Microwave-Assisted Batch and Continuous Flow Synthesis of

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Microwave assisted batch and continuous flow synthesis of palladium supported on magnetic nickel nanocrystals and their evaluation as reusable catalyst Arun V Nikam, Amol A. Kulkarni, and Bhagavatula L V Prasad Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00639 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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Crystal Growth & Design

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Microwave assisted batch and continuous flow synthesis of palladium supported on magnetic nickel nanocrystals and their evaluation as reusable catalyst

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Arun V. Nikam,†‡ Amol A. Kulkarni,‫ ‡’٭‬and Bhagavatula L. V. Prasad‫†’٭‬

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†

Physical/Materials Chemistry Division, and ‡Chemical Engineering & Process Development Division.

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CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road Pune 411 008, India

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E-mail: [email protected]. Tel: 91-20-25902153. Fax: 91-20-25902621.

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E-mail: [email protected]. Tel: 91-20-25902013. Fax: 91-20-25902636.

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Keywords: Bimetallic, Magnetic separation, Hydrogenation, Catalyst,

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Abstract

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Palladium nanocrystals (NCs) supported on nickel NCs (Pd/Ni) were synthesized in a continuous

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flow manner by the microwave-assisted method in the presence and absence of oleylamine.

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Parameters optimized for batch experiments were considered while performing continuous flow

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synthesis. The Pd/Ni NCs synthesized in the presence of oleylamine displayed good catalytic

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activity for hydrogenation of aromatic nitro compounds, and those bearing alkene, and alkyne

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moieties. The ferromagnetic character of the supporting nickel NCs allowed the recovery of the

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catalyst and these recovered catalysts could be reused several times.

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ACS Paragon Plus Environment

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1 Introduction

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Synthesis of nanocystals (NCs) that can be obtained as colloidal dispersions using bottom-up

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approaches has seen an explosion of efforts over the last few decades. These have afforded very

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reliable methods that allow us to tune the size, shape, and compositions of NCs precisely.1,

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However, there remains a great concern over the ability to translate these lab based methods for

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large scale synthesis. One of the main impediments in this direction is the fact that these

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reactions are highly sensitive, and thus slight fluctuations in any reaction parameters will have a

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significant impact on the product quality including size and shape. In this context, continuous

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flow synthetic methods offer great hope towards the controlled synthesis of NCs, providing

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multiple advantages over conventional batch synthetic methods.3, 4 These include improved level

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of control over reaction parameters leading to NCs with narrower size distributions and

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avoidance of batch to batch variations (better reproducibility). Finally, the continuous flow

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methods can easily be adapted for automation and could be scaled-up easily by numbering

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enabling rapid translation of laboratory scale efforts to manufacturing.5-7 Thus NC synthetic

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methods based on continuous flow are witnessing a great surge these days.

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Furthermore even amongst the various classes of NCs, bimetallic NCs have been getting

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significant attention due to their unique surface, electronic, catalytic and magnetic properties.8, 9

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Most of the bimetallic NCs are synthesized using batch processes that limit their application

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potential in industry due to scalability issue and there have been efforts to overcome this by

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continuous flow methods.10-12 When compared to single component systems like metal and

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semiconductor NCs, reports on the flow based synthesis of complex systems like bimetallic,

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core-shell and other heterostructures are rather sparse. One reason for this could be the necessity

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to use multiple reagents/precursors to synthesize these multicomponent systems. For successful


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translation of a batch synthetic method into a continuous flow based on the number of reagents

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involved should be kept bare minimum to avoid excessive downstream processing.

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With this background, we envisaged that microwave assisted flow synthesis could be one way

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to synthesize these complex bimetallic systems in a reliable way and that the same could be

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easily adapted to large scale synthesis. Microwave heating is a very efficient technique that

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consumes 72% less energy in comparison with conventional heating process.13 It is also known

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to enhance reaction kinetics and a continuous flow synthesis would assure reproducibility. Such

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a combination could lead to high throughput NC synthesis in a controlled manner and indeed

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there have been few reports on the continuous flow synthesis of NCs using microwave heating.

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Juwang et al. showed continuous synthesis of Au NCs using microwave assisted flow reactor.14

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Gregoery et al. used segmented flow microwave synthesize of size controlled PdSe NCs.

