Total oxidation of naphthalene at low temperatures using palladium nanoparticles supported on inorganic oxide-coated cordierite honeycomb monoliths

Article abstract of DOI:10.1039/c3cy00323j

A study on the preparation of thin films of ZSM-5 and BETA zeolites, and a SAPO-5 silicoaluminophosphate, supported on cordierite honeycomb monoliths by in situ synthesis was carried out for their use as catalyst supports. Furthermore γ-Al₂O₃ was also coated onto a cordierite honeycomb monolith by a dip-coating method for use as a standard support. Structured monolithic catalysts were prepared by impregnation of the aforementioned coated monoliths with polymer-protected Pd nanoparticles. The monolithic catalysts have been tested for the total oxidation of naphthalene (100 ppm, GHSV 1220 h ^-1). From the combined use of the zeolite with polymer-protected nanoparticles, enhanced catalytic properties have been found for the total abatement of naphthalene. The Pd/MBETA and Pd/MZSM-5 catalytic monoliths have shown excellent activity with a high degree of stability, even after undergoing accelerated ageing experiments.

Full text of DOI:10.1039/c3cy00323j

Catalysis
Science & Technology
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Total oxidation of naphthalene at low temperatures
using palladium nanoparticles supported on inorganic
oxide-coated cordierite honeycomb monoliths
Cite this: Catal. Sci. Technol., 2013,
, 2708
3
a
a
a
´
Francisco J. Varela-Gand ´ı a, Angel Berenguer-Murcia, Dolores Lozano-Castell o´ ,
a
b
b
Diego Cazorla-Amor o´ s,* David R. Sellick and Stuart H. Taylor*
A study on the preparation of thin films of ZSM-5 and BETA zeolites, and a SAPO-5 silicoaluminophosphate,
supported on cordierite honeycomb monoliths by in situ synthesis was carried out for their use as catalyst
2 3
supports. Furthermore g-Al O was also coated onto a cordierite honeycomb monolith by a dip-coating
method for use as a standard support. Structured monolithic catalysts were prepared by impregnation of
the aforementioned coated monoliths with polymer-protected Pd nanoparticles. The monolithic catalysts
Received 8th May 2013,
Accepted 4th July 2013
À1
have been tested for the total oxidation of naphthalene (100 ppm, GHSV 1220 h ). From the combined
use of the zeolite with polymer-protected nanoparticles, enhanced catalytic properties have been found for
the total abatement of naphthalene. The Pd/MBETA and Pd/MZSM-5 catalytic monoliths have shown
excellent activity with a high degree of stability, even after undergoing accelerated ageing experiments.
DOI: 10.1039/c3cy00323j
www.rsc.org/catalysis
However, for practical applications macro-structured catalysts
are preferred over powdered catalysts. For example, a monolithic
Introduction
Polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, reactor can involve a single block of material containing a large
are environmentally hazardous compounds produced as a result number of parallel channels, which are available in various
1
–5
23
of the incomplete combustion or pyrolysis of organic material.
sizes or shapes. Low pressure drop, uniform flow distribu-
Naphthalene is considered the least toxic and simplest molecule tion, the absence of hot spots and good tolerance to plugging by
of the PAHs, and thus it is used as a model compound for dust are essential requisites that ultimately lead to the use of
2
3,24
this group of pollutants. In the literature, a range of different catalytic monoliths.
technologies have been studied to reduce PAH emissions.
For gas phase applications, metal or
6
–13
ceramic honeycomb monoliths are standard support materials.
Among them, catalytic combustion is the most promising for In reference to ceramic monoliths, the support structure is
the removal of PAHs from polluted air streams, due to its lower made of a non-catalytic, thermally resistant material, typically
2
25
operating temperature and higher selectivity towards CO
2
.
cordierite, onto which a catalytic layer is deposited. The chemical
For catalytic total oxidation, the types of catalyst used are based composition of the cordierite is based on 2MgOÁ2Al O Á5SiO
2
3
2
to a large extent on metals such as Pd, Pt, Ru or Co supported and a cordierite monolith has excellent operational properties
1
4
2,15–19
on g-alumina by impregnation, metal oxide catalysts,
such as low pressure drop in the exhaust system, good thermal
mesoporous aluminosilicates with supported Pt or zeolites resistance, refractoriness, good wash-coat adherence and
2
0
21
22
26,27
ion-exchanged with Pt. Based on our previous studies, polymer compatibility between the wash-coat and the catalyst.
protected Pd nanoparticles supported on zeolites are one of Concerning the deposition method for active phases, different
the most promising types of catalyst for the total oxidation of strategies can be employed for initially coating the monolith
naphthalene, due to the low oxidation temperature reported channels with a support material, like alumina, silica, ceria and
(165–180 1C), and the high stability of the catalyst.
zeolites, followed by subsequent impregnation with an active
2
8
phase. In the case of zeolites or SAPO materials, there are two
ways to coat the monoliths: hydrothermal synthesis using
either direct synthesis or seeded growth, referred to as in situ
synthesis, and dip-coating or wash-coating, which consists of
deposition from a slurry of zeolite particles followed by a
a
Departamento de Qu ´ı mica Inorg a´ nica e Instituto Universitario de Materiales,
Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain. E-mail: cazorla@ua.es;
Fax: +34 96590 3454; Tel: +34 96590 3946
b
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
Park Place, Cardiff CF10 3AT, UK. E-mail: TaylorSH@Cardiff.ac.uk;
Fax: +44 (0)29208 74030; Tel: +44 (0)29208 74062
2
4
stabilizing thermal treatment.
