 and the zeolites acquired with the recycled further determined using the XPS techniques. As displayed in
mother liquid: ZSM-5-n (1–9) in Fig. 2). Combined with the Fig. 1c–e, the binding energy (BE) values of Si 2p (99.9–100.1 eV)
4
+
3+
elemental composition result, the XRD further demonstrates and Al 2p (70.9–71.1 eV) are associated with Si and Al in
3
4
that the ZSM-5 zeolites can be obtained via using the mother tetrahedral environments, respectively. In addition, the local
liquid as part of the raw materials and the synthetic route can coordination environments around Al were elucidated by
2
7
be applied for the green and economical production of ZSM-5 analyzing the Al MAS NMR spectra of the samples. As shown
zeolites. in Fig. 1f, the spectra of all the samples display an exclusive
Fig. 1 (a and b) XRD patterns of ZSM-5-0 and ZSM-5-n zeolites prepared with recycled mother liquid; and the (c) XPS survey spectra, (d) Si 2p XPS
27
spectra, (e) Al 2p XPS spectra, and (f) Al MAS NMR spectra of ZSM-5-n (n = 0, 1, 7, and 9).
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rod-shaped morphology, and the average particle size distribution
ranges from 0.3 to 0.5 mm. The significant reduction in particle
size may be attributed to the presence of more primary structural
units that were smaller than the self-made S-1 seeds in the mother
13,32,33
liquid,
process (detailed discussion is described in the mechanism
section). Furthermore, as presented in Table 1, the SiO /Al of
which act in the role of seeds during the synthesis
2
2 3
O
the samples prepared by the mother liquid increased compared
with ZSM-5-0, indicating the presence of unutilized Si or Al species
in the mother liquid.
The textural properties of ZSM-5 zeolites prepared with
recycled mother liquid were monitored by nitrogen adsorption
isotherms and Table 2 summarizes the corresponding textural
properties of these samples. According to IUPAC classification,
the isotherms of all the ZSM-5 samples exhibit type-I behavior
with only one high uptake at P/P
at 0.4 o P/P
0
o 0.02 and a long plateau
0
o 0.9, which is characteristic of Langmuir
adsorption owing to the filling of micropores, as shown in
Fig. S1a and b (ESI†).
Moreover, the uptake of the samples prepared by using the
recycled mother liquid is slightly higher than that of the fresh
catalyst ZSM-5-0 at high relative pressure, which can be
ascribed to the interparticle voids between the smaller particles.
As described in Table 2, as-prepared ZSM-5-0 with fresh raw
materials possesses the highest total and micropore surface
area. After the recycled mother liquid was used as part of the
raw materials, the total and micropore surface area of the
obtained ZSM-5 slightly declined, while their external surface
area shows an increasing trend. The possible reason is that the
relative crystallinity degree and particle size of the prepared
materials with the recycled mother liquid decrease, which is
consistent with the XRD and SEM results.
3
NH -TPD analysis was carried out to evaluate the surface
+
acidity of the ZSM-5 zeolites (H form). Fig. S1c and d (ESI†)
displays the NH -TPD profiles of ZSM-5 ion-exchanged directly
3
with HNO solution without pre-calcining. It is clear that the
3
profiles of the prepared samples exhibit two distinct desorption
peaks, the peak at the lower temperature belongs to the
Fig. 2 SEM images of ZSM-5-0 (a) and the zeolites acquired with the ammonia species either adsorbed (weakly chemisorbed) or held
recycled mother liquid: ZSM-5-n (b–j: 1–9, respectively).
in place by means of hydrogen bridging bonds (physisorbed),
whereas the other desorption peak at a higher temperature
peak at ca. 52 ppm, assigned to the tetrahedrally coordinated Al
species in the framework, while the peak at 0 ppm ascribed to
the octahedrally coordinated Al species was not observed in any
Table 2 Textural properties of ZSM-5-0 and ZSM-5-n prepared with the
recycled mother liquid
3
5
case. This observation indicated that almost all the Al species
were well incorporated into the framework of the ZSM-5
samples synthesized using the recycled mother liquid. As stated
above, these results are consistent with the fact that the Sample
2
À1
3
À1
Surface area (m g
)
Pore volume (cm g )
S
BET
S
Mic
S
ext
V
Total
V
Mic
V
Meso
proposed method can be successfully applied for the preparation
of ZSM-5, and the recycled mother liquid can also be reused as ZSM-5-1
part of the raw materials.
