Natarajan, Srinivasan

Neeraj, S.

Norquist, Alexander J.

Walton, Richard I.

English

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Haverford College Haverford Scholarship Faculty Publications Chemistry 2001 Direct in situ observation of increasing structural dimensionality during the hydrothermal formation of open-framework zinc phosphates Richard I. Walton Alexander J. Norquist Haverford College, anorquis@haverford.edu S. Neeraj Srinivasan Natarajan Follow this and additional works at: https://scholarship.haverford.edu/chemistry_facpubs Repository Citation Walton, Richard I., et al. "Direct in situ observation of increasing structural dimensionality during the hydrothermal formation of open-framework zinc phosphates." Chemical Communications 19 (2001): 1990-1991. This Journal Article is brought to you for free and open access by the Chemistry at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact nmedeiro@haverford.edu. Communicationwww.rsc.org/chemcommCHEMCOMMDirect in situ observation of increasing structural dimensionalityduring the hydrothermal formation of open-framework zincphosphatesRichard I. Walton,a Alexander J. Norquist,b S. Neeraj,c Srinivasan Natarajan,c C. N. R. Raoc andDermot O’Hare*ba School of Chemistry, Stocker Road, University of Exeter, Exeter, UK EX4 4QDb Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QRE-mail: dermot.ohare@chem.ox.ac.ukc Chemistry and Physics of Materials Unit, Jawaharla Nehru Centre for Advanced Scientific Research,Jakkur PO, Bangalore, 560 064, IndiaReceived (in Cambridge, UK) 27th June 2001, Accepted 10th August 2001First published as an Advance Article on the web 18th September 2001The first time-resolved in situ X-ray diffraction studies of thehydrothermal crystallisation of open-framework zinc phos-phates reveal a pathway of sequential crystallisation involv-ing formation of a metastable low dimensional chain phasebefore the growth of three-dimensional zeolitic archi-tectures.A great deal of effort is currently focussed on understanding thecrystallisation mechanism of microporous solids which canhave applications in shape-selective catalysis, separation sci-ence, and ion-exchange. It is hoped that an understanding ofmechanisms leading to the formation of these zeolite-like solidsat the atomic-scale will allow the rational synthesis of newmaterials.1,2 The common use of hydrothermal synthesisconditions in the preparation of such open-framework solidsrequires sealed thick-walled reaction vessels and it is only inrecent years that in situ probes of hydrothermal crystallisationhave been developed to follow reactions under these experimen-tally-challenging conditions.3 For example, in a study of thephosphates of gallium using energy-dispersive X-ray diffrac-tion (EDXRD) some of us have observed previously-unknown,transient phases prior to the onset of crystallisation of open-framework phases,4,5 and Taulelle et al. have identifiedoligomeric ‘building blocks’ in hydrothermal reaction solutionsusing NMR spectroscopy.6 The results of such studies providea foundation on which to build descriptions of reactionmechanisms.The first results of an in situ EDXRD study of thehydrothermal synthesis of zinc phosphates are described here.This large family of solids has recently been the focus of studyby some of us, and an Aufbau principle for the building up ofthree-dimensional structures from structures of lower di-mensionalities proposed.7 With an aim of gaining furtherinformation about crystallisation mechanisms it is crucial tostudy systems such as these using in situ measurements ofcrystallisation, since it has been established that on arresting ahydrothermal reaction by cooling to room temperature thematerial recovered is not necessarily representative of thematerial present under reaction conditions.4,5 The EDXRDmethod uses intense synchrotron X-rays which allows the use ofa laboratory-sized hydrothermal reaction vessel, mimicking theconditions employed in the synthetic laboratory and permittingrapid data collection ( < 1 min) for kinetic studies. An apparatusdeveloped over the past few years by O’Hare and co-workers8was used in the current work.