Solar Photothermochemical Reaction and Supercritical CO2

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Solar photo-thermochemical reaction and supercritical CO2 work up for a fully green process of preparation of pure p-nitrobenzyl bromide Milan Dinda, Supratim Chakraborty, Supravat Samanta, Chitrangi Bhatt, Subarna Maiti, Sandip Roy, Yogesh Kadam, and Pushpito Kumar Ghosh Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4019282 • Publication Date (Web): 09 Aug 2013 Downloaded from http://pubs.acs.org on August 16, 2013

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Environmental Science & Technology

Solar

photo-thermochemical

reaction

and

supercritical CO2 work up for a fully green process of preparation of pure p-nitrobenzyl bromide Milan Dinda,a Supratim Chakraborty,a Supravat Samanta,a Chitrangi Bhatt,b Subarna Maiti*,b Sandip Roy,c Yogesh Kadam c and Pushpito K. Ghosh*a,b a

b

c

AcSIR-CSMCRI, G. B. Marg, Bhavnagar –364 002, Gujarat, India

CSIR-Central Salt and Marine Chemicals Research Institute Department of Chemical Engineering, IIT Bombay, Powai, Mumbai 400076, India

E-mail: [email protected] (P. K. Ghosh); [email protected] (S. Maiti); Fax: +91-2782567562 KEYWORDS: Photo-thermochemical reactions, Solar-driven bromination, solar photothermochemical reactor, p-nitrobenzyl bromide, Supercritical CO2 extraction, Solvent-less reaction and work up

ABSTRACT. It has been reported by us recently that p-nitrobenzyl bromide (PNBBr) can be synthesized from p-nitrotoluene (PNT) in high isolated yield with respect to available bromine in 2:1 Br--BrO3- employed as brominating reagent.

The reaction was conducted in ethylene

ACS Paragon Plus Environment

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dichloride (EDC) and the substrate was taken in excess to suppress dibromo impurity formation. The product was “cold crystallized” from the reaction mass and the mother liquor was recycled in the subsequent batch thereby eliminating organic discharge. The present work attempts to further advance the synthesis of this commercially important molecule employed in protectiondeprotection strategies.

Herein its successful synthesis employing neat substrate and solar

radiation as the sole energy source to drive this photo-thermochemical reaction is reported. Further, 100% pure PNBBr could be isolated from the solid reaction mass in 87% yield by leaching out the excess substrate through supercritical CO2 (Sc-CO2) extraction. The reaction was therefore accomplished cleanly in all respects and with low carbon footprint. ■ Introduction

Most organic reactions require energy to drive them. The energy is required in the form of heat in thermochemical reactions or in the form of light in photochemical reactions.1-5 Organic reactions are also driven by microwave power, ultrasound energy, mechanical energy, etc.6-8 Such energy inputs most often have their origin in fossil fuels. Minimising the carbon footprint is an important objective of green chemistry besides atom efficiency, use of clean reagents and avoidance of effluents. This goal is all the more critical for commercially important organic molecules produced in substantial amounts. The use of solar energy in organic reactions is well known and attractive from this perspective. However, most of the reactions reported in the literature are either solar thermochemical or solar photochemical in nature.

Solar photo-

thermochemical reactions, i.e., those which are promoted by light and heat simultaneously derived from the sun, are less well studied.9 Bromine substitution at sp3 carbon is one such well known reaction, wherein photons are required for generation of Br·, while elevated temperatures


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promote the thermo-chemical propagation steps. Benzylic bromination is an important sub-set of such reactions.10 The synthesis of p-nitrobenzyl bromide (PNBBr) has been of particular interest to us in view of its wide deployment in the pharmaceutical industry for protection-deprotection of functional groups. The synthesis of the molecule from p-nitrotoluene (PNT) with zero organic discharge, employing 2:1 Br--BrO3- as the reagent (Eq 1), was reported by us recently.11a Thermal energy and light energy required for the reaction were supplied

through hot oil bath and tungsten lamp, respectively. In the present work these two energy sources were successfully replaced with a single renewable energy source, i.e., concentrated solar radiation.12 By performing the reaction with molten substrate, the need for organic solvent (EDC) was also eliminated. Further, the solid reaction mass was subjected directly to supercritical fluid extraction (SCFE) with CO2 to isolate the desired product in pure form thus avoiding organic solvent altogether in the process. ■ Experimental

2.1. Materials and methods All the reagents were of commercial grade and purified according to established procedure. 1H and

13

C NMR were recorded in CDCl3 with TMS as the internal standard using Model Bruker

Avance II spectrometer. FT-IR spectra were recorded in KBr using Perkin Elmer spectrometer. GC-MS data were acquired on GC-MS QP 2010 Shimadzu instrument. Melting point was


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recorded using Mettler Toledo instrument and the data were uncorrected. Experiments with artificial light were conducted using 25 watt LED, fluorescence and UV lamps. Construction of Solar photo-thermochemical reactors (SPTR) SPTR-1. A rectangular box (0.50m x 0.34m x 0.10m) made of wood was coated with matt black paint on the inner side. A magnetic stirrer was positioned in the box in such a way that uniform stirring could be insured. A 0.002 m thick detachable transparent commercial glass fixed to a teakwood frame was placed as cover over the box over a rubber gasket strip. The cover could be removed easily for cleaning purposes. The glass cover had a hole on its surface, from which the neck of the flask stuck out to enable addition of chemicals and drawing of samples. Two glass reflectors of 0.58 m x 0.44 m were positioned in a V-trough alignment on the two sides of the box in North–South direction. The angle of the reflectors could be adjusted to maximize solar radiation on the glass cover. A 20 watt PV module was positioned on top of the North side reflector in foldable manner. A 12 V, 0.21 A dc fan (Eiffel make) was fitted onto one of the walls of the box while a 0.04 m diameter opening with a flap was kept on the opposite wall to vent out excess trapped heat and thereby control the reaction temperature. The magnetic stirrer and fan were both operated with the same PV panel. The temperatures inside the box were measured with RTD thermocouples. The wind speed and ambient temperature were measured using a thermo-anemometer (Metershack, CEM DT-618B) having 0–5 ms-1 range and 0.01 ms-1 reading accuracy. The solar intensity during the reaction period was measured using an Eppley PSP pyranometer (sensitivity = 9.3μVW-1m2). SPTR-2. The parabolic dish concentrator shown in Figure 4 had an opening diameter of 70 cm and its interior surface was covered with 26 anodized aluminum reflector foils of symmetric


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trapezoid geometry, with the two bases and leg of each foil having the lengths 7.8 cm, 1 cm, and 38 cm, respectively. The geometrical concentration ratio was 8.22. A wooden rectangular box placed at the focus of the parabolic dish having l x b = 13 cm x 13 cm and height of 11cm was fabricated. The side facing the dish concentrator was made of toughened glass. The remaining interior of the box was painted black. One DC fan was fitted into the box for temperature control. A 250 mL double neck RB flask fitted with a mechanical stirrer was placed in the box with the necks protruding out. The scale up of reaction was conducted in this set up. General procedure for preparation of p-nitrobenzyl bromide in SPTRs The experiments in SPTR-1 and SPTR-2 were conducted in Bhavnagar, Gujarat, India (location: 21o 46’N, 72o 11’E). To conduct the reaction in the SPTR-1, a single neck RB flask equipped with a magnet was taken, with the neck sticking out from the glass cover. 10 g (73 mmol) of PNT (two-fold excess), 10 mL of water, 16.3 mmol of NaBr and 8.15 mmol of NaBrO3 (total 24.45 mmol active Br) were added into the flask. The contents were stirred with the built-in magnetic stirrer and 26 mmol of KHSO4 was added in lots once the PNT melted. The total reaction time was around 2.0-2.5 h. The flask was left standing to attain room temperature and 15-20 mL of water was added whereupon the oily organic mass solidified. The aqueous layer was drained out and the solid washed a couple of times with water. The solid was then ground, dried and analysed. The atmospheric pressure, ambient temperature, wind speed and humidity were measured during the reaction. For the scaled up study in SPTR-2, 100 g PNT (730 mmol) was added into a 2-neck flask followed by addition of 2:1 NaBr: NaBrO3 solid reagent (245 mmol of active bromine) and 40 mL water. After achieving molten conditions, ca. 100 mL of an aqueous solution containing 36 g KHSO4 (267 mmol; 1.1 eq with respect to active bromine) was added in lots and the reaction conducted as before.


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The experimental procedures followed for the reactions in Tables S5 and S6 are provided separately in the supporting information. Caution: Due care must be taken while undertaking the experiments with concentrated solar radiation. General procedure for isolation of p-nitrobenzyl bromide By Sc-CO2 extraction This separation process was carried out in the unit as described in Fig S1. Details of the extraction process are also provided in the Supporting Information. ■Results and discussion

The solar photo-thermochemical reactor of Figure 1 (SPTR-1) was fabricated to carry out the reaction of Eq 1 at the scale of 10 g of substrate (taken in twofold excess). The basic unit was

4

S

N

3

3

1

5

Figure 1. Laboratory scale solar photo-thermochemical reactor (SPTR-1) comprising a closed rectangular box with black absorber base and glass cover having suitably sized holes for projection of the neck(s) of the RB flask (1) resting on a magnetic stirrer (2). Glass reflectors (3) were used to concentrate the solar radiation and a 20 W PV panel (4) powered the magnetic stirrer and DC fan (5) employed for temperature regulation (see Experimental Section for details).


6


similar in design to V-trough solar cookers and was specially fitted with a PV panel which operated the in-built magnetic stirrer and fan for temperature regulation. Details of the unit are provided in the experimental section. Figure 2(A) shows the spectral profiles, along with relative intensities, of the global insolation and solar illumination as measured inside the SPTR-1. The latter was nearly two-fold higher. Figure 2(B) shows the absorption spectrum of the active

b

150

A 100

a.u

a

50

0 1.2 300

600

900

wavelength(nm)

1.0

0.8

Absorbance

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0.6

a

0.4

B

b

0.2

0.0 300

400

500

600

700

800

900

Wavelength(nm)

Figure 2. (A) Spectroradiometric plots showing intensity (a.u.) of (a) ambient (global) solar radiation and (b) concentrated solar radiation as seen inside the SPTR-1. (B) Uv-vis spectra of (a) active brominating agent employed in the present study; and (b) aqueous Br2. brominating agent generated upon addition of a small amount of KHSO4 (10-20 % of stoichiometric requirement) into an aqueous solution of 2:1 Br--BrO3-. The shoulder at 392 nm matched well with that of aqueous Br2. Thus the bromine generated in small amounts in the aqueous solution of active brominating agent was the photoactive species yielding Br radical.


7


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The proposed mechanistic scheme employing this reagent is shown in Scheme 1. As can be seen from Table S1, bromine atom efficiency is much higher for 2:1 Br--BrO3- than for liquid bromine. Moreover, it is safer to handle and can be prepared cost-effectively from the alkaline bromine intermediate of the “cold process” of liquid bromine manufacture.11b,c The reaction of Eq 1 was conducted next in the SPTR-1 of Fig 1. To avoid over-bromination which may lead to

Scheme 1: Proposed mechanism of thermo-photochemical benzylic bromination with 2:1:3 NaBr-NaBrO3-KHSO4. Eq 3 is the photo-initiation step, eqs. 4-6 represent the thermal propagation steps, and eq. 8 is the termination step. The net stoichiometry of the reaction is given by eq. 10. α,α–dibromo impurity formation, the substrate was taken in twofold excess with beneficial results (Table S2). Consequently, the yields referred to throughout the manuscript are with respect to the limiting reactant, i.e., the brominating reagent. The required amount of reagent was dissolved in 10 mL of water and added along with the substrate into a 100 mL RB flask equipped


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with a stir bar. The reactions were carried out on three consecutive days in the month of March. The solar global intensity was about 1000 Wm-2, the ambient temperature being 29-34oC and wind speed 1 ms-1. Figure 2 shows the temperature profile of the reaction mixture during the time period of 11:30 AM to 2:30 PM. Stoichiometric amount of KHSO4 taken in 2-3 mL of water was added gradually once the substrate began to melt (51-52oC). Addition of KHSO4 led to the development of a yellowish-orange colour due to liberation of small amounts of B2 (eq 2, Scheme 1) which was allowed to fade (Eqs 3, 6, 8; Scheme 1) before further addition of the acid. The reaction temperature continued to rise to ca. 90oC and then remained almost constant. Figure 3 shows the temperature profile of the reaction mixture along with data of global intensity and ambient temperature. The reaction was terminated after 2.5 h and the GC yield on reagent basis was found to be 93% (Entry 1, Table 1).

Since uncontrolled temperature rise may cause

hydrolysis of PNBBr to p-nitrobenzyl alcohol (Table S3), the reaction was repeated with operation of the PV-powered fan to induce convective heat loss. The average temperature of the reaction dropped to

Temperature T

105

1000

95 85

750

75

Ambient T o C

65

500 Reaction ToC

55

Global intensity

45

250

Solar intensity(W/m2)

115

35 25

1E-15

10

11

12

13

Time of day

Figure 3. Temperature and global insolation profile in SPTR-1 while conducting the reaction of Entry 1, Table 1.


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65oC, while the GC yield rose to 95% (Entry 2, Table 1). Note that when the RB flask was blackened to prevent exposure of the contents to light, the yield observed was only 19% (Entry 3, Table 1), underscoring the importance of the photo-thermochemical condition. Table 1. Reaction of Eq 1 in the SPTR-1 of Fig 1 Entry

PNT g/mmol

Br

T control

mmol through fan

Average

hν

reaction T/oC

Reaction

GC yield on

time/h

Br basis/ %

1

10/73

24.5

No

88

Yes

2.5

93

2

“

“

Yes

65

Yes

2.2

95

3

“

“

Yes

65

No

2.5

19

The reaction was scaled up subsequently to 100 g PNT scale (50 g on product basis) in the reactor assembly of Figure 4 (SPTR-2). In this configuration a 250 mL RB flask fitted with reflux condenser and overhead stirrer was confined in a box placed at the focus of a parabolic dish concentrator. Details are provided in the experimental section. The temperature within the box shot up to 106.2oC when it was positioned at the focus and there was no temperature regulation. The unit had to be de-focussed to maintain a reaction temperature near to 90-95oC. Under these conditions, the reaction could be carried out successfully (entry 1, Table 2). As in the case of SPTR-1, the temperature control was thereafter accomplished using a fan while positioning the unit at the focal point. The plots of global insolation, ambient temperature and


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2

3 1

Figure 4. Reaction assembly in solar parabolic dish concentrator with (1) anodized aluminium reflector (2) mechanical stirrer (3) Illuminated reaction RB flask inside a chamber at the focus. reaction temperature with fan control are shown in Figure 5. In this case also the reaction proceeded smoothly (entry 2, Table 2).

Table 2. Scale up of the reaction of Eq 1 in the assembly of Figure 4 Entry

PNT

Br

Temp. control

g/mmol

mmol

1

100/730

245

No

95

Yes

3.0

91

2

“

“

Yes

65

Yes

3.0

92

through fan

Average

hν

reaction T/oC

Reaction

GC yield on

time/h

Br basis/%


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100

Temp. T,oC

80

800

70

600

60

Rea. T

50

Amb. T

40

Intensity

400

Global Intensity,Wm-2

1000

90

200

30

0 11

12

13 Time of day

14

Figure 5. Temperature and global insolation profile in SPTR-2 during the reaction period on a typical day for which the ambient temperature is also shown. To extend the gains from avoidance of organic solvent in the reaction, workup of the reaction without use of any organic solvent was attempted. In particular, Sc-CO2 extraction was considered based on the difference in the solubility characteristics of PNT and PNBBr in hexane13 – the former being more soluble – and molecular weight differences between the two compounds. Table 3 provides data on the effect of temperature, pressure and time on the yield of PNBBr. As can be seen from the table, both compounds were leached out completely at 45oC temperature and 200 bar pressure, leaving no residue. Upon reduction of the pressure to 100 bar, the dissolution of PNT became more selective and 20% isolated yield of pure PNBBr was obtained. Further reduction of pressure to 75 bar along with reduction in the extraction time from 1 h to 20 min gave 46% yield of the desired product. Maintaining the pressure and extraction time constant thereafter, the temperature was lowered to 35 oC whereupon the yield increased to 87%. The purity by GC was 100% in all the cases. GC-MS of the reaction mixture


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Table 3. Optimization table for supercritical CO2 extraction for isolation of pure PNBBra Entry

Temperature

Pressure

Weight of reaction

Time

mass taken for

Isolated yield

Purity of

of PNBBr

PNBBr

g/(%)

(%)

extraction (oC)

(bar)

1

45

200

5

60

0.00 (0)

2

45

100

5

60

0.45 (20)

(g)

(min)

100 3

45

75

5

20

1.02 (46) 100

4 a

35

75

5

20

1.95 (87)

100

Detailed procedure provided in Supporting Information

Figure 6. GC-MS of (A) reaction mixture containing PNT and PNBBr in 2:1 molar ratio and (B) residue remaining in extractor after Sc-CO2 extraction experiment in entry 4, Table 3.

The

retention times of PNT and PNBBr were ca. 10.0 and 13.1 min, respectively (details in Supporting Information).


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and purified product obtained in entry 4, Table 3 are presented in Figure 6. Table 4 brings out the improvements effected in the present work in relation to the previous report. Due to the near quantitative bromine utilisation efficiency, there was negligible residual bromide in the aqueous Table 4. Comparison of the improvements made in the present study vis-à-vis Ref 11a

