Hydroconversion of Hydrocarbons over HZSM5 and Mo−HZSM5

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Hydroconversion of Hydrocarbons over HZSM5 and Mo-HZSM5 Catalysts: A FTIR and Flow Reactor Study Aı´da Gutie´ rrez-Alejandre, Horacio Gonza´ lez, and Jorge Ramı´rez* UNICAT, Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, UNAM, Ciudad Universitaria Me´ xico D.F. (04510), Mexico

Guido Busca† Laboratorio di Chimica delle Superfici e Catalisi Industriale, Dipartimento di Ingegneria Chimica e di Processo, Universita` di Genova, Piazzale J. F. Kennedy, I-16129, Genova, Italy

The interactions of n-heptane, benzene, and toluene with HZSM5 and Mo-HZSM5 catalysts were studied by FTIR spectroscopy. The results from the FTIR study on the interactions of single molecules with these catalysts are compared with the product distribution results obtained during the hydroconversion of a model mixture of n-heptane-benzene-toluene over HZSM5-alumina and Mo-HZSM5-alumina catalysts. The FTIR study indicates that there are interactions of heptane, benzene, and toluene with both the strongly acidic internal OH’s and the less acidic silanol groups in HZSM5. These interactions seem to be a precursor for the formation of carbenium ions, which are the active species in hydroconversion reactions.The incorporation of Mo into HZSM5 causes a strong decrease in the number of external silanols, suggesting that some of the impregnated Mo species are located at the external zeolite surface. It is also evident from the FTIR results that the incorporation of Mo does not entirely destroy or exchange the internal OH’s of HZSM5. The addition of Mo to the zeolite, in addition to causing a decrease in the overall conversion of heptane, causes a marked simplification in the pattern of the main reaction products. The cracking and alkylation functionalities of the catalyst are almost suppressed, whereas a significant increase in skeletal isomerization is observed. This suggests, in line with the FTIR experiments, that the incorporation of Mo into the HZSM5-alumina catalyst causes a decrease in the availability of the strong Brønsted acid sites located in the cavities of the zeolite. 1. Introduction Deep hydrodesulfurization of fluid catalytic cracking (FCC) gasoline, which contributes about 90% of the sulfur in the gasoline pool, is mandatory in view of stricter environmental regulations. However, during the hydrodesulfurization (HDS) of gasoline, octane is partly lost as a result of the hydrogenation of the existing olefins.1 A typical hydrotreated FCC gasoline formulation contains about 70 vol % paraffins and 20 vol % aromatics, with the remaining 10 vol % being mostly cycloparaffins. To recover or enhance the octane lost during HDS, the hydrodesulfurized FCC gasoline or a fraction of it must be processed over a selective hydroconversion zeolitic catalyst. The catalyst in the hydroconversion step must promote molecular rearrangements or transformations leading to high-octane molecules, i.e., skeletal isomerization, hydrocracking of nparaffins, aromatic and aliphatic alkylation and aromatization. Clearly, as for similar processes,2-4 the catalyst must also have shape selectivity to avoid the transformation of highly branched molecules that already have high octane. Because the catalyst will possibly work in the presence of low amounts of sulfur and nitrogen compounds, the use of noble metals such as Pt is not the best option. However, a molybdenum * Author to whom correspondence should be addressed. Tel.: 525-6225349. Fax: 525-6225366. E-mail: jrs@ servidor.unam.mx. † E-mail: [email protected].

metallic phase, which can work in the reduced and partially sulfided state, appears to be an interesting option. Although there is some knowledge concerning the chemistry and behavior of this hydroconversion catalyst, not all is well-known. In particular, it is important to study the interaction of paraffins and aromatics such as n-heptane, benzene and toluene with the HZSM5 and Mo-HZSM5 catalysts. It is also important to evaluate the changes in the product distribution caused by Mo incorporation that contribute to the research octane number (RON) and liquid yield when a mixture of paraffins and aromatics interacts with the catalyst under reaction conditions. In the present work, variations in the product distribution and liquid yield during the hydroconversion of a hydrocarbon mixture (74.0 wt % n-heptane, 12.8 wt % toluene, and 12.7 wt % benzene) over HZSM5-alumina and Mo-HZSM5-alumina catalysts are compared with the results from an FTIR study on the interaction of the single reactant molecules with HZSM5 and MoHZSM5. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The catalysts were prepared using HZSM5 zeolite (Si/ Al atomic ratio ) 27). The incorporation of the zeolite into the alumina matrix was achieved by using 10 wt % of the total Catapal B boehmite as binder. Peptization of the binder was achieved with formic acid (5 vol %).

