Catalyst sulfur uptake

The two patents described above were particularly important in the initiation of the developments of biodesulfurization catalysts. The bioreactor arrays required for operation and growth method constituted key elements in the following developments of the area, which would condition viability and successful path to industrialization. A sulfur bioavailability assay was incorporated into the screen for monitoring the sulfur uptake by the microorganisms, and the concept formed a claim in the patents [67,91], The objective... 

Two spectra of sulfided catalysts in Fig. 4.16 show that sulfur can also be detected, albeit with lower sensitivity, because of the Z2 dependence of the cross section in (4-6). Figure 4.17 gives the S/Mo atomic ratio as a function of sulfidation temperature. It indicates that sulfur uptake by M0O3 is already significant at room temperature and increases to the expected S/Mo ratio of 2 above 100 °C. Combination of these results with SIMS and XPS data have led to a detailed mechanism for the sulfidation of silica-supported molybdenum catalysts [21] (see also Chapter 9). 

The oxidation is catalyzed by various heavy metal ions such as Cu , Fe (hemin complex), Ni and Co and their complexes,and more importantly, the addition of these ions leads to the selective formation of disulfides without any overoxidized products. The cluster (Bu"4N)2[Fe4S4(SR)4], the analog of the active site of nonheme iron-sulfur proteins, catalyzed extremely smooth oxidation of thiols by oxygen to disulfides in acetonitrile at 0 C (equation 4), while in the case of FeCb or FeCb catalysts oxygen uptake was very slow." The catalysis by AI2O3 for aerobic oxidation is also common. Thus, by stirring thiols in benzene with exposure to air at room temperature for 4-6 h disulfides were obtained almost quantitatively except in the hindered case of Bu SH. 

With an H2S/H2 ratio in the gas of 1 ppb, the equilibrium surface coverage of nickel at 500°C is around 70%. This means that all sulfur in the feed is quantitatively adsorbed on the nickel catalyst of a prereformer. The result is not only the deactivation of the prereformer catalyst even at very low sulfur levels, but also the protection of downstream catalysts from poisoning. Sulfur uptake on the catalyst will initially take place as shell poisoning and because of pore diffusion restrictions, it may take years before sulfur reaches the center of the particle. ... 

The produced H2S then desorbes from the catalyst. Stored sulftir can in this way rapidly be released during retardation or acceleration, induced by changes in the air-fuel ratio in the exliaust gas. The fonnation of H2S can during short periods reach concentrations well above the human smell tlireshold of 0.02-0.03 ppm (5). This study was designed to investigate the sulfur uptake and release for differently deactivated catalysts and the influence of different reducing agents. 

One of the first radiotracer studies on sulfur uptake was that on a molybdenum disulfide catalyst.M0S2 was exposed to p S]H2S, admitted at 623K into the evacuated vessel containing the catalyst. ([ S]H2S in the following will be denoted for simplicity by H2 S, indicating labelled,... 

Sulfidation by H2 S, represented in Refe. 26, 40-44 is a widely used method for determining the sulfur uptake of catalysts, based on the radioactivity balance in between the introduced H2 S/H2 and the outflow-, or line-out gasesl l ... 

Besides H2S, sulfur uptake studies were carried out with labelled organic compounds, mostly with thiophene and dibenzothiophene (DBT). In these methods, the sulfur uptake is determined from the balance of radioactivity-loss of the S-labelled compound and the radioactivity of H2 S produced in HDS. It was established that C0M0/AI2O3 samples, sulfided with S-labelled thiophene pulses, contained sulfur about 2.5 times less than the same catalyst, presulfided with elemental sulfur at high pressure. S-uptake, determined by this method, represents uptake by active in HDS sites those sites that are non-participating in the reaction are involved due to the surface migration of sulfur. 

Labelled sulfur offers an excellent possibility for conclusions with respect to the mobility of catalyst sulfur, to determine its extent and to distinguish its different kinds (as reversibly adsorbed, or eluted with H2, or displaced by different other molecules, including S-containing ones, and that of sulfur exchange). There exist different non-isotopic methods for sulfur uptake determination these are different from sulfur mobility studies which is difficult to perform without applying isotope tracer. 

