Alkylation of phenol with cyclohexene on HBEA150: spectroscopic and

The gas-phase IR spectra of adsorbed cyclohexene over siliceous BEA (BAS: 0;

LAS: 0.03 mmol g^-1) and HBEA-150 (BAS: 0.19 mmol g^-1; LAS: 0.04 mmol g^-1) at 40

°C are shown in Fig. 4A-19. Cyclohexene was molecularly adsorbed on siliceous BEA, while alkenyl carbenium ions were observed when cyclohexene was adsorbed on HBEA-150 (see discussion in Section A5, Appendix). On acidic zeolites, surface alkenyl carbenium ion originating from the adsorption of olefin has been evidenced by IR or NMR spectroscopies.^[47,48] The alkenyl carbenium ions can form either via hydride transfer between olefin and the alkyl carbenium ion, or through hydride abstraction from olefin by LAS.^[49] Siliceous BEA and HBEA-150 contained similar amounts of LAS but only the latter showed bands of alkenyl carbocation. Thus, LAS-catalyzed hydride abstraction of cyclohexene, forming alkenyl carbenium ions, is unlikely on both samples.

0 2 4 6 8 10

0 0.2 0.4 0.6 0.8 1

Conversion rates of phenol (10-3(mol phenol) (mol H+)-1 s-1)

Cyclohexanol Conc. (mol L^-1)

- 144 - This, in turn, points to the other formation pathway for the alkenyl carbenium ion on HBEA-150, that is, hydride transfer between cyclohexene and the cyclohexyl carbocation (Fig. 4-7).^[47,49] To summarize, by proving the presence of cyclohexenyl carbocation on HBEA-150, we indirectly established the formation of the unstable cyclohexyl carbocation upon cyclohexene adsorption on HBEA in the gas phase.

Figure 4-7. Proposed intermediates of cyclohexene in gas-phase adsorption and liquid-phase alkylation on HBEA-150. Gas-phase adsorption was measured on an infrared spectrometer at 40 °C and the liquid-phase alkylation reactions were carried out at 120-160°C.

However, there was no evidence for the formation of cyclohexenyl cation during liquid phase reactions of phenol and cyclohexene, as inferred from the absence of observable cyclohexane in the products (cyclohexane would have been formed if hydride transfer between cyclohexene and cyclohexyl carbenium ion took place, as depicted in Fig. 4-7) and the absence of skeletal rearrangement (cyclohexenyl cation to 1-methycyclopentyl cation^[48]). The absence of hydride transfer between cyclohexene and cyclohexyl cation is expected, as the cyclohexyl carbenium ion is the direct electrophile which rapidly attacks phenol in phenol-cyclohexene alkylation (Fig. 4-7). The proposed mechanism and corresponding elementary steps are shown in Scheme 4A-2.

Gas-phase adsorption

Liquid-phase alkylation

- 145 - It was difficult to study the kinetics of phenol-cyclohexanol alkylation, not only because significant catalyst deactivation had occurred before alkylation started, but also because a number of species (phenol, cyclohexanol, cyclohexene and water) were already present at the initial stage of alkylation. As discussed above, cyclohexene/carbenium ion is the direct alkylating agent for the BAS-catalyzed alkylation of phenol with cyclohexanol. Thus, the kinetic study of alkylation was performed using equimolar amounts of phenol and cyclohexene over HBEA-150 in decalin at 120-150 °C (Fig. 4A-21). Two-ring products including 2-CHP, 4-CHP, 1-CC and CHPE were primary products, among which the selectivity to CHPE (O-alkylation) decreased with increasing temperature. C-alkylation at the ortho-position (2-CHP) was always faster than that at the para-position (4-CHP). The rate of C-alkylation was 3-5 times higher than the dimerization of cyclohexene.

For both C- and O-alkylation products, the reaction orders with respect to phenol and cyclohexene concentrations were determined to be ~ 1.0 and ~ 0.4 (Fig. 4A-22), respectively, at 120 °C where deactivation was least pronounced. The first order in phenol, along with its appreciable adsorption at the BAS as discussed in the previous section, indicates that phenol alkylation on HBEA occurs via an E-R type mechanism in decalin. The reaction order in phenol would be fractional if the kinetic mechanism were of L-H type. The fractional order in cyclohexene is also consistent with the above consideration and proves that cyclohexene reacts in an adsorbed state.

The initial formation rates of different phenol-cyclohexene alkylation products at 120–150 °C are compiled in Table 4-3. The Arrhenius plots are shown in Fig. 4A-23.

The apparent activation energies were comparable at 46–49 kJ mol^-1 for C-C bond coupling products, while being lower for the O-alkylation, only ~ 26 kJ mol^-1 (Table 4-3). For alkylation of phenol with cyclohexene in 1,2-dichloroethane (solvent) catalyzed by a variety of sulfonic resins, intrinsic activation barriers of 68, 74 and 20 kJ mol^-1 for 2-CHP, 4-CHP and CHPE, respectively, were obtained from fitting experimental results with an E-R model.^[50] In another report,³¹ an intrinsic activation energy of 70 kJ mol^-1 was obtained for liquid phase alkylation of benzene with octenes over HY zeolite also based on an E-R model. Considering that the apparent activation energy contains an negative enthalpy term related to olefin protonation (Ea,app = Ea,int + ΔHolefin,prot), the Ea,app

- 146 - of 46–49 kJ mol^-1 measured in this work for phenol alkylation appears comparable to those reported earlier for alkylation of benzene. Note that these activation energies for the formation of alkylation products were much lower than that for cyclohexanol dehydration, ~140 kJ mol^-1. Alkylation, however, did not proceed at a rate commensurate with this low activation barrier because of unfavorable entropy factors, i.e., bimolecular reactions lead to significant losses of entropy at the TS.

Table 4-3. Turnover frequencies and apparent activation energies for frequencies for the initial products of HBEA150-catalyzed alkylation of phenol with cyclohexene in decalin.^a

Products Rate^b

Temperature (°C) Ea

c

(kJ mol^-1)

120 130 140 150

1-cyclohexylcyclohexene TOF_1-CC (s^-1) 0.023 0.032 0.046 0.063 48

Cyclohexyl

phenyl ether TOF_CPE (s^-1) 0.22 0.28 0.33 0.39 26

2-cyclohexylphenol TOF_2-CHP (s^-1) 0.12 0.18 0.24 0.34 46

4-cyclohexylphenol TOF_4-CHP (s^-1) 0.085 0.11 0.16 0.22 49

a Typical conditions: cyclohexene (0.05 mol), phenol (0.05 mol), HBEA-150 (100 mg), decalin (100 mL), 5 MPa H2 (ambient temperature), stirred at 700 rpm; ^b TOF is determined as products formation rates (mol g^–1 s^–1) normalized to the concentration of total BAS (HBEA150). ^c Apparent activation barriers are determined from the Arrhenius plots for TOFs (a directly measured property).

4.3.5 Comparison of alkylation of phenol with cyclohexanol in decalin and in