Thiophene Hydrodesulphurization Research Articles

A comprehensive theoretical investigation on the thiophene hydrodesulphurisation mechanism over sulphided Co–Mo catalysts supported by ZSM-5, FAU, Beta and MCM-22 zeolites

ABSTRACT We have theoretically studied the hydrodesulphurisation (HDS) mechanism of thiophene and its hydrogenated derivatives over sulphided Co–Mo catalysts supported by four zeolites (ZSM-5, FAU, Beta, MCM-22) by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. Due to low energy barriers, the thiophene can be hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT). Direct desulphurisation (DDS) and hydrogenolysis desulphurisation (HYD) pathways were investigated in detail. The calculated results show that in DDS mechanism of thiophene, the rate-determining step is the C–S bond cleavage. In HYD mechanisms of 2,5-DHT, 2,3-DHT and THT, the rate-determining step is hydrogen transfer on ZSM-5, FAU and Beta zeolites except MCM-22 zeolite. The HYD of 2,3-DHT and THT should occur by the stepwise pathway. But for the 2,5-DHT, the concerted pathway is probably more favourable than the stepwise pathway. The reduced density gradient (RDG), differential charge density (DCD), and local orbital locator (LOL) maps show that for all transition states (TSs), there are complicated van der Waals (VDW) and electrostatic interactions between the organic fragment and the catalytic centre.

Rhenium Effect on the Performance of CoMoNi/γ‐Al2O3 Catalyst in Thiophene Hydrodesulphurization. Performance Evaluation and Process Kinetics

AbstractThe rhenium effect on the performance of CoMoNi/γ‐Al2O3 catalyst was studied in thiophene hydrodesulphurization. The unsulfided and freshly sulfided catalysts were characterized by N2 adsorption‐desorption, IR spectroscopy, X‐ray photoelectron spectroscopy (XPS) and total acidity. The effects of reaction temperature, pressure, and liquid hourly space velocity (LHSV) on thiophene HDS performance shown that the activity of the CoMoNiRe/γ‐Al2O3 catalyst for thiophene HDS was superior to that of the CoMoNi/γ‐Al2O catalyst, which was mainly ascribed to the higher acidic functional groups. Based on Langmuir‐Hinshelwood approach, a kinetic model for thiophene HDS over both catalysts have been developed. In this respect, two mechanisms have been considered, one hypothesizing that hydrogen is adsorbed on different active centers than thiophene (two‐centers mechanism) and the other considering one type of active centers. Based on statistical reasons, the kinetic model considering two types of active centers proved to be more adequate in representing the experimental results.

Comparative Study Between two Reaction Kinetic Mechanisms of Thiophene Hydrodesulphurization over CoMo /gama - Al2O3 Supported Catalyst

The kinetic study of the thiophene hydrodesulphurisation process was carried out for CoMo/gama-Al2O3 catalyst, at temperatures between 175 and 275 �C, pressure ranged from 30bar to 60 bar and the liquid hourly space velocity from 1h-1 to 4 h-1. For the reaction mechanism, the Langmuir-Hinshelwood-Hougen-Watson model (LHHW) was used and two kinetic models were proposed: the first model, that considered that H2 is adsorbed on a different type of active center than thiophene and the second model, that considered that the two reactants are adsorbed on the same type of active sites. The values obtained for the average relative error (ARE) and the correlation coefficient between the experimental and the calculated data (R2) indicate that the Langmuir-Hinshelwood model, describing the adsorption on two active sites, best describes the kinetics of the thiophene hydrodesulfurization reaction over CoMo/gama-Al2O3 tested catalyst.

Hydrodesulphurization of Thiophene over Co, Mo and CoMo /g-Al2O3 Catalysts

The hydrodesulphurization (HDS) of thiophene was performed on monometallic catalysts Co/g -Al2O3, Mo/g -Al2O3 and bimetallic CoMo/g -Al2O3. Experiments were carried out on a fixed bed catalytic reactor at 175-300�C, 30-60 atm, thiophene volume hourly space velocities of 1h-1- 4h-1 and molar ratio hydrogen/thiophene of 60/1. The thiophene conversion on the bimetallic CoMo/g -Al2O3 catalyst was higher than on Co/g -Al2O3 or Mo/g -Al2O3.

