Synthesis and functionalization of ordered mesoporous carbons supported Pt nanoparticles for hydroconversion of n-heptane

Article abstract of DOI:10.1039/c8nj02786b

A comprehensive study was performed on the spectroscopic and textural properties of ordered mesoporous carbon (OMC) of the CMK-3 type modified by acid oxidation using K₂S₂O₈ as a benign oxidant and nitrogen-doping by the aid of the polymerization of ethylenediamine and carbon tetrachloride inside the pore channels of SBA-15 hard template. The pristine, nitrogen-doped, and oxidized-ordered mesoporous carbons were used as supports to prepare 10 wt% platinum nanoparticles-loaded catalysts using ethylene glycol as a reducing agent. The catalytic behavior, mechanism, and influence of the surface functionalization of the ordered mesoporous carbon bifunctional catalysts toward the hydroconversion of n-heptane using a fixed-bed flow system operated under atmospheric pressure were investigated. The synthesized samples were characterized by various analytical and spectroscopic techniques. The mesostructural regularity corresponding to the hexagonal P6mm symmetry of the OMC-CMK-3 type was well-reserved even after surface modifications replicated from an SBA-15 template. H₂ pulse chemisorption and EDX mapping images confirmed differences in the Pt NPs contents and dispersion depending on the support composition. The catalytic activity results achieved were hand in hand with the proper balance between the acidity strength and Pt NPs dispersion degree.

Full text of DOI:10.1039/c8nj02786b

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Synthesis and functionalization of ordered
mesoporous carbons supported Pt nanoparticles
Cite this:DOI: 10.1039/c8nj02786b
for hydroconversion of n-heptane†
S. Said
A comprehensive study was performed on the spectroscopic and textural properties of ordered mesoporous
carbon (OMC) of the CMK-3 type modified by acid oxidation using K2S2O8 as a benign oxidant and
nitrogen-doping by the aid of the polymerization of ethylenediamine and carbon tetrachloride inside the
pore channels of SBA-15 hard template. The pristine, nitrogen-doped, and oxidized-ordered mesoporous
carbons were used as supports to prepare 10 wt% platinum nanoparticles-loaded catalysts using ethylene
glycol as a reducing agent. The catalytic behavior, mechanism, and influence of the surface functionalization
of the ordered mesoporous carbon bifunctional catalysts toward the hydroconversion of n-heptane using a
fixed-bed flow system operated under atmospheric pressure were investigated. The synthesized samples
were characterized by various analytical and spectroscopic techniques. The mesostructural regularity
corresponding to the hexagonal P6mm symmetry of the OMC-CMK-3 type was well-reserved even after
surface modifications replicated from an SBA-15 template. H2 pulse chemisorption and EDX mapping
images confirmed differences in the Pt NPs contents and dispersion depending on the support composition.
The catalytic activity results achieved were hand in hand with the proper balance between the acidity
strength and Pt NPs dispersion degree.
Received 5th June 2018,
Accepted 24th July 2018
DOI: 10.1039/c8nj02786b
rsc.li/njc
Ordered mesoporous carbon (OMC) has attracted substantial
research enthusiasm as a novel class of carbonaceous material
1. Introduction
Crude oil is a complex mixture made up of many thousands since it was first synthesized by Ryoo in 1999.3 OMC materials
of different molecules, and its natural state at the wellhead actually hold great potential to be applied for catalysis,4 electrochemistry,5
has limited value. A series of complex and expensive industrial and adsorption6 owing to their unique structural properties,
processes are thus needed to convert the crude oil into a useful such as large specific surface area, ordered channels, uniform
range of different products. Alkylation, reforming, isomerization, pore size, and mechanical and thermal stability. Ordered meso-
thermal, catalytic, and hydrocracking are the typical chemical porous carbons (OMCs) can be synthesized via two routes based
processing steps that are combined together to constitute petro- on template carbonization. The first one is called the ‘hard-
leum refining of the various streams coming from the distillation templating’ or ‘nanocasting’ multi-step route; in which the
units. Therefore, the aim of crude oil refining is to reduce the ordered mesoporous silica serves as a hard template, which is
volume of low value products and to ensure that as much value can impregnated with an appropriate carbon precursor (e.g., sucrose),
be created from the remaining marketable production. In order to carbonized, and finally the silica is removed by dissolving in HF.
enhance the octane level of gasoline and to improve the low- Such treatment results in a porous carbon material as a reverse
temperature performance of diesel or lubricating oils, branched replica of the silica template.7 The second method called ‘soft-
alkane isomers are some of the more desirable components.1 templating’ is a one-step method, where triblock copolymers act
n-Paraffin hydroisomerization is commonly conducted over bifunc- as the template. The interactions between a triblock copolymer
tional catalysts consisting of a hydrogenation/dehydrogenation and polymeric carbon precursor and further heating result in
function performed by a noble metal (Pt or Pd) supported on a degradation of sensitive copolymer molecules, obtaining ordered
support having acid sites for skeletal isomerization reaction.2
mesoporous carbon.8 However, the relatively inert nature and lack
of functional groups on the surface of pristine OMC materials
limit their practical applications. Many studies have been carried
out to find methods for the modification and functionalization of
OMC materials.9 Among these, post-synthesis acid oxidation has
been recognized as an effective way to introduce large amounts of
Refining Department, Catalysis Division, Egyptian Petroleum Research Institute,
Nasr City, Cairo 11727, Egypt. E-mail: ssm_epri83@yahoo.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj02786b
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oxygen-containing functional groups to the carbonaceous surface,10
The SBA-15 was prepared in a two-step pathway using
and heteroatom doping is also widely applied; for example, the tetraethylothosilicate (TEOS, Aldrich) as the silica source and
doping of nitrogen has a considerable effect on carbon by adjusting a nonionic surfactant ((EO)20(PO)70(EO)20, Aldrich) following
the geometric/electronic structure, altering the physical and the procedure described by Zhao et al.18 Briefly, 4 g of pluronic
chemical properties, especially the hydrophilicity of the OMC 123 was dissolved in 30 g of distilled water and 120 g of 2 M HCl
materials and the basicity, therefore enhancing the catalytic solution with stirring at 40 1C. Then, 8.50 g of tetraethylotho-
properties of the carbon materials.11 Meanwhile, a carbonaceous silicate (TEOS, Aldrich) was added into that solution with
surface enriched with heteroatom functional groups can be stirring at 40 1C for 20 h, followed by aging at 90 1C overnight
applied as a versatile platform for further modifications.12 None- without stirring. A white precipitate was recovered by filtration
theless, functionalization of the carbon surface can definitely and air-dried for 24 h. The template was removed from the
generate some problems too, such as structural damage and an as-synthesized mesoporous material by calcination at 500 1C
uneven distribution of the surface functional groups.13
for 6 h (heating rate = 1 1C minÀ1).
