catalysts

Review Review SBA-15 as a Support for Effective Olefin SBA-15 as a Support for Effective Olefin Metathesis Metathesis Catalysts Catalysts Hynek Balcar 1 and Jiˇrí Cejkaˇ 2,* Hynek Balcar 1 and Jiří Čejka 2,* 1 J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, v.v.i., Dolejškova 3, 1 J.182 Heyrovský 23 Prague Institute 8, Czech of Republic Physical Chemistry of the Czech Academy of Sciences, v.v.i., Dolejškova 3, 182 23 2 PragueDepartment 8, Czech of PhysicalRepublic and Macromolecular Chemistry, Faculty of Science, Charles University, 2 DepartmentHlavova 2030 of/ 8,Physical 128 43, and Prague, Macromolecular Czech Republic Chemistry, Faculty of Science, Charles University, Hlavova * 2030/8,Correspondence: 128 43, Prague, [email protected]; Czech Republic Tel.: 00420-732-804-674 * Correspondence: [email protected]; Tel: 00420-732-804 674


Received: 22 July 2019; Accepted: 22 August 2019; Published: 2 September 2019
 Received: 22 July 2019; Accepted: 22 August 2019; Published: date Abstract: Olefin metathesis is the catalytic transformation of olefinic substrates, finding a wide range ofAbstract: applications Olefin in metathesis organic synthesis. is the catalytic The mesoporous transformation molecular of olefinic sieve Santa substrates, Barbara finding Amorphous a wide (SBA-15)range of has applications proven to be in an excellent organic supportsynthesis. for metathesisThe mesoporous catalysts molecular thanks to its sieve regular Santa mesoporous Barbara structure,Amorphous high (SBA BET-15) area, has and proven large poreto be volume. an excellent A survey support of catalysts for metathesis consisting catalysts of (i) molybdenum thanks to its andregular tungsten mesoporous oxides on structure, SBA-15, highand (ii) BET molybdenum area, and large and ruthenium pore volume. organometallic A survey of complexes catalysts (Schrockconsisting and of Grubbs-type(i) molybdenum carbenes) and tungsten on SBA-15 oxides is provided on SBA-15, together and (ii) with molybdenum their characterization and ruthenium and catalyticorganometallic performance complexes in various (Schrock metathesis and Grubbs reactions.-type carbenes) The comparison on SBA with-15 is catalysts provided based together on other with supportstheir characterization demonstrates and the highcatalytic quality performance of the mesoporous in various molecular metathesis sieve reactions. SBA-15 T ashe an comparison advanced catalystwith catalysts support. based on other supports demonstrates the high quality of the mesoporous molecular sieve SBA-15 as an advanced catalyst support. Keywords: olefin metathesis; SBA-15; mesoporous molecular sieve; Mo and Ru hybrid catalysts Keywords: olefin metathesis; SBA-15; mesoporous molecular sieve; Mo and Ru hybrid catalysts.

1. Introduction: Olefin Metathesis and the Basic Properties of Santa Barbara Amorphous (SBA-15) 1. Introduction: Olefin Metathesis and the Basic Properties of Santa Barbara Amorphous (SBA-15) OlefinOlefin metathesis metathesis is isthe the catalytic catalytic transformation transformation of olefinic of olefinic substrates. substrates. In this In process, this process, C=C double C=C bonddouble splitting bond splitting is followed is followed by the combination by the combination of alkylidene of alkylidene molecular molecular fragments. fragments. This general This reaction general involvesreaction aliphaticinvolves aliphatic alkenes, cycloalkenes,alkenes, cycloalkenes, dienes, and dienes, polyenes, and polyenes, with or withoutwith or without functional functional groups (Schemegroups (Scheme1). 1).

2

SchemeScheme 1.1. AlkeneAlkene metathesismetathesis reaction.reaction.

VariousVarious namesnames areare oftenoften usedused forfor specificspecific typestypes ofof olefinolefin metathesismetathesis dependingdepending onon thethe substratesubstrate involvedinvolved in in the the reaction: reaction: (1) (1) cross-metathesis cross-metathesis (CM) (CM) for reactions for reactions between between two or two more or di morefferent different olefinic substrates,olefinic substrates, (2) ring-closing (2) ring metathesis-closing metathesis (RCM) for (RCM) reactions for involvingreactions dienesinvolving that dienes lead to that ring lead formation, to ring (3)formation, acyclic diene (3) acyclic metathesis diene (ADMET) metathesis for (ADMET) reactions for that reactions lead to linear that lead polyenes, to linear and polyenes, (4) ring-opening and (4) metathesisring-opening polymerization metathesis polymerization (ROMP) for transformations (ROMP) for transformations of cycloalkenes into of cycloalkenes polymers with into unsaturated polymers mainwith unsaturated chains. Because main of chains. its general Because character, of its general olefin metathesischaracter, olefin has a metathesis wide range has of applicationsa wide range inof organicapplications synthesis, in organic ranging synthesis, from the production ranging from of fine the chemicals production (e.g., of natural fine chemicals products, (e.g., biologically natural activeproducts, compounds, biologically pharmaceuticals), active compounds, to large-scale pharmaceuticals), industrial processes to large-scale (petrochemistry, industrial processes polymer synthesis).(petrochemistry, In 2005, polymer Chauvin, synthesis). Grubbs, andIn 2005, Schrock Chauvin, were awardedGrubbs, and the NobelSchrock Prize were “for awarded the development the Nobel Prize "for the development of the metathesis method in organic synthesis". Many important aspects

Catalysts 2019, 9, 743; doi:10.3390/catal9090743 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x, doi:FOR PEER REVIEW www.mdpi.com/journal/catalysts CatalystsCatalysts2019 2019,,9 9,, 743 x FOR PEER REVIEW 22of of 1820

of olefin metathesis and its applications have already been discussed in detail in several monographs of[1– the4]. metathesis method in organic synthesis”. Many important aspects of olefin metathesis and its applicationsOlefin metathesis have already catalysts been discussed are exclusively in detail based in several on the monographs following transition [1–4]. metals: Ti, Ta, Mo, W, Re,Olefin and metathesisRu, however, catalysts the importance are exclusively of Ti basedand Ta on is thevery following limited [5,6]. transition In addition, metals: carbenes Ti, Ta, Mo, of W,these Re, tra andnsition Ru, however,metals have the been importance shown to of act Ti andas active Ta is catalytic very limited species, [5,6 which]. In addition, exchange carbenes alkylidene of thesemolecular transition fragments metals through have been metallacyclobutane shown to act as active intermediates catalytic species,[7] (Scheme which 2). exchange alkylidene molecular fragments through metallacyclobutane intermediates [7] (Scheme2).

Catalysts 2019, 9, x FOR PEER REVIEWR 2 of 20 R R LnM of olefin metathesisLnM and its applications haveL alreadynM been discussed in detail in several monographs [1–4]. Olefin metathesis catalysts are exclusively based on the following transition metals: Ti, Ta, Mo, R´ R´ W, Re, and Ru, however, the importance of´ Ti and Ta is very´ limited [5,6]. In addition, carbenes of R´ R´ R R these transition metals have been shown to act as active catalytic species, which exchange alkylidene

molecularM = transition fragments metal, through Ln M= ligands= metallacyclobutane transition at transition metal, L metaln = ligandsintermediates at trans ition[7] (Scheme metal 2). Scheme 2. Mechanism of alkene metathesis. Scheme 2. Mechanism of alkene metathesis. R Most metathesis catalysts are water and oxygen sensitive,R thus, the metathesis reactionR is usually Most metathesis catalysts are water and oxygen sensitive, thus, the metathesis reaction is usually performed underLnM inert conditions. Traditionally,LnM metathesis catalysts areLn dividedM into two categories: (a)performed ill-defined under and (b)inert well-defined conditions. catalysts Traditionally, [8]. Ill-defined metathesis catalysts catalysts (historically are divided older) into include two categories: catalytic systems(a) ill-defined in which and metallocarbenes (b) well-defined are formed catalysts “in [8].situ” Ill in-defined a reaction catalysts between (historically components ofolder) the catalytic include catalytic systems in which metallocarbenes are formed “in situ” in a Rreaction´ between components´ of system and often the substrate itself. For example, in the formerly´ popular system WClR6 + tetraalkyltin, ´ R´ R tungstenthe catalytic carbenes systemR are and formed oftenR´ the “in substrate situ” in a itself. reaction For betweenexample, thesein the two formerly components. popular Similarly,system WCl in6 industrially+ tetraalkyltin, used tungsten systems carbenes based onare Mo,formed W, and“in situ” Re oxides in a reaction on silica between or alumina these two supports, components. active M = transition metal, Ln = ligands at transition metal metallocarbenesSimilarly, in industrially are formed used during systems the activation based on processMo, W, involvingand Re oxides substrate on silica molecules or alumina [9]. In supports, turn, the active metallocarbenes are formed during the activation process involving substrate molecules [9]. In group of well-defined catalystsScheme (developed 2. Mechanism later) includes of alkene stable metathesis. (isolable and storable) metal carbene complexesturn, the group that can of be well directly-defined used catalysts as catalysts (developed (Schrock later) W, Mo includes alkylidene stable complexes (isolable and [10], storable) Grubbs Rumetal carbenesMost carbene metathesis [11 complexes]). Typical catalysts that Schrock can are bewater and directly Grubbs and oxygen used carbenes as sensitive, catalys are depictedts thus,(Schrock the in metathesisW, Figures Mo alkylidene1 and reaction2, respectively. complexes is usually Theyperformed[10], Grubbs are used under Ru in carbenes organic inert conditions. solvents[11]). Typical (usually Traditionally, Schrock dried and andmetathesis Grubbs degassed) catalystscarbenes as homogeneous areare divideddepicted intocatalysts. in Figures two categories: The 1 and first 2, attempts(a)respectively. ill-defined to immobilize They and are(b) these well used- complexesdefined in organic catalysts on solvents silica [8]. surfaces (usually Ill-defined started dried catalysts soon and after degassed) (historically their discovery as older) homogeneous [12 include,13]. catalyticcatalysts.Initially, systems The microporous first in att whichempts silica metallocarbenes to immobilize or silica-alumina theseare formed werecomplexes used “in assitu” on supports silica in a reaction surfaces for molybdenum between started sooncomponents and after tungsten their of discovery [12,13]. oxidethe catalytic catalysts. system However, and often the the advantage substrate of itself. mesoporous For example, supports in the was formerly soon recognized.popular system Ookoshi WCl6 and+ tetraalkyltin, Onaka [14] tungsten studied 1-octene carbenes metathesis are formed over “in MoOsitu” 3insupported a reaction either between on commerciallythese two components. available silicaSimilarly, (Brunauer-Emmett-Teller in industrially used (BET)systems area based 619 mon2 /Mo,g) or W, on and laboratory Re oxides prepared on silica hexagonal or alumina mesoporous supports, silicasactive (HMS)metallocarbenes (d spacing are formed3 nm, during BET areas the betweenactivation 1230 process and involving 1450 m2/g). substrate The diff moleculeserences in [9]. both In 100 ≥ reactionturn, the rates group and of yields well of-defined the reactions catalysts were (developed enormous. later) Over includes low-surface stable silica, (isolable metathesis and products storable) Mo weremetal formed carbene in complexes negligible that amounts, can be whereasdirectlyRO used HMS as provided catalysts yields (Schrock up toW, 45%. Mo alkylidene In the same complexes reaction, Topka[10], Grubbs et al. [15 Ru] similarlycarbenes observed[11]). Typical that theSchrock activity and of Grubbs MoO3 supportedcarbenes are on depicted the mesoporous in Figures molecular 1 and 2, sievesrespectively. MCM-41, They MCM-48, are used and in Santa organic Barbara solvents Amorphous (usually (SBA-15) dried wasand higher degassed) than as that homogeneous of MoO3 on conventionalcatalysts. The silica. first att ThisRempts = enhancedt-Bu to (3.1), immobilize CMe activity2(CF was3these) (3.2) ascribed ,complexes CMe(CF to3)2 better (3.3)on silica, and MoO C(CFsurfaces3 dispersion3)3 (3.4) started on soon supports after withtheir largerdiscovery surface [12,13]. areas. Figure 1. Typical Schrock carbenes.