10.1021/ie000881i CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

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The remaining boehmite and the zeolite powders were added to the peptized boehmite gel and mixed thoroughly. The resulting paste was then extruded to form 0.1 × 0.2-cm cylindrical pellets. The final catalyst supports contained 10 wt % zeolite and 90 wt % alumina. The catalyst extrudates were dried (12 h at 393 K) and calcined (4 h at 773 K). Molybdenum was incorporated into the HZSM5 and HZSM5-alumina catalysts by pore volume impregnation of an aqueous solution of ammonium heptamolybdate of the appropriate concentration to obtain a solid with 6 wt % Mo. This catalyst was dried at 393 K for 12 h and calcined at 773 K for 4 h. The X-ray powder diffraction patterns of the HZSM5 and HZSM5-alumina samples with and without Mo were recorded in the 2° e 2θ e 70° range using a Siemens D5000 diffractometer with Cu KR radiation, a goniometer speed of 1.0°/min, and a graphite monocromator. The BET surface areas were determined by nitrogen physisorption at 77 K in a Micromeritics ASAP 2000 apparatus. IR spectra were recorded on a Nicolet Magna 750 Fourier transform instrument, using pressed disks of the pure catalyst powders, activated by outgassing the IR cell at 873 K. To improve resolution, the IR experiments on the interaction of hydrocarbon molecules with the catalysts were performed using pure HZSM5 zeolite with and without Mo. The adsorption experiments consisted of 3 min of contact of the activated catalyst with the hydrocarbon vapors (10 Torr), which allowed for saturation of the available surface. Next, outgassing was performed. Spectra were recorded before and after outgassing. A commercial HZSM5 catalyst (Si/ Al ) 23, Engelhard) in powder form was also used for comparison in the IR experiments. The liquids used in the IR experiments were from Carlo Erba (Milan, Italy). 2.2. Catalytic Experiments. The catalytic experiments were performed in a tubular stainless steel reactor. The hydrocarbon feed, consisting of 74.0 wt % n-heptane, 12.8 wt % toluene, and 12.7 wt % benzene, was added continuously by a Milton Roy high-pressure pump. Mass flow controllers were used to control the flows of hydrogen and nitrogen. Prior to the catalytic test, the catalysts were pretreated at 28 kg/cm2 with nitrogen (588 K, 1 h). The reaction was conducted at 588 K and 28 kg/cm2 using a liquid hourly space velocity (LHSV) of 2.5 h-1 and a hydrogen/hydrocarbon ratio of 356 m3 (STP) per cubic meter of liquid (2000 ft3/bbl). The liquid reaction products were analyzed at 2-h time intervals over a period of 6 h by gas chromatography, using a 50-m HP-PONA capillary column. 3. Results 3.1. XRD and Textural Characterization. Figure 1 shows the diffraction patterns of the different catalyst samples. The pattern of the HZSM5 zeolite used here (Figure 1a) corresponds well with that described in previous literature reports.5 When Mo is incorporated into the pure zeolite (Figure 1b) a new peak appears at 2θ ) 27.34°, which corresponds to the main reflection of MoO3 (JCPDS card 35-609); the other features of the diffractogram correspond to those observed for HZSM5, although they are less intense. On the other hand, Figure 1e shows that the pure alumina sample corresponds to a poorly crystallized γ-alumina phase. The HZSM5-alumina sample (Figure 1d) shows evidence of the zeolite and alumina phases according to their relative concentrations. When Mo is incorporated into

Figure 1. X-ray diffraction patterns of the samples: (a) HZSM5, (b) Mo-HZSM5, (c) Mo-HZSM5-alumina, (d) HZSM5-alumina, and (e) alumina. Table 1. Textural Properties of the Catalysts catalysts

surface area (m2/g)

pore volume (cm3/g)

Al2O3 HZSM5 HZSM5-Al2O3 Mo-HZSM5 Mo-HZSM5-Al2O3

250 425 275 399 250

0.43 0.26 0.39 0.20 0.31

HZSM5-alumina (Figure 1c), no reflections due to MoO3 are observed, indicating that the presence of alumina prevents the formation of large MoO3 particles. Table 1 shows the surface areas and pore volumes of the catalyst samples. The incorporation of Mo to the HZSM5 and HZSM5-alumina catalysts causes a small loss of surface area and pore volume, which could be associated with the partial blockage of the catalyst pores by the Mo species. In fact, the micropore area and pore volume drop by only about 14% when 6 wt % Mo is incorporated into the pure HZSM5 zeolite, indicating that most of the zeolite channels allow the diffusion of N2. 3.2. Catalytic Experiments. Preliminary data showed that no conversion of reactants takes place in the empty reactor or in the reactor filled with alumina, indicating that the catalytic activity observed with the HZSM5-alumina catalyst is exclusively due to the zeolitic component. Also, experiments carried out under the same conditions using a liquid feed containing only aromatics (i.e., pure toluene or benzene) did not give rise to significant conversion in the reactor filled with the HZSM5-alumina catalyst. On the contrary, nheptane was readily converted, producing mainly gaseous light hydrocarbons with small amounts of C5, C6, and C7 isomers and even smaller amounts of alkylaromatics (C8 and C9). Benzene and toluene were produced in trace amounts. These results indicate that cracking and aromatic alkylation are faster than aromatization and also that the transformation of aromatics using the mixture feed (heptane, toluene, and benzene) is mostly due to the bimolecular reactions of aromatics with the intermediates resulting from the transformations of heptane. Comparative experiments using only heptane or heptane plus aromatics (benzene and toluene) as the feed

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Table 2. Product Distribution for the Hydroconversion Experimentsa catalyst

HZSM5Al2O3

HZSM5Al2O3

Mo-HZSM5Al2O3

catalyst

HZSM5Al2O3

HZSM5Al2O3

Mo-HZSM5Al2O3

gas atmosphere

N2

H2

H2

gas atmosphere

N2

H2

H2

product

(wt %)

(wt %)

(wt %)

product

(wt %)

(wt %)

(wt %)

light products eC4 isopentane n-pentane 2-methyl-2-butene 2,3-dimethylbutane 2-methylpentane 3-methylpentane n-hexane methylcyclopentane 2,4-dimethylpentane benzene 2-methylhexane 2,3-dimethylpentane 3-methylhexane 3-ethylpentane n-heptane methylcyclohexane toluene 3-methylheptane n-octane

6.69 0.62 1.40 0.13 0.02 0.36 0.09 0.75 0.04 0.02 9.46 0.25 0.01 0.19 0.06 59.86 0.39 10.71 0.04 0.08

21.56 1.47 2.21 0.12 0.08 0.74 0.32 0.95 0.12 0.02 8.72 0.39 0.08 0.39 0.07 39.16 0.42 9.68 0.12 0.11

11.23 trace 0.03 trace trace 0.03 0.01 0.02 0.12 0.18 11.15 1.82 0.17 1.73 0.10 59.98 0.23 12.50 trace trace

ethylbenzene m-xylene p-xylene o-xylene isopropylbenzene n-propylbenzene m-ethyltoluene p-ethyltoluene 1,3,5-trimethylbenzene isobutylbenzene n-decane 1,2,4-trimethylbenzene 1-methyl-4-isopropylbenzene 1-methyl-3-n-propylbenzene 1-methyl-4-n-propylbenzene n-butylbenzene 1-methyl-2-n-propylbenzene dimethylethylbenzenes tetramethylbenzenes unidentified

