Chimie Douce Synthesis of Nanostructured Layered Materials - ACS

Chapter 16

Chimie Douce Synthesis of Nanostructured Layered Materials 1

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Doron Levin , Stuart L. Soled , and Jackie Y. Ying

Downloaded by INDIANA UNIV BLOOMINGTON on June 28, 2014 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch016

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Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge,MA02139 Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801 2

A class of layered ammonium transition-metal molybdate materials were derived by a novel room-temperature chimie douce synthesis technique using calcined layered double hydroxides (LDHs) as precursors. The compounds obtained are highly crystalline, and retain the rhombohedral symmetry of the LDH precursors. The host structure consists of distorted divalent cation octahedra which share edges to form layers perpendicular to the c axis, analogous to the LDH precursor. The tetrahedral molybdate species, however, are not merely intercalated within the interlayer domain, but are bonded to the layers themselves through shared Mo-O-M bonds, where M = Zn , Co , Cu , or Ni . This arrangement results in the formation of a net negative charge on the host structure, leading to incorporation of ammonium ions between the layers for charge balancing. The applicability of this novel synthesis route is dependent on the composi tion of the LDH precursor, and it appears that metastability in the calcined LDH favors conversion to this phase. 2+

2+

2+

2+

Pillared layered structures (PLS) are nanocomposite materials prepared by linking molecules to a layered host. These structures are an excellent example of materials by design. Research in this area has been focussed on the design of new materials by intercalating nanostructures into layered precursors, the key issue being the engineering of the interlayer space between these two-dimensional precursors. There is considerable interest in the synthesis of new materials having interlayer dimensions on the nanometer scale (2-3 nm) which are thermally stable. In general, ionic lamellar solids are preferred hosts for the preparation of pillared intercalates. Synthesis of PLS can be accomplished by modification of the host-structure chemical composition, chemical or structural modification of the guest species domains, or both. There has been extensive research into the preparation of PLS, primarily based on structural modification of the guest species domains. Examples include the pillaring of smectites, such as montmorillonite, 3

Corresponding author 0097-6156/96/0622-0237$15.00/0 © 1996 American Chemical Society

In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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by ion-exchange with polycationic species, e.g. Al -polyhydroxypolymer (Al 0 (OH) (H 0) ) (7-6), Zr-hydroxypolymers (Zr (OH) (H 0) ) (7,5), and other oligocations. One common feature of these materials, termed cross-linked smectite (CLS) molecular sieves, or pillared interlayered clays (PDJC), is that they were prepared without chemical modification of the host composition. Other examples of PLS prepared by modification of only the guest species domains include the pillaring of layered double hydroxides (LDHs), also known as hydrotalcite-like materials, by various anionic species. Examples include the pillaring of LDHs with polyoxometalate (POM) anions of [ X M ^ O J " " or Keggin ion type by direct anion exchange (9-14), or utilizing an organic anion-pillared precursor that was subsequently exchanged with the appropriate isopolymetalate under mildly acidic conditions (15-16). To the best of our knowledge, however, no new layered structure has been prepared by the simultaneous modification of both the chemical composition of the host structure and the guest species domain. In this chapter, we report a novel room temperature chimie douce synthesis technique which produced a new class of layered transition metal molybdate (LTM) materials using calcined LDHs as precursors. These new materials, while being related structurally to the L D H precursor, have undergone a modification of the host chemical composition, as well as complete transformation of the interlayer domain. 13

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Synthesis of L T M Materials The synthesis of a L T M material occurs via the following three steps: i) Synthesis of the L D H precursor ii) Preparation of a metastable mixed oxide phase by calcination of L D H precursor iii) Chimie douce reaction of the mixed oxide phase to form the L T M To illustrate these three steps, the synthesis of a Zn/Cu-LTM will be discussed. Synthesis and Structure of the LDH Precursor. The precursor for the synthesis of Zn/Cu-LTM was a Z n C u A l ( O H ) ( C O ) - z H O L D H . The L D H was synthesized by coprecipitation at constant pH, under conditions of low supersaturation. The precursor was prepared by adding a solution of Ζη(Ν0 ) ·6Η 0, Cu(N03) -6H 0 and Α1(Ν0 ) ·9Η 0 to a solution of K O H and Κ ^ ϋ 0 , at relative rates such that a constant pH of 9.0 was maintained. The temperature of precipitation was 55±2 °C. Following precipitation, the material was aged in the mother liquor overnight at 70 °C. The material was then filtered, washed, and dried at 110 °C at atmospheric pressure. The X ray diffraction pattern of this material is shown in Figure 1. The pattern was indexed using a hexagonal unit cell, with a = 3.08(3) À and c = 22.83(3) Â, deterrriined using a least squares method. The X R D pattern showed the presence of a secondary zincite (ZnO) phase. Peaks indicating the presence of this zincite phase are designated in Figure 1 with an asterisk. The presence of this secondary phase was expected due to the high zinc content (77), but proved to have no effect on the subsequent chimie douce synthesis. 0677

