(431d) Anthracene Aquacracking Using NiMo/SiO2 Catalysts in Supercritical Water Conditions

Anthracene aquacracking using NiMo/SiO₂
catalysts in supercritical water conditions

T.R. Reina ¹, P.Yeletsky ²,
J.M. Bermúdez ¹, P. Arcelus-Arrillaga ¹, V.A. Yakovlev ^2*,
M.Millan ^1*

¹Department
of Chemical Engineering, Imperial College London London SW7 2AZ, UK

² Boreskov Institute of Catalysis, Lavrentieva Ave. 5, Novosibirsk,
Russian Federation

 

*Corresponding authors: marcos.millan@imperial.ac.uk, yakovlev@catalysis.ru

Introduction

The
current status of world oil reserves is a contentious matter, but it is widely
accepted that conventional resources are dwindling and their reserves are less
easily accessible [1]. Therefore, the production of heavy crude oil (HCO),
which is the remnant of conventional oil has become more relevant and will
remain so in the foreseeable future [2]. In this sense, there is a need for
more efficient refining processes to transform HCO into lighter fuels. Conventional
processes for increasing the value of heavy oil fractions aim to increase the
H/C ratio of fuel, generating lighter fractions. However, this implies either
rejecting a large amount of the carbon in the feed as in thermal and catalytic
cracking processes, or using high pressure hydrogen, an expensive gas, in
hydrocracking processes [3].

As an
alternative, a fairly new approach to upgrade heavy oils consists in the
catalytic cracking in supercritical water (SCW) conditions using metal based
catalysts. This method takes advantage of the properties of SCW (T=374˚C,
P=220 bar and d=0.32 g/ml and above), in which water inverses its
properties as a solvent and became a non-polar solvent [4]. Under these
conditions, aromatic rings are well dissolved and dispersed in the reaction
media favoring the contact between reactants and catalysts particles.

On the
other hand, the chemistry of heavy oils is rather complex making difficult to
obtain deep comprehension of the process. Studies with model compounds
representing chemical structures found in heavy oils give relevant information
about the reactivity in the medium. In addition, the use of model compounds
facilitates to understand the role of the catalysts and to identify the most
likely reaction pathways.

In this
scenario, the aim of this work is to apply a series of NiMo/SiO₂
catalysts with different Ni/Mo ratio in the anthracene upgrading in SWC
conditions. A complete physico-chemical characterization of the fresh and spend
catalysts in order to correlate the catalysts' features with catalytic behavior
is also a subject of this study.

Experimental

The catalysts were prepared by sol-gel
method and partially reduced in H₂ flow at 550 ^oC for 1 h
followed by passivation with ethanol. Anthracene aquacraking experiments were
performed in a stainless steel batch reactor with a volume of 18 ml. The
operation of the reactor has been described elsewhere [3]. The reactor was
filled with 0.2 g of anthracene 98% (Sigma-Aldrich), 0.2 g of catalysts and the
amount of water required providing 230 bar pressure at 425 ºC. 

Gas products formed were analyzed in a
Perkin Elmer Clarus GC with TCD detector. Liquid products were separated from
the remaining water, extracted with a mixture of chloroform/methanol 4:1 and
then filtered. This liquid fraction was liquids in a Perkin Elmer
Clarus GC with FID detector and in a Varian Star 3400/Saturn 2000 GC/MS to
identify the main products obtained. The spent catalysts were recovered, dried
and prepared for post-reaction studies.

All the fresh samples have been fully
characterized by means of XRD, TEM, XPS, S_BET, H₂-TPR.
Some of these techniques were applied also to study the spent catalysts.

 

Results and Discussion

The nominal
compositions of the catalysts (if they were completely reduced to metallic Ni
and Mo) together with their textural properties and the anthracene conversions
are presented in Table 1.

Table 1.  Chemical composition (wt.%), textural properties of the
prepared catalysts (if they were completely reduced to metallic Ni and Mo) and
anthracene conversions

 

Sample	Ni (wt.%)	Mo (wt.%)	SiO2 (wt.%)	SBET (m2·g-1)	VPore (cm3·g-1)	Dpore (nm)	Anth. Conversion (%)
404020	40	40	20	87	0.17	8.1	46.7
305020	30	50	20	74	0.24	13.0	28.2
206020	20	60	20	68	0.25	14.9	35.0
107020	10	70	20	44	0.12	10.7	17.3

All the samples
are mesoporous materials with specific surface area and porosity governed by
the Ni/Mo ratio. In particular, the S_BET decreases when molybdenum
loading increases. Concerning the catalytic performance, all the solids are
active in the anthracene upgrading and their activity can be linked to the
catalysts composition. More precisely, the presence of Ni seems to be crucial
to achieve high conversions since the most active catalysts is the one with the
highest Ni content and the least active sample the one with the lowest Ni
loading.

