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Cu–Mo doped zeolite ZSM-5 catalyzed conversion of lignin to alkyl phenols with high selectivity† Cite this: DOI: 10.1039/c4cy01700e

Sunit Kumar Singh and Jayant D. Ekhe* Lignin conversion processes experience challenging issues including char formation, lower degree of depolymerization and lower product yield with no selectivity. Zeolite ZSM-5 was loaded with Cu and Mo using the reductive deposition–precipitation method. The metal loaded into ZSM-5 was found to be Received 19th December 2014, Accepted 8th January 2015 DOI: 10.1039/c4cy01700e www.rsc.org/catalysis

amorphous and nanostructured as confirmed by X-ray diffraction and transmission electron microscopy. The lignin conversion products were analyzed by gas chromatography-mass spectroscopy and electrospray ionization mass spectrometry. We demonstrate here the one-pot Cu/Mo loaded zeolite ZSM-5 catalyzed conversion of lignin into alkyl phenols with highly minimized char formation. The process yields alkyl phenols with high conversion (95.7%) and high selectivity of up to 70. 3% for a particular phenol.

Introduction With the projected depletion of fossil fuels, renewable sources of fuel and platform chemicals are the way to achieve a sustainable energy future. Sugars and vegetable oils are the source of first generation fuel and chemicals. However, they compete with the food chain for their feedstock and do not seem to be a sustainable option any longer. Developing second generation technologies based on cheaper and more abundant lignocellulosic biomass is a promising approach to achieve a sustainable energy future.1,2 This is due to the optimum availability of raw materials (e.g. straw, grass, wood, paper waste) and their renewability as compared to petroleum based resources. Another important factor in utilization of lignocellulose as a feedstock is its lower cost. Lignin constitutes ~15–30% by weight and ~40% of the energy of the lignocellulosic biomass. It is a complex amorphous polymer composed of phenylpropane type subunits linked by ether (C–O–C) and C–C linkages. The very large availability and phenylpropane skeleton structure of lignin suggest that it can be a valuable source of chemicals, particularly phenolics.2,3 However, lignin depolymerization with selective bond cleavage is the major challenge for converting it into liquid hydrocarbons and other value-added chemicals. Hence, its valorization via depolymerization and conversion to valuable chemicals is significant for effective utilization of biomass.

Department of Chemistry, Visvesvaraya National Institute of Technology, Nagpur-440010, M.S., India. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cy01700e

This journal is © The Royal Society of Chemistry 2015

Many methods have been reported for catalytic lignin transformations via pyrolysis, hydrocracking, acidic or basic hydrolysis, hydrogenolysis and oxidation with homo/electro/ heterogeneous catalysis.3 However, most of these methods result in either char formation, lower degree of depolymerization or lower product yield with no selectivity. Solvent based approaches have shown potential to stop or reduce char formation. Nucleophilic solvents like water and alcohols prevent re-polymerization by capping carbocationic centers generated during lignin depolymerization, thus minimizing char formation.4–10 Non-nucleophilic solvents, like hexane, do not stabilize the carbocations and hence char formation is difficult to avoid in such solvents. The utilization of externally added molecular hydrogen can be minimized or avoided by using hydrogen donating alcohols as solvents or co-solvents. Hydrogen donor solvents like methanol,4,5 ethanol,9,11 propanol,12 etc. have been utilized for lignin depolymerization coupled with hydrogenolysis/hydrogenation. Solvent combinations, viz. water–methanol,10 formic acid–ethanol,9,13 water–ethanol6,8 and water–dioxane,14,15 are also reported for lignin depolymerization and conversion to value added products. Catalysis has been considered an important technology in biomass and lignin conversion. Catalysts used in lignin depolymerization should promote high conversion and suppress char formation due to re-condensation reactions, while keeping the reaction severity under a permissible limit. Porous metal oxides and zeolites have been used in various hydrodeoxygenation reactions. Zeolite HZSM-5, a well known industrial petro-cracking catalyst, has been used as a hydrodeoxygenation catalyst in pyrolysis of lignin16 and biomass.17 It also catalyzes the depolymerization of lignin solubilized in acetone at 500–600 °C, producing high yields of deoxygenated aromatic hydrocarbons.18 Lignin model compounds have also

