Magnetic Silica Particles Functionalized with Guanidine Derivatives For
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www.nature.com/scientificreports OPEN Magnetic silica particles functionalized with guanidine derivatives for microwave‑assisted transesterifcation of waste oil Petre Chipurici1,6, Alexandru Vlaicu1,2,6, Ioan Călinescu1, Mircea Vînătoru1, Cristina Busuioc1, Adrian Dinescu3, Adi Ghebaur1,4, Edina Rusen1, Georgeta Voicu1, Maria Ignat5 & Aurel Diacon1* This study aimed to develop a facile synthesis procedure for heterogeneous catalysts based on organic guanidine derivatives superbases chemically grafted on silica‑coated Fe3O4 magnetic nanoparticles. Thus, the three organosilanes that were obtained by reacting the selected carbodiimides (N,N′‑ dicyclohexylcarbodiimide (DCC), N,N′‑diisopropylcarbodiimide (DIC), respectively 1‑ethyl‑3‑(3‑ dimethylaminopropyl) carbodiimide (EDC) with 3‑aminopropyltriethoxysilane (APTES) were used in a one‑pot synthesis stage for the generation of a catalytic active protective shell through the simultaneous hydrolysis/condensation reaction with tetraethyl orthosilicate (TEOS). The catalysts were characterized by FTIR, TGA, SEM, BET and XRD analysis confrming the successful covalent attachment of the organic derivatives in the silica shell. The second aim was to highlight the capacity of microwaves (MW) to intensify the transesterifcation process and to evaluate the activity, stability, and reusability characteristics of the catalysts. Thus, in MW‑assisted transesterifcation reactions, all catalysts displayed FAME yields of over 80% even after 5 reactions/activation cycles. Additionally, the infuence of FFA content on the catalytic activity was investigated. As a result, in the case of Fe3O4@ SiO2‑EDG, a higher tolerance towards the FFA content can be noticed with a FAME yield of over 90% (for a 5% (weight) vs oil catalyst content) and 5% weight FFA content. Biodiesel can represent a suitable renewable alternative for the direct replacement of standard diesel fuels derived from petroleum sources1,2. However, the techno-economic and environmental aspects of the production process must be continuously improved to assure the sustainability of biodiesel in the energy and transportation fuels sectors3–6. Tis can be achieved by decreasing the cost of feedstocks using cheaper raw materials7–9, through the increase of productivity by waste reduction and process integration strategies10–13, and the implementation of process intensifcation techniques14–17. Transesterifcation of vegetable oils using sodium or potassium hydroxide in homogenous catalysis is the preferred route for biodiesel production18. However, although the fast reaction rates this procedure presents some major drawbacks such as demands good quality vegetable oil (no water and low free-fatty acids content, usually < 0.5 wt%)19 to avoid the soap formation. Furthermore, the catalysts are non-recoverable requiring large volumes of water for their elimination, while corrosiveness afects downstream processing, thus limiting the economic efciency of the process 11. Te use of heterogeneous catalysis ofers a suitable alternative for a greener biodiesel synthesis route, facilitating catalyst separation and reutilization, thus, reducing the amount of waste produced20–22. Nevertheless, efcient recovery and longer catalyst lifetime of the catalyst remain challenges that need to be addressed. A good strategy to simplify the catalyst separation is to endow it with magnetic properties 23. 1Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania. 2National Research and Development Institute for Chemistry and Petrochemistry— ICECHIM, 202 Splaiul Independenţei, 060021 Bucharest, Romania. 3National Institute for Research and Development in Microtechnologies (IMT-Bucharest), 126 A, Erou Iancu Nicolae Street, P.O. Box 38-160, 023573 Bucharest, Romania. 4Advanced Polymer Materials Group, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania. 5Laboratory of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 700487 Iasi, Romania. 6These authors contributed equally: Petre Chipurici and Alexandru Vlaicu. *email: [email protected] Scientifc Reports | (2021) 11:17518 | https://doi.org/10.1038/s41598-021-97097-7 1 Vol.:(0123456789) www.nature.com/scientificreports/ Scheme 1. Reactions pathways for the preparation of the catalysts. Te use of heterogeneous superbases in the transesterifcation process has the advantage of high reaction rates under mild conditions as well as catalysts recycling due to facile separation from products24. Organic superbases such as guanidines25 have attracted increasing attention for application as catalysts in organic synthesis routes due to their potential functionality resulting from their extremely strong basicity. Tey are also promising active species due to the possibility for facile molecular modifcation, chemical grafing capacity, possible recyclability, and reduced toxicity. Examples of guanidine derivatives employed in the heterogeneous catalyzed transesteri- fcation of vegetable oils include both cyclic derivatives [i.e. 