Highly Efficient Ultralow Pd Loading Supported on MAX Phases for Chemoselective Hydrogenation

Article abstract of DOI:10.1021/acscatal.0c00082

Palladium is one of the most efficient metals for the hydrogenation of organic compounds. However, when molecules, such as nitroaromatics, with several reducible functionalities, are hydrogenated, Pd, like any other very active metal, such as nickel or platinum, often behaves unselectively. One strategy to render Pd more selective is to choose the proper support. Herein, we show that MAX phase powders of Ti₃SiC₂, Ti₂AlC, or Ti₃AlC₂ can chemoselectively hydrogenate 4-nitrostyrene to 4-aminostyrene, with 100% selectivity, at around 3-4% conversion. To boost the latter, we loaded Ti₃SiC₂ with 0.0005 wt % Pd and increased the conversion to 100% while maintaining the 4-AS selectivity at >90%. By optimizing the Pd loading, we were also able to increase the turnover frequency 100-fold relative to previous literature results. The identification of this highly efficient and chemoselective system has broad implications for the design of cost-effective, earth-abundant, nontoxic, metal catalysts, with ultralow noble metal loadings.

Full text of DOI:10.1021/acscatal.0c00082

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Article
Highly Efficient Ultra-low Loading Pd Supported on
MAX Phases for Chemoselective Hydrogenation
Mihaela-Mirela Trandafir, Florentina Neatu, Iuliana M Chirica, Stefan Neatu, Andrei
C. Kuncser, Elena I Cucolea, Varun Natu, Michel W Barsoum, and Mihaela Florea
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c00082 • Publication Date (Web): 24 Apr 2020
Downloaded from pubs.acs.org on April 25, 2020
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Highly Efficient Ultra-low Loading Pd Supported on MAX Phases for
Chemoselective Hydrogenation
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Mihaela M. Trandafir,1# Florentina Neațu,1# Iuliana M. Chirica,1,2 Ștefan Neațu,1 Andrei C.
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Kuncser,1 Elena I. Cucolea,3 Varun Natu,4 Michel W. Barsoum*4 and Mihaela Florea1*
1National Institute of Materials Physics, 405A Atomistilor Street, 077125 Magurele, Romania
2University of Bucharest, Faculty of Physics, 405 Atomistilor Street, 077125 Magurele,
Romania
3Research Center for Instrumental Analysis SCIENT, Petre Ispirescu Street, no. 1, 077167
Tancabesti, Ilfov, Romania
4Department of Materials Science and Engineering, Drexel University, Philadelphia,
Pennsylvania 19104, USA
# authors with equal contribution
ABSTRACT: Palladium is one of the most efficient metals for the hydrogenation of organic
compounds. But when molecules, such as nitroaromatics, with several reducible functionalities
are hydrogenated, Pd - like any other very active metal such as nickel or platinum - often
behaves unselectively. One strategy to render Pd more selective is to choose the proper support.
Herein, we show that MAX phase powders of Ti3SiC2, Ti2AlC or Ti3AlC2 can chemoselectively
hydrogenate 4-nitrostyrene to 4-aminostyrene, with 100 % selectivity. At around 3-4 %, the
conversions, however, are low. To boost the latter, we loaded Ti3SiC2 with 0.0005 wt.% Pd
and increased the conversion to 100% while maintaining the 4-AS selectivity at > 90%. By
optimizing the Pd loading, we were also able to increase the turnover frequency 100-fold
relative to previous literature results. The identification of this highly efficient and
chemoselective system has broad implications for the design of cost-effective, earth-abundant,
non-toxic, metal catalysts, with ultra-low noble metal loadings.
KEYWORDS: MAX phase, 4-nitrostyrene, chemoselective hydrogenation, low noble metal
amount, palladium
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INTRODUCTION
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Reduction with hydrogen, H2, of only one functional group when several functionalities are
present in a molecule is not trivial because the thermodynamic driving forces can be
comparable. One way to address this problem has been to use solid catalysts that can
chemoselectively orient the desired reactions. A chemoselective catalyst has to distinguish and
preferentially interact with one functional group, while avoiding the transformation of the
others. There are only few generally applicable catalytic systems for the selective reduction of
a nitro group in the presence of C=C, C=O or C≡N groups. Pd is a very efficient metal for the
hydrogenation of organic compounds, but is often unselective. Several strategies have been
tried, chief amongst them is the proper choice of support, that due to its architecture and
interaction with the noble metal increases selectivity. Another approach is to decrease of
number of active metal sites by adding additives such as oxides, other metals, etc., but in most
cases at the expenses of activity.1 For instance, to chemoselectively reduce the nitro group via
catalytic processes when carbon-carbon double bonds are present in the molecules is
considered a big challenge, since functionalized aniline derivatives resulting from
hydrogenation of nitro-compounds are key intermediates for a large palette of chemical
products with important industrial applications.2-4
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To date, and as far as we are aware, the chemoselectivity to aniline compounds is carried
out only in the presence of metals such as Pt, Au, Ru, Ni, Fe, Ag and Rh, as mono or bimetallic
catalysts.5-7 The best results are obtained using Au nanoparticles (NPs) deposited on a support,
or by using a bimetallic Au-Pt NPs supported on TiO2.8, 9 In particular, recently the
hydrogenation of 4-nitrostyrene (4-NS) to 4-aminostyrene (4-AS) has been achieved using
colloidal Co-Ru nanoparticles suspensions,10 Pt supported on a W-TiO2 modified with organic
thiol11, 12 or Pd nanosheets suspensions.13 However, the development of true heterogeneous
catalysts without the use of additives is highly desirable. In a recent review paper, it was shown
how chemoselectivity to nitro group reduction can be induced in unselective metals by
controlling the architecture of the metal catalysts8 and choosing the right support. In spite of
all efforts dedicated to this topic over the last few decades, chemoselectivities close to 100 %
have remained elusive over true heterogeneous materials.
