Catalysis Science & Technology

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Muthusamy Poomalai Pachamuthu,a Vinju Vasudevan Srinivasan,a. Rajamanickam Maheswari .... Cu Kα radiation (λ = 1.54 Å) operating at 30 kV and 15 mV.

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The impact of the copper source on the synthesis of meso-structured CuTUD-1: a promising catalyst for phenol hydroxylation† Muthusamy Poomalai Pachamuthu,a Vinju Vasudevan Srinivasan,a Rajamanickam Maheswari,ab Kannan Shanthi*a and Anand Ramanathan*b In the direct hydrothermal synthesis of copper containing mesoporous materials, various factors such as the Cu(II) 2+ ion distribution, the coordination environment and the quantity of Cu incorporation/grafting etc. influence catalytic activity. For these reasons, the copper source added during the synthesis plays a crucial role. In this work, three different CuTUD-1 materials with similar Si/Cu ratios (100) were synthesized with three different copper precursors (copper acetate (CuTA), copper chloride (CuTC) and copper nitrate (CuTN)). The physico-chemical properties of these CuTUD-1 materials were examined by XRD, N2 sorption studies, ICP-OES, Diffuse Reflectance UV–Vis, FT Raman spectroscopy, TPR and XPS characterization techniques. The results revealed that the distribution of isolated 2+ Cu species and oligonuclear CuO nanoclusters varied with different Cu sources. Copper nitrate yielded highly distributed isolated Cu

Received 24th June 2013, Accepted 17th September 2013 DOI: 10.1039/c3cy00432e www.rsc.org/catalysis

2+

sites, whereas oligonuclear CuO was observed as the major species when copper acetate

was used as the Cu source. These materials were tested as catalysts for liquid phase hydroxylation of phenol with H2O2. The significantly higher performance of the CuTN catalyst over that of the CuTC and CuTA catalysts was attributed to the larger amounts of active isolated Cu(II) species connected to the TUD-1 silica lattice. Moreover, the CuTUD-1 materials showed better catalytic activity than other copper based mesoporous systems (MCM-41, MCM-48 and SBA-15).

Introduction In the last two decades silica based mesoporous materials have been paid more attention due to their important role in various fields. Since the discovery of MCM-41,1 many mesoporous materials containing metal ions have been synthesized and utilized for various applications, particularly in catalysis.2–5 Generally, the active metal or non-metal cations (Mx+) are introduced into the SiO2 framework to make it catalytically active. However these direct synthesis methods are limited to loading only small amounts of Mx+ ions (typically with a Si/M ratio of approximately 30). A higher loading of metal ions leads to structural collapse or formation of a larger amount of crystalline metal oxides (MOx) in the ordered mesoporous materials.6 In this series of mesoporous materials, the recently added TUD-1 material has been studied as an excellent support for a

Department of Chemistry, Anna University, Chennai 600025, India. E-mail: [email protected] (K. S.); Tel: +91 984 014 6642 b Center for Environmentally Beneficial Catalysis (CEBC), The University of Kansas, Lawrence, KS 66047, USA. E-mail: [email protected]; Fax: +1 785 864 6051; Tel: +1 785 864 1631 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cy00432e

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various catalytically active species.7 Although TUD-1 possesses disordered pores, advantages such as its easy synthesis, interconnected narrow pore channels, tunable pore size, hydrothermal stability etc., have facilitated its application as a heterogeneous catalyst.7 Copper salts or copper complexes have been employed as active catalysts for oxidative transformation of organic compounds.8 Various methods have been explored to incorporate such active catalytic sites into heterogeneous supports.9–11 Since the active sites are the main factor determining a catalyst’s activity, a large number of these accessible sites might significantly enhance the activity of a catalyst. Besides, leaching of the active metal ions from the support must be prevented.12 Although it is difficult to incorporate a large quantity of divalent metal ions, such as Cu2+, into mesoporous silica, many research groups have worked to pack Cu2+ sites into silica frameworks, either as complexes or by direct incorporation.13–18 It is well known that the dispersion of Cu2+ active sites are influenced in the direct synthesis method by the structural nature of the copper precursors, the coordinative environment, the rate of hydrolysis and the solubility. Hence, the selection of the copper source is of prime concern in the synthesis of copper based mesoporous materials.

