Effect of Degassing Temperature on the Specific Surface Area of Some Metal Chelates

Introduction

Porous materials play a vital role in science and technology. Recent advancements in synthetic methods have enabled precise control over structure and characteristics of porous materials.^1,2 Over the past decade, research has focused on regulating pore dimensions, fostering advancements beyond conventional applications in adsorption and catalysis.^3-5 The surface chemistry of these materials, particularly metal chelate-based porous coordination polymers, offers a versatile tool to address emerging environmental concerns due to their unique properties.^6-9

The commercialization of automated equipment has led to a surge in the use of the physisorption of gases as a method for determining porosity. The porous coordination polymers must be subjected to sufficient degassing for the elimination of solvent molecules for high surface areas and persistent porosity.^10,11 Even while acknowledging the importance of degassing treatments done before gas adsorption measurements, only a very few researchers give importance to the conditions of outgassing treatments.^12-14 An improper activation temperature could cause the framework to collapse, or may cause an insufficient cleaning of the material surface.^13,15 The choice of temperature might be related to the thermal stability of the material under investigation, which can be obtained from corresponding TGA analysis. The effects of degassing temperature on metal chelates are, nevertheless, unreported in the literature. Thus, we have investigated the temperature of the degassing process on the surface area of the metal chelates, paying particular attention to their crystallinity and morphology. In this work, the gel grown coordination polymers CaADA, SrCaEDTA and NiZnEDTA were degassed at different temperatures to discard the adsorbed gases prior to the actual surface area measurements. The crystal structure and nonlinear optical properties of CaADA, SrCaEDTA and NiZnEDTA were reported earlier ^16-18, but this is the first report on their surface area measurements.

Materials and Methods

The surface area and porosity of the synthesized coordination polymers CaADA, SrCaEDTA, NiZnEDTA were analysed using nitrogen adsorption-desorption experiments. Nitrogen adsorption studies were done at 77 K with a Quantachrome autosorb iQ-MP-XR gas sorption analyser and analysed using ASiQwin software. For each gas sorption test, 50 mg of coordination polymers were utilised. The samples were degassed for 12 hours at various temperatures. The specific surface area was measured by the Brunauer-Emmett-Teller (BET) method.^9,12,19 The pore size was calculate by BJH technique.^20,21

Results and Discussion

Surface area and porosity measurements of CaADA

In our work, we have degassed CaADA at two different temperatures. First, the compound was degassed at 150^˚C for a period of 12 hrs and its specific surface area was calculated as 8.27 m²/g. The resulting isotherm (Fig.1) exhibits entirely reversible adsorption-desorption cycles. This shows that the interaction between N₂ and the CaADA was comparatively weak. In the isotherm of CaADA the adsorption and desorption branches coincide each other, indicates that no adsorbed nitrogen is present in the inner small pores when the pressure is released during the desorption phase.^19,22

Figure 1: Nitrogen adsorption–desorption isotherms for CaADA(1500C) at 77 KClick here to View table

Figure 2: BJH pore size distribution of CaADA(1500C).Click here to View Figure

The pore volume and pore diameter in CaADA was found to be 0.010 cc/g and 1.691 nm respectively. The pore distribution plot and nature of isotherm confirm that surface of coordination polymer consists of both micro and meso pores (Fig. 2) ^23,24.

When CaADA was degassed at 170^˚C there is an enhancement in the specific surface area (13.59 m²/g), pore volume (0.018 cc/g) and pore diameter (2.453 nm). An increase in surface area with increase in degassing temperature accounted due to the proper elimination of different impurities and moisture from the surface  of the coordination polymer¹⁰. Here the nature of hysteresis was like Type IV [9]( Fig. 3). The pore distribution plot (Fig. 4) and nature of isotherm confirm this coordination polymer as a mesoporous material ²⁴. Hence it can be inferred that nitrogen adsorption capacity of CaADA can be enhanced by degassing it at 170^˚C.

