M.F. Ribeiro

Structure-activity relationship in zeolites

Abstract In this review we will try to give an integrated view of the relation between the structure and the catalytic behaviour of zeolitic materials, covering both the compositional and the structural aspects. Due to the amount of work that has been done in this wide area of zeolite catalysis this will, necessarily, be an incomplete although, hopefully, unbiased work. The paper will refer, not only to the relation that can be observed with natural and as-synthesised zeolites, but also to the wide range of techniques that are currently available for the ‘tuning’ of the catalytic properties of zeolitic materials. A special emphasis will be placed in all the aspects concerning shape selective catalysis, which is certainly the most striking form of a structure-activity relationship in heterogeneous catalysis.

Acid-Catalysed Conversion of Saccharides into Furanic Aldehydes in the Presence of Three-Dimensional Mesoporous Al-TUD-1

The one-pot acid-catalysed conversion of mono/di/polysaccharides (inulin, xylan, cellobiose, sucrose, glucose, fructose, xylose) into 2-furfuraldehyde (FUR) or 5-hydroxymethylfurfural (HMF) in the presence of aluminium-containing mesoporous TUD-1 (denoted as Al-TUD-1, Si/Al = 21), at 170 ºC was investigated. Xylose gave 60% FUR yield after 6 h reaction; hexose-based mono/disaccharides gave less than 20% HMF yield; polysaccharides gave less than 20 wt % FUR or HMF yields after 6 h. For four consecutive 6 h batches of the xylose reaction in the presence of Al-TUD-1, the FUR yields achieved were similar, without significant changes in Si/Al ratio.

Production of biomass-derived furanic ethers and levulinate esters using heterogeneous acid catalysts

Mesoporous aluminosilicates of the type Al-TUD-1, prepared via “green”, low-cost, non-surfactant templating routes, are effective and versatile heterogeneous acid catalysts for the production of useful bio-based furanic ethers and levulinate esters, via the reactions of the biorenewable substrates 5-hydroxymethyl-2-furfural (Hmf) or furfuryl alcohol (FA) with aliphatic alcohols. The identification of reaction intermediates and products by comprehensive two-dimensional gas chromatography combined with time-of-flight mass spectrometry was carried out, giving mechanistic insights. Ethyl levulinate (EL) was formed from FA or Hmf as substrates, with higher EL yields being reached in the former case. Different types of alkyl levulinates may be synthesized from FA using Al-TUD-1 catalysts. On the other hand, 5-(ethoxymethyl)furan-2-carbaldehyde may be formed as the main product from Hmf. Modifications of the properties of Al-TUD-1 involved varying the Si/Al ratio and applying a post-synthesis acid treatment. The influence of these factors and of the reaction conditions on the catalytic reactions was investigated. The efficient regeneration and recyclability of Al-TUD-1 was assessed.

Zinc(II) phthalocyanines immobilized in mesoporous silica Al-MCM-41 and their applications in photocatalytic degradation of pesticides

In the present study the authors investigated a set of three new zinc(II) phthalocyanines (zinc(II) tetranitrophthalocyanine (ZnTNPc), zinc(II) tetra(phenyloxy)phthalocyanine (ZnTPhOPc) and the tetraiodide salt of zinc(II)tetra(N,N,N-trimethylaminoethyloxy) phthalocyaninate (ZnTTMAEOPcI)) immobilized into Al-MCM-41 prepared via ship-in-a-bottle methodology. The samples were fully characterized by diffuse reflectance-UV-vis spectroscopy (DRS-UV-vis), luminescence, thermogravimetric analysis (TG/DSC), N(2) adsorption techniques and elemental analysis. A comparative study was made on the photocatalytic performance upon irradiation within the wavelength range 320-460nm of these three systems in the degradation of pesticides fenamiphos and pentachlorophenol. ZnTNPc@Al-MCM-41 and ZnTTMAEOPcI@Al-MCM-41 were found to be the most active systems, with the best performance observed with the immobilized cationic phthalocyanine, ZnTTMAEOPcI@Al-MCM-41. This system showed high activity even after three photocatalytic cycles. LC-MS product characterization and mechanistic studies indicate that singlet oxygen ((1)O(2)), produced by excitation of these immobilized photosensitizers, is a key intermediate in the photocatalytic degradation of both pesticides.

Transformation of an ethylbenzene-o-xylene mixture on HMOR and Pt-HMOR catalysts. Comparison with ZSM-5 catalysts

Abstract The transformation of pure ethylbenzene and of an ethylbenzene (20wt.-%)- o -xylene (80 wt.-%) mixture is carried out on acid and bifunctional HMOR catalysts under the following conditions: fixed bed reactor, temperature 410°C, 0.96 MPa pressure of hydrogen and 0.24 MPa pressure of hydrocarbons, weight of hydrocarbon injected per weight of catalyst and per hour between 10 and 110. On both catalysts the main reaction of pure ethylbenzene is disproportionation, this reaction occurring mainly through diphenylethane intermediates. The main effect of platinum is to allow the bifunctional isomerization of ethylbenzene. In addition to disproportionation reactions, ethylbenzene in mixture with xylene is also transformed through similar mechanisms into dimethylethylbenzene (transethylation) and into ethylmethylbenzene (transmethylation). o -Xylene is rapidly transformed into m -andp-isomers. Large differences exist between MOR and ZSM-5 catalysts. ZSM-5 catalysts are very active for ethylbenzene dealkylation and C 8 naphthenes hydrocracking while MOR catalysts are more active for xylene isomerization and disproportionation, and ethylbenzene hydroisomerization. Disproportionation of ethylbenzene occurs mainly on H-ZSM-5 through a deethylation-ethylation mechanism and on HMOR through diphenylethane intermediates. Steric constraints on the formation of bulky bimolecular intermediates at channel intersections and diffusion limitations are responsible for the particular behaviour of ZSM-5 catalysts.

