Fuel 159 (2015) 837–844

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Enhanced production of high octane oxygenates from glyceroletherification using the desilicated BEA zeolite

http://dx.doi.org/10.1016/j.fuel.2015.07.0280016-2361/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 135 2525856; fax: +91 135 2660202.E-mail address: nvish@iip.res.in (N. Viswanadham).

Sandeep K. Saxena a,b, Ala’a H. Al-Muhtaseb b, Nagabhatla Viswanadham a,⇑a Refining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, Indiab Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Muscat, Oman

h i g h l i g h t s

� Desilication increase three-fold inmesopores and two-fold in porevolume of DSBEA.� The desilication method also resulted

in moderate increase in the strongacid sites.� DSBEA exhibited higher conversion,

higher selectivity to diesel miscibleoxygenates.� The mesopores in DSBEA especially

favored the highest selectivity toTTBG (74.6%).� Catalyst shows stable performance in

oxygenates production in studiedperiod of 36 h.

g r a p h i c a l a b s t r a c t

O

O

OCH3

CH3

CH3CH3

H3C

H3C

H3C

CH3H3C

HO

OH

OH

OH

CH3

CH3CH3+

GLYCEROL TBA TTBG (yield=74.6wt%)Mesoporous BEA

a r t i c l e i n f o

Article history:Received 18 May 2015Received in revised form 12 June 2015Accepted 9 July 2015Available online 17 July 2015

Keywords:Glycerol etherificationOxygenate fuelDesilicationMesoporesAcidity

a b s t r a c t

BEA zeolites possessing different properties in terms of porosity and acidity have been applied for cat-alytic conversion of low value glycerol into high octane oxygenate useful for fuel applications. The prop-erties of the materials have been characterized by X-ray diffraction (XRD), N2-sorption,Scanning/Tunneling Electron Microscopy (SEM/TEM), Temperature Programmed Desorption (TPD), FTIRspectroscopy and evaluated for their activity towards tert-butylation of glycerol. The desilication ofBeta zeolite (BEA) resulted in the creation of �20 nm range meso-pores responsible for a two-foldincrease in the pore volume without any structural damage to the zeolite framework. The desilicationmethod also resulted in increase of the strong acid sites measured by TPD. The desilicated BEA(DSBEA) zeolite with enhanced properties exhibited as high as 98% glycerol conversion with 99% selec-tivity to diesel miscible oxygenates i.e. mixture of di- and tri- ter-butyl glycerol (DTBG + TTBG). Theresults indicate that the space restrictions in the zeolite channels have been over ruled by the meso-porous BEA zeolite catalyst to produce high yields of TTBG. The DSBEA zeolite catalyst also exhibited sta-bility in catalytic performance with the reaction time and reaction cycles.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction such as lubricity, high flash point, close heat combustion, viscosity,

Recently, bio-diesel is emerging as a competitive alternative tothe traditional fossil fuel based diesel by virtue of its properties

limited exhaust emissions and superior cetane number [1–3].However, the unavoidable formation of huge glycerol bi-productlimits the fuel yields. Moreover, the glycerol component inbio-diesel polymerizes at high temperatures and part of it oxidizesto give toxic acrolein [3]. Among the various chemical routes forglycerol value addition, etherification stands best as it produces

838 S.K. Saxena et al. / Fuel 159 (2015) 837–844

oxygenate blending stock for diesel. Addition of glycerol etherenhances the cetane number and also reduces the fumes and par-ticulate matter, carbon monoxide emissions in the exhausts toimprove the total yield and quality of diesel by reducing emissionsthrough decreasing viscosity and cloud point [4]. Glycerol etherifi-cation was reported to give better conversion and product yieldswhen carried out with isobutylene rather than with tert-butylalcohol (TBA) [5]. However, the usage of isobutylene is expensiveand needs additional step of purification to separate it from theC4 mixture either by sulfuric acid extraction or by using molecularsieves. The acid catalysts like Amberlyst-15 are also known for effi-cient trimerization of isobutene [6]. Hence, the use of isobutyleneas etherifying agent to glycerol has strong possibility to produceoligomers and poly-oligomers that affect the yields and catalyststability [7]. The use of TBA on the other hand is reported to servethe purpose of both solvent and reactant to avoid technologicalproblems involved in the use of additional solvent and the draw-backs associated with three-phase system (mass transfer phenom-ena) [8,9]. When compared to the isobutylene the price andhandling of TBA is cheaper and unlike the refining originated iso-butylene TBA is a bio-origin as it is produced from starch or ligno-cellulosic biomass or wheat/straw fiber [10]. The tertiarybutylation of glycerol is aimed to produce di-tertiary butyl ether(DTBG) and tri-tertiary butyl ether (TTBG) as they can directlyblend into diesel while mono tri-tertiary butyl ether (MTBG) isvery less miscible in diesel. A three-step new route involving glyc-erol chlorination is reported by Canoira et al. for the production of1,3-di-tert-but-oxypropan-2-ol (DTBG) [11].

