The synthesis of zeolites under micro-gravity conditions: a review

Microporous and Mesoporous Materials 23 (1998) 119–136

The synthesis of zeolites under micro-gravity conditions: a review Eric N. Coker a,*1, Jacobus C. Jansen a, Johan A. Martens b, Pierre A. Jacobs b, Francesco DiRenzo c, Franc¸ois Fajula c, Albert Sacco, Jr.d a Faculty of Chemical Technology and Materials Science, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands b Department Interfasechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, (Leuven), Belgium c Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 34296 Montpellier, France d Department of Chemical Engineering, Northeastern University, 342 Snell Engineering Center, Boston, MA 02115, USA Received 26 August 1997; accepted 13 January 1998

Abstract The crystallisation of zeolites and other microporous materials in space has attracted some attention in recent years, in the search for larger or more ‘‘pure’’ and perfect crystals. Hydrodynamics and hydrostatics of zeolite synthesis solutions are strongly influenced by gravitational force, and the growth of zeolites in solutions which are effectively free from convection and sedimentation in micro-gravity may produce crystals of different character to those grown on Earth. The exact nature of the changes which occur on synthesising zeolites in micro-gravity depends upon the chemical and physical properties of the solutions as well as the ‘‘quality’’ of micro-gravity. The results from several experiments performed in space are summarised and discussed, and a few suggestions are made concerning possible improvements in experimental design and strategy. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Micro-gravity; Zeolite synthesis; Crystal growth

1. Introduction Inhomogeneities in the distribution of temperature or chemical composition within a fluid lead, under the influence of gravity, to convective currents. Under conditions of micro-gravity, such motions are minimal, and the fluid experiences little disturbance, thus offering a unique opportu* Corresponding author. 1Present address: BP Chemicals Ltd., Poplar House, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LL, UK. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 04 6 - 8

nity to grow (zeolite) crystals from a quiescent solution. The studies described in this review stem from the work of a number of researchers around the world and include details of several series of experiments, using various sets of hardware, conducted in micro-gravity. Owing to the relatively long periods of time required to synthesise zeolites with conventional heating, zeolite experiments are not performed on sounding rockets, but on orbital facilities, such as the US Space Shuttles, Soviet rockets and even satellites. In the earliest reported

June 1991, space shuttle STS-40, SLS-1 Sept. 1991, EURECA-1 satellite June 1992, space shuttle STS-50, USML-1

WPI, USA

Oct. 1995, space shuttle STS-73, USML-2

T.U. Delft, Netherlands

FAU MOR LTA FAU Beta MFI MFI

OFF, SOD LTA

ANA MOR GIS LTA

SAPO-5 MFI MFI Beta

FAU FAU Not known MFI, beta MFI

Zeolite type

100SiO :5Al O :17.5Na O:17.5K O:15TMAOH:1969H O, 348 K, 6 months 2 2 3 2 2 2 100SiO :119Al O :231Na O:aTEA:23 095H O, 369 K, 8 days 2 2 3 2 2 a=0, 252, 655, or 755 100SiO :28.6Al O :136Na O:57TEA:12 971H O, 378 K, 8 days 2 2 3 2 2 100SiO :5.88Al O :26.5Na O:23.53TEABr:2000H O, 448 K, 8 days 2 2 3 2 2 Si:Al=0.42, 0.44, or 1.53, TEA or BIS added, 373 K, 13 days Si:Al=1.75, TEA added, 373 K, 13 days Si:Al=10, TEA+ template, 393 K, 13 days Si:Al=2, TPABr template, 393 K, 13 days. 100SiO :aAl O :150Na O:400TPABr:18 000 H O, 448 K, 13 days 2 2 3 2 2 1: a=0.9 2: a=0.45

100SiO :71.3Al O :171Na O:441TEA:23 175 H O, 341 K, 12 days 2 2 3 2 2 100SiO :71.3Al O :171Na O:441TEA:23 175 H O, 340–341 K, 12 days 2 2 3 2 2 Not known Not known 100SiO :aTiO :bNH F:cTPABr:dH O:e (seeds), 443 K, 71 hours 2 2 4 2 1: a=0, b=10, c=100, d=3000, e=0 2: a=0, b=100, c=10, d=3000, e=0 3: a=10, b=150, c=100, d=3160, e=0.1% 4: a=10, b=10, c=100, d=3370, e=0.1% 5: a=0, b=600, c=50, d=10000, e=0 6: a=25, b=750, c=50, d=10000, e=1% Not known Si:Al=125, 423 K, 48 hours 100SiO :Al O :5Na O:10TPABr:8000 H O, 443 K, 48 hours 2 2 3 2 2 100SiO :aAl O :bB O :cNa O:dTEA+:e(OH )−: fH O, 423 K, 60 hours 2 2 3 2 3 2 2 1: a=1, b=0, c=5, d=35, e=44, f=2000 2: a=0.25, b=2.25, c=7.5, d=36, e=45, f=1300 3: a=4.3, b=0, c=5.5, d=109, e=22, f=2000 100SiO :1.15Al O :206Na O:3100H O, 423 K, 60 hours 2 2 3 2 2 100SiO :0.45Al O :33Na O:3000H O, 423 K, 60 hours 2 2 3 2 2 100SiO :5.5Al O :39Na O:3000H O, 423 K, 60 hours 2 2 3 2 2 100SiO :119Al O :231Na O:655TEA:23 095H O, 369 K, 72 hours 2 2 3 2 2

Compositiona, synthesis temperature, time

25 days

a few days

4 months a few days

110 days

Not known Not known 10 days 9 days

Not known Not known Not known 9 days Not known

Time lapseb

solutions solutions solutions solutions

Nutrients mixed in micro-gravity

Nutrients mixed in micro-gravity

Diffusion of unmixed nutrient pools Nutrients mixed in micro-gravity

Pre-mixed solutions

Pre-mixed Pre-mixed Pre-mixed Pre-mixed

Nutrients mixed in micro-gravity Nutrients mixed in micro-gravity Inconclusive Pre-mixed solutions Pre-mixed solutions

Comments

[3]

[36,38]

[39,40] [33]

[33,34]

[28] [28] [7,28] [28,31,32]

[2] [2] [27] [28,29] [28]

Ref.

aTEA=triethanolamine, TEA+=tetraethylammonium cation, TPABr=tetrapropylammonium bromide, TMAOH=tetramethylammonium hydroxide, BIS= 2,2-bis(hydroxymethyl )-2,2∞,2∞-nitrilotriethanol. bTime lapse=time period between preparation of experiment, i.e. loading the reactors with solution(s), and start of the experiment in micro-gravity.

