Fabrication of Au nanoparticles supported on one-dimensional (1D

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Fabrication of Au nanoparticles supported on onedimensional (1D) La2O3 nanorods for selective Esterification of Methacrolein to Methyl Methacrylate with Molecular Oxygen Bappi Paul, Rubina Khatun, Sachin Kumar Sharma, Shubhadeep Adak, Gurmeet Singh, Dipak Das, Nazia Siddiqui, Sonu Bhandari, Vedant Joshi, Takehiko Sasaki, and Rajaram Bal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05291 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Sustainable Chemistry & Engineering

Fabrication of Au nanoparticles supported on onedimensional (1D) La2O3 nanorods for selective Esterification

of

Methacrolein

to

Methyl

Methacrylate with Molecular Oxygen Bappi Paul,† Rubina Khatun, †Sachin K. Sharma,†Shubhadeep Adak,

†

Gurmeet Singh,

†

Dipak Das, † Nazia Siddiqui, † Sonu Bhandari, †Vedant Joshi, †Takehiko Sasaki‡ and Rajaram Bal†* *†Catalytic

Conversion & Processes Division, CSIR-Indian Institute of Petroleum Dehradun

248005, India. ‡Department

of Complexity Science and Engineering, Graduate School of Frontier Sciences,

The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561 *Corresponding authors. Fax: +91 135 2660202; Tel: +91 135 2525917 *E-mail: [email protected]

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ABSTRACT Here we demonstrate a simple and cost effectivesynthetic route for selective esterification of aldehydes with high productivity in the presence of molecular oxygen(O2) on gold supported lanthanum oxide (Au/La2O3) nanoparticle catalyst. Au nanoparticles with sizes of 2–7 nm supported on 1D La2O3 nanorods with diameters between 20 and 50 nm was synthesized by room-temperature surfactant-assisted single-step preparation method. The as-synthesized catalyst was thoroughly characterized by powder XRD, SEM, HR-TEM, H2-TPR, XPS, TGA/DTA, FTIR, BET, EXAFS and UV-visible spectroscopy. This prepared nanostructured catalyst was found to be highly effective in liquid phase synthesis of methyl methacrylate (MMA) via direct oxidative esterification of methacrolein (MA) with high turnover number of ∼1136. The effect of various reaction controlling parameters like reaction temperature, pressure and time of reaction were investigated and was studied. High methacrolein conversion of 89% and with high methyl methacrylate selectivity of 98% was attained without the use of any external additives. The synergistic effect between the surface AuNPs and La2O3 nanorods plays an important role towards the activity of the catalyst. KEYWORDS: Esterification; Methyl methacrylate; Nanorod; High selectivity. INTRODUCTION Esterification, one of the most essential and widely recognized organic synthetic routefrom small scale laboratories to industries because of their versatility and importance as a useful classes of compounds in fine and bulk chemicals.1,2 In recent years, a great deal of research effort has been devoted for the direct oxidative esterification of aldehydes with alcohols because of its cost-effective and environmental benefits on a commercial scale for the synthesis of esters.3 One of the most representative example is the aerobic oxidative coupling of methacrolein(MA) with methanol to form methyl methacrylate(MMA).4-7 Methacrylates an important


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building blocks for the production in textiles and plastics industries, also widely used for polymer dispersions for paints and coatings, and as an intermediates in the perfume industry.8,9 Currently, Acetone cyanohydrin (ACH) method is the industrially adopted process for the production of methyl methacrylate.10,11 However, this process has several drawbacks including utilization of toxic and corrosive hydrocyanic and acid raw material which are environmental unfavourable and long technological route (Scheme 1(a)). Therefore, many efforts have been ardent in developing a green an environmentally friendlymethod for the production of methyl methacrylate. Direct esterification of methylacrolein with methanol (CH3OH) in presence of molecular oxygen, over heterogeneous catalysts which avoids the use of toxic and corrosive compounds is highly desirable toward green, sustainable and economic processes (scheme 1(b)). O

NaOH, HCN H2SO4

CH3OH

NH3+HSO4-

O

+ CH3OH

(a)

O

O

O

+ NH4HSO4

O2

O

Catalyst

+ H2O

(b)

O

Scheme 1. Synthetic approaches for preparation of methyl methacrylate(a) Acetone cyanohydrin method and (b) oxidative coupling method. Recently, several catalytic systems have been developed counting both homogeneous and heterogeneous catalysts for oxidative esterification of MA.4-7,12 Preferably, a synthesis process using heterogeneous catalysts that does away with molecular oxygen for esterification of MA would be more environmentally friendly due to its easy separation and recycling. Among all the noble metals palladium and gold are the most widely used active metal for the oxidative esterification of alcohols and aldehyde with molecular oxygen.13-19 Supported Pd catalysts was proven to be an important 3 ACS Paragon Plus Environment