2
708 Catal. Sci. Technol., 2013, 3, 2708--2716
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In the present study, two different zeolites (BETA and ZSM-5)
Catalysis Science & Technology
After preparing the coated monoliths using two consecutive
and a silicoaluminophosphate molecular sieve (SAPO-5) have syntheses, the removal of the template was carried out by
been selected as coating materials for cordierite monoliths by calcination at the desired temperature for each material in a
À1
in situ synthesis. Furthermore, a commercial g-Al O has been muffle furnace under static air (heating rate 1 1C min ).
2
3
coated on the monolithic supports by the dip-coating method Finally, the total weight increase for both the first coating step
for comparative purposes. The active phase that will be performed and the whole coating process was measured. The
employed is polymer protected Pd nanoparticles, which have increase in mass is indicative of the loading and is expressed
shown good properties as catalysts for naphthalene total oxida- as wt%.
tion with the supports above, when they were used as powder
catalysts. Special attention has been focused on the stability following the methodology reported by Bueno-Lopez et al.,
Coating of the monoliths with zeolite BETA was carried out
2
2
27
and recyclability of the structured catalysts for extended time- but it was adapted to our larger monolith size. The details of
on-stream experiments.
the molar compositions of the synthesis gel are given in ref. 19.
The optimum conditions found for the synthesis of zeolite
BETA on cordierite monoliths were two consecutive syn-
thesis steps with a crystallization temperature of 132 1C for
Experimental
Catalyst preparation
48 hours. The BETA coated monoliths, named MBETA, were
Coating monoliths with BETA, ZSM-5, and SAPO-5. Cylindrical subjected to a calcination step to remove the template at 500 1C
cordierite monoliths (Corning) were used as supports (400 cpsi, for 6 hours.
length (l): 14 cm, diameter (d): 14 cm). These were cut into
ZSM-5 coated monoliths, which are called MZSM-5 in this
small cylindrical pieces (l: 1.6 cm, d: 1.4 cm, mass: B1.2 g). work, were obtained after two consecutive synthesis steps.
2
5
Prior to coating, all the small monoliths were blown with air to As mentioned previously by Ulla et al., the coating of high
remove the dust produced in the cutting process. A calcination aluminium content zeolites is hindered by Al-rich supports,
process in static air in a furnace at 800 1C for 2 hours (heating such as cordierite, and therefore it is necessary to combine two
À1
rate: 6 1C min ) was performed with the aim of removing synthesis protocols using materials with different Si/Al ratios.
organic impurities.
For this reason, the first-crystallization step was the prepara-
The growth of BETA and ZSM-5 zeolites and the SAPO-5 tion of a silicalite-1 coating, which is isostructural to ZSM-5, but
silicoaluminophosphate molecular sieve was carried out by with no aluminium content in its framework, in order to ensure
in situ synthesis on the cordierite monolith avoiding the use the crystallization and anchoring of the ZSM-5 zeolite onto the
of binders. The general experimental procedure for the syn- monolith. Thus, the synthesis was performed adapting the
thesis of the zeolites and the silicoaluminophosphate onto the methodology for the preparation of silicalite-1/carbon membranes
2
9
monolith was:
i) The cordierite honeycomb monoliths were wrapped with was obtained by adding 3.420 g of TEOS (tetraethoxysilane)
Teflon tape to prevent zeolite growth on the outer surface. dropwise into a 1 M tetrapropylammonium hydroxide TPA-OH
ii) The ‘‘teflonated’’ monoliths were placed in a stainless solution (4.920 g TPA-OH solution, in 31.570 g of distilled
reported by Berenguer-Murcia et al. The silicalite-1 solution
(
(
steel autoclave with a Teflon liner (V: 10 mL). A precursor water). The resulting solution was aged for 90 min and the
À1
gel/monolith weight ratio of 4 mL g was used for coating monoliths were coated with a layer of silicalite-1 after a crystal-
the monoliths.
iii) The autoclaves were placed horizontally in a convection step consisted of the preparation of a ZSM-5 zeolite coating.
oven and rotated during the whole growth process (v = 4 rpm) to The details of the experimental procedure used for the prepara-
lization time of 6 hours at 180 1C. The second crystallization
(
3
0
ensure a homogeneous growth of the solid film and prevent tion of ZSM-5 zeolite are given in detail elsewhere. According
2
7
blocking of the monolith channels. Four monoliths were to ref. 30, the crystallization conditions are 190 1C for 10 hours.
prepared per batch. Template removal was carried out by calcination in static air at
iv) The crystallization step was carried out at the desired 550 1C for 4 hours.
