Current Status of related R&D activityBenzoid compounds were introduced to a petrol for increasing of octane number instead of led compounds. But their toxicity and possible thermal destruction to carcinogenic polyaromatic products initiated research in development of new generation of petrol. This petrol should contain no led and benzoid components and posses high octane number. To solve this problem petrol should contain minimum of n-alkanes and maximum of iso-alkanes. Catalytic isomerization is used for this. Many disadvantages related to the use of liquid catalysts, namely H2SO4 or HF, mainly employed for catalytic alkylation have led to intensive research on solid catalysts. Fundamental expectations are connected with strong solid acids immobilised on inorganic carriers [1–4] in general. Titanium [5–6], binary and trinary metal oxides [7], zeolites [8] and mesoporous media [9] were tested as supports. Most researches were made on regular [10 – 13] and mesoporous [14] zirconium oxide, metal doped zirconia [8,15 – 17]. Sulphation of the support surface is used in most cases [18]. Other acids, like phosphoric [5], molibdo- and tungsto-phosphoric [19] were also studied as promoters. But most research was made on sulphated zirconia. According to proposed mechanism of short-chain alkan transformation the catalytic process is bifunctional and requires co-operation among neighbouring acidic groups in transfer of a hydron. Such co-operation is essential feature in the manifestation of superacidity. We demonstrated that on the surface of silica with covalently bonded groups despite to absence of long-distance order in the system of surface groups, the micro-order is present. It leads to formation of supramolecular structure organised from functional groups with similar orientated dipoles. Investigation of dielectric relaxation in such materials enables us to determine existence of such supramolecular structures by means of monitoring molecular mobility of dipoles depending on surface nature and particularly depending on the bonded group nature. Significant increase in dielectric losses from temperature higher then 248 Ê can be stipulated as melting of physically adsorbed water which is definitely present at silica gel. On surface of the hydrophobic silica two supramolecular structures generated by uno-dimentional physically adsorbed water and two-dimensional residual silanol groups co-exist. From this research can be sudgested that covalent immobilisation of functional groups will ensure higher stability of the catalysts and more freedom of acidic fragments for surface movements.
[1] Sommer J., Jost R., Hachoumy M. Catalysis
Today, 1997, 38(3), 309-319. |
