Exam 17 January 2011, questions PDF

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EXAM CATALYSIS FOR CHEMICAL January, 17th 2011: 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 4 questions on 4 pages The credit points you can earn are given after each question, in total 100 points Good luc...

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EXAM “HETEROGENEOUS CATALYSIS FOR CHEMICAL ENGINEERS” January, 17th 2011: 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 4 questions on 4 pages - The credit points you can earn are given after each question, in total 100 points - Good luck!! 1. While straight run jet and diesel fuel are usually of acceptable quality, the straight run gasolines typically require more processing to convert them into gasoline with a high Research Octane Number (RON) that meet the specifications for modern combustion engines. To increase the octane number of the gasoline pool, the Light Straight Run (LSR) gasoline fraction of oil is isomerized and in this way its RON increases from ~64 to ~ 80. This increase is achieved by isomerizing n-pentane and n-hexane, which are the main components of LSR, to the corresponding isopentane and mono- and dibranched hexanes, which have higher octane numbers. The industrial processes of nparaffins hydroisomerization need bifunctional catalysts, which present two kinds of sites with different functions: (i) an acid isomerization function and (ii) a hydrogenation–dehydrogenation function provided by a noble metal. Pt on acidic Mordenite (HMOR, 12MR zeolite) is one of the preferred catalysts for this conversion. For this catalyst, the hydroisomerization mechanism globally proceeds through the dehydrogenation of the alkane at the platinum sites to an alkene that reacts further on the acid site of the zeolite. Scheme 1 shows the reaction mechanism proposed by Weisz et al. in 1957:

Scheme 1.1.- Proposed reaction mechanism for the hydroisomerization of n-hexane. 1

Questions: a. Where is the acidity of the zeolite coming from? Describe a general procedure for the preparation of an acidic zeolite (10 points). b. At your lab, a series of Pt/HMOR catalyst with different loadings of metal and different Si/Al ratios on the zeolite has been prepared. Among them, catalyst A shows the best performance. Which characterization techniques would you propose in order to unravel the exact properties of the different catalysts? Give a brief explanation of the information that you would extract from every proposed technique. (30 points) 2. XPS is a technique that is often used to characterize catalysts after preparation a. Describe the physical principles of the technique (5 points) b. What information can be obtained with this technique? (5 points) c. 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? (5 points)

3. a. Ammonia TPD, IR-spectroscopy of CO adsorption, and IR-spectroscopy of pyridine adsorption are three different methods to characterize acidity of a solid. Describe the pros and cons of each one. (10 points) b. Explain the next statement: “for a negative reaction order an egg- yolk catalyst (figure d) is favorable”. (10 points)

Figure 3.1: Four types of active-phase distribution. a: uniform, b: egg-shell, c: egg-white, and d: egg-yolk

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Some researchers perform a basic catalysed reaction in the liquid phase, the Knoevenagel condensation of benzaldehyde (BA) with ethyl cyanoacetate (ECA) to ethyl (E)-αcyanocinnamate. New catalysts based on metal-organic frameworks (MOFs) with basic amino sites are being explored. The IRMOF-3DMF exhibits a 100% selectivity. The researchers want to know if the results they obtain represent the intrinsic performance before publishing.

The experimental data and conditions are given below. The reactor operation is batchwise with the catalyst suspended in a slurry, stirred by a magnetic flea. H

H C

CN

O

H2C

- H2 O

O

C

CN C

O C

C 2 H5

C 2 H5

C

+

O O

100 90 80 70 60

Y ie ld,+%

4.

50

+IR MO F 13 D MF +in+D MS O

40

+IR MO F 13 D MF +in+E tO H

30

+IR MO F 13 D MF +in+D MF +IR MO F 13 D MF +in+C y c lohe x a ne

20

+IR MO F 13 D MF +in+T o lue ne

10 0 0

10

20

30

40

50

60

70

80

90 100 110 120 130

T im e,+m in Conditions: Benzaldehyde (8 mmol) and ethyl cyanoacetate (7 mmol) in solvents (5 ml) at 313 K comparison between the performance of IRMOF-3DMF (0.18 g: corresponds to 0.18 mmol of -NH2 groups).

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Other data: Particle size Particle porosity tortuosity Particle density Liquid phase diffusivity Sherwood number Carberry number Weisz-Prater number

dp = 50 mm (can be considered spherical) ep = 0.5 τ = 1.5 ρp = 980 kg/m3 D = 10-9 m2/s k ⋅d Sh = f p D rvobs Ca = kf a ' cb Φ = ηφ2 =

rvobs ⋅ L2 Deff ⋅ cs

Questions: 1. How can you determine experimentally if external diffusion limitations are present in a solid catalysed reaction? Give two ways and explain briefly. (5 points) 2. How can you determine experimentally if internal diffusion limitations are present? Give two possibilities and explain briefly. (5 points) 3. Calculate the reaction rate based on the catalyst volume rvobs . Clearly state the dimensions used! (5 points) 4. In catalysis people like to express the activity of catalysts in terms of the turnover frequency, the number of mol reactant converted per mol active site per unit time. Calculate this TOF for the most active catalyst. (5 points)

5. Determine on the basis of the given data if external mass transfer was limiting, or if internal diffusion was limiting. Which experiment did you use for this? You may use Sh = 2. Can you tell why this is a reasonable assumption? (5points)

Good Luck!!!

- End of exam -

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