Kinetic studies for processes of liquid-phase alkylation of aromatics over solid acid catalysts

Abstract

The liquid-phase alkylation of aromatics has been effected traditionally with catalysis by inorganic acids or AlCl₃ and similar compounds. However the increasing importance of solid acid catalysts is changing the procedures in this field. One of the prerequisites for the development of relevant processes, and the design of reactors, is knowledge of the kinetic data in reactions catalyzed by solid acids. This review examines the information available on the kinetics of alkylations catalyzed by solid acids, mainly zeolites, supported metal chlorides and oxides, mesoporous (MCM) materials, and heteropolyacids, but also other less studied materials.

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Paolo Beltrame

Paolo Beltrame was born in Milano, Italy, in 1930. He received his “Laurea” in Industrial Chemistry from the University of Milano, in 1954. In 1955 he joined the same University as assistant professor. He carried out research work at University College London, UK, in 1959, and at University of California at Los Angeles, USA, in 1962. In 1970 he joined the University of Cagliari, Italy, as full professor. Currently he is full professor at the University of Milano, for Industrial Chemistry curricula. His current research interests are chemical kinetics and catalysis of reactions of industrial importance.

The alkylation of aromatics is a particularly important area of industrial chemistry. In searching for more environmentally benign replacements for traditional hazardous acid catalysts such as aluminium chloride, numerous solid acid catalysts have been suggested. Determining the most appropriate of these is very difficult and here the information available on the kinetics of these reactions is renewed in an attempt to help progress the development of new greener processes.

JHC

Introduction

The alkylation of aromatic compounds with olefins, halides or alcohols, traditionally catalyzed by the highly corrosive inorganic Brønsted and Lewis acids, can be carried out over solid acid catalysts, the advantages being easier handling, less equipment corrosion, few problems with regard to environmental disposal and toxicity, simple product separation, and catalyst recycling.

The mechanism proposed for these reactions is the general one for electrophilic aromatic substitution. In particular, it is proposed that the alkylating agent gives rise to an adsorbed organic cation. This may happen by protonation over protonating acids, producing, for instance, (C₆H₅CH₂⁺)_ads from benzyl chloride¹ or (C₆H₁₁⁺)_ads from cyclohexene² over a heteropolyacid, or by homolytic redox mechanism producing the cation from benzyl chloride over supported metallic oxides or chlorides such as Fe³⁺ chloride.^3,4 The next step is the reaction of the adsorbed cation with the aromatic reagent, present either in the fluid phase or itself adsorbed on the catalytic surface.

Our aim is to review liquid-phase alkylations over solid acid catalysts from the viewpoint of reaction kinetics. The articles on the subject found in the literature have been classified into four categories:

a) work including a discussion on mass transfer resistances and their weight in the process (the conclusions drawn are that such resistances are negligible), followed by a search for kinetic equations and an evaluation of the relevant parameters. The results in these cases can be reasonably considered as pertaining to intrinsic kinetics;

b) kinetic studies similar to those in (a), apart from the discussion on mass transfer resistances;

c) studies including the measurement of reaction rates, without a search for kinetic equations;

d) other articles containing kinetic data, although of less straightforward interpretation.

From the point of view of chemical engineering, that requires intrinsic kinetic equations, the first two categories are the most useful, although the results of type (b) may not be as reliable as those of type (a).

The reactions are usually carried out with an excess of aromatic reagent with respect to the alkylating agent, and in many cases it is the aromatic itself that is the reaction solvent; however in some cases there can be a different (usually non-aromatic) solvent.

This literature review has revealed great variety in the approaches to kinetic measurement, so that, given the differences in meaning of the parameters, it is not easy to make comparisons. Thus it has been considered important to unify, as far as possible, parameter definitions along the following lines.

