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Cu functionalized nano crystalline ZSM-5 as efficient catalyst for selective oxidation of toluene
Materials Today Chemistry 3 (2017) 37e48
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Cu functionalized nano crystalline ZSM-5 as efficient catalyst for selective oxidation of toluene N. Viswanadham a, *, Sandeep K. Saxena a, b, Ala'a H. Al-Muhtaseb b, ** a b
Refining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India Department of Petroleum and Chemical Engineering, College of Engineering, Sultan Qaboos University, Muscat 123, Oman
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 25 November 2016 Accepted 11 January 2017
Nano crystalline ZSM-5 (NZ) functionalized with Cu has been explored for the selective oxidation of toluene to benzoic acid for the first time, where a comparison is made with the traditional microcrystalline ZSM-5 (MZ) based catalyst having similar framework Si/Al ratios at organic solvent-less reaction conditions in presence of H2O2 as a green oxidant. The ZSM-5 catalysts exhibited oxidation property even in the absence of any Cu species, but with low toluene conversions and benzoic acid yields. Functionalization with Cu greatly enhanced the benzoic acid formation, especially on NZ loaded with 0.4 wt% of Cu (Cu-NZ) to produce 92 wt% of benzoic acid, by virtue of the presence of highly dispersed nano particles of CuO along with Cuþ2 ions on the high surface area, mesopore possessing NZ support, revealed from XRD, N2 adsorption-desorption, XPS, SEM, TEM, FITR, TPR and TPD analysis. © 2017 Published by Elsevier Ltd.
Keywords: Nano crystalline ZSM-5 Mesoporosity Toluene oxidation Cu loading Variable oxidation states
1. Introduction The volatile organic compounds (VOCs) emitted from industrial and transportation activities causes serious problems to human health by their toxic or malodorous nature and the formation of suspended particulate matter and photochemical smog. Catalytic oxidation of VOCs is an important process come up to address these problems, where complete oxidation of these compounds has been successfully achieved through catalytic combustion [1,2]. But the product obtained in this case is CO2 that does not have any commercial value. Moreover, this process generates a green-house gas that should be avoided to make the industrial processes environmentally benign. Partial and selective oxidation of such organic compounds into the useful intermediates is a green and profitable option, where development of an efficient catalyst for improving selectivity for the value-added products remains a major challenge for industry [3e6]. Traditional approaches for the reaction were based on applying the stoichiometric amounts of liquid oxidants such as potassium permanganate, but suffer from the limitations of low products selectivity [7]. Recently, several homogenous and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (N. Viswanadham), almuhtasebala@gmail. com (A.H. Al-Muhtaseb). http://dx.doi.org/10.1016/j.mtchem.2017.01.001 2468-5194/© 2017 Published by Elsevier Ltd.
heterogeneous catalyst systems have come up to address the basic problems involved in this process to make it green [8e10]. The subject still demands novel and practical inputs in the area of catalyst development for the selective oxidation of individual organic compounds so as to make it suitable for the industrial applications. Toluene is one of the substrates categorized as hazardous material. The chemical also identified to cause serious health problems such as nerve disorders, liver and kidney damage. These facts demand immediate attention for the green conversion of toluene into value added products. Toluene can be converted to oxygenated products such as benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate. Among them benzoic acid is very important due to its high demand in the current industrial practice such as intermediate in the productions of chemicals, plasticizers, dyestuffs, preservatives and retardants [11e13]. The commercial production of benzoic acid via the catalytic oxidation of toluene is achieved with cobalt acetate and bromide promoter in acetic acid using an oxidizing agent at high pressure [14]. Although complete conversion is achieved, the use of acidic solvents and bromide promoter results in difficulty in separation of catalyst from products along with other disadvantages caused by the large volume of toxic waste and equipment corrosion. Further, severe operation conditions adopted in vapor phase oxidation consumes large amount of energy which is not economical for
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industrial applications. Liquid phase oxidation of toluene is a better alternative as it is easy to operate and can achieve high selectivity under relatively mild reaction conditions. Many efforts have been made to improve the efficiency of toluene oxidation in the liquid phase reaction; however, most investigations were reported on homogeneous systems using volatile organic solvents [15e17]. Heterogeneous catalysts are preferred over homogeneous catalysts because these materials can be readily separated from the reaction mixture. Heterogeneous catalysts can also be easily used in flow reactors, facilitating the efficient productions of materials using continuous processes. Hence, operation of heterogeneous catalysts in liquid phase oxidation of toluene without solvent would make the process environment-friendly. The catalytic liquid phase oxidation mainly carried out using the oxidants such as H2O2, tertbutyl hydroperoxide, or O2. Here the selection of reagent is important to avoid the over oxidation of the substrate to avoid the formation of undesired CO2. Hydrogen peroxide with a high level of active oxygen is an efficient, inexpensive, mild and environmentally friendly reagent for oxidation (H2O and O2 are the byproducts) [18e20]. Bastock et al. have reported the oxidation of toluene to benzoic acid (75 wt% yield) in solvent free conditions using a commercial heterogeneous catalyst Envirocat EPAC promoted by a carboxylic acid at high temperature and atmospheric pressure [21]. Subrahmanyan et al. have performed toluene oxidation on vanadium substituted alumino-phosphate or alumino-silicate in solvent free conditions to produce benzaldehyde, benzoic acid and cresol depending on the nature of oxidizing agent used [22]. Thomas et al. have reported the solvent free aerobic oxidation of toluene using Fe-AlPO-5 zeolite catalyst to achieve 33% benzoic acid selectivity at ~40% toluene conversion [23]. Li et al. have also reported manganese oxide and copper manganese oxide as active catalysts for aerobic oxidation of toluene to benzoic acid in solvent free conditions [24]. Pt supported on zirconia support was also observed to give moderate conversions and benzoic acid selectivities in the liquid phase solvent free aerobic oxidation conditions [25]. Very recently, manganese oxide supported SBA-15 catalysts have been observed to facilitate solvent-free selective oxidation of toluene to produce benzaldehyde and benzoic acid depending on the concentration of the MnO species [26]. A common problem observed in most of the cases is the lower yields of benzoic acid along with considerable production of benzyl alcohol and benzaldehyde as side products. Present study reports the effective toluene oxidation property of a Cu functionalized Nano crystalline ZSM-5 possessing high external surface area and mesoporosity using H2O2 as green oxidation reagent to produce high yields of benzoic acid at solventfree reaction conditions at relatively higher reaction temperatures. Here the purpose of choosing high temperature is to minimize the formation of partial oxidation products such as benzyl alcohol and benzaldehyde while encouraging the formation of deep oxidation product benzoic acid. At higher reaction temperatures, the toluene in vapor phase can undergo vigorous interaction as the vapor diffusion in zeolite pores is high to exhibit higher conversions [12,21,24e26]. 2. Experimental
ammonium chloride (Aldrich) and copper nitrate (Merck Reagent Co. Ltd) were also purchased. 2.2. Synthesis of nano-crystalline ZSM-5(NZ) sample In this synthesis, aluminum nitrate solution was slowly added to pre-cooled tetra propyl ammonium hydroxide (TPAOH) solution, followed by the drop wise addition of tetraethyl ortho-silicate (TEOS). The components were mixed with constant stirring and the solution was left to hydrolyze at room temperature for 41 h. The gel thus obtained was heated at 80 C to evaporate water and ethanol formed during the hydrolysis to obtain a gel. The concentrated gel was charged in a Teflon lined autoclave for hydrothermal synthesis at 170 C for 48 h. Synthesized sample was filtered and washed with de-ionized water, followed by drying at 100 C and calcination at 500 C for 4 h under vacuum. The powder forms of the zeolites are converted into extrudates form by mixing with 40 wt% of pseudo boehmite (neutral) alumina followed by drying and calcinations [27]. 2.3. Preparation of Cu loaded ZSM-5 catalysts Incipient wet impregnation method is used for the loading 0.4 wt% of Cu on to the NZ and MZ, followed by drying at room temperature (25 C) for 12 h, heating at 100 C for 8 h and calcination at 500 C for 4 h in presence of air. The catalysts obtained have been denoted as Cu-NZ (0.4 wt% Cu) and Cu-MZ (0.4 wt% Cu) based on the nature of support. Much better performance in the reaction exhibited by Cu-NZ guided us for the detailed studies on the Cu-NZ catalyst. Further studies are conducted to check the effect of Cu concentration on NZ at below and above 0.4 wt% Cu loading. The catalysts are named as Cu-NZ1 (0.2 wt% Cu), Cu-NZ2 (0.6 wt% Cu) and Cu-NZ3 (0.8 wt% Cu). The calcined samples in metal oxide form were loaded in the reactor and all the catalysts were reduced in situ at 550 C under hydrogen pressure before using them for toluene oxidation reaction. 2.4. Physico-chemical characterization The Scanning Electron Microscope (SEM) images were recorded for obtaining particle morphology on JEOL JSM-7900F instrument Japan. Transmission Electron Microscopy (TEM) images were recorded on JEOL, JSM-2100F instrument, Japan. X-ray powder diffraction (XRD) patterns were measured on PANalytical Xpert PRO instrument, USA equipped with rotating anode and CuKa radiations. The Fourier transform infrared spectroscopy (FTIR) spectra of the zeolite samples were recorded on Perkin Elmer-spectrum two, USA. The BET surface area, pore size and pore volume measurements of all the zeolite based catalysts were carried out using a standard adsorption equipment (ASAP 2020, Micromeritics Instruments Inc., Norcross, GA, USA) using N2 gas (99.995% pure). The acidity of the catalyst was measured by temperature programmed desorption of NH3 (NH3-TPD) using a Micromeritics chemisorbs 2750 pulse chemisorption system. X-ray Photoelectron Spectroscopy (XPS) analysis was performed with a Kratos AXIS Ultra DLD apparatus, equipped with monochromated Al Ka radiation X-ray source.
