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Highly active Cu/MgO catalysts for selective dehydrogenation of benzyl alcohol into benzaldehyde using neither O2 nor H2 acceptor Ravi Kumar Marella, Chinna Krishna Prasad Neeli, Seetha Rama Rao Kamaraju and David Raju Burri*

Published on 25 June 2012. Downloaded on 25/06/2013 16:30:08.

Received 11th April 2012, Accepted 22nd June 2012 DOI: 10.1039/c2cy20222k Aiming at developing an efficient catalyst for selective dehydrogenation of benzyl alcohol into benzaldehyde in the absence of O2 and H2 acceptor, a series of Cu/MgO catalysts with Cu loading of 1, 3, 5, and 7% have been prepared, of which the 5% Cu/MgO catalyst in the gas phase exhibited a remarkable performance with 98% conversion and 97% selectivity and in support of the performance, the catalysts have been characterized by different techniques like BET surface area analysis, XRD, TEM, CO2TPD, N2O pulse chemisorption, XPS and TPR.

Introduction Selective oxidation of alcohols to aldehydes, in particular benzyl alcohol to benzaldehyde, is one of the pivotal processes because of its wider applications in cosmetics, perfumery, food and pharmaceutical industries. Traditionally, benzaldehyde is produced through stoichiometric oxidation of manganese and chromium salts or by hydrolysis of benzyl chloride and the oxidation of toluene in industrial processes.1 These oxidants are toxic and produce a large amount of hazardous waste, leading to severe environmental complications. In these environmentally concerning days, usage of molecular oxygen as an oxidant is under intensive investigation both in liquid and gas phase operations.2,3 However, in the presence of O2, overoxidation, explosion and flammability are some of the inevitable problems. When O2 is used in the dehydrogenation processes H2O forms as a by-product which often deactivates the catalyst and also requires tedious work up to purify the products. Alternatively, development of a catalyst that can be effective even in the absence of an oxidant and a H2 acceptor is particularly interesting from a practical and an environmental point of view because (i) it avoids the formation of H2O, (ii) it produces stoichiometric quantity of H2, which is a promising feedstock for energy generation, (iii) it improves the selectivity by suppressing the overoxidation products. Albeit the dehydrogenation of benzyl alcohol to benzaldehyde is thermodynamically limited, non-oxidative or oxidant-free dehydrogenation of alcohols has several practical advantages.

Recent reports have revealed that thermodynamic limitations can be alleviated by removing H2 from the system using an inert gas flow.4–7 In spite of having economical and environmental benefits in the non-oxidative dehydrogenation processes, information about gas-phase non-oxidative benzyl alcohol dehydrogenation processes is scarcely available.8 It is reported that MgO and HT supported Ag, Au and Cu catalysts are highly effective for oxidant-free dehydrogenation of alcohols.4–6 In our previous publications, using Cu/MgO catalysts hydrogenation, dehydrogenation and coupling of these two reactions are accomplished in the gas-phase in the absence of oxidants.9–14 Herein, the performance of Cu/MgO catalysts for the selective dehydrogenation of benzyl alcohol to benzaldehyde in the gas phase in the absence of an oxidant or a H2 acceptor has been delineated.

Experimental Preparation of catalysts The MgO support was prepared by a precipitation method using Mg(NO3)26H2O (M/s Alfa Aesar, USA, 99% pure) and K2CO3 (M/s SD Fine Chem Ltd, India, assay, 99.5%) as a precursor and a precipitating agent respectively. In a typical precipitation method, homogeneous aqueous solutions of both Mg(NO3)26H2O and K2CO3 were prepared by adding 10 times their volume of deionised H2O separately and the solutions were mixed slowly at a constant pH of 9 under vigorous stirring at room temperature. The resultant precipitate was thoroughly washed with copious amounts of deionised H2O, dried at 393 K for 12 h and calcined at 723 K for 6 h in air. Cu/MgO catalysts with 1, 3, 5, and 7% Cu loading (by weight) were prepared by impregnating the calcined MgO with Cu(NO3)23H2O (M/s Sigma-Aldrich, USA, 99.9%) aqueous solution as a precursor. The excess water was removed by evaporation, the samples were dried at 393 K for 12 h, calcined at 723 K for 6 h in air and designated as 1Cu/MgO, 3Cu/MgO, 5Cu/MgO, and 7Cu/MgO (the prefixed number represents the percentage of Cu loading (by weight) on the MgO support). Characterization of catalysts

