Chemical PapersDOI: 10.2478/s11696-014-0550-x

ORIGINAL PAPER

Mesoporous phosphated and sulphated silica as solid acid catalystsfor glycerol acetylation

aKhadijeh Beigom Ghoreishi*, bNilofar Asim, aMohd Ambar Yarmo,aMohd Wahid Samsudin

aSchool of Chemical Science and Food Technology, Faculty of Science and Technology, bSolar Energy Research Institute

(SERI), University Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia

Received 2 June 2013; Revised 11 December 2013; Accepted 16 December 2013

Sulphate- and phosphate-loaded silicas were synthesised using the sol–gel method with differ-ent sulphate and phosphate loadings. These catalysts were characterised using Fourier transforminfrared spectroscopy (FT-IR), the Brunauer–Emmett–Teller (BET) method and X-ray photoelec-tron spectroscopy (XPS). Acidity was measured using the temperature-programmed desorption ofammonia (TPD-NH3) method. The results showed that glycerol esterification with acetic acid con-version decreased as follows: α(H2SO4) (100 %) > α(H3PO4) (99 %) > α(silica loaded with 20 %sulphuric acid) (SS-20; 98 %) > α(silica loaded with 20 % phosphoric acid) (PS-20; 83 %). Thesestudies suggest that the solid acid catalytic activity in the esterification of glycerol is highly depen-dent on catalyst acidity strength, pore size and surface area.c© 2014 Institute of Chemistry, Slovak Academy of Sciences

Keywords: glycerol, acetylation, sulphated silica, phosphated silica, sol–gel, mesoporous

Introduction

Glycerol is the main by-product of oil trans-esterification with methanol and ethanol in the pro-duction of biodiesel. Any expansion in the biodieselmanufacturing industry inevitably leads to an increasein the amount of glycerol produced. Alternative meth-ods for overcoming this problem, as well as render-ing biodiesel generation more economical, have beeninvestigated (Behr et al., 2007; Johnson & Taconi,2007; Zhou et al., 2008; Adam et al., 2012). Recentresearch focused on the catalytic conversion of bio-glycerol into useful compounds, such as, 2-propanediol(I ), 1,3-propanediol, acrolein, glyceric acid, esters ofglycerol, etc. Esterification of glycerol with short-chain carboxylic acid (e.g. acetic acid) is one methodthat has proved to be promising in the conversionof glycerol into value-added chemical acetins, suchas monoacetin, diacetin and triacetin. These chem-icals have significant applications in cryogenics andthe manufacture of plastics, fuel additives, explosives,

plasticisers, softening agents, lubricants, emulsifyingagents, etc. In addition to the above, they are alsowidely used in the cosmetics, food and pharmaceuti-cal industries (Hofmann, 1985; Nabeshima, 1995; No-mura, 1995; Taguchi et al., 2000; Watanabe et al.,2005; Lal et al., 2006; Nebel et al., 2008; Meleroet al., 2009, 2010). Most research on glycerol acety-lation has been centred on how to obtain triacetinmore efficiently. However, the present work sought toachieve a high percentage of monoacetin compared todiacetin and triacetin, as monoacetin is widely usedas a raw material in the production of biodegrad-able polyesters and the other industrial applicationslisted above (Goncalves et al., 2008, 2011; Reddy etal., 2010; Dosuna-Rodríguez et al., 2011). Acetylationis an acid-catalysed reaction that introduces an acetylradical into an organic compound. Traditionally, min-eral acid catalysts, such as sulphuric acid, phosphoricacid or organic sulphonic acids, such as Twitchell-typereagents, were used as catalysts in the esterificationof glycerol with acetic acid. However, the use of such