Some authors give pseudo-first order rate coefficients with a dimension time⁻¹, while others give them with dimensions volume·mass⁻¹·time⁻¹. We have chosen to take eqn. (1) as a typical first-order rate equation:

r = k₁·C_alk·C_cat (1)

with r as mol l⁻¹·min⁻¹, C_alk (molar concentration of alkylating agent) as mol l⁻¹, C_cat (mass concentration of the catalyst in the suspension) as g l⁻¹, so that k₁ is given as l g⁻¹·min⁻¹. As a consequence, when it is necessary to convert pseudo-first order rate coefficients with dimension time⁻¹ into k₁ coefficients as l g⁻¹·min⁻¹, C_cat is evaluated from the experimental details given by the authors. Obviously, such coefficients must be taken as monoalkylation coefficients.

Other authors have chosen to give second-order rate coefficients. In these cases, we have chosen a kinetic equation for monoalkylation such as eqn. (2):

r = k₂·C_arom·C_alk·C_cat (2)

expressing r, whenever possible, as mol l⁻¹·min⁻¹, C_arom and C_alk (molar concentration of the aromatic and the alkylating agent, respectively) as mol l⁻¹, C_cat (mass concentration of the catalyst in the suspension) as g l⁻¹, so that k₂ is given as l² mol⁻¹·g⁻¹·min⁻¹.

In some cases, kinetic equations of the Langmuir–Hinshelwood type have been proposed. For monoalkylation such equations have the form of eqn. (3), i.e., they contain a numerator similar to eqn. (2) but also a denominator which includes one or more adsorption constants K_ads for one or more components of the reaction system.

where n = 1 or 2.

Analogous equations are employed for dialkylations and other reactions. It should be noted that in these equations the coefficient k_m is not a “true” rate coefficient because it also includes adsorption coefficients.

Finally, some authors were concerned about the deactivation rate of their catalysts, leading them to also study the kinetics of this phenomenon and to provide deactivation rate coefficients, with dimension time⁻¹. We have given such coefficients as k_d (min⁻¹). It is worth noting that they appear as first-order coefficients, but the relevant equations usually employ dimensionless variables so that the dimension of k_d does not give an idea of the actual form that the kinetic equation takes. Such forms will be specified when mentioning these deactivation coefficients.

In the cases of type (c), the measured rates given by the authors have been converted to units mmol g⁻¹·min⁻¹, for easier comparison.

As to the equipment for kinetic measurement, most articles mention stirred batch reactors, obviously slurry reactors, given the presence of solid catalyst. In some cases, when the nature of the alkylating agent and the reaction temperature required pressures higher than ambient pressure, stirred autoclaves were used. With gaseous alkylating agents, a semi-batch reactor was sometimes employed, maintaining a known C_alk value in the system.

2 Kinetic equations and kinetic parameters

Studies of type (a) were carried out at high stirring speed, checking, in most cases, that a further enhancement of the speed had no effect on the kinetics. In this way significant external mass transfer resistance was excluded. With regard to intraparticle resistance, care was taken (in most cases) to use very fine catalyst particles; in such cases very low Thiele modulus values and therefore catalyst effectiveness values close to unity can be calculated.

Table 1 shows typical results of studies of this type. The catalysts presented by the papers mentioned in Table 1 are zeolites (microporous crystalline aluminosilicates),¹⁷ amorphous silica-aluminas, supported metal oxides, HAlMCM-41 (mesoporous aluminosilicates presenting uniform channels from 15 to 100 Å in size),^18,19 sulfated zirconia supported on HMS (a mesoporous material similar to MCM-41),²⁰ and Nafion (a perfluorinated polymeric sulfonic acid)²¹ in the form of a Nafion–Silica composite.²²

Table 1 Evaluation of parameters within a kinetic equation, after tests about mass transfer resistance: typical results