Catalysis Laboratory, Indian Institute of Chemical Technology, Hyderabad-500607, India. E-mail: [email protected]; Fax: +91-40-27160921; Tel: +91-40-27191712

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The BET surface area was determined by N2 gas adsorption at 77 K using a SMART SORB 92/93 (M/s SMART Instruments, India) Catal. Sci. Technol., 2012, 2, 1833–1838

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under dynamic conditions. The X-ray diffraction analysis was made using an Ultima IV diffractometer (M/s Rigaku Corporation, Japan) with a nickel filtered Cu Ka radiation, operated at 40 kV and 20 mA in the 2y range of 2 to 801 having a scan speed of 21 min 1. X-ray photoelectron spectroscopy (XPS) analysis of the catalyst was carried out by a Kratos analytical spectrophotometer, with Mg Ka monochromated excited radiation (1253.6 eV). The residual pressure in the analysis chamber was around 10 9 mbar. The binding energy (BE) measurements were corrected for charging effects with reference to the C 1s peak of the adventitious carbon (284.6 eV). A Philips Tecnai F12 FEI transmission electron microscope (TEM) operating at 80–100 kV was used to record TEM images. The temperature programmed reduction (TPR), temperature programmed desorption of CO2 and N2O pulse chemisorption experiments were carried out on a home-made unit. Prior to conducting the TPR experiments, the samples were pre-treated at 473 K for 1 h in the flow of argon gas. After completion of pre-treatment, a gas mixture consisting of 5% H2 in argon gas was passed over the sample at a flow rate of 30 ml min 1 with a simultaneous ramping of temperature at a heating rate of 5 K min 1 from room temperature to 923 K. The hydrogen consumption was monitored using a thermal conductivity detector. The basic site distribution was measured by a well known CO2-TPD technique using a 10% CO2–He gas mixture. In a typical experiment, about 50 mg of the catalyst sample was pre-treated in flowing helium gas at 773 K for 1 h and allowed to cool to 373 K and at this temperature the pre-treated catalyst was exposed to a 10% CO2 in helium gas mixture with a flow rate of 20 ml min 1 for 30 min and subsequently the catalyst was purged with helium gas at 373 K for 1 h to remove the physisorbed CO2. The chemisorbed amount of CO2 which was desorbed in flowing helium gas with the flow rate of 20 ml min 1 from 373 to 1173 K with the heating rate of 5 K min 1 was measured. N2O pulse chemisorption experiments were carried out on a home-made pulse reactor. Typically, about 100 mg of the catalyst sample was reduced for 2 h at 523 K by flowing H2 gas followed by cooling to 363 K. Subsequently N2O was injected in pulses using a 6% N2O–He gas mixture at regular intervals until there is no change in the concentration of N2O at the outlet. Potassium content was estimated using ICP-OES (IRIS intrepid II, M/s Thermo Fisher Scientific) with a 2000 W RF generator operated at 27.12 MHz. Using commercially available potassium standards (10 ppm) catalyst samples were analyzed.

with a DB-5 capillary column (30 m L 0.32 mm id 1 mm ft). The H2O content in the product mixture was determined from KF titration using a 787 KF Titrino (M/s Metrohm).

