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Following the hydrothermal crystallisation of zeolites using time-resolved in situ powder neutron diffraction
Microporous and Mesoporous Materials 48 (2001) 79±88
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Following the hydrothermal crystallisation of zeolites using time-resolved in situ powder neutron diraction Richard I. Walton a,1, Ronald I. Smith b, Dermot O'Hare a,* b
a Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
Received 6 August 2000; received in revised form 14 November 2000; accepted 27 November 2000
Abstract A specially designed hydrothermal cell has been used for the study of the crystallisation of some zeolites from amorphous aluminosilicate gels by time-resolved in situ neutron diraction. The eect of sodium hydroxide concentration and temperature has been investigated. Following the reactions in situ has allowed the stability of zeolite phases to be monitored directly and eciently under real reaction conditions. At the lowest sodium hydroxide concentrations used and at the lowest temperature applied 80°C sodium zeolite A is the favoured product and after a period in excess of 10 h is the sole product. As the sodium hydroxide concentration or temperature is increased the initially formed sodium zeolite A is replaced by hydroxosodalite. At 140°C hydroxosodalite crystallises directly from the amorphous gel. By determining the changing areas of Bragg re¯ections with time, crystallisation curves can be obtained. The advantages of the in situ powder neutron diraction technique over analogous X-ray powder diraction methods are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Crystallisation; In situ neutron diraction; Sodalite; Zeolite A; Aluminosilicate
1. Introduction Microporous zeolites have now been widely studied and used in many industrially important areas for over 50 years. Despite the huge commercial value of both naturally occurring and synthetic zeolites, the formation mechanism of microporous materials under hydrothermal
* Corresponding author. Tel.: +44-1865-272600; fax: +441865-272690. E-mail address: [email protected] (D. O'Hare). 1 Present address: School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK.
conditions is poorly understood. A detailed understanding of hydrothermal crystallisation mechanisms would allow the outcome of new reactions to be predicted, in much the same way as the organic chemist is able to synthesise complex molecules by application of appropriate synthesis conditions. Such rational design in solid-state chemistry has not been achieved. In the case of zeolite synthesis, this predictive ability would have important implications, such as the ability to prepare new materials of desired pore size and shape for separation science, or with surface areas and chemical properties appropriate for a speci®c catalytic application [1]. The need to follow the crystallisation and behaviour of zeolites under
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R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
hydrothermal conditions, in order to gather mechanistic information, requires the construction of specialised apparatus to mimic real reaction conditions, and allow non-invasive study of reactions as they take place. This has been the focus of much attention in recent years, and progress in the area has been reviewed [2,3]. We have recently described the design and construction of a hydrothermal cell for the study of reacting mixtures of materials under hydrothermal conditions using time-resolved in situ neutron powder diraction [4]. Neutron diraction lends itself well for the use of large-volume reaction vessels, since many materials that have properties appropriate for the construction of pressure cells to operate at elevated temperature have very low neutron absorption and scattering cross-sections. Therefore high-resolution diraction data may be collected from materials under non-ambient conditions with no contribution from cell scatter. Our reaction cell is constructed from nullscattering Ti±Zr alloy (67.7 at.% Ti, 32.3 at.% Zr), and is the ®rst such apparatus to exploit fully the advantages of neutron diraction for the study of hydrothermal crystallisations. Only one study of hydrothermal reactions using neutron diraction has been described previously, but this did not overcome the problem of signi®cant cell scatter appearing in the data [5]. We have previously used the Oxford/ISIS reaction cell to study the hydrothermal formation of the ferroelectric material barium titanate [6], and now turn our attention to the study of some aluminosilicate zeolites, with the aim of gaining new information about their formation and behaviour under hydrothermal conditions. 2. Experimental section 2.1. The neutron diraction experiment We have described previously the design of the Oxford/ISIS hydrothermal reaction cell in some detail [4], so here give a brief description of its construction and operation. Fig. 1 shows a schematic of the hydrothermal reaction vessel. The cell is constructed from a 20 mm diameter Ti±Zr alloy
(67.7 at.% Ti, 32.3 at.% Zr) with 0.3 mm thick walls. The walls are internally coated with a thin 100 lm layer of gold metal to prevent corrosion by the chemical reagents used. The cell is heated by copper heating blocks above and below a 4 cm high window which is exposed to the neutron beam. Pressure is monitored continually during heating by means of a pressure transducer. Neutron diraction experiments were performed using the POLARIS diractometer of ISIS, the UK spallation neutron source at the Rutherford Appleton Laboratory. POLARIS is a high-¯ux, medium-resolution time-of-¯ight neutron diractometer [7]. Scattered neutrons are measured on POLARIS by up to four banks of detectors from a sample container in an evacuated tank. In the current work we have used data from the backscattering detector bank, which are least aected by attenuation eects of the solvent. This bank consists of 58 3 He tube detectors covering an angular range of 130±160°2h, allowing Bragg re to be measured. ¯ections of d-spacing 0.2±3.2 A Diraction patterns were measured every 15 min during the hydrothermal experiments. 