Search

By using isopropanol as alkylating agent in the direct alkylation of benzene to cumene, under properly selected conditions, performances comparable to those afforded by the current industrial use of propylene have been obtained. The reaction is carried out in the presence of a β-zeolite-based catalyst. Catalytic performances are influenced by temperature and pressure at fixed benzene to isopropanol molar ratio. In particular, catalyst activity and catalyst deactivation rate depend on the water content in the liquid phase fraction of the reaction mixture, as predicted by state equations for LV equilibria in the reacting system. The successful use of isopropanol as alkylating agent has paved the road toward a viable acetone recycle in phenol production, as foreseeable in case of acetone/phenol unbalanced demand.

Manufacturing methods of dimethylcarbonate (DMC) are examined, in particular those which have been industrially exploited. Apart from the old phosgenation process, two processes based on the oxy-carbonylation of methanol went on stream: the copper chloride catalysed, one step liquid-phase process, by EniChem, and the palladium catalysed, two steps gas-phase process, via methylnitrite, by UBE. Two further technologies are attractive for full-scale development in the next future: the gas-phase direct methanol oxy-carbonylation and the alkylenecarbonate transesterification process. In the last few years, the use of DMC in the chemical industry has considerably grown, due to its chemical properties and its non-toxicity, an outstanding example being represented by the non-phosgene production of aromatic polycarbonates. Other very promising fields of large scale DMC application are as solvent and as oxygenate in reformulated fuels.

Defective silicalite, an efficient and selective catalyst in the gas phase Beckmann rearrangement reaction, has been characterised by infrared spectroscopy and by molecular modelling techniques. We report a detailed IR study on the effect of outgassing treatments at increasing temperature on silanols bands and on framework modes. The effect of a temperature decrease up to 100 K (during the IR measurement) on the H-bonding interactions has also been investigated. The interaction of silanols with mesitylene, a probe molecule which cannot penetrate the channels, has been studied in order to distinguish between internal and external OH groups. Molecular mechanics and ab initio methods have also been used to model the structure and the vibrational features of a properly designed nest in order to support the assignments of the IR spectrum.

The room-temperature adsorption of NH{sub 3} on Ti-silicalite (TS-1) was studied by means of IR and XANES-EXAFS spectroscopies and by microcalorimetry. The propensity of framework tetrahedral Ti(IV) heteroatoms to expand their coordination shell upon adsorption of NH{sub 3} was monitored by the perturbation induced on the Ti-sensitive framework stretching band (960 cm{sup {minus}1}) and on the preedge peak at 4967 eV in XANES spectra. In both cases a modification of the local geometry of titanium sites upon adsorption of NH{sub 3} as an additional ligand was deduced. The amount of NH{sub 3} specifically adsorbed on the Ti(IV) sites was estimated by comparing volumetric data obtained for TS-1 with those obtained for a Ti-free silicalite taken as reference material. At p{sub NH{sub 3}} = 50 Torr, the number of NH{sub 3} molecules adsorbed per Ti atom was found to be close to 2, indicating that virtually all Ti atoms are involved in the interaction and have completed their 6-fold coordination shell. The molar heats of adsorption on the Ti(IV) sites were estimated following the same procedure. It was found that the molar heat of NH{sub 3} adsorption on titanium is higher (q{sub m} = 95--83 kJ/mol) than that measured for themore » silicalite matrix (q{sub m} = 66--58 kJ/mol). The interaction with ammonia was found to be essentially reversible at room temperature but for a significant amount of adsorbed species irreversibly held on titanium sites. The zero-coverage heat of adsorption was quite high (q {approx} 200 kJ/mol) not only for the first run of adsorption involving both irreversible and reversible interaction but also for the second run involving only the reversible component (q {approx} 130 kJ/mol). The evolution of the heat of adsorption with coverage was typical of heterogeneous surfaces; this is due not only to the presence of sites active toward ammonia on the silica matrix but also to the heterogeneous distribution of Ti(IV) sites, suggesting that a considerable number of framework sites (among the 12 available in the orthorhombic MFI framework) are occupied in a nearly equidistributed way. EXAFS measurements allowed the elongation of the Ti-O bond upon interaction with NH{sub 3} (0.05 {+-} 0.03 {angstrom}), the Ti-N distance (1.93 {+-} 0.03 {angstrom}), and the average number of adsorbed NH{sub 3} molecules per Ti site (1.9 {+-} 0.3) to be estimated. The results were in good agreement with the volumetric-calorimetric data. The effect of the pretreatment of TS-1 samples with ammonium acetate aqueous solution was also investigated by comparing the spectroscopic and the quantitative calorimetric results for both untreated and treated TS-1 samples.« less

Oxidative carbonylation of methanol to dimethylcarbonate catalysed by cobalt complexes is reported. Cobalt complexes with oxygen and or nitrogen donor ligands such as carboxylate, acetylacetonate, picolinate and Schiff bases are suitable catalysts. The oxidative carbonylation of methanol catalysed by cobalt complexes which has never been reported, affords dimethylcarbonate with remarkably high selectivities. Of the cobalt complexes, those with Schiff bases show the highest reactivity. The influence of co-solvents was also examined.

