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6 AZERBAIJAN CHEMICAL JOURNAL № 1 2023 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 546.92:546.171.1:546.264
PLATINUM - MESOPOROUS CARBON NITRIDE NANOCOMPOSITES: SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE IN PHENYLACETYLENE
HYDROGENATION
V.M.Akhmedov, N.E.Melnikova, G.G.Nurullayev, Vs.M.Ahmadov, D.B.Tagiyev
M.Nagiev Institute of Catalysis and Inorganic Chemistry, Ministry of Science and Education
of the Republic of Azerbaijan
advesv@gmail.com
Received 07.09.2022 Accepted 17.10.2022
The platinum nanocomposites were synthesized by chemical reduction of H2PtCl66H2O in situ with a methanol—water mixture using mesoporous graphitic carbon nitride as a stabilizing matrix and a catalyst support. The textural, morphological, and optical properties of the obtained platinum - mesoporous carbon nitride composites (Pt/mpg-C3N4) were studied. Pt/mpg-C3N4 has been developed as an effective heterogeneous catalyst for the gas and liquid phase hydrogenation of phenylacetylene. An efficient and versatile approach to the modification of platinum with various organic solvents and ligands for the selective hydrogenation of phenylacetylene to styrene in a flow gas-phase process is presented. The addition of pyridine, piperidine, or morpholine to the starting solution of phenylacetylene in hexane provides 95-100% conversion and 90-92% selectivity to styrene. A sharp increase in the selectivity to styrene without substantial changes in conversion is also observed when THF, ethanol, triethylamine, dioxane, or chloroform are used as diluents. The obtained Pt/mpg-C3N4 composites also demonstrated considerable catalytic activity in the selective hydrogenation of phenylacetylene to styrene at low temperatures in the liquid phase.
Keywords: mesoporous graphitic carbon nitride, platinum composite, heterogeneous catalysis, phenylacetylene, styrene, hydrogenation.
doi.org/10.32737/0005-2531-2023-1-6-21
Introduction
At present, most chemical processes in industry are carried out on metal-containing heterogeneous catalysts. In order to improve the catalytic activity of heterogeneous catalysts, much effort has been put into developing the desired structure, composition, morphology, defects, and surface chemistry of the heterogeneous catalysts. Among them, a significant place belongs to highly active and versatile Pt-containing heterogeneous catalytic systems. They are efficient in many practically important processes: crude oil cracking and reforming; hydrocarbon hydrogenation, dehydrogenation, and oxidation; photocatalytic decomposition of water, etc. [1, 2]. The results of numerous studies reported in scientific literature indicate that nanostructured catalysts are substantially superior in efficiency to their analogs prepared by known methods containing metallic macropar-ticles. The synthesis and studies of catalytic
systems incorporating platinum nanoparticles appear especially promising for expanding the scope of applicability of platinum catalysts [35]. The choice of support is important because the interaction between metal and active phase plays crucial role in the comprehensive performance of the catalysts [6].
In terms of environmentally friendly and sustainable development, it is desirable to choose more greener and efficient ways to modify nanostructured catalysts. In recent years, polymeric graphite-like carbon nitride (g-C3N4) has attracted considerable attention of researchers owing to its wide availability and unique structural and electronic properties. Carbon nitride can make a cost-effective and environmentally benign alternative to traditionally used of various metal supports of inorganic origin [7]. The two-dimensional g-C3N4 material has a layered structure is characterized by exceptionally high thermal (up to 6000C in air) and chemical stability in acids, bases, and organic solvents. It
possesses basic and semiconductor properties, catalytic action: without participation of any metal, g-C3N4 was used in both chemical and photochemical heterogeneous catalysis to activate benzene, carbon dioxide, and hydrogen molecules, to catalyze water splitting, and in some other processes [7-10]. The g-C3N4 surface is rich in nitrogen capable of forming coordination bonds. Therefore, it is an ideal support for attaching metals; in addition, the resulting support/metal system will also act as a tandem catalyst in which both the matrix and the catalytically active nanoparticle work effectively. Thus, the multifunctional properties of met-al/g-C3N4 nanocomposites provide grounds for designing more efficient multipurpose catalysts. However, conventional methods for the synthesis of g-C3N4 by thermolysis of nitrogen-rich inexpensive precursors (cyanamide, dicyandi-amide, urea, melamine, etc.) afford to obtain non-porous graphite-like materials of dense morphology, characterized by a small specific surface area (~10 m2/g), low electronic conductivity, and high band gap (~2.7 eV), 7 limiting their efficiency in catalysis. Therefore, it is important to modify the texture and electronic structure of g-C3N4. This problem is successfully solved by fabricating porous structures and by elemental doping [11, 12]. Mesoporous carbon nitride (mpg-C3N4) with a pore diameter of 2-50 nm has numerous advantages over dense g-C3N4. This material is characterized by high specific surface area, large uniform pore volume, high basicity and controlled electronic/atomic structure. Mesopores dictate the size and type of nanoparticles that are formed during the synthesis of composites. Owing to large surface area and large pore volume, particles are uniformly distributed over the surface. Presumably, a dual approach involving the simultaneous implementation of mesoporosity and the deposition of platinum nanoparticles on mpg-C3N4 would obtain heterostructures with high catalytic activity.
In this study, Pt nanocomposites with mpg-C3N4 were synthesized by chemical reduction of platinum ions with methanol in situ without the use of additional toxic or harsh re-
which give rise to its unique ducing agents or stabilization agents, which would contaminate the nanocomposite. The composition, structure, surface morphology, and optical properties of the resulting nano-composites were studied. The catalytic activity of the obtained Pt/mpg-C3N4 nanocomposites was evaluated in the hydrogenation reaction of phenylacetylene.