2.1. Materials 2.5. Catalyst evaluation studies Conventional H-ZSM-5 zeolite (SAR ¼ 30) was obtained from Suid Chemie India limited and termed as micro-crystalline zeolite (MZ), tetraethyl ortho-silicate (TEOS, AR), Tetra propyl ammonium hydroxide (TPAOH) solution, toluene (AR) and hydrogen peroxide were purchased from Merck Reagent Co. Ltd. Platinum tetra
The toluene oxidation with hydrogen peroxide was carried out without using any solvent in a PARR autoclave reactor 4848 equipped with four bladed pitched turbine impeller. In a typical reaction; 1.0 g of the catalyst, 25 ml of toluene (Merck -AR grade)
N. Viswanadham et al. / Materials Today Chemistry 3 (2017) 37e48
and 25 ml of 30 wt% hydrogen peroxide (Merck) with 25 ml deionized water. The aqueous hydrogen peroxide added drop-wise to avoid the immediate decomposition of hydrogen peroxide. The autoclave was pressurized with N2 gas upto 5.0 bar pressure. The reaction temperature was maintained at 180 C and reaction mixture stirred at 550 RPM for 4 h under nitrogen gas pressure. At the end of the reaction, the mixture was cooled upto room temperature (25 C) and the catalyst was separated by filtration, washed with ethanol, dried at 120 C and reused for four times. The reaction product was analyzed by using GC equipped with the DBWax column (30 m 0.53 mm 1.0 mm) and FID detector. 3. Results and discussion The present study is aimed to understand the effect of the nature of ZSM-5 supports, NZ and MZ, on the properties of Cu-loaded zeolites and their catalytic performances towards the oxidation of toluene. For this two catalysts Cu-MZ and Cu-NZ prepared by loading 0.4 wt% Cu have been thoroughly characterized and evaluated towards the oxidation reaction. 3.1. Physico-chemical properties of NZ, MZ, Cu-NZ and Cu-MZ samples Fig. 1 shows the X-ray diffraction patterns of Nano-ZSM-5 (NZ), Micro-ZSM-5(MZ), 0.4 wt% Cu loaded NZ and MZ zeolites (Cu-NZ and Cu-MZ). All the samples exhibit the typical diffraction peaks of ZSM-5 zeolite at 2q of 7.8 , 8.9 , 23.1 and 23.8 representing the crystal planes of (011), (020), (051) and (033) respectively [28]. Similar XRD patterns exhibited by the samples even after Cu loading (Cu-MZ and Cu-NZ) indicate that the structure of ZSM-5 support remains intact after the loading of Cu. No diffraction
Fig. 1. XRD patterns of MZ and NZ zeolites before and after Cu loading.
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patterns related to the presence of CuO observed in Cu-MZ and CuNZ samples suggest the presence of highly dispersed state of Cu species and the possible formation of smaller size aggregates of Cu which are too small to be detected by XRD. The structure morphology of the materials was characterized by SEM and TEM analysis. The SEM images (Fig. 2) clearly show that the zeolite crystal size has been successfully brought down from micrometer range crystal of MZ to nano meter range crystals of NZ. The TEM (Fig. 3) analysis shows the presence of 2 mm size crystals in MZ and ~30 nm size crystals in NZ to confirm the nano crystal structure of NZ. The Cu loaded zeolites (both Cu-NZ and Cu-MZ) did not show any prominent size particles of the Cu, supporting the presence of highly dispersed state of the Cu species in MZ and NZ samples. The XPS analysis was used for understanding the metal support interaction in these samples, where, the intensity and binding energy of Cu 2p of the samples have been measured (Fig. 4). In each case, the intensities have been normalised to the Si 2p peak area. Both Cu-MZ and Cu-NZ samples have shown two main peaks at B.E 933.4 eV and 953.2 eV in their Cu2p spectra related to Cu2p3/2 and Cu2p1/2, respectively. The intensities of both the peaks related to Cu species are much higher in Cu-NZ sample when compared to the Cu-MZ. Another difference in XPS spectra observed was the deconvolution patterns of the Cu2p3/2 peak. In case of Cu-NZ sample, the Cu2p3/2 peak can be further deconvoluted to two peaks; one at 933.4 eV and another at 935.5 eV. The higher binding energy feature (935.5eV) in the case of the Cu-NZ sample is indicative of the presence of a small amount of Cu(II) possibly in the form of isolated cations (Cuþ2). Hence, the two peaks appeared at 933.4 eV and 935.5 eV in Cu-NZ can be ascribed to the agglomerated CuO nano particles on the catalyst surface and Cu2þ ion coordinated with the zeolite perhaps at ion-exchange positions, respectively [29,30]. But, the absence of any diffraction patterns related to CuO particles observed in XRD and no visible particles observed in HRTEM analysis of the Cu-NZ sample suggests the Cu species in highly dispersed state with relatively smaller size particles of Cu (less than 3 nm) in this sample. This particular feature of deconvolution of Cu2p3/2 related to the Cu2þ species was absent in the Cu-MZ sample. This may be due to the relatively poor interaction of the Cu species with the MZ support. The superior interaction of NZ support with Cu species (Cu-NZ), especially in terms of the Cu2þ occupying ion-exchange positions of ZSM-5 framework, can be attributed to the easy accessibility of the active sites of hierarchically mesoporous nano ZSM-5 (NZ) for the interaction of Cu species in Cu-NZ. The Cu-MZ and Cu-NZ samples also showed difference in the H2-TPR patterns (Fig. 5). There are two distinguished reduction peaks observed for Cu-NZ at 232 C and 358 C related to the reduction of Cu in two different states, namely, dispersed CuO and Cuþ2 species located at ion-exchange positions [31]. But, in case of Cu-MZ the high temperature peak related to reduction of Cuþ2 was not prominent. This is in accordance with the XPS data indicating that the textural properties of nano crystals in NZ is responsible for the formation of significant amount of Cuþ2 species in Cu-NZ. The adsorption and desorption isotherm plots of both MZ and NZ samples were measured before and after the Cu loading to understand any changes in porous properties of these zeolites by Cu. The isotherms shown in Fig. 6 represent type IV isotherm with a steep jump in the loop volume at higher relative pressure caused by the filling of the pores in the zeolites. In case of NZ, there is a big adsorption loop appeared as compared to that of MZ. The prominent hysteresis loop appeared at high relative pressure (P/ P0 ¼ 0.9e1.0) of NZ is a clear indicative of the presence of an ample amount of mesopores, which are believed to be formed by inter crystalline voids of the nano crystals of the ZSM-5. The BJH pore size distribution patterns of the NZ further reveal the presence of
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Fig. 2. SEM images of the ZSM-5 and Cu-ZSM-5samples.
significant amount of mesopores (20e80 nm) in this sample (Fig. 7B), when compared to those in MZ (Fig. 7A). Hence, the NZ sample is expected to possess higher total surface area, external surface area, mesopore volume and total pore volume, which was indeed reflected in the measured properties (Table 1). Cu loading does not change the basic shape of the isotherms of the MZ and NZ samples (Fig. 6), but, there is a steep increase in the loop at 0.9e1.0 p/p0 related to the mesopores was observed in the Cu-MZ and Cu-NZ samples. This aspect is clearly visible from Fig. 7, where a significant increase in the mesopores of 5e80 nm was observed in NZ after Cu loading (Cu-NZ). The increase in mesopores of 30e90 nm was also observed in Cu-MZ, but the increase is not very significant in this case. In micro pore volume region all the samples exhibited the comparable pore volume plots (Fig. 8). The surface area, pore volume and average pore diameter values of the samples given in Table 1 clearly indicate that the textural properties of both MZ and NZ have improved after Cu loading, while this aspect is more pronounced in case of NZ. This phenomenon of increase in surface area and pore volume of the zeolites after metal loading can be explained by the effective interaction of the Cu species with the ZSM-5 framework structure. This phenomenon is significant especially in case of NZ catalyst. Hence, the increase in total pore volume of the samples after metal loading can be ascribed to the formation of mesopores by metal-support interaction. The situation suggests the state of highly dispersed Cu species on the NZ which does not cause any loss in pore volume by its physical occupation or filling of the zeolite pores of the resultant Cu-NZ sample. The proposed state of highly dispersed Cu species also explains why the particles of Cu species are not visible in the XRD patterns and TEM pictures of the Cu-NZ. Overall, the porosity patterns of the Cu loaded samples indicate that there is no pore occupation/blockage was occurred in both the zeolite samples by Cu loading.
The FTIR spectra of NZ and MZ samples were measured before and after Cu loading to understand the metal-support interactions in Cu-NZ and Cu-MZ samples. Fig. 9 shows the bands in both MZ and NZ near ~1080 cm1 (internal asymmetric stretch), ~790 cm1 (external symmetric stretch), ~540 cm1 (double ring vibration) and 450 cm1 (T-O bend) representing the MFI phase. The bands at 1068 cm1 to 1219 cm1 has been assigned to external linkages (between TO4 tetra hedral). The nano crystalline ZSM-5 (NZ) exhibited a prominent structure-sensitive asymmetric stretch vibrational band at this region when compared to the MZ due to the more exposed tetrahedral in the nano crystalline sample. The band at 3645 cm1 related to acidic (Al- and Si-bridged) hydroxyl group, 3450 cm1 related to silanol hydroxyl is also observed in the samples [32]. However, after Cu loading the band at 3645 cm1 (bridged eOH groups) in Cu-NZ was evidently disappeared as shown in Fig. 9, indicating the possible occupation of Cuþ2 ion by replacing the eOH group. This phenomenon was not observed in Cu-MZ suggesting the effective accessibility and interaction of Cu with NZ when compared to MZ. The IR in combination with XPS data indicates the presence of Cuþ2 at ion exchange positions (bridging hydroxyl group) of NZ in the Cu-NZ sample. The background spectra of all the zeolite samples are very similar. The ammonia desorption (TPD) of the both MZ and NZ samples were measured before and after Cu loading to understand the effect of Cu loading on the acid sites. TPD patterns of the MZ and NZ samples shown in Fig. 10A and B respectively indeed reveal the comparable acidity patterns of these two samples which are differ in crystal size but having same value of SAR, which support the crystal size of MZ is decease from micro level to the nano level without affecting the acidity. The TPD of each catalyst has been normalized to distinguish the relative peak areas of weak Bronsted acid from strong Lewis acid sites. Normally, strength of solid acid sites within TPD profiles can be classified by the temperature of
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Fig. 3. TEM images of the ZSM-5 and Cu-ZSM-5samples.
~420 C, which may be attributed to the desorption of two types of ammonia species adsorbed on weak acid (mostly Lewis acid) sites and strong acid (mostly Bronsted acid) sites, respectively. It can be observed that all the curves displayed a major desorption peak around 150 C, an indication of the weak (Bronsted and/or Lewis) sites present in the catalysts, which is consistent with typical ZSM5 peak. The loading of Cu on MZ caused slight decrease in the weak acidity but the strong acidity remains unchanged. In case of NZ, the Cu loading led to the reduction in weak acidity as well as strong acidity. The high temperature peak exhibited reduced intensities especially above 420 C along with a slight downshift of its position. This observation again suggests possible interaction of the Cuþ2 with the eOH groups (ion-exchange positions) in NZ sample and supports the findings of FTIR (decreased intensity at 3645 cm1) and XPS results (presence of Cuþ2 species) of Cu-NZ.