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Paper Phenol hydroxylation by hydrogen peroxide (H2O2) is a typical environmentally friendly catalytic reaction. The obtained products, catechol and hydroquinone (diphenols), are widely used as antioxidants, photographic chemicals, polymerization inhibitors, pesticides and flavoring agents. Although various types of catalysts have been studied for this reaction, copper based catalysts are fascinating due to their ability to precisely activate OH radicals and coordinate phenols.19–22 Very recently, copper containing mesoporous molecular sieves,23,24 a biopolymer based material (Cu(II) alginate)25 and a metal–organic framework26 were shown to exhibit good catalytic activity for this reaction. Zhang et al. reported that in mesoporous Cu/MCM-41 the Cu species and phenol hydroxylation activities vary with the preparation method.24 In our previous report we synthesized CuTUD-1 with different Si/Cu ratios (100 to 10) by using triethanolamine (TEA) as a bi-functional organic molecule (mesopore structure directing agent and metal ion complexing agent).27 It is well known that Cu(II) triethanolamine complexes are stable and highly active.28 In this work, we have investigated the effect of three different copper sources namely, copper nitrate, copper chloride and copper acetate on the synthesis of CuTUD-1 (Si/Cu = 100). The materials prepared were characterized by XRD, N2 sorption studies, ICP-OES, diffuse reflectance UV–Vis, EPR, FT Raman spectroscopy, SEM, HRTEM and H2-TPR studies. The catalysts’ activities were compared and the reaction conditions were optimized in detail.

Materials and methods Materials The chemicals used in this study include: tetraethyl orthosilicate (TEOS, 99%, Sigma Aldrich), copper nitrate (Cu(NO3)2·3H2O, Merck), copper chloride (CuCl2·2H2O, Sd fine), copper acetate (Cu(CH3COO)2·H2O, SRL), triethanolamine (TEA, SRL), tetraethylammonium hydroxide (TEAOH, Sigma Aldrich, 35%), cetyltrimethylammonium bromide (CTAB, Merck), Pluronic P123 (MW: 5800, Aldrich), hydrochloric acid (HCl, SRL), fumed silica (SiO2, Sigma Aldrich) and sodium hydroxide (NaOH, Merck).

Catalysis Science & Technology calcination at 600 °C for 10 h. The final grey colored CuTUD-1 solid samples are hereafter mentioned as CuTN (nitrate source), CuTC (chloride source) and CuTA (acetate source) respectively. Other Si/Cu ratios of 50, 25 and 10 were also prepared using copper nitrate for comparative studies. Cu-MCM-41 and Cu-MCM-48 with a Si/Cu ratio 50 were also synthesized as per the literature procedure using copper nitrate as a copper source.29,30 In addition, 2 wt% of Cu-SBA-15 and Cu-SiO2 were prepared using the incipient wetness impregnation method. Catalyst characterization X-ray powder diffraction was performed on a Bruker D8 using Cu Kα radiation (λ = 1.54 Å) operating at 30 kV and 15 mV. N2 physisorption isotherms and specific surface areas were measured using a Quantachrome porosimeter (Quantosorb SI). Diffuse reflectance (UV–Vis) spectra were recorded using a Bruker Tensor spectrometer using BaSO4 as a reference in the range of 200–800 nm. Raman spectroscopic measurements were carried out in the range of 200 to 800 cm−1 using a 1064 nm excitation source on a Bruker FT-Raman System 1000 R. The X-ray photoelectron spectroscopy (XPS) spectra were measured using an Omicron ESCA probe spectrometer with Al Kα X-rays (hv = 1486.6 eV) radiation. Electron spin resonance (ESR) spectra were recorded using a JEOL (JES FA-300) spectrometer at room temperature operating in the X-band region. Temperature programmed reduction (TPR) measurements were carried out using a Quantachrome ChemBET TPR/TPD chemisorption analyzer. For the sample analysis a 5 vol% H2 in Ar gas mixture was used. High resolution transmission electron microscopy (HRTEM) was carried out using a JEOL 3010 instrument with a UHR pole piece operated at an accelerating voltage of 300 kV. The sample morphologies were examined using SEM–EDAX imaging on a FEI Quanta FEG 200 high resolution scanning electron microscope. The elemental composition of the CuTUD-1 catalysts before and after phenol hydroxylation was analyzed using a ICP-OES Perkin Elmer Optima 5300 DV elemental analysis instrument. CuTUD-1 catalysed phenol hydroxylation