Reference	Aromatic (solvent)^a	Alkylating agent (pressure)^b	Reactor [rpm]^c	T/°C	Catalyst	Ccat/g l^−1	Parameter values (at temp. T,°C)
a If different from the aromatic itself.b If different from ambient pressure (1 atm = 101.325 KPa).c Stirrer speed, when specified.d Benzyl chloride.e For an equation −da/dt = kda^2, where a is the catalyst activity coefficient.f Methyl-tert-butyl ether, as a source of isobutene.g Reaction carried out in a continuous tubular reactor.h km1 and km2 correspond to the monobenzylation to 1-benzyl- and 2-benzyl-naphthalene, respectively.i Benzyl alcohol.
Pseudo-first-order kinetic equation (k1/l g^−1 min^−1) (kd/min^−1)
5	phenol	c-hexene	batch	100	HY, Si/Al = 25
	(dichlorobenzene)		[1000]			1.4	k1 = 0.0125
	(cyclohexane)					1.5	k1 = 0.0056
	(nitropropane)					1.7	k1 = 0.0011
6	biphenyl	BzCl^d	batch	80	HY, Si/Al = 7	5–13	k1 = 0.0010
	(cyclohexane)		[825]				kd = 0.25^e
7	benzene	BzCl	batch	60–80	In2O3/Hβ, Si/Al = 27	7.1	k1 = 0.090 (80)
	anisole		[high speed + N2 flux]	80			k1 = 0.014
Second-order kinetic equation (k2/l^2 mol^−1 g^−1 min^−1)
8	p-cresol	MTBE^f	autoclave	80–115	sulfated ZrO2/HMS	30.8	k2 = 0.000020 (100)
		(>1 atm)	[700]
9,10	benzene	1-dodecene	fixed bed	85–125	H3PW12O40/SiO2	^g	k2 = 0.000012 (100)
		(24 atm)
Second-order kinetic equation (k2/mol g^−1 min^−1 (mol fraction)^−2)
11	benzene	propene	fixed bed	200–240	CaHY	^g	k2 = 0.0024 (230)
		(40 atm)
Langmuir–Hinshelwood kinetic model (km/l^2 mol^−1 g^−1 min^−1) (Kads/l mol^−1)
12,13	biphenyl	BzCl	batch	80	amorphous	8–16	km = 0.00058
	(cyclohexane)		[825]		silica-alumina, Si/Al = 6.2		KBzCl = 10.6 (n = 1)
13	biphenyl	BzCl	batch	80	amorphous	5–16	km = 0.0021
	(cyclohexane)		[825]		silica-alumina, Si/Al = 2.7		KBzCl = 12.2 (n = 1)
	biphenyl	BzCl	batch	80	HAlMCM-41, Si/Al = 19	3	km = 0.0118
	(cyclohexane)		[825]				KBzCl = 6.53 (n = 1)
14	biphenyl	BzCl	batch	80	HAlMCM-41, Si/Al = 19	2–4	km = 0.0714
	(cyclohexane)		[825]				KBzCl = 24.3
							KBIP = 5.4 (n = 2)
							kd < 0.001^e
15	naphthalene	BzCl	batch	80	HAlMCM-41, Si/Al = 19.5	5	km1 = 0.46^h
	(cyclohexane)		[1000 + N2 flux]				km2 = 0.19^h
							KBzCl = 10.3
							KNA = 12.6 (n = 2)
16	naphthalene	BzOH^i	batch	80	Nafion-Silica	2–3	km = 0.50
	(cyclohexane)		[1000 + N2 flux]		composite (15%)		KBzOH = 110 (n = 2)

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The alkylating agents in Table 1 are, in many cases, benzyl chloride or benzyl alcohol, and, less commonly, olefins. Most of the studies were carried out at relatively low temperatures, in the 60–100 °C range, with catalyst doses of a few grams per liter.

Table 2 shows typical results of studies of type (b). The catalysts concerned include, besides the kind of solid acids already present in Table 1, K10 (an acid-treated montmorillonite clay of large surface area), Clayzic (zinc chloride supported on K10 montmorillonite),³³ a heteropolyacid H_0.5Cs_2.5PW₁₂O₄₀, and supported metal chlorides.