A theoretical study on reaction mechanisms and kinetics of thiophene hydrodesulfurization over MoS2 catalysts

In the present work, thiophene hydrodesulphurization (HDS) over the Mo-edge, the S-edge, and the Mo-S connection edge of MoS2 catalyst with 50% sulfur coverage was studied using first-principles based microkinetic modeling. Two parallel HDS routes, i.e., direct desulfurization (DDS) and hydrogenation (HYD) were taken into account. It has been found that the major reaction route of thiophene HDS on the Mo- and the Mo-S edges is temperature dependent. In the low temperature range of 500–600 K, the HYD route is dominant, leading to the C4H8 formation. As the temperature increases, the DDS route becomes competitive with the HYD route. At the temperature above 650 K, the DDS route will be the dominant HDS reaction route on the Mo- and the Mo-S edges. The DDS route leading to the formation of C4H6 is the major thiophene HDS reaction route on the S-edge in the entire temperature range of 500–750 K. The microkinetic modeling results show the overall HDS activity on the S-edge is lower than it on the Mo- and the Mo-S edges. The Mo-S edge also provides a preferential reaction pathway, which facilitates 2-hydrothiophene migration from the Mo-edge to the S-edge, followed by remaining elementary steps with lower activation barriers in the DDS route.

Relation between composition and morphology of K(Co)MoS active phase species and their performances in hydrotreating of model FCC gasoline

K-Mo/Al2O3 and K-CoMo/Al2O3 catalysts with various amount of potassium (up to 15wt. %) were synthesized using H3PMo12O40, CoCO3 and KOH. The catalysts were characterised by the following techniques: low-temperature N2 adsorption, Raman spectroscopy, temperature-programmed reduction, temperature-programmed desorption of NH3, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy. Synthesized catalysts were tested in hydrotreating of model fluid catalytic cracking gasoline containing 1000ppm of sulphur from thiophene and 36wt. % of n-hexene-1. It was found that potassium incorporation led to the strong changes in both physical-chemical characteristics and catalytic behaviour of sulphide catalysts. Using potassium favoured the growth of average slab length and sulphidation degree of metals with selective formation of CoMoS active phase. Addition of potassium also led to the partial poisoning of the active sites what resulted in decrease of catalyst activities compared with the reference Mo/Al2O3 and CoMo/Al2O3 samples. Olefin hydrogenation was more sensitive to potassium than thiophene hydrodesulphurization what resulted in the growth of the dependence of selectivity factor on potassium content in the catalysts. Obtained results allowed us to suppose that potassium partially inserts in sulphide slabs with changing the nature of active sites and probably with formation of new type of active “KCoMoS” sites.

Preparation of un-promoted molybdenum HDS catalysts supported on titania by equilibrium deposition filtration: Optimization of the preparative parameters and investigation of the promoting action of titania

The present work deals with un-promoted molybdenum hydrodesulphurization (HDS) catalysts supported on titania (anatase) which have been prepared by “Equilibrium Deposition Filtration” (EDF). The catalysts were characterized using various techniques (XRD, N2 adsoption–desorption isotherms, TPR, DRS and XPS) and evaluated at various temperatures and atmospheric pressure in a fixed bed differential reactor, using the HDS of thiophene as a probe reaction.The calcination of the carrier before its usage has been optimized. The adjustment of the impregnation parameters allowed regulating the Mo loading in the monolayer region and thus optimizing the catalytic performance. A linear decrease of the intrinsic activity with the Mo surface density and a concomitant volcano trend for the specific activity have been found.Monomer, monopodal Mo species are deposited on the anatase surface in the pH range 7.0–4.5 and polymer Mo species at pH 2. The former are transformed into tripodal ones after calcination. Thus, the monolayer capacity of the anatase surface is decreased leading to a partial association of these species into oligomer ones where the mean number of TiOMo bridges per Mo atom is lower than three. The extent of this transformation increased with the Mo surface density. The DRS, XPS and TPR results suggested that electron transfer takes place through the TiOMo bridges from the anatase surface oxygen to the Mo4d orbitals.The monomer tripodal Mo species are transformed into relatively small Mo sulphided edge clusters in tilted or perpendicular configuration bonded on the anatase surface by TiOMo and/or TiSMo bridges. The aforementioned transfer in the electronic density from the surface oxygens, and likewise from the surface sulfur, to Mo4d orbitals weakens the MoS bonds of the clusters and stabilizes S-deficient structures creating coordinative unsaturated active sites. This explains the very high intrinsic activity of these small Mo sulphided clusters.The oligomer Mo species, on the other hand, are transformed into relatively larger Mo S-deficient sulphided clusters, also with tilted or perpendicular configuration with respect to the anatase surface. The number the MoOTi and MoSTi bridges per Mo atom is smaller than that in the small Mo sulphided clusters, resulting to less coordinative unsaturated active sites per Mo atom. The above explained the aforementioned trends for the intrinsic and specific activities.

