Metal-Catalysed Reactions of Hydrocarbons / 00-front-matter

CONCERNING THE USEFULNESS OF MODELS AND MECHANISMS

The training of chemists inculcates a desire to interpret the phenomena of chemistry through the properties of individual atoms and molecules. To this end they have devised a variety of ways of symbolising and visualising their composition, size and shape. The purely symbolic method of identifying elements, while successfully distinguishing a hundred or so by means of at most two letters, requires subscripts and superscripts to define atomic mass, nuclear charge and oxidation state, but there is no means of showing size or chemical character.

Structural formulae of various degrees of sophistication may be used to show how atoms are linked in a molecule, what the bond angles and lengths are, and ultimately how orbitals are employed in bonding, but depictions of adsorbed species and surfaces processes of a very elementary kind are still often used, and all too frequently there is no diagram or sketch at all to show what is in the writer’s mind. This is a pity, because most chemists have pictorial minds, and a simple sketch can speak volumes. A flexible and informative symbolism for surfaces states and events is urgently needed, because our ability to think innovatively and imaginatively is limited by the techniques we have to express our thoughts. Words are very imperfect vehicles for ideas and emotions. Perception of the third dimension is helped by molecular graphics, but such displays are impermanent until printed, when the extra dimension is lost. Often there is no alternative to the use of some kind of atomic model to convey the structures of surface phases.

Our belief that we can meaningfully describe the transformations of molecules by a few squiggles on a sheet of paper is a major act of faith. Acts of faith have their place in science as in religion, and our ability to create a conceptual model or hypothesis is however no more than a set of statements, either formal or informal, that increases the probability of successfully predicting an event or the outcome from a given situation. Karl Popper asserted that no hypothesis can ever be proved correct; it only remains plausible as long as no evidence is found to contradict it. A few scientific ideas have graduated from speculation through theory to the status of immutable and universal law; the Periodic Classification of the Elements and General Theory of Relativity are two such, but unfortunately there is as yet little in catalysis of which we can say ‘It will always be thus’.

1.D. Rumsfeld: There are things we do not know we do not know (2003).

2.Catalysis by Metals (Preface), Academic Press: London (1962).

3.V. Ponec and G. C. Bond, Catalysis by Metals and Alloys, Elsevier: Amsterdam (1996).

4.See Article XIV of the Articles of Religion in the 1662 English Prayer Book.

4.1.2. Types of Unsaturated Hydrocarbon . . . . . . . . . . . . . . . . 154 4.1.3. The Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.2. The Chemisorption of Hydrocarbons: An Overview . . . . . . . . . . 156

4.3. The Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.4. Identification of Adsorbed Hydrocarbon Species . . . . . . . . . . . . 161 4.4.1. The Catalogue - or ‘The Organometallic Zoo’ . . . . . . . . . . 161 4.4.2. The π and di-σ Forms of Chemisorbed Alkenes . . . . . . . . . 169 4.5. Structures and Properties of Chemisorbed Hydrocarbons . . . . . . . . 176 4.5.1. Detailed Structures of Chemisorbed Alkenes . . . . . . . . . . 176 4.5.2. Structures of Chemisorbed Ethyne . . . . . . . . . . . . . . . . 178 4.5.3. Structures of Chemisorbed Benzene . . . . . . . . . . . . . . . 178 4.5.4. Heats of Adsorption . . . . . . . . . . . . . . . . . . . . . . . . 180 4.5.5. Characterisation by Other Spectroscopic Methods . . . . . . . . 186 4.5.6. C6 Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

4.6. Thermal Decomposition of Chemisorbed Hydrocarbons . . . . . . . . 186 4.7. Theoretical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . 190 4.8. Chemisorption of Alkanes . . . . . . . . . . . . . . . . . . . . . . . . 196 4.9. The Final Stage: Carbonaceous Deposits . . . . . . . . . . . . . . . . 197 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

