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Towards Platform Chemicals from Bio-based Ethylene: Heterogeneous Catalysts and Processes Vasile Hulea ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04294 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018
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ACS Catalysis
Towards Platform Chemicals from Bio-based Ethylene: Heterogeneous Catalysts and Processes Vasile Hulea* Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM-ENSCM, Matériaux Avancés pour la Catalyse et la Santé, 240 avenue du Professeur Emile Jeanbrau, CS 60297, 34296 Montpellier Cedex 5, France ABSTRACT: With stricter ecological regulation and reduction of fossil feedstock, the research works have been orientated to renewable resources. Various bio-based molecules were proposed for replacing the platform chemicals based on crude oil, but there are not yet well established processes for producing these chemicals at large scale. As a result, in the short to mid-term, a main impact will be expected from the production of bio-based bulk chemicals having identical structure with that of the today’s bulk chemicals. Among them, ethylene, which is a key intermediate for the production of platform molecules. The commercial method for producing ethylene is based on steam thermal cracking, but emerging method using alternative sources (natural gas, coal, biomass) are very promising processes. For example, the high-volume production of ethylene by ethanol dehydration became economically feasible application. This review summarizes the advances of catalysts, processes and fundamental understanding of reaction mechanisms in ethylene conversion to other high value hydrocarbons, including propylene, butenes and aromatics BTX.
KEYWORDS: ethylene, ethanol, propylene, butene, aromatics, dimerization, metathesis, heterogeneous catalysis.
1. INTRODUCTION
Nowadays, most of bulk chemicals, polymers and materials are produced using the large-volume unsaturated hydrocarbons, i.e. C2-C4-olefins and C6-C8 aromatics (benzene, toluene and xylenes [BTX]) as raw materials. These building block molecules are commonly obtained in very efficient processes, including steam cracking of saturated hydrocarbons, FCC, and catalytic reforming, from fossil-based feedstocks (particularly the crude oil).1 The nonrenewable sources will inexorably face the crisis of their limited reserves, and the total depletion is expected. Additionally, the growing awareness of the detrimental environmental effects related to the use of fossil resources has orientated the research works towards unconventional routes and raw materials for producing low olefins and aromatics. During the last decade various molecules such as 5hydroxymethyl furfural, furfural, xylose, levulinic acid, sorbitol, succinic acid or lactic acid, obtained from renewable sources, were proposed for replacing the platform chemicals based on crude oil.2-8 Unfortunately, there are not yet well established processes for producing these chemicals at large scale. As a result, in the short to mid-term, a major impact will be expected from the production of bio-based bulk chemicals having identical structure with that of the today’s bulk chemicals. The existing industrial infrastructure already optimized for the conversion of the crude oil fractions into chemicals and polymers could thus benefit future technologies based on alternative feedstocks. Numerous methods for converting the renewable sources (biomass) into light olefins and BTX aromatics via both ther-
mochemical and biochemical routes (chemicals, enzymes, and fermentative microorganisms) were investigated. For example, propylene can be obtained by the dehydration of 1- or 2propanol produced in fermentation processes.9 Biobutenes can be produced by dehydration of biobutanol, which is results through ABE fermentation of the biomass, using Clostridium species10 or by other fermentation processes of lignocellulosic and starchy biomass.11 To increase the butanol productivity, the organisms were modified by genetic techniques. On the other hand, according to the patent literature, various aromatic compounds have been obtained by glucose fermentation processes.12 High-temperature thermal processes can be used for lignin conversion in mono and polyhydroxylated aromatic compounds.13,14 Although promising results have been obtained, these applications are not yet mature technologies. However, in this field a major exception exists: the ethylene production from bioethanol, by catalytic dehydration. Ethylene produced by this way is a very promising bio-based molecule, mainly because its high-volume production by bioethanol dehydration became economically feasible. Additionally, ethylene is a key intermediate for producing other major platform molecules. The aim of this review is to provide an overview of the most plausible catalytic ways to platform molecules (propylene, butenes and aromatics) based on bio-based ethylene. Special attention will paid to the heterogeneous catalysts and processes involved. After a short presentation of the production and dehydration of bioethanol, the paper will focused on the major ways used for the manufacture of propylene, butenes and aromatics from bioethylene (Figure 1).
