JOURNAL OF MOLECULAR

CATALYSIS E L S E V I E R Journal of Molecular Catalysis 88 (1994) 343-358

Paraffin dehydrocyclization. Part 9. Conversion of n-octane with Pt-Sn catalysts at atmospheric

pressure

Ram Srinivasan and Burtron H. Davis* Center for Apphed Energy Research Universz~, of Kentucky, 3572 Iron Works Pike, Lexmgton, KY 40511, USA

( Received January 18, 1993; accepted November 5, 1993 )

Abstract

Tin alters the activity and stability of a series of Pt-alumina catalysts. Depending upon the support, dehydrocyclization may occur by a monofunctional metal catalyzed pathway or in combmation with a bifunctional pathway involving both metal and support. The mitial tin added by coimpregnation with l°t is found to decrease the activity of a catalyst based on acidic alumina but to promote the activtty of one based on a nonacidic support. When the catalyst based on an acidic alumina contains sufficient tin to eliminate the acidity of the support, and thereby the bifunctional pathway, the activity pattern vs. Sn/Pt ratio for the nonacidic and acidic based catalysts have a similar form. For similar Sn/Pt bulk compositions, a catalyst based upon coprecipitated tin-aluminum oxide differs signifi- cantly from the one based on an acidic or non-acidic alumina support. The results show that tin may serve ( 1 ) to enhance metal dispersion at lower loading, (2) to form a Pt:Sn = 1 : 1 alloy with a higher dehydrocyclization activity than Pt alone, ( 3 ) to modify the aromatics selectivity from the dehydro- cyclization of n-octane and (4) to poison acid sites and thereby eliminate the more rap~d pathway to produce aromatics.

Key words: debydrocychzatlon" octanes, platinum; tin

I. Introduct ion

The P t - S n system has been studied extensively, both f rom the standpoint o f the structure

of the catalyst as wel l as the reason for the superior catalytic properties. Davis uti l ized P t -

Sn complexes , including Pt3SnsC12o, in an organic solvent to decrease or e l iminate the

hydrolysis o f the tin compound used to prepare supported catalysts [ 1 ]. It was proposed

" Corresponding author.

0304-5102/94/$07.00 © 1994 Elsevier Sctence B.V. All rights reserved SSDI0304-5 102(93) E0280-T

344 R. Srimva~an, B H Darts/Journal of Molecular Caml'~sts 88 (1994) 343-358

that the reason for the superior activity of catalysts prepared in this manner was the formation o f a Pt-Sn alloy [2,3].

Burch and coworkers [4-7] reported on the use of Pt-Sn catalysts for hydrocarbon conversions. It was concluded that alloys of Pt and Sn were not formed so that this could not account for the changes in the catalytic properties imparted by Sn [4]. Burch and Garla concluded for these catalysts that: (i) n-hexane is isomerized by a bifunctional mechanism, (ii) benzene and methylcyclopentane are formed directly from n-hexane at metal sites, and (iii) the conversion of methylcyclopentane requires acidic sites [5]. It was concluded that the Sn(II) ions modified the Pt electronically with the result that self-poisoning by hydro- carbon residues is reduced. These later observations were the results of conversions at a pressure of one atmosphere.

V61ter and coworkers [ 8-15 ] have also reported extensively on the Pt-Sn catalyst system over a number of years. In an early study, tin was tbund to decrease both hydrogen adsorption and the rate of cyclohexane dehydrogenation but to increase the activity for n-heptane dehydrocyclization [8,9]. It was also reported that bimetallic catalysts can have both a positive and a negative shift of the activity and selectivity. The negative effect is connected with mild reaction conditions, and the positive effect with the severe deactivation reaction of aromatization. For dehydrocyclization the role of Sn is to decrease poisoning by carbo- naceous residues. Lank et al. [ 1 1 ] obtained similar results tbr n-hexane conversion using either an alumina supported or an unsupported Pt-Sn catalyst: thus, they attributed the impact of tin to a direct interaction with Pt, and not to an interaction of Sn with alumina. V61ter and Kiirschner [ 12] found that the addition of tin caused an increased production of benzene from methylcyclopentane, and at the same time a decrease in hydrogenolysis and deactivation, even though coke formation was not diminished from that of the Pt-alumina catalyst. V61ter and Ktirschner attributed these effects to Pt-Sn alloy formation. Pa~il et al. [ 14] reported evidence for the formation of hexatrienes during n-hexane conversion, and that the amount of the trienes was markedly higher for the bimetallic Pt-Sn or Pt-Pb catalyst. Lieske et al. [ 15] reported that the added tin increased the extent of coking by forming hexene but at the same time a larger part of the Pt remained free. They explained this by a drain-off effect of the alloying tin: on Pt-Sn/AlzO3 the coke precursors are more easily transferred to alumina. The hydrocarbon conversion studies by Volter and coworkers were at normal pressure.