Noble metal nanoparticles (NPs) are the most widely applied
2.1.2. Synthesis of pristine, oxidized, and nitrogen-
NPs in catalysis owing to their high activity and selectivity for functionalized OMCs. (a) OMC-CMK-3 was synthesized by
many chemical reactions.14 Specifically, the Pt noble metal, nanocasting the SBA-15 template using sucrose as the carbon
besides its hydrogenating/dehydrogenating function, has shown precursor following the method described by Vinu et al.19
outstanding catalytic performance as it can help alleviate bifunc- In short, about 0.9 g of sucrose (Sigma, 99.5%) was dissolved
tional catalyst deactivation by hydrogenating coke precursors in 0.1 g H2SO4 acid added drop wise to 1.0 g of SBA-15 under
during the petrochemical hydroconversion reaction.15 Further, stirring. The paste-like mixture was heated in an oven at 100 1C
downsizing Pt NPs increases their specific surface area, thus for 6 h and then the temperature was subsequently increased to
providing more active sites for catalytic reactions. Solvothermal 160 1C and maintained there for another 6 h. The brownish
approaches using ethylene glycol as the reducing agent are powder corresponding to the sucrose/silica composite was
frequently employed for the anchoring of Pt nanoparticles on to again impregnated with (0.75 g sucrose, 0.084 g concentrated
carbons.16
H2SO4, and 5 g H2O) and heated as indicated before.
Fernandes and coworkers17 used the Pt/CMK-3 catalyst, and The resulting black powder was then calcined in a tubular furnace
for comparison a commercial carbon surface supplied by at 900 1C for 5 h under a N2 atmosphere. The pitch black carbon
NORIT (GL-50), as a support but in order to increase its surface sample was treated with 5 wt% of HF (48%) to remove the silica
acidity, it was submitted to wet oxidation with 21 and 65 wt% template and washed 3 times with excess ethanol and then dried
HNO3 solutions. The Pt/CMK-3, Pt/(GL-50)(21) and Pt/(GL-50)(65) at 100 1C for 6 h in an oven. The obtained carbon sample was
catalysts were tested for the hydroisomerization of n-decane and donated as pristine-ordered mesoporous carbon (P-OMC).
the results showed identical conversions (8–10%) and product
(b) Oxidized-OMC was prepared following the method
distribution in the studied temperature range with a selectivity described by Barczak et al.10 First, 1.0 g pristine OMC was put
greater than 70% toward the formation of isomerization products. into a beaker filled with 1.75 M solution of potassium persulfate
The aim of the present research was to perform a compre- (KPS, 97%, CDH) in 2 M sulfuric acid (50 mL) and stirred for 24 h
hensive study on the textural and morphological properties of the at RT. After that, the mixture was filtrated and washed with water
functionalized ordered mesoporous carbon surface both from and ethanol. A black solid was obtained after drying at 80 1C
nitrogen-doping and by post-synthesis acid oxidation and to overnight. This oxidized sample is abbreviated herein as O-OMC.
investigate the influence of the surface modification potential-
(c) Nitrogen-functionalized OMC along with the aid of
ities as supports in the performance of Pt NPs/pristine, oxidized, ethylenediamine (EDA) and carbon tetrachloride (CTC) as
and nitrogen-functionalized OMCs as bifunctional catalysts for precursors20 was prepared as follows: SBA-15 (0.5 g) was well
n-heptane hydroconversion under atmospheric pressure using a stirred with a mixture containing EDA (1.35 g) and CTC (3 g);
fixed-bed flow system. To date the testing of functionalized the mixture was refluxed under stirring at 90 1C for 6 h. The
surface ordered mesoporous carbons in the hydroconversion resultant dark brown colored composite material was dried at
of n-heptane has not been reported, as far as we know, despite 100 1C for 12 h. The obtained material was pyrolyzed at 600 1C
the advantageous properties of ordered mesoporous carbon (heating rate 3.0 1C minÀ1) under nitrogen flow (50 mL minÀ1
)
materials. Thus, this is to the best of the author’s knowledge, for 5 h. The carbonized material was treated with 5 wt%
the first reported study investigating the use of surface function- hydrofluoric acid to dissolve the silica framework and the
alized ordered mesoporous carbon materials for this particular ordered mesoporous carbon nitride was recovered by filtration,
catalytic reaction.
followed by washing several times with ethanol and then drying
at 100 1C. This sample is abbreviated herein as N-OMC.
2.1.3. Preparation of Pt/OMCs catalysts. Pt nanoparticles
(Pt NPs) were loaded on the P-, O-, and N-OMCs by the aid of
an ethylene glycol (EG) reduction method.21 Here, 250 mg of
the OMCs was suspended in 25 mL EG under ultrasonic
2. Experimental
2.1. Mesoporous silica SBA-15 preparation
The mesoporous silica SBA-15 was used as a hard template for the stirring for 40 min. A given amount of the H2PtCl6/EG solution
synthesis of the ordered mesoporous carbon of the type CMK-3.
(Pt: 3.7 mg mLÀ1) was added in order to obtain the Pt/OMCs
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(pristine, oxidized, and N-functionalized) with 10 wt% metal 700 1C at a rate of 10 K minÀ1. The ammonia desorption signals
loading. Then, the pH of the solution was adjusted to 13 by were recorded with a TCD detector.
adding NaOH, followed by heating at 130 1C for 3 h under
Catalytic activity. The catalytic activity of the prepared Pt
flowing Ar. After cooling down to the room temperature, the nanoparticles-supported P-, O-, and N-OMCs was probed in
pH of the mixture was adjusted to 2 by adding 1.0 M HCl n-heptane hydroconversion. The catalytic experiments were
aqueous solution, which was then stirred for another 2 h to carried out in a fixed-bed continuous down-flow reactor operated
ensure that the Pt NPs were fully loaded on to the ordered under atmospheric pressure. Approximately 0.1 g of the catalyst
mesoporous carbons. Finally, the powders were filtrated, was loaded into the quartz tube and pretreated under a H2 flow
washed, and dried at 80 1C for 8 h in a vacuum oven.
at 450 1C for 2 h. The n-heptane was fed into the reactor using a
syringe pump. The reaction conditions used in this work were:
a H2 flow of 1.8 L hÀ1, molar ratio of H2/n-C7 of 14, and space
2.2. Physicochemical characterization
X-ray diffraction patterns of mesoporous samples were velocity of 1.5 hÀ1. The conversion was changed by varying
collected on a PAN analytical X’Pert Pro diffractometer using the reaction temperature from 300 1C to 500 1C. The liquid
Cu Ka (l = 0.1542 nm). The diffractograms were recorded in the products were analyzed on-line with a gas chromatograph
2y range of 0.5–701 with a 2y step size of 0.02 Å and a step time (Agilent Technologies-7890A) equipped with a flame ionization
of 0.60 s. Nitrogen adsorption–desorption isotherms were detector and a capillary column (HP-30m).
measured at À196 1C on a NOVA 3200 Unit, USA apparatus.
