Fabrication of Pd3?Beta for catalytic combustion of VOCs by efficient Pd3 cluster and seed-directed hydrothermal syntheses

Article abstract of DOI:10.1039/d0ra01576h

Subnanometric Pd clusters confined within zeolite crystals was fabricated using zeolitic seeds with premade [Pd₃Cl(PPh₂)₂(PPh₃)₃]⁺ clusters under hydrothermal conditions. Characterization of the Pd₃?Beta catalysts indicate that the Pd clusters confined in the channels of Beta zeolite exhibit better dispersion and stronger interaction with the zeolite support, leading to stabilized Pd species after heat treatment by high temperature. In the model reaction of toluene combustion, the Pd₃?Beta outperforms both zeolite-supported Pd nanoparticles prepared by conventional impregnation of Pd₃/Beta and Pd/Beta. Temperatures for achieving toluene conversion of 5%, 50% and 98% of Pd₃?Beta are 136, 169 and 187 °C at SV = 60 000 mL g^-1 h^-1, respectively. Pd₃?Beta could also maintain the catalytic reaction for more than 100 h at 230 °C without losing its activity, an important issue for practical applications. The metal-containing zeolitic seed directed synthesis of metal clusters inside zeolites endows the catalysts with excellent catalytic activity and high metal stability, thus providing potential avenues for the development of metal-encapsulated catalysts for VOCs removal.

Full text of DOI:10.1039/d0ra01576h

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Fabrication of Pd3@Beta for catalytic combustion
of VOCs by efficient Pd3 cluster and seed-directed
hydrothermal syntheses†
Cite this: RSC Adv., 2020, 10, 12772
a
Zhenglong Yang,a Yanbin Xu,a Yawei Shi,b Yongjie Shena
*
*
Wenjuan Sun,
and Guozhu Liu
c
Subnanometric Pd clusters confined within zeolite crystals was fabricated using zeolitic seeds with premade
[Pd3Cl(PPh2)2(PPh3)3]+ clusters under hydrothermal conditions. Characterization of the Pd3@Beta catalysts
indicate that the Pd clusters confined in the channels of Beta zeolite exhibit better dispersion and stronger
interaction with the zeolite support, leading to stabilized Pd species after heat treatment by high
temperature. In the model reaction of toluene combustion, the Pd3@Beta outperforms both zeolite-
supported Pd nanoparticles prepared by conventional impregnation of Pd3/Beta and Pd/Beta.
Temperatures for achieving toluene conversion of 5%, 50% and 98% of Pd3@Beta are 136, 169 and
187 ꢀC at SV ¼ 60 000 mL gꢁ1
h
ꢁ1, respectively. Pd3@Beta could also maintain the catalytic reaction for
Received 19th February 2020
Accepted 18th March 2020
more than 100 h at 230 ꢀC without losing its activity, an important issue for practical applications. The
metal-containing zeolitic seed directed synthesis of metal clusters inside zeolites endows the catalysts
with excellent catalytic activity and high metal stability, thus providing potential avenues for the
development of metal-encapsulated catalysts for VOCs removal.
DOI: 10.1039/d0ra01576h
rsc.li/rsc-advances
catalysts for VOC combustion. However, in order to achieve
good catalytic performance for the former two catalysts, a high
temperature (>600 ꢀC) must be performed. By contrast, sup-
1. Introduction
Breathing clean air is one basic demand for an ecological
environment or for human health and well-being. However,
volatile organic compounds (VOCs) from industrial emission
have been seen as a major constituent of air pollution and
gained much attention due to their effect on the formation of
suspended particulate, photochemical fog and haze. Thus,
various applicable techniques can be used for VOCs elimina-
tion, such as adsorption, membrane separation, condensation,
thermal incineration, biological oxidation and catalytic
combustion. Catalytic combustion is regarded as the most
important technology, where VOCs can be completely converted
into harmless products of CO2 and H2O at relatively low
temperatures (<400 ꢀC), especially for low VOC concentration
(<1000 ppm). Generally, transition-metal complexes, perov-
skites and supported noble metal are three categories of
ported noble-metal (e.g., palladium and platinum) catalysts
display excellent catalytic activity and metal stability for VOCs
combustion. Especially, zeolite-supported noble metal catalysts
combined with the advantages of the zeolites (particular porous
structure, large adsorption capacity and controllable acid
sites)1–4 and the noble metals (high low-temperature reactivity
and stability) were extensively investigated for its selective
adsorptive properties, low-temperature activity, high-
temperature stability and accessible active sites. Wang et al.
