Shape-Selective Catalytic Behavior of Pt-Porous Heteropoly

Chapter 26

Downloaded by UNIV OF PITTSBURGH on December 24, 2014 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch026

Shape-Selective Catalytic Behavior of Pt-Porous Heteropoly Compounds in Skeletal Isomerization of n-Butane Toshio Okuhara, Ryu-ichi Watanabe, and Yusuke Yoshinaga Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Effects of pore width of Pt-promoted porous materials including heteropoly compounds on selectivity in skeletal isomerization of n-butane have been studied. With 0.5wt%Pt-Cs H PW O which possesses mesopores with the width of about 5 nm, about 94% -selectivity was obtained in the presence of H at 573 K . On the other hand, an ultramicroporous heteropoly compound, 0.5wt%Pt -Cs H PW O , of which the pore-width is 0.43 - 0.50 nm, gave mainly propane as well as isobutane. Pore-width of Pt-zeolites also affected greatly the selectivity; Pt-H-ZSM-5 and H-ZSM-5 produced mainly small molecules such as ethane and propane, while Pt-HY gave isobutane with 80%-selectivity. These results demonstrated that the pore width of the microporous materials is a crucial factor influencing the selectivity of skeletal isomerization of n-butane due to the shape selectivity by the constrained pores. 2.5

0.5

12

40

2

2.1

0.9

12

40

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoints (1,2). Recently synthesized layered materials (3), microporous oxides and mixed oxides (4), and mesoporous silica (3) are also candidates for shape selective catalysts. Since heteropolyacids like H 3 P W 1 2 O 4 0 and its acidic salts are very strong acids and show high activities for various kinds of acid-catalyzed reactions (5), these materials have attracted much attention. Interestingly, porous heteropoly compounds were synthesized through partial substitution of C s for H of H 3 P W 1 2 O 4 0 (5-10). We further synthesized a Pt-promoted C S 2 . 1 H 0 . 9 P W 1 2 O 4 0 , which has only ultramicropores, having the width of about 0.5 nm (11). This bifunctional catalyst exhibits shape selectivity toward hydrogénation of alkenes and oxidation of hydrocarbons (11). Skeletal isomerization of «-butane to isobutane is an important industrial +

+

© 2000 American Chemical Society

In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

369

370 reaction, since isobutane is a feedstock for alkylation with butènes to C8 alkylates and synthesis of methyl terr-butyl ether. CS2.5H0.5PW12O40 was found to be active and selective than S04 "/Zr02 at 573 Κ (12). Furthermore, addition of Pt to CS2.5H0.5PW12O40 enhanced greatly the activity and selectivity for isomerization of «-butane in the presence of H2 (13,14). It was reported that the skeletal isomerization of «-butane over Pt-Cs2.5Ho.5PWi2Û40 proceeds through a bifunctional mechanism, in which «-butane is dehydrogenated to «-butènes over Pt, and «-butènes are converted to seobutyl carbenium ion intermediate (14). Furthermore, a unique function of proton has been proposed; protons present near the Pt particles play an important role in suppressing the hydrogenolysis on Pt, resulting in the selective skeletal isomerization (14).


2

To elucidate the factors controlling the selectivity of this reaction, we investigated the influence of the pore-width of porous catalysts on selectivity. The characteristics of Pt-promoted porous heteropoly compounds were examined by comparing the results with those of Pt-zeolites. Experimental Pt-promoted Cs H3- PWi2O40 (x = 2.1,2.2 and 2.5) were prepared by a titration method (11,15). For the former two, an aqueous solution of Pt(NH3)4(OH)2 (0.03 mol-dm"3) was added to an aqueous solution of H3PW12O40 (0.08 mol-dm"^) at room temperature. Then an aqueous solution of CS2CO3 (0.10 mol-dm"^) was added dropwise to the mixture at a rate of 0.1 cm^-min'l with vigorous stirring at room temperature. The obtained suspension was allowed to stand overnight at room temperature, and evaporated at 318 Κ to obtain solid. The Pt content of the solid was changed from 0.5wt% to 1.5wt% for Pt-Cs2.lHo.9PWi2040, and that of PtCS2.2H0.8PW12O40 was adjusted to 0.5wt%. These heteropoly compounds were pretreated in a flow of O2 at 573 Κ for 2 h. These catalysts are denoted as, e.g., 0.5wt%Pt-Cs2.1. 0.5wt%Pt-Cs2.5Hfj.5PWi2O40 was prepared using aqueous solutions of H2PtCl6, H3PW12O40, and CS2CO3 by the same preparation method (14). x

