Elemental Analysis of Zeolites

The following procedure utilizes Inorganic Ventures' UniSolv Acid Dissolution Reagents for the determination of major and minor elements in zeolites. This procedure replaces both the HF/boric acid digestion and the lithium borate fusion procedures. Accurate and precise data are obtained using conventional glass/quartz nebulizers, spray chambers, and torches via ICP. This procedure is applicable to the determination of aluminosilicates containing Al, Si, alkali earths, alkaline earths, rare earths, B, P, S, Cd, Cr, Co, Cu, Ga, Ge, Hf, In, Fe, Pb, Mn, Hg, Mo, Ni, Nb, Se, Ag, Ta, Sn, Ti, W, U, Zn, Zr, Pt, Pd, and Au. Sample preparations typically take less than five minutes.

Introduction

The elelmental analysis of zeolites for their major and minor components requires a sample preparation approach that will dissolve a wide variety of elements without volatilization or precipitation losses. The first of the two main approaches taken by the analytical community has been to perform a fusion with a mixture of lithium tetraborate / lithium carbonate followed by dissolution of the fuseate with dilute nitric acid. The second approach is to perform a digestion of the sample with aqua regia followed by dissolution in cold hydrofluoric acid (< 5°C) and chelation of the excess HF with boric acid.

Both preparations are followed by measurement using inductively coupled plasma emission spectroscopy. (ICPES). These procedures, which are state-of-the-art for quantitative zeolites elemental analysis, each required 1+ hours of sample preparation time and suffer from measurement problems associated with high solids containing solutions, such as torch and nebulizer fouling, high dilution factors, high Si blanks, and poor detection limits (approximately 100 ppm).

These notes describe a procedure for the elemental analysis of zeolites that has reduced the sample preparation down to the time it takes to weigh the sample, add reagents, and shake. It has also eliminated the measurement problems associated with solutions containing high solids.

Experimental

A.  Sample Preparation of Conventional Zeolites:

1. Weigh 80 to 100 mg (to the nearest 0.1 mg) of the zeolite sample into a 125 to 500 mL LDPE bottle that has been preweighed (note that zeolites are homogeneous by nature).
   
   NOTE:  Weighing from or into plastic containers using electronic balances may result in significant weighing errors due to electrostatic charges. It is best to weigh the sample by difference from a glass container.
2. The order of reagent addition is critical for steps 2 through 4. Add approximately 10 drops of deionized-distilled water and gently swirl to mix. It is important that the sample not be agglomerated and the surface be hydrated.
3. Add 10 mL of Solution UA-1 (CAUTION - contains HF) and 0.5 mL (10 drops) of concentrated 70% nitric acid. Cap the vessel and shake for 1 to 3 minutes.
   
   NOTE:  If the sample is a used catalyst and/or contains Pt, Pd, or Au then heating may be required. If this is the case, then quantitatively transfer the sample to a thick walled Teflon digestion vessel and immerse in boiling water for 15 minutes. For samples requiring digestion, cool and transfer the digestate to a preweighed LDPE bottle.
4. Add 50 mL of Solution UNS-1. Adjust the final weight of the solution from 100 to 500 grams with deionized-distilled water. Mix the solution well by inverting at least 50 times. The solution is now ready for analysis.

B.  Sample Preparation of Zeolites Containing Fluoride Insolubles:

1. Weigh 80 to 100 mg (to the nearest 0.1 mg) of the zeolite sample into a 125 to 500 mL LDPE bottle that has been preweighed.
   
   NOTE:  Weighing from or into plastic containers using electronic balances may result in significant weighing errors due to electrostatic charges. It is best to weigh the sample by difference from a glass container.
2. The order of reagent addition is critical for steps 2 through 4. Add approximately 10 drops of deionized-distilled water and gently swirl to mix. It is important that the sample not be agglomerated and the surface be hydrated.
3. Add 10 mL of Solution UA-4 (CAUTION - contains HF, HCl, and H₃PO₄) and 0.5 mL (10 drops) of concentrated 70% nitric acid. Cap the vessel and shake for 1 to 3 minutes.
   
