Tungstated Zirconia Catalysts Research Articles

NMR study of tungstated zirconia catalyst: acidic properties of tungstated zirconia and influence of tungsten loading

The concentration of Brönsted acid sites on tungstated zirconia (WZ) increased as tungsten increased (for tungsten loadings of 2.8, 5.3, and 9.6wt.%), while that of Lewis acid sites decreased, indicating that Brönsted acid sites were formed by the tungsten species and that the Lewis acid sites detected in 31P MAS NMR spectra of trimethylphosphine (TMP) were mostly due to Lewis acid sites on zirconia. WZ of 9.6wt.% tungsten content showed the same spectra as WZ of 16.6wt.% tungsten content, indicating that they have the same surface acid properties. Although the concentration of Brönsted acid sites on WZ increased with reduction treatment, Brönsted acid strength of WZ was lower than that of sulfated zirconia, even after reduction treatment. WZ of 16.6wt.% tungsten contained 0.07mmolg−1 of Brönsted acid sites. The number of Brönsted and Lewis acid sites corresponded to 3.3 and 10.0% of all the tungsten on the tungstated zirconia, and 6.1 and 18.3% of the estimated surface tungsten, respectively.

Structure, properties and roles of the different constituents in Pt/WO x/ZrO 2 catalysts

Six samples of platinum-promoted tungstated zirconia catalysts (Pt 0.5/WO x /ZrO 2) with W loadings between 6.5 and 12.5 wt.% are investigated using X-ray diffraction (XRD) and magnetic susceptibility measurements (4 to 350 K). Studies are also carried out on some of these samples after annealing them in air at temperatures up to 1000 °C. In the as-prepared samples, the absence of any lines due to WO 3, irrespective of W loading, suggests the highly dispersed state of WO 3. However, the dispersed WO 3 appears to be necessary to stabilize the tetragonal phase of zirconia ( t-ZrO 2). Ex-situ XRD studies show that on heating the samples to 1000 °C, the fraction of m-ZrO 2 (monoclinic) increases, with the simultaneous appearance of crystalline m-WO 3. This leads us to infer that the dispersed WO x species are associated with t-ZrO 2 only. By comparing the magnitude and the temperature variations of the magnetic susceptibility χ of the samples with those of Pt, α-PtO 2, Pt 3O 4, WO 3 and ZrO 2, we infer that Pt in the as-prepared catalysts is primarily in the oxidized form, α-PtO 2 and/or Pt 3O 4, relative magnitudes of the two oxides being dependent on the preparation procedures, thermal treatments and aging. The oxides are converted to Pt in reducing atmosphere.

Redox properties of tungstated zirconia catalysts: Relevance to the activation of n-alkanes

A tungstated zirconia catalyst containing approximately enough tungsten oxide to give a theoretical monolayer on the zirconia support was characterised by Raman spectroscopy, temperature-programmed reduction, EPR and IR spectroscopies. The data show that treatment of the catalyst in H2 at temperatures as low as 470 K led to partial reduction; EPR spectra indicated the formation of W5+ centres in two locations: in the bulk of tungstate clusters dispersed on the zirconia surface and at the surfaces of these clusters and consequently accessible to gas-phase reactants. When 1% n-pentane in N2 flowed through a bed of particles of the catalyst in an EPR tube at 523 K for 20 min, the following surface species formed: W5+ centres (shown by EPR); organic radicals (shown by EPR); and OH groups (shown by IR). The results indicate that n-pentane undergoes a homolytic C–H bond cleavage reaction such as the following followed by one-electron transfer steps that yield surface W5+ ions, OH groups, and chain carriers in the catalytic isomerisation of n-pentane, inferred to be carbenium ions. These processes are considered to be non-catalytic redox initiation reactions that explain the promoter role of the tungsten in the catalyst.

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO 3/ZrO 2, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO 4/ZrO 2) is compared with a Pt/WO 3/ZrO 2 catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO 3/ZrO 2 catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.

