Hydroconversion of a Model Mixture and Fluid Catalytic Cracking

Ind. Eng. Chem. Res. 2001, 40, 1103-1112

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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 Ingenierıá 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-benzenetoluene 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 (eC4), 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 treatment and use of the different petroleum cuts. In the particular case of gasoline, the main environmental restriction is placed on the sulfur content. In reformulated 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, * To whom correspondence should be addressed. Fax: (525) 56225366. E-mail: [email protected].

during the HDS process. This problem can be circumvented 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 selective hydroconversion catalyst must be able to emphasize, 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 hydroconversion 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-

10.1021/ie000351m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/20/2001

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Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001

ment in a liquid fuel, isomerization, cracking of nparaffins, alkylation, and production of aromatics can be considered. However, one must also take into account the restrictions in the aromatics content and that excessive cracking leads to the loss of liquid yield. Additionally, the catalyst must be able to operate in the presence of small amounts of sulfur and nitrogen compounds that will come from the preceding HDS process step. Clearly, the complexity of the hydroconversion reaction scheme increases substantially with the number of different components in the feed. It is for this reason that most of the previous basic studies have mainly analyzed the behavior of single-component reactions. Hydroconversion mechanistic and reaction pathway studies with paraffins lighter than n-heptane include n-butane,3,4 isobutane,5,6 and n-hexane.7-11 However, although the product distribution obtained using nheptane as the feed is more complex, it has been found more representative of the gasoline cuts and has been preferred.12-16 Hydroconversion studies with several components in the feed are scarce because of the complexity involved in the analysis of the possible reaction pathways.17-19 In the present study, the first of a series on selective hydroconversion of complex mixtures, we analyze the effects that changes in the concentration of the zeolite in a HZSM5-alumina catalyst cause on the reaction paths related to octane enhancement and liquid yield, during the hydroconversion of a mixture of hydrocarbons (n-heptane-benzene-toluene). The behavior of the synthetic model mixture will be contrasted with that of a real FCC gasoline feedstock. The catalyst used in the present study consisted of zeolite HZSM5 incorporated into an alumina matrix, in which the concentration of the zeolite was varied between 0 and 30 wt %. The case of a metal-containing bifunctional catalyst will be treated in a future study. In addition to the reaction study, the catalysts were characterized by X-ray diffraction (XRD), BrunauerEmmett-Teller (BET) surface area, and infrared (IR) spectroscopy. Experimental Section Preparation and Characterization of the Catalysts. The catalysts were prepared using a commercial HZSM5 zeolite (Zeolyst, Si/Al atomic ratio ) 80). The incorporation of the zeolite in the alumina matrix was achieved using boehmite (Catapal B) as the binder. A 10 wt % of the total boehmite was peptized using formic acid (5 vol %); the remaining boehmite and the total zeolite powders were mixed with the peptized boehmite gel and finally extruded to form 0.1 × 0.2 cm pellets. The catalyst extrudates were dried at 393 K for 12 h and calcined at 773 K for 4 h. Hereafter, the catalysts will be referred as HZSM5(x)-alumina, where x is the weight percent of zeolite in the catalyst. The powder XRD patterns of the γ-alumina, HZSM5, and different HZSM5-alumina samples were recorded in the range 2° e 2θ e 70° with a Philips PW 1050/25 diffractometer, using Fe-filtered Cu KR radiation (λ ) 1.5418 Å) and a goniometer speed of 2°/min. The IR spectra of the catalysts were recorded in a Nicolet Magna 750 Fourier transform instrument, using pressed disks of the pure catalyst powders, activated by outgassing at 748 K for 6 h in the IR cell. The BET surface

Figure 1. XRD patterns of the samples (a) HZSM5, (b) HZSM5(30)-alumina, (c) HZSM5(20)-alumina, (d) HZSM5(10)-alumina, and (e) alumina. Table 1. Characteristics of the Hydrotreated FCC Gasoline nominal boiling range, K specific gravity, g/cm3 total sulfur, ppm nitrogen, ppm research octane number (RON)