Table 2 Evaluation of parameters within a kinetic equation, without tests about mass transfer resistance: typical results

Reference	Aromatic (solvent)^a	Alkylating agent (pressure)^b	Reactor [rpm]^c	T/°C	Catalyst	Ccat/g l^−1	Parameter values (at temp. T, °C)
a See the corresponding footnote in Table 1.b See the corresponding footnote in Table 1.c See the corresponding footnote in Table 1.d See the corresponding footnote in Table 1.e Thermally activated.f Unspecified.g For an equation −da/dt = kda^3, where a is the catalyst activity coefficient.h Benzyl alcohol.i With N2 flux.
Pseudo-first-order kinetic equation (k1/l g^−1 min^−1) (kd/min^−1)
23	anisole	BzCl^d	batch	20–50	K10	30.3	k1 = 0.000043 (50)
	benzene	BzCl	batch	20–60	clayzic	35.7	k1 = 0.000104 (50)
					(unactivated)
24	benzene	BzCl	batch	40	clayzic^e	35.7	k1 = 0.00095
	chlorobenzene	BzCl	batch	40	clayzic^e	32.3	k1 = 0.00068
25	toluene	BzCl	batch	40	clayzic^e	30.8	k1 = 0.0319
	ethylbenzene	BzCl	batch	40	clayzic^e	30.8	k1 = 0.0015
26,27	benzene	1-dodecene	fluidized bed	70–90	HY, Si/Al = 1.6	^f	k1 = 1.12 (80)
							kd = 0.31^g
1	benzene	BzCl	batch	80	H0.5Cs2.5PW12O40	174	k1 = 0.027
28	benzene	1-dodecene	batch	100–140	HY, Si/Al = 2.5	5	k1 = 0.0049 (100)
	(decane)	(6–9 atm)	[500]
29	toluene	BzOH^h	batch	90	HAlMCM-41, Si/Al = ca. 10	8.7	k1 = 0.00017
			[+air flux]
30	benzene	BzCl	batch^i	80	TlOx20%/ZrO2	7.1	k1 = 0.0411
					(low surface)
	anisole	BzCl	batch^i	80	TlOx20%/ZrO2	7.1	k1 = 0.0287
					(low surface)
31	benzene	BzCl	batch^i	60–80	Ga2O3/HZSM-5, Si/Al = 31	7.1	k1 = 0.0210 (80)
32	benzene	BzCl	batch^i	60–80	InCl3/K10	7.1	k1 = 0.0737 (80)
	benzene	BzCl	batch^i	60–80	GaCl3/K10	7.1	k1 = 0.0451 (80)
	benzene	BzCl	batch^i	60–80	FeCl3/K10	7.1	k1 = 0.0772 (80)
	toluene	BzCl	batch^i	80	InCl3/K10	7.1	k1 = 0.0592
	mesitylene	BzCl	batch^i	80	InCl3/K10	7.1	k1 = 0.0521

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The most common alkylating agent present in Table 2 is again benzyl chloride, benzyl alcohol and olefins being less commonly employed. The reaction temperatures were low, even lower than those of Table 1.

The values shown in Tables 1 and 2 concern several different reactions, a fact that makes comparison difficult. The kinetic measurements (pseudo-first order coefficients) on benzene alkylation using benzyl chloride, carried out at 80 °C or at temperatures close to 80 °C, provide the largest data set, allowing comparison of the different catalysts. The order of decreasing catalytic activity appears to be the following:

In₂O₃/Hβ⁷ > FeCl₃/K10, InCl₃/K10³² > GaCl₃/K10³² > TlO_x/ZrO₂³⁰ > H_0.5Cs_2.5PW₁₂O₄₀¹ > Ga₂O₃/HZSM–5³¹ > Clayzic²⁴

with k₁-values differing by one order of magnitude from the first to the last member of the series. Note that, apart from one case, the mentioned catalysts are supported metal chlorides or oxides.

A few reactions have been carried out using a solvent different from the aromatic reagent. Eight different solvents were used in the reaction of phenol with cyclohexene over HY zeolite (Table 1), revealing that the alkylation rate had a peak for solvents of medium polarity.⁵ Since the measured parameter was a pseudo-first order coefficient with respect to C_alk, it could be expected that these reactions, having a lower C_arom value, would show lower k₁-values than reactions carried out with an aromatic excess, the excess acting as both reagent and solvent. Further examples are given by the reactions of biphenyl with benzyl chloride in cyclohexane⁶ (Table 1), and benzene with 1-dodecene in decane²⁸ (Table 2); both reactions employ a HY zeolite catalyst.