REACTIVITY AND MORPHOLOGY OFNi,Mo, ANDNi–MoOXIDE CLUSTERS SUPPORTED ON MCM-48 TOWARD THIOPHENE HYDRODESULPHURIZATION

In this paper, the morphology, chemical composition and reactivity of MCM-48 powders impregnated with Ni , Mo or both toward hydrodesulphurization (HDS) of thiophene were characterized. The reactivity of the catalyst was quantitatively compared with a standard industrial catalyst (from HaldorTopsoe, Denmark) and a novel WS2nanotube-based catalysts (from R. Tenne, Israel). Morphology and chemical composition were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and EDX elemental maps. Reactivity was determined in a gas-chromatograph based mini flow reactor using thiophene as a probe molecule. The sulfided MCM-48 supported Mo catalyst showed the largest HDS activity with turnover frequencies (TOF) about half as large as for the commercial system under the test conditions used here. Presulfiding did increase activity of all MCM-48 catalysts.

Effect of method of preparation on hydrodesulphurization activity of Co- or Ni-promoted MoS2/SBA-15 catalysts

Ordered mesoporous material SBA-15 was synthesized and used as a support for the preparation of molybdenum sulphide catalysts through precipitation from homogeneous solution (PFHS) technique with the Mo content varying from 2–12 wt%. The prepared catalysts were evaluated for thiophene hydrodesulphurization catalytic activities at 400°C. Catalysts prepared through PFHS method resulted in highly dispersed MoS2 catalysts, which were inferred from powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low temperature oxygen chemisorptions (LTOC) and BET surface area analysis. The relationship between XPS intensity ratio, oxygen chemisorption and catalytic activities is discussed in terms of highly dispersed nano particles of MoS2 and its consequence in accommodating more promoted atoms at the edge sites.

Novel MoO2/carbon hierarchical nano/microcomposites: synthesis, characterization, solid state transformations and thiophene HDS activity

Novel MoO(2)/C nano/microcomposites were prepared via a bottom-up approach by hydrothermal carbonization of a solution of glucose as a carbon precursor in the presence of polyoxometalates (POMs: phosphomolybdic acid [H(3)PMo(12)O(40)] and ammonium heptamolybdate tetrahydrate [(NH(4))(6)Mo(7)O(24)]·4H(2)O). The structural characterization by FT-IR, XRPD, SEM and TEM analyses revealed the controlled formation of hierarchical MoO(2)/C composites with different morphologies: strawberry-like, based on carbon microspheres decorated with MoO(2) nanoparticles; MoO(2)/C core-shell composites; and irregular aggregates in combination with ring-like microstructures bearing amorphous Mo species. These composites can be fine-tuned by varying reaction time, glucose/POM ratio and type of POM precursor. Subsequent transformations in the solid state through calcinations of MoO(2)/C core-shell composites in air lead to hollow nanostructured molybdenum trioxide microspheres together with nanorods and plate microcrystals or cauliflower-like composites (MoO(2)/C). In addition, the MoO(2)/C composite undergoes a morphology evolution to urchin-like composites when it is calcined under nitrogen atmosphere (MoO(2)/C-N(2)). The MoO(2)/C strawberry-like and MoO(2)/C-N(2) composites were transformed into Mo carbide and nitride supported on carbon microspheres (Mo(2)C/C, MoN/C, and MoN/C-N(2)). These phases were tested as precursors in thiophene hydrodesulphurization (HDS) at 400 °C, observing the following trend in relation to the thiophene steady-state conversion: MoN/C-N(2) > MoN/C > Mo(2)C/C > MoO(2)/C-N(2) > MoO(2)/C. According to these conversion values, a direct correlation was observed between higher HDS activity and decreasing crystal size as estimated from the Scherrer equation. These results suggest that such composites represent interesting and promising precursors for HDS catalysts, where the activity and stability can be modified either by chemical or structural changes of the composites under different conditions.