2.2. Material Aluminosilicate gels were prepared from amorphous alumina (Al2 O3 , synthesised by calcination of amorphous aluminium hydroxide hydrate (Aldrich) at 700°C for 2 h), fumed silica (SiO2 , particle size 0.007 lm, used as supplied from Aldrich), sodium deuteroxide solution (40% in D2 O as supplied by Aldrich) and D2 O (supplied by Fluorchem Ltd.). The deuterated reagents were necessary to reduce the large incoherent scattering of protons, and so give diraction data of lowered background. The Al2 O3 and SiO2 were shaken together to achieve intimate mixing, and stirred with the required amount of NaOD solution to produce a thick paste. Reaction mixtures containing 3 g of Al2 O3 were used and their total volume kept constant for all experiments. This was transferred to the hydrothermal reaction cell and heating commenced immediately. Desired temperature was reached typically within 5 min. Table 1 shows the compositions of the gels used, the
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
81
analysis. In the time-of-¯ight experiment, the d is spacing of a characteristic Bragg re¯ection (in A) related to time-of-¯ight (t in ls) by the equation: d 1:977 10 3 =L sin ht, where L is the total ¯ight path of the neutron from source to sample to detector element (in m) and 2h the Bragg scattering angle. For the backscattering bank on POLARIS detectors have L ranging from 0.65±1.35 m and 2h from 130° to 160°. Normalisation and conversion of time-of-¯ight to d-spacing is performed within the program Genie [8], with a routine written especially for the POLARIS instrument. In addition, this allowed output of data into a format appropriate for the pro®le ®tting program Fullprof [9] and also allowed Gaussian-®tting of Bragg re¯ections to be performed. 3. Results Fig. 1. A schematic of the Oxford/ISIS hydrothermal reaction cell for in situ neutron diraction. Table 1 Gel compositions and reaction conditions used in the in situ neutron diraction experiments Experiment
Gel composition
Temperature/°C
Final pressure/bar
1
Al2 O3 :2SiO2 :4NaOD:20D2 O Al2 O3 :2SiO2 :3NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :2.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O
100
1.64
100
1.62
100
1.60
100
1.65
80
1.32
120
2.14
140
3.04
2 3 4 5 6 7
temperature applied, and the maximum pressure attained during the reaction. 2.3. Data analysis Neutron diraction data from the POLARIS back-scattering detector bank were used in all data
Fig. 2a shows diraction data measured during Experiment 1 using a gel of composition Al2 O3 : 2SiO2 :4NaOD:20D2 O. The initial aluminosilicate gel is amorphous, and after heating for 45 min the ®rst Bragg re¯ections appear. The ®nal product of this reaction is identi®ed as hydroxosodalite, Na8 [Al6 Si6 O24 ](OH)2 nH2 O, however the ®rst Bragg re¯ections to appear are not due to this phase. Performing the experiment using less NaOD (Experiment 2) allowed the second phase present in the system to be identi®ed as sodium zeolite A (Fig. 2b) where even after 10 h of heating it is the sole product. At even lower NaOD concentrations (Experiment 4), the time taken for crystallisation to begin was considerably longer, and again after 10 h of heating only zeolite A is present. At intermediate NaOD concentrations (Experiment 3) the zeolite A that ®rst forms is present for a considerably longer period (3 h) before total transformation into hydroxosodalite is observed (Fig. 2c). Experiments were also performed at a number of temperatures using an initial gel of composition Al2 O3 :2SiO2 :3.5NaOD:20D2 O (Experiments 5±7, Table 1). Diraction data are shown in Fig. 3. It may be observed that increasing temperature has the same eect as increasing NaOD concentration, with crystallisation beginning after a shorter period
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R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
Fig. 2. Selected neutron diraction patterns measured during the heating of aluminosilicate gels of composition Al2 O3 :2SiO2 :xNaOD:D2 O at 100°C for (a) x 4, (b) x 3, and (c) x 3:5. The data presented represent a d-spacing range of 1±3 A. Two distinct crystalline aluminosilicates are identi®ed: hydroxosodalite (International Zeolite Association Code, SOD) and sodium zeolite A (LTA). See text for a discussion of conversion of time-of-¯ight into d-spacing.
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
83
Fig. 3. Selected neutron diraction patterns measured during the heating of aluminosilicate gels of composition Al2 O3 :2SiO2 :3.5 NaOD:D2 O at (a) 80°C, (b) 120°C, (c) 140°C.
of heating and zeolite A present for shorter periods of time before sodalite is the favoured product. At the highest temperature studied 140°C no zeolite
A is observed to form and hydroxosodalite crystallises directly from the amorphous starting materials.
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R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
analysis of the diraction data was performed. To improve the signal:noise ratio of the diraction data, four of the in situ data sets were summed and normalised to represent a total of one hour of data from hydroxosodalite. The structure of deuterohydroxosodalite, Na8 [Al6 Si6 O24 ](OD)2 2D2 O, has previously been determined using powder neutron diraction [11], and using the previously determined lattice constants we carried out Le Bail extraction of the integrated intensities (re®ned variables were lattice constants, peak shape and background terms). Fig. 6 shows the calculated diraction pattern obtained by whole pattern ®tting, along with the experimental data points. The signal:noise ratio of the data is not high, but it must be emphasised that the data were obtained from a sample of the aluminosilicate in sodium hydroxide solution at reaction temperature. A re was obtained ®ned cell parameter of 8.9311(3) A from the Le Bail extraction procedure (space group P-43n). 4. Discussion
Fig. 4. Typical Gaussian ®ts to Bragg re¯ections measured in situ, (a) the coincident (3 3 0) and (4 1 1) Bragg re¯ections of hydroxosodalite and (b) the coincident (6 4 4) and (8 2 0) peaks of zeolite A.
