Efficient Conversion of Cellulose to Levulinic Acid by Hydrothermal

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Efficient conversion of cellulose to levulinic acid by hydrothermal treatment using Zirconium dioxide as a recyclable solid acid catalyst Sunil Joshi, AMIT DADABHAU ZODGE, Kiran V Pandare, and Bhaskar D. Kulkarni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5011838 • Publication Date (Web): 23 May 2014 Downloaded from http://pubs.acs.org on June 7, 2014

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Industrial & Engineering Chemistry Research

Efficient conversion of cellulose to levulinic acid by hydrothermal treatment using Zirconium dioxide as a recyclable solid acid catalyst AUTHOR NAMES: Sunil S. Joshi,*a, Amit D. Zodge a, Kiran V. Pandare b, and Bhaskar D. Kulkarni a AUTHOR ADDRESS: a Chemical Engineering and Process Development Division, National Chemical Laboratory, Dr. Homi Bhaba Road Pune-411008, India. Fax: +91 20 25902621; Tel: +91 20 25902745; E-mail: [email protected] b

Polymer Science and Engineering Division, National Chemical Laboratory, Dr. Homi Bhaba

Road Pune-411008, India KEYWORDS: Cellulose, levulinic acid, 5-hydroxymethylfurfural, Zirconium dioxide, solid acid catalyst, hydrothermal

ABSTRACT: Conversion of cellulose into platform chemicals is essential for sustainable development of chemical industry. With this aim, single step hydrothermal conversion of cellulose to industrially important levulinic acid using zirconium dioxide as a catalyst has been investigated. A remarkably high yield (53.9 mol%) of levulinic acid with excellent accountability ◦

and total conversion of cellulose has been achieved at 180 C reaction temperature; reaction time

ACS Paragon Plus Environment

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of 3 hr with 2 wt% of catalyst and 2 g cellulose. Major products identified were levulinic acid (LA), formic acid (FA), 5-hydroxymethylfurfural (5-HMF), furfural and humins. Formation of humins during cellulose hydrolysis and dehydration was observed

and is also confirmed by

NMR and FTIR analysis. The effect of various reaction parameters such as temperature, time, substrate concentration and catalyst loading on conversion and selectivity has been studied. The catalyst has been regenerated and recycled several times without any loss of activity and selectivity.

1.0 Introduction Depleting resources of fossil fuels and rising prices of crude oil promote the use of renewable energy sources for sustainable development of chemical industry. The non-fodder component of the lignocelluloses is a promising feedstock and is available in bulk for recovery of industrially useful chemicals. The cellulose, hemicelluloses and lignins are three major components of biomass. Glucose is produced from cellulose and is considered as a renewable and useful source of platform chemicals, fuels, foods, and medicines.1-6 Several efforts have been made toward the degradation of cellulose to glucose using enzymes,6, 7 dilute acids,3, 8 and supercritical water.9-15 These processes have several disadvantages such as product-catalyst separation, corrosion, severe controls of enzymes, waste fluids and drastic reaction conditions.

The selective

conversion of cellulose into platform chemicals (e.g. LA) under mild reaction conditions would be more desirable than the current acid catalyzed reaction, high-temperature pyrolysis and supercritical processes.5 The use of mineral acid for such conversion leads to key environmental issues and generation of waste, Super-acidity of supercritical water causes corrosion of the


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reactor. Hence the process becomes uneconomical and demands huge capital investment. In attempts to solve these problems, Fukuoka et al.16 and Luo et al.

17, 18

reported a method using

heterogeneous catalyst for cellulose hydrolysis followed by hydrogenation using supported noble metal catalysts, such as Pt/Al2O3 and Ru/C in hot water under compressed H2. Various forms of zeolites and alumino-silicates catalysts have also been reported for such hydrogenation reaction.19, 20

Recent reports on hydrolysis of soluble oligosaccharides and starch showed the

use of sulfated mesoporous silica, H-form zeolites as a solid acid catalyst.20, 21 Ayumu Onda et al.22 studied hydrophobic zeolites with high Si/Al ratios and observed relatively high yield of glucose from cellulose under hydrothermal conditions. The sulfated zirconia and Amberlyst-15 catalysts showed higher activity than the H-form zeolite catalysts but conversion was found to be not more than 50% with yield 20-25%, however there were large amounts of SO4 2- ions and byproducts in the resultant solution. Brett L. Lucht et al. reported that hydrolysis of cellulose by Nafion SAC-13 and FeCl3 supported on amorphous silica was highly dependent upon the reaction temperature.23 Zhao et al. 24 studied the effect of a large number of metal chlorides on the conversion of sugars into 5-HMF in ionic liquids. Chromium (II) chloride was found to be effective and facilitate the conversion of glucose to 5-HMF to nearly 70% yield.25 Wang et al. 26 used sulfated TiO2 as a solid catalyst for hydrolysis of cellulose into LA and reported highest possible yield of 27.2 mol %. Van de Vyver et al.27 demonstrated that carbon nanofibers grown on Al2O3-supported Ni resulted in 47% yield of sugar alcohols from cellulose. A novel catalytic approach for the transformation of cellulose into methyl glucosides in methanol under relatively mild conditions was disclosed recently by Deng et al.28 Fukuoka et al.16 explained the reasons behind the low activity of solid acid catalyst that the solid to solid reaction is difficult and that protons are not freely available to interact with the oxygen atom in ether linkage of cellulose,


