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AZ9RBAYCAN KIMYA JURNALI № 4 2016
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UDC 541.49
NEW METAL-FREE CATALYSTS FOR THE SELECTIVE HYDROGENATION OF MULTIBLE BONDS iN AROMATiC HYDROCARBONS BASED ON GRAPHITIC
CARBON NITRIDES
V.M.Akhmedov, I.D.Ahmadov, H.G.Nurullayev, V.M.Ahmadov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
advesv@mail.com Received 27.06.2016
There has been demonstrated for the first time that polymeric carbon nitrides with graphitic structure can be successfully used as a metal-free catalyst for the selective hydrogenation of multiple bonds of aromatic compounds in vapor phase under atmospheric pressure of hydrogen. Graphitic carbon nitrides are capable to activate a hydrogen molecule and replace the metals for partial hydrogenation of phenylacetylene to styrene and phenol to cyclohexanone and cyclohexanol.
Keywords: graphitic carbon nitride, metal-free catalyst, phenylacetylene, phenol, hydrogenation, styrene, cyclohexanone, cyclohexanol
Polyaddition and polycondensation of some N-containing precursors (cyanamide, dicyandiamide, melamine etc.) lead to the formation of polymeric carbon nitrides with different structural and electronic properties [1]. Among them the carbon nitrides with graphite type modification (g-C3N4) are regarded to be the most stable allotropes. They have the correct electronic and microstructure, provide a suitable specific surface area. They exhibit high stability towards thermal (5500C in an air or inert gas atmosphere) and chemical influences (acids, bases and organic solvents), are resistant to oxidation. The uncondensed primary amino groups, as well as tertiary and aromatic amino groups of the three-s-triazine rings generate the Bronsted and Lewis base centers in the frame of
g-C3N4 [2].
Hydrogenation of carbon-carbon multiple bonds is one of the most important processes widely used in chemical industry. Currently, this type of reaction is carried out on a very large scale, using noble metals such as platinum, palladium and the first row transition metals such as nickel [3]. We have established
that g-C3N4 can replace metals for partial hydrogenation of multiple bonds in aromatic compounds in vapor phase under atmospheric pressure of hydrogen. In the present study a highly efficient catalyst on the base of synthesized polymeric carbon nitride has been developed for partial hydrogenation of phe-nylacetylene to styrene and phenol to cyclohexanone and cyclohexanol.
Experimental
g-C3N4 possessing high chemical and thermal stability can be made by condensation of different nitrogen-containing precursors [4]. The carbon nitrides were prepared by stepwise heating dicyandiamide or melamine and their combinations up to the temperatures between 490 and 5500C (Table 1). Depending on reaction conditions, a variety of polymeric graphitic carbon nitrides with different crystalline phase can be obtained. These as-prepared polymeric materials exhibit prolonged catalytic activity in partial hydrogenation of phenyl-acetylene to styrene and phenol to cyclohexa-none and cyclohexanol avoiding noble metals.
Table 1. The prepared polymeric graphitic carbon nitrides
Prepared samples of g-C3N4 Precursor Synthesis conditions Sbet , m2/g
I Dicyandiamide 300^400^500°C - 15h 5.8
II Melamine 350^400^490 ^5100C - 20h 5.3
III Melamine +Cyanuric acid 350^400^5500C - 25h 60.2
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NEW METAL-FREE CATALYSTS FOR THE SELECTIVE HYDROGENATION
In a typical synthesis, 10.0 g of precursor (or other precursors) was well powdered using a mortar placed in a semi-closed alumina crucible with a cover. The crucible was heated gradually to the certain temperature in the interval of 1525 h. After the reaction the alumina crucible was cooled to room temperature and then synthesized carbon nitride was collected and ground into powder, then were characterized by X-ray diffraction (Bruker-D2 Phaser, Germany), Fourier transform infrared spectroscopy (Nicolet-iS10, USA) and their specific surface areas were determined (Sorbi-MS, Russia). The X-ray diffraction pattern of this compound reveals a partly crystallized (about 40%) dimensional single phase with an interplanar distance equal to 3.24 A, which is consistent to the theoretical values predicted by Teter and Hemley for the graphitic form of carbon nitride [5]. The infrared spectrum performed on this phase exhibits features very similar to those of carbon nitride reported in literature [6]. The group of multiple bands in the 1700-1000 cm-1 spectral region is characteristic of s-triazine ring vibrations (C=N and C-N stretching modes).