The determination of the changing areas of Bragg re¯ections allows crystallisation curves to be obtained, since the intensity of a diraction peak is directly proportional to the amount of diracting material [10]. Fig. 4 shows typical Gaussian ®ts to selected Bragg re¯ections of the in situ diraction data and Fig. 5 crystallisation curves produced from the data shown in Fig. 2. For each zeolite studied all characteristic Bragg re¯ections were observed to appear at the same time, and their increase in intensity with time was identical. This immediately suggests that crystal growth is isotropic. Having identi®ed the crystalline aluminosilicates present during each reaction by the positions of their characteristic Bragg re¯ections further
The construction of a null-scattering hydrothermal reaction cell for neutron diraction has successfully allowed the crystallisation of zeolites to observed in situ in a non-invasive manner under real reaction conditions. The zeolites we have studied here are amongst the most widely studied of all zeolites. Sodium zeolite A was the ®rst synthetic zeolite to be prepared [12] and now ®nds widespread application as a laboratory drying agent, and as a detergent ``builder'' improving the action of household washing powders by exchanging magnesium and calcium ions by sodium [13]. Hydroxosodalite is one of a large number of materials, both naturally occurring and synthetic, that adopt the sodalite structure. The structure of a berylsilicate sodalite was ®rst elucidated in the 1930s by Pauling [14], and aluminosilicate sodalites have since been widely studied as a model zeolite structure, although of little practical application because of its small pore openings. The structures of the zeolite A and sodalite are closely related, both being constructed from what may be thought of as a fundamental zeolite structural
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
85
Fig. 5. Crystallisation curves determined by integration of Bragg re¯ections, (a) the growth of sodalite in Experiment 7, (b) the growth of zeolite A in Experiment 5, and (c) the changing amounts of zeolite A and hydroxosodalite in Experiment 3.
building unit, the b-sodalite cage [15]. The conversion of zeolite A to sodalite is a good example of
Ostwald's rule of successive crystallisations whereby a metastable phase ®rst crystallise followed by
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R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
Fig. 6. The observed (points) and simulated (line) neutron diraction pattern of hydroxosodalite measured in situ at 140°C.
the successive crystallisation of thermodynamically more stable phases [16]. Our in situ experiments highlight that reaction conditions must be carefully chosen in order to favour the product of zeolite A over sodalite. Small increases in NaOD concentration or in temperature result in the collapse of zeolite A into sodalite. We observe the formation of no other crystalline phases during the course of the transformation. Previous studies of the process have relied largely on quenching experiments, whereby reactions are removed from the oven after selected periods of heating, recovered by ®ltration and examined [17,18]. Such experiments showed that no other zeolites crystallise from the same reaction mixtures, and measurements of particle size distributions indicated that the transformation occurs by dissolution of the zeolite A and recrystallisation from solution, rather than by a direct solid-state transformation [17,18]. Our new results provide evidence that under reaction conditions no other phases crystallise during the transformation of zeolite A into hydroxosodalite. The ability to extract structural information from data measured in situ will be important in
understanding the properties of zeolites and their stability under non-ambient conditions. The re we have ob®ned cell parameter of 8.9311(3) A tained for hydroxosodalite is slightly higher than previously obtained by powder the value of 8.86 A neutron diraction [11]. This earlier value, however, had been obtained at 90°C, and we must expect some eect of thermal expansion for the sample we studied at 140°C. In addition, the previous structural study was performed on a sample of composition Na8 [Al6 Si6 O24 ](OD)2 2D2 O, but other authors have reported that extra water molecules can be incorporated into the structure to give materials of composition Na8 [Al6 Si6 O24 ](OD)2 nD2 O, with n up to 4, and a corresponding increase in room temperature cell parameter to up [19]. Our results are consistent with the to 8.92 A presence of an expanded sodalite framework incorporating extra water under reaction conditions. The use of diraction techniques for following hydrothermal crystallisations has only come about recently and work has so far largely concentrated on the use of X-ray diraction. Zeolite formation has naturally attracted some attention. Munn et al. ®rst described the use of energy-dispersive X-ray
