C-N bond hydrogenolysis of aniline and cyclohexylamine over TaO:X-Al2O3

Article abstract of DOI:10.1039/c6nj01076h

TaO_x grafted onto Al₂O₃ is investigated for C-N bond cleavage of amines under H₂ pressure. Heteroaromatics such as aniline are stoichiometrically denitrogenated at high temperature, while cyclohexylamine is catalytically denitrogenated. UV-visible and X-ray photoelectron spectroscopy indicate the formation of a stable Ta-N species in the former case.

Full text of DOI:10.1039/c6nj01076h

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C–N bond hydrogenolysis of aniline and
cyclohexylamine over TaO –Al O †
x
2 3
Cite this:DOI: 10.1039/c6nj01076h
ab
a
b
Mark Bachrach, Tobin J. Marks* and Justin M. Notestein*
Received (in Nottingham, UK)
7
th April 2016,
Accepted 23rd May 2016
DOI: 10.1039/c6nj01076h
www.rsc.org/njc
TaO
x
2 3
grafted onto Al O is investigated for C–N bond cleavage of benzenes, and cyclohexenes) as well as in situ X-ray absorption
amines under H
2
pressure. Heteroaromatics such as aniline are spectroscopy (XANES/EXAFS). The key step in the denitrogenation
stoichiometrically denitrogenated at high temperature, while process was proposed to be a partial hydrogenation of 2-propyl-
cyclohexylamine is catalytically denitrogenated. UV-visible and X-ray aniline to 1,6-dihydro-2-propylaniline by the Pd nanoparticle,
photoelectron spectroscopy indicate the formation of a stable Ta–N followed by transfer of this reactive intermediate to the metal
species in the former case.
oxide for deamination. We also showed that the MO
materials are significantly more active for denitrogenation than
Highly dispersed, supported Ta complexes have previously been bare Al
shown to carry out N–H and N–N bond activation under fairly Here, we further report on the deamination step with TaO
mild conditions. Basset et al. have shown that Ta–SiO derived as a model material in this series. This material is prepared by
from the organometallic precursor Ta(CH –CMe
x 2 3
–Al O
2 3
O .
x 2 3
–Al O
2
2
)
3 3
(Q CH– grafting Ta(acac)(OEt)
4
onto Al
2
O
3
at a 1.4 wt% Ta loading and
À1
CMe ) is able to cleave the N–H bond in NH (108 kcal mol
)
calcining the resulting complex in air at 500 1C. This yields an
3
3
1
,2
at room temperature as well as cleave the N–N triple bond in undercoordinated, isolated Ta(V)-oxo species supported on a
À1
2,3
2 2 3
N (226 kcal mol ) at 250 1C. Here we study related systems positively charged Al O surface as shown by our previously
for C–N bond cleavage. This reaction is of increasing importance in reported XANES/EXAFS, diffuse reflectance UV-visible spectro-
the hydrotreating process for removing heteroatom impurities scopy (DRUV-vis), and Zeta potential measurements. We now
(N,O,S) from crude fuels. Hydrodenitrogenation (HDN) is often compare the relative activity for C–N bond cleavage in the
studied with quinoline as a model reagent since it is representative deamination of cyclohexylamine, aniline, and pentafluoraniline,
4
–6
of a typical impurity within transportation fuel.
However, and postulate a deamination mechanism.
there are relatively few studies of HDN over well-defined model
The strength of the C–N bond impacts the minimum tem-
perature at which C–N bond cleavage occurs, the relative propensity
7
,8
catalytic materials.
Pd nanoparticles supported on Al
2
O
3
are catalysts for quino- for reactant desorption vs. conversion, and the ability of the material
line hydrogenation and C–N bond cleavage. We previously to turnover catalytically. Here, the material was treated under 10%
reported that adding low levels of Ti-, Ta-, or Mo-oxides to the /Ar for 2 h, followed by pulses of aniline, cyclohexylamine, or
Al support significantly increases the selectivity to aromatic pentafluoraniline in hexane at 120 1C under He until the surface
products (e.g. n-propylbenzene instead of n-propylcyclohexane).
