Hydrotreatmentand Hydrocrackingof Oil Fractions B. Delmon,G.F. Fromentand P. Grange(Editors) 9 1999ElsevierScienceB.V. All rightsreserved.
Hydrogen Spillover and hydrocracking,-hydroisomerization K. Fujimoto Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract It was found that proton on Br0nsted acid site of zeolite is easily exchanged with dihydrongen and that the desorption of adsorbed pyridine on Lewis acid site is promoted by gaseous hydrogen when zeolite carded noble metal or was hybridized with supported noble metal. Hydrogen in the gas phase was assumed to bedissociated into atomic fiydfogen on the noble metal and move onto zeolite, where hydrogen atom is converted to either proton (H § and hydride ion (H). It was claimed that the common characteristic feature of hydroconversion (hydrocracking, hydrotreating, hydroisomerization) catalysts is that the catalyst is composed of two functions: hydrogen activation and another one (acid, desulfurization et al.). Hydroisomerization catalyst, which is usually composed of solid acid and supported platinum on it, and has been thought to be a typical bi-functional catalyst, can be substituted by the physical mixture of Pt/Si02 or Pd/Si02 and zeolite. It was suggested that proton on acid activate paraffinic hydrocarbons to carbenium ion to catalyze isomerization or carcking reaction then the spilt-over hydride ion react with carbenium ion to make stable hydrocarbons.
1. INTRODUCTION Spilt-over hydrogen is known to have strong effect on catalytic reaction system on solid acid catalysts. Nakamura et al. found that hybrid catalyst, i.e. the physical mixture of Pt/SiO2 and HZSM-5, was very effective for the isomerization of n-pentane, n-hexane and other paraffinic hydrocarbons under hydrogen atmosphere [1-5]. They concluded that both high conversion and high selectivity were due to the effect of hydrogen spillover, and suggested that spilt-over hydrogen has two forms H § and H, and that the former regenerates Bronsted acid site while the latter stabilizes carbenium ion intermediate by its hydrogenation. Hattori et al. pointed out that hydrogen promoted the activity of cumene cracking o v e r Pt/SO42-ZrOz and inhibited its deactivation. It is expected that Br0nsted acid site generated from spilt-over hydrogen acts as the active site for the catalytic reaction . Hosoi et al. found that when Pt/SOa2-ZrO2 catalyst was used for skeletal isomerization of n-pentane in the presence of hydrogen, it showed not only high activity but also persistence of the activity for a long period, more than 1000 h. They explained that the hydrogen had the effect on the removal of coke formed during the reaction by hydrogenating it . It has been reported that spillover of hydrogen occurs even when physical mixture of supported metal catalyst and zeolite, namely, hybrid catalyst, is exposed to hydrogen atmosphere . Hydrogen molecule in gas phase is
38 dissociated on metal surface to atomic hydrogen at first and then migrates onto the support surface, and further, onto the surface of adjacent zeolite particles. Zhang et al. reported that pyridine chemisorbed on acid sites of zeolite could be hydrogenated into piperidine by spiltover hydrogen . The phenomena of spillover first noticed was the promoted decomposition of GeH4 on a Ge film with a Pt wire , the reduction of WO 3 to WO2 by hydrogen at room temperature for a mechanical mixture of WO3 with Pt/A1203  and the accelerated reduction of NiO by hydrogen when mixed with palladium or platinum . Als0, isotopic exchange of OH groups on Al:O3 due to spillover was reported in 1965 . Reverse hydrogen spillover on active carbon or zeolite was claimed as the key step of paraffin dehydrogenation . Important phenomena caused by spillover were summarized in a several reviews. They are (1) enhanced adsorption, (2) surface isotopic exchange, (3) bulk change, (4) strong metalsupport interaction (SMSI). Influences of spillover on catalytic process may be described as (1) spilt-over species keeps catalyst clean, (2) create or regenerate selective sites through a remote control mechanism, and (3) as a regult, catalytic reactions are accelerated and catalyst deactivation is inhibited, effectively. There are a lot of discussion about the nature of spilt-over hydrogen species, such as H atoms, radicals, H § and H ions, ion pairs, H3§ species or protons plus electrons [15,16]. Protons formed from spilt-over hydrogen are suggested to act as catalytic active site for acid catalyzed reaction [17-19]. The present authors have pointed out the possibility of the participation of H § and H which are produced from spilt-over hydrogen in the hydroisomerization or hydrocracking of aliphatic hydrocarbons over Pt or Pd-supp0rted zeolite or physically mixed Pt/SiOE-protonic zeolite system [1,5,20]. Roland et at. have given a clear proof for the electrical charge of the spilt-over