Samples were outgassed at 300 1C for 4 h prior to the nitrogen
adsorption measurements. The specific surface area was calcu-
lated using the Brunauer–Emmett–Teller (BET) method. The
pore size was obtained from the adsorption branch of the
nitrogen isotherms. The morphology of the prepared samples
3. Results and discussion
3.1. Structural and morphological characterization of the
SBA-15 template, P-OMC, O-OMC, and N-OMC supports
was observed on a Quanta 250 FEG field emission scanning Different analytical and spectroscopic techniques were used
electron microscope (HR-FESEM) using an accelerating voltage to study the physicochemical characteristics of the template
of 30 kV quipped with EDAX EDS and GIF (Gatan Image Filter). SBA-15 and the P-OMC, O-OMC, and N-OMC samples.
Raman spectra were recorded with an HR-UV 800 confocal
scanning spectrometer (Horiba Jobin Yvon), equipped with a
microscope and a 514.5 nm Ar+ ion laser as the excitation
source in the range of 400–2000 cmÀ1. Thermogravimetric
analyses (TGA & DSC) were performed using a SDTQ-600
The small-angle XRD (SAXRD) diffraction patterns were
recorded to check the structural ordering of the parent
SBA-15 employed as the hard template for the preparation of
P-OMC and N-OMC (Fig. 1). Well-resolved XRD peaks could clearly
be observed at 2y = 0.91, 1.51, and 2.41, which correspond to
(TA-USA) thermo balance instrument. The experiment was the (100), (110), and (200) diffractions of 2D hexagonal meso-
performed at 10 1C minÀ1 from room temperature to 800 1C structure, corresponding to the hexagonal P6mm symmetry, indi-
in nitrogen at a flow rate of 100 mL minÀ1. Pt dispersion was cative of the good quality of the template sample. The P-OMC and
evaluated by H2 pulse chemisorption at 25 1C on Chembet
3000 apparatus (Quantachrome, USA), through pulse titration
equipped with a TCD detector. Prior to the pulse chemisorption
experiments, all the samples were reduced under H2/Ar flow
(50 cm3 minÀ1) for 1 h at 350 1C. For each pulse chemisorption
analysis, about 40–50 mg of the sample was treated with an Ar
flow of 50 cm3 minÀ1 and pulses of 0.05 cm3 (10% H2 in Ar)
until saturation of the catalysts. To calculate the metal disper-
sion, an adsorption stoichiometry of H/Pt = 1 was assumed.22
Dynamic light scattering (DLS) was performed to determine the
particle size distribution of samples using a zeta-sizer nano
series HT (Nano-ZS) (Malvern Instruments). To prepare the DLS
samples, a dilute particle–tetrahydrofurane (THF) colloidal
mixture was ultrasonicated for 30 min. The measurement was
repeated at least 3 times for each sample to ensure the
reproducibility. The temperature programmed desorption of
ammonia (NH3-TPD) was carried out to characterize the acid
properties of the solid catalysts, providing information on the
acid density and acid strength distribution using a dynamic
chemisorption analyzer (B1 Cat II). Prior to NH3 adsorption at
200 1C for 0.5 h, the sample was pretreated in a helium flow at
550 1C for 0.5 h. After that, the physically adsorbed NH3 was
Fig. 1 SAXRD patterns of SBA-15, P-OMC, and N-OMC.
removed in a flow of helium for 0.5 h, followed by heating to
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N-OMC samples replicated from the SBA-15 template also showed defects in the carbon materials and hence depends on the
(100) diffraction peak at a relatively higher 2y of 2.71 and 1. 31, graphitization degree.26,27 Commercial graphite exhibits an
respectively, and less resolved higher order (110) and (200) ID/IG value of 0.366.28 The ID/IG ratio of P-OMC (1.14) was
(i.e., decreased unit cell dimension), indicating that the ordered higher than that of O-OMC (0.98) and N-OMC (1.04) and also
mesoporous structure with a two-dimensional (2D) hexagonal much higher than that of pure graphite, indicating that these
symmetry of the template was replicated into the carbon samples OMCs samples were not very well crystalized and had a low
with a little structural shrinkage or deterioration.23 A notable graphitization degree.29 The observation of the low graphitic
decrease in the intensity of the (100) diffraction peak of the structures was consistent with the wide-angle XRD results
P-OMC replicated sample was observed, indicating that the (Fig. 2A). It is well known that using sucrose as a hydrocarbon
high-temperature carbonization indeed had a substantial precursor leads to the formation of a non-graphitic carbon
influence on the self-assembly process causing shrinkage of structure after pyrolysis. Also, comparing the ID/IG values
the carbon frameworks leading to the lack of long-range among the three samples, it could be suggested that the surface
structural ordering.24
modification by oxidation generates tiny graphitic domains
Wide-angle XRD (WAXRD) and Raman spectroscopy were with edges serving as defects,30 indicating that the graphitiza-
carried out to study the graphitization and the prepared samples tion degree of O-OMC outweighs than that of both the P-OMC
orderliness degree (Fig. 2A and B). As shown in (Fig. 2A), the and N-OMC samples.
WAXRD of the SBA-15 hard template exhibited one broad hump
The change of morphology and the average particle size
at 2y = 201 due to amorphous silica. After nanocasting the during carbon replication were examined using FE-SEM images
(P-OMC), oxidation (O-OMC), and nitrogen-doping (N-OMC) and the micrographs are presented in Fig. 3. The SEM image of
samples, a broad peak at 211 and a slight one at 431 corres- the SBA-15 silica hard template shows a highly ordered array
ponding to graphite (002) and (101) planes of graphite were of a regular and uniform longitudinally arranged rod-like
observed. These peaks are characteristic of the partially graphitic morphology corresponding to the P6mm structure.31 The typical
wall structure, which confirmed the presence of small amounts hexagonal rod-like structure of SBA-15 was also manifested
of stacked crystalline graphite phase.25 From the XRD results, we on the SEM for all the carbon samples and exhibited well-
assumed that the pristine OMC and N-OMC structures were organized pores parallel to one another, confirming the good
reverse replicas of SBA-15. This indicates that the ordered replication process. The ordered rod-like morphology of all the
mesostructure was maintained even after the nanocasting carbon samples proved to be slightly damaged. However, the
process and thermal treatment at 900 1C. Upon modification SEM image of the O-OMC sample was composed of small
of the P-OMC sample surface by acid oxidation (K2S2O8/H2SO4), shaped rod-like particles that were agglomerated together,
a higher graphitization degree was attained, which could be indicating severe structural damage caused by oxidation of
distinguished by the increased intensity of the (100) plane at the P-OMC surface by using the strong acids.
