Zeolite MRs

The resolution of the zeolite MR image is 100 x 100 x 100 gm3 and has therefore reached the resolution limit that defines NMR microscopy. For the instrumentation used for this experiment, it will take at least a few milliseconds due to the ramping time of the field gradients. If the mean displacement of the xenon atoms during this experimental time scale reaches the dimension of the voxels or pixels, the resolution limit is reached. For instance, for the aerogel experiments in Figure... 

The zeolite membranes in MRs normally consist of a thin zeolite film on a porous support, typically a-Al203, stainless steel, or carbon. Generally speaking, it is more difficult to prepare defect-free zeolite membranes on tubular supports than on plate-shape supports due to the development of mechanical tensions during the drying/calcination steps that often lead to defects in the zeolite layer. However, most of the zeolite MRs are in a tubular configuration because of their higher specific area and... 

The majority of zeolite MR applications reported in the literature to date fall into the category of PBMRs. The reactor consists of a zeolite membrane with a conventional catalyst present in the form of a packed bed of pellets. The reaction takes place in the catalyst bed while the zeolite membrane serves mainly as a product separator (for H2 or H2O separation) [27] or a reactant distributor (for O2 distribution) [28]. Figure 3.5 illustrates a FAU-type zeolite PBMR combined with a packed bed reactor for dehydrogenation of cyclohexane [29]. Half of the catalyst is packed in the area upstream of the permeation portion to enhance the conversion, otherwise cyclohexane will preferentially permeate at the front end of the zeolite membrane, resulting in a decrease in conversion. 

Other reactions associated with water formation as a by-product, such as methanol synthesis by CO2 hydrogenation, can also be enhanced using zeolite MRs [42] ... 

In this section, an attempt is made to sketch the current status of zeohte MRs with respect to specific applications. The application of zeohte MRs is strongly related to the development status of zeolite membranes. Topics that are discussed are the most often studied reactions for zeolite membrane apphcations dewatering. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

Selectivity to p-isopropyl toluene being close to 30 % was achieved with SSZ-33, SSZ-35 and Beta zeolites. This is connected with the 12-MR channels in SSZ-33 and Beta. In the case of SSZ-35 the presence of 18-MR cavities decreased the differences in the rate of transport of individual isopropyl toluene isomers. In contrast, ZSM-5 zeolite behaves as para-selective catalyst in this alkylation reaction, the selectivity to p-isopropyl toluene reached 76 % after 180 min of T-O-S. 

In toluene disproportionation the highest toluene conversion was achieved over SSZ-33 due to a high acidity combined with 3-D channel system. High toluene conversion over SSZ-35 results from its strong acidity and large reaction volumes in 18-MR cavities. Toluene conversion in the alkylation with isopropyl alcohol is influenced by a high rate of competitive toluene disproportionation over SSZ-33. ZSM-5 exhibits a high p-selectivity for /7-isopropyl toluene, which seems to be connected with diffusion constraints in the channel system of this zeolite. 

The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

Until the recent discovery of UTD-1 and CIT-5, the largest pore zeolites known were composed of pore structures having 12-MRs or less. Many of these materials such as zeolite Y have enjoyed immense commercial success as catalysts (2). There is some evidence from catalytic cracking data that suggests the inverse selectivity found with the 12-MR pore ( 7.5 A) structure such as for SSZ-24 (Chevron) might be used to enhance octane values of fuel (3). However, small increases in pore size as well as variations in pore shape and dimensionality could further improve the catalysts. Pores with greater than a 12-MR structure might allow the conversion of... 

The 12-MR pores in the CIT-1 structure [International Zeolite Association (IZA) structure code, CON] are 6.8 x 6.4 A in diameter while the intersecting 10-MR pores are5.1x5.lA.B oth pore dimensions might be regarded as small for the ring size but interestingly molecular simulation of the pores between 0 and 200 K indicate the 10-MR pores can flex by >1 A (25). The potential catalytic activity of... 

ITQ-4, which has the formula Si32064.(C 4I I20NF)216, is another relatively new 12-MR ID channel-type zeolite that forms a pure silica polymorph (50-52). The structure of ITQ-4, which has been assigned the structure code IFR, is shown in Fig. 8. This view along the 001 direction reveals the distorted 12-MR pores that are... 

The zeolites discussed so far have all relied on exotic organocations to function as SDAs. However, sometimes the inorganic cations can have a greater influence as demonstrated by the 12-MR zincosilicate, VPI-8 (65-68). The synthesis of VPI-8 still requires an organic additive but its role as template may be as a void filler. A recently determined model for the structure of VPI-8 viewed along the 001 direction is shown in Fig. 10 (69). Much like SSZ-31, the structure of VPI-8 involves ID channels running in parallel that are defined by odd-shaped 12-MR structures (6.2 x 5.97 x 5.88 A). An unusual feature of this structure is the pinwheel building unit that is composed of four 5-MR stmctures surrounded by another four 5-MR structures. 

Although no zeolites have been characterized having accessible pores defined by >14-MR structures, there are the high silica phases SSZ-35, SSZ-44, and MCM-... 

---