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Biomass
Bioethanol
Bioethylene
Dimerization Dimerization/metathesis Oligomerization/cracking Aromatization
Butenes Propylene Aromatics
tigated.25,26 Lignocellulosic biomass is the most abundantly available raw material on the Earth for the production of bioethanol. Various technologies for its conversion into bioethanol, using thermochemical and biochemical thechnologies were described in recent reviews.27-29 The acid-catalyzed dehydration of ethanol (eq. 1) is one of the earliest processes used in the industry.30,31 C2H5OH
Figure 1. Biomass conversion into platform molecules 2. ETHYLENE: AVAILABLE BASIC CHEMICAL
With an annual production of about 140 million tons, ethylene (C2H4) is one of the key platform molecules. It is the raw material for producing various plastics, textiles, and many chemicals. Industrial processes involving ethylene include polymerization, oxidation, halogenation/hydro-halogenation, alkylation, hydration, oligomerization, hydro-formylation, etc. The commercial method for producing ethylene is based on steam thermal cracking of saturated hydrocarbons. Emerging method involving alternative sources (natural gas, coal, biomass) are investigated (Figure 2). Steam cracking
Oil derivatives Natural gas Coal Biomass
Syngas
Bioethanol
Methanol
Ethylene
Catalytic dehydration
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C2H4 + H2O (1)
H = 45.7 kJ mol-1
This reaction became economically viable due to the recent increased production of bioethanol. Industrial plants based on the bioethanol dehydration have been developed by different companies, including Braskem, Chematur, British Petroleum (BP), Dow, and Axens - Total - IFPEN.32,33 Typically, these technologies include two steps: the ethanol dehydration and the purification of ethylene.32,33 The original catalyst used in the industrial plants was an activated alumina-based material.34 American Halcon Scientific Design Inc. developed in the 1980s a dehydration process based on Syndol catalysts, consisting in Al2O3-MgO/SiO2. BP process (called Hummingbird) is based on a heteropolyacid catalyst and operates at 160–270 °C and 1–45 bar. Braskem produces since 2010 about 200 000 t a–1 ethylene from sugarcane based feedstock in an ethanol to ethylene plant. Bioethanol and steam in large extent are together injected in an adiabatic reactor. The Axens process (called Atol) operates with two fixed bed adiabatic reactors, at 400–500 °C, and produces 50 000–400 000 t a–1 ethylene. Using four adiabatic tubular reactors, the Chematur’s technology is able to produce up to 200 000 t a–1 ethylene. 4. CONVERSION OF ETHYLENE MOLECULES
Figure 2. Commercial and emerging methods for producing ethylene 3. BIOETHANOL: PRODUCTION AND DEHYDRATION TO ETHYLENE
The ethanol can be produced in both oil and biomass-based processes. It is remarkable to note that during the last decades, the production of ethanol from biomass has become increasingly efficient and competitive, and nowadays, 90% of the ethanol on the market is biomass-derived.15,16 The increase of the production of bioethanol (evaluated at more than 100 billion liters annually)17 is due to its use as a biofuel for substituting the fossil-based ones. Indeed, more than 67% of the bioethanol is used as biofuel, but its high-volume production at relatively low cost offer a real opportunity for its valorization as raw material for the production of various renewable value-added chemicals.18 Besides ethylene, other major molecules, including butadiene, acetaldehyde, butanol and ethyl acetate can be produced from ethanol.16 The conventional processes for the industrial production of bioethanol are based on sucrose (mainly from sugarcane) and starch (from cereals) in 1st generation processes (1G). This process may not be sustainable due to their food and feed value. Consequently, different options are currently explored to develop new technologies for bioethanol production from lignocellulosic materials and non-fermentable sugars. These are the 2nd generation processes (2G).19-24 In addition, 3rd generation processes (3G) based on algae and seaweeds are inves-
TO
PLATFORM
Butenes, propylene and BTX platform molecules can be obtained from ethylene according the reactions presented in Figure 1. The most important aspects about these processes, in the presence of heterogeneous catalysts will be discussed in this section. 4.1. Ethylene to butenes in heterogeneous catalysis