Oct. 1995, space shuttle STS-73, USML-2

WPI, USA

CIR, Oslo, Norway WPI, USA

Sept. Sept. Sept. Sept.

CSIC, Spain Degussa, Germany NCL, Japan ENSCM, Montpellier, France

CASIMIR-1 CASIMIR-1 CASIMIR-1 CASIMIR-1

1988, Soviet satellite Photon 1989, Soviet satellite Photon April 1990, space shuttle Sept. 1990, CASIMIR-1 Sept. 1990, CASIMIR-1

SPA KOMPOZIT, USSR SPA KOMPOZIT, USSR N.I.S.T., USA Eniricerche, Italy ENSCMu, Mulhouse, France

1990, 1990, 1990, 1990,

Flight date, mission

Affiliation

Table 1 Details of experiments to grow zeolites in micro-gravity

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experiments, simple autoclaves were employed, which were loaded with pre-mixed synthesis solutions. Relatively short heating periods were used, generally of two to five days. In the US ‘‘GASCAN-1’’ experiment [1], and some early Soviet experiments [2], autoclaves were used which were designed to hold a closed liquid, i.e. without any solution–gas interface, with the intention of reducing surface-tension-driven convection. An interesting development, on the USML-1 space shuttle mission in 1992, was the use of twocompartment autoclaves which allowed the nutrient solutions to be mixed together using four different mixing techniques when in orbit, immediately prior to heat-up. A variation on this theme was used for a 180 day experiment to grow zeolites at relatively low temperature aboard a retrievable satellite. The autoclave possessed two compartments separated by valves which, when automatically opened, allowed the nutrient pools to slowly diffuse together without forced mixing. Refined autoclave designs for the micro-gravity crystallisation of zeolites with, i.e. the capability of performing in situ processing, containing a closed liquid phase without gas, and allowing in situ spectroscopic observation are still not available. A recent advance, however, was the use of transparent autoclaves during the USML-2 space shuttle mission in 1995. These autoclaves were attached to the outer surface of the zeolite synthesis furnace and reached a temperature of 40°C, allowing the growth of zeolite A to be followed visually. Zeolites A, X, offretite, analcime, gismondine, mordenite, beta and MFI, as well as sodalite and SAPO-5 have been prepared in space, at temperatures between 60 and 180°C. While the aims of some experiments were the preparation of large zeolite crystals (e.g. for single-crystal X-ray structure determination and diffusion measurements), others studied morphological and compositional changes in catalytic materials. In some cases, relatively large size increases have been reported on performing the synthesis in micro-gravity, while in others, no increase in size, or even a decrease in size was found. However, only in a few studies are crystal size distributions presented. In some recent experiments, significant differences in external surface roughness has been found between crystals grown on Earth and those grown in micro-gravity

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[3]. The interpretation of the results of microgravity experiments is often hampered by the small amount of data available. Clearly, the behaviour of zeolite synthesis solutions in micro-gravity depends greatly upon the chemical and physical parameters of the mixture, as well as on the ‘‘quality’’ of the micro-gravity, i.e. the magnitude of the gravitational force (10−6–10−5g) and of random or systematic accelerations (10−4–10−2g). Thus the difference in the total set of parameters used on Earth and in space must be substantially smaller than the difference in gravity forces. Additional points worth attention for the successful operation of an experiment in micro-gravity are (i) stability of the precursor solutions over long periods of time, in case of launch delays, (ii) differences in the geometry of the autoclaves to be used in space, compared with those normally used, and the materials of their construction (to take advantage of surface tension effects), (iii) how the hardware, power consumption and dissipation and safety constrain the experiment. The aims of this review are to determine whether general rules can be extracted from the experiments carried out so far in the field of micro-gravity zeolite synthesis, to evaluate the experiments which have been conducted to date and to provide the impetus for improving efforts, hardware and experiments.

2. Experimental section Table 1 is a comprehensive list of the important synthesis parameters pertaining to the micro-gravity zeolite crystal growth experiments which are discussed in this review. The entries in the table are arranged chronologically, according to the start of the micro-gravity flight.

3. Theory: reasons for going into space Zeolite synthesis mixtures may be clear, misty, turbid, opal-like or even thick gel-like aqueous systems. In the course of the crystallisation, the starting reagents, in particular the silica species, reorganise via hydro-gel formation and/or dissolu-

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tion to form, ultimately, crystalline phases in a clear liquid. Under the influence of different gravitational forces, changes in convection (fluid motion) as well as sedimentation phenomena (motion of solid particles) might influence the zeolite crystallisation in various aspects. Fig. 1 is a simple schematic diagram illustrating some of the major factors in a zeolite synthesis mixture which may be influenced by changes in the gravitational force. Density-driven convective currents caused by thermal or compositional disequilibrium (in particular in non-clear synthesis mixtures) are strongly reduced under micro-gravity compared with normal gravity. The almost ideal static conditions in space compared with Earth should result in nucleation as well as crystal growth conditions which are (i) based on different equilibria of nutrients, see Fig. 1(b) [4], (ii) uniform over the synthesis volume and (iii) based on slow radial diffusion from a static nutrient pool to the growing crystal. For example, the relatively large gel spheres (up to 2 mm in diameter) [5] which can be formed easily on Earth by agglomeration of sol particles enhanced by collisions due to natural convection and segregation are hindered under micro-gravity conditions. 3.1. Fluid motion The hydrodynamic and hydrostatic behaviour of fluids differs greatly between micro-gravity and unit-gravity. Hydrodynamics refers to the motion of fluids while hydrostatics deals more with their steady-state behaviour. These subjects will be introduced briefly here; the reader is referred to Ref. [6 ] (and citations therein) for a more detailed discussion of these phenomena. The gravitational force is responsible for the convective flow of fluids, e.g. density- or buoyancy-driven convection. Fluids which are more dense than their surroundings, owing to a thermal dis-equilibrium (especially important during heat-up and cool-down stages) or inhomogeneous distribution of reagents throughout the vessel, are pulled towards the bottom of the vessel under the influence of gravity. The delivery of nutrients to growing crystals and nuclei (mass transport) may be considerably less efficient in micro-gravity than on Earth, owing