3.2. Solvent free oxidation of toluene
Fig. 4. XPS of the Cu loaded MZ and NZ zeolites.
desorption of NH3 as weak (120e300 C), moderate (300e500 C) and strong (500e650 C). For the MZ, NZ and 0.4 wt% Cu-loaded MZ (Cu-MZ) and NZ (Cu-NZ), the NH3- TPD peaks appeared at ~150 and
The performance of liquid-phase toluene oxidation of both CuNZ and Cu-MZ catalysts was studied with hydrogen peroxide (green oxidant) under solvent-free conditions at 180 C, 5 bar pressure (N2 gas) for 4 h, where the performance of these catalysts was compared with the metal-free zeolites (MZ and NZ) to understand the effect of Cu on oxidation activity (Table 2, entries 1e4). The conversion of toluene is moderate on metal-free ZSM-5 catalysts i.e MZ and NZ. But, the Cu loading increased the conversion of toluene significantly on both the zeolites (Cu-MZ and Cu-NZ). All
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Fig. 5. Temperature programmed reduction (TPR) patterns of the Cu loaded zeolites. Fig. 7. Effect of Cu loading on the mesopore size distribution patterns of MZ and NZ.
Table 1 Detailed physico-chemical properties of the various zeolite catalysts. Sample a
2
SBET (m /g) Micro pore Area (m2/g) External Surface Area (m2/g) b Total Pore Volume (cm3/g) c Micro pore volume (cm3/g) d Meso/Macro pore volume (cm3/g) e Median pore diameter (Å) Average particle size (Å) f Crystallinity (%)
MZ
NZ
Cu-MZ
Cu-NZ
299.74 204.02 95.72 0.1777 0.0825 0.0952 6.060 128.7 99.98
448.98 248.76 168.21 0.4880 0.1370 0.3510 6.349 120.92 99.97
432.89 291.10 141.79 0.2767 0.1179 0.1588 6.115 138.84 99.94
512.64 267.92 244.72 0.6297 0.1092 0.5205 6.671 137.10 99.97
a
BET surface area. Total pore volume taken from the volume of N2 adsorbed at P/P0 ¼ 0.9. c Micropore volume calculated from t-plot. d Meso/Macro pore volume calculated from total pore volume e micropore volume. e BJH adsorption average median pore diameter. f Crystallinity based on XRD analysis. CuMZ and CuNZ conating 0.4 wt% Cu metal. b
Fig. 6. Nitrogen adsorption-desorption isotherms of pure and Cu loaded zeolite catalysts.
the three partial oxidation products were observed in the reaction product, namely, benzyl alcohol, benzaldehyde and benzoic acid on the catalysts. The metal-free ZSM-5 catalysts (MZ and NZ) also exhibited toluene oxidation property, but the conversion of toluene and the yields of benzoic acid are low on these catalysts (entries 1 and 2). Functionalization with Cu greatly enhanced the toluene oxidation to give higher benzoic acid yields on Cu-MZ and Cu-NZ catalysts (entry 3 and 4). Between MZ and NZ, the latter exhibited higher toluene conversion and benzoic acid yields. The zeolites followed similar trend even after the Cu loading, where Cu-NZ exhibited superior performance to Cu-MZ in the reaction both in terms of toluene conversion and benzoic acid yields.
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Fig. 8. Effect of Cu loading on the micropore size distribution patterns of MZ and NZ.
Overall, the performance of the catalysts is in the order of MZ < NZ < Cu-MZ < Cu-NZ. It is clearly indicating that the nanocrystalline ZSM-5, with its hierarchical porosity created by zeolitic micropores in combination with additional mesopores, exhibits promising catalytic activity in toluene oxidation. The loading of 0.4 wt% Cu on the ZSM-5 further increased the yield of benzoic acid, both in case of micro-crystalline and nano-crystalline ZSM-5 supports. This has resulted in the simultaneous decrease in the yields of benzaldehyde and benzyl alcohol. The highest benzoic acid yields with negligible benzyl alcohol and benzaldehyde produced over Cu-NZ clearly indicate the effective oxydehydrogenation property of Cu species to facilitate the complete oxidation of toluene. The XPS and FTIR results of Cu-NZ also indicated the presence of some Cuþ2 ions in ion-exchange positions in addition to the highly dispersed CuO. The free Cuþ2 ions available for the effective interaction to oxidize the hydrocarbon intermediates during the conversion of toluene may be responsible for the superior performance of Cu-NZ catalyst. Earlier studies of Wang et al. also revealed the enhanced NO conversion and oxidation properties of Cu/SAPO-34 catalyst having both CuO nano particles and Cuþ2 ions [33]. These observations suggest two catalyst parameters responsible for the superior catalytic performance of the Cu-NZ in the toluene oxidation reaction; 1. role of nano size crystals of ZSM-5 and 2. role of Cu. The combined effect of nano crystallites of ZSM-5 with the oxidation property of loaded Cu is responsible for the highest toluene conversion of ~95% with as high as 92 wt% yield to benzoic acid obtained on Cu-NZ catalyst.
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Fig. 9. FTIR patterns of pure and Cu loaded zeolite catalysts.
3.3. Factors contributing to the superior oxidation property of CuNZ catalyst Cu is known for its variable valence and the resultant oxidation property is reflected in variety of oxidation reactions. Hence, the toluene oxidation property possessed by Cu-MZ and Cu-NZ catalysts can be ascribed to the redox properties of Cuþ2/Cu þ species facilitated at the reaction conditions. The superior performance of Cu-NZ is understood from the additional surface area, porosity and diffusivity operated in the hierarchically mesoporous nature of the nano crystalline ZSM-5. The interaction of Cu is also observed to be effective with the nano crystalline ZSM-5 (Cu-NZ), realized by considerable decrease in the band intensity of 3670 cm1 (related to bridging eOH groups) of NZ in FTIR due to the interaction of Cu ions with the framework eOH group of NZ during Cu loading. Further, the XPS and TPR results support the presence of the second form of copper (Cuþ2 at ion-exchange positions of ZSM-5) in significant amount, in addition to the highly dispersed CuO in Cu-NZ. This has resulted in the excellent oxidation property of the Cu-NZ, which indeed exhibited the highest oxidation property of toluene to yield benzoic acid. This phenomenon of the presence of bivalent copper (Cuþ2) species is less pronounced in Cu-MZ perhaps due to the inferior metal-support interaction occurred in the mere microporous micro crystals of MZ when compared to the corresponding meso-microporous nano crystals of NZ. On the other hand, the presence of inter-crystalline porosity, the higher external surface area in case of NZ perhaps responsible for the interaction of Cu species (Cuþ2) effectively at ion-exchange positions of in the CuNZ. Overall, the Cu-NZ catalyst of the present study could yield
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Fig. 11. Effect of Cu amount on the toluene oxidation performance of NZ catalyst.
Fig. 10. TPD patterns of pure and Cu loaded zeolite catalysts.
~92 wt% of benzoic acid, which is promising for the effective conversion of toluene to value added products useful for industrial applications at solvent-free conditions. After identifying the Cu-NZ (0.4 wt% of Cu on NZ) as promising oxidation catalyst, the effect of Cu concentration on the activity of
NZ has been studied, where the concentration of Cu has been varied below and above the 0.4 wt% of Cu. The catalysts are named as CuNZ1 (0.2 wt% Cu), Cu-NZ2 (0.6 wt% Cu) and Cu-NZ3 (0.8 wt% Cu). Fig. 11 shows the effect of Cu amount on the catalytic activity, where the amount of Cu loading was varied from 0.2 wt% to 0.8 wt% on the NZ support. The trends indicate that the conversion of toluene as well as benzoic acid yield was increased up to the 0.4 wt% Cu. Further increase of Cu did not cause any improvement in the catalytic performance (Table 2, entries 4e7). Detailed studies are also conducted to optimize the reaction temperature and reactor pressure for the production of benzoic acid (entries 4 to 14 in Table 2). Reaction temperature caused increase in the catalytic performance up to 180 C and then the activity was leveled above this temperature (Fig. 12A). The catalyst also exhibited better performance at above the atmospheric pressure with optimum activity at 5 bar pressure (Fig. 12B). Above this pressure there was no further improvement in the performance. . To explore its reusability, the Cu-NZ (0.4 wt% Cu) catalyst was separated out from the product mixture and reused for toluene conversion reaction after drying at 120 C for 4 h. The results shown in Table 3 indicated that the activity of the catalyst in terms of toluene conversion and benzoic acid yield was almost unaffected even at the third run. The toluene conversion was slightly declined at the fourth run, attributing to the possible decrease in amount of the catalyst recovered after the runs.
Table 2 Toluene oxidation performance of catalysts in solvent-free conditions. Entry
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Catalysts
MZ NZ Cu-MZ Cu-NZ Cu-NZ1 Cu-NZ2 Cu-NZ3 Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ
Temperature ( C)
180 180 180 180 180 180 180 120 160 180 200 180 180 180
Pressure (bar)
5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 0.0 2.5 7.5
Toluene conversion (wt%)
38.6 54.7 85.7 95.4 72.2 96.1 96.1 52.9 80.5 95.4 95.9 45.8 82.9 96.4
Product yield (wt%) Benzyl alcohol
Benzaldehyde
Benzoic acid
27.2 21.3 2.3 0.6 2.3 2.0 1.9 10.4 1.4 0.6 0.6 11.7 6.6 1.8
6.3 5.0 1.8 2.8 4.7 2.9 4.0 16.8 6.2 2.8 3.5 8.6 6.9 3.3
5.1 28.4 81.6 92.1 65.2 91.2 90.2 25.7 72.9 92.1 91.9 25.5 69.5 91.3
TON
TOF (s1)
17.3 90.0 2300 2594 3673 1528 1272 725 2057 2594 2593 721 1960 2577
0.0012 0.0062 0.159 0.180 0.255 0.106 0.088 0.050 0.143 0.180 0.180 0.050 0.136 0.178
Reaction Conditions: Reaction time ¼ 4 h; Feed ¼ 25 ml Toluene þ 25 ml H2O2 (30 wt%) þ 25 ml deionised water, Catalyst ¼ 1.0 g, Pressure with N2 gas Cu-NZ ¼ 0.4 wt% Cu, CuNZ1 ¼ 0.2 wt% Cu, Cu-NZ2 ¼ 0.6 wt% Cu, Cu-NZ3 ¼ 0.8 wt% Cu, Cu-MZ ¼ 0.4 wt% Cu.