431 compared with the P-OMC and N-OMC samples. In the
To verify the FE-SEM results, dynamic light scattering (DLS)
Raman spectra (Fig. 2B), two prominent peaks can be observed: measurements were carried out. The average particle size
one at 1321 cmÀ1, related to the vibration of carbon atoms of measurements are given in Table 1. As the mesoporous carbons
disordered graphite (D band) and another at 1578 cmÀ1 are hydrophobic in nature the aggregation of particles took
attributed to the vibration of sp2-bonded carbon atoms in the place easily in the polar solvent THF which was used to disperse
2D hexagonal lattice (G band).26 The intensity of the 1321 cmÀ1 the samples. The morphologies and dimensions of OMC were
line increases with the increase in carbon content within the relatively homogeneous. The DLS results revealed that the
samples and with the decrease in the graphite crystal size. The particle sizes of P-OMC, N-OMC, and O-OMC were B136, 239,
relative intensity of the D and G bands (ID/IG) is an indicator of and 807 nm, respectively (with a sharp distribution), which
the degree of in-plane defects crystalline dimensions and edge were all greater than that of SBA-15 (77 nm).
Fig. 2 (A) Wide-angle XRD patterns and (B) Raman spectra of the prepared samples.
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Fig. 4 N2 adsorption/desorption isotherms of SBA-15, P-OMC, O-OMC,
and N-OMC supports.
The porosity changes of the SBA-15 hard template, P-OMC,
O-OMC, and N-OMC were obtained from nitrogen adsorption/
desorption isotherms (Fig. 4), and the corresponding pore size
distribution curves were calculated from adsorption branch of
the isotherms by the BJH method (Fig. 4, inset). Also their
textural parameters are listed in Table 1. SBA-15 shows char-
acteristic a type IV adsorption/desorption isotherm with an
H1-type hysteresis loop, indicating capillary condensation
inside the uniform 2D cylindrical pores, which is a sure
indicator of the mesoporous nature of the hard template
material.32 The narrow H1 type hysteresis loop also indicated
a high degree of pore-size uniformity of SBA-15, as shown in the
pore size distribution of SBA-15 (Fig. 4, inset) with a sharp peak
at 7.5 nm, indicating a quite regular array of mesopores. The
P-OMC and N-OMC samples exhibited a type IV isotherm,
confirming the successful replication of mesoporosity from
Fig. 3 FE-SEM images of the SBA-15 hard template and the prepared the silica template (SBA-15) onto the P-OMC and N-OMC.
OMC samples.
However, they possessed a hysteresis loop of the H2 type located
at a smaller relative pressure range ( p/p0 B 0.4), suggesting the
Table 1 The structural textural parameters of the OMCs and Pt NPs-loaded OMCs samples
Average
particle sizea Surface area results
SBET (m2 gÀ1
)
Smicro (m2 gÀ1
)
Sext (m2 gÀ1
)
Vmicro (cc gÀ1
)
Vmeso (cc gÀ1
)
TVp (cc gÀ1
)
Dp (nm)
b
e
e
e
f
c
d
Sample
nm
SBA-15
P-OMC
77
136.2
450
1144
925
531
533
512
543
0
354
266
74
164
356
401
450
790
659
457
369
156
142
0
0.74
1.01
1.08
0.62
0.55
0.24
0.25
0.74
1.2
1.2
0.66
0.62
0.39
0.41
7.5
0.19
0.12
0.04
0.07
0.15
0.16
4.40
3.30
3.85
3.71
1.62
1.64
10% Pt/P-OMC(EG) 522.1
N-OMC 238.5
10% Pt/NOMC(EG) 141.8
O-OMC 806.7
10% Pt/O-OMC(EG) 462.1
a
b
c
d
By DLS. Total surface area calculated by BET method. The total pore volumes were estimated at P/P0 = 0.98. The mean pore size diameter of
e
the samples calculated from the BJH method. The micropore surface area (Smicro), external surface area (Sext), and micropore volume (Vmicro) of
the samples weredetermined using t-plot analysis. The mesopore volumes (Vmeso) were calculated by subtracting the micropore volume (Vmicro
f
)
from the total pore volume.
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4À
existence of disordered pores and a smaller pore size (FE-SEM tetrahedral unit SiO4 (at 786 and 1060 cmÀ1, respectively)
images, Fig. 3). The only exception was the nitrogen adsorption and the other one is assigned to the stretching vibration,
curve of the O-OMC sample with much lower N2-adsorbed dOH(Si–OH), of Si–O moiety of the silanol groups Si–OÁ Á ÁH on
amounts and a broad featureless hysteresis loop, which means the surface at 938 cmÀ1. One additional peak was evident at the
that a large number of micropores were blocked and/or low wave number corresponds to the same tetrahedral unit
damaged during the acid oxidation treatment of the P-OMC SiO44À, d(Si–O–Si) at 461 cm
surface. It is hypothesized that the oxidation of the surface spectra of the P-OMC sample (Fig. 5) revealed the absence of the
carbon species happens at the micropore/small mesopore bands associated with Si–O–Si at 1060, 938 and 786 & 461 cmÀ1
.
À1 36 On the other hand, The FT-IR
,
openings as a priority. As a result the oxygen-containing groups indicating that the samples were free from the mesoporous silica
attached to the pore openings can probably block the micro- template confirming that the structure of the OMC prepared
pores leading to a severe reduction of the measured pore surface sample came from the carbon and not from the template, with
area and volume.33 Using the Brunauer–Emmett–Teller (BET) the weak bands at around 1600 and 1550 cmÀ1 were character-
equation, the surface area and pore volume of SBA-15 and the istic for carbon materials.37 Also, the very broad band located at
OMCs were determined and the results are presented in Table 1. 3380 cmÀ1 became mild indicating fewer amounts of OH groups.
It should be noted that the carbon replica P-OMC sample The pristine mesoporous carbon was dominantly composed of
exhibited a high BET specific surface area (SBET = 1144 m2 gÀ1
)
carbon (B92 wt%) with only a small amount of H and O. Upon
and total pore volume (TVP = 1.2 cc gÀ1), which is consistent with oxidation of the pristine OMC surface, several new bands
the N2 isotherm curve. These parameters are comparable with appeared. First a new intense band at 1700 cmÀ1 was assigned
those published previously for OMC materials.34,35 Its porosity to the CQO stretching vibrations in lactone, carboxylic anhy-
constituted of both micropores (Vmicro = 0.19 cc gÀ1), which may dride, and/or carboxylic acid,38 indicating the generation of
be formed due to the high carbonization temperature (900 1C), a high density of carboxylic groups. Second, a new band at
and mesopores replicated from the SBA-15 template (Vmeso
=
1169 cmÀ1 was attributed to the C–O stretching vibrations in
1.01 cc gÀ1). After the modification of the P-OMC surface with ether, lactone, phenol, and/or carboxylic anhydride,39 implying
KPS and H2S2O4 treatment, the BET specific surface area the generation of many surface oxides. The strong band at
and the total pore volume values diminished from 1144 to 1561 cmÀ1 could be ascribed to symmetric COOÀ vibration
512 m2 gÀ1 and from 1.2 to 0.39 cc gÀ1, respectively (Table 1), and/or the C–C stretching vibrations in an aromatic ring coupled
suggesting that the oxidation treatment caused severe changes to highly conjugated keto–enol groups.40 In addition, the absence
in the morphology and consequently in the porosity. However, of the characteristically sharp band near 670 cmÀ1 ascribed to
2À
2À
compared to the template SBA-15 sample, the decreased pore S2O8 confirmed that the S2O8 ions had been thoroughly
volume and average pore diameter of N-OMC sample indicated washed away. Finally, the band centered at about 3374 cmÀ1
the successful incorporation of the N-containing organic became more pronounced, probably due to the contribution by
moieties onto the pore surface of the mesostructure.