(
temperature and time for each crystalline material.
SAPO-5 monoliths, named MSAPO-5, were prepared by
3
1
(v) After the hydrothermal synthesis, the autoclaves were adapting the procedure described by Campelo et al. for the
subjected to ‘‘fast cooling’’ using running tap water.
preparation of powdered SAPO-5. The crystallization was per-
(
vi) After the synthetic protocol, the coated monoliths were formed at 200 1C and crystallization times between 18 and
washed with abundant quantities of distilled water and dried at 24 hours were used for studying the coating process. The
00 1C overnight. All the monoliths were subjected to sonication calcination process for MSAPO-5 monoliths was performed at
for 6 hours to remove loosely adhered crystals and then, dried 600 1C for 6 hours.
again overnight in an oven at 100 1C. Coating of c-Al O . A coating of g-Al O onto selected honeycomb
1
2
3
2 3
Two consecutive synthesis steps were performed. Thus, after cordierite monoliths was performed by dip-coating and the
finishing the whole procedure described above the samples samples were labeled as Mg-Al O . Prior to the dip-coating step,
2
3
were again placed into an autoclave for a second synthesis the cordierite monoliths were wrapped with Teflon tape to
2 3
process using a fresh synthesis solution (i.e. points (ii) to (vi) avoid deposition of g-Al on their outer surface. Mg-Al O
2
O
3
were repeated).
monoliths were prepared following the procedure described by
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3
2
Villegas et al. The g-Al
2
O
3
powder was dispersed in HNO
3
Textural characterization of the coated monoliths and the
À1
(
g-Al –H O = 25 wt%, HNO
2
O
3
2
3
–g-Al
2
O
3
= 2 mmol g ) using a catalysts was carried out by means of adsorption of N
2
at À196 1C
high-shear mixer (ULTRA TURRAX T25, IKA Labortechnik). (Autosorb 6, Quantachrome). Prior to the adsorption measure-
Initially, the required amount of g-Al O powder was added to ments, the samples were outgassed under vacuum (10 mbar) at
À2
2
3
the acid solution for 10 min at 7000 rpm. After vigorous stirring 250 1C for 4 h to remove any adsorbed impurities. Surface area
14 000 rpm) for 5 min at room temperature, the stable sus- was calculated from nitrogen adsorption isotherms using the
pension was used for dip-coating. A series of 8 monoliths were BET equation (SBET). Total micropore volumes (VDR (N )) were
prepared with each dip-coating suspension, in order to check calculated by applying the Dubinin–Radushkevich (DR) equation
(
2
3
4
the reproducibility of the coated monoliths. The monoliths to the N
were immersed vertically into the suspension for 2 minutes and
the excess suspension was removed using a flow of compressed
2
adsorption data at À196 1C.
Catalyst performance
air. Finally, the monoliths were dried at room temperature Catalytic activity tests for naphthalene oxidation were carried
for 24 hours whilst continuously rotating horizontally to out in a fixed bed reactor (diameter = 1.6 cm). The feed stream
ensure optimum coating distribution. The procedure was per- consisted, in all cases, of 100 ppmv naphthalene in a mixture of
À1
formed twice with a fresh suspension. Mg-Al
2
O
3
monoliths 20% O and 80% He. The total flow was set to 50 mL min
2
À1
were calcined in static air at 600 1C for 4 hours (heating rate (GHSV = 1220 h ). Analysis of reactants and reaction products
À1
1
1C min ).
was performed by on-line gas chromatography using thermal
Deposition of Pd nanoparticles. Palladium nanoparticles conductivity and flame ionization detectors. The catalytic activity
protected by polyvinylpyrrolidone (PVP) were synthesized by was measured over the temperature range 100–200 1C in incre-
3
3
the reduction-by-solvent method as reported previously.
mental steps and temperatures were measured by a thermocouple
The catalysts used in this work were prepared by the impreg- placed in the catalyst bed connected to a PID controller. Data
nation method, as reported in our previous work for powdered were collected at each temperature after a stabilization time of
2
2
catalysts. The aforementioned coated monoliths were wrapped 20 minutes. Three analyses were made at each temperature to
with Teflon tape in order to fix them vertically in 10 mL Teflon ensure steady state was attained. Oxidation activity is expressed
liners and to prevent deposition of Pd nanoparticles on the as a yield of carbon dioxide. Furthermore, accelerated time-
outer surface of the cordierite monolith. The required concen- on-line experiments for long-term use of these catalysts were
tration of Pd nanoparticles, suspended in methanol, was added performed with the aim to determine potential deactivation.
based on the total amount of BETA, ZSM-5, SAPO-5 or g-Al O
For this purpose, the catalysts were subjected to reaction
coating, to yield a nominal 1 wt% of metallic loading with conditions at 250 1C for 48 hours.
