Jakkrapong Jitjamnonga,
Parinya Khongpromab,
Thanate Ratanawilaic and
Sukritthira Ratanawilai
*a
aDepartment of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand. E-mail: sukritthira.r@psu.ac.th
bAir Pollution and Health Effect Research Center, Prince of Songkla University, Songkhla, 90110, Thailand
cDepartment of Industrial and Manufacturing Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand
First published on 19th February 2024
Waste crude glycerol was successfully enriched and utilized as an inexpensive source for producing value-added chemicals, such as glycerol carbonate (GC) – a valuable compound with extensive industrial applications. The Li/MCM-41 heterogeneous catalyst was synthesized and used for the transesterification of enriched glycerol and dimethyl carbonate (DMC) to produce GC. The catalyst's physicochemical properties were characterized using thermogravimetric, Hammett indicator, inductively coupled plasma-optical emission spectroscopy, nitrogen adsorption–desorption, X-ray diffractometry, scanning electron microscopy, and Fourier-transform infrared spectroscopy analyses. Reaction conditions were optimized using response surface methodology and analysis of variance, yielding an accurate quadratic model to predict the GC yield under different transesterification variables. The results revealed that 5%Li/MCM-41 served as the optimal catalyst, achieving the highest TOF of 4.72 h−1. The DMC: enriched glycerol molar ratio had the greatest impact on the GC yield, with an R2 = 0.9743 and adjusted R2 = 0.9502. The optimal GC yield (58.77%) with a final purity of 78% was attained at a 5.15 wt% catalyst loading relative to the initial amount of enriched glycerol, DMC: enriched glycerol molar ratio of 4.24:
1, and a reaction temperature of 86 °C for 165 min. The 5%Li/MCM-41 heterogeneous catalyst could be reused for four cycles with a decreased GC yield from 58.77% to 45.72%. Thus, the Li/MCM-41 catalyst demonstrated a remarkable efficiency and potential as a heterogeneous catalyst for synthesizing GC. This method not only contributes to environmental sustainability by making use of a byproduct from biodiesel production but also aligns with the principles of a circular economy.
Crude glycerol, a prominent by-product of biodiesel production from alcohol transesterification with vegetable oils, animal fats, algae, and recycled restaurant grease, constitutes approximately 10 wt% of total biodiesel production and is commonly discarded as industrial waste.6–8 By 2025, the cumulative volume of crude glycerol is estimated to surpass an impressive 2.4 billion liters, presenting an enormous reservoir brimming with potential for refinement and conversion into value-added products. Consequently, the development of sustainable techniques for harnessing this cost-effective organic raw material becomes imperative.5 From an economic standpoint, this glut of crude glycerol has reduced its value and so it becomes crucial to efficiently utilize crude glycerol via its conversion to other value-added chemical products, thereby curtailing the level of waste generated during the biodiesel manufacturing process and fostering the economically and environmentally sustainable advancement of bio-refineries.
Crude glycerol derived from the biodiesel production process is typically comprised of approximately 50% or less glycerol, accompanied by a substantial quantity of methanol, water, fatty acid salts (soap), fatty acid methyl esters (FAMEs), free fatty acids (FFAs), and ash content.5 The presence of ash is attributed to catalyst residues and impurities originating from the oil during chemical reactions. The specific impurities found in waste crude glycerol are contingent upon the feedstock (natural oil sources and alcohol) utilized during the biodiesel production. Crude glycerol can be refined and distilled to yield pure glycerol. The general enrichment process for crude glycerol is comprised of three primary steps. Firstly, the removal of metals and soap occurs through a precipitation process during acidification, leading to the formation of metal salts and conversion of soap into FFAs. In cases where a base catalyst is utilized in the transesterification reactions, acid treatment is employed, whereas in reactions involving acid catalysts, alkaline treatment is applied. The excess alcohol in the crude glycerol mixture is then evaporated, refining the glycerol stream and enhancing the glycerol purity to around 95% (w/w).9,10 A glycerol purity of approximately 93.7% was obtained from crude glycerol through a process of acidification, solvent extraction, and purification from crude glycerol, but byproducts, such as water, glycerides, and ash, were also obtained.11 Similarly, a glycerol purity of roughly 93.34% was obtained using sulphuric acid (H2SO4) acidification, sodium hydroxide (NaOH) neutralization, solvent extraction, and enrichment, resulting in low levels of contaminants.12 The sequential saponification, acidification, neutralization, solvent extraction, and purification of crude glycerol led to glycerol with a 97.5% purity and minimal residual FFA, water, and ash contents.13
Glycerol serves as a prominent renewable raw material within the chemical industry, boasting a broad spectrum of applications as a raw material and an active solvent. Notably, glycerol carbonate (GC) has emerged as a substance of significant interest, particularly within the pharmaceutical and beauty sectors, where its value is greatly augmented by achieving high levels of purity.14,15 Furthermore, glycerol plays a pivotal role in various other domains. It finds extensive usage as an electrolyte in lithium (Li) and Li-ion batteries, as well as in applications like gas separation membranes, solvents, detergent compositions, chemical intermediates, polymers, and in constructing eco-composites.15–19
Synthesis of GC can be achieved through various chemical routes, including the transesterification reaction of glycerol with dimethyl carbonate (DMC),1,4,17,18 carbon monoxide (CO),20,21 carbon dioxide (CO2),22,23 phosgene, or urea.24–26 However, several challenges and limitations are associated with these routes. For CO, its toxicity, environmental pollution, catalyst poisoning, and explosive nature restrict its suitability as a reagent. The use of CO2 suffers from difficulties in breaking the carbon–oxygen double bond in CO2, resulting in low GC yields and relatively high production costs. The use of phosgene as a reagent poses hazards due to its high toxicity, corrosiveness, and pollution potential, making this route highly constrained. The reaction with urea necessitates expensive equipment due to the requirement for vacuum conditions for the continuous removal of the formed ammonia in order to shift the thermodynamic equilibrium. Consequently, this route yields low GC yields.27 A promising pathway involves the transesterification reaction of glycerol with DMC using either homogeneous or heterogeneous catalysts. This pathway is advantageous due to its mild reaction conditions and the ease of product separation. However, homogeneous catalysts pose challenges in terms of catalyst separation, purification, and reusability. Therefore, the utilization of heterogeneous catalysts shows more promise in driving more economical GC production, primarily due to their potential for reusability.
