Mahfoud Ziyad

Characterization of active sites on AgHf2(PO4)3 in butan-2-ol conversion

Abstract The silver–hafnium phosphate (AgHf 2 (PO 4 ) 3 ), belonging to the Nasicon-type structure, was synthesized by a sol–gel method and characterized by several techniques. The Ag + cations were found to be, as in zeolite frameworks, easy to reduce to metallic silver. This reduction was investigated by STEM/EDX which showed that, under an intense electron beam, the Ag + ions diffuse toward the surface of the sample and form metallic particles of sizes varying from 3 to 15 nm. The X-ray diffraction patterns confirmed that the reduction does not damage the phosphate structure. The Ag + ions were replaced in the structure by protons giving rise to (PO–H) acid groups. The catalytic behavior of AgHf 2 (PO 4 ) 3 was studied in butan-2-ol conversion. In the absence of O 2 , the reaction leads to an abrupt decrease of the dehydrogenation activity and a dehydration reaction that reaches a pseudo-stationary state. Concomittantly, the Ag + cations are reduced to Ag 0 . In the presence of O 2 , the dehydrogenation reaction undergoes a complex transitory state which can be decomposed into two steps. During the first one, the activity decreases in concert with the reduction of Ag + ions. In the second step it increases as the amount of oxidized silver species (Ag x O y ), which are the active sites for the alcohol dehydrogenation, increases. UV–VIS characterization of AgHf 2 (PO 4 ) 3 in dynamic conditions similar to those used in the catalytic tests confirmed that butan-2-ol reduces Ag + ions to metallic silver which, in the presence of O 2 in the reaction mixture, are oxidized to (Ag x O y ).

Hydrogenation of p‐Chloronitrobenzene over Nanostructured‐Carbon‐Supported Ruthenium Catalysts

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been used for the first time to support ruthenium nanoparticles for the hydrogenation of p-chloronitrobenzene (p-CNB) to produce selectively p-chloroaniline. The preparation of well-dispersed ruthenium catalysts from the [Ru(3)(CO)(12)] precursor required activation of the purified supports by nitric acid oxidation. The supports, purified and functionalized, and the supported catalysts have been characterized by a range of techniques. The catalytic activity of these materials for the hydrogenation of p-CNB at 35 bar and 60 °C is shown to reach as high as 18 mol(p-CNB)g(Ru)(-1) h(-1), which is one order of magnitude higher than a commercial Ru/Al(2)O(3) catalyst. Selectivities between 92 and 94 % are systematically obtained, the major byproduct being aniline.

Oxidative dehydrogenation of ethane on V- and Cr-based phosphate catalysts

Oxidative dehydrogenation of ethane to ethylene has been studied on V- and Cr-based phosphate catalysts at 550°C. yielding almost exclusively ethylene and COx. Zirconium hydrogenophosphates in their α or β phase were either submitted to cationic exchange of protons by VO2+ and Cr3+ salts or impregnated by such salts and then compared to (VO)2P2O7 and CrPO4 pure phases. VO2+ and Cr3+ on the αZr(HPO4)2·2H2O phase were observed to be more selective toward ethylene than the same cations deposited or exchanged on βZr(HPO4)2 and than the corresponding oxides V2O5 and Cr2O3 either pure (bulk) or deposited on supports as SiO2, ZrO2 or ZrP2O7. Moreover, (VO)2P2O7 (and to a lesser extent CrPO4) were observed to be even more selective toward ethylene at similar conversion levels. However, in terms of cation content per weight, the better yields of ethylene were obtained for αZrCrP and CrαZrP samples. The turnover number of ethane conversion per cation accessible was 0.26 min−1 for both V and Cr cations exchanged or supported on an αZrP phase. Ultraviolet (UV) and electron spin resonance (ESR) spectroscopic techniques, which are very sensitive to the oxidation state and environment symmetry of V and Cr cations, were used in addition to structural and morphological determination techniques such as X-ray diffraction (XRD), infrared (IR), BET, scanning electron microscopy (SEM), etc. Characterization was done after calcination in air at 500550°C and after catalytic testing at 550°C. All data allowed us to conclude that catalytic features are related to V and Cr local arrangements (small clusters or chain arrangements) and to the counter anion, O2− or PO43−. The best catalyst for ethane oxidative dehydrogenation consisted of VO2+ and Cr3− chains separated by PO43− anions, which are stronger bases in the sense of Pearson than O2− anions.