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)]26–31 or alkylguanidines32–35 [such as tetramethylguanidine (TMG)36–38]. Te aim of this study was the synthesis and characterization of heterogeneous catalysts based on organic guanidine derivatives superbases chemically grafed on silica-coated Fe3O4 magnetic nanoparticles. Te catalyst preparation involved the synthesis of a magnetic core and the generation of a catalytic active protective shell through the simultaneous hydrolysis/condensation reaction between tetraethyl orthosilicate (TEOS) and three guanidine derivatives silane precursors. Te silane precursors were obtained by reacting the selected carbodiim- ides (N,N′-dicyclohexylcarbodiimide (DCC)35, N,N′-diisopropylcarbodiimide (DIC), respectively 1-ethyl-3-(3- dimethylaminopropyl) carbodiimide (EDC) with 3-aminopropyltriethoxysilane (APTES). Te catalytic activity in conventional and MW-assisted conditions, respectively the catalysts reutilization characteristics for biodiesel production were evaluated at a temperature of 80 °C, a reaction time of 180 min and molar ratio oil/MeOH 1/6. Also, the infuence of free fatty acid content on the catalytic activity was also evaluated. Experimental Materials. Te tetraethyl orthosilicate (TEOS) (Aldrich), 3-aminopropyltriethoxysilane (APTES) (Aldrich), N,N′-dicyclohexylcarbodiimide (DCC) (Aldrich), N,N′-diisopropylcarbodiimide (DIC), 1-ethyl-3-(3-dimethyl- aminopropyl) carbodiimide (EDC) (Aldrich), cetyltrimethylammonium bromide (CTAB), lauric acid (Acros), sodium hydroxide (Fluka) were used without purifcation. Toluene (Merck) was used afer distillation and dry- ing on 3 Å molecular sieves. Commercial sunfower oil (Carrefour brand) and methanol (Aldrich) were used in the transesterifcation reactions without further purifcations. Methods. Synthesis of procedure for (1,3‑(dicyclohexyl) 2‑(3‑(triethoxysilyl)propyl) guanidine) (Si‑DCG), (1,3‑(diisopropyl) 2‑(3‑(triethoxysilyl) propyl) guanidine) (Si‑DIG) and (1‑ ethyl, 3 (3‑(3‑dimethylamino propyl)) 2‑(3‑(triethoxysilyl)propyl) guanidine) (Si‑EDG). Te synthesis procedure was based on a previous literature example35 and it was expanded for EDC and DIC. It involves the addition of amino group of APTES to the three carbodiimides derivatives (DCC, EDC and DIC) (Scheme 1). In a 50 mL round bottom fask, 10 mmoles of the starting material carbodiimide (2.1 g N,N′ diciclohexyl carbodimides (DCC) or 1.3 g N,N′-diisopropylcarbodiimide (DIC), or 1.55 g 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), respectively) were dissolved in 20 mL of dry toluene under a nitrogen blanket. To this solution 10 mmoles (2.21 g) of 3-aminopropyltriethoxysilane (APTES) were added and the mixture was kept Scientifc Reports | (2021) 11:17518 | https://doi.org/10.1038/s41598-021-97097-7 2 Vol:.(1234567890) www.nature.com/scientificreports/ under continuous stirring at 100 °C for 24 h. Te toluene was removed using a rotary evaporator at reduced pressure and yellowish oils were obtained and used directly for the preparation of the catalysts without any further purifcations. Synthesis procedure of the magnetic nanoparticles Fe 3O4. Te magnetic Fe3O4 nanoparticles were prepared by 31,39 a chemical co-precipitation method . 18 mmoles FeCl3·6H2O (4.864 g) and 8.9 mmoles FeCl2 (1.134 g) were dissolved in 100 mL of bidistilled water. Te solution was degassed by nitrogen bubbling for 15 min at room temperature under mechanical stirring (350 rpm) while being immersed in an ultrasonic water bath (40 kHz and 400 W). Te US power absorbed by the solution was determined calorimetrically as 15.2 W with a cor- responding power density of 0.152 W/mL). Ten, 10 mL of 25% NH4OH were added to this solution and the reaction mixture remained under mechanical stirring and nitrogen atmosphere at room temperature for 1 h. Te black precipitate was separated from the solution using magnetic decantation and was subsequently washed thoroughly (10 times) with deionized water and acetone. Te resulting black solid was dried under vacuum at room temperature for 5 h, yielding 2.10 g of magnetic nanoparticles. Synthesis procedure for the magnetic catalysts. In a 250 mL two-necked round bottom fask equipped with mechanical stirring, 0.55 g cetyltrimethylammonium bromide (CTAB) were dissolved in 200 mL of water. In the solution obtained 1 g of magnetic nanoparticles were dispersed to serve as core for the catalyst generation. Afer