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pointing to the generation of a large amount of H+ in the system. This may solve the problem of having freely available protons in the vicinity of the catalyst and the substrate. Palkovits et al.29 explained that optimization of solid acid catalysts by improving catalyst–support interactions. Review studies of Rackemann et al.30 expressed the concern about the development of more selective catalysts. Recently, series of solid acid catalysts prepared by precipitation and impregnation methods that involve the use of sulfuric acid31-33, the Brønsted acids or Lewis acids have been reported for such transformations. All these acids generate tremendous amounts of hazardous wastes. The rising environmental awareness has forced the chemical industry to take up preventive measures to minimize the polluting effluents by developing ‘Green Chemistry’ processes. Solid acid catalyst can be easily separated and recycled on activation. 34-39 Solid-acid catalysts are distinguished by Lewis and Brønsted acidity, the strength, number of these sites, and the morphology of the support (e.g., surface area, pore size). The literature reports the use of solid acid catalysts for cellulose conversion with poor yield and average selectivity for levulinic acid.23, 38-41 Girisuta et al.42 reported the use of ZSM-5 as a solid acid catalyst for conversion of glucose to LA and found that it is a promising catalyst for HMF to LA conversion and not favoring the glucose to LA conversion. Corma et al.43 used LZY zeolite for levulinic acid production from fructose and reported 43 mol% yields. Shimizu et al.

44

used

Amberlyst-15 with solvent DMSO to convert fructose in 100 mol% yield by use of slight vacuum to remove water which slows down the hydrolysis of HMF to LA and condensation reaction with poor selectivity. Heterogeneous catalysts with environmentally benign solvent makes the process and catalyst separation facile.30, 45, 46 Recently Weingarten et al. 47 reported two-step process for production of levulinic acid using solid acid catalyst. This two-step nature made the process tedious, time consuming and also make the use of high temperature range.


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Chareconlimkun et al.48 and Watanabe et al.

49

showed that ZrO2 has acidic and basic sites,

which are responsible for isomerization of glucose to fructose. Lin et al.

50

reported ZrO2

catalyzed aqueous phase partial oxidation (APPO) process for LA production with 51.9 % yield. Sulphated zirconia is an effective catalyst for producing 5 HMF from fructose in non-aqueous media but it is less active in water as a solvent.51 Weingarten et al.52 have used zirconium phosphate (ZrP) and tin phosphate with varying ratio of metal (IV) for conversion of cellulose to 5 HMF and Levulinic acid, the catalyst ZrP2 showed highest Levulinic acid selectivity of 22% with 64 % glucose conversion. It is known that the zirconium dioxide has acidity53, 54 this paper reports the use of zirconium dioxide as solid acid catalyst for conversion of cellulose to platform chemical such as levulinic acid in a single step by hydrothermal treatment. The effects of various reaction parameters on cellulose conversion, product distribution, the recovery and recycle of catalyst and catalyst characterization have been studied in detail. 2.0 Experimental section 2.1 Materials Microcrystalline cellulose (99 %), zirconium dioxide (99 %) , zirconium oxychloride and all the analytical standards such as LA (98%), standard sugar Kit ( 98 % ), furfural ( 99 % ), 5HMF ( 99 % ), cellobiose ( 98 % ), 1, 6 anhydro-β-D-glucose ( 97 % ) and 1, 2 dihydroxy acetone dimer ( 97 %) were procured from Sigma Aldrich and used without further puriﬁcation or treatment. Sodium hydroxide 52 % aqueous solution was procured from Fluka chemicals and used as such. De-ionized water generated by Siemens make Ultra Pure Water System (Model Ultra Clear TM TWF UV) with conductivity 0.055 µS/cm used for all the experiments. 2.2 Reactions


