Active temperature of catalyst

The ammonia synthesis reaction temperature depends on the active temperature of catalyst hence the process operator should know the active temperature of catalyst. Tables 8.3 and 8.4, and Fig. 8.4 show the relationships between the activity and temperature of ZA-5, A301, ICI74-1 and AllO-2 catalysts. 

Under the conditions listed in Table 8.3, the activity is close to the equilibrium ammonia concentration above 475°C, so the differences of activity between different catalysts become small. However, at lower temperatmes, especially at 350-450°C, the activity of ZA-5 is dramatically higher compared with that of AllO-2. During the design of the reactor and practical operations, the temperature of the catalyst bed should be set within the optimum range of temperatures of the catalyst. 

According to Table 8.3, to maintain the same ammonia concentration at the outlet, the active temperatures of different catalysts are shown in Table 8.5. The table shows that when achieving the same ammonia concentration at the outlet (16.68%), the active temperature of ZA-5 is 30°C or 40°C lower than that of ICI74-1 and AllO-2 respectively. The activity of ZA-5 at 400°C is equivalent to that of the AllO-2 at 440°C and ICI74-1 at 430°C respectively. In other words, if the reaction temperature of ZA-5 catalyst is reduced by say 40°C, it would still achieve the same ammonia concentration at the outlet as AllO-2 could at 440° C. We call this effect the temperature effect . The initial hotspot temperature (the highest temperature point in beds) of ZA-5 catalyst bed is 430-450°C, and the operational temperature range is 300 500°C. Therefore, ZA-5 is an excellent catalyst that could be used at a low temperature and within a wide temperature range. 

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The reaction temperatures of the catalyst bed are determined by the active temperature of catalysts. For the selection of the reaction temperature, not only the optimum reaction temperature but also the requirements of catalyst performance should be considered. In an industrial process, the temperature of catalyst beds should be especially controlled in the range of active temperature of catalysts. The inlet temperature of the catalyst bed should not be less than the initial active temperature of the catalyst. The highest bed temperature must not exceed the heat-resistance temperature of the catalyst. The characteristics of the operating temperatures of iron catalysts are shown in Table 8.7. 

The reactivity of the propagation centers in oxide polymerization catalysts depended on the nature of the transition metal, support, activation temperature of the catalyst, and type of reducing agent (168a). 

The optimal activation temperature of zeolites in the range 300-600°C was determined in preliminary experiments. The temperature 500°C was selected for all catalysts but for H-MFI-28 (400°C). DSC and TG analyses show that at 500°C, NH4+ forms of zeolites are totally converted into H-MOR-20 and H-FER-20, respectively. Table 2 shows alcohol conversion (XR0H), selectivity to ether (SROr), to olefin (Soiefm) and branched ethers (SrOR>), and ether yield (YROr) at 180°C after 6 h reaction course. 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Amorphous solids such as MgO and Mg-Al mixed oxides gave some interesting results in gas-phase hydrogen transfer reduction, though all these catalytic systems require activation temperatures of at least 500°C [16-18], In contrast, very few cases have been reported of hydrogen transfer reactions mediated by a supported metal catalyst, the most efficient being Ru(OI I)x/Al20, [19]. 

The entire spectrum of tools in synthetic inorganic chemistry, including high temperature methods, precipitation, solvothermal synthesis, sol-gel chemistry, chemical vapor deposition (CVD), and soft matter techniques adopted in part from organo-metallic chemistry, have been applied to synthesize the active mass of catalysts. Table 4.2.1 summarizes basic techniques. The preparation methods differ whether... 

Figure 1 show hydrogen conversions for dehydrocondensation at different temperatures of catalysts. It has been found that catalytic dehydrocondensation reaction displays the second order. Dehyd-rocondensation reaction rate constants are determined, and catalytic dehydrocondensation activation energies are calculated /, act = 28.1 -28.5 kJ/Mole. As a consequence, for anhydrous caustic potash and platinum hydrochloric acid application as the catalyst activation energies are almost the same. 

Solely a function of properties of reacting materials [e.g., concentration (activities, temperature, pressure, catalyst or solvent (if any)]. 

Catalyst activity does not depend very much upon the rate of heating within the range 25 275° per hour. In a set of experiments at an activation temperature of 300° with six different rates in this range, k increased from 31 to 45 with increasing rate of heating. In view of the scatter in Table IV we are not sure that the variation is significant. 

The discussion above has been in terms of results of activation temperatures between 215 and 300°. As may be seen from Table VIII, the fraction of multiply exchanged 2-hexene resulting from process (8) changes little until after an activation temperature of 300°. It then declines and becomes very small by 400° on both crystalline and amorphous catalysts. Process (5), formation of exchanged 1-hexene, declines steadily with activation temperature and becomes very small at 400°, particularly on amorphous catalysts. 