   NOTE:  If the sample is a used catalyst, has been ignited at > 800°C, and/or contains Pt, Pd, or Au then heating may be required. If this is the case, then quantitatively transfer the sample to a thick walled Teflon digestion vessel and immerse in boiling water for 15 minutes. For samples requiring digestion, cool and transfer the digestate to a preweighed LDPE bottle.
4. Add 100 mL of Solution UNS-2 which consists of two solutions, 2A and 2B; add 50 mL of 2A, mix, then add 50 mL of 2B. Adjust the final weight of the solution to 125 to 500 grams with deionized-distilled water, and mix well by inverting at least 50 times. The solution is now ready for analysis.

Analysis

1. When performing ICP analysis an all quartz/glass sample introduction system is strongly recommended for the optimum degree of precision and fast washout times. The use of HF resistant / plastic introduction systems and high dissolved solids nebulizers are not recommended. These introduction systems give poor precision due to the lack of wetting of the spray chamber and can result in poor accuracy.
2. Viscosity effects will be experienced unless all blank and standard solutions are prepared with the same volumes of Solutions UA-4, UNS-2A, and UNS-2B and adjusted to the same final weights and volumes as used for the samples. Although internal standards were not used in this study, their use would be helpful in cases where matrix matching is difficult or time consuming.
3. All data were obtained using a conventional radial view ICP equipped with a glass spray chamber, glass concentric nebulizer, and a quartz torch.

Results and Discussion

The dissolution of aluminosilicates in analytical laboratories is usually accomplished through the use of concentrated HF. During the past decade we have studied the dissolution of aluminosilicates and found that the overall dissolution results from parallel catalyzed and uncatalyzed reactions, and that the catalyzed reaction consists of HF attack at sites occupied by absorbed hydronium ions. Inorganic Ventures has designed wet chemistry HF mixtures which increased dissolution rates by an order of magnitude. The UA-1 and UA-4 solutions mentioned above were made from high purity electronic grade acids.

The presence of HF in the sample solution presents a safety hazard as well as a sample introduction problem. The hazards of allowing HF to remain in the final sample solution are obvious. The sample introduction problems have to do with HF attack upon the spray chamber and torch that are constructed of glass or quartz. Teflon or other plastic HF resistant sample introduction systems give poorer precision than glass/quartz due to their inferior drainage properties. Historically, chemists have eliminated the activity of HF through complexation with boric acid where one gram of boric acid is needed to complex one mL of 40% HF, forming monofluorobic acid. However, salting out at boric acid concentrations greater than 0.4 grams / 100 mL will occur resulting in large dilution factors. In addition, the concentric glass nebulizers have a life expectancy of approximately 30 seconds when used in the presence of monofluorobic acid. The presence of HF in the sample solution can also result in the formation of a volatile fluoride (H₂SiF₆), therefore giving false high results.

Inorganic Ventures has also developed a series of UniSolv neutralizing/stabilizing solutions referred to as UNS-1 and UNS-2A and 2B. These solutions contain a mixture of complexing and buffering agents which serve to:

1. deactivate the HF by increasing the pH to a value of 7.5 and 8.0 where HF (it is HF that attacks aluminosilicates, glass, quartz, etc. -- not F^-) does not exist while not creating a high hydroxide environment, which is corrosive to glass and will precipitate many of the elements;
2. maintain solubility of the sample through complexation.

Procedure A was applied to the analysis of conventional zeolites and zeolites containing a variety of elements that had been incorporated into the covalent and/or ionic structure. Table I shows these data as compared to a classical lithium tetraborate / lithium carbonate fusion performed in Au/Pt crucibles.

Table II shows data obtained on zeolites containing fluoride insoluble elements using procedure B as compared to the lithium tetraborate / lithium carbonate fusion.

Conclusions

The UniSolv reagents were designed to fill a need that exists in the analysis of materials requiring the use of HF in the sample preparation. The technique was found to be both rapid and inexpensive. It has been demonstrated that these procedures provide accurate and precise data for ICP analysis of zeolites without the need for fusion, microwave equipment, or HF resistant ICP introduction systems.