CC BY
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UDC 541.49
POLYMERIC GRAPHITIC CARBON NITRIDES AS METAL-FREE MULTIPURPOSE
ORGANIC CATALYSTS
V.M.Akhmedov
M.F.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
advesv@gmail.com Received
The development of new promising metal-free catalysts is of great significance in chemistry. This review paper presents a great advance for designing and developing highly efficient heterogeneous organic catalysts based on polymeric carbon nitrides. These challenging research areas have drawn increasing interests from many research groups including our own. Possibilities of the current trends of practical application of graphitic carbon nitride materials as effective photo-, electro- and chemical metal-free catalysts are briefly analyzed.
Keywords: graphitic carbon nitride, metal-free catalyst, oxidation, hydrogenation, oligomerzation, photochemical splitting of water, activation of benzene, activation of carbon dioxide.
Introduction
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. As an example, graphene, single layer graphite with close-packed conjugated hexagonal lattices, owing to its structural and electronic properties provides the prerequisites required for many technical purposes. Therefore, graphene has attracted a great deal of attention during recent years in the fields of microelectronic and optoelectronic devices [1], energy storage materials [2], catalysts [3], polymer composites [4] etc.
Polyaddition and polycondensation of some N-containing precursors (cyanamide, dicy-andiamide, melamine etc.) leads to the formation of polymeric carbon nitrides with different structural and electronic properties [5]. Among them the carbon nitrides with graphite type modification (g-C3N4) is regarded to be the most stable allotrope. It has been predicted to be structurally analogous to carbon-only graphite and have
attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. Compared with graphene, they have electron-rich properties, basic surface functionalities and H-bonding motifs due to the presence of N and H atoms. Their films consist of stacked as graphene two-dimensional (2D) crystals between a few and several hundreds of atomic layers in thickness. g-C3N4 and its modifications are have a high thermal and chemical stability and distinct to many other organic materials they have a high stability against oxidation up to 5000C in air, making handling them in air possible. It is thus regarded as a potential candidate to complement carbon in material applications.
g-C3N4 is a medium-bandgap semiconductor possessing an effective photo-, electro- and chemically catalytic performance in a broad variety of reactions and can be used as catalyst for the photochemical splitting of water, mild and selective oxidation and hydrogenation reactions, and as effective catalytic support (Figure 1) [6]. They also show unexpected catalytic activity for activation of benzene, oligomerization of nitriles, and the activation of carbon dioxide. Its catalytic performance is easily adjustable by modifying texture, optical, and electronic properties via nano-casting, doping, and copolymerization. The surface of g-C3N4 remains clean in contrast to conventional catalysts and it can provide a stable
catalytic action over a long period of time demonstrating a remarkable thermal stability.
Electronic ^n'' properties
' Л.Л
/ÑH \ Brönslcd basic N'<WN /' functions
JL4-. JL A
N^N N^N N^N
A A AX AX
N^N^N N^N^N N^N^N
. Л Л-А Л А А А Д..-Д:--,
N N I N. N N N N N \ N NH2
I — I I *........'
Lewis basic H-bonding
functions motif
Fig. 1. Multiple functionality of g-C3N4 surface [6].
Polymeric graphitic carbon nitride materials also tolerate functional groups and are therefore suited for multipurpose applications in biomass conversion and sustainable chemistry that make them excellent catalysts for green chemistry.