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been studied for hydrodeoxygenation over HZSM-5 and hydrogenation over Pd/C in one-pot to successfully produce cycloalkanes at 200 °C and 50 atm H2.19 Ni–Mo and Co–Mo supported on Al2O3, SiO2 and Al2O3–SiO2 are frequently used as hydrodeoxygenation catalysts in petroleum refineries.20 Besides this, Cu supported on metal oxides is also reported to catalyze oxidative or dehydrogenative transformations.21 In this work, we have studied the effect of Cu–Mo and Cu doped HZSM-5 as catalysts for lignin conversion into value added phenols. Recondensation of degradation products leading to charring is one of the major problems faced in the thermocatalytic conversion of lignin into low molecular weight hydrocarbons. Also, selectivity towards the products is another identified problem in effective and economic lignin conversion. Recondensation and selectivity problems, however, can be overcome by using a specific catalyst, additive and solvent selection in thermocatalytic conversion of lignin, which is therefore one of the prime objectives of this work.

Results and discussion Catalyst synthesis and characterization The detailed catalyst synthesis procedure is discussed in the experimental section. The synthesized HZSM-5 (Si/Al ratio 42) was loaded with Cu (5%) to prepare the Cu-ZSM-5 catalyst and with Cu (5%) /Mo (1%) to prepare the Cu/Mo-ZSM-5 catalyst. The loading was done using the reductive deposition– precipitation method. The XRD diffractograms of the synthesized catalysts are shown in Fig. 1. HZSM-5 showed diffractogram peaks typical for its MFI (Mordenite Framework Inverted) structure (Fig. 1B). Diffractograms of Cu/Mo-ZSM-5 and Cu-ZSM-5 showed no specific peaks for Cu and Mo. Instead, a broad hump in the region 2θ = 35–40° was observed indicating that the metal loaded is amorphous in nature (Fig. 1A). For the confirmation, both catalysts were analyzed by transmission electron microscopy (TEM). As the particles of HZSM-5 were nearly spherical with a size in the range of 2–10 μm, having no electron transparency, no clarity could be obtained for the size or morphology of the metal loaded on the zeolite (see Fig. S1 in the ESI†). However, for Cu/Mo-ZSM-5, TEM micrographs showed that the metal precipitated outside the zeolite (Fig. 2). These precipitates showed agglomerated particles having a particle size in the range 5–8 nm. The selected area electron diffraction (SAED) image of these agglomerates showed a typical diffused ring pattern (Fig. 2), indicating that the metal precipitates obtained in the synthesis procedure were amorphous and nanostructured. This ensures that the metal loaded on the zeolite should also be nanostructured, as also expected from the synthesis procedure followed. The zeolite was dispersed in an aqueous solution of the metal salts by stirring for 1 h. During this stirring, some of the metal ions remain adhered to the zeolite via ion-exchange, adsorption and diffusion into the pores along with the solvent. Subsequently, reduction of these metal ions was done using sodium borohydride (NaBH4) which renders a fast


Fig. 1 XRD diffractograms of A) metal loaded HZSM-5 and B) HZSM-5.

Fig. 2 TEM image and SAED pattern of Cu/Mo-ZSM-5.

reduction precipitation process, not allowing the metal particles to grow in size. These metal particles are finally converted into metal oxides by aerial oxidation and more significantly during the calcination process.