Among the cases listed in Table 2, some values are noteworthy: it is surprising how slow the reaction of toluene with benzyl alcohol over MCM-41²⁹ is, and how fast is that of benzene with 1–dodecene over HY;^26,27 in the latter case, the reason could be that the authors used a particularly efficient reactor.

Table 2 offers, for the same catalyst (Clayzic), the opportunity to compare an unactivated sample with a thermally activated one, under the same conditions:^23,24 activation increased the rate by more than one order of magnitude. The kinetic effect of alkyl substituents on benzene is less clear: with Clayzic as the catalyst, toluene and ethylbenzene reacted faster than benzene,^24,25 while with InCl₃/K10 as catalyst, toluene and mesitylene were found to be slightly less reactive than benzene.³²

One group of studies (Table 1) employed the kinetic model corresponding to eqn. (3). In these, no straightforward comparison of the different reactions can be made as the rates depend not only on the coefficients k_m in the numerator but also on the constants K_ads,j and the value of n in the denominator of the kinetic equations. However, it is possible to see that for the benzylation of biphenyl, the high-alumina silico-aluminate catalyst (Si/Al = 2.7) is more active than the low-alumina catalyst (Si/Al = 6.2),¹³ and catalyst HAlMCM-41 is more active than either of the previous ones.^13,14 For the naphthalene reactions, data for the same temperature and the same solvent are available, but different benzylating agents and different catalysts were used, so the figures in Table 2 cannot be compared directly. However, it has been reported that BzOH is much more reactive than BzCl on the Nafion-silica composite, while the chloride is more reactive than the alcohol on the MCM-41 catalyst.¹⁶

3 Other kinetic measurements

Table 3 gives results typical of the type (c) studies, where the catalysts of interest include, besides the kind of solid acids seen in the previous paragraph, also an ion-exchange resin (Amberlyst 15) and SAPO–11 (a component of a class of microporous silicoaluminophosphates).⁴⁵ The alkylating agents are benzyl chloride and benzyl alcohol, olefins, and adamantyl bromide. Note that the temperatures are often higher than those in Tables 1 and 2.

Table 3 Measured alkylation rates r (in most cases, initial rates)

Reference	Aromatic (solvent)^a	Alkylating agent (pressure)^b	Reactor [rpm]^c	T/°C	Catalyst	r/mmol g^−1 min^−1
a See the corresponding footnote in Table 1.b See the corresponding footnote in Table 1.c See the corresponding footnote in Table 1.d See the corresponding footnote in Table 1.e With catalyst deactivation due to olefin oligomers (coke); for a zero-order reaction of formation of % coke, measured kd = 0.056 min^−1 (80 °C).f Benzyl alcohol.g Adamantyl bromide.
34	benzene	BzCl^d	batch	95	sulfated ZrO2	r = 0.194
			[900]
2	m-xylene	c-hexene	batch	100	H0.5Cs2.5PW12O40	r = 0.41
	mesitylene	c-hexene	batch	100	H0.5Cs2.5PW12O40	r = 0.44
35	toluene	BzCl	batch	110	HY, Si/Al = 20	r = 1.12
			[700 + N2 flux]
36	benzene	1-dodecene	batch	100–130	HY, Si/Al = 2.7	r = 2.80 (100)
	(decane)	(6 atm)	[500]
37	o-xylene	styrene	batch	60–100	Amberlyst 15^e	r = 0.49 (80)
			[1500]
38	biphenyl	propene	stirred	250–300	SAPO-11	r = 0.015 (300)
	(decalin)	(>1 atm)	autoclave
	biphenyl	propene	stirred	250	H-Mordenite	r = 0.055
	(decalin)	(>1 atm)	autoclave
39	benzene	BzOH^f	batch	80	Nafion–Silica	r = 1.72
			[+N2 flux]		composite (13%)
	p-xylene	BzOH	batch	100	Nafion–Silica	r = 2.97
			[+N2 flux]		composite (13%)
40	naphthalene	BzCl	batch	40–70	Hβ, Si/Al = 13	r = 0.009 (70)
	(dichloroethane)
41	o-xylene	BzCl	batch	90–135	Hβ, Si/Al = 13	r = 0.21 (90)
42	toluene	Ad-Br^g	batch	111	Nafion–Silica	r = 3.38
					composite (13%)
43	benzene	1-dodecene	batch	80	Nafion–Silica	r = 0.080
					comp. (13%) [Type 1]
	benzene	1-dodecene	batch	80	Nafion–Silica	r = 1.08
					comp. (13%) [Type 2]
	benzene	propene	semi-batch	70	Nafion–Silica	r = 0.115
					comp. (13%) [Type 1]
	benzene	propene	semi-batch	70	Nafion–Silica	r = 0.72
					comp. (13%) [Type 2]
44	bromobenzene	BzCl	batch	130	HY, Si/Al = 2.8	r = 0.50
	bromobenzene	BzCl	batch	130	Hβ, Si/Al = 4.5	r = 0.40
	bromobenzene	BzCl	batch	130	Hβ, Si/Al = 13	r = 0.23
	bromobenzene	BzCl	batch	130	K10, Si/Al = 3.1	r = 0.62
4	benzene	BzCl	batch	80	FeCl3/K10	r = 24.2
			[+N2 flux]
	benzene	BzCl	batch	80	FeCl3/Si-MCM41	r = 19.8
			[+N2 flux]