Figueras and coworkers [ 16-18] have also investigated the catalytic activity of Pt-Sn catalysts for hydrocarbon conversions. For the conversion of n-heptane with Pt-Sn sup- ported on a non-porous Degussa alumina, the authors concluded that tin poisons the strong acid sites of the support and part of the platinum; these authors were surprised at the wide use of Pt-Sn based catalysts for commercial applications [ 16]. For methylcyclopentane conversion, Coq and Figueras [ 17,18] found that tin provides a stabilizing effect on Pt and imparts a decrease in the extent of hydrogenolysis. An enhancement in the aromatization of methylcyclopentane was observed as the tin content was increased: the aromatization passed through a maximum and then decreased with further increases in Sn/Pt ratios. The dehydrogenation of methylcyclopentane to methylcyclopentene remained unchanged as the Sn/Pt ratio was increased. These authors also conducted their studies at atmospheric pres- sure.

Dautzenberg et al. [ 19 ] tested a number of unsupported Pt-Sn alloys as well as a number

R. Srmtvasan, B.H Darts/Journal of Molecular Catalysis 88 (1994) 343-358 345

of alumina supported Pt-Sn catalysts, n-Hexane conversion was effected at atmospheric pressure for the unsupported alloy catalysts and for some supported catalysts: other studies with a supported catalyst were at 3 bar. These authors reported that the addition of tin decreased the amount of methylcyclopentane that was formed and that coke was dramatically reduced during the conversion of n-hexane.

Sexton et al. [ 20] also examined the activity of a series of Pt-Sn alumina catalysts with varying Sn/Pt ratios for the dehydrogenation of cyclohexane and the conversion of meth- ylcyclopentane. They found that the activity decreased as the Sn/Pt ratio increased. The selectivity for benzene formation from methylcyclopentane increased to a maximum at about 1.5 to 2.5 wt.% Sn (0.5 wt.% Pt ) and then declined. These conversions were conducted at normal pressures.

Beltramini and Trimrn [ 21 ] utilized Pt-, Sn- and Pt-Sn-supported on 7-alumina for the conversion of n-heptane at 500°C and 5 bar. They observed that during six hours of reaction time less coke per mole of heptane converted was deposited on the Pt-Sn-alumina catalyst than on a Pt-alumina catalyst: however, the total amount of coke formed during six hours was much greater on the Pt-Sn-alumina than on Pt-alumina. The addition of tin increased the selectivity of alkane dehydrocyclization. Since hydrocracking and isomerization activity of a Sn-alumina catalyst remained high in spite of coke formation, the authors concluded that there was little support for the suggestion that tin poisons most of the acid sites on the catalyst. These authors [22] also measured activity, selectivity and coking over a number of alumina supported catalysts, such as, Pt, Pt-Re, Pt-Ir, Pt-Sn and Pt-Ge. Coke formation was significantly reduced on these bimetallic catalysts compared to Pt-alumina; the results show that it is not only the amount but the location of coke that is important in determining catalyst performance. The greater efficiency of the bimetallics depended on both the amount of coke formed and its location.

Karpifiski and Clarke [23] compared the results of the conversion of n-pentane and n- hexane on Pt-Sn and Pt-Rh alloy films. In the temperature range of 320-400°C ( = 10 torr pressure with H2/hydrocarbon= 10/1), they found the ratio of 1,5 and 1,6 ring closure, $5/$6, was less than unity for pure Pt; as the Sn/Pt ratio increased the $5/$6 ratio increased to a maximum and then decreased. The authors concluded that surface carbiding [24] controlled $5/$6 selectivity for Pt; as the carbide was formed there was a shift from 1,5 to 1,6 ring closure. The addition of Sn decreased carbide formation; thus at low Sn/Pt ratios the value of $5/$6 increased. Thus, 1,5 cyclization selectivity patterns were interpreted in terms of the change in active site number, with possible modification by carbiding. Tin addition markedly decreased hydrogenolysis. Clarke et al. [ 25 ] also presented preliminary data obtained in a pulse reactor for Pt-Sn-sil ica catalysts.

Li and Klabunde [ 26] utilized a pulse reactor at 1 atm pressure. Pt and Sn were evaporated into a solvent at low temperature and then the solvent was allowed to warm to room temperature which resulted in the agglomeration of atoms to produce a dispersion of colloidal particles. These were then added to an alumina support. These catalysts were compared to conventional Pt-Sn-alumina catalysts for n-heptane conversion, The authors proposed that the presence of small amounts of Sn ° on the surface of Pt can cause both an increase in catalytic activity and a decrease in hydrogenolysis.

As a means of catalyst preparation Margitfalvi and coworkers, e.g., [ 27-30 ], have utilized a controlled surface reaction in which a volatile Sn (or Pt) compound is allowed to react

346 R. Srinivasan, B.H Darts/Journal of Molecular Catalyst6 88 (1994) 343-358

with Pt (or Sn) already present on a support. They utilized conventional and transient kinetic approaches to study the mechanism of hydrocarbon reactions on these catalysts; conversions were effected at atmospheric or lower pressures. These authors found a per- plexing variety of activity patterns, depending upon the manner and sequence in which Pt and Sn were added. Depending upon the preparation conditions, the added tin may either enhance or decrease Pt activity and increase or decrease the selectivity for hydrogenolysis [27].

