Titanium methyl tamed on silica: Synthesis of a well-defined pre-catalyst for hydrogenolysis of: N-alkane

Article abstract of DOI:10.1039/d0cc05816e

Alkylation of Ti(CH3)2Cl21 by MeLi gives the homoleptic Ti(CH3)42 for the first time in the absence of any coordinating solvent. The reaction of 2 with silica pretreated at 700 °C (SiO2-700) gives two inequivalent silica-supported Ti-methyl species 3. Complex 3 was characterized by IR, microanalysis (ICP-OES, CHNS, and gas quantification), and advanced solid-state NMR spectroscopy (1H, 13C, DQ, TQ, and HETCOR). The catalytic activity of the pre-catalyst 3 is investigated in low-temperature hydrogenolysis of propane and n-butane with TONs of 419 and 578, respectively. This journal is

Full text of DOI:10.1039/d0cc05816e

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Titanium Methyl Tamed on Silica: Synthesis of a Well-defined Pre-
catalyst for Hydrogenolysis of n-alkane
a
a
b
Aya Saidi, Walid Al Maksoud, Manoja K. Samantaray, *a Edy Abou-Hamad, and Jean-Marie
Received 00th January 20xx,
Accepted 00th January 20xx
Basset *a
DOI: 10.1039/x0xx00000x
Alkylation of Ti(CH3)2Cl2 1 by MeLi gives the homoleptic Ti(CH3)4 2
for the first time in the absence of any coordinating solvent. The
reaction of 2 with silica pretreated at 700 °C (SiO2-700) gives two
inequivalent silica-supported Ti-methyl species 3 (Scheme 2).
Complex 3 was characterized by IR, microanalysis (ICP-OES, CHNS,
and gas quantification), and advanced solid-state NMR
spectroscopy (1H, 13C, DQ, TQ, and HETCOR). The catalytic activity
intermediate by isotopically labeling the methyls attached to
metal center.9, 10 In this regard, we utilized various homoleptic
metal methyl homogeneous complexes, which are very rare,
very reactive in the homogeneous phase, and almost not
suitable for catalysis because of their unstable nature at room
temperature.11-14 To make them stable for catalysis, we used
Surface Organometallic Chemistry (SOMC) approach, which
stabilizes molecular species otherwise unstable in solution.
Metal alkyl can react with surface silanols and form a stable
catalyst at room temperature, which makes them
comparatively easier to use for catalysis.11, 15
of the pre-catalyst
3 is investigated in low-temperature
hydrogenolysis of propane and n-butane with TONs of 419 and 578,
respectively.
Conversion of inexpensive waxes, generated from the Fischer-
Tropsch process to valuable chemicals/fuels, is a value-added
process.1 Mainly, the hydrocracking process was used for the
Herein, we disclose for the first time the synthesis and
characterization of a homoleptic Ti(CH3)4 complex, its grafting
on silica partially dehydroxylated at 700 oC (SiO2-700),
characterization of the grafted material, and its application in
hydrogenolysis of propane and n-butane.
3
cracking of paraffinic waxes into useful chemicals.2, The
primary obstacles in this process are the high temperature of
reaction and the fast deactivation of the catalyst with the
formation of coke.4 Many efforts were undertaken to reduce
the temperature and carry out the conversion of waxes to
valuable chemicals effectively.5 Since the discovery of the well-
defined silica-supported Zr hydride, [(≡Si-O)3ZrH], which was
able to catalyze the hydrogenolysis of a given alkane to lower
homologs and the de-polymerization (Ziegler-Natta de-
polymerization) of LLDPE to fuel range alkanes at relatively low
temperature (150 oC), a new door was opened up for research
in this new field.6, 7
Synthesis of an ether bound (Et2O)Ti(CH3)4 without sufficient
proof is already known in the literature,16, 17 but for our catalytic
purposes, we wanted a homoleptic Ti without any coordination
of the solvent as the coordinated solvent makes them less
electropositive and eventually less effective for C-H bond
activation. The difficulty in preparing such metal complexes is
already reported in the literature18, 19 only partial methylated
species of metals of group 4 are known so far20-22. Therefore, we
choose a soluble metal precursor (TiCl4) and a non-coordinated
solvent (pentane and dichloromethane) for this reaction. It
appeared that the reaction was not straightforward and
required two steps (Scheme 1).