355-477 0.805 147 18 84.8

areas were determined by nitrogen physisorption at 77 K in a Micromeritics ASAP 2000 apparatus. Catalytic Experiments. The catalytic experiments were carried out in a tubular stainless steel reactor (1 cm diameter and 30 cm length). The feed (74.2 wt % n-heptane, 12.9 wt % toluene, and 12.7 wt % benzene) was added continuously by a Milton Roy high-pressure pump. The hydrogen flow to the reactor (40 mL/min) was regulated with a mass flow controller. Prior to the reaction, the catalyst was pretreated at 588 K for 1 h under a flow of nitrogen (50 mL/min). The reaction was conducted at 588 K and 28 kg/cm2, using a liquid hourly space velocity (LHSV) of 2.6 h-1 and a hydrogen/ hydrocarbon ratio of 2000 ft3 (STP)/barrel of liquid. The liquid reaction products were analyzed by gas chromatography, using a HP-5890-II chromatograph and a 50-m PONA capillary column. Product identification was achieved using chromatographic standards and mass spectrometry (HP-GCD Plus GI800B apparatus). Additional experiments were performed at two LHSV (1.3 and 2.6 h-1) with the remaining operating conditions the same as before. In these experiments a real hydrodesulfurized FCC gasoline was used as the feedstock. Desulfurization of the gasoline, prior to the hydroconversion step, was achieved with a conventional commercial CoMo/Al2O3 HDS catalyst. The main characteristics of the desulfurized FCC gasoline are shown in Table 1, and the corresponding PIONA analysis for this feedstock is presented later in Table 3. Results and Discussion Catalysts Characterization. Figure 1 shows the diffraction patterns of the different samples. Figure 1a corresponds to the pure HZSM5 zeolite, and the diffraction lines correspond well with those reported in the literature.20 Figure 1e corresponds to pure alumina and shows the existence of poorly crystallized γ-alumina. XRD of the HZSM5-alumina mixtures shows evidence of both phases according to their concentrations.

Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1105

Figure 2. IR spectra of the HZSM5(x)-alumina catalysts after activation at 748 K for 6 h: (a) HZSM5, (b) HZSM5(30)-alumina, (c) HZSM5(20)-alumina, (d) HZSM5(10)-alumina, (e) HZSM5(5)-alumina, (f) HZSM5(3)-alumina, (g) alumina. Table 2. Textural Properties of the Catalysts catalyst

surface area (m2/g)

pore volume (cm3/g)

mean pore diameter (Å)

Al2O3 HZSM5(3)-Al2O3 HZSM5(5)-Al2O3 HZSM5(10)-Al2O3 HZSM5(20)-Al2O3 HZSM5(30)-Al2O3 HZSM5

231 249 254 255 265 270 458

0.46 0.45 0.45 0.43 0.41 0.39 0.27

55 53 52 51 48 45 24

Table 2 shows the surface areas, pore volumes, and mean pore diameters of the catalyst samples. The results indicate that the incorporation of up to 10 wt % of zeolite into the alumina matrix was satisfactorily achieved without significant loss of the zeolite surface area. However, increasing amounts of the zeolite surface area were lost as the amount of zeolite increased to 20 and 30 wt %. In fact, at 30 wt % zeolite, almost 10% of the zeolite surface area was lost. Figure 2 compares the FTIR spectra of the HZSM5(x)-alumina catalysts with those of the pure HZSM5 and alumina samples. The pure alumina sample (Figure 2g) presents a very intense band near 3680 cm-1 and three additional weak absorptions at 3730, 3770, and 3790 cm-1 (shoulder). According to the literature,21 the bands at 3790 and 3770 cm-1 are assigned to terminal OHs over one tetrahedrally coordinated Al ion, in a nonvacant environment or near a cation vacancy, respectively. The band at 3730 cm-1 is assigned to a terminal OH over an octahedrally coordinated Al ion, while the band at 3680 cm-1 is assigned to bridging OHs. An additional band near 3590 cm-1, corresponding to triply bridging OHs, has been observed for similar catalysts;21 however, in our sample this band was not clearly identified. As can be observed in the spectra (Figure 2b-f), the four main IR bands observed for alumina are always present in the HZSM5(x)-alumina samples, because of the relatively high alumina content of these catalysts. The spectrum of pure HZSM5 (Figure 2a) agrees well with those reported previously for similar catalysts.22,23 The very intense band at 3612 cm-1 is associated with