Figure 3: Nitrogen adsorption–desorption isotherms for CaADA(170˚C) at 77 K.Click here to View Figure

Figure 4: BJH pore size distribution of CaADA(170˚C).Click here to View Figure

Effect of degassing

Fig. 5 and 6 display the IR spectra and PXRD patterns of CaADA at various degassing temperatures. It was noted that nature of bonds and the crystallinity remained unchanged during the degassing process.  The PXRD patterns of the compounds following activation at various temperatures show no obvious structural changes as the degassing was conducted below disintegration temperature, obtained from the TGA of the compounds.  SEM results of the formed crystals taken at various degassing conditions show that the appearance of the crystals was somewhat influenced by the activation temperature (Fig. 7).

Figure 5: FTIR spectra of CaADA (at 170˚C and 150˚C).Click here to View Figure

Figure 6: PXRD patterns of CaADA (at 170˚C and 150˚C).Click here to View Figure

Figure 7: SEM images of CaADA at different magnificationsClick here to View Figure

Surface area and porosity measurements of SrCaEDTA

We have degassed SrCaEDTA at 80^˚C and 90^˚C temperatures before the surface area measurements. When the compound was degassed at 80^˚C for a period of 12 hrs. The specific surface area was 8.78 m²/g. From Fig. 8, it is evident that, like CaADA, the interaction between N2 and the SrCaEDTA is relatively weak. In the isotherm, the desorption branch coincides with the adsorption branch. This indicates that the adsorbed nitrogen does exist in the interior narrow pores during the desorption process. Nature of adsorption-desorption graph and pore distribution diagram indicates that the coordination polymer is micro-porous in nature ²⁵.

Figure 8: Nitrogen adsorption–desorption isotherms for SrCaEDTA(80˚C) at 77 K.Click here to View Figure

The pore volume and pore diameter in SrCaEDTA was found to be 0.017 cc/g and 0.686 nm respectively. Pore size distribution is shown in Fig. 9.

Figure 9: BJH pore size distribution of SrCaEDTA(800C).Click here to View Figure

When SrCaEDTA was degassed at 90^˚C, there is an increase in the surface area (11.953 m²/g), pore volume (0.018 cc/g) and pore diameter (1.938 nm). Change in the degassing condition induced marked difference in surface area and pore width of SrCaEDTA. The desorption branch and the adsorption branch do not meet in the isotherm (Fig. 10). This is because the inner tiny pores of SrCaEDTA contain adsorbed nitrogen, which kinetically restricts when the pressure is released during the desorption process. The nature of the adsorption-desorption plot and pore distribution diagram (Fig. 11) show that the compound is mesoporous in nature²⁶.

Figure 10: Nitrogen adsorption–desorption isotherms for SrCaEDTA (90˚C) at 77 K.Click here to View Figure

Figure 11: BJH pore size distribution of SrCaEDTA (90˚C).Click here to View Figure

Effect of degassing

Identical IR spectra and PXRD patterns are produced by gel-grown SrCaEDTA that has been degassed at 80^˚C and 90^˚C for 12 hours (Fig. 12 and Fig.13). This indicates that activation temperatures had little impact on the crystallinity of SrCaEDTA. Rather, it had a considerable impact on its surface area, porosity, and textural qualities. As no additional peaks are seen in the IR and PXRD plots, the structures of the coordination polymer will probably stay intact. How ever the degassing temperature had influenced the BET surface areas along with the surface texture of SrCaEDTA (Fig 14).

Figure 12: FT-IR spectra of SrCaEDTA (at 90˚C and 80˚C).Click here to View Figure

Figure 13: PXRD patterns of SrCaEDTA (at 90˚C and 80˚C).Click here to View Figure

Figure 14: SEM images of SrCaEDTA at different magnificationsClick here to View Figure

Surface area and porosity measurements of NiZnEDTA

In this investigation, NiZnEDTA was degassed at two distinct temperatures. First, the substance was degassed at 80^˚C for 12 hours. The specific surface area of the compound was found to be 21.876 m²/g. The N₂ adsorption-desorption isotherms are reversible (Fig. 15). Therefore, N₂ interacts with the NiZnEDTA quite weakly and the gas was often adsorbed in the active site of the NiZnEDTA. The desorption branch coincides with the adsorption branch, indicating that adsorbed nitrogen doesn’t exist in the small pores of the interior during the desorption process. The pore volume and diameter were calculated to be 2.188 nm and 0.026 cc/g in close subsequence. Fig.16 shows the pore size distribution.

