A phosphorus-carbon framework over activated carbon supported palladium nanoparticles for the chemoselective hydrogenation of para-chloronitrobenzene

Article abstract of DOI:10.1039/c7cy00157f

A novel Pd-P-C framework structure was fabricated by supporting Pd on a P-doped carbon layer coated with activated carbon. A P-doped carbon layer was generated via calcination of sodium hypophosphite and ethanediol under inert gas atmosphere. The catalysts were characterized by Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) and were evaluated in the selective hydrogenation of p-CNB to p-CAN. The results indicate that the carbon layer generated via calcination of ethanediol presents a higher disordered structure and then the P-doped carbon layer becomes more ordered due to the formation of a P-C framework. Some electrons were transferred from C atoms adjacent to the P atoms to P atoms, which favors the formation of stable Pd-P species such as the Pd₁₅P₂ phase. Pd in the Pd-P-C framework structure possesses electron-rich properties resulting from electron transfer from C atoms to Pd atoms via P atoms, which induces the formation of electron-rich hydrogen (H^-) when hydrogen was absorbed on the Pd particles. The produced electron-rich H^- might prefer the nucleophilic attack on the nitro group rather than the electrophilic attack on the C-Cl bond. We suggest that it is responsible for the superior selectivity of up to 99.9% to p-CAN for the hydrogenation of p-CNB. The catalytic performance of the Pd particles supported on the P-doped carbon layer remains unchanged after five cycles indicating excellent stability.

Full text of DOI:10.1039/c7cy00157f

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Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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DOI: 10.1039/C7CY00157F
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Phosphorus-carbon framework over the activated carbon
supported palladium nanoparticles for the chemoselective
hydrogenation of para-chloronitrobenzene
Received 00th January 20xx,
Accepted 00th January 20xx
Chunshan Lu,a Mengjun Wang, Zhenlong Feng, Yani Qi, Feng Feng, Lei Ma, Qunfeng Zhang and
Xiaonian Li*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel Pd-P-C framework structure was fabricated by supporting Pd on the P-doped carbon layer coated on
the activated carbon. P-doped carbon layer was generated via the calcination of the sodium hypophosphite
and ethanediol under inert gas atmosphere. The catalysts were characterized by Brunauer-Emmett-
Teller(BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and X-ray
photoelectron spectroscopy (XPS) and were evaluated in the selective hydrogenation of p-CNB to p-CAN. The
results indicate that the carbon layer generated via the calcination of ethanediol presents the higher
disordered structure and then the P-doped carbon layer becomes more ordered due to the formation of P-C
framework. Some electrons were transferred from C atoms adjacent to the P atoms to P atoms, which favors
the formation of stable Pd-P species such as Pd15P2 phase. Pd in the Pd-P-C framework structure possesses
electron-rich property resulting from electron transfer from C atoms to Pd atoms via P atoms, which induces
the formation of electron-rich hydrogen (H-) when hydrogen was absorbed on the Pd particles. The produced
electron-rich H- might prefer the nucleophilic attack on the nitro group rather than the electrophilic attack
on the C-Cl bond. We suggest that it is responsible for the superior selectivity of up to 99.9% to p-CAN for the
hydrogenation of p-CNB. The catalytic performance of the Pd particles supported on the P-doped carbon
layer keeps unchanged after five cycles indicating the excellent stability.
Pd, Ni, Ru and Au.3,4
1. Introduction
Palladium has always been the focus in the hydrogenation
of aromatic halonitro compounds because of the outstanding
As one of the most essential fine chemicals and intermediates,
aromatic haloamines are intensively used in the synthesis of
organic dyes, perfumes, herbicides, pesticides, preservatives,
plant growth regulators, medicines and light sensitive or
nonlinear optical materials.1 The selective hydrogenation of
aromatic halonitro compounds to the corresponding aromatic
haloamines is considered as the most environment-friendly
and efficient method instead of conventional chemical
reduction involving the use of Fe, Na2S, hydrazine hydrate, etc.
However, up to now, it remains a major challenge to avoid the
undesired hydrodehalogenation because the catalytic
hydrodehalogenation rates of the C-X (Cl, Br) bond is enhanced
as a result of the electron donating effect of amino groups in
the aromatic ring.2 This experimental phenomenon can be
observed over almost all kinds of metal catalysts, such as Pt,
hydrogenation performance and moderate cost, although
palladium-based catalysts (with small Pd particle size) also
catalyse hydrodechlorination reactions for chlorinated organic
compounds compared with Pt and Ni catalysts.5,6 Numerous
research efforts have been devoted to the suppression
hydrodehalogenation over palladium-based catalysts. A high
selectivity of aromatic halomines could be obtained by
preparing
amorphous
alloys,7,8
synthesizing
metal
complexes,9,10 modifying the interaction between metal active
components and metallic oxide supports,11,12 modulating the
Pd particles size13 and so on. However, in order to meet the
requirements of large scale application, it is still highly desired
to improve the acid resistance for some metal oxide supports
and metal promoters, the reusability and the noble metal
availability of Pd supported catalysts, especially under the
condition of hydrochloric acid (HCl) produced via the
hydrodehalogenation reaction.