Recently, we observed that moving from metal neopentyl to
metal methyl, the reactivity in alkane metathesis reaction
increased several-fold.8 Additionally, it was comparatively easy
to characterize the catalyst and understood the possible
reaction mechanism by either DFT or by isolating the active
H3C
CH3
Ti
2
CH3
H3C
a. King Abdullah University of Science &Technology, Physical Science and
Engineering, KAUST, Thuwal, 23955-6900, Saudi Arabia. E-Mail:
-80 °C to -60 °C
Pentane
CH3Li
jeanmarie.basset@kaust.edu.sa, manoja.samantaray@kaust.edu.sa
b. Imaging and Characterization Core lab, King Abdullah University of Science and
Technology, KAUST, Thuwal, 23955-6900, Saudi Arabia
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
H3C
Cl
Cl
Cl
Cl
Cl
Cl
Pentane/dichloromethane
-80 °C to -40 °C
Ti
Ti
+
Zn(CH3)2
H3C
1
Scheme 1. Synthesis of TiCl2(CH3)2 1 and Ti(CH3)4 2.
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H3C
O
CH3
CH3
H3C
In the first step, the TiCl4 precursor is reacted with one
equivalent of Zn(CH3)2 in mixture of pentane and
H
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Ti
Ti
CH3
Pentane
-60°C, 4h
DOI: 10.1039/D0CC05816E
H3C
H3C
CH3
CH3
O
a
CH3
O
O
O
O
Ti
+
O
Si
+
O
O
dichloromethane (1/1) at a temperature range between -80 °C
to -40 °C for 4 hours to yield an orange solution of TiCl2(CH3)2.16
In the end, the reaction mixture was filtered, and the filtrate
was dried under vacuum. An aliquot of TiCl2(CH3)2 was taken in
an NMR tube and was mixed with CD2Cl2 and analyzed by liquid-
state NMR (1H, 13C, HSQC, and 35Cl) at -40 °C. 1H and 13C NMR
spectra (Figures S1A and B) display peaks at 2.6 ppm and 99.1
ppm, respectively, attributed to the hydrogen and carbon
atoms of the methyl group of Ti-CH3. This was further proved by
the correlation between both of the peaks observed in the
HSQC spectrum (Figure S1C). Additionally, 35Cl NMR was
conducted, and a peak at 601.1 ppm confirms the presence of
Ti-Cl (Figure S1D).23-25
Si
Si
O
Si
Si
Si
O
O
- CH4
O
O
O
O
O
O
O
O
O
3
Scheme 2. Reaction of Ti(CH3)4 with silica-700 at - 60 ᴼC.
To confirm the complete substitution of chlorine atom by
methyl ligand, we characterize the sample by 35Cl NMR (Figure
S2). The 35Cl spectrum of the complex does not show any
chlorine peak (which usually comes in between 500-800 ppm
depending upon the number of chlorine atoms attached to Ti 23,
24, 27) except that of the deuterated solvent CD2Cl2. This further
corroborates the successful synthesis of a homoleptic Ti(CH3)4
complex without any coordinated solvent.
In the second step, the TiCl2(CH3)2 reacted with an excess of
MeLi (50 % enriched) in pentane at a temperature range
between -80°C to -60°C for 3 hours, affording a red color
solution of the desired product Ti(CH3)4 (Scheme 1). The
complex is highly sensitive to oxygen, moisture, and
temperature; black fumes were observed immediately when
the temperature was increased above -60 °C, which makes it
very difficult to isolate the complex. Therefore, at the end of the
reaction, the reaction mixture is filtered and used immediately
for grafting on oxide surfaces.
Grafting of Ti(CH3)4 was performed in situ by reacting an excess
of 2 with partially dehydroxylated silica (silica pretreated at 700
oC, SiO2‑700) at −60 °C in pentane under an inert atmosphere of
argon for 4 hours (Scheme 2), followed by washing with
pentane three times to remove the excess of 2 and to dry under
high vacuum (10−5 mbar). In the end, a dark brown powder was
obtained. The grafting process generates the formation of two
environmentally different [(≡Si-O-)TiMe3] species resulting in
the interaction with the silanol groups and Si-O-Si bridges
(Scheme 2). As expected, from previous work carried out with
WMe6/silica, once grafted, the Ti complex is much more stable
and prone to more in-depth characterization.28, 29
In an NMR tube, 0.2 mL of TiCl4 mixed with one equivalent of
ZnMe2, pentane, and CD2Cl2 at -80 °C and let the reaction to run
for 1 hour, then an excess of 13C enriched MeLi was added to it,
and the reaction continued in the NMR tube for another 3 hours
at -60 °C. The progress of the reaction was analyzed by liquid-
The grafted complex 3 was characterized by FT-IR spectroscopy,
elemental analysis, methane quantification, and solid-state
NMR spectroscopy (1H, 13C, DQ, TQ, and HETCOR). The FTIR
spectrum was recorded at 25 °C (Figure S3); the IR peak of the
isolated silanols at 3747 cm−1 completely disappeared, and
groups of new bands appeared in 3005-2819 and 1454 cm−1
region. These bands are respectively assigned to the ν(CH) and
δ(CH) vibrations of the methyl ligands coordinated to titanium.