Initially, microporous silica or silica-alumina were used as supports for molybdenum and tungsten oxide catalysts. However, the advantage of mesoporous supports was soon recognized. Ookoshi and Onaka [14] studied 1-octene metathesis over MoO3 supported either on commercially Mo available silica (Brunauer-Emmett-TellerRO (BET) area 619 m2/g) or on laboratory prepared hexagonal mesoporous silicas (HMS) (d100 spacing ≥3 nm, BET areas between 1230 and 1450 m2/g). The

differencesR = t-Bu in (3.1), bothCMe reaction2(CF3 )rates(3.2), and CMe(CF yields3)2 (3.3)of the, and reactio C(CFns3)3 were(3.4) enormous. Over low-surface silica, metathesis products wereR = t- Buformed (3.1), CMein negligible2(CF3) (3.2) amounts,, CMe(CF3 )whereas2 (3.3), and HMS C(CF 3provided)3 (3.4) yields up to 45%. Figure 1. Typical Schrock carbenes. In the same reaction, Topka et al. [15] similarly observed that the activity of MoO3 supported on the Figure 1. Typical Schrock carbenes. mesoporous molecular sieves MCM-41, MCM-48, and Santa Barbara Amorphous (SBA-15) was

Initially, microporous silica or silica-alumina were used as supports for molybdenum and tungsten oxide catalysts. However, the advantage of mesoporous supports was soon recognized. Ookoshi and Onaka [14] studied 1-octene metathesis over MoO3 supported either on commercially available silica (Brunauer-Emmett-Teller (BET) area 619 m2/g) or on laboratory prepared hexagonal mesoporous silicas (HMS) (d100 spacing ≥3 nm, BET areas between 1230 and 1450 m2/g). The differences in both reaction rates and yields of the reactions were enormous. Over low-surface silica, metathesis products were formed in negligible amounts, whereas HMS provided yields up to 45%. In the same reaction, Topka et al. [15] similarly observed that the activity of MoO3 supported on the mesoporous molecular sieves MCM-41, MCM-48, and Santa Barbara Amorphous (SBA-15) was

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 20

Catalystshigher2019 than, 9, 743that of MoO3 on conventional silica. This enhanced activity was ascribed to better3 MoO of 18 3 dispersion on supports with larger surface areas.

Ph Ph

4.2 Hoveyda-Grubbs 1st 4.1 Grubbs 1st generation. 4.3 Grubbs 2nd generation. generation.

Ph

4.4 Hoveyda–Grubbs 2nd 4.5 Zhan catalyst RC 303 4.6 Grubbs 3rd generation. generation.

4.7 Catalyst with chelating carboxylate 4.8 Zhan catalyst RC 304

Cl- Cl-

a b

4.9 a, b Ammonium tagged Hoveyda–Grubbs 2nd generation.

FigureFigure 2. 2.Typical Typical Grubbs-, Grubbs and-, and Hoveyda–Grubbs-type Hoveyda–Grubbs-type carbenes carbenes used used in in hybrid hybrid catalyst. catalyst.

TheThe immobilization immobilization of of Schrock Schrock and and Grubbs Grubbs complexes complexes on on solid solid supports supports aimed aimed (i) (i to) to increase increase catalystcatalyst stability stability (especially (especially by preventing by preventing bimolecular bimolecular decomposition decomposition reactions), reactions), (ii) to improve (ii) to catalyst improve separationcatalyst separation from the reaction from the mixture, reaction andmixture, (iii) to and reuse (iii) the to reuse catalyst. the catalyst. The mesoporous The mesoporous character character of the supportsof the supports makes it possiblemakes it topossible introduce to i ratherntroduce bulky rather complexes bulky complexes into the pores into and the facilitates pores and the facilitates access of substrate molecules to the catalytic centers. The confinement of catalysts in pores of different sizes

Catalysts 2019, 9, 743 4 of 18 and shapes was also predicted to generate corresponding changes in catalyst selectivity. Therefore, various mesoporous sieves were used as supports to immobilize Schrock and Grubbs catalysts with varying degrees of success [16,17]. Santa Barbara Amorphous (SBA-15) [18] is a mesoporous silica with unique properties that allow its application as an advanced support for metathesis catalysts: uniform, hexagonally-ordered cylindrical mesopores (p6mm symmetry group), thick amorphous walls (3–6 nm [19]), hydrothermal 2 3 stability, large BET areas (SBET = 500–900 m /g), and large pore volume (up to 1.0 cm /g) [20]. The external surface area is usually 10% or less of the total surface [15,20]. Therefore, most of the area available for catalyst accommodation is in the mesopores. The mesopore diameter depends on the preparation method and typically ranges from 6 to 10 nm, although large-pore SBA-15 with a pore diameter up to 18 nm have been prepared using special swelling agents [19,21]. Such pores facilitate the placement of metathesis catalysts on the inner surface of SBA-15: for example, Hoveyda–Grubbs 2nd-generation catalyst 4.4 (see Figure2) has a molecular size of 1.76 1.35 1.05 nm [22], and substrate diffusion to the catalytic centers cause no serious hindrance × × to the reaction. Nevertheless, the existence of U-shaped mesopores was proven by Transition Electron Microscopy (TEM) [23] in large SBA-15 particles, which may have a negative effect on diffusion in the pores. Moreover, in many SBA-15 particles, micropores were observed in the walls, the amount, however, can be affected by template composition and by the calcination procedure [19]. The attachment of transition metal carbenes or their precursors on the surface can proceed in two ways: (1) by direct reaction with surface OH groups and (2) by functionalization of the surface with linkers, that is, special molecules designed for binding the support surface and the metal carbene (or its precursor). In both ways, the character and amount of surface OH groups are essential. In all-siliceous SBA-15, OH groups are weakly acidic (pKa about 4 [24]), and their amount depends on the preparation protocol and on the temperature treatment. For SBA-15 calcined at 550 ◦C, the number of silanol groups accessible to pyridine molecules (determined by NMR) initially reported was 3.7 groups per nm2 [24]. Later, van der Voort et al. [25] reported a total amount of OH groups in SBA-15 determined by IR of 3.5 mmol/g. However, a considerable part of these groups is buried in walls and inaccessible to reagents. The reported amount of OH groups capable of being grafted with trimethylsilyl groups is 1.8 mmol/g, which gives 1.7 OH group per nm2, with a slightly lower value (1.2 OH group per nm2) for the often used hexagonal MCM-41 and cubic MCM-48 particles with narrower pore sizes (2.9 and 2.8 nm of diameter, respectively). At high temperatures, when partial dehydroxylation occurs, the amounts of OH groups decrease. Thus, 1.56 mmol of accessible silanols per gram was reported for SBA-15 partially dehydroxylated at 500 ◦C in vacuo [26], and the amount of available OH groups dropped to 1.07 mmol/g for SBA-15 dehydroxylated at 700 ◦C[27]. In this case, entirely isolated OH groups occur on the surface, and they can be used to prepare “single-site catalysts” via methods of surface organometallic chemistry [28].