*Corresponding author, e-mail: khadijehbeigom@siswa.ukm.edu.my

ii K. B. Ghoreishi et al./Chemical Papers

catalysts came with significant environmental prob-lems, e.g. the generation of a substantial amount ofsalts during the neutralisation process and consider-able corrosion of the reactor (Alton Edward Baileyet al., 1979). To counter these drawbacks, solid acidcatalysts such as zeolite, niobic acid, Amberlyst�-15,heteropolyacids (tungstophosphoric acid, TPA) andsulphated zirconia are substituted for Brønsted acid(Goncalves et al., 2008; Ferreira et al., 2009a; Liao etal., 2009; Tuzen et al., 2009). Some significant advan-tages resulting from the utilisation of heterogeneousacid catalysts are a major proficiency in product sep-aration, high selectivity to main products, efficiencyin the conversion of glycerol, mild reaction conditions(i.e. lower temperatures and pressure) and simplicityof catalyst recovery. The catalysts promote the alco-hol nucleophilic attack by activating the protonationof the carbonyl oxygen on the carboxylic acid, therebyforming a tetrahedral intermediate. Eventually, thedisproportionation of this intermediate terminates inester production (Lotero et al., 2005). Many solid-acidcatalysts are used because of their appropriate prop-erties. As examples, zirconia-based catalysts demon-strated an 80 % selectivity to triacetin (Reddy et al.,2010), heteropolyacids (HPAs) encaged in USY zeo-lite catalyst displayed up to 60 % selectivity to di-acetin, with approximately 70 % glycerol conversion(Tuzen et al., 2009), SnCl2 was used as a catalystin glycerol acetylation to produce monoacetin (55 %)and diacetin (45 %) (Goncalves et al., 2011), sul-phated zirconia (SZ) presented a significant selectivityto monoacetin in mild conditions (Dosuna-Rodríguezet al., 2011), etc. The present work focused on thepreparation of sulphated silica (SS) and phosphatedsilica (PS) solid-acid heterogeneous catalysts. To doso, the sol–gel method was found to be the most ap-propriate in obtaining catalysts with large surface ar-eas and good anchoring of the sulphur and phosphorusspecies (Izumi et al., 1999). The sol–gel transition in-volves a number of complex processes of a chemicaland microstructural nature. Prior to formation of thegel, two main steps may be distinguished: (i) hydrol-ysis of the organometallic compound (tetraethoxysi-lane, TEOS) and; (ii) polycondensation of ethoxy(———Si—OEt) and silanol (———Si—OH) groups to formsiloxanes (———Si—O—Si———). In the sol–gel method, anumber of hydrolysis catalysts have been employed,for example, HCl, HNO3, H2SO4 and H3PO4. Silicais well-known to be an inert material, and the useof anions, such as SO2−4 , PO

3−4 have been found to

enhance its acidic and specific physico–chemical be-haviours, providing thermal stability and mesoporos-ity (Lion et al., 1990; Zhuang & Miller, 2001). In com-parison with other types of solid acid catalysts, sul-phated and phosphated silica render the acetylationreaction more selective to monoacetin under mild re-action conditions (i.e. shorter times and lower temper-atures) and to triacetin during reaction at higher tem-

Table 1. Amount of H2SO4 added in preparation of catalystsamples

H2SO4 H2SO4Catalyst samples

mass % g

SS-0 0 –SS-5 5 0.197SS-10 10 0.394SS-15 15 0.591SS-20 20 0.788

peratures (Dosuna-Rodríguez et al., 2011). The cat-alytic performances of SS and PS with varied sulphur-and phosphorus-loading percentages were assessed forglycerol esterification with acetic acid. Several param-eters, such as temperature, time and catalyst mass,were optimised to render the reaction more selectiveto monoacetin. In this work, unmodified and modifiedSiO2 samples with phosphate and sulphate ions wereprepared by the sol–gel method with the objective ofincreasing the acidic and textural properties of silicondioxide.

Experimental

Preparation of sulphated and phosphated sili-cas using sol–gel method

The catalysts were synthesised using the sol–gelmethod reported by Izumi et al. (1999). 2 g of TEOS(98 %; Aldrich, USA), used as a precursor for silica,was added to the mixture of butan-1-ol and water,with the mole ratios of butan-1-ol/TEOS (10 : 1) andH2O/TEOS (10 : 1) under vigorous stirring at ambi-ent temperature. The hydrolysis catalysts, H2SO4 andH3PO4, were added drop-wise to the solution, actingas both sulphating and phosphating agents as wellas the catalyst for polycondensation of the gel. Theamounts of sulphuric and phosphoric acid added fordifferent catalyst samples are shown in Tables 1 and 2.The hydrogel thus obtained was dried slowly at 100◦Cfor 3 h and then refluxed with methanol over 72 hfor complete dehydration. The resulting gel was thenannealed at 600◦C for 4 h under atmospheric condi-tions. The catalysts were prepared with varied load-ings of sulphuric acid and phosphoric acid onto silica(5 mass %, 10 mass %, 15 mass % and 20 mass %)as shown in Tables 1 and 2, and denoted as SS (sul-phated silica) and PS (phosphated silica). All chem-icals, unless otherwise stated, were purchased fromSigma–Aldrich (USA).