Catalyst synthesis CuTUD-1 with a Si/Cu ratio of 100 was prepared from three different copper sources using the sol–gel method according to the previously reported procedure,27 with a composition of 1 SiO2 : 0.01 CuO : 0.5 TEAOH : 1 TEA : 11 H2O. In a typical synthesis, 10 g of TEOS and 2 g of deionised water were mixed and stirred for 30 minutes. Calculated amounts of copper precursor (copper nitrate, copper chloride or copper acetate) dissolved in 1 g water were added to the above mixture during stirring. Then 7 g of TEA was added and stirred for 1 h. Finally, 10 g of TEAOH was added drop wise to the above mixture and stirring was continued for another 3 h. The obtained gel was aged at 30 °C and dried at 98 °C for 24 h each. The wet solid was kept at 180 °C in a Teflon coated autoclave for 8 h. The organic moieties were removed by

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The hydroxylation of phenol over copper containing catalyst was carried out in a two neck round bottom flask equipped with a water condenser, septum cap and magnetic stirrer. Prior to the runs, the catalysts were activated (pre-treated) for 2 h in air at 300 °C. In a typical run, 50 mg of CuTUD-1 catalyst was added to the flask. Then, 5 mmol of phenol in 10 ml of water was added to the flask. The reaction mixture was equilibrated at the desired temperature in an oil bath. Finally, H2O2 oxidant (5 mmol) was added drop wise to the reaction mixture with continuous stirring (~750 rpm) which was considered to be the start of the reaction. 0.5 ml liquid samples were then withdrawn at different time intervals and the products were identified and quantified using a Shimadzu high performance liquid chromatography (HPLC) equipped with an UV detector (254 nm) and a C18 column

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(Phenomenex Luna 5 μ C18(2) 100 Å). HPLC grade acetonitrile and water (1 : 1) mixture were used as the mobile phase (flow rate of 1 ml min−1, 0.5 μl sample injection at 40 °C). By using standard compounds the reactants and products were identified and quantified (see ESI†). The conversion of H2O2 (%) was calculated using the iodometry titration method.

(Scheme 1a) are the sources of isolated Cu species, whereas unbound surface Cu complexes (Scheme 1b) and binuclear complexes (Scheme 1c) mainly lead to formation of crystalline CuO species.

Results and discussion

The small angle XRD patterns of CuTA, CuTC and CuTN materials are shown in Fig. 1A. The CuT materials displayed a low intensity and broad peak at a small angle which confirms the meso-structured and disordered porous nature of these materials.27 The intensity of this peak varied with different copper sources suggesting that the disordered regular pore channel arrays were further influenced by the added Cu source. In addition, all three CuTUD-1 samples showed a broad peak between 2θ values of 10 and 30° in the wide angle XRD due to the amorphous nature of the mesoporous silica Fig. 1B. The presence of crystalline CuO in all three CuT materials, even at very low Cu loading (Si/Cu = 100), was evidenced from the low intensity reflections at 35.5 and 38.6° (002 and 111 respectively) suggesting the presence of either incorporated/grafted Cu2+ ions in the TUD-1 silica matrix or CuO species inside the pore channels. Due to the low signal to noise ratio, the average crystallite size of CuO was estimated (using the Debye–Scherrer method) after smoothing the signals. For the CuTA sample, the average CuO crystal size was estimated to be approximately 6 nm whereas for CuTN and CuTC it was calculated to be approximately 2–4 nm. The N2 physisorption isotherms of CuTA, CuTC and CuTN displayed a Type IV isotherm with H2 hysteresis (Fig. 2A), which confirmed the disordered and interconnected mesoporous nature of the material. The physical characteristics derived from the nitrogen sorption experiment, including the BET surface area, BJH pore size and pore volume, are tabulated in Table 1. The CuTUD-1 samples possessed a surface area in the range of 431–589 m2 g−1 with a pore volume in the range of 0.71–0.79 cm3 g−1. However the pore diameter distribution (Fig. 2B), determined using the BJH method is quite narrow. The CuTA samples showed the highest pore size with a broader distribution than CuTC or CuTN. The ICP-OES analysis (Table 1) of CuTUD-1 samples revealed that the Si/Cu atomic ratio is nearly the same as that in the

