Table 1 shows the chemical composition of the prepared materials. For Al-TUD-1 and CrAl-TUD-1 the atomic ratios agree roughly with those used in the respective synthesis mixtures; for Cr-TUD-1 the Si/Cr ratio of 150 is higher than the value of 100 used in the synthesis, suggesting that a fraction of the initial amount of chromium was not incorporated in the mesoporous silicate.

The powder X-ray diffraction (XRD) patterns for Al-TUD-1, Cr-TUD-1 and CrAl-TUD-1 showed one broad peak at low angles (ca. 1.5° 2θ for Cr-TUD-1 and CrAl-TUD-1, and 1.8° 2θ for Al-TUD-1 (inset of Figure 1) and a very broad peak centred around 25° 2θ (not shown), indicating that these materials are amorphous, but have mesostructured material characteristics [28,29,30,32,33,34]. No evidence of crystalline phases (e.g., alumina or chromium oxides) was detected in the patterns. These materials exhibit type IV N₂ adsorption-desorption isotherms at −196 °C, with a H2 hysteresis loop (Figure 2a), which is consistent with an interconnected (worm-like) mesoporous network characteristic of TUD-1 type materials [30,31,32,35,36,37]. The capillary condensation in the mesopores occurs in the relative pressure range of about 0.4–0.6 for Al-TUD-1 and CrAl-TUD-1, and 0.5–0.8 for Cr-TUD-1, above which the adsorption branch levels off (suggesting that the external surface area is minor). Similar results were reported previously for metal-incorporated TUD-1 samples [28,29,30,31]. The texture properties of Al-TUD-1 and CrAl-TUD-1 are similar; in comparison, the S_BET for Cr-TUD-1 is lower, and the PSD is wider and for greater values of pore widths (Table 1, inset of Figure 2a).

The zeolite BEA and the related ion-exchanged Cr-BEA possess similar Si/Al ratios; in the latter case, the Si/Cr ratio is 47. Based on the aluminium contents it is possible to estimate that the BEA/TUD-1 and Cr-BEA/TUD-1 composites possess ca. 26 wt.% BEA and 33 wt.% Cr-BEA, respectively. In the case of the chromium-containing materials, the Al/Cr ratio is slightly higher for Cr-BEA/TUD-1 than for Cr-BEA (6 and 4, respectively), possibly due to partial leaching of chromium from BEA during the preparation of Cr-BEA/TUD-1. The XRD diffraction patterns of BEA and Cr-BEA are similar suggesting that the crystalline structure was preserved during the ion-exchange procedure (Figure 1). The XRD patterns of BEA and the related composite BEA/TUD-1 are similar, showing the characteristic diffraction peaks of zeolite Beta at 2θ = 7–8° and 22.5° [38]; the same applies when comparing the data for Cr-BEA and Cr-BEA/TUD-1.

The sorption isotherms for BEA and Cr-BEA show a significant increase in N₂ uptake at p/p₀ < 0.01, typically associated with the filling of micropores, followed by a gradual increase in N₂ uptake as relative pressures approach unity (Figure 2), most likely due to multilayer adsorption on the external surface of the nanocrystallites. The related composite materials BEA/TUD-1 and Cr-BEA/TUD-1 exhibit type IV isotherms (Figure 2), typical of mesoporous materials, with a hysteresis loop at p/p₀ > 0.4, which is associated with capillary condensation/evaporation in mesopores; for the two materials the PSD curves lie in the range of pore widths 2.5–10 nm (Table 1).

The high-resolution (HR) TEM images of Cr-BEA (exemplified in Figure 3a) showed small crystallites with a size of about 20–30 nm and the lattice fringes characteristic of zeolite Beta. In the case of Cr-BEA/TUD-1, HRTEM characterisation showed a three-dimensional sponge- or worm-like mesoporous matrix with some dark gray domains that may be attributed to the embedded zeolite particles (Figure 3b), suggesting that Cr-BEA/TUD-1 is a composite of an amorphous mesoporous matrix and nanocrystalline Beta particles (isolated nanocrystals of ca. 20–30 nm, or aggregates of ca. 50–200 nm), which are fairly evenly distributed nanocrystals in the surrounding mesoporous matrix. The presence of small aggregates is in agreement with previous findings for a Beta/TUD-1 composite material with a zeolite loading of 40 wt.% [38,39], and can be attributed to the combined effect of the synthesis conditions and the high zeolite loading. These results are comparable to those previously reported by us for a composite material consisting of zeolite Beta and TUD-1 prepared in a similar fashion (and using the same commercial zeolite ammonia Beta powder) to that used in the present work [39].

