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Conversion of isobutylene to octane-booster compounds after MTBE phaseout: The role of heterogeneous catalysis Hilman Ibnu Mahdi, and Oki Muraza Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02533 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 8, 2016
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Conversion of isobutylene to octane-booster compounds after MTBE phaseout: The role of heterogeneous catalysis Hilman Ibnu Mahdi1, Oki Muraza1* 1
Chemical Engineering Department and Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran 30261, Saudi Arabia
Abstract
Over the last decades, the global demand of the production of MTBE was significantly decreased from 30 million tons per year (MTPY) in 1995 to 12 million tons per year (MTPY) in 2011 because of the MTBE phaseout in the United States and other countries. The MTBE was banned in some countries because it produces health and environmental pollution and contaminates water and air quality. After MTBE phaseout, isobutylene feedstock is used for the production another octane booster compound such as isooctane with more environmental friendly reputation. Isooctane is obtained over dimerization, trimerization, and oligomerization of isobutylene followed by a hydrogenation step. Dimerization of isobutylene to isooctane is a simple and low-cost effective technology using heterogeneous catalysts such as zeolites, metal oxides, and resins. Zeolites are potential catalyst with high selectivity, high stability, easy regeneration, and low-cost production. The conversion and selectivity can be increased with higher acid strength, larger dimensional channel, and higher pore size. USY zeolite (FAU topology) is a promising zeolite framework to be improved in dimerization of isobutylene. Further improvement on USY zeolite has been emphasized to develop better dimerization catalysts. Keywords: Isooctane; zeolite; isobutylene; heterogeneous catalysis. *Corresponding author. Tel.:+966 13 860 7612; e-mail address:
[email protected] 1
1.
Introduction 1.1 Conventional MTBE production for gasoline additive One of the most important applications of isobutylene is its usage as an octane booster
compound in the production of methyl tert-butyl ether (MTBE) 1. MTBE is produced from isobutylene and methanol using various catalysts number of gasoline
10
2-9
and further used to increase the octane
. The global capacities for MTBE production, which was started in Italy,
were increased from 50,000 tons/year
10
to 100,000 tons/year
11
. Its production significantly
increased to approximately 12 million tons per year (MTPY) in 1992, 20 MTPY in 1993 – 1994 11-13
, and 30 MTPY in 1995, where 53% was originated from USA capacity 10. However, in the last decade, the worlwide demand for MTBE production significantly
decreased from approximately 22 MTPY in 2000, 18 MTPY in 2003, and 17 MTPY in 2004 14 to 12 MTPY in 2011 15 because of the MTBE ban in the US and Canada 6, 14 as presented on Figure 1. MTBE demand is predicted to grow up to 2020 with average annual growing rate (AAGR) about 5.9% per year. Approximately, 62% of the worldwide demand for MTBE production is in the Asia Pacific region especially China 15. MTBE is commercially produced using variety of catalysts such as Lewatit K-2631 7, CVT resin, Amberlyst 15 (A-15)
16, 17
heteropolyacid (Dawson acid)
20
, Bowex 50w, Lewatit SPC 118
11, 18
, montmorillonites
19
and
based catalysts, clays treated using Al3+, Fe3+, Cr3+, Cu2+, Pb2+,
Ni2+, Co2+, Ca2+, and Na+ cations 5, fluorine-promoted SiO2-Al2O3 catalyst
21
, and sulfur
promoted ZrO2 catalyst 21. Development of MTBE production was also carried out using zeolites such as Y, ZSM-5, mordenite, Beta, NaA, and ZSM-11
22-25
to replace ion exchange resins as alternative catalysts
because of their superior performance
11
and highly selective catalysts even at low
2
isobutylene/methanol ratio
22
. The ZSM-5 and ZSM-11 are better than A-15 resin because of
better thermal stability, higher selectivity, and low sensitivity to methanol:isobutylene molar ratio
23
. Other zeolites such as boron pentasil zeolite and Beta zeolite with Si/Al ratio of 15.8
were applied in MTBE synthesis via in situ MAS NMR spectroscopy 26-28 and 13 X zeolite was utilized as an adsorbent in oxygenate removal for the production of MTBE over adsorptive separation 29. 1.2 Availability of feedstocks in MTBE plant The worldwide demand for isobutylene production has been dramatically increased from 10 MTPY
30, 31
to 30 MTPY
32
because it was widely utilized for many processes in the chemical
industry such as 41% for MTBE or ETBE, 47% for alkylate gasoline, 9% for synthetic rubber, and 3% for other products
30, 32-34
. Poly-isobutene, isoprene, butyl rubber, tert-butyl mercaptans,
tert-butanol, antioxidants, and isooctane were also produced from isobutylene 30, 35-40. At least, 650,000 barrels of isobutylene were produced in the USA in 2011 35, 41. Isobutylene was obtained over isomerization and catalytic dehydrogenation using butanes
42
with different
catalysts such as chromia alumina and zeolite membrane reactors (ZMR) treated by Pd/Ag Isobutylene was also mostly produced over skeletal isomerization of n-butenes
1, 44, 45
38, 43
.
over
various catalysts namely WOx/ γ-Al2O3 46, WOx/ ferrierite 47, MCM-22 48, 49, ZSM-22 50, Omega, SAPO 11, SAPO 34, ZSM-5 zeolite 51, and natural zeolite (clinoptilolite) 52-54. The performance of natural clinoptilolite zeolite can be improved over 1 N HCl in the synthesis of the dealuminated natural clinoptilolite zeolite at 120 oC for 1.5 h
55
. Other processes such as
dehydroisomerization of butanes over ZSM-5/Pt 56, H-TON/(Pt and Cu) 57, and H–Y/(Pt and Sn) 58
and direct conversion of n-butane over MCM-22/Pt 59 also produced isobutylene.
3
Similar with isobutylene, the production for methanol as a feedstock was also high
60, 61
.
Historically, the worldwide production of methanol was dramatically increased from 22.1 MTPY in 1991 60 to 47 MTPY in 2011 62 and it was utilized for feedstock of chemical reaction of 70%, 25% for MTBE, and 9% for acetic acid
61, 63
. The highest demand of methanol in the USA
happened in 1940s with more than 180 MTPY 64. Apart from its utilization as a feedstock in MTBE plant, methanol is also a promising automotive fuel in the future because of its unique physicochemical properties
65
.
Approximately, 85% of methanol was mixed with 15% of unleaded gasoline to the use in vehicle fuel
61, 65
. Methanol can be obtained from various sources namely natural gas, coal, petroleum,
and biomass over different catalysts such as Al2O3/Ni, Cu chromite, and ZnO/Cu
60, 66-71
.
Approximately, more than 75% methanol was currently produced over natural gas process 65, 72. The current demand of renewable sources for isobutylene production has been considered in sustainable production because of high pollution and limited fossil resources. One of the renewable sources applied to obtain a green isobutylene is isobutylene production via fermentation and using microorganism as reported by previous researchers 30, 73-80. Those reports show that isobutylene production could be obtained in the fermentative process where waste from industry was used as nutrient sources in the cultivation 76, 81.
1.3 Other prospective octane booster compounds in gasoline after MTBE phaseout MTBE was banned in the USA and Canada during the last decade because it results health and environmental pollution and leads to contamination in underground water quality because of nitrogen oxide, carbon monoxide, and hydrocarbons
13, 86
80, 82-85
and air
. Because of this
reason, the worldwide MTBE growth will be declined in the future. Hence, other potential
4
processes that can be developed from derivatives of isobutylene, other than MTBE, as the octane booster are considered. Ethyl tert-butyl ether (ETBE) and isooctane can be utilized as an octane booster to replace MTBE because these products are the more environmental friendly. ETBE is obtained from isobutylene synthesis with ethanol and isooctane is obtained from isobutylene synthesis with isobutane. Sensitivity analysis of ETBE synthesis can be modeled with the assistance of Aspen PLUS 87. The ETBE production has been studied by many researchers using various catalysts. Zeolites and resins utilized in MTBE synthesis were also used in the formation of ETBE. For instance, A-15, A-35, and ZSM-5 zeolite were used by Tau and Davis 88 as catalysts in the ETBE production. The ZSM-5 zeolite used in either MTBE synthesis reported by Chu and Kuhl
23
or
ETBE synthesis reported by Tau and Davis 88 resulted the same selectivity (100%). It means that ZSM-5 is a prospective catalyst to be used in the formation of ETBE replacing MTBE as an additive gasoline. Other catalysts used in ETBE synthesis were mordenite zeolite K-2631 resin
90
89
and Lewatit
. The Lewatit K-2631 resin resulted the higher activation energy than A-15
reported by Ancillotti et al 91 and Dowex M-32 resin reported by Kaitale et al 92. Even, this resin resulted the higher activation energy than the one over A-15 used in MTBE synthesis reported by Gicquel and Torck 93 and Ancillotti et al 91.
2.
Oligomerization of isobutylene Isobutylene produced over many reactions as above explained can be utilized in many
processes. One of the isobutylene derivatives is isooctene obtained over dimerization, trimerization, and oligomerization of isobutylene using various catalysts such as resins, substrates, and zeolites under different conditions as presented on Table 1 and Table S1.
5
Several catalysts used in dimerization of isobutylene were Ni-Al2O3 94, Lewatit K-2631 95, USY, beta, mordenite, ZSM-5, ferrierite (FER)
96-98
, nickel complex
99
, A-15
100, 101
, indion 125 resin,
and indion 130 resin 102. Trimerization is a reaction between diisobutylene (DIB, also known as isooctene) and isobutylene to produce triisobutylene (TIB) where the carbon atoms are between C8 and C12. The catalysts used in TIB synthesis were WOx/ZrO2
103
, nickel
99
, sulfated titania
104
, and cation
exchange resins such as A-15 101, A-35, A-31, and A-DT 105-107 Other catalysts used in this synthesis were zeolites such as USY (Si/Al ratio of 30), beta (Si/Al ratio of 9 – 19), mordenite (Si/Al ratio of 12.5), ZSM-5 (Si/Al ratio of 25), and FER (Si/Al ratio of 10) catalyst
80
97, 98, 105-107
. The high selectivity to trimers can be obtained over WOx/ZrO2 as a
calcinated at 550 – 800 oC
103
with stable process
80
. The selectivity of TIB was
increased with increased Ni loading 94, lower Si/Al ratio 97, higher reaction temperature 98, lower space velocity 107, and increased isobutylene conversion 105. However, the selectivity of TIB was decreased with higher pressure 101 and longer reaction time 97. Teraisobutylene (TEB) was obtained from oligomerization reaction between TIB and isobutylene with the number of carbon atoms of ˃ C12. The catalysts utilized in oligomerization of isobutylene were Na3HSiW12O40/SiO2 108, A-15, A-35 with acid capacity of 5.2 mequiv H+/g, A-DT with acid capacity of 3.1 mequiv H+/g, A-31 with acid capacity of 4.8 mequiv H+/g 105 and Dowex 50 with acid capacity of 5.02 mmoleqv H+/g 109. Zeolites such as USY (3D), Y (3D), beta (3D), mordenite (1D), ZSM-5 (3D) and FER (2D) with surface area of 400 – 720 m2/g 109
97, 98, 105,
were successfully utilized in the oligomerization reaction.
6
7
Table 1. Dimerization, trimerization, and oligomerization of isobutylene over different catalysts 94, 96-105, 108-111. Method
Phase
Dimerization Dimerization Dimerization Trimerization Trimerization Trimerization Trimerization Oligomerization Oligomerization Oligomerization
L L L L G–L G–L -
Temp [oC] 65 – 110 60 60 – 120 70 40 – 100 30 – 110 50 – 110 20 -
Time [h] 2 16 ~ 100 1 – 12 10 2 24 90 40
X [%] 5 – 95 8 100 – 10 100 100 91 ˃ 90 41 100 100
Sel [%] 94 – 16 ˃ 99 25 – 100 75 – 80 70 66 80 ~97 76 88
Catalyst
Modification
Ref.
Ni/Al2O3 Nickel Amberlyst 15 resin WOX/ZrO2 H-FER zeolite Amberlyst 35 resin Amberlyst 15 resin Na3HSiW12O40/SiO2 Sulfated titania (TiO2) Sulfated titania (TiO2)
The incipient impregnation method Aluminum alkyl-treated nickel (1.5 mol%) Synthesized over sodium hydroxide Calcinated at 700 oC Calcination at 550 oC for 8 h to form a proton Ion exchanged over ethanol to remove water Cleaning using acetone for 70 h Modified over Na+ ion in standard method Treatment via in situ over sulfuric acid Synthesized over tert-butanol and n-butoxide
94
L = liquid; G = gas; X = Conversion; Sel = Selectivity.
8
99 100 103 98 105 101 108 111 111
Other catalysts such as MFI (silicalite) with pore diameter of 500 nm in ZMR W2O3/Al2O3
112
, H4SiW12O40/SiO2
113
modified over Na+ cation
108
, and sulfated titania
84
,
104, 111
were also used in the synthesis. Sulfated titania (TiO2) catalysts with Lowry:Brønsted ratios of 1.25 – 3.65 were modified over sulfuric acid
111
and tetra propyl ammonium bromide (TPABr)
and tetra ethyl ortho silicate (TEOS) were utilized in the silicalite membrane synthesis with the typical synthesis of TPABr : TEOS : SiO2 : H2O : KOH : NaOH = 1 : 20 : 21 : 560 – 788 : 0.9 : 3 84, 114, 115
.
The selectivity of TEB was increased with higher Ni loading 94, increased Na+ cation content 108
, and higher isobutylene concentration
107
. However, the selectivity of TEB was decreased
with lower pressure 101 and space velocity 105, 107. The selectivity obtained over dimerization is the highest selectivity to DIB among TIB and TEB with the one of more than 99% (almost 100%) using nickel as a catalyst
99
. The
dimerization of isobutylene was carried out using various catalysts such as resins, substrates, and zeolites. In Table 2 and Table S2, we summarize previously reported dimerization of isobutylene over different resins and zeolites as catalysts where polar compounds were used as additives and different solvents were applied in different processes. Some metal oxide-based catalysts in dimerization were nickel complex treated over aluminum alkyl 99, NiSO4/γ-Al2O3 116, Al2O3 modified over ammonium phosphate 117, and nickel 94, 118, 119
. Nafion SAC-13 and Nafion NR-50 synthesized over H3PW12O40.nH2O were also used
in dimerization of isobutylene
120
. The conversion was increased from 3% to 100% because of
the presence of 0.003 moles of diethyl aluminum chloride (EtAlCl2) 118. However, the selectivity was significantly decreased from 98% to less than 1%. The commercial resins utilized in
9
dimerization reaction were mostly ion exhange resins
83, 121-128
such as A-15
83, 118, 127, 129, 130
, A-
36 126, A-35 83, 110, 130, A-IR-120 118, A-IR-118 118, Dowex 50 109, and Tulsion T-63 128, 131. Selectivity enhancers such as water, methanol, TBA, MTBE, isopropanol, and 2-butanol were utilized in the synthesis to increase the selectivity of DIB 133
7, 83, 100, 110, 121, 122, 124-126, 128-130, 132,
. The high selectivity in a reactive distillation was achieved because of the synergetic effect of
distillation and reaction 123 whereas the selectivity was not affected by the different particle sizes of catalysts 124. The selectivity enhancers have important impact on the selectivity and yield of DIB. The selectivity and yield of DIB without additives were 24% and 23%, respectively 125. However, the selectivity and yield of DIB were significantly increased using methanol and tert-butyl alcohol (TBA) where the selectivities and yields were 48% and 43% for methanol and 86% and 46% for TBA, respectively,
125
. The high selectivities and yields using methanol and TBA can be
achieved because of high polarity 130 whereas these polar components significantly decreased the activity of catalyst 100, 125. The effect of polar components such as methanol, ethanol, and isopropanol was also observed by Girolamo et al
110
. The highest selectivity to DIB was obtained using isopropanol
with the one of more than 90% for 3 h under alcohol:isobutylene molar ratio of 0.2 and isobutylene content of 45 – 50%. The selectivity to DIB obtained using ethanol was higher than using methanol with the ones of more than 80% for ethanol and less than 80% for methanol
110
.
The selectivity to DIB obtained over TBA was higher than over MTBE additive with the one of 80% for MTBE and 100% for TBA at 80 oC 125.
10
Table 2. Dimerization of isobutylene to isooctene over different catalysts 94, 99, 100, 110, 116-118, 120-126, 128-131, 134.
n-pentane m-xylene 2-methyl butane Isooctane Isooctane TBA–isooctane -
Temp [oC] 60 65 – 110 60 – 120 50 – 90 60 – 90 65 – 95 44 – 75 90 -
Pressure [atm] 20 21 10 25 15 ˃ 15 -
Time [h] Up to 30 2 2 11 -
Conversion and Selectivity [%] X = 2 – 5.5; S = 100 X = ˃ 95; S = 94 X = 16 – 70; S = 93 – 95 X = 10 – 20; S = 75 – 85 X = 99.58; S = 85.71 X = ~ 60; S = 91 – 94 X = 21 – 63; S = 83 – 98 X = 90 – 99; S = 80 – 95 X = 50 – 95; S = 85 – 96 X = 44; S = 74
Isopentane
60 – 90
13 – 15
-
X = 100 – 10; S = 24 – 100
100
Isopentane Isooctane Isopentane Organic n-decane
70 – 110 30 – 50 60 – 120 60 45
15 – 18 3.5 – 4.4 14.8 6–7 6–7
1–4 Up to 12
X = 30 – 70; S = 97 X = 22 – 98; S = 20 – 86 X = 40 – 60; S = 80 – 90 X = ~ 10; S = 100 X = 100; S = 99
126
Catalyst
System
Selectivity enhancer
Solvent
Al2O3-phosphated Ni/Al2O3 Nickel complex 1 NiSO4/γ-Al2O3 Tulsion T-63 Tulsion T-63 Ion exchange resin Ion exchange resin Ion exchange resin Beta – 30 zeolite
PFR BR BR BR Batch RD BR FBR RD PFR -
Amberlyst 15
CSTR and BR
Amberlyst 36 Nafion SAC Ion exchange resin Amberlite IR-120 Nickel salt
CSTR CM CSTR FBR and BR FBR and BR
Water and TBA TBA MeOH, TBA, MTBE and 2-butanol TBA TBA -
Note : S = selectivity; X = conversion; TBA = tert-butyl alcohol; MeOH = methanol; MTBE = methyl tert-butyl ether; CBV-760 = zeolite H-sdusy powder; RD = reactive distillation; CD = catalytic distillation; PFR = plug flow reactor; CM = catalytic membrane; FBR = Fixed bed reactor; BR = batch reactor; CSTR = continuous stirred tank reactor; TR = tubular reactor.
11
Ref. 117 94 99 116 131 128 121 122 123 134
120 124 118 118
The selectivity of DIB in dimerization of isobutylene was decreased over sodium content on resin based catalysts 124, 126, 135
. However, the selectivity can be increased using the polar components
, lower temperature
concentration 126
126
128
120, 125
, larger pore size of resins
127
, and smaller isobutylene
where the good selectivity was achieved using a medium crosslinked catalyst
. Another catalyst such as zeolite is also used in dimerization of isobutylene. Zeolites used in
the synthesis were Beta 109 with Si/Al ratios in the range of 10 to 150 134, mordenite with particle sizes of 20 – 35 mesh
109
, Y (FAU topology) with Si/Al ratios of 10.8 – 48
118
, and ZSM-5
136
with Si/Al ratios of 140 – 230 118. Physical properties of catalysts such as zeolites and resins used in dimerization of isobutylene are presented on Table 3 and Table S3. Zeolite Y has the largest specific surface areas of approximately 687 – 763 m2/g with acid capacity of 0.82 mequiv H+/g and pore diameter and pore volume of 3.61 nm and 0.17 cm3/g, respectively
96, 118
. Beta and
mordenite zeolites have surface areas of 650 m2/g and 500 m2/g, with pore diameter of 8.75 nm and 3.62 nm, respectively 96, 97. ZSM-5 has a smaller surface area of about 402 – 418 m2/g with Na2O contents of 0.03 – 0.05
118
. The effect of porosity of catalysts on the conversion and
selectivity of dimerization will be discussed on the next section (Section 3.3).
12
Table 3. Physical properties of catalysts used in dimerization of isobutylene 96-98, 118, 124, 126, 128, 130, 135-138. Properties cross-linking (%) exchange capacity (eq H+/kg) porosity (cm3/g) specific surface area (m2/g) pore diameter (nm) temperature stability (oC) particle size (mm) acidity (mequiv H+/g.dry) Si/Al ratio Na2O (%) moisture content (%) apparent density (g/cm3) pore volume (cm3/g)
T-63
Y-Z
A-15
P-CT 252
15 4.9 35 40 120 0.3 – 1.2 40 -
687 – 763 3.61 0.82 10.8 – 48 0.03 – 0.1 0.1654
H 4.81 0.30 45 – 157 30 – 34 120 0.63 – 1.25 4.75 53.1 0.77 0.3
M 5.40 132 39 130 0.78 5.40 0.2 – 0.5
T = tulsion; Z = zeolite; A = amberlyst; P = purolite; Y = FAU topology; H = high; M = medium.
13
The production of isooctene and isooctane can be produced over many of the presently existing commercial technologies in the available industries as presented on Table 4. Additives such as TBA, methanol, MTBE, and water were utilized in the process as a selectivity enhancer. Ion exchange resin was widely used as a catalyst by the most of technologies. However, Dimersol technology applied by IFP company used nickel complexes as a catalyst in the process to convert linear butenes to isooctene. This catalyst was synthesized using an organo chloro aluminate ionic liquids (OCAIL). The high selectivity of 92 – 93% was obtained without additive 100, 139. The obtained isooctene can be easily separated because of poorly soluble solution. Ortho phosphoric acid used as a catalyst by Nippon Oil’s Olefin Conversion Technology resulted the higher selectivity approximately 100% without selectivity enhancer 140
. The catalyst was synthesized using PAM supported on silica
140
100,
. Another catalyst beside ion
exchange resin utilized in isooctane production was a solid phosphoric acid (SPA) used by UOP’s InAlk technology 100, 130, 141, 142. The catalyst was treated using polyphosphoric acid (PA) where water, TBA, and methanol were used in the process to increase the selectivity of isooctene or isooctane. The commercial technologies using water or pure TBA as a selectivity enhancer and ion exchange resin as a catalyst were Neste Oil and KBR’s NExOCTANE technology UOP’S InAlk technology
100, 141, 142
100, 143, 144
, and Lyondell’s Alkylate 100 technology
100
,
with
deactivation of catalysts using sodium hydroxide solution, surface areas of 33 – 53 m2/g, pore diameters of 24 – 30 nm, and acid capacities of 4.7 – 5.4 mmol H+/g 126.
14
Table 4. Commercial technologies used to produce isooctane over the hydrogenation of isobutene dimers. Commercial Technology NExOCTANE CDIsoether
Industry Neste Oil and KBR CDTECH and Snamprogetti
Selectivity enhancer Water and TBA Water/TBA and MeOH/MTBE Water, TBA, and methanol
InAlk
UOP
OCT
Nippon Oil
-
IFP Lyondell
Water and TBA
Dimersol Alkylate 100
Catalyst
Synthesis
Ion exchange resin
Isooctane properties
Ref
-
RON 99.1
MON 96.3
SG 0.70
Ion exchange resin
-
100.2
100.3
0.72
83, 100, 145
SPA or SAR
SPA treated over PA
100
100
-
100, 130, 141, 142
Ortho phosphoric acid Nickel complexes Ion exchange resin
100, 143, 144
PAM supported on silica
-
-
100, 140
Treated using OCAIL treated over NaOH
-
-
100, 139 100, 126
KBR = Kellogg Brown and Root; CDTECH = catalytic distillation technologies; OCT = Olefin Conversion Technology; SPA = solid phosphoric acid; SAR = sulfonic acid resin; PA = polyphosphoric acid; PAM = phosphoric acid monomer; OCAIL = organo chloro aluminate ionic liquids; RON = Research Octane Number; MON = Motor Octane Number; SG = specific gravity.
15
The catalytic distillation technologies (CDTECH) and Snamprogetti’s CDIsoether technology used methanol or MTBE in addition into pure TBA to increase the selectivity of product
83, 100, 145, 146
and A-15 (pore diamater of 24 nm), A-35 (pore diamater of 20 nm), A-
XN1010 (pore diamater of 5 nm), and A-XE586 (pore diamater of 25 nm) as catalysts
83
where
dimerization and etherification were partially performed. The alcohols such as methanol or MTBE have important impact on isoether technology for isooctane synthesis via hydrogenation from dimerization reaction because of high polarity, shorter removing the reaction heat, and a poor control of temperature 83. Isooctane obtained over CDIsoether technology by CDTECH and Snamprogetti had research octane number (RON) and motor octane number (MON) of 100.2 and 100.3 and specific gravity of 0.72 83. Figure 2 shows a flowscheme isooctane production from isobutylene using NExOCTANE technology adapted from KBR technology. Isobutylene, which is used in dimerization, can be derived from fluid catalytic cracking (FCC) 149, 150
40, 147, 148
, naphtha and ethane steam cracking (SC)
, oxidative dehydrogenation of n-butene (n-butene ODH)
32, 151-155
, and skeletal
isomerization of n-butenes 44, 46-48, 50-52. The FCC produced 50.5 wt.% of C4 olefins containing 15 wt.% of isobutylene C4 olefins
32
40
147
using zeolites as catalysts
using various catalysts
and n-butene ODH produced 83.3 wt.% of
151, 153, 155, 156
. Approximately, 5.76 wt.% of C4 olefins was
produced over naphtha cracker 149 and 94 wt.% of C4 olefins was produced over the low severity SC process comprising 32 wt.% of isobutylene
32, 157
. The obtained C4 olefins were affected by
different temperatures 158, 159, the types of feedstocks 159, 160, and catalysts 159, 161. Production of isooctene was processed over dimerization of isobutylene and further hydrogenated to obtain isooctane using one of the commercial technologies such as NExOCTANE technology because of a low cost technology as presented on Figure 2. Isooctane
16
used as an octane booster compound in gasoline for fuel engines has octane number of 100 and n-heptane of 0 100. Isooctane production via hydrogenation of isooctene was performed in a three-phase reactor using a mixture of 2,4,4-trimethyl 2-pentene and 2,4,4-trimethyl 1-pentene with ratio of 1:4 and total concentration of 5 or 15 mol.% in the liquid phase, cyclohexane as a solvent, and Ni/Al2O3 catalyst with surface area of 108 m2/g and pore volume of 0.37 cm3/g at 35 – 95 oC and 10 – 14 atm for 5 – 12 h
162
. The obtained activation energies were 34 kJ/mol for 2,4,4-trimethyl 1-
pentene and 49 kJ/mol for 2,4,4-trimethyl 2-pentene
162
. Approximately, 60 – 65 kJ/mol of the
activation energy was obtained from etherification for 2,4,4-trimethyl-pentenes
163
using
methanol 164. The NExOCTANE technology was carried out in a fixed bed reactor in the liquid phase using water or TBA as a selectivity enhancer
100
and ion exchange resin as a catalyst with the
feedrate of approximately 13,700 lb/h at 57 oC and 30 atm feed component
119
119, 144
. Isopentane was an inert in the
. The catalysts used in the NExOCTANE technology had an excellent
performance and longer activity than others for saving cost. The NExOCTANE technology was operated and developed by Neste Oil and KBR technology. The NExOCTANE technology is chosen as a promising technology in the future for isooctane production and suitable for many processes such as FCC refinery, C4 olefins production, and isobutylene dehydrogenation process because it has many advantages. First of all, it is a low cost effective technology and appealing in the conversion of existing MTBE facilities. The second advantage, dimerization process and hydrogenation process were separated each other so the production of isooctene can be carried out without hydrogenation process in the NExOCTANE technology. It also produces the best
17
quality commercial product, high availability, and long catalyst age. In addition, it can be utilized by producer to the use of existing hydrogenation units or to mix isooctene into the gasoline. The mechanism of isooctane production obtained over dimerization of isobutylene is followed by hydrogenation process as presented on Figure 3 where the hydrogenation units were utilized to remove sulfur content to be less than 5 wppm with a reaction heat of about – 30,000 kcal/kmol because of high exothermic
165
. Catalysts are required in hydrogenation process to
fasten the reaction rate and shorten the reaction time and the catalysts mostly used in hydrogenation process were synthesized over nickel. The poisons on isobutylene have important impact on the catalytic reaction in the hydrogenation process 165.
3. Effect of heterogeneous catalysts properties in the oligomerization of isobutylene 3.1 Effect of different catalysts such as resins and zeolites Catalytic oligomerization of isobutylene to obtain DIB, TIB, and TEB was carried out using different catalysts such as resins and zeolites. Comparison of the effectiveness between resins such as A-15 and A-35 and zeolites such as Beta zeolite and ferrierite zeolite as catalysts in trimerization reaction was reported by Yoon et al
106
. The zeolites were calcinated to the proton
form and the resins comprised sulphonic acids. The conversion of isobutylene obtained over resins was very stable with the one of 100% up to 600 h where the performance of A-35 was better than A-15 because of higher acidity. The conversion obtained using A-35 was decreased over higher weight hourly space velocity (WHSV) for 20 h
107
. However, the conversion of
isobutylene obtained over zeolites showed the worse performance compared with those over resins. The one for Beta zeolite was 100% – 90% within 300 h and 100% – 80% for ferrierite zeolite up to 150 h 106.
18
Moreover, the selectivities of TIB obtained over resins were higher than the ones obtained over zeolites. The highest selectivities of TIB were approximately 80% for A-35, 75% for A-15, 70% for ferrierite zeolite, and 60% for Beta zeolite 106. The selectivity of TIB obtained using A35 and ferrierite zeolite was decreased because of higher WHSV
107
. For instance, the
selectivities of TIB obtained over A-35 were more than 70% with WHSV of 10 h-1, 60% with WHSV of 30 h-1, and less than 50% with WHSV of 50 h-1
107
. The advantages of resins used in
oligomerization as catalysts were long lifetime and high selectivity of TIB
106
. The deactivation
rate of resins was rapidly decreased with lower temperature as reported by Hauge et al 109. The conversion and selectivity of TIB in trimerization of isobutylene obtained using mordenite zeolite were smaller than those obtained using resins such as A-35, A-15, and A-DT where A-15 produced the highest conversion and selectivity of TIB because of the highest acid capacity 105. The resins were ion exchanged over ethanol to remove water content and mordenite zeolite was calcinated at 550 oC for 8 h. The conversion and selectivity of TIB obtained using mordenite zeolite were 70% and 16%, respectively, and the conversion and selectivity of TIB achieved using resins were 71% and 50% for A-DT, 90% and 64% for A-15, and 91% and 66% for A-35, respectively 105. The comparison between resins such as A-15 and Dowex 50 and zeolites such as ZSM-5, mordenite, beta and Y in oligomerization of isobutylene was reported by Hauge et al
109
. The
resins were slowly deactivated so that the conversion was slowly decreased with the one of from 70% to 35% for A-15. However, the zeolites were rapidly deactivated so that the conversions were sharply decreased with the ones of from more than 90% to less than 10% for zeolite Y, from more than 85% to less than 10% for Beta zeolite, from more than 60% to less than 10% for ZSM-5 and mordenite zeolite
109
. The A-15 showed a good stability
109
whereas the selectivity
19
over zeolites was low because of small pore size and strong acidity 106. The advantage of zeolites as catalysts is their regenerability over simple calcination. The shorter lifetime and the lower selectivity on zeolite catalysts in oligomerization reaction can be improved with meso- and macroporous sizes 106.
3.2 Effect of Topology (1-D versus 3-D) Topology has very important impact on catalyst design for dimerization, trimerization, and oligomerization of isobutylene. Various zeolite topologies were used in oligomerization of isobutene. Yoon et al
98, 106
studied the effect of different topologies in oligomerization of
isobutylene using Beta zeolite (BEA) zeolite (FER)
106
106
with three dimensional channels (3D)
with two dimensional channels (2D)
166
166
, ferrierite
, and mordenite zeolite (MOR)
98
with
one dimensional channels (1D) 166, The BEA and FER topologies were calcinated at 550 oC for 8 h and reactivation of aged zeolites was performed over calcination at 400 oC for 10 h
106
. The
MOR topology was heated at 300 oC for 10 h to remove water content 98. The selectivity to TEB obtained over BEA (3D) was the highest among the others and the one obtained over FER (2D) was higher than the one obtained over MOR (1D). The selectivity to TEB was in the order of BEA > FER > MOR because the dimension of BEA (3D) was the largest among FER (2D) and MOR (1D) and the dimension of FER (2D) was larger than the one of MOR (1D). The highest selectivity to TEB for BEA (3D) was more than 30% and the one for FER (2D) was approximately 20% 106. However, the selectivity to TEB obtained over MOR (1D) was less than 5%
98
. The BEA (3D) was slowly deactivated and had the highest selectivity of
TEB because of the largest channel system
106
. The higher selectivity of TEB can be achieved
over larger channel system.
20
Moreover, the conversion obtained over BEA (3D) was higher than over FER (2D) where the ones were approximately 99% for BEA (3D) and 83% for FER (2D) for 150 h
106
. The
conversion obtained over MOR (1D) was approximately 20% with the fastest reaction time of 12 h
98
. The zeolites with one dimensional channel (1D) such as mordenite zeolite (MOR)
SAPO-11 zeolite (AEL)
106
98
and
were rapidly deactivated because of the smallest channel systems.
Therefore, the conversion of isobutylene obtained using these zeolites was sharply reduced. The effect of topology (1-D versus 3-D) in trimerization of isobutylene using mordenite zeolite (MOR) with one dimensional channel (1D) and ZSM-5 zeolite (MFI) with three dimensional channels (3D) was reported by Yoon et al 98. The conversion on the MOR (1D) was sharply decreased approximately from 90% to 20% for 12 h with deactivation of approximately 6.4% per hour whereas the one on MFI (3D) was slowly decreased approximately from 60% to 40% for 12 h with deactivation of approximately 1.8% per hour
98
. The selectivity of TIB
obtained over MFI (3D) was higher than over MOR (1D) with the one of approximately 15% for MFI (3D) and less than 10% for MOR (1D)
98
. The MFI (3D) was slowly deactivated and had
higher selectivity of TIB because of the larger channel systems. The recent MFI synthesis was carried out using isopropylamine as an organic structure directing agent (OSDA) and 2-propanol as co-solvent 167. The (70%) selectivity of TIB obtained over the FER (2D) was much higher than the (10%) one obtained over MOR (1D) with the steady conversion (100%) for up to 12 h because of the larger channel system on FER (2D) 14.13, and c = 7.49 Å
168
98
. The FER (2D) had unit-cell dimensions a = 19.16, b =
and was also widely utilized as a catalyst in many applications as
previously reported by some researchers
15, 169-187
. The recent synthesis of FER topology zeolite
21
was carried out using various organic structure directing agents (OSDAs) at 60 – 200 oC with the fastest reaction time of 0.5 h and the longest reaction time of 336 h 188-200. The effect of topology (1-D versus 3-D) in trimerization of isobutylene using the BEA (3D) with Si/Al ratio of 12.5 and the MOR (1D) with Si/Al ratio of 12.5 was also studied
97
. In this
reaction, both catalysts were heated at 300 oC for 10 h to remove water content. The obtained result was the same with the result obtained by Yoon et al
98
where the zeolites having larger
dimensional channel produced the higher selectivity and the stable conversion during the reaction time. The selectivity of TIB obtained over the BEA (3D) was more than 60% for 12 h with the conversion of 100% up to 25 h 97. However, the smaller selectivity of TIB obtained over the MOR (1D) was less than 10% for 12 h and the conversion was rapidly decreased approximately from 90% to 20% for 13 h 97. The catalytic performance of the Beta zeolite (BEA) with Si/Al ratio of 12.5 was compared with the USY zeolite (FAU) with Si/Al ratio of 30 97 in trimerization of isobutylene where these zeolites have the same dimensional channel (3D) and the same pore size (12 membered rings) 166
. A result showed that the BEA (3D) had the remarkable performance compared with the FAU
(3D). The selectivity to TIB obtained over the BEA (3D) was higher than the one obtained over the FAU (3D). Moreover, the conversion on the BEA (3D) was very stable for 25 h whereas the one on the FAU (3D) was slowly decreased with the reaction time of 20 h. The selectivities of TIB were more than 60% for the BEA (3D) and more than 40% for the FAU (3D). The conversions were 100% for the BEA (3D) and from 100% to about 85% for the FAU (3D)
97
.
The FAU (3D) was ion exchanged over AgNO3 and Cu(NO3)2.3H2O at 25 oC for 24 h 201. The conversion and selectivity obtained over the BEA (3D)
97, 106
both in the trimerization
reaction and in the oligomerization reaction were the highest among the ones obtained over
22
MOR (1D) 97, 98, FER (2D) 106, FAU (3D) 97, and MFI (3D) 98. Therefore, the BEA (3D) was the most potential catalyst to use in the oligomerization of isobutylene to produce TIB and TEB because of the highest selectivity, high stability, low deactivation, easy regeneration, and stable conversion
97
. The performance of the BEA (3D) was decreased with the higher Si/Al ratio
97
.
Further improvement of BEA (3D) can be carried out over microwave synthesis on a borosilicate glass substrate
202
, using choline chloride/urea mixture as a deep eutectic solvent
203
, and the
incorporation of Mn into the framework of Beta zeolite 204. Even though, the MOR (1D) produced the lower selectivity of TIB and TEB than those produced over other catalysts because of the smallest dimensional channel. The MOR (1D) had a remarkable performance in dimerization of isobutylene to produce DIB, well known as isooctene. Dimerization of isobutylene was carried out using three different catalysts such as the MOR (1D), the FAU (3D), and the BEA (3D) with the same pore size of 12 member rings
97
.
The selectivities of DIB obtained over the BEA (3D) and the FAU (3D) were less than 10% and 40%, respectively. However, the selectivity of DIB achieved over the MOR (1D) was the highest selectivity of DIB with the one of approximately 90%
97
. The MOR can be dealuminated over
nitric acid (HNO3) 205, microwave 206, 207, and chemical reagents 208. The effect of topology (2-D versus 3-D) in dimerization of isobutylene using ferrierite zeolite (FER) with two dimensional channels (2D) and ZSM-5 zeolite (MFI) with three dimensional channels (3D) was also observed by Yoon et al 98. Both catalysts were calcinated at 550 oC for 8 h then converted into the proton form and then heated at 300 oC for 10 h. Even though, the FER (2D) showed a very stable conversion with the one of 100% up to 12 h, the selectivity of DIB obtained over FER (2D) was much smaller than over MFI (3D) because of the smaller dimensional channel. The selectivities of DIB obtained over FER (2D) and MFI (3D)
23
were more than 10% and 80%, respectively, after 10 h reaction time
98
ZSM-5 zeolite was carried out using both different organic templates
209-213
. The recent synthesis of and without organic
templates 214.
3.3 Effect of Porosity A better thermal stability, the high micropore surface area with pore diameter of 0.3 – 1.5 nm
215
and strong acid sites
216
, lead to zeolites are mostly used as catalysts in many reactions.
The different pore sizes of zeolite have significant impact on conversion and selectivity in dimerization, trimerization, and oligomeerization of isobutylene. For instance, Beta zeolite (BEA) with 12-member rings (MR-12) and ferrierite zeolite (FER) with 10-member rings (MR10) were applied in oligomerization of isobutylene to produce TEB with the reaction time of 600 h 106. The selectivity of TEB obtained using BEA (MR-12) was higher than using FER (MR-10) because the pore size of BEA (MR-12) was larger than the pore size of FER (MR-10). The highest selectivities of TEB were approximately 20% for FER (MR-10) and more than 30% for BEA (MR-12) 106. Moreover, deactivation in BEA (MR-12) was slower than the one in FER (MR-10) because of the larger pore size. It leads to the concentration on the FER (MR-10) was rapidly decreased. The conversion of isobutylene were approximately decreased from 100% to 90% for BEA (MR12) and from 100% to 80% for FER (MR-10) 106. The pore size had also significant impact on dimerization of isobutylene to produce DIB. Mordenite zeolite (MOR) with 12-member rings (MR-12), ferrierite zeolite (FER) with 10member rings (MR-10), and ZSM-5 zeolite (MFI) with 10-member rings (MR-10) were compared in this reaction to observe the catalytic performance of zeolites 98. The same trend was
24
obtained as in trimerization of isobutylene where the zeolites having the larger pore size showed the better performence. The selectivity to DIB on MOR (MR-12) was the highest among the one on FER (MR-10) and the one on MFI (MR-10) because of the largest pore size. The selectivities to DIB were approximately 90% for MOR (MR-12), 83% for MFI (MR-10), and less than 20% for FER (MR-10) after the reaction time of 10 h 98.
3.4 Effect of acidity Acidity affected the isooctene production obtained from dimerization, trimerization, and oligomerization of isobutylene using various catalysts such as substrates, resins, and zeolites. At Al2O3 catalyst synthesized over 1 – 6 % of ammonium phosphate, Lewis acid sites had more important impact on dimerization of isobutylene than Brønsted acid sites because of enough acid strength with high selectivity
117
. The higher catalysts stability was achieved with the higher
Lewis/Brønsted sites ratio in oligomerization of isobutylene using sulfated titania sol (TiO2) catalyst 111. The total acidities with Lewis/Brønsted sites ratios of 3.65, 1.82, and 1.25 were 280, 105, and 160 mmol/g, respectively. The conversion of isobutylene and selectivity of C 8 – C12 olefins using sulfated TiO2 synthesized over ammonium sulfate for 4 h were 100% and 88%, respectively, with the reaction time of 40 h 111. The effect of acidity on resins was reported by Yoon et al
105
in trimerization of isobutylene
using A-35, A-15, and A-DT resins. These resins were ion exchanged using ethanol to remove water content. The acid capacities on A-35, A-15, and A-DT resins were 5.2 mequiv.H+/g, 4.7 mequiv.H+/g, and 3.1 mequiv.H+/g. The conversions and selectivities to TIB obtained using the resins were 91% and 66% for A-35, 90% and 64% for A-15, and 71% and 50% for A-DT,
25
respectively
105
. The A-35 resin produced the highest conversion and selectivity because it has
the highest acid capacity 105. Zeolite was one of the most important catalysts in the industry because of its tunable acid sites 215, 216. The acidity of zeolites modified by nickel salts such as NiCl2, NiSO4, and NiCO3 has significant impact on catalytic activity, stability, and selectivity of zeolites 96. For instance, more active and selective to dimerization reaction was achieved over Y zeolite modified over NiCO 3 96
. The activity was significantly reduced over converting the most of Brønsted acid sites to
Lewis acid sites
217
and the unsaturated compounds were formed by the strong Lewis acid sites
(with the highest number of 0.923 mmol/g at 20 oC)
96
. The high selectivity to DIB in
dimerization reaction was probably enhanced because of Lewis acid sites having enough acid strength. However, the Brønsted acid sites were not observed
117
. Therefore, both Brønsted acid
sites and Lewis acid sites have important impact on dimerization reaction to isooctene. The effect of acidity on membrane zeolite beta (MZB) with SiO2/Al2O3 ratios of 60, 90, and 120 was also observed on the resulted selectivity to DIB in oligomerization of isobutylene at 100 – 150 oC
218
. The MZB-60 had the highest acid capacity among the MZB-90 and the MZB-120.
The acid capacities on MZB-60, MZB-90, and MZB-120 were 4.6 x 10-3, 3.07 x 10-3, 1.82 x 103
, respectively. The obtained selectivities to DIB were 92% for MZB-60, 84% for MZB-90, and
74% for MZB-120
218
. The higher acidity produced the higher selectivity. The MZB-60 resulted
the highest selectivity because of the highest acid capacity. The best conversion of isobutylene and high selectivity to TIB in trimerization of isobutylene using Beta-25 zeolite (SiO2/Al2O3 = 25) were achieved because of high Lewis:Brønsted site ratio with the selectivity to TIB of more than 50% at isobutylene WHSV of
26
10 h-1 for up to 100 h
97
. The Beta-25 zeolite had the better catalytic activity and higher
conversion of isobutylene than Beta-18 zeolite (SiO2/Al2O3 = 18) and Beta-38 zeolite (SiO2/Al2O3 = 38) with the high concentration of Lewis acid site
97
.
The acidity on commercial zeolites with different topologies such as USY zeolite (FAU) and Beta zeolite (BEA) influenced the obtained conversion of isobutylene. The numbers of Brønsted acid site and Lewis acid site on Beta zeolite were 247 µmol/g and 104 µmol/g at 200 oC, 204 µmol/g and 80 µmol/g at 300 oC, and 144 µmol/g and 67 µmol/g at 400 oC, respectively, with deactivation constant of 0.2060/h 219. However, the numbers of Brønsted acid site and Lewis acid site on USY zeolite were lower than on beta zeolite. The ones were 98 µmol/g and 61 µmol/g at 200 oC, 14 µmol/g and 29 µmol/g at 300 oC, and 0 µmol/g and 27 µmol/g at 400 oC, respectively, with deactivation constant of 0.6752/h. The conversion of isobutylene on USY zeolite was dramatically decreased from 43% to 5% for 30 h and the one on Beta zeolite was decreased from more than 45% to 10% for 30 h
219
. The deactivation of Beta zeolite was slower than USY
zeolite because of higher strength of acidity and lower deactivation constant
219
. The fast
deactivation on zeolites can be caused by blocking of the inner pores because of large oligomerization 109, 218.
3.5 Isooctane production via hydrogenation Another solution as replacing MTBE is isooctane derived from dimerization of isobutylene after hydrogenation using resin or zeolite as a catalyst in a MTBE reactor
132, 162, 165, 220, 221
. The
conditions of process such as temperature and pressure in the MTBE synthesis can be applied to isooctane process
146
. Therefore, the existing MTBE technology can be utilized for the
production of isooctane without significant changes 162. Dehydrogenation technology of paraffins
27
is an innovative technology to produce isooctane 146. Isooctane is considered as an octane booster compound because of high motor octane number (MON) and research octane number (RON) of 100 146, 162. However, the MON and RON for isooctene were 89 and 100, respectively 130. The hydrogenation kinetics of isooctene (2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2pentene) carried out to obtain isooctane (2,2,4-trimethylpentane) in a three-phase reactor at 35 – 95 oC and 9.8 – 39 atm was reported by Lylykangas et al
162
. In this synthesis, hydrogenation
study was performed in a three-phase Robinson-Mahoney reactor in the liquid phase using cyclohexane as a solvent and Ni/Al2O3 as a catalyst where the nickel-alumina catalyst had a high activity in the hydrogenation of isooctene process. Pd/Al2O3 can be also used as a catalyst in hydrogenation process
119, 222
. The molar fraction of 2,4,4-trimethyl-1-pentene was 0.673
162
.A
heat of the reaction in hydrogenation process was exothermic with the one approximately – 109.55 kJ/mol using RSTOIC Aspen model 119. Development of isooctane production over catalytic distillation process via hydrogenation process of isooctene was carried out by Goortani et al
119
. The reduced capital and operating
costs, beneficial production of higher efficiency, and decreased waste and recycle streams were achieved because of a combination of two or more unit processes to be single unit process. The catalytic distillation process was a green technology that was utilized to combine the functions of chemical reactor (catalytic reaction) and distillation column (separation of product from reactant) into a single unit process 222, 223 and produced higher conversion and selectivity 119. Composition of a mixture of isooctene to produce isooctane was adapted from the previous study as reported by Karinen et al
224
in equilibrium isomerization reaction of isooctene
production at 50 – 110 oC using methanol 225 and A-35 224 and Smopex-101 225 as catalysts where isooctene production was affected by isomerization of linear butenes using tert-butyl alcohol
28
and ZSM-5 zeolite synthesized over nickel as a catalyst
226
.
The formation of isooctene can be also carried out in a liquid phase using sulfonic acid resin
130
.
(TBA) as a selectivity enhancer
133
The data of isooctene production can be further applied for the development of isooctane production 162.
4.
Perspective on potential zeolite topologies for dimerization Dimerization of isobutylene to isooctene over various zeolites as catalysts was previously
reported by several researchers as presented on Table 5 and Table S4. Brown et al
227
studied
dimerization of isobutylene using wavelet shrinkage denoising combinated with time-varying flexible least squares over ZSM-5 zeolite with Si/Al in the range of 16 to 17 in a reactor at 100 o
C for 2 h. The activation energy of dimerization of isobutylene was 48.3 kJ/mol – 267 kJ/mol
with the average activation energy of 112 kJ/mol 227. Koskinen et al 228 reported isooctene synthesis using CO2 and propane as solvents in reaction medium over ZSM-5 and ZSM-23 zeolites as catalysts at 100 oC and 49 – 89 atm for 120 – 200 h. The ZSM-5 catalyst with Si/Al ratio of 23.8 had the total acidity of 380 µmol/g and the ZSM23 catalyst with Si/Al ratio of 19.7 had the total acidity of 690 µmol/g. The initial conversion obtained over ZSM-23 catalyst was more than 80%. However, the conversion obtained using CO2 as a solvent was approximately 56% for up to 200 h. It was higher than the one obtained using propane as a solvent with the conversion of 32% for up to 120 h. Stability of ZSM-23 was longer over CO2 solvent than over propane solvent
228
. Further improvement of ZSM-23 (MTT)
can be performed by microwave synthesis and alkaline treatment
229-232
stability of ZSM-23 (MTT) in hot water was affected by La and Ce
233
where hydrothermal
. The highest conversion
29
of isobutylene obtained using propane solvent and ZSM-5 catalyst was approximately 72% at 49 atm with WHSV of 25 h-1 for 1 min 228.
30
Table 5. Dimerization over zeolite based catalysts 96-98, 109, 218, 219, 227, 228, 234-236. Zeolite MZβ-60 MZβ-90 Beta USY USY HY-700 Mordenite ZSM-5 Mordenite USY MZβ-120 MSU-S/WBEA
Topology
MR
D
Si/Al
Phase
BEA BEA BEA FAU FAU FAU MOR MFI MOR FAU BEA BEA
12 12 12 12 12 12 12 10 12 12 12 12
3 3 3 3 3 3 1 3 1 3 3 3
30 45 45.7 2.6 30 1.4 12.5 25 12.5 30 60 43.2
Gas Gas Gas Gas Liquid Liquid Liquid Liquid Liquid Liquid Gas Gas
Surface area [m2/g] 455 396 811 867 750 382 500 425 500 720 237 718
Solvent n-butane n-butane n-butane n-butane n-butane n-butane -
Temp [oC] 100 – 150 100 – 150 60 60 70 70 40 – 100 40 – 100 40 – 100 40 – 100 100 – 150 60
Time [h] 96 96 30 30 20 6 Up to 25 1 – 12 1 – 12 Up to 25 96 30
Results [%] X = 66; S = 92 X = ˃ 30; S = 84 X = 47 – 10; S = 86 – 99 X = 43 – 5; S = 100 X = 85.3; S = 59.6 X = 74.1; S = 50.6 X = 90 – 20; S = ~ 90 X = 60 – 40; S = ~ 80 X = 90 – 20; S = ~ 90 X = 100 – 90; S = > 30 X = ˃ 30; S = 74 X = 55 – 35; S = 70 – 80
MR = member ring; D = dimension; X = conversion; S = selectivity; MZ = membrane zeolite; MSU-S/WBEA = meso structured aluminosilicates of beta seed.
31
Ref. 218 218 219 219 234 235 97 98 98 97 218 219
A novel reactor design namely catalytic membrane reactor utilized in oligomerization reaction in a gas phase over different Beta zeolites as catalysts was proposed by Torres et al
218
.
The Beta zeolites used in this method were membrane zeolite Beta-60 (MZB-60) with Si/Al ratio of 30, membrane zeolite Beta-90 (MZB-90) with Si/Al ratio of 45, and membrane zeolite Beta120 (MZB-120) with Si/Al ratio of 60. Aerosil 300 as a silica source, potassium hydroxide, and tetra ethyl ammonium hydroxide (TEAOH) as an OSDA were used in the synthesis. The typical synthesis had the molar ratio of Na2O : K2O : Al2O3 : SiO2 : (TEA)2O : H2O = 3.2 : 1.62 : 1 : x : 22.5 : 1080 with value of x = 60 for MZB-60, x = 90 for MZB-90, and x = 120 for MZB-120. The zeolites were ion exchanged by NH4NO3 solution. The catalytic reaction was carried out at 100 – 150 oC for 96 h. The conversion of isobutylene and selectivity of isooctene in the reaction using MZB-60 at 110 oC were 66% and 92%, respectively, and the selectivities on MZB-90 and MZB-120 were 84% and 74%, respectively with WHSV of 53 h-1 218. The catalytic performance of commercial zeolites such as Beta zeolite (Si/Al ratio of 45) and USY zeolite (Si/Al ratio of 2.5) were compered with mesoporous aluminosilicate of Beta zeolite (MSU-S/WBEA) and non-mesoporous aluminosilicate of Beta zeolite (MSU-S/SBEA) in oligomerization of isobutylene at 60 oC and 1 atm for 30 h 219. Beta seeds (Si/Al ratio of 44) was synthesized using tetra ethyl ammonium hydroxide (TEAOH) solution as an OSDA (organic structure directing agent) and dried at 100 oC for 6 h. Tallow tetra amine and HCl were used to obtain MSU-S/WBEA and digestion of beta seeds was carried out in the absence of surfactant as a mesoporous template at 150 oC for 20 h to obtain MSU-S/SBEA. The initial conversions were approximately 55% on MSU-S/WBEA, 51% on MSU-S/SBEA, 47% on Beta zeolite, and 43% on USY zeolite at 60 oC with isobutane/isobutene mass ratio of 50 wt./50 wt. At the same condition, the highest selectivities of isooctene on MSU-S/WBEA, MSU-S/SBEA, Beta, and USY zeolite were
32
approximately 80%, 79%, 99%, and 100%, respectively
219
. The selectivity of isooctene on
MSU-S/SBEA was stable because of the strong Brønsted acid sites. The commercial USY zeolite (Si/Al ratio of 30) was also treated over AlCl 3 and heated at 300 oC for 10 h
234
. The synthesis was carried out using n-butane solvent with isobutylene/n-
butane mass ratio of 50/50 (wt.%/wt.%) at 70 oC and 14.8 atm for 20 h. The USY zeolite and AlUSY zeolite (USY treated by AlCl3) were used in the synthesis as catalysts. The conversion and selectivity on USY zeolite were 85.3% and 59.6 wt.%, respectively, and the conversion and selectivity on Al-USY zeolite were 99.2% and 21 wt.%, respectively. The conversion on AlCl3 treated zeolite was higher than on commercial zeolite because of higher Lewis acid site/Brønsted acid site ratio. The commercial zeolite had Lewis acid site/Brønsted acid site ratio of 0.31 with 0.34 for Al-USY zeolite 234. USY, Beta, and mordenite zeolites were also used in dimerization reaction by Yoon et al 97. The synthesis was carried out in a liquid phase using n-butane as a solvent at 40 – 100 oC and 14.8 atm. Beta zeolite utilized was Beta-25 zeolite (SiO2/Al2O3 = 25) with surface area of 680 m2/g 97. The conversion on mordenite zeolite was reduced from approximately 90% for less than 5 h to approximately 20% for more than 10 h with the highest selectivity to DIB of approximately 90 wt.% for 12 h. Approximately, 100% of the highest conversion on USY zeolite was obtained for less than 5 h and the selectivity was more than 30 wt.% for 12 h. However, the conversion on Beta-25 zeolite was constant with the conversion of 100% up to 25 h. The selectivity to DIB was approximately 10 wt.% for 12 h 97. Yaocihuatl et al 96 utilized Beta (with Si/Al = 75), Y (with Si/Al = 2.6), and mordenite (with Si/Al = 45) zeolite for isooctene synthesis in a gas phase to reduce the effect of diffusion. These zeolites were modified over nickel salts such as NiCl2, NiSO4, and NiCO3 with different
33
concentrations (0 – 16.5 wt.%) and heated at 100 oC for 12 h and then calcinated at 500 oC for 5 h. Surface area of the zeolites were reduced after treated by the nickel salts. For instance, surface areas of beta, Y, and mordenite were 650, 660, and 500 m2/g, respectively. However, the ones were reduced to 437.5, 404.4, and 394.1 m2/g, respectively, after treated by NiSO4.6H2O. The highest conversion and selectivity to DIB obtained using Y zeolite (particle size of 0.3 mm) with 9% of nickel load at 25 oC were 0.7% and 0.98%, respectively 96. The more active and selective to dimerization process was obtained using Y zeolite synthesized over NiCO3. Dealumination of HY zeolite was carried out over calcination at 500 oC (HY-500), 600 oC (HY-600), and 700 oC (HY-700) for 12 h and then heated at 300 oC for 10 h
235
. The catalytic
reaction was performed in a liquid phase using n-butane solvent with isobutylene/n-butane mass ratio of 50/50 (wt.%/wt.%) at 70 oC for 6 h. The resulted conversions were 83.1% for HY zeolite, 91% for HY-500 zeolite, 98.4% for HY-600, and 74.1% for HY-700 and the obtained selectivities of isooctene for HY, HY-500, HY-600, and HY-700 zeolites were 32.5%, 26.3%, 16%, and 50.6%, respectively, with WHSV of 10 h-1 at 70 oC and 14.8 atm for 6 h 235. One of the potential catalysts in trimerization of isobutylene to isooctene is ferrierite zeolite (FER) because of high conversion and selectivity with the selectivity to DIB of more than 70 wt.% and the stable conversion of 100% for 12 h
98
. Because of the remarkable performance,
ferrierite zeolite was also used in dimerization of isobutylene. Comparison between resins and zeolites as catalysts used in oligomerization of isobutylene in a liquid phase using n-butane solvent at 30 – 70 oC and 10 atm was reported by Hauge et al 109. The resins used were A-15 and Dowex 50 and the zeolites used were H-Y, Beta, ZSM-5, and mordenite. The resins were heated in situ at 107 oC for more than 1 h and the zeolites were calcinated using dry air at 500 oC and 1 atm for 8 h. The conversions of the all catalysts were
34
decreased corresponding to the longer synthesis time. Even though, the A-15 resin had the best stability compared zeolites. A shortage of A-15 resin was smaller initial activity and lower initial conversion than Beta and H-Y zeolites. The initial conversions on Beta and H-Y zeolite were more than 80% and 90%, respectively. However, the one on A-15 resin was approximately 70% with WHSV of 60 h-1 at 40 oC and 10 atm for up to 5 h 109. Yoon et al
98
produced isooctene using three different catalysts such as ferrierite with Si/Al
ratio of 10, ZSM-5 with Si/Al ratio of 25, and mordenite with Si/Al ratio of 12.5. The synthesis was carried out in a liquid phase using n-butane solvent with WHSV of 2.5 h-1 – 20 h-1 at 40 – 100 oC and 14.8 atm. The ferrierite and ZSM-5 zeolites were calcinated at 550 oC for 8 h to create the proton form. The conversion on ferrierite zeolite was more much stable and higher than over ZSM-5 and mordenite zeolite, with approximately 20 – 90% over mordenite zeolite, 40 – 60% on ZSM-5 zeolite and 100% on ferrierite zeolite for 1 – 12 h. Nevertheless, the highest selectivity of dimer was achieved using mordenite and the smallest one was obtained using ferrierite zeolite. The selectivities on mordenite, ZSM-5, and ferrierite zeolite for 10 h were approximately 90 wt.%, 80 wt.%, and 10 wt.%, respectively. According to Table 5, the highest conversion and selectivity obtained over different zeolites in dimerization reaction of isobutylene were more than 60% 100% 90%
97
and 99%
97, 98
219
and 90%
for Beta, 100%
97, 98, 236
97
and 100%
219
for mordenite, and 100%
109
and above 80%
for USY, 98.4% 98
and 10%
98
235
98
for ZSM-5,
and 50.6%
235
for Y,
for ferrierite, respectively.
The best zeolite in dimerization process of isobutylene among the other zeolites was USY zeolite. In oligomerization of isobutylene to obtain isooctene (DIB), the USY zeolite was usually used without further purification
97, 234
and calcinated at 600 oC for 4 h
219
. Yoon et al
97
, for
35
instance, utilized the USY zeolite in trimerization of isobutylene. This zeolite was used without further purification and dehydrated at 300 oC for 10 h. However, the resulted selectivity to DIB was low with the one of less than 40 wt.% for 12 h
97
. The USY zeolite was then treated over
AlCl3 (Al-USY) as reported by Yoon et al 234 so that the conversion of isobutylene was increased from 85.3% to 99.2%. Nevertheless, the selectivity to DIB was dramatically decreased from 59.6 wt.% to 21 wt.% for 20 h
234
. The highest selectivity (100 wt.%) was achieved by Park et al
219
where the USY zeolite was calcinated at 600 oC for 4 h. However, the obtained conversion of isobutylene was sharply decreased from 43% to 5% and the reaction time was longer with time on stream of 30 h 219. Therefore, several exact strategies to optimize the performance of the USY zeolite in the reaction and synthesis of catalyst are required as presented on Figure 4. Organic template, shorter synthesis time, and low Lewis/Brønsted acid sites ratio are targeted to improve the performance of the USY zeolite in isooctane production so that dimerization of isobutylene using the USY zeolite can be further studied. The recent synthesis of USY zeolite was reported by some researchers using different organic templates 237-239. Imidazolium-based ionic liquid 239 and 3-(trimethoxysilyl)propyl hexadecyl dimethyl ammonium chloride (TPHAC)
237
can be, for
instance, utilized as a novel organic template to improve the performance of USY zeolite in dimerization of isobutylene. To obtain the higher selectivity to DIB, the low Lewis/Brønsted acid sites ratio should be applied as done by Yoon et al
234
. The shorter synthesis time leads to the
dimerization process to be more effective and promising reaction. These strategies proposed a novel development in the dimerization of isobutylene over USY zeolite by further research.
36
5.
Conclusions The production of isooctane can be obtained over many reactions such as dimerization,
trimerization, and oligomerization reactions via hydrogenation units using polar compounds such as water, methanol, TBA, MTBE, and 2-butanol as a selectivity enhancer. Dimerization is a strategic reaction to convert isobutylene, which was used as a feedstock in MTBE plant, to octane booster compound (isooctane). Heterogeneous resins, metal oxides, and zeolites were widely utilized as a catalyst in dimerization of isobutylene to isooctane using a simple and low cost effective technology. These catalysts have important impact on conversion of isobutylene and selectivity to isooctene in dimerization of isobutylene to isooctane. The conversion of isobutylene and selectivity to isooctene in dimerization reaction are also affected by several parameters such as dimensional channels, pore size (porosity on catalysts), acidity (Lewis and Brønsted acid sites), and reaction conditions. For instance, the conversion of isobutylene and selectivity to isooctene can be increased over larger dimensional channels, higher porosity, higher acid strength, lower temperatures, increased methanol:isobutylene molar ratio, and polar components. Nevertheless, the selectivity to isooctene in the dimerization reaction will be decreased over sodium content on resin based catalysts. Catalysts mostly used in the dimerization of isobutylene are commercial catalysts such as amberlyst resins, zeolite Beta (BEA), Y (FAU), ZSM-5 (MFI), mordenite (MOR), ferrierite (FER), and USY (FAU) zeolite. These zeolites are widely utilized in the synthesis as a catalyst because of easy regeneration, cheap catalysts, and more environmental friendly. The USY zeolite is one of the potential catalysts in isooctane production over dimerization reaction because of highest selectivity, better stability, high conversion, and easily regenerated catalyst. Therefore,
37
further development of the USY zeolite synthesis is required to improve the performance of the USY zeolite activity in dimerization of isobutylene to produce isooctane via hydrogenation process. Several strategies to improve the USY zeolite for commercial application in dimerization of isobutylene are expected to be developed in coming years, including organic template, shorter synthesis time, and low Lewis/Brønsted acid sites ratio.
Acknowledgement The authors would like to acknowledge the funding provided by Saudi Aramco for supporting this work through project contract number 6600011900 for the Center of Excellence in Nanotechnology at King Fahd University of Petroleum and Minerals.
38
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55
30
28
Million tons per year (MTPY)
26
24
22
20
18
16
14
12
10 1992
1993
1994
1995
1997
1998
2000
2001
2002
2003
2004
Year Figure 1. MTBE capacity and demand in the last decades [10–15].
2005
2011
Fluid catalytic cracking (FCC)
C4 olefins
Naphtha and Ethane steam cracking (SC)
C4 olefins
Oxidative dehydrogenation (ODH)
Dimerization
isobutylene
Skeletal isomerization of n-butenes
C4 olefins iso-octene
H2
Hydrogenation
iso-octane
As an octane booster
Figure 2. Flowscheme isooctane production from isobutylene using NExOCTANE technology (adapted from KBR technology).
CH3
H3C C
CH2
+
C
H2C
H3C
Isobutylene
Catalyst
CH3
H
H3C C
CH2
C
CH3
Isobutylene H
C
CH3
C
CH3
Catalyst
CH3
H3C
CH3 C
CH2
+
CH3 H C
CH3
CH3 H
C C H2
CH3
C
Catalyst
CH3
C
HC
H3C
Isooctene (2,4,4-trimethyl-2-pentene)
H3C
C
H3C
CH3 H H3C
CH3
HC
+
H
H3C
C
CH3
C
C CH2
CH3 H
Isooctene (2,4,4-trimethyl-1-pentene) CH3 H H3C
C
CH3
C
C CH2
CH3 H
CH3
CH3 H H3C
C
C
CH3
C
CH3
+ H
H
H2 (Hydrogenation)
H3C
CH3
C CH3
H3C
C
C
CH3 H
CH3
CH3 +
C
H
H
H2 (Hydrogenation)
CH2
Isooctene (2,4,4-trimethyl-1-pentene)
CH3
C
C
CH3
H
H
Isooctane
Isooctene (2,4,4-trimethyl-2-pentene) CH3 H
H
H3C
C CH3
H
CH3
C
C
H
H
CH3
Isooctane
Figure 3. Mechanism of isooctane production from dimerization of isobutylene followed by hydrogenation.
Novel organic template
Shorten synthesis time Al58Si134 O384 Low Lewis/Brønsted acid site ratio
USY zeolite
Figure 4. Strategies to improve USY zeolite for dimerization of isobutylene to isooctane.
C4 olefins
Dimerization
Isobutylene Organic template
Iso-octene
Shorten synthesis time Al58Si134 O384
USY zeolite H2
Hydrogenation
Iso-octane
Low Lewis/Brønsted acid sites ratio