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The total acidity of the zeolite samples was measured using a potentiometric method of titration with n-butylamine22 (Fig. S3 in the ESI†). The acidic strength and the total number of sites in the catalysts were found to decrease with metal loading, as evidenced by potentiometric measurements (Table 1). The decrease in number and density of acidic sites may be due to metal ions replacing the protons on the acidic sites. Metal loading over the zeolite also resulted in the decrease of the surface area and pore volume of the catalyst, as shown in Table 1. Catalytic studies The catalytic reactions were performed in a 100 ml SS316L reactor (Parr instruments Co., Illinois). In a typical reaction, 0.5 g of lignin, 0.125 g of catalyst, and 1.7 mmol of NaOH were dissolved in 60 ml of solvent (various water–methanol ratios were used as shown in Table 2). Methanol was used as the source of reductive equivalents via hydrogen transfer. The generation of hydrogen equivalents from methanol by reforming and water-gas shift reaction4,23 is shown in Scheme 1. The products in organic and aqueous phases were extracted using ethyl acetate (EtOAc). These EtOAc soluble products were qualitatively and quantitatively analyzed, as discussed in the experimental section. The results of the different reactions performed are shown in Table 2. The lignin conversion was observed to be high (>90%), except for the

conditions when either NaOH was absent (entry 6 – Table 2) or water or methanol alone was used as a solvent (entry 4, 5 – Table 2). As lignin is insoluble in water and partially soluble in methanol, NaOH was used to improve the solubility of lignin. Besides, in the absence of NaOH, an increase in char formation to about 20.7 wt% was observed. In the condition where only NaOH was used as a catalyst, the formation of hydrogenated products (other than phenols) was not observed.10 This clearly indicated the role of solid heterogeneous catalysts in the formation of hydrogenated products. We found water/methanol ratios 1 : 1 Ĳ30/30 ml ml−1) and 3 : 1 Ĳ45/15 ml ml−1) as suitable for the formation of monomeric products in higher proportions compared to other solvent ratios (Table 2). Cu/Mo-ZSM-5 was capable of catalyzing reactions leading to higher lignin conversion and higher monomeric products as compared to Cu-ZSM-5 (entry 1, 8 – Table 2). HZSM-5 catalyzed the reaction yielding the highest lignin conversion (98.5 wt%) along with a higher monomeric product yield (22.5 wt%). However, it resulted in product formation with no selectivity towards a particular phenol. Instead it showed a higher selectivity for mixed phenols and hydrodeoxygenated products like acyclic hydrocarbons and aromatics. Cu/Mo-ZSM-5 was efficiently (~98%) recovered after reaction and reused for 3 cycles with no loss of activity (see Table S2 in the ESI†). XRD diffractograms of the recovered HZSM-5 showed that the framework of the zeolite remains mostly stable with some signs of amorphization under the

Table 1 Total aciditya and surface area characteristics of synthesized catalysts

E0 (mV) Total acid strength Total number of acid sites (meq g−1) Density of acid sites (μeq m−2) BET surface area (m2 g−1) Average pore volume (cm3 g−1)

HZSM-5

Cu/ZSM-5

Cu–Mo/ZSM-5

115 vs 0.33 0.94 352.2 0.20

−8 w 0.29 0.93 308.8 0.21

−81 w 0.13 0.46 281.7 0.19

Determined by a potentiometric method, E > 100 mV (vs: very strong site), 0 < E < 100 mV (s: strong site), −100 < E < 0 mV (w: weak site), E < −100 mV (vw: very weak site). a

Table 2 Yields of products under different reaction conditions

Solvent ratioa Entry (ml ml−1)

Catalyst

NaOH (mmol)

Lignin conversionb (%)

Total EtOAc soluble products (wt%)

Monomeric productsc (wt%)

Oligomeric productsd (wt%)

Gaseous Char productse (wt%) (wt%)