Pairs A and B were assigned to adsorbed SO and sulfide bonded to ruthenium atoms, respectively. The activation of Ru catalysts was dependent on the heating temperature of catalysts. Pair B appeared when the catalysts were still inactive. Hence, ruthenium metal microparticles were first sulfided and then exhibited the catalytic reactivity to form elemental sulfur from SO. The difference of sulfidation temperature for the [Ru CjA iOj (503 K) and conv-Ru/TiO catalysts (573 K) may be originated from each Ru particle size. In general, smaller [RuJ cluster is more reactive than larger Ru particles (10 - 50A) of conventional catalysts [14]. Pair A was exclusively observed in addition to the weak shoulder peaks of Pair B (Fig. lA). Therefore, the surface during the catalysis of the SO + Hj reactions should be predominantly occupied by SO. The elementary step of SO dissociation to SO(ads) may be the rate-determining step of overall reaction. 

The first part has shown that photoreduction can not only replace thermal reduction for an activation method of catalysts but also give rise to higher activity than the thermal reduction. Reduction at low temperature in the photochemical method results in maintaining high coordinative unsaturation of a catalytically active site. 

FIGURE 68 The influence of Cr/silica activation temperature on catalyst porosity, activity, and the polymer elasticity (LCB). 

FIGURE 89 LCB contents of polymers made with two Cr/silica catalysts, which differed only in their degree of structural coalescence. LCB content responds independently to both the activation temperature of the catalyst, and also to its physical structure, as indicated here by the two separate lines. 

Cr/aluminophosphate catalysts respond to activation temperature in many of the same ways that Cr/silica does, and there are some differences too. The activity of the catalyst is generally increased at higher activation temperatures. Figure 175 shows how the kinetics of polymerization with a 0.8 P/Al catalyst responded to the activation temperature of the catalyst. There was little change in the overall shape of the kinetics profile only the height varied. The average activity of the catalyst improved when the activation temperature was raised from 300 up to 700 °C. 

The third series of experiments shown in Table 53 was carried out with the same catalyst, but polymerization was conducted at 95 °C with 8 ppm of BEt3 cocatalyst. As the activation temperature of the catalyst was raised from 500 to 900 °C, the MI of the polymer increased substantially and the MW declined. 

The breadth of the polymer MW distribution of these polymers is also shown in Table 53 as the polydispersity (MW/MN). All these values were high, indicating that Cr/A1P04 produces polymers with a broader MW distribution than does Cr/silica. In the first series, there was a narrowing of the polymer MW distribution as the activation temperature of the catalyst was raised from 400 to 800 °C, and then a major narrowing was observed after the catalyst was activated at 900 °C. 

The influence of the activation temperature of the catalyst can be seen in the data of Table 60. There, the melt indices are presented of polymers made with the catalysts activated at 600 vs. 800 °C. Fluoride treatment of the catalyst seems to compensate for the lower activation temperature, which suggests that it removes potential ligands. 

The initial temperature of catalyst activation can also influence the amount of in situ branching obtained in the polymer. This is in agreement with the olefin-generating behavior of the organochromium catalysts (Figures 185 and 192, Table 55). Table 67 shows an experiment in which Cr/silica-titania was activated at 800 °C or at 650 °C, and then it was reduced and tested for polymerization activity with 5 ppm triethylboron cocatalyst. The 800 °C catalyst resulted in significantly lower polymer density than the 650 °C catalyst. This derives from two causes. The 800 °C... 

Activity data of catalysts subjected to high temperature treatments at 700°C and 800°C and that of hydrothermally treated samples are given in Table 2. The percentage loss of catalyst activity during deactivation tabulated in Table 2 is a helpful index to rank catalysts with respect to their resistance to deactivation. 

CH4 reactions with CO2 or H2O on group VIII or noble metals (Ru, Rh, Pd, Ir, Pt) [1] form synthesis gas which is the precursor to valuable fuels and chemical compounds, as lirst shown by Fischer and Tropsch [2]. Due to the cost and availability of the nickel, compared to noble metals, Ni catalysts are used industrially. However, Ni-based catalysts tend to form inactive carbon residues that bloek the pores as well as the active sites of catalyst, and whose main activity is die formation of carbon filaments [3]. Therefore, the industrial methane steam reaction is usually performed under an excess of water to maintain the catalyst activity. Another alternative is the modification of the composition of the catalyst (generally Ni/Al203) by addition of a basic compound like MgO [4]. It is well known that the formation of NiO-MgO solid solution is easily favoured by calcining the mixed oxide at high temperatures [5] and much attention was devoted to its specific properties [6]. Parmaliana and al. 

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