Studies in Surface Science and Catalysis 100 CATALYSTS IN PETROLEUM REFINING AND PETROCHEMICAL INDUSTRIES

1995

Sponsored by the Kuwait Institute for Scientific Research, the Kuwait Foundation for the Advancement ofScience, the Kuwait National Petroleum Company, the Kuwait Petroleum Corporation, Kuwait University, the GulfCooperation Council, the Public Authority for Applied Education and Training, the Petrochemical Industries Company, and the Organization ofArab Petroleum Exporting Countries.

Studies in Surface Science and CatalysisAdvisory Editors: B. Delman and J.T. YatesVol. 100

CATALYSTS IN PETROLEUM REFINING AND PETROCHEMICAL INDUSTRIES 1995Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995

Editors

M. Absi-Halabi, J. Beshara, H. Oabazard and A. StanislausPetroleum, Petrochemicals and Materials Division, Kuwait Institute for Scientific Research, Kuwait

1996 ELSEVIER Amsterdam - Lausanne - New York - Oxford - Shannon - Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25~O.

Box 211, 1000 AE Amsterdam, The Netherlands

ISBN 0-444-82381-6 1996 Elsevier Science B.V. All rights reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

FOREWORD

The 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries was held in Kuwait during the period April 22-26, 1995, under the auspises of H.H. Sheikh Saad A1-Abdullah A1-Salem A1-Sabah, Kuwait's Crown Prince and Prime Minister. The 1st conference was also held in Kuwait in 1989. The present conference was scheduled to be held in 1993; however, it was postponed due to the events that encompassed Kuwait and the Gulf region in 1990-1991. The patronage of the conference, the organizing bodies, and the selective emphasis on the role of catalysts in the petroleum and petrochemical industries reflect the keen interest of the countries in the region in actively contributing to the development of these industries. Petroleum-related industries are the main economic activities of most countries in the region. The refining capacity in the Gulf Region exceeds 5 MM barrels/day and includes some of the most sophisticated petroleum refining schemes in the world. The basic petrochemical industry has been also growing steadily in the region since the early eighties. The conference was attended by around 300 specialists in the catalysis field from both academia and industry from over 30 countries. It provided a forum for the exchange of ideas between scientists and engineers from the region with their counterparts from the industrialized countries. A total of 62 scientific papers were presented. The papers were carefully selected to include a blend of fundamental and applied research, and industrial experience. Such a blend was thought to be essential for providing the participants from both industry and academia with a chance to become familiar with the challenges facing each group and the actions taken to meet them. A number of keynote speakers, carefully selected from high ranking officials, policy makers, and multinational company representatives, were also invited to address the conference. The keynote presentations, which are published as a separate volume by the Kuwait Institute for Scientific Research, provided the participants with an overview of the directions the petroleum and petrochemical industries will take over the next decade. The program of the conference included a field visit to one of Kuwait's most modem refineries. A trip was also organized to one of Kuwait's oil fields. The partipants had a chance to observe oil lakes and the extent of the damage incurred by the blowing up of Kuwait's oil wells. The success of the conference is perhaps difficult for the organizers to assess. However, the quality of the papers in this volume provides some indication. Another indication is the keen interest and encouragement expressed by numerous participants in attending the next meeting, which will be held in Kuwait in 1998.

The Editors

vi

P

R

E

F

A

C

E

Catalysis plays an increasingly critical role in modern petroleum refining and basic petrochemical industries. The market demands for and specifications of petroleum and petrochemical products are continuously changing. They have impacted the industry significantly over the past twenty years. Numerous new refining processes have been developed and significant improvements were made on existing technologies. Catalysts have been instrumental in enabling the industry to meet the continuous challenges posed by the market. As we enter the 21st century, new challenges for catalysis science and technology are anticipated in almost every field. Particularly, better utilization of petroleum resources and demands for cleaner transportation fuels are major items on the agenda. It is against this background that the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries was organized. The papers from the conference were carefully selected from around 100 submissions. The papers were refereed in terms of scientific and technical content and format in accordance with internationally accepted standards. They were a mix of reviews providing an overview of selected areas, original fundamental research results, and industrial experiences. The papers in the proceedings were grouped in the following sections for quick reference:-

Plenary Papers Hydroprocessing of Petroleum Residues and Distillates Fluid Catalytic Cracking Oxidation Catalysis Aromatization & Polymerization Catalysis Catalyst Characterization and Performance

The plenary papers were mostly reviews covering important topics related to the objectives of the conference. The remaining sections cover various topics of major impact on modern petroleum refining and petrochemical industries. A large number of papers dealt with hydroprocessing of petroleum distillates and residues which reflects the concern over meeting future sulfur-level specifications for diesel and fuel oils. The task of editing this volume was facilitated by the efforts of the International Advisory Committee and the Scientific Committee of the conference who reviewed all the papers. The editorial board gratefully acknowledge this effort; the cooperation, time and effort of all authors; and the management of the Kuwait Institute for Scientific Research for allocating the required resources to prepare the manuscript of this volume.

T h e

E d i t o r s

vii

TABLE OF CONTENTSForeword Preface Organizing Committees AcknowledgementsPLENARY LECTURES

v vi xi xii

Control of Catalyst Performance in Selective Oxidation of Light Hydrocarbons: Catalyst Design and Operational Conditions B. Delmon, P. Ruiz, S. R. G. Carrazan, S. Korili, M A. Vicente Rodriguez and Z. Sobalik Vanadium Resistant Fluid Cracking CatalystsM L. Occelli

1

27 49 65 7791

Metal Clusters in Zeolites: Nearly Molecular Catalysts for Hydrocarbon Conversion B. C. Gates Catalyst DeactivationD. L. Trimm

Preparation and Catalysis of Highly Dispersed Metal Sulfide Catalysts for HydrodesulfurizationY O"kamoto

New Developments in Olefin Polymerization with Metallocene CatalystsW Kaminislry and A. Duch

HYDROPROCESSING OF PETROLEUM RESIDUES AND DISTILLATES

New Developments in HydroprocessingJ. W M Sonnemans

99117

Optimizing Hydrotreater Catalyst Loadings for the Upgrading of Atmospheric ResiduesJ. Bartholdy and B. H Cooper

Hydrotreatment of Residuals Using a Special NiMo-Alumina CatalystA. Morales and R. B. Solari

125 135147

Residue Hydroprocessing: Development of a New Hydrodemetallation (HOM) Catalyst o. K Bhan and S. E. George Commercial Experience in Vacuum Residue HydrodesulfurizationH Koyama, E. Nagai, H Torii, andM Kumagai

Comparison of Operational Modes in Residue HydroprocessingM de Wind, Y Miyauchi, and K Fujita

157171

Mina Abdulla Refinery Experience with Atmospheric Residue Desulfurization (ARDS) A. AI-Nasser, S.R Chaudhuri and S. Bhatacharya. Cosmo Resid Hydroconversion Catalyst - Catalyst Combination TechnologyY Yamamoto, Y Mizutani, Y Shibata, Y KitouandH yamazaki

181

Influence of Catalyst Pore Size on Asphaltenes Conversion and Coke-Like Sediments Fomation During Catalytic Hydrocracking of Kuwait Vacuum Residues A. Stanislaus, M Absi-Halabi, and Z Khan Origin of the Low Reactivity of Aniline and Homologs in HydrodenitrogenationM Callant, K Holder, P. Grange, and B. Deln10n

189 199

viiiDeep HDS of Middle Distillates Using a High Loading CoMo Catalyst S. Mignard, S. Kasztelan, M Dorbon, A. Billon, and P. Sarrazin Environmentally Friendly Diesel Fuels Produced from Middle Distillates Generated by Conversion Processes R Zamfirache and 1 Blidisel Factors Influencing the Performance ofNaphtha Hydrodesulfurization Catalysts J. A. Anabtawi, S. A. Ali, M A. B. Siddiqui and S. M J. Zaidi Hydrocracking of Paraffinic Hydrocarbons over Hybrid Catalysts Containing H-ZSM-5 Zeolite and Supported Hydrogenation Catalyst 1 Nakamura and K Fujimoto Effect of Presulfiding on the Activity and Deactivation of Hydroprocessing Catalysts in Processing Kuwait Vacuum Residue M Absi-Halabi, A. Stanislaus, A. Qamra and S. Chopra Continuous Developments of Catalyst Off-Site Regeneration and Presulfiding P. Dufresne, F. Valeri, and S. Abotteen The Production of Large Polycyclic Aromatic Hydrocarbons During Catalytic Hydrocracking J. C. Fetzer Fouling Mechanisms and Effect of Process Conditions on Deposit Formation in H-Oil Equipment M A. Bannayan, H K Lemke, and W. K Stephenson Bed Expansion and Product Slate Predictions from H-Oil Process via Neural Netwrok Modelling E. K T. Kam, M M AI-Mashan, and H Dashti Renewed Attention to the EUREKA Process: Thermal Cracking Process and Related Technologies for Residual Oil Upgrading T. Takatsuka, R Watari and H HayakawaFLUID CATALYTIC CRACKING

