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AZ9RBAYCAN KIMYA JURNALI № 4 2014
UDC 541.49
METAL-FREE HETEROGENEOUS CATALYST FOR SELECTIVE HYDROGENATION OF PHENYLACETYLENE
Vagif M. Akhmedov, I.D. Ahmadov, H.G. Nurullayev, Vusal M. Ahmadov
M.F.Nagiyev Institute of Catalysis and Inorganic Chemistry of the National Academy
of Sciences of Azerbaijan
Recived 27.11.2014
Hydrogénation of carbon-carbon multiple bonds is one of the most important processes widely used in chemical industry. We have demonstrated for the first time that polymeric carbon nitrides can be successfully used as solid catalyst for the selective hydrogenation of phenylacetylene under mild conditions in the absence of any metal and metal oxides.
Keywords: nanostructuredpolymeric carbon nitrides, metal-free catalyst, phenylacetylene, hydrogenation.
Heterogeneous catalysis has a rich history of facilitating energy in the efficient selective molecular transformations and contributes to most of industrial chemical processes. Prevailing at the present time catalytic processes utilized in chemical industries use metals or metal oxides as catalysts. These systems are often high energy-consuming and not selective, wasting resources and producing greenhouse gases. Metal-free heterogeneous catalysis using carbon compounds is a potentially interesting alternative to some current industrialized chemical processes. Owing to their structural and electronic properties, polymeric nanocarbons and their nitrogen containing structural analogs provide the prerequisites required for heterogeneous catalysts. They have the correct electronic and microstructure, provide a suitable high specific surface area. Their surfaces remain clean in contrast to conventional catalysts and they can provide a stable performance over a long period of time demonstrating a remarkable thermal stability. High activity and increased selectivity make these compounds excellent catalysts for green chemistry. Due to the special semiconductor properties polymeric carbon nitrides (PCN) can act as a metal-free heterogeneous catalyst. PCN possessing high thermal stability can be made by condensation of different nitrogen-containing liquid precursors such as cyanamide, dicyandiamide, melamine or cy-anuric acid. Depending on reaction conditions, a variety of nanostructured materials such as na-noparticles or mesoporous powders can be obtained. They show unexpected catalytic activity for a variety of reactions, such as activation of benzene, oligomerization of nitriles, and also the activation of carbon dioxide.
We have demonstrated for the first time that PCN can be successfully used as solid catalysts for the selective hydrogenation of phenylacetylene under mild conditions in the absence of any metal and metal oxides. This communication highlights some recent experimental results of selective hydrogenation of phenylacetylene.
The reactants used were phenylacetylene, melamine, dicyanodiamide and n-heptane. High purity hydrogen (99.5%) was further purified by passing through a gas drying unit with a molecular sieve.
PCN was synthesized from melamine and dicyanodiamide [1]. In a typical synthesis, 5.0 g of melamine and 5.0 g of dicyanodiamide were well mixed using a mortar. After mixing the powder was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 5000C and held for 24 h at this temperature. After the reaction the alumina crucible was cooled to room temperature and the synthesized PCN was then collected and ground into powder. The prepared PCN was characterized by X-ray diffraction (Bruker-D2 Phaser, Germany) and Fourier transform infrared spectroscopy (Nicolet-iS10, USA). The X-ray diffraction pattern of this com-
VAGIF M. AKHMEDOV et al.
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pound reveals a partly crystallized (about 40%) bidimensional 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 [2]. The infrared spectrum performed on this phase exhibits features very similar to those of carbon nitride reported in literature [3]. 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 at atmospheric pressure in a flow type microreactor. The following experimental conditions were used for a typical run: 0.5 g of polymeric carbon nitride was loaded into the reactor tube (inner diameter 8 mm) and a thermocouple was placed at center of the catalyst bed to monitor the reaction temperature. The temperatures used were typically mild (50-1500C). The catalyst was treated at room temperature for 30 min in flowing hydrogen (60 cm3/min) and then heated to the reaction temperature. A typical experimental run consisted of passing the alkyne solution in heptane over the catalyst in a stream of hydrogen. All the reaction products were analyzed by chromatography equipped with FID (Agilent - 7820A) on a HP-5 capillary column 30 m long.