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Toshishinge et al. employed polyol process for continuous flow synthesis of monodisperse silver

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NCs by using AgNO3 and PVP as stabilizer.16 Recently, Kunal et al. attempted scalable synthesis

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of bimetallic RhAg alloy nanoparticles using microwave assisted continuous flow synthesis

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method.17 As can be noticed, most of these methods with microwave reactors were utilized for

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the preparation of monometallic and semiconducting NCs and the procedure involved for these

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single component system itself involves many reagents.18 Therefore adaption of the same

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protocols to make multicomponent system could be tedious. Addressing these issues here we

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disclose a simple method for the preparation bimetallic Pd/Ni NCs both in batch and continuous

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flow manner. The Pd@Ni core shell and PtNi alloy NCs have been attractive candidates due to

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their magnetic as well as catalytic properties.7, 19, 20

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Our endeavor started with the making of nickel (Ni) NCs both in batch and continuous flow

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techniques, as a first step. Next, we extended these protocols and developed a preparation


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method for bimetallic Ni-Pd NCs using microwave heating in a batch mode. We also

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demonstrate that, benzyl alcohol, which is used as a solvent due to its better microwave

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absorbance characteristics can act as a reducing agent under microwave conditions21, 22 to reduce

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Ni2+ ions. Finally, we translated these batch synthesis protocols into a continuous flow based

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ones and realized the synthesis of palladium NCs on nickel support in a sequential manner where

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the Ni NCs made in the first step are pumped continuously into the reactor along with palladium

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precursor solution independently. Accordingly, the reduction of Pd2+ ions in presence of Ni NCs

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led to the formation of palladium decorated Ni NCs. The utility of these bimetallic

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nanostructures as catalysts for the hydrogenation of nitroaromatic compounds, alkene and alkyne

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is also investigated. Ferromagnetic nature of these NCs enables their easier separation and

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recyclability.

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2 Experimental

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2.1 Chemicals and Materials, Nickel (II) acetate tetrahydrate and palladium (II) acetate

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(abbreviated as Ni(ac)2 and Pd(ac)2 respectively in rest of the manuscript), anhydrous benzyl

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alcohol (99.9% purity), oleylamine (70%) and diphenyl ether (99%) were purchased from Sigma

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Aldrich and used without any further purification. 5% and 10% Pd/C were received from Sigma

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Aldrich. Methanol and acetone were used from Thomas Baker Chemicals.

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2.2 Synthesis of nickel NCs without oleylamine

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2.2.1 Batch synthesis of nickel NCs

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62 mg of Ni(ac)2 (0.25 mmol) was mixed with 16 mL of anhydrous benzyl alcohol in a 25 mL

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round-bottom flask at room temperature and dissolved by sonication for 15 min and purged with


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N2 gas. NOTE: Ni(ac)2 is not very soluble in benzyl alcohol and the sonication resulted in only

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the dispersion of Ni(ac)2 in benzyl alcohol. The flask containing the reaction mixture was placed

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in a microwave oven (Ragatech Pvt. Ltd, India, 700 W, 2.45 GHz) for 8.5 minutes without any

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stirring. Measurement of exact reaction temperature was not possible in this case as the formed

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metal NCs got stuck to the temperature probe causing a spark. Hence the temperature raise with

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microwave heating was measured only with pure benzyl alcohol and temperature profile is

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shown in Figure S1. After the microwave irradiation, the reaction flask was removed from the

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microwave chamber and allowed to cool at room temperature. A black colored product was

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observed in reaction flask. Product was washed three times using mixture of acetone and

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methanol using sonication and separated by applying an external magnetic field. Supernatant of

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the solution was discarded after separating the product. The product was dried at 75 °C in an

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oven for 12 h. This product was identified to be nickel NCs by (powder X-ray diffraction)

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PXRD.

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2.2.2 Continuous flow synthesis of nickel NCs

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Flow synthesis of nickel NCs was carried out using a single continuous stirred tank reactor

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(CSTR -see schematic in Figure 4 A). 150 mL of Ni2+ solution was prepared by maintaining the

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aforementioned batch concentration. Peristaltic pump was used to inject Ni2+ solution at a flow

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rate of 2 mL/min into the CSTR of 20 mL volume placed in the microwave cavity. A condenser

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was joined to CSTR and N2 gas was purged in the reactor by using a dip-tube inserted through

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the condenser. Microwave irradiation was started before injecting the solutions. Black colored

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product was collected at flow rate 2 mL/min from the outlet of the reactor. After few minute of

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appearance nickel NCs, a sparking was observed inside the reactor.

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2.3 Synthesis of nickel NCs with oleylamine

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2.3. 1 Batch synthesis of nickel NCs with oleylamine

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62 mg of Ni(ac)2 (0.25 mmol) was added to 15 mL of anhydrous benzyl alcohol in a 25 mL

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round-bottom flask at room temperature and dispersed by sonication for 20 minutes. 1 mL of

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oleylamine was added to the above solution and kept for sonication for 5 min. It was noticed that

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adding oleylamine to Ni(ac)2 in benzyl alcohol makes a clear blue colored solution (see the

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supporting information Figure S2) unlike the case of Ni(ac)2 only in benzyl alcohol, where the

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Ni(ac)2 was not completely soluble in benzyl alcohol. N2 gas through this mixture for 15

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minutes. Subsequently, the flask containing the reaction mixture was placed in a microwave

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oven for 4 minutes without any stirring. After 4 minutes a black color product appeared and the

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flask was allowed to cool to room temperature. Separation and drying of the product was done as

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mentioned in 2.2.1.