Acidity and porosity properties of catalyst play important rolein bulky etherification reaction of glycerol with tertiary butyl spe-cies. The reaction steps involve the release of the Bronsted acidicsites (H+) into the reaction mixture and its reaction with OH groupof alcohol. The resultant protonated alcohol reacts with glycerol toform glycerol ethers [12], where the selectivity to bulky ethers willbe facilitated by acid strength as well as the porosity of the catalystmaterial. Amberlyst catalysts (Amberlyst-15, Amberlyst-35 andAmberlyst-36) exhibiting high acidity and porosity have been suc-cessfully applied for this reaction. However, the drawback of theresin is its thermal fragility, which may bring the corrosion prob-lems due to its decomposition at higher temperature.Furthermore the deactivated resin catalyst cannot be completelyregenerated. When the glycerol etherification reaction carried outwith TBA it produces water which can poison the acid sitesrequired for the etherification reaction [13]. This situationdemands the development of high temperature withstanding solidacid catalysts.

Zeolites have been considered to be potential substitute for thestrong acid ion exchange resins due to their outstanding propertiessuch as negatively charged tetrahedral framework, high thermalstability and tuneable properties such as porosity and acidity.However, the narrow zeolite pores also cause severemass-transfer constraints due to the poor diffusion of larger mole-cules in the narrow micropores of these materials that eventuallyleads to poor catalytic performance. Several supported acid cata-lysts like acid functionalized mesostructured silicas [14] solidheteropolyacids [15] and wide pore zeolite such as HY [16] andHBEA [17] have been used for etherification of glycerol. Amongthe various zeolite catalysts BEA zeolite shows promising resultsperhaps due to its intersecting channel structure and high externalsurface area required for the formation of bulky glycerol ethers.The preliminary studies conducted by Klepacova et al. on BEAand HY zeolites revealed the major hindrance in formation ofdesired tri-ether due to space restrictions in the micropores ofthese zeolites [18]. In order to overcome the space constraints, var-ious strategies have been adopted for the creation of larger pores inzeolites that include framework dealumination, carbon templating

and framework desilication [19,20]. However, obtaining a highlymesoporous BEA with good acidity and crystallinity which is suit-able for catalytic applications is still challenging.

Gonzalez et al. have conducted extensive studies on the effect ofvarious modification treatments on BEA zeolite to produce the diand tri ethers of glycerol. The selectivity to h-GTBE (DTBG + TTBG)was reported to be about 37% on fluorinated BEA while as high as75–91% selectivity is observed on sulfonic acid functionalized aero-gels, BEA, SBA-15 and HBS based catalysts [21–23]. The studies ofTuan et al. yielded highest glycerol conversion of 90% with 77.7%selectivity to DTBG + TTBG over p-toluenesulfonic acid catalystwhile the corresponding values on Amberlyst-35 are 59% and63.9% respectively [24]. Zhao et al. successfully applied rare-earthmodified BEA zeolite to obtain 65% DTBG along with 8% of TTBG at92% glycerol conversion using isobutylene as etherification agent[25]. In most of the cases the selectivity to the fuel blending oxy-genate (DTBG + TTBG) is limited to less than 90% and the selectivityto bulky TTBG is very low emphasizing the space constraints in thezeolite pore channels. Frusteri et al. used novel spherical silica sup-ported Hyflon catalyst for the etherification of glycerol using isobu-tene in a batch reactor and the product was tested towards fuelefficiency by engine testing. The highest yield of 85–90%DTBG + TTBG was observed on this catalyst with maximum TTBGyield of �35 wt% [26]. Our previous studies indicated the enhancedcatalytic activity of nano crystalline BEA towards the bulky etherproduction [17]. Present study is aimed to improve the yields ofglycerol ethers by modifying catalytic properties of BEA zeolite espe-cially in terms of acidity and porosity.

2. Experimental

2.1. Chemicals and catalysts

Glycerol (99%) and tertiary butyl alcohol (99%) were obtainedfrom Merck Chemicals. The BEA zeolite is obtained in powder formfrom the Sud-Chemie India Ltd., where as NBEA (nano crystallineBEA) and DSBEA (desilicated beta zeolite) were synthesized inlaboratory.

2.1.1. Synthesis of nano crystalline beta zeolite (NBEA)In a typical procedure of NBEA synthesis, sodium ion-free

homogenous gel was obtained by sequential mixing of silica sourceto quaternary ammonium salt followed by drop wise addition of alu-mina source at 0–2 �C temperature followed by hydrolysis of TEOSand heat treatment of the resultant gel as described elsewhere [27].