Elemental analysis of 3 was conducted and it gave 1.8% Ti, 1.3%
C and 0.33% H, with a ratio of C/Ti = 2.88 and H/Ti = 8.77. These
results further prove the formation of complex 3.
1
state NMR.(1H, 13C, and 35Cl). In the H and 13C NMR spectra
(Figures 1A and B), we observed that the peaks of the 1H and 13
C
were shifted from 2.6 and 99.1 ppm (as observed in the case of
TiCl2(CH3)2) to 1.99 and 72.9 ppm, respectively, for the newly
formed Ti(CH3)4 complex. This is understandable as complete
alkylation occurs on the Ti center causes an upfield shift of the
carbon and proton peaks of methyl ligands. The attribution of
the obtained chemical shifts in the liquid-state NMR is realized
by comparing them with those of the reported ether bound
methyl titanium chlorides16, 26
.
Figure 1. (A) 1H and (B) 13C NMR spectra of Ti(CH3)4 in CD2Cl2 at
213 K
2 | J. Name., 2012, 00, 1-3
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Furthermore, the structure of 3 was characterized by solid-state
NMR spectroscopy. The H magic-angle spinning (MAS) NMR
View Article Online
DOI: 10.1039/D0CC05816E
1
spectrum of 3 displays one signal at 2.0 ppm (Figure 2A) that
autocorrelated on the diagonals of both double-quantum (DQ)
and triple-quantum (TQ) NMR spectra at 4.0 and 6.0 ppm,
respectively (Figures 2B and C) which proves that they refer to -
CH3 group. The 13C cross-polarization magic-angle spinning (CP-
MAS) NMR spectrum shows a main peak at 76 ppm along with
a small signal at 61 ppm (Figure 2D) that correlate with the
1
proton at 2 ppm as indicated in the 2D H-13C HETCOR NMR
spectrum (Figure 2E). These peaks are assigned to the carbon
of the methyl of the titanium species 3 (both with and without
interaction with the oxygen atom of the siloxane bridge). This
type of interaction was already reported by our group with well-
defined [(≡Si-O-)TaCl2Me2] species.29 The 13C signal in solid-state
NMR is slightly shifted compared to the carbon peak in the
solution NMR spectrum from 72.9 to 76 ppm; this is
understandable as we have replaced at least one methyl group
with an electronegative oxygen atom.
triple-quantum (TQ) spectrum of 3 (Acquired on a 600 MHz
NMR spectrometer at 22 kHz MAS spinning frequency with a
back-to-back recoupling sequence, number of scans 128,
repetition delay 5 s, number of t1 increments 128), (D) 13C
CP/MAS NMR spectrum of 3 (Acquired on a 400 MHz NMR
spectrometer (9.4 T) with a 10 kHz MAS frequency, 20K scans, a
4 s repetition delay, and a 2 ms contact time. Exponential line
broadening of 80 Hz was applied before Fourier
transformation), (D) 2D 1H-13C CP/MAS dipolar HETCOR
spectrum of 3 (Acquired on a 400 MHz NMR spectrometer (9.4
T) with a 10 kHz MAS frequency, 2048 scans per t1 increment, a
4 s repetition delay, 64 individual t1 increments and a 0.2 ms
contact time).
Additionally, the structure of 3 was confirmed by the gas
quantification method after hydrolysis. The hydrolysis reaction
of 50 mg of complex 3 using degassed water produced 0.937
mmol/g of methane; theoretically, we should get 1.065
mmol/g, which is consistent with the formation of [(≡SiO-
)TiMe3] surface species.
After the characterization of complex 3, it was tested for
hydrogenolysis of propane and n-butane. In
a typical
experiment, complex 3 (0.088 mmol of Ti) was loaded in a
continuous flow reactor, and a mixture of propane or n-butane
(3.5 mL/min) and hydrogen (9 mL/min) was passed through the
catalytic bed at 180°C. All the reactions were conducted at the
atmospheric pressure (P= 1 bar). The conversion of the reactant
and the gas products were analyzed every 20 min by on-line gas
chromatography.