Assuming that
the upgrading process in supercritical water media may begin with a partial
oxidation of the rings leading to C-O bonds that are easier to crack than C-C
double bonds, the redox properties of the catalysts must be a relevant factor.
Indeed, some interesting results can be extracted from the H₂-TPR (Figure
1)

                                                                                                                                                              

 Figure 1. H₂-TPR profiles of the NiMo/SiO₂
catalysts

As seen in Figure
1 all the catalysts present a complex profile accounting for the
simultaneous reduction of NiO to Ni and several reduction steps of MoO₃.

Moreover this
reduction profile is influenced by the Ni and specially Mo particles size. XRD
data (not shown here) revealed that 404020 and 305020 catalysts are composed by
small Ni-Mo particles forming a solid solution while for the 206020 and 107020
some big molybdenum dioxide particles (ca. 30 nm) were observed. The
overall result is that only 404020 and 305020 samples achieved complete
reduction in the TPR experiment indicating that these solids present better
redox properties than the others. Furthermore, it seems very clear that Ni
assists Mo reduction shifting the reduction zones to lowers temperatures. The later
also may indicate an intimate Ni-Mo contact resulting in a positive effect in
terms of reducibility.

Very
interestingly Ni/Mo ratio also influences the liquid and gas products
distribution. Figure 2 shows liquids products distribution. Flourenone
and Xanthene are the dominant products for high Ni loadings in the catalysts
composition while some different products as for example Anthraquinone and Xantone
appear for the l07020 sample. In any case a diversity of liquids products
including oxidation products, ring opening molecules and hydrogenation products
were obtained.  

 

Figure 2. Liquids products distribution. Labels: 1 (404020); 2
(305020); 3 (206020); 4 (107020)

Regarding
the gaseous products, very interesting results were obtained (Figure 3). Irrespectively
of the catalysts composition, H₂ was the most abundant gas produced
in the reaction. Together with H₂ some CO, CO₂ and CH₄
were produced. Again, as observed for the liquid fraction, the gas product
distribution is controlled by the catalysts composition. In fact, some parallel
reactions taking place simultaneously may account for the obtained gases. For
instance, the water gas shift reaction and the CO and CO₂
methanation are plausible since Ni is a typical active phase for both
processes. Furthermore, partial oxidation of anthracene and cracking reactions
may happen according to the obtained liquids products.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                
                                   Figure 3 Gas products distribution. Labels:
1 (404020); 2 (305020); 3 (206020); 4 (107020)

 

 

Finally, the post reaction characterization
(not included in the abstract for sake of briefness) indicates a certain degree
of damage in the catalystsx structure. This observation opens up a new
challenge in terms of developing robust catalysts to be employed in upgrading
reactions in SWC conditions.

 

 

Conclusions

 

A series of NiMo/SiO₂ catalysts
has been successfully applied in the anthracene aquacraking in SWC conditions.
The samples present different structural and redox properties depending on the
Ni/Mo ratio. The best performances are obtained when small Ni-Mo particles are
in close contact and no segregation of MoO₂ is attained.

A variety of liquids products was obtained
including ring opening, oxygenated and hydrogenated molecules confirming the
successful upgrading of anthracene. Additionally, a valuable gas stream (rich
in H₂ with some CH₄) resulted from the process making it
more interesting in terms of energy efficiency. Both liquids and gaseous
products distribution are controlled by the Ni/Mo ratio that seems to be the
key parameter in the catalystsx design in order to maximize the overall
performance.

 

Acknowledgments

This
research was performed under the UNIHEAT project. The authors wish to
acknowledge the Skolkovo Foundation and BP for financial support.

References

(1)    
 Owen, N. A., Inderwildi, O. R. & King, D.
A. The status of conventional world oil reserves?Hype or cause for concern?
Energy Policy. (2010) 38 (8), 4743-4749.

(2)    
Hart Energy Research Group. Heavy Crude Oil:
A Global Analysis and Outlook to 2035. (2011) Hart Energy.

(3)    
J.L. Pinilla, P. Arcelus-Arrillaga, H. Puron,
M. Millan, Appl. Catal. A 459 (2013) 17-25

(4)    
H. Weingärtner, E.U. Franck, Angew. Chemie Int.
Ed. 44 (2005), 2672?2692.