o
o o
-J oo a>
ii'iiiiiiiili i
mil • 7 . \ / ^ ^ \ ?lU <o CKINETICS AND MECHANISM OF THE
DIRECT SYNTHESIS OF ORGANOCHLOROSILANES
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECH-NISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS PROF.DR.IR.H.VAN BEKKUM, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET
COLLEGE VAN DEKANEN TE VERDEDIGEN OP WOENSDAG 17 MAART 1976 TE 14.00 UUR
DOOR
M A R I O GUSTAAF ROGER TILLY DE COOKER
SCHEIKUNDIG INGENIEUR
GEBOREN TE SAS VAN GENT
BIBLIOTHEEK TU Delft P 1950 6249
649670
1976 /Q «rO OFFSETDRUKKIRIJ J.H PASMANS- UtN HAAG
CONTENTS
SAMENVATTING 5
SUMMARY 8
1 GENERAL INTRODUCTION 12
Silicones 12 Aim of this study 13
References and notes 15
2 THE REACTION OF A METAL CHLORIDE WITH SILICON AND ITS CATALYTIC AND PROMOTIVE ACTIVITY IN THE DIRECT
SYNTHESIS OF ORGANOCHLOROSILANES 16 2.1 Introduction , 16
2. 2 Exper imen ta1 17 2.2.1 Apparatus 17 2.2.2 Materials 18 2.3 Results and discussion 18
2.3.1 Experimental results 18 2.3.2 Promoters in the direct synthesis 22
2.3.3 Quantitative correlation of the results 22
2.4 Conclusions 23 References and notes 24
3 CADMIUM AS A CATALYST AND PROMOTER IN THE DIRECT
SYNTHESIS OF METHYLCHLOROSILANES 26
3.1 Introduction 26 3.2 Experimental 26
3.2.1 Apparatus 26 3-2.2 Analysis of reaction products 28
3.2.3 Materials used 28 3.2.4 Description of the experiments 29
3.2.5 Experiments performed 29 3.3 Discussion of the results 35
3.3.1 Reaction scheme of the direct synthesis 35 3.3.2 The role of the promoter in the direct
synthesis 36 3.3.3 The catalytic activity of cadmium 38
3.4 Conclusions 38
References 39
4 THE KINETICS OF THE DIRECT SYNTHESIS OF
METHYLCHLORO-SILANES 40 4.1 Introduction 40
4.2 Qualitative derivation of the kinetic model of
the propagation reaction 40 4.3 Experimental results and discussion 43
4.3.1 Quantitative description of the
4.3.2 The activation energy of the
propa-gation reaction 46 4.4 The kinetics of the initiation and termination
reactions 48 4.5 The secondary effects of the promoter CdCl2 51
4.6 Conclusions 54 References and notes 55
5 THE DIRECT SYNTHESIS OF METHYLDICHLOROSILANE AND
DIMETHYLCHLOROSILANE 57 5.1 Introduction 57 5.2 Theoretical aspects of the synthesis 57
5.3 Experimental 58 5.3.1 Experiments performed 58
5.3.2 Materials used and experimental
proce-dure 59 5.3.3 Identification and analysis of
reac-tion products 59 5.4 Results and discussion 60
5.5 Conclusions 63
References 63
6 THE KINETICS OF THE DIRECT SYNTHESIS OF
METHYLDICHLORO-SILANE AND DIMETHYLCHLOROMETHYLDICHLORO-SILANE 64
6.1 Introduction 64 6.2 Derivation of the kinetic model 64
6.3 Experimental 66 6.3.1 Experiments performed 66
6.3.2 Activation and deactivation of the
contact masses 67 6.3.3 The rate of formation of DH and MH if
no hydrogen is added 69
6.4 Results and discussion 69 6.4.1 Experiments in a fluid bed without
hydrogen 69 6.4.2 The activation energy of the DH + MH
synthesis 70 6.4.3 The rate of formation of DH + I-IH at
constant pressure of hydrogen 70 6.4.4 Kinetics of the synthesis of D and DH +
MH at varying pressures of
methyl-chloride and hydrogen 71
6.5 Conclusions 73
References 73
7 THE INFLUENCE OF OXYGEN ON THE DIRECT SYNTHESIS OF
METHYLCHLOROSILANES 74 7.1 Introduction 74 7.2 Experimental 74
7.2.2 Experimental procedure 75
7.3 Results and discussion 76
7.4 Kinetics 79 7.5 Conclusions 82
References 82
8 CADMIUM AS A CATALYST AND PROMOTER IN THE DIRECT
SYN-THESIS OF PHENYLCHLOROSILANES 83
8. 1 Introduction 83 8.2 Experimental 84
8.2.1 Apparatus 84 8.2.2 Materials used 85 8.2.3 Analysis of reaction products 85
8.2.4 Experimental procedure 87 8.2.5 Experimental conditions and experiments
performed 87 8.3 Results and discussion 89 8.4 The catalytic activity of metals other than copper
and cadmium 91 8.5 Conclusions 92
References 92
9 THE KINETICS OF THE DIRECT SYNTHESIS OF
PHENYLCHLORO-SILANES 94 9.1 Introduction 94
9.2 Theoretical aspects of the synthesis of
phenyl-chlorosilanes 94 9.2.1 Reaction scheme 94
9.2.2 Kinetic model 95
9.3 Experimental 96 9.3.1 Experimental procedure 96
9.3.2 Experimental conditions and experiments
performed 96 9.4 Results and discussion 97
9.5 Conclusions 101
References 101
10 THE CONTACT MASS AND ITS SURFACE: INVESTIGATIONS WITH A
SCANNING ELECTRON MICROSCOPE AND X-RAY ANALYSIS 103
10.1 Introduction 103 10.2 Experimental 103 10.3 Discussion of the results 104
10.3.1 Synthesis of methylchlorosilanes 104 10.3.2 Synthesis of phenylchlorosilanes 105
10.4 Conclusions 106 References and notes 106 Legends to figures of chapter 10 108
FINAL REMARKS 1 10
Synthesis of phenylchlorosilanes References and notes
APPENDIX: A GAS FLOW CALORIMETER AS AN INSTRUMENT IN KINETIC MEASUREMENTS
A.1 Introduction A.2 The calorimeter
A.2.1 Description of the calorimeter A.2.2 Properties of the calorimeter A.3 Experimental
A.3.1 The experimental equipment A.3.2 Materials used
A.3.3 Description of the experiments
A.3.4 The heat production in the calorimeter as a function of time
A.3.5 Mass balances References 1 10 111 112 112 112 112 113 1 14 114 115 116 116 118 118 LIST OF SYMBOLS 119
SAMENVATTING.
In dit proefschrift wordt een beeld geschetst van de reactie tus-sen gasvormig alkyl- of arylchloride met metallisch silicium, ook wel de directe synthese van organochloorsilanen genoemd. De synthese is zowel technisch als wetenschappelijk interessant in verband met de fa-bricage van grondstoffen voor de siliconenindustrie. Uitgaande van o.a kwalitatieve en semi-kwantitatieve thermodynamische beschouwingen wordt dit beeld ontwikkeld, waarbij een sterke analogic naar voren komt tussen de processen voor de bereiding van allerlei silanen. Grondgedachte is, dat de synthese bestaat uit een kettingreactie op het oppervlak van het vaste uitgangsmengsel van silicium, katalysator en promotoren, waarbij een voortdurende regeneratie van actieve reac-tie-intermediairen plaats vindt, leidend tot een ongehinderde voort-gang van het proces. Deze regeneratie vindt plaats in een propagatie-stap, waarin door chlooroverdracht van katalysator naar silicium het actieve oppervlakte-intermediair SiCl gevormd wordt.
Zonder toevoeging van een metallische katalysator is de synthese nauwelijks uitvoerbaar. Daar tijdens de synthese door interactie van katalysator met gasvormige reactant metaalchlorides gevormd kunnen worden (hetzij als reactie-intermediair, hetzij als produkt van een chemisorptie- of krakingsreactie) is een ongestoorde synthese slechts mogelijk, indien de chlooroverdracht van het metaalchloride naar al dan niet gesubstitueerd silicium kan plaatsvinden in elke reactiestap. Een noodzakelijke voorwaarde is dan, dat de Gibbs vrije energieveran-dering voor de reactie tussen het metaalchloride en silicium < 0 of slechts zwak positief is.
