Hydroconversion of a Model Mixture and Fluid Catalytic Cracking Gasoline for Octane Enhancement. Main Reaction Pathways over Monofunctional HZSM5(x)-Alumina Catalysts Horacio Gonza ´ lez and Jorge Ramı ´rez* UNICAT, Departamento de Ingenierı ´a Quı ´mica, Facultad de Quı ´mica, UNAM, Ciudad Universitaria, D.F. 04510, Me ´ xico Rene ´ Za ´ rate and Teresa Cortez Instituto Mexicano del Petro ´ leo, Eje Central Lazaro Cardenas #152, D.F. 07730, Me ´ xico The changes in RON and liquid yield during the hydroconversion of an n-heptane-benzene- toluene mixture were evaluated in a continuous high-pressure flow reactor using HZSM5(x)- alumina catalysts with variable contents of zeolite. The results from these experiments were compared with those obtained with a real fluid catalytic cracking (FCC) hydrotreated gasoline as the feedstock. An increase in the zeolite concentration altered the acid properties of the catalysts that showed a gradual increase in the intensity of the 3612 cm -1 IR band, associated with the internal strong Bro ¨nsted acid sites of the zeolite. The hydroconversion results show that increasing the zeolite content in the catalyst leads to a gradual increase in both the RON number of the liquid product and the amount of light hydrocarbons (eC 4 ), promoted mainly by secondary cracking and dealkylation reactions. Because of this opposite trend of RON and liquid yield, zeolite contents higher than 10 wt % led to an almost constant barrel-octane (RON × fractional liquid yield). The catalytic experiments with the synthetic mixture as the feedstock show that the main reactivity is associated with the cracking of n-heptane and with alkylation of the aromatics by olefins produced during n-heptane cracking. Additionally, other reaction pathways that lead to the production of small amounts of n-paraffins (other than n-heptane), isoparaffins, cycloparaffins, and, to a lesser extent, olefins are observed. The main reaction pathways leading to higher RON with the hydrotreated FCC gasoline as the feedstock seem to be similar to those observed with the synthetic mixture. Introduction The growing need for high-quality clean transport fuels has prompted the refining industry to look for new catalysts or process alternatives to optimize the treat- ment and use of the different petroleum cuts. In the particular case of gasoline, the main environmental restriction is placed on the sulfur content. In reformu- lated gasoline, the gasoline pool is constituted from different refinery streams, i.e., straight run gasoline, reformate, alkylate, oxygenates, and fluid catalytic cracking (FCC) gasoline. Of all of these streams, it is the FCC gasoline stream that contributes as much as 90% of the total sulfur in the gasoline pool. Therefore, the reduction of the sulfur content in the FCC gasoline stream is mandatory if one wants to make maximum use of this fraction and, at the same time, reduce substantially the level of sulfur in the gasoline pool. To achieve this task, two main alternatives seem possible: (i) hydrodesulfurization (HDS) of the FCC feed, which technically appears as the best option but requires high investment costs, and (ii) HDS of part or all of the FCC gasoline product fraction, which can be achieved at relatively low investment costs. However, the second alternative faces the problem of octane loss, because of the hydrogenation of the olefins present in the stream, during the HDS process. This problem can be circum- vented by the addition of a second catalyst bed or reactor where octane recovery or even enhancement can take place. It is this octane recovery/enhancement stage which has been the least studied, especially regarding the catalyst formulation. The requirements of the second stage catalyst, which hereafter we will call the selective hydroconversion catalyst, will depend on the type or fraction of naphtha to be processed, because the relative distribution of components in the naphtha (paraffins, olefins, and aromatics) will vary from one particular cut to another. Refineries operate with either a high or low octane/ liquids production ratio. Therefore, the required selec- tive hydroconversion catalyst must be able to empha- size, according to the particular refinery needs, either the production of octane without too much concern about the liquid yield or the preservation of the liquid yield with as much octane enhancement as possible. Clearly, the tuning of the catalyst formulation for each case will be different. In the design of the selective hydroconver- sion catalyst, one should aim, in general, at promoting the reactions that transform low octane molecules in the feed into higher octane products, preserving at the same time the liquid yield, that is, avoiding the production of gas (methane, ethane, etc.). According to previous studies, 1,2 among the main molecular rearrangements and transformations needed to achieve octane enhance- * To whom correspondence should be addressed. Fax: (525) 56225366. E-mail: jrs@servidor.unam.mx. 1103 Ind. Eng. Chem. Res. 2001, 40, 1103-1112 10.1021/ie000351m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/20/2001