Elsevier

Molecular Catalysis

Volume 513, August 2021, 111827
Molecular Catalysis

Cesium salt of iron substituted phosphomolybdate: Synthesis, characterization, room temperature hydrogenation of styrene and its mechanistic evaluation

https://doi.org/10.1016/j.mcat.2021.111827Get rights and content

Highlights

  • Cs salt of iron substituted phosphomolybdate was synthesized by one-pot method.

  • Insertion of Fe into lacuna & 3+ state was confirmed by FT-IR, 31P MAS NMR, ESR & XPS.

  • 5 mg of the active amount gives 99% conversion at RT in aqueous medium with high TON.

  • Easily recoverable, reusable without loss of the activity and viable for other substrates.

  • Mechanistic investigation carried out by ESR & D2O as a solvent.

Abstract

The present article elucidates one-pot synthesis of Cs salt of iron substituted phosphomolybdate, its characterizations and application for styrene hydrogenation. The synthesized material was characterized by elemental analysis, TGA, FT-IR, FT-Raman, ESR, XPS and XRD. A detailed study was carried out for maximum conversion of styrene to ethylbenzene by optimizing different parameters such as solvent, catalyst amount, hydrogen pressure, time and temperature. The catalyst shows high efficiency (99% conversion) with low catalyst amount (5 mg, Fe: 0.02 mol%), EtOH: water (1:1) as green solvent, low pressure (5 bar) in EtOH: H2O aqueous medium at room temperature only. The catalyst was easily recycled by simply centrifugation and reused for up to two cycles without any leaching or degradation. The present method is highly sustainable and green with no waste generation (E-factor = 0) with high Reaction mass efficiency (100%) as well as 100% atom economy. The role of water was confirmed by an isotope labeling experiment using D2O as a solvent and a mechanistic study was investigated by ESR.

Introduction

Transition metal substituted polyoxometalates (TMSPOMs), a sub-class of polyoxometalates (POMs), has gained more attention for various applications in interdisciplinary fields like medicine, magneto chemistry, electrochemical, materials science and catalysis due to their enthralling properties such as controllable shape and size, oxo-anionic nature, structural mobility, easy alteration of chemical composition, and design at a molecular level with an option in the tuning of redox/ acid-base properties [1,2]. Among various TMSPOMs, Transition metal substituted phosphomolybdate (TMSPMA) is most important due to its unique redox properties, as the nature of transition metal is expected to influence the same drastically [2], [3], [4]. At the same time, iron is the best candidate as it is very cheap, naturally abundant, nontoxic and unique magnetic properties [5,6]. However, it was found that in spite of the fascinating properties of phosphomolybdate (PMAs) and Fe, very few reports are available on iron substituted phosphomolybdate (PMo11Fe).

In 2001, Mizuno et al. [7] reported the synthesis of Cs3H1PMo11FeO39, Cs1.5Fe0.5PMo12O40, and Cs2(NH4)2PMo11.5Fe0.5O39, at pH 4.4 from PMo12O40 using lithium carbonate, followed by the addition of Fe(NO3)3.9H2O. The synthesized complex was characterized by X-ray fluorescence, TGA, FT-IR, BET and XRD and its catalytic efficiency was evaluated for selective oxidation of isobutene. Further in 2004, Mizuno group [8] synthesized Cs2.8H1.2PMo11Fe(H2O)O39.6H2O at pH 4.3 using the same method which is reported earlier and characterized by EDX, TGA, FT-IR, 31P NMR, ESR and XRD. Its catalytic activity evaluated for oxidative dehydrogenation of 2-propanol. In 2009, Rabia group [9] reported the synthesis of (NH4)6HPMo11FeIII(H2O)O39 from H3PO4, (NH4)2Mo2O7 and Fe(NO3)3 in which pH was maintained by using sulfuric acid and synthesis was carried out at 0 °C to avoid formation of 6-molybdometalate and NH4NO3 were added to precipitate of complex and characterized by a various analytical method such as Elemental analysis, FT-IR, DRS, 31P NMR and XRD. The catalytic activity of the synthesized complex was evaluated for the oxidation of propene. Later, in 2015, Wang et al. [10] reported the synthesis of Na6PMo11FeO40 by adjusting pH 6 of phosphomolybdic acid solution using NaHCO3, followed by the addition of iron nitrate solution and characterized by some basic techniques such as FT-IR and UV-Visible spectroscopy. The synthesized complex was studied for its inhibitory effect on mushroom tyrosine and as a preventive measure against the peroxidation of fresh-cut fruits and vegetables. In 2017, Rahimi et al. [11] synthesized Cs4H2PMo11FeO40.•6H2O using the same method reported by Mizuno group [8] and its catalytic efficiency was evaluated for selective adsorption of methylene blue dye form a mixture of organic dyes (methylene blue, methyl orange, and rhodamine-B).