2.1.2. Synthesis of desilicated beta zeolite (DSBEA)The mesoporous BEA zeolite material (DSBEA) is obtained by

sequential alkali and ammonium treatments conducted on parentBEA (Si/Al = 12.5) zeolite as per the detailed procedure describedelsewhere [28].

The powder forms of all the zeolite samples (BEA, NBEA andDSBEA) were shaped to extrudates so as to test them in a fixedbed continuous flow reactor. In a typical procedure, 3 g of dry zeo-lite powder is mixed with 2 g of alumina powder (pseudo boeh-mite binder) and the mixture was ground well to get uniformmixing, followed by drop wise addition of 3 vol% glacial acetic acidsolution (2.5 ml) with continuous grinding, till the formation of awet paste. Then the paste is allowed for peptization at room tem-perature (25 �C) for 2 h followed by wet extrusion of the resultantpaste through a 2 mm diameter size by lab-made hand metallicextruder. The wet extrudates are allowed to get dry at room tem-perature (25 �C) overnight (12 h) and heated at 100 �C for 7 h fol-lowed by calcination at 500 �C for 4 h in presences of air. Theextrudates after drying and calcinations are used for reaction.

INTE

NSI

TY (C

PS)

5 10 15 20 25 30 35 40 45 50 55 602θ

0

1000

2000

2 3 4 5

BEA

NBEA

DSBEA

DSBEA

Fig. 1. XRD patterns of various beta zeolite samples. ESI, Fig. S1 SEM images variousbeta zeolite samples.

S.K. Saxena et al. / Fuel 159 (2015) 837–844 839

2.2. Catalyst characterization

The morphology and crystal structures of zeolites were charac-terized by the X-ray diffraction (XRD) (Rigaku Dmax III B, Japan),scanning electron microscope (SEM) (Quanta 200f instrument,Netherlands) and transmission electron microscopy (TEM). TheBET surface area, pore size and pore volume measurements of allthe zeolite based catalysts were carried out using a standardadsorption equipment (ASAP 2010, Micromeritics InstrumentsInc., Norcross, GA, USA) using N2 gas (99.995% pure). The surfacearea, pore volume and pore size distribution were obtained by mea-suring the volume adsorbed at different P/P0 values and by applyingdifferent methods. Total pore volume was estimated by measuringthe volume of gas adsorbed at P/P0 of 0.99 whereas, t-plot methodwas used to calculate the micro pore surface area (0–20 Å) usingthe Harkins–Jura equation. The total micro pore volume (0–20 Å)and the micro pore size distribution were obtained by applyingthe Horvath–Kawazoe method (H–K). The Fourier TransmissionInfrared (FT-IR) spectra are used for confirmation of the functionalgroups in the BEA structure. The acidity of the catalyst was mea-sured by temperature programmed desorption of NH3 (NH3-TPD)using a Micromeritics chemisorbs 2750 pulse chemisorption sys-tem. 0.1 g sample was used for each TPD experiment.

2.3. Reaction and product analysis

The etherification reactions were carried out in two types ofreactors; (1) batch reactor and (2) fixed bed reactor (continuousflow reactor).

In batch reaction studies 5 g of glycerol and 0.38 g catalyst wereplaced in the reactor and outgassed of 16.1 g TBA. The reaction wasconducted at 90 �C at atmospheric pressure with continuous stir-ring and the reaction product is allowed to cool at room tempera-ture before taking for analysis.

In fixed bed reactor (220 mm length and 18.5 mm internaldiameter) 5 g of catalyst is loaded in the center of the reactorand the extrudates of a-alumina (inert material) were loadedabove (10 g) and below (10 g) the catalyst bed. The reaction is con-ducted with continuous flow of glycerol and TBA mixture usingEldex (France) syringe type feed pump. Glycerol etherificationreaction was performed on various catalysts in the presence ofN2 gas flow at the following reaction conditions; reaction temper-ature = 90–150 �C, pressure = atmospheric, WHSV = 2.3 h�1. Theproduct obtained at the end of the reactor was cooled with the helpof a water circulator. The conversion and product yields were qual-itatively analyzed by GC-mass spectrometer using 5890 series II GCand 5972 series mass selective detector, while for quantitativeanalysis the reaction product was analyzed by using GC equippedchemically bonded fused silica capillary column (50 m(L) � 0.32 mm (ID) � 0.2 lm (film thickness) with stationary phaseCP-Sil-5. The ionization source was electron impact at 70 eV andquadrupole analyzer. For line pressure 48 millitorr, helium gasused as carrier gas and PTGC condition were 40 �C for 5 min then5 �C/min up to 285 �C with split ratio 130:1. For the mass spectradata comparison Wiley 138 library was used. Since the reactanttertiary butyl alcohol was in excess, conversion was calculatedbased on the glycerol. After the catalyst deactivation it is furtherregenerated in presence of air at 500 �C.