the –COOH groups generated. The FT-IR spectra of the N-OMC
The surface composition of SBA-15, and the P-OMC, O-OMC, (Fig. 5) showed no peak associated with Si, revealing that the
and N-OMC samples was analyzed by their FT-IR spectra mesostructure was generated from the true replication process
(Fig. 5). The FT-IR results demonstrated a significant difference and the silica framework removal process using HF was highly
among the FT-IR spectra obtained for the prepared samples. efficient and successful. Also, a peak was observed at 1570 cmÀ1
There are two typical peaks that characterize the Si–OX stretch- corresponding to the CQN bond.41
ing range for SBA-15 as follows: nOH(H2O) 3380 cmÀ1, d
Fig. 6 demonstrates that there was no significant difference
OH(H2O) 1618 cmÀ1, which could be assigned to isolated among the TGA–DSC profiles obtained from SBA-15, and the
silanol groups and adsorbed water on the surface of the solid. N-OMC, pristine OMC, and O-OMC samples. As shown in Fig. 6,
Additionally, three bands are related to intrinsic vibrations, all the samples exhibit a small weight loss event below 100 1C,
symmetric ns(Si–O–Si) and asymmetric nas(Si–O–Si), of the corresponding to the removal of adsorbed water associated
with a DSC endothermic peak centered at around 160 1C.
The TG profile (Fig. 6) shows that the pristine OMC had a
weight loss, with B13 wt% in total up to 600 1C. However, after
oxidation of the P-OMC surface, the O-OMC sample showed a
considerable weight loss (Fig. 6). Three main weight loss stages
could be observed. The first one at 30–150 1C lost 11 wt%,
which was attributed to the dehydration of physisorbed water.
At 150–350 1C a second weight loss of B14% was observed
which probably was related to the decomposition of the func-
tional carboxylic and phenolic groups.42 The weight loss below
280 1C was ascribed to the decomposition of carboxylic groups,
while those from 280–350 1C were ascribed to the phenolic
groups, since the carboxylic groups can decompose at a lower
temperature than phenolic groups.43 The third stage with
Fig. 5 FT-IR spectra of the prepared OMCs.
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Fig. 6 TGA and DSC profiles of SBA-15 and the OMC prepared samples.
another 10 wt% weight loss occurred from 350–600 1C, which
could be attributed to the decomposition of lactone, quinine,
and anhydride groups.44 The high density of surface oxides of
O-OMC was further confirmed by the TGA profile. From the
TGA results, the functionalization of the OMC surface increased
the hydrophilicity of the modified carbon samples showing
a high affinity toward water adsorption because of the high
content of functional groups on the surface.
3.2. Characterization of Pt NPs-supported P-, O-, and N-OMC
catalysts
Fig. 7 XRD patterns of the Pt NPs-supported various OMCs.
The high surface area of OMC can provide more surface
anchoring sites or functional groups for the nucleation of metal
nanoparticles compared with conventional carbon blacks, such support and exhibited a higher density of platinum particles
as Vulcan XC-72R and Ketjen Black, and allows the preparation with an even distinguished agglomeration, which led us to
of metals-supported OMCs with a high dispersion degree. Also, postulate the formation of large particle Pt clusters with high
the uniform mesopore structure of OMC facilitates the diffu- dimensions; in contrast, the bright-field SEM images observed
sion of reactive molecules for hydroconversion reactions. In the for the Pt/O-OMC and Pt/N-OMC samples indicated an ordered
present work, P-, N-, and O-OMCs were efficiently used as array of mesoporous carbon nanorods ideally decorated with Pt
supports for the preparation of Pt NPs bifunctional catalysts nanoparticles, revealing that the surface modification either by
by using ethylene glycol as a reduction agent without any oxidation and/or nitrogen-doping were helpful for stabilizing the
surfactant or hydrothermal process. The total loading of Pt Pt NPs on the surface of OMC rods, which provoked not only a
was controlled to be as high as 10 wt%. A series of character- higher dispersion of Pt NPs on the functionalized carbon surface
ization methods were performed to confirm the successful but also an ordered structure, which could lead to the higher
loading of 10 wt% Pt/OMCs onto various OMCs samples and accessibility of these Pt nanoparticles toward reactants.
to investigate their physicochemical characterizations.
As described in Table 2, full energy dispersive X-ray spectro-
In the large-angle region of the XRD patterns (Fig. 7), all the scopy (EDX) attached with FE-SEM instrument analysis showed
Pt-supported P-, O-, and N-OMC samples showed a characteristic different wt% for all the Pt-supported OMCs, although all had
diffraction peak at 2y = 39.81 and less resolved two peaks at the same Pt loading of ca. 10 wt%. However, the Pt/P-OMC
around 46.21, and 67.81 assigned to the (111), (200), and (220) samples possessed a low Pt. content (8.85 wt%). This may be
planes, respectively, revealing the presence of Pt metal (Pt(0)) due to the weak interaction between Pt nanoparticles and the
NPs with a face-centered cubic (fcc) crystal phase structure, non-functionalized surface of the P-OMC sample; consequently,
indicating the effectiveness of Pt deposition to its metal state the Pt nanoparticles tended to aggregate on the external surface
by the EG reduction method. In addition, a broad diffraction area of P-OMC and form large Pt clusters (see SEM image, Fig. 8
peak at 251 was also observed corresponding to the graphite(002) with a magnification of 5 mm)—this was in accordance with the
plane of OMCs.
drop in the external surface area value from 790 to 640, the
The morphology and metal dispersion of Pt NPs were further increase of the average particle size from B136 to 522 (Table 1),
verified by FE-SEM measurements and the results are presented and constituent with the Pt average particle sizes (57 nm) as
in Fig. 8. The SEM image of the Pt/P-OMC sample had a estimated by H2-chemisorption analysis (Table 2) causing the
distinguishable morphology change compared with the P-OMC low dispersion (20%) for the Pt NPs metal displayed in the EDX
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Fig. 8 FE-SEM (left) and Pt EDX mapping images (right) of the Pt NPs-supported various OMCs.
