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Synthesis and characterization of composite catalysts Cr/ZSM-5 and their effects toward photocatalytic degradation of p-nitrophenol
⁎Corresponding author. Tel.: +20 0113157478; fax: +20 2 22629356. ibraheem_othman2002@yahoo.com (Ibraheem Ali)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Raw rice husks (white particles) were used to produce silica through submission to consecutive chemical treatment using NaOH and HCl solutions. The prepared silica was well incorporated with other components under hydrothermal conditions to synthesize ZSM-5. Cr framework-substituted ZSM-5 with constant Si/(Cr + Al) ratios have been synthesized hydrothermally and characterized by physicochemical methods, e.g. X-ray diffraction (XRD), FT-IR, UV–vis spectroscopy and N2 adsorption. FT-IR spectroscopy of Cr-substituted ZSM-5 shows new bands at 688 and 627 cm−1 due to extra framework chromium oxide. The replacement of Al3+ by Cr ion causes a shift of Si–O–T vibration to lower wave numbers with the appearence of a new band at 990 cm−1 in Cr-ZSM-5 samples which can be directly correlated with the entrance of chromium ions into the framework of the ZSM-5 lattice and the presence of Cr–O–Si linkage in the structures. UV spectroscopy showed absorption bands at 263 and 381 nm related to the tetrahedrally coordinated environment in the (Cr5+–O−1)∗ ← (O2−⚌Cr6+) charge transfer transition state of isolated Cr(VI) atoms inside the ZSM-5 matrix.
The photocatalytic activity of Cr incorporated ZSM-5 zeolite toward the degradation of p-nitrophenol (PNP) was well investigated at atmospheric pressure, 25 °C, with H2O2 as an oxidizing agent. The enhanced photocatalytic activity of 0.4CrZ is attributed to charge-transfer excited complex between Cr in zeolite along with PNP ligand in addition to higher surface area and high dispersion Cr in the framework comparatively. More information on local structures of metal oxides inside zeolites and their photocatalytic activities toward PNP were deduced, correlated and discussed.
Keywords
ZSM-5
Rice husk ash
Cr-ZSM-5
p-Nitrophenol
Surface texturing
1 Introduction
Nitroaromatic compounds, including nitrophenols, are widely distributed in the environment. P-nitrophenol (PNP) is an important member of the nitrophenol group. This chemical is a manufactured item that does not occur naturally in the environment (Sarasa et al., 1998). P-nitrophenol has applications in agriculture, dyes/pigments, engineering polymers and pharmaceuticals. P-nitrophenol is used as a fungicide for leather, production of parathion and organic synthesis. As fungicide, it is used to control fungal mold on leather. P-nitrophenol is used in specialty industry products used by the military. PNP is also used in the manufacture of acetaminophen, a non-aspirin pain reliever and as a raw material in the manufacture of certain dyes and pesticides. P-nitrophenol is also a major urinary metabolite of parathion and can be used as a biomarker of human exposure since it is a breakdown product of pesticides including parathion and fluoridifen (Sittig, 1985; O’Connor and Young, 1989).
Most nitrophenols, including p-nitrophenol, enter the environment during manufacturing and processing. It readily breaks down in surface waters but takes a long time in deep soil and in groundwater. P-nitrophenol is toxic to plant, animal and human health. Animal studies suggest that p-nitrophenol may cause a blood disorder (Spain, 1995). Acute exposure of p-nitrophenol may lead to methemoglobin formation, liver and kidney damage, anemia, skin and eye irritation, and systemic poisoning. According to the standards for chemical products in UK, the use of p-nitrophenol is no longer recommended (Munnecke, 1976; Stevens et al., 1991).
Various studies have reported that the photocatalytic reactivity of aromatic compounds can be affected by the number of substituents, their electronic nature, and their positions in the aromatic ring but only few of those studies correlate the measured photodegradation rate to some parameters characterizing the compound to degrade (Sangchakr et al., 1995; Assabane et al., 2000; Parra et al., 2002).