209

2 17 225

235

243 253 263 273 283

293

New Catalytic Technology for FCC Gasoline Sulfur Reduction without Yield Penalty U Alkemade and T. J. Dougan The Influence of Feedstocks and Catalyst Formulation on the Deactivation of FCC Catalysts R. Hughes, G. Hutchings, C. L. Koon, B. McGhee, and C. E. Snape Resid FCC Operating Regimes and Catalyst Selection P. 0' Connor and S. J. yanik Novel FCC Catalyst Systems for Resid Processing U Alkemade and S. Paloumbis Probing Internal Structures of FCC Catalyst Particles: From Parallel Bundles to Fractals R Mann and U A. EI-Nafaty Development of Microscale Acitivity Test Strategy for FCC Process Economics Enhancement O. H J. MuhammadOXIDATION CATALYSIS

303 313 323 339 355 365

Partial Oxidation of C2-C4 Alkanes into Oxygenates by Active Oxygen Generated Electrochemically on Gold through Yttria-stabilized Zirconia K Takehira, K Salo, S. Hamakawa, T. Hayakawa, and T. Tsunoda

375

ixThe Effects of Gas Composition and Process Conditions on the Oxidative Coupling of Methane over Li/MgO Catalyst S. M AI-Zahrani and L. Lobban Study on the Active Site Structure ofMgO Catalysts for Oxidative Coupling of Methane K Aika and T. Karasuda Various Characteristics of Supported CoPe on A1 20 3, Si02 and Si02-AI20 3 as Selective Catalysts in the Oxidative Dehydrogenation of Cyclohexene S. A. Hasan, S. A. Sadek, S. M Faramawy, and M A. Mekewi Dehydrogenation of Propane over Chromia/Alumina: a Comparative Characterization Study of Fresh and Spent CatalystsA. Rahman and M Ahmed

383 397

407

419 427

Deactivation Mechanism of a Chromia-Alumina Catalyst by Coke DepositionF Mandani, E. K T. Kam, and R Hughes

Investigation of Synthesis Gas Production from Methane by Partial Oxidation over Selected Sream Refonning Commercial CatalystsH AI-Qahtani

437

AROMATIZATION & POLYMERIZATION CATALYSIS

Aromatization of Butane over Modified MFI-Type Zeolite Catalysts T. Yashima, S. Ekiri, K Kato, T. Komatsu, and S. Namba Development of Light Naphtha Aromatization Process Using A Conventional Fixed Bed Unit S. Fukase, N Igarashi, K Kalo, T. Nomura, and Y: Ishibashi Improvement in the Perfonnance of Naphtha Refonning Catalysts by the Addition of Pentasil Zeolite J. N Beltramini and R Fang Zeolite Catalysts in Upgrading of Low Octane Hydrocarbon Feedstocks to Unleaded GasolineVG. Stepanov, KG. lone, andG. P. Snytnikova

447 455

465477

Catalysts for Cyclization of C6-AlkanesN Ph. Toktabaeva, G. D. Zakumbaeva, and L. B. Gorbacheva

483

High Quality Gasoline Synthesis by Selective Oligomerization of Light Olefins and Successive Hydrogenation T. Inui and J. B. Kim Hydrogenation of Aromatic Compounds Related to Fuels over a Hydrogen Storage Alloy S. Nakagawa, T. Ono, S. Murata, M Nomura, and T. Sakai A Theoretical Study of Ethylene Oligomerization by Organometallic Nickel Catalysts L. Fan, A. Krzywicki, A. Somogyvari, and T. Ziegler IFP-SABIC Process for the Selective Ethylene Dimerization to Butene-lF A. Al-Sherehy

489499 507

515

CATALYST CHARACTERIZATION AND PERFORMANCE

Cobalt Containing ZSM5 Zeolites - Preparation, Characterization and Structure SimulationA. Jentys, A. Lugstein, O. El-Dusouqui, H Vinek, M Englisch and J. A. Lercher

525535

Acid-base Property of Some Zeolites and their Activity for Decomposition of n-Hexane S. Tsuchiya

xReduction and Sulfidation Properties of Iron Species in Fe-Treated V-Zeolites for Hydrocracking CatalystsK Inamura and R Iwamoto

543

Preparation of Highly Active Zeolite-Based Hydrodesulfurization Catalysts: Zeolite-Supported Rh Catalysts M Sugioka, C. Tochiyama, F Sado, and N Maesaki High-Dispersed Supported Catalysts on Basis of Monodispersed Pt-Soles in Processes Reductive Transformation of Hydrocarbons N A. Zakarina and A. G. Akkulov Infrared Spectroscopy of CO/H2 Coadsorption on NilAl20 3 Hydrotreating Catalysts: Evidence for Perturbed Metal Sites M 1 ZakiList ofparticipants Author Index

551

559

569 579 595

xi ORGANIZING COMMITTEE Jasem AI Besharah Khaled A1 Muhailan Mamun Absi Halabi Abbas Ali Khan Anwar Abdullah Taher A1 Sahaf Mohammad Ali Abbas Abdul-Karim Abbas Bader AI Safran Faisal Mandani Hassan Qabazard Mubarak AI Adwani AI Tayeb Wenada Chairman Rapporteur Coordinator Member Member Member Member Member Member Member Member Member Member KISR KFAS KISR KFAS GCC KU KPC KNPC PIC PAAET KISR KISR OAPEC

INTERNATIONAL ADVISORY COMMITTEE Mamun Absi Halabi David L. Trimm Bernard Delmon Burce C. Gates Walter Kaminsky Yasuaki Okamoto Mario L. Occelli Henrik Topsoe Chairman Member Member Member Member Member Member Member Kuwait Australia Belgium USA Germany Japan USA Denmark

SCIENTIFIC COMMITTEE Taher A1 Sahaf Anthony Stanislaus Abdullah S. A1 Nasser Jaleel Shishtary Erdogan Alper Mustafa A. A. Gholoum Faisal Mandani Ezra Kam Chairman Rapporteur Member Member Member Member Member Member KU KISR Mina Abdulla~NPC Mina A1 Ahmadi/KNPC KU Shuaiba/KNPC PAAET KISR

. ~

Xll

ACKNOWLEDGEMENTS The Organizing Committee was deeply honored by the patronage of//. H. The Crown Prince and Prime Minister Sheikh Saad A1-Abdullah A1-Salem AI-Sabah, which reflects his keen interest in science and technology. The Committee is also grateful for the financial support of the Kuwait Institute for Scientific Research, the Kuwait Foundation for the Advancement of Science, the Kuwait National Petroleum Company, the Kuwait Petroleum Corporation, Kuwait University, the Gulf Cooperation Council, Public Authority for Applied Education and Training, the Petrochemical Industries Company and the Organization of Arab Petroleum Exporting Countries. The Committee would like also to express gratitude for the efforts of the Japan Petroleum Institute in coordinating and supporting the participation of prominent Japanese scientists in this event. The Committee would like also to extend its deep appreciation for the effort and time put forth by the distiguished keynote speakers, namely H.E. Mr. Hisham Al-Nazer, H.E. Mr. Erwin Valera, H.E. Mr. Lulwanu Lukman, Mr. Abdullatif AI-Hamad, Mr. Charles DiBona, Mr. John Yimoyines, Mr. J. Kent Murray, Mr. Mahmoud Yusef, Mr. Moayad Al-Qurtas, Mr. Khalaf A1-Oteibeh, Mr. Khaled Buhamra, and Mr. Nader Sultan. The Organizing Committee are also appreciative of the efforts of the members of the International Advisory Committee and the Scientific Committee for their thorough work in selecting and refereeing the submitted papers. The Committee also acknowledges the help and guidance provided by Elsevier Science Publishing Company and the advisory editors of this series in preparing this proceedings. We would like to thank our colleagues at the Kuwait Institute for Scientific Research, the Kuwait Ministry of Oil, and the chairmen and cochairmen of the sessions, who provided unlimited assistance at times when it was badly needed. Finally, we feel deeply indebted to the participants who enriched the meeting with their serious discussions till the end.DR. J A S E M B E S H A R A

