ARTICLE pubs.acs.org/IECR
Sulfonic Acid-Functionalized Catalysts for the Valorization of Glycerol via Transesterification with Methyl Acetate Gabriel Morales,*,† Marta Paniagua,† Juan A. Melero,† Gemma Vicente,‡ and Cristina Ochoa§ †
Department of Chemical and Environmental Technology, ESCET, and ‡Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, Mostoles, 28933, Spain § Thermochemical Processes Unit, Instituto IMDEA Energía, Mostoles, Madrid, 28933, Spain ABSTRACT: Sulfonic acid-functionalized mesostructured SBA-15 silicas have been demonstrated to be active for the catalytic transesterification of glycerol with methyl acetate to produce di- and triacetylglycerols. An optimization of the reaction conditions carried out with pharmaceutical glycerol by means of experimental design methodology showed that it is necessary to use a high methyl acetate to glycerol molar ratio (50:1) and a high catalyst loading (7.5 wt % based on glycerol) in order to obtain simultaneously very high glycerol conversion (99.5%) and high combined selectivity toward di- and triacetylglycerols (74.2%). In addition, the formation of nondesired byproduct was minimized at such reaction conditions. The activity displayed by arenesulfonic acid-functionalized mesostructured silica was comparable to that displayed by commercial catalysts such as the resin Amberlyst-70 or the composite Nafion-SAC-13. The acid strength of the catalytic sites, and in less extent their surface density, proved to be the most influential parameters in this reaction. Arenesulfonic SBA-15 also provided suitable conversions and selectivities with technical-grade glycerol, although not with crude glycerol due to the deactivating effect of salts.
1. INTRODUCTION Biodiesel is produced from the transesterification of oils and fats with methanol and currently represents one of the major biofuels used worldwide. The reaction is carried out industrially under base catalysis conditions, although acid catalysis is also possible and more convenient under certain circumstances.1 The reaction also affords glycerol as the main byproduct in a proportion of 10 wt %. Nowadays, one of the most challenging aspects of the biodiesel technology is related to the economical utilization of this glycerol. As produced, glycerol is about 80% pure, the main contaminants being soaps, salts, methanol, and water. Refinement steps are necessary in order to turn this crude glycerol into more usable and profitable grades, such as technical glycerol (>90% pure after a desalting process) and pharmaceutical glycerol (>99.7%, by distillation). Taking into account the present and future biodiesel production growth—the EU 2009/ 28/EC Directive establishes a 10% share of energy from renewable sources for transport in EU energy consumption by 2020—a dramatic rise in the availability of glycerol is expected. Currently, this chemical has over two thousand different industrial applications in as distinct fields as cosmetics, pharmaceutics, food and drinks, tobacco, paper, inks and printing, production of resins (phtalic and maleic alkyl resins) and cross-linked polyesters, and as a hydraulic agent.2 These sectors, however, will not be able to absorb all the production coming from biodiesel industry. As a consequence, the search for alternative glycerol uses has been object of extensive research during the last years. In this context, one explored option is the conversion of glycerol into oxygenates for biodiesel formulation. This application is extremely interesting because the resultant compounds could also be used as improvers of certain biodiesel properties.3 Furthermore, the glycerol-derived oxygenated compounds can be considered themselves as biofuels due to the biomass origin, increasing the r 2011 American Chemical Society
overall yield of the process and helping to meet the targets of the EU Directive. In this context, the transformation of glycerol by means of etherification with olefins or alcohols411 and acetalization with aldehydes or ketones1218 has been recently investigated. Likewise, the production of glycerol esters by acetylation, esterification, or transesterification with anhydrides, carboxylic acids or their methyl esters, respectively, is a remarked approach for the preparation of such glycerol derivates.1923 Special interest is focused on the preparation of mono-, di-, and triacetylglycerol, namely mono-, di- and triacetin. Diacetin and triacetin, with many different industrial applications in pharmaceutics, cosmetics, polymers, food additives, and