breakthrough in oxidative esterification of methacrolein.10,20 However, the present synthetic approaches suffers from numerous limitation such as low selectivity of MMA,use of liquid base additives and instability of α,β-unsaturated aldehydes. However, supported gold-based catalysts has attracted substantial interest because of their distinctive catalytic behaviours in various types of chemical reactions in term of selectivity.21,23 It is well documented that for a metal supported catalyst, the catalytic behaviour depends on the interactions between metals and supports which significantly govern its catalytic performances beside the property of the support, chemical state and size of Au particle. A wide variety of nanostructured materials like TiO2,24,25 Al2O3,26 CeO2,27-29 SiO2,30,31 Ga2O3,32 hydrotalcite33 and polymers34 etc., have been explored as supports for Au-based catalysts for its direct oxidative esterification of primary alcohols and aldehyde to methyl esters. However, in most cases, these supported catalysts suffers from various problems such as addition of liquid base additive, use of bulk amount of methanol, high energy consumption, generation of by-products, long reaction time and agglomeration of active metal (Au NPs) which lead to rapid drop/decrease in the catalytic activity during the course reactions. All these aforementioned limitations of literature reported provide enough scope for improvement in synthesis of a new catalytic system capable of exhibiting very strong performance. Taking into account the above scenarios, we report herein an in-situ synthesis of Au/La2O3 with controlled sizes and shapes using CTAB (cetyltrimethylammonium bromide) as surfactant and studies of their catalytic activity as an efficient and highly selective catalyst for esterification of MA with methanol to MMA with molecular oxygen and in absence of any additive. Moreover this supported catalyst was also explored for its catalytic performance in esterification of ethylene glycol.


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EXPERIMENTAL Catalyst preparation Gold supported Lanthanum oxide catalyst was prepared by surfactant assisted hydrothermal method by adapting our own synthesis method.35-36 The main advantage of this procedure is that the nanostructured material can be synthesized in large amount (up to 30 g) and highly reproducible. An amount of 13.02 g of lanthanum(III) nitrate hexahydrate (La(NO3)3•6H2O) and 0.345 g of chloroauric acid tetrahydrate (HAuCl4) were separately dissolved in a least amount of deionised water, where the coloured of the solution turns yellow. The resultant metal salt solutions were then mixed

together

under

stirring

condition.

Subsequently,

1.8g

of

cetyltrimethylammonium bromide(CTAB) was dissolved in an ethanol-water mixture (10 ml distilled water and 2 ml ethanol) and was added to the solution of premixed metal salt. The pH of the resultant solution mixture was raised to 8 by slow addition of ammonia (NH3), followed by addition of 0.1 g of hydrazine (NH2NH2), where the colour of the solution changed from yellow to reddish. The reaction mixture was stirred for 3 h and the resultant mixture was transferred to a stainless steel Teflon lined autoclave and kept in an oven maintained at 180 °C for 12 h. The material was collected and washed with water followed by ethanol and dried at 80 °C for 12 h. The as obtained solid powder material was calcined in a quartz tube inside a tube furnace for 4 h at 800°C in the presence of helium (10 ml min-1) to obtain powder of Au/La2O3. The loading of the gold nanoparticle was confirmed by ICP-AES analysis. The catalyst was also synthesized by impregnation method and co-precipitation methods.



Catalytic test of Aerobic esterification Oxidation esterification of methacrolein was performed in a 100 mL PTFE lined stainless-steel container in a high-pressure reactor furnished with a magnetic stirrer. The reactor was charged with 0.1 g of Au/La2O3 catalyst, 1.04 ml of methacrolein and 4 ml of methanol. Before starting the reaction, the reactor was purged with oxygen flow (10 ml/min) for two times, and each time was held for 3 min. The reaction vessel was pressurised with oxygen gas with a relative pressure of 0.2 MPa. and the reaction solution mixture was stirred (800 rpm) for 2 h at 70 °C. Small aliquots of the reaction mixture were taken carefully periodically for GC analysis. At the end of the reaction, the reactor was quickly cooled down to room temperature and the oxygen was depressurized slowly. The catalyst were separated by centrifugation and the products were analysed by Gas Chromatograph (GC, Agilent 7890B) connected with a HP-5 column. The Carbon balance for utmost of the experiments were found to be 98 ± 3%. A schematic figure of a complete reaction setup is presented at supporting information (Scheme S1). RESULT AND DISCUSSION Catalyst characterization

Figure 1. XRD patterns of (a) La2O3, (b) 1% Au/La2O3, (c) 2% Au/La2O3, (d) 3% Au/La2O3, (e) 2% Au/La2O3 (spent catalyst).(*-Au(200)) 6 ACS Paragon Plus Environment

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The amount of metallic gold present in the Au/La2O3 catalyst was projected by ICPAES. The crystalline phase and the nature of crystallinity of the sample was identified by powder X-ray diffraction pattern. The powder XRD pattern of La2O3 with different loading of Au NPs on lanthanum support is shown in Figure 1. The synthesized La2O3 nanoparticles (Figure 1a) displayed the peaks at 2θ values of 26.1°, 29.1°, 29.9°, 39.5° and 46.0° which confirms the formation of hexagonal structure of La2O3 (JCPDS File no. 83-1344). The XRD patterns showed the typical diffraction lines exclusively with the maximum intensity peak at a 2θ value of 29.9°, which correspond to the (011) plane of La2O3. No latticeplane of metallic Au or any other oxides of La2O3 were identified by XRD for 1 % and 2% Au/La2O3 (Figure 1(b,c)), indicating the very small Au-crystallites are highly dispersed over the La2O3 support. Moreover one additional peak at 44.3° corresponds to metallic Au(200) was observed for 3 % Au/La2O3 and that of spent catalyst which match well with the literature reported values (JCPDS File no. 89-3697) (Figure 1d and e). From the XRD pattern it is confirmed that the phase of the metallic Au remain intact even after five successive cycle. The crystallite size of the synthesized Au/La2O3 catalysts were calculated using Scherrer equation and the results are summarized in Table S1 in Supporting Information.