The MAX phases are ternary early transition metal carbides and/or nitrides with a layered
hexagonal crystal structure.14, 15 The moniker of these materials derives from their chemical
formula, Mn+1AXn, where M is an early transition metal, A is moslty a group 13 or 14 element,
X is carbon and/or nitrogen, and n=1, 2, 3 or 4. Most of these phases were discovered in the
sixties by Nowotny and co-workers, and lay dormant in the literature until the mid nighties
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when they were finally processed in fully, dense and predominantly single phase form which
in turn allowed their unusual properties to be documented.16 The original set numbered around
50;17 today over 150 phases are known to exist, with more being discovered on a routine basis.18
The MAX phases are unique in a sense that they combine properties of both metals and
ceramics. Similar to metals, they are readily machinable, possess high electronic and thermal
conductivities. In some cases, the electronic conductivties exceed those of the parent transition
metal. They are also damage and thermal shock tolerant.15 Like ceramics some are lightweight,
creep and oxidation resitant and refractory.
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Despite the fact that these phases were rediscovered over two decades ago and thousands
of studies on many of their properties have been explored, their use as catalysts had been totally
ignored until 2017, when Ng et al. showed the potential of Ti3AlC2 – a MAX phase - in selective
oxidation reactions.19 The useful catalytic actitvity was shown to most likely derived from a
very thin, presumably non-stoichiometric, Al and Ti mixed oxide surface layer containing
oxygen vacancies.
To the best of our knowledge, no hydrogenation reactions have been reported so far where
a MAX phase was used as either a catalyst or support. Herein, we explore the use of select
MAX phases, and more importantly as a support for Pd nanoparticles, NP, at exceedingly low
concentrations, for the chemoselective hydrogenation of a functionalized nitro-compound, 4-
NS. The product of interest, 4-AS, is an important platform molecule in organic synthesis for
a variety of agrochemicals and pharmaceuticals.2
In this study we report on a new generation of MAX-phase catalysts, in general, and Pd
NPs supported on Ti3SiC2 in particular, for the selective – in some cases 100 % – hydrogenation
of 4-NS to 4-AS with a turnover frequency (TOF) as high as 4.7103 h-1, which represents
roughly a 100-fold increase over the most selective catalyst reported to date in the literature
(See Figure 1a and Table S1). To also probe the generality of our approach we performed
catalytic hydrogenation on other molecules containing multiple functional groups, such as 3-
nitrostyrene (3-NS) and cinnamaldehyde. Additionally, a few runs were carried out where the
MAX-phase was used as support for Ni.
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Figure 1: a) Semi-log plot of our TOF results compared with some of the highest reported in
the literature for catalysts with 100% conversion and > 80% selectivities of 4-NS to 4-AS. Left
top inset shows 4-NS molecule; top right inset shows desired molecule, 4-AS. b) Reaction
pathways for hydrogenation of 4-NS.
RESULTS AND DISCUSSION
Catalytic activity of the MAX phases. The MAX phases were prepared by heating
elemental powders in a tube furnace under flowing argon, Ar, at 1500 °C for 3 h. X-ray
diffraction showed that in all cases the resulting powders were predominantly single-phase.
Three phases were synthesized: Ti3SiC2, Ti3AlC2 and Ti2AlC. The XRD patterns of the Ti3SiC2
powders is shown in Figure S1. Not surprisingly, given the ultra-low loading of Pd used herein,
impregnating these powders with Pd does not change the XRD patterns in any way (Figure
S1).
The chemoselective hydrogenation of 4-NS was performed in the presence of the MAX
phase using heptane as a solvent and hydrogen, H2 gas, as a reducing agent. The reaction was
performed at different temperatures, H2 pressures and reaction times. The reaction products
identified were 4-AS, 4-ethylnitrobenzene (4-EN) and 4-ethylaniline (4-EA) (see Figure 1b and
Figure S2). Blank tests, in the absence of the catalyst, were also performed and no conversion
of 4-NS was evidenced. Moreover, the carbon balance was always close to 100%, indicating
that no side reactions occurred.
Regardless of the MAX composition powders used, as shown in the top entries in Table 1,
we successfully hydrogenated 4-NS, with 100 % selectivity, into the product of interest, viz.
4-AS. This 100 % selectivity is reported for a MAX phase here for the first time. Similar
behavior was also reported for noble metal-free materials, such as graphene, which is able to
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hydrogenate acetylenes,20 due to the frustrated Lewis pairs or defects present in the material
which are able to heterolytically activate H2.21
The low conversions can be, at least partially, explained by the fact that the MAX phases
have a low H2 dissociation rate as evidenced by H2-TPD experiments that showed that not only
were very small amounts of H2 adsorbed on the MAX phase surfaces but that they also desorbed
at temperatures < 100 ℃ (Figure S3). Since the H2 activation usually takes place readily on
metals, it is reasonable to suppose that the presence of a thin native oxide layer – present here
- on the MAX phases' surfaces is responsible for the low adsorptions.