Cobalt speciation in cobalt oxide-apatite materials : structure-properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion

Impregnation of calcium hydroxyapatite by a solution of Co(II) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are isolated six-coordinated Co2+ ions. For Co content ≥0.4 wt%, the presence of tetrahedral Co2+ and octahedral Co3+ species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered CoxOy entities. For Co content ≥1.7 wt%, Co3O4 nanocrystals are generated, as evidenced by XRD and magnetic measurements.In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co2+ ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co2+ ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.

Oxidative dehydrogenation of ethane and propane over magnesium–cobalt phosphates CoxMg3−x(PO4)2

The magnesium–cobalt phosphates CoxMg3−x(PO4)2 belonging to the olivine-type structure were synthesized by coprecipitation and then investigated in the oxidative dehydrogenation (ODH) of ethane and propane. The best yields, with the exception of Co0.5Mg2.5(PO4)2, were achieved with the compositions ranging between 1≤x≤2.5. Magnesium phosphate Mg3(PO4)2 displayed no activity and pure cobalt phosphate Co3(PO4)2 was found to be the less active component of the solid solution. Comparison of the catalysts performances showed that they all have similar activity in ethane and propane ODH, albeit, they are more selective in propylene than in ethylene production. The CoxMg3−x(PO4)2 solid solution was also studied, for characterization purposes, in butan-2-ol conversion. The samples presented acid–base properties due essentially to the (PO–H) groups but they do not bear conventional redox centers. All the catalysts were active at low temperatures in the alcohol dehydration. The dehydrogenation activity versus the phosphates composition displayed two maxima around x=1 and 2, respectively. Similar striking behavior was also observed in ethane and propane ODH. UV-visible investigations of CoxMg3−x(PO4)2 showed, in agreement with the XRD data, that the Co2+ ions are distributed in the phosphate framework between six- and five-coordinated sites. The cobalt atoms in the five-coordinated sites Co(5) and their Co(5)–Co(5) interatomic distances were assumed to play the main role in the C–H bond activation and the appearance of maxima in the activity. Magnesium cations presumably intervene in acid–base properties of the samples and O2 activation. Characterization of the samples showed that they do not undergo any noticeable transformation after the catalytic tests.

Conversion of isopropyl alcohol over Ru and Pd loaded N-doped carbon nanotubes

Abstract Ru and Pd (2 wt%) loaded on pure and on N-doped carbon nanotubes (N-CNTs) were prepared and tested using the isopropyl alcohol decomposition reaction as probe reaction. The presence of nitrogen functionalities (pyridinic, pyrrolic, and quaternary nitrogen) on the nitrogen doped support induced a higher metal dispersion: Pd/N-CNT (1.8 nm)

Synthesis and properties of copper-hafnium triphosphate CuIHf2(PO4)3

Monovalent copper-hafnium triphosphate Cu{sup I}Hf{sub 2}(PO{sub 4}){sub 3} was obtained in a single phase using sol-gel type methods and solid state reactions. It was characterized by X-ray diffraction, {sup 31}P MAS-NMR and diffuse reflectance spectroscopies. Its crystallographic parameters were found to be similar to those of Nasicon type compounds Cu{sup I}M{sub 2}(PO{sub 4}){sub 3} (M = Ti, Sn, Zr). Moreover, results of the thermogravimetric analysis and EDX-STEM microscopy were consistent with the structure features and the existence of the reversible redox system Cu{sup 2+}/Cu{sup +} in the network of the phosphate. Under the electronic beam of the microscope, the Cu{sup +} ions diffuse throughout the conduction tunnels to give on the surface of the material crystallites and whiskers of metallic copper. This migration process does not affect notably the structure of the phosphate.