Catalysts tailored for the transesterification reaction are primarily comprised of inorganic bases with alkali or alkaline earth metals as the active species. The catalyst's surface contains basic sites responsible for initiating the transesterification process. Catalysts functionalized with metals, such as potassium (K),28,29 sodium (Na),30 Li,6,29 magnesium (Mg),28,29 strontium (Sr),28 and zirconium,22 exhibit an enhanced performance in GC production.
Mobil Composition of Matter no. 41 (MCM-41) has garnered considerable attention in the field of materials science. MCM-41 stands out for its uniform hexagonal straight channel structure, controlled mesoporous pore size (ranging from 1.5 to 20 nm), substantial pore volume (exceeding 0.6 cm3 g−1), high surface area (approximately 700–1500 m2 g−1), and exceptional hydrothermal, chemical, and mechanical stability. Consequently, MCM-41 finds wide-ranging applications, particularly in catalytic conversions involving large molecules. Nonetheless, siliceous MCM-41 lacks a sufficient number of basic sites. To address this, the catalytic potential of siliceous materials can be readily modified by introducing heteroatoms into the silicate framework, thereby enhancing their catalytic prowess within confined spaces.31 For instance, Li impregnation onto mesoporous MCM-41 enhanced its catalytic activity. They achieved a glycerol conversion level of 99% with a GC yield of 93% when maintaining a glycerol: DMC molar ratio of 3:
1, a catalyst loading of 13 wt%, and a temperature of 90 °C for 3 h.6 Cobalt oxide (Co3O4) supported on MCM-41 as a catalyst for the synthesis of GC via transesterification of glycerol and DMC gave a glycerol conversion level of 98.7% and GC yield of 94.1% under the optimized conditions of a DMC: glycerol molar ratio of 3
:
1, a catalyst dosage of 6 wt%, and a reaction time of 2 h at 90 °C.32 The efficacy of several active metals, such as Li, K, and barium (Ba), doped onto ash was shown for catalyzing GC synthesis via transesterification.29 In that study the active metals were doped at varying weight percentages using a straightforward wet impregnation method. Likewise, evaluation of the impact of K, Sr, and Mg on the catalytic stability of red mud in GC synthesis revealed that a 30 wt% K loading and calcination at 800 °C resulted in the most favorable catalytic performance due to the maximal surface concentration of active K2O.28
The synthesis process of glycerol carbonate is known for its high energy consumption and significant costs, necessitating optimization through the application of appropriate statistical tools. Specifically, the identification of optimal reaction parameters to maximize glycerol carbonate yield is crucial. Surprisingly, the utilization of response surface methodology (RSM) via central composite design (CCD), a statistical tool, has not been documented in existing research to the best of our knowledge. Hence, the implementation of RSM becomes imperative for optimizing the reaction factors and achieving a higher yield of glycerol carbonate. RSM techniques offer the advantage of identifying the true optimum point while accounting for interactions between factors, all while requiring fewer experimental runs. This approach has proven successful in numerous coagulation studies. Therefore, employing RSM in the optimization of glycerol carbonate synthesis can not only enhance efficiency but also provide valuable insights into the intricate relationships among reaction parameters, ultimately contributing to improved yields and cost-effectiveness.33–36
In the present work, crude glycerol was first enriched and then the enriched glycerol was used as a starting reactant for GC synthesis. The active species were incorporated onto the MCM-41 framework via impregnation. The research focused on investigating the impact of different types of active species (Li, Na, K, and Ba) loaded onto the MCM-41 support, with the goal of identifying the variant that produced the highest GC yield. Subsequently, the chosen catalyst was employed to optimize the transesterification reaction conditions via varying the catalyst loading (2–6 wt% relative to the initial amount of enriched glycerol), DMC: enriched glycerol molar ratio (1–5:
1), reaction temperature (70–90 °C), and reaction time (60–180 min) using RSM based on a CCD. The reusability of the catalyst was also evaluated.
