Zn-Modified CuCr2O4 as Stable and Active Catalyst for the Synthesis

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Zn modified CuCr2O4 as stable and active catalyst for the synthesis of 2,6dimethylpyrazine: Valorisation of crude glycerol obtained from bio-diesel plant KRISHNA VANKUDOTH, Vijay Kumar Velisoju, Naresh Gutta, Naveen Kumar Sathu, Hari Padmasri Aytam, Inkollu Sreedhar, and Venugopal Akula Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02594 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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Industrial & Engineering Chemistry Research

A direct correlation between Cu metal surface area and the 2,6-dimethylpyrazine rate is established over Zn modified CuCr2O4 catalyst. 169x190mm (96 x 96 DPI)

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Zn modified CuCr2O4 as stable and active catalyst for the synthesis of 2,6dimethylpyrazine: Valorisation of crude glycerol obtained from bio-diesel plant Krishna Vankudoth,†,‡ Vijay Kumar Velisoju,‡ Naresh Gutta,‡ Naveen Kumar Sathu,‡ Hari Padmasri Aytam,≠ Sreedhar Inkollu# and Venugopal Akula,*,†,‡ †

Academy of Scientific and Innovative Research, India.

‡

Catalysis Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of

Chemical Technology, Hyderabad, Telangana – 500007, India. ≠

Department of Chemistry, University College for Women, Osmania University, Koti,

Hyderabad - 500 095, Telangana, India. #

Department of Chemical Engineering, BITS Pilani Hyderabad Campus, Shameerpet

(Mandal), Hyderabad 500 078, Telangana, India.

ABSTRACT Modified CuCr2O4 catalysts were investigated for vapour phase conversion of crude glycerol and 1,2-propanediamine (1,2-PDA) to synthesize 2,6-dimethylpyrazine (2,6-DMP). Addition of Zn not only enhanced the activity of CuCr2O4 but also improved the stability of the catalyst. Introduction of Zn in to the CuCr2O4 matrix increased the copper metallic surface area of the catalyst. The rate of 2,6-DMP is found to be dependent on the Cu metal surface area. DRIFT spectroscopic data revealed that a high fraction of Lewis-acid-base pair sites are responsible for the selective formation of 2,6-DMP. A direct correlation between 2,6-DMP rate and the surface Cu is established.

Keywords: Crude glycerol; 2,6-DMP; DRIFTS; Promoters; Copper chromite 1 ACS Paragon Plus Environment

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INTRODUCTION In search of alternative fuels, the refinery and petrochemical industries are ended up with generation of large amounts of by products (which are identified as useful intermediates in some case) e.g. crude glycerol from the bio-diesel industry.1-3 As a result, the bio-diesel production is causing a concern due to such glycerol which poses threat, not only to the process economics but also from environmental standpoint. Hence valorisation of the crude glycerol (that contains numerous impurities: largely 70-80wt% H2O and traces of CH3OH, KOH and NaCl etc) into useful compounds is an area of interest. However, development of catalytic processes for the effective utilization of crude glycerol to produce fine chemicals becomes a promising approach wherein the catalyst system sustains in presence of impurities in the substrate.4-6 Plethora of literature is available on the utilization of such glycerol by catalytic methods for example dehydration, hydrogenation, oxidation, esterification and etherification etc., in order to make the biodiesel industry economically attractive.7-11 2-methylpyrazine (2-MP) an intermediate compound for the anti tuberculosis drug is being synthesized by cyclocondensation of ethylenediamine and 1,2-propanediol over Pd modified Zn-Cr-O catalyst.12 For safe disposal of crude glycerol we reported the synthesis of 2MP over ZnO-ZnCr2O4 catalyst in vapour phase conditions at ambient pressure.13-15 2,6DMP is an important compound widely used in agro-chemical industry and is also used as food flavouring agent.16 In a method Rizk et al. synthesized alkyl and substituted pyridines and pyrazines by intramolecular hydroamination in presence of p-toluenesulfonic acid.17 However, these methods produce unusual by-products.18 Li et al. reported the synthesis of alkylpyrazines over alumina supported zinc catalyst using glycerol and 1,2-PDA with a 2,6DMP rate of 2.59 µmol (gcat)-1 s-1.19 Recently we have reported the mechanistic aspect of dehydrocyclization process over metal chromites and mixed metal oxides with a 2,6-DMP rate of 3.83 µmol (gcat)-1 s-1.20-22

