Introduction
Long chain linear alkylbenzene (LAB) is one of critical chemicals since
its sulphonated products are the raw materials for industrial and
domestical detergents.1Commercially, the LAB is produced through the alkylation between benzene
with α-olefins (C8-C16), which is
catalyzed under the
HF
and AlCl3system.2 Notability, the
position of phenyl group in long chain α-olefin is crucial to
biodegradability and solubility, among various LAB isomers, the 2-LAB is
the most biodegradable and possesses the best
solubility.3 Therefore,
it’s highly desired to develop an efficiency catalysis technology to
improve the selectivity of 2-LAB isomer. However, the traditional HF and
AlCl3 catalysis technology for the production of LAB
brings some
severe
issues: 1) these homogeneous catalysts can generate a large number of
waste acids during the producing process, they also cause serious
corrosion towards reactor, and the separation with the reactants and
products is a tough energy-consuming
process.4 In addition,
the HF requires special handling and equipment, which further increase
cost and risk; 2) HF and AlCl3 catalysts exhibit low
2-LAB selectivity with ~15% and
30%,5 respectively.
This is far from the target that develops an environmental-friendly,
noncorrosive, reusable and green alkylation technology with desired
2-LAB selectivity.
To overcome these problems, a large number of clean technology has been
investigated based on the solid acid catalysts such as
zeolites,6,7ion exchange resin, 8metal oxides,2clay9 and heteropoly
acid.10 Among them, the
UOP company firstly realized the production of LAB in the industrial
scale based on the Detal technology by employing HF or
NH4F modified
SiO2-Al2O3 as the solid
catalyst,11 which open
a window for the development of green alkylation technology between
benzene with long chain α-olefins. Currently, the production of LAB
mainly depends on the HF, AlCl3 and Detal technology,
and the corresponding market share is ~75 %, 10 % and
15 %, respectively. Unsatisfactorily, the selectivity of 2-LAB is only
increased to 25 % through Detal
technology.12 Besides,
the acid strength of HF or NH4F modified
SiO2-Al2O3 is not enough
to catalyze the alkylation of benzene with long chain α-olefins, and the
reaction must be operated under the high temperature and pressure,
resulting in high cost compared with conventional HF technology.
Therefore,
it’s still highly expected to obtain an efficient green technology for
the production of 2-LAB.
Zeolites is a class of widely used solid catalysts with tunable acidity
and pore size, which is an ideal candidate for alkylation between
benzene with long chain linear α-olefins. In the past decades, great
efforts have been made to search suitable zeolites for 2-LAB
synthesis.13 The major
issues that prevent the successful industrialization of zeolite
catalysts for LAB process are the non-ideal distribution of LAB isomers
and deactivation of
zeolites.6 On the one
hand, the activity of zeolites is from acid sites that can catalyze
alkylation between benzene and α-olefins. Thus, zeolites must expose
more
accessible
acid sites in order to improve activity, the effective strategy for
improving accessible acid sites in zeolite framework is fabrication of
mesopore zeolites with open hierarchical channels. The mesopores not
only improve the accessibility of acid sites, but also enhance the
diffusion of coke precursor and increase the life time of catalysts. On
the other hand, the shape-selective catalysis for zeolites is mainly
based on the size or shape in sterically confined
environments.14 If the
mesopores are introduced into the framework or special fabrication of
layer zeolites with ultra-thin thickness along one crystal axis, the
shape-selective effect of zeolites is lineally decreased. Therefore,
it’s still a huge challenge to fabricate
suitable
zeolite catalysts for the production of 2-LAB.
In present research, the structure of MWW zeolite are optimized to solve
the above-mentioned issues. Dual
template strategy with conventional hexamethyleneimine (HMI) structure
directing agent and
long
chain quaternary ammonium surfactant is employed to delaminate the MWW
zeolites, which expect to expose the external acid sites and improve the
accessibility of acid sites. Our finds confirm that external surface
Brønsted acid sites can be systematically tailored by tuning the
hydrophobic chain length of surfactant. Moreover, the mono-layer
delaminated
MWW
zeolite is obtained by increasing the ratio between quaternary ammonium
surfactant and HMI. Computational results reveals that the external
isolated 12-ring hemicavities located on the upper and lower surface of
the MWW lamella can stabilize the 2-LAB due to the most appropriate
sterical configuration and highest binding energy with 12-ring
hemicavities, which
is
benefit to the formation of 2-LAB. Our study shows that
a regularly spaced MWW zeolite
nanosheets system can greatly improve the activity of catalysts in the
alkylation between benzene with 1-dodecene without sacrificing the
selectivity of 2-LAB. In addition, the catalytic performance of MWW
zeolite nanosheets are further maximized by altering the distribution of
Al species on the exterior surfaces and inhibiting the stacking of MWW
zeolite layers, the optimized MWW zeolite layers also show a superior
stability with
robust
selectivity of 2-LAB, which is highly desired and expected as an ideal
solid catalyst for the industrial production of 2-LAB.