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
diraction (EDXRD) for the in situ study of a zeolite crystallisation [20], and the method has been developed by O'Hare and co-workers in the last ®ve years to provide an apparatus which is now used by several groups [21,22]. Although the EDXRD method allows rapid collection of data (of the order of seconds), which provides excellent kinetic information, the intrinsic low resolution of the data means that little structural information is revealed. Norby and co-workers used angular dispersive X-ray diraction to follow hydrothermal reactions [23], and obtained data of higher resolution from zeolite crystallisations [24,25]. The technique, however, requires the use of very small reaction cells (glass capillaries) and which does not necessarily re¯ect real synthetic reaction conditions. In addition, in some cases problems in sampling a representative mixture of starting materials could possibly arise when very small volumes of solid and liquids must be taken, causing problems with data interpretation. Our ®rst experiments with time-of-¯ight neutron diraction have shown that it is possible to collect diraction data of high resolution, in a laboratory-sized reaction vessel with no background from the reaction cell. In order to improve the quality of data in the near future we will perform experiments on the new GEM diractometer at ISIS [26], which oers signi®cantly higher neutron ¯ux and a greater detector area compared to POLARIS. We are also planning to carry out experiments on the ®xed wavelength D20 diractometer at the Institut Laue Langevin, Grenoble, France [27], which oers similarly high ¯ux and a large-angle novel position-sensitive detector. 5. Conclusions We have demonstrated by the study of a simple aluminosilicate synthesis that neutron powder diraction can be a powerful technique in studying the crystallisation of zeolites under genuine laboratory reaction conditions in real time. Information is obtained about the course and time-scale of reaction in a direct and ecient manner. Crystallisation curves can easily be obtained and there exists the possibility of extracting detailed struc-
87
tural information form materials under genuine hydrothermal conditions. With the ongoing developments in neutron diractometers, with envisage being able to collect in situ diraction data of higher quality and in shorter periods of time. Acknowledgements We are grateful to the EPSRC for ®nancial support of this work and provision of beam-time at ISIS. We thank Dr. F. Millange for assistance with some of the data analysis. References [1] M.E. Davis, in: L. Bonneviot, S. Kaliaguine (Eds.), Stud. Surf. Sci. Catal.: Zeolites: A Re®ned Tool for Designing Catalytic Sites, vol. 97, 1995, p. 35. [2] A.K. Cheetham, C.F. Mellot, Chem. Mater. 9 (1997) 2269. [3] R.J. Francis, D. O'Hare, J. Chem. Soc. Dalton Trans. (1998) 3133. [4] R.I. Walton, R.J. Francis, P.S. Halasyamani, D. O'Hare, R.I. Smith, R. Done, R. Humpreys, Rev. Sci. Instrum. 70 (1999) 3391. [5] E. Polak, J. Munn, P. Barnes, S.E. Tarling, C. Ritter, J. Appl. Crystallogr. 23 (1990) 258. [6] R.I. Walton, R.I. Smith, F. Millange, I.J. Clark, D.C. Sinclair, D. O'Hare, Chem. Commun. (2000) 1267. [7] S. Hull, R.I. Smith, W.I.F. David, A.C. Hannon, J. Mayer, R. Cywinski, Physica B 180, 181 (1992) 1000. [8] W.I.F. David, M.W. Johnson, K.J. Knowles, C.M.M. Smith, G.D. Crosbie, E.P. Campbell, S.P. Graham, J.S. Lyall, Rutherford Appleton Laboratory Report RAL-86102, 1986. [9] J. Rodriguez-Carvajal, Satellite Meeting on Powder Diffraction, Abstracts of the XVth Conference of the International Union of Crystallography, Toulouse, 1990, p. 127. [10] H.P. Klug, L.E. Alexander, Diraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1974. [11] M. Wiebcke, G. Engelhardt, J. Felsche, P.B. Kempa, P. Sieger, J. Schefer, P. Fischer, J. Phys. Chem. 96 (1992) 392. [12] D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, T.L. Thomas, J. Am. Chem. Soc. 78 (1956) 5962. [13] A. Dyer, An Introduction to Zeolite Molecular Seives, Wiley, New York, 1988. [14] L. Pauling, Z. Kristallogr. 74 (1930) 213. [15] W.M. Meier, D.H. Olsen, C. Baerlocher, Atlas of Zeolite Structure Types, fourth ed., Elsevier, Amsterdam, 1996. [16] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, New York, 1982.
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[17] B. Subotic, D. Skritic, I. Smit, L. Sekovanic, J. Cryst. Growth 50 (1980) 498. [18] B. Subotic, L. Sekovanic, J. Cryst. Growth 75 (1986) 561. [19] J. Felsche, S. Luger, Ber. Bunsenges. Phys. Chem. 90 (1986) 731. [20] J. Munn, P. Barnes, D. Hausermann, S.A. Axon, J. Klinowski, Phase Transit. 39 (1992) 129. [21] J.S.O. Evans, R.J. Francis, D. O'Hare, S.J. Price, S.M. Clarke, J. Flaherty, J. Gordon, A. Nield, C.C. Tang, Rev. Sci. Inst. 66 (1995) 2442. [22] G. Muncaster, A.T. Davies, G. Sankar, C.R.A. Catlow, J.M. Thomas, S.L. Colston, P. Barnes, R.I. Walton, D. O'Hare, Phys. Chem. Chem. Phys. 2 (2000) 3523.