The dimerization of ethylene to butenes is a well-known reaction. For industrial applications, the challenge is to obtain selectively 1-butene, which is a major comonomer in polyolefin synthesis.35 The commercial processes for ethylene dimerization/oligomerization are based on homogeneous catalysis and organic solvents. During the last time complexes of various metals (Ni, Ti, Zr, Cr, Co, Fe) have been proposed as efficient homogeneous catalysts in this reaction.36-43 Today, the AlphaButol technology (AXENS/SABIC), using a homogeneous Ti(OBu)4 catalysts and AlEt3 co-catalyst, is the main commercialized process for producing 1-butene. This technology produces more than 700 kt a-1 1-butene, with a selectivity up to 93%.36,44,45 In line with the sustainable chemistry principles, the research attention has been focused to the development of heterogeneous processes for ethylene dimerization. Various families of solid catalysts were investigated: (i) complexes immobilized on polymers and oxides,36,46-56 (ii) metal organic frameworks (MOFs) and covalent organic frameworks (COFs) materials57-
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and (iii) nickel and palladium supported on inorganic porous materials.64-77 4.1.1. Ethylene dimerization on complexes supported on polymers and oxides In one of the earliest studies in the field, Peuckert and Keim,46 immobilized by impregnation the complexes Ph(Ph3P)Ni (Ph2PCHCOPh) and ( 3-C8H13)Ni(Ph2PCH2OO) on silica, silica-alumina and polystyrene and used these materials as catalysts for ethylene dimerization/oligomerization. The acidity of the supports has a very favorable effect on the dimerization rate, but it diminishes the selectivity to 1-butene due to the double-bond isomerization to 2-butenes. The catalysts obtained by binding the complexes on polystyrene are able to catalyze the oligomerization of ethylene to linear -olefins with 99% selectivity and rates > 3 mol C2H4 mol Ni-1 sec-1. Ligand leaching was observed when the catalytic reaction was carried out in slurry mode, in a solvent. Nesterov and Zakharov48 modified the silica or alumina supports with Ph2PCH2-CH2-Si(OE)3. The resulted supported phosphine was then used as a complexing agent for the bis(cyclooctadiene)nickel and triphenyl-phosphonium 2-oxo-2-phenylethylide. In this way, the homogeneous catalyst was chemically grafted on the support. The yield of linear -olefins was higher than 92%, but the amount of butenes in the oligomers (C4-C32) was maximum 19 wt.%. Smith et al.47 studied the ethylene dimerization on both unsupported and supported Ti(OEt)4 reduced by various metal alkyls. All of these compounds were effective catalysts for ethylene dimerization to 1-butene. Anchoring the complex to a carrier, such as silica, alumina, or aluminophosphate stabilized the active species, which produced better overall activity. Up to 8 mol of butene per mol-Ti per sec was obtained when silica and AlPO4 were used as supports. Angelescu et al.49 prepared various heterogeneous catalysts using Ni(4,4'-bipyridine)Cl2 as complex, AlCl(C2H5)2 as coactivator and molecular sieves (Y, L, mordenite, mesoporous MCM-41, silica–alumina) as supports. The activity and selectivity of these materials in the dimerization of ethylene were investigated at 25 °C and 12 bar of the monomer. The highest TOF value was 305 h-1 over MCM-41 based catalysts, while the highest selectivity to C4 (93%) was obtained with the complex supported on mordenite. In order to prepare heterogeneous catalysts based on Ni(MeCN)(BF4)2, de Souza et al.50,51 used Al-MCM-41 and Ni/Al-MCM-41 mesoporous materials as supports. These hybrid materials associated with AlEt3 were able to activate the ethylene oligomerization reaction at low temperature. In reaction tests performed at 40 °C and 9.8 bar, with catalysts having an Al/Ni molar ratio of 15, high selectivity in 1-butene (about 84%) has been obtained. Nickel- -diimine complexes are very well-known homogeneous catalysts for the ethylene dimerization/oligomerization. Rossetto et al.52,55 and Kumar et al.56 supported such complexes on silica and compared the catalytic behavior of these materials with that obtained in homogeneous mode. The heterogeneous catalysts exhibited lower activities compared to their homogeneous analogs, but they led to higher selectivities for butenes (around 100%) and 1-butene (> 90%).
