Narrow-pore catalysts

Fatemi et al. [54] also observed reaction rate hysteresis with temperature cycling by experiment. Thiophene hydrogenation was chosen as an example of a hydrotreating reaction and n-heptane was used as a solvent. Experiments were carried out for two Ni-Mo/y-AUOs samples first a narrow pore catalyst with BJH average pore radius of 26.9 A and secondly a wide pore catalyst with BJH average pore radius of 70.3 A. As shown in Figure 23.16, it was found that the narrow pore catalyst exhibits a more pronounced hysteresis loop than the wide pore catalyst. This difference could be explained since a larger volume of gas condenses and hquid evaporates from the catalyst with narrow pores, which are more susceptible to capillary condensation. 

Fatemi, S., Moosavian, M.A., Abolhamd, G., Mortazavi, Y., and Hudgins, R.R., Reaction rate hysteresis in the hydrotreating of thiophene in wide- and narrow-pore catalysts during temperature cycling. Can. J. Chem. Eng., 80, 231-238, 2002. 

A change during polymerization of outer surface of fragments from broad to narrow pores. Catalysts with broad pores are known to make high melt index polymer (1). It is reasonable to assume that the rather broad pore size distribution of the catalyst is not evenly distributed, but that there are regions of broad and of narrow pores. 

A.WIDE-PORE CATALYSTS B NARROW-PORE CATALYSTS... 

The nitrogen physisorption isotherm and pore size distributions for the synthesized catalysts are shown in Figs. 3 and 4. The Type IV isotherm, typical of mesoporous materials, for each sample exhibits a sharp inflection, characteristic of capillary condensation within the regular mesopores [5, 6], These features indicate that both TS-1/MCM-41-A and TS-l/MCM-41-B possess mesopores and a narrow pore size distribution. 

Consequently, catalysts with narrow pores should give lower yields of V than those with large pores. 

Thiophene (C4H4S) is representative of the organic sulfur compounds that are hydrogenated in the commercial hyditodesulfurization of petroleum naphtha. Estimate both the combined and effective diflfusivities for thiophene in hydrogen at 660 °K and 3.04 MPa in a catalyst with a BET surface area of 168 m2/g, a porosity of 0.40, and an apparent pellet density of 1.40 g/cm3. A narrow pore sized distribution... 

Fischer-Tropsch Synthesis Comparison of the Effect of Co-Fed Water on the Catalytic Performance of Co Catalysts Supported on Wide-Pore and Narrow-Pore Alumina... 

The above behavior of narrow-pore supported cobalt catalysts toward co-fed water can also be explained in terms of relative size of cobalt clusters, pore network of support, expected location of cobalt clusters within the pore network, and relative differences in the residence time of water vapor within and outside the... 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

The studies of Thomas and Raja [28] showed a remarkable effect of pore size on enantioselectivity (Table 42.3). The immobilized catalysts were more active than the homogeneous ones, but their enantioselectivity increased dramatically on supports which had smaller-diameter pores. This effect was ascribed to more steric confinement of the catalyst-substrate complex in the narrower pores. This confinement will lead to a larger influence of the chiral directing group on the orientation of the substrate. Although pore diffusion limitation can lead to lower hydrogen concentrations in narrow pores with a possible effect on enantioselectivity (see Section 42.2), this seems not to be the case here, because the immobilized catalyst with the smallest pores is the most active one. 

In another article by Corma et al. (178), ITQ-7, a three-dimensional large-pore zeolite, was tested as an alkylation catalyst and compared with a BEA sample of comparable Si/Al ratio and crystal size. The ratio of the selectivities to 2,2,4-TMP and 2,2,3-TMP, which have the largest kinetic diameter of the TMPs, and 2,3,3-TMP and 2,3,4-TMP, which have the lowest kinetic diameter, was used as a measure of the influence of the pore structure. Lower (2,2,4-TMP + 2,2,3-TMP)/ (2,3,3-TMP + 2,3,4-TMP) ratios in ITQ-7 were attributed to its smaller pore diameter. The bulky isomers have more spacious transition states, so that their formation in narrow pores is hindered moreover, their diffusion is slower. The hydride transfer activity, estimated by the dimethylhexane/dimethylhexene ratio,... 

