Catalytic behaviour of Pt or Pd metal nanoparticles–zeolite bifunctional catalysts for n-pentane hydroisomerization

Bifunctional catalysts based on acidified Mordenite or ZSM-5 and platinum or palladium as metal function, were tested for n-pentane hydroisomerization. Two methods were used to introduce platinum: wetness impregnation and microemulsion; palladium was introduced via an organometallic complex. A lower catalytic activity was obtained for palladium catalyst in comparison with platinum samples which is explained by the lower activity of Pd in the dehydrogenation reaction step. Different catalytic behaviour of systems based on Mordenite and ZSM-5 was attributed to zeolite pore structure. The uncompleted removal of surfactant during calcination could explain the lower activity showed by catalysts prepared by microemulsion.     

Zeolite beta catalysts for n-C7 hydroisomerization

Zeolite β with Si/2Al ratios of 60, 100, and 200 were synthesized using tetraethlammonium hydroxide (TEAOH) as the structure-directing agent (SDA) in the absence of alkali metal cations. Pt, Pd and Pt-Pd catalysts supported on the zeolite β samples were studied in n-heptane (n-C7) hydroisomerization. The Pt/β catalysts showed a higher catalytic activity than the Pd/β catalysts. For the Pt/β with a Si/2Al ratio of 100, its n-C7 conversion and selectivity of C7 isomers were observed to be 87.06% and 75.48% respectively at 250^∘C. The activity of n-C7 conversion was stable for at least 82 h. However, the selectivity of C7 isomers was gradually decreased with the reaction time. Experimental data also showed that the addition of Pd to catalyst Pt/β enhanced the n-C7 conversion, but lowered the selectivity of C7 isomers. Pd catalyst was also observed to minimize the formation of aromatics in comparison with Pt catalyst.     

n-Pentane Hydroisomerization on Pt Containing HZSM-5, HBEA and SAPO-11

Hydroisomerization of n-pentane over platinum promoted acids zeolites was studied. The effect of structure and acidity of the support was investigated at atmospheric pressure between 250 and 400 °C. Pt/HDBEA catalyst showed the best performance at 300 °C with high activity and selectivity to isopentane, due to its structure and a proper balance between acid and metallic sites. This catalyst has a high catalytic stability and regeneration under air flow after deactivation by coking, restores its activity and selectivity.     

Effect of hydrothermal treatment and ammonium ion incorporation in platinum-mordenite catalysis for n-hexane hydroconversion

Hydroconversion of n-hexane was studied on catalysts containing 0.25% Pt supported on H-mordenite (H-M) and NH₄-M. The H-M containing catalysts were Pt/H-M, Pt on steamed H-M (Pt/St H-M) and steamed Pt/H-M (StPt/H-M), whereas the NH₄-M containing catalysts were Pt/NH₄-M and StPt/NH₄-M. Steam-treatment of H-M containing catalysts enhanced the hydroconversion activity, whereas such treatment decreased the activity of the Pt/NH₄-M catalyst. The diffusion resistance parameter, i.e., the Thiele modulus, Φ_L, estimated for the reaction on the catalysts under study was found to increase with the increase in the catalytic activity, and both were found to decrease in the order:

Pt dispersion in the zeolite was not comparable with the catalytic activities of the H-M containing catalysts. The higher activity of the Pt/NH₄-M catalyst could be attributed to a higher Pt dispersion in the zeolitic channels, higher strength of the acid sites (determined by temperature programmed desorption (TPD) of ammonia) and higher diffusion limitation of the reactant in the catalyst pores.     

Hydroisomerization and hydrocracking of long chain n-alkanes on Pt/amorphous SiO2–Al2O3 catalyst

The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica–alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380°C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/n-alkane ratio of cracking products increased considerably with increasing conversion degree. At the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/n-alkane ratio. The conversion rate constants showed a considerable increase between n-C₁₆ and n-C₂₈, whereas a slight decrease between n-C₂₈ and n-C₃₆ was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was −1.1 ± 0.21 for n-C₁₆ and −0.66 ± 0.15 for n-C₂₈. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C₂₈ conversion was observed.     

