No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Assembly of the Presynthesized Crystalline AlPO4 Structure with Alumina and Its Promotion for Aromatic Hydrogenation
Abstract
To promote aromatic hydrogenation of distillates, a new composite support containing presynthesized crystalline AlPO4 structure was developed. This composite support causes easier reducibility of metal oxides and impedes nickel aluminate formation effectively compared with pure alumina counterparts. Moreover, the composite support exhibits higher surface area, larger pore volume, and better surface characteristic than the phosphoric acid-modified alumina to improve metal−support interactions and facilitate the polymeric tungsten species formation. The evaluation results show that the composite supported catalyst exhibits the highest conversion of tetralin, in comparison to the corresponding catalysts with pure alumina or phosphoric acid-modified alumina as supports.
... This difference probably affects the 2 International Journal of Chemical Engineering hydrogenation activity of catalyst and leads to the decrease of aromatics conversion. Generally, model compounds are composed of aromatics and inert hydrocarbons; for example, naphthalene dissolves in -hexadecane [16][17][18][19], -tridecane [20,21], -decane [22], -heptane [5,[10][11][12][13][14]23], and benzene [4] or tetralin dissolves in -dodecane [24], -decane [22,25,26], -heptane [6-9, 12, 15, 27, 28], -hexane [29], and cyclohexane [30,31]. The model compounds using light components as solvents are easier to vaporize and have lower dew point. ...
The effects of reactants' phase states (gas-liquid and single gas phase) on tetralin hydrogenation were investigated in the fixed bed. The kinetics of tetralin hydrogenation under different phase states was analyzed. Results showed that, without phase transition, the tetralin conversion increased with the rise of temperature. However, it decreased dramatically around the dew point of the feed at which the reactants' phase state transferred from gas-liquid phase into gas phase. It was also observed that the gas-liquid phase state was favorable to reduce the deactivation of catalyst in the tetralin hydrogenation.
A Ni–W loaded ETS-10/AlPO4-5/Al2O3 composite support catalyst was optimized and used in hydrodesulfurization (HDS) and hydrodearomatization (HDA) of Daqing FCC diesel feedstock. The result indicated that ETS-10 and AlPO4-5 showed positive synergism effect. The effects of operating conditions on its catalytic performance were investigated by using a 100 mL hydrotreating test unit. The catalyst showed a remarkable HDS conversion of 99.9% and a HDA conversion of 73.2%. A clean diesel product with ultra-low sulfur content (<1.0 μg/g) and very low polycyclic aromatic content (<2.0 wt.%) was obtained.
High surface area pure mesoporous aluminum-phosphorus oxide-based derivatives have been synthesized through an S(+)I(-) surfactant-assisted cooperative mechanism by means of a one-pot preparative procedure from aqueous solution and starting from aluminum atrane complexes and phosphoric and/or phosphorous acids. A soft chemical extraction procedure allows opening the pore system of the parent as-prepared materials by exchanging the surfactant without mesostructure collapse. The nature of the pore wall can be modulated from mesoporous aluminum phosphate (ALPO) up to total incorporation of phosphite entities (mesoporous aluminum phosphite), which results in a gradual evolution of the acidic properties of the final materials. While phosphate groups in ALPO act as network building blocks (bridging Al atoms), the phosphite entities become basically attached to the pore surface, what gives practically empty channels. The mesoporous nature of the final materials is confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N(2) adsorption-desorption isotherms. The materials present regular unimodal pore systems whose order decreases as the phosphite content increases. NMR spectroscopic results confirm the incorporation of oxo-phosphorus entities to the framework of these materials and also provide us useful information concerning the mechanism through which they are formed.
Since 1982, with the first report of molecular sieves with aluminophosphate lattices (the AlPO4 family), the compositional and structural diversity of AlPO4-based molecular sieves has continued to grow. It discusses that the synthesis of AlPO4-based molecular sieves has been achieved using hydrothermal synthesis techniques. Reaction mixtures containing All P, and one or more of 13 additional elements have been crystallized in the presence of organic amines or quaternary ammonium cations. These organic templating agents direct structure formation and are usually occluded in the voids of the crystalline microporous product. In addition to templating, other variables affecting synthesis are raw materials, composition of the reaction mixture, method of combining the reactants, crystallization time and temperature, and reaction pH. The first family of aluminophosphate-based molecular sieves designated AlPO4-n, contained Al and P in lattice T-sites. Subsequent efforts to incorporate other elements were successful and appropriate acronyms were introduced for each new family.
