Glycerol Acetylation with Propionic Acid Using Iron and Cobalt Oxides in Al-MCM-41 Catalysts

In this work, Al-MCM-41 molecular sieves were synthesized, containing iron and/or cobalt oxides, impregnated by incipient wetness method, characterized and applied as catalysts in the acetylation reaction of glycerol with propionic acid to produce green glyceryl propionate molecules of high commercial value. According to this, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Fourier Transform Infra Red (FT-IR), adsorption/desorption N2 isotherms, textural analysis, and Scanning Electron Microscope (SEM) analysis were recorded to evaluate the main characteristics of materials. The presence of Lewis and Brønsted acidic sites and catalysts surface area were observed as important key points to functionalize acetylation reaction. Thus, time reaction, temperature, and glycerol / propionic acid ratio varied to improve the most suitable reaction conditions and behaviors. As a result, glycerol conversion was above 96%, followed by 68% of selectivity to glyceryl monopropionate as well as the formation of glyceryl diand tripropionate and a small amount of ethylene glycol dipropionate as an undesired product. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Over the last decade, biodiesel has been suggested as a reliable renewable alternative for diesel fossil fuel; mainly due to reduced toxicity, exhaust emissions and enhancing positive carbon cycle for reduction of greenhouse gases. great application in cosmetics, personal hygiene, food, medicine, and tobacco sectors [7]. In terms of chemical transformation, it still presents limited applications, the main ones on explosives production, like nitroglycerin, and in the formation of alkyd resins [8]. There is a strong motivation to produce fuels from oxygenated molecules generated from biomass conversion [9,10].
Etherification reaction with alcohols, ketones, aldehydes or carboxylic acids are an important way to change glycerol molecule and promote the formation of chemical compounds that present better commercial and economic value, such as renewable fuels additives, surfactants, flavorings and solvents for use in medicine as well as anti-bacterial agents [11][12][13][14][15].
Functionalizing of glycerol with carboxylic acid, acetic acid or propionic acid, for example, produce glycerol esters that have been identified as valuable replacement of fuel additives that are related to depleted sources, price uncertainty and environmental concern of petroleum feedstock [9]. Mono-, di-and tri-acetyl glycerides (MAG, DAG, and TAG) are the main products from the acetylation/propylation of glycerol with acetic or propionic acids and application to fuel additives to improve cold flow, reduce viscosity in biodiesel use and antiknock additives of gasoline [9,13]. Moreover, these glycerol derived products are important building blocks of greener polyester for biorefinery industry [16].
Several studies have been published related to glycerol acetylation with acetic acid using heterogeneous catalysts that enrich both Brønsted and Lewis acidic sites. Acid zeolites, acid clays, modified molecular sieves, heteropoly acids, metal oxides, ion exchange resins, among others, with modifications in their chemical structures, are being responsible for increasing the number of acidic sites that were employed in the production of acetyl glycerides [17][18][19][20][21][22][23][24][25][26]. Heterogeneous acidic catalysts showed gains in glycerol functionalization towards derived products. Possibilities of better separation stages, reuse of catalysts or regeneration presents advanced to macroscale process in industry [11,27].
According to Trifoi et al. [28], industrial catalysts must have special requirements to the catalyst's projects, for example, it's not recommended to be very sensitive to poisoning and impurities, it's important to allow a huge capacity of production, get cheap structures and metals, have thermal and mechanical resistance and be recyclable. MCM-41 molecular sieves present all those characteristics, related to applications in industry, and not only added the possibility to incorporate trivalent atoms (Al, Fe, Ga) but also other transition metals (Sn, Zn, Ir) in the porous / walls of the MCM-41 that promotes Lewis and Brønsted acidic sites environment functionalizing reaction mechanisms and improving selectivity and yields [8,[29][30][31][32][33][34].
Some papers have highlighted the application of iron and cobalt oxides supported on zeolites or molecular sieves in the acetalization or acetylation of glycerol [21,[35][36]. These metals were able to modify and promote glycerol functionalization in esters of great commercial value.