This selectivity change results in a 10–15% reduction in the 10% H
amount of H required to remove the same number of N atoms. pentafluoroaniline) (Table 1).
O
2
2 3
O
8
is saturated. Temperature programmed reaction (TPRx) under
/N is carried out from 150–400 1C (or 500 1C for
2
2
2
3
In that report we suggested a mechanism for denitrogenation
with the Pd/MO materials based on our experiments with mol ) is relatively facile, and the vast majority (97%, Table 1)
Cleavage of the sp C–N bond in cyclohexylamine (87.6 kcal
À1 2
x
downstream intermediates (substituted anilines, cyclohexylamines, of the adsorbed reactant undergoes deamination before it can
desorb from the surface. As shown in Fig. 1 (also Table S1,
a
Department of Chemistry, Northwestern University, 2145 Sheridan Road,
ESI†), a very small percentage of the cyclohexylamine is dehydro-
Evanston, Illinois 60208, USA. E-mail: t-marks@northwestern.edu
genated to aniline, while C–N bond cleavage gives predominantly
b
Department of Chemical & Biological Engineering, Northwestern University,
cyclohexene (70%) and benzene (29%) with o1% cyclohexane
2
145 Sheridan Road, Evanston, Illinois 60208, USA.
E-mail: j-notestein@northwestern.edu
Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj01076h hexane ratio (36 : 1) approaches the equilibrium ratio (23 : 1),
evolving primarily between 250 and 300 1C. The benzene : cyclo-
9
†
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Table 1 Product distribution from amine desorption and denitrogenation
150–350 1C
400–500 1C
Reactant
desorption (%)
C–N bond
cleavage (%)
Reactant
desorption (%)
C–N bond
cleavage (%)
a
b
T range
molNH2/molTa
molHC/molTa
C
6
C
6
C
6
H
11–NH
–NH
–NH
2
3.0
99.5
95.4
96.4
0
0
o0.1
0.3
4.6
0.6
0.2
0
3
142
123
110
0.3
0
H
5
2
F
5
2
a
b
Total moles reactant desorbed over the entire temperature range, per mol Ta. Total moles C–N bond cleavage products (cyclohexene,
cyclohexane, benzene) detected over the entire temperature range, per mol Ta.
Scheme 1 Proposed steps in the stoichiometric denitrogenation of
aniline (left) and catalytic denitrogenation of cyclohexylamine (right).
Fig. 1 Temperature programmed reaction of cyclohexylamine over
TaO –Al from 150–400 1C. The y-axis spans three distinct scales.
0
Reactant amines are chemisorbed at the Lewis-acidic, d Ta center and
x
2 3
O
2 3
physisorbed on Lewis and Brønsted sites on the residual Al O surface.
Cyclohexylamine undergoes b-elimination to form a Ta-amido at low
temperatures and is displaced by other adsorbed cyclohexylamine substrate
however the cyclohexene produced exceeds the expected equilibrium
amount by orders of magnitude. This product distribution suggests direct sp C–N bond cleavage to give an unreactive Ta-imido; this occurs at
that cyclohexylamine undergoes b-elimination to cyclohexene as
2 3
molecules that migrate from the Al O surface. Aniline denitrogenates via
2
temperatures above which most of the aniline has desorbed.
the primary product (Scheme 1), which then slowly approaches
the benzene/ cyclohexene/ cyclohexane equilibrium. Direct C–N
bond hydrogenolysis is inconsistent with the high cyclohexene
The electron-withdrawing aromatic ring of pentafluoroani-
selectivity. In contrast, cyclohexylamine denitrogenation is catalytic, line and the geometric influence of the C–F groups further
1
0
with 110 molecules of product detected per Ta atom (Table 1), shorten and strengthen the C–N bond. As expected then, this
consistent with a mechanism where amines adsorbed on the molecule undergoes only intact desorption, with no C–N bond
bare alumina surface readily diffuse to the Ta sites where they cleavage products detected up to 500 1C.
undergo deamination.
Unreactive Ta-imido species were observed as products by
1
,3
3 2
In contrast to cyclohexylamine, aniline C–N bond cleavage Basset et al. in NH or N cleavage by supported Ta, and we
involves a significantly stronger sp C–N bond (102.6 kcal mol ).
