Improving the selectivity in hydrocracking of phenanthrene over mesoporous Al-SBA-15 based Fe-W catalysts by enhancing mesoporosity and acidity

Article abstract of DOI:10.1016/j.apcata.2015.10.051

Catalysts for heavy oil hydrocracking require an enhanced mesoporosity (higher pore diameters) and a moderate acidic function (mild acidity) to treat the bulky molecules present in this kind of feedstock and to yield middle distillates (MD). In this work, we have synthesized five different kinds of mesoporous silica based on SBA-15 by modifying some of the variables of their synthesis with the aim at enhancing mesoporosity. NH₄F and 1,3,5-trimethylbenzene (TMB) were used to modify the mesostructured arrangement in SBA-15. TMB modified SBA-15 materials exhibited the highest textural properties (2.16 cm³ g^-1 and 17 nm) in comparison to NH₄F modified silica (1.90 cm³ g^-1 and 10.8 nm) and pristine SBA-15 silica (1.03 cm³ g^-1 and 12 nm). The acidity of the SBA-15 based materials was modified by a post-synthesis "grafting procedure" to Si/Al molar ratios of 10, 25, and 40. SBA-15 based materials modified with both Al and TMB were used as supports for Fe-W sulfides. These catalysts were tested in the hydrocracking of phenanthrene. In general, all of the catalysts supported on Al-SBA-15 based materials were selective to the ring opening reaction of phenanthrene in contrast to the results obtained over a commercial Ni-Mo/γ catalyst. Such trend was associated to the presence of Br?nsted acid sites on the surface of the Al modified supports as shown 27 Al MAS NMR analysis.

Full text of DOI:10.1016/j.apcata.2015.10.051

Accepted Manuscript
Title: Improving the selectivity in hydrocracking of
phenanthrene over mesoporous Al-SBA-15 based Fe-W
catalysts by enhancing mesoporosity and acidity
Author: Jonatan R. Restrepo G. V ´ı ctor G.
Baldovino-Medrano Sonia A. Giraldo
PII:
S0926-860X(15)30228-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.051
APCATA 15627
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
25-6-2015
21-10-2015
31-10-2015
´
Please cite this article as: Jonatan R.Restrepo G., Victor G.Baldovino-Medrano,
Sonia A.Giraldo, Improving the selectivity in hydrocracking of phenanthrene over
mesoporous Al-SBA-15 based Fe-W catalysts by enhancing mesoporosity and acidity,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.10.051
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Improving
the
selectivity
in
hydrocracking
of
phenanthrene over mesoporous Al-SBA-15 based Fe-W
catalysts by enhancing mesoporosity and acidity
1
1
1
Jonatan R. Restrepo G. , Víctor G. Baldovino-Medrano , Sonia A. Giraldo *
1
Centro de Investigaciones en Catálisis (@CICATUIS), Escuela de Ingeniería
Química
Cra 27, Calle 9, Universidad Industrial de Santander (UIS) Bucaramanga,
Colombia
*
*
Corresponding author: Fax: +57 7 6344684
e-mail address: sgiraldo@uis.edu.co
1

GRAPHICAL ABSTRACT
2

HIGHLIGHTS

Al
modified
SBA-15
basedsupported
Fe-W
in
catalystsexhibitedremarkableselectivity
to
ring-openingproducts
2 3
thehydrocracking of phenanthrene, in comparison to Ni-Mo/ɤ-Al O
sulfidedcatalyst.



Enhancedmesoporosity
in
SBA-15
basedmesoporoussilicawaspossiblebyadding TMB (1,3,5-trimetilbenzene).
Theaddition of smallamounts of NH F duringthesynthesis of SBA-15
yieldedamorphoussilicawithwiderporesize.
Al post-synthesisincorporationmethodgeneratedacidity in SBA-15.
4
3

ABSTRACT
Catalysts for heavy oil hydrocracking require an enhanced mesoporosity (higher
pore diameters) and a moderate acidic function (mild acidity) to treat the bulky
molecules present in this kind of feedstock and to yield middle distillates (MD). In
this work, we have synthesized five different kinds of mesoporous silica based on
SBA-15 by modifying some of the variables of their synthesis with the aim at
4
enhancing mesoporosity. NH F and 1,3,5-trimethylbenzene (TMB) were used to
modify the mesostructured arrangement in SBA-15. TMB modified SBA-15
3
-1
materials exhibited the highest textural properties (2.16 cm g and 17 nm) in
3
-1
comparison to NH
4
F modified silica (1.90 cm g and 10.8 nm) and pristine SBA-
3
-1
1
5 silica (1.03 cm g and 12 nm). The acidity of the SBA-15 based materials was
modified by a post-synthesis “grafting procedure” to Si/Al molar ratios of 10, 25,
and 40. SBA-15 based materials modified with both Al and TMB were used as
supports for Fe-W sulfides. These catalysts were tested in the hydrocracking of
phenanthrene. In general, all of the catalysts supported on Al-SBA-15 based
materials were selective to the ring opening reaction of phenanthrene in contrast to
2 3
the results obtained over a commercial Ni-Mo/ɤ-Al O catalyst. Such trend was
associated to the presence of Brönsted acid sites on the surface of the Al modified
27
supports as shown Al MAS NMR analysis.
Keywords: Hydrocracking; molecular sieves; Fe-W catalysts; phenanthrene; Al-
SBA-15
4