Thus, a literature survey shows that there is neither detailed characterization data for PMo11Fe nor any report is available on hydrogenation of styrene using PMo11Fe even though the resulting product plays a pivotal role in the current chemical industries, for triggering a wide range of substrates commonly used in the production of either intermediates or finished products. Hydrogenated product, ethylbenzene has been used in gasoline, paints, inks, insecticides, carpet glues [12,13].

In this context, the present paper reports one-pot synthesis of caesium salt of iron substituted phosphomolybdate and its detailed characterization by various techniques such as elemental analysis, TGA, FT-IR, Raman spectroscopy, ESR, XPS, 31P MAS NMR and Powder XRD. Catalyst efficiency was evaluated for styrene hydrogenation in aqueous medium/ neat water under mild conditions using molecular H2 as a reducing agent. Various parameters like a catalyst to substrate ratio, pressure, solvent, time and temperature have been optimized for maximum% conversion. Also, heterogeneity test, recycling and reusability were studied under optimized conditions. The regenerated catalyst was characterized by EDX, FT-IR, Raman Spectroscopy and XPS to confirmed the stability of the catalyst. Green chemistry metrics were calculated and catalytic activity of the present catalyst was also compared with reported iron based catalysts. The possible reaction mechanism was investigated by ESR spectroscopy and D2O NMR study.

Section snippets

Materials

All chemicals used were of A.R. grade. Phosphomolybdic acid (H3PMo12O40), iron nitrate (Fe(NO3)3), CsCl, NaHCO3, styrene and dichloromethane (DCM)/ ethyl acetate obtained from Merck were used as received.

Synthesis of Cs salt of iron substituted phosphomolybdate (PMo11Fe)

Cs salt of iron substituted phosphomolybdate was synthesized by the reported method [8] with some modification. H3PM12O40 (1.825 g, 1 mmol) was dissolved in a minimum quantity of water and pH = 4.3 was maintained by the addition of saturated sodium bicarbonate (NaHCO3). The obtained solution

Characterization

Cesium salt of iron substituted phosphomolybdate was isolated and the remaining solution was filtered. The filtrate was analyzed for molybdenum gravimetrically [14]. The observed proportion of Mo in the filtrate was 0.5%, which corresponds to loss of one equivalent of Mo from H3PMo12O40.

Energy-dispersive X-ray mapping (EDX) shows (Fig. 1) the presence of Cs, P, Mo, Fe and O in synthesized materials. The observed EDX values of the elements (Cs:25.88%, P:1.27%, Mo:41.39%, Fe:2.08%, O:28.97%) were

Conclusion

We report here one-pot synthesis of Cs salt of iron substituted phosphomolybdate. The presence of all elements was confirmed by EDX mapping. Insertion of iron into lacuna by replacing one MoO unit of PMo12 with retention of Keggin unit was confirmed by FT-IR, Raman spectroscopy and powder XRD. ESR, XPS and 31P NMR analysis were carried out to confirm that Fe in a + 3 state. The catalyst gives 99% conversion with 5081 TON with a very less active amount of Fe (0.02 mol%) at RT. Moreover this

CRediT authorship contribution statement

Anjali U. Patel: Conceptualization, Validation, Writing – review & editing, Visualization, Supervision. Jay R. Patel: Methodology, Validation, Formal analysis, Investigation, Writing – original draft.

Declaration of Competing Interest

There are no conflicts of interest.

Acknowledgement

Ap and JP are thankful to Science and Engineering Research Board (SERB), Project No. EMR/2016/005718, New Delhi for the financial support. We are thankful to The Department of Physics, The Maharaja Sayajirao University of Baroda for FT-Raman analysis. Also, we are thank full to Mr. Bharat Padh, Research and Development, Rubamin Limited, Halol for the ICP analysis.

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