N. Viswanadham et al. / Materials Today Chemistry 3 (2017) 37e48
Fig. 12. Effect of reaction conditions on the oxidation performance of Cu-NZ (0.4 wt% Cu on NZ) catalyst. (A) Effect of reaction temperature, and (B) Effect of reaction pressure.
The Cu-NZ (0.4 wt%) catalyst obtained after the 4 reaction cycles was further characterized and compared with the fresh catalyst. The XRD patterns of both the fresh (Cu-NZ) and the spent Cu-NZ (SCu-NZ) catalysts are comparable (Fig. 13A). The FTIR spectra of fresh and spent catalyst also (Fig. 13B) shows similar features before and after reaction indicates the catalyst properties are retained during the reaction cycles and supports the catalyst performance data of the reused Cu-NZ catalysts towards benzoic acid production demonstrated in Table 3. However, the curve fitting XPS analysis of the spent Cu-NZ catalyst exhibited the presence of about 10% of Cu in the form of Cu2O (Cu+1) (Fig. 13C). The presence of Cuþ1 clearly suggests the reduction process of Cuþ2 occurred during the oxidation of toluene over the Cu-NZ catalyst. This observation proves the oxidation role of Cuþ2 species towards toluene oxidation. Then the question arises about the role of CuO nano particles
45
in the Cu-NZ catalyst? To answer this question luckily we have the Cu-MZ catalyst that has no Cuþ2 species but has only CuO nano particles, which has exhibited considerable toluene oxidation activity (Table 2). Hence, the dispersed nano particles of CuO are also considered effective for the toluene oxidation reaction. However, the superior performance of the Cu-NZ possessing both types of Cu species (CuO and Cuþ2) highlights the additional role of Cuþ2 to promote the oxidation reaction further. The inter conversions prevailed between Cuþ2/Cuþ1 species at reaction conditions may be responsible for higher conversions and product yields observed on this catalyst. Table 2 shows the TON and TOF of the all the catalyst samples studied for the toluene oxidation reaction (calculations given in ESI). The TON values are more suitable for the comparison of homogenous catalysts, while TOF is relevant to the heterogeneous catalyst systems such as the present catalyst system [34]. The TOF values are calculated for the catalysts used in the present reaction. The mille mole of Cu present in each catalyst has been considered for calculation of active sites, assuming all the Cu species are participating in the reaction [35e38]. Among the Cu loaded NZ catalysts, these values have been continuously decreased with increasing the Cu amount. Highest TOF values are exhibited at 0.2 wt% Cu loading on NZ, but the conversion of toluene is low on this catalyst. The second best catalyst observed in terms of TOF values is the Cu-NZ loaded with 0.4 wt% Cu which also exhibited higher conversion and product yields. This can be ascribed to the sufficient number of Cu species available on this catalyst when compared to the 0.2 wt% Cu loaded catalyst. Between Cu-NZ and Cu-MZ at similar Cu loadings of 0.4 wt%, the former exhibited superior TON and TOF values. Reaction temperature also influenced the values, where the TON and TOF values of the Cu-NZ catalyst was increased from 120 C to 180 C, but levelled off above this temperature. At 200 C, there was no increase in the TON and TOF values observed (Table 2). The Cu-free NZ and MZ catalysts have also exhibited toluene oxidation property but the conversions are low in these two cases (Entry 1 and 2 in Table 2). Since, these two are Cu-free catalysts, the acid sites present in them are considered as active sites for the reaction (as described in Scheme 1A and section 3.4). Hence, for these two catalysts, the TON and TOF values are calculated based on the acid sites. Overall, the present study reveals the advantages of Cu-NZ (0.4 wt% Cu on NZ) catalyst for effective oxidation of toluene to produce high yields of benzoic acid in terms of 1. easy handling and reusable zeolite 2. high conversions of toluene along with higher selectivity to benzoic acid, 3. higher stability in activity of the catalyst with minimized side product (Comparison with earlier reported catalysts is given in ESI, Table 1). 3.4. Toluene oxidation pathways on acidic and Cu functionalized MZ and NZ catalysts Overall the oxidation reaction of toluene was facilitated to different extents on 1) H-ZSM-5 (acid sites alone and no Cu
Table 3 Performance in reusability of Cu-NZ (0.4 wt% Cu on NZ) catalyst. Reusability (Cycle)
Toluene conversion (wt%)
1 2 3 4 5
95.4 95.4 95.3 94.0 92.2
Product yield (wt%) Benzyl alcohol
Benzaldehyde
Benzoic acid
0.6 0.6 0.5 0.6 0.5
2.8 2.8 3.0 3.2 3.5
92.1 92.1 91.8 90.3 88.2
Reaction Conditions: T ¼ 180 C; Reaction time ¼ 4 h; Pressure ¼ 5.0 bar (with N2 gas), Feed ¼ 25 ml. Toluene þ 25 ml H2O2 (30 wt%) þ 25 ml deionised water, Catalyst ¼ 1.0 g.
46
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Fig. 13. Comparison of the properties of the fresh and spent Cu-NZ (0.4 wt% Cu on NZ) catalysts. (A) XRD patterns (B) FTIR patterns and (C) XPS spectra.
species), 2) Cu-MZ (having dispersed CuO species) and 3) Cu-NZ (having dispersed CuO as well as exchanged Cuþ2 species) catalysts. The sequential reaction steps involved in the complete oxidation of toluene to benzoic acid over these catalysts systems in presence of H2O2 are given in Scheme 1. First step involves the addition of oxygen to toluene for the formation of benzyl alcohol which needs to undergo oxy-dehydrogenation in the second step to give bezaldehyde. The third step of benzoic acid production from benzaldehyde requires dehydrogenation reaction. Here, the three consequent steps involve three different types of oxidation. On MZ and NZ catalysts the reaction path follows Scheme 1A, where the oxidation product obtained is mostly benzyl alcohol (Table 2, entries 1,2) suggests the effective oxygen addition from H2O2 but lower oxy-dehydrogenation activity of acid sites to convert the benzyl alcohol to benzaldehyde. Since, the middle step involves dehydrogenation, it can be easily proceed on transition metal based active sites, where, Cu loaded ZSM-5 is expected to be much effective to facilitate this reaction. In case of Cu-MZ and Cu-NZ, the role of CuO can be represented as in Scheme 1B, where, CuO can act as an oxygen carrier from H2O2 to eCH2OH for effective conversion to produce benzaldehyde. On the other hand, the additional role of Cuþ2 species present in Cu-NZ may be explained by its polarization property. Since, the Cuþ2 occupies two eOH positions of the NZ (Scheme 1C), this ionic species can polarize the reactant molecules to enhance the reaction. This mechanism involves the formation of Cu þ species. The considerable amount of Cuþ is indeed observed in the XPS spectra of spent Cu-NZ catalyst (Fig. 13C) supports this reaction mechanism. This is also further reflected in the FTIR
spectra of spent Cu-NZ (Fig. 13B). We can recollect that the eOH vibration band at 3645 cm1 related to bridging eOH groups is present in the parent NZ sample before Cu loading, which was completely disappeared in Cu-NZ due to their occupation by Cuþ2 ions. As shown in Scheme 1C, the bivalent Cu species occupies two nearby eOH groups of ZSM-5. The conversion of this bi-occupant Cuþ2 to its lower single occupant oxidation state (Cuþ) can free away some of the eOH groups as shown in the Scheme 1C. This structure is indeed supported by the reappearance of the particular band at 3645 cm1 in the FTIR spectra of Cu-NZ spent catalyst (Fig. 13B) and further confirms the reactive polarizability role of Cuþ2 species (proposed Scheme 1C) in toluene oxidation reaction.
4. Conclusions By virtue of its higher external surface area, presence of intercrystalline voids and reduced diffusion path lengths, the nano crystalline ZSM-5 (NZ) has exhibited enhanced catalytic properties as compared to its corresponding micro-crystalline ZSM-5 (MZ). Cu loading has improved the toluene oxidation property of zeolite, where, the NZ functionalized with 0.4 wt% Cu (Cu-NZ) could produce highest benzoic acid yield of 92 wt% at optimized reaction conditions due to the presence of considerable amount of Cuþ2 along with highly dispersed state of CuO species. With its simple synthesis procedure, reproducibility and reusability, the Cu-NZ (0.4 wt% Cu on NZ) catalyst shows wide scope in expanding its applications for the other selective oxidation reactions.
N. Viswanadham et al. / Materials Today Chemistry 3 (2017) 37e48
47
H
CHO
CH2OH [O]
2CuO
Cu2O
[O]
H2O
H2O2
Scheme 1. Possible reaction pathways for the formation of benzoic acid on (A) NZ, (B) Cu-MZ and (C) Cu-NZ catalysts.
Acknowledgment Authors would like also to thank Prof. Chris Hardacre and Dr. Rebecca Taylor at the School of Chemistry and Chemical Engineering, Queen's University Belfast, for their generous contribution and support towards the XPS analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.mtchem.2017.01.001. References [1] C. Chen, F. Chen, L. Zhang, S. Pan, C. Bian, X. Zheng, X. Eng, F.S. Xiao, Importance of platinum particle size for complete oxidation of toluene over Pt/ZSM5 catalyst, Chem. Commun. 51 (2015) 5936e5938. [2] A.M. Carrillo, J.G. Carriazo, Cu and Co oxides supported on halloysite for the total oxidation of toluene, Appl. Catal. B 164 (2015) 443e452. [3] J. Shang, W. Sun, L. Zhao, W.K. Yuan, Liquid phase oxidation of alkyl aromatics at low oxygen partial pressures, Chem. Eng. J. 278 (2015) 533e540. [4] S.L.H. Rebelo, M.M.Q. Simoes, M.G.P.M.S. Neves, J.A.S. Cavaleiro, Oxidation of alkylaromatics with hydrogen peroxide catalysed by managanese (III) porphyrins in the presence of ammonium acetate, J. Mol. Catal. A 201 (2003) 9e22. [5] A.K. Suresh, M.M. Sharma, T. Sridhar, Engineering aspects of industrial liquidphase air oxidation of hydrocarbons, Ind. Eng. Chem. Res. 39 (2000) 3958e3997. [6] J. Thomas, R. Raja, G. Sankar, R.G. Bell, Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen, Nature 398 (1999) 227e230.