Table 2 Physical Properties of Pt-loaded OMCs samples
Characterization of metal function
Aciditye (mmol gÀ1
)
Pta (wt%)
Db (%)
SPt (m2 gÀ1
c
)
dPt (nm)
Weak
Medium
Strong
Total
d
Sample
10% Pt/P-OMC(EG)
10% Pt/N-OMC(EG)
10% Pt/O-OMC(EG)
8.85
9.38
9.97
20.02
44.79
27.54
4.38
10.38
6.78
56.56
25.28
41.13
—
0.117
0.053
0.284
0.074
0.181
—
0.251
0.398
0.248
0.442
0.632
a
b
c
d
Pt concentration determined by EDX. Dispersion by H2 pulse chemisorption. Platinum surface area by H2 pulse chemisorption. Average Pt
e
particle size by H2 pulse chemisorption. By NH3-TPD.
image mapping of the Pt element in Fig. 8. These results are the shrinkage of the average particle size to half compared to the
constituent with the drops in the SBET, TVp, and Dp values of the non-Pt-loaded samples (Table 1). Also, the aforementioned results
P-OMC support upon Pt loading (Table 1). As expected, in can justify the increase in textural parameters of the platinum-
comparison with Pt/P-OMC, Pt/O-OMC and Pt/N-OMC EXD supported N-OMC and O-OMC samples (Table 1) and were in good
analysis showed a relatively high Pt. content (9.97 wt% and agreement with the FE-SEM images.
9.38 wt%, respectively), close to the real loaded amount, which
Meanwhile, the N2 adsorption/desorption isotherms of
could inferred that the surface modifications by heteroatoms Pt-loaded OMCs samples exhibited similar shapes with those
incorporation (either N and/or O) into carbon materials led to of OMC supports of type IV with well-defined H2 hysteresis
enhanced p bonding and acted as anchoring sites for the loops, except for the decreased adsorption N2 quantity. Further-
metallic Pt NPs,45 and hence Pt was more homogenously more, it is interesting to point out that the pore size distribution
distributed in both the N-OMC and O-OMC matrix supports. The curves (Fig. S1, ESI†) obtained using the BJH method revealed
average platinum particle size obtained by H2-chemisorption that there was no displacement of the maximum position. This
decreased from 57 nm on P-OMC to 41 and 25 nm on O-OMC suggests that shrinkage of the average pore sizes was motivated
and N-OMC, respectively, facilitating the high dispersion of Pt by a partial pore plugging rather than pore blocking upon
nanoparticles with 45% and 28% (Fig. 8 and Table 2) and causing immobilization of the Pt NPs.
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More abundant defect sites on Pt/N-OMC with ID/IG = 0.99 Table 2 demonstrates that the 10% Pt NPs/P-OMC and 10% Pt NPs/
was confirmed by the Raman spectra (Fig. S2, ESI†) results, O-OMC catalysts had the highest content of medium acid site
allowing facile nucleation of Pt nanoparticles and leading densities with 0.284 mmol-NH3 gÀ1 and 0.181 mmol-NH3 gÀ1
,
to a higher Pt loading,46 matching the results estimated by respectively, while an opposite sequence was presented in terms of
H2-chemisorption. While ID/IG = 0.83 and 0.85 for the Pt/O-OMC strong acid sites, where the greatest value was for the 10% Pt NPs/
and Pt/P-OMC samples, respectively, indicating the graphitiza- O-OMC catalyst with 0.398 mmol-NH3 gÀ1 then the 10% Pt NPs/
tion degrees of the P-OMC and N-OMC increased upon loading N-OMC catalyst with 0.251 mmol-NH3
Pt NPs. 10% Pt NPs/N-OMC catalyst exhibited more weak acid sites
g
À1. Furthermore, the
Fig. S3 in the ESI† presents TG curves of the Pt NPs-supported compared to the 10% Pt NPs/O-OMC catalyst. A shift of the
samples in a nitrogen atmosphere, in the temperature range of strong acid sites NH3 desorption peak from 395 1C on 10% Pt
30–800 1C. The TGA plots demonstrate the similarity for all the NPs/N-OMC catalyst to 397 1C and 556 1C, indicating a major
Pt-supported various OMCs samples, differing only in the tem- increase in the acid site strength for 10% Pt NPs/O-OMC
peratures where the thermal event occurs. The TGA isotherms catalyst. It is proposed that weak-strength acid sites favor the
showed only one thermal event at B140 1C due to dehydration of skeleton isomerization reaction, while strong acid sites ones
the physisorbed water with a total weight loss of B43%, 20%, enhance the possibility of the cracking.
and 9% for the Pt/P-OMC, Pt/O-OMC, and Pt/N-OMC samples,
respectively. From the TGA curve, it can be concluded that the Pt
4. Hydroconversion of n-heptane
supported various OMCs to have a thermal stability up to 600 1C.
4.1. Effect of surface modification of the OMC support on the
hydroconversion reaction of n-heptane
The relationship between the acid sites density of the Pt
NPs-supported P-, O-, and N-OMC catalysts and their catalytic
activities were clarified by studying their NH3 adsorption/
desorption behaviors, and the results are shown in Fig. 9 and
summarized in Table 2.
The effect evolution of the functionalization of ordered meso-
porous carbon surface used as a support on the catalytic
performance of the bifunctional Pt NPs-supported P-OMC,
N-OMC, and O-OMC catalysts was investigated toward n-C7
hydroconversion using a fixed-bed reactor operated under atmo-
spheric pressure at a reaction temperature range of 300–500 1C,
and the results are illustrated in Fig. 10–13. The results cover
n-C7 hydroconversion, converted product distributions and the
selectivity. The major converted products were composed of:
(1) i-C7 isomers, (2) methyl cyclohexane, and (3) toluene.
As shown in Fig. 10, the three studied catalysts followed the
common behavior trend; where conversion increased with
raising the reaction temperature, regardless of the nature of
the carbon support or the dispersion of the Pt NPs degree.
Overall the three studied catalysts exhibited low catalytic activity
with n-C7 conversion: B7% at a reaction temperature below
350 1C. Nevertheless, the n-C7 total conversion increase rapidly
with the temperature afterwards for the Pt NPs-supported P-OMC
and O-OMC catalysts. Both catalysts exhibited a highest n-C7
TGA curves (Fig. S3, ESI†) show that the high content
of physisorbed water was removed at 100 1C in the presence
of N2 and the weight of the samples then kept steady. Thus the
adsorption of NH3 was conducted at 110 1C to highlight the
adsorption by the weak acid sites by eliminating the weak
physisorption. As shown in Fig. 9, the TPD-NH3 profile for Pt
NPs-supported P-OMC exhibited one sharp peak at ca. 352 1C,
ascribed to medium-strength acid sites. Meanwhile, the
TPD-NH3 profiles for the Pt NPs-supported N-OMC and
O-OMC catalysts showed three acid sites of different strengths
depending on the temperature that ammonia desorption takes
place. Below 200 1C is assigned to the weak acid sites peak,
while peak after 390 1C is due to strong acid sites, and the
intermediate peak is caused by medium acid sites. Overall, the
total acid sites density increases according to the following
order: 10% Pt NPs/O-OMC (0.632 mmol-NH3
Pt NPs/N-OMC (0.442 mmol-NH3
À1) 4 10% Pt NPs/P-OMC
(0.284 mmol-NH3
À1), suggesting the positive effect of OMC
g
À1) 4 10%
g
g
surface modification either by oxidation and/or by nitrogen-
doping, improving the acidity of the Pt NPs-supported catalysts,
since the catalyst acidity is determined mainly by the support.