3. Results and discussion

3.1. Physico-chemical properties of catalysts

The structural identification of the lab-synthesized nano crys-talline BEA (NBEA) and desilicated BEA (DSBEA) have been

confirmed by comparing their XRD patterns with those of the com-mercial BEA zeolite (Suid Chemie India Limited) which is used asbase material for desilication (Fig. 1). The wide angle XRD patternsof the samples indicate the presence of well crystalline structure ofthe BEA zeolite which is protected even after the severe desilica-tion treatments. Further, the low angle XRD patterns (Fig. 1 ininset) of the desilicated sample (DSBEA) shows the presence ofmesopores with the typical low angle XRD (100) reflection in therange 2h = 2–3� corresponds to the d-spacing of 3.3 nm. The mor-phology of the materials was examined by field emission scanningelectron microscopy (FE-SEM), transmission electron microscopy(TEM) and high resolution TEM (HRTEM). The SEM images(Fig. S1, ESI) suggest that the zeolite crystal size has been success-fully brought down from micro meter range of BEA to nano meterrange of NBEA that is expected to create inter-crystalline voids fall-ing in the range of mesopores in this sample. However, most of themesopores created in this sample (NBEA) are outside the crystaland are not expected to contribute significantly towards the cat-alytic activity. Different from this, the purpose of desilication isfor the creation of mesopores within the crystal of the BEA zeolite,which is indeed observed from HRTEM (Fig. 2) and N2-adsorptionmeasurements of DSBEA. This sample also exhibited decrease incrystal size and inter crystalline voids when compared to the par-ent BEA. The decrease in crystal size of the DSBEA zeolite may bedue to the treatments adopted in desilication procedure. TheTEM images also support the presence of �20 nm size mesoporesin the DSBEA sample (which were not present in the parentuntreated BEA sample). Overall the results indicated the significantdevelopment of meso pores in the desilicated BEA (DSBEA).

The porous properties of all the three samples were measuredby nitrogen adsorption–desorption isotherms (Fig. 3) belong totype IV isotherms with an adsorption–desorption hysteresis loopin the range of 0.7–1.0 P/P0. However, the shapes of the loops arenot similar in all these samples; BEA exhibits slightly flat type loopwith negligible amount of meso-pores, where a steep hump in hys-teresis loop representing considerable amount of meso pores isobserved in the P/P0 range of 0.8–0.9 only in sample NBEA andDSBEA. This type of loop is usually observed for larger mesopores.The pore size distribution of the BEA, NBEA and DSBEA samplemeasured by BJH method further reveals the formation of meso-pores in the NBEA (0.5014 cm3/g mesopore volume) and DSBEA

BEA

NBEA

DSBEA

Fig. 2. TEM of various beta zeolite samples.

840 S.K. Saxena et al. / Fuel 159 (2015) 837–844

sample (1.0977 cm3/g mesopore volume). Considerably highincrease in pore volume is observed in the mesopore region ofDSBEA as compared with NBEA and BEA samples, where a highpopulation of pores with pore diameter ranging from 2 to 50 nmwith peak height at �27 nm is observed in this sample (Fig. 3B).The micropore volume plots representing the zeolitic microporeswith pore diameter �6 Å of BEA, NBEA and DSBEA samples areobserved to be comparable (Fig. 3C). But, the micropore volumemeasured for the pores up to 20 Å is decreased while that of meso-pores is increased in the DSBEA sample (Table 1), that can beascribed to the conversion of larger micropores (larger than zeoliticpores, 6 Å) in the BEA sample into mesopores by desilicationmethod. These results of present study differing from thosereported on BEA zeolite synthesized using HF (in acidic medium).The resultant BEA zeolite was observed to be very sensitivetowards alkali treatment (during desilication) and frameworkdestruction was observed to occur in this zeolite even at lowerseverity conditions (differs from the present study) [29]. TheSi/Al ratio of the BEA taken for desilication treatment in the presentstudy was also much lower (Si/Al = 12.5) than the reported one(Si/Al = 35) and this factor may also contribute to the stability ofthe BEA zeolite of the present study during alkali treatments. Theoccurrence of desilication by alkali treatment followed by dealumi-nation during the calcinations of the resultant desilicated BEA wasalso reported by Gonzalez et al. [23].

Overall, the desilication procedure adopted in the present studyresulted in a more-than-two-fold increase in pore volume(1.1822 cm3/g) of the DSBEA zeolite sample (Table 1). Such as por-ous system possessing three types of pores together; zeolitic micropores, mesopores and macropores created by desilication exempli-fies the typical trymodal pore system which is desired for the cat-alytic applications due to its enhanced mass-transfer and diffusionproperties of the molecules involved in the ether formation fromglycerol.