†In the first zinc phosphate system studied we focussed on thereaction between piperazine phosphate ([C4N2H12][HPO4]·H2O, PIP-P) and zinc oxide in dilute hydrochloric acid solution.Fig. 1 shows a stack plot of the time-resolved EDXRD datameasured as a reaction mixture of composition ZnO+1.5HCl+1.75 PIP-P+100 H2O which was heated to 180 °C in anautoclave at ca. 5 °C min21 (detector angle 0.9325°). The use ofa three-element energy discriminating detector system9 allowsunambiguous identification of known crystalline phases, owingto the large amount of diffraction data that can be simultane-ously recorded.10 Characteristic Bragg reflections of thephosphate and amine source, PIP-P, are observed only duringthe first minute before disappearing; presumably this is due todissolution of this rather soluble species. Remarkably, weobserve that characteristic Bragg reflections of a known zincphosphate, [C4N2H12][Zn(HPO4)2(H2O)] (ZnPO–PIP-I),11 arepresent in the first spectrum recorded (31 °C), suggesting thatthis material forms on mixing the chemicals at room tem-perature. The compound has a 1-D ladder structure (see Table 1for descriptions of the phases discussed in this paper). Afterheating to 163 °C new strong Bragg reflections (of a phase notyet identified) are apparent, and these diffraction features, alongwith those of ZnPO–PIP-I, have constant intensity for 30 minduring which time a temperature of 180 °C is reached. Over aperiod of 10 min, these two phases and the starting material ZnOare replaced by two known zinc phosphates,[C4N2H12][Zn3.5(PO4)3(H2O)] (ZnPO–PIP-II)11,12 and[C4N2H12][Zn2(HPO4)2(H2PO4)2] (ZnPO–PIP-III),11 which arethe sole two products after 145 min of heating. ZnPO–PIP-IIand ZnPO–PIP-III both have three-dimensional frameworkstructures.On changing the amount of PIP-P we observed ratherdifferent behaviour. For a mixture of composition ZnO+1.5HCl+1 PIP-P+100 H2O, held at a constant temperature of 180°C, the ladder phase ZnPO–PIP-I is present in the first spectrumFig. 1 A stack plot of EDXRD data measured during the sequentialcrystallisation of zinc phosphates from the reaction mixture ZnO+1.5HCl+1.75 PIP-P+100 H2O.This journal is © The Royal Society of Chemistry 20011990 Chem. Commun., 2001, 1990–1991 DOI: 10.1039/b105659jDownloaded by Haverford College on 29/03/2013 16:18:49. Published on 18 September 2001 on http://pubs.rsc.org | doi:10.1039/B105659JView Article Online / Journal Homepage / Table of Contents for this issuerecorded, and almost immediately its Bragg reflections begin todecay. The ladder phase is consequently replaced by a knownzinc phosphate with a three-dimensional structure,[C4N2H12][Zn(H2O)Zn(HPO4)(PO4)] (ZnPO–PIP-IV).11 Thedecay of the precursor phase and growth of the product are notmirrored precisely: integrated peak intensities show that anappreciable amount of ZnPO–PIP-I is consumed before ZnPO–PIP-IV is observed. The decay and growth curves cross at a≈ 0.3 of product (a is the extent of reaction; see Fig. 2). Thissuggests that the transformation of the chain material to thethree-dimensional structure is a solution-mediated processinvolving dissolution of the first phase, rather than a solid-statetransformation.We have presented preliminary results of an in situ study ofthe formation of a large family of open-framework materials.The dissolution of starting materials and sequential crystallisa-tion of a number of open-framework solids was directlyobserved. In certain cases, crystalline materials with as-yetunknown structures form during the reactions. Given the hugenumber of zinc phosphate structures that can be formed fromreaction mixtures of only slightly differing composition, it is notunexpected to discover new metastable phases. The importanceof these crystallisation studies is that for the first