tifunctional materials were tested as heterogeneous catalysts in a continuous gas phase process for converting ethylene into 2butene by dimerization/isomerization reactions.53,54 The typical conditions were: 1 bar of ethylene, contact time of 8 s, ethylene flow of 23.1 NmL min-1. Both fixed and fluidized reactors were used in this process. In the fixed bed reactor, the catalyst lifetime was about 10 h, while at 19 °C, it was 120 h on stream, with a turnover number of about 53 000 molbutene molNi-1. The selectivity to 2-butene varied from 53-68 wt% (at 19 °C) to 85-92 wt% at 40 °C. The better heat elimination in the process performed in fluidized mode strongly increased the catalyst lifetime. Table 1 recapitulate the most important results obtained in the ethylene dimerization performed in the presence of various supported complexes catalysts, while the Figures 3 and 4 show the simplified structure of some Ni-MOF. 4.1.2. Ethylene dimerization on MOFs and COFs Metal-organic frameworks (MOFs) correspond to a wellknown family of crystalline porous materials. They 3D structure consists in metal ions (or clusters) connected by multitopic organic linkers.78 Using various metal and linkers, a large variety of MOF with interesting properties for many processes, such as gas separation/storage, and catalysis were synthesized. Additionally, new catalytic functions were created by post-synthetic modifications of MOF. Such a possibility is to introduce well-known homogeneous catalysts (active for the ethylene dimerization) into the MOF matrix. Thus, some MOF-based materials were developed and used as heterogeneous catalysts for the ethylene dimerization (Figures 3 and 4). In an one-pot postfuntionalization process, Canivet et al.57 anchored a nickel complex on (Fe)MIL-101 MOF (Figure 3). The resulted material showed high activity and good reusability as catalyst for the liquid phase ethylene conversion into 1butene. The best result (TOF = 10 455 h-1, selectivity to 1butene 95%) was obtained at 25 °C and 30 bar. Liu et al.58 synthesized mixed-linker MOFs like (Zn4O(BDC)x(ABDC)3-x), which were then functionalized with nickel species in order to obtained catalysts for ethylene oligomerization (Figure 3). These complexes showed superior properties in terms of stability and price than the catalysts obtained from IRMOF-3 by post-modification.
Figure 3. Simplified structure of Ni-MOFs with modified linkers: Ni-(Fe)MIL-101 (left), Ni-MixMOF (right). In part adapted from Ref. 152 with permission of The Royal Society of Chemistry.
Wasserscheid' research group used ionic liquids supported on silica for immobilizing cationic Ni-complexes. These mul-
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Table 1. Ethylene dimerization on supported complexes catalysts Complexes
Supports
Dimerization conditions
TOFa (h-1)
C4 selectivity
Ref.
Ph(Ph3P)Ni(Ph2PCHCOPh) ( 3C8H13)Ni(Ph2PCH2OO)
silica, silicaalumina polystyrene
50-75 °C, 60-70 bar C2H4, contact time 3-6 s
10 800
40-50 % C4, 5090% 1-C4
46
bis(cyclooctadiene)nickel, triphenylphosphonium 2-oxo-2phenylethylide
modified silica, alumina
batch reactor, toluene solvent, 10-50 bar C2H4, 70-80 °C
356
max. 19 % C4
48
Ti(OEt)4, Cocatalyst: AlEt3
silica, alumina, AlPO4
batch reactor, iso-butane solvent
57 600
99.6% C4, 98.9% 1-C4
47
Ni(4,4'-bipyridine)Cl2, Cocatalyst: AlCl(C2H5)2
Y, L, M zeolites, MCM-41, amorphous silica– alumina
batch reactor, 25°C, 12 bar
305
max. 93% C4
49
Ni(MeCN)(BF4)2; Cocatalyst: AlEt3
Al-MCM-41
40 °C, 10 bar of ethylene and Al/Ni molar ratio of 15
260
84%
50
2-(phenyl)amine-4-(phenyl)imine-2-pentene 2-(2,6-dimethylphenyl)amine-4(2,6dimethylphenyl)imine-2-pentene, Cocatalyst: Al2Et3Cl3