Selective hydrogenation of alkynes is of basic significance in the production of polymers, pharmaceuticals, and agrochemicals. For instance, the partial hydrogenation of alkyne impurities present in the raw materials is carried out in the course of industrial production of high-quality polymers based on ethylene and styrene [13]. These impurities impede raw materials storage and processing, especially in the case where the crude material is further subjected to a catalytic process when the impurities can react with catalysts of the polymerization process thus deactivating them or decreasing their activity. The catalysts based on Pd and Pt attract special attention due to their high reactivity in molecular hydrogen activation via dissociative adsorption [1-6]. A specific feature of these metals is the ability to absorb predominantly alkynes compared to alkenes [3, 14, 15, 16]. The main problem is the deep hydrogenation of alkynes to alkanes on these metals. Therefore, the development of new catalysts for the partial hydrogenation of alkynes with high selectivity and activity remains to be an urgent problem. Its solution can be either the dilution of the active metal with the second metal in order to decrease sizes of noble metal ensembles or the formation of small (1-1.5 nm) Pd (or Pt) clusters [17, 18, 19]. An alternative approach is based on the use of selective modifiers such as organic bases with some affinity to the catalyst surface [20]. It is experimentally proved that Pd is characterized by a higher selectivity in the partial hydrogenation of alkynes compared to Pt [21]. Nevertheless, in a series of recent studies a high selectivity is also achieved in the presence of the Pt-containing catalysts on the carbon (graphene-like) supports. Metallic nanoparticles of a specified size can be obtained and the in-
teraction of the nanoparticles with the support surface can be controlled using these methods. For example, the Pt nanoparticles immobilized on carbon nanotubes functionalized by various oxygen-containing groups manifested high catalytic activity and selectivity in the partial hydrogenation of phenylacetylene (PA): the conversion and selectivity to styrene (St) are 99% and 88%, respectively [22]. It was assumed that this can be related to a higher dispersity of Pt nano-particles, which favors the partial hydrogenation of PA. The Pt particles immobilized on carbon nanofibers and carbon nanotubes were also found to exhibit high catalytic activity and selectivity in the partial hydrogenation of acetylene [5]. The selectivity of the catalyst based on Pt and carbon nanotubes turned out to be much higher than the selectivity of Pt on carbon nano-fibers, which is probably a consequence of different electronic structures of the Pt complexes on the considered supports. The BASF concern used water-soluble commercially available N-hexadecyl-N-(2-hydroxyethyl)-N,N-dimethylam-monium dihydrophosphate (HHDMA) for the preparation of the catalysts based on Pt, Pd, and Pt-Pd crystallites 2-10 nm in size immobilized on active carbon, which exhibit high activity and selectivity in alkyne hydrogenation [20]. The differences in the catalytic characteristics of Pt-carbon nitride catalyst correlated with the relative accessibility of an organic substrate to an active site and with the adsorption, configuration, and strength depending on the sizes of an ensemble and surface potentials. A series of the catalysts with isolated Pd and Pt atoms that are active in the selective hydrogenation of diverse alkynes were described [23-26]. It was reported that isolated Pd atoms can be incorporated in pores of mesoporous carbon nitride (mpg-C3N4), which made it possible to prepare one-site Pd catalysts [25, 26]. These catalysts exceed in activity the traditional Pd-containing heterogeneous catalysts in C2H2 hydrogenation retaining a high selectivity to ethylene (>80-90%) and resistance to leaching/agglomeration during the whole process. We have previously found that polymeric carbon nitride involving no metal can catalyze the selective hydrogena-
tion of PA to St in the gas phase at 150-2500C and atmospheric pressure [27]. It is shown that mpg-C3N4 with a higher specific surface area over that of carbon nitride of dense morphology has prolonged catalytic activity and stability in the highly selective partial hydrogenation of PA to St (>94-95%). Continuing the studies in order to develop highly efficient catalysts of this process, we have tested obtained Pt/mpg-C3N4 composites which exhibited high catalytic activity in PA hydrogenation in the gas and liquid phases. In this work we demonstrated that the selectivity of PA hydrogenation on the Pt/mpg-C3N4 composites can be controlled by applying simple organic modifiers.
Experimental part
Materials
Solvents and reagents of analytical or chemical purity grade were used, and if necessary they were purified by standard methods prior to use.
Synthesis of Pt/mpg-C3N4 composites
In the typical synthesis, melamine (2 g) and cyanuric acid (2.05 g) were dissolved in DMSO (70 and 40 mL, respectively) under the ultrasonic treatment. After the complete dissolution, both solutions were mixed at ~200C to form a white precipitate. The latter was dried at 600C and calcined in a furnace under a nitrogen atmosphere gradually increasing the temperature to 5500C [28]. The mpg-C3N4 sample was powdered and used for the synthesis of composites with Pt. Reduction of ^PtCU^^O for the synthesis of Pt/mpg-C3N4 nanocomposites was carried out by methanol in situ in the presence of mpg-C3N4 [29] In order to prepare the catalyst samples with the Pt content 0.5, 1, and 3 wt.%, the calculated amounts of H2PtCl66H2O and mpg-C3N4 were added to a methanol-water (90:10, vol.%) mixture (50 mL), and the resulting mixture was ultra-sonicated for 30 min. The obtained mixture was transferred to a flask and heated to 60-650C with a reflux condenser with permanent stirring for 1 h. The mixture was filtered on a water-jet pump at ~200C. The formed solid residue was dried in a vacuum drying box at ~500C and
stored under an inert atmosphere. The prepared catalyst samples after analysis according to the reload of Pt (wt.%) were designated as Pt/mpg-C3N4-0.46, Pt/mpg-C3N4-0.91, and Pt/mpg-C3N4-2.86 (Table 1).
