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1.1.1 C4-Alkylation Fundamentals
Alkylation is a bimolecular nucleophilic substitution reaction wherein the olefin
molecule reacts with an iso-paraffin such as isobutane to form branched paraffins in
the presence of strong Bronsted or Lewis acid catalyst [6]. The acid catalyst helps
in forming carbocation intermediate, which undergoes rearrangement to form the
carbocation that is most stable and then the stabilized electron-deficient cation
reacts with electron-rich olefin to form large alkyl cation. Such a large cation eventually abstracts tertiary proton from isobutene and generates isoalkane/alkylates as
a product along with isobutane cation. Isobutane cation helps in propagating the
reaction further by reacting with another olefin molecule. The schematic diagram of
the detailed catalytic mechanism is given below using hydrofluoric acid (HF) as an
acid catalyst (Fig. 1). In the case of HF, fluoride acts as a stable anion and it donates
proton as a Bronsted acid source to form cation intermediate. However, for Lewis
acid-catalyzed alkylation, the metal complex-based cation coordinates with olefin
to generate carbocation intermediates, and thus the rest of the mechanism for forming alkylates remains the same.
1.1.2 Current/Conventional Alkylation Processes: Mineral Acids
Today, conventional alkylation processes use strong mineral acids such as sulfuric
(H 2 SO 4 ) and hydrofluoric acids as alkylation catalysts in refineries, of which 45% of
the world’s installed capacity is based on H 2 SO 4 and 55% is based on HF [7–13].
Since the reaction proceeds via carbocation intermediates, it can undergo several
side reactions by rearranging to various stable forms and results in the formation of
a complex product mixture (about 75–150 compounds) having a wide boiling range.
The reaction of isobutane with 1-butene and 2-butene leads to the formation of trimethyl pentanes (TMPs) in the presence of sulfuric and hydrofluoric acids with the
product having an octane number in the range of 90–98 (Table 3) [14]. However,
this requires the proper selection of feedstock, catalysts, and operating conditions.
The HF-based processes are more severe as compared to H 2 SO 4 -based processes
with respect to the reaction conditions. For achieving high-octane number with
lower acid consumption, it is required to maintain high surface area at the interface
of the catalyst and hydrocarbon along with continuous acid emulsion and an acid to
hydrocarbon ratio of 45:65 (v/v) [15]. The quantity and quality of products are governed by factors such as concentration of alkane, space velocity of olefin, temperature, concentration of catalyst, and mixing. Even though there are significant
technological advancements over the years for the production of alkylates using
mineral acid catalyst, the environmental concern still remains a major challenge
while using such strong liquid acids.
Emerging Trends in Solid Acid Catalyst Alkylation Processes
1.1.1 C4-Alkylation Fundamentals
Alkylation is a bimolecular nucleophilic substitution reaction wherein the olefin
molecule reacts with an iso-paraffin such as isobutane to form branched paraffins in
the presence of strong Bronsted or Lewis acid catalyst [6]. The acid catalyst helps
in forming carbocation intermediate, which undergoes rearrangement to form the
carbocation that is most stable and then the stabilized electron-deficient cation
reacts with electron-rich olefin to form large alkyl cation. Such a large cation eventually abstracts tertiary proton from isobutene and generates isoalkane/alkylates as
a product along with isobutane cation. Isobutane cation helps in propagating the
reaction further by reacting with another olefin molecule. The schematic diagram of
the detailed catalytic mechanism is given below using hydrofluoric acid (HF) as an
acid catalyst (Fig. 1). In the case of HF, fluoride acts as a stable anion and it donates
proton as a Bronsted acid source to form cation intermediate. However, for Lewis
acid-catalyzed alkylation, the metal complex-based cation coordinates with olefin
to generate carbocation intermediates, and thus the rest of the mechanism for forming alkylates remains the same.
1.1.2 Current/Conventional Alkylation Processes: Mineral Acids
Today, conventional alkylation processes use strong mineral acids such as sulfuric
(H 2 SO 4 ) and hydrofluoric acids as alkylation catalysts in refineries, of which 45% of
the world’s installed capacity is based on H 2 SO 4 and 55% is based on HF [7–13].
Since the reaction proceeds via carbocation intermediates, it can undergo several
side reactions by rearranging to various stable forms and results in the formation of
a complex product mixture (about 75–150 compounds) having a wide boiling range.
The reaction of isobutane with 1-butene and 2-butene leads to the formation of trimethyl pentanes (TMPs) in the presence of sulfuric and hydrofluoric acids with the
product having an octane number in the range of 90–98 (Table 3) [14]. However,
this requires the proper selection of feedstock, catalysts, and operating conditions.
The HF-based processes are more severe as compared to H 2 SO 4 -based processes
with respect to the reaction conditions. For achieving high-octane number with
lower acid consumption, it is required to maintain high surface area at the interface
of the catalyst and hydrocarbon along with continuous acid emulsion and an acid to
hydrocarbon ratio of 45:65 (v/v) [15]. The quantity and quality of products are governed by factors such as concentration of alkane, space velocity of olefin, temperature, concentration of catalyst, and mixing. Even though there are significant
technological advancements over the years for the production of alkylates using
mineral acid catalyst, the environmental concern still remains a major challenge
while using such strong liquid acids.
Emerging Trends in Solid Acid Catalyst Alkylation Processes