Langmuir 1994,10, 4167-4173
4167
Characterization of Microporous Solids by Adsorption: Measurement of High-ResolutionAdsorption Isotherms E. Maglara, A. Pullen, D. Sullivan, and W. C. Conner* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received April 29, 1994. I n Final Form: August 16, 1994@ The use of adsorption for the characterization of the micropores within zeolites was studied. An experimental procedure for the determination of adsorption isotherms in zeolites is demonstrated. A modified static technique was successfully employed for the acquisition of equilibrium adsorption data. Adsorption isotherms were measured over a variety of zeolites. Nitrogen at 77 K and argon at 77 and 87 K were compared as adsorbates. "he use of argon at 77 K is recommended, while nitrogen at 77 K can also be used and has advantages for adsorptionin the smallest pores (i.e., e 5 A) if the pressure is measured very accurately. We did not see any particular advantages of using argon at 87 K, as was suggested by prior investigators. Current commercial equipment does not accuratelymeasure high-resolution isotherms in a reasonable time.
Introduction The quantitative evaluation of the pore structure is crucial in the design and application of materials for adsorption and catalysis. The characterization of porous solids can be studied by volumetric sorption. All automated instruments currently available were developed to effectively measure adsorption a t pressures from 20 to 1000 Torr, and to optimize the analysis of mesoporosity by sorption ofnitrogen at 77 Kor argon at 87 K. However, the techniques by which measurements of adsorption are Torr (i.e., for adsorption made for pressures from 1 to in micropores) are less certain and need to be further studied. A relatively novel, automated, low-pressure sorption instrument (Omnisorp 360) has pioneered in the analysis of micropores. The technique has been named highresolution adsorption (HRADS).112 HRADS measures isotherms over a wide range of pressures, PIP0 5 1. Small increments in pressure (and thus amounts of adsorbent exposure)are required at the lower pressures in order to provide small differences in the data for volume adsorbed as a function of pressure, thus, high resolution. This technique appears very promising for the study of adsorption in microporous solids, and specifically in zeolites. We will first describe the techniques employed to measure high-resolution adsorption. We will then discuss the choices of adsorbent (Ar or Nz)and temperature (77 or 87 K). We will describe the experimental procedure that we employed. Our results will then be analyzed as to differences in the pressures where adsorption occurs for zeolites of different pore dimension and structure. Finally these results will be compared to the prior data. We find different conclusions as to the preferred adsorbent and conditions. We also find higher resolution with the improved measurements. Measurement Techniques. Venero and Chiou2 and Hathaway and Davis3measured adsorption isotherms on an Omnisorp 360 by using a continuous volumetric technique. Adsorbate gas is delivered to the sample which Abstract published inAduunce ACSAbstructs, October 1,1994. (1) Unger, K. K.; Mtiller, U. In Characterization of Porous Solids; Unger, K. IC,Ed.; Elsevier: Frankfurt, 1988. (2)Venero, A. F.; Chiou,J. N. Muter. Res. SOC.Symp. Proc. 1988, 111, 235. (3) Hathaway, P. E.; Davis, M. E. Cutul. Lett. 1990, 5, 333. @
is contained in a measured volume at a constant rate. The temperatures of the volumes within the delivery manifold and over the sample are maintained by temperature control and isothermal bath (such as immersion in liquid NZ or Ar). An electronic mass flow valve controls the sorbate gas flow. It is assumed that the flow rate is much lower than the rate of adsorption or desorption, and thus, equilibrium between the gas phase and the sorbed phase is assumed. However, this assumption is not necessarily correct particularly at low pressures (e.g., PIPo < where thermal equilibrium depends on transfer of the heat of adsorption through the gas at reduced pressures. Saito and Foley4 determined the adsorption isotherms in an Omnisorp 360 but employed a quasistatic technique. In this case the adsorbate gas is flowed into the sample cell for some time (30 s for their experiments). Then the system is allowed to equilibrate for 8 min before the next aliquot of adsorbate gas is admitted. How much time is required for equilibrium to be reached? Shouldn't the time required vary as the residual pressure changes? Currently, automated instruments employing adsorption of incremental doses of adsorbent that claim to perform high-resolution adsorption measurements do not allow the adsorption to vary the time between doses. In the state-of-the-art automated instruments, a personal computer controls the measurement, performs the data acquisition, and analyzes the gas volume adsorbed versus pressure data to calculate the surface area and pore size distribution. Adsorbent and Temperature. Argon and nitrogen have been extensively used as adsorbate gases in the characterization of porous solids by adsorption. Venero and Chiou2measured adsorption isotherms of both gases over three different zeolites: calcium A, ZSM-5, and sodium Y. Their data for the two adsorbates suggest that nitrogen adsorption a t 77 K is less suitable for the pore size characterization of zeolites. The adsorption of nitrogen at 77 K showed little differences between isotherms for A, ZSM-5, or Y zeolites. On the basis of these data, they suggested that nitrogen gave evidence for a strong interaction with zeolites, resulting in localized adsorption, due possibly to nitrogen's large quadrupole moment. This would make it difficult to differentiate among zeolites of different pore sizes. Argon has no quadrupole moment and would interact less strongly with (4) Saito, A.; Foley, H. C. AIChE J . 1991, 37, 429.
