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Title

Validation of BET Specific Surface Area for
Heterogeneous Ziegler‑Natta Catalysts based onα̲S‑plot

Author(s)Taniike, Toshiaki; Chammingkwan, Patchanee;
Thang, Vu Quoc; Funako, Toshiki; Terano, Minoru

Citation

Applied Catalysis A: General, 437‑438: 24‑27

Issue Date

2012‑06‑15

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/11451

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Toshiaki
Taniike, Patchanee Chammingkwan, Vu Quoc Thang,
Toshiki Funako, Minoru Terano, Applied Catalysis
A: General, 437‑438, 2012, 24‑27,
http://dx.doi.org/10.1016/j.apcata.2012.06.006

Description

Japan Advanced Institute of Science and Technology

Validation of BET Specific Surface Area for Heterogeneous
Ziegler-Natta Catalysts based on S-PlotToshiaki Taniike, Patchanee Chammingkwan, Vu Quoc Thang, Toshiki Funako, andMinoru Terano*

School of Materials Science, Japan Advance Institute of Science and Technology, 1-1 Asahidai,
Nomi, Ishikawa 923-1292, Japan*CORRESPONDING AUTHOR
Tel: +81-761-51-1620; Fax: +81-761-51-1625; E-mail: terano@jaist.ac.jp1

ABSTRACT: Specific surface area based on the BET method in N2 adsorption has historicallyposed poor correlation with olefin polymerization activity of heterogeneous Ziegler-Natta (ZN)
catalysts. We have investigated the validity of the BET surface area for ZN catalysts based onthe S-plot method, where non-porous core-shell MgO/MgCl2/TiCl4 catalysts were used as areference material. The S-plots for typical industrial ZN catalysts clarified the presence of twoclasses of micropores and resultant difficulty to isolate a multilayer adsorption region for the
BET method. The results cast doubt on the application of the BET method to evaluate thesurface area of heterogeneous ZN catalysts.

KEYWORDS: Ziegler-Natta polymerization; polyolefin; nitrogen adsorption; BET method;
S-plot2

1. Introduction
Specific surface area is one of the most fundamental characteristics of heterogeneous
catalysts, especially in terms of catalytic activity. The BET method [1] in N2 adsorption has
been routinely employed for this purpose. Similarly in heterogeneous Ziegler-Natta (ZN)
catalysts for industrial olefin polymerization, the specific surface area evaluated by the BET
method has been frequently attached as a kind of specs. However, it has been recognized thatthe BET surface area contains some intrinsic problem(s), probably arising from uncertainty inpore structures and/or the presence of internal donors that are more or less volatile. For example,
Marigo et al. showed quite poor correlation between two kinds of specific surface area
measured by the BET method and by small-angle X-ray scattering (SAXS) for ZN catalysts
prepared through precipitation of MgCl2-alcohol adduct [2]. The BET surface area was poorly
correlated with propylene polymerization activity, while the SAXS surface area gave some levelof correlation [2]. Rönkkö et al. reported that a self-supported ZN catalyst prepared by anemulsion technique [3] offered a good activity (~ several tens kg/g-cat·h) in propylene
polymerization in spite of extremely low BET surface area (~ 2 m2/g) [4]. Furthermore, thecatalyst underwent relatively homogeneous fragmentation during the polymerization, eventhough it was assumed in N2 adsorption that there were few spaces for reagents to diffuse inside
the catalyst particles [5].
In general, the application of the BET method requires careful attention in evaluating thesurface area of materials with unknown pore structures. Especially, the monolayer capacity (Vm)in the BET equation is not decided for microporous materials, thus leading to physicallymeaningless surface area. One of the most difficult cases is that a material apparently gives an3

