Quantitative determination of Bronsted acid site concentrations in calcined H-mordenite and H-ZSM-5 by means of conductometric titration and infrared spectroscopy reveals that the concentration of Bronsted acid sites present is lower than would be expected on the basis of the aluminum content. This discrepancy is attributed to dealumination and dehydroxylation processes occurring during calcination. A model is proposed according to which the degree of these processes strongly depends on the concentration of vicinal aluminum T sites in the zeolite: Bronsted acid sites associated with these T sites are lost, and the remaining Bronsted acidity is related to the concentration of the (remaining) isolated aluminum T sites. This model quantitatively correlates the experimentally determined concentration of Bronsted acid sites with the concentration of isolated aluminum T sites.

Over the past decades several new families of well-defined transition metal-based polymerization catalysts have been discovered. 1 Although these systems are generally referred to as "single-site" catalysts, this seems not to hold in all cases. 2 Deviations of the polydispersity (M w /M n ) of the produced (co)-polymers from the theoretical Flory-Schulz value of 2 have been attributed to polymer precipitation and heterogenization of the catalyst giving rise to "multiple-site" catalysis. 2a,c-f The copolymerization of ethene and carbon monoxide to a perfectly alternating polyketone, [CH 2 CH 2 C(O)] n , which is catalyzed by a cationic palladium catalyst, [L 2 PdR] + (X) -(L 2 ) bidentate ligand; X ) weakly or noncoordinating anion; R ) [CH 2 CH 2 C(O)] n OCH 3 or [C(O)CH 2 CH 2 ] n H (n g 0)), generally affords materials with M w /M n ≈ 2. 5-5. 3 This suggests that the initially single-site catalyst becomes a heterogeneous, multiple-site system during copolymerization, in line with the slurry nature of the polyketone manufacturing process. In this communication, evidence is provided from polymer end-group analysis that the running catalyst species, [L 2 PdR] + , exhibits two-stage kinetic behavior. This is ascribed to residence of the catalyst in two different states. Depending on the length of the growing copolymer chain R, the catalyst is either dissolved during copolymerization (at the shorter chains) or polyketone-supported (at the longer chains).Termination during CO/ethene copolymerization proceeds via two competing mechanisms, protonolysis, leading to the formation of a ketone end-group (K; eq 1) and alcoholysis, giving an ester end-group (E; eq 2). 1h,4 After termination via protonolysis a Pdmethoxy species is obtained that gives rise to the formation of an E end-group at the start of the next copolymer chain it produces (eq 3), whereas after alcoholysis a Pd-hydride species is obtained, giving a K end-group in the next chain (eq 4).According to this two-catalytic cycle model, three types of copolymer chains may be produced, those containing: (i) both an ester and a ketone end-group (EK), (ii) two ester end-groups (EE), and (iii) two ketone end-groups (KK). This model also predicts the formation of equal amounts of E and K and, consequently, equal amounts of EE and KK copolymer chains. 1h,5 However, in several cases the E/K ratio in the methanol-soluble oligomer fraction (filtrate) deviated from unity (GC-analysis). 6 For example, when Pd(bdompp)(TFA) 2 (bdompp ) 1,3-bis-(di-(o-methoxyphenyl)phosphino)propane) is employed as the catalyst, the E/K ratio in the filtrates of polyketone produced at 50 bar CO/ethene and 90°C amounts to 0.14, while no oligomeric EE-chains are detected (see Table 1). The E/K ratio in the precipitated polyketone did not significantly deviate from unity. As the latter fraction contains the large majority (>98%) of the chains formed, the E/K ratio of the total products is about unity, indicating that chain transfer occurs in a perfect fashion, that is, only via eq 1-4. These obser...

The Bronsted acidity of H-mordenite and H-ZSM-5 samples of varying proton concentration has been studied using aqueous conductometric titration, IR spectroscopy, and aqueous potentiometrictitration. Good agreement is observed between Bronsted acid site concentrations determined by conductometric titration and IR measurements, while indirect potentiometric titration affords acid site concentrations consistently lower than those measured using the conductometric technique. This finding is rationalized on the basis that, in a conductometric titration, all the accessible Bronsted acid sites are directly titrated, whereas in the potentiometric procedure utilized, only those protons which can be ion-exchanged out of the zeolite are titrated. After allowing for the presence of extraframework aluminum in the zeolites (determined by27Al NMR), the measured acidity for H-mordenite is found to increase linearly with increasing Al content within the range 0-1.5 mmol Al/g but appears to reach a limiting value at higher Al concentrations. For H-ZSM-5, the experimentally determined number of Bronsted acid sites is also found to be linearly dependent on the Al molar fraction within the range measured (0-1.20 mmol Al/g). For both series of zeolite samples, the measured acidity is generally found to be less than the theoretical maximum calculated on the basis of an H+/Al ratio of 1.

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051ChemInform Abstract (investigation of the X-irradiated compounds (I) and (II) at 100 K).