The effect of phosphorus on the structure of NiMo/Al2O3 hydrotreating catalyst precursors has been investigated. Calcined and reduced P/Al2O3, PNi/Al2O3, P-Mo/Al2O3, and PNiMo/Al2O3 (where the wt% P = 0.0 to 10.0, wt% Mo = 8.0 to 12.0, and wt% Ni = 4.0) have been studied using FT-IR, XRD, and 31P and 27Al MAS NMR techniques. Phosphoric acid reacts with alumina hydroxyls forming monomeric and polymeric phosphates. At the higher phosphorus loadings, aluminum phosphate is also formed. On calcined PNi/Al2O3, nickel phosphate is formed. This leads to a decrease in density of NO sites in the reduced state as measured by CO adsorption. The addition of up to 1.5 wt% P to Mo(8)/Al2O3 promotes the formation of octahedral molybdena on the alumina surface. However, the addition of > 2.0 wt% P results in the formation of bulk M003 and Al2(MoO4)3 in both PMo(8)/Al2O3 and PMo(12)/Al2O3. CO adsorption on reduced PNi(4)Mo(8)/Al2O3 samples shows that the presence of 0.5 wt% P causes a significant increase in the number of sites adsorbing CO. Increasing the P loading further causes a decrease in the number of adsorbing sites; this decrease can be attributed to the formation of either nickel phosphate or nickel molybdate.
The effect of phosphorus on the acidity and physical properties of a support of γ-alumina was studied, as a preliminary stage to understanding its role as an additive in hydrotreatment catalysts. The results showed that phosphorus was adsorbed on alumina following a Langmuir-type mechanism, in which a phosphate ion monolayer was formed at the surface. Equilibrium was strongly displaced towards adsorption. These results were verified using X-ray photoelectron spectroscopy. Phosphorus modified not only the acidity but also the physical properties of the alumina. From the experimental results obtained, an adsorption mechanism of phosphorus on alumina could be established.
High aromatic content in diesel fuel has been recognized both to lower the fuel quality and to contribute significantly to the formation of undesired emissions in exhaust gases [1, 2]. Because of the health hazards associated with these emissions, environmental regulations governing the composition of diesel fuels are being tightened in both Europe and the United States, leading to limitations on aromatics [3, 4].
A novel aluminophosphate microporous compound, denoted UHM-5, synthesized from an aqueous aluminophosphate gel at 150 °C in the presence of cyclohexylamine as the template, transforms upon calcination above 300 °C into AlPO4-5 molecular sieve.
The catalytic hydrocracking (HC) of diphenylmethane (DPM) and hydrodesulfurization (HDS) of dibenzothiophene (DBT) over Ni, Mo, and Ni–Mo sulfide catalysts supported on a mixed ultrastable Y (USY) zeolite and γ-Al2O3 were studied. The catalysts were characterized using NH3 temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), UV–Vis–NIR diffuse reflectance spectroscopy (DRS), high-resolution transmission electron microscopy (HRTEM), and chemical composition analysis. Because addition of zeolite to a conventional alumina support improves acidity, Ni, Mo, and Ni–Mo catalysts supported on the combined supports had much higher HC activity. Ni was found to be uniformly distributed throughout the catalysts; however, Mo preferentially entered the structure of γ-Al2O3 or was accommodated as oxide aggregates on γ-Al2O3, rather than associating with zeolite. Ni and Mo catalysts supported on γ-Al2O3–USY zeolite were good HDS catalysts and showed a shallow maximum in catalytic activity at a NiO and MoO3 content of 5 mol%. The higher activity at this content occurred because Ni or Mo species had higher surface concentrations, higher dispersion, and were more easily sulfided. Ni–Mo catalysts supported on γ-Al2O3–USY zeolite had high HDS activity, which showed a prominent maximum at a NiO/(NiO+MoO3) ratio of about 0.4, because at this ratio the surface species of Ni and Mo were well dispersed and more easily sulfided to form a Ni–Mo–S phase responsible for the high HDS activity. The Ni–Mo catalysts supported on γ-Al2O3–USY zeolite have slightly higher HDS activity than γ-Al2O3-supported Ni–Mo catalysts.
The effect of various impregnation procedures on the structuresof oxidic phosphorus-containing NiMo/Al2O3and CoMo/Al2O3catalysts was studied by quantitative31P and27Al solid state NMR. NH4H2PO4was used as the phosphorus source, since it reacts less strongly with the alumina support than does H3PO4. The presence of paramagnetic Co and Ni ions induces strong line broadening and a signal loss in the31P and27Al NMR spectra which can be used to deduce the distribution of Co and Ni over the catalyst surface. Large differences in this distributionexisted, depending on the paramagnetic ion and the impregnationprocedure. The addition of phosphorus led mainly to theformation of Co–Mo–P compounds in samples containing cobalt,whereas the creation of an aluminum phosphate phase was favoredin nickel-containing catalysts. An impregnation procedure inwhich Ni or Co and phosphorus are separately deposited on thecatalyst surface also promoted AlPO4formation, so that even inthe samples containing cobalt some aluminum phosphate isobserved. On the other hand, coimpregnation of Co or Ni andphosphorus resulted in catalysts which contained little aluminumphosphate. Possible consequences for hydrotreating reactions arediscussed.