the bridged framework hydroxyls of the Si-OH-Al type, which are responsible for the high acid strength in HZSM5 zeolites. A less intense band observed near 3745 cm-1 is due to silanol groups, analogous to SiOH on silica, located at the external surface of the zeolite. According to previous reports, these external silanol groups are still quite acidic; however, they are much less acidic than the internal ones.23 The weak band at 3725 cm-1 (shoulder) is associated with nonacidic silanols located in the internal pores such as those of silicalite.24 Finally, a very weak broad band in the 3700-3650 cm-1 region has been associated with the OHs of extraframework alumina. These data and the relative intensity of the above νOH bands characterize our sample as a typical HZSM5 zeolite with high Al content and without significant amounts of extraframework alumina. In the HZSM5(10)-alumina sample (Figure 2d), evidence of the 3612 and 3745 cm-1 bands due to the zeolite begins to be observed and their intensity increases with the zeolite content, signaling a clear increase in the number of acid sites. Hydroconversion of the n-Heptane-BenzeneToluene Mixture. Effect of the HZSM5 Concentration on the Product Distribution, Octane Number (RON), and Liquid Yield. The effect of the concentration of acid sites on the hydroconversion of the synthetic mixture was evaluated by changing the concentration of the zeolite (x ) 0, 3, 5, 10, 20, and 30 wt %) in the HZSM5(x)-alumina catalysts. Preliminary reaction tests carried out with the HZSM5(10)-alumina catalyst using the individual components of the feed showed that, at the conditions of the experiment (588 and 28 kg/cm2), the aromatics in the feed (benzene or toluene) did not undergo any transformation on this catalyst. In contrast, n-heptane transforms readily and yields mainly light C3-C4 hydrocarbons (15 wt %), C5 (3.7 wt %), C6 (2.5 wt %), C7 (1.5 wt %), and small amounts of alkylated aromatics, C8 (0.64 wt %) and C9 (0.2 wt %). Benzene, toluene, and heavy aromatics (C10 and C10+) were detected only as traces. The tests carried out with the total feed (n-heptanebenzene-toluene) over pure alumina indicated that in the absence of the zeolitic material no transformation takes place in any of the components of the feed. This indicates that under our experimental conditions the catalytic activity observed with the HZSM5(x)-alumina catalysts is exclusively due to the zeolitic component. The incorporation of HZSM5 into the catalyst leads to a gradual increase in the transformation of the three reactants in the feed when the zeolite content is varied from 3 to 30 wt % (Figure 3). The conversion order of the reactants was n-heptane > benzene > toluene. In this case, as in other studies previously reported,1 toluene, being intrinsically more reactive than benzene, reacts slower than the latter because of the diffusional effects in the channels of the HZSM5 zeolite. This suggests that the channel openings in HZSM5 can impose a stereospecific effect on alkylation reactions and that the rate of alkylation could be inhibited by the slow diffusion of some of the dialkylbenzene isomers.1 The changes in product selectivity with zeolite loading can be observed in Figure 4. In this figure, the reaction products have been grouped in order to facilitate the analysis of the results, and the selectivity has been defined as the weight percent of each product group

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Figure 3. Effect of the zeolite content on the conversion of ([) n-heptane, (9) benzene, and (4) toluene.

Figure 5. Effect of the zeolite content on ([) RON, (9) liquid yield, and (4) barrel-octane of the liquid product.

Figure 4. Effect of the zeolite content on the product selectivity: (O) eC4, (9) C5, (4) C6, (×) C7, (*) C8, (b) C9, (+) C10, (]) C10+.

divided by the total weight percent of the products and multiplied by 100. The light products include mainly C3 and C4 components, whereas paraffins, isoparaffins, and cycloparaffins constitute the groups C5, C6, and C7. The groups C8, C9, and C10 are mainly formed by alkylated aromatics. Finally the C10+ products are mainly aromatic products of 11 carbon atoms and other nonidentified products. As the HZSM5 concentration increases, the selectivity to light products (eC4) increases while that of the remaining products decreases, with the exception of the groups C8 and C10+ which present a small increase in the selectivity (Figure 4). Therefore, an increase in the number of zeolite acid sites in the catalyst promotes the cracking reactions responsible for the formation of gaseous products. The light products are initially formed from the cracking of n-heptane; however, at high zeolite contents, which imply higher conversion, there is also a contribution to the formation of light products from secondary reactions such as the subsequent cracking of isomers and alkylaromatics. These latter contributions are, in part, evidenced by the observed decrease in the selectivity to the C5, C6, C7, C9, and C10 groups with the zeolite content (Figure 4). High concentration of HZSM5 in the catalyst is in effect equivalent to an increase in the contact time of the reactants and products with the acid sites and is therefore contrary to the liquid yield preservation. The selectivity increase toward the C8 and C10+ groups at high zeolite contents