The large number of published works related the use of acetic acid in glycerol acetylation contrasts with small number of studies using propionic acid in reactions of glycerol propylation. This fact opens more horizons for scientific research. The oldest works found dated from the 1930s with homogeneous catalysis in obtaining mono-, di-and tri-propyl glycerides [37][38]. Cho et al. [39] carried out reactions for glycerol transformation with different dicarboxylic acids to obtain esters derived from glycerol. Rathod and collaborators [40] studied the esterification of propionic acid with isopropyl alcohol, observing feasibility in the production of its derived esters. Recently, Saengarum et al. [41] published scientific work which studied the glycerol propylation with propylene and 1butene hydrocarbons to obtain mono-, di-and tripropyl glycerol ethers to use as fuel additives. Commercial acid resins were employed as catalysts, corroborating that acid catalysis favored an increase in reaction yield.
Acidic routes favored acetylation glycerol reactions mostly by the use of acetic acid but rare published information presented glycerol acetylation with propionic acid, which characteristics could improve new green molecules. Iron and cobalt oxides present themselves as materials with a great acid capacity according to Pearson's theory of hard and soft acids, and this acid capacity of these metals has been little explored in the glycerol transformation.
In this context, our studies present novelty in perform acetylation reactions of glycerol with propionic acid, an important underexplored molecule to form mono-, di-, and tripropyl glycerides (MPG, DPG, and TPG). Our proposal is to functionalize this reaction with catalysts containing acidic characteristics promoted by Al-MCM-41 containing iron and cobalt oxides applied to improve glycerol chemistry transformations.
Briefly, sodium metasilicate (38 g) and aluminum sulfate (11.25 g and 2.25 g) were dissolved in a beaker with distilled water in magnetic stirrer for 1 h. Aluminum solution was slowly mixed with silicon solution and stirred by 30 minutes to homogenize. Sulfuric acid solution was added to mother liquor solution drop by drop until reach pH 10.5 resulting in a gel texture appearance in the solution. After pH adjustment, CTAB solution was prepared and added slowly to the gel mixture with mechanic stirrer keeping it in movement by an hour. The gel material formed was transferred to Tefloncoated autoclaves vessels to age the solution and kept in an air-drying lab oven at 145 °C for 40 hours. Then the solid product obtained was filtered, washed several times with distilled water and dried at 95 °C in an air-drying oven for 2 h. Then the sample was calcined in a muffle furnace at 550 °C for 15 h (0.7 °C.min -1 ramp rate) to remove the template.
To maximize Brønsted acidic sites in Al-MCM-41 an ion exchange procedure was done mixing the obtained white powder with a solu-tion of ammonium chlorate (1 M), twice times. This mixture was kept in magnetic stirring for 24 hours in room temperature. Solution was filtered, dried and calcined again at 550 °C for 4 h in air atmosphere.
Iron and/or cobalt metals were added to Al-MCM-41 materials by incipient wetness impregnation method described by Decyk et al. [43]. After determining the apparent volume of the Al-MCM-41 in a graduate beaker, a volume 10 times greater were used to prepare the solution. Iron(III) nitrate hexahydrate and/or cobalt(II) nitrate nonahydrate were dissolved in distillate water to provide 10 wt% total metal in catalysts. The solution was kept in magnetic stirrer for 1 hour and then the material temperature was raised to 70 °C for a period of 6 hours, until the material dried. After material was dried at 95 °C in an air-drying oven for 3 h and calcined in a muffle furnace at 550 °C for 4 h. Six different catalysts were able to test which codes were named Fe5, Fe25, Co5, Co25, FeCo5, and FeCo25.

Products Analysis
Analysis of the reaction products identified the Shimadzu QP-2010 Ultra gas chromatography with mass detector (GC-MS), using Rtx-Wax (Restek) column with 30 m  0.25 mm  0.25 m. The temperature of the injector chamber was 250 °C, varying the oven temperature from 40 °C to 200 °C with a rate of 10 °C.min -1 with hold for 9 minutes in final temperature. Split dilution was used at 1:60. External standard for GC analysis was used for the quantification of glycerol and benzaldehyde. The monoethylene glycol was used as internal standard diluted on methanol in the concentration of 100 mg.mL -1 , to products quantification.