[23] P. Norby, A.N. Christensen, J.C. Hanson, in: J. Weitkamp, H. Pfeifer, H.G. Kange, W. Hoelderich (Eds.), Stud. Surf. Sci. Catal.: Zeolites and Related Microporous Materials: State of the Art 1994, vol. 94, 1994, p. 179. [24] A. Gualtieri, P. Norby, G. Artioli, J. Hansen, Phys. Chem. Miner. 24 (1997) 191. [25] A. Gualtieri, P. Norby, G. Artioli, J. Hanson, Micropor. Mater. 9 (1997) 189. [26] W.G. Williams, R.M. Ibberson, P. Day, J.E. Enderby, Physica B 241 (1997) 234. [27] P. Convert, T. Hansen, A. Oed, J. Torregrossa, Physica B 241±243 (1998) 195.
www.elsevier.com/locate/micromeso
Following the hydrothermal crystallisation of zeolites using time-resolved in situ powder neutron diraction Richard I. Walton a,1, Ronald I. Smith b, Dermot O'Hare a,* b
a Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
Received 6 August 2000; received in revised form 14 November 2000; accepted 27 November 2000
Abstract A specially designed hydrothermal cell has been used for the study of the crystallisation of some zeolites from amorphous aluminosilicate gels by time-resolved in situ neutron diraction. The eect of sodium hydroxide concentration and temperature has been investigated. Following the reactions in situ has allowed the stability of zeolite phases to be monitored directly and eciently under real reaction conditions. At the lowest sodium hydroxide concentrations used and at the lowest temperature applied 80°C sodium zeolite A is the favoured product and after a period in excess of 10 h is the sole product. As the sodium hydroxide concentration or temperature is increased the initially formed sodium zeolite A is replaced by hydroxosodalite. At 140°C hydroxosodalite crystallises directly from the amorphous gel. By determining the changing areas of Bragg re¯ections with time, crystallisation curves can be obtained. The advantages of the in situ powder neutron diraction technique over analogous X-ray powder diraction methods are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Crystallisation; In situ neutron diraction; Sodalite; Zeolite A; Aluminosilicate
1. Introduction Microporous zeolites have now been widely studied and used in many industrially important areas for over 50 years. Despite the huge commercial value of both naturally occurring and synthetic zeolites, the formation mechanism of microporous materials under hydrothermal
* Corresponding author. Tel.: +44-1865-272600; fax: +441865-272690. E-mail address: [email protected] (D. O'Hare). 1 Present address: School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK.
conditions is poorly understood. A detailed understanding of hydrothermal crystallisation mechanisms would allow the outcome of new reactions to be predicted, in much the same way as the organic chemist is able to synthesise complex molecules by application of appropriate synthesis conditions. Such rational design in solid-state chemistry has not been achieved. In the case of zeolite synthesis, this predictive ability would have important implications, such as the ability to prepare new materials of desired pore size and shape for separation science, or with surface areas and chemical properties appropriate for a speci®c catalytic application [1]. The need to follow the crystallisation and behaviour of zeolites under
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 3 3 - X
80
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
hydrothermal conditions, in order to gather mechanistic information, requires the construction of specialised apparatus to mimic real reaction conditions, and allow non-invasive study of reactions as they take place. This has been the focus of much attention in recent years, and progress in the area has been reviewed [2,3]. We have recently described the design and construction of a hydrothermal cell for the study of reacting mixtures of materials under hydrothermal conditions using time-resolved in situ neutron powder diraction [4]. Neutron diraction lends itself well for the use of large-volume reaction vessels, since many materials that have properties appropriate for the construction of pressure cells to operate at elevated temperature have very low neutron absorption and scattering cross-sections. Therefore high-resolution diraction data may be collected from materials under non-ambient conditions with no contribution from cell scatter. Our reaction cell is constructed from nullscattering Ti±Zr alloy (67.7 at.% Ti, 32.3 at.% Zr), and is the ®rst such apparatus to exploit fully the advantages of neutron diraction for the study of hydrothermal crystallisations. Only one study of hydrothermal reactions using neutron diraction has been described previously, but this did not overcome the problem of signi®cant cell scatter appearing in the data [5]. We have previously used the Oxford/ISIS reaction cell to study the hydrothermal formation of the ferroelectric material barium titanate [6], and now turn our attention to the study of some aluminosilicate zeolites, with the aim of gaining new information about their formation and behaviour under hydrothermal conditions. 2. Experimental section 2.1. The neutron diraction experiment We have described previously the design of the Oxford/ISIS hydrothermal reaction cell in some detail [4], so here give a brief description of its construction and operation. Fig. 1 shows a schematic of the hydrothermal reaction vessel. The cell is constructed from a 20 mm diameter Ti±Zr alloy
