Surface Area Measurement

Introduction

Surface area is an attribute that is used by catalyst manufacturers and users to monitor the activity and stability of catalysts. There are different methods used to measure surface area and each method can yield different results. Most methods are based on the isothermal adsorption of nitrogen. Either a single point or multipoint method is used to calculate the surface area. At BASF, the multipoint Brunauer, Emmett and Teller (BET) method is used to measure total surface area of fresh and equilibrium moving bed and fluid catalytic cracking (FCC) catalysts. It is also used as a quality control tool during catalyst manufacture.

The data from the multipoint determination are used to calculate the matrix surface area by use of the t-plot method of Lippens and deBoers. The BET surface area minus the matrix surface area is considered to be the zeolite surface area.

Background

Surface area and pore size distribution are important attributes of fluid cracking catalysts. These attributes are measured by the use of nitrogen adsorption/desorption isotherms at liquid nitrogen temperature and relative pressures (P/Po) ranging from 0.05-1.0. A typical isotherm for a zeolitic FCC catalyst is shown in Figure 1. The large uptake of nitrogen at low P/Po indicates filling of the micropores (< 20 Angstrom) in the catalyst. The linear portion of the curve represents multilayer adsorption of nitrogen on the surface of the catalyst, and the concave upward portion of the curve represents filling of meso- (20 - 500 Angstrom) and macropores (>500 Angstrom). An entire isotherm is needed for one to calculate the pore size distribution of the catalyst. However, for a surface area evaluation, data in the relative pressure range of 0.05-0.30 are generally used.

Different analyses can be applied to these data to develop specific information. For example, application of the BET(Ref. 1) equation will provide total surface area of the catalyst, whereas, the t-plot of Lippens and deBoer(Ref. 2,3) is used to determine the surface area of the matrix portion of the catalyst.

This concept was discussed in several publications. Mieville(Ref. 4) used t-plot data on mixtures of Zeolon H and mesoporous silica gel. He concluded that the intercept from the t-plot gave the volume of micropores in the sample. His data for Zeolon content were consistent with calculated values.

Johnson(Ref. 5) used the surface area as a means of determining the zeolite content of a catalyst. He calculated the total surface area using the BET equation and the matrix surface from the t-plot. For an oxide-type catalyst, Johnson concluded that the ratio of t-area to BET area was 0.975 and recommended applying this correction to the t-area. The BET minus the corrected t-plot surface area was taken as the surface area allocated to the zeolitic component. The Johnson publication was the basis for developing ASTM Method D-4365, "Method for determining Zeolite Surface Area of a Catalyst" (Ref. 6).

Total Surface Area

The total to determine the method used to determine surface area, Table 1, showed that six of the respondents used a single point surface area of a catalyst can be measured by either a multipoint or single point technique. Results of a survey method, the remaining five respondents used a multipoint method. Most respondents used data points in the range of 0.05-.30 P/Po, only one used a P/Po below 0.05. In either the single point or multipoint method, the isotherm points are transformed with the BET equation :

where W is the weight of nitrogen adsorbed at a given P/Po, and Wm the weight of gas to give monolayer coverage and C, a constant that is related to the heat of adsorption. A linear relationship between 1/W[(Po/P)-1] and P/Po is required to obtain the quantity of nitrogen adsorbed. This linear portion of the curve is restricted to a limited portion of the isotherm, generally between 0.05-0.30. The slope and intercept are used to determine the quantity of nitrogen adsorbed in the monolayer and used to calculate the surface area. For a single point method, the intercept is taken as zero or a small positive value, and the slope from the BET plot used to calculate the surface area. The surface area reported will depend upon the method used, as well as the partial pressures at which the data are collected .

A comparison of multipoint and single point surface area data are shown in Figure 2. All data were collected on the same samples and instrument, the differences observed can only be attributed to method of calculation and relative pressure at which data were measured. All multipoint data were obtained at P/Po values of 0.08, 0.11, 0.14, 0.17 and 0.20. The single point data were obtained at P/Po value of 0.3. Directionally, the single point data are approximately 5% lower than the multipoint data .