Catalyst activity measurements

The esterification reactions were carried out in avacuum (666.61 Pa) within a temperature range of 50–

K. B. Ghoreishi et al./Chemical Papers iii

Table 2. Amount of H3PO4 added in preparation of catalystsamples

H3PO4 H3PO4Catalyst samples

mass % g

PS-0 0 –PS-5 5 0.202PS-10 10 0.405PS-15 15 0.607PS-20 20 0.810

110◦C with acetic acid-to-glycerol mole ratio (6 : 1) ina stirred batch reactor and under reflux conditions. ADean–Stark trap was attached to the 50 mL round-bottom flask in order to extract the water that deac-tivates the catalysts and causes a reversion of the es-terification reaction. In a typical experiment, 0.2 g ofthe catalyst was added to the miscible solution of reac-tants in a container with the stirring speed maintainedat 500 min−1. After cooling, the catalyst was sepa-rated from the solution mixture by centrifugation. Thereaction medium samples were analysed using a GC-FID (model 6890N; Agilent Technologies, USA) anda capillary column of HP-5 (30 m in length, 0.25 mmin internal diameter and 0.25 �m of film thickness).The GC injection port and the detector temperaturewere set at 240◦C and 260◦C, respectively. The initialcolumn temperature was set at 70◦C for 2 min andprogrammed from 70◦C to 150◦C for 1.5 min at a rateof 45◦C min−1 and from 150◦C to 180◦C at a rateof 8◦C min−1 and from 180◦C to 240◦C at a rate of35◦C min−1. The peak areas were used for quantifica-tion and construction of the calibration curve. For ev-ery analysis, the weighed amount of the collected sam-ple and 1 mL of hexan-1-ol solution (5000 mg L−1) asan internal standard were transferred to a volumetricflask and made up to 10 mL with absolute ethanol.The GC-FID chromatogram was fully in accordancewith previous research (Casas et al., 2012). The in-frared (IR) spectra of the samples in KBr pellets wererecorded at ambient temperature, using a Perkin–Elmer (USA) Paragon 2000 FTIR spectrometer, un-der atmospheric conditions. The surface area of thecatalyst was measured using the Brunauer–Emmett–Teller (BET) method (N2 adsorption) with a Geminiapparatus (Micromeritics 2010 Instrument Corpora-tion, USA). X-ray photoelectron spectra (XPS) wereacquired using a Kratos (UK; XSAM HS) spectrom-eter, equipped with a hemispherical electron analyserand MgKα (hν = 12536 eV, 1 eV = 1.6302 × 10−19 J)120 W X-ray source. The samples were analysed at3 × 10−7 kPa, using C1s line at 284.6 eV using ad-ventitious carbon as a reference for the binding ener-gies. The acidity of the catalytic materials was mea-sured by a temperature-programmed desorption ofammonia (TPD-NH3). These experiments were per-

Fig. 1. FT-IR transmittance spectra of SO2−4 (a), SiO2 an-nealing temperatures of: 800◦C (Line a), 600◦C (Lineb) and 400◦C (Line C) and; PO3−4 (b), SiO2 at dif-ferent annealing temperatures; 400◦C (Line a), 600◦C(Line b) and 800◦C (Line c).

formed in a gas-flow system equipped with a ther-mal conductivity detector (TCD). Prior to monitor-ing the TPD-NH3 profiles, the samples (50–60 mg)were placed in a U-shaped quartz reactor and heatedin a helium flow at 150◦C for 30 min. After coolingto 70◦C, the samples were exposed to ammonia andflushed with 50 mL min−1 helium for 60 min. TheTPD measurement was carried out with a heatingrate of 10◦C min−1 up to 600◦C under flowing he-lium (50 mL min−1). The desorption of ammonia wasmonitored using a TCD.