to the greatly reduced convective flows. Mass transport in micro-gravity relies principally upon diffusion of the reagents and Brownian motion of the particles. Relatively large concentration gradients may arise, and persist, in micro-gravity, which on Earth would cause convection to occur, removing or reducing the concentration gradient. Once these convective flows are reduced by going to micro-gravity, surface phenomena, such as Marangoni convection become increasingly important. Marangoni convection is driven by differences in surface tension along a free fluidfluid interface. In a closed vessel containing only a one-phase liquid or solution, and no gas, Marangoni convection should be absent. The total elimination of gas is a challenge to reactor designers, and also involves careful de-gassing of solutions prior to sealing the vessel. While Marangoni convection could be seen as inducing collisionbred nucleation (similar to density-driven convection), there is no direct evidence for any significant effect upon zeolite nucleation. However, in a few cases, micro-gravity zeolite crystallisations were reported to have been effected by Marangoni convection, i.e. by causing crystals to link together to form chains [7]. The hydrostatic pressure within a fluid is related to the density of the fluid and the magnitude of the gravitational force. Hence, the hydrostatic pressure within a fluid in micro-gravity can be much smaller than in the same solution on Earth. In the case of a typical hydrothermal zeolite synthesis, however, changes in the hydrostatic pressure due to gravitational differences may be small compared with the autogeneous pressure of the system. 3.2. Solid particle motion Sedimentation occurs on Earth because of differences in density between the amorphous solid hydro-gel (~1.1–1.2 g cm−3) or crystals (~1.7– 2.0 g cm−3) and the aqueous synthesis mixtures (~1.0 g cm−3). On Earth, aluminosilicate hydro-gel phases, which may form upon interaction of the aluminate and silicate solutions, behave differently than in micro-gravity. Under the influence of gravity, the hydro-gel phase is densified towards the bottom of the vessel, as the solid phase is more dense than

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(a)

(b)

(c) Fig. 1. Simplified schematic diagram showing some of the principal differences between micro-gravity and normal gravity, as related to a zeolite synthesis experiment: (a) qualitative representation of different concentration gradients, leading to differences in mass transfer; (b) processes involved in the evolution of zeolite crystals (adapted from Ref. [4]); (c) sedimentation and secondary nucleation.

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the surrounding solution. The inhomogeneously dispersed nutrient system thus formed can lead to the formation of additional, unwanted phases [8,9]. Under conditions of micro-gravity, however, the amorphous solid phase extends throughout the entire solution volume. The higher dispersity increases the solubility of the amorphous solid phase and reduces further the diffusion of nutrients through the liquid volume. It is worth mentioning that Robert et al. [10] consider crystal growth from a gel medium (albeit not an aluminosilicate gel ) under normal gravity as a model system for micro-gravity. For instance, the gel medium supported growing protein crystals (preventing sedimentation) and suppressed convection. In their experiments, however, the gel was inert, and was not consumed by the crystals, but simply pushed aside as they grew. Under normal gravity, crystals start to settle out from solution when they exceed the colloidal size of approximately 100 nm, while under micro-gravity, sedimentation is greatly reduced. The sedimentation of particles generates a dense layer of crystals at the bottom of the vessel. Diffusion of nutrients to the crystals is slightly hindered (owing to both the greater distance between the crystals and the bulk solution, and the shielding of crystals by those above them in the sedimented layer); however, convective flows on Earth make this effect quite small. Agglomeration and intergrowth of the crystals often occurs once they have settled under gravity, owing to the continuation of growth. The absence of sedimentation has been reported for a number of micro-gravity experiments, where the corresponding terrestrial samples were largely agglomerated. The settling of small crystals has been proposed to lead to the formation of a subsequent population of nuclei in the upper portion of the synthesis mixture on Earth, which also settle to the bottom of the reactor once they have reached a sufficient size [11]. This process is said to continue until the concentration of nutrients is reduced below some threshold. Since, in micro-gravity, sedimentation is minimised, the number of nuclei formed is hypothesised to be lower than in the corresponding terrestrial system. Inter-particle collisions, due to movement of crystals with the fluid or sedimentation, have been

hypothesised to result in the generation of secondary nuclei [12,13]. The secondary nuclei would compete for the available nutrients, yielding a product possessing a smaller average crystal size. If one considers a solid particle suspended in a viscous fluid with no free surface (i.e. no Marangoni convection) in micro-gravity, Brownian motion may be observed, the distance of movement being inversely proportional to the radius of the particle. In addition, random microaccelerations of the vessel containing the particle can induce movement of the particle. With the vessel attached to the walls of the spacecraft, any shocks due to adjustment of the spacecraft’s attitude, the operation of equipment or the movement of astronauts are translated to the vessel. Such movements of the particle, termed ‘‘inertial random walk’’ [14] or ‘‘g-jitter’’ are strongly dependent upon the size of the particle. Although the magnitude of these accelerations is several orders lower than that of Earth-gravity, it is generally several orders of magnitude stronger than micro-gravity. Regel et al. [15] have demonstrated that the inertial random walk coefficient varies with particle radius to the fourth power (i.e. the dependence on particle size is five orders of magnitude greater than with Brownian motion). For particles larger than 100 mm, as can be the case with the synthesis of large zeolite crystals, this effect may be very significant. When one considers a vessel containing more than one particle, the frequency of inter-particle collisions is strongly dependent upon the magnitude of the inertial random walk coefficient. Of course, such microaccelerations also occur in laboratories on Earth, but their effect is minimal compared with that of Earth-gravity. Under the above mentioned micro-gravity conditions, thus under relatively slow mass- and heattransfer processing, the size and perfectness of the crystals, the smoothness of their external surfaces and their chemical compositions may be expected to differ compared with terrestrially formed crystals. Evidence of these factors will be given in Section 4. While much is known regarding the behaviour of simple fluids or fluid mixtures in micro-gravity, zeolite synthesis solutions are generally far more

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complex, and their behaviour is not so well understood. Further research into the hydrodynamics of aluminosilicate solutions under hydrothermal conditions in space should allow more efficient utilisation of the micro-gravity environment. At present, experiments to grow zeolites in space are based more on intuition and results of prior experimentation and to a lesser extent on an understanding of how micro-gravity actually affects the synthesis.

