A Link between Reactivity and Local Structure in Acid Catalysis on Zeolites

Abstract

T he extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Brønsted acid sites encapsulated within subnanometer channels.We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD 4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH 3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD 4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH 3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.

Catalytic behavior and reaction routes of MEK oxidation over Pd/ZSM-5 and Pd–Ce/ZSM-5 catalysts

“…The scission products of CH 2 C(OH)CH 2 CH 3 (intermediate) are propionic acid and DK [40]. More DK appeared over the PCZ and CZ catalysts than over PZ, as shown in Table 4.…”

Identification of Active Surface Species for Friedel–Crafts Acylation and Koch Carbonylation Reactions by in situ Solid‐State NMR Spectroscopy

“…Then, 13 CH 3 13 COCl(1000 mmol/g zeolite; 13 C, 99 %; provided by Isotec) corresponding to a molar ratio of 13 CH 3 13 COCl/ Al = 0.5, was introduced onto the activated zeolite before heating the sample at 333 K for 2 h without degassing. The glass insert containing the sample was sealed and used for recording NMR spectra.…”

“…The results obtained provide direct experimental evidence of the formation acetyl species covalently bound to framework oxygen as the reactive intermediates in Friedel-Crafts acylation and Koch carbonylation reactions on acidic zeolites. Figure 1 shows the calculated 13 C NMR isotropic chemical shifts ( 13 C d) of the methyl and carbonyl groups of acetyl chloride adsorbed through hydrogen bonds onto the zeolite Brønsted acid site (Figure 1 a), and of the two possible species resulting from the subsequent reaction and release of hydrogen chloride: the covalent acetyl-zeolite (Figure 1 b) and the cationic acylium-zeolite (Figure 1 c) species. The 13 C NMR chemical shifts calculated for the acylium-zeolite complex shown in Figure 1 c agree fairly well with experimental values for this H 3 C À C + = O species on AlCl 3 , (carbonyl and methyl groups at d = 152 ppm and d = 14 ppm, respectively), which supports the validity of the theoretical models.…”

“…According to the theoretical chemical shifts shown in Figure 1, it should be possible to distinguish between the carbonyl groups of the two reaction intermediates (acetylzeolite at d = 169.7 ppm and acylium-zeolite at d = 154.6), as they are separated by more than 15 ppm and do not overlap with the reactants and products involved in the two reactions. As the 13 C NMR chemical shifts of the methyl group are less conclusive, for the sake of clarity we shall mostly focus on the carbonyl region of the 13 C NMR spectra. Figure 2 shows the carbonyl region of the 13 C MAS NMR spectra of the reaction mixture for the Friedel-Crafts acylation, 13 CH 3 13 COCl/anisole (1:1), adsorbed at room temperature on H-Beta zeolite (Figure 2 a) and after subsequent heating at 363 K for different periods of time (Figure 2 b,c).…”

“…As the 13 C NMR chemical shifts of the methyl group are less conclusive, for the sake of clarity we shall mostly focus on the carbonyl region of the 13 C NMR spectra. Figure 2 shows the carbonyl region of the 13 C MAS NMR spectra of the reaction mixture for the Friedel-Crafts acylation, 13 CH 3 13 COCl/anisole (1:1), adsorbed at room temperature on H-Beta zeolite (Figure 2 a) and after subsequent heating at 363 K for different periods of time (Figure 2 b,c). The spectrum of Figure 2 a, shows two carbonyl bands at d = 184 ppm and at d = 168 ppm.…”