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International audience; Adsorption of N-2 at 77 K and scanning electron microscopy have been used to measure the changes in the support morphology, at nano- and microscale level, along the processes involved in the preparation of a supported Pd catalyst: Pd deposition, doping, and thermal treatments. Among the investigated supports, viz., activated carbons, gamma-Al2O3, SiO2, and SiO2-Al2O3 (SA), the SA one was found particularly sensitive to these processes, as a result of its high plasticity and reactivity. Involved processes can be summarized as follows: (i) During the I'd deposition, the support itself is partially dissolved and removed as a result of both the basicity of the precipitating agent and the final washing. (ii) When the undoped sample is thermally treated up to 823 K, only modest phenomena are observed. (iii) Upon doping with potassium carbonate, the support dissolution continues, and the greater the carbonate concentration, the greater the dissolution extent. In this case the dissolved material is not removed, but reprecipitates (partially outside the pores), during the subsequent drying at 393 K. (iv) When doped samples are thermally treated, the reaction between carbonate and support causes the mobilization of the support itself, with sintering phenomena that can reach the total collapse of the porous structure. The starting temperature of the pore collapse decreases with increasing potassium carbonate concentration. The modification of the support influences, directly or indirectly, the surface properties and the availability of Pd particles that can be doped or even covered by materials from support and made more or less accessible or even inaccessible by pore narrowing, widening, or blocking.

A complete catalytic cycle was performed on CuCl2/Al2O3 catalyst for ethylene oxychlorination at 500 K. X-ray absorption near-edge spectroscopy, extended X-ray absorption fine structure, electron paramagnetic resonance, and IR of adsorbed CO were used to demonstrate that the ethylene oxychlorination reaction, C2H4+2HCl+1/2 O2→C2H4Cl2+H2O, follows a three-step mechanism: (i) reduction of CuCl2 to CuCl (2CuCl2+C2H4→C2H4Cl2+2CuCl), (ii) oxidation of CuCl to give an oxychloride (2CuCl+1/2 O2→Cu2OCl2), and (iii) closure of the catalytic circle by rechlorination with HCl, restoring the original CuCl2 (Cu2OCl2+2HCl→2CuCl2+H2O). The dispersing/sintering effect of the different reagents on the active phase has been also investigated.

Alumina supported CuCl2, the basic catalyst for ethylene oxychlorination, has been investigated by UV-Vis spectroscopy, EPR, EXAFS and XANES in a wide range (0.25-9.0 wt%) of Cu concentration. We have evidenced that, at low Cu content, the formation of a surface aluminate species takes place. The formation of this surface copper aluminate stops at 0.95 wt% Cu / 100 m2; at higher Cu concentrations excess copper chloride precipitates directly from solution during the drying step forming an highly dispersed CuCl2.H2O, phase, overlapping progressively the surface aluminate. Depletion tests and IR spectroscopy of adsorbed NO have demonstrated that the latter is the only active phase. A complete catalytic cycle has then been performed on CuCl2/Al2O3 catalyst. EPR, XANES and EXAFS, have been used to demonstrate that the ethylene oxychlorination reaction: C2H4 + 2HCl + ½ O2 --> C2H4Cl2 + H2O follows a three steps mechanism: (i) reduction of CuCl2 to CuCl (2CuCl2 + C2H4 --> C2H4Cl2 + 2CuCl), (ii) oxidation of CuCl to give an oxychloride (2CuCl + ½ O2 --> Cu2OCl2) and (iii) closure of the catalytic circle by rechlorination with HCl, restoring the original CuCl2 (Cu2OCl2 + 2HCl --> 2CuCl2 + H2O). Finally, we have shown that time resolved, in situ, spectroscopy is a very promising technique to investigate the interplay between catalyst activity and oxidation state of copper.

