CC BY
25
AZ9RBAYCAN KÍMYA JURNALI № 3 2016
175
UDC 542.943:544.47:546.215:546.26
CARBON NANO-DIMENSIONAL CATALYSTS FOR OXIDATION OF HYDROCARBONS BY HYDROGEN PEROXIDE (A REVIEW)
E.B.Zeynalov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Elzey@mail.ru Received 03.03.2016
The article reviews the versatility of hydrocarbons, including those of petroleum origin being oxidized by hydrogen peroxide in the presence of nanocarbon family with emphasis on their endless catalytic properties and applications. Virtually, all main contemporary articles covered by the Thomson Reuters electronic database have been considered.
Keywords: oxidation of hydrocarbons, hydrogen peroxide, carbon nanotubes, catalyst support, catalyst, degradation of aromatic compounds.
Introduction
The development of advanced catalysts plays a key role in the fields of materials science and engineering. Despite some excellent catalysts have been developed, considerable challenges remain to bring down the cost and increase the activity of catalysts. The emergence of carbon materials [i.e., activated carbon, graphite, fullerenes, carbon nanotubes (CNT), diamond, graphene, etc.] provides an excellent alternative to traditional catalysts [1-3] (Figure 1).
Fig. 1. Versatility of nanocarbon materials as alternative to traditional catalysis.
Considerable effort has been made to develop diverse carbon-based nanocatalysts, including heteroatom-doped carbon, carbon supported catalysts, carbon hybrids, and so on, providing the new class of metal free oxidation catalysts exhibiting excellent activity [4-15].
This analytical review describes the different types of carbon nanotubes used as a cata-
lyst for the oxidation of hydrocarbons. Mainly, the contemporary articles have been highlighted.
Carbon nanotubes supported catalysts
Several types of carbon materials, including activated carbon, polymer based carbon xerogels, multi-walled carbon nanotubes, nanodi-amonds, microdiamonds, graphite and silicon carbide were doped by gold using sol immobilization and double impregnation. Samples were characterized by N2 adsorption at tem-
perature programmed desorption, high-resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spec-trometry, high-angle annular dark-field imaging (Z-contrast), X-ray photoelectron spectroscopy and atomic absorption spectroscopy [16].
The obtained Au/nanocarbon materials were used as catalysts for the oxidation of cy-clohexane to cyclohexanol and cyclohexanone, with aqueous H2O2, under mild conditions. The most active catalyst was supporting gold nano-particles on carbon nanotubes, achieving an overall turnover number of ca. 171 and an overall yield of 3.6% after 6 h reaction time. These values are comparable to the industrial process (that uses Co catalysts and high temperature), but were obtained at ambient temperature with considerable low loads of catalyst (Au catalyst to
a
substrate molar ratio always lower than 1x
10),
which is of relevance for establishing a greener catalytic process for cyclohexane oxidation.
Moreover, a very high selectivity towards the formation of cyclohexanol and cyclohexanone was achieved, since no traces of by-products were detected. The promoting effect of pyrazine carboxylic acid was observed and an optimum peroxide-to-catalyst molar ratio was found to be 2x104. Further increase of the oxidant amount results in decreased yield due to over-oxidation reactions at higher H2O2 amounts. Catalyst recycling was tested up to six consecutive cycles for the most active catalytic system and it was found that the catalyst maintains almost the original level of activity after several reaction cycles (there was only a 6% drop in activity after the sixth cycle) with a rather high selectivity to cyclohexanol and cyclohexanone and with no catalyst leaching. The differences in activity for the other samples were explained in terms of gold nanoparticle size and the textal properties of the carbon support [16].
Highly efficient epoxidation of alkenes with H2O2 catalyzed by tungsten hexacarbonyl supported on multi-wall carbon nanotubes (MWCNTs) modified with 1,2-diaminobenzene is reported. The prepared catalyst, [W(CO)6@DAB-MWCNTs], was characterized by elemental analysis, scanning electron microscopy, FT-IR, and diffuse reflectance UV-Vis spectroscopic methods. The prepared catalyst was applied as an efficient catalyst for green epoxidation of alkenes with hydrogen peroxide in CH3CN. This heterogeneous metal carbonyl catalyst showed high stability and reusability in epoxidation without loss of its catalytic activity [17].