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The present investigation deals with synthesis of CuCr2O4 derived by sol-gel technique and the effect of modifiers such as Zn, Zr and Mg on the activity and selectivity for the conversion of crude glycerol and 1,2-PDA to 2,6-DMP. Among them the Zn modified CuCr2O4 showed tremendously higher 2,6-DMP rate. A detailed examination of the surface acidic and basic sites on the product distribution is emphasized by using pyridine and HCOOH adsorbed IR spectroscopy combined with metallic copper surface area of the catalyst measured by pulse N2O titration technique. The adsorption and spectroscopic data revealed that enhanced Lewis acidity and the stabilization of Cu0 species occurred on CuCr2O4 in the presence of Zn was mainly responsible for its stable activity. The fresh, reduced and some of the used catalyst were characterized by BET-SA, XRD, TEM, TPR, TPD of NH3, XPS, ESR, pyridine and HCOOH adsorbed DRIFT spectroscopy.

MATERIALS AND METHODS Preparation of catalysts, characterization and evaluation for the dehydrocyclization reaction The CuCr2O4 spinel was synthesized by sol–gel method.23 In a typical procedure, required amounts of Cu(NO3)2.3H2O and Cr(NO3)3.9H2O (at a desired mole ratio of Cu/Cr = 0.5) dissolved in 47 mL of ethanol at 60 °C followed by 1 mL of concentrated HNO3 was added to give a clear dark blue solution. About 18 mL Pluronic P123 was instantaneously added to the solution to turn out a dark green transparent gel formed within few minutes followed by drying at 120 °C for 15 h, and the resulting gel was calcined at 550 °C for 5 h. The powder XRD analysis of the sample showed CuCr2O4 phases. The Zn, Zr and Mg modified CuCr2O4 (Cu/Cr = 0.5) samples were prepared by simple wet impregnation method. In a typical procedure; required amount of metal nitrate [M(NO3)2.xH2O] (M = Zn and/or Mg)and Zr-oxy nitrate was taken to give respective metal wt% dissolved in distilled water in a 100 ml beaker and mixed with the requisite amount of CuCr2O4 obtained by sol-gel


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technique. The solutions were dried with constant stirring at 100 °C until the sample get dried and then kept for oven drying at 120 °C for about 24 h followed by calcination in static air at 550 °C for 5 h. The reactor dimensions and the complete details on experimental conditions maintained for the activity studies were included in the electronic supplementary information (ESI). RESULTS BET surface area and powder XRD analysis The BET surface areas of the 2wt% metal (M = Zn, Zr, Mg) modified CuCr2O4 samples showed not much change except the zirconia modified catalyst (Table 1). The XRD analysis of calcined samples exhibited only CuCr2O4 spinel phase [JCPDS # 72-1212] and the absence of diffraction lines due to ZnO, MgO and ZrO2 phases could be explained by a lower amount of this modifiers that are either highly dispersed or below the X-ray detection limit (Figure 1A). The XRD patterns of reduced samples displayed strong diffraction lines due to metallic copper [JCPDS # 04-0836] along with low intense CuCr2O4 phase (Figure 1B). These results indicated that copper species in a spinel structure are reduced to some extent and CuCr2O4 still present. No XRD peaks of species containing chromium appeared which suggests that the chromium is in highly dispersed and/or exist in an amorphous state. The crystal size of CuCr2O4 measured by Scherrer formula and the data is reported in Table 1. It is found that the FWHM of the diffraction peaks of CuCr2O4 in 2wt%Zn/CuCr2O4 sample is slightly broader explaining that the mean crystal size of CuCr2O4 species become smaller compared to CuCr2O4.



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Figure 1. XRD patterns of (A) calcined and (B) reduced; (a) CuCr2O4; (b) 2wt%Zn/CuCr2O4; (c) 2wt%Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples.