CHAPTER 5. INTRODUCTION TO THE CATALYSIS OF HYDROCARBON REACTIONS

5.1. The Essential Nature of Catalysis . . . . . . . . . . . . . . . . . . . . 210 5.1.1. A Brief History of Catalysis . . . . . . . . . . . . . . . . . . . . 210 5.1.2. How Catalysts Act . . . . . . . . . . . . . . . . . . . . . . . . . 211 5.1.3. The Catalytic Cycle . . . . . . . . . . . . . . . . . . . . . . . . 213 5.2. The Formulation of Kinetic Expressions . . . . . . . . . . . . . . . . . 214 5.2.1. Mass Transport versus Kinetic Control . . . . . . . . . . . . . . 214 5.2.2. The Purpose of Kinetic Measurements . . . . . . . . . . . . . . 215 5.2.3. Measurement and Expression of Rates of Reaction . . . . . . . 216 5.2.4. The Langmuir-Hinshelwood Formalism . . . . . . . . . . . . . 218 5.2.5. Effect of Temperature on Rate and Rate Constant . . . . . . . . 221 5.2.6. Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 5.2.7. Kinetic modelling . . . . . . . . . . . . . . . . . . . . . . . . . 225

5.3. The Concept of Reaction Mechanism . . . . . . . . . . . . . . . . . . 227 5.4. The Idea of the Active Centre . . . . . . . . . . . . . . . . . . . . . . . 229 5.5. The Use of Bimetallic Catalysts . . . . . . . . . . . . . . . . . . . . . 234 5.6. The Phenomenon of ‘Compensation’ . . . . . . . . . . . . . . . . . . 239 5.7. The Temkin Equation: Assumptions and Implications . . . . . . . . . 246 5.8. Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

5.8.1. Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 5.8.2. Use of Stable and Radioactive Isotopes . . . . . . . . . . . . . 249 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

CHAPTER 6. EXCHANGE OF ALKANES WITH DEUTERIUM

6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 6.2. Equilibration of Linear and Branched Alkanes with Deuterium . . . . 260 6.2.1. Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 6.2.2. Ethane and Higher Linear Alkanes . . . . . . . . . . . . . . . . 267 6.2.3. Higher Linear Alkanes . . . . . . . . . . . . . . . . . . . . . . . 271 6.2.4. Branched Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . 273 6.3. Equilibration of Cycloalkanes with Deuterium . . . . . . . . . . . . . 275

6.4. Interalkane Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 6.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

CHAPTER 7. HYDROGENATION OF ALKENES AND RELATED PROCESSES

7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 7.2. Hydrogenation of Ethene and Propene . . . . . . . . . . . . . . . . . . 297 7.2.1. Kinetics of Hydrogenation . . . . . . . . . . . . . . . . . . . . 297 7.2.2. Structure Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . 303 7.2.3. Ethene Hydrogenation on Bimetallic Catalysts . . . . . . . . . 306 7.2.4. Reactions of Ethene and of Propene with Deuterium . . . . . . 307 7.2.5. Reactions on Single Crystal Surfaces . . . . . . . . . . . . . . . 319

7.2.6. The Reaction Mechanism: Microkinetic Analysis, Monte

Carlo Simulation, and Multiple Steady States . . . . . . . . . . 321 7.2.7. Catalysis by Hydrogen Spillover and the Reactivity of

Hydrogen Bronzes . . . . . . . . . . . . . . . . . . . . . . . . . 325 7.3. Reactions of the Butenes with Hydrogen and with Deuterium . . . . . 328 7.3.1. The n-Butenes . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 7.3.2. The Single Turnover Approach . . . . . . . . . . . . . . . . . . 333 7.3.3. Isobutene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 7.3.4. Exchange Reactions between Alkenes . . . . . . . . . . . . . . 335 7.4. Reactions of Higher Alkenes with Hydrogen and with Deuterium . . . 336

7.5. Hydrogenation of Cycloalkenes . . . . . . . . . . . . . . . . . . . . . 338 7.5.1. Cyclohexene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 7.5.2. Other Cycloalkenes . . . . . . . . . . . . . . . . . . . . . . . . 339

7.5.3. Substituted Cycloalkenes: Stereochemical Factors . . . . . . . 340 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

CHAPTER 8. HYDROGENATION OF ALKADIENES AND POLY-ENES

8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 8.1.1. Types of Unsaturation . . . . . . . . . . . . . . . . . . . . . . . 357 8.1.2. Practical Applications of Selective Hydrogenation:

Outline of Mechanisms . . . . . . . . . . . . . . . . . . . . . . 359 8.2. Hydrogenation of 1, 2-Alkadienes (Allenes) . . . . . . . . . . . . . . . 360 8.2.1. Hydrogenation of Propadiene . . . . . . . . . . . . . . . . . . . 360 8.2.2. Hydrogenation of Substituted 1, 2-Alkadienes . . . . . . . . . . 362 8.2.3. Hydrogenation of Cumulenes . . . . . . . . . . . . . . . . . . . 365 8.3. Hydrogenation of 1,3-Butadiene . . . . . . . . . . . . . . . . . . . . . 365 8.3.1. General Characteristics of Butadiene Hydrogenation . . . . . . 365 8.3.2. Chemisorbed States of 1, 3-Butadiene . . . . . . . . . . . . . . 366 8.3.3. Hydrogenation of 1,3-Butadiene on Single Crystal Surfaces . . . 367

8.3.4. Hydrogenation of 1, 3-Butadiene on Supported

and Unsupported Metals . . . . . . . . . . . . . . . . . . . . . . 368 8.3.5. The Reaction of 1, 3-Butadiene with Deuterium:

Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . . 375 8.3.6. Hydrogenation of 1, 3-Butadiene by

Bimetallic Catalysts . . . . . . . . . . . . . . . . . . . . . . . . 379 8.4. Hydrogenation of Higher Alkadienes . . . . . . . . . . . . . . . . . . 382 8.4.1. Linear Alkadienes . . . . . . . . . . . . . . . . . . . . . . . . . 382 8.4.2. Branched Alkadienes . . . . . . . . . . . . . . . . . . . . . . . 386 8.4.3. Cycloalkadienes . . . . . . . . . . . . . . . . . . . . . . . . . . 388 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390

CHAPTER 9. HYDROGENATION OF ALKYNES

9.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 9.1.1. The Scope of the Literature . . . . . . . . . . . . . . . . . . . . 395 9.1.2. Industrial Applications of Alkyne Hydrogenation . . . . . . . . 396 9.1.3. The Chemisorbed State of Alkynes . . . . . . . . . . . . . . . . 397 9.1.4. The Origin of Selectivity in Alkyne Hydrogenation . . . . . . . 398 9.1.5. Interpretation of Results: Some Preliminary Comments . . . . . 399

9.2. Hydrogenation of Ethyne: 1, In Static Systems . . . . . . . . . . . . . 400 9.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 9.2.2. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 401

9.2.3. The Formation of Benzene from Ethyne . . . . . . . . . . . . . 407 9.2.4. The Reaction of Ethyne with Deuterium . . . . . . . . . . . . . 407 9.3. Hydrogenation of Ethyne: 2, in Dynamic System with Added Ethene . 411 9.3.1. Kinetics and Selectivity . . . . . . . . . . . . . . . . . . . . . . 411 9.3.2. Mechanisms and Modelling . . . . . . . . . . . . . . . . . . . . 415 9.3.3. Oligomerisation . . . . . . . . . . . . . . . . . . . . . . . . . . 417 9.3.4. Gaseous Promoters . . . . . . . . . . . . . . . . . . . . . . . . 417

9.4. Use of Bimetallic Catalysts for Ethyne Hydrogenation . . . . . . . . . 418 9.5. Hydrogenation of Higher Alkynes . . . . . . . . . . . . . . . . . . . . 421 9.5.1. Propyne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 9.5.2. The Butynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

9.5.3. Alkyl-Substituted Alkynes Having More Than Four

Carbon Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 9.5.4. Aryl-Substituted Alkynes . . . . . . . . . . . . . . . . . . . . . 428 9.5.5. Multiply-Unsaturated Molecules . . . . . . . . . . . . . . . . . 429 9.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

CHAPTER 10. HYDROGENATION OF THE AROMATIC RING

10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 10.1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438 10.1.2. Industrial Applications of Benzene Hydrogenation . . . . . . 439 10.2. Kinetics and Mechanism of Aromatic Ring Hydrogenation . . . . . . 440 10.2.1. Introduction: Early Work . . . . . . . . . . . . . . . . . . . . 440 10.2.2. Kinetics of Aromatic Ring Hydrogenation . . . . . . . . . . 441 10.2.3. Rate Expressions and Reaction Mechanisms . . . . . . . . . 446 10.2.4. Temperature-Inversion of Rates . . . . . . . . . . . . . . . . 448 10.2.5. Hydrogenation of Benzene Over Bimetallic Catalysts . . . . 450 10.2.6. Exchange of Aromatic Hydrocarbons with Deuterium . . . . 453 10.2.7. Hydrogenation of Benzene to Cyclohexene . . . . . . . . . . 457