Pd and PdAu catalysts supported over 3-MPTES grafted HMS used in the HDS of thiophene

Palladium and bimetallic palladium-gold catalysts were supported over mesoporous silica HMS grafted with variable amount of mercaptopropyl groups corresponding to different molar ratio of the SH group over the metal, molesSH/molesmetal=2, 4, 7, 9. The obtained catalysts, characterized by XRD and XPS techniques were tested in the hydrodesulphurization (HDS) reaction of thiophene. Generally speaking, all the bimetallic catalysts were more active as compared to the monometallic ones. The support modification affected the structure of the fresh (calcined) and reduced catalysts. Supporting the metals over the bare HMS, through deposition–precipitation with urea, produced a drastic decrease of the surface area at variance with the catalysts supported over the functionalized HMS which maintained fairly high surface areas. According to the XRD, the introduction of the SH groups contributed to the better dispersion of the PdO and metallic gold in the calcined samples. Upon hydrogen treatment the supported metals transformed into small particles of gold enriched alloy leading to high activity and lower catalyst deactivation. The promotion of the catalytic behaviour increased with the increase of the SH loading up to a molar ratio, molesSH/molesmetal=7. Above this value, the beneficial effect started diminishing. Based on the XPS and the XRD results, the catalytic behaviour was correlated with the structure and surface composition of the supported species, governed by the interaction between the metals and the mercaptogroups.

CoMo/Al2O3-SiO2 catalysts prepared by co-equilibrium deposition filtration: Characterization and catalytic behavior for the hydrodesulphurization of thiophene

Four CoMo catalysts supported on Al2O3-SiO2 mixed materials of varying SiO2 content (1.5, 10, 20 and 30% SiO2 w/w) were prepared following the co-EDF methodology. The catalysts were characterized using various techniques (BET, potentiometric mass titrations, XRD, DRS, XPS, LRS, TPR, TPD of ammonia and NO chemisorption). Two additional catalysts were prepared and characterized on two mixed supports, which contain 1.5 and 20% SiO2 w/w, following the conventional impregnation procedure. The hydrodesulphurization (HDS) activities of the catalysts studied were determined using thiophene as a probe molecule.The co-EDF catalysts were proved to be more active than the corresponding ones prepared following the conventional impregnation procedure. This was attributed to the relatively high dispersion of the Mo supported phase achieved by applying co-EDF. The following activity trend was obtained over the co-EDF catalysts, 20Si > 30Si > 1.5Si > 10Si, which indicates that the activity is maximized over the catalyst prepared on a mixed support containing 20% SiO2 w/w. This zigzag trend was explained in terms of two parameters, the ratio “tetrahedral (monomer) /octahedral (polymer) Mo species” and the amount of cobalt aluminate formed, both regulated by changing the composition of the mixed support.

Deep desulphurization of diesel fuels on bifunctional monolithic nanostructured Pt-zeolite catalysts