species, which was obtained through H-D exchange studies on the influence of a homogeneous magnetic field. The migration of spiltover hydrogen (deuterium) in Pt/NaY-HNaY catalyst was hindered, which was attributed to the influence of the Lorentz force on the electrically charged moving particles [21,22]. Pyridine is a typical organic base and can be chemisorbed on both BrCnsted (B) acid sites and Lewis (L) acid sites in zeolite catalyst while pyridinium ion and coordinately bonded pyridine complexes are formed on B and L sites, respectively-giving different m-adsorption bands on each occasion [23-25]. Zhang et al. reported that they found by FTIR that pyridine strongly adsorbed on acid sites of H-ZSM-5 was hydrogenated over Pt/H-ZSM-5 (0.5 wt%) and a Pt-Hybrid catalyst (a physically mixed catalyst with a weight ratio of Pt/SiO: (2.5 wt%) : H-ZSM-5 = 1:4) to adsorbed piperidine in the presence of gaseous hydrogen at around 473 K, whereas no such phenomena was observed on either H-ZSM-5 or Pt/SiO2126]. Y. Fan et al. reported that this hydrogenation rate was dependent not only on the nature of supported metals in hybrid catalyst system but also on the acidic strength of zeolite catalysts . This paper deals with the hydrogen spillover and its role in the catalytic hydroconversion.
In the. early period of spillover research, the main experimental method for detecting
39 spillover phenomenon is the adsorption technique, where much more amount of hydrogen, which should be adsorbed on supported metal, was adsorbed on metal-supported catalyst. Especially, for the carbon-supported system, the adsorption and the temperature programmed desorption technique revealed very clearly the reversible adsorption of hydrogen on active carbon through supported metal or metal sulfide and its participation ~ in the catalytic dehydrogenation. Also, the acceptor site of slSilt-over hydrogen has been concluded to be the hydrogen-unsaturated carbon (free radical). However, this method has not successively applied to solid acid such as zeolite, because of extremely small ~tmount of acceptor site. Other clear phenomenon about hydrogen spillover is the reduction of metal oxide at much lower temperature than metal free system in either metal-supported case or physically mixed metal-oxide system. This phenomenon is strongly related to the synergistic effect of cobalt or nickel for Co-Mo or Ni-Mo HDS catalyst. As it has been clearly demonstrated by Delmon et al., hydrogen which is spilt over at Co or Ni-site migrate from the site to Mo st~lfide site to react with sulfide ion and remove it as H2S to increase the sulfur deficiency on Mo, which is the active site of HDS reactionr This pl'fenomenon happens even when Co or Ni site is separated from Mo site by support. This concept also claimed for Pt-Mo system . One of the most important actions of spilt-over hydrogen is the generation and/or interaction with acid site. It has been reported that the protonic acid sites were generated and the Lewis acid sites were weakened on Pt/SO42-ZrO2 by heating in the presence of molecular hydrogen. It means that molecule on Pt, spillover of the H atom onto SOaZ-ZrO2 surface. It is suggested that the shift of the S=O stretching band to a lower frequency by heating in the presence of hydrogen is caused by the electron transfer from the spilt-over hydrogen atom to the Lewis acid sites. In the case of CoMo/SiO2+silica-alumina. (physical mixture), synergy effect in the selective cracking of diphenylmethane to benzene and toluene in the presence of H2 can be interpreted by the creation of BrCnsted acid site from the spillover hydrogen . Recently my group has studied the interaction between spilt-over hydrogen and acid sites on silica supported noble metal+zeolite hybrid catalysts by means of FTIR, especially, it was investigated the effect of hydrogen spillover on adsorbed pyridine over Pd/SiO2+H-USY hybrid catalyst. Figure l(a) shows the change of FTIR spectra when hydrogen gas was introduced to the sample with pyridine adsorption. The amount of pyridine left on B (1540 cm 1) and L (1450 cm "1) sites were 85% and 50% to saturated amount at 423 K. At initial stage of hydrogen introduction, L peak decreased and B peak increased. After B peak reached the saturation level, the peaks assigned to piperidine appeared. The results indicated that when vacant B sites exist, pyridine adsorbed on L site migrated to the B site. Figure l(b) shows the spectra of the sample with very low coverage of pyridine (B:55% and L: 25%). Pyridine migration from L to B was clearly observed and pyridine hydrogenation did not proceed at all even after 150-min hydrogen flow. This indicates that the spilt-over hydrogen promotes the migration or the desorption of pyridine on L acid site, suggesting that one of spilt-over hydrogen species can be hydride (H) ion, because this species is high L basic.