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2.3.2 Continuous flow synthesis of nickel NCs with oleylamine

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150 mL of Ni2+ solution with oleylamine was prepared by maintaining the aforementioned

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batch concentration in section 2.3.1. For flow synthesis of nickel same setup was used as

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mentioned in section 2.2.2. Peristaltic pump was used to inject Ni2+ solution at a flow rate of 4

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mL/min to maintain 5 min residence time in reactor. Product was collected at flow rate 4 mL/min

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from the outlet that is extended outside the microwave using another peristaltic pump operated in

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suction mode. Rest of the procedure is similar as mentioned in section 2.2.1. Interestingly no

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spark was observed in this case even if the reaction was continued for a long time (~40 min).

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2.4 Sequential reduction of Ni2+ and Pd2+

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2.4.1 Batch synthesis of palladium supported on nickel with oleylamine


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The same procedure was followed upto the formation of black colored nickel NCs in benzyl

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alcohol as in section 2.3.1. 8 mL of benzyl alcohol containing nickel NCs solution was used for

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the synthesis of Pd/Ni composite NCs. 4 mg of Pd(ac)2 was dissolved into 8 mL of diphenyl

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ether which was added into 8 mL of nickel NCs solution and completely mixed using sonication

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for 5 minutes. This mixture was kept under the microwave irradiation (700 W, 2.45 GHz) for 4

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minutes. Reaction solution was allowed to cool at room temperature. After cooling, NCs were

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separated by applying external magnet and washed three times using methanol and acetone and

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supernatant was discarded. Product was dried at 75 °C in an oven for 12h.

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2.4.2 Continuous flow synthesis of palladium on nickel support with oleylamine.

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For sequential reduction of metal ions, flow step as shown in Figure 4A was used. Nickel NCs

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were obtained in CSTR-I following the procedure as described in section 2.3.2. These nickel

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NCs entered into CSTR-II (24 ml volume) by over-flow at a flow rate of 4 mL/min. In CSTR-II,

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along with Ni NC dispersion, Pd(ac)2 solution of 4×10-3 M in diphenyl ether was injected

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through syringe pump at 2 mL/min flow rate. The flow rates of Ni NCs and Pd(ac)2 solution

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were adjusted in such a way that they experienced a 4 min residence time under microwave

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irradiation. As mentioned before, the product that formed was collected through the outlet

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outside the microwave by another peristaltic pump to ensure that there is no accumulation of

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reaction mixture in any CSTR. Separation and drying of this product was carried out as

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mentioned in section 2.4.1.

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2.5 General Procedure for hydrogenation of nitro compound:

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Hydrogenation of organic substrate was carried out in an SS316 high pressure autoclave

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reactor (Anton Parr, USA). In each case, 5 mmol of organic substrate was taken in 100 mL


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methanol as a solvent and 20 mg of Pd/Ni catalyst was dispersed in reaction mixture using

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sonication for 10 min. Reaction mixture was kept in the stainless steel reactor and sealed.

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Hydrogen gas was filled in reactor at 20 bar pressure and stirred at 1000 rpm. 2 mL of aliquots

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were removed after 5 min time intervals and analyzed on gas chromatography. After the reaction

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was completed, the catalyst was separated by applying an external magnet. Catalyst was washed

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using a mixture of 50% acetone and 50% methanol three times and dried in vacuum oven at 75

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˚C for 12 h and reused for the next reaction cycle.

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2.6 Characterization

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XRD patterns of nickel and Pd/Ni NCs were obtained with a Panalytical Xpert Pro PXRD

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operated at 40 kV and 30 mA using Cu Kα radiation. The shape and size of the NCs were

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analyzed by TEM (FEI-TechnaiTE-20 and JEOL JEM-2100F field emission transmission

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electron microscope) operated at 200 kV that are fitted with the an energy dispersive X-ray

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analysis (EDX) accessory for elemental mapping. TEM samples were made on a carbon coated

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TEM grid by putting a 7 µL drop of an as-prepared solution of nickel and palladium on nickel

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NCs. Yield and selectivity of hydrogenation reactions were measured using gas chromatography

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(Thermo Fischer trace GC Ultra Column HP-5 I.d. =0.25 mm , L=30 m). Elemental analysis was

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done by using AAS instrument (Agilent technology 240FS). Quantachrome Instrument version

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3.01 was used to measure the BET surface area.

