Crystal intracrystalline reaction

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Solid-state reactions are known from thermal intracrystalline conversions (isomerizations or loss of volatile fragments), photoreactions, gas-solid reactions, and solid-solid reactions. As all of these relate strictly to the crystal packing (unifying solid-state mechanism) and are not separated in the various sections. Also, nontopotactic (normal) and topotactic (very rare) reactions are not separated in different sections. 

Three obvious models which could describe the observed reaction rate are (a) concentration equilibrium between all parts of the intracrystalline pore structure and the exterior gas phase (reaction rate limiting), (b) equilibrium between the gas phase and the surface of the zeolite crystallites but diffusional limitations within the intracrystalline pore structure, and (c) concentration uniformity within the intracrystalline pore structure but a large difference from equilibrium at the interface between the zeolite crystal (pore mouth) and the gas phase (product desorption limitation). Combinations of the above may occur, and all models must include catalyst deactivation. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

This case study clearly illustrates the usefulness of the ZLD-TEOM technique in determining intracrystalline diffusivities in zeolites, provided that effects of other transport resistances such as the surface barrier are eliminated by varying the crystal size of the zeolites. The measured steady-state diffusivity can be directly used for predicting effects of diffusion in reactions catalyzed by zeolites. More important, the TEOM makes it possible to distinguish the deactivation caused by blockage of the active sites and by increased diffusion resistance caused by blockage of cavities or channels by coke. 

Reasons why reactions of solids may proceed more rapidly in a molten zone than within a crystalline reactant [75], include (i) relaxation of the regular stabilising intracrystalline forces (ii) establishment of a favourable configuration for chemical change may be possible due to mobility in a liquid but inhibited within a rigid crystal stmcture, and (iii) the influences of intermediates and impurities may be greater (or different) in a molten phase. 

The final stages of the above reaction overlapped with the onset of the nucleation and growth process that continued to complete the dehydration. Growth of three dimensional nuclei was confirmed microscopically. This second rate process was well described by the Avrami-Erofeev equation with = 2 and E, for crystals was 175 30 kJ mol (with a considerable scatter of data) below 460 K and a more reproducible reaction rate, with E, = 153 10 kJ mol, for powder. Above about 450 K there were some indications of intracrystalline melting of single crystals and the value of , increased markedly to 350 50 kJ mol (again with significant scatter of data). 

Each crystalline substance has a unique structure. Groups of compounds classified as isomorphous have similarities of lattice symmetry, but dimensions, and hence interionic forces, are different. Moreover, a particular substance can adopt alternative structures under changed conditions of temperature, pressure, crystallization conditions, presence of impurities, etc. Ordered packing, with symmetrical intracrystalline forces, appears to confer enhanced stability within the bulk solid so that decomposition processes usually occur at surfaces within a restricted reaction zone. Interfaces can be regarded variously as complex imperfections, zones of destabilizing strain, or (product) sites of catalytic activity. 

Zeolites are crystalline, microporous aluminosilicates with molecular-sized intracrystalline channels and cages. Guest molecules with molecular diameters smaller than zeolites (from 3 to 15 A) can enter the interior of zeoUte crystals (intercalation) giving rise to shape and size selective sorption and, consequently, highly selective reactions. 

Shape selectivity is in many cases not a result of sharp molecular sieving but of differences in intracrystalline mobility, due to steric hindrance. For example the para-isomers from alkylation followed by secondary isomerization inside zeolite crystals will preferably escape to the gas phcise. In principle the direction of diffusion processes is reversible. For this reason one can imagine an educt selectivity as reversion of the product selectivity described above. In this case from a mixture of isomers the reaction of the para isomer should be favoured. 