found that Ru/hierarchical HZSM-5 catalyst could act as both
adsorbent and catalyst for aromatic VOCs elimination under
low concentration levels.5 Besides, Yang et al. reported that the
hierarchical zeolite support could well improve the metal
dispersion, the ratio of metal active sites and the toluene
adsorption, and then the high catalytic performance during the
toluene catalytic combustion.6 However, the catalytic property
of conventional supported catalysts is still insufficient for the
industrial applications, because the supported metals remain
on the surface of zeolites as bulky particles and tend to sinter, or
agglomerate under reaction temperatures and pressures.
In order to make these metal catalysts more stable, zeolite-
encapsulated metal structure has been proposed by conning
metals inside the regular porous structure of zeolites. Recently,
various strategies for synthesizing these encapsulated materials
have been investigated, mainly including a ligand-stabilized
aSchool of Chemistry and Materials Science, Ludong University, 264025 Yantai,
Shandong Province, China. E-mail: sunwenjuan@ldu.edu.cn
bState Key Laboratory of Separation Membranes and Membrane Processes, School of
Environmental Science and Engineering, Tiangong University, Tianjin 300387, China
cKey Laboratory for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China.
E-mail: gliu@tju.edu.cn
† Electronic supplementary information (ESI) available: Synthesis information
and UV-Vis spectra of Pd3Cl; TEM micrographs of used catalyst; calculation of
kinetics parameters of rate constants, pre-exponential factor, activation energy
and turnover frequencies. See DOI: 10.1039/d0ra01576h
12772 | RSC Adv., 2020, 10, 12772–12779
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method,7–12 a mercaptosilane-assisted method,13,14 an inter- Pd3@Beta structure evolves from the seeds by a solution
zeolite transformations and uoride media method,15 and mediated mechanism,26 Pd clusters were conned inside the
a
precursor-stabilization method.16,17 These preparation zeolite crystals. The micropore-conned metal clusters will
methods usually lead to a narrow particle size distribution with possess high dispersion and thermal stability, thus ensuring
an average diameter around 1 nm and lager, which are greater long catalyst lifetime and excellent catalytic cracking
than most zeolites micropores. Moreover, these prepared cata- performance.
lysts have demonstrated excellent catalytic properties for cata-
lytic combustion of small organic volatiles. However, some
kinds of large organic volatiles such as aromatics with dimen-
2. Experimental section
2.1 Catalyst preparation
sions larger than zeolite micropores typically can not enter the
microchannels and thus deteriorate the accessibility to the
internal active sites in catalytic reactions. Therefore, it is also
a challenge to exploit methods for the synthesis of zeolite-
conned metal catalysts with improved mass transfer from
reactants and catalysts while inhibiting sintering/
agglomeration of metals.
Recently, Iglesia et al. encapsulated metal clusters in MFI
micropores by exchanging metal precursors within zeolite
precursor through hydrothermal treatment rst.14,15 Corma
et al. reported a novel method to encapsulate subnanometric
metal species into zeolites channels with the assistant of
dimethylformamide.18–21 This method provides a possibility for
the synthesis of thermal stable subnanometric metal catalysts
for the hydrogenation of alkenes and dehydrogenation of
propane to propylene. de Jong et al. reported the restriction of Pt
clusters through adding premade metal clusters to zeolitic
precursor to enhance their catalytic activity for dehydrogenation
reactions.22 Zhang et al. conned Pd nanoparticles inside FER
zeolite by using diethylenediamine palladium acetate as Pd
precursor, and showed the catalyst had excellent activity.23
These cases show the practicability of producing sub-
nanometric metal clusters conned in internal zeolite cavities
by premade metal complex with organic ligands.