2. Well-Defined Heterogeneous Olefin Metathesis Catalysts Prepared by Immobilization of Mo and Ru Organometallic Complexes on SBA-15 These catalysts combining inorganic support and organometallic catalysts are also known as hybrid catalysts. Well-defined olefin metathesis catalysts, i.e., Schrock and Grubbs-type carbenes were immobilized on various types of silica or silica-alumina supports ranging from high-surface microporous silica through mesoporous molecular sieves to micro- or mesoporous zeolites. Microporous silica was the most common type at first, but advances in the development of organized mesoporous silica (especially mesoporous molecular sieves MCM-41, MCM-48, and SBA-15) led to the increased number of applications of these materials as advanced supports. Recently, new types of zeolites (mesoporous, two-dimensional [29–31]) have been shown to be interesting supports for new metathesis heterogeneous catalysts (vide infra). Several review articles and book chapters have been devoted to this topic [16,17,32–36] and therefore we have exclusively focused on catalysts with a SBA-15 support and on their state-of-the-art features. Catalysts 2019, 9, 743 5 of 18 Catalysts 2019, 9, x FOR PEER REVIEW 5 of 20

Generally,Generally, transition metal carbene complexes complexes can can be be attached to to silica silica supports supports by by a) a) covalent bondsbonds and b) non-covalentnon-covalent interactions. The The following following strategies strategies have have been been developed developed for for this this immobilization:immobilization: (1) (1) direct direct grafting grafting of of the the complex complex on on the the support support through through a a reaction reaction of of ligands ligands of of the the carbenecarbene complex complex with with silanol silanol groups, groups, (2) (2) attachmen attachmentt of of the complex through linkers, i.e., molecules with a trialkoxysilyltrialkoxysilyl groupgroup that that is is able able to to react react with with silanol silanol groups groups of theof the support support on oneon sideone side and withand witha reactive a reactive group group that is ablethat to is coordinate able to coordinate a metal in a the metal complex in the (e.g., complex amino, (e.g., phosphino, amino, carboxylate)phosphino, carboxylon the oppositeate) on the site opposite –although site this –although method isthis more method material is more and material time consuming and time than consuming the others, than it theensures others, some it ensures distance some between distance the complex between and the the complex channel and wall the (proximity channel towall the (proximity surface may to a fftheect surfacethe catalytic may affect activity the of catalytic the immobilized activity of carbene the immobilized complex [carbene37]), and co (3)mplex attachment [37]), and via (3) non-covalent attachment viainteractions non-covalent (ionic, interactions hydrogen bond) (ionic, – this hydrogen approach bond) has become– this particularly approach has important become in particularly connection importantwith specific in connection Grubbs-type with complexes specific (videGrubbs infra).-type complexes (vide infra). StabilityStability is is the the key key characteristic characteristic of of every every catalyst. catalyst. In Inthe the case case of hybrid of hybrid catalysts, catalysts, this thismeans means not onlynot onlyis high is highTurnover Turnover number number (TON) (TON) achievable achievable but there but is therealso catalyst is also catalystreusability reusability and no leaching and no ofleaching metal (or of metalother (orcatalyst other residues) catalyst residues)into the reaction into the medium. reaction medium.A good catalyst A good should catalyst be should reusable be manyreusable time manys with times no decrease with no in decrease its catalytic in its activity, catalytic and activity, transition and transitionmetal leaching metal must leaching be negligible must be (nonegligible additional (no purification additional purificationof products offrom products metal or from other metal catalyst orother residues catalyst is needed, residues especially is needed, for pharmaceuticalespecially for pharmaceutical compounds). compounds). Such a good Such catalyst a good catalyst can also can be also applicable be applicable in in flow flow systems. systems. Unfortunately,Unfortunately, the application of well well-defined-defined heterogeneous catalysts catalysts in in flow flow reactors reactors is is still still rather rather rare,rare, and and data data on on leaching leaching and reusing are unavailable for many catalysts reported in literature. SchrockSchrock carbene complexes complexes 3.13.1and and3.3 3.3Mo( Mo(== CHCMe CHCMe2Ph)(2Ph)(=N=N-2,6--2,6-ii-Pr-Pr22CC66H3)(OR))(OR)22 (R(R = CMe33,, CMe(CFCMe(CF3))22)) were were grafted grafted on dehydrated mesoporousmesoporous molecularmolecular sieves sieves MCM-41, MCM-41, MCM-48, MCM-48, and and SBA-15 SBA- 15via via exchange exchange of of alkoxy alkoxy ligands ligands withwith surfacesurface hydroxyls (Scheme (Scheme 3)3)[ [38].38]. Hybrid Hybrid catalysts catalysts with with 1 wt% 1 Mowt% have Mo havebeen prepared. been prepared. The resulting The resulting alcohols alcohols were detected were detected by gas bychromatography gas chromatography (GC), albeit (GC), withoutalbeit without determining determining their quantity, their quantity, so the socomplexes the complexes could couldbe bound be bound to the to surface the surface by one by or one two or Motwo-O Mo-O-Si-bond(s).-Si-bond(s). In metathesis In metathesis of neat of neat 1-heptene, 1-heptene, the the hybrid hybrid cat catalystsalysts exhibited exhibited good good activity, activity, slightlyslightly increasing in the following order: MCM MCM-48-48 < MCMMCM-41-41 << SBASBA-- 15 15 (batch (batch reactor, reactor, TON TON ranging ranging fromfrom 1300 1300 to to 1700) 1700) with with negligible negligible Mo Mo leaching. leaching. In In comparison, comparison, the the same same complexes complexes immobilized immobilized on on dehydroxylateddehydroxylated silica [39] [39] ga gaveve monosiloxy monosiloxy surface surface complexes complexes with with a a well well-determined-determined structure structure and and with high activity in in propene metathesis metathesis (flow (flow reactor, reactor, TON about 188,000) 188,000) and and in in a a metathesis metathesis of of methylmethyl oleate (batch reactor, equilibrium after 24 h and TON = 2000).

C6H6, r. t.

R = CMe(CF3)2, CMe 3 SchemeScheme 3. 3. GraftingGrafting of of Schrock Schrock alkylidenes alkylidenes 3.33.3 andand 3.13.1 onon mesoporous mesoporous molecular molecular sieves.

RuRu carbenes (Grubbs(Grubbs-- and HoveydaHoveyda–Grubbs-type)–Grubbs-type) have have been immobilized on mesoporous molecularmolecular sieves sieves (and (and especially especially on onSBA-15) SBA- more15) more frequently frequently than Schrock-type than Schrock alkylidenes.-type alkylidenes. A possible A possiblereason is reason that Ru is that carbenes Ru carbenes are not are as not oxygen- as oxygen and- moisture-sensitive and moisture-sensitive as Schrock as Schrock alkylidenes alkylidenes and andtherefore therefore their their protection protection by inert by inert atmosphere atmosphere during during immobilization immobilization need need not be not so rigorous.be so rigorous. Some Someof these of these catalysts catalysts operate operate even even in a waterin a wa environment,ter environment, so they so they can can be appliedbe applied in thein the metathesis metathesis of ofwater-soluble water-soluble substrates. substrates. The The main main strategies strategies for for thethe immobilizationimmobilization ofof RuRu carbenes on inorganic supportssupports are summarized inin ReferencesReferences [[4040,41,41].]. TableTable1 1outlines outlines Ru Ru carbenes carbenes immobilized immobilized on on SBA-15. SBA- 15.Their Their immobilization immobilization approach approach is indicated is indicated together together with with their their main main characteristics. characteristics.

Catalysts 2019, 9, x FOR PEER REVIEW 6 of 20 Catalysts 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 20 CatalystsCatalysts 20192019, 9, x FOR, 9, 743PEER REVIEW 6 of 20 6 of 18

Table 1. Survey of Ru carbenes immobilized on Santa Barbara Amorphous (SBA-15) Table 1. Survey of Ru carbenes immobilized on Santa Barbara Amorphous (SBA--15) Table 1. Survey of Ru carbenes immobilized on Santa Barbara Amorphous (SBA-15) Loading, Leaching Carbene Loading, Leaching Number Carbene Immobilization Mode in wt % Activity in % of Starting Reusing Reference Number complexCarbene ImmobilizationImmobilization ModeMode Loading,inin wtwt %% Activityivity inin %% ofof StartingStarting ReusingLeaching Reference Numbercomplexcomplex Immobilization Mode Ru ActivityAmount of Ru Reusing Reference Complex in wt %Ru Ru Amount of Ru in % of Starting Amount of Ru By the linker 4.4 ByBy the the linker 0.85% TON in RCM = 1 4.4 0.85% TON in RCM = 2.3% 3 times [42] 1 4.4 4.4 Si Ag 0.85%0.85% TON in RCM = 2.3% 3 times [42] 1 1 4.5 Si Ag 0.65% 2900TON in RCM = 29002.3% 3 times 2.3%[42][42] 3 times [42] 4.5 4.5 SiSi Ag 0.65%0.65% 2900

By the linkers ByBy the the linkers

4.3 TON in RCM = 2 4.3 4.3 0.3–1.0% TON in RCM = 0.18–7.7% 6 times [43] 2 2 4.6 0.3–1.0%0.3–1.0% 2000 TON in RCM =0.182000–7.7% 6 times 0.18–7.7%[43][43] 6 times [43] 4.6 4.6 2000

ByBy the the linker By the linker TON in RCM = 3 4.7 0.27% TON in RCM = 2.7% 5 times [44] 3 34.7 4.7 Si 0.27%0.27% TON in TON RCM in = RCM = 21002.7% 5 times 2.7%[44] 5 times [44] 3 4.7 Si 0.27% 2100 2.7% 5 times [44][44] SiSi 2100

By the linker ByBy the the linker

4 44.8 4.8 1.07%1.07% RCM, ROMP RCM, ROMP0.1–0.38% 6 times 0.1–0.38%[45] 6 times [45] 4 4.8 1.07% RCM, ROMP 0.1–0.38% 6 times [45][45]

By the linker By the linker Metathesis By the linker Metathesis Metathesisand cross and- cross-metathesis of 5 times 5 timescummulative 5 4.8 0.3% and cross-- 5 times 0.5% [46] 5 4.8 0.3% metathesiscardanol, of TON up to0.5% 330 cummulative [46] TON = 1125 Catalysts 20192019,, 995,, xx FORFOR PEERPEER REVIEWREVIEW4.8 0.3% metathesis77 of ofof 2020 0.5% cummulativecummulative [46][46] cardanol, TON TON = 1125 cardanol,cardanol, TONTON TON = 1125 up to 330 up to 330 up to 330 6–30% 4.2 4.2 RCM TON = 6 6 0.5–1.7%0.5–1.7% RCM TON = 20 inaccording 1 cycle to 6–30%8 times according to pore[47][47] size 8 times [47] 4.4 4.4 20 in 1 cycle pore size

RCM TON = below detection 7 74.1 4.1 --- RCM TON = 20 in 1 cycle.5 belowtimes detection limit[48][48] 5 times [48] C44H99 20 in 1 cycle. limitlimit

4.4 TON up to 600 8 Linker free 0.29–1.0% 0.04%Flow in C H , 14% in DCM Flow conditions possible [49–53] 4.5 batch reactor 6 12 4.4 TON up to 600 0.04% in C66H1212,, conditionsconditions [49[49–53] 8 By meansLinker of Al modification free 0.29–1.0% Cumulative TON about 3000 94.5 4.4 1.5% batch reactor 14% in DCM Catalystpossible leaches in polar medium - [54] 4.5 of the surface batch(propene reactor metathesis)14% in DCM possible 10 4.9a Linker free 1.2% TON up to 3300 in RCM About 1% in DCM and EtOAc 5 times [55] TONCumulative up to 35000 in RCM, TON = More than 10 times 11 4.9b Linker free 0.1–1% About 1% [56,57] TON1600 about in cardanol Catalyst metathesis leaches suitable for flow condition By means of Al modification of 9 RCM4.4= ring closing metathesis, ROMP = ring-opening1.5% metathesis 3000 polymerization,(propene inin TON polarpolar= turnover number,-- DCM = [54]dichloromethane,[54] EtOAc = ethyl acetate. thethe surfacesurface metathesis) medium

TON up to About 1% in 5 times 10 4.9a Linker free 1.2% 3300 in RCM DCM and [55][55]

EtOAc TON up to More than 10 35000 in RCM, timestimes 11 4.9b Linker free 0.1–1% TON = 1600 in About 1% suitablesuitable forfor [56,57][56,57] cardanolcardanol flowflow metathesis conditioncondition

RCM= ring closing metathesis, ROMP= ring--opening metathesis polymerization, TON= turnover number, DCM= dichloromethane, EtOAc= ethyl acetate.

