Removal of BTX Compounds in Air by Total Catalytic Oxidation Promoted by Catalysts Based on SiO 2 ( 1x ) Cux

O objetivo deste estudo foi reduzir a emissão de compostos orgânicos como BTX (benzeno, tolueno e xilenos), devido à sua toxicidade e efeitos adversos ao meio ambiente. Neste sentido, os catalisadores (SiO2(1-x)Cux) foram desenvolvidos para apresentar elevada atividade catalítica na conversão de BTX em CO2 e H2O sem a necessidade do uso de solventes e materiais prejudiciais ao meio ambiente, com baixo custo de produção e excelente reprodutibilidade. O Cu foi impregnado (2,99; 5,01; 10,01 e 19,97%) em suportes (SiO2) por impregnação úmida a partir de nitrato de cobre(II). O estudo foi realizado na faixa de temperatura de 50-350 °C. Os catalisadores descritos nesse artigo apresentam elevada atividade catalítica na oxidação total de BTX. A conversão de benzeno neste estudo excedeu 85% a 150 oC.


Introduction
2][3][4] The potential pollutant gasoline is directly related to aromatic hydrocarbons, namely benzene, toluene and xylenes (known as BTX compounds or simply BTXs). 5,6ue to the high toxicity of BTX compounds and the resulting adverse effects in contaminated systems, the search for control and remediation procedures is of great importance.Catalytic oxidation has been recognized as an efficient method to reduce air pollution, particularly atmospheric concentrations of volatile organic compounds.
The advantages of catalytic oxidation include the fact that it can operate at relatively low temperatures and no byproducts are formed. 7,8ndreeva et al. reported studies on the effect of Au and V 2 O 5 on the oxidation of benzene in systems with Au/V 2 O 5 /TiO 2 and Au/V 2 O 5 /ZrO 2 .Wu et al. reported a study on the effect of the catalysts Au/ZnO, Au/Al 2 O 3 and Au/MgO on the oxidation of benzene, toluene and p-xylene and the most efficient was Au/ZnO.At 300 ºC the conversion of benzene slightly exceeded that of toluene (which was around 60%) while the conversion of p-xylene did not reach 35%. 9 The development of a catalyst with high specific activity at low temperatures, that is, high selectivity and low preparation cost, remains a challenging goal in the total oxidation of BTXs.Vol. 24, No. 10, 2013   In this context, the objective of this study was to investigate the capability of the base catalysts SiO 2(1-x) Cu x in relation to the total conversion of benzene, toluene and xylenes to CO 2 and H 2 O.

Preparation of catalysts
Four supported metal catalysts were prepared using silicon dioxide (SiO 2 , 70-230 mesh) (Vetec ® ) as the support.The precursor salt was copper(II) nitrate (Vetec ® ), which was dissolved in distilled water and added to the support.Subsequently, the materials were dried in an oven for 24 hours at 100 °C and then calcined in a muffle furnace with circulating air at a heating rate of 5 °C min -1 , starting at ambient temperature and increasing to 550 °C, this temperature being held for 4 hours.Samples were then crushed and sieved (to provide particles between 177 and 210 µm).The catalysts were identified as SiO 2(1-x) Cu x where x (mol%) is: SiO 2(0.97)Cu 0.03 ; SiO 2(0.95) Cu 0.05 ; SiO 2(0.9)Cu 0.1 and SiO 2(0.8)Cu 0.2 .

Characterization of the catalysts
The specific surface areas of the catalysts (S BET ) were determined using a Nova 2200e analyzer (Quantachrome Instruments ® ).The surface areas were calculated applying the Brunauer Emmett Teller (BET) method by isothermal gas adsorption/desorption of N 2 at 77 K using 40 points.The crystalline structures of the catalysts were analyzed by X-ray powder diffraction (Bruker-AXS Siemens ® ) with CuK α radiation (V = 40 kV; I = 30 mA).The morphology and microstructure of the catalysts were observed by scanning electron microscopy (SEM, JEOL JSM-6390LV) and field emission transmission electron microscopy (FETEM, JEOL JEM-1011).The chemical composition and metal content of the catalysts were analyzed by energydispersive X-ray spectroscopy (EDX).The binding energies of the elements were determined by X-ray photoemission spectroscopy (XPS) (VG Microtech 3000).