Beta nanozeolites were prepared by hydrothermal synthesis
with tetraethylammonium hydroxide (TEAOH) as the structure-
director. Typically, fumed silica (SiO2) solution (nSiO : nTEAOH
¼
2
5 : 1) and aluminum in TEAOH solution (nAl : nTEAOH ¼ 1 : 3)
were mixed on the scale of 35 to 1, according to our previous
research.27,28 The synthesis solution was pre-crystallization in
ꢀ
sealed autoclave at 115 C for one day. The obtained interme-
diate was denoted as pro-zeolitic seeds. The conventional Beta
zeolite was synthesized under similar conditions except for
ꢀ
crystallization at 115 C for 72 h.
Pd clusters xed inside of zeolite crystals were synthesized by
a two-step method. As a typical run for synthesis of the Pd3@-
Beta, the Beta zeolite seeds were impregnated with [Pd3-
Cl(PPh2)2(PPh3)3]+ (Pd3Cl, detailed synthesis steps were in
ESI†)29 by adding organic auxiliary of ethanol (nsi : nEtOH
7 : 5), followed by further crystallization for 48 h, drying at
¼
ꢀ
ꢀ
100 C overnight, treatment at 500 C for 6 h under vacuum.
Zeolite-supported Pd nanoparticles were obtained by incipient
wetness impregnation process. The Beta zeolites were impreg-
nated with Pd3Cl and PdCl2 solution under ultrasound for 0.5 h,
followed by evaporation to remove water, dryness at 80 ꢀC for 12 h,
treatment at 400 ꢀC for 4 h and reduction by hydrogen at 300 ꢀC
for 2 h. These prepared materials were coded as Pd3/Beta and Pd/
Beta, respectively. Effects of Pd loading amount on the toluene
conversion have been studied as shown in Table S1,† keeping
other experimental parameters constant. By varying the amount
of Pd from 0.1 to 0.9 wt% in Pd3@Beta catalyst, it is found that
0.5 wt% Pd loading is the optimum for the reaction, since there is
a balance between more Pd active sites and stable toluene mass
diffusion rate in zeolite support. Therefore, each catalyst has the
same metal loading of 0.5% (w/w).
In this work, inspired by the above idea, we report a novel
strategy to synthesize zeolite-conned subnanometric Pd cata-
lysts (Pd3@Beta, as shown in Scheme 1), by assembling metal
precursor clusters with protozeolitic seeds. As the staring
material, zeolite Beta with 12-membered ring (MR) pore nearly
˚
7.7 A can promote the mass transfer of large VOCs (like toluene)
and catalytic performance, and it can be used as a model.24,25 Pd
precursor clusters could interact with the silanol groups (Si–
OH) on the zeolitic units surface, and upon further wetness
impregnation treatment, they assemble into zeolite-supported
Pd cluster that function as zeolitic seeds. Because the
2.2 Characterizations
X-ray diffraction (XRD) patterns were determined on a Rigaku
D/MAX 2500 diffractometer (Cu Ka, l ¼ 0.154 nm) in the region
of 2q ¼ 5–50ꢀ. Transmission electron microscopy (TEM) images
were investigated on a JOEL JEM-1230 microscope. The metal
dispersion and particle size were distinguished through
a scanning transmission electron microscopy (STEM, Titan
cubed Themis G2 300, 300 kV). Textural properties of samples
(e.g., including specic BET surface area (SBET), external surface
area (Sext), mesopore volume (Vmeso), micropore volume (Vmic),
etc.) were collected from N2 adsorption/desorption isothermals
using an ASAP 2020 analyzer (Micromeritics) at ꢁ196 ꢀC. All
ꢀ
catalysts have to rst degass at 300 C for one day under high
Scheme
1 Illustrations of preparation procedures for Pd3@Beta
through premade Pd3Cl clusters.
vacuum. Sext and Vmic were obtained by the t-plot method. SBET
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and Vmeso were calculated by the Brunauer–Emmett–Teller zeolitic seeds and Pd clusters were rst synthesized. The
method and Barrett–Joyner–Halenda method, respectively. The premade Pd3Cl clusters are introduced by the impregnation
coordination states of Si and Al species were studied by 29Si CP/ method to obtain Pd-containing zeolitic seeds, and the incep-
MAS nuclear magnetic resonance (NMR, 59.6 MHz, 3.0 kHz) tion of Pd species is localized on the surface of zeolitic seeds.