Figure 15: Nitrogen adsorption–desorption isotherms for NiZnEDTA (80˚C) at 77 K.Click here to View Figure

Figure 16: BJH pore size distribution of NiZnEDTA (80˚C).Click here to View Figure

When NiZnEDTA was degassed at 90^˚C, an increase in surface area (25.970 m²/g), pore volume (0,032 cc/g) and pore diameter (2.789 nm) was observed. The obtained adsorption–desorption isotherms are reversible in nature (Fig 17). The pore distribution plot confirms the mesoporous nature (Fig 18) [27, 28]. The increase in surface area with increasing degassing temperature of the coordination polymer can be explained by the adequate elimination of moisture and other contaminants from its surface.

Figure 17: Nitrogen adsorption–desorption isotherms for NiZnEDTA (90˚C) at 77 KClick here to View Figure

Figure 18: BJH pore size distribution of NiZnEDTA (90˚C).Click here to View Figure

Effect of degassing

NiZnEDTA, that underwent degassing at 80^˚C and 90^˚C for 12 hours, failed to demonstrate any modification of the molecular structure as the IR and PXRD patterns persist unchanged (Fig.19 and Fig.20). The sample’s surface texture altered from normal, increasing the BET surface area in the process. Thereby, it can be said that for the chosen compounds, the activation temperature only had a bearing on the surface textural characteristics (Fig. 21)¹⁰.

Figure 19: FT-IR spectra of NiZnEDTA (at 90˚C and 80˚C).Click here to View Figure

Figure 20: PXRD patterns of NiZnEDTA (at 80˚C and 90˚C).Click here to View Figure

Figure 21: SEM images of NiZnEDTA at different magnifications. Click here to View Figure

Table 1 gives a comparison of surface area, pore diameter and pore volume of the three gel grown coordination polymers.

Table 1: Surface parameters of the three gel grown coordination polymers

The findings imply that before doing the BET surface area study, the coordination polymers are to be properly thermally degassed. We ascertained that although the crystallinity of the coordination polymers was not significantly altered by the activation temperature, but the surface morphology and textural characteristics were greatly affected by deactivation.

Conclusion

Nitrogen adsorption studies of the coordination polymers were conducted by degassing the sample at different temperatures for 12 hours. The degassing temperature had a marked effect on the magnitude of specific surface area values of the gel grown coordination polymers. When CaADA is degassed at 150^˚C and 170˚C for a period of 12 hr, specific surface area enhanced from 8.27 m²/g to 13.59 m²/g. SrCaEDTA degassed at 80˚C and 90˚C for a period of 12 hrs, the specific surface area changed from 8.78 m²/g to 11.953 m²/g. Nitrogen adsorption studies of NiZnEDTA was conducted by degassing the sample at 80˚C and 90˚C for 12 hours. The specific surface area of the compound was found to be 21.876 m²/g and 25.970 m²/g respectively. The results indicate that samples should be degassed before the BET surface area analysis. Lower degassing temperature or a shorter duration of degassing, will cause incomplete cleaning of the sample surface. The degassing temperature is a crucial factor while comparing the specific surface area of a compound from different sources.

Acknowledgment

The authors are thankful to the authorities of Sophisticated Analytical Instrumental Facilities (SAIF), Cochin University of Science and Technology, for providing the instrumental facilities. We also thank the Head, Department of Chemistry, University of Kerala, for providing an advanced analytical facility for BET studies.

Conflict of 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.

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