Figure 3. Evolution of conversion (Blue curve) and
corresponding TON over time (Red curve) for hydrogenolysis
reaction of (A) propane (B) n-butane using 3. Evolution of
product selectivities over time for hydrogenolysis reaction of (C)
propane (D) n-butane using 3.
We achieved a maximum conversion of 85.9% after 40 min and
a TON of 419 for propane hydrogenolysis and maximum
conversion of 99.8% and a TON of 578 for n-butane
hydrogenolysis at our contact time (Figures 3A and B).
During the hydrogenolysis reaction of propane, we observed
that the selectivities of methane and ethane remain similar
(50:50) (Figure 3C), which corroborates the previously reported
results that β-methyl transfer is only possible in the case of Ti.30
whereas, in the case of n-butane, methane, ethane, and
propane (Figure 3D) were observed. From the previously
reported data, we know that hydrogenolysis can take place
either by α-alkyl or β-alkyl migration, depending on the metal
atom.31, 32 Our results indicate that in our catalytic system, we
only observe β-alkyl transfer (as we observed ethane, propane,
along with methane instead of only methane) followed by the
formation of Ti(carbene)(alkyl) intermediate and finally,
hydrogenolysis to form lower alkanes (Scheme S1). In the
beginning, we observed that the selectivity for ethane is higher
than other products, and ethane was considered to be the
primary product. However, with time on stream, the evolution
Figure 2. Solid-state NMR characterization of grafted Ti(CH3)4
onto SiO2-700; (A) One-dimensional (1D) 1H MAS solid-state NMR
spectrum of 3 (Acquired on a 600 MHz NMR spectrometer (14.1
T) with a 22 kHz MAS spinning frequency, a repetition delay of
5 s, and 8 scans), (B) Two-dimensional (2D) 1H-1H double-
1
quantum (DQ) spectrum of 3, (C) Two-dimensional (2D) H-1H
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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13.
14.
M. K. Samantaray, S. Kavitake, N. Morlanes, E. Abou-
of methane and propane occurs. At the end of the reaction, we
observed that the selectivity for propane, methane, and ethane
remains similar. It is surprising to see that the selectivity for
propane is similar to that observed for ethane and methane. We
believe this is because of the low contact time of the reactant
to the catalyst, and the concentration of the propane is much
less to react back with the catalyst.
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DOI: 10.1039/D0CC05816E
Soc., 2017, 139, 3522-3527.
M. K. Samantaray, R. Dey, S. Kavitake, E. Abou-Hamad, A.
Bendjeriou-Sedjerari, A. Hamieh and J. M. Basset, J. Am.
Chem. Soc., 2016, 138, 8595-8602.
J. M. Basset, C. Coperet, D. Soulivong, M. Taoufik and J. T.
Cazat, Acc. Chem. Res., 2010, 43, 323-334.
S. Kleinhenz and K. Seppelt, Chem. Eur. J., 1999, 5, 3573-
3580.
H. Berthold and G. Groh, Z. Anorg. Allg. Chem., 1963, 319,
230-235.
H. Makio, T. Oshiki, K. Takai and T. Fujita, Chem. lett., 2005,
34, 1382-1383.
H. Makio, T. Ochiai, J.-i. Mohri, K. Takeda, T. Shimazaki, Y.
Usui, S. Matsuura and T. Fujita, J. Am. Chem. Soc., 2013,
135, 8177-8180.
Y. Gao, M. D. Christianson, Y. Wang, J. Chen, S. Marshall, J.
Klosin, T. L. Lohr and T. J. Marks, J. Am. Chem. Soc., 2019,
141, 7822-7830.
I. E. Nifant’ev, P. V. Ivchenko, V. V. Bagrov, S. M. Nagy, S.
Mihan, L. N. Winslow and A. V. Churakov, Organometallics,
2013, 32, 2685-2692.
D. Balboni, I. Camurati, A. C. Ingurgio, S. Guidotti, F.
Focante and L. Resconi, J. Organomet. Chem., 2003, 683, 2-
10.
15.
16.
17.
18.
19.
To our knowledge, we synthesized the first homoleptic Ti(CH3)4
complex without any coordinating solvent. The homogeneous
complex Ti(CH3)4 is very reactive and starts to decompose at a
temperature above -60 °C, which makes this complex very
difficult to study and use for any catalytic reaction. For the first
time, it was stabilized on partially dehydroxylated silica (SiO2-
700) and was characterized at the molecular level using advanced
solid-state NMR, IR and, gas quantification methods. The well-
20.