tobacco industry, have also been successfully tested as biodiesel additives.3,16,19 They can be produced in the acid-catalyzed reactions of glycerol with acetic acid and/or acetic anhydride, the use of the anhydride being more advantageous to achieve high selectivity toward the most preferred triacetin.1,23 They have been claimed to be valuable petrol fuel additives leading to either enhanced cold properties when blended with diesel-fuel,19 or antiknocking properties when added to gasoline.24 For the acid-catalyzed production of these acetylglycerols, many acid catalytic systems have been evaluated.1 In this context, sulfonic acid-functionalized mesostructured materials have demonstrated an excellent catalytic behavior in the transformation of glycerol into oxygenated compounds by means of esterification with acetic acid,20 etherification with isobutylene,11 and acetalization with acetone.18 These materials, featured by high surface area, large uniform pores, high thermal stability, and the capability to control the surface Received: November 22, 2010 Accepted: March 26, 2011 Revised: February 10, 2011 Published: April 06, 2011 5898
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Table 1. Physicochemical, Textural, And Acidity-Related Properties for Sulfonic-Acid-Modified Mesostructured Silicas textural properties
acid properties acid capacity (meq/g)
d100a (Å)
pore sizeb (Å)
BET area (m2/g)
pore volumec (cm3/g)
wall thicknessd (Å)
sulfure
Hþf
Pr-SBA-15
111
82
666
1.19
46
1.17
1.15
Ar-SBA-15
105
76
694
0.86
45
1.00
0.97
Ar-SBA-15(30)
111
73
492
0.53
55
2.37
1.90
sample
a
d (100) spacing, measured from small-angle X-ray diffraction. b Mean pore size (Dp) from adsorption branch applying the√BJH model. c The pore volume (Vp) was taken at P/Po = 0.975 single point. d Average pore wall thickness calculated by ao-pore size (ao = 2d(100)/ 3). e Sulfur content by elemental analysis. f Acid capacity defined as meq of acid centers per g of catalyst obtained by titration.
hydrophilic/hydrophobic balance as well as the strength and concentration of acid sites,25 appear as promising catalysts for this sort of acid-catalyzed reactions. In the particular case of the esterification of glycerol with acetic acid, the activity and selectivity of the silica-supported sulfonic acid catalysts have been shown to be comparable or even superior to other conventional acid catalysts (H2SO4, Amberlyst 15, and Nafionsilica composite SAC-13).20 Another strategy for the synthesis of glycerol esters is the transesterification of glycerol with methyl acetate, producing methanol as byproduct that can be recycled to the production of FAME. Methyl acetate can be directly obtained by esterification of acetic acid and methanol, but it is also a byproduct in the production of polyvinyl alcohol (PVA)—for every 1 kg of PVA, 1.68 kg of methyl acetate is produced. The worldwide production of PVA in 2006 was over 1Mt,26 which implies the generation of approximately 1.68 Mt of methyl acetate, whereas the production of glycerol in the UE estimated for 2010 is about 1 Mt. Thus, the use of methyl acetate in the transformation of glycerol appears as an interesting alternative for the production of glycerol acetates. It must be noted that the ability of methyl acetate as acyl acceptor in the interesterification of vegetable oils for the simultaneous production of FAME and triacetin has also been described in literature by means of base27 and enzymatic catalysis.28 Nevertheless, this reaction requires harsher reaction conditions than the conventional methanolysis processes and the final reaction product, consisting of 20 wt % triacetin in FAME due to stoichiometric reasons, does not fulfill the current European biodiesel fuel standard EN 14214.27 In this work, we have explored for the first time the synthesis of mono-, di- and triacetylglycerol via transesterification of glycerol with methyl acetate over different sulfonic acid-modified mesostructured silicas. Factorial design and response surface methodology have been used for the optimization of the reaction conditions. This technique is a powerful tool involving many advantages that have been described in previous works.29 In addition, the catalytic performance of these sulfonic acid-modified mesostructured silicas has been benchmarked with other commercial acid catalysts, also analyzing their reusability in the reaction. Finally, different types of glycerol grades—from crude glycerol to refined glycerol—were evaluated in this transesterification reaction.