1 2 3 4 5 6 7 8 9

Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu/Mo-ZSM-5 Cu-ZSM-5 HZSM-5

1.7 1.7 1.7 1.7 1.7 0 5.2 1.7 1.7

95.7 96.6 91.7 89.0 82.4 71.1 96 83.2 98.5

60.3 62.4 53.7 51.5 36.5 47 50.9 61.2 60.9

19.9 20.6 16.9 5.8 8.2 n.d. f n.d. 17.7 22.5

40.4 41.8 36.8 45.7 28.3 n.d. n.d. 43.5 38.4

35.4 34.2 38 37.5 45.9 24.1 45.1 22 37.6

30/30 45/15 15/45 0/60 60/0 30/30 30/30 30/30 30/30

0.23 0.29 0.27 7.4 8.7 20.7 0.47 0.53 0.3

a

Water/methanol ratio. b Lignin conversion is calculated by deducting the EtOAc insoluble products (i.e. residual lignin and char). Monomeric products = products quantified by GC analysis. d Oligomeric products = EtOAc soluble products – products quantified by GC analysis. e Gaseous products are calculated by deduction. f n.d. (not determined). Reaction conditions: lignin 0.5 g, 0.125 g of catalyst, solvent 60 ml, 220 °C, 7 h, inert atmosphere.

c



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Scheme 1 Generation of hydrogen equivalents from methanol.

reaction conditions used. The extent of amorphization increased with each reuse cycle. The catalyst lost its crystallinity after the third cycle and became amorphous, hence lost its activity in the fourth cycle. This amorphization also resulted in a decrease in the surface area and average pore volume of the catalyst (see Table S3 and Fig. S2 in the ESI†). Product analysis The products were analyzed qualitatively on a GC-MS/FID and the identified products were quantified. The selectivity towards a product or a group of products including monomeric products under different reaction conditions is shown in Table 3. The results showed that the highest selectivity of 70.3% for phenol, 3-methoxy, 2,5,6-trimethyl (PMT) was observed in the reaction catalyzed by Cu/Mo-ZSM-5 with a solvent ratio of 30/30 (entry 1 – Table 3). Along with PMT, 18.7% selectivity was observed for other phenols (alkylated). Selectivity for other aromatic, acyclic and cyclic compounds was very low as compared to HZSM-5 (entries 1, 9 – Table 3). As we changed the solvent ratio, the selectivity for PMT was decreased and no selectivity was observed in the case when pure water or methanol was used as a solvent (entries 4, 5 – Table 3). In the case where water or methanol was used as a solvent, the selectivity for a mixture of phenols was high (52.6% and 95.1%, respectively) but the yield of monomeric products was very low (as shown in Table 2, entries 4–5). Cu-ZSM-5 also showed comparable selectivity for PMT (entries 1, 8 – Table 3), but the selectivity for other phenols was low (12.5% as compared to 18.7% by Cu/Mo-ZSM-5). However, no selectivity for PMT was observed in the HZSM-5 catalysed reaction. Instead HZSM-5 showed a higher selectivity for mixed phenols than Cu-ZSM-5 and Cu/Mo-ZSM-5. The proportion of

acyclic and cyclic hydrocarbons was higher in the HZSM-5 catalysed reaction (entry 9 – Table 3). As the highest selectivity for PMT and other phenols was seen in the reaction catalyzed by Cu/Mo-ZSM-5 (entry 1, Table 3), the list of different products obtained in this reaction is shown in Table S4 (ESI†). The surface area and acidity of HZSM-5 were found to decrease with loading of Cu and Cu/Mo (Table 1). The total conversion in terms of extractible products in ethyl acetate was nearly the same but there is a significant change in the profile of the products formed. Therefore, the changed profile of products may be attributed to the loaded metal. The weight average molecular weight (Mw) and polydispersity index (PDI) of the EtOAc soluble products were determined by electrospray ionization mass spectrometry (ESI-MS).10,23 The ESI-MS spectra of the products from the Cu–Mo/ZSM-5 catalysed reaction are shown in Fig. 3. The mass distribution of the products was found to be in the range of 60–1000 m/z. The Mw of EtOAc soluble products from the HZSM-5 catalyzed reaction was found to be 448.0 g mol−1. The Mw further decreased to 311.9 g mol−1 when ZSM-5 was loaded with 5% Cu (Cu-ZSM-5). The lowest Mw (286.7 g mol−1) was found in the case where Cu/Mo-ZSM-5 (5% Cu, 1% Mo) was used as a catalyst. The Mw and PDI of the products formed in the reaction catalyzed by Cu/Mo-ZSM-5, Cu-ZSM-5 and HZSM-5 are shown in Table 4. The PDI in all the three catalytic conditions was found to be nearly the same and close to unity, which indicates the narrow distribution range of the products.