1
. INTRODUCTION
The increase in the demand of Middle Distillates (MD) production by refining heavy
oils remains a challenging task[1-4]. The efficiency of conventional catalysts
processing heavy oils is poor due to diffusional limitations. Therefore, researchers
have focused on developing materials with larger pores to use them as supports of
the catalysts aiming the conversion of heavy oils into MD[5-7].
It is possible to synthesize and modify silica based materials such as MCM-41
(Mobil Composition of Matter Nº 41), SBA-15 (Santa Barbara Amorphous Nº 5),
and MCF (Mesocellular Foam), and use them as catalytic supports for upgrading
heavy oils[8-11]. In particular, SBA-15 is a type of mesoporous silica, which
possesses wide pore volume, narrow pore size distributions, and better
hydrothermal stability as compared to, for example MCM-41[12]. Therefore, SBA-
1
5 is an interesting candidate for supported catalysts for hydroconversion of heavy
oils [12-14].SBA-15 is formed by a hexagonal array of tubular and uniform
cylindrical channels, formed in strong acid media during the synthesis of the
material thanks to the micelles built over triblock copolymers[9]. Such copolymers
act as both surfactant and a template during the synthesis of the material
[
10,15,16]. Aiming pore size widening, Zhao et al., studied the use of micellar
swelling agents to modify the textural and structural properties of SBA-15,
obtaining silica materials with larger pore sizes by using 1,3,5-trimethylbenzene
(TMB)[14, 15]. It has also been reported that the use of organic co-solvents such
as hexane or n-butanol can act a swelling agents during the synthesis of SBA-15,
hence widening pore size[8, 9,16]. On the other hand, an improvement on the
thickness of SBA-15 was reported after the addition of small quantities of inorganic
salts during the synthesis of the material [16-18], this feature makes SBA-15 a
specific mesostructured material.
Another issue when trying to apply SBA-15 as catalytic support for heavy oil
hydroconversion is its modest acidity[19]. Such low acidity is explained by the fact
that the structure of SBA-15 consists mainly of neutral Si atoms.To improve the
5

acidity of SBA-15 based supports, different metals such as Al, Ti, Zr, and Sn have
been used as promoters for the formation of Brönsted acid sites [20-22]. In the
case of aluminum, aluminum loaded SBA-15 materials have been prepared by
varying the Si/Al molar ratio of the materials, aiming to enhance the acidityof the
materials [23-26].
In this contribution, we investigated how to enhance both the porosity and acidity of
SBA-15 type materials in order to prepare catalysts for the hydroprocessing of
4
heavy crude oils. On the other hand, the influence of adding NH F and TMB (1,3,5-
trimethylbenzene) during the synthesis of SBA-15 on their structure and porosity
was analyzed. Five different kinds of SBA-15 based mesoporous silica were thus
obtained.Conversely, the acidity of selected SBA-15 based materials was modified
by aluminum grafting. SBA-15 based catalytic supports of different Si/Al molar
ratios: 10, 25, and 40 were thus synthesized. The active phase of the SBA-15 type
catalysts was based on a combination of Fe-W. In general, iron based active
phases have received less attention from researchers, despite of being cheaper
than conventional Ni and Co based catalysts[7, 27-31]. The synthesized catalytic
materials were tested in hydroprocessing of phenanthrene, as a model reaction for
processing vacuum gas oil (VGO). Catalysts were characterized by different
techniques: N
2
physisorption to calculate textural properties, XRD to study the
changes in the structure of the materials, SEM to observe changes in morphology,
2
7
Al MAS NMR to identify the Al species on the surface of the selected materials,
and PAD to studythe nature of OH groups on the surface of the materials and
overall acidity. Therefore, the effect of both, textural and acid properties of the Al-
SBA-15 based materials in hydroprocessing reactions is given.
6

2
2
. EXPERIMENTAL
.1Mesoporous silica synthesis
Five kinds of SBA-15 based materials were prepared by adapting the procedures
employed in some literature reports [8,15,17].Pluronic P123 (EO20PO70EO20
av=5800, Aldrich) was used as a structural direction agent (SDA), and
,
M
tetraethylortosilicate (TEOS, 98% Aldrich) as a silica source. Synthesis were
conducted in strong acid media conditions (pH<1) in accordance to the following
nominal molar ratio: 1.0 TEOS: 0.0169 P123: 4.42HCl:186 H
and TMB (1,3,5-trimethylbenzene) addition to the structure of SBA-15 was studied.
NH F (99% Aldrich) and TMB (98%, Alfa Aesar) were always added to the
2 4
O. The effect of NH F
4
synthesis mixture before TEOS incorporation.
In a typical synthesis of SBA-15, 10 g of P123 were completely dissolved in an
aqueous solution of 1.3 M HCl (37 vol. %, Merck) and under stirring (500 rpm).
Then, 24 mL of TEOS was added dropwise and kept under stirring for 1 h.
Subsequently temperature was increased to 311 K and thus kept during 24 h. The
milky suspension thus obtained was then transferred into a 500 mL PTFE vessel,
and placed inside a muffle furnace at 403 K for 24 h for the hydrothermal
treatment. Afterwards, the vessel was allowed to cool down. The recovered solid
product was filtered and washed with deionized water, and left to dry at ambient
conditions. The recovered powder was calcined, with a heating rate of 2K/minin a
flow of dry air (100 mL/min) at 773 K during 6 h in order to remove the template.
NH
4
Fmodified silica materials were synthesized by adding 0.10 g and 0.15 g of
F to the synthesis mixture before TEOS incorporation, those samples were
NH
4
labeled: SBA-F(0.10) and SBA-F(0.15), the value in brackets indicates the amount
of NH F added.
4
TMB modified silica materials were obtained adding the required amount of TMB to
get TMB:P123 mass ratios of 0.15 and 0.75 before dropwise TEOS incorporation
and kept under stirring for 1 h. Those materials were labeled: SBA-TMB(0.15) and
7

