Micropore size distribution by argon porosimetry for cesium hydrogen salts of 12-tungstophosphoric...

Microporous and Mesoporous Materials 26 (1998) 109–115

Micropore size distribution by argon porosimetry for cesiumhydrogen salts of 12-tungstophosphoric acid

Takashi Yamada, Ken-ichi Johkan, Toshio Okuhara *Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan

Received 8 June 1998; accepted 27 July 1998

Abstract

Micropore size distributions of Cs2.1H0.9PW12O40 and 0.5 wt% Pt-Cs2.1H0.9PW12O40 as well as H-ZSM-5, H-Y andAlPO4-5 as standard porous materials were analyzed by Ar porosimetry, assuming cylindrical pores consisting ofoxide ions. The calculated pore-widths of H-ZSM-5, HY and AlPO4-5 were in agreement with the pore-opening sizesdetermined by XRD. The Ar porosimetry demonstrated that Cs2.1H0.9PW12O40 and 0.5 wt% Pt-Cs2.1H0.9PW12O40possess only ultramicropores having widths of about 0.5 nm. Adsorption of various molecules revealed that the porewidths of these heteropoly compounds were in the range 0.43–0.59 nm, which is consistent with the results of the Arporosimetry. In conclusion, nearly uniformly sized micropores are formed on both Cs2.1H0.9PW12O40 and 0.5 wt%Pt-Cs2.1H0.9PW12O40. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Ar adsorption; Micropore size distribution; Microporous heteropoly compound; Molecular adsorption;Porosimetry

1. Introduction amount of surface protons among the acidic Cssalts [6,11,12]. More recently, we preliminarily

Porous heteropoly compounds [1–5] have poten- reported that Pt-promoted Cs2.1H0.9PW12O40tial for unique catalytic functions, since heteropoly showed an excellent reactant shape selectivity forcompounds exhibit high catalytic activities for hydrogenation and oxidation reactions [13,14].various kinds of reactions [6–10]. Particularly, However, the micropore size distribution ofCs2.1H0.9PW12O40 and Cs2.5H0.5PW12O40 are char- microporous heteropoly compounds has not beenacteristic. Cs2.1H0.9PW12O40 is microporous and fully elucidated yet. Recently, Saito and Foleyexhibits a reactant shape selectivity for acid-cata- [15,16 ] claimed that micropore size distributionslyzed reactions in solid–liquid reaction system of various zeolites and aluminophosphates were[3,4]. On the other hand, Cs2.5H0.5PW12O40 pos- reasonably estimated by Ar porosimetry, assumingsesses both micropores and mesopores and is very a cylindrical pore consisting of oxide ions and aactive for a variety of reactions due to the largest lumped parameter for adsorptive interaction. This

analysis provided reliable pore sizes of micro-porous materials such as H-ZSM-5, HY,* Corresponding author. Fax: +81 11 757 5995;

E-mail: [email protected] AlPO4-5 and VPI-5 [15,16 ].

1387-1811/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 98 ) 00221-2

110 T. Yamada et al. / Microporous and Mesoporous Materials 26 (1998) 109–115

In the present study, we attempted to reveal the 2.2. Adsorption isotherms of nitrogen and argonmicropore size distribution of the novel micro-

Both nitrogen and argon adsorption data wereporous materials, Cs2.1H0.9PW12O40 and 0.5 wt%collected using an adsorption system (BELSORPPt-Cs2.1H0.9PW12O40 by Ar porosimetry. By using28SA, BEL Japan Inc.) at liquid nitrogen (77 K)typical zeolites and AlPO4-5, the availability of Arand liquid argon temperatures (87 K), respectively.porosimetry was confirmed in the present study.Mesopore size distributions were calculated on theContrary to Ar porosimetry, N2 porosimetry wasbasis of N2 isotherm from the Dollimore–Healinapplicable for the micropores.(DH) method [18] with a program involving in theapparatus. In Ar adsorption experiments, usualstatic adsorption technique was used in the range2. Experimentalof the relative pressure (3×10−6–10−1 of p/p0) aswell as the full range (~1 of p/p0).2.1. Preparation of heteropoly compounds