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. Propylsulfonic acid-functionalized mesostructured silica (Pr-SBA-15) was synthesized following a previously reported procedure.30 Molar composition of the
mixture for 4 g of templating copolymer was 0.0369 tetraethylorthosilicate (TEOS, Sigma-Aldrich), 0.0041 mercaptopropyltrimethoxysilane (MPTMS, Sigma-Aldrich), 0.0369 H2O2, 0.24 HCl, and ∼6.67 H2O. Arenesulfonic acid-functionalized mesostructured silica (Ar-SBA-15) was obtained as described elsewhere.31 In this case, the molar composition of the mixture for 4 g of copolymer was 0.0369 TEOS, 0.0041 chlorosulfonyl-phenyl ethyltrimethoxy-silane (CSPTMS, ABCR), 0.24 HCl, and ∼6.67 H2O. The amount of sulfur-containing precursor in both materials (MPTMS and CSPTMS) has been established to be 10 mol % of total silicon species, as higher loadings can jeopardize the textural properties of the silica mesostructure. High-loading arenesulfonic acid-functionalized mesostructured silica (Ar-SBA-15(30)) was synthesized in order to increase the concentration of catalytic sites of the above-mentioned catalyst. The molar composition of the mixture for 4 g of copolymer was 0.0287 TEOS, 0.0123 CSPTMS, 0.24 HCl, and ∼6.67 H2O. The amount of precursor has been established to be 30 mol % of total silicon species, despite of the detrimental effect on the textural properties of such a high organic loading. Other commercial catalysts used in this work were NafionSiO2 composite (SAC-13) with resin content in the range of 1020 wt %, supplied by DuPont and an ionic-exchange sulfonic acid-based macroporous resin Amberlyst 70, supplied by Rohm and Haas. Both catalysts were ground to powder in order to minimize mass transfer limitations and thus avoid distortions in the catalytic results. Arenesulfonic and propylsulfonic-acid-functionalized nonordered silicas, under the commercial names SiliaBond Tosic Acid and SiliaBond Propylsulfonic Acid, were acquired from Silicycle directly in powder form. 2.2. Catalysts Characterization. Textural properties of the synthesized catalysts were obtained by means of nitrogen adsorption and desorption isotherms at 77 K using a Micromeritics TRISTAR 3000 system. BJH model was used for the analysis of data and total pore volume was taken at P/Po = 0.975 as a single point. Low-angle X-ray powder diffraction (XRD) was employed for determining structural ordering on a PHILIPS PERT apparatus using Cu KR radiation. Cationic-exchange capacities (acid capacity) were potentiometrically measured by titrating a suspension of catalyst in 2 M NaCl (aq) with dropwise addition of 0.01 M NaOH (aq). Sulfur and organic contents were determined by means of elemental analysis in a Vario EL III apparatus, and thermogravimetric analysis (SDT 2960 Simultaneous DSCTGA, from TA Instruments). Table 1 summarizes the most relevant physicochemical properties for the sulfonic acid-modified mesostructured materials. Data from XRD and nitrogen adsorption isotherms evidence 5899
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Table 2. Physicochemical Properties Corresponding to Commercial Sulfonic Acid-Based Catalystsa catalyst
a
acid capacity (meq Hþ/g)
BET area (m2/g)
pore size (Å)
pore volume (cm3/g)
max. operating temperature (°C)
Amberlyst-70
2.55
36
220
SiliaBond Propylsulfonic Acid
1.04
301 b
20200 b
0.44 b
>200
190
SiliaBond Tosic Acid
0.78
279 b
20200 b
0.38 b
>200
Nafion SAC-13
0.12
>200
>100
200
Properties provided by the suppliers. b Experimentally determined by N2 adsorptiondesorption isotherm at 77 K.