SBA-TMB(0.75), the value in brackets indicates the TMB:P123 mass ratio. The
other steps remained the same as SBA-15 preparation.
2
.2 Aluminum Incorporation into the SBA-15 type silica
Surface acidity of the supports was obtained by an Al post-synthesis grafting
procedure [29]. A 0.02 M ethanol (Merck, 98%) solution of aluminum isopropoxide
(
C
9
H
21AlO3, Merck 98%) was used as aluminum source. Powdered samples (5 g)
of SBA-15 and SBA-TMB(0.75) were grafted by soaking them in the C
solution, thence keeping the resulting suspension under constant stirring (500 rpm)
for 18 h. Grafting was performed atpH between 9.0-9.5 using NH OH (28 vol. %,
9 3
H21AlO
4
Aldrich, 98%). Appropriate amounts of the aluminum precursor were employed to
obtain materials with Si/Al molar ratios of 10, 25, and 40. The grafted materials
were filtered and washed with ethanol to remove residuafrom the aluminum
precursor. Then, the recovered materials were calcined indry air (100 mL/min) at
7
73 K for 6 h. Al-modified materials were labeled following the nomenclature: Al-
SBA-15-x and Al-SBA-TMB(0.75)-x, where “x” indicates the Si/Al molar ratio.
2
.3 Catalysts preparation
Fe-W catalysts supported on ɤ-Al
TMB(0.75)-x were prepared by the successive incipient wetness impregnation
method. Fe(NO .9H O (98%, Aldrich) and (NH O (98%, Aldrich)
2
3
O (Protocatalyse), Al-SBA-15-x and Al-SBA-
3
)
3
2
4 6 2 12 40. 2
) H W O H
aqueous solutions (30 % in excess of the required amount to fill pore volume of
catalysts) were used as Fe and W precursors, respectively. After each
impregnation step, materials were dried at 393 K (heating rate of 2K/min) during 12
h, and then calcined in dry air flow (100 mL/min) at 773 K during 6 h. Fe
2 3
O and
WO loadings were 3 wt. % and 15 wt. %, respectively. The thus obtained materials
3
8

were labeled as Fe-W/Al-SBA-15-x and Fe-W/Al-SBA-TMB(0.75)-x. All catalysts
were sieved downto a particle size range between 25 µm and 75 µm.
2
2
.4Characterization of supports and catalysts
.4.1 Textural characteristics
2
-1
The BET specific surface area (SBET, m g ) of the materials was determined by N
2
adsorption-desorption isotherms measured at 77 K in a 3FLEX apparatus from
Micromeritics.
Samples of 0.2 g of the materials were degassed in vacuum (3 Pa) at 393 K for 12
h before the analysis.
S
BET was calculated from data takenin the range of relative pressures
P/P )between 0.04 to 0.24 according to IUPAC recommendation[32].Pore size
distributions were estimated from N -DFT models provided by Micromeritics
(
0
2
depending on the geometry of the pores. For SBA-15 based materials, a model
based on cylindrical pores oxide surface, and for SBA-TMB(0.75) catalysts a model
based on slit pores was employed.Total pore volumes were calculated from the
0
amount of nitrogen adsorbed at a relative pressure (P/P ) of 0.9.
2
.4.2 XRD patterns
XRD patterns of the materials were recorded in a Bruker Advance diffractometer
with Da Vinci geometry using Ni-filtered CuKα1 radiation (40 kV, 30 mA)
instrument. Samples (0.2 g) were grinded in an agate mortar down to a particle
size of 38 µm. Low angle spectra were recordedto identify changes in the material
ordering pattern. 2θ° range was scanned between 0.5° and 8° with a step size of
0
.01526°, and a counting time of 0.4 s per step. Qualitative analysis of the
observed peaks was carried out by comparing the observed profile with the
9

2
diffraction profile reported in the database PDF-4+ ICDD (SiO -00-058-0344). Wide
angle analyses were performed for all catalysts in order to confirm the
occurrenceof the respective oxidized species of the metals incorporated. In this
case, 2θ° range was scanned between 10° and 80°.
2
.4.3 Scanning-electron-microscopy (SEM)
SEM measurements were made in a FEI Quanta 650 FEG apparatus operatedat
an electron voltage of 4.00 kV. Magnifications of3 µm, and 500 nm were made, as
depending on the size of the particle to be observed.
2
7
2
.4.4 Al MAS NMR measurements
27
Solid state Al MAS NMR measurements were conducted on a Bruker ADVANCE
00 WB III instrument at B = 9.4 T. The resonance frequency corresponding to
Al was 104.3 MHz. Allgrafted supports labeled as Al-SBA-15-x and Al-SBA-
4
0
2
7
TMB(0.75)-x were analyzed. The MestReNova 9.0 software was used to compute
the content of the incorporated aluminum species by a Gaussian integration of their
respective peaks.
2
.4.5 Proton affinity distributions (PAD)
Proton affinity distributions were computed from potentiometric titration performed
in a TitroLine 7000 (SI Analytics) titrator. Samples were grinded and sieveddown
to a particle size below 75 µm [28]. Samples of 0.05 gof the materials were added
to a0.01N aqueous solution of NaNO
for 30 min before performing titration. A0.1M aqueous solution of NaOH (Merck
9%) was used for titrations in the basic pH range. A volume of 0.03 mL of NaOH
was added every 90 s until reaching a pH of 10. Titration in the acidic pH range for
3
(99.5% Merck). The suspension was stirred
9
10

