117
framework acidity of zeolite limits hydride transfer step; therefore, it is essential to
tune the SAR ratio of the zeolite framework to maintain hydride transfer rate during
reaction [70, 71, 101–103]. Such a feature is imparted to the zeolite catalyst by
lowering the Si/Al ratio to an optimum level (Table 4). It is a well-known fact that
zeolite catalysts undergo rapid deactivation due to coke formation [17, 36, 85]
owing to the presence of olefinic precursors in constraint pore geometry. The deactivation rate is influenced by the temperature of the reaction as it affects the oligomerization, sorption property, and diffusion phenomena, which are major factors for
the formation of trimethyl pentanes (TMPs) in zeolite framework [7, 84, 104].
Reaction at low temperature favors the adsorption and oligomerization while
coke formation occurs at higher temperature [21, 37, 105]. All the aforementioned
side reactions occur along with alkylation and are unavoidable. Therefore, the acidity and the pore structure of the catalyst are important to increase the extent of
alkylation reactions and minimize side reactions over the zeolite catalyst [68, 106].
Therefore, the stability of the zeolite-based catalysts is a major hurdle toward the
successful implementation of such processes. The regeneration of the catalyst can
be done to overcome the loss due to catalyst deactivation. The literature suggests
that for solid catalysts to be competitive with existing processes based on H 2 SO 4 and
HF, the catalyst must survive multiple regeneration (as many as several hundred)
cycles [181]. To achieve this objective, the concept of wide pore and micro-meso
zeolites have been effectively used in developing zeolite-based catalysts.
Thus, research on zeolites with extra-large pores and/or with shorter diffusion
pathway is being carried out extensively. Frameworks like VPI-5, UTD-1, and
ECR-34 containing rings of 18, as well as 14 T-atoms, have been synthesized
[107–109]. However, these systems offer lower acidity, less thermal stability, and
Table 4 Comparison of product selectivities of Y zeolites with different silica to alumina
ratios [17]
Catalyst
Y-1
Y-2
Y-3
Y-4
Si/Al
5.4
13.6
35.3
62.2
C 5+ (wt%)
C 5 -C 7
24
19.2
10.5
13.3
C 8
59.5
54.4
49.3
39.6
C 9+
16.5
26.4
39.8
47.1
C 8 (wt%)
TMP
70.0
63.0
11.9
18.2
DMH
23.1
30.6
55.3
55.2
Olefins
6.9
6.4
32.8
26.6
TMP/DMH ratio
3.03
2.06
0.22
0.22
TMPs (wt%)
2,2,4-TMP
34.2
32.9
22.6
26.3
2,2,3-TMP
7.4
6.3
18.1
15.3
2,3,4-TMP
28.3
31.6
36.8
36.2
2,3.3-TMP
30.1
29.2
22.5
22.2
Emerging Trends in Solid Acid Catalyst Alkylation Processes
framework acidity of zeolite limits hydride transfer step; therefore, it is essential to
tune the SAR ratio of the zeolite framework to maintain hydride transfer rate during
reaction [70, 71, 101–103]. Such a feature is imparted to the zeolite catalyst by
lowering the Si/Al ratio to an optimum level (Table 4). It is a well-known fact that
zeolite catalysts undergo rapid deactivation due to coke formation [17, 36, 85]
owing to the presence of olefinic precursors in constraint pore geometry. The deactivation rate is influenced by the temperature of the reaction as it affects the oligomerization, sorption property, and diffusion phenomena, which are major factors for
the formation of trimethyl pentanes (TMPs) in zeolite framework [7, 84, 104].
Reaction at low temperature favors the adsorption and oligomerization while
coke formation occurs at higher temperature [21, 37, 105]. All the aforementioned
side reactions occur along with alkylation and are unavoidable. Therefore, the acidity and the pore structure of the catalyst are important to increase the extent of
alkylation reactions and minimize side reactions over the zeolite catalyst [68, 106].
Therefore, the stability of the zeolite-based catalysts is a major hurdle toward the
successful implementation of such processes. The regeneration of the catalyst can
be done to overcome the loss due to catalyst deactivation. The literature suggests
that for solid catalysts to be competitive with existing processes based on H 2 SO 4 and
HF, the catalyst must survive multiple regeneration (as many as several hundred)
cycles [181]. To achieve this objective, the concept of wide pore and micro-meso
zeolites have been effectively used in developing zeolite-based catalysts.
Thus, research on zeolites with extra-large pores and/or with shorter diffusion
pathway is being carried out extensively. Frameworks like VPI-5, UTD-1, and
ECR-34 containing rings of 18, as well as 14 T-atoms, have been synthesized
[107–109]. However, these systems offer lower acidity, less thermal stability, and
Table 4 Comparison of product selectivities of Y zeolites with different silica to alumina
ratios [17]
Catalyst
Y-1
Y-2
Y-3
Y-4
Si/Al
5.4
13.6
35.3
62.2
C 5+ (wt%)
C 5 -C 7
24
19.2
10.5
13.3
C 8
59.5
54.4
49.3
39.6
C 9+
16.5
26.4
39.8
47.1
C 8 (wt%)
TMP
70.0
63.0
11.9
18.2
DMH
23.1
30.6
55.3
55.2
Olefins
6.9
6.4
32.8
26.6
TMP/DMH ratio
3.03
2.06
0.22
0.22
TMPs (wt%)
2,2,4-TMP
34.2
32.9
22.6
26.3
2,2,3-TMP
7.4
6.3
18.1
15.3
2,3,4-TMP
28.3
31.6
36.8
36.2
2,3.3-TMP
30.1
29.2
22.5
22.2
Emerging Trends in Solid Acid Catalyst Alkylation Processes