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All experiments were conducted in a 300 ml capacity stainless steel Parr Instrument Company make autoclave. In a typical experiment, the required quantity of microcrystalline cellulose (~ 2 g), deionized water as solvents (100 ml), and solid acid catalyst (2 g) were charged into the reactor. The reaction temperature was maintained by using heating mantle and at stirring speed of 600 rpm for stipulated time. After completion of the reaction, the reactor contents were quenched by circulating chilled water through the internal cooling coil. The reaction mixture was filtered to separate the liquid products and solid catalyst. The filtrate was analyzed for the presence of monomers and oligomers by high performance anion exchange chromatography (HPAEC) and HPLC methods. The catalyst was dried in an oven at 100 oC for 12 hrs and weighed to estimate solid residue of the reaction. The catalyst recovered along with the reaction residue was activated in furnace at 400 oC for 4 hrs before reusing for the next recycle experiment. The effect of various reaction parameters such as time, temperature, catalyst loading, and substrate loading on conversion and selectivity was studied. Initially, the experiments were carried out at various temperatures ranging from 120 to 210 oC, and the best temperature was selected to study the effect of reaction time in the range 1 to 6 hrs, and catalyst loading from 0.3 wt % to 10 wt %. 2.3 Analytical Methods and Product Analysis Various sugars and oligomers formed during the de-polymerization were quantified by HPAEC method while LA, FA, 5-HMF, furfural, 1, 3 dihydroxyacetone dimer (1,3 DHAD) and mannose were quantified by HPLC method. Other dimers, trimers and oilgomers could be present in the product stream, but they were not detectable by the current methods of analysis. HPAEC: High Performance Anion Exchange chromatography analysis was done using Dionex ICS-3000 system equipped with an ED 50 electrochemical detector having internal


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amperometry, quaternary gradient pump GS 50 and Chromeleon software. A CarboPac PA-10 (4mm x 250 mm) analytical column was used for the separation of glucose and other sugars and oligomers using sodium hydroxide (100 mM) and gradient program of 90% water 10% NaOH solution as an eluent. HPLC: Reverse phase High Performance Liquid Chromatography (HPLC) analysis was done using Dionex Ultimate 3000 system with a binary gradient pump, auto sampler a UV / VIS detector and Chromeleon software. The analysis was carried out on a Thermo make C18 column (5µm x 4.6mm x 250 mm) using 1: 9 v/v acetonitrile: water and 0.1% formic acid (in each) pH= 2 as mobile phase with a flow rate of 1ml/min and a column temperature of 40 °C. External standard calibration method was used for quantification of various products. The yield of products on a molar base was calculated using the following equation, Conversion (%) = N – M / N ×100

Eq. no.1

Product yield (mol %) = 100M/N

Eq. no.2

where N is total amount (mol) of glucose monomer in cellulose charged in the reactor and M denotes the amount (mol) of products after reaction. 2.4 Catalyst Characterization FTIR: A Perkin Elmer Spectrum One FTIR was used for the characterization of the catalyst using KBr pallets, made by using 3-4 mg of catalyst sample in 200 mg of pre-dried KBr powder. The spectra were recorded in the range of 450–4000 cm−1. 13

C CPMAS: Samples were analyzed using a Bruker Cryomagnet BZH 300 MHz spectrometer

with a broad band solid state probe. The samples were packed into 3.2 mm silicon nitride rotors. The 13C CPMAS spectrum of all sample performed using set parameter such as Magic angle spinning was applied using 3.2 mm silicon nitride rotors at a frequency of 8 kHz.13C CPMAS


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experiments with arrays to optimize contact times for cross polarization were also run. Spectrum was collected with 2841 scans using the optimized contact times (0.5 ms) and relaxation delays 3 second for each sample. All samples were spun at room temperature 25 oC, which at a spinning speed of 8 kHz will cause the sample to experience a temperature of ∼28 oC in the probe. TGA: The thermal analysis of catalysts was performed using the Perkin Elmer TGA-7 model under N2 atmosphere at temperature 50-900 °C with a heating rate of 10 °C/min and loading 10 mg of the catalyst sample. XRD : The powder X-ray diffraction (XRD) patterns of the zirconia catalysts were measured by Rigaku Dmax 500 diffractometer using nickel filtered Cu Kα radiation (λ=1.814 Å). The sample was rotated in order to minimize textural effect. Diffractogram was recorded in a range between 10o to 80o of 2ϴ at a scanning rate of 0.06 o/s at a temperature of 25 oC. The mean size of the crystalline domain was estimated by Scherrer equation. The JCPDS database was used for the phase identification. TPD: The NH3-TPD measurements were done by using Micromeritics AutoChem II 2920 V3.05 instrument, a thermal conductivity detector was used for continuous monitoring of the desorbed ammonia and the areas under the peaks were integrated to estimate the amount of acid in the catalysts. The accurately weighed (100 mg) sample was heated at a rate of 10 oC/min up to 100 oC and kept for 20 min in a flow of helium gas (35 ml/min) to remove adsorbed species on the surface. The sample was cooled down to 50 oC in a flow of Helium gas,

followed by

adsorption of NH3 (10% NH3 gas in He, 50 ml/min) for 1 hr. The sample was flushed with Helium (50 ml/min) for 1 hr to remove the physically adsorbed NH3; the TPD data was recorded from 100 oC to 200 oC with a ramp of 10 oC/min.