a fresh sample was performed with a 0.1 M HCl (37 vol. %, Merck) until reaching a
pH of 3. The acquired data were used to construct a proton consumption function
(f(logk)) as a function of pH by proton balance [33-35].
Alumina acid sites distribution was proposed by Knözinger and Ratnasamy,
representing the OH groups configuration on its surface[35]. Hence, proton affinity
distributions were analyzed taking into account alumina acid sites contribution to Al
modified silica materials. The quantification of the identified OH groups was
performed by deconvolution of the peaks from the proton consumption curves. To
this end, theOriginPro 8.5.1 software was used, indicating the amount of acid sites
+
-1
[
mmol H g ] present in the samples. Trends in the quantification of the OH groups
derived from the PADs show mostly a qualitative value,being valid only for
comparing the series of catalysts prepared in this study.
2
.5 Catalytic Evaluation
Sulfided catalysts were tested in a 570 mL stainless steel batch reactor (Parr). The
reactor was loaded with 250 mL of the reaction feed composed of 4 wt. %
phenanthrene (Aldrich, 98%), 94 wt. % n-heptane (Merck, 98%); used as solvent
and 2 wt. % n-hexadecane (Aldrich, 98%) as an internal standard for
chromatographic analyses. Before the reaction, catalysts (1 g) were activated ex-
-1
2 2
situin a flow of 100 mL min of a mixture of H S/H (15/85 vol. %) at 673 K for 4 h.
The activated catalysts were addedto the liquid feed of the reactor, which was then
hermetically sealed. Afterwards, H was allowed into the reactor as to reach a
PH =2.2 MPaat room temperature. Then, temperature was increased (heating rate
2
2
of 2K/min) up to the reaction temperature 623 K, and under agitation (1000 rpm).
Once reaction temperature was reached, the registered reactor pressure was 11
MPa, and the first reaction sample was collected. This sample was considered as
corresponding to (zero) reaction time. Subsequently, samples were taken each 15
min for the first hour, each 30 min for the second hour, and each hour for the last
11

two hours. The volume of both reaction samples and its corresponding purges
were measured every time. Liquid reaction products of the reaction were analyzed
by GC-MS, using a GC-HP 6890instrument, equipped with an FID detector and an
HP-1 (100mx0.25mmx0.5 µm), and an HP-5 (30mx0.25mmx0.25µm) columns, MS
spectra were analyzed by a comparison with the W.8.1data basis, provided with
the Data Analysis Chemstation software from Agilent Technologies.
2 3
Commercial Catalyst Ni-Mo/ɤ-Al O Protocatalyse was also tested for comparison
purposes
2
2
.6 Expression of catalytic results
.6.1 Catalysts Activity
Reaction rates were calculated by a first order kinetic due to the high excess of
hydrogen[2,6,10,12,16,24]. Initial reaction rates were thus computed by initial
conversion data of phenanthrene corresponding to t= 0 min until t=60 min. The first
order equation (1) proposed by Gevert et al. was used[36]. This equationincludes a
correction factor due to the depletion in solution volume byrepetitive purges and
sampling during the experiments, as depicted in equation (2).
ꢃ
ꢄ
ꢏ
−
ꢀꢁ ꢂ ꢆ = ꢇ
ꢌꢍꢎ ꢑ ꢒꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓ(1)
ꢈꢉꢊꢋ
ꢐ
ꢃ
ꢅ
ꢋ
ꢏꢄ − ꢏ
ꢄꢕꢖ
ꢏ
ꢍꢎ ꢑ ꢒ = ꢔ
ꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓꢓ(2)ꢓ
ꢐ
ꢐꢄꢕꢖ
ꢄꢗꢖ
Where:ꢃ , ꢃ [g/mL] correspond to concentration of phenanthrene at each t time
ꢄ
ꢅ
when ꢘ samplesare taken and att=0 min of reaction, respectively, W [g] is the
weight of the loaded catalyst, ꢏ , ꢏꢄꢕꢖ[min] is the time after taking two consecutive
ꢄ
samples, andꢐꢄꢕꢖꢓ[mL] is the remaining liquid volume in the reactor after taking the
ꢘ − 1 sample.
12

2
.6.2 Selectivity
Selectivity towards 9,10-dihydrophenanthrene, 1,2,3,4-tetrahidrophenanthrene,
trans-stilben was computed from equation 3.
ꢚꢛꢀꢄ
ꢙꢄ =
(3)
ꢚꢛꢀꢈꢉꢊꢋꢕꢜꢊꢝꢞꢟꢄꢠꢋ
Whereꢓꢚꢛꢀ : are moles of I product (phenanthrene, trans-stilben, 9,10-
ꢄ
dihydrophenanthrene, 1,2,3,4-tetrahydrophenanthrene) andꢓꢚꢛꢀꢈꢉꢊꢋꢕꢜꢊꢝꢞꢟꢄꢠꢋ: are
moles of phenanthrene consumed duringthe reaction.
3
3
3
. RESULTS
.1Characterization of catalysts and supports
.1.1 Textural characteristics
Fig. 1, shows the N
Particularly, Fig. 1 (a) shows results for the synthesized silica materials, while Fig.
(b) displays N adsorption-desorption isotherms for the supported Fe-W oxide
2
adsorption-desorption isotherms for all synthesized materials.
1
2
precursors of the catalysts. In general, and according to the IUPAC classification,
all materials exhibited a type IV isotherm. This kind of isothermis typical of
mesoporous materials[32].
Fig. 1 (a) shows, a wider H1 type hysteresis loop for the samples SBA-15, SBA-
TMB(0.15) and SBA-TMB(0.75), For this kind of hysteresis, the adsorption and
desorption curves tend to be parallel between them at high relative pressures [37].
Accordingly, these materials possess tubular and uniform pores, as it is the case
for SBA-15[15].
The isotherm forSBA-F(0.15) sample displayed a narrower and not parallel H1 type
hysteresis loop at high relative pressures. Hence, it can be said that the addition of
4
NH F during the synthesis causes a loss in pore uniformity.This is confirmed by the
13

