Performance of spent sulfide catalysts in hydrodesulfurization of straight run and nitrogen-removed gas oils

After the test run of several months two kinds of commercial catalysts (NiMo/Al₂O₃ and CoMo/Al₂O₃) were examined in hydrodesulfurization (HDS) of straight run (SRGO) and nitrogen-removed gas oils, at 340 °C under 50 kg/cm² H₂. Hydrogen renewal between stages was attempted to show additional inhibition effects of the by-products such as H₂S and NH₃. Spent NiMo/Al₂O₃ and CoMo/Al₂O₃ catalysts showed contrasting activities in HDS and susceptibility to nitrogen species, according to their catalytic natures, compared to those of their virgin ones. HDS over spent NiMo/Al₂O₃ was significantly improved by removal of nitrogen species, while that over spent CoMo/Al₂O₃ was much improved by H₂ refreshment. The activity for refractory sulfur species such as 4,6-dimethyldibenzothiophene was reduced more severely than that for the reactive sulfur species such as benzothiophenes over spent catalysts. The effects of both two-stage hydrodesulfurization and nitrogen-removal were markedly reduced over the spent NiMo when compared with those over virgin NiMo one. The acidity of the catalysts was correlated with the inhibition susceptibility by nitrogen species as well as H₂S and NH₃. Spent catalysts apparently lost their activity due to the carbon deposition, which covered the active sites more preferentially. The spent NiMo catalyst carried more deposited carbon with larger C/H ratio and nitrogen content. Higher acidity was found to be present on the NiMo catalyst, but this was greatly decreased by the carbon deposition. Additionally, the reactivity of nitrogen species in HDS was briefly discussed in relation to the acidity of the catalyst and its deactivation by carbon deposition.     

CoMo/MgO–Al2O3 supported catalysts: An alternative approach to prepare HDS catalysts

Three different supports were prepared with distinct magnesia–alumina ratio x = MgO/(MgO + Al₂O₃) = 0.01, 0.1 and 0.5. Synthesized supports were impregnated with Co and Mo salts by the incipient wetness method along with 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA) as chelating agent. Catalysts were characterized by BET surface area, Raman spectroscopy, SEM-EDX and HRTEM (STEM) spectroscopy techniques. The catalysts were evaluated for the thiophene hydrodesulfurization reaction and its activity results are discussed in terms of using chelating agent during the preparation of catalyst. A comparison of the activity between uncalcined and calcined catalysts was made and a higher activity was obtained with calcined MgO–Al₂O₃ supported catalysts. Two different MgO containing calcined catalysts were tested at micro-plant with industrial feedstocks of heavy Maya crude oil. The effect of support composition was observed for hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodeasphaltenization (HDAs) and hydrodenitrogenation (HDN) reactions, which were reported at temperature of 380 °C, pressure of 7 MPa and space-velocity of 1.0 h⁻¹ during 204 h of time-on-stream (TOS).     

MgO-supported Mo, CoMo and NiMo sulfide hydrotreating catalysts

The most common preparation of high surface area MgO (100–500 m² g⁻¹) is calcination of Mg(OH)₂ obtained either by precipitation or MgO hydration or sol–gel method. Preparation of MoO₃/MgO catalyst is complicated by the high reactivity of MgO to H₂O and MoO₃. During conventional aqueous impregnation, MgO is transformed to Mg(OH)₂, and well soluble MgMoO₄ is easily formed. Alternative methods, that do not impair the starting MgO so strongly, are non-aqueous slurry impregnation and thermal spreading of MoO₃. Mo species of MoO₃/MgO catalyst are dissolved as MgMoO₄ during deposition of Co(Ni) by conventional aqueous impregnation. This can be avoided by using non-aqueous impregnation. Co(Ni)Mo/MgO catalysts must be calcined only at low temperature because Co(Ni)O and MgO easily form a solid solution. Literature data on hydrodesulfurization (HDS) activity of MgO-supported catalysts are often contradictory and do not reproduced well. However, some results suggest that very highly active HDS sites can be obtained using this support. Co(Ni)Mo/MgO catalysts prepared by non-aqueous impregnation and calcined at low temperature exhibited strong synergism in HDS activity. Co(Ni)Mo/MgO catalysts are much less deactivated by coking than their Al₂O₃-supported counterparts. Hydrodenitrogenation (HDN) activity of Mo/MgO catalyst is similar to the activity of Mo/Al₂O₃. However, the promotion effect of Co(Ni) in HDN on Co(Ni)Mo/MgO is lower than that on Co(Ni)Mo/Al₂O₃.     

