Verified Syntheses of Zeolitic Materials
2nd Revised Edition
The pH-value and its importance for the crystallization of zeolites
Hans Lechert
Institute of Physical Chemistry of the University of Hamburg
20146 Hamburg, Germany
1. Introduction
The alkalinity in a synthesis batch is one of the most important parameters
for the control of the crystallization of zeolites. It determines their composition
and is to a great extent responsible for the type of the crystallizing product.
[1-13]
Generally, the crystallization proceeds via the solution phase so that the species
of silicate, aluminate, and aluminosilicate in the solution are important for
the crystallization mechanism. The pH-value of the solution is determined by
the total alkali content and complicated buffering equilibria of the mentioned
species. [14, 15] Zeolites are usually synthesized in the presence of an amorphous
gel phase. The solubiity of this gel phase also depends on the alkalinity. It
assures the supersaturation for nucleation and growth processes.
The composition of synthesis batches can be described by the formula: MAlO2n[MmH4-mSiO4]
pH2O (1)The Îexcess alkalinity" (m) in this formula
is the difference between total alkalinity (MOH) and the alkali aluminate (MAlO2)
per mol of SiO2. m = (MOH - MAlO2)/SiO2 (2)In
systematic studies of the influence of the alkalinity on the product Si/Al-ratio,
m is generally used as a critical parameter. [1-4]
A detailed discussion of the formation of zeolites has been given recently by
Jansen [5] and by Feijen, et al. [6] In [7-12] thorough studies of the Si/Al
ratio and the rate constant of linear growth in their dependence on m and
n have been carried out for a series of zeolites.
The pH-value has been discussed in only a few papers in connection with the
parameters of the crystallization. Generally, the different zeolite types crystallize
within rather narrow ranges of pH. For faujasites, values between 12.3 and about
13.8 are observed. Robson has discussed procedures of a synthesis of NaY with
Si/Al = 5 at pH = 11. [12] Between 11.3 and 12.7 mordenite usually crystallizes;
at lower values ZSM-5 is obtained. [1,12] Donahoe and Liou have found a linear
dependence of the Si/Al ratio of phillipsite and merlinoite in the pH range
13.3-13.7 in crystallization experiments from clear solutions of systems with
an extremely high silica content. [13] In a series of papers, single values
of the pH or pH changes before and after crystallization are reported. These
differences are usually 1-2 units on the pH-scale.
Much work has been done in the analysis of the species in silicate and silico-aluminate
solutions. Important results have been obtained from NMR. [14-16] A summary
of this and other work can be found in the book of Engelhardt and Michel [16].
A thermodynamic analysis of these species has been undertaken by Guth, et al.
[17, 18]
Generally, it was found that in the pH range of the crystallization of zeolites,
the silicate is most probably present as [SiO2(OH)2]2-
or [SiO(OH)3]-. Only comparatively low concentrations
of higher condensed species are present at pH values above about 12.0. At lower
pH the concentration of dimers and four-membered ring species increases. This
range can be roughly identified with the crystallization region of the more
siliceous zeolites with five-membered rings in their structure.
The aluminate is generally present in very low concentrations and is often described
to be present as [Al(OH)3OSi(OH)3]-. [16,18]
A general survey of the pH-dependency of the hydrolysisequilibria of cations
has been given by Livage. [19] These data are important for syntheses with other
anions present beside aluminate and silicate or aluminate and phosphate. [20]
For a more detailed investigation of the pH in zeolite synthesis, we have done
thorough experiments on the connection of the pH with m and n and
the Si/Al ratio of faujasites within a broad range of compositions.
2. Remarks on the Measurement of pH
Direct control of the pH during the crystallization would be desirable especially
for industrial processes. However, this is difficult because zeolites crystallize
at rather high pH-values. For a measurement of these values the Pt|H2-electrode
or the glass-electrode can be used.
With the Pt|H2-electrode, in principle, very exact results can be
obtained as has been shown by Lagerström [21] and Ingri [22] in very careful
studies of the pH-dependency of the hydrolysis equilibria in silicate solutions.
However, the Pt|H2-electrode is not applicable for routine measurements
for automatic control of a crystallizing batch in an industrial reactor.
For measurements with the glass electrode, it must be taken into account that
this electrode responds to the concentration of the alkali ions as well as the
H3O+ present in the solution. This so-called Îalkaline
error" becomes important at pH > 11.0, depending on the sensitivity
of the electrode material. Data for different electrode materials are demonstrated
in [23]. The alkaline error increases with temperature. It is advisable to follow
the suggestions of the manufacturer of the electrode carefully. At high pH the
properties of the glass membrane of the electrode may change with time. An obvious
alternative is to take samples from the crystallizing batch and to carry out
the measurement of the pH at ambient temperature. This procedure is thoroughly
described in the literature.
3. Preparation of the Reaction Mixtures and pH-Measurements
Generally, the batches for zeolite crystallization were prepared by mixing a
silicate source and aluminate source and adjusting the excess alkalinity m
with a suitable quantity of alkali hydroxide. For the present study the
following sources were used:
Alumina source: 2.5 mole Al(OH)3 (Merck, reinst) and 5 mole NaOH
(Merck, reinst) in 1000g solution.