Mechanism of formation of different copper species During the synthesis of the CuTUD-1 catalyst, the formation and distribution of Cu species (isolated/grafted and oxide) followed an entirely different mechanism compared to other Cu-containing ordered mesoporous silicates. The general synthesis of metal containing TUD-1 utilizes triethanolamine (TEA) as a mesopore structure directing agent as well as a chelating agent for both the silicon and the metal ion species. Notably, this ‘atrane route’ yielded significantly higher amounts of active metal ion (Mn+) species in the silicate network.7 During the synthesis of CuTUD-1, triethanolamine chelates with the copper ions (Cu–TEA2+, log β = 4.3)31 which then interact with the silatrane molecules (Scheme 1). These complexes are strong enough to incorporate Cu2+ ions into the silica matrix during condensation/thermal treatment (see Fig. S1†). In addition, the various types of Cu complexes31 such as Cu–TEA2+, Cu(OH)2(H2O)TEA2+ and Cu2TEA2(OH)22+ account for the formation of isolated Cu and CuO species after the hydrothermal treatment and calcination process. Since the TUD-1 synthesis was carried out in basic conditions the formation of binuclear complexes can also be expected.32 Additionally, it is believed that mononuclear Cu complexes

Scheme 1 Some possible Cu complexes with silatrane formed during the CuTUD-1 synthesis, a) Cu–TEA complex connected to the silatrane layer, b) individual Cu complex on the surface; c) binuclear Cu complex.

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Catalyst characterization

Fig. 1 (A) Small angle and (B) wide angle XRD patterns of CuTA, CuTC and CuTN samples.

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Fig. 2 (A) N2 adsorption–desorption isotherms and (B) BJH pore size distributions of CuTA, CuTC and CuTN samples.

Table 1

Fig. 3

Diffuse reflectance UV–Vis spectra of the CuTA, CuTC and CuTN samples.

Fig. 4

FT Raman spectra of CuTA, CuTC and CuTN samples.

Physical properties of CuTA, CuTC and CuTN catalysts

Si/Cu (molar ratio) Catalysta

Synthesis gel

Calcineda

SBETb 2 −1 (m g )

VP,BJHc −1 (cc g )

dP,BJHd (Å)

CuTA CuTC CuTN

100 100 100

92 112 104

431 589 531

0.79 0.71 0.73

81 41 48

a

Elemental analysis by ICP-OES. b SBET = specific surface area. VP,BJH = total pore volume at P/Po = 0.98. d dP,BJH = BJH adsorption pore diameter.

c

synthesis gel, implying that all the copper ions were incorporated into the final solid. Additionally, disordered worm-hole like arrangements of mesopores were further evidenced in the HR-TEM image of a CuTN sample (ESI Fig. S2†). Further to this, the SEM images of the CuTUD-1 samples (ESI Fig. S3†) displayed the uneven size and shape of individual silica particles, and the corresponding Energy Dispersive X-ray analysis (EDAX) revealed the atomic percentage of Cu to be in the range of 1–1.2%. Three absorption peaks at 235, 310 and 350–800 nm were noticed in the diffuse reflectance UV–Vis spectra of the CuT samples (Fig. 3). The absorption peaks at ca. 235 and 310 nm are assigned to O2− → Cu2+ ligand to metal charge transfer (LMCT) and oligomers or aggregated CuO clusters respectively.27,33 In addition, a very broad peak above 400 nm was assigned to the pseudo-octohedral ligand oxygen environment of copper.17,18,23,24,33 Among the three samples, CuTA showed an intense absorption peak at 310 nm, which indicates the presence of oligonuclear CuO clusters18 and also provides indirect evidence for limited incorporation of Cu2+ into the silica matrix. Furthermore, for all the CuTUD-1 samples relative percentages of Cu species were estimated from the respective absorption peak areas. From these results, the amounts of isolated Cu2+ species were estimated to be approximately 65%, 42% and 30% respectively for the CuTN, CuTC and CuTA samples. Notably, approximately 36% of the oligonuclear and or aggregated Cu species were found in the CuTA sample. The FT Raman spectra of the CuTUD-1 samples are shown in Fig. 4. The Raman peaks at 295, 342 and 620 cm−1 are due