The FT-IR spectra of all three zeolite samples resemble similar-ity and confirm the functional groups in the BEA framework struc-ture (Fig. 4A). The presence of adsorption band at 560–575 cm�1, acharacteristic of BEA framework structure in NBEA and DSBEAsamples reveals the structural identity. The broad band at about3460 cm�1 was assigned to OH stretching and the one at1640 cm�1 to the bending vibration of the adsorbed water mole-cule. A band at 465 cm�1, characteristic to the pore opening, wasfound to shift to higher wave number of 472 cm�1 clearly supportsthe increase of pore opening in the DSBEA, which is in agreementwith the increase in average pore diameter (from 3.8 nm to10.5 nm) observed in studies. The bands in the range of 780–790 cm�1 and 1060–1090 cm�1 are attributed to the asymmetricand symmetric stretching vibrations of TO4 units of zeolite respec-tively [30,31]. Further, the asymmetric vibrational band is sensitiveto the framework Si/Al composition of the zeolite, where the

VOLU

ME

AD

SOR

BED

(cm

3 / g) S

TP

0.0

0.1

0.2

0.3

0.4

0 3 6 9 12PORE DIAMETER ( )

(C) DSBEA

BEA

NBEA

0.0

0.5

1.0

1.5

2.0

0 20 40 60 80 100

DSBEA

BEA

NBEA

0

200

400

600

800

0 0.3 0.6 0.9 1.2

PORE DIAMETER (nm)RELATIVE PRESSURE (p/p0)

POR

E VO

LUM

E (c

m3 /g

- nm

) (B)(A) DSBEA

BEA

NBEA

POR

E VO

LUM

E (c

m3 /g

-)

Fig. 3. (A) Nitrogen adsorption–desorption isotherms, (B) BJH mesopore distribution and (C) micropore size distribution patterns of BEA samples.

Table 1Physico-chemical properties of zeolite samples.

Properties BEA NBEA DSBEA

aSi/Al ratio 12.5 12.8 6.1bSBET (m2/g) 514.8 418.8 428.3cTotal pore volume (cm3/g) 0.5376 0.6755 1.1822dMicro pore volume (cm3/g) 0.1807 0.1741 0.0845eMeso pore volume (cm3/g) 0.3569 0.5014 1.0977fAdsorption average pore diameter (nm) 3.79614 6.2041 10.53945

a Si/Al ratio by EDX analysis.b BET surface area.c Total pore volume taken from the volume of N2 adsorbed at P/P0 = 0.995.d Micropore volume calculated from t-plot.e Mesopore volume calculated from total pore volume �micropore volume.f BJH adsorption average pore diameter.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600 800TEMPERATURE (0C)

BEA

DSBEA

NBEA

TCD

SIG

NA

L (a

.u.)

400900140019002400290034003900

WAVE NUMBER (Cm-1)

BEA

NBEA

DSBEA

TRA

NSM

ITTA

NC

E (%

)

(A)

(B)

Fig. 4. (A) FTIR and (B) TPD patterns of zeolite samples.

S.K. Saxena et al. / Fuel 159 (2015) 837–844 841

decrease of Si/Al ratio is observed to shift the vibration bandtowards the lower wave number side. The shifting of the particularband from 1090 cm�1 in case of BEA to 1080 cm�1 in DSBEA of thepresent study reveals the change of framework Si/Al through desil-ication. Such a change in framework composition is expected toinfluence the acidic property of the BEA zeolite.

The detailed acid strength distribution trends of BEA, NBEA andDSBEA have been measured by temperature programmed desorp-tion (TPD) of ammonia using Micromeritics 2750 pulse chemisorp-tion system. The TPD patterns of these samples are illustrated inFig. 4B, which exhibit two distinguished desorption peaks; a nar-row low temperature peak at around 150 �C and a broad high tem-perature peak at around 400–450 �C representing the weak andstrong acidity respectively. In the NBEA catalyst increase in theweak acidity along with decrease in intensity of strong acidity isobserved, while for DSBEA a slight increase in the intensity of weakacidity along with strong acidity was observed as compared to BEA

Table 2Detailed glycerol etherification studies.