Silica source: Water glass [Merck, sp. gr. = 1.37) or Silicic acid (Fällungskiesel
säure, Merck) and NaOH to give 273.5 g SiO2 in 1000 g solution.
The solutions were mixed at ambient temperature adding the silicate to the aluminate
and the additional NaOH. Most of the experiments (p = 400) were carried out
with a concentration of 50 g (AlO2- + n SiO2)
/ 1000 g H2O.
Regarding the general batch composition, MAlO2n(MmH4-mSiO4)
pH2O, the concentrations were adjusted to values of n = 2.0
- 14.0, m = 0.3 - 2.5 and p = 400. Some of the batches had lower
water contents of p = 195 and p = 260.
After 2 hours homogenization an appropriate amount of nucleation gel was added
to assure the crystallization of Y-zeolite, again stirred for half an hour and
then the pH measured. The nucleation gel had a composition of NaAlO2 7.5
(Na2.0H2.0SiO4) :155 H2O.
[11] The batch was then heated to the crystallization temperature of 88ºC.
After crystallization the samples were recovered and characterized as described
in [7-11]. The Si/Al-ratios of the products were determined by the EDAX method
in a Phillips SEM 515 and the EXAX-Analyzer PV 9900. The reaction mixtures had
pH values between 12.4 and 14.1. From all mixtures faujasite crystallized. The
pH measurements were done with WIWS 12 and a pH electrode belonging to this
apparatus.
Reproducible results could be obtained according to the following procedure:
- Calibrate at pH = 7.0 using a commercial buffer solution (Riedel de Haen).
- Calibrate at pH = 13.0 using a commercial buffer solution (Riedel de Haen)
and adjust the slope ofthe instrument.
- Read the pH of the sample. Usually a time of about 5 minutes was sufficient
to obtain a constant reading.
Between the different steps the electrode was cleaned with distilled water.
4. Discussion
In Fig. 1 the correlation of m and the measured pH values is demonstrated
for the different Si/Al rations n in the batch. The relation of both
quantities can be fitted by a logarithmic relation which is equivalent to a
titration curve of a weak acid with a strong base at the alkaline end. For n
<5, the curves for the different n are fairly close together.
For higher n, deviations are observed which are not demonstrated in Fig.
1. These n are often applied for crystallization of NaY
Click here for Figures 1 and 2
with high Si/Al ratios and also for crystallization of the more siliceous zeolites
as, e.g., mordenite or ZSM-5. m is a more sensitive measure for the alkalinity
than the pH value, apart from the advantage that it is given directly by the
batch composition.
Fig. 2 shows the dependency of the Si/Al ratio of the products on the measured
pH for different n in the batches. The crosses demonstrate results of
crystallization experiment leading to phillipsite and merlinoite carried out
by Donahue and Liou. [12]
The product Si/Al ratio decreases almost linearly with the pH if n is
held constant. For increasing n the slope of these straight lines increases.
The values for n = 6.0, 7.5 and 8.0 lie near the curve for n =
5.0. Formally, the curves for different n meet at Si/Al = 1.5 and about
pH = 13.5.
Extrapolating ihe curve of n = 5 and 6.0, 7.5 and 8.0 in Fig. 2 to pH
= 11.0 as it was suggested for the faujasite synthesis, a Si/Al ratio of about
4.0 can be obtained. [12]
Summarizing, Fig. 2 shows that for fixed n the Si/Al ratio of faujasites
is a unique function of the pH value in the solution phase. However, for a prediction
of the Si/Al ratio, the pH is not very suitable because exact pH values in the
batches can be expected only after a time-consuming aging procedure.
Therefore, we have tried to find a direct relation between the parameters n
and m of the batch composition and the final Si/Al ratio. [11] From
extensive experiments, including studies of the concentrations of OH- ions and
the silicate in the solution phase [7-10], a simple relation could be derived
which holds with great accuracy Si/Al =1+b|SiO2|sol/|OH-|sol (3)using
b = 2. Eq. 3 can be explained by a simple model of the formation of zeolites.
[24]
Like the pH values, the concentrations are not very suitable for practical use
in the prediction of the Si/Al ratios from the initial composition of the gel.
With some obvious assumptions, Eq. 3 has been changed to an empirical relation
containing only n and m as parameter with the constant b =2. Si/Al
= (b+m)n/(b+mn) (4)In Fig. 3 a large number Si/Al product ratios from batches
for different m with 1.4<n <14.0 are compared with values which
have been calculated using Eq. 4.
Click here for Figure 3
It can be seen that the Si/Al ratio of the crystallizing product can be predicted
by Eq. 4 over the whole range of n much more reliably than from the pH
values of the solution phase.
From extensive studies of literature data, especially from [1-4] and the first
edition of ÎVerified Syntheses," it could be seen that Eq. 4 can be used
successfully for all zeolites with four-and six-membered rings in their structure.
[24] As examples, ZSM-3, LTL, Rho and Offretite shall be mentioned. Data for
high silica zeolites like mordenite, ZSM-5 and Beta can be described
by Eq. 4 using larger values of b. [25]
Systematic arguments for a choice of b for a special zeolite are, however,
missing until now. Therefore, Eq. 4 must be regarded as an empirical relationship
at the present stage of discussion.