Changing the hydroisomerization to hydrocracking ratio of long chain alkanes by varying the level of delamination in zeolitic (ITQ-6) materials

Several delaminated ITQ-6 acid samples have been synthesized and tested in the isomerization of n-hexadecane. It has been found that the swelling time is a decisive parameter to prepare ITQ-6 catalytic materials with different ratios of micro to mesoporous surface area. n-Hexadecane hydroisomerization results indicate that the appropriate control of the delamination conditions can drive to obtain optimized ITQ-6 materials highly active for the hydroisomerization/hydrocracking of long chain alkanes to produce lube oils, diesels or gasolines.     

Improving then-pentane hydroisomerization of Pt/SO 4 2− SiO2) catalysts through mixing with ZrO2

The performance of Pt catalysts supported on sulfated zirconia-silica with different stoichiometries is investigated in then-pentane hydroisomerization reaction. Comparatively, with respect to the Pt/SO ₄ ^2– -SiO₂ or Pt/SO ₄ ^2– -ZrO₂ catalysts, the sulfated mixed oxides show an enhancement of the catalytic activity that increases with the content of ZrO₂, reaching its maximum at values between 10 and 15 wt% zirconia. The characterization of the samples reveals that at this stoichiometry occurs the highest H2-consumption of the samples as well as the top value of strong Brónsted acid sites according to the TPD-H₂ and FTIR measurements of absorbed pyridine respectively. That is, close to these percents of zirconia content one has a compound that is homogeneously mixed and above those values the segregations of the single oxides occur as verified by X-ray diffraction characterization.     

Effect of pillared clays on the hydroisomerization of n-heptane

Different montmorillonites and saponites were pillared with Al polyoxocations to obtain catalytic supports for the hydroisomerization of n-heptane. The catalysts were characterized by different techniques such as X-ray diffraction, elemental analysis and N₂ adsoprtion. The temperature-programmed desorption of ammonia indicated that pillared clays exhibited moderate and strong acid sites. The concentration of the acid sites depended on the starting clays as well as the type of the clays. The pillared saponites are more effective for the hydroisomerization of n-heptane at 300 °C, however, it decreased over the Al-pillared montmorillonites, and mainly cracking products were obtained.     

Influence of H4SiW12O40 loading on hydrocracking activity of non-sulfide Ni-H4SiW12O40/SiO2 catalysts

The effect of H₄SiW₁₂O₄₀ loading on the catalytic performance of the reduced Ni-H₄SiW₁₂O₄₀/SiO₂ catalysts for hydrocracking of n-decane with or without the presence of thiophene and pyridine is studied. The catalysts were characterized by BET, XRD, Raman, XPS, H₂-TPR, H₂-TPD, NH₃-TPD and FT-IR of pyridine adsorption. It was found that addition of H₄SiW₁₂O₄₀ to the system increases the catalytic activity and the promoting effect is a function of the H₄SiW₁₂O₄₀ loading. The best result was obtained on 5%Ni-50%H₄SiW₁₂O₄₀/SiO₂ catalyst which shows the highest activity for hydrocracking of n-decane and excellent tolerance to the sulfur and nitrogen compounds in the feedstock. The results showed that a suitable amount of H₄SiW₁₂O₄₀ loading on the 5%Ni/SiO₂ catalyst increases the amount of both hydrogen adsorbed and Brønsted acid and Lewis acid sites on the catalyst. The high catalytic performance of the catalyst can be related to the nature of H₄SiW₁₂O₄₀ and the proper balance between metal and acid functions.     