isotherm typical for the types II or IV in IUPAC classification but contribution from micropore
filling is hidden. The S-plot method is an empirical approach for the determination of a surface
area and the identification of pore structures [6], which utilizes the invariant nature of multilayer
adsorption of N2. Unlikely to the t-plot method [7], it requires only a non-porous ormacroporous reference material, whose surface chemistry resembles that of a sample of interest.
The adsorption isotherm of a reference material is used to construct an S curve as a function of
p/p0 and the isotherm of a sample of interest is plotted against the obtained S curve. Althoughan advantage of the S-plot method is for the validation of the BET surface area [8], it has been
never applied to heterogeneous ZN catalysts. The largest difficulty exists in the preparation of anon-porous or macroporous ZN catalyst as reference, since typical ZN catalysts consist ofirregular and hierarchical agglomeration of unknown building units bearing a variety of poresizes and shapes.
Recently we have prepared novel core-shell MgO/MgCl2/TiCl4 catalysts by chlorinating
single-crystal MgO nanoparticles with refluxing TiCl4, where the original MgO nanoparticles as
non-fragmentable core were thinly covered by TiCl4/MgCl2 catalytic overlayer [9]. Since thecatalysts were free from internal pores, a series of the core-shell catalysts with controlled
surface area were obtained from the MgO nanoparticles having different sizes. The state ofsupported Ti species studied with X-ray photoelectron spectroscopy was identical to that oftypical ZN catalysts (TiCl4 supported on MgCl2) irrespectively of the catalyst surface area.
Propylene polymerization with these catalysts firstly revealed a completely linear relationshipbetween the catalyst BET surface area and propylene polymerization activity, while the activity
was not dependent on the content of supported Ti species [9]. In this communication, the4non-porous feature of MgO/MgCl2/TiCl4 catalysts was exploited as a reference material for theS-plot method, and the validity of the BET specific surface area analysis in typicalheterogeneous ZN catalysts was firstly studied.

2. Experimental
2.1. Materials
The core-shell MgO/MgCl2/TiCl4 catalysts were synthesized according to our previous
literature [9]. Briefly, MgC2O4·2H2O prepared from Mg(NO)3·6H2O and (COOH)2·2H2O
solution in ethanol was calcined at 800, 750, or 650ºC, leading to single-crystal (i.e. poreless)
MgO nanoparticles with the crystallite size of 32.0, 19.5 or 12.8 nm, respectively (designated as
MgO1, MgO2, and MgO3). The crystallite sizes determined with the Scherrer equation for the(200) peak of MgO in X-ray diffraction coincided well with the sizes estimated fromtransmission electron microscope (TEM) and the BET specific surface area. The MgOnanoparticles were partially chlorinated by refluxing with TiCl4 for 2 h to obtain the core-shell
MgO1-3/MgCl2/TiCl4 catalysts with the sizes of MgO core kept almost unchanged after thechlorination [9]. The Ti contents were 0.88, 2.39, and 7.72 wt.-% for MgO1-3/MgCl2/TiCl4,
respectively, which increased as the catalyst surface area increased.
Two kinds of typical industrial ZN catalysts were prepared in this study. The first catalyst(designated as Cat-S) was synthesized based on precipitation from MgCl2-alcohol solution[10,11]. 10 g of MgCl2 co-dissolved by 52 ml of 2-ethylhexanol and 5 ml of di-n-butylphthalate(DNBP) in anhydrous decane was dropwisely added to 400 ml of TiCl4 kept at –15ºC for 1 h,
and heated at 110ºC. The resultant precipitate was washed with anhydrous heptane, again

5treated with 200 ml of TiCl4 at 110ºC and repeatedly washed with heptane to obtain the catalyst.
The second catalyst (designated as Cat-C) was obtained by chemically converting Mg(OEt)2
precursor into MgCl2 using TiCl4 [11,12]. 80 ml of TiCl4 was dropwisely added into 40 g ofMg(OEt)2 dispersed in 240 ml of toluene. The temperature was once increased up to 90C to
introduce 6 ml of DNBP, and then kept at 110C for 2 h. The obtained solid was washed withtoluene, additionally treated with 80 ml of TiCl4 in 200 ml of toluene at 110C for 2 h, andrepeatedly washed with heptane to yield the catalyst. The Ti contents were 2.1 and 2.7 wt.-% for
Cat-S and Cat-C, respectively.

