Polyol Synthesis via Ultrasonic Transesterification

Polyols are synthetic esters produced mainly via a transesterification of triglycerides from vegetable oils or animal fats. These polyols are raw material for the production of polyurethans, biolubricants and other cemicals. Ultrasonication is used to enhance transesterification reactions by applying intense shear forces and thermal energy. Ultrasound and its sonochemical effects supply reaction energy and help to overcome mass transfer limitations. Thereby, sonication significantly improves transesterification speed, yield, and overall efficiency.

Ultrasound-Assisted Transesterification

Transesterification reactions are one of the most important synthesis routes and are widely used as an effective technique for converting vegetable oils into petroleum products substitutes. Sono-synthesis (also sonochemical synthesis, which is chemical synthesis promoted via high-performance ultrasound), is well known for its beneficial effects on transesterification as well as other chemical processes.

Advantages of Ultrasonic Transesterification

  • Rapid conversion
  • More complete reaction
  • Less catalyst
  • Less unwanted by-products
  • Energy-efficient
  • Green chemistry

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Ultrasonication is known to improve transesterification reactions thereby giving e.g. higher methyl esters and polyols. Hielscher Ultrasonics manufactures industrial ultrasonic probes and reactors for high throughputs.

Ultrasonic reactor with 16,000 watts (4x 4000 watts probes) for sonochemically enhanced inline transesterification.

Sustainable Polyol Synthesis from Vegetable Oils using Ultrasound

Plant derived fatty acids, i.e. vegetable oils, are a widely available and renewable raw material and can be used for the preparation of biobased polyols and polyurethanes. The application of power ultrasound creates favourable sonochemical effects, which significantly accelerate the catalytic reaction of transesterification. Additionally, sonication enhances the yield of synthesized polyols as the intense mixing energy of acoustic cavitation overcomes mass transfer limitation. Ultrasonic transesterification reactions are well known to run efficiently with lower alcohol and catalyst as conventional transesterification reactions. This leads to an improved overall efficiency by ultrasonication.

Ultrasonic Synthesis of a Pentaerythritol Ester-Based Biolubricant

Ultrasonicator UP400St, which was used for the transesterification of vegetable oil to pentaerythritol ester.Pentaerythritol ester can be synthesized efficiently from rapeseed oil via two-step sonochemical process as demonstrated by the research team of Arumugam. In their optimization study, the researchers used the Hielscher ultrasonicator UP400St (see picture left). In the first sonochemically promoted transesterification, rapeseed oil is reacted with methanol to methyl ester. In the second transesterification step, the methyl ester reacts with xylene and a catalyst to pentaerythritol ester. The researcher focused on the optimization of ultrasonic process parameters in order to enhance yield and overall efficiency of the pentaerythritol ester synthesis under ultrasound. Improved yield of 81.4% of pentaerythritol ester was accomplished with an ultrasonic pulse of 15 s, an ultrasonic amplitude of 60%, a catalyst concentration of 1.5 wt%, and the reaction temperature of 100°C. For quality control, the sonochemically synthesized pentaerythritol ester was compared with synthetic grade compressor oil. In conclusion, the study suggests that the ultrasonically-promoted successive transesterification process is an efficient method to replace the conventional successive transesterification process for the synthesis of pentaerythritol ester-based biolubricant. The major advantages of the ultrasonic transesterification process are increased yields of pentaerythritol ester, a shortened reaction time and significantly lower reaction temperatures. (cf. Arumugam et al., 2019)

Ultrasonically intensified two-stage transesterification of rapeseed oil to the polyol esters such as pentaerythritol tetrastearate.

Ultrasonically intensified two-stage transesterification of rapeseed oil to the pentaerythritol ester.
(adapted from Arumugam et al., 2019)

Pentanal-Derived Acetal Esters via Ultrasonic Synthesis

The research team of Kurniawan synthesized three pentanal-derived acetal esters via sonochemical method employing the principles of green chemistry. Sonication was used to promote two chemical steps:

  1. Esterification of 9,10-dihydroxyoctadecanoic acid
  2. Acetalyzation of alkyl 9,10-dihydroxyoctadecanoate

In order to produce the esters of alkyl 9,10- dihydroxystearate two steps are required and yields of 67-85% were obtained. For efficiency evaluation, the sonochemical method was compared to the conventional reflux technique. Furthermore, homogeneous and solid acid catalysts, namely sulfuric acid (H2SO4), natural bentonite, and H-bentonite, were used to determine the influence and efficiency of various catalysts. It was found that sonochemical esterification of the acid-catalyzed by H-bentonite gave products in up to 70% yield in 3 times shorter reaction time than the reflux method, which is remarkable. The final acetalization step with n-pentanal in the presence of H-bentonite using ultrasonication afforded three pentanal-derived dioxolane derivatives in 69–85% yields, which are higher than the conventional method. The reflux method required a longer reaction time than the sonochemical method since ultrasonic synthesis required only 10-30min. Additionally to the significantly shorter reaction time under sonication, a remarkable yield of each ester was obtained using the sonochemical method.
The researcher also calculated that the energy requirements of the sonochemical reaction are approx. 62-times lower than that of the conventional method. This reduces costs and is environmental-friendly.
Examination of the physicochemical properties of each product revealed that methyl 8-(2-butyl-5-octyl-1,3-dioxolan-4-yl)octanoate is a potential novel biolubricant with the functionalities to substitute common lubricants. (cf. Kurniawan et al., 2021)

Ultrasonic reactor for chemical synthesis, e.g. transesterification, esterification, or acetylation processes.