Results and discussion Cu content in all the four catalysts is in accordance with the calculated amounts and also the negligible amount of potassium content is observed (o1 ppm). The BET surface areas of Cu/MgO catalysts displayed in Table 1 are in the decreasing order, but the decrease in the surface area with increase in Cu loading is marginal, which gives an impression that the Cu particles deposited on the surface of the MgO support are mostly in the nanometer range. However, the reason for marginal decrease in the surface area may be due to coverage of a small fraction of MgO support pores. The substantial increment in the Cu metal surface area (Table 1) with increase in Cu metal loading on the MgO support implies the fine and uniform distribution of Cu metal particles. Either increment in the Cu metal particles or their decrease in dispersion with increase in Cu metal loading on the MgO support (Table 1) is trivial, which reveals that the textural parameters of the support are well preserved in the Cu/MgO catalysts. Alternatively it can be understood that the Cu metal particles at all the loading are well distributed on the MgO support. XRD patterns for the reduced Cu/MgO catalysts are displayed in Fig. 1. The sharp diffraction peaks are observed for all the four catalysts at 2y = 36.9, 42.9, 62.3, 74.6, and 78.51, which are ascribed to the crystalline phase of the MgO support according to the JCPDS No. 4-829. Except the MgO crystalline phase, no Table 1 Physico-chemical characteristics of Cu/MgO catalysts Catalyst

SBETa (m2 g 1)

SCub (m2 g 1)

PCuc (nm)

DCud (%)

MgO 1Cu/MgO 3Cu/MgO 5Cu/MgO 7Cu/MgO

40 39 37 36 32

— 0.37 0.97 1.45 1.73

— 18.1 20.8 23.1 27.2

— 5.9 5.0 4.5 3.8

a

BET surface area determined from N2 gas absorption. b Cu metal surface area obtained from N2O pulse chemisorption. c Cu particle size determined from N2O pulse chemisorption. d Cu metal dispersion obtained from N2O pulse chemisorption.

Catalytic activity tests The catalytic experiments are carried out in a fixed bed reactor (14 mm id and 400 mm length) at atmospheric pressure under N2 flow. About 1 g of the catalyst is loaded and reduced in H2 flow at 553 K for 3 h. After catalyst reduction, the reaction temperature was set and the reactant (benzyl alcohol) was fed at a flow rate of 1 ml h 1 along with N2 gas flow at a rate of 900 ml h 1. The product was collected in an ice cold trap periodically for every 1 h and analysed by GC 17A (M/s Shimadzu) equipped with an FID detector and an OV-1 capillary column (30 m L 0.5 mm id 3 mm ft). The products were identified by GC-MS QP 5050A (M/s Shimadzu) equipped 1834

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Fig. 1 XRD patterns of reduced Cu/MgO catalysts.

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other crystalline phases of CuO, Cu2O, Cu0 are observed in the XRD pattern of the 1Cu/MgO catalyst, which implies that copper species present in 1Cu/MgO are in amorphous phase or below the detection limit of XRD. A new diffraction peak appeared in the XRD pattern of the 3Cu/MgO catalyst at 2y = 50.51. Intensity of this diffraction peak increased with increase in Cu loading. Evidently, it is the second highest diffraction peak characteristic of Cu0 in accordance with the JCPDS card No. 4-0836. Indeed, the 100% intense diffraction peak supposed to appear at 2y = 43.31 for metallic copper (JCPDS card No. 4-0836) is missing in the XRD patterns of Cu/MgO catalysts. This particular Cu0 characteristic peak may be merged with the highly intense diffraction peak (2y = 42.91) of the MgO support. To evaluate the reduction behaviour of CuO, the calcined CuO/MgO catalysts were examined by H2-TPR and the resultant reduction profiles are shown in Fig. 2. The reduction profiles of Cu/MgO show a single symmetric reduction peak. This reveals that CuO species present in the CuO/MgO catalyst are reduced completely into Cu0 (CuO + H2 Cu + H2O) in a single-step. The reduction profiles reveal that there is a striking balance between the Cu loading and the amount of H2 consumed. However, there is a marginal shift in temperature maxima towards higher temperatures from 600 to 615 K with increase in Cu loading on the MgO support, indicating the attainment of bulk CuO with increase in Cu loading.15 Temperatures programmed desorption of CO2 study was made to verify the basic characteristics of Cu/MgO catalysts and the resultant patterns are displayed in Fig. 3. The CO2 TPD pattern of the MgO support implies that there are 3 types of basic sites, such as moderate, strong and very strong. Similar kind of basic sites are observed in all the Cu/MgO catalysts except in the 5Cu/MgO catalyst. In the case of the 5Cu/MgO catalyst, most of the basic sites are weak in nature. The X-ray photoelectron spectrum of the reduced 5Cu/ MgO catalyst is displayed in Fig. 4, wherein, two distinct peaks corresponding to Cu 2p3/2 and Cu 2p1/2 are present at the binding energies of 934.7 and 954.4 eV respectively. There is a small peak at about 944 eV, which may be the Cu 2p3/2 satellite peak of the 5Cu/MgO catalyst, but the Cu 2p3/2 main