0.12 0.11 0.10 0.04 1.93 0.69 0.15 0.15 0.05 0.07 0.60 0.24 0.41 0.43 0.32 0.39 0.03 0.16 0.26 2.63

0.41 0.26 0.18 0.10 1.66 1.36 0.50 0.30 0.11 0.19 0.54 0.39 0.31 0.74 0.42 0.48 0.11 0.34 0.33 4.53

trace trace trace trace 0.06 0.05 trace trace 0.01 trace trace 0.01 0.01 0.02 0.02 trace trace 0.01 trace 0.50

liquid yield, % RON of the liquid productb a

93.3 33.0

78.4 45.4

88.7 29.5

Feed ) 74.0 wt % heptane, 12.7 wt % benzene, and 12.8 wt % toluene. b Feed RON ) 24.4, calculated according to ref 32.

showed higher production of olefins in the absence of aromatics. However, because the resulting paraffin-toolefin ratio was high with both feeds, hydrogen transfer and possibly also direct hydrogenation reactions must be important. However, as previously reported,6 the relative importance of the direct hydrogenation reaction depends on the hydrogen partial pressure. It is not clear whether the results from the IR study, performed with clean catalysts and in the absence of hydrogen, should be more closely related to those from the flow reactor study performed in the presence or absence of hydrogen. Clearly, the presence of hydrogen during reaction will help to maintain the catalyst surface free of contaminants such as coke and perhaps in a condition similar to that in the FTIR study. At the same time, the presence of hydrogen under the reaction conditions can promote Mo reduction. However, temperature-programmed reduction (TPR) experiments performed on the catalyst (results not shown) indicate that, for our catalyst, reduction of Mo at atmospheric pressure starts at about 673 K. Because the reaction was performed at 588 K, we expect only a small reduction of the Mo catalytic surface under the reaction conditions. In fact, in the IR experiments, the molybdenum oxide surface is also reduced to a small extent during the pretreatment at 873 K under vacuum. Therefore, the reaction behavior of the complex feed, n-heptanebenzene-toluene, with the catalysts was studied in three cases: (i) over HZSM5-alumina in the absence of hydrogen, (ii) over HZSM5-alumina in the presence of hydrogen, and (iii) over Mo-HZSM5-alumina in the presence of hydrogen. The product distribution analysis is given in Table 2. In the presence of hydrogen the monofunctional HZSM5alumina catalyst is by far the most active system, giving both higher conversions and lower liquid yields than the Mo-HZSM5-alumina catalyst, according to the high cracking activity exhibited by the monofunctional catalyst. It is also evident that the presence of hydrogen is beneficial for conversion. It seems likely that the role

of hydrogen is to maintain a low concentration of olefins, thus avoiding the polymerization reactions responsible for coke formation, and also to promote the hydrogenation of coke or surface coke precursors. The capability of HZSM5 to hydrogenate olefins has been previously demonstrated in the hydroconversion of ethylene.7 In the presence of nitrogen, the conversion of n-heptane over HZSM5-alumina is only 19.1% compared to 47.1% obtained in the presence of hydrogen. In line with this result, in the presence of nitrogen, the liquid yield increases and less than 7 wt % of the products are gaseous. The incorporation of Mo into the catalyst produces a strong detrimental effect on conversion and, consequently, an increase in the liquid yield. In all cases the main gaseous products are n-butane, isobutane and propane, with smaller amounts of olefins, ethane, and methane. This indicates that n-heptane cracking occurs mainly through a consecutive scheme involving the isomerization of the initial sec-heptyl carbenium ion, leading to 2-methyl-4-hexil or 3-methyl-5-hexil carbenium ions, followed by type C β-scission. This route will produce n-butane and propane as final products and has been documented in several previous publications.8,9 Over HZSM5-alumina in the presence of hydrogen, the predominant reaction products lighter than heptane are alkanes, with a predominance of linear butane, pentane, and hexane over the corresponding branched isomers. Only very small amounts of light olefins are detected, with predominance of butenes and 2-methyl2-butene. Some branched heptane isomers and small amounts of heptenes are also found. Among the products heavier than the reactants, a clear predominance is found for alkyl benzenes and alkyl toluenes. Detectable amounts of alkanes higher than heptane (>C7) are also found, predominantly linear isomers, with only trace amounts of the branched isomers. These higher paraffins might be the result of aliphatic alkylation reactions between olefinic fragments and carbenium ions adsorbed on the catalyst surface, both of them produced during heptane cracking.

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3487 Table 3. Product Selectivity at Similar n-Heptane Conversions in the Presence of N2 or H2a HZSM5alumina gas atmosphere

N2

C10 n-heptane conversion (%) benzene conversion (%) toluene conversion (%)

33.507 10.708 6.327 4.558 2.390 16.042 13.345 13.121 19.11 24.29 16.32

H2

Mo-HZSM5alumina H2

product selectivity (%) 40.235 10.520 6.976 5.874 2.775 16.618 13.650 3.348 19.43 18.50 11.01

68.625 0.339 1.011 25.807 0.066 0.846 0.406 2.897 18.94 10.82 2.35

a The selectivity was defined as the weight percent of each product group divided by the total weight percent of the products and multiplied by 100.

Figure 2. Product distribution for the C8+ group as a function of Mo loading (wt %) in Mo-HZSM5-alumina catalysts: (×) C8, (0) C9, (2) C10, (b) C10+.