Linet al. [31 ] reported that there are two types of sites on Pt-Sn-alumina catalysts: Mj sites adsorb hydrogen that can react with ethene while hydrogen adsorbed on M2 sites does not react with ethene. Aromatization and hydrogenolysis activity increased rapidly with an increase of M~ sites, while isomerization activity increased sharply with an increase in M~ sites. Zhang et al. [ 32 ] argued that the addition of Sn reduced the amount of carbon deposited on the metal surface.

Gault et al. [33] converted ~3C labeled hydrocarbons over catalysts containing 10% Pt and varying amounts of tin (0.2 to 5 wt.%) on an alumina support at low (5.5 torr) or atmospheric pressure. The support was an inert alumina and, even though the catalysts contained 1 to 1.2% C1, the authors did not consider the catalyst to have acidic character. They reported that the catalyst loaded with 10% Pt and small amounts of Sn had the same catalytic properties as very highly dispersed Pt-alumina catalysts.

Szirk~iny et al. [34] considered the formation of carbon deposits from hydrocarbons at normal pressure. They conclude that carbon deposition is decreased by the addition of tin but the rate of formation of carbon residues through a "polyene'" route is increased with the addition of tin. Wilde et al. [ 35 ], using normal pressure conditions, also emphasize the high activity and selectivity for dehydrogenation and dehydrocyclization by 1,6 ring closure due to lower carbon deposition on these catalysts.

For the most part, the data for alkane and cycloalkane conversion with Pt-Sn catalysts have been obtained at atmospheric pressure or lower reaction conditions. For commercial reforming operations, a much higher pressure is utilized. Thus, most of the activity data for Pt-Sn catalysts have been obtained at low pressures and it remains an open question as to how this data compares to that obtained under commercial reforming conditions. Further- more, even for the data obtained at atmospheric pressure the range of catalyst compositions and reaction conditions have led to many conflicting views of the role of Sn as outlined above. It is therefore of interest to generate data for a series of Pt-Sn catalysts utilizing both acidic and non-acidic supports at these lower pressure conditions with the view of resolving some of the conflicting views and for comparison with data obtained at higher operating pressures.

2. Experimental

The experimental details were given in previous pubhcations [ 2,3]. The silica (Davison Chemical Co., 340 m2/g) and acidic alumina (United Catalysts, Inc., 300 m2/g) supports were impregnated (or coimpregnated) with an acetone solution of chloroplatinic acid (or chloroplatlnic acid and stannous chloride to produce several Sn/Pt ratios), then dried in air at 120°C, and calcined in air at 400°C. A tin-aluminum oxide support (200 m2/g) was

R Srimvasan, B.H. Davis/Journal of Molecular Catalysis 88 (1994) 343-358 347

prepared by coprecipitation from a solution containing both tin chloride and aluminum chloride by the addition of ammonia. A non-acidic alumina was prepared by precipitation from a potassium aluminate solution using CO2 (200 m2/g after calcination at 600°C). These latter two supports were also impregnated with an acetone solution containing chlo- roplatinic acid or the appropriate mixture of chloroplatinic acid and stannous chloride. Each catalyst contained 1.0 wt.% Pt and the appropriate amount of Sn to give the indicated Sn : Pt ratio.

The catalyst was reduced at 500°C in-situ in a glass plug-flow reactor. Conversions were effected at atmospheric pressure without added hydrogen at 482°C. Conversions and product distributions were calculated from gas chromatographic data [2,3].

3 . R e s u l t s

3.1. Coprecipitated tin-aluminum oxide supported catalysts

The data in Fig. 1 show that tin acts as a promoter in both increasing the total conversion and in decreasing the rate of decline of total conversion. The time required for the total conversion to decline to a certain level (say 20% or 40%) increases as the Sn/Pt ratio increases.

The aromatic products are near an equilibrium composition for each of the three Sn : Pt ratios: 0 : 1, 1 : 1, and 3 : 1; in spite of this, there are differences among the aromatics produced by the three catalysts. For the Sn:Pt = 0 :1 catalyst an equilibrium aromatic composition was obtained except for possibly the initial sample. Thus, even though the activity of the metallic function declined drastically during the 8 h of operation, sufficient isomerization activity, presumably the bifunctional pathway involving alkane isomerization requiring support acidity, remained so as to produce nearly an equilibrium Cs-aromatic composition.

The S n : P t = 1:1 catalyst produced aromatic compositions further removed from an equilibrium composition than the other two catalysts based on a coprecipitated support.

120 °: . 7

~ I ~ AA 01 . I I I I 0 5 10 15 20 25 30

T IME ( H O U R )

Fig. 1. The conversion of n-octane with increasing time for Pt on coprec~p~tated tin-alumina support. ( • ) Sn/ Pt=0" (ll) Sn/Pt= 1" and I • ) Sn/Pt=3.