3.3. Terrestrial experiments In 1987, Sand et al. [12] reported the potential benefits of crystallising zeolites under micro-gravity conditions. Terrestrial experiments involving zeolite A synthesis mixtures which formed hydrogels, and to which triethanolamine had been added produced larger crystals than the corresponding mixtures without triethanolamine. The effect of triethanolamine was hypothesised to be at least two fold: (i) increasing solution viscosity and (ii) complexation of Al3+ in solution. Reducing the concentration of Al-oxyhydroxides (through complexation of Al3+) reduces the effective degree of supersaturation of the solution and hence the number of nuclei which form. In addition, the silica species in solution must compete with the complexing agent (triethanolamine) for free aluminium in solution. This further slows the formation of nuclei and most probably their growth. Increasing the viscosity of the solution reduces the convectional and diffusional mixing, and hinders the settling of crystals. This may, in itself, reduce nucleation as the suspension of the growing crystals and nuclei in solution assures continual consumption of nutrients, with accompanying reduction in the degree of supersaturation. However, later work showed that the hypothesised increase in viscosity was not significant [16 ]. Sacco et al. [12,17] quote the primary reasons for crystallising zeolites in space to be their suspension in the nutrient pool, reduction of fluid motion and the associated effect on heat and mass transfer, and the reduction of collision-breeding. Collision breeding occurs when crystals collide, driven by fluid motion or sedimentation, releasing microfragments of crystalline material which act as

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further nuclei. Three key issues in the growth of large zeolite crystals in space are identified: (1) nucleation should be controlled (i.e. kept at a low level ); (2) fluid motion should be minimal; (3) mass transfer of nutrients to the growing crystals should be maximal. Construction of the synthesis vessel from a nonwetting material (typically Teflon) may reduce the number of nuclei which are formed through interaction with the walls, although even such materials are able to attach relatively strongly to zeolites (e.g. preparation of zeolite membranes or films on Teflon supports [18]); such interactions are, however, physical and not chemical. The homogeneity of the synthesis mixture may be a key factor in producing large crystals, of a high phase purity. Depending upon the type of mixtures to be studied, some may require mixing immediately prior to hydrothermal treatment to avoid excessive spontaneous nucleation. Other mixtures may remain stable for extended periods of time after mixing. When the nutrients are to be mixed together immediately prior to hydrothermal treatment, i.e. in space, a careful study of the mixing characteristics of the reactor must be carried out to ensure adequate mixing can be achieved in micro-gravity [8,19]. Szostak et al. [20] suggest the synthesis of zeolites from clear (homogeneous) solutions as valuable micro-gravity experiments. The lack of amorphous hydro-gel phases means that intermediate crystalline or amorphous materials can more easily be identified. The homogeneous and dilute nature of such solutions would mean that segregation under the high-gravity forces during launch would not occur. Ostrach et al. [11,21–23] claimed that the mean size of zeolite crystals could be increased by the addition of extra nutrients during the synthesis. Terrestrial experiments on hydro-gel-forming zeolite A mixtures showed a 25–30% increase in size when the nutrient content was doubled at the end of the nucleation period. Subsequent investigations, however, have shown that the increase in size observed is predominantly due to agglomeration of crystals, rather than growth of larger individual crystals [24]. It was concluded that the

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size of individual zeolite A crystals could be increased by addition of nutrients, provided that the nucleation controlling triethanolamine was not present in the parent solution, but was present in the added solution(s). The final crystals, however, were not as large as those prepared when triethanolamine was added at the outset of the experiment, and to which no additions of nutrient were made [24]. Cundy et al. [25,26 ], while studying the synthesis of ZSM-5 in a semi-continuous reactor, were able to increase very effectively the size of seed crystals under appropriate conditions, e.g. from 0.7 mm up to 3 mm (a volume expansion by a factor of about 80) in 32 days at 90°C.

4. Results: experiments in micro-gravity The first reported attempts to grow zeolites in micro-gravity occurred in the late 1980s aboard Soviet retrievable capsules. Two experiments were performed, with the aim of growing large, perfect crystals of zeolite NaX [2]. A 300 cm3 capacity reactor constructed of stainless steel, Teflon and polyethylene was used which allowed mixing of the reagents in space and exclusion of a gas phase. One of the nutrient solutions was enclosed in a plastic bag which was punctured in micro-gravity, while mixing was performed by a motor-driven stirring device. To allow for the expansion of the liquid phase, in the absence of gas, a heat expansion compensator (bellows) was introduced to the reactor. Details of the important experimental parameters are given in Table 1. The first of the two experiments was flown in 1988 and the principal observations were that the micro-gravity synthesis had not reached completion in the 12 day period, while that performed on Earth had done. The Si/Al ratio of the crystalline portion of the product was higher in the micro-gravity sample (1.64) than in the terrestrial sample (1.27). This observation may be a consequence of the highly aluminous solution composition employed (Si/Al<1); silicate, being the limiting reagent, becomes depleted earlier in the synthesis than does aluminate. Crystals which had not finished growing could thus be expected to possess higher Si/Al

ratios than those which had reached completion. The second experiment, launched in 1989, used a slightly improved reactor design and the same solution composition. Operation at a higher temperature of 72–73°C instead of 68°C was planned, but, owing to a power failure, only the latter temperature was achieved. Again, the experiment in micro-gravity had not reached completion and the Si/Al ratio was higher for the space-grown sample (2.32) than its Earth-grown counterpart (2.22). In both experiments, FAU crystals of approximately 100 mm were obtained from the micro-gravity experiments, while those grown on Earth were slightly larger. In a very brief Science News report [27], mention was made of an experiment by workers at the National Institute of Standards and Technology in the US to grow zeolites on a space shuttle in 1990. No details are given of the type of zeolite grown, solution composition, size of crystals produced, etc. The most interesting result was the difference in morphology between Earth-grown and micro-gravity-grown samples; while the former formed as regular cubes, the latter were observed to be rod-like. No conclusions can be drawn from this, since no experimental details are known. 4.1. CASIMIR-1 mission A number of research groups from Europe and Japan participated in the CASIMIR-1 (CAtalyst Studies for Industry through MIcrogravity Research) mission in 1990 [28]. The mission employed a Soviet Resource-F recoverable rocket and included a facility dedicated to the growth of zeolites and related materials. The zeolite crystal growth facility consisted of four heated blocks, each containing seven autoclaves. Two blocks were heated to 423 K for 48 and 68 h, while the other two were heated to 443 K for 48 and 60 h. Of the autoclaves heated to 423 K, it was noticed that the heat-up profile in micro-gravity (ambient to 423 K in 2.5 h) was virtually identical to that on Earth, suggesting that heat transfer may be insignificantly affected by gravity in this type of reactor. Teflonlined stainless steel autoclaves were employed which possessed a Teflon piston device, designed to separate the solution from any gas phase while