Catalytic activity tests
The activity of the catalysts was measured in a fixed tubular quartz reactor under atmospheric pressure.The following conditions were chosen: catalyst volume 0.090 cm 3 , inlet benzene (Vetec ® ) concentration 2.5 g m −3 , toluene (Vetec ® ) concentration 0.65 g m −3 and o-xylene (Vetec ® ) concentration 0.45 g m −3 in air and temperature range 50-350 °C.The kinetic data were collected after at least 1 h on-stream at room temperature.The only products were CO 2 and H 2 O as determined by GC-MS and FTIR.The reactant and product mixtures were analyzed with an on-line gas chromatograph equipped with a FID detector and an HP-5 column.
The catalytic activity was expressed as the degree of conversion of the BTX compounds.The conversion of BTXs was calculated as follows in equation 1: ( where BTXs (%) = percentage of BTXs converted; [BTXs] in = input BTX concentration; [BTXs] out = output BTX concentration.

Structural and textural properties of the catalysts
The values for the surface area, pore volume and pore diameter of the catalysts are presented in Table 1.
As expected, there was a reduction in all of these parameters as the metal content increased.This reduction may be related to the high calcination temperature of 550 ºC (Table 1).The isotherms obtained were analyzed by comparison with the classification of the IUPAC (International Union of Pure and Applied Chemistry) and it was found that the catalysts have pores of regular cylindrical and/or polyhedral shape with open ends with type IV isotherms and type H1 hysteresis (Figure 1). 2 The X-ray diffractograms are shown in Figure 2, in which the diffraction peaks (2q = 33, 35, 38, 48, 54, 58, 62, 65, 66 e 68º) are assigned to the reflections for the tenorite (CuO) phase, a monoclinic crystal system (JCPDS data file 045-0937).The peak at 2q = 22º is characteristic of silicon oxide.The peaks are more intense for CuO (11-1) (2q = 35º) and CuO (111) (2q = 38º).Therefore, it can be concluded that the large crystal size, good stability and close contact between the Cu and SiO 2 particles favors the catalytic performance of the Cu/SiO 2 catalyst. 10,11rphology and chemical composition of catalysts Figure 3A-D shows the morphology of the Cu-SiO 2 catalyst particles in the shape of a hemisphere.The EDX analysis confirmed the presence of the element Cu, as shown in Table 1.The higher the copper content the lower the degree of metal dispersion in the catalyst due to the more pronounced formation of agglomerates in catalysts with higher metal content, as observed by SEM.
In Figure 4A-C copper oxide particles with a hemispherical shape and contact between the copper and silicon oxides at the perimeter can be observed.The size of copper-containing particles had a Gaussian distribution as represented in Figure 4D and the particle size distribution for each of the catalysts is shown in Table 1.The result shows a strong interaction between the metal and silicon oxides in the catalyst, where the particles appear to be faceted when adsorbed on the support, which could induce defects or the formation of active sites at which the reaction occurs easily, which favors the catalytic activity of the catalysts Cu/SiO 2 . 9

