Organic promoter-driven fast synthesis of zeolite beta and its acceleration mechanism

Article abstract of DOI:10.1039/c8ta07879c

Zeolite beta, an important catalytic material widely applied in the petrochemical field, is typically prepared under hydrothermal conditions. However, it usually suffers from the drawback of relatively long crystallization time. Herein, we show that the crystallization rate of zeolite beta can be accelerated by a factor of as much as about nine via the addition of the low-toxic and cost-effective pentacyclic lactam N-methyl-2-pyrrolidone (NMP) as a 'promoter'. Using characterization techniques, we demonstrate that the pentacyclic lactam induces the fast formation of secondary building units (SBUs) and enhances the formation rate of the activated TEA⁺ species responsible for the nucleation of beta crystals. More importantly, the as-synthesized beta exhibited high product yield (above 85 wt%) and good performance in the direct amination of isobutene (i-C₄H₈) to tert-butylamine reaction.

Full text of DOI:10.1039/c8ta07879c

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PAPER
Organic promoter-driven fast synthesis of zeolite
beta and its acceleration mechanism†
Cite this: J. Mater. Chem. A, 2018, 6,
24614
Dongpu Zhao,ab Weifeng Chu,ab Yanan Wang,ab Xiangxue Zhu,a Xiujie Li,a Sujuan Xie,a
a
a
a
a
*
*
Jie An, Wenjie Xin, Shenglin Liu and Longya Xu
Zeolite beta, an important catalytic material widely applied in the petrochemical field, is typically prepared
under hydrothermal conditions. However, it usually suffers from the drawback of relatively long
crystallization time. Herein, we show that the crystallization rate of zeolite beta can be accelerated by
a factor of as much as about nine via the addition of the low-toxic and cost-effective pentacyclic lactam
N-methyl-2-pyrrolidone (NMP) as a ‘promoter’. Using characterization techniques, we demonstrate that
the pentacyclic lactam induces the fast formation of secondary building units (SBUs) and enhances the
formation rate of the activated TEA+ species responsible for the nucleation of beta crystals. More
importantly, the as-synthesized beta exhibited high product yield (above 85 wt%) and good performance
in the direct amination of isobutene (i-C4H8) to tert-butylamine reaction.
Received 14th August 2018
Accepted 7th November 2018
DOI: 10.1039/c8ta07879c
rsc.li/materials-a
example, oxyanions (perchlorate, phosphate, nitrate, sulphate
and carbonate),17 hydroxyl free radicals (cOH),18,19 and seed
(including homogenous20–26 and heterogenous27–29 seeds) crys-
tals. Although these materials have led to a pronounced
reduction in synthesis time, they still suffer from the following
problems: limited acceleration effect, difficult to scale up, high
seed input30 and low product yield.8,20,22,26 Therefore, the effi-
cient production of zeolite beta with high yield remains a big
challenge.
Notably, the organic promoter (both organic amines and
quaternary ammonium salts)-driven fast synthesis of zeolite
beta has not yet been achieved and studied. Considering that
the secondary building units (SBUs) of zeolite beta contain
a considerable number of ve-membered rings (5Rs),9 and also
inspired by the seed-induced synthetic strategy,24 we propose
that some amphipathic organic molecules possessing a penta-
cyclic structure may also induce the formation of 5Rs in the
presence of TEAOH and promote the subsequent nucleation
and growth of beta crystals.
Introduction
Zeolite beta, with an intriguing three-dimensional pore system
composed of intersecting 12-ring channels,1 displays superior
performance as a solid acid catalyst in the chemical and
petrochemical industry2–6 due to its unique physico-chemical
properties. Currently, zeolite beta is synthesized primarily
under hydrothermal conditions, with tetraethylammonium
hydroxide (TEAOH) as the structure-directing agent (SDA), at
temperatures ranging from 100 ꢀC to 150 ꢀC for several days or
even weeks.7–11 Nevertheless, even when zeolite beta exhibits
excellent properties for a variety of catalytic reactions, its wide
applications are still limited due to its critical drawback of
relatively long crystallization time, especially its lengthy
induction period,12 resulting in both low production efficiency
and large energy consumption. In principle, increasing the
synthesis temperature can shorten the induction period of
zeolite beta greatly; however, this leads to the preferential
formation of impurity phases (including MOR8,13,14 and MFI10,15
)
and severe decomposition of TEAOH.16 Hence, exploring low-
cost promoters to accelerate the crystallization of zeolite beta
at relatively low temperatures is of great importance and needs
more attention for both fundamental research and industrial
applications.
Herein, we demonstrate for the rst time that the amphi-
pathic pentacyclic lactams 2-pyrrolidone, N-methyl-2-
pyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) can act as
promoters to induce the fast formation of 5Rs, and conse-
quently, the nucleation of zeolite beta is signicantly acceler-
ated. Beneting from the synergistic effects of TEAOH and these
pentacyclic lactams, highly crystalline zeolite beta with high
yield was synthesized within 12 h at 140 ꢀC, which was
approximately nine times faster than that without promoter.