Mesoporous molecular sieve MCM-41 supported Co–Mo catalyst for hydrodesulfurization of petroleum resids

In this work, we explored the potential of mesoporous zeolite-supported Co–Mo catalyst for hydrodesulfurization of petroleum resids, atmospheric and vacuum resids at 350–450°C under 6.9 MPa of H₂ pressure. A mesoporous molecular sieve of MCM-41 type was synthesized; which has SiO₂/Al₂O₃ ratio of about 41. MCM-41 supported Co–Mo catalyst was prepared by co-impregnation of Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄ followed by calcination and sulfidation. Commercial Al₂O₃ supported Co–Mo (criterion 344TL) and dispersed ammonium tetrathiomolybdate (ATTM) were also tested for comparison purposes. The results indicated that Co–Mo/MCM-41(H) is active for HDS, but is not as good as commercial Co–Mo/Al₂O₃ for desulfurization of petroleum resids. It appears that the pore size of the synthesized MCM-41 (28 Å) is not large enough to convert large-sized molecules such as asphaltene present in the petroleum resids. Removing asphaltene from the resid prior to HDS has been found to improve the catalytic activity of Co–Mo/MCM-41(H). The use of ATTM is not as effective as that of Co–Mo catalysts, but is better for conversions of >540°C fraction as compared to noncatalytic runs at 400–450°C.     

Preparation and characterization of heteropolyacid/mesoporous carbon catalyst for the vapor-phase 2-propanol conversion reaction

H₃PMo₁₂O₄₀ catalyst was chemically immobilized on the surface modified CMK-3 (SM-CMK-3) support as a charge compensating component, by taking advantage of the overall negative charge of [PMo₁₂O₄₀]³⁻. The supported H₃PMo₁₂O₄₀/SM-CMK-3 catalyst was characterized to have high surface area (≈1000 m²/g) and relatively large pore volume (0.83 cm³/g). The H₃PMo₁₂O₄₀/SM-CMK-3 catalyst was applied to the vapor-phase 2-propanol conversion reaction. The H₃PMo₁₂O₄₀/SM-CMK-3 catalyst exhibited higher 2-propanol conversion than the unsupported H₃PMo₁₂O₄₀ and the impregnated H₃PMo₁₂O₄₀ on CMK-3. Furthermore, the PMo₁₂/SM-CMK-3 catalyst showed the enhanced oxidation activity (acetone formation) and the suppressed acid catalytic activity (propylene formation) compared to the other two catalysts. It is believed that [PMo₁₂O₄₀]³⁻ species were chemically and finely immobilized on the SM-CMK-3 support as charge matching species, and thus, the PMo₁₂/SM-CMK-3 catalyst showed an excellent oxidation activity.     