Different from the observation on the absence of a correlation between sulfur uptake and HDS activity, a definite correlation was found (Fig. 6) between the amounts of exchangeable sulfur atoms (Sexc) and the thiophene HDS activity of the different catalysts, as mentioned before in the case of some other catalysts. This observation suggests some parallelism of exchangeable sulfur with the catalyst sulfur, and with the ratio of catalytic sites among sites of sulfur uptake. 

On alumina supported Pd and Pt, the Smob/Scat was 0.2 and 0.12 the amount of mobile sulfur was 3—3.9 x 10 S atoms/mg at 533K.I 1 According to the authors opinion, the difference between the molybdena based and the noble metal catalysts, is a consequence of the attachment of a part of sulfur to the alumina support of the noble metal catalysts. The data indicate that almost all sulfur accommodated on the noble metals was mobile, i.e., exchangeable and participated in the HDS reaction. This was different from the case with molybdena-based catalysts. This observation was confirmed recently that Smob/Pd(Pd- -Pt) ratio was 0.2, both for alumina supported Pd and Pd-Pt, irrespective of the H2S partial pressure and the amount of the total sulfur uptake. 

The conditions applied in different studies are substantially different. The strong and different pretreatment methods on sulfur uptake and thiophene HDS activity, for different catalysts effect, is demonstrated by a comparison of the effect of six different procedures applied for preparation and sulfidation of supported NiMoOx and two NiWOx samples. Note that even a 50K difference by similar pretreatment results in substantial differences in HDS activity, and the effect is different for different catalysts ... 

Sulfur uptake and exchange data 1 referred in Sec. 2.3 indicated substantial differences of alumina supported Pd and Pt, in comparison with the molybdena based catalysts. The mobile sulfur amounts were lower [0.25 Smob/Pd (or /(Pt)] than those experienced with the Mo and W based catalysts. Near to all sulfur, accommodated on the noble metal was mobile and — different from the Mo- and W-based catalysts — participated in the reaction the sulfur mobility was substantially higher due to the much lower S-Pd and S-Pt bond strengths. This indicates that the HDS mechanism, detailed above for Mo based catalysts cannot be valid for the noble metals. Again, the number of vacancies for bonding sulfur-organic compounds is not limiting, as in the case with monometallic Co and Ni. 

Figure 7. SEM and XRMA microphotographs of palladium catalysts supported on the amphiphilic resin made by DMAA, MTEA, MBAA (cross-linker) [30]. Microphotographs (a) and (b) show an image and the radial palladium distribution after uptake of [Pd(OAc)2] from water/acetone the precursor diffuses only into the outer layer of the relatively little swollen CFP after reduction the nanoclusters remain close to the edge of the catalyst beads. Microphotographs (c) and (d) show the radial distribution of sulfur and palladium, respectively, after uptake of [PdCU] from water after reduction palladium is homogenously distributed throughout the catalyst particles. This indicates that under these conditions the CFP was swollen enough to allow the metal precursor to readily penetrate the whole of polymeric mass. (Reprinted from Ref. [30], 2005, with permission from Elsevier.)...

Around 500 K, the catalyst consumes H2 in a sharp peak, while simultaneously H2S and some additional H20 are produced. Amoldy et al, assign the uptake of hydrogen and the evolution of H2S to the hydrogenation of excess sulfur formed via the decomposition of the oxysulfide species in (2-8) at low temperatures ... 

Reductive alkylation is an efficient method to synthesize secondary amines from primary amines. The aim of this study is to optimize sulfur-promoted platinum catalysts for the reductive alkylation of p-aminodiphenylamine (ADPA) with methyl isobutyl ketone (MIBK) to improve the productivity of N-(l,3-dimethylbutyl)-N-phenyl-p-phenylenediamine (6-PPD). In this study, we focus on Pt loading, the amount of sulfur, and the pH as the variables. The reaction was conducted in the liquid phase under kinetically limited conditions in a continuously stirred tank reactor at a constant hydrogen pressure. Use of the two-factorial design minimized the number of experiments needed to arrive at the optimal solution. The activity and selectivity of the reaction was followed using the hydrogen-uptake and chromatographic analysis of products. The most optimal catalyst was identified to be l%Pt-0.1%S/C prepared at a pH of 6. 

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