ZSM-5 zeolite has proved to be an efficient catalyst for catalytic degradation when supporting metal oxides because of its strong acidity, carbon–carbon bond scission and unique pore structure to reduce coke formation (Garforth and Dwyer, 2000). Redox compositions of metal oxides specifically those described by Zener (1951) as being capable for mobilizing electrons, and thus, generating the mobile-electron environment required for redox catalysis, have recently attracted much attention (Daifullah and Mohamed, 2004; Mohamed et al., 2005).
The incorporation of transition metal ions such as Ti and V makes zeolite oxidation catalysts which are active in liquid-phase reactions with hydrogen peroxide as an oxidant. However, due to the catalytic properties of chromium several attempts have been carried out to introduce chromium cations into zeolites (Huang et al., 1990; Sugimoto et al., 1992; Mambrim et al., 1993; Chapus et al., 1994). Sugimoto et al. (1992) reported that chromium atoms exist in a highly dispersed state on chromium-silicates, whereas Mambrim et al. (1993), and Chapus et al. (1994), claimed that Cr (III) is substituting Si (IV) in the silicalite skeleton. Cr containing microporous molecular sieves such as AlPOs (Lempers and Sheldon, 1996) and zeolite Y (Ryan et al., 1997) were found to be active and selective for the oxidation of olefins. However, one of the major problems with these catalysts is the leaching of active chromium from the matrix under the reaction conditions (Lempers and Sheldon, 1998). On the other hand, it is interesting to note that certain chromium incorporated mesoporous molecular sieves, e.g. Cr-MCM-41 were found to stabilize the active species and found that there is no leach out. Anpo and co-workers reported the photo-catalytic activity of chromium-incorporated HMS mesoporous material (Yamashita et al., 2004) and chromium-incorporated ZSM-5 zeolites (Yamashita et al., 2003) for selective oxidation. Results clearly showed the photocatalytic efficacy of CrIV in the framework of both silica and silica–alumina supports.
The purpose of this paper, was to synthesize ZSM-5 prepared by rice husk ash and modification of Al-ZSM-5 by substitution with Cr and their effects on ZSM-5 structure. These samples have been characterized by X-ray diffraction, FT-IR, UV–vis and N2 adsorption. Finally, the activities observed with the isomorphous substitution of Cr into the zeolite framework in the photocatalytic oxidation of P-Nitrophenol (PNP) by H2O2 is compared to what was observed when operating in the homogeneous phase in order to evidence the role of zeolite inthe substitution process and how it affects the oxidation process.
2 Experimental
The materials used were: RH powder (prepared in this work), sodium hydroxide pellets (AR 98%), aluminum sulfate [Merck, Al2(SO4)3·16H2O], Chromium sulfate [Merck, Cr2(SO4)3·6H2O], tetrapropylammonium bromide (TPABr, Fluka), n-propyl amine (n-PA, Merck), p-nitrophenol (Merck), hydrogen peroxide (H2O2, 30%), HCl and commercial H2SO4.
2.1 Silica preparation from rice husk
Dry raw rice husks (RHs) were sieved to eliminate residual rice and clay particles. They were well washed with distilled water, filtered, dried in air, and calcined at 750 °C for 6 h (white ash). 150 ml of 4 M NaOH solution was added to 12 g of calcined RHs and heated under reflux at 90 °C for 12 h. 20 ml of HCl (32%) was added to the aforementioned base dissolved RHs for complete precipitation. RHs were filtered, sample washed repeatedly with deionized water to remove chloride ions and finally dried in an oven at 120 °C for 6 h. The yield of silica in this sample ash was 42%. The chemical composition of rice husk ash is as follows: loss in ignition, 4.71; SiO2; 90.70; Al2O3, 0.13; Fe2O3, 0.06; TiO2, 0.015; CaO, 0.61; MgO, 0.25; Na2O, 0.09; K2O, 2.64; P2O5, 0.73, all numbers on wt.% basis.