The experiments performed with the HZSM5-alumina catalyst using nitrogen instead of hydrogen result in a much lower conversion of heptane and a small decrease in the conversions of the two aromatic reactants. However, over this monofunctional catalyst, the overall product distribution of the main reactions (cracking and aromatic alkylation) does not seem to be strongly affected by the presence or absence of hydrogen when analyzed at the same conversion level (see Table 3), with the exception of the formation of more heavy products. This observation suggests that the main effect of adding hydrogen to the feed is to limit the coking of the acid sites. The addition of Mo to the HZSM5-alumina catalyst, in addition to causing a decrease in the overall conversion of heptane in the presence of hydrogen, apparently causes a marked simplification in the pattern of the main reaction products (see Table 2). Also, a significant increase in skeletal isomerization products (isoheptanes) is observed, and no significant amounts of alkyl aromatics are found. The strong decrease of alkylation products with the incorporation of Mo could be due to a decrease in the availability of the strong Brønsted acid sites or to a hindering of the accessibility of the aromatics to the internal surface of the zeolite, caused by a partial blockage of the pore entrances by Mo. These possibilities are further investigated below. The more pronounced production of paraffin skeletal isomerization products might be due, in this case, to catalysis by molybdenum species or by the external zeolite OH’s, which have been characterized previously as terminal silanols with medium Brønsted acid strengths.10 An analysis of the reaction selectivities at similar n-heptane conversion (Table 3) confirms the increase in skeletal isomerization and the decrease in the formation of alkyl aromatics. Additionally, it is clear from Table 3 that the selectivity toward the formation of light products produced by the cracking reactions is significantly increased, suggesting that additional cracking is occurring on Mo centers. To determine whether the presence of Mo on the catalyst suppresses the aromatic alkylation reactions, additional experiments were performed following the evolution of alkyl aromatics with the Mo catalyst content. These results clearly show that the incorporation of amounts of Mo beyond 3 wt % cause a strong decrease in the production of alkyl aromatics (see Figure

2). Previous literature data11 indicating the formation of naphthalene over Mo-ZSM5 catalysts during the aromatization of methane might contradict our findings. However, these experiments were carried out at a much higher temperature (973 K) and with a Mo loading below 4 wt %. Because the experiments shown in Table 2 were conducted with a catalyst containing 6 wt % Mo, the decrease in the production of alkyl aromatics with respect to that for the Mo-free catalyst is not surprising and is in line with the results in Figure 2. 3.3. FTIR Analysis. The FTIR study was divided into two main parts. First, the different catalyst samples were characterized in the OH and low-frequency regions (2500-4000 and 500-2000 cm-1, respectively), and then the interactions of benzene, toluene, and n-heptane with the HZSM5 and Mo-HZSM5 catalysts were studied by looking at the OH and C-H stretching regions. These experiments should provide information on the changes occurring in the ZSM5 as a result of the incorporation of Mo and on the interactions of the hydrocarbons with the different OH groups in the HZSM5 zeolite. 3.3.1. FTIR Spectra of the Catalysts after Outgassing at 873 K. In Figure 3, left, the FTIR spectra of the HZSM5 and Mo-HZSM5 catalysts, with and without alumina matrix, are compared with that of a commercial HZSM5 sample. The spectrum of the commercial sample shows a weak sharp maximum at 3746 cm-1 and a very intense band at 3612 cm-1. This spectrum agrees well with those reported previously for the same type of catalyst.12-15 According to the literature, the latter band is associated with the strong Brønsted acidic OH’s of the HZSM5 zeolite, whereas the former band at 3746 cm-1 is due to the free silanol groups located at the external surface of the zeolite. These silanol groups have been previously characterized by us16 as still being quite acidic, although less so than the internal ones. In the middle between these two bands a small band, likely associated mainly with nonframework alumina, is also present. The spectrum of our HZSM5 sample is similar to that of the commercial sample, although it presents an additional weak band near 3680 cm-1, possibly also due to nonframework alumina. The spectrum of the Mo-HZSM5 sample is the worst in quality (higher noise) because of the lower light transmittance by the sample, but it evidently also shows a strong band at 3613 cm-1, due to the acidic internal

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Figure 3. FTIR spectra, in the 2500-4000 and 600-2200 cm-1 regions, of the activated (873 K, 1 h) HZSM5 and Mo-HZSM5 catalysts with and without an alumina matrix, compared with that of a commercial HZSM5 sample: (a) reference commercial HZSM5, (b) HZSM5, (c) Mo-HZSM5, (d) Mo-HZSM5-alumina.

HZSM5 OH’s. In contrast, for this sample, the band due to external silanols is not present. This suggests, in line with the previous experiments at different Mo loadings, that at least some of the Mo species are located at the external zeolite surface where silanols are suppressed and that the incorporation of Mo does not entirely destroy or exchange the internal OH’s of HZSM5. A similar behavior regarding the decrease of the external silanol band was reported previously when Mo was incorporated into HZSM5 zeolite.11 The IR spectrum of Mo-HZSM5-Al2O3 also shows the band of zeolitic OH, although the overall spectrum is partially obscured by the presence of the alumina OH’s. The spectra of the three catalytic materials in the spectral region 3000-1300 cm-1 are almost identical, showing that no absorptions attributable to Mo oxide species can be detected for Mo-HZSM5. However, in the lower frequency region (Figure 3, right), where the skeletal spectra of bulk HZSM5 is observed, the Mofree samples show two windows in the 950-820 and 780-600 cm-1 regions, whereas the Mo-ZSM5 sample only shows the latter window. It seems likely that for this latter sample, the first window is obscured by the Mo-O stretching modes, which can, in part, fall just in that region. 3.3.2. Benzene Adsorption and Transformation. In Figure 4, the effects of benzene adsorption on the spectra of the two HZSM5 zeolites used here and MoHZSM5 are shown in the O-H and C-H stretching regions. As discussed previously,16 benzene interacts strongly with the internal highly acidic OH’s of HZSM5. This interaction results in the formation of quite strong hydrogen bonds. Evidence for this is provided by the strong OH stretching band centered near 3250 cm-1 that arises from the shift of the stretching band of the internal bridged zeolite OH’s at 3617 cm-1. This band is very evident for both HZSM5 samples (Figure 4, b and d). The extent of the shift (∆ν > 350 cm-1) is