The yellow colour of all the chromium-containing samples suggests that the major species present was monochromate [40]. Accordingly, two very broad bands at ca. 275 (220–320 nm) and 365 (320–420 nm) in the DR UV-vis spectra (Figure 4) are assigned to O²⁻→Cr⁶⁺ charge transfer transitions of tetrahedrally coordinated isolated Cr^VI [30,40,41,42,43]. Similar results were reported previously for Cr-TUD-1 [30] and chromium-substituted BEA [42,43]. The shoulder at 450 nm (420–520 nm) may be due to dichromate or polychromate species [40,41], or, as proposed previously for Cr-TUD-1 [30], distorted isolated chromate species. The assignment of this band to octahedral Cr^III species (including Cr₂O₃-like clusters) can be excluded since no bands were detected at wavelengths greater than 600 nm, where octahedral Cr^III would be expected [30,41,42,43].

All reagents, solvents and standards were obtained from commercial sources and used as received: aluminum(III) isopropoxide (98%), tetraethylammonium hydroxide (TEAOH, 35 wt.% in water), tetraethylorthosilicate (TEOS, 98%), dimethylsulfoxide (DMSO, ≥99.9%), 1-butyl-3-methyl-imidazolium chloride ([bmim]Cl, ≥98%) and d-glucose from Aldrich, triethanolamine (TEA, 97%) and chromium nitrate (Cr(NO₃)₃·9H₂O, 99%) from ACRÓS, isopropanol (99%) and ethanol (99.8%) from Riedel-de Haën.

ICP-OES measurements for Si, Al and Cr (error ca. 5–10%) were carried out using a Horiba Jobin Yvon Activa M spectrometer, at the Central Laboratory for Analysis, University of Aveiro. Powder X-ray diffraction (XRD) data were collected at room temperature on a Philips X’Pert MPD diffractometer, equipped with an X’Celerator detector, a graphite monochromator (Cu-Kα X-radiation, λ = 1.54060 Å) and a flat-plate sample holder, in a Bragg-Brentano para-focusing optics configuration (40 kV, 50 mA). Samples were step-scanned in 0.04° 2θ steps with a counting time of 6 s per step. High-resolution transmission electron microscopy (HRTEM) was carried out on Hitachi H9000 and JEOL 2200FS instruments; the sample was prepared by spotting holey amorphous carbon film-coated 400-mesh copper grids (Agar Scientific, Stansted, UK) with a suspension of the solid sample in ethanol. The following textural parameters were estimated from N₂ adsorption isotherms measured at −196 °C using a Micromeritics Instrument Corp Gemini model 2380 (sample pre-treatment: 250 °C, vacuum): BET specific surface area (S_BET, calculated for relative pressures (p/p₀) in the range 0.02–0.10), pore volume (V_p, using the Gurvitch equation for p/p₀≅ 0.98), and BJH pore size distribution curve (PSD) calculated from the adsorption branch of the isotherm. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out under air, with a heating rate of 10 °C·min⁻¹, using Shimadzu TGA-50 and DSC-50 systems, respectively. The diffuse reflectance UV-vis (DR UV-vis) spectra were recorded using a Jasco V-560 spectrophotometer and BaSO₄ as reference. Attenuated total reflectance (ATR) FT-IR spectra were measured on a Mattson 7000 FT-IR spectrometer equipped with a Specac Golden Gate Mk II ATR accessory having a diamond top-plate and KRS-5 focusing lenses.

The aluminium and/or chromium-containing mesoporous silicas of the type TUD-1 were prepared by hydrothermal synthesis. In particular, Al-TUD-1 was prepared as described previously [20]. Specifically, TEOS (17.3 g, 83.0 mmol) was added to aluminium (III) isopropoxide (0.68 g, 3.33 mmol) dissolved in a mixture of isopropanol (6.5 mL) and ethanol (6.5 mL); tomic ratio Si/Al ≅ 25. After stirring for a few minutes, a mixture of TEA (12.51 g, 83.9 mmol) and water (9.4 g) was added, followed by addition of TEAOH (35 wt.% in water, 11.12 mL, 27.0 mmol) under vigorous stirring. The clear gel obtained was stirred at room temperature for 24 h and dried at 98 °C for 24 h, followed by hydrothermal treatment in a Teflon-lined stainless steel autoclave at 180 °C for 8 h. Finally, the solid was calcined at 600 °C in static air for 10 h (heating rate of 1 °C·min⁻¹).

