Roel Prins

Metal Phosphides: Preparation, Characterization and Catalytic Reactivity

The preparation, characterization, and catalytic activity of supported metal phosphides are reviewed. Reduction of metal compounds together with phosphate is a convenient method to prepare metal phosphides, but requires high temperature. Reduction with phosphite, hypophosphite, or phosphine and the plasma reduction of phosphate can be carried out at lower temperatures, which leads to smaller metal phosphide particles and more active catalysts. Organometallic routes allow the separate synthesis of metal phosphide nanoparticles, which have to be added to the support in a second step. LEED, STM, XPS, and DFT studies have shown that the surfaces of Ni2P reconstruct to P-rich surfaces. The investigation of metal phosphides as catalysts for hydrotreating reactions continues to be a topic of considerable research with recent advances realized in using bimetallic and noble metal phosphides to achieve high activities and tailored selectivities. Finally, hydrodeoxygenation catalysis over metal phosphides is a growing area of research given the need to develop catalysts for upgrading biomass to transportation fuels.Graphical Abstract

Hydrodesulfurization of 4,6‐Dimethyldibenzothiophene over Noble Metals Supported on Mesoporous Zeolites

The reduction of the sulfur content in gasoline and diesel fuel has been a subject of intense investigation in recent years because the sulfur level must in many countries be reduced to 10 ppm by the year 2010 for environmental reasons, while for fuel-cell applications the sulfur content should be below 0.1 ppm. To reach this low level, even highly refractory molecules such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) must be desulfurized. However, because of steric hindrance by the methyl groups adjacent to the sulfur atom, desulfurization of 4,6-DM-DBT mainly takes place after the molecule has first been hydrogenated. Therefore, the hydrogenating ability of the catalyst is of critical importance for deep hydrodesulfurization (HDS). Recent studies have shown that noble-metal-supported catalysts have much better hydrogenation performance than conventional metal sulfides in HDS, and may be used in the second reactor of a deep HDS process. Not only the active catalyst, but also the support plays an important role in the catalytic performance of catalysts. Acidic supports can increase the conversion of dibenzothiophene (DBT) and of 4,6-DM-DBT. One explanation for this is that they enable dealkylation and isomerization reactions of the alkyl substituents, which may transform refractory components into more reactive species and thus accelerate HDS. Moreover, acidic supports may also improve the catalytic activity of the catalyst particles. Since partial electron transfer can occur from the metal particles to acidic sites of the support, the resulting electron-deficient metal particles are deemed to have a better resistance to sulfur poisoning by decreasing the interaction with H2S. [5b,6] Another explanation for this improvement is the creation of a second hydrogenation pathway by spillover of hydrogen atoms from the metal particles to the aromatic sulfurcontaining molecules that are adsorbed on acidic sites in the vicinity of the metal particles. While the metal particles become poisoned by sulfur, they can still dissociate hydrogen molecules, and thus the hydrogenation pathway involving spillover would still be possible. It is well known that zeolites possess strong acidity, high stability, and a regular pore array, and are for these reasons applied in many industrial catalytic reactions. However, their small pore size means that relatively large molecules such as 4,6-DM-DBT cannot enter the pores; they can only react on the outer surface of the zeolites and cannot reach many active centers. A support with strong acidity and relatively large pores would, therefore, be preferred. The recent discovery of mesoporous zeolites with their hierarchical porosity and strong acidity opens the possibility of using them as supports in HDS. However, until now their use as a support in HDS has not been reported. Herein we report on Pt, Pd, and Pt-Pd catalysts supported on mesoporous Na-ZSM-5. The catalytic activity and selectivity of these catalysts were studied in the HDS of 4,6-DMDBT, and the hydrocarbon products as well as the hydrogenated intermediates were analyzed. Compared with conventional Na-ZSM-5or g-Al2O3-supported catalysts, the mesoporous Na-ZSM-5-supported catalysts exhibited much better catalytic performance for hydrodesulfurization. The powder XRD patterns of mesoporous Na-ZSM-5 (MNZ-5) and Na-ZSM-5 (NZ-5, see Figure S1 in the Supporting Information) show well-resolved peaks which are characteristic of the ZSM-5 zeolite structure. MNZ-5 exhibited a type IV N2 adsorption/desorption isotherm (see Figure S2 a in the Supporting Information) typical for mesoporous materials. In contrast, NZ-5 showed a type I isotherm, which is typical of microporous materials. Moreover, a uniform pore distribution centered at around 4.9 nm was observed for MNZ-5 (see Figure S2b in the Supporting Information). The detailed sorption data of both samples are listed in Table 1. The BET special surface area and mesoporous volume for MNZ-5 are 579 m g 1 and 0.44 cm g , respectively, much higher than those of NZ-5. The isomerization of 2-methyl-2-pentene (2M2P) is a good model reaction to evaluate the acidity of solid acids. The molar ratio of trans-3-methyl-2-pentene (trans-3M2P, obtained by shift of a methyl group) to transand cis-4methyl-2-pentene (transand cis-4M2P, respectively, obtained by shift of an H atom) in the product reflects the acidity of solid acids. The higher the molar ratio is, the stronger the acidity is. Table 1 shows that MNZ-5 resulted in a higher conversion in the isomerization of 2M2P than did NZ-5; this observation may be attributed to the high BET surface area of MNZ-5. The analysis of the Na content in the two zeolites showed that only 90 % of the Al atoms were charge-compensated by Na cations, so that about 10 % protons existed in NZ5 and MNZ-5, thus suggesting that they are indeed acidic supports. g-Al2O3, on the other hand, gave a lower conversion [*] Dr. Y. Sun, Prof. Dr. R. Prins Institute for Chemical and Bioengineering, ETH Z rich 8093 Z rich (Switzerland) Fax: (+ 41)44-632-1162 E-mail: prins@chem.ethz.ch