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Increasing the H2 pressure from 1.1 to 2.5 MPa increased the conversions slightly for all
samples (see entries 1-6 in Table 1), but they remained low, around 4%. Note that the selectivity
remained at ≈ 100% for Ti2AlC and Ti3SiC2 (entries 4 and 6 in Table 1). The high selectivity
to aniline is probably due to the preferential adsorption of nitro groups over C=C bonds on the
Ti containing support, such as TiO2, as demonstrated in previous reports,8 or due to the defects
present in MAX phases,18 which might heterolytically dissociate H2, thus hydrogenating
preferentially the polar group (nitro) found in 4-NS.22
Table 1. Catalytic performance of MAX phases-based catalysts for 4-NS conversion
Entry
Catalyst
H2 pressure
(MPa)
Conversion
(%)
Selectivity (%)
4-EN
4-AS
4-EA
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4
5
1.1
1.1
1.1
2.5
2.5
< 1
< 1
< 1
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100
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100
100
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Ti2AlC
Ti3AlC2
Ti3SiC2
Ti2AlC
Ti3AlC2
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0
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0
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8
2.5
2.5
2.5
Ti3SiC2
TiC
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0
100
51
-
0
49
-
0
-
MXene
-
Reaction conditions: 0.0134 mmol substrate; 4 mg catalyst; 3 mL of heptane; 24 h, 140 C, 2.5 MPa H2
To prove this, additional experiments were performed on bulk TiC (Alfa Aesar) and on the
corresponding MXene, Ti3C2Tz. The conversion of the TiC powders at ≈ 6% was comparable
to that of the MAX phases. At 51%, however, the selectivity was roughly half that of the MAX
phases (Table 1, entry 7). In the case of MXene no conversion was obtained (Table 1, entry 8).
Based on these results it was concluded, not unreasonably, that H2 dissociation was the rate
limiting step. If that were the case, then we conjectured that the presence of a metal capable of
activating H2 should increase our conversions. This is why we impregnated Ti3SiC2 with Pd.
Catalytic activity of Pd/MAX phases. The next challenge was to find the optimal Pd
quantity necessary, while maintaining a selectivity to 4-AS of close to 100%. To that effect the
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following very low Pd loadings were tested: 0.05 wt.% and 0.0005 wt.% The catalytic results
are summarized in Table 2.
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Table 2. Catalytic performance of Pd/MAX phases catalysts, prepared by different methods for 4-NS conversion
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Entry
Catalyst
Pd loading
(ICP-MS)
(wt.%)
Conv.
(%)
Selectivity (%)
4-EN
TOF
(ICP)
(h-1)
1.4*102
1.1*103
4.7*103
4-AS
4-EA
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2
3
0.05 wt.% Pd/Ti3SiC2
0.01060‡
0.00033‡
0.00014‡
100
100
100
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0
47
68
0
53
32
75
0.0005 wt.% Pd/Ti3SiC2
0.0005 wt.% Pd/Ti3SiC2_DP
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The addition of 0.05 wt.% or 0.0005 wt.% of Pd to Ti3SiC2 by impregnation increased the
conversion to 100%, but the process was no longer selective (Table 2, entries 1 and 2). The
advanced hydrogenation product, 4-EA was obtained via the formation of 4-EN (path b in
Figure 1b).
To broaden the scope of Pd loading, we changed the Pd supporting method, from
impregnation to deposition-precipitation (DP). Doing so improved the chemoselectivity to 4-
AS, while maintaining the total conversion of 4-NS. A selectivity of 25% for 4-AS and 75%
for 4-EA was thus obtained (Table 2, entry 3).
The ICP-MS analysis performed for the three catalysts (Table 2) demonstrated that
changing the technique also diminished the Pd content and the amount of Pd found in the
catalyst prepared by DP was three times lower than in the case of the impregnation method
(see Table 2, entry 3). Such behavior was expected, since the DP technique supposes free
adsorption of Pd onto the support from a higher volume solution and crucially also includes a
washing step, while impregnation has the advantage of using smaller volumes that enable the
deposition of higher Pd amounts onto the support and no washing step is needed. This is
supported by XPS analysis (Figure S4), in which the Pd was evidenced on the surface for the
sample prepared by impregnation, but not the surface prepared by the DP technique. For the
latter, the Pd was below the detection limits of our XPS (see Figure S4e).
It is important at this point to note that the deposition of Pd by the two methods chosen here
do not modify the MAX phase structure in any way (see Figures S1, S4). TEM images
confirmed that all the observed samples showed similar morphology and structure (thin sheets
of Ti3SiC2 (Figure 2). No changes in the MAX powders after impregnation, within TEM
resolution, were observed. X-ray mappings performed with a nm sized probe showed metallic
Pd nanocrystals with a size in the 10 nm range in the case of 0.05 wt.% Pd doped sample
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(Figure 3). In all other cases, no clear evidence in the TEM for Pd was found, most probably
because of its very low concentrations in the samples.
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Figure 2: HRTEM images for Ti3SiC2. Powdered sample has been observed generally as sheets
of Ti3SiC2 which tend to be in the same orientation. Results in (a)-(d) have been provided in
the g = (-111) zone axis orientation. Image (e) shows a thin sheet observed along c-axis as
identified by high resolution imaging. (f) Visualization for electronic and structural analysis
(VESTA) structural model of Ti3SiC2 on c zone axis perspective. (g) VESTA structural model
showing main lattice planes of Ti3SiC2 identified in electron diffraction patterns.
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Figure 3: Large area TEM image of 0.05% Pd/Ti3SiC2 showing numbered regions of interest
(ROI) 1 to 3. HRTEM image of Pd NP with Selected Area Diffraction inset on ROI 1 (Figure
3.1), image of Pd NPs cluster and attached X-ray spectra showing Pd peak on ROI 2 (Figure
3.2) and a high magnification image on the ROI 3 of Pd/Ti3SiC2 sheet (Figure 3.3) X-ray mapps
performed with a nm sized probe showed metallic Pd nanocrystals with a size in the 10 nm
range in the case of the 0.05 wt.% Pd doped sample.
With the purpose of preserving the preferential selectivity obtained on Ti3SiC2 and to better
control the amount of Pd, a third strategy was employed, viz. mechanically mixing Ti3SiC2
powders with those that were impregnated with Pd (viz. Pd/Ti3SiC2 catalyst). The schematic
preparation of mechanical mixture is depicted in Scheme 1 and described in the experimental
section. The exact compositions of the mixtures are listed in Table S2 and the results are listed
Table 3, entries 1-9.