(67.7 at.% Ti, 32.3 at.% Zr) with 0.3 mm thick walls. The walls are internally coated with a thin 100 lm layer of gold metal to prevent corrosion by the chemical reagents used. The cell is heated by copper heating blocks above and below a 4 cm high window which is exposed to the neutron beam. Pressure is monitored continually during heating by means of a pressure transducer. Neutron diraction experiments were performed using the POLARIS diractometer of ISIS, the UK spallation neutron source at the Rutherford Appleton Laboratory. POLARIS is a high-¯ux, medium-resolution time-of-¯ight neutron diractometer [7]. Scattered neutrons are measured on POLARIS by up to four banks of detectors from a sample container in an evacuated tank. In the current work we have used data from the backscattering detector bank, which are least aected by attenuation eects of the solvent. This bank consists of 58 3 He tube detectors covering an angular range of 130±160°2h, allowing Bragg re to be measured. ¯ections of d-spacing 0.2±3.2 A Diraction patterns were measured every 15 min during the hydrothermal experiments. 2.2. Material Aluminosilicate gels were prepared from amorphous alumina (Al2 O3 , synthesised by calcination of amorphous aluminium hydroxide hydrate (Aldrich) at 700°C for 2 h), fumed silica (SiO2 , particle size 0.007 lm, used as supplied from Aldrich), sodium deuteroxide solution (40% in D2 O as supplied by Aldrich) and D2 O (supplied by Fluorchem Ltd.). The deuterated reagents were necessary to reduce the large incoherent scattering of protons, and so give diraction data of lowered background. The Al2 O3 and SiO2 were shaken together to achieve intimate mixing, and stirred with the required amount of NaOD solution to produce a thick paste. Reaction mixtures containing 3 g of Al2 O3 were used and their total volume kept constant for all experiments. This was transferred to the hydrothermal reaction cell and heating commenced immediately. Desired temperature was reached typically within 5 min. Table 1 shows the compositions of the gels used, the
R.I. Walton et al. / Microporous and Mesoporous Materials 48 (2001) 79±88
81
analysis. In the time-of-¯ight experiment, the d is spacing of a characteristic Bragg re¯ection (in A) related to time-of-¯ight (t in ls) by the equation: d 1:977 10 3 =L sin ht, where L is the total ¯ight path of the neutron from source to sample to detector element (in m) and 2h the Bragg scattering angle. For the backscattering bank on POLARIS detectors have L ranging from 0.65±1.35 m and 2h from 130° to 160°. Normalisation and conversion of time-of-¯ight to d-spacing is performed within the program Genie [8], with a routine written especially for the POLARIS instrument. In addition, this allowed output of data into a format appropriate for the pro®le ®tting program Fullprof [9] and also allowed Gaussian-®tting of Bragg re¯ections to be performed. 3. Results Fig. 1. A schematic of the Oxford/ISIS hydrothermal reaction cell for in situ neutron diraction. Table 1 Gel compositions and reaction conditions used in the in situ neutron diraction experiments Experiment
Gel composition
Temperature/°C
Final pressure/bar
1
Al2 O3 :2SiO2 :4NaOD:20D2 O Al2 O3 :2SiO2 :3NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :2.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O Al2 O3 :2SiO2 :3.5NaOD:20D2 O
100
1.64
100
1.62
100
1.60
100
1.65
80
1.32
120
2.14
140
3.04
2 3 4 5 6 7
temperature applied, and the maximum pressure attained during the reaction. 2.3. Data analysis Neutron diraction data from the POLARIS back-scattering detector bank were used in all data
Fig. 2a shows diraction data measured during Experiment 1 using a gel of composition Al2 O3 : 2SiO2 :4NaOD:20D2 O. The initial aluminosilicate gel is amorphous, and after heating for 45 min the ®rst Bragg re¯ections appear. The ®nal product of this reaction is identi®ed as hydroxosodalite, Na8 [Al6 Si6 O24 ](OH)2 nH2 O, however the ®rst Bragg re¯ections to appear are not due to this phase. Performing the experiment using less NaOD (Experiment 2) allowed the second phase present in the system to be identi®ed as sodium zeolite A (Fig. 2b) where even after 10 h of heating it is the sole product. At even lower NaOD concentrations (Experiment 4), the time taken for crystallisation to begin was considerably longer, and again after 10 h of heating only zeolite A is present. At intermediate NaOD concentrations (Experiment 3) the zeolite A that ®rst forms is present for a considerably longer period (3 h) before total transformation into hydroxosodalite is observed (Fig. 2c). Experiments were also performed at a number of temperatures using an initial gel of composition Al2 O3 :2SiO2 :3.5NaOD:20D2 O (Experiments 5±7, Table 1). Diraction data are shown in Fig. 3. It may be observed that increasing temperature has the same eect as increasing NaOD concentration, with crystallisation beginning after a shorter period
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Fig. 2. Selected neutron diraction patterns measured during the heating of aluminosilicate gels of composition Al2 O3 :2SiO2 :xNaOD:D2 O at 100°C for (a) x 4, (b) x 3, and (c) x 3:5. The data presented represent a d-spacing range of 1±3 A. Two distinct crystalline aluminosilicates are identi®ed: hydroxosodalite (International Zeolite Association Code, SOD) and sodium zeolite A (LTA). See text for a discussion of conversion of time-of-¯ight into d-spacing.
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Fig. 3. Selected neutron diraction patterns measured during the heating of aluminosilicate gels of composition Al2 O3 :2SiO2 :3.5 NaOD:D2 O at (a) 80°C, (b) 120°C, (c) 140°C.
of heating and zeolite A present for shorter periods of time before sodalite is the favoured product. At the highest temperature studied 140°C no zeolite
A is observed to form and hydroxosodalite crystallises directly from the amorphous starting materials.