can be due to the formation of these products through secondary reactions such as alkylaromatics disproportionation and dealkylation or to consecutive condensation-cyclization-aromatization of olefinic fragments produced during n-heptane cracking, which is favored at high zeolite contents or high conversions. It is not quite clear why this behavior is only observed for groups C8 and C10+. To assess the effect that the change in the zeolite concentration has on the RON of the liquid product, Figure 5 presents the changes in RON with the zeolite concentration (dotted line). The RON was calculated according to a chromatographic method reported in the literature.25 The RON of the liquid product, when the zeolite concentration increases from 0.0 to 30.0 wt %, changes gradually from 24.4, that is, the RON of the feed, to 55.0, respectively. These results indicate that a significant increase in the RON of the liquid product is achieved with this catalyst. However, from the economical point of view, it is also clear that on this type of catalyst there is a competition between the cracking reactions, which leads to a loss of liquid yield, and the reactions leading to higher octane products (isomerization, alkylation, etc.). Therefore, there must exist an optimum value of the zeolite concentration, which gives the required balance between the gain in RON and liquid yield loss. Clearly, this value must be different for each feed composition. From the above results it appears that with the monofunctional catalyst used here, the main gain in the liquid product RON is related mainly to the aromatic alkylation reactions and not to the isomerization of n-heptane in the feed. It must be also taken into account that since the RON is calculated only from the liquid product and there is a loss of liquid in the hydroconversion process due to the production of light products, part of the RON increase is due to a concentration effect. As mentioned before, the important thing from the industrial point of view is to maintain a good balance between the octane gain and the liquid yield. In fact, it appears possible to reach high octane numbers in the liquid product but at the expense of a great loss of liquid. To analyze this problem in a practical way, refiners use the concept of barrel-octane, which is calculated as the product of RON multiplied by the fractional liquid yield.


Figure 6. Total light products (eC4) as a function of the HZSM5 content in the catalyst.

Figure 5 indicates quite clearly that the barrel-octane increases up to 10 wt % zeolite and then for zeolite contents beyond 10 wt % remains practically constant. Two opposite effects contribute to this result. As the zeolite content in the catalyst increases, we obtain a gradual increase in the RON of the liquid product, mainly because of the production of alkylaromatics, but at the same time there is a steady decrease of the liquid yield, because of the production of light products. Main Reaction Pathways. A more detailed analysis of the product distribution indicates that the complete reaction scheme is quite complex. Nevertheless, the main reaction paths, which give rise to the different product groups, are discussed below. Initiation Reactions. It is well-known that the transformation of hydrocarbons over acid catalysts proceeds through carbonium/carbenium ion chemistry.26-31 However, there are still some doubts regarding the initial steps in the paraffins cracking mechanisms, which should produce the first carbenium ions. Among the proposed mechanisms, the protolysis of an n-paraffin by a Brønsted acid site has been used to interpret the results for similar catalysts.7,11,15,32 In this mechanism, which was initially proposed by Haag and Dessau,7 a pentacoordinated carbonium ion is initially formed, followed by a dissociative step, which produces either a light alkane or molecular hydrogen and an adsorbed carbenium ion. As soon as they are formed, the starting carbenium ions are able to undergo different types of reactions (cracking, isomerization, dimerization, aliphatic alkylation, aromatics alkylation, etc.) that finally will produce the observed complex product mixture. As proposed recently,32 we can consider the hydroconversion reactions as a chain process: the chain initiation by protolysis, propagation by disproportionation (between an adsorbed carbenium ion and other molecules) or by β scission of a carbenium ion, and chain termination by desorption of olefins. These features of the hydroconversion process are presented later in a simplified way in Figure 9. In what follows we will analyze in more detail the possible reaction pathways that could explain the origin of each of the important product fractions observed in the experiments. Light Products eC4. The total light products reported in Figure 6 include both the light products (eC4) dissolved in the liquid mixture and the gas products obtained during the hydroconversion reactions. Among the light products, propane and n-butane were detected in higher percentages followed by isobutane and small amounts of butenes. Lighter products such as ethane and ethylene were found only as traces.

Figure 7. Main C5, C6, and C7 products as a function of the HZSM5 content in the catalyst: (]) isopentane, (9) n-pentane, (4) n-hexane, (×) methylpentanes, (b) cycloparaffins C6, (O) methylhexanes, (+) cycloparaffins C7.

The high amount of propane and butane in the product gas can be rationalized by a consecutive scheme involving an isomerization step of the initial sec-heptylcarbenium ion leading to 2-methyl-4-hexyl- or 3-methyl5-hexylcarbenium ions followed by C-type β scission. This route, which has been well documented in the literature,33,34 will produce n-butane and propane as final products. Other β-scission types involving the formation of di- and tribranched isomers are sterically suppressed in 10-membered-ring zeolites such as ZSM5. Isobutane evolution could come from a mechanism similar to the isomerization of paraffins through a protonated cyclopropane (PCP) intermediate, to give an isoparaffin and a linear olefin, as proposed by Sie.35 Other possibilities of the formation of isobutane would involve the formation of double- or triplebranched carbenium ions, which according to the literature are sterically hindered in 10-membered-ring zeolites such as ZSM533 or through a dimerization cracking mechanism.4 The high paraffin-to-olefin ratio observed in the gas product was related to the high hydrogen transfer activity of the HZSM5 zeolite. Furthermore, the high proportion of saturated products indicates that hydrogen transfer, which competes with aromatics or aliphatics alkylation reactions, is the fastest reaction. At low conversions, the light products are mainly the result of protolysis reactions; however, as the conversion increases, bimolecular disproportionation reactions become increasingly important, because of the high concentration of carbenium ions on the catalyst surface. In addition, it has been found that the bimolecular cracking mechanism contributes importantly to the overall cracking of n-paraffins over zeolite catalysts.7,36 Thus, both the monomolecular and bimolecular cracking mechanisms must be important in the (eC4) production. The observed increase in eC4 production at high zeolite contents, which means higher conversions, must be the result of secondary cracking and dealkylation of alkylaromatic products. As can be observed in Figures 7 and 8, the concentration trend of some isoparaffins and alkylaromatics presents the typical behavior of consecutive reactions. This can be rationalized by considering that while cracking is the secondary