respect to the zeolite, SAPO-5 or Al . To ensure the complete
2
3
2 3
O
immersion of the coated monoliths in the suspension addi-
tional methanol was added to give a final volume of 5 mL. The
Teflon liners filled with the monolith and Pd suspension were
placed in a thermostated bath, stirred at 60 rpm at room
temperature for two days. Subsequently, they were transferred
to an oven, where they were left at 60 1C in order to evaporate
the solvent and obtain the final structured honeycomb
monolith, labeled Pd/MBETA, Pd/MZSM-5, Pd/MSAPO-5 and
Results and discussion
Coated monoliths characterization
Table 1 shows the percentage of weight increase on coated
monoliths after the first and second coating steps. According to
2
7
Bueno-Lopez et al. in order to improve zeolite loading and
deposit a thin and homogeneous zeolite layer, autoclave rotation
during the hydrothermal treatment and a second synthesis step are
necessary. It is worth mentioning that both the supported zeolite
and SAPO-5 crystals (after the first step) and the zeolite and SAPO-5
layer (after the second step) are highly stable towards sonication,
indicating good adhesion of the zeolite to the monoliths surface.
Therefore, the error in the estimation of the weight increase after
two coating steps is reasonably low, considering the complex
process under study, including crystallization, cleaning and calcina-
tions steps. Mg-Al O monoliths, prepared in batches of 8 monoliths
2 3
Pd/Mg-Al O .
Characterization of the catalysts
ZSM-5 and BETA zeolites and the SAPO-5 silicoalumino-
phosphate molecular sieve supported on cordierite monoliths
were characterised by XRD, using a 2002 Seifert powder diffracto-
À1
meter. The scanning rate was 21 min and Cu-Ka radiation
was used.
2
3
The coated monoliths were characterized by SEM in a JEOL
microscope (model JSM-80). The monoliths were carefully cut
parallel to the monolith axis. This procedure allowed the
analysis of the surface of the monolith channels, and the
at a time, possess a high reproducibility with a weight increase and
standard deviation similar to observations by Villegas et al.
32
thickness and homogeneity of the coated layer. In the case of Table 1 Monolith weight increase (wt%) for single and two steps coatings
the Pd-containing catalysts, they were cut in the same manner
Coated monolith First coating step (wt%) Second coating step (wt%)
and analyzed by SEM in a HITACHI S-3000 microscope. Energy-
MBETA
MZSM-5
MSAPO-5
3.6 Æ 0.4
4.0 Æ 0.4
4.9 Æ 0.9
9.4 Æ 1.4
17.7 Æ 2.0
15.8 Æ 1.8
16.1 Æ 1.7
15.0 Æ 1.7
dispersive X-ray spectrometry (EDX) was used to ascertain the
Pd concentration along the channels by careful cutting of the
monolith along its length.
2 3
Mg-Al O
2
710 Catal. Sci. Technol., 2013, 3, 2708--2716
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Table 2 Porous texture characterization results of the coated monoliths
2
À1
3
À1
Sample
S
BET (m g
)
2
VDR (N ) (cm g )
Cordierite
MBETA
MZSM-5
MSAPO-5
1
0.00
0.05
0.02
0.02
100
35
35
8
a
Mg-Al
2
O
3
—
a
Not possible to determine.
contribution of the cordierite support to the adsorption proper-
ties of the coated monoliths. Table 2 summarizes the surface
2
areas (SBET) and the total micropore volumes (VDR(N )) calcu-
lated per unit weight of the monolith.
Fig. 1 XRD diffractograms of cordierite, coated monoliths and powder zeolites.
The peaks corresponding to the zeolite phases are marked with full dots.
In the case of Mg-Al
adsorption isotherm as the powder g-Al
2
O
3
it presents a similar nitrogen
used to coat the
2 3
O
X-ray diffraction was used to confirm the phase purity monolith, which indicates that the dip-coating procedure used
and crystallinity of the zeolite or SAPO layers coated on the to prepare this sample does not modify significantly the
cordierite monoliths after two synthesis steps. Fig. 1 compiles porosity of the supported g-Al
the diffractograms of uncoated cordierite and coated monoliths
In reference to zeolite and SAPO coated materials, MBETA,
MBETA, MZSM-5 and MSAPO-5). Powder zeolites and SAPO-5 MZSM-5 and MSAPO-5, the nitrogen adsorption isotherms also
diffractograms have been added to confirm the crystallinity of have the same shape, including the hysteresis loop, as the
O .
2 3
(
3
5
the coating layer; the diffractogram for the coated monoliths equivalent powdered samples prepared in previous work.
show the same characteristic peaks of the powdered zeolites To compare the specific surface area (SBET) area and micropore
3
5
(
indicated by full dots on the corresponding peaks). There- volume of the coated zeolites with the powdered zeolites, estima-
fore, it can be confirmed that the crystallinity of the zeolites are tions for the monoliths have been made considering the weight
maintained when they are supported on the monoliths.
of the zeolite coatings. These values have been excluded in this
In the case of MSAPO-5 monoliths, a synthesis time between paper for brevity, but it is evident that both BETA and SAPO-5
8–24 h was used. From the analysis of the XRD of the samples layers have very similar porosity to the powder materials, thus
1
prepared after 18, 20, 22 and 24 h (results not shown), it was practically they are unaffected by the in situ coating process.
confirmed that only the MSAPO-5 prepared after 18 h (XRD
On the other hand, the ZSM-5 layer, confirmed by X-ray
included in Fig. 1) showed the characteristic peaks of the SAPO-5 diffraction, shows a lower porosity than the one expected from
silicoaluminophosphate (AFI structure) without the appearance the comparison with a powder sample. The main reason is due
of other crystalline phases. Consequently, all the monoliths to the coating procedure used for this sample. As we have
synthesized for this work were prepared with a synthesis time mentioned previously, the coating of high aluminium content
of 18 h.
zeolites is hindered by Al-rich supports such as cordierite.