H2-temperature programme reduction (H2-TPR) analysis Reduction of the catalysts is a pre-requisite prior to the dehydrocyclization reaction, the reducibility of the modified CuCr2O4 catalysts are examined by H2-TPR (Figure 2). All the samples undergo single stage reduction showing the Tmax of CuCr2O4, Zn/CuCr2O4, Zr/CuCr2O4, and Mg/CuCr2O4 are 386, 368, 390 and 402 °C respectively due to incomplete reduction of copper chromite spinel phase to certain extent and/or the high valence chromium species.20,24 These results are in good accord with XRD analysis that showed both metallic Cu and CuCr2O4 phases after reduction. In addition, the reduction of modified CuCr2O4 samples occurred in a narrow temperature range, indicating that the copper and chromium species formed a single phase structure, which is good collaborate with the XRD results (Figure 1A). The hydrogen uptakes are in the following order: 2wt%Zn/CuCr2O4> CuCr2O4 > 2wt%Zr/CuCr2O4 > 2wt%Mg/CuCr2O4 (Table 1). The low temperature reduction signal in 2wt%Zn/CuCr2O4 sample can be explained by the presence of smaller crystallite size of CuCr2O4 compared to other samples (Table 1).


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Figure 2. TPR patterns of (a) CuCr2O4; (b) 2wt%Zn/CuCr2O4; (c) 2wt% Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples. Table 1. Physicochemical properties of the 2wt% metal (M = Zn, Zr, Mg) modified CuCr2O4 samples. Samples

a

b

d

H2 uptake (µmol g-1)

BET-SA (m2 g−1)

N2O uptake µmol/gcat

SCu (m2 g-1)

CuCr2O4

38.7

52.4

4.3

Crystallite size (nm) c CuCr2O4 (211 plane) 15.3

2Zn/CuCr2O4

33.7

55.0

4.5

12.3

265.4

2Zr/CuCr2O4

23.3

40.0

3.2

16.4

243.2

2Mg/CuCr2O4

35.6

28.3

2.3

17.1

236.7

a

248.4

b

BET–surface areas of the fresh calcined catalysts, Copper metal surface area measured by N2O decomposition studies, Crystallite size measured from XRD patterns of calcined CuCr2O4 samples using Scherrer formula with (211) plane, d H2 uptakes measured by TPR analysis calibrated with a standard H2-TPR of Ag2O. c

FT-IR analysis The FT-IR spectra shows a high frequency band at 614 cm-1 is assigned to the stretching vibration of tetrahedral M-O (M= metal) bond and the low frequency band around 513 cm-1 is due to octahedral M-O bond present in the CuCr2O4 spinel structure.25-27 Both the fresh calcined and reduced samples bands showed at 614 cm-1 and 513 cm-1, which are originated from Cu-O and Cr-O-Cu species in CuCr2O4 spinel (Figure 3).28 The XRD data also confirmed the formation of CuCr2O4 phase. However the sharp vibrational bands of copper chromite (513 cm-1 and 614 cm-1) in the fresh form became broader after reduction. 6 ACS Paragon Plus Environment


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Appearance of broad vibrational bands may be explained by the partial reduction of CuCr2O4 to form metallic copper along with some portion of unreduced CuCr2O4 phase. As a result; the vibrational band intensities either due to Cu-O or Cr-O-Cu species might have decreased.21-23 The fresh samples showed a band at 934 cm-1 due to chromium with higher oxidation state (Cr > 3 and < 6) likely to be in the form of Cu[Cr3+Crx+]O4 (3 < x < 6) species (Figure 3A). On the contrary the reduced samples manifested the absence of higher oxidation Cr species; oxidation state > 3 at 934 cm-1 (Figure 3B). Presence of spinel structure is also confirmed from the IR spectra of the reduced samples (Figure 3B).

Figure 3. FTIR spectra of (A) calcined and (B) reduced: (a) CuCr2O4; (b) 2wt%Zn/CuCr2O4; (c) 2wt%Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples. XPS analysis The Cu 2p XPS of the calcined and reduced CuCr2O4 samples shown in Figure 4; in which a broad peak observed at a binding energy (BE) of 935 eV, which is due to the Cu2+ species. A satellite peak at 940–946 eV was indicative of Cu2+ species in CuCr2O4 in both the calcined and reduced samples (Figure 4).29-31 The wide Cu 2p3/2 signals obtained for the