2
À1
2
propose (Scheme 1) to form similar Ta-imido and Ta-arylimido
Substoichiometric formation of exclusively benzene (0.3 per Ta, species during aniline denitrogenation. The used material from
Table 1) is detected at 400 1C, but the vast majority of the aniline aniline TPRx to 400 1C was subsequently studied by DRUV-vis
is desorbed intact as a broad feature at lower temperatures (Fig. S1, spectroscopy, X-ray photoelectron spectroscopy, and temperature
ESI†). Even at 400 1C, where 499% of the aniline had previously programmed reduction with H
desorbed, more aniline desorbs intact than is denitrogenated, species.
indicating that the surface species formed from aniline denitro- A Tauc plot of the direct band gap derived from the DRUV-vis
genation is chemically inert even at these conditions. Similarly, spectra (Fig. 2) shows that the used TaO –Al material has three
2
to provide evidence for such a
x
2 3
O
we previously observed catalytic denitrogenation of methyl- optical edges at 4.7 eV, 3.9 eV and 3.8 eV. All three edges are
cyclohexylamine but only stoichiometric denitrogenation of significantly lower than those of the freshly calcined material
2
-propylaniline over TaO
x
–Al
batch reactor. The lack of catalytic turnover even in the presence (bandgap = 8.8 eV). Bulk Ta
of excess reactant requires the formation of an inert surface is approximately the correct energy, but prior XANES/EXAFS
species as a co-product of aniline denitrogenation. showed no change in Ta coordination number indicative of the
2 3 2 2 3
O under high H pressures in a (5.4 eV). Al O is optically transparent to much higher energies
8
11
2 5
O has a bandgap of 4.1 eV which
1
2
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The lower energy transitions (edges below 4.0 eV) are then
assigned as ligand-to-metal charge transfer bands (LMCT) from
the imido or arylimido itself, as reported by Wolczanski et al.
Comparison of the XPS spectra of the freshly calcined TaO –Al O
x
2 3
material with the material from the aniline TPRx to 400 1C
shows the appearance of a N 1s peak at 399 eV on the shoulder
of the Ta 4p3/2 orbital peak at 404 eV (Fig. 3A). The Ta 4f5/2 peak
is also slightly redshifted (Fig. 3B) due to less energetically
demanding electron ejection in TaQNR (23.0 eV) than in TaQO
1
4,15
(23.5 eV).
Temperature programmed reduction (TPR) in H was also
2
carried out with the freshly calcined and used materials (Fig. 4).
Both traces show three broad reduction events. For the calcined
catalyst, the first reduction has an onset temperature of
B200 1C and a maximum near 300 1C, corresponding to peak
x 2 3
Fig. 2 Tauc plot for the direct band gap of TaO –Al O
materials after activity in cyclohexylamine denitrogenation. In earlier studies,8
calcination and aniline TPRx to 400 1C. Optical edges at 5.4 eV (calcined)
and 4.7 eV (after 400 1C reaction) correspond to alumina oxygen-to-Ta
transitions, while optical edges at 3.9 eV and 3.8 eV correspond to LMCT
transitions from the imido.
no reduction of the Ta oxidation state was observed at 275 1C in
, so this first feature that begins near 200 1C is assigned to
addition of H2 across the TaQO bond. Likewise, a second
feature with an onset of B500 1C corresponds to a similar
H
2
amount of H consumption, and is assigned to hydrogenolysis
aggregation of the TaO domains. Bulk Ta N has a much lower of the Ta–OH bond, giving a Ta hydride. Finally, the Ta atoms are
2
8
x
3 5
1
2
bandgap (2.1 eV) than bulk Ta
2
O
5
supporting the hypothesis reduced to Ta nanoparticles or isolated Ta(III) sites above B670 1C.
For the used catalyst, there is a significant shift in the
that the shift in bandgap from the higher bandgap in the freshly
calcined material to the lower bandgap in the used material temperature of the first reduction event, largely because the
indicates the transformation of isolated Ta–oxo to isolated material has already seen reducing conditions up to 400 1C.
Ta-imido sites. In contrast to these bulk materials, which do The onset of H uptake is at B450 1C, with a maximum of B530 1C.
