DOI: 10.1002/cssc.200((will be filled in by the editorial staff))
2
a
a
b
a
a
a,c
In order to design more sustainable processes for the alkylation of ketones, avoidance of both the use of atom ineffective leaving groups such as halogenides or boron and the use of noble metal based catalysts was attempted. For that purpose, high surface area titanium nitride was synthesized out of high surface area titanium oxide using cyanamide as a transcription agent. Both the resulting material and the initial oxide proved to be effective and versatile catalysts for the alkylation of ketones with alcohols. Interestingly, the nitridic catalyst yielded unsaturated compounds, while the oxide based catalyst mainly yielded saturated coupling products. Due to its metallic properties, TiN showed a strong tendency to catalyse the dehydrogenation of alcohols which then underwent aldol condensation with ketones. On the opposite, TiO
2
promoted the direct nucleophilic attack of ketones on alcohols.
Carbon-carbon bond forming reactions are fundamental in organic synthesis and for specialty chemistry. Despite the recent developments of organocatalysis, [1] most catalytic systems used to perform C-C couplings rely on rare (noble) metal complexes or metal nanoparticles [2, 3] and/or atom ineffective leaving groups such as halogenides or boron [4] . In a general effort to achieve greener and more sustainable syntheses, much interest has been focused on the use of alcohols as coupling educts. Alcohols are, indeed, easily accessible both from hydroformylation reactions on oil derivatives and from biomass. Moreover, water is the only byproduct of their use as electrophiles. Actually, recent papers nicely report on the coupling of alcohols with activated compounds like ketones, nitriles and malonates [5-7] . Nevertheless, these approaches remained dependent on the use of noble metals. ceramics (which is a required condition for the dehydrogenation step).
[10] Moreover TiN is thought to be a basic material, which is expected to trigger the aldol condensation step.
[11]
O O
Nb
2
O
5
Nb
2
O
5
,
R'
R"
R"
R'
R OH
-H
2
R
O
-H
2
O
R
Scheme 1. Proposed mechanism for the Nb
2
O
5
catalyzed alkylation of ketones.
Herein we report on a new synthesis of a high surface area
TiN like material and on its use as a catalyst for the alkylation of ketones with alcohols.
2.1 Description of the synthesised porous catalysts
In recent works attempting to screen the catalytic properties of group IV and V oxides nanoparticles we evidenced the activity of niobium oxide nanoparticles for the alkylation of ketones with alcohols [8] . However, these particles yielded complex reaction mixtures and failed to produce detectable amounts of alkylation products as soon as the em ployed alcohol wasn’t a benzyl alcohol derivative. The mechanism suggested for the formation of unsaturated alkylation compounds, inspired by the hydrogen borrowing mechanism proposed by Guillena et al ., [9] relies on the dehydrogenation of the alcohol to yield an aldehyde, which then reacts with the ketone in an aldol condensation (Scheme 1).
In order to improve the yield and the selectivity of this reaction, we were looking for ceramics which could trigger both the dehydrogenation and the condensation step. High surface area titanium oxide and titanium nitride are attractive candidates as titanium is cheap and known to easily produce Ti(III) species in
Even if some methods to produce high surface area TiN powders were already described [12] , a new general approach
[a] A. Fischer, P. Makowski, Prof. M. Antonietti, Dr. A. Thomas
Colloid Chemistry Department
Max-Planck Institute of Colloids and Interfaces
Research Campus Golm, 14542 Potsdam, Germany
Fax: (+) 49 331 567 9502
E-mail: arne.thomas@mpikg.mpg.de
[b] Dr. J.O. M üller
Department of Inorganic Chemistry
Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6,
14195 Berlin, Germany
[c] Dr. F. Goettmann
Laboratoire des Nanomatériaux Autoréparants
Institut de Chimie Séparative de Marcoule, ICSM, UMR 5257
Centre de Marcoule, BP 17171, 30207 Bagnols/Cèze, France
Fax : (+) 33 466 797 11
E-mail : frederic.goettmann@cea.fr
1

towards such materials was explored, starting from mesoporous titanium oxide using cyanamide (CA) or, more precisely, its condensation product, graphitic-C
3
N
4
(g-C
3
N
4
), as nitrogen source for the transformation of the oxide to the nitride nanostructure. g-
C
3
N
4 can easily be prepared by thermal condensation of cyanamide or dicyandiamide. The synthesis and catalytic applications of mesoporous C
3
N
4
were reported recently.