Glycerol conversion calculations, as well as the selectivity for desired products and the final reaction yield are defined by Equations 1 to 3 [44]. (1)

Reactional Experiments
The experiment was conducted through etherification of glycerol with propionic acid in 50 ml borosilicate glass batch sealed reactors, without use of organic solvents. Moreover, to control the reaction solution, externalcontrolled hotplates with external thermostat and a magnetic vigorous stirring system were used to avoid or diminish mass and heat transfers resistances. Besides that, the reactor was not attached to the reflux condenser in order to remove excess water from the reaction solution.
In a typical reaction procedure, 25 mmol of glycerol (2.3 g) was added with 125 mmol of propionic acid (9.26 g) (related to 1/5 glycerol / acid ratio). The use of 100 mg of catalyst per test was fixed and the reaction time varied from 2 to 4 hours. Besides that, five temperatures ranging from 90 °C until 140 °C, and the ratio of glycerol to propionic acid were observed always with acid excess in reaction medium. At the end of the reactions, an amount of material was filtered on qualitative filter papers (grade 4) with cotton, collected in 2 mL vials and stored in a freezer. All experiments were per-formed in duplicate form. Reaction conditions exploited was chosen according to literature researches [12,45].

Instrumentation
The X-ray diffraction (XRD) patterns of molecular sieves synthesized catalysts were obtained by using a Bruker powder diffractometer D2 PHASER model using Cunickel filter, 10 mA and 30 kV in the high voltage source, and scanning angle between 10° and 70° of 2θ with 0.02° steps per second. Adsorption/desorption experiments using N2 were carried out at 77 K on a Quantachrome NOVA 1200 instrument. Before each measurement the samples were first outgassed at 423 K for 12 h at 5×10 -3 Torr and then at room temperature for 2 h at 1×10 -5 Torr. The N2 isotherms were used to determine the specific surface areas, pore volume, and pore diameter using the BET equation and BJH theory. Infrared spectra were collected on a FTIR Bruker model Vertex 70 spectrophotometer, with KBr as dispersant and the spectra was recorded from 400 cm -1 to 4000 cm -1 with a resolution of 4 cm -1 and total of 10 scans. X-ray Fluorescence spectroscopy (XRF) analysis was recorded by Rigaku Supermini 200 using Pd X-ray tube with 50 kV voltage and 4mA. Catalysts were also characterized using scanning electron microscopy (SEM) with a Shimadzu scanning electron microscope model SSX-550 Superscan with an accelerating voltage of 30 kV and different magnifications.  terns exhibit typical MCM-41 signs with strong peak in the 2θ range of 2-3-2.6° due to (100) reflection lines but as larger amounts of aluminum are present in the structure this (100) reflection present peaks going broader and less intense. High Al content in MCM-41 contributes to diminish orderly the structural pore channels and the main intense peak shifts towards higher interplanar spacing and this behavior represents an increase in the pore size of the MCM-41 material [46][47]59]. This modified interplanar spacing can be explained about of replacement of shorter Si-O bonds (1.60 Å) by longer A1-O bonds (1.75 Å) in the MCM-41 structure [59]. Other Miller´s indices (110) and (200) also presents same behavior and the welldefined peaks disappears to form a broad peak in Si/Al in 1/25 and totally disappears in 1/5. The d100 spacing, lattice parameter (a0), and Scherrer calculated values are presented in Table 1.

Results and Discussion
In Figure 2, a typical iron(III) oxide (Fe2O3) and cobalt(II,III) oxides (CoO.Co2O3 → Co3O4) diffractogram peak patterns from Fe/Co-Al-MCM-41 samples are observed by the Miller indices. Spinel phase of cubic Co3O4 crystalline cell patterns are presented in the all cobalt synthesized catalysts and hematite hexagonal Fe2O3 crystalline cell patterns are showed in all iron synthesized catalysts [48][49]. No other phases/shift in peak positions were observed by XRD in this study which suggests that no extensive solid solution was formed between the two oxides as a result of this preparation catalyst impregnation method.