Matrix and Zeolite Surface Area

Total surface area is based on nitrogen gas adsorbed by the matrix as well as that condensed in the zeolite pores. Separation of the surface area attributed to these two components is made by the use of the t-plot method. The surface area determined by the t-plot is attributed to matrix, total minus matrix surface area assigned to zeolite.

The t-plot method is attributed to Lippens and deBoer(Ref. 2,3). They determined that the multi-layer adsorption curve for nitrogen at different pressures and constant temperature is identical for a wide variety of adsorbents, providing no capillary condensation occurs.

They refer to this curve as the universal multimolecular adsorption curve or t-curve. The experimental points of this t-curve were found to give good agreement with the isotherm equation of Harkins and Jura.

This equation is used in most applications for calculating t, the thickness of adsorbed gas as a function of nitrogen relative pressure.

Lippens and deBoer proposed plotting the volumes of nitrogen adsorbed (V) at different P/Po values as a function of t value from the above equation. For multimolecular adsorption, the experimental points should fall in a straight line and pass through the origin for a non-porous material. The slope (V/t) of this line and the relationship surface area, S, = 15.47 (V/t) gives the specific area of the catalyst in square meters per gram (mē/g). For a porous material, the line will have a positive intercept indicating micropores, or deviate from linearity suggesting filling of mesopores. For most materials, the linear portion of the curve between t = 3.5 to 6 Angstroms is used for determination of matrix surface area.

Confirmation of Theory

It was necessary to establish the quantity of nitrogen in terms of surface area that is attributed to the zeolite component of the catalyst. This surface area was considered to consist of two components the surface area attributed to condensation of nitrogen in the micropores of the zeolite, and the surface area (matrix) attributed to the nitrogen adsorbed on the external surface of the zeolite crystals.

To establish these parameters, the total, matrix and zeolite surface areas were determined from three samples prepared from a commercial sample of pure sodium Y zeolite. The zeolite Y was washed with water to a content of 12.5 wt% and a /molar ratio of 1.0 (Sample A). A portion was then exchanged with ammonium nitrate to reduce the content to 1.7% (Sample B). Finally, an aliquot of the ammonium zeolite was exchanged with rare earth to a level of 17.0% ReO (Sample C). All samples were evaluated for total and matrix surface area after degassing at 250 C for 4 hours. Surface area data, after adjustment for cation content are summarized in Table 2. These data show that the surface area will depend on the cation present in the zeolite, the sodium form having the highest total surface area of 806 mē/g, the ammonium form the lowest at 753 mē/g. The rare earth exchanged zeolite had an intermediate surface area. The matrix surface area ranged from 56 to 71 mē/g and the zeolite from 697 to 744, an average of 717mē/g.

A final confirmation of the procedure was made with a series of 24 catalysts of known zeolite content. Each evaluation was based on BET data and t-plot data at five relative pressure levels. A surface area of 717mē/g was assigned to the pure zeolite and used to calculate the zeolite content of the samples. Data for these samples, shown in Figure 3, show excellent agreement between the actual and measured values.

Conclusion

The surface area method is an acceptable procedure for determining the zeolite Y content in catalyst. It has proven to be a useful tool for research, production and troubleshooting commercial operations. The method is quantitative and other techniques such as X-Ray diffraction may be used to identify and verify the presence of zeolite.

References

1. Brunauer, Emmet, Teller, Journal of the American Chemical Society, Volume 60, 1938, p 309.

2. Lippens, B.C. and deBoer, J.H., Jour.Catal., 4, 319 (1965).

3. DeBoer, J.H. et al, Jour. Colloid Interface Sci., 21, 404 (1966).

4. Mieville, R.L.., Jour.Colloid Interface Sci., 41, 371(1972).

5.Johnson, M.F.L.., Jour. Catal., 52, 425 (1978).

6. ANNUAL BOOK of ASTM STANDARDS, Volume 05.03.