[14-17]
This material also proved able to act as both a nitrogen source and a structural confinement for the synthesis of titanium, vanadium and gallium nitride nanoparticles.
[18] Additionally, cyanamide was reported to be a useful nitrogen source for the conversion of various oxide nanoparticles into the corresponding nitrides.
[19] C
3
N
4
thus seemed to be an ideal candidate for the direct nitridification of porous TiO
2
.
X-100 as template. After extraction of the surfactant, mesoporous nano crystalline anatase powder was obtained (see Figure 1, P-
0.0).
The mesoporous titania powder was then infiltrated with different amounts of molten cyanamide at 50°C. (
Care is advised as titania catalyses the exothermic dimerisation of cyanamide at low temperatures ). Several experiments using cyanamide/TiO
2 mass ratios from 2 to 14 were carried out (see Table 1). The infiltrated materials were subsequently heated in a covered crucible up to 800°C under nitrogen. The resulting black powders were characterized by XRD, TEM, HRTEM, nitrogen sorption and elemental analysis. Figure 1 shows the XRD pattern of the mesoporous TiO2 powder and the powders resulting from the thermal treatment of the cyanamide impregnated TiO2.
Bulk mesoporous titanium oxide was synthesized by a typical hydrolysis condensation process using non-ionic surfactant Triton
Table 1.
Structure parameters of the mesoporous TiO2 and the different TiN like samples.
Sample
CA/TiO
2
Mass ratio
CA/TiO
2
Molar ratio
Crystallite size
(nm)
Carbon weight %
BET surface area (m 2 .g
-1 )
P-6
TiN
P-10
TiN
P-14
TiN
P-0
TiO
2
P-2
TiO
2
P-2.5
TiN
P-3.9
TiN
P-4.5
TiN
0
2
2.5
3.9
4.5
6
10
14
0
3.8
4.7
7.4
8.5
11.4
19
26.5
6.4
8.1
5.1
5.0
5.4
5.0
4.8
5
0
0.45
0.28
1.8
10.4
16.6
21.6
26.3
230
132
166
142
112
85
56
45
BJH ads. pore size (nm)
Pore Volume
(cm 3 .g
-1 )
6.0
4.6
6.9
6.1
5.3
5.3
6.6
7.2
0.29
0.28
0.28
0.22
0.17
0.17
0.21
0.18
Figure 1. XRD pattern of as-prepared mesoporous TiO
2
powder and of mesoporous TiN powder prepared using different CA/TiO
2
weight ratios.
(waterfall presentation with x shift of 10%)
The XRD patterns of the resulting black powders are in good agreement with the typical pattern of the TiN phase osbornite (except for P-2, which is still containing mostly anatase). Indeed the calculated cell parameter a=b=c = 4.21 Å fits with the expected database value of 4.23 Å for cubic titanium nitride, belonging to the space group Fm3m.
[21] No peaks corresponding to anatase were observed in the XRD patterns of samples prepared from CA/TiO2 mass ratios higher than 2, proving that a high TiN containing material can be produced using this synthesis.
[22] The fact that residual carbon
(as a side product of the nitridification reaction) is always detected, together with the fact that the surface of our material probably reoxidizes upon storing in air, does not allow speaking of a truly pure TiN material. However, for convenience reasons and as we do not expect any pronouned influence of the carbon shells, this material is still called a “TiNcatalyst”.
In case of sample P-2 the amount of cyanamide was not sufficient to convert the titanium oxide into titanium nitride completely. Experimentally it was found that about 2.35
2

equivalents, that mean a mass ratio CA/TiO2 of 2.5, are needed to assure the complete conversion of the titan species into TiN. This can be explained by a partial sublimation of the cyanamide during the heating process. Interestingly no mixed or intermediate phases of TiO
2
and TiN (such as Ti(O,N)) were detected along this synthetic pathway. The size of TiO
2
and
TiN crystals forming the walls of the mesoporous bulk materials was determined using the Scherrer equation. The detailed values for each sample are summarised in table 1. For as-made TiO
2
the crystallite size was estimated to be 6.4 nm.