Iron and cobalt oxides presented different crystallite dimensions calculated by Scherrer equation [50][51], while Co3O4 crystals showed increased dimensions if compared with Fe2O3 crystals (Table 1). This behavior is repeated even when the concentration of iron and cobalt elements are reduced by half in the catalysts FeCo5 and FeCo25. Unfortunately, no iron signal can be seen on FeCo25 sample patterns. Figure 3 reports the FT-IR spectra for all Fe and Co synthesized catalysts in 1800-400 cm -1 range. In all sample's curves, the band in 1640 cm -1 correspond to the O-H vibrations of silanol groups [52], while strong bands around 1230 and 1060 cm -1 correspond to ≡Si-O-Si≡ and Si-O-Si asymmetric stretching, typical for MCM-41. Curves A, C and E, which correspond to Fe5, FeCo5 and Co5 catalyst, have a structure with higher Al content, then this behavior  shifts in approximately 30 cm -1 (1060-1030 cm -1 ) the siloxane bands, this shifting band occurs due to SiO4 and AlO4 tetrahedral simultaneous vibrations [53]. The band around 970 cm -1 correspond to a vibration mode of SiO4 perturbed by the presence of neighboring Fe2O3/Co3O4 or Fe=O (OH) / Co=O (OH) group. The presence of this band is attributed to the formation of Si-O-Si linkages in metallosilicates [54]. Symmetric vibrations Si-O-Si signs appear near 800 cm -1 . The band at 450 cm -1 may be assigned to Si-O tetrahedral vibration [55]. The Fe-O-Fe stretching vibration, characteristic of Fe2O3 is observed at approximately 560 cm -1 and Co-O-Co stretching vibration, characteristic of Co3O4 is observed approximately 573 cm -1 and 670 cm -1 [56][57][58].
The results of the chemical analysis of the catalysts are given in the Table 2 and adsorption/desorption isotherms curves are presented in Figure 4. Classic MCM-41 molecular sieves isotherms type IV which represents mesoporosity materials are observed when Al/Si ratio are 1/25 which curves shows a smaller hysteresis behavior due to capillary condensation of N2 in the primary mesopores. Increasing the presence of Al in MCM-41 to provide Al/Si in 1/5 parts disorder the structure modifying interplanar spacing, lattice crystal parameters and had consequences for diminish strongly the surface area and pore volume. The enlargement of the unit cell observed by lattice crystal parameters by the presence of high Al content followed by addition of iron and/or cobalt oxides in the pores structure aimed to these behaviors in MCM-41 [59][60][61] a The t-Plot method were used to obtain micropores e meso-external surface area data. b BJH method were used to determine mean pore diameter and total pore volume. c Metal Fe and Co content were obtained by XRF analysis.    shows scanning electronic microscopy images (SEM) for all synthesized catalysts. All catalysts sample presented spherical small crystallites alternating between welldefined structures to greater particle nonuniform clusters. This morphology characteristics is typical for Al-MCM-41 and the more aluminum is inserted into the MCM-41 matrix structure itself, the greater the tendency to observe a more dispersed material containing a less defined morphology [52,[62][63][64].

Glycerol Etherification Reactions
Fe/Co-Al-MCM-41 catalysts were tested in the etherification reactions of glycerol with propionic acid. We considered the formation of Glyceryl 1-Monopropionate (MPG), Glyceryl 1,3-Dipropionate (DPG), and Glyceryl Tripropionate (TPG) as desired major products, besides that, the formation of Ethylene Glycol Dipropi-onate as undesired product. Diglycerol also could be formed but no GC-MS signal detected the presence on chromatograms (Figure 6).
In the first stage of experiments, all six catalysts were applied in reactions where the temperature was set at 110 °C, the glycerol/acid ratio was 1/5 (propionic acid excess) and the catalyst mass was 100 mg per test, all in duplicate. Catalysts were evaluated for 2 h and 4 h. Figure 7 and Figure 8 shows the reaction yield in relation to glyceryl 1-monopropionate (MPG) product, in addition to the selectivity for all the products observed.
