WO2021133301A1

WO2021133301A1 - Antimicrobial zeolitic materials with increased thermal resistance, having a hierarchical structure, comprising essential oils

Info

Publication number: WO2021133301A1
Application number: PCT/TR2020/051100
Authority: WO
Inventors: Burcu Akata Kurc; Melda ISLER BINAY
Original assignee: Orta Dogu Teknik Universitesi
Priority date: 2019-12-25
Filing date: 2020-11-13
Publication date: 2021-07-01

Abstract

The invention, is related to environmentally friendly materials that have been formed by increasing the antimicrobial effectiveness, antimicrobial efficiency and thermal resistance thereof, by encapsulating thymol and other similar organic molecules that are antimicrobial essential oils, that are not limited with a pore size, and that have been given a hierarchical structure by expanding their microporosities, into zeotype materials having a silicate structure with a low Si/Al ratio varying between 1-3 or titanium chain.

Description

ANTIMICROBIAL ZEOLITIC MATERIALS WITH INCREASED THERMAL RESISTANCE, HAVING A HIERARCHICAL STRUCTURE, COMPRISING

ESSENTIAL OILS

Technical Field

The invention is related to environmentally friendly materials whose antimicrobial effectiveness, antimicrobial efficiency and thermal resistance has been increased, by encapsulating the antimicrobial essential oils, that are not limited with a pore size, and that have been given a hierarchical structure by expanding their microporosities, into zeotype materials having a silicate structure with a low Si/Al ratio varying between 1-3 or titanium chain.

Known State of the Art (Prior Art)

Antimicrobial essential oils such as thymol, are organic molecules that have low physical and thermal resistance. In the prior art, following the process of encapsulating these essential oils into porous structures, it is known that their thermal and physical resistances increase. However, it is expected that the pore size range required for placement thereto, needs to be similar to or larger than the size of these molecules. As a result of the adsorption and other similar processes carried out on zeolite and zeotype materials that have not been pretreated in any way, essential oil molecules do not show their main effects such as being antimicrobial. Therefore, the antimicrobial efficiency and antimicrobial effectiveness time of the materials that have been prepared by this process are not sufficient.

As organic molecules such as essential oils, that is larger than the micro-porous structure of zeolite, they cannot be encapsulated inside the pores of a structure such as zeolite. Although, studies have been conducted that combine these molecules with zeolitic materials, none of the studies prove that these molecules are encapsulated in said pores. Therefore, as encapsulation cannot be carried out, the volatility of the organic molecules cannot be improved.

Inorganic elements such as silver, zinc and copper have been used as an antimicrobial additive in various sectors, for a long time, due to their antimicrobial efficiencies. However, in recent years, the usage of antimicrobial inorganic metal elements that are known to have toxic effects, such as silver have been prohibited by regulations and as a result, the usage of organic alternative antibacterial agents such as essential oils, that are extracted from plants have become prominent.

The main problem of using essential oils as antimicrobial agents is that these oils have unfavorable physical properties, such as low thermal resistance.

The methods of the prior art have first of all be tried on porous materials that do not have a hierarchical structure. Later on, in studies with similar aims, the organic molecules have been dissolved in a solvent medium and said molecules were then attached to a host material [EP2170043 A2]. As a result of using the encapsulation method by performing a mixing process in liquid phase in these and other similar studies, the rate of encapsulating the organic molecules into the pore was found to be very low.

Brief Description of the Invention and its Aims

The present invention is related to environmentally friendly materials that have been formed by increasing the antimicrobial effectiveness, antimicrobial efficiency and thermal resistances thereof by encapsulating thymol and other similar organic molecules that are antimicrobial essential oils, that are not limited with a pore size, and that have been given a hierarchical structure by expanding their porosities, into zeotype materials having a silicate structure with a low Si/Al ratio varying between 1-3 or titanium chain, that meet all of the requirements mentioned above, that eliminate all of the disadvantages and that bring about new advantages.

The primary aim of the invention is to ensure the development of a novel antimicrobial material that is more environmentally friendly and not harmful to health by encapsulating into a protective porous structure, natural and antimicrobial molecules that usually cannot be used in applications as they are difficult to process due to their thermal and volatility properties.

The materials developed with the invention are zeolitic materials having a hierarchical structure that are formed by being subjected different post-synthesis processes in accordance with the type of zeolitic materials. Essential organic molecules can be encapsulated into these hierarchical structured zeolitic materials.