2.1.1 Preparation of ZSM-5
The hydrogels of the following oxide molar compositions were prepared for the synthesis of ZSM-5 zeolite; comparable to those that can be found elsewhere (Ali et al., 2009; Othman, 2007), using the following molar ratios: A specific amount of NaOH was added to silica obtained from rice husk, in a small amount of H2O (∼40 ml) while stirring, followed by heating at 80 °C until a clear solution was reached. TPABr, on the other hand, was dissolved in a little amount of H2O (10 ml) under heating at 50 °C for 20 min. The solution of TPABr was added to sodium silicate solution with stirring for 15 min. n-Propyl amine (1 ml) was added as a mobilizing agent. The combined solution of sodium silicate and TPABr was added to a clear solution of aluminum sulfate (prepared by dissolving 0.5 g of aluminum sulfate in 10 ml of distilled water and 0.05 ml of concentrated H2SO4) with stirring for 30 min.
The pH of the mixture was adjusted at 11 by using NaOH (0.1 M) and H2SO4 (0.1 M) solutions. Finally, the mixture was hydrothermally treated at 150 °C in an oil bath, using stainless steel autoclaves, for 8 days. The autoclaves were removed at the specified time from the oil bath and quenched immediately with cold water. The solid product was filtered and washed with distilled water until the pH of the filtrate dropped to 8. The products were dried at 110 °C for 10 h, and then calcined at 550 °C for 6 h in an air oven. This sample was denoted as ZSM-5.
2.1.2 Preparation of Cr–ZSM-5 (built-in)
The synthesis procedure is described in molar ratio as follows: where: x was varied from 0.0 to 0.8 in each series.
The template tetrapropylammonium bromide (TPABr) as well as n-propyl amine as a mobilizing agent were mixed together with the sodium silicate and added to a clear solution of aluminum sulfate (∼5 ml). Solution of a calculated amount of Cr2(SO4)3. 6H2O was added dropwise to the previous solution simultaneously with constant stirring for 30 min. The pH of the mixture was adjusted to 11.0 by using sulfuric acid (0.1 M) or sodium hydroxide (0.1 M). The amorphous gel was formed and allowed to age for 1 h at room temperature. The reaction mixture was transferred to 300-ml stainless steel autoclaves and maintained in an oil bath at 150 °C autogenously under pressure. The autoclave was removed from the oil bath after 8 days and quenched in cold water for product identification. The solid products were separated by filtration. Excess alkali was thoroughly washed with water repeatedly until the pH of washing liquid was close to 8, and the products were dried in an oven at 120 °C for 10 h, then calcined at 550 °C for 6 h to remove the template. Its pale gray color was deeper by increase of Cr content. All series of samples were prepared from the gels with total SiO2/(Al2O3 + Cr2O3) molar ratio of 38. These samples were denoted as 0.2CrZ, 0.4CrZ, 0.6CrZ and 0.8CrZ depending on the ratio of Cr.
2.2 Instrumental techniques
The X-ray diffractograms of various zeolitic samples were measured by using a Philips diffractometer (PW-3710). The patterns were run with Ni-filtered copper radiation (Kα = 1.5404 Ǻ) at 30 kV and 10 mA with a scanning speed of 2θ = 2.5 °/min. The crystal sizes of the prepared materials were determined using the Scherrer equation. The instrumental line broadening was measured using a LaB6 standard. The crystallinity of the prepared samples was calculated using the ratio of the sum of the areas of the most intense peaks for modified AlZSM-5 samples (2θ = 20–25°) to that of the same peaks of the prepared AlZSM-5 and multiplying by 100.
The Fourier transform infrared (FT-IR) spectra were recorded on a Jasco FT-IR-40, single beam spectrometer with a resolution of 2 cm−1. The samples were ground with KBr (1:100 ratio) as a tablet and mounted to the sample holder in the cavity of the spectrometer. The measurements were recorded at room temperature in the region at 1400–400 cm−1.
UV–vis diffuse reflectance spectra of the samples were measured using a JASCO V-570 unit, serial No. 29635, at a scanning speed of 4000 nm/mm and a band width of 2 nm. The samples were measured in the wavelength range from 200 to 500 nm. The samples were prepared as self-supporting wafers and were recorded at room temperature.
