Student Number : 9502913H - PhD thesis - School of Chemistry - Faculty of Science / Alkyl-transfer (transalkylation, disproportionation) reactions of alkylaromatics were studied for the purpose of finding out the principles that governs them. Alkyl-transfer of simpler alkylaromatics ranging from mono to polyalkyl-benzenes and alkylnaphthalenes were studied in a fixed bed reactor system on solid acid catalysts (zeolites) at temperatures up to 400 °C. Results showed that alkyl-transfer reactions are reversible reactions with disproportionation favoured at lower temperatures while transalkylation seemed to be dominant at higher temperatures. The outlined mechanism showed that the catalyst pore sizes and the type of pores as well as the feed composition of binary mixtures play important roles in the transfer of alkyl groups between aromatic molecules. In alkyl-transfer reactions, the ease of conversion depends on the number of alkyl groups on the aromatic ring/s, the chain length, the type of alkyl substituent/s and the ring conjugation of the aromatic moiety. Zeolitic catalysts are rapidly deactivated by carbonaceous material deposition during alkyl-transfer reactions especially at higher temperatures while deactivation through molecular retention is dominant at lower temperatures. Nevertheless, zeolites can be regenerated by high temperatures in oxidizing atmospheres. Bulkier alkylaromatics (those found in coal and petroleum liquids) can be transformed through alkyl-transfer reactions if a suitable catalyst with the required strength and appropriate pore sizes can be developed, preferably a tri-dimensional arrangement as shown by the results of this study. Thus the alkyl-transfer process has promising future applications in petrochemical and related industries; especially those interested in the transformation of coal to chemicals.

Th. Univ.--Sci pétrolières--Univ. P. et M. Curie, 1991. / 1992 d'après la déclaration de dépôt légal. Résumé en anglais. Bibliogr. p. 158-163.

Benzene, toluene, and xylene are three basic raw materials for the production of most aromatic derivatives such as polyesters, plastics and detergents. Xylenes (p-, m- and o-) have the greatest market demand with an increasing annual rate of 6%. Owing to the availability of surplus toluene and low value of C9 aromatics, the transalkylation process is aimed at the production of more valued products, such as xylenes. Catalyst deactivation is a key challenge in transalkylation process. Using industrially relevant operating parameters, the transalkylation of 1,2,4-trimethylbenzene (TMB) with toluene was studied. The effect of zeolite structure and acidity, increased reaction pressure and temperature, and very low levels of platinum (Pt) impregnation has been investigated over both H-form and Pt-loaded zeolites: Beta, Mordenite (MOR), and Y. A fixed bed reactor was used at WHSV of 5 h-1, 400 oC, and a 50:50 wt. % toluene:TMB ratio with the order of activity after 50 h time-on-stream (TOS) of Y > Beta >> MOR at 1 bar. At elevated pressure (10 bar), all catalysts showed better performance with significant improvement in MOR as pore blockage was reduced and the order of activity was Beta > MOR > Y. With varying the Si/Al ratio for zeolites Beta (Si/Al = 12.5, 75 and 150) and Y (Si/Al = 2.6, 6, 15 and 30), the highest stability and xylenes yield were achieved over zeolite Beta with lowest Si/Al ratio at 41 wt. % conversion and 25 wt. % xylenes yield. In contrast, zeolites Y with Si/Al ratio of 2.6 showed the highest deactivation rate, whereas over Y zeolites with Si/Al = 6-30, the conversion was between 25-30 wt. % and xylenes yield around 11 wt. % after 50 h TOS. Incorporation of Pt (0.08 wt. %) further improved the activity of all catalysts with the highest conversion after 50 h TOS over Beta (62 wt. %) where Beta and MOR yielded similar levels of xylenes (40 wt. %). All catalysts were further optimized by reducing Pt levels whilst maintaining the desired stability and highest xylenes yield. In order to further develop a cost-effective and eco-friendly catalyst, the addition of alumina binder to Pt-Beta and the possibility of simplified regeneration of Beta/Pt-Beta catalyst were investigated. Firstly, the alumina binder reduced the conversion and xylenes yield, however, this reduction was small up to 40 wt. % added alumina binder (where xylenes yield only dropped to 35 wt. %). Secondly, the regeneration process was carried out using H2 only and up to four cycles (30 h TOS per cycle). The Pt-Beta catalyst found to be stable and the activity was fully restored by a hydrogenation process at 500 oC. However, the activity of Beta dropped gradually after each cycle suggesting that the H2 alone at 500 oC was insufficient in removing coke precursors. The drop in activity was attributed to the disappearance of Brà ̧nsted acid sites over the spent Beta catalyst due to the growth of coke molecules trapped in cavities leading to highly polyaromatic molecules blocking those active sites.

Zeolite structure can profoundly promote the activity of supported platinum. In addition, catalytic performances of Pt/ZSM-12 catalysts vary dramatically with platinum deposition procedure, namely ion exchange (IE) and impregnation procedure (IMP). Supported platinum prepared by IMP is more active than the Pt prepared by IE. The MCP/MCH ratio in benzene hydrogenation as an indication of bifunctional catalysis is significantly higher for IE Pt than IMP Pt. IE preparing platinum is located inside ZSM-12 pore and IMP preparing platinum is deposited on the external surface of ZSM-12. After steam treatment, it is found that Pt-atom perfectly migrates from internal channel to external surface and agglomerates into larger particle size for Pt(IE,0.100%,c) and Pt(IMP,0.123,a) catalysts. In contrast to the results of pure benzene hydrogenation at lower temperature (210¢J/240¢J), they are found that if all prepared various Pt/ZSM-12 catalysts were above the inversion temperature (Ti) then the benzene hydrogenation conversion over Pt(IE,0.100%,c) sample is higher than over Pt(IMP,0.123%,a) sample owing to latter provides less Pt-H+ active sites, as well as Pt(IMP,0.123%,a) sample is the most effective catalyst for toluene disproportionation and transalkylation with 1,2,4-trimethylbenzene. Owing to transformation generally is performed at higher temperature, such as above 400¢J, their operation temperatures are indeed above the inversion temperature (Ti) for all Pt/ZSM-12 catalysts. In situ comparing their benzene hydrogenation in transformation, including disproportionation and transalkylation, is suitable and valuable for understanding and determinating the characteristics of Pt/ZSM-12 zeolite catalysts. Relative conversion of benzene hydrogenation in transformation is the probe of characterizing the Pt-location onto ZSM-12 zeolite.