Zeolitic materials that have been encapsulated with essential organic molecules having a hierarchical structure, exhibit higher thermal resistance and more efficient antimicrobial properties in comparison to present materials. It is possible to prevent unfavorable physical properties, such as low thermal resistance which is the main problem in the usage of essential oils as antimicrobial agents, by encapsulating said organic materials into a hierarchical zeolitic structure.

Another aim of the invention is to provide the development of a natural alternative material that is environmentally friendly, instead of the materials that have been prepared with toxic inorganic elements, in order to provide antimicrobial efficiency.

With the invention, unlike the conventional zeolite-organic molecule mixing methods used in the prior art, for the first time, essential oil molecules with antimicrobial properties were encapsulated into zeolitic materials that still preserved their lattice structure although their pore volume was expanded. Therefore, they exhibit the following in comparison to similar porous structures that do not have a hierarchical structure, where the organic molecule is used on its own;

• Better antimicrobial efficiency,

• Higher thermal resistance and

• Longer antimicrobial effectiveness.

With the invention, the essential oil molecules, which exhibit antimicrobial properties have been encapsulated in a hierarchical zeolitic material, thus increasing their thermal resistance, for the first time, using the gas absorption technique. As a result, the use of organic volatile molecules as antimicrobial additives, which could not be utilized in applications due to difficulties in processing, has been made possible in various sectors.

Similar studies in the prior art comprise the step of removing the solvent from the solid-powder product. This step is formed of centrifuge and filtering stages. The encapsulation of the organic molecules into the host material has been carried out by means of the gas absorption technique used in the invention, without having to use the solvent or the process step and equipments that are required to remove the solvent. The application of the method is much easier, and it is cost effective.

Definition of the Figures of the Invention

The figures that have been prepared in order further describe the production method and the advantages of the antimicrobial zeolitic materials having a hierarchical structure comprising essential oils developed by this invention, have been defined below.

Figure 1: Production flow diagram of the hierarchical structured antimicrobial zeolitic materials comprising essential oils whose thermal resistance has been increased.

Figure 2: UV-Vis spectra of the hierarchical zeolitic material that includes thymol molecules (13X-H-thy) that have been placed therein by means of the gas absorption technique and by mixing with a fluid.

Figure 3: UV-Vis spectra of the hierarchical zeolitic material (30-ETS-thy), and the zeolitic material (ETS-thy) having thymol molecules placed therein.

Figure 4: Thermal resistance graphic of 13X-thy and 13X-H-thy.

Figure 5: Time dependent thymol release of ETS-thy and 30-ETS-thy.

Definitions of the parts/aspects/sections forming the invention

The parts and sections provided in the figures that have been prepared in order to further describe the production method of the hierarchical structured antimicrobial zeolitic materials comprising essential oils that have been developed by this invention, have each been numbered and the description of each reference number has been given below.

1. Hydrothermal zeolite 13X synthesis

2. Hydrothermal ETS- 10 synthesis

3. Expansion of the pores of the Hydrothermal zeolite 13X and giving it a hierarchical structure

4. Expansion of the pores of the Hydrothermal ETS- 10 and giving it a hierarchical structure

5. Encapsulating thymol and other similar molecules that are essential oils into hierarchical zeolitic materials.

Detailed Description of the Invention

The invention is the production method of an antimicrobial material production, which is provided by encapsulating at least one antibacterial essential oil molecule in the zeolitic material having a hierarchical porous structure, by means of a gas absorption technique in closed medium conditions, such that the dominant molecule is an essential oil molecule.

Figure 1 shows the production flow diagram of the hierarchical structured antimicrobial zeolitic materials comprising essential oils whose thermal resistance have been increased. The first stage of the production of the invention is the synthesis of the porous material that is to be used in the invention. The porous material used in the invention, are two types, being zeolites containing aluminum (Zeolite 13X) and zeotypes containing titanium (ETS-10).

One of these porous materials, which are zeolite 13X, is one of the most commonly used zeolites in the industry. Alumina silicate zeolite 13X synthesis is produced by following the well-known methods in the art. Following the aging process carried out by mixing, the mixture is reacted in an incubator at a predetermined temperature, which is 100°C. The reaction products, are separated from the liquid phase via the centrifuge method and the obtained crystals, are washed again with the centrifuge method using distilled water and are kept overnight in an incubator at a temperature range of 60-80 °C.