Fig. 2 depicts the SEM images of synthesized ZSM-5. It can
be found that ZSM-5-0 possesses an oval-like aggregate mor- ZSM-5-5
phology with a particle size of about 1.1–1.4 mm. After recycled
mother liquid was used as part of the raw materials for
ZSM-5-0
384
322
339
343
343
337
352
363
356
323
366
303
306
324
326
317
312
339
298
303
18
19
33
19
17
20
40
24
58
20
0.192
0.215
0.198
0.177
0.181
0.195
0.215
0.218
0.118
0.195
0.138
0.140
0.126
0.130
0.129
0.132
0.119
0.129
0.112
0.115
0.054
0.075
0.072
0.047
0.052
0.063
0.096
0.089
0.076
0.080
ZSM-5-2
ZSM-5-3
ZSM-5-4
ZSM-5-6
ZSM-5-7
ZSM-5-8
preparing ZSM-5, however, the obtained samples exhibit a ZSM-5-9
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2
9,36,37
represents the strong acid sites.
In addition, compared for the formation of nuclei. Then a new population of nuclei
with the ZSM-5-0, the acid amount of the samples synthesized (or nuclei breeding entities) is produced by an initial breeding
4
4,45
by the mother liquid slightly decreases in Table 3, which may mechanism at these sites.
(II) After nucleation, crystal
be due to the increase of SiO /Al O in these samples. The acid growth takes place on the seed crystal surface by consuming
2
2 3
strength of the strong and weak acid sites does not show an aluminosilicate precursors in the synthesis mixture. (III)
obvious trend of change (Fig. S1c and d, ESI†). When crystallization completes, the mother liquid compositions
FT-IR spectroscopy with the adsorption of pyridine was also contain primary structural units with a memory of the zeolite
performed to further investigate the nature and number of acid framework. Fig. S3(a) (ESI†) gives the XRD and FT-IR results of
sites (Brønsted and Lewis) of the prepared zeolite samples, as the dried mother liquid. The dried mother liquid does not show
presented in Fig. 3 and Table 3. Fig. 3a and b displays the the characteristic diffraction peaks attributed to the MFI
signals resulting from chemically adsorbed pyridine after topology of ZSM-5 zeolite. The FT-IR spectrum for the dried
À1
+
evacuation at 100 1C for all samples, the band at ca. 1545 cm
mother liquid is shown in Fig. S3(b) (ESI†). The band at
À1
was attributed to the ring vibration of protonated pyridine (PyH ) 450 cm is attributed to the T–O bending vibration of the
stemming from the interaction of pyridine and the Brønsted (B) SiO
acid, while the band at ca. 1455 cm was indicative of the vibration 798 cm
4
or AlO
4
internal tetrahedral units. The band near
À1
À1
is related to the external asymmetric stretching
of Py–L originating from the interaction of pyridine and the Lewis vibration of Si–O–T linkage. The existence of these two bands
À1
(
L) acid, and the band at ca. 1490 cm corresponds to the indicates that the primary ZSM-5 zeolite entities occurred in the
4
6,47
co-contribution of pyridine adsorbed over the Brønsted and Lewis mother liquid.
(IV) Upon adding the source of nutrients (part
38,39
40
acid sites.
According to the reported literature, a quantitative of the raw materials) to the mother liquid, the size of the ZSM-5
estimation of the spectra to determine the B and L acid sites was aggregates obtained sharply decreases compared to the fresh
performed. Apparently, the number of B and L acid sites of the sample due to the synergetic effect of the S-1 seed and the
10,32,33
ZSM-5 zeolites prepared by mother liquid significantly decreased as primary structural units in the mother liquid.
displayed by the declined peak intensity in comparison with ZSM-5- the ZSM-5 zeolites without self-made S-1 seeds were prepared
, their B acid site number is approximately 67–90% of that of via using the recycled mother liquid to verify the effect of
For comparison,
0
ZSM-5-0, as shown in Table 3. This indicates a decrease in the the primary structural units (Fig. S2, ESI†). These results
number of tetrahedrally coordinated framework Al due to the close demonstrate that the feasibility of the proposed novel approach
correlation between B acid sites and framework Al (generated from in this study could be attributed to the synergetic effect of the
41
Al–OH–Si). In addition, the vibration intensity of Py–L of the self-made S-1 seeds and the recycled mother liquid, which is
samples prepared by the mother liquid also reduced compared helpful to promote the formation of the small particle size.