![]() | ||
Fig. 1 (a) Three distinct layers of crude glycerol purification, (b) crude glycerol (c) enriched glycerol before activated carbon treatment, and (d) purified glycerol. |
For pH measurement, 1.00 ± 0.1 g of the sample was dissolved in 50 mL of deionized water and the pH value was assessed at ambient temperature using a digital pH meter (pH 150 Eutech model), which had been calibrated using buffer solutions of pH 4 and 7.37,38
The viscosity of the prepared samples was analyzed using a digital viscometer (LVDV-I Prime Model, Brookfield, USA) at 25 ± 0.5 °C.37,38
Ash content of the crude and enriched glycerol was measured by burning 1 g of sample in a muffle furnace at 750 °C for 3 h following ISO 2098-1972.37,38 The ash content was then calculated from eqn (1):
![]() | (1) |
The enriched glycerol content was determined using the titration method. Initially, the sample (0.5 g) was dissolved in distilled water to achieve a solution volume of 50 mL in a 500 mL Erlenmeyer flask. Approximately 5–6 droplets of bromothymol blue were added to the solution and the pH was adjusted to 2–4 by the addition of H2SO4 (1 M). Subsequently, the solution was neutralized (turns yellow) by the addition of NaOH (0.05 M). Distilled water was utilized as a blank sample for reference. Following this, 50 mL of sodium metaperiodate was introduced, and the resulting mixture was allowed to age in darkness for 30 min. Next, 10 mL of a 1:
1 (w/w) ratio mixture of ethylene glycol: distilled water was added and kept in darkness for 20 min,39,40 and then 300 mL of distilled water was added to this solution. Finally, NaOH (0.125 M) was slowly added drop by drop until the solution color changed from yellow to blue. The enriched glycerol content was then calculated using eqn (2):
![]() | (2) |
A series of MCM-41 supports doped with various metal types (Li, Na, K, and Ba) and metal loadings were prepared through incipient wetness impregnation. For instance, in the preparation of 5%Li/MCM-41, approximately 0.0805 g of the LiNO3 precursor was dissolved in 7.5 mL of distilled water and used to impregnate 1.5 g of dried MCM-41 support at ambient temperature for 24 h. The Li-impregnated MCM-41 sample was then dried at 105 °C overnight. Prior to the activity test in the transesterification reaction, the catalysts underwent calcination in a muffle furnace at 450 °C for 3 h at a heating rate of 5 °C min−1. A similar procedure was followed for the different metal types and metal loading levels (2–6 wt% relative to the initial amount of enriched glycerol), with the synthesized catalysts being designated as n%M/MCM-41, where n% represents the wt% of the loaded metal relative to the amount of metal and MCM-41 and M designates the type of metal.
The functional groups present in the samples were analyzed through Fourier-transform infrared spectroscopy (FTIR; Bruker Vertex70, Germany). The 1 mg of catalyst powder was mixed, pelleted with 99 mg of KBr powder, and compressed into a disk form. The FTIR spectrum was recorded across a wavenumber range of 4000–400 cm−1 with a resolution of 4 cm−1 in absorbance mode, and 32 scans were performed.
The morphology of the prepared catalyst was visualized using scanning electron microscopy (SEM; Apreo Model), with an accelerating voltage of 2.0 kV. Each sample was positioned on a stub, sputtered, and then placed in the SEM machine's sample holder. Images were acquired at a magnification of 50000×.
The thermal stability and decomposition properties of both the synthesized MCM-41 support and 5%Li/MCM-41 were evaluated using thermal gravimetric analysis (TGA; PerkinElmer, TGA8000 model, USA). Approximately 4–10 mg of the prepared sample was loaded into a platinum pan, and the analysis was conducted under a nitrogen (N2) atmosphere at a heating rate of 10 °C per minute, spanning a temperature range of 30 to 900 °C.
The surface area and pore size distribution of the prepared catalyst were characterized using N2 adsorption–desorption surface area analysis (ASAP2060 model, Micromeritics brand, USA). Prior to the test, any adsorbed water and volatiles on the sample surface were removed by heating the sample to 250 °C overnight under vacuum conditions. The Brunauer–Emmett–Teller (BET) method was used to determine the surface area (SBET) and the total pore volume (Vp), which was measured up to a relative pressure (P/P0) of 0.99.
The actual metal loading levels on the MCM-41 catalyst were characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer AVIO 500, USA). The prepared catalysts were digested in an aqua regia solution and subsequently diluted prior to characterization. The method was calibrated using known concentration standards, and the actual concentration values were then calculated from the standard curve.
The basic strength of the solid catalyst was determined using the Hammett indicator method using the following indicators: bromothymol blue (H_ = 7.2), phenolphthalein (H_ = 9.3), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4). Approximately 0.025 g of the catalyst was placed in a beaker and 5 mL of the selected indicator was added and thoroughly mixed for 2 h. The total basicity of the prepared catalyst was then determined via Hammett indicator titration. Initially, 0.1 g of the catalyst was mixed with 4 mL of methanol, and then 0.2 mL of phenolphthalein indicator was added. The titration was performed using benzoic acid (0.1 M) as a titrant, gradually added until the color transitioned from pink to colorless.
![]() | (3) |
Factor | Unit | Symbols | Coded factor levels | ||||
---|---|---|---|---|---|---|---|
−2 | −1 | 0 | +1 | +2 | |||
Catalyst loading | wt% relative to the initial amount of enriched glycerol | A | 2 | 3 | 4 | 5 | 6 |
DMC: enriched glycerol molar ratio | — | B | 1![]() ![]() |
2![]() ![]() |
3![]() ![]() |
4![]() ![]() |
5![]() ![]() |
Reaction temperature | °C | C | 70 | 75 | 80 | 85 | 90 |
Reaction time | min | D | 60 | 90 | 120 | 150 | 180 |
Upon completion of the reaction, the 5%Li/MCM-41 catalyst was recovered by filtration and washed with ethanol two to four times to eliminate adhered products like enriched glycerol and GC. Subsequently, the catalyst was dried in an oven at 105 °C overnight, preparing it for further utilization in subsequent experimental runs.