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BET: The BET measurements had been performed at -196 oC using Autosorb-1 Quantachrome apparatus with nitrogen as an adsorbate. The 10 mg of the sample was loaded and degassed initially at 250 oC for 1:30 hrs in the presence of helium with the flow of (20 ml/min).The linear BET equation was used to calculate the monolayer volume and surface of samples. XPS: X-ray photoelectron spectra (XPS) were recorded using a VG Scientific ESCA-3 Mk-II electron spectrometer fitted with an Al Ka source (soft X-ray source at 1486.6 eV, which is nonmonochromatic). The anode was operated at 120 W (12 kV, 10 mA) and the analyzer was operated at constant pass energy of 50 eV. The photoelectrons were collected at an electron takeoff angle of 60ϴ. The binding energy shifts due to surface charging were corrected using the C 1s level at 285 eV, as an internal standard. The XPS peaks were assumed to have Gaussian line shapes and were resolved into individual components by a non-linear least squares procedure after proper subtraction of the baseline. 3.0 Results and Discussion The de-polymerization of cellulose under hydrothermal conditions in the presence of zirconium dioxide as a solid acid catalyst has been studied in this work. The effect of water on cellulose hydrolysis has been extensively studied in the past.

55-57

It is known that water can act

as a reactant as well as catalyst during hydrolysis and dehydration reaction due to distinct properties of water at elevated temperature and pressure.58 Varhergyi et al.59 investigated the effects of water on cellulose decomposition and found that the decomposition was enhanced in the presence of water and cellulose is hydrolyzed in the presence of water to produce watersoluble products. Hence water has been used as solvent for de-polymerization of cellulose in this study. The effect of various reaction parameters such as reaction time, temperature, catalyst concentration etc. was investigated on cellulose de-polymerization using zirconia as a catalyst.


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In order to understand the role of catalyst in cellulose conversion under hydrothermal conditions, a blank run (without catalyst) was conducted by treating cellulose with water and was compared with catalytic run using zirconia as a catalyst under identical reaction conditions. The Table-1 clearly shows that cellulose conversion to LA and HMF significantly increases in the presence of catalyst zirconia. Water soluble fraction of the blank run yielded in mainly glucose and cellobiose around 18 mole % each, along with small levels of 5- HMF, LA and furfural accounting for less than 3 mole %. However, the catalytic run of cellulose showed 54 mol% yields of LA with 2-5 mole % of 5-HMF and furfural each, reducing sugars 3-5 %. In addition to this 25-40 wt % of darkbrown mass was also detected.

47

The experimental finding shows trace amount of LA was

formed in absence of the catalyst, however, in the presence of zirconia catalyst LA yield increased significantly up to 54 mol %. Thus, the hydrous zirconia catalyst showed considerably higher activity towards the formation of LA. Table-1: % yields of Blank and catalytic run of cellulose de-polymerization in water % Yield Products

5HMF

LA

1,3 Furfural Dextrose Cellobiose Fructose 1,6 Total AHGD DHAD

Blank Run

1.0

0.7

0.5

With Catalyst

1.7

53.9 5.5

18.0

18.0

0

0

0

38.2

0.7

0.1

1.8

1.2

0.8

65.7

Reaction conditions: 2 g microcrystalline cellulose, 2 wt% of catalyst in 100 ml deionized water; ◦

reaction temperature 180 C and reaction time 3 hr

3.1 Catalyst screening


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In order to understand the role of solid acid catalyst in cellulose conversion under hydrothermal conditions, various solid catalysts such as mesoporous materials, metal oxide and ion exchange resins having acidic properties were screened under identical reaction conditions. Some of the results are presented in Figure-1. It was observed that all these solid acid catalysts resulted into 100% cellulose conversion under the test conditions and gave formic acid, levulinic acid and furfural as the major products. The performance of zirconia was found to be excellent; giving highest yield for levulinic acid, and hence was selected for further investigations. 90 80 70

FA yield LA yield

60 50

5-HMF yield

%

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40

Furfural yield

30 20

Accountability , %

10 0

Figure 1 .Catalyst screening Reaction conditions: 2 g microcrystalline cellulose, 2 wt% of catalyst in 100 ml deionized water; reaction temperature 180 ◦C and reaction time 3 hr. Catalyst *ZrO2 is Sigma Aldrich make, catalyst #ZrO2 was synthesized in our lab from zirconyl chloride using standard procedure

62

,

while TPA/ZrO2 is 5% tungsto phosphoric acid loaded on Sigma Aldrich zirconia using procedure reported in the literature.63 Catalyst Zirconium dioxide synthesized in the lab was used for all reactions.