Performance of Pt/MgAPO-11 Catalysts in the Hydroisomerization of n-dodecane

MgAPO-11 molecular sieves with varying Mg contents synthesized by the hydrothermal method were used as supports for bifunctional Pt/MgAPO-11 catalysts. MgAPO-11 molecular sieves and the corresponding catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), temperature-programmed desorption of NH₃ (NH₃-TPD), differential thermogravimetric (DTG) analysis, temperature-programmed reduction of H₂ (H₂-TPR), H₂ chemisorption and catalytic reaction evaluation. The results indicated that the acidity generated via the substitution of Mg²⁺ for Al³⁺ in the framework increased with the Mg content. Acting as acidic components, the MgAPO-11 molecular sieves loaded with Pt were tested in the hydroisomerization of n-dodecane. Optimum isomer yield was obtained over the Pt/MgAPO-11 catalyst that had neither the highest acidity nor the highest Pt loading among the tested catalysts. In fact, the activity and the isomer yield both could attain a maximum on 0.5 wt.% Pt/MgAPO-11 catalysts with differing Mg contents. A lower Mg content resulted in an insufficient acidity, whilst a higher Mg content weakened the dehydrogenation/hydrogenation function of the Pt. These inappropriate balances between the acidic and the metallic functions of the catalysts would lead to low activities and isomer yields. On the other hand, the 0.5 wt.% Pt/MgAPO-11(3) catalyst was found to have a good balance between the acidic and the metallic functions, and thus exhibited both high activity and isomer yield in comparison with the conventional 0.5 wt.% Pt/SAPO-11 catalyst.     

Steam resistant CoLa-mordenite catalysts for the SCR of NOx with CH4

The stabilization of Co-mordenite catalysts through lanthanum exchange is reported here. The effect of exchange order and calcination conditions upon the reduction of NO_x to N₂ at 500 °C was tracked during 400 h on a stream containing NO_x, CH₄, O₂ and 10% H₂O. Both the fresh and used catalysts were characterized through TPR, Raman spectroscopy, FTIR spectroscopy using CO as probe molecule, and XPS. These techniques revealed that the CoLa-mordenite catalysts which were not affected by the severe hydrothermal treatment showed no sign of Co or La migration out of the exchange positions. Instead, those that rapidly deactivated showed the formation of cobalt oxides and, in some cases, the migration of the cations to other exchange positions. The presence of exchanged lanthanum seems to preserve the integrity of the zeolite structure preventing the migration of cobalt ions with the subsequent formation of cobalt oxides which favors the reaction of methane with O₂, thus decreasing N₂ production.     

Effect of hydrogen treatments on ZrO2 and Pt/ZrO2 catalysts

ZrO₂ and Pt/ZrO₂ catalysts have been investigated by TPR, hydrogen chemisorption, TPDH and in the conversion ofn-hexane. At high temperature, ZrO₂ takes up hydrogen. High temperature hydrogen treatment is a precondition of the catalytic activity in then-hexane conversion. Possibly, catalytically active acid sites are formed by this hydrogen treatment. The high temperature hydrogen treatment induces a strong Pt-ZrO₂ interaction.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

Highly selective and stable bimetallic catalysts supported on different materials for n-butane dehydrogenation

A study about the performance of Pt(0.3 wt%)Sn(0.3 wt%) catalysts supported on different materials in n-butane dehydrogenation is reported in this paper. The materials used as supports were γ-Al₂O₃, ZnAl₂O₄ spinel, MgAl₂O₄ spinel and spheres of α-Al₂O₃ with a washcoating of γ-Al₂O₃. The syntheses of both spinels leaded to very pure ZnAl₂O₄ and MgAl₂O₄ supports. The material prepared by washcoating showed the presence of an uniform and homogeneous layer of γ-Al₂O₃ (with a thickness between 12 and 18 μm) deposited on the spheres of α-Al₂O₃.The best behavior in activity, selectivity and stability through five severe cycles was achieved by bimetallic PtSn catalysts supported on MgAl₂O₄ spinel and on the material prepared by washcoating. The very good performance of these catalysts through the different cycles of reaction-regeneration can be due to the presence of metallic phases which preserve the strong intermetallic interaction along the different treatments, thus avoiding segregation processes.     