2
not appear to be a good spectroscopic fit for the formed By analogy to above, this feature is assigned as reduction of
structures, Wolczanski et al. reported a (silox)TaQNPh complex TaQNH and TaQNPh groups on the surface. The increase in
whose geometry is very similar to that drawn in Scheme 1 and temperature necessary for H2 addition across the Ta-imido
1
3
which has very similar optical features. This complex was relative to the Ta–oxo is attributed to the markedly higher
shown to have two primary features at 246 nm (peak maximum Ta-imido bond order compared to that of a Ta–oxo bond.1
at 5.0 eV) and 285 nm (peak maximum at 4.4 eV), very similar to A second feature immediately follows, which we assign to
the features in Fig. 3 with peak maxima at 4.9 eV (edge at 3.9 eV) hydrogenolysis of the remaining Ta–NHR bonds (amines and
and 4.3 eV (edge at 3.8 eV). The complex also shows a high amidos). Finally, reduction of Ta again has an onset of B680 1C.
energy feature at 215 nm (peak maximum at 5.8 eV) from the As expected from stoichiometry, all features are similar in area.
siloxide ligand, similar to the indicated feature with a peak
6,17
maximum at 5.7 eV (edge at 4.7 eV). We thus assign the high
energy feature in the experimental spectrum (edge at 4.7 eV) to
an LMCT from alumina oxygens to an isolated TaQNH or
TaQNPh center, which is shifted to lower energy than the
analogous interactions with a TaQO center (edge at 5.4 eV) in
the freshly calcined material, due to the significantly more
electron-donating character of the imido or arylimido group.
Fig. 3 Fitted XPS spectra of TaO
to 400 1C. (A) N 1s orbital (399 eV) and Ta 4p3/2 orbital (404 eV) and (B) Ta
f orbital.
x
–Al
2
O
3
after calcination and aniline TPRx
Fig. 4 Temperature programmed reduction in H
TaO –Al (bottom) and the material after aniline TPRx to 400 1C (top).
Note that after 800 1C, the x-axis corresponds to a 3 h isothermal hold.
2
of the calcined material
x
2 3
O
4
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Overall, these results illustrate how the dispersed metal PTFE as a perfect reflector. Optical edges were calculated based
2
oxide sites contribute to the cleavage of C–N bonds under on the intercept of the tangent line in a plot of [F(R) Â hn] vs. hn.
hydrogenolysis conditions. These results require that the direct
2
cleavage of the sp C–N bond in aniline proceeds via a different
mechanism than that of the sp C–N bond in cyclohexylamine.
Acknowledgements
3
Finally, this conclusion indicates that reactivity enhancements The authors acknowledge the ACS Petroleum Research Fund
in quinoline hydrodenitrogenation over Pd/TaO
00 1C are consistent with our previously-proposed mechanism JMN) and 86ER1311 (MB, TJM) for funding. The authors also
of partial ring hydrogenation over Pd, followed by deamination acknowledge N. M. Schweitzer and L. M. Savereide for technical
x 2 3
–Al O below and the DOE Office of Basic Sciences Grants SC-0006718 (MB,
4
2
assisted by the Ta sites, rather than direct sp C–N bond cleavage.
assistance. This work made use of the Keck-II facility (NUANCE
Center – Northwestern University), which has received support
from the W. M. Keck Foundation, Northwestern’s Institute for
Nanotechnology’s NSF-sponsored Nanoscale Science & Engineering
Center (EEC-0118025/003), both programs of the National Science
Experimental
Material synthesis
TaO –Al O was synthesized by our previously published Foundation; the State of Illinois; and Northwestern University. The
x
2 3
8
method. Briefly, Ta(acac)(OEt)
4
was grafted onto Al
2
O
3
in CleanCat Core facility acknowledges funding from the Department
A; 165 m g ) at a 1.4 wt% metal loading of Energy DE-FG02-03ER15457 used for the purchase of the
–Al
. The material was then calcined Altamira AMI-200.
for 6 h in air at 500 1C in a muffle furnace to form the TaO
Al species prior to loading in the reactor.