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to the Ag and Bg Raman active modes of CuO.34 Even at this low Cu loading (Si/Cu = 100), all the samples displayed CuO Raman active modes. This observation is consistent with the wide angle XRD and DRS UV–Vis analysis. The CuTA spectrum showed these peaks with high intensity compared to the other two CuT samples, providing evidence for a higher population of nanocrystalline CuO, suggesting that Cu species which were not incorporated in the silica matrix might have formed linear oligonuclear [Cuδ+⋯Oδ−⋯Cuδ+]n clusters (extra framework species). In particular free copper which readily converts to oxide is present in a higher proportion when the acetate source was used. Wang et al. have studied the influence of different copper sources, anions and cations, on the CuO dispersion and mesoporosity of SBA-15 and concluded that CuO dispersion favors the following anionic order: NO3− < Cl− < CH3COO− (lyotropic series).35 The ESR technique was employed to study the copper ion coordination environment, and the room temperature ESR spectrum of the CuTA, CuTC and CuTN samples are presented in Fig. 5. The signal intensity increased in the following order: CuTA < CuTC < CuTN. The principal g values calculated using the formula, g = hν/Bβ are consistent with the reported distorted octahedral environment of the Cu(II) ion.36 Bulk CuO is ESR silent, whereas both dispersed CuO

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Fig. 7 (A) XPS survey spectra and (B) Cu 2p core level binding energy spectra for the CuTN sample.

Fig. 5 ESR spectra of CuTA, CuTC and CuTN samples measured at room temperature.

and isolated Cu(II) sites are ESR active,15 and their signal intensities were affected by the presence of neighboring species.13 The lower signal intensity observed for the CuTA sample indicates the presence of a larger quantity of agglomerated CuO species. On the other hand, highly dispersed CuO or Cu(II) sites have been shown to be present in the CuTN sample. H2-TPR profiles of all the CuT samples (Fig. 6) showed two peaks in the temperature range of 100–320 °C. The low temperature reduction peak observed around 160 °C can be attributed to the one step reduction of isolated CuOx clusters to Cu0 and the partial reduction of isolated Cu2+ ions to Cu+.37 The second, broad, high temperature peak observed at 220–250 °C is associated with the reduction of highly dispersed CuO nanoparticles and/or oligomeric CuO clusters. The variation in the intensity of these peaks in CuTA and CuTC suggest that CuO clusters with variable sizes are predominant in these samples.18 To probe further, XPS was carried out to identify the oxidation state and coordination of the Cu ions. The XPS spectrum of CuTN (Fig. 7 and deconvoluted figure in ESI Fig. S4†) shows the characteristic spin–orbit split of Cu 2p3/2 and Cu 2p1/2 with binding energy values of 935.7 eV and 955.8 eV respectively. An asymmetric satellite peak at approximately

Fig. 6

H2-TPR profiles of CuTA, CuTC and CuTN samples.

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932.9 eV can also be seen. These results are consistent with SBA-16 containing Cu(II) and other copper based materials.19,23,24 Further to this, the asymmetric peak region can be divided into two peaks, the lower B.E. peak at 932.9 eV is assigned to dispersed CuO and higher energy region can be attributed to silica connected to Cu2+.23 These observations are in line with the DRS UV–Vis and EPR studies. From the characterization results discussed, it can be concluded that the largest quantity of isolated Cu(II) species are present in CuTN followed by CuTC and CuTA, whereas a higher concentration of oligonuclear and/or bulk CuO is found in CuTA followed by CuTC and CuTN.

Catalytic activity of CuTUD-1 in phenol hydroxylation In the CuTUD-1 catalyzed phenol hydroxylation (Scheme 2) catechol (CC), hydroquinone (HQ) and benzoquinone (BQ) are identified as major products. The reaction parameters including time, temperature, solvent and mole ratios of phenol and H2O2 were optimized in the presence of the CuTN catalyst. The catalytic efficiencies of CuTA, CuTC and other copper containing ordered mesoporous catalysts were then compared under these optimized conditions. The influence of reaction temperature on CuTN catalysis was studied from 30 °C to 80 °C and the results are shown in Fig. 8. Maximum catalytic activities were observed at the following reaction times 6 h, 4 h, 2 h and 1 h for the following reaction temperatures 30, 40, 60 and 80 °C respectively. In addition, the catalytic conversion of H2O2 over CuTN and/or decomposition of H2O2 at different reaction temperatures also influences the conversion of phenol. For instance, at 60 °C, when H2O2 conversion was 69% phenol conversion was observed to be 54%, whereas at 80 °C, the phenol conversion was only 62% even though H2O2 conversion was 100%.

Scheme 2 products.

Phenol hydroxylation over CuTUD-1 catalyst and possible reaction

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Fig. 8 The effect of reaction temperature on phenol hydroxylation with the CuTN catalyst.