Voor verschillende metaalchlorides wordt de reactie met silicium besproken. De starttemperaturen van deze reacties worden kwantitatief beschreven door een functie van de dampspanning en de gemiddelde Gibbs vrije vormingsenergie per chlooratoom van het metaalchloride. Tevens wordt de mogelijkheid besproken van beperkte reactie tussen metaal-chlorides en silicium, resulterend in de vorming van SiCln-intermedi-airen en onstabiele subchlorides. Naast de vorming van SiCl in een propagatiestap is dit een tweede vormingswijze van actieve reactie-in-termediairen, hetgeen tevens wordt gezien als de verklaring voor de promotorwerking van bepaalde metaalchlorides. Deze benaderingswijze impliceert voor een metaalchloride tevens de mogelijke overgang van promotor naar katalysator in de buurt van de starttemperatuur van de reactie met silicium. Voor cadmiumchloride is deze overgang inderdaad aangetoond bij 450 °C in zowel de synthese van methyl- als phenyl-chloorsilanen, waardoor dit metaal toegevoegd wordt aan de reeks reeds bekende katalysatoren voor de directe synthese. Er wordt nader inge-gaan op de combinatie koper(katalysator)/cadmiumchloride(promotor) bij de synthese van de methylchloorsilanen. Aan de hand van literatuurge-gevens en eigen experimenten in zowel een fluide als vast bed reactor wordt een reactieschema opgesteld, waarin de functies van de katalysa-tor en promokatalysa-tor duidelijk en logisch naar voren komen. De functie van de katalysator blijkt tweeledig. In de eerste plaats treedt interactie op met de gasfase, resulterend in chemisorptie van de gasvormige reac-tant en overdracht hiervan naar silicium. In de tweede plaats wordt uit het in de laatste propagatiestap gevormde katalysatorchloride het
nieuwe actieve intermediair SiCl gevormd. Deze tweede functie kan ge-heel of gedeeltelijk worden overgenomen door promotoren zoals CdCl2 en ZnCl2, waardoor de concentratie aan actieve reactie-intermediairen sterk toeneemt. Terminatie treedt op door reactie van intermediairen onderling; initiatie door vorming van nieuwe intermediairen via b.v. krakingsreacties. De ongehinderde voortgang van de propagatiereactie in aanwezigheid van promotoren uit zich onder meer in een hoge omzet-tingsgraad van silicium en een grote reactiesnelheid. Het reactiesche-ma wordt verder uitgewerkt tot een kinetisch model. Aangenomen wordt, dat de oppervlaktereactie tussen gechemisorbeerd methylchloride en een reactie-intermediair snelheidsbepalend is; de chemisorptie van methyl-chloride wordt beschreven als een Langmuir adsorptie op een adsorptie-plaats. Het resultaat is een snelheidsvergelijking, waarin de promotor-werking zich manifesteert als een ordeverlagend en reactiesnelheids-verhogend effect. De verschillende parameters in het kinetisch model worden bepaald met behulp van kwantitatieve experimenten, waarbij ge-bruik gemaakt wordt van een gasdoorstromingscalorimeter. Aandacht wordt verder besteed aan de verschillen in schijnbare
activeringsener-gie indien weinig of veel promotor aanwezig is en aan de kinetiek van de initiatie- en terminatiereacties. De experimenten zijn verricht bij een totaaldruk van 1 atmosfeer met koper als katalysator en ren tussen 310 en 340 °C. Voor cadmiumchloride is bij deze temperatu-ren de vorming van de niet stabiele katalysator uit de promotor aange-toond, hetgeen tevens bepaalde afwijkingen tussen kinetische experi-menten en model verklaart. Het afgeleide kinetisch model beschrijft de synthese belangrijk beter dan de tot nu toe in de literatuur gebruikte modelien.
Met behulp van de in de eerste hoofdstukken vergaarde kennis wordt vervolgens de directe synthese van methylchloorwaterstofsilanen nader onderzocht. De synthese van deze potentieel belangrijke grondstoffen voor de produktie van gemodificeerde siliconen met behulp van een HCl/ CH3CI of H2/CH3CI gasmengsel was in principe wel bekend, maar ging steeds gepaard met de vorming van grote hoeveelheden bijprodukt. Een reactieschema voor de vorming van Si-H bevattende verbindingen is nu uitgewerkt en de condities zijn vastgesteld, waarbij de hoogst moge-lijke opbrengst aan Si-H verbindingen gepaard gaat met een geringe vorming van bijprodukten. De synthese wordt uitgevoerd in een fluide bed met koper als katalysator, geringe hoeveelheden promotor en een gasmengsel bestaande uit waterstof en methylchloride. Bij een totaal-druk van 1 atmosfeer en temperaturen tussen 320 en 370 °C wordt een maximale opbrengst van ongeveer 80 mol% DH+MH verkregen. Later wordt, uitgaande van het reactieschema, een kinetisch model ontwikkeld voor de Si-H sjmthese. De overall reactiesnelheid kan gesplitst worden in verschillende, onafhankelijk van elkaar te beschrijven reacties, waar-van de D-vorming en de DH+MH-vorming de belangrijkste zijn. Ook de ac-tivering van de contactmassa kan gesplitst worden in acac-tiveringsfunc- activeringsfunc-ties behorende bij de verschillende vormingsreacactiveringsfunc-ties. De DH+MH-vor-mingsreactie wordt beschreven als een snelheidsbepalende stap op het oppervlak tussen gechemisorbeerd methylchloride en een Si-H binding bevattend reactie-intermediair; dit laatste wordt gevormd door reactie van H2 met o.a. CuCl en silicium, waardoor zowel rechtstreeks een Si-H
intermediair als via HCl een HSiCl of HSiCl2 intermediair ontstaat. De voor de afzonderlijke reacties bepaalde kinetische parameters zijn volledig in overeenstemming met de waarden daarvoor verkregen. Bij de experimenten worden de gassen zorgvuldig ontdaan van zuurstof, waarvan de storende werking kwantitatief wordt beschreven. Aanwezigheid van zuurstof heeft een sterk remmend effect op de reactiesnelheid en ver-mindert de converteerbaarheid van silicium.
De analogie tussen de synthese van phenylchloorsilanen en methyl-chloorsilanen met koper als katalysator en cadmiumchloride als promo-tor komt in het onderzoek zeer duidelijk naar voren. Verschillen in reactiegas (chloorbenzeen in plaats van methylchloride) en hogere tem-peraturen geven bij de synthese van phenylchloorsilanen echter aanlei-ding tot een moeizame vorming van een hoge concentratie aan actieve intermediairen en een grote hoeveelheid bijprodukten. Toepassing van een fluide bed is niet mogelijk vanwege het uitblazen van CdCl2. Ook in een vast bed verloopt de desactivering snel ten gevolge van o.a. verdamping van Cd en CdCl2 en de vorming van de intermetallische ver-binding Cu2Cd; zonder aanwezigheid van voldoende hoeveelheden promotor is de synthese niet mogelijk. Een reactieschema en kinetisch model worden opgesteld en met kinetische experimenten getoetst bij een to-taaldruk van 1 atmosfeer en temperaturen tussen 380 en 420 °C. Het ki-netisch gedrag blijkt volkomen vergelijkbaar met de kinetiek van de methylchloorsilanensynthese met CdCl2 als promotor. Over de al dan niet goede katalytische eigenschappen van zilver in de directe synthe-se van phenylchloorsilanen kan geen uitsluitsynthe-sel gegeven worden.
Informatie over de oppervlaktestructuur van contactmassa's als functie van o.a. omzettingsgraad van silicium is verkregen door middel van rasterelectronenmicroscopie. Op plaatsen waar Cu-katalysator op het siliciumdeeltje aanwezig is, treedt tijdens reactie een sterke vergroting en verruwing van het oppervlak op. Vooral in het eerste
stadium van de synthese breidt de actieve fase zich snel uit over het siliciumoppervlak. Ten gevolge van de opeenhoping van verontreinigingen wordt de actieve laag uiteindelijk bedekt door een niet reagerende
in-actieve laag. Afsplitsing van zowel in-actieve als inin-actieve lagen treedt op onder inwerking van CdCl2; een sterke reactivering van oude con-tactmassa's is hierdoor mogelijk. Bij concon-tactmassa's afkomstig van de synthese van phenylchloorsilanen werd met behulp van rontgenanalyse de intermetallische verbinding Cu2Cd aangetoond; ook door middel van energiedispersieve analyse met behulp van de rasterelectronenmicros-coop werden Cu/Cd-concentraten aangetoond op het oppervlak van de con-tactmassadeeltjes.
Met behulp van de beschreven achtergronden van de directe synthe-se van de methyl- en phenylchloorsilanen, de reactieschema's en de daaruit afgeleide kinetische vergelijkingen is een voorspelling moge-lijk van de realiseerbaarheid van de synthese van een bepaald produk-tenpakket. De optimale procescondities kunnen daarbij worden aangege-ven.
SUMMARY.
