N-hexane Isomerization Reaction Research Articles

Catalytic performance and kinetic modeling of n-hexane isomerization over phosphomolybdic acid (HPMo) combining palladium and platinum supported on metal-organic framework MIL-101(HPW)

A metal-organic framework MIL-101(HPW) was synthesized by hydrothermal reaction, and phosphomolybdic acid (HPMo) was synthesized by its precursor solution. Then (Pd + Pt)/HPMo/MIL-101(HPW) with different wt% were prepared by the impregnation method. The synthesized samples were characterized by XRD, FT-IR, NH3-TPD, N2 adsorption, FESEM, and TGA, and their performances were investigated in n-hexane isomerization. Under the optimized operating conditions (temperature of 180 °C, H2/nC6 molar ratio of 2, and WHSV of 3 h−1), the appropriate yield for the nC6 isomerization was provided. The (0.2%Pd + 0.3%Pt)/50%HPMo/MIL-101(HPW) sample showed a lifetime of 720 min with 92.5% iC6 selectivity and 75.3% nC6 conversion. Kinetic modeling of the n-hexane isomerization reaction was also performed, in which the classical bifunctional mechanism was used to calculate the kinetic parameters. The validity of the bifunctional behavior was proved by the high value of the statistical correlation coefficient of this model. The apparent activation energy of this mechanism was calculated at 42.5 kJ mol−1.

N-Hexane Isomerization in the Presence of Nanodispersed Catalysts Supported on Acid-Activated Montmorillonite in the Sodium Form

Catalysts based on a palladium sol supported on acid-activated montmorillonite in the Na form (Pd-sol/NaHMM) have been tested in the n-hexane isomerization reaction. An increase in the Pd content from 0.1 to 0.35% leads to an increase in the catalyst activity. The C6+ isomer selectivity remains stable and fairly high: it is 94.6–98.0% in the presence of the catalysts with a Pd content of 0.1 or 0.35%, respectively. The maximum conversion of n-hexane over the 0.35%Pd catalyst is 52.6% at a temperature of 400°C; the addition of mordenite leads to a slight decrease in conversion to 50.9%. The maximum conversion at 400°C in the presence of the 0.1%Pd catalyst and the mordenite-modified catalyst is 45.0 and 50.4%, respectively. The maximum yield of isomeric hexanes over the zeolite-free 0.35%Pd and 0.1%Pd catalysts is 44.9 and 39.3%, respectively. In the presence of the mordenite-containing 0.35%Pd/NaHMM+HM catalyst and the low-percentage 0.1%Pd/NaHMM+HM catalyst, the C6 isomer yield is 44.3 and 43.0%, respectively. The amount of hydrocracking products does not exceed 0.7%. The particle size of the Pd sols is 3.5–5 nm, as determined on a JEM 2100 high-resolution transmission electron microscope. The elemental composition of the Pd-sol/NaHMM catalysts containing different amounts of palladium and the catalysts modified with mordenite has been determined by energy dispersive X-ray fluorescence spectroscopy.

Influence of the zirconium precursor on the acidic and catalytic properties of sulfated zirconia catalysts prepared by sol–gel process

Sulfated zirconia catalysts were prepared using the sol–gel process. Hydrolysis water was produced in situ. The effects of the zirconium precursor on the physicochemical properties and the acidity of the solids have been investigated. The prepared catalysts were characterized by XRD, Raman spectroscopy, XPS, FTIR of adsorbed pyridine, physisorption of N2 at 77 K, and TPR–H2–MS. Pt-promoted catalysts were tested for n-hexane isomerization reaction and the influence of Pt amount on the stability was studied. Obtained solids exhibit low specific surface areas and narrow pore size distributions. In addition, the metastable tetragonal phase of ZrO2 was stabilized at a relatively high temperature. The acidity of catalysts prepared from different zirconium alkoxides and the magnitude of interaction between zirconia and sulfate species are compared and discussed.