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The structure of the L D H precursor is very similar to that of brucite, Mg(OH) , where octahedra of M g (6-fold coordinated to OH") share edges to form infinite sheets. These sheets are stacked on top of each other, and are held together by hydrogen bonding. Isomorphous substitution of a divalent cation in the lattice by a trivalent cation 2

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Figure 1. Powder X-ray diffraction pattern of (CO ) '4H O (* indicates zincite phase). 3 01125

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25 35 45 Two Thêta (degrees)

Zn Cu Al (OH) 0677

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having similar radius results in a positive charge generated in the hydroxy sheet. This net positive charge is compensated for by incorporation of hydrated anions in the interlayer region. _Synthetic LDHs formed by coprecipitation from aqueous media crystallize in the R3m space group with three double-layers present per unit cell. In general, the composition of the L D H precursors used for the synthesis of L T M materials can be represented by the formula: II

[Zn . . M Al (OH) ]-[(CO3)^.zH O], 0

n

x

y)

y

x

2

2

2+

2+

2+

where M is a divalent transition metal cation such as N i , Co , or C u . Typically, the trivalent cation substitution parameter, x, given by Al/(Zn + M + Al), is in the range of 0.2 to 0.33. The parameter y is constrained by the relationship (l-x-y)/x > 1, such that Zn is the dominant divalent transition metal cation. n

Preparation of Metastable Mixed Oxide Phase. The key step in the synthesis of the L T M phase is the preparation of the metastable mixed oxide phase. This mixed oxide phase is prepared by calcination of the L D H precursor. At temperatures between initial L D H decomposition and spinel phase formation, a series of metastable phases, both crystalline and amorphous, can be formed (18). The properties of these phases depend on the cations constituting the original L D H , preparation and thermal decomposition


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6.0

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— ι

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Time (min) Figure 2. Change in pH during chimie douce reaction.

conditions, and the presence of impurities (79). The process of thermal decomposition of a carbonate L D H consists of dehydration, dehydroxylation and decarbonation, yielding a mixed oxide phase at temperatures less than 550 °C. This mixed oxide phase, when examined by X-ray diffraction, shows only broad peaks corresponding to a zincite (ZnO) phase. At this stage of the synthesis, the presence of any secondary zincite formed during precipitation of the L D H becomes indistinguishable. The importance of having zinc as the dominant transition metal results from the necessity of having the mixed oxide phase with a wurtzite structure. The Zn/Cu/Al-LDH precursor was calcined for 3.5 hours to prepared the metastable mixed oxide phase.

Chimie Douce Synthesis of Zn/Cu-LTM. Following calcination, the mixed oxide phase was added to a room-temperature solution of ammonium heptamolybdate (0.05 M ΜογΟ^ " and 0.3 M NH ). Progress of the reaction was followed by monitoring the pH. A typical pH profile over the course of a reaction is shown in Figure 2. The time to complete the reaction is dependent on the temperature of the mixed oxide as it enters the solution, and is typically between 40 minutes and two hours. Following completion of the reaction, the product was recovered by filtration, washed with deionized water, and dried overnight at 110 °C. The product, called Zn/Cu-LTM, was a fine, pale cyan powder. 6

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Characterization of Zn/Cu-LTM A powder X-ray diffraction pattern of this material, collected on a Rigaku diffractometer operated at 30 kV and 40 mA with CuKa radiation, is shown in Figure 3. This diffraction pattern is significantly different from ZnO, and bares no resemblance to that expected from a L D H pillared with the heptamolybdate anion (20,21). This diffraction pattern is almost identical to that of the Zn-LTM phase (22). A least squares refinement of the peak positions gave a hexagonal unit cell of a = 6.104(9) Â, and c = 21.69(9) Λ. The crystal structure of the Zn/Cu-LTM is a derivative of the Zn-LTM, whose structure solution is reported elsewhere (22). The structure of the Zn-LTM was solved by Rietveld refinement of an isostructural ammonium nickel molybdate, (NH )HNi (OH) (Mo0 ) , prepared by precipitation, whose own structure was determined, ab initio, from powder synchrotron data (23). 4