Characterization of obtained Pt/mpg-C3N4 composites
The content of platinum was found by inductively coupled plasma atomic emission spectrometry on an Agilent 5110 ICPOES instrument. The specific surface area of the samples iSbet (Brunauer-Emmett-Teller method) was determined using data on nitrogen adsorption-desorption at 77 K using a Thermo Fisher Scientifi c automatic gas analyzer. The structural identity and the phase composition of samples was evaluated by powder X-ray diffraction on a D2 PHASER diffractometer (Bruker, Cu-Ka radiation, nickel fi lter) in the 20 range of 10-700C. The surface microstructure and morphology were examined by scanning electron microscopy (SEM) on a JSM-7600 F instrument (JEOL) in the LEI mode at an accelerating voltage of 15 kV and a working distance of 4.5 mm. FTIR spectra were recorded on a Varian 3600 FTIR spectrometer (Varian, Inc) in the frequency range of 400-4000 cm-1 in KBr pellets, the diffuse reflectance spectra were measured on a Specord 250 PLUS UV/Vis-spectrophotometer (Analytik, Jena) in the wavelength range of 200-1000 nm, and the photoluminescence (PL) spectra were recorded on a Varian Cary Eclipse spectrofluorimeter (Varian, Inc.) with emission wavelength of 400 nm.
Gas-phase hydrogenation of PA on Pt/mpg-C3N4 composites
Gas-phase hydrogenation of PA was carried out at 1500C in a flow type fixed-bed reactor. In the typical experiment, the preliminarily prepared powder of Pt/mpg- C3N4 (100 mg) was loaded between two layers of quartz wool in the reactor tube (length 250 mm, inner diameter 8 mm), which was placed in a temperature- maintained furnace equipped with an electric heater, a ventilator, and a thermocouple. The latter was placed at the center of the catalyst bed to monitor the reaction temperature. The temperature
was maintained with a Micromax microelectric regulator. The catalyst was treated for 30 min in a hydrogen flow at the reaction temperature. The starting reagent (solutions of PA with various diluents) was supplied with a microsyringe pump (New Era Systems Inc., USA). The liquid mixture was preheated in an evaporator and supplied to the reactor together with a H2 flow (or in a mixture of hydrogen with nitrogen) with a gas hourly space velocity (GHSV) of 2240 h-1.
Liquid-phase hydrogenation of PA on Pt/mpg-C3N4 composites
Liquid-phase hydrogenation of phenyla-cetylene was carried out in a 220 mL stainless steel autoclave. In a typical experiment 100 mg of Pt/ mpg-C3N4, 10 mmol phenylacetylene and 50 ml solvent were placed in the autoclave (Different organic solvents: n-hexane, cyclohexane, toluene, tetrahydrofuran, diethylamine, chloroform, dioxane were used as solvent). Then the autoclave was purged with 0.5-1.5 MPa hydrogen gas. The experiments were carried out in the temperature range 25-700C.
Analysis of the reaction products
The reaction products were analyzed on an Agilent-7820 A-FID gas-liquid chromatography using the 30-m HP-5 column. The conversion and yields of the reaction components (PA, St, and ethylbenzene (Eb)) were calculated from the calibration coefficients determined with allowance for the dependences of the chromatographic surface areas on the concentration. «-Octane was used as the internal standard. The reproducibility of the results of repeated experiments involving the catalyst samples from the same batch was ±5%. The conversion of PA (Y) and selectivity of hydrogenation (S) were determined by the following equations:
Y (%) = ([PA]in - [PA]out/[PA]in] ■ 100%,
S (%) = (NP/NPA)-100%,
where [PA]in and [PA]out are the contents of PA at the inlet and outlet from the column, respectively; NP and NPA are the number of moles of the obtained product P and PA, respectively.
Results and Discussion
Use of mpg-C3N4 as stabilizing ligand for platinum nanoparticles
In this study, we performed for the first time greener and efficient one-step synthesis of Pt/mpg- C3N4 composites. Metal nanoparticles were formed in situ on the porous mpg-C3N4 surface by reduction of platinum ions with methanol. The reducing agents normally used for the synthesis of metal composites (e.g., NaBH4, its mixtures with NaOH, hydrazine, etc.) are harsh and are often deposited on the substrate surface. In this respect, the reduction with alcohols has obvious advantages owing to its simplicity, the possibility of easy control of the course of the reaction, and the lack of difficulty in removing the reducing agent from the reaction medium. The mechanism of H2PtCl66H2O conversion in alcohols, in particular, in methanol and in a methanol-water mixture, was discussed in detail earlier [30]. It was shown that the course of synthesis of platinum nanoparticles correlates with the content of water in the reaction mixture. When water content is <10 vol.%, the reduction is substantially accelerated, i.e., water acts as a catalyst. The average diameter of Pt particles in colloid dispersions obtained by the reduction with alcohols is 2.7 nm and the particle size distribution is within 5 nm. Nanoparticles are rapidly aggregated in solutions to give 100-200 nm aggregates [31]; therefore, the presence of a stabilizing agent in the system is necessary. In this study, for the first time, the stabilizing function is performed by the matrix itself owing to the coordination of Pt to nitrogen-containing groups on the mpg-C3N4 surface. We synthesized Pt/mpg-C3N4 composite samples with platinum contents of 0.5, 1, and 3 wt. %. Table S1 presents data on the effect of the amount of platinum on the physical properties and textural characteristics of the obtained samples. Note that the weight fraction of the metal in all Pt/mpg-C3N4 nanocomposites was found to be lower than that calculated from the amounts of the precursors taken. This may be due to incomplete reduction of the precursors or to incomplete deposition of metal nanoparticles on the support surface, which is explained by the repulsion of the outer region of the nanoparticle solvation shell from the likely charged surface. As a result, some of Pt nanopar-
ticles that have not been attached to the support are lost during isolation of the composite [32].