Platinum Catalysts Supported on Iron-Pillared Sodium- and Calcium-Saturated Montmorillonite in n-Hexane Isomerization

The activity of 0.1%Pt and 0.35%Pt catalysts supported on iron-pillared Na- and Ca-saturated montmorillonite has been tested in the n-hexane isomerization reaction. It has been found that a decrease in the platinum content to 0.1% leads to the formation of high amounts of diisomeric hexanes and heptanes. The activity of the catalysts has been correlated with their physicochemical characteristics determined by the BET, ammonia TPD, electron microscopy, and Mossbauer spectroscopy methods. The high strength of the proposed catalysts, high yields of C6 and C7 di- and tri-isomers from n-hexane in the presence of catalysts with a low Pt content, the use of available and cheap natural materials, and a fairly high n-hexane conversion suggest that these catalysts can be used in the isomerization of low-octane light naphtha to high-octane isomers.

Investigation of Palladium Catalysts in n-hexane Isomerization Reaction

Pd/SO42-/ZrO2/Al2O3catalysts are prepared and investigated in reaction of isomerization of N-hexane in the work. For comparison Pt/SO42-/ZrO2/Al2O3 catalysts are investigated. It is established that in the field of low temperatures (140°C) the catalyst on the basis of palladium shows high rates comparable to the Pt-containing catalyst. At more high temperatures yielding of isomers for Pd/SO42-/ZrO2/Al2O3 catalyst are lower, than for Pt/SO42-/ZrO2/Al2O3. However, isomerization depth (the maintenance of a 2, 2-dimetilbutan in mix of C6 isomers) for the palladic catalyst in all studied range of temperatures exceeds for 6-10% a similar indicator for the platinum catalyst. For clarification of the role of condition of palladium the catalysts Pd/Al2O3, Pd/SiO2 were prepared and investigated. It was established that the best indicators are reached on the aluminum oxide systems in which palladium is in the oxidized state.

Comparative study of aerogels nanostructured catalysts: Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2−

A series of Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2− catalysts were prepared in one step by the sol–gel method and dried in hypercritical conditions of the solvent. The characteristic properties of those solids were investigated using many techniques: the XRD, the physisorption of N2, the IR, the UV–visible, and the X-ray photoelectron spectroscopy (XPS). Textural analysis reveals the mesoporosity of all the aerogel catalysts. Moreover, the addition of alumina to nickel sulfated zirconia at high calcination temperature increased twice the specific surface area, from 72 to 158 m2/g. XRD patterns show that nickel-promoted sulfated zirconia calcined at different temperature develops the tetragonal and the monoclinic ZrO2 phase, whereas the nickel sulfated zirconia alumina exhibits only the ZrO2 tetragonal phase. The addition of aluminum to nickel sulfated zirconia induces significant changes in symmetry of nickel by the migration of Ni ions from octahedral to tetrahedral coordination. XPS spectroscopy shows that the nickel in Ni/ZrO2–SO4 2− catalysts is more reducible than those in Ni/ZrO2–Al2O3–SO4 2−. Nickel sulfated zirconia catalyst exhibits higher activity than nickel sulfated zirconia alumina, in the n-hexane isomerization reaction.

The effect of extra framework species and acid properties of dealuminated zeolitic support on the catalytic behavior of palladium in n-hexane isomerization reaction

Isomerization of n-hexane was carried out over palladium based catalysts prepared by aqueous ion exchange with dealuminated NaY-zeolite support. The dealumination was realized using acid solutions of HNO3, HCl and H2SiF6 with different concentrations. The catalysts were characterized by nitrogen physisorption, XRD, 27Al and 29Si-MAS-NMR, the NH3-TPD, the H2-TPR and DRIFT. The catalytic reaction was performed in the temperature range between 498 and 548 K. The results showed that the isomer selectivity and catalytic activity of catalysts increased with increasing the Si/Al ratio of the support and, specially, with the presence of extra framework species which control the active Bronsted and Lewis acid sites.

Effect the solvent evacuation mode on the catalytic properties of nickel-modified sulfated zirconia catalysts: n-hexane isomerization

This work studies the effect of the evacuation mode of the solvent to optimize the catalytic performances of sulfated zirconia doped with nickel prepared by the sol gel method in one step. Aerogel and xerogel catalysts exhibit different textural, structural and catalytic properties at various calcination temperatures. Aerogels, obtained by drying under supercritical conditions of solvent, exhibit a developed specific surface area and stabilize zirconia tetragonal phase before heating and even at high calcination temperature. However, xerogels obtained by ordinary drying in an oven are amorphous and has a low surface area and weak porosity. XPS spectroscopy shows that the nickel in aerogels is more reducible than those in xerogels. Aerogels exhibit higher activity than the xerogels, in the n-hexane isomerization reaction.

