Probe molecules ammonia

A variety of methods to study the acidity of heteropoly compounds are reported in the literature in line with the study of the catalytic applications of the HPAs. These methods, mainly applied on Keggin type structures, evaluate the acid strength and the amount of molecules adsorbed with probe molecules (ammonia, pyridine) different from actual reactants. The studies that used actual reactant molecules, such as alcohols, determine the total amount of adsorption sites rather than the number of active acid sites at a certain temperature of reaction. In fact, the studies reported in the literature focused on the intermediate adsorbed species rather than on the in situ identification and quantification of the products of... 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

The main calorimetric studies on adsorption of water and ammonia on TS-1 and silicalite-1 have been reported by Bobs et al. [64,83,84,86], while other contributions came from the Auroux group [92] and Janchen et al. [93]. Cor-ma s group has investigated the interaction of water on zeolite [39]. The most important conclusion from the available literature is that calorimetric data require a very careful analysis, as probe molecules interact both with the silanols of the internal hydroxyl nests (see Sect. 3.8) and with Ti(lV) species. 

Acidity of both zeolites was investigated by adsorption of ammonia, pyridine, d3-acetonitrile and pivalonitrile ((CH3)3CCN) used as probe molecules followed by FTIR spectroscopy. All samples were activated in a form of self-supporting wafers at 450 °C or 550 °C under vacuum for 1 h prior to the adsorption of probe molecules. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

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... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

The choice of probe molecule for acidity determination is largely dictated by the sensitivity of the nuclei, and the high sensitivity of P nuclei has been exploited to quantify acidity in zeolites. The use of phosphorus containing bases as probe molecules to measure solid acidity circumvents insensitivity problems associated with (1.1% abundance) and N (0.4% abundance) in bases such as ammonia. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

The surface of alumina presents strong acid and basic sites, as demonstrated by the differential heats of adsorption of basic probe molecules such as ammonia [169- 171] and pyridine [169,172] or of acidic probe molecules such as SO2 [169,171] and CO2 [173,174]. Table 13.2 presents a survey of microcalorimetric studies performed for AI2O3. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

The influence of the nature of the aluminum source on the acidic properties of mesostructured materials (MCM41) has also been studied in the literature [244]. Microcalorimetry experiments using ammonia as a probe molecule have shown that Al insertion into the mesoporous silicate framework affected acid site strength and distribution in a manner controlled by the synthesis conditions (materials prepared... 

The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by 27A1 MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and 29Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. Re-inserted Al gives rise to Bronsted acidity. 

The coordinative and/or dissociative adsorption of various probe molecules has been used to characterize the surface properties of Ti02) which finds applications as a catalyst, photocatalyst, and sensor. Among the molecules used as probes, we mention CO (37, 38, 563-576), C02 (563, 565, 577), NO (578,579), water (580,581), pyridine (582,583), ammonia (584,585), alcohols (586, 587), ethers (including perfluoroethers) (588), ozone (589), nitrogen oxide (590), dioxygen (591), formic acid (592-594), benzene (584), benzoic acid (595), and chromyl chloride (596). 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

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. 

The same types of catalysts used for HDS are also used for hydrodenitrogena-tion, and ammonia is used as a probe molecule to characterize these catalysts. An IINS investigation of NH3 on partially desulfided RuS2 (25) showed that the chemisorption was dissociative, forming NH2 groups on the coordinatively unsaturated ruthenium sites. 

In this paper, we will discuss the use of temperature programmed desorption as a complementary technique for obtaining lattice concentrations for high-silica materials. While TPD of ammonia is commonly used as a measure of the number of acid sites in zeolites, this paper will discuss the use of the reactive probe molecules, 2-propanamine and 2-propanol. It has previously been demonstrated that well-defined adsorption complexes, with a stoichiometry of one molecule/Al atom could be observed in high-silica zeolites for a number of adsorbates. These complexes were found to be independent of Si/Al2 ratio for a series of H-ZSM-5 zeolites and were unaffected by... 

As alluded to above, probe molecules such as organic phosphates can also be used to probe Lewis acid sites. However, the most common probe molecules employed are nitrogen-containing species, in parhcular ammonia, alkylamines and pyridine. For the latter two either or N NMR techniques may be applied however, N NMR is generally preferred as the resultant spectra appear less complex and, as adsorphon to the surface occurs via the nitrogen atom, the observed resonance shifts are larger than they would be for spectra. The development of this method to investigate Lewis acidity of aluminas and siUca-aluminas has previously been discussed in detail by Eckert [196] and by Maciel and Ellis [183] and will only briefly be recapped here. 

From the Hterature it is apparent that microcalorimetry is very useful in providing informahon on the strength and distribution of acidic and basic sites of catalysts. The technique for determining the acid site distribution is quite well developed, especially if ammonia is used as the basic probe molecule. Moreover, the energehcs of surface reachons, including oxidahon and reduction of metal oxides, oxidahon of adsorbed hydrocarbons or hydrogen and decomposihon reachons can be determined direchy by calorimehy [4]. 

The technique has been fruitfully used to characterize acid and basic sites in many catalysts, in particular for zeoHtes and metal oxides [143]. It has also been applied for POMs [144]. It consists of measuring the differential heats of adsorption when adsorbing successive increments of a basic probe molecule such as ammonia or pyridine for acidity characterization or of an acid probe molecule such as GO2 or SO2 to characterize basicity. The technique produces a histogram of the acid-base strength as a function of coverage, in particular when heterogeneity in strength exists. The data should then be compared with ammonia or pyridine desorption data from IR and thermal desorption experiments (see above). 

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