A brief review of methods for the synthesis and modification of polymeric carbon nitrides
The history of carbon nitride polymers could trace back to 1834 when Liebig firstly prepared "melon", (C6N9H3), a carbon nitride derivative [7]. These chemicals were so inert, so insoluble, that deducing their structures was difficult. Interest in carbon nitrides - in particular its saturated, sp -hybridised, crystalline allotropes - was sparked in the 1980s when by the work of Cohen who theoretically predicted that the hardness of C3N4 phases (a-, P-, and cubic) "may be harder than diamond" [8]. Great efforts have been devoted to prepare such covalently bonded carbon nitride materials in recent years [5, 6]. However, instead of diamond-like C3N4 phases, in most cases, graphitic carbon nitride was synthesized and found to be the most stable allotrope at ambient conditions. While this "super-diamond" remains elusive, interest has spread to the graphitic forms of carbon nitride that have numerous potential uses in photo-, electro- and chemical catalysis. Graphitic carbon nitride polymers could be regarded as "doped" carbon-materials in which some carbon atoms in graphite are replaced by nitrogen atoms in a regular manner. They stand for a large family of related compounds (CxNy) in
general. It was suggested that C3N4 has five polymorphs, graphitic-C3N4, a-C3N4, P-C3N4, pseu-docubic-C3N4 and cubic-C3N4 [9]. Ideally, carbon nitrides only consist of carbon and nitride atoms, and no other atoms. Up to now, perfect g-C3N4 with structure shown in Figure 1 has not been prepared in experiment. An average C/N ratio of 0.72 is usually found (theoretical value of C3N4 = 0.75), as well as small but significant amounts of hydrogen (2%) from uncondensed amino functions. Based on these findings, it was assumed that a single structure probably cannot be regarded for C3N4, it is a mixture of polymers of different size and architecture in reality. Nevertheless, the unique conjugated structure had already make as-prepared graphitic carbon nitride polymer stable up to 5500C in air. The tri-s-triazine based structure of g-C3N4 was postulated on the basis of density-functional theory calculations to be more stable at ambient conditions [10].
As with graphite, due to the optimal packing of the van der Waa^ interaction between the individual layers g-C3N4 is insoluble in most solvents. The bandgap of the condensed graphitic carbon nitride is estimated to be 2.7 eV from its ultraviolet-visible spectrum, showing a strong absorption of the banned area about 420 nm [11]. The corresponding electronic structure of links makes g-C3N4 promising candidate for the transformation of energy systems, such as photo-electrochemical battery. High chemical and thermal stability makes such photo-electrochemical cells stable even in an oxygen atmosphere [12].
An arsenal of chemical methods for the synthesis of carbon nitride is very diverse [13]. Known to date synthetic procedures usually lead to the formation of g-C3N4. A relatively simple procedure of thermal self-condensation of small nitrogen-rich compounds (cyanamide, carbamide, dicyandiamide, melamine or their mixtures) in air at atmospheric pressure is regarded as the most appropriate method for the synthesis of the ideal layered g-C3N4 having a ratio of C/ N ~ 3A A possible route of formation of g-C3N4 based on cyanamide is represented in Figure 2 [6]. The successive condensation of small precursors for the synthesis of g-C3N4 allows the different degrees of polymerization which makes easy dop-
ing, transfer of charges and the stabilization of electrons and holes, making the material promising for a variety of catalytic schemes.
hi
400 425 450 475 500 525 550 575
«—dicyandiamidc cyan \ 1
amide ~ .melamine
inelam
ntelem ' —• ^
melamine chain
dimelen\
melon
\ melon " sheet
\
.CjN4
cx
Fig. 2. Calculated energy diagram for the synthesis of carbon nitride [6].
The starting precursor cyanamide is condensing into melamine. Further condensation can then proceed via the triazine route (dash-dot line) to C3N4, or melamine can form melem and then follow the tri-s-triazine route (dashed line) to form C6N8 [6].
The synthesized carbon nitrides by self-condensation of simple nitrogen-rich compounds usually are bulk materials with very low surface area. However, to expand the application possibilities of g-C3N4 as a catalyst or catalyst carrier requires increasing its surface area and related chemical functionalities. Some novel methods using various modifiers and templates were proposed in recent years enabling production of nano- and mesostructured g-C3N4 with high surface area and an increased number of active sites. A typical procedure for the synthesis of mesoporous carbon nitride (mpg-C3N4) of locally ordered structure and a large surface area with silica template is described in [6]. Electronic and morphological properties of bulk g-C3N4 can be also changed by exfoliation its nanosheets. Such sheets, like graphene possess unique structural and physicochemical properties, and are therefore of considerable interest to enhance the practical application of graphitic carbon nitride. It was shown that compared with the bulk carbon nitride the electron transfer resistance with nanosheets is reduced by 75% [14].The photocurrent can also be increased by protonation and involving of heteroatoms into the g-C3N4 molecule [15].