Nanostructured multiwalled CNTs and titanium dioxide composite catalyst was prepared by a modified acid-catalyzed sol-gel method. Pure anatase TiO2 and the CNT-TiO2 composite are tested in the photocatalytic degradation of four para-substituted phenols: 4-chlorophenol, 4-aminophenol, 4-hydroxybenzoic acid and 4-nitrophenol. The effect of several operational parameters on the photoefficiency of the composite catalyst is studied by using 4-chloro-phenol as model compound, namely catalyst loading, pH of the medium, hydrogen peroxide concentration, substrate concentration. A relationship between the Hammett constant of each ^ara-substituted phenolic compound and its de-
gradability by the photocatalysts is found. The effect of the carbon phase in the catalyst is ascribed to its photosensitizer action. A clear beneficial effect is observed for the degradation of 4-aminophenol and 4-chlorophenol. For the molecules with stronger electron-withdrawing (deactivating) groups, such as 4-hydroxyben-zoic acid and 4-nitrophenol, no synergy effect is observed [18] (Figure 2).
Fig. 2. Graphical abstract to the [18].
The methylene blue (MB) solution was used to examine the photocatalytic activity of Fe/TiO2/C and Fe/TiO2/MWCNTs composites. The photocatalytic activity of Fe/TiO2/C and Fe/TiO2/MWCNTs composites was found to be higher than that of the pure TiO2 and Fe/TiO2 under UV and visible light irradiation. In comparison of photocatalytic activity of Fe/TiO2/C and Fe/TiO2/MWCNTs composites, the photo-catalytic activity of Fe/TiO2/MWCNTs composites was slightly higher than that of Fe/TiO2/C due to strong electronic properties of MWCNTs. The photocatalytic activity of Fe/ TiO2/C and Fe/TiO2/MWCNTs composites was more progressed fast by powerful H2O2 assisting photo-Fenton reaction due to oxidation of Fe2+- Fe3+ under UV irradiation [19].
MWCNTs supported Fe2O3 catalysts were prepared by impregnating acid treated the mentioned one in Fe(NO3)3 solutions and calcined at 2000C. The catalysts were used to degrade phenol and phenolic contaminants (resor-cinol and ortho-chlorophenol) with H2O2 by Fenton reaction. The experimental conditions such as reaction temperature, catalyst load and H2O2 dosage on the degradation of phenol have been investigated [20].
More than 70% phenol has been degraded with Fe2O3/MWCNTs catalysts after 200 min, 20% higher than with bare Fe2O3, suggesting that the reaction performance could be enhanced by the presence of MWCNTs. This demonstrates that MWCNTs are good and promising supporting materials and catalysts for advanced oxidant process and other catalytic reactions as well [20].
Due to the availability and easy preparation, graphenes are currently under intense investigation for various applications in chemistry, including their use as metal-free catalysts. The presence in low percentage of heteroatoms on the graphene sheet ("doping") has become a general strategy to modify the electron density, electrical conductivity and other properties of graphenes. The influence of doping can be reflected on the use of these materials in catalysis.
After a brief introduction presenting the unique properties and features of graphenes and the reasons why they are suitable to be applicable in catalysis, the present review focuses on those reports describing the use of doped gra-phenes as metal-free catalyst or as support of metal nanoparticles, electro- and photocatalysis is excluded. Preparation methods of doped gra-phenes and adequate characterization techniques providing important information, particularly with respect to the active site in catalysis, are briefly presented before the main body of the review describing common features and relevant examples of the use of doped graphenes in catalysis. Also general traits of graphenes of support of metal nanoparticles have been commented [21]. The final section summarizes the main conclusions and provides our view future developments in the field (Figure 3).
Fig. 3. Graphical abstract to the [21].
The purpose of the present review is not to provide an exhaustive account of all the ex-
isting literature, but rather to introduce the reader in the opportunities and advantages that doped graphenes offer with regard to the use either as metal free catalyst or support of metal nanoparticles [21].