Scheme 1. Schematic of mechanical mixture preparation and its benefits.
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Table 3. Catalytic performance of Pd/MAX phases catalysts (mechanically mixed) for 4-NS conversion
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Entry
Catalyst
Pd loading
(ICP-MS)
(wt.%)
Conv.
(%)
Selectivity (%)
4-AS 4-EN 4-EA
TOF
(ICP)
(h-1)
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1
2
0.00002a
3.6*103
Pd/Ti3SiC2_mix1
Pd/Ti3SiC2_mix2, cycle 1
Pd/Ti3SiC2_mix2, cycle 2
Pd/Ti3SiC2_mix3
4
100
73
93
58
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8
7
0.00013a
2.8*103
59
3
4
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100
100
100
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4.7*103
0.00022a
2.8*103
0
42
90
3
5
6
0.00254a
4.7*102
Pd/Ti3SiC2_mix4
0
nd
nd
nd
nd
nd
nd
Pd/TiC_mix2, cycle 1
Pd/TiC_mix2, cycle 2
Pd/Ti2AlC_mix2
97
100
73
99
95
100
nd
7
-
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nd
8
52
27
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0
nd
9
Pd/Ti3AlC2_mix2
100
90
1
nd
10
11
Pd/Ti3C2Tz_mix2, cycle 1
Pd/Ti3C2Tz_mix2, cycle 2
-
5
nd
25
-
-
nd
Nd = not determined
The mechanically mixed Pd/Ti3SiC2_mix1 had the same catalytic behavior as Ti3SiC2
without Pd (Table 3, entry 1). The reason is mostly probably the almost negligible (0.00002
wt.%) amount of Pd. By slightly increasing the Pd amount to 0.00013 wt.%, the selectivity was
73% 4-AS and the conversion was 59% (Table 3, entry 2). Interestingly, after the second
reaction cycle of this sample, the catalytic performance improved, and the selectivity to 4-AS
reached 93%, at a conversion of 100% (Table 3, entry 3).
To test whether the “boomerang effect” was applicable here we carried out hot filtration
experiments. However, since the results before and after hot filtration were identical, we
23, 24
conclude that the well-known “boomerang effect”
is not responsible for the induction
period observed herein. We also checked the similarity with the Lindlar catalyst, for which the
selectivity is improved by protecting the highly active metals sites with other elements and
organic or inorganic compounds.25 In our case, we added small amounts of 4-AS and 4-EN in
the initial reaction mixture (Table S3, entries 1 and 2) and found that the conversion and
chemoselectivity decrease. These results suggest that our catalytic system does not behave like
Lindlar catalyst.26
The good results obtained after the second reaction cycle are thus most probably caused by
an induction period, that reflects the time needed for the promotion of a strong metal−support
interaction which is often invoked to be critical in catalysis.27 Also, there are different possible
explanations for the improvement of the chemoselectivity, not evidenced experimentally by us,
but sustained theoretically28 and experimentally29 by other studies: i) through electronic effects,
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Pd transforms into a more stable adsorption structure and a complex interplay of the electronic
interactions between the adsorbate molecules and the metal surface which can lead to changes
in the mechanism of H2 activation28 and/or ii) through geometric effects, the in situ reduction
of the Pd cluster size leading to a decreased number of adjacent adsorption sites.30 These
comments notwithstanding, more work is needed to better understand what is occurring as a
function of time/cycling of our system.
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Further increasing the Pd content (Table 3, entries 4 and 5) improves the catalytic
conversion, but the selectivity to 4-AS decreases to 58% and 10%, respectively. Apparently,
the Pd/Ti3SiC2_mix2 sample possesses the optimal Pd amount necessary to reach a conversion
of 100%, while maintaining the MAX phases’ selectivity to 4-AS at > 90%.
By assuming that all the Pd atoms were accessible to the reactants, we estimated the total
number of 4-NS moles transformed by moles of Pd per time, TOF, of all tested catalysts. The
TOF values for 4-NS hydrogenation varied from 102 to 103 h-1 (last column in Table 2 and 3).
These values are exceptionally high especially compared with literature data (see Table S1 and
Figure 1a). For the latter, we only report systems for which the selectivity was > 80%.
Catalyst recyclability and stability. Since, the catalytic stability is crucial in catalytic
processes, we tested the stability of Pd/Ti3SiC2_mix2 sample over 6 consecutive cycles. From
the results (Figure 4) it is obvious that our system is stable for at five reaction cycles.
Deactivation starts only during the sixth reaction cycle. A possible reason for the 20%
deactivation in the 6th cycle might be the partial blockage of the active centers with reaction
products, as evidenced by ATR-FTIR spectroscopy and XPS (Figures S5 and S6). Moreover,
the results of the leaching tests (compare entries 3 and 4 in Table S3) showed that 4-NS
hydrogenation stops when the catalyst is removed from the reaction media, indicating that no
Pd was solubilized in the reaction mixture and thus no homogenous reactions can occur. The
hot filtration experiments also indicate that Pd does not leach from the catalyst surface.
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Figure 4: Stability tests performed on Pd/Ti3SiC2_mix2 as catalyst for 4-NS hydrogenation, as
a function of cycle number.
It is also crucial to note that phenyl hydroxylamine, which is commonly obtained during 4-
NS hydrogenation reactions performed on oxide surfaces,8, 11 was not detected. As this
compound is a powerful explosive, its absence is necessary, and desirable, in industrial
applications/settings. Our results bode well for the use of this system commercially.