The general role of the OH- ion as a mineralizer can be partially
replaced by fluoride leading to a variety of new syntheses at lower pH values.
These syntheses are usually called Îlow alkaline syntheses" or Îfluoride
route." Most probably the F- increases the solubility of the
aluminate as could be shown, for example, in kinetic experiments of the crystallization
of Y zeolites. [26]
5. Conclusions
The composition and the ranges of stability of the zeolitic products depend
on the pH value, the alkalinity and the Si/Al ratio present in the batch. For
practical use in the planning of syntheses, the pH value is rather complicated
to handle. The empirical Eq. 4 proved to be suitable for a prediction of the
product Si/Al ratio from the batch composition with the restrictions discussed
for high silica zeolites. Eq. 4 includes the influence of the alkalinity m
as well as the Si/Al ration n of the batch.
Predictions of the kind of the crystallizing zeolite and also of the effect
of templates have not been possible thus far and remain a matter of experience.
6. References
[1] R. M. Barrer, Hydrothermal Chemistry of Zeolites, Acad. Press, London, 1981
[2] D. W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York,
London, Sydney, Toronto,1974
[3] Zeolite Synthesis, ACS Symposium Series 398, M. L Occelli, H. Robson
(eds.), Amer. Chem. Soc., Washington DC, 1989
[4] Molecular Sieves, Vol. 1, M. L Occelli, H. Robson (eds.), Van Nostrand
Reinhold, New York, 1992
[5] J. C. Jansen, in Introduction to Zeolite Science and Practice, H. van Bekkum,
E. M. Flanigen, J. C. Jansen (eds.), Elsevier, Amsterdam, 1989, p. 77
[6] E. J. P. Feijen, J. A. Martens, P. A. Jacobs, in Stud. Surf. Sd.
Catal. 84A, J. Weitkamp, H. G. Karge, H. Pfeifer, W. Holderich (eds.), Elsevier,
Amsterdam, 1994, p. 3
[7] H. Kacirek, H. Lechert, J. Phys. Chem. 79 (1975) 1589
[8] H. Kacirek, H. Lechert, J. Phys. Chem. 79 (1975) 1291
[9] H. Lechert, H. Kacirek, Zeolites 11(1991) 720
[10] H. Lechert, H. Kacirek, H. Weyda, in Molecular Sieves, M. L Occelli,
H. Robson, (eds.), Van -Nostrand Reinhold, New York, 1992, p. 494
[11] H. Lechert, P. Staelin, Ch. Kuntz, Zeolites 16 (1996)149
[12] H. E Robson, in Zeolite Synthesis, ACS Symposium Series 398 M. L Occelli,
H. Robson (eds.), Amer. Chem. Soc., Washington DC, 1989, p. 436
[13] R J. Donahoe, J. G. Liou, Geochim. Cosmochim. Acta, 49 (1985) 2349
[14] G. Harvey, L S. Dent Glasser, in Zeolite Synthesis, ACS Symposium Series
398, M. L Occelli, H. Robson (eds.), Amer. Chem. Soc., Washington DC, 1989,
p. 49
[15] A. T. Bell in Zeolite Synthesis, ACS Symposium Series 398 M. L Occelli,
H. Robson (eds.), Amer. Chem. Soc., Washington DC, 1989, p. 66
[16] G. Engelhardt, D. Michel, High Resolution Solid State NMR of Zeolites,
Wiley Sd. Pubi., Chichester (1987)
[17] J. L Guth, P. Caullet, A. Seive, J. Patarin, F. Delprato, in Guidelines
for Mastering the Properties of Molecular Sieves, D. Barthomeuf, et al (eds.),
Plenum Press, New York, 1990, p. 69
[18] P. Caullet, J. L Guth, Zeolite Synthesis, ACS Symposium Series 398,
M. L Occelli, H. Robson (eds.), Amer. Chem. Soc., Washington DC, 1989, p. 83
[19] J. Livage, in Stud. Surf. Sci. Catal. 85, J. C. Jansen, et al. (eds.),
Elsevier, Amsterdam, 1994, p. 1
[20] J. L Guth, H. Kessler, R Wey, in New Development in Zeolite Science and
Technology, Y.Murakami, A. Lima, J. W. Ward (eds.), Kodansha, Elsevier, Amsterdam,
1986, p. 121
[21] G.Lagerström.Acta Chim. Scand. 13 (1959), 722
[22] N. Ingri, Acta Chim. Scand. 13 (1959) 775
[23] D. A. Skoog, J. J. Leary, Principles of Instrumental Analysis, Saunders
College Publishing, Harcourt Brace College Publishers (1992) 498
[24] H. E. Robson, Verified Syntheses of Zeolitic Materials, Micropor. Mesopor.
Materials 22, H. Robson (ed.), Elsevier, Amsterdam, 1998, p. 495
[25] H. Lechert, Micropor. Mesopor. Mat., submitted for publication
[26] T. Lindner, H. Lechert, Zeolites 14 (1994) 582