The nitrogen adsorption isotherms of various zeolitic solids were measured at-196 °C using a conventional volumetric apparatus (Mohamed et al., 2005). Prior to the determination of the adsorption isotherm, the sample (0.1 g) was outgassed at 300 °C for 3 h under a reduced pressure of 10−5 Torr in order to remove moisture. The specific surface area (SBET) was obtained using the BET method (de Boer et al., 1964) while the micropore volume (VμP) and the external surface area (Sext) were obtained from the “t-plot” method (Van Dyck and Croitoru, 2007).
Chemical composition of rice husk ash was examined by X-ray fluorescence spectroscopy (XRF: Philips, PW1400).
2.3 Catalytic activity
2.3.1 Determination of PNP
The concentration of PNP was measured by UV spectrophotometer JASCO V-570 unit, serial No. 29635. After illumination, clear samples were obtained by filtering the solution using a Millipore filter (0.45 μm), then adjusted the pH, used for the analysis of PNP by measuring the absorbance at 400 nm.
2.3.2 Photocatalytic degradation experiment
The photoreactivity experiments were carried out in a cylindrical Pyrex glass reactor containing 0.20 g of catalyst and 250 ml of aqueous solution of p-Nitrophenol (PNP). The concentration of the organic compound was 5 × 10−3 M. Prior to irradiation, after adjusting pH, the suspensions were magnetically stirred in the dark for 60 min to establish the adsorption/desorption equilibrium of the PNP. The aqueous suspensions containing PNP were irradiated with constant aerating. At given irradiation time intervals, samples were taken from the suspension, and then passed through a 0.45 μm Millipore filter to remove the particles; the concentration of PNP in the filtrate was measured by applying the following equation. where Co is the original p-Nitrophenol (PNP) content and C is the retained PNP in solution.
3 Results and discussion
3.1 X-ray diffraction
XRD patterns of chromium substituted ZSM-5 and parent ZSM-5 samples (Fig. 1) showed typical lines of the parent aluminosilicate ZSM-5 synthesized using TPABr template, indicating the intact structure of ZSM-5 even after incorporation of Cr ions. Cr-ZSM-5 samples showed small particles of chromium silicate at d = 5.6297, 5.01, 4.624, 3.749, 3.3329 and 3.351 Å related to Cr-silicate (Chapus et al., 1994). This phase was highly intensified with increasing the Cr contents during the synthesis process. The effect of Cr loadings on crystallinity percentages, crystallites size, unit cell parameters and cell volume of ZSM-5 zeolite was depicted in Table 1. A monotonous decrease in crystallinity for 0.2 and 0.4CrZ samples was observed (62% and 59%, respectively). This was in line with increasing the lattice volume of the same samples when compared with that of the ZSM-5 strongly indicating the expansion of the ZSM-5 crystal lattice after the introduction of Cr element. Because the bond length of Cr–O is longer than those of Si–O or Al–O (Wang et al., 2013; Camblora et al., 1995), the incorporation of Cr ions into the zeolite framework sites will expand the zeolite lattice. Thus, the expansion of the ZSM-5 structure suggests that at the less dense part metal ions are incorporated into the framework sites. The value of average crystallites size of ZSM-5 crystals, calculated by the Scherrer equation, for 0.2 and 0.4CrZ samples measure, respectively, 75.01 and 66.04 nm that was the lowest between all samples. This demonstrates that incorporating Cr in ZSM-5 effectively prevents particle agglomeration, of this particular sample (0.4CrZ), allowing the material to maintain its dispersion. Accommodating small Cr particles in micropores can also cause expansion in unit cell and pore volume. Note: a, b, c are the lattice parameters (Å), V is the lattice volume (Å)3; a∗b∗c. D is the particles diameter (Å). The crystallinity of the prepared samples was calculated using the ratio of the sum of the areas of the most intense peaks for modified AlZSM-5 samples (2θ = 20–25°) to that of the same peaks of the prepared AlZSM-5 and multiplying by 100.