CHAIRMAN, ORGANIZING COMMITTEE

Catalysts in Petroleum Refining and Petrochemical Industries 1995

M. Absi-Halabi et al. (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

C O N T R O L OF CATALYST P E R F O R M A N C E IN SELECTIVE OXIDATION OF L I G H T H Y D R O C A R B O N S : C A T A L Y S T D E S I G N AND O P E R A T I O N A L CONDITIONS B. Delmon, P. Ruiz, S.R.G. Carraz~in, S. Korili, M.A. Vicente Rodriguez, Z. Sobalik Catalyse et Chimie des Mat6riaux Divis6s, Universit6 Catholique de Louvain, Place Croix du Sud 2/17 - 1348 Louvain-la-Neuve, Belgium This paper is an attempt to summarize the situation with respect to the selective catalytic oxidation of light alkanes using heterogeneous catalysts. Methane oxidation reactions and the oxidation of butane to maleic anhydride will only be alluded to occasionally, because they have been reviewed in detail in a large number of papers. We shall first show that it is still far from clear which are the families of catalysts to be used for the various reactions: mainly oxidative dehydrogenation or oxidation to oxygen-containing molecules of ethane, propane or isobutane. Much research is still necessary for understanding the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts inherited from the development in selective oxidation and ammoxidation of olefins are probably of little use. Conversely, much emphasis has to be laid on new data which opens promising perspectives, namely (i) the occurrence of cooperation effects between two (or several) separate phases and especially the role of spillover oxygen and the so-called "remote control" and (ii) the occurrence of homogeneous non-catalysed reactions which occur at temperatures only slightly higher than the catalytic ones and correspond to similar selectivities. This suggests that research on selective catalytic oxidation, to be effective, should be comprehensive: it should continue to involve a search for new active phases and efforts to improve the already known catalysts. But research should also include investigations on the role of spillover oxygen, the nature of this oxygen (more or less electrophilic), the donors that can generate it, and the way this spillover oxygen reacts with the catalytic surface. Research should also contemplate the problem of how homogeneous and heterogeneous reactions proceed simultaneously or consecutively. In parallel with these research lines, chemical engineering must develop new concepts and new reactors. Recent spectacular results in methane coupling or oxidative dehydrogenations show that considerable progress can be made if the problem of light alkane selective oxidation benefits from a multifacetted approach. 1. I N T R O D U C T I O N Making valuable products from light hydrocarbons is presently one of the major challenges for the petroleum and petrochemical industries. Among the various processes able to transform light hydrocarbons to useful products, catalysis has a major role to play. Conceptually, the cheapest and easiest route is through catalytic oxidation. The reason is that oxygen (pure or in air) is cheap and possesses the high reactivity necessary to activate saturated hydrocarbons. For that type of activation, heterogeneous and homogeneous catalysis are competing. Nevertheless, the preference in principle goes to heterogeneous catalysis, especially if very large quantities have to be transformed, as in the case of methane.

On the whole, a continuous progress towards a more selective oxidation of light saturated hydrocarbons is observed, and recent announcements demonstrate that dramatic progress can be made even in the very difficult case of methane activation, using either heterogeneous or homogeneous catalysts. The activation of light saturated hydrocarbons becomes increasingly more difficult as the molecules become smaller, with methane reactions being the most difficult to control. On the other hand, the occurrence of non-catalysed gas phase oxidation makes selectivity control very complicted. This is a problem common to almost all oxidations, unless one of the products is extremely stable 9 examples are unsaturated nitriles (e.g. acrylonitrile in the ammoxidation of propane) or maleic anhydride (in the oxidation of butane). There is a parallel trend in the changes of reactivity with molecular weight in catalytic and non catalytic (gas phase) oxidation. The challenge to catalysis to achieve selective reactions at lower temperature is thus equally important for all light hydrocarbons. The activation of very light hydrocarbons (propane, ethane and methane) in the presence of oxygen has been achieved only at temperatures substantially or much higher than those used in the reactions of other hydrocarbons. There is however little doubt that some mechanistic similitudes exist and that the vast body of knowledge accumulated on the reaction of other hydrocarbons (including unsaturated ones) with oxygen will be useful for improving the efficiency of these difficult reactions. Nevertheless, the outstanding commercial success of the oxidations and ammoxidations of light olefins and that of the oxidation of butane to maleic anhydride has directed the fundamental research of the largest number of investigators to topics which are probably not the most relevant to the new challenges set by the selective oxidation of light alkanes. A much broader approach has certainly to be taken, compared to that used in former investigations. It is the aim of this contribution to highlight a few promising directions for research in the area of selective reactions of light alkanes with oxygen (oxidation and oxidative dehydrogenation). We shall emphasize three aspects: (i) new concepts have been recently developed in a field which seemed to be well established, namely the catalytic oxidation of olefins and butane, but where new powerful methods of action have been discovered. We shall show that these new concepts are applicable to the catalytic oxidation of the light saturated hydrocarbons, namely containing from one to five carbon atoms. We shall present, in some cases for the first time, results which strongly suggest that a cooperation between distinct phases in oxidation catalysts could play an important role in the oxidation of light hydrocarbons, even perhaps in the coupling of methane. (ii) we shall suggest, on the basis of new results from our and other laboratories, that the intervention of non catalysed gas phase reactions must be accounted for and should be investigated carefully. (iii) we shall also show that catalyst discovery and development in the field of heterogeneous oxidation of light hydrocarbons should be accompanied by innovative developments on the chemical engineering side. Before examining specifically these points, we shall "set the stage", namely attempt to give an overview of the results published in literature on the selective reactions of light alkanes with oxygen. The largest part of the contribution will consist in a critical overview of the parameters traditionally believed to be crucial for activity and selectivity. We shall show that one parameter, which probably has the largest importance, has been almost completely forgotten: this is the ability for separate phases, inactive or poorly active, to enhance the activity of potentially active and selective phases, via an oxygen spillover process. Results will be presented which strongly suggest that the same sort of cooperation between phases can operate in the reactions of light alkanes. At the end, we shall suggest that the existence of gas phase oxidation reactions, the occurrence of the phase cooperation mentioned above and the other particularities of light alkane oxidation are about to trigger new developments in chemical engineering which will probably be as innovative and crucial for viable processes as the development of fluidized bed reactors for oxidation or ammoxidation, and riser reactors (in the

case of butane oxidation) has been during the remarkable development of catalytic oxidation in the last 25 years. 2 . C A T A L Y S T S A C T I V E IN T H E ALKANES W I T H OXYGEN SELECTIVE REACTION OF LIGHT

The variety of catalysts which have been claimed to activate light alkanes is very large. The only conspicuous exception concerns the reaction of butane to maleic anhydride; this is, however, a special case considering the high stability of the product, namely maleic anhydride. But this large diversity of formulations exists even in the ammoxidation of propane to acrylonitrile, although the product is also particularly stable in this case. It cannot be therefore concluded that given oxidation reactions take place only on a single family of catalysts. In what follows, we present a series of tables concerning various reactions of light alkanes with oxygen. We wish, however, to underline the fact that the data contained in the tables are by no means comprehensive. We have selected them in view of our objectives, namely (i) to underline the variety of formulations proposed for a single reaction, (ii) to extract from these data a few conclusions and (iii) to speculate on the possible importance of some parameters. We have avoided to overburden the tables with information on reaction conditions. These are indeed very different, and correlating them with catalyst composition has little usefulness for the moment (except perhaps for propane ammoxidation, where investigation is more advanced). We do not present data concerning either methane or butane. In the case of methane oxidation and oxidative coupling, innumerable articles (more than 1000) have been published, together with many review papers. Concerning butane, the numerous articles and review papers dealing with oxidation of maleic anhydride obscure the few scattered articles dealing with oxidative dehydrogenation; dehydrogenation of butane has mainly been done in reactions without oxygen. In the tables, we omit the chemical symbol of oxygen and list only the elements combined with oxygen in the catalysts, or oxygen when it is present in a phase indicated as such by the authors (e.g., supports: MgO, SIO2), except if there is good ground to believe that well defined metal oxide entities are crucial for catalytic activity (e.g., VO...). In addition to the systems listed in Table I for the oxidative dehydrogenation of ethane, other systems have been tested because they have proven to be active in other alkane oxidations; this is particularly the case of many catalysts used in the oxidative coupling of methane, VPO and magnesium phosphate catalysts (butane oxidation and propane dehydrogenation, respectively) and MoVO catalysts. Various zeolites have also been tested. This table, the largest to be presented here, perfectly illustrates the fact that no formulation seems convincingly better than the others. In the oxidative dehydrogenation of propane (Table II), the various magnesium vanadates have been the object of many studies, but other systems seem to have comparable performances (systems based on cerium, niobium, or vanadium, molybdates and noble metals on monoliths used with very short contact time). Because the direct dehydrogenation of isobutane to isobutene is now in operation industrially, it is not surprising that relatively few publications deal with the corresponding oxidative dehydrogenation to isobutene (about 20 in the past 6 years). On the whole, the catalysts used are similar to those mentioned in the previous tables: phosphates, chromates, molybdates. Active carbon has also been mentioned, but it is hard to imagine that the catalyst could work a long time in the presence of oxygen. Table III gives two examples of the results mentioned in literature. Mention has been made of the selective oxidation (yield = 65%) of isobutene on UV activated TiO2 [50].