Characterization of the obtained Pt/mpg-C3N4 composites
The nitrogen adsorption-desorption isotherms of the mesoporous matrix and the composites correspond to typical type IV isotherms according to the Brunauer-Emmett-Teller classification, which attests to the presence of mesopores in the materials [33]. The isotherm measured for the nanocomposite at elevated relative pressures is steeper than that of the initial matrix. This attests to a smaller pore volume and a smaller surface area of the composite in comparison with the matrix, since the support pores become occupied by the deposited Pt. However, the nanosized platinum occupies the pore space only partly and leaves them unplugged. An analysis of the pore size distribution curve calculated from desorption data shows that the pore size varies within 18-40 nm. The main bulk of the mesopores contains pores 22-35 nm in diameter.
The phase structure of the matrix and the composites was analyzed using the results of X-ray diffractometry (Figure 1), which confirmed the formation of graphite-like structures and composites based on them. All matrix and composite samples exhibit two peaks: the weak diffraction peak at 13.10C indexed in the (100) plane corresponds to an in-plane structural packing of tri-s-triazine units with a period of 6.91 A and the strong reflection at 27.40C indexed in the (002) plane corresponds to the interlayer packing of the conjugated aromatic systems with inter-layer spacing d = 3.26 A. These parameters are in good agreement with the accepted standards for the graphite structures (JCPDS Card No. 871526) and published data [34, 35].
When the metal is introduced into the mpg-C3N4 structure, the X-ray diffraction pattern changes. The formation of metal nanoparticles is confirmed by the corresponding peaks in the diffraction patterns. The Pt/mpg-C3N4-0.46 and Pt/mpg-C3N4-0.91spectral lines show additional characteristic peaks at 20 = 39.60, 46.05, and 67.180, which can be identified as X-ray diffraction from (111), (200), and (220) planes of Pt metal crystals, respectively (according to JCPDS Card No 65-2868). This indicates that nanoparti-
cles in the composite represent pure crystalline platinum with a face-centered cubic lattice [36]. In addition, the intensity of the XRD signals of Pt gradually increases with increasing Pt content in the samples. Three additional reflections confirm the deposition of platinum nanoparticles on the mpg-C3N4 layers and the formation of nanocom-posites. The positions of mpg-C3N4 peaks do not change as this takes place. Thus, platinum nano-particles are not incorporated into the carbon nitride structure, but are adsorbed on the surface, including the surface of the pores. According to powder X-ray diffraction data, the average size of platinum crystallites calculated by the Debye-Scherer formula is 30-39 nm [37]. As it can be identified from the SEM images of the surfaces of the initial matrix and nanocomposite (Pt/mpg-C3N4-2.86), the metal nanoparticles are uniformly arranged on the matrix nanosheets as inclusions (Figure 2 (a, b). The nanoparticle shape is nearly spherical, and their sizes vary in the range of 1843 nm depending on the Pt0 concentration.
The structural identity of the synthesized metal composites and the initial mpg-C3N4 was confirmed by FTIR spectroscopy (Figure 3). All spectra exhibit bands characteristic of pure carbon nitride. Small differences are due to the complex interaction between the matrix elements and the surface of a nanoparticle at the molecular level. In the 1460—1750 cm-1 range, all spectra exhibit strong bands at 1637, 1570, 1541, and 1458 cm-1, corresponding to the stretching vibrations of the conjugated C-N bonds in the heterocycle. The positions of these bands in the spectra of Pt/mpg-C3N4 -0.46, Pt/mpg-CsN4-0.91and Pt/mpg-C3N -2.86 coincide almost completely with their positions in the spectrum of the pure matrix.
The absorption bands at 809 cm-1 in the spectrum of the matrix and at 810 cm-1 in the spectra of the Pt/mpg-C3N4 -0.46, Pt/mpg-C3N4-0.91 and Pt/mpg-C3N4-2.86 composites are characteristic of the out-of plane C-N bending vibrations of heterocycles in the triazine unit [38]. In the 1230—1410 cm-1 spectral range, the bands at 1405, 1315, and 1235 cm-1 for the matrix; 1405,1315, and 1236 cm-1 for Pt/mpg-C3N4-0.46; 1406, 1315, and 1235 cm-1 for Pt/mpg- C3N4-0.91; and 1405, 1315, and
1235 cm-1 Pt/mpg- C3N4 -2.86 refer to the C-N(-C)-C or C-NH-C stretching modes [39]. The absorption bands in the 3100-3500 cm-1 range correspond to the O-H stretching modes of adsorbed water molecules and =NH or -NH2 stretching and bending modes. In the spectrum of the matrix, the residual amino components are recorded at 3252 and 3166 cm-1, while in the case of composites, the absorption maxima shift to longer wavelengths: up to 3254 and 3166 cm-1. They are present due to the incomplete condensation of organic precursors during pyrolysis [40]. The very weak band at 2175 cm-1, which characterizes the C=N or N=C=N stretching modes, indicates that trace amounts of precursor molecules have not completely cleaved during the pyrolysis [41]. Analysis of the Fourier transform IR spectra provides the conclusion that the major characteristic peaks of carbon nitride are retained in the spectra of all composite samples. This proves that the structural integrity and, hence, specific physico-chemical properties of the mesoporous matrix remain unchanged.