CrAl-TUD-1 was prepared using the following procedure: TEOS (17.3 g, 83.0 mmol) was added to aluminium(III) isopropoxide (0.509 g, 2.5 mmol) and chromium nitrate (0.333 g, 0.8 mmol) dissolved in a mixture of isopropanol (6.5 mL) and ethanol (6.5 mL); atomic ratios Si/(Al+Cr) ≅ 25, Si/Cr ≅ 100, Si/Al ≅ 33. After stirring for 30 min, a mixture of TEA (12.5 g, 83 mmol) and water (5.09 g) was added, followed by addition of TEAOH (35 wt.% in water, 17.07 mL, 41.5 mmol) under vigorous stirring. The clear gel obtained was stirred at room temperature for 24 h and dried at 98 °C for 24 h, followed by hydrothermal treatment in a Teflon-lined stainless steel autoclave at 180 °C for 8 h. Finally, the solid was calcined at 600 °C in static air for 10 h (heating rate of 1 °C·min⁻¹) to give a yellow powder.

To prepare Cr-TUD-1, a solution of Cr(NO₃)₃·9H₂O (0.38 g, 0.9 mmol) in water (2 mL) was added dropwise to TEOS (19.91 g, 95.6 mmol); atomic ratio Si/Cr ≅ 100. While stirring, a solution of TEA (14.4 g, 96.5 mmol) in water (3.6 mL) was added dropwise, followed by the dropwise addition of TEAOH (35 wt.% in water, 19.7 mL, 47.8 mmol). After stirring for 2 h, a clear and pale green solution was obtained. The mixture was aged at room temperature for 24 h, dried at 100 °C for 24 h, and subsequently subject to hydrothermal treatment in a stainless steel Teflon-lined autoclave at 180 °C for 8 h. Finally the solid was calcined at 600 °C for 10 h in air (heating rate of 1 °C·min⁻¹), to give a yellow powder.

The prepared materials were manually ground using an agate pestle and mortar and subsequently sieved to give a powder with particle sizes of less than 106 μm width.

Commercial zeolite ammonia Beta powder (NH₄-BEA, Zeolyst, CP814) was calcined at 550 °C (1°·min⁻¹) in static air for 10 h, to give BEA. To prepare Cr-BEA, an aqueous suspension of NH₄-BEA (2 g) in 200 mL of 0.01 M Cr(NO₃)₃·9H₂O was stirred at 25 °C for 2 h; the solid was then filtered and the ion-exchange procedure repeated twice. Finally, the exchanged solid was filtered, thoroughly washed with deionised water at 60 °C, and calcined at 500 °C for 6 h in air (heating rate of 1 °C·min⁻¹). The final solid was a yellow powder.

The composite BEA/TUD-1 was prepared as described previously using BEA zeolite [37]. A similar procedure was used to prepare the composite Cr-BEA/TUD-1. Firstly, BEA was ion-exchanged with chromium using the same procedure as that described above for Cr-BEA, excluding the final calcination step, giving a green powder. Subsequently, TEOS (5.34 g, 0.025 mmol) was added dropwise to a stirred suspension of this powder (1.00 g) in a mixture of TEA (3.85 g, 25 mmol) and water (2.99 g). Then, TEAOH (35 wt.% in water, 3.45 g, 7.5 mmol) was added to the suspension and stirring was continued for 2 h. The gel was aged at room temperature for 24 h, followed by drying at 100 °C for 24 h. The solid material was transferred to a Teflon-lined autoclave and heated at 180 °C for 8 h under static conditions. Finally, the solid was calcined at 600 °C for 10 h in air (heating rate of 1 °C·min⁻¹). The final solid was a yellow powder.

The prepared materials were manually ground using an agate pestle and mortar and subsequently sieved to give a powder with particle sizes of less than 106 μm width.

Batch catalytic experiments were performed under nitrogen in a tubular glass micro-reactor equipped with a valve for gas purging. In a typical procedure, glucose (Glu, 30 mg), powdered catalyst (30 mg), and [bmim]Cl (0.6 mL) were poured into the reactor. The reaction mixtures were stirred magnetically at 1,000 rpm and heated with a thermostatically controlled oil bath (120 °C). Zero time was taken to be the instant the micro-reactor was immersed in the oil bath. The reaction products were analysed using a Knauer K-1001 HPLC pump, coupled to a Knauer 2300 differential refractive index detector (for sugars) and a Knauer 2600 UV detector (280 nm, for Hmf). A PL Hi-Plex Ca 300 mm × 7.7 mm (i.d.) ion exchange column (Polymer Laboratories Ltd., UK) was used: the mobile phase was milli-Q water; flow rate 0.5 mL·min⁻¹; column temperature 80 °C. Authentic samples of reagents and products were used as standards for measuring calibration curves. Conversion (%) was calculated as 100 × (moles of Glu consumed)/(initial moles of Glu), and the Hmf yield (mol%) was calculated as 100 × (moles Hmf formed)/(initial moles of Glu). In order to check the data, the experiments were carried out in duplicate and the mean values were calculated.