Ultrasonic reactor with 4x 2000 watts probes (8kW) for sonochemical processes.

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Transesterification of Pentaerythryl Esters using Ultrasound

Pentaerythryl esters can be obtained from vegetable oils such sunflower, linseed, and jatropha oil. The research team of Hashem demonstrated the synthesis of biobased lubricants via a successive base-catalyzed transesterifications involving two transesterification steps. They demonstrated the feasibility of the synthesis using sunflower, linseed, and jatropha oil. In the first step, the oils were converted into the corresponding methyl esters. In the second process, the methyl esters were converted into pentaerythryl esters by the action of pentaerythritol as shown in the following scheme: (cf. Hashem et al., 2013)

After transesterification of vegetable oil to methyl ester, the methyl esters are converted into pentaerythryl esters by the action of pentaerythritol as shown in the scheme above.
(cf. Hashem et al., 2013)

The significantly reaction enhancing effects of ultrasonication on transesterification are scientifically proven and already since decades industrially adopted. The most prominent example for ultrasonically improved tranesterification is the conversion of oils and fats into fatty acid methyl ester (FAME), known as biodiesel.
Read more about the ultrasound-assisted transesterification of (waste) oils and fats into biodiesel!

Ultrasonic Probes and Reactors for Transesterification and other Chemical Syntheses

Hielscher Ultrasonics UIP1500hd is a powerful ultrasonic homogeniser that is used for batch and inline sonication.Hielscher Ultrasonics is your specialist when it comes to sophisticated high-performance ultrasonicators for sonochemical reactions. Hielscher designs, manufactures, and distributes high-power ultrasonicators and accessories such as probes (sonotrodes), reactors, and flow cells at any size and supplies chemical laboratories as well as chemical production facilities on industrial scale. From compact lab ultrasonic devices to industrial ultrasonic probes and reactors, Hielscher has the ideal ultrasonic system for your process. With long-time experience in applications such as sono-catalysis and sono-synthesis, our well-trained staff will recommend you the most suitable setup for your requirements.
Hielscher Ultrasonics manufactures high-performance ultrasonic systems of very high robustness and capable to deliver intense ultrasound waves since all Hielscher industrial ultrasonicators can deliver very high amplitudes in continuous operation (24/7). The robust ultrasound systems require almost no maintenance and are built to run. This makes Hielscher ultrasonic equipment reliable for heavy-duty applications under demanding conditions. Special sonotrodes for high-temperature or very harsh chemical are available too.
Highest Quality – Designed and Made in Germany: All equipment is designed and manufactured in our headquarter in Germany. Before delivery to the customer, every ultrasonic device is carefully tested under full load. We strive for customer satisfaction and our production is structured to fulfil highest quality assurance (e.g., ISO certification).

The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

Batch Volume Flow Rate Recommended Devices
1 to 500mL 10 to 200mL/min UP100H
10 to 2000mL 20 to 400mL/min UP200Ht, UP400St
0.1 to 20L 0.2 to 4L/min UIP2000hdT
10 to 100L 2 to 10L/min UIP4000hdT
n.a. 10 to 100L/min UIP16000
n.a. larger cluster of UIP16000

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Ultrasonic high-shear homogenizers are used in lab, bench-top, pilot and industrial processing.

Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers for mixing applications, dispersion, emulsification and extraction on lab, pilot and industrial scale.

Literature / References

Facts Worth Knowing

Polyol Synthesis Routes

Natural oil polyols (abbrev. NOPs) or biopolyols, are polyols derived from vegetable oils. Several different chemical routes are available to synthesize biopolyols. Biopolyols are mainly used as raw materials for the production of polyurethanes, but also goes into the productions of other products such as lubricants, elastomers, adhesives, artificial leather, and coatings.
Regarding polyols synthesis from vegetable oils various reaction methods such as epoxidation, transamidization and transesterification are available. For instance, rapeseed oil based polyol can be synthesized by partial epoxidation of the double bonds in fatty acid chains and overall opening oxirane rings by using diethylene glycol. Transamidization and transesterification of ester bonds of vegetable triglycerides can be run using diethanolamine and triethanolamine, respectively.

High performance ultrasonics! Hielscher's product range covers the full spectrum from the compact lab ultrasonicator over bench-top units to full-industrial ultrasonic systems.

Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers from lab to industrial size.