Alumina-supported cobalt-molybdenum catalyst for slurry phase Fischer–Tropsch synthesis

An evaluative investigation of the Fischer–Tropsch performance of two catalysts (20%Co/Al₂O₃ and 10%Co:10%Mo/Al₂O₃) has been carried out in a slurry reactor at 2 MPa and 220–260 °C. The addition of Mo to the Co-catalyst significantly increased the acid-site strength suggesting strong electron withdrawing character in the Co-Mo catalyst. Analysis of steady-state rate data however, indicates that the FT reaction proceeds via a similar mechanism on both catalysts (carbide mechanism with hydrogenation of surface precursors as the rate-determining step). Although chain growth, , on both catalysts were comparable (  0.6), stronger CH₂ adsorption on the Co-Mo catalyst and lower surface concentration of hydrogen adatoms as a result of increased acid-site strength was responsible for the lower individual hydrocarbons production rate compared to the Co catalyst. The activation energy, E, for Co (96.6 kJ mol⁻¹), is also smaller than the estimate for the Co-Mo catalyst (112 kJ mol⁻¹). Transient hydrocarbon rate profiles on each catalyst are indicative of first-order processes, however the associated surface time constants are higher for alkanes than alkenes on individual catalysts. Even so, for each homologous class, surface time constants for paraffins are greater for Co-Mo than Co, indicative that the adsorption of CH₂ species on the Co-Mo surface is stronger than on the monometallic Co catalyst.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

Combinatorial preparation and high-throughput catalytic tests of multi-component deNOx catalysts

A quaternary catalyst library of 56 samples comprising all combinations of four elements, viz. Ag, Co, Cu, In, with six equally spaced atomic fraction increments from 0 to 1 was prepared by impregnation of a proprietary mesoporous alumina support. Catalytic properties of the library were tested in the selective catalytic reduction (SCR) of NO_x by propane under lean conditions in the temperature range 400–500 °C. The catalytic data acquired by a parallel 64-channel microreactor system with automated time-of-flight mass spectrometric analysis have been evaluated regarding selectivity–compositional relationships, synergistic effects for NO_x conversion, and efficiency of propane utilization. Full conversion of NO_x is achieved over Ag–Co combinations at 450 °C with N₂ selectivities of more than 90% and reductant utilization of 20% in a feed of 1500 ppm NO, 1500 ppm propane and 5 vol.% O₂ (space velocity of 36,000 cm³ g_cat⁻¹ h⁻¹). For the single-component catalysts Ag/Al₂O₃, Co/Al₂O₃, Cu/Al₂O₃, and In/Al₂O₃, the state of the elements on the mesoporous alumina was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Cobalt forms a spinel-like cobalt aluminate phase whereas copper and indium are present as oxides with small sizes not detectable by XRD. Silver occurs in both metallic state and as Ag₂O, and forms Ag_n clusters of at least two different sizes, predominantly with diameters of about 30 nm. The conclusions are consistent with the reducibility of the single-component catalysts samples by H₂. Surface area measurements and pore size distributions revealed reasonable modifications of the textural properties. The main pore size of the alumina support is decreased from 7 to ca. 5 nm after loading of the active components.     

Hydrotreating of heavy vacuum gas oil (HVGO) on molybdenum and tungsten nitrides catalytic phases

A series of supported and unsupported Mo₂N and W₂N phases were synthesized by means of the treatment under ammonia atmosphere at 700°C of Mo and W oxides. The X-ray diffraction and electron microscopy techniques verified the formation of the Mo₂N and W₂N ceramic phases, while the N₂ adsorption (BET) was used to determine the surface areas, between 46–133 m²/g for Mo₂N (unsupported) and 81–101 m²/g for W₂N (unsupported). The supported phases had surface areas between 109–113 and 109–122 m²/g, for Mo₂N/Al₂O₃ and W₂N/Al₂O₃, respectively. The catalytic hydrotreating of a heavy vacuum gas oil (HVGO) derived from Maya crude (i.e. 2.21 wt.% S, 0.184 wt.% N₂) was performed on both, supported and unsupported Mo nitrides and W nitrides, which promoted the HDN reaction preferentially, up to 26.6% on Mo₂N/Al₂O₃ and up to 22.3% on W₂N/Al₂O₃, against 3.26% on the reference catalyst, i.e. CoMo/Al₂O₃ at 350°C and 80 kg/cm². Also, the rates for HDN increased with the crystallite size in the unsupported W₂N series. Also, the pore volume and mean pore diameters of the Mo₂N/Al₂O₃ and W₂N/Al₂O₃ series improve substantially with respect to the pure ceramic phases.     