x

0.2wt%Pt-H-ZSM-5 was prepared by an ion-exchange method using NH4ZSM-5 (from H-ZSM-5, SZ-820NAA, Tosoh Co.) and Pt(NH3)4(OH)2 (Strem Chemicals). 0.2wt%Pt-HY zeolites was also prepared by the ion-exchange method using NH4-Y (from H Y , JRC-Z-HY 4.8). These Pt-zeolites were calcined in air at 773 Κ for 4 h. Nitrogen adsorption-desorption isotherms were measured after the catalyst was pretreated at 573 Κ in a vacuum with an automatic gas adsorption apparatus (BELSORP 28SA, B E L Japan, Inc.). Adsorption of various molecules having different size was measured by using a microbalance (Shimadzu TG-30) directly connected to a high vacuum system (16). Adsorption temperatures are 295 Κ for methanol, benzene, and 1,3,5-trimethylbenzene, 77 Κ for N2 and 193 Κ for «-butane and isobutane. The relative pressure (p/po) used is 0.2, since near monolayer adsorption would be obtained around this relative pressure (17). Skeletal isomerization of «-butane was performed at 573 Κ with «-butane 5%, H2 50% (He balance) or «-butane 8%, H2 20% (He balance) after pretreatment in a flow of H2 at 573 Κ for 2 h (14). The total flow rate was changed from 10 cm -min" 3


1

371 to 40 cm^-min"! and catalyst weight was 0.2 ~ 2.0 g. The products were analyzed with a TCD gas chromatograph (Shimadzu GC 8A) with a column, VZ-10. Results and Discussion


2

0.5wt%Pt-Cs2.1 gave a Type I isotherm of N2 adsorption and had 61 m -g-l of BET surface area. Type I isotherm is usually observed for microporous materials (17). On the other hand, 1.0wt%, 1.5wt%Pt-Cs2.1 and 0.5wt%Pt-Cs2.5 showed Type IV isotherms which are applicable to mesoporous materials (17). Mesopore size distributions for these heteropoly compounds are provided in Fig. 1, where the distributions were derived from the desorption branch of the N2 isotherm by Dollimore-Heal method (18). Mesopores are defined as the pores having the width from 2 to 50 nm (19). It was found that no mesopore was present on 0.5wt%PtCs2.1. Therefore, we can conclude that 0.5wt%Pt-Cs2.1 possesses only micropores. On the other hand, 1.0wt%Pt-Cs2.1 and 1.5wt%Pt-Cs2.1 had mesopores with the widths of about 4.0 and 7.0 nm, respectively. Also for 0.5wt%Pt-Cs2.5, a relatively sharp peak due to mesopore was observed at about 5.0 nm. As was not shown in this figure, for 0.5wt%Pt-Cs2.2, only a small fraction of mesopores having the width less than 3.5 nm was detected (10). It was confirmed that when Pt-Cs2.5 was prepared from Pt(NH3)4(OH)2, the pore size was similar to that from H2PtCl6This indicates that the effect of the Pt materials on the pore structure was little in the case. Since the micropore size distribution cannot be reasonably determined from the N2 isotherm, we measured adsorption of molecules having different molecular size to estimate the micropore widths of these heteropoly compounds as well as Pt-zeolites. Table I compares the adsorption amounts measured by a microbalance connected directly to the ultrahigh vacuum system. The adsorption amounts were determined after the weight increases by the adsorptions with time became nearly zero. The adsorption amount is expressed by adsorption area calculated from the adsorption amount and molecular cross section area (see footnotes of Table I). The molecular cross section was estimated from the liquid density (17). A l l samples shown in Table I adsorbed appreciably N2 (molecular size (MS) = 0.36 nm) and methanol (MS = 0.40 nm). On 0.5wt%Pt-Cs2.1, a considerable amount of N2 and a small amount of w-butane (MS = 0.43 nm) were adsorbed, but the adsorption amounts of benzene (MS = 0.59 nm) and 1,3,5-trimethylbenzene (MS = 0.75 nm) were negligibly small. This result strongly supports that 0.5wt%PtCs2.1 has only ultramicropores and indicates that the pore-width is 0.43 ~ 0.50 nm. Ultramicropores are defined as pores having the width less than 0.7 nm (19). The adsorption areas of benzene and 1,3,5-trimethylbenzene on 0.5wt%Pt-Cs2.1 were about 3% that of the total surface area, showing that the external surface area of 0.5wt%Pt-Cs2.1 is very small. On 0.5wt%Pt-Cs2.5, all the molecules used in the present study were adsorbed appreciably and the adsorption areas are close to that of N2 (Table I). Thus, the pores of 0.5wt%Pt-Cs2.5 were larger than 0.75 nm if the micropores were present. As shown in Table I, 0.5wt%Pt-Cs2.2 has adsorption capacities of isobutane and