More importantly, the advantages of low cost, high stability and
easy separation of the pentacyclic lactams make them an
economical and recyclable promoter for the fast production of
industrial catalysts. Furthermore, the as-synthesized beta
To date, only a few inorganic materials have been found to
contribute to conversion of initial gel to crystalline zeolites, for
aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, Liaoning, China. E-mail: lyxu@dicp.ac.cn;
slliu@dicp.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta07879c
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catalyst also exhibited better catalytic performance in the direct NaOH and 6.45 g of TBABr were dissolved in 52 mL of deionized
amination of isobutene (i-C4H8) to tert-butylamine reaction water at room temperature with fast stirring, followed by the
compared with the commercial beta sample.
addition of 0 or 10 g of NMP, 20 g of silica sol and 5 mL of
deionized water. Then the resulting mixture was further stirred
for 1 h to obtain a homogenous gel. The nal gel was poured
into a 200 mL Teon-lined stainless steel autoclave and heated
Experimental section
Synthesis
ꢀ
from room temperature to 150 C for crystallization to occur.
Chemicals and reagents. The reagents used were fumed The synthesis was carried out under autogenic pressure with
silica (95% SiO2, Shenyang Chemical Co. Ltd, China), silica sol rotation at 30 rpm. Aer crystallization, the solid product was
(30% SiO2, Qingdao Haiyang Chemical Co. Ltd, China), sodium separated by centrifugation, washed with deionized water, and
aluminate (49% Al2O3, 38% Na2O, and 13% H2O, Zibo Tongjie dried at 100 ꢀC overnight.
Chemical Co. Ltd, China), aluminium sulfate (99% Al2(SO4)3-
$18H2O, Shandong Aluminum Co. Ltd, China), sodium
hydroxide (96% NaOH, Sinopharm Chemical Reagent Co. Ltd,
Characterization
China), tetraethylammonium hydroxide (TEAOH, 35% in water, Powder X-ray diffraction (XRD) analysis of the samples was
SACHEM), tetrabutylammonium bromide (TBABr, 99%, Tianjin performed on an Empyrean-100 X-ray diffractometer (PAN-
˚
Bodi Chemical Co. Ltd, China), 2-pyrrolidone (98%, Energy alytical B. V.) with Cu Ka radiation (l ¼ 1.5406 A) at 40 kV and 40
Chemical Reagent Co. Ltd, China), N-methyl-2-pyrrolidone mA in the 2q range of 5ꢀ to 50ꢀ. The relative crystallinity (RC) of
(NMP, 99%, Tianjin Kemiou Chemical Reagent Co. Ltd, zeolite beta was calculated using the ratio of the integrated
China), N-ethyl-2-pyrrolidone (NEP, 99%, Energy Chemical intensity of the peaks (2q ¼ 7.8ꢀ ꢂ 0.1ꢀ, 22.5ꢀ ꢂ 0.1ꢀ, 27.1ꢀ ꢂ
Reagent Co. Ltd, China) and N-isopropyl-2-pyrrolidone (NPP, 0.1ꢀ, and 29.6ꢀ ꢂ 0.1ꢀ) of the sample with respect to the well
99%, Beijing Ouhe Technology Co. Ltd, China). All reagents crystalline sample. The chemical compositions of the samples
were used as received without further purication.
were determined on a PANalytical Zetium X-ray uorescence
Synthesis of zeolite beta. Zeolite beta was hydrothermally (XRF) spectrometer. Nitrogen sorption isotherms were
synthesized following the approach described in the literature31 measured at ꢁ196 C on a Micromeritics ASAP-2020 analyzer.
ꢀ
with some modications. The starting gel was prepared with the Prior to the analysis, all samples were degassed at 350 ꢀC for 5 h
molar composition of
x
P/1.0SiO2/0.023Al2O3/0.048Na2O/ under vacuum at 10ꢁ3 Pa. Thermogravimetric analysis (TGA)
0.636TEAOH/15.6H2O, where x ranged from 0.00 to 3.00 and P was carried out with a Pyris Diamond TG/DTA analyser. All the
represented 2-pyrrolidinone, NMP, NEP or NPP. In a typical TGA experiments were carried out under air at a ow rate of 40
synthesis, 0.4 g of NaAlO2, 0.13 g of NaOH, 22.26 g of TEAOH mL minꢁ1 and ramp rate of 10 ꢀC minꢁ1. Fourier transform
and 5 mL of deionized water were successively added in infrared (FT-IR) spectra were obtained in the wavelength range
a 200 mL Teon-lined stainless steel autoclave at room of 400–1600 cmꢁ1 on a VERTEX 70 FT-IR spectrometer (Bruker
temperature under vigorous stirring until a clear solution was Company) using the KBr method. Low-magnication scanning
formed. Subsequently, a certain amount of lactam, 5.27 g of electron microscopy (SEM) observation was performed on
fumed silica and 4 mL of deionized water were added in a HITACHI SU1510 microscope operated at 15 kV using samples
sequence. Finally, aer stirring for 2 h, the resulting gel was coated with Pt. Field emission scanning electron microscopy
sealed and heateꢀd from room temperature to the desired (FESEM) was performed on a JSM-7800F electron microscope
ꢀ
temperature (120 C or 140 C). Crystallization was conducted (JEOL Company) with an operating voltage of 0.5 or 1 kV.