Fig. 2 TPR of calcined CuO/MgO catalysts.

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Fig. 3 CO2 TPD of Cu/MgO catalysts.

Fig. 4 XPS spectra of the reduced 5Cu/MgO catalyst.

peak supposed to appear at about 936 eV is absent, which may be merged with the 934.4 eV peak. If so, a small portion of Cu2+ may be present in the 5Cu/MgO catalyst. The appearance of the Cu 2p3/2 main peak at the binding energy of 934.3 eV demonstrates the presence of either Cu1+ or Cu0 in the catalyst. As binding energies of the Cu 2p3/2 main peak of both Cu1+ and Cu0 are very close (E1 eV), it is difficult to distinguish Cu1+ and Cu0 by XPS alone. Chang et al. analyzed both calcined and reduced CuO/SiO2 catalysts at different temperatures by XPS and reported that binding energies of the Cu 2p3/2 main peak are 936.4 eV for Cu2+ and 934.3 eV for Cu1+ or Cu0. The XPS results of the 5Cu/MgO catalyst are in agreement with the reported literature data.15 As shown in Fig. 5, the metallic nanoparticles of Cu0 are dispersed on the surface of the MgO support. Most of the Cu particles in the 5Cu/MgO catalyst are 8.6 nm in size. The histogram shown in Fig. 5 as inset reveals that the Cu metal particles are populated between 4 and 22 nm. The average particle size of the 5Cu/MgO catalyst obtained from N2O pulse chemisorption is 23.1 nm (Table 1), which is on the higher side comparatively. This inconsistency may be due to experimental error or the differences in the techniques used. Based on the industrial importance of benzaldehyde, selective dehydrogenation of benzyl alcohol to benzaldehyde without using either an oxidant or a hydrogen acceptor particularly in the gas phase is attempted. It is found that Cu/MgO catalysts are capable of catalyzing the dehydrogenation of benzyl alcohol Catal. Sci. Technol., 2012, 2, 1833–1838

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Fig. 5 TEM image of the 5Cu/MgO catalyst.