The electronic structure and optical properties of mpg-C3N4 and Pt-nanocomposites based on them were studied by diffuse reflectance electronic spectroscopy in the visible and UV ranges (Figure 4). In the case of mpg-C3N4, the absorption edge occurs at 440 nm. The spectra of the composites exhibit a red shift of the absorption edge by ~20-40 nm with increasing amount of platinum deposited on carbon nitride. The absorption intensity of the composites in the visible region (at X > 500 nm) obviously increases. Note that the increase in the amount of platinum in the composite is accompanied by increasing background absorption in this range. This is in good agreement with the color of the samples, which varies from yellow to nearly black. This fact is caused by direct absorption of light by dark platinum particles in the composite, which confirms the presence of platinum [42-44]. Thus, the Pt doping of the mesoporous matrix results in increasing absorption of visible light band gap for the composites. The charge separation efficiency and recombination rate of photogenerated electron-hole pairs were evaluated by measuring the PL at an emission wavelength of 400 nm. The PL spectra of mpg- C3N4 and
Pt/mpg-C3N4 are depicted in Figure 5. It is known that the PL intensity of mpg- C3N4 nanosheets is ~60% lower than that of g-C3N [45]. This indicates that the increased surface area and high porosity promote charge separation.
The emission peaks of the samples are located in the 400-600 nm wavelength range.
The PL spectrum of mpg- C3N4 exhibits an intense emission peak with a maximum at 431 nm, which is indicative of massive recombination of photogenerated charge carriers. When mpg-C3N4 is doped with Pt nanoparticles, the PL intensity decreases by ~50-60%.
Fig. 1. XRD patterns of mpg-C3N4 (1) and Pt/ mpg-C3N4 nanocomposites: 2 - Pt/ mpg-C3N4 - 0.46; 3 - Pt/ mpg-C3N4
-0.91; 4 - Pt/ mpg-C3N4 - 2.86.
a b
Fig. 2. Scanning electron microscopy images of Pt/mpg-C3N4 -2.86. SEM images of Pt/mpg-C3N4 -2.86 with increasing: a) x70000; b) x350 000.
*. I a™ 25® moo imo 500
Waveaumber, cm 1
Fig. 3. FTIR spectrums of mpg-C3N4 and the Pt/ mpg-C3N4 complexes recorded between 400 and 4000 cm" 1 - mpg-C3N4; 2 - Pt/ mpg-C3N4 - 0.46; 3 - Pt/ mpg-C3N4 - 0.91; 4 - Pt/ mpg-C3N4 - 2.86.
Fig. 4. Diffuse reflectance electronic spectra of mpg-C3N4 and nanocomposites: 1 - mpg- C3N4; 2 - Pt/ mpg-C3N4 -0.46; 3 - Pt/ mpg-C3N4 - 0.91; 4 - Pt/ mpg-C3N4 - 2.86.
Fig. 5. PL excitation spectra of mpg-C3N4 (1) and Pt/ mpg^N - 2.86 (2).
Catalytic properties of prepared Pt/mpg - C3N4 composites
In our previous work, we established for the first time that polymeric carbon nitride itself is capable of activating a hydrogen molecule without the presence of any metal, which was successfully used as a catalyst for the selective hydrogenation of PA in ST in the gas phase at atmospheric pressure [27]. We assume that the dual approach including simultaneous implementation of mesoporisity and deposition of platinum nanoparticles on mpg-C3N4 would furnish heterstructures with high catalytic activity in hydrogenation of PA. In this work, we studied the catalytic efficiency of the obtained Pt/mL/g-C3N4 composites in the hydrogenation of PA in the gas and liquid phases.
Hydrogenation of PA in the gas phase
Only the data on the gas phase hydrogenation of PA on the Au- and Pd-containing catalysts immobilized on alumina were published [46, 47]. In this work, the hydrogenation of PA on a platinum-containing catalyst in the gas phase was studied for the first time. The hydrogenation of PA was carried out at 1500C in a flow fixed-bed reactor under atmospheric pressure of hydrogen.
Experiments on hydrogenation were conducted at various molar ratios PA/H2. Solutions of PA in hexane with a concentration of 20-50 wt.% were used at the initial stage. From the data presented in Table 1, it follows that samples of Pt/mpg-C3N4 catalysts containing 0.46-2.86 wt. % Pt, have a high activity in the hydrogenation of PA, but low selectivity to St. The samples containing 0.91 and 2.86 wt.% Pt at the molar ratio H2/PA = 2 and the flow rate of PA equal to 0.02 mol h-1 almost completely convert PA to ethylbenzene (Eb), whereas 25-26% St are formed on the samples containing 0.46 wt.% Pt. The Pt/mpg-C3N4 -0.46 sample was chosen as an optimum catalyst for consecutive studies. The data presented in Table show that mpg-C3N4 itself without Pt hydrogenates PA with high selectivity for St and does not exhibit a catalytic effect in the hydrogenation of St. As the Pt content in the composites increases, the selectivity to St decreases and selectivity to Eb increases. This means that PA on Pt/mpg- C3N4 is subjected to the consecutive reduction to St, which is further hydrogenated to Eb. The PA concentration exerts a significant effect on the hydrogenation rate. The hydrogenation rate reaches 43.4 mol h-1 based on 1 g of platinum at the flow rate of a PA solution in
hexane from 0.005 to 0.02 mol h 1 and molar ratio H2/PA=2 on the Pt/mpg-C3N4 -0.46 catalyst. In this case, the full conversion of alkyne is observed and the selectivity to St increases from1.7 to 25.8%.
Catalytic activity of the Pt/mpg-C3N4 composites for PA h sure and various molar ratios PA/H2 a_
The data presented in Table also show that an increase in the H2/PA molar ratio favors a significant increase in the yield of Eb. This indicates the stimulation of deep hydrogenation by surface hydrogen excess.
ogenation in the gas phase at 1500C and atmospheric pres-
Catalyst Flow rate/mol h 1 Conversion (%) Selectivity (%)
PA H2 St Eb
mpg-C3N4b 0.02 0.04 35.9 94.2 5.8
mpg-Cs^ 0.02 0.04 No hydrogenation of St - -
Pt/ mpg-C3N4-2.86 0.02 0.04 100 2.9 97.1
Pt/ mpg-C3N4-0.91 0.02 0.04 100 6.7 93.3
Pt/ mpg-C3N4-0.46 0.005 0.01 100 1.7 98.3
0.01 0.02 100 5.7 94.3
0.015 0.03 100 20.6 79.4
0.01 0.03 100 2.2 97.8
0.01 0.04 100 traces 99.9
0.015 0.04 100 11.4 88.6
0.015 0.05 100 3.9 96.1
0.02 0.04 100 25.8 74.2
0.02 0.05 100 19.3 80.7
a Hexane as diluent; GHSV 2240 h-1; mpg-CsN^ 0.3 g; Pt/mpg-CsN^ 0.1 g; b Hydrogénation of PA; c Hydrogénation of St.