In this thesis the reaction of elementary silicon with a gaseous alkyl- or arylchloride is described. The reaction, which is called the direct synthesis of nrganochlorosilanes, is the most important process for the synthesis of the basic materials for the manufacture of sili-cones. The synthesis consists of a chain reaction on the surface of a solid mass composed of silicon, a catalyst and promoters and is char-acterized by a continuous regeneration of active reaction intermedi-ates (active centres) of the type SiCl. Regeneration of active centres occurs by transfer of chlorine from the catalyst to silicon and leads to the unimpeded progress of the synthesis. Starting from qualitative and semi-quantitative thermodynamic con'siderations, a sketch is pre-sented of the theoretical backgrounds of this chain reaction, whereby a strong analogy appears between the synthesis of various silanes.
Without addition of a metallic catalyst, reaction between the gaseous reactant and silicon is sluggish and is accompanied by the formation of many by-products. For these reasons a catalyst (usually copper) is always added to silicon in order to increase the reaction rate as well as the selectivity. Interaction of the catalyst with the reaction gas inevitably leads to the formation of metal chlorides in the form of reaction intermediates or cracking products. Unhampered reaction of the gaseous reactant and silicon occurs only if transfer of chlorine from metal chloride to (substituted) silicon is possible in every reaction step. In that case the Gibbs free energy change of the reaction of the metal chloride with silicon is less than zero or only slightly positive.
The threshold temperatures of the reactions between metal chlo-rides with silicon are quantitatively described. The formula is a function of the vapour pressure and the average Gibbs free energy of formation per chlorine atom of the metal chloride. The possibility of restricted reaction of metal chlorides with silicon is also discussed; this reaction leads to the formation of SiCl intermediates and unsta-ble metal subchlorides. In addition to the formation of SiCl interme-diates in a propagation step, this reaction serves as a parallel path for the formation of active centres and, as such, explains the promo-tive activity of metal chlorides such as ZnCl2 and CdCl2. Restricted reaction at certain conditions implies that the possibility exists for a metal chloride to change from a promoter to a metallic catalyst at or nearby the threshold temperature of its reaction with silicon. For CdCl2 this change is demonstrated at 450 °C in the synthesis of methyl-and phenylchlorosilanes.
In the synthesis of methylchlorosilanes attention is paid to the combination copper(catalyst)/cadmium chloride(promoter). From litera-ture data and from own experiments performed in a fluidized and fixed bed reactor a reaction scheme is developed. The role of the catalyst is twofold. In the first place, interaction with the gas phase occurs which results in chemisorption of the gaseous reactant on copper and transfer of the chemisorbed species to silicon. In the second place, active centres of the type SiCl are continuously regenerated by
reac-tion of the catalyst chloride with silicon in the last propagareac-tion step. This second role can be partially taken over by promoters, resulting in a further increase of the concentration of active centres on the surface of the contact mass. The unimpeded propagation reaction in the presence of promoters manifests itself via a high degree of conversion of silicon and a high reaction rate. Termination occurs by mutual reaction of active centres. Initiation (formation of active centres) occurs via cracking reactions for example.
From the reaction scheme a kinetic model is developed. The sur-face reaction of chemisorbed methyl chloride with an active centre is the rate determining step. It is assumed that chemisorption of methyl chloride proceeds via a Langmuir type adsorption on one surface site. In the rate equation the influence of promoters appears as a rate increasing and reaction order decreasing effect. The parameters in the kinetic model are quantitatively determined from kinetic experiments with a gas flow calorimeter. Attention is also paid to the apparent activation energy in experiments with different amounts of promoter and to the kinetics of the initiation and termination reactions. The experiments are conducted with copper as a catalyst at a total pres-sure of I atmosphere and temperatures of 310 - 340 °C. At these tem-peratures the formation of the unstable Cd-catalyst from the promoter CdCl2 is demonstrated, which accounts for at least part of the devia-tions occurring between the predicdevia-tions of the kinetic model and the experimental results. The kinetic model, presented in this thesis, is superior to the models proposed in the literature because it describes the reaction in a wider range of synthesis conditions.
The backgrounds of the synthesis being known from the previous chapters, emphasis is placed upon the synthesis of methylchlorosilanes containing a silicon-hydrogen bond. It is known that these substances, which are of importance for the production of certain classes of sili-cones, can be produced by reaction of a HCl/MeCl or H2/MeCl gas mix-ture with a Si/Cu contact mass. Large amounts of highly chlorinated by-products are, unfortunately, simultanuously formed.
A reaction scheme for the synthesis of Si-H containing products is developed, and the conditions are established at which a high pro-duction of these substances occurs with the formation of only minor amounts of by-products. The synthesis is conducted in a fluidized bed with copper as a catalyst, a low concentration of promoters and a gas mixture composed of methyl chloride and hydrogen. At temperatures of 320 - 370 °C and a total pressure of I atmosphere a maximum yield of about 80 mole-% DH+MH* is obtained.
From the reaction scheme a kinetic model for the synthesis of DH+MH* is developed. It is shown that the kinetics of the separate reactions which constitute the overall rate of production, i.e., the
reactions leading to the formation of D*, DH+MH and M+T** can be des-cribed independently. The overall activation of the contact mass dur-ing the synthesis can also be described by means of separate activa-tion funcactiva-tions corresponding to the separate reacactiva-tions. Formaactiva-tion of DH+MH proceeds via reaction of chemisorbed methyl chloride with an active centre; this reaction is rate determining. Active centres of the type SiH and HSiCl2 which are responsible for the formation of MH and DH, respectively, originate on the surface by reaction of H2 with, for example, CuCl and silicon. The kinetic parameters for the separate reactions agree with those obtained in previous chapters.
In the experiments the reaction gases are purified from oxygen, whose detrimental influence is investigated and quantitatively des-cribed. The presence of oxygen has a retarding influence on the reac-tion rate and decreases the degree of conversion of silicon.
From the experiments a clear analogy appears between the synthe-sis of methyl- and phenylchlorosilanes. In the synthesynthe-sis of phenyl-chlorosilanes, however, formation of active centres is far more diffi-cult than in the synthesis of methylchlorosilanes, resulting in the formation of a large amount of by-products and, if the synthesis is carried out without a promoter, also in a very low reaction rate. If CdCl2 is used as a promoter, it is not possible to conduct the synthe-sis in a fluidized bed because of the rapid loss of CdCl2 from the reactor. In a fixed bed deactivation also occurs rapidly as a result of the evaporation of CdCl2 and Cd and the formation of the inactive intermetallic compound Cu2Cd. A reaction scheme and kinetic model are developed, and the kinetic parameters are determined from experiments at a total pressure of 1 atmosphere and temperatures of 380 - 420 °C. The kinetics of the synthesis of phenylchlorosilanes with Cu as a catalyst are completely analogous to the kinetics of the synthesis of methylchlorosilanes. No decisive answer can be given concerning the catalytic activity of silver, however.
With a scanning electron microscope information is obtained con-cerning the surface structure of contact masses as a function of sili-con sili-conversion, type of promoter, etc.. During the synthesis, and especially in the first hours of reaction, the active Si/Cu surface spreads rapidly over the silicon particle, causing a great increase in the original specific surface of the contact mass. By accumulation of contaminants and cracking products, however, the active surface is eventually covered with an inactive layer. If CdCl2 is added as a pro-moter spalling of both active and inactive surface layers occurs. A strong reactivation of inactive contact masses is thus possible. By means of X-ray analysis it was found that in contact masses used for the production of phenylchlorosilanes, the intermetallic compound
* D = dimethyldichlorosilane
Cu2Cd occurs. By means of energy dispersive analysis with the scanning electron microscope, Cu/Cd concentrates on the surface of these con-tact masses were also observed.
With the knowledge of the mechanism of the direct synthesis of methyl- and phenylchlorosilanes, and with the resulting reaction schemes and kinetic equations known, the eventual realisation of a desired product mixture can be predicted. As such this dissertation serves to determine the optimum process parameters for the direct syn-thesis of organochlorosilanes.
1 GENERAL INTRODUCTION.
Organochlorosilanes, intermediates in the manufacture of sili-cones, are produced industrially by the direct synthesis which was independently discovered by Rochow [I] and Miiller [2] in the early 1940's. The process consists of the reaction of an alkyl (or aryl) chloride with a solid mass composed of silicon, a catalyst (usually copper) and promoters. The reaction is carried out in a fixed or flu-idized bed at high temperatures (250 - 600 °C, depending upon the nature of the gaseous reactant and desired reaction products) and pressures up to about 5 atm. The chief reaction products are dimethyl-dichlorosilane in the synthesis of methylchlorosilanes and diphenyldi-chlorosilane and trichlorophenylsilane in the synthesis of phenylchlo-rosilanes. Numerous by-products are also formed such as trichloro-methylsilane, trimethylchlorosilane, methane, benzene, etc. For a detailed description of the process the reader is referred to [3,4,5].
