N2, adsorption isotherms

Figure 2.5 N2 adsorption isotherms and schematized silicon dissolution (inset) upon alkaline treatment ofZSM-5 zeolites with different framework Si/AI ratios, highlighting the crucial role of framework aluminum.

Adequate description of many catalysts will require a large number of bits of data since they are usually rather complicated materials rather than simple chemicals. Attempts at tMs were just beginning by ICC 1, but now, one expects authors to give specific surface areas and some details of the porosity of their catalysts. Automation of the former tedious point by point measiirement of the N2 adsorption isotherm has greatly facilitated this. 

The BET surface area values are also reported with the distribution of porosity between microporosity (pore diameter <1.8 nm) deduced from N2 adsorption isotherms (t-curves) and mesoporosity (pore diameter > 1.8 nm). The following trend is observed for high atomic M/HPA ratio used for the precipitation, the precipitates exhibited high surface area mainly due to microporosity. However, depending on the nature of the coxmter cation and also of the previous ratio values, the textural characteristics were not similar. In particular, it is interesting to note the presence of mesopores for (NH4)2.4P, CS2.9P, CS2.7P and Cs2.4Si samples. 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

The N2 adsorption isotherms of the two Beta samples are depicted in Figure 1(b). The material prepared from silanized seeds exhibits a significant higher N2 adsorption compared to the Beta (0) sample, suggesting the presence of a secondary porosity. This is confirmed by the pore-size distributions derived from the Ar isotherms applying the... 

Figure 1. (a) XRD patterns (b) N2 adsorption isotherms at 77 K (c) Pore size distributions. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

The development of microporosity during steam activation was examined by Burchell et al. [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the N2 adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore structure characterization. 

Figure 1. N2 adsorption isotherms of precursors that were carbonised at 1273 K (left) and 973 K. (right)...

Porous texture characterization of all the samples was performed by physical adsorption of N2 at 77K. and CO2 at 273K, using an automatic adsorption system (Autosorb-6, Quantachrome). The micropore volume, Vpp (N2), was determined by application of Dubinin-Radushkevich equation to the N2 adsorption isotherm at 77K up to P/Po< 0.1. The volume of narrow micropores, Vnpp (DR,C02>, (mean pore size lower than 0.7 nm) was calculated from CO2 adsorption at 273 K. 

Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective singlelayer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin fihns prepared by the surface sol-gel process on 2-D substrates." " ... 

Figure 16. N2 adsorption isotherms for (a) CMS-CH2CH2COOH and (b) Co(III)-CMS5 and (c) Co(III)-CMS6.

Fig. 5.1 Left N2-adsorption isotherms ofgoethites synthesized at various temperatures right BET plots (Weidler, unpubL).

A well crystallized lepidocrocite with smooth edges (surface area 32.5 m g ) was nonporous and displayed a reversible type II N2 adsorption isotherm (Gomez-Villa-cieros et ah, 1984). Poorly crystallized, high surface area material also has a type II adsorption isotherm, but with B type hysteresis this material contained mesopores 2-20 nm across (Crosby et al., 1983 Madrid and De Arambarri, 1985). Lepidocrocite crystals with highly serrated terminals had a surface area of 67 m g of which 13% could be attributed to micropores ca. 1.5 nm across (Weidler, 1995). 

Weidler, P.G. (1997) BET sample pretreatment of synthetic ferrihydrite and its influence on the determination of surface area and porosity. J. Porous Materials 4 165-169 Weidler, P.G. Degovics, G. Laggner, P. (1998) Surface roughness created by acidic dissolution of synthetic goethite monitored with SAXS and N2 adsorption isotherms. J. Colloid Interface Sd. 197 1-8... 

The pure siliceous MCM-41 sample (reference) synthesized earlier by the same procedure [4, 5] showed the typical high surface area, well resolved [100], [110], [200] and [210] diffraction peaks in the XRD pattern and an N2 adsorption isotherm (IUPAC type IV) revealing a sharp inflection in the curve at ca. p/po=0.33 due to pore condensation typical for a narrow pore size distribution around a value of 28 A. The siliceous composite samples obtained, using combinations of the C6 and C 4 templates and different synthesis... 

Figure 4(a). N2 adsorption isotherm Figure 4(b). Pore size distribution... 

Figure 4. N2 adsorption isotherms of PSM and AMM-5 before and after compression at 200 MPa.

Figure 1. N2 adsorption isotherms and (inset) Horvath-Kawazoe pore size distribution for HMS and its functionalized analogs prepared by grafting and direct incorporation.

Silica MCM-41 was synthesized hydrothermally at 373 K for 7 days by using water glass and n-hexadecyltrimethylammonium bromide in a manner similar to that reported by Beck et al. [2]. The quality of MCM-41 prepared here was examined by the measurements of XRD, specific surface area and pore size distribution (calculated from N2 adsorption isotherm), and TEM. 

The structural quality of silica MCM-41 prepared here was very high. Because XRD showed the four clear peaks and N2 adsorption isotherm gave a high specific surface area (1040 m2 g-1) and a narrow pore size distribution. Moreover, TEM showed the honeycomb structure. 

The most reliable information about the mesoporous structure of solids comes from low-temperature nitrogen adsorption isotherms which enable the calculation of the specific surface area, pore volume, and pore size distribution Figure I shows the N2 adsorption isotherms of the purely siliceous MCM-41, niobium-containing MCM-41, and A1MCM-41 They are typical of reversible adsorption type IV and at relative low pressures (p/po < 0.3) are accounted for by monolayer adsorption of nitrogen on the walls of the mesopores. As the relative pressure increases (p/p0 > 0,3), the isotherm exhibits a sharp inflection, characteristic of the capillary condensation within uniform mesopores, where the p/po position of the inflection point is... 

From N2 adsorption isotherms, the surface area (Sbet) of the ASS is 680 m2 g 1 and its pore volume is 0.46 cm3 g, which is to be compared with a pore volume of 0.17 cm3 g 1 for the HZSM-5 samples. The average pore diameter of AAS is 50 A, whereas in ZSM-5 there is an intersecting network of straight and zigzag channels (average diameter 5.5 A), the cavities at the intersections being ca. 9 A in diameter. 

Figure 4.37 N2 adsorption isotherm (measured at 77 K) of the catalyst before and after the first run.

Analysis of Fractions. Surface areas and pore size distributions for both coked and regenerated catalyst fractions were determined by low temperature (Digisorb) N2 adsorption isotherms. Relative zeolite (micropore volume) and matrix (external surface area) contributions to the BET surface area were determined by t-plot analyses (3). Carbon and hydrogen on catalyst were determined using a Perkin Elmer 240 C instrument. Unit cell size and crystallinity for the molecular zeolite component were determined for coked and for regenerated catalyst fractions by x-ray diffraction. Elemental compositions for Ni, Fe, and V on each fraction were determined by ICP. Regeneration of coked catalyst fractions was accomplished in an air muffle furnace heated to 538°C at 2.8°C/min and held at that temperature for 6 hr. 

For obtaining more detailed information on the pore structure, pore size distribution (PSD) curves were determined by the application of the density functional theory (DFT) method to the N2 adsorption isotherms (Figure 3.20). The PSD curve of PFA-P7-H is very sharp and most of the... 

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