Numerous experimental data are available on the possibility to optimize the catalytic activity and selectivity of the catalysts based on noble metals for alkyne hydrogenation using various modifiers in the liquid phase [3, 12, 14, 17, 19, 22]. We found that the dilution of the starting PA with diverse solvents strongly affects the catalytic properties of Pt/mpg-C3N4 -0.46 in the gas-phase hydrogenation reaction. As shown in Figure 6, 100% conversions of PA and selectivity to St in a range of 25-26% were achieved using hexane as the diluent. When hexane was replaced by THF, EtOH, triethyla-mine, dioxane, or chloroform, the selectivity to St increased sharply, whereas the conversion of PA remained unchanged to a high extent. Experiments with the addition of EtOH, THF, and chloroform showed the selectivity to St equal to 91.1, 91.5, and 91.7%, respectively. Moreover, a selectivity of 92.2% to St was detected when using a mixture of EtOH and water as diluent.
The same phenomenon was observed for the liquid-phase partial hydrogenation of both terminal and internal alkynes in the presence of the heterogeneous catalysts based on Pd and other noble metals [26, 34]. It is well known that the nature of the solvent can strongly affect the reaction rate under homogeneous conditions. Acid and base catalysis is usually interpreted in terms
of polarity effects or solvent viscosity [48]. There were attempts to attribute effects of the diluents to their polarity and solubility of H2 [49, 50]. However, no systematic correlation was observed between the results of alkyne hydrogenation and solvent polarity or solubility of H2.
Ligands with electron-donor/acceptor properties, such as nitrogen- and phosphorus-containing compounds, are widely used in homogeneous catalysis to stabilize a series of catalyti-cally active nanoparticles (Pd, Ru, Ir, Rh, Au) by efficiently changing their catalytic characteristics during hydrogenation of polyunsaturated organic compounds [51]. The application of organic lig-ands of this type provides a new approach to tuning the catalytic characteristics of metallic nano-particles in heterogeneous catalysis as well. For example, the Pd/TiO2 catalyst modified by tri-phenylphosphine exhibits an enhanced selectivity in C2H2 hydrogenation due to the retardation of ethylene hydrogenation to ethane [52]. The Pt na-noparticles immobilized on silica, which is modified by cross-linked tri-phenylphosphine polymer (Pt/PSiO2), catalyze the liquid phase hydrogenation of PA providing 87.5% selectivity to St at a conversion of 72% [7]. The effect of modifiers was also illustrated using nitrogen-containing compounds to control selectivity of the liquidphase hydrogenation of alkynes in the presence of
the catalysts based on noble metals [3, 36, 37]. Using this approach, the BASF concern (Na-noSelectTM) developed a new palladium catalyst on the support modified by the N-ligand for the selective hydrogenation of alkynes [20].
In this work, we systematically considered for the first time the influence of various organic modifiers on controlling selectivity of the gasphase hydrogenation of PA on the Pt/mpg-C3N4 catalysts. We found that diverse nitrogen- containing additives substantially change the catalytic properties of the catalyst based on Pt/mpg-C3N4 in the gas-phase hydrogenation of PA.
As shown in Figure 7, the hydrogenation proceeds with 100% conversion and a selectivity of 90-91% with the addition of a small amount of pyridine (0.3-0.4 ml) to the initial mixture of PA in hexane. The addition of morpholine, piperidine, and triethylamine exerts a similar effect on the selectivity of the action of Pt/mpg- C3N4. Note that an increase in the con-
centration of the N-containing aromatic modifiers in the raw materials exerts a negative effect on the catalyst activity; while triethylamine can be applied even as a diluent of PA in hydrogenation (see Figure 4). We also performed the hydrogenation of PA on the PPh3Pt/mpg-C3N4, P(OPh)3 • Pt/mpg- C3N4, and bipyPt/mpg^N composites to study the effect of the ligands immobilized on the Pt/mpg-C3N4-0.46 surface.
As shown in Figure 8, the catalyst samples obtained by the immobilization of PPh3 and P(OPh)3 at the ratio L/Pt = 1 demonstrate a substantial increase in the selectivity of the reaction: the selectivity to St was 74 and 72.4%, respectively, at the full conversion of PA. The selectivity to St equal to 81% and 97% conversion of PA were achieved upon the addition of bipy under similar reaction conditions. The tested samples at the ratio Pt/L = 2 exhibited a low catalytic activity.
YorSl'i)
IConv.
St.
Eb.
Ill» III
HiMK Efaraw Cllcrofara THF Ethanol EtaotfWÜer R¡H Dirait
Fig. 6. Influence of various diluents on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in PA hydrogenation on Pt/mpg-C3N4 -0.46 at 1500C and atmospheric pressure; molar ratio H2/PA = 2; flow rate of PA 0.02 mol h-1, GHSV 2240 h-1.
I Com . I H St. H£b.
Oil
Pvxidine Morpholíne Pipíridine Triethanolamine
Fig. 7. Influence of nitrogen-containing additives on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in PA hydrogenation on Pt/mpg-C3N4-0.46 at 1500C and atmospheric pressure; molar ratio H2/PA = 2, hexane as diluent; flow rate of PA 0.02 mol h1, GHSV 2240 h1.