4
2,43
with ZSM-5-0. According to previous reports,
the vibration of
À1
Py–L at 1445 cm is caused by the tri-coordinated framework Al
due to the absence of octahedrally coordinated Al species in the
3.3 Catalytic performance
NMR results, and its amount might be associated with the The catalytic performances for the hydration of cyclohexene to
synergistic effect of extra silanols and Al ‘‘defect’’ sites.
cyclohexanol over the ZSM-5 samples synthesized with recycled
On the basis of the above results and discussion, the mother liquid are shown in Fig. S5 (ESI†) and Fig. 3c. The effect
probable formation mechanism of nanometer ZSM-5 aggregates of reaction time on the cyclohexanol yield was studied using the
with recycled mother liquid and seed-assistance is presented as ZSM-5-0 catalyst. As presented in Fig. S5 (ESI†), the yield of
follows (Fig. 4): (I) the external surface of the S-1 seed dispersed cyclohexanol increased with prolonging the reaction time from 0.5
in the initial gel will be partially dissolved to expose specific sites to 2 h, but the rate of increase gradually decreases. Therefore, one
hour was selected as the appropriate reaction time in this work.
Fig. 3c showed that the ZSM-5-0 synthesized with fresh raw
materials presented excellent catalytic activity and the cyclo-
Table 3 Acidic properties of ZSM-5-0 and ZSM-5-n synthesized with
hexanol yield was 8.49%. After the recycled mother liquid was
used as part of the raw materials, the obtained ZSM-5 samples
still maintained the comparatively high cyclohexanol yield,
only a slight decrease of yield was observed. This result
demonstrates that these materials obtained with the recycled
mother liquid exhibit a stable catalytic activity for cyclohexene
hydration. In addition, XRD and FT-IR characterization was
also performed to further investigate the structural stability of
the samples after the reaction (Fig. S4, ESI†). As shown in
Fig. S4a and b (ESI†), the samples after the reaction still
maintain the typical diffraction peaks assigned to the MFI
structure. Fig. S4c and d (ESI†) shows the FT-IR results for
recycled mother liquid
a
À1
b
À1
Distribution of acid sites (mmol g
)
Acidity (mmol g
)
Weak acid
Sample (170–200 1C)
Strong acid
(390–420 1C)
Total
Brønsted Lewis
ZSM-5-0 0.40
ZSM-5-1 0.36
ZSM-5-2 0.31
ZSM-5-3 0.33
ZSM-5-4 0.38
ZSM-5-5 0.35
ZSM-5-6 0.31
ZSM-5-7 0.32
ZSM-5-8 0.33
ZSM-5-9 0.34
0.47
0.45
0.32
0.39
0.43
0.40
0.41
0.41
0.42
0.43
0.87
0.81
0.63
0.72
0.81
0.75
0.72
0.73
0.75
0.77
0.4635
0.3092
0.3141
0.3556
0.3432
0.3276
0.3636
0.3578
0.3621
0.3378
0.0798
0.0460
0.0533
0.0621
0.0628
0.0483
0.0587
0.0522
0.0550
0.0473
the samples, all samples still showed characteristic adsorption
a
b
À1
Determined via NH
3
-TPD. Determined via Py-IR.
bands at around 450, 550, 800, 1093 and 1221 cm
.
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Fig. 3 (a and b) FTIR spectra of pyridine adsorbed on various catalysts, (c) the catalytic performances of various catalysts, and (d) the correlation between
the cyclohexanol yield and B acid amounts for various catalysts.
Fig. 4 A schematic depiction of the proposed crystallization behavior of ZSM-5 aggregates based on seed-assistance with recycled mother liquid.