Scheme 1. Main Reaction Products in the Glycerol Transesterification with Methyl Acetate
high mesoscopic ordering and high surface areas along with narrow pore size distributions around 78 nm (size enough to avoid the steric constraints imposed by the pore size when relatively bulky substrates such as glycerol derivates are considered). Standard materials, Pr-SBA-15 and Ar-SBA-15, with 10 mol % synthesis sulfur content, display high incorporation yields of the organic precursors, consistent with results previously reported.25 However, at higher loadings of sulfonic acid groups (i.e., 30 mol % for the sample Ar-SBA-15(30)), the formation of the mesostructure is impaired by the high degree of organic loading, leading to lower BET surface area and pore volume. As a consequence, a lower titration value than that expected is obtained, as deduced from the values of Hþ and sulfur content. This indicates that, for this sample, not every sulfur site within the material is a catalytically active acid site, most likely due to a partial lack of accessibility within the porous structure. Additionally, the average wall thickness is also higher as a consequence of the presence of a higher number of arenesulfonic moieties. Nevertheless, the sample Ar-SBA-15(30) still displays the highest amount of acid sites, 1.90 meqHþ g1, and hence it is deemed a valid catalyst to be evaluated in the present work. Furthermore, Table 2 summarizes some relevant properties corresponding to commercial sulfonic acid-based catalysts. In this case, the characterization is mostly provided by the suppliers (Rohm & Haas for the Amberlyst resins, DuPont for SAC-13 nanocomposite, and Sylicycle for the functionalized silicas). 2.3. Reaction Procedure. Crude, technical and pharmaceutical grade glycerols used in the present work were kindly
provided by Acciona Biocombustibles, from the biodiesel production plant in Caparroso (Navarra, Spain). The rest of the reagents used in the reactions and analyses, methyl acetate (99.8% purity), diacetylglycerol (50% purity), triacetylglycerol (99% purity), and 1,4-butanediol (99% purity), were acquired from Sigma-Aldrich. Monoacetylglycerol (40% purity) was supplied by Acros Organics. Scheme 1 is a representation of the transesterification of glycerol with methyl acetate, showing the main acetylation products: monoacetylglycerol (MAG), diacetylglycerol (DAG), and triacetylglycerol (TAG). It must be noted that MAG and DAG can include two isomers depending upon the acetylation position within the glycerol molecule. Hence, herein the terms MAG and DAG are intended to embrace all the possible mono- and diacetylated products, respectively. Among the products, DAG and TAG are the most interesting compounds from a fuel-application point of view. However, the presence of MAG in the final reaction product is undesired due to its relatively high solubility in water. Thus, the reaction conditions have to be established with the purpose of maximizing the production of the di- and triderivates. Transesterification runs were performed in liquid phase at temperatures ranging from 120 to 195 °C in a stirred stainlesssteel 100 mL autoclave under autogenous pressure (with an initial nitrogen pressure of 5 bar in order to ensure liquid phase for both reactants). The reaction temperature was controlled using a thermocouple immersed into the reaction mixture. Typically, 10:1 to 50:1 molar ratio of methyl acetate/glycerol and a catalyst loading ranging from 2.5 to 7.5 wt % based on 5900
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Figure 1. Effect of reaction temperature on glycerol conversion and selectivity to MAG, DAG, TAG and other nonidentified products over Ar-SBA-15. Reaction conditions: methyl acetate/glycerol molar ratio 30:1, 5 wt % catalyst based on glycerol, reaction time 4 h.
glycerol mass (total acid sites loading from 0.24 to 0.73 mmol Hþ) were used for the optimization of the reaction conditions with the catalyst Ar-SBA-15. For the screening of catalysts, and considering the differences in acid sites concentration for each catalyst, catalyst mass loading was varied to reach an equivalent total acid sites loading of 0.24 mmol Hþ. Reaction products were analyzed by gas chromatography (Varian 3900 chromatograph) using a CP-WAX 52 CB column (30 m 0.25 mm, DF = 0.25) and a flame ionization detector (FID). Under the analysis conditions, acetylation products were the only reaction products detected by GC. Catalytic results are shown either in terms of absolute conversion of glycerol or in terms of selectivity toward the different products. The quantification of the products was obtained by GC using commercial MAG, DAG, and TAG and pharmaceutical glycerol to obtain the corresponding response factors. The amount of “others” nondetected compounds under the analysis conditions, were obtained by closing the massbalance, as indicated in the following formulas. Glycerol conversion ðXG Þ ¼
reacted mol of glycerol initial mol of glycerol
Selectivity to MAG ðSMAG Þ ¼
formed mol of MAG reacted mol of glycerol
Selectivity to DAG ðSDAG Þ ¼
formed mol of DAG reacted mol of glycerol
Selectivity to TAG ðSTAG Þ ¼
formed mol of TAG reacted mol of glycerol
Selectivity to others ðSothers Þ ¼ 100 SMAG SDAG STAG
3. RESULTS AND DISCUSSION 3.1. Influence of Reaction Temperature and Time. As a previous step to the optimization of catalyst loading and methyl
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Figure 2. Evolution of glycerol conversion and selectivity to MAG, DAG, TAG, and other nonidentified products over Ar-SBA-15. Reaction conditions: methyl acetate/glycerol molar ratio, 30:1; 5 wt % catalyst based on glycerol; reaction temperature, 170 °C.
