Santiago Gómez-Quero

Exclusive production of chloroaniline from chloronitrobenzene over Au/TiO2 and Au/Al2O3.

The gas-phase continuous hydrogenation of p-chloronitrobenzene (p-CNB) over 1 mol% Au/TiO2 and Au/Al2O3 was compared for the first time. Both catalysts exhibit 100% selectivity in terms of -NO2 group reduction, resulting in the sole formation of p-chloroaniline (p-CAN). Au/TiO2 exhibited a narrower particle size (1-10 nm) distribution than Au/Al2O3 (1-20 nm) and a smaller surface-area-weighted mean Au size (6 nm versus 9 nm). Au/TiO2 delivered a higher specific hydrogenation rate (by a factor of up to four), a response that is discussed in terms of Au particle size and a possible contribution of the support to p-CNB activation. A CNB isomer reactivity sequence was established, that is, o> p> m, which is attributed to resonance stabilisation effects. The results presented establish a basis for the development of a sustainable alternative route for the production of haloamines.

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al2O3, TiO2, Fe2O3 and CeO2) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO2 bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO2 catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.

Alumina-Supported Ni-Au: Surface Synergistic Effects in Catalytic Hydrodechlorination

Catalytic gas‐phase hydrodechlorination (HDC) of 2,4‐dichlorophenol (2,4‐DCP) has been investigated over Ni/Al2O3 and Au/Al2O3 prepared by impregnation, and Au–Ni/Al2O3 prepared by reductive deposition of Au onto Ni. Catalyst activation by temperature‐programmed reduction is examined and the activated catalysts are characterized in terms of H2 chemisorption, XRD and TEM‐energy dispersive X‐ray (EDX) measurements. Ni/Al2O3 (<1–10 nm) and Au/Al2O3 (<1–15 nm) exhibit a relatively narrow metal size distribution while Au–Ni/Al2O3 bore larger particles (1–30 nm) with variable surface Ni/Au ratios. Au/Al2O3 exhibits low H2 uptake and low HDC activity to generate 2‐chlorophenol (2‐CP) as the sole product. H2 chemisorption on Au–Ni/Al2O3 was approximately five times lower than that recorded for Ni/Al2O3 but both catalysts delivered equivalent initial HDC activities. Ni/Al2O3 exhibits an irreversible temporal deactivation where partial dechlorination to 2‐CP is increasingly favored over full dechlorination to phenol. In contrast, thermal treatment of Au–Ni/Al2O3 in H2 after reaction elevates HDC activity with a preferential full HDC to phenol. This response is linked to a surface reconstruction resulting in a more homogeneous combination of Ni and Au. This result was also achieved by a direct treatment of Au–Ni/Al2O3 with HCl. A parallel/ consecutive kinetic model is used to quantify the catalytic HDC response.