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3. Results

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Our endeavor started with the preparation of nickel NCs using Ni(ac)2 as the precursor in a

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batch process. As mentioned previously Ni(ac)2 was not completely soluble in benzyl alcohol

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and formed only blue colored dispersion. When this solution was microwave irradiated a black


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colored product appeared after 8.5 min. On other hand, synthesis of Ni NCs with conventional

2

heating requires 6 h which clearly reveals that microwave enhances rate of formation of Ni NCs.

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This black precipitate could be easily separated from the solution using a magnet. The PXRD

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analysis of this black product (Figure 1A, Curve-I) was found to be in agreement with JCPDS

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file no.04-0850 that indicates the formation of metallic nickel. The crystallite size deduced from

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the 111 peak of nickel using Scherrer equation turned out to be 28 nm suggesting that nanosized

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crystals are formed. It may be noted here that in the present experimental recipe no external

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reducing agent has been added. Nanoscale crystals of nickel are known to be very reactive and

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are known to convert to their oxides if proper surface passivation is not carried out.23

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Interestingly, the nickel NCs prepared here turned out to be stable against oxidation and showed

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no NiO or any other impurity peaks as evidenced by the PXRD pattern recorded on NCs that are

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stored for a month at ambient condition (Figure 1A, Curve-II). However, the NCs were of

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different sizes and shapes and were polydispersed in nature. From the TEM image analysis we

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deduced the average size of the nickel NCs to be 33.4 ± 10 nm (Figure 1B and C). We next

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attempted to prepare nickel NCs in a continuous flow manner using the parameters (microwave

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power, time of irradiation, concentration of precursors etc.) optimized in the above mentioned

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batch process. However, during the flow synthesis, a spark was observed in CSTR after

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formation of nickel NCs. Due to safety concerns, microwave irradiation was stopped

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immediately and no variation in reactor shape helped in avoiding the spark. Therefore, to

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circumvent this issue, oleylamine was added along with Ni(ac)2 in benzyl alcohol and the

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reaction was also carried out under nitrogen flow. When Ni(ac)2-oleylamine mixture was

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irradiated with microwave under batch conditions black colored product started appearing after 4

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minutes, which is almost half of the time required for nickel NCs formation in the previous case


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without oleylamine. Here again, the PXRD pattern of as prepared product clearly indicated the

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formation of nickel NCs (see Figure 2A Curve-I) and even these NCs were stable for a month of

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storage (see Figure 2A Curve-II). The main difference, in the case of Ni NCs obtained in

4

presence of oleylamine, is that the average size of NCs (19.3±5.3 nm as calculated from TEM

5

images see Figure 2B and C) was much smaller even though non-uniformity in shape and size of

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the NCs was observed in this case too. We then attempted to make nickel NCs in presence of

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oleylamine in a continuous flow manner using the above parameters identified in the batch

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process. For this, the Ni(ac)2 + oleylamine mixture was injected into a CSTR at a flow rate of 4

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mL/min and was microwave irradiated. Black colored product was collected at flow rate 4

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mL/min from the outlet. Quite satisfyingly when the Ni(ac)2-oleylamine complex was subjected

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to microwave irradiation no sparking was observed even if the reaction was carried out for

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reasonably longer periods (40 min). The PXRD patterns recorded with the product obtained from

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the continuous flow method again indicated that the nickel in metallic state was formed (Figure

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3A Curve-I). The PXRD analysis of these NCs stored for a month also revealed that they are

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stable and do not undergo any decomposition (Figure 3A Curve-II). The TEM micrograph

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(Figure 3B and C) pointed that the particle shape is not well defined but the average size now is

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11 nm which is smaller than those obtained via the batch process. Having established the

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preparation of nickel in a batch as well as continuous flow methods we proceeded to the

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preparation of the Pd on nickel in a batch process by the sequential reduction of Ni2+ and

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Pd2+.ions. This was accomplished by the reduction of Ni2+ in the first step (in presence of

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oleylamine), followed by reducing Pd2+ in presence of nickel NCs obtained in the first step (See

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experimental section 2.4.1. for details). PXRD pattern of the magnetically separated NCs which

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were obtained from sequential reduction indicated the presence of palladium and nickel NCs


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(Figure 4B Curve-I). TEM image analysis of these composite NCs revealed the presence of ~5

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nm sized spherical palladium NCs anchored to the surface of ~11 nm nickel NCs (Figure 5A and

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B). Deposition of Pd on Ni NCs was confirmed by the mapping of elemental distribution of Ni

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and Pd using EDX (Figure 5C and 5D). From the AAS analysis the palladium loading on nickel

5

support was determined to be 7.8%.