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calcined sample could be fitted satisfactorily to one principal peaks after deconvolution as shown in Figure 4A. The predominant peak at 935.2 eV was assigned to Cu2+ of spinel CuCr2O4 spinel.32 The same sample after reduction in hydrogen at 450 °C for 3 h showed the presence of two peaks of different binding energies namely, 932.5 eV and 935.1 eV corresponding to metallic Cu and Cu2+ in CuCr2O4 respectively.29 This indicates the incomplete reduction of the CuCr2O4 under H2 activation which was also supported by XRD, FTIR studies. The signals corresponding to Cr 2p3/2 at 576.4 eV is due to Cr3+ and 578.3 eV is due to Cr6+ species in calcined samples (Figure 4A).24 The XP spectra corresponding to reduced CuCr2O4 and 2wt%Zn/CuCr2O4 (Figure 4B) illustrates the absence of Cr6+ at 578.3 eV indicating that the Cr6+ species are completely reduced to Cr3+ on the surface.14 Furthermore, the relative ratio of Cu2+/Cu0 is higher on CuCr2O4 (0.55) than 2wt%Zn/CuCr2O4 (0.24) sample; emphasizing a 20% excess reduction occurred on 2wt%Zn/CuCr2O4 under the reduction conditions adopted. This shows that Zn is favouring the low temperature reduction of Cu2+ species (Figure 2 and Table 1). Both calcined and reduced samples have shown two oxygen peaks; the high binding energy signal is due to surface oxygen and/or oxygen containing surface contamination, while the signal at low binding energy could be lattice oxygen.32,33



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Figure 4. X-ray photoelectron spectra of calcined (A) and reduced (B) of (a) CuCr2O4 and (b) and (b) 2wt%Zn/CuCr2O4 samples. Table 2. XPS peak area (%) of reduced samples calculated from full width half maximum (FWHM) of the corresponding signals. Samples

CuCr2O4

Binding energy (eV) of 2p3/2 Cu0 Cu2+ Cr3+ 932.20 935.36 576.30

2wt%Zn/CuCr2O4

932.34

935.41

576.27

a

XPS peak area (%) Cu0 Cu2+ 64.2 35.8 80.3

19.7

Cu2+/Cu0 0.55 0.24

Electron paramagnetic Resonance (EPR) analysis The room temperature EPR spectra of fresh calcined and reduced samples are presented in Figure 5. From Figure 5A, it is evident that the peak shapes of the signals attributed to Cu2+ are anisotropic with clearly defined ݃ region as expected for Cu2+ ions in surrounding with axial symmetry. The EPR signal (with g ~ 2.21) is due to the presence of isolated Cu species.34 The reduced samples showed highly symmetrical broad signals attributed to Cr3+ centred at 326.7 mT with corresponding ݃ ~ 1.97 is observed (Figure 5B). The peak to peak (∆HPP) line width is high in case of CuCr2O4 calcined than the 2wt%Zn/CuCr2O4 sample. Decrease in (∆HPP) peak to peak width is an indication of 9 ACS Paragon Plus Environment

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formation of large size particles, which in exact accordance with the XRD data (Table 1).35 Quite contract to this the EPR spectra of reduced samples showed the peak to peak width in CuCr2O4 is increased by the addition of Zn (Figure 5B), implying that a decrease in particle size of CuCr2O4 in 2wt%Zn/CuCr2O4 sample.

Figure 5. EPR spectra of (A) calcined and (B) reduced (a) CuCr2O4 and (b) 2wt% Zn/CuCr2O4 samples. TPD of NH3 studies The surface acidity of CuCr2O4 and the modified M/CuCr2O4 samples are examined by TPD of NH3 (Figure 6). Based on the NH3 desorption profiles, the acid sites are classified as weak (< 200 ᴼC), moderate (250-450 ᴼC) and strong (450-650 ᴼC).36 In the comparative analysis the zirconium modified CuCr2O4 sample exhibited a high ratio of acidic sites showing its character, while Mg modified CuCr2O4 showed lower acidity. The Zn modified CuCr2O4 showed an enhancement in the number of weak to moderate acid sites than the CuCr2O4 (Figure 6a, 6b). Li et al reported the role of weak to moderate acid sites on alumina



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supported zinc for 2,6-DMP production using glycerol and 1,2-PDA.19 Hence the role of Lewis or Brønsted acid sites are determined by pyridine adsorbed DRIFT spectroscopy.