Although titanosilicate ETS-10, has a smooth crystal structure, it still has faults in its structure. These faults that arise following synthesis, enables ETS-10 to easily transform into a hierarchical structure. Due to this reason, the second porous material that used in the invention has been determined as zeotype ETS-10. In this invention, the ETS-10 crystals are synthesized hydro thermally. The zeotype ETS-10 synthesis method known from the art is followed and after the required chemicals are mixed at the desired ratios, the mixture is subjected to reaction at a predetermined temperature of 230°C in an incubator. The reaction products, are separated from the liquid phase via the centrifuge method and the obtained crystals, are washed again with the centrifuge method using distilled water and are kept overnight in an incubator at a temperature range of 60-80 °C.

The second stage of the production of the invention is the expansion of the pores of the zeolitic material and therefore gaining a hierarchical structure. Zeolites that have a plurality of pore types in its structures are called hierarchical zeolites. Hierarchical zeolites, can be obtained through chemical procedures, following zeolite synthesis. By means of the final product obtained, besides increasing the inner crystal diffusion of partially larger molecules, the release of the diffusing molecule is also more controlled. In this invention the surface areas and pore volumes of the zeolite 13X and ETS-10 crystals that are synthesized, have been increased by subjecting said crystals to post-synthesis processes using different methods.

The washing of zeolite 13X in acid and base, are highly efficient methods that are commonly used in the prior art, which enable mesopores to form. One of the methods of providing hierarchical structure and expanding the pores of hydrothermal zeolite 13X is washing the synthesized zeolite 13X successively with aqueous KC1, NH₄CI and finally with Na₂H₂EDTA or NaOH solutions, at suitable temperatures, by mixing. In the invention, the same method has been followed and the zeolite 13X has been mixed and washed respectively with aqueous KC1 solution at 25-100 C for 30-180 minutes, following this it has been washed again for 30-180 minutes with aqueous NH4C1 solution, and in the final stage, it has been washed by mixing at 50-100°C for 1-6 hours with Na₂H₂EDTA or NaOH solution. After every process, the crystals are separated from the liquid phase via the centrifuge method again, and they are washed using distilled water and are kept overnight in an incubator at a temperature range of 60-80°C. The zeolite 13X that has not been subjected to any process is called “13X” and the zeolite 13X crystals that have been subjected to post-synthesis processes are called “13X-H”.

In order to expand the micropores of the synthesized hydrothermal ETS-10 and to give it a hierarchical structure, a different method from zeolite 13X has been used. It is known from the prior art that washing with aqueous hydrogen peroxide, breaks the titanium oxide chains within the ETS-10 crystals and it expands the pore volume. In the invention, hydrogen peroxide aqueous solutions that are 5-40% by mass are also prepared in the invention and some of the ETS-10 crystals that are produced in the initial stage are mixed at 5-40 minute time intervals in the prepared 5-40% hydrogen peroxide solution. The crystals that are obtained following the post-synthesis process, are again separated from the liquid phase via the centrifuge method and they are washed with ultra pure water and are kept overnight in an incubator at a temperature range of 60-80 °C. At the end of this step, ETS-10 crystals that have three different pore areas are obtained via the synthesized ETS-10 (i.e. ETS) and the ETS-10 crystals that have been obtained with the post-synthesis method. The ETS-10 crystals that are mixed for 10 minutes with 10% hydrogen peroxide aqueous solution are called “10- ETS” and the ETS-10 crystals that are mixed for 30 minutes with 30% hydrogen peroxide aqueous solution are called “30-ETS”.

In order to understand the effect of the process of gaining hierarchical structure to the structure of the pore, a BET adsorption- desorption analysis has been carried out. In Table 1, the BET analysis result is shown. As it can be seen in Table 1, the increasing of the chemical procedures has also increased the surface area, incrementally from 328,8 m 2 /g to 353,2 m 2 /g and then to 381,5 m /g (Table 1). The aqueous 10% hydrogen peroxide solution broke the titanium oxide chains and it has led to an increase in the micropore area, and it has also reduced the trace amount of mesopore area that should not be available in the structure that is formed as a result of the faults that have occurred during ETS synthesis. Additionally, the aqueous 30% hydrogen peroxide solution, has both increased the micro and the mesopore areas. The pore volume of the formed hierarchical ETS- 10 crystals has increased with direct proportion to the hydrogen peroxide solution density. The BET adsorption-desorption analysis method proved both the micropore and the mesopore formation via the post-synthesis method (Table 1).

Table 1: The surface areas and pore volumes prior to thymol loading of zeolitic materials (ETS) and hierarchical zeolitic materials (10-ETS and 30-ETS).