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Figure 8. Main C8+ products as a function of the HZSM5 content in the catalyst: (]) ethylbenzene, (0) xylenes, (4) isopropylbenzene, (×) n-propylbenzene, (b) ethyltoluenes, (O) trimethylbenzenes, (+) methyl-n-propylbenzenes, (9) n-butylbenzene, (*) isobutylbenzene, ([) C10+ products.

reaction for the isomers, dealkylation must be one of the possible secondary reactions for the alkylaromatic products. n-Paraffins. The linear paraffins found in the liquid product were mainly of the C5-C10 fraction. n-Pentane was produced in amounts higher than 2 wt %, followed by n-hexane (approximately 1 wt %), while n-octane and n-decane were found in percentages lower than 1 wt %. n-Nonane was detected only as traces. For both C5 and C6 groups, the experimental isoparaffins/normal paraffins ratio is lower than unity at low zeolite contents, indicating a clear predominance of linear paraffins over isoparaffins. However, this ratio increases with the zeolite content in the catalyst. For the C5 group (Figure 7), n-pentane, produced in higher amounts at low conversions, starts to decrease at higher conversions, because of cracking and isomerization reactions. A similar behavior is observed for n-hexane in the C6 group. However, in this case, n-hexane disappears faster than 3-methylpentane and 2-methylpentane, which results in an isoparaffins/normal paraffins ratio higher than unity at high zeolite contents. The products in the groups C5 and C6 seem to be mainly the result of primary n-heptane cracking reactions. At high conversions there might also be contributions from product disproportionation or dimerizationcracking reactions. This last mechanism occurs over strong acidic catalysts8,14,36,37 and has been used to explain the C5 and C6 production from n-paraffins cracking, a fact that is difficult to explain only by the classical β-scission mechanism. The bimolecular reactions between hydrocarbons from the gas phase and surface carbenium ions can result in the alkylation of a carbenium ion either by a reactant molecule or by an olefinic product, resulting in molecules larger than the feed. Moreover, the larger ions can rearrange into isomeric forms by hydride and methyl shifts, and then by successive β-scission and hydrogentransfer steps, they produce the observed complex spectrum of products, including the unexpected C5, C6, and C8+ paraffins and heavy C10+ products. In a related study using n-heptane as the test molecule, Meusinger

et al.36 found that, over HZSM5 and at similar operating conditions (high pressure and relatively low temperature), the dimerization-cracking mechanism was the dominant path in the conversion of n-heptane, giving important quantities of C5 and C6 products. As mentioned before, among the product n-paraffins with molecular weights higher than that of n-heptane, n-octane and n-decane were found in high percentages. As discussed above, these products could result from aliphatic alkylation reactions between adsorbed carbenium ions and olefins produced during the n-heptane cracking, followed by rearrangement and hydrogen transfer. Isoparaffins. The production of isoparaffins was limited to the C4-C8 range, in line with the restricted formation of bulky isomers into the HZSM5 pores. The isoparaffins found in higher amounts in the liquid product were isopentane, 2-methylpentane, and 3methylpentane (Figure 7). In contrast, although there was a high n-heptane concentration in the feed, the production of isomers from n-heptane was not favored on the monofunctional catalyst used here. The main C7 isomers were 2-methylhexane and 3methylhexane, both at concentrations lower than 1 wt %. Other isomers of the C7 group like 2,3-dimethylpentane, 3-ethylpentane, and naphthenic products were found in percentages lower than 0.3 wt %. These results indicate that, on this type of catalyst, the route of direct isomerization of n-heptane via PCP27,28 does not contribute in an important way to the RON increase. Several mechanisms have been reported explaining the formation of isoparaffins from n-paraffins. In some of them,38,39 the formation of branched products from long n-paraffins can only be understood if the skeletal isomerization of the straight-chain paraffin occurs before the cracking step. However, this classical mechanism involves the thermodynamically restricted formation of a primary carbenium ion, after β scission of a tertiary carbenium ion. To circumvent this problem, an alternative mechanism involving the formation of a PCP intermediary was proposed by Sie.35 In this mechanism the scission takes place in a lateral chain of the cyclopropane ring, resulting in a branched carbenium ion and a lineal olefin. Hydrogen-transfer reactions lead to the final branched paraffin. Clearly, other possibilities of formation of branched isomers C5-C7 could be through the direct isomerization of the linear carbenium ions produced during n-heptane cracking or through the dimerization-cracking mechanism14,36,37 involving C14type intermediaries. Among the isoparaffins heavier than the feed, only monobranched isooctanes were found but in amounts lower than 0.3 wt %. These products could be produced by aliphatic alkylation reactions. Olefins. The amount of olefins in the reaction product was, in general, small. Butenes, 2-methyl-2-butene, and heptenes were the olefins detected in higher percentages. In general, olefins are considered as reaction intermediates and are produced through carbenium ion desorption. The feed composition, operating conditions, and catalyst composition will dictate the percentage of olefins produced in the hydroconversion process. In the absence of a high hydrogenating-dehydrogenating function in the catalyst (i.e., Pt, Pd) as in this case, the equilibrium between olefins and paraffins will hardly be achieved, and so the amount of olefins will be smaller than their equilibrium composition.40