Figure 2. (a,b) SEM images and (c) SEM-EDS mapping of 2% Au/La2O3 catalyst. The scanning electron microscopy was performed to determine the morphology of as prepared Au/La2O3 catalysts. The SEM images of the catalyst shows rod like shape of 7 ACS Paragon Plus Environment


size 20-50 nm (Figure 2(a,b)). The EDS pattern disclosed the presence of lanthanum, oxygen and gold where no signature of impurities was detected in the spectra (Figure 2c). High resolution transmission electron microscopy (HRTEM) analysis was carried out to check the crystallite size of the particle and dissemination of the gold nanoparticles over nanocrystalline La2O3 (Figure 3). TEM image of La2O3 showed rod like morphology of size 20-50 nm where the spherical Au nanoparticle of size 2-7 nm are in close contact with each other. The lattice fringes analysis were found to be separated by 0.29 nm and 0.20 nm, probably due to (011) and(200) lattice plane of La2O3 and metallic Au respectively.

Figure 3. (a, b) TEM images (c) HRTEM image (lattice fringes) and (d) ED pattern of Au/ La2O3 nanoparticles.


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We did not observe any characteristic evidence for (111) plane neither in XRD nor in TEM (d spacing) for Au NPs. We believe that the exposure of (200) plane may be depend upon the synthesis method, as well as arrangement of active metals with support during the crystallization process. The statistical analysis of the La2O3 nanorod shows that maximum of the La2O3 nanorod are of size 45 nm. Moreover, the TEM image of the spent catalyst shows almost same shape and size of the catalyst even after five reuses. The dispersion of gold on the nanocrystalline La2O3 was confirmed by the elemental mapping of the nanostructured catalyst which demonstrated the homogenous dispersion of Au on nanocrystalline La2O3 support (Figure 4).

Figure 4. (a) STEM and STEM-elemental mapping of (b) La, (c) O and (d) Au in Au/La2O3 nanorod catalyst. The reducibility of the catalyst is strongly affected by loading of Au. H2-temperatureprogrammed-reduction (TPR) was employed in order to compare the reduction 9 ACS Paragon Plus Environment


behaviour and metal-support (Au–La) interactions in the catalysts as shown in Figure 5. The as-synthesized nanomaterial showed a broad high temperature reduction peaks around 680°C. It was also observed that with the loading of gold on the support the reduction temperature sifted to lower temperature. This observation demonstrated that small amount of Au enhanced the reducibility of support which confirm a strong interaction between the active metal (Au) and the support (La2O3). This phenomenon possibly takes place due to weakening of the surface oxygen bond in occurrence of the noble metal or the hydrogen(H2) spill over from the metal to the support.37-39

Figure 5. TPR profile of Au/La2O3 nanoparticles with different loading. The valence state of Au species in 2% Au/La2O3 was estimated by X-ray photoelectron spectroscopy (XPS). From the XPS spectrum of the prepared 2% Au/La2O3 catalyst we can see clearly see the presence of La, O, and of metallic Au (Figure 6).


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Figure 6. XPS spectra of (a) La 3d, (b) O 1s, and (c) Au 4f spectrum of Au/La2O3 catalyst. The XPS spectrum of Au 4f7/2 and Au 4f5/2 with binding energy of 83.5 and 87.1 eV respectively are shown in Figure 6c. The La 3d5/2 and La 3d3/2 spectra attributed to the binding energy at 834.3 and 851.1 eV with the shake-up peaks located at 838.0 and 851.1 eV, respectively, consistent with the reported results of La3+ in a La(III) oxidation state (Figure 6a).38,40 The O 1s spectrums could be de convoluted to three different peaks at the binding energies of 530.2, 530.9, and 532.3 eV, corresponding to the lattice oxygen (Olatt), hydroxyl oxygen (Ohyd) and physically adsorbed oxygen (Oads), respectively (Figure 6b).41 The corresponding XPS analysis of the spent catalyst confirms that the oxidation state Au 3d binding energy remains same during the catalysis (Figure S1 in Supporting Information).The electronic states of gold nanoparticles and the strong metal-support interaction (SMSI) was further explored, for which we have synthesized a series of supported Au catalysts with different sizes of Au NPs (2-50 nm) and metal-support interaction was studied. The different binding energies of Au 4f7/2 with different size of Au nanoparticles supported on onedimensional (1D) La2O3 nanorods are shown in Figure S2 in Supporting Information. The binding energy of all the nanoparticles with different size showed two separate peaks at 83.5 eV and 87.1 eV corresponds to Au 4f7/2 and Au 4f5/2. The Au 4f7/2 peak slightly shift to lower binding energies was observed for the sample having particle size of 2 and 5 nm. Which confirmed that there is a strong metal-support interaction 11 ACS Paragon Plus Environment