In contrast, for all TiN samples, considerably smaller crystallite sizes of approx. 5 nm were observed. This can be attributed to the higher density of the nitride compared to the oxide (d TiO
2
= 3.90 g.cm
-3 ; d TiN = 5.24 g.cm
-3 ).
The porosity of as-made TiO
2 and the resulting TiN powders were characterised by nitrogen- sorption measurements (Figure 2). All materials show type IV isotherms characteristic for mesoporous materials. The TiN powders have lower specific surface areas as compared to the mesoporous TiO
2
precursor, also owing to the higher density of
TiN. However, it was also observed that the surface areas of the mesoporous TiN samples depend on the amount of cyanamide used during the synthesis. Higher CA/TiO
2 mass ratios resulted in lower surface areas. This can be explained by increasing amounts of residual amorphous carbon in the samples. Indeed, elemental analysis measurements (Table 1) showed that with an increasing amount of cyanamide the carbon content of the resulting titanium nitride samples increased from close to zero up to 26 wt% of carbon. Thus, a pure mesoporous TiN could just be observed at lower CA/TiO
2 ratios (P-2,5) while higher cyanamide contents resulted in
TiN/C nano-composites.
Figure 2. Nitrogen sorption isotherms of the starting mp-TiO2 (▪-) and of the resulting mp-TiN materials made with different amounts of cyanamide
The average pore size calculated from the BJH analysis of the adsorption branch was determined to be 6 nm for the TiO
2 powder and varied from 7.2 nm to 5.3 nm for the different TiN powders (no trend can be observed concerning the CA/TiO
2 ratios, Table1). This shows that there is almost no change in the morphology of the powders during the TiO
2
to TiN transformation.
Figure 3 shows the TEM pictures of the starting mesoporous TiO
2 and one of the mesoporous TiN powders.
The overall particular framework described by Wang et al.
for the porous TiO
2
is obviously transcribed into in the resulting titanium nitride. The corresponding selected area electron diffraction patterns prove the presence of anatase and osbornite, respectively.
Figure 3. TEM pictures of mesoporous TiO2 precursor (P-0) (top) and the obtained osbornite TiN (P-2.5) (bottom).
Figure 4. HRTEM image of crystalline particles exhibiting the TiN (Osbornite) structure of the P-6 sample. The inset shows the calculated FFT of the indicated area. The particle is oriented in the [001] zone axis.
3

HRTEM measurements again underlined that the samples were constituted of a framework of well crystallized TiN nanoparticles. As shown on the Fourier transformation of the selected area on the HRTEM pictures the distance of 2.11 Å between the crystal planes corresponds quite well to the expected theoretical value of d[200] = 2.12 Å for a cubic osbornite titanium nitride, [23] as well as the measured angle of
90° corresponding fits to a cubic unit cell.
Tabl 2. Aldol reactions catalyzed by porous TiN [a]
Substrates Conv.
[b]
%
Products [c]
O
2.2 Alkylation of ketones.
Acetophenone
(500 mg)
85
+
Ph
> 99 % Traces
2.2.1 Aldol condensations.
Ph
O
Aldol condensation, the reaction between two aldehydes or ketones to form an enal, respectively an enone, is among the most useful C-C bond forming reactions in organic synthesis.
[24] Many materials proved able to catalyse this reaction including functionalized mesoporous silica [25] and oxidic bases.
[26] It was thus relatively straightforward to test, in a first step, our catalyst on this reaction.
Cyclohexanone
(500 mg)
Acetophenone
(120 mg)
Benzaldehyde
(110 mg)
95
100
O
> 98 %
> 90 %
As can be seen from table 2, mesoporous TiN can effectively act as a catalyst for the Aldol condensation. It has to be mentioned that even under our relatively harsh reaction conditions the reaction mixtures remained relatively simple.
Indanone
(140 mg)
Hexanal
(100 mg)
100 [d]
O
O
+
Bu
Bu
95 % 5%
Ph
Bu
2.2.2. Alkylation of ketones with alcohols.
We then tested our porous materials as catalysts on the direct alkylation of ketones with alcohols under similar conditions.