Figure 6. NH3-TPD of (a) CuCr2O4, (b) 2wt%Zn/CuCr2O4, (c) 2wt%Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples. Pyridine adsorbed DRIFT spectroscopy The adsorption bands at 1443 cm-1 is assigned to the pyridine adsorbed on Lewis acidic sites (LAS) and the band at 1485 cm-1 is due to both Brønsted and Lewis acidic sites. The existence of IR band at 1558 cm-1 is ascribed to the pyridine adsorbed on Brønsted acidic sites (BAS) (Figure 7).37 The DRIFT spectra show that the 2wt%Zn/CuCr2O4 sample exhibited higher Lewis acid sites than on CuCr2O4 and the Zr and Mg modified catalysts. The ratios of normalized relative intensities of bands due to LAS and BAS appearing in the region at 1443 cm-1 and 1558 cm-1 are measured (in order to compare the acidic strengths of CuCr2O4 and modified CuCr2O4 catalysts) which are found to be BAS1558/LAS1443 = 5.6; 3.0; 8.4 and 6.1 for CuCr2O4, Zn/CuCr2O4, Zr/CuCr2O4 and Mg/CuCr2O4 respectively. The


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DRIFTS data indicated a high portion of LAS on 2wt%Zn/CuCr2O4 compared to CuCr2O4 sample. The TPD of NH3 studies also revealed an enhanced acidity in the temperature region 300-450 °C which is found to be high on 2wt%Zn/CuCr2O4 compared to CuCr2O4 (Figure 6). We believe that these weak and moderate acid sites are attributed to LAS generated by the modification of CuCr2O4 with Zn.

Figure 7. Pyridine adsorbed DRIFT spectra of (a) CuCr2O4, (b) 2wt%Zn/CuCr2O4, (c) 2wt%Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples. HCOOH adsorbed DRIFT HCOOH adsorbed IR analysis is one of the useful techniques to determine the basicity of the catalysts. The adsorption band at 1620 cm-1 is ascribed to the surface basic sites strongly interacting with HCOOH and the 1584 cm-1 and 1677 cm-1 bands are emerged due to decomposition of surface formate species (Figure 8).38,39 The 1752 cm-1 band is due to formic acid associatively adsorbed on the weak basic sites. A band at 1284 cm−1 is assigned to HCOOH adsorbed on the CuCr2O4 surface (Figure 8). Adsorption of HCOOH on the basic sites can be illustrated as follows: 12 ACS Paragon Plus Environment


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HCOOH + Mn+-O2- →

HCOO- + Mn+-OH- (M = Cu and/or Cr)

Reduction of catalyst surface leads to dehydroxylation of metal oxides to form coordinatively unsaturated O2- ions i.e. basic sites. Therefore the basic sites on the metal or mixed metal oxide catalysts are in the form of Mn+−O2- species which will be converted to Mn+−OH− by the adsorption of HCOOH.39 The normalized relative peak intensity ratios of the bands at 1620 cm-1 is almost twice on Zn modified CuCr2O4 compared to the other samples. Quite contrast to this the CuCr2O4 showed the slightly higher number of weak basic sites (1752 cm-1 band) on its surface. Therefore, it is suggested that addition of Zn to CuCr2O4, the surface basic sites are increased (Figure 8a, 8b).

Figure 8. Formic acid adsorbed DRIFT spectra of (a) CuCr2O4, (b) 2wt%Zn/CuCr2O4, (c) 2wt%Zr/CuCr2O4 and (d) 2wt%Mg/CuCr2O4 samples.

Dehydrocyclization activity The dehydrocyclization of aqueous glycerol and 1,2-PDA carried out over M/CuCr2O4 (M = Zn, Zr, Mg) catalysts at 375 °C and the data is reported in Table 3.


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Conversion of glycerol and 1,2-PDA is high on 2wt%Zn/CuCr2O4 than the other samples. 2,6-DMP rate is higher than the other samples including the unmodified CuCr2O4. It appears that introduction of Zn drastically changed the acid-base characteristics of the CuCr2O4. As mentioned earlier, conversion of glycerol and 1,2-PDA is mainly depends upon the surface Lewis acidity. HCOOH adsorbed DRIFT spectra revealed the 2wt%Zn/CuCr2O4 sample showed high ratio of basic sites on the surface could be a possible reason for the higher 2,6DMP yields than the other promoters. It has been observed that addition of Zn to CuCr2O4 resulted in slightly enhanced metallic copper surface area of the catalyst.24 It has also been found that the 2,6-DMP rate is proportional to the metallic copper surface area of the catalysts (Figure 9).