acetate/glycerol molar ratio by means of experimental design methodology, preliminary studies were performed in order to establish adequate conditions for reaction temperature and time. Figure 1 depicts the effect of reaction temperature on glycerol conversion and selectivity to the different esters for a fixed reaction time of 4 h, whereas Figure 2 represents the kinetic curves for conversion and selectivity at a constant temperature of 170 °C. As expected, an increase of the reaction temperature promotes a faster transformation of glycerol, as evidenced by the glycerol conversion (below 80% at 120 °C and over 90% at 145 °C). Above 170 °C the conversion is practically total. From the point of view of selectivity, however, there is still a very pronounced effect: the higher the temperature, the lower the presence of MAG—reaching a value below 20% at 195 °C—and the higher the production of DAG and TAG. Unfortunately, the appearance of nonidentified byproduct derived from glycerol is also favored at high temperatures. These nonidentified products, denoted in the figures as “others”, likely come from the dehydration of glycerol—to yield acrolein, acetol, formaldehyde, etc.—which is especially promoted in the presence of acid catalysts at relatively high temperatures, and/or from the condensation of glycerol.32,33 In a similar way, at 170 °C the kinetic curves shown in Figure 2 indicate the presence of a serial reaction wherein glycerol is transformed into MAG, MAG into DAGs, which finally give TAG. In contrast, the selectivity toward the nonidentified compounds does not increase after 16 h, when no glycerol is left in the reaction medium. The reaction mixture rapidly evolves within the first 46 h. Beyond that time, only the selectivity toward the final product, TAG, keeps increasing up to values over 15% at 24 h. As shown in both figures, the transformation of glycerol is moderately fast and an adequate combination of temperature and time can result in values of glycerol conversion close to 100%. However, the selectivity to triacetin remains quite low even for long reaction time and high temperature. Furthermore, an increase in the selectivity to TAG appears to come necessarily accompanied by the appearance of nondesired byproduct. To increase the selectivity to TAG while keeping as low as possible the formation of nonidentified compounds two additional 5901
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reaction variables have been analyzed: the molar ratio of methyl acetate to glycerol and the catalyst loading. 3.2. Influence of Catalyst Loading and Methyl Acetate to Glycerol Molar Ratio. The influence of catalyst loading and methyl acetate to glycerol molar ratio was developed and optimized by following factorial design and response surface methodology.29 The experimental design applied to this study was a full 32 design (two factors, each one at three levels). Experiment corresponding to the central point of the design was repeated three times in order to determine the variability of the Table 3. Matrix of Experiments and Experimental Results for the Transesterification of Pharmaceutical Glycerol with Methyl Acetate over Arenesulfonic Acid-Modified Mesostructured Silica (Ar-SBA-15). [Temperature = 170 °C; Reaction Time = 4 h]a selectivity (%) run number MR C (wt %) IMR IC XG (%) MAG DAG TAG others 1 2
a
50:1 50:1
7.5 2.5
þ1 þ1 þ1 1
98.2 77.2
23.4 74.6
68.4 22.7
5.5 1.3
2.7 1.4
3
10:1
7.5
1 þ1
93.0
37.6
43.8
7.5
11.1
4
10:1
2.5
1 1
90.4
51.4
42.9
3.1
2.6
5
30:1
5
0
0
96.1
37.4
57.3
2.8
2.5
6
30:1
5
0
0
94.7
41.0
54.9
2.0
2.1
7
30:1
5
0
0
96.8
35.1
59.1
3.1
2.7
8
30:1
5
0
0
95.7
39.4
55.9
2.5
2.2
9 10
30:1 50:1
7.5 5
0 þ1 þ1 0
98.0 93.2
27.8 48.8
63.2 47.8
5.0 1.7
4.0 1.7
11
30:1
2.5
12
10:1
5
0 1 1
0
76.8
70.7
26.9
1.0
1.4
94.5
43.6
43.6
5.9
6.9
C, catalyst loading; MR, methyl acetate:glycerol molar ratio; I, coded value; XG, conversion of glycerol.