The production of the reactive oxidant (i.e., hydroxyl radical) by the Fenton-like reaction (Fe(III)/H2O2) was greatly improved by the addition of carbon materials such as powdered activated carbon (PAC) and carbon nanotubes (CNT). The presence of carbon materials enhanced the oxidative conversion of methanol into formaldehyde by the Fe(III)/H2O2 system. CNT exhibited a slightly higher activity than PAC [22].
In the presence of the carbon materials, the reduction of Fe(III) to Fe(II) was accelerated while the decomposition rate of H2O2 increased, which appears to be responsible for the enhanced oxidant production. The reducing power of carbon materials was also shown by the reductive conversion of polyoxomolybdate. Increasing the addition of carbon materials exhibited saturated (or slightly decreased) oxidant yields, while decreasing the H2O2 utilization efficiency. The enhanced oxidant production by carbon materials was more pronounced in the absence of dissolved oxygen, which was explained by the reactions of organo-radicals produced by methanol oxidation [22].
A new photocatalyst based on nano-sized TiO2 supported on single wall carbon nanotubes SWCNTs with tailored photocatalytic properties upon irradiation by both UV and solar simulated light was successfully employed for the degradation of a mixture of 22 organic pollutants in both ultrapure water and real secondary wastewater effluent [23].
First-order degradation rates showed that under UV irradiation nanosized TiO2 supported on SWCNTs is much more effective than conventional Degussa P25 for degradation of io-pamidol, iopromide, diatrizoic acid, diclofenac, triclosan and sulfamethoxazole in ultrapure water. For the remaining organics the degradation rates were comparable being in most of the cases Degussa P25 slightly more effective than nano-sized TiO2 supported on SWCNTs. Reactions performed in real secondary waste water effluent showed a general reduction of degrada-
tion rates. Specifically, such a reduction was in the range of 9-87% and 9-96% for the Degussa P25 and the nano-sized TiO2 supported on SWCNTs, respectively. Overall, the nano-sized TiO2 supported on SWCNTs under UV irradiation displayed comparable degradation rates with respect to convention Degussa P25. Under simulated solar irradiation the new prepared photocatalyst showed lower efficiency than Degussa P25 in ultrapure water. Such a gap was greatly reduced when the reactions were carried out in real secondary wastewater effluent. The nano-sized TiO2 supported on SWCNTs demonstrated to have the addition benefit to be easily removed from the aqueous solution by a mild centrifugation or a filtration step and, consequently, can be reused for a further photocataly-tic treatment batch. Therefore, the obtained results showed that new photocatalyst based on nano-sized TiO2 supported on SWCNTs has proved to be a promising candidate to be used in a photocatalytic based - advanced oxidation processes (AOP) and to be integrated with a biological step for the effective removal of emerging organic pollutants [23] (Figure 4).
>iySWCNT
Fig 4. Graphical abstract to the [23].
In this study heterogeneous Fenton oxidation using Fe3O4 amended onto multi-walled carbon nanotube (Fe3O4/MWCNTs) showed effective degradation of aqueous bisphenol A (BPA) [24]. The Fe3O4/MWCNTs exhibited an octahedron crystal structure of Fe3O4 (100-150 nm) which was well-dispersed onto the MWCNTs with little agglomeration. The Fe3O4/MWCNTs catalyst-driven Fenton oxidation achieved high removal of BPA (97% removal in 6 hours) under the selected operating conditions (pH of 3, 0.5 g catalyst/L, [H2O2MBPA] of 4 (mol/mol), 500C). The Fenton oxidation of BPA demonstrated similar removal of BPA at 0.5-1 g catalyst/L while
showing significant removal of BPA at the initial pH of 3. The [H2O2MBPA] of 4 in this study was found to be the cost-effective condition to achieve high removal of BPA at low concentration of H2O2. Besides, the intermediates and oxidation products produced by the Fenton oxidation of BPA at the [H2O2MBPA] of 4 did not show any biological toxicity. The [H2O2]:[BPA] ratio of 4 was much lower than that for other heterogeneous Fenton of BPA (54 mol H2O2/mol BPA) and comparable to those for homogeneous Fenton of BPA (2-9 mol H2O2/mol BPA). The Fenton oxidation rate of BPA using the catalyst was enhanced by the factor of 3.5 as the reaction temperature increased from 20 to 500C. The five cycles of the Fenton oxidation using the same catalyst resulted in the steady removal of BPA confirming high stability of the Fe3O4/MW CNTs catalyst over the multiple Fenton reactions. The scavenging tests of the hydroxyl radicals suggested that the hydroxyl radical-driven oxidation was the major step among the multiple reactions in the heterogeneous Fenton oxidation. The findings from this study suggest that the Fenton oxidation using Fe3O4/MWCNTs would highly efficient solution for the removal of bisphenol A from water [24].