Chemical modification is an effective way of settings the physico-chemical properties of graphitic carbon nitrides and expanding their application. It is possible to control both the surface texture and chemical properties, and electronic properties of carbon nitride by applying this method. Modification of carbon nitride is usually carried out by two ways. 1) Implantation of functional groups directly onto the surface of g-C3N4. 2) Feeding of additives into the system at the beginning of the synthesis and incorporation into the matrix during formation g-C3N4. g-C3N4 can be protonated by aqueous solutions HCl and H2SO4 at room temperature [14]. Sulphur, boron, fluorine and phosphorus can be doped at frame of carbon nitride [16]. Different metal ions doped (Li+, Na+, K+, Fe3+,
Al, Zn, Mn3+, Co3+, Ni3+, Cu2+, Eu etc.) carbon
nitrides were also synthesized [17]. Incorporation of transition metal cations such as Fe3 +, Mn3+, Co3+, Ni3+, Cu2 + at g-C3N4 frame can be used for expansion of the light absorption toward longer wavelengths and to reduce the recombination of photo-generated electrons [17]. Alkali metal ions such as Li +, Na +, K , coordinated g-C3N4 frame together with the ions Cl-, lead to a spatial distribution of charge carriers in areas intercalated areas [14]. Doping by rare earth metals, in particular Eu, leads to narrowing of bandgap zone of g-C3N4 [18].
Applications as non-metallic organic catalysts
Cheaper, clean, inert to the reaction medium impacts metal-free catalyst is today regarded as one of the most promising to solve the problem of effective photo-, electro- and chemical heterogeneous catalysis. Feature of non-metallic semiconductor polymeric graphitic carbon nitrides having an optical bandgap makes them a promising platform for the construction of the metal-free low cost green catalytic systems. They have the correct electronic and microstructure, 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 tri-s-triazine rings generate the Bronsted and
Lewis base centers in the frame of g-C3N4 (Figure 1) [6]. 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. The porous structure can increase the semiconductor surface area, which contributes to an enhancement in energy conversion efficiency [15, 16]. Thus, graphitic carbon nitrides can be regarded an as solid material having multifunctional surface with possibility of committing multipurpose choices of the photo-, electro- and chemical catalytic actions. Indeed, since it was reported by authors [6] that metal-free graphitic carbon nitrides can be used as effective catalyst for a variety of reactions, such as photo-catalytic water splitting process, activation of benzene and carbon oxide, Friedel-Crafts type reactions and oli-gomerization of nitriles, they have continuously attracted attention for the construction of low cost non-metallic heterogeneous catalysts.
Photocatalyst for water splitting. Hydrogen is believed to be the most promising clean energy and photo- and electrocatalytic splitting water is one of the most important pathways to produce hydrogen. The studies on heterogeneous photo-catalysis have mainly focused on the development of materials with a sufficiently small band gap zone and suitable band positions for water splitting applications [19]. Most of known conventional catalysts for the hydrogen production from water are exclusively metal-based, comprising metal oxides, nitrides, sulfides, and phosphides and their mixed composites. Searching catalytically active metal-free organic semiconductor materials for the effective water splitting, alternative to metal-containing catalysts becomes very important [20]. The carried out calculations by authors [11] provides evidence that nanostructured semiconductor materials based on g-C3N4 carbon nitride has the potential to function as a photocatalyst for the watersplitting reaction under visible-light irradiation. They suggest that the nitrogen atoms would be the preferred oxidation sites for H2O to form O2, whereas the carbon atoms provide the reduction sites for H+ to H2. The observed efficiency of g-C3N4 catalyst was approximately
0.1% with irradiation of 420-460 nm. The efficiency could be improved by 8.3 times by introducing mesoporosity into g-C3N4 [21]. These results open new opportunities for the search of energy production catalysts, using polymeric organic semiconductor materials. Doping g-C3N4 by hetero atoms offers an effective approach to modify the texture, optical and electronic properties, and photoreduction of g-C3N4. g-C3N4 with 3% fluorine content reduces the band gap 2.63 eV, improving the hydrogen evolution by 2.7 times [22]. Hydrogen evolution on the similarly modified S/g-C3N4 was 7.2-8.0 times greater than on the unmodified g-C3N4 [16]. Developed hybrid composite g-C3N4/GO-WO3 to increase photocatalyst efficiency by using reduced graphene oxide (GO) as an electron mediator [23]. Recently authors
[24] achieved that conversion of solar energy to hydrogen on a composite photocatalyst g-C3N4 and 2D dot-carbon (CD/g-C3N4) up to 2%. Prolonged experiments showed stable performance of the catalyst for 200 days.