Micropore size distributions were calculated onthe basis of Ar or N2 isotherms with an area-Cs2.1H0.9PW12O40 and Cs2.5H0.5PW12O40 wereaveraged filled pore model [15]. The relationprepared by a titration method using aqueousbetween relative pressure p/p0 and the size of thesolutions of H3PW12O40 and Cs2CO3 as describedcylindrical micropores was derived and the iso-previously [11,13]. Particularly, the conditions fortherms were transformed to a relation between thethe preparation of Cs2.1H0.9PW12O40 and 0.5 wt%volume adsorbed of Ar and the pore size. ThePt-Cs2.1H0.9PW12O40 were adjusted to themicropore size distributions were plotted as thefollowing: the concentrations of H3PW12O40 andrate of change in volume adsorbed with pore size.Cs2CO3 were 0.08 mol dm−3 and 0.10 mol dm−3,Physical parameters for the cylindrical pore modelrespectively, and the drop rate of Cs2CO3 was 0.used in the present study are listed in Table 1. The1 cm3 min−1.value of magnetic susceptibility (x, cm3),

In the cases of 0.5 wt% Pt-Cs2.1H0.9PW12O40 x=−1.3×10−29 cm3, was adopted according toand 0.5 wt% Pt-Cs2.5H0.5PW12O40, Pt(NH3)4 Saito and Foley [15]. The used equations are the(OH)2 (3×10−2 mol dm−3) was first added to the same as described in the literature [15]. A com-aqueous solution of H3PW12O40 at room temper- puter program made by us was used for theature to form yellow suspension. Then the aqueous calculation.solution was added dropwise to the suspension atroom temperature. Prior to use, these heteropoly 2.3. Adsorption of various moleculescompounds were pretreated in a vacuum at 523 Kfor 2 h after the calcination at 573 K in air. These Adsorption amounts of various moleculesheteropoly compounds will be denoted to, e.g. having different size were measured with a micro-Cs2.1 or 0.5 wt% Pt-Cs2.1. balance connected directly to a high vacuum

As reference, an H-ZSM-5 (HSZ-860HOA, system [20]. The molecules used are N2 [molecularTosoh Co., 410 m2g−1, Si/Al=36.7) was used. size (MS); 0.36 nm] [21], n-butane (MS=0.43 nm)AlPO4-5 (313 m2g−1) was synthesized by a method [21], benzene (MS=0.59 nm) [21] and 1,3,5-described in the literature [17], of which XRD trimethylbenzene (MS=0.75 nm) [22]. The rela-pattern of AlPO4-5 was similar to that in the tive pressure ( p/p0) was adjusted to 0.2. Theliterature. HY (JRC-Z-HY 4.8, Reference Catalyst adsorption was performed at 193 K for n-butaneof Catalysis Society of Japan, 607 m2 g−1) and and at 298 K for the other molecules for 2 h.Na-Y (HSZ-320 NAA, Tosoh, Co., 723 m2 g−1)were also used. Prior to use, these zeolites and 2.4. Other measurementsaluminophosphates were pretreated in the vacuumat 673 K for 4 h. As non-porous sample, SiO2 Surface area was estimated from N2 isotherm(Aerosil 300, Nippon Aerosil, 313 m2 g−1) was by BET method. XRD pattern was obtained by

using Rigaku Geigerflex 2027.used after the evacuation at 573 K for 4 h.

111T. Yamada et al. / Microporous and Mesoporous Materials 26 (1998) 109–115

Table 1Parameters for analysis of micropore size distribution [15,19]

Parameter Oxide iona Argon Nitrogen

Diameter (nm) 0.276 0.336 0.360Density (mol cm−2) 1.31×1015 8.52 ×1014 6.7×1014Polarizability (cm3) 2.50 ×10−24 1.63 ×10−24 1.46×10−24Magnetic susceptibility (cm3) 1.3 ×10−29 3.25 ×10−29 2.00 ×10−29Adsorption temperature ( K ) — 87 77

aCylindrical pore model [15].