![]() | (4) |
![]() | (5) |
![]() | (6) |
![]() | (7) |
Component | Commercial glycerol | Crude glycerol | Enriched glycerol | ||||||
---|---|---|---|---|---|---|---|---|---|
Ref. 30 | Ref. 37 | Ref. 38 | Ref. 37 | This work | Ref. 30 | Ref. 37 | Ref. 38 | This work | |
a Measured by titration.b Measured by Karl Fischer.c Calculated by subtracting the total of the amount of glycerol, water, and ash. | |||||||||
Glycerol (wt%)a | 99.9 | 99.98 | 98 min | 35.60 | 39.63 | 96.0 | 98.20 | 98.67 | 98.44 ± 0.11 |
Water content (wt%)b | 0.01 | 0.01 | 2.0 max | 9.38 | 7.26 | 1.30 | 0.63 | 0.42 | 0.54 ± 0.01 |
Ash content (wt%) | 0.0 | 0.002 | — | 4.73 | 5.77 ± 0.21 | 1.04 | 0.39 | 0.08 | 0.02 ± 0.03 |
MONG (wt%)c | 0.0 | 0.001 | 1.0 max | 50.29 | 47.34 | 1.09 | 0.78 | 0.83 | 1.00 |
Density (g cm−3) | 1.27 | — | — | — | 1.07 ± 0.03 | 1.26 | — | 1.26 ± 0.01 | |
pH | 6.97 | 7 | — | 9.6 | 10.32 ± 0.01 | 6.98 | 7.08 | 7.1 | 6.95 ± 0.02 |
Viscosity (cp at 25 °C) | — | — | — | — | 355.65 ± 2.22 | — | — | 823.30 ± 0.58 | |
Color | Clear | Clear | Clear | Dark brown | Dark brown | Clear | Clear | Clear | Clear |
The combined process of acidification, neutralization, and phase separation increased the glycerol content 2.4-fold from the initial 39.63 wt% of crude glycerol to 98.44 wt% in the enriched glycerol, with an elimination of the FFA, FAME, and MONG from the crude glycerol. However, the formation of water and salt during the neutralization reaction slightly decreased the glycerol purity.11 According to Kongjao et al., acidification to a pH of 1 increases the glycerol content and removes most residues, but increasing the pH during the acidification stage led to a significantly higher glycerol yield and reduced amounts of inorganic salt and FFA.12 The glycerol obtained in this study before the activated carbon treatment appeared as a yellow liquid (Fig. 1c). Finally, activated carbon was utilized to remove the color, and also eliminated residual fatty acids and other impurities with molecular sizes smaller than the pore sizes of activated carbon,11 to yield the clear enriched glycerol.
The reported enriched glycerol yield in this study was the average of over 30 batches, with some variation in individual yields being noted. Overall, the enrichment process resulted in a 62.7% glycerol yield with a 98.44% glycerol purity, as compared to the previously reported values of 98.20 wt%41 and 96.0 wt%.38
The crude glycerol often contains a considerable amount of water, which lowers its viscosity. However, during enrichment the water is separated from the glycerol, leading to a more concentrated glycerol solution with an increased viscosity.
The pH of the crude glycerol was 10.32, which was higher than that of the enriched glycerol at 6.95, due to the presence of residues, such as the alkaline catalyst, alcohol, and FAMEs, which can elevate the pH.37 These impurities, primarily composed of MONG, can result from the transesterification process used in the biodiesel production. Previously, a pH of around 9.6 was reported for crude glycerol due to the significant amount of MONG and associated impurities present in the sample.41,42
The crude glycerol had a lower density (1.079 g cm−3) than the enriched glycerol (1.26 g cm−3), which was again due to the extraction of residual wastes during the enrichment steps, including methanol, fatty acids, catalyst residues, water, and other compounds originally derived from the oil used.43 The density of the enriched glycerol closely matched that of the commercially available pure glycerol, indicating the success of the enrichment process in producing a high-quality product.
![]() | ||
Fig. 2 X-ray diffraction profile of (a) MCM-41; (b) 2%Li/MCM-41; (c) 3%Li/MCM-41; (d) 4%Li/MCM-41; (e) 5%Li/MCM-41; and (f) 6%Li/MCM-41. |
The FTIR profiles of MCM-41 and the 5%Li/MCM-41 catalyst were analyzed to investigate the stretching vibrational modes of various functional groups (Fig. 3). Across all catalyst spectra, the broad vibrational peaks observed at 3446.0–3447.2 and 1632.7–1636.0 cm−1 can be attributed to the hydroxyl groups linked with the silanol groups (Si–OH), arising from physically adsorbed moisture molecules on the catalyst surface.47 The vibrational peaks at 2918.6–2919.7 and 2850.5–2851.1 cm−1 are associated with C–H vibrations of the encapsulated surfactant molecules.36 The weak peak observed at 1465.7–1468.1 cm−1 is indicative of –CH2 groups.48 Peaks observed at 1088.8–1091.4 and 802.7–805.7 cm−1 are attributed to asymmetric stretching and bending of Si–O–Si bonds within the MCM-41 framework.49 The band at 464.4–467.6 cm−1 can be attributed to the vibration of Si–O bonds, indicating the mesoporous silica nature.48 Moreover, after Li doping onto the MCM-41 framework, a little bump peak appeared at 1383.6 cm−1 appeared, which was attributed to the N–O group of the LiNO3 precursor due to the presence of the metal oxide within the catalyst.6
The surface morphological characteristics of prepared catalyst at a magnification of 50000× are illustrated in Fig. 4. The morphological images of prepared catalyst shown the spherical particles.