348 R. Srinivasan, B.H. Davis/Journal of Molecular Catalysts 88 (1994) 343-358

Table 1 C8-aromatlc distributions for the dehydrocychzation of n-octane

Catalyst Time, h C~-Aromatlc. mol%

EB OX MX PX

Pt-AI203-UCI 1-3 14 26 38 22 3-7 12 24 50 14

Sn:Pt( 1' 1 ) -AI203-UCI 1 20 34 31 15 5-24 16 22 42 20

Sn: Pt(8 : 3 ) -AI203-UCI I 21 33 31 15 3-8 11 21 48 20

Sn : Pt(5 : 1 )-AI~O3-UCI 1 35 46 13 6 2 26 32 27 14

4 -8 19 23 39 19 Sn : Pt( 8 .1 )-AI~O~-UCI 1 26 43 20 1 I

7 26 36 25 13

14 27 33 26 14 20 32 33 24 1 I

Pt-AIzO3-COP ~ I 17 24 43 16 3-8 10 22 49 19

Pt-Sn( 1: I ) -AI ,O3-COP 1 28 40 20 12 10 28 34 26 12 20-30 30-34 28-24 27 12

Pt-Sn( 1 " 1 ) -AI203-COP 1 20 38 28 15 5-25 18 32-22 34-44 18

Pt-Sn( I : 1 ) Degussa 2 -4 14 23 48 15

Alumina only, prepared by same procedure as COP.

Initially the predominant aromatic products are those anticipated for direct six-carbon ring closure of n-octane. For the initial samples, about 70% of the Cs-aromatics were ethylben- zene (EB) and o-xylene (OX), the products expected for a direct six-carbon ring formation; the ratio of OX : EB is 1.5 : 1. With time, the aromatic distribution changes but does not attain an equilibrium composition even after 36 h (Fig. 2). With increasing reaction time there is a gradual decline in the fraction of OX and an increase in m-xylene (MX). The fraction of EB is essentially constant for about 25 h after which there is a gradual increase in EB. Likewise, p-xylene (PX) remains essentially constant during the 36 h run.

The aromatic products from n-octane conversion over the Sn : Pt = 3 : 1 approaches but does not attain an equilibrium value at longer time on stream. The failure to attain the equilibrium composition at longer time on stream is primarily due to the high fraction of EB. With this Pt/Sn catalyst, about 60% of the initial C8-aromatic products correspond to those expected for direct six-carbon ring closure; the initial OX/EB ratio was about 1.9. Thus, the initial OX : EB ratio approaches the value of 2; this is much greater than the value of 0.5 expected for a statistical distribution allowed by direct six-carbon ring formation. With time the amount of OX decreases with a corresponding increase in the amount of MX; the fraction of EB and PX remain essentially constant during the conversion.

R Srinivasan, B.H. Davts/Journal of Molecular Catalysis 88 (1994) 343-358 349

~)'°~ 000000 • imltmmm .~ :~ - == ~ o ~ ° o Oo

o z l0 - e ~ J ~ o ° o ° O O ° O ° o e ° ° oooooo

I I ] I L 0 5 I0 15 20 25 30

TIME ( H O U R )

Fig. 2 The Cs-aromatic distribution with increasing time for the Sn : Pt = 1 on the coprecipltated tin-alumina. ( • ) Ethylbenzene; (©) o-xylene; ([]) m-xylene; and ( • ) p-xylene.

o~ lOO

~ I I I I I I 0 1 2 3 4 5 6 7

T I M E ( H R )

Fig 3. The n-octane conversion with time for Pt-Sn supported on Degussa aluminum oxide C support. (O) Sn:Pt=0; ( • ) Sn:Pt= 1; ( • ) Sn:Pt=2.67;and ( • ) Sn:Pt=5.

3.2. D e g u s s a a l u m i n a s u p p o r t e d ca ta lys t s

The next series of catalysts to be considered are based upon a Degussa a lumina oxide C. This a lumina support is composed of non-porous spherical particles and has a surface area of 110 m2/g. T in clearly has an impact upon catalyst activity and stability (Fig. 3). In this instance the activity appears to first decline as the S n / P t ratio is increased to l : l , then increases with a further S n / P t ratio increase, and then declines again with a further increase. The Cs-aromatic distr ibution for the Sn : Pt = l : I ratio is close to an equi l ibr ium composi t ion (Fig. 4) . The aromatic distr ibutions obtained with the other S n : P t and the Pt only on the Degussa a lumina are similar to the one shown in Fig. 4.