X-ray powder diffraction patterns of ZSM-5, 0.2CrZ, 0.4CrZ, 0.6CrZ and 0.8CrZ.
Samples
D (Å)
Unit cell (Å)
Cell vol. (Å)3
Crystallinity %
c
b
a
Al-ZSM-5
99.28
19.652
19.818
13.363
5204.440
100
0.2Cr
75.01
19.990
19.960
13.468
5374.006
82
0.4 Cr
66.04
19.868
19.694
13.288
5199.333
85
0.6 Cr
68.76
20.036
19.767
13.496
5345.112
87
0.8Cr
69.86
20.351
20.270
13.402
5530.159
88
On the other hand, the 0.6 and 0.8CrZ samples presented the lowest lattice volume between all samples giving a criterion about decreasing the diffusion of Cr ions to proceed into compensating positions inside ZSM-5 channels. Accordingly, these samples showed crystallinity percentages of 73 and 83% which are more or less comparable to the one devoted for 0.4CrZ (59%). Given that the strongest lines of ZSM-5 (2θ = 8–10° and 22–25°) are enhanced in 0.6 and 0.8CrZ, hence the diffusion of Cr ions did not take place inside the zeolite channels otherwise a relative decrease in crystallinity could have been obtained as conceived for 0.2 and 0.4CrZ samples. Thus, the increase in crystallinity of the former sample could be caused by the presence of the chromium–silicate phase that have lines superimposed on those of ZSM-5. The result showed that Cr was present in internal surfaces inside channels substituting Na cations or in the framework Skeleton of ZSM-5 zeolite in 0.2 and 0.4CrZ where it migrates mostly to the outer surfaces of ZSM-5 structure in 0.6 and 0.8CrZ samples.
3.2 Framework structure
FT-IR spectra of zeolite lattice vibration modes and the corresponding Cr containing ones are depicted in Fig. 2 in the mid (1500–400 cm−1) range. All spectra of the samples show a typical ZSM-5 structure associated with minor changes because of Cr incorporation in ZSM-5. The absorption bands at 1221 and 1083 cm−1 correspond to TO4 asymmetric stretching vibration, while the other bands at 792, 547 and 451 cm−1 correspond to TO4 symmetric stretching, double ring and bending vibrations, respectively (Ali et al., 2009; Othman, 2007; Mohamed et al., 2005).
FT-IR absorbance spectra of ZSM-5, 0.2CrZ, 0.4CrZ, 0.6CrZ and 0.8CrZ.
In addition to this characteristic absorption, a small new band was observed at 990 cm−1 and appeared in all samples (Cr substituted zeolite). Recently, Boccuti et al. suggested this band observed at 960 cm−l in the framework of TS-1 could be attributed to a stretching mode of [SiO4] units directly bonded to a Ti4+ ion (Boccuti et al., 1989). More recently, Camblor et al. attributed this band to Si–O− defect groups associated with the presence of heteroatoms linked to the silicalite framework (Camblor et al., 1993). In the same behavior, we can assume that the band we observe at 990 cm−1 in Cr-ZSM-5 samples can be directly correlated with the entrance of chromium ions into the framework of ZSM-5 lattice and the presence of Cr–O–Si linkage in the structures. On the other hand the new bands at 688 and 627 indicate the presence of extra framework chromium oxide (Gadsden, 1975; Zhao and Wang, 2007).
3.3 UV-diffuse reflectance spectroscopy
The UV-diffuse reflectance spectra measured at room temperature for all Cr substituted and parent ZSM-5 samples shown in (Fig. 3) were recorded in order to obtain some information about their electronic properties. The parent ZSM-5 showed two peaks at 238 and 359 nm. The peak appearing at 359 nm is related to T2 transition which is assigned to Al–O units characterized by a highly charged oxygen atom and a highly electron-deficient aluminum atom compared to that of Al-units of the T1 transition (238 nm) charge-transfer processes (Ma et al., 2001).
UV–vis absorption spectra of ZSM-5, 0.2CrZ, 0.4CrZ, 0.6CrZ and 0.8CrZ.