Holmen and coworkers15-17 also observed a loss in activity when water was introduced to un-promoted and Re-promoted cobalt deposited on >-Al203. In a recent paper similar results were reported for Co Re supported on both narrow-pore and wide-pore y-Al203,18 and permanent deactivation was observed when the inlet ratio H20 H2 was 0.7. The same group reported that the rhenium-promoted catalysts lost activity more rapidly than their un-promoted counterparts.14-1619... 

Co/SiOz (Davisil634) CSTR None Narrow pore silica Catalyst . S BET = 380 m2/g, pore volume 0.64 cm3/g, aver, pore diam. 7.1 nm, DCo — 6.8%... 

The zeolite structure also plays a large role in RC product distribution. Weitkamp et al.62 conducted experiments with Pt/HZSM-5 catalysts, which have very narrow pore sizes when compared with other zeolites, such as USY or SAPO. They found that c is I trans-1,3-dime thylcyclopentane was formed, while 1,1 and 1,2-DMCP were not. This indicates that the more oval shaped 1,3-DMCP was able to diffuse through the pores, while the more bulky and spherical isomers were not, and thus not seen in the product distribution. In short, when compared with dealkylation to cyclohexane, ring contraction of MCH is a more effective pathway to yield higher ON products. However, in order to further improve the ON, ring-opening of the RC isomers may be necessary, as shown below. 

However, with respect to ee, the same catalyst immobilized on amorphous silica performed even better (conversion 72%, ee 92%) than the one immobilized on MCM-41. This example illustrates an important issue, i.e., OMS-based catalysts have to be compared with those based on amorphous silica or silica-alumina. If the amorphous materials perform as well or even better than the OMS materials, then there is no advantage in using the significantly more expensive OMSs. However, in those cases where the catalytic reaction benefits from the regular and well-defined pore systems of the OMS materials, such materials can be very attractive, e.g., for the conversion of bulkier molecules or to overcome transport limitations in more narrow pores. 

It will be demonstrated in this section that a narrow pore structure limits the reaction rate to an extent which casues the reaction rate to be either proportional to the square root of the specific surface area (per unit mass) or independent of it, depending on the mode of diffusion within the pore structure. Lest this departure of the reaction rate from direct proportionality with specific surface area might be thought to be accounted for in terms of a non-uniform distribution of surface energy over the catalyst surface, it should be pointed out that such in situ heterogeneity is usually only a small fraction of the total chemically active surface and cannot therefore explain the observed effects. 

The porous structure of either a catalyst or a solid reactant may have a considerable influence on the measured reaction rate, especially if a large proportion of the available surface area is only accessible through narrow pores. The problem of chemical reaction within porous solids was first considered quantitatively by Thiele [1] who developed mathematical models describing chemical reaction and intraparticle diffusion. Wheeler [2] later extended Thiele s work and identified model parameters which could be measured experimentally and used to predict reaction rates in... 

It was concluded at this point that zeolites with a very spacious pore system, such as faujasites or ZSM-20, are inappropriate catalysts for the isomerization of 1-methyl-naphthalene. Subsequently, a zeolite with much narrower pores was tested, viz. HZSM-5. Pertinent results are shown in Fig. 2. At 300 C, the conversion is low and even a temperature increase of 100 °C does not bring about a considerable increase in conversion. We presume that the reaction of 1-methylnaphthalene in HZSM-5 is controlled by diffusioiL There were practically no side reactions such as cracking, dealkylation or transalkylation, in other words XjMHp Y2.M.NP "e identical. This is at variance with the results of Matsuda et al. [21] who did observe some disproportionation on their H2SM-5 sample at 300 °C. More work is needed to elucidate the reasons for this different catalytic behavior of various samples of HZSM-5. As a whole, zeolite ZSM-5 was discarded at this stage due to its too narrow pore system. 

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