Results and discussion

Catalyst characterisation

Fig. 1 depicts the FT-IR transmittance spectra ofsulphated and phosphated silicas in the OH stretch-ing region annealed at 600◦C. A broad band be-tween 3300 cm−1 and 3500 cm−1 is assigned to O—Hstretching in H-bonded water. This band can be con-

iv K. B. Ghoreishi et al./Chemical Papers

Table 3. Textural properties of SS and PS samples

H2SO4 or H3PO4 BET surface area Pore volume Pore sizeSample

mass % m g−1 cm3 g−1 nm

SS-0 0 740 0.68 14.9SS-5 5 680 0.61 5.5SS-10 10 644 0.58 6.2SS-15 15 586 0.56 8.3SS-20 20 437 0.52 10.1PS-0 0 740 0.68 14.9PS-5 5 629 0.71 2.2PS-10 10 638 0.63 3.6PS-15 15 557 0.58 6.9PS-20 20 330 0.55 7.5

firmed through the band at 1635 cm−1 due to thescissor-bending vibration of the molecular water. Theintensity of these bands clearly declines in sulphatedsilica, indicating lower water desorption and dehydrox-ylation in this sample. The symmetric vibration ofSi—O appears at 800 cm−1 and the peak at approxi-mately 1100 cm−1 is attributed to the asymmetric vi-bration of silicon atoms in siloxane (Si—O—Si). TheFT-IR spectra of the systems show a peak at 1200–1100 cm−1 which can be assigned to the S——O group(Sunajadevi & Sugunan, 2004). The peak at approx-imately 1400 cm−1 suggests that the added sulphateexists as SO2−4 species. The peak at 1300–1400 cm

−1

in phosphated silica is the stretching frequency ofthe P—O bond and the 1100–1250 cm−1 peaks arethe characteristic frequencies of PO3−4 (Samantaray& Parida, 2001). A broad band is seen at 3468 cm−1;this is assigned to the νOH of molecular water. Thisabsorption is correlated to the deformation mode ob-served close to 1654 cm−1. A shoulder at 3662 cm−1

due to SiO—H silanols with hydrogen bonding is alsoobserved. At 600◦C, the high energy side decreasesin intensity and a new small band at 3771 cm−1 isattributed to free SiO—H silanols (Manríquez et al.,2004). When the sample was annealed at 800◦C, theOH bands intensity diminished considerably. On thesulphated sample, the OH absorption is still observedat 800◦C, indicating lower water desorption and de-hydroxylation in this sample (Aelion et al., 1950).The Si—O—Si stretching vibrations are observed at1070 cm−1 and 1212 cm−1, and the deformation modeat 460 cm−1 for the three samples. The band situatednear 945 cm−1 is due to the SiO—H flexion mode,while the band near 800 cm−1 is due to Si—O- flexionvibrations (Scherer, 1988).The textural properties of the catalysts, calcined

at 600◦C, obtained from the nitrogen adsorptionisotherms, are summarised in Table 3.Table 3 shows the specific surface area of the cat-

alysts which were determined by the BET method,along with the total pore volume and average porediameter of the catalysts. All the synthesised solids

Fig. 2. N2 adsorption/desorption isotherms of sol–gel; 1 –PO3−4 SiO2 (a), and SO

2−4 SiO2 (b) with: 2 – 5 mass %;

3 – 10 mass %; 4 – 15 mass %; 5 – 20 mass % of loading.