^a Has been measured with multipoint BET analysis. ^b Has been measured with SF method At the final stage of the production of the invention thymol and other similar molecules that are essential oils are encapsulated into hierarchical zeolitic materials that have different porous structures. In the invention carvacrol and other similar oils can be used as essential oils instead of thymol. The encapsulation process has been carried out with the gas absorption technique. This method is applied by keeping both the thymol molecules that are volatile and the prepared porous materials in a dessicator for two weeks. Thymol encapsulated ETS- 10 crystals have been named “ETS-thy”, hierarchical ETS- 10 crystals have been named “30-ETS- thy” and “10-ETS-thy”, zeolite 13X crystals have been named “13X-thy” and hierarchical zeolite 13X crystals have been named “13X-H-thy”. After the thymol was encapsulated into the hierarchically structured zeolitic material by means of the gas absorption technique or the mixing technique in the liquid medium in the desiccator, the amount of thymol in the produced material was measured with the UV-Vis device. It has been observed that the density of the characteristic thymol peak observed in the UV-Vis spectrum was much higher with the gas absorption technique when compared with the mixing technique in liquid medium. This analysis shows that more thymol can be encapsulated by means of the gas absorption technique (Figure 2). When thymol molecules are encapsulated zeolitic materials (ETS-thy) that do not have a hierarchical structure, in the gas phase, lower thymol absorption amount was determined by the UV-Vis device when compared to encapsulation of thymol into the hierarchically structured zeolitic materials (30-ETS-thy) in gas phase. In other words, the thymol absorption rate is increased by creating a hierarchical structure in zeolitic materials, and as a result an increase in the antimicrobial efficiency of the material is obtained (Figure 3).

An antimicrobial test has been applied with the disc diffusion method against Escherichia coli (E.coli), which is one of the most common pathogenic gram-negative bacteria, to zeolitic materials that are not hierarchical (ETS-thy) and to hierarchical zeolitic materials (30-ETS- thy) that have been loaded with thymol by means of the gas absorption technique. It has been observed that hierarchical zeolitic material (30-ETS-thy) causes more zone inhibition than zeolitic materials (ETS-thy) that are not hierarchical, in other words, it can be said that it provides higher antimicrobial efficiency (Table 2).

Table 2: The results of the antimicrobial zone inhibition test of the zeolytic material (ETS), hierarchical zeolitic material (30-ETS), thymol encapsulated zeolytic material (ETS-thy) and thymol encapsulated hierarchical zeolitic material (30-ETS thy).

The thermal resistance graph of zeolitic materials encapsulated essential oil molecules exhibiting antimicrobial properties placed via the gas absorption technique, therein has been formed. With the Thermogravimetric Analysis (TGA), it was observed that thymol molecules initially melted and then evaporated after 100 ⁰ C and that they left the medium rapidly and did not remain in the medium at all, at 170 ⁰ C. Although the non-hierarchical zeolitic material (13X-thy) lost 37% of its thymol molecules at 100 °C, that were present in its structure, it can still carry thymol molecules in its structure at even 200 °C. However hierarchical zeolitic materials (13X -H-thy) maintained 93% of its thymol molecules at 150 °C and it managed to maintain some of the thymol molecules in its structure at even 300 °C. This result shows that hierarchical zeolitic materials exhibited better thermal resistance in comparison to non-hierarchical zeolitic materials (Figure 4.)

Time dependent thymol release has been tested in both hierarchical and non-hierarchical zeolitic materials. In both samples that encapsulated thymol, it has been observed that time dependent thymol release was present. However while the hierarchical zeolitic materials (30- ETS-thy) lost 25% of the thymol in their structure at the end of the 25 hours, the non- hierarchical zeolitic materials (30-ETS-thy) lost 60% of their thymol at the end of the 25 hours. With this result it has been noted that thymol release was more controlled in the hierarchical zeolitic materials (30-ETS-thy) in comparison to the non-hierarchical zeolitic materials (ETS- thy). (Figure 5).

Claims

1. Antimicrobial material production method, characterized in that, at least one antibacterial essential oil molecule is encapsulated inside a zeolitic material having a hierarchical porous structure, by means of a gas absorption technique in closed medium conditions, such that the dominant molecule is an essential oil molecule.

2. Antimicrobial material characterized in that it is obtained by the method of claim 1.

3. Antimicrobial material according to claim 2, characterized in that the zeolitic material, is a zeotype having a silicate structure with a low Si/Al ratio varying between 1-3 or titanium chain.

4. Antimicrobial material according to claim 2, characterized in that the essential oil is thymol or carvacrol.

5. Antimicrobial material according to claim 3, characterized in that the zeolitic material is Zeolite 13X orETS-10.