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[20] V.R. Choudhary, D.K. Dumbre, B.S. Uphade, V.S. Narkhede, Solvent-free oxidation of benzyl alcohol to benzaldehyde by tert-butyl hydroperoxide using transition metal containing layered double hydroxides and/or mixed hydroxides, J. Mol. Catal. A 215 (2004) 129e135. [21] T.W. Bastock, J.H. Clark, K. Martin, B.W. Trenbirtha, Mild, solvent-free oxidation of toluene and substituted toluenes to their benzoic acids using carboxylic acid-promoted heterogeneous catalysis, Green Chem. 4 (2002) 615e617. [22] C. Subrahmanyam, B. Louis, B. Viswanathan, A. Renken, T.K. Varadarajan, Synthesis, characterization and catalytic properties of vanadium substituted mesoporous aluminophosphates, Appl. Catal. A 282 (2005) 67e71. [23] J.M. Thomas, R. Raja, Innovations in oxidation catalysis leading to a sustainable society, Catal. Today 117 (2006) 22e31. [24] X. Li, J. Xu, F. Wang, J. Gao, L. Zhou, G. Yang, Direct oxidation of toluene to benzoic acid with molecular oxygen over manganese oxides, Catal. Lett. 108 (2006) 137e140. [25] M. Ilyas, M. Sadiq, Oxidation of toluene to benzoic acid catalyzed by platinum supported on zirconia in the liquid phase-solvent free conditions, Catal. Lett. 128 (2009) 337e342. [26] W. Zhong, S.R. Kirk, D. Yin, Y. Li, R. Zou, L. Mao, G. Zou, Solvent-free selective oxidation of toluene by oxygen over MnOx/SBA-15 catalysts: relationship between catalytic behavior and surface structure, Chem. Eng. J. 280 (2015) 737e747. [27] V. Grieken, J.L. Sotelo, J.M. Menendez, J.A. Meler, Anomalous crystallization mechanism in the synthesis of nanocrytalline ZSM-5, Micropor. Mesopor. Mater 39 (2000) 135e147. [28] F. Bin, C. Song, G. Lv, J. Song, S. Wu, X. Li, Selective catalytic reduction of nitric oxide with ammonia over zirconium-doped copper/ZSM-5 catalysts, App. Catal. B 150e151 (2014) 532e543.
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Cu functionalized nano crystalline ZSM-5 as efficient catalyst for selective oxidation of toluene N. Viswanadham a, *, Sandeep K. Saxena a, b, Ala'a H. Al-Muhtaseb b, ** a b
Refining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India Department of Petroleum and Chemical Engineering, College of Engineering, Sultan Qaboos University, Muscat 123, Oman
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 25 November 2016 Accepted 11 January 2017
Nano crystalline ZSM-5 (NZ) functionalized with Cu has been explored for the selective oxidation of toluene to benzoic acid for the first time, where a comparison is made with the traditional microcrystalline ZSM-5 (MZ) based catalyst having similar framework Si/Al ratios at organic solvent-less reaction conditions in presence of H2O2 as a green oxidant. The ZSM-5 catalysts exhibited oxidation property even in the absence of any Cu species, but with low toluene conversions and benzoic acid yields. Functionalization with Cu greatly enhanced the benzoic acid formation, especially on NZ loaded with 0.4 wt% of Cu (Cu-NZ) to produce 92 wt% of benzoic acid, by virtue of the presence of highly dispersed nano particles of CuO along with Cuþ2 ions on the high surface area, mesopore possessing NZ support, revealed from XRD, N2 adsorption-desorption, XPS, SEM, TEM, FITR, TPR and TPD analysis. © 2017 Published by Elsevier Ltd.
Keywords: Nano crystalline ZSM-5 Mesoporosity Toluene oxidation Cu loading Variable oxidation states
1. Introduction The volatile organic compounds (VOCs) emitted from industrial and transportation activities causes serious problems to human health by their toxic or malodorous nature and the formation of suspended particulate matter and photochemical smog. Catalytic oxidation of VOCs is an important process come up to address these problems, where complete oxidation of these compounds has been successfully achieved through catalytic combustion [1,2]. But the product obtained in this case is CO2 that does not have any commercial value. Moreover, this process generates a green-house gas that should be avoided to make the industrial processes environmentally benign. Partial and selective oxidation of such organic compounds into the useful intermediates is a green and profitable option, where development of an efficient catalyst for improving selectivity for the value-added products remains a major challenge for industry [3e6]. Traditional approaches for the reaction were based on applying the stoichiometric amounts of liquid oxidants such as potassium permanganate, but suffer from the limitations of low products selectivity [7]. Recently, several homogenous and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (N. Viswanadham), almuhtasebala@gmail. com (A.H. Al-Muhtaseb). http://dx.doi.org/10.1016/j.mtchem.2017.01.001 2468-5194/© 2017 Published by Elsevier Ltd.
heterogeneous catalyst systems have come up to address the basic problems involved in this process to make it green [8e10]. The subject still demands novel and practical inputs in the area of catalyst development for the selective oxidation of individual organic compounds so as to make it suitable for the industrial applications. Toluene is one of the substrates categorized as hazardous material. The chemical also identified to cause serious health problems such as nerve disorders, liver and kidney damage. These facts demand immediate attention for the green conversion of toluene into value added products. Toluene can be converted to oxygenated products such as benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate. Among them benzoic acid is very important due to its high demand in the current industrial practice such as intermediate in the productions of chemicals, plasticizers, dyestuffs, preservatives and retardants [11e13]. The commercial production of benzoic acid via the catalytic oxidation of toluene is achieved with cobalt acetate and bromide promoter in acetic acid using an oxidizing agent at high pressure [14]. Although complete conversion is achieved, the use of acidic solvents and bromide promoter results in difficulty in separation of catalyst from products along with other disadvantages caused by the large volume of toxic waste and equipment corrosion. Further, severe operation conditions adopted in vapor phase oxidation consumes large amount of energy which is not economical for
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industrial applications. Liquid phase oxidation of toluene is a better alternative as it is easy to operate and can achieve high selectivity under relatively mild reaction conditions. Many efforts have been made to improve the efficiency of toluene oxidation in the liquid phase reaction; however, most investigations were reported on homogeneous systems using volatile organic solvents [15e17]. Heterogeneous catalysts are preferred over homogeneous catalysts because these materials can be readily separated from the reaction mixture. Heterogeneous catalysts can also be easily used in flow reactors, facilitating the efficient productions of materials using continuous processes. Hence, operation of heterogeneous catalysts in liquid phase oxidation of toluene without solvent would make the process environment-friendly. The catalytic liquid phase oxidation mainly carried out using the oxidants such as H2O2, tertbutyl hydroperoxide, or O2. Here the selection of reagent is important to avoid the over oxidation of the substrate to avoid the formation of undesired CO2. Hydrogen peroxide with a high level of active oxygen is an efficient, inexpensive, mild and environmentally friendly reagent for oxidation (H2O and O2 are the byproducts) [18e20]. Bastock et al. have reported the oxidation of toluene to benzoic acid (75 wt% yield) in solvent free conditions using a commercial heterogeneous catalyst Envirocat EPAC promoted by a carboxylic acid at high temperature and atmospheric pressure [21]. Subrahmanyan et al. have performed toluene oxidation on vanadium substituted alumino-phosphate or alumino-silicate in solvent free conditions to produce benzaldehyde, benzoic acid and cresol depending on the nature of oxidizing agent used [22]. Thomas et al. have reported the solvent free aerobic oxidation of toluene using Fe-AlPO-5 zeolite catalyst to achieve 33% benzoic acid selectivity at ~40% toluene conversion [23]. Li et al. have also reported manganese oxide and copper manganese oxide as active catalysts for aerobic oxidation of toluene to benzoic acid in solvent free conditions [24]. Pt supported on zirconia support was also observed to give moderate conversions and benzoic acid selectivities in the liquid phase solvent free aerobic oxidation conditions [25]. Very recently, manganese oxide supported SBA-15 catalysts have been observed to facilitate solvent-free selective oxidation of toluene to produce benzaldehyde and benzoic acid depending on the concentration of the MnO species [26]. A common problem observed in most of the cases is the lower yields of benzoic acid along with considerable production of benzyl alcohol and benzaldehyde as side products. Present study reports the effective toluene oxidation property of a Cu functionalized Nano crystalline ZSM-5 possessing high external surface area and mesoporosity using H2O2 as green oxidation reagent to produce high yields of benzoic acid at solventfree reaction conditions at relatively higher reaction temperatures. Here the purpose of choosing high temperature is to minimize the formation of partial oxidation products such as benzyl alcohol and benzaldehyde while encouraging the formation of deep oxidation product benzoic acid. At higher reaction temperatures, the toluene in vapor phase can undergo vigorous interaction as the vapor diffusion in zeolite pores is high to exhibit higher conversions [12,21,24e26]. 2. Experimental
ammonium chloride (Aldrich) and copper nitrate (Merck Reagent Co. Ltd) were also purchased. 2.2. Synthesis of nano-crystalline ZSM-5(NZ) sample In this synthesis, aluminum nitrate solution was slowly added to pre-cooled tetra propyl ammonium hydroxide (TPAOH) solution, followed by the drop wise addition of tetraethyl ortho-silicate (TEOS). The components were mixed with constant stirring and the solution was left to hydrolyze at room temperature for 41 h. The gel thus obtained was heated at 80 C to evaporate water and ethanol formed during the hydrolysis to obtain a gel. The concentrated gel was charged in a Teflon lined autoclave for hydrothermal synthesis at 170 C for 48 h. Synthesized sample was filtered and washed with de-ionized water, followed by drying at 100 C and calcination at 500 C for 4 h under vacuum. The powder forms of the zeolites are converted into extrudates form by mixing with 40 wt% of pseudo boehmite (neutral) alumina followed by drying and calcinations [27]. 2.3. Preparation of Cu loaded ZSM-5 catalysts Incipient wet impregnation method is used for the loading 0.4 wt% of Cu on to the NZ and MZ, followed by drying at room temperature (25 C) for 12 h, heating at 100 C for 8 h and calcination at 500 C for 4 h in presence of air. The catalysts obtained have been denoted as Cu-NZ (0.4 wt% Cu) and Cu-MZ (0.4 wt% Cu) based on the nature of support. Much better performance in the reaction exhibited by Cu-NZ guided us for the detailed studies on the Cu-NZ catalyst. Further studies are conducted to check the effect of Cu concentration on NZ at below and above 0.4 wt% Cu loading. The catalysts are named as Cu-NZ1 (0.2 wt% Cu), Cu-NZ2 (0.6 wt% Cu) and Cu-NZ3 (0.8 wt% Cu). The calcined samples in metal oxide form were loaded in the reactor and all the catalysts were reduced in situ at 550 C under hydrogen pressure before using them for toluene oxidation reaction. 2.4. Physico-chemical characterization The Scanning Electron Microscope (SEM) images were recorded for obtaining particle morphology on JEOL JSM-7900F instrument Japan. Transmission Electron Microscopy (TEM) images were recorded on JEOL, JSM-2100F instrument, Japan. X-ray powder diffraction (XRD) patterns were measured on PANalytical Xpert PRO instrument, USA equipped with rotating anode and CuKa radiations. The Fourier transform infrared spectroscopy (FTIR) spectra of the zeolite samples were recorded on Perkin Elmer-spectrum two, USA. The BET surface area, pore size and pore volume measurements of all the zeolite based catalysts were carried out using a standard adsorption equipment (ASAP 2020, Micromeritics Instruments Inc., Norcross, GA, USA) using N2 gas (99.995% pure). The acidity of the catalyst was measured by temperature programmed desorption of NH3 (NH3-TPD) using a Micromeritics chemisorbs 2750 pulse chemisorption system. X-ray Photoelectron Spectroscopy (XPS) analysis was performed with a Kratos AXIS Ultra DLD apparatus, equipped with monochromated Al Ka radiation X-ray source.