Silicones.
The most important and commercially most successful application of organochlorosilanes is their use as the starting materials for the production of silicones. Silicones are polymeric substances with an inorganic - Si-0 - skeleton and are characterized by, for example, excellent heat stability, high resistance to oxidation and unique sur-face properties such as strong water repellency. Silicones are prepared from the organochlorosilanes by hydrolysis and subsequent condensation of the silanols. Depending upon the number of chlorine atoms bound to silicon, dimers and linear or branched polymers are formed via, for example, reaction 1-1:
CH3
I
n(CH3)2SiCl2 + n H2O -*• 2n HCl + f Si - 0 }„ (1-1) CH3
Owing to the variety and wide variations of their properties, the silicones are employed in a wide range of functions in the chemical, automobile, aeronautical and numerous other industries in the form of silicone fluids, emulsions and compounds, resins, chemical intermedia-tes and elastomers. For an extensive survey of silicones, their proper-ties and applications the reader is referred to [4].
Due to their high price, world consumption of silicones is small as compared to many other synthetic polymers. The total world consump-tion of silicones in 1973, together with the world principal silicone producers, is shown in Table 1-1 [6]. For some silanes the import-prices for the Netherlands in 1973 are listed in Table 1-2 [7]. Con-cerning the world production of silicones, no data are available. The average price of silicone end-products in the years 1972-1975 can be estimated at about Dfl. 15,—/kg [8,9,10]. From Table 1-1 the total world production in 1973 can then be estimated at about 110,000 ton/ year. Silicone markets are growing at about 8%/year. It is expected
Table 1-1 World consumption of silicones in 1973 and principal silicone producers.
Consumption, Dfl. * 10~^ Principal producers
West Germany
France
United Kingdom
Others Western Europe United States Japan Other countries 168 108 84 102 840 240 60 1602 Bayer, Wacker, Goldschmidt Rhone-Pou lenc ICI General Electric, Dow Corning, Union Carbide, Stauffer Toshiba, Shin-Etsu Toray
Table 1-2 Import-prices of some silanes for the Netherlands in 1973.
Dfl/kg Dimethyldichlorosilane Trimethylchlorosilane Si lane Tetramethylsilane 3.20 6.40 30.--600.—
that 1981 capacity will have to be 80% greater than 1974 levels [21].
Aim of this study.
Notwithstanding the great scientific effort to elucidate the fundamentals of the direct synthesis of organochlorosilanes, little is as yet known concerning the actual mechanism by which the reaction proceeds. Various theories concerning the reaction mechanism have been put forward [11]. Hurd and Rochow [12] proposed that chemisorption of methyl chloride (and other organic chlorides) takes place on two copper
atoms. The formation of the reaction products then proceeds via chlo-rination of silicon by CuCl and subsequent methylation of the substi-tuted silicon by methyl radicals. Klebanskii and Fikhtengol'ts [13,14] assumed that dissociative chemisorption of methyl chloride takes place on the Si/Cu surface of the contact mass, the chlorine atom being attached to silicon and the methyl group to copper. Trambouze [15], Bazant [16] and Voorhoeve [17] also assumed a dissociative chemisorp-tion, the only difference being the fact that the chlorine atom is attached to copper and the methyl group to silicon. A more satisfactory explanation of the mechanism of the formation of organochlorosilanes
was put forward by Van Dalen [18]. Assuming that chemisorption of methyl chloride takes place on one Cu atom with chlorine attached to copper, selective formation of dimethyldichlorosilane takes place via a chain reaction on the surface with continuous regeneration of SiCl-intermediates. In recent years also Golubtsov [19] promulgated the idea that the synthesis proceeds via long chains on the surface. With-out examining the mechanism in detail, he assumed that in the formation of the end-products reaction intermediates of the type SiCl^ are
involved.
Owing to the complex nature of the reacting system, all of the above mentioned researchers can explain only some typical features of the synthesis in a limited range of synthesis conditions. The complex behaviour stems for a great part from the fact that the solid reactant consists of silicon particles with a metallic catalyst on the surface. During reaction silicon is continuously withdrawn from the particle's surface. Consequently, the composition of the reacting solid as a whole, and in particular the composition and structure of the active surface, changes appreciably as the reaction proceeds, and a constant reaction rate is therefore never achieved. Various researchers took this very difficult experimental problem into account by, for example, performing their measurements in a fairly stable region of the synthe-sis or by comparing the results of different experiments at a well defined reaction time (see, for example, [3]). In spite of these pre-cautions considerable scatter of the experimental results is still often encountered, which makes the interpretation of the data extremely difficult. Furthermore, a reliable comparison of the results of differ-ent researchers is often impossible because, in compatible experimdiffer-ents, the various reaction parameters such as the purity of the starting materials, the amount of catalyst and promoters used, the reaction tem-perature and the use of a fixed, stirred or fluid bed reactor etc. are rarely similar. Sometimes, unfortunately, the results are (or, at
least, seem to be) even contradictory.
In consequence of the above mentioned reasons, the direct synthe-sis of organochlorosilanes is still little understood and is often more an art rather than a scientifically well-founded working method: "Die erfolgreiche technische Durchfiihrung der Synthese ist, wie meistens bei Reaktionen im heterogenen System fest/gasformig, eine Angelegenheit der Erfahrung und der peinlichen Einhaltung sehr spezieller Arbeitsbedin-gungen" [4, p. 26]. The aim of this study is to change this unsatisfac-tory empirical approach to a more scientifically based approach by carefully studying the synthesis of silanes at well defined reaction conditions.
Accepting the idea of a chain reaction on the surface of the con-tact mass, the results of the authors' own experiments, in which emphasis is placed upon the kinetics of the synthesis of methyl- and phenylchlorosilanes, are compared with those of other investigators and critically discussed. In the experiments special attention is paid to the various factors which greatly influence the course of the syn-thesis and obscure correct interpretation of the data. Among these factors, the varying activity of the contact mass and the influence of oxygen, which is always present in the reaction gases as a contaminant,
are investigated. Because other researchers often paid little atten-tion to these factors, it is sometimes necessary to repeat experiments already described in the literature for reasons of comparison with own experiments.
In the experiments the use of a gas flow calorimeter is intro-duced as an instrument for measuring the kinetics of the synthesis of methylchlorosilanes. With the calorimeter various striking features of the direct synthesis can be observed which would go unobserved in other measuring techniques.
References and notes.
1. E.G. Rochow, J. Am. Chem. Soc., 67 (1945) 963. 2. R. Miiller, D.R.P. Anm. C57411 (1942).
3. R.J.H. Voorhoeve, Organohalosilanes: Precursors to Silicones, Elsevier, Amsterdam, 1967.
4. W. Noll, Chemie und Technologie der Silikonen, Verlag Chemie, Stuttgart, 1968.
5. C. Eaborn, Organosilicon Compounds, Butterworths Ltd., 1960. 6. Chimie-Actualites, febr. 27, 1974, p. 40.
7. Maandstatistieken CBS, 1973. 8. Soc. Elast., August 1972, p. 3,4. 9. Elec. News, Sept. 9, 1974, p. 56. 10. Chem. Week, 13 (1975) 21.
11. For an extensive discussion of the mechanisms, proposed in the literature, the reader is referred to [3].
12. D.T. Hurd and E.G. Rochow, J. Am. Chem. S o c , 67 (1945) 1057. 13. A.L. Klebanskii and V.S. Fikhtengol'ts, J. Gen. Chem. USSR, 26
(1956) 2795.
14. V.S. Fikhtengol'ts and A.L. Klebanskii, J. Gen. Chem. USSR, 24 (1957) 27.
15. P. Trambouze, Bull. Soc. Chim. France, (1956) 1756.
16. J. Joklik, M. Kraus and V. Bazant, Coll. Czech. Chem. Comm., 27 (1962) 974.
17. R.J.H. Voorhoeve, Thesis, Delft, 1964. 18. M.J. van Dalen, Thesis, Delft, 1971.
19. S.A. Golubtsov, K.A. Andrianov, N.T. Ivanova, R.A. Turetskaya, I.M. Podgornyi and N.S. Fel'dshtein, Zh. Obshch. Khim., 43 (1973) 2000.
20. V. Bazant, Pure and Applied Chem., (1969) 473. 21. Chemical Week, Nov. 5, 1975, p. 42.
2 THE REACTION OF A METAL CHLORIDE WITH SILICON AND ITS CATALYTIC AND PROMOTIVE ACTIVITY IN THE DIRECT SYNTHESIS OF ORGANOCHLOROSILANES.
2.1
Introduction.
