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Solid Acid Catalyzed Etherification of Glycerol to potential fuel additives Subhash B. Magar, Sumit Kamble, Govindraj T. Mohanraj, Sumit kumar Jana, and Chandrashekhar V. Rode Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02213 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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Solid Acid Catalyzed Etherification of Glycerol to potential fuel additives Subhash Magar,a,b,c Sumit Kamble,c Govindraj Thangaraj Mohanraj,b Sumit Kumar Jana,bChandrashekhar Rode c* a

Chemical Engineering Department, Pravara Rural Engineering College, Loni-413736, India.

b

Department of Chemical Engineering, BIT, Mesra-835215, India.

c

Chemical Engineering & Process Development Division, CSIR- National Chemical

Laboratory, Pune-41008, India. *Correspondence, E-mail: [email protected], Tel: +91202590 2349; Fax: +9120259 2621 E-mail addresses: [email protected] (Subhash Magar), [email protected] (Sumit Kamble), [email protected] (Govindraj Mohanraj), [email protected] (Sumit Kumar Jana)

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ABSTRACT: Various acidic clay catalysts have been explored for glycerol and tert-butyl alcohol (TBA) reaction. Complete conversion of glycerol could be achieved over montmorillonite KSF/O clay catalyst to form a product mixture containing mono, di and tri-tert-butyl glycerol ethers (MTBGE, DTBGE, and TTBGE). The different variables such as reaction time, reaction temperature, catalyst loading, and molar ratio of glycerol to tert-butyl alcohol were investigated in detail. X-ray diffraction, Thermo gravimetric analysis, Ammonia-Temperature programmed desorption, BET surface area measurements were performed for characterization of prepared catalyst. NH3-TPD revealed that Mont-KSF/O possessed highest acidity leading to its highest activity for etherification of glycerol. Mont-KSF/O could be successfully recycled for several times without losing its activity.

Keywords: Montmorillonite clay, etherification, oxygenated fuel additives, DTBGE, TBA.

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1. INTRODUCTION Eco-friendly, non-toxic and renewable sources of fuels, also known as fatty acid methyl esters are produced by trans-etherification of triglycerides derived from low carbon track animal fats or vegetable oils are emerging as alternative to petroleum fuels. These fuels can be biogenic which would reduce economic and environmental pressure with improvement in air quality, a cheaper product or better fuel properties 1-8. Worldwide use of biofuel is rapidly increasing which would reach to 20% by use of biodiesel on-road by 2020 9. Alongwith the production of biodiesel, glycerol being a main by-product, 10-12 its overproduction is a key challenge to deal with; as the growth of biodiesel industries is progressive in recent years. Hence, there is a need to convert it into value added chemicals by adequate technologies 13-14. ETBE (ethyl tert-butyl ether), MTBE (methyl tert-butyl ether), TAME (tert-amyl methyl ether) are known to be excellent diesel fuel additives. These additives derived from non-renewable petroleum fossil fuels are costly and cause environmental pollution, so bio-derived fuel additives serving alternative to the petroleum fuel additives. The United States has banned lead in the form of tetra ethyl lead as a fuel additive more than a couple of decades ago, which was then substituted with methyl tertiary butyl ether. Few years later, MTBE was also stopped due to its health hazards as it gets easily polluted with water 15. Addition of glycerol ethers to diesel improves the combustion efficiency of thermal combustion engine. These ethers become valuable diesel additives due to their excellent combustion properties, good blending ability and high cetane numbers. Moreover, these additives also reduce particulate matter and carbon monoxide emission from incomplete combustion of diesel. Due to its low solubility and poor stability because of decomposition and polymerization at high temperature, glycerol addition directly to diesel is not a viable option16. Conversion of glycerol

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to mono, di and tri-tert-butyl glycerol ethers by etherification is one of the promising processes among different strategies for valorization of glycerol

17

. This has been also studied by several

researchers with or without solvent and catalyzed by either homogeneous or heterogeneous catalysts18. Glycerol etherification has been reported with isobutylene 19-20 and tert-butyl alcohol 21-22

.

Mass transfer between glycerol and solvent strongly affects the performance of etherification process. From this perspective due to its high miscibility with glycerol, tert-butyl alcohol becomes the obvious choice for the etherification reaction, 23. While isobutylene having very low solubility, very expensive and requires high pressure to keep in liquid form which increases handling problems. Etherification of glycerol is an acid catalyzed reaction (Scheme 1) to produce the mixture of alkyl glycerol ethers such as MTBGE (mono-tert-butyl glycerol ether), DTBGE (di-tert-butyl glycerol ether) and TTBGE (tri-tert-butyl glycerol ether). The glycerol etherification to give DTBGE and TTBGE is more preferred to MTBGE as the later has hardly any solubility in diesel. Glycerol conversion to its ethers was initially attempted by conventional mineral acids which limit the application of the process due to cumbersome work up procedures and added pollutant salts in the environments. Among the solid acid catalysts, amberlyst-15 was well studied for the etherification of glycerol using isobutylene gas. Recently, various zeolites and montmorillonite K10 were functionalized with sulphonic acid for etherification of glycerol using isobutene to achieve complete conversion of glycerol with 79-91 % selectivity to higher ethers of glycerol 14. After that t-butyl alcohol was explored as a safe and in dual role as reagent as well as solvent for the etherification due to easy and stable carbocation formation in acidic medium.

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In continuation of our interest in bio-glycerol conversions to value added products, we explored here the use of commercially available montmorillonite clays as efficient solid acid catalysts for reacting glycerol with tert-butyl alcohol to give ether products. A variety of catalysts involving montmorillonite clay were screened for the glycerol etherification and conditions were optimized by studying the parameters effect over KSF/O catalyst. 2. EXPERIMENTAL 2.1. Materials. Montmorillonite KSF/O and other montmorillonite catalysts were purchased from Fluka, India. Glycerol (99.5 %) and t–butanol (99.7 %) were supplied by Ajax Chemicals (India). Mont K10 and Mont-Al (Aluminum-Pillared montmorillonite clay) were purchased from Fluka, India. 2.2. Experimental procedure. Glycerol (0.92 g) and tert-butyl alcohol (14.82 g) were added in a 100 mL glass reactor having a provision for and heating arrangement to desired temperature within 10 min and to withdraw initial sample. After addition of the catalyst (0.250 g, 27.17 wt %), reaction was continued at 110 o

C for 6 h. Then the reactor temperature was brought down to 25 oC. The reaction performance

was measured by collecting samples for analysis at specific intervals of time by GC (Varian 3600) organized with a capillary column (HP-FFAP 30m,0.53mm,1μm) having a stationary phase of polyethylene glycol and FID detector. The analytic parameters set were: Injection temperature; 300 oC, temperature of column ramped from 40 to 240 oC, detector temperature; 150 oC and carrier gas (N2; 30 bar). 2.3. Catalyst characterization. Crystalline structure of various clay catalysts samples by wide angle X-ray diffraction patterns (WAXRD) (Instrument: Analytical PXRD model X-Pert Pro-1712, where Ni filtered Cu-Kα

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radiation (λ = 0.154 nm) have been used as a source (current intensity, 30 mA; voltage, 40 kV) and a Xcelerator as a detector). The XRD analysis was done over a range of 2θ from 10o to 80o with a step time of 15 min and a step size of 0.214o/min. BET surface area measurement was performed on Micromeritics-2720 (chemisoft TPX) volumetric instrument. Samples were degassed to remove moisture at 200 oC with holding time of 20 min. The acidity of the catalyst was measured on NH3-TPD Micromeritics -2720(Chemisoft TPX). In order to estimate acidity with Brønsted and Lewis sites of the prepared samples, the NH3–TPD measurements were carried out by performing these following experiments (i) pre-treatment of the catalyst samples from room temperature to 200 oC under the helium flow rate of 25 mL min-1. (ii) ammonia adsorption at 50 oC (iii) ammonia desorption with an initial adsorption temperature of 25 oC, a heating rate of 10 oC min-1 and final temperature of 700 oC were used in analysis. Pore size analysis of montmorillonite clay catalysts was carried out by Thermo electronics mercury porosity meter, Italy, PASCAL 440 Series type instrument. Samples were characterized using mercury porosimetry by applying different pressure range to dip the sample in the mercury. The pressure required to impose mercury into the sample’s pores is inversely proportional to the size of the pores. Pore size distribution using mercury porosimetry with the principle of capillary law governing liquid mercury penetration into the small pores which has been expressed by using the Washburn equation31 given below: 1 D  ( )4 cos  P

(1)

Where, D is expressing the diameter of pore, P is the pressure has been applied, γ is stating the surface tension of mercury and φ is stating the contact angle between the mercury and sample, All dimension are in SI units. The volume of the mercury (V) being penetrated into the pores of

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the sample was measured directly as a function of the applied pressure. This P-V information has been used for determination of the pore structure31. The weight loss of the catalyst with temperature to 700 oC were observed by DTG-60, (Shimadzu) Japan, and TGA instrument with heating ramp of 10 oC min-1. The variation of reaction parameters including reaction time, temperature, substrate ratio, catalysts loading was studied and the results are discussed below with an error bar of ± 2-5 % (shown in the respective figures).The standard deviation was calculated using equation 2.



n 2  (X  X ) / N i i 1

(2)

Where  = Standard Deviation, X= Arithmetic Mean, N = Scaled by total number of points.

3. RESULTS AND DISCUSSION 3.1. Characterization of catalyst. 3.1.1. X-ray diffraction. The X-ray diffraction (XRD) patterns of KSF/O and Mont-K-10 in Figure 1 showed typical Qquartz phases by reflections at 2and 26.3o containing the silica-aluminate crystalline layered structure. In case of montmorillonite pillared Al, crystalline phase was totally absent which clearly indicated its amorphous nature (Figure 1c)

26-27

. Mont K10 showed (Figure 1b)

sharp reflections at 2θ = 12 and 20o which could be assigned to (002) and (110, 020) planes. The basal spacing of d001 observed was 1.47 nm (peak at 2θ ≈ 12°) which indicated the presence of 2:1 (T: O:T) structure. The reflection at 2θ = 26.3o was assigned to α-quartz phase. However, the intensity of reflections decreased significantly, particularly for reflection at 2θ = 36o. The

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diffraction patterns of all the samples confirmed the a typical smectite “montmorillonite” along with hkl reflections, other peaks were due to impurities like cristobalite and feldspar.28 3.1.2. Physico-chemical characterization. Table 1 presents BET surface area and the surface acidity of various montmorillonite clay catalysts. Among all the catalysts, Mont-Al showed the highest surface area of 250 m2 g-1 and the order of surface area values was found to be Mont-Al > Mont- K-10 > Mont-KSF/O. The order of acidity was found to be Mont-Al (0.005 mmol g-1) < Mont- K-10 (0.008 mmol g-1) < MontKSF/O (0.015 mmol g-1). This order of acidity played more predominant role in their activity than the surface towards etherification, as discussed later. The relative numbers of acid sites in Table 1 shows that both Mont KSF/O and Mont-Al possessed similar extent of 25 and 75% acid sites in low and high temperature regions, respectively. While, Mont- K10 possessed almost equal distribution of low and high temperature acid sites. 3.1.3. Thermogravimetry Thermogravimetric analysis was carried out to obtain weight loss using ratio of change in weight to the initial weight of the sample. Sudden drop in weight was observed for sample till 120 oC due to the moisture loss (Figure S1, ESI). Lower weight loss was observed in the range of 120 to 700 oC. It indicates that catalyst was quiet stable up to 700 oC. 3.1.4. Distribution of pore size. Distribution of pore size of montmorillonite KSF/O showed macroporous structure with an average pore radius of 616.043 nm and total cumulative volume of 345 mm3g-1 (Figure S2, ESI). 3.2. Catalyst Screening. Glycerol etherification with tert-butyl alcohol was studied over three different montmorillonite clay samples at 110 °C using glycerol: t- butanol mole ratio of 1:20 with the catalyst loading of

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0.250 g (27.17 wt %) for duration of 6 h. The performance of the catalyst was measured in terms selectivity of di and tri-tert-butyl glycerol ethers and glycerol conversion. Among the three samples, Mont-KSF/O catalyst showed the highest glycerol conversion due to Brønsted acidic sites having the maximum acidity of 0.015 mmol g-1 despite its lower surface area (128 m2 g-1) as compared with different catalysts. In all the cases MTBGE and DTBGE selectivities were higher as compared to TTBGE (Figure 2). Brønsted acid sites of our catalysts were found to be of crucial importance in the glycerol etherification. The low conversion and selectivity with MontAl was due to its lower content of acid sites (0.005 mmol g-1 NH3). Composition of various clay samples is presented in Table 2. Due to higher Si/Al ratio of Mont-Al as compared to Mont-K10 and Mont-KSF/O, the former was expected to have lowest acidity which was actually confirmed by NH3-TPD measurement (Table 1). However, inspite of having similar Si/Al ratio of MontK10 and Mont-KSF/O, due to the presence of S and Zr, the later was found to have the highest acidity. Although the surface area value for Mont KSF/O was the least, it showed the highest activity towards glycerol etherification in accordance with the order of acidity (Mont KSF/O > Mont K-10 > Mont-Al). As Mont KSF/O was found to be the best catalyst for etherification of glycerol with t-butanol, further studies on parameters optimization were carried out with the same catalyst. 3.2.1. Influence of reaction time. Since glycerol etherification using t-butanol is a stepwise process initiating with the formation of mono-ether followed by the production of di and tri-tert-butyl glycerol ethers thus, study of conversion of glycerol and selectivity of products against time becomes a very important aspect. As can be seen them in Figure 3, within first hour of the reaction time, glycerol was completely converted with the highest selectivity towards 87%, 9% and 3% of MTBGE, DTBGE and

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TTBGE, respectively. However, simultaneous decrease in the MTBGE and increase in DTBGE was observed after 12 h (Figure 3). This continued the sequential conversion of MTBGE to DTBGE as the reaction time progressed till 15 h and thereafter, the product distribution remained more or less constant. 3.2.2. Reaction temperature effect on activity. The influence of temperature on selectivity of reaction products and glycerol conversion was investigated over the range of 60 oC-150 oC with a mole ratio of t-butanol to glycerol as 20 and a catalyst amount of (0.250 g (27.17 wt %). As expected, the etherification was much slower at the lowest temperature of 60 oC giving the moderate glycerol conversion (34 %). Glycerol conversion and selectivity of products increased with increase in temperature before equilibrium approaching. The highest glycerol conversion was achieved at 110 oC. A significant increase in the selectivity of TTBG from 0 to 7 % and selectivity of DTBG from 0 to 22% were observed, while the reaction temperature being increased from 60 oC to 150 oC . Maximum selectivity to DTBGE achieved was up to 22 % at 110 oC in 6 h (Figure 4). 3.2.3. Substrate ratio effect on activity. In order to achieve higher selectivities to more substituted ethers like DTBGE and TTBGE effect of excess of TBA was investigated by varying the molar ratio of Glycerol: TBA in the range of 1:6 to 1:20. It was observed that the use of excess TBA promoted the formation of both DTBGE and TTBGE. Particularly, the enhancement in selectivity to DTBGE was almost two fold from 13% to 27 %, when the glycerol to TBA ratio was 1:20 (Figure 5) 3.2.4. Catalyst loading effect on activity. Loading effect of Mont KSF/O catalyst was studied at 110 oC with glycerol/TBA ratio of 1:20 (Figure 6). Under optimized reaction conditions catalyst weight in the reaction mixture was

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varied in from 100 mg to 350 mg. The selectivity of DTBG attained its maximum level of 30% over Mont-KSF/O with 250 mg loading. 3.2.5. Recycle of the catalyst. After end of the reaction, Mont KSF/O catalyst was filtered and being washed with methanol, then was dried at 100 oC for 2 h. After that, it was ready to reuse for the consequent run. Therefore, this catalyst can be recycled many times without any loss of activity (Figure 7). NH3TPD results of the used catalyst were also similar to those of the fresh catalyst (Table 1) 4. CONCLUSION Glycerol etherification with tert-butyl alcohol was effectively achieved in the presence various commercially available Mont-clay catalysts. Among the different screened catalysts, MontKSF/O actively catalyzed the etherification reaction due to its highest acidity as measured by ammonia-TPD (Mont-KSF/O > Mont-K-10 > Mont-Al). In spite of its lower surface area (MontKSF/O < Mont-K-10 < Mont-Al) it is active for production of DTBGE and TTBGE. The process was optimized by studying the influence of temperature, molar ratio (glycerol/TBA), catalyst loading and reaction time. It was found that the increase in temperature significantly affected the glycerol conversion and DTBGE formation. Almost complete glycerol conversion was achieved in 6 h, at 110 oC with 1:20 molar ratio of glycerol to TBA over (0.250 g (27.17 wt %) catalyst loading. Prolonged reaction time of 24 h led to maximum selectivity of ~ 33 % to DTBGE. The catalyst could be recycled efficiently with a consistent activity and selectivity.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Chandrashekhar Rode)

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ABBREVIATIONS MTBGE –Mono-t-butyl glycerol ether DTBGE –Di-t-butyl glycerol ether TTBGE –Tri-t-butyl glycerol ether REFERENCES (1)

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Figure Captions Figure 1. X-ray diffraction patterns of (a) Mont-KSF/O (b) Mont-K-10 (c) Mont-Al. Figure 2. Screening of different montmorillonite clays. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h. Figure 3. Effect of reaction time. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, time (2-24h), Mont.KSF/O loading (0.250 g, 27.17 wt %). Figure 4. Influence of reaction temperature. Reaction conditions: glycerol: t-butyl alcohol (1:20), temperature (60-150 oC), Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h. Figure 5. Influence of substrate ratio. Reaction conditions: glycerol: t-butyl alcohol (x:y), 110 o

C, Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h.

Figure 6. Influence of catalyst loading on conversion of glycerol and selectivity to ethers. Reaction conditions: glycerol: t- butanol (1:20), 110 oC, Mont KSF/O (0.250 g, 27.17 wt %), 6 h. Figure 7. Recycle study of Mont-KSF/O. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, Mont KSF/O (0.250 g, 27.17 wt %), 6 h.

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Figure 1. XRD analysis of (a) mont-KSF/O (b) mont-K-10 (c) mont-Al

Figure 2. Screening of different montmorillonite clays. Reaction conditions: glycerol tbutyl alcohol (1:20), 110 oC, Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h.

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Figure 3. Effect of reaction time. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, time (2-24h), Mont.KSF/O loading (0.250 g, 27.17 wt %).

Figure 4. Influence of reaction temperature. Reaction conditions: glycerol: t-butyl alcohol (1:20), temperature (60-150 oC), Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h.

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Figure 5. Influence of substrate ratio. Reaction conditions: glycerol: t-butyl alcohol (x:y), 110 o

C, Mont.KSF/O loading (0.250 g, 27.17 wt %), 6 h.

Figure 6. Influence of catalyst loading on conversion of glycerol and selectivity to ethers. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, Mont KSF/O (0.250 g, 27.17 wt %), 6 h. 17


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Figure 7. Recycle study of Mont-KSF/O. Reaction conditions: glycerol: t-butyl alcohol (1:20), 110 oC, Mont KSF/O (0.250 g, 27.17 wt %), 6 h.

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Scheme Captions Scheme 1. Reaction path for the glycerol etherification with t-butyl alcohol.

Scheme 1. Reaction path for the glycerol etherification with t-butyl alcohol.

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Table Captions Table1. BET Surface area and NH3-TPD characteristics of different clay catalysts. Table 2. Composition of clays used in study.

Table 1. BET Surface area and NH3-TPD of different clay catalysts. NH3 SBET, (m2/g)

Catalysts

Distribution of acidic sites %

Adsorbed

LT-Peak(150 OC) HT-Peak(400 OC)

(mmol/g)

Montmorillonite KSF/O

128

0.015

25

75

Montmorillonite KSF/O (used)

123

0.016

23

77

Montmorillonite K-10

230

0.008

52

Montmorillonite-Al

250

0.005

25

48

75

Table 2. Composition of clay samples used in this work Na

Mg

Al

Ca

Si

K

Fe

S

Zr

Ti

P

LOI

Ref.

MontKSF/O

------

3.05

10.32

-----

36.34

-----

5.76

22.40

22.13

------

-----

------

Mont-K10

2.18

2.81

17.64

5.01

56.47

4.11

11.6

------

------

------

-----

------

29

Mont-Al

0.13

3.31

12.10

1.76

64.42

0.17

1.48

------

-------

0.072

0.02

14.02

30

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Solid-Acid-Catalyzed Etherification of Glycerol to ... - ACS Publications

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