MCM-41
batch, toluene solvent, 10 °C, 5-15 bar ethylene, Al/Ni = 100-200
525
100% C4, 90% 1-C4
52
1,5-bis(phenyl)-pentanediiminedibromonickel(II); 1,5-bis(2,6dimethylphenyl)-2,4-pentanediiminedibromonickel(II); 1,5-bis(2,4,6trimethylphenyl)-2,4-pentanediiminedibromonickel(II), Cocatalyst: Al2Et3Cl3
silica
5-15 bar C2H4, Al/Ni = 30200
18 100
78-100% C4 8795% 1-C4
55
Ni-2-(decyloxy)phenyl)diphenylphosphine Ni-2-(2,6-dimethylphenoxy)phenyl) diphenylphosphine
ionic liquid containing silica
fixed bed reactor, 19-40 °C, 1 bar of ethylene, contact time of 8 s, volume flow of ethylene of 23.1 NmL min-1
888
max. 94% C4, 95% 2-C4
53
[(methallyl) Ni (2-methoxyphenyl) diphenylphosphine)] [SbF6]; [(methallyl) Ni (2-decyloxyphenyl) diphenylphosphine)] [SbF6]; [(methallyl) Ni (2-(2,6-dimethylphenoxy) phenyl) diphenylphosphine)] [SbF6]
ionic liquid containing silica
fluidized bed reactor, 20 °C, 1 bar of ethylene, contact time of 1.3 s, volume flow of ethylene of 5.1 NmL min-1
32 88
85% 2-C4
54
a
molethylene molMe-1 h-1
Figure 4. Simplified structure of Ni-MOFs with modified nodes: NU-1000-bpy-NiCl2 (left), Ni-NU-1000 (middle), Ni-MFU-4l (right)
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For different MOF compositions and reaction conditions, the selectivity to C4 varied from 35 to 92.7%, and the TOF values varied from 1 885 to 16 428 h-1. C6 and C8 olefins were obtained as main by-products. Liu et al.79 synthesized a novel Zn3(OH)2L2 MOF from dicarboxylate ligands with diimine (1,4-bis(4-(CO2HC6H4)-2,3-dimethyl-1,4-diazabutadiene). By a postsynthetic treatment using NiCl2, they obtained Zn3(OH)2(L1Ni)2, which was used as dimerization catalysts. A large number of oligomers in the range C4-C10+ were obtained. The selectivity to C4 strongly depended on the reaction parameter. It varied from 29.8 to 91.8% when the ratio Al/Ni increased from 200 to 1 500 (1 bar and 20 °C). The temperature and the pressure also affected the selectivity to C4. The highest selectivity was 87.4% at 20 °C (1 bar, Al/Ni = 1 000) and 87.6 at 25 bar (T = 20 °C, Al/Ni = 1 000). The effort to develop MOF catalysts for ethylene dimerization discussed in the above studies, has essentially focused on grafting well-known homogeneous catalysts on the organic linkers. Methods for introducing Ni active sites on the node of MOFs were also developed. Madrahimov et al.59 synthesized NU-1000-(bpy)NiII, a highly porous MOF material possessing, by anchoring well-defined (bpy)NiII moieties in the cluster nodes (Figure 4). Treatment with Et2AlCl afforded a singlesite catalyst with excellent catalytic activity for ethylene dimerization (intrinsic activity for butenes that is up to an order of magnitude higher than the corresponding (bpy)NiCl2 homogeneous analogue) and stability. The high porosity of this catalyst resulted in outstanding levels of activity at ambient temperature in gas phase ethylene dimerization reactions, both under batch and continuous flow conditions. Using the atomic layer deposition technique, Li et al.62 introduced uniformly and accurately large amount of Ni ions on the node of a Zr-based MOF, NU-1000 (Figure 4). The Ni-based MOF catalyst showed very high activity in the gas-phase hydrogenation reaction, but it was a modest catalyst for the ethylene dimerization. Low ethylene conversion (max 5%) and low C4 selectivity (max 46%) were obtained at 45 °C and pressure of 2 bar. The main product was C8 olefin, but other oligomers and even polymers were formed. In a recent study, the Dinca' group prepared catalytic materials by modifying the metal cluster of the MFU-4l MOF (Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5b],[4 ,5 -i])dibenzo[1,4]dioxin). One