2.2. Characterizations
The particle morphology of the above-mentioned catalysts was acquired by using scanningelectron microscope (SEM, Hitachi S-4100). Samples for SEM measurements were prepared ina glove bag under N2 atmosphere, then transferred to a deposition device (Hitachi E-1030 Ion
Sputter) for Pt-Pd coating, and finally transferred to a SEM chamber, during which the contactof samples with air was minimized [13].
N2 adsorption / desorption isotherms at 77 K were measured on Belsorp-max. Samples werefilled in a tube under N2 atmosphere and then outgassed for 2 h at 80C prior to themeasurements. In the S-plot method, the adsorption volume (Vads(p/p0)) normalized at Vads(0.4)
for a reference material is regarded as S(p/p0), which is used as a new x-axis to plot theadsorption isotherms for samples of interest. The discrete adsorption data for the reference werenumerically interpolated to generate a continuous S axis.

6

3. Results and discussion
Fig. 1 shows the N2 adsorption / desorption isotherms for Cat-S and Cat-C as typical ZN
catalysts. Their adsorption isotherms are apparently classified as the types IV and II,
respectively, even though contribution of micropore filling to the isotherms could not be judged
as was mentioned in the introduction. The BET surface area was determined to be 302 and 267
m2/g for Cat-S and Cat-C, respectively. The presence of a hysteresis loop is associated with thecapillary condensation taking place in mesopores, while the unrestricted sorption limit in type II
is generally observed for macroporous adsorbent. The hysteresis loop for Cat-S belongs to thetype H2, indicative of the presence of irregular mesopores in size and shape, made by
agglomeration of spheroidal particles [14]. On the other hand, the hysteresis loop for Cat-C
belongs to the type H3, typical for slit-shaped mesopores, whose sizes and shapes are again
non-uniform [12]. These suggestions for the shapes of mesopores are consistent with themorphology of the two catalysts: building units are spheroidal for Cat-S and lamellar for Cat-C
as can be seen in their cross-sectional SEM images (Fig. 2).
The core-shell MgO/MgCl2/TiCl4 catalysts were believed to keep the non-porous morphologyof the single-crystal MgO nanoparticles, since the crystallite dimension of the MgO core as wellas the particle surface area were almost kept unchanged after the formation of the TiCl4/MgCl2
catalytic overlayer [9]. Further, the perfectly proportional relationship between the BET surface
area and propylene polymerization activity [9] was a sort of indicator of the absence of internal
micropore structures (since the BET method does not correctly evaluate the surface area of
microporous materials). Thus, the core-shell MgO/MgCl2/TiCl4 catalysts were regarded as an
ideal reference material for ZN catalysts in the S-plot method.

