Catalyst pore architecture

Conversion of these coke precursors prior to the formation of the coke. It is to some extent debatable whether this can be done. Some improvements are possible using an improved catalyst pore architecture and acidity distribution. 

S.J Yanik, P. O Connor, D.H. Abner, and M.C. Friedrich. "FCC Catalyst Pore Architecture and Performance." 1991 AIChE Annual Meeting, 18-20 November 1992. 

Figure 7 Simplified Pore Architecture of fresh catalysts.

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Fouling can result in bigger differences in selectivity of various catalysts, because of changes in pore architecture [2, 10]. 

Figure 10 groups the parameters according to geometry, bulk defects, surface phenomena, and extrinsic modifications. The geometry of a catalyst particle is given by its size, its habitus (meaning the anisotropy or deviation from a spherical shape), and by its pore system. Only for micro-and mesoporous samples is XRD a sensitive tool to determine the pore architecture (Chen et al., 2005 Davidson, 2002 Li and Kim, 2005 Liu et al., 2002 Ohare et al., 1998). In many solids that are more compact than most catalysts, only secondary effects are related to the pores. 

Another effect of zeolite pore architecture on esterification is found in the lactonization of 15-hydroxypentadecanoic acid. With dissolved acid catalysts or with amorphous Si02-Al203, dimerization or polymerization are the dominant reactions, but when the reactant is adsorbed within the pores of a dealuminated HY zeolite, only the pentadecanolide is obtained (8) ... 

Since their discovery, microporous materials such as zeolites found major application fields in processes like separation, ion exchange and catalysis. Their uniform pore size and pore architecture are at the basis of separation processes whereas the chemical composition of these materials makes them unbeatable candidates to be used as a catalyst or an ion exchanger. Regardless of which process is used, the molecules engaged are adsorbed on the surface according to their molecular structure and properties. The bulkiness of the molecule compared to the pore size of the microporous material decides if or not the molecule can be trapped in the depth of the porous framework, thus there exists cases where molecules with larger diameters than the pore size are not able to enter the pores. This makes the microporous materials acting as a sieve in molecular level and they are hence referred to as molecular sieves. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

Planar faults are common in zeolites and related crystalline microporous solids. These can influence the sorptive characteristics in any one of several ways (i) they can have little influence on the overall accessibility or capacity, but alter the pore architecture, accessibility or difiusional constraints (ii) they can reduce the limiting dimensions of pore windows while leaving the tot pore volume unaffected (iii) they can block channels. Pores or pore access can also be blocked by detrital material such as alumina extracted from the framework, coke or sintered metal catalyst particles, immobile organic molecules or non-framework cations in blocking positions. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

Trichlorobenzene (TCB), a well known termite exterminator, is prepared selectively from 1,2-dichlorobenzene (DCB) using zeolite K-L as a catalyst and monochloroacetic acid as a promoter. An attempt has been made to apply the combination of molecular graphics, force field calculations and quantum chemical calculations to understand the mechanism of selective chlorination of 1,2-DCB to 1,2,4-TCB over K-L promoted by monochloroacetic acid. It was found that the zeolite lattice plays an important role in polarising the molecules. The peculiar "barrel shaped pore architecture allows zeolite L to act as a reactor vessel where monochloroacetic acid, chlorine and 1,2-DCB can be accommodated on a molecular level. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

The well-defined cages and pores of zeolites allow the application of these materials as molecular sieves and form-selective catalysts in heterogeneous catalysis. Xe NMR spectroscopy of xenon atoms adsorbed on zeolites is a suitable method for the characterization of the zeolitic pore architecture. The isotope Xe has a nuclear spin of 1=112, a natural abundance of 26.4% and, adsorbed on zeolites, a Ti relaxation time in the range between 10 ms and a few seconds. 

This review has presented an overview of the impact of tuning both the surface properties and pore architectures of solid acid and base catalysts on their performance in biodiesel synthesis. Plant-oil viscosity and poor miscibility with light alcohols continue to hamper the use of new heterogeneous catalysts for continuous biodiesel production from both materials and engineering perspectives. Thus, the design of... 

These data clearly indicate that the NiMCM-36 catalyst exhibits very interesting properties for ethylene oligomerization, by comparison with the microporous NiMCM-22 zeolite at both reaction temperatures (70 and 150°C, respectively). Compared with other catalysts, the NiMCM-36 behaviour is intermediate between Ni-exchanged dealuminated Y zeolite and Ni-exchanged mesoporous materials. Taking into account that the amount of Ni2+ sites is near the same for all samples (Table 1), in order to explain these differences in catalytic behaviors, two mains categories of properties could be considered (i) the concentration and strength of acid and nickel sites and (ii) the diffusional properties (determined by the size and the architecture of pores). 

Ary given catalytic material can be abstracted based on the same underlying similar architecture — for ease of comparison, we describe the catalytic material as a porous network with the active centers responsible for the conversion of educts to products distributed on the internal surface of the pores and the external surface area. Generally, the conversion of any given educt by the aid of the catalytic material is divided into a number of consecutive steps. Figure 11.13 illustrates these different steps. The governing transport phenomenon outside the catalyst responsible for mass transport is the convective fluid flow. This changes dramatically close to the catalyst surface from a certain boundary onwards, named the hydrodynamic boundary layer, mass transport toward and from the catalyst surface only takes place... 

This leads to a poor conversion and higher coke and fuel gas yields. In order to improve the conversion of large molecules, new modifications in the FCC Catalyst Architecture and Active Site Accessibility, are required in terms of Pore Size and Pore Acidity distribution. 

Figure 7 illustrates that there is an essentail difference in the pore size architecture between catalysts produced with this technology (Cat. C) and those produced with the other manufacturing technologies (Cat. A,B). With the Cat. C Technology (high meso pore activity catalysts) there is a shift towards more pores in the "larger" meso pore area (Table IV.)... 

The amount of soft" coke or hydrocarbons entrained to the regenerator without being stripped (3, 10] will have a significant effect on the overall coke selectivity and will depend on the surface area and pore size architecture of the aged catalyst [10, 49]. 

Zeolites are crystalline aluminosilicates found in nature and have been prepared synthetically since the 1800s. Their robust structure, porous architecture and internal acidic sites make them excellent sorption materials, ionic exchange materials and catalysts for industrial processes. There are numerous known zeolite structures that have been obtained naturally or prepared synthetically through various routes. In the simplest cases the porous architecture can be altered by exchanging different sized cations into the pores to vary pore diameter or volume. 

Table II (118) shows that the surface area, pore volume, and pore size of the deposited films vary consistently with the aging times. Thus the film structures may be tailored for such applications as surface passivation, sensors, membranes, or catalysts by a simple aging process prior to film deposition. In addition, multiple deposition schemes involving different compositions or structures or both allow the formation of complex layered architectures potentially useful for optics, electronics, or sensors.

MOR = 0.015. These results can be directly related to the micro-porous structure of the different catalysts, for which the pores of the tridimensional Y framework allow a readier diffusion of both substrate and product than those of the interconnected channels architecture of BEA and those of the bidimensional MOR framework. 

Microporous carriers as zeolites and molecular sieves are preferred catalyst carriers, because of the large variety of micrporous structures which are available and because of the regular pore or channel architecture of each of those materials. 

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