This observation clearly indicates the effect of temperature on H2O2 decomposition and the activity of the catalyst. As expected, increasing the reaction temperature caused an increase in the phenol conversion, whereas a maximum catechol selectivity of about 85% was achieved at 60 °C. At a higher temperature (80 °C), visible tar like products were detected in the reaction, this was due to extensive oxidation of benzoquinone and the yield of these products was estimated to be approximately 5% by weight.17,21 Similar observations were also reported for a metallic Cu nanoparticle catalyzed phenol hydroxylation.38 Considering the phenol conversion and catechol selectivity, 60 °C was selected as the optimum temperature for phenol hydroxylation using this catalyst. The molar ratio of H2O2 to phenol was varied from 0.5 to 2 and the results are displayed in Fig. 9. Near 95% H2O2 efficiency was achieved when the H2O2/phenol ratio was 0.5. Two fold and four fold increases in the H2O2 mole ratio increased the phenol conversion by approximately 10% only. A drastic decrease in CC selectivity with a concomitant increase in HQ and BQ selectivity was observed at a H2O2/ phenol ratio of 2. This could be due to over oxidation of

Catalysis Science & Technology formed products and formation of other unidentified products (mainly acetic and oxalic acids).39 Therefore, a mole ratio of H2O2/phenol ≤ 1 is favorable for H2O2 efficiency and CC selectivity. The effect of solvents has been shown to play a major role in phenol hydroxylation and product selectivity.17,33,40 The influence of polar protic and aprotic solvents on CuTN catalyzed phenol hydroxylation is presented in Table 2. Among the various solvents studied, phenol conversion follows the order: water > acetonitrile > ethanol > acetone. From the dielectric constants of the different solvents it can be inferred that the polarity of solvents influences the conversion of phenol. Moreover, water has been employed as a preferred solvent for phenol hydroxylation reactions, probably due to the ease of H2O2 activation at active sites which enhances the reaction rate.41 Table 3 shows the effect of reaction time on phenol hydroxylation using CuTA, CuTC and CuTN catalysts. Although a linear increase in H2O2 conversion was observed, no significant increase in phenol conversion was seen after 1 h of reaction when the CuTA and CuTC catalysts were used. In addition, these catalysts showed significantly worse performance (about 12–20% phenol conversion) than the CuTN catalyst. Furthermore, a relatively linear increase in phenol conversion was seen with the CuTN catalyst. This could be attributed to the presence of a higher number of active Cu sites on the CuTN catalyst compared to the CuTA and CuTC catalysts. Furthermore, 100% conversion of H2O2 was

Table 2 Effect of solvent on phenol hydroxylation using the CuTN catalyst

Dielectric constanta

XPhenol (%)

Selectivity (%)

Solvent

CC

HQ

BQ

Water Ethanol Acetone Acetonitrile

80 24.55 21 37.5

54 15 10 37

85 60 50 79

8 26 42 11

7 14 8 10

Reaction conditions: 5.0 mmol phenol, 5.0 mmol H2O2, 50 mg of CuTN, 10 ml solvent, 60 °C.a From macro.lsu.edu. Retrieved on 2013-01-26.

Table 3 Phenol hydroxylation reaction using CuTUD-1 catalysts prepared from different copper sources

Catalyst CuTA CuTC CuTN

Fig. 9 Effect of phenol : H2O2 mole ratio on phenol hydroxylation with the CuTN catalyst.

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Time (h)

XPhenol (%)

XH2O2 (%)

Selectivity (%) CC

HQ

BQ

1 2 4 1 2 4 1 2 4

35 34 36 37 37 43 42 54 56

38 49 100 60 74 100 46 69 100

72 65 58 68 60 64 78 85 71

13 15 18 16 17 16 11 8 14

15 20 24 16 23 20 11 7 15

Reaction conditions: 5.0 mmol phenol, 5.0 mmol H2O2, 50 mg of CuTUD-1, 10 ml water, 60 °C.