and 27Al MAS NMR (78.1 MHz, 8.0 kHz) spectroscopy, respectively. The above synthesized seeds are added into the Beta synthesis
The chemical states of Pd species were recorded with X-ray gel mixture with ethanol acting as solubilizer to enhance Pd3Cl
photoelectron spectrometer (XPS, Thermo ESCALAB 250Xi, Al stability in gel. Upon further hydrothermal time, the Pd
Ka). The overall Si/Al molar ratio and metal concentration were complexes interact with negatively charged Si–O–Al species27 via
obtained on an inductively coupled plasma-atomic emission electrostatic binding and the sol–gel reaction mixture crystal-
spectrometry (ICP-AES) analysis (ICPE9000). CO chemisorption lized around the crystal nucleus by a core–shell growth mech-
measurements were performed using a pulse method on an anism30 until the amorphous subunits completely converted
Altamira AMI-300 chemisorption analyzer. Before the test, into zeolite crystals. At this stage, most Pd species are encap-
approximately 50 mg specimen was reduced under hydrogen sulated into the zeolite framework by self-assembly. Upon
ꢀ
atmosphere at 450 C for 2 h. Aer the specimen was cooled to calcination under vacuum, the metallic subnanometric Pd
35 ꢀC under helium atmosphere, pulse chemisorption of CO was clusters were nally formed and conned inside the Beta
performed with 37.5% CO/62.5% He. The dispersion of Pd was zeolite. The specic feature of Pd clusters within the Beta zeolite
calculated by assuming that CO was adsorbed on Pd surface at on the properties and catalytic performance will be discussed
1 : 1 stoichiometry. Investigation on coke formation of these below.
catalysts were analysed via thermogravimetric analysis (TGA-50).
The structural properties for prepared Pd/Beta, Pd3/Beta and
Pd3@Beta catalysts were analyzed through a series of charac-
terization techniques. The XRD spectrums shown in Fig. 1
exhibit that all prepared zeolites have the characteristic
diffraction peaks of the zeolite Beta structure, indicating that
they are stable aer encapsulating Pd clusters into the zeolite
channel. In addition, no characteristic diffraction peaks are
detected for Pd and PdO because of its low loading or weak
crystallinity.
TEM images, Pd elemental maps, metal particle size distri-
butions and HAADF-STEM images of the synthesized catalysts
are shown in Fig. 2. All catalysts display regular crystalline
structure with noticeable crystal lattices. Pd/Beta support
possesses bipyramid morphology with particle size of 50 nm,
whereas Pd3@Beta support is comprised of small zeolite units
with particle size of 20 nm. As for Pd3@Beta catalyst, the
interparticle voids between the nanosized zeolite may generate
the intercrystalline mesopores. As shown in Fig. 2A, the Pd
clusters located inside the zeolite are homogenously dispersed
and distributed in the Pd3@Beta crystallites. The average
particle size of Pd species was obtained by calculating more
than 200 particles, as shown in Table 1. The existence and the
uniform dispersion of Pd clusters or individual Pd atoms can be
distinguished in the selected-area element mapping images
(Fig. 2B). It is can be seen that the mean sizes of Pd clusters of
2.3 Catalytic test for toluene combustion
All toluene combustion reactions were conducted in a quartz
xed-bed ow reactor (8 mm inner diameter). In order to
prevent the hot-spots phenomenon, 100 mg pressed catalyst
sample (20–40 mesh) was mixed with 2.0 g silicon carbide (SiC,
20–40 mesh) on the scale of 1 : 20, and then the mixture was
placed in a quartz reactor tube. Before each catalytic test, the
reactor was preheated to 100 ꢀC at heating rate of 2 ꢀC minꢁ1 for
1 h to reduce the absorbed residues and over-calculation of
toluene conversion. The inlet gas (79% N2 + 21% O2) with
1000 ppm of toluene was fed at a ow rate of 100 mL minꢁ1, i.e.,
a space velocity (SV) at 60 000 mL gꢁ1 hꢁ1. Both toluene and
oxidative products in outlet gas were obtained on a gas chro-
matograph (Bruker 456) equipped FID detector using a PONA
capillary column (50 m ꢂ 0.25 mm ꢂ 0.50 mm) for toluene, and
an infrared spectrum analyser (GXH-1050E) for CO2 and CO.