Table I. Ethane oxidative dehydrogenation to ethylene

CatalystCa-Ni ceramic foam monoliths + Pt, Rh, Pd Cd-La-A1 MgO based catalysts Ce2(CO3)3 Mo-Si, Si-W or P-W/A1203 Cr-Zr-P Li-Na-Mg Li/MgO Sr-Ce-Yb Na-Mn zeolites La203-B aF2 heteropolyacid Pt-cordierite (electrocatalytic) Mo-V-Nb-Sb Mo-V-Nb-Sb-M Na-K-Zr Li-Ti+Mo, Sn or Sb Li-Ti-Mn V-P-U Zn2TiO4+Bi Co-P+promoter Mo-Te Mo-Bi-Ti-Mn-Si Li/M~D+promoter

Conversion% 25 80

Yield%

Selectivity% 93.6 70 84 73.7 90 90 50-60 86 70

Ref.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

35 45 20-30 38 75-79 49 76.8 59.1 70 10.6 22-57 34 85 54.9 46.9 22.7 71.2 68.1 75

86.5 86-90 84.7 76-98 72 96.9 72-82 86 86.4 86.3 74.6 67.5 80.5 100 100 76

It seems that very few investigations concern the oxidation or oxidative dehydrogenation of C5 alkanes. Oxidative dehydrogenation of isopentane to isoprene has been mentioned. Two articles deal with MnO2, CoO/CaO3, NaOH/A1203, but in the presence of HI [51,52]; this obviously suggests the intervention of gas-phase reactions. The yields (Y) in isobutene were relatively high (e.g., Y = 50-60% with a selectivity of 65 to 95%). Pentane can also produce maleic anhydride and phthalic anhydride [53-57]. Considering in a general way the activation of light alkane by oxygen, the ammoxidation of propane has certainly not to be forgotten. This process is already under industrial development. If we try to get an overview of the recent work on the selective reactions of light alkanes with oxygen, two remarks may be made: 9 Several lines have been followed, all inspired by former successful lines of research. It is striking that the proposed catalysts are generally similar to those previously used in the selective reaction of alkanes with oxygen: oxidative coupling of methane or oxidation of butane to maleic anhydride. Many of them are also similar to catalysts used for the reactions of olefins with oxygen (molybdates) or for dehydrogenation without oxygen (chromium containing catalysts). Because of the success of vanadyl phosphate in butane oxidation, investigators tend to focus on vanadium containing catalysts also in the case of other alkanes. Nevertheless, the data available do not seem to exclude any other formulation. 9 On the other hand, the reaction of ethane, propane, isobutane, and pentanes with oxygen described until now are poorly selective at high, and even at moderate conversions. One cannot

exclude the empirical discovery of completely new catalysts with outstanding performances. However, a more systematic approach may also help find satisfactory catalysts. An in-depth understanding of the principles involved in catalytic selective oxidation is necessary to improve activity, selectivity and resistance to ageing of catalysts. This is true as well for the catalysts to be perhaps discovered as for those already cited. Table II. Propane oxidative dehydrogenation Reactant Product Catalyst Propane Propane Propene Propene Nb based catalysts VMg, VMg+Ag, Electrochemical pumping of oxygen (EOP) VMg and chloride of Cu +, Li +, Ag+, Cd2+ noble metals (Pt,Pd) on ceramic foam monoliths at short contact time, 5ms

Conversion % 7 10

Yield Selectivity Ref. % % 85 28 84, 86.9 29

Propane Propane

Propene Propene Ethene Propene Propene

23.1 100 65 (total olefins) 60 46 59 49 60 62 34.5 81.1 66.2 50 26.7 65

30 31

Propane Propane

Propane Propane Propane Propane Propane Butane Hexane Propane Propane Propane Propane Propane Propane Propane Propane Propane

Propene Propene Propene Propene Alkenes Propene Propene Propene Propene* Acrylonitrile** Propene Propene Propene Propene Propene

Co0.95MoO4 V/Mg= 2/1 2/2 2/3 VMgffiO2 NiMoO4 CeO2]2CeF3/Cs20 FeV-supported Nd203 Vanadate catalysts V-Fe-Nd-A1 VMg CeO~CeF3 (NH3)3PO4+ in(NO3)3+ Vanadyl phosphate NiMoOx (a=0.6-1.3; x=number determined by Ni or Mo valency) A1203 supported Pt/Cs/Sm MgV206 (50% V2Os+MgO calcined at 610 ~ CoMoO4/SiO2 NaHO/Na3VO4/A1

19 23 23 23 25 20 43 41.3 40.3 50 40.3 10 53.4 12 29

32 33

12.5 14.8 33.5

34 35 36 37 38 39 40 41 42 43

66.2 3 6.7 35* 36.7** 18.1

91 71 4.1 20.9 77.9 79.8

44 45 46 47

16.6

The next sections will therefore indicate some of these fundamental aspects and suggest the perspectives that some new f'mdings are opening. Table HI. Isobutane oxidative deh~,drosenation to isobutene. Catalyst Y203 + CeF3 Ni2P207 Zn2P207, Cr4(P207)3, M~2P207

Selectivity (S)high conversion S = 82 % S = 60-70 %

Ref.48 49 49

3. PARAMETERS TRADITIONALLY CONSIDERED IN S E L E C T I V E OXIDATION A very large amount of work has been devoted in the past to the oxidation of olefins Callylic" oxidation to unsaturated aldehydes) and butane (to maleic anhydride). This has led to the development of ideas and concepts which are quite naturally used in the new investigations concerning light alkanes. It is necessary to examine these ideas and concepts and to evaluate in a critical way their potential for discovering or improving catalysts in the new field that oxidation of light alkanes constitutes. This will be done here shortly on the basis of classical books or articles [53,58-62].

3.1.

Doping

The idea is to add foreign ions as a solid solution in already active oxide structures. This is logical. The oxidation of hydrocarbons involves oxygen from the catalyst lattice and replenishment of the latter by molecular oxygen after the hydrocarbon molecule has been dehydrogenated or oxidised. This is an oxido-reduction mechanism. Doping by elements of other valencies can in principle change the oxido-reduction level of the surface. More precisely, the really important parameters in the processes are the rates of (i) removal of oxygen by the reaction with the hydrocarbon and (ii) reoxidation by 02. In principle, doping can alter these rates, but very few measurements have been made along this line. Doping can also change surface acidity, a parameter essential for the activation of alkanes. Doping is certainly a good approach for modifying a catalyst. It should however be underlined that it has seldom been verified that the doping elements were really incorporated in the host oxide and did not spontaneously segregate out. There are indeed conspicuous instances of such segregations. For example, it had been claimed that antimony in solid solution in tin oxide SnO2 explained the high activity of Sb-Sn-O catalysts in oxidation. Actually, Sb has a strong tendency to segregate out of SnO2 during the catalytic reaction [63-65]. But in other reactions, there seems to be indeed an effect of doping elements to alter the extent of oxidationreduction in the near surface layers (e.g., cobalt in V-P-O catalysts) [66]. It is therefore advisable to use the doping elements in quantities compatible with complete solubility in the host oxide, and to check that they do not segregate during the catalytic reaction. Cobalt, mentioned above as a useful dopant, could exert a catastrophic effect if segregated as cobalt oxide, because of the high activity of the latter in complete oxidation.

3.2.

Supports

It seems that supports have been considered with much circumspection in the early days of allylic oxidation. Progressively silica began to be used, but it is considered as being generally inert, and permitting only a better dispersion or a higher mechanical strength. However, real supports are progressively appearing in the field of catalytic oxidation, as suggested by the tables presented above. A conspicuous and well known example is TiO2 as a support for V205. The advantage of using TiO2 (e.g., in o-xylene oxidation to phthalic anhydride) is probably not to give isolated surface vanadium atoms, but rather to stabilise islets of a sub-oxide of vanadium, V6013 over a broader range of oxido-reduction conditions [67-69]. This

stabilization has to be attributed to the strong interaction existing between vanadium oxide and the support. But another new factor should probably be taken into consideration. Surface mobile oxygen (spillover oxygen) has an important role in selective oxidation, as will be shown below. Silica is at the bottom of the scale with respect to oxygen surface mobility [70]. ZrO2 is much better, so could presumably be TiO2. We believe that supports could play a more important role in the oxidation of light alkanes than it did in allylic oxidation. But this role will be complex, and include better dispersion of the active phase, stabilisation of the selective phase, control of oxido-reduction, and/or facilitation of oxygen spillover.

3.3.

Epitaxy

Most active catalysts in oxidation contain several phases which act synergetically. This led to the widespread assumption that an epitaxy at the contact between two different phases was of crucial importance. This is undoubtedly a hypothesis to consider. The above example of V205fI'iO2 catalysts indeed suggests that a strong interaction between two phases could make one of them more stable, more active or more selective. But epitaxy should not be taken as a universal explanation, because there are very few proofs (if any) of such epitactic contact between the phases detected in allylic oxidation, even in the case of Sb204-FeSbO4 mixtures whose activity has often been attributed to epitaxy. The explanation of the activity of V-P-O catalysts has long been believed to involve such an epitaxy between two types of vanadium phosphates. But no such proof could be found [71,72]. The explanation of the activity of V-PO is now that a special local structure on the surface of vanadium pyrophosphate, namely twin flat pyramids in adjacent positions oriented in opposite directions is the active sites (fig. 1) [7375], and the epitaxy hypothesis is leaving the scene.

P

,,:,X,...

When the cooperating phases in catalytic oxidation have been found to be clearly \ / ;',, II ,,-'_/.-" i separated in no epitaxial 0 - , . I. , , ~ , O' o O I position, another traditional o .......' II.---'7 i q explanation was put forward, t o p j t namely that an element of one phase migrated to the other P t I .,tOP phase for making a \ : o ..-;, )j',,"'--.. I contamination layer of P , I o ~/-- i1-',, ,)o molecular thickness, or monolayer. The idea has been based on the observation that o . , ....... o MoO3 spreads spontaneously I 1 on 1,-A120 3 and, to a certain P P extent, on bismuth molybdates during calcination in air. But a Figure 1. Structure of vanadium pyrophosphate (VO)2P207. review of literature shows that MoO3 has much lower ability to spread on many other oxides [76]. A contaminating layer is intrinsically fragile, and stable only when its adhesion energy on the other phase is higher than the cohesion energy inside the bulk contaminant. The stability is extremely sensitive to the oxido-reduction conditions. A monolayer appearing upon calcination may not be stable in the conditions where catalysis takes place. A conspicuous example is the

.o

I I o~176 I , ~..... ,.;o-',,,-It'.,,

3.4. Formation of monolayers

"'o

v,,,- ..->:t'..--"

'[/P

v,,/

case of MoO3 mixed with Sb204. Even if dispersion of one element on the oxide of the other is realised, the contamination may disappear during catalysis [63,76]. The common teaching of section 3.3 and the present one, is that it is not excluded that epitaxy or mutual contamination could explain the high activity of oxidation catalysts, but that this has not been proven and that there are good reasons, experimental as well as theoretical, for thinking that such effects are not common.