Catalytic activity on other substrates. To explore the general utility of our optimized
catalyst - Pd/Ti3SiC2_mix2 – we attempted to hydrogenate 3-NS to 3-AS. The results (Table
S4, entry 1) show that 100% conversion is possible with a selectivity of 96% after the first
reaction cycle. Here again, unprecedented TOF values that are ≈ 100 fold higher than those
reported so far in the literature are obtained.6, 8, 31 Changing the position of functional groups
from para- to meta- on the aromatic ring improve the substrate conversion (see Table S4, entries
1, 2 and 4, 5) and also the selectivity. The electronic effects of the reactants could influence the
catalytic performance; similar results were observed on Ru based catalyst.32 Very interesting
results converting 4- or 3-nitrobenzaldehyde in the presence of our catalytic system were
obtained, i.e. reductive N-alkylation of nitrobenzaldehyde in one step, with a very high
chemoselectivity to the alkyl-derivative (96%), which proceed only after reduction of nitro-
group (see Table S4, entries 4 and 5).33 Several others nitro-olefins were tested and we
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identified that Pd/Ti3SiC2_mix2 was able to chemoselectively (83%) hydrogenate 4-
nitrophenol (Table S4, entry 6).
Corma et al.6 tested 5% Pd/C, among others, as catalyst in the hydrogenation of several
substituted nitro-compounds. It was shown that aliphatic molecules, such as 1-nitro-
1cyclohexene, are more difficult to be hydrogenated compared with aromatic nitro compounds.
It was also our case for other types of molecules containing C=C and nitro substituents as can
be observed in Table S4, entries 3, 7 and 8. The presence and/or the nature of the second
functional group influences the activity and the selectivity of the catalytic system and attention
must be paid to the type of solvent used or reaction temperature.
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We also tested optimized catalysts for the hydrogenation of cinnamaldehyde to
hydrocinnamaldehyde and/or cinnamyl alcohol (Table S4, entry 9). Our catalyst
chemoselectively hydrogenated cinnamaldehyde to hydrocinnamaldehyde with 80%
selectivity and 94% conversion. The TOF's were comparable with data presented in literature.34
In accordance with the literature,35 Pd deposited on different supports reacts much faster with
C=C bonds than with C=O bonds and this could explain why the selectivity to cinnamyl alcohol
is only 3%.
Overall, however, our results, obtained in conditions not yet optimized, reveal the potential
of Pd/MAX phase materials as chemoselective catalyst for nitro group hydrogenation.
When Pd is replaced by Ni, another nonselective hydrogenation metal, using as support the
most selective MAX powder, Ti3SiC2, the selectivity to 4-AS remained at 100% (Table S3,
entry 5). However, the conversions were around 1% most probably because the amount and
type of Ni and its oxidation state were not optimal for this reaction. It is reasonable to assume
here that, like for Pd, an optimum Ni loading also exists. This study is ongoing.
General assessments on the chemoselective hydrogenation behavior of Pd/MAX phase
The development of active and selective catalysts represents a crucial point in
chemoselective hydrogenations, and in this context beside the composition of a catalyst the
preparation method also has an influence over the reaction’s conversion/selectivity. Indeed, if
one compares catalysts with almost the same amount of Pd (~0.00014 wt.% by ICP-MS)
prepared by impregnation (Table 3, entry 2) and DP (Table 2, entry 3), the conversion increases
from 59% to 100% when using the catalyst prepared by impregnation vs. the DP method. TEM
investigations show Pd clusters formation for 0.05 wt.% Pd/Ti3SiC2 prepared by impregnation
(used in the mix samples), while for DP we assume, based on literature data,36 that the metal
loading is better dispersed (no clear evidence in TEM, due to technical limitations). Therefore,
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even though the Pd amount is comparable in both catalysts, the number of accessible sites
appears to be a function of the preparation method. Thus, by DP a good dispersion is obtained
leading to a higher number of accessible active sites and total conversion, while through
impregnation, larger clusters are formed limiting the number of accessible sites and
consequently the conversion (59 %).
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Usually, the improvement of the selectivity is accompanied by a decrease of catalytic
activity.37 Indeed, the results depicted in Table 3 (entries 1, 2, 4 and 5) underline the critical
importance of Pd content/dispersion onto a selective support and that exceeding an optimal Pd
value results in masking the selective effect of Ti3SiC2, leaving in place only a non-selective
Pd effect. For samples with the same degree of conversion (100%), large Pd clusters, from
impregnated sample for instance, transform unselectively 4-NS (Table 2, entry 2), while small
Pd particles, very well dispersed obtained by the DP method, produces 4-AS and 4-EA (Table
2, entry 3). Similar behaviour was observed for other different metal-support catalysts used in
the hydrogenation of 4-NS, when big metal clusters are formed during preparation the
selectivity diminishes.1, 38 This observation can be ascribed to the different ways of adsorption
and activation of H2. It is well known that H2 activation constitutes a critical step in
chemoselective hydrogenation reactions, and moreover, that homolytic activation of H2 is
promoted by larger metal particles, is which are not selective.39
In contradistinction, a higher metal dispersion to single atoms heterolytically cleave the H2
molecule into H+/H- pairs, which are more prone to hydrogenate the polar groups rather the
non-polar ones.37 The heterolytic cleavage of H2 into H+/H- pairs on Pd/support catalysts occurs
in a similar way as in the Shvo−Noyori catalysts and frustrated Lewis pairs, where the support
serves as a proton acceptor, while the Pd atom as the hydride acceptor.37
Our strategy to better control the Pd dispersion, by using mechanical mixing, led to a
selectivity of 73% (Table 3, entry 2) for an ultra-low amount of Pd (130 ppm). In this context,
we cannot exclude the generation of “single atom catalyst” that might occur, that could in turn
heterolytically activate H2, which has been demonstrated to be very effective for
chemoselective hydrogenations.37 Furthermore, Perez et al. using DFT calculations
demonstrate that atomically dispersed Pd atoms lead to high activity and selectivity due to a
facile hydrogen activation and reactant adsorption on these types of sites.40
As presented in Tables 1 and 3, the composition of the support, its surface and, most
probably, the metal-support interaction play an important role in this reaction. In comparison
with Ti3SiC2, TiC and Ti3C2Tz show a different behavior in hydrogenation of 4-NS when Pd
was deposited by the same method on its surface (see Table 3, entries 6 and 10) for 100% and
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90% conversion of 4-NS, respectively, crucially no 4-AS formed. The induction period was
also checked for these materials and instead of an improvement as in the case of Ti3SiC2 a
deactivation occurred; in the second cycle the conversion diminished drastically to 18% and
25% (Table 3, entries 7, 11) respectively, most probably due to a lower metal-support
interaction. These results alone imply that oxides formed during aerobic oxidation on the MAX
surfaces are responsible for the catalysis; no other interpretations are plausible.