The catalytic activities were estimated by the values of T5, T50
and T98, which were calculated with the temperature at 5%, 50%
and 98% of toluene conversion, respectively. Toluene conver-
sion can be computed from eqn (1):
Toluene conversion (%) ¼ 100(Cin,Tol ꢁ Cout,Tol)/Cin,Tol (1)
wherein Cin,Tol and Cout,Tol are concentration of toluene in the
feed and discharge gas, respectively. Then CO2 selectivity can be
computed using eqn (2):
CO2 selectivity (%) ¼ 100 nCO /(nCO + nCO
)
(2)
2
2
where nCO and nCO are the molar numbers of CO2 and CO,
2
respectively.
3. Results and discussion
3.1 Structural properties
The preparation process of Pd3@Beta is illustrated in Scheme 1.
To incorporate subnanometric Pd species into Beta crystals, Fig. 1 XRD spectrums for Pd3@Beta, Pd3/Beta and Pd/Beta catalysts.
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Fig. 2 (A) TEM images, (B) Pd elemental mapping images, (C) corresponding Pd cluster size distribution, (D) HAADF-STEM images, (E) HR-TEM
images and (F) schematic crystallographic of Pd clusters in Beta zeolite along the b-axis orientation for Pd3@Beta catalyst. TEM images and metal
particle size distributions of (G) Pd/Beta and (H) Pd3/Beta catalysts.
1.2 nm (Fig. 2C) in seed-directed Pd3@Beta sample are obvi- suggesting the formation of secondary mesopores related with
ously smaller than those of Pd3/Beta (3.5 nm, Fig. 2H) and the the spaces between zeolite crystal and particles. It is also shown
Pd/Beta (6 nm, Fig. 2G). It is worth mentioning that a majority of that the hysteresis loops of the Pd3@Beta shi towards lower
Pd clusters size between 0.3–0.8 nm can be seen here for relative pressure, meaning the formation of smaller mesopore.
Pd3@Beta (Fig. 2D). However, there are still few Pd clusters As shown in Fig. 3B, the pore size distribution of Pd/Beta is
larger than 2 nm can be observed in the sample. Notably, the extremely broad, from 4 to 20 nm. Among these catalysts,
sizes of ultra-small Pd clusters in the Pd3@Beta catalyst appear Pd3@Beta have the narrowest pore distribution around 3.0 nm,
smaller than those of Beta straight channels (0.77 ꢂ 0.67 nm) since the synthesis process make use of premade metal clusters
and tortuous channels (0.56 ꢂ 0.56 nm). Tomographic images
clearly show the micropores in Pd3@Beta sample and Pd clus-
ters can be found in it (Fig. 2E), suggesting the successful
encapsulation of Pd clusters inside of zeolite crystals (as illus-
trated in Fig. 2F). Since the connement of the Beta framework
could limit the size of metal nanoparticles, the Pd cluster with
less than 6 atoms can be contained.
N2 adsorption–desorption isotherms and the pore size
distribution curves for these prepared catalysts are presented in
Fig. 3, and the corresponding textural properties are listed in
Table 1. All samples exhibit typical isotherms of type-IV, indi-
Fig. 3 (A) Nitrogen physisorption isotherms at 77 K and (B) pore-size
cating the presence of mesoporosity. Pd3/Beta and Pd/Beta
distribution using NLDFT model of the adsorption isotherm for
Pd3@Beta, Pd3/Beta and Pd/Beta catalysts.
present hysteresis loops at relative pressure from 0.6 to 1.0,
Table 1 Textural properties, composition and Pd dispersion on Pd3@Beta, Pd3/Beta and Pd/Beta catalysts
Sample
Si/Ala
mPdb (wt%)
SBET (m2 gꢁ1
)
Sext (m2 gꢁ1
)
Vmeso (cm3 gꢁ1
)
Dc (%)
dCOd (nm)
dTEMe (nm)
Pd3@Beta
Pd3/Beta
Pd/Beta
36
35
35
0.46
0.47
0.45
718
626
578
336
182
120
0.53
0.35
0.28
57.3
28.9
16.7
1.7
4.9
6.6
1.2
3.5
6.0
a
Si-to-Al molar ratio measured by ICP-AES. b Pd content determined by ICP-AES analysis. c Metal dispersion obtained from CO chemisorption. d Pd
e
particle size calculated from CO chemisorption. Mean diameters by measuring at least 200 individual Pd particles from TEM images.
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