3 . 5 . Role of the traditional parameters A comprehensive view of the parameters playing a role in the selective oxidation reactions investigated until now is presented elsewhere [77]. When considering all the traditionally discussed parameters, it is clear that very few lines appear for controlling in a comprehensive way catalysts activity, selectivity and resistance to ageing. This is true even with the control of acidity. Removal of undesirable acidic sites leading to poor selectivity is possible to a certain extent [77]. But creating the acido-basic properties necessary for activating alkanes has not been possible until now. The idea which emerges from recent results is rather different, as the example of butane oxidation to maleic anhydride suggests. In full agreement with the new concepts developed in catalysis, the reaction takes place at special sites on the surface (e.g. the twin flat pyramid in VPO shown in fig. 1). This permits a special activated conformation of the reactant in the adsorbed state and makes possible the complicated concerted mechanism necessary for selective transformation. The emphasis is on surface structures, well determined at the atomic scale, which possess the adequate catalytic activity. This is obvious and should have been obvious for many years. What has been overlooked in the past is that surface structures do not necessarity reflect bulk structure: this result has been emphasized by the progress of surface science. Bulk structures, long range order or collective electronic behaviour influence only partially the structure and properties of the limited number of atoms in a special configuration which constitutes the active center. Another teaching of surface science has also been forgotten, namely that surfaces change according to the molecules they are contacted with on the one hand, and all other experimental conditions on the other hand. Position of doping elements, epitaxy, or monolayer depend crucially on all experimental conditions. The conclusion is thus that attention should be given to the local arrangement of limited numbers of atoms which permit the selective reaction and to mechanisms which maintain these structures intact in spite of the oxido-reduction process which continusouly tends to put this structure upside down. The next section will show some typical results of our work in reactions involving oxygen. These results strongly support the correctness of the above views. Our work has permitted to point to the crucial role of hydroxyls, an aspect almost completely ignored before, and to suggest the structure of molybdenum containing phases during catalysis. We have discovered a mechanism by which the steady-state surface can be controlled. The consequences of this discovery will be very briefly outlined. In the subsequent section, we shall suggest how a more comprehensive view of selective oxidations can foster progress in alkane activatien. This will be illustrated by some of our recent results.4. COOPERATION HYDROCARBONS STRUCTURES BETWEEN PHASES IN T H E WITH OXYGEN: CONTROL REACTION OF OF SURFACE

It is well known that the catalysts used for oxidation reactions such as those of propylene to acrolein, isobutene to methacrolein, or for ammoxidations (propylene to acrylonitrile, methylsubstituted benzenic rings to the corresponding aromatic nitriles) contain many components. This complexity in elemental composition is reflected by a complexity in phase composition.

The so-called "multicomponent catalysts" used in selective oxidation are oxides, and they represent the vast majority of catalysts used in this field. All multicomponent industrial catalysts contain several phases. We discovered about ten years ago that simple mechanical mixtures of two oxides had much better performances than those of the two constituents [63-69,7172,76,78]. This is illustrated by fig. 2 in the case of the oxidation of isobutene to methacrolein over mixtures of micron-size MoO3 and t~-Sb204 particles. All experiments were made with the same total quantity of catalysts. The arrows show the increase of yield compared to the simple addition of the individual contributions of the catalyst components. This phenomenon is due to the action exerted by surfaceIsobutene ~ Methacrolein mobile oxygen on the surface of one of the phases, which we call the acceptor (i.e., acceptor of surface-mobile, or spillover e s oxygen: this is MoO3 in the s example of fig. 2). Spillover (3 s oxygen Oso reacts with the surface of the acceptor and, thanks to this reaction, keeps the catalytic sites active and selective. 10 The other phase, often not active or poorly active catalytically, produces the Oso species. This is the donor of spiUover oxygen: aS b 2 0 4 is a typical donor. A comprehensive characterization l of the mixtures before and after catalytic test permitted to exclude any other explanation, such as mutual contamination, formation of new solid phases, bifunctional 0.0 0.5 1.0 catalysis, bulk diffusion, etc., in the majority of cases investigated Moo3 [63]. The occurrence of a surface (mass) migration of oxygen from a s eo ..oo3 donor (t~-Sb204) to an acceptor (MOO3) has beeen shown directly Figure 2. Synergy between o~-Sb204 and MoO3 particles using labelled oxygen [79-82]. in the selective oxidation of isobutene to methacrolein. The Another example, that of figure concerns yields (namely conversion x selectivity) in experiments where conversion was always below 25%. mixtures of o~-Sb204 with SnO2, The catalysts were prepared by mixing the powders of a- very conspicuously shows that Sb204 and MOO3, prepared separately, as a stirred the action of spillover (donated suspension in n-pentane, and evaporating n-pentane. The by a- S b 2 0 4) modifies the same overall weight of mixture was used for all selectivity of the active sites compositions and the experimental conditions were identical situated on SnO2 (the acceptor in [63,78]. the present case) (fig. 3).20

I

10

It had been believed for long that the best oxidation catalysts were oxides associating two or several elements in a ~. b,lethacrolein Isobutene given mixed oxide structure, like bismuth or iron molybdates. Fig. 4.a and 4.b [84,85] show that these compound oxides benefit from the flb contact with a donor of Oso (0t-Sb204 3O is a typical donor, as it has no activity of its own). The figures we present here are simplified, just showing that an important synergy (increase compared to the straight line joining the C~ (b two extreme points) occurs when the powders of the two compounds are C3 2O mixed with each other (simply by suspending them in n-pentane, agitating and evaporating n-pentane; please note that the same weight has been used in all experiments of the series). ~n \ We showed that the same 10 synergetic effect occurs in a broad variety of reactions: 9 oxidative dehydrogenation of butene (C4=) to butadiene (BDE) (fig. 4.c and 4.d) [85,86] 9 oxidation of alcohols: methanol to formaldehyde (fig. 4.e) [87], ethanol to 10 acetaldehyde (as shown in fig. 4.f) 0.5 [88]; an almost identical figure is sno 2 obtained when a-Sb204 is mixed with (mass) MoO3 instead of Fe2(MoO4)3 [87]) and , s.o 2 ethanol to acetic acid using a mixture of Figure 3. Synergy (selectivity) between 0~-Sb204 and three phases: MoO3 + SnO2 + a-Sb204 SnO2 particles in the selective oxidation of isobutene [88]oxygen-aided (fig. 4.g). transformation of to methacrolein. The preparation of the sample formamides to nitriles: an example mixtures and the experimental conditions are among more than 15 cases is shown in described in the legend of fig. 2. More details are fig. 4.f [89]; in that case, the selectivity found in the original articles [63-65,83]. remains always high, the most dramatic effect concerning activity. A very interesting observation is that the action of spillover oxygen protects the active phase from deactivation [63,90,91]. On the basis mainly of results obtained in the oxidation of isobutene to methacrolein, the oxidative dehydrogenation of butene to butadiene and the oxygen-aided dehydration of formamide to nitriles, it was possible to show that oxides present in catalysts are located on a scale reflecting donor-acceptor properties (fig. 5). Some oxides are essentially acceptors (e.g., MOO3, some tellurates)" they can potentially carry active and selective sites, provided they receive spillover oxygen. Others are essentially donors; a-Sb204, in this respect, is typical: it produces spillover oxygen but carries no sites active for oxidation. Other oxides have mixed properties. The acceptors are relatively covalent, the donors are more ionic [63,77].40_

