| Περίληψη: | A sustainable chemical industry cannot exist at scale without sustainable and renewable feedstocks. The utilization of minerals as “natural” catalysts for producing useful products through green processes, is very important for sustainable development in the frame of cyclic economy. Mordenite is a well-known zeolite material for its shape selectivity, making it a very important catalyst with several applications in the chemical industry.
In this Doctoral Thesis we have studied the natural mordenite TECHOSA-H2, which is originated from volcanic soils in Greek islands and has been activated by treatment with hydrochloric acid. The samples were characterized using various techniques (nitrogen physisorption at liquid nitrogen temperature, scanning electron microscopy – electron dispersive spectroscopy (SEM-EDS), transmittance electron microscopy (TEM), x-ray diffraction (XRD), thermogravimetric analysis (TGA), attenuated total reflection – furrier transform infrared (ATR-FTIR) and equilibrium pH measurements). Acid activation of the natural mordenite resulted to the emptying of the natural mordenite micropores by removal of the solid phases located inside them creating high surface area and surface acid sites without disturbing seriously the unique framework of the mineral.
Texture and acidity of the catalysts are crucial catalyst characteristics, which are expected to be affected by air calcination. The influence of air-calcination at 500oC for 2h on the physicochemical characteristics and catalytic performance of TECHNOSA-H2 was studied. It was found that air-calcination does not disturb the fibrous morphology of the material, but decreases drastically its acidity. The catalytic performance of the calcined materials was tested for the transformation of limonene to p-cymene. It was shown that the calcined materials are inferior to the non-calcined ones, proving the key role of acidity for the catalytic performance of mordenite catalysts for transformation of limonene to p-cymene.
In an effort to rationalize the high catalytic activity and selectivity of the acid-activated mordenite TECHNOSA-H2 for the transformation of limonene to p-cymene, we extended our study to natural montmorillonite which is a non-zeolite material. This investigation revealed that the acid treatment of montmorillonite leads also to an effective catalyst for the synthesis of p-cymene from limonene. The activation of natural montmorillonite by acid treatment, resulted to the removal of the sodium and calcium ions from the interlayer regions of the montmorillonite triple layers, without disturbing the structure of the triple layers themselves. Also, the method of acid treatment resulted in the increase of the specific surface area of the material from 62 to 155 m2g, increasing also considerably the acidity of natural montmorillonite. The acid activated montmorillonites can catalyze the transformation of the limonene into intermediate isomers and polymers. Indeed, limonene conversion to high added-value products greater than 90% was achieved at reaction temperature 100 °C and reaction time 20h over these catalysts. Comparing the catalytic behavior of the two catalyst materials originated from natural mordenite and natural montmorillonite, the beneficial role of the zeolite structure of the former on the p-cymene production was revealed.
Further investigation of the new applications of the TECHNOSA-H2 catalysts for the organic synthesis of high added value products, include the synthesis of two citral acetal derivatives named citral propylene glycol acetal (Citral PGA) and citral diethyl acetal (Citral DEA), which have outmost importance to the flavor and perfume industries due to their characteristic aroma and flavor. We proved that TECHNOSA-H2 can be used for the production of Citral PGA with yields of up to 50% in a reaction time equal to 20 minutes and a temperature of 60 oC. At the same time and reaction temperature, the production of 50% Citral DEA is achieved by the reaction of the citral with ethanol after mixing with TECHNOSA-H2. Since the synthesis of citral acetals typically involves homogeneous corrosive reagents, the substitution of these reagents for the TECHNOSA-H2 catalyst is a sustainable improvement to previously reported methods.
Reactions with terpenes that attract a lot of interest are the conversion of a-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. These two reactions draw a lot of attention not only because of the industrial importance of the products formed, but also because they are the two most frequently used catalytic test reactions for differentiating Brønsted and Lewis acidity in catalysts. The successful isomerization of a-pinene oxide over TECHNOSA-H2 for the one-step synthesis of campholenic aldehyde in yields up to 57% at 10min and room temperature was proved, as well as the one-step cyclization of citronellal over TECHNOSA-H2 to isopulegol in yields up to 57% at 140 oC and reaction time equal to 10min. Considering the product distribution in the two probe reactions studied and the available literature, we can conclude that the active sites on TECHNOSA-H2 are a combination of Lewis and Brønsted acid sites.