The preparation of Pt-zeolite catalysts, including choice of the noble metal precursor and loading (1.0–1.8 wt.%), was optimized for maximizing the catalytic activity in thiophene hydrodesulphurization (HDS) and benzene hydrogenation (HYD). According to data obtained by HRTEM, XPS, EXAFS and FTIR spectroscopy of adsorbed CO, the catalysts contained finely dispersed Pt nanoparticles (2–5 nm) located on montmorillonite and zeolite surfaces as: Pt 0 (main, ν CO = 2070–2095 cm −1), Pt δ+ ( ν CO = 2128 cm −1) and Pt 2+ ( ν CO = 2149–2155 cm −1). It was shown that the state of Pt depended on the Si/Al zeolite ratio, montmorillonite presence and Pt precursor. The use of H 2PtCl 6 as the precursor (impregnation) promoted stabilization of an oxidized Pt state, most likely Pt(OH) xCl y. When Pt(NH 3) 4Cl 2 (ion-exchange) was used, the Pt 0 and hydroxo- or oxy-complexes Pt(OH) 6 2− or PtO 2 were formed. The addition of the Ca-montmorillonite favoured stabilization of Pt +δ. The Cl − ions inhibit reduction of oxidized Pt state to Pt particles. The Pt-zeolite catalyst demonstrated high efficiency in ultra-deep desulphurization of DLCO. The good catalyst performance in hydrogenation activity and sulphur resistance can be explained by the favourable pore space architecture and the location and the state of the Pt clusters. The bimodal texture of the developed zeolite substrates allows realizing a concept for design of sulphur-resistant noble metal hydrotreating catalyst proposed by Song [C. Song, Shape-Selective Catalysis, Chemicals Synthesis and Hydrocarbon Processing (ACS Symposium Series 738), Washington, 1999, p. 381; Chemtech 29(3) (1999) 26].

Synthesis, Characterization, and Hydrodesulfurization Activity of New Mesoporous Carbon Supported Transition Metal Sulfide Catalysts

The potential of sulfided Mo, Co, and CoMo catalysts supported on two different carbon mesostructures by KAIST-1 and 3 (CMK-1 and 3) for hydrodesulphurization (HDS) of thiophene at specific conditions was explored to investigate the effect of different pore sizes, surface functional groups, metal dispersion, and presulfidation temperature. Co/CMK catalysts were superior to Mo/CMK catalysts on the basis of same metal loading. The synergistic effect was found at Co/Mo ratio of 5/5. Sulfidation on carbon supported catalysts was favored at lower temperature of 300 °C, whereas higher temperature of 400 and 500 °C showed a more favorable effect on alumina supported catalysts. CMK-3 supported CoMo catalysts were superior to all other catalysts due to larger pore size and higher acidic surface functional groups. CMK-3 oxidized with nitric acid showed even better performance for CoMo (5:5) supported catalysts since the increased surface functional groups might induce better dispersion leading to higher HDS activity.

Synthesis, Characterization, and Evaluation of Mesoporous MCM-41-supported Molybdenum Hydrotreating Catalysts

Mesoporous MCM-41 material with high surface area and narrow pore size distribution was synthesized and used as a support for Mo, CoMo, and NiMo catalysts. The molybdenum loading was varied from 2–14 wt% on MCM-41. On 10 wt% Mo/MCM-41, the promoter Co or Ni concentration was varied from 1–5 wt%. All the catalyst samples were characterized by surface area, low temperature oxygen chemisorption, x-ray diffraction (XRD), and temperature programmed reduction methods. Characterization results show that Mo is well dispersed on MCM-41 up to 10 wt%. The catalytic activities were evaluated for thiophene hydrodesulphurization (HDS), cyclohexene hydrogenation (HYD), and furan hydrodeoxygenation (HDO). All three catalytic functionalities vary in a similar manner to that of oxygen chemisorption as a function of Mo loading, indicating that there is a correlation between oxygen uptake and catalytic sites. The activities of these catalysts were compared with γ-Al2O3- and amorphous SiO2-supported catalysts. It was found that MCM-41-supported Mo catalysts displayed superior activities.

Synthesis and characterization of vanadium nanoparticles on activated carbon and their catalytic activity in thiophene hydrodesulphurization

Vanadium nanoparticles (∼7 nm) stabilized on activated carbon were synthesized by the reduction of VCl 3·3THF with K[BEt 3H]. This material was characterized by inductive coupled plasma-atomic emission spectroscopy (ICP-AES), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses. The catalytic performance of the carbon-supported vanadium was studied using thiophene hydrodesulfurization (HDS) as model reaction at 300 °C and P = 1 atm. The catalytic activity of the vanadium carbide phase on the activated carbon carrier was more significant than that of the reference catalysts, alumina supported NiMoS. The method proposed for the synthesis of such a catalyst led to an excellent performance of the HDS process.

