A new approach to polymer-supported phosphotungstic acid: Application for glycerol acetylation using robust sustainable acidic heterogeneous–homogenous catalyst
Graphical abstract
Introduction
The development of modern technologies for producing chemicals and green energy from sustainable resources has been quickly prompting throughout the world as a possible substitute for the petroleum based fuels because it is renewable, biodegradable, and biocompatible new energy sources [1]. Wide spread commercialization of biodiesel has in turn led to dissipate an oversupply of crude glycerol in the market as a byproduct until new technologies are created for glycerol by way of new product development. Glycerol is the main by-product in transesterification reaction of oil or waste oil with alcohols like methanol or ethanol. Since glycerol is functionalized molecule with three hydroxyl groups, it is considered as a potential renewable feedstock for numerous valuable products [2], [3]. Glycerol can undergo oxidation, hydrogenolysis [4], [5], carbonylation [6], [7], [8], esterification and etherification [9], etc., to yield useful commodity chemicals.
One of the important reactions is synthesis of acetin from esterification of glycerol with acetic acid, which are known valuable chemicals and fuel additives. In addition, the acetylated glycerol derivatives (monoacetain, MAG, diacetain (DAG), and triacetain (TAG)) have gained more attention to find better utilization of surplus glycerol resulted from the biodiesel transesterification processes. MAG is used in manufacturing of explosives and tanning agent as solvent for dyes. DAG is used as a solvent, plasticizer and softening agent. TAG is used as a solvent for diluting drugs and organic compounds. Furthermore, valorization of glycerol derivatives (DAG and TAG) as fuel additives can not only improving properties of diesel fuel by enhancing pour point, flash point and viscosity but also reduce particulate emissions and the cost of biofuel [3], [10].
The MAG, DAG and TAG can be catalytically formed by acetylation of glycerol with acetic acid or acetic anhydride with heterogeneous or homogeneous catalysts. Homogeneous acid catalysts such as p-toluene sulfonic acid, H3PO4 and HCl have been used for the glycerol acetylation. However, homogeneous catalysts cause several problems, including corrosion of the reactor, production of several toxic compounds and catalysts loss due to the difficulty of separation. To overcome these drawbacks, research interest has been mainly forced to discover eco-friendly and highly active heterogeneous catalysts, such as acid or base solid catalysts. In the last few years, several studies were reported by using varies solid acid catalysts such as Amberlyst-15, acidic mesoporous silica, sulfonated-carbon and dodecatungstophosphoric acid immobilized onto a silica for glycerol acetylation from reaction of glycerol with acetic acid [10], [11], [12], [13], [14]. Amberlyst-15 acid resin showed a conversion of 90% and selectivities of 54% and 13% toward DAG and TAG, respectively, after 10 min at 150 °C [11]. However, the main drawback of the Amberlyst-15 is its low thermal stability, which leads to corrosion problems caused by its decomposition, and therefore inhibits the possibility of regeneration [12]. Sulfonic acid functionalized mesostructured materials catalyst showed 90% of glycerol conversion with combined selectivity toward DAG and TAG of 85% at 125 °C after 4 h [10]. SO3H-carbon catalyst gave glycerol conversion (91%) and selectivities of 38% (MAG), 28% (DAG) and 34% (TAG) at reaction condition of 120 °C and 3 h of reaction time [13]. Silica supported dodecatungstophosphoric acid catalyst achieved 62% (DAG) and 3% (TAG) selectivities [14].
Supported heteropoly acids (HPAs), especially 12-tungstophosphoric acid (PTA), have become widespread materials to catalyze a wide variety of reactions because of their unique properties such as redox natures, super acidic properties (strong Brønsted acids) and the keggin structure, and can be used in homogeneous as well as heterogeneous catalysis [15], [16], [17]. However, HPA has several shortcomings such as thermal instability, low surface area (<10 m2/g), aggregating easily and separation problem from reaction mixtures. These problems can overcome by loading HPAs on the different supporters of stable structure and high surface area. Supported PTA has also been used for performing glycerol esterification reaction [18], [19], [20]. PTA supported on Cs-containing zirconia [18], activated carbon [14], cubic mesoporous silica [19] and niobic acid [20] have been reported for glycerol esterification. The disadvantages of these acidic solid catalysts such as limitation of pore-size, reuse and low selectivity toward the favored DAG and TAG materials are still unresolved issues for industrial applications due to leaching of PTA from the supports.
Despite the many investigations, the need for a supper acidic solid catalysts for glycerol upgrading and development of green technologies to obtain sustainable energy and valuable chemicals on their basis still exists. With these considerations in mind, we have tried to develop high acidic solid catalysts that are easy to prepare, cost-effective and free active sites leaching. In the present work, the authors report the synthesis of crosslinked polymer tailored inorganic acid through soft crosslinking agent as ethylenediamine. One amine of ethylenediamine could be quaternized with chloromethyl group belong to solid polymer and the remaining amine functional group could act as active sites to immobilize H3PW12O40 (PTA). In this way, both sintering and leaching of the PTA is eliminated and the properties for supporting active species are preserved. In addition, this study in particular investigates the influence of acidic properties, catalyst structure and reaction parameters on triacetin formation, and compares the results with other commercial catalysts. Further, to our best knowledge this is also the first account showcasing the application of PTA-polymer as stable recyclable catalysts for glycerol acetylation with high selectivity toward triacetin formation than others.
Section snippets
Materials
P-divinylbenzene (DVB, 85%), 4-vinylbenzyl chloride (VBC, 90%), cetrimonium bromide (CTAB, ≥99%), Benzoyl peroxide (BOP, 75%), ethylenediamine (ED, ≥99%), anhydrous 12-tungstophosphoric acid (PTA, 99.995%), neutral red (Dye content ≥90 %), glycerol (Gly, ≥99.5%), acetic acid (AA, ≥99.7%), potassium hydroxide (90%) and phenolphthalein were purchased from Sigma-Aldrich. The contents of DVB stabilizer was removed by several washing with a 1N NaOH solution followed by drying under sodium sulfate.
Results and discussion
One of the most promising approaches is to design high acidic heterogeneous–homogeneous catalyst containing H3[P(W3O10)4]− as active acidic moieties based on easy separating and recovering from the product mixture. Scheme 1 shows the procedures for the amine surface modification of PDVC and immobilization of H3[P(W3O10)4] onto the surface and pore of NH2-PDVC materials. The surface modification of PDVC was achieved by reacting the chloromethyl group of the VBC with one-NH2 group of ED under
Conclusions
In summary, the authors developed micro-mesopores H2N-PDVC polymer and reported the first successful immobilization of catalytically active acidic Keggin-type (H3PW12O40, PTA) material on the surface and pores of H2N-PDVC framework material. The resulting H2N-PDVC material is coated with a customized ratio of PTA through an innovative approach that increases the activity, durability and recyclability of the catalyst. PTA-H2N-PDVC can catalyze the glycerol acetylation with high-excellent
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