The direct synthesis of organochlorosilanes involves the use of catalysts which are present on the silicon surface as a metal (Ag, Cd) or as a component in an intermetallic compound (Cu in n-phase [1]). The action of the catalyst involves an interaction of the metal with the gas phase followed by chemisorption of the alkyl (or aryl) chloride and subsequent chlorination or alkylation (or arylation) of the silicon or other reaction intermediates present on the surface. It has been sug-gested that the reaction should be regarded as a chain reaction on the contact mixture surface, starting at or proceeding via active reaction centres which are continuously regenerated during the synthesis [2,3]. The active reaction centres may be unstable chemical intermediates of the type SiCln, RmSiCln or RmSi (n + m s 3) [30]. If the interaction of the metal with the gas phase results in a dissociative chemisorption of the gaseous reactant, or if cracking of the organic ligand takes place, then the metal chloride can be formed on the contact mixture surface. An example of the latter behaviour is illustrated in the synthesis of methylchlorosilanes with copper as a catalyst:
2 (MeCl)Cu •+ 2 CuCl + CH4 + H2 + C (2-1)
Furthermore, the metal chloride may be present on the surface as a reaction intermediate in the chain reaction. During the synthesis the reaction must not be interrupted at any stage, and this means that there must be the possibility of reaction between the metal chloride and the (substituted) silicon in every reaction step. In such a case the overall Gibbs free energy of reaction (AGj-) of reaction 2-2 should preferably be negative or at least only slightly positive (MCljj = metal chloride with n chlorine atoms bound to the metal).
m MCln + 2LI1 Si -* m M + 2LIlsici4 (2-2)
One has to keep in mind, however, that this is only a necessary condi-tion for the occurrence of reaccondi-tion and not a guarantee that reaccondi-tion will take place. Other factors, such as particle size of the reactants, can also influence the reaction kinetics; also, for one or several reaction steps, the free energy change may be far greater than zero, thus inhibiting the further course of the reaction. Summarising, every reaction step of the following type (equations 2-3 and 2-4) must be thermodynamically and kinetically possible in order to guarantee a rea-sonable reaction rate for the direct synthesis:
MCln + Si -> MCln-l + SiCl (2-3)
^ ^ i ^ ^ M c l : ; . , - 2 sici2 + ; ; ^ ; - ; (2-4)
con-tact mixture surface [28] has been demonstrated elsewhere [4,5]. Little is known about the reactions of a metal chloride and sili-con. It is, under the conditions of the investigation, a solid/solid or a solid/liquid reaction which may be accelerated by the transport of the metal chloride to the silicon via the gas phase. For these tions there is a reaction threshold temperature above which the reac-tion proceeds very rapidly, sometimes even violently. It should be emphasised that this threshold temperature is not the temperature of the beginning of the reaction but the temperature at which a marked rate of reaction can be observed [6,7]. The threshold temperature of a given system of reactants may depend on the history of the system, impurities in the reacting substances, impurities in the gas phase, etc. [6]. Furthermore, there is the complication that the oxide layer, always present on the silicon, may hamper the start of the reaction
[8].
Of the reactions under investigation, most is known about that between CuCl and silicon, because this reaction can be used to prepare a reactive contact mixture for the synthesis of silanes [1]. The reac-tion threshold temperature varies from 180 to 400 oc [7,9,10,11]; the usual temperature, using fresh CuCl and performing the reaction under a nitrogen atmosphere, is 300 °C [II]. Furthermore, the reaction thresh-old temperatures of PbCl2 and SnCl2 with silicon have been found to be about 360 and 350 oC, respectively [12]. Copper, lead and tin are known to be catalytically active in the direct synthesis of methylchlorosila-nes, each catalyst yielding a different main product [12]. Therefore, in order to find a theoretical basis for the choice of a catalyst for the direct synthesis of silanes, it was decided to investigate the reaction of some metal chlorides with silicon.
2.2 Expevimental.
2.2.1 Apparatus.
The equipment shown in Fig. 2-1 was used. Two thermocouples, one in the spherical shaped glass reactor A, the other in the reference reactor B, are connected in opposing polarity. The temperature of the system was measured by another thermocouple situated in reactor B. By measuring the heating curve of the system, the threshold temperature of an exothermic reaction can be detected accurately because the spherical shape of the reactor prevents the heat of reaction from dissipating easily, and a peak will occur in the thermogram on the recorder. The threshold temperature of an endothermic reaction is more difficult to determine in this way because the negative heat effect causes the reac-tion to slow down. When the heat producreac-tion of a reacreac-tion was negative, the reaction threshold temperature was operationally defined as the temperature at which the first reaction products appeared in the CO2/ acetone-cooled product vessel. Heating of the reactors was achieved using a fluidised bed of silicon. The reaction products were removed from the reactor with dry oxygen-free nitrogen.
Fig. 2-1 Apparatus for measuring the temperature of the start of the reaction between a metal chloride and silicon. A = reactor; B = reference reactor; C = fluidised bed of silicon; N2 = stream of purified nitrogen; a = product stream to product vessel; b = millivolt signal to recorder; Tc = thermocouple for temperature measure-ment.
2.2.2 Materials.
The silicon used in the experiments was technical grade. Its com-position is described in 3.2.3. Unless indicated otherwise, the parti-cle size of the silicon consisted of the sieve fraction 50 - 75 ym. The reference reactor was filled with silicon (10 - 15 g ) , the reactor itself was filled with 10 - 15 g of the silicon/metal chloride mixture. The metal chlorides used were fine powders (particle diameter < 50 ym) and were chemically pure (Merck). If necessary they were dried. CuCl was prepared from CuSO^, Cu and HCl; AgCl from AgN03 and HCl. Before heating, the mixture was dried at 50 - 100 °C during 16 h under a stream of purified nitrogen.
2.3 Results and discussion.
2.3.1 Experimental results,
To indicate the accuracy of the experiments some measurements of melting points are given in Table 2-1; there is good agreement with published values. Only in the case of ZnCl2 does disagreement exist. However, melting points of ZnCl2 ranging from 262 to 330 °C can be
found in the literature [15]. Some measurements of melting points are shown in Fig. 2-2.
Table 2-1. Measurements of melting points of metal chlorides.
MCl^ AgCl SnClj ZnClj CuCl SnClj * R e f . 13 ""n (g) 2.5 3.0 4.2 1.7 ,.7' b Ref Si (g) 10.0 10.0 10.0 10.4 14. dT/dt (°C/i«in) 2.5 0.9 2.0 2.1 M.p. (°C) 455 246 294 170 Lit. M.p. (°C) 454' 246* ^83" 172"
The most relevant data are summarised in Table 2-2. Some heating curves are shown in Fig. 2-3. RbCl2 and ZnCl2 were also investigated, but because the free energy change of reaction in the temperature region up to 500 °C is greater than zero, no reaction took place, as expected.
Fig. 2-2 Heating curves of SnCl2 and a mixture of SnCl2 and CuCl. a = SnCl2; b = SnCl2 + CuCl.
Under the conditions applied the chlorides of copper, CuCl and CuCl2, start reacting with silicon at about 300 oc. Mixing with SnCl2 reduces this temperature for CuCl to 250 °C, the lowest threshold tem-perature being 180 oc [9]. The product mixture resulting from the
reac-tion of the copper chlorides with silicon does not consist solely of SiCl^; appreciable quantities (10 - 20%) of Si2Cl6 and Si3Cl8 are also formed. The latter products were formed in all the reactions except that between CdCl2 and silicon, in which SiCl4 was formed almost
exclu-sively. Above 200 oC copper is used as a catalyst in the production of chlorosilanes above 260 oc in the production of methylchlorosilanes and above 400OC in the production of phenylchlorosilanes.
AgCl starts reacting with silicon at 430 oC; mixing AgCl with SnCl2 reduces the reaction threshold temperature of the AgCl reaction with silicon to 315 oc. At a temperature above 400 °C silver is known to be a catalyst in the direct synthesis of phenylchlorosilanes ([1, 16], 8.4). At lower temperatures pure silver can not be used as a cata-lyst because of the slow chlorine transfer to silicon.
PdCl2 and C0CI2, with reaction threshold temperatures of 350 and 430 OC, respectively, are not known to be catalysts for the direct syn-thesis, but are, in view of their threshold temperatures, possible cat-alysts for the production of phenylchlorosilanes.
HgCl2, which starts reacting at 226 oc, is not known to be a cata-lyst, and is probably too volatile to be used. SnCl2 (reaction thresh-old temperature 300 oc) is known to be a catalyst for the production of methyl- and phenylchlorosilanes above 300 °C ([12], 8.4). PbCl2
(reac-tion threshold temperature 390 "C) has been used as a catalyst for the production of methyl- and phenylchlorosilanes at temperatures above 400 oC ([12,23], 8.4). At lower temperatures PbCl2 is an inhibitor for the reaction [17].
Table 2-2. Summary of experimental data and results.