Proton MAS NMR spectroscopy has been used to investigate the influence of the impregnation procedure on the surface hydroxyl structure of dehydrated γ-Al2O3impregnated with varying amounts of molybdenum or phosphorus. Large differences were observed for catalysts prepared by coimpregnation, sequential impregnation, and sequential impregnation with intermediate calcination. A quantitative analysis of the spectra showed that molybdate ions preferentially reacted with basic OH groups at low molybdate loading, but also with acidic hydroxyl groups at higher. Phosphate ions adsorbed on both types of hydroxyl groups, even at low or moderate coverage, while at high surface coverage a nearly constant density of Al–OH and P–OH groups indicated AlPO4formation. Phosphorus and molybdenum were well dispersed after coimpregnation, whereas after sequential impregnation (phosphorus first) some pore-plugging occurred and Mo–O compounds adsorbed on underlying phosphates.
Four types of alumina and/or USY-zeolite-supported NiMo catalysts (NMA, NMAZ, NMACZ1, NMACZ2) were examined in an autoclave for HDS of straight run gas oil (SRGO; S = 11,780ppm) and its hydrodesulfurized gas oils (HSRGO; S = 340ppm) to simulate a first and second layer catalyst. NMACZ1 and NMACZ2 were physical mixtures of alumina and surface modified USY-zeolites with different amounts of alumina, while NMAZ is a physical mixture of alumina and USY-zeolite. ACZ series catalysts were prepared to obtain high HDS activity of NiMo on zeolite-containing supports without extensive cracking of hydrocarbons. USY-zeolite in NMACZ1 was coated with more alumina than that in NMACZ2, which showed the highest HDS activity in the first and second layers, achieving 34ppm sulfur (ppmS) for the first layer and almost complete removal of sulfur species in the second layer at 340°C. NMAZ showed high HDS activity for both layers. NMACZ1 showed medium HDS activity for the first layer, but very high HDS activity for refractory sulfur species in the second layer. A large amount of light hydrocarbons were produced over NMAZ through cracking of heavy hydrocatbon, while NMACZ1 and NMACZ2 produced lower amounts of light hydrocarbons, reflecting the greater amount of coating alumina. It must be noted that lower reaction temperatures of 330 and 320°C achieved almost completely 0ppm in the second layer, where the hydrocracking was very low level. Their clearly defined HDS activities against each layer may be due to the differences of MoS2 structures, support acidities and their surface areas, which are in turn due to morphologies of their supports.
A series of co-impregnated (pH 1) PNiW/Al2O3 catalysts of low tungsten content (1.6 W atoms/nm2) and varying phosphorus concentration (0–5.9 wt.−% P2O5) have been studied in the reaction of hydrogenation of naphthalene in a batch Parr reactor at 573 K and P = 44 atm. In addition to a previous characterization by XRD, DRS and XPS, the catalysts have been characterized here by DRS, FTIR spectroscopy in their oxide state and by FTIR spectroscopy of NO chemisorbed on the sulphided catalysts. A promoting effect of phosphorus is observed in all phosphorus containing samples. The catalytic activity vs. phosphorus content curve has a complex behavior passing through a maximum and a minimum. Phase transitions are observed in the samples with the change of the phosphorus content. The form of the catalytic activity curve is discussed in the light of these phase transitions and the structure and properties of the different phases. The highest catalytic activity is observed for the sample with 0.6% P2O5. This sample is also characterized by the highest amount of chemisorbed NO and by a local maximum in the relative number of terminal WO bonds. When strong overlapping of the bands is observed, the NO adsorption technique should be applied with care. Also, NO might adsorb on sites which are not active in the corresponding hydrotreatment reaction and the trend predicted by the total amount of chemisorbed NO should not necessarily coincide with the form of the catalytic activity curve. The catalytic activity curves indicate that during the reaction changes in the number of active sites or the formation of new active sites are possible.
The effect of the preparation method and of phosphorus concentration on the dispersion, the distribution and on the type of the surface species formed by the active components in a series of P-Ni-W/Al2O3, samples with varying phosphorus content was studied by X-ray diffraction, infrared, diffuse reflectance and X-ray photoelectron spectroscopies. The formation of a A1PO4 species is observed in all phosphorus-containing samples. The distribution of phosphorus in the co-impregnated P-Ni-W/Al2O3, system is in the form of a monolayer up to concentrations of 1 P at/nm2. The distribution of nickel in the co-impregnated samples is inhomogeneous and is accompanied by the formation of NiO crystallites. The state of nickel in the samples is strongly influenced by the presence of phosphorus. The increase of phosphorus concentration in the co-impregnated P-Ni-W/Al2O3 samples stimulates the formation of a NiWO4 phase. The distribution and the dispersion of tungsten in phosphorus-containing samples is inhomogeneous and WO3, and NiWO4 phases are observed depending on phosphorus concentration and on the presence of nickel in the samples.