Nitrogen adsorption was performed to analyze the porous According to Ulla et al. the first step in the hydrothermal
2
5
texture of the as-prepared materials. From the nitrogen isotherms synthesis of zeolite films is the formation of a precursor gel
(
see Fig. 2) it is possible to confirm that cordierite exhibits layer onto the substrate, which serves as the primary source of
negligible porosity; therefore, it can be assumed that there is no nucleation. However, the addition of Al to the solution accel-
erates gel layer formation, but delays the nucleation and
subsequent zeolite growth processes within it. Therefore, the
use of Al-rich synthesis gels has a direct influence on the kinetics
of the process and, as a consequence, high loadings of well-
crystallized and anchored zeolite films are difficult to obtain. In
a first synthesis, silicalite-1 is used to provide a suitable environ-
ment for the growth of a high aluminium content zeolite in the
second synthesis step. This first layer is partially dissolved
during the second synthesis, helping the nucleation and growth
of the second layer. Thus, this procedure may block or remove
part of the first zeolite coating producing a reduction in the
3
6
adsorption properties of the zeolite layer.
A detailed study of the different coated layers onto the
cordierite monoliths was carried out by SEM. Fig. 3 shows
the original uncoated monolith. The top view shows that a
cordierite monolith is a continuous support with macroscopic
Fig. 2 Nitrogen adsorption–desorption isotherms at À196 1C.
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for each type of zeolite, as has been reported in the literature
2
7,35
29,35
37,38
for BETA,
ZSM-5
and SAPO-5.
In reference to the
Mg-Al O , the slurry for dip-coating partially covered the surface
2
3
of the monoliths filling the external openings of the cordierite
monoliths.
When a second synthesis step was performed (see Fig. 4
right column), it is possible to cover all the surface of the
monolith with a complete and homogeneous layer of intergrown
crystals. MBETA and MZSM-5 crystals maintained their respective
shape. It must be pointed out that the MSAPO-5 monolith
presents a bimodal particle size distribution. The crystals
appearing after the first synthesis have increased in size up to
approximately 12 mm, and small SAPO-5 crystals have grown
Fig. 3 SEM images of the original uncoated cordierite honeycomb monolith
(
A) top view and (B) cross sectional view.
openings along the entire surface that will be accessible during
the zeolite coating process. From the cross sectional view it
can be established that the monolith walls are 150 mm wide and
have a macroporous character.
(
approximately 1 mm), filling the gaps between the larger crystals,
giving rise to a continuous layer. Finally, the Mg-Al monolith
prepared by the ‘‘dip-coating’’ method is formed by a homo-
geneous and continuous layer of Mg-Al over the entire mono-
2 3
O
Fig. 4 shows the top view of the coated monoliths with BETA,
ZSM-5, SAPO-5 and g-Al O , respectively. After the first synthesis
2
3
2 3
O
step (see Fig. 4, left column), MBETA, MZSM-5 and MSAPO-5
monoliths have crystals which do not cover all the surface of
the monoliths and, therefore, do not form a continuous layer.
These individual crystals, however will act as seeds for the second
in situ synthesis step performed. The individual crystals formed
onto the cordierite monoliths present the typical morphology
lith wall. In spite of the deposited monolith coating layer being
homogeneous, a significant quantity of g-Al O was preferentially
2
3
deposited on the corners towards the centre of the channels, and
small cracks were observed after the calcination step.
With the aim to analyze the thickness of the coated layer
onto the monolith walls, SEM images of the cross sectional view
are useful for this purpose (see Fig. 5). All samples in this study
prepared by the in situ synthesis method showed multiple
layers of zeolite crystals. There were no significant differences
between the zeolite coating or the SAPO-5 layer in the
centre and in the channel open ends, as well as along the
channels, indicating that the movement of the autoclaves
during synthesis results in a homogeneous growth along the
channel length.
Detailed analysis of SEM images has allowed us to conclude
that the MBETA monolith (Fig. 5A) is formed by a layer of inter-
grown zeolite crystals with an average thickness of 3 mm. Further-
more, the small crystal size of BETA zeolite permitted access to
the internal openings of the monolith walls, thus giving rise to
Fig. 5 SEM cross sectional images for the four prepared monoliths after the
Fig. 4 SEM top view images for the four prepared monoliths after the first
second coating step. (A) Sample MBETA, (B) sample MZSM-5, (C) sample MSAPO-5,
coating step (left column) and the second coating step (right column).