As promising and excellent electronic and/or structural
promoters, heteroatoms such as S, P and N have been widely
investigated in doped carbon materials (e.g. ordered
mesoporous carbon,14 carbon quantum dots,15 activated
carbon, carbon nanotubes16) and metal interstitial compounds
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(e.g. metal sulfides,17 metal phosphides,18 metal nitrides19). and heated from room temperature to 400
The doping of heteroatoms has significant influence on the of 10 /min, and then held that temperature for 3 h in a
℃ at a heating rate
℃
structure and electron distribution of the particles adjacent to flowing N2 gas with a flow rate of 30 mL/min. The obtained
them due to the difference in bond length and atomic size16 samples were washed with deionized water until pH of 7.0,
and rich electrons in their outer electronic orbit.20,21 and then suspended in 50mL deionized water under vigorous
Heteroatom-doped catalyst materials exhibit the enhanced stirring at 80℃ for 0.5 h. Subsequently, 2 mL of H2PdCl4
durability,21 basicity and p-donor capability, etc.22-24 These aqueous solution (0.05 g/mL) was added drop by drop into the
effective and attractive properties encourage researchers to suspension and stirred for 4 h. After the pH value was adjusted
explore the introduction of heteroatoms into carbon materials to 9.0 by the addition of NaOH aqueous solution, 2 mL of
or metal crystal for the various catalytic reactions. To date formaldehyde aqueous solution was dropped slowly into the
heteroatom-doped catalyst materials have been demonstrated suspension with vigorous stirring for
2 h. Finally, the
to improve catalytic performance in an increasing number of suspension was cooled down to room temperature and the as-
reactions, such as the hydrodeoxygenation,14 electrocatalytic prepared catalysts were filtered, washed with deionized water
oxygen reduction,15 photocatalytic selective oxidation,25 until pH of 7.0, and then dried in vacuum oven at 120
℃
for 4 h.
traditional selective hydrogenation19 and so on.
The nominal Pd loading for all catalysts were 2 wt%. The
In the case of selective hydrogenation of aromatic catalysts were labelled as Pd/C for those prepared without
halonitro compounds, the effect of heteroatoms on the phosphorus and ethanediol solution, Pd/C-EG for those
suppression hydrodehalogenation has been reported prepared in the presence of ethanediol solution and Pd/C-P-EG
previously. For example, S-, N- or P-containing compounds are for those with phosphorus and ethanediol solution. The
very effective for the selective hydrogenation reactions unreduced catalysts were labelled as Pd/C-b, Pd/C-EG-b and
whether they are used as inhibitors26,27 or ligand modifiers.28,29 Pd/C-P-EG-b accordingly.
However, they usually lead to decrease in the purity of the
product or the catalytic activity of catalyst.30 Cardenas et al.19
2.2 Catalytic hydrogenation tests
The hydrogenation of p-CNB was carried out in a 50 mL
stainless steel autoclave equipped with a flow accumulator for
the real-time record of H2 uptake, which was charged with the
desired amount of p-CNB and supported palladium catalyst
under solvent-free conditions. At first, the batch reactor was
purged with pure N2 and H2 five times respectively to replace
air and N2 in the system successively, and then heated to the
desired temperature with desired H2 pressure and stirring rate
(1200 r/min). After the complete conversion of the reactant,
the supported palladium catalyst was filtered from the mixture
of the organics and water for the next recycle use, and the
employed a temperature programmed treatment of MoO3 in
flowing N2+H2 to prepare β-phase molybdenum nitride (β-
Mo2N), which demonstrated a 100% selectivity in terms of -
NO2 group reduction in the continuous gas phase
hydrogenation of para-chloronitrobenzene (p-CNB). More
recently, Pd-S,31 Pt/H-NCNTs32 and Pd-gCN33 have been applied
to the selective hydrogenation of aromatic halonitro
compounds and show excellent catalytic performance. Despite
a larger atomic radius and higher electron-donating capability
of phosphorous, there are relatively few studies devoted to
the Pd catalyst promoted by phosphorous. Herein, we report a
novel P-doped carbon layer coated on the activated carbon
was fabricated by the calcination of the sodium hypophosphite
and ethanediol under inert gas atmosphere. Palladium
supported on the P-doped carbon layer interacts with P and
presents obvious electron-rich property as a result of the
electron transfer from C atoms to Pd atoms via P atoms. Pd-P
catalyst exhibits superior selectivity up to 99.9% with aromatic
haloamines, and excellent catalytic stability for the
chemoselective hydrogenation of p-CNB. The dechlorination
suppression mechanism was discussed at the end of this work.
corresponding aromatic haloamine was separated by
a
standing and layering process. The samples were periodically
withdrawn from the reactor and identified by GC–MS (Agilent
5973N) and analyzed by gas chromatography (Agilent 7890)
equipped with a FID detector and a capillary column HP-5
(30m*0.20mm*0.25μm). The quantitative analysis was
conducted by area normalization method.