Catalysts 2019, 9, x FOR PEER REVIEW 8 of 20

Immobilization usually proceeded by mixing (or refluxing) the suspension of a support (dehydrated, and/or surface-modified) with the solution of a carbene complex in an inert atmosphere. After a given time, the solid catalyst was isolated (by decantation, filtration, centrifugation), washed, and dried. The support architecture and main texture characteristics were preserved. Only a decrease Catalysts 2019, 9, 743 7 of 18 in SBET area was found in almost all cases. As examples, the N2 adsorption isotherms of SBA-15, 4.4 and 4.5 carbenes immobilized on SBA-15 using hexafluorocarboxylate linker–Table 1, entry 1 (denotedImmobilization as 4.4/SBA-15 usually and 4.5 proceeded/SBA-15) are by displayed mixing in (or Figure refluxing) 3. The S theBET areas suspension (m2/g), void of a volumes support (cm(dehydrated,3/g), and pore and/ ordiameter surface-modified) (nm) were 829, with 1.18, the solution and 6.6 offor a the carbene parent complex SBA-15, in 585, an inert 0.98, atmosphere. and 6.5 for 4.4After/SBA a given-15, and time, 591, the 0.97, solid and catalyst 6.5 for was 4.5/SBA isolated-15. (bySimilarly, decantation, 4.1 immobilization filtration, centrifugation), on SBA-15 using washed, an Nand-heterocyclic dried. The carbene support ( architectureNHC ) linker and (Table main 1, texture entry 7) characteristics led to a decrease were in preserved. SBET from Only734 to a 233 decrease m2/g and in pore size from 7.4 to 5.2 nm, thus highlighting that the mesoporous character was preserved in SBET area was found in almost all cases. As examples, the N2 adsorption isotherms of SBA-15, 4.4 [48].and 4.5Thecarbenes preservat immobilizedion of the SBA on SBA-15-15 structure using in hexafluorocarboxylate the corresponding hybrid linker–Table catalysts1, entrywas also 1 (denoted clearly shown by TEM [47,48]. 2 3 as 4.4/SBA-15 and 4.5/SBA-15) are displayed in Figure3. The SBET areas (m /g), void volumes (cm /g), and poreIn some diameter cases, (nm) OH weregroups 829, remaining 1.18, and on 6.6 the for thesurface parent after SBA-15, its functionalization 585, 0.98, and 6.5 by for linkers4.4/SBA-15, were coveredand 591, 0.97, with and Me 6.53Si for groups4.5/SBA-15. usually Similarly, using trimethylchlorosi4.1 immobilizationlane on or SBA-15 hexamethyldisilazane. using an N-heterocyclic This prevents these OH groups from reacting with Ru complexes, and/or with the substrate2 and increases carbene (NHC ) linker (Table1, entry 7) led to a decrease in SBET from 734 to 233 m /g and in pore thesize hydrophobicity from 7.4 to 5.2 of nm, the thus surface highlighting [42]. Selective that capping the mesoporous of OH groups character on the was external preserved surface [48 (before]. The linkerpreservation and/orof Ru the com SBA-15plex application) structure in the[48, corresponding58] was used to hybrid introduce catalysts catalytically was also active clearly Ru shown species by exclusivelyTEM [47,48 ].into the support channels.

Figure 3. Nitrogen sorption isotherms of the parent SBA-15 (square), 4.4/SBA-15 (circle), and 4.5/SBA-15 Figure(triangle). 3. Nitrogen Adsorption sorption branch isotherms filled, desorptionof the parent branch SBA-15 open (square), points. 4.4 For/SBA clarity,-15 (circle), 100 and and 150 4.5 cm/SBA3/g- 15(Standard (triangle). Temperature Adsorption and branch Pressure) filled, was desorption added to the branch isotherms open ofpoints.4.5/SBA-15 For clarity, and SBA-15, 100 and respectively. 150 cm3/g (STakentandard from T Referenceemperature [42 ]- and© Springer.Pressure) Reproduced was added with to the permission isotherms of Springer.4.5/SBA-15 and SBA-15, respectively. Taken from Reference [42]-© Springer. Reproduced with the permission of Springer. In some cases, OH groups remaining on the surface after its functionalization by linkers were coveredThe withbond Mebetween3Si groups the NHC usually linker using and the trimethylchlorosilane Ru atom (Table 1, entry or hexamethyldisilazane. 7) is very stable. Therefore, This theprevents hybrid these catalyst OH is groups reusable, from and reacting Ru leaching with Ru into complexes, liquid phase and /isor negligible. with the substrate Similarly, and carboxylate increases linkagethe hydrophobicity (Table 1, en oftries the 1 surface and 3) [is42 sufficiently]. Selective cappingfirm. However, of OH groups in both on cases, the external several surfacestep synthesis (before procedureslinker and/or are Ru necessary complex for application) linker preparation. [48,58] was On used the other to introduce hand, pyridine catalytically and phosphine active Ru specieslinkers areexclusively commercially into the available, support and channels. the preparation of hybrid catalysts with them is easy (Table 1, entries 2, 4, The5). Nevertheless, bond between the the catalytic NHC linker reaction and starts the Ru with atom pyridine (Table1 ,or entry phosphine 7) is very ligand stable. dissociation, Therefore, therebythe hybrid vacating catalyst a is place reusable, for substrate and Ru leaching coordination. into liquid This phase suggests is negligible. that the catalytically Similarly, carboxylate active Ru specieslinkage (Tableare not1, entries bound 1 to and the 3) surface is su ffi ciently during firm. the whole However, catalytic in both circle. cases, The several reusability step synthesis of the correspondingprocedures are hybrid necessary catalysts, for linker low preparation. Ru leaching, On and the split other (filtration) hand, pyridine tests, which and phosphine have confirmed linkers theare commerciallyreaction heterogeneity available, [59], and thehave preparation been explained of hybrid based catalysts on the withnotion them that is cataly easy (Tabletically1 ,active entries Ru 2, species4, 5). Nevertheless, are captured the by catalytic free linkers reaction during starts their with movement pyridine in or the phosphine channels ligand and that dissociation, this dissociation thereby- revacating-capturing a place process for substrate is repeated coordination. many times This during suggests the that reaction the catalytically [43]. The immobilization active Ru species of Hoveydaare not bound–Grubbs to the carbenes surface using during an theisopropoxys whole catalytictyrene linker circle. (Table The reusability 1, entry 6) of is the associated corresponding with a hybrid catalysts, low Ru leaching, and split (filtration) tests, which have confirmed the reaction heterogeneity [59], have been explained based on the notion that catalytically active Ru species are captured by free linkers during their movement in the channels and that this dissociation-re-capturing process is repeated many times during the reaction [43]. The immobilization of Hoveyda–Grubbs Catalysts 2019, 9, x FOR PEER REVIEW 9 of 20 similar problem: in the first metathesis step, this isopropoxy styrene ligand must be replaced by an alkylidene from a substrate according to the general mechanism of metathesis (see Scheme 2) and therefore the catalytic centres will lose their bond with the surface. This explains the relatively high catalyst leaching, which increases with the pore size [47]. “Linker free” immobilization of Hoveyda–Grubbs 2nd-generation carbenes on silica materials including SBA-15 (Table 1, entry 8) is a very advantageous method for the preparation of hybrid metathesis catalysts based on a simple mixing of a carbene solution and a silica support. This method was first described by Sels et al. [49], and the catalysts prepared using this approach turned out to be very active in all types of metathesis reactions. When used in a non-polar medium (e.g., cyclohexane), they behaved as true heterogeneous catalysts: they were reusable with very low Ru leaching and applicable in flow reactors as well. Hoveyda–Grubbs 2nd-generation carbenes with ionic tags (4.9a, b) provide hybrid catalystsCatalysts that 2019also, 9work, 743 in polar solvents such as dichloromethane or ethyl acetate 8 of 18 and that are highly active in the metathesis of various substrates (Table 1, entries 10, 11). Considering their stability, activity, and selectivity, these catalysts are among the best hybrid metathesis catalysts. An in-depth understandingcarbenes of using non-covalent an isopropoxystyrene interactions betweenlinker (Table the1 , carbene entry 6) complex is associated and with the a similar problem: in support in these cataltheysts first remains metathesis elusive. step, Although this isopropoxy originally styrene described ligand as must physisorption, be replaced the by an alkylidene from a strength of this bond suggestssubstrate that according stronger to interactions, the general mechanismsimilar to hydrogen of metathesis bonds, (see must Scheme come2) into and therefore the catalytic play. In this context, thecentres immobilization will lose their of Hoveyda bond with–Grubbs the surface. 2nd-generation This explains carben thee relativelyon SBA-15 high with catalyst leaching, which an Al-modified surfaceincreases (Table 1, withentry the 9) porewith sizethe proposed [47]. Al-Cl-Ru bond is inspiring. “Linker free” immobilization of Hoveyda–Grubbs 2nd-generation carbenes on silica materials 3. Application of SBAincluding-15-Based SBA-15Hybrid Catalysts (Table1, entry in Metathesis 8) is a very Polymerization advantageous Reactions method for the preparation of hybrid metathesis catalysts based on a simple mixing of a carbene solution and a silica support. This method Metathesis polymerization reactions (ADMET polymerization and ROMP) are still a domain of was first described by Sels et al. [49], and the catalysts prepared using this approach turned out to be homogeneous catalysts. Nevertheless, several successful attempts to use hybrid metathesis catalysts very active in all types of metathesis reactions. When used in a non-polar medium (e.g., cyclohexane), in ROMP are noteworthy. The Schrock Mo-alkylidene complex 3.3 grafted on SBA-15 (3.3/SBA-15) they behaved as true heterogeneous catalysts: they were reusable with very low Ru leaching and has been used in a ROMP of cyclooctene (COE) and norbornene (NBE) (Scheme 4) and delivered a applicable in flow reactors as well. Hoveyda–Grubbs 2nd-generation carbenes with ionic tags (4.9a, b) poly(COE) of Mw = 210,000 in 87% yield and a poly(NBE) of Mw = 1,800,000 in 60% yield [38]. During provide hybrid catalysts that also work in polar solvents such as dichloromethane or ethyl acetate and polymerization, the resulting polymers were continuously released into the liquid phase due to chain that are highly active in the metathesis of various substrates (Table1, entries 10, 11). Considering their transfer by the polymer. The molecular weight of the polymer could be effectively reduced by adding stability, activity, and selectivity, these catalysts are among the best hybrid metathesis catalysts. An 1-heptene as a chain transfer agent. The content of Mo in the isolated polymer was 6 ppm [60]. in-depth understanding of non-covalent interactions between the carbene complex and the support The Ru carbene precursors (RuCl2(p-cymene))2 and RuCl2(p-cymene)(PCy3), where Cy = in these catalysts remains elusive. Although originally described as physisorption, the strength of cyclohexane, immobilized on SBA-15 provided high-molecular weight poly(NBE) in high yields (in this bond suggests that stronger interactions, similar to hydrogen bonds, must come into play. In the former, after activation with (trimethylsilyl)diazomethane) [61,62]. Ru carbene 4.5 immobilized this context, the immobilization of Hoveyda–Grubbs 2nd-generation carbene on SBA-15 with an on SBA-15 via carboxylate linker produced a poly(COE) of Mw = 230,000 and a poly(NBE) of Mw = Al-modified surface (Table1, entry 9) with the proposed Al-Cl-Ru bond is inspiring. 2,200,000. The Ru content of the polymer was 16 ppm [42]. The same carbene complex 4.5 immobilized in “linker3.-free“ Application mode was of applied SBA-15-Based in ROMP Hybrid of COE, Catalysts yieldingin a polymer Metathesis of M Polymerizationw = 330,000 Reactions [51]. Large-pore supports were necessary for high polymer yields (Figure 4): the highest yield was obtained over a hybrid catalystMetathesis using polymerization an SBA-15 with reactions a pore (ADMET size of 11.1polymerization nm. Conversely, and ROMP) the are still a domain of premature end of this homogeneouspolymerization catalysts. over a catalyst Nevertheless, on ordinary several silic successfula particles attempts suggests to usethat hybrid the metathesis catalysts narrow pores are blockedin ROMP by growing are noteworthy. polymer chains. The Schrock Mo-alkylidene complex 3.3 grafted on SBA-15 (3.3/SBA-15) In all cases, polymerizationhas been usedover inhybrid a ROMP catalysts of cyclooctene provided polymers (COE) and with norbornene a high polydispersity (NBE) (Scheme 4) and delivered a poly(COE) of M = 210,000 in 87% yield and a poly(NBE) of M = 1,800,000 in 60% yield [38]. During index (Mw/Mn > 2), which is most likelyw associated with the slow reaction initiationw and with the intensive chain transfer.polymerization, The main advantage the resulting of applying polymers a hybrid were continuouslycatalyst was the released easy intoseparation the liquid phase due to chain of polymers from catalyststransfer and by the the preparationpolymer. The of molecular polymers weight with a of very the polymer low content could of be catalyst effectively reduced by adding Catalysts 2019, 9, x FOR PEER REVIEW 10 of 20 residues without requiring1-heptene additional as a chain purification. transfer The agent. catalysts, The content however, of Mo were in the not isolated reused. polymer was 6 ppm [60].