between the gold nanoparticles and La2O3 nanorod and efficient electron transfer from the conduction band of La2O3 nanorods and gold particles occurred.5,12 Similarly, the XPS spectra of Au 4f7/2 loaded on different support (CeO2, Al2O3, ZnO, MgO and La2O3) are shown in Figure S3 in Supporting Information. The XPS spectra shows similar pattern with slight negative shift towards lower binding energy for Au/ CeO2 and Au/La2O3 indicating a strong interaction between the active metal and the support which is also confirmed from the HR-TEM images. The electronic state of Au may be affected due to air (as no hydrogen is used in the process of synthesis) in particular for the Au/CeO2 which may be one of the reason for forming high interaction between metal and support. Moreover, Au/CeO2 is widely studied as catalysis due to its strong metal-support interaction (SMSI) properties between Au and CeO2.42,43 The uniformly distributed Au Particles over the CeO2 and La2O3 support indicating that the catalyst shows better metal-support interaction. Whereas no leaching of the Au after the catalytic activity is also the indication of the presence of strong interaction between the metal and support. In our present study the high dispersion of very small sized Au NPs supported on La2O3 nanorod leads to the strong metal-support interaction (SMSI) and easy accessibility of reactants on active catalytic sites for better catalytic activity. The metallic Au0 was identified to be active for oxidative coupling of aldehydes with methanol.3In comparison with that for bulk metallic Au0 (84.0 eV for Au 4f 7/2) reported in the literature, however, the binding energy (BE) value shifts to 83.5 eV for 2% Au/La2O3, indicative of the presence of the strong metal support interaction (SMSI).5,15On the other hand, the metal-support interaction (SMSI) of a material is highly dependent on its morphology and architecture.44,45In particular, 1D structures have more advantages such as large specific surface area, abundant interconnected pores for efficient mass transport/ diffusion, and rich accessible active sites.46,47Jiang 12 ACS Paragon Plus Environment

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et.al has also studied the surface area of 0D, 1D, 2D and 3D La2O3 where they also have found that 1D La2O3 shows higher surface area of 18.0 m2g-1 compared to 0D, 2D and 3D with surface area 7.2, 12.0, and 9.5 m2g-1 respectively.48Therefore, it is believed that La2O3 nanorod served as efficient platform to encode Au NPs over the rod which leads to easy accessibility of active catalytic sites with the reactants for the better catalytic activity.

Figure 7. FTIR diagram of (a) CTAB (b) uncalcined Au/La2O3 nanoparticle and (c) that of calcined Au/La2O3 nanoparticle catalyst. The surface-coordinating organic surfactant (CTAB) on the uncalcined material can be analysed by the FTIR studies (Figure 7). A comparison of the FTIR-spectra of uncalcined Au/La(OH)3 precursor with that of pure CTAB was studied, which confirmed the appearance and interaction of the template molecules with the La–O surface. For pure CTAB and uncalcined, the stretching vibrations of the ammonium group appeared at 1486 and 1472 cm-1 (νasym(CH3–N+)). The FTIR spectrum shows two sharp peaks at 2917 and 2842 cm-1, corresponds to the asymmetric and symmetric 13 ACS Paragon Plus Environment


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C–CH stretching vibrations of the methylene chains. It can be inferred that mutual interactions between CTAB and the surface of Au–La have taken place. An intense and sharp band at 3618.4cm−1 due to stretching and bending vibrations of O–H in La(OH)3. The bands presence at 3543cm−1 and 1631.3cm−1 can be attributed to the O– H vibration in absorbed water on the sample surface. When the material was calcined at 780°C all those typical frequencies of the surfactant was absent which confirm that the entrenched CTAB moieties was completely detached during the calcination. However, the absorption band at 656 cm−1 was observed even after calcination which refers to the characteristic peak of La-O. Moreover, the SEM-EDS spectra of the as synthesized catalyst (fresh catalyst) does not exhibit any peaks for C, N or Br, which suggest the elimination of the surfactant (CTAB) after calcination. The

surface-coordinating

organic

surfactant

CTAB

molecules

on

the

uncalcinedmaterial (Au/La(OH)3) was further confirmed by the TGA and DSC measurements. It can be seen from the curve (Figure S4 in Supporting Information) that the weight loss take place in five steps decomposition pathway. The first step the weight loss of about 5 % from 30-150°C, which may be due to the removal of physically adsorbed moisture in the precursor. The second, third and fourth step from 150-500 °C the weight loss of 25.3 % in indicate the decomposition of lanthanum oxide carbonate to La2O2CO3 and combustion of CTAB from the surface. In the last step the loss of weight ~7 % in the temperature range from 500-723 °C is due to decomposition of La2O2CO3 to La2O3 pure phase. The UV-Vis DRS absorption spectra of La2O3 and Au/La2O3 (after calcination) is shown in Figure S5 in Supporting Information. No absorption peak was observed for La2O3, however an absorption peak at 549 nm was observed which indicate the presence of metallic Au nanoparticles,


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which can be concluded that the presence of metallic Au nanoparticles supported over lanthanum oxide nanorod. To determine the textural properties such as surface area, pore volume and pore size of the calcined Au/La2O3 with different percentage of Au loading N2 physisorption measurements was conducted. The N2 adsorption–desorption isotherms of the samples possess high specific surface area and pore volume (Figure 8).