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The reactions considered in Table 3 were carried out at different temperatures and almost all differ one from the other, making it hard to compare the reaction rates. However for a common temperature an evaluation reveals the reaction rates of benzyl chloride with benzene and substituted benzenes to be similar (within an order of magnitude), apart from the case of the benzene reactions over supported FeCl₃ catalysts, where the rates were exceptionally high.⁴ Again, the lowest rates were measured for reactions in solutions having low values of C_arom, as for biphenyl + propene in decalin³⁸ and for naphthalene + BzCl in dichloroethane.⁴⁰

Needless to say, catalyst preparation matters. Table 3 shows two reactions of benzene with olefins,⁴³ where two types of Nafion–silica composite can be compared and it can be seen that they are of quite different activity, Type 2 being an order of magnitude more active than Type 1 for the same reaction at the same temperature.

In addition to the papers in Tables 1–3, other articles [type (d)] deal with the use of a heteropolyacid supported on silica,⁴⁶ Amberlyst 15 and analogous resins,^47–51 Clayzic,⁵² sulfated zirconia,^53–54 MCM–22,⁵⁵ zeolite Hβ,⁵⁶ and heteropolyacid supported on K10 clay.^57–59

The rates mentioned are those of forward alkylation reactions, as most authors gave no consideration to backward reactions, and those who did take them into account, for the alkylation of p-cresol with isobutene⁵¹ and that of benzene with propene,⁵⁶ found them to be negligible.

The values of the kinetic parameters (coefficients, adsorption constants, rates) given in Tables 1–3 are ascribed to a specified temperature. However, some kinetic measurements were carried out at different temperatures, and apparent activation energy values (E_A) have been reported.

Table 4 shows the E_A values available for the reactions mentioned in the previous Tables. They are presented separately for the reactions of Table 1 and 2 (in both cases mostly values from 10 to 20 kcal mol⁻¹), and Table 3. In the case of the reaction of o-xylene with BzCl over Hβ, the very low value (4.0 kcal mol⁻¹) is evidence of a diffusional control.

