Room Temperature Adsorption Measurements of Gases and Vapours on Molecular Sieves, Microporous Materials and Other Adsorbents

Room Temperature Option (P/N: 237-33000-00) for the Gemini analyzer is now available. This option allows you to measure gas and vapor adsorption on samples at temperatures in the range of -70°C to +100°C.

Introduction

The Room Temperature Option provides a faster, simpler way to characterize adsorbents intended for use in pressure-swing air separators, volatile organic chemical recovery systems, protective clothing, adsorption storage systems, air and water purifiers, selective adsorbent systems, and other similar applications.

The gases and vapors most frequently employed are nitrogen, oxygen, argon, CO2, low molecular weight hydrocarbons, SF6, and freons. The adsorbents are zeolites, activated carbons, silicas, aluminas, special types of clays, polymers, resins, and various carbonaceous materials.

To help ensure error-free measurements at or near room temperature, ensure that:

  • Once cleaned and prepared, the sample must never be exposed to air, other gases, or residual adsorbate which may have remained in the sample port from a previous analysis. Such exposure can preload the sample with gas or vapor prior to analysis, displacing some of the adsorbate during the actual measurement and causing an inaccurate adsorption volume to be registered. Short evacuations without heating just prior to analysis are ineffective in removing such recontamination on many of these strongly adsorbing materials.

  • An accurate and steady sample temperature is maintained. Adsorption tends to increase exponentially with decreasing temperatures; variations in excess of 0.1 °C adversely affect data accuracy and repeatability. While acceptable results for measurements requiring only a few hours can be obtained using only a water/crushed ice mixture or a Dewar flask of room temperature water, more elaborate means are necessary for analyses of longer duration or of the highest possible stability and precision.

Programming and Equipment Modifications

The Room Temperature Option allows you to:

  • remove residual adsorbate and air from the sample port after the sample has been installed
  • apply heating or more extensive vacuum treatment when required
  • report uptake as (a) weight percent adsorbate relative to the weight of the fully degassed sample, (b) micromoles adsorbate per gram of sample, or (c) standard cubic centimetres of adsorbate per gram of sample enter saturation pressures of up to 99.999 mmHg
  • record the actual temperature of the sample bath
  • specify the adsorbate to be used

Sample Tube Modifications

The sample tube should be modified as follows:

  • You must install a PrepSeal on the sample tube to prevent contamination of the sample

The following equipment is desirable:

  • A VacPrep 061 Degasser for degassing the sample
  • A commercial temperature controller/chiller for circulating an aqueous propylene glycol solution to the flask in which the sample tube is immersed

Verification Tests

Two classes of zeolitic materials were used to test the design of the Room Temperature Option:

  • A customer-supplied set of atmospheric gas separation zeolites
  • A commercially available pelleted 13X zeolite The same procedure was suitable for both types of zeolites. However, the durability of the customer supplied material upon severe or repeated preparation cycles was found to be inferior to the 13X material.

These zeolitic molecular sieves present some unique analytical challenges since they readily adsorb and retain many atmospheric gases and vapors at ambient temperature and pressure.

These gases and vapors are removed by outgassing while heating under vacuum in a two-step process, then transferred to the analyzer while still under vacuum.

This must be done with caution since heating too rapidly may volatilize the moisture present, causing it to alter the crystal structure which forms the small channels responsible for the strong and selective adsorption properties. Accordingly, a first interval of mild heat treatment under vacuum was used to remove the greater portion of water and a subsequent ramping to higher temperatures used to dislodge the remaining more strongly held water.

Typical Degas Protocol

An example of a typical degas protocol is:

The VacPrep 061 vacuum preparation and heating accessory was used to dry the sample at 110 °C for one hour, 350 °C for four to six hours, and finished at 400 °C for one hour.

A PrepSeal was installed on the sample tube opening, allowing sealing of the sample under vacuum to assure that the sample remained clean during transfer from the VacPrep to the sample port of the Gemini. The PrepSeal was opened after initial pumpdown.

Analyses were performed with nitrogen and later with oxygen in accordance with the run set outlined below:

Analysis Conditions

Evacuation time: 30
Pause during evac? No
Free space? Measure
Sample density: 1
Pressure table? Edit
First rel. pressure: 0.1
Last rel. pressure: 0.3
Number of points: 9
Adsorb pressure 1: 0.2
Adsorb pressure 2: 0.4
Adsorb pressure 3: 0.6
Adsorb pressure 4: 0.7
Adsorb pressure 5: 0.72
Adsorb pressure 6: 0.74
Adsorb pressure 7: 0.76
Adsorb pressure 8: 0.78
Adsorb pressure 9: 0.8
Number of points: 0
Analysis mode? Equilibrate
Equilibration time: 60
Scan rate: 10
System options:
Language? English
Report adsorbate as? 13//
Instrument ID: 1274
Date (DD/MM/YY): 23/10/97
Time (HH:MM:SS): 04:14:22
Request sample ID? Yes
Request sample wt? Yes
Request sat. prs.? Yes
Volume correction: 0
Report adsorbate as? cc/STP/g

Summary of Analysis Results

The customer-supplied material had exceptional selectivity in that several times the amount of nitrogen was adsorbed as compared to oxygen.

The uptake of the final fraction of nitrogen was slow and commensurately long equilibration times were used to ensure completion of uptake. When consistent preparation and running conditions were used, repeatability of about 2% for both nitrogen and oxygen uptake was achieved. Subsequent reuse of the same portion of material tended to show approximately a 10 to 15% decline in uptake per preparation/adsorption cycle. This is believed to have occurred due to disruption of the micropore structure by the strong heating necessary to dislodge contaminants from the pores.

The 13X material was much more stable toward reuse but since a large supply was available, a fresh portion was used for each analysis to eliminate any possibility of errors and variances from reuse. Ten runs were made with nitrogen yielding a standard deviation of uptake volume of 0.51% at 760 mmHg, 25 °C. The peak-to-peak range observed was 1.6% for nitrogen, and 2.4% for oxygen.

The data are reproduced in table 1.

Table 1. Adsorption Analysis Data

Sample Run

Nitrogen Volume Adsorbed (cc/g)

Oxygen Volume Adsorbed (cc/g)

1

7.49

2.51

2

7.52

2.49

3

7.44

2.50

4

7.52

2.49

5

7.43

2.48

6

7.51

2.47

7

7.54

2.52

8

7.55

2.53

9

7.52

2.52

10

7.47

2.51

Conclusions

The Gemini produces excellent results in agreement with vacuum micro-balance results on samples which traditionally have been gravimetrically analyzed for adsorption in the -70 to +100 °C temperature range. The key steps are:

  • prevent inadvertent recontamination of the sample during or after the transfer of the cleaned sample to the analysis station
  • maintain the sample at the exact temperature specified for the entire analysis
  • minimize the amount of operator time and effort required

Data can be presented in a choice of traditional formats.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.

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