9

11 Our work, and especially the comparison of results obtained with different types of reactions (see above) but using exactly the same catalyst mixtures, coupled with methods aimed at identifying active sites, also led to the demonstration that one of the consequences of the action of Oso was the creation or regeneration of acid hydroxyl groups (on MOO3) [63,92,93]. It was also shown directly that the deactivation and loss of selectivity of catalysts was associated with the fact that their surface got slightly reduced during the catalytic reaction. This does not occur when donors are present in the catalysts constituted of mixtures of donor and acceptor phases. The beneficial action of spillover oxygen is thus to keep the surface of the catalysts (acceptors) in a higher oxidation state [63,90,91,94,95]. All the phenomena observed can be explained by considering the full mechanism of the reaction, namely the simultaneous changes undergone by the reacting molecule and the acceptor part of the catalyst [91,94,95]. To make the argument as simple as possible, let us consider a very schematic structure of the surface of MoO3 (fig. 6). Octaedra composed of a central Mo ion and 6 oxygen ions surrounding it are the building blocks of the structure. They are normally linked together by comers, where an oxygen ion is shared by two neighboring octaedra: fig. 6 shows the real picture (a) together with the simplified representation we shall use in the following (b). The surface oxygens which react with the organic molecule might in principle be free "tips" (on top of our representation) or connecting O ions linking two surface octaedra. But theoretical and steric considerations [96] rule out the possibility that linking oxygens could come close enough to the hydrocarbon to react with it: only "tips" remain as likely candidates (fig. 7). The reaction of oxygen from the catalyst with the hydrocarbon thus brings about the formation of a reduced site which, in the MoO3 structure, corresponds to octaedra linked by one edge (namely by 2 oxygen ions, instead of one). We mentioned that acceptors not irrigated by Oso coming from donor tend to reduce. At the atomic scale, this means that oxygen is taken out of the surface by the hydrocarbon HC faster than molecular oxygen 02 from the gas phase can restore the corner-sharing structure (fig. 8). It ensues that the surface contains many more edge-sharing octaedra than corner-sharing ones. The role of Oso is to prevent this inbalance (fig. 9). The full argument is actually more elaborate and involves non-linear responses of the equilibrium as suggested in this figure [94,95]. The inbalance in the case where Oso is absent corresponds to a diminution of the number of active selective sites (the corner-sharing octaedra), and the appearance of non-selective sites (group of edge-sharing octaedra). The location of the acidic OH centers mentioned above is not yet clearly identified: they are likely to be present on the tip of a certain proportion of the corner-sharing octaedra at the surface of the catalyst. The transformation to edge-sharing pairs leads to their disappearance and the loss of activity. The accumulation of an excessive number of edge-sharing octaedra leads to bulk reduction and long-standing deactivation. This picture (or more precisely the complete elaborate picture resting on the ideas presented here in a schematic way) points to the necessity to have a well-defined architecture on the surface, which constitutes a demand for the elaborate concerted mechanism in selective oxidation. The conclusion is that spillover species permit that the correct coordination of atoms and groups of atoms at the surface of oxide catalysts be kept, thus permittting high activity and selectivity, and avoiding deactivation. The phenomenon by which a donor distinct from the real catalytic phase controls the catalytic properties of the latter is what we call a remote control.

12

d~(:3 (:3 I.,') (:b

(%1 X#,~#~oloS

(%) plo!,~0oo

N0 (~3,,,1.oII

. 0 -.~

Q)

(%! ,O!~!laalaS

...c. (b 0 r.j

I(:b',,~

\c5I

I

(%) X,z/A!laalaS

(%) Xl!A/,ZoalaS

/

\

-~ecb

9 r

c:b o,i

(%) ,OM!laalaS

(%) X,qA!~ooloS

13 Figure 4. Examples of synergy between phases in various oxidation reactions. The mixtures were made by suspending the starting powders in n-pentane and evaporation under stirring; rm is the weight ratio in the mixture of the oxide mentioned at the fight of the figure. 4a.: oxidation of isobutene to methacrolein on SnO2-Bi2MoO6 mixtures (460 ~ [84]. 4b.: oxidation of isobutene to methacrolein on a-Sb204-FeSbO4 mixtures (400 ~ [85]. 4c. oxidative dehydrogenation of 1-butene to butadiene on tx-Sb204-ZnFe204 mixtures (400 ~ BiPO4 has an effect almost identical to that of a-Sb204 [86]. 4d.: oxidative dehydrogenation of 1-butene to butadiene on BiPO4-Fe2(MoO4)3 mixtures (400 ~ [851. 4e.: oxidative dehydrogenation of methanol to formaldehyde on a-Sb204-MoO3 mixtures

(350 ~

[871.

4f.: oxidative dehydrogenation of ethanol to formaldehyde on a-Sb204-Fe2(MoO4)3 mixtures (350 ~ [881. 4g.: oxidation of ethanol to acetic acid on mixtures of a-Sb204, MoO3 and SnO2 (240 ~ MoO3 and SnO2 were mixed (mass ratio MoO3/(MoO3+SnO2)---0.4) before the addition of ot-Sb204 [88]. 4h.: oxygen-aided dehydration of N-ethyl-formamide to propionitrile on a-Sb204-MoO3 mixtures (370 ~ The selectivity of the reaction is higher than 98%. The figure presents the variation of propionitrile yield [891.

/9

~q,,+Figure 5. Donor-Acceptor scales for oxides used in selective oxidation (adapted from ref 63 or 77).

a

b

Figure 6. MoO3 octaedra and their normal linking by comers (or tips) (a). Picture b is the usual schematic representation of octaedra in the description of the structure.

14

12

corner sharing

"t:l

bi,,,,

"'tip'" vacancy

dedge sharing k not likely in oxidation catalysis "bridge'" vacancy

Figure 7. Representation of vacancies created by the reaction of an oxygen of the lattice with a hydrocarbon. As "tip" oxygens (corner oxygens above the surface) are the only ones accessible, at the exclusion of the bridging oxygens, the vacancies formed should be "tip" vacancies. The surface structure tends to spontaneously rearrange to create an edge sharing pair.

hydrocarbon

02Figure 8. Inbalance in the rates of the antagonistic reduction of the surface by the hydrocarbon reactant and the reoxidation by molecular 02 in selective oxidation.

15b . S u r f a c e kept more oxidised by spillover oxygen

a . Spontaneous ox,do-reduct,on state of s u r f a c e

Figure 9. Schematic representation of the surface at steady state a. when spillover oxygen is not present b. when spillover oxygen flows over the surface.5 . ROLE OF HOMOGENEOUS REACTIONS

Contrary to the case of olefins, homogeneous catalytic oxidations of light alkanes occur at temperatures similar to those of the catalytic reaction. This certainly led to misinterpretation of supposedly catalytic data in certain cases. Two examp!es will illustrate the role of homogeneous reaction: the oxidative dehydration of propane and the reactions of pentane with oxygen. Burch and Crabb investigated in detail the role of homogeneous and heterogeneous reactions in the oxidative dehydrogenation of propane [97]. The reaction needs a temperature about 130 ~ lower for the catalysed reaction, but the difference depends somewhat on the oxygen/hydrocarbon ratio. The quite unexpected result of Burch and Crabb is that there are similar conversion vs. selectivity relationships for both the homogeneous and most of the heterogeneous reactions [97]. The authors add that even the best catalysts are only as good as no catalyst at all (but at higher temperature in this last case). This could seem pessimistic, but does not exclude that other catalysts could give a decisive advantage to catalysed reactions. A very interesting finding can perhaps modify the vision we have presently of the reaction. In the case of the homogeneous reaction, we found that a partial pressure of water in the feed promotes propane conversion. Fig. 10 shows the dramatic difference [98]. This makes the performance of the homogeneous reaction at a given temperature very close to those of the catalysed reaction at this temperature. An interestiag observation is that the production of byproduct ethylene is very little affected by conversion and almost not at all by the presence of water [98]. Fig. 11 gives propene selectivity as a function of propane conversion [98]. This seems to exceed the performances indicated by Burch and Crabb. It is not yet known whether similar effects could take place in catalysed reactions.

16 ~ .9L.

100

Water added "Dry"

wq.

50

a..

480

530

580 Inlet temperature ~

Figure 10. Influence of water on the homogeneous oxidative dehydrogenation of propane: propane conversion. Quartz reactor: internal diameter 9.3 mm; length of the void zone: 7 cm; Feed: propane, 4% vol; oxygen 9.3% vol; when water added: 15% vol; the balance was helium; flow: 50 cm3.min -1 [98]. 100

Water added

50

~aa aa0 50 100 Conversion (%)Figure 11. Influence of water and temperature on the homogeneous oxidative dehydrogenation of propane: selectivity to propene. Conditions as in fig. 10 [98].

17 A new work based on old patented data and which adds much to the interest of homogeneous oxidation shows that propylene oxide can be formed in certain conditions [99]. With respect to heterogeneous or hetero-homogeneous reactions, a very special system, constituted of lithium hydroxide/lithium iodide melts gives considerably higher propene yields at higher propane conversion than other homogeneous reactions or reaction catalysed by solid catalysts [ 100]. It is therefore very difficult to take without restriction the pessimistic view of Burch and Crabb. But conversely, the last remark in their abstract is certainly very relevant: "A combination of homogeneous and heterogeneous contributions to the oxidative dehydrogenation reaction may provide a means of obtaining higher yields in propene" [97]. Another interesting case is that of n-pentane oxidation. The reaction has been studied in the presence of vanadium phosphate catalysts around 330 ~ [100-103]. Maleic anhydride and phthalic anhydride are produced. It should be mentioned, however, that the homogeneous reaction begins to be significant above 300 ~ (fig. 12). The extent of conversion increases with the oxygen partial pressure [104]. By using reactors with empty spaces of different volumes (lengths), it is possible to evaluate the relative influence of the heterogeneous and homogeneous reaction (table IV) [ 104]. The non-selective homogeneous reaction increases the n-pentane conversion, but the surprising finding is that the maleic/phthalic anhydride selectivity varies substantially. This suggests two conclusions. The first is that the homogeneous reaction can play an important role in the oxidation of n-pentane in the range of temperature where catalysts like VPO are active (around 350-400 ~ The second is that the occurrence of the homogeneous reaction in parallel with the heterogeneously catalysed one might modify selectivity. ~ 5O-20

~ 25

4

200

300

400

500 T ~

Figure 12. Non catalyzed reaction of n-pentane in an empty reactor (quartz; internal diameter 0.93 cm; length of the void zone: 7 cm; the rest of the reactor space is filled with SiC particles); gas feed: n-pentane 1% vol; 02:10 or 20% vol; balance: helium; total flow 30 cm3.min -1 [101].