Experiment Si MCI dT/dt T Reaction n start (g) (g) (°C/min) (°C) enthalpy 11- I 90.0' II- 2 13.0 II- 3 10.6 II- 4 10.2 I I - 5 10.0 I I - 6 10.2 I I - 7 10.0 I I - 8 10.0 I I - 9'' 10.0 11-10 10.5 11-11 10.0 11-12 10.7 11-13 10.4 11-14^ 7.0
The temperature of the start of the reaction was measured in the equipment described in Section 3.2.1.. Particle size of the silicon is 30-250 um.
CuCl-,2aq was dried in the reactor after mixing with silicon. Reaction of FeCl. with silicon takes place in two steps: the first reaction step ceases with the formation of Feci- (exotherm); further heating results in the formation of iron (endotherm). Particle size of silicon is 75-105 IJm. not dried during 16 h. Particle size of silicon is 75-105 Mm.
15.4 CuCl 2.0 C u C l j 2.2 CuCl2.2aq"' 3.0 F e C l j 3.0 C d C l j 3.0 P b C l j 2.0 P d C l ^ 2.5 A g C l 2.6 A g C l 2.5 C o C l j 3.0 S n C l j 2.7 H g C l j 1.7 C u C l 'l 7 S n C l ^ ' 3.0 A g C l '3.1 SnCl ' 2.0 2.5 1 5 2.0 1.8 1 .2 1 .3 2.5 2.2 1.2 0.9 2.0 2.1 2.7 300 3 1 0 295 240*^ 450"^ 480 390 350 456 430 432 300 226 250 315 n e g a t i v e n e g a t i v e n e g a t i v e n e g a t i v e p o s i t i v e p o s i t i v e p o s i t i v e n e g a t i v e n e g a t i v e n e g a t i v e n e g a t i v e p o s i t i v e n e g a t i v e n e g / p o s n e g / p o s
120 i^ 100 X ^^ 8 0 I t-60 40 2 0 0 280 320 360 400 440 480 ^^.— Temperature (°C )
Fig. 2-3 Heating curves of some metal chloride/silicon mixtures. a = CuCl/SnCl2; b = CuCl; c = PdCl2; d = AgCl/SnCl2; e = C0CI2; f = AgCl.
For the production of organochlorosilanes, iron is an interesting possible catalyst because it is always present in, the silicon as a con-taminant. For the reaction of FeCl3 with silicon a free energy change of reaction of -81 kcal/mole SiCl4 can be calculated (T = 500 K ) . Thus, viewed thermodynamically, FeCl3 can react completely with silicon to
form SiCl4. The standard free energies of formation of FeCl3(c) and FeCl2(c) are, however, -69.9 and -66.1 kcal/mole, respectively [18]. Apparently the greatest decrease in free energy in the reaction of FeCl3 with silicon will result from the conversion of FeCl3 to FeCl2
[27]. If there is a steady trend in the Gibbs free energy of reaction from the FeCl3 to FeCl2 conversion to the FeCl2 to FeCl conversion, then it is to be expected that reaction 2-5 will be rate limiting, because in this step the decrease in free energy will be low, and at lower temperatures the change in free energy of this step may even be positive.
Feci + SiCln -* Fe + SiCln+i (2-5)
Indeed, at 240 oc FeCl3 starts reacting exothermically with silicon, but the reaction stops with the formation of FeCl2; heating for some hours at 350 ^C does not cause further reaction. The endothermic reac-tion of FeCl2 with silicon occurs only at a temperature of 450 oc. Because of this, iron can not be used as a catalyst below about 450 °C, and even at 500 oc it proves to be a poor catalyst ([19], 8.4).
Another interesting metal is cadmium, because between 400 and 500 °C the sign of the free energy change of the reaction of CdCl2 with silicon changes from positive to negative. CdCl2 starts reacting with silicon at a temperature of 480 oC. At lower temperatures CdCl2 is known to be a very good promoter in the synthesis of organochlorosila-nes ([20,21,22], 3.3.2); at about 450 oc its action as a promoter
changes, and cadmium becomes a catalyst for the production of methyl-and phenylchlorosilanes (3.3.3, 8.3).
As can be seen from the experimental results, a clear qualitative relationship exists between the threshold temperature of the reaction of a metal chloride with silicon and the catalytic activity of the metal in the direct synthesis of organochlorosilanes; that is, the low-er the reaction threshold templow-erature, the lowlow-er the templow-erature at which the metal may become catalytic.
2.3.2 Promoters in the direct synthesis.
It has been shown above that for a metal to be catalytically active in the direct synthesis of silanes transfer of chlorine from the metal to silicon must be possible in every reaction step. If this is not the case, it may still be possible to transfer some chlorine to silicon in the initial reaction steps, because it is to be expected, as was shown for the FeCl3 + Si reaction (2.3.1)» that in these steps the greatest decrease in free energy can be obtained. Using CdCl2 as a pro-moter [29], for example:
CdCl2 + Si J CdCl + SiCl (2-6)
CdCl2 + SiCl J CdCl + SiCl2 (2-7)
CdCl + Si J Cd + SiCl (2-8)
In this way active reaction centres of the type SiCljj can be created, being starting centres of the chain reaction on the contact mixture surface. Also the promoting effect of ZnCl2 and SbCl3 can be explained by reactions of the type given above.
The existence of metal subchlorides of the type Cd2Cl2 and Zn2Cl2 in metal/metal chloride melts was reported in, for example, [24] and
[25,26]. The formation of the subchlorides is favoured by chloride acceptors such as CrCl3 [24] and AICI3 [26]. It is, therefore, very likely that also in the Si/Cu/MClji system the unstable subchlorides are formed during reaction with the gaseous reactant, silicon being a very strong chloride acceptor.
2.3.3 Quantitative correlation of the results.
A quantitative relationship has been established which relates the reaction threshold temperature of a metal chloride with silicon to a number of relevant physical properties of the substances. The most important properties besides, for example, particle size, are the vapour pressure and melting point of the metal chloride. A high vapour pressure and low melting point result in a good distribution of the metal chloride on the surface. Another important quantity is the change in Gibbs free energy of the system. If AGj- > 0 there will be little tendency for the reaction to proceed under the conditions of the direct synthesis.
A relationship has been found between the threshold temperature of the reaction of a pure metal chloride with silicon and the product of
the average standard free energy of formation per gram atom chlorine bound to the metal (AGj/Cl) times the temperature in K at which the vapour pressure of the metal chloride amounts to 1 Torr (see Fig. 2-4). If AGj. < 0 the reaction threshold temperatures can be found using equa-tion 2-9 (standard deviaequa-tion 32.3 °C).
T (OC) = 194.3 - 0.0072 x AG^/Cl x T(i Torr) (2-9)
The constants in this equation were calculated by means of the method of least squares.
Vapour pressures and AGj have been taken from ref. 13. AGf of PdCl2 and SnCl2 have been estimated from AH^. AG^ values of CUCI2, FeCl2 and FeCl3 have been taken from ref. 18. For the reaction of FeCl3 with sil-icon the AQ9 for transfer of the last chlorine atom has been used.
5 0 0 4 0 0 3 0 0 2 0 0 • 0 • A X
^
+ 9 1 O l / / y^ % 1 ^y^ 1-a G ( / C l X T V x l d ( Kcal*K/ Gram otom)
Fig. 2-4 Quantitative correlation of the reaction treshold tem-peratures of the reaction of metal chlorides with sil-icon. o = FeCl3; CuCl; X = PdCl2; FeCl 2; * = CdCl-= HgCl2; A CdCl-= CUCI2; + CdCl-= SnCl2; V CdCl-= = PbCl2; D = C0CI2; I = AgCl; • =
2.4
Conclusions.
It has been demonstrated that a relationship exists between the threshold temperature of the reaction of a metal chloride with silicon and the catalytic activity of the metal in the direct synthesis of chlorosilanes. A quantitative prediction of the reaction threshold tem-perature is possible if some basic properties of the metal chlorides are known. A possible catalyst can thus be selected on a thermodynamic basis, and the effect of a promoter can be explained on a similar basis. A pure metal may exhibit catalytic properties only if the chlo-rine transfer from the metal chloride to silicon is possible in every
reaction step. If this is not the case, the metal chloride may still be able to act as a promoter; active reaction centres of the type SiClj, are then created on the surface by partial chlorination of silicon.
References and notes.
1. R.J.H. Voorhoeve, Organohalosilanes: Precursors to Silicones, Elsevier, Amsterdam, 1967.
2. M.J. van Dalen, Thesis, Delft, 1971.
3. S.A. Golubtsov, K.A. Andrianov, N.T. Ivanova, R.A. Turetskaya, I.M. Podgornyi and N.S. Fel'dshtein, Zh. Obshch. Khim., 43 (1973) 2000.