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observed for all the catalysts at the end of a 4 h run. This clearly indicates the efficiency of the catalysts. Prolonging reaction time beyond 4 h leads to formation of a number of unidentified products (including tar) with all these catalysts. We also varied the amount of Cu present in the CuTN (Si/Cu = 100 to 10)27 used for phenol hydroxylation and the results are tabulated in Table 4. Phenol conversion increased with Cu loading up to a Si/Cu ratio of 20, however, after a further increase in Cu loading (Si/Cu = 10), the activity was found to be similar to CuTN (Si/Cu = 100). This could be attributed to larger quantities of bulk CuO (crystalline size ~ 10 nm) that can block the pores or active sites, thus impeding the contact between the substrate/oxidant and the active sites. However, finely dispersed CuO on MCM-48 and SBA-16 catalysts showed good hydroxylation activity.42,43 Our studies on CuO dispersed SBA-15 and SiO2 systems also showed hydroxylation with high BQ selectivity (see Table 4, entries 7 and 8). Nevertheless, the CuTUD-1 catalyst displayed higher catalytic activity and selectivity than the other Cu containing mesoporous and non mesoporous catalysts, which is mainly due to the 3D interconnected mesoporous structure and the presence of highly dispersed active Cu sites. Moreover, the isolated copper active sites are mainly grafted onto the internal surface of the pore walls where they activate H2O2 and phenol easily (Scheme 3). It is proposed that the reaction follows the Haber–Weis mechanism, similar to that reported for other Cu based materials (Scheme 3).17,24,38,44 In general, leaching of active sites is inevitable in oxidation catalysis (via solvolysis of the M–O–Si bonds) owing to the use of strong oxidants (e.g. H2O2).12 In order to examine Cu leaching from the CuTUD-1 samples, after the reaction hot-filtered samples were analysed using the ICP-OES method. Cu atomic weight losses of approximately 12, 20 and 8% were observed for CuTA, CuTC and CuTN samples respectively, suggesting that weakly bonded Cu species were leached (Table 5). To test the true heterogeneity of the reaction, the catalysts were hot-filtered after 30 min of reaction, then the reaction was continued in the hot filtrate for 4 h. As expected, no significant phenol conversion was observed, suggesting minimal catalytic activity. However, H2O2 decomposition due to heating was observed, which was noticeably lower compared to catalytic H2O2 conversion (see Table 3). Table 4 Comparison of catalytic activity of Cu based ordered and disordered mesoporous silicates for phenol hydroxylation

Selectivity (%) Catalyst (Si/Cu)

XPhenol (%)

CC

HQ

BQ

CuTN (104) CuTN (53) CuTN (21) CuTN (10) CuMCM-41 (50) CuMCM-48 (50) 2% Cu/SBA-15 2% Cu/SiO2

54 69 72 59 41 46 22 16

85 72 70 63 60 74 46 35

8 14 16 20 28 26 19 26

7 14 14 17 12 10 35 39

Reaction conditions: 5.0 mmol phenol, 5.0 mmol H2O2, 50 mg of catalyst, 10 ml water, 60 °C.

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Scheme 3 Proposed reaction pathway for phenol hydroxylation using the CuTUD-1 catalyst.

Table 5 Leaching studies for CuTA, CuTC and CuTN catalysts using a hotfiltration experiment

XH2O2 (%) Catalyst

Cu leachinga (%)

30 min

4h

CuTA CuTC CuTN

12 20 8

29 53 30

42 70 58

Reaction conditions: 5.0 mmol phenol, 5.0 mmol H2O2, 50 mg of catalyst, 10 ml water, 60 °C. Reaction mixture hot-filtered after 30 min of reaction.a Calculated from ICP-OES method.

Conclusions We have shown that the use of different copper sources can have a profound effect on the mesoporous structure of TUD-1, as seen from small angle XRD and N2 sorption studies. Two types of Cu species were observed in these materials, namely isolated Cu2+ and oligonuclear CuO species whose concentrations clearly depended on the Cu source. The use of copper nitrate as a source predominantly yielded isolated Cu2+ species with a lesser quantity of nanocrystalline CuO species. On the other hand copper acetate produced larger quantities of oligonuclear and/or bulk CuO species in TUD-1. Phenol hydroxylation using the CuTN catalyst showed better catalytic performance compared to the CuTA and CuTC catalysts due to the greater number of isolated Cu2+ active sites present. Also the small amount of Cu leaching (8%) and the hot-filtration studies confirmed that the catalyst was heterogeneous. Comparing the catalytic performance of CuTUD-1 with other supported systems, the phenol conversion and selectivity towards catechol were noticeably higher, which was attributed to higher amounts of framework Cu2+ species, dispersed CuO and the three dimensional porous nature of the material.

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Acknowledgements The author M. P. is thankful to DST-FIST, UGC for their instrument facility and financial support. The authors are thankful to Prof. B. Viswanathan, NCCR, IIT-Madras for providing laboratory and characterization facilities to carry out this work.

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