Photo-reduction of CO2. Due to exciting physical, chemical and electronic properties the composites based on g-C3N4 are expected to be also useful in the process of photo-reduction of CO2. Indeed, as shown recently, g-C3N4 synthesized from urea and melamine are enable to catalyze CO2 photo-reduction in NaOH solution
[25]. It is shown that CO2 reduction by g-C3N4 (urea) forms CH3OH and C2H5OH, while the g-C3N4 (melamine) produces only C2H5OH. S-containing carbon nitride synthesized from thiourea improves CH3OH yield by 1.4 times [26]. The hybrid composites graphene/g-C3N4 show high activity in the conversion of CO2 to CH4 in the presence of water vapor under illumination of daylight [27]. g-C3N4, obtained from urea, showed high catalytic activity in the reaction of CO2 cycloaddition to the epoxy substrates forming cyclic carbonate [28]. Mesoporous carbon nitride not only fixes CO2 in the form of carbonate, but also successfully activates CO. g-C3N4 initiates the reaction CO2 with benzene, producing phenol and carbon monoxide (Figure 3) [29]. It is assumed that the nitrogen-containing groups (NH2, C-NH-C) of g-C3N4 act as an organic base activating CO2 by nucleo-philic attack, similar to natural enzymes.
N^N
X iL
N^N N N^N N^N
M^IAN N^N^N
] i i 1
CO;
N^-N N^N^N'^O
N^N N^N
N^IAM N^IAM
^N^HJ^KAN^SJ^SAN''
Benzene
N
N^N
NAAn'^O ^IAn^N^ÎAO
X X XI.P
N^N N N^N N
-CO
'N' N^N
X iL
N^N N O
N^N
N^N^N
■ Phenol
N^N
JO
IAi-AN lAlAw 'NW^nW^N-'
Fig. 3. Estimated mechanism of phenol formation from CO2 and benzene on mpg-C3N4 [29].
Oxidation of hydrocarbons and alcohols. Catalytic oxidation is one of the most fundamental and important process for the production of many valuable chemical materials in industry. Current commercial oxidation processes are characterized by several significant drawbacks - low selectivity, low recycling of the catalyst, the negative impact of the toxic effects of the metal based catalysts on the environment [30]. Recent studies have shown that g-C3N4 and its modified forms is the most promising candidate for the development of a new generation of catalytic systems for non-metallic oxidation of alkanes, olefins and alcohols. One can expect that in the near future, the oxidation of various substrates on the g-C3N4 will receive broad practical application.
Recently, it was shown that g-C3N4 enriched with B and F promotes the oxidation of cyclohexane to cyclohexanone with a selectivity >90% [31]. Toluene is easily oxidized to benzaldehyde by mpg-C3N4 with 99% selectivity [32].