350 R. Srinivasan, B.H Darts/Journal of Molecular Catulysts 88 ¢ 1994) 343-358

50

4o

~ 30

&)~ 20 O Z

1o

[ ] [ ]

[ ] [ ]

O 0 0 ©

. I I I

0 I I I I

0 1 2 3 4

TIME (HR)

Fig 4 C8-aromat]c distributions with mcreasmg time for the products from the conversion of n-octane with a Sn : Pt = 1 on Degussa alumina oxide C f • ) Ethylbenzene, (O) o-xylene, ( [] ) m-xylene; and ( • ) p-xylene.

o~ 100 i 0 m 8o

u,l > z 6O 0 o

40 u.i z ,< I.- 20 o 9 e -

A

I I I I I 2 4 6 8 10

TIME (HR)

Fig. 5. n-Octane conversion w~th time for Sn/Pt on an ac~dtc Umted Catalysts, Inc. alumina. ( • ) Sn : Pt = 0: ( • ) Sn:Pt= 1; ( • ) Sn'Pt=2.67; (©) Sn:Pt=5' h and ((3) Sn:Pt=8

3.3. High surface area UCI alumina supported catalysts

The act ivi ty and stability for the catalyst series based on the UCI a lumina show the

fo l lowing trend: the S n / P t = 8 : 1 catalyst exhibits the s lowest decl ine in activity, this is

fo l lowed by the Pt without tin, and then the decl ine for the other three catalysts is in the

order Pt : Sn = 1 : 1, Pt : Sn = 5 : 1 --- Pt : Sn = 8 : 3 (Fig. 5 ). The convers ion after three hours

for catalysts with increasing S n / P t loading shows a rapid decrease, then a low level o f

convers ion at intermediate S n / P t ratios, and finally an increase in act ivi ty at a h igher S n /

Pt ratio (Fig. 6) . This trend contrasts with that of the catalysts based on Degussa A l u m i n u m Oxide C.

The Cs-aromat ic select ivi ty varies as the S n / P t ratio is varied. For the catalyst containing

only Pt, the Cs-aromat ics are near the composi t ion expected for a chemica l equi l ibr ium distr ibution during the course of the 7.5 h run. At longer reaction t imes for the Sn : Pt = 1 : 1

R. Srtmvasan, B.H Davis/JoutTzal of Molecular Catalysts 88 (1994) 343-358 351

100

z 80

O3

I-"

z 0 IZ: I.,U

Z 0 (J

,4,--

60

40

20

0 1:0 1:2 1:4 1:6 1:8 Pt:Sn

100o~

80 0 Z

m 6O =o

0 40 z

.-i 20 ro

"l- "n

0

Fig. 6. n-Octane conversion ( O ) at 3 h for the UCI alumina supported catalyst series and ( • ) at 2 h for the Degussa alumina catalyst series vs. Sn/Pt ratio.

50

40

! = 30 N

, o

~ z

10

0 I I I I

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4

TIME (HR) Fig 7. C8-aromatic distribution with Ume for a Sn:Pt = 8 on Umted Catalyst, lnc alumina, ( • ) Ethylbenzene" ( O ) o-xylene; (IS]) m-xylene: and ( 0 ) p - x y l e n e .

and 8 : 3 catalysts, an equilibrium distribution of Cs-aromatics is obtained; however, this is not the case at early times on stream. Ethyl benzene and o-xylene are formed as major products (55% of Cs-aromatics) in the initial conversion period with the Sn /P t=2 .67 catalyst but after two hours the Cs-aromatics are near an equilibrium distribution. This trend continues in a more exaggerated manner for the Sn : Pt = 5 : I catalyst ( Fig. 7). In the sample collected after one hour, ethylbenzene and o-xylene account for more than 80% of the Cs- aromatics. However, the amount of these two isomers decrease in the 2 and 3 h samples so that the samples collected at 4 h and later have essentially an equilibrium Cs-aromatic composition. This trend continues with increasing Sn : Pt ratio so that an equilibrium com- position of Cs-aromatics is not obtained even after 30 h of time-on-stream for the Sn : Pt = 8 : 1 catalyst (Fig. 7). Ethylbenzene and o-xylene are the dominant products during the entire 30 h run; however, there is an increase in m-xylene with a corresponding decrease in the amount of o-xylene during the first 15 h of operation; thereafter, there is little change in the composition of the C8-aromatic fraction.

352 R. Srmwasan, B.H. Davis~Journal of Molecular Catalysis 88 (1994)343-358

3.4. Silica supported catalysts

The activity of the Pt-Sn-SiO2 catalysts shows first an increase over that of Pt alone and then a decline as the Sn/Pt ratio increases further (Fig. 8). For the catalyst with Sn/Pt of 5 or 12, the low activity is maintained at essentially a constant level for ca. 8 h of reaction. For the catalyst with the two lower Sn/Pt ratios, the activity declines during a 7 h period to approach the activity level of the two catalysts with the higher Sn/Pt ratios. However, Sn is promoting the activity over that of Pt alone for the three lower Sn : Pt ratios.

For three of the catalysts, the C8 aromatics are those expected for direct six-carbon ring closure; however, the aromatic distribution does depend upon catalyst composition. For the Sn:Pt = 1 : 1 catalyst EB and OX comprise greater than 95% of the C8-aromatics; the remainder of the C8-aromatics are predominantly MX with PX being present in less than 0.1 wt.% (Fig. 9). For the two intermediate Sn/Pt catalysts ( S n : P t = 8 : 3 and 5: 1) the aromatics at first are predominantly EB and OX but gradually approach an equilibrium

o~=60 Z 0 ~s0 iv" LU > 4O Z 0 0 30

l.tl Z 20

I-- 0 10 0

|

c 0 I I ~ i I 0 2 4 6 8

I I I0 12

Sn/P t 14

Fag. 8 Conversion of n-octane at ( • ) 2, (m) 4 and I • ) 7 h reaction ume vs the Sn:Pt ratm for the sdica supported catalyst senes.

