Zeolites acidic properties

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Most of the microporous and mesoporous compounds require the use of structure-directing molecules under hydro(solvo)thermal conditions [14, 15, 171, 172]. A serious handicap is the application of high-temperature calcination to develop their porosity. It usually results in inferior textural and acidic properties, and even full structural collapse occurs in the case of open frameworks, (proto) zeolites containing small-crystalline domains, and mesostructures. These materials can show very interesting properties if their structure could be fully maintained. A principal question is, is there any alternative to calcination. There is a manifested interest to find alternatives to calcination to show the potential of new structures. 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Acidic properties of SSZ-33 and SSZ-35 novel zeolites a complex I.R. and MAS NMR study... 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

The analysis of the structural properties of zeolitic acid sites based on dipolar interactions has further improved the understanding of acidity. Grey and Vega were the first to apply the 1H 27A1 TRAPDOR technique [36]. The REAPDOR method was first applied by Kalwei and coauthors [236-238] on bare acid sites and also on zeolites loaded with probe molecules. These methods allow one to distinguish... 

Acidic ion-exchange resins, 10 485 Acidic oxides, 12 190-191 Acidic papers, preservation of, 11 414 Acidic properties, of titania-silica, 1 764 Acidic silicate solutions, 22 458 Acidic zeolites, 16 825 Acid industrial refractories, 21 515 Acidity, biofiltration system, 10 76-77 Acidity analysis, of water, 26 36-37... 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

Acidic properties of zeolitic materials have widely been investigated in the past fifteen years ( 1), particularly for Y-type zeolites. The presence of strong acid sites, as it is usually the case for acid zeolites, results in the presence of only weak basic sites, if any. Therefore, up to now, majority of the studies has dealt with the characterization of acid+rather than basic properties. The acid sites (Bronsted s H, Lewis Al) and basic sites (0, OH ) may be characterized directly by using physical... 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

NMR properties, 33 213, 274 in sheet silicate studies, 33 128-137 for zeolite acidity studies, 33 275-279 Hydrogen sulfide... 

Yamagishi, Y., Namba, S., and Vashima, T. (1989) Preparation and acidic properties of antimonosilicate with MFl structure, in Zeolites Facts, Figures,... 

Si NMR provides quantitative information about the framework composition, and framework Si/Al ratio is an important parameter used to tune the catalyst property. Zeolite acidity is directly related to the amount of framework Al. Framework Si/Al ratio can directly be obtained from just Si NMR alone. Si/Al ratio can be calculated from Si NMR intensities if the resonances due to different Q (nAl) species are well-resolved using Eq. (4.10), assuming there is no Al-O-Al bonds present ... 

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

Hence, water molecules enhance the acidic properties of the zeolite s Bronsted acids. Adsorbate-adsorbent interactions and, therefore, adsorbent selectivity and adsorbate mass transfer rates are altered due to water polarization. When developing an adsorbent to be used in a commercial adsorptive separation process, the water content of the adsorbent is adjusted to balance adsorbent selectivity and component mass transfer rate. 

Zeolites and other mesoporous materials are excellent catalysts for industrial and laboratory applications. Favourable characteristics are their capacity to immobihze homogenous catalysts rendering them heterogeneous, their thermal stability, and the ease of separation from the reaction products and reuse in hquid- and gas-phase conditions. The pore size and Brpnsted and Lewis acidic properties are determinant for their use as catalyst in the Beckmann rearrangement. Recently, a review on the use of zeolites and mesoporous materials in the Beckmann rearrangement was published. ... 

Vibrational spectroscopy of adsorbed probe molecules is one of the most powerful tools to assess the acidic properties of catalysts. Acidity studies of dealumi-nated Y zeolites (main active component of FCC catalysts) or other zeolitic catalysts are reported using mostly Fourier Transform Infrared Spectroscopy (FTIR) with CO adsorption at 77 K or FTIR-pyridine/substituted pyridines adsorption at 425 K [22-26]. FTIR acidity studies of commercial FCC catalysts are even more scarce... 

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