CC BY
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AZ9RBAYCAN KIMYA JURNALI № 2 2016
21
UDC 541.49
METAL-FREE GRAPHITIC CARBON NITRIDES AS A CATALYSTS FOR HYDROGENATION OF PHENOL
V.M.Akhmedov, I.D.Ahmadov, H.G.Nurullayev, V.M.Ahmadov
M.Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 11.01.2016
We have demonstrated for the first time that polymeric carbon nitrides with graphitic structure can be successfully used as a metal-free catalysts for the hydrogenation of phenol to cyclohexanone and cyclo-hexanol in vapor phase under atmospheric pressure of hydrogen.
Keywords: graphitic carbon nitride, metal-free catalyst, phenol, hydrogenation, cyclohexanone, cyclo-hexanol.
Introduction
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 cyclo-hexane and phenol by their oxidation and hydrogenation, respectively. Although cyclohex-ane oxidation dominates the market, because of cheaper raw materials, this process suffers from low products yields and complicated recovery/separation steps [1]. The hydrogenation of phenol remains competitive, offering better selectivity. This route can be realized either in vapor or in liquid phase [2]. Currently the hydrogenation of phenol is generally carried out in the vapor phase with supported palladium catalysts (Pd/C, Pd/AhO3, and Pd/NaY zeolite catalysts) [3]. The distribution of products from phenol hydrogenation is highly dependent on the type of catalyst and the properties of the support. Recently, numerous studies were focused on the development of new types of Pd catalysts for the liquid-phase phenol hydrogenation at relatively low temperatures. A remarkable example was reported by Han and co-workers showing high phenol conversion and high cyclohexanone selectivity for the reaction catalyzed by a dual-supported Pd Lewis acid catalyst in dichloromethane [4]. However, the presence of Lewis acid imposes severe limitations on the use of the catalyst in hydrogenation reactions, and the reaction conditions are not environmentally friendly. A novel Pd catalyst
supported on silica with 2D-rectangular meso-structure and helical channels resulting from microphase segregation of structure-directing cationic and non-ionic surfactants has been developed [5]. The conversion of phenol in this case dropped from 99% for the first run to 73%, while for the tenth run with the selectivity of cyclohexanone around 98%. The reasons for the decrease in phenol conversion may include the leaching of Pd and due to aggregation/agglomeration of Pd-nanoparticles. Recently, a novel catalyst containing Pd-nanoparticles for the selective hydrogenation of phenol has been made based on mesoporous graphitic carbon nitride (mpg-C3N4) as a support [6]. 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.
A key role of support in the hydrogenation of phenol has been shown in the previous investigations [4-6]. From this point of view, the high catalytic performance of Pd/mpg-C3N4 [6] 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 "nonpla-nar" adsorption of phenol, which finally gives rise to its fast and selective hydrogenation. In
fact, there are a number of arguments to define the exceptional impact of mpg-C3N4 specific structure not only to the activity catalyst metal center. Owing to the structural and electronic properties graphitic carbon nitrides provide the prerequisites required even for its independence from metal catalytic activity. If g-C3N4 as a semiconductor capable to photochemically overcome the endothermic character of the water splitting process, presumably, under certain conditions it can also chemically activate a hydrogen molecule and run the hydrogenation reaction. Indeed, we have demonstrated recently that g-C3N4 can be successfully used as effective catalyst for the selective hydrogenation of phenylacetylene to styrene in the absence of any metal and metal oxides [7]. Following a related strategy, we found that polymeric graphitic carbon nitrides are also capable to catalyze the phenol hydrogenation. This paper highlights the first experimental results of our study on phenol hydrogenation to cyclohexanone and cyclohexa-nol over metal-free g-C3N4.
Experimental
A g-C3N4 possessing high chemical and thermal stability can be prepared by condensation of different nitrogen-containing precursors. The carbon nitrides were prepared by stepwise heating urea, dicyandiamide or melamine and their combinations up to the temperatures between 490 and 5200C (Table 1).
Table 1. The prepared ] polymeric graphitic carbon nitrides
и 'ci •S 4> 1) M ft ° Й fin Precursor Synthesis conditions ^BETj m2/g
I Dicyandiamide 300^400^500°C, 15 h 5.8
II Melamine 350^400^490^510°C, 20 h 5.3
III Melamine+Cya-nuric acid 350^400^520^, 25 h 5.1
Depending on reaction conditions, a variety of polymeric graphitic carbon nitrides with different crystalline phase can be obtained. As is shown, these as-prepared polymeric materials exhibit prolonged catalytic activity in phenol
hydrogenation to cyclohexanone and cyclohex-anol avoiding noble metals.