It is well accepted that ZSM-5 zeolites as solid acid catalysts the recycled mother liquid is significantly smaller than ZSM-5-0
can provide enough protons for cyclohexene hydration, which (B1.1–1.4 mm). Small particles are beneficial to facilitate access
1
9
is considered to be an electrophilic addition reaction. Thus, to the active site for reactants and the diffusion of the products.
the B acid sites in the ZSM-5 zeolites are an essential property Therefore, these results further illustrated that preparing
for understanding the catalytic performances in the process of nanometer ZSM-5 aggregates from the recycled mother liquid
the hydration reaction, which has also been proved by the and seed-assistance is feasible.
correlation between the B acid sites and cyclohexanol yield as
shown in Fig. 3d. The number of B acid sites decreased by
4. Conclusions
10–30% on the basis of ZSM-5-0, but the cyclohexanol yield of
the samples prepared from the recycled mother liquid only In summary, ZSM-5 zeolites were successfully synthesized from
decreased by 1–6% of that of ZSM-5-0. This is due to the fact recycled mother liquid with a template-free method. The mother
that the particle size of the samples (B0.3–0.5 mm) prepared by liquid was recycled as H O and as part of the raw materials in the
2
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synthesis of ZSM-5 zeolites. With an increase in the number of
mother liquid recycling cycles, the Si, Al and Na levels in the
mother liquid gradually stabilize and the size of the nanometer
ZSM-5 aggregates synthesized using the mother liquid sharply
decreases compared to when using fresh catalyst owing to the
synergetic effect of the self-made S-1 seeds and the primary
7 J. Zhu, Z. Liu, A. Endo, Y. Yanaba, T. Yoshikawa, T. Wakihara
and T. Okubo, CrystEngComm, 2017, 19, 632–640.
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samples obtained from recycling nine times exhibit almost the
2004, 108, 20465–20470.
same catalytic performance as ZSM-5-0 prepared with fresh raw 11 M. Liu, H. Wei, B. Li, L. Song, S. Zhao, C. Niu, C. Jia,
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of nanometer ZSM-5 with 100% atom efficiency and may stimu-
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1
1
1
1
2
2
2
2
2
2
2
2
2
6 L. Bing, G. Wang, K. Yi, F. Li, J. Liu and F. Wang, Catal. Lett.,
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Author contributions
7 F. Duan, J. Li, P. Chen, J. Yu and R. Xu, Microporous
Mesoporous Mater., 2009, 126, 26–31.
8 H. Zhang, S. Mahajani, M. Sharma and T. Sridhar, Chem.
Eng. Sci., 2002, 57, 315–322.
Lukuan Zong designed the study, contributed to analysis, and
wrote the manuscript. Lukuan Zong and Hongxia Xu performed
the experiments. Huijuan Wei and Xiangyu Wang contributed
to performing the analysis with constructive discussion and
finalizing this paper. Lukuan Zong, Hongxia Xu, Xiangyu Wang
and Huijuan Wei revised and approved this paper.
9 X. Shan, Z. Cheng and P. Yuan, Chem. Eng. J., 2011, 175,
423–432.
0 M. Dyballa, O. Utz, B. Markus and W. Dai, Catal. Sci.
Technol., 2018, 8, 4440–4449.
1 I. Bakare, O. Muraza, M. Yoshioka, Z. Yamani and T. Yokoi,
Catal. Sci. Technol., 2016, 6, 7852–7859.
Conflicts of interest
2 K. Wang, X. Wang and G. Li, Catal. Commun., 2007, 8,
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.
3
24–328.
3 M. Farshadi and C. Falamaki, Chin. J. Chem. Eng., 2018, 26,
16–126.
1
4 Y. Hur, P. Kester, C. Nimlos, Y. Cho, J. Miller and
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5 Y. Wang, R. Wang, D. Xu, C. Sun, L. Nia, W. Fu, S. Zeng, S. Jiang,
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6 A. Ismail, R. Mohamed, O. Fouad and I. Ibrahim, Cryst. Res.
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7 J. Zhou, Z. Hua, X. Cui, Z. Ye, F. Cui and J. Shi, Chem.
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Acknowledgements
Financial support from the Innovation Fund for Elitists of
Henan Province, China (Grants No. 0221001200) and Beijing
Energy Engineering Technologies Co, Ltd is acknowledged.
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