K. B. Ghoreishi et al./Chemical Papers v

have a very high specific surface area and a meso-porous texture. In general, a strong correlation wasnoted between the textural properties and the sul-phate and phosphate contents of the catalysts. How-ever, in the PO3−4 SiO2 catalytic system, the surfacearea presented was lower than in the SO2−4 SiO2 sys-tem. The addition of sulphuric acid and phosphoricacid to the silica matrix led to a reduction in surfacearea and pore volume. The decreasing trend in sur-face area with an increase in sulphate- and phosphate-loadings may be due to blockage of the pores by theactive non-porous species that were formed. A similarresult was observed by Mukai et al. (2003) and Ferreiraet al. (2009b). Also, in both catalytic systems, it canbe observed that the pore size of silica first decreasedwith an addition of sulphuric or phosphoric acids inthe process of its preparation, and then the pore sizeincreased with an increase in the acid amount. Thesmallest pore sizes of 5.5 nm and 2.2 nm were achievedin SS-5 and PS-5, respectively. The BET isotherms ofsilica, PO3−4 SiO2, and SO

2−4 SiO2 are shown in Fig. 2.

The NH3 adsorption–desorption technique permitsdetermination of the strength of the acid sites presenton the catalyst surface, together with total acidity.The TPD-NH3 profiles of the catalysts are shown inFig. 3. The TPD profile of silica has is included forcomparison. The desorption peaks in maximum tem-perature ranges of 180–250◦C, 280–330◦C and 380–500◦C are normally attributed to NH3 chemisorbedon weak, medium and strong acid sites, respectively(Corma et al., 1994). In both the sulphated silica andthe phosphated silica, two desorption peaks were ob-served centred at 280◦C and 550◦C, which were at-tributed to the presence of moderate and strong acidsites. The TPD-NH3 patterns of SS-20 and PS-20 showthe highest levels in terms of total acidity in their rel-evant catalyst system. The total acidity results of thecatalysts are listed in Table 4.The similarity in the maximum desorption tem-

peratures implies that the distributions of acid sitestrengths for PS-20 and SS-20 are quite similar. As

Fig. 3. TPD-NH3 profiles of; 1 – SO2−4 SiO2 (a), and PO

3−4

SiO2 (b) with loadings of: 2 – 5 mass %; 3 – 10 mass %;4 – 15 mass %; 5 – 20 mass %.

stated in Table 4 and is obvious in Fig. 3, SS-20 ex-hibits the largest proportion of medium and strong

Table 4. Acidic properties of samples obtained from TPD of ammonia

Temperature at maximum/◦C Total NH3 desorbed/(�mol g−1)Sample H2SO4 or H3PO4/mass %

Peak 1 Peak 2 Peak 1 Peak 2

SS-0 0 – 450 – 250.18SS-5 5 271 453 140.89 280.59SS-10 10 280 531 334.95 388.22SS-15 15 277 539 409.67 465.81SS-20 20 247 550 436.21 479.34PS-0 0 – 450 – 250.18PS-5 5 225 544 114.61 220.63PS-10 10 271 512 322.17 337.86PS-15 15 274 523 380.97 395.23PS-20 20 287 543 418.01 446.12

vi K. B. Ghoreishi et al./Chemical Papers

acid sites of 436.21 mmol g−1 and 479 mmol g−1, re-spectively. The peak broadening in SO2−4 SiO2 andPO3−4 SiO2 is presumed to be the result of stronginteraction between the acid sites and the adsorbedNH3. It was noted that the levels of sulphate andphosphate ions involved determined the amount ofNH3 desorbed. The higher the amount of sulphateand phosphate ions, the higher the amount of NH3desorbed and, consequently, the higher the acidic ac-tivity of the catalysts.XPS was applied to verify the presence of sulphate

and phosphate on the surface of silica, as well as toidentify the oxidation state of the bonding characteris-tic of PO3−4 and SO

2−4 on silica. The C1s peak was set

at 284.6 eV in order to offset the electron charging forall spectra. The XPS findings on the SiO2, SS and PSsurfaces are shown in Figs. 4–7. The oxidation state ofSi in PS (Si2p, binding energy of 103.3 eV) exhibits a

Fig. 4. XPS wide-scan spectra of PS sol–gel (a), sol–gel silica(b) and SS sol–gel (c).

Fig. 5. XPS spectra of C1s (a), O1s (b), Si2p (c) and S2p (d) of SS.