Fig. 3 ESI-MS spectra of products from Cu–Mo/ZSM-5 catalysed reaction (reaction entry 1, Table 2).

Table 3 Selectivity towards monomeric products under different reaction conditions (as mentioned in Table 2)

Product selectivity (%) Entrya

Phenol,3-methoxy,2,5,6-trimethyl

Aromatics

1 2 3 4 5 8 9

70.3 62.3 63.4 n.f.b n.f. 69.2 n.f.

0.4 2.2 1.4 2.4 2.1 1.1 8.8

a

Acyclic compounds

Cyclic compounds

Phenols

Hydrocarbons

Oxygenates

Hydrocarbons

Oxygenates

Others

18.7 17.4 24.3 52.6 95.1 12.5 46.5

0.9 1.0 0.4 0.5 n.f. 0.4 12.4

1.3 1.8 0.8 9.7 n.f. 3.1 6.1

0.8 1.4 0.5 n.f. n.f. n.f. 3.1

5.1 7.2 5.2 n.f. 1.1 6.7 7.4

2.3 6.5 3.7 34.7 1.5 6.8 15.5

Entries are the same as in Table 2. b n.f. = not found.



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Table 4 Molecular weight of EtOAc soluble products from reactions catalyzed by different catalysts

Catalyst used a

Cu/Mo-ZSM-5 Cu-ZSM-5b HZSM-5c

Weight average molecular weight (Mw) (g mol−1)

Polydispersity index (PDI)

286.7 311.9 448.0

1.27 1.22 1.20


For reaction conditions see: a Entry 1, Table 2.

b

Entry 8, Table 2. c Entry 9, Table 2.

Barta et. al. reported a novel approach of catalytic disassembly of biomass derived lignin using a Cu doped porous metal oxide (Mg–Al oxide). The reactions performed at 300 °C for 24 h with methanol as a hydrogen donor solvent resulted in complete hydrogenolysis of lignin with aromatic ring hydrogenation. The products were mainly composed of monomeric substituted cyclohexyl derivatives with negligible aromatics.4 The same catalyst under lower reaction severity (180–220 °C, 12–20 h) in the presence of H2 showed the formation of aromatic products with selectivity towards a group of C9 catechols that could typically be derived from petroleum feedstocks.5 Our process utilizes methanol with water as a solvent at 220 °C for 7 h in inert atmosphere for selective formation of alkyl phenols, as shown in Table 3. Mechanistic hypothesis The possible mechanism for the formation of selective phenol PMT is shown in Scheme 2. The base catalyzed

Scheme 3 Product separation scheme.