Gas phase hydrogenation of maleic anhydride to tetrahydrofuran by Cu/ZnO/TiO2 catalysts in the presence of n-butanol

Cu/ZnO/TiO₂ catalysts were prepared via the coprecipitation method. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectrometry, temperature programmed reduction, and N₂ adsorption. The catalytic activity of Cu/ZnO/TiO₂ catalyst in gas phase hydrogenation of maleic anhydride in the presence of n-butanol was studied at 235–280 °C and 1 MPa. The conversion of maleic anhydride was more than 95.7% and the selectivity of tetrahydrofuran was up to 92.7%. At the same time, n-butanol was converted to butyraldehyde and butyl butyrate via reactions, namely, dehydrogenation, disproportionation, and esterification. There were two kinds of CuO species present in the calcined Cu/ZnO/TiO₂ catalysts. At a lower copper content, the CuO species strongly interacted with ZnO and TiO₂; at a higher copper content, both the surface-anchored and bulk CuO species were present. The metallic copper (CuO) produced by the reduction of the surface-anchored CuO species favored the deep hydrogenation of maleic anhydride to tetrahydrofuran. The deep hydrogenation activity of Cu/ZnO/TiO₂ catalyst increased with the decrease of crystallite sizes of CuO and the increase of microstrain values. Compensations of reaction heat and H₂ in the coupling reaction of maleic anhydride hydrogenation and n-butanol dehydrogenation were distinct.     

Solvothermal synthesis of vanadium phosphate catalysts for n-butane oxidation

In this paper, we have developed a simple, low-cost, template-free and surfactant-free solvothermal process for synthesis of vanadyl hydrogen phosphate hemihydrate (VOHPO₄·0.5H₂O) with well defined crystal size. The synthesis was performed by reaction of VPO₄·2H₂O with an aliphatic alcohol (isobutyl alcohol, 1-pentanol, 1-hexanol, 1-heptanol or 1-decanol). This afforded well crystallized VOHPO₄·0.5H₂O by solvothermal methods at 120 °C temperature. This new method significantly reduced the preparation time and lowered production temperature (50%) of catalyst precursor (VOHPO₄·0.5H₂O) when compared to conventional hydrothermal synthesis methods. By varying the reducing agent, the solvothermal evolution process from layered tetragonal phase VOPO₄·2H₂O to orthorhombic phase VOHPO₄·0.5H₂O was observed. It was found that the length of carbon chain in an alcohol in the solvothermal condition had a great impact on chemical and physical properties of resulting catalysts. Interestingly, there was no trace of VO(H₂PO₄)₂ an impurity noted to be readily formed under solvothermal preparation condition. Therefore, this study introduces a more facile synthetic pathway to V(III) compounds. In addition, the microwave-synthesized catalysts exhibited some properties superior to those of conventionally synthesized catalyst such as better stability, crystallinity, and catalytic activity in the production of maleic anhydride. The characterization of both precursors and calcined catalysts was carried out using X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), N₂ physisorption, temperature programmed reduction (H₂-TPR) and scanning electron microscopy (SEM). The XRD pattern of the active catalyst prepared by this solvothermal method confirmed the presence of smaller crystal size (between 6 and 13 nm along 0 2 0 planes) of vanadium phosphate catalyst with higher specific surface area. Finally, the yield of maleic anhydride was significantly increased from 29% for conventional catalyst to 44% for the new solvothermal catalyst.     

Steam reforming of n-hexadecane over noble metal-modified Ni-based catalysts

Steam reforming of n-hexadecane, a main constituent of diesel, over noble metal-modified Ni-based hydrotalcite catalyst was carried out in a temperature range of 700–950 °C, at an atmospheric pressure with space velocity of 10,000–100,000 h⁻¹ and feed molar ratio of H₂O/C = 3.0. The catalysts were prepared by a co-precipitation and dipping methods. The noble metal-modified Ni-based hydrotalcite catalyst displayed higher resistance for the sintering of active metal than the Ni-based hydrotalcite catalyst prepared by the conventional method. It was found that the Rh-modified Ni-based catalysts showed high resistance to the formation of carbon compared to Ni-based catalysts. The results suggest that Rh-modified Ni-based catalyst can be applied for the steam reforming (SR) reaction of diesel.     