2.1. Materials 2.5. Catalyst evaluation studies Conventional H-ZSM-5 zeolite (SAR ¼ 30) was obtained from Suid Chemie India limited and termed as micro-crystalline zeolite (MZ), tetraethyl ortho-silicate (TEOS, AR), Tetra propyl ammonium hydroxide (TPAOH) solution, toluene (AR) and hydrogen peroxide were purchased from Merck Reagent Co. Ltd. Platinum tetra
The toluene oxidation with hydrogen peroxide was carried out without using any solvent in a PARR autoclave reactor 4848 equipped with four bladed pitched turbine impeller. In a typical reaction; 1.0 g of the catalyst, 25 ml of toluene (Merck -AR grade)
N. Viswanadham et al. / Materials Today Chemistry 3 (2017) 37e48
and 25 ml of 30 wt% hydrogen peroxide (Merck) with 25 ml deionized water. The aqueous hydrogen peroxide added drop-wise to avoid the immediate decomposition of hydrogen peroxide. The autoclave was pressurized with N2 gas upto 5.0 bar pressure. The reaction temperature was maintained at 180 C and reaction mixture stirred at 550 RPM for 4 h under nitrogen gas pressure. At the end of the reaction, the mixture was cooled upto room temperature (25 C) and the catalyst was separated by filtration, washed with ethanol, dried at 120 C and reused for four times. The reaction product was analyzed by using GC equipped with the DBWax column (30 m 0.53 mm 1.0 mm) and FID detector. 3. Results and discussion The present study is aimed to understand the effect of the nature of ZSM-5 supports, NZ and MZ, on the properties of Cu-loaded zeolites and their catalytic performances towards the oxidation of toluene. For this two catalysts Cu-MZ and Cu-NZ prepared by loading 0.4 wt% Cu have been thoroughly characterized and evaluated towards the oxidation reaction. 3.1. Physico-chemical properties of NZ, MZ, Cu-NZ and Cu-MZ samples Fig. 1 shows the X-ray diffraction patterns of Nano-ZSM-5 (NZ), Micro-ZSM-5(MZ), 0.4 wt% Cu loaded NZ and MZ zeolites (Cu-NZ and Cu-MZ). All the samples exhibit the typical diffraction peaks of ZSM-5 zeolite at 2q of 7.8 , 8.9 , 23.1 and 23.8 representing the crystal planes of (011), (020), (051) and (033) respectively [28]. Similar XRD patterns exhibited by the samples even after Cu loading (Cu-MZ and Cu-NZ) indicate that the structure of ZSM-5 support remains intact after the loading of Cu. No diffraction
Fig. 1. XRD patterns of MZ and NZ zeolites before and after Cu loading.
39
patterns related to the presence of CuO observed in Cu-MZ and CuNZ samples suggest the presence of highly dispersed state of Cu species and the possible formation of smaller size aggregates of Cu which are too small to be detected by XRD. The structure morphology of the materials was characterized by SEM and TEM analysis. The SEM images (Fig. 2) clearly show that the zeolite crystal size has been successfully brought down from micrometer range crystal of MZ to nano meter range crystals of NZ. The TEM (Fig. 3) analysis shows the presence of 2 mm size crystals in MZ and ~30 nm size crystals in NZ to confirm the nano crystal structure of NZ. The Cu loaded zeolites (both Cu-NZ and Cu-MZ) did not show any prominent size particles of the Cu, supporting the presence of highly dispersed state of the Cu species in MZ and NZ samples. The XPS analysis was used for understanding the metal support interaction in these samples, where, the intensity and binding energy of Cu 2p of the samples have been measured (Fig. 4). In each case, the intensities have been normalised to the Si 2p peak area. Both Cu-MZ and Cu-NZ samples have shown two main peaks at B.E 933.4 eV and 953.2 eV in their Cu2p spectra related to Cu2p3/2 and Cu2p1/2, respectively. The intensities of both the peaks related to Cu species are much higher in Cu-NZ sample when compared to the Cu-MZ. Another difference in XPS spectra observed was the deconvolution patterns of the Cu2p3/2 peak. In case of Cu-NZ sample, the Cu2p3/2 peak can be further deconvoluted to two peaks; one at 933.4 eV and another at 935.5 eV. The higher binding energy feature (935.5eV) in the case of the Cu-NZ sample is indicative of the presence of a small amount of Cu(II) possibly in the form of isolated cations (Cuþ2). Hence, the two peaks appeared at 933.4 eV and 935.5 eV in Cu-NZ can be ascribed to the agglomerated CuO nano particles on the catalyst surface and Cu2þ ion coordinated with the zeolite perhaps at ion-exchange positions, respectively [29,30]. But, the absence of any diffraction patterns related to CuO particles observed in XRD and no visible particles observed in HRTEM analysis of the Cu-NZ sample suggests the Cu species in highly dispersed state with relatively smaller size particles of Cu (less than 3 nm) in this sample. This particular feature of deconvolution of Cu2p3/2 related to the Cu2þ species was absent in the Cu-MZ sample. This may be due to the relatively poor interaction of the Cu species with the MZ support. The superior interaction of NZ support with Cu species (Cu-NZ), especially in terms of the Cu2þ occupying ion-exchange positions of ZSM-5 framework, can be attributed to the easy accessibility of the active sites of hierarchically mesoporous nano ZSM-5 (NZ) for the interaction of Cu species in Cu-NZ. The Cu-MZ and Cu-NZ samples also showed difference in the H2-TPR patterns (Fig. 5). There are two distinguished reduction peaks observed for Cu-NZ at 232 C and 358 C related to the reduction of Cu in two different states, namely, dispersed CuO and Cuþ2 species located at ion-exchange positions [31]. But, in case of Cu-MZ the high temperature peak related to reduction of Cuþ2 was not prominent. This is in accordance with the XPS data indicating that the textural properties of nano crystals in NZ is responsible for the formation of significant amount of Cuþ2 species in Cu-NZ. The adsorption and desorption isotherm plots of both MZ and NZ samples were measured before and after the Cu loading to understand any changes in porous properties of these zeolites by Cu. The isotherms shown in Fig. 6 represent type IV isotherm with a steep jump in the loop volume at higher relative pressure caused by the filling of the pores in the zeolites. In case of NZ, there is a big adsorption loop appeared as compared to that of MZ. The prominent hysteresis loop appeared at high relative pressure (P/ P0 ¼ 0.9e1.0) of NZ is a clear indicative of the presence of an ample amount of mesopores, which are believed to be formed by inter crystalline voids of the nano crystals of the ZSM-5. The BJH pore size distribution patterns of the NZ further reveal the presence of
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Fig. 2. SEM images of the ZSM-5 and Cu-ZSM-5samples.
significant amount of mesopores (20e80 nm) in this sample (Fig. 7B), when compared to those in MZ (Fig. 7A). Hence, the NZ sample is expected to possess higher total surface area, external surface area, mesopore volume and total pore volume, which was indeed reflected in the measured properties (Table 1). Cu loading does not change the basic shape of the isotherms of the MZ and NZ samples (Fig. 6), but, there is a steep increase in the loop at 0.9e1.0 p/p0 related to the mesopores was observed in the Cu-MZ and Cu-NZ samples. This aspect is clearly visible from Fig. 7, where a significant increase in the mesopores of 5e80 nm was observed in NZ after Cu loading (Cu-NZ). The increase in mesopores of 30e90 nm was also observed in Cu-MZ, but the increase is not very significant in this case. In micro pore volume region all the samples exhibited the comparable pore volume plots (Fig. 8). The surface area, pore volume and average pore diameter values of the samples given in Table 1 clearly indicate that the textural properties of both MZ and NZ have improved after Cu loading, while this aspect is more pronounced in case of NZ. This phenomenon of increase in surface area and pore volume of the zeolites after metal loading can be explained by the effective interaction of the Cu species with the ZSM-5 framework structure. This phenomenon is significant especially in case of NZ catalyst. Hence, the increase in total pore volume of the samples after metal loading can be ascribed to the formation of mesopores by metal-support interaction. The situation suggests the state of highly dispersed Cu species on the NZ which does not cause any loss in pore volume by its physical occupation or filling of the zeolite pores of the resultant Cu-NZ sample. The proposed state of highly dispersed Cu species also explains why the particles of Cu species are not visible in the XRD patterns and TEM pictures of the Cu-NZ. Overall, the porosity patterns of the Cu loaded samples indicate that there is no pore occupation/blockage was occurred in both the zeolite samples by Cu loading.