Effect of the modified support γ-Al2O3-CaO on the structure and hydrodesulfurization activity of Mo and Ni-Mo catalysts

The effect of the support γ-Al2O3 modified with CaO by deposition–precipitation method on the structure, catalytic activity and selectivity of Mo and Ni-Mo catalysts for HDS was investigated. The resistance of the catalysts to deactivation by coke formation was also investigated. The samples have been characterized by means of XRD, DRS, XPS, Al27 NMR, TPR, and specific surface area and pore size distribution. The reaction of thiophene hydrodesulphurization, carried out at atmospheric pressure, was used as a model reaction for estimating the activity of the catalyst samples. The obtained results show that the applied method of modification with CaO leads to the formation of support with mesoporous structure and a considerable higher mechanical strength than that of the pure γ-Al2O3. Mo and Ni-Mo catalysts deposited on the modified support have higher HDS activity and resistance to coke formation. The presence of Ca2+ ions in the lattice of the support hinders the formation of crystallites of MoO3 and Al2(MoO4)3 phase. The higher catalytic activity of the samples with modified support is due to the higher degree of their sulfidation.

Effect of alumina–titania supports on the activity of Pd, Pt and bimetallic Pd–Pt catalysts for hydrorefining applications

A series of Pd, Pt and bimetallic Pd:Pt (4:1) based catalysts were supported on various alumina–titania mixed oxides. Their catalytic properties were probed on the thiophene hydrodesulphurisation (HDS). The supports were synthesised by the sol–gel method to obtain bulk mixed oxides. The Pd catalysts were tested in reduced and sulphided state, where the sulphided catalysts showed higher activity. While sulphided Pd increased its activity with titania content in the support, sulphided Pt proved to be more active when supported on the mixed oxide with the lowest titania concentration. The bimetallic catalysts presented the highest activity when supported on equimolar alumina–titania. A dependence of the deactivation rates with titania concentration in the mixed-oxide supports was detected for Pd and Pd–Pt bimetallic catalysts. On the other hand, almost no effect of the support composition on the Pt/alumina–titania deactivation rates was observed. The bimetallic catalyst activity and deactivation rates resemble closely those of the Pd monometallic catalysts suggesting a stronger influence of Pd on the performance of the bimetallic systems.

HDS of thiophene over CoMo/AlMCM-41 with different Si/Al ratios

A series of AlMCM-41 molecular sieves with different Si/Al ratios were synthesized followed by the deposition of cobalt and molybdenum oxides on these mesoporous supports by co-impregnation. Such materials were further calcined and catalysts with 15 wt.% of cobalt and molybdenum and a Co/(Co + Mo) atomic ratio of 0.30 were obtained. These materials were characterized by X-ray diffraction (XRD), transmission electron microscopy and selected area electron diffraction (TEM/SAED), X-ray fluorescence (XRF), and nitrogen adsorption. Hydrodesulphurisation (HDS) of thiophene was carried out at 350 °C in a fixed bed continuous flow micro reactor coupled on line to a gas chromatograph. The main XRD peaks of MCM-41 phase were observed in all samples and peaks due to MoO3 and CoMoO4 phases were also identified from XRD results. It was found that the as-synthesized catalysts presented reasonable conversion results for HDS of thiophene, when compared to other supported catalysts. The main products of HDS of thiophene were H2S, isobutene, 1-butene, n-butane, 2-butene-trans, and 2-butene-cis. It was observed that the reactivity of the as-synthesized catalysts is a direct function of the Si/Al ratio, nature and concentration of the active species on the mesoporous supports.

Characterization and HDS activities of mixed Fe–Mo sulphides supported on alumina and carbon

Alumina and activated carbon-supported mixed sulphides (FeMoS) were prepared as hydrotreating catalysts. Previous work had shown bulk Fe–Mo mixed sulphides to be promising catalysts for hydrotreatment. Characterizations of the solids by NH3-TPD proved to be a good technique to classify the solids according to their acidic strength. Thiophene hydrodesulphurization (HDS) and High vacuum gas oil (HVGO) hydrotreatment, performed at atmospheric pressure and high pressure respectively, were used as catalytic tests. Depending on the support, a more or less important synergetic effect is observed. The results are in agreement with a possible direct desulphurization process. The acidic strength plays an important role in determining the hydrogenolysis/hydrogenation ratio of the catalysts.