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 hours. After the reaction the alumina crucible was cooled to room temperature and then synthesized carbon nitride was collected and ground into powder, then characterized by X-ray diffraction (Bruker-D2 Phaser, Germany), Fourier transform infrared spectroscopy (Ni-colet-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 [8]. The infrared spectrum performed on this phase exhibits features very similar to those of carbon nitride reported in literature [9]. 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). It was observed that after prolonged use as catalysts at 190-2500C g-C3N4 powders were recovered and analyzed. The X-ray diffraction pattern of these fresh and used samples was almost identical.
Catalyst testing was conducted under an atmospheric pressure of hydrogen in a flow type microreactor. High purity hydrogen (99.5%) used for the phenol hydrogenation 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 phenol was studied in the temperature interval of 190-2500C. 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 experi-
mental run consisted of passing the 5 wt. % 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 chromatog-raphy equipped with FID (Agilent - 7820A) on a HP-5 capillary column 30 m long.
Results and discussion
The polymeric carbon nitrides with graphitic structure materials of Table 1 were tested as metal-free catalysts for the hydrogenation of phenol at temperatures ranging 190-2500C in a stream containing an excess of hydrogen with respect to the amount of phenol in solution. 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 2 demonstrates the relative activities of prepared samples.
Table 2. Hydrogenation of phenol on the polymeric graphitic carbon nitrides
ö oi Product distribution, %
£ ta
Catalyst r e H ^ tí o C 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+DCA 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; reac-
tant flow rate = 3 cm3/h;
DCA - diciandiamide, M - melamine.
All three prepared g-C3N4-based materials exhibit activity as hydrogenation catalysts of phenol to the mixture of cyclohexanon and cyc-lohexanol. The sample III has demonstrated much better activity as compared to the samples I and II (Table 1). The conversion of phenol in the presence of prepared samples increases along the temperature while selectivity of cy-clohexanon decreases significantly.
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) [10].
Electronic \ Bronstcd basic
properties / functions
Multiple functionality of g-C3N4 surface [10].
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. 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 solid material having multifunctional surface with possibility of committing multipurpose choices of the catalytic actions. Indeed, since it was reported by M.Antonietti et al. [10, 11] 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, oligomeri-zation of nitriles and Friedel-Crafts type reactions and, also for photo-catalytic water splitting, they have continuously attracted attention for the construction of low cost photo- and heterogeneous catalysts.
The 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 metals and in many cases, by the precious ones. As we have established earlier that g-C3N4 itself is capable to activate a hydrogen molecule and replace the metals for partial hydrogenation of triple bonds in phenylacetylene [7]. In the present study, a recyclable metal-free catalyst based on polymeric carbon nitride has been developed for hydrogenation of phenol to cyclohexanon and cyclo-hexanol.
It is still not clear how and which functionality of the carbon nitride surface interacts with hydrogen and phenol molecules leading to the formation of cyclohexanon and cyclohexa-nol. To understand the phenomenon better the known feature of so-called "frustrated Lewis acid-base pairs" it should be mentioned [12]. 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 et all. [13] concluded that the catalytic activity of metal-free graphene as hydrogenation catalysts of acetilene also would be the existence on the graphene layer of similar type of frustrated Lewis acid-base pairs 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 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 C=C bonds of phenol molecule.
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TORKiBiNDO METAL OLMAYAN QRAFÍT QURULU§LU KARBON NiTRiDLOR FENOLUN HiDROGENLOSMOSiNDO KATALiZATORLAR KÍMÍ
V.M.Ohmadov, LD.Ohmadov, H.Q.Nurullayev, V.M.Ohmadov
Biz ilk dafa olaraq gôstarmiçik ki, qrafit quruluçlu polimer karbon nitridlar metal içtiraki olmadan, qaz fazada va mülayim çaraitda phenolun tsirloheksanona va tsirloheksanola hidrogenlaçmasi ûçûn katalizator kimi istifada edila bilar.
Açar sözlar: qrafitaoxçar karbon nitridlar, metali olmayan katalizator, fenol, hidrogenh§m3, tsikloheksanon, tsiklo-heksanol.
ГРАФИТОПОДОБНЬК НИТРИДЫ УГЛЕРОДА, НЕ СОДЕРЖАЩИЕ МЕТАЛЛ, В КАЧЕСТВЕ КАТАЛИЗАТОРОВ ДЛЯ ГИДИРОВАНИЯ ФЕНОЛА
В.М.Ахмедов, И.Д.Ахмедов, ^Г.Нуруллаев, В.М.Ахмедов
Впервые показано, что полимерные нитриды углерода с графитовой структурой, не содержащие металл, могут быть успешно использованы в качестве катализаторов для гидрирования фенола до циклогексанона и циклогек-санола в паровой фазе при атмосферном давлении водорода.
Ключевые слова: графитоподобные нитриды углерода; катализатор, не содержащий металл; фенол; гидрогенизация; циклогексанон; циклогексанол.