The FTIR spectra of NZ and MZ samples were measured before and after Cu loading to understand the metal-support interactions in Cu-NZ and Cu-MZ samples. Fig. 9 shows the bands in both MZ and NZ near ~1080 cm1 (internal asymmetric stretch), ~790 cm1 (external symmetric stretch), ~540 cm1 (double ring vibration) and 450 cm1 (T-O bend) representing the MFI phase. The bands at 1068 cm1 to 1219 cm1 has been assigned to external linkages (between TO4 tetra hedral). The nano crystalline ZSM-5 (NZ) exhibited a prominent structure-sensitive asymmetric stretch vibrational band at this region when compared to the MZ due to the more exposed tetrahedral in the nano crystalline sample. The band at 3645 cm1 related to acidic (Al- and Si-bridged) hydroxyl group, 3450 cm1 related to silanol hydroxyl is also observed in the samples [32]. However, after Cu loading the band at 3645 cm1 (bridged eOH groups) in Cu-NZ was evidently disappeared as shown in Fig. 9, indicating the possible occupation of Cuþ2 ion by replacing the eOH group. This phenomenon was not observed in Cu-MZ suggesting the effective accessibility and interaction of Cu with NZ when compared to MZ. The IR in combination with XPS data indicates the presence of Cuþ2 at ion exchange positions (bridging hydroxyl group) of NZ in the Cu-NZ sample. The background spectra of all the zeolite samples are very similar. The ammonia desorption (TPD) of the both MZ and NZ samples were measured before and after Cu loading to understand the effect of Cu loading on the acid sites. TPD patterns of the MZ and NZ samples shown in Fig. 10A and B respectively indeed reveal the comparable acidity patterns of these two samples which are differ in crystal size but having same value of SAR, which support the crystal size of MZ is decease from micro level to the nano level without affecting the acidity. The TPD of each catalyst has been normalized to distinguish the relative peak areas of weak Bronsted acid from strong Lewis acid sites. Normally, strength of solid acid sites within TPD profiles can be classified by the temperature of
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41
Fig. 3. TEM images of the ZSM-5 and Cu-ZSM-5samples.
~420 C, which may be attributed to the desorption of two types of ammonia species adsorbed on weak acid (mostly Lewis acid) sites and strong acid (mostly Bronsted acid) sites, respectively. It can be observed that all the curves displayed a major desorption peak around 150 C, an indication of the weak (Bronsted and/or Lewis) sites present in the catalysts, which is consistent with typical ZSM5 peak. The loading of Cu on MZ caused slight decrease in the weak acidity but the strong acidity remains unchanged. In case of NZ, the Cu loading led to the reduction in weak acidity as well as strong acidity. The high temperature peak exhibited reduced intensities especially above 420 C along with a slight downshift of its position. This observation again suggests possible interaction of the Cuþ2 with the eOH groups (ion-exchange positions) in NZ sample and supports the findings of FTIR (decreased intensity at 3645 cm1) and XPS results (presence of Cuþ2 species) of Cu-NZ.
3.2. Solvent free oxidation of toluene
Fig. 4. XPS of the Cu loaded MZ and NZ zeolites.
desorption of NH3 as weak (120e300 C), moderate (300e500 C) and strong (500e650 C). For the MZ, NZ and 0.4 wt% Cu-loaded MZ (Cu-MZ) and NZ (Cu-NZ), the NH3- TPD peaks appeared at ~150 and
The performance of liquid-phase toluene oxidation of both CuNZ and Cu-MZ catalysts was studied with hydrogen peroxide (green oxidant) under solvent-free conditions at 180 C, 5 bar pressure (N2 gas) for 4 h, where the performance of these catalysts was compared with the metal-free zeolites (MZ and NZ) to understand the effect of Cu on oxidation activity (Table 2, entries 1e4). The conversion of toluene is moderate on metal-free ZSM-5 catalysts i.e MZ and NZ. But, the Cu loading increased the conversion of toluene significantly on both the zeolites (Cu-MZ and Cu-NZ). All
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Fig. 5. Temperature programmed reduction (TPR) patterns of the Cu loaded zeolites. Fig. 7. Effect of Cu loading on the mesopore size distribution patterns of MZ and NZ.
Table 1 Detailed physico-chemical properties of the various zeolite catalysts. Sample a
2
SBET (m /g) Micro pore Area (m2/g) External Surface Area (m2/g) b Total Pore Volume (cm3/g) c Micro pore volume (cm3/g) d Meso/Macro pore volume (cm3/g) e Median pore diameter (Å) Average particle size (Å) f Crystallinity (%)
MZ
NZ
Cu-MZ
Cu-NZ
299.74 204.02 95.72 0.1777 0.0825 0.0952 6.060 128.7 99.98
448.98 248.76 168.21 0.4880 0.1370 0.3510 6.349 120.92 99.97
432.89 291.10 141.79 0.2767 0.1179 0.1588 6.115 138.84 99.94
512.64 267.92 244.72 0.6297 0.1092 0.5205 6.671 137.10 99.97
a
BET surface area. Total pore volume taken from the volume of N2 adsorbed at P/P0 ¼ 0.9. c Micropore volume calculated from t-plot. d Meso/Macro pore volume calculated from total pore volume e micropore volume. e BJH adsorption average median pore diameter. f Crystallinity based on XRD analysis. CuMZ and CuNZ conating 0.4 wt% Cu metal. b
Fig. 6. Nitrogen adsorption-desorption isotherms of pure and Cu loaded zeolite catalysts.
the three partial oxidation products were observed in the reaction product, namely, benzyl alcohol, benzaldehyde and benzoic acid on the catalysts. The metal-free ZSM-5 catalysts (MZ and NZ) also exhibited toluene oxidation property, but the conversion of toluene and the yields of benzoic acid are low on these catalysts (entries 1 and 2). Functionalization with Cu greatly enhanced the toluene oxidation to give higher benzoic acid yields on Cu-MZ and Cu-NZ catalysts (entry 3 and 4). Between MZ and NZ, the latter exhibited higher toluene conversion and benzoic acid yields. The zeolites followed similar trend even after the Cu loading, where Cu-NZ exhibited superior performance to Cu-MZ in the reaction both in terms of toluene conversion and benzoic acid yields.
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Fig. 8. Effect of Cu loading on the micropore size distribution patterns of MZ and NZ.
Overall, the performance of the catalysts is in the order of MZ < NZ < Cu-MZ < Cu-NZ. It is clearly indicating that the nanocrystalline ZSM-5, with its hierarchical porosity created by zeolitic micropores in combination with additional mesopores, exhibits promising catalytic activity in toluene oxidation. The loading of 0.4 wt% Cu on the ZSM-5 further increased the yield of benzoic acid, both in case of micro-crystalline and nano-crystalline ZSM-5 supports. This has resulted in the simultaneous decrease in the yields of benzaldehyde and benzyl alcohol. The highest benzoic acid yields with negligible benzyl alcohol and benzaldehyde produced over Cu-NZ clearly indicate the effective oxydehydrogenation property of Cu species to facilitate the complete oxidation of toluene. The XPS and FTIR results of Cu-NZ also indicated the presence of some Cuþ2 ions in ion-exchange positions in addition to the highly dispersed CuO. The free Cuþ2 ions available for the effective interaction to oxidize the hydrocarbon intermediates during the conversion of toluene may be responsible for the superior performance of Cu-NZ catalyst. Earlier studies of Wang et al. also revealed the enhanced NO conversion and oxidation properties of Cu/SAPO-34 catalyst having both CuO nano particles and Cuþ2 ions [33]. These observations suggest two catalyst parameters responsible for the superior catalytic performance of the Cu-NZ in the toluene oxidation reaction; 1. role of nano size crystals of ZSM-5 and 2. role of Cu. The combined effect of nano crystallites of ZSM-5 with the oxidation property of loaded Cu is responsible for the highest toluene conversion of ~95% with as high as 92 wt% yield to benzoic acid obtained on Cu-NZ catalyst.
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Fig. 9. FTIR patterns of pure and Cu loaded zeolite catalysts.
3.3. Factors contributing to the superior oxidation property of CuNZ catalyst Cu is known for its variable valence and the resultant oxidation property is reflected in variety of oxidation reactions. Hence, the toluene oxidation property possessed by Cu-MZ and Cu-NZ catalysts can be ascribed to the redox properties of Cuþ2/Cu þ species facilitated at the reaction conditions. The superior performance of Cu-NZ is understood from the additional surface area, porosity and diffusivity operated in the hierarchically mesoporous nature of the nano crystalline ZSM-5. The interaction of Cu is also observed to be effective with the nano crystalline ZSM-5 (Cu-NZ), realized by considerable decrease in the band intensity of 3670 cm1 (related to bridging eOH groups) of NZ in FTIR due to the interaction of Cu ions with the framework eOH group of NZ during Cu loading. Further, the XPS and TPR results support the presence of the second form of copper (Cuþ2 at ion-exchange positions of ZSM-5) in significant amount, in addition to the highly dispersed CuO in Cu-NZ. This has resulted in the excellent oxidation property of the Cu-NZ, which indeed exhibited the highest oxidation property of toluene to yield benzoic acid. This phenomenon of the presence of bivalent copper (Cuþ2) species is less pronounced in Cu-MZ perhaps due to the inferior metal-support interaction occurred in the mere microporous micro crystals of MZ when compared to the corresponding meso-microporous nano crystals of NZ. On the other hand, the presence of inter-crystalline porosity, the higher external surface area in case of NZ perhaps responsible for the interaction of Cu species (Cuþ2) effectively at ion-exchange positions of in the CuNZ. Overall, the Cu-NZ catalyst of the present study could yield
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Fig. 11. Effect of Cu amount on the toluene oxidation performance of NZ catalyst.
Fig. 10. TPD patterns of pure and Cu loaded zeolite catalysts.
~92 wt% of benzoic acid, which is promising for the effective conversion of toluene to value added products useful for industrial applications at solvent-free conditions. After identifying the Cu-NZ (0.4 wt% of Cu on NZ) as promising oxidation catalyst, the effect of Cu concentration on the activity of
NZ has been studied, where the concentration of Cu has been varied below and above the 0.4 wt% of Cu. The catalysts are named as CuNZ1 (0.2 wt% Cu), Cu-NZ2 (0.6 wt% Cu) and Cu-NZ3 (0.8 wt% Cu). Fig. 11 shows the effect of Cu amount on the catalytic activity, where the amount of Cu loading was varied from 0.2 wt% to 0.8 wt% on the NZ support. The trends indicate that the conversion of toluene as well as benzoic acid yield was increased up to the 0.4 wt% Cu. Further increase of Cu did not cause any improvement in the catalytic performance (Table 2, entries 4e7). Detailed studies are also conducted to optimize the reaction temperature and reactor pressure for the production of benzoic acid (entries 4 to 14 in Table 2). Reaction temperature caused increase in the catalytic performance up to 180 C and then the activity was leveled above this temperature (Fig. 12A). The catalyst also exhibited better performance at above the atmospheric pressure with optimum activity at 5 bar pressure (Fig. 12B). Above this pressure there was no further improvement in the performance. . To explore its reusability, the Cu-NZ (0.4 wt% Cu) catalyst was separated out from the product mixture and reused for toluene conversion reaction after drying at 120 C for 4 h. The results shown in Table 3 indicated that the activity of the catalyst in terms of toluene conversion and benzoic acid yield was almost unaffected even at the third run. The toluene conversion was slightly declined at the fourth run, attributing to the possible decrease in amount of the catalyst recovered after the runs.