18 Table IV. Influence of the homogeneous reaction on the oxidation of n-pentane. The reactor was a U-tube (inner diameter 9.3 mm) in which a section of the length indicated in the table was left void. After this section, the reactant flow passed through a frit and the catalyst (0.2 g, bed height 3 mm). The remainder of the tube was f'tlled with carborandum. The catalyst was vanadium phosphate with P/V=l.26, surface area 44 m2.g -1. The gas composition (volume) was: pentane 0.7%; oxygen 20%; helium 5%; balance nitrogen. Total flow 30 cm3.min -1. (Hourly Space Velocity 6000 h-l). T = 375 ~ CTOT is the conversion obtained with the above arrangement (void section + catalyst). The homogeneous conversion CHOM was determined with the same empty section but without the catalyst (replaced by carborandum). SMA and SPA are the selectivities to maleic and phthalic anhydride, respectively [104]. Void section cm CHOM % CTOT % SMA % SPA % 0 0 27 62 34 0.2 12 33 37 20 0.5 27 60 22 6.5 1.5 35 68 20.4 5.0 These results question the validity of many previous results on catalytic oxidation of light alkanes. One should reassess the data concerning the relative reactivity of the various alkanes [105] and selectivity. The general conclusion of this section is that the problem of the selective oxidation of alkanes must unavoidably involve consideration of homogeneous reactions in parallel with the catalysed processes. This is obviously necessary for understanding the phenomena and progressing in the selection of better catalysts. If new processes are the goal of investigations, the interaction between homogeneous and heterogeneous processes must be taken into account. The kinetics will be different. The relative importance of the two kinds of phenomena, homogeneous and heterogeneous, depends necessarily on the shape and size of the catalyst, the form of the reactor, and the overall design of the reactor. Progress in the oxidation of alkane thus needs a comprehensive approach, where catalysis chemists and chemical engineers should work in fight cooperation. 6. CONTROL OF CATALYST ACTIVITY IN ALKANE OXIDATION There are very good reasons to believe that the new phenomena discovered in the selective oxidation of olefins, in oxidative dehydrogenations and the other reactions mentioned in section 4 also occur in the reactions of alkanes with oxygen. This clearly breaks open the way to a better control of these reactions. We have indeed shown that the concept of a control of catalytic activity thanks to the addition of a spillover oxygen donor applies to reactions of alkanes. A conspicuous case is the oxidation of butane to maleic anhydride. We have discovered that a typical oxygen donor, namely a-Sb204, acts synergetically with the VPO phases which are responsible for the reaction [72]. BiPO4, although less good for enhancing selectivity, substantially increases activity. Thermoreduction and thermoreoxidation measurements show that, as in the cases of section 4, the surface oxido-reduction is affected by the presence of a donor [72]. We speculate that spillover oxygen coming from a-Sb204 or BiPO4 protects the special structures necessary for the concerted reaction of butane to maleic anhydride on vanadium pyrophosphate (fig. 1). In a cooperative work of our laboratory with Mamedov and Baidikova, it was also demonstrated, for the first time, that 2-phase catalysts are more efficient than single phase ones in the oxidative coupling of methane [106]. The oxide catalyst contained bismuth and manganese, which can form a well defined phase, Bi2Mn4010. This phase decomposed partially to give a-Bi203 (and a-Mn203) during the catalytic test. Using a catalyst containing

19 mainly Bi2Mn4010 , the C2 yield slowly increased to a plateau in the course of the fast hour of reaction and a-Bi203 was simultaneously formed. A mixture of a-Bi203 and Bi2Mn4010 reached the steady-state activity in a short time, and this activity was higher than in the previous experiment. Higher yields were observed when intimately mixed a-Bi203 and a Bi-depleted phase, Bi2.xMn4010-y were present. This result leads us to speculate: on a possible control of another factor not yet mentioned in this article. Several oxygen species can be present on the surface of oxides: O2", 022", O', 02". Their respective surface concentrations depend on the nature of the oxide, gas partial pressure and temperature. These various species have different reactivities [61-63,77]. It is believed that 02- (nucleophilic) is necessary in aUylic oxidation, and that the other species (electrophilic) are detrimental, by bringing about complete oxidation. On the other hand, some of these electrophilic species are very likely necessary for removing the first hydrogen of the saturated hydrocarbons (oxidation of butane to maleic anhydride and selective reactions of methane with oxygen). We tentatively explain the results concerning methane oxidative coupling by supposing that a-Bi203 and Bi2-xMn4010-y are complementary in providing the fight surface oxygen species. Manganese oxides have a high activity for complete oxidation. This implies that they produce strongly electrophilic species. The presence of bismuth, together with manganese, in Bi2. xMn4010-y should diminish the aggressiveness of the electrophilic species: Bi203 is a good oxygen donor, which produces mild' (i.e., nuc!eophilic) oxygen. The combination could provide the adequate balance of the various oxygen species necessary for the oxidative coupling reaction [107]. Recent results of our laboratory also show that the kind of concepts we are developing applies to other reactions of alkanes. We selected the oxidative dehydrogenation of propane to propene. Based on previous investigation with pure magnesium phosphate phases [33], we mixed a-Sb204 with the pyrovanadate (Mg2V2OT, written here MgV2/2 in short) and the orthovanadate (Mg3V208 or MgV3/2). According to cases, the yield or the selectivity are improved [108]. If we refer to the remote control concepts and the various effects that spiUover produces, we can interpret the results in the following way: 9 spillover oxygen produced by a-Sb204 essentially creates additional sites of approximately the same selectivity (probably the same geometry) on magnesium pyrovanadate MgV2/2. 9 this spillover oxygen modifies favorably the selectivity of surface sites on magnesium orthovanadate MgV3/2 (probably by slightly modifying the surface structure). If we reason in this way, we may conclude, by reference to the donor-acceptor scale shown in fig. 5, that MgV 3/2 behaves as a typical acceptor, because its selectivity is increased by spillover oxygen. Along this line, MgV 2/2 should have a lesser degree of acceptor character and more of a donor character. If this was correct, mixing MgV 2/2 with MgV 3/2 would lead to a syngergetic effect. This is what we observe: the selectivity gets enhanced [ 109]. A similar reasoning had led us to the prediction that two VPO catalysts with different P/V ratios could act synergeticaUy in butane oxidation to maleic anhydride, and this was also verified [71]. Concluding, it seems that the concepts concerning cooperation between phases and the role of spillover oxygen can be extended to the field of selective reaction of light alkanes with oxygen. But the control is more subtle, because more reactive oxygen species are necessary. The challenge, for producing useful molecules from saturated hydrocarbons and oxygen, is to avoid complete oxidation to CO2 and H20. It seems that electrophilic species are necessary for the first step, probably the removal of the first hydrogen from the saturated molecule. But there should not be too large a quantity of these species on the surface, and their reactivity should not be excessive (O2-, O22-, O have certainly different electrophilicity and different reactivities). These electrophilic species are probably detrimental for the subsequent steps of the reaction. Then, nucleophilic species are necessary. They may be necessary just for diminishing the concentration of the harmful electrophilic species through mutual competition for sites on the surface. They are very likely necessary, as in the cases mentioned in section 4, for maintaining

20 the adequate oxidation state of the surface and, consequently, avoid the destruction of the arrangement of surface atoms demanded by the concerted mechanism necessary for selective reaction. They may also be necessary as reactant for certain steps. Although the demands concerning the active oxygen species seem conflicting, the experimental conditions can be selected to achieve a compromise. The oxidation of butane to maleic anhydride, widely industrialized now, shows that this compromise can be achieved and lead to economically attractive processes. Fortunately, experimental conditions do not constitute the only control parameter. SpiUover of oxygen can play a crucial role. This is what is observed in the examples mentioned above. Spillover takes place from an adequate donor to the active phase (or acceptor) namely VPO or MgVO in the case of butane or propane reactions, respectively, or possibly Bi2.xMn4010-y for methane coupling. In this context, the present situation suggests that research should be directed in priority along two lines. The first one would be to detect solids which, under given conditions, can develop the active and selective surface structures (the equivalent, for other reactions, of the inverted flat square pyramids necessary for butane oxidation to maleic anhydride). The second one would be to understand what kind of solids may generate the adequate spillover species in good proportion at adequate temperatures. The scales presented at the end of section 4 seem to concern essentially donors of nucleophilic spillover oxygen 02-. It can be expected that more ionic solids would produce more electrophilic species [63]. The higher the temperature necessary for the reaction, the more ionic will be the donors necessary for achieving the good balance of oxygen species.7. PROSPECTS: COMPREHENSIVE APPROACH TO F U N C T I O N A L I Z A T I O N OF L I G H T ALKANES BY R E A C T I O N OXYGEN