The UV-diffuse reflectance spectra of Cr-ZSM-5 samples with different Si/Cr ratio showed typical absorption peaks with λmax at 424, 381, 330 and 263 nm. The absorption peaks appears at 263 and 381 nm, which is assigned to the tetrahedrally coordinated environment in the (Cr5+–O−1)∗ ← (O2−⚌Cr6+) charge transfer transition state (Weckhuysen et al., 1996; Mambrim et al., 1993; Hamdy et al., 2006). This absorption thus confirms the presence of isolated Cr (VI) atoms inside the ZSM-5 matrix. This is in good agreement with the observed IR frequency at 990 cm−1, which is assigned to highly disperse isolated monochromate species (Weckhuysen et al., 1996).
A band around 440 nm characteristic of Cr (VI) polychromate is observed in Cr-ZSM-5, which suggests that part of chromium still exists in the form of the extra-framework Cr species (Wang et al., 2013; Weckhuysen et al., 1993). However, the CrZSM-5 samples change in color from pale green to yellow upon calcination. The former is due to the presence of trivalent chromium ions and the latter is due to the presence of hexavalent chromium ions, viz., chromate and/or polychromate ions.
3.4 Surface texturing
The results of the N2-adsorption data obtained for all samples including the specific surface area,SBET; total pore volume, VP; average pore radius, r−; micro and mesopores volumes and surfaces Vpμ, Vpwid,Sμ and Swid are collected in Table 2. Inspecting of the data compiled in this table reveals the following: (i) The SBET and St of various adsorbents are close to each other which justifies the correct choice of the t-curve used in analysis and indicates the absence of ultra-microporous substances; (ii) the values of external surface area of the various samples comprise values ⩽10% of the SBET, indicating the mesoporosity nature of these materials; (iii) the computed values of mean pore radius of different zeolitic samples, which are comparable to each other (20–25 Ǻ), show the dominance of mesoporosity. (ii) Both SBET and Vp of the investigated zeolite was found slightly increased (24 and 34%) respectively, in the lower substituted Al by Cr (0.4CrZ) but lower SBET and Vp with increased substitution (0.6CrZ, 0.8CrZ). The sample designated as 0.4CrZ measured maximum SBET and Vp and this finding might indicate the enforced location of the Cr species inside the framework structure of ZSM-5. On the other hand, decreasing the total pore volume of the samples (0.6CrZ and 0.8CrZ from replaced Al by Cr) when compared with the rest of the samples indicate the probability of the presence of the Cr species in separate phases probably as oxides or as chromium silicate species. Note: (SBET) BET-surface area; (St) surface area derived from V1−t plots; (Sext) external surface area; (Sμ) surface area of micropores; (Swid) surface area of wide pores; (Vptotal) total pore volume; (Vpμ) pore volume of micropores; (r−) mean pore radius.
Samples
SBET (m2/g)
St(m2/g)
S μ(m2/g)
Sext(m2/g)
Swid(m2/g)
r−(Å)
Vp total(cm3 g)
Vp μ(cm3/g)
Vpwid(cm3/g)
ZSM-5
660
658
593
34
67
21.0
0.554
0.498
0.056
0.2Cr
734
744
656
79
78
23
0.658
0.6576
0.0779
0.4Cr
817
825
760
76
57
21
0.7355
0.6121
0.0459
0.6Cr
673
685
622
67
46
25
0.614
0.572
0.042
0.8Cr
761
770
646
45
115
20
0.609
0.517
0.092
The V1−t plots of parent ZSM-5 and Cr substituted Al in ZSM-5 zeolite are shown in Fig. 4; those constructed depending on the values of C-constant in the BET equation, indicate intra-crystalline mesopores as illustrated from upward deviations shown for all samples. The Vl−t plots obtained for 0.2CrZ and 0.4CrZ samples show larger upward deviations (as noted in the t range 6–12.5 and 4–12.5 Å, respectively) when compared with those of the parent zeolite sample (t = 5.5–11 Å) proposing wider pore radius and pore volume. The latter result was confirmed by the position slope depicted for samples; in adsorption–desorption isotherms (not shown), instead of almost horizontal saturation plateau (characteristics of an ideal microporous structure) as well as decreasing the microporosity percentages. Decreasing the pore size of the 0.6 and 0.8CrZ samples when compared with the rest of the samples indicates the probability of the presence of Cr species as separate phases probably as oxides or as chromium silicate species.