The turnover numbers (TON) of phenol are 62.4 and 105.4 with a H2O2 efficiency of 28.9 wt.% and 48.9 wt.% for the Zr-Sil-2 samples A and B, respectively. A significant difference in the product distribution between these two runs is also observed. The catechol (CAT) to hydroquinone (HQ) ratios are 0.9 and 1.7 for Zr-Sil-2 (A) and (B) samples, respectively. A CAT/HQ ratio of 0.9 to 1.3 has been reported for titanium and vanadium silicate molecular sieves (TS-2 and VS-2) [13]. The samples synthesized using Zr(acac)4 show a nearly two fold activity in the reaction probably due to the smaller particle size. These results indicate that in the case of Zr-Sil-2 samples synthesized using ZrCU, the Zr " ions are well dispersed within the channels of the MEL structure while in the samples synthesized using Zr(acac)4, the hydroxylation occurs at the external surface as well, where a part of Zr species may be located. For small submicron crystals (<1 pm), external surface sites could be a significant fraction of the total surface area. If the external surface sites are catalytically either the same or more active than the intracrystalline active sites, then the shape selectivity of a zeolite could... 

Strong acid sites of the zeolite with and without silica binder were measured by the chemisorption of pyridine at 400 C. The acid sites were also measured in terms of the activity of the zeolite catalysts in acid catalyzed model reaction, disproportionation of toluene at 500 C. Acid sites on the external surface of zeolite crystals or intercrystalline acid sites of the zeolite catalysts were measured in terms of the iso-octane (which cannot enter in ZSM-5 zeolite channels even at 400°C [18, 19]) cracking activity at 400 C [11]. The results showing the influence of silica binder on both the intracrystalline and intercrystalline acidity of the zeolite catalyst are presented in Tables 1 and 2. 

As the catalyst ages, the light olefin yield and the selectivity both increase [127,129]. This appears to be related to the buildup of coke within the intracrystalline pores, which reduces both the intrinsic rate constant and the intracrystalline diffusivity [128,129]. Detailed measurements with different crystal sizes show that with increasing coke levels the diffusivity declines more rapidly than the rate constant, so that diffusional limitations become more pronounced as the catalyst ages. A high yield of light olefins requires that the dimethyl ether formed in the first step of the reaction be retained... 

For sufficiently small particles 0 0 and 1, so the measured rate constant approaches the intrinsic rate constant (k). By making replicate measurements under similar conditions, with different particle size fractions it is possible to determine both the intrinsic rate constant and the effective interparticle diffusivity. Haag [67] suggested that this approach could be used to determine intracrystalline diffusivities in zeolite crystals. A more complete experimental study in which the diffusivity of 2,2-dimethyl butane in HZSM-5 was determined both chromatographically and from measurements of the cracking rate under diffusion-limited conditions was reported by Post et al. [68] - see Fig. 11. This approach has the advantage that it makes steady-state rather than transient measurements, but it is limited to sorbates for which a suitable catalytic reaction occurs. 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

In most cases, catalysis on zeolites occurs inside the intracrystalline voids. Nevertheless, a catalytic role is sometimes attributed to the external surface of the crystals, which for many crystallographic directions consists of a collection of pore mouths. The possibility of catalysis at pore mouths was first discussed by Venuto [41]. Catalytic sites at the pore mouths can have a strength and a structural environment different from those within the intracrystalline cavities. In principle, situations may occur where the concentration of reactants is totally different at the pore mouths compared to the crystal interior. When intracrystalline diffusion is slow compared to the rate of the chemical reaction, only active sites near the external surface of crystallites may be responsible for catalysis. The absence of intracrystalline diffusion restrictions doesnot necessarily imply that the pore mouths are a-selective catalytic enviromnents. They may provide a local geometry which is different from that available inside the crystals. 

Zeolite catalysts are composed of silicon and aluminum oxides in a crystal structure that is permeated by Intracrystalline pores and cavities of precise and uniform dimensions. Chemical reactions occur primarily within these pores. If the Intracrystalline structure is chosen to have certain precise dimensions, the ease of accommodation of reactant and product molecules will depend critically on the shape of the molecules. It is thus possible to generate molecular shape selective catalysts (12-16). 

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