n n n n

Scheme 4. Ring-opening metathesisScheme 4. polymerizationRing-opening metathesis (ROMP) of polymerization cyclooctene and (ROMP) norbornene. of cyclooctene and norbornene.

The Ru carbene precursors (RuCl2(p-cymene))2 and RuCl2(p-cymene)(PCy3), where Cy = cyclohexane, immobilized on SBA-15 provided high-molecular weight poly(NBE) in high yields (in the former, after activation with (trimethylsilyl)diazomethane) [61,62]. Ru carbene 4.5 immobilized on SBA-15 via carboxylate linker produced a poly(COE) of Mw = 230,000 and a poly(NBE) of Mw = 2,200,000. The Ru content of the polymer was 16 ppm [42]. The same carbene complex 4.5 immobilized in “linker-free“ mode was applied in ROMP of COE, yielding a polymer of Mw = 330,000 [51]. Large-pore supports were necessary for high polymer yields (Figure4): the highest yield was obtained over a hybrid catalyst using an SBA-15 with a pore size of 11.1 nm. Conversely, the premature end of this polymerization over a catalyst on ordinary silica particles suggests that the narrow pores are blocked by growing polymer chains.

Figure 4. ROMP of cyclooctene over 4.5/SiO2 (stars), 4.5/MCM-41(squares), 4.5/SBA-15 d = 6.3 nm (circles), 4.5/SBA-15 d = 11.1 nm (diamonds). Cyclohexane, T = 13 °C, molar ratio cyclooctene/Ru = 500, c0cyclooctene = 0.6 mol/L, numbers at individual experimental points give Mw of polymer in thousands. Taken from Reference [51] © Elsevier. Reproduced with the permission of Elsevier.

4. Heterogeneous Olefin Metathesis Catalysts Consisting of Mo and W Oxides on SBA-15

MoO3 and WO3 deposited on silica or silica–alumina supports are some of the earliest metathesis catalysts. These relatively inexpensive catalysts are widely used, especially in large-scale petrochemical processes. With the discovery of new advanced supports, such as zeolites and mesoporous molecular sieves, molybdenum and tungsten oxides particularly attracted the attention of many research groups in their efforts to increase the effectiveness of commercially important catalysts [9,63]. Several studies showed that various mesoporous siliceous materials (MCM-41 [15,64,65], KIT-6 [66]), or MWW-zeolites [65,67], and MFI-zeolites [65,67,68] were substantially more active than ordinary silica or silica-alumina. However, the most commonly used support was SBA- 15 for its large pores and high BET areas. Table 2 outlines catalysts based on molybdenum and tungsten oxides supported on SBA-15.

Table 2. Survey of Mo and W oxide catalysts supported on SBA-15.

Method of W, Mo WOx,MoOx Species Number Catalyst Loading Substrate Activity Reference Source Preparation Wt % TOF = 0.003 thermal 1 MoO3 4–12 1-octene mol/molMo.s [15] spreading at 40 °C ammonium heptamolybdate 2 ion exchange 10.1 - - [69] (NH4)6Mo7O24.4H2O

TOF = 2.06 ammonium mmol/molMo.s 3 heptamolybdate ion exchange 9.7 propene [70] at 50 °C (NH4)6Mo7O24.4H2O

Catalysts 2019, 9, x FOR PEER REVIEW 10 of 20

n n Catalysts 2019, 9, 743 9 of 18 Scheme 4. Ring-opening metathesis polymerization (ROMP) of cyclooctene and norbornene.