Catalyst testing was conducted under an atmospheric pressure of hydrogen in a flow type microreactor. High purity hydrogen (99.5%) used for the hydrogenation of phenylacetylene and phenol was further purified by passing through a gas drying unit with a molecular sieve. The following experimental conditions were used for a typical run: 0.5 g of as-prepared g-C3N4 polymeric carbon nitride was loaded into the reactor tube (length of 250 mm and i.d. of 8 mm) and a thermocouple was placed at center of the catalyst bed to monitor the reaction temperature. The hydrogenation of phenylacetylene was studied in the temperature range of 150-2700C (for phenol 180-2700C). The catalyst was treated at room temperature for 30 min in flowing hydrogen (30 cm /min) and then heated to the reaction temperature. A typical experimental run consisted of passing the 5 wt. % phenylacetylene (or phenol) solutions in inert solvents (hexane, heptane and cyclohexane) over the catalyst under an atmospheric pressure of hydrogen. All the reaction products were analyzed by chromato-
graphy equipped with FID (Agilent - 7820A) on a HP-5 capillary column 30 m long. After prolonged use as catalysts g-C3N4 powders were recovered and analyzed. It was observed that the X-ray diffraction pattern of these fresh and used samples was almost identical.
Results and discussions
The selective hydrogenation of phenyl-acetylene: is of great practical importance in the production of the polymerization purity styrene. In the present study a highly efficient catalysts on the base of synthesized polymeric carbon nitrides have been developed for partial hydrogenation of phenylacetylene (Scheme 1) providing good to excellent conversion with remarkable selectivity (up to 98 - 99%) without additives (Table 2).
О
^C=CH + H
-2"
g-C3N4
/T^ch=CH2
(98.4%)
О
CH2-CH3
(0.6 %)
Scheme 1. Hydrogenation of phenylacetylene to styrene and to ethylbenzene.
Compared with classical Lindlar catalysts [3], the developed method is more advantageous due to effective catalyst recyclability in the metal-free system. This catalyst shows small differences in product distributions depending on the temperature: there was observed slight decreasing selectivity for the styrene by complete hydrogenation of phenylacetylene to ethylbenzene.
Table 2. Hydrogenation of phenylacetylene on graphitic carbon nitrides
C3N4 FA:H2:S(mol) T, 0C V, h-1 T, s KFA, % % SEB, %
I 150 1.0 4.5 33.2 99.4 0.6
I 200 1.0 4.5 58.7 99.1 0.9
I 1:1.2:1.7 250 1.0 4.5 77.4 98.7 1.3
II 250 1.0 4.5 76.1 99.1 0.9
III 250 1.0 4.5 76.7 98.7 1.3
I 1: 2.0:2.3 250 0.8 4.1 78.2 98.9 1.1
Catalyst - 0.5 g; hydrogen flow rate = 30 cm /min; reactant flow rate = 3 cm3/h.
The polymeric carbon nitrides with graphitic structure materials of Table 1 were also tested as metal-free catalysts for the hydro-
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genation of phenol at temperatures ranging 180-2700C in a stream containing an excess of hydrogen with respect to the amount of phenol in solution (Scheme 2).
Step 1
tast
Step 2
Scheme 2. Hydrogenation of phenol to cyclohexanon and cyclohexanol.
Cyclohexanone and cyclohexanol are the key intermediates in synthesis of s-caprolactam and adipic acid, which are basic materials for the industrial production of nylon-type polymers. The main routes for manufacture of these compounds are based on conversion of cyclohexane and phenol, by their oxidation and hydrogenation, respectively. Although cyclohexane oxidation dominates the market, because of cheaper raw materials, this process suffers from low products yields and complicated recovery/separation steps [7]. The hydrogenation of phenol remains competitive, offering better selectivity. Currently the hydrogenation of phenol is generally carried out in the vapor phase with supported palladium catalysts (Pd/C, Pd/A^Os, and Pd/NaY zeolite catalysts) [8]. Recently, a novel catalyst containing Pd-nanoparticles for the selective hydrogenation of phenol has been made by Antonietti et al. [9] using mesoporous graphitic carbon nitride (mpg-C3N4) as a support. This catalyst has shown high activity and promoted the direct hydrogenation of phenol to cyclohexanone under atmospheric pressure of hydrogen without additives using water as a clean solvent. However, an activity decrease in the conversion of phenol by leaching of the Pd nanoparticles from catalyst surface was observed. There remains great need to develop efficient and environmentally friendly catalysts for the phenol conversion to cyclohexanone and cyclohexanol.
The results of our study indicate that g-C3N4 functioned as a stable metal-free catalyst for the hydrogenation of phenol in the vapor phase. Table 3 demonstrates the relative active-ties of prepared samples.