evacuation at 423 K evacuation at 423 I(
1500 Wavenumbers , cm -~
1500 Wavenumbers ' c m -L
Fig. 1. FTIR spectra of pyridine adsorbed on Pd/SiO2 + USY hybrid catalyst. (a) Initial coverage of pyridine: B 85% L 50%, (b) B 55% L 25%. Condition of H 2 flow: 423 K, 15 ml/min, pure H 2.
a) L o w
HJ'H -USY X~
b) Fti~ coverage
Fig. 2. Model scheme of migration of pyridine adsorbed on L acid site to B acid site promoted by hydrogen spillover effect, and subsequent hydrogenation of pyridine to piperidine.
41 3. HYDROISOMERIZATION HYDROCARBONS
Hydroisomerization on supported and hybrid catalyst
It is well known that platinum supported acidic solid catalysts are effective catalysts under hydrogen atmosphere for the paraffin isomerization and that the role of supported platinum and acidic solid are: (1) dehydrogenation of n-paraffins to n-olefins on platinum, (2) isomerization of linear olefins to branched olefins on acid site and (3) its hydrogenation to iso-paraffins. Other interpretation of the platinum role is that the short distance between acid site and platinum, which hydrogenate olefins, is essential for the selective isomerization . In Table 1 it is seen that the hybrid catalyst composed of physically mixed and pressed fine powders of Pt/SiO2 or Pd/SiO2 and H-ZSM-5 show equivalent catalytic activity for n-pentane isomerization to that of Pt-suppQrted H-ZSM-5 in spite of that Pt/SiO2 or Pd/SiO2 shows negligible activity for either dehyhrogenation or isomerization under conditions sdopted. Also, it should be noted that the hybrid catalyst composed of mixed granules(Dp-lmm) of Pt/SiO z and H-ZSM-5 showed poor isomerization activity .
Table 1. Isomerization of n-pentane on ZSM-5 . Catalyst Conversion/%
Pt/SiO2 a) Pt]SiO 2 Pt/SiO 2 Pd/SiO 2 +H-ZSM-5 b) +H-ZSM-5 c) +H-ZSM-5 d)
i-C5 C6+ aliphatics
Reaction condition: 423 K, n-Cs:H2=0.1MPa:0.9 MPa, W/F=10 gh/mol. a) Pt (2.5 wt%)/SiO2 0.2 g, b) powdery mixture, Pt (2.5 wt%)/SiO2: H-ZSM-5=I:4, c) granular mixture (0.3 ram), Pt (2.5 wt%)/SiO2: H-ZSM-5=I:4, d) powdery mixture, Pd(2.5 wt%)/SiO2: H-ZSM-5=I:4. If the conventional working hypotheses are correct the hybrid catalyst containing Pd]SiO 2 should show no isomerization activity because Pd/SiO2 has no dehydrogenation activity under these conditions. Also the granular mixture of Pt/SiOz-H-ZSM-5 should show equivalent isomerization activity because the normal olefins formed on Pt/SiO2 can move quickly to the acid site on H-ZSM-5 through gas phase as shown in Fig. 3(a). However, experimental results
42 are not consistent with the expectations. If the hydrogen which has spilt over from Pt/SiO 2 or Pd/SiO2 to acid site act as an acid catalyst for the paraffin isomerization (Fig. 3(b)), all experimental results can be explained quite reasonably.