under simulated industrial conditions, i.e., autogenic pressure Transmission electron microscopy (TEM) and selected area
with rotation at 30 rpm. Aer various periods of crystallization, electron diffraction (SAED) were carried out on a JEM-2100
the autoclave was withdrawn and quenched with cold water. transmission electron microscope (JEOL Company) operating
The supernatant was carefully collected by centrifugation at 200 kV. All solid-state magic-angle-spinning (MAS) NMR
(10 000 rpm for twenty minutes). The solid product was ltereꢀ d, experiments were performed on an Agilent DD2-500 MHz
washed with distilled water several times and dried at 100 C. spectrometer. 27Al MAS NMR spectra were recorded at 130.32
For comparison, conventional zeolite beta was synthesized MHz with a p/12 pulse length of 3.2 ms, recycle delay of 2 s, and
without the “promoter” using a similar procedure.
Aer removing the organic species (calcined at 550 C for 6 recorded at 99.37 MHz with a p/2 pulse length of 5.0 ms, recycle
h), the Na+–beta zeolite was ion-exchanged with 1.0 mol Lꢁ1 delay of 4 s, and spinning frequency of 5 kHz. 13C MAS NMR
spinning frequency of 6 kHz. 29Si MAS NMR spectra were
ꢀ
+
+
ꢀ
NH4NO3 solution at 80 C for 2 h to replace Na by NH4 . This spectra were acquired at 125.7 MHz using a 4 mm MAS NMR
process was repeated three times, and the nal product was probe with a spinning rate of 5 kHz. Liquid-state 13C NMR
washed with deionized water, dried at 120 ꢀC and calcined in air spectra of the supernatants in D2O solutions were recorded on
at 550 C for 6 h.
an Agilent DD2-500 MHz spectrometer at a 13C frequency of
ꢀ
Synthesis of zeolite ZSM-11. Zeolite ZSM-11 was synthesized 125.7 MHz. The acidic properties of the samples were analyzed
according to our previous reported conditions32–36 with minor via temperature-programmed desorption of ammonia (NH3-
modication. The molar composition of the initial reaction gel TPD). In a typical experiment, 0.14 g sample was rstly pre-
was x NMP/1.0SiO2/0.15Na2O/0.016Al2O3/0.2TBABr/40H2O (x ¼ treated at 600 ꢀC for 0.5 h under a continuous He ow of 25
0.00 and 1.00). Typically, 1.09 g of Al2(SO4)3$18H2O, 1.25 g of mL minꢁ1 to remove any adsorbed impurity, then cooled to
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150 ꢀC and saturated with NH3. The TPD measurements of FESEM image (Fig. S1A†) indicated that the product was well
desorbed NH3 were conducted in owing He from 150 ꢀC to crystalline zeolite beta. However, in the absence of NMP, the
ꢁ1
ꢀ
ꢀ
600 C at a heating rate of 20 C min
.
material was still amorphous nanoparticles, as shown in Fig. 1B
and S1B.† These results suggested that the crystallization of
zeolite beta was truly accelerated by NMP in the presence of
TEAOH.
Catalytic tests
The direct amination of i-C4H8 to tert-butylamine was evaluated
in a stainless-steel xed-bed reactor. The catalyst (4 g, 20–40
mesh) loaded in the reactor was activated at 500 ꢀC for 1 h under
an N2 ow, and then, the temperature was cooled to 250 ꢀC.
Subsequently, liquid ammonia was pumped in to fully ll the
reactor before isobutylene was introduced. The reactions were
performed under the conditions of 250 ꢀC, 8.0 MPa, NH3/i-C4H8
molar ratio of 4.0, and WHSV(i-C4H8) of 0.5 hꢁ1. The reaction
products were analyzed on-line with an Agilent 7980B gas
chromatograph equipped with a ame ionization detector (FID)
and a PONA capillary column. The i-C4H8 conversion and
tert-butylamine selectivity were calculated according to our
previously reported method.34,37,38
Then, NMP was quantitatively added to investigate the
crystallization process of zeolite beta with the molar composi-
tion of
x NMP/1.0SiO2/0.023Al2O3/0.048Na2O/0.636TEAOH/
15.6H2O (x ¼ 0.00, 0.25, 0.50, 0.75, 1.00, 2.00 and 3.00). In the
absence of NMP, a very weak peak (at about 2q ¼ 22.5ꢀ) assigned
to poorly crystalline zeolite beta, as conrmed by XRD, emerged
at 100 h (Fig. S2A†). Aer 108 h, highly crystalline zeolite beta
was developed (Fig. S3 and S4†). While in the case with NMP,
the XRD patterns (Fig. S2B–G†) and SEM images (Fig. S5–S10†)
showed that primary beta crystals were already visible at 42 h,
24 h, 15 h, 10 h, 6 h and 12 h, and highly crystalline zeolite beta
was obtained at 44 h, 30 h, 18 h, 14 h, 12 h and 24 h for the NMP/
SiO2 molar ratios of 0.25, 0.50, 0.75, 1.00, 2.00 and 3.00,
respectively, as conrmed by the crystallization curves (Fig. 2A).