absence of the plateau at high relative pressures by increasing the amount of
NH F during the synthesis. For the case of SBA-F(0.10) silica, it is evidenced that
4
at high relative pressures, there was a partial deformation of the plateau as
compared to SBA-15 silica, suggesting the change in pore geometry at the same
4
rate NH F amounts increased.
Fig. 1 (b) shows narrower hysteresis loops for the supported Fe-W materials as
compared to the SBA-15 and SBA-TMB(0.75) catalytic supports, when metal
phases and aluminum species are added. Therefore, porosity of the oxide
precursor of the catalysts is different from the one of the catalytic supports.
Fig. 2 shows pore size distributions for all the synthesized mesoporous silica
materials. Gaussian and narrow pore sizes distributions were evidenced for SBA-
1
4
5 and silica materials modified with TMB rather than the modified with NH F,
indicating uniformity in pore sizes for TMB modified silica materials. In addition, all
materials exhibited the presence of pores in the range of microporosity.
2
-1
2 -1
Table 1 summarizes the calculated values of SBET (m g ),Smicro(m g ),NSBET, the
BET constant, pore diameter (P ), and pore volume (P ) for the synthesized
C
D
V
materials. CBET values were positive and lower than 300, which evidence for an
adequate fitting of the BET method for describingthe recorded isotherms.
Considering the SBA-15 material, all synthesized materials exhibited lower SBET
4
values. Among the prepared materials, those prepared with the addition of NH F
had the lowest SBET. Such trend allows to the collapse of mesostructured SBA-15
ordering pattern, which is notorious due to the change of the shape in the
4
hysteresis loop, when adding NH F.
Pore diameter and pore volume increased for SBA-TMB(0.15) and SBA-TMB(0.75)
as the same rate TMB:P123 mass ratio increased, Hence, the pore swellingeffect
of TMB was demonstrated. SBET area decreased as expenses at the increase of
pore diameter.For the samples, SBA-F(0.10) and SBA-F(0.15) materials, this
14

4
increase in pore diameter became remarkable with the highest amount of NH F.
However, by comparing SBA-F(0.15) with SBA-TMB(0.75), the effect of pore
swelling on SBET was less in TMB modified silica.Pore diameter and pore
volumewerethe highest for SBA-TMB(0.15). Consequently, this silicawas selected
for the Al incorporation procedure with SBA-15 for comparison purposes.
For the oxide precursor of the catalysts (Table 1), there was a decrease in SBET
with decreasing the Si/Al molar ratio of the materials. This decreasing is correlated
with narrower hysteresis loops in comparison to pristine SBA-15 and SBA-
TMB(0.75) supports. Pore diameter reported values remained slightly constant for
Fe-W/Al-SBA-15-x catalysts than Fe-W/Al-SBA-TMB(0.75)-x catalysts. Therefore,
as a result of metals impregnation and aluminum content, a decrease in porosity
for Al-modified supported catalysts is evidenced by the decrease in the specific
surface area at the same rate Al content increases.
In terms of the changes of pore volume in the catalysts, a decrease in the pore
volume with a decrease in the Si/Al molar ratio is evidenced as well. It may be
directly related to the highest amount of Al species for the Si/Al molar ratio of 10,
and it is more evident for Al-SBA-15-x supported catalysts instead of Al-SBA-
TMB(0.75)-x supported catalysts, because of the higher pore volume Al-SBA-
TMB(0.75) supports possess.
To confirm the presence of nanoparticles of Fe and W oxides inside the pores of
the SBA-15 based catalysts, normalized SBET values were calculated using the
following equation[8]:
ꢢꢣꢤꢥꢓꢦꢧꢏꢧꢀꢙꢨꢏꢨ
ꢡꢢꢣꢤꢥ
=
ꢭ
(4)
ꢢꢣꢤꢥꢓꢨꢩꢪꢪꢛꢫꢏꢨ ∗ (1 − ꢬ)
The normalized SBET is denoted as NSBET, and x is the weight fraction of the
metallic phases. If the metallic oxide component is distributed at the support
surface in the form of a close-packed monolayer of a given thickness, no pore-
15

blocking occurs. Therefore, the NSBET will decrease only as a result of the
narrowing of the mesopores. The introduction of Al species and Fe and W particles
caused in a reduction of pore volume, which resulted in the reduction of the
normalized surface area. NSBET values increased with increasing pore volumes.
The presence of microporosity is evidenced as well and reported in Table 1 by t-
plot (Harkins and Jura method) measured areas for all samples.
All of the synthesized SBA-15 based materials showed higher specific surface
2 3
areas and porosity as comparedto the commercial ɤ-Al O support.
3
.1.2 XRD patterns
Fig.3 (a) and Fig. 3 (b) depict low angle diffraction profiles for the synthesized
materials. For the SBA-15, Fig. 3 (a), three diffraction peaks, typical of this kind of
materials, at 2θº= 0.9, 1.5 and 1.9 corresponding to the (100), (110), and (200)
were identified. Such peaks are characteristic of the hexagonal symmetry of SBA-
1
5 with a space group p6mm[15].
4 :
For materials synthesized with NH F SBA-F(0.10) and SBA-F(0.15), XRD patterns
(Fig. 3a) show that only the peak attributed to the (100) plane is preserved. This
peak is comparatively less intense, wider, and is shifted to slightly lower 2θ values.
For TMB modified materials, a 0.3º leftwards displacement of the 3 peaks is
observed for SBA-TMB(0.75), which shows a structural change and rearrangement
of the pores for the modified silica supports.
According to Table 1. The d spacing corresponding to the plane (100) increases as
4
the amounts of NH F and TMB increase, thus indicating that the wall thickness of
the silica materials decreased as result of the increase in pore diameter.
Low angle diffraction patterns of Fe-W/Al-SBA-15 catalysts (Fig.3 b) exhibited no
changes as compared to the one obtained over the SBA-15 material (Fig.3 a); their
16