N-Hexane Isomerization on Ni-Pt/Catalysts Supported on Mordenite

The aim of this work was to evaluate the catalytic properties for n-hexane isomerization of bifunctional monometallic (Ni or Pt) and bimetallic catalysts (Pt-Ni), using HMOR zeolite as support. The method used for metal dispersion in the zeolite was competitive ion exchange using ammine complexes [Ni(NH3)6]Cl2 and [Pt(NH3)4]Cl2 as precursors. The catalysts were characterized by X-Ray diffraction, X-Ray energy dispersion spectroscopy, temperature-programmed reduction and transmission electron microscopy. The n-hexane isomerization reaction using the catalysts was carried out to evaluate the catalyst activity. The reaction was carried out in a fixed bed reactor operating at 250?C, 1 atm, H2/C6 = 9 molar ratio. The profiles obtained from TPR suggest that, for bimetallic catalysts, the presence of platinum facilitates the reduction of Ni2+ cations. The bimetallic catalysts presented a higher activity in the isomerization of n-hexane when compared to the monometallic ones, as well better stability as the Pt content in the solid increases.

A comparative study of n-hexane isomerization over solid acids catalysts: Sulfated and phosphated zirconia

Two series of zirconia based catalysts promoted with either sulfates or phosphates were prepared, calcined at different temperatures (600 and 700?C) and evaluated for the n-hexane isomerization reaction. The catalysts with different concentrations of sulfates or phosphates (4 or 10 wt. %) were characterized by BET, XRD, SEM methods, and total acidity was evaluated by using the Hammett indicators. Their final catalytic performances were correlated with their physical-chemical properties (surface, structural, textural and morphological). It was found that sulfated zirconia catalyst calcined at lower temperature showed the highest initial activity of all tested catalysts as the result of favorable total acidity, mesopore texture and structural properties. Somewhat lower activity of the sulfated catalyst calcined at higher temperature is related to the content of acid groups partially removed during thermal treatment, thus, lower total acidity, and also to less favorable textural and structural features. Negligible activity of phosphated zirconia catalysts is connected with low total acidity despite the positive status of particular property showing the complexity of the active phase/site formation in the catalyst.

Study of the Formation of WOx Nanostructures Supported on Zirconia Prepared by Different Synthesis Routes

AbstractDifferent synthesis procedures of Pt supported on tungstated zirconia catalysts (Pt/WOx-ZrO2) were investigated with the aim to elucidate the different WOx nanostructures developed on the zirconia surface depending on the preparation route. Pt/WOx-ZrO2 catalysts were synthesized by the coprecipitation and impregnation methods and pretreated by various procedures such as different calcinations temperatures or the use of reflux. The catalysts characterization was carried out by X-ray diffraction (XRD), Raman spectroscopy, transmission and scanning electron microscopy (TEM, SEM) and nitrogen physisorption, and the catalytic activity was evaluated in the n-hexane isomerization reaction. The results indicate that the development of active sites for isomerization of n-hexane is enhanced by the stabilization of the WOx nanostructures on the surface of zirconia, before the formation of the WO3 crystallites, and it largely depends on the synthesis method.

Characterization and reactivity of aerogel sulfated zirconia-ceria catalyst for n-hexane isomerization

Sulfated zirconia doped with cerium has been prepared by sol gel process using the supercritical evacuation way of the solvent. The physicochemical and catalytic properties of aerogel solids AZC were compared as a function of both the Ce/Zr molar ratio and the calcination temperature. The samples were characterized by N2 adsorption desorption, XRD, FTIR, RAMAN, XPS spectroscopy and TPD of NH3. The catalyst prepared with molar ratio Ce/Zr = 0.4 exhibits the high activity and selectivity toward isomerization product in the n-hexane isomerization reaction. This result is probably due to its important total acidity.