Table 4 Apparent activation energies for reactions listed in Tables 1–3

Reference	Reaction (parameter)^a	EA/kcal mol^−1
a Kinetic coefficient (k1, k2) or rate (r) on which the evaluation of EA was based.b Range of values, according to the product isomer.
from Table 1
7	benzene + BzCl over In2O3/Hβ (k1)	17.6
9,10	benzene + 1-dodecene over H3PW12O40/SiO2 (k2)	10.9–11.5^b
11	benzene + propene over CaHY (k2)	23.6
from Table 2
23	anisole + BzCl over K10 (k1)	15.6 ± 2.9
	benzene + BzCl over Clayzic (unactivated) (k1)	14.2 ± 2.2
26,27	benzene + 1-dodecene over HY (k1)	20.6
28	benzene + 1-dodecene over HY (k1)	12 ± 3
31	benzene + BzCl over Ga2O3/HZSM-5 (k1)	16.1
32	benzene + BzCl over InCl3/K10 (k1)	14.5
	benzene + BzCl over GaCl3/K10 (k1)	15.1
	benzene + BzCl over FeCl3/K10 (k1)	11.6
from Table 3
36	benzene + 1-dodecene over HY (r)	15
41	o-xylene + BzCl over Hβ (r)	4.0

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Table 5 shows the E_A values available for additional reactions. In many cases they fall in the 10 to 20 kcal mol⁻¹ range, but also values > 20 kcal mol⁻¹ have been reported. In two cases values as low as 7–8 kcal mol⁻¹ were measured, although the tests performed showed mass transfer resistances to be negligible.

Table 5 Apparent activation energies for other alkylations

Reference	Reaction	EA/kcal mol^−1
a For these reactions tests are reported showing that mass transfer resistances were negligible.
49	phenol + 1-dodecene over Amberlyst-15^a	7.4
56	benzene + propene over β zeolite^a	7.8
48	phenol + isobutene over Amberlyst-15^a	12.3 (to p-alkylate)
		13.8 (to o-alkylate)
46	benzene + 1-dodecene over H4SiW12O40	13.8
52	anisole + BzCl over Clayzic	15.5
55	benzene + propene over MCM-22	18.4
58	phenol + methyl tert-butyl ether over H3PW12O40/K10^a	18.5
59	benzene + 1-dodecene over H3PW12O40/K10^a	20.1
53	p-cresol + isobutene over sulfated zirconia^a	22.9
51	p-cresol + isobutene over Amberlyst-15^a	23.2
54	benzene + BzCl over sulfated zirconia^a	28.0

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Until now only the kinetic results for the mono-alkylation of aromatics have been considered; however, also kinetic data on consecutive dialkylation reactions are available. As shown in Table 6, the coefficient ratio for dialkylation and monoalkylation is in some cases < 1, in other cases ≅ 1, often > 1. The largest value was found during studies on the reaction of naphthalene with benzyl chloride over a MCM-41 catalyst: the second benzylation is reported as being much faster for 2-benzylnaphthalene than for the 1-benzyl isomer.¹⁵

Table 6 Ratio of kinetic coefficients for dialkylation and monoalkylation

Reference	Reaction	T/°C	Ratio (dialkyl/monoalkyl)
51	p-cresol + isobutene over Amberlyst-15	65	0.21
56	benzene + propene over β zeolite	100	0.45
53	p-cresol + isobutene over sulfated zirconia		0.65
46	benzene + 1-dodecene over H4SiW12O40	65–150	1
6	biphenyl + BzCl over HY	80	1
11	benzene + propene over CaHY	230	1.15
12,13	biphenyl + BzCl over amorphous silica-alumina
	having Si/Al = 6.2	80	2
	having Si/Al = 2.7	80	2.5
13,14	biphenyl + BzCl over HAlMCM-41	80	2.5
15	naphthalene + BzCl over HAlMCM-41
	for 1-benzylnaphthalene	80	1.4
	for 2-benzylnaphthalene	80	45
16	naphthalene + BzCl over Nafion–silica composite	80	4.6

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4 Catalyst stability

In several cases, the authors also investigated the possibility of re-using the catalyst one or more times after a (batch) run. Table 7 shows the average loss in activity per re-use, evaluated from tabulated data or plotted results.