372

Pore diameter, D / nm Figure 1. Pore size distribution curves for Pt-promoted heteropoly compounds derived from Dollimore-Heal method. a: 0.5wt%Pt-Cs2.lHo.9PWi2040, b: 1.0wt%Pt-Cs2.lHo.9PWi2040, c: 1.5wt%Pt-Cs2.lHo.9PWi2040, d: 0.5wt%Pt-Cs2.5Ho.5PWl204fj.


373 Table I. Adsorption of Various Molecules on Pt-promoted Porous Catalysts Catalyst Adsorption area of molecules Ν2^ MeOH η-butane isobutane benzene 1,3,5-TMB^ [0.75] [0.59] [0.50] [0.36] [0.40] [0.43] a

c

84 106 120 0.5wt%Pt-Cs2.5 133 131 149 4 20 22 0.5wt%Pt-Cs2.2 72 118 25 2 1 1 0.5wt%Pt-Cs2.1g 61 109 36 54 319 442 457 0.2wt%Pt-H-ZSM-5 405 621 10 541 520 0.2wt%Pt-HY 563 701 1050 Calculated from the adsorption amount and molecular cross section area; m^-g" . Molecular cross section areas used are 0.16 nm^ (N2), 0.18 nm^ (methanol), 0.32 nm^ («-butane and isobutane), 0.31 nm^ (benzene), and 0.41 nm^ (1,3,5-TMB). The figures in the brackets are molecular sizes in nm (20,21). ^BET surface area, Methanol. l,3,5-trimethylbenzene. 0.5wt%Pt-Cs2.5Ho.5PWi20400.5wt%Pt-Cs2.2H0.8PWi2O40. ê0.5wt%Pt-Cs2.1H0.9PW12O40. After 30 h. The value after 2 h was 8. e

f

h


1

c

f

d

e

h

benzene, while the adsorption amounts were somewhat small. Although the pore structure of 0.5wt%Pt-Cs2.2 is thought to be rather complex, a part of the pores has the width more than 0.59 nm. When these micropores were analyzed with Ar adsorption isotherm developed by Saito and Foley (22), the obtained results were consistent with the above data as will be described elsewhere (23). Figure 2 shows the time courses of skeletal isomerization of «-butane over 0.5wt%Pt-Cs2.5, 0.5wt%Pt-Cs2.2, and 0.5wt%Pt-Cs2.1 performed using 5% nbutane and 50% H2 at 573 K. The value of W/F was 40 g-h-(mol of feed gas)" , where W is catalyst weight (gram) and F is total flow rate (mol-h~l). The conversions on these catalysts decreased gradually at the initial stage of the reaction and reached nearly stationary states at 5 h. It was observed that the decreases in the conversion were smaller for 0.2wt%Pt-H-ZSM-5, 0.2wt%HY, and H-ZSM-5 than for the above heteropoly compounds. The conversion and selectivity were calculated on the basis of the data taken at 5 h of the reaction. While the starting material was different between Pt-Cs2.5 and Pt-Cs2.2 or Pt-Cs2.1, it was confirmed that the influence of the Pt material for Pt-Cs2.5 was little after the pretreatment with 02 (14). 1