Cycloparaffins. The cyclization of olefinic chains promoted by acid sites gives rise to the production of small amounts of naphthenic products. Among them, methylcyclopentane, dimethylcyclopentane, ethylcyclopentane, methylethylcyclopentane, and propylcyclopentane were found in higher percentages (Figure 7). However, the production of naphthenics was, in general, lower than the production of isoparaffins of the same carbon number, indicating that the cyclization reaction is slow compared to the isomerization or hydrogentransfer reactions. According to the literature,32,41 cyclization involves a “snake swallowing its own tail” mechanism, where the tail of a sufficiently long carbenium ion is attacked by its own charge center in a protonation reaction. After cyclization reactions, isomerization and ring contraction might play a role in producing the other naphthenes observed in the product distribution. Alkylaromatics. The groups C8, C9, and C10 (Figure 8) are formed mainly by alkylaromatics, which contribute in an important way to the RON of the liquid product. These alkylaromatics result mainly from reactions between the aromatics in the feed (benzene and toluene) and the olefinic fragments that come from n-heptane cracking. Ethylbenzene is the main product in the C8 group and can be produced through the direct alkylation of benzene by ethylcarbenium ions produced during n-heptane cracking. Also, the formation of ethylbenzene could result from the cyclization-aromatization of C8 olefins, formed via aliphatic alkylation reactions of carbenium ions with olefinic fragments produced during n-heptane cracking. Xylenes (ortho, meta, and para), on the other hand, could come from cyclization of C8 fragments, followed by hydrogen transfer, because the isomerization of ethylbenzene to xylenes occurs with difficulty over a purely acidic catalyst. In fact, over monofunctional ZSM5, ethylbenzene normally undergoes transalkylation and dealkylation reactions.42 The normalized xylenes distribution approaches the equilibrium composition as the zeolite content increases. This indicates that, despite the shape selectivity effects induced by HZSM5, at high conversions the reconversion of the para to the meta and ortho isomers plays an important role in the final xylene product distribution. Among the hydrocarbons present in the C9 group, n-propylbenzene and isopropylbenzene are the two major components. Isopropylbenzene is formed through the well-known mechanism that involves the attack of benzene by a secondary propylcarbenium ion. Figure 8 shows that isopropylbenzene is formed readily; however, its concentration reaches a maximum value and then is consumed by a consecutive reaction. This consumption of isopropylbenzene is explained by its transformation to n-propylbenzene, which increases steadily and also by a parallel route to o-ethyltoluene. The mechanism of the transformation of isopropylbenzene to n-propylbenzene, which at first sight appears unlikely, is based on the stronger basicity of the tertiary carbon in isopropylbenzene compared to the secondary carbon in n-propylbenzene. This last mechanism has been clearly explained in a previous study, when analyzing the transformation routes of isopropylbenzene over zeolitic catalysts.43 Moreover, it has also been reported that the transformation of iso- to n-propylbenzene is thermodynamically possible.44 This can explain why our results show that while isopropylbenzene