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analysis of the diraction data was performed. To improve the signal:noise ratio of the diraction data, four of the in situ data sets were summed and normalised to represent a total of one hour of data from hydroxosodalite. The structure of deuterohydroxosodalite, Na8 [Al6 Si6 O24 ](OD)2 2D2 O, has previously been determined using powder neutron diraction [11], and using the previously determined lattice constants we carried out Le Bail extraction of the integrated intensities (re®ned variables were lattice constants, peak shape and background terms). Fig. 6 shows the calculated diraction pattern obtained by whole pattern ®tting, along with the experimental data points. The signal:noise ratio of the data is not high, but it must be emphasised that the data were obtained from a sample of the aluminosilicate in sodium hydroxide solution at reaction temperature. A re was obtained ®ned cell parameter of 8.9311(3) A from the Le Bail extraction procedure (space group P-43n). 4. Discussion
Fig. 4. Typical Gaussian ®ts to Bragg re¯ections measured in situ, (a) the coincident (3 3 0) and (4 1 1) Bragg re¯ections of hydroxosodalite and (b) the coincident (6 4 4) and (8 2 0) peaks of zeolite A.
The determination of the changing areas of Bragg re¯ections allows crystallisation curves to be obtained, since the intensity of a diraction peak is directly proportional to the amount of diracting material [10]. Fig. 4 shows typical Gaussian ®ts to selected Bragg re¯ections of the in situ diraction data and Fig. 5 crystallisation curves produced from the data shown in Fig. 2. For each zeolite studied all characteristic Bragg re¯ections were observed to appear at the same time, and their increase in intensity with time was identical. This immediately suggests that crystal growth is isotropic. Having identi®ed the crystalline aluminosilicates present during each reaction by the positions of their characteristic Bragg re¯ections further
The construction of a null-scattering hydrothermal reaction cell for neutron diraction has successfully allowed the crystallisation of zeolites to observed in situ in a non-invasive manner under real reaction conditions. The zeolites we have studied here are amongst the most widely studied of all zeolites. Sodium zeolite A was the ®rst synthetic zeolite to be prepared [12] and now ®nds widespread application as a laboratory drying agent, and as a detergent ``builder'' improving the action of household washing powders by exchanging magnesium and calcium ions by sodium [13]. Hydroxosodalite is one of a large number of materials, both naturally occurring and synthetic, that adopt the sodalite structure. The structure of a berylsilicate sodalite was ®rst elucidated in the 1930s by Pauling [14], and aluminosilicate sodalites have since been widely studied as a model zeolite structure, although of little practical application because of its small pore openings. The structures of the zeolite A and sodalite are closely related, both being constructed from what may be thought of as a fundamental zeolite structural
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Fig. 5. Crystallisation curves determined by integration of Bragg re¯ections, (a) the growth of sodalite in Experiment 7, (b) the growth of zeolite A in Experiment 5, and (c) the changing amounts of zeolite A and hydroxosodalite in Experiment 3.
building unit, the b-sodalite cage [15]. The conversion of zeolite A to sodalite is a good example of
Ostwald's rule of successive crystallisations whereby a metastable phase ®rst crystallise followed by
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Fig. 6. The observed (points) and simulated (line) neutron diraction pattern of hydroxosodalite measured in situ at 140°C.
the successive crystallisation of thermodynamically more stable phases [16]. Our in situ experiments highlight that reaction conditions must be carefully chosen in order to favour the product of zeolite A over sodalite. Small increases in NaOD concentration or in temperature result in the collapse of zeolite A into sodalite. We observe the formation of no other crystalline phases during the course of the transformation. Previous studies of the process have relied largely on quenching experiments, whereby reactions are removed from the oven after selected periods of heating, recovered by ®ltration and examined [17,18]. Such experiments showed that no other zeolites crystallise from the same reaction mixtures, and measurements of particle size distributions indicated that the transformation occurs by dissolution of the zeolite A and recrystallisation from solution, rather than by a direct solid-state transformation [17,18]. Our new results provide evidence that under reaction conditions no other phases crystallise during the transformation of zeolite A into hydroxosodalite. The ability to extract structural information from data measured in situ will be important in
understanding the properties of zeolites and their stability under non-ambient conditions. The re we have ob®ned cell parameter of 8.9311(3) A tained for hydroxosodalite is slightly higher than previously obtained by powder the value of 8.86 A neutron diraction [11]. This earlier value, however, had been obtained at 90°C, and we must expect some eect of thermal expansion for the sample we studied at 140°C. In addition, the previous structural study was performed on a sample of composition Na8 [Al6 Si6 O24 ](OD)2 2D2 O, but other authors have reported that extra water molecules can be incorporated into the structure to give materials of composition Na8 [Al6 Si6 O24 ](OD)2 nD2 O, with n up to 4, and a corresponding increase in room temperature cell parameter to up [19]. Our results are consistent with the to 8.92 A presence of an expanded sodalite framework incorporating extra water under reaction conditions. The use of diraction techniques for following hydrothermal crystallisations has only come about recently and work has so far largely concentrated on the use of X-ray diraction. Zeolite formation has naturally attracted some attention. Munn et al. ®rst described the use of energy-dispersive X-ray