Fig. 8. Influence of the ligands strongly bound to platinum on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in PA hydrogenation on Pt/mpg-C3N4 -0.46 at 1500C and atmospheric pressure; L:Pt = 1:1; molar ratio H2/PA = 2, hexane as diluent; flow rate of PA 0.02 mol h1, GHSV 2240 h1.
It follows from the data obtained that the character of the effect of the organic modifiers on the catalytic efficiency of Pt/mpg- C3N4 in the gas phase does not substantially differ from specific features of their behavior observed for various Pd-containing catalysts in the liquid phase [20, 51-54]. We assume that similarly to the role of CO and NH3, in the case of using the Pd-containing catalysts, diverse polar diluents, such as EtOH, THF, chloroform, and others, do not directly participate in PA hydrogenation on Pt/mpg-C3N4: they form a dense layer and over the Pt surface, which prevents St to adsorb on the active site and H2 concentration to increase on the catalyst surface. Thus, deep hydrogenation is prevented. The increase in the selectivity of PA hydrogenation by the nitrogen-containing compounds can be explained by their ability to change the electron characteristics of the active metal. Pyridine, piperidine, and morpholine molecules act as o-donors and form weak coordination bonds with Pt and thus prevent the adsorption of St on the active site. Probably, the modification of the catalyst surface by ligands with electron-donor/electron-acceptor, or che-
lating properties is caused by strong electronic and steric effects. The yield of Eb increases due to the prolong residence of St on Pt particles.
The Pt/mpg-C3N4 -0.46 catalyst demonstrates high conversion (100%) and selectivity to St (91-92%) in PA hydrogenation in the presence of diverse organic modifiers in the gas phase at 1500C and atmospheric pressure. The catalytic activity of the developed catalyst exceeds 8 103 (mole of product) (mole of Pt)-1 h-1 under the studied conditions. The resistance of the new catalyst to metal loss was tested in the flow process (1500C, atmospheric pressure, THF as diluent) for 10 h.
As shown in Figure 9, neither conversion nor selectivity detected in the presence of the fresh catalyst did not decrease during experiment. This makes it possible to consider Pt/mpg-C3N4 to be an efficient and stable catalyst. Thus, a principal possibility was demonstrated for preparing the highly active catalyst of selective PA hydrogenation based on Pt using which the activity and selectivity compared to those for the widely used in practice Pd-containing catalysts can be achieved.
Fig. 9. Kinetics of changes in the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in PA hydrogenation on Pt/mpg-C3N4 -0.46 at 1500C and atmospheric pressure; molar ratio H2/PA = 2, THF as diluent; flow rate of PA 0.02 mol h-1, GHSV 2240 h-1.
Hydrogénation of PA in the liquid phase
Based on the high catalytic response of the resulting composites in the gas phase conversion of phenylacetylene, we also tested Pt/mpg-C3N4 as a liquid phase hydrogenation catalyst at lower temperatures. Hydrogenation of PA was carried out in a stainless steel autoclave. A solution of 10 mmol PA in cyclohexane was employed at the initial stage of study. The temperature from 25 to 700C and the pressure from 0.50 to 2.0 MPa were examined to investigate their influence on the catalytic performance. For a detailed study of the catalytic characteristics of the prepared Pt/mpg-C3N4, we tested all obtained three composites containing different amounts of platinum nanoparticles. It was shown that all samples containing 0.46-2.86 wt.% Pt, possess catalytic activity in the hydrogenation of PA at 500C As the Pt content in the composites increases, the selectivity to St decreases and selectivity to Eb increases. This means that PA on Pt/mpg- C3N4 is subjected to the consecutive reduction to St, which is further hydrogenated
to Eb. The Pt/mpg-C3N4 -0.46 sample was chosen as an optimum catalyst for consecutive studies. The influence of reaction temperature on efficiency of obtained composites in the hydrogenation of PA was studied in the range from 25 to 700C by using n-hexane as the solvent at 1.0 MPa hydrogen pressure. It can be seen from Figure 10, with an increase in the reaction temperature, the conversion of PA increases, but, as expected, the selectivity of PA for ST decreases significantly.
The pressure effect was examined for hydrogenation of PA in hexane at 500C in the range of 0.5-2.0 MPa. Figure 11 shows the conversions and selectivity's of hydrogenation of PA to St and Eb. The high pressure of H2, which increases the solubility of hydrogen in the liquid phase, significantly changes the distribution of the reaction products: in this case, the conversion of PA reaches 84%, although an increase in pressure has an unfavorable effect on the selectivity for St, reducing it by more than 14% at 2.0 MPa.
Com".
St.
lEb.
Y or S (%)
100
J d Ml
Fig. 10. Influence of reaction temperature on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in phenylacetylene hydrogenation on Pt/mpg-C3N4 -0.46._
■ conv. M St. H|M
100 1
Jill
0,5MPa IMPs l.SMPa 2 MPa
Fig. 11. Influence of hydrogen pressure on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in phen-yacetylene hydrogenation on Pt/mpg-QN -0.46 at 500C.
Fig. 12. Influence of various solvents on the conversion (1) and selectivity to styrene (2) or ethylbenzene (3) in phenyacetylene hydrogenation on Pt/mpg-C3N4 -0.46 at 500C.
■ Cobv. ■ St. H|Eb.
100 1
iiiii
The hydrogenation of PA on a Pt/mpg-C3N4 -0.46 was studied using various organic solvents at 500C and 1.0MPa. The corresponding conversion and selectivity are shown in Figure 12. Among the solvents tested, tetrahydrofuran gave high activity and selectivity toward styrene -97.5%. The resistance of the catalyst to metal loss was tested in tetrahydrofuran at 500C and 1.0MPa. The spent catalyst recovered by filtration and then reused seven times without apparent loss of activity and selectivity. Thus, the obtained Pt/mpg-C3N4 composites also demonstrated considerable catalytic activity in the selective hydrogenation of phenylacetylene to St at low temperatures in the liquid phase.