Dependencies of the stationary conversion on W/F are plotted in Fig. 3. The conversions increased linearly with the W/F for all catalysts up to at least 33%conversion which corresponds to 66% that in the thermodynamic equilibrium. The changes in the stationary selectivity as a function of the conversion are given in Fig. 4 for these Pt-porous catalysts. A slight decrease in the selectivity for 0.5wt%PtCs2.5 was observed, which is consistent with the previous result (19). Also in other cases in Fig. 4, the decreases in the selectivity to isobutane were not significant, while the selectivities were greatly different depending on the catalyst. Considering the trends in Fig. 4, the selectivity can be compared at a certain conversion in these conversion ranges.



ο

4^

U\

Ρ

g ο

s

1

Ο

ι—•

σ α

Ο

•

κ!

Ο

4^

3 to Ο

8

Ρ

ο w

χ α.

X

ρ bo

a to

ο

£

α» to

^

C3/

CD

ζ/ο

Crq

ο- Ρ

ΟΟ

5*ϊ

in

to

S»

to

I

Κ)

ρ

Ι ο

5»

ε

Ο

π •nrt ο

tO ΙΟ

3. ρ

pg

^ ίο

ο

II

CD

ο

â

Conversion / %

Οι

4^

^1

ρ

ο

£?8

οο ©" ο οο rt" ^ sr

II

5

H- ΙΟ

ιι Ο

Ρ

C Κ)

Ή p Ο S · * CTQ ο c

CD CL

I Ο

y

ο


375 The catalytic data at stationary states at 573 Κ and 0.5 atm of H2 are summarized in Table II. The catalytic activity estimated from the total reaction rate is in the order; 0.2wt%Pt-H-ZSM-5 > 0.5wt%Pt-Cs2.5 > 0.5wt%Pt-Cs2.2 > 0.5wt%Pt-Cs2.1 > 0.2wt%Pt-HY. The selectivity is in the order of Pt-Cs2.5 > 0.2wt%Pt-HY > 0.5wt%Pt-Cs2.2 > 0.5wt%Pt-Cs2.1 > 0.2wt%Pt-H-ZSM-5 > H ZSM-5. Thus the rate for the isobutane formation is the highest for 0.5wt%Pt-Cs2.5 (Table II). Table II demonstrates that the selectivity to isobutane greatly depended on the Cs content; 0.5wt%Pt-Cs2.1 (47.5%) < Pt-Cs2.2 (69.5%) < Pt-Cs2.5 (93.9%), where the figures in the parentheses are the selectivities to isobutane. Since the porewidth is in the order of 0.5wt%Pt-Cs2.1 < 0.5wt%Pt-Cs2.2 < Pt-Cs2.5, the different selectivity to isobutane is probably ascribed to the difference in the pore width. That is, the smaller micropores tended to produce the smaller molecules such as propane. It was previously reported that changes in the acid strength of the heteropolyacid were slight upon the substitution of C s for H (1,24). Thus the difference in selectivity among these heteropoly compounds is not due to the acid strength. The correlation between the selectivity to isobutane and the pore size of the catalysts is demonstrated in Fig. 5. +

+

Table II. Activity and Selectivity for Skeletal Isomerization of w-Butane over Pt-promoted Porous Catalysts in the Presence of 0.5 atm of H2 at 573 Κ 0

Rate

Catalyst

W/F

Shape-Selective Catalytic Behavior of Pt-Porous Heteropoly

Recommend Documents