4. K.A. Andrianov, M.V. Tikhomirov, V.I. Zubkov, V.K. Potapov and V.V. Sorokin, Dokl. Akad. Nauk SSSR, 194 (1970) 1077.
5. K.A. Andrianov, S.A. Golubtsov, M.V. Tikhomirov and V.I. Zubkov, Izv. Akad. Nauk SSSR, Ser. Khim., 2 (1973) 444.
6. P.P. Budnikov and A.M. Ginstling, Principles of Solid State Chem-istry, Reactions in Solids, McLaren, London, 1968.
7. H.W. Kohlschiitter and 0. Klump, Z. Anorg. Allg. Chem., 286 (1956) 193.
8. W. Noll, Chemie und Technologie der Silicone, Verlag Chemie, Weinheim, 1968, p. 26.
9. R. MUller and H. Gumbel, Z. Anorg. Allg. Chem., 327 (1963) 293. 10. H.W. Kohlschiitter, H. Schifferdecker and 0. Klump, Z. Anorg.
Allg. Chem., 238 (1956) 257.
11. R.J.H. Voorhoeve, Thesis, Delft, 1964, p. 32.
12. R.J.H. Voorhoeve and J.C. Vlugter, Rec. Trav. Chim. Pays-Bas, 82 (1963) 605.
13. Handbook of Chemistry and Physics, The Chemical Rubber Company, Cleveland, 49th ed., 1968.
14. G. Hermann, Z. Anorg. Chem., 71 (1911) 269.
15. E.M. Levin, C.R. Robbins and H.F. McMurdie, in M.K. Reser (Ed.), Phase Diagrams for Ceramists, The American Ceramic Society, 1964, and supplement 1969.
16. E.G. Rochow and W.F. Gilliam, J. Amer. Chem. S o c , 67 (1945) 1772.
17. I.V. Trofimova, N.P. Lobusevich, S.A. Golubtsov and K.A. Andrianov, J. Gen. Chem. USSR, 32 (1962) 835.
18. W.G. Davies, Introduction to Chemical Thermodynamics, W.B. Sanders, London, 1972.
19. L. Riccoboni and M. Zotta, J. Soc. Chem. Ind., London, 67 (1948) 235.
20. R.A. Turetskaya, S.A. Golubtsov, K.A. Andrianov, M.A. Luzganova and T.A. Tsvanger, Izv. Akad. Nauk SSSR, Ser. Khim., 8 (1966)
1442.
21. R.A. Turetskaya, S.A. Golubtsov, V.G. Dzvonar and M.A. Luzganova, J. Gen. Chem. USSR, 42 (1972) 1507.
22. H.F. Zock, Neth. Pat. Appl. 6600860, 1966.
23. K. Komatsu and M. Kuriyagawa, Japan Pat. 3772, 1956; Chem. Abstr., 51 (1957) 14802.
24. D.H. Kerridge and S.A. Tariq, J. Chem. Soc. (A), (1967) 1122. 25. J.D. Corbett, Inorg. Chem., 1 (1962) 700.
26. J.D. Corbett, W.J. Burkhard and L.F. Druding, J. Amer. Chem. S o c , 83 (1961) 76.
27. This trend also holds true for several other series of stable metal chlorides; for example: CuCl2 and CuCl, CrClo and CrCl2, the molybdenum chlorides etc..
28. In this thesis it is always assumed that the reaction intermedia-tes are bound to the surface. It has indeed been demonstrated by Fel'dshtein and Gorbunov et al (e.g.. Symposium International sur
la Chimie des Composes du Silicium, Resumes des Communications, Bordeaux, 1968, p. 81) that the direct synthesis must be regarded as a heterogeneous reaction which takes place solely on the sur-face of the solid reactant.
29. Voorhoeve (ref. 1, p. 132) gives the following definition of pro-moters: additives which, if present in small or very small
amounts, promote the working of the catalyst. Additives exceeding the amount of 1 wt% are called auxiliary chemicals. In this the-sis a different definition of a catalyst and promoter is used: catalysts are metals which, as the metal chloride or subchloride, are able to react with the reaction intermediates SiCl^ (n S 3) and are catalytically active in the direct synthesis of chloro-silanes; promoters are solid or liquid additives which, at reac-tion condireac-tions, are converted to stable metal chlorides on the contact mixture surface and promote the working of the catalyst by the formation of active reaction centres of the type SiCln. 30. The concept "active centre" can be only vaguely defined because
no detailed knowledge exists concerning the surface structure of the contact mass under reaction conditions. The concept, however vague, is widely used in the literature; see, for example, [3, 31 and 32].
31. V. Bazant, V. Chvalovsky and J. Rathousky, Chemistry of Organo-silicon Compounds, Publishing House of the Czechoslovak Academy of Sciences, Prague, 1965, p. 210.
32. M. Boudart, Kinetics of Chemical Processes, Prentice-Hall, Inc., 1968, p. 60.
3 CADMIUM AS A CATALYST AND PROMOTER IN THE SYNTHESIS OF METHYL-CHLOROSILANES.
3.1 Introduction.
In the direct synthesis of organochlorosilanes a catalyst is employed to decrease the reaction temperature and to increase the rate of reaction, the maximum degree of conversion of the silicon and the selectivity to the most desired product. In addition to the catalyst, a promoter is often introduced to effect an even better performance of the contact mass. The promoters which are commonly applied in the syn-thesis of methylchlorosilanes are zinc, aluminium and antimony [1,2a, 3] . In the synthesis of phenylchlorosilanes cadmium chloride and oxide (sometimes in combination with zinc and zinc oxide) have proved to be very effective [4,5], and in the synthesis of methyl- and ethylchloro-silanes cadmium chloride has also been used succesfully [6,7a]. If in the synthesis of methylchlorosilanes CdCl2 is added to the contact mass, the rate of reaction and the maximum degree of usage of silicon
increase, and the selectivity to D decreases, relative to the case in which zinc and aluminium are used as promoters [6].
Although promoters play a very important role in the direct syn-thesis, little is actually known about the mechanism by which they enhance the activity of the catalyst. In chapter 2 it has already been mentioned that the promotive activity of some metal chlorides can be
explained on the basis of the hypothesis that formation of active reac-tion centres of the type SiCln occurs as a result of the partial chlo-rination of the silicon. It follows from this hypothesis that the pro-motive action of a metal chloride may change into a catalytic action at or nearby the reaction threshold temperature of its reaction with sili-con. In this chapter experiments concerning the catalytic activity of cadmium in the direct synthesis of methylchlorosilanes are described which have been performed to verify this hypothesis. Further, because of the fact that quantitative experimental data concerning the promo-tive activity of CdCl2 are hardly available in the literature, also experiments concerning this subject have been performed. The experi-ments will be discussed in the light of a reaction scheme.
3.2 ExperimentaI.
3.2.1 Apparatus.
Part of the experiments was performed in the apparatus which is shown in Fig. 3-1.
Fig. 3-1 Laboratory equipment for fluid bed synthesis. 1. Cylin-der with nitrogen. 2. CylinCylin-der with methyl chloride. 3. BTS catalyst for oxygen removal. 4. Linde molecular sieve 4A for water removal. 5. Magnetic valve. 6. Cap-illary for oxygen injection. 7. Flowmeter. 8. Hersch-cell. 9. Flow meter. 10. Fluid bed with silicon. 11. Reactor. 12. Dust trap containing glass wool. 13. Dis-tillation column. 14. Discharge of silanes. 15. Dewar vessels with acetone/C02. 16. Vent. 17. Sample tube for gas analysis. 18. Dewar vessel for unreacted methyl chloride. 19. KOH pellets. 20. Gas meter. 21. Air for oxygen injection. 22. Heating water for reboiler. 23. Pressure air for fluid bed. T = temperature; P = pres-sure; M = manometer.
Methyl chloride and nitrogen were dosed via flow meters and freed from oxygen and water by passage through a column containing active copper
(BTS-catalyst, type R 3-11, from BASF) and a column containing a 4A molecular sieve. The silanes and unconverted methyl chloride were
par-tially separated by continuous distillation through a Vigreux column. The silanes and some of the methyl chloride were drained from the reboiler; the reflux was maintained by a Dewar vessel containing 15 kg of a C02/acetone mixture, which was sufficient to run the experiments for 18 h. The greater part of the unconverted methyl chloride was liquified in a second Dewar vessel (condenser) and stored in a contain-er with vacuum wall. Uncondensable gases wcontain-ere discharged via an eight-way valve and led off through a column containing KOH pellets to a gas meter. If necessary (chapter 7) oxygen could be injected in the feed gas through a calibrated stainless steel capillary, situated after the
BTS-catalyst and molecular sieve. Before the reactor part of the gas mixture (30 ml/min) was passed through a Hersch-cell (Fig. 3-2, [8]) to measure the oxygen concentration continuously. The cell was calibrated by electrolysis of water (assuming 100% efficiency), thus adding a known amount of oxygen to a known amount of methyl chloride.