Ni–W catalysts supported on commercial γ-alumina and silica displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni–W/SiO2 catalyst in toluene hydrogenation (HYD) was 6 times higher compared with Ni–W/Al2O3. The dearomatization performance of Ni–W/SiO2 catalyst was tested over a wide range of operation conditions with naphtha and middle distillates. 90% saturation of aromatics in FCC naphtha (340°C, LHSV of 1h−1) and 50% in Light cyclic oil (LCO) (360°C, LHSV of 1h−1) was achieved at 5.4MPa. In a two stage process with the same Ni–W/SiO2 and intermediate separation of hydrogen sulphide 90% saturation of aromatics in LCO was achieved at 320°C and total LHSV of 0.5h−1. At equal conditions, Ni–W/Al2O3 catalyst yielded 1.5–4 times lower total aromatics saturation.
Both the ethenyl and phenyl peroxyl radical absorb in the visible region in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were initially surprising. Ab initio determination of the electronic structure of the ground and excited states of the ethynyl, ethenyl, and phenyl peroxyl radicals provides a fundamental understanding of these electronic transitions. The electronic excited states of these radicals are low in energy because the pi-type open-shell orbital localized on the oxygen atoms in the ground state couples to a relatively low-energy pi or conjugated orbital system in the excited state. In the case of both the ethynyl and phenyl peroxyl radicals, the excitation of the C-C bond pi orbital is relatively high in energy, and the in vacuo prediction for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because all the radicals are polar in the ground state, and the dipole moment, in the relevant excited state is substantially larger. In addition, for the ethenyl peroxyl radical there are two isomeric forms whose ground states are close in energy, and the observed spectrum can be assigned to the convoluted spectra of both isomers.
The influence of various hydrothermal treatments on the physicochemical properties of pure H-mordenites and matrix-embedded H-mordenites was investigated. It was found that the crystallinity and the chemical composition of pure mordenite were strongly dependent on the ageing conditions. Embedding of mordenite had no influence on the extent of dealumination and prevented the mordenite structure from collapsing. This protecting effect of the matrix on the zeolite structure was not observed for a physical mixture MOR/matrix, thus suggesting that intimate contact between the zeolite and the matrix is necessary for such an effect to take place. This intimate contact MOR/matrix was confirmed by the formation of new chemical bonds between hydroxyl groups on the outer surfaces of the zeolite crystals and those on the surface of the embedding matrix.
Catalysts based on NiMo and Pt supported on the new delaminated ITQ-2 zeolite have been prepared and their catalytic properties evaluated for the mild hydrocracking (MHC) of vacuum gasoil and aromatic hydrogenation. The results were compared with those obtained using other conventional supports, e.g., silica, γ-alumina, amorphous silica–alumina (25 wt% Al2O3), and USY zeolite, all of which contain the same metal loading as the ITQ-2 material. In the case of MHC of vacuum gasoil, NiMo/ITQ-2 displayed a higher hydrocracking activity than NiMo/SiO2–Al2O3 and NiMoγ-Al2O3, and even higher activity than NiMo/USY in the range 375–425°C. Moreover, NiMo/ITQ-2 had a selectivity to middle distillates intermediate between those of NiMo/USY and NiMo/SiO2–Al2O3. For hydrogenation of naphthalene, Pt/ITQ-2 displayed higher activity than Ptγ-Al2O3 and Pt/SiO2-Al2O3 but lower activity than Pt/USY with similar Pt dispersion. This is explained by considering that some of the Pt centers located in the 10 MR channels of ITQ-2 are not accessible to the naphthalene molecules. Indeed, Pt/ITQ-2 is significantly more active than Pt/USY for the hydrogenation of benzene, which can access the metal sites in the 10 MR channels of the delaminated ITQ-2 zeolite. Furthermore, Pt/ITQ-2 gave the highest aromatic reduction when using a hydrotreated light cycle oil (HT-LCO) feedstock containing ca. 70 vol% total aromatics, 0.40 wt% S, and 480 ppm N. In this case, the larger external surface area of ITQ-2 as compared with USY may favor the hydrogenation of the voluminous aromatic molecules present in the HT-LCO feed. These results can be explained by the peculiar structure of the delaminated ITQ-2 zeolite, which combines the good activity of zeolites with the desired selectivity of amorphous catalysts, while minimizing the diffusional problems often encountered in microporous materials.
Cracking of tetralin and decalin was carried out over several zeolites to establish the effect of the pore topology of the catalyst on product distribution. These molecules were chosen as probe molecules, because they indicate which catalyst is the best for cracking or hydrotreating the light cycle oil (LCO) fraction, which is obtained directly from fluid catalytic cracking units. A set of zeolites with medium-sized (ZSM-5, MCM-22, ITQ-2), large (USY, Beta), and ultralarge pores (UTD-1), as well as a mesoporous MCM-41, were used as catalysts at 723 K. The results demonstrate that pore size and topology have a strong influence on diffusion, and consequently, on activity and selectivity in reactions such as ring opening, dealkylation, transalkylation, hydride transfer, and coke formation. UTD-1 generally has the highest activity per framework Al owing to the pore size and topology of this zeolite that enables flat molecules to diffuse easily inside the pores. According to the results, zeolites with medium-sized pores are adequate in combination with large-pore zeolites to crack naphthenes and fused aromatic–naphthenic rings, of the type present in LCO, to produce propene. Large-pore zeolites show good selectivity for naphthenic ring opening and appear to be better suited for hydrotreating LCO. Beta zeolite is a catalyst that is especially suitable for both processes.