into benzaldehyde to a greater extent with a minute quantity of toluene. Except toluene, no other by-products are noticed. The reported literature reveals that the usage of molecular oxygen as an oxidant has a pivotal role in the dehydrogenation of benzyl alcohol, particularly in the enhancement of conversion. It is reported that the conversion of benzyl alcohol on the BaPb0.6Bi0.4O3 perovskite catalyst is about 50% in the presence of O2, whereas in its absence the conversion is only 25%, i.e., the conversion is almost doubled in the presence of O2.8 Tang et al.16 reported that the conversion of benzyl alcohol is about 90% with 499% selectivity to benzaldehyde over the Cu–Mn/Al2O3 catalyst. Jia et al.17 also reported that the conversion of benzyl alcohol is nearly 100% with 96% selectivity of benzaldehyde over the Ag/HMS catalyst. Similar kind of high conversions and selectivities are reported over Au–Cu/SiO2 catalysts,18 but in all of these studies molecular oxygen was used as an oxidant. In a recent report, a hexagonal mesoporous K–Cu–TiO2 catalyst exhibited excellent catalytic activity for gas-phase oxidation of benzyl alcohol to benzaldehyde at a low temperature (483 K).19 Apart from using molecular oxygen, high temperature sensitivity and formation of by-products at 520 K are the disadvantages of this catalyst.17 From the activity point of view, the efficiency of this catalyst is on a par with the reported best catalysts even in the absence of an oxidant or a hydrogen acceptor. It appears that certain catalysts are highly active for the oxidation of alcohols to aldehydes in the presence molecular oxygen, but are not so effective under non-oxidative conditions. For instance, the gas phase oxidation of benzyl alcohol to benzaldehyde over alkaline earth metal promoted Ag/SiO2 catalysts exhibited very poor activity in the absence of O2. When benzyl alcohol, N2 and O2 are used, the yield of benzaldehyde is greater than 60%, when the flow of O2 is stopped the yield of benzaldehyde is negligible.20 Similar kind of catalytic studies can be found elsewhere.17,21 After recognizing the ability of Cu/MgO catalysts, the reaction parameters were optimized. To investigate the optimum loading of Cu on the MgO support, the dehydrogenation of benzyl alcohol to benzaldehyde was conducted on all the four catalysts (1Cu/MgO, 3Cu/MgO, 5Cu/MgO and 7Cu/MgO catalysts) in the gas-phase at 533 K in the inert atmosphere using nitrogen gas and the data are displayed in Fig. 6, which demonstrate that the increase in conversion of benzyl alcohol with increase in Cu loading from 1 to 5% is due to increase in the number of active sites available for participating in 1836


Fig. 6

Catalytic activity versus Cu metal loading.

the reaction. However, the increase in conversion with increase in Cu loading from 5 to 7% is marginal, implying that high metal content may lead to agglomeration of Cu metal particles (Table 1) and furthermore, selectivity to benzaldehyde decreased significantly due to simultaneous increase in the selectivity to toluene (22%). It can be ascertained that the active sites responsible for the dehydrogenation of benzyl alcohol and hydrogenolysis of benzyl alcohol are different. Among the Cu/MgO catalyst series, the 5Cu/MgO catalyst exhibited higher activity with 86% conversion of benzyl alcohol and 90% selectivity to benzaldehyde. The results demonstrate that there is a significant influence of the Cu loading on the MgO support. It seems that there is a striking balance between the conversion of benzyl alcohol and selectivity to benzaldehyde over the 5Cu/MgO catalyst. Hence, 5Cu/MgO is the best catalyst of the series. The TOF of the Cu/MgO catalysts and the yield of benzaldehyde with respect to Cu metal loading in Cu/MgO catalysts are displayed in Fig. 7, which reveals that the TOF and the yield of benzaldehyde are more or less similar in trend. There was a gradual increase in the TOF and the yield with increase in Cu metal loading up to 5%. However, beyond 5% Cu loading, the yield is slightly increased rather than levelling off, which may due to closeness in the particle size and dispersion (Table 1).

Fig. 7 The yield of benzaldehyde and its TOF against Cu metal loading.

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Scheme 1 Proposed mechanism for the dehydrogenation of benzyl alcohols to corresponding aldehydes.

Fig. 8 The catalytic activity against reaction temperature.

