Hydrogenation 1,3-Butadiene

This model reaction has been used for interpreting geometrical and electronic effects [7, 16-19]. As an example, we used the same reaction system described above however, with a palladium-supported catalyst, fed with butadiene and hydrogen at T =215 K, WHSV = 800 h . Results of dispersions, activity, and selectivity are presented in Table 3.5 [16]. 

The activity expressed as turnover frequency (TOF) shows that this reaction is structure sensitive (RSS). The literature presents ctMiflicting interpretations in this regard. Thus, the analysis of the performance of the catalysts will be made in terms of selectivity for the main products, as presented in Table 3.5. 

The catalyst supported on graphite (GR) showed the highest transicis ratio and a lower production of butane and 1-butene. According Boitiaux et al. [17], this performance is characteristic of metals with higher electron density, which disfavors the formation of the intermediate carbene. Regarding the supported catalyst on 

WHS V = 800 h tic selectivity ratio of trans-2-butene/cis-2-butene, SBA selectivity of butane, Si be/OE, selectivity of l-butene relative to the total butanes [16] 

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Butene is thought to be formed by the initial isomerization of the reactant to adsorbed 1,2-butadiene, followed by hydrogenation (see Fig. 26). This mechanism predicts a maximum at -d in the 2-butenes and at -d in 1-butene if the chance of surface species aquiring a deuterium atom, as opposed to a hydrogen atom, is high. Moreover, the appearance of trace quantities of 1,2-butadiene in the rhodium and iridium catalyzed reactions lends further support to this route. A check may be made, since, if the 1,2-butadiene hydrogenates to yield approximately equal proportions of cis-2-butene and 1-butene (see Section... 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Structure sensitivity of 1,3-butadiene hydrogenation was recently investigated by Rupprechter et al. [59] on well-defined Pd/Al203/NiAl(l 1 0) single crystals. 

Evidence for cobalt tr-butenyl and 7r-methylallyl intermediates in butadiene hydrogenations has been obtained using Raman spectroscopy (194), which could be a useful probe for catalytic reactions, especially in aqueous solutions. 

The report showed that, for 1,3-butadiene hydrogenation, the selectivities achieved with Pd(acac)2 dissolved in ionic liquids were similar to those observed under homogeneous conditions and were higher than under heterogeneous conditions (using Pd on carbon as the catalyst) or in neat 1,3-butadiene (Table 41.4). 

Much experience concerning the hydrogenation of conjugated dienes was obtained with butadiene hydrogenation. On Pt single crystals the reaction was found to be structure sensitive the activity sequence of different planes (marked with Miller s index) is... 

For 1,3-butadiene hydrogenation, the toxicity of sulfur is 3 (Fig. 13). which is lower than the toxicity for olefin hydrogenation. The hydrogenation of 1-butyne has also been studied for various ratios of sulfur over palladium. As was already published (86), the 1-butyne hydrogenation rate increases with time. The same effect has been observed on sulfided palladium. The turnover number is consequently presented for 1-butyne hydrogenation versus the sulfur content for various 1-butyne conversions (see Fig. 14). During the first minutes of reaction (0-25% conversion), the toxicity of sulfur appears close to 1 the rates are proportional to the free surface. However, at higher conversion, the rate becomes independent from the sulfur ratio. The toxicity is zero. 

Boitiaux et al. and Verna (61, 62) confirmed this variation of selectivity in the butadiene hydrogenation for the parallel reactions (1,2 and 1,4 addition) on presulfided palladium. Figure 16 shows the 2-butene/1-butene and the fraus-butene/c/s-butene ratios versus the sulfurization extent of the surface palladium. The sulfur decreases the trans/cis ratio and favors the 1,4 addition. 

EXAFS data characterizing Si02-supported molybdenum species made from [Mo2(allyl)4] have led to precise structural models, including those of surface dimers (with Mo-Mo bonds) and pair sites (without Mo-Mo bonds) (Iwasawa, 1987), but the structures seem to be sensitive to undefined chemistry of the Si02 support surfaces and could be difficult to reproduce. Catalytic data for ethylene hydrogenation and butadiene hydrogenation with these samples and with samples expected to have isolated Mo sites point to a catalytic role of the neighboring sites (Iwasawa, 1987). 

The objective of the work presented here is to combine activity studies using a model reaction with STM and AFM studies on model catalysts and to determine structural correlations between catalytic activity and morphology. Other characterization tools are also used to determine compositional effects induced by pretreatment or the reaction. The model reaction used is the hydrogenation of 1,3 butadiene hydrogenation due to its high reactivity on low-surface area Pd model catalysts and its well-studied mechanism [17-19]. X ray photoelectron spectroscopy (XPS) was used to determine surface composition. 

Activity measurement 1,3 butadiene hydrogenation was chosen as a probe reaction. The four major products of 1,3 butadiene hydrogenation, namely 1-butene, n-butane, cis- and trans-2-butene were monitored to study the pretreatment effect on the selectivity. Activity measurements were conducted after preheating the catalysts in-situ in various atmospheres without exposure to air. Two pieces of film catalysts were placed in a 0.8 cm ID quartz tube reactor and were pretreated as follows. 

Study of a) 8 h H2 activation treatment was carried out with two pieces of Pd/SiOj film catalysts (1.5 cm2 total) while two other pieces were activated in b) 4 h O2 and 4 h H2 at 450° C> prior to the activity measurements at 100° C with 1,3 butadiene hydrogenation. The H2 to... 

Figure 2 The selectivity plot of 1,3 butadiene hydrogenation over Pd/Si02- (+ H2 treated sample, x Q2+H2 treated sample, selectivity plots of H2 treated catalyst were interpolated)...

Table 1 Arrhenius, kinetic, and surface roughness parameters of 1,3 butadiene hydrogenation over Pd/Si02 thin film catalyst. ( ref 16, T=3Q0K, Ph2 = 0.7 kPa and Pc4H6 = 0.2 kPa) / denotes the surface-roughness-to-scan-area ratio.

Saint-Lager et al. (2005) PdsNi92 alloy Dynamics of reconstruction and segregation + + + Butadiene hydrogenation... 

Commission Recommendation of 29 April 2004 on the results of the risk evaluation and the risk reduction strategies for the substances acetonitrile acrylamide acrylonitrile acrylic acid butadiene hydrogen fluoride hydrogen peroxide methacrylic acid methyl methacrylate toluene trichlorobenzene (2004/394/EC) 30.4.2004, Official Journal of the European Union, EC, 2004, No.L.144. 

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