Catalysts Reaction conditions Glycerol conversion (wt%) Ethers selectivity (%)

Temperature (�C) Reaction time (h) Mono (MTBG) Di (DTBG) Tri (TTBG)

Batch reaction studiesBEA 90 8 96.5 39.4 44.2 16.4NBEA 90 8 98.2 30.3 48.9 20.8DSBEA 90 8 97.7 14.2 56.0 29.8

Fixed Bed reactor studiesBEA 90 4 85.4 10.8 41.6 47.6BEA 110 4 91.4 9.9 43.3 46.8BEA 130 4 91.0 6.4 45.2 48.4BEA 150 4 90.8 8.4 46.4 45.2NBEA 90 4 90.2 3.1 46.8 50.1NBEA 110 4 97.8 2.0 44.8 53.2NBEA 130 4 97.2 2.4 46.4 51.2NBEA 150 4 96.2 3.2 49.4 47.4DSBEA 90 4 88.6 1.4 30.2 68.4DSBEA 110 4 98.2 0.9 24.5 74.6DSBEA 130 4 98.0 1.4 25.2 73.4DSBEA 150 4 97.8 3.1 26.4 70.5

Catalyst = 5.0 gm; pressure = atmospheric (N2); TBA:glycerol = 4:1 (molar ratio), WHSV = 2.3 h�1.

842 S.K. Saxena et al. / Fuel 159 (2015) 837–844

sample. The increased strong acidity in DSBEA can be ascribed tothe removal of weakly acidic silanol groups from the zeolite duringthe desilication.

Overall, the characterization results indicated significantincrease in pore volume of BEA zeolite by creation of mesoporesranging from 10 to 50 nm along with small amount of macropores(>50 nm) through effective desilication method without loosingthe framework structure of the zeolite. The resultant DSBEA zeolitecatalyst possessing trimodal porosity (co-existence of micro, mesoand macro pores) is expected to facilitate facile diffusion of thebulky molecules with improved mass-transfer properties.Moreover, the sample also exhibited improved acidic propertiesrequired for acid catalyzed reactions. The etherification reactionof glycerol indeed requires large space (for the interaction of bulkyreactant molecules, bulky intermediates and the products) alongwith strong acid sites. The DSBEA catalyst possessing both theproperties is thus explored for the maximization of di andtri-tertiary butyl ether of glycerol.

3.2. Glycerol etherification studies

The catalytic activity studies are focused on etherification reac-tion of glycerol with tert-butanol. A general reaction scheme forthe glycerol etherification with TBA is illustrated in Fig. S2, ESI.1

The reaction products obtained are mono-tert-butyl glycerol ether(MTBG), di-tert-butyl glycerol ether (DTBG) and tri-tert-butyl glyc-erol ether (TTBG). The glycerol etherification performance of the cat-alysts was conducted in both batch reactor and fixed bed reactor toexplore the possibility of improving the product selectivities towardsdiesel miscible oxygenates namely, DTBG and TTBG.

3.2.1. Batch reactor studiesAll the catalysts exhibited more than 96 wt% glycerol conver-

sion (Table 2). Among the three catalysts, the selectivity to bulkyDTBG/TTBG is increased in the order of BEA < NBEA < DSBEA. Thehigher DTBG/TTBG selectivity exhibited by DSBEA can be ascribedto the large mesopore volume and internal spacing in the pores ofthis catalyst that is necessary for the formation of bulky product.Further, the DSBEA catalyst also exhibited the higher TTBG selec-tivity when compared to the other two catalysts. Thus, the highestamount of diesel miscible mixture of DTBG + TTBG (85.8 wt%) was

1 Electronic Supplementary information, ESI.

produced on the DSBEA catalyst. Still, there is a considerableamount of MTBG is formed on all the catalysts, including DSBEA.

Our earlier studies indicated the possible role of equilibriumlimitations in batch reactor for the formation of bulky DTBG andTTBG [17]. The reversible formation of DTBG and MTBG throughhydrolysis of TTBG may be responsible for the decrease in bulkyproduct selectivity. The shift in selectivity from TTBG/DTBG toMTBG in batch reactor operation can be ascribed to the presenceof huge amount of water bi-product stay in contact with the reac-tion product in batch reactor operation. In order to explore furtherincrease in DTBG/TTBG, the reaction studies are further conductedin continuous flow reactor, where the product once formed getsdesorbed out of the catalyst bed and hence not influenced by equi-librium. This situation is expected to improve the di and tri-etherselectivities.