Figure 4. ROMP of cyclooctene over 4.5/SiO2 (stars), 4.5/MCM-41(squares), 4.5/SBA-15 d = 6.3 nm (circles),Figure 4.4.5 ROMP/SBA-15 of dcyclooctene= 11.1 nm (diamonds). over 4.5/SiO Cyclohexane,2 (stars), 4.5/MCMT = 13-◦41(squares),C, molar ratio 4.5 cyclooctene/SBA-15 d /Ru= 6.3= 500,nm 0 c(circles),cyclooctene 4.5=/SBA0.6 mol-15 d/L, = numbers11.1 nm at(diamonds). individual Cyclohexane, experimental pointsT = 13 give°C, molarMw of ratio polymer cyclooctene/Ru in thousands. = Taken500, c0cyclooctene from Reference = 0.6 mol/L, [51 ]numbers©Elsevier. at individual Reproduced experimental with the permission points give of M Elsevier.w of polymer in thousands. Taken from Reference [51] © Elsevier. Reproduced with the permission of Elsevier. In all cases, polymerization over hybrid catalysts provided polymers with a high polydispersity index4. Heterogeneous (Mw/Mn > 2), Olefin which Metathesis is most likely Catalysts associated Consisting with theof Mo slow and reaction W Oxides initiation on SBA and-15 with the intensive chain transfer. The main advantage of applying a hybrid catalyst was the easy separation of MoO3 and WO3 deposited on silica or silica–alumina supports are some of the earliest metathesis polymers from catalysts and the preparation of polymers with a very low content of catalyst residues catalysts. These relatively inexpensive catalysts are widely used, especially in large-scale without requiring additional purification. The catalysts, however, were not reused. petrochemical processes. With the discovery of new advanced supports, such as zeolites and 4.mesoporous Heterogeneous molecular Olefin sieves, Metathesis molybdenum Catalysts and Consisting tungsten oxides of Mo particularly and W Oxides attracted on SBA-15 the attention of many research groups in their efforts to increase the effectiveness of commercially important catalystsMoO [9,3 and63].WO Several3 deposited studies on showed silica or silica–alumina that various mesoporous supports are somesiliceous of the materials earliest metathesis (MCM-41 catalysts.[15,64,65], These KIT-6 relatively [66]), or MWW inexpensive-zeolites catalysts [65,67], are and widely MFI- used,zeolites especially [65,67,68] in large-scalewere substantially petrochemical more processes.active than With ordinary the discovery silica or silica of new-alumina. advanced However, supports, the such most as commonly zeolites and used mesoporous support molecularwas SBA- sieves,15 for itsmolybdenum large pores and and tungsten high BET oxides areas. particularly Table 2 outlines attracted catalysts the attention based of omanyn molybdenum research groups and intungsten their eoxidesfforts tosupported increase on the SBA effectiveness-15. of commercially important catalysts [9,63]. Several studies showed that various mesoporous siliceous materials (MCM-41 [15,64,65], KIT-6 [66]), or MWW-zeolites [65,67Table], and 2. MFI-zeolitesSurvey of Mo [and65,67 W, 68oxide] were catalysts substantially supported more on SBA active-15. than ordinary silica or silica-alumina. However, the most commonly used support was SBA-15 for its large pores and high Method of W, Mo BET areas.WO Tablex,MoO2 outlinesx Species catalysts based on molybdenum and tungsten oxides supported on SBA-15. Number Catalyst Loading Substrate Activity Reference Source Preparation Wt % Table 2. Survey of Mo and W oxide catalysts supported on SBA-15. TOF = 0.003 thermal 1 MoO3 Method of Catalyst 4–12W, Mo Loading1-octene mol/molMo.s [15] Number WOx,MoOx Species Source spreading Substrate Activity Reference Preparation Wt % at 40 °C ammonium TOF = 0.003 1 MoO thermal spreading 4–12 1-octene mol/mol .s [15] heptamolybdate3 Mo 2 ion exchange 10.1 - at- 40 ◦C [69] ammonium heptamolybdate 2 (NH4)6Mo7O24.4H2O ion exchange 10.1 - - [69] (NH4)6Mo 7O24.4H2O TOF = 2.06 ammonium heptamolybdate 3 ion exchange 9.7 propene TOFmmol = 2.06/molMo .s [70] (NHammonium4)6Mo7O24.4H 2O mmol/molat 50Mo◦C.s 3 ammoniumheptamolybdate heptamolybdate ion exchange 9.7 propene 60% conversion [70] 4 wet impregnation 3 1-butene [71] (NH ) Mo O .4H O at 50at 450°C C (NH4)6Mo4 6 7O724.4H24 2O2 ◦ 1-octene, TOF = 0.008 MoO2(acac)2 thermal spreading, wet 5 4–12 1-dodecene, mol1-oct/molMo.s [72] MoO2(gly)2 impregnation 1-tetradecene at 40 ◦C 40% conversion 6 MoO3, MoO2(acac)2 thermal spreading 6 2-pentene [65] at 500 ◦C Catalysts 2019, 9, 743 10 of 18

Table 2. Cont.

Method of Catalyst W, Mo Loading Number WO ,MoO Species Source Substrate Activity Reference x x Preparation Wt % ammonium heptamolybdate 65% conversion 7 direct co- condensation 1.3–10.8 1-butene [71] (NH4)6Mo7O24.4H2O at 450 ◦C wet impregnation on ammonium heptamolybdate 65% conversion 8 alumina- modified ca. 6 1-butene [73] (NH ) Mo O .4H O at 90 C 4 6 7 24 2 surface ◦ ammonium metatungstate 10 wt% of WO 75% conversion 9 wet impregnation 3 2-butene [64] (NH ) H W O xH O (8% of W) at 550 C 4 6 2 12 40· 2 ◦ Sodium tungstate 5% and 9% of 87% conversion 10 direct co- condensation 2-butene [64,74] NaWO 2H O WO at 550 C 4· 2 3 ◦ 56.5% ammonium metatungstate 11 wet impregnation 6 2-pentene conversion at [65] (NH4)6H2W12O40 xH2O · 550 ◦C Sodium tungstate direct co- 92% conversion 12 Si/W=35 1-butene [75,76] NaWO 2H O condensation at 350 C 4· 2 ◦ acac = acetylacetonate, gly = glycolate, TOF = turnover frequency.

In principal, the catalysts were prepared using the following approaches: (a) thermal spreading, (b) wet impregnation, (c) ion exchange, and (d) hydrothermal direct co-condensation. (a) Thermal spreading is a very simple method consisting of heating a physical mixture of SBA-15 and the source of Mo to 500 ◦C for several hours in air (Table2, entries 1, 5, 6). (b) SBA-15 impregnation with an appropriate amount of a water solution of Mo or W compound followed by drying and calcination is a commonly used method for wet impregnation (Table2, entries 4, 5, 8, 9, 11). (c) In the ion exchange method (Table2, entries 2, 3), SBA-15 is modified by a reaction with trimethoxysilylpropylamine and then with HCl, followed by anion exchange with ammonium heptamolybdate in water. During calcination, the organic parts are removed, and MoOx species remain on the supports. According to the authors, this method provides a better control of the dispersion of Mo species on the surface than wet impregnation [77]. In all previous catalysts given in (a), (b), and (c), the mesoporous structure of the SBA-15 support was preserved. (d) The direct co-condensation method was used for the synthesis of Mo- and W-containing silica materials by adding a metal oxide source to the mixture for the hydrothermal preparation of SBA-15. Characterization using current physico-chemical methods showed that the resulting materials had hexagonal channels and textural parameters similar to SBA-15 and Mo- or W-oxide species dispersed in the silica framework (Table2, entries 7, 10, 12). All these systems are typical “ill-defined” catalysts. The real catalytically active centres are surface carbenes, which are formed “in situ” from their closest precursors on the surface and from substrate molecules. The number of surface carbenes was estimated as 2% of the original metal content [70]. The formation of the closest precursors of surface carbenes proceeds during the activation process (heating the catalyst to approximately 500 ◦C, usually in an inert atmosphere). Under the same conditions, catalysts are often regenerated. Isolated (-O-)2M(=O)2 species are usually considered the closest precursors of surface carbenes and therefore perfect dispersion of the metal oxide along the surface is an essential condition for catalyst activity [9,70,78]. In this regard, the high BET area of SBA-15 is an important advantage. The experimental work in this field was accompanied by a computational approach to the study with the aim to describe the surface process on a molecular level and to help in understanding the role of support [79]. In computational studies of Mo oxide species on silica surface DFT methods were usually used and silica support was approximated by a structure similar to β-cristobalite [80–82]. Catalytic activity depends on the (a) characteristics of the support (its texture parameters and acidity), (b) method of metal deposition and metal loading, (c) parameters of the activation process, (d) quality of the substrate, and (e) reaction conditions. This high number of variables complicates any comparison between activity data from different sources. Nevertheless, some general remarks can be made. (i) The acidity of the supports presumably increases the activity of the catalyst. The beneficial effect of Al modification of the SBA-15 (Table2, entry 8) was ascribed to the formation of Brønsted acid sites, which improve Mo dispersion. Conversely, acidity induces double bond isomerization Catalysts 2019, 9, x FOR PEER REVIEW 12 of 20

and textural parameters similar to SBA-15 and Mo- or W-oxide species dispersed in the silica framework (Table 2, entries 7, 10, 12). All these systems are typical “ill-defined” catalysts. The real catalytically active centres are surface carbenes, which are formed “in situ” from their closest precursors on the surface and from substrate molecules. The number of surface carbenes was estimated as 2% of the original metal content [70]. The formation of the closest precursors of surface carbenes proceeds during the activation process (heating the catalyst to approximately 500 °C, usually in an inert atmosphere). Under the same conditions, catalysts are often regenerated. Isolated (-O-)2M(=O)2 species are usually considered the closest precursors of surface carbenes and therefore perfect dispersion of the metal oxide along the surface is an essential condition for catalyst activity [9,70,78]. In this regard, the high BET area of SBA-15 is an important advantage. The experimental work in this field was accompanied by a computational approach to the study with the aim to describe the surface process on a molecular level and to help in understanding the role of support [79]. In computational studies of Mo oxide species on silica surface DFT methods were usually used and silica support was approximated by a structure similar to β-cristobalite [80–82]. Catalytic activity depends on the (a) characteristics of the support (its texture parameters and acidity), (b) method of metal deposition and metal loading, (c) parameters of the activation process, (d) quality of the substrate, and (e) reaction conditions. This high number of variables complicates any comparison between activity data from different sources. Nevertheless, some general remarks can be made. (i) The acidity of the supports presumably increases the activity of the catalyst. The Catalysts 9 beneficial2019 effect, , 743 of Al modification of the SBA-15 (Table 2, entry 8) was ascribed to the formation11 of 18of Brønsted acid sites, which improve Mo dispersion. Conversely, acidity induces double bond andisomerization alkene oligomerization and alkene oligomerization [67]. (ii) The dependence [67]. (ii) The of dependence activity on of metal activity loading on metal has a loading maximum: has ata maximum: high metal at concentrations, high metal concentrations, the dispersion the is dispersion imperfect, andis imperfect, a part of and the metala part formsof the metalmetal oxideforms microcrystals,metal oxide microcrystals, which do not which contribute do not tocontribute the catalytic to the activity. catalytic The activity. optimal The metal optimal loading metal usuallyloading rangesusually from ranges 4 wt% from to 84 wt%wt% ofto the 8 wt% metal of [ 72the]. (iii)metal Mo [72]. catalysts (iii) Mo usually catalysts operate usually at lower operate temperatures at lower thantemperatures W catalysts. than In W some catalysts. cases In (Table some2, cases entries (Table 1, 3, 5,2, 8),entries Mo catalysts 1, 3, 5, 8), were Mo activecatalysts at temperatureswere active at belowtemperat 100ures◦C. (iv)below In most100 °C. cases, (iv) MoIn most and Wcases, oxide Mo catalysts and W oxide were usedcatalysts in the were metathesis used in ofthe light metathesis olefins. of light olefins. Nevertheless, these catalysts were also active in the metathesis of longer chain alkenes Nevertheless, these catalysts were also active in the metathesis of longer chain alkenes C8–C14 (Table2, entriesC8–C14 1(Table and 5). 2, entries 1 and 5).