60

50

~ o ~ 4 0

.~ 2o

o z l o

0 0

I

0 0 0 0

= ." , . . .

I I

4 6 TIME (HR)

Fig. 9. Cs-aromat~c products for theconverslon of n-octane w~tha Sn. Pt= 1 on Day,son SiO: ( m ) Ethylbenzene: (O) o-xylene; ([]) m-xylene, and ( • ) p-xylene

R. Snmvasan, B.H. Davis/Journal of Molecular Catalysts 88 (1994) 343-358 353

composition with time of operation; the initial fraction of EB and OX are higher for the Sn: Pt = 8 : 3 catalyst than for the Sn :Pt = 5 : 1. For the Sn :Pt = 12 : 1 catalyst, the amount of EB is about the same as with the catalysts of lower Sn: Pt ratios; however, the amount of OX is much lower and MX much higher than was obtained for the two intermediate Sn/ Pt ratios. The C8-aromatics distribution for the Sn:Pt = 12:1 catalyst is similar to the one that is the Sn : Pt = 8 : 3 and 5 : 1 catalysts approach at longer time on stream. For the Pt- SiO2 catalyst ethylbenzene and o-xylene were the dominant C8-aromatics ( > 90%) and the OX : EB ratio was about 0.9 : 1. For the Sn : Pt = 1 : 1 catalyst the OX/EB ratio is approxi- mately 1.3 to 1.4 : 1. For the higher Sn: Pt ratio catalyst this ratio is smaller. It appears that the addition of Sn to a Pt-SiO2 catalyst alters the OX:EB ratio in much the same way it does when alumina is the support, and that is to favor OX over EB.

4. Discussion

The results for the Pt-SiO2 and Pt-Sn-SiO2 catalysts resemble those reported earlier for non-acidic alumina supported catalysts [ 1-3 ]. Adding tin to the Pt-SiO2 catalyst initially results in an increase in activity but with further increase in Sn/Pt ratio a maximum in activity is attained, and a further increase in the Sn/Pt ratio results in a decrease in catalyst activity. The two supports differ since the maximum activity was obtained for the non- acidic alumina support at a Sn/Pt ratio of about 4 whereas for the silica support the maximum occurred at a Sn/Pt ratio of 1, or less. The maximum in catalytic activity for the non-acidic alumina and silica supports appears to follow the pattern observed for Pt-Sn alloy formation for these catalyst series [36-44] . In general the characterization data showed that the dominant alloy was Sn: Pt--- 1 : 1 and that the maximum alloy formation, for a loading of 1 wt.% Pt that was used in both the activity and characterization studies, occurred at a Sn/Pt ratio of about 4 to 5 for a high surface area non-acidic alumina support but at a ratio of about 1 for a silica support. Thus, for both non-acidic supports, where the formation of aromatics is by only a metal catalyzed pathway, the addition of Sn increases both alloy formation and catalytic activity. Likewise, there is a selectivity change in the distribution of the C8-aromatics that are allowed by direct six-carbon ring formation, EB and OX, so the OX is favored with increasing Sn/Pt ratios. The data therefore lead us to the following conclusions:

1. The extent of formation of a Pt/Sn alloy increases with increasing Sn concentration at constant Pt loading.

2. The extent of n-octane conversion increases, and then decreases, with increasing Sn/ Pt ratio.

3. The increase in dehydrocyclization activity parallels an increase in Sn/Pt alloy for- mation and attains a maximum at about the same Sn/Pt ratio as where Pt is completely alloyed.

4. As the Sn/Pt ratio increases beyond that needed to convert the Pt to Pt-Sn alloy, the activity declines.

5. By inference, the Pt-Sn alloy is more active than Pt alone for single-function metal catalyzed dehydrocyclization, but the Pt-Sn alloy is " 'poisoned" by excess tin that could

354 R Srmn,asan, B.H. Daws/Journal of Molecular Catalysts 88 (1994) 343-358

produce a surface layer rich in Sn or by another alloy that is tin rich as, for example, Pt- Sn3.

