Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Introduction and explanatory notes

Accounts of synthesis experiments reported in the literature are nearly always cryptic leaving the reader who wants to repeat the experiment many choices of reagents and procedures. In most cases, there are multiple experiments producing similar products without a clear indication of which one the author(s) prefer. Characterizations of the products are often inadequate for an unambiguous choice for a new application.

Experts in the art of zeolite synthesis have learned to accept these ambiguities and persevere through early failures to reproduce the desired product in most, not all, cases. The neophyte may be less diligent; and early failures may be the end of the critical experiment leading to a new application. The IZA Synthesis Commission in preparing this volume seeks to improve the success ratio for synthesis experiments by encouraging the contributors to better define their experiments and adequately characterize their products.

The Organizing Committee, which preceded the Synthesis Commission, surveyed experts in the synthesis area and published an outline covering the essential points for reporting a zeolite synthesis [1]. The format for the recipes in this volume follows this outline. The table form is intended to assist the reader by placing the information in the same relative positions for all recipes. The table form supposes that all synthesis experiments follow the general sequence: batch preparation, crystallization, product recovery, and characterization.

1. Framework Type Codes

The three-letter codes (top line - far left) are arranged in alphabetical order as in the Atlas of Zeolite Structure Types [2]. They define the topography, but not the composition of the resulting phase. Unlike the Atlas, which has one entry for each code, this volume may contain multiple entries for a single code with differing Si/Al ratio products, products with differing T-atoms, or products of essentially the same composition produced by different synthesis procedures.

2. Product Name

The product name (top line - center) is the name by which the product is usually referred to in the literature. There may be several products of similar composition but different names. A more complete list of these names can be found in the Atlas section "Isotypic Framework Structures" for that framework type code. The choice among competing names has been left to the contributing author in most cases. The number of framework type codes is large but limited; the number of products or recipes is unlimited. As long as zeolite synthesis is an active field of research, there should be new recipes for later editions of this volume.

3. T-atom Composition

T-atom composition (top line - far right) refers to the elements which occupy tetrahedral positions in the framework and their relative numerical abundance (basis: 100 T-atoms). The values are based on the elemental analysis of the product of the recipe as given in Product Characterization. In most cases, values are rounded to integer values except where a minor Tcomponent has particular significance.

4. Contributed by

The name(s) indicate the person or persons who actually prepared the entry and is intended to identify the one most likely to respond to communications regarding the recipe. The entries are not intended for full scientific recognition for the research which produced the recipes; in most cases, recognition has already occurred elsewhere in the literature. Authors are identified by name only; academic titles and institutional affiliation are given in the contributors section. Single contributors are listed except where the authors expressly stated co-authorship.

5. Verified by

Verifiers are those Independent investigators who have reproduced the synthesis experiment and obtained a satisfactory product by their own evaluation. Again, only names are given here; for institutional affiliation, see the contributors list. Only those verifiers who responded affirmatively are listed here. Negative responders, those who replicated the experiment but obtained a product other than the desired phase, are acknowledged in the contributors section. These reports, both positive and negative, are part of the record of the recipe and are available on request. In many cases, the responses of verifiers have prompted changes in the recipes.

6. Type Material

Type material refers to contents of the unit cell as indicated by the elemental analysis. In most cases, the product has been washed and dried but not calcined. Thus the template is often a component of the product composition. Water contents of the products are not consistent; only in some cases has the synthesis product been equilibrated under controlled humidity.

7. Method

Method Cites the literature report on which the recipe is based, usually the authorÔs report but sometimes an earlier, more general reference. Patent references have been avoided unless they are specific. It is the intention of this volume that the reader be directed to the single recipe which gives the best chance of immediate success in the synthesis.

8. Batch Composition

Batch composition refers to the product of batch preparation stated in ratios of oxides, template molecules and neutralization products. The basis is usually a single formula weight of Al2O3 or another trivalent oxide; occasionally the base is SiO2 or P2O5

9. Source Materials

Source materials are those actual materials used to prepare the batch along with their purity and supplier. Generally the source materials are stated in the order in which they are used in preparing the batch. The authors have been encouraged to be specific as to the suppliers because many failures to replicate have been traced to the change of supplier for a source material, particularly in the case of silica or alumina. In most cases, the balance of the composition of the component is assumed to be H2O and should be included in calculating batch composition.

10. Batch Preparation

Batch preparation refers to actual quantities of materials plus the preparation steps used to prepare material for the crystallization step. The estimate of product yield is intended for the readerÔs convenience. For each step, the materials added and the order of addition are indicated within the brackets. Order of addition has been found to be critical in some cases. Instructions for completing the step follow the brackets. Combination at room temperature is contemplated unless otherwise stated. Completion of the batch preparation in a matter of minutes or of hours is expected unless delay is specifically required.

11. Crystallization

Crystallization refers to the experimental conditions and temperature profile which converts the finished batch to a product slurry of zeolite crystals in a "mother liquor." The containing vessel is assumed to be inert except in special cases. Accidental seeding by residues of earlier experiments has been shown to be a problem. If autoclaves or their liners are reused, theyshould be carefully cleaned. Rapid heat-up to the crystallization temperature is contemplated; rarely is the heat-up time a significant portion of the total treatment. Temperature fluctuations during treatment are to be expected.

Aging or incubation of the finished batch at ambient or some intermediate temperature is part of some treatments. Time / temperature tradeoffs are described in the literature; the intention here is to give the authorÔs best guess as to the optimum treatment. Monitoring the progress of crystallization can be instructive, but it is difficult in closed autoclaves at temperatures above 100║C. Rather than sample at temperature or cool, sample, and reheat, the usual approach is to divide the batch into several vessels and treat the aliquots for progressively longer times.