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allowing for solution expansion, and had internal volumes of 20–30 ml. The separation of any gas phase should prevent surface-tension-driven (Marangoni) convection. Separation of gas and liquid phases is relatively easy under Earth-gravity, where gas bubbles rise to the top of the vessel. Under conditions of micro-gravity, however, the density difference between gas and liquid phases provides virtually no driving force for separation. If gas bubbles form within a solution in microgravity, they will tend to stay immersed in the solution, and will not spontaneously segregate into a separate phase. Such gas bubbles may arise from the release of gases dissolved in the solution before sealing the vessels, or through chemical reaction (i.e. Hofmann degradation of TPA+ during the synthesis of MFI ). A summary of the results from each of the participating groups is given below. (1) Researchers from the National Chemical Laboratory for Industry in Japan prepared an experiment to grow ZSM-5 with a Si/Al ratio of around 50 [7]. Their solution composition (see Table 1) formed a clear solution which was deemed stable for four weeks after preparation. The crystallisations in space and on Earth (control experiment) were conducted at 443 K for 48 h in identical autoclaves. The sample grown in micro-gravity was powdery ZSM-5 with predominantly intergrown cubic crystals of ca. 10 mm while that grown on Earth was a film of crystals lining the bottom of the vessel. The near absence of sedimentation in space allowed the crystals to grow more as individual entities in their own nutrient pools. The yield of MFI from the micro-gravity experiment was about 12% higher than that on Earth and the concentration of nutrients remaining in solution after the synthesis in micro-gravity was correspondingly lower. Interestingly, the zeolite crystals grown in space were often linked to one another, forming chains. Surface analysis (energy-dispersive X-ray analysis) of successive crystals along one such chain showed a random variation of Si/Al ratio in the range 40–80. Furthermore, each of a number of crystals studied showed a steadily increasing Si/Al ratio from the outer surface to the centre of the crystal. The uniformity of size of the crystals

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within a chain suggests a similar growth period; however, the variation in surface Si/Al ratio suggests that the growth periods of the various crystals were not concurrent and/or the growing medium did not possess a homogeneous Si/Al distribution. Sano et al. suggest that the crystals stop growing once they reach a certain size (in this case, 10 mm). If one assumes that the solution in micro-gravity is completely free from convection (density-driven and surface-tension-driven), that Brownian motion and sedimentation of the crystals is minimal and that diffusion of nutrients is relatively slow, a cessation of growth can be envisaged once the pool of nutrients surrounding a crystal has been sufficiently exhausted. Thus, although the various crystals may begin to grow at different times, they will automatically stop growing once the critical size is reached; the composition of the solution phase at this time is reflected in the Si/Al ratio of the crystal surface. The linking of crystals is ascribed to the presence of surface-tension-driven (Marangoni) convection due to the presence of gas bubbles in the solution, which causes collisions between crystals. An alternative explanation may be the ‘‘inertial random walk effect’’ of Regel et al. [15]; as the crystals increase in size, they begin to move. The random walk coefficient increases with the radius of the crystal to the fourth power. (2) Researchers from Eniricerche, Milan [29] grew samples of MFI and BEA, possessing, respectively, titanium and boron as framework heteroatoms. After 9 days of ageing, the synthesis mixtures were crystallised at 423 or 443 K for 48 h. Titanium silicalite-1 ( TS-1) grew rapidly from a homogeneous solution, without the detection of any intermediate amorphous gel-like phase. Tetrapropylammonium hydroxide ( TPAOH ) acted as the template for TS-1 formation. The crystals produced in micro-gravity and on Earth were very similar in size and morphology (ca. 20 nm, globular agglomerates). Because of their small size, sedimentation could not be expected under Earth-gravity, and little difference could be expected on reducing the gravitational force. Titanium ZSM-5 (Al-containing TS-1) was prepared from two separate compositions. One was similar to that described above for TS-1, and produced similar products both in micro-gravity

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and on Earth (60–80 nm globular agglomerates). To the second composition was added tetraethylammonium hydroxide ( TEAOH ) to enhance the size of the crystals produced [30]. Although the initial solution was homogeneous, once the crystals formed and reached a certain size, they would begin to migrate under the influence of gravity on Earth. Thus collision-bred nuclei could be formed and compositional inhomogeneities develop under conditions of normal gravity. Samples grown on Earth exhibited broader distributions of particle sizes and smaller average sizes compared with their corresponding micro-gravity-grown samples. Crystals grown in space, which had a barrel-like shape, were mostly connected together as trains of approximately 10 mm in length, possibly because of the action of Marangoni convection, as seen by Sano et al. [7]. Those grown on Earth exhibited a variety of morphologies. Aluminium- and boron-containing zeolite beta (Al-B-BEA) crystallised slowly from a gel-forming system containing tetraethylammonium species. The presence of a hydro-gel during synthesis means that inhomogeneities within the mixture are more pronounced than in clear solutions. Shrinkage of the hydro-gel, through dissolution and Ostwald ripening results in settling under gravity and concomitant concentration gradients which cause convection. In micro-gravity these effects are greatly reduced; the amorphous solid phase remains dispersed throughout the mixture and thus dissolves more evenly. The system thus remains relatively homogeneous without large convective currents. While the crystals grown in micro-gravity are a similar size to those grown on Earth (ca. 10 nm), the respective yields were 22 and 7% ( X-ray diffraction ( XRD) analysis)—a consequence of the more rapid hydro-gel dissolution under microgravity conditions. (3) A group of researchers from the ENSCM, Montpellier [31,32] investigated four zeolite types: analcime, mordenite, gismondine and beta. The principal observations made were that crystallisation in micro-gravity is slower than on Earth and that convection-free systems (in micro-gravity) exhibited enhanced nucleation compared with control experiments. Seven autoclaves were employed each for the micro-gravity and the ground-based