![]() | ||
Fig. 4 SEM images at 50![]() |
The thermal behavior of MCM-41 and 5%Li/MCM-41 was analyzed using TGA (Fig. 5). The TGA profile of the MCM-41 support showed a two-stage thermal decomposition with the first stage up to 193.67 °C accounting for an approximately 2.55% mass loss, which was due to the desorption of adsorbed moisture from the pores of the MCM-41 framework. The second stage, above 193.67 °C, accounted for 1.59% mass loss and was due to the decomposition of organic impurities within the MCM-41 pores. The TGA profile of the 5%Li/MCM-41 catalyst also exhibited a significant stage at above 400–530 °C resulted in an approximately 1.35% mass loss, and might be attributed to the decomposition of organic impurities with the pores of this sample together with the salt decomposition.
The SBET, average pore diameter (dp), and Vp play essential roles in the activity of MCM-41 and 5%Li/MCM-41 catalysts. The N2 adsorption–desorption isotherms were characterized using liquid N2 at −195.85 °C (Fig. 6). Following the IUPAC nomenclature standards, the isotherms of the prepared catalysts were classified as type IV isotherms with H1 hysteresis loops6 (Fig. 6a and b). Both MCM-41 and 5%Li/MCM-41 isotherms exhibited very narrow loops confined within a relative pressure range (P/P0) of 0.5–1.0. The MCM-41 catalyst had a SBET of 642.05 m2 g−1, a Vp of 0.685 cm3 g−1, and an average dp of 4.27 nm, which is indicative of a well-defined mesoporous structure. It was previously shown that mesoporous catalysts exhibited high catalytic properties in the transesterification of enriched glycerol and DMC into GC, which was attributed to their ability to accommodate active metal species that enhance this catalytic activity.50
![]() | ||
Fig. 6 Nitrogen adsorption–desorption isotherm of (a) MCM-41; (b) 5%Li/MCM-41 and pore size distribution of (c) MCM-41; (d) 5%Li/MCM-41 and pore size. |
After incorporation of Li onto the MCM-41 framework, the SBET and Vp were slightly decreased to 504.12 m2 g−1 and 0.563 cm3 g−1, respectively, which is likely to be due to the incorporation of Li active metal onto the MCM-41 framework, whilst the dispersion of Li species within the mesoporous structure facilitates the transesterification reaction. However, the incorporation of the Li increased the average dp of 5%Li/MCM-41 to 4.47 nm, likely to be due to formation of metal oxide bond on the wall of MCM-41 framework.6
Catalyst | Basicity (mmol g−1) | Basic strength (H_) | GC yield (%) |
---|---|---|---|
a Reaction conditions: catalyst loading of 5 wt%, DMC to enriched glycerol molar ratio of 4![]() ![]() |
|||
MCM-41 | 1.11 | H_ ≤ 7.2 | 2.96 |
2%Li/MCM-41 | 1.45 | 7.2 ≤ H_ ≤ 9.8 | 17.24 |
3%Li/MCM-41 | 2.03 | 7.2 ≤ H_ ≤ 9.8 | 22.25 |
4%Li/MCM-41 | 2.49 | 7.2 ≤ H_ ≤ 9.8 | 29.30 |
5%Li/MCM-41 | 4.70 | 9.8 ≤ H_ ≤ 15 | 58.77 |
6%Li/MCM-41 | 5.05 | 9.8 ≤ H_ ≤15 | 47.27 |
Incorporation of an active metal (Li, Na, K, or Ba) at a 5 wt% metal loading into the MCM-41 framework, improved the transesterification reaction by introducing essential basic sites for GC synthesis. The resulting GC yields for 5%Li/MCM-41, 5%Na/MCM-41, 5%K/MCM-41, and 5%Ba/MCM-41 were 57.18%, 25.11%, 42.33%, and 30.87%, respectively (Fig. 7). Notably, the 5%Li/MCM-41 catalyst exhibited the highest catalytic activity (in terms of GC yield), which can be attributed to the strong ion size effect of Li.6 Consequently, the Li/MCM-41 catalyst emerged as a promising candidate for this transesterification reaction.
To explore the effect of the Li metal loading level on the MCM-41 framework, Li loading levels from 2 wt% to 6 wt% were examined (Fig. 8). As the Li metal loading increased from 2 to 5 wt%, the GC yield and enriched glycerol conversion rates were increased from 17.24% to 57.18% and from 18.94% to 57.35%, respectively. These findings imply that the incorporation of Li species into the catalysts enhanced their basicity, as seen in Table 3. These improved basicity properties are likely key factors contributing to the catalysts' enhanced performance compared to the MCM-41 framework. However, at a Li loading level of 6 wt%, there was a slight decrease in the GC yield to 47.27%. The exploration of catalyst performance involved TOF calculation. Elevating the active metal loading from 2 wt% to 5 wt% resulted in a TOF increase from 1.52 h−1 to 4.72 h−1. However, a subsequent increase to 6 wt% led to a TOF decrease due to the formation of a multilayer dispersion of Li metal on the MCM-41 support. This would lead to pore blockage or agglomeration of Li metal within the MCM-41 framework.6 Based on these observations, it was inferred that an optimal the highest GC yield and the 5%Li/MCM-41 catalyst was selected for determining the optimal conditions for GC synthesis.