Figure 8. (a) N2 adsorption-desorption isotherm and (b) pore size distribution curve of Au/La2O3 nanoparticles with different loading. It is obvious that the loading of Au on the support, we observed relatively difference in the surface area, pore size and total pore volume (Table 1). 1% Au/La2O3showed the largest surface area of 36.3 m2 g-1 with abundant small pores, 3% Au/La2O3 with less pores showed the smallest surface area as 20.4m2 g-1 with largest pore diameter (30.7 nm) and the widest pore size distribution. Table 1. Textural properties of different loading gold on Lanthanum support.

1% Au/La2O3

surface area (m2/g) 36.3

Pore Volumea (cm3/g) 0.3

Pore sizeb (nm) 16.2

2% Au/La2O3

29.9

0.2

19.2

3% Au/La2O3 20.4 b volume. Average pore diameter

0.2

30.7

Catalysts

a Pore



Page 16 of 40

Figure 9.k3-weighted Fourier transform (amplitude (solid curve) and imaginary part (dotted curve)) of Au LIII-edge EXAFS spectra of (a) fresh catalyst; (b) spent catalyst. Observed data: thick curves; fitting data: thin curves. The local structure of Au-atom of the fresh and spent Au/La2O3 catalyst was determined by Au LIII--edge extended X-ray absorption fine structure (EXAFS) spectra. Table 2 listed the structural parameters of the Au LIII--edge EXAFS data. The Au-Au bond distances in fresh and spent catalysts are 2.863 nm and 2.846 nm, respectively and the absence of Au-O bond specify the formation of Au(0) in both form of the catalyst (Figure 9). Table 2 EXAFS curve fitting parameters at Au-LIII edge of 2% Au–La2O3 Sample

Path

R (101nm)

CN

DW (105nm2)

∆k(10n m-1)

∆R (10-1nm) ∆E0 (eV)

Rf (%)

Fresh Catalyst

AuAu

2.863±0. 007

7.0±1.0

7.0±0.9

3 – 12

1.2 – 3.2

2.8±1.4

1.56

Spent Catalyst

AuAu

2.846±0. 007

6.6±0.9

6.6±0.8

3 – 12

1.0 – 3.2

2.8±1.4

1.68

R = bond length; CN = co-ordination number; ∆K: the range of wavenumbers used in the fitting; DW: Debye–Waller factor; ∆R: the range of bond distances used in the fitting; ∆E0: the shift of the edge-position; Rf: reliability factor; S0 2 = 0.74, S0 2: amplitude reducing factor.


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In agreement with the XPS spectra EXAFS fitting results also suggest that there is no significant change in oxidation state of gold in Au/La2O3 during esterification by molecular oxygen.

Scheme 2. Graphical presentation for formation of Au–La nanocomposites with various morphologies To explore the formation mechanism of 1D Au–La nanorods, a series of time-dependent experiments were performed. The experiment was performed both in absence and presence of CTAB and the final products were subjected to TEM analyses. The mechanism of formation of Au supported 1-D La2O3 nanorods can be explained on the basis of surfactant (CTAB) assisted nucleation growth study. In presence of surfactant, surface tension of the solution decreases which lowers the surface energy required to generate a new phase i.e. nucleation. According to crystallization nucleation – growth plot of LaMer, rate of nucleation increases with decrease of surface energy.49 Gibbs- Wulff theory explain equilibrium shape of crystal has minimum surface energy.49 When surface energy is isotropic the equilibrium shape will 17 ACS Paragon Plus Environment


be spherical because spherical shape has minimum surface area. 1- D nanostructured is formed when surface energy is anisotropic. To further explore the role of CTAB in the formation of 1-D nanorods of La2O3, a series of time dependent experiments with CTAB were performed (Scheme 2). The experiment was also performed in the absence of CTAB by keeping rest of the procedure same and final product was subjected to TEM analysis. We observed that without CTAB, particles are agglomerated with different shape and sizes. However when CTAB was added and the mixture was kept for 3 h in autoclave, (followed by calcination), very few rods were generated and the rest of sample was full of agglomeration, showing that the rods grew slowly from these aggregates. When the aging time was increase to 8 h, some agglomerate species were also observed in the TEM images but when aging time prolonged to 12 h, complete sample was converted into rod like structure. There for 12 h aging time was proved to be best optimum resident time for the growth of rod type morphology. The TEM images of the sample exhibited that almost homogeneously 1-D nanorods formed with diameter of 2050 nm grown in a definite direction. Moreover, when aging time further prolonged to 24 h, further crystal growth occurred and these nanorods are fused together by following Ostwald ripening process and formed larger, indefinite- shaped aggregates.50 It is well known fact that CTAB is a cationic surfactant having (C16) aliphatic carbon chain, which ionise completely into water. La(NO3)3.6H2O and HAuCl4 will ionised in the aqueous solution and then hydrolysed in the alkaline medium to form their corresponding hydroxides. Surfactant, Cetyl trimetyl triammonium bromide (CTAB) will form (CTA) + in the solution and a co-operative self-assembly between lone pair of oxygen atom and ionic part of the surfactant. When surfactant concentration become higher than critical micelle concentration, surfactant ions will start absorbing into the surface of La and form spherical micelles.51 It is well known phenomena that the curvature of ionic micelles can be turned into rod like 18 ACS Paragon Plus Environment

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micelle from spherical micelle.52 In our preparation method, the spherical micelle of La-OCTA were transformed into rods like micelle as a result of attractive electronegative interaction between CTA (cetyl trimethylammonium) stabilized lanthanum ions La-O-CTA and positively charged Au+ ions by anisotropic growth through selective adsorption into particular phase of lanthanum.51 Thus, the decrease in surface energy of lanthanum crystal in one direction will results the formation of 1-D nanorods. Catalytic esterification reaction After successful synthesis and characterization of nanostructured Au/La2O3, we have investigated the catalytic activity for esterification of methacrolein with CH3OH to methyl methacrylate in absence of any other additives. The catalytic esterification of methacrolein by methyl alcohol was evaluated over different loading of Au catalysts. The results showed that the 2% Au loading over La2O3 supp ort afforded highest MA conversion and MMA selectivity with acetal as the majorby-products (Figure 10).