Solid state double resonance NMR experiments on phosphorus-impregnated gamma-Al2O3 and amorphous AlPO4 have been conducted to investigate the interaction between the impregnated phosphorus and the gamma-Al2O3 surface. The P-31-Al-27 REDOR and TRAPDOR experiments have shown that most phosphorus is in close contact with aluminum, thereby excluding the possibility that stacked phosphate layers or bulk phosphates are formed when more phosphorus is impregnated than is necessary to cover the entire gamma-Al2O3 surface. The Al-27-P-31 experiments enabled the Al-27 spectrum of the aluminum atoms which interact with phosphorus to be separated from the spectrum of the bulk gamma-Al2O3. These double resonance experiments have shown that a layer of AlPO4 is indeed formed on the gamma-Al2O3 surface and that the structure of this layer, although similar, has a slightly higher degree of-ordering than the structure of amorphous AlPO4.
Two different nickel supported on alumina-pillared α-zirconium phosphate materials with metal loading of 20 and 30 wt% have been prepared. In both cases, before reduction, the typical lines of NiO are observed in the XRD patterns. The reduced catalysts exhibit similar metallic area and dispersion values. On the other hand, the catalyst with 20 wt% Ni has the highest surface acidity. Two other NiMo catalysts with 20 wt% Ni and 5 or 10 wt% Mo have been also synthesized. Their XRD patterns show the presence of the NiMoO4 spinel together with an excess of the NiO phase. XPS analysis reveals the existence, in both samples, of 51 and 76% Ni as spinel. After reduction, all these catalysts were tested in the hydrogenation and ring-opening of tetralin at 6.0 MPa. The catalyst with the lowest Ni content (20 wt%) is very active at 623 K, with a conversion of 85% and a high yield of decalins (40%) and cracking compounds (CC) (close to 45%). However, these catalysts strongly deactivate in the presence of 1000 ppm of dibenzotiophene (DBT) in the feed. The incorporation of molybdenum to these catalysts (Ni : Mo weight ratios of 20 : 5 and 20 : 10) favours the ring-opening to the detriment of the hydrogenation process, attributable to new acid sites associated with the molybdenum species. In addition, the NiMo-20 : 10 catalyst exhibits a high thiotolerance in the presence of 1000 ppm of DBT, maintaining its activity after 7 h of time on stream, with a conversion higher than 65% and with good yield of decalins (45%), and maintaining an interesting yield of cracking compounds (18%).
Porous aluminophosphates, which are useful as catalyst supports for polymerization, isomerization, or other hydrocarbon conversions, can be made by coprecipitation when an acidic solution of Al/sup 3 +/ and POâ/sup 3 -/ ions is neutralized. When the P/Al ratio in solution is equal to or greater than one, AlPOâ is obtained often as a crystalline material, leaving the excess phosphate in solution. However, when excess Al/sup 3 +/ is present in solution (P/Al < 1) then it also precipitates and the resulting support retains a similar P/Al ratio to that in solution. In this study the structure of such aluminophosphates has been examined by means of X-ray diffraction and high resolution solid state NMR spectroscopy using both ²â·Al and ³¹P nuclei. These materials are not simple coprecipitated mixtures of AlâOâ and AlPOâ. In fact, no evidence for the presence of either species was detected. Instead they appear to be amorphous structures in which the phosphate is randomly dispersed, and the aluminum exists in one octahedral and several different tetrahedral environments. Results from ethylene polymerization over these catalysts also support this view.
Two types of alumina–USY supported NiMo catalysts (NMAZ and NMACZ) were examined in an autoclave for HDS of straight run gas oil (SRGO; S=11780 ppm) and its hydrodesulfurized gas oils (HSRGO; S=340 ppm) to simulate a first and second layer catalyst. One of the supports was a physical mixture of alumina and USY type zeolite (AZ), while the other was surface modified USY type zeolite with alumina (ACZ). Their supports alone (AZ and ACZ) were examined also to clarify the function of zeolite support during hydrodesulfurization (HDS) of gas oil. NMAZ was found to be active enough for HDS of SRGO to reduce sulfur content to 242 ppm. The extensive cracking to produce light hydrocarbons is another characteristic of that catalyst. NMACZ was inferior to NMAZ in HDS of SRGO but superior in HDS of HSRGO in the presence of H2S and NH3, achieving 10.3 ppm of sulfur. No cracking was observed over NMACZ which is another advantage of this catalyst. Extensive HDN took place over NMAZ but only moderate HDN over NMACZ. NH3 TPD and Raman spectroscopy of oxide forms of the catalysts indicate lower acidity of both catalysts and slightly better dispersion of MoO3 over the NMACZ, which had lower surface area than NMAZ.Zeoite-containing support alone adsorbed and desulfurized refractory sulfur species, especially 4,6-dimethyldibenzothiophene contained in HSRGO. HDS is suggested to be favored over the zeolite-containing supports by such mechanisms, in addition to their surface acidity, to enhance the hydrogenation and resistivity against H2S inhibitions.
