A Novel Automated High Resolution Bet Apparatus

155

S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene © 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A [JOVEL AUTOt1ATED HIGH RESOLUTION BET APPARATUS II.J.ti.

Pieters, A.F. Venero

Omicron Technology Corporation

ABSTRACT A novel gas sorption apparatus will be presented for the precise uetermination of surface areas, pore size distribution, and chemisorption propc r t i e s . T'lre instrument is based on a new princinle, 1,000 over the most advanced units. The high resolution permits definition of new information of solid materials: Pores up to 6,000 ~

Extended range in pore size distribution. can routinely be defined by this technique.

The shape of the isotherm is defined by up to 1,000 data points. The pore size distribution and shape of this distribution can, therefore, be described in unique detail. t1icroporc volume of high surface area solids such as zeolites and activated carbons can be defined. For zeolites, this information can be used to calculate the percent crystallinity and average crystallite size. Cetailcd chemisorption inforLation and definition of the number of acid sites of solid acids.

INTRODUCTION Tile need to define the moruho l ogy and surface characteristics of solid materials has spurred the development of advanced techniques in microscopy and spectroscopy.

These methods in conjunction with

gas sorption are capable of resolving subtle differences in surface and bulk properties of solids. Gas

sorpt~on

·

systems are

typ~ca

.

11 y

stat~c

.

vo 1 ume t rr~ci

sys t ems (1) .

The experimental procedure to determine an adsorption isotherm requires the introduction of a known quantity of evacuated saluple(l). tion is measured.

ad!o~ate

to an

The change in pressure following each addi-

From this information, the amount adsorbed by

the sample can be determined.

The reverse procedure -- removal of

known quantities of adsorbate from a sample previously saturated with adsorbate -- generates the desorption isotherm. While these systems are adequate for surface area and pore size distribution measurements, the resolution of this technique does

156 not permit micropore analysis, definition of the shape of a pore size distribution or other studies requiring detailed information. A novel technique has been developed for the precise measurement of isotherms.

For nitrogen at liquid nitrogen temperature, the iso-

therm can be defined in partial pressure increments of 10- 5.

This

represents an improvement over conventional techniques by a factor

of 10 3.

High resolution measurement offers unique characterization

opportunities.

Examples will include micropore adsorption analysis,

zeolite crystallinity determination and definition of the shape of pore size distributions. EXPERIMENTAL The present study was carried out with the Omnisorp-360(T~1)(2) Analyzer from Omicron Technology Corporation (Figure 1).

The ins-

trument is based on a dynamic principle which allows the continuous introduction of the adsorbate to a sample.

In the adsorption mode,

the adsorbate is introduced by way of a micro-electronic mass-flow controller(3).

The flow controller introduces the adsorbate at a

constant rate, irrespective of the pressure in the sample holder. The value of the pressure is continuously monitored with a precision pressure transducer.

An on-line computer samples the pressure

values and provides unattended experimental control in addition to data reduction.

Experiments were carried out with ASTM standards

and controlled pore glasses CPG-75, CPG-120, and CPG-24A from Electro-Nucleonics, Inc. RESULTS AND DISCUSSION Definition of the sorption isotherms with the dynamic sorption instrument can generally be reproduced within 1%.

The advantage of

this technique is the high resolution with which data can be acquired, thereby improving significantly the quality of information derived from the sorption isotherms.

Critical to this experimental

approach is the ability to maintain equilibrium between the gas phase and the adsorbed gas on the solid throughout the analysis. This objective can be achieved by selecting the rate of introducing the adsorbate below the rate of adsorption.

It was found that for

determinations of the N isotherms essentially all materials can be 2 analyzed at a flow rate of 0.3 ml/min. This conclusion also proved valid for microporous materials such as zeolites and activated carbons.

Hhile the instrument allows these measurements at

much lower flow rates, no additional benefits are obtained.

157

5-&;'

i-&-~-~I

L

----_.

f--·· -·· .. 1

,

,

-_.

-----,,

, , ,!