indicative of a hydrogen bond with significant strength. On the other hand, the intensity of the band at 3617 cm-1 is substantially, but not completely, eliminated. This observation was taken as evidence of the unavailability of some of the internal acidic OH’s of HZSM5, likely because of steric hindrance,16 during interactions with rigid molecules such as benzene. The band component observed at 3510 cm-1, more evident for the commercial HZSM5 sample, is due to the shift of the silanol band originally at 3745 cm-1, as a result of the weaker although still quite strong H bonding (∆ν > 235 cm-1). These observations led us to conclude, in agreement with previous work,16 that some of the external terminal silanols, although less acidic than the internal bridging OH’s, still retain significant acid strength. These external silanols, which are more acidic than those present on silica, could well be the ones near aluminum cations. The mechanism for the increased acidity of terminal silanols near Al cations has been explained elsewhere.15,17 It is evident from Figure 4 and from other experiments in this work that the HZSM5 zeolite used here behaves similarly to the commercial sample used as a reference. The situation with the Mo-HZSM5 sample appears to be markedly different. In fact, in this case, upon benzene adsorption, the band at 3250 cm-1 is relatively much weaker, if it appears at all. Also, the main OH stretching component near 3500 cm-1 is hardly observed, whereas the band due to the acidic internal OH’s seems to be essentially absent. This was also confirmed by the subtraction spectra (spectrum before benzene adsorption subtracted from the spectrum after benzene adsorption, not shown). These data indicate that the benzene molecules do not reach the internal OH’s of Mo-HZSM5, possibly because the Mo species hinder the molecular diffusion of benzene into the zeolite cavities. On the other hand, the IR spectrum of benzene adsorbed on Mo-HZSM5 shows a significant spectral

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Figure 4. FTIR spectra of HZSM5 and Mo-HZSM5 zeolites in the 2500-4000 cm-1 region: (a) reference commercial HZSM5 activated at 873 K, 1 h; (b) reference commercial HZSM5, after benzene adsorption at RT; (c) HZSM5 activated at 873 K, 1 h; (d) HZSM5 after benzene adsorption at RT; (e) Mo-HZSM5 activated at 873 K, 1 h; (f) Mo-HZSM5 after benzene adsorption at RT. Table 4. Position of the IR Bands (cm-1) of Toluene and Benzene in the Liquid and Adsorbed States toluene liquid

adsorbed on Mo-ZSM5

3088 3064 3029 2979 2922 2874 1963

3086

1855 1803 1740 1605 1496 1462 1379 1175 1081 1030 729 694 a

adsorbed on HZSM5

benzene assignment

liquid

ν7a

2924 2870 1995

3087 3065 3030 2985 2925 2876 1956

ν2 ν20b νaCH3 νsCH3 ν19a + δsCH3 2ν5

3091 3071 3036 1960

1874

1888

ν10b + ν17a

1816

1820 1755 1603 1495 1457, 1463 1383 735

ν10a + ν17a ν10a + ν10b ν8a ν19a δCH2, δaCH3 δsCH3 ν9a ν18a ν18b ν11

1479 1036 673

3034

1605 1497 1460 1381

736, 726 696

697

adsorbed on Mo-ZSM5

adsorbed on HZSM5

3091

3092 3071 3038 1971

ν20 ν18 + ν19

νCH (FRa) comb. (FR)

ν1 + ν6 + ν19

comb. (FR)

ν5 + ν17

comb.

1830

ν10 + ν17

comb.

ν19

ring

ν18 ν11

δCH γCH

3041

2000 1980 1960 1880 1865

1481

691 679

1479 695

assignment

ν4

FR) Fermi Resonance.

perturbation with respect to that of the free molecule. In particular, we observe that components arising from the overtones involving the out-of-plane C-H deformation modes in the 2000-1600 cm-1 region are found at very high frequency with respect to those of the liquid (see Table 4). Also, the ν11 fundamental in the lowfrequency region, which is an out-of-plane C-H deformation mode too, is strongly shifted upward with

respect to the corresponding band for the liquid (see Table 4 and Figure 5). These perturbations are typically observed when the π-type electron cloud of benzene interacts strongly with electron-withdrawing centers. According to the absence of strong hydrogen bonding with the surface OH’s, it seems reasonable to propose that these perturbations are associated with the interaction of benzene with molybdenum centers.

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Figure 5. FTIR spectra of benzene interactions with both HZSM5 and Mo-HZSM5 catalysts in the 600-730 cm-1 region: (a) liquid benzene, (b) reference commercial HZSM5 after benzene adsorption at RT, (c) HZSM5 after benzene adsorption at RT, (d) MoHZSM5 after benzene adsorption at RT.

Additionally, the interactions of benzene with both HZSM5 and Mo-HZSM5 were studied at higher temperatures (up to 588 K). No new gas-phase species were found in these experiments, except for small amounts of CO (band with the typical PQR rovibrational contour, which consists of two maxima and one minimum at 2140 cm-1), which is likely a product of the oxidation of benzene by incompletely reduced Mo centers. This provides evidence for the reduction of Mo centers by benzene at high temperatures. Simultaneously, small amounts of carbonaceous species, associated with the main bands at 1605 and 1540 cm-1, are also formed at the surface. These bands can be ascribed to coke or cokeprecursor adsorbates. 3.3.3. Toluene Adsorption and Transformation. Studies analogous to those performed with benzene were performed with toluene, showing parallel results. In this case, the interaction of toluene with the surface OH’s of HZSM5 also gives rise to strong H bonding, which is responsible for the formation of a strong broad band near 3200 cm-1 (Figure 6, b and d). This occurs at the expense of a partial decrease of the band at 3617 cm-1, associated with the Brønsted acidic internal OH’s. The shift of this band (∆ν > 400 cm-1) upon interaction with toluene is even stronger than the shift upon its interaction with benzene (∆ν > 350 cm-1), in agreement with the higher electron density of the toluene ring due to the electron-donating effect of the methyl group. On Mo-HZSM5, this H bond is not found or is much less evident (Figure 6f). In this case, however, the bands for the adsorbed toluene species (Figure 7) show quite a strong perturbation (as deduced by the position of the out-of-plane deformation bands reported in Table 4), implying some kind of strong interaction. Thus, we can suppose that, also for toluene, the Mo species hinder the diffusion of the hydrocarbon into the zeolite pores

Figure 6. FTIR spectra of toluene interactions with HZSM5 and Mo-HZSM5 catalysts in the 2500-4000 cm-1 region: (a) reference commercial HZSM5 activated at 873 K, 1 h; (b) reference commercial HZSM5 after toluene adsorption at RT; (c) HZSM5 activated at 873 K, 1 h; (d) HZSM5 after toluene adsorption at RT; (e) Mo-HZSM5 activated at 873 K, 1 h; (f) Mo-HZSM5 after toluene adsorption at RT.