The conversion of ethylchloride into ethylene+HCl on pure and doped alumina supports and on CuCl 2 –Al 2 O 3 -based oxychlorination catalysts has been investigated by pulse reactor and FT-IR spectroscopy. FT-IR spectra of ethylchloride adsorbed on γ-Al 2 O 3 show weakly molecularly adsorbed species and ethoxy groups formed by nucleophilic substitution. Additionally, adsorbed diethylether is also observed. The analysis of the gas-phase species shows that ethoxy groups decompose, giving rise to ethylene at 523 K. Under the same conditions, gaseous HCl is also released from the surface and diethylether is also observed in the gas phase. Chlorination of alumina with HCl only partially hinders the dehydrochlorination mechanism occurring through ethoxy groups. Experiments performed on alumina doped with MgCl 2 and KCl indicate that the site reactivity scale for EtCl conversion to ethylene is the following: bare Al 2 O 3 >CuCl 2 on Al 2 O 3 >MgCl 2 on Al 2 O 3 >KCl on Al 2 O 3 . Doping with MgCl 2 and in particular with KCl limits the activity of the bare support in the dehydrochlorination. These data strongly support the previous proposal that the exposed alumina surface catalyzes the dehydrochlorination of EDC to VCM, with a consequent loss in selectivity in the oxychlorination reactor. The beneficial effect of doping with KCl and MgCl 2 is due to the reduced dehydrochlorination activity.

The effect of aging and heating treatments up to 500 K on alumina-supported CuCl2, i.e., the base catalyst for ethylene oxychlorination, has been investigated by UV‐vis spectroscopy, a solubility test, EXAFS, XRD, and EPR in a wide range (0.25‐9 wt%) of Cu concentration. It is shown that the catalyst undergoes significant changes with both time and thermal treatments, so accounting for some contradictory results reported in the literature. While the surface Cu aluminate (formed during impregnation) does not change upon aging and heating, supported CuCl2 (precipitated from impregnating solution during the drying process) undergoes a slow hydrolysis reaction with the formation of paratacamite and HCl. The HCl formed during the hydrolysis reacts with the alumina surface with the formation of >Al‐Cl species. Upon heating, the initially formed paratacamite can react with surface >Al‐Cl species with nearly total restoration of CuCl2, which is consequently the main species present on the catalyst at the beginning of the oxychlorination reaction. The obtained picture is able to explain the results emerging from activity tests on the whole set of catalysts, indicating that surface aluminate is not active and that the active phase is CuCl2. c ∞ 2000 Academic Press

CuCl 2 /Al 2 O 3 ethylene oxychlorination catalysts have been characterized by using IR spectroscopy of the surface hydroxy-groups and of adsorbed pyridine and CO 2 . The ethylene oxychlorination reaction to 1,2-dichloroethane has been investigated by using pulse reactor studies and in-situ IR experiments in static conditions. The results allowed emphasizing the negative role of uncovered alumina surface, evidenced by IR experiments, considered to be responsible for the dehydrochlorination of 1,2-dichloroethane with a consequent loss in selectivity that is very important in low Cu-loading samples.

The understanding, at the atomic level, of the role played by additives (dopants or promoters) in the chemistry of an industrial catalyst is a very complex and difficult task. We succeeded in this goal for the ethylene oxychlorination catalyst (CuCl(2)/gamma-Al(2)O(3)), used to produce dichloroethane, a key intermediate of the polyvinyl chloride chemistry (PVC). Among the most used additives for both fluid and fixed beds technologies (LiCl, KCl, CsCl, MgCl(2), LaCl(3), CeCl(4)) we have been able to highlight that KCl, and CsCl, forming in reaction conditions a mixed phase with CuCl(2), strongly modify the catalyst behaviour. In particular, these additives are able to displace the rate determining step from the CuCl oxidation (undoped catalyst) to the CuCl(2) reduction. This results from the decrease of the rate of the latter reaction, thus the overall activity of the system. For all remaining additives the rate determining step remains the CuCl oxidation, as for the undoped catalyst. These results have been obtained coupling the catalyst activity monitored with a pulse reactor working in both non-depletive and depletive modes with time resolved XANES spectroscopy performed under in operando conditions (i.e. coupled with mass spectrometry). Formation of CuK(x)Cl(2+x) and CuCs(x)Cl(2+x) mixed phases has been proved monitoring the Cu(II) d-d transitions with UV-Vis spectrometer and the CO stretching frequency of carbon monoxide adsorbed on reduced catalyst by in situ IR spectroscopy. Finally, of high relevance is the observation that the fully oxidized catalyst is inactive. This unexpected evidence highlight the role of coordinatively unsaturated Cu(I) species in adsorbing ethylene on the catalyst surface indicating that copper, in the working catalyst, exhibits a (I)/(II) mixed valence state.