(b) Spiilover Model
Powdery Mixture n-C 5 .
No significant difference
Granular mixture n-C 5
Powdery M'lxture _ . .H 2'
Granular mixture i-C 5
_ PH t~
Figure 3. Interpretation of catalytic activity on powdary mixture and granular mixture by bifunctional model (a) and spillover model. For example, the excellent activity of Pd/SiO 2 hybrid catalyst even with its negative dehydrogenation ability is quite understandable if it is assumed that the Pd site is the entrance of hydrogen from gas phase to acid site. Also, the poor catalytic activity of Pt/SiOz-H-ZSM-5 granular mixture could be attributed to that, the hydrogen species spilt over from Pt to SiO 2 surface has seldom chance to transfer to zeolite. The experiment of
means the switching of
atmosphere from hydrogen--'nitrogen-*hydrogen was designed and conducted with the two catalysts Pt/HZSM-5 and Pt-hybrid. Figure 4a, and 4b show the results. Under the atmosphere of H2, the conversion and i-pentane selectivity is kept high and stable both for Pt/HZSM-5 and Pt-hybrid, however, when gas flow stream was switched from H 2 to N2, the conversion on Pt/HZSM-5 increased dramatically, almost up to 100%, then deceased gradually. For Pthybrid catalyst, no such enhanced conversion was observed. In contract with the response in conversion, the selectivity lowered rapidly for both cases. The isomerization selectivity values
43 dropped from 98% to 10% just in 30 sec. At this stage, cracking reaction took place, bringing about the production of a large amount of C3, C4 paraffins. The reaction in N2 atmosphere was continued for about 1 h and then the gas flow was switched again from N 2 to H 2, the response of selectivity and activity was described as: the isomerization selectivities were restored rapidly for both catalysts whereas the recovery of conversion for the two c~ttalysts was quite different. The activity of Pt/HZSM-5 could be restored to about 80% of the initial level under Hz but the recovery rate was slower than that of selectivity, for Pt-hybrid, on the other hand, the activity did not recover anymore. Even after the carbon was completely~ removed by air calcination, the activity was not restored. However, if the deactivated catalyst was shaped again (including grinding and pressuremolding), the conversion and selectivity could be restored completely. The amounts of carbon deposited at different reaction stages were measured using the method mentioned above. The vertical lines in the same figures represent the results. Almost no carbon depositing was 100
(") 20 A C%
Time on stream (min) 250~
Time on stream (min)
deactivated in N., "
after removal of deposited surface carbon
Fig. 4 Effect of atmospheric gas on activity selectivity, coke and catalyst model.
44 detected in H2, but as long
N 2 was introduced, e/,en if only lmin, carbon depositing had
began and came up to 4.25% for Pt/HZSM-5 and 3.42% for Pt-hybrid in one hour. After gas flow was switched from removed.
N 2 to H 2
again, most of the surface hydrocarbon deposit could be
The model of different behavior of the hybrid catalyst is showr~ as in Fig. 4.The
deposited coke, which is formed during the reaction under N 2 should separate the particle of zeolite from Pt/SiO2. Therefore, even if the coke is removed by treating with either split-over hydrogen or air.
The lost contact can never restored and it is restored only when it is pressed
again. The newly postulated reaction mechanism of paraffin hydroisomerization isshown in Fig.5, which involves (1) the dissociation of gaseous hydrogen on metal site to atomic hydrogen and spillover to acid surface as H § and H-, (2) the activation of paraffinic hydrocarbon by H § to carbenium ion on acid site to result in the isomerization to branched carbenium ion and (3) the reaction of isomerized carbenium ion with-H to make isomerized paraffin. This explanation coincides with the fact no olefins are detected in the gas phase on hydroisomerization system even the catalyst is Pt- or Pd containing hybrid catalyst system and that even the hybrid catalyst which contain Pd/SiO 2 shows comparable activity.
