Exam 25 January 2012, questions PDF

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EXAM CATALYSIS FOR CHEMICAL Lecturers: Jorge Gascon, Freek Kapteijn, Patricia Kooyman January, 25th 2012: 14:00 until 17:00 (3 hours) Write your name and student number on each paper you use for answering the questions This exam counts in total 3 questions on 4 pages The credit points you can earn a...

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EXAM “HETEROGENEOUS CATALYSIS FOR CHEMICAL ENGINEERS” Lecturers: Jorge Gascon, Freek Kapteijn, Patricia Kooyman January, 25th 2012: 14:00 until 17:00 (3 hours)

- Write your name and student number on each paper you use for answering the questions - This exam counts in total 3 questions on 4 pages - The credit points you can earn are given after each question, in total 100 points - This exam represents 80% of your final mark. - Good luck!! 1.- On October 6 2010, the Royal Swedish Academy of Sciences announced its decision to award the 2010 Nobel Prize in Chemistry to Prof. Akira Suzuki and Prof. Norio Miyaura for their extensive work on “palladium-catalyzed cross couplings in organic synthesis”, one of the most important breakthroughs of the last decades in catalysis for C-C bond formation. The Suzuki Miyaura reaction is the organic reaction of an aryl- or vinyl-boronic acid with an aryl- or vinyl-halide catalyzed by a palladium(0) complex. It is widely used to synthesize poly-olefins, styrenes, and substituted biphenyls, and has been extended to incorporate alkyl bromides Although initially developed as a homogeneously catalyzed process, it was soon discovered that small Pd Nanoparticles are also excellent catalysts when immobilized in different supports. The nature of the support in heterogeneously catalyzed Suzuky Miyaura cross couplings is of the upmost importance, since sintering of nanoparticles may occur due to the continuous oxidation and reduction of the nanoparticles during the catalytic cycle (see scheme 1). The mechanism of the Suzuki reaction is best viewed from the perspective of the palladium catalyst. The first step is the oxidative addition of palladium to the halide 2 to form the organopalladium species 3. Reaction with base gives intermediate 4, which via transmetalation with the boronate complex 6 forms the organopalladium species 8. Reductive elimination of the desired product 9 restores the original palladium catalyst 1.

Scheme 1.1.- Proposed Mechanism for the Suzuki-Miyaura cross coupling reaction. 1

At your lab, a new catalyst based on Pd nanoparticles (np) embedded in sponge like amorphous silica has been developed: Pd nanoparticles of homogeneous size were prepared by PdCl2 reduction with hydrazine in an inverse micelle microemulsion. These Pd nanoparticles were stabilized with dodecanethiol and 3-mercaptopropyltrimethoxysilane. The latter ligands were co-condensed with tetraethyl- orthosilicate, yielding an amorphous, sponge-like silica (mesoporous) framework in which the Pd nanoparticles are embedded (see figure 1).

Figure 1.1.- Schematic representation of npPd capped with DT and MPMS and the npPd@SiO2 structure. The MPMS (thick lines) provides the organic links between the metal particles and the inorganic sponge-like silica. The resulting npPd@SiO2 exhibits significant catalytic activity for the demanding Suzuki coupling of electron-rich 4-bromoanisole and phenylboronic acid:

Scheme 1.2.- Suzuki coupling of 4-bromoanisole and phenylboronic acid. Reaction conditions: 4-bromoanisole (682 mg), phenylboronic acid (1.5 eq.), K2CO3 (3 eq.), DMF (4 mL). In spite of the promising initial results (the solid could be recovered and reused in consecutive slurry experiments), a gradual decrease in catalytic activity occurs, resulting in the total deactivation of the catalyst after 10 reaction cycles.

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Questions: a. Which techniques would you propose in order to fully characterize both the fresh and the deactivated catalyst? Enumerate the possible causes for catalyst deactivation and propose specific techniques to corroborate your hypothesis (30 points) b. Propose a laboratory scale reactor for testing the catalytic activity of different solids on the Suzuki Miyaura cross coupling. Explain your choice and the main limitations of the proposed experimental setup. (10 points) c. How can you determine experimentally if external diffusion limitations are present in a solid catalysed reaction. (5 points) d. How can you determine experimentally if internal diffusion limitations are present in a solid catalysed reaction. (5 points)

2. a. Explain the next statement: “for a negative reaction order an egg- yolk catalyst (figure d) is favorable”. (10 points)

Figure 2.1: Four types of active-phase distribution. a: uniform, b: egg-shell, c: egg-white, and d: egg-yolk b. Propose a preparation method for an egg-yolk Pt/Al2O3 catalyst using H2PtCl6 as Pt precursor. (10 points)

3.

a. Figure 3.1 displays 29Si MAS NMR spectra of 2 samples of zeolite FAU with different Si/Al ratios. One of the samples has a Si/Al ratio of 1 and the other a Si/Al ratio of 2. Identify each sample according to its NMR spectrum and explain your choice (10 points)

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Figure 3.1: 29Si MAS NMR spectra of two different FAU samples b. A supported metallic Cu catalyst shows in XRD (X-Ray diffraction) the expected diffraction pattern of metallic Cu. However, in XPS Cu2O is detected. Explain the difference. Which of these two analytical techniques is in your opinion more relevant to catalysis and why? (10 points) c. N2 physisorption at 77 K is commonly used for the characterization of porous materials. However, the use of N2 presents certain drawbacks, especially in the case of small pore materials. Explain the main drawbacks and propose alternatives to the use of N2 physisorption at 77 K. (10 points)

Good Luck!!!

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