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Our next and main task was to prepare the Pd on Ni composite in a continuous manner. The

7

setup used for sequential reduction of Ni2+ and Pd2+ is schematically represented in Figure 4A

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and the experimental details are provided in section 2.4.2. For this, the nickel NCs synthesized

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by microwave irradiation of Ni(ac)2-oleylamine mixture in a CSTR were directly pumped into

10

the second CSTR, which contained the Pd precursor solution. At the steady state the flow rates

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were fixed as 4 mL/min for nickel NCs solution and 2 mL/min for Pd precursor solution. The

12

product from the second CSTR was collected and the NCs were separated by applying external

13

magnet.

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These NCs were washed with methanol and acetone and could be easily re-dispersed in non-

15

polar solvents like toluene by sonication for 5 min. The PXRD pattern (Figure 4B Curve-II)

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obtained from this product contains a peak at 40.1˚ which confirms to the 111 plane of metallic

17

Pd in FCC lattice and the peaks at 44.5˚, 51.6˚ and 76.5˚ could be assigned to 111, 200, 220

18

planes corresponding to nickel metal. The palladium crystallite size calculated from Scherrer

19

equation is found to be ~3 nm. The TEM micrographs of Pd/Ni nanostructures acquired by

20

sequential reduction in a microwave assisted continuous flow manner shows Pd/Ni NCs

21

aggregates on the TEM grid (see Figure 5 E and F). Elemental mapping of the product obtained

22

shows Pd NCs supported on the Ni NCs (see Figure 5G and H). From the AAS analysis the Pd

23

loading was determined to be 6.8 %.


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4. Discussion

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Benzyl alcohol is known to possess good microwave absorption capacity and we have recently

3

shown that under microwave irradiation it also acts as a reducing agent.24 Our present

4

experimental results also support this contention as we could see the conversion of Ni2+ to Ni0,

5

when Ni2+ ions are heated with microwave in benzyl alcohol. However, when the nickel

6

precursor alone was heated in benzyl alcohol the NCs formed were found to be slightly larger in

7

size and poly-dispersed in nature. The non-uniformity in the shape of nickel NCs could be

8

attributed to the rapid reduction of Ni(ac)2 under microwave heating conditions. This is due to

9

the very fast volumetric heating that is caused by the microwave irradiation. To confirm this we

10

measured the rise in the temperature of benzyl alcohol under microwave irradiation. The results

11

indicate that the temperature rises from room temperature to 204 °C within 4 minutes (see

12

supporting information Figure S1) of microwave irradiation.

13

Another observation we made was that when Ni2+ ions are reduced in a continuous flow

14

manner without the addition of any stabilizer, sparks started to appear in the reactor after the

15

formation of nickel NCs. It is well-known that when a metal is placed in a microwave sparks get

16

generated due to charge accumulation and mobilization of electrons.25 These sparks can

17

deteriorate product quality and can lead to an explosion. We also noticed that during the flow

18

synthesis of Ni NCs in the absence of any stabilizer few NCs settled at the bottom of reactor and

19

some more were seen stuck to the wall. These could have led to the spark generation. One reason

20

for this de-stabilization of NCs is the improper dissolution of Ni(ac)2 in benzyl alcohol. This

21

could lead to inhomogeneous heating and larger crystal formation. As mentioned in the

22

experimental section addition of oleyamine to Ni(ac)2 helps its dissolution in benzyl alcohol. The

23

microwave irradiation of Ni(ac)2-oleylamine was also found to result in the formation of stable


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nickel NC dispersions. Though the NCs obtained in presence of oleylamine were still

2

polydispersed in nature the average size was significantly smaller than those obtained in the

3

absence of oleylamine. We attribute this to the formation of nickel NCs homogeneously

4

throughout the solution (due to the better solubility of Ni(ac)2 in presence of oleylamine) and

5

their better dispersion in the solvent, which helps in the avoidance of spark in the solution.26

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Once the problem of spark in the solution was resolved we could easily extend the procedure to

7

make Ni NCs into a continuous mode. Subsequently we extended this procedure to not only

8

prepare monometallic Ni NCs but also to prepare Pd NCs anchored on Ni NCs easily using a

9

sequential continuous flow method. For this the Ni NCs are made first in a CSTR which are then

10

drawn into the second reactor containing Pd precursor. The heating of Pd precursor in presence

11

of Ni NCs leads to the formation of Pd anchored on Ni NCs. The anchoring of the palladium on

12

nickel is due to secondary nucleation of palladium NCs on nickel support. Apart from the

13

palladium on nickel NCs several monometallic palladium NCs were also seen to form in this

14

case (see supporting information Figure S3). Many of these individual palladium crystals remain

15

in the supernatant when Pd/Ni NCs were separated by magnetic separation. It may be also

16

noticed that the NCs that were obtained via continuous flow methods were significantly smaller

17

in size. We ascribe this to better homogeneity maintained in reaction mixture due to i) the

18

presence of oleylamine and ii) the short residence time in the reactor.