Figure 10. Effect of loading on esterification of methacrolein to methyl methacrylate with molecular oxygen (a) methacrolein conversion and (b) methyl methacrylate selectivity. Reaction conditions: ethanol/MA molar ratio, 8:1; oxygen pressure, 0.3 MPa; reaction temperature, 70 °C, reaction time 2h. 19 ACS Paragon Plus Environment


Notably, 2 % Au/La2O3 shows very high MMA conversion of ~89 % with 98 % selectivity after 2.0 h of reaction time (Table 3). It is important to mention that the MMA selectivity remain almost same throughout the whole process of reaction. Taking into account the efficiency of 2 % Au/La2O3 nanostructured catalyst a sequence of catalytic experiments was performedto study the activity behaviour of MA conversion atdifferent temperature, pressure and time. The catalyst exhibits an MA conversion of 89% at 70 °C with the MMA selectivity of 98%.

Figure 11. Effect of temperature on esterification of methacrolein to methyl methacrylate with molecular oxygen. [ ] Conversion of methacrolein, [ ] selectivity of methyl methacrylate [ ] Yield of methyl methacrylate, Reaction conditions: methanol to MA molar ratio is 8:1; oxygen pressure is 0.3 MP and time 2h. It was observed that with increase of temperature from 50°C to 90°C the conversion of MA increases steadily but the selectivity of MMA decreases beyond 70°C due to the formation of acetal as the major by-product (Figure 11).The results also showed an increasing trend in methacrolein conversion with time (Figure 12). After 1.5 h of 20 ACS Paragon Plus Environment

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reaction time, we observe a sharp increase in the conversion of MA and selectivity of MMA. The results designated that after 2 h of reaction, the maximum selectivity of methyl methacrylate (∼98%) was achieved and further increase in time the selectivity of MMA is dropped.

Figure 12. Effect of reaction time on esterification of methacrolein to methyl methacrylate with molecular oxygen. [ ] Conversion of methacrolein, [ ] selectivity of methyl methacrylate [ ] Yield of methyl methacrylate, Reaction conditions: methanol/MA molar ratio, 8:1; oxygen pressure, 0.3 MPa; reaction temperature 70 °C. Similarly, the role of pressure on the reaction in the range of 0.1–0.5 MPa was examined and the equivalent results are depicted in Figure 13. When we conducted the reaction at atmospheric pressure and at 70 °C, we observed very deprived conversion of MA (11%). When the pressure was increased (O2), it was examined that upto 0.3MPa pressure, both the conversion and selectivity of MA and MMA increase monotonically. But, after 0.3 MPa pressure, we noticed increase in the conversion of 21 ACS Paragon Plus Environment


MA with decrease in the selectivity to MMA,which can attributed to the circumstance that, high pressure compelled to react MA, beyond its optimization reaction level.

Figure 13. Effect of pressure on esterification of methacrolein to methyl methacrylate with molecular oxygen. [ ] Conversion of methacrolein, [ ] selectivity of methyl methacrylate [ ] Yield of methyl methacrylate, Reaction conditions: methanol/MA molar ratio, 8:1; reaction temperature 70 °C; reaction time 2h. In order to explore the individual role of Au and La2O3 in its catalytic activity, a series of experiments were conducted. A blank run without the catalyst does not show any catalytic activity under thisstandard reaction conditions (Table 3, entry 12). The Au/La2O3 nanostructured catalyst synthesized by the present method shows high yield of MMA (Table 3, entry 6). When Au/La2O3nanostructured catalyst was prepared by impregnation method, it shows deprived catalytic activity due to formation of larger particle sizes and leaching of active metals component (Table 2, entry 5).Furthermore, the Au/La2O3 catalyst showed high TON of 1136.3 for MMA production, compared to 22 ACS Paragon Plus Environment