Table 2 Toluene oxidation performance of catalysts in solvent-free conditions. Entry
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Catalysts
MZ NZ Cu-MZ Cu-NZ Cu-NZ1 Cu-NZ2 Cu-NZ3 Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ Cu-NZ
Temperature ( C)
180 180 180 180 180 180 180 120 160 180 200 180 180 180
Pressure (bar)
5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 0.0 2.5 7.5
Toluene conversion (wt%)
38.6 54.7 85.7 95.4 72.2 96.1 96.1 52.9 80.5 95.4 95.9 45.8 82.9 96.4
Product yield (wt%) Benzyl alcohol
Benzaldehyde
Benzoic acid
27.2 21.3 2.3 0.6 2.3 2.0 1.9 10.4 1.4 0.6 0.6 11.7 6.6 1.8
6.3 5.0 1.8 2.8 4.7 2.9 4.0 16.8 6.2 2.8 3.5 8.6 6.9 3.3
5.1 28.4 81.6 92.1 65.2 91.2 90.2 25.7 72.9 92.1 91.9 25.5 69.5 91.3
TON
TOF (s1)
17.3 90.0 2300 2594 3673 1528 1272 725 2057 2594 2593 721 1960 2577
0.0012 0.0062 0.159 0.180 0.255 0.106 0.088 0.050 0.143 0.180 0.180 0.050 0.136 0.178
Reaction Conditions: Reaction time ¼ 4 h; Feed ¼ 25 ml Toluene þ 25 ml H2O2 (30 wt%) þ 25 ml deionised water, Catalyst ¼ 1.0 g, Pressure with N2 gas Cu-NZ ¼ 0.4 wt% Cu, CuNZ1 ¼ 0.2 wt% Cu, Cu-NZ2 ¼ 0.6 wt% Cu, Cu-NZ3 ¼ 0.8 wt% Cu, Cu-MZ ¼ 0.4 wt% Cu.
N. Viswanadham et al. / Materials Today Chemistry 3 (2017) 37e48
Fig. 12. Effect of reaction conditions on the oxidation performance of Cu-NZ (0.4 wt% Cu on NZ) catalyst. (A) Effect of reaction temperature, and (B) Effect of reaction pressure.
The Cu-NZ (0.4 wt%) catalyst obtained after the 4 reaction cycles was further characterized and compared with the fresh catalyst. The XRD patterns of both the fresh (Cu-NZ) and the spent Cu-NZ (SCu-NZ) catalysts are comparable (Fig. 13A). The FTIR spectra of fresh and spent catalyst also (Fig. 13B) shows similar features before and after reaction indicates the catalyst properties are retained during the reaction cycles and supports the catalyst performance data of the reused Cu-NZ catalysts towards benzoic acid production demonstrated in Table 3. However, the curve fitting XPS analysis of the spent Cu-NZ catalyst exhibited the presence of about 10% of Cu in the form of Cu2O (Cu+1) (Fig. 13C). The presence of Cuþ1 clearly suggests the reduction process of Cuþ2 occurred during the oxidation of toluene over the Cu-NZ catalyst. This observation proves the oxidation role of Cuþ2 species towards toluene oxidation. Then the question arises about the role of CuO nano particles
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in the Cu-NZ catalyst? To answer this question luckily we have the Cu-MZ catalyst that has no Cuþ2 species but has only CuO nano particles, which has exhibited considerable toluene oxidation activity (Table 2). Hence, the dispersed nano particles of CuO are also considered effective for the toluene oxidation reaction. However, the superior performance of the Cu-NZ possessing both types of Cu species (CuO and Cuþ2) highlights the additional role of Cuþ2 to promote the oxidation reaction further. The inter conversions prevailed between Cuþ2/Cuþ1 species at reaction conditions may be responsible for higher conversions and product yields observed on this catalyst. Table 2 shows the TON and TOF of the all the catalyst samples studied for the toluene oxidation reaction (calculations given in ESI). The TON values are more suitable for the comparison of homogenous catalysts, while TOF is relevant to the heterogeneous catalyst systems such as the present catalyst system [34]. The TOF values are calculated for the catalysts used in the present reaction. The mille mole of Cu present in each catalyst has been considered for calculation of active sites, assuming all the Cu species are participating in the reaction [35e38]. Among the Cu loaded NZ catalysts, these values have been continuously decreased with increasing the Cu amount. Highest TOF values are exhibited at 0.2 wt% Cu loading on NZ, but the conversion of toluene is low on this catalyst. The second best catalyst observed in terms of TOF values is the Cu-NZ loaded with 0.4 wt% Cu which also exhibited higher conversion and product yields. This can be ascribed to the sufficient number of Cu species available on this catalyst when compared to the 0.2 wt% Cu loaded catalyst. Between Cu-NZ and Cu-MZ at similar Cu loadings of 0.4 wt%, the former exhibited superior TON and TOF values. Reaction temperature also influenced the values, where the TON and TOF values of the Cu-NZ catalyst was increased from 120 C to 180 C, but levelled off above this temperature. At 200 C, there was no increase in the TON and TOF values observed (Table 2). The Cu-free NZ and MZ catalysts have also exhibited toluene oxidation property but the conversions are low in these two cases (Entry 1 and 2 in Table 2). Since, these two are Cu-free catalysts, the acid sites present in them are considered as active sites for the reaction (as described in Scheme 1A and section 3.4). Hence, for these two catalysts, the TON and TOF values are calculated based on the acid sites. Overall, the present study reveals the advantages of Cu-NZ (0.4 wt% Cu on NZ) catalyst for effective oxidation of toluene to produce high yields of benzoic acid in terms of 1. easy handling and reusable zeolite 2. high conversions of toluene along with higher selectivity to benzoic acid, 3. higher stability in activity of the catalyst with minimized side product (Comparison with earlier reported catalysts is given in ESI, Table 1). 3.4. Toluene oxidation pathways on acidic and Cu functionalized MZ and NZ catalysts Overall the oxidation reaction of toluene was facilitated to different extents on 1) H-ZSM-5 (acid sites alone and no Cu
Table 3 Performance in reusability of Cu-NZ (0.4 wt% Cu on NZ) catalyst. Reusability (Cycle)
Toluene conversion (wt%)
1 2 3 4 5
95.4 95.4 95.3 94.0 92.2
Product yield (wt%) Benzyl alcohol
Benzaldehyde
Benzoic acid
0.6 0.6 0.5 0.6 0.5
2.8 2.8 3.0 3.2 3.5
92.1 92.1 91.8 90.3 88.2
Reaction Conditions: T ¼ 180 C; Reaction time ¼ 4 h; Pressure ¼ 5.0 bar (with N2 gas), Feed ¼ 25 ml. Toluene þ 25 ml H2O2 (30 wt%) þ 25 ml deionised water, Catalyst ¼ 1.0 g.
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Fig. 13. Comparison of the properties of the fresh and spent Cu-NZ (0.4 wt% Cu on NZ) catalysts. (A) XRD patterns (B) FTIR patterns and (C) XPS spectra.
species), 2) Cu-MZ (having dispersed CuO species) and 3) Cu-NZ (having dispersed CuO as well as exchanged Cuþ2 species) catalysts. The sequential reaction steps involved in the complete oxidation of toluene to benzoic acid over these catalysts systems in presence of H2O2 are given in Scheme 1. First step involves the addition of oxygen to toluene for the formation of benzyl alcohol which needs to undergo oxy-dehydrogenation in the second step to give bezaldehyde. The third step of benzoic acid production from benzaldehyde requires dehydrogenation reaction. Here, the three consequent steps involve three different types of oxidation. On MZ and NZ catalysts the reaction path follows Scheme 1A, where the oxidation product obtained is mostly benzyl alcohol (Table 2, entries 1,2) suggests the effective oxygen addition from H2O2 but lower oxy-dehydrogenation activity of acid sites to convert the benzyl alcohol to benzaldehyde. Since, the middle step involves dehydrogenation, it can be easily proceed on transition metal based active sites, where, Cu loaded ZSM-5 is expected to be much effective to facilitate this reaction. In case of Cu-MZ and Cu-NZ, the role of CuO can be represented as in Scheme 1B, where, CuO can act as an oxygen carrier from H2O2 to eCH2OH for effective conversion to produce benzaldehyde. On the other hand, the additional role of Cuþ2 species present in Cu-NZ may be explained by its polarization property. Since, the Cuþ2 occupies two eOH positions of the NZ (Scheme 1C), this ionic species can polarize the reactant molecules to enhance the reaction. This mechanism involves the formation of Cu þ species. The considerable amount of Cuþ is indeed observed in the XPS spectra of spent Cu-NZ catalyst (Fig. 13C) supports this reaction mechanism. This is also further reflected in the FTIR
spectra of spent Cu-NZ (Fig. 13B). We can recollect that the eOH vibration band at 3645 cm1 related to bridging eOH groups is present in the parent NZ sample before Cu loading, which was completely disappeared in Cu-NZ due to their occupation by Cuþ2 ions. As shown in Scheme 1C, the bivalent Cu species occupies two nearby eOH groups of ZSM-5. The conversion of this bi-occupant Cuþ2 to its lower single occupant oxidation state (Cuþ) can free away some of the eOH groups as shown in the Scheme 1C. This structure is indeed supported by the reappearance of the particular band at 3645 cm1 in the FTIR spectra of Cu-NZ spent catalyst (Fig. 13B) and further confirms the reactive polarizability role of Cuþ2 species (proposed Scheme 1C) in toluene oxidation reaction.
4. Conclusions By virtue of its higher external surface area, presence of intercrystalline voids and reduced diffusion path lengths, the nano crystalline ZSM-5 (NZ) has exhibited enhanced catalytic properties as compared to its corresponding micro-crystalline ZSM-5 (MZ). Cu loading has improved the toluene oxidation property of zeolite, where, the NZ functionalized with 0.4 wt% Cu (Cu-NZ) could produce highest benzoic acid yield of 92 wt% at optimized reaction conditions due to the presence of considerable amount of Cuþ2 along with highly dispersed state of CuO species. With its simple synthesis procedure, reproducibility and reusability, the Cu-NZ (0.4 wt% Cu on NZ) catalyst shows wide scope in expanding its applications for the other selective oxidation reactions.
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H
CHO
CH2OH [O]
2CuO
Cu2O
[O]
H2O
H2O2
Scheme 1. Possible reaction pathways for the formation of benzoic acid on (A) NZ, (B) Cu-MZ and (C) Cu-NZ catalysts.
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