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Figure 7. FTIR spectra of toluene interactions with both HZSM5 and Mo-HZSM5 catalysts in the 600-760 cm-1 and 1300-2200 cm-1 regions: (a) liquid toluene, (b) reference commercial HZSM5 after toluene adsorption at RT, (c) Mo-HZSM5 after toluene adsorption at RT.

Figure 8. FTIR spectra of toluene interaction with Mo-HZSM5 in the 600-750 cm-1 and 1300-1700 cm-1 regions: (a) RT, (b) 523 K, (c) 588 K.

to reach the acidic internal OH’s but, at the same time, provide external sites for the strong adsorption of toluene. Experiments performed at higher temperatures allow some reactivity of toluene to be observed. In fact, upon adsorption of toluene on HZSM5, we observe the formation of gas-phase benzene, characterized by bands in the 3100-3000 cm-1 region and by the more typical bands at 1035 and 672 cm-1 (not shown). On Mo-HZSM5, we observe at 588 K the formation of gas-phase CO (not shown), due to reduction of molybdenum sites by

toluene, but also new weak bands at 673 and 748 cm-1 (Figure 8). The former band is certainly associated with benzene, showing that dealkylation and/or transalkylation occurs on both HZSM5 and Mo-HZSM5. The band at 748 cm-1 can tentatively be assigned to the presence of o-xylene (out-of-plane deformation). The formation of o-xylene and benzene can be explained assuming that toluene disproportionation occurs and that this reaction occurs on the outer surface of the MoHZSM5 crystals. In fact, it is well-known that o- and m-xylenes can actually be formed in the HZSM5 pores

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Figure 9. FTIR spectra of n-heptane adsorbed on Mo-HZSM5 in the C-H deformation region: (a) liquid n-heptane, (b) after n-heptane adsorption at RT, (c) after n-heptane adsorption at 523 K.

but do not leave them without first isomerizing into the less hindered p-xylene. This is the typical “product shape selectivity effect” that allows for the selective synthesis of p-xylene from toluene on ZSM5 catalysts. Thus, it seems quite reasonable to suppose that, in this case, Mo species located on the external surface of the zeolite are involved in the reaction. Also, upon hightemperature treatment (588 K), new bands, not due to toluene adsorbed as such, appear in the spectrum of the adsorbed species on Mo-HZSM5 (Figure 8). These bands are similar to, but more intense than, those observed for the benzene case at 1590 and 1525 cm-1 and can again be assigned to coke-precursor species. The more evident formation of these species from toluene than from benzene suggests that the methyl group of toluene might be involved. 3.3.4. n-Heptane Adsorption and Transformation. The IR spectrum of n-heptane adsorbed on HZSM5 (not shown) presents bands in positions similar to those observed for the pure compound. Therefore, it does not provide clear evidence of significant perturbations. In the C-H deformation region of the IR spectrum of heptane adsorbed at room temperature (RT) on MoHZSM5 (Figure 9b), we can detect the methyl and methylene deformations at 1468, 1460 (shoulder), 1379, and 1342 cm-1, almost the same as in the liquid. However, upon heating, strong reactivity is found. After contact with Mo-HZSM5 at 588 K, in the gas-phase spectrum (not shown), in addition to CO, we detect a CO2 band at 2340 cm-1 with the PR shape and a band at 1745 cm-1 that is certainly due to the CdO stretching of an aldehyde. We also find bands at 948 cm-1 (CH2 wagging of ethylene) and at 1035 and 673 cm-1, typically due to benzene. Nearly the same species, with additional cracking products such as propene and isobutene, have also been detected previously over pure HZSM5.12 It is therefore likely in this case that the n-heptane molecule actually manages to enter into the

zeolite cavity, despite the partial hindering effect due to the Mo species, according to its lower critical diameter with respect to aromatics. Additionally, in the spectrum of adsorbed heptane, a strong absorption centered again in the 1600 cm-1 region, with a strong decrease of transmittance, mainly for the Mo-HZSM catalyst, is found to grow in the spectrum of the catalyst treated at 523 K (Figure 9c). This is certainly associated with pronounced coking. It seems, consequently, that nheptane contributes to coking much more than toluene and benzene, possibly because of the higher reactivity of n-heptane. 4. Discussion The data reported above allow us to propose a picture for the behavior of HZSM5 and Mo-HZSM5 catalysts in the hydroconversion of an n-heptane-benzenetoluene mixture. The results of the catalytic experiments performed in the reactor show that, for the operating conditions of this work, the main reactivity is associated with the cracking of the n-heptane molecule and the alkylation of the aromatics by olefins produced during heptane cracking. These reactions are associated with strong Brønsted acidity18 and with well-known carbonium and carbenium ion chemistry.19-22 Additionally, we observe small amounts of skeletal isomerization products. This reaction is associated with acid sites of lower strength than is needed for cracking or alkylation.23 Our reaction data indicate that using hydrogen instead of nitrogen results in a strong increase in the catalytic activity of HZSM5 that is reflected both in a higher conversion of n-heptane and in a lower liquid yield. However, when analyzed at similar n-heptane conversions, the data indicate that the product selectivities in the two cases are not significantly different. This suggests that the main effect of hydrogen is to limit coke formation, thereby decreasing catalyst deactivation.