11


3.2 Effect of catalyst loading Catalyst amount plays an important role in the kinetics of transformation and that needs to be optimized to achieve maximum selectivity to the desired product. The experiments were performed by varying the catalyst loading in the range 0.5 to 10 wt % keeping other reaction conditions constant and the results are presented in Figure-2. The graph indicates that the yield of LA was higher for higher catalyst loading during the initial 3 hr of the reaction. It was observed that the yield of LA increased from 30 to 54 mole % as the catalyst loading was increased up to 2 wt. % . Further increase in the amount of catalyst increased the yield of FA while the LA yield and total accountability dropped significantly. Noticeable quantity of brown mass was observed in this reaction mixture, which was characterized by IR and NMR and showed close resemblance with humins as reported by Patil and Lund.57 The humins formation may occur due to aldol addition and condensation reactions of 5-HMF.

100 90 80

%

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70

Conversion, %

60

FA yield

50

LA yield

40

5-HMF yield

30

Furfural yield

20

Sugars yield

10

Accountability , %

0 0

2

4

6

8

10

Catalyst loading, %


12

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Figure 2 .Effect of catalyst loading Reaction conditions: 2 g microcrystalline cellulose, 100 ml deionized water; reaction time 3 hr, reaction temperature 180 oC 3.3 Effect of reaction temperature The reaction temperature is one of the important parameters in hydrothermal reaction as it influences greatly the properties of the solvent and hence modifies the rate and the mechanism of the reaction. The effect of reaction temperature on the cellulose conversion was studied in the range 120 to 210 oC. Figure-3 shows that the reaction temperature plays an important role in cellulose depolymerization leading to the formation of LA. Conversion of cellulose to LA was adversely affected at 120-160 oC with fall in yield of LA and slight increase in the yield of sugars, 5-HMF and furfural. This shows that 120-160 oC was unfavorable temperature range for dehydration reaction. When the reaction temperature was increased from 160 to 180 oC, there was a significant increase in the yield of LA with 100% conversion. Further increase in temperature enhances the rate of chemical reaction along with possibility of side reactions and increase in formic acid yield. When the reaction temperature was raised above 180 oC, the yield of LA dropped with significant decrease in total accountability. Glucose as well as 5-HMF was not detected above 200 oC. Formation of water soluble and insoluble humins47, 57 at elevated temperature could be the cause of low selectivity and total accountability. Therefore, increase in temperature found to be inappropriate condition for the selectivity of desired reaction and the ideal temperature was set to 180 oC for further experimentation.


13


100 Conversion, %

80

FA yield %

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60

LA yield 5-HMF yield

40

Furfural yield 20

Sugars yield Accountability , %

0 100

120

140

160

Temperature,

180

200

oC

Figure 3.Effect of reaction temperature Reaction conditions: 2 g microcrystalline cellulose, 2 wt % catalyst 100 ml deionized water, reaction time 3 hr 3.4 Effect of reaction Time In order to understand the effect of reaction time on the cellulose conversion, the experiments were conducted in the range 2 to 6 hr of reaction time. The results are presented in Figure-4, which shows that the reaction time has significant impact on the conversion of cellulose and selectivity to the monomers obtained. Reaction time of 2 hr showed 90% conversion with 26 mole % yield of LA and only 50 % monomers accountability. When the reaction time was increased from 2 to 3 hour, there was significant increase in the yield of LA with 100 % conversion. Beyond 3 hours of reaction time, total accountability and the LA yield was lowered along with increase in the yield of FA. These observations suggest that the dehydrated products may be unstable at elevated temperature and


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on prolonged reaction time it undergoes decomposition and other side reactions to form humins and char.57-60 . When the reaction was carried out for more than 3 hr, the glucose, cellobiose as well as 5-HMF were not detected. Thus, the prolonged reaction time severely affects the selectivity of the desired monomer and hence the optimum time was set to 3 hour in these experiments.

100 Conversion, %

80

FA yield %

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


60


40

Furfural yield 20


0 0

2

4

6

8

Reaction time, hrs

Figure 4. Effect of reaction time Reaction condition: 2 g microcrystalline cellulose, 2 wt % catalyst, 100 ml deionized water, reaction temp 180 oC 3.5 Effect of substrate concentration It is well known that in most of the cases rate of reaction increases on increasing the substrate concentration. To understand the effect of substrate concentration, experiments were conducted with substrate loading from 1 to 4 wt %. Figure-5 shows the effect of substrate loading on conversion and LA yield. It is observed that at 2 wt%. cellulose concentration, 100% conversion


15


and highest possible yield of LA (54 %) was obtained and on further increasing the concentration up to 3 wt %. the conversion dropped to 60 % This could be assigned to the availability of optimum surface area and active sites of the catalyst to bring the transformation, which resulted in high yield of LA with excellent accountability.

100 Conversion, %

80

FA yield %

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

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60


40

Furfural yield 20


0 0

1

2

3

4

Substarte conc.,wt. %

Figure 5. Effect of substrate concentration Reaction condition: 100 ml deionized water, 2 wt % catalyst, reaction time 3 hr, reaction temp 180 oC Therefore, increase in the substrate concentration beyond 2 wt % is unfavourable condition as far as total conversion and yield is considered. Hence optimum substrate concentration was set to 2 wt % with respect to catalyst amount in this experiment.