of 5 Zn2+ ions contained in the MOF cluster was exchanged by Ni2+, which became an isolated catalytic site for the ethylene dimerization (Figure 4).60 In a general manner, the catalytic behavior was excellent, but it strongly depended on the temperature, pressure and the methylaluminoxane (MAO) amount. To have a high activity, the optimum conditions were: 25 °C, 50 bar of ethylene and 100 equivalents of MAO in the reaction mixture. The highest TOF was 41 500 h-1, while the selectivity to C4 and 1-C4 was 97.4 and 92.0, respectively. The highest selectivity to 1-C4 was 96.2, obtained at 0 °C, 50 bar and 100 equivalents of MAO (TOF = 22600 h-1). Additionally, the Ni-MFU-4l showed high deactivation stability. In a more recent study, they examined the mechanistic aspects involved in the dimerization reaction.80 Combining various techniques such as isotopic labeling studies, mechanistic probes, and DFT computation the authors demonstrated that Ni-MFU-4l operated via the
Cossee-Arlman mechanism, which is also been implicated in homogeneous late transition metal catalysts (Figure 5). N N
N Ni
N
N
N
N
N Ni
Ni
H
Figure 5. Simplified Cossee-Arlman mechanism for ethylene dimerization over Ni-MFU-4l
Based on density functional theory calculations, Bernales et al.61 reported detailed aspects on several possible mechanisms involved in the ethylene dimerization with a mononuclear NiNU-1000 catalyst. This catalyst was compared to an isostructural mononuclear Co-NU-1000. Ni2+ and Co2+ ions were placed directly onto the NU-1000 node by atomic layer deposition method. The aim of the study was to understand how the coordination environment of the support influences the catalytic behavior of the two metals. Additionally, higher level multireference calculations on key transition states were presented to contribute to the rational design of new improved catalysts based on an understanding of key details of metal electronic structure. By combining the theoretic and the experimental data, two main conclusions were stated: (i) for both metal, the dimerisation occurs according to the Cossee-Arlman mechanism and (ii) Ni-based catalyst is more active than the catalyst containing Co ions. The behavior was attributed to transition state stabilization associated with an empty 3d orbital that hybridizes more readily with relevant carbon p orbitals in low-spin Ni-NU-1000 than in high-spin Co-NU-1000. All these studies demonstrate that metal nodes in MOFs mimic both functionally and mechanistically the homogeneous catalysts. Rozhko et al.63 supported Ni2+ species on COFs (covalent organic frameworks) consisting in triazine and imine linked frameworks. These linkers provide high concentration of nitrogen, as quasi bipyridine or diiminopyridine moieties. As a result, the COF supports coordinate large amount of NiBr2 on the porous framework. These materials were used as catalysts for the oligomerization of ethylene. At 25 or 50 °C and 15 bar, they exhibited behaviors similar to those of showed by the homogeneous counterpart. Selectivity to C4 up to 70% was obtained at 25 °C with a catalyst having an Al/Ni ratio of 100 and 4.7 wt% Ni. The main by-product was the C6 olefins. To summarize, Ni-based MOF/COF materials are promising heterogeneous catalysts for the ethylene dimerization (Tables 1 and 2). Most of them showed high activities and reasonable selectivities for butenes. However, the high selectivities were generally obtained with the lower active catalysts.
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Table 2. Ethylene dimerization by MOF- and COF-based catalysts Ni precursor/ complexes
MOF/COF
TOFa (h-1)
Dimerization conditions
C4 selectivity
Ref.