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To gain more information Raman spectroscopy was performed on Ti3SiC2 and TiC. The
spectra depicted in Figure 5 revealed the four characteristic Raman active modes of 2E2g, E1g
and A1g at 167 cm-1, 223 cm-1, 272 cm-1 and 358 cm-1, named 1, 2, 3, 4, related to C–Ti–
Si bonds for Ti3SiC2. These vibrations are comparable with those previously reported by some
of us.41 The vibration modes from 609 and 670 cm-1 are related to the C–Ti–C bonds. Crucially,
a supplementary Raman mode at 128 cm-1, which correspond to -SiO2 was observed in the
ternary MAX phase.42 The presence of SiO2 in the native oxide is not too surprising because
when Ti3SiC2 is oxidized at high temperatures, a duplex rutile and SiO2 scale forms.43 We
conjecture at this time that it is the presence of SiO2 that may help orient the 4-NS at the surface
of the support. A similar improved selectivity to 4-AS was also observed for other titanium-
modified SiO2 on which Pd was deposited.44 As already emphasized by Boronat et al.7 the
repulsion between the electrons from the aromatic ring and the oxygen electrons from the SiO2
orient the 4-NS perpendicularly to the surface, while the two oxygen atoms of the nitro group
bond to the Ti and Pd surface atoms.
Figure 5: Raman spectroscopy of Ti3SiC2 and TiC.
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The same 4-NS hydrogenation was also performed on Pd supported on Ti2AlC and Ti3AlC2
MAX phase powders. In this case the selectivity to 4-AS, was much reduced (Table 3, entries
8 and 9). Interestingly the selectivity to 4-EN was close to a 100% with a conversion of 100%
in the Pd/Ti3AlC2_mix2 case (Table 3, entry 9). Currently, we know that the oxides that form
on Ti3SiC2 surfaces (TiO2 and SiO2)43 are different than those that form on Ti3AlC2 (solid
solution between TiO2 and Al2O3),19 which probably lacks the ability to orient preferentially
the 4-NS to form 4-AS.
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Finally, it is reasonable to conclude that the high chemoselectivity of the Pd/MAX phase
catalyst can be attributed to the synergetic effect between the Pd nanoparticle size and
dispersion and the non-Ti containing oxides formed on the MAX phases that preferentially
activate the nitro group. If the Ti-containing oxides were solely responsible, then TiC should
have worked as well as the MAX phases, which is not observed.
Lastly, it is important to note that the results obtained herein are a strong function of the
nature and thicknesses of the native oxides that form on the MAX phases. This was made clear
to us when a second batch of Ti3SiC2 powders, nominally synthesized in exactly the same way
as the first, were found to have slightly different catalytic properties. Most probably the amount
of SiO2 formed on the surface is different and therefore the second batch behaved more like
TiC. We are currently trying to understand what is occurring. This observation brings up a
related important question - the answer of which is beyond the scope of this work – viz. what
is the relationship between the catalytic activity and the nature of the oxides that form on the
different MAX phases? How the nature of these oxides changes during catalysis is also an
important topic that we are currently exploring.
CONCLUSIONS
In conclusion, herein we show that the MAX phases Ti3SiC2, Ti3AlC2 and Ti2AlC are
selective in converting 4-NS to 4-AS, but the conversions are low. By using ultra-low amounts
of Pd, we were able to maintain the very high selectivity and concomitantly increase the
conversion due to the heterolytic activation of H2 and/or generation of single atom active
catalyst. With an optimal loading of very well dispersed 130 ppm Pd, we obtained a conversion
of 100% with a selectivity of 93% of 4-NS to 4-AS; higher Pd loadings result in a loss of
selectivity. The main role of the Pd is to dissociate/activate the H2 gas. And while Pd is not an
earth abundant element, the very small amounts needed together with the earth abundant and
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non-toxic nature of the elements in the MAX phases considered here suggests that the cost of
raw materials and/or toxicity should not be barriers to potential commercialization.
These reactions represent the first examples where select MAX phases, with ultra-low
amounts of Pd, can be used as catalyst in chemoselective hydrogenation reactions. Thus, this
could well be the starting point for designing of a new generation of chemoselective catalysts
in the hydrogenation of functionalized nitroderivatives, where the catalytic properties of the
MAX phases can be exploited.
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METHODS
Synthesis of MAX phase powders
The powders used for this work were: TiC, (Alfa Aesar, 99.5% 2 µm), silicon (Alfa Aesar,
99.5%, - 325 mesh), Ti (Alfa Aesar, 99.5%, - 325 mesh) and Al (Alfa Aesar, 99.5%, - 325
mesh).
The Ti3SiC2 powders were made by mixing TiC, Si and Ti powders in a molar ratio of
2:1:1, respectively. The mixed powders were ball milled for 24 h at 70 rpm using zirconia balls.