K. B. Ghoreishi et al./Chemical Papers vii

Fig. 6. XPS spectra of O1s (a), Si2p (b), C1s (c) and P2p (d) of PS.

slight shift compared to unmodified Si (Si2p, bindingenergy of 103 eV), indicating the replacement of oxy-gen with phosphorus. In the case of SS, this bindingenergy shifts to an even higher value of 103.7 eV, indi-cating an interaction between the sulphate anion andthe Si cation. The O1s in silica peaked at 529.5 eV,corresponding to the Si—O in SiO2. However, the O1sbinding energy of the PS occurred at 530 eV and wasascribed to the presence of the P—O—Si linkage. InSS, the peak was observed at 530.5 eV and attributed

to the presence of the S—O—Si linkage. The bind-ing energy for S2p3/2 at 168.9 eV is attributed toS—O bonds in SO2−4 species, indicating that the ox-idation state of sulphur in the sample is hexavalent(S6+) (Melada et al., 2004). The absence of a peakat approximately 163–164 eV reveals that there is noelemental sulphur or Si—S type species in the sample(Dutta et al., 1983). The bonding energy of 134.7 eV inthe phosphated silica is assigned to pentavalent phos-phorus (Splinter et al., 1996).

viii K. B. Ghoreishi et al./Chemical Papers

Table 5. Percentage of mass concentration (mass %) for C1s,O1s, Si2p S2p and P2p in catalysts

Mass concentration/mass %Type of catalyst

C1s O1s Si2p S2p P2p

SiO2 4.08 61.58 34.34 – –SS-20 2.86 63.72 21.49 11.93 –PS-20 3.71 67.64 18.63 – 10.02

Fig. 7. Effects of reaction time on conversion in glycerol acety-lation: • – SS-20; � – PS-20; × – H2SO4; ∗ – H3PO4;� – silica; – blank. Reaction temperature 110◦C, cat-alyst mass 0.2 g, AcOH/glycerol mole ratio 6 : 1.

Percentage of mass concentration (mass %) forC1s, O1s, Si2p. S2p and P2p in silica, SS-20 and PS-20 were obtained from XPS analysis and the resultsare given in Table 5.

Catalyst experiments

The acetylation of glycerol over sulphated silicaand phosphated silica catalysts led to the formation ofmonoacetin, diacetin and triacetin, which were anal-ysed using a gas chromatography-flame ionisation de-tector (GC-FID) and the capillary column of HP-5(Ghaziaskar et al., 2006). From the loaded silica cat-alysts, SS-20 and PS-20 were chosen as they exhib-ited maximum acidity in their catalyst group (referto TPD-NH3 results in Table 4). Fig. 7 compares theactivity of the catalysts for the acetylation reaction ofglycerol with acetic acid. For comparison, the profilesof H2SO4, H3PO4 and silica catalyst reactions, as wellas the profiles of a reaction without a catalyst (blank),are included.The glycerol conversion rate increased with the re-

action time for all the catalysts and, arranged accord-ing to catalyst activity, they are as follows: α(H2SO4)(with approximately 100 % conversion) > α(H3PO4)(99 %) > α(SS-20) (98 %) > α(PS-20) (83 %). Whena reaction occurred in the absence of a catalyst, the fi-

Fig. 8. Influence of reaction temperature on glycerol acety-lation over PS-20 (a) and SS-20 catalysts (b): –monoacetin; – diacetin; – triacetin; ∗ – conversion.Reaction time 5 h, catalyst mass 0.2 g, AcOH/glycerolmole ratio 6 : 1.

nal conversion was approximately 25 %, probably dueto the acidic properties of the acetic acid protons. Theuse of pure silica did not result in any significant im-provement in the conversion rate; this could be at-tributed to its low acidity compared to other catalystspecies (sulphated and phosphated silica). In brief,sulphated and phosphated silicas exhibited slightlylower catalyst activity within 5 h than the homoge-neous sulphuric acid and phosphoric acid. This high

K. B. Ghoreishi et al./Chemical Papers ix

Fig. 9. Effects of catalyst mass on glycerol acetylation over PS-20 (a) and SS-20 (b) catalysts: – triacetin; – diacetin;– monoacetin; � – conversion. Reaction time 5 h,

temperature 110◦C, AcOH/glycerol mole ratio 6 : 1.