Chemical vapor deposition (CVD) was used to fabricate as-prepared MWCNTs. CNTs/FeS Fenton-like catalyst was synthesized by oxidation-reduction-vulcanization method [25]. Furthermore, the catalyst was characterized by TEM, XRD, TG and other material analysis techniques. And then, the catalyst was used for the removal of ciproflox-acin from aqueous solution. The catalytic activity and catalytic mechanism were studied with concentration of H2O2, dosage of catalyst, concentration of ciprofloxacin and pH [25].
The results show that there are an optimum H2O2 concentration (20 mmol/L) and dosage of catalyst (10 mg) in CNTs/FeS Fenton-like catalytic reaction. The first order kinetics equation is more appropriate to describe the process of catalytic reaction. And the catalytic reaction has better suitability than previous study in wide range of pH values (pH=3-8). Meanwhile, CNTs/FeS Fenton-like catalyst has demonstrated good regenerated properties [25] (Figure 5).
Fig. 5. Graphical abstract to the [25].
Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant around the world. In the study the iron oxide decorated on a magnetic nanocomposite (Fe3O4/MWCNTs) being used as a heterogeneous Fenton catalyst for the degradation of TBBPA in the presence of H2O2 has been reported [26]. Fe3O4/MWCNTs was prepared by a simple solvothermal method, whereby an iron source Fe(Acac)3 and a reductant (n-octyl-amine) were allowed to react in n-octanol solvent. Monodisperse Fe3O4 nanoparticles of consistent shape were uniformly dispersed on the nanotubes. Samples were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area measurement, and vibrating sample magnetometry. The samples effectively catalyzed the generation of hydroxyl radicals (center dot OH) from H2O2, which degraded and subsequently mineralized the TBBPA. The whole process took four hours at near neutral pH. A degradation pathway for the system was proposed following analysis of intermediate products by gas chromatography-mass spectrometry. The quantification of Fe2+ and Fe3+ distribution before and after the recycling test of the composite were explored by X-ray photoelectron spectroscopy, in order to explain the stability and recyclability of the composite. Analysis of the results indicated that the magnetic nanocomposite is a potentially useful and environmentally compatible heterogeneous Fenton's reagent with promising applications related to pollution control [26].
Carbon nanotubes as catalysts itself
MWCNTs functionalized by different oxi-dants (HNO3/H2SO4, H2O2, O3 and air) have
been used as catalysts for the wet air oxidation of phenol [27].
To investigate the effect of the oxidation conditions on the structure of the functionalized MWCNTs, various characterization techniques, e.g., scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Fourier-transform infrared spec-troscopy (FTIR) and X-ray photoelectron spec-troscopy (XPS) have been used. The MWCNTs treated with O3 and H2O2 show higher amounts of oxygen-containing functional groups and car-boxylic acid groups, and a weaker acidic nature, in comparison with those treated with other oxidizing agents. All the functionalized MWCNTs exhibit good activity in the catalytic wet air oxidation (CWAO) of phenol. However, the MWCNTs treated with O3 show the highest activity with desirable stability in comparison with other functionalized MWCNTs, indicating that the functionalization of carbon nanotubes with O3 is a very promising strategy in synthesizing efficient catalysts for CWAO [27].