Fig. 3-2 Hersch-cell apparatus for measuring the oxygen concentration.
The reactor, i.d. 28 and length 600 mm [9], was made of glass and was situated in a fluidised bed containing silicon. The temperature in the synthesis reactor was kept constant to within 2 oc by means of a temperature control system employing a platinum resistance thermometer and was measured using chromel-alumel thermocouples.
3.2.2 Analysis of reaction products.
The product mixture, consisting of silanes and unconverted methyl chloride, was analysed by means of gas-liquid chromatography [2d]. Dimethyldichlorosilane was used as the internal standard. Separation took place on a 4 m column filled with nitrobenzene, 30 wt% on Chromo-sorb W, at a temperature of about 30 oc. Hydrogen, dried on a molecular sieve, was used as the carrier gas. Reaction products with a boiling point higher than D, a few percent of which were always present in the product mixture (as is normally the case [11]) were not analysed.
3.2.3 Materials used.
main impurities being 0.4% Fe, 0.1% Al and 0.3% Ca + Mg (by weight). Before use it was washed with destilled water, dried and treated with a magnet to remove part of the iron (resulting in an iron content of 0.2 - 0.3%). Because a number of the experiments was conducted in a fluidised bed, an easily fluidised mixture of silicon particles was used ([2a]; the particle size distribution in wt% was: 50 - 75 ym, 11.7; 75 - 105 ym, 36.9; 105 - 150 ym, 41.4; 150 - 210 ym, 10.0). Cop-per was introduced as pure CuCl, prepared by a standard method [10]. Cadmium chloride, zinc and aluminium were chemically pure and were added to the contact mixture as fine powders.
3.2.4 Description of the experiments.
Before each experiment the contact mass was prepared in situ. The components were mixed, stored in the reactor and dried at 50 - 150 oc for at least 3 h. Then the temperature was raised; reaction between CuCl and silicon took place at 250 - 330 oc (experiments III-l , -2 and -5), between CdCl2 and silicon at 470 oC (experiment 1II-3). Then the contact mass was heated for about 16 h under a stream of pure nitrogen before starting the experiment.
Some experiments were performed with a gas flow calorimeter. In these experiments the reaction rate is expressed as "C per unit weight of silicon. It can be shown that this temperature difference between sample and reference part of the calorimeter is proportional to the amount of methyl chloride reacting per unit of time, i.e., directly proportional to the reaction rate. If for the heat of reaction a value of -85 kcal/(mole Si) is used [7b], a temperature difference of 0.1 °C per 100 mg silicon corresponds to a reaction rate of 100 g MeCl/(kgSi/ h). The choice of these values facilitates comparison between kinetic studies performed in the usual apparatus (Fig. 3-1) and those performed in the calorimeter. A description of the calorimeter, its properties and use, and the experimental procedure can be found in the appendix.
3.2.5 Experiments performed.
Some typical experimental data have been summarised in Table 3-1. The composition of the contact mixtures and dust after the experiments and the contact masses used in the calorimeter can be found in Table 3-2 and 3-3, respectively.
Using zinc and aluminium as promoters (experiment III-l), the reaction rate and product composition exhibit the same features as have been described in [2c] (see also chapter 7). In experiment III-2 cad-mium chloride was added to the silicon/copper contact mass, and in this case the synthesis starts with a low selectivity to D. During the experiment the selectivity gradually increases until, at a silicon con-version of about 40%, the product mixture contains 90 mole-% of D. Hereafter the selectivity begins to decrease. The rate of reaction is increased by the addition of cadmium chloride. The reaction rate and product composition of experiments III-l and III-2 can be found in Figs. 3-3 and 3-4. In Fig. 3-3 the reaction rates have been converted
Table 3-1. Composition of contact mixtures before the start of the experiments and experimental conditions. Experiment i i i - i ^ 111-2^ 111-3^ 111-4" 111-5^ ' fluid Si (g) 90.0 90.0 94.7 94.2 45.0 b e d . " atmosphere. CuCl (g) 15.4 15.4 0 0 5.0 fixed CdClj (g) 0 3.0 20.2 0 2.0 Zn (g) 0.1 0 0 0 0 Al <g) 0.05 0 0 0 0
bed. The total pre
Temperature ("O 323 309 334-511 375-511 317-468 ssure in the ppm Oj in MeCl 4-8 1-10 2-10 5-7 25 reactor Duration (h) 61 77 30 25 13 MeCl (ml/ always amounted flow min at STP) 360 345 163 180 177 to about 1
Table 3-2. Composition (in wtZ) of contact mixtures and dust after the experiments. Experiment Cu Zn III-I cm dust III-2 cm dust II1-3 cm dust II1-4 cm III-5 cm dust 21.2 0.04 55.8 0.76 34.8 39.0 19.8 38.7 0.87 1.63 1.02 0.70 0.48 0.86 4.9 19.5 9.68 60.4 6.47 17.8 0 0 0 69 58 97 1 . 15 4.44 1 .55 3.24 1.45 0.52 1.68 3.05 2.31 0.11 0.81 0.24 0.59 0.02 0.03 cm • contact mixture.
Table 3-3. Composition (in wtZ) of contact mixtures used for calorimeter experiments. Contact mixture Cu Zn Al Fe I II 13.5 8.1 0.05 0.03 0.2 0.42 0.5 0.4 0.5 0.1 H 0.04 0.04
to the average temperature of experiment III-2 (309 °C) with an activa-tion energy of 26 kcal/mole (4.3.2).
20 4 0 6 0 " - Stiicon conversion ("/ol
20 4 0 6 0 - Silicon conversion (•/•)
Fig.3-3 Reaction rate of methyl chloride and silicon at 309 °C. o = without CdCl2
(experiment III-l); • = with CdCl2 (experiment
III-2).
Fig.3-4 Product composition as function of silicon conver-sion. o,« = D of experiment III-l and - 2 , resp.; +,x = T of experiment III-l and - 2 , resp.; A,V = M of experiment III-l and - 2 , resp..
The increase of the reaction rate when cadmium chloride is used as a promoter was confirmed by experiments with the gas flow calorimeter (Fig. 3-5; in chapter 4 this increase will be described quantitative-ly). This effect increases with the amount of promoter used, but tends to become stable at concentrations of ^ - 3% (Fig. 3-6). Apparently a surface coverage effect is involved.
r
60 — time (mm)
Fig.3-5 Reaction rate of methyl chloride and silicon (calo-rimeter experiments), a = sample I with 3% CdCl2, added after pre-treatment; b = sample I without CdCl2; temperature = 330 °C; pres-sure = 760 Torr. The lines in the Figures were calcu-lated from thermograms.
1 2 — CdCl2 (wt •/.)
Fig.3-6 Reaction rate of methyl chloride and silicon as function of wt% CdCl2 (calo-rimeter experiments). Sample I. Temperature = 330 oc; pressure = 760 Torr. The reaction rates were calcu-lated from thermograms after 325 min of reaction.
The influence of cadmium chloride on the maximum degree of usage of silicon in a fixed bed reactor is shown in Fig. 3-7. Without the use of CdCl2 the reaction rate already becomes very low at 30% silicon conver-sion; by addition of CdCl2, however, a degree of usage of silicon can be achieved (Z 70%) which is normally encountered only in a fluid bed synthesis ([2c], chapter 7).
0 I 1 I I I I L_ 0 20 40 60
^ » Silicon conversion (°/<.)
Fig. 3-7 Reaction rate of methyl chloride and silicon as func-tion of silicon conversion (calorimeter experiments). Temperature = 330 oc; pressure = 760 Torr. Lower line: sample II without CdClo; time of reaction 4320 min. Upper lines: sample II with 3% CdCl2, added after pre-treatment. Time of reaction 1590 (63.3% Si conversion) and 1050 min. The lines were calculated from thermo-grams .
In experiment III-3 cadmium was used as a catalyst. Before the start of the experiment, cadmium chloride was converted to metallic cadmium by reaction with silicon (the threshold temperature was 470 oc). After heating the contact mixture for 16 h at 300 oc, methyl chloride was introduced into the reactor at 344 oc. In the first hours some cracking occurred, and about 200 mg of methylchlorosilanes were formed. When the reaction had stopped, the temperature was first raised to 417 Oc, but it was necessary to increase this to 450 oc to achieve renewed reaction. Finally the temperature was raised to and kept at 511 °C. A decrease and subsequent increase of the temperature showed that 450 °C was a reproducable temperature value for the start of the formation of silanes. After conducting the experiment for 30 h the contact mass was