Characterization of WOx/ZrO2by Vibrational Spectroscopy andn-Pentane Isomerization Catalysis

Tungstated zirconia catalysts containing WO3loadings ranging from 3.6 to 23.9 wt% were prepared by refluxing an aqueous suspension of hydrous zirconia containing appropriate amounts of metatungstate, drying, and calcinating at 923 K or 1098 K. The acidic properties of the samples were characterized by low-temperature infrared spectroscopy with adsorbed CO as a probe molecule. Both Lewis and Brønsted acid centers with enhanced acid strength were created as the WO3loading increased. The Lewis sites are coordinatively unsaturated surface Zr4+ions. Their density decreased significantly as the WO3loading approached saturation, and simultaneously the surface ZrOH groups disappeared. These observations are consistent with high (perhaps monolayer) dispersion of the surface tungstate. The Brønsted acid strength also increased with increasing WO3loading up to saturation and remained constant at higher loadings. These results show that large WO3domain or cluster sizes are required to create strong Brønsted acidity. Zr-heteropolytungstates containing charge-compensating protons are proposed as surface structures to account for the observations. The catalytic activities of these materials forn-pentane isomerization at 523 K in a flow reactor were found to parallel the measured Brønsted acid strengths. However, because an induction period of increasing catalytic activity was found, followed by catalyst deactivation, it is suggested that the detected Brønsted acidity may not simply account for the catalytic activity. Catalytic andin-situRaman spectroscopic evidence indicates the formation of carbonaceous surface deposits during the induction period. These may be converted into catalytic sites and ultimately into deposits that cause catalyst deactivation.

Alcohol Dehydration Reactions over Tungstated Zirconia Catalysts

The behavior of tungstated zirconia (WZ) toward dehydration of 1-propanol, 2-propanol, andtert.-butanol was compared with that of H-mordenite. For 1- and 2-propanol, the desorption temperatures of olefinic products formed over the WZ catalyst were found to be very close to those observed in H-mordenite; however, the zeolite catalyst had a much larger tendency to retain (or to readsorb) olefins in the pores in the form of oligomerization products, as evidenced by the detection of them/e=65 signal (cyclopentadienyl cation) during the temperature-programmed reaction of 2-propanol. The Brønsted acid sites of the zeolite underwent rapid H+/D+exchange when 2-propanol deuterated in its hydroxyl group was used. This allowed the olefinic products to incorporate deuterium during reaction. On the other hand, the WZ catalyst did not effect the H+/D+exchange of propene formed from CH3–HCOD–CH3, presumably because olefin readsorption onto hydrated W6+sites does not take place. The structure of the WZ catalyst was monitored by EXAFS and XRD.

Effect of preparation method and selective poisoning on the performance of platinum supported on tungstated zirconia catalysts for alkane isomerization

The reactions of n-butane with hydrogen at 573 K were used as chemical probes to study the performance of platinum supported on tungstated zirconia (PtWZ) prepared by sequential impregnation of tungsten (W) and platinum (Pt) precursors. The effect of Pt and W loading and calcination and reduction temperatures on the n-butane isomerization activity and selectivity was determined. N-propylamine poisoning experiments show that the isomerization and homologation (C 3 + C 5) pathways are affected more dramatically than the hydrogenolysis routes because the former are exclusively acid-catalyzed reactions.

A study of platinum supported on tungstated zirconia catalysts

The reaction of n-butane with hydrogen at 573 K was studied over platinum supported on tungstated zirconia (PtWZ). A higher Pt dispersion increases the selectivity toward hydrogenolysis products and overall activity. The % coke after 90 min on stream at a H 2/n-C 4 ratio of 3 was found to be less than 0.1 wt.-% in all cases. The presence of Brønsted acid sites upon reduction is confirmed by diffuse reflectance infrared spectroscopy of adsorbed pyridine. The WO 3/ZrO 2 (WZ) support consists of a mixture of tetragonal and monoclinic zirconia and tetragonal tungsten oxide regardless of the presence of platinum.