WO3/TiO2−Al2O3 catalyst powders with titania−alumina mixed oxides synthesized by a sol−gel procedure have been prepared by dry impregnation. The surface structure of the resulting materials has been investigated by IR and Raman spectroscopies in the skeletal region and IR spectra of adsorbed water and ammonia. It is concluded that the largely predominant tungsten species, when these are below the monolayer coverage and in dry conditions, on all the supports studied, are constituted by monooxo wolframyl species which are coordinatively unsaturated and act as strong Lewis acid sites. The overall coordination around tungsten is consequently 4 and/or 5. By adsorption of water, the overall coordination of tungsten grows and the site behaves as a strong Brønsted acid site. The vibrational behavior of such species suggests that the WO vibrators are uncoupled, which means that they belong to isolated molecular units anchored to the surface by W−O−(Ti,Al) bonds, without significant extent of W−O−W bridges. The nature of the support surface significantly modifies the strength of the WO bond, which is indicative of the electronic state of tungsten. This is attributed to the different basicity of the surface oxide ions that act as the ligands of the wolframyl ion. The support surface hydroxy groups apparently do not play an important role in anchoring the wolframyl species. The “monolayer capacity” of the supports apparently depends quite strongly on the support nature, and it seems that care should be taken with its calculation, to compare on the same basis the results arising from different laboratories.
When mesoporous silica doped with zirconium with a MCM-41-type structure as a support was used, three different supported nickel catalysts were prepared by means of incipient wetness impregnation using different nickel salt solutions. Impregnation with aqueous or ethanolic nickel nitrate solutions gives rise to large Ni particles, whereas with an aqueous nickel citrate solution, the formation of small particles with a high metallic area is observed. Therefore, the use of nickel citrate as the nickel source effectively inhibits the aggregation of nickel particles. This catalyst showed the highest catalytic activity in the hydrogenation and ring opening of tetralin at a moderate temperature (648 K) and high hydrogen pressure (6 MPa), exhibiting a THN conversion of 90% with a high yield of hydrogenation products (53.8%) and cracking compounds (36.1%).
The 27Al NMR and 31P NMR MAS spectra of four crystalline aluminophosphate molecular sieves, AlPO4-5, AlPO4-11, AlPO4-17, and AlPO4-31 were studied at 4.7 T and compared to some nonmicroporous materials, AlPO4-quartz, metavariscite, and AlPO4-tridymite. The molecular sieve spectra are generally consistent with their known framework structures constructed of alternating AlO4 and PO4 tetrahedra; however, the 27Al NMR chemical shift range was wide and asymmetrical lines and multiple peak maxima were observed. Both as-synthesized and calcined forms were examined and in one case (AlPO4-17) adsorbed water reversibly resulted in a chemical shift into a region previously found from octahedral Al in aluminophosphates and herein from metavariscite. High fields (9.4 T) and {1H-27Al} cross-polarization techniques helped assign the most unusual 27Al NMR chemical shifts to the result of secondary interactions of framework Al with occluded template or H2O within the micropores. Quadrupole effects are so severe in AlPO4-quartz that even at 9.4 T a residual powder pattern persists under MAS conditions. All the 31P NMR MAS results are consistent with tetrahedral phosphorus.
The 1 H MAS-NMR and low-temperature oxygen chemisorption have indicated the monolayer loading of WO 3 on AlPO 4 as 9 wt %. The presulfided WO 3 /AlPO 4 catalysts have shown improved HDS activity compared to conventional η-Al 2 O 3 supported catalysts
Characteristics of zeolite β in stable aqueous colloidal solution were measured by cryo-TEM, DLS, SAXS and after separation from solution by SEM, HR-TEM. Twenty-nanometer spherical zeolite crystals were measured in equilibrium with flocculates. Mixing of colloidal zeolite solution with aluminogel yielded coagulation of both materials. The mass of zeolite adsorbed by aluminogel increased with decreasing pH. After calcination, pellets of the composite material at zeolite loading below 60 wt % contained separated nanocrystals of 10−15-nm zeolite β stabilized in the mesopores of alumina matrix. The imbedded zeolite had high structure order and acidity. No blocking of the zeolite micropores in composite materials was detected. The activity of zeolite β nanocrystals (imbedded in alumina matrix) in cumene cracking twice that of bulk nanozeolite clusters.