On determining the nuclearity of iron sites in Fe-ZSM-5—a critical evaluation

In order to interpret catalytic data on iron zeolites in terms of structure-activity relationships, reliable characterisation methods are needed. In particular, the nuclearity of the iron species is an important issue, since it is often invoked to explain catalytic activity. In the present contribution, we address the problem of the nuclearity of the iron species in Fe-ZSM-5 by a combination of techniques, that is, UV-Vis, EXAFS, Magnetic Circular Dichroism (MCD) and magnetisation measurements. Based on an in-depth analysis of these data, we show that some of the current interpretations of UV-Vis and EXAFS spectra need to be revised.

EXAFS study of the local structure of Ni in Ni-MoS2/C hydrodesulfurization catalysts

To study the local structure of the Ni promoter atom, the Ni and Mo K edge EXAFS spectra of Ni-MoS2/C hydrodesulfurization catalyst were measured in an in-situ EXAFS cell at 77 K. The Ni atom is situated in a square pyramid of five S atoms at a distance of 2.21 Å from the S atoms. In addition an EXAFS contribution due to a Mo atom at 2.82 Å from the Ni atom could be identified. This local structure indicates that the Ni atoms are situated on top of the S4 squares at the MoS2 edges in millerite-type Ni sites. The Ni atoms are situated in the planes of the Mo atoms and not in the intercalation plane midway between successive MoS2 sandwich layers.

Effect of metal oxide additives on the CO hydrogenation to methanol over Rh/SiO2 and Pd/SiO2

Catalysts were prepared from ultra pure SiO2, Pd and Rh nitrates and chlorides, and by doping with Al, Fe, Na, K or Ca nitrate. The activities and selectivities of the Pd and Rh catalysts were investigated at 553 K, H2/CO=2 or 3 and 2.5 or 4 MPa respectively. Additives had a strong influence on the catalytic properties. The doping with alkali and alkaline earth oxides led to a strong suppression of the CO dissociation. Particularly basic additives, such as Ca, had a strong promoting effect on the methanol production. This may confirm that the formation of methanol occurs through formate intermediates.