Table 3 summarises some of the results we obtained.
Table 3.
Alkylation of ketones with alcohols catalyzed by TiN.
[a]
[a] In a typical reaction 50 mg of titanium nitride was added the corresponding pure ketone or to a mixture of ketone and aldehyde in 2 ml xylene and heated to 150°C for 48 h. [b] Determined by GC with mesithylene as an internal standard. [c] The indicated percentage corresponds to the relative amount of compound within the reaction products as determined by GC. [d] Calculated on the basis of the aldehyde
Substrates TiO
2
Conv
(%) [b]
Products [c]
TiN
Conv
(%) [b]
Products [c]
O O
1
Acetophenone
Benzyl Alcohol
100 100
> 95 % > 90 %
O O O O
2
Acetophenone
Benzyl Alcohol [d]
90 40
O
70 % 30 %
O
10 % 90 %
O
3
Acetophenone
4-Methybenzyl
Alcohol
100 31
> 95 %
90 % 10%
O
O MeO
4
Acetophenone
4-Methoxybenzyl Alcohol
100
OMe
100
> 95 %
OMe OH
10 % 90 %
OMe
4

O O
5
Acetophenone
1-Phenylethanol
99 0 /
40 % 60 %
O
6
Acetophenone
Hexanol
41 0 /
< 95 %
O O
7
Acetophenone
Cyclohexanol
10 0 /
65 % 35 %
O O
8
9
10
Indanone
Benzyl Alcohol
100
Cyclo-hexanone
Benzyl Alcohol
50
Hexanal
Benzyl Alcohol
100
Ph
100
O
> 95 %
O
Ph
O
Ph
Traces 65 % 15 %
Bu Bu Bu
O
O
5 % 95 %
Ph
100
99
Ph
> 95 %
O O
Ph
15 % 85 %
Bu
O
> 90 %
Bu
Ph
[a] In a typical reaction 50 mg of titanium dioxide or nitride were added to a 2 ml solution of 1 mmol ketone and 1 mmol aldehyde in xylene and heated to 150°C for 48h.
[b] Determined by GC on the basis of the alcohol conversion with mesitylene as an internal standard. [c] The indicated percentage corresponds to the relative amount of compound within the reaction products. [d] Reference test with 10 mg of catalyst .
As can be seen, a large variety of ketones could be alkylated in moderate to high yields under the chosen conditions. The products obtained using TiN as a catalyst are, to a large extend, α,ß-unsaturated ketones or aldehydes. This confirms that TiN is able to catalyse this alkylation similarily to
Nb
2
O
5
. Nevertheless, it is remarkable that in the present case the reaction products are much cleaner than in the case of niobium oxide. The scope of alcohols, which could be used as electrophiles under those conditions seems to be limited to benzyl alcohol derivatives (such as benzyl alcohol, table 3, entry 1, 4-methylbenzylalcohol, table 3, entry 3, or 4methoxybenzyl alcohols, table 3, entry 4) while the range of useful ketones is much broader. Acetophenone, indanone and cyclohexanone yielded the corresponding alkylation products with benzyl alcohol to more than 90 % (table 3, entries 1, 8 and
9 respectively).
Surprisingly, mesoporous TiO
2
, meant as a reference sample, not only proved to be active for the same type of reactions but featured a broader scope of applications. Indeed, it was also able to promote the alkylation of numerous ketones with less active electrophiles such as aliphatic alcohols (as exemplified by hexanol, table 3, entry 6) or even secondary alcohols (such as 1-phenylethanol or cyclohexanol, table 3, entries 5 and 7, resp.).
Contrary to what is observed with TiN, saturated alkylation products are also found, in some cases even as the major product. The comparison can be exemplified by the alkylation of acetophenone with benzyl alcohol (Table 2, entry 1). On one side; TiN yielded 90 % of 1,3-diphenylpropen-1-one, whereas on the other side, TiO2 gave 95 % of 1,3-diphenylpropan-1one. This evidences that, besides the dehydrogenationcondensation mechanism occurring in the TiN catalysis, TiO
2
is also able to catalyse another process, yielding saturated compounds. A putative mechanism for this process involves the concomitant deprotonation of the ketone and activation of the alcohol, as depicted in scheme 2. This would be very similar to the mechanism postulated by Sanchez et coll . for the mesoporous ZrO2 catalysed hydroformylation of alkenes and alkynes.