In paper 1 we have shown that the structural properties of silicalites depend upon the preparation procedure. In particular Na- and Al-free sample (S) prepared following a specifically designed method is characterized by the presence of internal atomic-dimension defects (nanodefects and microcavities). On the basis of spectroscopic data and computer graphic simulations, it is shown that the microcavities derive from one or more missing [SiO 4 ] units and can contain up to four OH groups per missing tetrahedron

We present a methodology that allows us to extract quantitative information from EXAFS data collected on complex samples where the absorbing species is present in more than one phase. We have chosen as our case study the CuCl2/Al2O3 material, which represents the basic catalyst for the ethylene oxychlorination reaction (a fundamental intermediate step in PVC production). In previous studies [J. Catal. 2000, 189, 91, and J. Catal. 2000, 189, 105], it has been shown that three different copper species are present on CuCl2/Al2O3a Cu-aluminate phase, a highly dispersed copper chloride phase, and an aggregated paratacamite (Cu2(OH)3Cl) phase, whose relative fraction depends on copper loading, sample aging, and heating conditions. In this work, we extract from the EXAFS spectra the fraction of the three phases on a quantitative ground. A study of the corresponding XANES spectra allows us to qualitatively support the EXAFS results. This methodology represents a generalization of the standard EXAFS procedure that...

CuCl 2 on γ-alumina, the base catalyst for oxychiorination reactions, has been investigated by several techniques as a function of Cu loading, sample aging and heating. The results point out the formation of a surface Cu aluminate species and a highly dispersed CuCl 2 . The latter slowly hydrolyzes under aging, but CuCl 2 is restored under heating and is the active species in oxychiorination as demonstrated by activity tests.

CuCl 2 /Al 2 O 3 based ethylene oxychlorination catalysts have been characterized by using JR spectroscopy of the surface hydroxy-groups and of adsorbed pyridine and CO 2 . The ethylene oxychlorination reaction to 1,2-dichloroethane and the dehydrochlorination of ethylchloride (chosen as a probe molecule) have been investigated by using both pulse reactor measurements and in-situ JR experiments in static conditions. Experiments performed over doped alumina and doped CuCl 2 /Al 2 O 3 confirmed that the exposed support surface can be responsible of the dehydrochlorination of 1,2-dichloroethane (EDC) to vinyl chloride monomer (VCM), with a consequent loss in selectivity in the oxychlorination reactor. Doping alumina with MgC1 2 and in particular with KC1 limits the activity of the bare support in this side-reaction.

The influence of the synthesis parameters over the physico-chemical features of poroussilica gels derived from tetraethyl orthosilicate has been studied using nitrogen adsorption and FT-IR. Under acid conditions long shaped siloxane particles were produced and microporous amorphous silicas with high surface area were obtained. These samples showed a relatively high concentration of H-bonded and/or “internal” hydroxy groups. The base-catalyzed samples exhibited features of colloidal gel and mesoporous amorphous materials were obtained. The structure is characterized by relatively low surface area with a monomodal porosity. These silicas have an high concentration of the free surface silanol groups. The acid-catalyzed samples resulted completely inactive in the ammoximation reaction, while the base-catalyzed silicas were active in the same reaction.

The synthesis of hydroxylamine by oxidation of ammonia with hydrogen peroxide on titanium based catalysts is reported. Titanium silicalite is the best catalyst for the reaction. The influence of some reaction parameters on the main and side reactions is discussed and the reaction network is proposed. The role of titanium is pointed out by results of spectroscopic studies.

In a series of combined adsorption, reactivity and spectroscopic experiments in solution with ketones of different size and shape, the fundamental role of the hydroxylamine pathway in the cyclohexanone ammoximation is established. In presence of H 2 O 2 and NH 3 , framework Ti(IV) forms octahedrally distorted aquohydroperoxo and ammonia-hydroperoxo mixed complexes: the latter one being the precursor of the hydroxylamine formation. This intermediate immediately reacts with cyclohexanone in the channels or diffuses to the external solution where it gives oximation reaction with larger ketones which could not penetrate the channels.