N2 adsorption–desorption isotherms of ZSM-5, 0.2CrZ, 0.4CrZ, 0.6CrZ and 0.8CrZ.
3.5 Photocatalytic activity
The degradation of PNP as a model reaction was studied to investigate the photocatalytic activities of different ratios of Cr in ZSM-5 samples under UV irradiation. The changes in the concentration of PNP recorded during UV irradiation at specific time intervals are shown in Fig. 5. Control experiment shows that PNP is not degraded in the dark or under UV light in the absence of catalysts for 1.5 h. It can be seen that the conversion of PNP was higher when UV-light irradiation was applied in the presence of 0.4CrZ sample and H2O2, i.e. the conversion of PNP reached 90% after 75 min UV-light irradiation. These results clearly indicate that Cr-ZSM-5 can absorb UV light and act as an efficient photocatalyst under UV light irradiation.
The efficiency of PNP decolorization and photodegradation on 0.4CrZ catalyst; Experimental conditions: pH 5, reaction volume 250 ml, catalyst content 0.2 g, reaction time 60 min and initial PNP concentration 5 × 10−3 M.
Fig. 6 shows the different chromium substitutions in ZSM-5 varied from 0.2CrZ to 0.8CrZ in the degradation reaction. This figure shows that Cr containing ZSM-5 at the 0.4CrZ substitution was the most active catalyst. The decrease in activity with an increase in Si/Cr ratio may be attributed to the decrease in the level of Cr3+ and Cr6+ species in Cr-ZSM-5 catalysts. DR-UV–vis spectra of the catalysts with different Si/Cr ratios also show that there is a corresponding decrease in the intensity of the absorption bands corresponding to Cr3+ and Cr6+ species. Based on the above fact, it is suggested that this reaction requires the co-existence of Cr3+ and Cr6+ which is more at the 0.4CrZ sample than at other samples.
The efficiency of PNP photodegradation over various catalysts. Experimental conditions: pH = 5, reaction volume 250 ml, catalyst content 0.2 g, reaction time 60 min, initial PNP concentration 5 × 10−3 M.
The samples containing high loading Cr (0.6CrZ to 0.8CrZ) as well as containing Cr2O3 showed low catalytic activity. This may indicate that Cr is active when incorporated inside zeolite (heterogeneous phase) and its share in the reaction activation may come following that of ZSM-5.
On the other hand, charge-transfer process on M–O moieties (of tetrahedral structure) may involve an electron transfer from the O−2 to M+ ions forming charge-transfer excited triple state: Thus, adsorption of molecules with electron donating nature and those with electron accepting nature either within zeolite or on its external surface can lead to the CT complexes due to attractive force between the donor–acceptor pairs (Ali et al., 2009; Dzwigaj et al., 2000). Thus, a remarkable enhancement in photocatalytic decomposition of PNP is attained by such synergism. This highlights that the photocatalytic oxidation of the PNP on 0.4CrZ is primarily due to the charge transfer relating an electron transfer from the O2− of SiO4 unit to the coordinated Cr ions.