intensity and shape of the diffraction peaks were very similar. This evidences that
the addition of aluminum by grafting as well as the impregnation of the Fe-W
metaldid not destabilize the crystalline structure of the SBA-15 base material.
For catalysts prepared on Al-SBA-TMB(0.75)-x is possible to observe in Fig. 3 (b) a
decrease in intensity of the three diffraction peaks. It is more apparent for Fe-W/Al-
SBA-TMB(0.75)-x catalyst, which possesses a greater amount of aluminum
content. Fe-W/Al-SBA-TMB(0.75)-x catalyst showed almost the same diffraction
pattern than SBA-TMB(0.75) support (Fig. 3a). Based on the fact that the metal
contentis the same for all six catalysts, the change in the intensity is attributed to
the aluminum incorporation for the lower Si/Al molar ratio. The formation of Al
species at higher rates on the surface of the catalysts by decreasing the Si/Al
molar ratio diminished the intensity of the XRD signals. However, the ordering
pattern was also kept as the same Al-SBA-15-x catalysts, since the original peaks
position remained unaltered.
Fig. 4 shows the wide angle diffraction patterns for Fe-W/Al-SBA-15-x and Fe-
W/Al-SBA-TMB(0.75)-xoxide precursors of the catalysts. Fe-W/Al-SBA-15-x
2
catalysts showed only the characteristic hump of SiO material in the 2θº range
centered between 19º and 27º [8]. The high surface area of silica support favors
dispersion of the Fe-W phases, suggesting that either metals were homogeneously
dispersed on the support, or their compositions were below the detection limit of
the X-ray signals (< 4 nm). The absence of XRD signals in wide angle region
indicates that the particle size of metals is below the coherence length of X-ray
scattering.
Nevertheless, Fe-W/Al-SBA-TMB(0.75)-25 and Fe-W/Al-SBA-TMB(0.75)-40
catalysts exhibited other two peaks in 2θº of: 33.2 and 24.4, which are attributed to
poorly crystalline oxidized species of Fe
2 3 3 3
O and WO , respectively. The WO
crystallite size as calculated by Scherrer’s equation is reported in Table 1, showing
17

that at higher Si/Al molar ratios, bigger WO
3
particles are formed on the surface of
supports.
the Al-SBA-TMB(0.75)
18

3
.1.3 Scanning-electron-microscopy (SEM)
Results in Fig. 5 shows the fibrous hexagonal morphology of the SBA-15
material synthesized herein. Results are similar to those previously reported in
literature[15].
4
The effect on morphology by the addition of NH F in the synthesis of SBA-15 is
illustrated in Fig.6.The increase of NH F content led to the formation of semi-
4
spherical (peanut-shaped) particles. Spherical particles are typical of
amorphous silica, and here they are associated to the collapse of the SBA-15
structure caused by the pores swellingduring synthesis.The interactions fluoride
ionswith the P123 micelles changes the latter copolymer in the formation of
well-developed silica particles as it has been previously stated by several
authors [8, 9].
Fig.7, featuring SEM pictures for SBA-TMB(0.75), shows the formation of well-
defined spherical particles with an increase in the TMB: P123 mass ratio. This
result evidences that adding more quantity of TMB leads to swell pore diameter
in SBA-15 as it has been previously reported[11, 15].
Recently, the 2D to 3D transition from SBA-15 to MCF(Mesocellular Foam)
particles with the increase in the TMB in the mixture synthesis was given in
open literature[38]. In this case, spherical particles with different particle size
were achieved, indicating that there is an optimal mass ratio and a maximum
load of TMB to expand the pores [15, 16]. Spherical particles observed
resemble MCF particles [38-40]. Based on results obtained herein, it can be
stated that fibrous SBA-15 morphology was changed to spherical morphology
by the addition of TMB and NH F.
4
3
.1.4 Protonaffinitydistributions (PAD)
Fig. 8 shows PAD curves for the catalysts prepared with the Al-SBA-15-x andAl-
SBA-TMB(0.75)-x materials. Peaks associated to type III (pH< 3.5) and IIA
(3.5<pH< 5.5) alumina acid sites, silanol (pH=6.8), and a mixed group (silanol
and siloxane, pH>9) bonds for both kinds of catalystswere evidenced.
19