3. Results and discussion The isotherms for H-ZSM-5 and NaY are givenin Fig. 3. H-ZSM-5 (Si/Al=36.7) gave an isothermsimilar to that for an H-ZSM-5 (Si/Al=23.5)3.1. Pore size distribution by ar adsorption isotherm

Fig. 1 shows the adsorption–desorption iso-therms of Ar for the typical porous materials inthe full range of pressure (~1 atm). Cs2.1 showeda Type I isotherm (a Langmuir-type isotherm),indicating that Cs2.1 is a microporous material[23,24]. On the other hand, Cs2.5 gave a Type IVisotherm, which is usually observed for meso-porous materials [23,24]. A hysteresis loop wasobserved for the Ar isotherm. These isotherms forthese Cs salts were nearly identical to those ofN2 [4]. It was confirmed that 0.5 wt%Pt-Cs2.1H0.9PW12O40 and 0.5 wt% Pt-Cs2.5H0.5PW12O40 gave Ar isotherms similar to those forCs2.1 and Cs2.5 shown in Fig. 1, respectively.Therefore, 0.5 wt% Pt-Cs2.1H0.9PW12O40 and0.5 wt% Pt-Cs2.5H0.5PW12O40 are classified intomicroporous and mesoporous materials,respectively.

H-ZSM-5, Na-Y and HY zeolites also showedType I isotherms, the adsorption amounts beinggreatly larger than those of the Cs salts. Thedesorption branches were mostly identical to thoseof the adsorption branches for these zeolites.

Fig. 2 provides the detailed Ar isotherms ofCs2.1 and Cs2.5 in the lower pressure range( p/p0=10−6−10−1), in which the isotherms aredisplayed as semi-log plots. A gentle transition ofadsorption was observed at a relative pressure ofabout 10−5 for Cs2.1. A nearly monotonical

Fig. 1. Adsorption–desorption isotherms of Ar for microporousincrease in the adsorption amount was observedmaterials: #, $ — Cs2.1H0.9PW12O40; %, & —

for Cs2.5. The isotherms for 0.5 wt% Pt-Cs2.1 and Cs2.5H0.5PW12O40; ), n — NaY; and 6, + — H-ZSM-5.0.5 wt% Pt-Cs2.5 were essentially the same as those Open and closed symbols are adsorption and desorption

branches, respectively.of Cs2.1 and Cs2.5, respectively.


3.2. Micropore size distribution

The micropore size distributions of zeolites andAlPO4-5 derived from the analysis of Ar adsorp-tion by the Saito–Foley method are given in Fig. 4.The calculated effective diameters are summarizedin Table 2. H-ZSM-5 gives a peak at 0.51 nm forpore-width. Considering that the pore size ofH-ZSM-5 determined by XRD, 0.53×0.56 nm, thepeak at 0.51 nm corresponds to the pore-width ofH-ZSM-5. This value is in accordance with theprevious results [16 ].

As shown in Fig. 4 and Table 2, HY andAlPO4-5 gave the peaks at 0.73 and 0.63 nm,respectively. Since HY has the opening size of0.74 nm, the detected peak at 0.73 nm reflects theaperture size of the pore. Although, for AlPO4-5,Fig. 2. Ar adsorptions at low pressure ranges for: #,

Cs2.1H0.9PW12O40; and %, Cs2.5H0.5PW12O40. the observed peak, 0.63 nm, is smaller than thestructural size, 0.73 nm [25], this result is consis-tent with that of Saito and Foley [16 ]. Theypointed out that the calculated pores of alumino-phosphates tended to be smaller than the structuralpores [16 ], while the reason for the deviation isnot elucidated. As was not shown in Fig. 4, it wasconfirmed that non-porous SiO2 did not give anypeak in the micropore region. Conclusively, thepresent analysis of Ar adsorption gave the reliabledata on the pore size of the zeolites. The present

Fig. 3. Ar adsorption at low pressure ranges for: ), NaY; and6, H-ZSM-5.

reported elsewhere [16 ], in which two transitionswere detected at the relative pressures ( p/p0) ofabout 3.4×10−5 and 2×10−3. On the other hand,NaY gave an inflection point at about 10−3 ofp/p0. For AlPO4-5 and HY zeolites, the Ar adsorp- Fig. 4. Micropore size distributions for zeolites and AlPO4-5tion isotherms were also confirmed to be nearly derived from Ar adsorption isotherm: a, H-ZSM-5; b,

AlPO4-5; c, NaY; and d, HY.identical to those reported previously [16 ].