Phenol and its derivatives are widely used in the manufacture of various chemical materials. Currently, most phenol is produced from benzene by the three-step cumene process,
which generates high amounts of waste. Recently, it was shown that g-C3N4 is an active photo-catalyst for the highly selective oxidation of benzene to phenol under mild conditions [33]. B- and F-doped g-C3N4 demonstrated increased activity under irradiation with visible light (X>420 nm) at 600C using H2O2 as an oxidant for the conversion of benzene to phenol.
g-C3N4 also is an effective catalyst for the selective oxidation of alkenes. The oxidation of cyclohexene by molecular oxygen is characterized with exceptionally high selectivity towards cyclohexenon. The model systems that have been developed in recent years, based on metal-loporphyrins due to oxidative degradation of the metalloporphyrin catalysts makes them impractical for routine oxidative catalysis. Formation of allyls - 2-cyclohexene-1-one and 2-cyclohexene-2-hydroperoxide on g-C3N4 indicates that attack the activated C-H bond is more preferable than the attack of C=C double bond [34].
The oxidation of alcohols to aldehydes and ketones is another process that is of fundamental importance in both the laboratory and industry [35]. Photocatalysis system based on polymeric carbon nitride materials has been also extended
for the oxidation of alcohols. It is shown that mpg-C3N4 acts as an effective catalyst for the oxidation of alcohols to aldehydes/ ketones by photochemical activation of oxygen. Oxidation of benzyl alcohol to benzaldehyde follows with 57% conversion and more than 99% selectivity under visible light irradiation for 3 hours at 1000C [36].
Conversion of biomass. Nowadays, the study directed to obtaining liquid fuel and important intermediates based on renewable raw materials being actively developed [37]. The successful solution of this problem is related to the catalytic dehydration of biomass to 5-hydroxy-metilfurfural (HMF), which envisaged as the main raw material for the manufacture of the most important technical products such as high-effective fuels, polymers, pharmaceuticals, fertilizers and others. However, due to the complexity
the isolation and purification, HMF produced today by the acidic dehydration of fructose is an overly expensive raw material. Progress in this direction is to create an integrated chemical process where all steps take place in the "one-pot". The authors of [38] using protonated g-C3N4(H+) for the dehydration of fructose, and V/g-C3N4, catalyzing the successive oxidation of HMF managed to conduct direct transformation of fructose to 2.5-diformilfuran (DFF) in the "one-pot", with a yield of 45% (Figure 3).
We have investigated the process of fructose conversion to HMF in ionic liquids using g-C3N4 and its modified forms. According to our unpublished data, g-C3N4 exhibits a catalytic activity with a yield in the range of 5% at 1000C. Conversion of fructose on g-C3N4 /H+ increases to 36-40%, and on g-C3N4 (H+) /CrCfe to 60%.
i-c3n4(h+
Fig. 3. Conversion of fructose to HMF and DFF, catalyzed by the modified g-C3N4 [38].
Hydrogenation Reaction. Hydrogenation of carbon-carbon double and triple 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.
Styrene (ST) is a valuable monomer used in the production of synthetic resins and plastics. It is produced by dehydrogenation of ethylbenzene (EB). Phenylacetylene (FA) is also formed in this process as a byproduct, which worsens polymerization catalyst activity of styrene and has a negative impact on the quality of the resulting polystyrene. For this reason, phe-nylacetylene must be selectively hydrogenated to styrene or ethylbenzene to produce styrene of polymerization purity. Recently, we have found that g-C3N4 can replace metals for partial hy-
drogenation of triple bonds in phenylacetylene [39]:
O-CH CHt Och=CH & 0CH-"CH' ■
It was demonstrated that a highly efficient catalysts can be prepared on the base of polymeric carbon nitrides synthesized from urea, dicyandiamide or melamine and their combinations at the temperature interval of 490-5200C. Catalyst testing was conducted under an atmospheric pressure of hydrogen in a flow type microreactor. These as-prepared polymeric materials exhibit prolonged catalytic activity in partial hydrogenation of phenylacety-lene to styrene in the temperature interval of 150-2700C providing high conversion with re-
markable selectivity (up to 95-97%) without additives (Table 1). Compared with classical Lindlar catalysts [40] the developed method is more advantageous due to effective catalyst re-cyclability on the metal-free system.
