M.A. Golubeva and A.L. Maximov
Applied Catalysis A, General xxx (xxxx) xxx
measured to identify the surface chemical composition and valence
states. The XPS survey spectrum of the NiP-H3PO2 (Fig. S17a) shows the
presence of Ni, P, O, C elements. The XPS survey spectrum of the NiP-
TOP (Fig. S18a) shows the presence of Ni, P, O, C and Fe elements.
The impurity of Fe could be explained by leaching of reactor material
under the reaction conditions. The XPS spectra shows Ni and P reduced
and oxidized species related to nickel phosphide and nickel phosphate
respectively. It is suggested that small positive charge of Ni and small
negative charge of P occur due to electron density transfer from Ni to P
[35]. The magnitude of Ni changes as follows: Ni2P > Ni12P5>Ni [36,
37]. However, it cannot be reliably stated, which phase of phosphide
was formed, with slight differences in the binding energy values. The
values of 853.5 eV and 853.7 eV of binding energy are corresponded to
Niδ+ chemical state in nickel phosphide in the NiP-H3PO2 (Fig. S17b)
and NiP-TOP (Fig. S18b) respectively [30]. Binding energies of 857.1
and 856.8 eV refer to nickel in the oxidized state which can be associated
with nickel phosphate in the NiP-H3PO2 and the NiP-TOP respectively
[30,36,37]. Binding energies of 862.2 and 862.1 eV refer to shake-up
satellite of Ni2+ in Ni2p/3 region in the NiP-H3PO2 and the NiP-TOP
respectively [30,36,37]. The peaks observed at 129.7 eV and 129.9 eV
are attributed to Pδꢀ in the NiP-H3PO2 and the NiP-TOP respectively.
Binding energies of 129.7 eV and 133.7 eV in NiP-H3PO2 are corre-
sponded to Pδ- in phosphide and P5+ species related to phosphate
(Fig. S17c). Binding energies of 129.9 eV and 133.8 eV in the NiP-TOP
are also related to reduced Pδꢀ and oxidized P5+ species (Fig. S18c) [35].
hydrogenation and into furan by decarbonylation [18,43]. For the
NiP-TOP two routes of furfural conversion are observed. The results of
catalytic tests of the NiP-TOP are presented in Fig. 4. Proposed reaction
pathways are presented in Fig. 5. Furan was obtained from furfural at all
reaction temperatures over metal sites of catalyst. With increasing
temperature, the furan selectivity decreased. The highest selectivity for
furan was 44 % at 250 ◦C after 3 h of reaction. Furfuryl alcohol was an
intermediate product and was converted into 2-methylfuran over metal
sites by hydrogenolysis [44]. 2-methylfuran was a main product at all
temperatures and reaction time. Thus, the conversion to furfuryl alcohol
prevailed and with increasing temperature the conversion to furan was
inhibited more. Full conversion of furfural reached after 3 h at 250 ◦C,
after 1 h at 300 ◦C and after 0.5 h at 350 ◦C. With increasing temperature
2-pentanone was detected among the reaction products. It was gener-
ated from 2-methylfuran by ring-opening hydrogenation reaction over
metal sites [45]. The highest selectivity of 2-methylfuran were 77 % at
350 ◦C after 3 h of the reaction. Jimenez-Gomez et al. [18] reported
about the obtaining of 2-methylfuran and furan among the reaction
products with 75.4 % and 24.5 % of selectivity respectively over
Ni2P/SiO2 at 190 ◦C at full conversion. However, with increasing tem-
perature to 210 ◦C 2-methylfuran selectivity decreased to 15.8 %, furan
selectivity increased to 44.7 % and tetrahydrofuran was obtained with
38.4 % of selectivity.
The results of catalytic tests of the NiP-H3PO2 are presented in Fig. 6.
Product selectivity in ethanol as a solvent was lower than in toluene. It
can be explained by the presence of numerous competing reactions.
Proposed reaction pathways are presented in Fig. 7. Furfuryl alcohol was
the one product formed from furfural in the first stage. Thus, furfural
decarbonylation to furan in ethanol was totally inhibited. Products with
the highest selectivity of 40 % and 38 % in ethanol were ethyl levulinate
and 2-methylfuran respectively. Full conversion of furfural reached after
1 h at 250 ◦C and 300 ◦C and after 0.5 h at 350 ◦C. Ethyl levulinate was
obtained from furfuryl alcohol by Brønsted acid-catalyzed ring-opening
hydrogenation and following esterification [46,47]. It was mentioned
about acid-catalyzed conversion of furfural into ethyl levulinate [48,
49]. Apparently it also happens through furfuryl alcohol formation
catalyzed by Lewis acid sites [47].With increasing temperature
γ-valerolactone and ethyl valerate were detected in the reaction me-
dium. Ethyl valerate selectivity was higher than γ-valerolactone selec-
tivity. There are two possible pathways of ethyl levulinate conversion
into γ-valerolactone. The first is the formation of angelica lactone as an
intermediate product by the deethoxylation over Lewis acid sites and
following hydrogenation over metal sites. The second is the formation of
ethyl 2-hydroxyvalerate over metal or Lewis acid sites and following
deethoxylation over Lewis acid sites [50,51]. Hydrogenolysis of ethyl
2-hydroxyvalerate into ethyl valerate is proceeded over metal sites [51].