19

After establishing the preparation of Pd/Ni composites by batch and continuous flow methods

20

in presence of oleylamine we proceeded to test their catalytic activity using the standard

21

nitrobenzene hydrogenation reaction. The two catalysts prepared in the batch and continuous

22

flow manner were named as Pd/Ni-B and Pd/Ni-CNF respectively. The hydrogenation reactions

23

were carried out with pure hydrogen gas at room temperature at 20 bar hydrogen pressure in


13


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Page 14 of 26

1

methanol. All the experiments were carried out using the Pd/Ni NCs prepared in presence of

2

oleylamine. We also compared the activity of Pd/Ni-B and Pd/Ni-CNF catalysts with

3

commercial 5% palladium on carbon (Pd/C) and 10% Pd/C catalysts (see Figure 6A). It may be

4

noticed that a complete conversion of nitrobenzene occurs within 30 min with the Pd/Ni-B

5

catalyst. This (time taken for completion of the reaction) was same as that observed for 10%

6

Pd/C (700-800 m2/gm). On other hand, when the same reaction was conducted for the same time

7

with Pd/Ni-CNF (surface area: 18 m2/gm) and 5% Pd/C (surface area 700-1000 m2/gm) catalysts

8

the conversion was 63% and 76%, respectively. From these results, it may be concluded that

9

catalyst prepared by our method (especially the Pd/Ni-B catalyst) displays reasonably good

10

catalytic activity. It may be worth mentioning here that though the Pd loading in our Pd/Ni-B

11

catalyst is slightly lower than that of the commercial 10% Pd/C, these two display similar

12

efficiencies. Since Pd/Ni-B catalyst shows better catalytic activity compared to Pd/Ni-CNF

13

catalyst, we used the same for hydrogenation of several other substrates as mentioned in Table 1.

14

As can be noticed from the table all the substrates got converted to their hydrogenated products

15

within a reasonable period of time. The NMR spectral analysis of the products recorded from the

16

samples prepared from crude products obtained from these substrates without any purification

17

(supporting information Figure S4) clearly proves that the products are devoid of any impurities.

18

As mentioned previously the presence of Ni imparts ferromagnetic nature to the catalyst

19

allowing its easy separation using applying external magnetic field. Accordingly, we performed

20

the recyclability study employing the magnetically separated catalysts again using the

21

hydrogenation of nitrobenzene as the standard reaction. Quite gratifyingly 66% conversion of

22

nitrobenzene occurred within 30 min even after third cycle (Figure 6B).

23


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1


5. Conclusions

2

Nickel and Pd/ Ni NCs have been successfully synthesized using a microwave assisted method

3

in which benzyl alcohol acts as a solvent as well as reducing agent. In presence of oleylamine,

4

nickel NCs could be obtained in a very short time of 4 min. We could extend this procedure to

5

prepare nickel supported palladium nanocomposite via both the batch processes and continuous

6

flow manner. This resulted in a reproducible synthesis protocol for these bimetallic NCs. The Ni

7

supported Pd nanomaterial was found to be an active catalyst for the hydrogenation of alkene,

8

alkyne and aryl nitro compounds. The catalyst used could be easily retrived by applying external

9

magnet. The flow synthesis approach reported here is easy to scale-up and can be used for other

10

supports such carbon, metal oxide, and polymer as well to prepare supported metal catalysts.

11 12

6. Acknowledgements

13

A.N. acknowledges CSIR New Delhi for a Senior Research Fellowship. We thank the IPC

14

Department IISc for helping us with TEM analysis. AAK and BLVP acknowledge the support

15

from CSIR’s XIIth Five Year Plan Project (CSC0123 and CSC0134). We thank Dr. Ramana for

16

help with catalysis experiments. Financial assistance from DST (GoI) is also acknowledged.

17 18

7. References

19

References

20 21 22 23 24

(1) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.M.; Hyeon, T., Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mat. 2004, 3, 891-895. (2) Sun, Y.; Xia, Y., Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176-2179.