Investigation of CoNiMo/Al2O3 catalysts: Relationship between H2S adsorption and HDS activity

Sulfidation of trimetallic CoNiMo/Al₂O₃ catalysts was studied by thermogravimetry at 400 °C under flow and pressure conditions. Results were compared with those obtained on prepared and industrial CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts. The amount of sorbed H₂S on the sulfided solids was measured at 300 °C in the H₂S pressure range 0–3.5 MPa at constant H₂ pressure (3.8 MPa). The adsorption isotherms were simulated using a model featuring dissociated adsorption of H₂S on supported metal sulfides and bare alumina. The amount of sulfur-vacancy sites could thus be determined under conditions close to industrial practice. A relationship with activity results for thiophene HDS and benzene hydrogenation was sought for.     

Selective oxidation of ammonia over copper-silver-based catalysts

A novel catalyst based on copper-silver was developed to solve the contradiction between the high conversion temperature of Cu-based catalyst and low N₂ selectivity of Ag-based catalyst during selective oxidation of ammonium gas. The Cu-Ag-based catalyst (Cu 5 wt.%-Ag 5 wt.%/Al₂O₃) displayed a relatively low complete conversion temperature (<320 °C) with a high N₂ selectivity (>95%). Increasing loading of Cu and Ag decreases N₂ selectivity. The low N₂ selectivity of Ag-based catalyst is possibly related to the formation of Ag₂O crystals. Improvement of N₂ selectivity of Ag-based catalyst was obtained by doping Cu to decrease crystallized Ag₂O phase. The temperature programmed reaction (TPR) data show that N₂O is the main byproduct of oxidation of ammonia at temperature lower than 200 °C. Two bands of nitrate species at 1541 and 1302 cm⁻¹ were observed on Ag 10 wt.%/Al₂O₃ at the temperature higher than 250 °C, which indicates the formation of NO_x during the selective catalytic oxidation of ammonia. No nitrate species was observed on Cu 10 wt.%/Al₂O₃ and Cu 5 wt.%-Ag 5 wt.%/Al₂O₃, while only one nitrate species (1543 cm⁻¹) existed on Cu 10 wt.%-Ag 10 wt.%/Al₂O₃. We proposed that mixing Ag with Cu inhibited the formation of NO_x during the selective catalytic oxidation of ammonia over Cu-Ag/Al₂O₃.     

DRIFT study of temperature programmed desorption of NO adsorbed on Co–Mo/Al2O3 sulfided at high pressure

In order to make clear the coordinatively unsaturated sites (CUS) of Co–Mo/Al₂O₃ sulfided at high pressure, the temperature programmed desorption of NO adsorbed on Co–Mo/Al₂O₃ sulfided at high pressure was studied by DRIFT method. The intensity of two IR bands (1835 and 1785 cm⁻¹) of adsorbed NO on Co–Mo/Al₂O₃decreased simultaneously up to 393 K. The higher frequency band disappeared at 393 K, while the lower frequency band remained even at 403 K. In the case of Mo/Al₂O₃, the intensities of two bands appeared at 298 K decreased monotonously with increasing temperature, and disappeared simultaneously over 433 K. In the case of Co/Al₂O₃, two bands disappeared simultaneously over 393 K. These results suggest that two kinds of nitrosyl species are formed on Co–Mo/Al₂O₃. One is dinitrosyl species adsorbed on CUS of Co, and the other is unidentified nitrosyl species. Comparing DRIFT spectra of Co–Mo/Al₂O₃ with those of a physical mixture of Mo/Al₂O₃ and Co/Al₂O₃, it is also suggested that the formation of the latter one correlates with the interaction between Co and Mo in Co–Mo/Al₂O₃. The unidentified nitrosyl species might be the key to explain the dependency of DRIFT spectrum of adsorbed NO on the pressure of sulfiding.     