Table 7 Re-use of the catalysts and loss of activity

Reference	Reaction	Catalyst	No. of re-uses	Average loss of activity per re-use
a when recycling is effected at reaction temperature
16	naphthalene + BzOH	Nafion–silica composite	3	negligible^a
31	benzene + BzCl	Ga2O3/HZSM–5	3	negligible
32	benzene + BzCl	InCl3/K10	5	∼1%
7	benzene + BzCl	In2O3/Hβ	5	∼2%
7	benzene + BzCl	InCl3/Hβ	5	∼5%
54	benzene + BzCl	sulfated ZrO2–Fe2O3	3	∼4%
58	phenol + MTBE	H3PW12O40/K10	3	∼5%
59	benzene + 1-dodecene	H3PW12O40/K10	2	∼6%
8	p-cresol + MTBE	sulfated ZrO2/K10	2	∼18%
37	o-xylene + styrene	Amberlyst–15	5	∼18%
41	o-xylene + BzCl	Hβ	3	∼23%
4	benzene + BzCl	FeCl3/K10 or
		FeCl3/SiMCM–41	1	40–50%
54	benzene + BzCl	sulfated ZrO2	1	>87%

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A different way of testing catalyst stability is to use the catalyst for continuous runs over a period of at least several hours. Runs in the CSTR mode revealed amorphous low-alumina silicoaluminate (Si/Al = 6.2)¹² and HAlMCM-41¹⁴ to have constant behaviour in the reaction of biphenyl with benzyl chloride; an analogous reaction using naphthalene again evidenced the stability of HAlMCM-41¹⁵ and, in the reaction of naphthalene with benzyl alcohol, stability was also found for the Nafion–silica composite.¹⁶ In contrast, the same kind of test showed a slight loss in activity for amorphous high-alumina silicoaluminate (Si/Al = 2.7).¹³ Runs in a fixed bed continuous reactor, for the reactions of benzene with ethene or propene, revealed MCM-22 to be stable.⁵⁵

In other cases, the catalysts underwent regeneration and were then proved to be re-usable. This was the case of Hβ⁴⁴ and Amberlyst-15.⁴⁸

Among the reactions taken into account, some of the kinetic models of the alkylation processes included catalyst deactivation. In these cases, a deactivation rate coefficient k_d was evaluated. The values reported in Tables 1–3 correspond to different kinetic equations, so their numerical values have to be interpreted. By integrating the kinetic equations, it can be seen that:

for HY (Si/Al = 7) in the benzylation of biphenyl,⁶ where k_d = 0.25 min⁻¹, this means that over 60 min the activity is lowered from 100% to about 6%;

for HAlMCM-41 in the same reaction,¹⁴ where k_d < 0.001 min⁻¹, this means that over 60 min the activity is lowered from 100% to a fraction > 94%;

for HY (Si/Al = 1.6) in the reaction of benzene with 1-dodecene,^26,27 where k_d = 0.31 min⁻¹, this means that over 60 min the activity is lowered from 100% to 16%.

In the case of o-xylene reacting with styrene over Amberlyst-15,³⁷k_d = 0.056 min⁻¹, the coke formation measured revealing that about 3% coke is deposited on the catalyst over 60 min.

5 Conclusions

The kinetic parameters for many monoalkylation reactions, over different types of solid acids, often at 40 to 100 °C, are reported in the scientific literature. In several cases, also apparent activation energy has been measured. In the present review, research papers including a discussion on resistances to mass transfer have been indicated.

Aromatics like benzene, toluene, the xylenes, biphenyl, the phenols and anisole are the ones given the most consideration. The most common alkylating agents are benzyl chloride, olefins and alcohols. The most commonly employed catalysts are zeolites, supported metal chlorides and oxides, mesoporous (MCM) materials and heteropolyacids, these are followed by sulfated zirconia, ion-exchange resins, Nafion derivatives, and other minor items.

Data available on the kinetics of successive dialkylation reactions are also presented, the kinetic parameters usually being of the same order as those of the first reaction.

The catalysts were often tested for their stability, particularly keeping in mind their recycling for re-use: in several cases this was found possible, though usually with some loss of catalytic activity. Given the practical importance of catalyst re-use, further work is needed in this field.

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