3.6 Catalyst recycle studies The recovery and reusability of the catalyst are obviously the most important factors from process economic point of view for the production of LA. Recycle of catalyst, prolonged thermal


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stability and ease of separation are the key characteristics of environmental benign catalyst and the hydrous zirconia was not an exception for this. After the first run of the reaction the catalyst was separated from the liquid phase by filtration and recycled. The studies showed that the catalyst do not show similar activity; this could be due to deposition of organics such as humins formed during the reaction. It was observed that the catalyst regain its complete activity on calcination.33 The recovered catalyst was activated at 400 oC for 4 hr to remove the deposited humins and organic matter. Further runs of the reaction were performed by using previous run activated catalyst. The results are shown in Figure-6; it shows that no loss of the catalytic activity and selectivity was observed and possible to recycle the catalyst several times

100 Conversion, %

80

FA yield %

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


60


40

Furfural yield 20


0 0

2

4

6

Recycle runs

Figure 6. Effect of catalyst recycles Reaction condition: 2 g microcrystalline cellulose, 2 wt % catalyst,100 ml deionized water, reaction time 3 hr, reaction temp 180 oC The thermal stability and reusability of the catalyst for cellulose de-polymerization was also confirmed by TGA, XRD and TPD analysis. The XRD characterization of the fresh and used catalyst for 1st, 2nd and 5th run of the reactions shows no change in the peak position and the


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profile, while TPD analysis indicated marginal change in the acidity of the catalyst confirming that the catalyst does not undergo any changes during the de-polymerization of cellulose. 3.7 Mass Balance Material balance is one of the important parameter in all chemical transformations. Catalytic de-polymerization of cellulose accounts for 65% yield of water soluble products. It is well known that mechanistically LA and FA are formed in 1:1 proportion. Among aqueous products LA is the major product with 53.9 % yield. Dehydration products 5-HMF and furfural were 1.7% and 5.5 % respectively. All known reducing sugars contributes 4.5 % yield. It is known that two types of humins, water soluble and water insoluble were formed during the catalytic hydrolysis -dehydration process of cellulose or sugars in water as a solvent.54 Dry solid known as water insoluble humins obtained from recovered catalyst and precipitated humins from prolong kept aqueous phase / aqueous reaction mixture

were 7.2 % and 18.6 % respectively making total

mass balance 91%. Solid state NMR and FTIR study showed no structural difference for both types of humins. Unidentified products may be dimers or oligomers and observed to be around 8.5 % for this reaction.50

3.8 Mechanism of Catalytic de-polymerization of cellulose to LA The conversion of cellulose to LA is a complicated multistep process, the polymer chains of cellulose break-down into oligomers and monomer fragments such as cellobiose, glucose etc. Further, the glucose is decomposed to 5-HMF and furfural. The dehydration of 5-HMF leads to LA and FA. 45 Scheme-1 shows various products formed after de-polymerization of cellulose. Several metal chlorides as well as sulfated solid acid 33, 46 have shown precise catalytic effects on the conversion of cellulose and glucose via partial complex formation, such complex may


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occur due to the formation of oxo complex between free hydroxyl groups on cellulose with zirconium hydroxide.53-55 In particular the exceptional effectiveness of zirconium hydroxide in the conversion of cellulose not only reflected in the present investigation for the LA formation in water, but also in Li’s work for the 5-HMF production in ionic liquid and Lu Lin’s work with metal chloride.33,

46

Zirconia in hydroxide form can have partial coordinating abilities with

hydroxyl groups of the reactant similar to Li’s et al.33 observation for CrCl3 in [C4mim]Cl form [C4mim]n[CrCl3+n] complexes. The zirconia in hydroxide form can form partial oxo complex that facilitate cellulose hydrolysis followed by glucose dehydration

53-55

and the reaction

mechanism could be similar to that in ionic liquids and CrCl3. 16, 46 A plausible reaction mechanism for the zirconium hydroxide catalyzed hydrolysis of cellulose to LA in water is thus proposed, and is as shown in Scheme -2. In the first step of mechanism, hydrous zirconia forms oxo complex with glycosidic oxygen of cellulose.55 Such complex between zirconia and oxygen could lower the energy of activation. Lewis acidity would help the breakdown of the glycosidic linkage, which can undergo reversion, epimerization and dehydration reactions. The oxo complex also facilitates the glucose degradation via mutarotation of the α-anomer of glucose to the β-one through hydrogen bonding. It is well known that α and β form of glucopyranoside undergoes isomerization to form unstable 5-HMF and which on dehydration reaction can get converted into LA and FA. Similar findings were also reported by Lincai Peng et al. 46 and Amarasekara, A.S. et al.56