Fig. 10 Conversion of n-heptane as a function of the reaction tempera-
Fig. 9 NH3-TPD profiles of Pt NPs supported various OMCs.
ture over Pt NPs-supported P-, O-, and N-OMC catalysts.
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and O-OMC catalysts, the turnover frequency (TOF) was thus
calculated. The TOF is defined as the no. of moles of n-heptane
converted/platinum metal site surface area estimated from the
dispersion degrees, which are directly related to the average
size of the Pt NPs (Table 2)/second. The TOF calculated values
for n-heptane conversion as a function of the different modified
catalyst support OMC surfaces are shown in Fig. 11. It was
clearly observed that n-heptane at 350 1C had very different TOF
numbers depending on the dispersion degree of the Pt NPs.
Fig. 11 illustrates that n-heptane hydroconversion increased
with the decrease in the dispersion degree of Pt NPs metal
supported on different catalyst supports, similar to the conversion
trend in Fig. 10. Thus one can assert that the superior catalytic
performance of the Pt/P-&O-OMC catalysts compared with that of
Pt/N-OMC mainly originated from the improved medium acid
sites on the P-, and O-OMC catalyst support, which is favorable for
Fig. 11 Effect of different Pt NPs dispersion degrees of the supported
P-, N-, O-OMC catalysts on the TOF of n-heptane at reaction temperature
of 350 1C.
conversion of above 40% at the reaction temperature of 500 1C. maintaining the well-presented synergetic interaction between the
While, the Pt NPs-supported N-OMC catalyst showed extremely Pt NPs metallic clusters and acid sites.
low catalytic activity as the n-C7 conversion of monotonic
increased with the reaction temperature, reaching 8% total showed qualitatively similar types of n-C7 converted products,
conversion at 500 1C. including: (a) isoheptanes, (b) cyclization products (methycyclo-
The Pt NPs-supported P-OMC, O-OMC, and N-OMC catalysts
It is generally recognized that when paraffin hydroconversion hexane), (c) aromatic products (toluene), and (d) trace amounts
is performed over solid supported noble metal bifunctional of cracked products (light hydrocarbons) that appeared only at
catalysts, an adequate ratio between the metal sites and the 500 1C (Fig. 12). This may be owing to the mesoporous
surface acid sites of the support surface is critical to obtain high character of the OMC supports that promotes the occurrence
catalytic performance.47 The differences in the catalytic perfor- of more than one reaction on the acid sites before reaching the
mance of the three catalysts toward n-heptane hydroconversion hydrogenating sites; thus allowing the formation of bulky
were consistent with the characterization results of the Pt metal molecules. The cyclization of n-C7 into methyl cyclohexane
dispersion degree and the acidity strength. The catalytic perfor- occurred at a low reaction temperature (at the beginning of
mance over the Pt/P-, O-, and N-OMC bifunctional catalysts reaction), i.e., it is a primary product. Its yield trended upward
correlated positively with the medium-strength acid density on to a maximum with 8.2%, 5.7%, and 1.4% at 400 1C over the Pt
the catalyst surface in the order of P-OMC 4 O-OMC 4 N-OMC NPs-supported P-OMC, O-OMC, and N-OMC catalysts, respec-
(Table 2), which was required to provide easier activation for n-C7 tively, and then decreased with the temperature in parallel with
conversion. In general, it is necessary for the supported catalysts the increase in the aromatic product. The behavior of the
to hold sufficient acid sites with considerable acid strength for isomerization reaction (i-C7 production) followed the same
achieving high catalytic activity, which clarified the relatively low pattern of methycyclohexane, i.e., i-C7 yield formation depends
catalytic activity of Pt/N-OMC compared with the Pt NPs-supported on the reaction temperature. They passed through a maximum at
P-OMC and O-OMC catalysts. The sequence of the catalytic activity 350 1C, as clearly shown in Fig. 12, and then decreased with the
performances of the prepared catalysts as a function of the reaction temperature due to the thermodynamic limitation and
medium acid site strength values varied according to the following the higher activation energy for the isomerization reactions.48
order: Pt/P-OMC (46%) 4 Pt/O-OMC (41%) 4 Pt/N-OMC (8%).
This maximum was clearly observed for the Pt NPs-supported
To investigate the effect of different Pt dispersion degrees on functionalized ordered mesoporous carbon surfaces (N-OMC and
the different catalytic performances of the prepared Pt/P-, N-, O-OMC) catalysts. In fact the methyl cyclohexane and i-C7 yield
Fig. 12 Converted product distributions and i-C7 isomers selectivity over: Pt NPs-supported (a) P-OMC (b) O-OMC, and (c) N-OMC catalysts.
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Fig. 13 (A) Selectivity of the different product distributions (B) ratio between isomerization and the sum of cyclization and dehydrocyclization
(Is/(Cy + DCy) over the three studied catalysts.
formation curves quickly reached a maximum and then fell to metal (Table 2) achieved for the Pt NPs-supported N-OMC
zero (suggesting that both products are very reactive intermediates). catalyst. The formation of methyl cyclohexane products due
However, the aromatization reaction (toluene formation) was to the cyclization reaction of n-heptane with 55%, 47%, and
predominant at high reaction temperatures over all the studied 39% selectivity was obtained over the Pt NPs-supported P-OMC,
catalysts (Fig. 12). The yield of toluene was as high as 41% and O-OMC, and N-OMC catalysts matches the variations of medium-
37% at 500 1C over the Pt NPs-supported P-OMC and O-OMC strength acid site density (mild acidity) over the three studied
catalysts, respectively, and then decreased to 6% at 500 1C upon catalysts. Having the highest strong acid site strength density, the
using the Pt NPs-supported N-OMC catalyst.