Novel and effective copper-aluminum propane dehydrogenation catalysts

The importance of solid catalysts for converting petro- and bulk chemicals is reflected in the sheer magnitude of their market size: catalysts’ sales topped nine billion dollars in 2009.[1] This large value mirrors also the increasing academic interest in heterogeneous catalysis research.[2] As far as bulk chemicals (such as ethene, propene, and their derivatives) are concerned, there is a strong demand for clean and inexpensive catalysts and synthesis processes.[3] There are two types of commonly used dehydrogenation catalysts: supported Cr oxides[4] and Pt-based[5] systems. The problem is that these catalysts are typically either rare and costly, or hazardous. Moreover, they perform well only at high temperatures[6] (typically at 500–600 °C) due to thermodynamic limitations.[7] Optimization studies have led to the inclusion of several promoters of which tin, especially in the combination with platinum as Pt–Sn/Al2O3 is one of the most popular. We report here the discovery of a new alternative catalyst for propane dehydrogenation which does not contain noble or hazardous metals. It is an oxidized porous Cu–Al alloy with a structure that is similar to Raney-type metals.[8] The Raney process, patented by Murray Raney in 1925 and commercialized by W.R. Grace & Co.,[9] is one of the most successful routes for making porous metals. The problem is that this process requires extreme conditions that often restrict the final outcome at the nanometric scale. Here we opted for a different approach, applying a modified version of the ultrasound pore formation method that we have recently reported: high-power ultrasound.[10] In a typical synthesis, Cu and Al beads are melted by using an electric arc. The resulting cake is then pulverized and sonicated in water. This gives a highly porous material containing pores predominantly at the micro-scale (see Figure 1 a and 1 b for a representative example; catalyst D).[11] Figure 1 c and 1 d shows transmission and scanning electron micrographs of such a catalyst. We hypothesize that the sonication creates pores in the Al component followed by surface oxidation (compare the XRD profiles (a) and (b) in Figure 2), whereas Cu supplies the active centers for the catalysis (vide infra). The thickness of the ultra thin oxide layer was estimated by using 3D field ion microscopy as less than 2.0 nm.[10a] Figure 1 Porous Al–Cu alloy D N2 adsorption/desorption isotherms (a), pore size distributions (b), scanning electron micrographs (c) and transmission electron micrograph (d). Figure 2 X-ray diffraction patterns of the porous Al-Cu catalyst D before (a) and after (b) ultrasound treatment (the JCPDS-ICDD standards are also included for ease of comparison). We prepared a series of catalysts with different Cu content (Table 1, entries 1–4), and found that using 25 wt % Cu (catalyst D) gave the most promising results. This catalyst was then activated under different conditions in an effort to optimize the preparation recipe (see entries 4–6). We see a reduction of conversion in the first minutes, probably reflecting some initial sintering and coke deposition (see Figure 3).[7], [12] After this short deactivation period, the catalyst maintains its steady-state activity (all values hereafter refer to the steady-state period). Our catalyst gave reasonable propane consumption rates already at 550 °C (see Table 1). Note that all the reactions gave very good reproducibility (±7 % for different samples from the same catalyst batch). However, if we look at the theoretical phase diagram of Al–Cu, we see that it shows an eutectic point at 548 °C.[13] True, our catalyst is not a pure Al–Cu alloy (since at least its surface is passivated with an oxide layer; see Figure 2). Nevertheless, we hypothesized that a partial melting occurs during the pre-treatment at 600 °C (and possibly even during the reaction at 550 °C). Even if only part of the catalyst were melting, it would be perforce the active part. This is because the first sites that would melt would be the high-energy kinks and breaks where catalysis usually happens.[14] Indeed, when we compared samples A and B that had less Cu but a larger particle size (typically>150 μm), we saw that these were more active than those with more copper but smaller sizes. To check this hypothesis, we prepared another batch of the same catalyst D, but this time activated at 400 °C (all other conditions identical). We then ran the dehydrogenation again, this time 200 degrees lower (i.e. at 350 °C). Excitingly, as Figure 3 shows, this catalyst gave greater conversions, reaching a stable 4 % on stream. This is equivalent to a constant rate of 0.83 mol h−1 g−1. This result is all the more remarkable considering the temperature difference: a 200 °C offset would be expected to slow down the reaction by approximately an order of magnitude (all other known catalysts are inactive under these conditions). For comparison purposes, we tested a standard Pt–Sn/Al2O3 catalyst under similar conditions. This catalyst has been shown in the available literature as the best in terms of activity/selectivity/stability for propane dehydrogenation.[15] Under the same reaction conditions, Pt–Sn/Al2O3 was practically inactive at 350 °C and gave less than 1 % conversion (<0.2 mol h−1 g−1) at 550 °C. Searching the literature, we did not find any reports on propane dehydrogenation over Cu/Al2O3. But, we note the increase in rate quoted by Sokolova et al.[16] when adding Cu to Pt/Al2O3. Table 1 Composition, surface area and initial dehydrogenation rate for Al–Cu catalysts A–D. Figure 3 Temporal propane conversion for catalyst D at 350 °C (•) and 550 °C (○). Inset: relationship between the catalyst space time (i.e. amount of catalyst per propane molar flow pass) and initial propane conversion. Note: the ... In conclusion, we show here that high-power ultrasound is a green chemistry tool for the synthesis of porous copper–aluminum frameworks stabilized by metal oxide. Furthermore, this material is inexpensive (production expenses are approximately 3 € per liter) and the method can be easily scaled-up by using different sonotrodes (or a series of them), as these may vary widely in size and shape. These new porous materials (or “metal sponges”) have an alloy bulk and an oxidized surface, and can catalyze propane dehydrogenation at low temperatures. Thanks to their high activity and because they contain no noble metals, they open exciting opportunities in low-temperature dehydrogenation catalysis for making bulk chemicals.