concentration reaches a maximum, the concentrations of n-propylbenzene and o-ethyltoluene increase steadily with the zeolite concentration. Another possible route for the formation of n-propylbenzene from isopropylbenzene has been reported in the literature and would be through the rearrangement of the ring isopropyl group to form n-propylbenzene by a cracking-realkylation reaction.45 Other important compounds in this group are the o-, m-, and p-ethyltoluenes and trimethylbenzenes, which increase with the zeolite content and have their origin in the alkylation, isomerization, transalkylation, and disproportionation reactions. Products arising mainly from aromatic alkylation reactions form the group of C10 hydrocarbons. Tetramethylbenzenes, diethylbenzenes, dimethylethylbenzenes, and n-decane are also produced to a lesser extent. As can be observed, most of these latter compounds tend to a constant composition with the amount of zeolite (high conversions of heptane). This behavior is probably due to thermodynamic limitations and shape selectivity effects. High molecular weight products that appear in low concentrations such as undecane, indans, and naphthalenes (Figure 8) form the group C10+. The data reported above allows us to propose a simplified picture for the behavior of HZSM5(x)alumina catalysts in the hydroconversion of the nheptane-benzene-toluene mixture. The catalytic experiments show that, at the operating conditions of this work, the main reactivity is associated with the cracking of the n-heptane molecule and with the alkylation of the aromatics by olefins produced during n-heptane cracking. Additionally, we also observe the production of small amounts of n-paraffins, isoparaffins, cycloparaffins, and, to a lesser extent, olefins. Some of these reactions are associated with strong Brønsted acidity and with the well-known carbonium and carbenium ion chemistry.27,28,30,31 The main reaction pathways that give rise to the different product groups are shown in Figure 9. The reaction system initiates with the protonation of nheptane on a Brønsted site to produce a pentacoordinated carbonium ion that most probably dissociates to produce a sec-heptylcarbenium ion. At low conversions the main source of carbenium ions would be n-heptane protolysis. However, at high conversions different reactions such as aliphatic and aromatic alkylation, β scission, isomerization, and cyclization will contribute to the diversity of carbenium ions adsorbed on the catalyst surface. In the final step of the reaction scheme, reactions such as hydrogen transfer or desorption lead to the different final products. When hydrogen transfer takes place, the products will be saturated hydrocarbons such as paraffins, isoparaffins, and cycloparaffins. If, on the other hand, a desorption of the hydrocarbon takes place leaving the proton on the surface, olefins or alkylaromatics can be produced depending on the type of carbenium ion precursor. For example, isomerization of a lineal C7 carbenium ion followed by a hydrogentransfer reaction will lead to i-C7 as the reaction product. At the same time, as a result of the hydrogentransfer reaction between a hydrocarbon and the carbenium ion, a new carbenium ion is formed and incorporated to the carbenium ion pool on the catalyst surface. Hydroconversion of Hydrotreated FCC Gasoline. To assess the performance of the monofunctional

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Figure 10. Hydroconversion of the synthetic feed: ([) RON of the feed, (9) RON of the liquid product, (4) % liquid yield.

Figure 9. Simplified scheme of the main reaction pathways during the hydroconversion of the synthetic mixture. Table 3. Product Distribution for the Hydroconversion of Hydrotreated FCC Gasoline composition (wt %)

paraffins isoparaffins olefins naphthenes aromatics heavy products nonidentified RON liquid yield, % barrel-octane

FCC gasoline feeda

FCC gasoline after hydroconversion over HZSM5(10)-aluminab

10.8 29.9 0.4 14.8 34.5 7.2 2.3 84.8 100 84.8

8.59 30.8 1.63 11.9 37.8 7.05 2.1 88.5 92.3 81.7

a After HDS over a commercial CoMo/Al O HDS catalyst. 2 3 Operating conditions: T ) 598 K, P ) 28 kg/cm2, H2/HC ) 4000 ft3/barrel, LHSV ) 1.3 h-1.

b

catalyst HZSM5(10)-alumina with a real feed, an industrial FCC gasoline sample (Table 3) was treated with a conventional HDS commercial catalyst to give a substantially desulfurized product, which was then used as the feedstock in the hydroconversion catalytic tests at different LHSV. Prior to analyzing the results with the real feed, we will for the sake of comparison outline the behavior in RON and liquid yield observed with the synthetic feed. The results of RON and product liquid yield versus LHSV using a synthetic mixture as the feed are shown in Figure 10. In this case we observe an increase of 16 units in the RON and a decrease of 15% in the liquid yield as the LHSV is varied from 1.3 to 2.6 h-1. These

Figure 11. Hydroconversion of the hydrotreated FCC gasoline: ([) RON of the feed, (9) RON of the liquid product, (4) % liquid yield.

