Interaction and thermal stability of carboxymethyl cellulose on α-Fe2O3(001) surface: ReaxFF molecular dynamics simulations study

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Highlights

  • Chemisorption of carboxymethyl cellulose on α-Fe2O3 (001) surface.

  • Bonding interaction with the oxygen in –CH2OCH2- like unit of carboxymethyl cellulose.

  • Carboxymethyl cellulose remains attached to hematite surface up to higher temperature.

  • Effect of moisture and temperature on interaction nature.

Abstract

Reactive (ReaxFF) molecular dynamic simulations were performed to elucidate the nature of interaction between hematite and carboxymethyl cellulose (CMC) considering effect of moisture and temperature. Simulations showed that the presence of moisture prohibited CMC to interact directly with hematite surface. However, the moisture helps to disperse CMC along the hematite surface thus maximizing the interaction. In dry condition, it was found that the negatively charged oxygen present in the functional groups and in CMC backbone (–CH2OCH2–) take part to form chemical bond with the positively charged surface Fe. The bonding interaction with polymer back bone is a new finding from this simulation. The binding energy estimated as −56.2 kcal/mol clearly indicates chemisorption with bond length ∼2.00 Å. Upon heating up the complex in presence of atmospheric oxygen it was observed that CMC decomposed at high temperature rather than desorbed from the surface. This study clearly shows that CMC is a suitable binder for iron ore pelletization and will pave the way for more rationale design of organic binders.

Introduction

A better understanding on the interaction of organic polymer with iron oxide surface would benefit a range of applications, from stabilizing iron oxide nanoparticles for use as biomarkers and catalysts [1] to extracting iron ore for steel production [2,3] and to protecting steel surfaces for corrosion inhibition [4]. In recent times there has been a lot of focus on the study of organic polymers due to their outstanding binding properties with mineral and metal surfaces. Organic polymers are being considered as binders for iron ore pelletization as alternate to the traditionally used bentonite as organic binders do not leave any residue during the pelletizing process and do not have to be removed in the iron making process by addition of fluxes as in the case for bentonite. For this purpose, the usage of several organic polymers as a full or partial replacement to bentonite in iron ore pelletization have been explored. The application of organic polymers or binders in iron ore pelletization or agglomeration is limited due to the lack of stability of organic polymers at very high temperatures. Therefore, it is very important to understand the interaction phenomena of organic polymers with iron oxide surfaces and the changes in the interaction pattern with change in temperature. Mostly, carboxymethyl cellulose (CMC), Peridur C-10 and Peridur CX3, organic polymers, domestic CMC, Na lignosulphonate (NLS), starch and glue have been considered for iron ore pelletization [[5], [6], [7], [8], [9]]. Sivrikaya et al. comparatively studied bentonite and certain organic binders namely CMC, Peridur C-10, and Peridur CX3 with magnetite concentrates [8]. They observed that the organic binders achieved the wet and dry (673 K) pellet properties.

Recent experimental studies showed formation of co-ordinate covalent bond between organic molecule and the metal oxide surface [10,11]. More specifically, the experimental study by Lu et al. showed bonding interaction between polysaccharides and magnetite surface [10]. These studies suggested that CMC can interact with iron oxide surface through chemical interaction and hydrogen bonding. Again, CMC in combination with CaCO3 nano particle showed better binding ability for pelletization of magnetite particles [12]. Recently, CMC based binder was used in producing iron ore pellets and reported improvement in the compressive strength of wet and dry pellets [13]. Despite all these studies it is not clear how the interaction varies with the change in the backbone of the polymer structure or change in functional groups. Moreover, how moisture can affect the interaction is not yet clear particularly with the variation in temperature.