Kinetics of propane dehydrogenation over Pt–Sn/Al2O3

The kinetics of the gas-phase dehydrogenation of propane over Pt–Sn (1 : 1 mol ratio) supported on Al2O3 was investigated for the first time over the high range of reactant/products partial pressures (up to 0.875 atm). The Pt precursor was reduced to metallic form after a temperature-programmed reduction (TPR) at 873 K. X-ray photoelectron spectroscopy (XPS) analysis suggests that a Pt–Sn surface alloy forms, decreasing the H2 adsorption on the Pt–Sn sites (5.6 nm in average size) relative to monometallic Pt (6.8 nm). We performed kinetic studies in the absence of mass/heat transfer limitations. The incorporation of Sn into Pt in Pt–Sn/Al2O3 enhanced the catalytic activity and stability when compared to Pt/Al2O3. We attribute this response to the surface electronic interaction between Pt and Sn. The initial propane consumption rate increases with the partial pressure of propane and decreases with the partial pressure of propene, while varying that of hydrogen has a negligible effect. Applying the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach, the most likely kinetic model is non-dissociative adsorption of propane with simultaneous release of H2, where surface reaction is the rate-limiting step. Our results of the kinetic aspects provide practical insights relevant to propane dehydrogenation.

Gold nano–particles supported on hematite and magnetite as highly selective catalysts for the hydrogenation of nitro–aromatics

The catalytic action of nano–sized Au particles supported on hematite (Fe2O3) and magnetite (Fe3O4) is compared in the continuous gas phase hydrogenation of p–chloronitrobenzene and m–dinitrobenzene. The catalysts were prepared by deposition–precipitation and have been characterised in terms of BET/pore volume, powder X–ray diffraction (XRD), temperature programmed reduction (TPR), H2 chemisorption, high–resolution transmission electron microscopy (HRTEM) and X–ray photoelectron spectroscopy (XPS) measurements. XRD confirmed the formation Fe2O3, which was transformed into Fe3O4 during TPR to 673 K with a concomitant decrease in BET area and pore volume. Post–TPR to 423 K, Au/Fe2O3 exhibited well dispersed pseudo–spherical Au particles with mean diameter = 2.0 nm. HRTEM and XPS demonstrate the encapsulation of Au in the Fe3O4 matrix after TPR to 423 K, which inhibited hydrogenation rate. Thermal treatment to 673 K resulted in the segregation of Au on the Fe3O4 surface and the formation of nano–scale particles with mean diameter = 4.0 nm. Similar activities were recorded over both Au/Fe2O3 and Au/Fe3O4 with exclusive nitro–group reduction to yield p–chloroaniline and m–nitroaniline, a response that is discussed in terms of Au electronic character.

Nano-scale Au supported on Fe3O4: characterization and application in the catalytic treatment of 2,4-dichlorophenol

Catalytic hydrodechlorination (HDC) is an effective means of detoxifying chlorinated waste. Gold nanoparticles supported on Fe(3)O(4) have been tested in the gas phase (1 atm, 423 K) HDC of 2,4-dichlorophenol. Two 1% w/w supported gold catalysts have been prepared by: (i) stepwise deposition of Au on α-Fe(2)O(3) with subsequent temperature-programmed reduction at 673 K (Au/Fe(3)O(4)-step); (ii) direct deposition of Au on Fe(3)O(4) (Au/Fe(3)O(4)-dir). TEM analysis has established the presence of Au at the nano-scale with a greater mean diameter (7.6 nm) on Au/Fe(3)O(4)-dir relative to Au/Fe(3)O(4)-step (4.5 nm). We account for this difference in terms of stronger (electrostatic) precursor/support interactions in the latter that can be associated with the lower pH point of zero charge (with respect to the final deposition pH) for Fe(2)O(3). Both catalysts promoted the preferential removal of the ortho-Cl substituent in 2,4-dichlorophenol, generating 4-chlorophenol and phenol as products of partial and total HDC, respectively, where Au/Fe(3)O(4)-step delivered a two-fold higher rate (2 × 10(-4) mol(Cl) h(-1) m(Au)(-2)) when compared with Au/Fe(3)O(4)-dir. This unprecedented selectivity response is attributed to activation of the ortho-C-Cl bond via interaction with electron-deficient Au nanoparticles. The results demonstrate the feasibility of a controlled recovery/recycling of chlorophenol waste using nano-structured Au catalysts.