catalyst activity corresponds to the wide surface areaand reasonable acidic properties of heterogeneous cat-alysts.The effects of reaction temperature on the ester-

ification of glycerol over SS-20 and PS-20 were eval-uated at temperatures varying from 50◦C to 110◦C;the results are shown in Fig. 8. As expected, in bothcases the conversion of glycerol increased with temper-ature. However, the conversion rate was higher withSS-20 due to its higher acidity. Selectivity was also

Fig. 10. Re-usability of SS-20 (a) and PS-20 (b) in esterifica-tion of glycerol with acetic acid: × – first use; ∆ –second use; ◦ – third use. Temperature 110◦C, cata-lyst mass 0.2 g, AcOH/glycerol mole ratio 6 : 1.

affected by an increase in reaction temperature. Forinstance, the SS-20 catalyst selectivity to monoacetinat a lower temperature was marked, while there wasno trace of triacetin. As the temperature increased,so did the selectivity towards diacetin and triacetin.Similar selectivities and activities were achieved by us-ing PS-20 as a catalyst. It is noteworthy that PS-20exhibited more selectivity to triacetin at 110◦C thanSS-20, while SS-20 indicated significant selectivity tomonoacetin at 50◦C. Clearly, as shown in Fig. 8, bothcatalysts presented considerable activity even at lowreaction temperatures (Chimienti et al., 2001).The effects of catalyst mass on glycerol acetylation

were also investigated. A range of 0.1–0.4 g of cata-lyst was used, and the reaction temperature was main-tained at 110◦C for 5 h. An increase in catalyst mass(up to 0.2 g for SS-20 and 0.3 g for PS-20) led to aslight increase in glycerol conversion (Fig. 9). Hence, itis clear that catalyst loadings of above 0.2 g and 0.3 gfor SS-20 and PS-20, respectively, did not improve therate of reaction. The investigation also revealed that,with SS-20, a small amount of catalyst was sufficient toattain maximum conversion. It is also significant that,in the SS-20 catalyst reaction, an increase in mass ofthe catalyst from 0.1 g to 0.4 g led to a decrease in the

x K. B. Ghoreishi et al./Chemical Papers

selectivity to monoacetin, while the selectivity to di-acetin and triacetin increased. An increase in the massof PS-20 did not indicate any regular trend in selec-tivity towards the products of the acetylation reactionIn order to study the catalytic stability, the SS-20

and PS-20 catalysts were re-used. Each consecutivecycle was carried out under the same reaction condi-tions and with the same catalyst sample (reaction tem-perature 110◦C, catalyst mass 0.2 g, AcOH/glycerolmole ratio 6 : 1). After each reaction cycle, the cata-lysts were washed with hexane and dried overnightat 100◦C prior to re-use (Petchmala et al., 2010).Fig. 1 shows that the conversion of glycerol decreasedslightly in the second cycle; however, similar catalyticactivities were observed in both catalytic systems afterthe second use. The slight deactivation of the SS-20and PS-20 catalysts could be explained by sulphateand phosphate loss from the catalysts’ surface (leach-ing of catalysts to the liquid phase).

Conclusions

A series of sulphate and phosphate loading in silicawith varied loading masses from 5 g to 20 g was per-formed to prepare solid acid catalysts by the sol–gelmethod. Their acidic strengths, evaluated by the am-monia adsorption method, revealed that the SS-20 andPS-20 catalysts had the highest acidic levels in theirrelevant catalyst series, with SS-20 recording higheracidic properties than PS-20. The catalytic activitiesof SS-20 and PS-20 were examined by acidic glycerolesterification at 110◦C with 5 h of reaction time andAcOH/glycerol mole ratio 6 : 1. The results suggestthat the esterification activity was related not only tothe acidity of the catalysts but also to such parametersas temperature, time and catalyst mass. Sulphatedand phosphated silicas were found to be highly ac-tive and selective in the esterification of glycerol evenat low reaction temperatures and small amounts ofcatalyst.

Acknowledgements. The authors would like to thank Uni-versity Kebangsaan Malaysia for their funding (Code UKM-ST-07-FRGS0002-2008 and UKM-ST-06-FRGS0147-2010)and the staff of the School of Chemical Sciences and Food Tech-nology.

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