2 3
and (D) sample Mg-Al O .
2
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Table 3 Porous texture characterization results of the catalytic monoliths
2
À1
3
À1
Sample
S
BET (m g
)
2
VDR (N ) (cm g )
Pd/MBETA
Pd/MZSM-5
Pd/MSAPO-5
95
35
10
7
0.05
0.02
a
—
a
Pd/Mg-Al
2
O
3
—
a
Not possible to determine.
analysis performed on the four catalysts (Pd/MBETA, Pd/MZSM-5,
Pd/SAPO-5 and Pd/Mg-Al ), the general conclusion was that the
2 3
O
amount of Pd in all the monoliths was around 1 wt% with respect
to the coating layer and the Pd was uniformly distributed along
the channels.
Nitrogen adsorption was performed on the Pd–monolith cata-
Fig. 6 SEM cross sectional images for BETA zeolite monolith with an internal lysts in the same manner as the coated monoliths. Table 3
opening completely filled with BETA zeolite.
includes the BET surface areas (SBET) and the total micropore
2
volumes (VDR(N )). Comparing the results with those obtained for
the coated monoliths (see Table 2), it can be established that the
catalyst preparation method has affected the adsorption properties
of the coated layer on the Pd/MBETA and Pd/MZSM-5 and
a zeolite deposit formed by the intergrowth between the crystals
in the internal and external surfaces, producing a three dimen-
sional network of the zeolite layer that results in high mechanical
stability. Fig. 6 shows, as an example, a cross sectional view of
a monolith wall, where the BETA zeolite layer was deposited on
the cordierite wall, together with an internal opening filled with
zeolite crystals.
In the case of MZSM-5 monoliths, the coating layer consists
of intergrown crystals with an average thickness of 6 mm. It is
worth mentioning that the previous silicalite-1 crystals deposited
during the first synthesis cannot be observed. As explained
previously, these crystals act as seeds for the deposition of a
stable ZSM-5 layer and are partially dissolved during hydro-
thermal synthesis. MSAPO-5 monoliths are formed by a coated
layer of intergrown crystals with a bimodal particle size distri-
bution with a layer thickness of around 12 mm. To conclude, for
the monoliths prepared by in situ synthesis, it is important to
note that MZSM-5 and MSAPO-5 only exhibit the crystallization
of individual zeolite crystals in the internal openings without
filling the porosity within the monolith walls, contrary to the
behaviour observed for the MBETA monolith.
Pd/Mg-Al
2 3
O only to a minor extent. Only in the case of the
2
7
Pd/MSAPO-5 catalyst was there a significant reduction of the N
2
adsorption capacity compared to the MSAPO-5 monolith before
impregnation with Pd nanoparticles. From the N adsorption
2
isotherms (results not shown), it is possible to confirm that,
after impregnation with the Pd-based nanoparticles, the shape
of the isotherms is the same as for the coated monoliths.
Comparing these results with powder catalysts Pd/BETA and
2
2
Pd/ZSM-5 prepared previously, the decrease in SBET is lower in
the coated monoliths. This behavior could be tentatively assigned
to the location of deposited Pd nanoparticles. When a powder
catalyst is prepared, the Pd nanoparticles are located over all of
the zeolite crystals homogeneously. On the contrary, Pd nano-
particles in the coated monolith are predominantly distributed
on the surface of the coating layer, due to the highly dense and
compacted layers formed, as corroborated by EDX. Similar to the
22
Pd/SAPO-5 powdered catalyst, the catalytic monolith shows the
same decrease in apparent surface area.
2
2
As mentioned in our previous work, the synthesized Pd
colloid shows very low polydispersity with a particle size of
2 3 2 3
Analyzing the g-Al O layer on Mg-Al O monoliths (see Fig. 5),
a variation in thickness has been found. The thickness of the
alumina wash-coating varies from 3 to 6 mm on the left side of the
monolith to 16 mm on the other end of the opening, whilst it was
about 12 mm in the middle of the channel as determined by SEM.
This behaviour can be attributed to the immersion–extraction
rate of the monolith from the slurry and the final step of blowing
with air that may alter the coated layer at one of the ends.
1
.8 Æ 0.3 nm. In the case of the prepared monoliths it is not
possible to observe the Pd dispersion using TEM, but deposition
of polymer protected Pd nanoparticles did result in a noticeable
change of Pd particle size according to our previous studies,
so it is expected that the particle size increases when the polymer
protected Pd nanoparticles are supported onto the coated monolith.