Nests of OH groups localized at defective positions inside the silicalite framework can react with chromic acid to give grafted chromate (and/or dichromate) species. Successive reduction in CO yields mainly Cr(II) species anchored to the framework, which upon CO and/or NO dosage form well-defined polycarbonylic, polynitrosylic, and mixed CO/ NO complexes fully characterized by IR spectroscopy. The grafted Cr(II), and possibly Cr(III), ions are efficient centres for ethylene polymerization. Time-resolved IR spectra show that the polymeric chains formed inside the silicalite channels are short and have spectroscopic features different from those of the longer chains formed on the external surface.

As a part of a research devoted to selective oxidation of organic compounds, we have studied the synthesis of cyclohexanonoxime by direct ammonia oxidation in presence of cyclohexanone. By using titanium silicalite as catalyst the reaction took place in a smooth and selective way. The effect of the most important experimental conditions was investigated. The role of the catalyst and some possible reaction mechanisms were discussed. Our results provided a promising starting point for the development of a new technology for cyclohexanonoxime synthesis.

Toluene disproportionation was performed at 300°–360°C in a fixed-bad reactor (carrier H 2 ) over samples of HZSM-5 differing in surface acidity, organic compound used for the synthesis, crystal size and procedure used for decationation. Xylene isomers in the product were in equilibrium ratios. The catalyst activity was found to be clearly dependent on surface acidity. An interpretation is given, in terms of a slow toluene disproportionation within the zeolite pores, followed by fast xylene isomerization on the external surface. Measurements of the reaction rate at 300°C over one of the catalysts confirmed a kinetic model as previously suggested.

The activity and selectivity to para -ethyltoluene in toluene alkylation with ethanol have been correlated to the structural and morphological features of the ZSM-5 and ZSM-11 zeolites employed as catalysts. In particular, as the particle size increases and correspondingly the surface area outside the zeolite channels decreases, lower activities and higher para -isomer selectivities (up to 90%) are obtained. This behavior has been interpreted by supposing a two-step mechanism: in the first step, para -isomer forms by toluene alkylation with high selectivity inside the zeolitic channels (shape selectivity), while in the second step the isomerization toward the thermodynamic equilibrium of the three isomers proceeds just on the external surface

A new synthesis method of ZSM-5 zeolites, developed by us, by starting from microspheroidal silica impregnated with an aqueous solution of tetrapropylammonium bromide, NaAlO2 and NaOH, has been compared with a traditional one, starting from colloidal silica. In both cases a reaction mechanism consisting of an initial crystallization of an Al-poor product, followed by a slow rearrangement of the Al into the crystalline lattice, has been suggested. In the case of our new technique the total time of synthesis, to obtain the best catalyst for the m-xylene isomerization (nucleation + crystallization + Al rearrangement), is much shorter than in the case of the traditional method.

The role of the external surface of the zeolite catalysts is examined by comparing our results in styrene oxide isomerization and toluene alkylation with those reported in other published works. As a conclusion the external surface of zeolite crystals must be regarded as catalytically active and its role can be useful to explain the catalytic behaviour of the zeolites. Nevertheless the effect of the external active sites is not sufficiently depicted and further investigations appear to be necessary.

The strongly acidic Bronsted sites of H-ZSM-5 can be quantitatively exchanged with monovalent copper ions by reaction with CuCl at 573 K, as evidenced by the disappearance of the characteristic IR bands of bridged OH groups. Characterization of the Cu-ZSM-5 samples prepared following this route by means of UV-Vis-NIR (diffuse reflectance) and photoluminescence spectroscopies confirms that the protons are substituted by Cu+ ions, which are isolated and located in a few, structurally well defined sites easily accessible to ligand molecules. These Cu+ ions are highly coordinatively unsaturated and can form Cu+ (CO)n (n=1, 2 or 3) carbonylic and Cu+ (NO)n (n=1 or 2) nitrosylic complexes upon dosage of carbon monoxide or nitric oxide at 77 K. The Cu+ (NO)2 dinytrosylic complexes are unstable at room temperature and evolve with formation of nitrous oxide, NO2− and oxidized CuIINO species. This behaviour strongly supports the hypothesis that a redox mechanism is operating in the nitric oxide decomposition reaction leading to nitrogen and oxygen.