[27, 28]
Ph
O
Ph
OH
O
Ti
O
Ti
O
Ti
-H
2
O
Ph
O
Ti
Ph
O
O
Ti
O
Ti
+ H
2
O
Ph Ph
OH
Ti
OH
O
O
Ti
O
Ti
Scheme 2. Proposed mechanism for the TiO
2
catalysed alkylation of ketones.
5

In this contribution, we have described the synthesis and applications of a new sustainable catalytic system, high surface area scaffolds based on TiN. The convenient and simple synthesis of the catalyst was based on graphitic carbon nitride, being at the same time the nitrogen source throughout nitridification of titanium oxide, while its beneficial mechanical properties prevented the collapse of the pores.
It was also evidenced that this material was able to catalyze various types of aldol condensations, where the activity of these catalysts originates from a combination of Lewis acidic and Brönsted basic properties. Both mesoporous TiO
2
and TiN could be used as valuable alternatives to noble metal complexes in such C-C bond forming reactions, while especially the alkylation of ketones with alcohols appeared to be a very attractive reaction option. The required reaction conditions are harsher than the one reported for noble metal catalysts, for example, by Kwon et al .
[5] Nevertheless, this draw-back might be balanced by the relative abundance of titanium compared to palladium. Conceptually interesting, the ability of titanium based ceramics to catalyze the C-C coupling of alcohols and ketones seems to take advantage of multiple site processes, whereas many progresses in heterogeneous catalysis in recent years relied on site isolation.
[29-31]
Synthesis of high surface area TiN.
Synthesis of Bulk mesoporous Titanium Oxide
All chemicals were used as received. The mesoporous TiO
2
was prepared as already described by Wang and coworkers
[20]
by hydrolysis condensation using tetrabutyl orthotitanate (TBOT,
Aldrich) as a metal precursor and Triton X-100 (TX-100, 4-
(C
8
H
17
)C
6
H
4
(OCH
2
CH
2
) n
OH, n~10, Aldrich) as the structuring agent.
In a typical procedure TBOT (6.8 g = 19.98 mmol) and TX-100
(6.46 g = 10 mmol) were mixed with absolute ethanol (38 g = 825 mmol) under vigorous stirring. Water (168 g = 9333 mmol) was then added drop-wise under continuous stirring, which induced the immediate precipitation of a white solid. The molar composition of the reaction mixture was: TBOT: TX-100: EtOH: H
2
O
~2:1:82.5:933. The mixture was aged at room temperature for 24h and at
80°C for another 24h without stirring. The supernatant solution was removed and the rest of the mixture was centrifuged.
The obtained white powder was then washed under stirring with twice 40 mL water followed by twice 40 mL of EtOH. Finally the powder wa s dried over night under vacuum at 100°C.
Synthesis of the mesoporous Titanium Nitride
The mesoporous Titanium nitride powders were prepared by using the above described mesoporous TiO
2
powder and cyanamide (CA,
NH
2
CN, Aldrich) as nitrogen source. In a typical synthesis, 100 mg of the TiO
2 powder were mixed with a given amount of molten CA for the high amounts or CA dissolved in 0.5 mL of water for the small amounts in a covered crucible, in order to penetrate the porous network of the TiO
2
.. The amount of CA was varied in order to vary the CA/TiO
2
mass ratio x from x = 2 to 14. The covered crucible was then heated to 800°C under nitrogen for 3 h with a heating ramp of 3.3 K/min, in a chamber kiln from Nabertherm
GmbH. The resulting black powders were labelled P-x. Attention is recommended as high surface area TiN proved to be very pyrophoric! The oven has to be totally cooled down (T<40°C) before removing the samples. An additional passivation step with some % of oxygen in the nitrogen flux can be advantageous to yield stable powders, which do not need to be stored or manipulated under inert gas.
Characterization of the powders.