Catalysts that have both iron oxides and cobalt oxides were shown to be more active compared to catalysts that contain only one of the metal oxides. Catalysts with cobalt oxides have higher conversion values compared to catalysts with iron oxides. Catalysts with a high aluminum content (Al/Si = 1/5) showed lower conversion values compared to catalysts with a lower concentration of this metal in the matrix of MCM-41 (Al/Si = 1/25). The available surface area for catalysis is an important factor for a better reaction performance and this characteristic proved to be more relevant, even with the presence of a higher concentration of aluminum atoms in the framework or extraframework. The insertion of Al can offer more Lewis acid sites to the catalysts surface and alters the structure of silicates to offer Brønsted sites according to the Löwenstein rule [65][66]. MPG product proved to be the main molecule obtained with selectivity between 78 to 90% and, as the conversion values increase, also does the formation of DPG and TPG, but on a still low scale, up to 15% selectivity for DPG and only 5% for TPG. The by-product Ethylene Glycol Dipropionate was also observed in up to 5% selectivity. Despite the lower conversion value, Fe5 catalyst was the only material that favored the formation of TPG, under the conditions presented. It is possible that the more acidic characteristics of the catalyst together with the hard acid dynamics of iron oxide may favor the achievement of TPG. After 4 hours of reaction, yields above 60% were obtained in acetylation of glycerol with propionic acid, at a temperature of only 110 °C, which shows that the use of the molecular sieve Al-MCM-41 containing iron and cobalt is important to functionalize glycerol towards values green products. It is observed that there is a junction between the catalysts acidic characteristics associated with the surface area availability for chemical contact of the catalysis sites with the reagents ( Table 2). The synthesis of Al-MCM-41 and its modification to maximize acidic sites of both Lewis and Brønsted sites are shown to be present in all catalysts. However, the insertion of iron and cobalt metals is the key point for the increase conversion and yields ( Figure  7). The concept of acidity provided by transition metals is best explained by the Theory of Hard and Soft Acids by Ralph Pearson [67]. Pearson defined species more polarizable as soft (acid or base) and less polarizable as hard (acid or base). The hardness or softness value can be determined using the values of ionization potential and electronic affinity of these species [68]. XRD analysis showed the presence of Fe 3+ and Co 3+ /Co 2+ in the form of oxides of different nanometric sizes (36-108 Å, Table 1) and the trivalent metals are classified as hard acids (higher acidity index) and the divalent metal as a characteristic intermediate acid. In this sense, the presence of these metals is strongly beneficial in the glycerol acetylation with propionic acid.

Temperature Reaction Dependence
In this second stage, the reaction temperature varied, while the other parameters were kept constant, to observe what would be the reaction behavior with increasing temperature. Moreover, only the catalysts that showed the highest reaction yield in the first test stage were used: Fe25, Co25, and FeCo25. It was observed that the reaction depends strongly of the increase of the temperature from 100 °C (Figure 9), and glycerol conversion tends to stabilize close to 95% from 130 °C. In this way, a temperature of 140 °C did not prove to be advantageous for significantly increasing of glycerol conversion values. Catalysts containing cobalt showed to be more active than iron, reaching higher conversion values as the temperature increased.
The maximum MPG yield is reached at 130 °C (approximately 68%) and has a slight decline at 140 °C ( Figure 10). Again, a greater formation of TPG (13.1%) occurs with the use of the catalyst containing iron (Fe25), a behavior observed in the first stage of experiments that can be explained by Pearson's theory, previously analyzed.
Zhang et al. [36] investigated the use of M-AlPO4 zeolite containing some transition metals, including cobalt, in the acetalization reaction of glycerol with acetone. Even having a behavior of a smaller surface area and lower acidic sites characteristics compared to the best catalyst used, the cobalt containing catalyst proved to be a material capable of offering the second-best reaction yield, under the investigated conditions (yield 67%). This result shows to be important for the use of cobalt as an active site in glycerol acetalization. A similar result was observed for the same reaction using cobalt catalysts in layered double hydroxide (LDH), where a yield of around 70% of reaction was achieved [35].
Gonzales-Arellano et al. [21] performed glycerol acetalization with levulinic acid using Zr-SBA-16 containing different Zr levels. At 100 °C and 15 hours of reaction, his best results reached around 70% conversion, 77% selectivity for monoglycerides and 23% for diglycerides. A behavior similar to the data presented in this work. It is worth noting that the appearance of triglycerides occurred at a higher temperature after the conversion approached 100%, which demonstrates a behavior of occurring consecutive reactions as the glycerol is depleted as a limiting reagent in the process.