Catalysts consisting of CoMo supported on zeolite NaY, γ-Al203 and SiO2 have been characterized in their oxidic form using various techni
Nickel-tungsten supported on zirconium-doped mesoporous silica catalysts was prepared by using different methodologies and successfully tested at the hydrogenation and ring opening of tetralin. The preparation method has an important influence on surface properties such as dispersion, location, and reducibility of NiW species as deduced from H2-TPR, H2 chemisorption, FTIR spectra of CO adsorbed, and XPS studies. Thus, when nickel, in the form of nickel citrate, is first incorporated followed by tungsten, a catalyst is obtained with the highest acidity and the best catalytic performance, yielding a full conversion of tetralin (100%) as well as high yields of hydrogenation (42.7%) and ring-opening (56.1%) products under mild conditions (6.0 MPa of H2 and 375 °C). This catalyst exhibits a good thiotolerance in the presence of 300 ppm of dibenzothiophene in the feed, maintaining a high catalytic activity in the hydrogenation and ring-opening reactions after 6 h on stream.
Ni–W catalysts were prepared by impregnation of commercial -alumina and silica supports. The sulfidation, performed directly after drying at 100C, yielded fully sulfided Ni–W species on both supports (SEM-EDAX, XPS, XRD). At optimal metals loading (50 wt% NiO + WO3, Ni/W = 2), the sulfided catalysts had similar texture (N2 adsorption) and displayed similar activity in dibenzothiophene hydrodesulfurization (DBT HDS), while the activity of the Ni–W/SiO2 catalyst in toluene hydrogenation (HYD) was six times higher than that of Ni–W/Al2O3. This is due to the more than two times higher WS2 slabs stacking number in Ni–W/SiO2 compared with Ni–W/Al2O3 (XRD, HR-TEM), yielding stronger adsorption of toluene (TPD).
The reaction mechanisms of tetralin as a model compound of heavy petroleum fractions were investigated with the aim of improving bifunctional USY zeolite-based catalysts for heavy-oil hydrocracking. The major reaction path in the initial period was found to differ from that later in the reaction. In the initial reaction period, over USY catalysts with and without NiW sulfide, the reaction began with the formation of phenylbutyltetralin from two tetralin molecules by electrophilic aromatic substitution. Phenylbutyltetralin subsequently decomposed into benzene and octahydro tricyclic aromatic compounds. These reactions were catalyzed without the involvement of gaseous hydrogen and very likely lead to coke formation by chain reactions. Later in the reaction, tetralin and the above-mentioned heavy compounds were gradually hydrocracked over the bifunctional USY-supported NiW catalyst. These results indicated that the hydrogen supply from NiW sulfide to the acid site was not fast enough to prevent retrogressive reactions of partially hydrogenated polycyclic aromatic compounds. It was inferred that a closer relationship between the active sites of hydrogenation and cracking was needed for the improvement of bifunctional catalysts in heavy-oil upgrading.
Temperature-programmed reduction patterns are reported of NiO/Al2O3, WO3/Al2O3, and NiOWO3/Al2O3 catalysts, and of bulk oxides relevant to these catalysts. At loadings below 10% NiO and 19% WO3 nickel and tungsten are present as disperse species and no bulk oxides are found on the support. Tungsten forms a stable monolayer species which is extremely difficult to reduce. At high temperatures of reduction tungsten metal is formed without the intermediate formation of WO2. Several nickel species were detected in the supported catalysts, and their characters and amounts depend on the loading and the temperature of calcination. After calcination at low temperatures and at low loadings (2% NiO) nickel is incorporated in the surface layers of the support; at higher loadings a NiO like species is formed which is more difficult to reduce than bulk NiO because of the interaction with the aluminium ions of the support. A high temperature of calcination favours the formation of a diluted spinel phase at the expense of the surface phases. In NiOWO3/Al2O3 catalysts a fraction of nickel is present in a mixed NiWOAl phase which is formed by incorporation of nickel in a tungsten containing surface layer. At high temperatures of calcination some disperse NiWO4 is formed. It is shown that there are two causes for a strong interaction between nickel species and the support: incorporation of nickel ions in the surface layers of the support during impregnation, and solid-state diffusion during calcination of the catalysts. The nickel containing species which are the precursor to the active phases in sulfided HDS catalysts are identified as the nickel species in the surface layers of the support in NiO/Al2O3 samples, and the NiWOAl phase in NiOWO3/Al2O3 catalysts.
Supported Mo–sulfide catalysts were structurally characterized by means of transmission electron microscopy (TEM), dynamic oxygen chemisorption (DOC), and EXAFS. The catalysts show the well-known MoS2 slab structures with a multilayered morphology in the case of Mo/SiO2 and Mo/ASA. The MoS2 edge dispersion was evaluated from the TEM micrographs. While sulfidation of Mo/Al2O3 results in a highly dispersed, mostly single–layered MoS2 phase, a decreased metal–support interaction (NTA addition) or use of supports with a lower metal–support interaction leads to a higher stacking degree concomitant with a loss in edge dispersion. Combined TEM and DOC results reveal that Mo/C has the highest MoS2 dispersion. Reaction rate constants corrected for MoS2 dispersion for hydrodesulfurization (HDS) and hydrogenation (HYD) of thiophene and dibenzothiophene and HYD of toluene were measured. In general, HYD rates increase with an increasing stacking degree attributed to a less hampered planar adsorption geometry of reactants on multilayered MoS2. In contrast to the HDS rate constant of the small thiophene molecule, the DBT HDS rate constant is also strongly dependent on the stacking degree. It is concluded that perpendicular adsorption via sulfur is favored for HDS of thiophene, while in the DBT case a planar adsorption geometry is preferred. Carbon is the preferred support for the HYD of thiophene and toluene, most likely due to the large fraction of corner sites. A real support effect is also found for Mo/ASA, which exhibits low intrinsic HDS activities compensated by very high HYD activity. The present results indicate that the selectivities for hydrodesul-furization and hydrogenation can be fine-tuned by the morphology of the MoS2 phase and the choice of support.
