Luis A. Villaescusa

Synthesis of all-silica and high-silica molecular sieves in fluoride media

Recent advances in the synthesis of all-silica and high-silica crystalline molecular sieves in fluoride media, with special regard to low framework density phases, are presented. The fundamental differences between the synthesis in hydroxide and fluoride media with respect to the properties of the materials obtained and the phase selectivity of the crystallization are discussed.

High silica zeolites with three-dimensional systems of large pore channels

Abstract Several high silica zeolites with three-dimensional systems of large pore channels have been prepared in aqueous fluoride media: Beta, ITQ-7, ITQ-10 and ITQ-14. Pure silica beta can crystallize using up to 17 organic cations considerably different in size, shape, rigidity and C/N ratios. There are clear differences in the powder X-ray diffraction patterns of calcined beta materials prepared with different cations, suggesting structural differences between them. However, this technique cannot be readily used to get further insights into these differences. 19 F MAS NMR spectroscopy shows the existence of fluoride occluded in double four ring cages in most of the different betas, thus demonstrating for the first time the real occurrence in these intergrown materials of polymorphs different from A and B. Double four ring cages are also present in ITQ-7 and in the new ITQ-10 and ITQ-14 zeolites.

The widespread occurrence of negative thermal expansion in zeolites

Powder X-ray and neutron diffraction studies of the calcined siliceous zeolites ITQ-7, ITQ-9 and CIT-5, and the aluminophosphates AlPO4-31 and Mg-doped AlPO4-17 have been carried out as a function of temperature in order to determine their thermal expansivity properties. Of these, ITQ-7, ITQ-9 and MAPO-17 show negative thermal expansivity over a wide temperature range, in common with several other previously studied microporous materials. CIT-5 and AlPO4-31 are anomalous in showing an expansion on heating. A detailed study of CIT-5 highlights the key structural parameters which determine its positive expansivity. From expansivity data currently available on seventeen microporous materials, it is clear that negative thermal expansivity must be considered the norm rather than the exception. Positive expansivity is encouraged by the structure having a relatively high framework density and a one-dimensional channel system.

Synthesis and structure of fluoride-containing GeO2 analogues of zeolite double four-ring building unitsElectronic supplementary information (ESI) available: X-ray diffraction data. See http://www.rsc.org/suppdata/cc/b2/b207374a/

Double four-membered ring (D4R) anionic building units [Ge8O12(OH)8F]- have been synthesised hydrothermally and their structures solved from powder X-ray diffraction.

Strong negative thermal expansion in the siliceous zeolites ITQ-1, ITQ-3 and SSZ-23

Strong negative thermal expansion has been found for the pure SiO 2 zzeolites ITQ-1 (α v =–12.1×10 –6 K –1 ), SSZ-23 (α v =–10.3×10 –6 K –1 ) and ITQ-3 (α v =–11.4×10 –6 K –1 ) from 50 to 500 °C and above.

Hybrid functionalised mesoporous silica–polymer composites for enhanced analyte monitoring using optical sensors

We report herein a new optical sensor for the colorimetric monitoring of aqueous amines designed by anchoring a suitable reactand on a hybrid material that was further incorporated on a polymeric matrix. An azo chromophore containing a trifluoroacetophenone moiety was used as signalling reporter. In this system colour modulation is a consequence of the reversible reaction of the amine with the trifluoroacetyl group that results in the formation of a hemiaminal. Based on this approach but at the same time trying to enhance selectivity the chromoreactand was anchored in a first step onto suitable mesoporous nanoscopic hydrophobic pockets. As a mesoporous system we selected a UVM-7 solid (a MCM41-type material) containing a homogeneous distribution of pores of about 3 nm and a specific surface area of over 1000 m2/g. This material was chemically modified to yield a trifluoroacetylazobenzene indicator dye within the pores, resulting in the hybrid material S3. It was found that whereas the analogous trifluoroacetylazobenzene dye in solution gives an unspecific response, the hybrid material S3 shows enhanced features in terms of selectivity, only reacting with the not-too-large but lipophilic enough aminesn-octylamine and n-decylamine. The material S3 reacts with neither small hydrophilic amines (due to the hydrophobicity of the inner pores in S3) nor very large aliphatic amines (due to the size of the pore). In the second part of this work a composite material (P–S3) was prepared by mixing hydrophilic polyurethane polymer and S3. The X-ray diffraction patterns of P–S3 displays the characteristic intense peak of the MCM-41 type mesoporous material and TEM images show a homogeneous dispersion of the nanometric mesoporous particles into the polymeric matrix. The measurements were carried out in a flow-through module connected to an optical fibre via a Y-probe at pH 9.5 (borax buffer). The P–S3 composite retains the favourable features in terms of selectivity shown by S3 and only displays a clear change in colour for n-octylamine or n-decylamine, but no response to the remaining linear primary amines. The response time of the sensor P–S3 to solutions of these amines was in the range of a few seconds, the regeneration time was between 5 to 10 min and the detection limit was 6 × 10−4 mol L−1. The sensor response was reproducible and reversible.

The location of fluoride and organic guests in ‘as-made’ pure silica zeolites FER and CHA

The structure determination of as-made silica zeolites CHA and FER prepared in the presence of fluoride ions has been accomplished using microcrystal X-ray diffraction at a synchrotron source. In both cases, the location of the fluoride ions has been determined. Fluoride ions are incorporated into small double six-ring cages in the CHA framework, and the organic guests have been located in the larger chabazite cages. In contrast to two previous single-crystal X-ray diffraction studies of as-made siliceous FER prepared in related ways fluoride ions are incorporated into the framework and are found occluded inside small cages in the structure.

Hybrid materials with nanoscopic anion-binding pockets for the colorimetric sensing of phosphate in water using displacement assays.

Mesoporous amino-functionalised solids containing certain dyes have been used as suitable anion hosts in displacement assays for the colorimetric signalling of phosphate in water.

Efficient boron removal by using mesoporous matrices grafted with saccharides

Highly efficient boron removal from water was achieved by using mesoporous silica materials functionalised with saccharides.

Synthesis, characterization and control of faulting in STF/SFF topologies, a new family of intergrowth zeolites

A new family of zeolites, which can be described as intergrowths between the STF and SFF structure types has been prepared. The degree of faulting between the two end-member structures can be varied across the full range of possibilities, from almost pure STF through to almost pure SFF. This control over the faulting in the materials is accomplished by carefully changing the synthesis conditions and by addition of fluoride ions as a mineralising agent, but for the first time using the same structure directing agent and the same framework composition in all preparations. The materials have been characterized by a number of techniques, including X-ray diffraction, transmission electron microscopy and MAS that supports the intergrowth nature of the samples.