21.
22.
defined silica-supported pre-catalyst
3
was used for
hydrogenolysis of propane and n-butane with a TON of 419 and
578, respectively. Other aspects of this complex are under study
in our laboratory.
Conflicts of interest
There are no conflicts to declare.
23.
24.
M. A. Fedotov, O. L. Malkina and V. G. Malkin, Chem. Phys.
Lett., 1996, 258, 330-335.
A. J. Rossini, R. W. Mills, G. A. Briscoe, E. L. Norton, S. J.
Geier, I. Hung, S. Zheng, J. Autschbach and R. W. Schurko,
J. Am. Chem. Soc., 2009, 131, 3317-3330.
Y. Saito, Can. J. Chem., 1965, 43, 2530-2534.
S. Berger, W. Bock, G. Frenking, V. Jonas and F. Mueller, J.
Am. Chem. Soc., 1995, 117, 3820-3829.
E. S. Blaakmeer, F. J. Wensink, E. R. H. van Eck, G. A. de Wijs
and A. P. M. Kentgens, J. Phys. Chem. C, 2019, 123, 14490-
14500.
M. K. Samantaray, R. Dey, E. Abou-Hamad, A. Hamieh and
J. M. Basset, Chem. Eur. J., 2015, 21, 6100-6106.
Y. Chen, E. Callens, E. Abou-Hamad, N. Merle, A. J. P. White,
M. Taoufik, C. Coperet, E. Le Roux and J. M. Basset, Angew.
Chem. Int. Ed., 2012, 51, 11886-11889.
C. Rosier, G. P. Niccolai and J. M. Basset, J. Am. Chem. Soc.,
1997, 119, 12408-12409.
Notes and references
1.
2.
3.
4.
5.
6.
C. Bouchy, G. Hastoy, E. Guillon and J. A. Martens, Oil Gas
Sci. Technol., 2009, 64, 91-112.
C. Gambaro, V. Calemma, D. Molinari and J. Denayer, AIChe
J., 2011, 57, 711-723.
J. Lee, S. Hwang, J. G. Seo, S. B. Lee, J. C. Jung and I. K. Song,
J. Ind. Eng. Chem., 2010, 16, 790-794.
J. W. Gosselink and W. H. J. Stork, Ind. Eng. Chem. Res.,
1997, 36, 3354-3359.
T. Hanaoka, T. Miyazawa, K. Shimura and S. Hirata,
Catalysts, 2015, 5, 1983-2000.
J. Corker, F. Lefebvre, C. Lecuyer, V. Dufaud, F. Quignard,
A. Choplin, J. Evans and J. M. Basset, Science, 1996, 271,
966-969.
25.
26.
27.
28.
29.
30.
31.
7.
8.
V. R. Dufaud and J. M. Basset, Angew. Chem. Int. Ed., 1998,
37, 806-810.
M. K. Samantaray, E. Callens, E. Abou-Hamad, A. J. Rossini,
C. M. Widdifield, R. Dey, L. Emsley and J. M. Basset, J. Am.
Chem. Soc., 2014, 136, 1054-1061.
S. Norsic, C. Larabi, M. Delgado, A. Garron, A. de Mallmann,
C. Santini, K. C. Szeto, J. M. Basset and M. Taoufik, Catal.
Sci. Technol., 2012, 2, 215-219.
V. Polshettiwar, F. A. Pasha, A. De Mallmann, S. Norsic, J.
Thivolle-Cazat and J. M. Basset, ChemCatChem, 2012, 4,
363-369.
32.
9.
Y. Chen, E. Abou-hamad, A. Hamieh, B. Hamzaoui, L.
Emsley and J. M. Basset, J. Am. Chem. Soc., 2015, 137, 588-
591.
10.
N. Maity, S. Barman, E. Callens, M. K. Samantaray, E. Abou-
Hamad, Y. Minenkov, V. D'Elia, A. S. Hoffman, C. M.
Widdifield, L. Cavallo, B. C. Gates and J. M. Basset, Chem.
Sci., 2016, 7, 1558-1568.
11.
12.
M. K. Samantaray, E. Pump, A. Bendjeriou-Sedjerari, V.
D'Elia, J. D. A. Pelletier, M. Guidotti, R. Psaro and J. M.
Basset, Chem. Soc. Rev., 2018, 47, 8403-8437.
M. K. Samantaray, V. D'Eia, E. Pump, L. Falivene, M. Harb,
S. O. Chikh, L. Cavallo and J. M. Basset, Chem. Rev., 2020,
120, 734-813.
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