Obviously, as the amount of NMP increased in the gel, the
crystallization time decreased initially and then increased aer
the optimum concentration. This is probably due to the
decrease in alkalinity of the synthetic gel caused by the reaction
of TEAOH with excess NMP (see below, Fig. 4E). Then both the
depolymerization and subsequent co-condensation of silicates
and aluminates species were slowed. Notably, the crystallization
rate at NMP/SiO2 ¼ 2.00 was approximately nine times faster
than that without promoter, and even superior than the seeding
method26 and promoter oxyanions.17 The detailed chemical
composition and porosity data of the well crystalline zeolite beta
synthesized with various NMP/SiO2 ratios are summarized in
Table S1.†
Results and discussion
Demonstration of the acceleration effects of the amphipathic
pentacyclic lactams
To investigate the effects of the amphipathic pentacyclic lac-
tams on zeolite beta crystallization, we rstly used a low-toxic
and cost-effective pentacyclic lactam, NMP (Scheme 1). The
samples with initial molar compositions of x NMP/1.0SiO2/
0.023Al2O3/0.048Na2O/0.636TEAOH/15.6H2O (x ¼ 0.00 and
ꢀ
0.50) were hydrothermally treated at 140 C for 60 h. Dramati-
cally, in the presence of NMP, the XRD pattern (Fig. 1A) and
Furthermore, given that reducing the crystallization
temperature could slow down the crystallization of zeolites39
and suppress the decomposition of TEAOH,16 we further
studied the crystallization behaviors of the initial mixtures (with
NMP/SiO2 ¼ 0.00 and 2.00) at a lower crystallization tempera-
ture (120 ꢀC versus 140 ꢀC). Under the NMP-free conditions,
zeolite beta crystals were observed until 192 h, and well
Scheme 1 Chemical structures of 2-pyrrolidone, NMP, NEP and NPP.
Fig. 1 XRD patterns of zeolite beta synthesized with ((A) x ¼ 0.50) and without ((B) x ¼ 0.00) NMP at 140 ꢀC for 60 h.
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Fig. 2 (A) Crystallization curves of zeolite beta synthesized with NMP/SiO2 molar ratios of 0.0, 0.25, 0.50, 0.75, 1.00, 2.00, and 3.00 at 140 ꢀC. (B)
Crystallization time (h) of fully crystalline zeolite beta synthesized with NMP/SiO2 molar ratios of 0.00, 0.25, 0.50, 0.75, 1.00, 2.00, and 3.00 at
140 ꢀC.
crystalline crystals were obtained when the reaction time was while a more uniform morphology could be obtained when
prolonged to 240 h (Fig. S11A,† XRD and Fig. S12A,† SEM NMP was employed.
ꢀ
ꢀ
image) at 120 C; and at 140 C, beta crystals were well devel-
Besides NMP, the acceleration effects of some other pentacy-
oped aer 108 h (Fig. S2A,† XRD). With the assistance of NMP, clic lactams, such as 2-pyrrolidinone, NEP and NPP (Scheme 1)
well crystalline beta crystals were readily obtained aer 24 h were also investigated with the starting molar composition
ꢀ
ꢀ
(Fig. S11B and S12B†) and 12 h at 120 C and 140 C, respec- of 0.50P/1.0SiO2/0.023Al2O3/0.048Na2O/0.636TEAOH/15.6H2O at
tively. To the best of our knowledge, this is the shortest crys- 140 ꢀC. As shown in the XRD patterns (Fig. 3A and B) and FESEM
tallization time for zeolite beta obtained at such a low images (Fig. S13A and B†), highly crystalline beta crystals were
crystallization temperature.22,40 These results clearly demon- obtained when the reaction time reached 18 h and 21 h for 2-
strated that the acceleration effect of NMP increased with pyrrolidinone and NEP, respectively. However, when NPP was
a decrease in temperature. Consequently, the crystallization used, zeolite beta was observed aer 108 h (Fig. 3C), which was
rate of zeolite beta at 120 ꢀC with NMP/SiO2 ¼ 2.00 was even 4.5 comparable to that with no promoter. The weaker acceleration
times faster than that at 140 ꢀC with no NMP. The SEM images effect of NPP than 2-pyrrolidinone, NMP and NEP probably arose
(Fig. S3, S9 and S12†) showed that the crystal size of these from its high hydrophobicity (obvious stratication phenomenon
samples was comparable to each other at the same temperature, could be observed in the supernatant, Fig. S14†), leading to few
Fig. 3 Crystallization processes of zeolites beta at 140 ꢀC using 2-pyrrolidinone (A), NEP (B) and NPP (C) as promoters. Molar gel composition:
0.50P/1.0SiO2/0.023Al2O3/0.048Na2O/0.636TEAOH/15.6H2O.