Effect of isopropanol aging of Zr(OH) 4 on n-hexane isomerization over Pt-SO 42−/Al 2O 3–ZrO 2

Two Pt/SO 4 2−/ZrO 2–Al 2O 3 (denoted as PSZA and I-PSZA) catalysts were prepared by using Zr(OH) 4 as precursor aged in different conditions, including conventional aging and isopropanol aging. The textural properties, crystalline structure, surface acidity and reduction behaviours of the as PSZA and I-PSZA catalysts were comparatively investigated by N 2 adsorption isotherms, XRD, TG–DSC, pyridine-FTIR and H 2-TPR techniques. Catalytic tests showed that, using isopropanol aged Zr(OH) 4 as ZrO 2 precursor, in the n-hexane isomerization reaction, the catalytic activity and stability of the catalyst could be greatly increased and the selectivity of 2,2-DMB was enhanced by more than two times. It was found that isopropanol aging of Zr(OH) 4 modified the textural properties and inhibited zirconia phase transformation and stabilized the tetragonal ZrO 2 crystalline structure. On the I-PSZA catalyst, more acid centers were formed in comparison with that presented on the PSZA catalyst. Our experimental results show that the surface acidity, textural properties and phase composition of the Pt/SO 4 2−/ZrO 2–Al 2O 3 catalysts play an important role in n-hexane hydroisomerization.

Temperature imposed textural and surface synergism affecting the isomerization activity of sulfated zirconia catalysts

Using sulfuric acid as the sulfating agent, two catalyst series were obtained from hydroxide and nitrate precursor with a sulfate loading identical to commercial sulfated hydroxide, i.e., 4.2 mass%. After calcination at 500, 600 and 700?C, all nine samples had various contents of residual sulfates depending on the origin of the catalyst. Accordingly, their surface properties were different, which, together with various textural properties, govern the formation of the active phase and their catalytic activity in the n-hexane isomerization reaction. The dominant activity and yield of mainly mono-branched isomers were attained in reaction at 200?C with a commercially sulfated zirconia catalyst calcined at 500?C. Among the SZ catalyst series synthesized from hydroxide and nitrate, the second according to its activity profile was similar to that of the commercially sulfated one, while samples originating from hydroxide showed some activity only after calcination at 600?C. This is due to the poorer textural properties of the hydroxide series, necessitating a higher calcination temperature in order to promote the simultaneous decomposition of S-containing species and their re-adsorption into the zirconia matrix following interaction and active phase formation. It seems that the tetragonal zirconia phase was not responsible for the catalytic activity but a synergistic effect of the textural properties of the samples and the sulfate loadings, which determine different acid strengths on the catalyst surface.

A new activation process of bimetallic catalysts and application to the n-hexane isomerization

Bimetallic catalysts applied to the n-hexane isomerization reaction were prepared with a constant molar content of 130 μmol Me/gcat of Ni and Pt and characterized in its active form by X-ray absorption near edge spectroscopy (XANES), transmission electron microscopy (TEM), temperature programmed reduction and hydrogen chemisorption techniques. The metal was dispersed in a HUSY zeolite matrix in order to provide both types of sites, metallic and acid, to obtain a bifunctional catalyst. The catalytic activity was evaluated using the catalyst activated at different temperatures following a new activation process, named “fast activation method” and the profile pointed out a maximum catalytic activity when the catalyst was activated at 400 °C. XANES results showed that all samples present similar electronic properties in the studied conditions, despite the differences found in the average particle diameter obtained by TEM, suggesting that the catalytic activity is related to the metal/acid balance.

Sulfated zirconia grafted on a mesoporous silica aerogel: Influence of the preparation parameters on textural, structural and catalytic properties

Silica supported sulfated zirconia catalysts were synthesized via a new method by grafting sulfated zirconia on the surface of a silica aerogel previously prepared. The main parameters studied in this work were the S/Zr, Zr/Si molar ratios and the support nature. The synthesized solids were characterized using XRD, N 2 physisorption at 77 K, TG-DTA/SM, sulfur chemical analysis and adsorption–desorption of pyridine followed by infrared spectroscopy. These solids were tested in the n-hexane isomerization reaction. Two types of mesopores were observed on the silica aerogel. This mesoporosity was affected depending on the preparation parameters. The increase of the Zr/Si molar ratio induces the decrease of the size of zirconia particles deposed on the support. In this case, appreciable amounts of sulfur are retained with the presence of a relatively strong Brönsted and Lewis acid sites on the catalyst surface. A high density of Brönsted sites seems to be interesting in the n-hexane isomerization reaction.