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. Catalysis is dominated by metals and in many cases, by the precious ones. We have established that PCN can replace metals for partial hydrogenation of triple bonds in acetylene compounds. In the present study a highly efficient catalyst on the base of synthesized polymeric carbon nitride has been developed for partial hydrogenation of phenylacetylene providing good to excellent conversion with remarkable selectivity (up to 97%) without additives. Compared with classical Lindlar catalysts [4], the developed method is more advantageous due to the use of mild reaction conditions and effective catalyst recyclability on the metal-free system.
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Hydrogenation of phenylacetylene vs. temperature on free-metal polymeric carbon nitride: 1 - styrene selectivity,
2 - conversion of phenylacetylene,
3 - ethylbenzene selectivity; hydrogen flow rate = 60 cm3/min, reactant flow rate = 0.05 cm3/min.
Temperature, 0C
Figure demonstrates the activity and selectivity of PCN in the partial hydrogenation vs. temperature. It was observed that PCN possesses extremely high hydrogenation activity of triple bonds in phenylacetylene molecule. PCN is so reactive that noticeable conversion of triple bonds takes place even below 500C. This catalyst shows significant differences in product distributions depending on the temperature: there was observed slight decreasing selectivity for the styrene by increasing the process temperature. These results show that metal-free polymeric carbon nitrides can also be used for hydrogenation of alkenes.
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METAL-FREE HETEROGENEOUS CATALYST FOR SELECTIVE
Since PCN possesses high thermal and chemical stability, it is a valuable and useful material for some other practical applications. These findings also provide potential opportunities for the advanced design of metal-free heterogeneous nanostructured catalysts.
REFERENCES
1. Kenichi Katsumata, Ryosuke Motoyoshi, Nobuhiro Matsushita, Kiyoshi Okada // J. Hazardous Materials. 2013. V. 260. P. 475-482.
2. Teter D M. and Hemley R.J. // Science. 1996. V. 271. P. 53-55.
3. Khabashesku V.N., Zimmerman J.L. and Margrave J.L. // Chem. Mater. 2000. V. 12. P. 3264-3270.
4. Lindlar H., Dubuis R. // Org. Synth. Coll. 1973. V. 5. P. 880-893.
FENÎLASETÎLENÎN SELLEKTÎV HÎDROGENL3§M3SÎ UÇUN METAL ÎÇTÎRAK ETM3Y3N HETEROGEN KATALÎZATOR
Vaqif M.3hmadov, Î.D.3hmadov, H.Q.Nurullayev, Vtisal M.3hmadov
Muxtalif doymamiç karbohidrogenlarin hidrogenlaçmasi kimya sanayesinda geniç istifada olunnan an muhum proseslardan biridir. Biz ilk dafa olaraq gôstarmiçik ki, polimer karbon nitridlar metal içtiraki olmadan, mulayim çaraitda phenilacetyleni selektiv hidrogenlaçdirmak uçun heterogen katalizator kimi istifada edila bilar.
Açar sôzfor: nano-quruluçlu polimer karbon nitridhr, metali olmayan heterogen katalizator, fenilasetilen, hidrogenhçmd.
ГЕТЕРОГЕННЫЙ КАТАЛИЗАТОР, НЕ СОДЕРЖАЩИЙ МЕТАЛЛ, ДЛЯ СЕЛЕКТИВНОГО ГИДРИРОВАНИЯ ФЕНИЛАЦЕТИЛЕНА
Вагиф М.Ахмедов, И.Д.Ахмедов, H-Г.Нуруллаев, Вусал М.Ахмедов
Гидрирование углерод-углеродных кратных связей является одним из наиболее важных процессов, широко используемых в химической промышленности. Мы впервые показали, что полимерные нитриды углерода, не содержащие металл, могут быть успешно использованы в качестве твердых катализаторов для селективного гидрирования фенилацетилена в мягких условиях.
Ключевые слова: нано-структурные полимерные нитриды углерода, несодержащий металл катализатор, фенилацетилен, гидрирование.
