Titanium-on-silica catalyst

Buijink, J.K.F., van Vlaanderen, J.J.M., Crocker, M., and Niele, F.G.M. (2004) Propylene epoxidation over titanium-on-silica catalyst-the heart of the SMPO process. Catal. Today, 93-95,199-204. 

FIGURE 13.7 Representation of titanium-on-silica catalyst in SMPO as a range of silica-supported—and therefore stabilized—ultrafine titania particles containing catalytically active, coordinatively unsaturated titanate sites. 

Of course, PO is a highly reactive molecule and the by-product chemistry, often involving the titanium-on-silica catalyst, including isomerization, dimerization, hydrolysis, and alcoholysis, has also been (re)investigated. Not surprisingly, PO yield loss (and MPC, via adduct formation) and the separation of PO from trace components with similar boiling points, such as lower aldehydes, are the main byproduct issues here. Due to space limitations, this chemistry will not be discussed further here. 

Phenol and the ethylphenols are formed in both the oxidation and epoxidation sections. The Lewis acidic titanium-on-silica catalyst is implicated in the heterolytic cleavage of EBHP to phenol and acetaldehyde, and the hydroxylation of EB to... 

More pragmatically, one can attempt to address the question of the benefits of nanostructured catalysts by comparison against the properties of similar catalysts prepared by traditional methods. There are a couple of well-known reactions that are thought to be catalyzed by atomically dispersed metals on supports [95]. Epoxi-dation of olefins by titanium on silica is one [96, 97]. 

FIGURE 13.3 Proposed most active site in SMPO titanium-on-silica epoxidation catalyst (R = MesSi) Si denotes Silica. Reprinted with permission from [1]. Copyright (2004) Elsevier. 

Figure 3.9 Ethylene polymerization rates for chromium on silica (o) and for chromium/ titanium on silica (square symbol). Ethylene flow rate in arbitrary imits per gram of catalyst. Reprinted from [11] with permission from John Wiley and Sons.

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

SMPO [styrene monomer propylene oxide] A process for making propylene oxide by the catalytic epoxidation of propylene. The catalyst contains a compound of vanadium, tungsten, molybdenum, or titanium on a silica support. Developed by Shell and operated in The Netherlands since 1978. 

The ability of titanium-grafted silicas in catalyzing the epoxidation with TBHP of fatty compounds was first tested on two pure Qg monounsaturated FAMEs methyl oleate (ds-9-octadecenoate Scheme 12.1) and methyl elaidate (trans-9-octadecenoate) [49]. In both cases, selectivity to 9,10-epoxystearate was very high (>95%) and the reaction was fully stereospecific, confirming that epoxidation with titanium catalysts and TBHP proceeds via a non-radical mechanism with retention of configuration at the C=C bond. Ti-MCM-41 was more active than Ti-SiC>2 (Fig. 12.1). Actually, methyl oleate was almost completely converted after... 

Titanium Alkoxides Silica-supported titanium(IV) alkoxides and Ti-silicalite are industrial epoxidation catalysts [53-56] and have been applied in deperoxidation reactions [57]. Computational and EXAFS data [53, 54] as well as spectroscopic investigations on the surface species [58] have indicated that the dominant active surface species is a four-coordinate trisUoxy complex [(=SiO)3TiOH] [59] whose coordination shell expands to six-coordinate during catalysis [60]. 

Among the large bulk of work regarding titanium(IV) silsesquioxanes, specific studies have been focused on synthesizing close molecular models of the active sites of titanium heterogeneous epoxidation catalysts in siUcaUte and in mesostruc-tured MCM-41 silica (Figure 14.3 gives some selected examples) [54, 61-66]. 

Ermakova and co-workers manipulated the Ni particle size to achieve large CF yields from methane decomposition. The Ni-based catalysts employed for the process were synthesized by impregnation of nickel oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium dioxide, zirconium oxide and magnesia). The optimum particle size (10 0 nm) was obtained by varying the calcination temperature of NiO. The 90% Ni-10% silica catalyst was found to be the most effective catalyst with a total CF yield of 375 gcp/gcat- XRD studies by the same group on high loaded Ni-silica... 

Gas-phase epoxidation of propylene with 02/H2 mixtures was accomplished over Ag1267 or Au1268 catalysts dispersed on TS-1 or other Ti-containing supports and Ti-modified high-silica zeolites.1269 Sodium ions were shown to be beneficial on the selectivity of propylene epoxidation with H202 over titanium silicalite.1270 A chromia-silica catalyst is active in the visible light-induced photoepoxidation of propylene by molecular oxygen.1271... 

Therefore, in such heterogeneous polymerizations, almost all industrial catalysts are supported, for example on silica, whereas the typical Ziegler s titanium catalysts are by definition supported on magnesium chloride. These catalysts are adsorbed at the surface or incorporated into the crystal structure of the support. Other catalysts, such as Phillips chromium catalysts, can be coupled at the support surface by a chemical bond. 

The synthesis of methyl /-butyl ether (MTBE) from isobutylene and methanol on TS-1 has been investigated. This reaction is catalyzed by acids and the industrial production is carried out with sulfonic acid resin catalysts. It has been reported that at 363-383 K the reaction proceeds in the presence of the acidic HZSM-5, but also on TS-1, which is much more weakly acidic. However, the characterization of the catalysts used is not completely satisfactory for instance, the IR spectra reported do not show the 960-cm 1 band that is always present in titanium-containing silicas. It is therefore possible that the materials with which the reaction has been studied are not pufe-phase TS-1. The catalytic activity for MTBE synthesis is, in any case, an interesting result, and further investigations with fully characterized catalysts are expected to provide a satisfactory interpretation of these results (Chang et al., 1992). 

Finally, the concept was broadened by supporting these titanium silsesquiox-anes on silica remarkably, the heterogeneous silica-supported catalysts displayed an epoxidation activity per mole of titanium (94% TBHP conversion after 3 h) similar to that of the homogeneous titanium-silsesquioxane a2b4 complexes, although with a lower selectivity towards 1,2-epoxyoctane (92%). These heterogeneous catalysts did not leach active species and proved to be recyclable [45]. 

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