Table 1. Hydrogenation of phenylacetylene on graphitic carbon nitrides; catalyst - 0.5 g; hydrogen flow rate = 30 cm3/min; reactant flow rate = 3 cm3/h. Samples from: I - di-ciandiamide; II - melamine; III - dicyandiamide+melamine
№ FA:H2:S, mol C3N4 T, 0C V, h-1 T, s Conv. FA, % % >5EB, %
Solvent - «-heptane
1 I 150 1.0 4.5 33.2 99.4 0.6
2 1:1.2:1.7 I 200 1.0 4.5 58.7 99.1 0.9
3 I 250 1.0 4.5 77.4 98.7 1.3
4 II 250 1.0 4.5 76.1 99.1 0.9
5 III 250 1.0 4.5 76.7 98.7 1.3
6 1: 2.0:2.3 I 250 0.8 4.1 78.2 98.9 1.1
Recently, we have also found that g-C3N4 can replace metals for the hydrogenation of phenol to the mixture cyclohexanone and cy-clohexanol, which are the key intermediates in synthesis of s-caprolactam and adipic acid, basic materials for the industrial production of nylon - type polymers [41]. 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 [42]. 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/Al2O3, and Pd/NaY zeolite catalysts) [43]. Recently, a novel catalyst containing Pd-nanoparticles for the selective hydrogenation of phenol has been made by authors [44] 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 nanopar-ticles from catalyst surface was observed. There remains great need to develop efficient and en-
vironmentally friendly catalysts for the phenol conversion to cyclohexanone and cyclohexanol.
The results of our study indicate that the prepared from urea, dicyandiamide or melamine and their combinations, used for the partial hydrogenation of phenylacetylene carbon nitrides with graphitic structure can be also functioned as a stable non-metallic catalyst for the hydrogenation of phenol to cyclohexanone and cy-clohexanol in the vapor phase at temperatures ranging 190-2700C:
The conversion of phenol in the presence of prepared samples increases along the temperature while selectivity of cyclohexanon decreases significantly (Table 2).
Table 2. Hydrogenation of phenol on the polymeric graphitic carbon nitrides; catalyst - 0.5 g; hydrogen flow rate = 30 cm3/min; reactant flow rate = 3 cm3/h, DCA- dici-andiamide, M - melamine_
Catalyst T, 0C Conversion, % Product distribution, %
cyclo-hexanon cyclohexanol
DCA 190 10.4 83.4 16.6
200 25.7 80.1 19.9
220 50.4 74.6 25.4
250 71.3 66.3 33.7
M 190 16.8 74.8 25.2
200 23.6 70.4 29.6
220 41.8 67.3 32.7
250 68.4 59.6 40.4
M+DCA 190 33.3 87.5 12.5
200 55.3 75.8 24.2
220 76.9 71.1 28.9
250 97.4 48.6 51.4
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" [45]. 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 al. [46] concluded that the catalytic activity of metal-free graphene as hydrogenation cata-
lyst of acetylene 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 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. Accordingly, the reaction mechanism should involve the uptake of H2 on the defects of g-C3N4 surface that subsequently would transfer to the compounds with double and triple bonds.
Friedel-Crafts type reactions. Friedel-Crafts reactions are widely used in industry. These processes are usually catalyzed by Lewis acids. It has been shown recently that in principle, the same results can be obtained using mesoporous mpg-C3N4 as catalyst having strongly expressed nucleo-philic character [6, 47]. The texture and surface basic centers of mpg-C3N4 can be modified by changing the weight ratio of template/monomer. The highest activity exhibited mpg-C3N4/0.5 synthesized from cyanamide and mpg-C3N4/0.7 synthesized from guanidine hydrochloride. It is found that the maximum conversion is achieved in heptane - 80% at 900C. The principle of nucleophilic activation has been extended also on alcohols and carboxylic acids in the Friedel-Crafts reactions. Alkylation of benzene yields mesitylene with 100% selectivity at 20% conversion. Reaction with ethanol gives 16% para-diethyl benzene for 24 hours and with a isopropanol 13% of cymene is produced at relatively mild conditions. Benzaldehyde was synthesized in the reaction of benzene and formic acid with a yield of 100% and 100% selectivity.