control experiments. Once in stable orbit, the autoclaves were heated to 423 K where they were kept for 60 h. The control autoclaves were simultaneously subjected to an effectively identical temperature program. Analcime and mordenite (one sample of each) were grown from nominally clear solutions, while gismondine (one sample) and zeolite beta (four samples) crystallised from a hydrogel phase. Analcime and mordenite, which grew from clear solutions, both nucleated more profusely in space than on Earth, as evidenced by the smaller and more numerous crystals obtained from the microgravity experiment. The analcime crystals from space possessed an average volume (or mass) some 20 times less than the corresponding Earth-grown crystals while the average volume ratio for the two mordenite samples was around 2. Two possible explanations were given: (1) The release from solution of gas bubbles in micro-gravity is much slower than on Earth, thus a larger gas–liquid interface may exist in space. Evaporation of solution at the interface increases the supersaturation of the solution phase close to the gas–liquid boundary, increasing the tendency for nucleation to occur. (2) Slower growth rates in space mean that the depletion of the nutrient pool around growing crystals/nuclei is less rapid. Thus, under microgravity conditions, larger volumes of the synthesis mixture may remain at relatively higher degrees of supersaturation for longer than the corresponding solution on Earth. The higher supersaturation naturally leads to a higher degree of homogeneous nucleation in the micro-gravity experiment. None of the samples which grew from a hydrogel phase (gismondine and beta) showed any remarkable change in nucleation frequency or crystal size on reducing gravity. Crystals of zeolite beta, where tetraethylammonium ions acted as structure-directing agents, grew as much as 20% larger under conditions of micro-gravity. The precise effect of micro-gravity on these systems has not been elucidated yet. Of the four compositions for zeolite beta, one contained boron and was a less dilute solution; this was the only beta sample

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to approach complete crystallisation in the allotted heating time. In all zeolite beta samples, the degree of crystallinity attained in the micro-gravity experiment was lower than that attained under Earthgravity after the same heating period. The slower crystal growth rate in space, often observed before, is due to reduced convection and mass transport rates. Peak-width analysis of the XRD patterns of beta samples indicated that one of the samples grown in micro-gravity possessed significantly better crystallinity than its Earth-grown counterpart. The composition used for that particular sample was one which was known to produce crystals rich in defects on Earth. Mordenite crystals grown in micro-gravity were flattened in the 010 direction compared with those grown on Earth. Such effects are usually caused by changes in the ratios of growth rates of the various crystal faces. The difference observed here can be attributed to the slower mass transfer and steeper concentration gradients surrounding the crystals which are inherent in the relatively convection-free solution in micro-gravity. The chemical composition of crystals grown in space were not significantly different from those grown on Earth. Similarly, lattice parameters determined through X-ray powder diffraction measurements were effectively invariant between corresponding pairs of micro-gravity and Earthgravity samples. (4) Researchers from ENSCMu, Mulhouse [28] grew silicalite-1 and Ti-silicalite-1 in micro-gravity (total: 7 samples) from both clear solution and gel-forming systems at 443 K for 71 h. No difference was found in the size or morphology of crystals whether grown in micro-gravity or on Earth for any of the compositions. The chemical compositions of the products were essentially unaffected by the gravity parameter. Additionally, 29Si and 19F MAS-NMR spectra were found to be invariant between samples grown on Earth and the corresponding ones from micro-gravity. This implies that the local ordering of framework atoms is unaffected by gravity variations for the compositions studied. (5) A sample of SAPO-5 was prepared in microgravity by workers from CSIC, Valencia [28]. A two-phase liquid system was used, comprising the

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silica source (tetraethylorthosilicate, TEOS) dissolved in n-hexanol and the alumina and phosphate sources in an aqueous medium. When silicon species migrate into the aqueous phase, hydrolysis occurs, providing silicate species to support crystal growth. The migration across the phase boundary acts as a slow-release mechanism, providing a very slow influx of silicate. This two-phase system was chosen in an attempt to alter the silicon distribution within the framework and, hence, alter the catalytic properties of the crystals compared with those grown on Earth. Two different compositions were flown: one containing a surfactant, cetyltrimethylammonium bromide (CTAB), and one without. The surfactant was included to enhance the mixing between aqueous and organic phases. Crystallisation took place at 443 K for 48 h. Samples grown in space were found to be up to 10% more crystalline compared with those grown on the ground, while impurity phases were not present in any of the samples. Crystals from both samples prepared in micro-gravity were approximately 25% larger than their terrestrial counterparts. While the crystals prepared without CTAB were well-defined hexagonal platelets 1–3 mm in diameter, those grown in the presence of CTAB were spherical aggregates of small crystals. Chemical analysis revealed a slight enrichment of the micro-gravity samples in silicon (replacing phosphorus) compared with the terrestrial samples. Calorimetric investigation of the adsorption of NH onto the acidic sites of the samples showed 3 a slightly lower acidic strength for the microgravity samples. The authors attributed this to a less homogeneous silicon distribution in the microgravity sample, producing silicon-rich zones in the crystals, which possess lower acidity than zones where silicon atoms do not exist in next-nearestneighbour positions. (6) Scientists from Degussa, Germany grew two samples of ZSM-5 on the CASIMIR-1 mission. The majority of crystals grew in contact with the walls of the vessel, but those which grew in free solution were considerably larger. Although particle size analysis showed particles of up to several hundred mm to be present, these were agglomerates of smaller crystals. Particle size analysis revealed in all cases a tri-modal distribution of crystal