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(3) Xu, L.; Srinivasakannan, C.; Peng, J.; Zhang, D.; Chen, G., Synthesis of nickel nanoparticles by aqueous reduction in continuous flow microreactor. Chem. Eng. Process. Process Intensification 2015, 93, 44-49. (4) Zhang, L.; Xia, Y., Scaling up the production of colloidal nanocrystals: Should we increase or decrease the reaction volume? Adv. Mat. 2014, 26, 2600-2606. (5) Wong, W. K.; Yap, S. K.; Lim, Y. C.; Khan, S. A.; Pelletier, F.; Corbos, E. C., Robust, non-fouling liters-per-day flow synthesis of ultra-small catalytically active metal nanoparticles in a single-channel reactor. React. Chem. Eng., 2017. (6) Roberts, E. J.; Habas, S. E.; Wang, L.; Ruddy, D. A.; White, E. A.; Baddour, F. G.; Griffin, M. B.; Schaidle, J. A.; Malmstadt, N.; Brutchey, R. L., High-Throughput Continuous Flow Synthesis of Nickel Nanoparticles for the Catalytic Hydrodeoxygenation of Guaiacol. ACS Sustain. Chem. Eng., 2016, 5, 632-639. (7) Niu, G.; Zhou, M.; Yang, X.; Park, J.; Lu, N.; Wang, J.; Kim, M. J.; Wang, L.; Xia, Y., Synthesis of Pt–Ni Octahedra in Continuous-Flow Droplet Reactors for the Scalable Production of Highly Active Catalysts toward Oxygen Reduction. Nano Lett. 2016, 16, 3850-3857. (8) Ahrenstorf, K.; Albrecht, O.; Heller, H.; Kornowski, A.; Görlitz, D.; Weller, H., Colloidal synthesis of NixPt1− x nanoparticles with tuneable composition and size. Small 2007, 3, 271-274. (9) Zhang, D.; Wu, F.; Peng, M.; Wang, X.; Xia, D.; Guo, G., One-Step, Facile and Ultrafast Synthesis of Phase-and Size-Controlled Pt–Bi Intermetallic Nanocatalysts through ContinuousFlow Microfluidics. J Am.Chem. Soc. 2015, 137, 6263-6269. (10) Zhang, L.; Niu, G.; Lu, N.; Wang, J.; Tong, L.; Wang, L.; Kim, M. J.; Xia, Y., Continuous and scalable production of well-controlled noble-metal nanocrystals in millilitersized droplet reactors. Nano Lett. 2014, 14, 6626-6631. (11) Duraiswamy, S.; Khan, S. A., Droplet‐Based Microfluidic Synthesis of Anisotropic Metal Nanocrystals. Small 2009, 5, 2828-2834. (12) Eluri, R.; Paul, B., Synthesis of nickel nanoparticles by hydrazine reduction: mechanistic study and continuous flow synthesis. J. Nanopart. Res. 2012, 14, 800. (13) Ashley, B.; Lovingood, D. D.; Chiu, Y.-C.; Gao, H.; Owens, J.; Strouse, G. F., Specific effects in microwave chemistry explored through reactor vessel design, theory, and spectroscopy. Phy. Chem. Chem. Phy. 2015, 17, 27317-27327. (14) Bayazit, M. K.; Yue, J.; Cao, E.; Gavriilidis, A.; Tang, J., Controllable synthesis of gold nanoparticles in aqueous solution by microwave assisted flow chemistry. ACS Sustain. Chem. Eng., 2016, 4, 6435-6442. (15) Hostetler, E. B.; Kim, K.-J.; Oleksak, R. P.; Fitzmorris, R. C.; Peterson, D. A.; Chandran, P.; Chang, C.-H.; Paul, B. K.; Schut, D. M.; Herman, G. S., Synthesis of colloidal PbSe nanoparticles using a microwave-assisted segmented flow reactor. Mater. Lett., 2014, 128, 5459. (16) Nishioka, M.; Miyakawa, M.; Kataoka, H.; Koda, H.; Sato, K.; Suzuki, T. M., Continuous synthesis of monodispersed silver nanoparticles using a homogeneous heating microwave reactor system. Nanoscale, 2011, 3, 2621-2626. (17) Kunal, P.; Roberts, E. J.; Riche, C. T.; Jarvis, K.; Malmstadt, N.; Brutchey, R. L.; Humphrey, S. M., Continuous Flow Synthesis of Rh and RhAg Alloy Nanoparticle Catalysts Enables Scalable Production and Improved Morphological Control. Chem. Mat. 2017, 29, 43414350.


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Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25