Q743-7463l94/241Q-4167$Q4.5QlQ 0 1994 American Chemical Society
Maglara et al.
4168 Langmuir, Vol. 10,No.11, 1994 zeolites if their hypothesis is correct. Differences of up to 1order of magnitude in relative pressure were found for similar amounts ofAr adsorption on these zeolites. Thus, Venero and Chiou proposed argon as the best adsorbate for the determination of an isotherm over zeolites. The initial “high-resolution”measurements of physical adsorption in zeolites were conducted with automated equipment employing a quasistatic technique using Omicron equipment (currently Coulter). Gas (Nz or Ar) is flowed for a period of time into a fixed volume containing the sample at 77 or 87 K, and then the flow is stopped for 8 min while the adsorption is allowed to approach equilibrium. Subsequent doses of adsorbinggas are added by this stop-flow technique. The pressure was detected with a diaphragm 10Torr (maximum)pressure transducer to quantify adsorption at low pressures. A 1000 Torr transducer is employed for measurement of higher pressures to saturation.
Experimental Procedure In general, the major difficulties in the measurement of adsorption isotherms at low pressures (on the order of Torr in the case of micropores) is the addition of small amounts of adsorbate gas and the accurate measurement of the resulting equilibrium pressures. In order to overcome these difficulties, we developeda modified statictechnique. The differencebetween our technique and the technique followed by Saito and Foley, Choi and Venero, or Hathaway and Davis is that we actually dosed the adsorbate gas into the sample cell through a Valco dosing valve. In this way we knew exactly the amount of adsorbate gas admitted into the system. The dosing volume was 0.36 SCC. Each dose correspondst o increases in relative pressures (PIPo)of less than per dose for Ar at 87 K or N2 at 77 K. The system was then allowed to equilibrate for a variable time before the next aliquot of adsorbate was added. Helium is first added into the system t o determine the “dead volume” of the sample and sample cell from the relationship between pressure vs volume added. The dead volume refers to the volume of the sample holder excluding the sample itself. Mer the evacuation of the helium, the adsorbate gas addition follows. Note that helium is run into the system continuously through a flow controllerfor the determination ofthe dead volume in order to reduce the measurement time. A n added advantage of the exposure to helium prior to the adsorption measurement is that a large heat transfer coefficient of helium assures rapid equilibration between the sample and the liquid nitrogen or argon temperature baths. When the adsorbate gas is dosed into the sample cell, the pressure goes up. As adsorption takes place, the pressure decreases. The system is allowed t o equilibrate before the next plug of adsorbate is dosed. Equilibrium is reached, and the pressure no longer changes with time. The time required for equilibrium to be achieved varies from more than 30 min for the initial doses at residual pressures below Torr to less than 5 min for pressures approaching 1Torr (PIP0 The size of the dose is varied throughout the measurement of the isotherm. Initially, doses in increments less than Torr (PIP0 < are added. If equal size pulses were added (asthey are in automated equipment), over 1000 doses would be required to span the pressures toPIPo = lo-sinincrements ofPIPo < 10-6, as required for the initial doses. If equal times were allowed for equilibrium between doses of adsorbent, 1000 increments of equilibrium time would be needed. We vary the size of the doses by up to an order of magnitude (increasing with pressure) and the time for equilibrium (decreasing with pressure) to assure accuracy in the equilibrium pressure and to minimize the total time of the measurement. Conventionalautomated systems employ a 10-Torrand a 1000Torr pressure transducer for the measurement of pressures at equilibrium. The transducer manufacturers claim accuracy down to