Effect of the properties of hydrous zirconia prepared under various hydrothermal conditions on the isomerization activity of Pt/WO3-ZrO2

A series of hydrous zirconia samples were prepared by hydrothermal method and the effects of the properties of hydrous zirconia on the catalytic activity of Pt/WO₃-ZrO₂ in the hexane isomerization were investigated. The catalysts were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and differential thermal analysis (TDA), H₂-temperature programmed reduction (H₂-TPR) and NH₃-temperature programmed desorption (NH₃-TPD). The results showed that the hydrothermal treatment under different times and pH values led to remarkable changes in the properties (such as hydroxyl group, ordering degree and thermal stability) of hydrous zirconia. Moreover, the isomerization activity of Pt/WO₃-ZrO₂ varied distinctly with the hydrothermal treatment condition of hydrous zirconia. The correlation between the properties of hydrous zirconia and the isomerization activity of the catalyst was primarily established. It was proposed that the isomerization activity was strongly dependent on the stability and ordering degree of hydrous zirconia, while it was irrelevant to the amount of hydroxyl groups in hydrous zirconia.     

Ni–Pt/H-Y Zeolite Catalysts for Hydroisomerization of n-Hexane and n-Heptane

Ni–Pt/H-Y zeolite catalysts with different Ni contents were prepared and applied to the hydroisomerization of n-hexane and n-heptane in the temperature range 225-375 °C. ESCA studies show the complete reduction of Ni species up to 0.3 wt% Ni addition over 0.1 wt% Pt/H-Y and further addition leads to the occurrence of unreduced nickel species as NiAl₂O₄. A TEM study shows the formation of bimetallic (Ni–Pt) particles of nanoscale size and the average particle size is found to increase with increasing Ni loading. Acidity measurements by NH₃-TPD and pyridine-adsorbed FTIR spectroscopy show the increasing occupation of acid sites by the added nickel when increasing the nickel loading. The catalytic activity of Ni–Pt/H-Y zeolite and Pt/H-Y catalysts was compared and it was found that addition of Ni up to 0.3 wt% increases the n-hexane and n-heptane conversion, multibranched isomer selectivity and sustainability of the catalysts due to better metal-acid synergism, complete reduction of Ni species and the formation of catalytically active Ni–Pt bimetallic particles. Further Ni addition leads to a decrease in conversion and multibranched isomer selectivity and an increase in the cracked products, which may be due to the presence of unreduced Ni species and pore blockage by larger-sized bimetallic particles formed.     

Effect of metal/acid balance in Pt-loaded large pore zeolites on the hydroisomerization ofn-Hexane andn-heptane

Large-pore zeolites H-Beta, H-Mordenite and H-Omega were loaded with platinum and applied for the hydroisomerization of n-hexane andn-heptane. The catalytic activity of Pt-loaded zeolite showed good correlation with the effective acidity probed by pyridine rather than with the total acidity probed by ammonia when equal amounts of metal sites are present. The selectivity to multibranched isomers over three different catalysts was found dependent upon the conversion ofn-paraffin. At low conversion, the selectivity to multibranched isomers was higher over Pt-load-ed zeolites with high acid strength. At high conversion level, however, the selectivity was mainly governed by the metal/acid balance. On H-Mordenite and H-Omega, large amount of strong acid sites are located in the small pore or cage. In Pt/H-MOR, the metal/acid balance was poor because there was detrimental loss of metal sites caused by isolation of Pt in the side pockets of 8-MR and/or pore blockage of the linear 12-MR channel. Even though the acidity of H-Beta was very low compared to H-MOR and H-Omega, Pt/H-Beta yielded the best performance for the hydroisomerization ofn-paraffin because of better metal/acid balance in Pt/H-Beta.