Figure 9. Relationship between Cu metal surface area and the 2,6-DMP rate against the Zn loading on CuCr2O4 catalysts. In comparative analysis, Mg modified CuCr2O4 showed slightly high glycerol conversion than the Zr doped sample. However, a slightly higher rate of 6-HMP is observed over Mg modified CuCr2O4 (Table 3). This could be explained possibly due to a higher proportion of basic sites on the catalyst surface. The role of modifier on the product 14 ACS Paragon Plus Environment


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distribution is discussed based on the acid-base strengths of the catalysts measured from the relative ratios of the peaks due to Brønsted and/or Lewis (acid-base) sites obtained from the pyridine and HCOOH adsorbed DRIFT spectroscopy. Table 3. Dehydrocyclization of aqueous glycerol and 1,2-PDA over reduced M/CuCr2O4 catalysts; catalyst weight ~ 0.05 g diluted with ~ 0.150 g of α-Al2O3; reaction temperature = 375 °C; GHSV at 40.25 mL gcat-1 s-1. Catalyst

Xglycerol

X1,2-PDA

CuCr2O4

15.2

2Zr/CuCr2O4

b

Selectivity (%) S2,6-DMP

S6-HMP

15.4

84.6

7.3

5.1

8.7

9.3

81.7

7.5

2Mg/CuCr2O4

10.2

11.5

76.8

2Zn/CuCr2O4

25.3

26.7

5Zn/CuCr2O4

37.7

7Zn/CuCr2O4

a

6-HMP

3.0

7.2

0.62

5.5

5.3

4.03

0.37

9.2

7.2

6.8

4.4

0.53

96.1

1.8

0.8

1.3

13.5

0.25

39.3

97.3

1.2

0.5

1.0

20.5

0.32

23.2

25.1

98.1

0.9

0.3

0.7

12.6

0.11

10Zn/CuCr2O4

14.4

16.7

98.8

0.7

0.3

0.2

7.9

0.05

c

20.2

21.5

83.7

9.3

2.0

5.0

9.3

1.0

12.3

14.7

76.3

13.4

4.7

5.6

5.2

0.63

5Zn/CuCr2O4 5Zn/CuCr2O4

S2,6-DMPip

Sothers

Rate / 10-6 mol (gcat)-1 s-1 2,6-DMP

d a

Conversion (%)

b

Others include 2,5-DMP and 5-HMP. Rate of 2,6-DMP and 6-HMP measured with respect to glycerol conversion and their respective selectivity. cCrude glycerol composition: 9.8 mmol glycerol +9.8 mmol 1,2-PDA + 0.2 µmol KOH+ 0.3 mmol CH3OH + 198 mmol H2O. Crude bio-glycerol received during biodiesel production from Jatropha oil (Jatropha Curcas) that contained about 13 wt% of glycerol, 5 wt% KOH, 7 wt% NaCl and ∼75 wt%H2O. d

Using a simulated bio-glycerol, the Zn modified sample showed a rate of 9.3 µmol (gcat)-1 s-1. Whereas with the glycerol mixture obtained from a bio-diesel plant, the Zn/CuCr2O4 demonstrated reduced 2,6-DMP rate of about 5.2 µmol (gcat)-1 s-1. The drastic fall in 2,6-DMP rate could be explained due to KOH and NaCl present in the feed, which reacts to block the surface active sites. But the selectivity towards 2,6-DMP is well maintained (~ 76.3%). These results however further supported from the surface mechanism which suggests that presence of high proportion of acid sites leads to 2,6-DMP and a relatively lower acid sites or a high proportion of basic sites leads to 6-HMP. Since the crude glycerol contains large amount of KOH; the surface basic sites may be affected or enhanced as a result the 6-HMP selectivity is higher compared to the clean glycerol as a substrate. The decrease in activity can also be due to the presence of large excess of NaCl which seems to 15 ACS Paragon Plus Environment