PAD analysis of the SBA-15 based silica materials modified with Al was
performed considering the contributions of the alumina and silica acid sites to
the overall acidity and acid strength. Type III alumina acid sites refers to
octahedral aluminum linked to hydroxyl group, meanwhile Type IIA alumina acid
sites refers to tetrahedral aluminum linked to the hydroxyl group on its
surface[35]. The latter OH groups represent the strongest acid sites for alumina
supports.
PAD deconvolution as reported in Table 2, show that the acidity of the materials
increases with the Si/Al molar ratio of the materials. Similar results have been
reported in literature for zeolites[41]. For the TMB supports there is an increase
in the concentration of silanol and of the mixed group. This group is similar to
the IA group in terms its acidity [42].
Al-SBA-15-40 and Al-SBA-TMB(0.75)-25 supports also presented a higher
concentration of type III and type IIA acid sites in comparison to other SBA-15
based supports grafted with Al. A leftwards displacement of 1.3 in pH of the
peak associated to silanol group (pH=6.8) for these supportswas observed.
Taking into account that Brönsted acid sites are formed by the introduction of
aluminum in the terminal silanol bonds of the surface of silica, such a
displacement evidencesgeneration of Brönsted acid sites with increasing the
Si/Al molar ratio.
Regardingacid sites distribution of the oxide precursors of the catalysts, an
increase in the concentration of weak acid sites (pH>9) related to the mixed
group was only evidenced when the active metals were incorporated (Fig. 8 b).
This result indicates that the incorporation of Fe-Wdecreases the acid strength
of the catalysts (Table 2).
27
3
.1.5 Al MAS NMR solid measurements
27
Figs. 9, shows the
Al MAS NMR spectraforAl-SBA-15-x and Al-SBA-
TMB(0.75)-x. These materials exhibit two clear signals at a chemical shift, ∂ = 0
ppm ascribed to AlO octahedral coordination group, which is characteristic of
the presence of extra structural aluminum species and other signal at ∂ = 52.9
ppm corresponding to AlO tetrahedral aluminum species[43].
6
4
20

The area of each Al peak was computed by a Gaussian integration of the peaks
section 2.4.4). In all cases, samples prepared with a Si/ Al molar ratio of
5exhibited a greater incorporation of Al within the lattice of SBA-15 supports,
(
2
as evidenced by the intensity of the signal corresponding to tetrahedral
aluminum coordination in Fig. 10.This result confirms the formation of Brönsted
acid sites forall catalysts as in agreement to proposed PAD results previously
commented (Fig. 8).
Octahedral aluminum species were found in higher proportion on theAl-SBA-
TMB(0.75)-x supports with a Si/Al molar ratio of 40 support (Fig. 10b). Such a
change might be associated to the changes in the morphology, which went from
fibrous to spherical silica afterthe addition of TMB. In this sense, the surface of
the SBA-15 based materials seems to be influencing the kind of Al species
introduced to the framework of the material.
The post synthesis method of incorporation or "chemical grafting" proposed to
incorporate on the silica framework aluminum species was satisfactory to
generate the acid strength distribution to the material on the surface of the
catalysts supported on these modified silica.
These results are consistent with the PAD curves, which showed that the
materials with higher content of type III and IIA acid sites were Al-SBA-15-25
and Al-SBA-TMB(0.75)-25. It is known that the tetrahedral Al coordination is a
precursor of Brönsted acid sites and octahedral for Lewis acid sites [7, 27-30,
4
4,45].
21

3
3
.2 Catalytic Evaluation
.2.1 Catalysts Activit
Table 3, represents the change in the activity of the catalysts evaluated.
The commercial Ni-Mo/ɤ-Al
2
O
3
catalyst exhibited the highest initial rate
-
1
-1
constant (kPhen= 2.16 mL g min ). Among the SBA-15 based catalysts
prepared herein, the catalyst Fe-W/Al-SBA-TMB(0.75)-10was more
active in comparison to the other Al-modified TMB catalysts and Al-SBA-
1
5 supported catalysts.
3
.2.2 Catalysts selectivity
Fig. 11 depicts the iso-conversion selectivity towards condensable products.
Selectivity is analyzed at a conversion of 7.3 %. In particular the selectivity to
9
,10-dihydrophenanthrene, 1,2,3,4-tetrahydrophenanthrene and trans-stilben is
analyzed. The commercial catalyst Ni-Mo/ɤ-Al as well as for Fe-W/ɤ-
Al were selective tohydrogenated (Fig. 13). Fe-W/Al-SBA-15-x and Fe-W/Al-
2
O
3
2
O
3
SBA-TMB(0.75)-x sulfided catalysts were highlyselective to trans-stilben, which
is a product of aromatic ring opening reactions. In general all Fe-Wsulfided
catalysts showed no significant differences in selectivity.
4
4
. DISCUSSION
.1Effect of the textural properties in the HCK of phenanthrene
Fig. 12(a), shows the catalytic activity of all SBA-15 based catalysts,
demonstrating the effect of specific surface area in the catalytic activity of the
sulfided catalysts. The catalysts,which showed the highest activity were: Fe-
W/Al-SBA-15-40 and Fe-W/Al-SBA-TMB(0.75)-10sulfided catalysts.
For both series of sulfided catalysts, opposing trends in catalytic activity by SBET
were evidenced. Sulfided catalysts supported on Al-SBA-15-x exhibited that
catalytic activity increased at the same rate as SBET. Nevertheless, as indicated
in Fig. 12 (b), sulfided catalysts supported on Al-SBA-TMB(0.75)-x did not
22