All three prepared g-C3N4 based materials exhibit activity as hydrogenation catalysts
of phenol to the mixture of cyclohexanon and cyclohexanol. The sample III has shown much better activity compared with the samples I and II (Table 1). The conversion of phenol in the presence of prepared samples increases along the temperature while selectivity of cyclohexa-non decreases significantly.
Table 3. Hydrogenation of phenol on the polymeric
Catalyst Tempera- Conver- Product distribution, %
ture, 0C sion, % cyclohexanon cyclohexanol
190 10.4 83.4 16.6
DCA 200 25.7 80.1 19.9
220 50.4 74.6 25.4
250 71.3 66.3 33.7
190 16.8 74.8 25.2
M 200 23.6 70.4 29.6
220 41.8 67.3 32.7
250 68.4 59.6 40.4
190 33.3 87.5 12.5
M+CA 200 55.3 75.8 24.2
220 76.9 71.1 28.9
250 97.4 48.6 51.4
Catalyst - 0.5 g; hydrogen flow rate = 30 cm /min; reactant flow rate = 3 cm3/h. (DCA - diciandiamide, M -melamine, CA - cyanuric acid)
There has been shown a key role of support in the hydrogenation of multiple bonds in aromatic compounds [9]. From this point of view, the high catalytic performance of Pd/mpg-C3N4 can be attributed to the special semiconductor feature of the support and its connection with metal, which leads to additional electronic activation of Pd nanoparticles and a "nonplanar" adsorption of phenol, that finally gives rise to its fast and selective hydrogenation. In fact, there are a number of arguments to define the exceptional impact of g-C3N4 specific structure not only on the activity catalyst metal center. Owing to the structural and electronic properties graphitic carbon nitrides provide the required prerequisites even independence from metal catalytic activity. If g-C3N4 as a semiconductor is capable to photochemically overcome the endo-thermic character of the water splitting process, presumably, under certain conditions it can also chemically activate a hydrogen molecule and run the hydrogenation reaction. Indeed, as described above, g-C3N4 can be successfully used as effective catalyst for the selective hydrogenation of phenylacetylene to styrene in the absence of
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NEW METAL-FREE CATALYSTS FOR THE SELECTIVE HYDROGENATION
any metal and metal oxides. Following a related strategy, we found that polymeric graphitic carbon nitrides are also capable to catalyze the phenol hydrogenation.
Feature of g-C3N4 makes it a promising platform for the construction of the metal free low cost green catalytic systems. They have the correct electronic and microstructure, provide a suitable specific surface area (Figure 1) [10]. They exhibit high stability towards thermal) and chemical influences (acids, bases and organic solvents). The uncondensed primary amino groups, as well as tertiary and aromatic amino groups and the Bronsted and Lewis base centers generate in the frame of g-C3N4. Moreover, the electron rich aromatic tri-s-triazine rings are able to activate the corresponding substrates by the donor-acceptor interactions. It should be also taken into account the propensity of g-C3N4 to form the hydrogen bonds. Thus, graphitic carbon nitrides can be regarded as an solid material having multifunctional surface with possibility of committing multipurpose choices of the catalytic actions. Indeed, since it was reported in [10] that metal-free graphitic carbon nitrides can be used as effective catalyst for a variety of reactions, such as activation of carbon oxide and benzene, oligomerization of nitriles and Friedel-Crafts type reactions and, also for photo-catalytic water splitting, they have continuously attracted attention to develop effective photo- and heterogeneous catalysts.
Electronic properties.
.AA.
,'NH
nV^N
A'X.
Bronsted basic functions
JL4-. X X
N^N N^N N^N
II 1 JU JL iL JL
. X X-X X X X x
N N i N . N N N N N NH2 •
I - I I -........'
Lewis basic H-bonding
functions motif
Fig. 1. Multiple functionality of g-C3N4 surface [10].
It is still not clear how and which functionality of the carbon nitride surface interacts with hydrogen and phenol (or phenylacetylene)
catalyzing the hydrogenation reaction. To understand the phenomenon better it should be mentioned the known feature of so-called "frustrated Lewis acid-base pairs" [11]. These discrete organic molecules comprising Lewis acid-base pairs separated at a distance can activate hydrogen molecules and act as hydrogenation catalysts. On the basis of these results A. Primo at al. [12] concluded that the catalytic activity of metal-free graphene as hydrogenation catalysts of acetylene also would be the existence of similar type of "frustrated Lewis acid-base pairs" on the graphene layer. As in the most heterogeneous catalysts, surface terminations and defects seem to be the real active sites, whereas crystalline perfection only contributes to the bulk properties, such as the graphitic structure, high thermal and chemical stability, and semiconductor electronic feature. As pointed out above, g-C3N4 exhibits an appropriate microstructure as graphene with surface defects at a distance and contains additionally nitrogen atoms for electron localization or for anchoring the active sites. Presumably, activation of H2 and phenylacetylene (or phenol) on g-C3N4 would also take place as it occurs in the type of molecules having "frustrated Lewis acid-base pairs" by polarization of H2 [12]. Accordingly, the reaction mechanism should involve the uptake of H2 on the defects of g-C3N4 surface that subsequently would transfer to the aromatic compounds with multiple carbon-carbon bonds.