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block the active sites during the course of reaction. In our earlier tests we haven’t found the influence of other impurities other than KOH and NaCl such as methanol on the rate of 2,6DMP or 6-HMP.21 The influence of gas hourly space velocity (GHSV) on conversion of glycerol and 1,2-PDA is studied in the range of 48.3 to 64.4 mL gcat-1 s-1, at 375 °C (Table S1). Upon increasing the space velocity both the glycerol and 1.2-PDA conversions are decreased which is probably due to reduction in residence time of substrates. At lower GHSV, catalyst particles were partially wetted; therefore, there would be direct contact of reactant and catalyst surface as result an increase the reaction rate is observed. However, the selectivity of 2,6-DMP is maintained around 90% even at very high space velocity. DISCUSSION The high proportion of 2,6-DMP on metal doped CuCr2O4 samples can be explained by the primary reaction step i.e. dehydration of glycerol to hydroxyacetone (HA) that occurs on surface acidic sites and subsequent cyclocondensation of HA with 1,2-PDA to produce 2,6-DMP (Scheme S1). The higher catalytic performance of 2wt%Zn/CuCr2O4 may be explained by higher number of acid sites present on the catalyst surface (Figure 7). Hence the nature of acidic sites (Lewis and Brønsted) on M/CuCr2O4 is examined by pyridine adsorbed DRIFTS (Figure 7). The DRIFTS data indicated a high proportion of LAS on 2wt%Zn/CuCr2O4 compared to CuCr2O4 sample (Figure 7). From our earlier mechanistic studies it is inferred that, the basic sites are expected from the reduced copper species upon exposure to steam at and above 300 °C.21 However, the surface hydroxy groups on the catalyst surface may be undergo condensation to form coordinatively unsaturated Mn+−O2- species, during the reduction at 450 °C; which will be converted to Mn+−OH− by the adsorption of HCOOH.39 The HCOOH DRIFT spectra showed



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very strong band at 1620 cm-1 over 2wt%Zn/CuCr2O4 which is very weak on CuCr2O4 although an equimolar dose of HCOOH is injected (Figure 8). Addition of Zn to CuCr2O4 resulted in slightly enhanced metallic copper surface area of the catalyst (Figure 9).39 The enhanced Cu metal surface area can be explained by the interaction of Zn particles with surface chromium species to form ZnCr2O4, as a result of it segregated copper species might have formed on the catalyst surface. Formation of ZnCr2O4 is thermodynamically more favourable than the zirconium chromite and the magnesium chromite. A straight line between TOF and SCu should be obtained if the catalyst activity is only dependent on SCu.41 However, in the current study there is a general trend of increasing TOF as the SCu is increased, this further indicates that catalyst activity is only related to SCu (Figure S3). The N2O uptakes measured for the various loadings of Zn revealed that the 5wt%Zn loaded CuCr2O4 sample displayed enhanced Cu metal surface area (Table S2). Further increase in Zn loading leads to a decrease in surface Cu metal area. The TEM images of calcined CuCr2O4, 2wt%Zn/CuCr2O4, 2wt%Zr/CuCr2O4 and 2wt%Mg/CuCr2O4 samples are presented in supporting information (Figure S6). It can be seen that mixed oxides aggregate together and form agglomerates with an average particle size of 17 nm, 12.7 nm, 20.7

nm

and

23.4

nm

in

CuCr2O4,

2wt%Zn/CuCr2O4,

2wt%Zr/CuCr2O4

and

2wt%Mg/CuCr2O4 samples respectively (Figure S6 a-d), which are in close correlation with the XRD results (Figure 1A) and crystal sizes measured by Scherrer formula (Table 1). The dehydrocyclization activity data over various Zn loaded CuCr2O4 (2wt%, 5wt%, 7wt%, 10wt%) indicated that the 2,6-DMP rate is slightly increased from 13.5 µmol gcat−1 s−1 to 20.5 µmol gcat−1 s−1, upon increasing the Zn loading (from 2 to 5wt%). Further increase in Zn loading leads to decrease in the rate of 2,6-DMP to 7.9 µmol gcat−1 s−1 at a Zn loading of 10wt%. Although reaction carried with simulated crude glycerol mixture that contained KOH and methanol (expected from the bio-diesel plants); the 5wt%Zn/CuCr2O4 showed promising