The effect of phosphorus on the HDN reaction of piperidine, decahydroquinoline and ortho-propylaniline over Ni-MoS2/Al2O3 catalysts

The hydrodenitrogenation (HDN) of piperidine, decahydroquinoline (DHQ) and orthopropylaniline (OPA) has been studied over NiMo(P)/Al2O3 catalysts at 593 K and 3.0 MPa in order to understand the effect of phosphorus on the elementary HDN reaction steps. Phosphorus exhibited a negative effect on the HDN of piperidine and DHQ, both on the C-N bond cleavage reaction and on the subsequent hydrogenation reaction of alkene to alkane. A P/Al2O3 catalyst showed no HDN activity at all, neither with piperidine, nor with DHQ. A positive effect of phosphorus was observed in the HDN of OPA, where hydrogenation of the aromatic ring is needed and is rate limiting. It is suggested that introduction of phosphorus to NiMo/Al2O3 catalysts on the one hand decreases the available support surface area, and as a consequence the dispersion of the Ni-Mo-S phase and thus the capacity for C-N bond breaking and olefin hydrogenation. On the other hand, phosphorus induces either new or more active sites for the hydrogenation of aromatics.

An EXAFS study of the spreading of MoO3 on the surface of γ-Al2O3

Abstract An EXAFS analysis has been carried out of physical mixtures of MoO 3 with γ-Al 2 O 3 prior to and after thermal treatment at 720 K in the absence and presence of water vapor. The degree of order in the MoO 3 /Al 2 O 3 samples decreases during calcination. This loss of long-range order is even more pronounced in the presence of water vapor than in its absence. The local order around molybdenum after thermal treatment in a dry atmosphere still resembles that in MoO 3 , while significant structural modifications are induced when water vapor is present. After calcination a new Mo- X distance was observed which may correspond to a MoAl contribution in a MoOAl configuration. The EXAFS results support earlier conclusions from ion scattering and Raman spectroscopy studies on identical samples, which suggests a spreading Of MoO 3 over the surface of alumina as a result of solid-solid wetting.

Facile Preparation of Ni2P with a Sulfur-Containing Surface Layer by Low-Temperature Reduction of Ni2P2S6

Preparation of Ni2P by temperature-programmed reduction (TPR) of a phosphate precursor is challenging because the P-O bond is strong. An alternative approach to synthesizing Ni2P, by reduction of nickel hexathiodiphosphate (Ni2P2S6), is presented. Conversion of Ni2P2S6 into Ni2P occurs at 200-220 °C, a temperature much lower than that required by the conventional TPR method (typically 500 °C). A sulfur-containing layer with a thickness of about 4.7 nm, composed of tiny crystallites, was observed at the surface of the obtained Ni2 P catalyst (Ni2P-S). This is a direct observation of the sulfur-containing layer of Ni2P, or the so-called nickel phosphosulfide phase. Both the hydrodesulfurization activity and the selective hydrogenation performance of Ni2P-S were superior to that of the catalyst prepared by the TPR method, suggesting a positive role of sulfur on the surface of Ni2P-S. These features render Ni2P-S a legitimate alternative non-precious metal catalyst for hydrogenation reactions.

X-ray absorption investigations on Ti-containing zeolites

Fluorescence X-ray absorption experiments were carried out on titanium containing zeolites. The edge- and EXAFS-regions proved that the coordination of the Ti-sites of TS-1 and [Ti]ZSM-5 are similar, although no quantitative data could be obtained for the coordination numbers. EXAFS analysis of a [Li, Ti]Y zeolite, containing 12% Ti, showed that most of the titanium has a rutile-like coordination and therefore is not likely to be present as part of the framework.

Catalytic vapour-phase nitration of benzene over modified Y zeolites: influence of catalyst treatment

Abstract The catalytic vapour-phase nitration of benzene with aqueous nitric acid has been carried out on modified Y zeolites. The influence of various ultrastabilization and activation procedures on the catalytic activity and stability of the materials was investigated. The modified Y zeolites were characterized by physico-chemical methods, and correlations between catalyst properties and catalytic performance are discussed. It was found that modified Y zeolites are active catalysts for the vapour-phase nitration of benzene, and that both nitrobenzene space time yield and catalytic stability are strongly dependent on the preparation procedure. Thus, stable catalysts for the vapour-phase nitration of benzene were obtained by acid treatment of low sodium ultrastabilized Y zeolites.