In order to determine the optimal amount of photocatalyst, a series of experiments with varied amounts of the 0.4CrZ catalyst have been conducted. The amount of the photocatalyst was varied between 0.05 and 0.5 g/dm3 of 0.4CrZ. The catalyst was added to 250 ml of 5 × 10−3 M PNP, stirred and irradiated for 60 min. The samples were taken from the suspension, passed through a 0.45 μm Millipore filter to remove the particles, the concentration of PNP in the filtrate was determined at once. The influences of the mass of 0.4CrZ on the degradation efficiency of PNP are shown in Fig. 7. It was observed that the degree of photocatalytic efficiency of PNP solution increased with increasing the loading catalyst amount, till 0.2 g. The most effective decomposition of PNP (92%) was observed with the catalyst amount equal to 0.2 g after which (0.25 to 0.5 g of catalyst loading) the photocatalytic efficiency decreases. This phenomenon caused by the so-called shielding effect, i.e. exceeding the optimal amount of the suspended 0.4CrZ particles increases the complex formation with the PNP and thus blocking off the available surface active sites. Increasing the suspended particles of Cr containing zeolites reduced the adsorption from solution due to the expected crowding of the PNP molecules during the adsorption process. Many authors have investigated the reaction rate as a function of catalyst amounts under different experimental conditions (Mengyue et al., 1995; Othman et al., 2006). They concluded that the increase of catalyst amounts promote the existence of the presence of parallel paths associated with catalyst degradation during the catalytic cycle (e.g. dimerization reaction) (Salama et al., 2009), and thus, increases as the concentration of the amount of the catalyst increases. However, attaining complete degradation of the PNP at specific catalyst concentration (0.2 g) nullifies the possibility of the presence of competing paths for the reaction and indeed suggests a facile pathway for the degradation of such a PNP.
Effect of photocatalyst content of 0.4CrZ on the decolorization of PNP. Experimental conditions: pH = 5, T = 25 °C, time = 60 min, volume = 250 ml, initial PNP concentration 5 × 10−3 M.
The degradation efficiency of PNP on 0.4CrZ at different pH levels was shown in Fig. 8. The pH of the solution was adjusted with dilute NaOH and HCl solutions. The different concentrations of acid or base have been chosen in order to add the minimum quantity of these species to avoid the volume change of the reaction mixture. It was found that the degradation of PNP over 0.4CrZ was highly pH dependent, with the degradation efficiencies increased with decreasing pH values. This is evidence that the contribution of the leaching chromium activity to the CrZSM-5 activity in the organic oxidation is negligible all along the catalyst use. Thus, most part of chromium in CrZSM-5 exists in a stable form, does not leach from the catalyst and is active in the organic oxidation by hydrogen peroxide during manifold use of the catalyst. It is likely that this part of chromium exists in the framework position of zeolite; however, the role of di-/oligonuclear iron complexes, consisting of zeolite, in the H2O2 decomposition reaction is unclear and cannot be revealed from the given experimental data (Canali and Sherrington, 1999). The main reason of different initial pH values of the suspensions assigned to the content of aluminum also influences the catalyst acidity (Kuznetsova et al., 2004).
Effect of pH on the photodegradation percentages of PNP on 0.4CrZ. Experimental conditions: reaction volume 250 ml, catalyst content 0.2 g, reaction time 60 min, initial PNP concentration 5 × 10−3 M.
4 Conclusions
Silica derived from rice husk ash has been used to synthesize ZSM-5 zeolite with an Si/Al ratio of 38. Cr-substituted ZSM-5 has also been hydrothermally synthesized by using TPABr template conditions by replacing Al with Cr at constant SiO2/(Al2O3 + Cr2O3) molar ratios.
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The replacement of Al3+ by Cr with larger ionic radius causes a shift of Si–O–T vibration to lower wavenumbers. The presence of tetrahedral Cr has been confirmed by UV–vis spectroscopy.
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By comparing the lattice volume of the Cr-substituted ZSM-5 samples with that of no iron, one observes that the lattice volume increase in the case of 0.2CrZ and 0.4CrZ samples pointing to the presence of some Cr species in a highly dispersed state inside zeolite channels. The recognized increase in lattice volume was due to increasing the accessibility of Cr ions into ZSM-5 channels exchanging Na ions located therein.
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The Cr-substituted ZSM-5 has been used as a catalyst in the photocatalytic degradation of p-nitrophenol. 0.4CrZ was found to be the most active sample in the degradation of PNP in accordance with the higher dispersion of Cr and the larger value of its BET surface area. It seems that the photocatalytic degradation of PNP can be accelerated by various Cr species specifically those in framework positions and those exhibiting finally dispersed Cr2O3 species.
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