SBA-15 catalyst

    Vapor phase esterification of levulinic acid over ZrO 2 /SBA-15 catalyst E. Siva Sankar, V. Mohan, M. Suresh, G. Saidulu, B. David Raju, K.S. Rama Rao PII: DOI: Reference:

S1566-7367(15)30108-4 doi: 10.1016/j.catcom.2015.10.013 CATCOM 4471

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

29 July 2015 15 September 2015 5 October 2015

Please cite this article as: E. Siva Sankar, V. Mohan, M. Suresh, G. Saidulu, B. David Raju, K.S. Rama Rao, Vapor phase esterification of levulinic acid over ZrO2 /SBA-15 catalyst, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.10.013

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ACCEPTED MANUSCRIPT Vapor phase esterification of levulinic acid over ZrO2/SBA-15 catalyst

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E. Siva Sankar, V. Mohan, M. Suresh, G. Saidulu, B. David Raju, K. S. Rama Rao* Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,

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Hyderabad, India-5000071. E-mail: [email protected] & [email protected]

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40-27160921; Tel: +91-40-27193163

Fax: +91-

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Abstract:

ZrO2/SBA-15 catalysts with different ZrO2 loadings prepared by impregnation method

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were tested for the esterification of levulinic acid in a fixed bed reactor in vapor phase conditions

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at atmospheric pressure for the first time and found that the catalyst with 7 weight percent ZrO2

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exhibited 100% conversion of levulinic acid and 96% selectivity to methyl levulinate due to the presence of more number of moderate acidic sites. It is interesting to observe the formation of γ-valerolactone as the alcohol chain length increases at the expense of alkyl levulinate, through MPV reduction mechanism. Key words

ZrO2/SBA-15, esterification, levulinic acid, alkyl levulinate, -valerolactone, MPV reduction

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ACCEPTED MANUSCRIPT 1 Introduction: Levulinic acid is an inexpensive renewable resource, obtained through the acid hydrolysis

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of pentose’s and hexoses [1, 2], and is considered as a promising chemical for the production of

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chemicals, fuels and fuel additives. This molecule has been the subject of focus in recent times

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owing to presence of both highly reactive functional groups [1, 3-5]. In recent times, catalytic transformations of levulinic acid to valuable compounds including alkyl levulinate,

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γ-valerolactone and others have been extensively reported [2, 5-10]. To obtain these valuable

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compounds, various methods like hydrogenation, esterification, reductive amination, oxidation, etc., can be applied. Amongst, esterification of levulinic acid seems to be important because the

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compounds synthesized through this methodology find applications in flavouring agents,

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plasticizers, solvents and fragrances etc., and as fuel additives [6-9]. The esterification of levulinic acid has been commonly performed in presence of inorganic acids like H2SO4 [9-14].

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The disadvantages associated with the use of inorganic acids are their corrosiveness, lack of recyclability and separation problems after the reaction. In this scenario, many groups tried the application of solid acids such as heteropoly acids, zeolites and sulphated metal oxides for the esterification of levulinic acid [6-9]. Most of the catalysts for the esterification of levulinic acid were operated in batch conditions. In this scenario, there is room for the design and development of heterogeneous catalysts for the continuous processes. The advantages of continuous process are no solvent usage, ease of catalyst separation and can be operated for longer hours. Herein, we report esterification of levulinic acid over mesoporous ordered SBA-15 supported ZrO2 catalysts in vapor phase at atmospheric pressure.

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ACCEPTED MANUSCRIPT 2 Experimental 2.1 Catalyst preparation

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SBA-15 designated hereafter as ‘S’ was synthesized using a procedure described

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elsewhere [15, 16, 17]. The synthesized SBA-15 was used as a support for the preparation of ZrO2 /SBA-15 catalyst. ZrO2/SBA-15 catalysts with various zirconia loadings were prepared by

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impregnation method using zirconium acetyl acetonate as a precursor. These catalysts were

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designated as 3ZS, 7ZS and 10ZS respectively where the numerical stands for the weight percent of ZrO2. The composition of ZrO2 was calculated by using ICP-OES iCAP6500 DU

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(M/s.Thermo Scientific, USA) instrument. 2.2 Catalyst characterization

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Textural properties of SBA-15, ZrO2 and ZrO2/SBA-15 catalysts were measured by means of nitrogen adsorption and desorption measurements at liquid nitrogen temperature on a

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Quadrasorb SI system (M/s. Quantachrome Instruments, USA). BJH method was applied to get the pore size distributions. Structural characterization was obtained on a Ultima IV X-ray powder diffractometer, (M/s. Rigaku Corporation, Japan) with a scanning step of 0.02° using Ni filtered Cu Kα radiation (λ= 1.5406 Å) with a scan speed of 4° min−1 and a scan range of 10 – 80° at 40 kV and 20 mA. The detailed characterizations of the catalysts were described in electronic supplementary material. 2.3 Catalytic activity The vapor phase esterification was performed in fixed-bed quartz reactor (14 mm id and 300 mm long) at atmospheric pressure. 0.5 g of the catalyst and same amount of the quartz particles were mixed and loaded at the centre of the quartz reactor and flushed at 523 K for 1 h in N2 flow of 1800cm3h-1. The liquid feed with required molar ratios of LA and alcohol (1: 7)

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ACCEPTED MANUSCRIPT (unless specified) were continuously fed at a flow rate of 1cm3h-1 (unless specified) using a syringe pump (M/s. B. Braun, Germany). The product mixture was collected in an ice cooled

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trap at regular intervals and analyzed by a FID equipped GC-17A (M/s. Shimadzu Instruments,

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Japan) with EB–5 capillary column (30 m x 0.53 mm x 5.0  m) and confirmed by GC-MS, QP5050 (M/s. Shimadzu Instruments, Japan) with EB–5 M/s. capillary column (30 m x 0.25 mm x

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0.25  m)

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3 Results and discussion 3.1 X-ray Diffraction Analysis

SBA-15 and ZrO2/SBA-15 catalysts (Figure-1) exhibited a well-

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XRD patterns of

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resolved pattern with a sharp reflection at a 2 value of around 0.9° and another two weak peaks

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at around 1.6° and 1.7° which are in line with the earlier reports [15,18]. The intensities of these peaks are found to decrease as ZrO2 loading increases. The estimated unit cell parameters of a

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hexagonal lattice, d (100) are shown in Table 1. Interestingly, larger unit cell parameters are observed for the ZrO2 incorporated samples which may due to the incorporation of ZrO2 species into the framework of mesoporous SBA-15 [19, 20]. Reflections due to ZrO2 are absent in the wide angle XRD patterns (inset of Figure-1) indicating the amorphous nature of ZrO2 in all the ZrO2/SBA-15 catalysts. In order to confirm the presence of ZrO2, these catalysts were subjected to ICP-OES and found 2.7, 6.8 and 9.3 weight percent of ZrO2 in 3ZS, 7ZS and 10ZS catalysts respectively which clearly indicates the closeness of ZrO2 content with the theoretical values. 3.2 Nitrogen adsorption-desorption studies N2 adsorption-desorption isotherms of SBA-15 and ZrO2/SBA-15 catalysts are shown in Figure-2. All the N2 adsorption-desorption isotherms except parent ZrO2 are found to be of Type IV in nature as per the IUPAC classification and show evidence of a H1 hysteresis loop

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ACCEPTED MANUSCRIPT corresponding to mesoporous solids [21].

Besides, the adsorption branches of isotherm

demonstrated a sharp inflection at a relative pressure of 0.7 < P/P0 < 0.85 which is due to the

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capillary condensation in the ordered mesopores [22]. Furthermore, the position of the inflection

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point is obviously associated to a diameter in the mesopore range, and the sharpness of these steps specifies the evenness of the mesopore size distribution [22, 23]. The pore size distribution

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curves (inset of Figure-2) indicates that majority of the pores are concentrated at around 7 nm

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even though this value is different from the average pore size (~6 nm). Generally, the pore wall thickness of SBA-15 type mesoporous materials will be lower than average pore size, but there

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are instances where both values are more or less same [24- 26 ].

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3.3 UV-vis Diffuse Reflectance Spectroscopy (UV-vis-DRS)

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The UV-vis-DRS patterns of ZrO2/SBA15 catalysts (ES1) show an absorption band in the

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range of 205−215 nm) [20, 26−30]. This absorption band could be ascribed to ligand-to-metal charge transfer (LMCT i.e., from an O2− to an isolated Zr4+ ion in a tetrahedral configuration). This peak is due to the presence of isolated Zr4+ ions in the SBA-15 mesoporous frame work [20]. In addition, a diverse broad band of low intensity at ∼280 nm was observed in all the ZrO2/SBA15 samples which might be due to the highly distributed ZrO2 nanoparticles [24, 25, 29]. Furthermore, an additional band in the vicinity of 230 nm appeared in the UV-vis-DRS patterns of 7ZS and 10ZS samples could be ascribed to the presence of bigger ZrO2 particles (may be below the XRD detection limit) in these samples [26, 27, 30]. 3.4 Temperature programmed desorption of ammonia From the temperature programmed desorption of ammonia profiles of the catalysts (ES 2), the acid site population can be classified as weak (<523 K), medium (523 - 673 K) and strong (>673 K) acidic sites [5]. ZrO2 possesses very strong acid sites. The strong acid peak identified 5

ACCEPTED MANUSCRIPT in the TPD pattern of SBA-15 material could be ascribed to the surface hydroxyl groups [31, 32] while diverse patterns are appeared in the case of zirconia incorporated SBA-15 materials. The

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total acidity of SBA-15, ZrO2, 3ZS, 7ZS and 10ZS were found to be 130, 312, 177, 276, and

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493 µmole g-1 respectively. 7ZS catalyst possesses more number of moderate acidic sites. 3.4 SEM and TEM Analysis

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The SEM and TEM images of SBA-15 and 7ZS shown in Figure-3 afford the direct proof

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of presence of well organized hexagonal array of mesopores and hexagonal channels [15]. Even after incorporation of ZrO2, the morphology of materials remains intact. In addition to SEM and

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TEM, low angle XRD and N2 sorption results also confirmed the retention of mesoporous structure even after the incorporation of ZrO2.

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4 Catalytic performances

4.1 Effect of ZrO2 loading on the activity

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Along with the esterification reaction, other pathways leading to angelica lactones and -valerolactone are also possible (Scheme-1, ES 3). The reaction may proceed according to a mechanism proposed by Fang Su et.al [33]. Based on this, the reaction mechanism (Figure-4) may follow the adsorption of carbonyl group of acid followed by generation the carbocation, further attack of nucleophile (alcohol) on the carbocation and then dehydration to form the resultant esterification product. The results on the influence of ZrO2 loading on the esterification of levulinic acid (LA) with methanol (MeOH) with liquid flow rate of 1cm3h-1 over 0.5 g catalyst were displayed in Table 2. The conversion of levulinic acid over SBA-15 and ZrO2 (commercial) are 38 and 94.5% respectively. The selectivity of methyl levulinate (ML) over SBA-15 and ZrO2 (commercial) are 80 and 69% respectively. Furthermore, the ZrO2 was also prepared in the same manner as those 6

ACCEPTED MANUSCRIPT of zirconia incorporated SBA-15 catalysts and the activity of this catalyst, ZrO2 (Prepared) for the esterification reaction, was noticed to be around 17 % conversion of LA and 82 % selectivity

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to ML. The deposition of ZrO2 onto the mesoporous silica (SBA-15) shows greater selectivity

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towards ML due to not only the presence of ZrO2 in dispersed form but also enhanced acidity obtained through the incorporation of ZrO2. 10ZS catalyst showed lower selectivity towards the

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desired product presumably due to stronger acidity (as evidenced from the NH3 TPD). The high

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acidity of 10ZS catalyst facilitates the dehydration to yield angelica lactones (6%) rather than the esterification.

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Since the 100% LA conversion with close ML selectivity were observed over 3ZS, 7ZS and 10ZS catalysts, the reaction was performed at higher flow rates. At a flow rate of 5 cm3h-1

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over 0.5g of catalyst, 85, 92, 91, 73 and 24 % LA conversions with ML selectivity’s of 84, 95, 90, 68, and 45 over 3ZS, 7ZS, 10ZS, ZrO2 (commercial), and SBA-15 respectively were

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observed. Further increase in the liquid flow rate to 7 cm3 h-1 was found to be useful to distinguish the catalytic activity. Under these conditions, The LA conversion of 72, 87 and 81 % with ML selectivity of 69, 81 and 74% were observed over 3ZS, 7ZS and 10ZS catalysts respectively. This is a clear indication that 7ZS is superior in terms of LA conversion and selectivity to ML compared to other catalysts. 4.2 Effect of molar ratio The influence of molar ratio of MeOH to (LA) to yield ML over 7ZS catalyst was illustrated in ES 4. Increase in methanol content increases the yield of ML and reaches 96% yield at LA: MeOH ratio of 1:7. Further increase in MeOH results in the formation of γ-valerolactone because the excess methanol serves as a hydrogen source. Around 3 and 7% selectivity to GVL was noticed when the molar ratio is 1:9 and 1:11 respectively. Hence, one can understand that

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ACCEPTED MANUSCRIPT the excess methanol results in the formation of GVL. Small amount of dimethylether due to intermolecular dehydration is also obtained. Even though esterification of LA is a reversible

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reaction, presence of large amount of alcohol favours the formation of ester. On the other hand,

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the silica based materials, in particular SBA-15 could play an important role in stabilising ZrO2 because of its high surface area, tunable pore size and thermal stability [34, 35]. It is well known

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from the literature that the ZrO2 catalysts can also act as a transfer hydrogenation catalyst [27,

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30]. 4.3 Effect of reaction temperature

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The effect of temperature over 7ZS for the esterification reaction can be seen in ES 5. The conversion of levulinic acid is almost complete at all temperatures. The diverse product

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distribution (formation of side products such as angelica lactone and gamma valerolactone) is noticed as the temperature increases. These studies confirm that 523 K (near to the boiling point

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of levulinic acid) is an optimum temperature for the esterification reaction. 4.4 Influence of time on stream

The results on time on stream studies over 7ZS catalyst were presented in Figure-5. The results indicate a slight decline in the selectivity to ML, may be due to the accumulation of carbonaceous species on the catalyst surface thereby decreases the availability of acid sites. In order to know the reason for decline in the selectivity during the time on stream, TPD analysis of spent catalyst was performed. The results indicate a decrease in acidity from 276 (in the fresh catalyst) to 230 µmole g-1 in the spent catalyst. Hence, one can understand that the decrease of catalytic performance presumably due to coking of the catalyst. It was observed that there was no change in the SBA-15 structure from XRD results (ES.7).

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ACCEPTED MANUSCRIPT 4.5 Effect of various alcohols 7ZS catalyst was tested for the esterification with various alcohols and the results were

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presented in ES 6. As the chain length of alcohol increases, the decrease in selectivity of alkyl levulinate results. The presence of alcohols other than methanol demonstrates the obvious

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formation of γ-valerolactone through the catalytic transfer hydrogenation via Meerwein–

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Ponndorf–Verley (MPV) mechanism. Among the propyl alcohol isomers, isopropyl alcohol is

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better hydrogen source via MPV reduction to yield γ-valerolactone. The reason may lie in the fact that the longer the alcohol chain, the more readily the hydrogen can be transferred from the

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alcohol molecule to levulinic acid molecule leading to forming more - valerolactone. 5 Conclusions

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Among the ZrO2/SBA-15 catalysts synthesised by impregnation method 7ZS is found to be better for esterification of LA due to the presence of more number of moderate acidic sites.

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This is the first report on the use of solid acid catalysts for continuous process in vapour phase with limited alcohol source. As alcohol chain length increases, we observed the formation of γ-valerolactone through the catalytic transfer hydrogenation via MPV reduction. The slight decrease in selectivity towards ML during the time on stream could be ascribed to the accumulation of minor by-products (α, β- angelica lactones) through the condensation.

Acknowledgments The authors, SSE, MV, MS and GS are grateful to the University Grants Commission and Council of Scientific and Industrial Research, New Delhi, India respectively for the award of fellowship and the services provided by

the Analytical Division, CSIR-IICT is greatly

acknowledged.

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ACCEPTED MANUSCRIPT Figure Captions Figure-1

Low angle XRD pattern of parent SBA-15 and ZrO2/ SBA-15 and in inset figure

N2 adsorption-desorption isotherms for parent SBA-15 and various ZrO2/SBA-15

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Figure-2

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wide angle XRD patterns of ZrO2/SBA-15

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samples and in inset figure pore size distribution of parent SBA-15 and various

Figure-3

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ZrO2/SBA-15

SEM images of (a) parent SBA-15 and (b) 7ZS, TEM images of (c) parent SBA-

Figure-4

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15 and (d) 7ZS

Plausible reaction mechanism for esterification of levulinic acid over ZrO2/SBA-

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Influence of time on stream of esterification of levulinic acid with methanol.

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Figure-5

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Reaction conditions: Catalyst Weight: 0.5g, LA: MeOH =1:7 reaction temperature: 523 K, N2 flow: 1800cm3h-1, Liquid feed flow: 1cm3h-1

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( m2/gm)

(nm)

(cc/gm)

S

715

5.60

1.01

3ZS

712

5.59

1.09

7ZS

701

5.57

10ZS

663

5.86

ZrO2

32

8.79

d100d

tf =ao-Dp

9.20

10.62

5.02

9.79

11.31

5.72

0.97

9.75

11.26

5.69

0.94

9.89

11.42

5.56

0.08

-

-

-

nm

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Vp c

BET Surface area

d

f

b

Average Pore size

c

Total Pore Volume

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a

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Dpb

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a0 e

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SBET a

Catalyst

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Table-1: Physico-chemical properties of ZrO2/SBA-15 catalysts

Periodicity of SBA-15 derived from a low angle XRD

Pore wall thickness

14

e

Unit cell length

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Catalyst

Conversion of LA (%)

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Table-2: Effect of ZrO2 loading on esterification of levulinic acid with methanol Selectivity to ML (%)

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flow: 1800cm3h-1, Liquid feed flow: 1cm3h-1

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3ZS 100 89 7ZS 100 96 10ZS 100 92 ZrO2 (Prepared) 17 82 ZrO2 (commercial) 94.5 69 S 38 80 Reaction conditions: Catalyst weight: 0.5g, LA: MeOH =1:7 reaction temperature: 523 K, N2

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Graphical Abstract

Vapour phase esterification of levulinic acid over ZrO2/SBA-15 catalyst

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E. Siva Sankar, V.Mohan, M.Suresh, G.Saidulu, B.David Raju, K.S.Rama Rao* Inorganic and Physical Chemistry Division, CSIR-Indian Institute of ChemicalTechnology,

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Hyderabad, India-5000071. E-mail: [email protected]; Fax: +91-40-27160921; Tel: +91-40-

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27191712

7 weight % ZrO2 / SBA-15 catalyst exhibits good activity in the esterification of levulinic acid under vapor phase conditions

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Highlights

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 Impregnation method was adopted to prepare ZrO2/SBA-15 catalysts

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 Levulinic acid esterification in vapour phase for the first time was tested

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 Catalyst with 7 wt% ZrO2 loading exhibited good activity to yield methyl levulinate  Steady activity was found over 7 wt% ZrO2 catalyst during 10h time on stream study

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 Increase in the alcohol chain length favors the formation of γ-valerolactone

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