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Crystal structure of Zn/Cu-LTM The framework of the Zn/Cu-LTM consists of stacks of sheets built up from edge-sharing transition-metal octahedra to which tetrahedral molybdate groups are bonded. The zinc and copper atoms defining these layers are located at site 9(e), which can vary in occupancy from V2 to 1 (23). If the occupancy of this site is one, the arrangement of the transition metal atoms can be considered as a pattern of two alternating strings, one being M - M - M , as in L D H , the other being M - d - M , where M may be Zn or Cu, and • represents an ordered cation vacancy. This ordered cation vacancy is independent of the occupancy of the 9(e) site. As the occupancy of this site deviates from unity, additional disordered vacancies appear in the sheet. The coordination of oxygens about the zinc and copper atoms is distorted octahedral, with a tetragonal contraction along the C axis running through O - M - O , where M may be Zn or Cu, and Ο is a bridging oxygen. The zinc and copper atoms are linked to each other through double oxygen bridges. Each octahedron shares edges with four adjacent octahedra, thereby creating sheets of octahedra perpendicular to the c axis, analogous to the L D H precursor. The absence of an atom at the origin generates ordered vacancies in the sheet. These vacant octahedral sites are capped, both above and below, by tetrahedral molybdate groups. These tetrahedron above and below the octahedra sheet are related to each other by the center of inversion at the origin. The tetrahedra share the three oxygens forming the base with the zinc-oxygen octahedra, each oxygen in the base being shared with two different octahedra. Therefore, each molybdate tetrahedron shares corners with six different zinc octahedra, generating the hexagonal arrangement shown in Figure 4. 4

The three transition-metal molybdate layers defining the unit cell are stacked at a separation of c/3, analogous to the rhombohedral L D H precursor. In these layers, the position of the ordered vacancy capped by the molybdate groups follows the sequence A - B - C - A (where A , B , and C are the three threefold axes at x,y = 0,0; 2/3,1/3; and 1/3,2/3). This three layer arrangement is shown in Figure 5. Between each layer, in the space defined by the apical oxygens of six tetrahedral molybdates, lie ammonium ions. The positions of the nitrogen atoms follow the sequence C-A-B-C. The ammonium ions do not serve to connect the array of layers in the [00/] direction, the distance between the


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10

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40 50 Two Theta (degrees)

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Figure 3. Powder X-ray diffraction pattern of Zn/Cu-LTM.


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Figure 4. Basai plane of Zn/Cu-LTM, viewed along [001 ].


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Figure 5. Crystal structure of Zn/Cu-LTM, viewed along [010], showing three layer arrangement in polyhedron representation.


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nitrogen and the bridging oxygen in the adjacent layer being too long for hydrogen bonding. The vertical distance between the apical oxygen of the molybdate tetrahedra and the bridging oxygen of the adjacent zinc layer suggests that these two oxygens are involved in a hydrogen-bonding mechanism that serves to connect the layers (23). Figure 6 illustrates a three-dimensional polyhedron representation of the crystal structure of Zn/Cu-LTM.


Discussion The crystal structure of Zn/Cu-LTM allows for an interesting comparison to its L D H precursor. Fundamentally, both structures have, as a basic framework, sheets of zinc, copper (and aluminum) octahedra running perpendicular to the c axis. The type and occupancy of the octahedral sites however, differ in the two structures. In the L D H framework, the metal cations are situated at site 3(a), and are fully occupied, such that a equals the cation-cation distance within the sheet (Figure 7(a)). However, in the L T M structure, the metal cations are situated at site 9(e), and can have variable occupancy. Consequently, a for the L T M structure depends on the Mo-Mo distance, a distance equivalent to that between consecutive ordered vacancies at the origin (Figure 7(b)). Considering the case of a fully occupied 9(e) site in a L T M structure, there would be three metal cations per layer in the unit cell (Figure 7(b)). If we define a supercell of the L D H with a - 2 a , c' = c , the area of the unit supercell would be almost identical to that of the L T M cell. In this equivalent area supercell however, there would be four metal cations per layer (Figure 7(c)). Therefore, the chimie douce synthesis reaction, while reconstructing the fundamental layered nature of the structure from the metastable zincite phase, resulted in a reduction of the number of cations in octahedral coordination in the layers. 0

y

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A possible mechanism leading to this reduction in the number of cations has been determined on the basis of elemental analysis of the L T M phases, and Al M A S - N M R data on the metastable mixed oxide phases (22). Elemental analysis had shown that the final product contained less than 1% A l . This suggested that, on reaction with the ammonium heptamolybdate solution, the A l in the zincite phase left the solid phase and entered into solution. T h e A l M A S - N M R analysis of a calcined Z n A l - L D H showed a very high proportion (> 80%) of tetrahedrally coordinated A l (22). In comparison, Al M A S - N M R analysis of a calcined M g A l - L D H showed a relatively low proportion (

Chimie Douce Synthesis of Nanostructured Layered Materials - ACS

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