Conclusions
In this review, we briefly summarized the latest research efforts on the development of g-
C3N4 based photo- electro- and chemical organic green catalysts regarded as one of the most promising to solve the problem of effective metal-free heterogeneous catalysis. Graphite-like carbon nitride having a 2D-structure is an organic semiconductor. It is non-toxic, due to strong covalent bond between carbon and nitrogen is stable in acidic and alkaline media and can withstand temperatures of up to 5500C, which makes possible the use this material under conditions which are not suitable for the operation of presently known organic semiconductors. g-C3N4, possessing the functions of Bronsted and Lewis bases, rich in electronic centers and easily forms a hydrogen bond, tolerate functional groups in the reaction medium and therefore can be considered as an environmentally friendly multifun-tional green catalyst for the implementation of many important chemical synthesis. Its catalytic performance is easily adjustable by modifying texture, optical, and electronic properties via nanocasting, doping, and copolymerization. Layering g-C3N4 to 2D sheets like graphene is a promising way to improve its chemical catalytic activity. Creating new structures, such as hybrids of g-C3N4 with graphene, improves the unique electronic, optical, mechanical and chemical properties, which presents a significant opportunity for the use them as multifunctional catalysts. It is highly interesting that many reactions that use catalytic nature of g-C3N4, based on multi N-C=N binding motif, appear as mimic of biochemical processes, like the catalytic function of natural enzymes.
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torkíbíndo metal OLMAYAN QRAFÍT QURULUSLU POLÍMER KARBON nítríd osasinda
ÇOXFUNKSiYALI ÜZVÍ KATALÍZATORLAR
V.M. Ohmadov
Tarkibinda metal olmayan yeni perspektiv katalizatorlarin hazirlanmasi böyük ahamiyyata malikdir. Bu maqalada polimer karbon nitridlar asasinda son illarda tarkibibda metal olmayan yüsak aktivliya malik üzvi heterogen katalizatorlarin hazirlanmasi sahasinda alda olunan naticalar müzakira edilir. Bu perspektiv elmi istiqamat dünyanin bir sira aparici tadqiqat markazlarinda intensiv inkiçaf etdirilir. Maqalada qrafit-quruluçlu karbon nitridlar asasinda hazirlanan katalizatorlarin foto-, elektro- va kimyavi kataliz kimi mühüm sahalarda istifada imkanlari analiz olunur.
Açar sözlzr: selen, qrafit polimer karbon nitridlar, metal olmayan heterogen katalizator, oksidhçma, hidrogenhçma, oliqomerl3§m3, suyun fotokimyavi parçalanmasi, benzolun aktivh§masi, karbon qazinin aktivh§masi.
ГРАФИТОПОДОБНЫЕ ПОЛИМЕРНЫЕ НИТРИДЫ УГЛЕРОДА, НЕ СОДЕРЖАЩИЕ МЕТАЛЛ, В КАЧЕСТВЕ МНОГОЦЕЛЕВЫХ ОРГАНИЧЕСКИХ КАТАЛИЗАТОРОВ
ВЖ.Ахмедов
Обсуждаются результаты, достигнутые в последние годы в области разработки новых перспективных высокоэффективных безметаллических органических катализаторов на основе полимерных нитридов углерода. Анализируются это научное направление, привлекшее возрастающий интерес многих исследовательских групп, а также актуальные направления практического применения графитоподобных нитридов углерода в качестве эффективных фото-, электро- и химических катализаторов.
Ключевые слова: графитоподобный нитрид углерода, безметаллические катализаторы, окисление, гидрирование, олигомеризация, фотохимическое расщепление воды, активация бензола, активация углекислого газа.