Table 2Calculated pore width on the basis of the area-averaged cylindrical pore model

Calculated pore width (nm) Pore diameter by X-ray (nm)Porous materials Ar N2

H-ZSM-5 0.51 0.51 0.53 ×0.56AlPO4-5 0.63 0.45 0.73NaY 0.68 — 0.74HY 0.73 — 0.74Cs2.1H0.9PW12O40 0.52 — —0.5 wt% Pt-Cs2.1H0.9PW12O40 0.52 — —Cs2.5H0.5PW12O40 0.52–0.8 — —0.5 wt% Pt-Cs2.5H0.5PW12O40 0.52–0.8 — —

analysis is, thus, applicable to the microporous We already reported by the adsorption of vari-ous molecules that Cs2.1 and 0.5 wt% Pt-Cs2.1heteropoly compounds. On the other hand, the

calculated pore-sizes for H-ZSM-5 and AlPO4-5 were ultramicroporous [4,14]. The ultrami-cropores are defined as pores having the width lessby N2 adsorption were much smaller than the

structural ones (Table 2). Fig. 5 provides the than 0.7 nm [26]. Typical results for the adsorptionof molecules having different size are summarizedmicropore size distributions calculated on the basis

of Ar adsorption for these heteropoly compounds. in Table 3. Cs2.1 and 0.5 wt% Pt-Cs2.1 did notadsorb benzene (MS=0.59 nm), while N2 (MS=Cs2.1 and 0.5 wt% Pt-Cs2.1 have peaks at

0.52–0.53 nm, while the peaks were relatively 0.36 nm) and n-butane (MS=0.43 nm) wereadsorbed, indicating that Cs2.1 and 0.5 wt%broad. In the cases of Cs2.5 and 0.5 wt% Pt-Cs2.5

(Fig. 6), rather wide distributions were observed. Pt-Cs2.1 have only ultramicropores with a width0.43–0.59 nm.In addition, the micropores of Cs2.5 and 0.5 wt%

Pt-Cs2.5 were found to be larger than those of Besides Cs2.1 and 0.5 wt% Pt-Cs2.1, Cs2.5 and0.5 wt% Pt-Cs2.5 are fascinating catalysts, becauseCs2.1 and 0.5 wt% Pt-Cs2.5 [14].

Fig. 6. Micropore size distribution derived from Ar adsorptionFig. 5. Micropore size distributions derived from Ar adsorptionfor Cs2.1H0.9PW12O40 (solid line) and 0.5 wt% for Cs2.1H0.5PW12O40 (solid line) and 0.5 wt%

Pt-Cs2.5H0.5PW12O40 (dotted line).Pt-Cs2.1H0.9PW12O40 (dotted line).


Table 3Adsorption of molecules having different size

Catalyst Adsorption amounta (mmol g−1)N2b [0.36 nm] n-butanec [0.43 nm] Benzenec [0.59 nm] 1,3,5-TMBc [0.75 nm]

Cs2.1H0.9PW12O40 584(57) 135(26) 10(2) 6(2)0.5 wt% Pt-Cs2.1H0.9PW12O40 625(61) 187(36)d 5(1) 9(2)

aThe figures in the parentheses are adsorption areas (m2 g−1) calculated from molecular cross-section areas and the adsorptionamounts. The figures in the brackets are the size of the molecule.bFrom BET method.cp/p0=0.2.dAfter 30 h. The value at 2 h is 42(8).