results indicate that, although there is a significant decrease in the product liquid yield, the positive effect of the RON dominates the final value of the barreloctane, which is greater than that of the feed. When using hydrotreated FCC gasoline as the feed (Figure 11), the observed changes in RON and liquid yield follow trends similar to those of the synthetic feed; however, the changes are less drastic. At the high residence time in the reactor (LHSV ) 1.3 h-1), we observe an increase of 2 RON units and, at the same time, the liquid yield diminishes 5.7%. Thus, in this case the liquid yield loss is dominant. However, the RON and liquid yield trends for the synthetic mixture and the real feed are similar. These changes are the result of variations in the product group distributions (see Table 3). Analysis of Table 3 indicates that for the FCC gasoline the RON enhancement comes mainly from the cracking of paraffins and naphthenes and from the formation of aromatics (mostly alkylated aromatics). Additionally, we observe that transformation of isoparaffins is almost absent and olefins are formed to a small extent. These results indicate that the main reaction pathways in the hydroconversion process leading to higher RON with the synthetic mixture and with the real feed are similar. Clearly, the reaction paths for the real feed are more difficult to analyze in detail, because of the large number of components present in such feedstocks. Conclusions From the above results we can extract the following conclusions:


With the HZSM5(x)-alumina monofunctional catalysts, it is possible to increase the RON of the synthetic mixture and of the hydrotreated FCC gasoline used in this work. The gradual increase in the zeolite content from 0 to 30 wt % in the HZSM5(x)-alumina catalyst produces a gradual increase in the RON of the liquid product. However, the liquid yield drops with the zeolite content, causing the barrel-octane to remain almost constant for zeolite contents higher than 10 wt %. The analysis of the reaction products of the catalytic experiments shows that, with the synthetic mixture as the feedstock, the main reactivity is associated with the cracking of the n-heptane molecule and with the alkylation of the aromatics present in the feed by olefins produced during n-heptane cracking. The production of small amounts of n-paraffins, isoparaffins, cycloparaffins, and, to a lesser extent, olefins is related to the complex scheme of carbenium ion reactions taking place on the catalyst surface and which are associated with the strong Brønsted acidity of the HZSM5 zeolite. The analysis of the changes in product distribution of the FCC gasoline indicates that the main reaction pathways leading to higher RON with the FCC gasoline as the feedstock seem to be similar to those observed with the synthetic mixture used here. Acknowledgment H.G. thanks CONACyT and DGAPA-UNAM for the grant received. We also acknowledge the financial support from PEMEX-Refinacioń, IMP, and DGAPAUNAM programs. Thanks are also given to Leticia Banõs (IIM-UNAM) for the XRD characterizations. Literature Cited (1) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape Selective Catalysis in Industrial Applications, 2nd ed.; Marcel Dekker: New York, 1996. (2) Scherzer, J.; Gruia, A. J. Hydrocracking Science and Technology; Marcel Dekker: New York, 1996. (3) Krannila, H.; Haag, W. O.; Gates, B. C. Monomolecular and Bimolecular mechanisms of paraffin cracking: n-Butane Cracking Catalyzed by HZSM-5. J. Catal. 1992, 135, 115. (4) Trung Trang, M.; Gnep, N. S.; Szabo, G.; Guisnet, M. Isomerization of n-butane over H-mordenites under nitrogen and hydrogen: Influence of the acid site density. J. Catal. 1998, 174, 185. (5) Lombardo, E. A.; Hall, W. K. The mechanism of Isobutane cracking over amorphous and crystalline aluminosilicates. J. Catal. 1988, 112, 565. (6) Corma, A.; Miguel, P. J.; Orchilles, A. V. The role of Reaction Temperature and Cracking catalyst Characteristics in Determining the Relative rates of protolytic Cracking, chain propagation and hydrogen transfer. J. Catal. 1994, 145, 171. (7) Haag, W. O.; Dessau, R. M. Duality of Mechanism for AcidCatalyzed Paraffin Cracking. Proceedings of the Eighth International Congress on Catalysis, Berlin, 1984; Dechema: Frankfurtam-Main, Germany, 1984; Vol. 2, p 305. (8) Abbot, J.; Wojciechowski, B. W. Catalytic Reactions of n-hexane on HY zeolite. Can. J. Chem. Eng. 1988, 66, 825. (9) Wielers, A. F. H.; Vaarkamp, M.; Post, M. F. M. Relation between properties and performance of zeolites in paraffin cracking. J. Catal. 1991, 127, 51. (10) Lukyanov, D. B.; Shtral, V. I.; Khadzhiev, S. N. A kinetic Model for the Hexane Cracking Reaction over HZSM-5. J. Catal. 1994, 146, 87. (11) Babitz, S. M.; Williams, B. A.; Miller, J. T.; Snurr, R. Q.; Haag, W. O.; Kung, H. H. Monomolecular cracking of n-hexane on Y, MOR and ZSM5 zeolites. Appl. Catal. 1999, 179, 71.

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Received for review March 30, 2000 Revised manuscript received September 13, 2000 Accepted October 4, 2000 IE000351M

Hydroconversion of a Model Mixture and Fluid Catalytic Cracking

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