The powders were then placed in an alumina tube furnace and heated under flowing Ar at
1500°C for 3 h. The heating and cooling rates were set at 5°C min-1. The resulting porous
blocks were ground to a powder using a drill press. The milled powders were passed through
a 400 mesh (particle size < 38 µm) sieve for further experiments.
The same procedure was used to make Ti3AlC2 and Ti2AlC powders. In this case the
TiC:Al:Ti ratios used were 2:1.05:1 and 1:1.1:1, respectively.
Catalyst synthesis
Three methods were used to support Pd on Ti3SiC2.
i) Wet Impregnation Method
Pd was supported on Ti3SiC2 by a wet impregnation method. The required amount of the
Pd precursor (Pd (II) acetate) was dissolved in methanol and the MAX powder was then added.
The system was stirred at 40 °C for 16 h, before drying the mixture at 60 °C, under vacuum
(10-4 MPa), for 5 h.
In convert the acetate to Pd supported on the MAX powders, the dried mixture was annealed
in a 10 mL min-1 flow of 5 vol % H2 in Ar at 200 °C at a heating rate 10 °C min-1 for 3 h. The
catalyst was collected after cooling to RT and subsequent gradual exposure to air. Materials
containing several Pd loadings (0.0005 wt. % and 0.05 wt. %) were prepared. The catalysts in
this case are henceforth referred to as 0.0005% Pd/Ti3SiC2 and 0.05% Pd/Ti3SiC2, respectively.
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Palladium (0.05 wt. %) was impregnated on other two MAX phases - Ti2AlC and Ti3AlC2
- employing the same procedure as for the 0.05 wt. % Pd/Ti3SiC2 powders.
A 0.05 wt. % Ni/Ti3SiC2 was also prepared also using wet impregnation method,
starting from Ni(II) nitrate hexahydrate as precursor. The final material was annealed in 5 vol
% H2 in Ar flow (10 mL min-1) at 450 °C at a heating rate 10 °C min-1, for 3 h.
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ii) Deposition-Precipitation Method
Palladium was also supported on Ti3SiC2 using a deposition-precipitation method, for a
calculated concentration of 0.0005 wt. % Pd. The required amount of Pd (II) acetate was
dissolved in 5 mL of methanol and the MAX powder was added to the mixture. A 25% NH4OH
solution was then added, drop wise, until a pH 9 was reached at which time the mixture was
stirred at 40 °C for 16 h. The material was thoroughly washed with distilled water and the
material was dried, at 60 °C, under vacuum, for 5 h. The obtained supported Pd precursor was
reduced in a 10 mL min-1 flow of 5 vol % H2 in Ar at 200 °C using a heating rate 10 °C min-1
for 3 h. The catalyst was collected after cooling to RT and slowly exposed to air. The final
material will henceforth be referred to as 0.0005% Pd/Ti3SiC2_DP.
iii) Mechanical Mixing
The Pd content of the 0.05 wt. % impregnated catalyst powders was further diluted by hand
mixing 0.05%Pd Ti3SiC2 powders with extra pure Ti3SiC2 powders, in the ratios listed in Table
S2. These samples were denoted Pd/Ti3SiC2_mix1, Pd/Ti3SiC2_mix2, Pd/Ti3SiC2_mix3 and
Pd/Ti3SiC2_mix4. Their composition and calculated percentage of Pd, based on our ICP-MS
results are also listed in Table S2. For the most part the Pd content measured by ICP described
further.
The same procedure was employed for the preparation of Pd/Ti2AlC_mix2 and
Pd/Ti3AlC2_mix2 with the proportions listed in Table S2.
Catalytic tests
The catalytic runs were performed in a 20 mL stainless steel autoclave, equipped with a
magnetic stirrer. In a typical experiment, the autoclave was loaded with 4 mg of catalyst, 0.0134
mmol of substrate and 3 mL of solvent (heptane).
The autoclave was purged 3 times with H2 in order to remove any residual air and then
pressurized with hydrogen to 2.5 MPa. The reaction was carried out at 140 ºC for different
times. At the end of the reaction, the autoclave was cooled down to RT and the H2 pressure
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was slowly released. The catalyst was separated from the reaction media and the products from
the liquid phase were analyzed by GC-MS. Data reproducibility was checked by performing
each experiment in duplicate. The obtained results were within ± 5 % of each other.
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Stability tests
The stability test was carried out for 4-NS hydrogenation (0.0134 mmol) in the presence of
Pd/Ti3SiC2_mix2 catalyst (4 mg), using heptane as solvent (3 mL), at 140 ºC, 2.5 MPa H2
pressure, for 24 h per run. For each run, the catalyst was separated by filtration from the reaction
mixture and reused in the follow up run. A total of six reaction cycles were carried out.
Leaching tests
Leaching tests were performed to check if any Pd dissolves in the reaction mixture and to
determine if a homogeneous reaction takes place. The procedure was the following: the catalyst
from the reaction mixture obtained after the first reaction cycle was filtered at RT and at high
temperature (hot filtration) using filter paper and the remaining filtrate was again pressurized
with H2 and subjected to the reaction without catalyst. The results are displayed in Table S3,
entries 3 and 4, from which we conclude that no leaching of the catalyst occurred.
Analysis of reaction products
Separation and determination of the analytes was performed by gas chromatography (GC)
and a quadrupole mass spectrometer (MS) (Shimadzu GC-MS-QP2010 Ultra GC-MS)
equipped with a ShinCarbon ST Micropacked column (2 m x 1 mm, OD 1/16″, mesh 100/120,
temperature limits up to 300 °C).
The GC conditions for the sample analysis were: Injector temperature: 275 °C; Carrier gas:
helium; Pressure: 120.0 kPa; Linear velocity: 52.9 cm/s; Oven temp.: 40 °C (hold 1 min.) to
50 °C at 2 °C min-1, then to 200 °C at 40 °C min-1 then to 280 °C at 15 °C min-1 (hold 20 min).