After realizing the best catalyst (5Cu/MgO) of the Cu/MgO catalyst series, other reaction parameters were optimized. Initially, to verify the nature of reaction, an experiment was conducted without the catalyst using quartz pieces as inert medium in the reactor, and no products were detected. Hence, a homogeneous surface/volume effect was ruled out. Later on, an in situ activity measurement was made using a glass microreactor interfaced with a TCD equipped GC as reported elsewhere22 to authenticate the formation of hydrogen along with benzaldehyde. The results reveal that hydrogen is being produced in the absence of O2 from benzyl alcohol over the 5Cu/MgO catalyst. Formation of H2O in the hydrogenolysis of benzyl alcohol to toluene was confirmed by KF titration. Since 5Cu/MgO is the best catalyst of the Cu/MgO catalyst series, the influence of reaction temperature on the catalytic activity of this catalyst is studied and the results are displayed in Fig. 8. The conversion at 513 K is about 71%, which increased gradually with rise in reaction temperature and attained a maximum of 98% at 573 K and levelled off beyond this temperature. Surprisingly, with increase in reaction temperature from 513 to 573 K, the selectivity to benzaldehyde increased from 84 to 97%, but at 593 K selectivity dropped down to 94%, which is due to deviation in the mechanism from dehydro-

Fig. 9 The catalytic activity against time-on-stream.

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genation to hydrogenolysis. Hence, the optimum reaction temperature is 573 K in order to maintain the maximum conversion (98%) of benzyl alcohol towards the production of benzaldehyde selectively (97%). To understand the life of the 5Cu/MgO catalyst, time-onstream study was made at 573 K for 10 h and the results are displayed in Fig. 9, which reveals that there is no substantial loss in activity. The abundant literature reveals that the acid–base properties of the catalysts play a prominent role in the dehydrogenation of alcohols.23,24 Addition of bases as co-catalysts or generation of basic sites in the catalyst facilitates the dehydrogenation of alcohols significantly. It is reported that Pd supported on basic supports such as MgO or hydrotalcite is highly effective in the dehydrogenation of alcohols, whereas the same Pd supported on alumina, silica and zeolite b is ineffective towards the dehydrogenation of alcohols.25 Since Cu/MgO is an efficient catalyst, it can be understood that the basic sites required for the adsorption of alcohols and abstraction of hydrogen from benzyl alcohol are available in the MgO support as shown in Scheme 1. It is reported that the basic sites can exhibit nucleophilic activity and abstraction of proton from alcohol to form a negatively charged alkoxide intermediate.6,26–28 The elimination of b-H from the alkoxide produces benzaldehyde.

Table 2 Activity of the 5Cu/MgO catalyst for the dehydrogenation of benzyl alcohol and its derivatives to corresponding aldehydes S. No. Substrate

Conversion (%) Selectivity (%)

1

98

97

2

96

97

3

97

96

4

91

95

5

95

95

Weight of the catalyst = 1 g, temperature = 573 K, pressure = 1 atm, substrate = 1 ml h 1, carrier gas (N2) = 900 ml h 1.


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Finally desorption of adsorbed hydrogen from the Cu/MgO catalyst surface produces molecular hydrogen (Scheme 1). Since the hydrogenolysis of benzyl alcohol to toluene is much favoured on the 7Cu/MgO catalyst (Fig. 6), we further studied the activity of the 7Cu/MgO catalyst. The conversion of benzyl alcohol at 533 K is about 89% with the selectivities to benzaldehyde and toluene being 78% and 22% respectively. When the temperature is increased to 593 K the conversion (90%) is more or less constant, but there is a considerable decrease in the selectivity to benzaldehyde and simultaneous increased in the selectivity to toluene to 31%. It is already been reported that the transformation of benzyl alcohol into toluene proceeds via a hydrogenolysis mechanism29,30 which is shown below. C6H5–CH2–OH + H2 - C6H5–CH3 + H2O To evaluate the scope of the reaction, the oxidation reactions of p-methyl alcohol, p-methoxy benzyl alcohol, m-methoxy benzyl alcohol, m-phenoxy benzyl alcohol were further studied (Table 2). As above, all the substrates consistently underwent dehydrogenation selectively to the corresponding aldehydes in high yields. In summary, it is demonstrated that Cu/MgO catalysts exhibited overwhelming performance in dehydrogenation of benzyl alcohol to benzaldehyde in the gas-phase without using either an oxidant or a hydrogen acceptor. The 5Cu/MgO catalyst is proved to be an excellent catalyst of the series with 98% conversion and 97% selectivity.

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