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The FTIR data provide evidence of the interaction of heptane, benzene, and toluene with both the strongly acidic internal OH’s and with the less acidic external silanol groups in HZSM5. This interaction involves, in the case of n-alkanes, mainly the C-C single bonds, which are the most electron-dense and, consequently, the most σ-basic.23,24 In the case of aromatics, this interaction is even stronger and involves H bonding with the π-type electron cloud. A detailed analysis of the IR spectra obtained upon adsorption of the different hydrocarbon molecules allowed us to conclude, in agreement with NMR spectroscopy data,25 that the observed interaction is a precursor for the formation of carbenium ions, such as alkyl carbenium ions from alkanes and arenium ions from aromatics. These carbenium ions are, according to recent theoretical treatments,26,27 transition states rather than intermediates, so they are not sufficiently stable species to be detected spectroscopically. The alkyl carbenium ions should mainly arise from the intermediate formation of carbonium ions resulting from the interaction of the surface OH’s with the C-C single bonds of paraffins. The catalytic data show that the addition of molybdenum to the catalyst strongly reduces the cracking and alkylation activities of the catalyst (Table 2). Actually, the alkylation activity is, under our conditions, associated with the cracking activity, because the olefins needed for alkylation are formed from n-heptane cracking. In any case, the loss of the cracking and alkylation activities should result either from the disappearance or significant weakening of the zeolite acid sites or from their unavailability. The active sites for such interactions, i.e., the internal bridging OH’s, which display very strong Brønsted acidity, are still present in the MoHZSM5 catalyst, as shown by the IR spectra (Figure 3). However, they are not perturbed significantly by the hydrocarbon adsorbates because the band at 3250 cm-1, which is the result of the perturbation, is, if present at all, far less intense than on the Mo-free sample (Figures 4 and 6). This indicates that, after incorporation of Mo into the catalyst, the internal bridging OH’s are no longer able to interact with the reactant molecules, in particular with the aromatics. This has been rationalized by assuming either that the Mo species enter the HZSM5 cavities and make their size smaller, hindering reactant diffusion into the cavities, or that Mo is deposited mainly at the external zeolite surface, partially blocking the channel entrance. The comparison of the IR spectra of HZSM5 and Mo-HZSM5 (Figure 3), showing that the external silanol groups are no longer apparent after Mo impregnation, suggests that at least some of the Mo species lie on the external zeolite surface. The partial anchoring of Mo on the external zeolite surface is in line with previous findings.11,28 Interestingly, Mo-HZSM5 seems instead to have an enhanced activity for n-heptane skeletal isomerization, decreasing the formation of alkyl aromatics and cracking products with respect to HZSM5. The usually accepted mechanism for acid-catalyzed alkane skeletal isomerization involves the initial formation of secondary carbenium ions from the corresponding carbonium ions. The secondary carbenium ions should undergo transposition to tertiary carbenium ions, which would later be hydrogenated by hydrogen-transfer reactions.15,18 The cracking and alkylation mechanisms also go through the initial formation of a secondary carbenium ion, followed by a β-scission step leading to a small olefin and a

carbenium ion, which can alkylate an aromatic molecule. This route, however, is substantially suppressed by Mo incorporation that partially blocks the access to the strong internal acid sites that promote the cracking and alkylation reactions. Alternatively, skeletal isomerization can also arise from metal centers.29,30 The behavior of the catalyst strongly supports the idea that, in the case of Mo-HZSM5, the active sites for skeletal isomerization are Mo centers. The tendency of Mo, in a reduced state, to produce isomers has been observed during the isomerization of cyclopropane over Mo/Al2O3 catalysts.31 Thus, the result of Mo addition is to decrease access to the strong acid sites in HZSM5 and to introduce partially reduced Mo centers that are capable of carrying out isomerization reactions. The result of this is a decrease in cracking activity, an increased production of branched paraffins, and a small but significant decrease in the amount of free benzene, without any significant increase in the total aromatic content of the liquid product. The final result is therefore a better liquid yield in the process and an improvement of the RON number of the product mixture compared to that of the feed, although it is lower than that produced by the metal-free HZSM5. 5. Conclusions Mo incorporation into the HZSM5 causes a significant decrease of the IR signal of external silanols, whereas at least some of the internal bridging OH’s remain. This suggests that some of the Mo species are located on the external zeolite surface and that the incorporation of Mo does not entirely destroy or exchange the internal OH’s of HZSM5. From the FTIR data, we can conclude that there are interactions of heptane, benzene, and toluene with both the strongly acidic internal OH’s and with the less acidic silanols groups in HZSM5. These interactions seem to be a precursor for the formation of carbenium ions, which are the active species in hydroconversion reactions. The catalytic experiments indicate that for the HZSM5alumina sample in hydrogen atmosphere, the catalytic activity was higher than in nitrogen atmosphere. This could be due to coke formation on the catalyst surface in the latter case. The addition of Mo to HZSM5, in addition to causing a decrease in the overall conversion of heptane, causes a marked simplification in the pattern of the main reaction products. The cracking and alkylation functionalities of the catalyst are almost suppressed, whereas a significant increase in skeletal isomerization is observed. This suggests that, for the Mo-HZSM5-alumina catalyst the availability of strong Brønsted acid sites decreases because of the presence of Mo, as was observed in the FTIR experiments. Acknowledgment We thank CONACyT (Me´xico), CNR (Italy), DGAPAUNAM, and the IMP FIES program for financial support. One of us (H.G.) acknowledges the scholarships from DGAPA-UNAM and CONACyT. We thank also L. Ban˜os for the XRD work. Literature Cited (1) Krenzke, L. D.; Kennedy, J. E.; Baron, K.; Skripek, M. Hydrotreating technology improvements for low-emissions fuels. Presented at the NPRA Annual Meeting, San Antonio, TX, March 1996; Paper AM-96-67.