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activity (Table 3). When we have used the crude bio-glycerol procured from biodiesel production plant Jatropha oil (Jatropha Curcas); a drastic decrease in the conversion and selectivity is observed and the rate of 2,6- DMP is decreased by four times. The drastic fall of 2,6-DMP rate is due to high amount of KOH present in the crude bio-glycerol (Table 3). The stability of the CuCr2O4 and 5wt%Zn/CuCr2O4 catalyst was studied for a period of 26 h at a GHSV of 40.2 mL gcat-1 s-1. The rate of 2,6-DMP over CuCr2O4 is dropped from 8.7 µmol gcat−1 s−1 to 5.2 µmol gcat−1 s−1 after 10 h of continuous operation and maintained almost constant activity for another 16 h (Figure S4). Whereas, the 5wt%Zn/CuCr2O4 showed decline in the 2,6-DMP rate in the first 8 h and showed stable activity of 20.5 µmol gcat−1 s−1 (Figure S4). These results thus suggested that Zn not only increasing the dehydrocyclization activity of CuCr2O4 but also stabilizing the catalyst which showed a stable activity. The pyridine adsorbed DRIFT spectra revealed the presence of higher number of Lewis acid sites on 2wt%Zn/CuCr2O4 sample (Figure 7). In the comparative analysis the Mg modified CuCr2O4 showed slightly high glycerol conversion than the Zr doped sample. However, a slightly higher rate of 6-HMP is observed over Mg modified CuCr2O4 (Table 3). This could be explained possibly due to a higher proportion of basic sites on the catalyst surface. The CHNS (Table S3) analysis showed about 2-3% of carbon deposition over the catalysts recovered after 6 h of continuous operation, indicating that there is not much carbon loss even after a prolonged period of time.20 The XRD analysis of the CuCr2O4 and 5wt%Zn/CuCr2O4 catalysts recovered after 26 h reaction time reported in Figure S5. The normalized XRD spectra confirm the Cu0 and CuCr2O4 species. The crystal size of metallic Cu and CuCr2O4 species are increased to 16.4 to 15.3 nm respectively, compared to their fresh form (Table 1). These results thus emphasized that fall in dehydrocyclization activity of CuCr2O4 may be due to increase in crystal size of Cu. In contrast the used 5wt%Zn/CuCr2O4 showed broad diffraction lines than CuCr2O4 used sample. The extent of



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fall in the rate of 2,6-DMP is lower on 5wt%Zn/CuCr2O4 compared to the CuCr2O4. Hence it can be concluded that introduction of Zn is favourable to improve the stability of CuCr2O4 catalyst and prevent it from sintering of surface copper species. CONCLUSIONS XPS showed addition of Zn to CuCr2O4 enhanced the number of surface copper metallic sites. TPD of NH3 indicated the increase in weak to moderate acid sites in the presence of Zn on the CuCr2O4 surface. The introduction of Zn up to 5wt% increased the dispersion of copper and the Cu0 surface area is observed from N2O titration studies. The Zn modified CuCr2O4 not only increased the activity but also improved the catalyst stability. The pyridine IR studies exemplified that a high ratio of Lewis acid sites on Zn/CuCr2O4 can be a reason for the high activity of the catalyst compared to CuCr2O4. The HCOOH DRIFT spectra revealed that strong basic sites are responsible for the dehydrogenation of 2,6dimethylpiperazine to form 2,6-DMP. Finally it can concluded that a pair of weak to moderate Lewis acid and strong basic sites are required for the selective synthesis of 2,6DMP an important compound in the agro and food industry by the utilization of crude bioglycerol obtained from the bio-diesel plant. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: The catalyst preparation, experimental analysis, catalytic activity discussed using adsorption and spectroscopy techniques like BET-SA, XRD, TPR, FTIR, XPS, NH3 TPD, pyridine adsorbed and formic acid adsorbed DRIFT spectroscopy. Author Information Corresponding author 19 ACS Paragon Plus Environment

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* E-mail address: [email protected], Tel.:+91-40-27193510, Fax: +91-40-27160921. Notes The authors declare no competing financial interest.

Acknowledgments The authors AV sincerely thank DST New Delhi for funding through India-Australia (AISRF program). One of the authors VK thanks the CSIR New Delhi for the award of fellowship. References 1.

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Zn-Modified CuCr2O4 as Stable and Active Catalyst for the Synthesis

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