The situation is more complex when an acidic alumina support is employed since n- octane dehydrocyclization is more rapid for the bifunctional pathway than by the mono- functional metal catalyzed pathway [45 ]. Thus, the results for the high area alumina support are considered first. For this support a series of catalysts were prepared so that the Sn/Pt ratio varied from 0 to 8. As shown in Fig. 6, there is a decline in activity (the three hour activities are compared in Fig. 6) as the Sn/Pt ratio increases from 0 to 5; it is postulated that this decline is due to poisoning of the acid sites of the support by the added tin so that the fraction of aromatics formed by the more rapid bifunctional pathway is decreased, or essentially eliminated. The Cs-aromatic distribution is in agreement with this view since the Pt-AI203 catalyst ( S n / P t = 0 ) produced a product distribution corresponding to a chemical equilibrium composition. As the Sn/Pt ratio increased to Sn : Pt = 5 : 1, initially the two aromatics formed by direct six-carbon ring closure, EB and OX, dominated but with increasing reaction time the Cs-aromatic distribution approached an equilibrium com- position. This data is consistent with tin eliminating the acidity due to the alumina support so that the majority of the Cs-aromatics are the result of an increasing contribution of n- octane aromatization by the metal function pathway with increasing Sn/Pt ratios; however, as the catalyst ages the fraction of Cs-aromatics formed by the bifunctional pathway increases. Thus, it appears that the following reaction pathways are applicable:

~ e t a ~ j EB + OX

n - C a

n- C8 = = = H ÷

b n - C 8 + i - C 8

R R H~

[ R . .

E B , O X , M X , PX

meta I 2 E B , O X , M X , P X

(R, R = H,CH 3 or C2H5]

Finally, with the S n : Pt = 8 : 1 catalyst the activity increases from the low level of a catalyst with the ratio Sn : Pt = 5 : 1. This activity is primarily due to the Pt/Sn alloy and produces Cs-aromatics that are predominantly EB and OX over the entire reaction time (30 h).

The catalyst characterization data indicate that with the high surface area alumina a Sn : Pt ratio greater than 5 : 1 is required to attain the maximum XRD intensity of Pt-Sn alloy. Thus, the characterization data indicate that alloy formation increases with increasing Sn/

R. Srintvasan. B H. Darts/Journal of Molecular Catalysts 88 (1994) 343-358 355

Pt ratios, but for this catalyst the increase in alloy formation parallels a decrease in the catalytic activity. It therefore appears that a portion of the tin present in this catalyst reacts with the alumina support to decrease the activity by eliminating or poisoning the acid sites more rapidly than it combines with Pt to form an alloy.

The catalyst characterization data [ 37,46 ] for the catalysts based on the Degussa alumina follow the trend exhibited by the catalysts based on the high surface alumina. However, with the lower surface area, non-porous alumina, the Pt is converted to the maximum amount of Pt-Sn alloy at a lower Sn : Pt ratio than was the case for the high surface area UCI alumina supported catalysts. Thus, the initial decrease in activity followed by increased conversion occurs as the Sn : Pt ratio is varied from 0 to 2.67 : 1 rather than the range of 0 to 5 : l that was observed for the UCI support. As with the Pt-Sn-SiO2 catalyst series, the very high Sn:Pt ratios produce a decline in the catalytic activity due to Sn ° covering the Sn/Pt alloy(s). In the case of the catalysts based upon the Degussa alumina, the C8- aromatics do not exhibit as clear-cut alteration in the C8-aromatics as was the case with the catalysts based on the UCI alumina support. However, the Cs-aromatic products from Sn/ Pt = 0 have the equilibrium composition. The aromatics for the Sn:Pt = 2.67:1 catalyst contain about 70% EB plus OX as was observed for Sn : Pt = 5 : l for the UCI catalyst. The Cs-aromatic products for the low activity Sn : Pt = 5 : 1 on Degussa alumina catalyst contains about 30% EB ( vs. about 10% at equilibrium); however, it appears that the xylene isomers are near the equilibrium value.

The activity of the catalysts based on the coprecipitated tin-aluminum oxide (Sn /AI oxide) exhibits a different trend than the two catalyst series prepared by impregnation of an alumina support, both in alloy formation based upon the characterization data and in activity changes with increasing Sn:Pt ratios. For a given Sn:Pt ratio, based upon bulk analyses, the coimpregnated catalyst had a significantly larger fraction of Pt present in an alloy form than for the catalyst based upon the coprecipitated Sn/A1 oxide. This indicates that the surface Sn concentration in the coprecipitated Sn/AI oxide is much lower than on a catalyst with the same bulk composition that was prepared by coimpregnation with Pt and Sn. Thus, as the Sn:Pt ratio, based on bulk analysis for the coprecipitated tin-aluminum oxide, increases the surface Sn concentration is not sufficiently high to form a significant amount of Pt-Sn alloy. The data also indicate that the surface Sn concentration is too low to reduce the catalyst acidity by a significant amount. Energy dispersive X-ray (EDX) analysis indicated that both Sn and Pt could be detected for the UCI based catalyst but only Pt could be detected for the Sn/AI oxide based catalyst; this is consistent with the dominant fraction of Sn being present in the bulk of the Sn/A1 oxide catalyst [38]. The catalytic activity for the coprecipitated catalyst increases with increasing Sn : Pt ratios (based on bulk composition). Presumably the beneficial role of Sn in the coprecipitated catalyst is to aid in maintaining Pt dispersion so that dehydrocyclization is effected by the bifunctional pathway.