Saengarum et al. [41] compared Amberlyst-15 resin with commercial sulfated B100 and B200 resins by applying them in the etherification of glycerol with propylene, a molecule of size close to propionic acid. At 100 °C and 8 hours of reaction, only Amberlyst-15 resin was able to convert glycerol into mono-, di-, and triglycerol ethers with 60% conversion. As the reaction time was extended to 16, 24, and 72 hours, the sulfated resins also showed the same products reaching up to 72% conversion. The acid capacity of Amberlyst-15 favored the results obtained, but the deterioration of its molecular structure associated with the formation of coke in its pores significantly reduced its use in sequential reactions.
It is observed with the investigated works and with the experiments carried out (Figure 9 and 10) that as the reaction exhausts glycerol, DPG and TPG tends to appear in greater quantities as the reaction time and increasing temperature. The preference for the formation of the MPG is given by the easier supply of glycerol in the initial stages of reaction. When glycerol is depleting from reaction medium, the excess of propionic acid continues to react with MPG to form DPG and, likewise, to produce TPG. Acidic surface capacity associated with the availability of catalytic sites of the materials, are catalyst highlighted keys in the formation derived products from glycerol and propionic acid.

Reagents Ratio Dependence
Glycerol acetalization reactions with propionic acid were also carried out by changing the concentrations of reagents, keeping the temperature constant at 110 °C, as well as other Figure 11. Glycerol conversion (%) and products yields (%) in different glycerol / propionic acid ratios and catalysts. constant parameters. Reactions containing the excess acidic conditions in the Glycerol / Acid ratio in 1/3, 1/5 and 1/7 were analyzed.
It is noted that the increase in propionic acid concentration shifted the reaction to the products (Figure 11), increasing the glycerol conversion for all catalysts tested and favoring the consecutive reactions to form DPG and TPG. It can also be seen that the Co25 catalyst favored the formation of MPG more, reaching a yield of up to 66% with a corresponding selectivity of 81.8% (Figure 12). Fe25 catalyst was more inclined to form TPG, but was less effective in converting glycerol, which reduced its final yield values. The FeCo25 catalyst proved to be more versatile in forming DPG, but its high conversion was responsible for having the best reaction yield values. As the general characteristics of all catalysts are very close, the differences in selectivity and final reaction yield are also similar. However, all materials tested were capable of transforming glycerol and forming MPG, DPG, and TPG, important green platform molecules capable to improve renewable energy technologies and new products.

Conclusions
Synthesis and characterization of Al-MCM-41 molecular sieves, impregnated with iron and/or cobalt, was carried out and applied as catalysts in the glycerol acetylation reaction with propionic acid to the product glyceryl mono-, di-, and tri-propionates. XRD analysis identified Fe2O3 and Co3O4 oxides with nanometric dimensions and FT-IR showed shift bands by the high presence of aluminum in silicate Al-MCM-41 characteristics and the presence of all structures indicate Lewis and Brønsted acidic sites in the synthesized catalysts. XRF adsorption/desorption N2 isotherms and textural analysis presented MCM-41 molecular sieves behavior with micropores and mesopores differences according with Al content in the structure and also Fe/Co mass percentage. SEM images showed the presence of spherical small crystallites alternating between well-defined structures until nonuniform clusters. This characteristic view is caused by the high presence of aluminum which increase less defined morphology in MCM-41.
Glycerol acetylation reactions with propionic acid formed mostly glyceryl propionates (mono, di and tri). High yields and selectivity values were obtained in the reactions reaching 79% yield and 90% to MPG selectivity in 110 °C condition. Increasing temperature to 130 °C and the rise of propionic acid concentration were favorable to increase the glycerol conversion up to 95% and maximize selectivity to MPG up to 68%. As the reaction time and temperature advances, the formation of DPG and TPG is observed and, consequently, the decrease of MPG as formed products. In general, catalysts containing iron oxide and a high concentration of aluminum in the chemical structure obtained lower glycerol conversion values, but showed a slightly higher selectivity for DPG and TPG, when compared to catalysts containing cobalt and with lower aluminum content in the structure. The highest reaction yields were obtained with the use of catalysts containing cobalt and iron-cobalt, however more MPG was formed. The acidic materials characteristics associated with the surface area of the pores proved to be determinant for increasing productivity in the formation of glyceryl propionates.