Catalyst acid-base sites

Some observations indicate that the zeolite framework is a prerequisite for good catalyst performance via physical constraints and proximity of acid-base sites within the molecular sieves environment.431 Other observations show that rather microporosity plays an important role in the alkylation.432... 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

G. M. Zhidomirov, and V. B. Kazansky, Quantum-chemical cluster models of acid-base sites of oxide catalysts, Adv. Cat. 34, 131-201 (1986). 

Quantum-Chemical Cluster Models of Acid-Base Sites of Oxide Catalysts... 

Some preliminary runs were made on the Al O support in the absence of Pt. Heating this alumina without CO pulsing showed a small peak appearing near 100°C. This was identified as N adsorbed as an impurity from the carrier gas. CO completely displaces the on pulsing at room temperature and, as previously reported, probably results from the presence of Lewis acid-base sites gn the alumina (8). The amount of this peak was approximately 10 moles per g of Al O which represents less than 5% of the CO desorbing from a well dispersed Pt catalyst of 0.4 wt% loading. Runs without the in-situ H O and 0 trap had shown that C0 desorbed with the CO. However, with the In-situ trap minimal CO was observed. This was confirmed by a pair of runs shown in Figure 1 with and without Al O (CO2 trap) placed at the exit of the desorption tube. 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

The Knoevenagel reaction [3] is one of the most important C-C bond-forming reactions available to synthetic chemists. It is widely used in the synthesis of important intermediates or end-products for perfumes [4], pharmaceuticals [5], e. g. antihypertensive and calcium antagonists [6], and polymers [7]. The reaction is catalyzed by bases, acids, or catalysts containing acid-base sites [8], e. g. bases such as ammonia, primary and secondary amines and their salts [1], and Lewis acids such as CUCI2 [9], ZnCl2 [10], and Sml3 [11]. 

Taking into account that the cooperation of Lewis acid-base sites is advantageous for the coupling reaction of CO and epoxides. Yuan et al. [38] attempted to use the combination Mg(OH)Cl with KI as heterogeneous catalyst for the carboxylation of epoxides and found that this... 

The catalyst surface may contain acid and base sites that can interact together. On a certain surfaces the acid or base behaviour may prevail even if both sites are always present. In a catalyst characterisation, it is very useful to define the nature (Lewis or Bronsted, see par. 4.3) of these sites, their density, location, distribution and strength. Generally, an acid site is defined as a site that can react with a base and, on the contrary, a base site is one that can react with an acid. The above information (acid-base sites density) can be obtained by performing a chemisorption measurement using an acid (such as CO2, SO2) or base gas (such as dry ammonia) while their strength could be measured by temperature programmed techniques (desorption). 

Mesoporous materials can also be used as base catalysts— for example, when the negative charge on a mesoporous aluminosilicate is compensated by metal ions such as sodium (Na) or cesium (Cs). Amines anchored on mesoporous silica can also be used as base catalysts. Successful utilization of basic sites requires the total absence of acid sites since the two functions tendto drive reactions through different pathways. There are, however, some cases where adjacent acid/base sites are desirable. 

Notably, the inhibiting effect of NH3 on the Fast SCR activity at low temperature is not due to the ammonia competitive chemisorption on the catalytic sites, but occurs because ammonia captures a key intermediate in an unreactive form. In this respect, one way to partially prevent this undesired effect is to modify the equilibrium of ammonium nitrate dissociation, e.g., by increasing the temperature or by decreasing the gas-phase ammonia concentration. Since the blocking effect is related to the acid properties of the formed nitrates, another possibility to moderate its negative impact on the Fast SCR reactivity at low T would be to modify the catalyst acid/base properties in order to favor the interaction of ammonia with the catalyst sites rather than with the nitrates [5]. 

Catalyst BET surface area (m -g ) Acid/base site ratio... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Dehydrogenation processes for acetone, methyl isobutyl ketone [108-10-1], and higher ketones (qv) utilizing, in one process, a copper-based catalyst have been disclosed (18,19). Dehydrogenation reaction is used to study the acid—base character of catalytic sites on a series of oxides (20,21). 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

---