References
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3. Lindlar H., Dubuis R. Palladium Catalyst for Partial Reduction of Acetylenes // Org. Synth. Coll. 1973. V. 5. P. 880-893.
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5. Teter D.M., Hemley R.J. Low-Compressibility carbon nitrides // Science. 1996. V. 271. P. 53-55.
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Amorphous Carbon Nitride // Chem. Mater. 2000. V. 12. P. 3264-3270.
7. Castellan A., Bart J.C.J., Cavallaro S. Industrial production and use of adipic acid // Catal. Today. 1991. V. 9. P. 237-254.
8. Neri G., Visco A.M., Donato A., Milone C., Malentacchi M., Gubitosa G. Hydrogenation of phenol to cyclohexanone over palladium and alkali-doped palladium catalysts // Appl. Catal. A. 1994. V. 110. P. 49-59.
9. Wang Y., Yao J., Haoran L., Su D., Antonietti M. Highly Selective Hydrogenation of Phenol and Derivatives over a Pd@Carbon Nitride Catalyst
in Aqueous Media // J. Am. Chem. Soc. 2011. V. 133 (8). P. 2362-2365.
10. Zhong J., Chen J., Chen L. Selective hydrogenation of phenol and related Derivatives // Catal. Sci. Technol. 2014. V. 4. P. 3555-3569.
11. Welch C., Stephan D.W. Activate H2 through heterolytic cleavage // J. Am. Chem. Soc. 2007. V. 129. P. 1880-1881.
12. Primo A., Neatu F., Florea M., Parvulescu V., Garcia H. Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation // Nat. Commun. 2014. P. 1-9.
AROMATÍK BÏRLOÇMOLORDO MULTÍ RABÍTOLORÍN SELEKTÍV HÍDROGENLO^MOSÍ ÛÇÛN TORKIBiNDO METAL OLMAYAN QRAFÍT TÍPLÍ KARBON NÍTRÍD OSASÍNDA YENÍ
KATALÍZATORLAR
V.M.Ohmadov, LD.Ohmadov, H.Q.Nurullayev, V.M.Ohmadov
ilk dafa olaraq gôstarilmiçdir ki, qrafit tipli polimer karbon nitridlar metal içtiraki olmadan, qaz fazada va normal tazyiqda aromatik birlaçmalarda multi ikiqat rabitalari selektiv hidrogenlaçdira bilar. Qrafit tipli karbon nitridlar hidrogeni aktivlaçdirmaya va fenilasetileni stirola va fenolu tsikloheksanona va tsikloheksanola selektiv hidrogenlaçdirmaya gadirdir.
Açar sözlzr: qrafit tipli karbon nitrid, metali olmayan katalizator, fenilasetilen, fenol, hidrogenh§m3, stirol, tsikloheksanon, tsikloheksanol.
НОВЫЕ КАТАЛИЗАТОРЫ ДЛЯ СЕЛЕКТИВНОГО ГИДРИРОВАНИЯ КРАТНЫХ СВЯЗЕЙ В АРОМАТИЧЕСКИХ СОЕДИНЕНИЯХ НА ОСНОВЕ ГРАФИТОПОДОБНЫХ НИТРИДОВ УГЛЕРОДА
В.М.Ахмедов, И.Д.Ахмедов, Г.Г.Нуруллаев, В.М.Ахмедов
Впервые было показано, что полимерные нитриды углерода с графитоподобной структурой, не содержащие металл могут быть успешно использованы в качестве катализатора для селективного гидрирования кратных углерод- углерод связей в ароматических соединениях в паровой фазе и при атмосферном давлении водорода. Графитоподобные нитриды углерода способны активировать молекулы водорода и заменять металл для селективного гидрирования фенилацетилена до стирола и фенола в циклогексанон и циклогексанол.
Ключевые слова: графитоподобный нитрид углерода, катализатор без металла, фенилацетилен, фенол, гидрогенизация, стирол, циклогексанон, циклогексанол.