5.5. Comparison Comparison of of SBA-Based SBA-Based Olefin Olefin Metathesis Metathesis Catalyst Catalyst with with Catalysts Catalysts using using other other Advanced Advanced SupportsSupports (Mesoporous(Mesoporous MolecularMolecular SievesSieves andand Zeolites)Zeolites) SystematicSystematic studiesstudies comparing comparing the the activity activity of catalystsof catalysts based based on SBA-15 on SBA with-15 thatwith of that catalysts of catalysts using otherusing mesoporousother mesoporous molecular molecular sieves sieves and/or an zeolitesd/or zeolites under under the same the conditionssame conditions are rather are rather rare. Most rare. existingMost existing reports reports focus on focus Ru hybridon Ru hybrid catalysts. catalysts. Using phosphine Using phosphine linkers, linkers, Hoveyda–Grubbs-type Hoveyda–Grubbs carbene-type 4.8carbenewas immobilized4.8 was immobilized on several on all-siliceous several all mesoporous-siliceous mesoporous molecular sieves molecular differing sieves in architecture differing in 2 andarchite porecture size: and(i) pore hexagonal size: (i) hexagonal channel-like channel MCM-41-like MCM (SBET-41= 972(SBET m= 972/g, md 2=/g,4.0 d = nm),4.0 nm), (ii) (ii) SBA-15 SBA- 2 (15SBET (SBET= =766 766 m m/2g,/g, dd = 6.26.2 nm), nm), (iii) (iii) cubic cubic three three-dimensional-dimensional (3D) (3D) pore pore-like-like MCM MCM-48-48 (SBET (S =BET 756= m7562/g, 2 2 md =/ g,6.0d nm),= 6.0 and nm), (iv) and cubic (iv) cage cubic-like cage-like SBA-16 ( SBA-16SBET = 796 (S mBET2/g,= d796entrance m = /4.7g, d nm).entrance In RCM= 4.7 of nm). N,N In-diallyl RCM- of2,2,2 N,N-diallyl-2,2,2-trifluoroacetamide-trifluoroacetamide (DAF) (Scheme (DAF) 5), the (Scheme initial5), reaction the initial rate reaction and/or rate conversion and /or conversion achieved achievedincreased increased with the withpore thesize pore of the size catalyst of the support catalyst supportin the following in the following order: MCM order:-41< MCM-41 SBA-16<

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Scheme 5. Ring-closing metathesis (RCM) of N,N-diallyl-2,2,2-trifluoroacetamide. Scheme 5. Ring-closing metathesis (RCM) of N,N-diallyl-2,2,2-trifluoroacetamide.

Figure 5. RCM of N,N-diallyl-2,2,2-trifluoroacetamide (DAF) with catalysts 4.8 (), 4.8/SBA-15 0 Figure( ), 4.8 5./MCM-48 RCM of (N,NN), -4.8diallyl/SBA-16-2,2,2 (-Htrifluoroacetamide), and 4.8/MCM-41 (DAF) (). with 30 ◦ C,catalysts Ru/DAF 4.8 =(1:100,), 4.8/SBA toluene,-15 ( c), 4.8(DAF)/MCM= 0.15-48 ( mol),/ L.4.8 Taken/SBA- from16 ( Reference), and 4.8 [/MCM45] ©Elsevier.-41 (). Reproduced30 ◦C, Ru/DAF with = the1:100, permission toluene, ofc0 Elsevier.(DAF) = 0.15 mol/L. Taken from Reference [45] © Elsevier. Reproduced with the permission of Elsevier.

Similar effects of support pore size on the reaction rate were observed in metathesis of methyl oleate (Scheme 6) over 4.6 immobilized by phosphine linkers on MCM-41 (d = 4.0), and two SBA-15 supports with different pore diameters (d = 6.9 and d = 11.1 ): the reaction rates (at inflexion points of conversion curves) increased with the pore size (Figure 6) [43].

Scheme 6. Metathesis of methyl oleate.

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Scheme 5. Ring-closing metathesis (RCM) of N,N-diallyl-2,2,2-trifluoroacetamide.

Figure 5. RCM of N,N-diallyl-2,2,2-trifluoroacetamide (DAF) with catalysts 4.8 (), 4.8/SBA-15 (), 0 Catalysts4.82019/MCM, 9, 743-48 (), 4.8/SBA-16 (), and 4.8/MCM-41 (). 30 ◦C, Ru/DAF = 1:100, toluene, c (DAF) 12= of 18 0.15 mol/L. Taken from Reference [45] © Elsevier. Reproduced with the permission of Elsevier.

SimilarSimilar eeffectsffects ofof supportsupport porepore sizesize onon thethe reactionreaction raterate werewere observedobserved inin metathesismetathesis ofof methylmethyl oleateoleate (Scheme(Scheme6 6)) over over 4.64.6 immobilizedimmobilized byby phosphinephosphine linkerslinkers onon MCM-41MCM-41 ((dd= = 4.0),4.0), andand twotwo SBA-15SBA-15 supportssupports withwith didifferentfferent porepore diameters diameters ( d(d= = 6.96.9 andandd d= = 11.1 ):): the reaction rates (at inflexioninflexion pointspoints ofof conversionconversion curves)curves) increasedincreased withwith thethe porepore sizesize (Figure(Figure6 )[6) 43[43].].

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SchemeScheme 6.6. Metathesis ofof methylmethyl oleate.oleate.

60

50

40

30

20

conversion, %

10

0 0 20 40 60 80 100 120 time, min

Figure 6. Effect of catalyst pore size on conversion in the metathesis of methyl oleate. Toluene, 60 ◦C, 0 RuFigure/MeOl 6.= Effect1:250, of c catalyMeOl =st 0.15pore mol size/L, on catalysts conversion4.6/ SBA-15in the metathesis (d = 6.9 nm) of methyl (H), 4.6 oleate./SBA-15LP Toluene, (d = 6011.1 °C, nm)Ru/MeOl ( ), 4.6 =/MCM-41(4.0 1:250, c0MeOl = nm) 0.15 (mol/L,). Taken catalysts from 4.6/ ReferenceSBA-15 [(43d =] ©6.9American nm) (), 4.6 Chemical/SBA-15LP Society (d = 11.1 (ACS). nm) Reproduced(), 4.6/MCM with-41(4.0 the permission nm) (). of Taken ACS Publications. from Reference [43] © American Chemical Society (ACS). Reproduced with the permission of ACS Publications. Similar trends were also reported for linker-free immobilized Hoveyda–Grubbs carbene 4.5. In RCM of ( )-β-citronellene and metathesis of 1-decene, the initial reaction rates increased with the pore − diameter in the following order: 4.5/MCM-41 (d = 4.0 nm) < 4.5/MCM-48 (d = 6.0 nm) < 4.5/SBA-15 (d = 6.9 nm) < 4.5/SBA-15LP (d =11.1 nm) [51]. The comparison between SBA-15 particles with the zeolites MCM-22, MCM-36,50 and MCM-56 as supports for ammonium-tagged carbene 4.9a is given in Reference [83] for metathesis of methyl oleate (Figure7). The initial reaction rate of the catalyst based on SBA-15 was higher than that of catalysts based on zeolitic supports. 40

30

20

oleate conversion, % 10

0 0 50 100 150 200 250 300 time, min

Figure 7. Self-metathesis of methyl oleate over 4.9a /SBA-15 (■), 4.9a /MCM-22 (▲), 4.9a /MCM-56

(▼), and 4.9a /MCM-36 (●). Toluene, 60 °C, molar ratio oleate/Ru = 250, cMeOl =0.15 mol/L. Taken from Reference [83].

The characteristics of the support significantly affected not only the activity but also the selectivity of the catalyst. In ADMET of 1,9-decadiene (Scheme 7) over 4.5 on MCM-41 (d = 4.0 nm), SBA-15 (d = 6.9), and SBA-15LP (d = 11.1 nm) [51], the composition of oligomeric products significantly varied with the catalyst support used. When increasing the pore diameter, the selectivity to dimer decreased, and the selectivity to higher oligomer increased. In ring-opening/ring-closing metathesis of cyclooctene (Scheme 8) over 4.4 linker-free immobilized on various supports, the catalysts on SBA-15 and MCM-41were the most active. However, they strongly differed in selectivity.

Catalysts 2019, 9, x FOR PEER REVIEW 14 of 20

60

50

40

30

20

conversion, %

10

0 0 20 40 60 80 100 120 time, min

Figure 6. Effect of catalyst pore size on conversion in the metathesis of methyl oleate. Toluene, 60 °C, Ru/MeOl = 1:250, c0MeOl = 0.15 mol/L, catalysts 4.6/SBA-15 (d = 6.9 nm) (), 4.6/SBA-15LP (d = 11.1 nm) (), 4.6/MCM-41(4.0 nm) (). Taken from Reference [43] © American Chemical Society (ACS). Reproduced with the permission of ACS Publications.

Catalysts 2019, 9, 743 13 of 18

50

40

30

20

oleate conversion, % 10

0 0 50 100 150 200 250 300 time, min

Figure 7. Self-metathesis of methyl oleate over 4.9a /SBA-15 (), 4.9a /MCM-22 (N), 4.9a / MCM-56 (H),

andFigure4.9a 7./MCM-36 Self-metathesis (). Toluene, of methyl 60 ◦ C,oleate molar over ratio 4.9a oleate /SBA/Ru-15 =(■250,), 4.9acMeOl /MCM=0.15-22 mol(▲),/L. 4.9a Taken /MCM from-56 Reference(▼), and [4.9a83]. /MCM-36 (●). Toluene, 60 °C, molar ratio oleate/Ru = 250, cMeOl =0.15 mol/L. Taken from Reference [83]. The characteristics of the support significantly affected not only the activity but also the selectivity of theThe catalyst. characteristics In ADMET of of the 1,9-decadiene support significantly (Scheme7) affectedover 4.5 noton MCM-41 only the (d activity= 4.0 nm), but SBA-15 also the (dselectivity= 6.9), and of the SBA-15LP catalyst. ( dIn= ADMET11.1 nm) of [ 511,9],-decadiene the composition (Scheme of 7) oligomeric over 4.5 on products MCM-41 significantly (d = 4.0 nm), variedSBA-15 with (d the= 6.9), catalyst and support SBA-15LP used. (d When= 11.1 increasing nm) [51], the the pore composition diameter, the of oligomericselectivity to products dimer Catalystsdecreased,Catalystssignificantly 2019 2019 and, ,9 9, ,x variedx FOR theFOR selectivityPEER PEER with REVIEW REVIEW the tocatalyst higher support oligomer used. increased. When increasing In ring-opening the pore/ring-closing diameter, metathesisthe selectivity1515 of of of20 20 cycloocteneto dimer decreased, (Scheme8 )and over the4.4 selectivitylinker-free to immobilized higher oligomer on various increased. supports, In ring the-opening/ring catalysts on SBA-15-closing InandInmetathesis thethe MCM-41were casecase ofof of SBASBA cyclooctene- the-15,15, most ththee polymerpolymer active. (Scheme However, waswas 8) formed formed over they 4.4 withwith stronglylinker 45%45%- free selectivity. diselectivity.ff ered immobilized in selectivity. InIn turn,turn, on inin various Inthethe the casecase case supports, ofof of MCMMCM SBA-15,- -41,41, the nothenocatalysts polymer polymer on was wasSBA formedformed,formed,-15 and with MCM andand 45% higherhigher-41were selectivity. oligomersoligomers the most In turn,(from (fromactive. in 66 However, the toto 20 case20 monomericmonomeric of they MCM-41, strongly units)units) no polymer differed prevailedprevailed in was withselectivity.with formed, 65%65% selectivityandselectivity higher [52]. [52]. oligomers (from 6 to 20 monomeric units) prevailed with 65% selectivity [52].