Et2AlCl
(Fe)MIL-101
20 920
batch reactor, 25 °C, 30 bar C2H4, contact time 3-6 s
95% 1-C4
57
Et2AlCl
MixMOF
16 428
batch reactor, toluene solvent, 20 bar C2H4, 20-40 °C
max. 92.7% C4
58
Cocatalyst
Zn4O(BDC)x(ABDC)3-x 1,4-bis(4-(CO2HC6H4)-2,3dimethyl-1,4-diazabutadiene
Et2AlCl
Zn3(OH)2L2
non reported
batch reactor, toluene solvent, 0-60 °C, 1-25 bar
99.6% C4, 98.9% 1C4
79
Ni(DME)Br2
Et2AlCl
Zr6O4(OH)4(bpydc)6
25 000
batch mode, cyclohexane, 59 bar C2H4, 55 °C
non reported
81
NiCl2
Et2AlCl
NU-1000-bpy
12 080
room temp., 15 bar, batch reactor, heptane solvent,
93-95% C4, 85-88% 1-C4
59
NiCl2
Et2AlCl
NU-1000-bpy
140
gas flow reactor, 15 bar
83% C4, 70% 1-C4
59
Et2AlCl
NU-1000
1 080
flow reactor, 45 °C, 2 bar
46%
62
MAO
MFU-4l
max. 41 500
0-25 °C, 5-50 bar, batch, toluene solvent
max. 96.2% 1-C4
60
Et2AlCl
NU-1000
216
flow reactor, 45 °C, 2 bar
95% C4, 70% 1-C4
61
Et3Al
COF
275
batch mode, heptane solvent, 15 bar, 25 or 50 °C
max. 70% C4
63
Ni(NO3)2
NiBr2
a
molethylene molNi-1 h-1
Some of the catalysts generate heavier oligomers and even polymers, which lead to the catalyst deactivation. These catalysts require the use of aluminoxane activators or cocatalysts in large excess and the presence of solvents or reactants as a bulk liquid phase. These systems also exhibit rapid deactivation and require complex regeneration protocols to avoid structural and catalytic degradation, thus limiting their use in practical large-scale oligomerization processes. The CosseeArlman mechanism seems to be the most appropriate for the couple MOF catalyst-ethylene dimerization. 4.1.3. Nickel supported on inorganic porous materials For developing more efficient and stable catalytic materials for the ethylene dimerization/oligomerization, without any cocatalysts, mineral porous materials, including supported NiO,
Ni-exchanged zeolites, sulfated-alumina, amorphous silicaalumina and mesostructured silica-alumina have been extensively examined during the past decades. Some of first Nibased heterogeneous catalysts have been described in review articles written by Skupinska82 and Al-Jarallah et al.36 in the early 1990s. An extended overview of the research and knowledge on the Ni-based materials used as heterogeneous oligomerization catalysts has been provided more recently by Finiels et al.74 Oligomerization catalytic materials and processes conducted in static, flow, and slurry mode, at different reaction parameters, have been discussed. The general conclusions were as follows: (i) the Ni-exchanged materials, containing isolated Ni ions show higher catalytic properties compared to those of the NiO-based catalysts; (ii) the catalytic sites implicated in the ethylene oligomerization are most likely the isolated Ni+ and dehydrated Ni2+ ions; (iii) the chemical and textural properties
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of the catalyst strongly affect its behaviour; (iv) the product distribution depends essentially on the ratio between the nickel and acid sites, as well as on the reaction parameters. Some of these reviewed aspects will be succinctly detailed here below. The recent research on the textural design of Ni-exchanged porous catalysts and dimerization mechanistic aspects will be also examined. The discussion will focused on the crucial parameters affecting the performances of the Ni-based catalysts. NiO on silica was the original Ni-based heterogeneous catalysts for ethylene dimerization/oligomerization. Morikawa first showed that nickel oxide supported on Kieselguhr catalyzed the ethylene conversion to butenes even at room temperature.83 Then, Bailey and Reid64 from Phillips Petroleum Company reported that NiO/silica gel it is an efficient catalyst at temperatures up to 150 °C. C4, C6 and C6 olefins were the major products. Ozaki and Shiba,84 as well as Uchida and Amai85 showed that nickel oxide supported on silica is able to catalyze the ethylene dimerization at room temperature. Matsuda et al.86 reported that the activity of the NiO species was improved by the acidic properties of the support. In a series of studies, Sohn et al. [see ref. 74] studded the dimerization of the ethylene at room temperature catalyzed by NiO with various supports, including TiO2 and Al2O3-TiO2 modified with WO3, ZrO2 and La2O3-ZrO2 modified with WO3, ZrO2 modified with MoO3, TiO2 and ZrO2 treated with H2SO4, TiO2 promoted with H2SO4, H3PO4, H3BO3, and H2SeO4. The same group prepared NiSO4/support heterogeneous dimerization catalysts. Typically these materials were prepared by impregnation using nickel sulfate and various supports such as -Al2O3, SiO2-Al2O3, ZrO2, Fe2O3, Fe2O3ZrO2, CeO2-ZrO2, TiO2-ZrO2. The catalytic activity of both supported NiO and NiSO4 materials were evaluated in static mode, with low amount of ethylene. Under these conditions, no other oligomers than C4 were obtained. The catalysts containing isolated Ni2+ ions were prepared by ionic exchange of various microporous and mesoporous aluminosilicates. The general method used for preparing the Niexchanged catalysts is illustrated in Figure 6. Si
Si
Si
Al O
+
1) exchange / NH4
O
O
Si
Si
Si
Al O
O
O
2+
+
O Al
Niδ
+
Na
Si
Niδ
+
+
H
3) 550 °C / air
O
O Si
2) exchange / Ni
+
Na
+
Na
O Al
aluminosilicate support
Al
O
O Si
Si
Al
Ni-exchanged catalyst
Figure 6. General protocol for preparing Ni-exchanged porous catalysts
Usually, the starting material is an aluminosilicate in Na form, which is successively transformed into ammonium and nickel form by ionic exchange with ammonium nitrate and nickel nitrate, respectively. The resulting sample is calcined at 550 °C to obtain a bifunctional catalyst with both Ni and acid sites. As showed by FTIR measurements, when the catalytic material is thermally treated, the Ni2+ species are reduced to Ni+ ions.87,88 Thus both Ni+ and dehydrated Ni+2 species can be considered as active sites in oligomerization.