10.3. Hydrogenation of Alkyl-Substituted Benzenes . . . . . . . . . . . . . 458 10.3.1. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . . 458 10.3.2. Stereochemistry of the Hydrogenation of

Alkyl-Substituted Benzenes . . . . . . . . . . . . . . . . . . 460 10.4. Hydrogenation of Multiple Aromatic Ring Systems . . . . . . . . . . 461 10.4.1. Polyphenyls . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 10.4.2. Fused Aromatic Rings: (1) Naphthalene . . . . . . . . . . . . 461 10.4.3. Fused Aromatic Rings: (2) Multiple Fused Rings . . . . . . . 466 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

CHAPTER 11. HYDROGENATION OF SMALL ALICYCLIC RINGS

11.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 11.2. Hydrogenation and Hydrogenolysis of Cyclopropane . . . . . . . . . 477 11.2.1. Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 11.2.2. The Reaction of Cyclopropane with Deuterium . . . . . . . . 481 11.2.3. Reaction Mechanisms . . . . . . . . . . . . . . . . . . . . . . 482 11.3. Hydrogenation of Alkylcyclopropanes . . . . . . . . . . . . . . . . . 484 11.3.1. Mono-alkylcyclopropanes . . . . . . . . . . . . . . . . . . . 484 11.3.2. Poly-alkylcyclopropanes . . . . . . . . . . . . . . . . . . . . 488 11.3.3. The Cyclopropane Ring in More Complex Hydrocarbons . . 490

11.4. Hydrogenation of Cyclopropanes Having Other

Unsaturated Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 11.5. Hydrogenation of Alkylcyclobutanes and Related Molecules . . . . . 494 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

CHAPTER 12. DEHYDROGENATION OF ALKANES

12.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 12.2. Dehydrogenation of Acyclic Alkanes . . . . . . . . . . . . . . . . . . 504 12.2.1. Introduction: Alkane Chemisorption . . . . . . . . . . . . . . 504 12.2.2. Supported Platinum and Platinum-Tin Catalysts . . . . . . . 505 12.2.3. Other Metals and Modifiers . . . . . . . . . . . . . . . . . . 507 12.2.4. Kinetics and Mechanism . . . . . . . . . . . . . . . . . . . . 508 12.3. Dehydrogenation of Cycloalkanes . . . . . . . . . . . . . . . . . . . 509 12.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 12.3.2. Reaction on Pure Metals . . . . . . . . . . . . . . . . . . . . 510 12.3.3. Reaction on Bimetallic Catalysts . . . . . . . . . . . . . . . . 512

12.4. The Chemisorption of Hydrogen on Platinum . . . . . . . . . . . . . 514 12.5. The Formation, Structure, and Function of Carbonaceous Deposits . 516 12.6. The Homologation of Methane . . . . . . . . . . . . . . . . . . . . . 519 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520

CHAPTER 13. REACTIONS OF THE LOWER ALKANES WITH HYDROGEN

13.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526 13.1.1. A Short Philosophical Digression . . . . . . . . . . . . . . . 526 13.1.2. Alkane Hydrogenolysis: General Characteristics . . . . . . . 527

13.1.3. Problems in Studying Reaction Kinetics . . . . . . . . . . . . 528 13.1.4. Ways of Expressing Product Composition . . . . . . . . . . . 530

13.2. Hydrogenolysis of the Lower Alkanes on Single Metal Catalysts:

Rates, Kinetics, and Mechanisms . . . . . . . . . . . . . . . . . . . . 531 13.2.1. The Beginning . . . . . . . . . . . . . . . . . . . . . . . . . . 531 13.2.2. Kinetic Parameters . . . . . . . . . . . . . . . . . . . . . . . 531 13.2.3. Mechanisms and Kinetic Formulations . . . . . . . . . . . . 540 13.2.4. A Generalised Model for Alkane Hydrogenolysis . . . . . . 549 13.2.5. Alkane Hydrogenolysis on Metals Other than Platinum . . . 552