Oxidation state and binding energy of the catalysts
The XPS analysis (Figure 5) yielded the values for the bond energy of the electrons O 1s (532.4 eV), Si 2p 1/2 2p (103.3 eV) and Cu 2p 3/2 (933.4 eV) before and after the catalytic reaction.][14][15] It was observed that the value obtained for the binding energy of the electron Si 2p 1/2 is consistent with that reported in the literature, suggesting that in these catalysts all of the silicon is present in the form of Si(IV). 13,16,17The value for the binding energy of Cu 2p 3/2 in the catalysts studied was 933 eV which, on-comparing with results reported in the literature, is attributed to the presence of copper(II) oxide.This value is reported as 933.4 eV in the literature. 12, 16 18, 19talyst performance for BTX (benzene, toluene and o-xylene) oxidation Figure 6 shows the conversion of BTXs as a function of the reaction temperature for the catalysts studied.
The catalyst SiO 2(0.9)Cu 0.1 showed higher catalytic activity toward the BTX compounds.The catalytic activity of the catalyst SiO 2(0.8)Cu 0.2 may be related to the excessive    formation of agglomerated particles on the surface, reducing the metal dispersion, according to Table 1.As expected the catalyst SiO 2(0.97)Cu 0.03 had a lower catalytic activity than the other catalysts tested, due to its lower metal content.
On comparing the catalytic activity of the Cu/SiO 2 products with other catalysts described in the literature that are highly active in BTX oxidation (Table 2), it can be seen that the T benzene50 temperature (benzene conversion up to 50%) is 106 °C for SiO 2(0.9)Cu 0.1 , 124 °C for SiO 2(0.95) Cu 0.05 and 162 °C for SiO 2(0.97)Cu 0.03 .These values are comparable with those reported in literature for the catalyst Cu/g-Al 2 O 3 ,    3,9,20,21 The results of these experiments revealed that the SiO 2 support has no activity and reports in the literature for other catalysts indicate that interaction between Cu and SiO 2 could improve the catalytic activity of Cu/SiO 2 in relation to BTX oxidation. 9,21ccording to the results shown in Figure 6, the activity of the Cu/SiO 2 catalyst decreases in the order benzene > toluene > o-xylene.The conversion of benzene is higher than those of toluene and o-xylene in the same concentrations and reaction conditions.This indicates that the catalytic activity in relation to aromatic compounds is highly dependent on the relative adsorption strength of the model compounds, the strength of the weakest carbonhydrogen bond in the structure and the ionization potential of the methyl derivatives. 9,11,12t is well known that the oxidation of BTX compounds promoted by solid oxide catalysts can proceed by the Mars-van Krevelen mechanism, in the key steps are the supply of oxygen by the reducible oxide, the introduction of the oxygen (originating from the oxide lattice) into the substrate molecule and the re-oxidation of the reduced solid by the oxygen-containing gaseous phase, which is the rate-determining step of the reaction. 9,22,23ccording to the Mars-van Krevelen mechanism, the BTX oxidation rate is determined by the concentration of BTX chemisorbed onto the Cu-containing particles close to the periphery of the Cu-SiO 2 interface.The active oxygen species formed on the metal-support interface or on the support close to the metal directly participate in the catalytic oxidation, and the metal coated on the support serves as the adsorption site for BTXs.The catalytic cycle involves the chemisorption of BTX onto Cu-containing particles, migration of the chemisorbed BTX to the Cu-SiO 2 interface, O 2 activation on the defect sites in the SiO 2 or at the Cu-SiO 2 interface, the formation of active oxygen species and the reaction between the chemisorbed BTX and active oxygen species at the interface. 9,20,24,25

Conclusions
The SiO 2(0.9)Cu 0.1 product exhibits higher activity compared with other catalysts reported in the literature (5% and 15% Cu/g-Al 2 O 3 , 5% Cu/TiO 2 (rutile), 1.5% Au/ZnO and 3% Au/V 2 O 5 /TiO 2 ) with regard to BTX oxidation.The benzene conversion for the SiO 2(0.9)Cu 0.1 catalyst exceeded 85% at 150 °C, which is comparable to the values for 1.5% Au/ZnO and 3% Au/V 2 O 5 /TiO 2 .The high catalytic activity could be attributed to the high stability observed due to a greater energy of interaction between the Cu-SiO 2 catalysts in this study.Thus, the catalysts of the base Cu/SiO 2 proved to be highly efficient in the conversion of BTX compounds.

Figure 6 .
Figure 6.BTX conversion as a function of reaction temperature for different catalysts under varying reaction conditions: benzene 2.0 g m 3 , toluene 0.5 g m 3 , o-xylene 0.4 g m 3 .Total flow rate: 20 ml min -1 .

Table 1 .
Textural data for the supported copper catalysts Vp = pore volume; Dp = pore diameter; a determined by XRD; b obtained by HRTEM; c calculated from semi-quantitative EDX data; d calculated using formula(Wu et al. 2011): D Cu = 6n s M Cu /r Cu N A d Cu , where n s is the number of Cu atoms at the surface per unit area (1.7 x 10 19 m -2 ), M Cu is the molar mass of copper (63.546 g mol -1 ), r Cu is the density of copper (8.92 g cm -3 ), N A is Avogadro's number (6.023 x 10 23 mol -1 ) and d Cu is the average Cu particle size (obtained by HRTEM).

Table 2 .
Comparison of data published in the literature with those obtained in this study