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Page 20 of 37

Scheme 1: Hydrolysis and dehydration reactions of cellulose


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Scheme 2: Proposed mechanism for the ZrOH4 [IV] catalyzed conversion of cellulose to LA under hydrothermal conditions 4.0. Catalysts characterization 4.1 XRD analysis The X-ray diffraction (XRD) patterns of zirconia catalyst from different runs are shown in Fgure-7. The recycled catalysts were calcined at 400 oC for 4 hours before taking XRD to remove organic residue deposited on the catalyst surface. The figure also shows XRD profiles of zirconium oxide before and after the first and fifth runs. The formation of tetragonal ZrO2 phase was observed in zirconium oxide. It shows high crystallinity. The crystallite sizes of zirconium oxide 16 nm and 17 nm were determined from the half width of corresponding peaks using


21


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Page 22 of 37

Scherrer equation. In the case of the regenerated zirconium catalyst after first (b) and fifth (c) run, it can be observed from Figure-6 that the tetragonal ZrO2 phase is preserved throughout all the recycle runs. The crystallite size of zirconium oxide changed slightly after the catalyst reuse cycle, as shown in Figure-7 these results show that the ZrO2 crystallization structure remains unchanged. The results also indicate that zirconium oxide is thermally stable before and after the reaction.

Figure 7. XRD pattern of catalyst (a) Fresh zirconium oxide ;(b) zirconium oxide after 1st reuse; (c) zirconium oxide after 5th reuse .a, b and c were calcined at 400 oC for 4 hr 4.2 BET surface areas The BET surface areas of catalyst before and after the reactions are listed in Table-2. The specific surface area of the calcined zirconium oxide catalysts were determined by BET method. As it can be observed that the BET surface areas of 1st and 5th run catalyst were marginally lower as compared to fresh ZrO2 catalyst. The repeated use of catalyst changes the porosity


22

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which in turn affects the pore volume and pore size and could be the reason of marginal decrease in the surface area. Catalyst BET surface area (m2/g) Total pore volume (cc/g) Pore Dia (Å) ZrO2

3.71

0.0389

389

1st Run

3.58

0.0359

359

5th Run

3.52

0.0352

352

Table 2. BET surface area, pore volume and pore size of catalyst 4.3 NH3-TPD Measurement Catalyst

Total NH3 desorbed (mmol/g)

Peak temperature (◦C)

Lit. Ref.

ZrO2(IV)

0.4048

180

This study

Zr(OH)4

0.611*

100

43

SO4 2−/ZrO2

0.57

180,566

41

SO42−/ZrO2 0.49 after 5th reuse

189, 521

41

Table-3: NH3-TPD of catalysts *acidity was determined by titration with n-butyl amine using methyl red indicator It has been reported in the literature that zirconia in hydrous as well as in sulfated form show the similar Lewis acidity. 23 The TPD profiles of desorbed ammonia (NH3) on various samples of catalyst from literature and our experiment is shown in Table-3, the desorption temperature indicates the acid strength of the catalyst. Quantity of adsorbed NH3 was estimated by integrating the areas under the peaks. Amount of acid was corresponding to the amount of adsorbed


23


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NH3.These finding indicate that zirconia with Lewis acidity in hydrous form could be acting as solid acid catalyst and facilitate cellulose hydrolysis and dehydration reactions. 4.4 XPS analysis The major reason for conducting XPS analysis was to obtain information regarding to chemical nature and composition of element present in the fresh and regenerated zirconium catalyst. Calcination of the catalyst was performed before the analysis to avoid interference of impurity such as humins deposition. Table-4 shows the elemental composition of the fresh and regenerated zirconia catalyst and confirmed by conducting XPS analysis. It can be seen that the calcined zirconia at 400 oC consists of only Zr and oxygen elements. Spent and regenerated catalysts showed nearly similar binding energy and intensity. These observations support the unchanged chemical environment of the catalyst even after the repeated use. XPS analysis of fresh, first recycle and 5 th recycled catalyst shows that Zr 3d photoelectron peaks at 183 eV for Zr 3d5/2 line and 187 eV for Zr 3d3/2 line however, the binding energy of the Zr 3d5/2 line in pure ZrO2 sample ranges from 183 to 187 eV. 23, 60 The complex nature of the O 1s photoelectron spectrum could be due to overlapping effects of trapped hydroxy or intervention form atmospheric oxygen.

XPS analysis study showed not much variation in

elemental composition and there by activity of catalyst on repeated use. Atomic percentage and Lewis acidity available during the chemical reaction remains constant. Catalyst

Zr (%) O (%)

Fresh ZrO2

22.3

77.7

ZrO2 after 1st reuse 21.9

78.1

ZrO2 after 5th reuse

77.3

19.6

Table 4.Compositions by XPS analysis for fresh and regenerated zirconia catalysts.