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NPP functioning in the crystallization process of zeolite beta. in this section. As a proof-of-concept study, the crystallization
These results suggested that the acceleration abilities of these processes of the samples synthesized with the starting
pentacyclic lactams increased in the order: NPP z none ꢃ molar composition of x NMP/1.0SiO2/0.023Al2O3/0.048Na2O/
NMP z NEP < 2-pyrrolidinone, and their hydrophilicity also 0.636TEAOH/15.6H2O (x ¼ 0.00 and 2.00) and crystallization
ꢀ
played a signicant role in determining the crystallization rate.
temperature of 140 C were investigated in detail.
Apart from zeolite beta, similar results were also observed in
Fig. 4 provides the 13C NMR spectra of the supernatants and
the case of another 5Rs-rich zeolite, ZSM-11, using NMP as the solid products synthesized with NMP. In the presence of
a promoter. As seen from Fig. S15,† the initial diffraction peaks NMP, before hydrothermal treatment, the spectrum of the
assigned to zeolite ZSM-11, were rst detected at 24 h and 33 h supernatant only exhibited peaks associated with aqueous TEA+
for the samples synthesized with and without NMP, respec- and NMP species. Upon heating 3 h, some weak peaks (Fig. 4A,
tively. Also, 27 h were sufficient to obtain fully crystalline zeolite marked as ①, ②, ③, ④, and ⑤) assigned to trace quantity of
ZSM-11 with NMP, while 39 h were required for the NMP-free alkali-catalyzed ring-opening product of NMP appeared.41 Two
synthetic system, as shown in the SEM images (Fig. S16†). additional weak signals (Fig. 4A, marked as I and II) attributed
This result suggested that the present synthetic strategy may be to trimethylamine (TEA), the Hoffmann degradation product of
also capable of promoting the crystallization of other 5Rs-rich TEAOH at high crystallization temperature,42,43 were detected
zeolites.
aer hydrothermal treatment for 6 h. Upon heating between 9 h
Investigation of the acceleration mechanism. As is known, and 12 h, the peak intensities presented above only increased
zeolite crystallization is a typical two-step process, i.e., nucle- slightly while lowering the OHꢁ concentration (Fig. 4E) in the
ation and crystal growth.19 The crystallization curves shown in synthesis mixture,12,44 conrming that both NMP and TEAOH
Fig. 2A showed that the induction period under NMP- were relatively stable under highly alkaline synthetic condi-
containing conditions was much shorter than that with no tions. However, only organic TEA+ species existed in the washed
NMP; however, the slopes of the crystallization curves for all products, as shown in the solid-state 13C NMR spectra (Fig. 4B),
cases were similar. These results suggested that NMP plays an implying that only TEA+ species were indeed effective in the
important role in accelerating nucleation but not crystal crystallization process of zeolite beta. Similarly, in the absence
growth. Therefore, more attention is paid to the nucleation of NMP, the Hoffmann degradation of TEAOH also occurred
stage for understanding the unique acceleration effect of NMP and increased with time (Fig. S17A†), and only TEA+ was found
Fig. 4 (A) Liquid-state 13C NMR spectra of the supernatants. (B) Solid-state 13C NMR spectra of the solid products. (C) Liquid-state 13C NMR
spectra of aqueous TEAOH, NMP, and TEA. (D) Chemical structures of TEAOH, NMP, TEA, and the ring-opening product of NMP. (E) Alkali-
catalyzed ring-opening reaction of NMP and the Hoffmann degradation of TEAOH.
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in the solid product (Fig. S17B†), while more TEA was detected temperature process III (>350 ꢀC) was attributed to TEA+
in the nal supernatant. These results indicated that the addi- balancing the negatively charged framework Al sites
tion of NMP could decrease the Hoffmann degradation of (Al(OSi)ꢁ4 ). Obviously, as observed in Fig. 5, the distribution site
TEAOH by shortening the crystallization time, while it did not of TEA+ was signicantly inuenced by NMP.
direct the formation of zeolite beta.
When the synthesis was performed without NMP, the
Fig. 5 and Table 1 summarize the thermal analysis results of occluded TEA+ species were mainly used to balance the frame-
the products synthesized with and without NMP. A similar work Al sites; while, when NMP was added, the occluded TEA+
variation tendency was observed for the occluded TEA+ species, species were mostly used to balance the framework silanol
i.e., the quantity of the occluded TEA+ increased with an defects, implying that the sample synthesized with NMP
increase in synthesis time. However, compared with the sample possessed more connected defects. This is mainly due to the
synthesized without NMP, the combination rate of aluminosil- fact that the addition of NMP accelerated the crystallization
icates with TEA+ was notably faster for its counterpart, and this greatly, resulting in the formation of more connectivity
was further veried by the weight loss of the samples synthe- defects.22,46
sized with different NMP/SiO2 molar ratios at 140 ꢀC for 3 h
Meanwhile, 27Al and 29Si solid-state NMR spectroscopy were
(Fig. S18†). Considering that NMP was undetectable in the solid also employed to trace the chemical environment evolution of
product, one possibility was that the NMP molecules might the aluminosilicates during the crystallization. As shown in
interact with TEAOH (Fig. 4E), leading to the release of the Fig. 6A, the Al environment in the presence of NMP was similar
activated TEA+ species, and thus the combination of TEA+ with to that reported in the absence of NMP.9,26 In fact, only one peak
aluminosilicates could be accelerated.