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dimension; the largest due to crystals grown in solution, the middle mode due to those grown on the walls of the vessel and the smallest fraction believed to be due to secondary nucleation phenomena. Median sizes were found to be 20 to 40% larger for the space samples compared with those from the Earth; however, part of this increase may be due to crystal agglomeration. The crystals grew, in all cases, in a prismatic form, but with a low aspect ratio of 1. 4.2. US experiments A number of zeolite crystal growth experiments have been flown in space by the group at Worcester Polytechnic Institute, USA. The first of these was a simple experiment to grow zeolite A using premixed nutrient solutions in two 10 ml autoclaves within a ‘‘GASCAN’’ (Get Away Special Canister) on space shuttle mission STS-40 [1,33,34]. The GASCAN is a self-contained unit with a volume of 56 l, weighing a maximum of 90 kg, possessing an internal power supply and control systems which can be activated in orbit by a single switch. The solution composition used was one which formed a hydro-gel on mixing and which contained triethanolamine to control nucleation and slow the rate of growth. Although the mixed solutions were shown to be stable for 30 days prior to hydrothermal treatment, a launch delay of 110 days was experienced. Once activated, the autoclaves were heated to 369 K for 72 h under autogeneous pressure. Comparison of the crystals grown in microgravity with those from the ground control experiment (also aged 110 days) showed them to be identical, containing 25–35 mm intergrown cubes. It was hypothesised that (pre-)nucleation occurred, and reached completion, during the 110 day ageing period. Thus the same number of nuclei were present in the micro-gravity and ground control experiments even before heating the autoclaves. The importance of being able to mix the nutrient solutions immediately prior to hydrothermal treatment (at least for compositions where nucleation occurs at ambient temperatures) was thus realised. Autoclaves were developed, which allow the aluminate and silicate solutions to remain separated until ‘‘activation’’ in orbit. The disadvantage of

such autoclaves is the uncertainty in their mixing abilities, their relatively small internal volume (compared with the overall volume, owing to the internal mixing mechanism), the presence of metal components and the necessity of leaving an air gap inside to allow for thermal expansion of the fluid. To ensure that nutrient mixing would be sufficient, a number of transparent replicas of the autoclave were used for visual inspection of the mixing during parabolic flights (short periods of low gravity) and in the glove box facility on the space shuttle prior to activation of the crystal growth autoclaves. In addition, an extensive investigation of solution homogeneity within the autoclaves after various mixing protocols was performed by non-invasive NMR imaging [19]. The second experiment of the Worcester Polytechnic group was flown as part of the payload on the USML-1 ( United States Micro-gravity Laboratory 1) space shuttle mission, STS-50, in 1992 [33,34]. The experiment utilised 38 of the mixing autoclaves in a dedicated furnace [35] with three temperature zones of 369 K (zeolite A, 24 samples), 378 K (zeolite X, 12 samples) and 448 K (mordenite, 2 samples). Once in orbit, the autoclaves were activated by an astronaut (involving the turning of a screw mechanism on each autoclave a number of times with a powered screwdriver) before placing in the furnace and heating. The heat-up profile of the space shuttle furnace was used as the basis for operating the groundcontrol experiment, started a few days later, using an identical set of autoclaves, protocols and furnace. The furnace remained on for a period of eight days. The zeolite A samples which were prepared without the presence of triethanolamine were essentially the same, whether grown on Earth or in micro-gravity, showing that without the control of nucleation, such systems do not produce larger crystals in space. When triethanolamine was present in the mixture, crystals grew to an average size of 10–25% larger in micro-gravity than on Earth, with apparently fewer intergrowths, and without the [110] crystal facets observed in the Earth-grown samples. Electron micrographs and particle size distributions for these samples can be seen in Figs. 2 and 3, respectively. Zeolite X samples, also prepared in the presence of triethano-

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Fig. 3. Particle size distributions of the samples shown in Fig. 2.

Fig. 2. Zeolite A crystals grown in micro-gravity during the USML-1 space shuttle mission ( FLIGHT ), and on Earth during the corresponding control experiment ( TERRESTRIAL)

lamine, grew up to 50% longer (apex-to-apex) in micro-gravity than on Earth. The SEM photographs ( Fig. 4) and particle size distributions (Fig. 5) show clearly the increase in size for the micro-gravity sample. In some cases, the samples also contained small quantities of zeolite P, which is a common contaminant in products from compositions aimed at large zeolite X crystals. Both the zeolite A and zeolite X samples grown in micro-gravity were found to have slightly higher Si/Al ratios than their terrestrial counterparts. These were measured by both X-ray photoelectron spectroscopy and electron microprobe analysis (through the cross-section of the crystals). The

smaller unit cell sizes ( XRD powder patterns) and lower Brunauer–Emmett–Teller (BET ) surface areas (N or CO adsorption) of samples grown 2 2 in space suggest that they may contain fewer lattice defects than those grown on Earth. The mordenite crystals obtained from the ground control experiment were intergrown to form a plug of dense material as a result of sedimentation under gravity, while those from micro-gravity were individual entities. The size and morphology of the crystals was similar for both samples. The USML-2 mission, launched in October 1995, utilised the same hardware as USML-1, but involved a wider range of zeolite synthesis experiments, including zeolites A, X, beta and MFI [36 ]. An alternative ‘‘nucleation suppressant’’, 2,2bis(hydroxymethyl )-2,2∞,2∞-nitrilotriethanol (BIS), as proposed by Morris et al. [37], was used in place of triethanolamine in some compositions. The results of these experiments have not yet been published [38]. In the form of a collaboration between the

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Fig. 5. Particle size distributions of the samples shown in Fig. 4.

Fig. 4. Zeolite X crystals grown in micro-gravity during the USML-1 space shuttle mission ( FLIGHT ), and on Earth during the corresponding control experiment ( TERRESTRIAL)

European Space Agency ( ESA) and NASA, researchers at Delft University of Technology, The Netherlands, also participated in the USML-2 Zeolite Crystal Growth experiment. The central ‘‘zone’’ of the furnace [35] used by Worcester Polytechnic Institute was allocated to ESA. Two samples of ZSM-5 were prepared (see Table 1), the detailed analysis of which is still in progress [3]. Preliminary data indicate that the most striking difference between ZSM-5 crystals grown on Earth and in micro-gravity is the smoothness of their external surfaces. Fig. 6 shows atomic force microscope images of a pair of ZSM-5 crystals of similar size and morphology grown on Earth and

in space, respectively. It can be seen that the Earthgrown crystal has a much rougher external surface. Morphological differences were also seen; the micro-gravity samples contained a larger proportion of single crystals than the terrestrial ones (as evidenced by single-crystal XRD of many crystals). Also, the terrestrial samples contained relatively more prismatic crystals while the space samples contained more cubic-type crystals. 4.3. The EURECA satellite Scientists from the Center for Industrial Research in Norway [39,40] conducted an experiment to grow zeolite offretite under conditions of micro-gravity aboard the EURECA-1 mission. The EURECA retrievable satellite was launched aboard a space shuttle on July 31st, 1992, and remained in orbit for an extended period, allowing a growth period of 5–6 months under conditions of micro-gravity. A reactor possessing three cham-