/ ]

f :

,, ,

,

0: c :,

-{2)

Fig 1. Schematic Diagram of Dynamic Sorption Unit FC Flowcontroller, Sample located in Liquid Nitrogen LN, C on-line Computer

,

t...... _ .........

LZY - 52

u

--.... u u

Fig 2. Adsorption Isotherm of zeolite

-

o

w

dl

Ir

o

l/l

o

«

w

z::>

o

....I

>

sr02-AL203 ASTM

Q.

Iro 0'-

<,

s Q.

Fig 3. Pore Size Distribution . of ASTM SiO NI0074 Z-Al Z0 3 ;

\.

• >.

;

\

-,.................. RPCA)

158

A simple test allows verification of equilibrium conditions.

To

this end, a solenoid is closed between the flow controller and pressure transducer.

Closing this solenoid interrupts the further ad-

dition of adsorbate.

Under equilibrium conditions, no pressure

change should be observed. ADSORPTION Measurements of the surface area of samples based on the adsorption isotherm can generally be made with a reproducibility of 1%. 2/gl For low surface area materials (below 10 m the reproducibility is frequently somewhat less.

In general, one finds, however, that

the main source of errors is in the determination of the dry weight of samples.

Table 1 represents results obtained for some well es-

tablished reference materials.

The adsorption isotherms can be de-

fined at intervals which are determined by the resolution of the pressure transducer.

In the case of nitrogen adsorption at liquid

nitrogen temperature, this resolution is about 10- 5 pip.

High reso-

lution is advantageous for the analysis of microporous material, e.g., zeolites, activated carbons, etc., as well as for chemisorption studies and pore size distribution analysis. TABLE 1

Comparison of Surface Area Value * by Dynamic Sorption With Accepted Values. MATERIAL

Ot1tHSORP-360

ACCEPTED VALUE

ASTM KAOLIN (M16384l

10.8 +- 0.4

11.2 +- 0.6

ASTM ALUMINA (N1013l)

160 +- 2.0

160 +- 4.2

CPG-75 Controlled Pore Glass

161.8 +- 0.8

162

SILICA-ALUMINA

549 +- 5

555

*

2/g Units in m +- standard deviation. For zeolites the heat of adsorption within the zeolite structure

is far greater than the adsorption on the external surface of the crystallites.

This difference in heat of adsorption results in a

characteristic isotherm, Figure 2, which shows that the adsorption is substantially taking place at very low partial pressures. curate determination of the adsorbed volume in the zeolitic

An ac-

159

structure can be used to define the crystallinity.

The adsorption

method is to be preferred over X-ray analysis which is inaccurate at low crystallinity values while relative peak intensities can shift as a result of subtle changes in preparation or composition. A first approximation of the micropore volume in zeolites can be obtained with at-plot analysis(4l

A more regorous treatment is

based on the pore size analysis as a function of the partial pres(6) I n accor d ance w~t. h t h i~s approac h, "~t was f oun d t h at

sure (5)

for zeolite Y, the saturation of the micropore volume is at 3 7 x 10- p/po. In Table 2, a comparison of the crystallinity determined by X-ray vs. micropore adsorption is made. TABLE 2 ZEOLITE CRYSTALLINITY ANALYSIS X-RAY DIFFRACTIOtJ VS. HICROPORE VOLUME ANALYSIS (Analysis Based on Common Reference Hateriall X-P..AY

MICROPORE ANALYSIS ADSORPTION

11

8 48 65

Lf1

64 83 91 96 97 98 98 98

80 100

90

99 96 100 102

In both adsorption and X-ray analysis, the same reference material was used, which was assigned a crystallinity value of 100.

In

Table 2 the X-ray analysis is based on the integration of the three most prominent peaks.

The crystallinity data in this Table would

be somewhat different if the analysis was based on the single most prominent peak or if a broader range of peaks were used.

The ad-

sorption analysis is more precise in distinguishing materials at high crystallinity and more absolute at low crystallinity uhere X-ray peaks are described on a broad amorphous background. The average crystallite size can be determined from the adsorption isotherm by analyzing the external surface area.