Conclusions
The Pt nanoparticles immobilized on mesoporous graphitic carbon nitride by chemical reduction of H2PtCl6*6H2O in situ with a methanol-water mixture showed high catalytic activity of obtained composites in hydrogenation of PA in the gas phase at 1500C and atmospheric pressure. High conversions of PA and selectivity to St (up to 100% and 91-92%, respectively) were achieved on the model Pt/mpg-C3N4-0.46 catalyst modified by the available organic modifiers. The catalytic activity of the developed catalyst for PA hydrogenation in the gas phase is higher than 8*103 (mole of product) (mole of Pt)-1 h-1. The stability of the new catalyst was tested in the flow process (1500C, atmospheric pressure, THF as diluent) for 10 h. No decrease in the conversion of PA and selectivity to St was observed, indicating a high effi-
ciency of the developed catalyst. The obtained Pt/mpg-C3N4 composites also demonstrated considerable catalytic activity in the selective hydrogenation of phenylacetylene to St at low temperatures in the liquid phase.
The results obtained can be used in the development of heterogeneous catalysts based on polymeric carbon nitride, which acts as an efficient and environmentally friendly matrix and stabilizing ligand, and possibly as a cocata-lyst for the selective hydrogenation of polyun-saturated hydrocarbons.
Acknowledgments
This work was supported by the National Academy of Sciences of Azerbaijan (Priority Project for Fundamental Research and Development: grant number - 7/3 /2018).
The authors contributed equally to this paper: conceived the experiments and wrote the paper, discussed the results and the manuscript.
This paper does not contain descriptions of studies performed by the authors that involve humans or use animals as objects. The authors declare no conflict of interest in financial or any other sphere.
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PLATINUM-MESGMOSAMOLl KARBON NÍTRÍD NANOKOMPOZlTLORl: SiNTEZi, QURULUSU VO FENILASETÍLENÍN HÍDROGENLO§MOSÍNDO KATALITIK XASSOLORl
V.M.Ohmadov, Н.Е.Мелникова, G.G.Nurullayev, Vs.M.Ohmadov, D.B.Tagiyev
Mezomasamali qrafit karbon nitridi stabillaçdiriri matris va katalizator daçiyicisi kimi istifada etmakla H2PtCl6^6H2O-nun metanol va su qariçigi ila in situ kimyavi reduksiyasi yolu ila platinin nanokompozitlari (Pt/mpg-C3N4) sintez edilmiçdir. Alinmiç platin- kompozitlarinin (Pt/mpg-C3N4) morfoloji va optik xassalari tadqiq edilmiçdir. Pt/mpg-C3N4 fenilasetilenin qaz va maye faza hidrogenlaçmasiprosesinda heterogen katalizator kimi tadqiq edilmiçdir. Pt/mpg-C3N4-in katalitik xassalarina müxtalif halledcilarin va liqandlarin tasiri ôyrnilmiçdir. Fenilasetilenin heksanda mahluluna piridin, piperidin va ya morfolinin alava edilmasi 95-100% çevrilma va stirol göra 90-92% selektivlik tamin edir. THF, etanol, trietilamin, dioksan va ya xloroform halledicilar kimi istifada edildikda, çevrilmada ahamiyyatli dayiçikliklar olmadan stirol ûçûn seçiciliyin kaskin artmasi mûçahida olunur. Pt/mpg-C3N4 kompozitlari maye fazada açagi temperaturda fenilasetilenin stirola seçici hidrogenlaçdirilmasinda da ahamiyyatli katalitik aktivlik nûmayiç etdirmiçdir.
Açar sözlzr: mezo masamali karbon nitridi, platin kompozitlari, heterogen kataliz, fenilasetilen, stirol, hidrogenla§ma.
ПЛАТИНОВО-МЕЗОПОРИСТЫЕ НАНОКОМПОЗИТЫ УГЛЕРОДА: СИНТЕЗ, ХАРАКТЕРИСТИКА И КАТАЛИТИЧЕСКИЕ СВОЙСТВА В ГИДРИРОВАНИИ ФЕНИЛАЦЕТИЛЕНА
В. М. Ахмедов, Н. Е. Мельникова, Г.Г. Нуруллаев, Вс. М.Ахмедов, Д.Б.Тагиев
Нанокомпозиты платины были синтезированы методом химического восстановления H2PtCl6^6H2O in situ смесью метанола и воды с использованием мезопористого графитового нитрида углерода в качестве стабилизирующей матрицы и носителя катализатора. Исследованы текстурные, морфологические и оптические свойства полученных композитов платина - мезопористый нитрид углерода (Pt/ mpg-C3N4). Pt/mpg-C3N4 был разработан как эффективный гетерогенный катализатор для газофазного и жидкофазного гидрирования фенилацетилена. Представлен эффективный и универсальный подход к модификации платины различными органическими растворителями и лигандами для селективного гидрирования фенилацетилена в стирол в проточном газофазном процессе. Добавление пиридина, пиперидина или морфолина к исходному раствору фенилацетилена в гексане обеспечивает конверсию 95—100% и селективность по стиролу 90—92%. Резкий рост селективности по стиролу без существенного изменения конверсии наблюдается и при использовании в качестве разбавителей ТГФ, этанола, триэтиламина, диоксана или хлороформа. Полученные композиты Pt/mpg-C3N4 также продемонстрировали значительную каталитическую активность в реакции селективного гидрирования фенилацетилена в стирол при низких температурах в жидкой фазе.
Ключевые слова: мезопористый нитрид углерода, композиты платины, гетерогенный катализ, фенилацети-лен, стирол, гидрирование.