The data for the four catalyst series present a consistent picture for the role of Sn in Pt- A1203 alkane dehydrocyclization catalysts. Presumably the conclusions given below are applicable as a guide to defining alkane dehydrocyclization pathways for the low pressure naphtha reforming operations: 1. For a catalyst based upon a non-acidic support, the addition of Sn to give the lower Sn : Pt

ratios causes the activity to increase to a maximum, and beyond this composition an increase in the Sn/Pt ratio results in a decrease in catalytic activity. The Sn : Pt ratio that

356 R. Srintvasan, B.H. Darts/Journal of Molecular Catalysts 88 (1994) 343-358

produces the maximum activity will depend upon the support material ( silica, alumina, etc.) and on the surface area of the support. For a silica support the maximum activity is attained at a lower Sn :P t ratio than for alumina of a similar surface area. With each support material, the Sn : Pt ratio for maximum activity increases with increasing surface area.

2. The major fraction (90% or greater) of the C8-aromatics are formed by a direct six- carbon ring formation for catalysts based on a non-acidic support. Thus, for n-octane the major products are EB and OX; the ratio of OX : EB increases from about 1 : 1 or even 1 : 2 for the catalyst containing Pt only to about 2 : 1 for the Sn : Pt = 1 : I alloy. This change to favor the OX aromatic isomer is considered to be because the Pt in the alloy is less electron deficient and this favors the aromatic that is formed by an adsorption that favors the weaker secondary C - H bonds of n-octane and leads to OX over the adsorption that involves the stronger primary C - H bond that must be ruptured if EB is produced.

3. For the catalyst prepared by coimpregnation of an acidic alumina the activity initially decreases with an increase in Sn :P t ratio (Figs. 3 and 6). The initial decline is followed by an increase in catalytic activity to attain a maximum and this is then followed by a decline in activity with increasing Sn : Pt ratios. The latter part of the activity vs. Sn : Pt ratio curve is similar to the one obtained with the non-acidic support that has been displaced to a higher Sn /P t ratio. Thus, the initial decline in activity is considered to result from elimination of acid sites of the support as Sn is added to increase the Sn : Pt ratio; as the acid sites are eliminated the bifunctional pathway is eliminated. When most of the acid sites have been eliminated, further increases in the Sn : Pt ratio results in an activity pattern that is similar to that of the catalysts that are based on a non-acidic support. The Sn :P t ratio for attaining maximum or minimum activity depends upon both the

Acidic Alumina Support

BifuncUonal Pathway Monofunctlonal Metal Pathway X X

Elimination of Formation of Excess Sn ° to

Acidic Support "Non-Acidic" Support X Isomerizatlon, Aromatics by Direct C~-rlng

Equilibrium Formation Cs-Aromatlcs

Sn/Pt Fig. 10. Schematic for the conversion of n-octane w~th catalysts based upon acidic alumina with increasing Sn" Pt ratios

R. Srimvasan. B.H. Davzs / Journal of Molecular Catalysis 88 (1994) 343-358 357

support material and its surface area just as was observed for the non-acidic support. The activity pattern can be summarized by the schematic shown in Fig. 10.

4. The C8-aromatics for the lower S n : P t ratio catalysts based on an acidic a lumina have essential ly an equi l ibr ium composi t ion that results from the isomerizat ion of n-octane to iso-octanes through a bifunct ional pathway as well as the acid catalyzed isomerizat ion of the a lkylcyclopentenes to produce alkylcyclohexenes. When most, or all, of the acid sites have been e l iminated by Sn, the Cs-aromatic distr ibution resembles those obtained by a S n / P t alloy on a non-acidic support.

5. For P t -Sn catalysts based on a coprecipitated Sn/A1 oxide, a measurable amount of alloy is not formed [46] . Addit ional ly, most of the Sn appears to be located within the bulk of the support [46] so that the concentrat ion of surface Sn is too low to el iminate an appreciable amount of acidity even for a catalyst with a Sn : Pt ratio of 3 : 1. The activity for these catalysts is therefore bel ieved to be due to a bifunct ional mechanism. The role of tin in these catalysts is bel ieved to be due to the main tenance of small Pt crystals dur ing catalyst usage. The retardation of Pt agglomerat ion may be by a mechan i sm as proposed by McVicker et al. [47] where the surface Sn ions provide traps to greatly retard Pt atom migration. The Cs-aromatics formed from the three catalyst composi t ion of this catalyst series have a composi t ion that is essential ly an equi l ibr ium distribution, as expected for aromatics produced by a bifunctional .

6. The activity patterns with increasing Sn : Pt ratios, the dependence of m ax im um activity on the support material, and the dependence of the m a x i m u m on support surface area are consistent with extensive characterization data with these catalysts [36--44,46] which show a similar pattern for the dependency of Sn : Pt alloy = 1 : 1 upon the support, surface area and Sn: Pt ratio. The P t -Sn alloy is more active for direct s ix-carbon ring formation than Pt alone.

7. The addit ion of tin does not decrease the amount of alkyl cyclopentanes formed. Instead, the addit ion of tin increases the relative rate of alkyl cyclopentane hydrogenolysis to produce iso-alkanes.

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