2.3 Catalyst characterization
X-ray diffraction (XRD) measurements of the catalysts were
performed using an X’Pert PRO diffractometer (PNAlytical Co.)
equipped with a Cu Kα radiation source that was operated at
60 kV and 55 mA. Diffraction patterns were collected at a
scanning rate of 2°/min and with a step of 0.02°. The mean size
of Pd particles was calculated from the (111) plane using the
Scherrer equation.
2. Experimental Details
2.1 Catalyst preparation
The addition of phosphorus to the activated carbon was
carried out by the incipient wetness impregnation. 5 g
activated carbon made by France Arkema CECA (Particle Size:
250-300 mesh, N2-BET: 980 m2/g, Ash content: 3%) was
impregnated with 12.5 mL of NaH2PO2 (Macklin Company, AR,
99.0%) and ethanediol solution (0.0133 g/ml, nP : nPd = 2 : 1).
The size and morphology of Pd particles on the surface of
catalysts were also analyzed by transmission electron
microscopy (TEM) using a Tecnai G2 F30 S-Twin Microscope
(Philips-FEI Co., Netherlands). At least 200 individual Pd
particles were counted for each catalyst and the mean Pd
particle size of the catalyst, ds, was calculated by the following
equation: ds=Σnidi3/Σnidi2, where visible particle size di on the
After dried at room temperature and 120
℃ for 5 h
respectively, the samples were placed in a quartz tube reactor
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micrographs was measured by a computerized system and ni is seen that palladium nanoparticles were uniformly distributed
the number of particles of diameter di and Σni > 200. on the surface of Pd/C-P-EG in Fig. 1(c), compared with those
XPS analysis was performed by Kratos AXIS Ultra DLD of Pd/C and Pd/C-EG. According to the distribution histogram,
spectrometer with monochromatized aluminum X-ray source Pd/C-P-EG has a narrower particle size distribution and smaller
(1486.6 eV). The operating pressure in the analysis chamber average particle size. The sizes of up to about 80% particles are
was maintained below 6 ×10-9 Torr. Wide-scan spectra in the in range of 5.0–10.5 nm and the average particle size was 8.7
binding energy range of 1100–0 eV were recorded in a step nm, much smaller than 14.1 nm of Pd/C and 14.3 nm of Pd/C-
size of 1 eV with a pass energy of 50 eV. High resolution EG. From HRTEM image of Pd/C-P-EG in Fig. 1(d), large Pd
spectra of elemental signals were recorded in a step size of particles clearly show wrinkled crystal structure which was
0.05 eV with a pass energy of 20 eV. The calibration of binding rarely observed in the Pd catalysts supported on the activated
energy (BE) of the spectra was referenced to the C1s electron carbons. Only with images, it is very difficult to determine
bonding energy at 284.8 eV arising from adventitious carbon. whether they are palladium phosphides as the lattice distances
This reference gives binding energy values a precision of ±0.02 of Pd0 [PDF 01-087-0637] and palladium phosphide [PDF 00-
eV. After the linear baseline for non-metal element signals 031-0937] are almost identical. However, the EDX mapping for
(using Shirley baseline for metal element signal) was Pd/C-P-EG in Fig. 1(e) clearly reveals that P element is
subtracted, curve fitting was performed using the non-linear uniformly distributed throughout the entire nanostructure and
least-square algorithm assuming a Gaussian peak shape. The slightly gathered on the region of Pd particles. In addition, the
surface atomic composition ratio was calculated by using the phosphorus peaks are observed in the EDX spectra of Pd/C-P-
Schofield sensitivity factors after the normalization of each EG catalyst. These results confirm the existence of phosphorus
individual peak’s area.
inside the Pd/C-P-EG catalyst.
BET surface area and porous parameters of the samples
The XRD patterns of Pd/C, Pd/C-EG and Pd/C-P-EG catalysts
were determined by nitrogen physical adsorption at 77 K. are shown in Fig. 2. The XRD characteristic diffraction peaks of
Before the measurements, a 50-mg sample was degassed at these catalysts at 2θ =39.9o, 46.5o and 68.1o are ascribed to
523 K for 12 h under vacuum. Nitrogen adsorption isotherm Pd(111), Pd(200) and Pd(220) crystal facet respectively,
was then performed using
a Micromeritics ASAP 2020
instrument. The surface area was calculated by BET equation.
The pore distribution was calculated by the Horvath-Kawazoe
(HK) and Barrett-Joyner-Halenda (BJH) method from the
adsorption branch of isotherms.
Raman spectra were taken on powder samples on a
Perkin–Elmer 400F Raman spectrometer with 785 nm red laser
irradiation.
3. Discussion and Results
TEM technique is a direct method to observe the morphology
and size distribution of catalyst particles. Fig. 1 shows the TEM
images of Pd/C, Pd/C-EG and Pd/C-P-EG catalysts. It can be
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