3.2.2. Etherification studies in fixed bed reactorThe glycerol etherification reactions are carried out in fixed bed

down flow reactor at various reaction temperatures ranging from90 to 150 �C at atmospheric pressure. The TBA/Glycerol molar ratioof 4:1 is selected as the optimum reactant ratio based on the bestproduct selectivities obtained at this condition (Fig. S3, ESI1).Table 2 shows performance of various catalysts for glycerol ether-ification reaction, where the glycerol conversions are between 85and 98 wt% depending on the reaction conditions and catalystswith DTBG and TTBG being the major product, followed byMTBG. With the increase of reaction temperature from 90 �C to110 �C, the Glycerol conversion as well as ether selectivity isincreased. However, above 110 �C, the selectivity towards desiredDTBG and TTBG is not increased anymore. On the basis of detailexperimental results illustrated in Table 2, the 110 �C temperaturecondition seems to be best for achieving highest selectivity toDTBG and TTBG ethers on all the catalysts. Though the two cata-lysts, BEA and NBEA exhibited similar performance trends, theresults shown in Table 2 suggest the better performance of NBEAover BEA in terms of retaining DTBG/TTBG selectivity. Among thethree catalysts, the NBEA and DSBEA outperformed the BEA cata-lyst in terms of glycerol conversion and ether selectivity. At theseconditions glycerol conversion achieved is about 97–98 wt% onNBEA/DSBEA catalysts with as high as >98% selectivity toDTBG + TTBG ethers. On the other hand DSBEA catalyst showsthe higher performance over BEA and NBEA in terms of bulky etherproduct (TTBG) selectivities. The increasing TTBG selectivity overthe three catalysts is in the order of BEA < NBEA < DSBEA. The

REACTION TIME IN HOURS

0

20

40

60

80

100

Fresh catalyst 1 Cycle 2 Cycle 3 Cycle50

60

70

80

90

100

MTBG DTBGTTBG Glycerol conversion

CO

NVE

RSI

ON

/SEL

ECTI

VITY

GLYC

ERO

L CO

NVER

SION

(wt%

)

ETH

ER S

ELEC

TIVI

TY (%

)

REACTION ACTIVITY PATTERNS

(A)

(B)

Fig. 5. Performance of DSBEA catalyst with respect to (A) time-on-stream and (B)regeneration cycles.

S.K. Saxena et al. / Fuel 159 (2015) 837–844 843

highest conversion of 98 wt% with 74.6% selectivity to bulky TTBGproduct was observed on DSBEA, which indeed explains the role ofmesopores in this sample. The TTBG selectivity obtained in the pre-sent study is highest ever reported on the solid acid catalysts to thebest of our knowledge. Further, the selectivity to diesel miscibleproduct (DTBG + TTBG) is as high as 99% on this catalyst at98 wt% glycerol conversion. Overall, the desirable Glycerol conver-sion with excellent product selectivity towards high octane oxy-genates with significant increase in bulky TTBG has beenobserved on DSBEA catalyst. The yield of diesel miscible oxygenate(DTBG + TTBG) is also highest on DSBEA which is attractive for fuelapplications.

3.2.3. Factors responsible for higher performance of DSBEAAcidity and porosity are the two important factors that are

expected to govern the reactivity of glycerol towards etherificationand the extent of etherification at three hydroxyl groups in theglycerol. In solid acid catalysts, especially in case of microporouszeolites, though the acidity is sufficient for the glycerol conversion,the limitations in the space available for the formation of bulkymolecules limit the formation of desired bulky ethers i.e. DTBGand TTBG. Hence, most of the cases the product obtained is domi-nant in undesired MTBG having poor fuel qualities (Table S11). Theacidity improved catalyst systems such as Sulphonated aerogel,Sulphonated SBA-15, fluorinated H-BEA, rare-earth exchangedBEA and silica supported Hyflon have been successfully employedfor the production of high octane ethers (DTBG + TTBG) with highselectivity 92.3%. Among the three ether products, The TTBG con-centration is always observed to be low concentrations. Even onhierarchical BEA zeolite system, the TTBG formation was reportedto be in negligible amounts (Table S11). The highest selectivitytowards bulky TTBG reported so far on modified BEA catalysts is36%, while on silica supported Hyflon the value is 53.6%.(Table S11). The much higher selectivity of 74.6% towards TTBGand as high as 99% selectivity to high octane bulky DTBG + TTBGproduct obtained on DSBEA of the present study clearly envisionsthe occurrence of breakthrough improvement in the catalytic prop-erties of the BEA zeolite.

The glycerol etherification reaction studies indicate more than90% conversion on all the three catalysts. The conversion of glyc-erol is almost completed (�99%) on DSBEA catalyst. The higherconversion of glycerol obtained on DSBEA can be ascribed to theimprovement in acidity of the BEA zeolite. The Si/Al ratios of thesamples also support this aspect as the ratio is decreased to halfthe value of the desilication (Table 1), which means the increaseof Al per gram of catalyst related to the acidity of the catalystresponsible for the catalytic activity. The TPD data indeed indicatesthe presence of highest strong acidity in DSBEA that supports thepositive role of decrease in Si/Al as responsible factor for achievinghigher glycerol conversions on DSBEA catalyst. The DSBEA catalystalso exhibited the highest selectivity towards the high octane oxy-genate (DTBG + TTBG) (99%). Interestingly, the selectivity towardsTTBG is significantly increased on DSBEA catalyst. The TTBG selec-tivity is as high as 74.6% on DSBEA when compared to BEA andNBEA catalysts. This clearly envisions the role of enhanced porosityin DSBEA in facilitating bulky etherification reactions to give TTBG.The data given in Table 1 indeed reveals the two fold increase intotal pore volume of the DSBEA along with nearly three foldincreases in the mesopore volume. The average pore diameter isalso increased up to three times showing almost 10.5 nm porediameter in the DSBEA. Hence, this enhanced porosity of DSBEAcan be attributed to be responsible for the enhanced TTBG productselectivity obtained on this catalyst in the glycerol etherificationreaction. Such a porous system of DSBEA with tri-modal porosity;zeolitc micropores, mesopores and larger macropores is expectedto possess facile molecular diffusion and improved catalyst life.