at d ¼ 53.7 ppm, corresponding to four-coordinated Al
The differential thermogravimetric (DTG) analysis (Fig. 5B species,9,26 was present throughout the crystallization process.
and D) provided more insight into the distribution sites of the No signal of six-coordinated Al species, generally appearing at
TEA+ species. As assigned by A. Corma,45 the weight loss in d ¼ 0 ppm,9,26 was observed even in the amorphous products.
process I (<150 ꢀC) could be assigned to the desorption of This was likely because the added aluminate species immedi-
adsorbed water; the complex process II (150–350 ꢀC) corre- ately dissolved and reacted with silicates, and then became
sponded to the decomposition of TEA+ balancing at the aluminosilicates with four-coordinated Al.9
framework silanol defects (^Si–Oꢁ/HOSi^), and the high
Fig. 5 TG and DTG curves for the samples synthesized without (A and B) and with NMP (C and D), respectively.
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Table 1 Solid yields and chemical compositions of the products synthesized for different periods of timea
Synthesis
time (h)
Solid
Si yield
(wt%)
Al yield
(wt%)
TEA+ weight
loss (wt%)
H2O
Sample
Si/Al
yieldb (wt%)
weight loss (wt%)
NMP/SiO2 ¼ 0.00
0
3
60
102
108
0
3
6
9
12
17.1
3.9
5.9
25.6
2.4
25.1
2.00
17.4
30.5
47.3
55.9
24.9
34.1
62.7
84.4
31.9
11.2
64.0
84.8
85.4
71.4
71.0
86.7
99.6
99.9
5.47
7.24
15.31
15.75
19.12
7.52
14.38
16.13
19.28
19.72
2.81
8.85
3.34
2.39
1.02
3.02
3.44
2.89
2.07
1.55
19.5
33.0
49.3
56.9
27.0
36.6
64.7
85.8
7.8
12.0
17.0
7.6
8.5
13.7
17.5
NMP/SiO2 ¼ 2.00
a
Si and Al contents were quantied by XRF, while TEA+ and H2O amounts were determined by TGA. b Calculated based on oxides (excluding TEA+,
Na2O and H2O).
The typical 29Si NMR spectra are shown in Fig. 6B and C. In obviously slower than that in the case of NMP (Fig. S19†). These
the presence of NMP, the initial product was rich in Q4(0Al) results demonstrated that the presence of the promoter NMP
silicon species (Si(OSi)4) since the spectrum showed a stronger signicantly accelerated both the formation of Q4(0Al) species
signal around d ¼ ꢁ111 ppm and a weaker shoulder signal and their condensation into crystalline zeolite. This was similar
around d ¼ ꢁ103 ppm. Aer hydrothermal treatment for 3 h, to the performance of promoter phosphate ions observed by R.
the signal shied to d ¼ ꢁ103 ppm, which was assigned to the Kumar in the synthesis of silicate-1.17
dissolution of silica into Q3(0Al) ((SiO)3SiOH or (SiO)3SiOꢁ) or
To unveil the hydrothermal transformation process assisted
Q4(2Al) (Si(AlO)2(SiO)2) species.9 Upon heating 6 h, the slight by NMP, we recorded the evolution of structural subunits using
increase in the signal intensity at about ꢁ111 ppm was ascribed FT-IR spectroscopy. Compared to the FT-IR spectra of TEA+,9
to the increase in Q4(0Al) species, suggesting the condensation NMP (Fig. S20A†) and the corresponding calcined samples
of silicates. Further hydrothermal treatment from 7.5 h to 10.5 h (Fig. S20B and C†), the FT-IR bands at 1172 and 1300–
caused a sharp increase in the signal at about ꢁ111 ppm 1500 cmꢁ1 observed for the as-synthesized products could be
together with a rapid decrease in its shoulder, indicating the assigned to TEA+ (Fig. 7). Notably, no band assigned to NMP
formation of a more ordered structure and the condensation of molecules was observed for all the as-synthesized samples
silicates during the crystal growth period. However, in the throughout the crystallization, which agrees well with the 13C
absence of NMP, the formation rate of Q4(0Al) species was MAS NMR results (Fig. 4B). In addition, the band at 955 cmꢁ1
Fig. 6 27Al (A) and 29Si (B) NMR spectra of the samples synthesized with (NMP/SiO2 ¼ 2.00) NMP and 29Si NMR spectra of the samples
synthesized without (NMP/SiO2 ¼ 0.00) NMP (C).