(18) Harada, M.; Cong, C., Microwave-assisted polyol synthesis of polymer-protected monometallic nanoparticles prepared in batch and continuous-flow processing. Indust. Eng. Chem. Res., 2016, 55, 5634-5643. (19) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Lee, J.; Hyeon, T., Designed synthesis of atom-economical Pd/Ni bimetallic nanoparticle-based catalysts for sonogashira coupling reactions. J. Am. Chem. Soc., 2004, 126, 5026-5027. (20) Ding, J.; Bu, L.; Guo, S.; Zhao, Z.; Zhu, E.; Huang, Y.; Huang, X., Morphology and Phase Controlled Construction of Pt–Ni Nanostructures for Efficient Electrocatalysis. Nano Lett., 2016, 16, 2762-2767. (21) Bilecka, I.; Niederberger, M., Microwave chemistry for inorganic nanomaterials synthesis. Nanoscale 2010, 2, 1358-1374. (22) Bilecka, I.; Elser, P.; Niederberger, M., Kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles in benzyl alcohol. ACS Nano 2009, 3, 467477. (23) Yan, J.-M.; Zhang, X.-B.; Han, S.; Shioyama, H.; Xu, Q., Synthesis of longtime water/air-stable Ni nanoparticles and their high catalytic activity for hydrolysis of ammonia− borane for hydrogen generation. Inorg. chem., 2009, 48, 7389-7393. (24) Nikam, A. V.; Arulkashmir, A.; Krishnamoorthy, K.; Kulkarni, A. A.; Prasad, B., pHdependent single-step rapid synthesis of CuO and Cu2O nanoparticles from the same precursor. Cryst. Growth & Des., 2014, 14, 4329-4334. (25) Shanmugharaj, A.; Ryu, S. H., Excellent electrochemical performance of graphene-silver nanoparticle hybrids prepared using a microwave spark assistance process. Electrochim. Acta 2012, 74, 207-214. (26) Mallikarjuna, N. N.; Varma, R. S., Microwave-assisted shape-controlled bulk synthesis of noble nanocrystals and their catalytic properties. Crystal growth & design 2007, 7, 686-690.

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Page 18 of 26

1 2

Figure 1. X-ray diffraction pattern of NCs prepared without stabilizer (Curve-I) nickel NCs,

3

(Curve-II) nickel NCs stored for a month. B) And C) TEM images of Ni NCs prepared without

4

stabilizer.

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1

2 3

Figure 2. X-ray diffraction pattern of NCs prepared in presence of oleylamine (Curve-I) nickel

4

NCs, (Curve-II) nickel NCs stored for a month. B) and C) TEM images of Ni NCs prepared

5

using oleylamine in batch mode.

6 7


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Page 20 of 26

1 2

Figure 3. X-ray diffraction pattern of Ni NCs prepared with oleylamine using flow synthesis

3

(Curve-I) nickel NCs, (Curve-II) nickel NCs stored for a month. B) and C) TEM images of Ni

4

NCs prepared using in continuous flow mode.

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1 2

Figure 4. A) Schematic of continuous flow setup for sequential reduction of Ni(II) and Pd(II)

3

precursors. B) PXRD pattern for Pd/Ni prepared in batch (Curve-I) and continuous flow method

4

(Curve-II).

5 6


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Page 22 of 26

1 2

Figure 5. A) and B) TEM images, (C- D) EDX mapping of elemental distribution for C) Ni and

3

D) Pd for Pd on Ni prepared in batch in presence of oleylamine. And E) and F) TEM images, (G-

4

H) EDX mapping of elemental distribution for G) Ni and H) Pd for Pd on Ni prepared using

5

continuous flow method in presence of oleylamine.

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1 2

Figure 6. A) Comparison of conversion efficiency of nitrobenzene to aniline with 7.8% PD/Ni

3

catalyst and commercial 5% Pd/C and 10% Pd/C catalysts. B) Conversion efficiencies of the

4

same reaction with recovered catalysts.

5


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Table 1. Hydrogenation of nitro compounds, alkene and alkyne. Entry Substrate Product Yield Time NO2

NH2

100a

30 min

100 b

60 min

100a

180 min

3

100a

5 min

4

100a

5 min

1 NO2

NH2

2 CH3

CH3

2 3 4

Reaction condition: 5 mmole of substrate, 20 mg of Pd/Ni-B catalyst, room temperature, 20 bar pressure, and methanol as solvent. aPd/Ni-B, bPd/Ni-CNF.

5


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For Table of Contents Use Only

1

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Microwave assisted batch and continuous flow synthesis of palladium supported on magnetic nickel nanocrystals and their evaluation as reusable catalyst †

Arun V. Nikam,†‡ Amol A. Kulkarni,‫ ‡’٭‬and Bhagavatula L. V. Prasad‫†’٭‬ Physical/Materials Chemistry Division, and ‡Chemical Engineering & Process Development Division. CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road Pune 411 008, India E-mail: [email protected]. Tel: 91-20-25902153. Fax: 91-20-25902621. E-mail: [email protected]. Tel: 91-20-25902013. Fax: 91-20-25902636.

12

Synopsis:

13

Magnetically recoverable Pd/Ni nanocatalyst for hydrogenation prepared using microwave

14

assisted flow and batch chemistry in short time by sequential reduction of metal precursors.

15 16 17


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The preparation of palladium nanocrystals (NCs) supported on nickel (Pd/Ni) in a continuous flow manner by the microwave-assisted method is demonstrated. 378x200mm (96 x 96 DPI)


Page 26 of 26

Microwave-Assisted Batch and Continuous Flow Synthesis of

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