Structure analysis of the novel microporous aluminophosphate IST-1 using synchrotron powder diffraction data and HETCOR MAS NMR

Abstract A combination of advanced powder diffraction and NMR techniques have allowed the structure of the novel microporous aluminophosphate IST-1 ( | (CH3NH2)4(CH3NH3+)4(OH−)4 | [ Al12P12O48 ] ) to be elucidated. The framework structure was determined in the non-centrosymmetric space group Pca21 (a=9.61523(1) A, b=8.67024(1) A, c=16.21957(2) A) from high-resolution synchrotron powder diffraction data using the program FOCUS. Extra framework species were then located on difference electron density maps. A hydroxyl group was found to bridge between two of the framework Al atoms, and one methylamine species, presumably protonated, could be located in the channels where it H-bonds to three framework oxygens. The most unusual feature of the structure is the second methylamine molecule, which bonds directly to a framework Al atom. The structure is entirely consistent with 31P and 27Al MAS NMR studies, which showed there to be three P (all 4-coordinate) and three Al (one 4-, one 5- and one 6-coordinate) sites, and with 13C MAS NMR, which showed there to be two different types of methylamine species in equal amounts. Assignment of the 31P, 27Al and 13C MAS NMR signals could be deduced from the crystallographic data,31P-27Al HETCOR spectra and ab initio calculations.

Synthesis and characterization of new CoAPSO-40 and ZnAPSO-40 molecular sieves. Influence of the composition on the thermal and hydrothermal stability of AlPO4-40-based materials

Abstract Highly crystalline CoAPSO-40 and ZnAPSO-40 have been synthesized and characterized by powder XRD, TG/DSC, SEM, EDX, 27Al, 31P and 29Si solid-state NMR, diffuse reflectance UV–VIS spectroscopy, FT IR and catalytic tests using the m-xylene isomerization and n-heptane cracking as model reactions. The simultaneous incorporation of silicon and a divalent metal in the framework of the AFR structure results in the generation of Bronsted acid sites similar to those found in SAPO-40 and MeAPO-40. The catalytic activity of the samples for acid catalyzed reactions is higher than that of SAPO-40 with a homogeneous framework silicon distribution. The framework incorporation of cobalt or zinc results in a decrease of the thermal and hydrothermal stability of the metal bearing AFR framework, when compared with AlPO4-40 and SAPO-40.

Study of catalytic properties of SAPO-40

A series of SAPO-40 samples with Si atomic fraction from 0.06 to 0.12 has been synthesised. The study of their catalytic properties has been evaluated by comparison with SAPO-37, USHY and mordenite, using as model reaction the cracking of n-heptane. The catalytic activity of SAPO-40 samples depends on the Si content, and is probably associated with the non-uniform distribution of silicon. SAPO-40 showed a stronger acidity than SAPO-37 with a similar Si content. SAPO-40 is more resistant to deactivation by coke than the studied mordenite sample. Its selectivity for cracking products is comparable to USHY, but hydrogen transfer is lower for SAPO-40 than for USHY and mordenite.

Thermal and hydrothermal stability of the silicoaluminophosphate SAPO-40

Abstract Thermal and hydrothermal stability of SAPO-40 were studied by X-ray powder diffraction (XRD) and magic-angle spinning nuclear magnetic resonance (MAS-NMR). For the X-ray study, the hydrothermal treatments were carried out, in situ, in a high-temperature chamber. The SAPO-40 structure shows a remarkable stability to high temperatures under both dry and humid air. At high temperatures a reorganisation of silicon occurs, but the long-range order is only lost above ca. 1000°C. Changes in the XRD pattern induced by water at room temperature are completely reversible upon re-heating at 200°C, as well as the changes in the coordination and local environments of aluminium and phosphorus observed by NMR. For silicon however, some changes, that are observed upon hydration, remain after dehydration.

Al-MCM-41 systems exchanged with alkali–metal cations: FT-IR characterization and catalytic activity towards 1-butene isomerization

Abstract Starting from an as-synthesized Na-bearing MCM-41 sample with Si/Al=20, several Cs or K exchanged samples have been obtained. All samples, both washed after exchange to remove the excess alkali–metal salt and unwashed, were characterized by studying in the IR the adsorption of CO at a nominal temperature of 77 K, and of CO 2 at room temperature. On unwashed samples, after thermal treatment at 500 °C, a basic alkali-oxide phase and a surface silicate were detected. In contrast, washed samples did not exhibit basic character and only Lewis acidic cationic centers were detected, the strength of which is close to that of alkali-cation exchanged ZSM-5 systems. Unwashed samples were inactive in the isomerization of 1-butene. Washed samples converted 1-butene to cis/trans -2-butene, with a ratio close to one (the presence of isobutene was negligible), as typical of mildly acidic Bronsted centers, which are probably present even after exchange: indeed, a sample of Al-MCM-41 in the protonic form, tested as reference, gave the same reaction with highest activity.