. -sc, ss,on
1l +H-so ~ ~+H+so, .--I-t2
+H'so +H+so, -H2
Fig. 5 Reaction model of n-pentane hydrocracking over Pd/SiO2-DAM hybrid catalyst
3.2 Hydrocracking of n-Heptane on Pd containing mordenite Hydrocracking of normal paraffins on metal-supported zeolites, which only
45 paraffins as primary products was studied. Fig. 6(a) shows the changes of catalytic activities of a veriety of catalysts containing Pd/SiO2 and/or H-M(mordenite) for n-heptane hydrocracking.
Pd/H-M, which is a typical
dual functional catalyst, showed excellent activity for the hydrocracking. activity of H-M was not affected by the atmosphere and decreased quickly. little activity for both dehydrogenation and cracking of n-C7H16.
The catalytic Pd/SiOz showed
On the other hand, the
o Pd-hybrid H2~)
9 Pd-hybrid N21) ~' 50 o
[5 DAM H22)
A Pd/SiO 2 H22)
0.5 1 1.5 2 2.5 Time on Stream (h)
O Pd/DAM H22)
563 K, H2/n-CT--9, 1.1 MPa, 11 Pd/SiO2:DAM=I:I, W/F=2.4 g h mol-1, tool-1
W/F=I.2 g h
Fig. 6 Hydrocracking of n-neptane over Pd-DAM hybrid catalyst; a) conversion as a function of time on stream, b) C-number (TOS=2.5 h), c) distribution of C4 hydrocarbons formed in the hydrocracking of n-C7 (TOS=2.5 h). catalytic activity of a hybrid catalyst comprising Pd/SiO2 and H-M was the highest and its activity was kept constant under hydrogen atmosphere while it was much lower and decreased
46 quickly under nitrogen atmosphere.
This phenomenon clearly shows that the presence of
hydrogen is essential to generate hydrocracking activity. It is well known that the supported platinum shows a high catalytic activity for the dehydrogenation of paraffin whereas the supported palladium does not.
The results shown
in Fig. 6(a) suggest that the dehydrogenation activity of supported metal is not essential for the appearance of the paraffin hydrocracking activity, but the hydrogen migration from Pd/SiOz or supported palladium to acid site should be essential for the high and stable catalytic activity. The characteristic feature of the product distribution is that the reaction products of Pdhybrid catalyst system are only isomerized heptane and propane and equimolar amount of isobutane (small amount of n-C4Hlo was formed), whereas the products ,on ,H-M alone distributed from C3 to C9 and Ca products contained all kind of paraffins and olefins as shown in Fig. 6(b).
The wide product distribution for H-M system should be attributed to the
reaction path comprising oligomerization of cracked fragments and cracking of the oligomers. In hydrocracking of normal paraffin with metal supported acid catalyst, the iso/normal ratios in the paraffinic products generally exceed the thermodynamic equilibrium(Fig. 6(c)). It proves that at least some of the branched paraffins are primary products of the cracking and notthe result of the post isomerization.
This is particularly true in the case of C4, since n-
butane cannot be isomerized under typical hydrocracking conditions with a zeolite catalyst. One probable path is the skeletural isomerization of n-paraffin to branched paraffin and its cracking.
Fig. 7 shows the results of hydrocracking kinetics of C7 isomers.
dimethylhexane was more reactive than n-neptane was less reactive than the other branched isomers.
The reactivities of the C7 isomers can be explained by stability of the
corresponding carbenium ion.
These facts indicate that n-neptane is isomerized to 2-
methylhexane and then it is further isomerized to 2,4-dimethylpentane and both of then were cracked to give propane and iso-butane.
It has been suggested that the formation of
multibranched isomers from the feed and cracking are consecutive reactions .
of a normal paraffin must thus proceed through the stage of formation of monobranched isomers such as 2-methylhexane, dibranched isomers such as 2,4-dimethylpentane and finally cracked.