7

Fig. 3 displays the N2 adsorption / desorption isotherms of the core-shell MgO/MgCl2/TiCl4
catalysts. The MgO1/MgCl2/TiCl4 catalyst having the largest particle size exhibits the type II
adsorption isotherm without prominent rise at p/p0 < 0.1 and with a marginal hysteresis at p/p0 >0.9, which indicates the dominant presence of macropores (> 50 nm). These macroporesabsolutely belong to inter-particle pores since their sizes are above the particle size of 32 nm.
The other two catalysts with smaller particle sizes showed similar isotherms except downward
shift of the hysteresis closure point due to smaller inter-particle pore sizes. The BET surface
area was evaluated to be 33, 100 and 149 m2/g for MgO1-3/MgCl2/TiCl4, respectively.
Although it appeared that all the core-shell catalysts were free from pore filling at least up to
p/p0 = 0.4 (the BET method utilizes 0.05 < p/p0 < 0.35), we have employed the adsorption
isotherm of the MgO1/MgCl2/TiCl4 catalyst to construct an S curve due to its largest
inter-particle pores, where the S value was set to unity at p/p0 = 0.4. Fig. 4 summarizes S-plots
for all the catalysts. The S-plots for the smaller core-shell MgO2,3/MgCl2/TiCl4 catalysts were
substantially linear up to S = 1.5 (corresponding to p/p0  0.66), even though the smallest
MgO3/MgCl2/TiCl4 exhibited a hysteresis around the corresponding pressure. This result
indicates that most of the N2 uptake arose from monolayer and multilayer adsorption of N2. In
this way, the non-porous feature of the core-shell MgO/MgCl2/TiCl4 catalysts held irrespectivelyof their particle sizes. The surface area calculated from the gradients, 99 m2/g for
MgO2/MgCl2/TiCl4 and 151 m2/g for MgO3/MgCl2/TiCl4, well coincided with the BET surface
area (given above), validating the BET method for the core-shell system. On the other hand, theS-plots for the two typical ZN catalysts were far from being linear: the gradient once decreasedaround 0.1-0.3 of S, then increased from ca. 0.7 of S and finally decreased gradually from ca.

8

1.0 of S. These results suggest the presence of at least two steps of pore filling, one at very low
S associated with pores of molecular dimensions and the other at ca. 0.7-1.0 of S associatedwith pores involving quasi-multilayer formation [12]. Thus, we found with the aid of the S-plot
method that typical ZN catalysts contain at least two classes of micropores, and it is not possible
to isolate a multilayer adsorption region from the pore filling (critical for the BET method).
These results clearly cast doubt on the validity of the BET surface area measurement for ZN
catalysts, which is believed as one of the reasons why the BET surface area has posed poorcorrelation with olefin polymerization activity [2,4,15] in contrast to the perfectly linear
relationship observed for the core-shell MgO/MgCl2/TiCl4 catalysts [9].

4. Conclusions
In order to identify an origin of poor correlation between the BET specific surface area andolefin polymerization activity of heterogeneous ZN catalysts, the validity of the BET method
was examined by means of the S-plot method. A standard S curve was constructed from
non-porous core-shell MgO/MgCl2/TiCl4 catalysts, whose BET surface area had exhibited
perfectly linear correlation with propylene polymerization activity [9]. The N2 adsorption
isotherms for two kinds of typical industrial ZN catalysts were apparently classified into thetypes II and IV, to which the BET method is usually applicable. However, the S-plots revealed
the presence of two classes of micropores, making the isolation of a multilayer adsorption
region for the BET method almost impossible. It was concluded that the validity of the BET
specific surface area of heterogeneous ZN catalysts is questionable, which is believed as one of
the causes for the reported poor correlation with olefin polymerization activity.

9References[1] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309319.
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Fundamentals of Adsorption, Engineering Foundation, New York, 1987, pp. 8998.
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11

Figure captions
Fig. 1. N2 adsorption / desorption isotherms for (open / filled circles) Cat-S and (open / filled
triangles) Cat-C

Fig. 2. Cross-sectional SEM images of (upper) Cat-S and (lower) Cat-C. The images in the left
side are magnified in the right side.

Fig. 3. N2 adsorption / desorption isotherms for (open / filled circles) MgO1/MgCl2/TiCl4, (open/ filled triangles) MgO2/MgCl2/TiCl4, and (open / filled diamonds) MgO3/MgCl2/TiCl4

Fig. 4. S-plot for (circle) MgO1/MgCl2/TiCl4 as reference, (triangle) MgO2/MgCl2/TiCl4,(diamond) MgO3/MgCl2/TiCl4, (square) Cat-S, and (positive sign) Cat-C

12

Fig. 1.

Fig. 2.

13

Fig. 3.

Fig. 4.

14