Elemental analysis of the samples to determine the nitrogen and carbon content of the powders was performed on a Vario
Elementar instrument from Elementar Analysensysteme. X-Ray diffraction (X-Ray) measurements were carried out on a D8 WAXS system from Bruker. The X-Ray tube, operated at 40 kV and 40 mA, emitted Cu-
Kα radiation (λ=0.154 nm) that was monochromatized by a multilayer Göbel mirror. A energy diepersive detector (Sol-x, Briker) was used to ensure low background noise via the exclusion of inelastic scattered photons.
Samples were finely grinded and measured on silicon or deeped plastic sample holders. The mesuremets were performed in reflection geometry as coupled θ-2θ scans with a beam slit of 2 mm. N
2
adsorption-desorption measurements were carried out at
77K using a Quantachrome Autosorb gas sorption system.
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models were used to determine the specific surface areas and respectively the pore sizes of the samples. Before the measurements, the samples were degassed at 100°C under vacuum for at least one night. The stability of the samples toward the degassing was confirmed by X-Ray diffraction. No change in the x-Ray diffraction pattern before and after the degassing could be observed. Transmission electron microscopy (TEM) images were obtained on an OMEGA EM 912 microscope from Zeiss with an acceleration voltage of 120 kV. Samples were measured on carbon coated copper grids (400 mesh). The powders were grinded, dispersed in acetone and one drop of the dispersion was deposited on the grid. Finally high resolution transmission electron microscopy (HRTEM) picture was recorded on a Phillips
TEM/STEM CM 200 FEG transmission electron microscope equipped with a Field-emission gun to study the morphology and microstructure of the TiN particles. The acceleration voltage was set to 200 kV. To avoid any misinterpretation of the image contrast, all investigations were carried out on particles without underlying carbon film.
Catalytic tests.
As our aim was to access cheap and easy to handle catalysts, the obtained powders were neither stored nor handled under inert gas.
The used chemicals were employed as received and the solvents were not further purified. More over the scaling up of the synthesis of mesoporous TiN (in order to be able to run a large number of catalytic tests) proved easier with the procedure with the highest initial Cyanamid/TiO
2
ratio. We thus used the powders resulting from this synthesis (P-14) as a catalyst even if they featured a large amount of residual carbon as discussed in the text. As preliminary tests, the activity of our TiN powder was tested in aldol condensation and further compared to the activity of mesoporous
TiO
2
in the alkylation of ketones with alcohols.
Aldol condensation.
In a typical reaction, 50 mg of mesoporous TiN (P-14) were weighted in a SCHOTT screw capped glass tube (160 mm length, about 10 mm inside diameter). The wanted amount of the pure ketone or alternatively a 2 ml xylene solution of ketone and aldehyde was added. The tube was closed and heated to 150°C for 48h. The products were analyzed and quantified by GC-MS
(Agilent Technologies, GC 6890N, MS 5975).
Alkylation reactions.
In a typical reaction 50 mg of titanium dioxide or nitride were weighted in a similar screw capped glass tube (160 mm length, about 10 mm inside diameter). 2 ml of a solution of 1 mmol of
6

ketone and 1 mmol of alcohol in xylene were added. The tube was closed and heated to 150°C in an oil bath (only 20 mm of the tube were in contact with the oil) without stirring. The products of the reaction were analyzed by GC-MS (Agilent Technologies, GC
6890N, MS 5975) and 1H NMR (Bruker DMX 400).
The Max-Planck Society is gratefully acknowledged for financially supporting this work within the frame of the
ENERCHEM project house.
C-C coupling group IV ceramics mesoporous titanium nitride heterogeneous catalysis.
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[31] J. M. Notestein, A. Katz, Chem. Eur. J. 2006 , 12 , 3954-3965.
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Almost as useful as palladium but 500 times less expensive: mesoporous
TiO
2
and TiN proved able to catalyze the alkylation of ketones with alcohols, a reaction which was limited to noble metal based catalysts.
Anna Fischer, a Philippe Makowski, a
JensOliver Müller, b Markus Antonietti, a
Frederic Goettmann a,c * and Arne
Thomas.
a *
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High surface area TiO
2
and TiN as noble metal free catalysts for the C-C coupling of alcohols and ketones
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