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(2) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape-Selective Catalysis in Industrial Applications, 2nd ed.; Marcel Dekker: New York, 1996. (3) Chen, N. Y.; Garwood, W. E.; Heck, R. H. M-Forming Process. Ind. Eng. Chem. Res. 1987, 26, 706. (4) Scherzer, J.; Gruia, A. J. Hydrocracking Science and Technology; Marcel Dekker: New York, 1996. (5) Olson, D. H.; Haag, W. O.; Lago, R. M. Chemical and Physical Properties of the ZSM-5 Substitutional Series. J. Catal. 1980, 61, 390. (6) Meusinger, J.; Liers, J.; Mosch, A.; Reschetilowski, W. Cracking of n-Heptane on Metal-Free H-ZSM-5 Zeolite at High Hydrogen Pressure. J. Catal. 1994, 148, 30. (7) Kanai, J.; Martens, J. A.; Jacobs, P. A. On the nature of the active sites for ethylene hydrogenation in metal-free zeolites. J. Catal. 1992, 133, 527. (8) Martens, J. A.; Parton, R.; Uytterhoeven L.; Jacobs, P. A.; Froment, G. F. A comparison of platinum/ZSM-22, platinum/ ZSM-5 and platinum/USY zeolite catalysts. Appl. Catal. 1991, 76, 95. (9) Lugstein, A.; Jentys, A.; Vinek, H. Hydroconversion of n-heptane over Co/Ni-containing HZSM5. Appl. Catal. A: Gen. 1997, 152, 93. (10) Trombetta, M.; Busca, G. On the characterization of the external acid sites of ferrierite and other zeolites. J. Catal. 1999, 187, 521. (11) Jun-Zhong; Zhang; Long, M. A.; Howe, R. F. Molybdenum ZSM-5 zeolite catalysts for the conversion of methane to benzene. Catal. Today 1998, 44, 293. (12) Quin, G.; Zheng, L.; Xie, Y.; Wu, C. On the framework hydroxil groups of H-ZSM-5 zeolites. J. Catal. 1985, 95, 609. (13) Kustov, L. M.; Kazansky, V. B.; Beran, S.; Kubelkova`, L.; Jiru´, P. Adsorption of Carbon Monoxide on ZSM-5 Zeolites. Infrared Spectroscopic Study and Quantum-Chemical Calculations. J. Phys. Chem. 1987, 91, 5247. (14) Zecchina, A.; Bordiga, S.; Spoto, G.; Scarano, D.; Petrini, G.; Leofanti, G.; Padovan, M.; Otero-Arean, C. Low-temperature Fourier transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite. J. Chem. Soc., Faraday Trans. 1992, 88, 2959. (15) Trombetta, M.; Busca, G.; Rossini, S.; Piccoli, V.; Cornaro, U.; Guercio, A.; Catani, R.; Willey, R. J. FT-IR Studies on Light Olefin Skeletal Isomerization Catalysis. III. Surface Acidity and Activity of Amorphous and Crystalline Catalysts Belonging to the SiO2-Al2O3 System. J. Catal. 1998, 179, 581. (16) Trombetta, M.; Armaroli, T.; Gutie´rrez-Alejandre, A.; Ramı´rez, J.; Busca, G. An FT-IR study of the internal and external surfaces of HZSM5 zeolite. Appl Catal. A: Gen. 2000, 192, 125. (17) Trombetta, M.; Busca, G.; Lenarda, M.; Storaro, L.; Pavan, M. An investigation of the surface acidity of mesoporous Alcontaining MCM-41 and of the external surface of ferrierite through pivalonitrile adsorption. Appl. Catal A: Gen. 1999, 182, 225.

(18) Wojciechowski, B. W.; Corma, A. Catalytic Cracking Catalysts, Chemistry and Kinetics; Marcel Dekker: New York, 1986. (19) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; John Wiley and Sons: New York, 1995. (20) Brouwer, D. M.; Hogeveen, H. Electrophilic substitutions at alkanes and in alkylcarbonium ions. Prog. Phys. Org. Chem. 1972, 9, 179. (21) Brouwer, D. M. Reactions of alkylcarbenium ions in relation to isomerization and cracking of hydrocarbons. In Chemistry and Chemical Processes; Prins, R., Schuit, G. C. A., Eds.; Sythoff and Noordhoff: Alphen aan den Rijn, The Netherlands, 1980. (22) Vogel, P. Carbocation Chemistry. In Studies in Organic Chemistry; Elsevier: Amsterdam, The Netherlands, 1985; Vol. 21. (23) Olah, G. A. Carbocations and electrophilic reactions. Angew. Chem., Int. Ed. Engl. 1973, 12, 173. (24) Sommer, J.; Jost, R.; Hachoumy, M. Activation of small alkanes on strong 319 solid acids: Mechanistic approaches. Catal. Today 1997, 38, 309. (25) Haw, J. F.; Xu, T. NMR Studies of solid Acidity. Adv. Catal. 1998, 42, 115. (26) Kazansky, V. B.; Frash, M. V.; van Santen, R. A. Quantum chemical study of the nonclassical carbonium ion-like transition states in isobutane cracking on zeolites. In 11th International Congress on Catalysiss40th Anniversary; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Eds.; Elsevier: Amsterdam, 1996. (27) Bates, S. P.; van Santen, R. A. The molecular basis of zeolite catalysis: A review of theoretical simulations. Adv. Catal. 1998, 42, 1. (28) Wang, D.; Lunsford, J. H.; Rosynek, M. P. Characterization of a Mo/ZSM-5 catalyst for the conversion of methane to benzene. J. Catal. 1997, 169, 347. (29) Maire, G. L. C.; Garin, F. G. Metal catalysed skeletal reactions of hydrocarbons. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 6, p 161. (30) Keller, V.; Barath, F.; Maire, G. Catalytic activity of reduced MoO3/R-Al2O3 for hexanes reforming. II. Catalytic activity and mechanistic approach using 13C tracer studies and probe molecules. J. Catal. 2000, 189, 269. (31) Lo Jacono, M.; Hall, W. K. Cyclopropane Reactions over Reduced Molybdena-Alumina. J. Colloid Interface Sci. 1977, 58(1). (32) Walsh, R. P.; Mortimer, J. V. New way to test product quality. Hydrocarbon Process. 1971, Sept, 153.

Received for review October 13, 2000 Revised manuscript received February 5, 2001 Accepted May 12, 2001 IE000881I