the former was highly active for various kinds of the titration, microcrystallites of Cs3PW12O40having about 10 nm in diameter are first formedacid-catalyzed reactions and the latter selective for

skeletal isomerization of n-butane, respectively in the solution and then these crystallites arethickly covered by H3PW12O40 remaining in the[11,27]. The Ar isotherms for Cs2.5 (Fig. 1) and

0.5 wt% Pt-Cs2.5 showed that these are meso- solution. Upon the evaporation, large aggregates(secondary particles) are formed and at the sameporous. However, the steep increase at the very

low pressures (<0.05 of p/p0) suggests that these time interparticle voids are generated. The adductformed from Cs3 and H3PW12O40-overlayer hashave micropores at the same time. As reported

previously [13], Cs2.5 and 0.5 wt% Pt-Cs2.5 the composition of Cs2HPW12O40 and its aggre-gates (Cs2HPW12O40) obtained after the drynessadsorbed larger molecules such as 1,3,5-

trimethylbenzene (MS=0.75 nm) of which the are non-porous large particles. Probably, the inter-stices between the primary particles are occupiedamounts were comparable to that of N2 in the

unit of the adsorption area. These results suggest by very small particles or amorphous phase ascements [29].that the size of micropores of Cs2.5 if present is

probably larger than 0.75 nm. In the present study, The pores of Cs2.1 and Cs2.5 are probablyinterparticle voids between the microcrystallites.it was demonstrated that Cs2.5 and 0.5 wt%

Pt-Cs2.5 possess micropores as shown in Fig. 6 When the Cs content (Cs/polyanion) exceeded to2, the salts became porous and the pore widthand the pores have wide distributions from 0.5 nm

to more than 1.4 nm, which does not conflict with increased as the Cs content increased. The meso-pores of Cs2.5 correspond to the voids betweenthe adsorption data in Table 3. Since

Dollimore–Heal analysis of N2 desorption iso- the microcrystallites themselves. We speculate thatthe micropores for Cs2.1 (probably for Cs2.5) aretherm for Cs2.5 showed the presence of mesopores

with a width of 4.0–4.5 nm [4]. Therefore, the spaces formed between the crystal faces of theprimary crystallites, that is, the spaces correspondpores of Cs2.5 and Pt-Cs2.5 have bimodal distribu-

tions of the pores, in which micropores and meso- to voids formed by crystallographic mismatching[30].pores coexist.

The pores of these heteropoly compounds arenot intracrystallite pores, but intercrystallites pores[4,12]. As described previously [26 ], these com- 4. Conclusionspounds consist of small crystallites (~10 nm insize), of which crystallites themselves are non- Micropores of novel porous materials, Cs

hydrogen salts of H3PW12O40, were characterizedporous [27]. The interparticle voids correspond tothe pores between the microcrystallites [27]. by Saito–Foley analysis of Ar adsorption isotherm.

This analysis of Ar gave reliable pore size distribu-For the microstructure of Cs hydrogen salts ofH3PW12O40, we proposed a model [28,29]. During tions for Cs2.1, 0.5 wt% Pt-Cs2.1, Cs2.5 and


[8] Y. Izumi, K. Urabe, M. Onaka, Zeolites, Clay and0.5 wt% Pt-Cs2.5. It was concluded that Cs2.1 andHeteropoly Acid in Organic Reactions, Kodansha, Tokyo0.5 wt% Pt-Cs2.1 possess only the micropores withand VHC, Weinheim, New York, 1992.

widths of 0.51–0.53 nm, which is consistent [9] Y. Ono, in: J.M. Thomas, K.I. (Eds.), Zamaraev,with the adsorption data of various molecules. Perspectives in Catalysis, Blackwell, London, 1993,

pp. 341.This distribution is responsible for the shape selec-[10] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171.tive catalysis of Cs2.1 and 0.5 wt% Pt-Cs2.1. On[11] T. Okuhara, T. Nishimura, M. Misono, J. Mol. Catal. 74the other hand, Cs2.5 and 0.5 wt% Pt-Cs2.5 have

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