In this paper [28] MWCNTs were directly used as catalysts for the hydroxylation of aromatic hydrocarbons at low temperature 50-700C. Without the assistance of any solvent or additive, high selectivity was still obtained. The catalysts were characterized by X-ray powder diffraction, infrared spectra, Raman spectra, and transmission electronic microscopy. These results prove that MWCNT was a highly-active, highly-selective and well-reproductive heterogeneous catalyst. The curved sp2-hybridized carbon surfaces of MWCNT played an important role in these selective catalytic reactions. The reactions were proposed to occur via an oxene attacking process. The active oxygen species were generated through the interaction between the hydrogen peroxide and the MWCNTs and subsequently consumed by the
aromatic hydrocarbons. This process was repeated in the catalytic reactions [28].
The effect of hydroxyl radical (*OH)-assisted oxidation of MWCNTs as catalysts for oxidative dehydrogenation of ethylbenzene (ODEB) to styrene was studied [29]. Fourier transform infrared spectroscopy (FTIR). Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results indicated that the presence of hydroxyl radicals during UV/H2O2 oxidative treatment introduced significantly the amount of hydroxyl and carbonyl groups onto the surface of carbon nanotubes, leading to an improvement in the catalytic behavior. In the present work UV/H35 MWCNTs obtained the best catalytic behavior with 91% styrene selectivity and 47% ethylbenzene conversion at 4000C under conventional heating.
Transmission electron microscopy (TEM) of UV/H35 MWCNTs showed the formation of new sp-carbon layers on the wall of nanotubes after the stability test [29].
The Fenton reaction is widely used for remediation of waste water and for the degradation of organic pollutants in water. Currently, there is considerable interest to convert the classical Fenton reaction, which consumes stoichiometric amounts of iron(II) salts, into a catalytic process that is promoted by a solid. The analytical review describes the work that has used carbonaceous materials either directly as catalysts or, more frequently, as a large-area support for catalytically activated transition metals or metal-oxide nanoparticles. The interest in this type of catalyst derives from the wide use of carbon in conventional water treatments and the wide applicability of the Fenton reaction. After two general sections that illustrate the scope and background of Fenton chemistry, the review describes the activity of activated carbon in the absence or presence of metal-containing particles. The last sections of the review focus on different types of carbonaceous materials, such as carbon nanotubes and diamond nanoparti-cles. The review concludes with a section that anticipates future developments in this area, which are aimed at overcoming the current limitations of low activity and occurrence of metal leaching [30] (Figure 6).
Carbon based materials
* <ф
iites ^^
HO
Active sites (i.e. Cu, Fe, Au)
Fig. 6. Graphical abstract to the [30].
Commercial multi-walled carbon nano-tubes with different properties (three samples from Sigma-Aldrich - SA1, SA2 and SA2-H (SA2 modified by hydrothermal treatment with concentrated sulfuric acid), one sample from Nanocyl - NC; and two samples from Shenzhen Nanotech - SZ and LSZ were tested as catalysts in wet peroxide oxidation. Phenol was selected as model compound since it represents a class of noxious compounds for human health and for the environment and, due to this, phenol is typically considered in wastewater treatment studies.
The experiments were carried out under the following intensified conditions: phenol concentration - 4.5 g/L, hydrogen peroxide concentration - 25 g/L, catalyst load - 2.5 g/L, pH= 3.5, r=353 K and x = 24 hours. The results demonstrated that phenol is poorly adsorbed in this type of carbon materials (11% as maximum when using the NC sample). However, in the catalytic experiments, complete removal of phenol is achieved when using some of the carbon nanotubes (SA1, NC and SA2), together with a remarkable total organic carbon removal (77, 69 and 67%, respectively). These materials have the less pronounced acidic character, which is often considered favorable for oxidation reactions in advanced oxidation processes and may explain the higher performance of SA1, NC and SA2 regarding the other materials. Leaching of Fe species into the solution was also observed in all cases (that can also have some influence on the degradation of phenol), SA1 leading to the highest concentration of Fe species leached (26 mg/L), followed by SA2 (2 mg/L) and NC (1 mg/L) [31].
Considering the lower Fe leaching levels observed for SA2 and NC, these catalysts were then tested in consecutive reusability cycles.