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bare Au or La2O3 catalyst (Table 2, entry 3 and 4). The reasons for excellent catalytic activity of Au/La2O3 prepared by the present method is believed to be due to higher specific surface area and higher dispersion of very small sized Au NPs supported on La2O3 leading to easy accessibility of reactants on active catalytic sites. The loading of the Au percentage over the support has a significant effect on the catalytic conversion and selectivity. When the Au loading was 2 % (2% Au/La2O3) the catalyst showed 89 % conversion with 98 % MMA selectivity, while on increasing Au content to 3 % the MA conversion decrease to 86 % and selectivity of MMA decrease to 97 % (Table 3, entry 10 and 11). Moreover, the particle size of Au NPs can also influence the catalytic performance. To confirm the influence of gold on the catalytic performance, we have synthesized a series of supported Au catalysts with different sizes of Au NPs (2-50 nm) and their catalytic activities were also tested (Figure S6 in Supporting Information). It was observed that with the increase in the particle size from 2 to 5 nm conversion of methacrolein increases (Table S2 in Supporting Information). The reasons for increase in conversion is believed to be due to high dispersion of very small sized Au NPs supported on La2O3, leading to easy accessibility of reactants on active catalytic sites. When the particle size increases to 10 nm the conversion decreases due to formation of larger particle sizes the metal support interaction is low and the number of active Au sites present in the catalyst are also low. Further increase in the particle size the conversion and selectivity decreases. Probably, large and nonuniform sizes of these catalysts limit their accessibility towards the reacting substrates. So, it is clear that the activity of the catalyst depends upon the size of the Au particle. Therefore, nanocrystalline Au with size between 2 to 50 nm directly influences the activity of nanoparticles for esterification of methacrolein to methyl methacrylate. Using our reported procedure, we have also synthesized Au nanoparticles on various



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support like CeO2 (FigureS7 in Supporting Information), ZnO (Figure S8 in Supporting Information), MgO (Figure S9 in Supporting Information) and Al2O3 (Figure S10 in Supporting Information) to study the effect of support on catalytic conversion of methacrolein, where we found that the activity is very low (Table S3 in Supporting Information). So it can be concluded that La2O3 prepared by the present method is a good support for the gold catalyst for the esterification of methacrolein to methyl methacrylate. We strongly believe that a synergistic effect between that nanocrystalline La2O3 and 2–5 nm Au particles directly influences the catalytic esterification process. Thus, taking into account of all the reaction parameter and 2 % Au/La2O3 as a standard catalyst the scope of the reaction was explored next. The results are summarized in Table 4. It was observed from the table that the substrate with electron donating substituent promotes the esterification reaction, whereas the electron withdrawing substituent retards the formation of ester. Table 3. Activities of the different catalysts for esterification of aldehydes Entry

1 2 3 4 5 6 7 8 9 10 11 12

Selectivity SPc (%)

Catalyst

Aucom La2O3 com Auus La2O3 us Au/La2O3imp 2% Au/La2O3 nano catalyste 2% Au/La2O3 nano catalystf 2% Au/La2O3g 2 % Au/La2O3h 1% Au/La2O3 3% Au/La2O3 No Catalyst

CTb (%)

MMA

Acetal

Other

18 21 27 32 23 89

12 42 7 56 62 98

78 52 77 39 38 -

88

98

92 96 54 86 8

99 99 51 97 -

10 6 16 5 2

Yield Y Ad (%) 2.16 8.82 1.89 17.92 14.26 87.22

Turnover number (TON) 28.14 114.9 24.6 233.4 185.7 1136.3

-

2

86.24

1123.5

40 2 -

1 1 9 1 -

91.08 95.04 27.54 83.42 -

1186.6 1238.2 358.8 1086.8 -


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a

Reaction conditions: substrate (methacrolein) = 1.04 ml, methanol (4 ml), methanol: MA molar ratio, 8:1; oxygen pressure, 0.3 MPa; catalyst = 0.1 g, gold loading = 2 wt%, time = 2 h, temperature = 70 °C.b CT: conversion of methacrolein based on GC results = [moles of methacrolein reacted/primary moles of methacrolein used] × 100. c SP: selectivity of methyl methacrylate was calculated by moles of product formed/ moles of methacrolein converted; dYA (Yield of methyl methacrylate) = conversion × selectivity/100. e∼Au nanoparticles supported on La2O3. fCatalyst after 5 reuse; Au/La2O3 nanocatalyst. gWhen methanol: MA molar ratio 20:1. hWhen methanol: MA molar ratio 40:1; com = commercial; us = bare Au and La2O3 prepared using our synthesized method; imp = impregnation. Table 4. Activities of the Au/La2O3 nanostructure catalyst for oxidative esterification of aldehydes with alcoholsa Entry

Aldehyde

O

1

b

Alcohol

Product

Methanol

O

Conversion (%)/ Selectivity (%)

Yield (%)

89/98

87.2

46/97

44.6

62/97

60.1

O

79/97

76.6

O

58/98

56.8

56/97

54.3

58/97

56.2

O

78/98

76.4

O

58/99

57.4

O

72/96

69.1

54/97

52.3

O O

Ethanol

O

2

O O

n-butanol

O

3

O O

4

O

Methanol O

O

5

Ethanol O

O

Isopropyl alcohol

O

6

Me Me

O O

7

O 8

9

MeO

Cl

O

MeO

O

Methanol Cl

Cl O

11

Ethanol

MeO O

10

O

Methanol

MeO O

Me

O

Propyl alcohol

Ethanol

O O

Cl

a

Reaction conditions: aldehyde 15 mmol, Au/La2O3(Au 2 %) in alcohol (4 mL), O2 0.3 MPa) at 70 °C for 2 h. bDetermined by GC analysis was performed using tetradecane as an internal standard.