time we haveobserved under real reaction conditions the sequential crystal-lisation of materials with increasing dimensionality. Recently ithas been shown that a ladder zinc phosphate, similar to thatobserved as a transient phase in our in situ studies, can be usedas a solid precursor to a number of other layered and 3-Dphases.13 Combined with our in situ results, such studies of thereactivity of the ZnPO phases provide convincing evidence forthe Aufbau principle proposed recently for the building up of3-D zinc phosphate structures.7 We are currently extending ourin situ studies to survey other ZnPO/amine systems in order tounderstand more fully the nature of the structural transforma-tions taking place.We thank the EPRSC for funding and provision of beam-timeat SRS, Daresbury Laboratory.Notes and references† EDXRD experiments were performed on Station 16.4 of the DaresburySynchrotron Radiation Source (I ~ 200 mA, ~ 2 GeV synchrotron). Station16.4 was illuminated with X-rays in the energy range 5–120 keV from a 6T superconducting wiggler insertion device. Diffracted X-rays weremeasured using a three-element germanium solid-state detector; thisallowed access to three d-spacing ranges, dependant on the angle to whichthe detector was set. The delay between mixing reagents, sealing in thehydrothermal autoclave and beginning data collection was of the order of 5min.1 M. E. Davis, Stud. Surf. Sci. Catal., 1995, 97, 35.2 R. J. Francis and D. O'Hare, J. Chem. Soc., Dalton Trans., 1998,3133.3 R. I. Walton and D. O'Hare, Chem. Commun., 2000, 2283.4 R. J. Francis, S. O'Brien, A. M. Fogg, P. S. Halasyamani, D. O'Hare, T.Loiseau and G. Férey, J. Am. Chem. Soc., 1999, 121, 1002.5 R. I. Walton, T. Loiseau, D. O'Hare and G. Férey, Chem. Mater., 1999,3201.6 F. Taulelle, M. Haouas, C. Gerardin, C. Estournes, T. Loiseau and G.Férey, Colloids Surf. A, 1999, 158, 299.7 C. N. R. Rao, S. Natarajan, A. Choudhury, S. Neeraj and A. A. Ayi, Acc.Chem. Res., 2001, 34, 80.8 J. S. O. Evans, R. J. Francis, D. O'Hare, S. J. Price, S. M. Clarke, J.Flaherty, J. Gordon, A. Nield and C. C. Tang, Rev. Sci. Instrum., 1995,66, 2442.9 S. L. Colston, S. D. M. Jacques, P. Barnes, A. C. Jupe and C. Hall, J.Synchrotron Radiat., 1998, 5, 112.10 G. Muncaster, A. T. Davies, G. Sankar, C. R. A. Catlow, J. M. Thomas,S. L. Colston, P. Barnes, R. I. Walton and D. O'Hare, Phys. Chem.Chem. Phys., 2000, 2, 3523.11 C. N. R. Rao, S. Natarajan and S. Neeraj, J. Am. Chem. Soc., 2000, 122,2810.12 K. O. Kongshaug, H. Fjellvåg and K. P. Lillerud, J. Mater. Chem., 1999,9, 3119.13 A. Choudhury, S. Neeraj, S. Natarajan and C. N. R. Rao, J. Mater.Chem., 2001, 11, 1537.Table 1 Zinc phosphates observed in situ in the ZnO/PIP-P/HCl (aq) systemCompound (dimensionality) FormulaMiller index, observed d (expected)/c/ÅLowest EDXRD detector at 0.9325°ZnPO–PIP-Ia (1-D) [C4N2H12][Zn(HPO4)2(H2O)] 011, 7.77 (7.733)020, 7.05 (7.012)ZnPO–PIP-IIa,b (3-D) [C4N2H12][Zn3.5(PO4)3(H2O)] 200, 7.82 (7.813)110, 7.30 (7.300)111, 6.74 (6.717)ZnPO–PIP-IIIa (3-D) [C4N2H12][Zn2(HPO4)2(H2PO4)2] 110, 9.285 (9.272)111¯, 6.37 (6.325)ZnPO–PIP-IVa (3-D) [C4N2H12][Zn(H2O)Zn(HPO4)(PO4)]2 110, 9.35 (9.327)111¯, 7.73 (7.703)020, 7.47 (7.444)111, 6.99 (6.954)200, 5.96 (5.983)a Ref. 11 b Ref. 12. c Bottom EDXRD detection.Fig. 2 Integrated peak intensities of ZnPO–PIP-I and ZnPO–PIP-IV duringcrystallisation from the reaction mixture ZnO+1.5 HCl+1 PIP-P+100 H2O at180 °C.Chem. Commun., 2001, 1990–1991 1991Downloaded by Haverford College on 29/03/2013 16:18:49. Published on 18 September 2001 on http://pubs.rsc.org | doi:10.1039/B105659JView Article Online

Direct in situ observation of increasing structural dimensionality during the hydrothermal formation of open-framework zinc phosphates

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Direct in situ observation of increasing structural dimensionality during the hydrothermal formation of open-framework zinc phosphates

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