Detector: MS (EI); Ion source temp.: 200 °C; Interface temp.: 250 °C; Detector voltage: 0.8
kV.
Activity results are expressed in terms of 4-NS conversion (moles reacted over the initial
number of moles) and selectivity to 4-AS (moles of 4-AS produced over the moles of reacted
4-NS). TOF represents the total number of moles transformed per mole of Pd per second.
Characterization methods
XPS measurements were performed using a Kratos Ultra DLD Setup spectrometer using
the Ka and Al–Kα (1486.74 eV) radiation produced by an X-ray source operating at a total
power of 300 W (12.0 kV × 25 mA) and a vacuum of ≈ 1 × 10-4 MPa.
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The H2 temperature programmed desorption experiments (H2-TPD) were performed using
a Porotec TPDRO 1100 device. Prior to the adsorption step, approximately 50 mg of sample
was pretreated for 1 h at 200 °C in a He gas flow to ensure a clean surface, after which it was
cooled down to RT also in a He gas flow. A 5 vol. % H2-He mixture was passed over the sample
with a flow rate of 50 mL min-1 for 1 h. This was followed by He gas passed for 10 minutes
through the sample to again clean the surface and the temperature was linearly increased by 10
°C min-1 to 800 °C. The quantification of the H2 released during desorption process was carried
out by using the equipped thermal conductivity detector of the TPDRO device and a calibration
curve.
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The XRD measurements were performed using a Bruker-AXS D8 Advance diffractometer
equipped with a LynxEye 1D detector and Cu-Kα (0.1541 nm) radiation source and a
scintillation counter detector. The diffraction patterns were recorded at an 2θ angle in the range
of 9-90°, with a step size of 0.02° and a rate of 1.2° min-1. For the identification of the XRD
phases present in the samples, the Powder Diffraction File from the International Centre for
Diffraction Data (PDF-ICDD) was used.
The FTIR spectra were acquired using a Perkin Elmer Spectrum BX II apparatus, equipped
with a Pike-MIRacle ATR attachment having a diamond/ZnSe crystal plate. The spectra were
recorded in attenuated total reflection (ATR) mode, with a resolution of 4 cm−1. A total of 128
scans were collected. The investigated wavenumber domain was set in the range of 500-4000
cm−1.
The Raman spectra were recorded in the range between 50 and 2000 cm-1, using a HORIBA
Jobin-Yvon LabRAM HR evolution spectrometer equipped with an air cooled CCD and a He-
Ne laser with a wavelength of 633 nm. The spectra were recorded in the extended scan mode
with acquisition time of 5x60 s.
All TEM observations were been performed with a JEOL2100 system, equipped with X-
Ray spectrometer and ASTAR crystallographic tool developed by NANOMEGAS.
A Perkin Elmer NexION 300S ICP Mass Spectrometer, MS, was used for the determination
of the Pd content. The instrument was equipped with Pt sampling and skimmer cones, a PFA
nebulizer, quartz spray chamber, quartz torch and PC3 Peltier Cooler inlet system. The
instrument also includes a hyper skimmer to focus the ion beam. The ICP-MS system was
operated using a collision cell with kinetic energy discrimination (KED), which utilizes He gas
flow (5.4 mL min-1) for an effective removal of most common polyatomic interferences.
Samples were measured in triplicate. The analyte mass (m/z) - 106Pd was acquired. The
equipment was calibrated in the range 0.05 – 4.00 µg L-1 Pd. The reference material used for
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calibration was a 1000 µg mL-1 Pd pure single-element standard in 10% HCl, supplied by
Perkin Elmer. All glassware used for the analysis was cleaned with 5% HNO3 solution and
rinsed with ultrapure water.
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AUTHOR INFORMATION
Corresponding Author
*Email: mihaela.florea@infim.ro and barsoumw@drexel.edu
ORCID
Mihaela Trandafir: 0000-0001-9001-8738
Florentina Neatu: 0000-0002-7885-436X
Stefan Neatu: 0000-0002-2450-0291
Mihaela Florea: 0000-0002-6612-6090
Author contributions
F.N., M.W.B., and M.F. conceived the project. V.N. prepared the MAX phases, M.M.T.
prepared the materials containing Pd and performed the catalytic tests reactions and TPD
studies, A.K. performed the TEM experiments, I.M.C performed the FTIR and Raman
spectroscopies and interpretation, E.I.C. performed the ICP experiments, F. N., S.N. and
M.M.T. performed XPS and XRD analysis and analyzed the data. All the authors wrote the
paper. All authors read and approved the final version of paper.
SI include experimental procedures, characterization data, GC-MS chromatograms
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
MWB and VN were funded by NSF (DMR 1740795). FN, MMT, SN, MF acknowledge
the funding from Ministry of Research and Innovation, CNCS-UEFISCDI, project number PN-
III-P1.1-TE-2016-2116 and Romanian Ministry of Research and Innovation through the Core
Program PN19-030101. This work was also supported by a grant of the Romanian National
Authority for Scientific Research and Innovation, CCCDI – UEFISCDI, project number
111/2019, ERANET-M.-CATALEAST-1, within PNCDI III.
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83x44mm (96 x 96 DPI)
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4-AS
4-NS
4-EA
4-EN
Figure 1: a) Semi-log plot of our TOF results compared with some of the highest reported in the literature for
catalysts with 100% conversion and > 80% selectivities of 4-NS to 4-AS. Left top inset shows 4-NS molecule; top
right inset shows desired molecule, 4-AS. b) Reaction pathways for hydrogenation of 4-NS.
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Mix catalysts
Selectivity
Higher activity
Selectivity
Higher activity
Scheme 1. Schematic of mechanical mixture preparation and its benefits.
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