This value

is obtained from the isotherm beyond the point where the micropore structure is saturated.

This portion of the isotherm represents

the adsorption of both the amorphous phase and the external surface area of the zeolite crystallites.

For highly crystalline

160

°v

SI/AL

Fig 4. Comparison of 2 Pore Size Distributions, ASTM Si0 NI0074 2-Al 20 J

c,

0::

00 -<;

a,

>

'"

o

~

\.

...,

"'''-----30

Be>

40

RPw

PORE VOLUME DISTRIBUTION FOR AST" REFER:N::""::E:::L e

F::::7::::::A120. PO CORP (ASTH FIRST DRAFT)

.l UO (A5TH FIRST DRAFT) x ARea
18

Z8

38

i Fig 5. Round Robin results of ASTM Committe D-J2 on Catalysts on SiO?-Al?03 NI0074. Results to be published.

S8

AVERAGE PORE RADIUS, (ANGSTROMS)

CPG 859

;

~}

"

i.

":: : • ! :

Fig 6. Pore Size Distribution of a mixture of 3 controlled Pore Glasses. CPG 75, CPG 120 and CPG 240.

:~

,, .

s'

:j 00

., f:

I

~J

100

300

400

500

600

RPw

700

800

900

1000

161 materials, the contribution of the amorphous phase can be neglected. The external area can, therefore, be used to define an average crystallite size.

Approximating the crystallite shape as cubic par-

ticles, the average size "L" can be obtained from the simple expression: 6

d

L = ~

density; A - external area

DCSOIlPTIOU Pore size distribution analysis of silica-alumina ASTt1 N10074 is shown in Figure 3.

High resolution analysis not only defines the

position of the most prominent pore size, but also defines the shape of the pore size distribution.

This result is compared with an in-

dependent analysis performed with a fresh sample on a separate instrument (Figure 4).

Recently, a round-robin analysis by the ASTH

D-32 Committee on Catalysis resulted in a to-be-published analysis, shown for comparison in Figure 5.

An example of the application of

the dynamic sorption system for a synthetic mixture of three controlled pore glasses is given in Figure 6.

From this Figure, the

three materials can readily be distinguished.

An expanded view of

the pore glass contributinf, a main peak at around 150 ~ shows the complexity of this material more clearly.

(Figure 7)

In a separate

analysis of this pore glass (CPG-240J, one can observe six distinct peaks, Figure 8. Activated carbons and coke often present complex internal structures.

Figure 9 presents a detailed view of two closely spaced

pores in a coke sample. COlJCLUS ron The dynamic sorption system offers an improvement in resolution for sorption analysis by a factor better than 10 3 over conventional systems.

The improved resolution permits a detailed analysis of

microporous materials, offers improved values for chemisorption analysis and a detailed view of the shape of the pore size distribution.

162

Fig 7. Expanded Pore Size Distribution of CPG 240 based on data in Figure 6 .

............

~oo

110

120

..

::.:....

....

130

140

150

160

170

180

190

200

RPw

CPG-240

o

'"

Fig 8. Detailed analysis of CPG 240 Pore Size Distribution

..

~

......

~ .:.: ;,:~....... :~i :

A;{j

r :

;

O,.LO---"£"00~--

"2~OO:;;;----::C=----:±-=-

400

RPw

COKE

Fig 9. _P~re'Size Distribution of Coke showing 2 closely spaced pores.

o . .1.15

16

........_ 17

........_ 18

........_

19

-rr20

~__,

21

RPw

22

........-r-v-r-rr-:

23

24

25

163

REFERENCES 1 2 3 4 5 6

S.J. Gregg and K.S.W. Sing. Adsorption, Surface Area and Porosity. Academic Press, London, 1982, 283 pp. Patent Pending Patent Pending M.F.L. Johnson, I. CATAL., 52, 425-431 (1978) D. Dollimore and G.R. Heal, I. APPL. CHEM., 14, 1964, 109-114 G. Horvath and K. Kawazoe, I. CHEM ENG. JAPAN, 16, 6, 1983, 470-475