2
2,33
Catalytic performance in the oxidation of naphthalene
Monolithic catalysts characterization
The catalytic activity of naphthalene oxidation was measured
After impregnation, the monolithic catalysts were analyzed by for the four monolithic catalysts. Three reaction cycles were
XRD and no appreciable changes were observed in the prepared performed with each catalyst in order to ensure the stability of
catalysts (results not shown). To analyze the distribution and the catalysts. This was achieved by incrementally increasing the
loading of Pd nanoparticles on the monolith, Energy-dispersive reaction temperature to the desired set point and then cooling
X-ray spectrometry (EDX) was used to measure the quantity of down to room temperature in order to start the next catalytic
Pd along the channels of the monolith. From the individual cycle. Additionally, it is noteworthy that no CO was detected as
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a reaction product in any of the experiments performed. Previous
to testing the monolithic Pd catalysts, the coated monoliths
(
MBETA, MZSM-5, MSAPO-5 and Mg-Al O ) were tested in the
2 3
oxidation of naphthalene and, in general, none of the coated
monoliths was active at the temperature range studied (i.e. between
1
00 and 200 1C). For example, the MBETA monolith was active
at temperatures above 300 1C reaching a 45% conversion of
naphthalene to CO at 350 1C.
Fig. 7 shows the variation of the catalytic activity (expressed
as conversion towards CO ) for naphthalene oxidation, as a
2
2
function of the reaction temperature for the four Pd–monolith
catalysts. After the first cycle, outstanding catalytic performance
has been observed for the Pd/Mg-Al O and Pd/MSAPO-5 cata-
2
3
lysts, whilst Pd/MZSM-5 and Pd/MBETA were able to reach full
naphthalene conversion at higher temperatures. It is worth
mentioning that the catalytic activity remained unchanged after
three cycles, maintaining high naphthalene oxidation activity to
CO
Table 4 shows the temperature required for total naphthalene
oxidation to CO2 for each monolithic catalyst over the three
2
without any evidence for catalyst deactivation.
2
2
cycles studied. Comparing these results with our previous work
a small increase in the temperature needed to reach total
oxidation is observed for the zeolite-coated monolith (BETA
and ZSM-5). This can be due to the dense thin layer of the
zeolite that may have an influence in naphthalene adsorption–
desorption, thus having some effect on the kinetics of the
process. Analyzing the CO conversion at 150 1C in the mono-
2
lith samples (see Table 5) and comparing with our previous
2
2
work, the catalytic activity of the monoliths at 150 1C shows a
similar trend. The zeolite catalyst showed an increase of activity
from the first to the third cycle, whilst the Pd/MAl O showed
2 3
reduced activity after three cycles. The activity is slightly lower
for the catalysts in monolith form with respect to the powder
form, and this is most probably due to increased diffusion
limitations with the monoliths. Nevertheless, these are minor
differences that show that the catalysts based on Pd nano-
particles supported on the coated-monoliths have a similar
2
2
activity to the powder catalysts.
It is also useful to start to probe the stability of the active
catalysts. An accelerated experiment to study possible deactiva-
tion as a function of time was conducted at 250 1C for 48 h.
Fig. 8 shows the time-on-line data for the four Pd–monolith
catalysts. It was evident that the Pd/MBETA and Pd/ZSM-5
samples were stable for at least 48 h and there is no evidence
of catalyst deactivation over the testing period. The yield of CO2
at the beginning of the experiment was 100% and it was
not diminished after 48 h. Contrary to these two catalysts,
Pd/MSAPO-5 and Pd/Mg-Al
the first hours, but their conversion to CO
0% for Pd/MSAPO-5 and Pd/Mg-Al , respectively, indicating
2
O
3
exhibit a conversion of 100% during
2
dropped to 92% and
9
2 3
O
possible deactivation, probably induced by the agglomeration
of the Pd nanoparticles. Therefore, Pd/MBETA and Pd/MZSM-5
Fig. 7 Variation of the catalytic activity for naphthalene total oxidation
2
(expressed as yield to CO ) as a function of reaction temperature over the four
are the most suitable catalysts for naphthalene removal. In these catalysts. (A) Pd/MBETA, (B) Pd/MZSM-5, (C) Pd/MSAPO-5, and (D) Pd/Mg-Al O .
2
3
cases, the rugosity of the prepared zeolite layer prevents the
agglomeration of the Pd nanoparticles. These observations are in
agreement with those for the powder catalyst.
These results help to assess the viability of the monolith-
based catalysts for naphthalene removal by catalytic oxidation,
2
2
2
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Table 4 Temperature required for naphthalene total oxidation for the different honeycomb monoliths (400 cps) wash-coated with g-Al
catalysts for the three oxidation cycles tested
2 3
O
and
used as supports for noble metal catalysts (Pd or Pt, 0.1 mol%),
metal oxides (MnO CuO , 10 mol%) and combinations of noble
Temperature (1C) for total conversion to CO
2
x
x
metals with metal oxides. Analyzing the T50 temperature for the
fresh monolithic catalyst, the Pd based catalyst was the most
Catalyst
Cycle 1
Cycle 2
Cycle 3
1
1
1
1
%Pd/MBETA
%Pd/MZSM-5
%Pd/MSAPO-5
195
195
180
165
180
180
165
165
180
180
165
165
active (T50 = 200 1C), whilst the metal oxides alone, MnO
x
and
CuO , were able to reach a conversion of 50% at 365 and 420 1C,
x
%Pd/Mg-Al
2
O
3
respectively. It was pointed out that impregnation of the metal
oxides with Pd or Pt significantly decreased T50 for the resulting
catalysts, however, they are still higher than those obtained in
this study at least by 50 1C. To conclude, all our monolithic
catalysts show T50 values of around 150–160 1C after 3 cycles,
indicating the benefits of the use of PVP polymer protected
Cycle 3 nanoparticles with more controllable properties. Furthermore,
2
Table 5 CO conversion at 150 1C for the four catalysts and during the three
cycles tested
2
CO conversion (%) (T = 150 1C)
Catalyst
Cycle 1
Cycle 2
in our studies total oxidation of naphthalene is reached at
lower temperatures.