2
À1
toluene (Selecto Al
2 3
O
to form Ta(acac)(OEt)
2
2 3
O
x
–
2 3
O
Notes and references
Hydrodenitrogenation
TaO –Al (300 mg) was loaded into a quartz u-tube reactor in
an Altamira AMI-200 reactor system. The material was calcined
at 500 1C under 25 sccm 10% O /Ar for 2 h. 300 mL reactant
cyclohexylamine, aniline, or pentafluoraniline) was then slowly
1
P. Avenier, A. Lesage, M. Taoufik, A. Baudouin, A. De Mallmann,
S. Fiddy, M. Vautier, L. Veyre, J. M. Basset, L. Emsley and
E. A. Quadrelli, J. Am. Chem. Soc., 2007, 129, 176–186.
x
2 3
O
2
2 Y.-R. Luo, Comprehensive Handbook of Chemical Bond Energies,
CRC Press, Boca Raton, 2007.
(
injected over an hour under He at 120 1C. The pentafluoroaniline
was diluted in hexanes for injection since it is a solid at room
temperature. The reactor was then purged with He at 120 1C for
an additional 2 h to allow for loosely physisorbed reactant to be
3 P. Avenier, M. Taoufik, A. Lesage, X. Solans-Monfort, A. Baudouin,
A. De Mallmann, L. Veyre, J. M. Basset, O. Eisenstein, L. Emsley
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removed from the material. The reactor was then heated in 10%
À1
H /N
2 2
at 30 sccm by stepwise ramps at 5 1C min and then 3 h
holds at 150, 200, 250, 300, 350, and 400 1C for the aniline and
cyclohexylamine reactions, and 450 1C and 500 1C for the
pentafluoroaniline reaction. All products were quantified with
a Pfeiffer process mass spectrometer. m/z assignments: cyclo-
hexylamine = 99, aniline = 93, pentafluoraniline = 183, penta-
fluorocyclohexylamine = 189, cyclohexene = 82, benzene = 78,
cyclohexane = 84, pentafluorocyclohexene = 172, pentafluoro-
cyclohexane = 174, pentafluorobenzene = 168.
9 G. G. Janz, J. Chem. Phys., 1954, 22, 751.
1
1
0 P. Wojciechowski, J. Fluorine Chem., 2013, 154, 7–15.
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Materials characterization
Temperature programmed reduction used 1.00 g material in an 12 M. Harb, P. Sautet, E. Nurlaela, P. Raybaud, L. Cavallo,
Altamira AMI-200 with a TCD detector. Reductions were run
K. Domen, J. M. Basset and K. Takanabe, Phys. Chem. Chem.
Phys., 2014, 16, 20548–20560.
À1
with a 3 1C min ramp under 10% H /N until 800 1C and then
2
2
held at 800 1C for 3 h. An acetone/dry ice bath trapped H O, 13 E. B. Hulley, J. B. Bonanno, P. T. Wolczanski, T. R. Cundari
2
NH
measured H
.1. XPS was performed on a Thermo Scientific ESCALAB 250XI
with Al K-alpha radiation with a flood gun. Samples were 15 W. J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J. N. Kondo,
3
, and other condensables to ensure that the TCD only
consumption. Data were analyzed in OriginLab 14 Q. S. Gao, S. N. Wang, Y. C. Ma, Y. Tang, C. Giordano and
M. Antonietti, Angew. Chem., Int. Ed., 2012, 51, 961–965.
and E. B. Lobkovsky, Inorg. Chem., 2010, 49, 8524–8544.
2
9
referenced to carbon with a binding energy of 284.8 eV. Samples
were fitted to a Gaussian in OriginLab 9.1. UV-visible spectra
M. Hara, M. Kawai, Y. Matsumoto and K. Domen, J. Phys.
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from 800–200 nm were collected on a Shimadzu 3600 equipped 16 T. R. Cundari, Chem. Rev., 2000, 100, 807–818.
with a Harrick Praying Mantis Diffuse Reflectance accessory. 17 T. I. Gountchev and T. D. Tilley, J. Am. Chem. Soc., 1997, 119,
Kubelka Munk pseudoabsorbances were calculated relative to
12831–12841.
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