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Fig. 7 FT-IR spectra of the samples synthesized without (left: NMP/SiO2 ¼ 0.00) and with (right: NMP/SiO2 ¼ 2.00) NMP.
for the calcined sample synthesized with NMP at 140 ꢀC for 12 h crystallization time (Fig. 2). Combined with the results from the
was substantially stronger than that in the sample synthesized thermal analysis and 13C NMR spectra, the increase in 5Rs
without NMP at 140 ꢀC for 108 h, further conrming that concentration at NMP/SiO2 ratios less than 2.00 can be ascribed
a considerable amount of connectivity defects existed in the to the fast conversion of the initial gels into 5Rs subunits assisted
product synthesized with NMP.47
by the increase in TEA+ species; whereas, the subsequent
In the presence of NMP, the FT-IR spectrum of the product decrease mainly resulted from the decreased pH of the synthetic
before heating showed was similar to that of fumed silica. Aer gel caused by the over-introduction of NMP. These results
3–6 h of heating, obvious absorption bands assigned to 5Rs (at conrmed that the polymerization of aluminosilicates and their
567 cmꢁ1
)
and TEA+ (at 1172 and 1300–1500 cmꢁ1) were incorporation with TEA+ and the formation of 5Rs were
48,49
observed, suggesting the formation of 5R (alumino)silicates and dramatically accelerated by adding an appropriate amount of
their rapid incorporation with the TEA+ species. With the NMP.
formation of weak crystalline beta crystals (hydrothermal
The chemical composition evolution of the solid product
treatment for 7.5 h, which was identied by XRD), the band at with crystallization time was monitored by XRF measurement
567 cmꢁ1 became sharper and narrower (because the 5Rs in and summarized in Table 1. In the presence of NMP, a signi-
zeolite beta were more rigid than that in the amorphous solid), cant alteration in composition occurred during the rst 3 h of
and a very weak band at 520 cmꢁ1 appeared. Further prolonging hydrothermal treatment. The drastic decrease in the Si/Al molar
the crystallization time from 9 h to 12 h, the intensities of these ratio could be ascribed to the fast depolymerization of fumed
bands became more pronounced, along with the splitting of the silica into soluble precursors (mono-, di-, tri- and tetra-meric
band at 450 cmꢁ1 into two bands at 426 and 465 cmꢁ1, and the silicate species)17 and the following co-condensation of silicate
appearance of a new band at 1218 cmꢁ1, indicating that well and aluminate,9 which could be deduced from the changes in Si
crystalline zeolite beta was emerging.13,50
and Al yields. Meanwhile, increasing amounts of TEA+ and H2O
In comparison with the sample synthesized without NMP, were incorporated in the solid product at this stage, indicating
the FT-IR spectrum of the sample synthesized with NMP was the formation of hydrated, amorphous TEA+-aluminosilicate
obviously different aer 3 h of hydrothermal treatment. For the composites.
sample synthesized with an NMP/SiO2 ratio of 2.00, the bands at
Further prolonging the crystallization time up to 6 h, the Si
1300–1500 and 1172 cmꢁ1 associated with TEA+ and 567 cmꢁ1 and Al yields and the occluded TEA+ species increased slightly
assigned to 5Rs were apparently stronger. A similar phenom- in the product, suggesting the gradual condensation of alumi-
enon was also observed in the samples synthesized with various nosilicate species and their combination with TEA+ during the
NMP/SiO2 ratios (Fig. S21†). It was noteworthy that with an initial nucleation stage. A rapid increase in the Si/Al ratio and Si
increase in the amount of NMP in the gel, the 5Rs concentration yield and a decrease in the amount of occluded H2O were
in the corresponding sample increased linearly initially and observed during the crystal growth stage with crystallization
then decreased slightly aer the optimum concentration (NMP/ time ranging from 6 h to 12 h.
SiO2 ¼ 2.00), which showed the reverse order with their
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Similar variation trends were observed for the samples
synthesized without NMP at different crystallization stages. The
notable differences mainly appeared in the early stage of crys-
tallization, i.e., at a crystallization time of 3 h. Compared with
its counterpart, the occluded TEA+ (14.38 wt% versus 7.24 wt%)
and Si/Al molar ratio (7.6 versus 3.9) were about two times
higher for the sample synthesized with an NMP/SiO2 ratio of
2.00. These results demonstrated that the presence of NMP
signicantly accelerated both the co-condensation of silicate
and aluminate and their combination with TEA+, which is in
good accordance with the observation from FT-IR. In addition,
it could also be found that the nal product yield (85.8 wt%
versus 49.3 wt%) and Si/Al ratio (17.5 versus 12.0) in the presence
of NMP were remarkably higher than that without NMP. The
main reason for this is the decrease in the gel pH caused by the
reaction of NMP with TEAOH (Fig. 4E), which enhanced the
incorporation of silicon into the zeolite framework.12
In addition, to understand the formation process of zeolite
beta assisted by NMP, time consumption was also investigated
via XRD, FESEM, and TEM. As indicated by XRD, no crystalline
product was detected in the rst 3 h, and the crystallinity of
zeolite beta increased sharply from 6 h to 12 h (Fig. 2 and S2F†).