THE WITH

Letting light alkanes react selectively to give valuable products is one of the main goals of petroleum chemistry nowadays. If the selective oxidation of methane is considered, this even appears by far as the most important issue in the very next years. This is clear when remembering that methane represents about one-third of the hydrocarbon resources of the world during this decennia. It is therefore not surprising that all chemists and particularly catalysis chemists have devoted much effort to functionalize methane and the light alkanes. Progress since the industrialization of the butane to maleic anhydride until 1994 has been extremely modest. It is therefore worthwhile to assess critically the approach taken by the various investigators. In the present article, we suggested some critical considerations. But one aspect was almost left aside until now, namely the role of chemical engineering. We shall now attempt to suggest how the various pieces of science are probably assembling together and are progressively unveiling a new, more comprehensive and more realistic approach. The functionalization of light alkanes and particularly their reactions with oxygen necessarily involve, roughly speaking, both the chemical and the chemical engineering aspects (in addition, of course, to economic considerations and the now associated environmental aspects). The chemical approach itself is composed of two distinct but narrowly interconnected lines: the purely catalytic and the homogeneous aspects. The latter is obviously of considerable importance as commented above and proven by the case of methane oxidative coupling. But it is striking that, even in methane coupling, an overwhelming fraction of research has been directed to the discovery of new catalysts (perhaps over 90%) with only a very small fraction trying to take homogeneous phenomena into account. The progress has been deceivingly modest. This has allowed respected scientists, even industrial scientists, to discourage further research on the topic. They were right in mentioning that the results obtained were very far from being economically attractive. But, instead of discouraging research, they should have spurred research, while specifying "on different lines". Among these lines, new developments in chemical engineering were obviously to be considered. The growing importance of chemical

21 engineering is clear in all the field of catalysis, as shown by the overview of the new catalytic processes developed in the world during the 80's [ 110]. It should have been perceived as still more proeminent in selective reactions of alkanes with oxygen, just if one had considered the possibility that homogeneous reactions could occur in conditions identical, or very close to, those of catalysis. It is therefore easy to predict that the research and pre-development work aimed at alkane functionalization using oxygen should incorporate in comparable amount various ingredients. Recent developments announce these changes. These ingredients are: 9 continuation of the approach traditionally taken in catalysis, namely search for new phases able to permit the initial attack of alkanes by some form of oxygen; 9 the new approach described in section 5, considering the role of surface-mobile oxygen in catalysis and the special reactivity of such species when produced by separate phases (donors); 9 the understanding of homogeneous reactions: initiation in the gas phase or on the catalyst surface, propagation in the gas phase, inhibition of propagation thanks to radical trapping on adequate surfaces, etc.; 9 the design and building of new types of reactors and equipment, in order to compensate for poor conversion (if high selectivities are desired), to cope with homogeneous reactions and, probably, to permit extremely fast reactions. In this last section of our contribution, little has to be added concerning the first and second points, which have already been discussed in detail. Following Mamedov [ 111], we wish to underline the role of the reactive atmosphere in selective oxidation. A new result obtained with gold deposited in a proper way on TiO2 supports this assertion [ 112]. The authors show that C3 and C4 hydrocarbons can be selectively oxidised at very low temperatures (50-80~ using simultaneously molecular oxygen and hydrogen: examples are propane to acetone and isobutane to tert.butanol with selectivities of, respectively, 14.6 and 46% (at, understandably, low conversions). In this context, we remark that the role of water (steam) in selective reactions with oxygen has not been given proper attention in general. The role of CO2 should perhaps be also studied. A systematic search for oxides able to donate the appropriate spillover oxygen species at high temperatures is highly desirable. Very recent results certainly reinforce the conclusion that the occurrence of homogeneous oxidation reactions of light alkanes must be considered with attention. The example of ethane discussed above shows that the homogeneous reaction can be as selective, or almost as selective, as the catalysed one [97]. In the reported experiments, the homogeneous reaction was controlled by none of the techniques well known in the field of combustion and radical gas phase reactions (artificial genesis of radicals, trapping of radicals, presence of foreign inert molecules, etc.). Oppositely, the catalysts used for comparison were the result of a selection and some optimisation. This could suggest that simple homogeneous reactions might be the basis of economically viable processes in the future. Other very recent results reinforce the validity of this prospect, like the recent observation that the gas phase reaction of methane with oxygen can give methanol in selectivities exceeding 30% at methane conversion of 5 % [113]. If we now consider the role of chemical engineering, the impressive results of Huff and Schmidt cited above demonstrate that employing a type of reactor not used previously in oxidation and very short residence time can lead to promising prospects [31]. But chemical engineering is not only the science of reactors. It has to consider the whole plant. Two very recent results dramatically demonstrate that integrating recycle and separation features with a catalytic reactor lead to very impressive yield. Tonkovich et al. reached a 50% yield in C2 hydrocarbons in the oxidative coupling of methane using a moving bed reactor, thus permitting a sort of chromatographic separation [114]. The problem indeed is the high reactivity of ethylene compared to CH4. But the reaction of methane to ethylene can be extremely selective at very low methane conversions. Considering these particularities, the group of the University of Patras led by C.G. Vayenas achieved an ethylene yield of 85% (calculated on the carbon contained in CH4) [115]. The key to success is highly selective adsorption of ethylene, ethane and CO2 on a 5A molecular sieve from which they are periodically released. Conversion

22 is kept very low, and the non-reacted methane recycled. It is remarkable that the catalyst is not one of those giving the best performances in conventional reactors. In addition to silver on which oxygen is "pumped" electrolytically, the authors used a Sn203-CaO-Ag catalyst using conventionally molecular oxygen from the gas phase (both catalysts gave approximately the same results). Not only does the reactor concept offer new perspectives, but the composition of the catalyst used suggests other developments. This spectacular result obtained in the most extensively investigated reaction, namely the oxidative coupling of methane, is a clear indication that we are still at the beginning of really innovative research in the functionalization of light alkanes. This is also a clear indication that the discoveries triggering these innovations will involve all the relevant scientific fields, catalysis as well as homogeneous reactions, discovery of new catalysts as well as new che~,ical engineering concepts. Finally, we would like to underline that this article is not intended to be a comprehensive review paper. Many extremely valuable contributions have not been cited. We would particularly like to give credit to the recent papers to be found in references 116 to 123. REFERENCES M. Zhang, J. Liu, Ch. Liu, R. Lan, L. Ji and X. Chen, J. Chem. S,c., Chem. Commun., 19 (1993) 1480. M. Huff and L.D. Schmidt, J. Phys. Chem., 97 (1993) 11815. F. Qiu and Sh. Lu, J. Nat. Gas Chem., 1 (1992) 341. R. Burch and E.M. Crabb, Appl. Catal., 97 (1993), 49. E.M. Kennedy and N.W. Kant, Appl. Catal., 87 (1992) 171. S.A. Tungaratova, B.M. Shingisbaev, A.S. Sass, A. Savereva, K. Zh. Serikpaeva and N.M. Popova, Izv. Akad. Nauk. Resp. Kaz., 4 (1992) 26. M. Loukah, G. Coudurier and J.C. V6drine, Stud. Surf. Sci. Catal., 72 (1992) 191. H.M. Swaan, A. Toebes, K. Seshan, J.G. Van Ommen and J.R. Ross, Catal. Today, 13 (1992) 629. S.J. Conway and J.H. Lunsford, J. Catal., 131 (1991) 513. O.J. Velle, A. Andersen and K.J. Jens, Catal. Today, 6 (1990) 567. S.A. Dzhamalova, I.A. Guliev, S.R. Mirzabekova and A. Kh. Memedov, Neftekhimiya, 29 (1989) 780. S.N. Vereshchagin, L.I. Baikalova and A.G. Anshits, Izv. Akad. Nauk. SSSR, 8 (1988) 1718. X. Zhou, H. Wan and Q. Cal, CN Patent 1069907A (1993). M. Kutyrev, F. Cavani and F. Trifiro, Eur. Patent 544372A1 (1993). J.J. Font, J. Josephus, M. Horward and T. Lomas, Eur. Patent 332289A2 (1989). T. Mazanec and T.L. Cable, GB Patent 2203446A1 (1988). J.H. McCain, Eur. Patent Appl. 166438A2 (1986). J.H. McCain, U.S. Patent 4524236A (1985). A.D. Eastman, J.P. Guillory, Ch. F. Cook and J.B. Kimble, U.S. Patent 4497971A (1985). J.B. Kimble, U.S. Patent 4476344A (1984). A.D. Eastman and J.B. Kimble, U.S. Patent 4450313A (1984). P.R. Blum and E.C. Milberger, U.S. Patent 4410752A (1983). J.H. Kolts, U.S. Patent 4368344A (1983). A.D. Eastman, U.S. Patent 4368346A (1983) Union Carbide, U.S. Patent 4250346A (1981). Union Carbide, Ger. Offen. 2644107 (1977). S.J. Conway, D.J. Wang and J.H. Lunsford, Appl. Catal., 79 (1991) L1. J.R.H. Ross, R.H.H. Smits and R. Seshan, Catal. Today, 16 (1993) 503.

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