(CH2)6 n-1 n catalystcatalyst (CH2)6 n-1 n (CH2)6 (CH2)6 n n SchemeScheme 7. 7. Acyclic Acyclic diene diene me metathesismetathesistathesis (ADMET) (ADMET) of of 1,9 1,9-decadiene.1,9--decadiene.decadiene.

nn

polymer cyclic oligomers polymer cyclic oligomers SchemeScheme 8. 8. Ring Ring-openingRing--opening/ringopening/ring/ring-closing--closingclosing metathesis metathesis of of cyclooctene. cyclooctene. In contrast to hybrid Ru catalysts, in which SBA-15 proved its superiority, in oxidic catalysts, i.e., InIn contrast contrast to to hybrid hybrid Ru Ru catalysts, catalysts, in in which which SBA SBA--1515 proved proved its its superiority, superiority, in in oxidic oxidic catalysts, catalysts, i.e., i.e., supported MoO3 and WO3 , MCM-41 usually gave rise to more active catalysts than SBA-15. An early supportedsupported MoO MoO3 and and WO WO3,,MCM MCM--4141 usually usually gave gave rise rise to to more more active active catalysts catalysts than than SBA SBA--15.15. An An early early study on 1-octene metathesis over MoO3 on3 mesoporous molecular sieves [15] showed that the activity studystudy on on 1 1--octeneoctene metathesis metathesis over over MoO MoO3 onon mesoporous mesoporous molecular molecular sieves sieves [15] [15] showed showed that that the the of MoO3/MCM-413 was higher than that of MoO3/SBA-15,3 at least at loadings from 4 wt% to 8 wt% Mo. activityactivity of of MoO MoO3/MCM/MCM--4141 was was higher higher than than that that of of MoO MoO3/SBA/SBA--15,15, at at least least at at loadings loadings from from 4 4 wt% wt% to to 8 8 In 2-pentene metathesis over both Mo and W oxides supported on MCM-41, higher conversions were wt%wt% Mo. Mo. In In 2 2--pentenepentene metathesis metathesis over over both both Mo Mo and and W W oxides oxides supported supported on on MCM MCM--41,41, higher higher reached at 500 C than over catalysts based on SBA-15 despite the larger pore diameter of SBA-15 [65]. conversionsconversions were were◦ reached reached at at 500 500 °C °C thanthan over over catalysts catalysts based based on on SBA SBA--1515 despite despite the the larger larger pore pore Similarly to 2-butene metathesis, the conversion achieved with WO3/MCM-41 (8 wt% of W) was3 higher diameterdiameter of of SBA SBA--1515 [65]. [65]. Similarly Similarly to to 2 2--butenebutene metathesis, metathesis, the the conversion conversion achieved achieved with with WO WO3/MCM/MCM-- than that with WO3/SBA-15 at all temperatures3 from 450 to 550 ◦C[64]. Such results were usually 4141 (8(8 wt%wt% ofof W)W) waswas higherhigher thanthan thatthat withwith WOWO3/SBA/SBA--1515 atat allall temperaturestemperatures fromfrom 450450 toto 550550 °C°C [64].[64]. explained as a consequence of the increased specific area and improved oxide dispersion on the surface SuchSuch resultsresults werewere usuallyusually explainedexplained asas aa consequenceconsequence ofof thethe increasedincreased specificspecific areaarea andand improvedimproved 12 14 oxideoxide dispersiondispersion onon thethe surfacesurface ofof MCMMCM--41.41. InIn turn,turn, thethe metathesismetathesis ofof longlong chainchain alkenesalkenes (C(C12, , CC14)) showedshowed better better results results when when using using catalysts catalysts on on SBA SBA--1515 [72]. [72]. In In this this case, case, the the advantage advantage of of a a large large pore pore sizesize prevailed. prevailed.

6.6. Conclusions Conclusions and and Perspectives Perspectives SBASBA--1515 is is an an excellent excellent suppo supportrt for for metathesis metathesis catalysts catalysts thanks thanks to to its its thermal thermal and and chemical chemical stability,stability, well well--defineddefined structure, structure, and and large large pores. pores. Especially Especially in in hybrid hybrid catalysts catalysts prepared prepared by by immobilizationimmobilization of of well well--defineddefined Mo Mo and and Ru Ru carbenes, carbenes, these these properties properties have have proved proved particularly particularly usefu useful.l. AlthoughAlthough somesome RuRu carbenescarbenes havehave alsoalso beenbeen immobilizedimmobilized onon otherother materialsmaterials suchsuch asas MetalMetal--OrganicOrganic FrameworkFramework MOFMOF [84],[84], graphenegraphene [85,86],[85,86], glass,glass, wool,wool, andand paperpaper [87],[87], thethe datadata reportedreported soso farfar showshow thatthat SBA SBA--1515 is is the the most most promising promising support support for for effective, effective, stable, stable, and and widely widely applicable applicable catalysts. catalysts. InIn our our review, review, we we focused focused on on olefin olefin metathesis metathesis catalysts catalysts using using SBA SBA--1515 as as a a support. support. New New mmetathesisetathesis catalystscatalysts havehave beenbeen rapidlyrapidly developeddeveloped inin recentrecent years,years, andand severalseveral modernmodern MoMo andand RuRu carbenescarbenes are are currently currently available. available. However, However, these these carbenes carbenes were were not not immobilized immobilized at at all all or or were were immobilizedimmobilized onon currentcurrent silicasilica only.only. ThisThis remainsremains aa greatgreat challenge,challenge, especiallyespecially thethe immobilizationimmobilization ofof newnew carbenes carbenes with with high high stereoselectivity stereoselectivity [88] [88] and and even even enantioselectivity enantioselectivity [89] [89] as as homogeneous homogeneous catalysts.catalysts. Another Another major major challenge challenge is is the the development development of of heterogeneous heterogeneous catalysts catalysts for for alkyne alkyne and and enyneenyne metathesis. metathesis. For For example, example, Bu Buchmeiserchmeiser has has recently recently developed developed silica silica--supportedsupported NHC NHC Mo Mo alkylidynealkylidyne complexes complexes used used in in the the metathesis metathesis of of various various 2 2--alkynesalkynes [90]. [90]. For For development development of of these these new new hybridhybrid catalysts catalysts highly highly effective effective in in metathesis metathesis of of a a variety variety of of substrates, substrates, SBA SBA--1515 can can be be a a good good cho choice.ice.

AuthorAuthor Contributions:Contributions: conceptualizationconceptualization HH.B.B. . andand JJ.C.C.,. , originaloriginal draftdraft preparationpreparation HH.B.B.,. , reviewreview andand editingediting JJ.C..C.

Funding:Funding: JCJC acknowledgesacknowledges thethe CzechCzech ScienceScience FoundationFoundation (Project(Project ExProExPro 1919--27551X)27551X) andand OPOP VVVVVV “Excellent“Excellent ResearchResearch Teams”, Teams”, project project No. No. CZ.02.1 CZ.02.1.01/0.0/0.0/15_003/0000417.01/0.0/0.0/15_003/0000417 – –CUCAM.CUCAM.

ConflictsConflicts of of Interest: Interest: TheThe authors authors declare declare no no conflicts conflicts of of interest. interest.

ReferencesReferences

Catalysts 2019, 9, 743 14 of 18

of MCM-41. In turn, the metathesis of long chain alkenes (C12,C14) showed better results when using catalysts on SBA-15 [72]. In this case, the advantage of a large pore size prevailed.

6. Conclusions and Perspectives SBA-15 is an excellent support for metathesis catalysts thanks to its thermal and chemical stability, well-defined structure, and large pores. Especially in hybrid catalysts prepared by immobilization of well-defined Mo and Ru carbenes, these properties have proved particularly useful. Although some Ru carbenes have also been immobilized on other materials such as Metal-Organic Framework MOF [84], graphene [85,86], glass, wool, and paper [87], the data reported so far show that SBA-15 is the most promising support for effective, stable, and widely applicable catalysts. In our review, we focused on olefin metathesis catalysts using SBA-15 as a support. New metathesis catalysts have been rapidly developed in recent years, and several modern Mo and Ru carbenes are currently available. However, these carbenes were not immobilized at all or were immobilized on current silica only. This remains a great challenge, especially the immobilization of new carbenes with high stereoselectivity [88] and even enantioselectivity [89] as homogeneous catalysts. Another major challenge is the development of heterogeneous catalysts for alkyne and enyne metathesis. For example, Buchmeiser has recently developed silica-supported NHC Mo alkylidyne complexes used in the metathesis of various 2-alkynes [90]. For development of these new hybrid catalysts highly effective in metathesis of a variety of substrates, SBA-15 can be a good choice.

Author Contributions: Conceptualization H.B. and J.C., original draft preparation H.B., review and editing J.C. Funding: JC acknowledges the Czech Science Foundation (Project ExPro 19-27551X) and OP VVV “Excellent Research Teams”, project No. CZ.02.1.01/0.0/0.0/15_003/0000417 –CUCAM. Conflicts of Interest: The authors declare no conflicts of interest.

References

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