A study on the preparation of supported metal oxide catalysts using JRC-reference catalysts. I. Preparation of a molybdena–alumina catalyst. Part 1. Surface area of alumina

This is the first report of a group study on the preparation of a MoO₃/Al₂O₃ catalyst to find predominant preparation parameters for better and reproducible catalyst preparations. Variously prepared MoO₃/Al₂O₃ catalysts possessing 13 wt% MoO₃ were subjected to multiprong characterizations and catalytic tests. It was found that the surface area of the support was the most predominant preparation parameter for the dispersion of Mo oxide species; the dispersion increased as the surface area of the support increased. The formation of crystalline MoO₃ was observed at a surface Mo concentration higher than 3.2 Mo nm⁻². With sulfided MoO₃/Al₂O₃, it was established that the dispersion of Mo sulfide species increased with increasing surface area of the support and was in proportion to that of Mo oxide precursor species. The hydrodesulfurization activity of sulfided MoO₃/Al₂O₃ was proportional to the NO adsorption capacity. It is suggested that a homogeneous distribution of Mo oxide species is attained by an equilibrium adsorption technique. However, it was revealed that the surface area of the catalyst and Mo distribution were considerably modified by preparation parameters, such as drying processes, other than the surface area.     

Hydrodesulfurization over noble metals supported on ZSM-5 zeolites

Pt/HZSM-5 showed high and stable catalytic activity for the hydrodesulfurization of thiophene at 400°C and its catalytic activity was higher than that of commercial CoMo/Al₂O₃ catalyst. Pt/HZSM-5 zeolite was not poisoned by hydrogen sulfide in the hydrodesulfurization of thiophene and hydrocracking of hydrocarbons. The catalytic activity of Pt/HZSM-5 decreased with increase of SiO₂/Al₂O₃ ratio in HZSM-5. The Brønsted acid site of HZSM-5 and spillover hydrogen formed on Pt particle in Pt/HZSM-5 catalyst play an important role for the hydrodesulfurization of thiophene.     

Reactivity of olefins in the hydrodesulfurization of FCC gasoline over CoMo sulfide catalyst

To achieve selective hydrodesulfurization (HDS) of fluid catalytic-cracked (FCC) gasoline for producing sulfur-free gasoline (S < 10 ppm), the reactivity of various olefins contained in FCC gasoline on CoMoP/Al₂O₃ sulfide catalysts was investigated. Isomerization of the CC double bond from the terminal position to an internal position was observed. The steric hindrance around the CC double bond suppresses the reactivity of olefin hydrogenation. The sulfidation temperature of the catalyst has a major influence on olefin hydrogenation active sites. Addition of the appropriate amount of cobalt (Co/Mo ratio approximately 0.6) contributes to the suppression of olefin hydrogenation at high reaction temperature (260 °C). From the comparison of catalytic performance and characterization of our CoMoP/Al₂O₃ catalyst with an analogous commercial catalyst, it is suggested that the hydrogenation of olefins depends not only on the state of the Mo CUS but also on the steric effects of both olefin structure and MoS₂ crystalline structure.     

Effect of Pt on the water resistance of Co-zeolites upon the SCR of NOx with CH4

The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al₂O₃ on the selective catalytic reduction (SCR) of NO_x with CH₄ under dry and wet reaction stream. After being reduced in H₂ at 350°C, the PtCo bimetallic zeolites showed higher NO to N₂ conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH₄ = 1000 ppm, O₂ = 2%), the NO conversion dropped to zero over Co_2.0Mor at 500°C and GHSV = 30,000 h⁻¹, whereas it is 20% in Pt_0.5Co_2.0Mor. In Pt/Co/Al₂O₃ the NO_x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Fer, 13 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Mor, 4.33 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0ZSM-5 and 0.5 × 10⁻⁴ s⁻¹ for Pt/Co/Al₂O₃. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co²⁺ and Pt²⁺ at exchange positions. Co° seems not to participate as an active site in the SCR of NO_x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO₂ reaction, the activation of CH₄ and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