depolymerization of lignin results in the release of monomeric and dimeric products (step A). The monomeric products are demethoxylated in water, resulting in the formation of polyphenols. These polyphenols further undergo side chain removal on the Brønsted acidic sites of HZSM-5 (step B). The polyphenols are hydrogenated over Cu–Mo oxides supported on ZSM-5 to form cyclohexanol type of structures. These cyclohexanols may undergo dehydration on the Brønsted acidic sites of HZSM-5, forming cyclohexenols (step C).12,19 These cyclohexenols are then methylated on the Lewis acidic sites of the zeolite and further re-aromatized to form methylated phenols or alkyl phenols (step D). The ring hydrogenation followed by a dehydroxylation (RHD) mechanism is supported by the fact that the Ar–OH bond dissociation enthalpy (465 kJ mol−1) is higher than the C–O bond dissociation enthalpy in aliphatic and cyclic compounds (99%, Merck), ammonium heptamolybdate tetrahydrate (>99%, Merck), ethyl acetate (Lichrosolv, Merck), tetrahydrofuran (Lichrosolv, Merck), methanol (Emparta, Merck), NaOH (≥97%, Merck) and H2SO4 (97%, Merck) were used as received. Kraft lignin was isolated from industrial black liquor (BILT paper industry, India). The biomass feedstock used for paper making is hardwood. Black liquor was acidified with 50% H2SO4 with vigorous stirring, and then the precipitated lignin was washed thoroughly with distilled water and dried in an oven at 80 °C for 3 h. The lignin was further purified using 1,4-dioxane. Purification with 1,4-dioxane ensures that the lignin is free of carbohydrate impurities.24 The solvent was recovered and pure lignin was used in the experiments. Proximate analysis of lignin showed 3.96 wt% moisture, 9.60 wt% ash, 45.10 wt% volatile matter and 42.8 wt% fixed carbon. Elemental analysis showed 59.3 wt% carbon, 7.1 wt% hydrogen, 0.1 wt% nitrogen, 0.24 wt% sulphur and 33.26 wt% oxygen (by difference). The weight average molecular weight (Mw) of kraft lignin6 is reported to be ~5000 g mol−1 with a polydispersity index of 5.9. Catalyst synthesis Zeolite HZSM-5 was synthesized according to the procedure previously described.10 To prepare the Cu/Mo ZSM-5 catalyst, 0.24 g of ammonium heptamolybdate tetrahydrate and 2.6 g of copper sulphate pentahydrate were dissolved in 50 ml of deionized water (solution 1). 13 g of HZSM-5 powder was added to it and stirred vigorously at 500 rpm under ice cold conditions for 2 h. Sodium borohydride solution (solution 2) was prepared by adding 0.75 g of NaBH4 and 0.1 g of NaOH into 50 ml of deionized water. Solution 2 was added to solution 1 slowly at a rate ~ 2 ml min−1 and the stirring was continued for 1 h. Then, the mixture was centrifuged to separate the precipitate and washed with deionized water 3 times followed by drying in a hot air oven at 90 °C for 4 h. After drying, calcination was done at 500 °C for 3 h. The same



method was followed for preparing Cu-ZSM-5 in the absence of ammonium heptamolybdate. Catalyst characterization The XRD patterns of the samples were recorded on a PANalytical – X'Pert PRO in a 2θ range from 5° to 90° (step size 0.017°, step time 20 s) using CuKα radiation (1.54 Å) at 40 kV and 100 mA. The morphology of the catalyst was studied by scanning electron microscopy (FEI Quanta 450 FEG) and transmission electron microscopy (PHILIPS, CM200). The surface area, pore volume and average pore size were characterized and determined by N2 adsorption–desorption (Micromeritics ASAP 2010, Norcross, GA, USA). The total acidity of the zeolite samples was measured using a potentiometric method of titration with n-butylamine.22 n-Butylamime (0.1 N, 0.05 mL) was added to a dispersion of 0.15 g of zeolite sample in 90 mL of acetonitrile. This system was kept under steady stirring for 3 h. The suspension was then titrated using a base solution volume of 0.05 mL each time. The time elapsed before making a potential measurement was 2 min. The electrode potential variation (mV) was measured with a Toshcon CL46+ digital pH/mV meter. The total number of acid sites per gram of catalyst was estimated from the total amount of base added to reach the plateau in the potential vs. milliequivalent Ĳn-butylamine) per gram of catalyst curve, and the acid site density was calculated by considering the apparent surface area value of the corresponding zeolite sample. Catalytic reactions Lignin (0.5 g), solvent (60 mL) and catalyst (0.125 g) were placed in a 100 mL stainless steel SS316 reactor (Parr Instruments Co., Illinois, USA). The reactor was sealed after purging it with argon 3–5 times to expel air. The reaction was performed at 220 °C. When the desired reaction time was reached, the vessel part of the reactor was quenched in water. The products from the reactor were washed with 20 mL of water and centrifuged at 2500 rpm for 15 minutes to separate the water soluble and water insoluble products. The products in organic and aqueous phases were extracted with ethyl acetate (EtOAc). The catalyst, char, unconverted lignin and high molecular weight fragments of lignin were separated as EtOAc insoluble products. Unconverted lignin and high molecular weight fragments of lignin were solubilized in tetrahydrofuran (THF) to separate char and the catalyst (as lignin is completely soluble in THF). The product separation is shown in Scheme 3. The yields were calculated by measurement of weights of the products after solvent evaporation in a vacuum rotary evaporator. The yields were defined as shown in eqn (1)–(6): 

Lignin Conv.  %   1 

w  EtOAc insoluble  Catalyst used  



E  wt%  

w  Initial Lignin 

w( EtOAc solubles) w( Initial Lignin )

 100

 100 (1)  (2)


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w  THF solubles  w  Initial Lignin 

100

(3)

 w THF insolubles   w Catalyst added    100   w  Initial Lignin   

C  wt%   


G (wt %) = 100 − (E + T + C) Product selectivity  %  

w  Product 

w  Total Products 

(4)

(5)  100

(6)

where, E = EtOAc soluble products, T = THF soluble products, C = char and G = gaseous products. Product characterization The EtOAc soluble products were qualitatively and quantitatively analyzed on a GC-MS-FID instrument (Thermo Scientific, Trace-1310/Thermo ISQ MS detector) equipped with a TRACE TR-5MS capillary column (30 m × 0.25 mm × 0.1 μm) and helium as a carrier gas. The column was initially kept at 50 °C for 2 min, then was heated at a rate of 8 °C min−1 to 290 °C, and maintained for another 5 min. In MS, the ion source temperature was kept at 230 °C and operated in full scan mode with a mass range 40–500 amu. The sample (0.5 μL) was injected with an autosampler in a split mode with a column flow of 1.2 ml min−1 and a split flow of 30 ml min−1. Phenanthrene was used as an internal standard. Products were identified by comparison with pure compounds. The sample (2 mg mL−1) was dissolved in acetonitrile– methanol (1 : 2) and analyzed by full-scan electrospray ionisation mass spectrometry Ĳm/z range from 60 to 2000 with 1 scan s−1) using a Waters, Micromass Q-TOF Micro with the Waters Alliance 2795 separation module (Waters Corporation, Milford, MA). The system consisted of a Waters 1525 μ binary HPLC pump with a mobile phase degassing unit, a quadrupole-time of flight mass spectrometer and MassLynx™ 4.0 software. Samples of 20 μL were injected by the autosampler and placed in the mass spectrometer with 70 cm of PEEK tubing (i.d. of 0.18 mm) without separation on a chromatographic column. Each sample was analyzed 5 times. The mobile phase consisted of 80 : 20 methanol/water, using a flow rate of 0.3 mL min−1. Both positive and negative electrospray ionization were used to detect the different compounds. In all cases, identical methods of extractions and analysis conditions were maintained.

Acknowledgements The first author, Sunit K. Singh is thankful to the Director of the Visvesvaraya National Institute of Technology, Nagpur, for providing the necessary facilities and financial support. We are thankful to SAIF Chandigarh for providing the XRD and ESI-MS facility, SAIF Chennai for providing the SEM-EDS facility and SAIF IIT Bombay for providing the TEM facility.


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28 A. Gardziella, L. A. Pilato and A. Knop, Phenolic Resins: Chemistry, Applications, Standardization, Safety and Ecology, Springer Verlag, 2000.


27 L. R. Rudnick, Lubricant Additives: Chemistry and Applications, CRC press, Taylor and Francis group, LLC, 2nd edn, 2009.