The conversion of γ-valerolactone into ethyl valerate which is catalyzed
by Brønsted acid sites is proceeded through the formation of ethyl
pentenoate [52]. The highest value for ethyl valerate was 26 % and was
obtained after 6 h of the reaction at 350 ◦C. Tetrahydro-2-methylfuran
could be obtained from 2-methylfuran by hydrogenation, from tetra-
hydrofurfuryl alcohol by hydrogenolysis and from γ-valerolactone by
hydrodeoxygenation [53]. However, tetrahydrofurfuryl alcohol was not
detected among the reaction products. The pattern of γ-valerolactone
transformation into tetrahydro-2-methylfuran is not traced in present
work. Thus, tetrahydro-2-methylfuran was obtained from 2-methylfuran
by hydrogenation over metal sites [43]. With increasing reaction time
selectivity of tetrahydro-2-methylfuran increased and the highest value
of 33 % was obtained at 300 ◦C after 6 h of the reaction. Moreover,
2-(diethoxymethyl)furan and 5,5-diethoxy-2-pentanone were detected
in the reaction medium. They are acetals of ethanol with furfural and
4-oxopentanal respectively [54]. 4-oxopentanal could be obtained by
hydrogenation of carboxylic group in levulinic acid, but it was not
detected among the products. With increasing temperature the yields of
acetals increased first, and then decreased. These acetals were formed
over Brønsted or Lewis acid sites of catalyst [54–56]. In addition,
3.1.4. NH3 temperature-programmed desorption
The NH3-TPD desorption curves of the NiP-H3PO2 and the NiP-TOP
samples obtained at 350 ◦C after 6 h are presented in Fig. S19. For
both of the catalysts only the weak acid sites were detected. It was
–
assigned to the P OH groups [38]. The desorption peak for the
NiP-H3PO2 and NiP-TOP catalysts was observed at around 168 ◦C and
138 ◦C respectively. The amount of the desorbed ammonia was 45 and
6 μmol/g respectively. Thus, the NiP-H3PO2 acidity was higher than the
NiP-TOP acidity. It can be due to the fact that acidic component was
involved to the synthesis of the NiP-H3PO2 and only neutral components
were involved to the synthesis of the NiP-TOP. Nickel phosphate is
formed as a by-product of the hypophosphorous acid decomposition
[39] and contributes to the higher acidity of the NiP-H3PO2 sample
compared to the NiP-TOP.
3.2. Hydrogenation of furfural
Mentioned catalytic systems were tested in the hydrodeoxygenation
of furfural. Earlier similar systems were successfully tested in the
hydrodeoxygenation of guaiacol [14], palmitic and stearic acids [15]. In
present work, the first catalytic system consists of Ni (II) 2-ethylhexa-
noate and TOP, furfural as a substrate and toluene as a solvent (the
NiP-TOP series of catalysts were obtained), the second catalytic system
consists of Ni(OAc)2, H3PO2, furfural and ethanol (the NiP-H3PO2 series
of catalysts were obtained). Both of systems were active in the hydro-
processing of furfural. Type of reaction products and their distribution
firstly depended on used catalytic system and solvent, secondly on re-
action temperature and time. The transformations occurring with
furfural and its derivatives using the catalytic systems proceed with the
participation of metal and acid sites. Ni in Ni2P is responsible for
–
transformations over metal sites. P OH groups on the catalyst surface
and in H3PO2 are Brønsted acid sites [32,40]. Ni2+ and Niδ+ are Lewis
acid sites [35,40,41].
Mavrikakis and Barteau were reported about two types of in-
termediates, formed on the metal surface as a result of aldehyde
adsorption.
η
1(O) aldehyde intermediate configuration is bonded to the
surface via oxygen atom,
η
2(C,O) aldehyde intermediate configuration is
bonded via carbon and oxygen atoms [42].
η
2(C,O) intermediate is more
stable and it is typical for the adsorption on the Ni-containing surface.
This intermediate can be converted into furfuryl alcohol by
5