Standard run | Catalyst loading (wt%) | DMC: enriched glycerol molar ratio | Reaction temperature (°C) | Reaction time (min) | GC yield (%) | |||||
---|---|---|---|---|---|---|---|---|---|---|
Coded | Real | Coded | Real | Coded | Real | Coded | Real | Observed | Predicted | |
1 | −1 | 3 | +1 | 4![]() ![]() |
−1 | 75 | −1 | 90 | 36.77 | 36.71 |
2 | +1 | 5 | +1 | 4![]() ![]() |
−1 | 75 | −1 | 90 | 43.62 | 44.25 |
3 | +1 | 5 | −1 | 2![]() ![]() |
−1 | 75 | +1 | 150 | 44.15 | 43.08 |
4 | +1 | 5 | −1 | 2![]() ![]() |
−1 | 75 | −1 | 90 | 42.08 | 43.00 |
5 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | +2 | 180 | 53.75 | 54.28 |
6 | +1 | 5 | −1 | 2![]() ![]() |
+1 | 85 | −1 | 90 | 37.78 | 38.72 |
7 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 54.90 | 55.31 |
8 | +1 | 5 | +1 | 4![]() ![]() |
−1 | 75 | +1 | 150 | 46.82 | 46.92 |
9 | −1 | 3 | −1 | 2![]() ![]() |
−1 | 75 | −1 | 90 | 46.34 | 44.32 |
10 | −1 | 3 | −1 | 2![]() ![]() |
+1 | 85 | −1 | 90 | 41.09 | 40.53 |
11 | 0 | 4 | 0 | 3![]() ![]() |
+2 | 90 | 0 | 120 | 48.77 | 47.72 |
12 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | −2 | 60 | 48.16 | 47.65 |
13 | −1 | 3 | −1 | 2![]() ![]() |
−1 | 75 | +1 | 150 | 45.94 | 45.82 |
14 | −1 | 3 | +1 | 4![]() ![]() |
−1 | 75 | +1 | 150 | 41.31 | 40.81 |
15 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 55.64 | 55.31 |
16 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 55.75 | 55.31 |
17 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 56.46 | 55.31 |
18 | +1 | 5 | +1 | 4![]() ![]() |
+1 | 85 | +1 | 150 | 57.18 | 59.65 |
19 | 0 | 4 | 0 | 3![]() ![]() |
−2 | 70 | 0 | 120 | 37.72 | 38.78 |
20 | −1 | 3 | +1 | 4![]() ![]() |
+1 | 85 | +1 | 150 | 55.40 | 54.03 |
21 | +1 | 5 | +1 | 4![]() ![]() |
+1 | 85 | −1 | 90 | 54.86 | 54.52 |
22 | +2 | 6 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 51.33 | 49.70 |
23 | 0 | 4 | +2 | 5![]() ![]() |
0 | 80 | 0 | 120 | 46.25 | 45.02 |
24 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 55.61 | 55.31 |
25 | +1 | 5 | −1 | 2![]() ![]() |
+1 | 85 | +1 | 150 | 41.64 | 41.25 |
26 | −2 | 2 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 43.76 | 45.40 |
27 | −1 | 3 | +1 | 4![]() ![]() |
+1 | 85 | −1 | 90 | 45.96 | 47.48 |
28 | −1 | 3 | −1 | 2![]() ![]() |
+1 | 85 | +1 | 150 | 44.67 | 44.49 |
29 | 0 | 4 | 0 | 3![]() ![]() |
0 | 80 | 0 | 120 | 53.51 | 55.31 |
30 | 0 | 4 | −2 | 1![]() ![]() |
0 | 80 | 0 | 120 | 32.99 | 34.23 |
Source | Sum of squares | Degree of freedom | Mean square | F-value | p-value | Remark |
---|---|---|---|---|---|---|
Mean vs. total | 67233.21 | 1 | 67233.21 | |||
Linear vs. mean | 388.30 | 4 | 97.08 | 2.52 | 0.0666 | |
2FI vs. linear | 305.25 | 6 | 50.87 | 1.47 | 0.2417 | |
Quadratic vs. 2FI | 623.56 | 4 | 155.89 | 67.17 | <0.0001 | Suggested |
Cubic vs. quadratic | 28.88 | 8 | 3.61 | 4.26 | 0.0358 | Aliased |
Residual | 5.93 | 7 | 0.8470 | |||
Total | 68585.14 | 30 | 2286.17 |
The ANOVA statistical analysis was used to reveal the significant process parameters (Table 6). The p-values of this model were less than 0.0001, indicating the model terms were significant. The coefficient of the linear terms (A, B, C, and D) interactive terms (AB and BC), and quadratic terms (A2, B2, C2, and D2) were significant. The F-value of model is 40.54, indicating that it is statistically significant. This demonstrates that the model has a 0.01% chance of producing an F-value this large due to noise. Moreover, the lack of fit F-value is 2.90, implying that it is not significant model. There is only a 12.60% chance due to noise. The coefficient of determination (R2) explained the variability for the GC yield was 97.43%. Furthermore, the predicted determination coefficient (predicted R2) and adjusted determination coefficient (adjusted R2) were observed to be 86.81% and 95.02%, respectively, representing that the predicted and actual values were well-fitting, and indicating a statistically significant correlation between the GC yield and the four variable factors that affect it. The predicted versus actual values plot on the GC yield is illustrated in Fig. 9. A linear relationship between the actual and predicted values means was observed, indicating that the predictive model captured the underlying patterns in the data and is able to accurately predict the GC yield based on the input variables. The predicted equation for the GC yield is displayed in eqn (8):
GC yield (%) = 55.31 + 1.07A + 2.70B + 2.24C + 1.66D + 2.21AB − 0.1231AC − 0.3569AD + 3.64BC + 0.6494BD + 0.6119CD − 1.94A2 − 3.92B2 − 3.02C2 − 1.09D2 | (8) |
Source | Sum of squares | Degree of freedom | Mean square | F-value | p-value | Remark |
---|---|---|---|---|---|---|
Model | 1317.11 | 14 | 94.08 | 40.54 | <0.0001 | Significant |
A | 27.71 | 1 | 27.71 | 11.94 | 0.0035 | Significant |