Catalytic activity. Most of oligomerization studies with Niexchanged catalysts have been carried out at temperatures lower than 160 °C, at pressures between 25 and 40 bar, in well-mixed G-L-S slurry reactors (at constant ethylene pressure, in a solvent) or G-S dynamic (flow) reactors. The nature of the catalyst and reaction conditions strongly affect the oligomerization results, such as activity, productivity and oligomer distribution (Table 3). Indeed, the mesopores are beneficial to the diffusion of higher product molecules inside the pore system, leading to a less deactivation rate and a higher activity (Figure 7).73
Figure 7. Productivity in ethylene oligomerization (in goligomers gcatalyst-1 h-1) over Ni-exchanged aluminosilicates (1.5-2 wt% Ni) with different topologies and pore sizes. Reaction conditions: 150 °C, 35-40 bar, batch mode, 1 h of reaction (according to data from refs. 73,89-93)
For example, in a continuous stirred tank reactor (CSTR), at only 30 °C, Ni-MCM-41 catalyst displayed a constant high activity and selectivity to C4 during 170 h on stream.94 Similarly, in a fixed-bed flow reactor, at 150 °C, Ni-SBA-15 showed very high stability for more than 80 h on stream.73 In contrast, the micropores in Ni-zeolites were rapidly blocked with heavy products and thus the catalysts suffered severe deactivation.89,90 Lallemand et al.90 evaluated by TG analysis the amount of heavy products immobilized on the catalysts during the oligomerization reaction. The mass loss of the purely microporous Ni-MCM-22 was of 21.0 mg goligomers-1 gcatalyst-1, while that of the microporous-mesoporous Ni-MCM36 was only 1.6 mg goligomers-1 gcatalyst-1. The effect of the porosity in the ethylene oligomerization has been also evaluated on the Ni-based clay catalysts.95 The catalytic behaviour (activity, selectivity, stability) of the catalysts strongly depended on their textures and acidic properties. Thus, Ni-exchanged K10 clay, which combines high pore accessibility and lower acid site density, exhibited superior catalytic behaviour compared to Ni-PILC sample. It is important to note that the Ni-exchanged inorganic porous materials showed excellent stability and catalytic behavior after regeneration at high temperature. This is the main advantage relative to the less stable organic catalysts, such as immobilized complexes, COF and MOF.
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Table 3. Comparison of catalytic performances in the oligomerization of ethylene of Ni-exchanged aluminosilicates with different topologies Catalyst
Pore diameter Ni T (nm) (wt%) (°C)
P (bar)
TOF (h-1)b
Activity (g g-1 h-1)a
Oligomers (wt%)
Ref.
C4
C6
C8
C10+
36 67 81 81 45 45 40 42 41 56
21 10 5 8 25 33 33 37 37 31
25 14 13 6 15 15 16 14 15 10
18 9 1 5 15 7 11 7 7 3
Reaction in batch mode Ni-Y (Si/Al=6) Ni-USY (Si/Al=30) Ni-MCM-22 Ni-MCM-36