correlate with the increase in SBET. Thus, catalysts activity was not directly
influenced by the porosity of this kind of TMB modified supports.
To provide a better explanation to this behavior wide angle XRD patterns (Fig.
4
) made it possible to establish that catalysts with higher specific surface area,
but higher pore volumes have enabled the formation of larger particles of the
metallic phase (Table 1). Therefore, the catalytic activity with TMB modified
catalysts was diminished despite of their larger pore sizes.
On the other hand, SBET offers the required platform for homogeneous
dispersion of metal nanoparticles. However, the effective surface area required
for metal dispersion suffered at the expense of increasing pore diameter. The
decrease in the mesostructured ordering pattern of SBA-15 by the addition of
TMB to expand its pores in Al-SBA-TMB(0.75)-x supported catalysts as shown
in Figs. 2b and 3 indicates less uniformity on pore size distribution and pore
structure, and thus decreasing SBET while widening pore size
4
.2Effect of acid properties in the HCK of phenanthrene
The mechanism of hydrocracking reactions is bi-functional[46], which indicates
that the occurrence of mild acid sites on the catalysts’ surface influences the
rate of reaction as the same selectivity does. For that reason, the effect of Si/Al
molar ratio on the performance of catalysts is elucidated in Fig. 13. For sulfided
catalysts supported onAl-SBA-15-x, the highest Si/Al molar ratio leaded to the
highest activity.According with PAD analysis, overall acidity is the highest for
the highest Si/Al molar ratio (25, and 40).
For sulfided catalysts supported on Al-SBA-TMB(0.75)-x, PAD analysis given
(Table 2) specified that when Fe-W oxidized species were introduced on the
supports, overall acidity was changed. Fe-W/Al-SBA-TMB(0.75)-10 revealed the
highest content of acid sites. As a result, the lowest Si/Al molar ratio with TMB
modified sulfided catalysts yielded in the highest catalytic activity results.
On the other hand, the influence of acid strength is showed in Fig. 14. The
formation of ring opening compounds such as trans-stilbencan be attributed to
the effect of Al incorporation into the lattice of SBA-15 based supports, as a
23

result of the slight enhancement and fitting in selectivity for the catalysts
prepared with the Si/Al molar ratio of 25, as compared to octahedral aluminum,
which revealed no correlationin sulfided catalysts selectivity as shown in Fig.
1
5. Therefore, it is established the effect of tetrahedral Al incorporation in the
selectivity of phenanthrene hydrocracking reaction. Tetrahedral aluminum
species are precursors of Brönsted acid sites, which in terms of acidity are
stronger than Lewis acid sites related to octahedral aluminum species.
A similar behavior was also evidenced by Dai et al., who tested Ni-W/Al-SBA-15
catalysts in the hydrocracking of a VGO, demonstrating that Al-SBA-15
catalysts induced high selectivity to MD and rupture compounds as a result of a
higher tetrahedral aluminum incorporation [27].
27
The previousresults were directly correlated with the computed values of Al
NMR spectra, which evidenced that at the highest tetrahedral Al ions were
incorporated to catalyststo Si/Al molar ratio of 25 (Fig. 12). PAD deconvolution
data as reported in Table 2, also confirmed that the catalysts with higher
content of type III acid sites are the catalysts the highest Si/Al molar ratio. In
that sense, tetrahedral aluminum incorporation generates the strong acid sites
contribution to the overall acidity of the catalysts and supports. Brönsted acidity
is formed by this kind of Al incorporation, as it has been reported by several
authors, it influences isomerization and cracking reactions [20-23, 43, 47,48].
4
.3 Effect of the Fe-W active phases in the HCK of phenanthrene
The effect of Fe-W active phase is evidenced by the changes in catalytic activity
when incorporated on Al-modified SBA-15 based catalysts. As shown in Table
-
1
-1
3
, catalytic activity withAl-SBA-TMB(0.75)-10 support is 0.12 mLg min in
-
1
-1
comparison to 0.84 mL g min when Fe-W active phases are impregnated for
the same support. Therefore, as evidenced by several authors [23-26], ametallic
phaseis required to direct the reaction to the desired products, and also to
increase the activity and product yield.
In this case, the catalysts based on a Fe-W active phase exhibited a different
product distribution as compared to conventional sulfide Ni-Mo/ɤ-Al
2
O
3
catalyst.Fe-W catalysts supported on Al incorporated silica were selective to
ring opening compounds in the hydrocracking of phenanthrene (trans-stilben). It
is noteworthy that Fe-W/ɤ-Al
2
O did not show the same product distribution,
3
confirming that Al-modified silica supports enhance the production of such
compounds, which is associatedto the acid sites distribution therein.
24

5
. CONCLUSIONS
The synthesis parameters in SBA-15 mesostructured silica preparation were
modified. This modification led to get larger pore diameter mesoporous silica by
adding either a micellar expansion agent such as TMBor an inorganic salt such
NH
change morphology from a hexagonal array of tubular channels into spherical
nanoparticles, meanwhile adding small amounts of NH F yielded inan
amorphous silica materialwith higher pore diameters, but there was a significant
4
F. The swelling effect of TMB expanded micelles to form the pores to
4
4
loss in the specific surface area. Therefore, NH F additions disfavored ordering
at the initial synthesis temperature.
A series of Al-SBA-15 mesoporous materials were synthesized by a post-
synthesis aluminum grafting procedure. The incorporation of Al species led to
an enhancement of the concentration of Brönsted and Lewis acid sites, which
was correlated to the formation of tetrahedral and octahedral aluminum surface
species. Therefore, generating stronger acid sites on the SBA-15 based
materials.
Fe-W/Al-SBA based catalysts displayed high selectivity towards the ring
opening of phenanthrene due to their Brönsted acid sites. It must be remarked
that though the catalytic activity of the Fe-W/Al-SBA based catalysts was lower
than that of conventional Ni-Mo/Al
2
O , the latter was only selective to partial
3
hydrogenation products. Therefore, Fe-W/Al-SBA based catalysts are promising
candidates for hydrocracking reactions.
6
. Acknowledgements
This work was possible due to the financial support of the VIE-UIS in the frame
of the project “Diseño de catalizadores para hidrocraqueo de
fraccionestipogasóleo y estudio del efecto de moléculasnitrogenadas”, code
1
2
329. Jonatan R. Restrepo thanks to COLCIENCIAS for its “JovenInvestigador
012” fellowship.
25

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List of Figures
Fig. 1. N adsorption-desorption isotherms, (a) silica materials, (b) catalysts
2
29