(264f) Deoxygenation of a Biofuel Model Compound (2-Methyltetrahydrofuran) On Transition Metal Phosphides

Second
generation bio-fuels are attracting interest as they are produced from non-food
sources such as waste products from agriculture, forestry, or paper manufacture.
These biomass sources have several advantages over conventional fossil feedstocks
for their abundant availability, extremely low sulfur and nitrogen content,
localized production, and reduction in greenhouse gas emissions (due to the CO₂
consumption of biomass). The average composition of raw oil from pyrolysis of
biomass has a high oxygen content (35-50 wt%) that results in low heating value, immiscibility with
hydrocarbon fuels, high acidity, and chemical and thermal instability. The
oxygen compounds can be removed through hydrodeoxygenation (HDO) reactions
which entail cleavage of C-O bonds in the presence of hydrogen to form
hydrocarbons and environmentally benign water. Reactions in HDO are similar to those
in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes because
they all involve the breaking of carbon-heteroatom bonds.  The similarity of HDO with HDS and HDN potentially
allows integration of biomass conversion with the existing petroleum refining
infrastructure for processing and transportation. Among new catalyst
compositions that have been explored in hydrotreating processes, phosphides of
transition metals stand out as potential substitutes for conventional sulfides.
For example, nickel phosphide, cobalt phosphide, tungsten phosphide and
molybdenum phosphide are highly active for HDS and HDN of petroleum feedstocks.
This study has two objectives, first, to compare phosphides (Ni₂P/SiO₂
and WP/SiO₂) prepared by the reduction of phosphate and
phosphite precursors and second, to evaluate their activity in the
hydrodeoxygenation of the bio-oil model compound, 2-methyltetrahydrofuran
(2-MTHF). The latter is a five-atom saturated ring compound that has a methyl
group which allows distinguishing between the ring-opening products and gives
mechanistic information.

Transition
metal phosphides, Ni₂P and WP,supported on a high
surface area fumed silica (Cab-osil ® EH5) provided
by Cabot Corp. were synthesized using the phosphate and phosphite methods.
Equal moles of metal per gram of support (1.16 mmol/g
support) were used. The phosphate method (method A) produces phosphides via
phosphate precursors which contain phosphorus at an oxidation state of +5.
Meanwhile, the phosphite method (method I) produces phosphides via phosphite
precursors which have phosphorus at a lower oxidation state of +3. This lower
oxidation state is expected to make the precursors potentially reducible at a
lower temperature.

The
catalysts were characterized with temperature programmed reduction,
BET surface area measurement, CO chemisorption, X-ray diffraction and X-ray
photoelectron spectroscopy. The phosphite precursor of nickel phosphide was
reduced at a lower temperature (560^oC) than the phosphate precursor
(590^oC). A similar trend was observed for tungsten phosphide in
which the phosphite precursor was reduced at a lower temperature (580^oC)
than the phosphate precursor (600^oC). The BET surface areas range
from 149 to 217 m²/g, significantly lower than the area of the silica
support (350 m²/g). Catalysts from the phosphate
method have a lower surface area than those from the phosphite method, probably
due to the double sintering from calcination and reduction of the former.

Hydrodeoxygenation
of 2-methyltetrahydrofuran (2-MTHF) was carried out in a packed-bed reactor as
a function of temperature at atmospheric pressure. Quantities of catalysts
loaded into the reactor were equivalent to 30 umol of active surface metal
atoms (from CO uptake measurements). The gas-phase feed contained 3.2% of 2MTHF
in the H₂ stream. The catalytic activities of all phosphides at
different temperatures were presented as turnover frequencies (TOF) based on the
same number of CO chemisorption sites. In both cases, nickel phosphide was more
active with higher TOF values than tungsten phosphide. Between each method,
nickel phosphide by the phosphate method had a TOF value about 1.4 times than
that of the phosphite method while tungsten phosphide had practically the same
TOF value for both methods.

The
main HDO products of supported Ni₂P were pentane and butane whereas the
products of supported WP were mostly pentenes and pentadienes, suggesting different reaction pathways on
these catalysts. Contact time studies were carried out to identify the
intermediate products and determine reaction pathways. Quantities of catalysts
loaded into the packed-bed reactor were equivalent to 5 umol
of active surface metal atoms. For nickel phosphide from both methods, the
phosphate method and the phosphite method, the product selectivities
were similar with pentenes as primary products,
2-pentanone as a secondary product, and pentane as a final product; a reaction
network involving surface intermediates was proposed in Scheme 1. In contrast, for
tungsten phosphide the product selectivities depended
greatly on the preparation method.  With
WP/SiO₂ (phosphite method) pentenes were
produced prior to pentadienes indicating sequential
hydrogen removal on a single site (Scheme 2). With WP/SiO₂
(phosphate method) pentadienes were produced prior to
pentenes, indicating a surface intermediate reacting
on two active sites (Scheme 2). This result was consistent with the lower P
content of the phosphate-method catalyst which would leave more open metal
sites.  

Scheme 1: HDO reaction network of 2-MTHF on Ni₂P/SiO₂

Scheme 2: HDO reaction network of
2-MTHF on WP/SiO₂; (I) ? Phosphite method, (A) ? Phosphate method