The upgrading of Pseudocumene (1,2,4-TMB) to more valuable methylbenzenes over acidic zeolites with variable acidic and diffusive properties is studied, using reactivity tests and IR spectroscopy. The concurrent reactions investigated include isomerization and transalkylation. At low conversion levels the isomer distribution in C 8 , C 9 and C 10 products is not thermodynamically controlled, being different over different zeolite frameworks, indicating shape selectivity. Variable temperature experiments allow to identify the location of active sites for the different reactions on different catalysts, and the nature of shape selectivity.

Oxidative carbonylation of alcohols is mainly exploited for the production of carbonate and oxalate esters. Most important is the manufacture of dimethyl carbonate (DMC), an intermediate in the non-phosgene route to aromatic polycarbonates. Two processes are industrially adopted. In the one-step EniChem process, DMC is produced by copper chloride catalyzed, liquid-phase methanol oxy-carbonylation. In the two-step Ube process, methanol is first converted to methyl nitrite, which in turn undergoes palladium-catalyzed, gas-phase carbonylation to DMC. Further potentially important industrial applications of oxidative carbonylation relate to the production of diphenyl carbonate, carbamate, and acrylate esters. Keywords: oxidative carbonylation; dimethyl carbonate; dialkyl oxalate; alkyl nitrite; copper catalyst; palladium catalyst; industrial processes

In this work we present an exhaustive, experimental and theoretical, investigation on the structural, vibrational, electronic, and energetic properties of TS-1 catalyst. The perturbation induced by adsorption of water and ammonia ligands, which represent two key molecules in the industrial processes catalyzed by TS-1, is also discussed in great detail on both experimental and computational ground. Theory and experiments allow us to present a picture able to describe in a satisfactory way the interaction of both molecules with Ti(IV) sites and with the hosting siliceous matrix.

A thorough analysis of the vibrational features of the titanium silicalite-1 (TS-1) catalyst is presented, based on quantitative IR measurements, Raman and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm(-1) associated with titanium insertion into the zeolite framework. The enhancement of the intensity of the 1125 cm(-1) feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of resonant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theory provide the basis for the assignment of the main vibrational contributions and for the understanding of Raman enhancement. The resonance-enhanced 1125 cm(-1) mode is unambiguously associated with a totally symmetric vibration of the TiO(4) tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as a totally symmetric stretching of the four Si-O bonds pointing toward Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO(4) units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxygen atom belongs to an OH group, such as in terminal tetrahedra of cluster models and in real defective zeolites. Asymmetric SiO(4) and TiO(4) stretching modes appear above and below 1000 cm(-1), respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm(-1) (in both SiO(4) and TiO(4)) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features associated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO(4) modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO(4) vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti caused by water adsorption quenches the UV-Raman enhancement of the 1125 cm(-1) band.

The results of an investigation on catalytic systems arising from the electrochemical activation of salts and complexes of several transition metals for the carbonylation of methanol to dimethylcarbonate (DMC) are reported. Metals were tested as inorganic salts or complexes with ligands such as 2,2′-bipyridine (bipy), 1,6-bis (2-oxyphenyl)-2,5-diaza-1,5-hexadiene (salen), 2,4-pentanedionate (acac), triphenylphosphine (TPP) in methanol at room temperature saturated with carbon monoxide at atmospheric pressure. Best faradic yields vs DMC were observed as follows: CuCl(bipy): 84.8%; PdCl 2 (bipy): 64.0%; CoCl 2 : 26.0%; RhCl 3 : 25.0%; AgBF 4 (bipy): 10.2%; AuCl 3 : 9.4%; Cr(acac) 3 : 6.7%; PtCl 2 (bipy): 2.1%; NiCl 2 : 1.8%. The influence of the ligand on faradic yields has been also investigated.

The reaction of imidazole (IM) and its 1-methyl (1-MIM) and 4-methyl (4-MIM) derivatives with p-nitrophenyl propionate has been investigated in benzene at 25°. Under pseudo-first-order conditions, the observed rate law is kψ=k2[4-MIM]2 and kψ=k2[IM]2+k3[IM]3; 1-MIM is a catalyst of the reaction between 4-MIM or IM and the ester. The state of aggregation of IM and 4-MIM and the high kinetic order are taken as evidence of a main mechanism involving associated imidazole and ester to give a tetrahedral anion (paired to an imidazolium cation) as the intermediate whose breakdown is rate-limiting.