Static treatments or only modest or intermittent agitation is the usual case. Continuous agitation may be required for specific preparations.

12. Product Recovery

Product recovery refers to the procedure for separating the desired product from the byproducts of the crystallization process. Most zeolite products are micron-sized crystals which are easily filtered while the "mother liquor is a solution of excess alkali silicate, excess template, etc. Very fine product crystals may require centrifugation for good product recovery. For alkaline synthesis, the pH drops as the washing proceeds; pH = 10 for the final wash is usually sufficient. For fluoride synthesis or AlPO4-type materials, other criteria for adequate washing are required.

Although most zeolite products are water stable, prolonged washing can produce subtle changes in their composition. Hydrolysis may replace cations with H30+ salt or template inclusions may be reduced or eliminated. Some investigators prefer to wash with dilute NaOH rather than pure water. In general, washing conditions must be considered part of the synthesis.

Drying usually is accomplished in a laboratory oven at ~100C. It is good technique to equilibrate the dried sample at a constant 50% humidity to make it stable to handling in laboratory air. Yield here is expressed as percent of theoretical yield based on the limiting component (usually Al2O3 or SiO2). In the literature, yield is sometimes expressed as percent by weight based on the finished crystallization batch.

12.1 Flocculation [3]

Sometimes flocculation, a method of agglomerating fine particles to filterable size, is advantageous. An example of an organic flocculant is a detergent-type molecule, which adsorbs with the hydrophilic end on the hydrophilic zeolite particle surface, with the hydrophobic end extending into the aqueous medium. The thus generated hydrophobic particles coagulate to form flocs or flocks, which can be filtered and washed on the filter with water.

Before applying such an organic flocculant, the alkalinity of the crystallized reaction mixture needs to be reduced. The application of an electrolyte, such as NaCl, as a flocculant, however, has the disadvantage that colloidal silica present in the mother liquor is coagulated as well, so that the crystallinity of, for example, zeolite alpha will be <90%. If this is acceptable, NaCl is added with mild stirring (magnet bar) until, after turning off the stirrer, flocs become visible, first where the meniscus meets the glass. The flocculated product will settle, and the supernatant liquid can be decanted. The sediment may be filtered, but washing with water causes the flocs to disintegrate, and the crystallites will pass the filter again. Washing, however is not necessary. Instead, the filter cake is reslurried, and now an organic flocculant, such as Betz No. 1192, which is added in small portions of a 0.2% solution, until complete flocculation is observed, can be applied. The thus flocculated product can be filtered and washed with water.

If coagulation of the colloidal silica is to be avoided, the strongly diluted crystallized reaction mixture can be left undisturbed for settling, if necessary, for as long as a few days, or centrifuged. The supernatant solution is cautiously decanted from the sediment. If complete settling is not achieved, the small amount of solids left in suspension may be sacrificed. The sediment is then reslurried and flocculated with an organic flocculant, such as Betz No. 1192, filtered and washed, as described above

13. Product characterization

Product characterization identifies the crystalline product and compares its properties to those of known standards. For this volume, basic characterizations are the X-ray diffraction pattern, elemental analysis and crystal size and habit from SEM. For particular applications, several other characterizations might be desired, such as sorptive capacity, ion exchange properties, thermal analysis, nuclear magnetic resonance, etc. Not many authors report their products in such detail, and in some cases it is difficult to obtain data reproducible in another laboratory. Secondary characterization, when provided, are reported in the Notes section.

14. XRD

XRD refers to the principal phase as identified by comparison of its x-ray diffraction pattern with those in the literature. Unit cell parameters are usually given. When competing crystalline phases have been identified from extraneous lines, they are indicated plus an estimate of amorphous material from the background intensity.A reference pattern for the product in the "as synthesized" is attached. In some cases a second pattern of the calcined product is provided. Some of the calcined materials, particularly AlPO4 and GaPO4, are moisture-sensitive. For other cases the calcined material is virtually identical in the XRD pattern to the as-synthesized sample. In such cases no XRD trace of the calcined product is given. A separate article describes the instrument conditions for recording the XRD patterns.

15. Elemental Analysis

Elemental analysis gives ratios of metal cations present usually expressed as the ratios of their oxides. The editor prefers the direct analytical result (weight percent of the element or its oxide based on the dry sample). Most authors give ratios of the oxides based on one formula weight of Al2O3 or SiO2. In most cases, these were determined by inductively coupled plasma emission spectroscopy. in some cases, the content of water or template molecules in the product as indicated by thermal analysis is also included.

16. Crystal Size and Habit

Crystal size is an estimate of the crystallite size and/or the aggregate particle size. Habit is a qualitative description of the sample as observed in the SEM.

17. References

References indicate the primary literature report on which the recipe is based plus selected general references recommended by the author. This list is intentionally limited and is intended to start the userÔs search of the literature, not complete it.

18. Notes

The notes give additional instructions or information which the author believes helpful to the reader but which do not fit into the recipe format. The additional instructions are intended to substitute for a private conversation with the author before the reader/user begins the synthesis experiment. It is potentially the most valuable part of the contribution.

References

[1] H. Robson, Zeolites, 13 (1993) 399
[2] W. M. Meier, D. H. Olson, Ch. Baerlocher, Zeolites, 17 (1996) 1
[3] G. Kuhl, personal correspondence