The acid-basic properties of the catalyst was also explored using NH3-TPD and CO2-TPD analysis. The surface acidity and basicity of catalysts played an important roles in the above catalytic process. The loading of Au particles changes the acid-base properties of the support. The NH3-TPD was studied to explore the surface acidic sites and the strength of the catalysts. As we can see from Figure S11 in Supporting Information the gold supported lanthanum oxide showed two decomposition peaks in TPD spectra. The decomposition below 250 °C could be attributed to the decomposition of the ammonia from the week acidic sites. On the other hand the catalyst decomposition in the range between 300 °C to 400 °C could be attributed to the decomposition of the ammonia from the medium acidic sites. It was observed that with increasing Au loading the week acid site increases. Similarly supports with different morphologies shows week to medium acidity and the data are summarised in Table S4 in Supporting Information. The CO2-TPD spectra for different Au catalysts are shown in Figure S12 in Supporting Information. All the catalyst showed the presence of week and moderate basic strength. Similar to NH3-TPD, the TPD profile of CO2-TPD shows increase in the desorption strength, showing the increasing basicity with increase in Au loading. Comparative studies show that the gold supported lanthanum oxide catalysts shows the presence of both acidic and basic sites. The presence of acidic and basic sites also influences the catalytic activity in the esterification reaction as shown in Table S4 in Supporting Information. The conversion increases with increasing Au loading. We believe both basicity and formation of SMSI, probably plays and an important role in the catalytic performance. The 2 % Au/La2O3 nanostructured catalyst exhibited the best performance owing to the presence of strong basic sites and the strong interaction between the metallic Au and La2O3, leading to an anchoring effect, which enhance the


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stability of the nanostructured catalyst and promote the oxygen mobility and activate the methanol, respectively.12 In addition to methacrolein esterification, the efficiency of the catalyst was also tested for the esterification of ethylene glycol (EG) to afford the corresponding esters. Oxidative esterification of EG was carried out in presence of methanol at 100 °C, under an oxygen (0.3 MPa) for 2 h, to obtain 97% selectivity of methyl glycolate and 19% conversion of ethylene glycol (Scheme 3). HO

O

Au/La2O3 OH

+ CH3OH

O2 ,

0.3MPa

HO

O

Scheme 3. Esterification of ethylene glycol with methanol in presence of O2 On the basis of methacrolein conversion over different loading of Au catalyst were studied to find the order of the reaction. It was observed that at different time conversion increase linearly demonstrating that the esterification of MA fellows first order rate equation (-ln(1-x) (where x is the MA conversion) (Figure S13 in Supporting Information).

Figure 14 Rate constant vs. reciprocal of reaction temperature over different loading of Au catalysts. 27 ACS Paragon Plus Environment


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Taking into account for the linear correlation between lnk and temperature of the reaction (where K is the reaction rate constant). The apparent activation energy of the oxidative esterification of MA determined from Arrhenius equation is found to be about 18.75 KJ/mol for 1% Au/La2O3, 15.9 KJ/mol for 2% Au/La2O3 and 16. 54 KJ/mol for 3% Au/La2O3 respectively (Figure14). These aforementioned results further confirmed that 2% Au/La2O3 shows lower activation energy for oxidative esterification of MA with excellent catalytic performance compare to other two catalyst. O O O

CH3OH

MMA

O OH

MA

Hemiacetal

O O Acetal

Scheme 4.Proposedmechanism pathway for esterification of methacrolein

The possible reaction mechanism for the oxidative esterification of methacrolein(MA) to methyl methacrylate in presence of molecular oxygen over gold based catalysts has been

investigated

recently,

which

involved

two

step

mechanism

(Scheme

4).5,12,15Firstly, the aldehyde couples with methanol viacondensation reactions between aldehyde and alcohol to form hemiacetal followed by removal of H from the hemiacetal to produce ester. During the reaction, the oxygen atom and the basic side of the support play an important part in esterification of methacrolein. In this present study the importance of basic support, which enhance the formation of the intermediate hemiacetal which lead to the high activity. The strong metal support


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interaction between Au and la2O3, which enhanced the stability. The rod shape morphology for Au/ la2O3withhighactive sites activate the methoxy species and indorse the reactive oxygen species through oxygen spillover.53,54 Thus the

2 %

Au/La2O3nanostructured catalyst exhibited the best performance owing to the strong basic sites and the strong interaction between the metallic Au and La2O3, leading to an fastening effect, which enhance the stability of the nanostructured catalyst and promote the oxygen mobility and activate the methanol, respectively. After accomplishment of the esterification reaction, the solid nanostructured catalyst was collected from the reaction followed by washing thoroughly with ethanol and reused for multiple cycles to confirm the productivity of the catalyst. The activity of the recovered nanostructured catalyst after 5 successive runs did not show any significant decline in the catalytic activity (Figure15).

Figure 15. (a) Recyclability test of Au/La2O3 on esterification of methacrolein to methyl methacrylate with molecular oxygen. [ ] Conversion of methacrolein, [ ] selectivity of methyl methacrylate, [ ] Yield of methyl methacrylate, (b) Sheldon’s hot filtration test. Reaction conditions: methanol/MA molar ratio, 8:1; oxygen pressure, 0.3 MPa; temperature, 70 °C; time 2h. The TEM image of the spent catalyst after five conjugative cycle showed that the shape and size ofnanocrystalline material remain similar that of the fresh catalyst 29 ACS Paragon Plus Environment


(Figure S14 in Supporting Information). The catalyst after recycling for five times shows presence of nearly same amount of Au and La conforming the true heterogeneity of the nanostructured catalyst. Negligible amount of leaching of Au and La was detected (

Fabrication of Au nanoparticles supported on one-dimensional (1D

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