Butenko Eleonora, Kapustin Alexey

Azov State Technical University, Mariupol, Ukraine

Changes in the structure of layered double hydroxides during the adsorption of organic compounds

Keywords: Mg_xAl_y(OH)_z layered double hydroxides, polycrystal X-ray diffraction, specific surface area.

Layered double hydroxides (LDHs) are used as sorbents, catalysts, and carriers for catalytic species. In general, they are natural or synthetic minerals with part of their cations being replaced isomorphically with cations of higher oxidation states, which results in the formation of positively charged two-dimensional layers containing solvate molecules and counterions capable of anion exchange [1]. LDHs are solid bases [2]. Since there are both Brønsted and Lewis sites, anion exchange processes occur in the interlayer internal space of LDHs [3]. The interlayer distance depends on the nature of anions and solvate molecules as well as the degree of isomorphic substitution. When the interlayer distance changes, LDH properties also change (for example, sorption capacity changes because of the changing number of available active sites). A study of the LDH structure and its changes during the sorption of various compounds provides conclusions about the mechanism of these processes and prospects of creating new materials with desired properties.

This work reveals regular changes in the structure of synthetic Mg_xAl_y(OH)_z hydrotalcites of various composition, which occur during the adsorption of amyl alcohol, naphthalene, and b-naphthol. The hydrotalcite structure is shown in Fig. 1.

Fig. 2. – Structure of LDHs

Experimental. Mg_xAl_y(OH)_z samples were prepared according to the procedures described in [4,5].

X-ray diffraction was performed on Siemens D-500 (СоK_a1-radiation, l = 1.789 Å) and DRON-UM1 diffractometers (СuK_a-radiation, Ni-filter).

The specific surface area of the samples was determined from the low-temperature nitrogen adsorption measurements followed by the processing of the obtained data using the BET method.

Results and Discussion. A study of the sorption of С₅Н₁₁ОН amyl alcohol on Mg_xAl_y(OH)_z samples has shown that the sorption process consists of two stages. At the beginning, apparent dynamic equilibrium is established rapidly (in tens of minutes). However, on long keeping (48 hours) the degree of sorption increases significantly, and, what is more important, it is accompanied by structural transformations of the samples. Let us divide them according to their duration and nominally call them dynamic and static periods.

As table 1 shows, at the first stage of the sorption there are no structural changes in the samples.

Sorption capacities (C) and interlayer distances (d) in Mg_xAl_y(OH)_z during the sorption of amyl alcohol at dynamic and static periods

Mg/(Mg+Al)	0,52	0,72	0,81	0,86
Cdyn., meq./g	0,075	0,081	0,041	0,036
Cstat.., meq./g	0,38	0,41	0,10	0,08
dorig., Å	3,038	3,036	3,045	3,058
ddyn., Å	3,038	3,038	3,042	3,060
dstat., Å	8,515	8,477	8,644	8,832

However, after the second stage the interlayer distance increases by about 2.8 times. So, at the initial moment of the penetration of amyl alcohol molecules they seem to be adsorbed physically; they are parallel to the main plains, thus, blocking the neighboring active sites. At the second stage, due to the chemisorption (ionic exchange processes) the intercalated molecules are reoriented, and they expand the hydrotalcite inner space. This property is typical of many organic compounds, and the extent of changes in the LDH interlayer distance depends on the length of the hydrocarbon radical, its configuration, and orientation in the interplanar space [6].

In order to study the effect of the structure and size of the adsorbed molecules on the structural characteristics of LDHs, aromatic compounds were used: С₁₀Н₈ naphthalene and С₁₀Н₇ОН b-naphthol. The main difference between them is the nature of the sorption. Physical adsorption is typical of naphthalene, while in the case of b-naphthol, the sorption causes the anion exchange reaction due to its active hydroxyl group. In the first case, the activity and selectivity of the sorption is determined by the sorbent specific surface area and pore sizes, while in the second case, by the concentration of active sites on the sorbent surface and their accessibility to the substrate molecules.

Fig. 2 shows typical X-ray patterns of starting hydrotalcite and the samples with sorbed naphthalene and b-naphthol.

If we compare these X–ray patterns, it is evident that when both naphthalene and b-naphthol are sorbed, the hydrotalcite structural parameters change, as it could be seen in the highlighted inset. For b-naphthol there are larger changes in the structural parameters, which is not surprising because of its possible chemisorption. Changes that are caused by the physical adsorption of naphthalene are likely to be attributed to a large size of its molecules.

Along with the changes in the crystal structure parameters during the sorption of naphthalene and b-naphthol, there are also changes in the microstructure of the samples. Namely, their specific surface area changes. As Table 2 shows, though this value has a non-monotonic dependence on the composition, it generally increases as aluminum isomorphically substitutes for magnesium.

Table 2

Mg_xAl_y(OH)_z specific surface area

Mg/(Mg+Al)	Specific surface area of samples, m^2/g
	original	with b-naphthol	with naphthalene
0,52	162,4	155,7	156
0,72	14,4	14,4	14,4
0,81	9,2	12	19,59
0,86	22	28	34,7

On the contrary, after the sorption of naphthalene and b-naphthol the most evident relative changes in the Mg_xAl_y(OH)_z specific surface areas occur in the samples with a low concentration of aluminum.

Conclusions. It is shown that there is a difference in changes in the structure of LDHs during the sorption of organic compounds with regard to the ionic exchange and physical sorption. Moreover, it is established that the structure of the organic molecule depends on changes in the size of the interlayer distance in LDHs.

REFERENCES

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