1
1
1
1
%Pd/MBETA
%Pd/MZSM-5
%Pd/MSAPO-5
5
4
9
19
16
33
53
25
17
39
43
Concerning the zeolite coated monoliths, a synergistic effect
between using zeolites (BETA or ZSM-5) prepared by an in situ
methodology with PVP–Pd nanoparticles has been found.
The catalytic monoliths are able to produce total oxidation of
naphthalene at low temperatures, together with naphthalene
removal from the gas stream by adsorption at lower tempera-
ture where the nanoparticles are not yet active. Secondly, the
time-on-line experiments have shown that the catalysts are
stable and thus should be regarded as suitable for the total
oxidation of naphthalene in a practical application.
%Pd/Mg-Al
2
O
3
38
Conclusions
Zeolites BETA and ZSM-5, and a silicoaluminophosphate mole-
cular sieve, SAPO-5, have been coated on cordierite honeycomb
monoliths (400 cpsi) by an in situ synthesis method. Two
synthesis steps were required in order to coat the cordierite
monoliths. The amount of zeolite introduced during the first
single-step synthesis cannot completely cover the monolith
2
Fig. 8 Conversion of naphthalene to CO as a function of time-on-line at 250 1C
for the four monolithic catalysts.
but a critical comparison with monolith-based catalysts from surface, but it is used to seed the second synthesis step, which
2
1
the literature should also be considered. Neyestanaki et al.
produces homogeneous and continuous thin zeolite films.
deposited ZSM-5 on cordierite monolith substrates by means of Furthermore, the BETA coating layer is formed by a compact
a ball-milled zeolite–water slurry method (monolith characteristics three-dimensional zeolite network conferring strong zeolite
4
00 cpsi, d = 21.8 mm, h = 20 mm), by dip-coating and drying anchoring to the cordierite structure. In general, the prepara-
several times, until a wash-coated loading between 13.5 and 30.8% tion reproducibility of the coated materials was very high.
and Pd, loaded by classical methods, was achieved. All the All the coated monoliths were impregnated with Pd–PVP-
monolithic catalysts were tested for the removal of the emissions protected nanoparticles and were used as catalysts for the total
of pollutants from the combustion of biofuels in a simulated oxidation of naphthalene. All the catalysts showed high activity
mixture, which contained naphthalene (50 ppm), methane, CO, for naphthalene conversion to CO , with total conversion taking
2
À1
CO
2
and O
2
, with a balance of N
2
(GHSV = 20 000 h ). As place at 165 1C after three cycles for Pd/MSAPO-5 and Pd/Mg-Al
2 3
O
expected, an increase in wash-coating loading resulted in and 180 1C for Pd/MBETA and MSAPO-5 monoliths. All the
decreased light-off temperatures of all the pollutants. Focusing catalysts possess high stability to temperature cycling, because
on the monolith with similar characteristics (0.95% Pd/NaZSM-5) their catalytic activity remained unchanged after testing for
to our monolithic catalyst, the T50 was 246 1C and 243 1C for several oxidation cycles. Time-on-line experiments have been
coated monoliths with an amount of ZSM-5 of 14.6%wt and used to test the stability of the catalysts. The Pd/MBETA and
2
4.3%wt, respectively.
Pd/MZSM-5 samples were stable after accelerated ageing time-
3
9
Ferrandon et al. studied the total oxidation of mixtures with on-line for 48 h at 250 1C. However, despite being the most
different concentrations of the same aforementioned pollutants, active after three cycles, Pd/SAPO-5 and Pd/Mg-Al
including 50 ppm of naphthalene (GHSV = 23 000 h , catalyst decreased catalytic activity for the conversion of naphthalene
volume 6.5 mL). The monoliths used were composed of cordierite to 95%, due to possible deactivation after ageing. It has been
2
O
3
, exhibited
À1
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Paper
established that the use of Pd-based nanoparticles supported 18 D. R. Sellick, A. Aranda, T. Garc ´ı a, J. M. L o´ pez, B. Solsona,
on BETA and ZSM-5 zeolites supported on honeycomb mono-
liths is a very interesting option for the abatement of PAHs by
catalytic oxidation.
A. M. Mastral, D. J. Morgan, A. F. Carley and S. H. Taylor,
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19 T. Garcia, D. Sellick, F. Varela, I. V ´a zquez, A. Dejoz,
S. Agouram, S. H. Taylor and B. Solsona, Appl. Catal., A,
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013, 450, 169.
Acknowledgements
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