The saturation of aromatic compounds in distillate fractions and in particular in diesel fuel has received considerable attention in recent years. A high aromatic content is associated with poor fuel quality, giving a low cetane number in diesel fuel and a high smoke point in jet fuel. There is also evidence that particulate emissions in diesel exhaust gases correlate with the aromatic content of the fuel. New legislation has been introduced to limit aromatics in diesel fuel and this has led to new catalyst and process developments for aromatic saturation. This paper gives an overview of these developments. The types of aromatic compounds found in distillate streams are described, and the kinetics of both single (model) compounds and groups of compounds as found in industrial feedstocks are discussed. Both supported metal sulfide and supported noble metal catalysts are used industrially and the paper outlines the role of the active species in these catalysts and compares reaction conditions used for each. The tolerance of different catalyst systems towards sulfur and nitrogen in the feed is dealt with in some detail. Commercial processes employ either single- or dual-stage catalyst systems depending on the nature of the aromatic saturation catalyst. The paper considers the merits of different process configurations. The paper concludes with a brief survey of possible future applications for distillate aromatic saturation catalysts.
This paper is a selective review on design approaches and associated catalysis and chemistry for deep desulfurization and deep dearomatization (hydrogenation) of hydrocarbon fuels, particularly diesel fuels. The challenge for deep desulfurization of diesel fuels is the difficulty of removing the refractory sulfur compounds, particularly 4,6-dimethyldibenzothiophene, with conventional hydrodesulfurization processes. The problem is exacerbated by the inhibiting effect of polyaromatics and nitrogen compounds, which exist in some diesel blend stocks on deep HDS. With the new Environmental Protection Agency (EPA) Tier II regulations to cut the diesel sulfur from current 500 ppmw down to 15 ppmw by June 2006, refineries are facing major challenges to meet the fuel sulfur specification along with the required reduction of aromatics contents. The principles and problems for the existing hydrodesulfurization processes, and the concepts, advantages and disadvantages of various new approaches will be discussed. Specifically, the following new design approaches for sulfur removal will be discussed: (1) novel catalysts for ultra-deep hydrodesulfurization under conventional HDS process conditions; (2) new design concept for sulfur-tolerant noble metal catalysts for low-temperature hydrogenation; (3) new desulfurization process by sulfur adsorption and capture under H2; (4) new desulfurization process by selective adsorption at ambient temperature without H2 and a related integrated process concept; (5) oxidative desulfurization in liquid-phase; and (6) biodesulfurization.
The influence of phosphate on the acidity and thermal stability of γ-alumina and alumina supported hydrotreating catalysts has been investigated. The results from the temperature-programmed desorption of ammonia showed that the addition of phosphate significantly alters the acid strength distribution of γ-alumina. While the number of strong acid sites is reduced, the concentration of medium strength acid sites is progressively increased with increasing phosphate level. Phosphate ion has a very significant effect in improving the thermal stability of γ-alumina with regard to sintering and phase transition to α-alumina, but, in the presence of molybdena it loses the stabilizing function. Calcination of molybdena based catalyst extrudates in their oxidic form (MoO3/γ-Al2O3, MoO3-PO4/γ-Al2O3, NiO-MoO3-PO4/γ-Al2O3) at high temperatures results in considerable reduction of the mechanical strength. The possible role of MoO3 in disrupting the stabilizing effect of phosphate ion with regard to the sintering and phase transformation behavior of alumina support is discussed.
Two series of phosphorus containing NiW/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5. The influence of the phosphorus concentration and the preparation procedure on the structure and the dispersion of the compounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases the amount of Ni2+(Oh) ions in the oxide form of the samples. This effect of phosphorus is better expressed when phosphorus is first introduced to alumina, followed by co-impregnation with nickel and tungsten. Surface AlPO4 is formed in the phosphorus containing samples irrespective of the preparation procedure. Phosphorus is distributed as a monolayer up to concentrations of 1.3 P at/nm2 in both series of catalysts. It was also found that the increased phosphorus content in the samples leads to an increased degree of polymerization of the tungsten species via WOW bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of tungsten species shows that it increases from 10Åfor the phosphorus free sample up to 20–25Åfor the samples with high phosphorus content. The relationship between the structure of the phosphorus promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed.