Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Kinetics, Catalysis, and Reaction Engineering
PHENOL HYDRODEOXYGENATION OVER A REDUCED AND SULFIDED NiMo/#-#l2O3 CATALYST Chrysovalantis Templis, Constantinos Revelas, Anestis Papastylianou, and Nikos G. Papayannakos Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06465 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
PHENOL HYDRODEOXYGENATION OVER A REDUCED AND SULFIDED NiMo/γ-Αl2O3 CATALYST
1 2 3 4 5 6 7 8
Chrysovalantis C. Templis, Constantinos J. Revelas, Anestis A. Papastylianou, Nikos G. Papayannakos*
9
Corresponding Author
School of Chemical Engineering, National Technical University of Athens, 9, Heroon Polytechniou Str., Gr-15780 Zografos, Athens, Greece.
10 11 12 13
*Tel.: (+30) 210 7723239, fax: +30 210 772 3155. E-mail:
[email protected] (N.G. Papayannakos).
14
The authors declare no competing financial interest.
15 16 17
Abstract
18
sulfided form was investigated. Crushed particles (0.165-0.315 mm) were employed to
19
determine the intrinsic reaction kinetics in either case. The internal diffusion phenomena
20
within the catalytic particle of commercial dimensions and forms were also studied and the
21
phenol effective diffusion coefficient and effectiveness factor for both forms of the catalyst
22
were determined.
23
The sulfided form was active to phenol conversion at higher temperatures in comparison to
24
the reduced form. As indicated from the data acquired by phenol, cyclohexanol and benzene
25
hydrotreatment experiments, phenol hydrodeoxygenation over the reduced and sulfided
26
NiMo/γ-Al2O3 catalyst follows both parallel pathways: the direct deoxygenation (DDO) and
27
the step-wise hydrogenation (HYD). The step-wise hydrogenation (HYD) pathway is
28
favoured under the tested conditions. In the presence of the reduced catalyst cyclohexanol
29
and cyclohexane are the main products and the selectivity towards cyclohexanol is high at
30
low temperatures. In the presence of the sulfided catalyst, cyclohexane was the main
31
hydrotreatment product together with a small amount of benzene.
32 33 34 35 36 37 38
Conflict of interest
Phenol hydrodeoxygenation over a commercial NiMo/γ-Al2O3 catalyst in reduced and
Keywords Hydrodeoxygenation, phenol, NiMo/γ-Al2O3, kinetics, internal mass transfer phenomena.
1 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
39
Page 2 of 42
1. INTRODUCTION
40
Cellulosic biomass can be converted into transportation fuels through three major pathways:
41
syngas production by gasification, bio-oil production by pyrolysis or liquifaction and
42
aqueous sugar production by hydrolysis1.
43
Bio-oil is considered as an alternative to petroleum-based fractions for the production of a
44
wide range of fuels and high value-added chemicals. The major organic compounds of the
45
bio-oils are oxygenates like acids, alcohols, ethers, ketones, aldehydes, phenols, esters,
46
sugars, furans, and nitrogen compounds. The phenolic compounds (phenol, guaiacol and
47
other substituted phenol compounds) are formed by decomposition of lignin. The
48
penetration of bio-oil in the market of transportation fuels requires its upgrading to increase
49
heating value, thermal stability, and reduce viscosity and the content of oxygenates. There
50
are two possible ways to upgrade bio-oil1,2 : (i) treatment with zeolites at atmospheric
51
pressure and high temperatures and (ii) catalytic hydrodeoxygenation (HDO) under high
52
pressure. The latter is currently extensively examined as it is based on the well known
53
hydrotreatment process of petroleum fractions and appears to be very effective for the bio-
54
oil upgrading. Phenols are considered as model compounds of bio-oil oxygenates and they
55
have received considerable attention because of their low reactivity in the HDO process3.
56
Two main pathways have been reported so far concerning phenol hydrodeoxygenation: the
57
direct deoxygenation (DDO) and the step-wise hydrogenation (HYD). Different reaction
58
conditions or catalysts favor one of the two pathways4.
59
In DDO mechanism, the carbon-oxygen bond breaks immediately leading to the formation
60
of benzene and water i.e. oxygen is removed from the very first step, while the aromaticity
61
of the molecule is preserved. Cyclohexane and/or cyclohexene can be formed with further
62
hydrogenation of the aromatic ring. On the other hand, in HYD mechanism the consecutive
63
hydrogenation leads to the saturation of the phenolic compound forming cyclohexanol
64
and/or cyclohexanone. Again, depending on the level of hydrogenation, cyclohexene and/or
65
cyclohexane can also be formed as final products.
66
Various catalytic systems have already been reported in literature for phenol hydrotreatment.
67
Sulfided NiMo/γ-Al2O35-9 and sulfided ΝiW/γ-Al2O36 catalysts have been tested. Reduced
68
ΝiMo catalysts have been tested on supports γ-Al2O36,7, ΤiO27 and ΑC10. Monometallic Ni
69
catalysts supported on SiO211-15, on HZSM-516,18, on Al2O312,14,17,18,19, on mixed Al2O3 and
70
HZSM-516,18 have also been tested. 2 ACS Paragon Plus Environment
Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
71
The reported reaction temperature varried within the range 150-350 oC, the tested pressure
72
varried within the range 1-100 bar, and for the studies using fixed beds the WHSV varried
73
within the range 0.5-107.5 h-1.
74
For the hydrodeoxygenation of phenol over Ni containing catalysts, the most commonly
75
referred deoxygenated products in the studies are benzene, cyclohexene and cyclohexane.
76
The oxygenated products are cyclohexanol and cyclohexanone. In some studies they
77
coexist11,16, while in some others only cyclohexanol was detected17. Depending on the metal
78
and substrate active sites more products have been reported. Methylcyclopentane production
79
by isomerisation of cyclohexane is referred in literature6,7,20. Moreover, traces of C12
80
bicyclic products and C18 tri-cyclic products, oxygenated or non oxygenated, such as
81
cyclohexyl
82
dicyclohexyl ether13 and diphenylether9 can be derived from alkylation of oxygenated and/or
83
non-oxygenated products.
84
The high activity of NiMo/Al2O3 in hydrogenation processes, especially at low
85
temperatures, is reported by Gandarias et al.6 and Platanitis et al.7. Moreover, Platanitis et
86
al.7 have noticed that over a lab made NiMo/Al2O3 catalyst, the phenol HDO activity is
87
higher over the reduced form than the one exhibited using the sulfided form. In that study
88
the reduced catalyst was tested in a stand-alone process while the sulfided form was tested at
89
conditions of simultaneous HDO and HDS. Gandarias et al.6 have also reported that another
90
NiMo/Al2O3 catalyst was less active in the sulfided state in comparison to the reduced state.
91
Several researchers use pseudo-first-order kinetics to determine reaction constants as a
92
means of assessing catalyst activity for phenol conversion7,11,21,22,23. Shin and Keane11,
93
Viljava and Krause21 and Yang et al.22 report that the first-order consideration for phenol
94
conversion showed good fitting to the experimental data. Platanitis et al.7 carried out phenol
95
hydrotreatment experiments over NiMo on various substrates and assuming pseudo-first
96
order kinetics for phenol conversion, they calculated activation energies in the range 47-104
97
kJ/mol. Activation energies for the individual steps of the phenol hydrodeoxygenation
98
reaction scheme for other catalytic systems are in the range 30-150 kJ/mol16,21,24. Bjelic et
99
al.25-27 presented a microkinetic model for eugenol hydrodeoxygenation including various
100
catalytic systems and the activation energies of the individual steps lies within the range 30-
101
150 kJ/mol.
cyclohexane8,20,
biphenyl20,
cyclohexylbenzene20,
cyclohexylphenol8,
3 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 42
102
To the best of our knowledge, kinetic studies including the possible routes for product
103
formation over NiMo/γ-Al2O3 during phenol hydrotreatment do not exist. The special
104
interest in NiMo/Al2O3 catalysts lies in the fact that they are widely available and used in the
105
petroleum industry for hydrotreatment of light to heavy fractions, they are relatively cheap
106
and they appear very active and attractive for HDO applications. The aim of this
107
communication is to model the kinetics of phenol hydrodeoxygenation over a commercial
108
nickel-molybdenum catalyst supported on γ-alumina, in both the reduced and the sulfided
109
form. A reaction scheme that includes the detected products is proposed for each form of the
110
catalyst and kinetic equations for the individual steps of the reaction scheme are provided.
111
Moreover, the estimation of the diffusion effects in the commercial catalyst particles is
112
presented, which allows the calculation of global reaction rates that can be incorporated in
113
continuum models for reactor simulation, scale-up and follow-up studies. In this way, the
114
calculation of product composition and yields will provide full information on reactor
115
performance for any catalyst size and reactor scale for a wide range of operating conditions.
116
The power law kinetic model is used to relate reaction rates, reaction conditions and species
117
concentration. This type of kinetics allows accurate calculation of reaction rates and
118
effectiveness factors and is suitable for reactor simulation and scale-up studies as well as for
119
the study of the internal diffusion effects.
120
2. Experimental
121
The experiments were carried out in a mini scale unit. A reactor tube of 2.1 mm internal
122
diameter, coiled in spiral form, was loaded with the commercial NiMo/γ-Al2O3 catalyst. The
123
coils of the spiral reactor are horizontal with a small inclination 3-4 deg. When running
124
experiments to determine the intrinsic reaction kinetics, the reactor was loaded with 0.25 g
125
of crushed catalyst with dimensions in the range of 0.160-0.315 mm. The catalytic beds of
126
crushed particles were diluted with inert ceramic granules of the same size with the crushed
127
catalyst, with a volume ratio of catalyst to inerts 1 : 0.5. When running experiments to study
128
the internal mass transport phenomena, the reactor was loaded with 0.31 g of 4-lobe catalyst
129
particles, at their commercial dimensions, creating a spiral string bed28. The diameter of the
130
circumscribed circle around the particle cross section was 1.2±0.1 mm, the lobe diameter
131
0.49±0.03 mm and the mean particle length 2.7 mm. The string beds of the catalyst particles
132
in their commercial dimensions were diluted with cylindrical ceramic particles of 1.5 mm in
133
diameter by adding an inert particle in between of two catalyst particles. In all experiments
134
the liquid and gas were introduced cocurrently in upflow mode. 4 ACS Paragon Plus Environment
Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
135
The typical commercial catalyst used contained 2.6% wt. Ni and 12.3% wt Mo. The catalyst
136
specific surface area was equal to 185 m2/g calculated with the Brunauer-Emmett-Teller
137
method. The total pore volume was equal to 0.61 cm3/g and the average pore size was equal
138
to 12.3 nm.
139
The liquid feed of the phenol hydrotreatment experiments consisted of 1% wt phenol diluted
140
in n-dodecane. WHSV varied between 11 and 36 gfeed gcat-1 h-1. The experiments with the
141
reduced catalyst were performed at three pressures 20, 30 and 40 bar and four temperatures
142
within the range 130-170 oC. The experiments with the sulfided catalyst were performed
143
under constant pressure 30 bar and within the temperature range 200-250 oC. In all cases,
144
the H2 feed flow rates were high enough (gas to liquid flow rate ratio G/L>420 Nl/l) to
145
ensure negligible external mass transfer effects and hydrodynamics effects. This has been
146
verified by conducting experiments varying the G/L ratio from 150 Nl/l to 900 Nl/l, because
147
this ratio is the controlling parameter of the two phase flow characteristics29,30. It was proved
148
that for G/L>420 Nl/l conversion and selectivities were practically independent from G/L.
149
Hydrotreatment experiments of the products benzene and cyclohexanol were also performed
150
in order to investigate the reaction scheme for phenol hydrodeoxygenation over the
151
employed catalyst. For the investigation of the matrix effect on phenol HDO,
152
hydrotreatment experiments were also conducted using n-hexane as solvent.
153
Standard experiments were repeated at regular time intervals for the determination of
154
catalyst activity level. The analysis of liquid samples at steady state conditions was
155
conducted by gas chromatography by means of a Shimadzu GC2010 gas chromatographer
156
supplied with an FID detector and a chromatography column DB-5. The length of the
157
column was 30 m and its diameter 0.25 mm. Another chromatograph, DANI GC 1000,
158
supplied with an FID detector and a DB-624 column of 30 m in length and 0.32 mm in
159
diameter was used for the analysis of benzene and cyclohexane.
160
The non-sulfided catalyst was reduced before use with hydrogen at 10 bars and gas flow rate
161
of 6 Nl/h. The thermo-program of reduction included an initial temperature increase from 20
162
oC
163
latter period.
164
For the preparation of the sulfided catalyst form, the catalyst was initially dehumidified in
165
situ under hydrogen flow rate of 1 Nl/h for 2 h at 100 °C and 30 bar. Then, the temperature
166
was raised first from 100 to 300 °C at a rate of 20 °C/h and second from 300 to 340 °C at a
up to 350 oC with a rate of 45 oC/h, a period of 4 hours at 350 oC and cooling after the
5 ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 42
167
rate of 10 °C/h. Sulfidation was continued for 4 h at 340 °C. For all sulfidation phases, a
168
hydrogen flow rate of 1-1.25 Nl/h and a liquid flow rate corresponding to WHSV 16 h-1
169
were applied. The sulfur-containing liquid feed (1.5% wt. sulfur in n-dodecane) was spiked
170
with Sulfrzol. All chemicals were of high purity (≥99%). Dodecane was supplied from Alfa
171
Aesar, hexane and cyclohexanol were supplied from Merck, benzene was supplied from
172
Lach-Ner, phenol, cyclohexene and cyclohexane were supplied from Chemlab. Hydrogen
173
was of high purity (>99%).
174
3. Results
175
3.1 Reduced crushed NiMo/γ-Al2O3 catalyst
176
3.1.1 Activity -Selectivity
177
During phenol hydrodeoxygenation experiments over the reduced NiMo/γ-Al2O3 catalyst,
178
the main products detected within the studied conditions are cyclohexane and cyclohexanol.
179
Very small yields of cyclohexene (