Biodiesel From Moroccan Waste Frying Oil: The Optimization of Transesterification Parameters Impact of Biodiesel on the Petrodiesel Lubricity and Combustion

The transesterification reaction of Moroccan waste frying oil (WFO) by methanol, using sodium hydroxide and potassium hydroxide as homogeneous catalysts, was studied. The waste oil was recovered from Moroccan popular restaurants after repeated deep-frying of spiced fishes. The effects of methanol/oil molar ratio (5:1–12:1), catalyst concentration (0.5–2 wt%), reaction temperature (30–65°C), and type of catalyst were investigated. The extent of the transesterification reaction was followed by gas chromatography (GC) and 1H-NMR spectroscopy, determining the concentration of methyl esters at different reaction times and triglyceride conversion respectively. The optimal reaction conditions for the transesterification of WFO were found at the reaction temperature of 65°C, reaction time of 60 min, molar ratio of methanol to oil equal to 7:1, and in presence of NaOH as catalyst. The corresponding maximum ester yields were 93.5 wt% for the studied WFO. The lubricity test showed that 1 vol% of biodiesel in petrodiesel leads to the value fixed by the European regulation, namely 460 μm. The optimized conditions were used to produce biodiesel at small scale. The resulting product was tested in a diesel electricity generator engine, which operated in real conditions. The results showed that biodiesel combustion leads to a higher concentration of CO and a decrease in NOx emission as compared with a petrodiesel-fuelled engine. An optimization of the operating parameters of the engine would guarantee lower CO emissions in conformity with literature and regulations.

Structural and spectroscopic properties of calcium-iron Ca9Fe(PO4)7 phosphate

Abstract Fresh (f-) and reduced (r-) Ca 9 Fe(PO 4 ) 7 whitlockite-like compounds were prepared and characterized by X-ray diffraction (XRD), IR, temperature programmed reduction and oxidation (TPR/TPO), UV-visible, electron paramagnetic resonance (EPR), and 57 Fe Mossbauer spectroscopy. TPR/TPO, XRD, and IR investigations showed that cycles of reduction and oxidation reversibly affected the iron cations without noticeable modification of the structure. The reduction by H 2 was accompanied by a protonation of the sample and a slight increase of the unit-cell parameters. It was observed that the reduction of Fe 3+ to Fe 2+ never exceeded 91%. The simulation of f-Ca 9 Fe(PO 4 ) 7 EPR spectra and the Mossbauer investigations confirmed that the Fe 3+ cations are located in highly symmetrical sites. These techniques also revealed that in r-Ca 9 Fe(PO 4 ) 7 the residual ferric ions are in a rhombic symmetry and the Fe 2+ ions are in octahedral sites slightly distorted by the Jahn-Teller effect.

Synthesis and properties of a new copper(II)-hafnium phosphate Cu0 5Hf2(PO4)3

Abstract Phosphates of general formula M 0.5 Hf 2 (PO 4 ) 3 with M=Cd 2+ , Ca 2+ , Sr 2+ and Cu 2+ were prepared by coprecipitation and characterized by several physical techniques. The compounds containing Cd 2+ , Ca 2+ , Sr 2+ belong to the Nasicon-type structure, whereas Cu 0.5 Hf 2 (PO 4 ) 3 exhibited substantially different DRX patterns. Combined temperature programmed reduction (TPR) and temperature-programmed oxidation (TPO) showed that the copper in Cu 0.5 Hf 2 (PO 4 ) 3 was distributed between two energetically different sites in proportions respectively equal to 40 and 60%. Electron Paramagnetic Resonance (EPR) investigations confirmed the TPR/TPO results and revealed that the two sites hosting the Cu 2+ ions are of orthorhombic symmetry. Moreover, the Cu 2+ ions might be reduced by hydrogen to Cu + . These results were also supported by the UV–visible studies that showed the disappearance, under reducing conditions, of the band corresponding to crystal field transitions of Cu 2+ ions and the emergence of a new peak attributed to the transitions between (3d) 10 and (3d) 9 (4s) 1 Cu + levels. At the same time, IR spectroscopy confirmed that protons entered the open lattice framework of the material and gave rise to a new protonated phase containing monovalent copper Cu 0.5 I H 0.5 Hf 2 (PO 4 ) 3 . This redox process was proven to be reversible without any subsequent change in the network of the phosphate.