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diraction (EDXRD) for the in situ study of a zeolite crystallisation [20], and the method has been developed by O'Hare and co-workers in the last ®ve years to provide an apparatus which is now used by several groups [21,22]. Although the EDXRD method allows rapid collection of data (of the order of seconds), which provides excellent kinetic information, the intrinsic low resolution of the data means that little structural information is revealed. Norby and co-workers used angular dispersive X-ray diraction to follow hydrothermal reactions [23], and obtained data of higher resolution from zeolite crystallisations [24,25]. The technique, however, requires the use of very small reaction cells (glass capillaries) and which does not necessarily re¯ect real synthetic reaction conditions. In addition, in some cases problems in sampling a representative mixture of starting materials could possibly arise when very small volumes of solid and liquids must be taken, causing problems with data interpretation. Our ®rst experiments with time-of-¯ight neutron diraction have shown that it is possible to collect diraction data of high resolution, in a laboratory-sized reaction vessel with no background from the reaction cell. In order to improve the quality of data in the near future we will perform experiments on the new GEM diractometer at ISIS [26], which oers signi®cantly higher neutron ¯ux and a greater detector area compared to POLARIS. We are also planning to carry out experiments on the ®xed wavelength D20 diractometer at the Institut Laue Langevin, Grenoble, France [27], which oers similarly high ¯ux and a large-angle novel position-sensitive detector. 5. Conclusions We have demonstrated by the study of a simple aluminosilicate synthesis that neutron powder diraction can be a powerful technique in studying the crystallisation of zeolites under genuine laboratory reaction conditions in real time. Information is obtained about the course and time-scale of reaction in a direct and ecient manner. Crystallisation curves can easily be obtained and there exists the possibility of extracting detailed struc-
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tural information form materials under genuine hydrothermal conditions. With the ongoing developments in neutron diractometers, with envisage being able to collect in situ diraction data of higher quality and in shorter periods of time. Acknowledgements We are grateful to the EPSRC for ®nancial support of this work and provision of beam-time at ISIS. We thank Dr. F. Millange for assistance with some of the data analysis. References [1] M.E. Davis, in: L. Bonneviot, S. Kaliaguine (Eds.), Stud. Surf. Sci. Catal.: Zeolites: A Re®ned Tool for Designing Catalytic Sites, vol. 97, 1995, p. 35. [2] A.K. Cheetham, C.F. Mellot, Chem. Mater. 9 (1997) 2269. [3] R.J. Francis, D. O'Hare, J. Chem. Soc. Dalton Trans. (1998) 3133. [4] R.I. Walton, R.J. Francis, P.S. Halasyamani, D. O'Hare, R.I. Smith, R. Done, R. Humpreys, Rev. Sci. Instrum. 70 (1999) 3391. [5] E. Polak, J. Munn, P. Barnes, S.E. Tarling, C. Ritter, J. Appl. Crystallogr. 23 (1990) 258. [6] R.I. Walton, R.I. Smith, F. Millange, I.J. Clark, D.C. Sinclair, D. O'Hare, Chem. Commun. (2000) 1267. [7] S. Hull, R.I. Smith, W.I.F. David, A.C. Hannon, J. Mayer, R. Cywinski, Physica B 180, 181 (1992) 1000. [8] W.I.F. David, M.W. Johnson, K.J. Knowles, C.M.M. Smith, G.D. Crosbie, E.P. Campbell, S.P. Graham, J.S. Lyall, Rutherford Appleton Laboratory Report RAL-86102, 1986. [9] J. Rodriguez-Carvajal, Satellite Meeting on Powder Diffraction, Abstracts of the XVth Conference of the International Union of Crystallography, Toulouse, 1990, p. 127. [10] H.P. Klug, L.E. Alexander, Diraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1974. [11] M. Wiebcke, G. Engelhardt, J. Felsche, P.B. Kempa, P. Sieger, J. Schefer, P. Fischer, J. Phys. Chem. 96 (1992) 392. [12] D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, T.L. Thomas, J. Am. Chem. Soc. 78 (1956) 5962. [13] A. Dyer, An Introduction to Zeolite Molecular Seives, Wiley, New York, 1988. [14] L. Pauling, Z. Kristallogr. 74 (1930) 213. [15] W.M. Meier, D.H. Olsen, C. Baerlocher, Atlas of Zeolite Structure Types, fourth ed., Elsevier, Amsterdam, 1996. [16] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, New York, 1982.
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[23] P. Norby, A.N. Christensen, J.C. Hanson, in: J. Weitkamp, H. Pfeifer, H.G. Kange, W. Hoelderich (Eds.), Stud. Surf. Sci. Catal.: Zeolites and Related Microporous Materials: State of the Art 1994, vol. 94, 1994, p. 179. [24] A. Gualtieri, P. Norby, G. Artioli, J. Hansen, Phys. Chem. Miner. 24 (1997) 191. [25] A. Gualtieri, P. Norby, G. Artioli, J. Hanson, Micropor. Mater. 9 (1997) 189. [26] W.G. Williams, R.M. Ibberson, P. Day, J.E. Enderby, Physica B 241 (1997) 234. [27] P. Convert, T. Hansen, A. Oed, J. Torregrossa, Physica B 241±243 (1998) 195.