Furthermore, shape selectivity is a very important property of a zeolite catalyst. In order to test the catalytic action of the acid-activated mordenite TECHNOSA-H2 in reactions that reveal shape selectivity properties between stereoisomers, we applied TECHNOSA-H2 over the E- and Z- isomers of citral. It was shown that TECHNOSA-H2 is an effective catalyst for the cyclization of neral i.e. the E-isomer of citral to afford p-cymene, dehydro p-cymene and terpinolene in 60min at 130 oC. However, when TECHNOSA-H2 is applied, geranial i.e. the Z-isomer of citral, remains intact. This proves the shape selectivity of the TECHNOSA-H2 catalyst over the E- and Z- isomers of citral. Considering these results, TECHNOSA-H2 would be potentially a valuable catalyst for applications where the selectivity of the catalyst over only specific isomeric forms of a molecule is required.
Finally, in the present doctoral thesis, we studied the potential of employing the natural mordenite catalyst TECHNOSA-H2 for the synthesis of ethers from alcohols. For this investigation we studied the synthesis of symmetric and non-symmetric ethers utilizing furfuryl alcohol as the staring material. In addition, we studied the synthesis of cyclic ethers from diols with two tertiary hydroxyl groups, as this synthesis has been a challenge so far. For this investigation, we utilize the diol sclareol as the starting material.
Both symmetric and non-symmetric ether derivatives of furfuryl alcohol can be synthesized using TECHNOSA-H2 catalyst. More specifically, the synthesis of difurfuryl ether and difuryl methane was achieved in yields 30% and 11%, respectively, by the conversion of furfuryl alcohol over TECHNOSA-H2 at 130 oC in 3 hours. In the presence of the same catalyst, using furfuryl alcohol and ethanol as reactants, a yield of furfuryl ethyl ether of 60% in 3 hours and 150 °C, without high polymer production can be achieved.
With regards to the synthesis of cyclic ethers, the water elimination of sclareol is accelerated over the strong acid catalyst TECHNOSA-H2. However, in absence of a solvent or in the presence of an inappropriate solvent this elimination is random leading in variety of dehydration products. The mechanism of the catalytic dehydration reaction of sclareol was investigated and it was shown that sclareol can be dehydrated leading to either the formation of a cyclic ether which is a mixture of Manoyl oxide C13 epimers or the formation of several alcohols.
To control the product distribution towards the desired compound, a solvent-driven selectivity of the natural mordenite acid catalyst is proposed. The combination of TECHNOSA-H2 catalyst with a glyme type solvent increases the yield of the cyclic ether manoyl oxide. The key to success for this controllable system is the use of the solvent polarity as an effective strategy to regulate the linkage between sclareol and the acid catalyst and modify the inter-molecular distances. In contrary with prior art on the synthesis of manoyl oxide where long reaction times, multistep processes or toxic reagents are involved, our new “TECHNOSA-H2–Glyme” system provides a sustainable alternative to the already mentioned processes. This new system offers high specificity substrate dehydration in a one-step process, where short reaction times (t=10min), low temperatures (T=135 oC) and atmospheric pressure conditions are only involved, resulting to yields in manoyl oxide of up to 90%. Our work provides a step forward towards the sustainable synthesis of 13R-Manoyl oxide as a precursor of ambrox and a promising option for application in the synthesis of forskolin where the total biosynthetic pathway for forskolin synthesis can be replaced by a process which combines a chemical catalytic step for the production of 13R-manoyl oxide, followed by a biosynthetic pathway for the synthesis of forskolin from 13R- manoyl oxide.
Based on the findings of the current doctoral thesis, it is proven that TECHNOSA-H2 can be a very promising catalyst for various applications in organic synthesis, while some of these applications include isomerization and etherification reactions, as well as reactions where shape selectivity between reactants is desired. Finally, the combination of TECHNOSA-H2 with a suitable solvent can affect the composition of the reaction product mixture.
|
|---|