Pt NPs-supported O-OMC catalyst exhibited the highest selectivity
Regardless of the carbons support, it could be observed that toward the dehydrocyclization of n-heptane into toluene forma-
the isomerization and cyclization reaction of n-C7 to i-C7 and tion, with 22% selectivity at the same reaction conditions.
methyl cyclohexane were catalyzed at lower reaction tempera-
The histogram in Fig. 13B displays the ratio between iso-
tures (300–400 1C), while the dehydrocyclization reaction of merization and the sum of cyclization + dehydrocyclization
n-C7 into toluene was activated at higher reaction temperatures (IS/Cy + DCy) products at the same reaction temperature,
(400–500 1C). Changes in the activity due to isomerization, 350 1C, and at total conversion of B6.7%, 5.8%, and 3.1% for
cyclization, and dehydrocyclization can be related to the effect the Pt NPs-supported P-OMC, O-OMC, and N-OMC studied
of the OMC support functionality and hence the variation of the catalysts, respectively. As shown, all the bifunctional catalysts
density, acid strength, and the Pt NPs metal dispersion. A poor emphasized different (IS/Cy + DCy) ratio value magnitudes;
Pt NPs metal dispersion and a moderate acidic strength are the highest value was obtained for both the Pt NPs-supported
indicative of high methyl cyclohexane and toluene products N-OMC and O-OMC catalyst samples and particularly for the
formation, i.e., cyclization and dehydrocyclization over the Pt NPs-supported N-OMC, indicating the dominancy of isomer-
isomerization reaction for the Pt NPs-supported P-OMC ization products over the cyclization + dehydrocyclization reac-
and O-OMC catalysts. Whereas the Pt NPs-supported N-OMC tions. On the other hand, the lowest value was observed over the
bifunctional catalyst had an adequate balance between the Pt NPs-supported P-OMC bifunctional catalyst, highlighting the
weak acidic strength and Pt NPs metal sites dispersion that fast desorption of cyclization + dehydrocyclization over isomeriza-
facilitated the occurrence of the isomerization reaction, result- tion reactions. These differences may be explained in terms of the
ing in the highest isomerization yield, which represents variation of the acid site strength of the catalysts; particularly, the
the most valuable products compared with cyclization and presence of more weak acid sites is necessary for achieving high
dehydrocyclization products yield formation.
catalytic activity in isomerization reactions.
The hydroconversion of n-C7 was accompanied by a remark-
ably low isomer selectivity. The low selectivity toward i-C7 at
4.2. Mechanistic proposal for n-heptane hydroconversion
high reaction temperatures (4350 1C) may be caused by the From the identified converted product resulting from the hydro-
strong tendency of i-C7 consumption in the formation of conversion of n-C7 analyzed above, one can conclude that n-heptane
cyclization (methyl cyclohexane) and dehydrocyclization (toluene) was transformed over the Pt NPs-supported P-OMC, N-OMC, and
products. The histogram in Fig. 13A represents the converted O-OMC catalysts following consecutive and parallel interconnected
product distribution selectivity over the three studied catalysts at different reactions, namely, isomerization (IS), cyclization (CY), and
a reaction temperature of 350 1C. The three studied catalysts were dehydrocyclization (DCY) as proposed in Scheme 1.
able to form isomers, methycyclohexane, and toluene products. It
The adsorption of carbon atoms on Pt NPs metal led to the
was clarified that the isomerization is the main reaction; with more formation of a heptene-like intermediate (QC7), in agreement
than 40% selectivity over Pt NPs-supported N-OMC catalyst, but this with most of the published results.49 This species may be
decreases to about 30% and 26% over the Pt NPs-supported transformed in the following pathways:
O-OMC and P-OMC catalysts, respectively. This is substantially
(i) Cyclization with C1–C6 ring closure on the medium-strength
consistent with the combination of the more weak-strength acid sites of the carbon support forming methylcyclohexane, which
acid site density and the homogeneous dispersion of Pt NPs may desorb to the product phase or successively be dehydrogenated
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Scheme 1 Reaction pathway of n-heptane hydroconversion.50
by Pt NPs metal on the surface and irreversibly transformed into
toluene (Scheme 1). This path is very likely, as the appearance of from 57 nm on P-OMC to 25 nm on the N-OMC sample.
ꢀ Decreased the average size of the Pt NPs metal particles
methycyclohexane as a primary product suggests that it is formed
The catalytic activities of PtNPs/P-, N-, and O-OMCs cata-
directly from the cyclization of n-heptane through a C1–C6 ring lysts were tested for n-heptane hydroconversion at a reaction
closure skeletal rearrangement on the acid sites of the carbon temperature range of 300–500 1C, and H2/n-C7 of 14 using a
support, which is favored over platinum metal sites50 and due fixed-bed flow system operated under atmospheric pressure.
to the high toluene selectivity over the Pt NPs-supported P-OMC, All the catalysts displayed identical converted products
N-OMC, and O-OMC catalysts, respectively.
composed of i-C7, methyl cyclohexane and toluene, ensuring
(ii) The (QC7) undergoes skeletal rearrangement with a the occurrence of parallel and consecutive interconnected
bond shift isomerization reaction into i-C7 that requires only cyclization (CY), dehydrocyclization (DCY), and isomerization
weak acid site strength.
(IS) reactions of n-heptane hydroconversion over the Pt NPs-
supported various OMCs catalysts. A high catalytic activity in
terms of n-heptane hydroconversion was obtained over the
Pt NPs/P- and O-OMC catalysts, which was correlated with the
more medium-strength acid sites density, which is essential
for activation of the n-heptane reactant. On the other hand,
a high selectivity toward i-C7 yield formation was attained over
the Pt NPs/N-OMC catalyst, establishing a proper balance
between the Pt NPs metals dispersion degree and the more
weak acid sites density over this catalyst. The value of the
IS/Cy + DCy was high for the Pt NPs/N- and O-OMC catalysts
compared to that of Pt NPs/P-OMC under the same reaction
conditions, demonstrating the positive effect of function-
alization of the OMC surface on obtaining isomers that are
more valuable products than methyl cyclohexane and toluene
products.
5. Conclusions
The synthesized P-, N-, and O-OMC support sample character-
ization analysis assured that the structural and the morpho-
logical aspects of the SBA-15 silica hard template were faithfully
transferred to mesoporous carbons by a successful replication
process. Here, 10 wt% Pt NPs were loaded on to P-, N-, and
O-OMC supports with the aid of the ethylene glycol (EG)
reduction method, where the detection of Pt metal (Pt(0)) NPs
having a face-centered cubic (fcc) crystal phase structure, reveled
the accomplished Pt deposition to its metal state by the EG
reduction method. The Pt NPs/P-, N-, and O-OMC samples were
analyzed by various spectroscopic techniques and the results
emphasized the outstanding roles of nitrogen and oxygen func-
tionalities on the OMC surface in terms of the following:
ꢀ Enhancement of Pt NPs dispersion by providing p bonding,
which act as anchoring sites for the metallic Pt NPs. Pt NPs
dispersion increased from 20% on P-OMC to 45% on the N-OMC
support.
Conflicts of interest
There are no conflicts to declare.
ꢀ Increased the total acid sites density and variation in the
acid site strength.
Acknowledgements
ꢀ Pt NPs were ideally decorated on the ordered array of meso-
porous carbon rods of N- and O-OMC samples, whereas they formed This research was supported by the Egyptian Petroleum Research
large clusters on the surface of P-OMC, as assessed by FE-SEM.
Institute (EPRI).
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