B | 174.69 | 1 | 174.69 | 75.28 | <0.0001 | Significant |
C | 119.93 | 1 | 119.93 | 51.68 | <0.0001 | Significant |
D | 65.97 | 1 | 65.97 | 28.43 | <0.0001 | Significant |
AB | 78.46 | 1 | 78.46 | 33.81 | <0.0001 | Significant |
AC | 0.2426 | 1 | 0.2426 | 0.1045 | 0.7509 | |
AD | 2.04 | 1 | 2.04 | 0.8781 | 0.3636 | |
BC | 211.78 | 1 | 211.78 | 91.26 | <0.0001 | Significant |
BD | 6.75 | 1 | 6.75 | 2.91 | 0.1088 | |
CD | 5.99 | 1 | 5.99 | 2.58 | 0.1290 | |
A2 | 103.24 | 1 | 103.24 | 44.49 | <0.0001 | Significant |
B2 | 214.77 | 1 | 214.77 | 181.74 | <0.0001 | Significant |
C2 | 249.35 | 1 | 249.35 | 107.45 | <0.0001 | Significant |
D2 | 32.44 | 1 | 32.44 | 13.98 | 0.0020 | Significant |
Residual | 34.81 | 15 | 2.32 | |||
Lack of fit | 29.69 | 10 | 2.97 | 2.90 | 0.1260 | Not significant |
Pure error | 5.12 | 5 | 1.02 | |||
Cor total | 1351.92 | 29 | ||||
R2 = 0.9743, adj R2 = 0.9502 |
The combined influence of the four variables on the GC yield is visualized in Fig. 10. This was achieved by constructing perturbation graphs, which enabled a comparative analysis of the effects of the process variables at specific points within the design range.51 The four parameters under investigation were factor A (catalyst loading level: 4 wt% relative to the initial amount of enriched glycerol), factor B (DMC: enriched glycerol molar ratio: 3:
1), factor C (reaction temperature: 80 °C), and factor D (reaction time: 180 min). These variables were identified as the controlling factors in determining the GC yield. In a perturbation plot, the steepness of the slope generally signifies the magnitude of a factor's impact on the yield, with a steeper slope indicating a more substantial effect compared to a flatter slope. As per the findings of the study, the perturbation diagram unveiled an inflection point in relation to the DMC: enriched glycerol molar ratio. This indicated that this particular parameter exerted a more pronounced influence on the GC yield compared to the other parameters. Following this, the sequence of impact was observed to be as follows: DMC: enriched glycerol molar ratio, reaction temperature, reaction time, and catalyst loading level.
![]() | ||
Fig. 10 Perturbation plot including catalyst loading (A), DMC to enriched glycerol molar ratio (B), reaction temperature (C), and reaction time (D). |
Predicted GC yield (%) | Experimental GC yield (%) | Error (%) |
---|---|---|
a Reaction conditions: catalyst loading of 5.15 wt%, DMC: enriched glycerol molar ratio of 4.24![]() ![]() |
||
59.75 | 58.77 | 1.64 |
The three-dimensional (3D) surface diagrams (Fig. 11) offer a comprehensive visualization of how the operational factors interact with each other. Varying the catalyst loading level (A) from 2–6 wt% relative to the initial amount of enriched glycerol and the DMC: enriched glycerol molar ratio (B) from 1:
1 to 5
:
1 on the GC yield, with a constant reaction temperature (80 °C) and reaction time (180 min), is shown in Fig. 11a–c. In chemical processes, the catalyst is pivotal, expediting product formation and lowering the reaction's activation energy. From the 3D surface diagrams, it is evident that increasing the catalyst loading from 2 wt% to 4 wt% (relative to the initial amount of enriched glycerol) led to a corresponding rise in the GC yield from 45.40% to 55.31%. This increase in yield can be attributed to the greater number of Li active sites present, which play a vital role in facilitating the transesterification reaction.19 The lower catalyst concentrations often resulted in a diminished GC yield due to the scarcity of active sites. Hence, the catalyst's activity is closely linked to its active sites during the reaction. Conversely, when the catalyst loading level was elevated to 6 wt% (relative to the initial amount of enriched glycerol), it potentially led to pore blockage and increased mass transfer resistance between the reactants and catalyst phases, resulting in a decline in the GC yield.4,30
The molar ratio of DMC to enriched glycerol has proven to be crucial for achieving the highest yield in the transesterification reaction of glycerol. The influence of the DMC to enriched glycerol molar ratio (1:
1 to 5
:
1) on the GC yield (under a constant reaction temperature of 80 °C, catalyst loading level of 4 wt% (relative to the initial amount of enriched glycerol), and reaction time of 180 min) is illustrated in Fig. 11a, d and e. The GC yield was predicted to be 34.20% at a DMC: enriched glycerol molar ratio of 1
:
1 and markedly increased when raising the DMC: enriched glycerol molar ratio to 3
:
1. This is because an excess of DMC is necessary to facilitate the reaction in the forward direction.48 However, increasing the molar ratio to 5
:
1 did not enhance the GC yield; instead, it decreased to 45.02% owing to the poor miscibility of reactants at high concentrations.4
