Acetonitrile acid sites

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Two types of probe molecules have been used for the detection of Lewis and Bronsted acid sites. The first involves the adsorption of relatively strong basic molecules such as pyridine, ammonia, quinoline, and diazines. The second kind involves the adsorption of weak base molecules such as CO, NO, acetone, acetonitrile, and olefins. The pioneering works of Parry27 and Hughes and... 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

Since the acidity of porous materials is important in catalytic applications, a characterization of this interesting property is carried out by adsorption of the probe molecule acetonitrile CD3CN. Acetonitrile-d3, a weak base, can be applied to investigate Bronsted and Lewis acid sites and to discriminate between both types of sites [9,10], The analysis is based on the study of the C=N stretching region by infra-red spectroscopy. 

After evacuation at 25°C (spectrum C), the physisorbed fraction of acetonitrile-d3 and also the H-bonding effects with the silica surface -OH have disappeared, while the Bronsted acidity is still present (2309 cm 1). Subsequent evacuation at 60°C does not change the intensity of the Bransted acidity (spectrum D). Even at 120°C and at 150°C Bransted acid sites are still detected (spectra E, F). Therefore, it can be concluded that the Al-PCH is characterized by an important Bransted surface acidity. This type of acidity is expected since the initial Na+ ions on saponite have been replaced by surfactant cations and then by protons upon destruction of the surfactant through calcination. Besides, by the grafting of Al-species onto the support, Si-(OH)-Al bonds have been created, giving rise to the band at 2309 cm 1 indicative of Bransted acidity [10]. 

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

Acetonitrile is also an interesting molecule for probing acid sites in catalysts. 

The weakly basic probe molecules most commonly used are the following sulphur compounds such as H2S, unsaturated hydrocarbons such as ethylene, carbon monoxide and dcu-terated acetonitrile. They are used to detect the strongest acid sites of the solid under study. In these cases protonation does not occur but there is formation of species linked by hydrogen bonding. 

The catalytically active sites of isomorphous substituted MFI structures have been characterized by inirared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH, adsorption, the NH, TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from Al>Fe>In> >silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample. 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

For these reasons, the effect of support on Ni based catalysts is better shown when comparing the MEA selectivity at low acetonitrile conversions (Table 2). The improvement of primary amine selectivity upon Mg addition could arise from a modification of the acido-basic properties of the support surface. To check any differences in these properties, the acid sites were probed by TPD of NH3 and adsorption of MEA followed by calorimetry. 

Broensted and Lewis acid sites in crystalline phosphates, silicates and in gels with moleculeu sieve properties were studied by IR-spectroscopy. Two types of bridged hydroxyls were fovmd in SAPO-5 which were accessible to adsorbed molecules and were able to interact with ethylene. Lewis sites in metallophosphates, zirconosilicate and in Ti-containing silica gel were observed which did not interact with weak bases (CO, hydrogen) but formed strong complexes with acetonitrile. They were supposed to be framework metal ions in tetrahedral coordination. 

Unlike weak bases adsorption of acetonitrile reveals Lewis acid sites responsible for the IR-band at 2310-2305 cm (Fig.3b). The appearance of... 

The concentrations of Broensted and Lewis acid sites were determined after adsorption of da-acetonitrile followed by FTIR spectroscopy on a Nicolet... 

The interaction of deuterated and chlorinated acetonitrile, CD3CN and CCI3CN, respectively, with Bronsted acid sites of H-ZSM-5 and H,D-ZSM-5 and the thus-induced changes in the IR spectra were interpreted by Pehnenschikov et al. [657] in the frame of the resonance theory of the A-B-C triplet, developed for molecular H-complexes. The approach was similar to that of water adsorption (vide supra). Kotrla and Kubelkova [729] discussed in great detail the spectral features observed on the adsorption of deuterated acetonitrile, designated as... 

In particular, the combined application of acetonitrile and the bulkier adamantanecarbonitrile (diameter greater than 0.6 mn) in FTIR experiments proved to be suitable for discriminating internal from external acid sites of medium pore zeolite crystallites such as ZSM-5, chabazite, erionite and ferrierite. For the same purpose, Trombetta et al. [695] employed 2,2-dimethylproprionitrile (pivalonitrile, PN) instead of adamantanecarbonitrile. They studied B- and L-sites on the (external) surface of ferrierite and the total surface of [Si]MCM-41 and [ Al] MCM-41. Upon adsorption of PN, the C = N band was shifted from 2236 to about 2250 cm. The weak B-sites of [Al]MCM-41 interacted with pivalonitrile and were also associated with an OH band at 3745 cm. ... 

Acetonitrile is also an interesting molecule for probing acid sites in catalysts [39,47-49]. It is a weak base, so no protons are abstracted and actual hydroxyl groups can be observed. It also allows the investigation of both Lewis and Bronsted acidities. While it is normally considered to be a weak base, it actually has a moderately high proton affinity (798 kJ mof, compared to 857 kJ mof for ammonia and 773 kJ mol for methanol). Other nitriles and alcohols have also been used to probe the acid sites of catalysts [50,51],... 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

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