The FESEM images (Fig. S22†) showed that primary particles
with a diameter of about 10–17 nm initially formed at room
temperature, which gradually evolved into larger secondary
particles with a diameter of about 25–35 nm aer 6 h. At 7.5 h,
partial secondary particles aggregated into bits of spherical-like
crystalline nuclei. More and larger crystals were formed when
the crystallization time was prolonged to 9 h. When the crys-
tallization proceeded to 10.5–12 h, the amorphous ingredients
completely disappeared together with the formation of clean
and isolated crystals as the nal products.
According to TEM (Fig. S23†), the intermediates obtained in
the rst 3 h were completely amorphous. Small crystallites
(nuclei) embedded in the amorphous matrix were observed at 6 h,
which gradually grew into large isolated crystals together with the
consumption of surrounding amorphous agglomerates by
extending the reaction time from 7.5 h to 12 h. It is very inter-
esting to note that despite being composed of many nanocrystals
(less than 30 nm), each particle obtained at 10.5 h and 12 h was
a single crystal. As shown in Fig. S22,† the selected area electron
diffraction (SAED) patterns of these particles exhibited
highly discrete diffraction spots. Also, the corresponding high-
resolution transmission electron microscopy (HRTEM) images
showed clear lattice fringes with consistent orientations over the
entire image region, conrming that each particle was a single
crystal rather than a random aggregation of nanocrystals.44,51–53
Evaluation of catalytic performance. Tert-butylamine is an
important chemical raw material and pharmaceutical inter-
mediate, and the direct amination of i-C4H8 to tert-butylamine
over solid acid–base catalysts has been reported as an envi-
ronmentally benign and economical process. Zeolites ZSM-5
and beta have been found to be superior catalysts to other
materials.54,55 Also, the chemical company BASF commercial-
ized the beta catalyst for the direct amination of i-C4H8 process.
However, this reaction is performed under harsh conditions of
ꢀ
270 C, 28.0 MPa, and NH3/i-C4H8 molar ratio of 1.5, and the
conversion of i-C4H8 was 22.67 wt%.56 Here, we evaluated the
catalytic performance of the fast-synthesized beta catalyst
(denoted as *beta, obtained at 140 ꢀC for 12 h with an initial gel
composition
of
2.00NMP/1.0SiO2/0.023Al2O3/0.048Na2O/
0.636TEAOH/15.6H2O) for this reaction under relatively mild
ꢀ
conditions of 250 C, 8.0 MPa, NH3/i-C4H8 molar ratio of 4.0,
and WHSV(i-C4H8) of 0.5 hꢁ1. For comparison, a commercial
beta sample (denoted as C-beta, provided by Fushun Petroleum
Company, China, and its XRD pattern and FESEM image are
shown in Fig. S24†) possessing a similar Si/Al ratio, micropore
property (Table 2), crystal size (about 250–300 nm) and acidity
with that of *beta, was chosen as reference catalyst and tested
for this reaction under the same conditions. As determined by
the N2 sorption isotherms (Fig. S25†), *beta had a larger
external surface area than C-beta (211 versus 117 m2 gꢁ1).
Fig. 8 presents the i-C4H8 conversion and tert-butylamine
selectivity of *beta and C-beta with continuous operation for
100 h. As can be seen, both catalysts showed stable i-C4H8
conversion and tert-butylamine selectivity during the catalytic
processes. Most of the products were tert-butylamine (>99%)
and the trace components were C8 hydrocarbons. Only a small
amount of oligomerization and isomerization products were
detected, but the selectivity of both was lower than 0.1%. This is
in good agreement with the cases of H-ZSM-11, H-FAU and H-
MOR zeolites.34,38 In contrast, the fast-synthesized *beta
exhibited higher i-C4H8 conversion than C-beta (about 17.0%
versus 14.0%). Since these two catalysts possess similar acidity
(Fig. S26†) and micropore property, the excellent amination of i-
C4H8 to tert-butylamine performance of *beta could be attrib-
uted to its larger external surface area because a relatively rich
external surface generally improves the diffusion properties and
endows a catalyst with more accessible or available active sites
for reactant adsorption and conversion.44,57
Besides, aer the reaction, the changes in the physico-chemical
properties of these two catalysts were also investigated. As shown
in Table 2 and Fig. S27,† even aer 100 h of continuous reaction,
the structural, compositional and morphological properties of
Table 2 Structural, compositional and textural properties of the fresh and spent catalysts
Sample
Si/Al molar ratio
SBET (m2 gꢁ1
)
Sext (m2 gꢁ1
)
Smicro (m2 gꢁ1
)
Vtotal (cm3 gꢁ1
)
Vmicro (cm3 gꢁ1
)
Vmeso (cm3 gꢁ1
)
C-betafresh
C-betaspent
*Betafresh
*Betaspent
18.84
19.03
18.12
18.43
640
568
713
610
117
150
211
191
523
418
502
419
0.414
0.501
0.544
0.534
0.204
0.164
0.202
0.170
0.210
0.337
0.342
0.365
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This research was supported by the National Natural Science
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