13.3. Structure-Sensitivity of Rates of Alkane Hydrogenolysis . . . . . . . 552 13.4. Selectivity of Product Formation in Alkane Hydrogenolysis . . . . . 555 13.5. Mechanisms Based on Product Selectivities . . . . . . . . . . . . . . 562 13.6. Hydrogenolysis of Alkanes on Ruthenium Catalysts . . . . . . . . . 565 13.7. Effects of Additives and the Strong Metal-Support Interaction

on Alkane Hydrogenolysis . . . . . . . . . . . . . . . . . . . . . . . 569 13.8. Hydrogenolysis of Alkanes on Bimetallic Catalysts . . . . . . . . . . 574 13.8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 574 13.8.2. Metals of Groups 8 to 10 plus Group 11 . . . . . . . . . . . . 575 13.8.3. Metals of Groups 8 to 10 plus Groups 13 or 14 . . . . . . . . 578

13.8.4. Platinum and Iridium plus Zirconium, Molybdenum,

and Rhenium . . . . . . . . . . . . . . . . . . . . . . . . . . 579 13.8.5. Bimetallic Catalysts of Metals of Groups 8 to 10 . . . . . . . 583 13.9. Apologia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583

CHAPTER 14. REACTIONS OF HIGHER ALKANES WITH HYDROGEN

14.1. Introduction: Petroleum Reforming and Reactions of Higher

Alkanes with Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . 592 14.1.1. The Scope of This Chapter . . . . . . . . . . . . . . . . . . . 592 14.1.2. Bifunctional Catalysis: Principles of Petroleum

Reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 14.1.3. Reactions of the Higher Alkanes with Hydrogen . . . . . . . 596 14.1.4. The Scope and Limitations of the Literature . . . . . . . . . 597 14.1.5. The Principal Themes . . . . . . . . . . . . . . . . . . . . . . 598

14.2. Reactions of Higher Alkanes with Hydrogen: Rates and

Product Selectivities . . . . . . . . . . . . . . . . . . . . . . . . . . . 599 14.2.1. Activities of Pure Metals . . . . . . . . . . . . . . . . . . . . 599 14.2.2. Effect of Varying Conversion . . . . . . . . . . . . . . . . . . 601

14.2.3. Reactions of Linear Alkanes with Hydrogen . . . . . . . . . 602 14.2.4. Reactions of Branched Alkanes with Hydrogen . . . . . . . . 609 14.2.5. Reactions of Cyclic Alkanes with Hydrogen . . . . . . . . . 616 14.2.6. The Environment of the Active Site: Effect of ‘Carbon’ . . . 621 14.3. Mechanisms of Alkane Transformations . . . . . . . . . . . . . . . . 624 14.3.1. A General Overview . . . . . . . . . . . . . . . . . . . . . . 624 14.3.2. Mechanisms of Skeletal Isomerisation . . . . . . . . . . . . . 625 14.3.3. Dehydrocyclisation . . . . . . . . . . . . . . . . . . . . . . . 628

14.4. Structure–Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 629 14.4.1. Reactions on Single-Crystal Surfaces . . . . . . . . . . . . . 629 14.4.2. Particle-Size Effects with Supported Metals . . . . . . . . . . 630 14.5. Modification of the Active Centre . . . . . . . . . . . . . . . . . . . . 634 14.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 14.5.2. Metal Particles in Zeolites . . . . . . . . . . . . . . . . . . . 634 14.5.3. Platinum-Rhenium Catalysts . . . . . . . . . . . . . . . . . . 635

14.5.4. Modification by Elements of Groups 14 and 15 and

Some Others . . . . . . . . . . . . . . . . . . . . . . . . . . . 637 14.5.5. Other Bimetallic Catalysts . . . . . . . . . . . . . . . . . . . 639 14.5.6. The Role of Sulfur . . . . . . . . . . . . . . . . . . . . . . . 643 14.5.7. Metal-Support Interactions . . . . . . . . . . . . . . . . . . . 644 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647