Reduction of the toxicity of tobacco smoke with catalytic materials: Study of the influence of SBA-15 synthesis variables, time and temperatura of the hydrothermal treatment.

Effect of Temperature and Time of the Hydrothermal Treatment in the SBA-15 Synthesis Process on the Structure and Textural Properties and the Ability to Reduce the Evolution of Tars in Tobacco Smoking" of and of order to determine the of of The of is not significative but a a not frequently the aparent the most relevant with the on smoking experimentos. the effect of reduce the supernatant liquor also has been and the results have shown that the material properties remain practically unchanged.


Introduction
The main objective of the present work is to modify the time and temperature of the hydrothermal treatment during the synthesis of the SBA-15 for studying the influence of the corresponding properties developed on the emission of toxic compounds when smoking their mixtures with 3R4F reference tobacco. Nevertheless, we have not found commercial availability of SBA-15 synthesized modifying these variables and with different textural properties. Consequently, if such a study is to be developed, the corresponding samples must be "ad hoc" synthetised and characterized to relate the properties with the effect on tobacco smoke. On another hand, the hydrothermal treatment is usually carried out in autoclaves or tapered vessels. This fact may involve high cost of the equipment required for industrial applications, since large volume reactors capable of standing the developed pressures would be used. Thus, we have also considered the effect of removing the supernatant liquid before the hydrothermal treatment in order to explore the possibility of reducing the volume of the reactors, both objectives contributing to optimize and simplify the SBA-15 synthesis conditions.

Definition
During the last decades, great efforts have been made to reduce the consumption of tobacco, however, due to the great difficulty of completely eradicating tobacco consumption, different approaches to reduce the number of toxic compounds in tobacco smoke have attracted great attention. In this sense, the use SBA-15 has been studied as a tobacco additive yielding high reductions of toxic compounds. Generally, its synthesis process involves 4 principal steps, i.e.: pre-treatment, hydrothermal treatment, drying and calcination. Lots of papers can be found on the effect of the synthesis conditions on the properties of the material, mainly during the first and second steps (i.e.: ). The textural properties of the catalysts are likely to affect its behaviour un every particular application. Nevertheless, no studies from other research groups have been found on the influence of the different properties developed by the material on its effect in modifying the chemistry and reducing the presence of toxicant compounds in tobacco smoke.
This article is associated with the MDPI "Catalysts" journal, which belongs to a Special Issue "SBA-15 and Catalysis", Materials obtained have been characterized by 77 K N isotherm, XRD, SEM and apparent density. Figures 1, 2 and 3 and Table 1  It can be concluded that the synthesized materials have developed typical properties of SBA-15 materials and the evolution of these characteristics is in accordance with the properties described by numerous authors.

Smoking experiment results
Mixtures were prepared by thoroughly mixing manually the tobacco (3R4F reference tobacco from Kentucky University) with around 4.8 weight % catalyst. Gas and condensed fractions were analysed. TPM, nicotine, CO and CO results are shown in Tables 2 and 3. Figures 3 and 4 show the reductions obtained in the different families of compounds considered in the gas and TPM fractions. The effect of removing part of the reaction medium is very small, even certain improvement is observed. This low effect also had been observed in the textural properties developed by the catalysts. According to this, the most of the mother liquor can be eliminated during the hydrothermal treatment obtaining similar results, which is a very important feature for reducing the cost of the equipment and simplifying the process.
On the other hand, increasing the temperature results in a decrease in the reduction ability of nicotine and TPM, while carbon oxides show a maximum at 100 ºC. Although the sample synthesized at 130 ºC presented slightly lower results in gases and similar in TPM, it is possible to affirm that all samples show similar and very large reduction capacity for most compounds.
For the compounds in gas fraction, no clear trends are observed but all the catalysts obtained are very effective in reducing tobacco smoke compounds. In fact, most families show reductions around 70 % in TPM compounds, carbonyls and phenolic presenting higher values.
A clear growing trend in the reductions is observed, both in the gas and TPM fractions, when increasing the time of hydrothermal treatment. However, the results obtained at 15 h of hydrothermal treatment are very similar to, or even better than, those obtained at 24 h tH. These results can be relevant when considering a large-scale production process.

Conclusions
SBA-15 synthesized in this study show good behaviour as catalysts for reducing the toxicity of tobacco smoke.
Furthermore, reducing the amount of mother liquor during the hydrothermal treatment leads to very good results. This would allow the use of smaller volume equipment which is especially relevant when considering that such equipment must stand the pressure generated in the process. On the other hand, good results have also been obtained for the proposed application at all temperatures studied, therefore, the lowest of these (80 ºC) could be chosen for large-scale manufacturing. Both issues could have important implications for reducing the manufacturing costs of these materials in large-scale processes. It has been observed that, the apparent density of these powder materials is a very relevant property in the application we are studying since it may be related, in addition to the morphology of the particles, to the way the catalyst particles contact the tobacco strands.
Finally, one aspect that we would like to highlight is that the use of these materials may help in matching the recent suggestion of the FDA to reduce nicotine levels in cigarettes to 0.5 mg/g of tobacco filler, in order to reach the objective of minimizing levels of addiction.