Ref 11a

Present work

Reaction in dissolved state with EDC as

Reaction with molten substrate without use of

solvent

organic solvent

Reaction at reflux temperature; heat

65-90oC reaction temperature achieved with

energy through oil bath

solar radiation

Photo illumination through tungsten

Use of photon flux in solar radiation

lamp Stirring with conventional magnetic

Stirring with in-built PV-powered magnetic

stirrer

stirrer

High

bromine

utilization

efficiency

Reagent maintained the same and high GC yield

using 2:1 bromide-bromate reagent

achieved

Activation of reagent by H2SO4

Activation with the milder acid KHSO4

3:1 substrate to reagent ratio taken to

Ratio maintained the same and impurity avoided

suppress dibromo impurity 70-80% pure product recovery from

87% isolated yield of pure product from solid

liquid reaction mass through chilling

reaction mixture through Sc-CO2 extraction

Direct recycle of mother liquor

Direct recycle of solid PNT-PNBBr leached out during Sc-CO2 extraction


14

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layer after reaction. Although in the present study this layer contained a mixture of salts (Na2SO4 and K2SO4), pure aqueous K2SO4, useful as agricultural nutrient, would be obtained with KBr-KBrO3 as reagent in place of NaBr-NaBrO3, leading to complete greening of the process. Reactions under solar photo-thermochemical conditions are distinctly advantageous, no doubt, but alternative light source must be supplied when solar insolation is poor.

PNT

bromination was attempted with LED, fluorescent light and UV light. As can be seen from Table S4, all of these light sources gave PNBBr in good yields (86-89%). The reactions were also faster than with the tungsten lamp used previously. Whereas the above study was focused on PNBBr only, the utility of the SPTR-1 was demonstrated for a number of other reactions which are summarised in the supporting information (Tables S5 and S6). In the case of benzylic substrates (Table S5), the reactions in all cases were conducted without organic solvent and the GC yields were in the range of 87-99%. p-Methoxy benzyl bromide, employed as common protecting group for alcohol and carboxylic acid, could also be obtained in good (91%) yield (entry 8, Table S5). Though not attempted, Sc-CO2 separation may be applicable in these cases also. Reactions of alkane substrates with the above reagent, employing EDC as solvent and MnO2 as a catalyst14, were also found to be feasible in the SPTR-1 (Table S6). Thus the methodology may be of general interest for a wide variety of bromination reactions at sp 3 carbons. ■Acknowledgement

The reviewers are acknowledged for several helpful suggestions. CSIR India is acknowledged for supporting the study as part of an in-house laboratory project. We thank Mr. P. Patel, Mr. J.


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N. Bharadia and Dr. P. S. Bapat for fabrication of the solar devices, the Discipline of Analytical Science and Centralized Instrument Facility for analytical support, and Mr. R. Nair for assistance with data collection. Dr. B. Ganguly is acknowledged for helpful discussions on mechanistic aspects and computation of dipole moment values. MD, SC and SS thank CSIR for their research fellowships. ABBREVIATIONS: PNT, p- nitrotoluene; PNBBr, p-nitrobenzyl bromide; EDC, ethylene dichloride; SPTR, Solar photo thermochemical reactor; Sc CO2, Supercritical carbon dioxide; SCFE, Supercritical fluid extraction ; GCMS, Gas chromatography-Mass spectroscopy; ■Supporting Information. Experimental details of Sc-CO2 extraction unit and process; Tables on the various bromination reactions conducted; GC-MS and NMR data. This material is available free of charge via Internet at http://pubs.acs.org.

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (P. K. Ghosh); [email protected] (S. Maiti); Fax: +91-2782567562 ■REFERENCES

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Standalone Laboratory Scale Device for Conducting Solar-Driven

Organic Reactions Promoted by Elevated Temperature, Light and Agitation and Process thereof. Indian Patent (provisional) Appln. No. 2117/DEL/2012 dated 9 July, 2012. (13) Semi-empirical calculations employing AMI Quantum chemical method gave dipole moment values of 5.73D and 4.78D for PNT and PNBBr, respectively, whereas solubility characteristics suggest that the polarity of PNT is lower than that of PNBBr. (14) (a) Nishina, Y.; Morita, J.; Ohtani, B., Direct Bromination of Hydrocarbons Catalyzed by Li2MnO3 under Oxygen and Photo-Irradiation Conditions. RSC Advances 2013, 3, 2158-


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2162. (b) Nishina, Y.; Hachimoto, H.; Kimura, N.; Miyata, N.; Fujii, T.; Ohtani, B.; Takada, J., Biogenic Manganese Oxide: Effective New Catalyst for Direct Bromination of Hydrocarbons. RSC Advances 2012, 2, 6420-6423.


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Solar photo-thermochemical reaction and supercritical CO2 work up for a fully green process of preparation of pure p-nitrobenzyl bromide Milan Dinda, Supratim Chakraborty, Supravat Samanta, Chitrangi Bhatt, Subarna Maiti, Sandip Roy, Yogesh Kadam, and Pushpito K. Ghosh


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