The effect of the reaction temperature (70–90 °C) and catalyst loading on the reaction (with a fixed DMC: enriched glycerol molar ratio of 3:
1 and reaction time of 180 min) is illustrated in Fig. 11b. The GC yield increased noticeably when increasing the temperature from 70 °C to 80 °C. This effect is presumably attributed to the temperature rise enhancing the dispersibility and contact between the reactants. Additionally, the reduced viscosity of the reaction mixture facilitates improved mixing between the enriched glycerol and DMC, ultimately resulting in an increased reaction rate.53 However, further increasing the temperature to 90 °C reduced the GC yield (Fig. 11b, d and f), suggesting that the decarboxylation reaction occurs at high temperatures, leading to the formation of glycidol.52 Another contributing factor is the loss of DMC through evaporation, especially given that the boiling temperature of DMC is close to 90 °C. High temperatures can have a negative impact on chemical reactions, such as promoting unwanted side reactions (e.g., decarboxylation), and so decrease the effectiveness of catalysts and reduce their ability to facilitate the desired reactions.4
The reaction time was evaluated within the range of 60–180 min while keeping the other three variables constant (Fig. 11c, e and f). The GC yield increased gradually as the reaction time increased, and reached a maximum of 55.31% at 120 min. However, the yield then slowly decreased to 54.28% at 180 min, which could be attributed to the occurrence of side reactions involving the products, such as the formation of glycidol from GC. After a certain time period, the reaction progresses towards glycidol selectivity.53
![]() | ||
Fig. 12 Possible reaction mechanism of enriched glycerol and DMC on 5%Li/MCM-41 catalyst for glycerol carbonate synthesis via transesterification. |
The ICP-OES analysis revealed that the concentration of Li active species of the 5%Li/MCM-41 was decreased from 3440.00 ± 20 ppm (first run) to 490.30 ± 11.9 ppm after the fourth run (Fig. 14). Thus, the reduced activity of the catalyst over four runs was likely to be due to leaching of the Li active species.
The basicity (Table 8) was markedly declined from 4.70 mmol g−1 (fresh catalyst) to 2.82 mmol g−1 after the fourth cycle. This decline suggests a potential gradual reduction in active basic sites, likely due to the leaching of Li during successive applications. The basic strength (Table 8) consistently fell within the range of 9.8 ≤ H_ ≤ 15 for both the fresh and secondarily reused catalysts. However, for the third and fourth cycles, it decreased to 7.2 ≤ H_ ≤ 9.8. This implies that while the number of basic sites may decrease upon reuse, the remaining sites retain a relatively stable strength. A noteworthy finding is the impact of LiNO3 doping on the fourth reused catalyst, resulting in a substantial increase in basicity to 7.62 mmol g−1. This suggests that LiNO3 addition has a rejuvenating effect on the catalyst's basic sites, potentially enhancing their activity. The basic strength remained within the range of 9.8 ≤ H_ ≤ 15, indicating that the added LiNO3 contributes to stronger basic sites.
Catalyst | Basicity (mmol g−1) | Basic strength (H_) |
---|---|---|
a Reaction conditions: catalyst loading of 5.15 wt%, DMC to enriched glycerol molar ratio of 4.24![]() ![]() |
||
5%Li/MCM-41 (fresh) | 4.70 | 9.8 ≤ H_ ≤ 15 |
5%Li/MCM-41 (2nd reused) | 4.27 | 9.8 ≤ H_ ≤ 15 |
5%Li/MCM-41 (3rd reused) | 3.73 | 7.2 ≤ H_ ≤ 9.8 |
5%Li/MCM-41 (4th reused) | 2.82 | 7.2 ≤ H_ ≤ 9.8 |
5 wt% of LiNO3 doped on 5% Li/MCM-41 (4th reused) | 7.62 | 9.8 ≤ H_ ≤ 15 |
The surface morphological characteristics of 5%Li/MCM-41 (4th reused) and 5 wt% of LiNO3 doped on 5%Li/MCM-41 (4th reused) at a magnification of 50000× is exhibited in Fig. 4c and d. The morphological images consisted of spherical particles.
Feed glycerol | Catalyst | DMC: enriched glycerol (molar ratio) | Catalyst loading (wt% of enriched glycerol) | Temperature (°C) | Time (min) | GC yield (%) | Glycerol conversion (%) | Selectivity (%) | Reference |
---|---|---|---|---|---|---|---|---|---|
a Note: C-glycerol is commercial glycerol. | |||||||||
C-glycerol | LiNO3 | 2![]() ![]() |
5 | 95 | 240 | 15.16 | 15.81 | 95.89 | 40 |
C-glycerol | 3Ca–La | 5![]() ![]() |
10.8 | 90 | 90 | 74.0 | 94.0 | 78.72 | 51 |
C-glycerol | Hydrotalcites-Ni | 3![]() ![]() |
10 | 100 | 120 | 55.0 | Not reported | — | 52 |
C-glycerol | 3![]() ![]() ![]() ![]() |
5![]() ![]() |
15 | 90 | 90 | 56.0 | 96.0 | 58.33 | 53 |
C-glycerol | MgO | 5![]() ![]() |
15 | 75 | 90 | 12.1 | 12.4 | 97.58 | 54 |
Enriched glycerol | 5%Li/MCM-41 | 4.24![]() ![]() |
5.15 | 86 | 165 | 58.77 | 59.05 | 99.53 | This study |
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