results and to assess the experimental error. As response variables the glycerol conversion, XG, and the selectivity toward the different products, SMAG, SDAG, STAG, and Sothers, were selected taking into account the main objective of achieving high XG while minimizing SMAG and Sothers. As mentioned above, MAG derivate would not be suitable as a diesel component owing to its relatively high water solubility. From the above preliminary studies, temperature and time were established at 170 °C and 4 h. Thus, the factors chosen for the optimization were the methyl acetate to glycerol molar ratio, MR, and the catalyst loading, C. The lower and upper levels for molar ratio were 10:1 and 50:1, and the levels for catalyst loading were 2.5 and 7.5 wt % based on glycerol mass. In this way, the standard experimental matrix for the design is shown in Table 3. Columns 4 and 5 represent the 0 and (1 encoded factor levels on a dimensionless scale, whereas columns 2 and 3 represent the factor levels on a natural scale. Experiments were randomly run to minimize errors due to possible systematic trends in the variables. The table also includes the experimental results of the response variables, that is, glycerol conversion and selectivity toward the different products. From the matrix generated by the experimental data, and assuming a second-order polynomial model, eqs 112 were obtained by multiple regression analysis (Table 4). The statistical model is obtained from encoded levels giving the real influence of each variable on the process and the technological model is obtained from the real values of the variables. Consequently, the influence of variables on the responses is discussed using the statistical models shown in eqs 16 (Table 4). The arithmetical averages and standard deviations of the response were calculated from the central-point replicas: glycerol conversion (95.8 ( 1.8%); selectivity to MAG (38.2 ( 2.8%); selectivity to DAG (56.8 ( 2.9%); selectivity to TAG (2.6 ( 1.2%); selectivity to “others” (2.4 ( 1.8%). All the standard deviations were below 3% so that the experimental error can be regarded as not excessively significant.
Table 4. Predictive Equations Obtained by Design of Experimentsa
a C, catalyst loading; MR, methyl acetate/glycerol molar ratio; I, coded value; XG, conversion of glycerol; S, selectivity to the different products (MAG, DAG, TAG, and others). The models include only the significant terms.
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Figure 3. Response surfaces for glycerol conversion (A) and selectivity to MAG (B), DAGþTAG (C), and other nonidentified products (D) over Ar-SBA-15 predicted by the models.
Statistical analysis of the studied experimental range identifies the catalyst loading as the most important factor in the glycerol conversion response, the second factor in importance being the quadratic effect thereof. As expected, this factor has a positive effect on the glycerol conversion, since an increase in the catalyst loading produces an increase in the conversion of glycerol. Nevertheless, the quadratic effect of this factor has a significant negative influence on the glycerol conversion. This indicates that the increase in the catalyst loading does not produce a constant rise in the glycerol conversion, which can be attributed to a significant curvature effect at the higher levels of catalyst loading. In addition, the molar ratio-catalyst loading interaction has a significant positive influence on the glycerol conversion. As shown in Figure 3A, the influence of methyl acetate/glycerol molar ratio in the conversion of glycerol is strongly dependent on catalyst concentration. At low catalyst loading, an increase of the molar ratio yields a decrease in XG. In contrast, as the catalyst concentration reaches higher values, the effect of the molar ratio on the conversion of glycerol becomes practically negligible. Thus, from the point of view of glycerol conversion, it is necessary to work at the highest catalyst loading (7.5 wt %) and at methyl acetate/glycerol molar ratios over 30. At these conditions, the glycerol conversion predicted by the nonlinear model (eqs 1 or 7) ranges from 96.3% to 99.5%. Since the higher esters (DAG and TAG) are the most suitable products for use as diesel or biodiesel components, the optimum conditions in terms of selectivity are those which result in the
Table 5. Composition of the Different Grades of Glycerol Evaluateda ash (wt %) glycerol
purity
water
grade
(wt %)
(wt %)
NaCl
othersb
MONGc wt %)
pharmaceutical
99.9
0.1