Kinetic modeling of the hydrotreatment of light cycle oil and heavy gas oil using the structural contributions approach

The kinetics of the hydrodesulfurization of light cycle oil (LCO) and heavy gas oil (HGO) over a CoMo/Al₂O₃ catalyst were investigated in a perfectly mixed flow reactor with stationary basket of the Robinson-Mahoney type at temperatures of 330, 310 and 290 °C, H₂/HC molar ratios of 2.8, 3.6 and 7.2 and a pressure of 65 bar. Hougen-Watson type rate equations were derived for the conversion of dibenzothiophene, substituted (di)benzothiophene and their products. To avoid having to deal with a huge number of model parameters, a methodology based upon structural contributions was applied. In the absence of own kinetic data on key components a number of kinetic and adsorption parameters were taken from published work on a very similar catalyst. For a given value of H₂/HC only a small number of experiments was required to determine the value of the very complex denominators DEN_σ and DEN_τ appearing in the rate equations for the hydrodesulfurization of LCO and of HGO and of their evolution with the conversion of the feedstock. With the rate equations constructed in this way the calculated total conversion of DBT, its conversion into biphenyl and into cyclohexylbenzene were in excellent agreement with the experimental values.     

Porous structures constructed from [SiMo12O40] and [SiW12O40] Keggin units

Three compounds, K₂(H₂O)₄H₂SiMo₁₂O₄₀ · 7H₂O (1), K₂Na₂(H₂O)₄SiW₁₂O₄₀ · 4H₂O (2), and Na₄(H₂O)₈SiMo₁₂O₄₀ · 6H₂O (3) have been synthesized and structurally characterized by single-crystal X-ray analysis, IR, and thermogravimetry. Compounds 1 and 2 both show the high symmetry trigonal space group P3₂21 and a novel 3D network structure. The Keggin anions [SiM₁₂O₄₀]⁴⁻(M = Mo, W) are linked by potassium or sodium cations to generate hexagon-shaped channels along the c-axis, in which water molecules are accommodated. Compound 3 is tetragonal, space group P4/mnc constructed from [SiMo₁₂O₄₀]⁴⁻ anions and Na ions.     

Effect of the order of potassium introduction on the texture and activity of Mo/Al2O3 catalysts in water gas shift reaction

The effect of deposition and order of potassium introduction on the texture and activity of Mo/γ-Al₂O₃ catalysts in water gas shift (WGS) reaction was investigated. The samples were synthesised by incipient wetness impregnation of the carrier with aqueous solutions of the corresponding salts followed by drying and calcination after each deposition step. The prepared catalyst precursors were sulphided at 400°C for 2 h with 6% H₂S in H₂ before testing in WGS reaction in a glass flow apparatus at 400°C under atmospheric pressure.

The results show that potassium deposition alone on the bare γ-Al₂O₃ (sample K/Al₂O₃) decreases the specific surface after calcination by blocking the constrictions between the pores in the primary porous texture. In the WGS reaction conditions part of the pores are deblocked and a redistribution in the pore volumes occurs.

The deposition of the Mo (sample Mo/Al₂O₃) also results in a decrease in both specific surface and total pore volume with respect to the bare support. However after catalytic activity test no substantial changes in its texture were observed.

The addition of K to the Mo (sample KMo/Al₂O₃) leads to nonuniformity in distribution of molybdenum–oxygen entities due to partial migration of the MoO_x species to the external surface. The specific surface is not changed during the reaction test.

The deposition of Mo on K/Al₂O₃ contributes to the uniform distribution of oxomolybdenum species in the porous texture of the support. This uniformity is preserved to a high extent in the catalytic reaction as well. The activity in the synthesised samples in the WGS reaction decreases in the order MoK/Al₂O₃ > Mo/Al₂O₃ > KMo/Al₂O₃.