In order to check the stability in activity of the desilicated BEA,the long time performance of this catalyst towards the glyceroletherification reaction has been compared with that of the parentBEA catalyst. The data shown in Fig. 5A indicates a slight decreasein the glycerol conversion, but the selectivity towards the bulkyoxygenates (DTBG + TTBG) is almost maintained in the studiedperiod of 36 h. The observed decrease in conversion on DSBEAmay be due to the deactivation of the active sites in this catalystby coke. The BEA catalyst also exhibited the decrease in glycerolconversion, but the decrease is much higher on this catalyst espe-cially after 10 h reaction time. The selectivity to the bulky oxy-genates (DTBG + TTBG) is also decreased significantly after 20 hreaction time. The performance patterns of DSBEA and BEA sug-gests that the presence of higher Al in the desilicated catalyst(DSBEA) may be responsible for its higher glycerol conversion,while the enhanced porosity especially the mesopores created inDSBEA is responsible for the increased bulky product selectivity.Due to this reason, though there is a slight decrease in the glycerolconversion occurred on DSBEA with reaction time, the selectivitytowards bulky products is not affected to the similar extent(Fig. 5A). Further, the decrease in glycerol conversion on DSBEAis much lower compared to that of BEA. This may be due to thepresence of high amount of Al in the former catalyst. At the samelevel of deactivation (reaction conditions), the higher Al containingDSBEA can have the possibility of exhibiting higher acidityrequired for the glycerol conversion. Overall, the better glycerolconversion and bulky product selectivity observed at initial as wellas long reaction time periods on DSBEA catalyst when compared toits parent BEA, can be ascribed to the higher Al concentration andhigher meso porosity present in the desilicated (DSBEA) catalyst.Here, the higher Al content (Si/Al = 6.1) in DSBEA also hints thepossibility of any hydrothermal instability. Further, the decreasein glycerol conversion exhibited by DSBEA with reaction time

844 S.K. Saxena et al. / Fuel 159 (2015) 837–844

indicates the coke deactivation of the active sites in the catalystwhich requires frequent regenerations of this catalyst. In order tocheck the suitability of the catalyst for reaction–regeneration oper-ations, the reaction was conducted on the regenerated DSBEA cat-alyst. As shown in Fig 5B, the DSBEA catalyst also exhibited thereproducible catalytic activities up to three regeneration cycles(each cycle time is 8 h). Though the Si/Al ratio of the DSBEA islower (Table 1), the value is comparable with that of the ultrastable Y (USY) used in Fluid catalytic cracking (FCC) and thatmay be the reason for the stable performance of the DSBEA catalystof the present study.

Overall, the desilication method adopted on BEA, in the presentstudy, incurred enhanced catalytic properties such as huge amountof mesoporosity and strong acidity responsible for the improvedglycerol conversions with shifting of product selectivity towardsdesired high octane diesel miscible oxygenate (DTBG + TTBG) suit-able for fuel applications.

4. Conclusions

Breakthrough increase in the production of high octane oxy-genate bio-fuel blending stock consist of DTBG + TTBG has beensuccessfully achieved from glycerol by catalytic modification ofBEA. Among the three Bea zeolites, the desilicated BEA (DSBEA)with its more than two-fold increase in pore volume, mesoporesand acidity could produce high yields of bulky diesel miscible oxy-genate product (DTBG + TTBG) in glycerol etherification reactionwith tertiary butyl alcohol. The studies also indicate the significantscope of the DSBEA for bulky organic transformations related tobio-mass conversion, fine chemical synthesis and production ofmiddle distillate range hydrocarbons through alkylation. TheTTBG produced in the present study can be used as the high oxygencontaining octane enhancer for fuel applications.

Acknowledgements

Authors are thankful to catalyst characterization group at IIPand TEM, GC-Mass groups at Sultan Qaboos University, Oman.The research work is supported by CSIR, India under 12th FYP.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2015.07.028.

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