Fig. 7. Graphical abstract to the [31].
SA2 showed a superior performance than NC, but temperature-programmed desorption as well as thermogravimetric analysis suggested that the carbon material is oxidized by hydrogen peroxide at the employed conditions and/or that carboxylic acids are adsorbed on the catalyst surface after consecutive runs (mainly after the first use). However, only a slight decrease in the catalyst activity was observed [31].
Conclusions
1. The analytical reviewing shows that carbon nanotubes (CNTs) doped by metals and its oxides repeatedly increase the catalytic activity of the tubes, allowing to conduct the oxidation reactions even at room temperature with high selectivity on end-products.
2. CNTs act as strong promotors of decaying hydrogen peroxide (H2O2) to Reactive Oxygen Species (ROS) to catalyze the oxidation of phenolic compounds.
3. The catalysts maintain the original level of activity after many reaction cycles.
4. Thus, CNTs can be offered as active catalysts for total degradation of oil contaminants by H2O2, being on the way of solving the definite environmental problem.
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KARBOHÍDROGENLORÍN HÍDROGEN PEROKSÍDLO OKSÍDLO§MOSÍ Ü£ÜN NANOÖLCÜLÜ
KARBON KATALIZATORLARI
E.B.Zeynalov
Maqalada karbon nanoborularinin (KNB) i§tiraki ila karbohidrogenlarin hidrogen-peroksidla oksidla§masina aid Thomson Reuters elektron bazasina daxil olan an müasir na§rlarin icmali göstarilmi§dir. icmalda hidrogen-peroksidla karbohidrogenlarin oksidla§masinda katalizator kimi KNB istifadasinin iki aspekti nazardan kegirilmi§dir - 1) metal katalizatorlari ügün aktiv da§iyici va 2) katalitik sistemlarin asas komponenti kimi. Göstarilmi§dir ki, KNB-nin tarkibina metallarin va metal oksidlarinin daxil edilmasi, borularin katalitik aktivliyini dafalarla artiraraq, reaksiyani hatta otaq temperaturunda son mahsullara göra yüksak selektivlikla aparmaga imkan verir. KNB hidrogen-peroksidin oksigenin aktiv hissaciklara pargalanmasinin güclü promotoru kimi va aromatik birla§malarin, asasan fenollarin effektiv katalizatorlari kimi i§tirak edirlar. Belalikla, KNB bir sira malum ekoloji problemlarin hallinda neft girklandiricilarinin hidrogen-peroksidla pargalanmasi ügün aktiv katalizator kimi istifada oluna bilar.
Agar sözlzr: karbohidrogenlsrin oksidh§masi, hidrogen-peroksid, karbon nanoborulari, katalizator da§iyicisi, katalizator, aromatik birÍ3§m3Í3rin pargalanmasi.
УГЛЕРОДНЫЕ НАНОРАЗМЕРНЫЕ КАТАЛИЗАТОРЫ ДЛЯ ОКИСЛЕНИЯ УГЛЕВОДОРОДОВ
ПЕРОКСИДОМ ВОДОРОДА
Э.Б.Зейналов
Представлен обзор современных публикаций по окислению углеводородов пероксидом водорода в присутствии углеродных нанотрубок (УНТ), охватываемых электронной базой данных Thomson Reute. Рассмотрены два аспекта использования УНТ в качестве катализаторов окисления углеводородов пероксидом водорода: а) как активной подложки для металлических катализаторов и б) как основного компонента каталитической системы. Показано, что допирование УНТ металлами и оксидами металлов многократно увеличивает каталитическую активность трубок, позволяя вести реакции окисления пероксидом водорода даже при комнатной температуре с высокой селективностью по конечным продуктам. УНТ выступают как сильные промоторы распада пероксида водорода на активные формы кислорода и эффективные катализаторы окисления ароматических соединений, в частности, фенолов. По полученным результатам УНТ могут быть предложены в качестве активных катализаторов разрушения нефтяных контаминантов пероксидом водорода.
Ключевые слова: окисление углеводородов, пероксид водорода, углеродные нанотрубки, каталитическая подложка, катализатор, деструкция ароматических соединений.