Adsorption of organic surfactant such as carboxylic acid, amid mono-glyceride and hexanamide on α-Fe2O3(001) surface was studied using density functional theory (DFT) and force field based molecular dynamics in absence of any solvent where strong chemisorption was observed [14,15]. Experimental measurements and DFT calculations also showed that in the absence of solvent, stearic acid was capable of forming chemisorbed, crystalline adsorbed layers on hematite [14,16]. Nguyen et al. performed DFT calculations and showed that water can be adsorbed on the α-Fe2O3(001) surface at room temperature. This observation is consistent with previous experimental findings [17]. Nature of adsorption of various alkanes on different surfaces, e.g, Fe(110), FeO(110), and Fe2O3(001) were studied using DFT and molecular dynamics (MD) simulations using COMPASS force filed at 150 K [18]. The study showed that the interaction depends on the molecular chain length on both iron and its oxide surfaces.

Computational studies reported so far using DFT provide the static interaction of the functional group with the surface. But how the interaction can be affected with molecular structure or molecular weight or both and by temperature are not reported in the literature. Again, how moisture can control the adsorption process or what happens upon heating the adsorbed complex are not reported so far. Furthermore, static calculations would be useful for small molecules with one active site as other part of the molecule is not interacting with surface strongly. But while a polymer with many active sites is under consideration, static calculation is will not be effective as there is no temperature effect. Hence, MD simulations with temperature effect could be a good choice to explain this interaction. But in that scenario DFT MD is not a feasible choice as it is computationally expensive for this large model system. As an alternative, one can consider force field, say, reactive force field (ReaxFF) [19,20], based method from where one may gain fundamental understanding for a reasonably larger model system. ReaxFF describes bond-breaking and bond-formation events based on bond-orders applied over the valence atomic interaction terms. In ReaxFF, general relationship between (i) bond distance and bond order and (ii) bond order and bond energy are maintained which leads to proper dissociation of bonds of two separated atoms. Therefore, it's a suitable force filed which can take care of high temperature effect where reactions may occur for reasonably large model system. ReaxFF is applied intensively to study hydrocarbon systems and on carbon based nanostructures [[21], [22], [23], [24], [25], [26], [27]]. Additionally, since ReaxFF includes van der Waals (vdW) interaction appropriately, it is suitable to study material aggregation. vdW interaction is crucial to study adsorption process on metal surface [28]. ReaxFF has also been used successfully to study iron oxide (Fe2O3) surfaces computationally [29]. Moreover, the ReaxFF can take care of water to metal oxide surface interaction appropriately [30]. In the present simulations, we consider water explicitly so that the model mimic more realistic physical condition [31]. Hence, ReaxFF MD simulations is an appropriate method and was used in this study to elucidate the interaction between CMC and α-Fe2O3(001) surface at different temperatures in addition to effect of moisture. We have also studied the effect of heating the adsorbed complex in presence of oxygen as it is pertinent to various industrial applications [13].

Section snippets

Computational details

We considered relatively large molecule (C30H50O21), as shown in Fig. 1, to represent the CMC polymer so that the effect of polymer shape/structure can be captured in addition to the effect of functional groups in the adsorption processes. The CMC molecule was optimized at the B3LYP/6-31G(d, p) level of theory available in Gaussian09 suite of computational package [32] and used as the starting CMC structure for ReaxFF MD simulations. Frequency calculation also performed at the same DFT level of

Results and discussions

The CMC molecule considered in this study is sufficiently large to understand the effect of the molecular weight, functional group and molecular architecture on the adsorption process. Both the CMC molecule and bulk Fe2O3 were optimized using ReaxFF method. ReaxFF optimized Fe–O bond length obtained was 1.99 Å for bulk Fe2O3 which is slightly larger than the experimental value of 1.95 Å. It is expected that the thermal fluctuations in the polymer structure as well as in the hematite surface

Conclusions

The adsorption properties of carboxymethyl cellulose (CMC) on α-Fe2O3 (001) surface was studied using reactive force field (ReaxFF) based molecular dynamics simulations. Calculated binding energies show that CMC strongly interacted (chemisorption) with the iron ore surface. The adsorption energy varied with functional group, adsorption sites at the impact point, orientation with respect to surface, coordinate along surface (top site vs hollow site). It was found that CMC can adsorb on the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

B.S. is thankful to Dr. S. Chandra, Chief, R&D, Tata Steel, India for his continuous encouragement during this research. B.S. is very much thankful Prof. A. C. T. van Duin, Pennsylvania State University, USA for his helpful suggestion.

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