In hygrocracking, reaction path which include hydrogen are shown in Fig. 5.
Both proton and hydride ion should be participated in either the activation of hydrocarbon molecule or stablization of acrbenium ion. Hydride ion reacts with the cationic-cracked products on zeolite while and olefinic-cracked products are hydrogenated on palladium to be converted into the less reactive smaller paraffins. Isobutylene, which is one of a pair of the primary cracked products of the 2-methylhezane or 2,4-dimethylpentane hydrocracking, will be hydrogenated to isobutane over palladium catalyst in the presence of hydrogen.
1t30 90 ~, 80
100 90 ~..,--80
"~ 6o o 50 9 40 o 30 20 10 0
"~ 60 50 40
3o r,.) 20 1o o 0
W/F(g h tool -l )
W/F(g h rnol1 )
 Conversion(%}' 9 Sel. of Cracking(%) A Sel. of Isomerization(%) 523 K, H2/n-C7---9, I.I MPa, Pd/SiOz:DAM=I'I, Fig. 7 Hydrocracking of C7 isomers over Pd-DAM hybrid catalyst. 3.3'
Effect of hydrogenation
activity on isomerization
Fig. 8 shows the effect of catalyst composition on the activity and selectivity of the hydroconversion of n-hcxane on Pd/SiOz - H-M hybrid catalyst. As apparently from the data in Fig 8, Pd/SiO2 shows no activity and H-M shows quite low activity. However, their physical mixture shows much higher activity. With increasing Pd/SiO2 content the selectivity of hydroisomerization is increased, while the hydrocracking selectivity increases with increased mordenite content. In terms of TOF (based on acid site), cracking reaction is almost independent on the catalyst composition, whereas that of isomerization reaction is markedly accelerated by increasing Pd/SiO 2 content. These facts suggest that the increased amount of hydrogen promotes the isomerization reaction, while it does not promote the cracking. This concept is also supported by the effect of hydrogen pressure on the hydroconversion on the Pd/SiO2-hybrid catalyst as shown in Fig. 9. In case of the catalyst with the composition of Pd/SiO2 to mordenite ratio is 1:4, the cracking rate increases quickly with the increase in hydrogen pressure to reach maximum at about 1 MPa and then decreases while the isomcrization rate increases mononously up to 2.5 MPa. These phenomena also happens with thc catalyst of highcr Pd/SiO2 contcnt (Pd/SiO2" moredcnite is 3:1), but the maximum rates appcar at lower hydrogen pressure, probably because of higher hydrogen supply.
explained reasonably by the reaction model which include hydrogen spillover (H § and H) shown in Fig. 5. At the initial stage of the reaction, n-paraffin is first activated to secondary carbcnium ion and then isomcrized to branched carbcnium ion (tertiary carbenium ion).
this carbcnium ion reacts with H to form stable paraffin, the reaction is isomcrization.
48 However, the carbenium ion is not stabilized by H , i t reacts further along the O-scission to make cracking product. Thus, hydrogen species (H) on the acid site promote isomerization. It is clear that split-over hydrogen is essential for the activation of paraffins and control the acid-catalyzed reaction. 4. CONCLUSION It is darified that hydrogen spillover onto zeolite for either B-acid site and L-acid site and that the role or noble metal and gaseous hydrogen in the hydroconversion of paraffinic hydrocarbones on metal-supported solid acid.
The similar model should be applied for the
practical hydroconversion catalysts. 280~C, 1MPa, W/F=1 ~40 = 30 O
r,..) lO . , . . . . . l. . . . . . . . .
i. . . . . . . . .
I. . . . . . . . .
I. . . . . . . .
9 ~. ....
~---~.___._____~ 1 2 Hydrogen pressure/MPa
9 Pd/SiO2:Mordenite=l:3 20 PdySiO 2
Mordenite content (%) Mor.
Fig. 8 Hydrocracking of n-hexane over mordenite hybrid catalysts
9 Pd/SiO2: Mordenite =4:1 Fig. 9 Effect of hydrogen pressure on hydroisomerization and hydrocracking of n-decane
49 5. R E F E R E N C E S
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