Zeolite supported Pt-Ni catalysts in n-hexane isomerization

Bifunctional catalysts containing 1–4 wt.% of Ni and Pt in a metallic form supported in an ultrastabilized HUSY zeolite were prepared by competitive ion exchange method. The catalysts were characterized by ICP and TPR techniques to obtain metal contents and conditions for the reduction steps, respectively, and evaluated in the n-hexane isomerization reaction. The Pt monometallic catalysts were more stable, active and selective in this reaction than that containing only Ni. However, it was observed that maintaining the same total metal loading (1 or 2 wt.%) and varying the proportion of these metals, the Pt-Ni bimetallic catalysts containing 20–30% Pt presented higher activity in the mentioned reaction than the Pt monometallic catalyst. In addition, the bimetallic catalysts were more selective to the formation of di-branched alkanes, that is, products with higher octane number.

Migration and oxidation of tungsten species at the origin of acidity and catalytic activity on WO3-ZrO2 catalysts

A WO3-ZrO2 catalyst, prepared by a precipitation method, was annealed at 560, 700 and 800 °C with a view to examine the nature of WOx species using infrared and Raman spectroscopy, X-ray photoelectron spectroscopies (XPS), high resolution and energy filtered transmission electron microscopy (HRTEM and EFTEM). When calcining at 560 °C, a large amount of W atoms remain in W5+ oxidation state, in strong interaction with the ZrO2, forming ZrOW bonds. Only a little fraction of the W atoms was expelled to the surface as isolated WOx species, which are responsible for the Brönsted acid sites. After calcining at 700 °C, all W atoms migrated to the surface, remaining in full W6+ oxidation state, forming ca. 0.5 nm nanoclusters. In these nanoclusters, all W atoms are exposed on the surface and promote acid sites, mainly Brönsted acidity. When the sample was calcined at 800 °C, the tungsten nanoclusters grew to partially reduced ca. 3.0 nm WOx domains. These WOx domains showed very low acid sites density and strength, however, they showed the highest catalytic activity in n-hexane isomerization reaction; supporting the hypothesis that redox properties of WOx domains are responsible for this reaction.

Characterization of sulfated TiO2 prepared by the sol–gel method and its catalytic activity in the n-hexane isomerization reaction

Sulfated TiO2 was prepared by a single-step sol–gel method, in which the hydrolysis and sulfation occur in the same step, varying some conditions of the preparation method. The characterization was carried out by thermogravimetric analysis, infrared absorption spectroscopy, Raman spectroscopy, pyridine adsorption infrared spectroscopy for the determination of the Brönsted acid sites/Lewis acid sites ratio, determination of the specific surface area (BET method), average pore diameter (BJH method) and catalytic activity in the n-hexane isomerization reaction. In the infrared spectra it was verified that the sulfate is coordinated to the titanium in a chelate form, while in the Raman spectra it was observed that the samples are in an anatase crystalline form. Catalytic tests of the n-hexane conversion at 100 and 200 °C were carried out. At 100 °C, it was observed that the selectivity for isomerization and catalytic stability are higher than at 200 °C, however, at 100 °C the conversion is lower than at 200 °C. Among the structural and textural properties investigated, it was found that the samples which had greater average pore diameters and a higher Brönsted acid sites/Lewis acid sites ratio were the ones which showed greater catalytic activity.

N-Hexane Isomerization over NiO/SO2-4/ZrO2Catalysts. Effect of Nickel Loading

The effect of nickel addition to the sulfated ZrO2 catalysts for n-hexane isomerization was investigated. A series of catalysts with different nickel concentration (from 0 to 9.6 wt%) were synthesized by supporting nickel on sulfated zirconia by incipient wetness method. After a calcination procedure the catalysts were characterized by X-ray diffraction, nitrogen physisorption (BET method) and its acidic properties were determined by NH3 adsorption. (TPD) and FT-IR of pyridine adsorption. The n-hexane isomerization reaction was carried out in a fixed bed microreactor at 673 K under 345 kPa of total pressure. Results showed that nickel content between 1.5 and 4.5 wt% improved the catalytic activity and favored the formation 2,3-dimethylbutane, which is a highly desired product. X-ray diffraction results showed no evidence of any NiO phase at NiO concentration below 9.6 wt% suggesting a NiO phase highly dispersed on the surface of ZrO2–SO2− 4 with crystallite sizes lesser than 3nm. Catalytic activity and 2,3-dimethylbutane selectivity seems to be correlated with the NiO well dispersed phase rather than with their acidic properties.