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4.5 Thermo gravimetric analysis (TGA) Further the catalysts were characterized for their thermal properties. Figure-8 shows TGA of Zirconia in oxide form superimposed with un-calcined zirconia in hydrated form of run-1 to run3. Zirconia in oxide form shows excellent thermal stability, however catalyst recovered from run -1 and run -2 show first weight loss around 100 oC due to presence of moisture and second weight loss at 330 oC could be due to the decomposition of deposited organic matter on the catalyst. This transition is absent in standard zirconia. Catalyst from Run-3 shows first weight loss around 100 oC due to moisture. At 353 oC a maximum weight loss was observed for this sample. This weight loss was much larger in comparison to run 1 and run 2. This could be attributed to the decomposition of deposited, unconverted, reaction products, humins and de-hydroxylation process to form ZrO2 from hydroxy form. Above 390 oC the heat effect of the phase transformation from amorphous zirconia to tetragonal zirconia and from tetragonal zirconia to monoclinic zirconia could take place. Similar findings were also reported by Xinhua Qi.51 Run-3 shows maximum deposition of humins as the catalyst for run-3 was directly used from run -2 without activation and calcination hence showed almost 40% weight loss. This shows that the deposition on catalyst increases with recycle runs. This could be the reason for fall in activity and hence accountability. Hence activation (at 400 oC for 4 hr) of the catalyst after the reaction is necessary in order to regain catalyst performance.


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Figure 8. TGA profile of un-calcined zirconia recycled catalyst 4.6 FTIR and 13C CPMAS study Main objective to conduct FT-IR and

13

C CPMAS study was to confirm the formation of

humins during the LA production from cellulose hydrolysis and dehydration. The humins deposition was studied after every run. For this purpose the catalyst of first run was directly used for the second and third run without activation. Figure-9 shows FTIR spectra of recycled zirconia catalyst for runs 1-3 superimposed with standard zirconia. The bands at 1630-1700 cm-1 in the run-1 recycle catalyst is due to the carbonyl group, the broad band around 3300 cm-1 attributed due to hydroxy and absorption bands at 1607, 1235, 1162, 1033, 1059, 897, and 743 cm-1 are vibrations associated with either furan ring or the hydroxymethyl group of the humins structure. Intensities of these absorption bands increase with progressive recycling indicating the increased deposition on catalyst with every cycle. The CPMAS spectra shows broad and strong signal in the region 100-160 ppm


26

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attribute to C=C in conjugation (108-119) and

=C-O-C= (150-155 ppm) from furan ring in

humin structure. The weak signal at 174 and 208 ppm could be due to dalkyl ketone and carbon in aromatic carbonyl attributes to carbonyls from humin. The structures of humin formed in our reactions resembles to the structure proposed by Horvat et al. 51, 57, 61 In addition to this there are some evidences for the reaction between vinyl hydroxy and carbonyl giving rise to cyclic ether linkage (1230, 1033 cm-1 in FTIR and around 25 ppm in CPMAS).

Std Zirconia Run - 1

989

Run-2 3202 2922

%T

Run - 3

1607 1696 1693 1607

3339

1208 1059 1301 1033 1246 1303 897

744

1235 1637 1429

1317 1372 1162

2902 3350

741

743 743 1112

1059

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 -1

600 450

cm

Figure 9. FTIR profile of un-calcined zirconia recycled catalyst 5.0 Conclusions The present work demonstrates that zirconia under hydrothermal conditions is very effective for hydrolysis of cellulose and subsequent conversion to LA in a single step. Experimental results showed that total conversion of cellulose with 53.9 mol% yield of LA under the optimal


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Page 28 of 37

reaction conditions can be achieved, which is the best result reported so far in the literature. The yield of LA is significantly influenced by the reaction temperature, time, substrate concentration and amount of catalyst. The zirconia catalyst is recoverable from the resulting product mixture and can be reused multiple times after calcination without any loss of the catalytic activity. IR and CPMAS studies supports the formation of humins from HMF and 2,5-dioxo-6hydrohexanal as proposed by Horvat by aldol condensation mechanism. It seems under hydrothermal conditions, the hydroxy form of zirconia shows similar Lewis acidity as other reported solid acid catalysts and facilitate cellulose hydrolysis and dehydration reactions. A plausible reaction mechanism for the zirconium hydroxide catalyzed hydrolysis of cellulose to LA in water has been proposed We proposed that hydrous zirconia, an environmentally benign catalyst with higher activity and ample Lewis acidity could be used for variety of chemical transformations where acidity plays an important role.

ASSOCIATED CONTENT The following material is supplied as supporting information. Calibration curves, for quantification, mass balance, XPS and CPMAS of fresh and recycled catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Sunil S. Joshi


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Chemical Engineering and Process Development Division, National Chemical Laboratory, Dr. Homi Bhaba Road Pune-411008, India. Fax: +91 20 25902621; Tel: +91 20 25902745; E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All the authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by CSIR-NCL and we are thankful for financial support. REFERENCES 1.

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Efficient Conversion of Cellulose to Levulinic Acid by Hydrothermal

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