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Fig. 6. Atomic force microscope images of the external surfaces of ZSM-5 crystals grown on Earth ( left) and in space, aboard the USML-2 space shuttle mission (right).

bers, arranged in series and separated by valves, was used. The middle chamber (300 ml ) contained sodium aluminate, NaOH, KOH and tetramethylammonium hydroxide dissolved in water, while the two end chambers (500 ml each) contained the silica source dissolved in a solution of NaOH and KOH. The reactors (one for launch into space and an identical one for the ground-based experiment) were loaded with the appropriate solutions 4 months prior to the initiation of the experiments. The experimental procedure was a little unusual for zeolite syntheses in that the solutions were not forcibly mixed, but were allowed to diffuse slowly together under micro-gravity. A relatively low temperature of 348 K was used (owing to a power constraint), and the valves separating the chambers opened slowly over a 21 h period in order to disturb the solutions as little as possible. The experiments were terminated in mid-January 1993 and the reactors were opened in September 1993. Apart from offretite, sodalite was also present in high yield (offretite:sodalite#40:60) in both the terrestrial and the micro-gravity reactors. Chemical analysis of the products revealed no significant difference between samples grown on Earth and those grown in space. While the middle chamber contained no solid material after the micro-gravity experiment, amorphous solid was detected in the central chamber of the terrestrial reactor. This is

ascribed to the faster mixing of the solutions (owing to convection) on Earth, introducing sufficient silica into the middle chamber to allow formation of an amorphous hydro-gel. Chemical analysis of the solutions remaining in the three chambers after reaction (isolated by the valves on completion of the experiment) verified that transport of materials between the various chambers was a slower process in micro-gravity than on Earth. The terrestrial offretite crystals were approximately spherical with a diameter less than 0.5 mm, while those grown in micro-gravity were rod-like, with dimensions of about 0.6×2 mm.

5. Outlook Based upon the results discussed above, Table 2 lists some possible improvements to experimental design and strategy which would lead to a better utilisation of the potential benefits of micro-gravity to zeolite crystal growth. If one is to learn about the mechanisms of zeolite growth, the experiments should be made as simple as possible, with few variables. A multitude of processes occur during the synthesis of zeolites, and a complex synthesis system may result in blurring of the effects of micro-gravity. In microgravity experiments, it is important to ensure that

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Table 2 Possible improvements to experimental and strategic design $ $ $ $ $ $ $ $

Simplest possible experiments, where the micro-gravity parameter is not masked by other effects Simplest possible autoclaves In situ processing (i.e. nutrient mixing) Closed liquid phase (no gas bubbles) In situ observation (i.e. spectroscopic, light scattering) No chemical additives which are not absolutely necessary for zeolite formation More complete listings of results Research consortia instead of point groups

the gravitational force is the only parameter which is altered compared with the terrestrial control experiments. For instance, minute changes in the chemical composition of the solutions, the concentration of impurities or dust and differences in the autoclaves between the space and terrestrial experiments can have marked influences on the products. With the above comments in mind, the development of more sophisticated autoclaves, allowing, e.g. total absence of a gas phase, automated mixing of two nutrient pools prior to or during heat-up and fast heating and cooling rates would allow the experimenter to reduce to a minimum the factors which may mask the effects of changing the gravitational force. Autoclaves which allow in situ spectroscopic observation would be valuable for improving our understanding of the behaviour of zeolite synthesis solutions in micro-gravity.

6. Summary The most common differences which are observed between zeolite syntheses carried out on Earth and in space relate to: (1) the reduced rate of mass transfer, because of reduced convection in micro-gravity. Crystallisations in space commonly proceed more slowly than on Earth, often resulting in a reduced yield or incomplete conversion of nutrients into zeolite if the synthesis time is not sufficiently long. However, if the synthesis time in micro-gravity is extended sufficiently, greater conversion may be achieved compared with the corresponding terrestrial experiment

owing to suspension of the crystals in the nutrient pool. (2) the reduced tendency for sedimentation in micro-gravity. Terrestrial syntheses of large crystals commonly result in films or plugs of intergrown material in the bottom of the reactor, which are avoided in micro-gravity. In order to grow large zeolite crystals in microgravity, one must start with a composition which undergoes controlled (i.e. suppressed ) nucleation. ‘‘Normal’’ zeolite synthesis compositions which nucleate profusely yielding crystals from tens of nm to several mm in size cannot be expected to yield crystals much larger in space. The greatest success in terms of the growth of large crystals has been achieved through the use of hydro-gel-forming solutions in combination with nucleation suppression through complexing agents, or other additives. Increases in crystal size by up to 50% in linear dimension have been reported for such systems in space compared with those on Earth [33]. In similar syntheses where nucleation is not controlled, however, no significant increase in size has been reported. Decreases in unit cell parameters and lower BET surface areas have been reported for zeolites A and X grown in space and the difference ascribed to the presence of fewer framework defects in the micro-gravity samples [33], which could be expected from a more static system. Similarly, peak widths of XRD patterns of zeolite beta samples have, on occasion, been found to be narrower from syntheses carried out in microgravity, suggesting a less faulted structure than terrestrial beta [32]. Changes in growth rates and nutrient concentrations on performing a synthesis in space as opposed to on Earth have been seen to induce changes in crystal morphology. Examples include ZSM-5 (cubic+small prismatic crystals from space; large prismatics+cubic crystals on Earth), mordenite (space crystals flattened in 010 direction compared with terrestrial ones) and offretite (rod-like crystals from space; spherical ones on Earth). In a number of cases, bulk Si/Al ratios have been reported to be higher in samples grown in micro-gravity than in their terrestrial counterparts.

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One of the most striking differences seen on performing syntheses in space has been the smoothness of the external surfaces of crystals. For instance, ZSM-5 crystals grown on the USML-2 mission were found to have significantly smoother external surfaces compared with their terrestrial counterparts [3]. The implications for shape-selective catalysis are important, especially with small particles, which are preferred for kinetic reasons, where the external surface may be a significant fraction of the total surface.

Acknowledgement The authors thank NIVR and the European Space Agency for funding of the RADIUS (Research Association for the Development of the Industrial Utilisation of Space) zeolite programme, which includes the Katholieke Universiteit Leuven and ENSCM Montpellier and is based at Delft University of Technology, under the direction of which this review was written. We would also like to thank NASA for allowing some of the results herein to be published.

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